Fluvial-lacustrine sequence stratigraphy, provenance, ichnology, and sandstone reservoir modeling of the tertiary Uinta and Duchesne River Formation, northern Uinta Basin, Utah

The Tertiary Uinta and Duchesne River Formations exhibit spectacular outcrop exposures in the Uinta Basin, northeastern Utah. This paper documents four different geological topics/subjects resulting from field and laboratory studies: 1) fluvial-lacustrine sequence stratigraphy, 2) source-to-sink fluvial system, 3) ichnology and paleoenvironment implications, and 4) sandstone reservoir models and characterization. Chapter 1 highlights a sequence stratigraphic framework and basin-scale facies architecture of the Duchesne River Formation. An upward-fining sequence of the lower three members was heavily influenced by uplift in the Uinta Mountains. Its internal fluvial-lacustrine deposits show marked contrasts between the western and eastern part of the basin due to irregular allogenic controls of tectonic subsidence and water discharge (climate and source terrain input controls). Chapter 2 highlights a source-to-sink fluvial system of the basal member of the Duchesne River Formation, which preserves a high net-sand-to-gross-thickness ratio (NTG) system in the western sink (basin) and a low NTG system in the eastern sink. Petrographic data and drainage patterns indicate a high discharge from multiple source terrains with a long sediment transport along the E-W basin axis in the western part of the basin. These factors were important for development of large-volume and high-quality (porous) fluvial sandstone reservoirs in the sink. Chapter 3 focuses on distinct trace fossil assemblages within the fluvial-lacustrine sequence of the uppermost Uinta and the overlying Duchesne River Formations. The study demonstrates the important relationships of depositional facies and trace fossils: 1) lacustrine deposits with the dominant horizontal grazing trace fossil assemblage, 2) fluvial deposits with the dominant insect trace fossil assemblage, and 3) transitional (wetland) deposits with intermediate trace fossil assemblage. Chapter 4 emphasizes the outcrop-based geological/reservoir modeling of fluvial and lacustrine deposits of the Uinta and Duchesne River Formations. The study provides statistical inputs of fluvial channel geometry for reservoir modeling applications, and demonstrates which stochastic modeling techniques best represent observed depositional patterns derived from outcrop data. The Uinta and Duchesne River Formations exhibit the important aspects of coarse-grained deposits in the late-stage lacustrine basin fill.

FLUVIAL-LACUSTRINE SEQUENCE STRATIGRAPHY, PROVENANCE, ICHNOLOGY, AND SANDSTONE RESERVOIR MODELING OF THE TERTIARY UINTA AND DUCHESNE RIVER FORMATION, NORTHERN UINTA BASIN, UTAH by Takashi Sato A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Master of Science in Geology Department of Geology and Geophysics The University of Utah May 2015 Copyright © Takashi Sato 2015 All Rights Reserved The University of Utah Graduate School STATEMENT OF THESIS APPROVAL The following faculty members served as the supervisory committee chair and members for the thesis of____________ Takashi Sato____________________________ Dates at right indicate the members' approval of the thesis. ________ Marjorie A. Chan____________________, Chair Dec 12, 2014 Date Approved ________ Allan A. Ekdale____________________ , Member Dec 15, 2014 Date Approved ________ Lisa Stright________________________ , Member Dec 16, 2014 Date Approved The thesis has also been approved by______ John Bartley Department/School/College of Geology and Geophysics and by David B. Kieda, Dean of The Graduate School. Chair of the ABSTRACT The Tertiary Uinta and Duchesne River Formations exhibit spectacular outcrop exposures in the Uinta Basin, northeastern Utah. This paper documents four different geological topics/subjects resulting from field and laboratory studies: 1) fluvial-lacustrine sequence stratigraphy, 2) source-to-sink fluvial system, 3) ichnology and paleoenvironment implications, and 4) sandstone reservoir models and characterization. Chapter 1 highlights a sequence stratigraphic framework and basin-scale facies architecture of the Duchesne River Formation. An upward-fining sequence of the lower three members was heavily influenced by uplift in the Uinta Mountains. Its internal fluvial-lacustrine deposits show marked contrasts between the western and eastern part of the basin due to irregular allogenic controls of tectonic subsidence and water discharge (climate and source terrain input controls). Chapter 2 highlights a source-to-sink fluvial system of the basal member of the Duchesne River Formation, which preserves a high net-sand-to-gross-thickness ratio (NTG) system in the western sink (basin) and a low NTG system in the eastern sink. Petrographic data and drainage patterns indicate a high discharge from multiple source terrains with a long sediment transport along the E-W basin axis in the western part of the basin. These factors were important for development of large-volume and high-quality (porous) fluvial sandstone reservoirs in the sink. Chapter 3 focuses on distinct trace fossil assemblages within the fluvial-lacustrine sequence of the uppermost Uinta and the overlying Duchesne River Formations. The study demonstrates the important relationships of depositional facies and trace fossils: 1) lacustrine deposits with the dominant horizontal grazing trace fossil assemblage, 2) fluvial deposits with the dominant insect trace fossil assemblage, and 3) transitional (wetland) deposits with intermediate trace fossil assemblage. Chapter 4 emphasizes the outcrop-based geological/reservoir modeling of fluvial and lacustrine deposits of the Uinta and Duchesne River Formations. The study provides statistical inputs of fluvial channel geometry for reservoir modeling applications, and demonstrates which stochastic modeling techniques best represent observed depositional patterns derived from outcrop data. The Uinta and Duchesne River Formations exhibit the important aspects of coarse­grained deposits in the late-stage lacustrine basin fill. iv TABLE OF CONTENTS ABSTRACT...................................................................................................................................... iii LIST OF TABLES........................................................................................................................ viii LIST OF FIGURES.......................................................................................................................... ix ACKNOWLEDGEMENTS.............................................................................................................. xii Chapters 1. FLUVIAL-LACUSTRINE FACIES ARCHITECTURE AND SEQUENCE STRATIGRAPHY OF THE TERTIARY DUCHESNE RIVER FORMATION, UINTA BASIN, UTAH............ 1 1.1 Abstract......................................................................................................................... 1 1.2 Introduction................................................................................................................... 2 1.3 Geological Context and Previous Work...................................................................... 3 1.4 Methods..................................................................................................................... ....5 1.5 Lithofacies and Facies Associations............................................................................6 1.5.1 Lithofacies...................................................................................................... 7 1.5.2 Facies Association 1 (FA1): Amalgamated Braided Fluvial Channels...... 7 1.5.3 Facies Association 2 (FA2): Extensive Flood Plain and Stacked Broad Fluvial Channels.............................................................................................11 1.5.4 Facies Association 3 (FA3): Extensive Flood Plain and Isolated Small Streams...................................................................................................................... 12 1.5.5 Facies Association 4 (FA4): Alluvial Fan Complex......................................14 1.5.6 Facies Association 5 (FA5): Dry and Wet Flood Plains and Fluvial Channels.....................................................................................................................15 1.5.7 Facies Association 6 (FA6): Extensive Lacustrine Deposits.......................17 1.6 Regional Facies Architecture and Paleocurrent.................................................... ...18 1.6.1 Regional Facies Architecture........................................................................ 18 1.6.2 Paleocurrent and Fluvial Style...................................................................... 23 1.7 Proposed Scenario of Duchesne River Sequence................................................ 23 1.7.1 Stage 1 - D b .................................................................................................. 24 1.7.2 Stage 2 - D d .................................................................................................. 27 1.7.3 Stage 3 - D l................................................................................................... 28 1.8 Discussion................................................................................................................... 29 1.8.1 Upward-fining Succession and Tectonics along the Sevier FTB............. 29 1.8.2 Controlling Factors on the Duchesne River Sequence.............................. 29 1.8.3 Comparison with Fluvial-Lacustrine Sequence Stratigraphic Models....... 31 1.9 Conclusions................................................................................................................ 31 1.10 References............................................................................................................... 32 2. SOURCE-TO-SINK FLUVIAL SYSTEMS FOR SANDSTONE RESERVOIR EXPLORATION: EXAMPLE FROM THE BASAL BRENNAN BASIN MEMBER OF TERTIARY DUCHESNE RIVER FORMATION, NORTHERN UINTA BASIN, UTAH......37 2.1 Abstract....................................................................................................................... 37 2.2 Introduction..................................................................................................................38 2.3 Geological Context..................................................................................................... 38 2.3.1 Geological Setting........................................................................................... 38 2.3.2 Previous Studies............................................................................................. 41 2.4 Methods.......................................................................................................................41 2.4.1 Regional Stratigraphic Study......................................................................... 42 2.4.2 Petrographic Study........................................................................................ 42 2.5 Results and Interpretations........................................................................................44 2.5.1 Basin-scale Facies Architectures.................................................................. 44 2.5.2 Sandstone Compositions and Provenances............................................... 48 2.6 Synthesis..................................................................................................................... 52 2.6.1 Source-to-Sink Fluvial Systems and Controlling Factor.............................52 2.6.2 Fluvial Sandstone Reservoir Exploration......................................................54 2.7 Conclusions..................................................................................................................57 2.8 References.................................................................................................................. 58 3. TRACE FOSSILS AND FLUVIAL-LACUSTRINE ICHNOFACIES OF THE EOCENE UINTA AND DUCHESNE RIVER FORMATIONS, NORTHERN UINTA BASIN, UTAH......................................................................................................................................62 3.1 Abstract....................................................................................................................... 62 3.2 Introduction................................................................................................................ 63 3.3 Geological Context..................................................................................................... 63 3.3.1 Geological Setting.......................................................................................... 63 3.3.2 Previous Studies............................................................................................ 66 3.4 Methods...................................................................................................................... 66 3.5 Observed Trace Fossils and Paleoenvironmental Interpretations........................67 3.5.1 Uppermost Uinta Formation...........................................................................68 3.5.2 Brennan Basin Member (Db).........................................................................72 3.5.3 Dry Gulch Creek Member (Dd)...................................................................... 75 3.5.4 Lapoint Member (Dl).......................................................................................78 3.6 Synthesis and Discussion.......................................................................................... 