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Calibration of the Jurassic time scale Pálfy, József 1997

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CALIBRATION OF T H E JURASSIC TIME SCALE  by  JOZSEF P A L F Y  Diploma in Geology, Eotvos University, Budapest, 1986 M.Sc., The University of British Columbia, 1991  A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF T H E REQUIREMENTS FOR T H E D E G R E E OF DOCTOR OF PHILOSOPHY in T H E F A C U L T Y OF G R A D U A T E STUDIES D E P A R T M E N T OF E A R T H A N D O C E A N SCIENCES (GEOLOGY)  We accept this thesis as conforming to the required standard  T H E UNIVERSITY OF BRITISH C O L U M B I A September 1997 © JozsefPalfy, 1997  In presenting this thesis in partial fulfilment  of the  requirements  for an advanced  degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department  or  by  his or  her  representatives.  It  is understood  that  copying or  publication of this thesis for financial gain shall not be allowed without my written permission.  Department of i T A l T H  A * *  OCE/\h/  The University of British Columbia Vancouver, Canada Date  DE-6 (2/88)  12.  -SePTEMtifcTA-  IW  jqgNESS  ABSTRACT Current time scales disagree on age estimates for Jurassic stage boundaries and carry large uncertainties. The U-Pb or Ar- Ar dating of volcaniclastic rocks of precisely known biochronologic age is the preferred method to 40  39  improve the calibration. Eighteen new U-Pb zircon dates were obtained on volcaniclastic rocks from Jurassic volcanic arc assemblages of the North American Cordillera. The volcanic rocks are interbedded in fossiliferous marine sedimentary rocks, which in turn were dated by ammonite biochronology at the zonal level. In the Queen Charlotte Islands, a tuff layer directly beneath the Triassic-Jurassic boundary was dated at 200±1 Ma. Lowermost Jurassic (middle and upper Hettangian) volcaniclastic rocks from Alaska yielded ages of 200.8+2.7/-2.8 Ma, 197.8±1.0 Ma, and 197.8+1.2/—0.4 Ma. An ash layer near the base of the Middle Toarcian Crassicosta Zone in its type section in the Queen Charlotte Islands was dated at 181.4+1.2 Ma. The other new dates furnish additional time scale calibration points. The biostratigraphy of measured sections contribute to the understanding of North American Jurassic ammonite successions, especially for the Hettangian of Alaska. In an attempt to quantify biochronologic correlation uncertainty, the computer-assisted Unitary Association method was used to correlate a Toarcian North American regional ammonite zone with the northwest European standard. The correlation uncertainty between the two regions is not more than ±1 standard subzone. A radiometric age database consisting of 50 U-Pb and Ar- Ar ages was compiled to construct a revised 40  39  Jurassic time scale. Apart from the newly obtained U-Pb ages, several recently reported Cordilleran dates are included with revised biochronologic ages. Additional dates were compiled from previous time scales and recent literature. Only a few boundaries are dated directly. The chronogram method was applied for thefirsttime to estimate all Early and early Middle Jurassic chron boundaries, as well as late Middle Jurassic substage boundaries and Late Jurassic stage boundaries. The most significant improvement concerns the Pliensbachian and Toarcian, where six consecutive chron boundaries were determined. The derived chron durations vary between 0.4 and 1.6 Ma. Their disparities argue against the assumption of equal duration of chrons or subchrons, which was used as a basis for  ii  interpolation in several previous time scales. Interpolation based on magnetochronology improved chronogram estimates for the latest Jurassic, where the isotopic database remains too sparse. The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous): 144.8 +2.61-2.1 Ma; Tithonian: 151.5 +1.0/-1.4 Ma; Kimmeridgian: 154.7 +1.0/-0.9 Ma; Oxfordian: 156.6 +2.0/-2.7 Ma; Callovian: 159.7 ±1.1 Ma; Bathonian: 166.0 +0.6/-5.4 Ma; Bajocian: 174.0 +1.0/-7.3 Ma; Aalenian: 178.0 +1.0/-1.5 Ma; Toarcian: 183.6+1.6/-1.1 Ma; Pliensbachian: 192.0 +3.8/-5.2 Ma; Sinemurian: 197.0 +1.2/—4.2 Ma; Hettangian (Triassic-Jurassic): 201±1 Ma. The revised time scale was used to assess the timing of mass extinction events and subsequent biotic recoveries. Marine and terrestrial extinctions at the end of the Triassic both occured at about 201 Ma, later than suggested in most previous time scales. During the Pliensbachian-Toarcian mass extinction, elevated extinction rates were sustained between 185.7 and 181.4 Ma and apparently followed by an immediate recovery. The hypothetical 26 Ma extinction periodicity is plausible between the end-Permian and end-Triassic, between the Pliensbachian-Toarcian and Callovian, and among the Tithonian, Cenomanian-Turonian, and end-Cretaceous events. However, the spacing of the end-Triassic and Pliensbachian-Toarcian events is less than 20 Ma, significantly less than predicted by the periodicity model.  iii  TABLE OF CONTENTS ABSTRACT  :  TABLE OF CONTENTS  »v  LIST OF TABLES  viii  LIST OF FIGURES  «  LIST OF PLATES  xi  ACKNOWLEDGEMENT  xii  DEDICATION  xiii  CHAPTER 1. INTRODUCTION  1  1.1. INTRODUCTORY STATEMENT  1  1.2. OBJECTIVES  2  1.3. METHODS  2  1.4. PRESENTATION  3  CHAPTER 2. DEVELOPMENT OF THE JURASSIC GEOCHRONOLOGIC SCALE  5  2.1. INTRODUCTION  5  2.2. DATABASE AND PROCEDURES IN TIME SCALE CALIBRATION  8  2.3. KEY AREAS OF PROGRESS AND CONSTRUCTIONAL DIFFERENCES BETWEEN SCALES  9  2.3.1. Advances in isotopic dating  9  2.3.2. Selection of dates  11  2.3.3. Errors and uncertainties in data  14  2.3.4. Boundary estimation and mathematical data manipulation  16  2.3.5. Interpolation techniques  17  2.4. DIRECTIONS IN CURRENT AND FUTURE TIME SCALE RESEARCH  19  2.5. SUMMARY  21 iv  CHAPTER 3. A U-Pb AGE FROM THE TOARCIAN (LOWER JURASSIC) AND ITS USE FOR TIME SCALE CALIBRATION THROUGH ERROR ANALYSIS OF BIOCHRONOLOGIC DATING 3.1. INTRODUCTION  23 23  3.2. GEOLOGIC SETTING  -.  3.3. U-Pb GEOCHRONOMETRY  24 27  3.3.1. Methodology  27  3.3.2. Results  27  3.4. BIOCHRONOLOGY  29  3.4.1. The ammonite fossil record  29  3.4.2. Reliability of observed ranges  29  3.4.3. Biochronologic correlations  33  3.4.4. The effect of taxonomic noise  39  3.5. DISCUSSION  41  3.6. CONCLUSIONS  42  CHAPTER 4. INTEGRATED AMMONITE BIOCHRONOLOGY AND U-Pb GEOCHRONOLOGY FROM A BASAL JURASSIC SECTION IN ALASKA  44  4.1. INTRODUCTION  44  4.2. GEOLOGIC SETTING AND PREVIOUS WORK  46  4.3. HETTANGIAN-SINEMUR1AN BIOCHRONOLOGY  48  4.3.1. Local ammonite ranges  49  4.3.2. Correlation and biochronologic dating  54  4.3.3. Taxonomic remarks  57  4.4. U-Pb GEOCHRONOLOGY  63  4.4.1. Analytical methods  63  4.4.2. Age determinations  63  4.5. DISCUSSION  67  4.6. CONCLUSIONS  71  v  CHAPTER 5. NEW U-Pb ZIRCON AGES INTEGRATED WITH AMMONITE BIOCHRONOLOGY FROM THE JURASSIC OF THE CANADIAN CORDILLERA 5.1. INTRODUCTION  72 '.  72  5.2. REGIONAL GEOLOGICAL SETTING  73  5.3. DATING METHODS  74  5.3.1. Ammonite biochronology  74  5.3.2. U-Pb geochronology  76  5.4. INTEGRATED BIO- AND GEOCHRONOLOGICAL RESULTS  76  5.4.1. Sinemurian  77  5.4.2. Pliensbachian  86  5.4.3. Toarcian  87  5.4.4. Aalenian  89  5.4.5. Bajocian  90  5.4.6. Bathonian  91  5.5. DISCUSSION  :  97  CHAPTER 6. U-Pb DATING THE TRIASSIC-JURASSIC MASS EXTINCTION BOUNDARY 6.1. INTRODUCTION  99 99  6.2. A NEW U-Pb AGE NEAR THE T-J BOUNDARY IN THE QUEEN CHARLOTTE ISLANDS  100  6.3. BIOCHRONOLOGY OF THE T-J BOUNDARY IN THE QUEEN CHARLOTTE ISLANDS  103  6.4. OTHER U-Pb AGES AROUND THE T-J BOUNDARY  104  6.5. THE AGE OF THE T-J BOUNDARY  106  6.6. TIMING THE END-TRIASSIC MASS EXTINCTION  107  6.7. SPACING OF MASS EXTINCTIONS  107  6.8. CONCLUSIONS  108  CHAPTER 7. A U-Pb AND Ar- Ar TIME SCALE FOR THE JURASSIC 40  39  110  7.1. INTRODUCTION  110  7.2. METHODS  Ill  vi  7.3.  CHRONOSTRATIGRAPHIC FRAMEWORK  112  7.4.  THE ISOTOPIC AGE DATABASE  113  7.5.  COMMENTS ON ITEMS USED IN THE ISOTOPIC DATABASE  115  7.6.  COMMENTS ON ITEMS USED IN EARLIER TIME SCALES BUT REJECTED IN THIS STUDY  128  7.7.  DIRECT DATING OF STRATIGRAPHIC BOUNDARIES  131  7.8.  CHRONOGRAM ESTIMATION OF BOUNDARIES  132  7.9.  ADJUSTED STAGE BOUNDARY ESTIMATES  137  7.10.  LATEST JURASSIC STAGE BOUNDARY ESTIMATES THROUGH MAGNETOCHRONOLOGIC INTERPOLATION  138  ,7.11. THE JURASSIC TIME SCALE 7.12.  139  DISCUSSION  140  CHAPTER 8. TIME CONSTRAINTS ON JURASSIC MASS EXTINCTIONS AND RECOVERIES  143  8.1. INTRODUCTION  143  8.2.  THE END-TRIASSIC MASS EXTINCTION AND EARLIEST JURASSIC RECOVERY  144  8.3.  THE PLIENSBACHIAN-TOARCIAN EVENT.:  145  8.4.  LATER JURASSIC EVENTS  145  8.5.  SPACING OF JURASSIC AND OTHER MESOZOIC MASS EXTINCTIONS  146  CHAPTER 9. SUMMARY  149  REFERENCES CITED  153 -i<-  APPENDIX 1. INPUT OF COMPUTER-ASSISTED BIOCHRONOLOGIC CORRELATION USING BioGraph  171  Al. 1. LIST OF AMMONOID TAXA AND THEIR CODES  171  A 1.2.  173  AMMONOID DISTRIBUTION IN REPRESENTATIVE TOARCIAN SECTIONS  APPENDIX 2. CORRELATION TABLES USING UNITARY ASSOCIATIONS (BioGraph OUTPUT). 176  vii  LIST OF TABLES Table 2.1. Boundary ages and duration of the Jurassic Period in the early time scales  6  Table 2.2. Summary of dates obtained from the Guichon Creek batholith  14  Table 3.1. U-Pb zircon analytical data of sample PCA-YR-1  28  Table 3.2. List of stratigraphic sections and sources of BioGraph input  35  Table 3.3. Synonymies of species of Hildoceras  40  Table 4.1. U-Pb zircon analytical data  64  Table 4.2. Comparison of ammonite identifications in Imlay (1981) and in this report  68  Table 5.1. Summary of biochronology, geochronology, and location of samples  78  Table 5.2. U-Pb analytical data  83  Table 6.1. U-Pb analytical data  102  Table 7.1. Listing of selected critical isotopic ages  116  Table 7.2. Chronogram estimates of the initial boundary of chrons, substages and stages  134  viii  LIST O F FIGURES  Figure 2.1. Synopsis of the Jurassic part of time scales with assigned stage boundary ages  7  Figure 2.2. The number of critical Jurassic dates used in some key time scales  9  Figure 2.3. Summary of geochronometric data from the Guichon Creek batholith Figure 2.4. Proportion of dates obtained by different isotopic methods  11 13  Figure 3.1. Location map of the Yakoun River section  25  Figure 3.2. Litho- and biostratigraphy of the Toarcian of the Yakoun River section  26  Figure 3.3. U-Pb concordia diagram for zircons from sample PCA-YR-1  28  Figure 3.4. Collection levels, observed ranges, and estimated maximum ranges in the Yakoun River section  32  Figure 3.5. Global latest Early to early Late Toarcian ammonoid taxon ranges  37  Figure 3.6. Reproducibility of the 40 Unitary Associations in Toarcian ammonite faunas  38  Figure 3.7. Simulation of the effect of 25% taxonomic noise on the unitary associations  40  Figure 3.8. Comparison of boundary age estimates and numerical mid-point of the Toarcian  42  Figure 4.1. Location map of the Puale Bay section  45  Figure 4.2. Normal fault separating the lowest Jurassic from uppermost Triassic rocks  47  Figure 4.3. Measured lowest Jurassic stratigraphic section and ammonite ranges at Puale Bay, Alaska  50  Figure 4.4. Biochronologic correlation chart of Hettangian ammonite zonations proposed for key regions  55  Figure 4.5. Ranges of Hettangian ammonite taxa known from Puale Bay as established in other regions  56  Figure 4.6. One cm thick crystal tuff layer near the top of the Kamishak Formation  65  Figure 4.7. U-Pb concordia diagrams for samples from the Hettangian section at Puale Bay  66  Figure 4.8. Comparison of Hettangian age estimates in recent time scales and the new U-Pb dates Figure 5.1. Index map of U-Pb sample localities  70 73  Figure 5.2. North American composite ammonite zonal scheme for the Sinemurian through Bajocian  75  Figure 5.3. U-Pb concordia diagrams of samples 1-13  79  Figure 5.4. Measured or schematic stratigraphic sections showing U-Pb samples and fossil collections  81  Figure 6.1. Locality index map of Triassic-Jurassic boundary sections in the Queen Charlotte Islands  ix  100  Figure 6.2. U-Pb concordia diagram of zircons from a tuff layer immediately below the T-J boundary  101  Figure 6.3. The U-Pb dated tuff layer in the T-J boundary section on Kunga Island  104  Figure 6.4. Chronogram estimation of the T-J boundary age based on 13 U-Pb dates  106  Figure 7.1. Early and Middle Jurassic ammonite biochronological units of North America  114  Figure 7.2. Chronogram plots of chronostratigraphic boundary ages  135  Figure 7.3. Comparison of the proposed Jurassic time scale with other major time scales  142  Figure 8.1. Periods between mass extinction events in the Mesozoic and at its boundaries  147  x  LIST OF PLATES Plate 4.1. Middle Hettangian ammonite fauna of Puale Bay  52  Plate 4.2. Late Hettangian through Early Sinemurian ammonite fauna of Puale Bay  53  Plate 5.1. Late Bathonian ammonoids of the McDonell Lake and Zymoetz (Copper) River sections  93  Plate 5.2. Late Bathonian ammonoids of the McDonell Lake and Zymoetz (Copper) River sections  94  xi  ACKNOWLEDGEMENT The nature of this thesis project required an unusual degree of collaboration with numerousfieldgeologists, geochronologists, and paleontologists to whom I'm grateful for their advice, help, and continuing interest. The opportunity to interact with such a diverse group of knowledgeable colleagues tremendously enhanced my learning experience. Firstly, I thank Paul Smith, my thesis supervisor, for suggesting a rewarding (and frustrating at times, I might add) research project, providing the paleontological laboratory facilities and generous funding, accompanying me to thefield,and discussing and reviewing thesis drafts and various other manuscripts. Jim Mortensen efficiently introduced a paleontologist to the world of U-Pb geochronology, allowed me to use the facilites of the Geochronology Laboratory, answered my many simple-minded questions throughout the project, and somehow found time for coming to the field with me. Bob Anderson is thanked for providing logistical support through the Geological Survey of Canada for field work during the summer seasosns, acting as a liaison to the Geochronology Laboratory of the GSC, his keen interest in the project, and insightful and thorough reading of thesis drafts. Howard Tipper's immense experience in the Jurassic geology and biostratigraphy of western Canada was an invaluable resource in selectingfieldareas for dating and interpreting results. Access to his lab at the GSC was most useful. Besides, his wisdom, encouragement and benevolence throughout the project is much appreciated. Kurt Grimm is thanked for his advice and intellectual stimulation as a member of the supervisory committee. Rich Friedman taught me everything I needed to know about zircon sample processing, mineral separation, zircon picking, abrasion, and data reduction, and did it patiently several times over. Rich kept my samples going through the pipe and his ever-careful job in chemistry and mass spectrometry was instrumental in the analytical work for this project. Randy Parrish (formerly at GSC) participated in the project by collecting zircon samples in the Queen Charlotte Islands and producing quality geochronological results reported in Chapter 3. Vicki McNicoll and Mike Villeneuve (GSC) produced some of the analytical results reported in Chapter 5. In variousfieldareas throughout the Cordillera, Chris Ash (BCGS), Randy Enkin (GSC), Charlie Greig (GSC), Craig Hart (Yukon Geoscience), Julie Hunt (Yukon Geoscience), Peter Lewis (UBC), Gary Payie (BCGS), Don Maclntyre (BCGS), and Graham Nixon (BCGS) provided much useful logistical assistance and shared their knowledge of the respective areas. Fiona Childe (UBC), Larry Diakow (BCGS), Carol Evenchick (GSC), Janet Gabites (UBC), Gary Johannson (UBC), Brian Mahoney (UBC), Mitch Mihalynuk (BCGS), and JoAnne Nelson (BCGS) contributed unpublished isotopic ages or information on the geological setting of dated or potentially dateable units. The isotopic database compilation benefited from discussions with and unpublished information from Bart Kowallis (Brigham Young U.), Paul Olsen (Lamont), and Fred Peterson (USGS). Beth Carter, Fabrice Cordey, Russell Hall (U. of Calgary), Genga Nadaraju (UBC), Mike Orchard (GSC), and Tim Tozer (GSC) helped in micro- or macrofossil identification and/or gave advice on biochronology. Among my paleontologist colleagues, Giselle Jakobs dererves special thanks for sharing her knowledge on the biostratigraphy of the Yakoun River and Diagonal Mountain localities and her help in the identification of Toarcian ammonites and collecting in the field. Studentfieldassistants Rocio Lopez, Sean Galway, Tanya Mauthner, and Keegan Schmidt gave help in sample collecting. Fellow graduate students at UBC, especially Mark Caplan, Ben Edwards, Bo Liang, and Hristo Stoynov are thanked for fruitful discussions, their help with computers, and etc. Attila Voros and Istvan Matskasi granted me an extended study leave from the Hungarian Natural History Museum to undertake PhD studies at UBC. In addition, Attila also kept me in touch with the Hungarian geological community. A number of friends gave me invaluable moral support in difficult times when the project was in limbo. Thank you Ian, Naomi, D6ra, and all of you I can't list here. My stay at UBC was an endeavor for the whole family. To prioritize graduate school and parenting has proved to be a most delicate task in wich all of us had to make sacrifices. The biggest of hugs goes to my wife, Maria Mayer-Palfy, for her tremendous support, coping with my absences, just keeping sane in crazy times, and enduring my seemingly lifelong learning.  xii  "Science hath bitter rootes but it bearath sweet fruits. "  Janos Apaczai Csere, 17th century Hungarian scholar  "...here: see this stone, from up there? try as you might, you can't; to show all this in fine detail — there is no such instrument. "  Miklos Radnoti, 20th century Hungarian poet [Translation by George Emery]  DEDICATION  To my children, Marton, Mate, Lilla, and Aron, who all took their first steps on this very special campus.  xiii  CHAPTER 1  INTRODUCTION  1.1.  INTRODUCTORY STATEMENT Time is a fundamental dimension of geology. There are numerous methods for dating rocks and geological  events. Primarily, time can be measured in million years (Ma) or expressed in terms of a standard chronostratigraphic framework. Geochronologic, or time scales represent the integration of these two methods whereas numeric ages are assigned to stratigraphic boundaries. The subject of the present study is the time scale calibration for the Jurassic Period. There are two fundamental observations that warrant this study (dubbed as the "Cordilleran Jurassic Calibration Project): (1) that the available and widely used time scales for he Jurassic are conflicting and imprecise, and ( 2 ) that the volcanosedimentary sequences of the North American Cordillera have the potential to improve the time scale through furnishing new tie points. Indeed, the Jurassic time scale is inferior to most other post-Paleozoic periods in precision and accuracy. Time scales are built from calibration or tie ponts, i.e., radiometric ages of known stratigraphic position. In the North American Cordillera, volcanic arc assemblages are widespread therefore it is possible to radiometrically date volcanic and volcaniclastic strata that are interbedded with biochronologically dated fossiliferous sedimentary rocks. An improved time scale will find its application in a diverse suite of Jurassic research. From a Cordilleran perspective, it is particularly important for better correlation of magmatic, sedimentary and metamorphic events thereby refining the geologic history of Cordilleran terranes. As Jurassic rocks are hosts to numerous mineral deposits, mineral exploration will also benefit from establishing more realistic linkages of known ages of mineralization events, sedimentary and magmatic host rocks, etc.  1  1.2.  OBJECTIVES The primary objectives of this study are as follows:  (1) To review the available Jurassic time scales, assessing their methodology, strengths, and deficiencies. (2) To identify the most suitable geo- and biochronological methods for modern time scale research and to obtain reliable calibration points. (3) To develop methods to estimate the uncertainty of biochronologic dating and correlation between North American and northwest European zonations. (4) To compile an updated global database of acceptable and stratigraphically well-constrained Jurassic isotopic ages. (5) To construct a Jurassic time scale based on the above, attempting to refine resolution to the zonal level. (6) To assess the implications of timing constraints provided by the above scale on major Jurassic bio-events.  1.3. METHODS In the course of the present study, the following methods were employed: Literature review - An extensive literature review was carried out to summarize the state of affairs in Jurassic time scale research and to select Cordilleran sampling sites for integrated radiometric and ammonite biochronologic dating. Preference was given to sections with a potential or proven record of zonal resolution ammonite biochronology. Ammonite biochronology - The standard chronostratigraphy of the Jurassic is based on ammonoid biochronology, therefore its use to establish biochronologic ages was preferred wherever practical. Ammonite and other faunas were collected from measured sections wherever feasible. After identification, we followed the practice of using regional or local North American zonations that are correlated with the northwest European standard. Error estimation of biochronologic dating - To make a rigorous assessment of correlation uncertainty, we employed a model calculation of range extension and the computer-assisted Unitary Association method to determine a realistic error range.  2  U-Pb geochronology - For U-Pb zircon geochronology, the most felsic, crystal-rich volcaniclastic units were selected for sampling in thefield.Depending on the lithology and ease of collecting, 5 to 30 kg samples were taken, preferably from measured sections. Procedures for mineral separation, selection of grains, chemistry, mass spectometry, data reductions and interpretation follow those outlined by Mortensen et al. (1995) with additional remarks given in later chapters. Time scale construction - Steps of building the time scale include initial screening and compilation of a database, selection of suitable dates as direct boundary estimates, calculation of chronograms for stage and, if possible, zonal boundaries, adjustments of calculated boundary ages, and magnetochronologic interpolation for the Late Jurassic stage boundaries.  1.4. PRESENTATION The main part of this dissertation consists of seven chapters (Chapters 2-8) that are self-contained research articles. Chapters 2 and 3 have been published whereas Chapters 4 to 8 are to be submitted for publication in various earth science journals. This format facilitates rapid communication of the main results to the scientific community. However, it also results in some inevitable repetition, especially in the introduction and, to a lesser degree, in the discussion sections. Care was taken to minimize the redundancy, although some overlap was unavoidable. Chapter 2 provides a review of existing Jurassic time scales with emphasis on their different methodologies and concomitant discrepancies. By demonstrating the deficiencies of available scales, we establish the need for the present study. The analysis of previous work helps define the goals and methods to be followed here. Chapter 3 is centered around a single U-Pb age obtained from the type section of a North American regional standard ammonite zone. It is presented separately as an example for a high-quality calibration point, on the strength of a precise isotopic age with zonal biochronologic constraints. A case study in analysis of biochronologic dating error is also given here. Chapters 4 and 5 contain the bulk of the new U-Pb ages obtained in the course of this study from Alaska and British Columbia (three and thirteen, respectively). All isotopic dates are integrated with ammonite biochronology.  3  Chapter 6 reports another new U-Pb age that has the singular importance of providing a direct date for the Triassic-Jurassic boundary. It is considered together with twelve other U-Pb ages from the uppermost Triassic and lowermost Jurassic to define the age of this boundary that is also marked by a severe mass extinction event. Chapter 7 represents the culmination of this study whereby a comprehensive Jurassic isotopic database is presented and is used to construct a revised time scale, arguably the most important outcome of this study. Chapter 8 is a preliminary assessment of the timing of major Jurassic and other Mesozoic biotic events (mass extinction and subsequent recoveries) in the light of the new time scale. It is only one of the several examples of how the revised time scale might shed new light on problems of Jurassic earth history. Exploring more of such avenues of research will be fruitful but are beyond the scope of this study.  4  CHAPTER 2 DEVELOPMENT OF THE JURASSIC GEOCHRONOLOGIC SCALE  2.1.  INTRODUCTION  Time is a fundamental dimension o f geologic processes and phenomena. Stratigraphers and geoscientists in general interested in the Jurassic (or any other geologic period) need a chronometrically calibrated chronostratigraphic scale which facilitates correlation o f rocks and events dated by various methods. It is also necessary for measuring rates o f geologic processes. Conclusions in a wide range o f problems are affected by the accuracy and precision o f the time scale used. For example, in an active margin setting the correlation o f isotopically dated magmatic and metamorphic events with biochronologically dated sedimentary events depends on the reliability o f time scale. A l s o , calculation o f sedimentation rates, sedimentary cycle duration, purported  -  periodicity in extinction and other phenomena, subsidence or uplift rates, plate velocity vectors, etc. needs accurate measurements o f time, again dependent on the time scale. The nomenclature o f different kinds o f time scales is extensively discussed by Harland (1978, see also Harland et al., 1990). The focus o f this review is the calibration o f the chronostratigraphic scale made up o f standard subdivisions with the linear chronometric scales which uses M a (million years) units. The resulting calibrated scale is termed the geochronologic scale, sometimes also referred to as the numeric or geologic time scale. Over the past 40 years the Jurassic chronostratigraphic scale has remained relatively stable except for the status o f the Aalenian and the subdivisions o f the latest Jurassic. Even at the zonal level o f standard ammonite biochronology, refinements (e.g. Cope et al., 1980) since Arkell's synthesis (1956) have been minor compared with developments in the field o f isotopic dating and the emergence o f new concepts and methods o f time scale calibration.  Published in 1995 under the same title in Hantkeniana, v. 1, pp. 13-25. 5  Barrell 1917 Minimum Maximum  Holmes 1937  Holmes 1947  Holmes 1959  Kulp 1961  Jurassic/Cretaceous boundary  120  150  108  127  135+5  135  Triassic/Jurassic boundary  155  195  145  152  180+5  181  37  25  45  46  Duration of Jurassic  40  Table 2.1. Boundary ages and duration of the Jurassic Period in the early time scales.  Many geologic time scales have been published since Barrell's (1917) first attempt at systematically estimating the ages of geologic system boundaries (including those of the Jurassic). The early scales (Barrell, 1917, Holmes, 1937, 1947, 1959, Kulp, 1961) were based on very limited numbers of isotopic dates and provided estimates for only the upper and lower boundaries of the period (Table 2.1). Beginning with Harland et al. (1964), the next generation of researchers attempted to assign numeric age estimates to stage boundaries based on a growing database of primarily K-Ar ages (e.g. Van Hinte, 1976, Armstrong, 1978), although some workers (e.g. Lambert, 1971 and Fitch et al., 1974) rejected this approach as inappropriate given the available dataset. Modern time scales published between 1982 and 1994 (see Fig. 2.1) are based on a much larger, although still insufficient, isotopic age database which includes a slowly increasing proportion of A r - A r and U 40  39  Pb ages. Many of these time scales introduce some kind of statistical treatment of accepted isotopic data and express the uncertainties of boundary age estimates. Some of these time scales are extensively referred to in the recent geologic literature (especially Harland et al., 1982 superseded by Harland et al., 1990, Palmer, 1983, and to a lesser extent Kennedy and Odin, 1982, Haq et al., 1987, 1988). Time scales which oversimplified the problem (Salvador, 1985) or concentrated on the mathematical aspects only (Carr et al., 1984, Bayer, 1987) did not gain widespread acceptance, nor did those based on reinterpretations of pre-existing datasets (e.g. Westermann, 1984, 1988, Hallam et al., 1985, Menning, 1989). The Soviet time scales were not widely available and thus had relatively little impact in the western geoscience literature (e.g. Afanasyev and Zykov, 1975, Afanasyev, 1987). 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I I III  Ll  io 111  O  -H  i-  OS  I  CN  o  A s illustrated in F i g . 2.1, the wide range o f available time scales disagree significantly in their boundary estimates. Where uncertainties are quoted, they often exceed the duration o f adjacent stages. The aims o f this paper are to (1) provide some background regarding the construction o f time scales; (2) review the historic development o f Jurassic time scales; (3) present the variety o f modern time scales and analyse their differences and limitations; and (4) discuss current and future avenues o f research which may lead to improved accuracy. It is hoped that Jurassic stratigraphers w i l l benefit from an increased awareness o f the differences in the available time scales and w i l l be more critical in their use.  2.2.  DATABASE AND PROCEDURES IN TIME SCALE CALIBRATION  A calibrated time scale depends on biostratigraphically constrained isotopic dates. The following types o f dates are c o m m o n l y used, listed here in order o f decreasing frequency in most scales: (1) ages o f intrusive rocks whose stratigraphic constraints are established from cross-cutting and superpositional relationships with fossiliferous sedimentary rocks; (2) ages o f authigenic glauconite found in fossiliferous sedimentary rocks; (3) ages of volcanic flow or pyroclastic layers intercalated in fossiliferous sedimentary sequences. Isotopic age determinations applicable for time scale construction carry three kinds o f uncertainties (e.g. O d i n , 1982): analytical uncertainty (measurement error o f the isotopic age determination), geochemical uncertainty (the possibility that the measured apparent age differs from the true age due to disturbance o f the isotopic system), and stratigraphic uncertainty (the width o f the stratigraphic interval to which the sample is confined, coupled with potential correlation error). Every isotopic age determination must be evaluated for analytical precision, geochronologic accuracy, and quality o f biochronologic constraints. The construction o f a time scale which best represents this screened database can be achieved by statistical or graphical methods, or intuitively, i.e. by judging and weighting each date individually. If direct isotopic dating o f stratigraphic boundaries is not available, as has so far been the case for the Jurassic, then interpolation between calibration points becomes necessary.  8  2.3. K E Y AREAS OF PROGRESS AND CONSTRUCTIONAL DIFFERENCES BETWEEN SCALES  Successive amendments of the geologic time scale reflect progress in certain key areas of building the scale and the differing methodologies of research groups. An analysis of these factors is attempted in the following. There has been a slow but steady increase in the number of useful Jurassic isotopic dates (commonly called "items" following Harland et al., 1964) (Fig. 2.2). Armstrong (1978) pioneered a computer database for efficient management of large number of isotopic dates, a practice followed by subsequent workers who relied on comprehensive and original databases with the intention of mathematical treatment of dates (Harland et al., 1982, 1990, Gradstein etal., 1994).  2.3.1. Advances in isotopic dating A succinct overview of progress in geochronometry can be found in Armstrong (1991). Developments in this field were reflected in the time scales. The earliest scales (Barrell, 1917, Holmes, 1937, 1947) relied mostly on age determinations of uranium minerals. In the 1950's, the K-Ar method became available and for decades it was the most frequently used dating method. The databases used in the most recent attempts at time scale calibration  50  45  40  36 30  30  20  10  10  Holmes 1959  Kulp 1961  12  Harland et al. 1964 1982  Odin Harland Gradstein 1982 et al.1990 et al.1994  a  Figure 2.2. The number of critical Jurassic dates used in some key time scales built on comprehensive isotopic databases. seven additional glauconite dates listed but not used in final calculations. a  9  (Harland et al., 1990, Gradstein et al., 1994) accumulated during decades of work, and include a significant number of ages obtained during the 1960's and 1970's. (The historically oldest item (PTS76) still used by Harland et al., 1990 and Gradstein et al., 1994 is based on an age determination reported in 1958.) Most of these early data were obtained using the K-Ar method along with a lesser number of Rb-Sr analyses. Complicating the early time scale compilations was the need to recalculate ages because of inconsistent decay constants. Following the adoption of standard decay constants by the IUGS Subcommission on Geochronology (Steiger and Jager, 1977), this problem was resolved. Over the last ten years the U-Pb and A r - A r methods emerged as superior in precision to the K-Ar, Rb-Sr, 40  39  and other methods. They also offer the advantage of interpretation of the geological meaning of the ages obtained. The detection of radiogenic daughter isotope loss greatly reduces the geochemical uncertainty of these ages. The U-Pb dating of accessory minerals, most notably zircon, is one of the most accurate isotopic dating methods presently available. The closing temperature of zircon is estimated to be >800 C° (Heaman and Parrish, 1991), thus it retains the radiogenic Pb isotopes even in upper amphibolite grade metamorphism. Recent advances contributing to the increased accuracy of the method include new analytical procedures lowering the analytical Pb blank (Heaman and Parrish, 1991); the introduction of air abrasion to reduce or eliminate discordance due to postcrystallization Pb loss (Krogh, 1982); and the use of an ultra-clean  2 0 5  P b tracer (Parrish and Krogh, 1987).  Minerals other than zircon can also be used for U-Pb dating, e.g. badelleyite, an accessory mineral in mafic rocks (for a Jurassic example see Dunning and Hodych, 1990), and the relatively rare accessory mineral monazite (Parrish, 1990). Monazite has been successfully used (for a Paleozoic example see Roden et al., 1990) in cases where inheritance of older xenocrystic zircon complicates the interpretation of U-Pb zircon data, as inherited monazite appears to be extremely rare. The A r - A r dating technique was introduced in 1966 by Merrihue and Turner, and its application became 40  39  more widespread by the early 1970's (McDougall and Harrison, 1988). Nevertheless the Harland et al. (1990) time scale lists only three A r - A r dates (as opposed to 30 conventional K-Ar ages) for the Jurassic. It is now well 4 0  3 9  established that the age spectra generated by step-heating technique can be used to interpret the thermal history of rocks. Ar loss or gain can therefore be detected and a true cooling age can be inferred. Among the datable, common rock-forming minerals, hornblende has the highest closure temperature (>500 °C). The precision of A r - A r 40  39  technique approaches that of the U-Pb chronometer, attaining ±1-2 Ma for Jurassic samples in favourable circumstances.  10  30  K-Ar dates  10  - i — i — ' — i — — i 165 170 175 180 1  i— i 185 190  i 195  i 200  205  210  215 220Ma  Rb-Sr dates U—Pb date  Mortimer et al. (19  Figure 2.3. Summary of geochronometric data from the Guichon Creek batholith (British Columbia, Canada) (modified after Mortimer et al., 1990). Selected dates from this dataset were used in different time scales for constraining the Triassic/Jurassic boundary (see Table 2.2). K-Ar dates (published between 1960 and 1979, and unpublished data) are summarized on a histogram with a 5 Ma class interval, reported analytical errors are typically ±8 Ma. Two Rb-Sr dates (published in 1969 and 1979) and one U-Pb date are plotted with their respective error bar. Sources of data are listed in Mortimer et al. (1990).  As an example, dates obtained from the Guichon Creek batholith (British Columbia, Canada) are illustrated here. This intrusion is perhaps the most geochronometrically investigated igneous body in the Canadian Cordillera (Mortimer et al., 1990). The histogram of Fig. 2.3 summarizes the distribution of apparent K-Ar ages reported between 1960 and 1979 which are compared with two Rb-Sr isochron ages and a recent U-Pb zircon age. It is evident that (1) the nearly concordant U-Pb age is analytically precise, (2) the majority of the K-Ar ages are too young, and (3) at least one (the more precise) of the two Rb-Sr isochron ages is also too young, casting doubt on the reliability of those geochronometers.  2.3.2. Selection of dates  It is clear from the criteria listed above that not every isotopic date is suitable for use in time scale calibration. Lambert (1971) pointed out that each Jurassic item quoted by Howarth (1964) and listed in Harland et al. (1964) can be criticized on one or more grounds. The greatest risk posed by minerals with low closure temperature is their susceptibility to loss of radiogenic daughter isotopes in post-crystallization geological processes. The result is a young apparent age which is hard to detect especially using the K-Ar method. It became apparent that glauconite ages often tend to be younger than their high-temperature counterparts (e.g. Obradovich, 1988). However, Odin II  advocated the use o f glauconite arguing that its geochemical behaviour can be independently evaluated (Odin, 1982). Glauconitic sediments are abundant in the European Jurassic and most glauconite dates are biochronologically tightly constrained (e.g. Fischer and G y g i , 1989), therefore their use in time scale studies is tempting. In the time scales o f Kennedy and O d i n (1982), Afanasyev and Z y k o v (1975), Haq et al. (1988), O d i n and Odin (1990), and O d i n (1994) glauconite ages dominate the Late Jurassic dataset. Other workers, however, gradually abandoned the use o f glauconite ages. Harland et al. (1982) qualified some glauconite ages as too young and later treated them as m i n i m u m ages only (Harland et al., 1990). Gradstein et al. (1994) list glauconite ages but reject all o f them for their final calculations. The 10 to 15 M a discrepancy in the Jurassic/Cretaceous boundary estimates between the two types o f scales (see Fig. 2.1) is clearly the result o f the inclusion or rejection o f glauconite dates. Apart from the glauconite question, the current scales are not selective with respect to the dating method or dated mineral used. There is a significant time lag between major advances in geochronometry and their utilization for time scale studies. U - P b and  4 0  Ar-  3 9  A r dates still represent only a minor fraction o f items used in most recent  time scales ( F i g . 2.4). Moreover, the all-inclusive nature o f databases, advocated by Harland et al. (1982, 1990) and also adopted by Gradstein et al. (1994) for sake o f relative stability in numeric boundary estimates, dampens the effect o f new dates with superior precision and accuracy. It is revealing that the most recent time scales contain items gleaned from late 1950's-early 1960's publications in their data suite. A more radical approach, such as Obradovich's (1993) exclusive use o f A r 4 0  3 9  A r ages with high analytical and stratigraphic precision for the  Cretaceous time scale, has not yet been taken for the Jurassic. The example o f the Guichon Creek batholith can be used again to illustrate this point. Dates from this pluton have been used in time scales since the work o f Holmes (1959), because the emplacement o f the batholith is thought to be near the Triassic/Jurassic boundary. Successive attempts at dating the batholith, their representation in various time scales, and their effect on the placement o f the Triassic/Jurassic boundary are listed in Table 2.2. It is notable that (1) the more recently obtained ages tend to be geologically older then previous ones; (2) some time scales (e.g. Gradstein et al., 1994 using sources published more than 20 years earlier) rely on outdated information; (3) the U - P b age should be preferred over K - A r or R b - S r ages. The last point has not been considered in any existing Jurassic time scale but it may indicate that K - A r or R b - S r ages are best treated as m i n i m u m ages only.  12  fGradstein etal. 1994  '///// / / / / / / //////////////,I  ''''/////  Hariandetal. 1990  / / / / / / /  /  /  /  /  U-Pb  K-Ar(ig)'////.  /  / / /  /  /  /  /  /  /  K-Ar(ig) / / / /  /  K-Ar(gl)  -  U-Pb  Odin 1982 / / / /  K-Ar(ig) / / / ' / / / / , K-Ar(gl)  / / / / , , , , / / /  y<<<Rb-Sr  I Hariandetal. 1982  V//// ' / / / / / K-Ar(ig) ///////////////I y  '/////  y  y////  /  \  ' / / , K-Ar(gi)  Harland etal. 1964 /  K-kmy////  / / / / / /  //////////////  /  /  /  /  /  /  /  /  /  /  /  /  /  /  /  ' ' ' ' K-Ar(gl)  Kulp 1961  y///// //////////////,  /  K-Ar(ig)  /  /  /  /  /  /  /  '  /  '  '  /  /  / / / / / // ./ / // // , , K-Ar(gl)'>> /  /  /  '  /  /  .  Holmes 1959 /  /  / / / / / / /  /  0 0  - ('9>  K Ar  /  K  -  A  r  ( ' 9 ) / / / / /  /  /  I  I  I  20  40  60  0  K  " (gl) Ar  G Rb-Sr  /  / / ^ / /  R  b  _  S  r  :::  I 80 H Ar-Ar  100% § U-Pb  Figure 2.4. Proportion of dates obtained by different isotopic methods used in some widely cited time scales. gl - glauconite age; ig - age determined on igneous mineral.  The elimination of biochronologically well constrained glauconite dates increases the weight of ages from intrusive rocks whose stratigraphic position is often poorly bracketed although their isotopic age can be precise. Multi-phase intrusions, whose emplacement can span long intervals of time, are common in the geologic record. The large percentage of plutonic ages in the database of every time scale is accountable for the large uncertainties attached to stage boundary estimates. The listing of inconsistent interpretations of the stratigraphic constraints on the Guichon Creek batholith (Table 2.2) reveals the problems associated with intrusive bodies and their serious effect on boundary age estimates.  13  TIME SCALE  ITEM CODE  1  QUOTED AGE QUOTED CHRONOSTRATIC POSITION  METHOD PUBLICATION YEAR OF QUOTED SOURCE OF A G E 2  Holmes, 1959  2  TRIASSIC/ JURASSIC BOUNDARY ESTIMATE  180  Upper Triassic or Lower Jurassic  1959  K-Ar  180±5  Kulp, 1961  62  181±5  post-Carnian, pre-Bajocian  1960  K-Ar  181  Harland etal., 1964  10(Tozer& Hacker)  185, 186  Norian to Bajocian  1961  K-Ar  190-195  Harland & Francis 1971  366 (Lambert)  200±2  post-early Upper Triassic, pre-Middle Jurassic  1969  Rb-Sr  200?  