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Lithostratigraphy, palynostratigraphy, and sedimentology of the Northern Skeena Mountains and their implications… Cookenboo, Harrison Owen 1989

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LITHOSTRATIGRAPHY, PALYNOSTRATIGRAPHY, AND SEDIMENTOLOGY OF THE NORTHERN SKEENA MOUNTAINS AND THEIR IMPLICATIONS TO THE TECTONIC HISTORY OF THE CANADIAN CORDILLERA By HARRISON OWEN COOKENBOO B.Sc. Duke University 1981 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS OF THE DEGREE OF MASTER;? OF SCIENCE  in  THE FACULTY OF GRADUATE STUDIES  Department of Geological Sciences  We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA April 1989 Harrison Owen Cookenboo, 1989  In  presenting  degree at the  this  thesis  in  University of  partial  fulfilment  of  of  department  this thesis for or  by  his  or  scholarly purposes may be her  representatives.  permission.  Department The University of British Columbia Vancouver, Canada  for  an advanced  Library shall make it  agree that permission for extensive  It  publication of this thesis for financial gain shall not  DE-6 (2/88)  requirements  British Columbia, I agree that the  freely available for reference and study. I further copying  the  is  granted  by the  understood  that  head of copying  my or  be allowed without my written  ABSTRACT:  The Skeena Mountains of north-central British Columbia contain exposures of more than 4500 m of Jura-Cretaceous strata deposited in the Bowser Basin. These strata were deposited as a result of tectonism that formed the Canadian Cordillera, and serve as a record of the tectonic history of the Cordillera.  The strata of the north-central Bowser Basin have been the subject of an integrated lithostratigraphic, palynostratigraphic and sedimentologic study in order to better understand the depositional history of the basin and the tectonic history of the Cordillera. The study area is in the northern Skeena Mountains, approximately coincident with boundaries of the Groundhog coalfield.  Four hthostratigraphic units are exposed in the study area. The upper three units, consisting of over 2000 m of strata, are well exposed, and are formally named herein, from oldest to youngest, the Currier, McEvoy and Devils Claw Formations. Underlying the Currier Formation is the informally named Jackson unit, which comprises the oldest exposed strata in the study area. Together, the Jackson unit and Currier Formation comprise the Bowser Lake Group. The McEvoy and Devils Claw Formations unconformably overlie the Bowser Lake Group and correlate with the Skeena Group.  Palynostratigraphy has been used successfully to correlate and date the sediments. Marine macrofossils are rare above the Jackson unit, necessitating use of palynomorphs to date the strata. Recovery of palynomorphs from anthracite rank coal measures has proven possible by extended treatment in Shulze's solution. The Currier Formation is suggested to be Late Jurassic (Oxfordian to Kimmeridgian or Tithonian) in age. The overlying McEvoy and Devils Claw Formations are entirely Cretaceous in age. The McEvoy Formation extends from the late Barremian or Aptian to the middle or late Albian, and the Devils Claw Formation spans the middle or late Albian to the Cenomanian.  iii  Two sedimentologically distinct deltaic sequences have been interpretedfromthe lithofacies associations. Both delta sequences probably accumulated in relatively shallow water, suggesting shallow shelf deposition. The older deltaic sequence occurs in the Currier Formation, and is analogous to facies of the modern Mississippi delta. The younger deltaic sequence encompasses strata of the McEvoy and Devils Claw Formation, and is more analogous to the coarse grained delta of the Copper River. The two delta sequences are separated by a hiatus of 20 to 35 million years. Sediment provenance, interpreted from paleocurrent data and clast lithologies, appears to have remained the same during deposition of both deltaic sequences. Paleocurrent indicators point to a source to the east-northeast, and the dominance of chert suggests a dominantly sedimentary source terrane, likely the Cache Creek Group. North American provenance is consistent with paleocurrent indicators and clast lithologies.  The tectonic history of the Cordillera is related to the depositional history of the Bowser Basin. Subsidence, as indicated by sediment accumulation, occurred during the Middle and Late Jurassic and probably decreased near the end of the Jurassic, leading to a period of tectonic quiescence in the Cordillera. Subsidence resumed near the end of the early Cretaceous, leading to another period of deltaic sedimentation. This second deltaic sequence, represented by strata of the McEvoy and Devils Claw Formations, is coarser than the Late Jurassic delta, which suggests a more rugged and proximal source area.  iv TABLE OF CONTENTS  ABSTRACT  ii  LIST OF FIGURES  viii  ACKNOWLEDGMENTS CHAPTER I: INTRODUCTION AND REGIONAL GEOLOGY  xi 1  INTRODUCTION  1  REGIONAL GEOLOGY  2  Canadian Cordillera  2  Suspect terranes  4  Bowser Basin  6  Sustut Group  8  REFERENCES CITED CHAPTER II: LITHOSTRATIGRAPHY  10 13  ABSTRACT  13  INTRODUCTION  14  FORMATIONS  18  Currier Formation  18  Definition  18  Type section  19  Lithology  23  Age  24  McEvoy Formation  25  Definition  25  Type section  26  Lithology  28  Age  29  V  Devils Claw Formation  30  Definition  30  Type section  31  Lithology  31  Age  33  RELATIONSHIP TO STRATA IN SOUTHERN BOWSER BASIN 33 ACKNOWLEDGMENTS  36  REFERENCES CITED  37  CHAPTER III: PALYNOSTRATIGRAPHY  40  ABSTRACT  40  INTRODUCTION  41  PREVIOUS STUDIES  43  TECHNIQUES  44  STRATIGRAPHY  45  PALYNOASSEMBLAGES AND CHRONOLOGY  47  ASSEMBLAGES  47  Assemblage #1  47  Assemblage #2  49  Assemblage #3  50  Assemblage #4  50  Assemblage #5  51  DISCUSSION  53  Marine influence  53  Depositional sequences  53  Hiatus  55  Tectonic implications  56  CONCLUSIONS  58  vi REFERENCES CITED  60  CHAPTER IV: SEDIMENTOLOGY  63  ABSTRACT  63  INTRODUCTION  64  STRATIGRAPHY AND STRUCTURE  66  Setting  66  Stratigraphy  68  Structure  69  LITHOFACIES  71  Currier Formation Lithofacies associations  74 75  Transitional deltaic lithofacies association Interpretation McEvoy Formation Lithofacies associations Siltstone lithofacies association  76 81 83 83 85  Coarse conglomerate lithofacies association Basal conglomerate lithofacies Interpretation Devils Claw Formation Lithofacies associations  89 90 91 92 93  Channelized conglomerate lithofacies association  95  Stackedfining-upwardconglomerate lithofacies association Interpretation PROVENANCE  96 99 100  vii DEPOSITIONAL SYSTEM  103  Currier Formation  103  Skeena Group  104  TECTONIC IMPLICATIONS  106  REFERENCES CITED  109  REFERENCES CITED  113  APPENDIX I  120  Sample 9-1  120  Sample 12-5  120  Sample 19-6  122  Sample 19-4  122  Sample 19-1  124  Sample 10-8  124  Sample 16-7  125  Sample 8-2  126  Sample 8-3  126  Sample 8-7  127  Sample 8-9  127  Sample 8-14  128  Sample C-l  129  Sample C-3  130  viii  LIST OF FIGURES 1. The five tectonic provinces of the Canadian Cordillera.  3  2. Jura-Cretaceous sedimentary basins.  3  3. Locations of different stratigraphic studies in the Bowser and adjacent Sustut Basins.  9  4. Index map to study area.  4  5. Groundhog coalfield.  15  6. Composite stratigraphic column.  17  7. Lithologies and stratigraphic correlations of measured sections.  20  8. Lithologies and stratigraphic correlations of measured sections.  21  9. Laterally extensive shales, siltstones, sandstones, coals and ironstones of the Currier Formation .  22  10. Shales, siltstones, and minor sandstones of the McEvoy Formation.  27  11. Lens shaped massive arenites of the upper McEvoy Formation.  27  12. Interbedded limestones, siltstones, shales, and fine-grained sandstones in the McEvoy Formation.  28  13. Type section of the Devils Claw Formation.  32  14. Locator map of the Bowser Basin.  42  15. Lithostratigraphic units •. 16. Palynoassemblages. 17. Regional lithostratigraphic correlations.  46 48 52  ix  18. Location map. 19. Locator map for the study area in the Skeena Mountains of northern British Columbia. 20. Jura-Cretaceous basins of the Intermontane Belt. 21. Stratigraphic units of the Bowser Basin. On the left are widely recognized groups, and on the right are the lithostratigraphic units within the study area. 22. Looking northwest along the southwestern limb of the Beirnes Synclinorium. 23. Cartoon of approximate outcrop extents for the four lithostratigraphic units in the study area. 24. Yellow brown concretionary layer in shale bed. 25. Typical strata of the transitional deltaic lithofacies association of the Currier Formation. Arrows point upward in direction of repeated but poorly defined coarsening trends. 26. Thick recessive siltstones of the McEvoy Formation. Resistant cliff to the right is the lowermost Devils Claw Formation. 27. Tree trunk in life position in the McEvoy Formation. 28. Typical well defined coarsening upward sequences from the siltstone lithofacies of the McEvoy Formation. Arrows point in the direction of coarsening trend. A single fining upward sandstone that breaks up the middle of this section may represent crevasse splay channel. 29. High ridge of exposed Devils Claw Formation on the northeast limb of the Beirnes Synclinorium. 30. Stacked examples of typicalfiningupward conglomerate units in the upper Devils Formation. 31. Chert pebble-cobble conglomerate of the Devils Claw Formation, with typically poorly sorted clasts.  X  32. Large scale scour on the overturned base of a massive conglomerate 33. Rose diagrams of paleocurrent direction indicators measured from planar crossbeds, a) The entire data set showing a general southwest orientation; b) The Bowser Lake Group Measures; c) The Skeena Group measures.  xi  ACKNOWLEDGMENTS  This study was supported by NSERC grant A7337 (Bustin). The support and advice of Dr. R.M. Bustin made this project possible.  I gratefully acknowledge additionalfieldand logistical support provided by the Mount Klappan crew of Gulf Canada Resources Limited, and enthusiasticfieldassistance provided by C. Bryan and M. Gant.  I owe many thanks to Dr. G. E. Rouse both for fossil identification and geologic encouragement. Drs. W. C. Barnes and P. Smith are both gratefully acknowledged for their comments on various parts of this thesis which have led to material improvements in the final product.  1  CHAPTER I: INTRODUCTION AND REGIONAL GEOLOGY  INTRODUCTION  Sediments preserved in Jurassic and Cretaceous basins of Western Canada are of great significance in understanding the tectonic evolution of the Canadian Cordillera. These sediments were deposited as a result of the tectonic events that formed the Cordillera, and record the orogenic history in their lithologies, ages, and depositional environments. Detailed lithostratigraphic, biostratigraphic and sedimentologic study of these Western Canadian basins are required to evaluate and constrain the widely touted models of allochthonous terranes and accretionary tectonics to the formation of the Canadian Cordillera.  One of the largest and least studied of the Jura-Cretaceous basins is the Bowser Basin, located in northern British Columbia, which has previously received little detailed stratigraphic and sedimentological study. The most detailed stratigraphic studies within the Bowser Basin to date have been in Lower and Middle Jurassic rocks (Tipper and Richards, 1976; Thomson et al, 1986). Reported herein are the results of a recently conducted detailed lithostratigraphic, biostratigraphic, and sedimentologic study of strata exposed in the north-central Bowser Basin. This study has formed the basis for a reconstruction of the depositional history of the north-central Bowser Basin, and regional correlations to strata elsewhere in the Bowser Basin, and the Cordillera.  The study is presented in the form of three independent articles, preceded by a review of the regional geology. Each article is designated a separate chapter in the thesis. The first article formalizes the stratigraphy of the north-central Bowser Basin, and closely follows Cookenboo and Bustin (in press). The second article details the biostratigraphy of the study area, with particular  2 attention to palynology. The final article describes and interprets the lithofacies of the rocks, and discusses the relationship of the depositional environments to the tectonic history.  REGIONAL GEOLOGY:  Canadian Cordillera  The wide variety of rocks exposed within the Canadian Cordillera reflect a complex geologic history, much of which is recorded in contemporaneous sedimentary deposits. Based on structural style and lithologic associations, the rocks of the Canadian Cordillera have been divided into five northsouth trending tectonic provinces, which are, from east to west, Rocky Mountain Belt, Omineca Crystalline Belt, Intermontane Belt, Coast Plutonic Complex, and Insular Belt (Wheeler and Gabrielse, 1972; Fig. 1). Plutonic and metamorphic rock exposures are concentrated in the Omineca Belt and Coast Plutonic Complex, suggesting great uplift and erosion, and making these provinces sediment source areas. Thick sedimentary sequences accumulated during Cordilleran tectonism in the Insular, Rocky Mountain, and Intermontane Belts.  In the Intermontane Belt separate Jurassic and Cretaceous basins preserve detritus derived from formation of the Cordillera. Among the Jura-Cretaceous basins are the Bowser, Tantalus, Sustut, Nechako, Tyaughton, and Methow Basins (Fig. 2). Each of the Jura-Cretaceous basins are characterized by sedimentary rocks deposited with angular unconformity on older Mesozoic sedimentary and volcanic strata. These basins unconformably overlie older plutonic and volcanogenic strata, and hence have been called successor basins (Eisbacher, 1974).  Figure 2. Jura-Cretaceous sedimentary basins of the Intermontane Belt.  4  Suspect terranes  Much of the Mesozoic and older strata underlying the Cordillera west of the Rocky Mountain Trench has been termed "suspect" terrane, meaning fault bound blocks with little if any relationship to neighboring terranes (Monger and Ross, 1971). A variety of paleomagnetic, paleobiogeographic, structural, and mineralogic arguments have been cited to support the theory that many suspect terranes are allochthonous and have been subjected to significant motion relative to the North American craton both prior to and after amalgamation and accretion (Coney et al., 1980). Although most of these studies agree on northward displacement of terranes, the distances and timing of movement vary widely from study to study. The following brief review of accretionary tectonics as applied to the Canadian Cordillera is not designed to be comprehensive, but merely to describe the most commonly invoked concepts and to set the stage for this study.  The Stikine terrane is the largest of the suspect terranes, and is composed of Mississippian and Permian volcaniclastic, volcanic, and carbonate rocks (Coney et al., 1980). The Stikine terrane, also known as Stikinia, is nearly coincident with the boundaries of the Intermontane Belt, and underlies at least parts of each of the successor basins mentioned earlier. Stikinia amalgamated with the Cache Creek, Quesnellia, and Eastern terranes prior to accretion with North America by the Middle Jurassic, based on the occurrence of detrital lithologies in the Bowser Basin that were sourced from the Cache Creek terrane (Monger et al., 1982). The amalgamated Stikinia terrane is known as Stikinia superterrane, or Terrane I (Monger et al., 1982).  West of the Coast Plutonic Complex, a second major group of amalgamated terranes, consisting largely of the Alexander and Wrangellia terranes, is known as the Wrangellian superterrane,  5 or Terrane II (Monger et al., 1982). This set of terranes underlies the Insular Belt, and may have formed prior to amalgamation with Stikinia. The Gravina-Nutzotin terrane consisting of Upper Jurassic and Lower Cretaceous flysch and melange deposits, overlaps the Alexander and Wrangellia terranes, constraining their amalgamation by the Late Jurassic (Monger et. al., 1982). Recent work suggests that the Alexander and Wrangellia terranes may have been adjacent for much longer. Both the Alexander and Wrangellia terranes are intruded by the Banard Glacier pluton, which has been dated radiometrically as Middle Pennsylvanian, suggesting that Wrangellia and Alexander terranes must have amalgamated prior to the Middle Pennsylvanian (Gardner, et al., 1988).  Wrangellia superterrane may have amalgamated with Stikinia superterrane prior to the accretion of either with North America. Timing of the amalgamation of the two super terranes is constrained by overlapping Jura-Cretaceous lithologies in the Tyaughton-Methow Trough which occurred prior to deposition of the Early Cretaceous Jackass Mountain Group (Kleinspehn, 1985). Recent radiometric dating from the Coast Plutonic Complex has led van der Heyden (1989) to conclude that Wrangellia and Stikinia have been in close to the same relative positions at least since the Early Jurassic, and possibly much longer.  Accretion of the two amalgamated superterranes to the North American Craton has been postulated to have occurred from as early as the Middle Jurassic to as late as post-Cretaceous (Coney et al, 1980; Panuska, 1985).  Additional right lateral motion following terrane accretion has been proposed by a number of workers in various parts of the Cordillera. Paleomagnetic data from spatially separated areas of British Columbia consistently suggest an origin far to the south of present positions (Monger and Irving, 1980; Irving et al, 1985). Mesozoic volcanogenic strata of north-central British Columbia yielded paleomagnetic results consistent with their formation 1300 km south of their present position (Monger and Irving, 1980). Irving et al. (1985) report paleomagnetic evidence that British Columbia was  6  displaced 2400 km to the north since the mid-Cretaceous, while Umhoefer (1987) cites paleomagnetic evidence which suggests that the Insular, Intermontane, and Omineca Crystalline Belts moved as a unit more than 2400 kilometres northward between the Late Cretaceous and the Paleocene (85 Ma to 57 Ma). In the Wrangell Mountains of Alaska, Panuska (1985) cites paleomagnetic evidence for 3500 km of northward displacement. Recent work in the Carmacks Group in the northern Intermontane Belt suggests 1500 plus or minus 950 km of northward displacement between the latest Cretaceous and the Eocene (Marquis and Globerman, 1988). Geologic evidence of major dextral transcurrent displacement along the Rocky Mountain Trench and several parallel lineaments presented by Gabrielse (1985), supports more than 900 kilometres of northward translation after accretion.  Bowser Basin  The Bowser Basin is a 60 000 square kilometre Jura-Cretaceous depocenter located in northern British Columbia. A composite stratigraphic column more than 4500 m thick consisting of volcanic, volcaniclastic, and sedimentary rocks from Early Jurassic (Pliensbachian) to Late Cretaceous (Cenomanian) age, is exposed in the northeast part of the basin. The upper part of this section, exposed in the area of the headwaters of the Skeena, Nass, Klappan, and Spatsizi Rivers, constitutes the subject of this study.  The Bowser Basin was infilled by an overall coarsening upward sequence of sediments grading from fine marine clastic and detrital volcanics in the Early Jurassic to marine shales in the Middle Jurassic. The coarsening trend continued with deposition of deltaic marine and nonmarine sandstones and shales in the Late Jurassic and culminated with coarse nonmarine deposits in the Early to midCretaceous. Exposures of Lower and Middle Jurassic rocks in various parts of the Bowser Basin have been described previously (Fig. 3; Tipper and Richards, 1976; Thomson et al, 1986), and a formal stratigraphic column has been defined up to the earliest Late Jurassic. North of the study area, Thomson et al. (1986) have divided up to 730 m of Lower Jurassic (Lower Pliensbachian) to Middle  7 Jurassic (Lower Bajocian), dominantly fine clastic and detrital volcanic sediments, of the Spatsizi Group into the Joan, Wolf Den, Melisson, Abou, and Quock Formations.  In the southern Bowser Basin, a thick assemblage of Lower (Sinemurian) to Middle Jurassic (Lower Callovian) volcanic and sedimentary rocks has been assigned to the Telkwa, Niltkitkwa, and Smithers Formations of the Hazelton Group (Tipper and Richards, 1976). The Bowser Lake Group either conformably or unconformably overlies the Spatsizi and Hazelton Groups (Thomson et al, 1986; Tipper and Richards, 1976; Duffel and Souther, 1964).  Overlying the Hazelton Group in the southern Bowser Basin, Tipper and Richards (1976) described two major stratigraphic subdivisions of clastic sedimentary rocks. The lower of the two, the Bowser Lake Group, is composed of Middle Jurassic (Upper Bajocian) to Upper Jurassic (Oxfordian or Kimmeridgian?) marine and nonmarine sediments with minor volcanics. The Bowser Lake Group was divided by Tipper and Richards (1976), into the Ashman Formation composed of marine shales, and younger strata including the Trout Creek assemblage composed of sandstones, conglomerates, siltstones, and minor coals. More recently, exposures of the Ashman Formation have been reported at the northern edge of the Bowser Basin by Gabrielse and Tipper (1984), who conclude that the formation is probably continuous under younger beds exposed in the center of the basin.  In the southeastern part of the Bowser Basin sediments correlative to the Bowser Lake Group were described by Duffel and Souther (1964) as being composed of "marine and freshwater shales, argillites, greywackes, sandstones and conglomerates" of Jurassic to Early Cretaceous age. A lower Oxfordian index fossil, the ammonite Cardioceras, and a suite of plant fossils were used to date the section. Duffel and Souther (1964) originally named these rocks Bowser Group, but they were later  I  8 renamed the Bowser Lake Group by Tipper and Richards (1976) due to prior usage of Bowser Group in Alaska.  The Skeena Group overlies the Bowser Lake Group (Tipper and Richards, 1976), and is composed of Lower Cretaceous (Hauterivian) to mid-Cretaceous (Albian or Cenomanian) marine and nonmarine elastics and volcanics. The Skeena Group has not been formally divided and is unconformably separated from the Bowser Lake Group by a hiatus of Kimmeridgian to Early Cretaceous age (Tipper and Richards, 1976). The Skeena Group is lithologically similar to the underlying Bowser Lake Group and is characterized by thin- to-thick coals and the presence of detrital mica. Tipper and Richards (1976) postulated that deposition of the Skeena Group occurred in a different basin than the Bowser Lake Group, because the Skeena Group sediments were deposited across the Skeena Arch, which defines the southern margin of the Bowser Basin.  