81 3.7 Conclusions.................................................................................................................82 3.8 References.................................................................................................................. 83 4. FLUVIAL AND LACUSTRINE SANDSTONE RESERVOIR MODELS AND CHARACTERIZATION: EOCENE UINTA AND DUCHESNE RIVER FORMATIONS, NORTHERN UINTA BASIN, UTAH...................................................................................... 86 4.1 Abstract.......................................................................................................................86 4.2 Introduction................................................................................................................. 87 4.3 Geological Context......................................................................................................88 4.3.1 Geological Setting........................................................................................... 88 4.3.2 Previous Studies............................................................................................. 89 4.4 Data Collection and Methods..................................................................................... 90 4.5 Facies Classification and Outcrop Interpretation..................................................... 92 4.5.1 Sedimentary Facies Classification and Gamma Ray Log.......................... 92 4.5.2 Outcrop Interpretation (Sequence Stratigraphic Framework and Zonation)................................................................................................................... 94 4.5.3 Translation into Outcrop Reference............................................................. 97 4.6 Evaluation of Reservoir Modeling Techniques.........................................................99 4.6.1 Geomodel Generations by Three Different Techniques............................. 99 4.6.2 Geomodel Comparisons and Evaluations (Static Connectivity Analysis). 101 4.6.3 Strengths and Weaknesses of Examined Modeling Techniques............. 105 4.7 Discussion..................................................................................................................105 vi 4.8 Conclusions................................................................................................................ 106 4.9 References................................................................................................................. 107 Appendices A. PREVIOUS STUDIES AND GEOLOGICAL AGE.............................................................. 111 B. MEASURED SECTIONS.......................................................................................................116 C. LIST OF SANDSTONE SAMPLES AND RESULTS OF THIN SECTION AND QEMSCAN ANALYSIS............................................................................................................................120 D. DETAILED PROCEDURE OF QEMSCAN AUTOMATED DISAGGREGATED COUNTS................................................................................................................................122 E. DATA IN DIGITAL FORMAT (DVD)....................................................................................126 vii LIST OF TABLES 1.1. Summary of Duchesne River Lithofacies.............................................................................. 8 1.2. Summary of Duchesne River Facies Associations.............................................................. 9 2.1. Duchesne River Facies Associations................................................................................. 45 4.1. Interpreted Facies Classifications and Descriptions..........................................................93 4.2. Geomodels Examined......................................................................................................... 100 C.1. List of Sandstone Samples and Results of Thin Section and QEMScan Analysis........ 121 D.1. Comparison between Thin Section Point Counts and QEMScan Automated Disaggregated Counts........................................................................................................ 123 LIST OF FIGURES 1.1. Geological map of the Uinta Basin........................................................................................4 1.2. Geological map of the Duchesne River Formation and surrounding area........................ 6 1.3. Facies association FA1 at MS28.......................................................................................... 10 1.4. Facies association FA2 at MS33...........................................................................................12 1.5. Facies association FA3 at MS14...........................................................................................13 1.6. Facies association FA4 at MS01........................................................................................... 15 1.7. Facies association FA5 at MS15........................................................................................... 16 1.8. Facies association FA6 at MS06 and MS26.........................................................................18 1.9. E-W regional correlations of composite sections A to G......................................................19 1.10. N-S geological cross section along MS01 - MS05 - MS03................................................. 21 1.11. Paleocurrent data (total 264 measurements) plotted as rose diagrams with average directions (blue arrows), schematic fluvial channel styles, and stacking patterns of Db............................................................................................................................................ 24 1.12. Proposed tectonic-driven sequence stratigraphic framework for the Duchesne River Formation................................................................................................................................25 1.13. Three-staged evolutionary paleogeographic scenario of the upward-fining sequence of the Duchesne River Formation..............................................................................................26 2.1. Geological map and geologic column of the Uinta Basin...................................................39 2.2. Geological map of the Duchesne River Formation and surrounding area.........................40 2.3. E-W regional correlations of composite sections A to G..................................................... 43 2.4. Net-sandstone-to-gross-thickness ratio (NTG) map and schematic fluvial styles of Db........................................................................................................................................... 49 2.5. Ternary QFL(R) plots showing sandstone compositions of the Duchesne River Formation................................................................................................................................50 2.6. Paleocurrent data from Db (plot a) and longitude versus percent rock fragments of grains (plot b).....................................................................................................................................51 2.7. Thin section petrography of sandstone samples from Db...................................................52 2.8. Sequence stratigraphic framework of the Duchesne River Formation and controlling factors.....................................................................................................................................53 2.9. The tectonic-driven sequence stratigraphy with three-stages for the Duchesne River upward-fining sequence......................................................................................................... 55 2.10. Modern precipitation in and around the Uinta Basin.......................................................... 56 3.1. Six continental ichnofacies (a to f) and marginal lacustrine Skolithos ichnofacies in a continental depositional setting..............................................................................................64 3.2. Trace fossil (ichnongenus) compositions of continental ichnofacies models.................... 64 3.3. Index map and schematic geologic column showing the Paleogene sequence of the Uinta Basin........................................................................................................................................65 3.4. Geological map of the Duchesne River Formation and surrounding area....................... 67 3.5. E-W regional correlations of composite sections A to G showing the stratigraphic framework and detailed basin-scale facies architectures of the uppermost Uinta and Duchesne River Formations................................................................................................... 68 3.6. The uppermost Uinta Formation at MS24, showing stacked microbial carbonate mounds with stromatolitic structures.................................................................................................... 69 3.7. Trace fossils observed in the uppermost Uinta Formation at MS24.................................. 70 3.8. Trace fossils observed in the uppermost Uinta Formation at MS24 and MS28................70 3.9. Paleoenvironmental reconstruction and trace fossil assemblage of the uppermost Uinta........................................................................................................................................ 71 3.10. The basal member of the Duchesne River Formation (Db) at MS33 showing typical lithofacies and biofacies........................................................................................................ 73 3.11. Trace fossils observed in Db (1)............................................................................................73 3.12. Trace fossils observed in Db (2)............................................................................................74 3.13. Trace fossils observed in Db (3)........................................................................................... 74 3.14. Paleoenvironmental reconstruction and trace fossil assemblage of Db...........................76 3.15. The second member of the Duchesne River Formation (Dd) at MS26 showing typical lithofacies and biofacies......................................................................................................... 76 3.16. Trace fossils observed in the western part of Dd (1)...........................................................77 3.17. Trace fossils observed in the western part of Dd (2 )..........................................................77 3.18. Paleoenvironmental reconstruction and trace fossil assemblage of Dd...........................79 3.19. The third member of the Duchesne River Formation (Dl) at MS06 showing typical lithofacies and biofacies........................................................................................................ 79 3.20. Trace fossils observed in in the western part of Dl.............................................................80 3.21. Paleoenvironmental reconstruction and trace fossil assemblage of Dl..............................81 x 4.1. 4.2. 4.3. 4.4. 4.5. 4.6. 4.7. 4.8. 4.9. 4.10. 4.11. 4.12. 4.13. B.1. B.2. B.3. D.1. D.2. 3.22. Synthesis of sequence stratigraphic framework and trace fossil occurrences of the uppermost Uinta and Duchesne River Formations..............................................................82 Index map and schematic geologic column showing the Paleogene sequence of the Uinta Basin.......................................................................................................................................89 Blacktail outcrop photo...........................................................................................................91 Idealized parasequences..................................................................................................... 94 Surface (MS-1) to subsurface (six wells) gamma ray correlations in a 30 km west-east section.................................................................................................................................... 95 Sequence stratigraphic framework of the Blacktail outcrop................................................96 Blacktail outcrop interpretation...............................................................................................97 Quantitative sandbody geometry data extracted from the fluvial unit (Db)....................... 98 A pixel-based geomodel based on the outcrop interpretation............................................ 98 Examples of geomodels generated by a) indicator kriging (IK), b) and c) sequential indicator simulation (SIS) scenario 1, and d) and e) SIS scenario 2 ..............................101 Examples of geomodels generated by object-based (OB) stochastic modeling............ 102 Five patterns of well deployment and locations of 17 wells, with schematic connected and unconnected fluvial channels..............................................................................................102 Static connectivity analysis (plots of well patterns A to E versus connectivity)...............104 Comparisons of the static connectivity curve of the outcrop reference with average curves of geomodels/realizations by SIS scenario 1, SIS scenario 2, and OB modeling.......... 104 Measured sections, MS01 to MS12....................................................................................117 Measured sections, MS13 to MS24....................................................................................118 Measured sections, MS25 to MS35....................................................................................119 An example (sample 5) of postprocessing (Method 1) with three stages (a, b, and c) of QEMScan automated disaggregated count....................................................................... 