Armstrong, 1978  366 475  200±2 200±5  Carnian-Pliensbachian brackets  not given  Rb-Sr  ~211  Armstrong, 1982  209 (oldest) 203 (mean)  post-Norian, Hettangian or older  1969  K-Ar K-Ar  208  NDS 177 Odin, 1982 Kennedy & Odin, 1982 (Armstrong)  205 (mean) 205±20  Late Norian to pre-Sinemurian  1967-1973 1979  K-Ar Rb-Sr  204±4  Carnian-Pliensbachian brackets  (1969)  Rb-Sr  213  3  Harland etal., 1982  PTSS 366  195.77±2  Harland etal., 1990  NDS 177 (A366)  205±2.5  Norian-Hettangian brackets  (1969-1973)  K-Ar  208  Gradstein et al., 1994  284 (NDS 177)  205±2.5  Norian-Hettangian brackets  (1969-1973)  K-Ar  205.7±4.0  210±3  post-Carnian (possibly postNorian), pre-Pliensbachian (possibly pre-Sinemurian)  Mortimer etal., 1990  U-Pb  4  5  Table 2.2. Summary of d a t e s obtained from the Guichon Creek batholith (British Columbia, Canada) and their role in successive time scales with reference to the Triassic/Jurassic boundary age estimate. 'Name of abstractor or reference to item used in previous time scales given in parentheses; parentheses indicate that no direct source given but reference made to an item used in previous time scales; approximate reading from graph, no numeric value given in text; R e c a l c u l a t e d in Harland et al. (1982) using standard decay constants in Steiger and J age not referred to in Gradstein et al., 1994 or any other time scale. 2  3  5  2.3.3. Errors and uncertainties in data The three main sources of uncertainties related to items used in time scale studies were mentioned above. Quantification of uncertainties in form of error limits is required if errors on mathematically derived boundary age estimates are to be rigorously calculated. Errors need to be taken into account even if an intuitive method is preferred when each date is individually evaluated and weighted. Any isotopic age determination is subject to analytical error. It is generally reported at la (68% confidence) or 2a (95% confidence) level. Although this is common practice now, many early dates used in even modern scales do not have published error limits.  14  a g  The geochemical uncertainty, i.e. the possibility that the apparent age may deviate from the true age o f the dated rock, cannot be quantified. It is dealt with at the selection o f acceptable isotopic dates when for example, glauconite ages are treated as m i n i m u m ages only (Harland et al., 1990) or rejected altogether as suspect (Gradstein et al., 1994; see discussion above). A s noted earlier, none o f the Jurassic scales acknowledges that U - P b and 3 9  4 0  Ar-  A r analyses can prove the closed system behaviour o f the isotopic system, thereby m i n i m i z i n g the geochemical  uncertainty. The stratigraphic uncertainty also poses serious problems. Chronostratigraphic assignment o f intrusive rocks is problematic and c o m m o n l y even the tightest constraints leave an interval o f several stages, as demonstrated above by the example o f the Guichon Creek batholith (Table 2.2). Here the overlying sediments were dated as Early Jurassic based on the bivalve Weyla (Frebold and Tipper, 1969). The upper bracket o f the batholith's emplacement was arbitrarily interpreted as Hettangian, Sinemurian, or Pliensbachian in different time scales, without due consideration to the Hettangian to Toarcian age range o f  Weyla.  Although Late Pliensbachian  ammonites were subsequently found as the oldest fossils in overlying sediments (Tipper in M o n g e r and M c M i l l a n , 1984), this is not reflected in the time scales o f Harland et al. (1990) and Gradstein et al. (1994) which cite a Hettangian upper bracket. Most authors are satisfied to accept the reported stage-level chronostratigraphic brackets o f isotopic ages and they take them at face value i m p l y i n g that biochronologic correlation is free o f error. Most biostratigraphers, however, w o u l d agree that chronologic correlation between stratigraphies o f different fossil groups and separate biogeographic provinces is not straightforward. O n l y Harland et al. (1990) attempted to quantify this kind o f chronostratigraphic uncertainty as "fossil error" by arbitrarily assigning an additional 2.5 M a error to all Jurassic ages. A s acknowledged by Smith (1993), it is an incorrect overestimation o f the uncertainty which was chosen only for simplicity and in lack o f other available error estimation method. Another example from near the Triassic/Jurassic boundary further illustrates the difficulties faced. Hodych and D u n n i n g (1992) obtained a U - P b age from a basalt flow within the non-marine, Upper Triassic to Lower Jurassic N e w a r k Supergroup in N o v a Scotia. The Triassic/Jurassic boundary within the N e w a r k Supergroup is established not far below the dated basalt, based on palynology and vertebrate biostratigraphy. However, correlation with standard chronostratigraphy is debated (Hallam, 1990). Clearly, the usefulness o f this analytically precise date depends on the estimation o f the uncertainty involved in correlating non-marine faunas and floras with the standard ammonite zonation.  15  2.3.4. Boundary estimation and mathematical data manipulation  After building the dataset o f acceptable isotopic dates, the stage and period boundary ages need to be estimated. W i t h the "geochronologic approach" (Odin, 1994), the boundary ages are derived from intuitive judgement and weighting o f each individual data point. A s there are often conflicts between isotopic ages and their chronostratigraphic position, an element o f subjectivity is introduced in resolving the disagreements. Pre-1982 time scales were all constructed in this manner, and Odin's research group still favours this method (Kennedy and Odin, 1982, O d i n and O d i n , 1990, O d i n , 1994). W i t h an increase in the number o f dates used, the intuitive method becomes more difficult and the need arises for mathematical treatment o f data. On the other hand, a large dataset is needed to statistically validate such procedures. A r m s t r o n g (1978) graphically plotted all o f the relevant dates accepted by h i m , thus provided a visual means o f evaluating consistency and choosing the best fit for boundary ages. This was a step towards removing potential bias by using the dataset collectively. There have been three mathematical methods applied to the boundary age estimation problem. The formulae and detailed descriptions are not reproduced herein, only the key features are discussed. Harland et al. (1982, 1990) used the chronogram method which was originally introduced by C o x and Dalrymple (1967) to calculate the age o f magnetic polarity reversals from the available isotopic dates. The boundary is approximated by an Eft) error function w h i c h is calculated at regularly placed trial ages (for details see Harland et al., 1990, p. 106). In the calculation only the inconsistent dates are considered, i.e. those where the isotopic age and the trial age for stage boundary are in conflict given the known chronostratigraphic age o f the sample. The standard deviation o f the date is taken into account, therefore analytically precise ages are more heavily weighted regardless o f their geochemical uncertainties. A chronogram is graphically produced by plotting E, the error function against t, the trial age. The ideal chronogram curve is parabolic and assumes its minimal value at the boundary age. The error range is established statistically non-rigorously as the time interval where £ < £  m i n  +l.  Agterberg (1988, 1994) noted that although the chronogram method does provide unbiased boundary age estimates, it does not use the dataset efficiently inasmuch as it excludes consistent dates which carry important information. He proposed using the m a x i m u m likelihood method which was shown to be superior to the chronogram method by obtaining boundary age estimates with lower error limits from the same data set. A s with the chronoa ram method, the m a x i m u m likelihood method is also based on the assumptions that the true ages o f samples are randomly distributed and their error distribution is normal. Gradstein et al. (1994) used the maximum likelihood method and noted that it yielded significantly better results (i.e. smaller standard deviations) than the 16  chronogram when the available data was sparse, such as for the Jurassic. It is, however, not fully documented whether in the case o f small sample size the assumption o f normal distribution o f samples remains warranted. Furthermore, Gradstein et al. (1994) ignore the chronostratigraphic error in their time scale construction, which is partly responsible for their smaller boundary age errors as compared with the Harland et al. (1990) scale. Carr et al. (1984) also noted that a weakness o f the chronogram method was that the consistent dates were ignored in the calculation. They proposed a linear regression model instead, based on the assumption that the relative stratigraphic position o f isotopically dated samples can be expressed numerically and the plot o f isotopic ages versus stratigraphic position is expected to be a straight line. A serious practical limitation to this method is that only the ages o f stratigraphically precisely constrained samples can be used, not many o f which are available in the Jurassic. In fact, Carr et al. (1984) were only able to calculate the boundary ages for the Jurassic period but not for its stages. Theoretically, the quantification o f stratigraphic position requires the assumption o f constant rate o f the geological process used in the linearization. A s it w i l l be discussed below, most such assumptions are controversial.  2.3.5. Interpolation techniques  Whether using a statistical approach or intuitive evaluation o f data for best fit, establishing a satisfactory boundary age requires a m i n i m u m number o f good quality, consistent dates from both side o f the boundary. L a c k i n g adequate control for boundaries to be calibrated, estimates must be made by interpolation between some anchored calibration points using a linearizing operator based on a geologic process with assumed constant rate. Barrell (1917) had only one Tertiary and one Late Paleozoic isotopic date to calibrate the intervening Mesozoic. His approach, followed in the subsequent time scales o f Holmes including his last attempt (1959), was to use cumulative sediment thicknesses as a proxy for time. Although the underlying assumption o f constant sedimentation rate cannot be defended, this method allows a crude estimate o f relative duration o f geologic periods. A n attempt to revive the concept by Smith (1993) suggests that a new look at this method taking into account recent research on measuring stratigraphic completeness (e.g. Sadler and Strauss, 1990) could yield more sophisticated results. For interpolation within a period, Harland et al. (1964, 1982) assumed equal duration o f constituent stages. The influential Harland et al. (1982) scale used no Jurassic tie-points (i.e. all Jurassic stage boundary chronograms were rejected as having error ranges in excess o f 5 M a ) . Interpolation between the Anisian/Ladinian and A p t i a n / A l b i a n tie points was used to determine all Jurassic stage boundaries. Chronostratigraphic subdivisions are  17  based on faunal evolution but the usage o f stages records historical developments and tradition in stratigraphy. It is therefore less unrealistic to assume an equal duration for biochronozones (ammonite zones for the Jurassic) in which case a constant rate o f evolution is implied. This method was introduced by V a n Hinte (1976) and later followed by Kent and Gradstein (1985, 1986, for Hettangian to Oxfordian only) and Harland et al. (1990). The latter scale used five tie-points for interpolation o f Jurassic stages: the chronogram ages to define Norian/Rhaetian, Hettangian/Sinemurian, and Oxfordian/Kimmeridgian boundaries which are supplemented by the mid-Bajocian and Tithonian/Berriasian pseudo-tie-points. Westermann (1984, 1988) carried the argument further by developing the scaled equal subzone interpolation method which considers the ammonite subzone as a basic unit and scales the duration o f a subzone at 0.75 o f that o f an undivided zone. His time scale is based on the re-apportion o f Jurassic according to this method, using the period boundary ages proposed by Palmer (1983). (However, after scaling and rounding, the sum o f Westermann's (1984, fig. I) stage durations exceeds the total duration o f Jurassic in Palmer (1983) by 1.5 M a . ) Hallam et al. (1985) also used a similar, subzone based interpolation scheme for their Jurassic time scale. A fundamental criticism leveled against these methods is that the assumption o f constant rate o f evolution is unwarranted (e.g. House, 1985). Direct isotopic dating o f ammonite zones is not yet available for the Jurassic but Cretaceous data o f Obradovich (1993) points to widely disparate zonal durations. A possible independent method o f estimating zonal durations utilizes orbitally forced sedimentary cycles. House (1985) suggested that M i l a n k o v i t c h cycles could be demonstrated in the rhythmically-bedded lowermost Jurassic Blue Lias and the Upper Jurassic K i m m e r i d g e C l a y o f England and the recognition o f cycles could have chronostratigraphic application. Schwarzacher (1993) pointed out that House's (1985) calculations supporting cycle durations within the M i l a n k o v i t c h range first used ammonite zonal durations derived from the equal zone assumption, before the oversimplified counting o f bed couplets was in turn used to provide arguments against the equal zone durations. Subsequent studies confirmed the presence o f cycles in the Blue Lias utilizing sophisticated filtering and spectral analysis (Weedon, 1985, Smith, 1990). Smith's (1990) data, derived from the cyclostratigraphic correlation o f three biostratigraphically well-controlled sections, are strongly suggestive o f disparate ammonite zonal durations in the Hettangian and earliest Sinemurian. Based on the number o f cycles represented, he found an order o f magnitude difference between some zones, independent o f the absolute cycle length. Similar disparity in ammonite zonal length was also suggested for the Cenomanian (Cretaceous) based on M i l a n k o v i t c h cycles studied by Gale (1990). Lastly, another possible linearizing operator for interpolation is the width o f sea-floor magnetic anomalies. The oldest k n o w n preserved oceanic crust is Callovian in age, therefore the post-Middle Jurassic geochronologic.  18  time scale can be calibrated using the magnetic polarity time scale. A thorough review o f the topic can be found in H a i l w o o d (1989). If a constant spreading rate is assumed, then the width o f sea floor magnetic anomalies is proportional to the duration o f the respective magnetochrons. The spacing o f the Hawaiian sequence o f oceanic magnetic lineations (Larson and Hilde, 1975) was used for Late Jurassic interpolation in the D N A G time scale (Palmer, 1983, Kent and Gradstein, 1985, 1986) and by Gradstein et al. (1994). It is interesting, although perhaps coincidental, that the proportions o f Late Jurassic stages in these magnetochronologically interpolated time scales are remarkably similar to the proportions obtained by Westermann (1984, 1988) using the equal scaled subzone method. C u b i c spline smoothing was proposed as an additional mathematical technique to improve interpolation (Agterberg, 1988, 1994). The time scale o f Gradstein et al. (1994) uses this method which offers several advantages over the simple linear interpolation: it takes into account the standard error o f tie-points, reduces the asymmetry o f unit length on the two sides o f tie-points, and reduces the uncertainty o f intermediate boundaries, especially for stages with large error. Despite the availability o f these techniques, Odin (1994 and references therein) argued against interpolation and used it only to complement his "geochronologic approach" when the lack o f isotopic dates otherwise precluded the assignment o f a boundary age (e.g. for his Sinemurian/Pliensbachian, Pliensbachian/Toarcian, Toarcian/Aalenian, and Oxfordian/Kimmeridgian boundaries).  2.4.  DIRECTIONS IN CURRENT AND FUTURE TIME SCALE RESEARCH  The relatively poor quality o f calibration in the Jurassic has been repeatedly pointed out by various time scale authors (e.g. Harland et al., 1990, Gradstein et al., 1994, O d i n , 1994). Its primary cause is the scarcity o f adequate critical dates from this period. Therefore significant improvement o f the Jurassic time scale cannot be achieved without new isotopic dates. Statistical data analysis and interpolation w i l l be relegated to secondary importance as the pool o f acceptable ages increases. Recently the gap between the resolution o f biochronology and isotopic dating has been closing with the advent o f accurate and precise U - P b and  4 0  Ar-  3 9  A r dates with uncertainties near ± 1 M a , thus comparable with the  average ammonite zonal duration (e.g. Westermann, 1984). It is expected that zonally constrained high-precision dates w i l l become available, as it has been the case for the Cretaceous (Obradovich, 1993). Dating o f volcanic layers intercalated with fossiliferous sediments should be given priority, since glauconite is generally rejected as an  19  unreliable geochronometer yielding anomalously low ages (e.g. Obradovich, 1988) and plutonic rocks cannot generally have tight enough stratigraphic brackets required for zonal resolution. The U - P b and  4 0  Ar-  3 9  A r dating  methods yield superior analytical precision with the benefit o f offering an internal checks for closed system behaviour. Their use reduces or eliminates the geochemical error, i.e. the apparent versus true age dilemma plaguing other dating methods. In this light the traditional time scale database needs to be re-evaluated and the bulk o f old K - A r and R b - S r ages replaced (rather than supplemented) by newly obtained U - P b and  4 0  Ar-  3 9  A r ages.  Volcanosedimentary successions with the potential for stratigraphically well-constrained dateable horizons are scarce in Europe but known to exist abundantly in areas o f Jurassic active margins, e.g. in the North American Cordillera and the Andes. Dedicated studies towards improving the Jurassic time scale are now underway in Canada (Palfy et al., 1995). A prerequisite for such projects is the development o f regional ammonite zonal schemes outside Europe (e.g. Hillebrandt et al., 1992, Smith et al., 1994). The increase in precision o f intercontinental correlation w i l l render isotopic age determinations from different parts o f the world much more useful for calibration. If chronostratigraphic age assignments are made at the zonal level, the need for an accurate representation o f correlation uncertainty increases. Promising recent research in quantitative biostratigraphy (e.g. taxon range confidence intervals: Strauss and Sadler, 1989, Marshall, 1994) may be adapted and further developed to assess uncertainties o f zonal correlation. Alternatively, a semi-quantitative uncertainty assessment may prove satisfactory, where expert judgement is used to numerically express the reliability o f biochronologic correlation to the standard scheme. Refinements in the Sr isotope reference curve for the Jurassic enable correlations comparable to the subzonal level o f resolution at the steepest parts o f the curve (Jones et al., 1994a, b). A s an independent method, it can corroborate biochronologic correlation or substitute it where zonal fossils are not available to determine the chronostratigraphic age o f radioisotopically dated samples. Developments in cyclostratigraphic analysis appear promising for interpolation below the stage level when it remains necessary. Where the completeness o f record and the orbital forcing o f sedimentary cycles can be documented, relative zonal durations can be independently assessed.  20  2.5.  SUMMARY  The building blocks o f geochronologic scales are isotopic dates o f known chronostratigraphic age. Plutonic rocks are frequently dated isotopically but their chronostratigraphic position is difficult to establish and commonly poorly constrained. Glauconite, an authigenic mineral occurring in sedimentary rocks, can also be isotopically dated, but the apparent age is commonly anomalously low. The biochronologic age o f the sampled horizon, however, can be very precisely known. Volcanosedimentary sequences provide opportunities for isotopic dating o f samples from intercalated volcanic rocks. In many cases, fossils from adjacent sediments tightly bracket the chronostratigraphic age o f the sample. The database o f critical age determinations has been growing gradually, allowing the estimation o f Jurassic stage boundaries for the first time in 1964 (Harland et al., 1964). In the past 30 years numerous amendments were published, resulting in a perplexing array o f time scales available for the Jurassic. The significant discrepancies o f the proposed stage boundary ages and their quoted uncertainties (which often exceed the stage durations) may confuse time scale users, who cannot assume that the most recently published scale is necessarily the most reliable. A n obvious reason for repeated revisions o f time scales is that new isotopic dates become available and technologic advances increase their analytical precision. Yet the differences between time scales cannot be explained by these factors alone. Divergent approaches regarding the selection o f data, handling o f uncertainties, procedures for boundary age estimation and interpolation all contribute to the disagreement among time scales. A m o n g the major modern works (published in 1982 or later), Odin's successive time scales (Kennedy and O d i n , 1982, O d i n and O d i n , 1990, O d i n , 1994) represent the "geochronologic approach", whereby each date is individually evaluated, the boundary ages are intuitively selected to provide a subjective best fit for the dataset, uncertainties are assigned manually, and interpolation is used, i f at all, only as a last resort where critical data are lacking. Glauconite ages, especially common for the Late Jurassic, are weighted heavily, resulting in younger boundary estimates than in most other scales. Harland et al. (1982, 1990) treat glauconite dates as m i n i m u m ages but their database is conservative in preserving most items from previous compilations. A l l accepted dates are handled equally by using the chronogram method for calculating boundary ages and estimating their associated error range. O n l y the best constrained chronogram ages are accepted as tie-points, the intervening stage boundaries are interpolated assuming equal duration o f stages or zones. The D N A G time scale (Palmer, 1983, Kent and Gradstein, 1985, 1986) uses "hand-picked" tie-points and interpolation based on sea-floor magnetic anomalies for the K i m m e r i d g i a n - T i t h o n i a n and the equal-zone-duration 21  method for the older part of the Jurassic. Gradstein et al. (1994) append the isotopic database of Harland et al. (1990) but reject the use of glauconite ages. The maximum likelihood method is used to obtain first estimates for stage boundaries, then cubic spline smoothing is employed to refine the interpolation results of equal-subzoneduration (for the Early and Middle Jurassic) and sea-floor magnetochronology (for the Late Jurassic) methods. The framework of Jurassic isotopic ages is still too sparse for reliable and precise calibration. Existing scales do not reflect recent directions in geochronometry which increasingly employ the U-Pb and A r - A r methods for 40  39  precise and accurate dating. Perhaps the most promising approach to improve the Jurassic geochronologic scale is the construction of a scale from U-Pb and A r - A r ages which are directly constrained at the zonal level. 40  39  22  CHAPTER 3  A U-Pb AGE FROM THE TOARCIAN (LOWER JURASSIC) AND ITS USE FOR TIME SCALE CALIBRATION THROUGH ERROR ANALYSIS OF BIOCHRONOLOGIC DATINGt  3.1. INTRODUCTION  The paradox of Jurassic geochronology is that the biochronologic framework was laid out in the mid-19th century (Oppel, 1856-1858) and has been continuously refined (see Callomon, 1995 for one of several recent reviews) but its calibration with isotopic ages is still poor (Palfy, 1995). Recently published time scales (Gradstein et al., 1994; Gradstein et al., 1995; Haq et al., 1987; Haq et al., 1988; Harland et al., 1990; Kent and Gradstein, 1985; Kent and Gradstein, 1986; Odin, 1994; Palmer, 1983) suggest conflicting boundary age estimates and stage durations. Where uncertainties are quoted (Gradstein et al., 1994; Gradstein et al., 1995; Harland et al., 1990; Palmer, 1983), the boundaries of the Toarcian appear the most poorly constrained of all the Jurassic stages. Underlying this problem, there is a scarcity of isotopic dates (eight items listed in Gradstein et al., 1994; Gradstein et al., 1995; Harland et al., 1990), of which all except one are K-Ar or Rb-Sr ages with inferior precision and accuracy. Also, most dated samples are stratigraphically poorly constrained and none of them is unambiguously confined to the Toarcian stage.  t Published in 1997 under the same title by Palfy, J., Parrish, R.R., and Smith, P.L., Earth and Planetary Science Letters, v. 146, pp. 659-675. The U-Pb analysis was done by R.R. Parrish who is also the primary author of section 3.3. All other parts of this chapter were written by the present author. 23  To improve the calibration of the Jurassic time scale, concerted effort is underway in the Canadian Cordillera (Palfy et al., 1995). U-Pb dating of volcanic units is preferred where fossiliferous volcanosedimentary sequences allow independent ammonite biochronologic dating at the zonal level. We report a new, precise (± error is less than 1% at 2a level) U-Pb zircon age from the Queen Charlotte Islands. The dated sample is biochronologically constrained at the zonal level. Moreover, it was obtained from the type section of three consecutive Middle and Upper Toarcian North American standard ammonite zones (Jakobs et al., 1994). To demonstrate the usefulness of this date as a time scale calibration point, we rigorously assess the ammonite fossil record of the section using classical confidence intervals for taxon ranges (Marshall, 1990; Springer and Lilje, 1988), the global biochronological correlations using the unitary association (UA) method (Guex, 1991), and the effect of fossil identification errors using model calculations. These methods serve to evaluate the biochronologic dating error that is not easily quantifiable. In previous time scales it was either not entered into calculations (Gradstein et al., 1994; Gradstein et al., 1995) or overestimated (Harland et al., 1990). Conclusions derived here can be extended to the use of other dates from the North American Cordillera as calibration points for the Jurassic time scale.  3.2. GEOLOGIC SETTING  The studied section is exposed on the left bank of Yakoun River on Graham Island, Queen Charlotte Islands, British Columbia (Fig. 3.1). Jurassic rocks of the archipelago are assigned to Wrangellia, one of the major tectonostratigraphic terranes comprising the North American Cordillera (Monger et al., 1991). The Jurassic stratigraphy of Wrangellia is characterized by arc-related volcanosedimentary sequences, although the Lower Jurassic Kunga and Maude groups are predominantly sedimentary on the Queen Charlotte Islands. The Yakoun River section is the designated type section of the Whiteaves Formation within the Maude Group (Section 15 in (Cameron and Tipper, 1985)). The stratigraphy of the section is shown on Fig. 3.2. The Whiteaves Formation consists of dark, poorly bedded concretionary mudstone with minor sandstone interbeds and rare, thin bentonitic ash layers. It records deposition in a marine basin of moderate water depth (Cameron and Tipper, 1985). There is no apparent gap within the exposed section. The Whiteaves Formation is comformably overlain by thin-bedded to massive sandstone of the Phantom Creek Formation although the basal part of that unit is largely concealed.  24  Figure 3.1. Location map of the Yakoun River section on Graham Island, Queen Charlotte Islands. Grid reference for the U-Pb sample locality is UTM Zone 9, 681500E 5921830N.  25  Legend  P  tuff layer  massive sandstone  mudstone, shale [ with calcareous concretions  bedded sandstone  LU UJ  tz o  SECTION  c  CD CD  •Q  0)  70  I  H  -3  CD  covered  Q-  CD  CD ~  °-  3  Iiw  E CD  50 —\  CO  :£  c ~ cn CD  Q.  t:  CD • Q  CD  CO T3  b.E  •§ ci . cn  ' O  CO CO CD C -D  C/3  CD  <D  LU CD CD ^ ^ CD O  30 •  O  CL  1 CD  3  CD »~ CD  _CD  O o  CD  t .  CO  O  CD CO  CJ  £ E  O  co  CD  m o co  CD v, CD  £  co  5  co fc  < I  UJ — I  Y A K O U N RIVER  O O ra E ^, -c Q.  is  II l i s CD  c  s  a.  CD  C 3 O  is CD C -C co CD  ma. a. a.  CD CD •Q  CD C  o CD CD  1 M CO CD CD J-  £ 8- 8 CO TO Q)  o: x  CL  c  CD  c "a  I  — *>  "O co CO CD CO 3 C c t CD CD CD CO C DD E ^ C O .g CD O)  : JCO CO ^ CD 3 CD  !0 TS  CO CO  CD CO • CO  CD  £ C CD CJ  O  ^ O  is -J  8  CD  8  i  o  c  «I E  a.  2  E  *  co — (3  o CD  O  is  -c  20  '£3  —  a3  f  a.  ID  Figure 3.2. Litho- and biostratigraphy of the Toarcian of the Yakoun River section (modified after Jakobs et al., 1994). "Z" denotes the level of U-Pb zircon sample. Ammonite ranges (vertical lines) with collection levels (horizontal bars) are shown for the Planulata, Crassicosta, and Hillebrandti zones only (after Jakobs, in press (1997)). Dotted lines denote imprecisely located early collections.  26  3.3. U-PB GEOCHRONOMETRY  3.3.1. Methodology Sample PCA-YR-1 was collected from a 10-cm thick, clay-rich, white gritty ash bed interbedded within black mudstone of the Whiteaves Formation in the Yakoun River section. The sample was excavated with a knife and spoon and stored in a moist state. In the laboratory, the sample was blended into a slurry, allowed to settle, and the fines decanted. The washed concentrate was further ultrasonically agitated and cleaned, passed through heavy liquids, and separated into aliquots with different magnetic susceptibilities. Euhedral needles of zircon were identified in the least magnetic material, hand picked, photographed, and abraded (Krogh, 1982) for U-Pb analysis. Crystals selected were reasonably homogeneous, but allowing for the possibility that xenocrysts or transported zircons may have been present, the first set of analyses were of single grains weighing only a few micrograms. The concordance and reproducibility of these analyses established a high probability of a single population of zircon, and additional analyses consisted of up to 10 grains to achieve somewhat smaller U-Pb analytical errors. The methods and decay constants used in U-Pb analyses are described elsewhere (Parrish et al., 1987; Roddick et al., 1987; Steiger and Jager, 1977). Table 3.1 presents these data and errors (Roddick, 1987), and they are shown graphically on Fig. 3.3. U and Pb blanks were approximately 0.2 and 3 picograms, respectively.  3.3.2. Results  Six analyses of single or multiple grains of abraded zircons all produced consistently concordant and overlapping error ellipses on the concordia diagram (Fig. 3.3). In Jurassic zircons the abundance ratio of Pb/ Pb 206  206  is about 20, therefore the  238  207  Pb/ U age from single grain analyses is inherently more precise than the  207  235  Pb/ U  age and it reliably estimates the crystallization age when concordance of multiple analyses is demonstrated and the 206  Pb/  238  U ages form a tight cluster (Mundil et al., 1996). The strong abrasion of the grains (Krogh, 1982), their 206  mutual concordance, and agreement in their  238  Pb/ U ages definitively argue against any subsequent Pb loss in the  analyzed crystals. The weighted mean of Pb/ U ages based on all six fractions is 181.2±0.6 Ma (2a). We take a 206  238  more conservative approach by selecting the three most precise analyses (A-l, B-3, C-2), and noting their concordance, we use the mutual overlap of their P b / U ages as the best estimate of the true age and uncertainty 206  238  and assign 181.4±1.2 Ma (2a) as the crystallization age of zircons.  27  u  Fraction** wt* (Hg)  2.5  A-l,s*  (ppm)  455.8  1.0 1021  A-4,s* A-7  11.0 1.0  B,s* B-3  ,  3  /  7  186.4 266.5 406.9  1.0 1533  C-2,s* a  Pb  c  206  pb  d  204p7  (ppm)  12.80 28.73 5.291 7.567  p c , e Th/U b  206  p b /  23  8 u  /  207  p b /  23  5 u  / 207  p b /  2 0 6 / 206 p b  113 354 100  4.4 18.9 10.9 5.8  0.30 0.32 0.34 0.33  u  K  207  p b /  23  5 u  corr.  age (Ma) #  coef.  207  pb/  206  age ( M a )  0.02854±0.34  0.1973±0.83  0.05014±0.73  181.4±1.2  182.8=1=2.8  0.49  202±34  0.02839±0.32  0.1970±2.56  0.05033±2.39  180.5=1=1.2  182.6=1=8.6  0.57  210±UO  0.02853±1.18  O.I980±2.70  0.05034±2.24  181.3±4.2  183.4=1=9.1  0.58  211=1=104  0.02862±0.56  O.I842±4.02  0.04668±3.79  181.9=4=2.0  171.7±I2.7  0.47  0.04948±0.43  181.4±1.2  180.6±1.6  0.50  1711=20  0.04966±0.86  181.4±l.4  181.3±3.2  0.45  179±40  11.71  1094  2.5  0.39  0.02853±0.33  0.1947±0.49  42.22  393  7.2  0.25  0.02855±0.40  0.1955±0.97  33±182  s, single grain; * , weight estimated.  b  Weighing error: 0.001 mg.  c  Radiogenic Pb.  d  Measured ratio, corrected for spike and Pb fractionation of 0.09%±0.03%/AMU.  e  Total common Pb in analysis corrected for fractionation and spike.  /  238  age ( M a )  (Pg)  477  pb/  Corrected for blank Pb and U , and model common Pb composition (Stacey and Kramers, 1975) for 181 Ma Pb; errors are 1 standard error of the mean in percent. g  Corrected for blank and common Pb; errors are 2 standard errors of the mean in Ma.  Table 3.1. U-Pb zircon analytical data of sample PCA-YR-1  0.024 I 0.165  •  ' 0.185  i  207  :  ' 0.205  '  1  0.225  235 Pb/  U  Figure 3.3. U-Pb concordia diagram for zircons from sample PCA-YR-1, a volcanic ash layer in the Whiteaves Formation at Yakoun River. Errors are shown at 2a (95%) level. Error ellipses with thick outline denote single grain analyses, grey shading indicates fractions not used in final age interpretation (see text for details and Table 3.1 for analytical data). 28  s  pb  3.4. BIOCHRONOLOGY  Biochronologic dating is based on sequences of associations of taxa that have been shown to maintain their stratigraphic relationships over a wide geographic area. This approach is well-suited to ammonoids and has been successfully used in developing zonations for the North American Lower Jurassic (Jakobs et al., 1994; Smith et al., 1988). The use of the new U-Pb date as a time scale calibration point requires correlation between the secondary standard North American and the primary standard northwest European zonations (see (Callomon, 1984) for discussion). Here we develop such a correlation and apply additional methods to test for biochronologic error.  3.4.1. The ammonite fossil record The Whiteaves Formation in the Yakoun River section is richly fossiliferous. The most common macrofossils are ammonites with a total of 119 collections obtained through initial reconnaissance work (Cameron and Tipper, 1985) followed by threefieldseasons of careful systematic sampling (Jakobs, in press (1997)). This collection history and the abundance of fauna distinguishes the Yakoun River section as one of the best studied Early Jurassic ammonite localities in North America. Well-preserved ammonites occur most commonly in non-septarian calcareous concretions, the majority of which are distributed randomly in the section while some form distinct layers. Although rare, ammonites were also found crushed within the shale [G. Jakobs, pers. comm. 1996]. The uniform lithology throughout the Whiteaves Formation suggests that there is no significant preservational bias affecting the vertical distribution of fossils which was carefully measured and documented (Jakobs, in press (1997); Jakobs etal., 1994). Five regional standard ammonite zones were proposed for the Toarcian of North America (Jakobs et al., 1994). All of them are recognizable in the Yakoun River section which serves as the stratotype for the Planulata, Crassicosta, and Hillebrandti zones (Jakobs et al., 1994). The ash layer yielding the analyzed U-Pb sample is by definition assigned to the lower part of Crassicosta Zone. Therefore the following discussion is concerned with the Crassicosta and the adjacent Planulata and Hillebrandti zones only.  3.4.2. Reliability of observed ranges  The regional standard zonal scheme for the Toarcian (Jakobs et al., 1994) is based on a literal reading of observed vertical ranges of ammonites. It is widely accepted, however, that the fossil record is incomplete (Paul,  29  1982) and it underestimates the true ranges (Signor and Lipps, 1982). Our aim in assessing the robustness of ammonite zones as defined in the Yakoun River section is twofold: to use rigorous statistical methods to test the validity of zones and to compare the empirical data derived from successive collecting campaigns with the predictions from statistical methods. The following calculations are based on published fossil occurrence data (Jakobs, in press (1997); Jakobs et al., 1994). The simplest way of providing a maximum estimate of the completeness of the ammonite record is to calculate a "hit/miss" ratio, where the hits are confirmed occurrences and the misses are collection levels with no record of a given taxon within its known vertical range (also termed virtual occurrences (Guex, 1991)). For the 54 levels reported within the three zones, there are 74 hits and 110 misses suggesting that the maximum average probability of finding a species in a fossiliferous horizon within its vertical range is no better than 40%. It can be argued that the average probability is biased towards lower values by rare taxa which are given little weight in devising biozonations. Of the 22 taxa considered, only six (Rarenodia planulata, Phymatoceras crassicosta, P.  hillebrandti, P. cf. pseudoerbaense, Leukadiella ionica, and Denckmannia tumefactd) were found at more tha levels. Indeed, these six taxa score a hit/miss ratio of 51/54 giving a nearly 50% chance of being found. Not surprisingly, the three zonal index species among them score the highest (hit/miss=36/20, chance of being found=64%). It is evident that misses would occur beyond the lowest and highest hits, i.e. the observed ranges are shorter than the true local ranges. To estimate the true ranges, gaps separating occurrence levels must also be considered (Paul, 1982; Springer and Lilje, 1988). Fig. 3.2 is a re-plotting of observed ranges (Jakobs et al., 1994) considering all available information on vertical distribution (Jakobs, in press (1997)). The distribution of gap lengths in the fossil record is shown to have Dirichlet distribution and classical confidence intervals can be calculated for estimating the true stratigraphic ranges (Marshall, 1990; Springer and Lilje, 1988). These statistics are only valid if the fossil distribution is assumed to be random. As pointed out above, there is no appreciable preservational bias or change in sedimentation rate in the rather uniform lithologic sequence of the Whiteaves Formation. Likewise, collection bias can also be discounted as collection intensity was constant during the first field season and collecting strategy remained systematic (although slightly biased towards parts of the Planulata and Hillebrandti zones) in the following two campaigns [G. Jakobs, pers. comm. 1996]. Extensions of the observed ranges for the six common taxa were calculated at the 95% confidence level (Marshall, 1990) and plotted on Fig. 3.4. Note that the length of the range extension is a function of the length of the observed range and the number of occurrences within it. The statistical meaning of the range extensions is that there is less than 5% probability of finding a given species beyond the limits of the extended range. For the three 30  zonal index species, occurrences are separated according to the year of their first finding and changes in cumulative observed ranges are also plotted. The following observations can be made: (1) For commonly occurring ammonite taxa collected through systematic collecting effort, range extensions at the 95% confidence are not justifiable. A lesser degree of confidence would better agree with range extensions realized through further collecting but its approximate level needs to be established empirically through tests on a larger database. (2) The Planulata, Crassicosta, and Hillebrandti zones are defined as assemblage zones (Jakobs et al., 1994). In practice the definition of their boundaries at the stratotype depends, to a large degree, on the range of their zonal index species. The Planulata and Hillebrandti zones are based on robust evidence from the distribution of their respective zonal indices. Their range extension even at the 95% confidence level would not modify the zonation significantly. The Crassicosta Zone, on the other hand, is less well-defined by the range of its zonal index but is well-constrained by the sub- and superjacent zones and the cohort of co-occurring taxa. (3) Of the six range end-points of other relatively common species, only one would require modification of zonal assignments if the range extension at the 95% confidence level is considered. (4) The isotopically dated tuff layer (within the observed range of Phymatoceras crassicosta) falls very near to the top of the extended range (at the 95% confidence level) of Rarenodia planulata, therefore its assignment to the Crassicosta Zone is undoubted.  31  60  TOP OF WHITEAVES FORMATION  7T  T3 C CO  ZONE* Q Z <  •0  -£  a"  50  3 C ro Q. 40  •C CO CO v, CD O O  CD LU  ro •E • -C  0,  CO  CO  o o  to  O  CO CO  T)  o c  O  2  CD  CO CO <  CJ CO  i-.  CD DC  ro  o  CD CJ  o ro E  •c 0,  30  7>  I,  Z < _l CL  20  CD CO  c  <D <0  cu  CD O tl 3 CD CO  ro 3 CD -J  10m  .  •9 ro u CD  from base  co ro CD CJ  o ro E  CL  E 3 MU  O  •3  c c co E  UJ CO  z UJ z <  CJ  c  CD  Q  ±3-  Figure 3.4. Collection levels (arrowheads), observed ranges (unfilled boxes), and estimated maximum ranges at 95% confidence level (shaded boxes) of common ammonite species (i.e., occurring at more than two levels) from the Planulata, Crassicosta, and Hillebrandti zones in the Yakoun River section. Changes in observed and estimated ranges after successive collection years are shown for the three zonal index species (Rarenodia planulata, Phymatoceras crassicosta, and P. hillebrandti). Occurrence data from (Jakobs, in press (1997)). "Z" denotes the level of U-Pb zircon sample. See text for discussion. 