Sustut Group  Upper Cretaceous to Eocene rocks of the Sustut Group unconformably overlie the Bowser Lake Group and older strata north and east of the study area (Eisbacher, 1974). The Sustut Group has been divided into the Tango Creek (Cenomanian to Turonian) and Brothers Peak Formations (Eocene and possibly Paleocene; Eisbacher, 1974). Unpublished palynological analysis of samples collected from the Sustut Basin, however, identified only Upper Cretaceous (Campanian to Maastrichtian) palynomorphs from the Sustut Group (G. Rouse, pers. comm., in McKenzie, 1985). Outliers of the Tango Creek Formation as old as mid-Albian age have been reported in the Bowser Basin (Evenchick, 1987). The outliers appear to be partly contemporaneous with the Skeena Group, but the overall  relationship of the Tango Creek Formation to the Skeena Group is unclear and requires more detailed study.  The rocks exposed in the study area are marine and nonmarine sediments and form the top of an overall coarsening upward sequence that filled the Bowser Basin from the Early Jurassic to the midCretaceous. Detailed descriptions reported herein have led to the identification of two successive and distinct coarsening upward trends within the general coarsening upward sequence. These coarsening upward sequences are coeval with the Bowser Lake Group and the Skeena Group as defined by Tipper arid Richards (1976) in the southern Bowser Basin.  Figure 3. Locations of different stratigraphic studies in the Bowser and adjacent Sustut Basins: A) Tipper and Richards (1976); B) Duffel and Souther (1964); C) Thomson et al. (1986); Eisbacher (1974) studied the northern and southern Sustut Basins; and McKenzie (1985) studied the southern Sustut Basin.  10  REFERENCES CITED Coney, P. J., Jones, D. L., and Monger, J. W. H. 1980. Cordilleran suspect terranes. Nature, v. 288, 329-333. Cookenboo, H. O., and Bustin, R. M. In press. Jura-Cretaceous (Oxfordian to Cenomanian) stratigraphy of the north-central Bowser Basin, northern British Columbia. Canadian Journal of Earth Sciences. Duffel, S., and Souther, J. G. 1964. Geology of Terrace map-area British Columbia. Geological Survey of Canada Memoir 329,117 p. Eisbacher, G. H. 1974. Evolution of successor basins in the Canadian Cordillera. In Modern and Ancient Geosynclinal Sedimentation. Society of Economic Paleontologists and Mineralogists, Special Publication 19, p. 274-291. Evenchick, C. A. 1987. Stratigraphy and structure of the northeast margin of the Bower Basin, Spatsizi map area, north-central British Columbia. In Current Research, Part A, Geologic Survey of Canada, Paper 87-1A, p. 719-726. Gabrielse, H., and Tipper, H. W. 1984. Bedrock geology of Spatsizi map area (104H): Geological Survey of Canada. Open File 1005. Gabrielse, H. 1985. Major dextral transcurrent displacements along the Northern Rocky Mountain Trench and related lineaments in north-central British Columbia. Geological Society of America Bulletin, v. 96, p. 1-14. Gardner, M. C, Bergman, S. C, Cushing, G. W., MacKevett, E. M. Jr., Plafker, G., Campbell, R. Dodds, C. J., McClelland, W. C, and Mueller, P. A. 1988. Pennsylvanian pluton stitching of  11  Wrangellia and the Alexander Terrane, Wrangell Mountains, Alaska. Geology, v. 16, p. 967971. Irving, E. 1979. Paleopoles and paleolatitudes and speculations about displaced terrains. Canadian Journal of Earth Sciences, v. 14, p. 669-694. Irving, E., Woodsworth, G. J., and Wynne , P. J. 1985. Paleomagnetic evidence for displacement from the south of the Coast Plutonic Complex, British Columbia. Canadian Journal of Earth Sciences, v. 22, p. 584-598. Kleinspehn, K. L. 1985. Cretaceous sedimentation and tectonics, Tyaughton-Methow Basin, southwestern British Columbia. Canadian Journal of Earth Sciences, v. 22, p. 154-174. Marquis, G., and Globerman, B. R. 1988. Northward motion of the Whitehorse Trough: paleomagnetic evidence from the Upper Cretaceous Carmacks Group. Canadian Journal of Earth Sciences, v. 25, p. 2005-2016. McKenzie, K. J. 1985. Sedimentology and stratigraphy of the southern Sustut Basin, north central British Columbia. MSc thesis, University of British Columbia, Vancouver, 120 p. Monger, J. W. H., Price, R. A., and Tempelman-Kluit, D. J. 1982. Tectonic accretion and the origin two major metamorphic and plutonic welts in the Canadian Cordillera. Geology, v. 10, p. 7075. Monger, J. W. H., and Irving, E. 1980. Northward displacement of north-central British Columbia. Nature, v. 285, p. 289-294. Monger, J. W. H., and Ross, C. A. 1971.. Canadian Journal of Earth Sciences, v. 8, p. 259-278. Panuska, B. C. 1985. Paleomagnetic evidence for a post-Cretaceous accretion of Wrangellia. Geology, v. 13, p. 880-883. Thomson, R. C, Smith, P. L., and Tipper, H. W. 1986. Lower to Middle Jurassic (Pliensbachian to Bajocian) stratigraphy of the northern Spatsizi area, north-central British Columbia. Canadian Journal of Earth Sciences, v. 23, p. 1963-1973. Tipper, H. W., and Richards, TA. 1976. Jurassic stratigraphy and history of north-central British Columbia. Geological Survey of Canada, Bulletin 270,73 p. Umhoefer, P. J. 1987. Northward translation of "Baja British Columbia" along the Late Cretaceous to Paleocene margin of Western North American. Tectonics, v. 6, p. 377-394. van der Heyden, P. A. H. 1989. U-Pb geochronometry of the Coast Plutonic Complex, 53°N to 54°N,  British Columbia, and implications for the Insular-Intermontane Superterrane boundary. PhD. Thesis, University of British Columbia. Wheeler, J. O., and Gabrielse, H. coordinators 1972. The Cordilleran structural province. In Price, R A., and Douglas, R. J. W., eds. Variations in tectonic styles in Canada: G A.C. Special Paper 11 p. 1-81.  13 CHAPTER II: LITHOSTRATIGRAPHY  JURA-CRETACEOUS (OXFORDIAN TO CENOMANIAN) STRATIGRAPHY OF THE NORTHCENTRAL BOWSER BASIN, NORTHERN BRITISH COLUMBIA  ABSTRACT  Three new formations of Late Jurassic and Early to mid-Cretaceous age are proposed for a more than 2000 m thick section of Jura-Cretaceous rocks exposed in the north-central Bowser Basin. The Currier Formation (Oxfordian to Kimmeridgian or Tithonian) consists of 350 m to over 600 m of interbedded shales, siltstones, sandstones, coals and ironstones. The McEvoy Formation (late Barremian or Aptian to as young as Albian) consists of 400 to more than 800 m of siltstones and shales with minor sandstones, thin coals, limestones and conglomerates. The Devils Claw Formation (in part mid- to late Albian to Cenomanian) consists of 350 m to at least 600 m of strata characterized by thick pebble and cobble conglomerates, with associated coarse sandstones and minor siltstones and shales.  Two successive coarsening upward sequences are identified in the study area. Thefirstbegins with Middle Jurassic marine shales of the Jackson unit grading upwards to coarser Upper Jurassic facies of the Currier Formation. The Currier Formation is conformably or unconformably overlain by siltstones and shales of the Lower Cretaceous McEvoy Formation, which forms the base of a second coarsening upward sequence. Conglomerates appear with increasing frequency in the upper McEvoy, and are the dominant lithology of the overlying Devils Claw Formation. The contact between the McEvoy and Devils Claw Formations is gradational. The Devils Claw Formation forms the top of the second coarsening upward sequence.  The Currier Formation (Late Jurassic) is equivalent to the upper units of the Bowser Lake Group. The McEvoy and the Devils Claw Formations (Barremian to Cenomanian) are coeval with the  14 Skeena Group (Hauterivian? to Cenomanian). A probable unconformity separating the Upper Jurassic Currier Formation from the Lower Cretaceous McEvoy Formation correlates with a hiatus in the southern Bowser Basin, and probably represents a regional unconformity.  INTRODUCTION  A thick succession of Middle Jurassic to mid-Cretaceous (used herein for Hauterivian to Ceomanian age strata) strata in the Groundhog Coalfield of the north-central Bowser Basin has been the subject of detailed stratigraphic and sedimentologic studies conducted over the past several years (Bustin and Moffat, 1983; Moffat et al., 1988). These studies provide the basis for a formalized stratigraphy proposed herein. The study area is 4000 km located within the watersheds of the Skeena, Nass, Klappan, and Spatsizi Rivers in northern British Columbia (Figs. 4,5).  This paper proposes to elevate to formation rank three previously described (Bustin and Moffat, 1983) informal stratigraphic units of the north-central Bowser Basin. The informal names of "Currier unit", "McEvoy unit" and "Devils Claw unit" are herein replaced by Currier Formation, McEvoy Formation, and Devils Claw Formation, respectively. Ongoing sedimentologic studies conducted since 1983 have demonstrated the regional utility of the units described by Bustin and Moffat (1983) and amply justify the elevation of these units to formation rank. Correlations between described sections are based on lithologic descriptions paleobotanical analysis. Palynomorphs recovered from the study area and identified by G. E. Rouse (pers. comm., 1983) have significantly  Figure 4. Index map to study area. Insert of Bowser Basin displays cartoon of extent of exposures of the Devils Claw, McEvoy, and Currier Formations.  Figure 5. Groundhog coalfield. The boundaries have not been defined, but are generally considered to coincide with the Skeena, Nass, Klappan and Spatsizi watersheds. Lines A-Q and R-AA indicate stratigraphic sections shown in Figures 7 and 8 (adapted from Bustin and Moffat, 1983).  LEAF 16 OMITTED IN PAGE NUMBERING.  FEUILLET 16 NON INCLUS DANS LA PAGINATION.  National L i b r a r y of Canada  Bibliotheque nationale du Canada  Canadian Theses S e r v i c e .  Service des theses canadiennes.  17  North-Central  Central  DEVILS CLAW FORMATION  Studied  South  Cenomanian  UJ  > < CC  m y  FORMATION Q.  <° 3 o  I— < EC 0- (3 OT  Barremian Hauterivian  0  Valanginian Berriasian  TROUT CREEK ASSEMBLAGE  FORMATION  ASHMAN  Aptian  OT O  CURRIER  AND Y O U N G E R FACIES  JACKSON ASHMAN UNIT  ASHMAN FORMATION  FM  'QUOCKVM. ABOU FM. MELISSON FM. WOLF DEN FM. JOAN FM.  "SMITHERS FM. NILKITKWA FM. TELKWA FM.  Tithonian  DC  Kimmeridgian  ai UJ OT ¥  o -  Oxfordian  1  CD  _ UJ N  Callovian  0.  3  O  C t  1 9  P  e S e n t  S t U d y a r C a  a  n  d S 0 U t h e r n  s t u d  y  _i on  Si  _l  Ul -1  >_i  <  UJ.  Ti  r  O  <  2  ( PPe and Richards,  r-  Ul  Baiocian Aalenian Toarcian Pliensbachian  tt  >-  Battionian  Figure 6. Composite stratigraphic column in the Bowser Basin comparing northern study area W T Tune scale , ffollowing 2' ' ' 1976). Palmer (1983).  o  Albian  is  FORMATION  UJ 3  SCALE  Turonian  Not  McEVOY  £  TIME  UNITS  •US  ROCK  RE AC  North  BASIN  IDD  BOWSER  o OT OT  < rx  18 FORMATIONS Currier Formation  Definition  The Currier Formation consists of interbedded shales, siltstones, sandstones and coals exposed between the Kluatantan River on the east, the Nass River valley on the west, the Spatsizi Wilderness Provincial Park on the north, and Mount Jackson to the south. The name Currier is retained from prior informal usage as the Currier unit (Bustin and Moffat, 1983; Moffat, 1985; Moffat et al., 1988). The Currier Formation conformably overlies dominantly marine shales with minor sandstones assigned to the Jackson unit (Bustin and Moffat, 1983; Moffat, 1985). The definition of the Currier Formation essentially follows Moffat's (1985) description in the northern Groundhog coalfield, but includes some prodelta strata including sandstones, and minor conglomerates previously assigned to the upper Jackson unit. Thick coals in the upper Jackson include the potentially economically significant strata of the Klappan Coalfield (Chapt. III). The Currier Formation is unconformably overlain by siltstones, shales, sandstones, and thin coals of the McEvoy Formation described later.  The Currier Formation is estimated to range from 350 m to over 600 m in thickness. A true thickness is difficult to establish in most areas due to a lack of exposed upper and lower contacts. A complete section 600 m thick is described below as the type section. The Currier Formation is the youngest unit of Jurassic rocks present in the study area. As described later, the Currier Formation can be correlated to the Bowser Lake Group strata in the southern Bowser Basin.  East of the Kluatantan River, the Currier is not identified and a coarser-grained facies, referred to as the Prudential unit (section Y, Figs. 5, 8; Bustin and Moffat, 1983), occupies a similar stratigraphic position. The Prudential unit, named for an exposure on Prudential Mountain, is estimated to be 400 m to 500 m thick, although no complete section is known. More detailed  19  stratigraphic study is required to determine the exact relationship between the Prudential unit and the rocks of the Bowser Lake Group.  The contact of the Currier with the underlying Jackson is gradational and is defined as the change from transitional marine facies to a series of coarsening upward, deltaic sequences from 5 m to 25 m thick (Bustin and Moffat, 1983). In the area near Mount Klappan, the contact is marked by the last appearance of Jackson conglomerate (Moffat, 1985).  The contact between the Currier and the overlying McEvoy Formation is represented by appearance of thin- to thick-bedded, light grey, fine sandstones of limited lateral extent, thick siltstones, shales, and occasional pebbly conglomerates, as well as a marked reduction in coal. The contact is probably unconformable, as detailed later. In the area of the type section, the contact is picked as the first occurrence of a poorly sorted, clast supported, massive-pebbly conglomerate.  Type section  The Currier Formation type section is a 600 m thick exposure on a north-south trending ridge crest (Fig. 9), located south west of Mount Klappan, between the headwaters of the Little Klappan  River, Tahtsedle Creek, and Didene Creek (section F, Figs. 5, 7; 57° 11' 30" N. lat. and 129° 0' 50" long.). Both upper and lower contacts are exposed in this section. The section forms an ascending series of bluffs with generally good exposures, separated by dip and talus slopes characterized by covered section and poor exposure. Low lying alpine vegetation of heather and grass is present over some of the section.  Figure 7. Lithologies and stratigraphic correlations of measured sections. Section line is shown Figure 5 (adapted from Bustin and Moffat, 1983).  21  PRUDENTIAL UNIT  Prudential Mtn.  P? !  Limestone  S3  Coal  1  Z  Conglomerate S3  Shale  i^j  Siltstone  I  I Sandstone S  facies change  m  Taylor Peak  Figure 8. Lithologies and stratigraphic correlations of measured sections. Section line is shown in Figure 5. The Dave Creek section is modified from drill hole data presented by Tompson et al. (1970; adapted from Bustin and Moffat, 1983).  Figure 9. Laterally extensive shales, siltstones, sandstones, coals and carbonates of the Currier Formation exposed in a series of coarsening upward bluffs (from type section).  Lithology  The lithology of the Currier Formation consists of alternating shales, siltstones, sandstones, coals and carbonates. The rocks are thin- to thick-bedded and typically weathered medium grey to black, and are laterally extensive over 10's to 100's of metres. Some beds form laterally extensive, coarsening upward sequences (Fig. 9). Commonly included among the shales and siltstones are strongly lithified carbonates. The carbonates are reddish-brown to yellow-brown on weathered surfaces and black on fresh surfaces.  The sandstones are dark grey, chert lithic wackes to litharenites (Moffat, 1985), and are generally sub-angular, and very fine- to fine-grained. Beds usually have sharp contacts and locally erosive bases. Cross-bedding is rarely seen at the bottom of the section but becomes common at the top of the formation.  Shales are typically black to dark grey and vary gradationally between laminated shale and homogeneous mudstone facies. Beds exhibit both sharp and gradational contacts. The black shale facies are usually parallel laminated, although they sometimes exhibit soft sediment deformation structures. The mudstones are often highly carbonaceous and typically contain plant fossils.  Siltstones weather grey to brown, and are lithologically variable, ranging from arenaceous to argillaceous siltstones. Interbeds of either shales or sandstones are common.  At (east eight seams of coal occur in the Currier Formation, ranging from 0.5 m to 4.0 m thick (Bustin and Moffat, 1983). Some seams are sheared, oxidized in part and locally cut by quartz and  24  carbonate veins. Coals are anthracite and meta-anthracite in rank, with maximum random vitrinite reflectance values up to 5.8% (Bustin, 1984).  Carbonates layers are nodular and 5 to 60 cm thick. The carbonates weather reddish-brown to yellow-brown in well developed layers. The carbonates are most often found at or near the top contact of shale or siltstone beds. The nodules and layers are very strongly lithified and resistant to weathering. Fresh surfaces are black and fracture sub-conchoidally. Plant fossils and trace fossils are commonly preserved imprinted on the surfaces of the carbonate nodules.  Age  A probable age of Late Jurassic has been established for the Currier Formation based on plant fossil assemblages (Bustin and Moffat, 1983) that includes Ctenis borealis, Cladophlebis vaccensis,  Pityophyllum nordenskioldi, Coniopteris cf. hymenophylloides, Ginkgoites digitata, Baiera lindle  Sphenobaiera angustiloba, Ctenophyllum pachynerve, Nilssonia parvula, and N. orientalis var. mi  (Rouse, pers. comm., 1988). Cladophlebis vaccensis, Coniopteris cf. hymenophylloides, Ctenoph  pachynerve, and Nilssonia parvula have been found in the Jurassic of Douglas County, Oregon (Ward, 1905).  Additional support for a Late Jurassic age is provided by the recovery of dinocysts and palynomorphs of Oxfordian to as young as Tithonian age (Rouse, pers. comm., 1988). Characteristic species include Nannoceratopsis sp, Pareodinia minuta, and Pareodinia cf. ceratophora.  Palynostratigraphy is discussed in detail a later section of this thesis (Chapt. III).  The Late Jurassic age established for the Currier in the study area, coupled with a Barremian age 35 m from the base of the McEvoy Formation (discussed later) indicates that a significant interval of latest Jurassic and earliest Cretaceous time may not be represented by strata in the study area.  25 Either reduced sedimentation rates, or one or more unrecognized unconformities could explain the missing interval. The latter explanation is considered more likely, given that a hiatus has already been recognized in the southern Bowser Basin (Tipper and Richards, 1976) in equivalent strata.  McEvoy Formation  Definition  The McEvoy Formation is defined for an interbedded sequence of siltstones and shales with minor limestones, coals, sandstones, and conglomerates that crops out between the Nass and Kluatantan Rivers, and between Tahtsedle Creek and the Groundhog Range (Fig. 10). The McEvoy Formation conformably or paraconformably overlies the Currier Formation and is conformably overlain by massive conglomerates of the Devils Claw Formation. East of the Kluatantan River the McEvoy Formation is not recognized.  The thickness of the McEvoy Formation is highly variable. In the north, near Tahtsedle Creek, it is about 400 m to 600 m thick, whereas to the south, at McEvoy Ridge, it is 800 m thick. To the east, at Ranger Creek and Jenkins Creek, it is on the order of 500 m thick.  The McEvoy Formation was initially described as the McEvoy unit by Bustin and Moffat (1983; Moffat et al., 1988). East of the Skeena River the lower part of the McEvoy unit is considered to be eroded or to change facies to a coarser grained interval informally referred to as the Prudential unit by Bustin and Moffat (1983). As described later, the McEvoy Formation is in part time equivalent to the Skeena Group described from the southern Bowser Basin by Tipper and Richards (1976). This correlation agrees with that proposed by Koo (1986).  26 The McEvoy Formation can be distinguished from the underlying Currier Formation by the paucity of sandstones and coals and occurrence of thick siltstones and shales and minor limestones of the McEvoy Formation. The McEvoy Formation can be distinguished from the overlying Devils Claw Formation by the occurrence of massive conglomerates in the latter. In the central and southern part of the coalfield, between the Skeena and Nass Rivers, thin conglomerates occur in the middle and upper parts of the McEvoy Formation. The contact between the McEvoy and Devils Claw Formations in this area is picked at the first occurrence of massive conglomerates greater than 5 m thick and the paucity of siltstones and shales. The contact is arbitrary inasmuch as the conglomerates are channel deposits and thus change thickness and facies rapidly.  In the region of Mt. Klappan, in the northern part of the Groundhog coalfield, the McEvoy Formation is about 800 m thick. Here the McEvoy is divisible into two informal units (Moffat, 1985): 1) a lower unit of about 500 m thick composed of light- to medium- grey, coarse grained arenaceous sandstone and greywackes and two major conglomerate channels horizons up to 10 m thick; and 2) an upper unit about 300 m thick comprised of a basal 13 m thick chert pebble conglomerate with interbeds of coarse grained sandstones and numerous thick and massive bedded arenites and greywackes and lenses or thin beds of limy mudstones and calcareous siltstones (Fig. 11).  Type section  The type section of the McEvoy Formation is an 800 m thick section at McEvoy Ridge, southwest of Devils Claw Mountain (section O, Figs. 5, 7, 8; 56° 52' 10" W. lat. and 128° 25' 50" long.). The lower contact of the McEvoy Formation with the Currier Formation is covered. The upper conformable contact with the Devils Claw Formation is well exposed. Within the type section the central part of the McEvoy Formation is repeated by a minor reverse fault.  Figure 11. Lens shaped massive arenites of the upper McEvoy Formation exposed southwest of Mt. Klappan.  