124 Crossplots of grain type proportions from QEMSCan (X-axis) with three different processing methods (M1, M2, and M3) and proportions from thin section examination (Y-axis)................................................................................................................................125 xi ACKNOWLEDGEMENTS This research project has been achieved through the support of many individuals. In particular, my heartfelt appreciation goes to my supervisory committee, Marjorie Chan, Allan Ekdale, and Lisa Stright, whose comments and suggestions were highly insightful and constructive. I express my gratitude to the geology faculty of the University of Utah, including Erich Petersen, Cari Johnson, and Lauren Birgenheier for their useful comments on this project. Stephen Hasiotis at the University of Kansas provided input on the continental trace fossils. Wil Mace and Quintin Sahratian provided technical help with thin section and QEMScan preparation. Douglas Sprinkel at the Utah Geological Survey was a great supporter and provided valuable insight on the Uinta Basin geology. I acknowledge the Ute Indian Tribe, the Bureau of Land Management in Vernal Ouray National Wildlife Refuge, Bill Barrett Corporation, and Owl and the Hawk who provided field permissions. CHAPTER 1 FLUVIAL-LACUSTRINE FACIES ARCHITECTURE AND SEQUENCE STRATIGRAPHY OF THE TERTIARY DUCHESNE RIVER FORMATION, UINTA BASIN, UTAH 1.1 Abstract Continental sequence stratigraphy in dynamic upstream environments can be complex due to the interplay of source tectonics, climate change (global and local), and topography. The Tertiary Duchesne River Formation represents the last stage of Lake Uinta intermontane basin fill, surrounded by sediment source mountain ranges of the Uinta Mountains to the north and Sevier Fold Thrust Belt (FTB) to the west. Excellent basin-scale exposures allow vertical and lateral characterization of facies architectures to interpret controlling mechanisms in the upstream environments. The four members of the Duchesne River Formation are distinctive lithological units. The lower three members comprise a typical upward-fining fluvial sequence (unconformity-bounded) from the basal coarse-grained unit into overlying fine-grained units. The fourth (uppermost) coarse-grained member records the onset of another upward-fining cycle. The sequence stratigraphy at these member scales was primarily tectonic-driven, due to uplift of the Uinta Mountains, which was similar to, but smaller than the main Laramide events that produced the nearby series of Paleogene lacustrine basins. Internally, the Duchesne River Formation records a distinct change in fluvial - lacustrine styles between the western and eastern part of the basin, demonstrating the variable allogenic controls of tectonics (subsidence) and discharge (local climate and source terrain input) within the basin. Specifically, the western high NTG (degradational) fluvial system of the basal member was controlled by high discharge due to a wet climate and two source terrain inputs 2 (Uinta Mountains and Sevier FTB), whereas the eastern low NTG (aggradational) fluvial system was controlled by low discharge due to a dry climate and single source terrain input (Uinta Mountains). The development of lacustrine environments of the third member in the west was controlled by differential tectonic subsidence in the basin. The Duchesne River Formation of the Uinta Basin preserves a valuable example of an upstream sequence, and demonstrates how internal facies architectures at the basin-scale evolved by allogenic controls. 1.2 Introduction Concepts of continental sequence stratigraphy are important to aid in exploration of lacustrine basins that may have lacustrine source rocks and fluvial reservoir rocks. However, sequences and facies in dynamic upstream environments are controlled by complicated and interdependent allogenic factors such as tectonics, climate change (global and local), and topography. Early studies in fluvial sequence stratigraphy mainly emphasized sea level controls in marine - coastal systems (Posamentier and Vail 1988; Wright and Marriott 1993; Shanley and McCabe 1994). However, some workers downplayed the influence of sea level changes in upstream environments (e.g., Schumm 1993; Shanley and McCabe 1994; Dalrymple et al. 1998). Still others adopted different terminology for fluvial systems tracts on the basis of change in accommodation (Currie 1997) and stacking patterns (Legaretta and Uliana 1998), which provided a descriptive mechanism to apply the sequence stratigraphic concepts to fluvial deposits even if the driving mechanism is unclear. Fluvial sequence stratigraphy is commonly assessed and discussed by separating upstream controls and downstream controls (e.g., Blum and Tornqvist 2001; Catuneanu 2006; Holbrook et al. 2006). In upstream environments where sea or lake level fluctuations (i.e., downstream control) do not influence the sequence development, tectonics and climate change are interpreted as the main controlling factors on fluvial sequences (e.g., Catuneanu 2006). However, since both tectonics (accommodation control) and climate (discharge control) could be variable laterally within a continental basin, resulting large-scale fluvial-lacustrine facies and stacking patterns might significantly differ even within the coeval units. 3 The purpose of this paper is to: 1) document major lithological architecture and facies of the Duchesne River Formation in vertical and lateral extents across the Uinta Basin, 2) build the regional sequence stratigraphic framework, and 3) assess how tectonic (accommodation) and climatic and source terrain (discharge) controls are reflected in large-scale (>1,000 m) vertical successions and dramatic, basin-scale (>130 km) lateral facies changes. This is an unusual geological example with sufficient vertical and lateral exposures to demonstrate these scales of change. The sequence stratigraphic approach in this paper is based on the recognition of major sequence boundaries (unconformities). Although many different sequence stratigraphic models and systems tracts for alluvial strata have been proposed (summarized in Gibling et al. 2011), the sequence boundary is the only universal surface among these models. Classifications of systems tracts based on accommodation (e.g., low and high accommodation) or stacking patterns (e.g., degradation and aggradation) do not fit with the stratigraphic framework of the Duchesne River Formation, because differences in accommodation and stacking pattern occur even within one coeval unit. Although some measured sections do show individual parasequences (i.e., cyclicity at the scale of tens of meters), here we focus on the large-scale facies change that provides the basic sequence stratigraphic framework. 1.3 Geological Context and Previous Work The Uinta Basin contains thick Paleogene continental deposits of the Wasatch, Green River, Uinta, and Duchesne River formations in ascending order (Fig. 1.1). The upper three of the formations comprise a typical upward-shallowing/coarsening lacustrine basin filling (Visher 1965; Picard and High 1972; Lambiase 1990) preserving the following generalized depositional environments: extensive basinal to marginal lacustrine (Green River Formation), lacustrine-deltaic and fluvial mixed/transitional (Uinta Formation), and fluvial (Duchesne River Formation) (Fig. 1.1). A series of lake basins emerged in the present central Rocky Mountain region in Montana, Wyoming, Utah, and Colorado during the Laramide orogeny in the latest Cretaceous to early Paleogene (Dickinson et al. 1988), including Lake Uinta (situated around the present Uinta Basin). The lacustrine organic-rich shale deposited in Lake Uinta (i.e., Green River 4 Figure 1.1. Geological map of the Uinta Basin. a) The generalized geological map, modified from Andersen and Picard (1974), Bryant et al. (1989), Bryant (1992), Hintze et al. (2000), and Sprinkel (2006 and 2007). Regional dip is to the north and formations get progressively younger toward the Uinta Mountains. The basin is surrounded by high mountain ranges of the Uinta Mountains to the north and the Sevier Fold Thrust Belt (fTb ) to the west. The map of Laramide lake basin system is from Dickinson et al. (1988). b) A schematic geologic column shows the Paleogene sequence of the Uinta Basin (modified from Hintze et al. 2000). T2 to T4 exhibits a typical upward-coarsening/shallowing lacustrine basin-fill succession. shales) is a renowned, world-class, hydrocarbon source rock. In this intermontane lacustrine basin, most regional stratigraphic studies focused on the Green River Formation (e.g., Keighley et al. 2003). In contrast, the overlying Uinta and Duchesne River Formations have received much less attention despite their good exposures, probably due to their lesser economic significance. The Duchesne River Formation derived sediment from adjacent active source mountain range(s) of the Uinta Mountains in the north (Andersen and Picard 1974; Bruhn et al. 1986) and possibly the Sevier Fold Thrust Belt (FTB) in the west. The paleoenvironmental setting was very 5 far (700+ km) from any marine influence (e.g., Blakey 2011) and shows no evidence of large-scale terminal lake development during its history (e.g., Franczyk et al. 1992). The formation is comprised primarily of braided and meandering fluvial deposits, with some minor lacustrine deposits. It is subdivided into four members: Brennan Basin (Db), Dry Gulch Creek (Dd), Lapoint (Dl), and Starr Flat (Ds) members in ascending order (Andersen and Picard 1972) (Fig. 1.2). The lower three members generally comprise an upward-fining succession of sandstone-dominated Db to mudstone-dominated Dl. The uppermost member (Ds) is rich in sandstone and conglomerate. The mudstone-dominated Dl contains abundant tuff or tuffaceous beds, with K-Ar ages of ~40 Ma reported from tuffs at the base of this member (McDowell et al. 1973; Andersen and Picard 1974; Prothero and Swisher 1992; Kelly et al. 2012) (see the detailed nomenclatural history and the geological age of the Duchesne River Formation in Appendix A). 1.4 Methods Field studies broadly examined the Duchesne River Formation throughout its E-W and N-S exposure in the Uinta Basin. Methodologies included basin-scale examinations from aerial imagery as well as specific field measured sections with tape, Jacob staff, and a laser range finder, as well as gigapan photography. A total of 35 locations of measured geological sections (labeled MS01 to MS35 in Fig. 1.2) covered a total of 2,750 m in stratified length (described at minimum resolution of 10-20 cm). The measured sections strategically covered major member boundaries and represent the regional facies architecture of the Duchesne River Formation. The sections were grouped along north-south trending composite sections lettered A to G (Fig. 1.2) to construct a regional stratigraphic framework. The member thickness and stratigraphic positions of acquired measured sections at each composite section were controlled by the modified geological map (Fig. 1.2) along with structural strikes and dips. A total of 441 paleocurrent measurements were acquired throughout all four members of the formation (all detailed measured sections with paleocurrent data are in Appendix B). 6 Figure 1.2. Geological map of the Duchesne River Formation and surrounding area. Regional dip is to the north and the Duchesne River members (Db: Brennan Basin Member, Dd: Dry Gulch Creek Member, Dl: Lapoint Member and Ds: Starr Flat Member) get progressively younger toward the Uinta Mountains. The location of 35 measured sections (MS) are marked by black circles and composite sections A to G (black lines) are shown on the map. The map is modified after Andersen and Picard (1974), Rowley et al. (1985), Bryant et al. (1989) and Sprinkel (2006 and 2007). 1.5 Lithofacies and Facies Associations Twelve lithofacies and six facies associations that characterize the Duchesne River Formation are described and interpreted in this section. A typical facies association is usually a group of associated sedimentary facies at scales of tens of meters representing a specific depositional environment or a related succession (e.g., Allen and Johnson 2010; Aswasereelert et al. 2012; Kukulski et al. 2013). The facies associations in this study are nearly an order of magnitude greater in vertical thickness (e.g., hundreds of meters), and thus these are more akin to "large-scale facies associations", which are used to express the basin-scale facies architecture. 7 1.5.1 Lithofacies Twelve basic lithofacies fall into broad categories of a conglomerate, five sandstone, four mudstone, a limestone, and a tuff lithofacies (detailed in Table 1.1, arranged in order of approximate decreasing grain size). Each lithofacies is distinguished with a combination code of lithology and key features as described below. The first capitalized letter of lithofacies code represents a primary lithology (i.e., C = conglomerate, S = sandstone, M = mudstone, L = limestone, T = tuff/tuffaceous clastics). The second lower-case letter(s) represents key features for the later environmental interpretations such as geometries or body shapes for sandstones and conglomerates (i.e., c = channelized, ta = tabular, th = thin-layered) and colors for mudstones (i.e., r = red, y = yellow, g = green/gray). In addition, numerical characters are added to lithofacies Sc to distinguish different connected sandbody dimension/size (largest Sc1 to smallest Sc3) and Mg to distinguish different internal sedimentary structures (i.e., Mg1: mottled, Mg2: massive or laminated). These subcategories are helpful to distinguish six facies associations described in the following section. Although these twelve lithofacies are individually distinctive (Table 1.1), because of the large-scale emphasis of the fluvial-lacustrine facies architecture, this paper herein focuses on the facies associations. 