32  3.4.3. Biochronologic correlations  Traditional ammonite biochronology was founded and proved extremely powerful within single bioprovinces (e.g., Boreal northwest Europe, Tethys) where it is possible to establish reproducible high resolution units (faunal horizons). Limitations and difficulties are imposed by facies dependence and limited paleobiogeographic distribution of taxa resulting in complex time/space distribution patterns overprinted by the vagaries of fossil record (i.e., preservation and collection biases). Correlation across biogeographical boundaries is often controversial, as exemplified by long-standing debates on Tethyan/Boreal correlation in the Toarcian of Europe (Elmi et al., 1994). The traditional approach is based on expert judgement in emphasizing the correlation value of certain ammonoid taxa and their associations at the expense of others when discrepancies in the first and last appearance datums (FAD/LAD) are detected. A traditional solution for the correlation of North American Toarcian ammonite zones with northwest European standard zones (sensu (Dean et al., 1961)) equates the Planulata Zone with the Bifrons and basal Variabilis zones, the Crassicosta Zone with the rest of the Variabilis Zone, and the Hillebrandti Zone with the Thouarsense Zone (Jakobs et al., 1994). It is acknowledged that (1) correlation is hampered by the absence of key European taxa in North American faunas (e.g., Hildoceras and Haugia, to which index species of the Bifrons and Variabilis zones belong); (2) North American faunas have greater similarity to Tethyan and South American ones than those of Boreal northwest Europe; (3) in North America, the FAD and/or LAD of several important genera (e.g., Phymatoceras, Podagrosites, Peronoceras, Mercaticeras) are anomalous with respect to those found elsewhere (Jakobs et al., 1994). Consequently, a cautious approach to correlation needs to integrate global stratigraphic distribution from all faunal provinces, should seek maximum ranges of taxa, and is expected to result in a correlation scheme with apparent resolution sacrificed for increased reproducibility and confidence. The amount of available data calls for quantitative treatment which also provides the advantage of eliminating potential bias introduced by subjective judgements. There is a variety of quantitative biostratigraphic techniques available and the choice depends on the nature of data and the expected outcome (Edwards, 1982). In our case, the data are obtained from measured sections in different sedimentary basins and bioprovinces, hence the total number of taxa is high (>100) while the number of common taxa is often low. Also, fossil distribution among sections may be non-random due to faunal migration. As the timing of migration events is not known independently, maximum ranges are sought to use for correlation. Among the most widely used and tested quantitative methods, graphic correlation seeks maximum ranges but is not practical beyond a single sedimentary basin (Mann and Lane, 1995). The probabilistic ranking and scaling method 33  also works best within a single basin/bioprovince where it produces average ranges with the maximum likelihood of FAD/LAD sequence while assuming random distribution of fossils (Agterberg, 1990). The unitary association (UA) method (Guex, 1991) appears to be best suited to our problem. It furnishes maximum ranges based on a deterministic approach and it does not require random fossil distribution or homogeneity of source data. It applies the rigor of graph theory to the familiar concept of Oppelian assemblage zones thus it retains the philosophy underlying traditional ammonite biochronology. The pitfalls of intuitive correlation schemes are minimized. The algorithmic formulation of the UA method resulted in an efficient computer program, the BioGraph (Savary and Guex, 1991). The UA method was found to efficiently construct biochronologically meaningful zonations from complex data (Baumgartner, 1984). Notably, it was successfully used for correlation between different bioprovinces (Baumgartner, 1993) and was demonstrated to closely reproduce the ammonite zonation developed by traditional methods in northwest Europe (Dommergues and Meister, 1987). We used the following procedure for computer-assisted biochronologic correlation. Conventional correlation of the Planulata, Crassicosta, and Hillebrandti zones (Jakobs et al., 1994) was accepted as a guide. The scanning range was set to latest Early to early Late Toarcian. All established ammonite provinces were considered and the available literature was culled for representative sections from each province. The selection was based on evidence that collections were made from measured sections spanning several zones with no indication of condensed horizons or reworking, that the ammonite fauna is abundant and diverse, and that local ranges are welldocumented. Sources with sound taxonomic documentation (preferably with illustration) were chosen if possible. No adequate sections meeting the above criteria were located from the western Pacific and Arctic provinces. Beside the Yakoun River section, data from one other North American, four South American, five western Tethyan and five northwest European sections were compiled (see Table 3.2 for details of sources and remarks). Among the taxa occurring, only species of Ammonitina reported from two or more localities were used. Polyplectus and Pseudolioceras, two long-ranging genera with little stratigraphic value, were omitted. Limited attempt was made to homogenize taxonomy (see discussion below), mainly to consider synonymies of taxa reported from the Yakoun River (Jakobs, in press (1997)). Composite genus ranges were added for each section to mitigate the effect of paleobiogeographic differences at the species level. A conservative generic classification scheme (Donovan et al., 1981) was adopted with the exception of recognition of Rarenodia (Jakobs, in press (1997)) and the combining of Porpoceras with Peronoceras to avoid confusion stemming from different opinions as to their species content. The 103 taxa and the stratigraphic ranges in each section are listed in Appendix 1.  34  REFERENCE  LOCALITY NORTH AMERICA 1. Yakoun River, British Columbia  Jakobs (in press, 1997); Jakobs et al. (1994)  2. Joan Lake, British Columbia  Jakobs (in press, 1997); Jakobs et al. (1994)  WESTERN TETHYS 3. Valdorbia, Umbria, Italy Cresta et al. (1989); Gallitelli Wendt (1970) (As bed-by-bed correlation between the two sources are ambiguous at some levels, data from the two sources was first processed by BioGraph and the resulting taxa ranges with UA treated as levels were entered into the main BioGraph input file.) 4. Djebel-es-Saffeh (Section 3B), Djebel Nador area, Algeria  Elmi et al. (1974)  5. Djebel-es-Saffeh (Section 2), Djebel Nador Elmi et al. (1974) area, Algeria (Supplementary section to Djebel-es-Saffeh 3B, mainly to better document the ammonite fauna from the lower part of the succession.) 6. Paghania, Greece  Kottek (1966)  7. Monte di Civitella, Umbria, Italy  Venturi (1975)  NORTHWEST EUROPE 8. St-Paul des Fonts, Aveyron, France  Guex (1972)  (Ammonite faunas of horizons, said to represent one to three beds, are given and entered here as levels.) 9. Camplong, Aveyron, France  Guex (1975)  10. Ricla and La Almunia, Iberian Range, Spain  Goy and Martinez (1990)  11. Anse Saint-Nicolas, Vendee, France  Gabilly (1976)  (From the vicinity of Thouars, the classical type area for the Toarcian, this section contains the best and most diverse ammonoid fauna.) 12. Ravenscar and Whitby, Yorkshire, England Dean (1954); Howarth (1962); Howarth (1992) (Data compiled following the bed-by-bed correlation between the two sections (Howarth, 1992).)  documented  SOUTH AMERICA 13. Quebrada El Bolito, northern Chile  Hillebrandt and Schmidt-Effing (1981)  14. Quebrada Yerbas Buenas, northern Chile  Hillebrandt and Schmidt-Effing (1981)  15. Quebrada Larga, northern Chile  Hillebrandt and Schmidt-Effing (1981)  16. Rio del Toro, northern Chile Hillebrandt and Schmidt-Effing (1981) (For all South American section, emended taxonomy was used (Hillebrandt, 1987).)  Table 3.2.  List of stratigraphic sections and sources of BioGraph input.  These data were entered into and processed by the BioGraph program (Savary and Guex, 1991). Sections within each province were ordered with priority given to the most species-rich and complete one. As the resolution of contradictory stratigraphic relationships may be affected by the order in which the data are processed (Guex, 1991), all six permutations of North American, Tethyan and northwest European sections were tried with the least  35  informative South American sections consistently entered last. The output of 40 successive UA (Fig. 3.5), however, was not sensitive to the permutation. The UA assigned to specific beds or collection levels in the source sections were determined from the correlation tables produced by BioGraph (Appendix 2). Thus at the Yakoun River, the base of the Crassicosta Zone corresponds to UA 22-23, the base of the Hillebrandti Zone to UA 33, and the isotopically dated tuff layer to UA 22-25. Standard subzonal and zonal boundaries reported from the analyzed northwest European sections were used to determine the maximum permissible extent of these units in terms of the UA scheme (Fig. 3.6). (Lacking a universally accepted Toarcian standard zonation for northwest Europe, we adopt a recently updated scheme (Elmi et al., 1994) which differs only slightly from a more traditional zonation (Dean et al., 1961). Notably, the subzones of the Bifrons Zones are based on the species sequence of Hildoceras, the Variabilis Zone is subdivided into three subzones, and the subzonal scheme within the Thouarsense Zone is revised, increasing the correlation potential (Elmi et al., 1994).) The most likely range, as expressed by the overlap or minimum required range of UA from the analyzed sections, was also inferred for the standard units. It is evident from Fig. 3.6 that individual UA are seldom reproducible in more than one or two sections and across bioprovinces. However, their groupings which correspond to traditional zones or subzones, are commonly present in several sections from different provinces. Only four UA are unambiguously identifiable in North America and they are not directly correlatable with other provinces. Fig. 3.6 is used to determine the most likely and the maximum permissible correlations of the Crassicosta Zone (and the isotopically dated level within it) with the northwest European standard zonation. The beginning of the zone appears equivalent or marginally older (but cannot be younger) than that of the standard Variabilis Subzone (Variabilis Zone). It could also be correlative with (but not older than) the later part of Semipolitum Subzone (Bifrons Zone). The beginning of Hillebrandti Zone lies most likely within the latest Variabilis Zone (Vitiosa Subzone) but it could be as old as the Illustris Subzone (middle part of Variabilis Zone) or as young as the earliest Bingmanni Subzone of the Thouarsense Zone. The isotopically dated tuff layer is constrained to the Semipolitum through Illustris subzones (inclusive), with the earlier part of the Variabilis Zone being its most likely age.  36  1111111111222222222233333333334 1234567890123456789012345678901234567890  U  A  I  I  II 1  I  I I I  I I I I  II I  I I 1 1 I  I  I I  I  I I I  I I I I  I I I  I 1 1 I 1 1  I I I I I I I II  I I I I II  II  TAXA Hildaites serpentiniformis Hildaites serpentinus Nodicoeloceras crassoides Hildaites Dactylioceras Nodicoeloceras Harpoceras Hildaites propeserpentinus Hildoceras sublevisoni Hildoceras Catacoeloceras Hildoceras caterinii Harpoceras falcifer Peronoceras Mercaticeras hellenicum Phymatoceras elegans Catacoeloceras ghinii Hildoceras bifrons Mercaticeras umbilicatum Catacoeloceras crassum Mercaticeras mercati Mercaticeras Pseudomercaticeras Phymatoceras crassicosta Phymatoceras Dactylioceras athleticum Dactylioceras commune Hildoceras tethysi Hildoceras crassum Hildoceras lusitanicum Frechiella subcarinata Hildoceras semipolitum Hildoceras apertum Harpoceras subplanatum Leukadiella Rarenodia planulata Mercaticeras dilatum Phymatoceras rude Zugodactylites braunianus Peronoceras verticosum Peronoceras subarmatum Peronoceras fibulatum Pseudomercaticeras rotaries Pseudomercaticeras frantzi Phymatoceras robustum Peronoceras vorticellum Hammatoceras Peronoceras crassicostatum Peronoceras planiventer Peronoceras pacificum Phymatoceras narbonense Leukadiella ionica Peronoceras vortex Collina gemma Collina  I I I  Phymatoceras pseudoerbaense Mercaticeras tyrrhenicum Peronoceras desplacei Phymatoceras erbaense Peronoceras moerickei Pseudomercaticeras latum Brodieia bayani Brodieia Catacoeloceras dumortien Harpoceras subexaratum Collina linae Paroniceras Brodieia alticarinata Collina mucronata Haugia navis Denckmannia malagma Denckmannia tumefacta Denckmannia Brodieia clausa Haugia variabilis Haugia  I I I I I I I  - t - H -  - H -  -M-f-  I II I I I  I I I I  i i i i i i  H—I—h-  Pseudogrammoceras fallaciosum Pseudogrammoceras Brodieia primaria Brodieia gradata Phymatoceras venustulum Phymatoceras speciosum Haugia illustris Paroniceras sternale Haugia phillipsi Pseudogrammoceras aratum Pseudogrammoceras subregale Podagrosites Hammatoceras costatum Podagrosites latescens Haugia vitiosa Grammoceras Pseudogrammoceras btngmanni Hammatoceras porcarellense Pseudogrammoceras doerntense Grammoceras striatulum Grammoceras thouarsense Phymatoceras hillebrandti Hammatoceras insigne Esericeras fascigerum Hammatoceras praefallax Hammatoceras speciosum Pseudolillia emiliana  Figure 3.5. Global latest Early to early Late Toarcian ammonoid taxon ranges relative to the 40 Unitary Associations produced by the BioGraph program.  37  SECTIONS  1111111  1234567890123456  II I III  I J_ I I  I  I I I  I  I I  i  I  1  i N Am.  Tethys  II I NW Europe  S Am.  Figure 3.6. Reproducibility of the 40 Unitary Associations recognized in latest Early to early Late Toarcian ammonite faunas analyzed using the BioGraph program. The left side of diagram shows the occurrences of UA (solid bars) in the 16 selected sections (listed in Table 3.2). Plotted on the right side are minimal (solid bars) and maximal (hatched bars) groupings of UA corresponding to the northwest European standard zones and subzones (Elmi et al., 1994) and their correlation with the North American Crassicosta Zone (box) and the isotopically dated level (gray shaded) within it.  38  A s expected, the quality o f U A based correlation decreases towards either end o f the scanning range (i.e., earliest Bifrons Zone and latest Thouarsense Zone) where there is insufficient superpositional control deduced from the raw data. W i t h this caveat, the correlative o f the base o f Planulata Zone ( U A 7 - 1 4 ) appears to lie somewhere between the late Sublevisoni and early Semipolitum subzones (Bifrons Zone) The top o f Hillebrandti Z o n e ( U A 38) shows robust correlation with the Thouarsense Subzone but correlation with any o f the subzones (Bingmanni through Fallaciosum) o f the Thouarsense Zone is permitted.  3.4.4. The effect of taxonomic noise In addition to differences between observed and true fossil ranges and correlation uncertainty , a third potential source o f error in biochronologic dating is the uncertainty o f fossil identification. G i v e n the variety o f authors and their differing opinions on species, taxonomic noise (i.e., random inconsistencies in identifications) is expected in the source data for biochronologic correlation. The following example is given to illustrate the effect o f high levels o f taxonomic noise. The stratigraphically important genus  Hildoceras was  recently revised (Howarth, 1992). Four widely  distributed species were accepted with many other nominal species synonymized (Table 3.3). Tabulated are the revisions made to figured specimens from sources used in our correlation database ( E l m i et al., 1974; G a b i l l y , 1976; Gallitelli Wendt, 1970; Guex, 1972; Kottek, 1966). In approximately 2 5 % o f the cases, the original author's identification is revised in addition to reassignments o f synonymized nominal species. In three cases, specimens originally figured under one name are assigned to two different species by the reviser. It follows that even with amply illustrated material, the stratigraphic distribution data cannot be unambiguously revised without a comprehensive review o f all considered specimens, a daunting task not attempted here. If we adopt this high figure o f 2 5 % erroneous identifications at the species level, we can then examine the effect o f this taxonomic noise on the U A correlation method. The simplest model calculation is to construct U A from two species ( A and B ) , both represented by one specimen at two levels o f a hypothetical stratigraphic section (Fig. 3.7). The stratigraphic ranges o f A and B can be concurrent, overlapping, or exclusive. There is a single U A in case o f concurrent or overlapping ranges whereas two successive U A are identified for exclusive ranges. Let one specimen i.e., one quarter o f the total, be misidentified ( A erroneously called B or B called A ) . In each case, there are four permutations to place the misidentified specimens. Recording the changes in U A and the proportion o f number o f levels with erroneously assigned U A provides a measure o f sensitivity o f the U A method to taxonomic  39  Hildoceras laticosta BELLINI (=H. sublevisoni FUCINI; H. caterinii MERLA)  H. lusitanicum MEISTER  H. bifrons (BRUGUIERE)  (=H. apertum GABILLY) (=H. graecum RENZ; H. crassum M1TZ0P0UL0S; H. tethysi GECZY)  H. semipolitum BUCKMAN  (=H. angustisiphonatum PR1NZ; H. semicosla BUCKMAN)  ? H. semipolitum (Kottek, 1966) H. semipolitum (Kottek, 1966) H. semipolitum (Guex, 1972) H. graecum sublevisoni (Kottek, < > 1966) H. semicosta (Guex, 1972) ? Hildaites levisoni (Gallitelli H. graecum graecum (KottekH. semicosta (Guex, 1972) < > Wendt, 1970) 1966) H. angustisiphonata (Guex, H. bifrons (Guex, 1972) H. graecum lusitanicum (Kottek, H. sublevisoni (Elmi et al., 1974) 1972) 1966) H. bifrons (Elmi et al., 1974) H. bifrons (Elmi et al., 1974) H. sublevisoni (Gallitelli Wendt, H. sublevisoni (Gabilly, 1976) 1970) < > H. bifrons var. angustisiphonata H. sublevisoni (Guex, 1972) H. apertum (Elmi et al., 1974) H. caterinii (Gabilly, 1976) H. bifrons bifrons (Kottek, 1966) (Elmi et al., 1974) ? H. graecum (Guex, 1972) ? H. lusitanicum (Guex, 1972) H. bifrons walcoti (Kottek, 1966) H. semipolitum (Elmi et al., 1974) H. bifrons bifrons (Gallitelli H. semipolitum (Gabilly, 1976) H. lusitanicum (Elmi et al., 1974) Wendt, 1970) H. bifrons angustisiphonatum H. tethysi (Gabilly, 1976) (Gallitelli Wendt, 1970) H. crassum (Gabilly, 1976) H. lusitanicum (Gabilly, 1976) H. semipolitum (Gallitelli Wendt 1970) H. apertum (Gabilly, 1976) H. bifrons (Gabilly, 1976)  Table 3.3. Synonymies of species of Hildoceras figured in sources used in our biochronologic correlation database (Elmi et al., 1974; Gabilly, 1976; Gallitelli Wendt, 1970; Guex, 1972; Kottek, 1966), as recognized in the revision of Howarth (Howarth, 1992). Subjective junior synonyms are given in parentheses in the heading (Howarth, 1992). Bold face indicates disagreement between the reviser and the original author after reassignment of synonymized nominal species. Arrows mark specimens originally figured under one name that are now assigned to two different species (Howarth, 1992).  CORRECT ID  25% ID  UA  UA  A A  B B  1 1  B B A B  1 1  ERROR  CONCURRENT RANGES UA UA  A B B B  1 1  A A  A B  UA  1 1  A A  B A  1 1  1 1 1  B A A A  ©  1 1 1 1  B A  OVERLAPPING RANGES  A A  B B  1 1 1  B B B A  © © 1  B A B B  1 1 1  A A  A B  1 1  EXCLUSIVE RANGES  B B A A  2 2 1 1  B B B A  B B  2 2  ©  A  1  8  1 1 1 1  A B  A A  A A  2  © 1 1  Figure 3.7. Simulation of the effect of 25% taxonomic noise on the unitary associations (UA). Left column: stratigraphic relationships of species A and B known from two levels in each section and derived UA. Other four columns: permutations of one misidentified specimen (in bold italic type). Heavy border denotes modification in UA pattern compared with left column, circled UA number denotes levels of critical error. See text for discussion 40  noise. We shall only consider the critical errors where the misidentification leads to the addition of artificial UA or to a shift of UA boundary. If the result is the merger of two UA, the net effect is a decrease in resolution and loss of correlation power without introduction of correlation error. As shown on Fig. 3.7, there are two permutations for the overlapping ranges and two for the exclusive ranges where a total of five levels suffer critical error in UA assignment. A comparison with the total number of levels considered in all scenarios (36) reveals that at the 25% taxonomic noise level, the critical stratigraphic error is less than 14%. A remarkable property of the UA method is that increase in the density or complexity of source data greatly reduces the sensitivity to taxonomic noise. In the model calculation, it can be demonstrated that a twofold increase in the number of occurrence levels (from two to four) leads to less than 3% critical stratigraphic error at the same 25% taxonomic noise level. Furthermore, we note that misidentification most often occurs among morphologically similar forms that are often phylogenetically closely related and have similar stratigraphic distributions. Taxonomic mistakes involving taxa with concurrent ranges will not introduce stratigraphic error. The amount and complexity of data used in the world-wide correlation together with the foregoing considerations lead us to regard the contribution of taxonomic noise to biochronologic dating error as negligible.  3.5. DISCUSSION  The results of the correlation using the UA method can be compared with those obtained traditionally (Jakobs et al., 1994). The boundary of Planulata and Crassicosta zones, previously correlated with the early part of Variabilis Zone, is now allowed to fall within the interval of Semipolitum to Variabilis subzones, with the greatest likelihood indeed near the base of the Variabilis Zone. The boundary of Crassicosta and Hillebrandti zones, previously equated with the base of the Thouarsense Zone, is now allowed to fall within the interval of Illustris Subzone to the base of Thouarsense Zone, most likely within the Vitiosa Subzone. As demonstrated above, the maximum uncertainty in biochronologic age assignment of the isotopically dated sample is ± one standard subzone. It is superior to all previously available dates which are constrained at the stage level at best. The precision of the U-Pb age also surpasses that of any other isotopic age from, or near, the Toarcian. Thus the U-Pb age from near the middle of Middle Toarcian at Yakoun River serves as an important new calibration point for the Jurassic time scale. When plotted against the major recently published and widely used time scales, the new U-Pb date of 181.4±1.2 Ma falls within the Toarcian in all except the DNAG (Kent and Gradstein, 1985; Kent and Gradstein, 1986; Palmer, 1983) scale (Fig. 3.8). The mid-point of Toarcian is within the error of our date in the EXX88 (Haq etal., 1987; Haqetal., 1988) and GTS89 (Harland etal., 1990) scales only. 41  The new date is significant in that it, together with other newly obtained Early Jurassic isotopic ages, will help greatly reduce the large uncertainty associated with previous stage boundary estimates. Rigorous error estimates are only given in GTS89 (Harland et al., 1990) and MTS (Gradstein et al., 1994; Gradstein et al., 1995). Considering the error ranges, in GTS89 (Harland et al., 1990) the base of the Toarcian can be as young as 172 Ma and the top as old as 188.5 Ma, both outside the error limits of the new U-Pb date. In MTS (Gradstein et al., 1994; Gradstein et al., 1995), the top of the Toarcian can be as old as 184.1 Ma which is in conflict with the U-Pb date reported here.  175 Ma  179  180-  ±3.5" •  1  7  8  °  ±  1  0  -  5  ^EXX88HGTS89 186  ±3.5  185187  190-  NEW U-Pb DATE MID-TOARCIAN  175  q 187.0  ±15  ODIN 184  180.1  181.4±1.2  MTS 189.6  ±17  ±4.0  ±4.0  A DNAG 193  ±14  195.  Figure 3.8. Comparison of boundary age estimates and numerical mid-point (marked with arrowheads) of the Toarcian in some recent time scales and the new U-Pb date from the midToarcian of Queen Charlotte Islands. Sources of time scales quoted: DNAG (Kent and Gradstein, 1985; Kent and Gradstein, 1986; Palmer, 1983); EXX88 (Haq et al., 1987; Haq et al., 1988); GTS89 (Harland et al., 1990); Odin (Odin, 1994); MTS (Gradstein et al., 1994; Gradstein et al., 1995).  3.6. CONCLUSIONS  We report a newly obtained U-Pb zircon age of 181.4± 1.2 Ma (2a) from the Queen Charlotte Islands. The interpreted age is based on concordant and overlapping analyses of single-crystal as well as multi-grain fractions. The dated sample was collected from a bentonitic ash layer within the Toarcian Whiteaves Formation in the Yakoun River section. The ash layer lies within the North American standard Crassicosta Zone as defined by ammonite biostratigraphy (Jakobs et al., 1994). The Yakoun River is the designated type section of the Crassicosta Zone as well as the subjacent Planulata and superjacent Hillebrandti zones (Jakobs et al., 1994). We statistically analyzed the quality of the ammonite fossil record in this section. All three zones are assemblage zones by definition but in practice are delimited at the stratotype by their respective index species. The collection density of Rarenodia planulata hillebrandti  and Phymatoceras  is adequate in that their range extension using 95% confidence intervals would not modify the 42  placement of the zonal boundaries significantly. Collection data from three successive field seasons (Jakobs, in press) suggest that generally much less than 95% confidence level is required when proposing range extensions for commonly occurring ammonite taxa. The assignment of the dated tuff layer to the lower part of Crassicosta Zone would not change even when range extensions at the 90% confidence level are considered. Biochronologic correlation was done using the computer-assisted unitary association (UA) method (Guex, 1991). Ammonite local range data from representative North American, western Tethyan, northwest European, and South American sections were processed to provide unbiased estimates of global maximum taxon ranges and a sequence of UA. The UA framework was then used to establish the maximum extent of permissible correlation between the secondary standard North American and the primary standard northwest European zonations. Correlation of the Crassicosta Zone is bracketed by the Semipolitum Subzone (late Bifrons Zone) and Bingmanni Subzone (early Thouarsense Zone). In particular, the dated tuff layer cannot be older than the Semipolitum Subzone or younger than the Illustris Subzone (Variabilis Zone). Random misidentifications are known to occur in the world-wide dataset used for correlation. However, the UA method is shown to respond to taxonomic noise by loss of resolution rather than erroneous correlation. Thus we regard the proposed correlation as conservative best estimates. With less than ±1% (2a) error in the isotopic age and a maximum of ±one standard subzone uncertainty in biochronologic correlation, the U-Pb age from the Yakoun River section serves as an important calibration point for the Early Jurassic time scale. A comparison with recently proposed time scales reveals that only minor adjustments are required for the Toarcian stage boundary age estimates but the associated uncertainties can be greatly reduced. U-Pb dating of volcanic horizons within fossiliferous sequences in the North American Cordillera holds promise for providing more useful calibration points for the Jurassic time scale.  43  CHAPTER 4  INTEGRATED AMMONITE BIOCHRONOLOGY AND U-Pb GEOCHRONOLOGY FROM A BASAL JURASSIC SECTION IN ALASKA  4.1.  INTRODUCTION  Lowermost Jurassic rocks are rare world-wide and their dating and correlation is often difficult. Fossiliferous marine sediments are scarce and ammonites are known from only a few localities. The Triassic-Jurassic boundary is marked by one of the five major Phanerozoic mass extinction events but its age and the timing of post-extinction recovery is poorly constrained. Numerical estimates of the age of the Triassic-Jurassic boundary vary widely between 213 and 200 Ma for time scales published in the last 15 years (Palfy, 1995). The main problem, the small number of relevant isotopic ages is attributable to the scarcity of datable and stratigraphically well-constrained volcanic rocks near the boundary. The Cordilleran Jurassic Calibration Project is an initiative to improve the Jurassic time scale through U-Pb dating of volcanic units whose age is well-constrained by ammonite biochronology from the North American Cordillera (Palfy et al., 1995). To refine the time scale near the Triassic-Jurassic boundary, we studied the Puale Bay section on the Alaska Peninsula (Fig. 4.1) because it was long known to contain fossiliferous uppermost Triassic and lowermost Jurassic sedimentary strata and volcanics (Capps, 1923; Smith and Baker, 1924; Martin, 1926; Kellum et al., 1945). The monographic treatment of its Hettangian ammonoids by Imlay (1981) indicated the presence of earliest Jurassic faunas. Moreover, Newton (1989) suggested a continuous Triassic-Jurassic transition, a situation that is only known at a handful of localities world-wide (Hallam, 1990).  44  Figure 4.1. Location map of the Puale Bay section.  45  The studied section is located on the southeastern shore of Puale Bay, on the rugged coastline of the Alaska Peninsula, across the Shelikof Strait from Kodiak Island (Fig. 4.1). A wave-cut intertidal platform and adjacent coastal bluffs provide excellent exposures. Beds dip moderately to the northwest and their continuous erosion by waves aid macrofossil collecting. In 1995, we examined the Triassic-Jurassic transition, collected ammonoids and other macrofauna from 26 levels in the Hettangian-Sinemurian strata, measured the basal Jurassic stratigraphic section, and sampled potentially zircon-bearing volcanic units for U-Pb dating, In this report we: (1) document the Early Jurassic ammonoid succession in the Puale Bay section based on the new collections; (2) establish a biochronologic framework to constrain the dated volcanic units at the zonal level; (3) report three new zircon U-Pb ages; (4) discuss the significance of new data in comparison with recent time scales; and (5) reconsider the status of the Puale Bay section as a Triassic-Jurassic boundary section.  4.2.  GEOLOGIC SETTING AND PREVIOUS WORK  South-central Alaska is a collage of tectonostratigraphic terranes. The three largest, the Alexander, Peninsular, and Wrangellia terranes are thought to have amalgamated into the Wrangellia composite terrane prior to their accretion to North America in the Cretaceous (Nokleberg et al., 1994 and references therein). The Puale Bay area forms part of the Peninsular terrane, also known as the Alaska Peninsula terrane (Wilson et al., 1985). Its Triassic and Jurassic stratigraphy records the geological evolution of a volcanic island arc (Wang et al., 1988) that is closely linked to Wrangellia on the basis of shared shallow marine carbonate, clastic, and volcanic (mainly volcaniclastic) sequences. The Puale Bay section was recognized as one of the most complete and relatively undeformed Triassic and Jurassic successions in south-central Alaska. Early work is summarized by Imlay and Detterman (1977). Based on a comprehensive evaluation of all Early Jurassic ammonite collections made in the area (including neighbouring Alinchak Bay), Imlay (1981) demonstrated the presence of Hettangian rocks. The underlying Upper Triassic sequence was studied in detail by Wang et al. (1988). A complete (Upper Triassic to Upper Jurassic), measured stratigraphic section is found in Detterman et al. (1985). The most recent stratigraphic summary is given by Nokleberg et al. (1994).  46  The Upper Triassic Kamishak Formation is the oldest Mesozoic unit at Puale Bay. It comprises some 700 m of shallow marine, biogenic carbonate that is, in its upper part, interbedded with basaltic volcanic rocks (Wang et al., 1988). Wilson and Shew (1992) obtained an imprecise whole rock K-Ar age of 197±12 Ma (la) for the basalt. Dark grey, organic-rich, thin-bedded siliceous limestone in the upper part of the formation yielded ammonites (Metasibirites, Rhabdoceras), bivalves (Monotis spp.), and the hydrozoan Heterastridium that indicate a Late Norian age (Silberling in Detterman et al., 1985, Wang et al., 1988, Newton, 1989, and this study). Above the fossiliferous Norian beds, there is some 50 m of strata apparently devoid of macrofossils that contain only trace fossils (Newton, 1989). We found the lowest Jurassic ammonoid-bearing beds immediately above a fault of unknown (but probably not large) offset (Fig. 4.2). There is only a slight and gradual change upsection manifested in the decrease of carbonate content. Rocks on two sides of the fault are nearly identical. The HettangianSinemurian section above the fault was measured in detail and is shown on Fig. 4.3. It contains an abrupt transition from fine-grained, calcareous sediments to andesitic volcaniclastic rocks. The latter is referred to as the Talkeetna Formation, a widespread, predominantly volcanigenic unit of the Peninsular Terrane (Nokleberg et al., 1994). In the Puale Bay section, we suggest drawing the base of the Talkeetna Formation at the lowermost massive green  Figure 4.2. Normal fault separating the lowest Jurassic (Middle Hettangian) ammonitebearing beds to the left from unfossiliferous, lithologically very similar, presumably uppermost Triassic rocks to the right.  47  tuff. This differs from the traditional approach of approximately equating the Triassic-Jurassic system boundary with the Kamishak-Talkeetna formation boundary by arbitrarily dividing the non-volcanigenic sedimentary sequence (Nokleberg et al., 1994 and references therein). The lower 120 m of the Talkeetna Formation consists of mainly green tuff, volcanic breccia and agglomerate with minor volcanigenic sandstone. The rocks are interpreted as subaqueous deposits of a major volcanic episode in moderate proximity to the eruptive centres. Higher up, there is some 200 m of predominantly green-grey, coarse tuffaceous sandstone that is commonly thick bedded to massive and locally cross-stratified. Shale interbeds occur only rarely. The abundant volcanic detritus was likely derived from the active parts of the Talkeetna island arc. The Hettangian-Sinemurian section is truncated by a fault that juxtaposes the Talkeetna Formation and the several hundred metres thick, dark shale sequence of the Kialagvik Formation. The oldest ammonites from the latter unit occur merely 6 m above the fault. From this level Imlay (1981) identified Haugia cf. compressa of Middle Toarcian age that was later revised as Pleydellia maudensis, a guide ammonite of the latest Toarcian Yakounensis Zone (Jakobs et al., 1994; Jakobs, in press (1997)). Our collection of Pleydellia sp. confirms the presence of uppermost Toarcian. Beds a few metres upsection from this uppermost Toarcian level yielded Tmetoceras, a characteristic Aalenian genus. Stratigraphically younger parts of the Puale Bay section are beyond the scope of this study.  4.3.  HETTANGIAN-SINEMURIAN BIOCHRONOLOGY  Ammonoids are the predominant megafossils in the Lower Jurassic section at Puale Bay. More than one hundred specimens were collected from 26 levels (numbered from the base up in Fig. 4.3) in a measured stratigraphic section. Bivalves, plant fossils, and nautiloids also occur but they do not rival the abundance and stratigraphic importance of ammonoids and are not discussed here. The ammonoids are moderately to poorly preserved internal molds that suffered post-depositional compression to varying degrees. In most cases, much important morphologic information such as whorl cross section, ventral features, and suture line are lost. The set of characteristics available for identification is commonly restricted to volution, whorl expansion rate, and ornamentation. The less than ideal preservation necessitates the frequent use of open nomenclature (Bengtson, 1988), in several cases only a group of taxa can be given which share the observed characters but their differences cannot be resolved in the absence of preserved distinguishing features. In some cases comparable species have been attributed to more than one genus. We indicate this by a question mark appended to the genus considered the most likely. The low 48  morphologic diversity among Hettangian ammonoids, especially the narrow range of ornamentation (Liang, 1994) hinders precise identification. Homeomorphy is common among Hettangian ammonoids that record post-extinction recovery from limited root stocks (Tozer, 1971). In case of heterochronous homeomorphs, care was taken to avoid using stratigraphic inferences in the identification. All possible homeomorphs known from the HettangianSinemurian were considered to ensure unbiased biochronologic dating.  4.3.1. Local ammonite ranges  Observed ranges of ammonite taxa are shown on Fig. 4.3. Detailed taxonomic remarks are given below in section 4.3.3. Good ammonite biostratigraphic control exists in three intervals of the Hettangian-Sinemurian section. Calcareous mudstones of the basal 55 m above the putative Triassic-Jurassic boundary fault yield the most abundant and diverse ammonoid fauna (Level 1-14). Kammerkaritesl cf.frigga (PI. 4.1, fig. B, E, F, H) is the most common form in the lowest 30 m (Level 1-11). Discamphiceras occurs throughout with D. cf. silberlingi (PI. 4.1, fig. A, D) appearing first (Level 2-12) and apparently replaced by D. aff reissi (PI. 4.1, fig. C) (Level 13-14). Less common but stratigraphically important forms include, in order of their first appearance, K. ex gr. megastoma (PI.  4.1, fig. R), Saxoceras? sp. (PI. 4.1, fig. G, J), S.l ex gr. portlocki (PI. 4.1, fig. I), Pleuroacanthites ex gr. mulleri (PI. 4.1, fig. L), and Mullerites cf. pleuroacanthitoides (PI. 4.1, fig. N, O). Several poorly understood forms such as Franzicerasl sp. (PI. 4.1, fig. M), a psiloceratid (PI. 4.1, fig. P, Q), Euphyllitesl sp. and a lytoceratid (PI. 4.1, fig. K) occur in the upper part of the interval. The second fossiliferous interval occurs in the basal 60 m of sandstone that is separated from the lower fossiliferous unit by more than 100 m of dominantly volcaniclastic strata. A single specimen of Sunrisitesl sp. (PI. 4.2, fig. A, B) was recovered near the lowest epiclastic beds (Level 16). It is overlain by a moderately diverse assemblage of Badouxia (showing an apparent succession of B. canadensis (PI. 4.2, fig. I) followed by B.  columbiae (PI. 4.2, fig. G)), Eolytoceras cf. tasekoi (PI. 4.2,fig.D, H), Paracaloceras (including P. rursicostatum (PI. 4.2, fig. C, J)) and a single large schlotheimiid specimen.  49  ^  -V  ^  KEY sandstone  O LLl UJ >  U26  <  CO  <h: u  volcanic breccia  =>  CO  I"  andesitic tuff  Ct  LU Z  2 3  '_L.'. J calcareous siltstone  (A  o  ? o o o  .25  z  <  E  300  h-24  to  s  CD  ••2 5  CU  E  •a  8  |o  S <u  <o  <u o o  o  o  c .« -2 73 ro aj  •8 GO CO  z  UJ I Q  < z  < o  A to c ^ ° s •H 3 O oa CO  <  r - 23  or o  • 22 • 21  200  • 20 • 19 • 18  i— LU UJ  CO  2  V  O  -I  • u u> o -r •a A 8 £ ro O  §• 3-s  tv. co <D  5 a  22 •rH CQ  cq co  P  5 .  £4-  • 17 • 16  t rr  UJ  DL  0_  * 95JP3 * 95JP4 * 95JP5 c  'VV\  <  2, §  100  CD  z  UJ X  u  <0  ;S c :  * 95JP1  13 12 • 1110 9 •6 •5  y: <  X CO  •J-v  'v  I£  CN  14  0m  13CD  -8  0  ' V V V V V  9  c  r-15 ' V V V V V v  UJ _l Q  to  1 E £  !tn« a, of 0)  0)  o  •  Q.  ^ cv.  to v  5  O)  ' V V \ I V V V  •2 C  €  E  4J  IB q <0  2  a) cu 0 X u 8 ^ R to -X to ^ a> CDB 42 -a "5.  °> IS C  "3  to  ^  1 to  to >-l o fl .<oo o <u tu o  fl  •H  go  5  5  to  5  S  I  I  3-4 • 1-2  Figure 4.3. Measured lowest Jurassic stratigraphic section and ammonite ranges at Puale Bay, Alaska. Taxa identified at the species level or assigned to a well-defined group of species are shown in normal print. Bold typewriter font denotes confidence in genus level identification only or cases where closely comparable species are allocated to different genera. Normal typewriter font marks the most tentative assignments when a genus is not or only provisionally suggested. s 50  Plate 4.1. (On p. 52.) Middle Hettangian ammonite fauna of Puale Bay. All specimens are 95% of natural size. Collection levels are shown on Fig. 4.3. Specimens are deposited in the type collection of the Department of Earth and Ocean Sciences, University of British Columbia, under the type numbers with the prefix UBC. A, D:  Discamphiceras cf. silberlingi Guex A: Level 7, UBC 018; D: Level 12, UBC 021.  B, E, F, H: Kammerkaritesl cf. frigga (Wanner) B: Level 5, UBC 019; E: Level 10, UBC 022; F: Level 8, UBC 023; H: Level 7, UBC 025. C:  Discamphiceras aff. reissi (Tilmann) Level 13, UBC 020.  G, J:  Saxocerasl sp., G: Level 8, UBC 024.; J : Level 4, UBC 027.  1:  Saxocerasl ex gr. portlocki (Wright) Level 6, UBC 026.  K:  Lytoceratid indet. Level 14, UBC 028.  L:  Pleuroacanthites ex gr. mulleri Guex  Level 9, UBC 029 (note parabolic nodes). M:  Franzicerasl sp.  Level 8, UBC 030. N, O:  Mullerites cf. pleuroacanthitoid.es Guex  Level 11, UBC 031 and UBC 032. P, Q:  Psiloceratid indet. Level 9 UBC 033 (counterparts of the same specimen, note nodes on inner whorls).  R:  Kammerkarites ex gr. megastoma (Gumbel)  Level 3 UBC 034.  51  Plate 4.1  Plate 4.2  Plate 4.2. Late Hettangian through Early Sinemurian ammonite fauna of Puale Bay. All specimens natural size. Collection levels are shown on Fig. 4.3. Specimens are deposited in the type collection of the Department of Earth and Ocean Sciences, University of British Columbia, under the type numbers with the prefix UBC. A, B:  Sunrisitesl sp., Level 16, UBC 035, (counterparts of the same specimen);  C, J:  Paracaloceras cf. rursicostatum Frebold, C: Level 18, UBC 036; J: float specimen, UBC 043;  D, H:  Eolytoceras cf. tasekoi Frebold D: Level 19, UBC 037; H: Level 19, UBC 041 (nucleus only);  E, F:  Badouxial sp. E: Level 22, UBC 038; F: Badouxial sp., Level 21, UBC 039;  G:  Badouxia columbiae (Frebold), Level 23, UBC 040;  I:  Badouxia canadensis (Frebold), Level 18, UBC 042;  K:  Arnioceras sp., Level 9, UBC 044 (inner whorls).  53  The third ammonite-bearing interval is found within the top 80 m below the fault contact with Toarcian strata where sparsely fossiliferous sandstone and minor shale interbeds yield a monogeneric Arnioceras fauna (PI. 4.2, fig. h K).  4.3.2. Correlation and biochronologic dating The standard chronostratigraphy of the Jurassic is based on the northwest European ammonite succession. North American Early Jurassic ammonite faunas are different enough from the northwest European ones to warrant independent regional standard zonations, as demonstrated for the Pliensbachian and Toarcian (Smith et al, 1988; Jakobs et al., 1994). To date, no regional North American zonal scheme has been proposed for the Hettangian but local biostratigraphy has recently been elaborated for two areas with the most complete fauna) succession: Nevada (Guex, 1995) and the Queen Charlotte Islands (Tipper and Guex, 1994). Wefirstcompare the Alaskan faunas to these areas to which they exhibit great faunal similarity. Next we consider the South American and Alpine regional zonal schemes that are useful for correlation on the basis of a host of common taxa. Finally, we attempt a correlation with the standard chronozones (i.e., northwest European zones) through a web of interregional correlation and a few direct links. Fig. 4.4 shows a global compilation of Hettangian ammonite zonations and their approximate correlation. The stratigraphic distribution of the Alaskan Hettangian taxa known from other regions is summarized in Fig. 4.5. Vertical ranges are shown at zonal resolution except for Nevada and the Queen Charlotte Islands, where data are available to further confine ranges to certain parts of zones. The Alpine zonation (Warmer, 1886) is based on condensed sequences that may not necessarily represent all the time of other units shown as their correlatives. It is evident that the lower fauna from Puale Bay correlates with Middle Hettangian units world-wide. It may be further constrained to its lower part (i.e., Portlocki Subzone equivalents). Direct correlation with northwest Europe using Saxoceras ex gr. portlocki and Kammerkarites ex gr. megastoma corroborates this conclusion. The only species that apparently contradicts this correlation is Mullerites pleuroacanthitoides, which is only known from a single bed in Nevada representing a slightly higher stratigraphic position within the Middle Hettangian.  