Abstract Stratigraphy and Palynology of the Albian
Total Page：16
File Type：pdf, Size：1020Kb
ABSTRACT STRATIGRAPHY AND PALYNOLOGY OF THE ALBIAN-CENOMANIAN DAKOTA FORMATION AND MOWRY SHALE, UINTA BASIN, UTAH AND COLORADO by Justin Scott Pierson The Albian-Cenomanian Dakota Formation and Mowry Shale of the Uinta Basin consist of an overall transgressive sequence of continental and marine strata that were deposited in the medial part of the Cordilleran foreland-basin system. However, complex stratigraphic relationships within the Dakota Formation and Mowry Shale, as well as complexities between both older and younger strata, have led to different interpretations regarding the large-scale stratigraphic architecture of the basin. In order to provide a better understanding of the overall stratigraphic relationships between the Albian-Cenomanian rocks of the region, palynological samples from the Dakota Formation, Mowry Shale, and overlying units were collected from the northern and southern margins of the Uinta Basin. When combined with preexisting biostratigraphic studies and regional stratigraphic correlations of the Dakota-Mowry interval, this data can be used to construct a unifying stratigraphic model for Dakota and Mowry deposition. Results of this study indicate that Dakota Formation contains three, unconformity-bound depositional sequences. The first Dakota sequence is late Albian in age and was deposited above a regional unconformity that can be traced throughout the Uinta Basin. The second Dakota sequence was deposited above a second regional unconformity that formed during latest Albian time. In the northern part of the Uinta Basin, nonmarine deposition of second Dakota sequence was confined to the late Albian. Early Cenomanian-aged marine deposits of the overlying Mowry Shale comprise the upper parts of the sequence. In the southern part of the basin, the second Dakota sequence is entirely nonmarine. A regional decrease in relative sea level at the end of early Cenomanian time resulted in the development of a regional unconformity throughout the study area. In the southern Uinta Basin, incised fluvial channel complexes of the third Dakota sequence lie above the unconformity. The third Dakota sequence in this region is capped by the middle Cenomanian-Turonian Tununk Member of the Mancos Shale In the northern part of the basin, nonmarine deposits of the third Dakota sequence are absent. In this region the unconformity is situated at the erosional contact between the Mowry Shale and the middle Turonian Tununk Member. STRATIGRAPHY AND PALYNOLOGY OF THE ALBIAN – CENOMANAIN DAKOTA FORMATION AND MOWRY SHALE, UINTA BASIN, UTAH AND COLORADO A Thesis Submitted to the Faculty of Miami University in partial fulfillment of the requirements for the degree of Master of Science Department of Geology by Justin Scott Pierson Miami University Oxford, Ohio 2009 Advisor________________________ (Dr. Brian Currie) Reader_________________________ (Dr. Jason Rech) Reader_________________________ (Dr. Ellen Currano) TABLE OF CONTENTS INTRODUCTION 1 METHODS 6 GEOLOGIC SETTING 7 REGIONAL STRATIGRAPHY 8 Northern Uinta Basin First Dakota Sequence 8 Second Dakota Sequence 9 Mowry Shale 11 Southern Uinta Basin First Dakota Sequence 12 Second Dakota Sequence 14 Mancos Shale Tununk Member 15 DEPOSITIONAL AGE 17 PREVIOUS AGE DETERMINATIONS 17 Northern Uinta Basin Dakota Formation 17 Mowry Shale 17 Mancos Shale Tununk Member 18 Southern Uinta Basin Dakota Formation 18 Mancos Shale Tununk Member 18 NEW BIOSTRATIGRAPHIC DATA 20 Northern Uinta Basin Steinaker Reservoir Location 20 Mowry Shale 20 Southern Uinta Basin Westwater Creek and Westwater Creek East Locations 20 Dakota Formation 23 Mancos Shale Tununk Member 23 Agate Wash Location 24 Dakota Formation 26 Mancos Shale Tununk Member 26 Yellow Cat Location 27 Dakota Formation 27 ii TABLE OF CONTENTS –Continued Green River East Location 27 Dakota Formation 27 Mancos Shale Tununk Member 30 Cottonwood Wash Location 30 Mancos Shale Tununk Member 30 Trapp Springs Well Core 30 Dakota Formation 32 Mancos Shale Tununk Member 33 BIOSTRATIGRAPHIC SYNTHESIS 33 STRATIGRAPHIC IMPLICATIONS 35 Dakota Formation and Mowry Shale 35 Mancos Shale Tununk Member 35 UINTA BASIN CORRELATIONS 37 REGIONAL STRATIGRAPHIC EVOLUTION 40 CONCLUSIONS 45 REFERENCES 46 APPENDIX A. Detailed palynological results 52 Plate 1. Detailed palynology for the Steinaker Reservoir Location 53 Plate 2. Detailed palynology for the Westwater Creek and Westwater 54 Creek East Location Plate 3. Detailed palynology for the Agate Wash Location 55 Plate 4. Detailed palynology for the Yellow Cat Location 56 Plate 5. Detailed palynology for the Green River East and Cottonwood 57 Wash Locations Plate 6. Detailed palynology for the Trapp Springs Core Location 58 iii LIST OF TABLES 1. Uinta