1.5.2 Facies Association 1 (FA1): Amalgamated Braided Fluvial Channels 1.5.2.1 Description FA1 is dominated by strongly amalgamated channelized sandstones of Sc1, which exhibits a high net-sandstone-to-gross-thickness ratio (NTG) (0.75 at MS28). FA1 is composed of three lithofacies (Table 1.2, Fig. 1.3): a) Sc1, fine- to coarse-grained, yellowish and reddish gray, poor- to well-sorted, channelized, and trough cross-stratified sandstones with strongly amalgamated bodies (with apparent connected bodies over lateral distances of >1,000 m); b) Mr, clay- to silt-sized, red, massive or mottled mudstone with common vertical and semivertical burrows; and c) Sth, poor- to well-sorted, thin-layered (commonly < 1m), massive or trough cross-stratified (occasionally indistinct) sandstone and siltstone with common intensive bioturbation. Trace fossils are common in FA1 although less abundant than in FA2 and FA5. Table 1.1. Summary of Duchesne River Lithofacies Lithofacies Name Code Grain Size Color Sorting Shape Sedimentary Structures Other Features Upward-fining package of conglomerate-sandstone Cc (Cgl) Granule to boulder (max>1 m) Varicolored Very poor Channelized (max 10 m thick) or lenticular Structureless or imbricate structures Clast-supported, well-rounded to sub-angular clasts Cc (Ss) Very fine to very coarse, occasionally silty Yellowish white, yellowish gray Poor to moderate Channelized or lenticular Commonly trough cross-stratified Occasionally calcareous Amalgamated channelized sandstones Sc1 Fine to coarse, occasionally very fine, partly angular granules Yellowish gray, reddish gray Poor to well Channelized, strongly amalgamated (>1,000 m lateral apparent connected bodies) Trough cross­stratified, rip-up clasts Noncalcareous to slightly calcareous, common iron concretions Stacked broad channelized sandstones Sc2 Fine to coarse, occasionally very fine, partly angular granules Light gray, yellowish gray Poor to moderate Channelized, stacked/ amalgamated (>100 m lateral apparent connected bodies) Trough cross­stratified, rip-up clasts Commonly calcareous, uncommon lateral accretion features Isolated and narrow channelized sandstones Sc3 Fine to coarse, partly angular granules Light gray, grayish white, yellowish white Poor to well Channelized, isolated (<100 m lateral apparent connected bodies} Trough cross­stratified Commonly calcareous Thin-layered sandstone and siltstone Sth Silt, fine to medium, occasionally coarse Red, grayish white, greenish gray, light gray, yellowish gray Poor to well Thin-layered (<1 m thick) Massive or trough cross-stratified (occasionally indistinct), occasional rip-up clasts Commonly calcareous, common intensive bioturbation Tabular sandstone Sta Very fine to medium, occasionally silty Light gray, yellowish gray, red Well Tabular (>1,000 m lateral apparent connected bodies (maximum)) Massive, rippled, parallel and wavy laminated, planar and trough cross­stratified Calcareous or noncalcareous, common bioturbation, common carbonaceous materials Red mudstone/silty mudstone Mr Clay to silt Red N/A N/A Massive or mottled Occasional slickensides, common vertical and semi­vertical burrows (filled with white, calcareous siltstones) Yellow mudstone My Clay to silt Yellow, brown N/A N/A Mottled, common relict bedding Green/gray mudstone Mg1 Clay to silt Dominantly green and gray, partly yellow, purple and red N/A N/A Mottled Occasional thin carbonaceous m udstones/materials, occasional intensive horizontal to oblique gypsum veins Green, gray, and dark gray mudstone Mg2 Clay to silt Dominantly green, gray and dark gray N/A N/A Massive or laminated Occasional thin siltstone and carbonaceous mudstone layers Limestone Lth Clay (calcilutite) Tan, brownish gray and gray N/A Thin-layered (<40 cm thick) Massive Commonly fossiliferous (gastropods and bivalves) Tuff and tuffaceous bed T Clay to silt, partly sandy White, light gray N/A Tabular, channelized, lenticular Massive Occasionally rich in biotite 8 Table 1.2. Summary of Duchesne River Facies Associations FA # Facies Association Member Occurrence Facies Components Ss/Ms Ratio Apparent Lithofacies Code Interpretation Trace Fossils Sandbody Dimensions Amalgamated Braided Fluvial Channels Db (western part of basin), Ds Sc1 Amalgamated braided fluvial channels r2o? a : t<r>U ro Cl W c o EEoO cro T3 c 3 _Q < FA 1 Sth Overbank deposit, typically pedogenically-altered 75/25 (MS28) > 1,000 m (MS28) Mr Well-drained flood plain paleosol Extensive Flood Plain Sc2 Braided and sinuous fluvial channels FA 2 Db, Dd and Dl (central-eastern part of basin) Sc3 Isolated small stream channel 50/50 (MS33) > 100 m (MS33) and Stacked Sth Overbank deposit, typically pedogenically-altered Broad Fluvial Channels Mr Well-drained flood plain paleosol My Moderately-drained flood plain paleosol Extensive Sc3 Isolated small stream channel FA Flood Plain Db (eastern Sth Overbank deposit, typically pedogenically-altered 15/85 < 100 m 3 and Isolated part of basin) Mr Well-drained flood plain paleosol (MS14) (MS14) Small Steams My Moderately-drained flood plain paleosol FA 4 Alluvial Fan Complex All members Cc Alluvial fan channel/lobe 70/30 (northern margin of basin) Mr Well-drained flood plain (interchannel) paleosol (cgl+ss/ms) n/a Mg1 Playa or wetland deposit in the distal fan (MS01) Sc2 Braided and sinuous fluvial channels Dry and Wet Flood Plains Sta Marginal lacustrine deltaic deposit > 100 m FA Dd (western Sth Overbank deposit, typically pedogenically-altered 27/73 (Sc2), 5 and Fluvial part of basin) Mr Well-drained flood plain paleosol (MS15) > 1,000m Channels My Moderately-drained flood plain paleosol (Sta) Mg1 Poorly-drained wetland or shallow lacustrine deposit Sc3 Isolated small stream channel Extensive Lacustrine Deposits Sta Marginal lacustrine deltaic deposit FA Dl (western Mr Well-drained paleosol 5/95 n/a 6 part of basin) Mg2 Lacustrine deposit (MS06) Lth Lacustrine deposit T Ash fall and reworked deposit Abbreviations: FA = Facies Association, MS = Measured Section 6 10 Figure 1.3. Facies association FA1 at MS28. a) Outcrop interpretation shows amalgamated channelized sandstone bodies (Sc1) highlighted in yellow. b) Lithofacies Sc1 and underlying Mr and Sth. c) Mottled and burrowed structures of lithofacies Mr and adjacent Sth (massive). d) Representative portion of MS28 shows the detailed descriptions of lithofacies Sc1, Mr, and Sth. Lithofacies descriptions (generalized) and codes are in Table 1.1. 1.5.2.2 Interpretation FA1 represents a fluvial style of widespread multiple interweaving fluvial channels (i.e., braided channels of Sc1), punctuated by dry flood plain environments (Sth and Mr). Lithofacies Sc1 (amalgamated channelized sandstones) indicates traction transport mainly in the upper part of the lower flow regime. Lithofacies Mr (red mudstone/silty mudstone) indicates suspension deposition followed by pedogenic alterations with well-drained conditions (e.g., Kraus 2002; Atchley et al. 2004; Kraus and Hasiotis 2006). Lithofacies Sth (thin-layered sandstone and siltstone) indicates traction transport usually followed by pedogenic alterations. The lower abundance of trace fossils than FA2 and FA5 might reflect frequent destruction of traces due to repetitive cut-and-fill patterns of the amalgamated fluvial channels (Sc1). 11 1.5.3 Facies Association 2 (FA2): Extensive Flood Plain and Stacked Broad Fluvial Channels 1.5.3.1 Description FA2 is dominated by stacked channelized sandstones of Sc2 and red-colored mudstones of Mr, which exhibits a moderate NTG (0.5 at MS33). Overall these channelized sandstones (Sc2) are less connected than strongly amalgamated channelized sandstones (Sc1) of FA1. FA2 is composed of five lithofacies (Table 1.2, Fig. 1.4): a) Sc2, fine- to coarse-grained, light and yellowish gray, channelized, and trough cross-stratified sandstones with stacked/amalgamated bodies (with apparent connected bodies over lateral distances of >100 m) and uncommon lateral accretion features; b) Sc3, fine- to coarse-grained, light gray and grayish/yellowish white, channelized, and trough cross-stratified sandstones with isolated narrow bodies (with apparent connected bodies under lateral distances of <100 m); c) Sth; d) Mr; and e) My, clay- to silt-sized, yellow to brown, mottled mudstone with common relict bedding. FA2 has abundant trace fossils such as root structures (rhizoliths) in mudstones and a variety of meniscate backfill burrows and nesting structures both in mudstones and sandstones. 1.5.3.2 Interpretation FA2 represents a depositional environment of extensive dry flood plains (Mr, My, and Sth) with mixed braided, meandering, and isolated small river systems (Sc2 and Sc3). Lithofacies Sc2 (stacked broad channelized sandstones) and Sc3 (isolated and narrow channelized sandstones) both indicate traction transport mainly in the upper part of the lower flow regime. Uncommon lateral bar accretion features of Sc2 indicate some rivers were at least more sinuous than those of FA1. Lithofacies My (yellow mudstone) indicates suspension deposition followed by pedogenic alterations with moderately-drained conditions (e.g., Atchley et al. 2004; Kraus and Hasiotis 2006). The abundance of trace fossils in this facies association indicates prosperous organic communities under moderately prolonged stable conditions and high preservation potential of organic traces due to the aggradational stacking pattern (i.e., episodic burial without destroying traces). 12 portion of of MS33 Sc2 (stacked broad channelized 33 ): medium-to coarse-grained, angular granules in part, light gray, yellow in part, hard, poor to moderately sorted, channelized shape, stacked /amalgamated, trough cross­stratified, rip-up clasts My (yellow ms): clay-sized, yellow to brown, moderately hard, mottled l^ilcgl I Iss | Sits BBjMs(red) ^ Ms(others) "color1 Unclear < slope-forming) 1 ____I unit (Inferred lithology) _ j Trough cross bedding • # Rip-up clast Q including >granule- 0 sized grain 'U Burrow 1 1 Undifferentiated ' ' mottled structure Slickensides (red ms/silty ms): clay-sized, red, mottled, slickensides (thin-layered ss and sits): silt to very fine-grained, greenish gray and grayish white, moderately hard, thin-layered, calcareous Figure 1.4. Facies association FA2 at MS33. a) Outcrop interpretation shows stacked broad channelized sandstones (Sc2) and isolated and narrow channelized sandstones (Sc3) highlighted in yellow. b) Lithofacies Sc2 and underlying Mr, and Sth. c) Representative portion of MS33 shows the detailed descriptions of lithofacies Sc2, Mr, and Sth. Lithofacies descriptions (generalized) and codes are in Table 1.1. 1.5.4 Facies Association 3 (FA3): Extensive Flood Plain and Isolated Small Streams 1.5.4.1 Description FA3 is dominated by red-colored mudstones of Mr, which exhibits a low NTG (0.15 at MS14). FA3 is composed of four lithofacies (Table 1.2, Fig. 1.5): a) Sc3; b) Sth; c) Mr; and d) My. The difference between FA2 and FA3 is the absence of board channelized sandstones of Sc2 (i.e., only small and isolated channelized sandstones of Sc3 occur in FA3). This facies association tends to form very muddy, poorly exposed, slope-forming "badlands" outcrops. FA3 has moderate amounts (lesser amounts than FA2 and FA5) of trace fossils. 13 Figure 1.5. Facies association FA3 at MS14. a) Outcrop interpretation shows isolated and narrow channelized sandstones (Sc3) highlighted in yellow. b) Lithofacies Sc3 and underlying Mr and Sth. c) Representative portion of MS14 shows the detailed descriptions of lithofacies Sc3, Mr, and Sth. Lithofacies descriptions (generalized) and codes are in Table 1.1. 1.5.4.2 Interpretation The absence of Sc2 (stacked broad channelized sandstones) and the dominance of mudstones (Mr) indicates a depositional environment of extensive dry flood plains (Mr, My, and Sth) with only isolated small streams (Sc3). The low abundance of trace fossils in FA3 compared to FA2 an FA5 could be resulted from a sampling (data collection) bias due to poorly exposed conditions of this muddy facies association. 14 1.5.5 Facies Association 4 (FA4): Alluvial Fan Complex 1.5.5.1 Description FA4 is dominated by the conglomeratic lithofacies Cc, which exhibits a high percentage of coarse-grained deposits (e.g., the ratio of conglomerate/sandstone and mudstone is 70:30 at MS01). FA4 is composed of three lithofacies (Table 1.2, Fig. 1.6): a) Cc (thick upward-fining package of mixed conglomerate-sandstone), poor-sorted, granule- to boulder-size (max 1 m), structureless or imbricate conglomerates with channelized or lenticular shaped bodies (max 10 m thick), and very fine- to very coarse-grained, trough cross-stratified sandstones with channelized or lenticular shaped bodies; b) Mr; and c) Mg1, clay- to silt-sized, dominantly green and gray to partly yellow, purple and red, mottled mudstone with thin carbonaceous (e.g., fossil plants/woods) mudstone layers, and intensive gypsum veins. Trace fossils are scarce in this facies association, although there are large rhizocretes at one locality (MS01). 