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Provenance constraints on Early Jurassic evolution of the northern Stikinian arc : Laberge Group, Whitehorse… Johannson, Gary G. 1994

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P R O V E N A N C E C O N S T R A I N T S O N E A R L Y J U R A S S I C E V O L U T I O N O F T H E N O R T H E R N S T I K I N I A N A R C : L A B E R G E G R O U P , W H I T E H O R S E T R O U G H , N O R T H W E S T E R N B R I T I S H C O L U M B I A by G A R Y G . J O H A N N S O N B . S c , The University of British Columbia, 1991 A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S D E P A R T M E N T O F G E O L O G I C A L S C I E N C E S We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F BRITISH C O L U M B I A October 1994 © G a r y G . Johannson, 1994 In presenting t h i s thesis i n p a r t i a l f u l f i l l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or pub l i c a t i o n of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of C-^rWrrJ ^o^^rOh The University of B r i t i s h Columbia Vancouver, Canada u A B S T R A C T The Lower to Middle Jurassic Inklin Formation forms part of the Laberge Group in northwestern B.C. which links the Stikine and Cache Creek terranes along much of the length of the Whitehorse Trough. Ammonite biochronology of the Inklin Formation in the Atlin Lake area constrain the age of the succession there to a range of Early Sinemurian to Late Pliensbachian and provide the temporal framework for analyzing petrofacies and interpreting basin history using paleocurrent data, ternary plots, and conglomerate clast age and provenance. Inklin Formation sediments record a complex arc-basin evolution during the Early Jurassic characterized by episodic flank uplifts, rejuvenated volcanism, and strike-slip influence on sedimentation. Strong temporal trends in both paleocurrent patterns and sandstone-conglomerate petrofacies are evident and allow definition of three discrete phases in basin-fill history. QFL, QmPK, and QpLvLs sandstone ternary plots, conglomerate clast trends and provenance, paleocurrent patterns and measured sections together indicate a regime of stable tectonics characterized by relative volcanic quiescence and low sedimentation rates prevailed through most or all of the Sinemurian. Sinemurian sandstone and conglomerate petrofacies record a transitional arc provenance derived from an older volcanic pile, flanking coastal sediments, and arc roots to the southwest. Progressive incision of the Late Triassic arc initiated in latest Triassic or earliest Jurassic time transported predominantly Upper Triassic elastics of mixed volcanoplutonic and sedimentary provenance initially eastward into the basin via turbidity currents where sediments were dispersed in a generally northwesterly direction, reflecting longitudinal fan-systems prograding north/northwesterly along the axial trend of the basin. Pliensbachian petrofacies record two episodes of distinct provenance and indicate the establishment of a new sedimentation regime. Lower Pliensbachian sandstone and conglomerate petrofacies indicate progressive arc dissection was reversed by a major magmatic episode accompanied by widespread rejuvenated volcanism in the arc segment, causing a strong provenance shift to volcanogenic sources. The volume of broadly coeval volcaniclastic debris deposited during this sub-stage delineates a distinct depositional episode in Early Jurassic basin-fill history and demonstrates provenance from a nearly undissected magmatic arc. i i Ill Early Pliensbachian paleocurrent patterns record a fundamental change in paleoflow systems from predominantly southwest-derived longitudinal paleoflow to opposed bidirectional radial or transverse paleoflow systems. The initiation of southwest-directed paleoflow systems during the Early Pliensbachian indicates a change in basin morphology with the development of an outer basin margin and may represent an ephemeral or episodic ridged forearc phase in Early Jurassic basin history. Late Pliensbachian sediments record a new phase of arc evolution in which active tectonics replace volcanism as the first-order controls on sedimentation. Sandstone and conglomerate petrofacies record an abrupt shift from a volcanogenic provenance in Early Pliensbachian time to a mixed volcanoplutonic provenance dominated by granitic material in the Late Pliensbachian, indicating a rapid transition to a dissected arc provenance. Accelerated uplift of segments of the arc massif accompanied major intra-arc strike-slip faulting that led to rapid arc dissection and exhumation of comagmatic plutons. The resulting influx of granitic detritus delineates another distinct depositional episode in Early Jurassic basin-fill history and includes a significant component derived from Pliensbachian plutons (U-Pb: 186.6 +0.5/-1.0 Ma) to the southwest, underlining dramatic rates of uplift and arc incision during this time. High sedimentation rates prevailed and provided the impetus for a major progradational-aggradational pulse towards the northeast which subdued outer margin topography and changed former (Early Pliensbachian) bidirectional paleoflow systems to southwest-derived paleoflow systems. i i i iv T A B L E O F C O N T E N T S Abstract i i Table of Contents iv List of Figures vi i List of Tables x List of Appendices x i Acknowledgements x i i C H A P T E R 1 I N T R O D U C T I O N 1 1.1 Location of the Study Area 1 1.2 Previous Work 3 1.3 Objectives 4 1.1 Methods 5 1.5 Regional Geological Setting 6 1.5.1 Introduction 6 1.5.2 Stikine Terrane 6 1.5.3 Cache Creek Terrane 9 1.5.4 Nisling Terrane 10 1.5.5 Laberge Group 11 C H A P T E R 2 B I O C H R O N O L O G Y A N D B I O S T R A T I G R A P H Y 15 2.1 Introduction 15 2.2 Ammonite Biochronology 15 2.3 Biochronology at Atlin Lake 19 2.4 Geochronology 33 2.5 Paleobiogeography 36 2.6 Discussion 40 2.6.1 Ammonite Taxonomy 40 2.6.2 General Discussion 43 P A R T 2: A N A L Y S I S OF FOSSILIFEROUS C O N G L O M E R A T E C L A S T S 46 2.7 Introduction 46 2.8 Lithological and Faunal Descriptions 48 2.9 Discussion and Conclusions 57 2.9.1 Sedimentology and Paleoenvironmental Interpretations 57 2.9.2 Tectonic Implications 59 C H A P T E R 3 C O N G L O M E R A T E A N A L Y S I S 61 3.1 Introduction 61 3.2 Methods 62 3.3 Sinemurian Conglomerate 67 3.3.1 General Description and Interpretation 67 3.3.2 Clast Composition 74 V 3.4 Lower Pliensbachian Conglomerate 79 3.4.1 General Description and Interpretation 80 3.4.2 Clast Composition 83 3.5 Upper Pliensbachian Conglomerate 87 3.5.1 General Description and Interpretation 87 3.5.2 Clast Composition 89 3.6 Provenance 91 3.7 Conglomerate Petrofacies 94 3.8 Temporal Trends: Tectonic Implications 97 CHAPTER 4 SANDSTONE PETROGRAPHY 104 4.1 Introduction 104 4.2 Methods 105 4.3 Detrital modes: Introduction 110 4.4 Sinemurian Sandstone 110 4.4.1 Quantitative Detrital Mode Data 110 4.4.2 Qualitative Petrographic Observations and Interpretations 111 4.4.3 Discussion 113 4.5 Lower Pliensbachian Sandstone 122 4.5.1 Quantitative Detrital Mode Data 122 4.5.2 Qualitative Petrographic Observations and Interpretations 123 4.5.3 Discussion 126 4.6 Upper Pliensbachian Sandstone 128 4.6.1 Quantitative Detrital Mode Data 128 4.6.2 Qualitative Petrographic Observations and Interpretations 129 4.6.3 Discussion 131 4.7 Petrofacies Summary 133 4.7.1 Sinemurian 133 4.7.2 Lower Pliensbachian 134 4.7.3 Upper Pliensbachian 135 4.8 Provenance 136 4.8.1 Q F L Plots 136 4.8.2 QmPK, QpLvLs Plots 137 4.9 Discussion 151 CHAPTER 5 PALEOCURRENT ANALYSIS 154 5.1 Introduction 154 5.2 Methods 158 5.3 Paleocurrent Data 161 5.3.1 Sinemurian 161 5.3.2 Lower Pliensbachian 164 5.3.3 Upper Pliensbachian 166 5.4 Discussion 169 5.4.1 Sinemurian 169 5.4.2 Lower Pliensbachian 170 5.4.3 Upper Pliensbachian 175 5.5 Summary and Conclusions 178 vi CHAPTER 6 DI AGENESIS AND THERMAL HISTORY 179 6.1 Introduction 179 PART 1: T H E R M A L HISTORY 180 6.2 Introduction 180 6.3 Methods 180 6.4 Thermal Maturation Data 181 PART 2: DIAGENESIS 184 6.5 Introduction 184 6.6 Diagenetic Framework 186 6.7 Shallow Burial Diagenesis 189 6.8 Intermediate/Deep Burial Diagenesis 189 6.8.1 Authigenic Feldspar 189 6.8.2 Laumontization 193 6.8.3 Late-stage Calcite 194 6.8.4 Chloritization 195 6.9 Discussion 204 CHAPTER 7 DISCUSSION AND CONCLUSIONS 208 7.1 Biochronology 208 7.2 Sedimentology and Stratigraphy 210 7.2.1 Provenance and Basin Setting 210 7.2.2 Formation Thickness: 213 7.3 Paleotectonic Reconstruction 215 7.3.1 Sinemurian 215 7.3.2 Early Pliensbachian 216 7.3.3 Late Pliensbachian 217 7.4 Terrane Linkages 218 7.5 Speculations on the Whitehorse Trough '. 221 7.6 Summary 225 REFERENCES 226 APPENDICES 236 PLATES 276 1 - 8 Ammonite Taxa 276 9 -10 Benthonic (Bivalve) Taxa 295 vii L I S T O F F I G U R E S Figure 1.1. Map showing location of study area and regional setting of the Whitehorse Trough 2 Figure 1.2. Distribution of the five morphogeological belts of the Canadian Cordillera 7 Figure 1.3. Generalized geological map of the northern Canadian Cordillera 8 Figure 1.4. Generalized stratigraphy of the Whitehorse trough in the Atlin Lake area 11 Figure 1.5. Interpreted stratigraphic relationships and terrane linkages of the Intermontane Belt 13 Figure 2.1. Ammonite zonal scheme for the Pliensbachian of North America 17 Figure 2.2. Regional assemblage zones for the Sinemurian of the Queen Charlotte Islands 18 Figure 2.3. Location map for fossil (ammonite) sites 21 Figure 2.4.1. Early Sinemurian (Griffith Island) measured stratigraphic section 24 Figure 2.4.2. Late Sinemurian (southeast Atlin Lake) measured stratigraphic section 25 Figure 2.4.3. Pliensbachian (Sloko Island) measured stratigraphic section 26 Figure 2.4.4. Pliensbachian (Sloko Island) measured stratigraphic section 30 Figure 2.5. Concordia plots for radiometric (U:Pb) age determinations 35 Figure 2.6. Sinemurian paleobiogeographical ammonite faunal realms 37 Figure 2.7. Pliensbachian paleobiogeographical ammonite faunal realms 38 Figure 2.8. Composite depth assemblage zones of Taylor (1982) 59 Figure 3.1. Location map of Inklin Formation pebble-count sites 64 Figure 3.2. Sinemurian Type 1 debris flow conglomerate 68 Figure 3.3.A. Plastically-deformed intraclast in Sinemurian Type 1 debris flow conglomerate 70 Figure 3.3.B. Sinemurian Type 1 conglomerate facies association 70 Figure 3.4. Clast fabric and grading in Sinemurian Type 1 conglomerates 72 Figure 3.5. Sinemurian Type 2 conglomerate 73 Figure 3.6. Generalized facies model for resedimented conglomerates 73 Figure 3.7. Temporal clast trends in Inklin Formation conglomerates at Atlin Lake 76 Figure 3.8. Sinemurian conglomerate clast content 77 Figure 3.9 Pliensbachian Type 3 pebble conglomerate 81 Figure 3.10.A. Inverse to normally graded Lower Pliensbachian Type 4 conglomerate 84 Figure 3.10.B Lower Pliensbachian volcanic conglomerate clasts 84 Figure 3.11. Pliensbachian Type 3 conglomerate clast content 86 Figure 3.12.A. Olistolith in Upper Pliensbachian Type 1 debris flow conglomerate 90 Figure 3.12.B. Upper Pliensbachian granitoid cobble-boulder conglomerate 90 Figure 3.13. Conglomerate petrofacies fields for Inklin Formation stratigraphic sub-sets 95 Figure 3.14. Ternary temporal clast trend diagram of Sinemurian conglomerate beds 98 Figure 3.15. Ternary temporal clast trend diagram of Lower Pliensbachian conglomerate beds 99 Figure 3.16. Ternary temporal clast trend diagram of Upper Pliensbachian conglomerate beds 100 Figure 3.17. Ternary temporal clast trend diagram of stratigraphic sub-set means of Inklin Formation conglomerate 101 Figure 4.1. Sandstone classification scheme 107 Figure 4.2. A - C : Photomicrographs of volcanic quartz grains 114 Figure 4.3. A . Photomicrographs of plutonic quartz grains 115 Figure 4.4A-B. Photomicrographs of polycrystalline quartz aggregate 116 Figure 4.4C: Photomicrograph of perthitic orthoclase grain in Upper Pliensbachian sandstone 116 Figure 4.5. Photomicrographs of microlitic textures in andesitic lithic grains in Sinemurian sandstone 117 Figure 4.6A-B. Photomicrographs of volcanic lithic grains 118 Figure 4.7. Photomicrographs of granitoid lithic fragments 119 Figure 4.8. Photomicrographs of granitic lithic fragment in Upper Pliensbachian sandstone 120 Figure 4.9.A.B. Photomicrographs of oscillatory zoning in plagioclase phenocryst in volcanic lithic grain 125 Figure 4.9C. Photomicrograph of porphyritic rhyodacite lithic fragment 125 Figure 4.10A. Photomicrograph of granitic lithic grain in Upper Pliensbachian sandstone 130 Figure 4.10B. Photomicrograph of granophyric texture in Pliensbachian monzogranite clast 130 Figure 4.11. QFL ternary plot of Sinemurian sandstone detrital modes 139 Figure 4.12. Q F L ternary plot of Lower Pliensbachian sandstone detrital modes 140 Figure 4.13. Q F L ternary plot of Upper Pliensbachian sandstone detrital modes 141 Figure 4.14. QFL ternary plot of mean detrital modes for Inklin Formation sandstone suites 142 Figure 4.15. QmPK ternary plot of Sinemurian sandstone partial detrital modes 143 Figure 4.16. QmPK ternary plot of Lower Pliensbachian sandstone partial detrital modes 144 Figure 4.17. QmPK ternary plot of Upper Pliensbachian sandstone partial detrital modes 145 Figure 4.18. QmPK ternary plot of mean partial modes for Inklin Formation sandstone suites 146 Figure 4.19. QpLvLs ternary plot of Sinemurian sandstone partial detrital modes 147 .Figure 4.20. QpLvLs ternary plot of Lower Pliensbachian sandstone partial detrital modes 148 Figure 4.21. QpLvLs ternary plot of Upper Pliensbachian sandstone partial detrital modes 149 Figure 4.22. QpLvLs ternary plot of mean partial modes for Inklin Formation sandstone suites 150 Figure 5.1. Generalized submarine fan model of Walker (1978) 156 Figure 5.2.. Facies classification scheme for deep-marine sediments 157 Figure 5.3. Turbidite (Bouma) sequence model 158 Figure 5.4.. Ripple cross-lamination in T c e and T D ce beds 159 Figure 5.5. Sinemurian paleocurrent patterns 163 Figure 5.6A. Mega-ripple foresets in Lower Pliensbachian composite sandstone bed 165 Figure 5.6B. Facies class D sedimentary sequence 165 Figure 5.7. Early Pliensbachian paleocurrent patterns 167 Figure 5.8. Late Pliensbachian paleocurrent patterns 168 Figure 5.9. Flume simulation of a natural density current reflection 174 Figure 5.10. Sloko Island Kunae Zone (Late Pliensbachian) paleoflow vectors 177 Figure 6.1. Plot of Inklin Formation thermal samples: ^ values versus increasing age 183 Figure 6.2. Schematic diagenetic sequence for arc-derived sandstones 188 Figure 6.3A. Early diagenetic calcareous concretions in Pliensbachian sandstone 190 Figure 6.3B. Laumontized Lower Pliensbachian sandstone 190 Figure 6.4. Photomicrographs of albite overgrowths and albitization on feldspar grains 196 Figure 6.5. Photomicrographs of quartz and feldspar overgrowths 197 Figure 6.6. Photomicrograph of complex replacive diagenetic fabrics on plagioclase grain 198 Figure 6.7. Photomicrograph of impure K-spar replacive cement 199 Figure 6.8. Photomicrographs of preferential and complete calcite replacements of feldspar grains in Sinemurian sandstone 200 Figure 6.9. Photomicrographs of calcite pore-fill in Upper Pliensbachian sandstone 201 Figure 6.10. Photomicrographs of multiple diagenetic fabrics in Sinemurian sandstone 202 Figure 6.11. Photomicrographs of cryptocrystalline intergrowths of microspar and chlorite 203 Figure 6.12. Generalized paragenetic sequence of authigenic mineral phases in Inklin Formation sandstones 206 Figure 6.13. Generalized schematic depiction of authigenic mineral phases in Inklin Formation sandstones by stratigraphic interval 206 Figure 7.1. Generalized stratigraphic column of the Inklin Formation at Atlin Lake 209 Figure 7.2. Schematic depiction of hypothesized tectonic setting for the Early Jurassic northern Whitehorse Trough 212 Figure 7.3. Schematic depiction of Laberge Group stratigraphy at Atlin Lake 220 Figure 7.4. Morphotectonic features of forearc basin types 222 Figure 7.5. Ridged forearc basin types 222 L I S T O F T A B L E S Table 2.1. A . Ammonite biostratigraphy of the Inklin Formation: Atlin Lake 22 Table 2.I.B. Ammonite biostratigraphy of the Inklin Formation: Atlin Lake 23 Table 2.2. Ammonite and bivalve identifications of Inklin Formation fossiliferous conglomerate clasts 47 Table 3.1. Inklin Formation conglomerate pebble-count data 65 Table 3.2. Inklin Formation conglomerate types 66 Table 4.1. Inklin Formation sandstone point-count data 108 Table 4.2. Inklin Formation sandstone recalculated point-count data 109 Table 6.1. Thin-section age assignments and % diagenetic calcite 185 xi L I S T O F A P P E N D I C E S Appendix 1. Fossil I.D.s, GSC loc. nos. & U T M coordinates for Atlin Lake fossil collections 236 Appendix 2. Fossil I.D.s, GSC loc. nos. & U T M coordinates for Atlin Lake clast collections 240 Appendix 3. Inklin Formation Conglomerate Clast Lithologies 241 Appendix 4. Paleocurrent Data 244 Appendix 5. Thermal Data 267 Appendix 6. Field Station Map 273 Appendix 7. Miscellaneous Sample List and U T M Coordinates 274 X l l A C K N O W L E D G E M E N T S The interest and assistance of many people have contributed to the completion of this thesis project. First, I would like to express special appreciation to my thesis supervisor, Dr. Paul Smith, for providing the opportunity and financial support to undertake this project. I am especially grateful for the latitude which Paul allowed me in shaping the direction of this thesis, in spite of any unvoiced misgivings he may have entertained concerning certain avenues of study well outside his research interests. His support, patience and enthusiasm for the project have contributed greatly and helped make the experience a rewarding one, both intellectually and personally. I owe a great deal to Dr. Steve Gordey of the Geological Survey of Canada for providing the logistical and financial support to conduct fieldwork during two summers. His support and assistance in other matters, as well as the challenge of thought-provoking discussion, is also appreciated. Dr. Howard Tipper's contributions warrant special mention; first, for suggesting the study area and introducing me to the regional geology, and secondly, for providing many hours of fascinating geological and paleontological discussion. M y development as a geologist has been greatly enhanced by the experience. I benefitted from many stimulating discussions with Craig Hart of the Canada-Yukon Geoscience Office on the geology of the Laberge Group. His interest and assistance, including the sharing of some unpublished geochronological data, is greatly appreciated. Mitch Mihalynuk's (BCGS) contribution of thoughts, suggestions and maps is also appreciated. I would like to thank Dr. William Barnes (U.B.C.) and Dr. Brian Ricketts (G.S.C.) for their critical reviews of portions of this manuscript. Dr. R . M . Bustin (U.B.C.) is thanked for analyzing thermal samples for vitrinite reflectance. I would like to extend a special note of appreciation to my colleagues in the Jurassic Biostratigraphy group at the University of British Columbia. Genga Nadaraju, Jozsef Palfy, and Bo Liang were all generous with their time and expertise. Genga and Jozsef were most helpful with confirmations of ammonite identifications and also provided stimulating discussion on varied paleontological and geological topics. Bo was of considerable assistance in solving computer problems and kindly provided access to the A M M O N data-base. Financial support for this project came from Natural Science and Engineering Research Council of Canada grants to Dr. P.L. Smith, and the Energy, Mines, and Resources Ministry of the federal goverment through Dr. S.P. Gordey. Finally, I reserve my deepest expression of gratitude for Kelly, who not only provided superb field assistance in 1993 but also endured endless hours of thesis preparation with patience, love and support. INTRODUCTION 1 C H A P T E R 1 I N T R O D U C T I O N 1.1 L O C A T I O N O F T H E S T U D Y A R E A The location of the study area is found in the Atlin Lake region of northwestern British Columbia (Fig. l.l).The study area itself lies mainly within the boundaries of Atlin Lake Wilderness provincial park in the southern part of the lake and covers over 500 square kilometres, much of which is covered by the lake. The area covered by the study is largely contained within the 1:50,000 Teresa Island (104 N/5) and Sloko Lake (104 N/4) map sheets and extends westward into the southwest corner of the Edgar Lake (104 M/8) map sheet. Access to the region and town of Atlin is by vehicle along the gravel Atlin Lake road from a junction at Jake's Corner on the Alaska Highway roughly 200 kilometres east of Whitehorse. There are no roads within park boundaries; consequently, access to the study area is by boat, floatplane or helicopter. Atlin Lake claims the distinction of being the largest natural lake in B.C. and is a narrow body of water some 127 kilometres (85 miles) long and 3 to 8 kilometres (2-5 miles) wide. The lake sits at an elevation of 677 metres (2200 feet), and contains some large, and many small, islands. The study area lies within the Intermontane morphogeological Belt and is rugged, mountainous country. The heavily glaciated Boundary Ranges of the Coast Mountains flank the lake along its southwest margin with the Sloko and Johnson Ranges flanking the eastern lake margin. The Llewellyn Glacier extends from the southwest tip of Atlin Lake almost to the coast at Juneau, Alaska, making it one of the largest icefields on the continent. The lake imposes considerable physical challenge on those working from small boats due to strong winds and sudden windstorms which are a common summertime occurrence, and can generate waves in excess of 5 feet high. Extensive exposures of Laberge Group strata are found along the shores of southern Atlin Lake and its numerous islands, where low- to high-relief (few metres to tens of metres) outcrop form small bluffs and cliffs. The outcrop is often fronted by rocky beaches, especially early in the field-season when lake Figure 1.1. Map showing location of study area and regional setting of the Whitehorse Trough (Inklin Formation) in the Atlin Lake area (modified after Wheeler et al., 1991). INTRODUCTION 3 waters are at their lowest, which provide excellent access to the rocks. Details of the shore-line strata are well-exposed due to the action of wind and waves, making the area particularly well-suited to sedimentological study. Away from the shores outcrop is limited and generally poorly exposed due to extensive bush cover. 1.2 P R E V I O U S W O R K The earliest recorded geological observations of the Whitehorse trough were made by G . M . Dawson in 1887 during an expedition up the Yukon River (Dawson, 1889). The Klondike Gold Rush of 1896 and a major gold discovery on Pine Creek in the Atlin area in 1898 provided the impetus for the early geological investigations that followed. Extensive reconnaissance mapping was conducted in the region during the early part of the century, as well as evaluation of mineral properties, particularly in the southern Yukon. Gwillim's (1901) extensive reconnaissance in the Atlin area resulted in the first geological map of northwestern British Columbia that includes the Whitehorse trough. The Geological Survey of Canada figures prominantly in these early efforts (e.g., McConnell, 1906, 1909; Cairnes, 1906, 1908, 1910, 1912, 1913; Cockfield and Bell; 1926) which led to publication of the first geological map of the Whitehorse Mining District (Cockfield and Bell, 1926). Studies of the regional geology in and around the Whitehorse Trough continued through the 1930's, 1940's, 1950's, and early 1960's, resulting in a number of bedrock maps and geological reports (e.g. Bostock, 1936; Lees, 1934; Bostock and Lees, 1938; Cockfield and Bell, 1944; Aitken, 1959; Wheeler, 1961; Mulligan, 1963). The last major mapping effort involving the Whitehorse Trough by the Geological Survey of Canada was conducted south of the study area in the Tulsequah region by Souther (1971). In the late 1970's more detailed geological mapping was carried out in the Atlin and Tagish Lakes area by Bultman (1979) as part of a Ph.D. thesis. More recently, a new mapping initiative by the British Columbia Geological Survey began in the northwest region of B.C. during the late 1980's. Some of this work involved regional mapping of Whitehorse Trough strata in adjacent regions outside the park boundaries and has resulted in the publication of a number of 1:50,000 open file maps and field reports (e.g., Mihalynuk and Rouse, 1988; Mihalynuk and Mountjoy, 1990; Bloodgood and Bellefontaine, 1990; Mihalynuk et al., 1992). INTRODUCTION 4 1.3 OBJECTIVES A thorough basin analysis involves numerous components of study in order to fully characterize the depositional history of a particular basin. Elements of basin analysis include such diverse topics as biochronology, stratigraphy, structure, sedimentology, provenance, thermal maturation, and diagenesis. The most important product of basin analysis is the documentation of the paleogeographic evolution of the sedimentary basin of interest (Miall, 1984). In this study, the author has utilized many of these approaches in order to characterize the depositional history of the Whitehorse Trough and unravel the tectonic evolution of the arc/basin system. In particular, the following objectives are addressed with regard to deposition of the Inklin Formation: 1) Define the biochronology of the Inklin Formation at Atlin Lake by studying its ammonites and use the results to establish a biostratigraphic framework for provenance studies. 2) Utilize Inklin Formation conglomerates to define and interpret temporal clast trends in the context of arc evolution. 3) Attempt specific provenance linkages through study of the petrology of conglomerate clasts. 4) Define and interpret the depositional history of the Inklin Formation through petrographic study of sandstones. 5) Use paleocurrent indicators to establish regional paleoflow patterns to aid in provenance determinations and to interpret the geometry and orientation of sediment dispersal systems. 6) Establish a framework of thermal and diagenetic history through thermal maturation studies and petrographic analysis of authigenic minerals. 7) Characterize and define major aspects of Early Jurassic arc evolution using the results of this study and attempt to reconcile fundamental differences in basin classification between the Whitehorse Trough in the study area and in the south-central Yukon (i.e., backarc (Bultman, 1979) versus forearc (e.g., Tempelman-Kluit, 1979) respectively). ) INTRODUCTION '5 1.4 METHODS Prior to the commencement of fieldwork in June 1992 the author conducted a literature search for relevent information on the Whitehorse Trough and Lower Jurassic ammonites. Information was gathered from a variety of sources including Geological Survey of Canada publications (memoirs, current research, open-file maps), British Columbia Geological Survey publications (fieldwork reports, open-file maps), general literature (e.g., journals) and thesis work. A complete air photo reconnsaissance of the study area was conducted for the purpose of assessing potential section measurement localities and identifying probable faults. Fieldwork was conducted during two summers; three months in 1992 and a month of follow-up work in 1993. A n inflatable Zodiac boat with outboard engine provided transport and access to extensive outcrops along the shores and many islands of the lake. The mapping approach was primarily a detailed one with reconnaissance or regional mapping restricted to areas of poor outcrop exposure or limited access. A primary emphasis on sedimentology concentrated data collection on pertinent sedimentological details that include measurement of paleocurrent indicators, bed thickness trends and geometry, grain-size and composition, sedimentary structures (e.g. ripple cross-lamination, sole marks, loads) and textures (e.g., sorting, imbrication), conglomerate pebble-counts, clast-size determinations (maximum and modal), and clast collections. A fundamental component of this study was the establishment of a high-resolution biostratigraphic framework; consequently, significant efforts were expended locating and collecting ammonites. In particular, the location of ammonites was a prerequisite for detailed section measurements. Additional fieldwork objectives included collection of carbonaceous material for thermal maturation studies, sampling of appropriate lithologies for micro-fossils and collection of rock samples for radiometric (U-Pb) dating. Laboratory studies initially concentrated on confirming ammonite identifications made in the field and, where preservation allowed, refining identifications to the species level. A n emphasis on provenance directed ensuing laboratory studies into thin-section analysis of modal mineralogy (point-counts) and qualitative provenance indicators using a petrographic microscope, and petrological study of conglomerate clast collections. INTRODUCTION 6 1.5 REGIONAL GEOLOGICAL SETTING 1.5.1 INTRODUCTION Regionally, the study area is found within the northern Intermontane Belt which narrows to the north and essentially terminates in the west-central Yukon with the exception of a thin sliver which extends northwestward into Alaska (Fig. 1.2). The northern termination is enveloped by rocks of the Omineca Belt, while to the west plutonic and metamorphic rocks of the Coast Belt mark the western boundary of the Intermontane Belt. In the study area the Intermontane Belt is represented by the Stikine terrane which narrows to roughly 35 kilometres wide and the Cache Creek (Atlin) terrane. In the immediate vicinity of Atlin Lake are four major tectonostratigraphic elements, consisting of the northern Cache Creek terrane to the east, Stikine and Nisling terranes to the west, and the Laberge Group of the Whitehorse trough. The Whitehorse Trough is an arc-marginal marine basin of Mesozoic age that forms a northwest-trending synclinorium, extending 700 kilometres from the south-central Yukon into northern British Columbia (Fig. 1.3). The Whitehorse Trough and associated arc, the Late Triassic Stikinian arc (i.e., Stuhini Group in B.C. - Lewes River Group in the Yukon), comprise northern Stikinia. The Whitehorse Trough delimits the eastern extent of Stikinia along its western margins in the northern Canadian Cordillera, where its strata overlie the major tectonic boundary between the Stikine and Cache Creek terranes. 1.5.2 STIKINE TERRANE The northern segment of Stikinia differs significantly from southern portions of this accreted arc terrane. In the south and central regions of the terrane, Stikinian basement rocks are formed of Paleozoic volcanic and marine sedimentary successions (Stikine Assemblage) which are overlain by Triassic to Jurassic volcanosedimentary arc assemblages. However, Stikinia north of 59° latitude apparently lacks the Paleozoic basement and is comprised entirely of Late Triassic and younger arc volcanics, plutonic roots, and arc-marginal sediments (e.g., Hart et al., in press). Rocks of the Stikine terrane lie along the western boundary of the Laberge Group, where they are found mainly in structural contact along two main regional faults; the King Salmon Fault for most of the British Columbian portions of the Whitehorse Trough, and the Llewellyn Fault for northernmost British INTRODUCTION 7 - 4 9 ° G S C EXPLANATION Accreted superterranes Pericratonic and displaced terranes Figure 1.2. Distribution of the five morphogeological belts of the Canadian Cordillera (from Gabrielse et al., 1991). INTRODUCTION 8 Figure 1.3. Generalized geology of the northern Canadian Cordillera. The Whitehorse Trough (Laberge Group) straddles the B.C. - Yukon border in the N W quadrant of the province (modified after Wheeler and McFeely, 1991). INTRODUCTION 9 Columbia and parts of the southern Yukon (Fig. 1.3)(Wheeler and McFeely, 1991). The Llewellyn Fault is a major dextral transcurrent extension of the King Salmon fault (Mihalynuk and Rouse, 1988) and forms the regional basin-bounding structure in the study area. Stikinia is comprised of Upper Triassic volcanic and clastic rocks of the Stuhini Group in the Atlin Lake area. Stuhini Group equivalents north of the British Columbia-Yukon border are known as the Lewes River Group (Wheeler, 1961). The thickness of the Stuhini succession is quite variable in the study area with maximum thickness estimated to be in the range of 3000 to 3500 metres (Bultman, 1979; Mihalynuk and Rouse, 1988). Units of the Stuhini belt in northern British Columbia have been correlated with similar Stuhini Group rocks to the south in the Tulsequah area (Bultman, 1979) and consist of a succession that spans the range between mafic submarine volcanics, to coarse arc-derived alluvial (to marginal-marine) elastics and shallow-marine carbonates (Bultman, 1979). The Stuhini Group unconformably overlies pre-upper Triassic metamorphic and plutonic rocks of the Coast belt. The Stuhini Group in the study area consists of a basal unit dominated by sandstone and conglomerate of mixed volcanic, plutonic and metamorphic provenance, succeeded by a transitional volcanogenic clastic unit which is in turn overlain by a thick volcanic and volcaniclastic unit of intermediate to basic composition. (Units A , A B , and B respectively of Bultman, 1979). The upper divisions of the Stuhini Group are formed of a distinctive olive green to maroon volcanic unit (Unit C of Bultman, 1979). The Stuhini group is unconformably overlain by Upper Triassic (Norian) shallow marine carbonates that form a prominent belt of outcrop along the southwest margin of Atlin Lake. These carbonates (up to 300 metres thick) have been correlated with the Norian Sinwa Formation to the south (Bultman, 1979) and are included as part of the Stuhini Group. 1.5.3 C A C H E C R E E K T E R R A N E The Cache Creek Terrane (Monger et al., 1982) represents a major tectonostratigraphic entity of the Intermontane Belt that is characterized by rocks of oceanic affinity. The terrane is found along the eastern boundary of Stikinia throughout much of the length of British Columbia and the southern Yukon. The northern Cache Creek Group is known as the Atlin terrrane. There, greater stratigraphic continuity is preserved than in southern British Columbia. INTRODUCTION 10 Monger (1975) has described the stratigraphy of the most complete section (Nakina subterrane) known in the Cache Creek Terrane. Monger's (1975) stratigraphic succession consists of a basal unit composed dominantly of Mississippian to Pennsylvanian oceanic basalts (Nakina Formation). This formation is overlain by and interfingers with radiolarian chert, clastic sediments, minor carbonates, and volcanics of the Kedahda Formation. The upper unit is comprised of a thick (~ 2000m) shallow-marine carbonate sequence (Horsefeed Formation) of Late Mississipian to Late Permian age. Locally, Triassic to Lower or Middle (?) Jurassic radiolarian chert and elastics unconformably overlie the carbonates (Monger et al., 1991; Cordey et al., 1991). Ultramafic rocks that are generally spatially associated with Nakina Formation basalts form linear bodies that range from a few metres to tens of kilometres in length, placing them among the largest ultramafite occurrences in British Columbia (Wheeler and McFeely, 1991). These bodies have been interpreted as oceanic basement to the Nakina Formation emplaced by thrust faulting (Monger, 1977; Ash and Arksey, 1990). The Atlin terrane is characteristically fault-bounded (Monger, 1975); to the southwest the Nahlin Fault marks the tectonic boundary with the Stikine terrane, onto which it was thrust in Middle or Late Jurassic time (Monger, 1984), mainly along this major regional fault. The Nahlin fault forms the northeast boundary of the Laberge Group along most of the length of the Whitehorse Trough, including the study area (Fig. 1.3). 1.5.4 NISLING T E R R A N E West of Stikinia, the Nisling Terrane forms a northwest-trending belt that extends from south of Atlin Lake into the west-central Yukon. The Nisling Terrane is a metamorphosed continental margin assemblage in fault contact with the western boundary of Stikinia. In British Columbia Nisling rocks are found in fault contact with Laberge Group strata north of the study area, where rocks of the Stikine terrane structurally pinchout along the Llewellyn Fault. The Boundary Ranges metamorphic suite, which forms part of the Nisling assemblage, appears to be unconformably overlain by Laberge group strata in northwestern British Columbia (Mihalynuk and Rouse, 1988). INTRODUCTION 11 1.5.5 L A B E R G E G R O U P The Laberge Group overlies the calc-alkaline arc assemblage of the Late Triassic Stuhini Group through most of the length of the Whitehorse trough in B.C. In the Yukon the Laberge Group overlies the Lewes River Group (equivalent to the Stuhini Group) (Wheeler, 1961, Tempelman-Kluit, 1985). South of the study area, in the Tulsequah region and elsewhere, the Inklin Formation disconformably overlies carbonates of the Lake Triassic Sinwa Formation (Souther, 1971;) which forms the upper division of the Stuhini Group along much of the western Whitehorse trough. The generalized stratigraphic relationships of the Laberge Group in the study area are shown in Figure 1.4. E A R L Y T E R T I A R Y S L O K O G R O U P Volcanogenic rocks of andesitic to rhyolitic composition: includes volcanic flows, breccias, tuffs, ignimbrites, minor conglomerate and volcanic sandstone. Angular unconformity E A R L Y J U R A S S I C L A B E R G E G R O U P Inklin Formation: Feldspathic to lithic sandstone (sub-wacke, greywacke, and arenite), dacitic to rhyodacitic tuff and resedimented pyroclastic debris, rhythmic thin-bedded siltstone, argillite and argillaceous mudstone, minor conglomerate. Disconformity L A T E TRIASSIC STUHINI G R O U P Undifferentiated units of intermediate to basic volcanic flows, breccia, tuff, agglomerate, and volcaniclastic conglomerate, sandstone, minor argillite. Succeeded in upper divisions by micritic shallow-marine carbonate of Norian age ('Sinwa Formation'). Figure 1.4. Generalized stratigraphy of the Whitehorse trough in the Atlin Lake area. INTRODUCTION 12 Cairnes (1910) originally used the term Laberge Group to denote a northwest-trending belt of conglomerate, sandstone and argillite in the Whitehorse trough. In the Tulsequah area, Souther (1971) recognized two distinct facies and subdivided the Laberge Group in northern B.C. into two constituent formations; 1) the Takwahoni Formation, a proximal shallow-marine volcanosedimentary succession, and 2) the Inklin Formation, a deep marine clastic succession. Although the two formations are broadly interpreted as nearshore-offshore facies equivalents (Souther, 1971) they appear to be only partly coeval. Basal Inklin Formation stata is biostratigraphically constrained at an age of Early (?) Sinemurian in the Tulsequah area (Souther, 1971) and Early Sinemurian the Atlin Lake area (Mihalynuk and Mountjoy, 1990; Johannson, 1993); however, ammonites collected from basal strata of the Takwahoni Formation are of Pliensbachian age (Frebold, 1964; Souther, 1971; Smith et al., 1988). Uppermost strata of the Takwahoni Formation in the Tulsequah area contain ammonites of Bajocian (Middle Jurassic ) age (H.W. Tipper - pers, comm., 1993). Previous biostratigraphic determinations of Laberge Group stata in northwestern B.C. and south-central Yukon have indicated ages that range from Sinemurian to Bajocian; however, the accuracy of much of this earlier work is now suspect and should be considered unreliable (H.W. Tipper - pers. comm., 1994). Consequently, until these earlier ammonites collections are re-examined, the youngest confident upper age constraints for the basinal Inklin facies of the Laberge Group come from more recent work and are Late Pliensbachian (Mihalynuk and Rouse, 1988; Johannson, 1993; C. Hart - pers. comm., 1993). Estimates of the thickness of the Laberge Group have ranged widely since the group was first recognized (e.g., Cairnes, 1912; Cockfield and Bell, 1926; Wheeler, 1961; Souther, 1971). More recent estimates range from ~ 2000 metres (Eisbacher, 1974) to as high as 7000 metres (Bultman, 1979). Thickness is difficult to estimate due to both the tectonic history and lithostratigraphy of the Laberge Group. Structural complications in the form of folding, thrusting, and later (i.e., Eocene) extensional faulting have produced a highly disrupted stratigraphy in many areas of the Whitehorse trough (Bultman, 1979). This, combined with a monotonous and repetitive lithostratigraphy characterized by rapid lateral facies changes and a lack of good marker beds, undermine accuracy and has undoubtedly contributed to INTRODUCTION 13 B Middle \ Jurassic to \ Lower Cretaceous COAST BELT GB 1 N T E R M O N T A N E 1 E L T S T I K I N I A Whitehorse Trough SW of King Salmon Fault - -8 • - " ', C A C H E C R E E K Whitehorse Trough NE of King Salmon Fault WT Cache Creek Terrane :-!'''GC~" mainly southwesterly directed-folds and thrust faults O M I N E C A B E L T OB QUESNELLIA Lower and Middle Jurassic Upper Triassic: Upper Carnian and Norian Upper Middle and (?) Lower Upper Triassic Mississippian to Middle Triassic Lower Paleozoic in OB LABERGE 'VTAKWAHONL' GROUP . KAC'.KS » f i£ ; ; ; ^ r ">c>LNKiL* t F A C j E S ; NAZCHA SHONEKTAW ' SINWA FORMATION STUHINI iGROUP ^_^j-S"s |s s s s s s s s s s s s s s s s ^~r&&sSSS f s s s s s s s s s s s s s s s s s s s s s s s s s j s s s s s s s s s s s s s s s s deformation, blueschist s metamorphic * Triassic '. and (?) older : granites: interval of metamorphism, . deformation and Intrusion O —i < ASSEMBLAGE may include Jurassic strata in SE part of area undifferentiated Paleozoic; older part/distal miogeoclinal; younger part volcanogenic Figure 1.5. Schematic depiction of interpreted stratigraphic relationships and terrane linkages in northwestern British Columbia and south-central Yukon (from Monger et al., 1991). INTRODUCTION 14 the wide range of estimated thickness for the group. In addition to these substantial obstacles, the lack of good biostratigraphic control in previous studies adds further uncertainty to the reliability of estimates. Claims of continuous sections in the range of 3000 to 3500 metres for Laberge group strata have been made (e.g., Bultman, 1979; Dickie, 1989) although the author is unaware of any documented section measurements to support these claims; however, the Laberge Group is unquestionably a thick sedimentary succession that appears to exceed 3 kilometres and is likely substantially thicker. The Laberge Group is of particular tectonic and stratigraphic importance in the northern Cordillera as it is widely interpreted to represent an overlap assemblage that stratigraphically links Late Triassic and older arc assemblages of the Stikine Terrane to the west with Paleozoic to Early Jurassic oceanic assemblages of the Cache Creek Terrane to the east (e.g. Wheeler, et al, 1988; Gabrielse and Yorath; 1992). In the southern Whitehorse trough the two constituent formations of the Laberge Group are interpreted to represent two different provenance sources; 1) the Takwahoni Formation unconformably overlies predominantly Upper Triassic rocks of the Stikinian arc (i.e., Stuhini Group) and is apparently wholly derived from the west, and 2) the Inklin Formation, which is interpreted to overlie mainly Cache Creek basement and believed to be derived predominantly from the east and partly from the west (Fig. 1.5) (e.g., Thorstad and Gabrielse, 1986; Gabrielse and Yorath; 1992). Although there is some evidence to support this interpretation in southern portions of the Whitehorse trough (e.g., Thorstad and Gabrielse, 1986), there is no documented evidence to support a similar interpretation in the northern Whitehorse trough. Consequently, an understanding of the age, sedimentology and stratigraphy of the Inklin Formation is essential in deciphering Cache Creek, Stikine, and Quesnel (?) terrane interactions in the northern Cordillera during Early Jurassic time. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 15 C H A P T E R 2 B I O C H R O N O L O G Y A N D B I O S T R A T I G R A P H Y 2.1 I N T R O D U C T I O N The main purpose of this chapter is to outline the biochronology of the Inklin succession at Atlin Lake and establish the biostratigraphic framework upon which the provenance studies in following chapters are based. The Atlin Lake area is particularly well-suited to biostratigraphic applications due to the presence of numerous macrofossils and extensive shore-line exposures of marine strata. The regional structural history of the Inklin Formation has produced a disrupted stratigraphy. Consequently, there are no complete sections of the Inklin Formation, although conformable sections in excess of one kilometre thick can be found at Atlin Lake. In order to piece together the stratigraphy of the Inklin succession the ability to establish good biostratigraphic control becomes critical. Megafauna collected in this study represent many faunal groups and include ammonites, nautiloids, belemnoids, bivalves, corals, brachiopods, crinoids and fish. Fossils are generally moderately to well-preserved though often flattened in the case of ammonites. Ammonites are found predominantly in muddy facies throughout the Inklin succession where yields range from sparse to highly abundant. The ammonites are by far the most numerous and important fossils in the study area and form the basis for biostratigraphic determinations. 2.2 A M M O N I T E B I O C H R O N O L O G Y Ammonites have long been recognized as a most important faunal group for biostratigraphic determinations in Mesozoic strata. Ammonite zonal schemes have been in existence for almost two centuries and have subsequently undergone numerous modifications and refinements that continue today. Jurassic ammonite successions have a long history of study with the first zonal scheme dating back to the mid-nineteenth century (Quenstedt, 1856-58; Oppel, 1856-58). A hundred years later the standard zones of northwest Europe were well-established for the Lower Jurassic (Dean et. al., 1961). In recent decades detailed stratigraphic work in North America revealed inconsistencies regarding application of the B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 16 northwest European zonal scheme and led to the erection of zonal schemes for North America (e.g., Frebold, 1967; Smith et. al., 1988). The current state of ammonite biochronozones for the Lower Jurassic of North America is a combination of well-established standard ammonite zones and regional assemblage zones. The ammonite zonal schemes relevant to this study represent the Sinemurian and Pliensbachian stages of the Early Jurassic. Smith and others (1988) have formally described standard ammonite zones for the Pliensbachian stage of North America and have established 5 biochronozones defined by type sections (Fig. 2.1). Regional assemblage zones for Sinemurian ammonites have been described by Palfy (1991; et al., in press) and include 5 biochronozones (Fig. 2.2). The Canadensis Zone, which spans the Hettangian-Sinemurian boundary, is a formally described standard ammonite zone (Frebold, 1967). The application of ammonite biochronozonal schemes permits a high degree of biostratigraphic resolution when preservation is adequate for genus and species determinations. The duration of Jurassic ammonite zones have been estimated from European ammonite successions and range from 0.6 to 1.8 M a (Kennedy and Cobban, 1976; Westermann, 1984; Hallam et al., 1985). Presumably, ammonite biochronozones for North America are of similar duration. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 17 Z O N E S A M M O N I T E T A X A U.SIN. IMLAYI WHITEAVESI F R E B O L D I K U N A E C A R L O T T E N S E L.TOAR. echioceratids Gemmeltaroceras spp. Polymorphites confusus Miltoceras aff. sellae Tropidoceras aff. erythraeum Pseudoskirroceras imlayi Tropidoceras spp. Metaderoceras evolutum Tropidoceras flandrini Tropidoceras actaeon Phricodoceras cf. taylori Tropidoceras masseanum Acanthopleuroceras whiteavesi Dubariceras silviesi Metaderoceras aff. muticum Acanthopleuroceras aff. start// Liparoceras (Becheiceras) bechei Metaderoceras mouterdei Reynesocoeloceras spp. Dubariceras ireboidi Uptonial so. mm Dayiceras sp. Aveyroniceras spp. Hyperderoceras? sp. Prodactylioceras aff. davoei Aveyroniceras colubriforme Fanninoceras fannini Aveyroniceras cf. inaequiornatum Fontanelliceras sp. Fuciniceras aff. intumescens Arieticeras spp. -Protogrammoceras spp. Fanninoceras spp. Fietdingiceras pseudolieldingi Fanninoceras latum Arieticeras aff. algovianum Fietdingiceras aff. sygma Leptaleoceras atf. speciosum Reynesocoeloceras cf. indunese Protogrammoceras ci. lusitanicum Protogrammoceras varicostalum - Fanninoceras crassum Fanninoceras n.sp. / Leptaleoceras aff. accuratum Arieticeras cf. ruthenense Protogrammoceras cf. isseii ill . Ill • Fanninoceras kunae Amaltheus stokesi Amaltheus margaritatus Reynesoceras ragazzonii Protogrammoceras paltum Protogrammoceras pectinatum Tiltoniceras propinquum Fanninoceras cartottense Lioceratoides spp. Amaltheus viligaensis Protogrammoceras allifordensis dactylioceratids Figure 2.1. Ammonite zonal scheme for the Pliensbachian of North America depicting standard zones and relative stratigraphic ranges for important Pliensbachian ammonites. From Smith et al., 1988. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 18 STAGE HETT. SINEMURIAN ? PLI. X \ Z O N E TAXA Canadensis "Coroniceras" Arnouldi Varians Harbledownense Recognitum Imlayi Schlotheimia n. sp . Alsatites cf. proaries Badouxia oregonensis Badouxia canadensis Vormiceras cf. supraspiratum • Badouxia cf. occidentals Eolytoceras ? guexi ? Vermiceras ex gr. coregonense Eolytoceras tasekoi Angulatjceras cf. ventricosum Sulciferites cf. trapezoidalis Metophioceras cf. rursicostatum Sunrisites senililevis Arietitinae gen. et sp . indet. Lytoceras s p p . • Ectocentrites ? sp. Metophioceras s p p . Badouxia cotumbiae Metophioceras aff. rotarium Badouxia ? sp. Badouxia aff. bccidentatis Lytoceras sp. Sulciferites marmoreus Sulciferites ? sp. Vermiceras sp. • PfiyBoceras sp. Juraphyllites sp . Juraphyllites cf. transytvanicus Amioceras sp. Coroniceras ? s p p . • • Amioceras arnouldi Amioceras ex gr. m e n d a x Amioceras cf. s p e c i o s u m Amioceras ex gr. ceratrtoides Amioceras mtserabDe Audaxtytoceras aff. audax Juraphyllites cf. Bmatus Adnethiceras cf. adnethicum Amioceras cf. densicosta Angulatjceras spez ianum • • • • Amioceras cf. oppeii Tragolytoceras ? sp . Asteroceras aff. margarita E p o p N o c e r a s aff. cai inatum Asteroceras cf. varians • Asteroceras saftriense ? H y p asteroceras ? sp. Oxynotjceras c l . s impsoni Plesechioceras yakounense Plesechioceras ? cf. aWavikense • — Tetraspidoceras sp. Glevlceras cf. s u b g U b a E a n u m Tetraspidoceras paciftcum Plesechioceras ? harbledownense Paltechioceras cf. romankujm • • • Paltechioceras aff. b o e h m i Paltechioceras cf. rothptetzi Paltechioceras sp. Posldonotjs semiplicata Crucilobiceras ? sp. • • Juraphyllites aff. nardti Radstockiceras ex gr. numismale Tetraspidoceras recognitum Oxynoticeratidae gen . et sp . indet. GemmeDaroceras sp. • • Miltoceras sp. Pseudoskirroceras imlayi Figure 2.2. Regional assemblage zones for the Sinemurian depicting zonations and relative stratigraphic ranges of latest Hettangian to earliest Pliensbachian ammonites of the Queen Charlotte Islands. From Palfy, 1991. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 19 2.3 B I O C H R O N O L O G Y A T A T L I N L A K E A total of 85 ammonite collections was taken from the study area (see Fig. 2.3 for localities) and individually may consist of anywhere from a single specimen to a few dozen specimens. These collections were supplemented by a number of others collected by H.W. Tipper during the summers of 1984 and 1985 (see Appendix 1 for details). Ammonites were found in outcrop, in talus, in conglomerate clasts, and as float but the majority were collected from talus at the base of outcrop. Due to steep bedding and/or low-relief outcrop, many of the ex situ talus collections are believed to occur within a few metres or less of their actual stratigraphic positions and for the purposes of this study provide biostratigraphic resolutions equivalent to in situ collections. Ammonite taxa of the Inklin Formation at Atlin Lake represent Early Sinemurian through Late Pliensbachian faunas. Representative ammonite taxa were not collected for all the biochronozones of the Sinemurian and Pliensbachian stages. No ammonites of basal Sinemurian age (Canadensis Zone) were found in the study area nor were taxa of the succeeding Coroniceras Zone collected from strata, but a conglomerate clast of shallow-marine origin containing numerous specimens of Coroniceras was collected from an Upper Pliensbachian unit, indicating the presence of equivalent basinal strata in or adjacent to the study area. This clast represents the oldest stratigraphic interval identified in this study. The succeeding Arnouldi and, to a lesser extent, Varians zones are well-represented at Atlin Lake, but Upper Sinemurian strata containing diagnostic ammonites are rare. Only one locality of Harbledownense Zone age was identified (Loc. 12) and no ammonite taxa of the uppermost Sinemurian (Recognitum Zone) were found, although conglomerate clasts of shallow-marine origin containing Harbledownense Zone fauna (e.g. Paltechioceras) are relatively common in some Upper Pliensbachian conglomerates. The apparent paucity of Upper Sinemurian strata may simply be the result of collection failure, however, more fundamental causes such as poor exposure of relevant intervals or local unconformities may be a factor. Both Lower and Upper Pliensbachian strata are widespread in the study area and form the bulk of exposures. Pliensbachian ammonite faunas representing 4 of the 5 standard zones for the stage were collected at Atlin Lake. Unequivocal basal Pliensbachian Imlayi Zone faunas were found at only one locality on southeast Atlin Lake (Loc. 35), although two other collections lacking zonal resolution contain B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 20 Tropidoceras sp. (Loc. 31) which first appears in the Imlayi Zone and Metaderoceras evolutum (Loc. 73) which ranges through the Imlayi and Whiteavesi Zones and may represent strata of this age. Both the succeeding Whiteavesi and Freboldi Zones of the Early Pliensbachian are well-represented in the study area. Over 30 ammonite collections representing the basal Late Pliensbachian Kunae Zone were taken, however, no ammonites diagnostic of the upper biochronozone (Carlottense Zone) were identified in this study, suggesting strata of this age are either not exposed in the study area or have been removed by erosion. Genus and species identifications are listed by locality in Tables 2.1 A and 2.IB. GSC locality numbers and collections are contained in Appendix 1. Figures 2.4.1 to 2.4.4 are stratigraphic columns depicting sedimentological details and occurrence of ammonites in selected measured sections that represent most of the Inklin succession at Atlin Lake. 0 • s9!i!