28 Lithology  At all localities the McEvoy Formation is composed of monotonously interbedded thin- to thick-bedded dark grey siltstones, shales, sandstones, with minor coals and limestones. The siltstones and shales are well indurated, and occur in intervals up to 40 m thick. They are locally carbonaceous and grade to coal. The sandstones are massive or planar and trough cross-bedded and fine- to coarsegrained. Mineralogically they are composed mainly of chert, volcanic rock fragments and minor feldspar. Many of the thicker sandstones are in channels. Limestones, although a minor component, occur throughout and characterize the succession (Fig. 12). They occur in tan weathering beds up to 0.5 m thick or as concretionary horizons. Locally root casts and carbonaceous partings pervade the limestones. Coals vary in thickness from partings of a few centimetres thick to seams up to 0.5 m thick and make up a very small proportion of the succession. Conglomerates occur throughout the McEvoy unit between the Nass and Skeena Rivers but are more abundant in the upper McEvoy where it is transitional to the overlying Devils Claw unit.  Figure 12. Interbedded limestones, siltstones, shales, and fine-grained sandstones in the McEvoy Formation near Devils Claw Mountain. The resistant light-coloured bed is limestone. Hammer for scale (from Bustin and Moffat, 1983).  29  The conglomerates are up to 5 m thick, commonly have an erosional base and are massive. Some conglomerates fine upwards to coarse-grained sandstones and pinch out laterally. The conglomerates are composed mainly of sub-rounded to rounded, moderately well sorted, chert and volcanic pebbles and cobbles in a coarse grained sand matrix. Granodiorite clasts are present in the conglomerates throughout the McEvoy but are exceedingly rare in the lower part and are only slightly more abundant in the upper part. No trends in the abundance of other clasts were noted.  In the southeastern part of the coalfield, at Moss Mountain, the McEvoy Formation, is cleaved. In other parts of the coalfield cleavage is absent or only weakly developed.  Age  For the most part palynomorphs and macrofossils from the McEvoy Formation have not been diagnostic for age determinations. A palynomorph assemblage 35 m from the base of formation near Mt. Klappan yielded Tsugaepollenites mesozoicus, Cyathidites minor, Cedriptes cretaceous,  Spheripollensporites scabratus, Podocarpidites multessimus, Parvisaccites radiatus, Pluricellaes  psilatus, Pseudoceratium pelliferum and Paleoperidinium caulleri. This assemblage has been assigned  late Barremian to Aptian age (G. Rouse pers. comm., 1988). In addition b e l e m n i t e s , which occur in the lower part of the McEvoy Formation, indicate the strata are no younger than Barremian or older than Toarcian (H. Tipper pers. comm., 1982 in Bustin and Moffat, 1983).  Section higher in the McEvoy Formation has yielded palynomorph suites including the diagnostic palynomorphs Tricolpites minutus, Tricolpites crassimurus, Tricolpites micromunus,  Clavatipollenites minutus, Lycopodiumsporites crassatus, L. crassimacerius, Distaltriangulisporite costatus, Verrumonocolpites conspicuus, and the dinocysb Paleohystrichophora paucispina,  30  Gonyaulacysta cf. helicoides, Diconodinium pusillum, and Pseudoceratium regium and are dated f  the mid to late Albian. The palynology is discussed in detail in a later section of this thesis (Chap. III).  The mid- to late Albian age of the overlying Devils Claw Formation, in conjunction with the Barremian age from near the base of the McEvoy Formation and Albian age from higher in the Formation, suggests the McEvoy Formation ranges from the late Barremian or Aptian to the mid- to late Albian.  Devils Claw Formation  Definition  The Devils Claw Formation is named for a succession of massive conglomerates and minor sandstones, siltstones and shales which overlies the McEvoy Formation and crops out between the Nass and Skeena Rivers and Tahtsedle Creek and the Groundhog Range. East of the Kluatantan River and west of the Nass River the Devils Claw Formation is not preserved.  The Devils Claw Formation conformably overlies the McEvoy Formation. The contact between the Devils Claw and the underlying McEvoy Formation has been selected at the first occurrence of thick sequences of conglomerate. In the northern portion of the Groundhog coalfield the contact is defined by a laterally persistent, dark red-brown weathered, massive and resistant chert pebble conglomerate (Moffat et al, 1988). The Devils Claw is the youngest unit in the study area. The Formation varies from 300 m thick at Devils Claw Mountain to about 600 m thick at Mt. Gunanoot.  The Devils Claw Formation was previously referred to informally as the Devils Claw unit (Bustin and Moffat, 1983; Moffat et al, 1988). As described later, the Devils Claw Formation is time equivalent to strata of the Skeena Group in the southern part of the Bowser Basin (Tipper and  31  Richards, 1976) and may in part be equivalent to the Tango Creek Formation of the Sustut Basin to the east and south of the study area (Bustin and McKenzie, in press).  Type section  The Devils Claw Formation is a cliff forming unit and many of the better exposed sections are not readily accessible. For the type section, an exposure of Devils Claw Formation on Devils Claw Mountain is proposed (Figs. 2, 4, 5; 56° 52' 10" W. lat. and 128° 25' 50" N. long). Here a fairly resistant sequence of strata about 300 m thick crops out on the south facing slope. The contact with the underlying McEvoy Formation is well exposed and the basal Devils Claw Formation is perfectly exposed. The upper part of the type section is similarly well exposed but access is more difficult. A thicker but less accessible exposure of the Devils Claw Formation may be found on the slopes and peaks of Mount Gunanoot north of the proposed type section.  Lithology  At Devils Claw Mountain and adjacent areas, the Devils Claw Formation comprises predominantly massive, thick bedded, variably sorted, pebble-cobble conglomerates with rare lenticular sandstones. The conglomerates are light grey or vari-colored and composed of well-rounded and wellsorted light green and white chert, with minor volcanic, and quartz clasts. An increasing percentage of quartz pebbles occurs upwards through the section. Granitic clasts occur locally, particularly in the upper part of the Formation. The conglomerates are massive or locally trough cross-bedded. The sandstones are thin to thick bedded, medium to coarse grained and composed primarily of quartz, and chert with minor carbonaceous fragments. The lower part of the Devils Claw Formation includes thick sequences of siltstones, shales, minor sandstones, and coals interbedded with massive conglomerates. At the top of the Devils Claw Formation conglomerate is more prevalent and, at the type section on  32 Devils Claw Mountain, the top 93 m is comprised of massive conglomerate with no observed interbeds (Fig. 13).  Figure 13. Type section of the Devils Claw Formation exposed at the top of Devils Claw Mountain.  33 Age  A palynomorph assemblage from a carbonaceous shale in the middle part of the Devils Claw Formation included fern spores and gymnosperm and angiosperm pollen and dinocysts. The main species of palynomorphs recovered are Pareodinia-2, Pareodinia-1, Dictyotosporites complex, Deltoidospora hallei, D. psilostoma, Cicatricosisporites hallei, C. sp. cf. Anemia exilioides,  Lycopodiumsporites crassimacerius, Converrucosisporites sp., Cycadopites ovatus, Alisporites m  Liliacidites reticularis, Retimonocolpites sp., Tricolpites crassimurus, Clavatipollenites minutus a Liliacidites sp. The presence of the angiosperm pollen Tricolpites crassimurus, Liliacidites reticulates  and Clavatipollenites minutus and the absence of tricolporate pollen suggest a late middle Albian age for part of the Devils Claw Formation.  The palynomorphs Psilatricolporites prolatus and Grammonocolpites asymmetricus, collected  from a black shale higher in the Formation, indicate that deposition continued until at least the Cenomanian (G. Rouse, pers. comm., 1988).  RELATIONSHIP TO STRATA IN SOUTHERN BOWSER BASIN  The Upper Jurassic and Lower Cretaceous strata described by Tipper and Richards (1976) in the southern Bowser Basin are of generally similar lithology and are roughly coeval to strata exposed in the study area. In both the southern and the northern parts of the basin, a coarsening upward trend from Middle Jurassic marine shales to Oxfordian and Kimmeridgian or Tithonian deltaic facies is overlain by a second coarsening upward trend in the Lower Cretaceous.  Based on the similarity of the lithologies and age, the rocks described in the study area can be correlated with the other strata in the Bowser Basin. The oldest rocks exposed in the study area are thick (up to 2000 m) marine shales of the Jackson unit (Bustin and Moffat, 1983). The upper Jackson  34  unit contains prodelta sandstones, conglomerates, and coals, in addition to shales (Moffat, 1985). Moffat (1985) described the upper Jackson unit as approximately 200 m thick. Biostratigraphic data from this study (Chapt. Ill) indicate that the upper Jackson unit may actually be much thicker than 200 metres, although the true thickness is still not known. Based on an assemblage of ammonites and bivalves found near Mount Klappan, the Jackson unit has previously been dated as Callovian to Oxfordian in age (Moffat et al., 1988), and correlated with the lower shale unit of the Bowser Lake Group (Moffat, 1985), termed the Ashman Formation. Palynostratigraphy from coal measures near Mount Klappan also yield a Callovian age (Chapt. III). The Ashman Formation was originally defined in the southern Bowser Basin, and was dated from Late Bajocian to the late Early Oxfordian based on an assemblage of marine bivalves and ammonites (Tipper and Richards, 1976).  Although the Jackson unit is generally coeval with the Ashman Formation, the correlation between top of the Jackson unit and the top of the Ashman Formation is uncertain. Stratigraphic, lithologic, and paleontologic considerations indicate that coarser facies of the upper Jackson unit may correlate in part with an undefined lower portion of the Trout Creek assemblage. The Trout Creek assemblage, which was defined in the southern Bowser Basin by Tipper and Richards (1976), conformably overlies the Ashman Formation. The contact of the Trout Creek assemblage with the Ashman Formation was dated as the late Early Oxfordian (Tipper and Richards, 1976) and therefore generally younger than the Jackson unit. The Trout Creek assemblage consists of Oxfordian to Kimmeridgian sandstones, conglomerates, siltstones, and minor coals, similar lithologically and paleontologically to the upper Jackson unit and the Currier Formation. The uppermost Jackson unit may therefore correlate with an undefined lower portion of the Trout Creek assemblage.  Eisbacher (1981) previously correlated the Oxfordian Trout Creek beds of the southern Bowser Basin with a facies dominated by massive conglomerates that is assigned here to the  35 Cretaceous aged Devils Claw Formation. The new correlation matches upper Jackson and Currier Formation conglomerates with similar facies in the Trout Creek assemblage.  The Currier Formation correlates with the Trout Creek assemblage, excluding the lower portion as mentioned above, and forms the upper facies of the Bowser Lake Group in the study area. Both the Currier Formation and the Trout Creek assemblage are Late Jurassic in age and have similar lithologies. Additionally, a probable unconformity separates both units from overlying Lower Cretaceous rocks. A hiatus of Kimmeridgian to Early Cretaceous (Hauterivian?) time unconformably separates the Bowser Lake Group from the overlying Skeena Group (Tipper and Richards, 1976). The lithology of the Skeena Group consists of interbedded marine and nonmarine sediments and volcanics. The sediments are interbedded greywackes, sandstones, shales, and conglomerates with minor or major coal seams, and are thus similar to the Lower Cretaceous rocks of this study. Differences that are noted in lithology, such as the presence of fine flakes of detrital mica in the sediments, and locally variable volcanics of the Skeena Group (Tipper and Richards, 1976), are attributable to different sources for the northern and southern ends of the basin.  The contact of the Bowser Lake Group with the overlying Skeena Group was described by Tipper and Richards (1976) as a hiatus in the southern Bowser Basin. Because no age younger than Late Jurassic has been determined for the Currier, and the base of the McEvoy has yielded a Barremian fossil assemblage, an unconformable relationship coincident with the hiatus to the south is suggested in the study area. Disharmonic folding has been proposed (Bustin and Moffat, 1983) to explain angular relationships between the Currier and the McEvoy seen in the central basin, but given the timing of the hiatus reported in the south and the similar relationship in the north, a regional unconformity is believed to be the more probable interpretation.  The overall similarity of stratigraphic succession and timing of the Bowser Lake and Skeena Groups, as well as the probable unconformity separating them, is consistent throughout the Bowser  Basin, and may reflect basin wide timing of tectonism. Post-Kimmeridgian/pre-Albian contractional deformation and erosion of Bowser Lake Group rocks has been reported north of the study area (Evenchick, 1987). More detailed stratigraphic and sedimentologic studies are necessary to clarify the relationship between the northern and southern parts of the Bowser Basin, as well as the tectonic influences on sedimentation in the basin.  ACKNOWLEDGEMENTS  This study was supported by NSERC grant A7337 (Bustin). We gratefully acknowledge additional field and logistical support provided by the Mount Klappan crew of Gulf Canada Resources Limited, and enthusiastic field assistance provided by C. Bryan.  I We thank Dr. G. E. Rouse for fossil identification and Dr. P. Smith for helpful comments and suggestions on an earlier draft of this paper.  37  REFERENCES CITED Buckham, F., and Latour, B. A. 1950. The Groundhog coalfield, British Columbia. Geological Survey of Canada, Bulletin 16,82 p. Bustin R. M., and Moffat, 1.1983. Groundhog Coalfield, central British Columbia: reconnaissance stratigraphy and structure. Bulletin of Canadian Petroleum Geology, v. 31, p. 231-245. Bustin, R. M. 1984. Coalification levels and their significance in the Groundhog Coalfield, north-central British Columbia. International Journal of Coal Geology, v. 4, p. 21-44. Bustin, R. M., and McKenzie, K. J. In press. Stratigraphy and depositional environments of the Sustut Group, southern Sustut Basin, north central British Columbia. Canadian Society of Petroleum Geology.  Coney, P. J., Jones, D. L., and Monger, J. W. H. 1980. Cordilleran suspect terranes. Nature, v. 288 329-333. Dawson, G. M. 1901. Summary report on the operations of the Geological Survey of Canada. Summary Report 1900, p. 16.  Dowling, D. B. 1915. Coal fields of British Columbia. Geological Survey of Canada, Memoir 69, p. 189222. Dupont, V. H. 1901. Report of an exploration on the upper part of the Stikine River to ascertain the feasibility of a railway. Department of Railways and Canals, Canada, Annual Report July 1, 1899 to June 30,1900, Part 1, p. 152-155. Eisbacher, G. H. 1974a. Sedimentary and tectonic evolution of the Sustut and Sifton Basins, northcentral British Columbia. Geological Survey of Canada, Paper 73-31, 57 p. ,1974b. Evolution of successor basins in the Canadian Cordillera. In Modern and Ancient Geosynclinal Sedimentation. Society of Economic Paleontologists and Mineralogists, Special Publication 19, p. 274-291. ,1981. Late Mesozoic - Paleogene Bowser Basin molasse and Cordilleran tectonics, western Canada. In Miall, A. D. (Ed.), Sedimentation and Tectonics. In Alluvial Basins. Geological Association of Canada, Special Paper 23, p. 125-151. Evenchick, C. A. 1987. Stratigraphy and structure of the northeast margin of the Bower Basin, Spatsizi map area, north-central British Columbia. In Current Research, Part A, Geologic Survey of Canada, Paper 87-1A, p. 719-726.  38 Survey of Canada. Open File 1005. Koo, J. 1986. Geology of the Klappan Coalfield in northwestern British Columbia. British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, Paper 1986-1, p. 225-228. Leach, W. W. 1910. The Skeena River district. Geological Survey of Canada, Summary Report 1909, p. 63-64. Malloch, G. S. 1912. Notes on the Groundhog Coalfield Basin, Skeena district, British Columbia. Transactions, Canadian Mining Institute, v. 15, p. 278-281. , 1914. The Groundhog Coal Field, British Columbia. Geological Survey of Canada, Summary Report 1912, p. 69-101. McKenzie, K. J. 1985. Sedimentology and stratigraphy of the southern Sustut Basin, north central British Columbia. MSc thesis, University of British Columbia, Vancouver, 120 p. Moffat, I. W., 1985. The nature and timing of deformational events and organic and inorganic metamorphism in the northern Groundhog Coalfield: implications for the tectonic history of the Bowser Basin. Ph.D dissertation, University of British Columbia, Vancouver, 204 p. Moffat, I. W., Bustin, R. M., and Rouse, G. E. 1988. Biochronology of selected Bowser Basin strata: tectonic significance. Canadian Journal of Earth Sciences, v. 10, p. 1571-1578.  Monger, J. W. H., Price, R. A., and Tempelman-Kluit, D. J. 1982. Tectonic accretion and the origin of two major metamorphic and plutonic welts in the Canadian Cordillera. Geology, v. 10, p. 7075. Palmer, A. R. 1983. The decade of North American geology 1983 geological time scale. Geology, v. 11, p. 503-504.  Price, R. A. 1981. Cordilleran cross-section, Calgary to Victoria. In Thomson, R. I., and Cook, D. G., (Eds.) Field guide to geology and mineral deposits. Calgary 1981 annual meeting Geological  Association of Canada/ Mineralogical Association of Canada/ Canadian Geophysical Union, p. 261 LVTfl. Richards, T. A., and Gilchrist, R. A. 1979. Groundhog coal area, British Columbia. Geological Survey of Canada, Paper 79-lb, p.411-414. Souther, J. G., and Armstrong, J. E. 1966. North Central belt of the Cordillera of British Columbia. In Tectonic History and Mineral Deposits of Western Canada. Canadian Institute of Mining and Metallurgy, Special Volume 8, p. 171-184. Thomson, R. C, Smith, P. L., and Tipper, H. W. 1986. Lower to Middle Jurassic (Pliensbachian to Bajocian) stratigraphy of the northern Spatsizi area, north-central British Columbia. Canadian Journal of Earth Sciences, v. 23, p. 1963-1973. Tipper, H. W., and Richards, T A. 1976. Jurassic stratigraphy and history of north-central British Columbia. Geological Survey of Canada, Bulletin 270, 73 p. Ward, L. F. 1905. The status of the Mesozoic floras of the United States. Monographs of the United States Geological Survey, v. 48, 616 p. Wheeler, J. O., and Gabrielse, H., coordinators 1972. The Cordilleran structural province. In Price, R. A., and Douglas, R. J. W., eds. Variations in tectonic styles in Canada: G A.C. Special Paper 11 p. 1-81.  40  CHAPTER III: PALYNOSTRATIGRAPHY  MIDDLE JURASSIC TO EARLIEST LATE CRETACEOUS PALYNOASSEMBLAGES OF THE NORTH-CENTRAL BOWSER BASIN AND THEIR BIOSTRATIGRAPHIC IMPLICATIONS  ABSTRACT  Five palynoassemblages have been recognized in Jura-Cretaceous strata of the north-central Bowser Basin. Each assemblage spans approximately one stage between the Callovian (Middle Jurassic) and the Cenomanian (Late Cretaceous). Two of the five assemblages are Jurassic and three are Cretaceous. The Jurassic assemblages extend from the Callovian to the Oxfordian and possibly to the Kimmeridgian or Tithonian, and have been found exclusively in the Currier Formation and Jackson unit (informal). The strata bearing the Jurassic assemblages are assigned to the Bowser Lake Group. The Cretaceous assemblages span the upper Barremian or Aptian to the Cenomanian, and are encountered in strata of the McEvoy and Devils Claw Formations. These strata are correlated to the Skeena Group, based on contemporaneous deposition and lithologic similarity. A regional hiatus of between 20 and 35 million years separates the Jurassic and Cretaceous assemblages.  All palynomorph assemblages examined from the Callovian through the Late Albian contain dinocysts, in addition to pollen and spores, indicating that partially marine conditions persisted during deposition of the Jackson unit, Currier Formation, McEvoy Formation and lower beds of the Devils Claw Formation.  Ages assigned to the palynomorph assemblages are sufficiently precise to permit inter-regional stratigraphic comparison and provide constraints to Cordilleran tectonic development. Similarity of timing of deposition and lithology of mid-Cretaceous age sediments in the Bowser Basin and east of the Rocky Mountains supports the possibility that no more than minor lateral movement between  41 Stikinia and the North American craton has occurred since the Early Cretaceous. Consideration of timing and lithology of Jura-Cretaceous deposits in the Queen Charlotte Basin, to the west, similarly suggests that the stratigraphy of the Insular Belt and Stikinia is consistent with the two regions having been in close to the same relative position since the Late Jurassic.  INTRODUCTION  Establishment of a workable stratigraphic framework in the Bowser Basin has been hampered by lack of distinctive lithologic markers and absence of time sensitive macrofossils, combined with intense structural and thermal alteration (equivalent to anthracite and meta-anthracite rank coals). Ammonite correlations have been used successfully to date formations in Lower and Middle Jurassic marine sediments (Thomson et al., 1986), but in overlying Upper Jurassic and Cretaceous strata, preserved macrofossil suites are dominated by a long ranging flora that is of little value for regional correlation. Limited previous palynomorph analysis (Rouse, in Bustin and Moffat, 1983; and Cookenboo and Bustin, in press) has demonstrated the potential for greatly increased biostratigraphic precision over that given by macrofossils.  Palynostratigraphy has the potential to improve correlations in transitional strata lacking marine macrofossils, but has in the past usually proven unsuccessful in anthracite and meta-anthracite diagenetic grade rocks. Intensive maceration techniques, often including very long exposure times to Shulze's solution, have been successfully employed in this study to allow identification of carbonized palynomorphs. Recovered palynomorphs have been grouped into time diagnostic assemblages which greatly facilitate correlations both locally and regionally.  