1.5.5.1 Interpretation FA4 is interpreted to represent an alluvial fan (Cc) with relatively narrow interchannel (Mr) and playa/wetland environments (Mg1). Structureless conglomerates in the lower portion of lithofacies Cc indicate debris flows. These vertically transition to imbricated conglomerates and trough-cross stratified sandstones in the upper portion indicating traction transport (Nemec and Steel 1984), with considerable variations in paleocurrent directions (e.g., NW to E paleoflow at MS01 and MS22). The mixed transportation mechanisms and radial paleocurrent indicators suggest very high-energy seasonal to perennial gravel-bed river processes, and episodic and repetitive avulsions and lobe switching (e.g., Crews and Ethridge 1993). Mg1 (green/gray mudstone) indicates suspension deposition followed by pedogenic modifications under poorly-drained (wet) conditions (e.g., Kraus 2002; Atchley et al. 2004; Kraus and Hasiotis 2006). 15 Figure 1.6. Facies association FA4 at MS01. a) Outcrop interpretation shows distinctive conglomerates and sandstones (Cc) highlighted in yellow. b) Granule- to boulder-size conglomerate of lithofacies Cc. c) Sandstone (including tar) of lithofacies Cc and overlying Mr (mostly covered). d) Representative portion of MS01 shows the detailed descriptions of lithofacies Cc. Lithofacies descriptions (generalized) and codes are in Table 1.1. 1.5.6 Facies Association 5 (FA5): Dry and Wet Flood Plains and Fluvial Channels 1.5.6.1 Description FA5 is dominated by red-colored mudstones of Mr and green/gray-colored mudstones of Mg1 with scattered stacked channelized sandstones of Sc2, which exhibits a moderate NTG (0.27 at MS15). FA5 is composed of six lithofacies (Table 1.2, Fig. 1.7). In this facies association, lithofacies Sc2, Sta, Mr, and My, which are constituents of FA2, coincide with lithofacies Mg1 and minor Sta (tabular sandstone). Lithofacies Sta is characterized by very fine-to medium-grained, well-sorted, tabularly bedded (50 to 200 cm thick), massive, rippled (wave and current) or trough cross-stratified sandstones with common bioturbation and 16 Figure 1.7. Facies association FA5 at MS15. a) Outcrop interpretation shows an upward-coarsening succession of FA5 with stacked broad channelized sandstones (Sc2) highlighted in yellow. b) The lower part of an upward-coarsening succession comprised of lithofacies Mg1 and Mr. c) Representative portion of MS15 shows the detailed descriptions of lithofacies Sc2, Sth, Mr, and Mg1. Lithofacies descriptions (generalized) and codes are in Table 1.1. carbonaceous/woody materials. Some thick sandstones of Sta are traceable laterally at scales of thousands of meters. Trace fossils including rhizoliths and meniscate backfill burrows are abundant in this facies association. 1.5.6.2 Interpretation FA5 represents a depositional environment of extensive alluvial plains accompanying wetland and shallow lacustrine conditions. In this facies association, there is a common challenge in interpreting continental depositional environments due to extensive post-depositional pedogenic processes that modify or destroy indications of the original depositional environments (Hasiotis 2000; Retallack 2001). Mg1 generally exhibits mottled structures, indicating pedogenic alterations. Nevertheless, upward-coarsening successions, which are 17 comprised of basal gypsiferous and partly carbonaceous green/gray mudstones (Mg1), alternating red mudstones (Mr) and thin-layered sandstones (Sth), and capped channelized sandstones (Sc2), can represent shallow lacustrine-fill succession (Fig. 1.7). Lithofacies Sta (tabular sandstone) indicates several sedimentation processes (e.g., oscillatory flow, sandy gravity flow). Minor occurrences of this lithofacies also suggest some short-lived lacustrine conditions. 1.5.7 Facies Association 6 (FA6): Extensive Lacustrine Deposits 1.5.7.1 Description FA6 is composed of four fine-grained and two coarse-grained lithofacies (Table 1.2, Fig. 1.8), and is dominated by green/gray-colored mudstones of Mg2. FA6 exhibits an extremely low NTG (0.05 at MS06). The four fine-grained lithofacies are: a) Mr, red mudstone; b) Mg2, clay­sized, dominantly green, gray and dark gray, massive or laminated mudstone occasionally including thin siltstones and carbonaceous mudstones; c) Lth, clay-sized (calcilutite), tan, very hard, thin-layered limestone including gastropods and bivalves; and d) T, clay- to silt-sized, occasionally sandy, white to light gray, soft, massive, tabular or lenticular tuff or tuffaceous mudstone/siltstone occasionally rich in biotite. The two sandstone lithofacies are: a) Sc3, isolated small channelized sandstones; and b) Sta, tabular sandstones. Trace fossils are sparse in this facies association, although some U-shaped burrows and horizontal traces occur in lithofacies Sta (tabular sandstone). 1.5.7.2 Interpretation FA6 represents extensive lacustrine environments indicated by dominant Mg2 (green, gray and dark gray mudstone) and occurrences of Sta (tabular sandstone) and Lth (limestone) (Fig. 1.8). Lithofacies Mg2 indicates suspension deposition (lacking any pedogenesis feature such as mottled and slickenside structures). Lithofacies Lth suggests deposition in shallow and quiet (sediment-starved) water. Lithofacies T indicates ash fall or reworked ash fall deposits. Common occurrences of red mudstones (Mr) indicate oxidizing conditions of exposure, thus the 18 Figure 1.8. Facies association FA6 at MS06 and MS26. a) Outcrop photo shows a typical FA6 succession at MS06. b) Lithofacies Mg2 and Lth. c) Lithofacies Lth including a shell of bivalves at MS06. d) Lithofacies T (biotite-rich tuff) at MS06. e) Lithofacies Sta with wave ripples at MS26. f) Representative portion of MS06 shows the detailed descriptions of lithofacies Mg2, Lth, and T. g) Representative portion of MS26 shows the detailed descriptions of lithofacies Sta. Lithofacies descriptions (generalized) and codes are in Table 1.1. lacustrine environments were relatively shallow, as well as periodic and not long-lived. 1.6 Regional Facies Architecture and Paleocurrent 1.6.1 Regional Facies Architecture E-W basin-wide regional correlations of composite sections are presented in Figure 1.9. Distinct sequence boundaries are recognized at the bases of members Db and Ds. This section describes and interprets boundaries, internal facies (facies association) architectures, and thickness changes of the Duchesne River members. 19 Figure 1.9. E-W regional correlations of composite sections A to G (location of cross section in Fig. 1.2). Paleocurrent data at measured section locations (vertical bars with numbers) are shown as rose diagrams. The stratigraphic datum is set at the base of Dl, which can be regarded as a nearly isochronous boundary (~40Ma). Lithology classifications represent the dominant or representative lithology and are generalized for this scale of correlation. Lithological interpretations between measured sections (detailed sections in the Appendix 2) are schematic. The architecture of facies associations (FA1-FA6) is shown in the upper-right inset panel. Note the significant contrast of facies (facies association) between the western and eastern portion. Composite section abbreviations: BTMN; Blacktail Mountain North, SBM; Steamboat Mountain, TNE; Talmage NE, RC; Red Cap, BSLNW; Big Sand Lake NW, ANE; Altonah NE, CW; Cottonwood Wash, UE; Upalco E, BK; Bucher Knife, MR; Monarch Ridge, JSF; John Starr Flat, ID; Independence, RVE; Roosevelt E, RVNE; Roosevelt NE, R; Randlett, HH; Halfway Hollow, LM; Little Mountain, OE; Ouray E, HSB; Horseshoe Bend, BZ; Bonanza, RW: Red Wash. 20 1.6.1.1 Brennan Basin Member (Db) The basal Brennan Basin Member (Db) of the Duchesne River Formation is characterized by channelized sandstones interbedded with red fine-grained rocks (Andersen and Picard 1972). It consists of fluvial facies associations of FA1, FA2, and FA3 in the E-W regional section (Fig. 1.9). This member has a sharp contact with the underlying Uinta Formation, particularly at several locations (e.g., MS13, MS24) in the mid-western part of the basin. In these locations, dominant green mudstones and conspicuous stromatolitic limestones of the Uinta Formation that clearly indicate a lacustrine environment are overlain by amalgamated channelized sandstones (lithofacies Sc1) of the Db fluvial environment (FA1). Thus, the base of Db marks a sequence boundary that represents an abrupt basinward shift of facies. This sequence boundary becomes gradually obscure to the west (MS28) where both Db and the Uinta Formation are dominated by sandstones, and to the east (MS03, MS10, and MS23) where both Db and the Uinta Formation are dominated by mudstones (Fig. 1.9). Mudstone colors are important to help trace the contact (i.e., sequence boundary) of Db (red) with the Uinta Formation (green/gray) at these locations. This sequence boundary forms an angular unconformity in the northern margin of the basin, where Db overlies the older rocks (Anderson and Picard 1972; Campbell and Ritzma 1979). For example, the FA4 (alluvial fan complex) of Db unconformably overlies the Cretaceous Mesaverde Group along Asphalt Ridge (see N-S regional section of Fig. 1.10). Db exhibits a significant contrast of facies; sandy FA1 in the west and muddy FA2 and FA3 in the east. The difference in the total thickness between the west (~400 m) and east (~600 m) indicates a lower aggradation rate (i.e., more bypassing and/or degradation) of FA1 in the west and a higher aggradation rate of FA2 and FA3 in the east, even though the E-W cross section shows some exaggeration due to the intertonguing relationship at the upper member boundary (Fig. 1.9). 1.6.1.2 Dry Gulch Creek Member (Dd) The second Dry Gulch Creek Member (Dd) of the Duchesne River Formation is characterized by red and green/gray fine-grained rocks with interbedded sandstones (Andersen 21 Figure 1.10. N-S geological cross section along MS01 - MS05 - MS03 (location of cross section in Fig. 1.2). Paleocurrent data at measured section locations are shown as rose diagrams. The architecture of facies associations is shown in the upper-left inset panel. FA4 (alluvial fan complex) commonly occurs throughout all the members in the north (i.e., foothills of the Uinta Mountains). Note that Db is juxtaposed with the Cretaceous Mesaverde Group in the north where the Tertiary Uinta and Green River formations are completely eroded out. Composite section abbreviation: AR; Asphalt Ridge, TW; Twelvemiles Wash, LM; Little Mountain, R; Randlett, HH; Halfway Hollow. and Picard 1972). It is composed of fluvial - lacustrine facies associations of FA5 and FA2 in the E-W regional section (Fig. 1.9). It has a conformable contact with the underlying Db, and the basal beds interfinger upsection to the east of Roosevelt (Bryant et al. 1989). The contacts are nearly isochronous to the west of MS15 near Roosevelt as the basal green/gray mudstones (lithofacies Mg1) are widely traceable. Although Dd exhibits a significant contrast of facies, red and green/gray mudstones with interbedded sandstones of FA5 (wetland) in the west and red mudstones with interbedded sandstones of FA2 (dry alluvial plain) in the east, there is no significant difference in formation thickness between the west and east. 22 1.6.1.3 Lapoint Member (Dl) The third Lapoint Member (Dl) of the Duchesne River Formation is characterized by dominant green/gray mudstones and minor red fine-grained and coarse-grained rocks (Andersen and Picard 1972). It is composed of facies associations of FA6 and FA2 in the E-W regional section (Fig. 1.9). The base of this member is defined by the occurrence of extensive bentonitic fine-grained beds (lithofacies T), and thus is regarded as a nearly isochronous boundary (Andersen and Picard 1972). Tuffs were presumably sourced from volcanoes in the Wasatch Range, East Tintic Mountains, and Oquirrh Mountains to the west (Bryant et al. 1989). Dl shows contrasting facies associations that are green/gray mudstone-dominated FA6 (lacustrine) in the west and red mudstone-dominated FA2 (dry alluvial plain) in the east (Fig. 1.9). Correspondingly, there is also a remarkable difference in the total thickness where the west is several hundred meters thicker than the east (Fig. 1.9). 1.6.1.4 Starr Flat Member (Ds) The uppermost Starr Flat Member (Ds) of the Duchesne River Formation is characterized by dominant conglomerates and sandstones with lesser amounts of fine-grained rocks (Andersen and Picard 1972). It is composed of facies associations FA1 and FA4. It has a sharp contact with the underlying Dl at the type locality (MS09) where sandstone-dominated FA1 of Ds overlies green/gray mudstone-dominated FA6 (lacustrine) of Dl (Fig. 1.9). This basal contact indicates an abrupt basinward shift of facies (i.e., sequence boundary). In some other areas to the west (e.g., MS32) and east (e.g., MS02, MS18), sporadic conglomeratic FA4 (alluvial fan) of Ds are observed. Bryant et al. (1989) noticed that conglomeratic facies of Ds unconformably overlie the underlying Duchesne River members in some areas. However, it is difficult to assign these outcrops to Ds and make basin-scale correlations with confidence because of their patchy distribution caused by modern erosion, limited exposure by vegetation, and lithological similarities to the overlying Bishop Conglomerate. 23 1.6.2 Paleocurrent and Fluvial Style Newly acquired paleocurrent measurements show dominant southerly transport that confirms earlier reports by Warner (1965, 1966) and Andersen and Picard (1974). However, examination of paleocurrents of the fluvial channel dominated member Db shows significant features that assist in interpreting the paleodrainage patterns (Fig. 1.11). The western half of the basin tends to have more east or southeast directed flows, whereas the central-eastern part of the basin shows more south to southwest directed flows. Flow directions in the eastern part of the basin are more variable. Correspondingly, there is a remarkable contrast of fluvial styles between the western and eastern portions of the basin (Fig. 1.11): a high NTG amalgamated channel system of FA1 in the western part and a lower NTG isolated channel system of FA2 and FA3 in the eastern part. A possible scenario is that in the western part of the basin, more confined and packed eastward axial drainage systems developed, while the east had less confined and relatively isolated drainage patterns. Notably well-sorted and quartz-rich sandstones in the eastern part of Db-FA1 (e.g., MS24, MS13) suggest their long transport from the west and support this scenario. 