|B00| IISSOJ uonewjoj mp|U| zz ain6!j 0 suoijeooi isep sejousp xyajj suoi}BOO| HSSOJ, £661- J 0 J loquuAs suojjeooi ||ssoj 2661- ->°J "oqwAs > W Pi „ II 5*1 00 8 3 5 S 1 § 3 2 o • 5- 5. B » a" c/3 O T3 o ro 5! 2. 5* ft 3 w L S. N -'• O 00 s S ' Is > a. II -13 > 3 o c o 0> CD J 5 a. 5 2 R O CD 3 * 9; o> r o II CD a oo £ OP cr o S-J? o | Stratigraphic section | Locality Number Gen. et sp. indet. Fanninoceras (Charlotticeras) sp. Fanninoceras spp. Amaltheus stokesi FontanelUceras spp. FontanelUceras juliae FontanelUceras cf. fontanellense Leptaleoceras spp. Leptaleoceras accuratum Fuciniceras ? sp. Protogrammoceras spp. Protogrammoceras nipponicum Arieticeras spp. Arieticeras algovianum Reynesoceras spp. Reynesoceras inaequiornatum Reynesoceras italicum Reynesoceras colubriforme Reynescoeloceras spp. Cymbites laevigatus Holcophylloceras spp. Phylloceras spp. Dubariceras freboldi Dubariceras silviesi Metaderoceras spp. Metaderoceras mouterdei Metaderoceras talkeetnaense Metaderoceras evolutum Olstoceras spp. Olstoceras compressum Acanthopleuroceras thomsoni Acanthopleuroceras whiteavesi Tropidoceras actaeon Polymorphites? sp. Miltoceras sp. • Pseudoskirroceras sp. Oemmellaroceras spp. Juraphyllites? sp. Paltechioceras harbledownense Epophioceras spp. Asteroceras ci. saltriense Asteroceras varians Arnioceras spp. Arnioceras angusticostatus Arnioceras cf. oppeli Arnioceras sp. A Arnioceras speciosum Arnioceras arnouldi Arnioceras ceratitoides Arnioceras miserabile Locality Number Stage/Zone — • — • • • 9 • tsJ W • • • • OJ •0- • • • • •£» Lft • • • • Lrt ON 0"> LP -O • • • • • - J OO • • OO • • vo • • • o • - •-o • a cn OJ • • U> > W it • • • ' It > n • • t-/> > • • ON > < - > 00 • 00 > • VO 1 AV o • t-J O 1 AV NJ • r-J 1 AV • K ) to 1 AV to u> • • u Cn K) -U -• • < W N> • • • • • • K < P to ON • to ON < •J to • ^ < oo • t-J oo 1 AV to • • • u> • • © 1 EP Ul • • Ul to 1 w? • • • • u> • u> s • • • • -U> ON • o\ U> -O • Ul U> 00 • oo T l u> N© • • u> NO •fl cc o • • o • f l H • • • f l O • • to H • • • & - • • • £ O • • z Os • • • • ON - J • • • • • 4V -o -U oo • • 4k 00 • • vo © • • O • Stratieranhic section Locality Number Gen. et sp. indet. Fanninoceras (Charlotticeras) spp. Fanninoceras spp. Amaltheus stokesi FontanelUceras spp. FontanelUceras juliae FontanelUceras cf. fontanellense Leptaleoceras spp. Leptaleoceras accuratum Fuciniceras ? sp. Protogrammoceras spp. Protogrammoceras nipponicum Arieticeras spp. Arieticeras algovianum Reynesoceras spp. Reynesoceras inaequiornatum Reynesoceras italicum Reynesoceras colubriforme Reynescoeloceras spp. Cymbites laevigatus Holcophylloceras spp. Phylloceras spp. Dubariceras freboldi Dubariceras silviesi Metaderoceras spp. Metaderoceras mouterdei Metaderoceras talkeetnaense Metaderoceras evolutum Olstoceras spp. Olstoceras compressum Acanthopleuroceras thomsoni Acanthopleuroceras whiteavesi Tropidoceras actaeon o f 0 •3' a-1 Vi V Miltocerassp. Pseudoskirroceras sp. Gemmellarocerds spp. Juraphyllites? sp. Paltechioceras harbledownense Epophioceras spp. Asteroceras cf. saltriense Asteroceras varians Arnioceras spp. Arnioceras angusticostaus Arnioceras cf. oppeli Arnioceras sp. A Arnioceras speciosum 1 Arnioceras arnouldi ; Arnioceras ceratitoides Arnioceras miserabile Locality Number 1 Stage/Zone to to > td r w w 7? « £ I  —. K 00 5 ' a S o P 3 3 o 3 3 3 03 H2. • g' 5' 5 3 " i 3 o 3 w o ft 3! 2. 5" CD 0) | Stratigraphic Section | Locality Number Gen. et sp. indet. Fanninoceras (Charlotticeras) spp. Fanninoceras spp. Amaltheus stokesi Fontanelliceras spp. Fontanelliceras juliae Fontanelliceras cf.fontanellense Leptaleoceras spp. Leptaleoceras accuratum Fuciniceras ? sp. | Protogrammoceras spp. | Protogrammoceras nipponicum \ Arieticeras spp. Arieticeras algovianum Reynesoceras spp. 1 Reynesoceras inaequiomatum \ Reynesoceras italicum Reynesoceras colubriforme Reynescoeloceras spp. Cymbites laevigatus Holcophylloceras spp. Phylloceras spp. \Dubariceras freboldi Dubariceras silviesi Metaderoceras spp. Metaderoceras mouterdei \ Metaderoceras talkeetnaense \ Metaderoceras evolutum \ Oistoceras spp. Oistoceras compressum Acanthopleuroceras thomsoni \ Acanthopleuroceras whiteavesi \ Tropidoceras actaeon \ \ Polymorphitesl sp. | \ Miltoceras sp. | | Pseudoskirroceras sp. j | Gemmellaroceras spp. j | Juraphyllites? sp. j | Pahechioceras harbledownense | \ Epophioceras spp. j | Asteroceras cf. saltriense j Asteroceras varians 1 Amioceras spp. j Amioceras angusticostatus Amioceras cf. oppeli j Amioceras sp.A j Amioceras speciosum | Amioceras amouldi | Amioceras ceratitoides j 1 Amioceras miserabile \ Locality Number | Stage/Zone • to • 1 K? Ul • • v/l J> • • 1 K? • X o\ • Ov IIP • • • K? 0  0  LP • • • Ti 8 • • 8 EP 5 • C\ to •fl Ov <*> • • Os • f l £ • • 2 W? m • • Si !K • • $ Ti Ov -J • • • -J. Ti ON oo • • o\ o  Ti 0\ VO • • • • • .Ov vO c • • • -J o Ti ^ • -J |LP to • to 1 EP w • a • • • -O • -j in >fl -J • o -J • -J -J < oo " 3 -o oo ES -J VO • • • •o vo > 0  o • • • • 0  o > 00 • • • 0  oo to • • • 0  to • f l 0  l*J • • • • • • • 09 • f l 0  • o  Ti 0  • • 0  *n Stratigraphic Section Locality Number Gen. et sp. indet. Fanninoceras (Charlotticeras) spp. Fanninoceras spp. Amaltheus stokesi Fontanelliceras spp. Fontanelliceras juliae Fontanelliceras cf.fontanellense Leptaleoceras spp. Leptaleoceras accuratum Fuciniceras ? sp. Protogrammoceras spp. Protogrammoceras nipponicum Arieticeras spp. Arieticeras algovianum Reynesoceras spp. Reynesoceras inaequiomatum Reynesoceras italicum Reynesoceras* colubriforme ' Reynescoeloceras spp. Cymbites laevigatus Holcophylloceras spp. Phylloceras spp. Dubariceras freboldi Dubariceras silviesi Metaderoceras spp. Metaderoceras mouterdei Metaderoceras talkeetnaense Metaderoceras evolutum Oistoceras spp. Oistoceras compressum Acanthopleuroceras thomsoni Acanthopleuroceras whiteavesi Tropidoceras actaeon 0 f 1 g-1 Vt •a Miltoceras sp. Pseudoskirroceras sp. I Gemmellaroceras spp. Juraphyllites? sp. Pahechioceras harbledownense Epophioceras spp. Asteroceras cf. saltriense Asteroceras varians Amioceras spp. Amioceras angusticostatus Amioceras cf. oppeli Amioceras sp.A Amioceras speciosum Amioceras amouldi Amioceras ceratitoides Amioceras miserabile Locality Number 1 Stage/Zone 5' cc 3 c Q. S ' (3 co cr o E3-8 £ o O 0> fij" ? oo .5 eta *1 £ I  a. m o to B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY AMMONITES L E G E N D Sandstone Rhythmic siltstone and fine sandstone Rhythmic silty argillite and mudstone Conglomerate Tuff and resedimented pyroclastic debris Muddy carbonate Slump or muddy debris flow deposit • 6)J 45 Fossil Locality • In situ • Ex situ 111 0 < i i an E C CO • _i a c 0 Kl (fl c a >" c .5 E C to 0) 15 _J 1 r 3 5 e » E ti s s 0 1 FIGURE 2.4.1: Section 2 - Southeast Atlin Lake. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 25 ui 0 < (0 LU 0) c 0 IM 3 a c AMMONITES I AMMONITES Column 1 31 [=1 3: i—11 i Clay rn c vf "i—r~i c vc Gravel Column 2 3 31 ^3 1 i—r~r Clay m c T—I—I c vc Gravel Silt Sand Silt Sand F I G U R E 2.4.2: Section 1 - Griffith Island B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 26 FIGURE 2.4.3A: Section 3 - Sloko Island, Columns 1 and 2. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 27 FIGURE 2.4.3B: Section 3 - Sloko Island, Columns 3 and 4. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 28 FIGURE 2.4.3C: Section 3 - Sloko Island, Columns 5 and 6. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 29 FIGURE 2.4.3D: Section 3 - Sloko Island, Columns 7 and 8. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 30 F I G U R E 2AAA: Section 4 - Copper Island, Columns 1 and 2. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 31 FIGURE 2.4.4B: Section 4 - Copper Island, Columns 3 and 4. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY F I G U R E 2.4.4C : Section 4 - Copper Island, Column 5. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 33 2.4 GEOCHRONOLOGY Geochronological-biochronological calibrations embodied in the geological time-scale are critical to a host of geological interpretations. This is particularly true of the Jurassic of the Canadian Cordillera which was a period of important regional tectonism. Time-scale revisions are a matter of course as new data become available and have led to a number of revised geological time-scales in recent decades (e.g., Palmer, 1983; Harland et al., 1990). Geochronological data from this study illustrates the need for revision of the Lower Jurassic segment of the time-scale of Harland and others (1989) currently favored by many Cordilleran workers. Samples were collected from Upper Pliensbachian strata at two localities and U-Pb analyses performed on zircon content by workers at the Geological Survey of Canada Geochronology Laboratory in Ottawa. Each sample was collected from biostratigraphically well-constrained strata of Kunae Zone age. The sample sites are found within measured sections on Sloko and Copper Islands and are shown in Figure 2.3 (Localities 43 and 66 respectively). The measured sections correspond to Section 3: Sloko Island (Figs. 2.4.3A-D: columns 1 to 8) and Section 4: Copper Island (Figs. 2.4.4A-C: columns 1 to 5). The stratigraphic level at which the sampled units occur is shown in Figure 2.4.3C (column 5) and Figure 2.4.4B (column 4). The Sloko Island sample is a granitic conglomerate clast which yielded a precise crystallization age of 186.6 + 0.5/-1 M a (Fig. 2.5A). This age determination should be considered a minimum age of crystallization (J. Mortensen - pers. comm., 1994). The granitic clast was obtained from a conglomerate unit found - 1 5 0 metres above a sharply defined Lower-Upper Pliensbachian boundary marked by the first appearance of Fanninoceras with Dubariceras freboldi. The first appearance of Fanninoceras defines the base of the Late Pliensbachian substage (see Fig. 2.1) while Dubariceras freboldi is considered diagnostic of the Freboldi Zone of the Early Pliensbachian and is rarely found to occur with Fanninoceras. This co-occurrence provides a high-resolution marker for the Early-Late Pliensbachian substage boundary. The Copper Island sample was collected from a sequence of water-lain lapilli tuff and resedimented pyroclastic debris and yielded a U-Pb age of 186 ± 1 M a (Fig. 2.5B). The tuff unit occurs in a thick, coarse clastic sequence - 3 0 0 metres above a well defined Lower-Upper Pliensbachian boundary and is bracketed by Reynesoceras inaequiornatum and Protogrammoceras sp. 260 metres below and Amaltheus stokesi 210 B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 34 and 260 metres above. Both these species have very short ranges in the Kunae Zone; Reynesoceras inaequiornatum is confined to the base of the Kunae Zone (see Fig. 2.1: Aveyroniceras inaequiornatum) and Amaltheus stokesi is confined to the lower third of the zone. Both samples were collected from strata deposited as part of a major progradational and aggradational episode characterized by high sedimentation rates and are interpreted to occur in strata temporally close (i.e., in the order of magnitude of 10 5 years) to the Early-Late Pliensbachian boundary. The relatively precise ages of both samples and their tight biostratigraphic constraints makes them prime candidates for refinement of the Pliensbachian substage boundary. Some discrepancy between the absolute age of the upper Pliensbachian stage boundary given by Harland and others (1989) and data from this study is evident. In their time-scale calibration the Pliensbachian-Toarcian boundary is placed at 187.0 Ma; however, data from this study indicates that this stage boundary age assignment more accurately represents the Early-Late Pliensbachian substage boundary. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 35 B) GGAJ-92-127 0 028 1 1 1 1 1 1 1 0.1925 0.1975 0.2025 0.2075 Figure 2.5. Concordia plots for radiometric (U:Pb) age determinations. A : Orthoclase-megacrystic biotite-hornblende monzogranite clast (sample GGAJ-92-71) from Upper Pliensbachian (Kunae Zone) conglomerate on Sloko Island. B: Lithic-crystal dacitic lapilli tuff (sample GGAJ-92-127) from Upper Pliensbachian (Kunae Zone) section on Copper Island. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 36 2.5 P A L E O B I O G E O G R A P H Y The ammonite taxa collected at Atlin Lake furnish insight into the paleobiogeography of the northern Stikinian arc during Early Jurassic time. Ammonite faunal realms and provinces have been described for Jurassic time in Europe and North America (q.v., Hallam, 1973; Westermann, 1984) and provide a paleontological means of assessing relative and absolute terrane positions that is independent of other lines of study such as paleomagnetic determinations. The degree of ammonite provinciality varies considerably during the Jurassic but for the most part can be resolved at the stage or substage level into discrete biogeographic components with some overlap. Of relevance to this study are faunal realms described for Lower Jurassic strata of North America (e.g., Taylor et al., 1984). Ammonite provinciality is relatively poorly developed during Sinemurian time. The Sinemurian of North America is divided into Tethyan and East Pacific Realms (Taylor et. al., 1984)(Fig. 2.6) with pandemic forms dominating stage taxa. Sinemurian collections from the Inklin succession at Atlin Lake are composed entirely of pandemic forms such as Arnioceras, Asteroceras, Epophioceras, and Paltechioceras and provide no useful information on paleobiogeography during this stage. A single exception consisting of a provisionally identified juraphyllitid (locality 11) suggests a Tethyan influence. The Early Pliensbachian of North America also displays weakly-developed ammonite provinciality. Only a Tethyan Realm is currently defined for this substage which lacks a northern faunal realm equivalent to the Boreal Realm established by Late Pliensbachian time (Fig. 2.7)(Taylor et al, 1984). Characteristic Tethyan forms such as Metaderoceras, Dubariceras, Reynescoeloceras, and Miltoceras are found in the Inklin succession at Atlin Lake with Metaderoceras and Dubariceras particularly abundant. Cosmopolitan forms such as Tropidoceras and Acanthopleuroceras are also widespread in the study area. Although considered a characteristically Tethyan form at the generic level, the two species of Dubariceras found at Atlin Lake are restricted to the western Americas (Thomson and Smith, 1992) and may be considered as forming the nucleus of an informal East Pacific realm during most of the Early Pliensbachian. Thus, the northern Stikinian arc appears to be clearly allied with North America by no later than the Early Pliensbachian. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 37 Figure 2.6. Present distribution of Tethyan and East Pacific Realm ammonite faunas for the Sinemurian of western North America. A : Early Sinemurian; B: Late Sinemurian. From Taylor et. al., 1984. Ammonite provinciality is much more strongly developed during the Late Pliensbachian substage and permits the recognition of 3 distinct faunal realms, all of which overlap to some degree (Fig. 2.7). A circum-polar Boreal Realm is distinguished on the basis of its characteristic low-diversity amaltheid-dominated taxa, in contrast to the high-diversity hildoceratid-dominated taxa of the Tethyan Realm (Taylor et a], 1984). There is a marked biogeographic separation of the two realms that is particularly evident in Europe (Howarth, 1973) but also found in the Canadian Cordillera (Tipper, 1981; Smith and Tipper, 1986). The two realms are separated by a transitional zone of mixed fauna. A n East Pacific Realm is recognized on the basis of the occurrence of the genus Fanninoceras, which is restricted to the western Americas (Taylor, 1984). B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 38 Figure 2.7. A : Present distribution of Tethyan Realm ammonite faunas for the Early Pliensbachian of western North America. B: Present distribution of Tethyan, Boreal, and East Pacific Realm ammonite faunas for the Late Pliensbachian of western North America. Square hatched pattern indicates transitional zone of mixed faunal affinity between Tethyan and Boreal Realms. From Taylor et. al., 1984. Late Pliensbachian ammonite taxa in the study area are dominated by Tethyan forms such as Arieticeras, Reynesoceras, Fontanelliceras, Fuciniceras, and the primarily, but not exclusively, Tethyan forms Protogrammoceras and Leptaleoceras. Fanninoceras is a moderately widespread, though never abundant, co-occurrence in the Upper Pliensbachian Inklin succession and confirms an East Pacific longitude. The occurrence of Fanninoceras at Atlin Lake represents the most northern locality at which this genus has been found on the inboard terranes (i.e., Stikinia and Quesnellia)(H.W. Tipper - pers. comm., 1994). The Boreal form Amaltheus is also found in Upper Pliensbachian strata at Atlin Lake but is B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 39 generally not widespread. It is noteworthy that Amaltheus was found only in Upper Pliensbachian strata in the northern half of the study area and is apparently absent in the southern half. Collection failure is a distinct possibility that could explain this difference; however, given that nearly half of the collections taken from Upper Pliensbachian strata on Copper Island contain amaltheids while a similar number of Late Pliensbachian collections from Sloko Island contain no amaltheids, the present geographic distribution of Amaltheus suggests it is, at least in part, a biogeographic rather than solely a collection artifact. Three of the five amaltheid occurrences are found with hildoceratids (± Fanninoceras), however, two occurrences on Copper Island are monospecific, including one collection taken from a highly abundant horizon containing hundreds of specimens. This latter collection represents the most strongly Boreal locality yet identified in the Late Pliensbachian of British Columbia (H.W. Tipper, P.L. Smith -pers. comm., 1992). The presence of amaltheids clearly establishes a mixed zone faunal affinity for Late Pliensbachian ammonite taxa at Atlin Lake. Atypical occurrences of monospecific amaltheids indicate this region of the northern Stikinian arc occupied a paleogeography very close to the transition from mixed faunal realm to Boreal Realm by early Late Pliensbachian time. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 40 2.6 DISCUSSION 2.6.1 Ammonite Taxonomy Arnioceras: Arnioceras exhibits a number of readily apparent morphological characteristics that permit easy recognition of this genus. It is typified by evolute forms possessing sharp keels often flanked by sulci. Prominent generic characteristics are smooth innermost whorls succeeded at various diameters by simple sharp ribbing which is most often straight but may range from gently prorsiradiate to gently rursiradiate. In some species ribs project forward onto the venter. Arnioceras is a genus plagued by oversplitting ( > 50 species) so that species assignments can be difficult. In this study a total of seven species are distinguished, most of which occur in the Arnouldi Zone on Griffith Island (Section 1: Fig. 2.4.1). In most cases species assignments are unambiguous; however, in the case of Arnioceras sp. A (Plate 1: Figs. 13, 14) there are some unusual features apparent. In the two best whole specimens there is sufficient preservation of these features to distinguish them from other species of Arnioceras. They are characterized by whorls that enlarge rapidly for Arnioceras, giving them a whorl-shape that approaches mid-volute. In this respect, as well as in rib density, they are similar to A. ceratitoides; however, their other rib characteristics appear to set them apart from this species. The ribbing of the two specimens of Arnioceras sp. A consists of moderately dense, slightly sinuous wiry ribs which trend rectiradiately to slightly prorursidiately on the lower flank and flex to become gently rursiradiate on the mid and upper flank. The ribs appear to project only weakly on to the venter or not at all. The slightly sinuous character of the ribbing sets these specimens apart from other species of Arnioceras. It may be that these flexous ribs are the product of sexual dimorphism within the Arnioceras ceratitoides species group or some other similar species, although sexual dimorphism has not been documented for this genus. Alternatively, there may be sufficient distinction in the ribbing alone to warrant the erection of a new species but the poor preservation of critical ventral features preclude this. Consequently, these specimens are assigned a nominal species designation. Reynesoceras: Species of this genus are common in strata of Kunae Zone age at Atlin Lake. Three species have been identified and include Reynesoceras inaequiornatum, R. colubriforme, and R. italicum. These species have been previously published under the genus Aveyroniceras which is now considered a B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 41 macroconch and a junior synonym of Reynesoceras (Sestini, 1975; Braga; 1983; Meister, 1989). It should be noted that species of Aveyroniceras listed in the Pliensbachian ammonite zonal scheme of Smith and others (1988)(Fig 2.1) have since been reassigned to the Reynesoceras genus (P.L. Smith - pers. comm., 1994). A single specimen with unusual ribbing characteristics for dactylioceratids of Pliensbachian age was collected at Atlin Lake. This specimen (Plate 5, Fig. 10) is an evolute, slowly expanding form with gently rounded flanks and what appears to be a broad rounded venter, although the top of the venter is not visible. Ribbing is moderately dense with strong, straight ribs that trend rectiradiately to gently prorsiradiately on the lower flanks of the outer whorl. On the mid to upper flank ribs are prominently bifurcated with irregularly interspersed, simple ribs occurring every third or fourth rib. Prominent swellings and sporadic tubercules are associated with the point of bifurcation but ribs appear to be predominantly non-tuberculate. Inner whorls are poorly preserved but appear to have prorsiradiate, strong, non-tuberculate, non-bifurcating ribs. Ribs on the outer whorl project strongly onto the venter and appear to continue across the venter where preservation is poor. The presence of common bifurcating ribs suggests an affinity to the Toarcian genus Dactylioceras but the association with other Pliensbachian forms constrains this specimen to an age of latest Early Pliensbachian. This specimen was found in beds containing a prolific ammonite fauna that includes Dubariceras freboldi, Reynesoceras inaequiornatum, Reynesoceras italicum, Cymbites laevigatus, Gemmellaroceras spp., Protogrammoceras sp., and Fanninoceras ? sp. This co-occurrence indicates an uppermost Freboldi Zone stratigraphic level. The pronounced bifurcating ribs set this specimen apart from other dactylioceratids of this age; however, the general morphological characteristics (e.g., non-bifurcate ribs on inner whorls, sporadic tubercules) of the specimen and their stratigraphic level suggest a relationship with Reynesoceras. Consequently, this specimen is given provisional generic assignment to Reynesoceras and almost certainly represents a new species. Fanninoceras: Species of Fanninoceras are not abundant in the Inklin succession at Atlin Lake but have a widespread distribution throughout the study area in beds of Kunae Zone age. In most specimens poor preservation of ribbing characteristics does not permit species assignments, but the general lack of B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 42 discernible ribbing suggests a species with subdued ornamentation such as Fanninoceras fannini accounts for the majority of Kunae Zone occurrences. The presence of a superficially similar form, Cymbites laevigatus, in the Pliensbachian succession at Atlin Lake raises the possibility of mistaken generic assignments in flattened specimens with poor preservation. At least one other species of Fanninoceras is present in Kunae Zone strata, however, due to the generally poor preservation of this material, no attempt is made to discriminate species. Consequently, all specimens of Fanninoceras (excluding Charlotticeras discussed below) are grouped together under Fanninoceras spp. in Tables 2.1 A , 2. IB, and Appendix 1. A n exception to the general lack of rib preservation are two specimens that compare well with figured specimens of the new subgenus Charlotticeras (Smith and Tipper, in review) and have been assigned to this subgenus. The first specimen (Plate 8, Fig. 13) was found in beds at the top of a measured section (Section 3; Fig. 2.4.3D) that contains numerous hildoceratids (e.g., Protogrammoceras sp., Leptaleoceras sp., Arieticeras sp., FontanelUceras sp.) and moderately abundant dactylioceratids (i.e., Reynesoceras sp.) which tend to characterize the lowermost Kunae Zone (Smith et. al., 1988). This specimen most resembles Fanninoceras {Charlotticeras) carteri but the poor preservation precludes confident species assignment. The second specimen (Plate 8, Fig. 14) occurs in a section containing Amaltheus stokesi as well as numerous hildoceratids that include Arieticeras algovianum, Protogrammoceras sp., and Leptaleoceras sp., indicating the lower Kunae Zone. The biostratigraphic constraints for these two specimens indicate that species of Charlotticeras first appeared in northern Stikinia during early Kunae Zone time. Protogrammoceras: A number of species of Protogrammoceras occur in the Kunae Zone at Atlin Lake. Species assignments for this genus can be difficult and even generic assignment is not always unequivocal due to the similarity of Fuciniceras which also ranges through the Kunae Zone. Because Protogrammoceras was predominantly found in association with other genera and species clearly of Kunae Zone age, species assignments for this genus were seen to provide no refinement of biostratigraphic resolution. Consequently, with the exception of Protogrammoceras nipponicum, species assignments were not attempted for the majority of Protogrammoceras specimens. In addition to Protogrammoceras nipponicum, at least two other species of Protogrammoceras (Plate 8: Figs. 1, 3) are present. The latter are grouped under Protogrammoceras spp. in Tables 2.1 A , 2.IB and Appendix 1. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 43 Miscellaneous Taxa: A number of unusual co-occurrences were found in a prolific collection taken from Locality 83 and warrant further mention. In addition to the specimen discussed under Reynesoceras, numerous specimens of Cymbites laevigatus (Plate 6: Figs. 3, 5) were also found in association with this well-constrained uppermost Freboldi Zone fauna. This species is previously known only from Late Pliensbachian localities in the Cordillera. This new occurrence in the Inklin succession extends the stratigraphic range of the species in North America down from the Kunae to the upper Freboldi zone. Elsewhere in the world, C. laevigatus ranges as low as the Sinemurian Turned Zone. Another unusual occurrence abundant at Locality 83 consist of species of Gemmellaroceras. These specimens are very small evolute forms less than 1.5 cm. in diameter that possess coarse simple ribbing (Plate 6, Figs. 1, 2). They were not identified in any other Freboldi Zone collections but are similar to specimens found to occur in Freboldi Zone strata in the Queen Charlotte Islands (Smith and Tipper, in review). The abundance of these small ribbed species of Gemmellaroceras in this uppermost Freboldi Zone collection and their apparent absence lower in the Freboldi Zone suggests that proliferation of these species is a latest Early Pliensbachian event in the northern Stikinian arc. These species were also not identified in any Kunae Zone collections, suggesting a relatively short stratigraphic range. A few specimens of the long-ranging genus Holcophylloceras are also found at Locality 83. These specimens display the prominent, slightly sigmoidal, negative constrictions characteristic of the genus (Plate 6: Fig. 4) and are unremarkable but for their relative rarity in the Inklin succession at Atlin lake. This genus was encountered at only two localities (Locs. 83 & 5) in the study area . 2.6.2 General Discussion The depositional setting of the Inklin Formation precludes the erection and description of formal stratigraphic units within the Inklin succession at Atlin Lake. Submarine fans are dynamic sedimentary environments characterized by rapid lateral and down-fan facies changes and dominated by repetitive sandstone and mudstone lithologies that generally lack distinguishing features. Consequently, it is not possible to correlate even biostratigraphically well-constrained coeval strata across the study area. In a few cases a particularly distinctive conglomerate unit could be traced for a few kilometres but the monotonous B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 44 lithological nature of the majority of strata rendered confident correlation impossible. To compound the inherent difficulty of correlating disrupted submarine-fan strata, multiple coeval fan systems were found to be operating during at least Pliensbachian time (Ch. 5, p. 175), producing interfingering and overlapping sediment bodies that cannot be correlated except in a local sense. Ammonites from the Inklin Formation exposed at Atlin Lake constrain the age of the succession there to a range of Early Sinemurian to Late Pliensbachian. This contradicts the findings of Bultman (1979) who used fossil collections taken predominantly from strata exposed on Atlin Lake to interpret a stratigraphic range for the Inklin Formation that spans latest Triassic to Middle Jurassic. The latest Triassic age determination is the result of Bultman's (1979) erroneous correlation of Late Triassic Stuhini Group strata with the Takwahoni Formation of the Laberge Group. A Middle Jurassic age is also untenable for Inklin strata at Atlin Lake and appears to be the result of incorrect generic assignments for ammonites collected and identified by Bultman (1979). Fossils of marine organisms, most notably ammonites, are found throughout the entire Inklin succession at Atlin Lake and confirm the marine nature of this sedimentary package. Bultman (1979) claimed that the highest parts of the Inklin Formation contain coal fragments and used this to support his suggestion that higher stratigraphic levels were deposited, at least locally, in non-marine to marginal marine environments. Evidence from this study has shown (Ch. 6) that high-rank coalified wood fragments occur in all stratigraphic levels of the Inklin Formation and are, in fact, markedly'more abundant in Sinemurian strata. Souther (1971) also observed a concentration of coalified wood fragments and other plant debris in basal strata (i.e., Sinemurian) of the Inklin Formation in the Tulsequah area. The presence of terrestrial organic detritus in the Inklin succession at Atlin Lake is unrelated to depositional environment and reflects only the availability of woody material transported off-shelf by turbidity currents. Ammonites in the study area are typically found in thin, rhythmically bedded, fine-grained strata that include fine sand-mud and silt-mud couplets, thin and thick irregular to planar silt-mud laminae, and graded stratified muds. A l l of the above are formed by suspension deposition from the bulk volume of low-density turbidity currents or the tail-end of high-density turbidity currents and may represent a variety of depositional environments (Lowe, 1982; Pickering et. al., 1986). These include medial-fan interchannel B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 45 and channel settings, and lower fan-fringe to basin-plain settings. Ammonite yields are typically low for Sinemurian strata in the study area, in contrast to many Pliensbachian collections. One may speculate whether this is a result of primary biological factors, collection failure, or preservation potential related to depositional environment. A factor which may be of relevance to this question is the sedimentary context of Sinemurian muddy sequences. These often form thick sequences (> 100 metres) with minor sand interbeds that are interpreted as representing predominantly lower fan-fringe and basin plain environments. It is noteworthy that similar lithofacies in Lower and Upper Pliensbachian strata, where ammonite yields are higher, are predominantly of medial fan affinity (i.e., interchannel to lobe fringe), suggesting that proximity to shelf locales may be a factor in the numbers of ammonites transported down-fan. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 46 PART 2: ANALYSIS OF FOSSILIFEROUS CONGLOMERATE CLASTS 2.7 INTRODUCTION Fossiliferous conglomerate clasts can be used as multi-purpose tools that permit paleoenvironmental reconstructions, provenance determinations, and provide insight into regional tectonic regimes. The occurrence of fossiliferous carbonate clasts in an Upper Pliensbachian (Kunae Zone) debris flow appear to provide the only physical evidence of Early Jurassic shallow marine environments in the study area. A small suite (N=14) of clast samples was collected from four adjacent sites and analyzed for lithological and faunal content. Detailed descriptions of lithology and faunal content are dealt with in the following section. Collection sites are shown in Figure 2.3. Table 2.2 summarizes clast taxa. Clasts were collected both from outcrop and talus at four different sites located in a group of small islands near the present N E basin 'margin', all within a 1 km radius. Clasts from outcrop range in size from 0.5 to > 2.5 m and occur as large sub-angular to sub-rounded rectangular boulders or 'blocks'. Stratigraphic and structural context, as well as lithological characteristics indicate a high probability that the four sites are correlatives of the same depositional event. A l l sites are situated very close to (-200 m) and equidistant from a major northwest-trending syncline axis, indicating a common stratigraphic horizon. Lithological (clast and matrix composition) features of this distinctive debris flow unit are virtually identical between the four localities, as are their stratigraphic context which is within massive thick-bedded sandstone facies of mid-fan channel affinity. One locality (Cl-2) is well-constrained biochronologically, bracketed by numerous specimens of Protogrammoceras and Arieticeras (loc. 4-5) of Late Pliensbachian Kunae Zone age. If not actual correlatives, the four sites are most certainly at least broadly coeval (i.e., Kunae Zone). The presence of an Arieticeras ? fragment (falcoid, bisulcate keel) in an incompetent olistostromal (?) greywacke at one site (C7-C12) also indicates a Late Pliensbachian age for this unit. Although accurate generic identifications for bivalves were not possible in many cases, sufficient distinctions in shell morphology and ornamentation were apparent to allow for approximations of generic diversity. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 47 Stage/Zone H EJ LS LS H? w EJ EJ EJ ES LS LS w C? H LT LT A Locality Number C l C2 C3 C4 C5 C6 C7 C8 C9 CIO C l l C12 C13 C14 C15 C16 C17 M Metophioceras ? sp. •> M Coroniceras sp. • 0 Epophioceras sp. • N Gleviceras sp. • I Paltechioceras harbledownense • • T Paltechioceras spp. •> • • • E Oxynoticeras sp. • Acanthopleuroceras thomsoni • T Oistoceras compressum • • A Metaderoceras talkeetnaense • X Metaderoceras mouterdei • A Dubariceras silviesi • • Echioceratid ammonite • M Monotis subcircularis • I Posidontis semipllcata • S Weyla bodenbenderi • • • • c Weyla alata • E Weyla ? sp. • L Gressyla sp. • • L Phoiadomya sp. - • A Pleuromya sp. • • • N Modiolus sp. • • • • • E Protocardia sp. • •> O Goniomya sp. • U Pinna sp. • • • s Entolium sp. • • Trigonia ? • T Pentacrinus sp. • A Halorella sp. • X Gastropods • • A Belemnoids • • Locality Number C l C2 C3 C4 C5 C6 C7 C8 C9 CIO C l l C12 CI3 C14 C15 C16 C17 T A B L E 2.2. Taxa contained in Inklin Formation conglomerate clasts. E /L = Early/Late; T = Triassic; J =Jurassic; Stage symbols: N = Norian; S = Sinemurian; P = Pliensbachian. Zone symbols: C = Coroniceras; A = Arnouldi; V = Varians; H = Harbledownense; I = Imlayi; W = Whiteavesi. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 48 2.8 LITHOLOGICAL AND FAUNAL DESCRIPTIONS Clasts were examined as handsamples using hand-lens and binocular microscope. Rock names were derived using Embry and Klovan's (1971) modification of the original classification scheme by Dunham (1962) for carbonate rocks. For brevity the term bioclast is used in following descriptions when referring to the skeletal component, but is not intended to convey any genetic connotation (i.e., no implied mechanical deposition). C l . Sample GGAJ-93-4-FA (talus) - Skeletal mudstone Color: Light buff to rusty brown weathering, dark grey fresh. Composition: A silty carbonate mud transitional to a muddy carbonate silt forms bulk of rock (> 90%). Silt component is dominantly very fine carbonates with sparse carbonaceous material and rare siliciclastics. Bioclasts are minor (~ 10-15%) and composed of small bivalves and ammonites. Texture: Homogeneous (bimodal), well-sorted with sparse bioclasts floating in matrix. In-situ bivalve component composed of tiny forms (< 0.5 cm) locally concentrated in small pockets, whole » fragments, articulated » disarticulated. Large ammonites (7-> 20 cm) with unusual preservation features: one genus is pyritized, the other with opalescent shell material. Fauna: Ammonites include two Late Sinemurian Harbledownense Zone specimens: Paltechioceras harbledownense and Oxynoticeras sp. At least 3 indeterminate genera of bivalves are present and are strongly dominated by unornamented infaunal forms although rare byssally attached forms are also found (Lima?). Interpretation: Textural and compositional features indicate a deeper water setting, likely below storm-wave base, suggesting outer shelf to slope environments. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 49 C2. Sample GGAJ-93-4-FB (talus) - Skeletal mudstone Color: Light to medium buff brown weathering, dark greyish black fresh. Composition: Matrix (> 90 %) is a relatively clean carbonate mud with a minor very fine silt component composed of carbonates and very sparse siliciclastics (primarily biotite). Minor bioclasts are composed entirely of small bivalves. Sparse plant material is also present. Texture: Homogeneous, well-sorted matrix with minor bioclasts. Bivalves range from < 1 cm to ~2 cm (av. 1.5 cm) and are found concentrated in small pockets. A l l specimens are whole with articulated » disarticulated. Fauna: A monospecific population of in-situ bivalve 'nests' composed of epifaunal/free-lying? pectenids with small auricles, thin shells, prominent growth lines and no ribbing identified as Posidontis semiplicata (M. Aberhan - pers. comm., 1994). This species ranges from the Late Sinemurian to the Late Pliensbachian. Interpretation: Textural and compositional features (abundant well-sorted mud matrix, monospecific fauna) indicate a deeper water setting unaffected by event deposition (i.e., below storm-wave base and without bottom current influence). C3. Sample GGAJ-92-58-FA-1 (talus) - Stratified bioclastic grainstone Color: Medium buff brown weathering, medium tan fresh. Composition: Fine carbonate sand and bioclastic debris. Sparse siliciclastics (< 2 %). Texture: Relatively homogeneous, highly porous, permeable clastic/bioclastic texture. Thinly interbedded (1-2 cm) alternating moderately sorted fine carbonate sand and sorted fine (range: mm's to < 2 cm) shell hash. Fauna: Varied mixture of bivalves, crinoids, gastropods, belemnites, and ammonites (in decreasing order of abundance), dominated by epifaunal bivalves (including Weyla and other numerous Pectenids). Strong ornamentation is the norm. The poorly preserved ammonite appears to be of echioceratid affinity, suggesting a Late Sinemurian age. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 50 Interpretation: Depositional processes are purely mechanical and indicate a moderately high energy environment, probably above (or near) normal wave base or subject to strong bottom currents. C4. Sample GGAJ-92-58-FA-2 (talus) - Sandy skeletal (molluscan) rudstone Color: Dark buff brown weathering, medium greyish brown fresh. Composition: Bioclasts are dominant, with medium to thick-shelled bivalves and subordinate ammonites. Matrix is an important but subordinate component (~ 40%) composed of fine silty carbonate sand. Plant fragments are ubiquitous but minor, siliciclastics sparse. Texture: Heterogeneous, very poorly sorted. Loosely framework supported by large (7+ cm) to small (< 1 cm) pelecypod fragments, valves and ammonites. Fragments > whole, disarticulated » articulated. Fauna: Bivalves include Pinna and Weyla bodenbenderi?. Approximations of diversity are undermined by limited clast size but number 4-5 genera which display common ornamentation (including numerous Pectenids). A poorly preserved evolute keeled echioceratid ammonite resembles Pahechioceras, indicating a Late Sinemurian age. Interpretation: Textural features indicate a mixed origin, moderate to low energy environment above storm-wave base where both biogenic and mechanical processes are significant contributors to volume. C5. Sample GGAJ-92-58-FB (talus) - Silty pelecypod rudstone Color: Buff brown weathering, medium grey fresh. Composition: Bioclasts (bivalves, minor ammonites) form major component of rock with shells mainly moderate to thin. Matrix is an important subordinate component (40-50%) composed of muddy fine lime silt containing sparse siliciclastics (quartz, biotite). Plant fragments are ubiquitous but minor. Texture: Heterogeneous, very poorly sorted. Pelecypod valves and fragments are strongly dominant and are mainly in the range of 1.5-2.5 cm with rare shells to ~5 cm. Framework texture is packed with whole valves » fragments and disarticulated > articulated. Fauna: Moderately diverse bivalve community (> 8 genera including the dominant genus Protocardia (M. Aberhan - pers. comm., 1994), Pholadomya, Modiolus, Pinna, Goniomya and Trigonia?) includes B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 51 Pholadomyoids, Mytiloids, Pectenids, and probably Corbiculids, Trigonids and Pteroids. Infaunal/semi-infaunal forms dominate (esp. Protocardid) but byssally-attached forms are not uncommon. Ornamentation is subdued in most genera. Ammonites are of probable Late Sinemurian affinity and include 3 genera: Gleviceras sp., Paltechioceras? sp.,and Epophioceras! sp. Interpretation: A strong generic dominance (Protocardid) plus other textural features such as whole/fragment ratios, presence of mud, and rare siliciclastics indicates a major component of bioaccumulation. A low-energy environment below storm-wave base is indicated. C6. Sample GGAJ-92-58-FC (outcrop) - Silty mudstone Color: Buff brown to rusty buff weathering, medium grey fresh. Composition: Carbonate mud dominates (> 85%), with minor (~ 10%) fine siliciclastic and carbonate allochems, and sparse bioclasts (ammonites). Siliciclastics include (in decreasing order): quartz, feldspar, hornblende, biotite, and muscovite. Texture: Homogeneous, moderately sorted. Siliciclastics are mainly uniformly distributed silt to fine sand particles (carbonate particles subordinate) with minor small pockets of up to ~ 20% medium-coarse siliciclastic detritus. Ammonites are sparse but ubiquitous and generally < 2 cm but are found up to 6 cm. Fauna: Ammonites represent an Early Pliensbachian Whiteavesi Zone fauna. At least 3 genera are present and include Dubariceras silviesi, Metaderoceras mouterdei, and Olstoceras compressum. Interpretation: Textures indicate a deeper water setting (probably below storm-wave base), suggesting an outer shelf to slope locale. Cl. Sample GGAJ-92-57-FC (outcrop) - Muddy gastropod rudstone Color: Buff brown to rusty buff weathering, dark greyish black fresh. Composition: Bioclasts composed almost entirely of gastropods (>95%) with rare small pelecypods. Minor matrix component (30-40%) is a silty to sandy carbonate mud. Siliciclastics are absent. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 52 Texture: Relatively homogeneous (bimodal), poorly sorted. Framework composed almost entirely of packed gastropods (>95%) ranging from 2-3 cm to < 4 mm. Gastropods display a strong preferred orientation. Fauna: Gastropods represent a low diversity/high abundance colony. Two genera are present: a weakly ornate larger (2-3 cm) genus dominates with a small strongly ornate genus (Pyrgotrochus?) subordinate. Moderate to thick shells are common. An Early Jurassic age is probable. Interpretation: A strong preferred orientation among the gastropods and absence of other faunal groups, in conjunction with a silty mud matrix, indicates an in situ bioaccumulated deposit subject to predominantly low to moderate energy conditions, consistent with depths below normal-wave base. C8. Sample GGAJ-92-57-FA-1 (outcrop) - Muddy pelecypod floatstone-rudstone Color: Buff to medium dark brown weathering, dark grey fresh. Composition: Pelecypod bioclasts, ranging from thin, fine (<1 cm) to thick, large (4-5 cm) varieties. Matrix forms a major proportion of the rock (60%) and is composed of a silty to sandy carbonate mud. Other minor but ubiquitous clasts include: mineral grains (mostly quartz), lithic fragments (green to dark grey aphanitic volcanics), and plant fragments (mostly 'sticks' to 1 x 8 cm). Texture: Heterogeneous, very poorly sorted, loosely 'packed', marginally 'grain'-supported texture. Finer bioclasts are dominant, coarser subordinate. Whole valves are common but subordinate to fragments, disarticulated » articulated. Articulated specimens are largely confined to very small or juvenile bivalves. Siliciclastics are sparsely interspersed throughout matrix and locally concentrated in small pockets. Fauna: A moderately diverse bivalve community (> 7 genera including Modiolus, Entolium, Gressyla and Weyla bodenbenderi?) is represented. Epifaunal forms (including numerous Pectenids) are marginally dominant to infaunal forms. Moderate to thin-shelled bivalves are marginally dominant to thick-shelled varieties. Moderately ornate bivalves are common. Clast age is pre-Late Pliensbachian Early Jurassic and probably of Sinemurian or Early Pliensbachian age. Interpretation: Texture and composition is consistent with a quiet water environment subject to periodic influxes of terrigenous and bioclastic detritus. The common occurrence of volcanic clasts confirms B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 53 mechanical processes form a significant (but subordinate) component of deposition and indicates an environment above storm wave base. C9. Sample GGAJ-92-57-FA-2 (talus) - Silty skeletal floatstone Color: Rusty buff to rusty weathering, medium grey fresh. Composition: Mixed bioclasts (bivalves, minor crinoids) range from thick, large varieties to small thin fragments. Matrix is a sandy carbonate silt with minor mud and forms up to ~ 50% of rock volume. Common siliciclastic (10-20%) mineral grains dominated by quartz with minor feldspar (?) and rare euhedral biotite. Small plant fragments are also common. Texture: Heterogeneous, very poorly sorted, fine granular matrix. Bioclasts are dominantly bivalve fragments ranging from > 6 cm to < 4 mm. Detrital siliciclastics are of silt to fine sand grade. Fauna: Bivalves include Weyla bodenbenderi (M. Aberhan - pers. comm., 1994). Also present are common crinoid ossicles of Pentacrinus. A pre-Late Pliensbachian Early Jurassic age is indicated. Interpretation: Dominant mechanical processes appear to characterize the depositional texture. A n environment of moderately high energy is indicated, possibly within normal wave base or affected by strong bottom currents. CIO. Sample GGAJ-92-57-FB (outcrop) - Skeletal (molluscan) floatstone Color: Buff brown to rusty weathering, dark greyish black fresh. Composition: Bioclasts range from thin, fine (< 0.5 cm) to thick, large (5+ cm) pelecypod fragments and valves, dominated by medium to thick-shelled varieties. Other clasts include minor (-5%) black muddy bioclastic intraclasts, sparse plant fragments (up to 1 x 5 cm), and rare siliciclastics. Matrix forms a major proportion of rock volume (60-70%) and is composed of a silty carbonate mud. Texture: Extremely poorly sorted heterogeneous mixture of silty mud, large bivalves and fragments. Bioclasts have an essentially bimodal occurrence: large pelecypod valves and fragments (disarticulated > articulated), and fine shell fragments (often in local small 'packed' pockets). B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 54 Fauna: Assemblage is strongly dominated by a moderately diverse (> 6 genera) bivalve community with sparse small gastropods and rare ammonites. Infaunal forms are dominant with the major proportion composed of large, thick-shelled specimens of Pleuromya. Two poorly preserved ammonites suggest a possible Early Sinemurian age (Metophioceras ? - H . Tipper, pers. comm.). Interpretation: The prevalence of one genus (Pleuromya) as well as other textural features (sorting, abundance of mud in matrix) indicates a major degree of bioaccumulation in this 'unit'. A low-energy environment below normal-wave base and above storm-wave base is consistent with the textural and compositional features. C l l . Sample TD-84-06-F (talus) - Silty molluscan floatstone Color: Buff to medium dark brown weathering, dark grey fresh. Composition: Bioclasts range from thin, fine (< 0.5 cm) to thick, large (5+ cm) pelecypod fragments and valves, dominated by medium to thick-shelled varieties. Other clasts include sparse plant fragments and rare siliciclastics. Matrix forms a major proportion of rock volume (~ 60%) and is composed of muddy fine lime silt containing sparse siliciclastics (quartz, biotite). Texture: Heterogeneous, very poorly sorted. Clast texture consists of whole bivalves and fragments suspended in a muddy lime silt matrix. Pelecypod valves and fragments are strongly dominant and are mainly in the range of 2 to 4 cm with rare shells to ~5 cm. Framework texture is loosely 'packed' with whole valves » fragments and articulated > disarticulated. Fauna: A moderately high diversity bivalve community dominated by infaunal forms. Over 7 genera including Weyla bodenbenderi and W. alata? (M. Aberhan - pers. comm., 1994), common Gressyla, Pleuromya, Modiolus, and Entolium) are represented, with minor gastropods (Pyrgotrochus?) and ammonites. The ammonites are echioceratids identified as Paltechioceras sp. and Echioceras? sp., giving an age of Late Sinemurian. Interpretation: The prevalence of whole infaunal forms (mainly Gressyla and Pleuromya), as well as other textural features such as poor sorting, presence of mud in matrix, and rare siliciclastics, indicate a B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 55 major degree of bioaccumulation in this 'unit'. A low-energy environment below normal-wave base and above storm-wave base is consistent with these textural and compositional features. C12. Sample GGAJ-93-1 -FB (talus) - Muddy skeletal floatstone-rudstone Color: Medium buff to tan brown weathering, dark grey fresh. Composition: Matrix is dominant (-50-60%) and composed of carbonate mud with abundant very fine silt. Fine particles include common carbonate, carbonaceous material, minor oxidized material and sparse siliciclastics. Bioclasts are an important subordinate component (-30-40%) with abundant bivalves, common ammonites, plant material, and rare gastropods. Texture: Heterogeneous, very poorly sorted. Bivalves range from < 0.5 to 3 cm with whole > fragments, disarticulated > articulated. Stick fragments to > 5 cm are moderately common. Fauna: Ammonites are of probable Late Sinemurian affinity (Pahechioceras?). Bivalves represent > 5 genera (incl. Eopecten?, Pinna?) with pectenids subordinate to infaunal forms. Interpretation: Bioaccumulation is the strongly dominant depositional mode but mechanical processes are a significant minor component. A low energy environment below normal wave base and influenced by storm-waves or bottom currents is indicated. C13. GGAJ-93-12-FA (outcrop) - Skeletal mudstone-floatstone Color: Light to medium buff brown weathering, dark grey fresh. Composition: Silty carbonate mud dominates (-75%) with a ubiquitous minor, very fine silt to fine sand grade component of bioclastic and siliciclastic detritus. Fine particles are dominated by cabonates with subordinate siliciclastics (quartz > biotite) and minor carbonaceous material. Minor bioclasts (~ 20-25%) are strongly dominated by ammonites with minor belemnoids, bivalves and sparse plant fragments.. Texture: Relatively homogeneous, moderately sorted. Very fine silt sparsely interspersed throughout matrix with very fine to fine sand locally concentrated in small pockets. Very common tiny blebs ( « 1 mm) of pyrite are uniformly disseminated throughout matrix and comprise a few %. Ammonites are common throughout and range from < 1cm to > 8 cm with whole forms » fragments. Bivalves range from B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 56 a few mm to > 5 cm, whole » fragments, articulated » disarticulated. Plant fragments include 'sticks' and leaf/grass impressions to 4+ cm. Fauna: Ammonites comprise a moderately diverse Early Pliensbachian Whiteavesi Zone fauna. At least 4 genera are represented and include Dubariceras silviesi, Acanthopleuroceras thomsoni, Metaderoceras cf. talkeetnaense, and Olstoceras compressum. Bivalves are dominantly unornamented infaunal forms (Pleuromya?