Basin outcrop and well locations 3 iv LIST OF FIGURES 1. Location map of the study area 2 2. Time stratigraphic chart for Albian-Cenomanian time 5 3. Outcrop correlation in the northern Uinta Basin 10 4. Outcrop correlation in the southern Uinta Basin 13 5. Steinaker reservoir outcrop measured section 21 6. Westwater Creek and Westwater Creek East outcrop measured sections 22 7. Agate Wash outcrop measured section 25 8. Yellow Cat outcrop measured section 28 9. Green River East and Cottonwood Wash outcrop measured section 29 10. Trapp Springs Core description and geophysical log 31 12. North-to-south surface-to-subsurface cross-section 38 13. Stratigraphic evolution of the Dakota Formation and Mowry Shale 41-42 v ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Brian Currie, for all of the insight, time, effort, and guidance that he offered during the course of this project. Also, I would like to thank Mary and Steve McPherson for opening their home while I was conducting field work and for Mary’s technical guidance throughout this project. Thanks also go to Jared Gooley for assisting me in the field and to the Utah Geological Survey for providing funding for this research. Finally, I would like to thank my parents for their unending support through all of my academic career. vi INTRODUCTION The Dakota Formation and Mowry Shale of eastern Utah and western Colorado consists of an overall transgressive sequence of continental and marine strata that was deposited in the medial part of the Cordilleran foreland basin system during Albian-Cenomanian time (~105-96 Ma) (Currie, 2002). Surface exposures of these units occur in areas north and south of the Uinta Basin (Figure 1, Table 1). Previous studies of the Dakota Formation and Mowry Shale in the region have focused on the sedimentology, depositional environments, and regional stratigraphy (Young, 1960; Vaughn and Picard, 1976; Ryer, et al, 1987; Molenaar and Wilson, 1990). However, complex stratigraphic relationships within the Dakota Formation (Currie, 2002), between the Dakota Formation and the Mowry Shale (Quigley, 1959; Molenaar and Cobban, 1991), and between the Dakota/Mowry and the overlying Mancos Shale, have led to differing interpretations regarding the basin-scale stratigraphic architecture of these units. For example, Molenaar and Cobban (1991) postulated that a regional unconformity identified at the base of the Dakota in the southern part of the Uinta Basin correlates with an unconformity between the Mowry Shale and overlying Mancos Shale in the northern Uinta Basin. These same authors concluded that the Dakota Formation in the northern and southern parts of the basin makes up two separate depositional units, and that the older Dakota unit to the north is conformable with the underlying Cedar Mountain Formation. They also suggested that the upper parts of the older Dakota unit identified in the subsurface of the Uinta Basin correlate with the Mowry Shale, but that outcrop exposures of the Dakota in the southern parts of the basin are younger than the Mowry. Currie (2002), however, hypothesized the Dakota Formation in the northern parts of the Uinta Basin is separated from the underlying Cedar Mountain formation by a regional unconformity. He also speculated that the Dakota Formation in the northern basin contains a second intra-formational unconformity that divides the unit into distinct upper and lower depositional sequences, and that the two unconformities merge into a single unconformity at the base of the unit in the southern part of the basin. Currie (2002) concluded that the Dakota Formation exposed in outcrops along the southern part of the Uinta basin is laterally equivalent with the Mowry Shale to the north. One reason for disparate regional correlations of these units is that most of the chronostratigraphic control used by previous workers was based on molluscan biostratigraphic 1 R.103W R.01E R.20E R.01W R.25E North ID A NV B C A T.01N B WY T.01S T.05S Salt Lake City Vernal Vernal 1 T.05N 40 191 D E Green F 40 River 2 3 UT CO AZ NM T.01N 4 40°T.01S T.06SExplanation Albian-Cenomanian 5 T.11S T.11S outcrops A Measured section location & number Uinta 6 7 1 Well location & number 129 9 8 13 Trapp Spring well location 11 10 Highway 12 T.05S Line of section, Figure 3 Line of section, Figure 4 13 T.15S Basin Line of section, Figure 11 30 miles 14 50 kilometers 15 16 R.03W 17 T.10S 191 6 T.01N T.20S G K H Green River 39° 70 San Rafael Swell J R.104W R.26E R.19E R.14E 110° I 109° Figure 1. A) Uinta Basin study area location, eastern Utah and western Colorado. Grey shading shows location B. B) Simplified geologic map of the study area showing Albian-Cenomanian outcrop belt, measured section locations, and stratigraphic cross section lines for Figure 4, Figure 5, and Figure 11. See Table 1 for