1.7 Proposed Scenario of Duchesne River Sequence The basin-scale stratigraphic architecture (Fig. 1.9) shows distinct vertical and lateral (west to east) facies changes. A tectonic-driven sequence development scenario can explain the evolution of the Duchesne River late basin-fill (Fig. 1.12, Fig. 1.13). Although a tectonic force (uplift) is the ultimate control on the large-scale sedimentary packages, irregular accommodation and discharge controls within the basin (both caused in response to a tectonic uplift event) and resultant lateral facies changes are discernable. This section presents a three-staged evolutionary scenario (Fig. 1.12, Fig. 1.13) for the upward-fining sequence of members Db (stage 1), Dd (stage 2), and Dl (stage 3). A traditional terminology scheme of sequence stratigraphy for these stages (i.e., LST: lowstand systems tract for stage 1, TST/HST: transgressive/highstand systems tract for the combined stages 2 and 3) is adopted in this study. The systems tracts here are based on the relative position of water table depth (Fig. 1.12). Thus, 24 Figure 1.11. Paleocurrent data (total 264 measurements, magnetic declination: +11° used for corrections) plotted as rose diagrams with average directions (blue arrows), schematic fluvial channel styles, and stacking patterns of Db. More east and southeast flows in the western portion are possibly indicative of a confined eastward axial drainage system, while the eastern portion suggests an isolated and unconfined southward drainage system. These two systems possibly met near around the south of Roosevelt - Fort Duchesne and flowed south along the present-day basin axis. LST represents the dominant fluvial environment where the water table is low, and TST/HST marks dominant wetland to lacustrine settings where the water table is rising or high. The term "base level" is not used in this study, since the definition of base level at fluvial environments is debatable (Schumm 1993; Dalrymple et al. 1998; Catuneanu 2006). The base-level or a graded equilibrium profile in upstream environments are the result of complex allogenic controls such as 1) tectonics (uplift and subsidence), 2) flow energy/slope, and 3) discharge (e.g., Holbrook et al. 2006). Therefore, the relative water table level change in this paper is also a consequence of these allogenic controls. 1.7.1 Stage 1 - Db The first stage (Db) is marked by a basal sequence boundary and initial deposits comprising an upward-fining sequence (Fig. 1.12). There is a clear indication of the Uinta 25 Figure 1.12. Proposed tectonic-driven sequence stratigraphic framework for the Duchesne River Formation. The Duchesne River sequence development is primarily triggered by uplift(s) in the Uinta Mountains and possibly in the Sevier FTB. The upward-fining sequence in the western part of the basin started with high energy sedimentation on the steep slope (stage 1), followed by a lower energy fluvial system accompanying wetland conditions on the gentle slope (stage 2). Then an extensive lake system emerged as a result of differential subsidence (stage 3). In contrast, the eastern part of the basin predominantly exhibits mixed braided, meandering, and isolated fluvial channel systems with gradually decreasing coarse-grained deposits upward (stage 1 to stage 3). Here, the relative water table level change is a consequence of three major allogenic controls of tectonics, flow energy/slope, and discharge (see text). Mountains uplift because of an angular unconformity in the northern margin of the basin at the beginning of this stage (Fig. 1.10). Correspondingly, the southern part of basin also exhibits a distinct basinward shift of facies (i.e., sequence boundary) shown by the lacustrine deposits of the Uinta Formation overlain by the fluvial deposits of Db. There is a possibility that the Sevier FTB also activated at the same time (as discussed in the later section). These Uinta and Sevier FTB uplifts would induce the following environmental changes: 1) destruction of accommodation space in the proximal part of the basin, 2) higher discharge and sediment influx from uplifted mountain range(s), and 3) formation of steep and unstable slopes around the basin boundary fault. These changes are reflected in the deposition (progradation) of the fluvial facies associations (FA1, FA2, FA3) of Db and the cessation of lake deposition (Fig. 1.12). The internal difference in basin-wide facies association between the west (FA1) and the 26 Figure 1.13. Three-staged evolutionary paleogeographic scenario of the upward-fining sequence of the Duchesne River Formation. Stage 1 is characterized by: uplift(s) possible in two source terrains; formation of an angular unconformity (SB); and development of a confined, high NTG braided fluvial system in the western part of the basin due to a high discharge from two source terrains (Uinta Mountains and Sevier FTB). In stage 2, retreat of sediment entry points in the alluvial fan facies, a decrease in discharge and sediment influx, and a possible differential subsidence caused development of low NTG fluvial systems on wet (west) and dry (east) alluvial plains. In stage 3, further retreat of sediment entry points and differential subsidence allowed development of an extensive lake system in the western part of the basin. Note the significant difference in facies, thickness, and stacking pattern between the west and east due to irregular allogenic controls (discharge and tectonic subsidence) within the basin. 27 east (FA2 and FA3) can be explained by local topographic and climatic factors. Specifically, the two clastic source terrains (Uinta Mountains and Sevier FTB) in the west could have caused a higher discharge (Fig. 1.13). The channel systems might have been confined (topographically controlled) along the east-west trending basin-axis in the western part of the basin. These controls resulted in the western high NTG braided river system (FA1) with repetitive cut-and-fill (degradational) patterns and frequent avulsions. Concurrently, the eastern part of the basin probably received a lower discharge due to a single source terrain (Uinta Mountains) and low channel confinement (resulting in lower NTG aggradational stacking patterns) (Fig. 1.13). It should be noted that there is a climatic contrast even in the present, modern-day Uinta Basin and surrounding ranges. The western area currently has a wetter climate and higher precipitation because it is surrounded by higher ranges of the Uinta and Wasatch Mountains, while the eastern area has a drier climate and lower precipitation (Greer 1981; Jensen et al. 1990; Gillies and Ramsey 2009). Although the modern Green River flowing across the eastern Uinta Mountains gives a significant amount of discharge into the eastern dry Uinta Basin (Fig. 1.1), this large drainage system opened in the late Miocene or early Pliocene time (Hansen 1986) and did not exist in the Late Eocene. Thus, the Db climatic contrast discussed here was probably strongly affected by local tectonics (i.e., climatic feedback mechanism by mountain range formation) rather than global climatic dynamics. The name of systems tract LST was adopted for this stage on the basis of the relative low water table level that is recorded in the dominance of fluvial environments. 1.7.2 Stage 2 - Dd The second stage (Dd) is the transitional phase between stage 1 (Db) and stage 3 (Dl). A lower energy (relatively sinuous) fluvial system developed on the gentle slopes due to retreat of sediment entry points in the alluvial fan facies and a decrease in discharge and sediment influx as a result of erosional lowering of the source mountain ranges. The internal lateral facies change of Dd between the west (FA5) and east (FA2) probably reflects tectonic and/or climatic controls (Fig. 1.12). The wetter facies of FA5 in the west could be a response to 1) differential 28 tectonic subsidence and formation of a depression area in the west, and/or 2) continuous high discharge from multiple source terrains to the west. Total sediment package during this transitional stage is thinner than other stages, and thus the time period of this phase might be shorter than other stages. This is a possible reason why there is no distinct difference in thickness between the western and eastern part of the basin. The name of systems tract TST/HST was adopted for this stage on the basis of the relatively higher water table level. 1.7.3 Stage 3 - Dl The third and youngest phase of the upward-fining sequence is characterized by fine­grained sediments (Fig. 1.12). The dominance of fine-grained deposits suggests a further retreat of sediment entry points and alluvial fan facies due to the possible lowering of source mountain ranges. The lateral facies change from wetter/lacustrine FA6 in the west to drier/fluvial FA2 in the east reflects continuous tectonic and climatic controls from stage 2. The development of a more extensive lake system of FA6 in the west is largely due to differential subsidence (i.e., faster accommodation space development). A significant thickness difference between the west (thick) and the east (thin) within Dl (Fig. 1.9) could be caused by a thrust-loading mechanism, which is a classic subsidence concept in Laramide basins (Beck et al. 1988). The uplift(s) of the Uinta Mountains (i.e., reactivation of the basin boundary fault) to the north and possibly the Sevier FTB to the west could produce higher subsidence in the western part of the basin due to a higher thrust-loading in the west (Fig. 1.13). In the Uinta Basin, numerous tuff/tuffaceous beds in this stage suggest contemporaneous volcanic activity presumably in the mountain ranges (Wasatch Range, East Tintic Mountains, and Oquirrh Mountains) to the west (Bryant et al. 1989). Although this volcanic activity might have occurred far away from the Uinta Basin and the tectonic impact on the basin is unrecognizable, there was surely some change in clastic source materials. The high water table position of the lacustrine system suggests a TST/HST for this final and last stage in the upward-fining sequence. 29 1.8 Discussion 1.8.1 Upward-fining Succession and Tectonics along the Sevier FTB The Eocene intermontane system in central Utah shows a common pattern of upward-fining succession that suggests a strong relationship to a regional tectono-sedimentary regime (i.e., uplift in the Sevier FTB). Lake Flagstaff is another Eocene lake system/basin that developed in the present central Utah along the Sevier FTB (e.g., Stanley and Collinson 1979; Zawiskie et al. 1982; Davis et al. 2009). The Eocene Crazy Hollow Formation consists of fluvial channel and flood plain deposits (Willis 1994; Weiss and Warner 2001), and unconformably overlies lacustrine deposits of the Green River Formation in the Flagstaff Basin (Weiss 1982). The Crazy Hollow Formation is in turn conformably overlain by the Aurora (Bald Knoll) Formation, which is more lacustrine dominated (McGookey 1960; Williams and Hackman 1971). Willis (1988) acquired biotite K-Ar ages of 38.4±1.5 to 40.5±1.7 Ma from the upper part of the Aurora Formation, which is similar and correlatable to the K-Ar ages of ~40 Ma (by several researchers as noted above) from the tuff at the base of Dl. Collectively, this upward-fining succession has a very similar profile (i.e., distinct sequence boundary at base and gradual change from fluvial to shallow lacustrine upwards), with a similar age to the upward-fining Duchesne River sequence. Thus, it is possible that the Sevier FTB activated contemporaneously with the Uinta Mountains at the beginning of the Duchesne River deposition, and it simultaneously triggered the development of the upward-fining Crazy Hollow - Aurora sequence as well as the Duchesne River sequence. 1.8.2 Controlling Factors on the Duchesne River Sequence In proximal continental environments, it is broadly accepted that an upward-fining sequence is controlled by source uplift that initially brings in coarse-grained sediments on steep slopes and the fine-grained sediments ensue later due to source area retreat and relief decline (e.g., Mack and Ramussen 1984; Catuneanu and Elango 2001). Here, we examine three major controlling factors, 1) tectonics (accommodation), 2) flow energy/slope, and 3) discharge, on the three-staged Duchesne River sequence (Db, Dd, Dl) and fluvial - lacustrine styles (Fig. 1.12, Fig. 30 1.13). The relative water table level curve in Figure 1.12 is the result of these three allogenic controls, and thus represents dominant depositional environments (i.e., LST: fluvial, TST/HST: wetland to lacustrine settings). All three major controlling factors were in response to the initial tectonic pulse (uplifts) in the Uinta Mountains and possibly in the Sevier FTB (see descriptions for each factor below). Other major allogenic contributions (i.e., global climate changes) would likely be completely masked or overpowered by the strong, local tectonic driving force in the Uinta Basin. 