TECTONIC PROCESSES AND CONTINENTAL HYDROCLIMATE DURING THE LARAMIDE DEFORMATION IN THE CENTRAL ROCKY MOUNTAINS by MIN GAO Presented to the Faculty of the Graduate School of The University of Texas at Arlington in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY THE UNIVERSITY OF TEXAS AT ARLINGTON May 2017 Copyright © by Min Gao 2017 All Rights Reserved ii Acknowledgements I would first like to express my deep appreciation to Dr. Majie Fan for her continual guidance and support throughout the course of my research and the preparation of my dissertation. I appreciate the great amount of time and patience that Dr. Fan has put on teaching me field geology and lab techniques, inspiring my research thoughts, and helping me improve writing. I would also like to thank the members of my dissertation committee, Drs. Asish R. Basu, Elizabeth M. Griffith, William A. Griffith, and James P. Grover, for their time, comments, and guidance throughout the preparation and review of this dissertation. A special thanks is extended to Dr. Robert Moucha for his contribution and helpful discussion in the first chapter. Special thanks also goes to Patricia Garay, Jackie Garcia and Lu Zhu for their great help in the field or in the laboratory. Great thanks to my parents, Kehong Zhao and Mingqian Gao, my sister, Jing Gao, and my best friend, Longfei Wu for supporting and encouraging. I also wish to thank Lu Zhu, Junjie He, Mingjia Ma, Jenna West, Ohood Al Salem and Zhiye Gao for being good friends and great support during my time at the University of Texas at Arlington. This research was supported by grant PRF–54673–DNI8, a UTA research enhanced grant, and two Geological Society of America student research grants. April 17, 2017 iii Abstract TECTONIC PROCESSES AND CONTINENTAL HYDROCLIMATE DURING THE LARAMIDE DEFORMATION IN THE CENTRAL ROCKY MOUNTAINS Min Gao, PhD The University of Texas at Arlington, 2017 Supervising Professor: Majie Fan The central Rocky Mountains in western U.S.A. is a geologic province formed by the low-angle subduction of the Farallon oceanic plate under the North American continental plate during the latest Cretaceous–early Eocene Laramide orogeny. The tectonic processes and mechanism, the paleotopography and depositional environment, and the evolution of continental hydroclimate during the Laramide deformation remain controversial. This dissertation examines the tectonic processes of the Laramide orogeny throughout the entire central Rocky Mountains, and paleotopography, depositional processes, and paleoclimate in the Greater Green River Basin in southwestern Wyoming. The first project of this study uses a 2D flexural subsidence modeling method to explore the mechanism that links the surface deformation pattern to deep mantle processes during the Laramide deformation. The stiffness of Wyoming lithosphere decreased spatially iv from northeastern Wyoming (Te=32–46 km) to southwestern Wyoming (Te=6–9 km), while varied slightly in each basin during the ~30 Myr duration of the Laramide deformation. We attribute this southwestward weakening pattern to bending stresses, crust–mantle decoupling and end load effect. We also suggest a two-stage Laramide deformation based on the accelerated uplift of Laramide mountains during the early Eocene. The second project combines lithofacies analysis, sandstone petrography and oxygen isotope paleoaltimetry to examine the depositional environment, sediment provenance, and paleotopography of the early Paleogene Greater Green River Basin, southwestern Wyoming. The reconstructed paleoelevation of the Uinta Mountains to the south of the Greater Green River Basin was at least 3.5 km during the earliest Eocene. Surface uplift of the Uinta Mountains during the earliest Eocene resulted in changes in drainage pattern, depositional environment and sediment sources in the Greater Green River Basin. The third project uses multiple climate proxies including bulk organic carbon 13 isotope record (δ Corg), paleosol morphology, atmospheric CO2 concentration (pCO2) and mean annual precipitation (MAP) to study the continental hydroclimate characteristics in the Greater Green River Basin during the late 13 Paleocene-early Eocene. Two negative δ Corg excursions were recognized, representing two early Paleogene hyperthermals: PETM and ETM-2. The climate in the basin was generally warm and humid during late Paleocene-early Eocene, but transient drying occurred during the extreme hot hyperthermals. v Table of Contents Acknowledgements ................................................................................................ iii Abstract .................................................................................................................. iv List of Figures ......................................................................................................... x List of Tables ........................................................................................................ xii Chapter 1 Introduction ............................................................................................ 1 Chapter 2 Southwestward weakening of Wyoming lithosphere during the Laramide orogeny ............................................................................................. 7 Abstract ............................................................................................................... 8 2.1 Introduction ................................................................................................... 9 2.2 Geologic setting .......................................................................................... 11 2.2.1 Regional tectonics ................................................................................ 11 2.2.2 Basin stratigraphy and age constraints ................................................. 13 2.3 Methodology ............................................................................................... 15 2.3.1 Decompaction ...................................................................................... 16 2.3.2 2D flexural subsidence modeling ......................................................... 17 2.4 Load heights and lithospheric stiffness ....................................................... 21 2.5 Discussion ................................................................................................... 22 2.5.1 Southwestward weakening of Wyoming lithosphere .......................... 22 2.5.2 Implications for the geodynamics of Laramide deformation ............... 28 2.6 Conclusions ................................................................................................. 32 vi Acknowledgements ........................................................................................... 33 References ......................................................................................................... 33 Figures and tables ............................................................................................. 43 Chapter 3 Depositional environment, sediment provenance and oxygen isotope paleoaltimetry of the early Paleogene Greater Green River Basin, southwestern Wyoming, U.S.A. ................................................................ 53 Abstract ............................................................................................................. 53 3.1 Introduction ................................................................................................. 54 3.2 Regional geology ........................................................................................ 56 3.3 Lithofacies and depositional environments ................................................ 59 3.3.1 Fort Union Formation: Sandy Meandering River Association .................................................................................................... 60 Description ................................................................................................ 60 Interpretation ............................................................................................. 61 3.3.2 Wasatch Formation: Sandy Braided River Association ....................... 63 Description ................................................................................................ 63 Interpretation ............................................................................................. 64 3.4 Sandstone petrography ................................................................................ 65 3.4.1 Methods and Description ..................................................................... 65 3.4.2 Interpretation ........................................................................................ 67 3.5 Oxygen isotope paleoaltimetry ................................................................... 69 vii 3.5.1 Methods ................................................................................................ 69 3.5.2 Results .................................................................................................. 70 3.5.3 Interpretation ........................................................................................ 72 Screening carbonate diagenesis ................................................................ 72 Presence of dolomite ................................................................................. 75 Carbonate and water δ18O values .............................................................. 76 High earliest Eocene paleorelief between the Uinta Mountains and