54  The second fauna contains East Pacific elements to such extent that there is a conspicuous lack of direct links to European faunas. Nevertheless, a placement within the Late Hettangian through Hettangian-Sinemurian boundary interval is undoubted. Sunrisites, occurring in the lowest bed of this fauna, suggests the lower part of the Upper Hettangian as the genus appears at this level in North and South America. Somewhat higher, the association of Badouxia,  Eolytoceras,  and  Paracaloceras  is characteristic of the Canadensis Zone (Frebold, 1967; Palfy et al.,  1994) but thefirstrepresentatives of these genera may appear earlier (Tipper and Guex, 1994; Guex, 1995). Correlation of the Canadensis Zone is difficult. Several East Pacific workers favor its placement straddling the Hettangian-Sinemurian stage boundary (Taylor, 1990; Riccardi et al., 1991; Palfy et al., 1994). The correlation of this zone with the Alpine Marmorea Zone is more conclusive but the position of the Marmorea Zone itself is. debated (Guex and Taylor, 1976; Bloos, 1983; Taylor, 1986). Recent studies by Bloos (1994; 1996) provide evidence supporting the placement of Marmorea Zone in the Hettangian. In Alaska as well as in the Queen Charlotte  ?  MGIAN  Ang  ra ro  — I  23 16 Complanata Marmorea  _i  LU  38 Paracaloceras  S.  Megastoma  Portlocki  w o c ra  0_  33  Euphyllites  32  42 "Curviceras" 54  41  53 P. rectocostatum  Psiloceras  Planorbis NW EUROPE  55  43  occidentalis  Calliphyllum 11  S. montana  P. doetzkirchneri  P. polymorphum  1  56  44  D. reissi  21  Johnstoni  52 P. primocostatum  • 51  P. pacificum  ALPS  marmorea  Franziceras  proaries  P. mulleri  12  57 S.  Franziceras^S E.  X  ?  45 Canadensis  36 A.  13  sunrisense  22  Laqueus  '(/) ra  ?  37  15 Extranodosa 14  w o  ?  NEVADA  P. tilmanni  QUEEN CHARLOTTE IS.  S AMERICA  Figure 4.4. Biochronologic correlation chart of Hettangian ammonite zonations proposed for key regions. Only approximate correlation is implied and the Hettangian-Sinemurian boundary is not precisely located outside northwest Europe. Zone reference numbers are the same as in Fig. 4.5. Compiled from the following sources: Northwest Europe - Dean et al. (1961), Alps - Wahner (1886), Nevada - Guex (1995), Queen Charlotte Islands - Tipper and Guex (1994), South America -Hillebrandt (1994).  55  NW EUROPE  ALPS  NEVADA (G95)  QUEEN CHARLOTTE IS. S AMERICA  Figure 4.5. Ranges of Hettangian ammonite taxa known from Puale Bay as established in other regions. Numbered biostratigraphic units are the same as in Fig. 4.4. All taxa listed in the taxonomic remarks as included in the identified species groups are considered. Lighter shade denotes taxa of uncertain generic assignment. Solid bars indicate taxa occurring in the lower faunal level, open bars indicate taxa from the middle faunal level at Puale Bay. Genus abbreviations: B: Badouxia; D: Discamphiceras; E: Eolytoceras; F: Franziceras; K: Kammerkarites; P: Paracaloceras; P: Pleuroacanthites; Pc: Paracaloceras; S: Saxoceras.  M: Mullerites;  Sources: G95: Guex (1995); GF90: Guerin-Franiatte (1990); H90: Hillebrandt (1990); H94: Hillebrandt (1994); L52: Lange (1952); P94: Palfy et al. (1994); R91: Riccardi et al. (1991); R93: Rakiis (1993a); TG94: Tipper and Guex (1994); W: Warmer (1882-1898). 56  Islands, the first appearance of Badouxia columbiae postdates that of most other elements of the Canadensis Zone and thus it can be pragmatically used to approximate the base of the Sinemurian. The third fauna contains Arnioceras only, A.cf. arnouldi being the only species identified. It is a guide fossil of the Arnouldi Assemblage recognized in the Lower Sinemurian of the Queen Charlotte Islands (Palfy et al., 1994). Although Arnioceras is known to range up to the lower part of the Upper Sinemurian, monogeneric faunas typically occur in the upper Lower Sinemurian. The global record of this cosmopolitan genus also suggests that the standard Semicostatum Zone is the most likely correlative of this fauna.  4.3.3. Taxonomic remarks  As full taxonomic treatment is not intended here, taxa are listed alphabetically. The generic and even higher level classification of Hettangian ammonites is far from settled (for two different approaches, see Donovan et al., 1981; Guex, 1987). For practical reasons we mainly follow the scheme used by Guex (1980; 1987; 1995) as it is conveniently applicable to the Nevadan and Alpine faunas to which our Alaskan material shows remarkable affinity. Uncertainty in generic assignment of early schlotheimiids reflects the fact that critical ventral features are rarely preserved in the Alaskan material.  Arnioceras cf. arnouldi (Dumortier)  A large specimen exceeding 25 cm in diameter was found in situ (level 24) on a bedding surface in the intertidal platform where its removal was not feasible. It is identical in coiling and curved rursiradiate ribbing to A. arnouldi known from the Queen Charlotte Islands (Palfy et al., 1994). A smaller, fragmentary specimen (PI. 4.2, fig. K), is also referred to Arnioceras.  Badouxial sp. (PI. 4.2, fig. E, F) Several specimens from levels 19-22 are comparable to B. columbiae but differ in the occasional presence of bifurcating ribs on the inner whorls (see PI. 4.2, fig. F), a feature common in Schlotheimia or other schlotheimiids. The venter of the slightly crushed specimens is not well preserved but it doesn't appear to show a schlotheimiidtype, well-defined sulcus or abrupt termination of ribs such as seen on a specimen of Schlotheimia sp. figured by Imlay (1981, pi. 2, fig. 16-17) from Puale Bay.  57  Discamphiceras aff. reissi (Tilmann) (PI. 4.1, fig. C) Two crushed, weakly ornamented specimens from levels 13-14 are more involute than D. cf. silberlingi and show weaker and less regular ribbing. They closely resemble D. aff. reissi from Nevada as figured by Guex (1995, pi. 2, fig. 13-19). The typical D. reissi itself is distinguished by its regular, more prominent ribs which persist to the upper flank as shown by the holotype (Tilmann, 1917, pi. 21, fig. 4) and other South American specimens (Hillebrandt, 1994, pi. 1, fig. 12; Quinzio Sinn, 1987, pi. 1, fig. 12, pi. 2, fig. 1). We note that there are transitional forms between D. aff. reissi and D. silberlingi in both the Nevadan (Guex, 1995, pi. 15, fig. 13-16) and Alaskan material.  Discamphiceras cf. silberlingi Guex (PI. 4.1, fig. A, D) Four crushed specimens from levels 2, 7, and 12 show midvolute coiling and slightly prorsiradiate ribs that are strongest on mid-flank and fade towards the venter. This form appears to be the most common species of D. in the section and similar specimens were referred to D. cf. toxophorum by Imlay (1981, pi. 1, fig. 3, 4, 8-10) who also discussed their affinities to other species of Discamphiceras from the Eastern Alps. As Guex (1995) noted, at least some of the specimens figured by Imlay can be identified with D. silberlingi, a species originally described from Nevada. We favor the assignment of the Alaskan form to D. silberlingi while emphasizing its similarity to the Alpine species. The South American D. reissi (see below) is also closely related (see Hillebrandt, 1994, pi. 1, fig. 13).  Eolytoceras cf. tasekoi Frebold (PI. 4.2, fig. D, H) This species is common in the middle part of the section (levels 18-21) where it is represented by small, presumably juvenile specimens. Widely spaced constrictions, faint and irregular ribbing, and an evolute coiling with moderate expansion rate suggest assignment to E. tasekoi.  Euphyllites! sp. Several crushed specimens of small size from levels 12-13 resemble Euphyllites in their evolute coiling, moderate expansion of whorls, and lack of ornamentation except for nodes on the nucleus. Positive indentification is precluded by the poor preservation and lack of larger specimens. The least involute and most weakly ornamented Discamphiceras species are also morphologically similar but differ from Euphyllites by their lack of nodes on the 58  innermost whorls. Our form also resembles to a specimen from Puale Bay figured by Imlay (1981, pi. 1, fig. 2) as Psiloceras cf. planorbis that differs in being more evolute.  Franzicerasl sp. (PI. 4.1, fig. M) Four crushed specimens from levels 8, 10, and 11 show moderately evolute coiling and simple, straight, rectiradiate ribs with rounded profile that fade approaching the ventro-lateral margin. Stratigraphically unbiased identification of such simple forms at the given state of preservation is difficult. Similar morphologies occur amon early Hettangian Psiloceras (especially P. polymorphum Guex) and Caloceras (especially C. peruvianum Lange, see Tilmann, 1917, pi. 22, fig. 2-3, and Riccardi, 1991, pi. 4, fig. 3-5, and C. crassicostatum Guex, 1995, pi. 5, fig 1, 2, 5-10), middle Hettangian Franziceras (e.g., F. ruidum Buckman), and the late Hettangian Sunrisites. The specimen with the best preserved nucleus shows tuberculate innermost whorls, a common feature of Franziceras (e.g., F. coronoides Guex (Guex, 1989; Guex, 1995)) and Psiloceras but not in Caloceras or Sunrisites.  Kammerkaritesl cf. frigga (Wanner) (PI. 4.1, fig. B, E, F, H)  Several tens of mostly crushed specimens of Kammerkarites cf. frigga were collected as the most common ammonoid in the basal part of the Jurassic section (levels 1,4-8, and 10-11). Their size does not exceed 30 mm in diameter. The shell is evolute and strongly costate, the ribs are projected forward near the ventro-lateral margin. This form was identified as Waehneroceras cf. W. tenerum by Imlay (1981, p. 30, pi. 2, fig. 1-6). W. tenerum is abundantly illustrated from the middle Hettangian of Nevada (Guex, 1995, pi. 10, fig. 21-42) where it is said to be identical to the material from its type locality in the Eastern Alps (e.g., figured under Teneroceras in Lange, 1952, pi. 12, fig. 1-16). The Alaskan specimens differ in their sharper rib profile and more pronounced forward projection of ribs. A closer comparison can be made to K. frigga, known from the middle Hettangian Megastoma Zone of the Eastern Alps (Warmer, 1882-1898; Lange, 1952), Western Carpathians (Rakus, 1993b), and Nevada (Guex, 1995) Also comparable are several specimens from the South American Middle Hettangian Reissi Zone which were referred to Storthoceras sp. (Hillebrandt, 1990, pi. 3, fig. 16-18) and Curviceras sp. (Hillebrandt, 1994, pi. 1, fig. 3). Strongly resembling the Alaskan K. cf. frigga are specimens from northeast Siberiafiguredas Primapsilocerasprimulum Repin (Polubotko and Repin, 1972, pi. 1, fig. 1-2, especially fig. 2). It was suggested that this species represents a basal Jurassic horizon below the Planorbis Zone (Repin, 1988), a claim refuted by 59  Guex and Rakiis (1991) who showed that Primapsiloceras primulum can be regarded as a juvenile or microconch Kammerkarites, consistent with a reassignment of Psiloceras suberugatum, an index ammonite from higher levels in northeast Siberia, to Pleuroacanthites.  Kammerkarites ex gr. megastoma (Gumbel) (PI. 4.1, fig. R) A single large body chamber fragment was recovered from level 3. The entire specimen would have a diameter of about 30 cm and it shows straight, slightly prorsiradiate ribs that fade above mid-flank. Several species of Kammerkarites have similar morphology at a comparable diameter. These include K. megastoma (e.g., Guerin-Franiatte, 1990, pi. 12, fig. 4), K. longipontinum (Oppel) (e.g., Guerin-Franiatte, 1990, pi. 15, fig. 2, Riccardi et al., 1991,fig.4/7-8), and K. armanense (Repin) (Repin, 1988, pi. 1,fig.7). K. haploptychus (Wahner) is also closely related but it differs in its ribs persisting higher on the upper flank (e.g., Warmer, 1882, pi. 17,fig.1-5, Guex, 1995, pi. 15,fig.1-2).  Lytoceratid indet. (PI. 4.1,fig.K) Only one quarter of a body chamber of a large specimen (D > 13 cm) is available from level 14. Only faint, irregular ribbing is seen together with widely spaced flares, one of which terminates in an indistinct node.  Mullerites cf. pleuroacanthitoides Guex (PI. 4.1,fig.N, O) This species is described in detail by Guex (1980; 1995). The larger of the two Alaskan specimens from level 11 clearly shows a change in ribbing style from straight and rectiradiate to aborally curved and possesses several fine, broadly parabolic striae superimposed on ribs of the same trajectory. There is an apparent increase in the expansion rate of the last whorl of the Alaskan specimen, a change not seen in the Nevadan type material. It may be the result of post-depositional distortion and differential preservation of the body chamber.  Paracaloceras cf. rursicostatum Frebold (PI. 4.2,fig.C, J) A few poorly preserved fragments from level 18 and a float specimen show evolute coiling, carinate-bisulcate venter, and coarse, dense, rursiradiate ribbing that suggest comparison with P. rursicostatum, a common species at numerous other localities thoughout the East Pacific realm.  60  Pleuroacanthites ex gr. mulleri Guex (PI. 4.1, fig. L) A single specimen of small size (D = 20 mm) from level 9 shows highly evolute coiling, well-developed nodes on the innermost whorls and irregular ribbing interspersed with prominent parabolic nodes between shell diameters of 10 and 20 mm, consistent with its assignment to Pleuroacanthites. Similar sized specimens of P. mulleri Guex, known only from the middle Hettangian of Nevada, bear strong resemblance to the Alaskan specimen (Guex, 1995, p. 43, especially pi. 24, figs. 5-6 and 11-12). The Alaskan form is also close to small specimens of P. biformis (Sowerby) from the Megastoma Zone of Austrian Alps (Wanner, 1894, especially pi. 5,figs.3-6; Lange, 1952, p. 93, pi. 11,fig.4), and the Hettangian of West Carpathians (Rakus, 1993b, p. 13, pi. 6,fig.7). Distinction between P. mulleri and P. biformis is based on features of the adult body chamber (Guex, 1980; Guex, 1995) which is not preserved in our specimen.  Psiloceratid indet. (PI. 4.1,fig.P, Q) One specimen from level 9 appears very similar to Imlay's (1981, pi. 1,fig.2) Psiloceras ci. planorbis. It shows strongly nodose inner whorls, a feature common to Psiloceras, Pleuroacanthites, and other psiloceratids (Guex, 1995).  Saxocerasl ex gr. portlocki (Wright) (PI. 4.1,fig.I) Three incomplete, crushed internal molds from levels 6 and 7 differ from K. cf. frigga in their higher whorl and nearly straight, rectiradiate ribbing. Such forms from Puale Bay were previously referred to Waehneroceras portlockiby Imlay (1981, pi. 2,fig.7, 10-11, 14-15). There are several closely related species known from the Eastern Alps such as S. extracostatum (Wanner) (a possible synonym of S. portlocki, according to Donovan, 1952, and Imlay, 1981) and S.panzneri (Warmer). We note that the latter species (e.g., Wanner, 1882, pi. 15,fig.1-2), Lange (1952, pi. 16,fig.12-14)) is closer to our form in its high whorls, while there may be a continuum of whorl \  height distribution and species distinction on this basis would remain arbitrary. A recent treatment of the portlocki species group from France by Guerin-Franiatte (1990) includes "Waehneroceras"prometheus (Reynes) (lectotype refigured in Guerin-Franiatte (1990, pi. 12,fig.2)) which our form also closely resembles. Assignment of these species to Saxoceras follows Guex (1995, p. 35).  61  Saxoceras? sp. (PI. 4.1, fig. G, J)  This form, occurring in levels 4, 6, and 8, is distinguished from other representatives of Saxoceras and/or Kammerkarites by its marked coarsening of ribbing on the body chamber. Two specimens from Puale Bay figured by Imlay (1981, pi. 2, fig. 12, 13) as Waehneroceras cf. portlocki closely match our form but clearly differ from similar-sized specimens which do not show an abrupt change in rib density. A specimen from the Eastern Alps figured by Warmer (1882, pi. 16, fig. 1) as Aegoceras n. f. indet., cf. extracostatum shows abrupt coarsening of ribs although at somewhat larger diameter. Guerin-Franiatte (1990) mentions some change in ribbing of "Waehneroceras"portlocki but it appears to be less pronounced than in the Alaskan form. This form may be a new species but better preserved material is needed for a definitive treatment.  Schlotheimiid indet.  One very large specimen (D > 40 cm) from level 22 that resembles both "Charmasseiceras" and the "marmorea" group (sensu Bloos, 1988) in its inner whorls. The last whorl becomes almost completely smooth.  Sunrisites? sp. (PI. 4.1, fig. A, B)  A single specimen from level 16 shows evolute coiling and coarse, blunt, rectiradiate ribbing. It is morphologically similar to Franzicerasl sp. discussed above, although they are clearly separated stratigraphically and occur within different ammonite assemblages. The lack of tuberculation of the inner whorls serves as a distinguishing feature. The ribs are less flexuous and more widely spaced than in S. sunrisense Guex or S. hadroptychum (Warmer). Reasonable comparison can be made with S. peruvianum (Lange) that was originally assigned to Caloceras but is now placed within Sunrisites based on its stratigraphic position within the upper Hettangian (Hillebrandt, 1994). Two specimens figured as Psiloceras (Franziceras) sp. from northern Alaska by Imlay (1981, pi. 1, fig. 11, 15-16) were assigned to Sunrisites by Guex (1995) and are close to the Puale Bay specimen differing only by their rectiradiate ribbing trend. Comparable coarse ribbing also occurs in Nevadan specimens referred to Alsatites nigromontanus (Gumbel) (Guex, 1995). The latter species is said to show transitional forms towards Sunrisites (Warmer, 1886, Guex, 1995).  62  4.4.  U-Pb GEOCHRONOLOGY  4.4.1. Analytical methods  U-Pb analytical work was done at the Geochronology Laboratory of the University of British Columbia. Zircon was obtained from 8 to 25 kg samples using crushing, grinding, wet shaking, and heavy liquid separation. Fractions were hand-picked based on differences in magnetic susceptibility, size, and crystal morphology. Whenever possible, the least magnetic and best quality grains (i.e., free of cracks, inclusions, and visible cores or zoning) were used for the analyses. In all but one sample with the smallest zircon yield (95JP4), the fractions were strongly air-abraded using the technique of Krogh (1982) to minimize or eliminate the effect of surface-correlated Pb-loss. Details of techniques employed in chemistry and mass spectrometry are given by Mortensen et al. (1995). During the course of this study, U and Pb procedural blanks were 0.6-2.5 and 5-9 pg, respectively. Blank isotopic compositions, crucial in calculation of Pb/ U and 207  235  207  Pb/ Pb ages, were carefully monitored. 206  Analytical data are reported in Table 4.1. The age calculations are based on the decay constants recommended by Steiger and Jager (1977); the correction for initial common Pb follows the model of Stacey and Kramers (1975). The analytical errors are propagated through the age calculations using the method of Roddick (1987). In some analyzed zircon fractions, U-Pb systematics are affected by Pb-loss and inheritance. The rationale for assignment of the crystallization age and its associated error at the 2 sigma level is described separately for each sample.  4.4.2. Age determinations  The lowermost sample (95JP1) was collected from a conspicuous, 1 cm thick, pink, coarse-grained crystal tuff layer interbedded with the siliceous marl sequence (Fig. 4.6). A small, less than 10 kg sample yielded abundant and excellent quality zircons. The apparently homogenous population consisted of pale brown, mainly euhedral, multifaceted, doubly terrminated prisms with varying aspect ratio, ranging from stubby to needle-like grains. Seven of the analyzed nine fractions are concordant to nearly concordant (Fig. 4.7A). The P b / U ages 206  2j8  range from 194.7 to 198.3 Ma and many of them overlap. Slight Pb loss not entirely removed by air-abrasion is indicated for these fractions, perhaps with the exception of fraction A that is concordant and yields the oldest 206  238  Pb/  U age (198.3±0.7 Ma). Notably, fractions E and F, abraded to a lesser degree, yielded the youngest  °Pbr"U a ges.  iU  Since the concordance and age of Fraction A has not been duplicated, we calculated a seven63  fraction discordia line through the origin ( M S W D = 1 . 9 8 ) that yields an upper intercept age o f 2 0 0 . 4 ± 2 . 7 M a and only requires a 0.1 M a extension to include the  2 0 6  Pb/  2 3 8  U age o f A and its range o f error. Thus a conservative best  estimate for the age o f the rock is 200.4+2.7/-2.8 M a . Fraction J was not considered as it is reversely discordant and it clearly represents an inferior analysis. Fraction D differs from all other fractions by its older  2 0 6  Pb/  2 3 8  U age that is  interpreted as a result o f small amount o f inherited older Pb component, likely as undetected, cryptic cores. Unequivocal evidence for inherited or xenocrystic zircon w i l l be presented below for samples stratigraphically higher in the section. O n this basis, fraction D was also omitted from the Final age calculation. Sample / traction  1  Wt mg  U ppm  Pb>  Pb*2  ppm  2(Mpb  Pg  Calculated ages (±2a,Ma)  Isotopic ratios (±la,%)«  5»»Pbi %  2W,Pb/23»U  2«pb/238U  2(]7Pb/2»'.|»b  95JP1 A[>134, Nl,e+p+s, s-a] B[>134, Nl,e+s+t, s-a] D[>134, N2, p+e, s-a] E[<74, Nl,n+e, m-a] F [74-134, Nl,n+e, m-a] G [74-134, N l , p , s-a] H [74-134, N2, p+e, s-a] I[>134, N2, e+p, s-a] J [74-134, N2, p+e,s-a]  0.175 0.095 0.143 0.062 0.083 0.125 0.177 0.111 0.054  233 277 261 324 239 268 318 245 308  7 9 8 10 7 8 10 7 9  9493 4331 7505 4360 1868 3787 6776 3428 1907  8 12 10 9 21 17 16 15 17  8.4 8.9 8.9 10.1 9.4 9.1 9.5 8.7 8.8  0.03124 ±0.17% 0.03105 ±0.10% 0.03206 ±0.25% 0.03089 ±0.10% 0.03066 ±0.11% 0.03097 ±0.16% 0.03104 ±0.11% 0.03091 ±0.10% 0.03044 ±0.26%  0.2156 ±0.16% 0.2147 ±0.21% 0.2227 ±0.29% 0.2136 ±0.20% 0.2117 ±0.24% 0.2144 ±0.23% 0.2149 ±0.20% 0.2131 ±0.19% 0.2084 ±0.38%  0.05006 ±0.08% 0.05015 ±0.12% 0.05038 ±0.11% 0.05016 ±0.12% 0.05006 ±0.15% 0.05020 ±0.13% 0.05021 ±0.10% 0.05000 ±0.12% 0.04964 ±0.32%  198.3 197.1 203.4 196.1 194.7 196.6 197.0 196.2 193.3  ±0.7 ±0.4 ±1.0 ±0.4 ±0.4 ±0.6 ±0.4 ±0.4 ±1.0  197.9 202.2 212.4 202.4 198.0 204.3 204.8 195.2 178.0  ±3.6 ±5.5 ±5.0 ±5.5 ±7.0 ±6.1 ±4.8 ±5.6 ±14.8  95JP3 A[>104, N5, p+eq, s-a] B [74-104, N5,p+t, s-a] C [<74, N5, ah+sh, s-a] D[>104, N5, p,s-a] E[<104, N5, p+sh,s-a]  0.026 0.043 0.041 0.004 0.006  295 193 149 306 472  9 8 5 10 32  2322 2596 3050 257 1068  7 8 4 10 12  10.9 10.7 11.0 11.3 4.1  0.03122 0.03997 0.03477 0.03111 0.07033  ±0.10% ±0.10% ±0.11% ±0.19% ±0.14%  0.2171 ±0.18% 0.3335 ±0.22% 0.3160 ±0.21% 0.2124 ±1.1% 0.6546 ±0.36%  0.05044 0.06051 0.06592 0.04952 0.06750  198.2 252.7 220.3 197.5 438.2  ±0.5 ±0.5 ±0.5 ±0.8 ±1.2  215.2 621.8 803.8 172.7 853.3  ±9.3 ±6.1 ±5.4 ±45 ±12.3  95JP4 » A [74-134, N2, eq, n-a] B [<74, N2, eq, sh, n-a]  0.032 0.045  398 239  24 15  735 1466  64 30  11.4 8.5  0.05727 ±0.25% 0.06274 ±0.10%  0.5875 ±0.36% 0.6524 ±0.22%  0.07440 ±0.20% 0.07542 ±0.13%  359.0 ±1.7 392.2 ±0.8  1052.3 ±8.1 1079.6 ±5.4  95JP5 A[>134, Nl,el+p, s-a] B[>134,Nl,p, s-a] C[>134, Nl,s+p, s-a] F[>134,Nl,p+el, s-a] G [104-134, N l , p , s-a] H[>134,Nl,p+el,s-a] I [104-134, NI, p+s, s-a]  0.121 0.295 0.381 0.230 0.265 0.165 0.104  98 113 88 142 140 113 102  3 4 3 5 4 4 3  2381 4253 3181 3457 5162 2289 990  10 15 21 19 14 16 21  11.9 11.6 10.8 11.8 11.7 11.6 11.5  0.03119 0.03128 0.03112 0.03109 0.03112 0.03079 0.03113  0.2146 ±0.23% 0.2160 ±0.20% 0.2153 ±0.21% 0.2150 ±0.22% 0.2152 ±0.20% 0.2135 ±0.26% 0.2149 ±0.32%  0.04990 0.05009 0.05017 0.05016 0.05015 0.05030 0.05007  198.0 198.6 197.6 197.4 197.6 195.5 197.6  190.2 199.3 203.1 202.5 202.1 208.7 198.2  1  8  ±0.17% ±0.14% ±0.13% ±0.97% ±0.30%  1 0  ±0.09% ±0.09% ±0.10% ±0.12% ±0.11% ±0.14% ±0.14%  ±0.15% ±0.12% ±0.12% ±0.12% ±0.11% ±0.19% ±0.24%  ±0.4 ±0.4 ±0.4 ±0.5 ±0.4 ±0.5 ±0.5  ±6.9 ±5.4 ±5.7 ±5.8 ±5.1 ±8.6 ±11.1  ' A l l zircon fractions. Listed in brackets: Grain size range in microns; Side slope of Franz magnetic separator (in degrees) at which grains are non-magnetic (N) or magnetic (M), using 20° front slope and 1.8A field strength; Grain character: b=broken pieces, e=elongate, eq=equant, n=needles, p=prismatic, s=stubby, t=tabular ah=anhedral, sh=subhedral; Degree of air abrasion: n-a=non-abraded, l-a=lightly abraded, m-a=moderately abraded, s-a=strongly abraded. ^Radiogenic Pb •^Measured ratio corrected for spike and Pb fractionation of 0.0043/amu± 20% (Daly collector) and 0.0012/amu ± 7% (Faraday collector). ^Total common Pb in analysis based on blank isotopic composition ^Radiogenic Pb ^Corrected for blank Pb, U , and initial common Pb based on the Stacey and Kramers (1975) model at the age of the rock or the P b / P b age of the fraction. 207  7  206  Location: SE shore of Pulae Bay, Map Karluk C-4/5, U T M 6400950N, 357630E  8  Location: SE shore of Pulae Bay, Map Karluk C-4/5, U T M 6400950N, 357630E  9  Location: SE shore of Pulae Bay, Map Karluk C-4/5, U T M 6400950N, 357630E  10  Location: SE shore of Pulae Bay, Map Karluk C-4/5, U T M 6400950N, 357630E Table 4.1. U - P b zircon analytical data  64  Figure 4.6. One cm thick crystal tuff layer (Sample 95JP1) interbedded with the siliceous marl near the top o f the Kamishak Formation. Shotgun for scale (120 cm in length) at left center.  Three samples were collected from the main volcaniclastic unit. The lowermost o f these, 95JP5, is from a nearly 6 m thick bed o f green to red, unaltered, crystal-rich tuff. C o m m o n phenocrysts include plagioclase, biotite, and hornblende. Abundant gem-quality zircon was recovered from this sample. Colorless to pale brown, euhedral, doubly-terminating prismatic grains form a single population ranging continuously from needles with simple, square cross-section to more multifaceted, stubby crystals. Six o f the analysed seven fractions intercept or touch the concordia line and overlap one another (Fig. 4.7B). Fraction H that yielded the youngest  2 0 6  Pb/  2 3 8  U age is slightly discordant, likely due to Pb loss not completely  removed by abrasion. The other six fractions yield a weighted mean  2 0 6  Pb/  2 3 8  U age o f 197.8±0.4 M a . A l l o w i n g for  the possibility o f slight Pb loss in some o f these fraction, we extend the error to include the oldest  2 0 6  Pb/  2 j 8  U age o f  the concordant fraction B ( 1 9 8 . 6 ± 0 . 4 ) . Thus the best age estimate o f this sample is 197.8+1.2/-0.4 M a . Sample 95JP4 was collected from a green, resistant, 10 cm thick, likely somewhat reworked tuff layer that contains rare plagioclase and biotite phenocrysts, devitrified glass and lithic fragments. A 20 k g sample yielded less than 0.1 m g zircon that was divided into two fractions. Both contained subhedral grains o f good quality with no visible cores or zoning. A b r a s i o n was prohibited by the small quantity o f grains. Both fractions show a strong inherited Pb component ( F i g . 4 . 7 C ) . A discordia line yields a lower intercept o f 1 1 3 ± 2 9 M a . The deviation from the  65  95JP1 (Middle Hettangian)  Interpreted age: 200.4 +2J/-2.8 Ma 0.205  0.210  95JP3 and 95JP4 combined (Middle to Late Hettangian)  Interpreted age (95JP3): 197.8 ± 1.0 Ma  95JP5 (Middle to Late Hettangian)  Interpreted age: 197.8 +1.2/-0.4 Ma  20 7  PW  235  U  Figure 4.7. U-Pb concordia diagrams for samples from the Hettangian section at Puale Bay. A: Sample 95JP1, Middle Hettangian, crystal tuff from near the top of Kamishak Formation (?); B: Sample 95JP5, Middle to Late Hettangian, tuff from the Talkeetna Formation; C: Samples 95JP3 and 95JP4, tuff from the Talkeetna Formation. 66  expected crystallization age could result from Pb loss and/or mixing of inherited Pb of different ages. Results from our other samples suggest that Pb loss is expected for unabraded zircons. The upper intercept of 1270±68 Ma points to a Proterozoic zircon component that was not detected as cryptic cores or xenocrysts. The stratigraphically highest sample, 95JP3, was collected from massive, soft, poorly lithified, plagioclasephyric water-laid tuff with green, somewhat altered groundmass. Zircon in this sample was not abundant but the recovered material was sufficient to analyze five fractions. Colorless to pale brown grains of good to excellent clarity were separated into fractions of euhedral or dominantly subhedral to anhedral crystals. Fractions A and D, both containing euhedral prisms, are overlapping and slightly discordant to concordant whereas the other three fractions reveal variable amount of inherited Pb. It is possible that some resorbed grains are xenocrysts or cryptic zircon cores remained visually undetected. When plotted on a composite concordia diagram (Fig. 4.7C), these three fractions and the two discordant fractions of sample 95JP4 fail to define a single chord. It is therefore likely that mixing of inherited Pb of varying ages occured in both samples. A bounding chord through fractions A, D, and C defines an older upper intercept age of 2775±30 Ma. The other bounding chord through A, D, and E yields an upper intercept age of 1094±30 Ma. The crystallization age of the tuff is estimated from the  Pb/ U age of  fractions A and D that.are virtually free of inheritance and Pb loss. An estimate of 197.8±1.0 Ma is derived from the weighted mean Pbr U age with its error extended to encompass the error range of both fractions. U0  4.5.  J5  DISCUSSION  In this study we identified 20 ammonite taxa from new collections as compared to the nine taxa reported by Imlay (1981) whose work is based on all previous collections from the same area. Table 4.2 summarizes the revised taxonomy in comparison with that of Imlay (1981). Only two previously known species have not been found in the new collection. Laqueoceras cf. sublaqueus is a serpenticone alsatitid characteristic to the upper part of the Liasicus Zone or its equivalents. Schlotheimia sp., with the genus ranging from the Middle through the Late Hettangian, can also be easily accommodated within the stratigraphic framework outlined above. The biochronologically most important revision concerns Psiloceras cf. planorbis in Imlay (1981). Similar, midvolute unormanented forms occurring in the lower fauna are now tentatively interpreted as Euphyllites? sp. Also similar is a large, smooth whorl fragment at the top of the lower fauna that is probably a lytoceratid. Both of these identifications are in accordance with the Middle Hettangian age assignment. There appears to be no firm evidence for the presence of Early Hettangian faunas. The youngest proven Triassic is the Norian Cordilleranus 67  Zone and the oldest proven Jurassic is equivalent to the Liasicus Zone. Between the two at Puale Bay, there are several tens of metres of strata containing only trace fossils (Newton, 1989) and a fault. The presence of Middle Hettangian ammonites in the lowest beds of the hanging wall and the lack of significant lithological change across the critical interval suggest that deposition may have been continuous throughout the Triassic-Jurassic boundary but the basal Jurassic is faulted out. The Puale Bay section is therefore ruled out as a continuous Triassic-Jurassic boundary section. However, outcrops at neighbouring Alinchak Bay should be scrutinized as they may contain an uninterrupted record spanning the boundary.  IMLAY 1981  THIS REPORT Eolytoceras cf. tasekoi  —  Pleuroacanthites  —  ex gr. mulleri  lytoceratid indet.  ? Psiloceras ct. P. planorbis  Discamphiceras  cf. silberlingi  Discamphiceras  aff. reissi  Euphyllites?  D. cf. D.  toxophorum —  ?Psiloceras  sp.  cf. P. planorbis  psiloceratid indet. Kammerkarites?  —  Waehneroceras  cf. frigga  Kammerkarites  ex gr.  cf. W. tenerum  —  megastoma  Saxoceras?  ex gr. portlocki  Waehneroceras  cf. W. portlocki  Saxoceras?  sp.  Waehneroceras  cf. W. portlocki  Franziceras?  —  sp. Schlotheimia  —  sp.  schlotheimiid indet..  —  Mullerites cf.  —  pleuroacanthitoides Laqueoceras  —  Sunrisites? sp. Badouxia  Badouxia  canadensis  Arnioceras  canadensis  —  sp.  Paracaloceras  sublaqueus  —  B. columbiae Badouxia?  cf. L. —  cf.  Paracaloceras  rursicostatum  cf.  rursicostatum  Arnioceras cf. A. densicosta  arnouldi  Table 4.2. Comparison of ammonite identifications in Imlay (1981) and in this report.  68  The detailed documentation of ammonoid ranges in the Puale Bay section is an important step toward the development a regional standard zonation for the North American Hettangian. The main stratigraphic relationships known from Nevada and the Queen Charlotte Islands were upheld. Our new observations underscore the need for the study of several sections before the biochronologic significance of certain taxa can be fully understood. The lower fauna is not easily amenable to subdivision yet it contains elements of four local zones from Nevada (Guex, 1995) and two from the Queen Charlotte Islands (Tipper and Guex, 1994). The occurrence of Discamphihceras cf. silberlingi below D. aff. reissi is the reverse of the succession in Nevada. There is a more substantial overlap in the ranges of Kammerkarites, Saxoceras and Franziceras than found elsewhere. Euphyllites, if correctly identified, occurs above Mullerites. The succession of Sunrisites'? and Badouxia in the second fauna supports the idea that this lineage is the most useful for subdividing the Late Hettangian and the Hettangian-Sinemurian transition in North America. The isotopically dated levels are firmly constrained by the ammonite biochronology. Sample 95JP1 comes from within the lower fauna, i.e., it belongs in the Middle Hettangian (Liasicus Zone equivalent). This thin tuff layer predates the onset of a major volcanic phase that is dated by the other three samples (JP955, JP954, and JP953). They are not older than Middle Hettangian (Liasicus Zone equivalent) and not younger than Late Hettangian (Angulata Zone equivalent). The isotopic ages of the Middle Hettangian and the Middle or Upper Hettangian samples overlap within error therefore the duration of these Hettangian ammonite zones cannot be directly estimated but were likely short. The new U-Pb ages do not correspond to the Hettangian in most of the recently published time scales, although they fall within the range of error proposed for the terminal Hettangian boundary (Fig. 4.8). Based on the new results, the Hettangian-Sinemurian boundary cannot be older than 199 Ma. The age of the Triassic-Jurassic boundary cannot be directly derived from our data. Many time scales (e.g., Harland et al., 1990, Gradstein et al., 1994) use interpolation based on the equal duration of biochronologic units. This method is ill-suited for the period of biotic recovery following the end-Triassic mass extinction events. Disparate zonal durations in the Hettangian, i.e., the brevity of the Planorbis Zone compared to the Liasicus Zone, were demonstrated using Milankovitch cyclicity (Smith, 1990). Accepting that the Planorbis Zone is not longer than average, the Triassic-Jurassic boundary is likely to fall in the 200-205 Ma interval. This estimate is younger than those in most current time scales with the exception of Odin's (1994) (Fig. 4.8). It is also compatible with the 201-202 Ma boundary age suggested from the Newark Basin (Olsen et al., 1996b) through the integration of cyclostratigraphy (Olsen et al.,  69  195 Ma  Middle to Late  95JP1  200  ODIN  205 4  204  ±9  DNAG 208  203  ±3  203.5 ±6.3  EXX88  201.9 ±3.9  MTS 205.7 ±4.0  GTS89  I  95JP3 95JP5  200 +4/-? 201 ±3.5  I  Middle NEW HETTANGIAN U-Pb DATES  208.0 ±7.5  ±9 210 ±3.5  210  Figure 4.8. Comparison of Hettangian stage boundary age estimates in recent time scales and the new Hettangian U-Pb dates reported here. Sources of time scales quoted: DNAG (Palmer, 1983; Kent and Gradstein, 1985; Kent and Gradstein, 1986); EXX88 (Haq et al., 1987; Haq et al, 1988); GTS89 (Harland et al., 1990); Odin (Odin, 1994); MTS (Gradstein et al., 1994; Gradstein et al., 1995).  1996a), U-Pb dating (Dunning and Hodych, 1990; Hodych and Dunning, 1992) and palynostratigraphy (Fowell et al., 1994). The presence of inherited Proterozoic zircon in the Talkeetna Formation is documented here for the first time. Five fractions in two of the dated samples (95JP3 and 95JP4) contained a strong inherited zircon component. The lack of co-linearity in the discordant fractions is probably attributed to the mixing of inherited zircons of different ages. The two bounding chords (Fig. 4.7C) suggest a range from at least late Middle Proterozoic (1094±30 Ma) to Late Archean (2775±30 Ma) upper intercept ages. The admixing of such old zircon requires the proximity of evolved Precambrian crust during the magmatic processes, a scenario that is not compatible with some current tectonic models. On the basis of elemental abundances, Barker et al. (1994) found the Talkeetna volcanics transitional between tholeiitic and calc-alkaline type and proposed an intra-oceanic volcanic arc as their likely tectonic setting. In another geochemical study, DeBari and Sleep (1991) documented high-Mg and low-Al "bulk arc" composition for the Talkeetna arc and suggested that the magma was sourced from a mantle wedge rather than a subducting plate. A new tectonic model for the Talkeetna arc should account for the observed zircon inheritance pattern while remaining compatible with the geochemical affinities.  70  4.6.  CONCLUSIONS  The lowest Jurassic ammonite volcano-sedimentary sequence of the Puale Bay section was successfully dated using ammonite biochronology and U-Pb geochronometry. The oldest Jurassic ammonites in the Puale Bay section are of Middle Hettangian age (Liasicus Zone, probably Portlocki Subzone equivalent). No record of basal Hettangian was found and the Triassic-Jurassic transition is probably missing locally due to a small fault. The andesitic volcanism recorded in the Talkeetna Formation started during or immediately after the Middle Hettangian and was terminated, at least locally, before the end of Hettangian. Hettangian and Early Sinemurian ammonite faunas are closely comparable with those of the other major North American localities in Nevada and the Queen Charlotte Islands permitting global correlation at approximately the zonal level. Three new U-Pb zircon dates from the stratigraphically tightly constrained Hettangian volcanic units furnish calibration points for the Jurassic time scale. A thin tuff layer from within the correlatives of the Middle Hettangian Liasicus Zone is dated at 200.8+2.7/-2.8 Ma. Tuffs of the Talkeetna Formation bracketed by Middle and Late Hettangian ammonites yield crystallization ages of 197.8+1.2/-0.4 Ma and 197.8±1.0 Ma. Zircon inheritance patterns reveal the proximity of the Talkeetna arc to evolved, Proterozoic to Late Archean crustal components that is not compatible with current tectonic models. The new dates suggest that the Hettangian-Sinemurian boundary is younger than 199 Ma and its age is overestimated in nearly all currently used time scales. The age of the Triassic-Jurassic boundary is likely between 200 and 205 Ma, younger than most current estimates.  71  CHAPTER 5  NEW U-Pb ZIRCON AGES INTEGRATED WITH AMMONITE BIOCHRONOLOGY FROM THE JURASSIC OF THE CANADIAN CORDILLERA  5.1.  INTRODUCTION  More than a dozen time scales published in the last 15 years provide different age estimates for Jurassic stage boundaries. Even the most widely used time scales (Palmer, 1983; Haq et al., 1988; Harland et al., 1990; Gradstein et al., 1994), however, commonly disagree considerably and contain large uncertainties. The flaws are attributed to the relatively small number and uneven temporal distribution of isotopic ages used, the predominance of K-Ar ages, the frequent use of minerals with low isotopic closure temperature in dating, and the large proportion of ages obtained from stratigraphically poorly-constrained plutonic rocks (Palfy, 1995). The Cordilleran Jurassic Calibration Project was initiated to improve the time scale through generation of new calibration points (Palfy et al., 1995). Integrated ammonite biochronology and U-Pb geochronology was adopted as a combination of one of the best fossil and isotopic dating techniques presently available that is well-suited to the abundant volcanosedimentary arc sequences in the Canadian Cordillera. The usefulness of this approach is demonstrated by Palfy et al. (1997). Results of a study using a similar approach in Alaska are presented in Chapter 4. Here we report 13 new U-Pb ages obtained from volcaniclastic units that are well constrained by ammonite biochronology. This concerted dating effort is unprecedented for the Jurassic in that it attempts to systematically generate time scale calibration points for much of the Early and Middle Jurassic and uses zonal resolution in biochronologic dating.  72  5.2.  REGIONAL GEOLOGICAL SETTING  Target sections forfieldwork and sample collecting were identified in Stikinia and Wrangellia, two of the largest volcanic arc-type Cordilleran tectonostratigraphic terranes (Fig. 5.1). A voluminous Lower to Middle Jurassic calc-alkaline volcanic suite and associated marine sedimentary rocks make up the Hazelton Group (Tipper and Richards, 1976), a key stratigraphic unit of Stikinia. Local lithostratigraphy varies along the length and width of the terrane as summarized by Marsden and Thorkelson (1992) who interpret the wide volcanic belt as the result of paired subduction-related arc volcanism and sedimentation in a common back-arc basin. This basin, the Hazelton Trough, contains an intermittent and scattered ammonite record that nevertheless represents most zones of the Late Sinemurian through Late Bajocian interval. The waning of widespread Hazelton volcanism corresponded to the inception of the Bowser basin, a successor basin of the Hazelton Trough. This depocenter accumulated thick Middle Jurassic to mid-Cretaceous molasse shed from the emerging Cordillera (Ricketts et al., 1992). The Ashman Formation, the lowermost unit of Bowser Group, still contains a minor volcaniclastic component. The cessation of volcanism in the Late Jurassic restricts our study to Early and Middle Jurassic time.  Figure 5.1. Index map of U-Pb sample localities with relation to the major accreted terranes of the Canadian Cordillera. A - Ashman Ridge; B - Mount Brock range; C - Zymoetz (Copper) River; D - Diagonal Mountain; M - McDonell Lake; R - Rupert Inlet; Te - Telkwa Range; To - Todagin Mountain; Tr - Troitsa Ridge. 73  In Wrangellia, Jurassic volcanic arcs are recorded in the Lower to Middle Jurassic Bonanza Group volcanic rocks on Vancouver Island (Jeletzky, 1976) and Middle Jurassic Yakoun Group of the Queen Charlotte Islands (Cameron and Tipper, 1985). The only Wrangellian sample reported in this paper is from the Harbledown Formation, a thin-bedded Sinemurian shale unit that contains thin tuff layers and is interpreted a distal marine facies equivalent of parts of the Bonanza Group volcanic rocks (Muller et al., 1974).  5.3.  