SURFACE AND SUBSURFACE ANALYSIS OF THE MIDDLE JURASSIC EXETER SANDSTONE IN NORTHEASTERN NEW MEXICO by KEVIN BUGEL, B.S. A THESIS IN GEOLOGY Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved Accepted May, 1991 ACKNOWLEDGEMENTS I would like to express my appreciation to ...• ....., I J- ! ) , • v I Dr. George Asquith for his suggestion of the thesis topic and for his assistance and support throughout the course of this study. The assistance and suggestions of Dr. Thomas M. Lehman and Dr. Alanzo D. Jacka were also appreciated. I would like to thank New Mexico Bureau of Mines and Mineral resources for their funding of the field work and use of their geologic resources facility. The financial assistance of the Adobe Chair for both field expenses and film processing was also acknowledged. Also greatly appreciated was the field assistance of Tom Richardson and Jeff Over. I would like to thank Mike Gower for his participation in the thin section preparation. I am grateful to my parents Tom and Donna Bugel for their unlimited support and for believing in me. I would also like to thank the residents within the study area for their interest and cooperation. Special thanks are extended to the Oberg and Miera families for their hospitality and friendship. ii TABLE OF CONTENTS Page . ACKN'OWLEDGEMENTS • •••••••••••.••••••••••••••••.••... 11 ABSTRACT . ..............•........................... v .. LIST OF FIGURES . .................................. V11 CHAPTER I. INTRODUCTION. 1 Purpose of Study.................... 1 Methods of Study.................... 2 Location of Study Area.............. 3 Physical Geography.................. 3 II. STRATIGRAPHIC RELATIONSHIPS.............. 7 General. 7 Type section. 13 Regional Stratigraphy............... 14 Regional Depositional Setting....... 16 III. SEDIMENTARY FACIES ANALYSIS.............. 21 General. 21 Fac1.es. 1............................ 22 Characteristics................ 22 Depositional Systems........... 27 Facies 2............................ 32 Characteristics................ 32 Depositional Systems........... 39 Facies 3 . 4 0 Characteristics. 4 o Depositional Systems........... 41 IV. PETROLOGY. 57 General. 57 Mineral Composition And Texture..... 58 Gra1n. S1ze.. 62 Cements. 6 6 iii Page v. PALEO-WIND ANALYSIS. • • • • • • • • • • . • • • . 78 General .................. 78 Discussion ............... 78 VI. DEPOSITIONAL ENVIRONMENTS. 83 General .............. 83 Lithostratigraphic Relationships .. 84 Exeter Geometry .................. 92 VII. SUMM.A.RY •••••••••••••••••••••••••••••••••• 97 REFERENCES ..........•...•.....•.................... 100 APPENDICIES APPENDIX A ........•............................ 108 APPENDIX B ••••••••••••••••••••••••••••••••••••• 174 APPENDIX C ••••••••••••••••••••••••••••••••••••• 184 APPENDIX D ••••••••••••••••••••••••••••••••••••• 192 iv ABSTRACT The Middle Jurassic Exeter Sandstone in northeastern New Mexico is a medium- to fine-grained, moderate to well sorted quartzose sandstone. This sandstone can be subdivided into three eolian facies: 1.) cross-stratified dune facies; 2.) horizontally stratified interdune and extradune facies; and 3.) massive (bioturbated) interdune and extradune facies. These three eolian facies are complexly interbedded throughout the study area. The cross-stratified dune facies consists of large sets (up to 4.6 meters ) of tabular and trough cross­ strata with grainflow and grainfall laminations. The horizontally stratified interdune and extradune facies display horizontal to low angle (<10°) stratification that commonly exhibit bimodal grain size distributions. Similar bimodal grain size distributions have been described in both ancient and modern interdune lag or reg deposits. The massive (reworked) interdune and extradune facies consist of burrowed and bioturbated sandstones with minor wave ripples, soft sediment deformation, and desiccation polygons. The association of these internal features with the massive facies suggests an intermittent interdune or extradune pond or playa depositional environment. The three eolian facies of the Exeter Sandstone in northeastern New Mexico display criteria similar to v those described in the Permian White Rim Sandstone. An erg margin depositional setting for the Exeter Sandstone is also supported by regional paleo-wind studies on Jurassic eolian transport systems. Paleo-wind analysis of the Exeter Sandstone suggests windflow toward the south-southwest and to the northeast with a regional resultant vector of N.79° E. The large variations are typical of arc spreads found in modern and ancient eolian deposits of crescentic dunes. The absence of readings to the northwest suggests that the paleo-wind direction was from the northwest. This regional paleo­ wind assessment is in agreement with the mid-Jurassic paleo-wind trends of other Jurassic eolian deposits. Isopach maps of the Exeter reveal a series of southeasterly trending isopach thicks and thins. This geometry is interpreted to represent a series of sand ridges (draa?) and depressions (interdraa corridors?) which were influenced by pre-Exeter paleotopographic features. vi LIST OF FIGURES Figure Page 1.1 Location of Study Area ••...