Palynomorphs recovered from the study area in the northern Skeena Mountains (Fig. 14) have been grouped in five diagnostic assemblages which provide important new insights into the depositional and tectonic history of the Bowser Basin. Assemblages recognized in this study can be divided into  Figure 14. Locator map of the Bowser Basin in north-central British Columbia. The study area located in the northern Skeena Mountains.  43 separate Jurassic and Cretaceous age ranges. The younger age range spans the Late Barremian to the Cenomanian and is correlative with the Skeena Group, and the older extends from the Callovian to the Kimmeridgian or Tithonian in the Jurassic and is correlative with the Bowser Lake Group.  Between 20 and 35 million years separates the younger Jurassic assemblages from the oldest Cretaceous assemblage. This time gap in the sedimentary record is interpreted as the local expression of a regionally correlative hiatus. No unconformable surface is recognized in association with the hiatus in the study area. Elsewhere in the Bowser Basin as well as in other Jura-Cretaceous depocenters in the Cordillera, the latest Jurassic and earliest Cretaceous is represented by erosional unconformity, very low sedimentation rate, or hiatus.  PREVIOUS STUDIES  Diverse floral suites have been reported from the study area (Buckham and Latour, 1950; Bustin and Moffat, 1983; Moffat, et al, 1988; and Cookenboo and Bustin, in press), but all of the forms described are long ranging and are largely facies associated. Ages based solely on plant macrofossils have been, therefore, general and sometimes speculative.  Buckham and Latour (1950), in a study on the Groundhog coalfield report a plant fossil suite of sixteen species (unspecified) assigned to the Lower Cretaceous by W. A. Bell.  Suites of floral macrofossils identified by G. E. Rouse (in Moffat, et al, 1988; Cookenboo and Bustin, in press) include the following species: Ctenis borealis, Cladophlebis vaccensis, Pityophyllum  nordenskioldi, Coniopteris cf, hymenophylloides, Ginkgoites digitata, Baiera lindleyana, Spheno  angustiloba, Ctenophyllum pachynerve, Nilssonia parvula, and N. orientalis var.. None of the abov  species is restricted to either the Upper Jurassic or the Lower Cretaceous. Nilssonia parvula, which was reported from the Upper Jurassic of Douglas County, Oregon by Ward (1905), was previously  44  considered a Late Jurassic marker. However, Nilssonia parvula is contemporaneous with tricolpate angiosperm pollen, and middle Albian age dinocysts in samples from this study, therefore the range of N. parvula extends at least to the mid-Cretaceous.  Moffat (1985) and Moffat et al. (1988) used marine macrofossils and a single palynomorph suite to partially constrain the ages of the oldest and youngest units exposed in the study area. Ammonites and bivalves collected from the upper Jackson unit (informal name from Bustin and Moffat, 1983), which is the oldest unit exposed in the study area, were assigned a pre-Oxfordian,  possibly Callovian, age (P. Smith, C. Stelck, and R. Thomson, in Moffat, 1985; and Moffat, et al, 1988) Palynomorphs recovered from near the base of the Devils Claw Formation, the upper unit in the study area, were identified as middle to late Albian age (G. Rouse in Moffat, 1985; Moffat, et al, 1988). A palynomorph suite from near the top of the section, first reported in Cookenboo and Bustin (in press) and detailed in this study, indicates deposition continued until at least Cenomanian time.  Eisbacher (1981) reported K-Ar dates from detrital micas in Skeena Group sediments in the southern Bowser Basin of 151 Ma , giving an absolute maximum age of the Late Jurassic (Tithonian; Palmer, 1983).  TECHNIQUES  Samples chosen for palynomorph analysis were collected from very fine grain rocks in sections measured as part of a detailed stratigraphic and sedimentologic study reported in Chapters II and IV respectively of this thesis. Fine grain shales directly overlying coal seams proved the best for recovery  45 of palynomorph suites. Samples were collected from the freshest material available in order to avoid surface oxidation.  The sediments in the study area were exposed to high paleotemperatures, estimated at over 200°C (Bustin and Moffat, in press) which has rendered all organic material black. Clearing palynomorphs sufficiently for identification required use of intensive and time consuming maceration techniques. Standard preparation techniques [HF to dissolve silicates, HC1 to dissolve carbonates, and Shulze's solution (100% nitric acid with added potassium chlorate crystals) to oxidize organic matter] were used, but maceration required close monitoring often over extended periods (Rouse, in press). Exposure time to Shulze's solution ranged from 0 to 45 hours (fresh Shulze's every 6 hours) before final slides were made.  The wide variation in time in Shulze's solution is attributed to a combination of many variables, including amount of in situ oxidation of the sample, either at the time of burial or since exposure, differing compositions of palynomorphs recovered, and differences in thermal maturity level of individual samples.  STRATIGRAPHY:  Four lithostratigraphic units have been recognized in the study area. From oldest to youngest, they are the informal Jackson unit, and the Currier, McEvoy and Devils Claw Formations (Fig. 15). The lithostratigraphic units shown in figure 15 are described in detail in Chapter. II. Brief descriptions of the four lithostratigraphic units follow:  The Jackson unit is a thick marine shale unit that coarsens up in the upper part to include prodelta and delta front conglomerate, sandstone, siltstone, and thick coals. No base to the Jackson is known in the study area, but estimates of total thickness range up to 2000 metres (Moffat, 1985). The  to  o  w u  < h  W  K  BOWSER  BASIN  Sustut Group Skeena Group  U  Bowser Lake Group Spatsizi  K  Group/  Hazelton  Group  Takla Group  STUDY  AREA  Devils  CJaw _Fm._ McEvoy F m . Currier  Fm.  Jackson unit/ Ashman Fm.  Figure 15. Lithostratigraphic units in the study area and their relation to regional stratigraphy.  47  Currier Formation consists of 350 m to more than 600 m of interbedded shales, siltstones, sandstones, coals and carbonate layers. The McEvoy Formation consists of 600 m to 800 m of siltstones and shales with minor sandstones, thin coals, limestones and conglomerates. The Devils Claw Formation consists of 300 m to 550 m of strata characterized by thick pebble and cobble conglomerates, with associated coarse sandstones and minor siltstones and shales.  PALYNOASSEMBLAGES AND CHRONOLOGY  Palynomorphs recovered from the study area were identified by G.E. Rouse (pers. comm., 1988), and grouped into assemblages of diagnostic forms based on previous reports in the literature. Five distinct assemblages of diagnostic palynomorphs have been recognized, each with a distinct age range. The assemblages and their relation to the lithostratigraphic units in the study area are represented in figure 16 and described individually in the next section.  Recovered palynomorph assemblages, combined with previously published marine macrofossil dates, suggest two discrete depositional sequences (Fig. 16). The first extends from at least the Callovian to the Kimmeridgian or Tithonian in the Jurassic, and the second spans the late Barremian or Aptian to the Cenomanian in the Cretaceous (Fig. 16). Separating the two depositional sequences is a significant time period represented by either a hiatus, or sharply decreased sedimentation rates.  ASSEMBLAGES  Assemblage #1 -  Assemblage #1 comprises the diagnostic palynomorphs Psilatricolporites prolatus and c£ Granamonocolpites asymmetricus.  Cenomanian age has been interpreted for Assemblage #1, based on  the occurrence of both of the above forms in the Cenomanian of Minnesota (Pierce, 1961).  48  is  &3  CO to 3 in  e 5» 3  3=8 •M s  s.5 S iIl l' s  O  3|  S  3^  ? i O <u H  " 8-  i«  "I 2  S o> o> ~»  o 3 3  a Q) o Co co 8 00,0,0,0,  .So-^puLcuco -J  0,5? Co  cu  Cla  'ION  m  Devi or  OT fi  3  O >  (0 • 1—<  <  fi CO  o  3 fi  ^ fi cO  .2  c  ca  B CP Si SH  o fi CO  ^  (OT • i-H  CO  1—I  <!  fi  -4->  G  CD W OT  OT OT  fi  fi  CP O  W  a  CD OT OT <!  <  0  CO  OR  CD OT OT  6 cd  CD  • 1—1  b—1  o <:  CD  a  B  OT  =tt=  cd (—I  i—i  OT  ^ Gt-  C i E~  =*fc  CD  cd  CD  °>  8 o 3 P a 3 . 3 5 -b 3  CO  oo  cd  •3u g'-s  -a 8  L.  =«=  CD  a -3«  9 e'  •i> o a r  : e g. o s..  =tfc  °  d  • B  »««sli"f l i t  8«  8  a l l  32  •s-I  Q  C35  cO PQ  C  cO •i-H _>  CP  3  cO  K  03  fifi CO  cO cn  2  &0 fi CO 1-—H cO >  cp CP CO CO  fl  c cO c  '5ii T3  'C  o fl  CP  CM  in  a 6 2  s  fi fi cO 2  O  o  X  o  CO  o  fi  o fl <0  cn  CO  co  3IPPTH  snoaoTeq.aj:3  O T S S T 9  JETTLf  Figure 16. Palynoassemblages constructed from diagnostic palynomorphs and the lithostratigraphic units they were collected from are related to geologic time scale after Palmer (1983). The break between the McEvoy and Currier Formations is interpreted as a regional hiatus.  49  Tricolporate pollen are first encountered in the Lower Shaftsbury Formation of Western Canada of late Albian age (Singh, 1971). Assemblage #1 has been recovered from one sample of Devils Claw Formation.  Assemblage #2 -  Assemblage #2 comprises the diagnostic palynomorphs Tricolpites minutus, T. crassimurus, T. micromunus, Clavatipollenites minutus, Lycopodiumsporites crassatus, L. crasimacerius, Distaltriangulisporites costatus, Verrumonocolpites conspicuus, and the dinocysts Paleohystrichophora paucispina, Gonyaulacysta cf helicoides, Diconodinium pusillum, and Pseudoceratium regium. In  addition to the diagnostic palynomorphs listed above, a number of long ranging dinocysts were identified in each sample, suggesting that deposition was at least partly marine. Tricolpites crassimurus, Clavatipollenites minutus, and Lycopodiumsporites crassatus were reported from the late middle Albian  of the Peace River Formation (Singh, 1971). Assemblage #2 was compiled from four samples in the upper to middle McEvoy Formation and is interpreted to be mid to late Albian in age.  The dinocyst Paleohystrichophora paucispina may be indicative of an older subdivision of Assemblage #2. The palynomorph has previously been only reported from the mid Albian (Eisenack, 1972). In the study area, Paleohystrichophora paucispina was found only in the stratigraphically lowest sample bearing Assemblage #2. The presence of Paleohystrichophora paucispina which is restricted to the mid-Albian in only the lowest part of Assemblage #2 suggests that further divisions of this assemblage may be possible in the future.  Assemblage #2 extends up into the lower part of the Devils Claw Formation. Moffat et al. (1988) report the recovery of Tricolpites crassimurus, and Clavatipollenites minutus from a single sample of Devils Claw strata taken low in the Devils Claw Formation.  50 Assemblage #3 -  The palynomorphs Cedripites cretaceus, and Podocarpidites multessimus, and the dinocysts Pseudoceratium pelliferum, Pseudoceratium gochtii,  and Paleohystrichosphaeridium brevispinosa,  comprise Assemblage #3. The above forms have been reported from late Barremian and Aptian aged strata. To date, three samples of lower McEvoy Formation shale have yielded Assemblage #3 palynomorphs.  The ages assigned Assemblages #2 and #3 suggests the McEvoy Formation ranges from late Barremian to mid to late Albian in age. The presence of dinocysts in Assemblage #3 as well as Assemblage #2 suggests the McEvoy Formation is at least partly marine throughout.  Assemblage #4 -  The most diagnostic forms found in Assemblage #4 are the dinocysts Nannoceratopsis pellucida  and Pareodinia minuta which have ranges of Oxfordian and Oxfordian to Tithonian  respectively. Ceratosporites rotundiformis, found in association with the above forms, is also a useful Jurassic marker. Assemblage #4 has been found in three Currier Formation samples, ranging from near the base of the formation to within two hundred metres of the top. Pareodinia minuta was found without Nannoceratopsis pellucida in the stratigraphically highest sample of Currier Formation examined, suggesting that the Currier Formation may range from Oxfordian to Kimmeridgian or Tithonian in age.  Assemblage #4 of the Currier Formation is separated from late Barremian/Aptian Assemblage #3 of the McEvoy Formation by less than 200 metres of Currier Formation strata. In a  51 continuous exposure south of the Little Klappan River (section 8 in Fig. 17), no structural breaks were found between strata containing Assemblage #3 and #4.  Assemblage #5 -  Assemblage #5 comprises the dinocysts Gonyaulacysta cf. crassicomuta, Meiourogonyaulax cf.  callomonii, Chytroeisphaeridia c£ chictydia, Tenua cf. varispinosa, and Tenua granulata. Assembla  #5 is from upper Jackson roof shales sampled from Gulf Canada Drill Hole 8615 core located in coal measures northeast of Mount Klappan. The above palynomorphs have been reported from the Callovian of Western Canada (Pocock, 1970 and 1972). The dinocysts Valensiella ovula, Tenua cf.  evittii, Gonyaulacysta eisenacki, Scriniodinium d.parvimarginatum, and Gonyaulacysta jurassica  longicornis are also found in Assemblage #5 but are longer ranging Jurassic forms.  The interpreted Callovian age, coupled with the locally marine depositional environment implied by the diverse assemblage of dinocysts collected from roof shales, strongly suggests that the coal measures were deposited contemporaneously with thick marine shales of the Ashman Formation reported elsewhere in the Bowser Basin (Tipper and Richards, 1976; Evenchick, 1987). The coal bearing strata are interpreted as coastal peat swamps which were accumulating at the margins of the basin simultaneously with the deposition of thick shales in more marine settings toward the middle of the basin. Further work directly tieing the marine macrofossils used to date the Ashman Formation to the microfossils reported herein is necessary to clarify the relation between the Ashman Formation and the coal bearing strata.  P a l y n o s t r a t i g r a p h i c Correlations Northern Skeena Mountains  SE >  A  DEVILS CLAW  A s s e m b l a g e #1  A s s e m b l a g e #2  MCEVOY  LT  A s s e m b l a g e #3 «3>  —  &  Assemblage  CURRIER  #4 JACKSON  UNIT  CIDRE  Assemblage #5 0 m  Figure 17. Regional lithostratigraphic correlations and their relation to palynoassemblages identified in this study (see location map Fig. 18).  s o  Figure 18. Location map for above cross section (Fig. 17).  53 DISCUSSION  Marine influence  The presence of dinocysts in nearly all assemblages strongly suggests marginal marine conditions persisted in deposits from the Callovian to the late Albian. Assemblage #1 (Cenomanian age) is the only assemblage from which dinocysts have not been recovered to date. Dinocysts originate from dinoflagellates, and although some freshwater dinoflagellates are known (Tschudy and Scott, 1969) the wide variety of recovered dinocysts in this study almost certainly reflects a marine origin. Floral micro- and macrofossils, coal, and wood fragments are also present, clearly denoting intermittent terrestrial conditions or very close proximity to terrestrial sites.  Palynomorph suites were recovered from fine grained shales, in many locations directly overlying coal seams. The intimate juxtaposition of marine dinocysts with high sulphur terrestrial coal (Bustin, 1984) implies subsiding coastal coal swamps repeatedly drowned by marine transgression.  Depositional Sequences  Interpreted ages of the recognized palynoassemblages point to the presence of two successive depositional sequences. Sequence is used here in the sense of Sloss (1950) for depositional units bounded by major regional unconformities, and is equivalent to the term synthem proposed by the International Stratigraphic Code (1976). Diagnostic palynomorphs demonstrate that the depositional sequences are of significantly differing ages, although no disconformable contact has been recognized  54 in the field. Within each sequence, deposition was continuous, with no more than minor discontinuities which are beyond the resolution given by the present sample density.  The lithostratigraphic units which make up the depositional sequences and their characteristic palynoassemblages are described below.  The Jackson unit and Currier Formation comprise the older depositional sequence, and are characterized by assemblages #4 and #5. These assemblages are entirely Jurassic in age, extending from the Callovian to the Oxfordian and perhaps to as young as the Tithonian.  The McEvoy and Devils Claw Formations comprise the younger depositional sequence and are characterized by assemblages #1, #2, and #3. These three assemblages span the late Barremian or Aptian to the Cenomanian (mid-Cretaceous in this study). A regional hiatus of 20 to 35 million years, discussed in the next section, is interpreted to separate the Jurassic and Cretaceous sequences.  The two depositional sequences described above correlate both lithostratigraphically and chronologically to the Bowser Lake and Skeena Groups described by Tipper and Richards (1976), in the southern Bowser Basin (Chapter. II). Previously, all the rocks in the study area have been assigned to the Bowser Lake Group, due to apparent lithologic and stratigraphic continuity (Bustin and Moffat, 1983; Moffat et al., 1988). The International Stratigraphic Guide (Hedberg, 1976) states, however,  "...the union of adjacent strata separated by regional unconformities or major hiatuses into a single lithostratigraphic unit should preferably be  55 avoided, even if no more than minor lithologic differences can be found to justify the separation."  In light of the age difference between the Currier and the overlying McEvoy Formation, Bowser Lake Group is reserved here for the Currier Formation and older rocks.  It is preferred here to include the McEvoy and Devils Claw Formations in the Skeena Group because of nearly contemporaneous timing of deposition, generally similar lithology, and the desire to keep new names to a minimum. The Skeena Group, as described by Tipper and Richards (1976) in the southern Bowser Basin is lithologically and stratigraphically similar to the McEvoy Formation, except for the presence of abundant mica and some detrital volcanic units in Skeena Group sandstones which most likely reflect local provenance. Whatever name is ultimately accepted, the McEvoy and Devils Claw Formations are not time equivalent to the Bowser Lake Group, and must be included under different lithostratigraphic terminology,  Hiatus  The Jurassic and Cretaceous assemblages are separated by a time break of 20 to 35 million years interpreted as a regionally significant hiatus. The hypothesized hiatus extends from the Late Jurassic to the late Barremian or Aptian. The total time from the Callovian to the Cenomanian is roughly 65 to 75 million years (Fig. 16), therefore perhaps one third to one half of the geologic history recorded in the study area is actually represented by this hiatus.  Supporting the recognition of an earliest Cretaceous hiatus is the fact that a very distinctive palynomorph assemblage characteristic of Neocomian (earliest Cretaceous) aged sediments and widely  56 documented in the literature is completely missing from samples in this study. The Neocomian palynoflora is quite distinctive and readily identified if present (G. Rouse, pers. comm., 1988).  No strata of earliest Cretaceous age have been identified within the Bowser Basin. Palynomorph sample density of this study constrains any earliest Cretaceous strata to less than two hundred metres out of a total section studied exceeding 2000 metres in thickness (Cookenboo and Bustin, in press). Two hundred metres thickness should be regarded as an absolute maximum, and in all probability, earliest Cretaceous strata, if present at all, are much thinner.  Samples from an over one thousand metre thick continuous exposure southwest of Mt. Klappan in the northern Skeena Mountains (section #8 of Fig. 17) indicate that Assemblage #3 (late Barremian or Aptian age) replaces Assemblage #4 (Late Jurassic age) somewhere within a two hundred metre thick interval containing the Currier and McEvoy Formation contact. The contact is probably coincident with the interpreted hiatus, based on stratigraphic and sedimentologic characteristics. The contact is marked by subtle change in lithofacies, and was originally described as gradational and conformable (Bustin and Moffat, 1983). However, at least locally the change from Jurassic Assemblage #4 to mid-Cretaceous Assemblage #3 is associated with a distinctive conglomerate to pebbly sandstone zone 6 to 10 metres thick (Fig. 17). This conglomerate zone, although not everywhere present, has been correlated in association with the first appearance of Assemblage #3 for more than 40 kilometres in the study area (Fig. 18) and is the basal conglomerate for the McEvoy Formation (Chapt. IV).  Tectonic implications  Regional correlations that cross terrane boundaries can help constrain timing of accretionary tectonic events. Because allocthonous or "suspect" terranes are defined on the basis of stratigraphy that  57 is sharply different from adjacent areas, stratigraphy that correlates across terrane boundaries can be used to argue a minimum age for amalgamation or accretion.  By comparing the timing and lithologies of sedimentary deposits in the Bowser Basin with other basins of the Cordillera, important time constraints to the Jura-Cretaceous tectonic history can be resolved. Such comparisons involving the study area have only become possible with the relatively precise dates yielded by the palynostratigraphy documented in this study.  A brief review of the regional extent of the hiatus described above (implying possible tectonic relations) is followed by a specific comparison of the timing and lithology in basins both west of the Coast Plutonic Complex and east of the Rocky Mountains. The tectonic control may be reflected in a similarly timed magmatic lull noted throughout the Canadian Cordillera for more than ten million years in the Late Jurassic and Early Cretaceous (Armstrong, 1988). In all locations cited, the hiatus separates Jurassic and Cretaceous regressive units composed predominantly of marine to marginal marine sediments..  