/Gressyla?, Modiolus?) with minor pectenids (Eopecten?). Interpretation: Textural and compositional features indicate an essentially bioaccumulated unit deposited in a moderately low to low energy environment, most likely below storm-wave base. C14. Sample GGAJ-93-12-FB (outcrop) - Muddy carbonaceous floatstone Color: Light to medium buff brown weathering, dark greyish black fresh. Composition: Matrix is dominant (-60%) and is composed of very fine to fine silty mud. Bioclasts' are strongly dominated by plant material (-35%) with minor ammonites, belemnoids, and bivalves. Siliciclastics are sparse with euhedral biotite » quartz /feldspar. Texture: Heterogeneous, very poorly sorted. Crudely stratified with stratification defined by plant material. Plant fragments range from fine (< 1 cm) to coarse 'sticks' ( > 4 x 20 cm) and display high thermal maturation (semi-anthracite to anthracite rank). Fauna are strongly dominated by whole ammonites that range from < 1 cm to > 6 cm. Minor belemnoids (> 14 cm) and rare bivalves a few cm long were also observed. Fauna: Ammonites are predominantly the Early Sinemurian forms Coroniceras and possibly Arnioceras? Bivalves are infaunal forms (Modiolus?) lacking ornamentation. Interpretation: Texture and composition indicate periodic influxes of terrigenous organic material into a low energy environment. A quiet water setting below normal wave base and above storm-wave base (or channel/bottom-current influenced) is indicated. B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 57 2.9 DISCUSSION AND CONCLUSIONS While shelf reconstructions are not possible from such a limited suite of samples, they can provide valuable insight into aspects of their depositional setting. A spectrum of shallow marine environments is indicated by lithological characteristics, ranging from moderately high energy to varying degrees of low energy. Their faunal content indicates a thriving benthos and allows for reasonable speculation on some environmental conditions. The age of the clasts is relevant to paleoenvironmental interpretations, as well as functioning as indicators of substage regional tectonic regimes. 2.9.1 SEDIMENTOLOGY AND PALEOENVIRONMENTAL INTERPRETATIONS The sedimentological and faunal characteristics of the fossiliferous clasts are typical of particular shelf environments and provides insight into former basin margin settings. These characteristics are typical of relatively shallow (< 100 m) quiet water settings subjected to minor periodic storm-induced influxes of bioclastic and siliciclastic allochems. The 'framework' component is strongly dominated by a moderately diverse bivalve fauna that is interpreted to be of a largely in situ nature. Clast specimens exhibit a predominantly restricted range of sedimentological characteristics that indicate related shallow marine depositional environments. Rock textures exhibit very poor sorting and composition consists primarily of silty carbonate mud and shelly fauna. The carbonates contain a significant, often dominant, skeletal component and textures range from rudstones to mudstones with varying amounts of carbonate mud, silt, sand and minor siliciclastics composing matrix. Skeletal components are strongly dominated by bivalves (> 90 %) with minor gastropods, crinoids, belemnites and ammonites. The biogenic framework includes both allochemic and in situ bioaccumulated components with an emphasis on in situ in most cases. Only one sample (C3) is of a purely bioclastic nature. Molluscan wackestones are a characteristic Mesozoic shelf facies with molluscan and coral biostromes also common (Carozzi, 1989). Most samples in this study may be characterized as molluscan wackestones/floatstones and packstones/rudstones and are products of relatively low energy environments below normal wave base. The very poor sorting (especially in the skeletal component) and presence of mud in these rocks is typical of in situ accumulations (Blatt, 1992). Both the skeletal and matrix components contain varying degrees of clastic input,ranging from significant (C8) to minor (CIO), B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 58 probably from periodic storm-generated influxes of bioclastic and terrigenous detritus, indicating environments above (or near) storm-wave base. These types of deposits often accumulate in local topographic lows but may also form small topographic highs as lime mud mounds of a biostromal nature (Wilson and Jordan, 1983). The lack of stratigraphic context for these deposits limits interpretation and introduces a rather speculative note; however, the range of faunal and sedimentological characteristics suggest both settings may be represented in the sample suite. The fact that these fossiliferous carbonates are common in the debris flow, and many apparently coeval samples possess subtle differences with respect to matrix and faunal content, indicates relatively widespread carbonate-dominated shelf environments. Given that each clast may be considered a 'grab' sample unlikely to represent the full generic diversity of the particular environment, it seems reasonable to conclude that samples containing a conservatively estimated 6 or more genera are examples of high diversity bivalve paleocommunities. This assumption allows certain conclusions to be drawn with respect to the paleobathymetry of their environments, in accordance with the depth zonation scheme of Taylor (1982). With the possible exception of a few samples (i.e., C l , C2, C6, C14), all samples may be assigned to Taylor's (1982) Composite Assemblage zones B and C (Fig. 2.8). Those samples interpreted to be high diversity bivalve communities possess many of the faunal characteristics (diversity and abundance, shell size, thickness, and ornamentation), as well as the appropriate sedimentological characteristics to satisfy the major criteria of Taylor's zonation scheme. A number of these samples (C8, C9, C3, C4, C15) are interpreted to occupy positions within Composite Assemblage zone B and possess a greater mechanical component than other samples (C7, CIO, C5, C12) which have strong faunal and sedimentological affinities to zone C l . A few samples ( C l , C2, C6, C14) appear to occupy a position offshore from the other samples, possibly zone C2 or D. A relatively shallow paleobathymetry is implicit in Composite Assemblages B and C, and Hallam (1976) suggests it is in the order of less than 50-100 m, with diversity and abundance falling off sharply below these depths. The fossiliferous carbonate clasts range from Lower Sinemurian to Lower Pliensbachian, indicating the likelihood that a relatively stable shallow marine environment favorable to at least local carbonate accumulation existed throughout much of this time. Environmental conditions conducive to benthic life B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 59 are indicated by the moderately diverse and abundant bivalve fauna. Bivalve communities range from those dominated by infaunal burrowers (especially Pholadomyoids) with minor byssally-attached forms (mainly Pectinids) to communities dominated by epifaunal forms with subordinate burrowers. Some speculations on paleoenvironment are possible and are as follows; generally normal salinity and C O M P O S I T E A S S E M B L A G E S A m m o n i t e s B e l e m n i t e s Naut i l idS T c r e b r o t u l i d s Rhynchoneli idS. Ina r t i cu la te ' S r a c h i o p o d s Benthonic Bivalves "Of lshore" B i v a l v e s S c a p h o p o d s G a s t r o p o d s S e r p u l i d s C r i n o i d s c 0 C(l) | C(2) 0(1) |0<2) gravel & ' -c sand s i l t -very fine ~~ very l ine-coarse sand ^ O F F S H O R E mud(lominoted)"~ sand NEAHSHOHE Figure 2.8. Composite Assemblage Zones of Taylor (1982) depicting nearshore to oflshore distribution and abundances (bar thickness) of Jurassic shallow-marine invertebrates. moderately to well-oxygenated waters appear to prevail as indicated by the diversity and abundance of bivalves i n most samples, as well as the presence of stenohaline organisms such as crinoids, belemnites, and ammonites in others. Some indications of possibly fluctuating conditions might be the apparent absence of brachiopods and corals (only one isolated "button' coral found in matrix), strong generic (bivalve) dominance in some samples (C4, C9), general matrix color (often dark grey/black, suggesting possible semi-anoxic substrate conditions), and lack of trace fossils. Bioturbation structures are largely absent where other sedimentological and faunal characteristics provide the expectation of some degree of soft-bodied infauna. This suggests bottom-water (or substrate pore-water) conditions inimicable to such infauna, however alternately, the abundance of burrowing bivalves in the muddy carbonates may be responsible for the destruction of these structures, i f they did indeed exist. 2.9.2 T E C T O N I C I M P L I C A T I O N S The presence of late Lower Pliensbachian shallow marine carbonates in an early Upper Pliensbachian (Kunae Zone) debris flow conglomerate provides some insight into the regional tectonic regime. The fact that "young' strata are being eroded implies some degree of active uplift along the basin B I O C H R O N O L O G Y A N D BIOSTRATIGRAPHY 60 margin although controls may be eustatic rather than tectonic. Speculations on possible tectonic influences on regional sedimentation must be considered highly tenuous i f based on isolated occurrences from a particular stratigraphic horizon. However, significant erosion of the shallow marine succession is indicated by the co-occurrence of common Upper Sinemurian carbonate 'blocks' and olistoliths along with Early Sinemurian and Early Pliensbachian clasts in the same debris flow. The relative percentages of dated clasts is indicative of the degree of erosion by Late Pliensbachian time. Of those clasts firmly dated, over 50% are Late Sinemurian, outnumbering combined Early Sinemurian and Early Pliensbachian clasts by over 2:1. This suggests that Upper Sinemurian strata were the primary locus of shelf erosion at this time, with most of the Early Pliensbachian shallow-marine succession already removed and locally the Upper Sinemurian as well, allowing some erosion of Lower Sinemurian strata. It should be noted that the Pliensbachian stage is generally characterized by a high siliciclastic input (Ch. 4, Ch. 5) compared with the Sinemurian and a reduction in shallow-marine carbonate environments due to clastic dilution seems likely to have occurred. This may account for the low frequency of Lower Pliensbachian clasts in the debris flow and the observation that only environments reflecting outer shelf depositional settings are represented by Lower Pliensbachian clasts. The data and interpretations are strengthened when viewed in a regional context. The evidence for accelerated tectonic uplift in Late Pliensbachian time is supported by the regional eustatic regime, which was a period of widespread transgressive seas throughout much of the northern and central Cordillera (H.W. Tipper, 1993: pers. comm.). The fact that significant relative uplift was being generated during a regional highstand indicates that the progressive erosional stripping of shelf facies occurring by early Late Pliensbachian time was tectonically driven. Independent lines of evidence from study of sandstones (Ch. 4) and conglomerates (Ch. 3) support this interpretation. In summary, these fossiliferous carbonate clasts provide valuable information in a dual capacity; they furnish the only physical evidence of Lower Jurassic shelf facies in the study area, indicating the existence of a stable (regional?) carbonate-dominated shelf along the basin margin during at least Sinemurian and possibly Early Pliensbachian time; and their age relationships indicate active erosion was well underway by the early Late Pliensbachian, suggesting some degree of tectonic uplift by that time. C O N G L O M E R A T E A N A L Y S I S C H A P T E R 3 C O N G L O M E R A T E ANALYSIS 61 3.1 INTRODUCTION Analysis of Inklin Formation conglomerates was conducted for the purpose of provenance determinations and consisted of two primary objectives. The first aim was to attempt specific provenance linkages with existing formations through detailed analysis of clast collections. The second objective was to determine whether any temporal clast trends exist in the Inklin Formation at Atlin Lake and to delineate and characterize such trends as may be present in an attempt to unravel the tectonic and magmatic history of the Lower Jurassic Stikinian arc. In addition to the primary objectives, the sedimentary context and features of conglomerates were of interest from a sedimentological perspective and findings are included in the analysis. Conglomerates provide valuable insight into regional tectonic regimes and often furnish the best record of provenance, at times containing the only lithological evidence of source areas completely removed by erosion. Distinct temporal trends in clast composition and proportion have emerged from this study and allow insight into aspects of arc evolution during the Early Jurassic. Fundamental to the analysis of conglomerates was good biostratigraphic control furnished by ammonites. Seventeen of the samples studied are constrained at the zonal level of resolution based on the ammonite biochronozone schemes of Smith et al. (1988) and Palfy (1991). The remaining samples have at least sub-stage stratigraphic resolution (e.g., Lower Pliensbachian). Although the sample suite is relatively small (N=24) their tight age constraints provide the opportunity to delineate temporal trends with respect to clast lithologies with some confidence. Resedimented deep-marine conglomerates and conglomeratic beds of the Laberge Group are found in highly variable proportions throughout the length of the Whitehorse trough. To the north of the study area in the Whitehorse region coarse polymictic conglomerates of Early Jurassic age form thick successions that C O N G L O M E R A T E A N A L Y S I S 62 have been subdivided from coeval basinal strata into the Conglomerate Formation (Tempelman-Kluit, 1985). Similar thick conglomerate sequences are not found in the Atlin Lake area. In northern B.C. the Laberge Group has been subdivided into the Takwahoni and Inklin Formations with the Takwahoni Formation interpreted to be the coarser proximal equivalent to the basinal Inklin Formation (Souther, 1971). Although the Takwahoni Formation contains a much higher proportion of conglomerates than the Inklin Formation it does not approach the volume of the Conglomerate Formation. It also appears to be only partly coeval with the Inklin Formation as its basal strata represents only Late Pliensbachian and later time (H. Tipper - pers. comm., 1992). The Takwahoni Formation is nowhere exposed at Atlin Lake although previous work had indicated the presence of this formation on southwest Atlin Lake (Bultman, 1979). However, these green and maroon largely monolithic volcanic conglomerates and agglomerates appear identical to Triassic Stuhini Group rocks (H. Tipper - pers. comm., 1992) and are found (structurally?) underlying Upper Triassic Sinwa Formation-correlative carbonates (Bultman, 1979). Further, these rocks are nowhere found in contact with Inklin Formation sediments at Atlin Lake and thus are dismissed as correlatives of the Takwahoni Formation, being instead considered of Stuhini Group affinity. Conglomerates comprise only a minor volume of the Inklin Formation at Atlin Lake. They are found scattered throughout the succession but only occasionally form units thicker than a few metres. They are most commonly associated with thick sand sequences but occur in a variety of facies including thin-bedded muddy turbidites. Inklin Formation conglomerates exhibit a variety of internal textures and fabrics, and include lensoid (channelized) and thin, tabular sheet geometries. Conglomerates in the study area are characteristically polymictic although relative clast modes vary widely through the Inklin succession. Some of these features appear to be stratigraphically confined and are discussed in more detail in following sections. 3.2 METHODS The sample suite is divided into three stratigraphic sub-sets corresponding to Sinemurian (n=9), Lower Pliensbachian (n=8), and Upper Pliensbachian (n=7)(see Table 3.1). The Sinemurian sub-set is composed of six late Lower Sinemurian (mainly Amouldi Zone) and three late Upper Sinemurian units C O N G L O M E R A T E A N A L Y S I S 63 (Harbledownense Zone plus undifferentiated units); the Early Pliensbachian sub-set contains at least one Whiteavesi Zone unit, three others of probable Whiteavesi Zone age but possibly Imlayi Zone, and four undifferentiated Early Pliensbachian units; and the Upper Pliensbachian sub-set is composed entirely of Kunae Zone age units. Pebble-count localities are shown in Figure 3.1. Both qualitative and quantitative conglomerate data were collected. The quantitative approach was largely conducted in the field on outcrop and consisted of clast counts, modal clast size and maximum clast size determinations. The techniques used for these procedures can be found in many sedimentary petrology textbooks (q.v. Graham, 1988; Blatt, 1992). For the purposes of clast counts only basic lithologic divisions were used (i.e., plutonic, volcanic, sedimentary, and limestone) with no attempt at subdivision, as weathering on outcrop sometimes rendered more detailed divisions both time-consuming and uncertain. In only one case (GGAJ-92-179) was weathering too intense for confident determination of these basic divisions in the field. In this case a random collection of clasts (n=54) was obtained from outcrop and counted in the lab after processing to expose 'fresh' surfaces (including slabbing). The average count consisted of 95 clasts. The qualitative aspect of analysis was largely conducted in the lab using clast collections obtained at clast count sites. Clasts were collected in a semi-qualitative fashion in an attempt to sample all available clast lithologies. Clasts were processed to expose fresh surfaces (mainly by slabbing) and then studied using a binocular microscope and hand-lens. In most cases accurate identifications based on their compositional and textural features were obtained. A n average of 36 clasts was collected from each outcrop. Inklin Formation clast lithologies are listed by age in Appendix 3. For the sake of brevity, the terminology for unconsolidated sediments (i.e. gravel, sand, silt, mud) is often used in the following sections instead of their more cumbersome lithified equivalents. For ease of reference, conglomerates have been grouped into types based on their internal textures, clast fabrics and other features (see Table 3.2 for details). These should be considered as broadly inclusive genetic divisions that reflect the primary flow processes responsible for their deposition rather than rigorous detailed sub-divisions. The classifications imply no specific lithofacies association. Figure 3.1. Inklin Formation pebble-count localities.Letter prefix = unit age, first number = conglomerate type, second number = unit. E/L = Early/Late, S = Sinemurian, P = Pliensbachian. (E.g., LP-2-3 = Late Pliensbachian - Type 2 - unit 3). C O N G L O M E R A T E A N A L Y S I S 65 L U OO LU CO C M CO CO cn co C O CM C O CO C O CM LO C M C O L O CM CO C O C M 00 CO CM C M co CM in C O CM CO 00 00 LO 00 00 00 co C O 00 CO CO CM CO LO CO CO C/3 C O — es H C O N G L O M E R A T E A N A L Y S I S CU o. >> H u a a o U ri 3 « 5) X °E ** v H 2 -2 CO cd 3 6 3 3 <*> is § « .2 € 1 & o 3 en •3 S3 OX) E a o p Z cw 'C •a « « g o u .a 5 5 B o w . 3 V 43 M pa V -«-» Si la E a, •2 ti"1 e o U 6 CD •s g cn C 2 I « CD "g I .a s OT 3 I -a T3 "« "3 O . O <L> o T3 I § § 0 PH CO JD CD Q. o 5 © A C/J o 6 T3 1 CD -a cs & 3* u m 3 P .2 5 .s 0 0 1 o O CD « O C cu o T 3 O T J -!3 c« cu PH 3 < •5 .s «, •7-1 I S Cft 2 g 8 ~"' 13 ' > .2 ^ I & o a -a IS o o cj CO I I O" CJ 0) 1) i_ CO 8 8 CU CD 3 2 CD O 0- X> + i n CQ <L> •S e o CO '6b' CD T3 O (L> OT cn 3 I I O cn CD 1 § , TS - a "O a « in I 6 % 3 s 5 CD T3 CD & g 1 cd X> S £ s i CO QU 3 X ) CD O . e -S 2 .3 g tL, X ) o <U CO g ST>I CD o T 3 O o CO I •3 CD 3 o o> 3 CJ .3 -8 vi o o .9 "8 O T 3 CD T 3 ca CD is o -o 3 o CJ CO 3 x> o CD g I 3 CD CD 3 2 x> 3 eu o O C O N G L O M E R A T E A N A L Y S I S 67 3.3 S I N E M U R I A N C O N G L O M E R A T E Sinemurian conglomerates display a number of distinctive features with respect to facies association, internal texture and fabric, and clast composition that together allow them to be confidently distinguished from other conglomerates in the succession at Atlin Lake. The most diagnostic criteria relate to clast compositions and percentages, in particular their sedimentary and carbonate content. In most cases it was not possible to determine lateral variations within conglomerate units due to the nature of the outcrop. The outcrops are generally steeply dipping, extensive shoreline exposures with bedding often oriented roughly perpendicular to the shore. They usually present steep topographic profiles, forming small bluffs and cliffs ranging from a few metres to a few tens of metres high. The tops of most exposures are almost always capped by bush so that there is usually little or no exposure of outcrop away from the shore. 3.3.1 General Description and Interpretation The first type of conglomerate (designated Type 1A) is volumetrically the most important Sinemurian conglomerate in the study area and can be considered unique to the Sinemurian section through a combination of clast content, internal texture and fabric, and facies association. These units comprise over 50% of all Sinemurian conglomerates encountered in the study area. They are characterized by thick beds that range from about 2 to 18 metres, and a generally disorganized ungraded internal fabric. The matrix is always mud-rich, ranging from muddy sands through silty muds and forms between 40 and 90 % of unit volume (Fig. 3.2). Some of the clast-poor units are transitional to pebbly mudstones but these varieties are not included in the data-set. Consequently, Type 1A conglomerates are dominantly matrix-supported but also include more complex units capped by clast-supported fabrics. Type 1A conglomerates are generally poorly to non-sorted with clast sizes ranging from fine pebbles to boulders and blocks in excess of 1.5 metres. These beds rarely show any grading or stratification although very thick beds may display a succession of internal fabrics that include normal coarse tail grading in the upper divisions. A n example of this type is an 18 metre thick unit on a scoured base with the lower 9 metres composed of a chaotic, muddy, matrix-supported, unsorted cobble-boulder conglomerate with abundant intraformational clasts displaying soft-sediment deformation. These latter clasts form the entire boulder C O N G L O M E R A T E A N A L Y S I S Figure 3.2. Thick Lower Sinemurian Type 1 debris flow conglomerate overlying muddy distal turbidite strata on planar base. Note dominance of muddy matrix and absence of clast fabric or internal organization. C O N G L O M E R A T E A N A L Y S I S 69 component and much of the cobbles along with igneous and carbonate clasts, and are often angular to rounded clasts of dislocated and balled fine-grained strata with original laminae sometimes strongly deformed (Fig. 3.3A). Pockets of chaotic slump folds are found within the muddy matrix. This thick basal division is capped by a 2 metre layer of matrix-supported, poorly-sorted pebble-cobble conglomerate that crudely grades upward into a well-graded, moderately sorted, clast-supported coarse to fine pebble conglomerate layer at the top. This upper division is about 4 metres thick with clasts dominated by well-rounded sub-spherical igneous and carbonate rocks, and displays a sharp planar upper contact with the overlying finely laminated muddy silts and sands. These thick, complex units generally show gradational or blurred transitions with no apparent grain-size breaks or scours between internal divisions, indicating they were formed by a single depositional event. Whether or not Type 1A conglomerates contain an upper normally graded division, they all display some internal evidence of wet-sediment deformation. Basal contacts range from planar to irregular low-relief scours but are generally incised though not always obviously erosive (Fig. 3.2, 3.3B). The dominant facies asssociation for these units is overlying thick sequences of fine-grained, thin rhythmically bedded muddy turbidites of fan-fringe and basin plain affinity (Fig. 3.3B). For the majority of Type 1A conglomerates the underlying muddy sequences commonly display features associated with gravity-induced submarine slumps or slides such as slump folds, dislocated or contorted strata, and sparse dispersed igneous pebbles and small cobbles. This underlying soft-sediment deformation ranges from mild to chaotic. The conglomerates may be sharply overlain by similar undeformed muddy turbidites or may pass gradationally into medium to thick-bedded massive or pebbly sandstones. Textural and compositional features of Type 1A conglomerates clearly indicate deposition by cohesive mass-flow processes (Lowe, 1982; Pickering et al., 1986) and are interpreted as debris flow deposits. The intimate association of some Type 1A conglomerates with matrix-supported to clast-supported graded caps implies a causal link. It is possible that debris flows and slumps were triggered by loading and shear induced from the passage and deposition of high-concentration gravelly turbidity currents (Kelling & Stanley, 1976). C O N G L O M E R A T E A N A L Y S I S Figure 3.3. A : Dislocated and plastically-deformed intraclast in lower division of a Sinemurian Type 1 debris flow conglomerate. B : Thick Early Sinemurian Type 1 debris flow conglomerate overlying gently incised muddy turbidite sequence. Basal strata are interpreted to represent a lower fan fringe setting. C O N G L O M E R A T E A N A L Y S I S 71 A variant of Type 1 conglomerates require sub-division on the basis of their matrix and clast fabric. Three units (GGAJ-92-50/52/53)(designated TypelB) possess a 'greywacke' matrix dominated by fine to medium grained sands. They are thick (4 to > 15 metres), poorly sorted, mainly clast-supported pebble-cobble conglomerates with a largely disorganized fabric that may locally display poorly developed normal grading. The thickest unit is capped by a moderately developed, distribution graded upper division a few metres thick which is absent in the other two units but present some Type 1A conglomerates(Fig. 3.4A). The base is always erosive with low to high-relief scours and is commonly dominated in the basal 1 to 2 metres by tabular cobble-grade intraclasts with a well-developed bedding-parallel alignment showing a-axis imbrication (Fig. 3.4B). Tops are gradational into coarse massive sands. A l l units overlie thick muddy sequences, two of which were observed to contain slump folds. Two of these units had sufficient exposure to display an irregular lensoid bed geometry and are channelized deposits. These features are consistent with non-cohesive mass-flow processes (Lowe, 1982; Pickering et al., 1986) and are interpreted as products of gravelly cohesionless debris flows. The maximum clast size of Type 1 deposits on rare occasions exceeds 2 metres and boulders in the range of 1 metre are fairly common. These large clasts sizes are universally composed of intraformational sediments, many of which bear evidence of soft-sediment deformation. The second common conglomerate type (designated Type 2) is not found associated with submarine slumps. Nine units of this type were encountered in the study area and are relatively thin, ranging from a few decimetres to rarely in excess of 4 metres with most beds generally between 2 and 3 metres thick. They are predominantly normally graded, clast-supported coarse pebble to small cobble conglomerates with a moderately mud-poor coarse to fine sandy matrix (Fig. 3.5). Normal grading of both coarse tail and distribution types are present in these units. Type 2 conglomerates are dominantly moderate to poorly sorted units lacking internal stratification although weak planar stratification was noted in the top of two units displaying normal distribution grading into pebbly sands. The bases are commonly erosive with low-relief scours while upper contacts may be either gradational into coarse sands or sharp planar surfaces overlain by fine-grained sediments. In those few cases where units could be traced for a few tens of metres, both lensoid and tabular geometries were observed, indicating both channeling and spillover C O N G L O M E R A T E A N A L Y S I S Figure 3.4. A : Composite Sinemurian Type 1 debris flow conglomerate. Note the transition (arrow) from disorganized, muddy matrix-supported cobble conglomerate to normally-graded clast-supported upper division. B : Tabular cobble-grade intraclasts dominate clasts in basal divisions of some Sinemurian Type 1 debris flow conglomerates. Intraclasts are aligned parallel to bedding and display a-axis imbrication. Center scale = 5cm. C O N G L O M E R A T E A N A L Y S I S 73 Figure 3.5. Sinemurian Type 2 conglomerate displaying moderate sorting and normal grading. Arrow indicates direction of tops. GRADED-STRATIFIED NO INVERSE GRADING S T R A T . , C R O S S - S T R A T . IMBRICATED G R A D E D - B E D o o o o o o C> O <=> O o O &C?c? <2) NO INVERSE GRADING NO S T R A T . IMBRICATED I N V E R S E - T O N O R M A L L Y GRADED NO STRAT. IMBRICATED THESE THREE MODELS SHOWN IN S U G G E S T E D RELATIVE POSITIONS DOWNCURRENT DISORGANIZED-BED NO GRADING NO INVERSE GRADING NO S T R A T . IMBRIC. R A R E Figure 3.6. Four models for resedimented (deep-water) conglomerates proposed by Walker (1975).(The three models with clast fabrics are shown in their inferred relative downcurrent positions. This sequence does not imply a specific fan depositional setting. C O N G L O M E R A T E A N A L Y S I S 74 (sheet flood) processes are responsible for bed geometries. The clast-supported fabric, moderate sorting and normal grading that characterize Type 2 conglomerates identify the flow processes responsible for their deposition. They are generated from gravelly, high-density, cohesionless turbidity currents and represent the downslope or distal expression of the gravel sedimentation wave (Lowe, 1982). The absence of inverse grading, moderate sorting and common coarse tail grading indicates the absence of a traction carpet and deposition by direct suspension sedimentation (Lowe, 1982; Pickering et al., 1986). These units correspond to the normally-graded and graded-stratified conglomerate facies of Walker (1975, 1984) which is interpreted to occur downstream of the inverse-to-normally-graded conglomerate facies where tractive processes are operative. The common facies association of Type 2 conglomerates with generally mud-dominated sequences tends to support Walker's (1975, 1984) suggestion that deposits of this type are distal deposits (Fig. 3.6). Although clast imbrication is often associated with the normally graded facies (Walker, 1975), it is largely absent or at best poorly developed in Type 2 conglomerates. This general absence of clast fabric can be attributed to the dominance of well-rounded sub-spherical clasts in these units. 3.3.2 Clast Composition Sinemurian conglomerate clast modes are diagnostic features that permit the distinction of these units from all others in the succession at Atlin Lake. Clast lithologies found in Sinemurian conglomerates are listed in Appendix 3. No attempt was made to estimate percentages of modal sub-groups (i.e., dacite from rhyodacite, etc.) so that the observations made here on relative proportions are of a purely qualitative nature. The lack of quantitative data to support these observations weakens their usefulness but they may still serve as useful indicators of provenance and support interpretations based on clast mode data. Figure 3.7 depicts relative clast modes and temporal trends of Inklin Formation conglomerates. Details of normalized clast modes and pebble-count data are found in Table 3.1. Sinemurian conglomerates are generally dominated by igneous rocks with the primary clast lithology composed of volcanic varieties. Clast percentages of volcanic rocks range from 33 to 56% and average 43%. Plutonic rocks are always subordinate to minor, ranging from a low of 11% to as high as 31% with C O N G L O M E R A T E A N A L Y S I S 75 an average of 21%. The Sinemurian volcanic clast suite displays a wide range of composition and textures and is the most heterolithic in the Inklin succession. Rhyolitic through basaltic compositions are present but usually minor end-members. Volcanic textures include aphanitic and micro-phyric varieties through to porphyritic and pyroclastic rocks. Qualitative observations permit the establishment of some basic trends with respect to volcanic modes. Pyroclastic rocks form only minor to trace amounts of the volcanic suite, with the vast bulk of volcanic modes comprised of intermediate, massive, aphanitic to micro-phyric/phyric varieties and more felsic fine to medium grained porphyritic rocks. Significantly there is no clear dominance by either textural variant in Sinemurian conglomerates. Both form an appreciable portion of the volcanic clasts and either may be marginally dominant in a particular unit. The aphanitic and micro-phyric varieties are dominantly andestic to basaltic in composition but include minor more felsic variants such as latites and rare trachytic rhyolites. They are commonly medium to dark green, dark grey to black, and dark brownish to rusty red (Fig. 3.8A). The color of these latter types is attributed to oxidization of abundant ferro-magnesian minerals and suggests some degree of sub-aerial exposure during transport. These fine-grained volcanics tend to show higher degrees of alteration than the more felsic porphyries. The fine to medium grained porphyritic varieties contain an appreciable andesitic component but are dominated by more felsic compositions. Hornblende-feldspar (± biotite), biotite-feldspar, and quartz-feldspar porphyritic dacites are most common with more felsic variants such as biotite-quartz-feldspar porphyritic rhyodacites ('quartz-eye' porphyry) generally minor. Pyroclastics appear to be all dacitic in composition and range from crystal and crystal-lithic dust tuffs through crystal-lithic fine lapilli tuffs. Plutonic rocks are generally a relatively minor clast mode and are often subordinate to limestone, and to a lesser degree sedimentary, clasts. Plutonic modes are dominated by leucocratic varieties of mainly felsic to intermediate compositions. The most common varieties are biotite (± hornblende) quartz monzonites and monzodiorites but the plutonic suite also includes monzonites, quartz monzodiorites and monzogranites. The plutonics tend to be quartz-poor even when falling in the quartz monzonite/quartz monzodiorite field, so that granitic rocks such as monzogranite form a minor component of the plutonic suite. Plutonics are 76 u 93 £ c c o B 0 o LL, a -a a 5 2 0 r U s u C O N G L O M E R A T E A N A L Y S I S Figure 3.8. A : Heterolithic volcanic clasts dominate Sinemurian conglomerates. Also visible are larger tabular sedimentary intraclasts displaying a-axis imbrication (1), Sinwa Formation carbonate clasts (2), and granitoid cobble (3). B: Distinctive grey carbonates (1) and sedimentary intraclasts (2) are ubiquitous clast components characteristic of Sinemurian conglomerates. C O N G L O M E R A T E A N A L Y S I S 78 generally smooth and well-rounded sub-spherical clasts but tend to be highly weathered and/or altered. No mafic varities such as diorites or gabbroic rocks were identified in Sinemurian conglomerates. Two sedimentary clast types are distinctive and ubiquitous components of Sinemurian conglomerates. The first component is comprised of grey microcrystalline carbonates (Fig. 3.8B). These tend to be well-rounded sub-spherical to prolate clasts and range from 6% (atypical low) to 31%. These carbonates can dominate the fine to coarse pebble fraction in some units but also occur up to boulder-grade sizes. A mean value of 20% for these carbonate clasts makes them an important subordinate, sometimes secondary, clast mode in Sinemurian conglomerates - an abundance confined to the Sinemurian, making this a unique feature of these units. They weather shades of light cream to medium grey and light to dark tan and brown greys, and are commonly medium to dark grey or tan grey on fresh surfaces. The carbonates are characteristically stylolitic with parallel to oblique arrangements of fine black planar to dentate ('seismic') stylolites. They often contain abundant white veins of sparry (recrystallized) calcite which sometimes show a sub-perpendicular relationship to the stylolites. The carbonates are dominantly microcrystalline limestones lacking obvious bioclasts but are also found as sparsely to moderately bioclastic skeletal wackestones, floatstones and, more rarely, rudstones. Bioclasts tend to be highly recrystallized fragments of bivalves, brachiopods, corals, bryozoans and crinoids. Rare atypical examples include one monospecific molluscan rudstone representing an in situ accumulation of the Late Norian bivalve Monotis subcircularis collected from an Upper Sinemurian (Harbledownense Zone) conglomerate, and one skeletal packstone/rudstone composed of shell fragments and abundant crinoid ossicles collected from an undifferentiated Sinemurian unit. A second sedimentary component unique to the section is composed of generally distinctive intraformational clasts. A range of sedimentary clasts spanning massive greywacke through finely laminated mudstone is found but angular to plastically deformed silty to sandy lime mudstone slabs and clasts are most abundant (Fig. 3.8B). These fine calcareous elastics form 4% (atypically low) to 36% of Sinemurian conglomerates with a mean of 15 % and are an important subordinate clast mode. These are strongly dominated by buff-weathering silty lime mudstones and calcareous laminated mudstones and siltstones with the pervasive bioturbation characteristic of muddy Sinemurian sediments (see Johannson, C O N G L O M E R A T E A N A L Y S I S 79 1993 for lithological description). Most of these bioturbated silty lime mudstones are identical to thin resistant interbeds scattered throughout the Sinemurian succession in thick muddy sequences where they are a minor but distinctive and volumetrically substantial component. These clasts have predominantly angular, tabular shapes and may be concentrated in basal zones of some units where they can be dominant constituents (Fig. 3.4B). In thick debris flows they are sometimes plastically deformed but are more commonly found as coarse pebble- through boulder-grade tabular slabs in both Type 1 and 2 conglomerates. A strong tendency to larger sizes makes them a relatively sparse constituent of the pebble fraction. This common clast morphology can be attributed to early lithification resulting from abundant early calcite cements, allowing their preferential survival during debris flow transport where associated laminated muddy sediments are often chaotically contorted and/or have primary laminations completely obliterated, forming the muddy matrix of these units. There is a definite bimodal distribution with respect to clast shape that is a function of composition and transport history. Arc-derived igneous clasts and carbonates predominantly have smooth, well-rounded, sub-spherical to prolate shapes while intraformational sedimentary clasts usually have an angular tabular shape, reflecting their entrainment during the flow event and subsequent short transport history. The well-rounded sub-spherical shape of the volcanic clasts indicates appreciable hydraulic transport before deposition, suggesting a source in the volcanogenic uplands of the arc. 3.4 LOWER PLIENSBACHIAN CONGLOMERATE Lower Pliensbachian conglomerates are markedly different from those of Sinemurian age with respect to their clast composition, internal texture and facies association. The contrast in style and content is strikingly abrupt and represents a discrete phase of arc evolution and sedimentation history in the Atlin Lake area. Coarse conglomerates of this age are exceedingly rare in the study area with only two units of coarse pebble to cobble-grade encountered. Fine pebble conglomerates through conglomeritic to pebbly sands are relatively common in the Lower Pliensbachian section where all conglomerate units are strongly dominated by volcanic materials. The abrupt shift in conglomerate style and content may be in part an artifact of data collection as no conglomeritic units of known basal Pliensbachian (i.e., Imlayi Zone) age C O N G L O M E R A T E A N A L Y S I S 80 were identified but independent lines of evidence from study of sandstone petrography (Ch. 4), paleocurrent patterns (Ch. 5), and stratigraphic sequences all support a primary causative mechanism as the source of the marked differences between Sinemurian and Lower Pliensbachian conglomerates 3.4.1 General Description and Interpretation The Lower Pliensbachian sample suite consists of eight units. The fine pebble conglomerates analyzed in this study consist mostly of clast-supported units and comprise six of the beds analyzed for clast content. Only one matrix-supported unit is included in the pebble counts and is clast-rich (~ 40%) although the full spectrum of textural variants from pebbly sandstones to grits and sands is generally present in this facies association. The fine pebble conglomerates account for over 80% of Lower Pliensbachian conglomerate encountered in the study area and are combined into one group (designated Type 3) partly as a consequence of their similar facies association rather than purely on the basis of their internal features. Sufficient distinctions exist within this group to allow further sub-division, for example clast-supported from matrix-supported conglomerates; however, such detailed sub-divisions were felt to be unwarranted because of the small sample suite and emphasis on provenance in this study. There are many similarities within the group and in fact the differences are largely the result of gradational processes within single flow events, producing a spectrum of related conglomeritic to pebbly sand lithologies from similar depositional processes. These conglomerates are generally moderately to poorly sorted and always contain a matrix of medium to coarse sand. Moderate to poor normal grading is the norm but ungraded units are also present. Normal distribution grading is somewhat more common than coarse-tail grading. These units are commonly thin, ranging from a few decimetres up to 2 metres. They occur as coarse-tail basal 'lags' in thick sand beds and as stacked repetitions with abrupt normal grading in very thick amalgamated beds (Fig. 3.9). More rarely, very thick beds are produced from a single high-volume gravelly grain flow as encountered at one locality (GGAJ-92-62). There, a 12 metre bed displaying good normal distribution grading fines upward from a basal medium pebble conglomerate through a fine pebble to granule conglomerate across 10 metres into coarse grits at the top. Figure 3.9. Typical occurrence of Type 3 pebble conglomerate forming thick series of stacked beds. This example is an Upper Pliensbachian unit. Note the abundance of white pebbles which are predominantly of granitoid composition. C O N G L O M E R A T E A N A L Y S I S 82 Stratification was observed in a number of the fine pebble conglomerates and is predominantly planar. It is most clearly seen in matrix-supported units where stratification is usually defined in upper bed divisions by strings of pebbles and granules. Imbrication appears to be either largely absent or only weakly developed in all these units - a probable consequence of the dominance of sub-angular to sub-rounded sub-spherical clast shapes rather than primary flow processes. The fine pebble conglomerates are found within thick sand sequences that commonly exceed 100 metres. Interbedded shales are uncommon in these sequences and thin (generally centimetre to decimetre scale). These coarse-grained sand bodies are rarely seen in three dimensions but cross-sections show dominantly very thick-bedded units with planar contacts so that these sequences are interpreted to be mainly sheet sands produced by both high-volume single grain flows and amalgamation of individual graded and non-graded beds. The facies association for these thick coarse sands corresponds to the pebbly-sandstone and massive sandstone facies of Walker (1984) which generally reflects mid-fan lobe settings. There appears to be a gradation between the coarser pebbly-sandstone facies and the fine pebble conglomerates. Despite the variations among type 3 conglomerates, they are all interpreted to result from internal variations of similar processes during flow evolution of mass-flow events. These beds are produced by high-concentration gravelly turbidity currents with deposition primarily by direct suspension sedimentation but also probably from traction carpets beneath similar turbidity curents in the case of ungraded and stratified units. (Lowe, 1982; Pickering et al., 1986). Many of the fine pebble conglomerates encountered in the Lower Pliensbachian section have the textural and compositional characteristics of epiclastic mass-flow deposits. They often exhibit a restricted heterogenous clast composition that is strongly volcanic in origin. Clasts are generally angular to sub-angular with minor degrees of rounding, indicating minimal epiclastic reworking. Some of these highly tuffaceous volcanic pebble conglomerates are transitional to water-lain coarse lapilli tuffs. In the case of the two coarser conglomerates, the coarse pebble conglomerate is a moderately sorted, clast-supported unit a few metres thick (> 3 metres - top covered) that differs in no significant way from Sinemurian Type 2 conglomerates except in its facies association which is within a thick sequence of thick C O N G L O M E R A T E A N A L Y S I S 83 and very thick-bedded massive to pebbly sandstone. The cobble conglomerate is unique in a number of respects. It represents the only occurrence of a coarse conglomerate of appreciable thickness encountered in the Lower Pliensbachian section and is the only conglomerate in the Inklin succession at Atlin Lake to display well-developed inverse-to-normal grading (designated Type 4). This is a very thick-bedded unit that grades into a coarse massive sandstone - total bed thickness, including sandstone, was not measured but the conglomerate section which dominates was 20 metres thick. This conglomerate has an erosive base with low relief scours and is clast-supported in its entirety except for a thin zone on top at the transition to pebbly sand. The inversely graded portion is poorly sorted and shows a gradual increase in clast size (to medium to coarse cobble-grade with minor boulders) from the bottom up to a height of about two-thirds of the conglomerate thickness (Fig. 3.1 OA). The matrix is formed of a muddy coarse sand. The inversely graded division passes rather abruptly into a 7 metre division of moderately sorted, normally graded coarse pebble-cobble conglomerate with a sandier matrix. The inversely graded division shows a weakly developed current-parallel a-axis imbrication. Inversely-to normally graded conglomerates are produced from rapid deposition by frictional freezing of a high concentration traction carpet beneath a high density turbidity current (Lowe 1982; Pickering et al., 1986). This unit corresponds to Walker's (1977) inverse-to-normally graded facies which is suggested to be a 'proximal' deposit: however, there is no evidence at this locality to indicate a setting any more proximal than other conglomerates in the Lower Pliensbachian succession. 3.4.2 Clast Composition The clast content of Lower Pliensbachian conglomerates differs markedly from the Sinemurian. Three prominent trends are apparent: 1) a large increase in the volcanic content; 2) a significant decrease in plutonic material; and 3) an abrupt termination of carbonates as a clast source (Fig. 3.7). Volcanic materials are the strongly dominant clast mode in all Lower Pliensbachian conglomerates (Fig. 3.1 OB). The volcanic content ranges from a low of 69% to a high of 94% with a mean value of 82% for these conglomerates. A rather heterolithic range of volcanic lithologies is present in the two coarse conglomerates while the majority of fine pebble conglomerates tend to show dominance by compositionally restricted varieties (Figs 3.10B, 3.11 A) . C O N G L O M E R A T E A N A L Y S I S 84 Figure 3.10. A : Inverse to normally graded Lower Pliensbachian Type 4 conglomerate. Outcrop in foreground (arrow) is the normally graded division. Strata are overturned. Hammer circled for scale. B : Typical content of Lower Pliensbachian conglomerate is strongly dominated by volcanic clasts. Note the absence of granitoid clasts in both cases. C O N G L O M E R A T E A N A L Y S I S 85 The coarse conglomerates display a range of volcanic lithologies similar to Sinemurian units in some respects. The range spans dacitic to rhyodacitic tuffs and porphyries to aphanitic basaltic andesites. Many of these clasts have a fresher appearance than Sinemurian volcanics, especially more felsic varieties. In general the fine-grained andesitic rocks tend to be highly weathered/altered and exhibit a similar range of compositions and textures to those found in Sinemurian conglomerates, indicating their likely derivation from the same sources as Sinemurian equivalents. There is a higher proportion of rhyodacitic to dacitic rocks than in Sinemurian conglomerates. Many textural and compositional variants are evident and range from massive phyric to microphyric volcanics, a variety of pyroclastic equivalents (lithic-crystal, crystal-lithic, and crystal dust tuffs), to coarse porphyries of hypabyssal origin. Rhyodacites are uncommon but more abundant than in Sinemurian conglomerates. Phenocryst phases in this sub-group are always strongly dominated by feldspar with generally minor amounts of one or more of the following; biotite, hornblende, quartz. Pyroxene is occasionally present but generally sparse or absent. Fine pebble conglomerates are strongly dominated by dacitic to rhyodacitic varieties (Fig. 3.11 A) . This sub-group is the generally dominant volcanic clast mode in the Early Pliensbachian. Phenocryst phases are described above and show a considerable range of textural variants within the restricted composition. Some of these rocks are unwelded crystal and crystal-lithic tuffs. A distinctive minor component of the coarser rhyodacitic porphyries occasionally contain K-spar megacrysts in the range of 1 to 2 centimetres. These latter varieties become quite common in Upper Pliensbachian units. Plutonic rocks generally form a very minor component of clast modes. Plutonic content ranges from a low of 3% to a high of 24%, with a mean value of 10% for Lower Pliensbachian conglomerates. Plutonics are always less than 10% in fine pebble conglomerates and only form appreciable minor amounts in the two coarse units. Sedimentary clast content ranges from 3 to 12% and rarely exceeds 10%. The mean value for this clast type is 7% in Lower Pliensbachian conglomerates, a value half that of Sinemurian conglomerates and somewhat lower than Upper Pliensbachian conglomerates. These tend to be finer-grained elastics that lack any distinguishing features and are almost certainly of intraformational origin. C O N G L O M E R A T E A N A L Y S I S 86 Figure 3.11. A : Lower Pliensbachian Type 3 conglomerate displaying dominance of dacitic to rhyodacitic porphyrite volcanic clasts. Arrows point to examples. Matrix is formed of dacitic resedimented pyroclastic debris composed primarily of broken and whole euhedra of plagioclase, hornblende, quartz and biotite (in decreasing order of abundance). B: Example of Upper Pliensbachian Type 3 conglomerate. Granitoids (white) form a prominent component of clasts. C O N G L O M E R A T E A N A L Y S I S 87 The carbonates common in Sinemurian units are usually absent in Lower Pliensbachian conglomerates but can occur in trace amounts. One large boulder-sized clast of this type was found in the inversely graded conglomerate and would be unremarkable but for its faunal content. This dark grey skeletal rudstone represents an in situ accumulation of monospecific brachiopods of unusually large size. The genus is Halorella, a Triassic Rhynchonellid brachiopod particularly characteristic of the Norian stage (Ager, 1973). A range of juvenile through mature forms is found in this close-packed brachiopod 'paleonest' with some specimens attaining long dimensions in excess of 10 centimetres - a size that is unusually large for brachiopods, including the Hallora genus (J. Palfy - pers. comm., 1994). 3.5 U P P E R P L I E N S B A C H I A N C O N G L O M E R A T E Upper Pliensbachian conglomerates represent a new phase in the depositional history of the Inklin Formation at Atlin Lake. A dramatic shift in conglomerate clast style and content from those of the Lower Pliensbachian is evident. These conglomerates record a rapid transition from volcanically dominated to plutonically dominated source areas with the abrupt appearance of coarse cobble-boulder conglomerates containing abundant granitoid clasts (Fig. 3.7). These units are the most conspicuous, although volumetrically minor, members of the major Late Pliensbachian progradational megasequence generated by uplift and erosion. 