DEPOSITIONAL ENVIRONMENTS, SEDIMENTOLOGY, AND STRATIGRAPHY OF THE DOCKUM GROUP (TRIASSIC) IN THE TEXAS PANHANDLE by BRENT A. MAY, B.S. A THESIS IN GEOSCIENCE Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved Accepted August, 1988 ACKNOWLEDGMENTS I would like to extend my appreciation to Dr. Thomas Lehman for suggesting this project and chairing the thesis committee. Dr. Lehman's advice in the field and during the writing of this study was invaluable. Financial assistance could not have obtained without his help. Dr. Sankar Chatterjee and Dr. Alonzo Jacka offered enlightening discussions and helpful editorial comments for this project. Invaluable field assistance was given by Dick Record and Kerry "Crockett" Howard. Special thanks goes to Susan Ensenat for her helpful questions and observations in the field. Dr. Necip Guven and Darrell Brownlow gave generous assistance for the interpretation of X-ray diffraction patterns. Thanks goes to my parents for providing food, shelter, and moral support during research in the Palo Duro Canyon area. I would like to thank Edwin and Claudine May for shelter and excellent food while conducting field work in the Tule Canyon area. Appreciation is extended to Dave May for the use of a field vehicle along with computer hardware, software, and advice. The Texas Parks and Wildlife Department was gracious enough to allow this study to be conducted in the Palo Duro Canyon and Caprock Canyons State Parks. Funding for this project was granted by the American Association of Petroleum Geologists and the Geological Society of America. This project is dedicated to the memory of my grandfather, David Ross Sherman. ii CONTENTS ACKNOWLEDGMENTS ii ABSTRACT vi LIST OF FIGURES vii I. INTRODUCTION 1 Objectives and Study Methods 1 Study Area, General Geology, and Stratigraphy 2 Previous Works 10 II. STRATIGRAPHY 13 Introduction 13 Biostratigraphy 14 Tecovas Formation 23 Aeolian unit 24 Channel sandstones 27 Paleosols 29 Multi-colored mudstones 33 Upper mudstone 36 Trujillo Formation 38 Stratigraphic Problems 44 Lithostratigraphic correlation 47 Naming inconsistencies 48 Informal subdivisions 52 m III. SEDIMENTARY FACIES 53 Introduction 53 Aeolian Faci es 54 Dune sub-facies 56 Interdune sub-facies 59 Fluvial Channel Facies 60 Thalweg sub-faci es 61 Channel-fill sub-facies 61 Point bar sub-facies 66 Abandonment sub-facies 78 Small-scale channel deposits 80 Overbank Faci es 85 Levee sub-facies 85 Crevasse splay sub-facies 86 Sheet f 1 ow sub-f aci es 89 Floodplain sub-facies 92 Valley-Fill Facies 93 IV. SANDSTONE PETROGRAPHY 101 Methods 101 Sandstone Mineralogy 101 Sandstone Classification 107 Cements 119 IV V. PALEOCURRENT ANALYSIS 123 Methods 123 Previous Analyses 126 Results 129 VI. DEPOSITIONAL ENVIRONMENTS 138 Introduction 138 Current model 138 An Alternate Depositional Model 140 Stream patterns 140 Depositional history 141 Overbank environments 147 Paleogeomorphology 150 Paleoclimate 152 Problematic Deposits 153 Lacustrine interpretation 154 Fluvial overbank interpretation 160 Source of volcanic ash 162 Fluvial and Deltaic-Lacustrine Comparisons 163 Point bars and sheetflows versus deltas 163 Fluvial overbank versus lacustrine 164 Paleocurrent data versus sandbody geometry 167 Suggestions for Further Research 168 VII. CONCLUSIONS 170 BIBLIOGRAPHY 172 ABSTRACT The Triassic Dockum Group consists of nonmarine "red beds" exposed around the Southern High Plains of Texas and New Mexico. The Dockum Group is subdivided into two formations: the lower Tecovas Formation and the upper Trujillo Formation. Both formations are comprised of conglomerate, sandstone, siltstone, and mudstone. Sandstones from the two formations are readily distinguished petrographically. Aeolian, fluvial channel, fluvial overbank, and valley-fill facies are present in the Dockum Group. Dune and interdune sub-facies are recognized in the aeolian facies. The fluvial channel facies is divided into thalweg, channel-fill, point bar, and abandonment sub-facies. Levee, crevasse splay, sheetflow, and floodplain sub-facies are identified in the fluvial overbank facies. Valley-fill facies are exposed at three localities in the study area. Extrabasinal conglomerates are associated with the valley-fill deposits. Paleocurrent data yield a grand vector mean of N 19 W for Dockum sandstones. This value compares well with previous paleocurrent analyses. The prevailing depositional model for the Dockum indicates the presence of deltaic, lacustrine, and fluvial environments. A "Dockum lake" is postulated to have formed in the relict Midland Basin. This study proposes a wholly fluvial setting for the Dockum Group. Deposits of low and high sinuosity streams dominate Dockum deposystems. Lacustrine deposits reflect only small local lakes or ponds. No deltaic deposits are present. VI ^^^^aP^fflSSSH»W^^iM^D^BBB^IB^H®i«!»Sl!e«™iL3K!a'aaiE5^^ FIGURES 1.1 Outcrop areas of the Dockum Group and location of the detailed outcrop studies in this report 3 1.2 Location of detailed outcrop studies in the Palo Duro Canyon area 6 1.3 Location of detailed outcrop studies in the Caprock Canyons area 8 2.1 Correlation of Triassic continental deposits in the United States 15 2.2 Divisions of the Alpine and Germanic sequences and phytosaur zonations in Germany 18 2.3 Phytosaur zonation and correlation of North American Triassic strata 21 2.4 Correlation of measured sections of the Dockum Group .. In Pocket 2.5 The unconformable Permo-Triassic contact 25 2.6 Aeolian sandstone at the base of the Tecovas Formation .... 