The Early Cretaceous hiatus documented above correlates both within the basin and regionally in other successor basins and the Western Canadian Basin (cf. Tipper and Richards, 1976; Jeletzky, 1978; Stott, 1982). The absence of early Cretaceous strata have been attributed to a hiatus or erosion between the Bowser Lake and Skeena Groups in the southern Bowser Basin (Tipper and Richards, 1976), below the Jackass Mountain Group in the Tyaughton Trough (Jeletzky, 1978), and Methow Basin (Trexler and Bourgeois, 1985; Kleinspehn, 1985), and below the Blairmore, Bullhead and Fort St. John Groups in the Western Canadian Basin (Stott, 1982). Similarly, the basal Cretaceous strata (Longarm Formation) of the Queen Charlotte Basin unconformably overlie the Middle Jurassic  58 Moresby Group or unnamed Late Jurassic sandstones and conglomerates (Cameron and Hamilton, 1988).  Widespread correlation of a similarly timed hiatus in lithologically similar strata of the Intermontane, Insular and Rocky Mountain Belts is consistent with both regions responding to the same tectonic influences during the Late Jurassic and Early Cretaceous. A profound change in tectonic regime from subsidence during the Late Jurassic to stability during the latest Jurassic and earliest Cretaceous and back to subsidence in the mid-Cretaceous is apparently a Cordillera wide phenomenon. The period of stability, inferred from the regional hiatus, is contemporaneous with a Cordillera wide magmatic lull based on extensive geochronometry (Armstrong, 1988). The roughly correlative stratigraphy reported from the North American craton across the Intermontane Belt to the Queen Charlotte Islands is consistent with all three regions being in nearly the same relative positions since at least the Late Jurassic.  CONCLUSIONS  1) . Five distinct palynoassemblages have been recognized in the north-central Bowser Basin, each yielding ages of nearly stage precision.  2) . The five palynoassemblages divide into Cretaceous and Jurassic sets associated with different lithostratigraphic units. Assemblages #1 (Cenomanian), #2 (mid to late Albian) and #3 (late Barremian to Aptian) are entirely Cretaceous, and have only been recovered from the Devils Claw and McEvoy Formations, which are herein considered to be part of the Skeena Group. Assemblages #4  59 (Oxfordian to Kimmeridgian or Tithonian), and #5 (Callovian) are no younger than Jurassic and are restricted to the Currier Formation and the Jackson unit, and are part of the Bowser Lake Group.  3) . Separating the Jurassic and Cretaceous assemblages is a hiatus of 20 to 35 million years, which encompasses all or part of the Tithonian, Berriasian, Valanginian, Hauterivian, and Barremian Stages. The hiatus is regionally important, and has been reported elsewhere in the Bowser Basin and in other basins of the Cordillera.  4) . Partially marine conditions, indicated by the presence of dinocysts, probably prevailed throughout the succession, at least to the upper Devils Claw Formation (Assemblage #1). Assemblage #1 of Cenomanian age is the only assemblage lacking dinocysts.  5) . Timing of deposition in the study area is similar to surrounding areas and consistent with the possibility that Bowser Basin may have been formed in nearly the same position relative to both the North American craton to the east and the Insular Belt to the west as it is found today.  60  REFERENCES CITED Armstrong, R. L. 1988. Mesozoic and early Cenozoic magmatic evolution of the Canadian Cordillera. Geological Society of America Special Paper 218, p. 55-91. Buckham, F., and Latour, B. A. 1950. The Groundhog coalfield, British Columbia. Geological Survey of Canada, Bulletin 16, 82 p. Bustin R. M., and Moffat, 1.1983. Groundhog Coalfield, British Columbia: reconnaissance stratigraphy and structure. Bulletin of Canadian Petroleum Geology, v. 31, p. 231-245. Bustin, R. M., and Moffat, I. In press. Semianthracite, anthracite, and meta-anthracite in the central Canadian Cordillera: their geology, characteristics and coalification history. Cookenboo, H. O., and Bustin, R. M. In press. Jura-Cretaceous (Oxfordian to Cenomanian) stratigraphy of the north-central Bowser Basin, northern British Columbia. Canadian Journal of Earth Sciences. Eisbacher, G. H. 1981. Late Mesozoic - Paleogene Bowser Basin molasse and Cordilleran tectonics, western Canada. In Miall, A. D. (Ed.), Sedimentation and Tectonics. In Alluvial Basins. Geological Association of Canada, Special Paper 23, p. 125-151. Eisenack, A., and Cramer, F. H. 1973. Katalog der fossilen Dinoflagellaten, Hystrichospharen, und verwandten Mikrofossilien; Band III, Acritarcha, 1. Teil, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, 1104 p. Evenchick, C. A. 1987. Stratigraphy and structure of the northeast margin of the Bower Basin, Spatsizi map area, north-central British Columbia. In Current Research, Part A, Geologic Survey of Canada, Paper 87-1A, p. 719-726. Evitt, W. R. 1969. Dinoflagellates and other organisms in palynological preparations. In Tschudy R. H. and Scott, R. A. (eds.) Aspects of Palynology. Wiley-Interscience, p. 439-479. Hedberg, H. D. (ed.) 1976. International stratigraphic guide. John Wiley and Sons, 200 p. Jeletzky, J. A. 1978. Causes of Cretaceous oscillations in sea level in Western and Arctic Canada and some general geotectonic implications. Geological Survey of Canada Paper 77-18, 44 p. Kleinspenh, K. L. 1985. Cretaceous sedimentation and tectonics, Tyaughton-Methow Basin, southwestern British Columbia. Canadian Journal of Earth Sciences, v. 22, p. 154-174. Koo, J. 1986. Geology of the Klappan Coalfield in northwestern British Columbia. British Columbia  61  Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, Paper 1986-1, p. 225-228. Moffat, I. W. 1985. The nature and timing of deformational events and organic and inorganic metamorphism in the northern Groundhog Coalfield: implications for the tectonic history of the Bowser Basin. Ph.D dissertation, University of British Columbia, Vancouver, 204 p. Moffat, I. W., Bustin, R. M., and Rouse, G. E. 1988. Biochronology of selected Bowser Basin strata: tectonic significance. Canadian Journal of Earth Sciences, v. 10, p. 1571-1578. Palmer, A. R. 1983. The decade of North American geology 1983 geological time scale. Geology, v. 11, p. 503-504. Pierce, R. L. 1961. Lower Upper Cretaceous plant microfossils from Minnesota. University of Minnesota, Minnesota Geological Survey Bulletin, v. 42,68 p. Pocock, S. A. J. 1970. Palynology of the Jurassic sediments of Western Canada. Part 1-Terrestrial species. Palaeontographica, Abt. B, v. Ill, p. 1-95, pi. 1-3. Pocock, S. A. J. 1972. Palynology of the Jurassic sediments of western Canada. Part 2. Marine species. Palaeontographica, Abt. B, v. 137, p. 85-153, pl.22-29. Rouse, G. E. In press. Palynological dating and paleoreconstruction of the gold and silver orebodies in the western Cordillera of Canada, United States, and Mexico. Ore Geology Reviews, Elsevier. Singh, C. 1971. Lower Cretaceous microfloras of the Peace River area, northwestern Alberta; Research Council Alberta Bulletin 28,542 p. Sloss, L. L. 1950. Paleozoic stratigraphy in the Montana area. American Association of Petroleum Geologists, v. 34, p. 423-451. Stott, D. F. 1982. Lower Cretaceous Fort St. John Group and Upper Cretaceous Dunvegan Formation of the foothills and plains of Alberta, British Columbia, District of Mackenzie and Yukon Territory. Geologic Society of Canada Bulletin, v. 328,124 p. Thomson, R. C, Smith, P. L., and Tipper, H. W. 1986. Lower to Middle Jurassic (Pliensbachian to Bajocian) stratigraphy of the northern Spatsizi area, north-central British Columbia. Canadian Journal of Earth Sciences, v. 23, p. 1963-1973. Tipper, H. W., and Richards, T. A. 1976. Jurassic stratigraphy and history of north-central British Columbia. Geological Survey of Canada, Bulletin 270, 73 p. Trexler, J. H., and Bourgeois, J. 1985. Evidence for mid-Cretaceous wrench faulting in the Methow  basin, Washington. Tectonics, v. 4, p. 379-394. Ward, L. F. 1905. The status of the Mesozoic floras of the United States. Monographs of the United States Geological Survey, v. 48,616 p.  63  Chapter. IV. SEDIMENTOLOGY  SEDIMENTOLOGY OF JURA-CRETACEOUS STRATA OF THE BOWSER BASIN, NORTHCENTRAL BRITISH COLUMBIA: IMPLICATIONS FOR CORDILLERAN TECTONIC HISTORY  ABSTRACT:  Lithofacies associations have been described from over 2000 m of Jura-Cretaceous strata exposed in the Skeena Mountains of north-central British Columbia. The strata are part of the Bowser Basin, a Jura-Cretaceous basin in-filled by detritus derived from the uplift of the Canadian Cordillera. The lithofacies associations are interpreted as a succession of fine- and coarse-grained deltaic and fluvio-deltaic deposits. Changes in lithofacies associations through time relate to the tectonic history of the Cordillera.  Four lithostratigraphic units are recognized in the study area. From oldest to youngest, the units are the informally named Jackson unit, and the Currier, McEvoy and Devils Claw Formations. These lithostratigraphic units were deposited as a result of progradation and aggradation of two regionally correctable regressive depositional sequences, one of Middle to Late Jurassic age and the other of mid-Cretaceous age. The Jurassic regressive sequence is equivalent to the Bowser Lake Group and consists of rocks of the Currier Formation and older units. The McEvoy and Devils Claw Formations, which are correlative with the Skeena Group, comprise the younger sequence. A regionally correctable hiatus separates the two depositional sequences.  The described lithofacies associations are characteristic of individual lithostratigraphic units. The Currier Formation is characterized by a transitional deltaic lithofacies association, which includes marsh, crevasse splay, interdistributary bay, distributary and prodelta deposits. The transitional deltaic lithofacies association is analogous to depositional environments described from the modern  64 Mississippi River delta. The lithofacies associations of the McEvoy and Devils Claw Formations are interpreted as coarse-grained fluvio-deltaic deposits, analogous to the modern Copper River delta in southeastern Alaska.  The clast lithologies are dominated by chert, which indicates an uplifted sedimentary source and paleocurrent indicators suggest that the source was to the east. The sediment source area appears to have remained the same for all Bowser Basin deposits, suggesting that the source must have been a large area. The sedimentary origin, easterly direction and large volume of detritus, point to North America as the likely provenance. North American provenance has important implications to Cordilleran tectonic history, because it implies that the Bowser Basin, and underlying Stikinia, must have been adjacent to the continent since at least the Middle Jurassic.  INTRODUCTION  Thick sedimentary strata exposed in the Skeena Mountains are of critical importance to understanding the tectonic history of the Canadian Cordillera. These strata comprise part of the Bowser Basin, a more than 4500 m thick accumulation of sedimentary rocks exposed over much of north-central British Columbia. Bowser Basin sediments were deposited as a direct consequence of tectonism involved in the formation of the Cordillera and therefore their depositional history serves as a record of Cordilleran tectonic history. Understanding the depositional history of the Bowser Basin has been the primary goal of an integrated lithostratigraphic, biostratigraphic and sedimentologic study conducted in the Skeena Mountains over the last two years (Cookenboo and Bustin, in press; Chaps. II and III of this thesis). This study has produced insights into timing and environment of deposition of Bowser Basin sediments that significantly constrain tectonic development of the Cordillera.  The study area is located in the northern Skeena Mountains, 240 kms north-northeast of  1  Prince Rupert, British Columbia (Fig. 19). The area covers approximately 4000 km in north-central  Figure 19. Locator map for the study area in the Skeena Mountains of northern British Columbia.  British Columbia within the watersheds of the Skeena, Nass, Klappan, and Spatsizi Rivers. The boundaries of the study area approximately coincide with the Groundhog coalfield, the largest anthracite deposit in Canada.  The strata exposed in the study area are the upper 2000 m out of a total section of more than 4500 m of Bowser Basin sediments deposited between the Middle Jurassic and the earliest Late Cretaceous. The Bowser Basin sediments unconformably overlie thick Lower and Middle Jurassic volcanic strata that form the base of the Bowser Basin. The upper sedimentary strata of the Bowser Basin are well exposed in the study area.  STRATIGRAPHY AND STRUCTURE:  Setting  The Jura-Cretaceous sediments of the Bowser Basin correlate with strata of a number of spatially separate Intermontane basins (Fig. 20). The observation that Bowser Basin sediments correlate with other areas of the Cordillera is not new (Eisbacher 1974c; Tipper 1981). Eisbacher (1974c) considered the Bowser Basin to be the type area for Bowser Assemblage rocks that occur in the Tyaughton, Tantalus, and Dezadeash basins. Tipper (1981) recognized a widespread Middle Jurassic shale episode in the Cordillera covering the Intermontane basins as well as parts of Vancouver and the Queen Charlotte Islands. Tipper (1981) also recognized a widespread Albian (midCretaceous) marine transgression that covered much of central British Columbia. Detailed  67  Figure 20. Jura-Cretaceous basins of the Intermontane Belt.  68  stratigraphic and sedimentologic study of the Bowser Basin reported later in this paper sheds new light on these correlations.  Palynostratigraphic evidence presented in Chapter III of this thesis indicates that coastal to marginal marine depositional conditions persisted during most if not all of Bowser Basin sedimentation in the study area. Dinocysts, which are usually of marine origin, have been recovered from all but the stratigraphically highest palynology sample examined. The same samples also contain spores and pollen, and are closely associated with coal and leaf fossils, which strongly point to terrestrial environments. The close association of marine and terrestrial biota is consistent with a marginal marine setting.  Lithostratigraphic and palynostratigraphic evidence presented in Chapters II and III of this thesis, and reviewed briefly below, demonstrate that deposition occurred in two successive periods, one in the Late Jurassic and the second in the mid-Cretaceous.  Stratigraphy  For nearly a century, geologists have studied various rocks in the Bowser Basin, and have described those rocks using a plethora of stratigraphic names. The most useful and widely recognized stratigraphic division recognizes six major rock groups (Fig. 21). The oldest strata assigned to the Bowser Basin are Late Triassic basaltic and andesitic volcanics of the Takla Group (Tipper and Richards, 1976). Overlying the Takla Group are Early to Middle Jurassic volcanic and volcaniclastic rocks. In the south along the Skeena Arch, these rocks are referred to as the Hazelton Group and in the north similar rocks exposed along the Stikine Arch are assigned to the Spatsizi Group .  Beginning in the Middle Jurassic clastic sedimentary deposits filled the Bowser Basin. These sediments lie unconformably above the Hazelton and Spatsizi Groups, and have previously been  69  divided into two groups: 1) the Middle to Late Jurassic aged Bowser Lake Group; and 2) the midCretaceous age (Hauterivian to Cenomanian) Skeena Group (Tipper and Richards, 1976). Separating the two groups is a regionally correlatable hiatus. The groups are depositional sequences in the sense of Sloss (1950), who defined sequences as depositional units separated by major regional unconformities. Recognition of two successive depositional sequences has important implications for the depositional history of the study area. Bowser Basin deposition ended by the earliest Late Cretaceous and was succeeded by deposition of the Sustut Group in the Sustut Basin to the east of the Bowser Basin (Bustin and McKenzie, in press).  Four lithostratigraphic units are exposed in the study area (Fig. 21). The upper three units are, from youngest to oldest, the Devils Claw, McEvoy and Currier Formations, and are fully described in Cookenboo and Bustin, (in press; and Chap. II). The oldest strata are less well exposed and informally referred to as the Jackson unit (Bustin and Moffat, 1983). Only the upper Jackson unit is well exposed in outcrop.  In the study area, the Jackson unit and Currier Formation comprise the Bowser Lake Group and the McEvoy and Devils Claw Formations comprise the Skeena Group (Chapt. III).  The ages of each unit are detailed in Chapter III, as determined by palynology. A brief summary of the lithostratigraphy and palynostratigraphy is provided for each formal unit later, followed by description of the recognized lithofacies associations.  Structure  The Bowser Basin has been subjected to tectonic deformation resulting in numerous thrust faults and folds. The faulted and folded strata makes correlation between exposures difficult and directly controls the outcrop pattern. The dominant structural feature in the study area is a northwest-  70  STUDY AREA  BOWSER BASIN  Sustut Group  Devils Claw F m .  Skeena Group  McEvoy F m .  Bowser Lake Group  C u r r i e r Fm.  Spatsizi  Group/  Hazelton  Group  Takla Group  N  Jackson unit/ Ashman Fm.  Figure 21. Stratigraphic units of the Bowser Basin. On the left are widely recognized groups, and on the right are the lithostratigraphic units within the study area.  71 southeast trending broad synclinorium located between the Skeena and Nass rivers, referred to as the Beirnes Synclinorium (Eisbacher, 1974b; Fig. 22). The Devils Claw Formation is exposed along the axis of the synclinorium, with progressively older units exposed in concentric belts around its margins (Fig. 23).  The dominant trend of folds and thrust faults is northwest-southeast, approximately paralleling the axis of Beirnes Synclinorium. Total structural shortening of 35% has been estimated perpendicular to the northwest-southeast structural trend (Moffat, 1985). This dominant structural trend was attributed to the earliest and most intense phase of deformation in the area (Moffat, 1985). Detailed structural mapping in the area surrounding Mt. Klappan suggests a second and less intense phase of deformation that resulted in broad folds trending nearly perpendicular to the first phase (Moffat, 1985).  Moffat concluded that deformation was entirely post-depositional and therefore post-Albian in age. Structural mapping in Bowser Basin sediments north of the study area by Evenchick (1987), however, has disclosed post-Kimmeridgian and pre-Albian deformation. Pre-Albian deformation is also recorded by a widespread angular unconformity in the Sustut Basin (Eisbacher, 1974a). PreAlbian deformation in the immediate vicinity of the study area suggests that the Currier Formation and Jackson unit may have been deformed prior to deposition of the McEvoy and Devils Claw Formations.  LITHOFACIES  Distinctive lithofacies associations recognized in the Currier, McEvoy and Devils Claw Formations are described below. The lithofacies associations record a succession of deltaic and fluvio-  73  Figure 23. Cartoon of approximate outcrop extents for the four lithostratigraphic units in the study area.  74  deltaic depositional environments that become progressively more coarse and fluvially dominated through time.  Deltaic facies described below differ significantly between the Currier Formation and the McEvoy and Devils Claw Formations. The Currier Formation is interpreted to be generally analogous to major progradational modern deltas such as the Mississippi River delta. The McEvoy and Devils Claw Formations are also interpreted to be deltaic, but are more coarse grained than most modern deltas. The McEvoy and Devils Claw Formation delta system has similarities to the modern Copper River delta of southeastern Alaska, although the analogy is imperfect. Interpretation of the McEvoy and Devils Claw Formations is made following description of lithofacies associations for both formations, because essentially the same depositional system is thought to be responsible for both. This is consistent with the McEvoy and Devils Claw Formations being part of the same depositional sequence.  Currier Formation:  The Currier Formation comprises shales, siltstones, sandstones, coals and rare conglomerates exposed in the watersheds of the Skeena and Nass Rivers. The best exposures are at the northeast and southwest ends of the Beirnes Synclinorium where overlying McEvoy and Devils Claw Formations have been removed by erosion. Continuity between the northern and southern areas of exposure is considered likely due to lithostratigraphic similarities (Chapter. II of this thesis), although correlation is not directly demonstrable due to cover of overlying McEvoy and Devils Claw Formations in the center of the Beirnes Synclinorium and recent alluvium deposits in the river valleys. The thickness of the  75 Currier Formation ranges from 350 m at the southern end of the Beirnes Synclinorium (Bustin and Moffat, 1983) to more than 600 m in the north (Cookenboo and Bustin, in press).  The Currier Formation is Late Jurassic in age, spanning the Oxfordian to Kimmeridgian or Tithonian stages (Chapter III). The Currier Formation gradationally overlies prodelta and delta front lithofacies characteristic of the upper Jackson unit (Moffat, 1985), and is overlain by the Cretaceous aged McEvoy Formation (Chapter II).  Lithofacies Associations  Lithofacies characteristic of a variety of marginal marine, deltaic depositional environments are interbedded in a regressive sequence. The different facies occur in a complex arrangement described together here as the transitional deltaic lithofacies association.  Diverse plant macro- and microfossils occur in fine grained beds of the Currier Formation. These plant fossils provide evidence for both age and depositional environment. Well preserved leaves of 11 species of plants (see Chap. II), including conifers, ginkgos, cycads and ferns, and wood fragments, including sticks and stumps, are common, especially in the upper beds of the formation. Palynomorph samples have yielded a variety of dinocysts, which indicate deposition was partially marine (Chap. III). Time distinctive dinocysts date the section as Oxfordian to as young as Tithonian. Marine macrofossils of Oxfordian age have been recovered in the upper Jackson unit, but are generally lacking in the Currier Formation (Moffat, 1985). Preferential preservation of marine dinocysts over marine macrofossils reflects carbonate dissolution of macrofossils by groundwater. Such environmental requirements suggest a relatively humid terrestrial environment, and are consistent with the occurrence of abundant plant fossils and coals. The variety and abundance of leaves, trees, and  76  dinocysts in conjunction with a lack of marine macrofossils suggests lush coastal forests and marshes alternating with near-shore shallow marine deposition.  