PETROGRAPHY AND GEOCHEMISTRY OF DOLOMITES IN THE LOWER CRETACEOUS EDWARDS FORMATION, TAYLOR COUNTY, TEXAS by CYNTHIA LYNN WELCH, B.S. A THESIS IN GEOSCIENCE Submitted to the Graduate Faculty of Texas Tech University m Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved May, 2001 ACKNOWLEDGEMENTS I extend my gratitude to both the National and the Southwest Section chapters of AAPG and to Texaco Exploration and Production for theu assistance in fimding my graduate work. I would also like to thank the Department of Geosciences at Texas Tech University for the innumerable education opportunities I have received. I would like to thank my advisor, George B. Asquith for his guidance, constant enthusiasm, and most of all his love of Geology that has inspired me from the first day I sat in his classroom. Thank you to Dr. Calvin G. Barnes for his advice and guidance during both my undergraduate and graduate careers. Teachers, in the true sense of the word, are rare. Thank you to my graduate committee: Dr. George B. Asquith, Dr. Calvin G. Barnes, Dr. Moira K. Ridley, and Dr. Haraldur Karlsson for all of their comments and support in developing this thesis. I would like to thank Mike Gower for the preparation of my thin sections and his overall ability to do anything asked of him. Thank you to Rusty Winn for his invaluable tutorials in the isotope lab, James Browning for his assistance in the isotope lab, and to Melanie Barnes for her assistance with the trace element analysis. Thank you to the Department of Biological Sciences Electron Microscopy Laboratory at Texas Tech University for providing facilities for SEM analysis and to Dr. Mark Grimson for assistance in the SEM lab. 1 would like to thank my family for their constant role as my support system. My parents, Tom and Sharon Welch have always supported me without hesitation and 11 encouraged me to achieve my goals without limitation. Thank you to my sister, Kathleen Welch for her amazing strength and constant belief in my fiiture. I would like to thank Jana Moore for her encouragement, inexhaustible patience, and enduring love. Finally, I dedicate this thesis to the memory of my Grandmother, Margaret Welch who would have been dancmg at my graduation if she were able. HI TABLE OF CONTENTS ACKNOWLEDGEMENTS ii ABSTRACT vi LIST OF TABLES ix LIST OF FIGURES x CHAPTER I. INTRODUCTION 1 Purpose of Study 1 Previous Work 3 Dolomitization 4 Methods of Study 9 II. STRATIGRAPHY AND REGIONAL DEPOSITIONAL SETTING 11 Regional Stratigraphy 11 Regional Depositional Setting 11 III. PETROGRAPHY 17 General Information 17 Detailed Descriptions and Photomicrographs 20 IV. SCANNING ELECTRON MICROSCOPY ANALYSIS 33 Purpose of Analysis 33 Detailed Descriptions and SEM Photos 33 IV V. MULBERRY CANYON AREA DEPOSITIONAL HISTORY 42 VI. ISOTOPE, MAJOR ELEMENT, AND TRACE ELEMENT GEOCHEMISTRY 55 Analytical Methods 55 Models of Dolomitization 68 Results 76 Interpretation of Geochemistry 80 VII. DL\GENETIC HISTORY AND CONCLUSIONS 92 Penecontemporaneous Dolomitization Event 92 Subtidal Dolomitization Event 94 Leaching Event 96 Porosity-filling Dolomite Event 97 Diagenetic Model 99 VIII. SUMMARY 100 REFERENCES 102 APPENDIX A. X-RAY DIFFRACTION ANALYSIS CALIBRATION CURVES 107 B. ISOTOPE, MAJOR ELEMENT, AND TRACE ELEMENT DATA 109 ABSTRACT The Lower Cretaceous (Edwards) Mulberry Canyon Section is located in Taylor County, Texas and consists of two tidal flat-capped parasequences that contain three stages of dolomite. Of the three stages of dolomite present, previous work has established the probable environment and conditions for the formation of the first stage of dolomite, however, the environment and conditions for the other two stages has not been conclusively established. The first stage is fine-grained (<10}am), matrix-selective, often- laminated dolomite that has replaced the micrite in unfossiliferous mudstones (tidal flats). The second stage is fine-grained (<10nm), matrix-selective dolomite that has replaced the micrite in fossiliferous subtidal wackestones/packstones. The third stage is coarse­ grained (10-240}am) dolomite that is filling or partially filling fossil-moidic porosity along with equant calcite in the fossiliferous subtidal wackestones/packstones. The purpose of this study is to use petrographic (petrography, scanning electron microscopy, and x-ray diffraction) and geochemical (major element, trace element, and isotopic) analyses to: (1) establish the depositional history of the Mulberry Canyon Section, (2) document the three stages of dolomite present in the section, and (3) establish the environment and conditions of dolomitization for each stage present in the Mulberry Canyon Section. A hypersaline penecontemporaneous origin for the dolomitized tidal flats is