26 2.7 Typical sandstone from the Tecovas Formation 28 2.8 Typical paleosol unit occurring in a basal aeolian sandstone unit 30 2.9 Manganese nodules occurring in the paleosol unit 31 2.10 Silcrete horizon occurring in a paleosol unit 32 2.11 Capitol Peak in Palo Duro Canyon 34 2.12 Typical red and yellow mudstones in the Tecovas Formation 35 2.13 The upper sandstone of the Tecovas Formation 37 2.14 Interbedded sandstones and mudstones of the Trujillo Formation 39 vii 2.15 Generalized cross-section of the Dockum Group along the eastern "Caprock" escarpment showing major sandstone sequences; contacts between the Permian, Tecovas, Trujillo, and Tertiary; and underlying Paleozoi c structural features 42 2.16 Comparison of the stratigraphic nomenclature applied to the Dockum Group in Texas 45 2.17 Correlation of the lower Dockum Group with Triassic deposits in New Mexico and the Colorado Plateau 50 3.1 Example of the dune and interdune sub-facies in aeolian deposits of the Dockum Group 57 3.2 Trough cross-beds with re-activation surface 63 3.3 Example of the channel-fill sub-facies, thalweg sub-facies, and floodplain sub-facies 64 3.4 Example of stacked channel-fill sub-facies 67 3.5 Lateral accretion surfaces in point bar sub-facies 69 3.6 Example of point bar sub-facies 71 3.7 Outcrop drawing of a cross-sectional view of a channel deposit exhibiting point bar sub-facies 73 3.8 Example of a channel deposit with point bar sub-facies ... 75 3.9 Clay plug filling a stream channel 79 3.10 Example of abandonment sub-facies and a capping channel- fill sub-facies 81 3.11 Example of a small-scale channel deposit 83 3.12 Example of levee sub-facies within floodplain sub-facies 87 3.13 Example of crevasse splay sub-facies and floodplain sub-facies 90 3.14 Outcrop drawing of a composite valley-fill sequence 96 3.15 Slumped sandstone boulders at the base of a valley- fill sequence that has eroded into channel abandonment sub-facies 98 viii 3.16 Multiple stream channel facies within a valley-fill sequence 99 4.1 Photomicrograph of polycrystalline quartz grain 102 4.2 Photomicrograph of a typical orthoclase grain which is slightly altered and shows no twinning 104 4.3 Foliated quartz-mica schist rock fragment and muscovite grain 105 4.4 Photomicrograph of a squashed claystone fragment 106 4.5 Photomicrograph of a rounded zircon grain 108 4.6 Photomicrograph of a vertebrate bone fragment in calclithic conglomerate 109 4.7 Mineralogical composition of Dockum sandstones showing compositional groups A, B, and C discussed in text Ill 4.8 Photomicrograph of a typical Tecovas sandstone sample .... 114 4.9 Photomicrograph of a typical Trujillo sandstone sample ... 116 4.10 Photomicrograph of a calclithic conglomerate 118 4.11 Photomicrograph of clay coating surrounding a monocrystalline quartz grain 120 4.12 Photomicrograph of pore-filling kaolinite-dickite booklets 122 5.1 Trough cross-bedding in plan view 124 5.2 Trough cross-bedding in cross-section oblique to paleo-flow 125 5.3 Rose diagrams displaying paleocurrent directions from sandstones in the Trujillo Formation 127 5.4 Paleocurrent data from Boone's "meanderbelt system" in Tule Canyon 130 5.5 Rose diagrams generated from paleocurrent data collected from Trujillo Formation sandstones 133 5.6 Rose diagrams constructed for the Trujillo Formation, Tecovas Formation, and the Dockum Group total 135 ix Sv^ESs^^ssaH 6.1 Generalized cross-section from the sediment source area to the Dockum Basin 142 6.2 Photomicrograph of Tecovas silcrete 145 6.3 X-ray diffraction pattern of a red overbank mudstone sample from the Trujillo Formation 148 6.4 X-ray diffraction patterns of yellow smectite-rich mudstone from a problematic deposit in the Tecovas Formation .... 155 6.5 Possible "lungfish burrows" within smectite-rich mudstones of the Tecovas Formation 158 6.6 Gray mudstone and a rare concretionary carbonate in a pond deposit 161 6.7 Fine-grained epsilon cross-beds 165 CHAPTER I INTRODUCTION ^ The Triassic Dockum Group is composed of nonmarine "red beds" exposed around the Southern High Plains of Texas and New Mexico. The Dockum crops out beneath the "Caprock" of the High Plains in many places. The abundance and quality of exposures allow an excellent opportunity to study the origin of these deposits.