3.5.1 General Description and Interpretation The Upper Pliensbachian sample suite consists of 7 units. Two of these units are moderately sorted, normally graded, clast- to matrix-supported Type 3 fine pebble conglomerates essentially identical to Lower Pliensbachian equivalents except in clast composition (Figs. 3.9, 3.1 IB). Thin Type 3 beds are relatively common in thick sand sequences but appear to be less abundant than in the Lower Pliensbachian section. Another member of the Upper Pliensbachian sub-set is a clast-rich (~ 35%), matrix-supported Type 1A muddy debris flow unit. This is a very thick-bedded unit, exceeding 15 metres at its thickest point (base covered). This distinctive unit outcrops across a roughly one square kilometre area, exhibiting both high-relief scoured and planar, bedding-parallel, basal contacts. Bed geometry appears to be an irregular lensoid shape with the unit everywhere overlain by very thick-bedded, massive coarse sands. C O N G L O M E R A T E A N A L Y S I S 88 It is an extremely unsorted, generally disorganized bed, although a crude upper and lateral dilution of clasts was observed at one outcrop (Site LP-1A-3). Clast content and size make this a unique unit in the Inklin succession at Atlin Lake. Clast content, which includes unusually fossiliferous shallow marine intraclasts, is discussed in the following section, but the size of some 'clasts' is also remarkable. Large olistostromal blocks of semi-lithified and lithified sediments are common in two localities (Fig. 3.12A) (Sites LP-1A-3/4). These are dominated by tabular blocks of incompetent coarse and pebbly sandstone that were found up to 10 x 2 metres. The largest olistostrome found in this unit is a bedding-parallel fine-grained slab 4 metres thick that could be traced for 40 metres before dipping under cover. This unit forms a particularly spectacular example of a debris flow in the Inklin succession due to its thickness and unusual clast content but it is the only significant example of a Type 1A conglomerate identified in the Upper Pliensbachian section and represents a rare catastrophic event. Similar units are believed to form only a minor component of the section. The conglomerates most characteristic of the Late Pliensbachian, accounting for over 60% of all conglomerate beds in the section, are formed of coarse pebble-cobble to cobble-boulder, clast-supported units. They form relatively thick beds ranging from 1 to 6 metres but can form thicker successions of amalgamated units in excess of 12 metres. Matrix is always composed of mud-poor medium to gritty sands and, in one case, fine pebbles and grits. They commonly show scoured erosive bases and tops gradational into coarse sands. Units generally have moderate to good normal distribution grading and less commonly crude coarse-tail grading, although occasionally thicker units have a generally disorganized clast fabric. In ungraded beds thin zones of crude normal grading are commonly developed at the top and thin zones of apparent inverse grading were occasionally present at the bottom. It was not clear whether this latter feature was the result of primary flow or the indistinguishable amalgamation of a coarser bed on pebbly or conglomeratic sands. Most of these conglomerates, especially thicker amalgamated units, are clearly channelized but some thinner units display a relatively tabular geometry, suggesting they are overbank spillover sheets. They are non-stratified and tend to be weakly or non-imbricated. The predominantly well-rounded sub-spherical clast shape is probably a factor in the poor development of imbrication. C O N G L O M E R A T E A N A L Y S I S 89 With the exception of atypical ungraded beds and thicker amalgamated units, these coarse conglomerates differ in no significant way from Sinemurian Type 2 conglomerates except in their facies association. These beds are also the product of direct suspension sedimentation from coarse gravelly high-concentration turbidity currents but differ in their relative fan setting. They are always overlain, and commonly underlain, by very thick-bedded massive to graded sands, indicating a medial fan environment. This is interpreted to represent both mid-fan channel and lobe sequences with amalgamated units apparently confined to channels. Amalgamated successions of normally graded beds are also the product of Type 2 depositional processes. Ungraded disorganized beds suggest deposition from channelized sandy cohesionless debris flows with final rapid sedimentation by frictional freezing (Lowe, 1982; Pickering et al., 1986). 3.5.2 Clast Composition Upper Pliensbachian conglomerates record another significant provenance shift that is distinct from the preceding volcanically-dominated Lower Pliensbachian provenance. The most prominent feature is a dramatic increase in plutonic modes (Fig. 3.7). Plutonic content ranges from 38% to 67% in these conglomerates, making them an important subordinate to primary clast mode. The mean value is 49% for Upper Pliensbachian plutonic modes. The plutonic clast suite is not lithologically diverse, displaying a rather restricted compositional range which is strongly dominated by rocks of granitic composition. These range from quartz syenites and alkali feldspar granites to granodiorites and quartz monzodiorites. The most common lithology is granite and monzogranite with subordinate quartz monzonite. The majority of quartz monzonites and quartz monzodiorites are quartz-rich varieties transitional to monzogranite and granodiorite. Biotite is the dominant mafic phase with hornblende often subordinate or absent. Distinctive K-spar-megacrystic hornblende-biotite monzogranites/granites and quartz monzonites are a conspicuous component of the granitoids with megacrysts occasionally up to 5 centimetres long (Fig. 3.12B). Volcanic lithologies are strongly dominated by rhyodacitic to dacitic tuffs and porphyries that differ in no significant way from Lower Pliensbachian equivalents except in relative proportions. Rhyodacite porphyries are conspicuous components of Upper Pliensbachian volcanic modes and show a range of textural variants that represent hypabyssal rocks transitional into shallow plutonic rocks. These porphyries C O N G L O M E R A T E A N A L Y S I S 90 Figure 3.12. A : Upper Pliensbachian Type 1 conglomerate with olistolith of coarse-grained greywacke oriented parallel to bedding. Block is ~ 40 meters long and appears to have undergone significant rotation during transport as internal bedding of olistolith is highly discordant with unit bedding. Pogo stick (1.5 m) circled for scale. B : Upper Pliensbachian cobble-boulder conglomerate with abundant granitoid clasts. The plutonic clast suite is dominated by compositionally restricted varieties of monzogranite, many of which are K-spar megacrystic (circle). A significant portion of these varieties are derived from 'young' Pliensbachian plutons. C O N G L O M E R A T E A N A L Y S I S 91 tend to be coarser grained than those in the Lower Pliensbachian, generally medium to coarse grained varieties that are commonly K-spar megacrystic. The coarser varieties tend to be closely packed porphyries with little groundmass (often < 10%) and contain pink K-spar megacrysts that can exceed 3 centimetres. Like Lower Pliensbachian conglomerates, carbonates are generally absent or only trace components in these units. Sedimentary clasts include a wide range of coarse-grained through muddy and calcareous lithologies. Clast content ranges from 4 to 17 % with an Upper Pliensbachian mean of 10 %. In general terms, the lithologies represented are unremarkable in that similar equivalents can be found within the Upper Pliensbachian section and lack distinctive lithological features. Some exceptions exist as shallow-marine rocks are included in this clast mode. A suite (n = 14) of highly fossiliferous muddy carbonates collected from four localities is particularly distinctive and warrants further discussion. Lithologies include mud-free bioclastic grainstone, muddy molluscan floatstones and rudstones, and skeletal mudstones. The faunal content of these clasts was found to consist of dominantly in situ accumulations of bivalves representing Early Jurassic shallow-marine (~100 metres depth) bivalve paleocommunities. A detailed analysis of the lithological characteristics and faunal content of these distinctive clasts is found in Chapter 2 (Part 2). 3.6 PROVENANCE Specific provenance linkages are not possible for most Inklin clast lithologies. Volcanic clast lithologies in particular appear to preserve the only record of a spectrum of volcanic rock units now completely removed by erosion. One notable exception that can be firmly tied to Inklin Formation conglomerates is the Nordenskiold dacite (Cairnes, 1910). This member occurs throughout the northern British Columbian and Yukon regions of the Whitehorse Trough as generally thin and minor interbedded water-lain tuffs and highly tuffaceous epiclastics within Laberge Group stratigraphy. Zircon dates from this study and others (Hart et al., in press) place an upper age constraint of ca. 186 (± 1) M a on these volcanics, the majority of which are somewhat older (i.e., Lower Pliensbachian) in the study area. Sub-aerial correlatives of the Nordenskiold dacite appear to be rare in the Whitehorse Trough region. The intraformational character of these units confirms the coeval volcanic origin of similar textural variants that C O N G L O M E R A T E A N A L Y S I S 92 dominate the Lower Pliensbachian, and parts of the Upper Pliensbachian, section as clasts. A volcanic lithology conspicuous by its complete absence in the study area is the coarse-grained augite porphyries of the Stuhini Group in B.C. and the Povoas Formation in the Yukon, which forms the volcanic part of the underlying Lewes River Group in the Whitehorse region. These distinctive Triassic volcanic rocks are found in conglomerates throughout Laberge Group strata in the Whitehorse region, particularly high in the succession (early Middle Jurassic)(Dickie, 1989) but as low as Sinemurian and Hettangian (?) (C.Hart -pers.comm, 1994). They are also relatively common south of the study area in Whitehorse trough strata in the Tulsequah region (Souther, 1971). Fine-grained augite porphyries do occur in conglomerates at Atlin Lake but are sparse and uncommon constituents while the characteristically coarse-grained equivalents common in adjacent strata to the north and south are absent. The absence of these distinctive volcanic clasts in coeval conglomerates a short distance away in the Atlin Lake area is a mystery but illustrates the potential influence of local point sources on clast trends along the arc strike. Similar difficulties arise when attempting provenance linkages with plutonic clasts. There are no good candidates in the region that fulfill both age and compositional requirements for Sinemurian plutonic clast lithologies. These are dominated by more felsic relatively quartz-poor compositions, mostly monzonites and quartz monzonites with minor granitic rocks (monzogranite) and quartz monzodiorites. A string of Late Triassic to earliest Jurassic plutons has been interpreted as the likely source for most Laberge Group plutonic clasts in the Whitehorse region (Hart et al., in press); however, these plutons are all of an intermediate to more mafic composition not generally found in Sinemurian conglomerates, so that they appear unlikely to have been major sources for Sinemurian plutonic modes. No likely provenence linkages can be conjectured for the dominant Sinemurian plutonic clast lithologies at this time. Specific provenance linkages are possible for a significant component of the Upper Pliensbachian plutonic clast suite. A zircon date from a granitic clast collected from a Kunae Zone age conglomerate on Sloko Island yielded a date of 186.6 (+ 0.5/- 1) M a (Ch. 2, p. 34, Fig. 2.5). This orthoclase-megacrystic biotite-hornblende monzogranite clast is a particularly abundant lithology within the Upper Pliensbachian plutonic suite. The Little River Batholith, which forms part of the Long Lake Suite in the Yukon, displays strong similarities with respect to composition, texture (e.g., commonly K-spar-megacrystic), and age, C O N G L O M E R A T E A N A L Y S I S 93 which is identical to the dated clast in this study (C. Hart, unpub. data). The Upper Pliensbachian plutonic clast suite is strongly dominated by felsic compositions, mainly granite/monzogranite and quartz monzonite, with only minor granodiorite/quartz monzodiorite. The strong compositional and textural similarity of these fresh-looking granitoids suggests a oogenetic link, perhaps as different phases of the same magmatic episode. Given that the Little River Batholith is believed to have a number of closely related phases (C. Hart, pers. comm. 1994), this is a reasonable speculation. Also, there is little evidence of provenance from intermediate Late Triassic to earliest Jurassic plutons in Upper Pliensbachian conglomerates in the study area, suggesting that most of the granitoids may be derived from younger plutons of Pliensbachian age. The provenance of the stylolitic grey carbonates ubiquitous in Sinemurian conglomerates and present in trace amounts in the younger succession can be firmly established. The majority of the carbonates are macroscopically identical to Upper Triassic carbonate units exposed along the southwest margin of the basin at Atlin Lake that have been correlated with the Sinwa Formation (Bultman, 1979). A number of these clasts have been processed for microfossils by other workers and generally yield conodonts of Late Norian age (H.W. Tipper - pers. comm, 1992; M . Mihalynuk - pers. comm., 1993). This is the same age as that determined from conodonts in the carbonate units of Sinwa Formation affinity exposed along southwest Atlin Lake (H.W. Tipper - pers. comm, 1992) and supports a firm provenance linkage to the Stikinian 'Sinwa Formation' for the majority of these carbonate clasts. The relatively high percentage of carbonate clasts demonstrates the importance of these Upper Norian carbonates as a source for Sinemurian conglomerate and is a unique feature of Sinemurian units. Other sedimentary clasts form a minor, and in the case of Sinemurian conglomerates sometimes subordinate, clast mode in all Inklin Formation conglomerate. A wide range of lithologies is represented, spanning the range from coarse- through fine-grained clastic to impure muddy carbonate rocks. The vast majority of these clasts are interpreted to be intraformational in origin. Definitive evidence to support the interpretation is often lacking although identical i f non-descript lithologies are common in the succession. However, in a number of instances unusually distinct lithologies and, more rarely, fossiliferous clasts can be clearly linked to the coeval stratigraphy. The distinctive buff-weathering bioturbated lime mudstone C O N G L O M E R A T E A N A L Y S I S 94 clasts common in Sinemurian conglomerates are one example. The source of these unusual intraclasts is confined to the Sinemurian section where they occur as minor but distinctive thin interbeds. In some cases the intraformational clasts are of shallow-marine origin and equivalent units are not found in the basinal stratigraphy of the Inklin Formation. The suite of highly fossiliferous muddy carbonate clasts of shallow-marine origin collected from an Upper Pliensbachian unit provide an example. Ammonites allowed accurate dating of the clasts and showed that shallow-marine strata of Early Sinemurian, Late Sinemurian, and Early Pliensbachian age were clast sources (Ch. 2, Part 2). 3.7 C O N G L O M E R A T E P E T R O F A C I E S The strong temporal trends evident in Inklin Formation conglomerates at Atlin Lake can be used to delineate three distinct conglomerate petrofacies corresponding to Sinemurian, Early Pliensbachian and Late Pliensbachian. Fields depicting the conglomerate petrofacies are shown in Figure 3.13. where they may be described as delineating relatively discrete petrofacies with minimal overlaps. There probably exists greater overlap than that shown as one might reasonably expect more transitional units in the Inklin succession than apparently occur in this data-set. Details of the petrofacies were dealt with in clast composition sections and are summarized below. Sinemurian: The presence of ubiquitous Upper Norian 'Sinwa Formation' carbonates is a diagnostic criterion of Sinemurian conglomerates everywhere in the study area. These carbonates are always present in appreciable minor amounts, rarely less than 10% and most commonly in the range of 20% or greater. The presence of distinctive buff-weathering, pervasively bioturbated sedimentary intraclasts is another unique feature of Sinemurian conglomerates. They occur in highly variable proportions but are always present, usually as a conspicuous minor to subordinate clast mode. The source of these unusual intraclasts is confined to the Sinemurian section. Volcanic clast modes contain no unique lithologies but show a higher proportion of fine-grained andestic rocks than elsewhere in the Inklin succession. Volcanics are the dominant clast mode but usually comprise less than 50% of total clasts. Plutonics are a minor clast mode and dominated by monzonites and quartz monzonites that tend to be moderately altered. C O N G L O M E R A T E A N A L Y S I S 95 Figure 3.13. Ternary tectonic discrimination diagram depicting fields for conglomerate petrofacies of Inklin Formation stratigraphic sub-sets. Poles represent normalized clast modes for plutonic (P), volcanic (V), and sedimentary (S) clasts (After Dickie and Hein, 1993). C O N G L O M E R A T E A N A L Y S I S 96 Lower Pliensbachian: Abundant volcanics of dacitic to rhyodacitic composition strongly dominate Lower Pliensbachian clast modes. Volcanics always form considerably more than 50% of total clasts and usually 80% or more. Upper Norian carbonates are conspicuous by their general absence and are never found in more than trace amounts. The content of intraformational sedimentary clasts are the lowest in the succession (mean=7%) and display no distinguishing characteristics. Plutonic clasts are essentially the same as those in the Sinemurian but generally occur in much lower amounts, usually forming less than 10% of most units. Upper Pliensbachian: The plutonic content of Upper Pliensbachian is considered a diagnostic feature of these units. Plutonic clasts are much more abundant than elsewhere in the succession and are always an important secondary or primary clast mode, sometimes forming greater than 50% of total clasts. The plutonic clast suite is strongly dominated by granitioid lithologies. In particular, distinctive K-spar-megacrystic monzogranites of Pliensbachian age are abundant and tell-tale indicators of Upper Pliensbachian units. Associated cogenetic coarse-grained hypabyssal rocks are also relatively abundant volcanic components and often K-spar-megacrystic as well. Volcanic clasts are strongly dominated by tuffaceous to porphyritic dacites and rhyodacites that are identical to Lower Pliensbachian equivalents and are important secondary to primary clast modes in Upper Pliensbachian conglomerates. As in Lower Pliensbachian conglomerates, Upper Norian carbonates are absent or only trace components. Sedimentary clasts are minor components and are largely intraformational. Their lithologies are generally unremarkable with the exception of highly fossiliferous shallow-marine muddy carbonates which were found only in the Upper Pliensbachian section. C O N G L O M E R A T E A N A L Y S I S 97 3.8 TEMPORAL TRENDS: TECTONIC IMPLICATIONS Conglomerate petrofacies reflect Early Jurassic arc/basin evolution, recording details of the tectonic history in distinct depositional episodes. Normalized pebble-count data can be treated in a similar fashion to sandstone point-counts and plotted on a ternary tectonic discrimination diagram using plutonic, volcanic and sedimentary poles (Dickie & Hein, 1993). Three tectonic discrimination fields representing successive stages of progressive arc dissection (i.e., undissected-, transitional-, and dissected-arc) are used to depict provenance of clasts. When the time dimension is added through biostratigraphic control of conglomerate bed age, temporal clast trends can be plotted and used to characterize the relative stage of arc evolution at particular points in time. A n idealized progressive arc-dissection sequence begins with abundant sedimentary clasts being generated from coastal sediments by flank uplifts, followed by increasing volcanic content as the arc becomes progressively incised and ending with clast content dominated by plutonics derived from exposed arc roots. Sinemurian conglomerates reflect a largely transitional arc provenance as indicated by conglomerate petrofacies. Although volcanic rocks dominate clast modes, plutonics, sedimentary intraclasts and Upper Triassic carbonates are important minor to subordinate modes, indicating a degree of arc incision consistent with a largely inactive transitional arc segment. Ubiquitous Upper Triassic carbonates reveal a frontal arc that had undergone an extensive flank uplift such that these older units were a locus of active erosion by Sinemurian time. Broadly coeval Upper Triassic volcanic units are the probable source for most of the heterolithic volcanics in Sinemurian conglomerates as incision of the older volcanic pile had progressed to the level of the plutonic arc roots. The ternary tectonic discrimination plot shows Sinemurian points spread across the transitional and flank uplift fields and supports the interpretation (Fig. 3.14). The dramatic change of clast content in Early Pliensbachian conglomerates records a distinct phase in arc evolution. The strong increase in volcanic materials of restricted composition and the stratigraphicalty abrupt termination of Upper Triassic carbonates as a clast source together provide an explanation. Removal of the carbonate source area by erosion is not a viable explanation as the Sinwa Formation is presently exposed as a regionally mappable unit along much of the Whitehorse trough. A plausible explanation involves renewed subsidence as a result of the generation of extensive subaerial cover by new pyroclastic C O N G L O M E R A T E A N A L Y S I S 98 Figure 3.14. Temporal clast trend diagram depicting plot of Sinemurian conglomerate beds. Poles represent normalized clast modes for plutonic (P), volcanic (V), and sedimentary (S) clasts. Total bed number = 9 (After Dickie and Hein, 1993). C O N G L O M E R A T E A N A L Y S I S 99 Figure 3.15. Temporal clast trend diagram depicting plot of Lower Pliensbachian conglomerate beds. Total bed number = 8 (After Dickie and Hein, 1993). C O N G L O M E R A T E A N A L Y S I S 100 Figure 3.16. Temporal clast trend diagram depicting plot of Upper Pliensbachian conglomerate beds. Total bed number = 7 (After Dickie and Hein, 1993). Figure 3.17. Temporal clast trend diagram depicting plot stratigraphic sub-set means for Inklin Formation conglomerate. Arrows point in a younging direction. Total beds = 24 C O N G L O M E R A T E A N A L Y S I S 102 blankets and the accompanying rapid influx of volcaniclastic sediments produced by voluminous rejuvenated volcanism. Taken together these features indicate that the Lower Pliensbachian section represents a new phase of active arc-building. The ternary plot shows Lower Pliensbachian points forming a relatively tight cluster near the bottom of the flank uplift field (Fig. 3.15), a position consistent with a largely undissected-arc provenance. The actual boundary between the two stratigraphically abrupt and distinct sedimentation styles must occur somewhere between latest Sinemurian (post-Harbledownense Zone) and earliest Pliensbachian (pre-Whiteavesi Zone) in the study area. Upper Pliensbachian conglomerates record another significant shift in temporal clast trends. The ternary diagram shows Upper Pliensbachian points fall mainly in the dissected-arc field and spread intothe transitional-arc field (Fig. 3.16), revealing a relatively advanced stage of pluton exposure consistent with a dissected-arc provenance. The relatively abrupt influx of abundant coarse granitoid clasts indicates a primary tectonic role in their appearance, given that this occurs during a relative highstand throughout the north and north-central Cordillera (H.W. Tipper - pers. comm., 1993). Therefore, it seems probable that major uplift and rapid erosion driven by tectonic forces are responsible for the advanced pluton exhumation occurring by early Late Pliensbachian time, rather than progressive unroofing of the arc or eustatic controls. The fact that young plutons of Pliensbachian age are involved indicates dramatic rates of arc incision and unroofing, suggesting an accelerated tectonic episode. The role that intra-arc strike-slip tectonics play in this arc dissection may be major. Granitic plutons of similar age and composition to Upper Pliensbachian clasts are interpreted to have undergone strike-slip displacements along the Tally-Ho Shear Zone during the Early Jurassic (C. Hart - pers. comm., 1994). This regional structure is on trend with the basin-bounding Llewellyn Fault on southwest Atlin Lake and is believed to be a probable extension of it (GHart - pers. comm., 1993). Major intra-arc strike-slip 'transform' faults are common in subduction zones with oblique convergence (Jarrad, 1986; Sylvester, 1988) and major uplift commonly accompanies these strike-slip displacements (Sylvester, 1988). Similar processes appear likely to have been operative in the northern Stikinian arc given the paleotectonic reconstructions that invoke highly oblique convergence during the Early Jurassic (q.v., Engebretson, 1985; Umhoefer, 1989; Irving & Wynne, 1991) coupled with the presence of long-lived arc-parallel regional C O N G L O M E R A T E A N A L Y S I S 103 structures, some of which appear to have been active during this time and may well represent reactivated intra-arc transforms. It is here postulated that intra-arc strike-slip tectonics of this type played a major role in the rapid arc dissection occurring by Late Pliensbachian time. In summary, Inklin Formation pebble-count data allows delineation of three distinct depositional episodes in Early Jurassic basin-fill at Atlin Lake. Conglomerate petrofacies and ternary diagram depictions reveal a temporal pattern inconsistent with progressive arc dissection. A ternary clast trend plot of conglomerate means indicates relatively abrupt provenance shifts during Early Jurassic time (Fig. 3.17) which is consistent with episodic basin-fill events. A major magmatic episode with attendant rejuvenated volcanism during Early Pliensbachian time produced the first depositional episode, causing significant reversal of the progressive arc dissection initiated in Late Triassic time, followed by a second depositional episode in the Late Pliensbachian characterized by abundant granitic material - a sedimentary artifact reflecting rapid pluton exhumation likely driven, at least in part, by intra-arc strike-slip tectonics. Similar temporal patterns have emerged from analysis of sandstone detrital modes and, in conjunction with conglomerates clast trends, provide a picture of complex arc/basin evolution during the Early Jurassic characterized by episodic flank uplifts, rejuvenated volcanism, and strike-slip-influenced rapid arc dissection. SANDSTONE P E T R O G R A P H Y CHAPTER 4 SANDSTONE PETROGRAPHY 104 4.1 INTRODUCTION Petrographic analysis of Inklin Formation sandstones was conducted with two objectives in mind. The primary aim of the study was to refine temporal clast trends noted in study of conglomerates with a view to providing superior delineation of depositional episodes in the context of arc evolution. A n important secondary objective concerns fundamental basin classification. Previous work has suggested a backarc setting for the Inklin Formation in the Atlin Lake area (Bultman, 1979; Eisbacher, 1974), whereas more recent work in the southern Yukon has consistently resulted in interpretations of a forearc setting for Laberge Group strata there (q.v., Tempelman-Kluit, 1979, Hansen, 1988; Dickie, 1989). As both backarc and forearc basins exhibit different provenance characteristics through time, petrographic analysis has potential to discriminate subtle mineralogical differences that can serve as provenance indicators. There is a large body of literature that deals with sand/sandstone compositional variations and their relation to plate tectonic setting through the employment of ternary tectonic discrimination diagrams (e.g., Dickinson, 1970; Dickinson and Suczek, 1979; Dickinson et al, 1983; Dickinson & Valloni, 1980; Valloni & Maynard, 1981; Valloni, 1985; Marsaglia & Ingersoll, 1992). Syntheses of modern and ancient sandstone detrital modes by Dickinson (1970), Dickinson and Suczek (1979) and Dickinson and others (1983) resulted in the definition of characteristic compositional fields with respect to provenance on ternary diagrams. It has become standard practice in many regional studies to plot petrographic data on these diagrams to help elucidate both tectonic setting and compositional trends through time. The sedimentary fill of arc-marginal basins may record the entire magmatic and tectonic evolution of ancient island arc systems. Often sediments may record the only evidence of discrete phases of arc growth SANDSTONE P E T R O G R A P H Y 105 and erosion. Detailed petrographic 'mapping' of the sedimentary rocks has been shown to be an effective method for unraveling complex arc/basin histories. It has great potential to delineate depositional episodes and is a fundamental tool for paleotectonic reconstructions of arc terranes. The compressive tectonic regimes in which volcanic island arcs evolve and are ultimately preserved as accretionary elements produce disrupted volcano-sedimentary successions where petrographic trends can be difficult to delineate with accuracy. This poses the greatest obstacle for application of petrographic mapping. Good biostratigraphic control is fundamental to an analysis of this nature. Numerous ammonite collections from the study area provide the biostratigraphic control necessary for the definition of temporal petrofacies. Over 80 ammonite collections representing Early Sinemurian through Late Pliensbachian time were obtained, the majority of which were determined to zonal stratigraphic resolutions through identification of ammonite genera and species. This high resolution stratigraphy allowed the division of sandstone samples selected for the petrographic analysis into three stratigraphic sub-sets. 4.2 M E T H O D S Thin-sections were cut from 30 biostratigraphically constrained sandstone samples and sub-divided into three separate suites representing the Sinemurian, Early Pliensbachian, and Late Pliensbachian. Seven samples are included in the Sinemurian suite; four are of Early Sinemurian age (Arnouldi Zone), two are of early to middle Late Sinemurian age (Varians & Harbledownense(?) Zones), and one is interpreted to be of an undifferentiated, possibly Late, Sinemurian age. Eleven samples are included in the Early Pliensbachian suite. Most of these samples lack zonal resolution and were correlated on the basis of their occurrence in strata containing the ammonite genus Metaderoceras, species of which range throughout the Early Pliensbachian and only rarely into the earliest Late Pliensbachian. This genus is considered diagnostic of an Early Pliensbachian age. Where species identifications were possible greater stratigraphic resolution was determined for some samples and are as follows; one sample is of Whiteavesi Zone age along with two others of probable Whiteavesi Zone age, and one is of Imlayi to Whiteavesi Zone age. The remaining seven samples have only sub-stage resolution (e.g., Lower Pliensbachian) and include two samples of interpreted, rather than biostratigraphically-constrained, age. The Late Pliensbachian suite is composed of twelve SANDSTONE P E T R O G R A P H Y 106 samples, nine of which have zonal (Kunae zone) resolution. Two of the remaining three are from undifferentiated Late Pliensbachian strata but are of probable Kunae zone age. One sample was collected from a structurally complex package containing ammonites that range through the Freboldi (upper Early Pliensbachian biochronozone) into the Kunae zone and is interpreted to be of a probable Late Pliensbachian age. Age assignments for individual samples are found in Table 4.1 with point-count data. In keeping with standard procedures for petrographic analysis of sedimentary rocks (Dickinson, 1984; Valloni, 1984), sandstones of similar grain-size were selected to achieve consistency and allow valid comparison of sample suites. Medium to coarse-grained sandstones were chosen, however, fine to medium-grained sandstones are also included in the suites as minor components. Petrographic analysis of thin-sections utilized quantitative (i.e., point-counts) and qualitative (e.g., discrimination of plutonic from volcanic quartz) approaches. Point-counting was conducted using a Swift Model F point-counter. A total of 300 points was counted for each thin-section in order to obtain statistically reliable results (Van der Plas & Tobi, 1965). A maximum grid spacing was used that resulted in a traverse of the entire slide. Only sand-sized (> 0.0625 mm) grains intersected by the cross-hairs were counted although the total count probably includes some coarse silt grains as well. Detrital categories are based on the scheme of Dickinson (1970; Dickinson & Suczek, 1979) (see Table 4.1). Samples were stained with sodium cobaltinitrite (for potassium feldspar) to facilitate distinction of feldspars in thin-section. The Gazzi-Dickinson point-counting method was employed in order to reduce the compositional dependence on grain-size (Tables 4.1 and 4.2: see Ingersoll et al., 1984 for discussion). This method differs from the traditional method primarily by classifying sand-sized monomineralic crystals and other grains occurring in larger lithic fragments in the appropriate crystal or grain category, rather than in the category of the larger lithic fragment. For example, a volcanic lithic fragment with sand-sized plagioclase phenocrysts wil l be classified as P when the cross-hairs intersect the crystal rather than Lv. This method also eliminates the traditional category for plutonic lithic fragments. Because of the abundance of dacitic porphyry fragments in many samples (especially Lower Pliensbachian) the L values for those samples are SANDSTONE P E T R O G R A P H Y not truly represenative of the lithic content, with a significant proportion of the P values derived from Lv phenocrysts. This is a minor drawback when compared to the advantages of the method which give more uniform results for all grain sizes and allow quicker counts with less ambiguity, especially for diagenetically altered, poorly-sorted sandstones. Sandstones were classified using a modified version (Carozzi, 1993) of the scheme proposed by Pettijohn et al. (1987), which i n turn is a modification of the scheme presented by Dott (1964) (Fig. 4.1). In this scheme most of the Inklin Formation sandstones are not classified as wackes as matrix content rarely exceeds 15%; however, many of the sandstones have a matrix content in the range of 10% so that they are better characterized as sub-wackes rather than arenites. Figure 4.1. Sandstone classification scheme modified after Dott (1964) and Pettijohn et al. (1987) (from Carozzi, 1993). SANDSTONE P E T R O G R A P H Y 108 Table 4.1: Point-count Data Sinemurian Sample No. Age Qm Qp P K Lv Ls L m M D Misc. Total GGAJ-92-16-1 E. Sin.-A. 44 0 154 43 52 5 1 0 1 13 300 GGAJ-92-27-1 E. Sin.-A. 43 2 178 39 38 0 0 0 0 18 300 GGAJ-92-37-2 E. Sin.-A. 37 2 172 31 52 1 0 0 5 6 300 GGAJ-92-181-1 E. Sin.-A. 56 4 163 26 45 1 0 1 4 8 300 GGAJ-92-33-1 L? Sin. 43 4 164 41 41 2 0 1 1 5 300 GGAJ-92-177-1 L . Sin.-V. 64 3 140 62 21 3 0 1 6 10 300 GGAJ-92-49-1 L . Sin.-H? 74 6 116 38 54 7 1 0 4 11 300 Mean Count 52.3 3.0 155.4 40.2 43.6 2.7 — 0.4 7.0 . . . — Mean Volume% 17.4 1.0 51.8 13.4 14.5 0.9 — 0.1 2.3 . . . — Lower Pliensbachian GGAJ-92-55-2 E? Plien. 22 0 118 15 129 0 1? 0 15 11 300 GGAJ-92-63-3 E. Pli.-I/W 32 0 156 4 52 1 0 5 46 8 300 GGAJ-92-65-2 E. Plien. 7 0 147 4 98 0 0 3 41 10 300 GGAJ-92-78-1 E. Plien. 42 1 187 12 55 2 1? 1 4 9 300 GGAJ-92-79-3 E. Pli.-W. 54 0 160 2 74 2 0 6 2 3 300 GGAJ-92-80-1 E. Pli.-W? 45 2 110 5 102 1 0 9 26 8 300 GGAJ-92-80-2 E. Pli.-W? 37 2 183 4 60 3 0 0 11 20 300 GGAJ-92-89-1 E. Plien. 51 3 185 21 38 1 0 0 1 7 300 GGAJ-92-114-1 E. Plien. 36 0 188 6 40 1 0 2 31 6 300 GGAJ-92-132-2 E? Plien. 41 0 198 8 48 2 0 0 3 9 300 GGAJ-92-185-1 E. Plien. 27 1 121 3 130 1 0 0 17 7 300 Mean Count 35.8 0.8 159.4 7.6 75.1 1.3 — 2.5 17.9 . . . . . . Mean Volume% 11.9 0.3 53.1 2.5 25.0 0.4 — 0.83 6.0 . . . — Upper Pliensbachian GGAJ-92-58-2 L . Pli . -K? 72 2 138 57 30 1 Tr 0 0 6 300 GGAJ-92-59-3 L . Pl i . -K. 106 6 78 72 23 2 1 7 7 8 300 GGAJ-92-84-4 L . Pl i . -K. 68 2 169 20 36 1 1 0 3 4 300 GGAJ-92-91-1 L . Pl i . -K. 105 4 98 70 13 1 0 4 5 6 300 GGAJ-92-92-2 L . Pl i . -K. 91 6 98 43 47 4 Tr? 1 10 13 300 GGAJ-92-94-2 L . Pl i . -K. 71 3 182 18 23 2 Tr 0 0 8 300 GGAJ-92-105-1 L . Pl i . -K. 84 2 141 53 9 0 0 9 2 10 300 GGAJ-92-110-1 L . Pl i . -K. 81 8 93 75 21 4 0 6 4 9 300 GGAJ-92-111-1 L . Pl i . -K. 78 3 102 86 14 4 0 4 8 9 300 GGAJ-92-127-1 L . Pl i . -K. 54 2 148 12 57 0 0 5 22 3 300 GGAJ-92-136-1 L . Pli . -K? 75 3 120 74 20 5 Tr? 2 1 7 300 GGAJ-92-155-1 L? Plien. 57 1 168 49 22 1 0 0 1 5 300 Mean Count 78.5 3.5 127.9 61.0 26.3 2.1 — 3.2 5.3 . . . — Mean VoIume% 26.2 1.2 42.6 20.3 8.8 0.7 — 1.1 1.8 . . . . . . Table 4.1: Counted Parameters Q m = monocrystalline quartz Ls = sedimentary lithics Qp = polycrystalline quartz (incl. chert) L m = metamorphic lithics P = plagioclase feldspar M = phyllosilicates (mainly biotite) K = potassium feldspar D = dense minerals (mainly hornblende) Lv = volcanic-hypabyssal lithics Misc. miscellaneous and unidentified Table 4.1 age assignments: E. /L =Early/Late; Sin.=Sinemurian; Pli./Plien.=Pliensbachian; A.=Arnouldi Zone; V=Varians Zone; H=Harbledownense Zone; I =Imlayi Zone; W=Whiteavesi Zone; K=Kunae Zone. SANDSTONE P E T R O G R A P H Y Table 4.2: Recalculated Parameters Sinemurian Sample No. Q F L Qm P K Q P L v Ls V / L P/F GGAJ-92-16-1 15 66 19 18 64 18 0 90 10 0.90 0.78 GGAJ-92-27-1 15 74 11 17 68 15 5 95 0 0.95 0.82 GGAJ-92-37-2 13 69 18 15 72 13 4 94 2 0.95 0.85 GGAJ-92-33-1 16 69 15 17 66 17 9 87 4 0.87 0.80 GGAJ-92-49-1 27 52 21 32 51 17 10 79 11 0.79 0.75 GGAJ-92-177-1 23 69 8 24 53 23 11 78 11 0.78 0.69 GGAJ-92-181-1 20 64 16 23 66 11 8 90 2 0.90 0.86 Mean 18.4 66.1 15.4 20.8 62.9 16.3 6.7 87.6 5.7 87.7 0.79 Lower Pliensbachian GGAJ-92-55-2 8 46 46 14 76 10 0 99 1 0.99 0.89 GGAJ-92-63-3 13 64 23 17 81 2 0 98 2 0.98 0.98 GGAJ-92-65-2 3 59 38 4 93 3 0 100 0 1.0 0.97 GGAJ-92-78-1 14 67 19 17 78 5 2 93 5 0.95 0.93 GGAJ-92-79-3 18 56 26 25 74 1 0 97 3 0.97 0.99 GGAJ-92-80-1 18 43 39 28 69 3 2 97 1 0.97 0.96 GGAJ-92-80-2 13 65 22 17 82 1 3 92 5 0.92 0.98 GGAJ-92-89-1 18 69 13 20 72 8 7 91 2 0.90 0.90 GGAJ-92-114-1 13 71 16 16 83 1 0 98 2 0.98 0.99 GGAJ-92-132-2 14 69 17 17 80 3 0 96 4 0.96 0.96 GGAJ-92-185-1 10 44 46 18 80 2 1 98 1 0.98 0.98 Mean 12.9 59.4 27.7 17.5 78.9 3.6 1.4 95.6 2.4 0.96 0.96 Upper Pliensbachian GGAJ-92-58-2 25 65 10 27 52 21 6 91 3 0.91 0.71 GGAJ-92-59-3 39 52 9 42 30 28 19 72 9 0.72 0.52 GGAJ-92-84-4 23 64 13 26 66 8 8 90 2 0.90 0.89 GGAJ-92-91-1 37 58 5 38 36 26 22 72 6 0.72 0.58 GGAJ-92-92-2 33 49 18 39 42 19 11 82 7 0.82 0.70 GGAJ-92-94-2 25 66 9 26 67 7 10 83 7 0.81 0.91 GGAJ-92-105-1 30 67 3 30 50 20 18 82 0 0.82 0.72 GGAJ-92-110-1 32 60 8 33 37 30 24 64 12 0.64 0.55 GGAJ-92-111-1 28 66 6 30 38 32 14 67 19 0.67 0.54 GGAJ-92-127-1 21 58 21 25 69 6 3 97 0 0.97 0.93 GGAJ-92-136-1 26 65 9 28 45 27 11 71 18 0.71 0.62 GGAJ-92-155-1 20 72 8 21 62 17 4 92 4 0.92 0.77 Mean 28.3 61.8 9.9 30.4 49.5 20.1 12.5 80.3 7.1 0.80 0.70 Table 4.2: Recalculated Parameters Q = Qm + Qp = (QFL % Q = 100Q/(Q + F + L)) F = P + K = (QFL % F = 100F/(Q + F + L)) L = Lv + Ls + L m = (QFL % L = 100L/(Q + F + L)) QpLvLs % Qp = 100Qp/(Qp + Lv + Ls) QpLvLs % Lv = 100Lv/(Qp + L v + Ls) QpLvLs % Ls = 100Ls/(Qp + Lv + Ls) QmPK % Qm = 100Qm/(Qm + P + K ) QmPK % P = 100P/(Qm + P + K) QmPK % K = 100K/(Qm + P + K) P/F = Plagioclase/Total feldspar V / L = Volcanic lithics/Total lithics SANDSTONE P E T R O G R A P H Y 110 4.3 DETRITAL MODES: INTRODUCTION Inklin Formation sandstones are by definition both texturally and compositionally immature. The textural immaturity is manifested by their generally coarse grain size, moderate to poor sorting, angular to poorly rounded grains, and the ubiquitous presence of interstitial detrital matrix. The compositional immaturity is shown by a dominant plagioclase feldspar detrital mode, subordinate quartz content, and highly variable lithic content. In general, Inklin Formation sandstones may be characterized as lithic-rich to lithic-poor feldspathic arenites with subordinate feldspar-rich lithic arenites and minor feldspathic wackes. Normalized detrital modes range from 43% to 71% for feldspars, 3% to 39% for quartz, and 3% to 46% for lithic fragments in the 30 thin sections point-counted. A number of distinct temporal trends with respect to detrital modes have emerged from the petrographic study and are discussed in detail in following sections. 4.4 SINEMURIAN SANDSTONE 4.4.1 Quantitative Detrital Mode Data Feldspar is the dominant framework grain in Sinemurian sandstones. Normalized feldspar content ranges from 52% to 74% with a mean value of ~ 66% (Table 4.2). Plagioclase dominates the feldspar component although detrital K-spar is present in all samples to a varying degree. P/F (plagioclase/total feldspar) ratios are somewhat variable as a result, ranging from 0.69 to 0.86 with a mean of 0.79 (Table 4.2). Detrital K-spar averages about 13% by volume of Sinemurian samples, ranging from a low of - 8% to occasionally in excess of 20%. Normalized quartz values range from 13% to 27% with a mean of ~ 18% for the suite. Quartz ranks as an important secondary component in most of these samples, though is sometimes subordinate to lithics. The ratio of feldspar to quartz ranges from a high of more than 5:1 to about 2:1 with a mean ratio of ~ 3.5:1. Fine polycrystalline quartz aggregates (Qp) form about 1% by volume of Sinemurian samples. The proportion of lithic fragments in Sinemurian sandstones ranges from a low of - 8% to 21% with a mean is about 15% (Table 4.2). Volcanic varieties dominate lithic fragments, however sedimentary lithics form about 6% of the total lithic content. The sedimentary contribution to lithic modes is reflected in the variable Sinemurian V / L ratios which range from 0.78 to 0.95. The mean Sinemurian V / L ratio is - 0.88. SANDSTONE P E T R O G R A P H Y 111 4.4.2 Qualitative Petrographic Observations and Interpretations Feldspar: Plagioclase grains are composed of two different components: (1) broken and whole euhedra, and (2) mechanically abraded anhedra. Either type may be dominant in a particular sample but typically anhedra are more common. Oscillatory zoning is sometimes present but generally sparse. This may be in large part a diagenetic artifact as the feldspar component in Sinemurian sandstones is typically more highly altered than that in younger units and may obscure recognition of this feature. Grain shapes range from very angular (euhedra) to sub-rounded. A detrital context is clear for most K-spar which dominantly occurs as irregular, mainly sub-angular grains but is also found in plutonic lithic fragments. Orthoclase is the dominant K-spar mineral species. Microcline is also present but appears to form only a minor portion of the K-spar component. Rare myrmekitic feldspar was also identified. Plagioclase euhedra are of volcanic derivation and represent the tuffaceous component of plagioclase. Mechanically abraded plagioclase anhedra lack genetic indicators with the exception of sparse oscillatory zoning. The presence of minor ubiquitous detrital K-spar, as well as other indicators such as the general dominance of plutonic quartz, implies a similar source for much of the plagioclase anhedra. However, the tuffaceous component is ubiquitous in all Sinemurian sandstones, as shown by common broken euhedra, and may form a dominant portion of plagioclase. Relative proportions of plutonic/volcanic plagioclase are impossible to ascertain and more reliance is given to plutonic/volcanic quartz ratios and relative abundance of plutonic lithic fragments as indicators of provenance. Quartz: Monocrystalline quartz has two different qualitative components: (1) quartz grains of volcanic origin and, (2) grains of plutonic derivation. The characteristics of high-temperature volcanic quartz are well-documented (Hughes, 1982; Pettijohn et al, 1987; Carozzi, 1993) and include the following criteria: monocrystalline, limpid quality (i.e., relatively inclusion-free) with straight extinction, distinctive euhedral bipyramidal form which is commonly rounded and/or embayed due to partial magmatic resorption, and often with attached aphanitic microcrytalline or glassy blebs (Fig. 4.2). Petrographic features associated with plutonic varieties include the predominance of weakly to strongly undulose extinction, often of a semi-composite to composite nature, and ubiquitous fluid inclusions (Fig. 4.3). SANDSTONE P E T R O G R A P H Y 112 Volcanic quartz grains are predominantly very angular to sub-angular in shape. The plutonic quartz occurs as variably sized irregular monomineralic grains that tend to show somewhat greater textural maturity than volcanic quartz grains. Some of the plutonic quartz grains display Boehm and deformation lamellae. Most of the quartz in Sinemurian samples displays petrographic features associated with plutonic rocks so that plutonic quartz is interpreted to be generally dominant to volcanic varieties; however, quartz interpreted to be of volcanic derivation is usually an important subordinate component of the quartz grains and can sometimes dominante. A sparse but persistent occurrence in Sinemurian samples is fine polycrystalline quartz aggregates. (Fig. 4.4A) that display a range of textures. With the exception of aggregates composed of a few (< 4) larger anhedra, these quartz aggregates have been classified as polycrystalline quartz (Qp). There are two basic types: anhedral masses composed of randomly associated smaller and larger (i.e., polymodal) anhedra, and aggregates displaying very small anhedra of restricted, almost unimodal size. Grain contacts within the aggregates are very irregular (including 'embayed') and are generally sharp but sometimes diffuse and even appearing sutured. Random extinction is the rule for these aggregates. Lithic Fragments: Sinemurian volcanic lithics are dominated by microlitic grains that display hyalopilitic, trachytic and lesser intersertal textures (Figs. 4.5). Micro-phyric to porphyritic varieties with microlitic groundmass are somewhat subordinate to purely microlitic varieties in a number of samples and are generally hyalocrystalline although sub-hyalocrystalline textures are not uncommon. Phyric/porphyritic phenocrysts are largely plagioclase with minor amounts of hornblende and pyroxene. Quartz was rarely seen as a phenocryst phase. Another common volcanic lithic texture is felsitic varieties with groundmass formed of anhedral microcrystalline granular mosaics (Fig. 4.6A). These are often aphyric but more commonly contain variable degrees of phenocrysts that tend toward hyalocrystalline proportions. This latter group is somewhat problematic due to the uncertainty as to whether the groundmass is primary or diagenetic. As primary textures, and some clearly are, they are indicative of mainly silicic volcanic rocks associated with dacitic and rhyodacitic provenance (Dickinson, 1970). Aphyric varieties resemble some cherts and this appearance may be derived from silicification of felsitic volcanic fragments which can be indistinguishable from chert of normal sedimentary origin (Blatt, 1967). However, close SANDSTONE P E T R O G R A P H Y 113 examination of most usually reveals either of the following: the presence of sparse tiny euhedral microlites which is diagnostic of a volcanic origin (Carozzi, 1993), or the presence of more than one mineral phase that superficially are petrographically similar (i.e., mainly quartz and feldspar)(Fig. 4.6A). Sodium cobaltinitrite staining confirms the presence of common K-spar in many of the latter types. Another qualitative distinction that distinguishes Sinemurian volcanic lithics from Lower Pliensbachian L v suites is the relatively higher degree of alteration observed in the Sinemurian section. Plutonic fragments are present in most Sinemurian samples and are of granitoid composition, generally occurring as polymineralic grains composed of quartz, plagioclase and K-spar, or quartz and K -spar (Fig. 4.7A). The K-spar in these fragments is generally orthoclase, as is the detrital K-spar in Sinemurian samples. Fine polycrystalline quartz aggregates were observed in a few of the plutonic grains. A lithic suite conspicuous by its apparent absence is carbonate lithic fragments. Carbonate clasts derived from erosion of the Upper Triassic Sinwa Formation (Ch. 3, p. 93) in the Sinemurian sandstones. Evidence of carbonate grains is inferred to be obliterated by pervasive overprints during deep burial diagenesis that includes extensive spotty replacements by late-stage diagenetic calcite. 4.4.3 Discussion Plagioclase dominates the feldspar component of Sinemurian sandstones although detrital K-spar is present in all samples in variable amounts. The presence of common detrital K-spar, as well as other indicators such as the general dominance of plutonic quartz implies a similar plutonic source for much of the plagioclase. A tuffaceous component is also present in all samples and may form an appreciable or even dominant portion of plagioclase in Sinemurian sandstones. This is especially true in some samples that are considerably more tuffaceous than the majority. The variability in K-spar content can be attributed to varying degrees of volcanic/plutonic admixture. P/F (plagioclase/total feldspar) ratios are somewhat variable as a result and display values consistent with a transitional arc provenance (Dickinson, 1970). Both orthoclase and microcline are commonly found as anhedra in granitic rocks with orthoclase particularly common in fairly shallow intrusives of Phanerozoic age (Nesse, 1986). The presence of rare myrmekitic feldspar is also indicative of moderate to shallowly SANDSTONE P E T R O G R A P H Y 114 Figure 4.2. A - C : Photomicrographs of euhedral quartz grains displaying characteristic features of volcanic origin. Note bipyramidal shape, limpid quality, and unit extinction. A : Crossed polars, B : Plane light. C : Crossed polars. D: Photomicrograph of quartz phenocryst in volcanic lithic grain. Note rounded euhedral bipyramidal shape with prominent embayment. Plane light. SANDSTONE P E T R O G R A P H Y 115 0.2 nun Figure 4.3. A : Photomicrograph of plutonic quartz grain. Note abundant inclusions and semi-composite extinction. Crossed polars, B: Plane light. C : Granitoid plutonic lithic grain formed of quartz and plagioclase with pericline twinning. Note undulose extinction (arrow). Crossed polars. D: Plutonic quartz grain (arrow) with abundant inclusions and inclusion trains. Plane light. SANDSTONE P E T R O G R A P H Y 116 Figure 4.4. A : Photomicrograph of polycrystalline quartz aggregate of plutonic derivation displaying composite extinction. Arrow points to undulatory extinction visible in larger sub-grain in upper right of grain. Crossed polars; B : Plane light. C : Out-sized perthitic orthoclase grain in Upper Pliensbachian sandstone. Plutonic derivation is evident from enclosed hornblende crystal. Plane light. SANDSTONE P E T R O G R A P H Y 117 Figure 4.5. Photomicrograph of microlitic textures in andesitic lithic grains in Sinemurian sandstone. Cryptocrystalline groundmass is typically pervasively chloritized suggesting a relict glassy component. A : Trachytic texture. Crossed polars, B : Plane light. C : Herringbone texture. Crossed polars. D : Random arrangement of microlites. Plane light. SANDSTONE P E T R O G R A P H Y 118 Figure 4.6. A: Photomicrograph of altered volcanic lithic grain with authigenic microcrystalline 'groundmass' resembling chert. In this example remnant phenocrysts (arrow) clearly reveal the volcanic origin. Crossed polars. B : Polymineralic nature of microcrystalline 'groundmass' is evident in plane light. C : Dacitic volcanic lithic grain with microlitic groundmass. Phenocrysts are feldspar (mainly plagioclase) and hornblende. Crossed polars, D: Plane light. SANDSTONE P E T R O G R A P H Y 119 Figure 4.7. A : Photomicrograph of granitoid lithic fragment in Sinemurian sandstone. Grain is composed of anhedral quartz aggregate and euhedral feldspar (K-spar ?) crystal. Crossed polars, B : Plane light. C : Granitoid lithic fragment in Upper Pliensbachian sandstone composed of quartz and altered feldspar (plagioclase ?). Crossed polars, D: Plane light. SANDSTONE P E T R O G R A P H Y 120 Figure 4.8. A : Photomicrograph of plutonic lithic fragment of granitic composition in Upper Pliensbachian sandstone. Grain is dominated by polymodally-sized quartz anhedra with albitized plagioclase and K-spar ? anhedra (arrows). Crossed polars, B: Plane light. SANDSTONE P E T R O G R A P H Y 121 emplaced plutons which are inferred to be the source of the detrital K-spar. The quartz content of Sinemurian samples is quite variable both qualitatively and quantitatively, ranging from minor to subordinate as a detrital mode and displaying variable proportions of both plutonic and volcanic quartz. These attributes are consistent with a mixed volcanoplutonic provenance and indicate derivation from a transitional arc. Although plutonic quartz is generally dominant, volcanic quartz can be sometimes dominant and in one sample this dominance was found to be pronounced, suggesting that coeval volcanism may have been a sporadic contributor to Sinemurian basin-fill. Some of the plutonic quartz grains display Boehm and deformation lamellae which is indicative of high strain regimes, suggesting some of this quartz is from deeper crustal levels. Sparse but persistent fine polycrystalline quartz aggregates are difficult to confidently identify as plutonic because of the similar appearance of some high-rank metamorphics. Criteria such as more elongate anhedra, tendency to show sub-parallel extinction and dominance of sutured or polygonized grain boundaries can be used to distinguish metamorphic from plutonic aggregates but are not unequivocal in the absence of a preferred planar fabric (Carozzi, 1993). These aggregates typically lack any preferred fabric and are attributed to plutonic rather than metamorphic sources. This interpretation is supported by the sparse occurrence of similar polycrystalline aggregates enclosing K-spar and plagioclase anhedra that are of obvious plutonic derivation. No polycrystalline quartz of unequivocal metamorphic origin was identified although two samples contained a Qp grain with appreciable textural differences that are suggestive of a high-rank metamorphic origin. Based on the petrographic evidence metamorphic sources must be considered conjectural in Sinemurian time. Other sparse but persistent fine polycrystalline grains found throughout the Inklin Formation are those that resemble some cherts. In his thesis, Bultman (1979) makes reference to common chert clasts in Inklin sandstones but evidence from this study indicates these are largely erroneous identifications. The misidentification of micro/cryptocrystalline felsitic volcanic fragments as chert is not uncommon in petrographic analyses of volcaniclastic sediments (Pettijohn et al, 1987; Carozzi, 1993). Petrographic evidence from this study indicates a felsic volcanic origin for the majority, whether as primary felsic grains SANDSTONE P E T R O G R A P H Y 122 or devitrified tuffs (Dickinson, 1970; Pettijohn et al, 1987). Consequently, sedimentary chert is rare or absent in the succession at Atlin Lake. The proportion of Sinemurian lithic fragments is fairly constant in these samples with a mean roughly half of the Lower Pliensbachian mean. As in the rest of the succession, volcanic varieties dominate lithic fragments: however, sedimentary lithics, though sparse, are more common here than elsewhere - a feature also noted in Sinemurian conglomerates (Ch 3, p. 78). The sedimentary influence contributes to the minor variability seen in V / L ratios which are intermediate between Early and Late Pliensbachian means. Another qualitative distinction that distinguishes Sinemurian volcanic lithics from Lower Pliensbachian suites is the relative degree of alteration. Sinemurian volcanic lithics tend to be considerably more altered than those in the Early Pliensbachian but because Sinemurian sediments are more altered in general it is impossible to determine whether this is a paleo-weathering or diagenetic artifact. Given the general dominance of plutonic material in most samples and the relatively high initial matrix contents of more tuffaceous units, one suspects many of these lithics are paleovolcanics (derived from erosion of old volcanic suites). 