DATING METHODS  5.3.1. Ammonite biochronology Ammonite biochronology is preferred as the method to date sedimentary rocks intercalated with the isotopically-dated volcanic rocks because: (1) ammonites, along with biostratigraphically less useful bivalves, are the most common macrofossils in Cordilleran Jurassic strata; (2) standard stratigraphic units in the Jurassic are based on ammonite biochronology; (3) ammonite faunal successions of the Cordillera have been intensively studied and regionally applicable standard or preliminary zonal schemes are now available for most parts of the Jurassic. The Sinemurian through Bajocian North American ammonite biochronologic scheme used in this study (Fig. 5.2) comprises formal regional standard zones and informal assemblages based on local studies or less wellunderstood faunas awaiting further studies. Sources of compilation include Palfy et al. (1994) for the Sinemurian, Smith et al. (1988) for the Pliensbachian, Jakobs et al. (1994) for the Toarcian, Poulton and Tipper (1991) for the Aalenian, Hall and Westermann (1980) and Hillebrandt et al. (1992) for the Bajocian. The Bathonian and Callovian ammonite successions are less well understood and no regionally applicable and widely accepted zonal scheme is available. An excellent summary of faunal associations of this age is given by Callomon (1984) and recent updates are found in Arthur et al. (1993) and Poulton et al. (1994). The following broader units are distinguished: the Early Bathonian based on Iniskinites, Parareineckeia and perisphinctids; the Middle/Late Bathonian based on early representatives of Kepplerites along with Iniskinites, the first Lilloettia, and perisphinctids, the Early Callovian with Cadoceras comma, coarsely ribbed Kepplerites spp., and Lilloettia spp. The Middle Callovian with its Cordilleran index ammonite Cadoceras stenoloboide is beyond the stratigraphic range covered in this report.  74  SUBSTAGE  STAG  HI  Epizigzagiceras Assemblage  IAN  L  NORTH AMERICAN AMMONITE ZONES OR ASSEMBLAGES  Rotundum Zone  O  Oblatum Zone  O  < m  Kirschneri Zone  E  Crassicostatus Zone Widebayense Zone Howelli Zone  < z  Scissum Zone  UJ _]  Westermanni Zone  RCI  z < <  O H  L  Yakounensis Zone Hillebrandti Zone  M E  NVIHC  L  Crassicosta Zone Planulata Zone Kanense Zone Carlottense Zone Kunae Zone  <  Freboldi Zone  V)  Whiteavesi Zone  OQ  z Ul _l  E  Imlayi Zone  IURIAN  Q.  Tetraspidoceras Assemblage  L  Asteroceras varians Assemblage  2 Ul  z  55  Plesechioceras? harbledownense Assemblage  Amioceras amouldi Assemblage  E  Coroniceras Assemblage Canadensis Zone  Figure 5.2. North American composite ammonite zonal scheme for the Sinemurian through Bajocian. See text for explanation and sources of compilation.  Correlation with the northwest European, primary standard zonal scheme is discussed in the sources quoted and not dealt with here, except where warranted by new data. Zonal assignments are based on the published ranges of ammonites found in the studied sections. Where fossils occur only either below or above a U-Pb dated volcanic unit, the temporal proximity of fossils to the dated stratum is inferred from the stratigraphic relationships.  75  5.3.2. U-Pb geochronology U-Pb zircon dating was carried out in the Geochronology Laboratory of the Department of Earth and Ocean Sciences, University of British Columbia (all samples except for those listed below) and in the Geochronology Laboratory of the Geological Survey of Canada (samples 2 and 5). Zircon was obtained from 15 to 40 kg samples using conventional crushing, grinding, wet shaking, and heavy liquid separation. Fractions were hand-picked based on differences in magnetic susceptibility, size, and crystal morphology. Whenever possible, the least magnetic and best quality grains (i.e., free of cracks, inclusions, and visible cores or zoning) were used for the analyses. Most fractions were air-abraded using the technique of Krogh (1982) to minimize or eliminate the effect of surface-correlated Pb loss. Non-abraded fractions were sometimes used as a necessity dictated by the small amount of zircon available (e.g., samples land 9) or to assess the amount Pb loss and to help constrain the Pb loss chord (samples 7 and 11). Details of techniques employed in chemistry and mass spectrometry at UBC are given by Mortensen et al. (1995) and those used at the GSC are discussed by Parrish et al. (1987). During the course of this study, U and Pb procedural blanks were 0.6-2.5 and 5-9 pg, respectively. Blank isotopic compositions, crucial in calculation of Pb/ U and Pb/ Pb ages, were carefully 207  235  207  206  monitored. The age calculations are based on the decay constants recommended by Steiger and Jager (1977); the correction for initial common Pb follows the model of Stacey and Kramers (1975). The analytical errors are propagated through the age calculations using the method of Roddick (1987). In the analyzed samples, zircon UPb systematics are commonly affected by Pb loss and/or inheritance. The rationale for assignment of the crystallization age and its associated error at the 2 sigma level is described individually in each case.  5.4.  INTEGRATED BIO- AND GEOCHRONOLOGICAL RESULTS  In the following section we present the results in order of oldest to youngest stages. Biochronology, geochronology, and location of samples are summarized in Table 5.1. For each sample, a brief description of the local geology and the studied section, the ammonite biochronology, the U-Pb systematics, and a discussion of the age assignment are given. The U-Pb analytical data are listed in Table 5.2. In many cases ammonite data from the sections under discussion have already been published. However, it is necessary to illustrate newly discovered Bathonian faunas.  76  5.4.1. Sinemurian  Sample 1. Tuff layer in Harbledown Formation, near Rupert Inlet, northern Vancouver Island  A predominantly dark shale sequence is exposed in a quarry east of the head of Rupert Inlet on northern Vancouver Island (see Table 5.1 for locality information) and was assigned to the Harbledown Formation and dated as Sinemurian based on the presence of Arnioceras (Haggart and Tipper, 1994). Stratigraphic relationships with the Triassic (and possibly basal Jurassic) Parson Bay Formation and Lower to Middle Jurassic Bonanza Group are discussed in Nixon et al. (1994). Sample 1 was collected from one of several, few centimetre thick, light grey, reworked air-fall tuff layers interbedded with thin-bedded, dark shale and siltstone. Our new ammonite collections yielded Arnioceras cf. arnouldi 2.5 m below the dated tuff layer and Arnioceras cf. oppeli another 5 m farther downsection. Both species range from the late Early Sinemurian Arnouldi Assemblage to the early Late Sinemurian Varians Asemblage (Palfy et al., 1994). In the Queen Charlotte Islands where the ammonite succession is better known, the absence of associated other ammonites (e.g., asteroceratids) characterizes the Arnouldi Assemblage. Although no ammonites were recovered from above the tuff layer, no lithologic break or other indication of a gap was observed between the tuff and the fossiliferous beds. Ammonite zones in the Harbledown Formation in its type area on Harbledown Island (Crickmay, 1928), as well as in the temporally and lithologically equivalent Sandilands Formation on the Queen Charlotte Islands (Cameron and Tipper, 1985), are typically represented by several tens of metres of strata. The isotopically dated tuff is therefore assigned to the Arnouldi or possibly to the Varians Assemblage. Approximately 45 kg of processed rock yielded less than 0.5 mg of zircon. An apparently homogenous population consisted of mostly subhedral, prismatic grains of excellent clarity. One of the two analyzed fractions, B is slightly reversely discordant (Fig. 5.3A), likely due to analytical problems in measuring Pb in the small 2C7  amount of available material and the sensitivity of results to blank Pb composition (see Table 5.2 for analytical data). Significant Pb loss is evident in the non-abraded fraction A. It is likely that the moderate abrasion of fraction B did not remove all the surface-correlated Pb loss, therefore we interpret its Pb/ U age of 191.3±0.4 Ma as the 206  minimum age of the tuff.  77  238  .a  E  E  E  I  LU  E  E  c  c  c  mi  mi  mi  Q_  ro  S  —  C) oo  T  CD  ro ro CM CD  ro  2  CO  CO  oS  +  +1 CO  CD  cn  55 =3  E  CU  OJ c  tt) to CL Q.  C CD  E  E  ro CU c <Z>  _i  CO CO  CO CO  cb  +  Tf  iri 00  o 00  co  +  ro CO  in o  cb  CD  00 cri  +  CM  r-  CD  +i  cn  E Tf  3  E  CJ)  E ro 2 co  +  l<  c  J CM + CD CN CO  ^  £=  ro  UJ  ro O  Q.  ro O  CL  s  •—^  < CO  < CO  ro ro >  ro CD ro CD > >  c  o  ro  2 CN  Tf  ro o p  CU  CO CO  —  Tf  2  •P  <  _i  ro  !c  o  0) c C/>  _i  —  3  CU c  ro  2 oo o  o  E „ c  ro  ro 2ro  < cn c  N  o  T3 CU  N  2  iS  CQ  2>  JS 0.  ro  o J3 LL  =j c=  LU  '  = cu  o .o 2, 'ro  m  3 o  CO  £ CO  c  CU  *-(-»  a  X  o _o  X)  c  cs _o o c o  E CU  o  CU  3 o  <  CL>  oa  <  CU  CU CU  LU^  S  o  o  fU  -o  xi  =5  -S  -S  N p o  N  JD CU  _o N  "o m B  <D  ro  a o  N  CU  ro >-  3 o  X  ro ro  o  UL  CO  IS C4-  O  CD CD •g CU  Q.  CC »*o  UJ o  cn CO CO o co cn  Q:  ro  c  ro  CO  t—  I-  O  O OO O  ro E  tt>  cu  < o  CO o CQ  o m CO m  in. 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Tf  CO  H  o CN  o o CO o  LU I LU  f^  cn  Q  CD  CD  Tf  5 CO  CO CO CL  00  o  CO CL ~3  TT  CL -3  CD  CD  cn  CD  CD  O)  o  CM  CO  co r-.  in cn CL  CD  co  CL  CN  CN  5 CO  cn  LU CO CD  Q  Tf  Q  Tf  CD  r-  —) —) —) —} —} —i Tf Tf CO CO m in in cn  Q.  ro C O  Q:  o t-  L/6  .— CD  2  2  E  cu  o  8 w ° ^  03  4HJ  ro  "g  b:  £^  cu D) C  4 hi  3  CU  CU D) c ro  in  CD  oo CL -3  m  CO CO  < I cu c  o  IS1  Figure 5.3. Continued on following page. 79  0.10  0.14  0.18  0.16  Figure 5.3 (this and preceding page). U-Pb concordia diagrams of samples l—13.  80  0.18  (A)  A s h m a n Ridge @ A1  CD  ro to co J> co .E < W  300 m  c <B ro g. "ir Q -  Jf / v v v Vv Vv V V V  v  ly v v v v v v v v v v \ V V V V V V V V V v \ V V V V V V V V V V ' V V V V V V V V V v \ 7 V V V V V V V V V v \ V V V V V V V V V V  Troitsa ridge  (B)  -Z-Z-Z-+- @Tr13  120 +:  U-Pb 3  -J  co 2 o •£ c .  x  100  4  io " 1  H - CQCTMO  V  v  \J  V V V V V V V V V V v v v v v V V V v v •> V V V V V V V V V V ^ V V V V V V V V V V V V V V V  K U-Pb 9  v  200 m  \J \J \J ' v  S'S •S b oUj b  LU  ^Tr9 3 co  (  ""  v v  o —  v  k  U-Pb 0  CO ° CD CO  c c co  IO ?.1= LU , b LU  100 m  -C  „  50  J7 A t 7 7 £ . ? W  t. 1  U-Pb 2  7  io [A A A A A A l A A A A A A  o o  JTr3 }Tr2  20 m  20 - r (C)  @Tr1  <* CO CO ir£ CD CD O CD p  10 m  McDonell Lake  i—rrrr  @ M3  2 a C co  30  ® M2  i_: "cB Cn.O  £ U-Pb 11 20  @ M1  co — <2 cL (b co co tnco <o t CDC co  & CD  CO CN. .CO  10 m  .S>  QCO-J S o "> co o o - J <D r~  ;0  '•+3 '-+3 • —  42  6  §£' : § CD, O CD  I  co  a ST. 'fi<D c: a  o  CO  0)  &s  £ o co c CD o o c  Figure 5.4. Continued on following page. 81  (E)  (F)  Telkwa Range V V V V V |y v v v v \A  150  A  A  |A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A A A  A  A  A  A  A  A  A  A  A  A  A  H  60  A  A  .1I?  U-Pb5  k  |A  , Zymoetz (Copper) River  co to = w  50 4  •S & o 3  - @ C2  40 ^  <  j-@  30  CO  0  •SE •« t3 ° o £ CD co E  100 m  E  20 H  0)  CL  Q_  O cu  Q.  a a  5  II  • • •  C1  -c .CD  CO  CO  -2 "ra  CO  C:  Q.Q.  U-Pb 13  o <u  21  CD O O  c  5  OJ  OJ  o  o  10 m  £°  5<3  CD O O  c  LEGEND limestone shale siltstone A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  sandstone sandstone coquina conglomerate  (D)  1/ V  10 m  V V V <\ A A  V  A  A  A  A  A  /I  A  A  A  A  A  \  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  /  A  A  A  A  A  /  ra |  welded tuff  A  h l\ /  U-Pb 7 V V '/  \— @ Dactylioceras spp. V  V  V  V  V V  V  V V  V  V V  V  ' V  V  volcanic flow  '  volcanic breccia  |y v v v v v| v v v v v  \J  densely welded tuff  I, 7  c ,_  CU  volcanic tuff (non-welded)  A A A A A A A A A A  v v v v v|  c <5 o o N S 8,!° CD  tuffaceous sandstone  West of Mount Brock  \i  V V V V V '/ V V V V V |  dyke  Figure 5.4. Measured or schematic stratigraphic sections showing U-Pb samples and fossil collections. A - Ashman Ridge; B - Troitsa ridge; C - McDonell Lake; D - West of Mount Brock; E - Telkwa Range; F - Zymoetz (Copper) River 82  Sample / Fraction  1  Wt  U  Pb*2  2(>6p3 2(Mpb  D  Pb«  2(>8p5  Isotopic ratios (±la,%)'>  0  2(»pb/2'»U  Calculated ages (±2c,Ma) <•  2(rtpb/235(J  207pb/206Pb  20f,pb/" U 8  2(npb/2()6pb  mg  PPm  ppm  0.014 0.011  230 132  6 4  829 527  6 5  7.8 11.4  0.02501 (0.14) 0.03012 (0.11)  0.1742 (0.47) 0.2054 (0.60)  0.05052 (0.41) 0.04947 (0.57)  159.2 (0.4) 191.3 (0.4)  219.2 (19) 170.2 (27)  0.028 0.036 0.034  1360 1959 2127  43 61 65  731 608 584  102 221 235  .14.1 13.8 14.4  0.02980 (0.11) 0.02949 (0.14) 0.02886 (0.13)  0.2061 (0.27) 0.2063 (0.31) 0.1998 (0.33)  0.05015 (0.21) 0.05074 (0.24) 0.05020 (0.26)  189.3 (0.4) 187.4 (0.5) 183.4 (0.5)  202.1 (9.8)  0.104 0.154 0.096 0.065 0.070  157 176 186 172 146  5 5 6 5 4  5705 2956 4165 994 2595  5 18 8 21 7  8.4 8.9 9.0 8.8 9.8  0.03026 (0.15) 0.03026 (0.11) 0.03001 (0.11) 0.02956 (0.19)  0.05029 (0.14) 0.04988 (0.13) 0.05018 (0.12) 0.04969 (0.25)  192.1 (0.6) 192.2 (0.4) 190.6 (0.4) 187.8(0.7)  208.3 (6.7) 189.6(6.0) 203.4 (5.7)  0.02824 (0.18)  0.2098 (0.23) 0.2081 (0.22) 0.2076 (0.21) 0.2025 (0.35) 0.2030 (0.26)  0.05213 (0.15)  179.5 (0.6)  0.024 0.040 0.018 0.024 0.027 0.021  322 303 464 519 237 353  9 9 14 16 7 9  752 918 764 946 1379 743  18 24 21 24 9 17  11.1 10.2 11.2 11.3 10.1 10.6  0.02805 (0.13) 0.03000 (0.11) 0.03036 (0.16) 0.02961 (0.11) 0.02968 (0.18) 0.02607 (0.43)  0.1934 (0.50) 0.2067 (0.43) 0.2092 (0.51) 0.2040 (0.43) 0.2037 (0.29) 0.1802 (0.53)  0.05001 (0.44) 0.04997 (0.37) 0.04997 (0.44) 0.04997 (0.37) 0.04977 (0.22) 0.05012 (0.29)  178.3 (0.4) 190.5 (0.4) 192.8 (0.6) 188.1 (0.4) 188.5 (0.7) 165.9(1.4)  195.4 (21) 193.5(17) 193.8 (20) 193.5(17) 184.2 (10) 200.7 (13)  0.038 0.049  7 7 7  2479 779 541  7 30 28  9.5 9.6 9.8  0.03005 (0.09) 0.03012 (0.13) 0.03018 (0.15)  0.2080 (0.29) 0.2074 (0.25) 0.2078 (0.59)  0.05021 (0.26) 0.04996 (0.18) 0.04993 (0.52)  190.9 (0.4) 191.3 (0.5) 191.7 (0.6)  204.6 (12) 193.0 (8.5) 191.6 (24)  0.1988 (0.20)  183.2 (0.4) 180.7(0.4) 185.6 (0.5) 185.4 (0.4)  196.1 194.4 180.6 191.1  186.7(4.8) 188.8(4.7) 191.5 (4.4)  Pg  %  Sample 1 (95JP36) A[<100,N2, b+e, n-a] B[<100, N2, sp,m-a] Sample 2 (93JP31) A[<62,Nl,sp, l-a] B[<62,Nl,p, l-a] C [ < 6 2 , N l , p , l-a]  229.1 (11) 204.4 (12)  Sample 3 (93JP32) A B C D E  [74-134, N20, ep, s-a] [74-134, N20, ep, s-a] [74-134, N20, sp, s-a] [74-104, N2, sp, s-a] [44-74, N2, ep, s-a]  Sample 4 (93JP47) A [>134, b+p+t, a] B [104-134, b+sp, m-a] C [74-104, b+sp, m-a] D [74-134, b+sp, m-a] E [74-134, N5, sp, m-a] F [74-134, N5, sp, a] Sample 5 (93JP48) A[>74, N l , p , s-a] B[>74,Nl,p, s-a] C[>74,Nl,p, s-a]  180.7 (11.6) 291.2 (6.9)  0.031  231 248 248  Sample6(96JP14) A[>104, N5, sp, s-a] B [74-104, N l . s p , s-a] C [74-104, N l , s p , s-a] D [>74, N5, sp, s-a]  0.061 0.081 0.082 0.118  640 474 334 754  19 14 10 23  2709 1084 1003 7855  26 65 51 21  13.5 12.7 10.5 14.4  0.02882 (0.11) • 0.02843 (0.11) 0.02921 (0.14) 0.02917 (0.10)  0.1960 (0.27) 0.2001 (0.30) 0.2008 (0.17)  0.05002 (0.12) 0.04999 (0.19) 0.04969 (0.20) 0.04992 (0.09)  Sample 7 (94JP61) A [<134, N2, eq+p, n-a] B[>134, N2, eq+p, n-a] C [>100,N2, eq+ep, s-a] D[>134, N2, e+eq, s-a]  0.218 0.306 0.227 0.165  504 820 822 367  14 23 24 10  6586 7097 13257 4136  30 63 25 26  11.7 10.8 10.8 9.8  0.02805 (0.10) 0.02800 (0.14) 0.02835 (0.15) 0.02838(0.10)  0.1927 (0.19) 0.1925 (0.22) 0.1951 (0.22) 0.1949 (0.20)  0.04982 (0.10) 0.04987 (0.10) 0.04992 (0.09) 0.04980 (0.11)  178.4 178.0 180.2 180.4  Sample 8 (95JP73) A [>100, sp, a] B [74-104, sp+eq, a] C [74-104, sp+eq, a] D [<104, sp, a]  0.020 0.015 0.023 0.082  225 180 242 195  6 5 7 5  1050 282 590 2053  7 17 17 13  16.5 16.0 17.5 17.2  0.02589 (0.11) 0.02586 (0.17) 0.02722 (0.13) 0.02559(0.12)  0.1772 (0.41) 0.1744 (0.82) 0.1863 (0.46) 0.1753 (0.23)  0.04966 (0.34) 0.04891 (0.71) 0.04964 (0.37) 0.04968 (0.16)  164.7 (0.4) 164.6(0.6) 173.1 (0.4) 162.9 (0.4)  179.0(16.2) 143.4 (33.8) 178.4(17.5) 180.3 (7.7)  [>74, N5, sp, m-a] [44-74, N2, sp+e, n-a] [44-74, N2, sp+n, n-a] [44-74, N5, sp+e, n-a]  0.046 0.033 0.031 0.045  285 384 398 344  8 7 7 7  1263 785 935 1090  17  17.3 14.2 14.3 15.3  0.02551 (0.10) 0.01716(0.11) 0.01767 (0.12) 0.02000 (0.10)  0.1739 (0.29) 0.1152 (0.43) 0.1182 (0.42) 0.1373 (0.34)  0.04943 (0.21) 0.04871 (0.36) 0.04849 (0.35) 0.04979 (0.27)  162.4(0.3) 109.7(0.2) 112.9 (0.3) 127.7(0.3)  168.4 (9.9)  18 15 18  Sample 10(95JP68) A [74-104, eq+sp, m-a] B [74-104, eq+sp, l-a] C [<74, eq+sp, l-a] D[<104, N2, p, s-a]  0.021 0.029 0.046 0.030  151 163 261 134  4 4 7 4  380 496 533 480  14 15 36 14  15.4 16.5 18.6 16.1  0.02510 (0.16) 0.02438 (0.15) 0.02423 (0.15) 0.02627 (0.13)  0.1713 (0.64) 0.1688 (0.51) 0.1664 (0.46)  0.04950 (0.55) 0.05021 (0.43) 0.04979 (0.35) 0.04946 (0.44)  159.8 155.3 154.3 167.2  171.5 (26.0) 204.5 (19.8) 185.4(16.6) 169.9 (20.7)  0.090 0.122  187 237  11.8 13.0 13.6 14.8 14.4 13.1 12.9 12.8 8.9 11.8 10.5  0.02485 (0.13) 0.02479 (0.14) 0.02497 (0.12) 0.02465 (0.11) 0.02482 (0.12)  0.1685 (0.37) 0.1672 (0.24) 0.1702 (0.24) 0.1667 (0.31) 0.1695 (0.21)  0.04938 (0.17) 0.04891 (0.38) 0.04955 (0.15) 0.04953 (0.21) 0.04919(0.30) 0.04891 (0.18) 0.04945 (0.16) 0.04905 (0.22) 0.04953 (0.11)  154.5 (1.8) 158.6 (0.4) 156.9 (0.3) 159.7 (0.4) 150.2 (0.3) 157.6 (0.4) 158.2 (0.4) 157.8(0.4) 159.0 (0.4) 157.0 (0.3) 158.0 (0.4)  82.8 (132) 179.4 (9.9)  0.02464 (0.13) 0.02508 (0.13) 0.02357 (0.11) 0.02474 (0.12)  0.1594 (3.2) 0.1706 (0.32) 0.1677 (0.27) 0.1691 (0.45) 0.1610(0.24) 0.1690 (0.27)  0.030 0.070 0.076 0.071 0.191  411 36 31 8 21 10 17 9 9 18 17  0.04767 (2.7) 0.04967 (0.21)  262 123 237 239 290 230 216 158 249  82 1298 1537 738 1851 1661 833 2823 2921 993 4513  0.02426 (0.86) 0.02491 (0.16)  0.115 0.030 0.111 0.045  5 6 7 3 6 6 7 6 5 4 6  0.063 0.084  203 178  5 5  1231 1445  17 17  12.0 12.1  0.02596 (0.12) 0.02510(0.11)  0.1823 (0.28) 0.1715(0.26)  0.05093 (0.20) 0.04958 (0.18)  165.2 (0.4) 159.8 (0.3)  237.7 (9.2) 175.2 (8.2)  0.013 0.015  218 273  6 8  409  12 10  13.1 12.7  0.02588 (0.16) 0.02656 (0.14)  0.1777 (0.72) 0.1861 (0.49)  0.04982 (0.65) 0.05082 (0.42)  164.7 (0.5) 169.0 (0.5)  232.7 (19.4)  (0.4) (0.5) (0.5) (0.4)  (5.6) (8.7) (9.1) (4.0)  185.7(5.1)  Sample 9 (94JP95) A B C D  Sample 11 (95JP78-79) 78A [74-134, n, s-a] 78B [74-134, ep, s-a] 78C [74-134, ep, s-a] 78D [<74, n, s-a] 78E [<74, sp+b, n-a] 78F [74-104, N2, n, s-a] 78G [74-104, N2, p, s-a] 78H [74-104, N l , p , s-a] 79A [>74, ep, s-a] 79B [>74, N20, n, s-a] 79C [>74, N20, sp, s-a] Sample 12 (95JP60) A[<104, N2, sp, s-a] C [<74, N2, sp, m-a] Sample 13 (95JP84) A [>104, sp+eq, s-a] B [<74, sp+eq, l-a]  715  83  0.1792 (0.51)  (0.5) (0.4) (0.4) (0.4)  134.1 (17.2) 123.4(16.7) 185.1 (12.5)  165.8 (8.0) 143.6(17.7) 173.8(7.0) 172.9(10.0) 156.9(14.0) 143.8(8.5) • 169.2 (7.4) 150.4(10.3) 172.9 (5.4)  186.4 (30.4)  1 All zircon fractions. Listed in brackets: Grain size range in microns; Side slope of Franz magnetic separator (in degrees) at which grains are non-magnetic (N) or magnetic (M), using 20° front slope and 1.8A field strength; Grain character: b=broken pieces, e=elongate, eq=equant, n=needles, p=prismatic, s=stubby, t=tabular; Degree of air abrasion: n-a=nonabraded, a=abraded, l-a=lightly abraded, m-a=moderately abraded, s-a=strongly abraded. ^Radiogenic Pb ^Measured ratio corrected for spike and Pb fractionation of 0.0043/amu± 20% (Daly collector) and 0.0012/amu ± 7% (Faraday collector). ^Total common Pb in analysis based on blank isotopic composition ^Radiogenic Pb ^Corrected for blank Pb, U, and initial common Pb based on the Stacey and Kramers (1975) model at the age of the rock or the Pb/ Pb age of the fraction. M7  20(i  Table 5.2. (Continued from preceding page) U-Pb analytical data.  Sample 2. Rhyolite tuff in Telkwa Formation, Ashman Ridge, Smithers map area  The section on Ashman Ridge is one of the most complete and best known Jurassic successions within Stikinia in north-central British Columbia (Tipper and Richards, 1976). Its lower part comprises a thick pile of volcanic and volcaniclastic strata of the Telkwa Formation and it contains the oldest known fossiliferous Jurassic sedimentary rocks in the area (Fig. 5.4A). A detailed description is given in Palfy and Schmidt (1994). A densely welded, rhyolite to dacite crystal-lithic tuff unit near the base of the exposed section was sampled for U-Pb dating (Sample 2). It lies more than 200 m below a locally very fossiliferous, calcareous tuffaceous sandstone (Level A l on Fig. 5.4A) which in turn overlies a thick-bedded, micritic limestone both of which appears to record the only short-lived episode of marine sedimentation in the Telkwa Formation in the Ashman Ridge area. The ammonite fauna is composed exclusively of asteroceratids (Asteroceras c f , A. aff. margarita, and Aegasterocerasl aff. jeletzkyi, all figured in Palfy and Schmidt (1994)), indicating the early Late Sinemurian Varians Assemblage. The Telkwa Formation appears to represent geologically rapid accumulation of predominantly subaerially deposited volcanic and volcaniclastic rocks (Tipper and Richards, 1976), hence the age of the isotopically dated tuff unit is considered to be early Late Sinemurian or slightly older. No precise age assignment is possible because all three zircon fractions analyzed are discordant to varying degrees and do not form a linear array (Fig. 5.3B). The older  207  Pb/  206  P b age of fraction B suggests the presence of 207  minor inherited cores or xenocrysts undetected by visual inspection of the grains. The  206  Pb/  Pb ages of fractions  A and C agree well and a regression line fit through them is consistent with recent Pb loss. A weighted mean 207  206  Pb/  Pb age of these two fractions, 202.9±7.5 Ma, is interpreted as the maximum age of the tuff. A minimum  age of 189.3±0.4 Ma is assigned based on the  2 0 6  Pb/  2 3 8  U age of the least discordant fraction A .  84  Sample 3. Andesite tuff in Telkwa Formation, Ashman Ridge, Smithers map area  Sample 3 was also collected from the Ashman Ridge section described above. The sampled level is 210 m stratigraphically above Sample 2, near the top of a 75 m thick, massive, purple dacite to andesite flow unit that immediately underlies the asteroceratid-bearing sedimentary rocks described above (Level A l on Fig. 5.4A). Thus the biochronological upper age constraint of the two samples are identical: the early Late Sinemurian Varians Assemblage. A moderate amount of good quality zircon with prismatic habit was recovered from the sample. The five 206  analyzed fractions do not permit a precise age assignment. There is a good agreement in the 207  fractions A and B as well as the  238  Pb/ U ages of  206  Pb/ Pb ages of B and D that converge around 192 Ma. This is regarded as a  minimum age. Pb loss effects are clearly indicated despite the strong abrasion but the lack of colinearity of discordant fractions also suggests minor inheritance of older Pb in fractions A , C, and D.  Sample 4. Dacite tuff in Telkwa Formation, southern Telkwa Range, Smithers map area  A more than 150 m thick section comprising volcanic, volcaniclastic, and carbonate rocks of the Telkwa Formation is exposed in the southern Telkwa Range (Fig. 5.4E). The geology of the area is discussed by Desjardins et al. (1990) and the section is described in detail by Palfy and Schmidt (1994). Sample 4 was obtained from a maroon, crystal-lithic dacite tuff unit in the lower part of the section. Paltechioceras cf. boehmi occurs in sparsely fossiliferous, micritic to bioclastic limestones both below and above the tuff. P. cf. rothpletzi and P. ex gr. aplanatum was collected some 15 m above the tuff. The ammonite fauna unequivocally indicates that the isotopically dated level is within the Upper Sinemurian Harbledownense Assemblage. Zircons from Sample 4 were colourless to pale brown, mostly stubby prismatic or tabular grains. The six analyzed fractions clearly define a discordia line indicating recent Pb loss (Fig. 5.3D). Analytical problems encountered during mass spectrometry of fractions E and F render them less reliable, as manifest in the slight reverse discordance of E. Fractions A through D form a linear array and yield a weighted mean 207  194.0±9.1 Ma. Considering the  2 0 6  Pb/  2 3 8  p b /  206  p b  Q  f  U age of 192.8±0.6 Ma from the most concordant fraction C as a minimum  age, we interpret 194.0+9.1/-1.8 Ma as the age of the tuff.  85  Sample 5. Rhyolite in Telkwa Formation, southern Telkwa Range, Smithers map area  Sample 5 was collected from near the top of the same section that yielded Sample 4 (Fig. 5.4E). More than 100 m of epiclastic and pyroclastic strata separate the sampled rhyolite flow unit from the underlying fossiliferous limestone of Late Sinemurian (Harbledownense Assemblage) age described above. The stratigraphic distance does not necessarily represent significant difference in age as sudden facies changes and rapid deposition of thick volcanigenic strata are typical in the proximal volcanic facies of the Hazelton Group (Tipper and Richards, 1976). Faunas younger than Late Sinemurian are not known from the Telkwa Formation. Regionally, the oldest faunas reported from the overlying Nilkitkwa Formation are of Early Pliensbachian (Whiteavesi Zone) age (Tipper and Richards, 1976). Therefore the age of the rhyolite is regarded as Late Sinemurian (Harbledownense Assemblage) or slightly younger but no younger than the Early Pliensbachian Whiteavesi Zone. ^06  238  •  The three zircon fractions analyzed form a tight cluster with overlapping " Pb/ U ages (Fig. 5.3E). Fractions B and C are concordant while fraction A is only slightly discordant but overlaps with B. We interpret the 206  age of the rhyolite as 191.5±0.8 Ma, based on the  238  Pb/ U ages of B and C.  5.4.2. Pliensbachian Sample 6. Dacitic lapilli tuff in Hazelton Group, Todagin Mountain, Spatsizi map area A well-exposed section of Lower Jurassic volcanic, volcaniclastic and sedimentary rocks occurs southwest of Todagin Mtn, east of Kinaskan Lake (Ash et al., 1996). Locally, the upper part of the Hazelton Group consists of a bimodal, basaltic to rhyolitic volcanic suite with common, predominantly fine-grained, sedimentary intercalations (Ash et al., 1997). Ammonites from the sequence suggest a Pliensbachian age (Tipper in Ash et al., 1996) whereas a spatially closely related felsic volcanic unit yielded a preliminary U-Pb age of 181.0 +5.9/-0.4 Ma (Ash et al., 1997). Subsequent detailed field work for this study aimed at sampling and dating felsic volcanic rocks unambiguously bracketed by ammonite-bearing strata. Sample 6 was collected from a waterlain, quartz-phyric lapilli tuff that locally contained rip-up clasts from the underlying mudstone. The section is structurally intact and the tuff is underlain and overlain by shale, siltstone, and sandstone of undoubted Freboldi Zone age, proven by the zonal index Dubariceras freboldi. Additional taxa, Metaderoceras sp. and Oistocerasl sp. found immediately  86  above the tuff, confirm the late Early Pliensbachian age assignment. The fauna is closely similar to that known from elsewhere in the Spatsizi area (Thomson and Smith, 1992). Sample 6 yielded a more than average quantity of zircon for volcanic rocks. Quality was variable and visually the best grains were included in fractions C and D, of which D had high-U content and contained slightly magnetic zircons. As expected, fraction C is the most concordant fraction with a slight reverse discordance while fraction D ^06  238  is only slightly discordant and the two error ellipses nearly overlap (Fig. 5.3F). The " Pb/ U ages of these fractions are in good agreement at 185.6±0.6 Ma that it can be argued to represent the true crystallization age. The 207  obvious Pb loss in fractions A and B and the somewhat older weighted mean  207  Pb/  206  206  Pb/ Pb ages of A , B, and D suggest caution as the  P b age of all fractions is 191.7±4.9 Ma (i.e., outside the error of the oldest  2 0 6  Pb/  2 3 8  U ages).  The corresponding discordia line is poorly fitted (MSWD=3.0). We derive a conservative age estimate of 206  185.6+6.1/-0.6 Ma by placing greater confidence in the weighted mean  207  Pb/  206  238  Pb/ U ages but extending the error range to include the  P b age.  5.4.3. Toarcian  Sample 7. Crystal tuff in Hazelton Group, west ofMount Brock, Spatsizi map area  One of the few places in the Hazelton Trough where a substantial pile of Toarcian volcanic rocks is exposed is at and near Mount Brock in the northwest corner of the Spatsizi map area. This volcanic unit within the Hazelton Group is informally known as the Mount Brock volcanics (Thorkelson et al., 1995). On an unnamed mountain 10 km northwest of Mt. Brock, the volcanic rocks interfinger with fossiliferous sedimentary rocks (Smith et al., 1984; Thomson et al., 1986; Read and Psutka, 1990). Part of the section measured on the west-facing mountainside is shown in Fig. 5.4D; further details warranted on the merit of its ammonite record that spans the Pliensbachian/Toarcian boundary will be published elsewhere. Out of three sampled volcanic units here, only the stratigraphically highest yielded interpretable U-Pb results. Sample 7 was taken from a 1.5 m thick, felsic, nonwelded, waterlain, feldspar-phyric crystal tuff. The sampled interval lies about 8 m above a 3-4 m thick, remarkably fossiliferous bioclastic limestone bed (Fig. 5.4D). Earlier collections by Read and Psutka (1990, G S C Loc. C-90698) yielded Dactylioceras, identified either as D. cf. commune (Jakobs et al., 1994) or D. cf. pseudocommune, D. cf. simplex, and D. cf. kanense (Jakobs, 1992). Our new collection and its evaluation confirm the latter identifications and the concomitant age assignment to the earliest Toarcian Kanense Zone. It is further  87  supported by the finding of Lioceratoides propinquum, a guide fossil of the Pliensbacian/Toarcian transition in North America, at a lower level. Fossiliferous sedimentary intercalations become sparse within a thick sequence of andesitic volcanic rocks that overlie the dated tuff. Two fossil collections are used to provide an upper age bracket. One contains bivalves and an ammonite originally identified as  Polyplectus sp., on which a Middle Toarcian age  assignment was based (H.W. Tipper, pers. comm., 1996). The other one, a new collection farther upsection, yielded  Dactylioceras sp., whose range is mutually exclusive with Polyplectus. Assuming no structural repetition, this finding suggests an Early to early MiddleToarcian (Kanense to Planulata Zone) age for the entire succession, that can be reconciled with the first collection if its diagnostic ammonite is re-interpreted as an indeterminate harpoceratid. Sample 7 yielded abundant, clear, equant to elongate zircons that are relatively coarse for volcanic rocks and have a high U content (up to 850 ppm). Two unabraded fractions (A and B) and two abraded fractions (C and D) mutually overlap (Fig. 5.3G). The slight discordance and younger  Pb/  U age of the unabraded fractions indicate  that only mild Pb loss affected these zircons. Fraction D touches the concordia curve at a  2 0 6  180.4±0.4 Ma. Fraction C is slightly discordant but overlaps fraction D, which allow their 206  regarded as an estimate of the unit's age. Despite the small difference in  Pb/  206  2 3 8  Pb/  U age of  238  U ages to be  238  Pb/  U ages of abraded and unabraded  fractions, all four fractions fit on a well-defined discordia line that intercepts concordia at an age of 188.4±2.5 Ma. This is significantly older than the  2 0 6  Pb/  2 3 8  U age of the nearly concordant fraction D. The dilemma of this age 206  interpretation is therefore to reconcile these two ages. Our solution is to take the error to the weighted mean  207  Pb/  206  238  Pb/  U age of D and extend its  P b age, yielding a 180.4+8.0/-0.4 Ma age for the tuff.  Sample 8. Tuff layer in Smithers Formation, Diagonal Mountain, McConnell Creek map area  A thick succession of colour-banded fine elastics of the Hazelton Group near Diagonal Mountain resembles and is partly correlative with units known from elsewhere in the Hazelton Trough, including the Yuen Member of the Smithers Formation (Tipper and Richards, 1976) and the Quock Formation (Thomson et al., 1986). The geology of the area is discussed by Evenchick and Porter (1993). Ammonite biostratigraphy of the Diagonal Mountain area is subject of an on-going study by G. Jakobs who carried out extensive field work and detailed collecting from measured sections (Jakobs, 1993). The dated zircon samples are calibrated to her sections and we use her unpublished data complemented by our own results from collections made in the immediate vicinity of U Pb samples. Publication of the ammonite biostratigraphy and taxonomy is forthcoming by G. Jakobs.  88  Sample 8 was collected from a weathered and altered tuff layer within the thick succession of colour-banded fine elastics (informally known as "pyjama beds") of the Hazelton Group 4 km SSW from the peak of Diagonal Mtn. A richly fossiliferous bed some 25 m below the dated tuff yielded Yakounia cf. silvae (Jakobs, pers. comm., 1995) and Pleydellia sp., clearly indicating the uppermost Toarcian Yakounensis Zone. From approximately 8 m above the sampled tuff, sonninid ammonites were recovered (Jakobs, pers. comm., 1995). Similar, poorly preserved sonninids were also found in the talus. While the poor preservation precludes a precise identification, an Early Bajocian age is suggested. The biochronologic age of the sampled tuff layer is thus bracketed between the latest Toarcian and Early Bajocian. Sample 8 yielded a small number of subequant to stubby prismatic, euhedral to subhedral zircon crystals. It was split into three small fractions (A-C) and one larger fraction (D) that were all strongly abraded. Results from the smallest fraction (B) are reversely discordant, analytically inferior, and not considered further. Fractions A , C, and D define a discordia line with good fit (MSWD=0.03) that defines an upper intercept at 179.8 M a (Fig. 5.3H). The discordance is attributed to Pb loss, a common phenomenon in the Diagonal Mtn. samples. Our best estimate of the unit's age is 179.8±6.3 Ma, the weighted mean  207  Pb/  206  P b age of fractions A , C, and D.  5.4.4. Aalenian  Sample 9. Rhyolitic tuff in Smithers Formation, Troitsa Ridge, Whitesail Lake map area  A ridge west of Troitsa Peak, Whitesail Lake map area, is the only known section in the Canadian Cordillera where a succession of all three Aalenian ammonite zones is present (Poulton and Tipper, 1991). The upper part of the Hazelton Group (Smithers Formation) exposed here contains minor felsic volcanic units interbedded in a predominantly sedimentary sequence (Fig. 5.4B). A description of the general stratigraphy of the area is given by Diakow and Mihalynuk (1987). Our attempt to isotopically date a volcanic unit from all three Aalenian ammonite zones only gave meaningful results, albeit of poor quality, for the highest zone. Sample 9 is collected from an 8 m thick, soft, well-bedded, felsic, waterlain crystal-lithic tuff interbedded with brown, fossiliferous siltstone and finegrained sandstone. Ammonites found immediately below include Erycitoides howelli and E. cf. teres whereas E. cf spinosum and E. sp occur some 25 m above. These levels also fall into the vertical range of Tmetoceras scissum and T. kirki. These assemblages unequivocally establish the late Aalenian Howelli Zone as the age of the tuff.  89  Sample 9, 25 kg in weight, yielded a small amount of fine-grained, clear, prismatic to needle-like zircons. It was subdivided into four fractions, of which only one (fraction A) was abraded. This fraction is only slightly discordant whereas the three unabraded and very fine-grained fractions exhibit significant Pb loss (Fig. 5.31). The 207  Pb/  206  P b ages of fractions B and C suggest a nearly complete resetting in-Cretaceous time, presumably due to the  emplacement of nearby plutons of the Coast Plutonic Complex. The  207  Pb/  206  P b ages of fractions A and D,  however, are within error and allow the calculation of a weighted mean age of 174.8±16.0 Ma which can be further refined from the  Pb/  U age of fraction A as a minimum age to derive a best age estimate of 174.8+16.0/—12.4  Ma.  5.4.5. Bajocian Sample 10. Tuff layer in Smithers Formation, Diagonal Mountain, McConnell Creek map area  On a ridge 4.5 km SS W of Diagonal Mtn., color-banded fine elastics interbedded with rare tuff layers are exposed. These strata, informally referred to as "pyjama beds", are assigned to the upper part of the Hazelton Group. (Refer to Sample 8 above for details of the geology of the area.) A fossiliferous bed some 30 m below the  Leptosphinctes (Prorsisphinctes)  zircon sample was first located by G. Jakobs who identified  cliffensis,  and  Stephanoceras  cf.  meseres, L. cf.  sp. (Jakobs, pers. comm., 1996). Elsewhere in the section, in which ammonites occur  only sparsely, poorly preserved sonninids and stephanoceratids were found. The age of the sampled tuff layer is early Late Bajocian based on the co-occurrence of perisphinctids and stephanoceratids, characteristic of the Rotundum Zone (Hall and Westermann, 1980). Sample 10 was collected from a 5 cm thick, blue-grey, clay-rich tuff layer. Only a small amount of zircon, comprising mostly fine, subequant to stubby prismtic crystals, was recovered from a 25 kg sample. Four very small fractions were analyzed. A and D were strongly abraded whereas B and C were only moderately abraded. As in the 206  other samples from the Diagonal Mountain area, Pb loss is evidenced by the younger  238  Pb/  U ages of the less  abraded fractions (Fig. 5.3J). Due to the small size of the fractions and the relatively large amount of common Pb present, the  206  Pb/  204  P b ratio is less than 600 and all analyses are rather imprecise. The 207  error, mainly because of their large individual errors. The 207  agreement at about 170 Ma. The  iU,  207  Pb/  206  P b ages are within  206  Pbr°Pb  ages of fractions A and D are in good  206  Pb/  Pb age of fraction C is somewhat older, and that of B is significantly  older, perhaps indicating minor inheritance. A three-point regression (involving fractions A , C , and D) yielded a 177.7±11.3 Ma upper intercept age, whereas a line fit through only fractions A and D yielded 170.5±15.8 Ma. 90  Fraction D is concordant with a  2 0 6  Pb/  2 3 8  U age of 167.2±0.4 Ma. We combine the  fraction with the more conservative three-point weighted mean  207  Pb/  206  2 U 6  Pb/  2 J 8  U age of the concordant  P b age to assign a best age estimate of  167.2+10.5/-0.4 Ma.  5.4.6. Bathonian Sample 11. Reworked tuff in Ashman Formation, McDonell Lake, Smithers map area  At the east end of McDonell Lake, a logging roadcut exposes a short section of tuffaceous sandstone of the Ashman Formation (Fig. SAC). The locality is about 20 km SE of Ashman Ridge, the type section of the formation (Tipper and Richards, 1976). Outcrops of Bajocian to Oxfordian fossiliferous strata are disrupted by block faults in the vicinity of McDonell Lake (Frebold and Tipper, 1973). Within the sequence of well-bedded, fossiliferous, green to buff, lithic sandstone, a 2 m thick interval was identified as waterlain, strongly reworked tuff or tuffaceous sandstone, based on the abundance of feldspar and the presence of glass shards. Two samples, (Field No. 95JP78 and 95JP79) were taken from adjacent beds that have no demonstrable difference in biochronologic age.  Iniskinites  sp. and perisphinctids (PI. 5.1, Figs. 6, 9) occur both immediately below and above the isotopically  dated tuff. Ammonites recovered from above the tuff include  Kepplerites  ex gr.  tychonis  (PI. 5.1, Fig. 5),  Kepplerites sp. (PI. 5.1, Fig. 1), Parareineckeial sp. (PI. 5.2, Fig. 4), Lilloettia cf. lilloetensis (PI. 5.2, Fig. 10), and Xenocephalites cf. vicarius  (PI. 5.2, Fig. 6). The assemblage best agrees with Fauna B6 of Callomon (1984).  Perhaps the most stratigraphically diagnostic form among them is K. ex gr.  tychonis that allows  correlation with the  basal Upper Bathonian where the genus first appears, represented by similarly finely ribbed forms throughout its area (Callomon, 1984). occurrence with  Parareineckeia was thought  Iniskinites, Lilloettia,  and  to be restricted to the Bajocian (Callomon, 1984) but its co-  Kepplerites  suggests that it ranges up to the Bathonian.  Approximately 25 kg samples from sites 95JP78 and 95JP79 yielded enough zircon to analyze three and five fractions, respectively. To better resolve the age of the rock, another 25 kg sample from site 95JP78 was processed and three additional fractions were selected and analyzed based on the experience gained from the earlier analyses. Analytical results from the two samples are combined in Sample 11 as they represent adjacent and essentially coeval beds.  91  EXPLANATION OF PLATES 5.1 AND 5.2 (on following two pages) All figures are 95% of natural size. Figured specimens are housed in the type collection of the Geological Survey of Canada (GSC). For each specimen the type number is followed by the field number of locality (referred to in text and figures) and the G S C locality number.  Plate 5.1. (On p. 93) Late Bathonian ammonoids of the McDonell Lake and Zymoetz (Copper) River sections.  Fig. 1.  Kepplerites sp., McDonell Lake, G S C 111520, 95JP82,  Fig. 2.  Perisphinctid sp. A , Zymoetz (Copper) River, G S C 111521, 95JP83, G S C Loc. C-176588.  Fig. 3.  "Choffatia"  sp., Zymoetz (Copper) River, G S C 111522, 95JP85, G S C Loc. C-176590.  Fig. 4.  "Choffatia"  sp., Zymoetz (Copper) River, G S C 111523, 95JP87, G S C Loc. C-176592.  Fig. 5  Kepplerites ex  gr.  tychonis  G S C Loc. C-176587.  (Ravn), McDonell Lake, G S C 111524, 95JP82, G S C Loc. C-  176587. Fig. 6.  Perisphinctid sp. B, McDonell Lake, G S C 111525, 95JP81, G S C Loc. C-176586.  Figs. 7-8.  Kepplerites  Fig. 9.  Perisphinctid sp. B, McDonell Lake, G S C 111527, 95JP81, G S C Loc. C-176586.  Fig. 10.  Kepplerites ex gr. loganianus (Whiteaves), Zymoetz  s p , Zymoetz (Copper) River, G S C 111526, 95JP87, G S C Loc. C-176592.  (Copper) River, G S C 1 1 1528,  95JP87, G S C Loc. C-176592.  Plate 5.2.  (On p. 94.) Late Bathonian ammonoids of the McDonell Lake and Zymoetz (Copper) •  River sections.  Fig. 1.  Iniskinites cf. martini (Imlay), Zymoetz  (Copper) River, GSC 111529, 95JP83, GSC Loc.  C-176588. Figs. 2-3.  Xenocephalites cf. vicarius  Imlay, Zymoetz (Copper) River, G S C 111530, 95JP87, G S C  Loc. C-176592. Fig. 4.  Parareineckeial  Fig. 5  Iniskinites  or  s p , McDonell Lake, GSC 111531, 95JP82, G S C Loc. C-176587.  Lilloettia sp. (inner whorls),  McDonell Lake, G S C 1 1 1532, 95JP81, G S C  Loc. C-176586. Fig. 6.  Xenocephalites  cf.  vicarius  Imlay, McDonell Lake, G S C 1 1 1533, 95JP81, G S C Loc. C-  176586. Fig. 7.  Cadoceras cf. moffiti Imlay, Zymoetz (Copper) River, G S C 111534, 95JP83, G S C Loc. C176588.  Fig.8.  Lilloettia cf. lilloetensis  Crickmay, Zymoetz (Copper) River, GSC 1 1 1535, 95JP86, G S C  Loc. C-176591. Fig.9.  Iniskinites  or  Lilloettia  sp. (inner whorls), McDonell Lake, GSC 1 11536, 95JP81, G S C  Loc. C-176586. Fig. 10.  Lilloettia cf. lilloetensis  Crickmay, McDonell Lake, GSC 1 11537, 95JP81, G S C Loc. C-  176586.  92  Plate 5.1  Plate 5.2  Zircon populations from the two samples were homogenous, dominated by morphologically simple, euhedral crystals ranging in aspect ratio from subequant to elongate, needle-like grains. Apart from fraction E, all other fractions were moderately to strongly abraded. The discordance of unabraded fraction E demonstrates that minor Pb loss affected the zircons (Fig. 5.3K). Fraction A is an inferior analysis with an unusually high amount of common Pb and is not considered further. The other fractions form a cluster centered around the most concordant fraction G that yielded a  2 0 6  Pb/  2 3 8  U age of 158.2±0.4 Ma. It is within error with the  2 0 6  Pb/  2 3 8  U ages of fractions F  and H that together with G , were also subjected to the greatest amount of abrasion. Slight discordance and reverse 206  discordance of other fractions with similar  238  Pb/  ^07  U ages suggest that the measurement of"  235  Pb/  U ratio in small  fractions is less reliable. To account for the possibility of any surface-correlated Pb loss not completely removed by abrasion, we extend the error range of the  2 0 6  Pb/  2 3 8  U age of fraction G to include the oldest apparent  2 0 6  Pb/  2 3 8  U age  of fraction D which suggests an interpreted age of 158.2+1.9/—0.4 Ma.  