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Fluvial Evolution Between the Salt Wash and Brushy Basin Members of the Upper Jurassic Morrison Formation, South-Central Utah

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Fluvial Evolution Between the Salt Wash and Brushy Basin Members of the Upper Jurassic Morrison Formation, South-Central Utah Jhei i thIB w2!$ suypofttcl., 11tt Rward EiVfin 1 i:1t Jtts rci:tJYUJY1h l'Utot:hef' oubtattdiYi,.g si-udtttt · ii thz bto1o,BB .De:yaA1ttz:itt 1 lkt\Jiti11t,,y J 1dtr1:M$ofa1 1)u11itJt FLUVIAL EVOLUTION BETWEEN THE SALT WASH AND BRUSHY BASIN MEMBERS OF THE UPPER JURASSIC MORRISON FORMATION, SOUTH-CENTRAL UTAH A THESIS ( SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Riyad Abdulrahim Ali-Adeeb IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE June 2007 Copyright© Riyad Abdulrahim Ali-Adeeb 2007 ABSTRACT Abrupt changes in fluvial deposition between the Salt Wash.Member and Brushy Basin Member of the Upper Jurassic Morrison Formation, in western interior North America, have been documented across a seemingly basin-wide depositional unconformity that resulted from poorly understood base level changes. Detailed field observations, paleopedologic analysis, and petrographic analysis of proximal deposits in the Henry · Basin in south-central Utah, yield no depositional hiatus in the proximal basin during formation of the unconformity in the distal basin. Rather, the proximal stratal architecture suggest continuous deposition between the two members that likely resulted from a combination ofrelatively high basin subsidence rates and an increase in gravel fraction in the sediment source. Computed subsidence rates in the southern Henry Basin from recent radiometric dates document a time-averaged subsidence rate of 0.121 mm per year, while petrographic evidence suggests an up-section increase in chert-rich sediment source. Collectively these resulted in starving the distal basin of sediment and the formation of a distal depositional hiatus, while the proximal basin accumulated prograding sediments. A south-to-north transect near the western margin of the Morrison Basin documents a rapid pinch out of 150 meters of fluvial gravelly-sandstones of the Salt Wash Member between the southern Henry Basin to the south and the Emery High region 7 5 miles to the north. Mature paleosols at the base of the Morrison Formation in the north suggest that the lack of Salt Wash Member deposits were due to non-deposition rather than to post depositional erosion or incision. This suggests that syndepositional basin subsidence in the south channeled deposits there, while regions to the north underwent little to subsidence and deposition. The disparity in basin subsidence rates between closely spaced regions in proximal regions, combined with petrographic evidence of increased gravel-fraction in the sediment, suggests that tectonic thrusting to the west may have contributed an important role in producing continuous aggradation of sediment in the proximal basin, while contemporaneously starving the distal basin and forming a depositional unconformity. Incised valley-fill conglomerates at the contact between the two members also suggest that the change in sedimentation was influenced by a drop in base level that followed deposition of the Salt Wash Member. 1 ACKNOWLEDGMENTS I dedicate this thesis to my wife, Jessica, who shares my passion for the Geological Sciences, who provided me with incredible support, and helped me begin, and complete this project. I would like to thank my advisor, Dr. Timothy M. Demko, for sharing with me his passion for Mesozoic stratigraphy on the Colorado Plateau and the wonderful challenges in unraveling the mysteries of the dinosaur-rich Morrison Formation. I would also like to thank my thesis committee members, Dr. John Swenson for introducing me to the wonders of numerical basin modeling, Dr. Pat Farrell for her support in understanding the relevance of paleosols in the Morrison Formation, and Dr. Michael Jackson, for his insight and guidance of the paleomagnetic analyses in this project. I would also like to acknowledge the help and support of the faculty and staff of the Institute for Rock Magnetism at the campus of the University of Minnesota, Twin Cities. I would also like to thank my colleagues Ryan Erickson, Nick Freiburger, and Joseph Beer for their insights, feedback and support for this project, and Eric Tharalson for his assistance in the field. In addition, many faculty members at the University of Minnesota Duluth provided much support and guidance throughout the duration of this project, including Dr. Richard W. Ojakangas for his help and feedback with petrographic analysis in this project. Funding for this project was provided by the American Association of Petroleum Geologists, the Colorado Scientific Society, the University of Minnesota Graduate School, the College of Science and Engineering, and the Department of Geological Sciences at the University of Minnesota Duluth. 11 TABLE OF CONTENTS ABSTRACT ........................................................................................... i ACKNOWLEDGMENTS ..................................................................... ..... ii TABLE OF CONTENTS .......................................................................... iii LIST OF FIGURES ................................................................................. v LIST OF TABLES ................................................................................. vii 1. INTRODUCTION ...................... ........................................................... 1 1.1 Purpose ........................................................ ........... ·............... 1 1.2 Background ............................................................................. 1 1.3 Approach ................................................................................ 4 2. GEOLOGIC BACKGROUND ........................................................ ......... 5 2.1 Overview ....... ·.................. , ...................................................... 5 2.2 Tectonic Setting ......................................................................... 5 2.3 Paleogeography ....................................................................... 10 2.4 Paleoclimate .......................................................... ; ................ 12 2.5 Stratigraphy of the Morrison Formation ................... ................ : ...... 12 2.6 Age of the Morrison Formation .................................................... 17 2.7 Paleosols in the Morrison Formation .............................................. 18 2.8 Evolution of the Morrison Rivers .... ....... ... ... ................................. 21 2.9 Magnetostratigraphy of the Morrison Formation ................................ 22 2.10 Base Level ............................................................................ 23 3. METHODS ........... ....................................................................... ... 27 3.1 Measured Sections ................................................................ .. 27 3.2 Paleocurrents ................ : ...................................... ............ ..... 29 111 3.3 Petrography ........................................................................... 30 3.4 Paleomagnetism ...................................................................... 30 4. RESULTS ....................................................................................... 37 4.1 Lithology and Stratigraphy ........... .............................................. 37 4.2 Facies Architecture .................................................................. 65 4.3 Paleocurrents ......................................................................... 70 4.4 Petrography ........................................................................... 73 4.5 Paleomagnetism ...................................................................... 78 5. INTERPRETATIONS ....... .................................................................. 84 5.1 Depositional History Reconstruction .............................................. 84 5.2 Basin Subsidence ..................................................................... 90 5.3 Unconformity Paleosols ............................................................. 91 5.4 Provenance ............................................................................ 93 5.5 Paleorn:agnetic Interpretation ....................................................... 96 6. DISCUSSION ................................................................................. 101 7. CONCLUSION ................................................. : ..... ........................ 115 APPENDICES ... : ................................................................................ 116 I. Measured Sections and Paleocurrents ........................................... 116 II. Petrographic data ................................................................... 143 III. Magnetostratigraphic data ............. ............................................ 147 REFERENCES ................................................................................. .. 150 IV LIST OF FIGURES Figure 1.1 Field study area and distribution of Morrison Formation outcrops ............... 2 Figure 2.1 Tectonic setting of western North America during the Late Jurassic Period ... 6 Figure 2.2 Subsidence in the Henry Basin ....................................................... 9 Figure 2.3 Paleogeography of the Colorado Plateau during the Jurassic Period ........... 11 Figure 2.4 Stratigraphic nomenclatures of Upper Jurassic Strata on Colorado Plateau .. 14 Figure 2.5 Age of the Morrison Formation ..................................................... 19 Figure 2.6 Magnetostratigraphic framework of the
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