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The Distribution and Origin of Silcrete in the Ogallala Formation, Garza County, Texas

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The Distribution and Origin of Silcrete in the Ogallala Formation, Garza County, Texas The Distribution and Origin of Silcrete in the Ogallala Formation, Garza County, Texas by Zaneta Larie McCoy, B.S., B.A. 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 SCIENCE Approved Thomas Lehman Chair of Committee Eileen Johnson Haraldur Karlsson Peter Holterhoff Peggy Gordon Miller Dean of the Graduate School August, 2011 © 2011, Zaneta Larie McCoy, B.S., B.A. Texas Tech University, Zaneta L. McCoy , August 2011 Acknowledgements I would first like to thank my wonderful and thoughtful husband, Travis. Your unconditional love, support, and humor have brought me through some hard times. I would not be where I am without you and our fur-babies, Wednesday, Katt, and Bear. I would like to extend my gratitude and appreciation to my thesis advisor, Dr. Tom Lehman, and my thesis committee, Drs. Eileen Johnson, Hal Karlsson, and Peter Holterhoff. Dr. Lehman, your educational guidance, which often took the form of weekly pep-talks and humor-laden e-mails, has proved to be invaluable in the course of this research. The extent of your knowledge in geology and, for that matter, silcretes, which apparently only six people in the whole world care about, still astounds me. Dr. Lehman, I guess you are one of those six people, afterall. Dr. Johnson, your financial support during my time at Texas Tech is greatly appreciated. Also, your constructive criticism and knowledge of Southern High Plains archaeology has proved invaluable in finishing this project. Dr. Karlsson, thank you for your help in stable isotope geochemistry and hydrous minerals. Dr. Holterhoff, thank you so much for your positive attitude and open mind. Your enthusiasm for geology is infectious. I am indebted to my parents, Efrain and Sandra Martinez, and my parents-in-law, Mark and Vicki McCoy. Mom and Dad, thank you for your love and encouragement ii Texas Tech University, Zaneta L. McCoy , August 2011 these many years, and well, raising me. You guys are the best. Mark and Vicki, thank you so much for your love and support. I feel lucky to count you as my family. I would like to thank James Browning, the unsung hero of the Geosciences Department. James, your ninja skills in thin-section production are unmatched. I would especially like to thank my Lubbock family, Nolwenn Coint and Joseph Schubert. Nolwenn, ma biche, thank you for always being there when I needed to talk about the perils of geochemistry, the weirdness that is sedimentary petrology, or the latest Harry Potter flick. You are a true friend and confidant. Joseph, although we started off as acquaintances many years ago in San Antonio, you’ve become one of my best friends. I could not imagine the world without you in it. Thank you to my field buddies, Dr. Stance Hurst and Katherine Bell Ehlers. Stance, you’re a great field manager and an even better friend. Katherine, thank you for being a great friend and for making those long, hot days in the field tolerable. I would finally like to thank Dr. Zach Sharp and Justin Dodd at the University of New Mexico’s Stable Isotope Lab for letting this opal-fiend tinker with their beautiful laser fluorination line. And, even though I did vacuum away one of my samples (accidently), it was still an awesome experience. This research was funded by the Museum of Texas Tech University and a Texas Tech University internal grant through the Research Development Fund Grant Competition and represents part of the ongoing Lubbock Lake Landmark regional research into grasslands hunter-gatherers and adaptation to ecological change. iii Texas Tech University, Zaneta L. McCoy , August 2011 Table of Contents Acknowledgements ...................................................................................... ii Abstract .......................................................................................................... vi List of Tables .................................................................................................. vii List of Figures ................................................................................................. viii Chapter I. Introduction ............................................................................................. 1 Ogallala Formation Silcretes ..........................................................................1 The Nature of Silcretes ...................................................................................2 Silcretes on the Great Plains ..........................................................................5 II. Regional Geology and Silcrete Exposures ........................................... 6 Geologic Setting .............................................................................................6 Silcrete Exposures...........................................................................................9 III. Previous Research on Silcretes ............................................................. 20 Fabric Classifications ......................................................................................20 Genetic Classifications ...................................................................................22 IV. Petrography ............................................................................................ 29 Methods .........................................................................................................29 General Petrology ..........................................................................................29 iv Texas Tech University, Zaneta L. McCoy , August 2011 Petrography ...................................................................................................32 Interpretation .................................................................................................61 V. Mineralogy and Geochemistry ............................................................ 65 Sample Preparation .......................................................................................65 Oxygen Isotope Thermometry........................................................................78 Interpretation .................................................................................................89 VI. Archaeological Significance .................................................................. 93 Background ....................................................................................................93 Ogallala Silcrete as a Lithic Resource .............................................................97 Macroscopic Identification of Ogallala Silcrete Types ...................................106 Potential for Isotopic Discrimination of Lithic Materials ...............................108 VII. Discussion .............................................................................................. 110 Silica Paragenesis ...........................................................................................110 Genetic Classification of Ogallala Silcretes ....................................................116 Possible Silica Sources ....................................................................................121 VIII. Summary and Conclusions .................................................................. 127 References ..................................................................................................... 129 Appendix: Thin Section Data ...................................................................... 137 v Texas Tech University, Zaneta L. McCoy , August 2011 Abstract Silica-cemented sandstone, conglomerate, and caliche intervals (“silcretes”) occur at multiple levels within the Ogallala Formation in Garza County, Texas. The distribution, stratigraphic position, petrology, and isotopic geochemistry of Ogallala silcretes are of interest because stratigraphic constraints indicate that they must have formed very recently, and at low temperature (2 to 17˚C) under groundwater conditions. Silicification of these silcretes, mainly by opal-CT, began with: 1) precipitation within intergranular pore spaces; 2) silicification by replacement of microcrystalline calcite; and 3) silica precipitation within remaining voids. The sequence of silica precipitation within voids was from opal-CT to lussatite to chalcedony to mega-quartz. The Ogallala silcretes likely formed through the mixing of groundwater and lake water (Pleistocene Spring Creek Lake) that created an environment conducive to the precipitation of opal-CT. vi Texas Tech University, Zaneta L. McCoy , August 2011 List of Tables 4.1 Thin section identification to corresponding bed and location ........................30 4.2 An overview of the outcrop location and the type of silicification...................30 4.3 Point count data for Ogallala silcrete samples .................................................35 5.1 Loss-on-ignition data for the three silica samples (Macy locality 71) ..............66 5.2 Stable oxygen isotopic compositions of opal-CT samples (Macy locality 71) and other chert samples from the study area ....................75 5.3 Stable carbon and oxygen isotopic compositions of lacustrine carbonate and caliche samples .........................................................................79 5.4 Summary of temperature estimates for opal-CT precipitation ........................86 5.5 Temperature estimates for the Pleistocene Spring Creek lacustrine carbonate sample LC-1 .....................................................................................92
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