1.8.2.1 Tectonics Initial source uplift(s) in the Uinta Mountains to the north and possibly the Sevier FTB to the west caused a low accommodation space in the proximal part of the basin and induced the progradation of fluvial environments (cessation of the lake environment of the Uinta Formation) in stage 1 (Db). The late-stage thrust-loading (i.e., differential subsidence) caused a high accommodation space in the western part of the basin and induced development of lacustrine environment in stage 3 (Dl). 1.8.2.2 Flow Energy/Slope Initial high flow energy from uplifted mountain range(s) gradually decreased over time due to reduced relief and source area retreat (following models by Mack and Ramussen 1984; Catuneanu and Elango 2001). This trend resulted in the overall member-scale, upward-fining sequence from stage 1 (Db) to stage 3 (Dl). 1.8.2.3 Discharge The geographical difference in discharge (high discharge in the west and low discharge in the east) was attributed to local climate (climatic feedback mechanisms from mountain range uplifts) and source terrain input. This difference induced contrasting fluvial styles and stacking patterns of high NTG / degradation in the west and low NTG / aggradation in the east, during stage 1 (Db) (Fig. 1.13). 31 1.8.3 Comparison with Fluvial-Lacustrine Sequence Stratigraphic Models The upward-fining sequence within the Duchesne River Formation is similar to the fluvial sequence stratigraphic models by Wright and Marriot (1993), Shanley and McCabe (1994), Currie (1997), and Legaretta and Uliana (1998). Their models commonly show a vertical profile with facies change from basal confined amalgamated channels (degradation) into upper unconfined isolated channels (aggradation). However, the Duchesne River example shows gradation from an amalgamated channel (degradational) system (FA1) into an isolated/unconfined channel (aggradational) system (FA2, FA3, and FA5), not only in the vertical succession, but also in the lateral and coeval successions. Thus, this Duchesne River study provides a comprehensive three-dimensional picture of how irregular allogenic controls such as tectonics (differential subsidence) and discharge (different local climate and source terrain input) within a continental basin can affect the resultant basin-scale lateral facies variations of fluvial - lacustrine deposits. The western portion of fluvial-lacustrine architecture of the Duchesne River Formation can also be explained by a lacustrine sequence stratigraphic model of Carroll and Bohacs (1999). Their lake-basin type classification system is based on the interaction of the rates of sediment and water supply (mostly climatic) with potential accommodation (mostly tectonic). Potential accommodation space was overwhelmed by sediment and water supplies during the stage 1 (Db) of the Duchesne River sequence, resulting in fluvial sedimentation. By contrast, these factors of accommodation and sedimentation were more equal or balanced due to an increase in potential accommodation where the differential subsidence led to the lake development in the western part of the basin during the stage 3 (Dl). 1.9 Conclusions This outcrop-based sequence stratigraphic study of the Tertiary Duchesne River Formation of the Uinta Basin reveals the basin-scale (>130 km) architecture of fluvial - lacustrine facies associations: FA1- amalgamated braided fluvial channels, FA2- extensive flood plain and stacked broad fluvial channels, FA3- extensive flood plain and isolated small streams, 32 FA4- alluvial fan complex, FA5- dry and wet flood plains and fluvial channels, and FA6- extensive lacustrine deposits. The member-scale upward-fining sequence (unconformity-bounded) was likely triggered by the uplift(s) of the Uinta Mountains and possibly the Sevier FTB. A tectonic-driven sequence stratigraphic scenario with three stages (stage 1: Db, stage 2: Dd, stage 3: Dl) is proposed to decipher the evolution of the Duchesne River late basin-fill. The significant facies and thickness variations between the western and eastern part of the basin record irregular allogenic controls of tectonics and discharge within the basin. Specifically, a high NTG (degradational) fluvial system (FA1) in the west in stage 1 reflects a higher discharge due to a wet climate and two source terrain inputs (Uinta Mountains and Sevier FTB), whereas a low NTG (aggradational) fluvial system (FA2 and FA3) in the east reflects a lower discharge due to a dry climate and single source terrain input (Uinta Mountains). The development of thick lacustrine deposits (FA6) in the west during final stage 3 reflects differential tectonic subsidence in the Uinta Basin. The Duchesne River example provides a comprehensive picture of the complex sequence stratigraphic framework of upstream environments, and demonstrates how internal facies architectures at the basin-scale evolve by allogenic controls stemming from tectonic uplifts. These kinds of remarkable outcrop field exposures are valuable for testing and refining models of continental sequence stratigraphy, with applications to explorations in similar fluvial - lacustrine systems in continental basins. 1.10 References Allen, J.L., and Johnson, C.L, 2010, Facies control on sandstone composition (and influence of statistical methods on interpretations) in the John Henry Member, Straight Cliffs Formation, Southern Utah, USA: Sedimentary Geology, v. 230, p. 60-76. Andersen, D.W., and Picard, M.D., 1972, Stratigraphy of the Duchesne River Formation (Eocene-Oligocene?), northern Uinta basin, northeastern Utah: Utah Geological and Mineral Survey Bulletin, v. 97, 29 p. Andersen, D.W., and Picard, M.D., I974, Evolution of synorogenic clastic deposits in the intermontane Uinta Basin of Utah, in Dickinson, W.R., ed., Tectonics and Sedimentation: SEPM Special Publication, v. 22, p. 167-189. 33 Aswasereelert, W., Meyers, S.R., Carroll, A.R., Peters, S.E., Smith, M.E. and Feigl, K.L., 2013, Basin-scale cyclostratigraphy of the Green River Formation, Wyoming: Geological Society of America Bulletin, v. 125, p. 216-228. Atchley, S.C., Nordt, L.C., and Dworkin, S.I., 2004, Eustatic control on alluvial sequence stratigraphy: A possible example from the Cretaceous-Tertiary transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A.: Journal of Sedimentary Research, v. 74, p .391-404. Beck, R.A., Vondra C.F., Filkins, J.E., and Olander, J.D., 1988, Syntectonic sedimentation and Laramide basement thrusting, Cordilleran foreland; Timing of deformation: Geological Society of America Memoirs, v. 171, p. 465-488. 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Jensen, D.T., Bingham, G.E., Ashcroft, G.L., Malek, E., McCurdy G.D., and McDougal, W.K., 1990, Precipitation pattern analysis Uinta Basin-Wasatch Front, Report to Division of Water Resources, State of Utah under Contract Number 90-3078: Office of the State Climatologist, Utah State University, Logan, Utah, 41 p. Keighley, D., Flint, S., Howell, J., and Moscariello, A., 2003, Sequence stratigraphy in lacustrine basins: a model for part of the Green River Formation (Eocene), southwest Uinta Basin, Utah: Journal of Sedimentary Research, v. 73, p. 987-1006. Kelly, T.S., Murphey, P.C., and Walsh, S.L., 2012, New records of small mammals from the middle Eocene Duchesne River Formation, Utah, and their implications for the Uintan- Duchesnean North American Land Mammal Age transition; Paludicola, v. 8, p. 208-251. Kraus, M.J., 2002, Basin-scale change in flood plain paleosols: Implications for interpreting alluvial architecture: Journal of Sedimentary Research, v. 72, p. 500-509. 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Legaretta, L., and Uliana, M.A., 1998, Anatomy of hinterland depositional sequences: Upper Cretaceous fluvial strata, Nequen Basin, west-central Argentina, in Shanley, K.W., and McCabe, P.J., eds., Relative Role of Eustasy, Climate and Tectonism in Continental Rocks: SEPM Special Publication, v. 59, p. 83-92. Mack, G.H., and Rasmussen, K.A., 1984, Alluvial-fan sedimentation of the Cutler Formation (Permo-Pennsylvanian) near Gateway, Colorado: Geological Society of America Bulletin, v. 95, p. 109-116. McDowell, F.W., Wilson, J.A., and Clark, J., 1973, K-Ar dates for biotite from two paleontologically significant localities: Duchesne River Formation, Utah, and Chadron Formation, South Dakota: Isochron/West, v. 7, p. 11-12. McGookey, D.P., 1960, Early Tertiary stratigraphy of part of central Utah: American Association of Petroleum Geologists Bulletin, v. 44, p. 589-615. Nemec, W., and Steel, R.J., 1984, Alluvial and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits, in Koster, E.H., and Steel, R.J., eds., Sedimentology of Gravels and Conglomerates: Canadian Society of Petroleum Geologists Memoir 10, p. 1 -31. Picard, M.D., and High, L.R., 1972, Criteria for recognizing lacustrine rocks, in Rigby, J.K., and Hamblin, W.K., eds., Recognition of ancient sedimentary environments: SEPM Special Publication, v. 16, p. 108-145. Posamentier, H.W., and Vail, P.R., 1988, Eustatic controls on clastic deposition Il-sequence and systems tract models, in Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., Van Wagoner, J.C., eds., Sea Level Changes-An Integrated Approach: SEPM Special Publication, v. 42, p. 125-154. Prothero, D.R. and Swisher, C.C., 1992, Magnetostratigraphy and geochronology of the terrestrial Eocene-Oligocene transition in North America, in Prothero, D.R., and Berggren, W.A., eds., Eocene-Oligocene climatic and biotic evolution: Princeton University Press, p. 46-74. Retallack, G.J., 2001, Soils of the past, 2nd edition, Blackwell Science, Oxford, 404 p. Rowley, P.D., Hansen, W.R., Tweto, O., and Carrara, P.E., 1985, Geologic map of the Vernal 10 x 20 quadrangle, Colorado, Utah, and Wyoming: U.S. Geological Survey Miscellaneous Investigations Series I-1526. Schumm, S.A., 1993, River response to baselevel change: Implications for sequence stratigraphy: The Journal of Geology, v. 101, p. 279-294. Shanley, K.W., and McCabe, P.J., 1994, Perspectives on the sequence stratigraphy of continental strata: American Association of Petroleum Geologists Bulletin, v. 78, p. 544­568. Sprinkel, D.A., 2006, Interim geologic map of the Dutch John 30' x 60' quadrangle, Daggett and Uintah Counties Utah, Moffat County, Colorado, and Sweetwater County, Wyoming: Utah Geological Survey. 36 Sprinkel, D.A., 2007, Interim geologic map of the Vernal 30' x 60' quadrangle, Uintah and Duchesne Counties, Utah, and Moffat and Rio Blanco Counties, Colorado: Utah Geological Survey. Stanley, K.O., and Collinson, J.W., 1979, Depositional history of Paleocene-lower Eocene Flagstaff Limestone and coeval rocks, Central Utah: American Association of Petroleum Geologists Bulletin, v. 63, p. 311-323. Visher, G.S., 1965, Use of vertical profile in environmental reconstruction: American Association of Petroleum Geologists Bulletin, v. 49, p. 41-61. Warner, M.M., 1965, Cementation as a clue to structure, drainage patterns, permeability, and other factors: Journal of Sedimentary Petrology, v. 35, p. 797-804. Warner, M.M., 1966, Sedimentational analysis of the Duchesne River Formation, Uinta Basin, Utah: Geological Society of America Bulletin v. 77, p. 945-957. Weiss, M.P., 1982, Relation of the Crazy Hollow Formation to the Green River Formation, Central Utah, Overthrust Belt of Utah: Utah Geological Association Publication, v. 10, p. 285-289. Weiss, M.P., and Warner, K.N., 2001, The Crazy Hollow Formation (Eocene) of central Utah: Brigham Young University Geology Studies, v. 46, p. 143-161. Williams, P.L., and Hackman, R.J., 1971, Geology, structure, and uranium deposits of the Salina Quadrangle, Utah: U.S. Geological Survey IMAP 591. Willis, G.C., 1988, Geologic map of the Aurora quadrangle, Sevier County, Utah: Utah Geological and Mineral Survey, Map 112, 21 p. Willis, G.C., 1994, Interim geologic map of the Richfield quadrangle, Sevier County, Utah: Utah Geological Survey Open-file Report 309, 73 p. Wright, V.P., and Marriott, S.B., 1993, The sequence stratigraphy of fluvial depositional systems: the role of floodplain sediment storage: Sedimentary Geology, v. 86, p. 203-210. Zawiskie, J., Champman, D., and Alley, R., 1982, Depositional history of the Paleocene- Eocene Colton Formation, north-central Utah, in Nielson, D.L., ed., Overthrust Belt of Utah: Utah Geological Association Publication, v. 10, p. 273-284. CHAPTER 2 SOURCE-TO-SINK FLUVIAL SYSTEMS FOR SANDSTONE RESERVOIR EXPLORATION: EXAMPLE FROM THE BASAL BRENNAN BASIN MEMBER OF TERTIARY DUCHESNE RIVER FORMATION, NORTHERN UINTA BASIN, UTAH 2.1 Abstract The Tertiary Duchesne River Formation represents the late-stage fill of the Uinta Basin, which preserves an upward-fining fluvial sequence. Extensive outcrop exposures of the Tertiary Duchesne River Formation can be traced from source to sink, and comprise a valuable example of basin-scale facies architectures to evaluate controlling factors on the depositional history. Distinct basin-scale facies change of the sandstone-dominated basal member of the Duchesne River Formation exhibits a high net-sandstone-to-gross-thickness (NTG) braided river system in the western and a low NTG braided, meandering, and isolated small river system in the eastern portion of the basin. Extensive paleocurrent data show overall southerly transport largely derived from the Uinta Mountains in the north. However, many southeastward flows and texturally and compositionally mature (quartz-rich) sandstones in the western part of the basin suggest the influence of long transportation from a different source terrain, specifically the Sevier Fold Thrust Belt to the west. The multiple transport patterns and petrographic data indicate that the high-discharge drainage system along the E-W basin axis in the western part of the basin was important for development of a large-volume and high-quality (porous) reservoir