Transitional Deltaic Lithofacies Association  Coal  Coals in the Currier Formation are generally less than 1.5 m thick, although one seam of 4 m was reported by Bustin and Moffat (1983) east of Mt. Gunanoot. Compositional characteristics, including sulfur and ash content, support coastal marsh depositional environment. Sulfur content, which generally increases in coals deposited in association with marine water (Teichmuller and Teichmuller, 1975) , averages 1.2% from 61 samples of Groundhog coal analyzed by Buckham and Latour (1950). Pyrite occurs in euhedral crystals, framboids and on cleat surfaces. Coals have an ash content that is only rarely low and grades into carbonaceous shale. Coal seams commonly overlie coarsening upward sandstones, suggesting accumulation in marshes formed in association with an active sediment transport system. In addition, shales directly overlying coal seams (roof shales) have yielded marine dinocysts (Chap. II). High sulfur and ash content, and the presence of dinocysts in adjacent shales, are all factors that suggest marsh deposition in a coastal environment.  Shales  The fine grained clastic facies are described below as two ideal end members of a continuum of beds resulting from a variable depositional regime. Two end member facies of shales are recognized: 1) dark grey to black carbonaceous shale which commonly has layers rich in plant remains and little recognizable internal structure, and which is interpreted to represent fine sediment accumulation in interdistributary bays and lagoons; and 2) black laminated shale with flat-lying brown silty laminations varying in thickness from less than 3 mm to more than 1 cm, and generally lacking in  77 fossils. Preservation of fine laminae indicates minimal bioturbation, which may result from either rapid burial or anoxic bottom waters. Rapid sedimentation is more likely given the associated depositional environments which point to active sedimentation in shallow marine deltaic conditions. The black laminated shales are also commonly lacking in palynomorphs and leaf fossils, suggesting that sedimentation may have been rapid, allowing little opportunity for accumulation of organic remains. The laminated shales are analogous to prodelta shales of the modern Mississippi River, which preserve laminae due to rapid burial (Coleman, 1982).  Diagenetic carbonate layers within the shale facies are common. These layers are up to 1 m thick (although more usually 20 to 40 cm thick) and commonly form laterally continuous beds that are traceable across even extensive outcrops (Fig. 24). Less well developed yellow-brown horizons consist of separate ellipsoidal concretions. During growth of concretions, one concretion may impinge on the next, eventually forming a continuous bed. The presence of a gradational set of increasingly well developed carbonate layers in Currier Formation sediments supports a diagenetic origin. The carbonates weather yellow-brown, are very resistive, and commonly have very well preserved plant fossil impressions on their surfaces. Fresh surfaces are black and featureless. The yellow-brown layers most commonly occur at the top of shale beds, possibly related to short lived exposure surfaces. Well preserved fossil leaves commonly associated with carbonates in the study area are consistent with subaerial exposure for at least some layers. Similar carbonates resulting from early diagenesis are widely reported from rapidly deposited, organic rich fine grained deposits (cf. Hallam, 1967; Curtis and Spears, 1968). The breakdown of organic matter provides an abundance of dissolved HCO^ necessary for formation of diagenetic carbonate. Similar carbonate layers occur throughout  78  Figure 24. Yellow brown concretionary layer in shale bed.  79 the McEvoy and Devils Claw Formations.  Sandstones  Sandstones 1 to 3 m thick are interbedded with shale or siltstone and commonly coarsen upward. The finer grained interbeds are up to 40 cm thick and concentrated near the base. Interbedded sandstone is replaced by massive fine grained sandstone beds at the top. Intervals of finer grained strata generally between 4 and 8 m thick separate one coarsening upward sandstone from the next. Coarsening upward sandstones suggest active filling of interdistributary bays and are interpreted as crevasse splay deposits. The interval between sandstones of 4 to 8 m probably roughly reflects water depths in the interdistributary bays. Sandstones 1 to 3 m thick with sharp and erosive basal contacts which may contain shale rip-up clasts also occur, but less commonly than coarsening upward sandstones. The erosive bases and rip-up clasts composed of locally derived shale pebbles suggest channel deposits, perhaps crevasse channels leading from distributaries to interdistributary bay fill deposits.  Thicker, highly resistant and well sorted sandstones also occur in the Currier Formation. These resistant sandstones range from 6 to 15 metres in thickness, commonly have erosive bases, and mayfineupward. The sandstones are compositionally similar to the more common and thinner sandstones described above. The greater thickness of these sandstones appear to relate them to a distinctly larger stream system. These sandstones are therefore interpreted to reflect deposits of the major distributary channels which feed the crevasse splay systems interpreted for thinner sandstones described above.  The sandstones arefineto veryfinegrained, with moderately to well sorted and sub-angular to sub-round textures, and are composed dominantly of rock fragments with lesser amounts of quartz and minor feldspar. Rock fragments are dominantly chert. The sandstone classification system of Folk  80 (1968) which treats chert separately from quartz is used herein, because of the large portion of chert. By this system, the sandstones are chert-arenites. Sub-angular to sub-round nature of grains is consistent with a proximal source area. The mature composition of the sandstones, contrasts with the roundness, but probably reflects source area composition more than transport distance. Extensive diagenetic replacement and alteration of grains makes identification of original constituents problematic, and may result in under-estimation of feldspar.  Conglomerates  Resistant chert pebble conglomerates 5 to 15 m thick, occur, but only infrequently in the Currier Formation. Conglomerates are generally widely spaced (separated vertically by more than 200 m of strata), and are more common in the lower Currier Formation. These conglomerates are mostly clast supported, with rounded, and well sorted chert pebble clasts in a fine chert sand matrix. Clasts are usually between 5 and 25 mm across with larger pebbles concentrated at the base. Maximum observed clast size is less than 5 cm. In general, pebbles are smaller, more rounded, and better sorted than in the McEvoy and Devils Claw Formation conglomerates. Beds are laterally extensive on the order of 100's of m and some have erosive bases which cut down as much as 1.5 m down into underlying mudstones. A 15 m thick conglomerate bed exposed on a cliff face south of Mt. Klappan extends to the west for more than 300 m, at which point pebbles are replaced by fine sandstone which has eroded into black laminated shale facies. To the east conglomerate thins rapidly and is replaced by sandstone which is continuous to the end of the outcrop (approx. 200 m). The conglomerates are similar in scale, and as in the previous example, may be laterally gradational with thick sandstones. The fining upward nature of the conglomerate, and its deep erosion into fine grained deposits suggests distributary channel deposition, similar to channel sands from the modern delta of the Mississippi River (Coleman^ 1982). A conglomerate 3 m thick on Mount Klappan east of the Spatsizi River coarsens upward from sandstone to pebbly sandstone and finally clast supported pebble conglomerate.  81  The coarsening upward conglomerate indicates aggradation, and is similar to distributary mouth bar deposits of the Mississippi delta (Coleman, 1982)  Interpretation  Lithofacies recognized in this association are common to a variety deltaic depositional environments including prodelta, crevasse splay, distributary channel and mouth bar, interdistributary bay or lagoon, and marsh. The various lithofacies occur in a complex, but generally coarsening upward sequences up to 30 m thick, suggesting a prograding depositional system (Fig. 25). The marginal marine setting indicated by the occurrence of both marine and terrestrial fossils supports a deltaic interpretation for this lithofacies association.  Analogous facies have been described in the modern Mississippi delta for each of the Currier Formation lithofacies (Coleman, 1982). Clays that are similar to the carbonaceous black shale in the Currier Formation accumulate in interdistributary bays in the Mississippi delta. Black laminated shales have their analogs in the prodelta muds of the Mississippi,which are rapidly deposited and therefore sedimentary structures are not disturbed by bioturbation.  Sandstones and the rare conglomerates of the Currier Formation also have their analogs in Mississippi delta sediments. The most commonly occurring sandstones in the Currier Formation are 1 to 3 m thick and coarsen upward, similar to bay fill sands in the Mississippi delta. Erosive based sandstones of the Currier Formation are analogous to crevasse splay channels. Thicker sandstones (up to 15 m thick) and conglomerates of the Currier Formation are analogous to distributary channels and mouth bar. These coarser deposits occur in both aggradational coarsening upward beds and sharp based, erosive beds, similar to distributary channel and mouth bars deposits in modern deltas.  Figure 25. Typical strata of the transitional deltaic lithofacies association of the Currier Formation. Arrows point upward in direction of repeated but poorly defined coarsening trends.  83 The conglomerates are coarser than deposits of most modern rivers, including the Mississippi and Niger (Allen, 1965; Coleman, 1982), but this is believed to be a reflection of different material transported in bedload rather than a fundamental difference in depositional processes. This point will be discussed at greater length in the development of a depositional model for the McEvoy and Devils Claw Formations later.  McEvoy Formation  The McEvoy Formation is dominated by recessive siltstones interbedded with fine to very fine grained sandstones and shales exposed over a wide area of the Groundhog coalfield. The formation is exposed from south of the Little Klappan river in the north to McEvoy Ridge in the south (Fig. 26). In the Beirnes Synclinorium, the McEvoy Formation is overlain by the Devils Claw Formation, but good exposures run the length of the Skeena and Nass River valleys in the study area, and extend east of the Skeena to the Kluatantan River. Complete sections of the McEvoy Formation are unknown but the total thickness, based on correlations between several sections, is estimated to exceed 800 m.  McEvoy Formation exposures are recognized and correlated in thefieldbased on subtle lithologic differences from the underlying Currier Formation, including the occurrence of less coal, thicker siltstones, and more plant fossils. The McEvoy Formation is overlain gradationally by the Devils Claw Formation.  The McEvoy Formation was deposited during the mid-Cretaceous, and unconformably overlies the Late Jurassic Currier Formation. Palynomorphs indicate ages extending from the late Barremian or Aptian to the middle or late Albian for the McEvoy Formation (Chap. III).  Lithofacies Associations  Figure 26. Thick recessive siltstones of the McEvoy Formation. Resistant cliff to the right is the lowermost Devils Claw Formation.  85 Three lithofacies associations have been recognized in the McEvoy Formation: 1) a siltstone lithofacies association which is interpreted as distal coarse grained delta deposition; 2) a resistant coarse conglomerate lithofacies association, which is interpreted as coarse grained distributary channel deposit; and a 3) a basal conglomerate lithofacies, named for its coarse grain size and occurrence in the lowermost McEvoy Formation.  A wide variety of pollen, spores and marine dinocysts have been recovered from shales of the McEvoy Formation (Chap. III). Among the pollen are some of the earliest angiosperm species, including monolcolpate and tricolpate forms, supporting the interpreted age range of late Barremian to middle or late Albian (Chap. III). Plant macrofossils are also abundant in fine grained facies, including leaves of ferns, ginkgos, conifers and cycads (Chap. II). In addition, tree trunks up to 60 cm in diameter and 4 m long occur in growth position, attesting to periods of rapid burial (Fig. 27).  Siltstone lithofacies association  The siltstone lithofacies association is the major facies of the McEvoy Formation, comprising most of the recessive exposures that characterize the formation. The lithofacies association is dominated by coarsening upward sequences from 2 to 6 m thick composed of siltstone and interbedded black shale, fine to very fine sandstone and minor thin coal. Coarsening upward sequences are stacked repeatedly to form sets commonly exceeding 30 m in thickness (Fig. 28). The sets are separated by fining upward sandstones and thick black shales.  A typical complete coarsening upward sequence (CUS) consists of a black shale bed (with carbonaceous shale or thin coal at the base) that grades up to dark gray to gray siltstone, which in turn grades up into very fine grained sandstone. Dark gray to gray siltstone comprises more than 70% of the typical CUS, although some sequences are dominated by shale and others by sandstone. Sandstones capping the sequence are 1 to 3 m thick, silty and very fine grained near the base, becoming  86  Figure 27. Tree trunk in life position in the McEvoy Formation.  Figure 28. Typical well defined coarsening upward sequences from the siltstone lithofacies of the McEvoy Formation. Arrows point in the direction of coarsening trend. A single fining upward sandstone that breaks up the middle of this section may represent crevasse splay channel.  0  m  88  better sorted upward. The capping sandstones are chert-arenites, with sub-angular to sub-rounded grains. Each CUS is bounded by sharp contacts at the top and base and displays gradational contacts internally. Often only two lithologies (shale and siltstone or siltstone and sandstone) are present in individual sequences. The stacked CUS are analogous to interdistributary bay fill facies associated with crevasse splays of the modern Mississippi River (Coleman, 1982).  Resistant fining upward sandstone beds up to 6 m thick occur interbedded with the coarsening upward sequences. These sandstones are irregularly spaced 10 to 70 m apart vertically, and tend to be lens shaped with erosive bases. Rip-up clasts of shale are common in the lower portions of these sandstones. The lower 1 to 3 m of the fining upward sandstones are very thick bedded, fine grained and well sorted and grade upwards to thin bedded, very fine grained, moderately sorted to poorly sorted and silty sandstone. Sandstones are fine to very fine grained chert-arenites of similar composition to those in the Currier Formation. Planar cross-bed sets 25 to 75 cm thick are common in the very thick bedded basal portion, and trough and ripple cross-laminations are common in the somewhat finer upper portions.  Channel deposition is interpreted for the fining upward sandstones based on occurrence of erosive bases, rip-up clasts, and lens shapes. Stream channel depth is roughly estimated to be on the order of 2 to 3 m based on thickness offiningupward sandstones. These relatively small channels may reflect a system of subsidiary distributary streams leading from a major channel or crevasse to the coarsening upward sequences of interdistributary bay fill facies.  Thick black shales occur interbedded with the coarsening upward sequences, much as the fining upward sandstones. These black shales are between 3 and 15 m thick, and interbedded with argillaceous siltstones, thin carbonaceous layers, and narrow (usually less than 10 m wide) sandstone lenses. Plant fossils are common, especially in the carbonaceous layers. Lamination is uncommon in this shale facies. The black shales are interpreted to have been deposited in marshes or  89 interdistributary bays protected from currents, due to the occurrence of fine elastics and organic matter.  Coarse conglomerate lithofacies association  The coarse conglomerate lithofacies association is characterized by resistant lens shaped and laterally extensive conglomerates, some of which can be traced in continuous outcrop for over 2 km. This facies consists of resistant chert pebble-cobble conglomerates interbedded with medium to coarse sandstones and minor siltstone, shale and coal. The conglomerates reach 30 m in thickness, although 10 to 15 m is more common. Two or three of the most laterally extensive coarse conglomerates occur in a zone approximately 200 m thick in the middle of the McEvoy Formation. The conglomerates have been correlated lithologically and palynologically more than 20 km across the study area. The more lens shaped conglomerates are usually less than 10 m thick and occur widely scattered throughout the McEvoy Formation.  Conglomerates are characteristically clast supported, and poorly sorted chert pebble-cobble conglomerates composed of rounded to well rounded clasts and a minimal amount offineto medium grained sandstone matrix. Maximum clast size reaches 10 cm. Conglomerate clasts are more than 95% chert, and most are shades of gray, green, white, or black. Colors are gradational between green, gray and white, making color determination subjective and not easily repeatable, but the black pebbles are distinctive in outcrop. Counts of more than 500 adjacent pebbles in McEvoy strata near Devils Claw Mountain reveal that black chert comprises 20% of the total. Variation in black chert percentage may have provenance significance, but the present data base is too limited and further counts are needed.  The coarse conglomerate lithofacies association is interpreted as distributary channels and bars of a large delta system. Deltaic deposition is supported by the presence of both dinocysts and  90 abundant terrestrialfloralremains, which points to a marginal marine environment, and relatively rapid accumulation of associated thick coarsening upward fine grained deposits. Channels in the coarse conglomerate facies were apparently much deeper and more powerful (conglomerate/sandstone fining upward beds up to 10 m thick) than in the siltstone lithofacies association. Large, high energy rivers within a rapidly aggrading system of smaller streams in a coastal position suggest a distributary system of rivers leading to the smaller aggrading crevasse splays and interdistributary bay fill deposits of the siltstone lithofacies association.  Basal conglomerate lithofacies  A conglomerate and pebbly sandstone facies of unusual character has been correlated for more than 40 km at the base of the McEvoy Formation. The facies occurs as a bed of matrix supported chert granule-pebble conglomerate, with poorly sorted sub-rounded clasts, which ranges from 6 to 10 m thick.  Clast composition is more varied than in other conglomerates in the study area. Chert pebbles predominate, but rounded sandstone and white quartz pebbles also occur. The matrix is fine to medium grained sandstone, which comprises approximately 50% of the bed. Pebbly sandstone of similar composition to the matrix replaces conglomerate locally. Clast supported conglomerate interbeds up to 25 cm thick and of limited lateral extent occur locally, generally concentrated near the base of the conglomerate facies. The largest pebbles are usually less 2 cm across, with none larger than 4 cm observed.  Although conglomerate that fits the description of the basal conglomerate lithofacies is not unique in either the McEvoy or Devils Claw Formations, it is unusual. The combination of unusual character and position near the base of the McEvoy Formation distinguishes this conglomerate from the coarse conglomerate facies, and may give this bed important tectonic significance.  91 Interpretation  The lithofacies associations of the McEvoy Formation are interpreted below in relation to analogous depositional environments. The basal conglomerate lithofacies association is comparable to (and contemporaneous with) the Cadomin Formation of the Western Canadian Basin. The siltstone and coarse grained lithofacies associations are typical of deltaic deposition, and are analogous to lithofacies described for both common deltas such as the modern Mississippi and Niger deltas and coarse grained deltas like the Copper River delta in southeastern Alaska (Galloway, 1976).  The basal conglomerate facies records the initial rapid progradation over Bowser Lake Group strata (including the Currier Formation) and is interpreted to mark the onset of renewed tectonic activity following a regional hiatus (Chap. III). This facies is sedimentologically and stratigraphically similar to the Cadomin Formation of northeastern British Columbia and Alberta (Varley, 1984). The Cadomin Formation is also a basal conglomerate of probable Barremian or Aptian age deposited on an unconformity surface (Stott, 1967). Varley (1984) interprets the Cadomin Formation to be a proximal to distal alluvial braidplain deposit, based in part on regional variations in clast texture. The basal McEvoy conglomerate is texturally quite similar to the distal braidplain facies of Varley (1984), and may represent a similar depositional environment.  The siltstone and coarse conglomerate lithofacies associations of the McEvoy Formation suggest deposition in a marginal marine, coarse grained delta environment. Deposition in a marginal marine setting is supported by the association of abundant dinocysts with fossil leaves and trees. The siltstone lithofacies association is interpreted as deposits of subsidiary streams leading from the coarse grained distributary system. The characteristic fine grained coarsening upward sequences and interbedded marsh and small stream facies that comprise the siltstone lithofacies association are typical of modern deltas such as the Mississippi and Niger (Allen, 1965; Coleman, 1982). The coarse  92 conglomerate lithofacies association suggests high energy channel deposits, and is consistent with major rivers carrying a coarse clastic sediment load and.  