Sample 12. Tuff layer in Smithers Formation, Diagonal Mountain, McConnell Creek map area  Sample 12 was collected from a 10 to 20 cm thick, cream-colored tuff layer interbedded in a thick succession of dark mudstone of the Ashman Formation approx. 2.4 km SW of Diagonal Mountain. (Refer to Sample 8 above for the geology of the area.) Ashman Formation strata are tightly folded in the sampling area, rendering stratigraphic observations difficult. collections yielded 1996).  Iniskinites  Iniskinites  Iniskinites  cf.  robustus  sp. was collected both below and above the zircon sample. Earlier  below and Adabofoloceras or  Lilloettia above  (G. Jakobs, pers. comm.,  is taken to indicate the Late Bathonian as the most likely age of the sample although precise  correlation around the Bathonian/Callovian transition is difficult (Callomon, 1984; Poulton et al., 1994). This sample yielded a small amount of fine-grained zircon. Only two fractions yielded usable analyses. 207  Fraction A is discordant with an old  206  Pb/  Pb age suggesting inheritance whereas fraction C lies near the  concordia curve (Fig. 5.3L). A two point discordia line yields a lower intercept age of 158.4±0.8 Ma and an upper intercept age of 1.33±0.14 Ga. As the experience with the other samples from Diagonal Mountain (Sample 8 and 10) demonstrated, the possibility of Pb loss not removed by moderate abrasion of fraction C cannot be ruled out. Thus the lower intercept age is interpreted as a minimum estimate of the unit's age. The maximum estimated age is determined by the  207  Pb/  206  P b age of this fraction, 175.2±8.2 Ma.  95  Sample 13. Reworked tuff layer in Ashman Formation, Zymoetz River, Smithers map area  Richly fossiliferous shallow marine sandstone of the Ashman Formation is exposed along a new logging road 2.5 km northeast of the confluence of Limonite Creek and Zymoetz (Copper) River. According to Tipper and Richards (1976), this locality lies close to the southern limit of Bathonian/Callovian marine deposits in the Bowser basin. Another section at Tenas Creek occupies an analogous stratigraphic and paleogeographic position and its stratigraphy and ammonite fauna is discussed in detail by Tipper and Richards (1976) and Frebold (1978), respectively. Only one fossil collection from Zymoetz River is mentioned by Frebold (1978) and that outcrop of the "Copper River fossil beds" was described in detail by Jakobs (1986).The new roadcut provides a fresh outcrop of the fossiliferous marine sedimentary rocks that overlie (presumably with discomformity) maroon tuffs that likely belong the Red Tuff Member of the Nilkitkwa Formation. An 8 cm thick, pale blue, tuffaceous, clay-rich layer containing rare marine fossils was identified as a reworked volcanic ash. It occurs 8.5 m above the base of the sandstone and it was sampled for U-Pb dating. Figure 5.4F shows the measured section and the vertical distribution of ammonoids. The assemblage undoubtedly indicates the Bathonian/Callovian transition. In the framework of North American faunal successions given by Callomon (1984), the assamblage recovered immediately below the tuff, including Cadoceras cf."moffiti (PI. 5.2, Fig. 7),  Iniskinites cf. martini (PI. 5.2, Fig. 1), and Xenocephalites cf. vicarius, clearly corresponds to  Fauna B7(a) that is correlated with the Late Bathonian. Among others,  "Choffatia" sp. (PI. 5.1, Fig. 3, 4),  Kepplerites ex gr. loganianus (PI. 5.1, Fig. 10), K. sp. (PI. 5.1, Fig. 7-8), and Lilloettia cf. lilloetensis (PI. 5.2, Fig. 8) were collected from above the tuff layer. These can be accommodated in either the latest Bathonian or the Early Callovian (Faunas B7-B8 of Callomon (1984), see also Poulton et al. (1994)). Recent studies in Mexico (Sandoval et a l , 1990) and South America (Riccardi et a l , 1991 and references therein) suggest that Lilloettia, represented by species closely similar to  L. lilloetensis, is most abundant in the latest Bathonian Steinmanni Zone and ranges up to  the basal Callovian Vergarensis Zone. The South American biochronology employs common taxa with the Mediterranean province for correlation with the northwest European standard zonation. The Boreal genera  Cadoceras and Kepplerites offer another means of correlation via Alaska, northern Canada, and Greenland, confirming the Late Bathonian-Early Callovian age. An approximately 40 kg sample (Sample 13) yielded less than 1 mg of fine-grained zircon. The grains were clear, colourless, containing only a few small, clear inclusions, and ranging in morphology from subequant to elongate with a majority of stubby prismatic crystals. The available material was split into two very small fractions. Fraction A marginally intersects concordia whereas the analysis of fraction B strongly indicates the presence of 96  inherited zircon component (Fig. 5.3M). A line drawn through the two points yields a poorly constrained Proterozoic upper intercept age of 1.30+0.69/-0.55 Ga. The lower intercept at 162.6+2.9/-7.0 Ma is interpreted as the best estimate of the age of the tuff. It should be noted, however, that the relatively imprecise analyses of small 207  amounts of zircon are sensitive to the blank Pb correction which strongly affects the apparent 207  Pb/  206  235  Pb/  U and  P b ages. Moreover, the slight discordance of fraction A could alternatively be explained by minor Pb loss  not eliminated by the strong abrasion. More analyses, requiring re-sampling of an even larger amount of rock, would be needed to better resolve the age of the tuff layer.  5.5.  DISCUSSION  Several recent studies demonstrate that U-Pb dating of volcaniclastic rocks intercalated with fossiliferous strata offers one of the best techniques for time scale calibration (Tucker and McKerrow, 1995; Mundil et al., 1996). Data presented here underscores that volcanic arc terranes provide a suitable geological setting for such studies. In volcanosedimentary sequences, the ratio of volcaniclastic and marine sedimentary rocks is a function of proximity to the eruptive centre. We found that integrated isotopic and biochronologic dating can be used successfully in moderately proximal sections (as in Samples 4, 5, 6, 7, and 11, see also Samples 2 and 3 for pitfalls) where of substantial volcaniclastic units and ammonite-bearing sedimentary rocks are equally abundant. Dating of thin ash beds deposited in a distal setting is promising as there is often a continuous fossil record for zonal biochronology. However, U-Pb dating is often plagued by low zircon yields (see Samples 1,8, 10, 12, and 13). In these cases the capability of analyzing single grains, not attempted in this study, is essential to produce sufficiently precise ages (Mundil et al., 1996; Palfy et al., 1997). Interpretation of U-Pb ages has often proved difficult due to complex isotopic systematics characterized by Pb loss and inheritance. Pb loss was most pronounced in samples in proximity to the Coast Plutonic Complex and in the thin ash layers. Inheritance patterns in Early and Middle Jurassic samples from Stikinia reveal the presence of slightly older zircon components in Sample 2 and strong Proterozoic (approx. 1.3 Ga) inheritance in two upper Middle Jurassic samples (12 and 13). In the first case the minor inherited component may have derived from underlying volcanic strata of the Upper Triassic Stuhini Group or its comagmatic plutons (Thorkelson et al., 1995). Our finding of Proterozoic inheritance is compatible with most tectonic models that suggest accretion of Stikinia to North America by late Middle Jurassic time. We did not find any Proterozoic component in older Hazelton arc volcanic rocks.  97  A measure of reliability of our data is their internal consistency whereby isotopic and biochronologic age relationships are in agreement. The two most precise isotopic dates (samples 5 andl 1) have less than 1.5% 2c> error ranges. These new calibration points far exceed the quality of those in most recent time scales (Harland et a l , 1990; Gradstein et a l , 1994). The next class comprises samples 4, 6, 7, and 10 where a relatively large positive error results from the conservative use of the weighted mean  207  Pb/  206  P b ages to complement the age of a concordant  fraction if that could not be duplicated. The 2a error range in these cases is 3 to 7%. Comparable precision is 207  obtained using a weighted mean  206  Pb/  Pb age for Sample 8 and a lower intercept age for Sample 13. These six  items meet or exceed the average precision of isotopic dates used in previous time scales in addition to being superior to them in their zonal biochronologic constraints. Four of the remaining five isotopic dates reported here are minimum ages only (Samples 1, 2, 3, and 12) and another one is of poor precision (Sample 9). However, on the strength of their precise stratigraphic age, they also provide useful constraints for the time scale. The Early and Middle Jurassic portion of the time scales of Harland et al. (1990) and Gradstein et al. (1994), the ones that employ the most comprehensive isotopic dataset, are based on 25 and 30 ages, respectively. The 13 new dates reported here along with many more obtained recently by other Cordilleran workers therefore represents a substantial addition to the database. A revision of the Jurassic time scale is presented in Chapter 7.  98  CHAPTER 6  U-Pb DATING THE TRIASSIC-JURASSIC MASS EXTINCTION BOUNDARY  6.1. INTRODUCTION  The Triassic-Jurassic (T-J) boundary is marked by one of the five biggest mass extinction events (Raup and Sepkoski, 1982) but it is, perhaps, the most poorly understood (Hallam, 1990). In addition to the widely debated causation, its timing is also poorly constrained. T - J boundary age estimates in widely used time scales vary between 210±3.5 (Haq et al., 1988) and 203±3 Ma (Odin, 1994). Such disagreement is attributed to the paucity of calibration points. Precise U-Pb data near the boundary exist only from basalts of the continental Newark Supergroup (Dunning and Hodych, 1990; Hodych and Dunning, 1992). This was used to postulate the boundary at 201-202 Ma (Olsen et al., 1996) but correlation of marine and continental sequences has not been independently and unequivocally demonstrated (Hallam, 1990). The quandary is avoided if precise isotopic dating is applied in conjunction with biochronology of marine fossils that allow unambiguous correlation with the T - J system boundary. Here we present such results from the Queen Charlotte Islands, western Canada, where a new U-Pb date was obtained from immediately below the boundary. This date is pooled with 12 other recently obtained and pertinent U-Pb dates to provide a tightly constrained estimate of the age of the T - J boundary. The updated time frame help focus discussions concerning the end-Triassic mass extinction event and the spacing of mass extinctions.  99  6.2.  A NEW U-Pb A G E NEAR THE T - J BOUNDARY IN T H E QUEEN C H A R L O T T E ISLANDS  The preferred method for dating a stratigraphic boundary is to locate isotopically datable volcanic or volcaniclastic rocks interbedded in a continuous and fossiliferous section across the boundary. The Sandilands Formation in the Queen Charlotte Islands is a sequence of thin-bedded shale and siltstone (Cameron and Tipper, 1985) that ranges in age from the Norian to the early Pliensbachian (Desrochers and Orchard, 1991; Palfy et a l , 1994). It records an apparently uninterrupted T - J transition exposed at two locales: Kunga Island and Kennecott  100  Point (Tipper et a l , 1994) (Fig. 6.1). Light-colored layers causing a characteristic banded appearance were suggested of volcanic ash origin (Cameron and Tipper, 1985). Immediately below the T - J boundary on Kunga Island, a 3.5 cm thick tuff layer was located and sampled (Sample K M ) . The undulatory but sharp base and top, somewhat graded texture, and the presence of crystals and small mudstone rip-up clasts indicate a reworked ash bed. U-Pb zircon geochronometry was carried out in the Geochronology Laboratory of the University of British Columbia utilizing standard procedures in mineral separation, zircon chemistry, mass spectrometry, and data reduction (Mortensen et a l , 1995). U and Pb laboratory blanks were approximately 1 and 8 picograms, respectively. Zircons were subdivided into four fractions (Table 6.1). Fraction A is slightly reversely discordant, fractions B and C are slightly discordant and overlapping, while fraction D is squarely concordant (Fig. 6.2). The lack of additional concordant analyses hinders an unambiguous interpretation of the age. Some Pb loss is indicated by the 206  23S  206  195 to 200 M a range of calculated ™Pbr U ages. The clustering of  238  Pb/  U ages of three out of four fractions  around 198 and 200 Ma indicates that in these cases Pb loss was almost completely eliminated by the strong  0.032  Figure 6.2. U-Pb concordia diagram of zircons from a 3.5 cm thick tuff layer immediately below the T - J boundary, Sandilands Formation, Kunga Island. A homogenous population of ca. 0.5 mg zircon was recovered from a 10 kg sample. All grains were smaller than 104 urn, colorless, of excellent clarity without visible cores or zoning, and containing very rare inclusions only. Morphologies ranged continuously from anhedral and slightly resorbed, oval to elongate grains to euhedral, simple, stubby prismatic to elongate needle-like or tabular crystals. 101  "Wl  Sample / Fraction  1  mg  fj  Pb* »<>Pb> Pb 2  ppm ppm  2  2M  4  ""Pb  2  Pb  pg  %  Calculated ages (±2o,Ma) ' f  Isotopic ratios (±lo,%)  s  Mpb/^U  "Pb/^U  2ll7  Pb/ "'Pb 2,  2,  "'Pb/ "U 2,  2  ?b/ Pb  " Pb/ U 7  m  2,5  m  KI-1 A [74-104, N l , eh+sh, sp+t, s-a]  0.060  144  4  976  17  6.5  0.03072 (0.11)  0.2107 (0.30)  0.04974 (0.23)  195.1(0.4)  194.1(1.1)  182.9 (10.8)  B [74-104, Nl,sh+ah, ov+el, s-a]  0.058  165  5  2513  8  7.0  0.03127 (0.10)  C [<74, N2, sh+eh, sp+n, s-a]  0.090  166  5  3181  9  7.1  0.03125 (0.10)  0.2166(0.24)  0.05025 (0.18)  198.5 (0.4)  199.1(0.9)  206.7 (8.3)  0,2166 (0.20)  0.05029(0.14)  198.3 (0.4)  199.1(0.7) 208.3 (6.5)  D [74-104, N2, sh+eh, sp]  0.058  143  4  1639  10  6.4  0.03144 (0.10)  0.2172 (0.25)  0.05011(0.19)  199.6 (0.4)  199.6(0.9) 200.0 (8.7)  Table 6.1.  U-Pb zircon analytical data of sample KI-1, collected on the southeast shore of Kunga Island, 1:50 000 map area N T S 103B/13-14, U T M 327280E, 5848350N. ^ A l l zircon fractions. Listed in brackets: Grain size range in microns; Side slope of Franz magnetic separator (in degrees) at which grains are non-magnetic (N) or magnetic (M), using 20° front slope and 1.8A field strength; Grain character: eh=euhedral, sh=subhedral, ah=anhedral, n=needles, sp=stubby prismatic, t=tabular, el=elongate, ov=oval; Degree of air abrasion: s-a=strongly abraded.  2  Radio genie Pb  ^Measured ratio corrected for spike and Pb fractionation of 0.0036/amu± 20% (Daly collector). 4  Total common Pb in analysis based on blank isotopic composition  ^Radiogenic Pb ^Corrected for blank Pb, U, and initial common Pb based on the Stacey and Kramers (1975) model at 200 Ma.  abrasion. Therefore 199.6±0.4 Ma, the oldest  Pb/ U age given by the concordant fraction D, can be regarded as  the crystallization age of the sample. An alternative interpretation relies on the calculated  207  Pb/  206  P b ages that fall  between 183 and 208 Ma. If fraction A is dismissed due to its reverse discordance, the other three fractions yield a weighted mean  207  Pb/  206  P b age of 205.7±4.8 Ma.  The two interpretations disagree beyond their respective error. The weighted mean  207  206  Pb/ Pb age is regarded  208  as less reliable because in Jurassic zircons the abundance of  207  Pb is more than 20 times greater than that of Pb.  Consequently, the measurement of the latter is inherently less precise and the calculated  2 0 7  Pb/  2 3 5  U and  207  Pb/  206  Pb  ages are strongly dependent on the composition of blank and initial common Pb used in corrections. Also, the discordia line and its upper intercept are not well constrained due to the small spread of fractions B, C, and D. There are additional geological arguments to help interpret the U-Pb systematics. The thermal history of the Sandilands Formation on Kunga Island is determined from conodont alteration index values between 4.5 and 5.5, indicative of temperatures in the 245-515°C range (Orchard and Forster, 1991) that is compatible with the results of vitrinite reflectance studies (Vellutini and Bustin, 1991). The likely event responsible for heating is the emplacement of nearby Middle Jurassic plutons of the Burnaby Island Plutonic Suite that consists of high-level plutons with rapid cooling (Anderson and Reichenbach, 1991). The Sandilands Formation remained well below the 102  >800°C closure temperature of zircon (Haeman and Parrish, 1991) throughout its history thus any Pb loss should be attributed to recent processes. The low U content (140-170 ppm) and the excellent quality of zircons supports the interpretation that the surface-correlated Pb loss was mild and, in case of fraction D, completely removed by abrasion. To de-emphasize the apparent but not reproduced precision, we round up the age and error to assign 200± 1 Ma as the age of the tuff layer.  6.3.  BIOCHRONOLOGY OF T H E T - J BOUNDARY IN T H E QUEEN C H A R L O T T E ISLANDS  The T - J boundary sections on Kunga Island and Kennecott Point in the Queen Charlotte Islands are recognized through integrated biochronology using ammonites, radiolarians, and conodonts (Orchard, 1991a; Carter, 1993; Carter, 1994; Tipper et a l , 1994; Carter et a l , in press). A summary of these detailed studies help provide a biochronologic framework for the isotopically dated tuff. Sampled levels, their diagnostic fossils and zonal assignments are shown on Fig. 6.3. In the critical interval, radiolarian biochronology is the most complete. The tuff occurs very near the top of the latest Triassic  Globolaxtorum tozeri Zone (equivalent  of the ammonite  standard Crickmayi Zone), based on an abundant and diverse fauna (Carter, 1993). The last appearance of the zonal index (at site R4, Fig. 6.2) is 4 m above the tuff (total thickness of the zone is 26 m) whereas the next higher collection (R5) 2.5 m farther upsection yields an impoverished and markedly different assemblage with  Pantanellium tanuense,  clearly indicative of the Early Hettangian (Carter, 1994). Conodont faunas recovered from  C1-C5 are of low diversity but diagnostic to the Rhaetian. The highest collection, 4 m above the tuff, yielded  Neogondolella which ammonite  in the Kennecott Point section occurs slightly above  Choristoceras nobile,  Misikella posthernsteini and the  both guide fossils of the terminal Triassic (Orchard, 1991a; Tipper et a l , 1994).  No Late Triassic ammonoids have been found yet in the Kunga Island section. The oldest Jurassic ammonites recovered 18 m or more above the tuff (A1-A3, Fig. 6.2) include  Waehneroceras  and  Discamphiceras  early Middle Hettangian (Tipper in Carter et a l , in press). In summary, the tuff lies immediately below the T - J boundary defined by the disappearance of conodonts and a marked change in radiolarian faunas.  103  suggesting  g  to  CO CD  CU  100  A3 A2  -4  I-  o  03 CO  c  CD  CO CO CO CD  c  A1  CD  CD CD O .O  c  CO  CO  c  CD CD  90 - 4  -c  £ -c S . <u CD O c ^ .!2  9 - S Q  h  R5 R4, C5 R3  80 -I 2 0 0 ± 1 Ma  U-Pb R2, C4  o  c  CD N O  a co CD  CO CO (0  o  D> O CD  o  CL Q.  70  -Q  H  o  C3 C2  D  3  c to 0)  CD  or  Q.  E  CD C CD  O)  CD  60  R1, C1  H  a a  CO  o c o  Cn  R0 h  R00  50 m (from base)  Figure 6.3. The U-Pb dated tuff layer in the T - J boundary section on Kunga Island. Key radiolarian (R), conodont (C), and ammonite (A) taxa and section measurements are compiled from Carter (1993), Tipper et al. (1994), and Carter et al. (in press).  6.4.  OTHER U-Pb AGES AROUND T H E T - J BOUNDARY  Several other U-Pb dates help date the T - J boundary. Some of them (items 2 and 5 below) have been used in previous time scales (e.g., Harland et a l , 1990; Gradstein et a l , 1994) whereas items 1, 3, and 4 are used as calibration points for the first time. (1) Puale Bay, Alaska Peninsula: Three U-Pb ages from tuffs within a well-documented ammonite-bearing basal Jurassic section are reported in Chapter 4. A Middle Hettangian tuff layer is dated at 200.8+2.7/-2.8 Ma and Middle to Late Hettangian tuffs yield crystallization ages of 197.8+1.2/-0.4 Ma and 197.8±1.0 Ma. The significance of these calibration points is discussed in detail in Chapter 4 and not repeated here. 104  (2) Newark Supergroup: Three U-Pb dates for mafic, rift-related volcanic and hypabyssal rocks of eastern North America. Dunning and Hodych (1990) report a zircon and baddeleyite age of 200.9±1.0 Ma on the Palisades sill and a zircon age of 201.3± 1.0 Ma on the Gettysburg sill in the Newark Basin. The sills are thought to be feeders to the Orange Mountain basalt, the lowermost extrusive rocks which immediately overlies the purported T - J boundary based on palynological and vertebrate faunal evidence (Olsen et al., 1987; Fowell and Olsen, 1993). From the Fundy Basin, Hodych and Dunning (1992) obtained a U-Pb zircon age of 201.7+1.4/-1.1 Ma from the North Mountain basalt whose base lies some 20 m above the T - J boundary, also defined by palynology and vertebrate biostratigraphy (Olsen et al., 1987). (3) Goldslide intrusions, northwest British Columbia: The Biotite porphyry, the oldest of three phases of the mineralized Goldslide intrusions on Red Mountain near Stewart (Rhys et al., 1995), yielded a 201.8+0.5 Ma U-Pb date on zircon (Greig et al., 1995). Only this older phase is affected by pervasive cleavage related to a regional deformation event near the T - J boundary (Greig et al., 1995). A few tens of km to the north, the youngest deformed strata contain the Rhaetian ammonoid  Choristoceras  (Jakobs and Palfy, 1994). Triassic radiolaria  ranging in age from Ladinian-Carnian to Norian were recovered from strata cut by the Biotite porphyry (F. Cordey, pers. comm. 1996). Rhys et al. (1995) obtained a U-Pb zircon age of 197.6+1.9 Ma from the Goldslide porphyry, the youngest post-kinematic phase. A tuff 8 km to the south that is considered correlative to the top of the Red Mountain succession yielded a U-Pb age of 199+2 Ma (Greig and Gehrels, 1995). On Red Mountain, abundant peperitic structures and strongly disrupted country rocks with evidence of soft-sediment deformation suggest that at least some of the Goldslide intrusions intermingled with unlithified, wet sediments and are nearly coeval with their country rocks (Greig and Gehrels, 1995; Rhys et al., 1995). We recovered an indeterminate ammonite of Early Jurassic aspect along with the bivalve Oxytoma stratigraphically above the syn-sedimentary intrusions. In Europe, Oxytoma is known as low as the Upper Triassic but in the Eastern Pacific it first appears in the Lower Jurassic with common occurrences in the Hettangian (Aberhan, 1994). (4) Griffith Creek volcanics, northwest British Columbia: Thorkelson et al. (1995) obtained two U-Pb ages (205.8+0.9 and 205.8+1.5/-3.1 Ma) from a volcanic unit that is tightly folded and overlies a conglomerate containing Upper Triassic limestone pebbles and Norian clastic strata of the Stuhini Group. Folding predated deposition of the younger, Lower Jurassic Cold Fish volcanics and records another example of the regional deformational event near the T - J boundary (Thorkelson et al., 1995) discussed above.  105  (5) Guichon Creek batholith, south-central British Columbia: K - A r and Rb-Sr dates obtained from this composite intrusion influenced the placement of the T - J boundary in most time scales compiled since 1959. The current best estimate of the crystallization age is a U-Pb zircon age of 210±3 Ma (Mortimer et a l , 1990). The dated sample is not closely associated with fossiliferous strata but sedimentary intercalations within the Nicola Group that are crosscut by the batholith range up to the Middle Norian (Frebold and Tipper, 1969). A minimum age for the intrusion is no older than Pliensbachian based on ammonites from the onlapping Ashcroft Formation. Therefore the age of the Guichon Creek batholith is not considered to provide a first-rate calibration point.  6.5. T H E A G E OF T H E T - J BOUNDARY  The chronogram method (described in detail by Harland et a l , 1990) was used to provide an unbiased estimate for the age of the T - J boundary. The calculation is based on the eight U-Pb ages from above the boundary and five ages (including the newly reported one) from below as discussed earlier. The chronogram yields 200.9±0.6 Ma as the age of the boundary (Fig. 6.4). Because the new date reported here heavily controls the chronogram and may marginally err on the young side, we round this value to 201±1 Ma. This estimate is younger than most previous ones or lies at the young end of their error range. It is also more precise than any other preCretaceous boundary estimate.  CD  3  ro >  o  o LU  0 204  203  202  201  200  199  198  Trial age (Ma) Figure 6.4. Chronogram estimation of the T - J boundary age based on 13 U-Pb dates from the terminal Triassic and basal Jurassic (see text for a list and discussion). Previous estimates given by widely used time scales are 208±7 (Palmer, 1983), 210±3.5 (Haq et a l , 1988), 208±7.5 (Harland et a l , 1990), 203±3 (Odin, 1994), and 205.7±4.0 (Gradstein et a l , 1994). 106  6.6. TIMING T H E END-TRIASSIC MASS EXTINCTION  To establish synchroneity of extinctions in the marine and terrestrial realms is critical to assessing the endTriassic event. Correlation of a continental T - J boundary defined by palynomorphs and vertebrates (Olsen et al., 1987; Fowell and Olsen, 1993) with the marine, ammonite-based chronology is generally assumed but not infallible (Van Veen, 1995). The Kunga Island and Newark Supergroup U-Pb ages from immediately below the marine and above the continental boundary, respectively, agree within error. This provides strong independent evidence that the end-Triassic extinctions are indeed synchronous. This conclusion is implicit in our chronogram calculation which incorporates marine and terrestrial biochronologic ages at face value. Fixing the T - J boundary at 201±1 Ma implies that the Norian-Rhaetian interval may be longer than previously thought: The Cordilleran dates suggest a duration of at least 4 Ma or perhaps as much as 9 Ma. There are several fossil groups, mainly shelf dwellers such as ammonites, bivalves, and conodonts, whose severe endTriassic extinction was preceded by an apparently protracted decline in diversity and abundance (Hallam, 1996). Such trends are clear in the North American Cordillera (Newton, 1983; Orchard, 1991b; Tozer, 1994) although the role of the Signor-Lipps effect (Signor and Lipps, 1982) has not been thoroughly evaluated yet. On the other hand, marine microplankton (radiolarians and calcareous nannofossils) shows an abrupt faunal turnover at the boundary (Carter, 1994; Bown, 1996), similar to the continental record of vertebrates and palynomorphs (Olsen et al., 1987; Fowell and Olsen, 1993). This duality in temporal extinction patterns, emphasized by the relative length of the "twilight zone" for the shelf-dwelling groups, may imply a complex interplay of causal agents in the extiction. Parallel biological responses to a "press event" and a "pulse event" would suggest that a long-term environmental crisis selectively affecting shelf habitats was punctuated by a geologically rapid event that decimated the already stressed taxa and marine planktic and terrestrial ecosystems alike.  6.7. SPACING OF MASS EXTINCTIONS  The controversial hypothesis of a 26 Ma periodicity in recurring mass extinction events claimed by Raup and Sepkoski (1984) is often criticized on the basis of inadequate time scales (Hoffman, 1985; Heisler and Tremaine, 1989). A n update of the time series analysis using the time scale of Harland et al. (1990) eliminates periodicity in  107  some Jurassic extinction peaks while confidence in remaining periodicities is low due to the large uncertainties of Triassic and Jurassic time scale (Sepkoski, 1996). Based on our estimates, the age of the end-Triassic mass extinction is 201±1 Ma. The preceding extinction peak is at the end-Permian. Tuff layers at the Permian-Triassic boundary in southern China are A r - A r dated at 4 0  3 9  249.9±1.5 and 250.0±1.6 M a (Renne et a l , 1995) and U-Pb dated at 251.2±3.4 Ma (Claoue-Long et a l , 1991) that suggest ca. 250 Ma as the boundary age. The estimated interval between the P-T and T - J events is 49 Ma, only slightly shorter than the double phase length (52 Ma) predicted by the periodic extinction model. The next extinction peak in the Jurassic, originally recognized in the Pliensbachian (Raup and Sepkoski, 1984), is now interpreted as a protracted extinction event that culminated in the Early Toarcian (Little and Benton, 1995). In the Canadian Cordillera, ammonite biochronologically controlled U-Pb dates bracket the interval of elevated extinction rate from 186±1 (Johannson and McNicoll, in press) and 184.7±0.5 (Maclntyre et a l , 1997) (Late Pliensbachian) to 181.4±1.2 Ma (Palfy et a l , 1997) (mid-Toarcian). Therefore the next extinction event was over no more than 20 Ma after the T - J event, at odds with the predictions of the 26 Ma periodicity.  6.8. CONCLUSIONS  The end-Triassic mass extinction is one of the five most important Phanerozoic mass extinction events. Deciphering the dynamics of this mass extinction requires an accurate time frame but available age estimates of the Triassic-Jurassic system boundary in various time scales are conflicting and imprecise. A new U-Pb zircon age of 200±1 Ma from a tuff layer immediately below the Triassic-Jurassic boundary in the Queen Charlotte Islands, western Canada, is well-constrained by integrated biochronology of radiolarians, conodonts, and ammonites in an apparently continuous, marine succession. The new date, pooled with 12 other U-Pb ages from uppermost Triassic and basal Jurassic rocks, is used to construct a chronogram that yields 201±1 Ma for the age of the TriassicJurassic boundary, several million years younger than suggested by most time scales. The close agreement of isotopic ages from marine (Queen Charlotte Islands) and continental (Newark basin) boundary sections suggests that extinctions in the marine and terrestrial realms were synchronous. The newly recognized extended duration of the terminal Triassic emphasizes the duality of extinction dynamics: Decline of some shelf-dwelling fossil groups (ammonoids, conodonts, and bivalves) which were decimated by the mass extinction event may have occurred over a few million years interval, suggesting a protracted environmental crisis whereas the marine microplankton (e.g.,  108  radiolarians) and terrestrial vertebrates and plants show an abrupt turnover at the boundary. Recent advances in dating the Permian-Triassic boundary and the Early Jurassic permit re-evaluation of the spacing of the end-Triassic and neighbouring mass extinction events. The interval between the end-Permian and end-Triassic events is close to two phase lengths as predicted by the 26 Ma periodicity model. However, the Late Pliensbachian-Early Toarcian event followed less than 20 Ma after the end-Triassic crisis.  109  CHAPTER 7  A U-Pb AND A r - A r TIME SCALE FOR THE JURASSIC 40  7.1.  39  INTRODUCTION  Geochronologic or time scales express the estimated numerical ages of chronostratigraphic units in millions of years. Jurassic chronostratigraphic units are defined on ammonite biochronology based on the well-established zonal standard from northwest Europe. In contrast to the high resolution of biochronology, the Jurassic time scale remained less well-calibrated than most other periods. Conflicting and imprecise estimates of the Jurassic time scales derive from the scarcity of biochronologically well-constrained isotopic ages and the preponderance of lowtemperature K - A r and Rb-Sr dates of poor accuracy and precision (Palfy, 1995). In the North American Cordillera, systematic effort was made to generate new calibration points by integrating ammonite biochronology of marine sediments and U-Pb zircon dating of interbedded volcanic or volcaniclastic rocks (Palfy et al., 1995). The result is 18 new calibration points (Palfy et al., 1997, and Chapters 3-6). Additional radiometric ages, also useful for time scale calibration, are reported in other recent studies. The revised Jurassic time scale presented here differs from the previous because it (1) is more selective in database compilation by employing high precision U Pb and A r - A r ages only; (2) uses zonal level biochronology and attempts to estimate chron boundary ages; (3) 4 0  3 9  rejects scaling based on the assumption of equal duration of biochronologic units and minimizes the use of interpolation. The proposed time scale is compared with previous ones, of which the most frequently cited scales are abbreviated as follows: N D S (Odin, 1982), D N A G (Palmer, 1983; Kent and Gradstein, 1985), EX88 (Haq et a l , 1988), GTS89 (Harland et al., 1990), OD94 (Harland et al., 1990), and MTS94 (Gradstein et al., 1994; Gradstein et al., 1995).  110  7.2.  METHODS  The revised Jurassic time scale is the result of combining methods that were successfully employed in previous work with new approaches made possible by recent advances in geochronometry and biochronology. Typically, previous time scale calibrations have utilized three different aproaches (Odin, 1994): (1) manual construction that considers each relevant isotopic date individually and weighs them subjectively to arrive at best boundary estimates (as in OD94); (2) statistically-oriented methods that treat each accepted date equally and derive the boundary estimates mathematically (as in GTS89 and MTS94); and (3) reliance on a select set of a few dates judged most reliable and determination of the intervening boundaries by interpolation assuming either an equal duration of biochronologic units or a constant spreading rate deduced from oceanic magnetic anomalies (e.g., EXX88). Some combination of the above is more common in recent works (GTS89, MTS94). Manual construction lacks rigourous methodology and reproducibility but offers flexibility. This method would suffice if each boundary had available isotopic age constraints, a situation not yet attainable for the Jurassic. Statistical methods offer the advantage of handling large numbers of dates efficiently and producing reproducible results. Two methods are tested and available: the chronogram method (Harland et a l , 1990) and the maximum likelihood method (Agterberg, 1988). They are similar in assuming a random distribution of isotopic ages and their results converge for densely sampled intervals (Agterberg, 1988). We prefer to use the semi-rigorous chronogram method as it can accommodate, after a slight modification, isotopic ages with asymmetric error that are common among the U-Pb ages. The maximum likelihood method, although elegant, statistically solid and rigorous in error propagation, is too restrictive in requiring Gaussian errors for the source data. In previous time scales, interpolation arose from the sparseness of available isotopic ages. The combination of magnetochronology and biochronology provides a powerful tool that allows the interpolation of boundaries if some magnetochrons are directly dated isotopically and a constant spreading rate is assumed during the formation of oceanic magnetic anomalies (Hailwood, 1989). This assumption appears to be tenable for the Late Jurassic (Channell et a l , 1995). As no oceanic crust older than Callovian is preserved, the method is not applicable for the Early and Middle Jurassic. In that interval, most scales use some form of biochronologically-based interpolation, assuming equal duration of chrons or subchrons (Harland et a l , 1990; Gradstein et a l , 1994). This method is inadequate based on three independent lines of evidence which suggest widely disparate durations for Mesozoic ammonite zones: (1) direct high-resolution isotopic dating of Triassic (Mundil et a l , 1996) and Cretaceous (Obradovich, 1993) ammonite zones; (2) Milankovitch cyclostratigraphy of Jurassic (Smith, 1990) and Cretaceous  111  (Gale, 1995) ammonite zones; and (3) the width of oceanic magnetic anomalies calibrated to Late Jurassic ammonite zones (Ogg et al., 1991; Ogg and Gutowski, 1996). The need for interpolation is eliminated if reliable isotopic ages are available from each zone or stage. The most promising new development in time scale studies is the use of high-precision U-Pb and A r - A r dating on 4 0  3 9  volcanic flows and volcaniclastic layers from biochronologically well-dated sections (e.g., Obradovich, 1993; Mundil et al., 1996). The present study uses this approach in a systematic effort to generate critical U-Pb ages for the Jurassic (Palfy et al., 1997, Chapters 3-7). A conservative, all-inclusive database compilation is advocated in GTS89 "... that does not exclude any generally accepted data [and therefore] introduces considerable stability into the time scale" (Harland et al., 1990). We note that GTS89 and MTS94 include K - A r ages that were produced as early as 1959, whereas there is an emerging consensus among geochronologists that U-Pb and A r - A r systems are the most reliable and precise 4 0  3 9  geochronometers currently available. We chose to emphasize accuracy over stability, therefore we give priority to U-Pb and A r - A r ages and omit ages derived from materials with low closure temperature. Such a radical w  v  approach was already pursued for the Cretaceous (Obradovich, 1993).  7.3.  CHRONOSTRATIGRAPHIC FRAMEWORK  The Jurassic chronostratigraphy is traditionally based on the sequence of northwest European ammonite faunas. A n apparently well-established, hierarchic scheme of stages, zones, subzones and, for many intervals, horizons has been developed, although none of the Jurassic stage boundaries has been formally defined yet by means of Global Boundary Stratotype Section and Point (Page and Melendez, 1995; Remane, 1996). Nevertheless, considerable consensus exists regarding the placement of most boundaries. This study relies heavily on North American data, mainly because the European successions generally lack isotopically datable horizons. It is therefore practical to use a regional ammonite biochronologic scheme, that is readily applicable to constrain the majority of the isotopic dates and correlatable to the northwest European standard. Such North American ammonite biochronologic standards have recently been developed for the Pliensbachian (Smith et al., 1988), Toarcian (Jakobs et al., 1994), Aalenian (Poulton and Tipper, 1991), and Bajocian (Hall and Westermann, 1980; Hillebrandt et al., 1992) stages. Well-documented local zonations also exist for the Hettangian (Tipper and Guex, 1994) and Sinemurian (Palfy et al., 1994). Because their non-standard units  112  (i.e., informally defined assemblages) have proved to have widespread applicability, herein we treat them equivalent to zones (chrons). Correlation to the northwest European standard zonation was carefully considered by the respective authors of North American stage zonations and is compiled here in Fig. 7.1. A case study of the chronostratigraphic error introduced by interregional correlation demonstrates it to be no more than one subchron (Palfy et a l , 1997). Endemism of ammonite faunas increases from the Late Bajocian onward. The Bathonian through Oxfordian faunal succession is increasingly well-understood (Callomon, 1984; Poulton et a l , 1994) even though no formal regional zonation has been proposed yet. Precise correlation, however, is hampered by significant differences between North American and European faunas, therefore only substage level subdivision is applied here. The Kimmeridgian and Tithonian is not subdivided here due to their general scarcity of ammonites in North America.  7.4.  T H E ISOTOPIC A G E DATABASE  Our database of critical isotopic ages is derived from three sources: (1) U-Pb ages from the North American Cordillera (either produced as part of this project or obtained by other workers and the biochronologic constraints critically reviewed and/or revised by us); (2) U-Pb or A r - A r ages culled from the databases of previous time 4 0  3 9  scales (mainly from GTS89 and MTS94); and (3) recently reported U-Pb or A r - A r ages from outside the 4 0  j 9  Cordillera that have not been used in time scale calibration before. Only ages with adequately documented analytical data are included (i.e., dates appearing only in abstracts are not included), with the exception of some recently obtained Cordilleran ages that were made available by our colleagues through personal communications and are currently being prepared for publication. The isotopic ages were screened for accuracy, precision, and quality of chronostratigraphic constraints. Accuracy is adequate if reproducibility is demonstrated and/or the error assignment is conservative. A few unresolved dates that exhibit loss of radiogenic daughter products contribute only minimum ages for boundary estimation. O f the U-Pb ages, only datasets incorporating multiple fraction analyses are considered. The threshold  113  STAGE  BAJOCIAN  L  NORTH AMERICAN AMMONITE CHRONS  NW EUROPEAN AMMONITE CHRONS  Epizigzagiceras  Parkinsoni Garantiana  Rotundum  Subfurcatum  Oblatum  Humphresianum  Kirschneri E  Crassicostatus  TOARCIAN  AALENIAN  Widebayense Howelli  L M  SINEMURIAN  PLIENSBACHIAN  E L  E  L  E  Sauzei Laeviuscula Discites Concavum Murchisonae  Scissum Westermanni  Opalinum  Yakounensis  Levesquei  Hillebrandti Crassicosta  Thouarsense Variabilis  Planulata  Bifrons Falciferum  Kanense  Tenuicostatum  Carlottense Freboldi  Spinatum Margaritatus Davoei  Whiteavesi  Ibex  Imlayi  Jamesoni  Tetraspidoceras Plesechioceras? harbledownense  Raricostatum Oxynotum  Asteroceras varians  Obtusum  Arnioceras arnouldi  Turneri Semicostatum  Coroniceras  Bucklandi  Kunae  HETTANGIAN  Canadensis  Figure 7.1.  Pseudaetomoceras  Angulata  Franziceras  Liasicus  Euphyllites  Planorbis  Psiloceras  Early and Middle Jurassic ammonite biochronological units of North America and their  correlation with the northwest European standard. Compiled from Tipper and Guex (1994), Palfy et al. (1994), Smith et al. (1988), Jakobs et al. (1994), Poulton and Tipper (1991), Hall and Westermann (1980), and Hillebrandt et al. (1992). 114  of acceptable precision is generally ±5 Ma (2a), less precise dates are only exceptionally included where warranted by the lack of better data. Our maximum allowable error is lower than the 6 Ma (2a) average precision in the dataset of MTS94, the most recently produced time scale. Chronostratigraphic constraints are obtained using a variety of methods but preference is given to ammonite biochronology. Assignment of age brackets is based on the tightest possible biochronologic constraints and they may be complemented with reasonable geologic age inferences. Isotopic ages are considered critical to the boundary estimation if they are at least bracketed by adjacent stages. Through the use of zonal biochronology, the average chronostratigraphic precision is significantly improved, although exceptionally a few loosely constrained ages need to be included where better dates are unavailable. Table 7.1 lists the selected critical ages arranged by stages. Also included are several Late Triassic ages and one Early Cretaceous age that help constrain the lower and upper boundary of the Jurassic, bringing the total number of critical dates used to 56. A detailed discussion of the isotopic age and chronostratigraphic constraint of each database item is given below, followed by an annotated list of U-Pb and A r - A r ages that were used in 4 0  3 9  GTS89 and/or MTS94 but are rejected in this study.  