4.5 LOWER PLIENSBACHIAN SANDSTONE 4.5.1 Quantitative Detrital Mode Data The feldspar component is dominated by plagioclase. Normalized feldspar content ranges from 43% to 71% with a mean value of ~ 59% (Table 4.2). These values are lower than those found in Sinemurian and Upper Pliensbachian sandstones due to the higher lithic content of Lower Pliensbachian rocks. Detrital K -spar ranges from sparse to absent and is a distinguishing feature of Lower Pliensbachian sandstones. Mean detrital K-spar is less than 3% by volume of these rocks and consequently P/F (plagioclase/total feldspar) ratios approach unity in all samples. Normalized lithic contents are always a significant component of the rock's volume, ranging from a low of 13% up to 46 % (Table 4.2). Volcanic lithics completely dominate lithic fragments. Sedimentary lithics are absent or present in only trace amounts, averaging less than 2% of the lithic category. The mean lithic content for Lower Pliensbachian sandstones is approximately 28%. SANDSTONE P E T R O G R A P H Y 123 The quartz content of Lower Pliensbachian sandstones is distinct with respect to its modal abundance. Normalized Q values range from 3% to 18%, with a mean value of approximately 13% (Table 4.2). Feldspar to quartz ratios are the highest in the Inklin succession, ranging from roughly 3:1 to 20:1 with a mean of about 4.5:1. Polycrystalline quartz is generally absent or a rare trace component, averaging a small fraction of 1% in Lower Pliensbachian sandstone. 4.5.2 Qualitative Petrographic Observations and Interpretations Feldspars: The feldspar component is strongly dominated by plagioclase. Texturally, most of this material is composed of single crystal euhedra and broken euhedra, indicating a volcanic origin. Oscillatory zoning is quite common and embayments due to magmatic resorption are also visible on some crystals. Grain shape is commonly angular to very angular. A significant subordinate component is composed of mechanically abraded anhedra. Detrital K-spar ranges from sparse to essentially absent. The identification a broken euhedral K-spar crystal in one sample, as well as rare sand-sized K-spar phenocrysts in volcanic lithic fragments, suggests that felsic volcanics are the source of some of the Lower Pliensbachian detrital K-spar. The K-spar mineral species were not determined but the occurrence of K-spar as volcanic phenocryst phases suggests some are sanidine. Ambiguity about the genetic source (i.e., volcanic vs. plutonic) of some anhedra and alteration effects rendered compositional determinations somewhat equivocal, however a number of suitable, relatively unaltered, clearly volcanic plagioclase crystals were measured for extinction angles on a universal stage. Plagioclase compositions ranged from A n I 9 to A n 3 6 and are consistent with felsic volcanic source rocks. Lithic Fragments: The lithic fragments are composed almost entirely of volcanic material that ranges in composition from intermediate to felsic. This material is dominantly dacitic to rhyodacitic in composition and is often relatively unaltered compared with paleovolcanics in thin-section. Sodium cobaltinitrite staining reveals that K-spar is a ubiquitous component of the cryptocrystalline groundmass in many of these volcanics. SANDSTONE P E T R O G R A P H Y 124 The groundmass textures in the volcanic lithic grains range through microgranular-felsitic, crypto/microcrystalline, microlitic, and glassy. Microgranular-felsitic and cryptocrystalline grains are almost always phyric to porphyritic with phenocrysts generally in hyalocrystalline proportions (Fig. 4.9A). These are more abundant than in Sinemurian sediments and contain dominantly plagioclase phenocrysts with subordinate to minor mafics (mainly hornblende ± pyroxene and biotite), lesser quartz and rare K -spar. Microlitic varieties are also common though generally subordinate and display mainly pilotaxic and trachytic textures with minor intersertal textures. These are also generally phyric or porphyritic with similar phenocryst phases and proportions as microgranular-felsitic and cryptocrystalline varieties (Fig. 4.6C). Classification of the volcanic fragments is hampered by their small (sand grade) size. For example the distinction between dacite and andesite is based on the presence or absence of quartz phenocrysts. However quartz phenocrysts are a minor phenocryst phase in dacites (often < 10 %) so they are unlikely to be present in most sand-sized lithic fragments. Groundmass textures (conspicuous plagioclase microlites) indicate that many fragments are of either dacitic or andesitic composition (Hughes, 1982) but the presence of occasional quartz phenocrysts in some, and their strong textural similarity to those fragments without visible quartz suggests that most or all of the lithics with microlitic groundmass are dacites rather than andesites (Fig. 4.6C, 4.9A). The same reasoning applies for classification of felsitic textural variants that are a common component of Lower Pliensbachian sandstone. These lithic types display a K-spar-rich mesostasis which predominates over plagioclase microlites in the groundmass - a textural feature associated with rhyodacites (Hughes, 1982). Again, groundmass texture and the presence of quartz phenocrysts in some are used to classify this volcanic lithic 'suite' as rhyodacites (Fig. 4.9C). Quartz: Qualitatively, detrital quartz in Lower Pliensbachian sandstone differs significantly from the rest of the Inklin succession. In these rocks volcanic quartz is always strongly to wholly dominant and is typically angular to very angular. The overwhelming predominance of this qualitative feature is unique to Lower Pliensbachian samples and is a strong provenance indicator underlining the relative degree of volcanic input. Plutonic quartz is present in varying small amounts in most samples but is always a minor to sparse component of the total quartz. Another noteworthy feature is the essential absence of SANDSTONE P E T R O G R A P H Y 125 Figure 4.9. A : Photomicrograph showing normal oscillatory zoning in plagioclase phenocryst contained in hyalocrystalline volcanic lithic grain found in Lower Pliensbachian tuffaceous sandstone. Crossed polars, B : Plane light. C : Porphyritic rhyodacite lithic fragment in Lower Pliensbachian sandstone. Note bipyramidal quartz phenocryst and plagioclase phenocryst with resorbed margins. Crossed polars. SANDSTONE P E T R O G R A P H Y 126 polycrystalline quartz in Lower Pliensbachian sandstones. Polycrystalline quartz content in Inklin sandstones is strongly dominated by plutonic varieties and their absence in Lower Pliensbachian sediments is further evidence of minor plutonic input. 4.5.3 Discussion Feldspar content is lower than those found in Sinemurian and Upper Pliensbachian sandstones and this is largely due to the higher lithic content of Lower Pliensbachian rocks. The dominance of plagioclase in the feldspar component in Lower Pliensbachian sandstones is pronounced. The compositions of the plagioclase are consistent with felsic volcanic products. Detrital K-spar ranges from sparse to absent and is a distinguishing feature of Lower Pliensbachian sandstone. Consequently, P/F (plagioclase/total feldspar) ratios approach unity in all samples - a feature indicative of an undissected-arc provenance and unlike the variability of P/F ratios displayed by Sinemurian and Upper Pliensbachian samples (Table 4.2). Various degrees of mechanical transport of plagioclase anhedra renders a clear volcanic context for these crystals uncertain, however, the ubiquity of broken and whole euhedra in general, as well as the dominance of volcanic quartz, indicates a strong pyroclastic volcanic provenance, suggesting much of the anhedra are from similar volcanic sources. A plutonic influence is evident from the small amount of plutonic quartz grains in most samples and consequently, a plutonic source is inferred for a portion of mechanically abraded anhedra. The plutonic contribution is most probably quite small to absent in highly tuffaceous units as indicated by both the relative rarity of detrital K-spar and plutonic quartz, and the abundance of euhedral plagioclase. Texturally, the plagioclase component can be characterized as generally tuffacous, indicating its derivation from largely volcanic sources. The lithic content of Lower Pliensbachian sandstone is much higher than elsewhere in the section and consequently these sandstones include the only feldspathic lithic arenites identified in the Inklin Formation. Lithic fragments are always a significant component of the rock's volume with mean lithic content twice the Sinemurian mean and triple that of Upper Pliensbachian sandstones. The proportionally higher lithic content of these rocks can be attributed to the input of coeval volcaniclastics during this period. It must also be remembered that lithic values are systematically low due to the abundance of porphyritic volcanic clasts SANDSTONE P E T R O G R A P H Y 127 which are sometimes counted as P (plagioclase) or more rarely Qm (monocrystalline quartz) rather than L v (volcanic lithic fragment) by the Gazzi-Dickinson method of point-counting. The lithic fragments are composed almost entirely of volcanic material so that V / L ratios approach unity in all Lower Pliensbachian sandstones. This is one of the diagnostic criteria for undissected-arc provenance outlined by Dickinson (1970). The composition of the volcanic lithics ranges from intermediate to felsic and likely represents both paleovolcanic and neovolcanic (derived from broadly coeval volcanism) materials; however, the dominance of dacitic and rhyodacitic compositions suggests a genetic relationship for this volcanic suite. The relatively fresh appearance, similar textures, and similar compositions of these latter varieties supports an interpretation of broadly coeval volcanism as the source of the dacitic and rhyodacitic volcanic lithics. The quartz content of Lower Pliensbachian sandstones is distinctive both with respect to its modal abundance and composition. The volume of quartz in these rocks can be characterized as consistently lower than other parts of the succession, although there is overlap with some Sinemurian units. Qualitatively, the predominance of volcanic quartz is unique to Lower Pliensbachian samples and can be considered a diagnostic criterion for their correlation in the Atlin Lake area. Even though certain Sinemurian samples can also have a dominant volcanic quartz content they are always accompanied by a significant subordinate plutonic component. The mean ratio of normalized feldspar to quartz in Lower Pliensbachian sandstones is over twice the Upper Pliensbachian mean but is still misleadingly low due to the much greater importance of lithics in Lower Pliensbachian sediments which skews F/Q values toward artificial lows. Lower Pliensbachian sandstones with higher quartz contents may reflect volcanic eruptions of rhyodacitic composition and/or intense reworking of pyroclastic materials of similar composition while those with lower contents probably reflect largely dacitic pyroclastic events. Some minor plutonic contribution is also apparent and may be a larger factor in some stratigraphic horizons. Plutonic sources of Qp exposed during the Sinemurian and Late Pliensbachian are interpreted to have been largely unavailable during the Early Pliensbachian due to extensive surface cover of pyroclastic blankets generated by broadly coeval volcanism. SANDSTONE P E T R O G R A P H Y 128 4.6 U P P E R P L I E N S B A C H I A N S A N D S T O N E 4.6.1 Quantitative Detrital Mode Data Feldspar constitutes the dominant framework grain in all Upper Pliensbachian samples. Normalized feldspar content ranges from 49% to 72%, with a mean value of ~ 62%. Plagioclase is the dominant constituent of the feldspars however K-spar is an important subordinate component in most samples. There is dramatic increase in detrital K-spar in Upper Pliensbachian sandstones. Detrital K-spar averages about 20% by volume of Upper Pliensbachian samples and can be as high as 32% in some. P/F ratios range from a high of 0.93 in highly tuffaceous units to as low as 0.52 in plutonic dominated units. The mean P/F value is 0.70 for Upper Pliensbachian sandstones. Upper Pliensbachian sandstones are significantly more quartzose than any other part of the succession. Even highly tuffaceous samples had at least 20% quartz content due to the addition of some granitic material, although minor primary water-lain tuffs had values similar to the Early Pliensbachian. Of the 12 samples point-counted, quartz contents ranged from 20% to 39% with a mean of ~ 28%, which is more than double that of the Early Pliensbachian. Quartz ranks as an important secondary component in these sediments and combined with feldspar accounts for more than 80% of the rock volume in almost all samples. The ratio of feldspar to quartz ranges from slightly more than 3:1 to almost 1:1 with a mean of ~ 2:1 - a value considerably lower than elsewhere in the succesion. Another minor but persistent variety of detrital quartz are polymineralic anhedral aggregates that display a range of textures similar to those described in the Sinemurian (Fig. 4.4A). This type of quartz averages over 1% by volume of Upper Pliensbachian sandstone. The lithic content of Upper Pliensbachian sandstone is the lowest in the succession, ranging from a low of 3% to atypically 21% in highly tuffaceous units. The mean is slightly less than 10% which is approximately a third of the Early Pliensbachian mean. Plutonic lithic fragments of granitic composition are common in most samples (Fig. 4.7C) but are not included in lithics by the Gazzi-Dickinson point-counting method. SANDSTONE P E T R O G R A P H Y 129 4.6.2 Qualitative Petrographic Observations and Interpretations Feldspar: As with all Inklin Formation sandstones, plagioclase dominates the feldspar component in Upper Pliensbachian sandstones; however, these rocks differ greatly in the degree of dominance. Detrital K-spar is a common subordinate component of the total feldspar. The K-spar is dominantly orthoclase although microcline is a persistent minor occurrence. Both typically occur as mechanically abraded anhedra or in plutonic lithic fragments where some euhedral faces are found (Fig. 4.7C). Detrital orthoclase dominates the largest grain sizes and are common as outsized particles (Figs. 4.4C, 4.1 OA). Perthitic textures are not unusual in either K-spar though less common in microcline. Granophyric textures were occasionally seen in orthoclase and myrmekitic textures in granitic lithic fragments. Orthoclase-hosted incipient to highly resorbed plagioclase anhedra (plus occasional mafics) are common in some samples (Fig. 4.1 OA). The abundance of plutonic K-spar in many samples directly implies a similar source for the bulk of the plagioclase. However, mild to moderate admixture from volcanic sources is indicated by the occurrence of plagioclase with oscillatory zoning, as well as broken euhedra and volcanic lithic fragments. Certain tuffaceous units can be strongly dominated by plagioclase with these features. Quartz: Quartz content in Upper Pliensbachian sandstones is qualitatively distinctive. The detrital quartz typically displays the petrographic features associated with plutonic varieties. A n appreciable minor portion of this quartz is found in plutonic lithic fragments where it occurs with plagioclase and potassium feldspars. There is a pronounced dominance of quartz with these features in most of the samples analyzed, in striking contrast to Lower Pliensbachian sandstones; however, volcanic quartz is always present and can be an important subordinate component. Tuffaceous units are dominated by volcanic quartz but this is atypical for most Upper Pliensbachian sandstone. As noted in Sinemurian samples, quartz grains with Boehm and deformation lamellae are a sparse but persistent occurrence. Plutonic quartz occurs as variably sized monomineralic grains and tends to be less angular than volcanic varieties Polycrystalline anhedral aggregates are a persistent minor occurrence in Upper Pliensbachian sandstones and more numerous than elsewhere in the Inklin succession. Textures are similar to those described in Sinemurian sandstones and are also largely attributed to plutonic sources. This interpretation is supported by the occurrence of similar quartz aggregates in plutonic lithic fragments (Fig. 4.8). Rarely SANDSTONE P E T R O G R A P H Y 130 Figure 4.10. A : Photomicrograph of out-sized plutonic lithic grain composed primarily of perthitic orthoclase. Note enclosed resorbed plagioclase anhedra (arrow). Crossed polars. B: Granophyric texture of quartz and K-spar intergrowths (arrow) in monzogranite clast in Upper Pliensbachian conglomerate. Clast is of Pliensbachian age. (U:Pb - 186.6 +0.5/-1.0 Ma.). This texture is indicative of shallow emplacement of plutonic body. Crossed polars. SANDSTONE P E T R O G R A P H Y 131 these quartz aggregates display the petrographic features associated with metamorphic varieties, including a preferred planar fabric, so that some metamorphic input is indicated; however, unequivocal quartz-mica tectonites of high-rank metamorphic origin are rare and are believed to form a very small portion of these polycrystalline aggregates. Lithic Fragments: Plutonic lithic fragments of granitic composition are common in many samples (Figs. 4.7B, 4.8, 4.10A) and would form an important subordinate component of the normalized lithics but are not included in the lithic category by the Gazzi-Dickinson point-counting method. The plutonic lithics appear to be entirely of granitic composition and are generally fresh in appearance. K-spar is the dominant component in most fragments and occurs with either quartz or quartz and plagioclase anhedra. Biotite and hornblende are minor accessory minerals sometimes present. The quartz in these fragments may be monocrystalline or fine polycrystalline aggregates similar to those counted as Qp (Fig. 4.8). This latter occurrence identifies these plutonics as the probable source of the detrital polycrystalline quartz. Granophyric and myrmekitic textures were observed in these granitic lithic fragments. The lithic category is dominated by dacitic to rhyodacitic volcanic fragments that are identical to those found in the Lower Pliensbachian section and require no further description. Tuffaceous units display relatively fresh volcanic lithics but generally they tend to be somewhat more altered than in Lower Pliensbachian sandstone. This is unlikely to be a diagenetic artifact as these sediments are younger and possibly reflects paleo-weathering effects. Another minor difference is the higher proportion of sedimentary lithics. V / L ratios for Upper Pliensbachian sandstones are quite variable but lower in general than elsewhere in the succession. The sedimentary lithics are dominantly muddy fine siltstones and are likely intraclasts of Pliensbachian age. 4.6.3 Discussion The feldspar component of Upper Pliensbachian sandstones is distinctive and sets these rocks apart from the rest of the Inklin succession. Detrital K-spar is ubiquitous and forms an important secondary component of the total feldspar - a feature unique in Inklin Formation sandstones in the study area and in stark contrast to the K-spar-poor sediments of the Lower Pliensbachian.. Detrital K-spar composes as much SANDSTONE P E T R O G R A P H Y 132 as one-third of the rock volume in some stratigraphic horizons, averaging about 20% of Upper Pliensbachian sandstones and is a useful diagnostic criterion for distinguishing them from Lower Pliensbachian feldspathic sandstones. The mean P/F value is 0.70 for Upper Pliensbachian sandstones - a value significantly lower than Sinemurian and Lower Pliensbachian sediments and one consistent with a dissected arc provenance. Textures observed in thin-sections indicates provenance from granitic plutons emplaced at shallow crustal levels and, more specifically, the dominance of orthoclase among the coarse out-size grain fraction suggests a provenance linkage with orthoclase-megacrystic granites of Pliensbachian age (Ch. 3, p.92). Given the abundance of K-spar of clear plutonic derivation, a similar provenance is inferred for a significant, possibly major, portion of the plagioclase in Upper Pliensbachian sandstones. Some volcanic input is generally present as indicated by the occurrence of broken and whole plagioclase euhedra and anhedra with oscillatory zoning. Tuffaceous units can be dominated by plagioclase with these features and are interpreted to represent sporadic pyroclastic events that are broadly coeval with deposition of these units. However even highly tuffaceous units contain some evidence of plutonic admixture so that a portion of the plagioclase is likely derived from plutonic sources as well. In sum, it is plutonic sources that supplied the majority of the feldspar component in Upper Pliensbachian sediments. The abundance of K -spar in Upper Pliensbachian sandstone provides clear evidence of the importance of plutonic sources and underlines the dissected nature of the arc in Late Pliensbachian time. Quartz ranks as an important secondary component in these sediments, averaging one-quarter of Upper Pliensbachian detrital modes and can form greater than one-third of the rock volume in some stratigraphic horizons. Thus, Upper Pliensbachian sandstones are significantly more quartzose than any other part of the succession. Mean quartz content is more than double that in Lower Pliensbachian rocks and this attribute furnishes another useful criterion for the distinction between Upper and Lower Pliensbachian sandstone. The mean ratio of feldspar to quartz is considerably lower than elsewhere in the Inklin succession due to the importance of granitic plutons as primary clastic sources. Polycrystalline quartz aggregates with similar textures to those observed in plutonic lithic fragments are interpreted to be of dominantly plutonic derivation; however, textures indicative of a metamorphic SANDSTONE P E T R O G R A P H Y 133 origin were observed in a few grains so that some minor metamorphic input is indicated. The most likely source for these metamorphic grains is from exposure of the metamorphic envelopes that enclose batholiths in the roots of the magmatic arc (Dickinson, 1982). This is consistent with deep incision of the magmatic arc apparent from plutonic quartz and feldspar abundance. The normalized lithic content of Upper Pliensbachian sandstone is the lowest in the succession. The mean is approximately a third of the Early Pliensbachian mean, and this relatively low value can be attributed to the waning volcanic influence. The similar mineral abundance and textures of ubiquitous granitic lithic fragments are reflected in the composition of Upper Pliensbachian detrital modes and provide a clear provenance linkage to a dissected arc. 4.7 P E T R O F A C I E S S U M M A R Y 4.7.1 Sinemurian Unlike Lower and Upper Pliensbachian sandstones, Sinemurian samples do not exhibit the same strong suite of distinct petrographic features that allow clear definition of separate petrofacies. This may be due in part to the small sample suite (n=7) which spans both Early and Late Sinemurian time. This more diffuse resolution (compared with Early and Late Pliensbachian suites) may conceivably 'smear out' distinct sub-stage temporal trends to some degree. Another factor may be the greater degree of diagenetic alteration these sediments have undergone, rendering certain petrographic distinctions more difficult and equivocal. However, despite these factors, sufficient distinctions do exist to allow for their discrimination. In general, the Sinemurian samples show evidence of a greater degree of volcanoplutonic mixing than the rest of the succession and it is noteworthy that means for petrographic parameters are almost always consistently intermediate in value between Lower and Upper Pliensbachian means. Sinemurian sandstone can be distinguished from Lower Pliensbachian sandstone on the basis of detrital mode abundances which include lower lithic content, much higher K-spar content, generally higher quartz content, lower mafic mineral content, and more subtle distinctions among trace components such as greater abundance of polycrystalline quartz and sedimentary lithic fragments. Discrimination between Sinemurian and Upper Pliensbachian sandstone can be resolved by the higher lithic, lower quartz, and SANDSTONE P E T R O G R A P H Y 134 lower K-spar content of Sinemurian sandstone. Qualitative discriminators include less plutonic quartz, plutonic lithic fragments, and felsic volcanic material in Sinemurian sandstone. As a general rule, Sinemurian sandstone exhibits a greater degree of diagenetic alteration than both Lower and Upper Pliensbachian sandstone. 4.7.2 Lower Pliensbachian Lower Pliensbachian sandstone can be distinguished from other parts of the Inklin succession on the basis of their detrital modes as well as a number of qualitative features. Lower Pliensbachian sandstone are distinct by virtue of their low quartz content, dominance of volcanic quartz, high volcanic lithic content, and paucity of detrital K-spar. Lower Pliensbachian sandstones are dominantly tuffaceous plagioclase feldspathic arenites (and 'sub-wackes') but include a significant component of dacitic volcanic lithic arenites and minor plagioclase feldspathic greywackes. A n important subordinate component of the section includes water-lain lithic-crystal dacitic to rhyodacitic lapilli tuffs that grade into variably reworked resedimented pyroclastic debris and highly tuffaceous epiclastic deposits. In general terms, Lower Pliensbachian sandstone can be characterized as tuffaceous or volcaniclastic sandstone. The detrital mineralogy of all samples is composed predominantly of volcanically derived components of plagioclase feldspar, lithic fragments, and quartz. The volcanic nature of the plagioclase is clearly evident in thin-sections where the ubiquity of broken and whole euhedral crystals, as well as common oscillatory zoning, is seen. The quartz grains display similar genetic textures, being strongly dominated by relatively inclusion-free crystals with straight extinction. Polycrystalline quartz of either metamorphic or plutonic origin is essentially absent. Lithic fragments are almost entirely volcanic in origin. Further evidence of neovolcanic provenance is found in the abundance of mafic minerals. The mafic content (mainly hornblende with minor pyroxene and biotite) is on average two and three times that of Sinemurian and Upper Pliensbachian sediments respectively, and can equal or exceed the quartz content of the sandstone (Table 4.1) - a modal attribute noted in many volcanic sandstones (Dickinson, 1970). This component often shows less alteration compared with other sections. SANDSTONE P E T R O G R A P H Y 135 4.7.3 Upper Pliensbachian Upper Pliensbachian sandstones represent another distinct petrofacies and contain features unique to this part of the succession. Detrital modes reflect the same pronounced influx of granitic material so evident in coeval conglomerates (Ch 3, p. 89), consequently these sandstones contain the highest proportions of quartz and K-spar in the succession at Atlin Lake. They are characteristically medium to coarse lithic-poor arkosic sub-wackes and many samples can be characterized as quartzofeldspathic sandstones. In most cases they are moderately to strongly dominated by plutonic material although a number of highly tuffaceous units, including water-lain lapilli tuff, are found in the section. These are identical to Early Pliensbachian equivalents and, similarly, can possess matrix contents of less than 5%, so that feldspathic volcanic arenites are not uncommon, although minor in volume. Feldspars of mixed volcanic-plutonic provenance are the dominant detrital mode with quartz being an important subordinate mode and lithics generally minor. The combined petrographic attributes of Upper Pliensbachian sandstone provide a strong body of evidence indicating a dissected arc provenance for these sediments. The abundance of detrital K-spar and plutonic lithic fragments, and the relatively high quartz content dominated by plutonic varieties are diagnostic attributes of the Upper Pliensbachian section and stand in marked contrast to the petrographic characteristics of Lower Pliensbachian sandstone. SANDSTONE P E T R O G R A P H Y 136 4.8 P R O V E N A N C E Ternary Diagrams Ternary plots of Inklin Formation detrital modes graphically illustrate strong temporal trends that reveal significant events in Lower Jurassic tectonic and magmatic evolution of the northern Stikinian arc. QFL, QmPK, and QpLvLs ternary diagrams reveal a complex arc/basin evolution punctuated by strong provenance shifts. Details are discussed below. 4.8.1 Q F L Plots Sinemurian points plot mainly in the far left transitional arc field near the boundary between dissected arc and basement uplift (Fig. 4.11). Qualitative petrographic indicators already discussed reveal relatively advanced volcanoplutonic mixing in most samples and confirm a transitional arc provenance for this suite. Early Pliensbachian points plot entirely within the transitional arc field - a position inconsistent with the interpretation of a strong volcanic provenance (Fig. 4.12). Qualitative petrographic evidence clearly shows a strong to wholly dominant volcanic provenance for these rocks and supports the interpretation of a largely undissected arc provenance for Lower Pliensbachian sediments. Some explanation of this apparent discrepency is required. A combination of two influences cause the high feldspar to lithic ratio which shifts the suite to the plagioclase pole and away from the undissected arc field on the QFL diagram: (1) the predominance of large-volume crystal-rich (which can be > 60% by volume of dacitic pyroclastics) dacitic/rhyodacitic pyroclastics over associated flow rocks, and (2) intense reworking of the volcaniclastic debris. In most cases, volcanic provenance in magmatic arcs is from pyroclastic blankets rather than lavas (Dickinson, 1970) and some associated sands of undissected arc provenance show strong concentrations of plagioclase grains relative to lithic fragments due to intense reworking (Dickinson & Suczek, 1979). In all other respects the petrographic features of these rocks strongly conform to diagnostic criteria for sediments of undissected arc provenance (Dickinson & Suczek, 1979). Late Pliensbachian points plot mainly in the basement uplift field to the left of the dissected arc field (Fig. 4.13). This is consistent with arkosic sands derived mainly from the plutons of deeply dissected SANDSTONE P E T R O G R A P H Y 137 magmatic arcs which are gradational to similar sands derived from basement uplifts exposing granite and gneiss within continental blocks (Dickinson et al, 1983). Extensive batholithic exposure is implicit. The QFL plot of mean values for the three sandstone suites show a shift from a transitional arc provenance in the Sinemurian back towards an undissected arc provenance in the Early Pliensbachian (Fig. 4.14). This was followed by a dramatic shift to a deeply dissected arc provenance in the Late Pliensbachian. 4.8.2 QmPK and QpLvLs Plots QmPK and QpLvLs plots supplement QFL diagrams by depicting partial modes of mineral grains alone (QmPK) and polycrystalline lithic fragments (QpLvLs). Recent work by Marsaglia and Ingersoll (1992) that concentrates on magmatic-arc provenance has refined the model of Dickinson & Suczek (1979) and Dickinson and others (1983). Their results stress the importance of oblique subduction and/or intra-arc strike-slip tectonics in producing sand with dissected-arc provenance, and they have defined a compositional field on QmPK diagrams that distinguishes between strike-slip continental arcs and continental arcs without strike-slip influence. In the scheme of Marsaglia and Ingersoll (1992), continental arcs are distinguished from intra-oceanic arcs by possessing a component of either granitic basement, accreted terranes, or a combination of both. The delineation of a strike-slip continental arc field is an effort to incorporate the growing recognition of the fundamental role of trench-linked intra-arc transform faults play in the evolution of arcs and their associated basins in zones of oblique convergence (e.g., Christie-Blick and Biddle, 1985; Woodcock, 1986; Sylvester, 1988; Jarrad, 1986; Karig etal, 1986; Sarewitz and Lewis, 1991), and refers to continental-arc systems affected by periods of intra-arc or forearc strike-slip movement or oblique subduction (Marsaglia and Ingersoll, 1992). Sinemurian points fall mainly within the continental arc field but do spill over into the strike-slip continental arc field, reflecting their largely transitional arc provenance (Fig. 4.15). Early Pliensbachian points are largely confined to the continental arc field but plot to the right of Sinemurian points, reflecting the paucity of K-spar (and hence plutonic influence) in these sediments (Fig. 4.16). Late Pliensbachian points occupy much of the strike-slip continental arc field, plotting well above and to the left (i.e., towards the K pole) of the other two suites in general as a consequence of their strong dissected arc provenance SANDSTONE P E T R O G R A P H Y 138 (Fig. 4.17). The mean QmPK diagram shows the same significant reversal seen in the Q F L mean plot with the additional refinement of illustrating the significant influence of strike-slip tectonics on Late Pliensbachian sedimentation (Fig. 4.18 ). QpLvLs plots show similar petrographic trends. Early Pliensbachian points are tightly clustered at the L v pole (Fig. 4.20), reflecting their strong volcanic provenance while Late Pliensbachian points display considerable spread and maximum distance from the Lv pole (Fig. 4.21), with Sinemurian points occupying an intermediate position (Fig. 4.19). The distance of QpLvLs means from the Lv pole is a largely a function of relative Qp content which, in the case of Inklin sandstones, is essentially of plutonic origin and therefore an indicator of relative plutonic influence (Fig. 4.22). SANDSTONE P E T R O G R A P H Y 139 Figure 4.11. Detrital modes of Sinemurian sandstone plotted on Dickinson et. al.'s (1983) Quartz-Feldspar-Lithics (QFL) ternary diagram for sandstones. Tectonic discrimination fields indicate provenance of sandstone samples. SANDSTONE P E T R O G R A P H Y 140 Figure 4.12. QFL plot of detrital modes for Lower Pliensbachian sandstone samples (After Dickinson etal., 1983). SANDSTONE P E T R O G R A P H Y 141 Figure 4.13. QFL plot of detrital modes for Upper Pliensbachian sandstone samples (After Dickinson et.al., 1983). Figure 4.14. QFL plot of mean detrital modes for Inklin Formation sandstone suites. Arrows indicate mean temporal trends and point in a younging direction (After Dickinson et.al., 1983). SANDSTONE P E T R O G R A P H Y 143 Figure 4.15. Partial modes of Sinemurian sandstone plotted on Marsaglia and Ingersoll's (1991) ternary diagram for arc-derived sandstone. Scheme utilizes Monocrystalline quartz-Plagioclase feldpar-Potassium feldspar (QmPK) detrital modes to discriminate arc provenance and tectonic influence (i.e., strike-slip transforms and/or oblique subduction). SANDSTONE P E T R O G R A P H Y 144 Figure 4.16. QmPK plot of partial modes for Lower Pliensbachian sandstone samples (After Marsaglia and Ingersoll, 1991). Figure 4.17. QmPK plot of partial modes for Upper Pliensbachian sandstone samples (After Marsaglia and Ingersoll, 1991). Figure 4.18. QmPK plot of mean partial modes for Inklin Formation sandstone suites. Arrows indicate mean temporal trends and point in a younging direction (After Marsaglia and Ingersoll, 1991). SANDSTONE P E T R O G R A P H Y 147 Figure 4.19. Partial modes of Sinemurian sandstone plotted on ternary diagrams utilizing Polycrystalline quartz-Volcanic lithics-Sedimentary lithics (QpLvLs) (After Dickinson and Suczek, 1979). SANDSTONE P E T R O G R A P H Y Figure 4.20. QpLvLs plot of partial modes for Lower Pliensbachian sandstone samples. SANDSTONE P E T R O G R A P H Y Figure 4.21. QpLvLs plot of partial modes for Upper Pliensbachian sandstone samples. SANDSTONE P E T R O G R A P H Y 150 Figure 4.22. QpLvLs plot of mean partial modes for Inklin Formation sandstone suites. Arrows indicate mean temporal trends and point in a younging direction. SANDSTONE P E T R O G R A P H Y 151 4.9 DISCUSSION AND CONCLUSIONS Qualitative and quantitative petrographic data provide ample evidence for the definition of distinct temporal petrofacies within the Inklin Formation at Atlin Lake. Representative petrofacies depict strong provenance shifts in the Inklin succession and delineate relatively discrete depositional phases that are related to episodic magmatic and tectonic processes within the arc. Ternary plots of the Sinemurian suite confirm what qualitative petrographic evidence illustrates - a relatively advanced degree of volcanoplutonic mixing in most samples. The moderate variability of P/F and V / L ratios are entirely consistent with a transitional-arc provenance and reflect the progressive unroofing of the Late Triassic arc initiated in pre-Sinemurian time. The consistently intermediate values of detrital modes and associated ratios demonstrates that the Sinemurian arc's relative state of dissection falls somewhere between the largely undissected Early Pliensbachian arc and the deeply dissected Late Pliensbachian arc. Qualitative petrographic evidence indicates granitic plutons emplaced at moderate to shallow depths in the arc's crust were an important source of detrital modes. Provenance linkages to specific source plutons were not undertaken; however, a string of presently small Late Triassic granitic and intermediate plutons are interpreted to be the probable detrital sources for Late Triassic (ca. 215 - 208 Ma) Laberge Group clasts found in conglomerates of Hettangian (?) to Pliensbachian age in the southern Yukon (Hart et al, in press). Plutons of similar age are the inferred source for Sinemurian plutonic modes. Erosion of older Late Triassic (?) volcanics plus possibly coeval volcaniclastics are important primary to subordinate sources of Sinemurian elastics. Lower Pliensbachian petrofacies are characterized by their volcanigenic origin. Both ternary plots and qualitative data demonstrate the abundance of volcaniclastic detritus and relative paucity of plutonic material. Both P/F and V / L ratios consistently approach unity in the Early Pliensbachian suite - a feature considered diagnostic of volcanic provenance (Dickinson, 1970; Dickinson, 1983). In addition to P/F and V / L ratios, more qualitative features such as relative abundance of volcanic quartz and broken/whole feldspar euhedra observed in Lower Pliensbachian sandstones fulfill the major criteria considered diagnostic of undissected-arc provenance (Dickinson, 1970; Dickinson & Suczek, 1979; Dickinson, 1982; Pettijohn et al, 1987). SANDSTONE P E T R O G R A P H Y 152 The most important products of island arc volcanism are dacitic to rhyolitic pyroclastics (Carozzi, 1993) and are the inferred source for much of the Lower Pliensbachian basin-fill. The volcaniclastics are interpreted to be broadly coeval in large part, as indicated by the highly tuffaceous and epiclastic nature of many of these units, as well as their relative absence in Sinemurian sediments. Most of the highly tuffaceous units display abnormally low initial matrix contents for turbiditic sandstones and, although a sedimentary context is clear in most, often show minimal reworking and admixture from non-volcanic sources. Because reworking and redistribution of pyroclastic material is common in active arcs (Pettijohn et al, 1987), it is often difficult to discriminate between true water-lain tuffs and tuffaceous sandstones petrographically as the one grades insensibly into the other. A minority are true primary water-lain tuffs but many of the highly tuffaceous units have a similar genesis from pyroclastic flow and fall-out processes with short 'parking' periods on adjacent shelves before resedimentation. These latter deposits may be characterized as resedimented pyroclastic debris. Similar age and composition allow correlation of Early Pliensbachian basin-fill to equivalent pyroclastic and epiclastic deposits in the Yukon known as the Nordenskiold dacite (Cairnes, 1910). However, the volume of tuffaceous Lower Pliensbachian sediments indicates that erosional stripping and resedimentation has led to an underestimation of the regional extent of the Nordenskiold dacite and co-genetic deposits at Atlin Lake. Over a kilometre-thick section of broadly coeval dacitic pyroclastic detritus was deposited during Early Pliensbachian time, requiring a thickness and extent of volcanic cover that could only be found on an active undissected arc segment. Upper Pliensbachian petrographic data confirm a deeply dissected-arc provenance for these sediments. Both ternary plots and qualitative evidence demonstrate the abundance of plutonic quartz and K-spar in most of the suite. The high variability displayed by P/F and V7L ratios is fully consistent with definitions of dissected-arc provenance (Dickinson, 1970; Dickinson & Suczek, 1979; Dickinson, 1982; Pettijohn et al, 1987). The presence of rare high-rank metamorphic clasts in these quartzofeldspathic sands is also indicative of the advanced degree of pluton exhumation by this time. Qualitative evidence indicates moderate to shallowly emplaced granitic plutons are the primary source of elastics. However, in addition to the Late Triassic plutonic suite, younger plutons of Pliensbachian age SANDSTONE P E T R O G R A P H Y 153 are significant contributors to Late Pliensbachian basin-fill (Ch. 3). A zircon date from an Upper Pliensbachian conglomerate clast places an upper age constraint of - 186 M a on these younger plutons. This orthoclase-megacrystic biotite-hornblende monzogranite displays textures indicative of moderate to shallow emplacement commonly recognized in Upper Pliensbachian plutonic lithic and mineral grains (Fig. 4.1 OB). Perhaps most noteworthy is the common presence of fine intergranular polycrystalline quartz mosaics. Specific provenance linkages are not unequivocal but are provisionally made with reasonable confidence for these younger plutons. A number of thin-sections and handsamples of the Little River Batholith, which forms part of the Long Lake Suite in the Yukon, were analyzed and reveal strong similarities with respect to composition and macro/microtextures (e.g., orthoclase-megacrystic/granophyric textures/common intergranular polycrystalline quartz aggregates). The age of the Little River Batholith is the same (C. Hart, unpub. data) as the dated clast from this study. Consistent with ternary plots (QmPK) showing a strike-slip continental arc provenance, these younger plutons were likely offset by strike-slip displacements along the Tally-Ho Shear Zone during Early Jurassic time (C. Hart, pers. comm. 1994). Tuffaceous and pyroclastic units are found in the Upper Pliensbachian section and are derived, at least in part, from broadly coeval volcanic eruptions rather than solely from erosion of Lower Pliensbachian volcaniclastics as indicated by a zircon date from a dacitic lapilli tuff unit which yielded an age of 186 (± 1) M a (Ch. 2, Fig. 2.5). However such units are minor in volume in the Upper Pliensbachian section, indicating the waning influence of volcanism on sedimentation. The presence of coeval volcaniclastic units can be reconciled with a dissected-arc provenance as arc volcanism commonly continues in mature magmatic arcs even as dissection is exposing older plutonic roots (Dickinson & Suczek, 1979). P A L E O C U R R E N T A N A L Y S I S 154 C H A P T E R 5 P A L E O C U R R E N T ANALYSIS 5.1 INTRODUCTION The collection and interpretation of paleocurrent data is an important component of basin analysis and fundamental to both provenance studies and sedimentation history. In this study, paleocurrent data was collected and analyzed for the main purpose of provenance determinations. Subsidiary goals include attempts to characterize regional paleoflow patterns through time and space to provide insight into the evolution of regional sedimentation patterns and submarine fan morphology. Paleocurrent data can provide insight into local or regional paleoslope direction, sediment dispersal patterns, and location of source areas. Even the geometry and trend of lithologic units and macroforms (e.g., channel, lobe, individual fans) of submarine fan complexes can be elucidated by paleocurrent analysis (Miall, 1984), however this type of definition becomes increasingly difficult where the stratigraphy is structurally dismembered and/or outcrop exposures are limited. Good biostratigraphic control provides an effective means to overcome the difficulties associated with structural complications. In this study, biostratigraphically well-constrained strata permit collection of paleoflow indicators of known age range and establish the temporal framework necessary for detailed analysis of paleocurrent data, allowing the sub-division of data into stratigraphic sub-sets. This type of age control provides additional benefits beyond the obvious ones to a study of this nature, and has enabled the delineation of some coeval macroform elements, including individual fan systems. A number of factors affect the consistency, variance and 'modality' of paleocurrent data including depositional environment, bedform hierarchy, and sampling scale (Miall, 1984). The larger the sampling area or the thicker the section sampled, the greater the number of depositional events contributing to the data set and, consequently, the greater the directional variance. This generalization reflects the increase in variance resulting from the significant time dimension contained in grouped paleocurrent data from ancient P A L E O C U R R E N T A N A L Y S I S 155 sedimentary environments, and can be systemized by the concept of bedform hierarchy (Miall, 1984). Flow fields and their associated sedimentary structures can be ranked in orders of magnitude into three fundamental groups classified by Mial l (1984) as microforms, mesoforms, and macroforms that are dependant upon time scale and physical scale (Miall, 1984). The small-scale structures measured in most studies, including this one, are found in microforms where structures are controlled by turbulent eddies in the fluid boundary layer, and mesoforms where structures are produced by larger scale processes such as sediment gravity flows. Sedimentary structures produced by these processes represent time scale variations in the order of hours to seconds. These structures generated in geologically instantaneous time periods are used to define macroforms (e.g., channels, lobes, individual fans) representing long-term accumulation of sediments in response to major tectonic and geomorphic controls. The influence of depositional environment is another major factor affecting directional variance and for this reason the importance of interpreting paleocurrent data in the context of its lithofacies association is often stressed in paleocurrent analysis (Potter and Pettijohn, 1977: Mial l , 1984). In the case of turbidite systems directional variance is enhanced by the physical dynamics of turbidity currents and their relation to lithofacies. Phenomena genetically related to turbidity current evolution commonly result in high variability in paleoflow indicators from discrete flow events. A n example of this effect is found in the causes of paleocurrent variability in middle fan facies. Two main factors are responsible; the first is the result of channel morphology which may often manifest irregular, meandering or even braided trends in submarine fans that can be oriented perpendicular to regional fan slope; the second cause comes from overflow deposits in the interchannel areas which wil l have paleocurrents directed away from adjacent channels. Thus, both these lithofacies will contain paleocurrent indicators with significantly different paleoflow vectors although they may have been generated by the same turbidity current. A rigorous correlation of individual data sets with specific genetic lithofacies has not been undertaken in this analysis, however the dominant lithofacies and interpreted environments from which data of stratigraphic sub-sets were taken is noted in following sections (Figs. 5.1, 5.2). P A L E O C U R R E N T A N A L Y S I S 156 FEEDER C H A N N E L DEBRIS DISORGANIZED-BED FLOWS CONGLOMERATES SLUMPS S L O P E INTO B A S I N CONGLOMERATES: INVERSE-TO-NORMALLY GRADED GRADED-BED GRADED-STRATIFIED THIN BEDDED TURBIDITES ON LEVEE PEBBLY SSTS. MASSIVE SSTS. BASIN P L A I N THIN BEDDED NO R E L A T I V E S C A L E IMPLIED Figure 5.1. Generalized submarine fan model proposed by Walker (1978) showing morphological elements and depositional settings of turbidite fans. Lithofacies are shown in their inferred position on the fan. P A L E O C U R R E N T A N A L Y S I S 157 C L A S S G R O U P F A C I E S A G R A V E L S , A1 D I S O R G A N I Z E D M U D D Y G R A V E L S , G R A V E L L Y M U D S A 2 O R G A N I Z E D & P E B B L Y S A N D S I E3P pa i 2 £ B S A N D S B1 D I S O R G A N I Z E D B2 O R G A N I Z E D S A N D - M U D C1 D I S O R G A N I Z E D C C O U P L E T S & M U D D Y S A N D S C2 O R G A N I Z E D D S I L T S , D1 D I S O R G A N I Z E D S I L T Y M U D S & S I L T - M U D D 2 O R G A N I Z E D C O U P L E T S E 1 D I S O R G A N I Z E D E M U D S & C L A Y S E2 O R G A N I Z E D F 1 E X O T I C C L A S T S C H A O T I C D E P O S I T S C O N T O R T E D & F2 D ISTURBED S T R A T A BIOGENIC O O Z E S & A R L S G G 1 B I O G E N I C O O Z E S , H E M I P E L A G I T E S Q 2 HEMIPE L A GIT E S TVS e?l as & C H E M O G E N I C D E P O S I T S G 3 C H E M O G E N I C D E P O S I T S Figure 5.2. Facies classification scheme for deep-water sediments (From Pickering et. al., 1986). P A L E O C U R R E N T A N A L Y S I S 158 5.2 M E T H O D S Paleocurrent measurements were collected throughout the study area from strata which span the stratigraphic range of the Inklin Formation in the At l in Lake area. Several hundred measurements are usually considered the minimum necessary for a thorough analysis of a basin (Miall , 1984) and over 650 measurements were taken in this study, most of which are from strata that are biostratigrapbically well-constrained. Small-scale sedimentary structures are generated as a consequence of waning flow in turbidity currents and form part of the Bouma sequence (Fig. 5.3). Partial Bouma sequences are a common feature in submarine-fan settings and provided the main focus for data collection. The vast majority (> 600) were taken from unidirectional indicators and include ripple CTOss:laminations, trough cross-bedding, climbing ripples, and flute casts. The partial Bouma sequences T c e and T b c e are commonly associated with various sub-facies of mid-fan, lower-fan, fan-fringe, and basin plain affinity and often display well-developed fine ripple cross-laminated foresets in the T c division (Fig. 5.3). This is the primary source of the paleocurrent data. Bi-directional paleocurrent indicators were taken from groove casts. Grain Size Bouma (1962) Divisions Interpretation • D 3 E Pelite Pelagic sedimentation or fine grained, low density turbidity current deposition D Upper parallel laminae ? —Sand-I to C Ripples, wavy or convoluted laminae Lower part of Lower Flow Regime B Plane parallel laminae Upper Flow Regime Plane Bed • • . * . - - * . - • X c c cr (to granule at base) A Massive, graded ? Upper Flow Regime Rapid deposition Figure 5.3. Ideal sequence of sedimentary structures (Bouma sequence) in a turbidite bed (from Boggs, 1987). P A L E O C U R R E N T A N A L Y S I S 159 Figure 5.4.. A : Thin pinch and swell T c e bed displaying ripple cross-lamination in sequence of Sinemurian graded, stratified muddy siltstone. The pervasive bioturbation is a characteristic feature of fine-grained Sinemurian sediments. B: Thin Tbce bed with well-developed ripple cross-lamination in Upper Pliensbachian muddy sequence dominated by silt-mud laminae. P A L E O C U R R E N T A N A L Y S I S 160 The strike and dip of bed foresets were measured from three-dimensional exposures with a Brunton compass. Corrections for bed-tilt, and azimuth and vector-mean determinations for each site were performed by computer using templates written for this purpose for use in Excel and Lotus 1-2-3 spreadsheet programs (Mustard, 1991). Calculation of vector magnitude, standard deviation, variance, and Rayleigh significance were also performed for each data set and are included in Appendix 4. Corrections for fold-axis tilts were not required due to the low plunge angles (< 10° ) of fold axes in the study area. Paleocurrent data has been sub-divided into three stratigraphic sub-sets corresponding to Sinemurian, Lower Pliensbachian, and Upper Pliensbachian. Paleocurrent data was obtained from 36 separate localities across the entire study area. The majority of the localities occur in strata that is biostratigraphically well-constrained. Only 8 localities of the total are of interpreted age, however, most of these correlations were made with a high degree of confidence. An effort was made to collect paleocurrent measurements in a systematic fashion consistent with statistically accepted procedures for paleocurrent data collection (see Pettijohn and Potter, 1977) from each outcrop. These procedures require a minimum of ~ 20 measurements from each locality with no more than 3 measurements per individual bed. In a number of cases strict adherence to these procedures was not possible due to limited exposures. The majority of these