The Copper River delta of southeastern Alaska is in many ways a modern analog to the McEvoy Formation, although significant differences exist. The Copper River delta is a humid region, coarse grained delta which has pebbly, coarse sand filled distributaries in its proximal facies, and fine to very fine grained muddy sands in its distal portions (Galloway, 1976). Marshes and mudflats also exist in the proximal reaches (Galloway, 1976). The Copper River delta was classified by Galloway (1976) as a "fan-delta", but McPherson et al. (1987) suggest that the term fan-delta be restricted to alluvial fans that prograde into standing water, and that the Copper River delta is better referred to as coarse grained braid-delta. The term coarse grained delta is used herein to describe the McEvoy and Devils Claw Formation depositional system because, although their deltas were clearly coarse grained, there is little direct evidence of whether braided river conditions prevailed or not.  Although analogous in many respects to the Copper River delta, the setting of the McEvoy Formation differs in that the Copper River deposits on to a 100 to 200 m deep shelf and is restricted to a limited area by nearshore mountains. Abundant well preserved leaf fossils and thin nonmarine coals suggest that the McEvoy Formation was probably deposited in shallower water, and no evidence exists to suggest the deposition was laterally restricted by topography. Deposition in shallow marine waters unrestricted by local topography may lead to the mixing of coarse distributary deposits within thick interdistributary siltstones by encouraging progradation and delta switching.  Devils Claw Formation:  The Devils Claw Formation is dominated by massive cliff forming conglomerates, which hold up many of the higher peaks and ridges of the Skeena Mountains. Interbedded with the conglomerates are sandstones, siltstones, shales and coals. The formation is exposed between the headwaters of the  93 2 Skeena and Nass rivers for over 1500 km from Tahtsedle Creek in the north to Devils Claw Mountain in the south. In the central portion of the Beirnes Synclinorium, the Devils Claw Formation exceeds 600 m in thickness (Fig. 29). Access is limited to some of the higher exposures by steep cliffs and permanent snow fields.  Observed macrofossils are entirely floral, and include cycads, Metasequoia, and ferns leaves similar to those found in the McEvoy Formation. Log casts up to 60 cm in diameter and 3 m in length are preserved, especially in the lower parts of conglomerate beds.  Microfossils from the Devils Claw Formation include spores, pollen, and dinocysts (Rouse in Bustin and Moffat, 1983; Chap. Ill of this thesis), indicating at least parts of the Devils Claw Formation are marine or transitional marine deposits. Palynomorph assemblages described in Chapter III date the Devils Claw Formation as mid or late Albian to Cenomanian in age.  Lithofacies Associations  Two lithofacies associations are recognized in the Devils Claw Formation: a channelized conglomerate facies association and a stacked fining upward conglomerate facies. The channelized conglomerate lithofacies occurs in the lower 100 to 150 m of the formation, and the stacked fining upward conglomerate lithofacies association comprises the thicker upper portion. The lithofacies associations described below are interpreted as coarse grained fluvio-deltaic deposits. The Devils Claw conglomerates are in many ways similar to the conglomerates described for the McEvoy Formation. The Devils Claw Formation, however, is dominated by conglomerates, whereas the McEvoy is dominated by the siltstone lithofacies association, and may be more similar to upper delta plain facies of coarse grained delta deposition. The stacked fining upward conglomerates may represent fluviodeltaic deposition on a braid-plain.  Figure 29. High ridge of exposed Devils Claw Formation on the northeast limb of the Beirnes Synclinorium.  95 Channelized Conglomerate Lithofacies Association  The channelized conglomerate lithofacies association comprises a lower Devils Claw zone 100 to 150 m thick that gradationally overlies the McEvoy Formation. Conglomerates in this facies are generally lens shaped and commonly display erosive bases and sharp, flat tops and erode down into siltstone and fine to veryfinegrained sandstones equivalent to McEvoy Formation siltstone lithofacies association. Erosive bases cut down as much as 6 m into interbedded sandstone and siltstone.  Chert pebble conglomerate lenses up to 10 m thick and 10's to 100's of m wide cut down into interbedded fine to very fine grained sandstone and siltstone. Conglomerate lenses grade laterally over distances of tens of metres from clast supported pebble sizes into conglomerate with interbedded fine to medium grained gray sandstone. Sandstone interbeds lacking pebbles makes up approximately 50% of each lens and can reach 2 m in thickness.  Maximum clast size is less than in the overlying stacked conglomerate lithofacies association. In some channels, most of the clasts are rounded and moderately sorted chert pebbles, measuring between 0.5 and 1.5 cm across. The pebbles are concentrated in clast supported beds up to 50 cm thick. Scattered pebbles are also present in sandstone and sandy conglomerate. Less common are large pebble clasts (to 6 cm), which are concentrated at the base of the conglomerate lenses.  The channelized conglomerate lithofacies association, with its characteristic lens shaped conglomerates that cut down extensively into underlying siltstone facies, suggests rapidly prograding channels. The channels are near marine influence, as suggested by the presence of dinocysts in the lower Devils Claw Formation, and therefore are suggested to be coarse grained delta distributary deposits. This lithofacies association may reflect progradation of coarse grained delta distributary facies over the interdistributary bay fill deposits of that dominate the McEvoy Formation.  96 Stackedfining-upwardconglomerate facies association  Stacked fining-upward conglomerates with interbedded sandstones, siltstones, shales and minor coals comprise the dominant lithofacies association of the Devils Claw Formation. The stacked conglomerates form resistant cliffs that are laterally traceable for more than a kilometre where exposure length and accessibility permits. Airphoto interpretation indicates lateral conglomerate continuity may be on the order of 5 to 10 km. Aggregate thickness of this lithofacies exceeds 500 m.  A typical fining-upward unit (Fig. 30) consists of a basal chert pebble-cobble conglomerate gradationally overlain by sandstone, siltstone, and carbonaceous shale. Coals up to 1 m thick commonly cap the units. Individual fining upward units, where completely preserved, range from 25 to 30 m thick. Conglomerate commonly comprises between 20% and 70% of afiningupward unit, although some sequences are entirely conglomerate. Continuous conglomerates up to 65 m thick exist, which are divided into 20 to 25 m intervals of clast supported conglomerate separated by 1 to 2 m of sandy conglomerate or pebbly sandstone. The conglomerates are therefore considered to be stacked, incompletely preserved deposits of the same processes responsible for the complete fining upward sequences.  The conglomerates are clast supported, composed of moderately to poorly sorted and well rounded chert pebble and cobble clasts (Fig. 31). Maximum observed clast size is 13 cm, with small cobbles of 6 to 8 cm common. Matrix is medium to coarse grained sandstone. Sandstone interbeds are generally lens shaped and less than 1 m thick, although pebbly sandstone beds of several metres occur. Massive cross-beds in conglomerates occur in sets more than 3 m thick. The sets are composed of layers approximately 20 cm thick. The cross-bed layersfineupward with cobbles concentrated in the lower 5 cm, and are concave up.  97  Figure 30. Stacked examples of typical fining upward conglomerate units in the upper Devils Formation.  99  The base of conglomerates is locally erosive, cutting down more than 3 m into underlying fine sediments over lateral distance of less than 50 m. Scour grooves 50 cm to 1 m deep at the base of the conglomerate are exceptionally well exposed in overturned bedding in Section 18, east of the Nass River (Fig. 32). Cobbles up to 13 cm in length are concentrated at the base of large scour grooves, attesting to strong traction currents. The powerful erosive currents inferred for this lithofacies strongly suggests channel deposition, but the very wide lateral extents suggests that the channels were not entrained, consistent with coastal fluvio-deltaic deposition.  Interpretation  The Devils Claw Formation was apparently deposited in depositional continuity with the underlying McEvoy Formation. The Devils Claw Formation contains dinocysts (at least in the lower portions) suggesting marginal marine deposition, and plant fossils and coals, pointing to terrestrial environments, like the McEvoy Formation. Lithologically, the Devils Claw is also similar to the McEvoy Formation, with both containing thick chert pebble-cobble conglomerates, although the conglomerates are much more common in the Devils Claw Formation. Age of the Devils Claw Formation is also consistent with depositional continuity with McEvoy Formation. The Devils Claw Formation has an age range of middle or late Albian to Cenomanian, based on palynoassemblages (Chap. Ill), which is only slightly younger than most McEvoy Formation deposition, which occurred in the middle or late Albian. Given the lithologic, and palynostratigraphic similarities, the Devils Claw Formation may have had a similar depositional environment.  The lithofacies associations of the Devils Claw Formation are interpreted to reflect coarse grained deltaic and fluvio-deltaic deposition. Thick sequences of shallow marine conglomerates have been thought most likely associated with deltaic systems (Nemec and Steel, 1984). The proximal distributary channel facies of the Copper River delta (Galloway, 1976), consisting of pebbles and sands, are suggestive of the channelized conglomerate lithofacies association of the Devils Claw Formation.  100  The stackedfining-upwardconglomerate lithofacies association displays sedimentologic characteristics of both fluvial and deltaic deposition. Thefiningupward trend is common to braided gravel deposits such as the Donjek (Williams and Rust, 1969), Durance and Ardeche Rivers (Doeglas, 1962), but the Devils Claw conglomerates are orders of magnitude thicker and more laterally extensive and are more suggestive of deltaic deposition.  Distributaries of coarse grained deltaic and fluvio-deltaic facies similar to the Copper River (Galloway, 1976) are consistent with the scale and geometry of Devils Claw Formation conglomerates. Coastal or marginal marine deposition is indicated by the occurrence of dinocysts in at least the lower Devils Claw Formation, as well as the accumulation of coal which most commonly is associated with flat-lying deposits. The stacked fining upwards conglomerate lithofacies association is more fluvial in character than the underlying channelized conglomerates, and may indicate progradation of fluviodeltaic deposits over deltaic facies below.  PROVENANCE  Provenance from an uplifted terrane to the east has previously been inferred for Bowser Basin sediments (Eisbacher, 1974c and 1981), and is supported by lithologic and paleocurrent data reported in this study. An uplifted sedimentary source terrane is suggested by the dominance of chert in clasts of conglomerates and sandstones (Folk, 1968). Eisbacher (1974c) suggested Cache Creek Group, which comprises thick Paleozoic and Mesozoic bedded chert sequences (Cordey, et al, 1988), as the source for chert. Mesozoic uplift of the Omineca Belt may have caused erosion of sedimentary terranes, including the Cache Creek Group.  Sandstone and conglomerate clast compositions are dominated by chert in all stratigraphic units, suggesting that the source terrane may have remained the same. Maximum clast size, however, increases significantly from the conglomerates of the Currier Formation to the Conglomerates of the  101  Devils Claw Formation. Clasts in Currier Formation and the basal conglomerate lithofacies association of the McEvoy Formation are less than 6 cm across, but by the middle Albian deposition of most of the McEvoy Formation, maximum clast size increases to over 10 cm. Devils Claw Formation conglomerates contain even larger cobbles, reaching at least 13 cm across. The increasing clast size supports an more rugged and proximal source area in the middle to late Albian than in the Late Jurassic and perhaps even the Late Barremian or Aptian.  Paleocurrent directions were measured from planar and trough cross-bed sets in both sandstones and conglomerates as part of this study. Measurable cross-bed occur rarely in the lower parts of the Currier Formation, but become increasing common higher in the Currier Formation and in the overlying McEvoy and Devils Claw Formations. Planar cross-beds make up the bulk of the data, and are considered in more detail below.  Figure 33a shows a rose diagram of 177 measured planar cross bed sets. Current directions are dominantly towards the southwest, which is consistent with an eastern source area. Scatter is wide, however, and not significantly different from a uniform distribution. The results suggest a generally south and west direction of sediment transport persisted for Bowser Lake and Skeena Group deposition.  Division of the paleocurrent data into Bowser Lake Group and Skeena Group sets yields a sharper picture of depositional environment and paleocurrent direction. Both groups yield data sets that significantly differ from an uniform distribution (Mardia, 1972) and suggest a weak cluster distribution (Woodcock, 1977) at the 95% confidence level.  The Bowser Lake Group set consists of 67 measurements and shows a strongly bipolar distribution (Fig. 33b). One group of measurements points east-northeast and the other points westsouthwest. A bipolar distribution has been attributed to marine reworking of sands (Potter and  North  a Bowser Lake Group North  Single Line Show* Vector hean Skeena Group North  Figure 33. Rose diagrams of paleocurrent direction indicators measured from planar cross-beds, a) Theentire data set showing a general southwest orientation; b) The Bowser Lake Group Measures; c) The Skeena Group measures.  103 Pettijohn, 1977), and are consistent with marine influence in the Currier Formation. Perhaps because of poor preservation of cross-beds in the Currier Formation, herringbone cross-beds are rarely observed. Given the overall trend of current measures and other provenance considerations such as chert clasts in sandstones and conglomerates, the west southwest group is interpreted to reflect transport and the east-northeast group is assigned to marine influence.  The Skeena Group set of 110 measurements exhibits a single, dominantly southwest trend, suggesting a more fluvially dominated depositional environment than prevailed during Bowser Lake Group deposition (Fig. 33c). Lack of marine reworking of Skeena Group sediments is reflected in preservation of well defined coarsening and fining upward trends, and is consistent with more rapid subsidence and higher energy rivers than in the older Bowser Lake Group. Lithofacies of the McEvoy and Devils Claw Formations, especially above the basal McEvoy conglomerate lithofacies, are consistent with rapid subsidence and high energy.  DEPOSITIONAL SYSTEMS:  Currier Formation Depositional System  A model for the Currier Formation depositional system can be constructed from the above considerations. The Currier Formation was deposited as a prograding delta system into a shallow marine shelf environment. The source area was to the east, consistent with provenance from uplifted sedimentary terranes along the Omineca Belt.  The Currier Formation lithofacies point to deltaic deposition in a marginal marine setting. The transitional deltaic lithofacies association of the Currier Formation includes a variety of facies common to modern deltas. Lithofacies interpreted as marsh, crevasse splay, interdistributary bay,  104  distributary channel and mouth bar, and prodelta deposits are analogous to facies described from the modern Mississippi River delta (Coleman, 1982).  Currier Formation deposition appears to have occurred generally in less than 15 m of water based on the maximum thickness of the coarse distributary deposits. Shallow water deposition is consistent with the combination of terrestrial and marine biota and coal seams.  Deposition of prograding deltaic deposits into a shallow water, marginal marine setting suggests shelf sedimentation. The older marine shales of the Bowser Lake Group, which include turbidites and sub-sea fan deposits (Eisbacher, 1974b), may represent initial construction of the shelf, which culminated in the shallow water deposition of the Currier Formation. Reports of lithologically similar Bowser Lake Group rocks (described as the Bowser Group by Duffel and Souther, 1964) with a similar plant fossil suite from the western edge of British Columbia, near Terrace, suggests shallow water deposition was widespread in the Bowser Basin during the Late Jurassic. Deposition of the Currier Formation may have occurred on a broad shelf that stretched across the Bowser Basin.  Skeena Group Depositional System  Modern analogs for McEvoy and Devils Claw deposition are imperfect. The reason may be that very few modern rivers deliver bedload beyond the shoreline to deltaic depositional environments, and fluvial transport of gravels occurs mostly in the bedload. There is evidence that prior to the Holocene sea level rise, however, many major rivers delivered bedload to their deltas, and at least some carried gravels. The Mississippi River today deposits no coarser thanfinesand in its delta, but gravels occur in pre-Holocene submarine fan channels (Bouma and Coleman, 1984), demonstrating that rudaceous material passed through the delta. The lower Rhone River today carries little material  105  coarser than sand size, but during the last glaciation apparently carried gravels to the shoreline (Oompkens, 1970).  Modern examples of coastal gravel deposits are dominated by fan-deltas and beach deposits, both specialized depositional environments not likely to produce thick, laterally extensive deposits such as the McEvoy and Devils Claw Formations. One example of a large modern coarse grained delta described in the literature is the Copper River delta in southeastern Alaska. The Copper River is a major sediment transport system, delivering to its delta 1/4 the volume of sediment carried by the Mississippi River. Included in this load is a significant amount of coarse grained material, including gravels.  Lithofacies of the Copper River delta described by (Galloway, 1976) are sufficiently similar to those of the McEvoy and Devils Claw Formations to suggest that similar depositional were responsible. The proximal channel deposits of the Copper River include coarse pebbly sands, and the distal channel deposits are fine to very fine grained sands and silts (Galloway, 1976). Marshes and mudflats also exist in the proximal reaches of the Copper River delta (Galloway, 1976). The siltstone lithofacies association of the McEvoy Formation is similar to distal deposits, marshes, and mudflats of the Copper River delta. The coarse conglomerate lithofacies association of the McEvoy Formation, and the channelized lithofacies association of the lower Devils Claw Formation are suggestive of the channel deposits of the proximal Copper River delta. The stacked fining upward lithofacies of the upper Devils Claw Formation are suggestive of large, fluvially dominated channel deposits and may reflect coastal rivers or distributaries.  The occurrence of thick fine grained and coarse grained lithofacies in the McEvoy and Devils Claw Formations suggests a depositional system similar to the Copper River, but prograding onto shallower and broader shelf. The Copper River Delta is up to 150 m thick, and is prograding out over marine muds on a 100 to 200 m deep shelf (Galloway, 1976). The McEvoy and Devils Claw  106  Formations were deposited in shallower marine waters, and may have prograded much farther and faster than the Copper River delta, leading to the interbedding of fine and coarse grained lithofacies associations.  Interpreting the McEvoy and Devils Claw Formations as coarse grained delta deposits is consistent with the marginal marine setting suggested by the occurrence of dinocysts, the broad lateral extent of many of the conglomerates, and the great overall thickness of the formations. Deltaic deposition is also supported by the occurrence of coals, which tend to form in flat lying areas.  Tectonic implications:  The depositional history of the Bowser Basin has important implications for the tectonic development of the Cordillera. One of the most important constraints to the development of the Cordillera is provided by sediment source area. An uplifted sedimentary provenance to the east is indicated for sediments in the Bowser Lake and Skeena Groups, which constrains Stikinia to have been adjacent to North America since before the Middle Jurassic.  The thick sedimentary strata of the Bowser Basin accumulated during two periods of subsidence. Subsidence in the Middle and Late Jurassic allowed the accumulation of thick marginal marine deposits of the Bowser Lake Group. Deltaic deposition interpreted for the Currier Formation suggests shallow shelf deposition at the culmination of the Bowser Lake Group regression. Relative subsidence rate may have decreased during deposition of the Currier Formation, because sediments apparently became shallower than in lower Bowser Lake Group deposits.  Cessation of subsidence by the latest Jurassic is indicated by regionally correlatable hiatus revealed by palynostratigraphy (Chap. III). Lack of evident erosion and resumption of marginal marine sedimentation in the mid-Cretaceous suggests that the hiatus represents a time of tectonic  107  quiescence that persisted until at least to the late Barremian. A contemporaneous magmatic lull has previously been reported throughout the Cordillera by Armstrong (1988) based on extensive radiometric dating.  