7.5.  COMMENTS ON ITEMS USED IN T H E ISOTOPIC DATABASE  The time scale significance of newly determined items that were generated through this project are discussed in detail in previous chapters. These iclude items 5 (Chapter 6), 9-11 (Chapter 4), 14-18, 22, 29, 31, 36, 40 (Chapter 5), and 30 (Palfy et a l , 1997, Chapter 3).  Item 1 — Guichon Creek batholith  A U-Pb zircon age of 210±3 Ma was obtained by Mortimer (1990) and interpreted as the crystallization age of the Guichon Creek batholith. See discussion on p. 106 (Chapter 6). K - A r and Rb-Sr minimum ages averaged around 205-209 Ma from the Guichon Creek batholith appear as NDS 177 (Armstrong in (Odin, 1982). The dates are subsequently used as 205±2.5 Ma (1 sigma) with NorianHetangian brackets in GTS89. MTS94 cites the same data without considering the more precise and accurate U-Pb age used here.  115  NO ON ON  eu Xi cu  V)  o  ON ON  u z w w tt. w as  73  & 2 ^  ON ON  o o >. 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Items 4 and 13 — Goldslide intrusions, Red Mountain, Item 12 — Cambria Icefield  See p. 105 (Chapter 6).  Items 6-8  — Newark Supergroup basalts  Seep. 105 (Chapter6).  Item 19 — Joan Lake  The U-Pb age of 193.9+1.1/-4.5 Ma was obtained from the Joan Lake area (northwestern British Columbia) by Thorkelson et al. (1995). The sample was collected near the top of a thick welded rhyolite tuff unit which is disconformably overlain by the fossiliferous Joan Formation that contains an abundant ammonite fauna of Early Pliensbachian (Whiteavesi Zone) age (Thomson and Smith, 1992). A time gap of undetermined duration is suggested by the erosional surface and the basal conglomerate at the contact between the two formations. The biochronological data therefore provide an upper bracket to the isotopic age: the Cold Fish volcanics at Joan Lake is Whiteavesi Zone (Early Pliensbachian) or older. Thorkelson et al. (1995) dated three other samples from the Cold Fish Volcanics in the Spatsizi River map area. Rhyolite sills and a dyke yielded U-Pb ages of 193.7±1.9, 194.8+1.0/-3.0, and 196.6+1.6 Ma, respectively, all within error with the Joan Lake rhyolite tuff. In MTS94, Gradstein et al. (1994, Item 276) use a weighted mean of three of these ages arbitrarily simplified to symmetric errors and interpret it as Early Pliensbachian. Although there are limited occurrences of fossiliferous Lower Pliensbachian sediments interbedded with volcanic rocks elsewhere in the map area (Thomson et a l , 1986), the evidence from the Joan Lake section itself doesn't justify this restrictive interpretation.  118  Item 20 — Chuchi intrusion  In the Mt. Milligan map area in northern Quesnellia, the locally fossiliferous Chuchi Lake Formation of the Takla Group is intruded by small igneous bodies. One of them, a monzonite intrusion near the BP-Chuchi property, was dated as 188.5±2.5 Ma by U-Pb method (Nelson and Bellefontaine, 1996). The crystallization age is defined by two concordant and overlapping titanite fractions and the lower intercept of a discordia line through two zircon fractions containing small amounts of inherited Proterozoic Pb component. Field observations (e.g., wet sediment deformation near the intrusive contacts, predominance of sills, cross-cutting relationships) suggest syn-sedimentary (i.e., pre-lithification) emplacement of this high-level intrusion (Nelson and Bellefontaine, 1996). Sedimentary beds in the area, likely correlative with the sedimentary rocks intruded by the dated monzonite body (Nelson and Bellefontaine, 1996), yielded ammonites  (Leptaleoceras, Arieticeras,  and Amaltheus) of the Late Pliensbachian  Kunae Zone (Tipper in Nelson and Bellefontaine, 1996). In one section, however, an Early Pliensbachian (Whiteavesi Zone) ammonite fauna was recovered from a thin and stratigraphically lower sedimentary unit (Tipper in Nelson and Bellefontaine, 1996). A conservative interpretation is to bracket the crystallization age between the Whiteavesi and Kunae zones.  Items 21, 23, and 24 — Atlin Lake  The fossiliferous Lower Jurassic Laberge Group is well exposed on the shores of Atlin Lake in northwestern British Columbia and contains minor volcanic rocks (Johannson et a l , in press). A tuff layer (informally assigned to the Nordenskiold volcanics) yielded a U-Pb zircon date of 187.5± 1 Ma (Item 21, Mihalynuk and Gabites, pers. comm 1996). Ammonite collections stratigraphically below  talkeetnaense, Dubariceras  cf.  (Tropidoceras actaeon)  silviesi, Acanthopleuroceras  cf.  and above  thomsoni)  (Metaderoceras  cf.  the tuff indicate the Whiteavesi Zone  (Early Pliensbachian) (Johannson, 1994). On Copper Island, another crystal tuff layer stratigraphically higher was U-Pb dated at 186.0±1 Ma (Item 24, Johannson and McNicoll, in press). This sample is also tightly constrained by ammonite biochronology. Diagnostic taxa, indicative of the Late Pliensbachian Kunae Zone, include  colubriforme and Arieticeras from  below and  Leptaleoceras, Arieticeras,  tuff (Johannson, 1994).  119  and  Reynesoceras  Protogrammoceras  from above th  Another U-Pb age of 186.6+0.5/-1.0 Ma was obtained from a granitoid boulder within a conglomerate on Sloko Island (Item 23, Johannson and McNicoll, in press). The conglomerate is also of Kunae Zone age based on the ammonite fauna (Reynesoceras, Leptaleoceras, Protogrammoceras, Fuciniceras, Arieticeras) recovered from adjacent finer-grained sedimentary rocks (Johannson, 1994). Assuming geologically rapid uplift and erosion, the U-Pb date provides a useful maximum age for the Kunae Zone (Johannson et al., in press).  Item 25 — Skinhead Lake  A U-Pb age of 184.7+0.5 Ma was obtained by M. Villeneuve (in Maclntyre et al., 1997) from zircon in a rhyolite tuff near Skinhead Lake (west of Babine Lake, northwestern British Columbia). Immediately above the dated unit, from a lens of fossiliferous sandstone within the volcaniclastic package we collected a small ammonite fauna consisting of Arieticeras and Fanninocerasl indicative of the Late Pliensbachian Kunae Zone.  Item 26 — Whitehorse  A dacite tuff assigned to the Nordenskiold volcanics is U-Pb dated at 184.1 +4.2/—1.6 Ma near Whitehorse, Yukon (Hart, in press). It is interbedded in a section of fossiliferous Laberge Group sediments. Arieticeras occurs a few meters below the tuff whereas Arieticeras and Amaltheus cf. stokesi was collected upsection indicating the presence of the Upper Pliensbachian Kunae Zone. The section is discussed in detail along with illustration of the ammonite fauna in Palfy and Hart (1995).  Item 27 — Eskayporphyry  A sill-like feldspar porphyry intrusion in near the Eskay Creek gold mine in the Iskut River area (northwestern British Columbia) was dated by Macdonald et al. (1992) who reported a U-Pb zircon age of 186+2 Ma. It was recalculated and reinterpreted as 184+5/-1 Ma (Childe, 1996). The porphyry intrudes fossiliferous mudstone that yielded Lioceratoides propinquum, Protogrammoceras cf. kurrianum and other hildoceratids characteristic to the Pliensbachian/Toarcian transition (Nadaraju, 1993). The ammonoids range from the topmost  120  Pliensbachian Carlottense Zone to the basal Toarcian Kanense Zone, although the absence of  Dactylioceras  favors  an assignment to the Carlottense Zone, for which the age of the intrusion is regarded as a minimum age.  Item 28 — McEwan Creek pluton  The quartz monzonite McEwan Creek pluton intrudes Lower Jurassic and older strata in the northwest part of the Spatsizi River map area (northwest British Columbia) and was dated by U-Pb method (Evenchick and McNicoll, 1993). The reported age of 183.5±0.5 M a is based on the  2 0 6  Pb/  2 3 8  U age of the more precise of two  concordant and overlapping zircon fractions. Two titanite fractions also analyzed from the same sample are perfectly concordant and overlapping at a  2 0 6  Pb/  2 3 8  U age of 183.0±0.5 Ma. Although the zircon and titanite ages are  within error and the somewhat younger age of titanite may be explained by its lower closure temperature, we take a more conservative approach in assigning a crystallization age of 183.2±0.7 Ma based on the weighted mean 206  238  Pb/  U age of the three most precise and concordant fractions. The youngest stratigraphic unit intruded by the pluton is the Mount Brock volcanics (Evenchick and  McNicoll, 1993), the youngest volcanic member of the Hazelton Group in the Spatsizi area (Thorkelson et a l , 1995). Critical fossil localities providing constraints on the age of the Mount Brock volcanics are found in intercalated marine sedimentary rocks (Read and Psutka, 1990). Early and middle Toarcian ammonites are reported in all previous studies (Read and Psutka, 1990; Evenchick and McNicoll, 1993; Thorkelson et a l , 1995). Our new fossil collections from the area west of Mount Brock indicate that volcanism started in latest Pliensbachian time (see also Thomson, 1986), and the evidence for middle Toarcian needs revision. Dactylioceras, commonly occuring in (but not restricted to) the lower Toarcian was found stratigraphically above the only collection that had suggested a middle Toarcian age in previous work, based on the identification of  Polyplectus  and  Dactylioceras  Polyplectus  sp. As the ranges of  are mutually exclusive, it is possible that the specimen in question actually  represents some other, morphologically similar harpoceratid ammonite. In conclusion, we favor an interpretation that Mount Brock volcanism (and emplacement of the perhaps co-magmatic McEwan Creek pluton) is not younger than early Toarcian.  121  Item 32 — Julian Lake dacite  A thick pile of felsic volcanics with locally interbedded marine sedimentary rocks occur in the Salmon River Formation near Julian Lake (Iskut River area, northwestern British Columbia). A dacite flow near its base yielded a U-Pb zircon age of 178±1 Ma (P. Lewis and J. Mortensen, personal communication 1996). Hyaloclastite observed at the flow top points to submarine emplacement, in turn suggesting that the sedimentary rocks are not significantly younger than the dated flow that they directly overlie. These volcanic sandstones are locally fossiliferous and yielded a diverse ammonoid fauna  {Yakounia silvae, Pleydellia cf. maudensis, P. cf. crassiomata, Phymatocera  sp.) clearly indicating the uppermost Toarcian Yakounensis Zone. Several hundreds of metres upsection, another dacite flow yielded an age of 172.3±1.0 Ma (P. Lewis and J. Mortensen, personal communication 1996). No identifiable fossils have been found in the upper part of the section, therefore the latter date can only be used with a latest Toarcian lower bracket and a Bajocian upper bracket, inferred from regional geology as the age of cessation of Salmon River felsic volcanism.  Item 33 — Treaty Ridge  The Treaty Ridge section is an important reference section for the Hazelton and overlying Bowser Lake groups in the Iskut River map area (northwestern British Columbia) (Lewis et al., 1993). Four separate samples from the upper felsic unit of the Salmon River Formation have been recently dated by the U-Pb method. Complex isotopic systematics that suggest presence of Late Triassic xenocrystic zircon and/or Pb-loss hampered interpretation of data for one sample (McNicoll and Anderson, personal communication 1995). Only one sample yielded concordant analyses giving an interpreted age of 177.3±0.8 Ma (Friedman and Anderson, personal 207  communication 1996). A weighted mean  206  Pb/  Pb age of two discordant but apparently inheritance-free fractions  gives a 178±12 Ma age for another sample (Friedman and Anderson, personal communication 1996). Fossiliferous sediments yielded ammonites both below and above the felsic volcanic unit (Lewis et al., 1993; Nadaraju, 1993; Jakobs and Palfy, 1994). The Upper Aalenian Howelli Zone is documented by the presence of  Erycitoides mudstone.  cf.  howelli, Pseudolioceras cf. whiteavesi, Tmetoceras  Sonninial  sp.,  Stephanoceras  cf.  kirki,  and  Leiocerasl  sp. in the underly  sp. and Zemistephanus sp., collected from siltstone overlying the dated  volcanic unit, assign an Early Bajocian upper age limit to the volcanism. 122  Items 34-35 — Eskay rhyolite  A flow-banded rhyolite unit that underlies the locally mineralized argillite at the Eskay Creek gold mine (Iskut River area, northwestern British Columbia) was sampled for U-Pb zircon dating on the east limb of the Eskay anticline. Childe (1996) obtained an age of 174+2/-1 Ma.  Erycitoides  cf.  howelli,  the index ammonite of the  late Aalenian Howelli Zone, was collected from above the rhyolite on the same limb of the anticline, less than 3 km away from the U-Pb sampling site. The rhyolite is therefore Upper Aalenian or older. No precise lower bracket can be assigned. The latest Pliensbachian (or possibly earliest Toarcian) ammonites listed for the Eskay porphyry (Item 27) are the youngest fauna from underlying units in the area. Regionally, felsic volcanism correlative to the Eskay rhyolite is not known to begin prior to the latest Toarcian (see Item 32). Another sample from the same rhyolite unit on the west limb of the Eskay anticline yielded an age of 175±2 Ma (Childe, 1996). No ammonites have been found near this sample site but radiolarians identified from drill core obtained from the overlying argillite are dated as Aalenian to possibly early Bajocian (Nadaraju, 1993). The local correlation of the rhyolite and argillite units is well-documented by detailed geological mapping and further supported by the statistically indistinguishable U-Pb dates (Childe, 1996) and concordant ammonoid and radiolarian ages. Therefore the same stratigraphic constraints are applied to both isotopic dates.  Item 37 — Gunlock  The Middle Jurassic Carmel Formation in Utah contains numerous ash beds several of which have been recently dated using the A r - A r single crystal laser probe method (Kowallis et a l , 1996). One of them is 4 0  3 9  published in detail and thus included in our database: a 166.3±0.8 Ma sanidine age from the upper part of the Carmel Formation near Gunlock. (Kowallis et a l , 1993). Characteristic bivalves allow correlation of the Carmel Fm. with the more fossiliferous Sliderock and Rich members of the Twin Creek Limestone that yielded ammonites of late Early to Late Bajocian age (Imlay, 1967; Imlay, 1980). Younger K - A r (161.2±1.8 Ma (la)) and Rb-Sr (162±6.5 Ma (la)) ages from the Carmel Formation were reported as N D S 102a and N D S 102b and also used in GTS89.  123  Item 38 — Burnaby Island Plutonic Suite  Plutonic rocks assigned to the Burnaby Island Plutonic Suite (BIPS) yielded several U-Pb dates from the Queen Charlotte Islands. They range in age between 172 and > 158 Ma (Anderson and Reichenbach, 1991). We use here the four dates (Poole Point: 168+4/-1 Ma; Shields Bay: 168±4 Ma; Rennell Sound: 164+4/-2 Ma (Anderson and Reichenbach, 1991); and Cumshewa Head: 167±2 Ma (Anderson and McNicoll, 1995)) which form a tight cluster in the older part of the age range of the BIPS. The youngest country rocks cross-cut by BIPS intrusions are Bajocian volcano-sedimentary strata of the Yakoun Group. Ammonoid biochronology of the Yakoun Group elsewhere in the Queen Charlotte Islands suggests an age range of Widebayense to Oblatum zones (Hall and Westermann, 1980; Poulton et al., 1991b). It is possible that the Yakoun Group volcanics represent extrusive equivalents of BIPS plutons. In the future, isotopic dating of the Yakoun Group volcanics holds promise for contributing to better constraints on the Bajocian time scale. At present, we use a pooled age of 168±4 Ma for early BIPS plutons as a minimum constraint for the undivided Bajocian stage. The late Bathonian and younger age of the Moresby Group (Poulton et al., 1991b) which disconformably overlies the Yakoun Group and is not known to be cross-cut by BIPS is used to provide a minimum age for the BIPS, although the oldest strata found directly deposited on BIPS rocks are Early Cretaceous in age (Anderson and Reichenbach, 1991).  Item 39 — Harrison Lake  Mahoney et al. (1995) obtained a U-Pb zircon age of 166.0±0.4 Ma from a rhyolite near the top of the Weaver Lake Member (Harrison Lake Formation) exposed on Echo Island in Harrison Lake, southwestern British Columbia. No age diagnostic fossils have been recovered from sedimentary interbeds within the Weaver Lake Member (Arthur et al., 1993). The youngest fossil known from the underlying Francis Lake Member is  Erycitoides? sp. and Tmetoceras scissum of  Late Aalenian age. Both Arthur et al. (1993) and Mahoney et al. (1995)  speculate that the Weaver Lake Member is as young as Early Bajocian or possibly even younger. The Harrison Lake Formation is unconformably overlain by the Mysterious Creek Formation which yielded a rich Early Callovian ammonite fauna (Arthur et al., 1993). The unconformity separating the units represents a regional deformation event and strongly suggests that the Weaver Lake Member is significantly older than Early Callovian.  124  The isotopic age is further supported by another two, nearly identical U-Pb ages from related rocks in the area. A rhyolite dyke from strata correlative to the upper part of the Weaver Lake Member near the Seneca mineral deposit yielded an age of 165.9+6.4/-0.3 Ma (McKinley, 1996) and a comagmatic quartz-feldspar porphyry stock (Hemlock Valley stock) was dated at 166.0±0.4 Ma (Mahoney et a l , 1995).  Items 43-44 — Chacay Melehue  Two tuff layers from the Chacay Melehue section in the Neuquen Basin, Argentina, were dated at 160.5±0.3 Ma and 161.0±0.5 Ma by zircon U-Pb method (Odin et a l , 1992). Both dates are lower intercept ages, the first one is corroborated by one concordant fraction. The section has a well-documented ammonite fauna that is dominated by endemic South American forms but allows correlation with the European standard. The lower sample is very near to the boundary of the regional Steinmanni and Vergarensis zones that is equated to the Bathonian/Callovian boundary (Riccardi et a l , 1991). The upper sample was collected near the boundary between the Bodenbenderi and Proximum zones that approximately corresponds to the middle of the Boreal Calloviense Zone or the Mediterranean Gracilis Zone in the upper part of the Lower Callovian (Riccardi et a l , 1991).  45 — Tsatia Mountain  Lenses of boulder conglomerate within the Bowser Lake Group in the Bowser Basin (northwestern British Columbia) locally contain dacite clasts, one of which was dated by U-Pb zircon method. Ricketts and Parrish (1992) obtained a precise age of 160.7±0.7 Ma based on analysis of three concordant fractions. The dated clast was recovered from a biostratigraphically well-constrained section on Tsatia Mountain. An Early Callovian Cadoceras fauna was found below the conglomerate whereas overlying deposits yielded Stenocadoceras of Middle Calovian age and still younger assemblages higher upsection (Poulton et a l , 1991a; Poulton et a l , 1994). The dacite clast therefore cannot be younger than Middle Callovian.  125  Item 46 — Josephine ophiolite  U-Pb ages from plagiogranites in the Josephine ophiolite have been used in previous time scales. GTS89 (Item HS1) uses an age of 157±2 Ma as Oxfordian or older. The original data appear in Saleeby (1982) as analyses of two different samples (one single-fraction and one based on two fractions of unabraded zircons). A later revison of one of these dates to 162±1 Ma (Saleeby, 1987) is not considered in GTS89 or MTS94 (Item 253, labelled as HS2). A full documentation is now available (Harper et a l , 1994), showing that the interpreted age is the  2 0 6  Pb/  2 3 8  U  age (with 1 sigma error) of the subsequently analyzed, concordant, abraded fraction. We assign a more conservative age estimate to samples when no duplicate concordant fraction is available and there is evidence of Pb-loss. A weighted mean  207  Pb/  206  P b age from the two fractions (sample A88Z) combined with the  2 0 6  Pb/  2 3 8  U age  of the concordant one as a minimum age gives 162+7/-2 Ma as the crystallization age. The choice of a larger positive error is also warranted when an independent A r - A r plateau age of 165±3 Ma (1 sigma) obtained from a 4 0  3 9  high-level gabbro intrusion 1 km from the U-Pb dated plagiogranite (Harper et a l , 1994) is considered. Furthermore, Wright and Wyld (1986) reported a U-Pb age of 164±1 Ma from the Devils Elbow ophiolite remnant thought to be correlative to the Josephine ophiolite. The Josephine ophiolite is overlain by the Galice Formation which yielded bivalves perisphinctid ammonites, and radiolarians (e.g.,  Mirifusus)  {Buchia concentrica),  indicating an Oxfordian age (Imlay, 1980; Pessagno and  Blome, 1990).  Item 47 — Rogue Formation tuff breccia  A U-Pb zircon age of 157±1.5 Ma (la) was obtained from tuff breccia in the Rogue Formation (Klamath Mountains, northern California) (Saleeby, 1984; Harper et a l , 1994). Analytical data have not been published. Considering the scarcity of Late Jurassic data, we include this date with a qualifier as a minimum age. It is warranted as Pb-loss is documented as a common phenomenon affecting the U-Pb systematics of Jurassic zircons from the area (Saleeby, 1987). The Rogue Formation is overlain by the sparsely fossiliferous Galice Formation which yielded bivalves  (Buchia concentrica) and radiolarians (e.g., Mirifusus)  1980; Pessagno and Blome, 1990).  126  indicating an Oxfordian age (Imlay,  Item 48 — Hotnarko volcanics  Van der Heyden (1991) obtained a preliminary U-Pb age of 154.4±1.2 Ma on a sample from the Hotnarko volcanics in the eastern Coast Belt in west-central British Columbia. Volcaniclastic rocks near the base of the succession yielded  Anditrigonia aff. plumasensis  (identification by T. Poulton) which ranges from the Callovian  through the middle Oxfordian. It thus provides a lower stratigraphic bracket for the dated volcanic rocks whereas geologic evidence suggested that the entire succession may have been deposited within a brief volcanic episode (van der Heyden, 1991).  Item 49 — Tidwell Member (Morrison Formation)  Three sanidine A r - A r single-crystal laser fusion ages (154.8±1.2, 154.8±1.1, and 154.8±2.8 Ma) and one 4 0  3 9  plateau age (154.9±1.0 Ma) are reported by (Kowallis et al., in press) from an ash bed in the Tidwell Member (lower part of the Morrison Formation) sampled in two sections of Utah. All analyses determine the age of the same ash bed and are remarkably concordant therefore we use the most precise of them in our database. The youngest cardioceratid ammonites from the underlying Redwater Shale (Stump Formation) are Middle Oxfordian (Imlay, 1980; Callomon, 1984). The base of the Morrison Formation marks a regional unconformity. Ostracods and charophytes (Schudack et al., in press) as well as palynomorphs (Litwin et al., in press) suggest that the lower part of the Morrison Formation, including the Tidwell Member, is Kimmeridgian (likely Early Kimmeridgian) in age. Magnetostratigraphic studies also reveal correlation between the reversal sequence in the Morrison Formation and the Kimmeridgian oceanic magnetic anomaly pattern (Steiner et al., 1994).  Items 50-55 — Brushy Basin Member (Morrison Formation)  Six A r - A r laser fusion ages for plagioclase from volcanic ash layers in the Brushy Basin Member 4 0  3 9  (Colorado Plateau) range between 153 and 148 Ma (Kowallis et al., 1991). This dataset is now superseded by seven more precise A r - A r sanidine laser fusion ages from different ash layers in five section of the upper part of the 4 0  3 9  Brushy Basin Member (Kowallis e t a l , in press). Six of them (150.3±0.5, 150.2±1.0, 149.3±1.1, 149.3±1.0, 149.0±0.8, and 148.1±1.0 Ma) are consistent with their relative stratigraphic position within the unit and are  127  preferred. All of the dated ash layers occur in the upper part of the unit, above the level marking a characteristic change in clay mineralogy (Kowallis et a l , in press). Correlation based on ostracods and charophytes suggests the isotopically dated interval to be Kimmeridgian in age in its lower part, possibly ranging to the Tithonian near the top (Schudack et a l , in press). Palynological results are less conclusive but corroborate the placement of these strata to the Kimmeridgian and Tithonian (Litwin et a l , in press). We infer an age restricted to the Kimmeridgian for the lowest two dated ashes.  Item 56 — Grindstone Creek tuff  A precise and well-documented U-Pb zircon age of 137.1 + 1.6/-0.6 Ma was obtained from two altered crystal tuff layers within a fossiliferous Upper Jurassic to Lower Cretaceous mudstone succession (Great Valley Sequence) in California by Bralower et al. (1990). The dated levels are assigned to  pacifica bivalve  zones and the  Assipetra infracretacea (NK-2A)  Buchia uncitoides  and  B.  nannofossil subzone, both indicating a Late  Berriasian age. Correlation with DSDP sites containing integrated magnetostratigraphic and nannofossil records permited their assignmnent to the CM16 or CM16n magnetochrons (Bralower et a l , 1990). Channell et al. (1995) revised the ranges of critical nannofossil taxa and suggest that the dated level can represent the CM16-CM15 magnetochrons, still within the Upper Berriasian. This date is used in MTS94 as Item 232.  7.6.  COMMENTS ON ITEMS USED IN EARLIER TIME SCALES BUT REJECTED  IN THIS STUDY  Item HLB1 (GTS89) and Item 269 (MTS94)  The quoted A r - A r age is 185.0±3.0 Ma (1 sigma) with Pliensbachian-Toarcian stage brackets. The 4 0  3 9  original source (Hess et a l , 1987) reports 11 ages, ranging from 190 to 180 Ma, from a 2000 km area in the 2  northern Caucasus underlain predominantly by thick volcanosedimentary sequences. No detailed stratigraphy was reported but sedimentary rocks generally associated with the volcanics were said to contain rare Pliensbachian bivalves and brachiopods as well as Toarcian ammonites. We conclude that the coarse stratigraphic resolution of  128  this dataset does not warrant its inclusion in the time scale calibration and the simple averaging of the 11 different ages is inadequate for inclusion in our database.  Item 251 (MTS94)  MTS94 quotes an age of 155.3±3.4 Ma as an A r - A r age for Oxfordian oceanic crust recovered from 4 0  3 9  DSDP Site 765. However, it is clear from the original source (Ludden, 1992) that the age in question was obtained using the K - A r method, therefore we exclude it from our database. See also remarks in section 7.10.  Item 258 (MTS94)  Gradstein et al. (1994) quote an A r - A r age of 166.8±4.5 Ma obtained by Pringle (1992) as an age for 4 0  3 9  Callovian oceanic crust recovered from DSDP Site 801. In Pringle's (1992) account, this is the age of tholeiitic M O R B basalt recovered from the base of the drillhole. It is overlain by alkaline off-ridge basalt that yielded an 4 0  A r - A r age of 157.4±0.5 Ma. Magnetostratigraphic evidence suggests that the site was drilled into the Jurassic 3 9  Quiet Zone some 450 km away from anomaly M37, the oldest known preserved oceanic magnetic lineation thought to be Callovian in age. At DSDP Site 801, original radiolarian biostratigraphy from the oldest overlying sedimentary rocks indicated a latest Bathonian to earliest Callovian age (Matsuoka, 1992). This interpretation was challenged by Pessagno (1996) who considered the fauna as middle Oxfordian and argued that the younger isotopic age supports this assignment. Considering the controversy that surrounds this dataset, we chose not to include it in the present compilation.  NDS184 (revised in GTS89) and Item 272 (MTS94)  Both GTS89 and MTS94 quoted an A r - A r age of 197.6±2.5 Ma from the Toodoggone volcanics in 4 0  3 9  British Columbia, based on an undocumented revision of NDS184 (Armstrong in Odin, 1982). The stratigraphic age given is Sinemurian to Toarcian but the date is rejected on the basis of a lack of analytical documentation. Clark and Williams-Jones (1991) recently obtained a fully documented A r - A r age of 195.1±1.6 from the 4 0  3 9  Toodoggone volcanics but there is no fossil data available to bracket the age of these mainly subaerial volcanic 129  rocks. Although it can be reasonably correlated with the early phase of Hazelton Group volcanism recorded elsewhere (e.g., Telkwa Formation, Cold Fish volcanics), it is unuseable for time scale calibration.  Item SCH1 (GTS89) and Item 245 (and 247?) (MTS94)  GTS89 used a U-Pb age of 153±1 Ma in the Kimmeridgian. MTS94 revised the error to ±3 Ma and lists apparently the same age twice, the second time with Oxfordian-Kimmeridgian brackets. In fact this is not a U-Pb age itself but rather a "conservative estimate of the age of the Nevadan deformation" (Schweikert et a l , 1984). It is based on several, poorly documented or single-fraction early-determined U-Pb ages pooled together with K - A r dates. These items are rejected as a composite of questionable veracity and validity to time scale calibration.  Item HS2 (GTS889) and Item 244 (labelled as HS1 in MTS94)  GTS89 quoted a U-Pb age of 150.5±2 Ma as Kimmeridgian or younger. Evidently the quoted date derived from a combination of two single-fraction U-Pb ages of unabraded zircons (Saleeby et a l , 1982; Harper, 1984): a 151±3 Ma age obtained from a dyke intruding the Josephine ophiolite and a second 150±2 Ma age from a sill intruding the overlying Galice Formation. These ages are rejected as suspect because Pb-loss is documented from several other samples from the same dataset in subsequent work on abraded zircons (Saleeby, 1987; Harper et a l , 1994).  Item HMP2 (GTS9) and Item 239 (MTS94)  The Tithonian or older U-Pb age of 152.5±2 Ma used in GTS89 and MTS94 presumably represents a mean of  2 0 6  Pb/  2 3 8  U and  2 0 7  Pb/  2 3 5  U ages reported by Hopson (1981). This date doesn't satisfy our criteria for inclusion in  the database as it is based on the analysis of a single fraction.  130  Item HMP3 (GTS9) and Item 240 (MTS94)  The Tithonian or older U-Pb age of 154±2 Ma used in GTS89 and MTS94 is apparently the W z u  J  3  U age  of a slightly reversely discordant fraction reported by Hopson (1981). We reject this date as being based on the analysis of a single fraction where reverse discordance likely indicates analytical problems.  Item HMP4 (GTS89) and Item 255 (MTS94)  20f3  The quoted Oxfordian or older U-Pb age is 162±2 Ma, apparently the younger  238  Pb/  U age of the two  unabraded fractions analyzed by Hopson (1981). This fraction is slightly reversely discordant and the other fraction yielded a  207  Pb/  206  P b age of 184 ±10 M a which is outside the error of the  2 0 6  Pb/  2 3 8  U age of 165±2 Ma. This  discrepancy of apparent ages points to complexities in the U-Pb systematics. This date is rejected as the true crystallization age cannot be unambiguously resolved from the published analyses.  Item HMP1 (GTS89) and Item 254 (MTS94)  The quoted Oxfordian or older U-Pb age is 161±2 Ma based on data of Hopson (1981). O f the two analyzed unabraded fractions, one yielded a significantly older apparent  207  Pb/  206  P b age outside the error of the  2 0 6  Pb/  2 3 8  U  age. We interpret this as an indication of Pb-loss, inheritance, or both. On the same grounds as in the previous item, we reject this date as the true crystallization age cannot be unambiguously resolved from the published analyses.  7.7.  DIRECT DATING OF STRATIGRAPHIC BOUNDARIES  Ideally, the age of a biochronologically-defined boundary is determined by isotopic dating of a volcanogenic layer situated at or in the immediate vicinity of the boundary. This situation occurs rarely in the Jurassic. The Triassic-Jurassic boundary is directly dated in eastern North America and the Queen Charlotte Islands (Chapter 6). In the Newark Basin, U-Pb ages of 200.9±1.0 M a (Item 8) and 201.3±1.0 Ma (Item 7) (Dunning and Hodych, 1990) were obtained on sills that are thought to be feeders to the Orange Mountain basalt, the lowermost extrusive rock which lies immediately above the Triassic-Jurassic boundary defined by palynology and vertebrate  131  biostratigraphy (Olsen et al, 1987; Fowell and Olsen, 1993). From the Fundy Basin, a U-Pb zircon age of 201.7+1.4/—1.1 Ma (1992) is reported from the North Mountain basalt whose base lies about 20 m above the Triassic-Jurassic boundary. A tuff layer immediately below the conodont- and radiolarian-defined TriassicJurassic boundary in the Queen Charlotte Islands yielded a U-Pb age of 200±1 Ma (Item 5) (Palfy et al, in review). These four dates are reconciled to a 200.9±0.6 Ma boundary estimate, that is rounded to 201±1 Ma to deemphasize apparent precision in the presence of slightly conflicting ages (see Chapter 6). A volcanic ash layer directly above the base of the Middle Toarcian Crassicosta Zone, a regional standard ammonite zone for North America, is dated by U-Pb method at 181.4±1.2 Ma in the Queen Charlotte Islands (Palfy etal, 1997). A third directly dated level is the boundary of the South American Steinmanni and Vergarensis chrons that is equated to the Bathonian-Callovian boundary (Riccardi et al, 1991). A tuff layer located at this boundary in the Chacay Melehue^ section (Neuquen Basin, Argentina) yielded a U-Pb zircon date of 161.0±0.5 Ma (Odin et al, 1992).  7.8.  CHRONOGRAM ESTIMATION OF BOUNDARIES  In the absence of direct isotopic dating, the ages of a stratigraphic boundary can be estimated using dates from adjacent units. The chronogram method, described in detail in GTS89 (Harland et al, 1990), calculates the E error function value for trial ages in a time window scanned for possible boundary ages. Taking into account the relevant isotopic dates, their stratigraphic position below or above the boundary in question, and their plus and minus errors, it provides a semi-rigorous measure of compatibility of the data with the trial ages. Following GTS89, the best chronogram estimate is defined by the minimum value of E or the mean of a range where E = 0, whereas the endpoints of the error range around the best estimates are taken where E = E +1. We use here a min  slightly modified formula that accommodates asymmetric errors, which are common among interpreted U-Pb ages: E = E(Y -te) /S 2  i  +  2 Yi  + 2(O -t ) /S 2  i  e  2 0i  where t is the trial age, Y, are the isotopic ages stratigraphically younger than the boundary for which Y > t , S e  s  e  + Yi  are their plus 2a error, Oj are the isotopic ages stratigraphically older than the boundary for which 0, < t , S" are e  0i  their minus 2a error. We calculated chronograms for each chron boundary in the Hettangian through Bajocian and each substage boundary in the Bathonian through Callovian. As not all isotopic dates have zonal or substage resolution 132  constraints (not even attempted for the Oxfordian through Tithonian), stage boundary chronograms were also calculated (Table 7.2). These may be different from the chronogram of the earliest chron of the stage if significant dates lack zonal constraints (e.g. the Hettangian). The chronogram method assumes a random distribution of dates within the stratigraphic intervals whose boundaries are sought. Therefore the directly determined boundary ages listed above are omitted from the calculation of those boundaries and accepted if they overlap with the error range of the respective chronogram. If a unit lacks critical dates confined to it, its chronogram tends to converge to the next younger unit. Fig. 7.2 thus shows only those meaningful chronograms that have no identical counterpart for a younger stratigraphic boundary. The assumption of random distribution may prove to be unfounded for stage chronograms if dates are crowded in some parts and are missing from other parts of the unit. This is demonstrated for the Sinemurian, Pliensbachian, Aalenian, Bajocian, and Bathonian and its consequences are discussed next.  133  Minimum  +Error  -Error  (2a)  (2a)  Error range  1.3  1.3  2.6  1.3  1.3  2.6  3.7 2.4  3.0 4.4  6.7 6.8  5.1  7.5  5.1 5.1 4.5 4.5 4.5  7.5 7.5 9.0  Initial boundary  Maximum  41  198.7  200.0  197.4  Franziceras  42  198.7  200.0  197.4  Pseudoaetomoceras  43 51 52  195.8 195.8  199.5 198.2  192.8 191.4  195.8  198.2  2.4 2.4 2.4 4.5 4.5 4.5  Code  Chron, substage, STAGE Psiloceras/Euphyllites  Canadensis Coroniceras/Arnouldi  Imlayi Whiteavesi  53 54 55 61 62  195.8 195.8 191.3 191.3 191.3  198.2 198.2 195.8 195.8 195.8  190.7 190.7 190.7 186.8 186.8 186.8  Freboldi Kunae Carlottense Kanense Planulata  63 64 65 71 72  186.8 185.7 184.1 183.7 182.8  188.6 186.3 185.1 185.1 185.1  185.0 184.9 182.5 182.5 180.2  1.8 0.6 1.0 1.4 2.3  1.8 0.8 1.6 1.2 2.6  3.6 1.4 2.6 2.6 4.9  Crassicosta Hillebrandti  73 74  180.1 180.1  180.8  176.6 176.6  0.7  3.5  4.2  180.8  0.7  3.5  4.2  Yakounensis  75  180.1  180.8  176.6  0.7  3.5  4.2  Westermanni/Scissum  81  ' 177.6  179.0  176.5  1.4  1.1  2.5  Howelli  82  177.6  179.0  176.5  1.4  1.1  2.5  Widebayense Crass icostatus Kirschneri Ob latum Rotundum Epizigzagiceras Middle/Late Bathonian  91 92 93 94 95 96 102  170.6 170.6 170.6 170.6 170.6 166.1 164.3  175.0 175.0 175.0 175.0 175.0 174.1 166.4  165.5 165.5 165.5 165.5 165.5 160.6 160.6  4.4 4.4 4.4 4.4  5.1 5.1 5.1  9.5 9.5 9.5  5.1 5.1 5.5 3.7  9.5 9.5 13.5 5.8  Early Callovian  111  159.7  160.8  158.6  1.1  1.1  2.2  Middle Callovian Late Callovian  112 113  156.6 156.6  158.6 158.6  153.9 153.9  2.0 2.0  2.7 2.7  4.7 4.7  HETTANGIAN  40  201.4  202.3  198.3  0.9  3.1  4.0  SINEMURIAN PLIENSBACHIAN  50  198.2  2.4 4.5  5.4  195.8  192.8 186.8  3.0  60  195.8 191.3  4.5  9.0  TOARCIAN AALENIAN  70 80  183.6 177.6  185.2 179.0  182.5 176.5  1.6 1.4  1.1 1.1  2.7  BAJOCIAN  171.0  175.0  166.7  4.0  BATHONIAN  90 100  164.5  167.1  160.6  2.6  4.3 3.9  8.3 6.5  CALLOVIAN OXFORDIAN  110 120  159.7 156.6  160.8 158.6  158.6 153.9  1.1 2.0  1.1  2.2  2.7  4.7  KIMMERIDGIAN TITHONIAN BERRIASIAN  130 140 150  154.7 143.6  155.7 151.7 149.1  153.8 136.4  1.0 8.1 6.5  0.9 7.2  1.9 15.3  7.1  13.6  Varians Harbledownense Tetraspidoceras  Table 7.2.  142.6  135.5  4.4 8.0 2.1  9.0 9.0  2.5  Chronogram estimates of the initial boundary of chrons, substages and stages. Chrons  with poor stratigraphic record (Euphyllites, Coroniceras, and Westermanni) were combined with adjacent chrons. Bold face designates good quality chronograms that are different from the next younger stratigraphic boundary and the error range is less than 5 Ma.  134  7.9.  ADJUSTED STAGE BOUNDARY ESTIMATES  Stage boundaries are of particular interest, therefore their age estimates are considered individually. Chronogram estimates may be biased if no sufficient data exist in the vicinity of the boundary. This is indicated if the chronogram of the earliest chron is identical to any or both adjacent chrons. While chronogram maxima and minima are retained as valid, reasonable adjustments in the best estimates are made with respect to the critical data. As discussed above, two stage boundaries are dated directly. The base of the Hettangian fixed at 200.9±0.6 Ma based on the four direct ages is in good agreement with the chronogram age derived from the other pertinent dates. However, the base of the Callovian is pegged by a single date (161.0±0.5 Ma, Item 43) that is consistent with a slightly younger date (Item 44) produced by the same authors (Odin et a l , 1992) from the same section but is in conflict with another relatively precise age (Item 40) from the Bathonian. As a result, the chronogram minimum takes an unusually high value (>21) and in this case we choose to define the error range where E = 2 E  min  . Such a  chronogram age of 159.7±1.1 Ma overlaps with the directly determined boundary age and is our preferred estimate. In the Early Jurassic, the chronogram age of the Pliensbachian-Toarcian boundary (183.6+1.6/-1.1 Ma) is tightly controlled as there is a series of good quality chronograms for the neighbouring chrons. The HettangianSinemurian and the Sinemurian-Pliensbachian boundaries are less well constrained. The Canadensis Zone, likely to span Hettangian-Sinemurian the boundary, is here arbitrarily classified as Sinemurian but this does not affect the chronogram. Early and early Late Sinemurian ages are all minimum ages causing an asymmetric chronogram. The chronogram estimate of 195.8+2.4/-3.0 is adjusted by shifting the best estimate up to 197.0+1.2/-4.2 Ma. Such reapportioning is also supported by observed sediment thicknesses in many Lower Jurassic sections. The chronogram of the Sinemurian-Pliensbachian is identical to the initial Whiteavesi chron boundary and is not well constrained. On the other hand, the chronogram of the Late Sinemurian Harbledownense chron only differs from the initial Sinemurian boundary by a lower minimum. No data exist from the Tetraspidoceras and Imlayi chrons adjacent to the Sinemurian-Pliensbachian boundary. A more balanced allocation of time requires downward adjustment of the Harbledownense chron and upward adjustment of the initial Pliensbachian. A possible less conservative interpretation of Item 17, the main control on the Late Sinemurian, would also suggest a younger age (192-194 Ma) for the Harbledownense chron. Therefore we pick 192 Ma (with errors adjusted to +3.8/-5.2 Ma) for the best estimate of the age of the Sinemurian-Pliensbachian boundary.  137  In the Middle Jurassic, the tight Toarcian-Aalenian boundary chronogram is identical to the middle-late Aalenian boundary chronogram calling for an upward shift in age. We propose a boundary age of 178.0 Ma (with errors adjusted to +1.0/-1.5 Ma). The Aalenian-Bajocian and Bajocian-Bathonian boundary chronograms are less tightly constrained and are also controlled by dates from the younger half of the stages. Their best estimates and maximum ages are only marginally older than those of the Late Bajocian Rotundum Zone and the Middle-Late Bathonian, respectively. Hence an upward adjustment of boundary ages is likely to produce more realistic allocation. We propose 174 Ma for the initial Bajocian boundary, corresponding to the oldest trial age with an error function value of zero. The duration of the late Aalenian Howelli Chron thus appears to be 3.6 Ma, conspicuously longer than any other chron. There are three controlling dates from the Howelli Zone (items 33-35) and none from the adjacent zones, therefore the unusually long chron duration may be an artifact of a slight inaccuracy in some of the isotopic dates. The preferred estimate for the Bajocian-Bathonian boundary is 166 Ma.  7.10.  