data-sets (see Appendix 4) display relatively low variance which allows their use in a statistically valid context (Graham, 1988). However, data-sets which fail to satisfy statistical criteria are still useful paleoflow indicators when they conform to general paleoflow trends, and have been incorporated into the regional data-set, despite their statistical 'weakness'. As with all individual paleocurrent data-sets in this study, these latter data-sets all (with only one exception) satisfy the Rayleigh significance test at the 95% confidence level. Although the majority of plotted current roses could be qualitatively characterized as displaying a dispersed unimodal paleoflow, a significant number display weak to moderate bimodality. In cases of bimodality or polymodality the use of vector statistics is inappropriate because it assumes a unimodal distribution. Methods have been proposed to deal with treatment of bimodal and polymodal distributions (Graham, 1988). For bimodal distributions the problem is easily resolved by partitioning the two modes and calculating separate vector means. This operation was performed in those cases with weak to moderate P A L E O C U R R E N T A N A L Y S I S 161 bimodality (i.e., > 15% of n) and results are displayed with appropriate current roses in Figures 1, 2, and 4. This refinement did not significantly affect associated primary vector means. The factors that affect directional variance such as the influence of depositional environment, sampling scale, and stratigraphic scale operate to a significant degree in this study. Stratigraphic subsets (e.g., Lower Pliensbachian) typically span hundreds of meters and distances of over 30 kilometers may separate paleocurrent measurements within the same subset. Lithofacies sampled within stratigraphic sub-sets may span basin-plain to mid-fan channel depositional environments. Given the inherent variability of deep-sea fans in general and of small-scale bedforms (which are the predominant source of paleocurrent data in this study) in particular, the moderately high variance values found within some sites are not considered to be statistically meaningful in the context of sub-stage regional sedimentation. Add to this the presence of multiple coeval fan systems and the significance of statistical measures such as variance begin to lose any real value in evaluating the data. It becomes necessary to attempt the grouping of data within the stratigraphic subsets to achieve an internal coherency that makes geological sense. For these types of reasons little weight has been given to certain statistical measures such as variance and standard deviation in analysis of the data. 5.3 P A L E O C U R R E N T D A T A 5.3.1 Sinemurian The majority of the Sinemurian paleocurrent data was collected from thin-bedded muddy turbidites of facies class D (Figs. 5.4A, 5.6B). This facies class (Pickering et al., 1986) contains sediments that are dominantly silt and clay grade and is similar to Facies DI and D2 of Mutti and Ricci Lucchi (1972). Sediments in this class include those transported and deposited from the bulk volume of low-density turbidity currents and tail-end of high-density turbidity currents in particular (Pickering et al., 1986). In general, Sinemurian strata are dominated by this lithofacies type and are interpreted as representing mainly lower fan fringe and basin plain depositional environments, although in some cases interchannel overbank deposits are indicated by sedimentological evidence. P A L E O C U R R E N T A N A L Y S I S 162 The grand vector mean for Sinemurian paleocurrent indicators is 26° N E (n=142) and indicates a source to the southwest. This grand mean can be subdivided into two semi-discrete components; an axial component (vector mean=3°, n=100), representing the dominant longitudinal northerly paleoflow (i.e., 'distal' fan) and an easterly paleoflow component (vector mean=75°, n=42) that may represent initial flow direction in the sediment dispersal system (i.e.,1 medial' fan) (fig. 5.5). No rigorous criteria (e.g., facies association) are applied to the subdivision of axial from normal paleoflow vectors. In some cases there is sedimentological evidence of more medial fan settings to support the subdivision of normal from axial paleoflow (e.g., GGAJ-92-177); however, in other cases it is the relative physical orientation of individual vector means that provide the basis for the distinction. The differences may possibly be the products of differential structural rotations so certain assumptions are necessary to justify the subdivisions. For the purposes of this analysis, the author has assumed that the effects and degree of any structural rotations on paleocurrent data are generally evenly distributed across the study area and hence, orientation of paleoflow vectors are paleogeographic artifacts and not artifacts of differential structural rotations. Mean paleoflow vectors from individual sites exhibit a predominance of flow along or sub-parallel to the basin's axial trend (Fig. 5.5). Sites that do not conform to the strong axial flow patterns have vector means that are highly oblique to the basin margin normal. There are some exceptions to the dominant pattern which may represent more medial lithofacies (e.g., interchannel environments) that would be expected to display vector means close to the basin margin normal. The dominant paleoflow is in a northerly direction and is largely unimodal with a few minor exceptions, indicating a regional paleoslope down to the north. The minor bimodality evident at two sites roughly parallels the axial flow pattern. The southerly directed current rose petals, as well as a single current rose, are interpreted as belonging to the same depositional system as the predominant flow trend and most probably represent the influence of local topography and channel switching in a longitudinal fan system rather than generation from a northeast-derived system. Figure 5.5. Map showing Sinemurian paleocurrent patterns. P A L E O C U R R E N T A N A L Y S I S 164 5.3.2 Lower Pliensbachian Lower Pliensbachian paleocurrent measurements were collected from a wide variety of lithofacies. Paleocurrent data derived from the southwest occur primarily in thin-bedded muddy turbidites of lower fan fringe affinity as well as in thick muddy (interchannel) overbank deposits (Figs. 5.4B, 5.6B)(i.e., facies class C2.3, D2.1/2/3: thin-bedded graded sand-mud/silt-mud couplets and thin to thick silt-mud laminae -Bouma T c . e , T D ce divisions)(Pickering et al., 1986). An important subordinate component was collected from sandy mid-fan fining-upward sequences with well-developed current structures generated by high-concentration turbidity currents and traction bed-load (e.g., trough cross-bedding, climbing ripples) that are interpreted as mainly channel deposits. Paleocurrent measurements derived from the northeast occur almost entirely in thick-bedded sandy facies of mid-fan channel and lobe affinity (Fig. 5.6A). The majority are taken from the thinner-bedded upper sections of fining-upward channel sequences and topmost rippled (i.e., Bouma T c ) divisions within thick-bedded units of mid-fan lobe affinity but data include a minor component from finer interchannel and fan-fringe settings (Fig. 5.6B). The Lower Pliensbachian stratigraphic subset is composed of a total of 268 paleocurrent measurements, which is subdivided into two separate systems; the first is derived from the southwest and displays strongly developed radial paleoflow with a vector mean of 66° (n=154); the second is apparently a northeast-derived system with similar radial paleoflow and a vector mean of 220° (n=l 14) (Fig. 5.7). Early Pliensbachian time marks significant change in regional sedimentation patterns with the development of a strong paleoflow component from the northeast. The regional paleocurrent patterns also differ markedly from the Sinemurian with a strong shift away from dominantly axial paleoflow. The paleocurrent patterns from both sides of the basin display a general paleoflow that is largely normal to moderately oblique to the basin axis. This represents a dramatic shift in the orientation and direction of sediment dispersal systems in the basin. In addition to this major change in regional paleoflow it is also observed that bimodality within individual sites is absent or only weakly developed. P A L E O C U R R E N T A N A L Y S I S 165 Figure 5.6. A: Mega-ripple foresets in Lower Pliensbachian graded composite sand bed with southwest paleoflow (northeast study area). Truncated mega-ripple foresets are overlain by thin Tbce division with no obvious grain-size break, suggesting a single flow event rather than amalgamation. Pulsed turbidity currents or reflected turbidity currents which generate multiple wave-fronts from single mass flow events may produce this sequence. B: Typical facies class D sediments include fine sand/silt-mud couplets, and thick to thin silt-mud laminae. This example is from a Lower Pliensbachian sequence. P A L E O C U R R E N T A N A L Y S I S 166 5.3 .3 Upper Pliensbachian The source of Upper Pliensbachian paleocurrent data is similar to Lower Pliensbachian data with respect to the diversity of lithofacies from which it was gathered. Approximately half of the measurements are from muddy facies (Figs. 5.4B, 5.6B), predominantly interchannel environments but also lower fan fringe settings (i.e., Facies Class D: Bouma T c e , T ^ c e divisions)(Pickering et al., 1986), with the remainder collected from sandier mid-fan channel and lobe sequences. Paleocurrent control is relatively poor in the northeastern half of the basin due largely to the paucity of measureable paleoflow indicators in known Late Pliensbachian strata, which are dominated by massive thick-bedded sands of mid-fan affinity (mainly partial Bouma sequences T a e & T a ( , e ) . The Upper Pliensbachian stratigraphic sub-set consists of 199 measurements confined to one ammonite biochronozone (the Kunae Zone)(Fig. 5.8). The majority (n=133) represent southwest-derived transverse fan systems with well developed radial paleoflow which have a grand vector mean of 49°. A subordinate component with a strong southeasterly axial paleoflow (vector mean=151°: n=43) has been subdivided from the main body of data. Southwesterly paleoflow indicators were measured at one locality on Copper Island (Stn. GGAJ-92-126: vector mean=288°: n=23) but cannot be considered unequivocal evidence of a northeast source given the absence of other similar Upper Pliensbachian paleoflow indicators. However, in light of Lower Pliensbachian paleocurrent patterns, an interpretation of apparent northeast derivation seems both reasonable and warranted. Upper Pliensbachian paleocurrent patterns display a marked change from those of the Lower Pliensbachian, indicating another significant temporal shift in sediment dispersal patterns. Absent is the strong southwesterly paleoflow component common during Early Pliensbachian time. Only the Copper Island site contained apparent northeast-derived paleoflow indicators, suggesting that, although transport systems from this source appear to still be operating, they were not important sediment sources by Late Pliensbachian time. The dominant paleocurrent trend is strongly axial normal and of southwest provenance. A minor but significant component (> 20%) of the Upper Pliensbachian paleocurrent data are parallel to the basin axial trend. The direction of this axial paleoflow is dominantly to the southeast which is opposite to the axial paleoflow which dominated the Sinemurian. P A L E O C U R R E N T A N A L Y S I S 167 Figure 5.7. Map showing Early Pliensbachian paleocurrent patterns. P A L E O C U R R E N T A N A L Y S I S 168 Figure 5.8. Map showing Late Pliensbachian paleocurrent patterns. P A L E O C U R R E N T A N A L Y S I S 169 5.4 DISCUSSION 5.4.1 Sinemurian: Sinemurian paleocurrent patterns show no evidence of paleoflow from the northeast and appear to represent a sediment dispersal system derived from the southwest. Evidence from sandstone and conglomerate petrofacies also supports the interpretation of a paleoflow derived entirely from the southwest (Ch. 3, Ch. 4). Because exposures of Sinemurian strata are generally confined to an axial 'belt' through the west-central region of the study area and are essentially absent from the northeast 'half of the basin, the possibility that these patterns may be an artifact of data collection must be considered. The dominant facies association (i.e., distal fan) of most Sinemurian outcrops must also be considered as both these factors provide the basis for different interpretations. No proximal Sinemurian facies were identified and the overwhelming majority of paleocurrent measurements were taken from large muddy turbidites exposures of distal fan affinity (i.e., lower fan fringe and basin plain settings). One might argue that, in the presence of regional contour currents, deflection of low concentration turbidity currents could produce the axial flow patterns seen regardless of fan geometry or their initial paleoflow (i.e., southwest vs. northwest). This possibility has been considered and eliminated for two main reasons. The first reason is lithological in nature. The strata from which the data were collected display an abundance of sedimentary features commonly associated with turbidity currents (e.g., thin rhythmic bedding, normal and reverse grading, load structures). The partial Bouma sequences T c . e and T j j c e are characteristic of Sinemurian strata and are contained in lithofacies sequences of obvious turbidity current origin (i.e., facies class C2.3, D2.1/2/3: thin-bedded graded sand-mud/silt-mud couplets and thin silt-mud laminae)(Pickering et al., 1986). No contourites were identified in these sequences. The second reason relates to sedimentation dynamics in arc-marginal basins. Unlike large ocean basins where regional contour currents are often an important influence on basin/slope sedimentation, contour currents are generally not well-developed in relatively narrow arc-marginal basins and are unlikely to be a significant factor in sediment dispersal in such settings (B. Ricketts - pers. comm., 1993). Sinemurian paleoflow vectors display a pattern that is dominantly directed parallel or sub-parallel to the basin axial trend. This type of paleocurrent pattern is characteristic of longitudinal rather than P A L E O C U R R E N T A N A L Y S I S 170 radial/transverse fan geometry. Longitudinal fan geometry is common in arc-marginal turbidite fan settings (Dickinson and Seely, 1979). In many arc-related basins, where deep-marine sedimentation takes place in troughs oriented parallel to tectonic strike, sediment gravity flows, particularly low-viscosity turbidity currents like those responsible for most Sinemurian sequences sampled, often emerge from submarine fans and turn 90° to flow longitudinally down the regional paleoslope (Miall, 1984). There are many examples of this type of pattern in the literature (q.v., Ricci-Lucchi, 1975). The minor bimodality apparent within the axial flow trend component is consistent with longitudinal fan systems where local influences (especially topography) can cause ephemeral reversals of the dominant controls of directional progradation such as paleoslope or regional gyres. Thus, the geometry and orientation of Sinemurian vector means is interpreted as representing a longitudinal fan system(s) where regional paleoslope is the dominant control on the direction of sediment dispersal in the basin. Compared with the Pliensbachian, there is a relative lack of basin-margin normal paleoflow vectors in the Sinemurian subset which may be in large part an artifact of facies association. Vectors with this type of orientation would be expected in proximal and medial fan regions (i.e., before the 90° turn of turbidity currents downfan). The general paucity of proximal and medial fan deposits in the Sinemurian section (and hence their relative non-representation in the subset) could account for the strong bias towards axial flow components. The absence of such vectors does not weaken the interpretation of a dominantly longitudinal sediment transport system during Sinemurian time. On a more general scale, it has been noted that paleocurrent data tend to form a set pattern when seen over appreciable areas (Potter and Pettijohn, 1977). Regionally, the overall Sinemurian paleocurrent pattern may be characterized as generally parallel with some divergence and is consistent with longitudinal fan systems. 5.4.2 Lower Pliensbachian: The strong axial-normal paleocurrent patterns evident in Lower Pliensbachian strata are more consistent with a radial or transverse fan geometry than a longitudinal one and are interpreted to represent the basinward progradation of bidirectional radial or transverse fan systems. Both the development of opposing paleoflows as well as the shift from an axial flow trend underline the fundamental change in sedimentation dynamics occurring in the Early Pliensbachian. The P A L E O C U R R E N T A N A L Y S I S 171 relatively abrupt establishment of this different sedimentation regime argues for a geologically dynamic mechanism. Petrographic evidence has shown the Early Pliensbachian to be a period of rejuvenated volcanism which can account for the increased sediment load during this time and progradation of fans from the southwest (Ch. 4); however, this mechanism in itself is inadequate to explain the development of a northeast-derived paleoflow. The bidirectional progradation of submarine fans during the Early Pliensbachian appears episodic in nature. The apparent absence of a northeast source during Sinemurian time supports this interpretation. The existence of a northeast sediment source in what is widely interpreted to be a forearc basin (Templeman-Kluit 1979; Hansen, 1988; Dickie, 1989) poses potential difficulties with existing interpretations that need to be addressed. A number of explanations could potentially account for this significant shift in paleocurrent patterns. The first set of explanations deals with systems having true opposed bidirectional paleoflow. Two possibilities exist and each is a function of basin genesis and type. The most common case of opposing paleoflows in an arc-marginal basin occurs in backarc basins and needs no further explanation. It should be added that there is no evidence from this study other than the paleocurrent patterns to support a backarc basin setting. The second scenario is somewhat atypical and requires subaerial exposure of an outer forearc ridge. A specific forearc basin type is implicit in the latter case and necessitates classification of the basin as either a narrow or broad ridged forearc basin (see Fig. 7.5)(Dickinson and Seely, 1979). Independent lines of study (i.e., petrographic) fail to provide convincing evidence favoring either scenario. The lack of a significant component of northeast-derived paleoflow in the Sinemurian and Late Pliensbachian could be an artifact of data collection, however, in itself this temporal trend is suggestive. From the paleocurrent data the strong shift to opposed paleoflows during Early Pliensbachian time appears to define a relatively discrete phase in regional sedimentation. If the apparent temporal trends in paleocurrent patterns are real and not collection artifacts, then a reasonable interpretation is one involving episodic uplift of an outer forearc ridge. This latter scenario can be reconciled with the petrographic data which indicates no significant quantitative mineralogical or qualitative differences between north-east and south-west derived sediments. P A L E O C U R R E N T A N A L Y S I S 172 During this extended period of voluminous volcaniclastic sedimentation, it is possible that a portion of these sediments accumulated in considerable thickness along outer rise/ridge margins as sediment drapes or ponds. It is also possible that sediment bypass through discrete delivery points along the outer rise allowed dispersal and incorporation of volcaniclastics into trench slope/trench deposits and hence into the subduction complex itself (Dickinson, 1982). A number of tectonic processes are capable of generating episodic uplift along the outer forearc rise/ridge (e.g., subduction of seamounts, changes in relative plate trajectories or subduction rates) and could have produced sufficient uplift to cause cannibalization of these deposits. The composition of sands delivered in this fashion to outer rise regions and subduction complexes should be indistinguishable from coeval sands deposited in the adjacent forearc basin (Dickinson, 1982). Modern island arc systems with forearc basins bounded by subaerial outer forearc ridges have been documented but are atypical. Both the Banda arc (Indonesia) and Lesser Antilles arc (Barbados/Caribbean) systems have morphotectonic features potentially analogous to the Stikinian arc segment of this study (q.v., Bowin et al., 1980; Torrini et al., 1985; Speed et al., 1989). Episodic uplift of the Barbados accretionary prism produced a volumetrically important sediment source during a later phase of the Tobago Trough's depositional history (Speed et al., 1989). The accretionary prism contains turbiditic forearc-basin strata that has been tectonically shouldered by the arcward migration of the prism crest (Torrini et al., 1985), producing an 'outboard' clastic source with similar composition to the original 'inboard' source. The Banda arc system is even more unusual in that it possesses both an inner arc and an outer arc of deformed sedimentary and metamorphic rock separated by a deep trough (Weber Trough) that receives sediments from both sides (Bowin et al., 1980). The outer arc is a submarine ridge that locally rises above sea-level to form islands, some of which are unusually large (e.g., Timor, Seram). Although young volcanoes are generally absent in the outer Banda arc, they are found in a few locations (Bowin et al., 1980) and demonstrate the potential for complex magmatic and tectonic developments in arc systems. Although purely speculative, either of the above scenarios could serve as potential analogs for the arc segment of this study. There is an alternate explanation for Lower Pliensbachian paleocurrent patterns for which it is not necessary to invoke a northeast source. The phenomena of deflected and reflected turbidity currents in P A L E O C U R R E N T A N A L Y S I S 173 ancient and modern deep-sea environments has been amply proven in recent years (q.v., Hiscott and Pickering, 1984; Pickering and Hiscott, 1985; Pantin and Leeder, 1987; Marjanac, 1990; Kneller et al., 1991; Haughton, 1994) and provides a mechanism capable of producing reversed paleoflow in narrow elongate basins. The most fundamental requirement for this process is morphological in nature and simply requires a relatively narrow elongate basin with a confined basin floor. The Whitehorse Trough appears to fulfill this basic requirement, at least at Atlin Lake where present basin width is less than 30 kilometers. Even the most generous palinspastic reconstruction would produce a paleo-basin of less than 100 kilometers wide in this region. Given the strong transverse paleoflow from the southwest and the predominance of deposits from high-concentration sediment gravity flows that characterize the Lower Pliensbachian succession, it is plausible or even likely that many of these flows traversed the entire basin width. In the presence of an outer forearc rise or ridge with some substantial topographical relief, reflections of the flows are a natural consequence. This has been conclusively demonstrated both in laboratory studies (Pantin and Leeder, 1987; Kneller et al., 1991) and by careful analysis of paleoflow indicators within ancient sandy turbidite beds found in narrow elongate basins (Hiscott and Pickering, 1984; Pickering and Hiscott, 1985; Marjanac, 1990; Kneller et al., 1991; Haughton, 1994). Deflections and reflections are produced when the head of the turbidity current encounters topographical obstructions such as outer basin margins and propagate normal to the strike of the obstacle, even in cases of oblique initial flow directions (Kneller et al., 1991)(Fig. 5.9). These reflections generate reverse paleoflow back along the initial flow path and produce thin to thick turbidite beds with composite internal organization that may be complex to simple. Descriptions of sedimentological features of thick turbidites (Pickering and Hiscott, 1985; Marjanac, 1990; Haughton, 1994) are similar to those observed in parts of the Lower Pliensbachian section, especially in the northeast study area where most of the southwest paleoflow indicators were collected. Thick and very thick turbidite beds containing alternations of partial Bouma sequences with subtle grain-size breaks and/or erosive internal contacts ( e.g., T a , then Tjj C , then T a b) were occasionally encountered and attributed in the field to amalgamation. It was noted that these beds lacked any basal mud intraclasts within the internal divisions that were common in other amalgamated beds and throughout the section in general, perhaps indicating that other processes (i.e., reflected flows) were P A L E O C U R R E N T A N A L Y S I S 174 responsible for these composite beds. Thick composite turbidite beds containing a reflected component may lack good measureable paleoflow indicators within some or all the internal divisions thus rendering their recognition difficult. Such beds could easily be mistakenly attributed to more common processes like bed amalgamation or even 'vacillitory' (i.e., pulsed) turbidity currents (Larue et al., 1988). PLAN VIEW Figure 5.9. Sketch of flume experiment designed to simulate a natural density current meeting a shallow ramp (15°) obliquely at 45°. The progressive expansion of the head of the flow is shown for 4 steps, with the position of two solitary waves generated by collapse of denser fluid off the ramp shown for step four. Note the reversal of flow vectors. From Haughton (1994). Because these thick beds were generally poor candidates for paleocurrent indicators most were not-measured and when they were, only the upper rippled divisions were used; consequently, there are no data to support the existence of reflected turbidity currents although contrary indicators were occasionally noted at various localities where data collection proceeded upsection through a series of beds. However, given the narrow basin morphology in the context of a ridged forearc type, the predominance of large volume high-density flows and the similarity of some beds to published descriptions of reflected flows, it is possible that reflections of turbidity currents are partly or wholly responsible for apparent northeast-derived paleoflow. In this scenario the paleocurrent patterns become a local artifact without any regional provenance significance. P A L E O C U R R E N T A N A L Y S I S 175 On balance, the evidence favors a separate fan system derived from the north-east and is permissive of an outer forearc ridge uplift as a potential source of the elastics, although one cannot discount the potential role for reflected flows in generating the paleocurrent patterns. In either case an outer forearc rise or ridge of substantial relief is implicit. Possible genesis from the north-east margin of a back-arc basin cannot be unequivocably eliminated on the basis of the data in this study but is believed to be unlikely. Biostratigraphic resolution is generally inadequate (potentially spanning two ammonite biochronozones at many sites) across the study area to delineate major coeval macroform elements such as individual fan systems with any confidence beyond the basics of apparent northeast-derived versus southwest-derived systems. Multiple fan systems are implied by both the high sediment volume and areal coverage, and undoubtedly exist but cannot be recognized on the basis of paleocurrent data. 5.4.3 Upper Pliensbachian: The strong axial-normal orientation of Late Pliensbachian transport systems indicates radial or transverse fan geometry. Paleoflow generated from the southwest is largely or wholly responsible for paleocurrent indicators. Apparent northeast-derived paleoflow indicators are rare, suggesting that causal mechanisms operating in the Early Pliensbachian were a minor and waning influence on basin sedimentation by Late Pliensbachian time. The general paleocurrent pattern is consistent with dominantly transverse fan systems prograding to the northeast. The component of southeasterly-directed axial paleoflow is opposite to the axial paleoflow which dominated the Sinemurian and may be related to a southeasterly-prograding longitudinal fan system. Although it is possible that this is purely the result of local influences on sediment dispersal, it may indicate a fundamental change in regional paleoslope. Regional current directions can be reversed by changes in paleoslope caused by the tilting of basins (Miall, 1984). Although the data-base is sparse it is consistent with proposed models for the tectonic evolution of the Whitehorse trough that call for transpressive closure with southward propagation (Hansen, 1988; Dickie, 1989; Tempelman-Kluit, 1979). At least three, and possibly, four separate fan systems are indicated by the paleocurrent data. Submarine fans are dynamic sedimentary environments subject to considerable temporal variation with respect to the locus of deposition via channel avulsion, lobe switching, etc. Thus, distinction of discrete fan systems P A L E O C U R R E N T A N A L Y S I S 176 requires more than mere geographic separation in the scale of a few to a few tens of kilometers. In this case, paleontological data provides the basis for the interpretation. Biochronological constraints are very good in all four cases, with sub-zonal resolution. Three localities are involved: Sloko, Copper, and Teresa Islands, and at each one a sharp Early-Late Pliensbachian (Freboldi/Kunae Zones) boundary was identified. The majority of the data was collected in relatively close proximity to this boundary, ranging from a few tens (muddy facies) to about 100 meters (sandy facies). Given the high sedimentation rates of turbidite fan systems in general (vertical aggradation rates in modern fan systems range from 100-200 mm/1000 yrs to 11,000 mm/1000 yrs: Pickering et al., 1989) and the Upper Pliensbachian progradational fan system in particular, deposition of the strata from which the data was collected may be considered as coeval. In a measured section on Copper Island opposed paleoflow vectors were collected. Thus two separate transverse fan systems are indicated; a medial southwest-derived system prograding over distal northeast-derived fan deposits (lower fan-fringe or basin-plain). Alternately, i f turbidity current reflections are responsible for the lone northeast-derived paleoflow vector, then only one fan system is required. Approximately nine kilometers north on Teresa Island an axial paleoflow vector mean indicates a southeast-prograding longitudinal fan of the same age. Some ten kilometers south of Copper Island, Sloko Island paleoflow vectors display transverse fan geometry. However, highly oblique northerly paleoflow vectors in correlative strata ~ 3 km northeast of Sloko Island on Bastion Island suggest a possible downfan axial flow which is consistent with a longitudinal fan system (Fig. 5.10). This fan system can be considered distinct from the Copper Island fan system on the basis of faunal content. Both islands contain numerous fossiliferous horizons with abundant ammonites. Nearly half of the Late Pliensbachian fossil collections (n=9) from Copper Island contain the Boreal (circum-polar faunal province) ammonite Amaltheus stokesi which are absent from coeval Late Pliensbachian collections (n=14) from Sloko Island. Although collection failure provides a possible explanation for this discrepency, given the number of ammonite collections taken from coeval strata on both islands, these significant faunal differences imply at least some geographic separation and supports the interpretation of two separate fan systems. P A L E O C U R R E N T A N A L Y S I S 177 Figure 5.10. Kunae Zone (Late Pliensbachian) paleoflow vectors in the vicinity of Sloko Island. Based purely on the geometry of the vector means, three of the four represent transverse fans systems prograding across the basin's axial trend, while the fourth is apparently a longitudinal fan system prograding to the southeast. It is possible that this latter system may represent a highly divergent sector of a transverse fan although the spread of nodes about the vector mean indicates otherwise. In either case temporal and spatial relationships support its distinction from the adjacent Copper Island fan system. Within each separate fan 'domain' the general sediment dispersal pattern appears to be divergent (Potter and Pettijohn, 1977), although only Sloko Island contains enough separate broadly coeval paleocurrent sites to quantify this conclusion (Fig.5.7). It is worth noting that, with one minor exception, there is no bimodality, although a number of current roses display significant nodes divergent from the dominant flow. This is consistent with discrete unimodal sediment transport systems and to be expected in submarine fans. In a more regional context, Upper Pliensbachian paleocurrent data may be characterized as forming a generally convergent pattern. This can be attributed to coalescence of the separate fan systems. P A L E O C U R R E N T A N A L Y S I S 178 5.5 S U M M A R Y A N D C O N C L U S I O N S Early Jurassic paleocurrent indicators in the study area display atypical patterns for a forearc basin. Paleocurrent data indicates the development of a northeast clastic source during the Early Pliensbachian. From a basin morphological perspective, this indicates one of two alternatives: (1) the paleocurrent patterns reflect an increased sediment input from a pre-existing northeast basin margin, in which case a backarc basin is indicated; or (2) the patterns reflect development of a new northeast basin margin, in which case uplift of an outer forearc ridge is indicated. Given that no appreciable bidirectional paleoflow was found to occur in the Inklin succession outside the Lower Pliensbachian, only the second alternative can be supported on the basis of the paleocurrent data in this study and is the interpretation of the writer. By invoking an outer forearc ridge it becomes necessary to consider the potential for reflected turbidity currents to generate apparent northeast-derived paleoflow. The basin morphology envisaged by an interpretation of a relatively narrow ridged forearc basin permits the possibility, or even likelihood, that such processes may be a factor in regional paleoflow patterns and thus must be considered a testable alternative to the interpretation proposed here. Sediment dispersal during the Sinemurian is dominated by paleoflow along the axial trend of the basin, indicating predominantly longitudinal fan systems. By contrast, both Early and Late Pliensbachian sediment dispersal is dominated by paleoflow normal or highly oblique to the axial trend, indicating radial or transverse fan systems. This evolution of fan morphology is attributed to fundamental changes in sediment supply - from a relatively low-volume regime during most of the Sinemurian to a high-volume regime during the Pliensbachian. Pliensbachian paleocurrent data indicates multiple prograding radial/transverse fan systems were in operation in the study area by this time. Three or four separate coeval fan systems are interpreted for Late Pliensbachian time and similar numbers presumably existed in the Early Pliensbachian. A consequence of progradation was the coalescence of coeval fan elements. DIAGENESIS A N D T H E R M A L HISTORY C H A P T E R 6 D I A G E N E S I S A N D T H E R M A L H I S T O R Y 179 6.1 INTRODUCTION The purpose of this study is to attempt to characterize the diagenetic and thermal history of Inklin Formation sediments in the study area. A rigorous and comprehensive analysis of this nature would require a number of techniques such as S E M and electron microprobe analysis that were not employed in this study and are beyond the scope of this thesis. The data and observations contained herein should be considered as a 'reconnaissance' study that establish a framework upon which further study may be based. This study has two separate components which furnish quantitative and qualitative data upon which to base interpretations; (1) quantitative data obtained from thermal samples run for vitrinite reflectance, and (2) semi-qualitative petrographic observations of diagenetic mineral phases and fabrics from thin-section analysis. By itself, the small thermal sample suite does not provide a statistically sound basis upon which to base interpretations of thermal history. The usefulness of this data-set would be rather limited i f not for the high resolution biostratigraphy that provides a temporal framework for the analysis. Evidence to support interpretations based upon the thermal maturation study can be found in petrographic analysis of diagenetic artifacts in Inklin Formation sandstones. This aspect of the study, an outgrowth of petrographic analysis of sandstone detrital modes, focuses on identification of authigenic minerals that can be related to existing schemes of burial diagenesis and was not concerned with quantitative determinations of their volume percentages. Taken together, these two components provide a sound base upon which to construct a framework of diagenetic and thermal history. The chapter is divided into two sections and arranged so that supporting evidence provided by petrographic determinations and interpretations of diagenetic history follow presentation of quantitative thermal data and interpretations. DIAGENESIS A N D T H E R M A L HISTORY 180 P A R T 1: T H E R M A L H I S T O R Y 6.2 I N T R O D U C T I O N Vitrinite reflectance remains the most widely used, reliable and precise method of quantifying organic diagenesis and calculating the maximum paleotemperatures sediments have experienced during their burial history. However, a number of uncertainties exist when attempting to reconstruct the thermal history of ancient sedimentary sequences, most notably the the steepness of the paleogeothermal gradient. Such thermal reconstructions are beyond the scope of this study but the data-set does provide some fundamental criteria for assessing burial history. Determinations of modern geothermal gradients have demonstrated that temperature variations (with depth or with temperature at fixed depths) are more complicated than might be assumed, even over relatively small areas (Blatt, 1992). The conclusion to be drawn from modern studies is that paleotemperature values obtained from ancient sedimentary sequences are valid only for very restricted geographic areas at specific points in time (Blatt, 1992). Consequently, paleotemperature determinations from this study, based on restricted selective sampling across the stratigraphy, establish a basic framework of thermal history for Inklin Formation sediments at Atlin Lake, but cannot be considered representative of Lower Jurassic Whitehorse Trough strata in general without further study. Highly carbonaceous sandstone beds to thin (cm scale) coaly horizons are found scattered throughout the entire Inklin Formation stratigraphy at Atlin Lake. These horizons provided the targets for collection of thermal maturation data via analysis of their vitrinite content. Sampling was done from selected stratigraphic intervals throughout the study area and span the stratigraphic range of the Inklin Formation. The age of the samples can be resolved to at least a sub-stage stratigraphic level (e.g. Early versus Late Sinemurian). Samples from sedimentary sequences lacking biostratigraphic controls have ages assigned on the basis of stratigraphic position and lithological characteristics. 6.3 M E T H O D S The sample suite (n=16) was 'high-graded' in the lab and only those samples (n=8) with sufficient coaly material to be manually 'picked' were processed. Processed samples were run by Dr. Marc Bustin for vitrinite reflectance. A n average of 59 measurements of vitrinite particles were taken per sample. Statistical summaries and histograms of random reflectance data are listed in Appendix 5. The samples range from DIAGENESIS A N D T H E R M A L HISTORY 181 the Lower Sinemurian to the Upper Pliensbachian (Kunae Zone). Although the sample suite is quite limited in size it does provide reasonable estimates of the maximum paleotemperatures to which these sediments were subjected. Maximum diagenetic temperatures were calculated from vitrinite reflectance data using the the relationship: ln(Ro) = 0.00959(Tmax) -1.42 (Barker, 1988: from Blatt, 1992). No attempt is made to estimate duration of heating for Inklin sediments. Recent studies have questioned the importance of time (i.e., heating duration) in the thermal maturation of sedimentary organic matter (q.v., Barker, 1989; Price, 1983). Barker's (1989) comprehensive study concluded that O M (organic matter) stabilizes with respect to temperature after about 10 6 to 10 7 years in burial diagenesis and therefore T m a x alone is sufficient to characterize thermal maturation without considering functional heating duration. 6.4 T H E R M A L M A T U R A T I O N D A T A The majority of the samples (n=6) have a coal rank ranging from high volatile bituminous C (R m = 0.60)(Rm=mean %RQ) to low volatile bituminous (Rm=1.62). The remaining two samples have a coal rank of semi-anthracite (Rm=2.01 & 2.36). These two occur in Upper Pliensbachian and Upper Sinemurian/Lower Pliensbachian sequences respectively and are both anomalously high and out of sequence with the stratigraphic order of the other values. These samples (GGAJ-92-06-T/110-T) were collected from localities approximately 3 kilometres apart: the first (06-T) from the southern tip of Griffith Island and the second (110-T) from the northeast side of Bastion Island. On a small island between the two localities (approximately 2 km N N W of the Bastion Island site and 1 km SW of the Griffith Island site) a granitic body was found intruding Inklin sediments. The extent of the intrusive body could not be determined due to poor exposure; however, a highly visible rusty, baked, and locally silicified zone extends for over 60 metres in the flanking sediments on the west side of the island. This highly weathered, friable coarse-grained granitic rock likely belongs to the post-tectonic suite of Cretaceous and early Tertiary intrusive bodies mapped as the Whitehorse Trough intrusives by Bultman (1979). These intrusions range from quartz diorite to granite in composition and vary greatly in size and shape (Bultman, 1979). The anomalously high R,,, values of these two samples can be attributed to their proximity to the granitic body which produced a locally high geothermal gradient. Qualitative microscopic features also suggest intrusive DIAGENESIS A N D T H E R M A L HISTORY 182 thermal overprints for at least one of these two samples (R.M. Bustin - pers. comm. 1993). Consequently, these two Rn, values do not represent regional paleotemperatures related to burial history and are not considered in the following treatment. When the remaining Rn, values are arranged in stratigraphic order (Upper Pliensbachian to Lower Sinemurian) they display the progressive increase with increasing age to be expected from burial history (Fig.6.1). Only one sample (59-T) of Late Pliensbachian (Kunae Zone) age was analyzed, from which the lowest R m value of the sample suite was obtained (Rm=0.60), corresponding to a maximum paleotemperature of ~ 95° C. Two Early Pliensbachian samples were taken from a measured section at Janus Point approximately 200 metres apart (56-T & 184-T) and produced identical R m values (R m= 0.85) which corresponds to a maximum paleotemperature of ~ 131°C. The next sample in the sequence (119-T) was collected at southeastern Atlin Lake from a structurally complex package where it was found in structural contact with Sinemurian sediments and in close proximity to sediments of earliest Early Pliensbachian (Imlayi Zone) age. It is interpreted to be of Sinemurian age, most probably Late Sinemurian but possibly of earliest Pliensbachian age, and produced a R m value of 1.46, corresponding to a maximum paleotemperature of ~ 188° C. Sample 177-T is of latest Early Sinemurian age (Arnouldi to Varians Zone) and produced a R m value of 1.55 which corresponds to a maximum paleotemperature of ~ 194° C. The final sample in the sequence (45-T) was collected on northern Griffith Island downsection of an Early Sinemurian (Arnouldi Zone) sequence from which it was separated by a covered interval that probably concealed a fault. Strong similarities in lithology and attitude across the suspected fault suggest minor stratigraphic offset and support an interpretation of an Early Sinemurian age for this sample. The Rm value obtained from this sample was 1.62, corresponding to a maximum paleotemperature of - 198° C. In summary, the Rn, values range from a high of 1.62 in the Sinemurian section to a low of 0.60 in the Upper Pliensbachian section, indicating maximum paleotemperatures attained ~ 200° C in the basal Inklin Formation and were ~ 100° C in the upper parts of the section. These values show a positive correlation with increasing age (Fig. 6.1), a relationship consistent with a burial history curve, and indicate deep burial for lower sections of the Inklin Formation. Estimates of absolute burial depths from these values must be considered somewhat speculative without knowledge of paleogeothermal gradients and DIAGENESIS A N D T H E R M A L HISTORY 183 duration of heating. Also, there is no question that thermal overprints from intrusive events are responsible for the local thermal maturation anomalies in this data set. To what degree the intrusive events may have elevated the regional geothermal gradient is impossible to say. However, despite these uncertainties, when compared to thermal studies in similar tectonic settings (Galloway, 1974; Helmold & van de Kamp, 1984;) and assuming high geothermal gradients of 40°C/km, maximum paleotemperature values from this study indicate burial depths in excess of 5 kilometres for Sinemurian sections. Supporting evidence for this conclusion can be found in the diagenetic fabrics observed in thin-sections which are discussed in the following section. Rm \s. Relative Age 2 1.8 59-T 56-T 18 4 -T 119-T 177-T 45 -T Late Ear ly Ear ly Late Ear ly Ea r ly? H iens . Piiens. Pliens. S in . S in . S in . Relative Age Figure 6.1. Plot of Inklin Formation thermal samples: R,,, values versus increasing relative age. DIAGENESIS A N D T H E R M A L HISTORY 184 P A R T 2: D I A G E N E S I S 6.5 I N T R O D U C T I O N Inklin Formation sandstones have a complex diagenetic history that is in part a function of their provenance. Volcanogenic sandstones have an inherently high chemical reactivity due to the abundance of unstable volcanic lithic fragments and plagioclase feldspar as primary detrital modes. Consequently these sediments are generally characterized by high proportions of altered feldspars and lithic grains, and often display complex diagenetic fabrics reflecting the multiple diagenetic phases that accompany progressive burial. Constructing a detailed diagenetic history for these deeply buried volcaniclastic sediments is not the purpose of this study: the aim is to establish a general diagenetic framework from which supporting evidence may be drawn for interpretations of burial history. A total of 39 thin-sections was examined semi-qualitatively for their diagenetic features. The sample suite contains 6 samples of interpreted age but the remainder are biostratigraphically well-constrained with the majority resolved to zonal stratigraphic levels and the rest to at least sub-stage levels. These were divided into three stratigraphic sub-sets corresponding to Sinemurian (n=10), Lower Pliensbachian (n=14), and Upper Pliensbachian (n= 15). Age determinations for individual samples are listed in Table 6.1. A range of diagenetic features is visible in all samples and records the multiple diagenetic regimes to which these sediments have been subjected. In some cases late stage diagenetic overprints are so pervasive that they largely obliterate evidence of earlier diagenetic phases. Unraveling this complex diagenesis requires a demanding petrographic analysis not undertaken by the writer; however, identification of key diagenetic features can provide insight into burial history and permit reasonable estimates of burial depths. This study uses diagenetic artifacts as indicators of relative burial depths attained by the Inklin stratigraphic succession at Atlin Lake. For this purpose it is sufficient to document the maximum (i.e. latest) diagenetic mineral phase present without much consideration given to the relative degree and 'style' of development of preceding diagenetic stages. Diagenetic stages cannot be calibrated to absolute depths due to the number of variables that affect the diagenetic history of sediments; however, they can function as crude yardsticks of DIAGENESIS A N D T H E R M A L HISTORY 185 Table 6.1 Sample No. Age % C c Sinemurian GGAJ-92-16-1 E. Sin.-A. 10 GGAJ-92-27-1 E. Sin.-A. 15 GGAJ-92-37-2 E. Sin.-A. 15 GGAJ-92-181-1 E. Sin.-A. 3 GGAJ-92-33-1 L . Sin? 5 GGAJ-92-177-1 L . Sin.-V. 7 GGAJ-92-49-1 L . Sin.-H? 10 GGAJ-92-21-1 E. Sin.-A? 20 GGAJ-92-47-2 E? Sin. 25 GGAJ-92-37-1 E. Sin.-A. 10 Lower Pliensbachian GGAJ-92-55-2 E? Plien. 1 GGAJ-92-63-3 E. Plien.-I/W Tr GGAJ-92-65-2 E. Plien. Tr GGAJ-92-78-1 E. Plien. Tr GGAJ-92-79-3 E. Plien.-W. Tr GGAJ-92-80-1 E. Plien.-W? 5 GGAJ-92-80-2 E. Plien.-W? 20 GGAJ-92-89-1 E. Plien. 3 GGAJ-92-114-1 E. Plien. Tr GGAJ-92-132-2 E? Plien. 2 GGAJ-92-185-1 E. Plien. 20 GGAJ-92-69-1A E. Plien. Tr GGAJ-92-123-1 E. Plien.-I. 45 GGAJ-92-115-1 E. Plien.-W? 8 Sample No. Age % C c Upper Pliensbachian GGAJ-92-58-2 L . Plien.-K? 5 GGAJ-92-59-3 L . Plien.-K. 10 GGAJ-92-84-4 L . Plien.-K. Tr GGAJ-92-91-1 L . Plien.-K. Tr GGAJ-92-92-2 L . Plien.-K. Tr GGAJ-92-94-2 L . Plien.-K. 2 GGAJ-92-105-1 L . Plien.-K. 5 GGAJ-92-110-1 L . Plien.-K. Tr GGAJ-92-111-1 L . Plien.-K. 2 GGAJ-92-127-1 L . Plien.-K. 1 GGAJ-92-136-1 L . Plien.-K? Tr GGAJ-92-155-1 L? Plien. Tr GGAJ-92-189-1 L . Plien.-K. Tr GGAJ-92-59-2 L . Plien.-K. 10 GGAJ-92-171-1 L? Plien. 3 Table 6.1. Stratigraphic sub-sets of analyzed thin-sections showing individual age assignments and visually estimated % of late-stage diagenetic calcite. Age assignments: prefix E. /L. = Early/Late; Sin. = Sinemurian; Plien. = Pliensbachian; A = Arnouldi Zone; V = Varians Zone; H = Harbledownense Zone; I. = Imlayi Zone;W = Whiteavesi Zone; K = Kunae Zone. Cc = Calcite; Tr = Trace (< 1 %) DIAGENESIS A N D T H E R M A L HISTORY 186 burial depth. Identification of diagenetic stages can be used to determine to what degree sediments have advanced along the continuum of diagenetic processes and hence whether they have undergone shallow, intermediate, or deep burial. 