Additional support for a tectonicalfy stable period is provided by sedimentation in the Tyaughton Trough. For most of the Late Jurassic and through the mid-Cretaceous, sedimentation in the Tyaughton Trough is correlative with that in the Bowser Basin, but whereas a hiatus is found in the Bowser Basin, in the Tyaughton Trough sedimentation is continuous. The early Cretaceous sedimentation of the Tyaughton Trough, however, is composed of thin elastics and thick shell lags characteristic of very low sedimentation rates, and is consistent with lack of clastic input (J. Garver, pers. comm., 1989).  Cretaceous subsidence, beginning in the late Barremian and extending to the Cenomanian, lead to accumulation of the McEvoy and Devils Claw Formations. The coarse grained deltaic and fluvio-deltaic depositional environments interpreted for the McEvoy and Devils Claw Formations suggests that the source area was more proximal and rugged than during deposition of the underlying Currier Formation. The increased clast size noted in stratigraphically higher conglomerates in both the McEvoy and Devils Claw Formations may also reflect a more proximal source area. A more proximal source area is consistent with pre-Albian tectonic shortening of Bowser Lake Group reported in studies to the east and north (Eisbacher, 1974a; Evenchick, 1987). Subsidence rate probably accelerated through mid to late Albian time, as indicated by the accumulation of thicker and coarser marginal marine sediments than in the pre-Albian lower McEvoy Formation.  Onset of the major period of tectonic activity in the Coast Plutonic Complex (Van der heyden, 1989) to the west corresponds to the late Albian and Cenomanian age of the Devils Claw Formation, and may relate to the end of the Bowser Basin as an active sedimentation center. Uplift of the incipient Coast Mountains may have cut the Bowser Basin off from the sea and caused subsidence to  cease by late Albian or Cenomanian time. Subsequent late Cretaceous deposition in the Sustut Basin to the east of the Bowser Basin is entirely fluvial in origin, and may have been sourced from unroofed Bowser Basin strata (Bustin and McKenzie, in press).  109  REFERENCES CITED  Armstrong, R. L. 1988. Mesozoic and early Cenozoic magmatic evolution of the of the Canadian Cordillera. Geological Society of America Special Paper 218, p. 55-91.  Bouma, A. H., and Coleman, J. M. 1984. Lithologic characteristics of the Mississippi Fan. American Association of Petroleum Geologists Bulletin. Abstract, v. 68, p. 456.  Buckham, F., and Latour, B. A. 1950. The Groundhog coalfield, British Columbia. Geological Survey of Canada, Bulletin 16,82 p.  Bustin R. M., and Moffat, 1.1983. Groundhog Coalfield, central British Columbia: reconnaissance stratigraphy and structure. Bulletin of Canadian Petroleum Geology, v. 31, p. 231-245.  Bustin, R. M. 1984. Coalification levels and their significance in the Groundhog Coalfield, north-central British Columbia. International Journal of Coal Geology, v. 4, p. 21-44.  Bustin, R. M., and McKenzie, K. J. In press. Stratigraphy and depositional environments of the Sustut Group, southern Sustut Basin, north central British Columbia. Canadian Society of Petroleum Geology.  Coleman, J. M. 1982. Deltas. International Human Resources Development Corporation. 124 p.  Cookenboo, H. O., and Bustin, R. M. In press. Jura-Cretaceous (Oxfordian to Cenomanian) stratigraphy of the north-central Bowser Basin, northern British Columbia. Canadian Journal of Earth Sciences.  Cordey, F., Mortimer, N., Dewever, P., and Monger, J. W. H. 1987. Significance of Jurassic radiolarians from the Cache Creek terrane, British Columbia. Geology, v. 15, p. 1151-1154.  Curtis, C. D., and Spears, D. A. 1968. The formation of sedimentary iron minerals. Economic Geology, v. 3, p. 257-270.  Doeglas, D. J. 1962. The structure of sedimentary deposits of braided rivers. Sedimentology, v. 1, p. 167-190.  Duffel, S., and Souther, J. G. 1964. Geology of Terrace map-area British Columbia. Geological Survey of Canada Memoir 329,117 p.  Eisbacher, G. 1974a. Sedimentary history and tectonic evolution of the Sustut and Sifton Basins, northcentral British Columbia. 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B. 1915. Coal fields of British Columbia. Geological Survey of Canada, Memoir 69, p. 189222.  Duffel, S., and Souther, J. G. 1964. Geology of Terrace map-area British Columbia. Geological Survey of Canada Memoir 329,117 p.  Dupont, V. H. 1901. Report of an exploration on the upper part of the Stikine River to ascertain the feasibility of a railway. Department of Railways and Canals, Canada, Annual Report July 1, 1899 to June 30,1900, Part 1, p. 152-155.  Eisbacher, G. 1974a. Sedimentary history and tectonic evolution of the Sustut and Sifton Basins, northcentral British Columbia. Geologic Survey of Canada, Paper 73-31,57 p.  , 1974b. Deltaic sedimentation in the northeastern Bowser Basin, British Columbia. Geologic Survey of Canada, Paper 73-33,13 p.  , 1974c. Evolution of successor basins in the Canadian Cordillera. In Dott, R. H., and Shaver, R. H., (eds.) Modern and Ancient Geosynclinal Sedimentation. 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Tectonic accretion and the origin of two major metamorphic and plutonic welts in the Canadian Cordillera. Geology, v. 10, p. 7075.  Nemec, W., and Steel, R. J. 1984. Aluviall and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits. In Koster, E. H., and Steel, R. J. (eds.) Sedimentology of Gravels and Conglomerates. Canadian Society of Petroleum Geologists Memoir 10, p. 175-187.  Oertel, G,. and Curtis, C. D. 1972. Clay-ironstone concretion preserving fabrics due to progressive compaction. Geological Society of America, v. 83, p. 2597-2606.  Palmer, A. R. 1983. The decade of North American geology 1983 geological time scale. Geology, v. 11, p. 503-504.  Panuska, B. C. 1985. Paleomagnetic evidence for a post-Cretaceous accretion of Wrangellia. Geology, v. 13, p. 880-883.  Pierce, R. L. 1961. Lower Upper Cretaceous plant microfossils from Minnesota. University of Minnesota, Minnesota Geological Survey Bulletin, v. 42,68 p.  Pocock, S. A. J. 1970. Palynology of the Jurassic sediments of Western Canada. Part 1-Terrestrial species. Palaeontographica, Abt. B, v. I l l , p. 1-95, pi. 1-3.  Pocock, S. A. J. 1972. Palynology of the Jurassic sediments of western Canada. Part 2. Marine species. Palaeontographica, Abt. B, v. 137, p. 85-153, pl.22-29.  Potter, P. E., and Pettijohn, F. J. 1977. Paleocurrents and basin analysis. Springer-Verlag New york. 425 p.  Price, R. A. 1981. Cordilleran cross-section, Calgary to Victoria. In Thomson, R. I., and Cook, D. G., (Eds.) Field guide to geology and mineral deposits. Calgary 1981 annual meeting Geological  118 Association of Canada/ Mineralogical Association of Canada/ Canadian Geophysical Union, p. 261LVIII.  Richards, T. A., and Gilchrist, R. A. 1979. Groundhog coal area, British Columbia. Geological Survey of Canada, Paper 79-lb, p.411-414.  Rouse, G. E. In press. Palynological dating and paleoreconstruction of the gold and silver orebodies in the western Cordillera of Canada, United States, and Mexico. Ore Geology Reviews, Elsevier.  Singh, C. 1971. Lower Cretaceous microfloras of the Peace River area, northwestern Alberta; Research Council Alberta Bulletin 28,542 p.  Sloss, L. L. 1950. Paleozoic stratigraphy in the Montana area. American Association of Petroleum Geologists, v. 34, p. 423-451.  Souther, J. G., and Armstrong, J. E. 1966. North Central belt of the Cordillera of British Columbia. In Tectonic History and Mineral Deposits of Western Canada. Canadian Institute of Mining and Metallurgy, Special Volume 8, p. 171-184.  Stott, D. F. 1968. Lower Cretaceous Bullhead and Fort St. John Groups, between Smoky and Peace Rivers, Rocky Mountain Foothills, Alberta and British Columbia. Geological Survey of Canada, Bulletin 152, 279p.  Stott, D. F. 1982. Lower Cretaceous Fort St. John Group and Upper Cretaceous Dunvegan Formation of the foothills and plains of Alberta, British Columbia, District of Mackenzie and Yukon Territory. Geological Survey of Canada, Bulletin 328,124 p.  Teichmuller, M., and Teichmuller, R. 1975. The geological basis for coal formation. In Stach, E. (ed.), Coal Petrology. Gebruder Borntragger, Berlin, 428 p.  Thomson, R. C, Smith, P. L., and Tipper, H. W. 1986. Lower to Middle Jurassic (Pliensbachian to Bajocian) stratigraphy of the northern Spatsizi area, north-central British Columbia. Canadian Journal of Earth Sciences, v. 23, p. 1963-1973.  Tipper, H. W. and Richards, T A. 1976. Jurassic stratigraphy and history of north-central British Columbia. Geological Survey of Canada, Bulletin 270,73 p.  Tipper, H. W. 1981. The allocthonous Jurassic-Lower Cretaceous terranes of the Canadian Cordillera and their relation to correlative strata of the North American craton. In Westermann, G. E. G.  119 (ed.), Jurassic-Cretaceous biochronolgy and Paleogeography of North America. Geological Association of Canada Special Paper 27, p. ID-120.  Trexler, J. H., and Bourgeois, J. 1985. Evidence for mid-Cretaceous wrench faulting in the Methow basin, Washington. Tectonics, v. 4, p. 379-394.  Umhoefer, P. J. 1987. Northward translation of "Baja British Columbia" along the Late Cretaceous to Paleocene margin of Western North American. Tectonics, v. 6, p. 377-394.  van der Heyden, P. A. H. 1989. U-Pb geochronometry of the Coast Plutonic Complex, 53°N to 54°N, British Columbia, and implications for the Insular-Intermontane Superterrane boundary. PhD. Thesis, University of British Columbia.  Varley, C. J. 1984. Sedimentology and hydrocarbon distribution of the Lower Cretaceous Cadomin Formation northwestern Alberta In Koster, E. H., and Steel, R. J. (eds.) Sedimentology of Gravels and Conglomerates. Canadian Society of Petroleum Geologists Memoir 10, p. 175187.  Ward, L. F. 1905. The status of the Mesozoic floras of the United States. Monographs of the United States Geological Survey, v. 48,616 p. Wheeler, J. O., and Gabrielse, H., coordinators 1972. The Cordilleran structural province. In Price, R. A., and Douglas, R. J. W., eds. Variations in tectonic styles in Canada: G A.C. Special Paper 11 p. 1-81.  Williams, P. F., and Rust, B. R. 1969. The sedimentology of a braided river: Journal of Sedimentary Petrology, v. 39, p. 649-679.  Woodcock, N. H. 1977. Specification of fabric shapes using an eigenvalue method. Geological Society of America Bulletin, v. 88, p. 1231-1236.  120 APPENDIX  The following shale samples were collected from measured sections as part of the sedimentologic study of the north-central Bowser Basin. Location, palynomorph content, interpreted age and relation to stratigraphy of each sample is described in detail.  Sample 9-1:  57° 11* 02"; 128° 57 12"  Black shale collected from Devils Claw Formation strata south of Mount Klappan, on the east side of a north - south trending valley (Fig. 1) yielded the following suite:  Schizaeceae spore Alisporites bilateralis Psilatricolporites prolatus cf. Granamonocolpites asymmetricus  Based on the presence of Psilatricolporites prolatus and cf. Granamonocolpites asymmetricus, which occur in the Cenomanian of Minnesota (Pierce, 1961), this sample has been assigned to Assemblage #1. Tricolporate pollen isfirstencountered in Western Canada and the Eastern United States in late Albian aged strata (Singh, 1971), which supports a post Albian age for the sample.  The stratigraphic position of the sample is consistent with the interpreted age. The shale is 80 m above a cliff forming, 35 m thick chert pebble-cobble conglomerate typical of the Devils Claw formation. Although the Devils Claw Formation is usually preserved at the tops of ridges, this outcrop is in the downthrown block of a major high angle fault which was previously identified by Koo (1986).  Sample 12-5:  57o 57' 09"; 128° 31* 30"  121  Roof shale collected in the upper McEvoy Formation (Chapter. II), on a northeast trending ridge, north of Beirnes Creek (Fig. 1) yielded the following pollen and dinocyst assemblage:  Araucariacites australis  Monocolpate Alisporites (Podocarpidus) ellipticus Cycadopites ovatus Tsugaepollenites canadensis Verrucosisporites sp. Alisporites rotundus Osmundacidites wellmanii Biretisporites potoniaei ?dinocyst cf. Apteodinium (coarse Apteodinium sp.) Pseudoceratium regium  This assemblage is considered to be most likely mid- to late Albian age. The dinocyst Pseudoceratium regium has been previously reported in the Harmon member of the Peace River Formation and the Lower Shaftsbury Formation, which are mid and late Albian age, respectively (Stott, 1982).  The paleontological age interpretation is supported by stratigraphic considerations. The sample is from black roof shale in dominantly fine grain strata 50 m below the base of a cliff forming chert pebble conglomerate that marks the base of the overlying Devils Claw Formation. The Devils Claw Formation has elsewhere yielded a palynomorph suite of late mid-Albian age (Rouse, in Moffat, 1985, and Moffat et al., 1988), which included tricolpate pollen not present in this sample. Lack of  122  tricolpate pollen and presence of dinocysts points to marine deposition for at least part of the upper McEvoy Formation.  Sample 19-6:  57° 06' 40"; 128° 42' 54"  A sample from a conglomerate rich section of the middle McEvoy Formation exposed on a north east trending ridge south of the headwaters of the Skeena River yielded the following palynomorph assemblage:  Tricolpate Tricolpites minutus Tricolpites crassimurus Verrumonocolpites conspicuus (3) Cycadopites follicularis Osmundacidites wellmanii  This assemblage is interpreted to be late middle Albian age and is assigned to Assemblage #2. Tricolpate and monocolpate pollens are both present in this sample. In Alberta, monosulcate pollen first appears in Barremian and Aptian strata and tricolpate pollen is first reported in the (Singh, 1971).  Sample 19-6 is from near the middle of the McEvoy Formation in a section dominated by cliff forming conglomerates. These strata, which has been correlated more than 20 kilometres along the Skeena River, is stratigraphically lower than sample 12-5 (described earlier), which is also middle to late Albian age.  Sample 19-4:  57° 06' 43"; 128° 42' 56"  Two hundred metres below sample 19-6 on the sameridge,a roof shale yielded the following palynomorph suite:  Cyathidites minor Multicellaesporites sp. cf. Clavatipollenites (5) elongate fungal fil. Osmundacidites wellmanni cf. Tricolpites sp. Deltoiospora minor Ovalipollis sp. Tricolpites micromunus (3) Clavatipollenites minutus Alisporites bilateralis Baculate monocolpate cf. Deflandrea limpida Veryhachium sp. Gonyaulacysta cf. helicoides Diconodinium pusillum Chytroeisphaeridia sp. Paleohystrichophora paucispina  The above assemblage has been interpreted to be middle Albian age and is assigned to  Assemblage #2, based on the forms Tricolpites micromunus, Clavatipollenites minutus, Gaunyaulacy cf. helicoides, Diconodinium pusillum, and Paleobystrubophorapaucispina. Paleobystrubophora paucispina is restricted to the middle Albian, and may indicate that this sample is slightly older than  124 other samples in Assemblage #2, although more detailed work is needed to clarify this relationship. The sample is also interpreted to be marginal marine, due to the presence of both pollen and dinocysts.  Macrofossils including Cladophlebis vaccenensis and Nilssonia parvula were common in sample 19-4 in addition to the above palynomorphs. The presence of Nilssonia parvula in Albian strata demonstrates that the form is not restricted to the Jurassic as previously believed, and therefore is of little value for dating as the other fossil plants.  Sample 19-1:  57° 06' 44"; 128° 42' 58"  This sample was collected from the base of the same ridge that yielded samples 19-6 and 19-4 above. The following palynomorphs were identified: Alisporites minutus Lycopodiumsporites austroclavatidites Osmundacidites wellmanii Pseudoceratium pelliferum P. gochtii cf. Paleohystrichosphaeridium brevispinosa The dinocysts Pseudoceratium pelliferum, P. gochtii, and cf. Paleohystrichosphaeridium brevispinosa are all consistent with an interpreted age of late Barremian or Aptian. Singh has reported Pseudoceratium pelliferum from the middle Albian of western Alberta, suggesting that this assemblage might be only slightly younger than those found in samples 19-6 and 19-4.  Sample 10-8:  57° 08' 49"; 127° 46' 31"  A roof shale collected from a north-south trending ridge south of the confluence of Tahtsedle Creek and the Spatsizi River yielded the following palynomorph suite:  125  Osmundacites wellmanii Osmundasporites Deltoidospora diaphana cf. Clavatipollenites sp. (3) Diconodinium pusillum Gonyaulacysta sp. Lycopodiumsporites crassatus  Based on the identification of Diconodinium pusillum, and Lycopodiumsporites crassatus, this suite is interpreted to be middle to late Albian age and is assigned to Assemblage #2.  Sample 16-7:  56° 51' 07"; 128° 23' 11"  This sample was taken north of Currier Creek, on the south flank of Devils Claw Mountain. The following palynomorphs were identified: Cycadopites follicularis Cycathidites minor cf. Tricolpites crassimurus cf. Verrumonocolpites conspicuus cf. Clavatipollenites Liliacidites Lycopodiumsporites crassimacerius cf. Distaltrianglisporites costatus Biretisporites potoniaeae Osmundacidites wellmanii fOukisporites foveolatus Pseudoceratium pelliferum  126 Gonyaulacysta sp. cf. Epilidosphaeridia spinosa cf. Canningia sp. In addition to the above forms, an 11 micron monocolpate angjosperm pollen was recovered. The most likely age for this assemblage is middle Albian.  Samples 8-2, 8-3,8-7,8-9, and 8-14:  Five samples from a northwest trending ridge southeast of Mount Klappan are described below from oldest to youngest.  Sample 8-2:  57° 12' 02"; 129° 01* 13"  This sample is from the base of the ridge, within the upper Jackson unit. The plant fossils Cladophlebis vaccensis, C. waltoni, and Pityophyllum nordenskioldi were identified in addition to following palynomorphs:  Gonyaulacysta sarjenti Imbatodinium kondratjevi ?Chytroeisphaeridia sp.  The black shale which yielded the above fossil assemblage is considered to be an Upper Jurassic transitional marine sediment. No stage name significance can be assigned to this sample.  Sample 8-3:  57° 11' 40"; 129° 0' 55"  Upper Jackson black shale 50 m above sample 8-2 yielded the following palynomorph suite:  127 Ceratosporites rotundiformis cf. Nannoceratopsis pellucida Ablosporites sp. Pareodinia minuta  The dinocysts Nannoceratopsis pellucida and Pareodinia minuta indicate an Oxfordian age for the sample. In light of the Upper Jurassic marine determination for sample 8-2, 50 m below, an Oxfordian marine depositional environment is consistent.  Sample 8-7:  57° 11* 30"; 129° 0' 50"  Sample 8-7 is in the base of the Currier Formation, 300 m above sample 8-3. The following forms were noted:  cf. Paleoperidinium sp. Nannoceratopsis sp. (3) Verrucate spore unknown dinocyst  The dinocyst Nannoceratopsis sp. has a reported range from Toarcian to Kimmeridgian age. Based on the forms in this sample, the deposit can be classified as marine shale of Jurassic age. Given an Oxfordian age in sample 8-3, an Oxfordian to Kimmeridgian age is most probable.  Sample 8-9:  57° 11' 30"; 129° 0' 50"  A black shale near the middle of the Currier Formation, 200 m above sample 8-3 (above), yielded the following palynomorph suite:  128 Converrucosisporites utriculosus Reticulatisporites sp. Pareodinia minuta Pareodinia cf. ceratophora reticulate monolete  Like the samples above from this section, sample 8-9 contains marine Jurassic forms. Pareodinia minuta has been reported from sediments of Oxfordian to Tithonian age. The middle of the Currier Formation may therefore be as old as Oxfordian or as young as Tithonian.  Sample 8-14:  57° 11' 25"; 129° 0' 35"  Sample 8-14 is located 35 m above the base of the McEvoy Formation and 200 m above sample 8-9. The following palynomorph suite was recovered:  Tsugaepollenites mesozoicus Pluricellae sporites (5) Cyathidites minor (3) Cedripites cretaceus (2) Spheripollenites scabratus Podocarpidites multessimus Parvisaccites radiants Pseudoceratium pelliferum (4) cf. Paleoperidinium caulleri  In addition to the identified forms above, an unknown dinocyst, cf. angiosperm pitting and a foram were noted. This suite is interpreted to be upper Barremian or Aptian in age and is assigned to  Assemblage #3. This sample has marine affinities, as demonstrated by the presence of forams and dinocysts.  Core Samples:  Roof shales collected from a drill core in the Klappan coal measures, northwest of Mount Klappan (DH 8516) yielded two samples. Both samples are from the upper Jackson unit, as informally described by Moffat (1985).  Sample C-l:  57° 15'; 128° 55'  Gulf Canada Drill Hole 8615 was sampled between 43 m and 56 m. The following palynomorph forms were identified:  Trilete spore Deltoidospora fulva Cyathidites sp. Cyathidites australis Gleicheniidites senonicus c£ Tenua cf. Tenua sp. Valensiella ovula Tenua cievittii Chytroeisphaeridia sp. Gonyaulacysta cf. crassicomta Gonyaulacysta eisenacki Tenua cf. varispinosa  130  Scriniodinium cf. parvimarginatum Gonyaulacysta jurassica var. longicornis  The dinocysts Gonyaulacysta cf. crassicomta, and Tenua cf. varispinosa are the most time restrictive out of the above suite, both having only been reported from the Callovian (Pocock, 1972). Reported ranges for Valensiella ovula, Tenua cf. evittii, Gonyaulacysta eisenacki, Scriniodinium cf. parvimarginatum, and Gonyaulacysta jurassica var. longicornis all overlap the Callovian stage, thus supporting a Callovian age for the sample. The presence of both dinocysts and palynomorphs indicate marginal marine depositional conditions. This sample is assigned to Assemblage #5.  Sample C-3:  57° 15'; 128° 55'  The second sample from Gulf Canada drill hole 8615 was taken 20 m above sample C -1, and yielded the following dinocysts:  Gonyaulacysta eisenacki Meiourogonyaulax cf. callomonii Chytroeisphaeridia sp. Chytroeisphaeridia c£ chictydia Chytroeisphaeridia sp. A Chytroeisphaeridia sp. B Nannoceratopsis sp. Tenua granulata  The above suite is interpreted as Callovian age, based on the identification of Gonyaulacysta  eisenacki, Meiourogonyaulax cf. callomonii, Chytroeisphaeridia cf. chictydia, and Tenua granulata Gonyaulacysta eisenacki, which was also found in sample C-l, is known from Bathonian to Upper  Oxfordian sediments, but each of the other forms has only been reported from the Callovian. This sample, like sample C-l, is assigned to Assemblage #5.  The interpreted Callovian age is consistent in both core samples. The coal measures are clearly older than the other samples examined in this study.  

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