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Sedimentology of the Cretaceous-Paleogene Boundary Interval in the Tornillo Group of West Texas by Jacob Cobb, B.S. A Thesis In Geosciences Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCES Approved Thomas Lehman Chair of Committee Dustin Sweet Paul Sylvester Mark Sheridan Dean of the Graduate School August, 2016 Copyright 2016, Jacob Cobb Texas Tech University, Jacob Cobb, August 2016 ACKNOWLEDGMENTS I would like to thank Dr. Lehman for making this study possible and for taking me on as his student, and for all his help and support along the way. Thanks to my parents for their support, and my dad for all his help out in the field. I would like to thank my fiancé for all of her support through this process. This research was also aided by Texas Tech Geosciences faculty, as professors and students helped support and guide me through my research. Special thanks to Nick Miller for providing help and knowledge on the picking of charophytes. I would like to thank Mathew Pippin for all the support, and help with SEM imaging. To Tom Shiller, Zach Wilson, Jenna Kohn, Jenna Hessert, Kevin Werts, and Ethan Backus I appreciate all of the support. Special thanks goes to Ethan Backus for his help throughout this research. Dr. Sweet and Dr. Sylvester your time, help, and support meant a great deal to me and went a long way in this research. I would like to thank Melanie Barnes for her help, support, and also the use of her lab. Thanks to Dr. Hetherington and Dr. Zhoa for use of the SEM. I also would like to thank my God for providing me this opportunity and seeing me through. ii Texas Tech University, Jacob Cobb, August 2016 TABLE OF CONTENTS ACKNOWLEDGMENTS ................................................................................................... ii ABSTRACT ........................................................................................................................ v CHAPTER 1: INTRODUCTION ....................................................................................... 1 GEOLOGY OF THE TORNILLO GROUP ................................................................... 3 The Tornillo Basin ....................................................................................................... 3 The Tornillo Group ...................................................................................................... 4 THE K/PG BOUNDARY IN BIG BEND ...................................................................... 7 The Dawson Creek Exposure ...................................................................................... 9 The Rough Run Amphitheater ................................................................................... 10 The Rough Run East Exposure .................................................................................. 13 THE K/PG BOUNDARY AROUND THE GULF OF MEXICO ................................ 14 CHAPTER 2: GEOLOGY OF THE ‘ROUGH RUN EAST’ EXPOSURE ...................... 28 Stratigraphic Section at Rough Run East ................................................................... 29 The ‘Odd Conglomerate’ ........................................................................................... 33 CHAPTER 3: PALEONTOLOGY OF THE ‘ROUGH RUN EAST’ EXPOSURE ......... 46 Sites #1 through #4 .................................................................................................... 46 Site #5 ........................................................................................................................ 47 Site #6 ........................................................................................................................ 49 CHAPTER 4: SANDSTONE PETROGRAPHY AND GRAIN MORPHOLOGY ......... 55 Sandstone Petrography .............................................................................................. 56 Grain Morphology ..................................................................................................... 60 CHAPTER 5: GEOCHRONOLOGY ............................................................................... 87 Tom’s Top ................................................................................................................. 88 CHAPTER 6: DISCUSSION ............................................................................................ 98 Correlation of Rough Run East and Dawson Creek sections ..................................... 98 Paleontological and Isotopic Age Constraints on the K/Pg Boundary ...................... 99 Deposition of the ‘odd conglomerate’ ..................................................................... 102 iii Texas Tech University, Jacob Cobb, August 2016 Significance of Charcoal .......................................................................................... 106 Sand Grain Morphology .......................................................................................... 108 Significance of Shocked Quartz .............................................................................. 110 Interpretation of the ‘odd conglomerate’ ................................................................. 111 Comparison with modern tsunami deposits ............................................................. 112 Comparison with proximal K/Pg boundary Sites .................................................... 115 Future work .............................................................................................................. 118 CHAPTER 7: CONCLUSIONS ..................................................................................... 121 BIBLIOGRAPHY: .......................................................................................................... 124 APPENDIX: .................................................................................................................... 137 iv Texas Tech University, Jacob Cobb, August 2016 ABSTRACT A recently discovered outcrop of the Tornillo Group at Rough Run Creek in Big Bend National Park provides the best exposure of the Cretaceous/Paleogene (K/Pg) boundary interval thus far known in the Big Bend region. An unusual conglomerate bed is exposed here, and vertebrate fossils found above and below the conglomerate, as well as charophyte oogonia found in the bed itself, indicate that it was deposited at or around the time of the K/Pg boundary. The unusual conglomerate appears to occupy the base of a broad paleo-valley incised into underlying strata. The conglomerate bed is generally less than a meter in thickness, but includes very large boulders, and is generally much coarser grained than conglomerates found in typical fluvial channel deposits in the Tornillo Group. Its composition is also unusual, and consists of rounded sandstone clasts, abraded pedogenic carbonate nodules, mudstone ‘rip-up’ clasts, and charcoal, all derived from local sources. The conglomerate was deposited during or shortly after motion on a small fault that was buried during sedimentation, and at least three successive episodes of sediment transport occurred under rapid (upper flow regime) conditions, with clasts transported only a short distance prior to deposition. Sand grains from the matrix of the conglomerate and from a sandstone layer at its top include quartz grains with planar deformation features, and spherules with ‘splash’ and ‘ablated’ morphologies. These unusual aspects of the conglomerate suggest that it may record the Chicxulub impact event. The very coarse conglomerate reflects a series of brief, high velocity flow events, perhaps recording the transit of tsunami waves upstream several hundred kilometers from the coastline at that time. If this interpretation is correct, then v Texas Tech University, Jacob Cobb, August 2016 the conglomerate bed may provide the first documentation of this event in a proximal (within several thousand km from Chicxulub impact) non-marine setting. Other well studied proximal K/Pg boundary sites around the Gulf of Mexico and Caribbean are exclusively in marine deposits. Other known continental K/Pg boundary sites are in distal settings where only a thin atmospheric fall-out layer is recorded. Further detailed studies of the chemistry, mineralogy, and precise age of the conglomerate are necessary to determine whether it actually records the K/Pg boundary event. vi Texas Tech University, Jacob Cobb, August 2016 List of Tables 4.1. Petrography of Rough Run Creek East Region Sandstone Units.…………………....71 4.2. Petrographic data of time periods within the Tornillo Basin………………………...72 5.1. U/Pb zircon Concentrations at Tom’s Top.……………………………….................92 5.2. Zircon standard GJ tabulated data……………….…………………………………..93 5.3. Zircon standard Plesovice tabulated data……………………………………………94 vii Texas Tech University, Jacob Cobb, August 2016 List of Figures 1.1. Geology and structure of the Laramide Tornillo Basin…………………...………….19 1.2. Stratigraphy of the Tornillo Group in Big Bend……………………….……………..20 1.3. Stratigraphy of the K/Pg boundary interval in the Tornillo Group on Dawson Creek ……………………………………………………………………………………...21 1.4. Stratigraphy of the K/Pg boundary interval in the Tornillo Group at the Rough Run Amphitheater…..………………………………………………………………...22 1.5. Geologic map of “Rough Run Amphitheater”………………………………………..23 1.6. Geologic map of south bank of Rough Run Creek.…………………………………..24 1.7. Exposures at “Rough Run East” showing position of the odd conglomerate………...25 1.8. Map of the Gulf of Mexico at end of Cretaceous time, showing comparison
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