LATEST JURASSIC STAGE BOUNDARY ESTIMATES THROUGH MAGNETOCHRONOLOGIC INTERPOLATION  The lack of isotopic ages from definitively dated Tithonian rocks renders chronogram estimation of the Kimmeridgian-Tithonian and the Tithonian-Berriasian boundaries extremely imprecise. Consequently, we employ magnetochronologic interpolation similar to that discussed in detail in GTS89 and MTS94. For the JurassicCretaceous transition, we use the magnetic anomaly block model derived from the Hawaiian lineation set that was shown to best approximate a constant spreading rate (Channell et a l , 1995). The oldest Cretaceous isotopic date available to anchor the magnetochronology is 137.1 + 1.6/—0.6 Ma (Item 56) that is indirectly correlated to the Late Berriasian M16 chron using nannofossils (Bralower et a l , 1990). A subseqent revision suggests that a broader correlation with the M16-M15 interval is more appropriate (Channell et a l , 1995). Anchoring the Jurassic side of the lineation set is more controversial. MTS94 uses direct dating of M26r at 155.3+3.4 Ma in the Argo Abyssal Plain (Ludden, 1992). This in fact is a minimum K - A r age of a celadonite vein therefore we did not include it in our dataset (see p. 129). From the same site, incremental heating A r - A r dating of basalt from M25-26 did not 4 0  3 9  yield a plateau age, only a disputable total fusion age of 155+6 Ma (2a) was obtained (Ludden, 1992).  138  Instead we rely on the chronogram estimate of the Oxfordian-Kimmeridgian boundary (154.7+1.0/-0.9 Ma). Magnetochronologic correlation of land-based, ammonite-constrained sections with the oceanic magnetic anomalies allow the placement of this boundary at the base of M25n (Ogg and Gutowski, 1996). As the isotopic ages controlling the chronogram are not precisely constrained (Items 48-55), allowance needs to be made for correlation uncertainty in the chronogram age. We regard M29 near the base of the Late Oxfordian (Ogg and Gutowski, 1996) as a maximum, based on the maximum age of the Morrison Formation by ostracods (Schudack et a l , in press) and ammonoids from underlying strata (Imlay, 1980; Callomon, 1984). The interpolation yields 151.5+1.0/-1.4 Ma for the Kimmeridgian-Tithonian boundary and 144.8+2.6/-3.7 Ma for the Tithonian-Berriasian boundary. The error limits reflect a combination of the numeric errors of the Oxfordian-Kimmeridgian boundary chronogram and the Berriasian isotopic age and the associated biochronologic/magnetochronologic correlation uncertainties.  7.11.  T H E JURASSIC TIME SCALE  In summary, the initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous)  144.8+2.6/-3.7 Ma  Tithonian  151.5 +1.07-1.4 Ma  Kimmeridgian  154.7 +1.0/-0.9 Ma  Oxfordian  156.6 +2.0/-2.7 Ma  Callovian  159.7+1.1 M a  Bathonian  166.0 +0.6/-5.4 Ma  Bajocian  174.0+1.0/-7.3 Ma  Aalenian  178.0 +1.0/-1.5 Ma  Toarcian  183.6 +1.6/—1.1 Ma  Pliensbachian  192.0 +3.8/-5.2 Ma  Sinemurian  197.0+1.2/-4.2 Ma  Hettangian (Triassic-Jurassic)  201 ± 1 Ma  In addition, Early and Middle Jurassic chron boundary ages were also estimated. Although this list remains incomplete, the following initial chron boundaries (other than those coinciding with stage boundaries) are known with less than 5 M a range of error: 139  Howelli (Aalenian)  177.6+1.4/—1.1 Ma  Yakounensis (Toarcian)  180.1+0.7/-3.0 Ma  Crassicosta (Toarcian)  181.4+1.2 Ma  Planulata (Toarcian)  182.8+2.3/-2.6  Carlottense (Pliensbachian)  184.1+1.0/-1.6Ma  Kunae (Pliensbachian)  185.7+0.6/-0.8 Ma  Freboldi (Pliensbachian)  186.8+1.8 Ma  Franziceras (Hettangian)  198.7+1.3 Ma  7.12.  DISCUSSION  The Jurassic time scale developed here differs from its predecessors in several aspects of methodology. It is based exclusively on U-Pb and Ar- Ar ages. Although it was not an explicit criterion for inclusion in the 40  39  isotopic database, all ages used were reported after 1990. Despite such restrictions, there are 50 Jurassic isotopic ages used in this study, slightly exceeding the similar number in previous scales (e.g, 45 in GTS89 and 43 in MTS94). Both GTS89 and MTS94 entered the errors at the 1 a confidence level whereas the more conservative 2a level is used throughout this study. Improvement is most significant in the Early Jurassic portion of the time scale. Through direct estimates of the base of the Hettangian and the Crassicosta Chron and eight additional, precise (i.e., error range is less than 5 Ma) chronogram boundary ages, nearly half of the 20 chrons now have acceptable boundary estimates. Direct estimation of zonal duration was attempted for parts of the Pliensbachian and Toarcian only. From the Freboldi to the Crassicosta chrons, a sequence of five well-constrained chronograms and a directly dated boundary permit us to propose the following best estimates for zonal duration: Freboldi: 1.1 Ma, Kunae: 1.6 Ma, Carlottense: 0.5 Ma, Kanense: 0.8 Ma, and Planulata: 1.4 Ma. Although the errors of boundary ages clearly indicate that chron durations are also subject to error, the observed more than three-fold difference among the chron durations is unlikely to be accounted for by random error. Therefore Jurassic ammonite chrons appear to be of disparate length. Such a conclusion seriously undermines earlier time scales which were founded on interpolation based on the assumption of equal duration of ammonite chrons. Fig. 7.3 presents a comparison of the proposed time scale with the five scales most widely used in recent works. Key observations are as follows: (1) The Jurassic Period had a duration of approximately 56 Ma, shorter 140  than previously estimated; (2)The Triassic-Jurassic boundary (201 Ma) is younger than previously thought; (3) There is a good agreement on the Jurassic-Cretaceous boundary age among the proposed scale (145 M a ) , D N A G , GTS89, and MTS94, whereas the same boundary is too young in EXX88 and OD94 which use K - A r glauconite ages; (4) The Late Jurassic appears to be significantly shorter (11.8 Ma) than the Early and Middle Jurassic epochs (23 and 21.4 Ma, respectively); (5) Re-apportioning of time in the Early and Middle Jurassic is a direct consequence of not using interpolated stage durations. As demonstrated here, the new generation of time scales need to be based exclusively on U-Pb and A r 4 0  3 9  A r ages to benefit from the improved precision and accuracy offered by these two dating methods. Improved  statistical techniques can be sought to refine age estimates while accommodating asymmetric, non-Gaussian errors frequently encountered in U-Pb dating. However, the goal of systematically defining chron boundary ages can only be achieved through the acquisition of still more calibration points, ultimately eliminating the need for interpolation. The dating of volcanic flow or pyroclastic units within fossiliferous sequences is proved to be the most successful way of obtaining calibration points. An increasingly more refined time scale offering consistently high precision and resolution at the zonal level is shown in this study to be a realistic goal.  141  DNAG  EXX88  GTS89  Ma  MTS94  OD94  130  TTH  135  ±5  TTH KIM  140  +7/-1  KIM TTH  OXF  TTH  ±11.3  KIM OXF CAL  CLV BTH  CLV  —  AAL  SIN HET  BAJ  170  AAL  175  AAL  ±4 0  180  TOA TOA  i  ±4.0  PLB ±3.9  SIN +4/-?  HET  185  PLB 190  PLB  SIN  SIN HET  1  195  1  200  -4.2  ±3.9 t 3  HET , ±4X  HET ,  165  +0.6 ±4.0  PLB  ±fi 25  HET  ±3.a  ±4.0  TOA  SIN  160  BAJ  AAL±10.5  ±15  SIN  CLV  BTH  +4/-3  ±7.5  PLI  KIM  BTH  CLV  ±2  ± 3 4  TOA  PLB  ±3.6  ±2  BAJ  AAL  150 155  BTH  BAJ  TOA  ±32  OXF  1 6 5  ±6.8  TOA  KIM  CLV  BAJ  BAT AAL  ±3.0  ±6  BTH BAJ  TTH  TTH  OXF ±5  KIM  OXF±75  145  ±2.6  ±9  ;205  ±7  ' 210 ;  Figure 7.3. Comparison of the proposed Jurassic time scale with other major time scales ( D N A G , E X X 8 8 , GTS89, MTS94, OD94). Stage abbreviations follow those in GTS89. Error bars for stage boundaries in the new scale are shown on the right.  142  215  CHAPTER 8  TIME CONSTRAINTS ON JURASSIC MASS EXTINCTIONS AND RECOVERIES  8.1. INTRODUCTION  Mass extinction events (MEE) have been in the focus of heightened paleontological research since the theory of extraterrestrial impacts leading to extinctions was documented by Alvarez et al. (1980). In attempting to understand the dynamics of M E E , attention and research effort has also been directed to the processes of biotic recovery following M E E (e.g. Hart, 1996). Basic questions concerning the dynamics of extinction and recovery, such as rates of diversity change, taxon origination, and extinction, can only be quantified using an accurate time scale. The detection of fine-scale patterns were previously prohibited by time scales that were based on the assumption of equal duration for biochronologic units. True zonal resolution time scales such as the one attempted here are needed to model recovery processes. The assessment of extinction periodicity, a pattern first documented by Raup and Sepkoski (1984), is also dependent on the time scale used. Stage level time series analysis of a compiled global taxonomic dataset demonstrates extinction peaks that surpass background extinction among both marine animal genera and families (Sepkoski, 1996). The end-Triassic crisis represents a first-order M E E whereas second order peaks corresponding to the Pliensbachian, Callovian, and Tithonian were also identified. We discuss below the implications of the amended Jurassic time scale, outlined in the previous chapter, for the end-Triassic and Jurassic M E E . Revised timing constraints help provide insights into  143  the following problems: (1) the timing and gradual vs. sudden nature of M E E ; (2) the timing of biotic recoveries; and (3) the spacing of M E E within the Jurassic and, in a broader context, the Mesozoic.  8.2. T H E END-TRIASSIC MASS EXTINCTION AND EARLIEST JURASSIC RECOVERY  The end-Triassic M E E is one of the five most severe biotic crises during the Phanerozoic. Recent summaries are given by Hallam (1990; 1996a). The age of the Triassic-Jurassic boundary, marked by this M E E , is shown to be 201±1 Ma (see chapters 6 and 7). A detailed discussion of this event and the implications of the revised age is also given in Chapter 6. The earliest Jurassic recovery is the most significant evolutionary radiation during the Jurassic. Unfortunately, the temporal framework outlined here remains inadequate to unambiguously describe the repopulation of niches vacated after the end-Triassic M E E . Besides the age of the Triassic-Jurassic boundary, only the initial boundary of the Franziceras chron (i.e., the beginning of the Middle Hettangian) is defined satisfactorily with the chronogram method as 198.7±1.3 Ma. The duration of the Early Hettangian (i.e., Planorbis standard chron) is therefore best estimated at 2.3 Ma but the error ranges indicate that this chron could also possibly be very short. Control over the younger Hettangian and Sinemurian chrons remains too weak to assess the recovery pattern. Owing to the sparsity of basal Jurassic rocks worldwide, earliest Jurassic faunas are not well known but appear to be cosmopolitan and of low diversity. Compilations of diversity changes exist for the ammonoids (Hallam, 1996b; Smith and Liang, in review), bivalves, and brachiopods (Hallam, 1987). Ammonoids, the best studied fossil group, suffered near-extinction at the Triassic-Jurassic boundary and show extremely low diversity at all taxonomic levels in the Planorbis Chron. The same holds true for the other groups as well. Thus, this zone can be regarded as the post-extinction lag period. Delayed recovery, known to be especially pronounced in the Early Triassic (Hallam, 1991), therefore may also characterize the Planorbis Chron. One of the simple and popular models describing evolutionary radiations is the logistic growth model predicting initial exponential growth in diversity, later dampened to approach an equilibrium state (Walker, 1985). Graphically it is manifest in a sigmoidal growth curve. Assuming that rates of taxonomic evolution, as expressed in diversity change, are reflected in the duration of  144  empirical biochronological units, the duration of the first few chrons is critical to determine the shape of the diversity growth curve. The greatest increase in supraspecific ammonoid diversity is confined to the Middle Hettangian which is therefore expected to be shorter than the Early Hettangian. Isotopic age constraints are not precise enough yet to substantiate this model. In fact, our dating results remain also compatible with alternative results of independent zonal duration estimates derived from Milankovitch cyclostratigraphy in southern England. Based on presumed cycle counts, Smith (1990) found the Planorbis Chron only about half as long as the next younger Liasicus Chron. One caveat is that the Triassic-Jurassic boundary as defined by the incoming of Psiloceras in England may be facies controlled thus this biozone may not represent the entire earliest Jurassic locally (Hallam, 1995).  8.3. T H E PLIENSBACHIAN-TOARCIAN EVENT  Raup and Sepkoski (1984) originally recognized a second order extinction peak in the Pliensbachian. Hallam (1986) argued that this event in fact occurred during the Early Toarcian and was regional in extent, in response to the anoxic event in epicontinental northwest Europe and the western Tethys (Jenkyns, 1988). Aberhan and Fursich (1995) noted that the extinction is recorded in South America thus it is not restricted to a single region. Recent detailed analyses at zonal level temporal resolution suggest that the Late Pliensbachian-Early Toarcian event spans five standard ammonite chrons across the Pliensbachian-Toarcian boundary (Little and Benton, 1995; Little, 1996). This coincides with the best controlled interval in our time scale which indicates that the elevated extictintion rate was sustained for 4.3+1.8/-2.0 Ma. Backward smearing in the fossil record (Signor and Lipps, 1982) is an alternative explanation to account for a protracted and apparently gradual extinction pattern but, in this case, the broad extinction plateau suggests a truly long-term event. There appears to be an immediate recovery after the Pliensbachian-Toarcian event, even if it failed to restore the diversity of ammonites and other groups to the Late Pliensbachian level (Hallam, 1996b; Smith and Liang, in review). Although not very well constrained, the average zonal duration for the Middle and Late Toarcian recovery interval (1.1+0.9/—0.7 Ma) is statistically indistinguishable from that of the preceding crisis (0.9+0.3/-0.4 Ma).  145  8.4. LATER JURASSIC EVENTS  The percent extinction curves of Sepkoski (1996) contain significant maxima in the Callovian and the Tithonian, similar in magnitude to the Pliensbachian. However, significant extinction in the Callovian is only discernible in filtered global data containing only "well-preserved" genera and it has not yet been recognized in local or regional studies (Sepkoski, 1996). There is one study which suggests elevated Ir levels at the CallovianOxfordian boundary in Poland (Brochwicz-Lewinski et a l , 1986). Our best age estimate for the CallovianOxfordian boundary is 156.6+2.0/-2.7 Ma. The Tithonian extinction peak is displayed at both genus and family level, in filtered or unfiltered global data (Raup and Sepkoski, 1984; Sepkoski, 1996). Nevertheless, it remains controversial because marked extinction appears to be restricted to bivalves and some other benthic groups in Europe. Its global extent has not been demonstrated. Changes in ammonoid faunas are less pronounced at the Jurassic-Cretaceous boundary (Hallam, 1986). Our estimate for the Tithonian-Berriasian boundary is 144.8+2.6/-3.7 Ma. Clearly, the significance and dynamics of these events are poorly understood. Aggravating the problem is the lack of finer, zonal resolution in our later Jurassic time scale. Much more work is needed to establish a better temporal framework for the purported Callovian and Tithonian extinction events.  8.5. SPACING OF JURASSIC AND OTHER MESOZOIC MASS EXTINCTIONS  The spacing of the end-Triassic and the Pliensbachian-Toarcian events was briefly discussed in Chapter 6. Here we develop a broader framework to discuss all Jurassic events discussed above, along with other major events in the Mesozoic or at its boundaries. Liberating the underlying time scale from the effect of interpolation based on the assumption of equal duration for zones/subzones removes any possible autocorrelation of the alleged extinction periodicity and biochronologies. Apart from our Jurassic time scale, we use estimates from direct dating of the following mass extinction boundaries: Permian-Triassic: 250 Ma (Claoue-Long et a l , 1991; Renne et a l , 1995), Cenomanian-Turonian: 93 Ma (Obradovich, 1993; Kowallis et a l , 1995), Cretaceous-Tertiary: 65 Ma (Sharpton et a l , 1992; Swisher et a l , 1992; Krogh et a l , 1993).  146  Cretaceous-Tertiary (K-T)  •  CenomanianTuronian (CEN-TUR)  23 [26]  •  Tithonian (TTH)  80 [78]  52 (521  •  Callovian (CLV)  9 2 [104]  6 4 [78]  1 2 [26]  •  PliensbachianToarcian (PLB-TOA)  117-120 [130]  89-92 [78]  37-40 [52]  25-28 [26}  136 [156]  108  5 6 [78]  4 4 [52]  [130]  16-19 [26]  •  185 [208]  157 [182]  105 [130]  93 [104]  65-68 [78]  49 (521  K-T  CENTUR  TTH  CLV  PLBTOA  T-J  P-T  65  93  145  157  182-185  201  250 Ma  end-Triassic (T-J) end-Permian (P-T)  A  •  •  Cretaceous-Tertiary (K-T)  •  CenomanianTuronian (CEN-TUR)  28 [26]  Tithonian (TTH)  80 (78] ii 52 [52]  Callovian (CLV)  9 2 [104]  6 4 [78]  1 2 [26]  •  PliensbachianToarcian (PLB-TOA)  117-120 [104]  89-92  37-40  [78]  [26]  25-28 [26J  136 [130]  108 (104]  56 (52]  4 4 [52]  16-19 [26]  •  185  157 (156]  105 (1041  93 [104]  65-68 [78]  49 [52J  K-T  CENTUR  TTH  CLV  PLBTOA  T-J  P-T  65  93  145  157  182-185  201  250 Ma  end-Triassic (T-J) end-Permian (P-T)  [1823  B  • •  •  •  Figure 8.1. Periods between mass extinction events in the Mesozoic and at its boundaries. For comparison, predicted values derived from the 26 Ma periodicity hypothesis (Raup and Sepkoski, 1984) are given in brackets. Bold face font in shaded box denotes good agreement between the model and actual values. A: Model periods (in brackets) based on a three period lengths duration of the Jurassic. B: Model periods (in brackets) based on a two period lengths duration of the Jurassic. See text for discussion. 147  A comparison of the actual period between extinction events with their predicted spacing based on the 26 Ma periodicity model (Raup and Sepkoski, 1984) is presented in Fig. 8.1. It allows a simple test of the model, without resorting to sophisticated mathematical tools. At both the young and the old ends of the studied interval, the Tithonian and Cretaceous as well as the end-Permian and end-Triassic events appear to fit the periodic model. The spacing of the two intra-Jurassic events, the Pliensbachian-Toarcian and the Callovian is also approximately 26 Ma. However, when compared to the younger and older events, these two extinctions appear consistently out of phase compared to the predicted periodicity. Curiously, equating the whole Jurassic to two period lengths instead of three restores the apparent periodicity among the pre-Jurassic and terminal to post-Jurassic events (see Fig 8. IB). The statistical significance of this pattern remains to be evaluated.  148  CHAPTER 9  SUMMARY  There are more than a dozen different time scales published in the last 15 years that provide age estimates for Jurassic stage boundaries. Their discrepancies and the large uncertainties hamper their use and warrant a new attempt to produce a refined time scale. A major weakness of the available scales is the small number and often low quality of isotopic ages used, among which K - A r and Rb-Sr dates of questionable reliability predominate. A further problem is the frequently imprecise stratigraphic constraints on many plutonic rocks dated. Recent advances in radiometric dating established the U-Pb and A r - A r methods as the two most precise 4 0  3 9  and accurate dating methods currently available. The use of these techniques to date volcaniclastic rocks of precisely known stratigraphic age promises to yield useful new calibration points, as proved by recent studies from other systems. Samples for U-Pb zircon dating were collected from Stikinia, Wrangellia, and Peninsular terranes of British Columbia and Alaska. In these Jurassic arc assemblages, the sampled volcaniclastic rocks vary from proximal pyroclastic flows to distal air-fall tuffs and occur interbedded in fossiliferous marine sediments that are dated by ammonite biochronology. The challenge of such an integrated dating approach is that thin ash layers, frequently found in abundantly fossiliferous sections, generally contain only a small amount of zircon, whereas proximal pyroclastics are more likely to yield abundant zircon but the surrounding sedimentary facies are often poorly fossiliferous. We report 18 new U-Pb ages from Jurassic rocks integrated with zonal biochronologic dating of under- and overlying strata. In the Queen Charlotte Islands, a tuff layer immediately below the Triassic-Jurassic boundary was  149  dated at 200±1 Ma. Lowermost Jurassic (middle and upper Hettangian) volcaniclastics from Alaska yielded ages of 200.8+2.7/-2.8 Ma, 197.8±1.0 Ma, and 197.8+1.2/-0.4 Ma. An ash layer near the base of the Crassicosta Zone (Middle Toarcian) in its type section in the Queen Charlotte Islands was dated at 181.4±1.2 Ma. The other new dates also furnish important time scale calibration points. The ammonite biostratigraphy from sections measured in the course of this study provide important contributions for the North American Jurassic, especially from the Hettangian of Alaska. Theoretical biochronological investigation is centered around the quantification of correlation uncertainty. Statistical range extension based on the number and density of occurrence levels in the observed ammonoid ranges was considered but the 95% confidence level is empirically shown to be too high for a well-studied ammonite locality. The computer-assisted Unitary Association method was used to identify global maximum ranges of Toarcian ammonoid taxa and determine the most likely and the maximum permissible correlation of a North American regional ammonite zone with the northwest European standard. The Unitary Associaton method is not corrupted significantly by the taxonomic noise in the database. The correlation uncertainty between the two regions is not more than ± 1 standard subzone. A radiometric age database was built for the construction of the revised Jurassic time scale. Apart from the 18 newly obtained U-Pb ages, sampling sites of several recently reported Cordilleran ages were studied in order to improve biochronologic constraints. Additional dates were compiled from previous time scales and recent literature. The database consists of 50 U-Pb and A r - A r ages with a precision of ±5 Ma (2a) or better that are 4 0  3 9  confined to no more than two adjacent stages, preferably expressed at zonal resolution. For the first time for the Jurassic, the calibration of chron boundaries was attempted. Direct dates are available for the Triassic-Jurassic boundary and the initial boundaries of the Crassicosta chron and the Callovian stage. The chronogram method was used to estimate all Early and early Middle Jurassic chron boundaries, late Middle Jurassic substage boundaries, and Late Jurassic stage boundaries. Chronogram estimates are meaningful only when controlling ages are available for a given stratigraphic unit, therefore the chronogram is different from those of the adjacent units. The most significant improvement concerns the Pliensbachian and Toarcian, where six consecutive chron boundaries are determined. The derived chron durations are disparate, varying between 0.4 and 1.6 Ma. The assumption of equal duration of chrons or subchrons, used for interpolation in several previous time scales, is therefore not considered valid. For Early and Middle Jurassic stage boundaries, we use the appropriate 150  chronogram estimate if it is unique and corresponds to the chronogram of the earliest chron of the given stage, otherwise adjustments are made and justified on a case by case basis. The latest Jurassic isotopic database remains too sparse, therefore chronogram estimates can be improved using interpolation based on magnetochronology. The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous)  144.8 +2.6A-3.7 Ma  Tithonian  151.5+1.0/-1.4 Ma  Kimmeridgian  154.7 +1.0/-0.9 Ma  Oxfordian  156.6 +2.0/-2.7Ma  Callovian  159.7 ±1.1 Ma  Bathonian  166.0 +0.6/-5.4 Ma  Bajocian  174.0+1.0/-7.3 Ma  Aalenian  178.0+1.0/-1.5 Ma  Toarcian  183.6 +1.6/—1.1 Ma  Pliensbachian  192.0 +3.8/-5.2 Ma  Sinemurian  197.0+1.2/-4.2 Ma  Hettangian (Triassic-Jurassic)  201 ±1 Ma  In addition, the following initial chron boundaries (other than those coinciding with stage boundaries) are known with better than 5 Ma range of error: Howelli (Aalenian)  177.6+1.4/-1.1 Ma  Yakounensis (Toarcian)  180.1 +0.7/-3.0 Ma  Crassicosta (Toarcian)  181.4+1.2 Ma  Planulata (Toarcian)  182.8 +2.3A-2.6  Carlottense (Pliensbachian)  184.1 +1.0/—1.6 Ma  Kunae (Pliensbachian)  185.7 +0.6/-0.8 Ma  Freboldi (Pliensbachian)  186.8+1.8 Ma  Franziceras (Hettangian)  198.7+1.3 Ma  151  This study demonstrates that a new generation of time scales should only employ reliable and precise U-Pb and A r - A r dating integrated with zonal biochronologic constraints. Although still more calibration points are 4 0  3 9  needed, a time scale for chron boundaries is a realistic goal, partially accomplished herein. One application of the revised time scale is a re-evaluation of the timing of mass extinction events and subsequent biotic recoveries. The Triassic-Jurassic boundary at 201 Ma is younger than suggested by most previous time scales. The end-Triassic marine and terrestrial extinctions appear to be simultaneous. The latest Triassic (Norian-Rhaetian) was a several million years long period of decline for ammonoids, bivalves, and conodonts, while marine microplankton and terrestrial vertebrates and palynomorphs show an abrupt change at the boundary. The duality in temporal extinction patterns may imply an interplay of of long-term environmental crisis severely affecting shelf habitats and an abrupt terminal Triassic event. The timing of the recovery is still not well constrained. The Planorbis Chron, interpreted as a post-extinction lag period of global low diversity, has an estimated duration of 2.3 Ma but its error range also permits a much shorter length. The second-order Pliensbachian-Toarcian mass extinction, recently shown to span five standard ammonite zones (Little and Benton, 1995), coincides with the best controlled interval of the new time scale. Elevated extinction rate was sustained for 4.3+1.8/-2.0 Ma, between 185.7 and 181.4 Ma. It appears to be followed by an immediate recovery, and average zonal durations before, during, and after the event are indistinguishable. The nature of the Callovian and Tithonian mass extinctions remains controversial. The 26 Ma periodicity of mass extinctions (Raup and Sepkoski, 1984), a theory criticized on the grounds of inaccurate time scales, was assessed using the revised Jurassic time scale and recent dating of other Mesozoic mass extinctions. 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Significance of xenocrystic Precambrian zircon contained within the southern continuation of the Josephine ophiolite: Devils Elbow ophiolite remnant, Klamath Mountains, northern California. Geology, 14: 671-674.  170  APPENDIX 1  INPUT OF COMPUTER-ASSISTED BIOCHRONOLOGIC CORRELATION USING BioGraph  A l . l . LIST OF AMMONOID TAXA AND THEIR CODES  OOBa OIBb OlOBc 02Bg 03Bp 04B  Brodieia  05Cc 06Cg 07C  Catacoeloceras Catacoeloceras Catacoeloceras  Brodieia Brodieia Brodieia Brodieia Brodieia  Collina 09Cg 090C1 Collina 091 C m Collina Collina 10C HDa 12Dc  alticarinatus bayani clausa gradata primaria  crassum ghinii  gemma linae mucronata  Dactylioceras Dactylioceras  athleticum commune  230Hp 24Hp 26Hs 27H 28Hf 29Hsx 30Hsp 31H  Hammatoceras  Harpoceras Harpoceras Harpoceras  praefallax speciosum  falcifer subexaratum subplanatum  Harpoceras  32Hi  Haugia  320Hn 33Hp  Haugia  34Hva 35Hvi 36H  porcarellense  Hammatoceras Hammatoceras Hammatoceras  illustris navis  Haugia Haugia  phillipsi variabilis  Haugia Haugia  vitiosa  Hildaites  Dactylioceras  37Hp 38Hsf  Hildaites  14Dm  Denckmannia  malagma  39Hs 40H  15Dt  Denckmannia  tumefacta  16D  Denckmannia  41Ha  apertum bifrons caterinii crassum lusitanicum semipolitum sublevisoni tethysi  13D  Hildaites  19Gs  Grammoceras  striatulum  47Hsl  20Gt  Grammoceras  thouarsense  48Ht  Hildoceras  21G  Grammoceras  49H  Hildoceras  22Hc  Hammatoceras  costatum  50Li  Leukadiella  23Hi  Hammatoceras  insigne  51L  Leukadiella  17Ef  Esericeras  fascigerum  18Fs  Frechiella  subcarinata  171  serpentiniformis serpentinus  Hildaites Hildoceras Hildoceras Hildoceras Hildoceras Hildoceras Hildoceras Hildoceras  42Hb 43Hca 44Hcr 45H1 46Hsp  propeserpentinus  ionica  54Mm 540Mt 55Mu 56M  Mercaticeras dilatum Mercaticeras hellenicum Mercaticeras mercati Mercaticeras tyrrhenicum Mercaticeras umbilicatum Mercaticeras  74Pp 75Pro 76Pru  78P  Phymatoceras pseudoerbaense Phymatoceras robustum Phymatoceras rude Phymatoceras speciosum Phymatoceras venustulum Phymatoceras  58Nc 59N  Nodicoeloceras crassoides Nodicoeloceras  79P 790P1  Podagrosites Podagrosites latescens  60Ps 61P  Paroniceras sternale Paroniceras  82Pa 83Pb 84Pd  62Pd  85Pf 86Ps 87P  640Pp 641Pp 65Ps 66Pve 67Pvx 68Pvc 69P  Peronoceras desplacei Peronoceras fibulatum Peronoceras moerickei Peronoceras planiventer Peronoceras pacificum Peronoceras subarmatum Peronoceras verticosum Peronoceras vortex. Peronoceras vorticellum Peronoceras  Pseudogrammoceras aratum Pseudogrammoceras bingmanni Pseudogrammoceras doerntense Pseudogrammoceras fallaciosum Pseudogrammoceras subregale Pseudogrammoceras  88Pe  Pseudolillia emiliana  93 Pf 94P1 95 Pr 96P  Pseudomercaticeras frantzi Pseudomercaticeras latum Pseudomercaticeras rotaries Pseudomercaticeras  70Pc 71 Pel 72Per 720Ph 73Pn  Phymatoceras crassicosta (^cornucopia) Phymatoceras elegans Phymatoceras erbaense Phymatoceras hillebrandti Phymatoceras narbonense  97Rp  Rarenodia planulata  98Zb  Zugodactylites braunianus  52Md 53Mh  63Pf 64Pm  77Ps 770Pv  172  A1.2. AMMONOID DISTRIBUTION IN REPRESENTATIVE TOARCIAN SECTIONS  Taxon codes are followed by range expressed in bed or level numbers (Sources, additional locality information, and notes are listed in Table 3.2, p. 35)  Section 1: Yakoun River Base: 19 Top: 130 OOBa 15Dt 16D 20Gt 21G 23Hi 27H 30Hsp 31H 50Li 51L 56M 64Pm 66Pve 69P 70Pc 720Ph 72Per 74Pp 76Pru  69-69 39-63 39-63 130-130 84-130 110-130 110-130 21-21 21-21 28-38 28-38 69-69 58-58 23-24 23-58 55-84  78P 79P 97Rp  Section 3: Valdorbia Base: 1 Top: 20 OOBa OlOBc OIBb 02Bg 04B 05Cc 06Cg 07C 090C1 09Cg 10C 18Fs 230Hp 27H 28Hf 29Hsx 31H  78P 790P1 79P 93Pf  91-130 77-95 37-71 77-83 37-130 84-130 84-130 55-66  96P 97Rp  55-66 19-55  Section 2: Joan Lake Base: 1 Top: 7  4-7 6-6 3-3  14-20 12-15 13-17 16-17 12-20 2-2 2-11 2-11 11-16 11-17 10-17 4-6 20-20  76Pru 770Pv  4-12 15-18 15-19  77Ps 78P 85Pf 86Ps 87P 94P1 95Pr 96P 97Rp  5-11 7-7 3-14  54Mm  1-1  50Li  2-2  55Mu  51L 641Pp 64Pm  2-2 2-2  56M  2-5 2-14  58Na 59N  3-3 3-4  60Ps 61P 62Pd  17-19 15-19 11-16  60-70 77-94 60-94 69-70 60-61 60-64  96-96 91-97  10C 13D 27H 31H 42Hb 45H1 46Hsp 47Hsl 49H  52Md 540Mt 54Mm  93-103  55Mu 56M  4-5  5-5 1-1  07C 13D 28Hf 31H 37Hp 38Hsf 39Hs 40H 45H1 47Hsl 49H  Section 4: Djebel-esSaffeh (3B)  44Hcr 45H1 46Hsp 47Hsl 48Ht 49H 50Li 51L  16D 31H 40H  Section 5: Djebel-esSaffeh (2) Base: 60 Top: 103  52Md 53 Mh  09Cg  6-9 2-3 4-4 2-9 6-10 4-10  87P  49-50 49-50  4-4  2-8  4-4  2-19 13-19 18-19 13-19 9-14 10-13 3-14  86Ps  60-70 96-103 91-95 91-103 97-97 91-91  OlOBc 02Bg 04B  39Hs 40H 41Ha 42Hb  5-5  66Pve 69P 720Ph  15-16 9-14 11-16 8-16 3-4 2-8 7-19 7-17  19-20 2-2 9-14 2-19 1-1 1-1 4-4  15Dt  4-5 2-2 2-5 6-7  63Pf 65Ps 67Pvx 69P 70Pc 71 Pel 72Per 75Pro  Base: 10 Top: 54 45-47 47^17 45^18 45-45 45^15 22-28  45-45 10-37 29^13 20-24 44^14 11-20 11-44 45^15  Section 6: Paghania Base: 1 Top: 78 OOBa OIBb  78-78 33-76  04B 05Cc 06Cg 07C  33-78 5-34  09Cg 10C 13D 27H 28Hf 31H  5^t9 1-68 33-69 33-69 1-51 74-74 8^14  20-20  37Hp  1-73 1-1  40H  1-1  55Mu  31-31 12-12 12-45 21-34  46Hsp 47Hsl 49H  32-36  56M 59N 61P 75Pro 78P  40^10  52Md 53Mh 54Mm  33-^9 22-22 29-69  52Md 53Mh  173  45^5 45^5  1-31 1-36  55Mu  20^10  16D  16-18  85Pf  18-44  56M  4-69  18Fs  A-A  87P  1-45  72Per 75Pro  33-39 11-30  28Hf  1-2 3-10  88Pe  44^8  76Pru 78P 93Pf 94P1 95Pr  55-71 11-77 7-32 33^14 3-52  Section 10: Ricla and La Almunia  96P  3-68  1-10 14-14 16-18 14-16 14-18 3-13 3-3 8-13 1-2  Section 7: Monte di Civitella Base: 1 Top: 32 OIBb 04B 18Fs 27H 28Hf 31H 40H 42Hb 46Hsp 47Hsl 540Mt 54Mm 55Mu 56M 59N 69P 71 Pel 72Per 770Pv 77Ps 78P 93 Pf 94P1 95Pr 96P  28-29 28-29 11-11 12-31 24-26 24-26 3-16 25-25 14-29 2-20 21-21 16-27 17-25 7-27 1-18 11-11 9-9 28-31 31-31  30Hsp 31H 320Hn 32Hi 34Hva 36H 42Hb 45H1 46Hsp 47Hsl 49H 52Md 56M 59N 60Ps 61P 620Pc 640Pp 66Pve 67Pvx 68Pvc 69P 71 Pel 73 Pn 75Pro 76Pru 78P 93Pf 96P 98Zb  1-13 12-13 12-13 10-15 17-17 17-17 9-9 9-9 7-7 7-7 7-7 7-18 6-6 7-10 6-11 14-15 6-11 16-16 16-16 4-4 •  86Ps 87P  213-225 211-263  88Pe  273-279 193-201  93 Pf 94P1 96P  165-165 165-201  Base: 55 Top: 307 03 Bp 04B 07C 10C 13D 14Dm 15Dt 16D 17Ef 22Hc 230Hp 23Hi 24Hp  197-213 197-213 165-229 165-165 101-133 177-185 209-233 177-233 241-245 231-237 287-287 265-275 281-307  26Hs 27H 28Hf  265-283 231-307 101-127  30Hsp 31H 33Hp 34Hva 35Hvi 36H 38Hsf 40H 41Ha  159-159 101-159 234-234 173-195 231-233 173-234  42Hb  84-84 55-113  Section 11: Ains Saint-Nicolas Base: 44 Top: 118 07C 11 Da 12Dc 13D 14Dm 15Dt 16D 17Ef 20Gt 21G 23Hi 27H 28Hf 30Hsp 31H 320Hn 32Hi 33Hp 34Hva 35Hvi 36H  80-80 56-56 56-58 56-58 82-84 90-94 80-94 109-109 109-109 109-109 118-118 113-118 44-56 67-68 44-76 84-84 85-86 90-90 79-84 96-96 79-96 58-64  135-149 143-169  41Ha 42Hb  59-77  43Hca 44Hcr 45H1  48-48 55-58  31-31 9-32 19-30  Section 9: Camplong Base: 1 Top: 68  43Hca 44Hcr 45H1  31-31 15-24 15-31  04B 15Dt 16D  1-1 1-2 1-20  17Ef  46Hsp 47Hsl 48Ht 49H  123-123 127-133 129-143 165-179 115-127 127-127 115-179  58-61 78-84  • 46Hsp 47Hsl 48Ht 49H  46-51 52-57 44-84  Section 8: St-Pauldes-Fonts  19Gs  20-20 18-24  58Nc  71-77  58Nc  44-44  20Gt  17-20  59N  71-89  59N  Base: 1 Top: 18  21G  17-24  61P  173-183  62Pd  44-51 70-70  22Hc 23Hi  1-1  62Pd 67Pvx  159-159  68Pvc  76-76  20-42  69P 70Pc 73 Pn 75Pro  3-3 1-3  78P 790P1  237-261  69P 70Pc 73 Pn 75 Pro 82Pa 83 Pb 84Pd  48-90  42-60 34-56 20-60 1-2  159-163 159-163 169-169 155-159 161-169 155-169  1-32 1-48 3-12  79P 82Pa 83 Pb  223-261 211-217 235-237  13-17  85Pf  255-263  OIBb 03Bp 04B 050Cd 07C 090C1 091 Cm  16-16 15-18 14-18 12-16 10-18 14-15 16-18  24Hp 26Hs 27H 32Hi 35Hvi 36H  09Cg  7-11 7-18  790P1 79P  1-3  83 Pb 84Pd  10C 13D 15Dt  17-17  174  70-90 65-66 70-70 92-92 100-100 105-105  85Pf  109-109  86Ps 87P  92-96 92-109  Section 12: Ravenscar and Whitby  46Hsp 47Hsl 49H  Base: 16 Top: 78  63Pf 65 Ps 66Pve  050Cd 05Cc  54-58  45-55 45-58 07C 091 Cm 48-49 48-49 10C HDa 27-28 12Dc 18-25 13D 18-28 23-23 18Fs 19Gs 75-77 20Gt 75-75 73-77 21G 30Hsp 32-42 31H 32-42 62-62 32Hi 34Hva 68-68 36H 58-72 30-51 42Hb 45H1 16-28  67Pvx 68Pvc 69P 84Pd 87P 98Zb  48-50  9-9 8-8 8-8 8-8  620Pc 640Pp  9-9 9-9  19-26 19-51 29-32 29-31 42-42  Base: 7 Top: 10  42^14  10C  10-10  65 Ps  8-8  31H 38Hsf 39Hs 40H 58Nc 59N 62Pd  7-10  69P 74Pp 75Pro  8-9 10-10 9-9 10-10 9-11  42^12 29^14 77-77 77-78 30-32  Section 13: Quebrada El Bolito Base: 6 Top: 9 31H 56M 641Pp 65Ps 69P 72Per 78P  10C 31H 40H 56M  Section 14: Quebrada Yerbas Buenas  6-8 6-6 7-7 6-6 6-8 9-9 8-9  641Pp 64 Pm 65Ps 66Pve 69P  7-7 7-7 7-7 7-7 7-7 8-8 9-9 10-10 8-8 9-9 8-10  Section 15: Quebrada Larga Base: 8 Top: 11  77Ps 78P  Section 16: Rio del Toro Base: 7 Top: 9 10C 29Hsx 31H 69P 78P  7-7 7-7 7-7 7-7 7-9  APPENDIX 2  CORRELATION TABLES USING UNITARY ASSOCIATIONS (BioGraph OUTPUT)  First column: level or bed number; second column: U A range (see Fig. 3.5 for definition of U A )  Section 1: Yakoun River 130: 129: 128: 127: 126: 125: 124: 123: 122: 121: 120: 119: 118: 117: 116: 115: 114: 113: 112: 111: 110: 109: 108: 107: 106: 105: 104: 103: 102: 101:  38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 38-38 37-38 37-38 37-38 37-38 37-38 37-38 37-38 37-38 37-38  100: 37-38 99: 37-38 98: 37-38 97: 37-38 96: 37-38 95: 37-37 94: 37-37 93: 37-37 92: 37-37 91: 37-37 90: 33-37 89: 33-37 88: 33-37 87: 33-37 86: 33-37 85: 33-37 84: 33-33 83: 14-25 82: 14-25 81: 14-25 80: 14-25 79: 14-25 78: 14-25 77: 14-25 76: 4-33 75: 4-33 74: 4-33 73: 4-33 72: 4-33 71: 13-27 70: 13-27 69: 20-24 68: 13-27  67: 66: 65: 64: 63: 62: 61: 60: 59: 58: 57: 56: 55: 54: 53: 52: 51: 50: 49: 48: 47: 46: 45: 44: 43: 42: 41: 40:  13-27 13-27 13-27 13-27 22-27 22-27 22-27 22-27 22-27 22-23 22-27 22-27 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22 22-22  34  39: 38: 37: 36: 35:  22-22 13-18 13-18 12-18 12-18  Section 3: Valdorbia  176  33 32 31 30 29 28 27 26 25 24  12--18 12 -18 12 -18 12 -18 12 -18 12 -18 12 -18 7--22 7--22 7--22 8--14  23 22 21  8--14 7--22 7--14  20  7--22  19  7--22  Section 2: Joan Lake 7  37--38  6 37--38 5 22--23 4 15--23 3 7- 22 2 12 -14 1  1--11  20: 35--35 19: 30--30 18: 30--30  17: 16: 15: 14: 13: 12:  28-28 26-27 26-27 24-24 24-24 23-24 11 19-19 10 18-18 9 18-18 14-14 14-14 12-13 7-8 7-7 4-4 4-14 1-1  Section 4: Djebel-esSaffeh (3B) 50: 49: 48: 47: 46: 45: 44: 43: 42: 41: 40: 39: 38: 37: 36: 35: 34: 33: 32: 31: 30: 29:  30-33 30-33 16-35 26-27 23-27 23-23 5-24 4-21 4-21 4-21 20-21 4-21 4-21 4-21 4-21 4-21 4-21 4-21 4-21 4-21 4-21 4-21  28: 27: 26: 25: 24:  4-16 4-16 4-16 4-16 5-8  23:  5-8  22: 21:  5-8 5-8  20:  5-8  19: 4-15 18: 4-15 17: 4-15 16: 4-15  Section 6: Paghania  24  15 14  4-15  13 12  4-15 4-15 2-15  78: 20-35 77: 16-35 76: 16-28  22 21 20  1-30  75: 74: 73: 72: 71: 70: 69: 68: 67: 66: 65: 64: 63: 62: 61: 60: 59: 58: 57: 56: 55: 54: 53: 52: 51: 50: 49: 48: 47: 46: 45: 44: 43: 42: 41: 40: 39: 38: 37: 36: 35: 34: 33: 32:  19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1  11 10  Section 5: Djebel-esSaffeh (2) 103 102 101 100 99: 98: 97: 96: 95: 94: 93: 92: 91: 90: 89: 88: 87: 86: 85: 84: 83: 82: 81: 80: 79: 78: 77: 76: 75: 74: 73: 72: 71: 70: 69: 68: 67: 66: 65: 64: 63: 62: 61: 60:  23  4-15  5-8 5-8 5-8 5-8 5-8 5-8 7-8 5-8 4-15 4-15 4-15 4-15 4-10 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 3-20 1-30 1-30 1-30 1-30 1-30 1- 30 2 -2 2-2 1-11 1-11 1-11 1-11 1-1 1-1 1-1 1-1 1-1  16-28 16-28 16-28 16-28 16-25 16-25 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-24 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 16-16 10-16  10-15 10-15 10-10 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 10-15 2-15 2-2  Section 7: Monte di Civitella  31: 10-15 30: 10-15  32 4 - 38 31 26-26 30 14-27 29 16-24 28 16-24 27 11-24 26 11-20 25 11-20 24 11-20 23 11-21 22 11-21 21 14-17 20 11-15 19 11-15 18 11-15 17 11-15 16 11-11 15 11-11 14 11-11 13 11-11 12 11-11 11 5- 11 10 4-11  29: 10-15  9  28: 10-15  8  27: 10-15 26: 10-15 25: 10-15  7 6 5  177  4-11 4-11 4-11 2-11 2-11  4: 2--11 3: 2--11 2: 2--15 1: 1--25  Section 8: St-Pauldes-Fonts 18: 17: 16: 15: 14: 13: 12: 11: 10: 9 8 7  27-28 28-28 27-27 25-25 23-23 17-21 17-21 12-21 12-14 12--12 12--14  12--14 6 10--14 5 7- 14 4 8- 9 3 7- 8 2 3--15 1 3--15  Section 9: Camplong 60: 59: 58: 57: 56: 55: 54: 53: 52: 51: 50: 49: 48: 47: 46: 45: 44: 43: 42: 41: 40: 39: 38: 37: 36: 35: 34:  40^40 40^10 40-40 40-40 40-40 40-40 40^10 40^40 40-40 40-40 40-40 40-40 40^10 40^10 40^10 40^10 40-40 40^10 40-40 40-40 40-40 40-40 40-40 40-40 40-40 40-40 40-40  33: 32: 31: 30: 29: 28: 27: 26: 25: 24: 23: 22: 21: 20: 19: 18: 17: 16: 15: 14: 13: 12: 11: 10:  38^10 38-39 38-39 38-39 38-39 38-39 38-39 38-39 38-39 38-39 38-39 38-39 38-39 39-39 36-39 36-39 36-36 36-36 36-36 36-36 36-36 34-34 34-34 34-34  9 34- -34 8 34- -34 7 34- -34 6 34--34 5 34--34 4 34--34 3 34--34 2 32--32 1 32--32  Section 10: Ricla and La Almunia 307: 306: 305: 304: 303: 302: 301: 300: 299: 298: 297: 296: 295: 294: 293: 292: 291: 290: 289:  40^10 40-40 40-40 40-40 40^10 40-40 40^10 40^10 40-40 40^0 40-40 40-40 40-40 40-40 40-40 40-40 40-40 40-40 40^10  288: 287: 286: 285: 284: 283: 282: 281: 280: 279: 278: 277: 276: 275: 274: 273: - 272: 271: 270: 269: 268: 267: 266: 265: 264: 263: 262: 261: 260: 259: 258: 257: 256: 255: 254: 253: 252: 251: 250: 249: 248: 247: 246: 245: 244: 243: 242: 241: 240: 239: 238: 237: 236: 235: 234: 233: 178  40^10 40-40 40-40 40-40 40-40 40^10 40-40 40^10 40-40 40-40 40-40 40-40 40^10 40-40 40-40 40^10 40^10 40-40 40^10 40-40 40-40 40-40 40-40 40^10 11^10 24^10 24-40 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 32-39 39-39 39-39 39-39 39-39 39-39 32-39 32-39 32-39 34-34 34-34 34-34 32-33 33-33  232: 231: 230: 229: 228: 227: 226: 225: 224: 223: 222: 221: 220: 219: 218: 217: 216: 215: 214: 213: 212: 211: 210: 209: 208: 207: 206: 205: 204: 203: 202: 201: 200: 199: 198: 197: 196: 195: 194: 193: 192: 191: 190: 189: 188: 187: 186: 185: 184: 183: 182: 181: 180: 179: 178: 177:  33-33 33-33 31-33 31-31 31-31 31-31 31-31 31-31 31-31 31-31 30-31 30-31 30-31 30-31 30-31 30-30 30-30 30-30 30-30 30-30 30-30 30-30 25-30 25-30 25-30 25-30 25-30 25-30 25-30 25-30 25-30 25-27 25-27 25-27 25-27 25-27 23-27 23-27 23-27 23-27 23-27 23-27 23-27 23-27 23-27 23-27 23-27 23-24 23-24 23-24 23-24 23-24 23-24 23-24 23-24 23-24  176: 23-24 175: 23-24  120:  3-15  64  1-11  119:  3-15  1-11  73: 72:  174: 23-24  118: 117: 116: 115: 114:  3-15  63 62  1-11  71:  61 60 59 58 57 56  1-11 1-11 1-11 1-11 1-11 1-11  55  1-11  173: 23-24 172: 5-24 171: 5-24 170: 5-24 169: 10-21 168: 10-21  113: 112: 111:  3-15 3-15 3-15 3-16 3-11 3-11 3-11 3-11 3-11 3-11 3-11  167: 166: 165: 164: 163:  10-21 10-21 15-21 10-21 12-21  162: 161: 160: 159: 158: 157: 156: 155: 154:  12-21 12-21 12-21 14-14 12-14 12-14 12-14 12-14 4-21  153: 152: 151: 150: 149: 148: 147: 146:  4-21 4-21 4-21 4-21 6-7 6-7 6-7 6-7  145: 144:  6-7 6-7  143: 142: 141:  6-7 6-7 6-7  140: 139: 138:  6-7 6-7 6-7  137: 136: 135: 134:  6-7 6-7 6-7 5-8  81 80 79 78  1-11 1-11  133: 132: 131: 130:  5-8 5-8 5-8 5-8  77  1-4,  76 75 74  129: 128: 127:  5-8 5-8 5-7  73 72 71  1-4 1-4 1-4 1^1  126: 125:  3-15 3-15  70  1-11  69  1-11  124:  3-15  123: 122: 121:  3-3 3-15 3-15  68 67 66  1-11 1-11  110: 109: 108: 107:  106: 3-11 105: 3-11 104: 3-11 103: 3-11 102: 3-11 101: 3-11 100: 1-11 99: 1-11 98- 1-11 97- 1-11 96 1-11 95 1-11 94 1-11 93 1-11 92 1-11 1-11 91 90 1-11 89 1-11 88 1-11 87 1-11 86 85 84 83 82  65  1-11 1-11 1-1 1-11 1-11 1-11 1-11  1^1 1-4  1-11 1-11  Section 11: Ains Saint-Nicolas  4-21 4-21  4-21 70: 14-21 69: 4-21 68: 7-14 67: 7-14 66: 12-14 65: 12-14 64: 6-7 63: 6-7 62: 6-7 61: 6-7 60: 6-7  118: 117: 116: 115: 114: 113: 109. 108 107 106  38-40 11-40 11-40 11^10 11^10 11-40 39-39 24^10 24-40 24-40  59: 58: 57: 56: 55: 54: 53: 52: 51: 50:  6-7 6-6 5-6 5-5 5-7 5-7 5-7 5-7 3-15 3-15  105 104 103 102 101 100 99: 98 97:  36-36 24-40 24^10 24-40 24^10 34-34  49: 48: 47: 46: 45: 44:  3-15 3-3 3-15 3-15 3-20 3-4  24-40 24^10 24^10  96: 33-33 95 30-33 94: 30-33 93 30-33 92: 30-30 91 23-33 90: 29-29 89: 23-29 88 23-29 87: 23-29 86: 27-29 85 27-29 84: 23-23 83 23-24 82 23-24 81 23-24 80: 23-24 79: 23-24 78: 5-24 77: 4-21 76: 11-14 75: 4-21 74: 4-21 179  Section 12 Ravenscar Whitby 78: 77: 76: 75: 74:  24^10 36-36 36-39 36-39 33-39  73: 33-39 72: 23-34 71: 23-34 70: 23-34 69: 23-34 68: 23-27 67: 23-34 66: 23-34 65: 23-34 64: 23-34 63: 23-34 62: 27-32 61: 23-34 60: 23-34 59: 23-34 58: 23-27  57: 56: 55: 54: 53: 52:  17-27 17-27 17-22 17-22 4-22 4-22  51: 50: 49: 48: 47: 46: 45: 44: 43: 42: 41:  4-21 5-21 21-21 21-21 4-21 4-21 4-21 12-21 12-21 12-14 7-14  40: 7-14 39: 7-14  38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22  7-14 7-14 7-14 7-14 7-14 7-14 9-9 9-9 9-9 9-24 5-5 5-5 5-8 5-6 5-6  21  5-6 5-6 5-6  20  5-6  19: 18: 17: 16:  5-6 5-6 5-8 5-8  Section 13: Quebrada El Bolito 9: 8: 7: 6:  14--37 4--29 12 -14 9--24  Section 14: Quebrada Yerbas Buenas 10: 15-23 9: 12-14  180  8: 14-24 7: 1-1  Section 15: Quebrada Larga 11: 4-38 10: 26-27 9: 12-12 8: 9-11  Section 16: Rio del Toro 9: 4--38 8: 4--38 7: 18 -24  

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