suggested by the presence of mudcracks, tepee structures, possible algal laminations, modified cubic pyrite crystals, and by their association with the Kirschberg Evaporite. VI The geochemical data supports a hypersaline penecontemporaneous origin for the tidal flat dolomite. Values for 6^^0 of both tidal flats (6'^O=-0.9 to -0.25foPDB, 5^^C=0.6 to 1.9%oPDB) have a more hypersaline signature when compared to Lower Cretaceous marine (6^^0=-4.4%oPDB, 5^^C=2.4%oPDB) and meteoric (6^^0= -8.8%PPDB, 5^^C=l.l%oPDB) values determined m this study. Values for 6^^C of both tidal flats 1 ^ (5 C=0.6 to 1.9%oPDB) vary in a possible mixing fi^ctionation trend suggesting (along with the presence of modified pyrite) bacterial processes such as methanogenesis and/or sulfate reduction in the tidal flats. Na/Ca versus Mg/Ca, and Sr/Ca values of the tidal flat dolomitized mudstones exhibit a slight positive correlation, which suggests an evaporative association and supports the isotopic data. The tidal flat hypersaline brines sank, moved seaward (seepage reflux), and mixed with marine water. This mixed hypersaline and marine fluid was responsible for selective dolomitization of the underlying subtidal wackestones/packstones matrix. The similarity in isotope and trace element geochemistry between the dolomitized mudstones (tidal flats) and subtidal dolomitized matrix indicates similar dolomitizing fluids were responsible in both stages of dolomitization. Oxygen isotope values (6'^0=-3.7 to -0.996oPDB) of the dolomitized subtidal wackestone/packstone matrix below the upper tidal flat have a marine signature, suggesting a marine influence in this stage of dolomite. A slight positive correlation in Na/Ca versus Mg/Ca, and Sr/Ca values of the subtidal dolomitized matrix suggests evaporite association and, therefore, an association with the hypersaline brines responsible for the dolomitization of the tidal flats. Vll Coarse-grained dolomite filling or partially filling moldic porosity exhibits marine to meteoric isotopic values (6'*0=-7.2 to -4.7%,PDB, 8^^C=-0.3 to +0.5%<,PDB). hi addition to isotopic evidence, the coarse-grained dolomite exhibits similar concentrations of Mn, Fe, and Sr (Mn=124ppm, Fe=1098ppm, and Sr=276ppm) to the meteoric values (Mn=l 17ppm, Fe=863ppm, and Sr=192ppm) suggestmg that mixed waters (marine and meteoric; Dorag dolomitization) were responsible for this last stage of dolomite. Vlll LIST OF TABLES 3.1 Percentage Dolomite in Mulberry Canyon Section Samples 17 6.1 Isotopic Results from Alizarin Red-S Treated and Untreated Calcite Standard 60 6.2 Major and Trace Element Results from Alizarin Red-S Treated and Untreated Calcite Standard 62 6.3 Diagnostic Geologic, Petrographic, and Geochemical Characteristics of Different Dolomitization Models 74 6.4 Geologic, Petrographic, and Geochemical Results for the Three Types of Dolomites Present in the Mulberry Canyon Section 90 B. 1 Marine and Meteoric Calcite Fraction Isotope Values for Samples from Fluvanna Quarry 110 B.2 Marine and Meteoric Trace Element Values for samples from Fluvaima Quarry 110 B.3 Dolomite Fraction Oxygen and Carbon Isotope Values for the Mulberry Canyon Section Samples 111 B.4 Calcite Fraction Oxygen and Carbon Isotope Values for the Mulberry Canyon Section Samples 112 B.5 Concentrations of Ca, Mg, Na, and Sr for Samples in the Mulberry Canyon Section 113 B.6 Concentrations of Fe, Mn, Ba, and Zn for Samples in the Mulberry Canyon Section 114 B.7 Concentrations of Al and K for Samples in the Mulberry Canyon Section 115 IX LIST OF FIGURES 1.1 Map Illustrating General Location of Mulberry Canyon Section 2 1.2 Seepage Reflux, Evaporative Pumping, and Mixing Zone (Dorag) Models for Dolomitization 7 2.1 General Stratigraphic Nomenclature of the northem Edwards Plateau 12 2.2 Location of Study Area, the Lower Cretaceous Shelf Edge, and the Concho Arch 13 2.3 Paleoenvironmental Map of Texas During Lower Cretaceous Time 15 3.1 Stratigraphic Column of Mulberry Canyon Section 18 3.2 Photomicrographs of Units 1 through 3 21 3.3 Photomicrographsof Units 3 through 7 23 3.4 Photomicrograph of Unit 8 and Outcrop Photographs of Units 8 and 9 26 3.5 Photomicrographs of Units 9 through 12 28 3.6 Photomicrographs of Units 13 and 14 31 4.1 SEM Photographs of Units 1 through 3 34 4.2 SEM Photographs of Units 4 through 8 36 4.3 SEM Photographs of Units 8 through 11 38 4.4 SEM Photographs of Units 11 through 14 40 5.1 Mulberry Canyon Section Stratigraphic Column with Depositional Environments 43 5.2 Lithofacies Distribution During Deposition of Unit 1 45 5.3 Lithofacies Distribution During Deposition of Units 2 through 4 45 5.4 Lithofacies Distribution During Deposition ofUnits 6 through 10 46 X 5.5 Lithofacies Distribution During Deposition of Unit 11 46