6.6 D I A G E N E T I C F R A M E W O R K Significant contributions have been made to our understanding of diagenesis in volcaniclastic sandstones by Galloway, (1974; 1979), Surdam and Boles (1979), Helmold and van de Kamp (1984), Surdam and Crossey (1987), and Lee (1992). Although there is continuing debate over the relative importance of temperature versus other controls such as pore-fluid composition, fluid flow, and detrital mineralogy during burial diagenesis, it is now well established that there is a sequential temporal progression of diagenetic changes that occur during burial of arc-derived volcaniclastic sediments. Surdam and Boles (1979) reviewed the diagenetic reactions occurring in volcaniclastic sandstones and concluded that these reactions occur over a broad range of temperature (10 - 200+° C) and pressure (up to 3 kilobars). They document three basic types of reactions which are placed in temporal sequence and divide these diagenetic processes into early and late stages. Early diagenesis is characterized by hydration and carbonatization reactions followed by late diagenetic dehydration reactions but considerable overlap between reaction types may occur, depending upon numerous factors such as modal mineralogy, pore-fluid composition, permeability, and geothermal gradient which may vary from basin to basin. More recent reaction studies have indicated a highly reactive diagenetic zone occurring at temperatures ranging from 80 to 110° C in sand/shale sequences and demonstrate the interdependence of such ubiquitous processes as clay mineral diagenesis, laumontite formation, albitization, and carbonate and quartz cementation (Surdam & Crossey, 1987). The generalized four stage diagenetic sequence used below was proposed by Galloway (1974, 1979) for volcaniclastic arenites in active margin basins where major diagenetic events are a function of thermal factors related to burial depth and variations in fluid composition (Fig. 6.2). Similar schemes use the same diagenetic sequence with only minor variations (q.v., Surdam & Boles, 1979; Helmold & van de Kamp, 1984; Surdam & Crossey, 1987). Most of the diagenetic features observed in Inklin Formation sandstones DIAGENESIS A N D T H E R M A L HISTORY 187 are similar to those described by Dickinson (1970), Galloway (1974, 1979), Surdam and Boles (1979), and others (Helmold & van de Kamp, 1984; Surdam & Crossey, 1987; Lee, 1992). The first stage involves both hydration and carbonatization reactions forming early zeolites and local sparry calcite pore-rim and pore-fill cements. This phase begins at the sediment-water interface and extends to a few hundred metres. The second stage involves the formation of clay rims from chemical alteration of unstable grains and develops with increasing depth of burial on the order of 300 to 1200 metres (Carozzi, 1993). These clay minerals (commonly chlorite and smectite) are ubiquitous as coats and rims on volcaniclastic grains (Lee, 1992). The onset of siliceous overgrowths such as quartz, albite and K -spar can occur during this stage but is generally a later diagenetic process. The third stage corresponds to a second cementation episode and generally occurs at depths between 1000 and 3000 metres. Dehydration reactions form authigenic zeolite (laumontite) or a second layer of well-crystallized phyllosilicate (usually chlorite or smectite)(Galloway, 1974, 1979; Surdam & Boles, 1979). This stage also includes albitization of detrital plagioclase and detrital and diagenetic K-spar. Stage four represents still greater depths of burial and is characterized by the formation of a variety of replacement, overgrowth, and dissolution features which create complex diagenetic fabrics. Calcite replacements of feldspars, volcanic lithics, and matrix occurs in a spotty but appreciable fashion and locally replaces zeolites such as laumontite. Albitization of detrital feldspars is commonly observed in deeply buried sandstones, as are grain (mainly volcanic lithics) and matrix replacements by chlorite, epidote and chert (Surdam & Boles, 1979). The mineral textures chosen as diagnostic of an authigenic origin follow those outlined by Helmold and van de Kamp (1984) and include the following: overgrowths (especially on quartz, plagioclase and K-spar grains); cements; sutured boundaries; intergrowths of euhedral quartz and clay minerals with delicate fabrics; pore-linings and pore-fillings of clay minerals; grain fractures healed in situ by quartz and albite; albitization of detrital feldspars; and replacement of detrital grains by calcite and laumontite.In keeping with Helmold and van de Kamp's criteria, sericite and sausserite alterations of plagioclase, which may have formed in situ and are present in Inklin sediments to variable minor degrees, are not considered diagnostic of a diagenetic origin. It should also be noted that, although reference is made to the presence of of laumontite in this analysis, the large majority of zeolite occurrences in sedimentary rocks typically DIAGENESIS A N D T H E R M A L HISTORY 188 involve crystals that are far too small to allow identification with a petrographic microscope (Nesse, 1986), and Inklin sediments are no exception. The presence of zeolites in these rocks is based on macroscopic and microscopic evidence, and supported by earlier work (Bultman, 1979). Laumontite is the characteristic zeolite mineral of volcanogenic and plagioclase-rich sediments with a deep burial history (Madsen & Murata, 1970; Galloway, 1979; Surdam & Boles, 1979; Helmold & van de Kamp, 1984; Lee, 1992) and it is highly probable that this is the zeolite phase present. Early Calcite Pore Fill Clay Rims O Phyllosilicate S Pore Fill S Zeolite jij Pore Fill ^ Late Calcite 3 and Siliceous W Overgrowths Mechanical Cnishing % POROSITY Well-sorted, medium to fine-grained sandstones (average and range) 0 10 20 30 40 1 I l l l DEEP DEPTH-TEMPERATURE SHALLOW HOT ^ COOL Depositional Surface DEEP DEPTH-TEMPERATURE SHALLOW HOT ^ COOL 1 i i i i i i i i i 1 hv 1 £ 1 i - ^ 1 © ' ' i © Figure 6.2. Sequential development of diagenetic features in arc-derived sandstones based on paragenetic sequence and/or first occurrence in wells. The vertical bars denote interpreted depth ranges of major diagenetic events (specific events defining stages are numbered). Modified after Galloway (1979). DIAGENESIS A N D T H E R M A L HISTORY 189 6.7 S H A L L O W B U R I A L D I A G E N E S I S A distinctive early diagenetic feature common in thick-bedded sandstones of the Inklin Formation is the abundance of sandy calcareous concretions. These features represent the common occurrence of localized early calcite cements. On a macroscopic scale, the concretions occur in perfectly spherical, oblate and prolate shapes with the latter being most common (Fig. 6.3A). They range from small cobble-size to occasionally in excess of 2 metres and can display a wide range within a single bed but are generally found within a moderately restricted range of decimetre scale in single beds. They are most often found as minor bed elements (< 10% by volume) but can comprise in excess of 20% of total bed volume in some occurrences. These concretions commonly occur in both Lower and Upper Pliensbachian strata but are also occasionally found in Sinemurian beds. Their confinement to thick/very thick-bedded sand-dominated sections suggests the concretions are a lithofacies-dependent early diagenetic feature. This early calcite is characteristically formed of clean spar and is difficult to distinguish petrographically from late-stage calcite which can have an identical appearance, although macroscopic evidence (i.e., sandy calcareous concretions) clearly indicates its presence in these rocks. Another early diagenetic feature that is ubiquitous in Inklin sediments is the presence of thin clay coats and rims on framework grains. They occur as continuous coats on detrital grains (except at points of grain contact), discontinuous rims and patches between detrital grains, and can also be found separating detrital grains of plagioclase and quartz from overgrowths, indicating their earlier formation. 6.8 I N T E R M E D I A T E / D E E P B U R I A L D I A G E N E S I S 6.8.1 Authigenic Feldspar Albitization: Some degree of albitization has affected the entire stratigraphic sequence at Atlin lake, whether as albite overgrowths on detrital feldspar, albitization of detrital plagioclase, or both. The albitization varies within stratigraphic subdivisions but a general trend of increasing albitization is apparent. Albitized detrital plagioclase grains are characterized by a murky brown appearance in plane light (Fig. 6.4) resulting from numerous 'dusty' inclusions (Boles, 1982), and act as hosts for albite overgrowths (Fig. 6.5C). Incipient albitization is also relatively common on the detrital K-spar ubiquitous DIAGENESIS A N D T H E R M A L HISTORY 190 Figure 6.3. A : Photograph of sandy calcareous concretions produced by local early diagenetic calcite cement. Concretions are a common feature in thick-bedded Pliensbachian sandstone. B: Photograph of characteristic mottled appearance of laumontized sandstone. This feature is especially prominent in Lower Pliensbachian stratigraphic horizons. DIAGENESIS A N D T H E R M A L HISTORY 191 in Upper Pliensbachian sandstones (Figs. 4.4C, 6.4D). Albitized plagioclase grains are a ubiquitous feature of the entire Inklin succession and can range from incipient to pervasive even within the highest stratigraphic subdivisions, indicating that factors other than burial depth (i.e. temperature) are important controls on this diagenetic process. It was observed that albite overgrowths were less common in Upper Pliensbachian samples than in Lower Pliensbachian samples, suggesting that albite overgrowths post-date albitization of detrital plagioclase. Albite (+ quartz) fracture fills and veinlets were observed in a few Lower and Upper Pliensbachian samples (Fig. 6.4C). Increased albitization cannot be confidently determined for most Sinemurian samples because of extensive replacements by late stage diagenetic calcite, chlorite and quartz, but presumably exists as a function of their deeper burial. In Lower Pliensbachian samples and, to a lesser degree, Upper Pliensbachian samples albitized plagioclase grains may be partially replaced by a variety of diagenetic mineral phases that include laumontite, epidote (+ pumpellyite), calcite, quartz and chlorite (Fig. 6.6). Authigenic Potassium Feldspar: Staining of thin-section offcuts for potassium feldspar (K-spar) reveals the presence of common though minor authigenic K-spar in many of the sandstones studied. The effects are most pronounced in Lower Pliensbachian and Sinemurian sediments but are also present in Upper Pliensbachian strata. The most common occurrence is as finely crystalline interstitial cements and pore-fills where they are found predominantly as irregular cryptocrystalline fibrous aggregates that resemble chert (Fig. 6.7). These interstitial varieties are very similar in texture to those described by Sibley (1978). Examination of stained offcuts with a binocular microscope clearly shows the majority of K-spar cements partially or completely enclosing detrital plagioclase and, to a lesser extent, quartz grains. They are also found as overgrowths and partial replacements on detrital feldspars. Detrital cores generally preserve relict textures such that distinction between plagioclase and K-spar hosts can usually be accomplished -overgrowths and replacements occur on both species. Both types of occurrences together are not volumetrically important, comprising at most a few percent. Both interstitial varieties and partial grain replacements appear to be generally impure, being often intergrown with other authigenic mineral phases that include quartz and possibly albite (Fig. 6.7). Both DIAGENESIS A N D T H E R M A L HISTORY 192 authigenic K-spar and albite are predominantly untwinned and consequently appear identical in unstained thin-sections. The small size of overgrowths and crystals precludes petrographic determination of the optical properties necessary to distinguish between the two so that it was not possible to estimate relative proportions. Although textural relationships are unclear in some respects, authigenic K-spar formation clearly post-dates early clay rim/coat formation and appears to be broadly coeval with some authigenic quartz and albite formation. As temperature appears to have its most important effect in switching authigenic feldspar formation from K-spar to albite (Kastner & Seiver, 1979) and relatively high temperatures have been attained in deeper sections, it is possible that albite replacements may directly replace earlier authigenic K -spar. It was noted that incipient albitization had affected some K-spar overgrowths, indicating that at least some albitization post-dates diagenetic K-spar formation. Partial calcite replacements of K-spar overgrowths were also noted. It would appear that authigenic K-spar cements and replacements occupy an intermediate position in the diagenetic sequence undergone by Inklin sediments and are probably initiated prior to the onset of albitization but have considerable overlap with the albitization window. Partial K-spar pseudomorphic replacements of plagioclase grains were observed but due to the inability to clearly distinguish petrographically between authigenic K-spar and albite it is unclear as to whether this is the preferential mode of K-spar replacement; however, stained offcuts show that partial pseudomorphic replacements occur predominantly on plagioclase. Where replacement is pervasive the feldspar species of the precursor host grain is undetermined. Given the abundance of partial replacements on plagioclase however, it seems reasonable that precursor hosts were plagioclase rather than K-spar. This is consistent with the general rule that authigenic K-spar is far more common than albite in volcaniclastic sands where the detrital feldspar component is usually strongly plagioclase-dominated (Kastner & Seiver, 1979). This conforms to the observation that authigenic K-spar is most abundant in Sinemurian and Lower Pliensbachian sediments which both contain considerably higher volcanogenic content than Upper Pliensbachian sediments. DIAGENESIS A N D T H E R M A L HISTORY 193 6.8.2 Laumontization Laumontite can often be recognized in the field by the characteristic mottling it imparts to the rock i f it is present in sufficient amounts (Madsen and Murata, 1970). This macroscopic feature was observed on numerous occasions during fieldwork in 1992, before the author was aware of its significance (Fig. 6.3B). It was found to occur almost exclusively in tuffaceous Early Pliensbachian sediments and was such a recognizable feature that it was used as a mapping tool in the latter stages of fieldwork. This laumontization is a pervasive feature in many Early Pliensbachian stratigraphic horizons. Its stratigraphic effects range from well-bedded sequences in excess of 100 metres down to bed scale thicknesses. At a number of localities the laumontization could be clearly seen to be stratigraphically confined in thick sections of otherwise similar, non-laumontized sandstones, suggesting that factors other than burial depth are important controls on the diagenetic alteration. Recent studies indicate that important controls on laumontite distribution appear to be pore-fluid chemistry and post-compaction permeability variations between petrologically similar sandstones (Helmold & van de Kamp, 1984; Crossey et al., 1984). This would appear to be a factor in the distribution of laumontite in Inklin sediments, as laumontite is generally stratigraphically confined to highly tuffaceous units that have atypically high porosity and permeability for turbiditic sandstones. Laumontization of detrital plagioclase varies from incipient to complete replacement and is the most common mode of occurrence of laumontite (Fig. 6.6). Detrital plagioclase hosts generally display varying degrees of albitization, suggesting albitization precedes laumontite formation to some degree although there is probably considerable overlap in both these processes. It also occurs as interstitial patches in some samples and can be found displacing biotite along cleavage. The laumontite varies in abundance from trace amounts to a significant minor proportion of the whole rock (> 10%). These latter occurrences appear to be confined to highly tuffaceous units or sequences. The bulk of this alteration is largely confined to Lower Pliensbachian highly tuffaceous units but also affects parts of the Sinemurian section, though to a lesser degree. This latter observation may be in large part a diagenetic artifact, as laumontite wi l l alter to calcite and quartz under the proper conditions during deep burial (Surdam & Boles, 1979; Helmold & van de Kamp, 1984). There is also petrographic evidence DIAGENESIS A N D T H E R M A L HISTORY 194 that suggests partial replacement of laumontite by calcite, quartz and epidote in Lower Pliensbachian sediments as well although the textural relationships are unclear. It is essentially absent from the Upper Pliensbachian section, with the exception of a few bed-scale occurrences in tuffaceous units near the base of the section, suggesting that the top of the laumontite zone roughly coincides with the Early-Late Pliensbachian boundary or, alternately, that Upper Pliensbachian strata generally lack sufficient permeability for laumontite formation. 6.8.3 Late-stage Calcite Deep burial stage 4 diagenetic calcite replacements are found throughout the succession but vary considerably in intensity. Table 6.1 lists visually estimated calcite content for each thin-section. A general trend of increasing late-stage calcite replacements with increasing age and depth of burial is apparent although there is no linear increase with stratigraphic position. As a general rule Upper Pliensbachian strata exhibit the lowest amounts of diagenetic calcite while Sinemurian strata exhibit the highest but exceptions to the rule exist. Mean diagenetic calcite content for the stratigraphic sub-sets illustrate this general trend with values of 12%, 7%, and 3% for Sinemurian, Lower Pliensbachian, and Upper Pliensbachian respectively. Sinemurian strata display extensive spotty diagenetic calcite that ranges from incipient to pervasive replacements of both matrix (i.e. late stage 'cements') and detrital grains which can form up to 25 % by volume of the sample (Fig. 6.8). Grain replacements affect mainly plagioclase but also components of volcanic lithic fragments. Late stage diagenetic calcite rarely forms less than 5% of these rocks and is generally in the range of 10 to 20% by volume. This degree of intensive diagenesis is largely confined to Sinemurian strata with few exceptions. The late-stage calcite is often murky, containing visible impurities resulting from the incomplete replacement of detrital grains and matrix, and relict mineral textures such as albite twinning in plagioclase are usually visible (Fig. 6.8B). Individual crystals can be large clean spar or fine murky equant microspar, giving the calcite a granular texture. Lower Pliensbachian strata display similar textures with respect to late-stage calcite but generally lack the volume of diagenetic calcite found in Sinemurian sandstones. There are a few exceptions to this trend, most notably one sample with ~ 45 % calcite by volume in an earliest Lower Pliensbachian (Imlayi Zone) DIAGENESIS A N D T H E R M A L HISTORY 195 sample and two others with ~ 20% calcite; however, the majority contain only trace amounts or at most a few percent calcite. Upper Pliensbachian sandstones display variable amounts of diagenetic calcite but in general they are less affected by this process than Lower Pliensbachian samples. Two samples range as high as 10% but the majority contain only trace or small amounts (i.e. 1-3%) of diagenetic calcite. In these rocks relatively clean spar dominates over impure microspar and is mainly confined to interstitial locations as cements in most samples (Fig. 6.9). Some of this calcite is clearly of a late-stage replacement origin and much is likely late-stage replacive cements and pore-fill. Early diagenetic calcite is known to occur in some of these rocks, however, and it is often difficult to estimate the relative contributions of early versus late diagenetic calcite. 6.8.4 Chloritization Another diagenetic feature that characterizes Sinemurian strata is the degree of late chloritization. This refers to the chloritization that post-dates ubiquitous early clay coats and rims on detrital grains. Although variable degrees of similar chloritization affect the entire stratigraphy, nowhere else is it as extensive and pervasive as in Sinemurian sediments. It is ubiquitous as matrix replacements where it can be intimately intergrown with late-stage calcite (and/or quartz), occurs as complete and partial grain replacements (mainly in volcanic lithics), and also occurs as spotty partial replacements intergrown with other authigenic replacements on detrital plagioclase (Figs. 6.10, 6.11). As a diagenetic mineral phase it is subordinate only to the pervasive late-stage calcite and can be occasionally dominant, generally forming a few to as much as 10% of the rock volume. This subordinate relationship and relatively large volume is confined to the Sinemurian section with rare exception. It is also commonly associated with spotty opaques (pyrite?) both in matrix and grain replacements (Fig. 6.11). Textural fabrics such as intergrowths with late-stage calcite indicate a broadly coeval late diagenetic formation which is consistent with the paragenetic sequence outlined by Surdam and Crossey (1987), where chlorite replacements co-occur with ferroan carbonate replacements during late diagenesis. DIAGENESIS A N D T H E R M A L HISTORY 196 Figure 6.4. A : Photomicrograph of albite overgrowth and in situ 'healing' of fractured plagioclase grain (1). Adjacent plagioclase grain has partial albite replacement as well as overgrowth (2). Note optical discontinuity. Crossed polars, B: Plane light. C: . Albite fracture-fill. Crossed polars. D: Patchy brownish discolourations visible in plane light are the effects of moderately advanced albitization on feldspar (mainly plagioclase) grains. Large grain in center displaying less alteration is perthitic orthoclase (O). Plane light. DIAGENESIS A N D T H E R M A L HISTORY 197 Figure 6.5. A : Photomicrograph of quartz overgrowth (arrow) on quartz grain. Note optical discontinuity., Crossed polars, B : Plane light. C : Clear albite (Ab) (and K-spar ?) overgrowths (arrows) on intensely altered plagioclase grains. Overgrowths post-date earlier alteration of host grains. Note optical discontinuity. Crossed polars, D: Plane light. DIAGENESIS A N D T H E R M A L HISTORY 198 Figure 6.6. A : Photomicrograph of complex replacive fabrics on plagioclase core involving mainly granular to prismatic laumontite (arrow) plus other cryptocrystalline diagenetic minerals. Crossed polars, B: Plane light. Figure 6.7. A: Photomicrograph of cryptocrystalline to microcrystalline anhedral aggregate of impure K -spar replacive cement. Note chert-like appearance. Crossed polars, B: Plane light. DIAGENESIS A N D T H E R M A L HISTORY 200 Figure 6.8. A : Photomicrograph of preferential spotty calcite replacements and microfracture fill on plagioclase phenocryst in volcanic lithic grain. Crossed polars. B: Near-complete replacement of feldspar grain (plagioclase?) in Sinemurian sandstone by late-stage calcite spar and granular micro-spar. Arrows point to remnant feldspar. Crossed polars. DIAGENESIS A N D T H E R M A L HISTORY 201 Figure 6.9. A: Photomicrograph of calcite pore-fill and replacements in Upper Pliensbachian sandstone. Replacive nature of 'cement' is evident from relationship with central grain which 'floats' in sparry calcite. Euhedral feldspar overgrowth (arrow) on albitized feldspar crystal predates late calcite cement. Crossed polars, B: Plane light. DIAGENESIS A N D T H E R M A L HISTORY 202 Figure 6.10. A : Photomicrograph of multiple diagenetic fabrics in Sinemurian sandstone. 1: chloritized volcanic lithic fragment, and advanced chlorite replacement of grains and matrix; 2: incipient to advanced calcite replacements on feldspar grains, 3: complex multimineralic replacements on (volcanic?) lithic fragment includes albite, quartz, chlorite and calcite. Crossed polars, B : Plane light. DIAGENESIS A N D T H E R M A L HISTORY 203 Figure 6.11. A: Photomicrograph of cryptocrystalline intergrowths of granular microspar and chlorite on matrix (1) and grains (2). Note association of opaque mineral with pervasive chlorite replacement of grain on lower left (3). Crossed polars, B: Plane light. DIAGENESIS A N D T H E R M A L HISTORY 204 6.9 DISCUSSION Petrographic study documents the following diagenetic features in Sinemurian and Lower Pliensbachian sediments: clay sheath replacements of devitrified volcanic glass, early phyllosilicate cements and clay coats (includes chlorite and mica), authigenic K-spar cements and grain replacements, albitization and overgrowths on detrital plagioclase grains, replacive and interstitial laumontite, quartz overgrowths (Fig. 6.5A)and spotty replacements (including chalcedonic cements), incipient to pervasive calcite grain replacements and late-stage cements, incipient to complete chloritization of matrix and grains, epidote/clinozoisite spotty replacements on detrital grains and alteration of mafic minerals, and development of apparent pressure solution textures. Upper Pliensbachian sediments in general have undergone a milder diagenesis although most of the above diagenetic features are present in all Upper Pliensbachian samples and all are present in some. This would appear to indicate that these sediments have entered the same diagenetic window as more deeply buried sections but remained at shallower levels within it for much shorter periods of time, consequently diagenetic effects are less pronounced and often incipient. For example, thermal data indicates burial temperatures in the range of 100° C, so laumontite is consequently either sparsely distributed or absent. The fact that laumontite is present in Upper Pliensbachian strata even in small amounts and is common in Lower Pliensbachian and, to a lesser extent, Sinemurian strata provides insight into the relative burial depths attained by the Inklin succession. Laumontite is diagnostic of the zeolite facies, which is the lowest grade of metamorphism characteristic of volcaniclastic sediments. While it can be used as an index of depth of burial (Pettijohn et al., 1987), assignment of absolute depth ranges is rendered equivocal due to the uncertainty concerning past geothermal gradients during burial diagenesis and post-diagenetic events related to accretion. However it is generally considered indicative of relatively deep burial diagenesis. The effects of diagenetic alteration are most pronounced in Sinemurian and Lower Pliensbachian strata where alteration of feldspars and lithic grains can be intense. Upper Pliensbachian strata contain the only samples with high proportions of relatively unaltered feldspar, although even in this suite alteration can range from incipient to pervasive. The most common types of alteration affecting feldspars are albitization, laumontization, and calcite replacements but more complex multi-phase authigenic replacements involving DIAGENESIS A N D T H E R M A L HISTORY 205 intergrowths of various combinations of quartz, chlorite, epidote and opaques can be found throughout the Inklin succession and affect lithic grains as well, particularly in Sinemurian and Lower Pliensbachian sediments. These diagenetic artifacts are intermediate and deep burial indicators. Albitization of detrital feldspar is a diagenetic process commonly observed in sandstones that have been deeply buried (Surdam & Crossey, 1987). Numerous studies have documented the trend of increasing albitic content of detrital feldspars with increasing burial depths and some have delineated depth zones marking the onset of incipient albitization ranging from about 800 m. (Lee, 1992) to > 2 kilometres (Boles, 1982). It is clear from petrographic evidence that uppermost Inklin sediments have passed through threshold burial depths required for albitization of detrital feldspar. Calcite replacements of framework grains (mostly plagioclase feldspar) are most pronounced in Sinemurian and Lower Pliensbachian sediments although similar replacive fabrics are relatively well advanced in a few Upper Pliensbachian samples. Minor late-stage calcite cements are found in most Upper Pliensbachian samples but framework grains remain largely unaffected by this process beyond some incipient to spotty replacement. Petrographic evidence shows that Sinemurian and Lower Pliensbachian sediments, as well as parts or all of the Upper Pliensbachian section, have undergone Stage 4 diagenetic events, indicating that all of the Inklin Formation has been subject to deep burial conditions during its diagenetic history. A generalized paragenetic sequence for authigenic mineral phases identified in Inklin Formation sandstones is shown in Figure 6.12. It depicts the relative succession of diagenetic events observed in this study. Considerable uncertainty is attached to the start and end points of some diagenetic event bars, particularly those that are broadly coeval, as a result of two factors: (1) textural relationships between certain authigenic mineral phases were sometimes unclear and, (2) the diagenetic history of sediments from similar statigraphic levels can vary as a result of factors other than burial depth alone. Relative proportions of diagenetic components in the Inklin succession are shown in Figure 6.13. DIAGENESIS A N D T H E R M A L HISTORY 206 Diagenetic Minerals Increasing Burial Depth 1> Early Calcite Cements 1 Clay Coats Albite Quartz Overgrowths _ Plagioclase Overgrowths K-spar Cements and Overgrowths 2 Phyllosilicate Pore-fill Laumontite 3 Late Calcite Cements And Replacements 4 Late Chlorite Cements and Replacements 9 Iron Oxides - ? — ? -Figure 6.12. Generalized paragenetic sequence of authigenic mineral phases i n Inklin Formation sandstones. Numbers 1-4 correspond to diagenetic stages of Galloway (1979). Diagenetic Event Upper Pliensbachian Lower Pliensbachian Sinemurian Albitization Quartz Overgrowths Plagioclase Overgrowths K-spar Cements and Overgrowths Phyllosilicate Pore-fill Laumontization Late Calcite Cements And Replacements Late Chlorite Cements and Replacements o 1 9 Iron Oxides i Figure 6.13. Generalized schematic depiction of authigenic mineral phases in Inklin Formation sandstones by stratigraphic interval. Relative abundance of diagenetic components is indicated by width of bars. DIAGENESIS A N D T H E R M A L HISTORY 207 Diagenetic fabrics in uppermost Inklin sediments (Upper Pliensbachian - Kunae Zone) at Atlin Lake indicate that the top of the preserved section has attained at least the onset of stage 4 diagenesis which is an intermediate to deep burial diagenetic artifact (Fig. 6.13). Depth ranges affixed to the successive relative burial realms vary somewhat according to one's definition and are somewhat arbitrary but fairly consistent broad overlaps from numerous sources permit crude estimates of burial depths for Inklin sediments. A n interpretation of at least intermediate burial depths is justified by diagenetic fabrics and minerals (e.g., albitization, siliceous (quartz, albite, K-spar) overgrowths, incipient to advanced late-stage calcite replacements) and pressure solution textures in Upper Pliensbachian sediments which suggests a depth range of 2 to 4 kilometres, depending upon geothermal gradient. This range is based upon figures taken from various schemes and case studies (Galloway, 1979; Surdam & Boles, 1979; Helmold & van de Kamp, 1984; Surdam et al., 1989: Lee, 1992) and incorporates extremes of assumed paleogeothermal gradients given by Surdam and others (1989). The range is consistent with thermal data from Upper Pliensbachian strata and should be considered conservative. Therefore, a threshold burial depth of 2 kilometres would appear to be required to produce the diagenetic fabrics in Upper Pliensbachian sediments and an estimate of 3 to 4 kilometres burial for these sediments is considered reasonable by the writer. Given that the entire Inklin stratigraphic succession at Atlin Lake is estimated to be in the range of 3 to 4 kilometres thick (Ch.7, p. 214), then basal (Sinemurian) sections have undergone burial to depths in the range of 5 to 7 kilometres. Sinemurian thermal data are permissive of estimates in this range. It should be stressed that these are crude but conservative estimates with minima reflecting paleogeothermal gradients considerably higher than those commonly found in such settings. The data and observations in this study provide fundamental information on the diagenetic history of Inklin Formation sediments and permit initial estimates of burial depths, however, further study is required to fully address questions of regional burial history. Much work remains to be done on thermal maturation studies in the region before a data-base of regional application can be compiled. A good starting point for such studies would be in the Atlin Lake area where biostratigraphic control provides the temporal framework for selection of appropriate sampling targets. DISCUSSION A N D CONCLUSIONS 208 C H A P T E R 7 D I S C U S S I O N A N D C O N C L U S I O N S 7.1 B I O C H R O N O L O G Y The age of the Inklin succession at Atlin Lake is well-constrained by ammonite biochronology to a range of Early Sinemurian (Coroniceras? to Amouldi zone) to Late Pliensbachian (Kunae Zone)(Fig. 7.1). This is the longest firmly documented succession of Inklin strata in the Whitehorse trough, although it appears to represent only basal to intermediate stratigraphic levels of the formation. Previous biosrratigraphic determinations have indicated upper age ranges from Toarcian to Bajocian (Middle Jurassic) for Inklin strata in other parts of the northern Whitehorse trough; however, the accuracy of this earlier work is now suspect and must be considered unreliable (H.W. Tipper- personal communication 1994). Upper age constraints for the partly coeval Takwahoni Formation in the Tulsequah area extend the stratigraphy there into the Bajocian stage (H.W.Tipper-personal communication 1994) and presumably coeval strata were deposited in the basinal facies represented by the Inklin Formation. Whether or not higher stratigraphic levels (i.e., Toarcian to Bajocian ) of the Inklin Formation are preserved in the Whitehorse trough remains to be determined. The paleobiogeography of the northern Stikinian arc segment of this study can be approximated using ammonite taxa documented at Atlin Lake. Sinemurian paleobiogeography is inconclusive due to the pandemic nature of ammonite taxa collected in this study; however, there is some indication of Tethyan influence (i.e., rare juraphyllatid ammonites) and an East Pacific locality is presumed on the basis of Early Pliensbachian faunal affinities. A strong Tethyan influence is apparent from Early Pliensbachian ammonite taxa and proximity to North America is indicated by abundant specimens of East Pacific affinity. A clear link to cratonal North America can be made by early Late Pliensbachian time with the occurrence of locally strong Boreal faunal components, placing this segment of the northern Stikinian arc on the periphery of the northern Boreal Faunal Realm during the Kunae Zone. This sequence suggests northward displacement during Pliensbachian time which is contrary to the southward displacements invoked for the Early Jurassic by current paleotectonic models based on paleomagnetic data. DISCUSSION A N D CONCLUSIONS 209 .2 JE o ra _a (A >600m C 0 CL n >1000 m i n 13 31 LEGEND Sandstone Tuff and resedimented pyroclastic debris m Rhythmic silty argitlite Rhythmic siltstone and fine sandstone • Conglomerate > 500 m DESCRIPTION SINEMURIAN: Rhythmic thin-bedded siltstone, silty argillites and argillaceous mudstones (sequences commonly exceed 100 m), predominantly fine to medium-grained feldspathic greywacke and sub-wacke, minor conglomerate. LOWER PLIENSBACHIAN: Volcanogenic feldspathic to lithic sandstone (includes fine-grained to pebbly sub-wacke, arenite, minor greywacke), dacitic to rhyodacitic tuff, resedimented pyroclastic debris, rhythmic thin-bedded fine sandstone, siltstone, and argillaceous mudstone, minor volcanic conglomerate. UPPER PLIENSBACHIAN: Volcanoplutonic quartzofeldspathic to lithic sandstone (includes fine-grained to pebbly sub-wackc, arenite, minor greywacke), minor to subordinate tuffaceous rocks (includes dacitic to rhyodacitic tuff and resedimented pyroclastic debris), rhythmic thin-bedded fine sandstone, siltstone, and mudstone| minor volcanoplutonic conglomerate. ;isv.s (s rnscs Figure 7.1 Generalized stratigraphic column of the Inklin Formation at Atlin Lake. Stratigraphic thicknesses shown are not estimates of total thickness but represent minima based on measured sections. Not to scale. DISCUSSION A N D CONCLUSIONS 210 7.2 S E D I M E N T O L O G Y A N D S T R A T I G R A P H Y 7.2.1 Provenance and Basin Setting: The Laberge Group represents a Lower to Middle Jurassic succession of arc-marginal sediments deposited on an arc constructed mainly in Late Triassic time. In the study area only the Sinemurian and Pliensbachian stages of the Lower Jurassic are exposed or preserved. Details of the arc/basin evolution during this period have emerged from this study and can be divided into three main phases corresponding to Sinemurian, Early Pliensbachian and Late Pliensbachian time. Significant provenance shifts evident from study of conglomerates and sandstones delineate these phases and provide constraints on the Early Jurassic magmatic and tectonic evolution of this arc segment of the northern Stikine terrane. Specific provenance linkages can be made for the Inklin succession at Atlin Lake and support detrital sources derived wholly from the southwest margin of the basin. Unequivocal unidirectional provenance in an arc-marginal basin indicates a forearc basin setting; however, evidence from this study is equivocal. The existence of a strong paleoflow apparently derived from the northeast lends support to earlier interpretations favoring a back-arc setting for this region of the arc (Bultman, 1979; Eisbacher,1974). For a number of reasons, this paleocurrent data fails to provide convincing evidence of an independent clastic source along the northeast basin margin. The first reason relates to the paleocurrent data itself. There is a lack of continuity in the northeast-derived paleoflow that is inconsistent with a backarc setting. No evidence of paleoflow from the northeast was found in Sinemurian strata nor is there data to support a significant northeast source during Late Pliensbachian time. This observation leads to the conclusion that northeast-derived paleoflow was ephemeral and, hence, more likely the result of episodic processes related to tectonics or sedimentation dynamics rather than fundamental basin setting. Another reason to discount a backarc setting comes from independent lines of study related to provenance. Quantitative and qualitative modal mineralogical data (Ch. 4) from sandstone collected from Lower Pliensbachian strata derived from both of the apparently opposed paleoflow systems shows no significant, or even discernible, differences in modal mineralogy or qualitative discriminators. Evidence from analysis of conglomerates gives similar results. Upper Pliensbachian conglomerates near the northeast basin margin contain granitic, volcanic and hypabyssal clasts that are lithologically identical to clasts found DISCUSSION A N D CONCLUSIONS 211 in coeval strata clearly derived from the southwest. From a provenance perspective, the data from sandstone and conglomerate analysis requires one of the following scenarios: 1) first-order derivation from a single source (forearc) terrane; or 2) first-order derivation from two opposed sources of essentially identical lithology. In the first scenario, petrographic and petrologic data can be reconciled with apparent episodic bi-directional paleoflow in one of two ways: 1) regional uplift of an outer forearc rise or ridge tectonically 'shoulders' forearc strata basinward, cannibalizing and resedimenting broadly coeval forearc sediments; or 2) apparent northeast-derived paleoflow is generated by reflection of high velocity, large-volume, high-density turbidity currents from 'distal' basin topographic obstructions such as an outer forearc rise or ridge. A combination of both processes may also be conjectured. In any case, the original provenance of the sediments would be from a single source terrane to the southwest, with ultimate deposition reflecting a second-order derivation largely confined to Early Pliensbachian time. In the second scenario, the requirement of two distinct opposed, yet lithologically identical, source areas poses geological difficulties. Marsden and Thorkelson (1993) have proposed a model for paired opposing subduction zones in the Stikinian arc during Lower Jurassic time. They argue that this type of plate configuration could produce the similar coeval volcanic belts that flank the Hazelton Trough, a backarc basin that received volcaniclastic detritus from both sides. A similar type of plate configuration could conceivably fulfill the geological requirements of the second scenario; however, in the absence of any supporting evidence this is highly conjectural. Further, this scenario cannot be reconciled with paleocurrent data indicating an episodic or ephemeral northeast-derived paleoflow system. In light of the conflict between data indicating identical lithological provenance and apparently ephemeral bi-directional paleoflow systems, a backarc setting is improbable for the Whitehorse Trough in the study area. Consequently, the Inklin Formation is interpreted to represent a forearc basin setting in the Stikinian arc segment of this study (Fig. 7.2). It should also be noted that there is no evidence of sediments derived from an accretionary complex (e.g., chert) in strata apparently derived from the northeast, as would be expected as outer basin flanks are progressively uplifted and erosion exposes the underlying accretionary complex. However, this observation is not inconsistent with stratigraphic range of DISCUSSION A N D CONCLUSIONS 212 Figure 7.2. Schematic depiction of hypothesized tectonic setting for the northern Whitehorse trough. Figure shows relationship of oceanic basement to the Laberge Group in a speculative residual forearc basin setting. In this scenario forearc basin strata overlie mainly trapped oceanic or transitional crust of uncertain affinity. A rising imbricate stack incorporating slices of Cache Creek crust could potentially act as a provenance source later in the basin's history. the Inklin Formation. The stratigraphic intervals exposed at Atlin Lake (i.e., Sinemurian and Pliensbachian) represent only basal and intermediate stratigraphic levels of the entire Inklin succession, which is believed elsewhere in the Whitehorse Trough to range as high as Aalenian and Bajocian time (e.g., Poulton and Tipper, 1991). Evidence of clastic input from an exposed accretionary complex might reasonably be expected to occur later in the basin's history. The distribution of Upper Pliensbachian sandstones in the QmPK tectonic discrimination diagram of Marsaglia and Ingersoll (1991) clearly indicates a strike-slip influence on arc dissection and sedimentation. Further evidence of rapid arc dissection is found in the abundance of young (i.e., 186 M a ) granitic clasts in Upper Pliensbachian conglomerates. Taken together, the data indicate that major strike-slip faulting and the accompanying uplift were an important, possibly dominant, control on the rapid unroofing of arc plutons in Late Pliensbachian time. The Llewellyn fault is a regional structure interpreted to be a major dextral transcurrent extension of the King Salmon Thrust (Mihalynuk and Rouse, 1988) and which may represent the intra-arc transform fault responsible. Although clear evidence of motion along this structure is DISCUSSION A N D CONCLUSIONS 213 constrained at post-100 M a (L.D. Currie - pers. comm., 1994), the high preservation potential of intra-arc transforms and their propensity for reactivation is consistent with repeated episodes of motion during subsequent crustal convergence and regional tectonic events that can modify and/or overprint their original genesis (Eisbacher, 1985). Indications of earlier motion along the Llewellyn Fault have come from recent regional mapping efforts such as those of Mihalynuk and Rouse (1988) who observed stratigraphic evidence which they suggest is indicative of syn-deposkional movement along this structure in Early Jurassic time. 7.2.2 Formation Thickness: By piecing together a number of biostratigraphically well-constrained measured sections the stratigraphic thickness of the Inklin succession at Atlin Lake can be estimated. Sinemurian strata are the most poorly constrained with three measured sections representing the Arnouldi and Varians zones. Approximately 250 metres of strata represent fan-fringe and basin-plain deposition within that span of time and it is unlikely that the strata represent the entire zones. Two other sections of 360 and 200 metres were measured in strata interpreted to be of Sinemurian age and represent lower fan-fringe and medial fan settings respectively; however, uncertainty as to their stratigraphic level in the Sinemurian succession does not permit their use in stratigraphic compilation. One sequence of Harbledownense Zone (Upper Sinemurian) strata was estimated to be in excess of 200 metres thick and is comprised mainly of lower fan fringe strata but contains a number of debris flow conglomerates. No basal Sinemurian (Canadensis and Coroniceras Zones) or uppermost Sinemurian (Recognitum Zone) strata were identified in the study area but presumably, at least the Coroniceras and Recognitum Zones form part of the Sinemurian succession. In total, approximately 500 metres of Lower and Upper Sinemurian strata representing parts (or all ?) of half of the Sinemurian biochronozones are known in the study area. Another 560 metres of measured Sinemurian strata may be partly or wholly coeval with biostratigraphically constrained Sinemurian sections. Given the relatively low sedimentation rates that appear to have prevailed for much of Sinemurian time and the known thickness of parts of the Sinemurian succession, an estimate of 1000 to 1200 metres is considered reasonable for this part of the Inklin succession at Atlin Lake. DISCUSSION A N D CONCLUSIONS 214 A number of measured sections represent parts of the Early Pliensbachian succession. A section measured on Sloko Island contains 435 metres of Lower Pliensbachian strata representing mainly the Freboldi Zone and possibly upper portions of the Whiteavesi Zone. The upper 200 metres of the Lower Pliensbachian segment is comprised of muddy sediments interpreted to represent interchannel to lower fan-fringe environments. This muddy sequence alone represents a pre-compactional sequence in excess of 600 metres and presumably coeval sandy equivalents were considerably thicker. Given the predominance of thick sand sequences in Lower Pliensbachian strata, an estimate of 500-600 metres is considered conservative for the Freboldi Zone. Whiteavesi Zone strata appear to have the highest proportion of primary tuffs (i.e. bona fide Nordenskiold Dacite) and this zone is interpreted to represent the thickest section of the Early Pliensbachian succession. Stratigraphic thickness in excess of 600 metres to as much as 800-1000 metres is estimated for the Whiteavesi Zone. Basal Pliensbachian Imlayi Zone strata are poorly constrained in the study area but presumably are at least a few hundred metres thick. A measured section of undifferentiated Lower Pliensbachian strata nearly a kilometre thick occurs south of Janus Point and is interpreted to represent mainly Whiteavesi Zone deposition but may include significant portions of Imlayi Zone strata. In total, a stratigraphic thickness of approximately 1500-2000 metres is indicated for the Early Pliensbachian succession. Late Pliensbachian sedimentation is characterized by coarser clastic deposition that reflect the highest sedimentation rates in the Inklin succession at Atlin Lake. Two measured sections with sharply defined Early-Late Pliensbachian boundaries on Copper and Sloko Islands contain 570 and 535 metres of Upper Pliensbachian (Kunae Zone) strata, respectively. Tight biostratigraphic brackets on Copper Island indicate that the 570 metres of section were deposited within the lower third of the Kunae Zone. Even assuming reduced sedimentation rates for the remainder of the Kunae Zone not represented in the section, a stratigraphic thickness well in excess of one kilometre is indicated for the Kunae Zone. In summary, the total thickness of the Inklin succession at Atlin Lake is estimated to be at least 3000 metres and more probably between 3500 to 4000 metres. DISCUSSION A N D CONCLUSIONS 215 7.3 P A L E O - T E C T O N I C R E C O N S T R U C T I O N The principal variations in detrital modes of arc-derived sandstone suites lie along a continuum between lithic-rich sands derived mainly from volcanic sources and sands rich in feldspar and quartz derived mainly from plutonic sources (Dickinson, 1982). Temporal clast trends in conglomerates reflect a similar spectrum of variation with clast suites ranging from end-members derived mainly from volcanic sources and flanking coastal sediments to those derived primarily from plutonic arc roots (Dickie and Hein, 1992). When viewed in the context of these relations, both the quantitative and qualitative petrographic evidence supports the following paleotectonic reconstruction. 7.3.1 Sinemurian: The Sinemurian represents a relatively quiescent phase in the arc's history. Progressive incision of the Late Triassic arc initiated in latest Triassic or earliest Jurassic time transported elastics of mixed volcanoplutonic and sedimentary provenance initially eastward into the basin via turbidity currents where sediments were dispersed in a predominantly northwesterly direction. The prevailing paleoflow systems operating during Sinemurian time reflect longitudinal fan-systems prograding north/northwesterly along the axial trend of the basin with regional paleoslope the probable first-order control on the direction of sediment dispersal. A relatively stable tectonic regime is indicated for this period. Evidence of low sedimentation rates are found in the reduced stratigraphic thicknesses of the Sinemurian section, the extensive development of lower fan-fringe and basin-plain environments and the abundant, pervasive bioturbation of Sinemurian sediments which is not found elsewhere in the Inklin succession at Atlin Lake. The Sinemurian arc was flanked by a shallow-marine shelf dominated at least locally by carbonate depositional environments as indicated by the abundance of Sinemurian (especially Upper Sinemurian) muddy carbonate clasts in Upper Pliensbachian conglomerates, supporting an interpretation of low sedimentation rates. Well-oxygenated waters prevail in both shallow and deep marine settings as shown by a thriving shallow-marine benthos and the prolific deep-marine soft-bodied infauna. Late Triassic sources account for the majority of Sinemurian basin-fill. By Sinemurian time the arc had undergone sufficient uplift and incision to expose older Late Triassic moderate to shallowly-emplaced granitic plutons, however, volcanic cover generated mainly in Late Triassic time was still extensive enough to be a major contributor to basin-fill. Volcaniclastics were probably derived from a number of quiescent (and possibly sporadically DISCUSSION A N D CONCLUSIONS 216 active) erosively incised volcanic fields along the arc segment. Upper Norian (Late Triassic) carbonates of Sinwa Formation affinity were extensively exposed along the southwest basin margin flanks and a site of active erosion during the Sinemurian. This regime of stable tectonics characterized by relative volcanic quiescence and low sedimentation rates prevailed through most or all of the Sinemurian and represents a transitional arc phase in the progressive dissection of the Late Triassic arc. 7.3.2 Early Pliensbachian: Early Pliensbachian time saw a dramatic shift in the nature of arc evolution. The onset of a major magmatic episode, accompanied by widespread rejuvenated volcanism in the arc segment, caused a strong provenance shift to broadly coeval volcaniclastics. Voluminous eruptions of dacitic and rhyodacitic pyroclastics supplied the bulk of detritus deposited during this time. Deposition occurred mainly by high-density grain flows of resedimented pyroclastic debris and epiclastics but includes a significant minor component of primary pyroclastic air-fall. The mixture of primary pyroclastics, coeval resedimented pyroclastic debris, and broadly coeval, as well as older, epiclastics shed from volcanogenic highlands produced highly tuffaceous sands. The Early Pliensbachian succession represents a major episode of accelerated basin-fill, producing as much as a two kilometre thick section of volcanogenic sediments. Sub-aerial pyroclastic blankets and associated flow-rocks generated continuous or semi-continuous volcanic fields that masked the Late Triassic volcanics, subjacent plutons and carbonates exposed for much of the Sinemurian. This volcanogenic influx terminated inner forearc flank uplift/erosion and caused rapid subsidence of the inner flanks, abruptly shutting off the Upper Norian carbonates as a clast source. Minor plutonic materials could be derived from reworked Sinemurian elastics or restricted local plutonic exposures such as in deep gorges. The high volume of broadly coeval volcaniclastics recorded in Early Pliensbachian sediments qualifies this period as a new constructional phase of arc-building and demonstrates provenance from a nearly undissected magmatic arc. The high sedimentation rates that were a consequence of the combination of abundant pyroclastic influx and increased topographical relief caused a fundamental change in sediment dispersal systems from predominantly longitudinal fans to radial or transverse fans prograding northeast across the basin axis. The initiation of southwest-directed paleoflow systems during the Early Pliensbachian indicates a change in basin morphology with the development of an outer basin margin. Uplift of an outer forearc rise or ridge DISCUSSION A N D CONCLUSIONS 217 produced by crustal flexure from increased sediment loading and/or tectonic processes related to subduction are the probable cause of morphological changes. Evolution of an ephemeral ridged forearc basin phase is indicated. Clastics transported by southwesterly-directed paleoflow systems are of second-order derivation generated from cannibalization and resedimentation of broadly coeval forearc strata involved in the outer flank uplift and/or reflection of high-velocity, high-density turbidity currents off the outer basin flank high. 7.3.3 Late Pliensbachian: Late Pliensbachian sediments record a new phase of arc evolution in which active tectonics replace volcanism as the first-order controls on sedimentation. A n abrupt shift from a volcanogenic provenance in Early Pliensbachian time to a mixed volcanoplutonic provenance dominated by granitic material in the Late Pliensbachian highlights the fundamental change in processes governing clastic sources. The constructional relief generated by Early Pliensbachian volcanism was augmented by an episode of uplift which increased topographical gradients and accelerated arc erosion. Rates of uplift must have been dramatic given that they were occurring during a time of widespread transgressive seas and had exhumed young comagmatic plutons of Pliensbachian age (~ 186 Ma) by earliest Late Pliensbachian time. This major uplift provided the impetus for a major progradational/aggradational episode in Late Pliensbachian time. A n example of the high sedimentation rates which characterize Late Pliensbachian basin-fill history is found in the measured section on Copper Island (Fig. 2.4.4) where a post-compactional sand sequence almost 600 metres thick was deposited during a fraction (i.e., < 1/3) of Kunae Zone time -this fraction representing a time-slice with a likely duration on the order of magnitude of 10 5 years. A n intra-arc transform fault(s) sliced through portions of the arc and initiated rapid arc dissection by mobilizing crustal segments of the arc. This mechanism, in conjunction with accompanying uplift, led to rapid exposure of moderate to shallowly emplaced Pliensbachian granitic plutons which became important clast sources along this arc segment during early Late Pliensbachian time. The driving mechanism of intra-arc strike-slip tectonics is most likely related to changes in oblique convergence rates and/or vectors along the subduction zone. Sand-rich predominantly radial or transverse fans prograded across the relatively narrow basin. Outer basin margins no longer acted as significant clastic sources and/or topographical obstructions to high-DISCUSSION A N D CONCLUSIONS 218 density grain-flows, suggesting that much of the available accommodation space had been rapidly infilled by Upper Pliensbachian sediments. Former basin-plain environments are over-ridden by prograding sands and are not developed during this time. By early Late Pliensbachian time the effects of uplift and erosion had produced a deeply dissected Late Pliensbachian arc characterized by extensive exposures of Late Triassic (?) and Early Jurassic granitic plutons capped only locally by isolated volcanoes. Sparse high-rank metamorphic grains are probably derived from locally exposed metamorphic envelopes of the arc core. Intra-arc strike-slip tectonics play a significant role in rapid arc dissection and pluton exhumation. 