•.............. 4 1.2 Prominent Physiographic Features of Northeastern New Mexico ••................. 5 2.1 Schematic diagram of the mid-Jurassic system showing the underlying and overlying relationships .•...•............. 8 2.2 Digitized induction and compensated neutron density log illustrating the Jurassic subsurface stratigraphic relationships .... 9 2.3 Unconformable lower contact between the mid-Jurassic Exeter and the Upper Triassic DocJcum Group.............................. 10 2.4 Late Paleozoic Uplifts and Basins in New Mexico and Colorado ••.......•.•....... 17 2.5 Regional reference map of the major tectonic elements of northeastern New Mexico.................................... 18 3.1 Cross-stratification in Facies 1 ......... 23 3.2 Coarse-grained laminations ••..•..•....... 26 3.3 Scour and fill structure in Facies 1 ..... 28 3.4 Horizontal stratification in Facies 2 .... 33 3.5 Oscilation-ripples in float in the base of unit 2 (Facies 2) at Cimarron East (measured section 26) ...•................. 35 3.6 Barite lenses in unit 3 (Facies 2) at Gallegos South (measured section 2) .•..... 36 3.7 Clay laminations in Facies 2 ............. 37 3.8 Bioturbated sediments of Facies 3 ........ 42 vii Figure Page 3.9 Bioturbated sediments in Facies 3 .•....... 44 3.10 Burrows in the bioturbated sediments in Facies 3 at Trujillo road cut (measured section 14) .•......•.............•.......• 46 3.11 Disturbed bedding in Facies 3 ............ 48 3.12 Load structures at the contact between Facies 3 and the Triassic Dockum Group at the Cimarron East Section (measured section 26) .•..••.•....•••...••........... 49 3.13 Megapolygonal mud crack casts measuring 90 centimeters across at the Miera Ranch (measured section 8) • . • . • . 50 3.14 Probable water escape structures .•........ 51 3.15 Clay rip up clasts in Facies 3 at the basal contact with the Triassic Dockum Group at the Ray Ranch (measured section 10)............................... 53 3.16 Discontinuous clay laminations in Facies 3 at the Miera Ranch (measured section 8)................................ 54 4.1 Compositional classification of the Exeter Sandstone in northeastern New Mexico.................................... 59 4.2 Photomicrograph of a polycrystalline quartz grain (center) ...........•......... 60 4.3 Photomicrograph of twinned plagioclase feldspar grains and fine grained quartz grains.................................... 61 4.4 Rock fragments in the Exeter Sandstone .... 63 4.5 Photomicrograph illustrating bimodal grain size distributions in the Exeter Sandstone. • . • • . 65 viii Figure Page 4.6 Characteristic bimodal grain size distributions in interdune deposits ....•.. 67 4.7 Photomicrograph of kaolinite cement filling primary intergranular pore space and displaying its characteristic "booklet" morphology ..................... 69 4.8 Cutans in the Exeter Sandstone •........... 71 4.9 Photomicrograph displaying poikilotopic carbonate cement filling the intergrainular pore space ................ 73 4.10 Carbonate cements in the Exeter Sandstone. • • . • . • . • • . • • . • . • . 7 4 4.11 Photomicrograph of well developed quartz overgrowths. • . 77 5.1 Paleo-wind pattern for the Exeter Sandstone in northeastern New Mexico ..... 79 5.2 Paleogeographic map for Middle Jurassic time depicting cross bed resultants .•..... 81 6.1 Schematic diagram and cross-sections of dune, interdune and extradune deposits .... 84 6.2 Cross-section A-A' illustrating the intertonguing relationships of Facies 1, 2, and 3 in the east-central part of the study area............................ 86 6.3 Cross-section B-B' illustrating the intertonguing relationships of Facies 1, 2, and 3 from the eastern to the western part of the study area ................... 88 6.4 Cross-section C-C' illustrating the intertonguing relationships of Facies 1, and 2 in the southern part of the study area. • • • • • • . • • • . • . • . • . 9 0 ix Figure Page 6.5 Isopach map of the Middle Jurassic Exeter Sandstone in northeastern New Mexico (hand contoured) . • • . • • • • . • . • • . • • • • . 94 6.6 Isopach map of the Middle Jurassic Exeter Sandstone in northeastern New Mexico (contoured by computer program Surfer version 4.0 using the spline curve method) . • • . • . • . 95 X CHAPTER