7.4 TERRANE LINKAGES Whitehorse trough strata of the Laberge Group have came to be known as the Inklin overlap assemblage that is thought to link Late Triassic and older arc assemblages of Stikinia to the west with mainly Paleozoic oceanic assemblages of the Cache Creek Terrane to the east (Wheeler et al, 1988). Contacts with the Cache Creek Terrane are of a predominantly (or entirely?) structural nature along the eastern margin of the Whitehorse trough, consequently, clear stratigraphic evidence that documents deposition of bona fide Cache Creek lithologies in the Inklin Formation is relatively sparse. The Inklin Formation is presumed to overlie mainly Cache Creek basement but, while there is evidence of this in southern portions of the Whitehorse trough, there is no evidence in northern portions including the study area (Fig. 7.3). Some of the best evidence of Cache Creek basement comes from the Cry Lake map area where the King Salmon Assemblage, a volcanosedimentary arc succession interpreted to form part of the Cache Creek Terrane, is exposed (Monger, 1974; Thorstad and Gabrielse, 1986 ). The three formations which it comprises are: 1) the Kutcho Formation - an Upper Triassic volcanic succession of bimodal mafic and felsic calc-alkaline volcaniclastic arc rocks; 2) Upper Triassic (Norian) Sinwa Formation - a shallow-marine carbonate sequence; and 3) the Inklin Formation. This assemblage is interpreted to represent a Late Triassic arc built upon Cache Creek oceanic basement (Thorstad and Gabrielse, 1986) and presumably was a west-facing arc with the Inklin Formation derived from erosion of the arc and transported eastward to be deposited as forearc strata. What is not clear is how the Inklin 'overlap assemblage' can form a continuous stratigraphic unit straddling an active subduction zone between Stikinia and the Cache Creek Terrane, DISCUSSION A N D CONCLUSIONS 219 unless one assumes subduction had ceased by Early Jurassic time and juxtaposition of opposed Stikinian and Cache Creek forearc basins had occurred. Evidence from the Cry Lake area cannot be reconciled with what is known of the Early Jurassic tectonic, magmatic, and basin-fill history of the Whitehorse trough in northern Stikinia. The strong disparity in Inklin Formation provenance between the two regions indicates major differences in arc and basin evolution. Clearly, it is inappropriate to use the King Salmon Assemblage as an analog or model for Laberge Group deposition along the length of the Whitehorse trough, although this appears to be the case in recent regional syntheses in which the Inklin Formation is stated to be predominantly derived from the east (Gabrielse and Yorath, 1992)). Evidence from this study clearly indicates the Inklin Formation is derived predominantly or even wholly from the southwest, at least during Sinemurian and Pliensbachian time. Clast provenance and to a lesser extent, paleocurrent data provide strong support for a Stikinian provenance. There is no data from this study to support any provenance linkage to the Cache Creek Terrane during Sinemurian and Pliensbachian time. Upper Pliensbachian conglomerates near the northeast basin margin contain distinctive granitic and volcanic-hypabyssal clasts that are identical to those found in coeval units clearly shown to be derived from the Stikinian arc (Ch. 3, p. 93). Some unusual fossiliferous muddy carbonates found in northeastern units are lithologically identical to stratigraphic units underlying the Nordenskiold dacite along the Lewes River arc flank in the south-central Yukon (C. Hart - personal communication; 1994). This does not necessarily imply that provenance linkages between the Cache Creek and Stikine terranes did not exist at this time but it does indicate that any provenance linkages between the terranes in the northern Cordillera were tenuous or incipient in Sinemurian and Pliensbachian time. It may well be that evidence of Cache Creek provenance is found higher in the Inklin succession than is preserved in the study area. In light of these conclusions, it is suggested that the use of the term overlap assemblage (sensu stricto) for the Inklin Formation may be inappropriate and misleading for portions of the northern Whitehorse trough. With regard to provenance linkages with the Nisling Terrane to the west, little can be said from the data gathered in this study. Metamorphic clasts are exceedingly rare in Inklin Formation conglomerate at DISCUSSION A N D CONCLUSIONS 220 Atlin Lake. If the Nisling Terrane was in contact with the western margin of Stikinia by Sinemurian or Pliensbachian time, it evidently lacked sufficient relief to act as a clast source. Figure 7.3. Schematic depiction of Laberge Group stratigraphy at Atlin Lake. The top of the Laberge Group is not found in the study area. The Laberge Group is interpreted to disconformably overlie the Upper Triassic Stuhini Group along the southwest basin margin. Basement relations with the Cache Creek Group are not known in the region - those shown here are based on the interpreted relations of basal Laberge Group (Inklin Formation) in the southern Whitehorse trough and may not be applicable to the northern Whitehorse trough. DISCUSSION A N D CONCLUSIONS 221 7.5 SPECULATIONS ON THE WHITEHORSE TROUGH Dickinson and Seely (1979) suggest that the presence of thin crust of oceanic and transitional character between the arc and trench is a prerequisite for deep water or large net subsidence in forearc regions. Consequently, it is their belief that deep or thick forearc basins such as the Whitehorse trough are most probably initiated as residual forearc basins and that these may evolve into composite basins through time (Fig. 7.4), as they infer is the case with prominent forearc basins in the Sunda and Banda arcs. Residual basins which originate in deep water may later become composite basins with a variety of configurations which can include ridged varieties (Fig. 7.5)(Dickinson and Seely, 1979). The structural high of the outer forearc rise is, along with the arc massif, a belt of uplift and thus a potential provenance area for the forearc basin. Protracted or episodic growth of the subduction complex can lead to the development of a ridged basin morphology. Such a progression can be envisioned for the Whitehorse trough. The thickness of strata within the trough and its dominantly marine character indicates a likely origin as a deep-water residual forearc basin. Evolution into a composite basin with a ridged morphology is suggested by Early Pliensbachian paleocurrent patterns and sandstone petrofacies in the study area. One may only speculate on the basement character of the northern Whitehorse trough at this point in time, but as acquisition of data proceeds it may soon be possible to address some of these fundamental questions. Although never stated explicitly, the Whitehorse trough is conventionally thought of and treated as a single coherent stratigraphic entity along its entire present strike length of ~ 700 km. This is in large part due to the broadly coeval nature of the strata deposited therein (although strata age range remains poorly constrained in significant portions of the Whitehorse trough) and the similarity of its stratigraphic and structural relationships along both margins. While the former basin paleomorphology may be indecipherable due to extensive structural disruption, it is apparent from its present geographic extent that it represents a depocenter of at least a few hundred kilometres length. The underlying assumption that Laberge Group strata represent a stratigraphically continuous body of sediments may a simplistic one in light of what is known of sedimentary basin dynamics in active arc systems, as well as from emerging data on the Early Jurassic tectonic and sedimentary history of the Laberge Group. It is difficult to reconcile the disparity in clast provenance along arc strike length with a DISCUSSION A N D CONCLUSIONS 222 Figure 7.4. Forearc basin types of Dickinson and Seely (1979). RIDGED FOREARCS Figure 7.5. Configurations of modern ridged forearc basins (from Dickinson and Seely 1979). DISCUSSION A N D CONCLUSIONS 223 single depositional unit. For example, clasts of Cache Creek provenance are documented in Inklin strata in the southern Whitehorse trough (Thorstad and Gabrielse, 1986) while similar provenance appears to be lacking in the northern Whitehorse trough. There, clasts are derived from western sources, at least during Sinemurian and Pliensbachian time, as in the south-central Yukon where, for instance, Upper Triassic Povoas Formation volcanic clasts are ubiquitous (q.v., Dickie, 1989) yet these same distinctive clasts are largely absent in westerly-derived coeval strata a short distance south in the study area (ch. 3, p. 92). Paleoflow systems appear to differ significantly along strike as well - from wholly westerly-derived in the Yukon (Dickie, 1989; Hart et al., in press) to westerlyrderived and episodically easterly-derived in the study area (Johannson, 1993; ch. 5), to predominantly easterly derived in the south (Thorstad and Gabrielse, 1986; Monger et al., 1992). One can argue that differing clast provenances merely reflect the influence of local sources upon deposition and this is undoubtedly true; however, it seems likely that more fundamental underlying causes are responsible for some of these differences. An awareness of the complex variations and behavior of modern arc systems is critical to paleotectonic analyses of ancient accreted-arc terranes. These complexities are perhaps best exemplified by the arcs of Indonesia and surrounding regions. Numerous studies in that region document major changes in the character of magmatic arcs along strike (e.g., Sunda and Banda Arcs), complex evolution of accretionary complexes (e.g., Sunda and Banda outer forearc ridges), discontinuous fore-arc basins (e.g. Banda forearc), and complex syn-and post-depositional structural histories (e.g., Sumba Island-Banda Arc, Marinduque Basin-Philippines) (q.v., Karig et al. 1980, Bowin et al., 1980; Hamilton, 1979, Sarewitz and Lewis, 1991). These features are all products of the active tectonism and magmatism that record the complex interactions of mega- and micro-plates in the region (Hamilton, 1988). Despite controvers/ over various interpretations, it is obvious that long-lived, steady-state subduction systems are atypical and that the history and kinematics can vary dramatically along strike in continuous arc complexes. Most arc-trench systems commonly include transform motion along arc trends and thus arc systems are often affected by strike-slip and oroclinal deformation (q.v., Ingersoll, 1988; Jarrad, 1986; Hamilton, 1988; Beck, 1983). The potential for complex evolution of arc systems is implicit, especially in zones of oblique convergence such as that interpreted for the Jurassic Cordillera (e.g., Engebretson et al., 1985) and acquires likelihood the DISCUSSION A N D CONCLUSIONS 224 larger and longer-lived the arc system. It is possible in such settings to envision a scenario in which a long narrow depocenter oriented along tectonic strike such as the Whitehorse trough becomes segmented into sub-basins during periods of its depositional history. Structural basin highs may form in response to regional or local tectonic events, creating sub-basins that may reflect predominantly local provenance and tectonic history. In this scenario a long, major depocenter like the Whitehorse trough may have semi-discrete segments that record slightly or even significantly different sedimentary histories from closely related, broadly coeval sub-basins. While, in broad terms, this may still be considered as a single depositional system there may be considerable differences in depositional history along strike. One may speculate on the applicability of a similar scenario for portions of the Whitehorse trough during its Early to Middle Jurassic history. The comparison of the Indonesian region and the Mesozoic Cordillera has been made before (q.v., Silver and Smith, 1983; Coney et al., 1980) but should be re-emphasized. While the details may have no broad applicability, the complex interplay of various crustal elements that include arcs and their associated marginal basins serves to illustrate the disparate and varied tectonic, magmatic and sedimentary histories within and between arc systems closely related in time and space. The Intermontane Belt is composed of two major, broadly coeval arc terranes (i.e., Quesnel and Stikine terranes) separated by the oceanic Cache Creek terrane which appears to also have contained an oceanic arc(s) (Thorstad and Gabrielse, 1986). These systems were interactive and evolving in a regime of obliquely convergent tectonics for much of their early Mesozoic history (q.v., Engebretson et al., 1985) and undoubtedly have complex histories. A extensive body of work already exists on many aspects of Intermontane terrane interaction (see Gabrielse and Yorath, 1992 for recent syntheses), but much remains to deciphered. The tectonostratigraphic significance of the Whitehorse trough in general, and the Laberge Group in particular, is an important element in deciphering terrane interactions in the northern Cordillera. The interpretation that the Laberge Group represents an overlap assemblage along the entire length of the Whitehorse trough may be simplistic and serves to mask the apparent complexity that attended its local and regional evolution. Whether the Laberge Group truly represents an overlap assemblage (sensu stricto) in DISCUSSION A N D CONCLUSIONS 225 the entirety of its stratigraphic and paleogeographic coverage is a question that remains to be answered. The evidence of this study would suggest that it is not, at least during its early history. 7.6 SUMMARY The northern Stikinian arc segment of this study underwent a complex tectonic and magmatic evolution during Early Jurassic time. Basin-fill records distinct depositional episodes punctuated by abrupt provenance shifts. Progressive arc dissection initiated in latest Triassic to earliest Jurassic time was interrupted by a major magmatic episode which culminated in widespread rejuvenated volcanism in the Early Pliensbachian. The volume of broadly coeval volcaniclastic debris deposited during this sub-stage delineates a distinct depositional episode in Early Jurassic basin-fill history and indicates a constructional phase in arc evolution, reversing the earlier progression toward a mature dissected magmatic arc. Renewed subsidence of the inner arc flanks was accompanied by broad uplift of outer basin margins as a consequence of tectonic processes related to subduction and/or sediment loading. These events caused a fundamental change in paleoflow systems from predominantly southwest-derived longitudinal paleoflow to opposed bi-directional radial or transverse paleoflow systems. The generation of a substantial northeast basin margin may represent an ephemeral or episodic ridged forearc phase in Early Jurassic basin history. The Late Pliensbachian marked a change from a regime dominated by volcanic processes to one dominated by active tectonics. Accelerated uplift of segments of the arc massif accompanied major intra-arc strike-slip faulting that led to rapid arc dissection and exhumation of comagmatic plutons. Continuing uplift in the Late Pliensbachian maintained high topographical gradients and the resulting influx of granitic detritus delineates another distinct depositional episode in Early Jurassic basin-fill history. Granitic elastics include a significant component derived from Pliensbachian plutons, underlining dramatic rates of uplift and arc incision during this time. 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A P P E N D I X 1 236 APPENDIX 1 INKLIN F O R M A T I O N A M M O N I T E S Loc.# Field # G S C # Eastings Northings Ammonite Identifications Age 1 GGAJ-92-76-F C-203263 578100 6578425 Fontanelliceras cf.fontanellense ? L . Plien. - K 2 GGAJ-92-63-FA C-203257 572125 6578250 Protogrammoceras nipponicum, Fontanelloceras cf. fontanellense, Arieticeras sp., Fuciniceras sp. Protogrammoceras sp. L . Plien  - K 3 GGAJ-92-63-FB C-203257 572125 6578250 Metaderoceras evolutum, M. talkeetnaense ?, M. cf. mouterdei ?, Tropidoceras cf. actaeon, Dubariceras cf. silviesi, Pseudoskirroceras sp. E. Plien. - W 4 GGAJ-92-66-FA C-203258 572925 6577600 Amaltheus stokesi, Fanninoceras sp., Protogrammoceras sp., Leptaleoceras sp. L . Plien. - K 5 GGAJ-92-66-FB C-203259 572925 6577600 Arieticeras algovianum, Protogrammoceras sp., Fanninoceras (Charlotticeras) sp.,Holcophylloceras sp. L . Plien. - K 6 GGAJ-92-57-F C-203243 571175 6577700 Arieticeras ? sp. L . Plien. 7 GGAJ-92-59-FA TD-85-F-30A C-203252 C-117204 571625 6578475 Metaderoceras talkeetnaense, M. evolutum, Acanthopleuroceras whiteavesi, Dubariceras silviesi, Gemmellaroceras sp., Coeloceras? sp. E. Plien. - W 8 GGAJ-92-59-FB C-203253 571650 6578475 Leptaleoceras cf. accuratum, Protogrammoceras sp., Arieticeras ? sp. L . Plien. - K 9 GGAJ-92-59-FC C-203254 571625 6578475 Dubariceras cf. freboldi, Oistoceras ? sp. E. Plien. - F 10 GGAJ-92-79-F C-203264 570225 6576300 Metaderoceras evolutum, Acanthopleuroceras cf. whiteavesi, Dubariceras cf. silviesi E. Plien. - W 11 GGAJ-92-43FA/B C-203226 568955 6574025 Juraphyllites ? sp., Weyla sp. Sin. - Plien. 12 GGAJ-92-52-F C-203236 568950 6574390 Pahechioceras cf. harbledownense L . Sin. - H 13 GGAJ-92-08-FA C-203201 566130 6574650 Amioceras miserabile, Amioceras sp. A E. Sin. - A 14 GGAJ-92-08-FB C-203202 566130 6574650 Amioceras miserabile, Amioceras sp. A , A. cf. speciosum E. Sin. - A 15 GGAJ-92-08-FC C-203203 566130 6574650 Amioceras miserabile, A. cf. oppeli E. Sin. - A 16 GGAJ-92-08-FD C-203204 566130 6574650 Amioceras cf. amouldi, A. cf. miserabile E. Sin. - A 17 GGAJ-92-08-FE C-203205 566130 6574650 Amioceras cf. speciosum E. Sin. - A 18 GGAJ-92-08-FF C-203206 566130 6574650 Amioceras cf. ceratitoides E. Sin. - A 19 GGAJ-92-27-F C-203222 566650 6574420 Amioceras sp., A. cf. speciosum ? E. Sin. - A? 20 GGAJ-93-7-FB C-203335 566785 6574150 Amioceras sp. E. Sin. - A APPENDIX 1 237 Loc.# Field # G S C # Eastings Northings Ammonite Identifications Age 21 GGAJ-93-7-FA C-203334 566950 6573925 Arnioceras cf. arnouldi E. Sin. - A 22 GGAJ-92-26-F C-203221 566950 6573730 Arnioceras sp. E. Sin. - A 23 GGAJ-92-07 C-203209 566800 6572875 Dubariceras silviesi, D. freboldi E. Plien. - F 24 GGAJ-92-14-FA C-203211 567090 6571925 Arnioceras sp., Epophioceras sp. E-L . Sin. - V 25 GGAJ-92-14-FB C-203212 567050 6571875 Arnioceras cf. ceratitoides, A. cf. miserabile,, A. sp. indet, Asteroceras cf. saltriense, Epophioceras sp., Gen. et sp. indet. E-L . Sin. - V 26 GGAJ-92-14-F C-203214 566940 6571850 Asteroceras cf. saltriense E-L . Sin. - V 27 GGAJ-92-14-FC C-203336 566940 6571850 Arnioceras cf. speciosum E-L. Sin. - V 28 GGAJ-92-22-F C-203331 567600 6570690 Arnioceras cf. speciosum E. Sin. - A ? V 29 GGAJ-92-111-F C-203296 565875 6568100 Reynesoceras cf. italicum, Protogrammoceras sp., Arieticeras cf. algovianum, Fanninoceras sp. L . Plien. - K 30 GGAJ-92-106-F C-203292 565300 6568000 Leptaleoceras cf. accuratum, Protogrammoceras sp, Fuciniceras ? sp. L . Plien. - K 31 GGAJ-92-114-F C-203297 566925 6566325 cf. Tropidoceras sp. E. Plien. 32 GGAJ-92- 115-F C-203298 566550 6565650 Metaderoceras cf. talkeetnaense, cf. Tropidoceras cf. actaeon E. Plien. - W? 33 GGAJ-92-116-FA C-203299 566325 6564750 Metaderoceras cf. talkeetnaense, Dubariceras cf. silviesi, Acanthopleuroceras cf. thomsoni, cf. Oistoceras sp. E. Plien. - W 34 GGAJ-92- 116-FB C-203300 566700 6564700 Tropidoceras actaeon E. Plien. - W 35 GGAJ-92-123-F C-203301 568125 6558025 Metaderoceras evolutum, M. sp., Gemmelloceras sp., Miltoceras sp., Polymorphites! sp. E. Plien. -1 36 GGAJ-92-89-F C-203276 562550 6562600 Metaderoceras cf. talkeetnaense E. Plien. - W? 37 GGAJ-92-102-BF C-203332 562725 6563050 Dubariceras cf. freboldi E. Plien. - F 38 GGAJ-92-83-F C-203265 563000 6562850 Dubariceras freboldi E. Plien. - F 39 GGAJ-92-83-FA C-203266 563150 6562800 Dubariceras freboldi, Metaderoceras talkeetnaense, E. Plien. - F 40 GGAJ-92-83-FB C-203267 563150 6562800 Dubariceras freboldi, Metaderoceras talkeetnaense, E. Plien. - F 41 GGAJ-92-83-FC C-203268 563150 6562800 Dubariceras freboldi, Phylloceras sp. E. Plien. - F 42 GGAJ-92-83-FD C-203269 563250 6562810 Dubariceras freboldi, Fanninoceras sp., cf. Leptaleoceras sp. L . Plien. - K 43 GGAJ-92-84-F TD-34-31aF C-203270 C-117318 563675 6562800 Leptaleoceras sp., cf. Reynesoceras sp., cf. Arieticeras sp., Protogrammoceras sp. L. Plien. - K 44 GGAJ-92-87-F C-203275 563175 6563550 Reynesoceras cf. colubriforme, Protogrammoceras cf. nipponicum, Arieticeras cf. algovianum L . Plien. - K 45 GGAJ-92-91-FA C-203278 562400 6563700 Protogrammoceras sp., cf. Fuciniceras sp. L . Plien. - K APPENDIX 1 238 Loc.# Field # G S C # Eastings Northings Ammonite Identifications Age 46 GGAJ-92-91-FB C-203279 562375 6563650 Reynesoceras colubriforme, Reynesoceras italicum, Protogrammoceras sp., Leptaleoceras accuratum L . Plien. - K 47 GGAJ-92-91-FC C-203280 562300 6563650 Reynesoceras colubriforme, Protogrammoceras nipponicum, P. sp., Arieticeras sp., Fontanelliceras sp. L. Plien. - K 48 GGAJ-92-91-FD C-203281 562300 6563650 Reynesoceras italicum, Reynesoceras cf. colubriforme L . Plien. - K 49 GGAJ-92-92-F C-203282 562125 6563775 Reynesoceras cf. colubriforme, Protogrammoceras sp., cf. Fontanelliceras sp. L. Plien. - K 50 GGAJ-92-93-F C-203284 562600 6563700 Fontanelliceras sp., cf. Protogrammoceras sp. L . Plien. - K 51 GGAJ-92-94-F C-203285 562150 6564175 Fanninoceras (Charlotticeras) cf. carteri L . Plien. - K 52 GGAJ-92-95-F C-203286 562750 6564700 Fuciniceras ? sp. L . P l i en , -K? 53 GGAJ-92-104-F C-203333 562625 6565200 cf. Reynesoceras sp., cf. Leptaleoceras sp. L . Plien. - K 54 GGAJ-92-98-F C-203287 562900 6564710 Protogrammoceras sp., cf. Arieticeras sp. L . Plien. - K? 55 GGAJ-92-100-F C-203288 563525 6564250 Gen. sp. indet. L? Plien. 56 GGAJ-92-86-F C-203274 563575 6563875 Gen. sp. indet. L? Plien. 57 GGAJ-92-103-AF C-203290 561750 6566200 Protogrammoceras cf. nipponicum ?, cf. Arieticeras sp., cf. Fanninoceras sp. L . Plien. - K 58 GGAJ-92-103-BF C-203291 561975 6566325 Reynesocers ? sp. L. Plien. - K? 59 GGAJ-92-133-F C-203308 559700 6569300 Arieticeras cf. algovianum .Protogrammoceras sp., Fontanelliceras juliae, Leptaleoceras ? sp. L . Plien. - K 60 GGAJ-92-135-F C-203309 557775 6570275 Reynescoeloceras sp., Reynesoceras ? sp. Plien. - F?K 61 GGAJ-92-138-FA C-203310 556900 6572300 Dubariceras ? sp. E. Plien. - F? 62 GGAJ-92-13 8-FB C-203311 556825 6572350 Dubariceras ci. freboldi E. Plien. - F 63 GGAJ-92-13 8-FC C-203312 556775 6572525 Dubariceras freboldi, Reynescoeloceras sp. E. Plien. - F 64 GGAJ-92-125-F C-203302 557975 6572750 Tropidoceras actaeon, Metaderoceras sp. E. Plien. - W 65 GGAJ-92- 126-F C-203303 558275 6572925 Reynesoceras inaequiomatum, Protogrammoceras sp. L . Plien. - K 66 GGAJ-92-127-F C-203304 558375 6573225 Reynesoceras colubriforme, cf. Arieticeras sp. L. Plien. - K 67 GGAJ-92-127-FA C-203305 558400 6573325 Protogrammoceras sp., Arieticeras sp., Leptaleoceras sp. L . Plien. - K 68 GGAJ-92- 127-FB C-203306 558475 6573450 Amaltheus cf. stokesi, cf. Protogrammoceras sp. L . Plien. - K 69 GGAJ-92-127-FC C-203307 558525 6573500 Amaltheus stokesi, Leptaleoceras cf. accuratum ?, Protogrammoceras sp., Arieticeras sp., cf. Fanninoceras sp. L . Plien. - K 70 GGAJ-92-143-F C-203313 559375 6573025 Leptaleoceras cf. accuratum, Fuciniceras ? sp. L . Plien. - K APPENDIX 1 239 Loc.# Field # G S C # Eastings Northings Ammonite Identifications Age 71 GGAJ-92-144-F C-203330 559700 6572975 Protogrammoceras sp. L . Plien. -72 GGAJ-92-145-F C-203328 559925 6572975 Metaderoceras cf. talkeetnaense ? E. Plien. - W? 73 GGAJ-92-146-F C-203329 560350 6572875 Metaderoceras cf. evolutum E. Plien. - W? 74 GGAJ-92- 147-F C-203314 560850 6573450 Tropidoceras actaeon, Acanthopleuroceras cf. thomsoni, Metaderoceras talkeetnaense, Oxytoma inequivalvis E. Plien. - W 75 GGAJ-92-148-F C-203315 561025 6574325 Dubariceras cf. freboldi E. Plien. - F 76 GGAJ-92-149-F C-203316 561500 6573975 Amaltheus stokesi L . Plien. - K 77 GGAJ-92- 177-F C-203320 562975 6576175 Asteroceras varians, Arnioceras cf. arnouldi L . Sin. - V 78 GGAJ-92- 178-FA C-203321 563050 6576300 Epophioceras ? sp. E. Sin. - A 79 GGAJ-92-178-FB C-202322 563175 6576400 Arnioceras miserabile, A. ceratitoides, Gen. et sp. indet. E. Sin. - A 80 GGAJ-92-179-F C-203323 563475 6577275 Arnioceras miserabile, Arnioceras ceratitoides, Arnioceras angusticostatus, Arnioceras cf. oppeli E. S in . - A 81 GGAJ-92- 189-F C-203326 566290 6579825 Arieticeras algovianum, Leptaleoceras accuratum, Protogrammoceras sp., Fanninoceras ? sp. L . Plien. - K 82 GGAJ-92-172-F C-203327 561150 6582825 Reynesoceras cf. inaequiornatum, Dubariceras freboldi, Metaderoceras cf. talkeetnaense E. Plien. - F 83 GGAJ-92-164-F 84- TD-67F 85- TD-F21 C-203319 C-117344 C-117179 557000 6579450 Dubariceras freboldi, Reynesoceras italicum, Reynesoceras inaequiornatum, Reynesoceras ? sp., Cymbites laevigatus, Gemmellaroceras sp., Protogrammoceras sp., Holcophylloceras sp., Fanninoceras ? sp., FontanelUceras ? sp. E. Plien. - F 84 GGAJ-92-153-F C-203317 557950 6573075 Amaltheus stokesi L . Plien. - K 85 GGAJ-92-160-F C-203318 554975 6576175 Reynescoeloceras sp., Metaderoceras talkeetnaense E. Plien. - F A P P E N D I X 2 240 A P P E N D I X 2 F O S S I L I F E R O U S C O N G L O M E R A T E C L A S T S Loc.# Field # G S C # Eastings Northings Fauna Age C l GGAJ-93-4-FA C-203258 572925 6577600 Pahechioceras harbledownense, Oxynoticeras sp., Lima? sp. L . Sin. - H C2 GGAJ-93-4-FB C-203258 572925 6577600 Posidontis semiplicata L . Sin. - L . Plien. C3 GGAJ-92-58-FA-1 C-203248 571675 6577325 Weyla sp., echioceratid ammonite, gastropods L? Sin. C4 GGAJ-92-58-FA-2 C-203248 571675 6577325 Pahechioceras sp., Weyla bodenbenderi?, Pinna sp., belemnoid L . Sin. - H? C5 GGAJ-92-58-FB C-203249 571675 6577325 Protocardia sp., Pholadomya sp., Modiolus sp., Pinna sp., Goniomya sp., Trigonia? sp., Gleviceras sp., Pahechioceras? sp., Epophioceras? sp. L . Si . C6 GGAJ-92-58-FC C-203250 571675 6577325 Dubariceras silviesi, Metaderoceras mouterdei, Oistoceras compressum, Protocardia ? sp. E. Plien. - W C7 GGAJ-92-57-FC C-203245 571175 6577700 Gastropods (Pyrgotrochus ?) E. Jurassic C8 GGAJ-92-57-FA-1 C-203243 571175 6577700 Weyla bodenbenderi?, Gressyla sp., Modiolus sp., Entolium sp. E. Jurassic C9 GGAJ-92-57-FA-2 C-203243 571175 6577700 Weyla bodenbenderi, Pentacrinus sp. E. Jurassic CIO GGAJ-92-57-FB C-203244 571175 6577700 Pleuromya sp., Metophioceras? sp. E? Sin. C l l TD-84-06-F C-117453 571175 6577700 Pahechioceras sp., Echioceras? sp., Gresslya sp., Pleuromya sp., Weyla bodenbenderi, W. alata?, Modiolus sp., Entolium sp. L . Sin. - H C12 GGAJ-93-1-FB C-203243 571175 6577700 Pahechioceras? sp., Eopecten? sp., Pinna? sp. L . Sin. - H? C13 GGAJ-93-12-FA C-203337 571775 6578175 Dubariceras silviesi, Acanthopleuroceras thomsoni, Metaderoceras talkeetnaense?, Oistoceras compressum, Pleuromya? sp., Modiolus? sp. E. Plien. - W C14 GGAJ-93-12-FB C-203337 571775 6578175 Coroniceras cf. bisculatum?, Modiolus? sp., belemnoid E. Sin. C15 GGAJ-92-52-FA C-203236 568950 6574390 Pahechioceras harbledownense L . Sin. - H C16 GGAJ-92-52-FB C-203237 568950 6574390 Monotis subcircularis L . Tri. - L . Norian C17 GGAJ-92-146-FB C-203329 560350 6572875 Halorella sp. L . Tri. -Norian? APPENDIX 3 APPENDIX 3 INKLIN FORMATION CLAST LITHOLOGIES S I N E M U R I A N Volcanic Clasts 1. Hornblende-feldspar latite 2. Green andesite 3. Rhyodacitic biotite-quartz-feldspar ('quartz eye') porphyry 4. Dark grey microdioritic andesite 5. Dacitic crystal dust tuff (crystal-lithic) 6. Quartz-feldspar dacite porphyry 7. Microporphyritic andesite/latite 8. Rhyodacite 9. Biotite-hornblende-feldspar dacite porphyry 10. Biotite-feldspar dacite 11. Rhyolite-trachyte 12. Muscovite-feldspar rhyodacite 13. Dacitic crystal-lithic fine lapilli tuff 14. Hornblende-feldspar dacite 15. Aphyric andesite Plutonic Clasts 1. Monzonite 2. Quartz monzonite(±biotite) 3. Quartz monozodiorite 4. Monzogranite 5. Biotite quartz monzogranite 6. Biotite-hornblende quartz monzodiorite 7. Biotite-hornblende quartz monzonite Sedimentary Clasts 1. Finely laminated black silty mudstone 2. Massive fine siltstone 3. Impure (muddy) dark grey microcrystalline limestone 4. Fine sandy mudstone 5. Massive silty lime mudstone 6. Dark grey pervasively bioturbated silty lime mudstone 7. Fine grained greywacke 8. Medium brown crystalline limestone 9. Medium brownish grey bioclastic microcrystalline (recrystallized) limestone 10. Light brownish grey bioclastic micritic wackestone 11. Medium-dark brown crystalline limestone 12. Recrystallized crinoidal packstone (rare) 13. Molluscan (Monotis) rudstone (rare) 14. Light brown chert (rare) APPENDIX 3 242 L O W E R P L I E N S B A C H I A N Volcanic Clasts 1. Porphyritic hornblende-feldspar dacite 2. Porphyritic biotite-feldspar dacite 3. Andesite-microphyric/packed-massive 4. Hornblende-plagioclase dacite 5. Hornblende-plagioclase dacite 6. Trachytic dacite/andesite (latite) (micro-phyric) 7. Hornblende-biotite-feldspar dacitic porphyry 8. Felsic mylonite (rare) 9. Hornblende-feldspar dacite porphyry 10. Massive andesite 11. Rhyodacitic quartz-feldspar lithic-crystal lapilli tuff 12. Andestitic/dacitic hornblende-feldspar crystal dust tuff 13. Biotite-quartz rhyodacite porphyry (hypabyssal) 14. Rhyolitic crystal-lithic coarse lapilli tuff 15. Rhyodacite quartz porphyry (quartz > 20%) Plutonic Clasts 16. Biotite monzodiorite 17. Biotite-hornblende quartz monozonite 18. Biotite monzogranite Sedimentary Clasts 19. Laminated black lime mudstone (with stratifugic pyrite) 20. Recrystallized bioclastic floatstone 21. Silty lime mudstone 22. Calcareous black muddy siltstone 23. Brachiopod (Hallora) rudstone (rare) U P P E R P L I E N S B A C H I A N Volcanic Clasts 1. Hornblende-feldspar dacite porphyry 2. Biotite-feldspar dacite porphyry 3. Augite-plagioclase porphyitic andesite 4. Feldspar fine crystal lapilli tuff 5. Rhyolitic quartz-feldspar porphyry 6. Rhyodacitic biotite-feldspar porphyry (megacrystic K-spar) 7. Biotite-hornblende-quartz-feldspar rhyodacite (quartz-eye) porphyry 8. Dacitic crystal-lithic lapilli tuff 9. Biotite-feldspar rhyodacite porphyry 10. Rhyolite porphyry 11. Rhyodacitic biotite-quartz-feldspar fine crystal lapilli tuff 12. Biotite-feldspar dacite Plutonic Clasts 1. Biotite quartz monzonite 2. Biotite monzogranite 3. Biotite quartz monozodiorite 4. Hornblende-biotite granodiorite 5. Hornblende quartz monozodiorite 6. Hornblende-biotite monzogranite APPENDIX 3 243 7. Biotite granite 8. K-spar megacrystic biotite granite 9. Quartz syenite 10. Alkal i feldspar granite 11. Biotite-hornblende granodiorite 12. Hornblende-biotite quartz monzonite Sedimentary Clasts 1. Black muddy limestone 2. Finely laminated black lime mudstone 3. Medium grey massive microcrystalline limestone 4. Laminated black silty limestone 5. Stratified muddy siltstone 6. Dark grey microcrystalline limestone 7. Laminated silty microcrystalline limestone 8. Dark grey massive silty lime mudstone 9. Graded muddy brown silt limestone 10. Light brownish grey microcrystalline limestone 11. Muddy molluscan rudstone 12. Silty skeletal floatstone 13. Muddy gastropod rudstone (rare) 14. Stratified bioclastic grainstone ('coquina') (rare) 15. Skeletal mudstone 16. Muddy carbonaceous floatstone (rare) 17. Gritty to pebbly incompetent green greywacke 18 Grey chert (rare) APPENDIX 4 244 Interval Sci e «= mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct D i r e c t INTERVAL # % Radius (mm) GGAJ-92-13 A 33 7 7 >0 30 3 19 22 11 15 15 31 60 2 13 18 SE. ATLIN L K . 16 20 20 61 90 2 13 18 12 32 32 91 120 0 0 0 mid SINEMURIAN 3 50 50 121 150 l l l l i i i i i i 0 Varians Zone 12 67 67 151 180 0 i i i i 0 12 69 69 181 210 i i i i i i i i i i 0 10 284 284 211 240 i l l ! i i i i i 0 15 306 306 241 270 0 0 0 17 326 326 271 300 1 6 13 27 329 329 301 330 4 25 25 21 330 330 331 360 4 25 25 18 335 335 missed = 0 26 338 338 n= 16 23 352 352 14 356 356 Vector 358 ### Mean ### ### Vector Magnitude (%] l l l l l l 77 Rayleigh Significance- 7.23E-05 Circular Standard Dev llllll 38 Circular Variance » 1439 Interval Scate = 5 mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct D i r e c t INTERVAL # % Radius (mm) GGAJ-92-13B 7 126 126 >0 30 l l l l i I I I ! ® 0 10 159 159 31 60 IIIII llllll 0 SE. ATLIN L K . 8 185 185 61 90 1111 IIIII 0 91 120 IIIII i i i i i 0 mid SINEMURIAN 121 150 l l l l i i i i i 29 Varians Zone 151 180 l l l l i i i i i 29 181 210 i 33 29 Bimodal 211 240 0 0 0 P a r t i t i o n 241 270 0 0 0 271 300 0 0 0 301 330 0 0 0 331 360 0 0 0 mxssed = 0 n= 3 Vector ### ### Mean ### 157 Vector Magnitude (%] llllll 91 Rayleigh Significance = 8.19E-02 Circular Standard Dev fflllll 22 Circular Variance = 501 A P P E N D I X 4 245 SITE GGAJ-92-45-1 G r i f f i t h Island E? SINEMURIAN | Interval Scale = mmmm mm PALEOCURRENT Rounded ROSE Interval | Dip Di r e c t D i r e c t INTERVAL 111!! I ! ! ! ! Radius ( m) 8 7 7 >0 30 l i l l l : 11114! 18 14 14 14 31 60 l l l l l l l l l l l l 11 9 16 16 61 90 l i l l l l i l l l 0 11 38 38 91 120 l l l l l l i i i i i 0 13 256 256 121 150 l l l l l l l l l l l l 0 8 294 294 151 180 l i i i i l i l l l 0 19 296 296 181 210 i i i i i i i i i i 0 19 303 303 211 240 l l l l l l l i l l l 0 ! 18 309 309 241 270 l l l l l l l i l l l 11 14 312 312 271 300 ! ! ! ! § • l i l l l 15 21 314 314 301 330 i i i l l i 1 1 1 ! 34 11 315 315 331 360 l l l l l l i i i i 24 22 316 316 missed = l l l l l l 14 320 320 n= i i i i i 18 322 322 16 322 322 Vector 329 ### 30 323 323 Mean ### ### 16 336 336 23 338 338 Vector Magnitude (%) IIIII! 86 13 341 341 Rayleigh Significance = 8.22E-08 28 343 343 Circular Standard Dev l l l l l l ! ! 29 20 360 360 Circular Variance = 847 interval Scale — mmmm mm PALEOCURRENT Rounded ROSE Interval 1 SITE Dip Direct D i r e c t INTERVAL i l l l i l l l Radius (mm) GGAJ-92-45-1B 12 146 146 >0 : 30 i l l l Q i 0 6 151 151 31 i l l l i l l l IIIII 0 GRIFFITH ISLAND 12 167 167 61 i l l I I I I l i l l l ; 0 24 177 177 91 i l l H i l l l i l l l 0 E? SINEMURIAN 7 182 182 121 i l l l l l l l l l l l l l l 22 151 180 3 60 39 181 210 1 20 22 Bimodal 211 240 l l l l l l l l l l l l 0 P a r t i t i o n 241 270 l i l l l i i i i i 0 271 300 i S i l l i l i ! 0 301 i n S i l l IIIII 0 331 360 l i l i 0 0 missed = i l l ® n= 1111! Vector ### ### Mean ### l i l i Vector Magnitude (%) l l l l l l l l ? Rayleigh Signil icance = 9.06E-03 Circular Standard Dev 11 Circular Variance -- i i i i i i i i i APPENDIX 4 246 S I T E G G A J - 9 2 - 5 5 E . A T L I N L K . E? P L I E N S . Interval Scate = mm PALEOCURRENT Rounded ROSE Interval | D i p D i r e c t D i r e c t INTERVAL # % Radius (mm) 23 175 175 >0 30 l l l l i 0 0 64 196 196 31 60 l l l l i 0 0 41 202 202 61 90 l l l l i 0 0 27 203 203 91 120 l i l p IIIII 0 59 218 218 121 t i l i i i i i i i i 0 31 218 218 151 180 i i i i i l i i l 19 12 280 28,0 181 i l l i i i i i H i ! 33 211 i l l l l l l l l ; i l l ! 27 241 270 l l l l i 0 0 271 300 l i i l 14 19 301 i l l 0 0 0 331 360 0 0 0 m i s s e d = 0 n= 7 V e c t o r ### ##/ Mean 211 ### Vector Magnitude {%] l l l l l l S7 Rayleigh Significance: 4.96E-03 Circular Standard Dev i l l ! ! ! 28 Circular Variance — 7S4 | Interval Scale — mm PALEOCURRENT Rounded ROSE Interval | S I T E D i p D i r e c t D i r e c t INTERVAL i i i i i 111! ! Rodius (mm) G G A J - 9 2 - 5 6 8 74 74 >0 30 i i i i ; i i i i 0 19 88 88 31 60 l l l l i l l l l l l : 0 JANUS POINT 23 105 105 61 90 l l l l l l l l l l i 20 17 108 108 91 120 3 i l l l 24 E . P L I E N S . 27 113 113 121 150 l l l l l l l l l l i 20 28 126 126 151 180 l l l l i I l l l 28 21 136 136 181 210 l i l i l l l l i 14 24 159 159 211 240 l l l l i i i i i i ; 0 18 159 159 241 270 l l l l i l l l l l l 0 23 166 166 271 300 l l l l l l l l l l l l 0 10 171 171 301 330 i i i i l i i l 14 21 202 202 331 360 0 0 0 13 329 329 m i s s e d = n= 0 13 V e c t o r ### ### Mean ### i i i i i Vector Magnitude (%) l l l l l l l § 3 Rayleigh Significance: 6.13E-03 Circular Standard Dev i i i i i i 49 Circular Variance = 2408 APPENDIX 4 247 Interval Scale = mm PALEOCURRENT Rounded ROSE Interval | S I T E D i p D i r e c t D i r e c t INTERVAL ill!! Radius (mm) G G A J - 9 2 - 6 1 12 77 77 >0 i i i IIII® I i i i i 0 9 83 83 31 111 lilll Illi: 0 N E . A T L I N L K . 17 130 130 61 111 i l l ! llllll 29 25 143 143 91 11! III! llllll 0 L . P L I E N S . 20 171 171 121 t i ! lilll Hill 29 Kunae Zone 14 341 341 151 i l l lilll lilll 20 181 i l l i l l i i l l ! 0 211 240 l i l i i l l ! 0 241 270 lilll l i l i 0 271 300 0 llllll 0 301 i l l lilll lili; 0 331 360 i i i i 17 20 m i s s e d = 0 n= l i l i V e c t o r ### ### Mean *## 110 Vector Magnitude (%) i i i i i 56 Rayleigh Significance = 1.52E-01 Circular Standard Dev §111111; 53 Circular Variance = 2843 Interval Scale <=••;• 5 mm PALEOCURRENT Rounded ROSE Interval | S I T E l i i i i i i D i r e c t D i r e c t INTERVAL Illi llllll Radius (mm) G G A J - 9 2 - 6 3 4 21 21 >0 30 llllll iiiii: 18 19 80 80 31 60 11111 i l l i 0 N E . A T L I N L K . 25 110 110 61 90 i i i i lilll 18 13 139 139 91 120 i l l i lilll 18 L . P L I E N S . 23 148 148 121 150 l i l i 25 25 Kunae Zone 17 152 152 151 180 illlll llllll 18 10 208 208 181 210 i i i i 1:13 18 59 341 341 211 240 l i l i i i i i 0 241 270 i i i i l i l i 0 271 300 IIIII lilll 0 301 330 0 l i l i 0 331 360 1 lilll 18 m i s s e d = i i i i n= Il li V e c t o r ### ### Mean ### 118 Vector Magnitude (%) illlll 44 Rayleigh Significance = 2.08E-01 Circular Standard Dev 1111111 60 Circular Variance s= 3622 APPENDIX 4 248 Interval Scale = • : mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct Direct INTERVAL # % Radius (mm) GGAJ-92-79A 22 110 110 >0 30 0 0 0 17 119 119 31 60 0 0 0 NE. ATLIN LAKE 29 122 122 61 90 0 0 0 21 146 146 91 120 2 11 16 E. PLIENS. 44 153 153 121 150 2 11 16 Whiteavesi Zone 13 170 170 151 180 S l l l l i 26 17 173 173 181 210 6 I l l l 28 16 179 179 211 240 4 •11 23 15 180 180 241 270 0 I i i i 1 0 11 181 181 271 300 l l l l i ; i l l l 0 12 184 184 301 330 i i i i i i i i 0 21 188 188 331 360 l l l l i l l l l l l 0 20 189 189 missed = I I S 22 189 189 n= l l l l i 24 194 194 6 211 211 Vector ### ### 25 215 215 Mean ### 177 4 215 215 25 217 217 Vector Magnitude (%] i l l l l l ! 86 Rayleigh Significance: 8.12E-07 Circular Standard Dev #11111 29 Circular Variance = 860 . - . . 1 Interval Scale = mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct Direct INTERVAL # % Radius (mm) GGAJ-92-79B 17 8 8 >0 30 l 20 22 11 31 31 31 60 l 20 22 NE. ATLIN LAKE 27 69 69 61 90 i 20 22 22 304 304 91 120 0 0 0 E. PLIENS. 12 339 339 121 150 0 0 0 Whiteavesi Zone 151 180 0 0 0 181 210 0 0 0 Bimodal 211 240 0 0 0 Partition 241 270 0 0 0 271 300 0 0 0 301 330 1 20 22 331 360 1 20 22 missed = 0 n= 5 Vector ### I I I I6 Mean ### ### Vector Magnitude (%) P i l l 74 Rayleigh Significance: 6.35E-02 Circular Standard Dev §11111 40 Circular Variance — 1635 APPENDIX 4 249 | Interval Sea e = mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct Direct INTERVAL # l i l l l Radius (mm) GGAJ-92-83FD 11 55 55 >0 30 0 l i i i o i 0 23 57 57 31 60 2 l l l l l l 17 SLOKO ISL. 26 74 74 61 90 4 i i i i 24 25 87 87 91 120 l i l l l i i i i 31 L. PLIENS. 20 90 90 121 150 i i i i 1 1 1 ! 26 Freboldi-Kunae 15 90 90 151 180 l l l l l l I l l i : 0 Zone boundary 24 95 95 181 210 l i l i 0 0 34 108 108 211 240 l i l l l 0 0 14 110 110 241 270 0 I l l l l 0 28 111 111 271 300 0 0 0 18 117 117 301 330 0 0 0 22 119 119 331 360 0 0 0 17 119 119 missed = 0 11 121 121 n= 18 19 125 125 9 128 128 Vector ### ### 23 140 140 Mean ### 106 28 149 149 Vector Magnitude (%] l l l l l l i i i i i i i 9 | Rayleigh Significance 3 81E-07 Circular Standard Dev l l l l l l 23 Circular Variance = 547 Interval Scale « 1 mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct Direct INTERVAL # % Radius (mm) GGAJ-92-87 27 2 2 >0 30 9 50 35 12 8 8 31 60 i l l l l 28 26 SLOKO ISL. 18 9 9 61 90 l i l l l 11 17 16 11 11 91 120 I i i i i i i i 0 L. PLIENS 19 17 17 121 150 0 i l l i 0 Kunae Zone 12 18 18 151 180 0 l i l i ! 0 7 18 18 181 210 0 l i l i 0 12 26 26 211 240 0 0 0 25 28 28 241 270 0 0 0 14 32 32 271 300 0 0 0 19 35 35 301 330 0 0 0 16 35 35 331 360 2 11 17 14 36 36 missed = 0 13 57 57 n= 18 22 61 61 18 75 75 Vector ### 25 20 353 353 Mean ### ### 20 359 359 Vector Magnitude (%] l l l l l l 93 Rayleigh Significance 1.62E-07 Circular Standard Dev 111!!; 19 Circular Variance ~ 376 APPENDIX 4 250 ill«lli mm PALEOCURRENT R o u n d e d ROSE Interval | SITE D i p D i r e c t D i r e c t INTERVAL iiii IIIII Radius {mm) GGAJ-92-83 11 25 25 >0 HIS! l l l l i 1119] 15 12 46 46 31 60 1 ;!!!!; 15 SLOKO ISLAND 21 86 86 61 Illl lllli 18 21 13 88 88 91 120 3 llllll 26 E . PLIENS. 13 98 98 121 1 1 1 Illl illl 30 F r e b o l d i Z o n e 18 115 115 151 i i i iiii iiii! 0 15 120 120 181 i i i 0 0 0 26 122 122 211 ill; IIIII 0 0 11 123 123 241 270 lllli iiiii; 0 24 124 124 271 300 0 lllli 0 9 136 136 301 ill; Illl lllli 0 331 iii iiii iiii 0 m x s s e d = ii!ii : n i l l i i i i V e c t o r ### ### M e a n ### illl -Vector Magnitude (%) iliiiiii i i l l l l l l! Rayleigh Significance = 4.04E-04 Circular Standard Dev =111111 31 Circular Variance = illllllll SITE GGAJ-92-88 SLOKO ISL. E . PLIENS. F r e b o l d i ? Z o n e | Interval Scale = mmmm mm PALEOCURRENT R o u n d e d ROSE Interval | D i p D i r e c t D i r e c t INTERVAL lllli illlll Radius ( m) 4 71 71 >0 30 iiiii III©; 0 14 79 79 31 60 IIII llllll 0 14 84 84 61 90 t i l l ! illl 18 20 93 93 91 120 IIIII lllli 23 22 100 100 121 150 iiii lllli 33 19 102 102 151 180 iiiii 22 23 12 104 104 181 210 111! 0 . 0 17 117 117 211 240 iiiii 0 0 13 121 121 241 270 iiiii 0 0 12 122 122 271 300 IIIII 0 0 17 123 123 301 330 lllli 0 0 14 124 124 331 360 IIIII 0 0 21 128 128 m i s s e d = 0 26 129 129 n= 23 15 144 144 5 148 148 V e c t o r ### IIIII 13 149 149 M e a n ### 126 17 150 150 13 154 154 Vector Magnitude (%) illiil iliiiiii? 23 156 156 Rayleigh Significance: 1 86E-08 16 160 160 Circular Standard Dev llllll! 27 17 164 164 Circular Variance illliiiii 19 180 180 APPENDIX 4 251 Interval Scale = 5 mm P A L E O C U R R E N T R o u n d e d R O S E Interval | S I T E D i p D i r e c t D i r e c t I N T E R V A L # % Radius {mm) G G A J - 9 2 - 9 1 11 13 13 >0 3 0 2 2 5 25 3 0 30 30 31 6 0 1 13 18 S l o k o I s l a n d 27 36 36 61 9 0 l l l l l l i i i i 0 14 92 92 9 1 1 2 0 3 l i l i 31 L . P L I E N S . 16 113 113 121 1 5 0 0 11111 0 K u n a e Z o n e 14 1 1 4 114 151 1 8 0 i i i i i i l l i 25 23 153 153 181 2 1 0 0 0 0 2 1 154 154 2 1 1 2 4 0 0 0 0 2 4 1 2 7 0 0 0 0 2 7 1 3 0 0 0 0 0 301 3 3 0 0 0 0 3 3 1 3 6 0 0 0 0 m i s s e d = 0 n= 8 V e c t o r ### 9 0 M e a n ### ### Vector Magnitude (%1 l l l l l l 63 Rayleigh Significance: 4.19E-02 Circular Standard Dev i l l l l 49 Circular Variance ~ 2383 Interval Scale = mm P A L E O C U R R E N T R o u n d e d R O S E Interval J S I T E D i p D i r e c t D i r e c t I N T E R V A L # I l l l l Radius (mm) G G A J - 9 2 - 9 4 19 1 1 >0 i i i i i i i 11121: 24 14 4 4 31 l ! l i l i i i i i i 12 S L O K O I S L . 18 8 8 61 1 1 1 ! l l l l l l I I I ! 21 15 13 13 9 1 § 1 1 I l l l l i i i i 24 L . P L I E N S . 14 55 55 121 i l l I l l l l l l 12 K u n a e Z o n e 21 77 77 1 5 1 1 8 0 0 l l l l l l 0 19 79 79 1 8 1 1 1 1 i l l l l I I I ! 0 20 89 89 2 1 1 i l l i i i i l l l l l l ! 0 6 93 93 2 4 1 2 7 0 i l l i I I I ! 0 17 104 104 2 7 1 3 0 0 i i i i i l l ! 0 18 1 1 4 1 1 4 3 0 1 3 3 0 0 l i l l l ! 0 22 1 1 8 1 1 8 3 3 1 3 6 0 4 IIII 24 21 134 1 3 4 m i s s e d = 0 18 3 3 3 3 3 3 n = i i i i i 10 334 3 3 4 15 343 343 V e c t o r ### 11147 ; 11 349 349 M e a n ### ### Vector Magnitude (%) 11111 59 Rayleigh Significance: 2.68E-03 Circular Standard Dev §11!!! 51 Circular Variance = 2646 APPENDIX 4 252 S I T E GGAJ-92-100 E . S l o k o I s l . E . P l i e n s . 1 Interval Scale - mmm mm PALEOCURRENT R o u n d e d ROSE Interval | D i p D i r e c t D i r e c t INTERVAL iiiii llllll Radius (mm) 15 16 16 >0 30 1112: ilia; 32 20 29 29 31 60 llllll •IIIII 0 25 62 62 61 90 llllll 1141 32 15 70 70 91 120 llllll Illl 0 10 328 328 121 150 0 Illl 0 151 180 llllll IIIII 0 181 210 0 llllll 0 211 240 0 lit! 0 241 270 0 Illl! 0 271 300 0 fill 0 301 330 1 i l l l 22 331 360 0 i l l l 0 m i s s e d = I I I I I n= lllli V e c t o r ### III30: M e a n ### ### Vector Magnitude (%) = 81 Rayleigh Significance = 3.74E-02 Circular Standard Dev § 1 1 1 1 34 Circular Variance — 1181 Interval Scate = 5 mm P A L E O C U R R E N T R o u n d e d R O S E Interval | D i p D i r e c t D i r e c t I N T E R V A L i i i i llllll Radius (mm) 14 15 15 >0 iii; IIIII 1 1 2 1 : 25 27 72 72 31 111 i i i i llllli 0 17 79 79 61 illl lllli i l l l 35 18 320 320 91 i l l lllli l i i l 0 121 111 l i i l lllli 0 151 180 llllli IIIII 0 181 111 l i i l Illl 0 211 111 l i i l iiiii 0 241 270 l i i l i i i i 0 271 3Q0 l i i l i i i i 0 301 111 IIII i i i i 25 331 360 i i i i Illl! 0 m i s s e d = i l l ® n= i i i i V e c t o r ### 1:35 M e a n ### iiiii Vector Magnitude (%) lillll 68 Rayleigh Significance- 1.58E-01 Circular Standard Dev = 45 Circular Variance — 2055 S I T E GGAJ-92-103A NW. S l o k o I s l . L . P l i e n s . APPENDIX 4 253 Interval Scale = 5 mm PALEOCURRENT Rounded ROSE Interval | SITE l l l l l l l i l D i r e c t D i r e c t INTERVAL l i l i I l l l l Radius (mm) GGAJ-92-110 20 5 5 >0 i i i i i 11127 26 19 13 13 31 i l l l i l i I i i i i 0 Bast ion I s land 19 22 22 6 1 111! l i l i i i i i 0 21 124 124 9 1 11! 0 1 1 1 ! 0 L . P l i e n s . 16 304 3 0 4 121 i l l I l i l l l 15 Kunae Zone 17 324 324 151 1 8 0 l l l l l l 1 1 1 ! 0 28 3 3 0 3 3 0 181 111 l i l i i l l ! 0 10 3 3 0 3 3 0 2 1 1 l l ! I I I ! 1111! 0 14 343 343 241 11! i i i i i l l ! 0 19 346 3 4 6 2 7 1 11! i i i i 1 1 1 ! 0 23 347 347 3 0 1 11! i l i l I l l i 30 3 3 1 i l l lili! l i l i 26 missed = i i i i i n= I I Vector 349 ### Mean ### ### Vector Magnitude (%) i l l i i l i 77 Rayleigh Significance- 1.40E-O3 Circular Standard Dev 38 Circular Variance = 1432 Interval Scale <= mmmm mm PALEOCURRENT Rounded ROSE Interval | SITE Dip D i r e c t D i r e c t INTERVAL l i l l l I l l l l Radius ( m) G G A J - 9 2 - 1 1 1 27 2 2 >0 i i i ililiSi i i i i i 22 27 22 22 3 1 111! 4 i l l l l 26 Bast ion I s land 20 25 25 61 1 1 ! i i i i i i 1 1 1 ! 0 15 35 35 91 11! i i i i i I l l i 0 L . P l i e n s . 17 41 41 1 2 1 11! i l l i i 0 0 22 56 56 1 5 1 180 l i l i I l l l l ! 0 Kunae Zone 22 56 56 1 8 1 2 1 0 0 0 0 20 2 8 0 2 8 0 2 1 1 2 4 0 0 0 0 15 282 282 2 4 1 2 7 0 0 0 0 13 3 1 1 3 1 1 2 7 1 3 0 0 2 1 3 18 17 3 1 7 3 1 7 3 0 1 3 3 0 4 27 26 13 322 322 3 3 1 3 6 0 2 1 3 18 . ,34 324 324 missed = 0 22 342 342 n= 15 24 343 343 Vector 352 ### Mean ### ### Vector Magnitude (%) 111111 i i i i i i i t i Rayleigh Significance: 3.63E-04 Circular Standard Dev i l l l l ! 42 Circular Variance = 1740 APPENDIX 4 SITE GGAJ-92-116A SE.ATLIN LAKE E . PLIENS. W h i t e a v e s i Z o n e | Interval Scale = mm PALEOCURRENT R o u n d e d ROSE Interval | D i p | 17 D i r e c t D i r e c t INTERVAL # i i i i Radius (mm) 31 31 >0 30 l l l l i 0 6 33 33 31 60 i l l ! l l l l i 25 17 39 39 61 90 9 I l l l 27 28 44 44 91 120 i l l l IIIII 25 12 50 50 121 150 IIIII i i i i 20 20 50 50 151 180 i i i i i l l l 9 9 50 50 181 210 i i i i 0 0 22 59 59 211 240 0 l l l l l i 0 22 61 61 241 270 0 0 0 48 66 66 271 300 0 0 0 37 70 70 301 330 0 0 0 18 70 70 331 360 0 0 0 35 74 74 m i s s e d = 0 19 77 77 n= 31 16 82 82 57 83 83 V e c t o r ### 76 45 83 83 M e a n ### ### 35 91 91 19 94 94 Vector Magnitude (%] llllll 84 14 102 102 Rayleigh Significance: 2.55E-10 10 102 102 Circular Standard Dev ill!!! 31 20 108 108 Circular Variance = 960 12 109 109 29 110 110 26 119 119 1 19 125 125 8 128 128 1 6 145 145 26 146 146 4 147 147 32 164 164 APPENDIX 4 Interval Scale = 5 mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct D i r e c t INTERVAL # % Radius (mm) GGAJ-92-116B 29 188 188 >0 30 0 0 0 2 208 208 31 60 I l l l l 0 0 SE. ATLIN LAKE 5 230 230 61 90 i l l i 0 0 16 238 238 91 120 i l l i i l l l l 0 E. PLIENS. 26 296 296 121 150 l l l l l l 1 1 1 ! 0 Whiteavesi Zone 151 180 0 0 0 181 210 2 40 32 Bimodal 211 240 2 40 32 P a r t i t i o n 241 270 0 0 0 271 300 1 20 22 301 330 0 0 0 331 360 0 0 0 missed = 0 n= i l l s Vector ### ### Mean 230 ### Vector Magnitude (% l l l l l l 81 Rayleigh Significance: 3.67E-02 Circular Standard Dev i l l l l 34 Circular Variance = 1166 Interval Scale = mmmm mm PALEOCURRENT Rounded ROSE Interval | SITE Dip Direct D i r e c t INTERVAL 11111 11111 Radius ( m) GGAJ-92-147B 22 3 3 >0 i l l I l l l l : ; ;:;!5A 35 23 6 6 31 i l l i IIII i l l i 0 COPPER ISLAND 21 9 9 61 1 1 1 i l l l l l i l i 0 26 11 11 91 i i i i i i l l l i l ! 1 1 1 ! 0 E. PLIENS. 24 12 12 121 i l l i i i i : I I I I ! 0 Whiteavesi Zone 35 15 15 151 180 I l l l l m i l 0 13 24 24 181 210 1 1 1 ! IIIII 0 Bimodal 14 284 284 211 240 I i i i i 1 1 1 ! 0 P a r t i t i o n 20 317 317 241 270 l i l i IIIII 0 16 341 341 271 300 i i i i i 13 1 30 349 349 301 330 i 7 13 23 356 356 331 360 5 36 30 11 357 357 missed = 0 19 359 359 n= 14 Vector 356 ### Mean ### ### Vector Magnitude (%) i i i i l l 91 Rayleigh Significance: 9.13E-06 Circular Standard Dev l i i i i i i 23 Circular Variance — 519 APPENDIX 4 S I T E G G A J - 9 2 - 1 4 7 A C O P P E R I S L A N D E . P L I E N S . W h i t e a v e s i Z o n e 1 Interval Scale = mmm mm P A L E O C U R R E N T R o u n d e d R O S E Interval | D i p D i r e c t D i r e c t I N T E R V A L IIII % Radius (mm) 27 98 98 >0 30 Illl® 1111:0: 0 2 1 99 99 3 1 60 0 l l l l i 0 17 1 0 3 1 0 3 6 1 90 • I t • i i 0 18 1 0 3 1 0 3 9 1 1 2 0 l l l l i 26 25 13 1 0 5 1 0 5 1 2 1 1 5 0 I I I ! i i i i 32 9 1 0 9 1 0 9 1 5 1 1 8 0 7 l l l l i 22 8 1 1 0 1 1 0 1 8 1 2 1 0 l l l l i 1 4 19 17 1 1 3 1 1 3 2 1 1 2 4 0 IIIII 0 0 6 1 1 4 1 1 4 2 4 1 2 7 0 i i i i 0 0 10 1 2 5 1 2 5 2 7 1 3 0 0 i i i i IIII 0 25 126 1 2 6 3 0 1 330 0 6 0 24 132 1 3 2 3 3 1 3 6 0 0 l l l l i 0 15 133 1 3 3 m i s s e d = Illl® 30 133 1 3 3 n= 11135 23 133 1 3 3 16 1 3 3 1 3 3 V e c t o r ##/ ### 16 1 3 3 1 3 3 M e a n ### i l l i 31 1 3 6 1 3 6 21 1 3 9 1 3 9 Vector Magnitude (%) :;illll 62 49 1 4 2 1 4 2 Rayleigh Significance = 1.52E-06 18 1 4 3 1 4 3 Circular Standard Dev = 50 17 146 1 4 6 Circular Variance = 2457 8 146 1 4 6 18 154 1 5 4 •—• 16 1 6 0 1 6 0 13 164 1 6 4 4 6 166 168 1 6 6 1 6 8 18 171 1 7 1 17 177 177 7 184 1 8 4 8 186 1 8 6 12 188 1 8 8 11 192 1 9 2 17 2 0 1 2 0 1 APPENDIX 4 Interval Scale = mm PALEOCURRENT Rounded ROSE Interval | SITE Dip D i r e c t D i r e c t INTERVAL i i i i 111! ! Radius (mm) GGAJ-92-35 15 20 20 >0 1130: l l l l i 11113! 18 11 151 151 31 111 I l l l l l l l l l 0 GRIFFITH ISL. 9 187 187 61 I l l l l l l l l i !!I!I 0 16 242 242 91 111 I l l l § 1 1 1 0 E.PLIEN? 18 259 259 121 i l l 0 l i i l 0 25 270 270 151 180 I l l l ; l l l l i 18 14 271 271 181 111 l l l l l l 13 18 21 310 310 211 240 i i i i i i 0 0 241 270 l i i l i i i i 31 271 300 i 13 18 301 330 i 13 18 331 360 0 0 0 missed = 0 n= 8 Vector ### ### Mean 257 ### Vector Magnitude (%) 1111111 50 Rayleigh Significance- 1.31E-01 Circular Standard Dev Sissis! 57 Circular Variance i l i l i i l i Interval Scale = 5 mm PALEOCURRENT Rounded ROSE Interval J Dip Direct D i r e c t INTERVAL # i i i i Radius (mm) 20 22 22 >0 30 IIII: I i i i i 19 G.C 42 42 31 60 llllll! I l l l 19 14 70 70 61 i l l lllli llllll 33 28 76 76 91 i n 1 I l l l 19 G.C 77 77 121 i l l I l l l llllll 0 19 106 106 151 180 llllll lllli! 0 11 193 193 181 i n iiii I l l l ! 19 211 i l l i l l l ! 0 0 241 270 lllli 0 0 271 300 Illl! 0 0 301 i l l lilH! 0 0 331 360 0 IIIII 0 missed = IIIIO! n= 7 Vector ### 75 Mean ### ### Vector Magnitude (%) = 69 Rayleigh Significance - 3.60E-02 Circular Standard Dev 45 Circular Variance t= 1989 SITE GGAJ-92-119 SE. A t l i n L k . E . P l i e n s . APPENDIX 4 S I T E G G A J - 9 2 - 1 2 0 S E . A t l i n L k . E . P l i e n s . Interval Scale = mm P A L E O C U R R E N T R o u n d e d R O S E Interval | D i p D i r e c t D i r e c t I N T E R V A L # % Radius (mm) 2 0 22 22 11111 3 0 1 2 0 22 2 8 76 76 3 1 60 0 0 0 3 1 77 77 6 1 111! 2 4 0 32 19 1 0 6 1 0 6 9 1 i l l l l l l l l Illi 22 1 1 1 9 3 1 9 3 1 2 1 illi I l l l l i l l ! 0 1 S 1 1 8 0 i l l i i l l ! 0 1 8 1 i l l I l l l l i l l ! 22 2 1 1 2 4 0 i l l i 1 1 1 ! 0 2 4 1 2 7 0 0 1 1 1 ! 0 2 7 1 3 0 0 0 0 0 3 0 1 3 3 0 0 0 0 3 3 1 3 6 0 0 0 0 m i s s e d = 0 n= 5 V e c t o r ### 8 7 M e a n ### ### Vector Magnitude |%] I l l l l 61 Rayleigh Significance; 1.54E-01 Circular Standard Dev illll! 50 Circular Variance s= 2503 Interval Scale = mmmm mm R O S E Interval | I N T E R V A L l i l l l I l l l l Rodius (mm) >0 30 Illi® i i i i ! 0 3 1 111; i l l l l ! i i i i 0 6 1 I l l i l i l i i i i i 0 9 1 i i i i 111! l i l i 0 1 2 1 i l l i 111! i i i i 0 1 5 1 1 8 0 111! l i l i 0 1 8 1 111 0 l i l i 0 2 1 1 III IIIII I l l i 29 2 4 1 2 7 0 111! i l l i : 0 2 7 1 3 0 0 i 3 3 29 3 0 1 3 3 0 0 0 0 3 3 1 3 6 0 1 3 3 29 m i s s e d = 0 n= 3 V e c t o r 2 9 1 ### M e a n ### ### Vector Magnitude (%] 11111 65 Rayleigh Significance: 2.86E-01 Circular Standard Dev 111! 48 Circular Variance — 2277 S I T E G G A J - 9 2 - 1 2 3 S E . A T L I N L K . E . P l i e n s . I m l a y i Z o n e P A L E O C U R R E N T D i p 1 7 1 7 1 1 D i r e c t 2 2 6 2 9 3 3 5 0 R o u n d e d D i r e c t 2 2 6 2 9 3 3 5 0 APPENDIX 4 S I T E G G A J - 9 2 - 1 2 5 C O P P E R I S L A N D E . P L I E N S . W h i t e a v e s i Z o n e P A L E O C U R R E N T D i p 32 25 20 38 27 28 32 30 27 38 26 19 29 26 36 36 32 18 26 26 30 41 32 D i r e c t 2 6 7 11 12 13 13 19 23 26 27 28 30 31 38 54 76 339 344 354 355 359 360 R o u n d e d D i r e c t 2 6 7 11 12 13 13 19 23 26 27 28 30 31 38 54 76 339 344 354 355 359 360 Interval Scale = mm R O S E Interval | I N T E R V A L # l i i l Radius (mm) >0 30 12 mm 36 31 60 4 I l l l 21 61 90 1 4 10 91 120 0 I l l l 0 121 150 l i l l i 1 1 1 ! 0 151 180 § 1 1 ! i i i i 0 181 210 l l l l i l l l l l l ! 0 211 240 l l l l i i i i i 0 241 270 0 0 0 271 300 0 0 0 301 330 0 0 0 331 360 6 26 26 m i s s e d = 0 n= 23 V e c t o r ### 16 M e a n ### ### Vector Magnitude (%] l l l l l l 93 Rayleigh Significance: 2.11E-09 Circular Standard Dev l l l l l l ! 19 Circular Variance — 376 Interval Scale = mm P A L E O C U R R E N T R o u n d e d R O S E Interval 1 D i p D i r e c t D i r e c t I N T E R V A L l l l l l l l l l l l l Radius (mm) 7 20 20 >0 30 IIIII2 III!® 14 18 24 24 31 60 l l l l l l i i i i 27 23 31 31 61 90 I l l l ! l l l l i 18 18 39 39 91 120 5 I l l l ! 23 22 46 46 121 150 I I I I I IIIII 14 18 52 52 151 180 i i i i I l l l 18 26 53 53 181 210 i 4 10 20 54 54 211 240 l l l l l l ! 4 10 26 58 58 241 270 0 0 0 28 87 87 271 300 0 l l l l i ! 0 34 89 89 301 330 0 l l l l i ! 0 29 90 90 331 360 0 I I I ! 0 19 94 94 m i s s e d = 0 16 96 96 n= i i i i 27 108 108 27 111 111 V e c t o r ### ### 26 112 112 M e a n ### 93 22 124 124 17 138