system. This example demonstrates the importance of deciphering the complex input of multiple source terrains for exploration of fluvial sandstone reservoirs in the sink. 38 The Uinta Basin, a Laramide intermontane lake basin, is a prolific hydrocarbon province containing a world-class lacustrine source rock (i.e., Green River shale). The lacustrine basin-fill sequence has very good exposures at the basin margins, and thus comprises a valuable field analog for growing exploration efforts in more challenging lacustrine basins (e.g., deep-seated lacustrine basins on Atlantic margins, intracratonic rift basins in Africa). The Duchesne River Formation is an uppermost unit of the Lake Uinta basin-fill sequence that reveals detailed basin-scale fluvial - lacustrine facies architectures. There is a remarkable difference in fluvial sandstone reservoir facies and property between the western and eastern part of the basin within the basal member (Db). The purpose of this chapter is to document the multiple fluvial sedimentary processes from source terrains to the basin (sink). Two approaches, basin-scale field surveys and petrographic studies, are taken to decipher the mechanism of development of suitable fluvial sandstone reservoir facies in the sink. 2.3 Geological Context 2.3.1 Geological Setting The Uinta Basin, situated in northeastern Utah (Fig. 2.1), is a part of the Laramide basin system that emerged during the latest Cretaceous to early Paleogene (Dickinson et al. 1988). After the Cretaceous Western Interior Seaway receded, Laramide basement-involved uplifts caused several segmented intermontane lake basins in the present Rocky Mountain region covering Montana, Wyoming, Colorado, and Utah (e.g., Dickinson et al. 1986, 1988). The Uinta Basin is surrounded by the Uinta Mountains to the north, the Sevier Fold Thrust Belt (FTB) to the west, Douglas Creek arch to the east, and the Uncompahgre uplift and San Rafael Swell to the south. The basin shows an asymmetric basin shape that is northerly bounded by a high-angle reverse fault at the foothills of Uinta Mountains (e.g., Fouch 1975; Bruhn et al. 1983, 1986). The Paleocene-Eocene lacustrine basin-fill sequence is comprised of the Wasatch (fluvial), Green River (lacustrine), Uinta (fluvial-lacustrine), and Duchesne River (fluvial) 2.2 Introduction 39 Figure 2.1. Geological map and geologic column of the Uinta Basin. a) Geological map of the Uinta Basin. Regional dip is to the north and formations get progressively younger toward the Uinta Mountains. The basin is surrounded by high mountain ranges of the Uinta Mountains and Sevier Fold Thrust Belt (FTB). The map of Laramide lake basin system is from Dickinson et al. (1988). The geological map is modified from Andersen and Picard (1974), Bryant et al. (1989), Bryant (1992), Hintze et al. (2000), and Sprinkel (2006 and 2007). b) Schematic geologic column showing the Paleogene sequence of the Uinta Basin (modified from Hintze et al. 2000). T2 to T4 exhibits a typical upward-coarsening/shallowing lacustrine basin-fill succession. formations (Fig. 2.1) in ascending order. The upper three of these formations exhibit a typical upward shallowing or coarsening succession commonly observed in lacustrine basin-fill settings (Visher 1965; Picard and High 1972; Lambiase 1990). This basin-fill sequence is unconformably overlain by the Oligocene Bishop Conglomerate (Fig. 2.1). The Duchesne River Formation is exposed in the northern part of the Uinta Basin (Fig. 2.1). It consists of four members, the Brennan Basin (Db), Dry Gulch Creek (Dd), Lapoint (Dl), and Starr Flat (Ds) members in ascending order (Fig. 2.2). The lower three members comprise an upward-fining sequence with a sandstone-dominated basal member (Db), a transitional second member (Dd), and a mudstone-dominated third member (Dl). It is noted that Dl is rich in 40 Figure 2.2. Geological map of the Duchesne River Formation and surrounding area. Regional dip is to the north and the Duchesne River members (Db: Brennan Basin Member, Dd: Dry Gulch Creek Member, Dl: Lapoint Member, and Ds: Starr Flat Member) get progressively younger toward the Uinta Mountains. The locations of 35 measured sections (MS), sandstone samples for thin section (yellow triangles) and for QEMScan (light blue triangles), and composite sections A to G (black lines) are shown on the map. The map is modified after Andersen and Picard (1974), Rowley et al. (1985), Bryant et al. (1989), and Sprinkel (2006 and 2007). tuff/tuffaceous beds throughout the basin, and a basal tuff of this member is a good stratigraphic time-marker (K-Ar ages of ~40 Ma reported by several researchers, McDowell et al. 1973; Andersen and Picard 1974; Prothero and Swisher 1992; Kelly et al. 2012) (see the detailed discussion on the geological age in Appendix A). The uppermost member (Ds) consists mainly of coarse-grained rocks (e.g., sandstones and conglomerates), indicating the onset of another upward-fining fluvial cycle. The Uinta Basin contains not only conventional hydrocarbon resources but also significant unconventional resources such as oil shales (e.g., Cashion 1964; Vanden Berg 2008), tar sands (e.g., Campbell and Ritzma 1979; Ritzma 1979; Blackett 1996), and gilsonites (e.g., Cashion 1967; Boden and Tripp 2012). In particular, the Duchesne River Formation contains significant amounts of tar sands at Asphalt Ridge situated in the eastern margin of the 41 basin (Fig. 2.2), as first studied in detail by Spieker (1931). This oil is sourced from the underlying Green River Formation (Covington 1957, 1963, 1964; Kayser 1966; Thomas et al. 1977; Hatcher et al. 1992). 2.3.2 Previous Studies There are not many stratigraphic and sedimentological studies of the Duchesne River Formation compared with the well-studied underlying Green River Formation, which attracts more researchers due to its economic significance. The latest stratigraphic nomenclature for the four members of the Duchesne River Formation was defined by Andersen and Picard (1972) (detailed nomenclatural history in Appendix A). Rowley et al. (1985) and Bryant et al. (1989) were the first to regionally map the four members defined by Andersen and Picard (1972). Andersen and Picard (1974) is the only study which examined sandstone compositions of the Duchesne River Formation. They noticed a geographical difference in sandstone compositions (i.e., rich in quartz in the western part of basin and rich in rock fragments in the eastern part of the basin). This chapter presents additional petrographic analyses focusing on the sandstone-dominated basal member of the Duchesne River Formation in order to recognize the mechanism of this geographical difference in sandstone compositions and also to examine the relationship with sandstone properties (porosity). 2.4 Methods Two approaches were taken to decipher the source-to-sink fluvial depositional system of the Duchesne River Formation. Basin-wide field surveys contributed to the construction of the regional stratigraphic framework along with detailed internal facies architectures and interpretation of paleodrainage patterns. Petrographic studies on representative sandstone samples from all over the basin provided supportive data for sediment sources, transportation distance, and sandstone property as described in detail below. 42 2.4.1 Regional Stratigraphic Study Thirty-five measured sections (MS01 to MS35) covering the distribution of the Duchesne River Formation (a total of 2,970 m, which includes the uppermost part of the Uinta Formation) and 441 paleocurrent measurements (264 measurements from the basal member) were acquired (Fig. 2.2). Then N-S trending composite sections A to G are correlated by referring to several geological maps and structural dips and strikes. It should be noted that the geological map is modified after Andersen and Picard (1974), Rowley et al. (1985), Bryant et al. (1989), and Sprinkel (2006, 2007). Although the latest regional 1° x 2° geological map of the Salt Lake City quadrangle (Bryant 1992) shows the "Undivided Duchesne River Formation" covering the broad area of the western part of the basin, this division obviously includes older units that are equivalent to the Uinta and Green River formations. For this region (to the west of Duchesne, UT), this study adopt the outline of the Duchesne River Formation by Andersen and Picard (1974), which is more suitable for sequence stratigraphic interpretation. E-W regional correlations of composite sections A to G are shown in Figure 2.3. The stratigraphic datum is set at the base of Dl (K-Ar ages of ~40 Ma from basal Dl tuffs as noted above). Lithological classifications presented in this correlation are the dominant or representative lithology. For the broad correlation, each lithology is generalized (e.g., a sandstone lithology mostly represents thicker channel type sandstone). Lithological interpretations between measured sections (MS locations on Figure 2.2 and detailed sections in Appendix B) are schematic. Nevertheless, expansive outcrops greatly support the interpretations presented here. 2.4.2 Petrographic Study Twenty fine- to medium-grained sandstone samples were collected over the basin in this study (Appendix C, Fig. 2.2) to supplement the large amounts of data (79 sandstone composition data) in the previous work by Andersen and Picard (1974). Nine thin sections (8 samples from Db, 1 sample from Dd) were studied in detail. We followed the Gazzi-Dickinson approach (Gazzi 1966; Dickinson 1970; Ingersoll et al. 1984) for these nine thin sections, counting a total of 500 grain counts per section. In this method, any mineral >0.0625 mm is 43 Figure 2.3. E-W regional correlations of composite sections A to G (location of cross section in Fig. 2.2). Paleocurrent data (magnetic declination: +11° used for corrections) at measured section locations (vertical bars with numbers) are shown as rose diagrams. The stratigraphic datum is set at the base of Dl (~40Ma). The thickness at each composite section is estimated by the modified geological map and structural dips. Lithology classifications represent the dominant or representative lithology and are generalized for this scale of correlation. Lithological interpretations between measured sections (detailed sections in the Appendix B) are somewhat schematic. The architecture of facies associations on this section is shown in the upper-right panel. Note that the significant contrast of facies (facies association) between the eastern and western portion. Abbreviation: BTMN; Blacktail Mountain North, SBM; Steamboat Mountain, TNE; Talmage NE, RC; Red Cap, BSLNW; Big Sand Lake NW, ANE; Altonah NE, CW; Cottonwood Wash, UE; Upalco E, BK; Bucher Knife, MR; Monarch Ridge, JSF; John Starr Flat, ID; Independence, RVE; Roosevelt E, RVNE; Roosevelt NE, R; Randlett, HH; Halfway Hollow, LM; Little Mountain, OE; Ouray E, HSB; Horseshoe Bend, BZ; Bonanza, RW: Red Wash. 44 counted as an individual grain component such as quartz and feldspar, even if this mineral forms a part of volcanic or sedimentary rock fragments. On the other hand, previous petrographic work by Andersen and Picard (1974) followed the classification of sandstone framework constituents of Folk (1968). However, there is no significant difference in the resultant compositions between the two approaches in sandstone samples of this study because rock fragments with such large (> 0.0625 mm) minerals/grains are quite minor and the majority of the fragments are composed of carbonates and cherts, as described in detail below. In addition, QEMScan automated disaggregated counts (e.g., method described in Allen et al. 2012) were conducted for 20 sandstone samples to supplement the thin section data. Nine of the 20 samples were the same as those used for the above-mentioned thin section analysis (Appendix C), which were included to ensure the consistency of the results between the QEMSCan and petrographic point counting methods. Thus, 11 of 20 samples (10 samples from Db, 1 sample from Dl) are used to supplement or fill in gaps between thin section data points. The biggest advantage of QEMScan automated disaggregated count is to shorten the analytical time, although the duration depends on the resolution and the number of particles to be counted. The detailed procedure of the QEMScan automated disaggregated count is summarized in Appendix D. 2.5 Results and Interpretations 2.5.1 Basin-scale Facies Architectures Chapter 1 of this thesis presented a detailed facies analysis and defined six facies associations, FA1 to FA6, in the Duchesne River Formation (Fig. 2.3, Table 2.1). Here, we present an abbreviated description of the Duchesne River members for presenting the context of depositional style and petrographic analysis. 2.5.1.1 Brennan Basin Member (Db) The basal Brennan Basin Member (Db) of the Duchesne River Formation is characterized by channelized sandstones interbedded with red fine-grained rocks (Andersen Table 2.1. Duchesne River Facies Associations FA Facies Member Facies Components Ss/Ms Apparent Sandbody Dimensions # Association Occurrence Lithology Sedimentary Structure Interpretation Fossils Ratio Amalgamated Braided Fluvial Db Amalgamated channelized ss Trough cross-stratified Amalgamated multiple interweaving fluvial channels 2 o to ra a . W c o EEo O cra -o c3 ■Q < FA 1 (western part of basin), Ds Red ms/silty ms Massive or mottled, usually in discontinuous/lenticular shape Well-drained flood plain paleosol 75/25 (MS28) > 1,000 m (MS28) Channels Thin-layered ss and slts Massive or trough cross-stratified (sometimes indistinct) Overbank deposit, often pedogenically-altered Red ms/silty ms Massive or mottled, usually in continuous shape Well-drained flood plain paleosol Extensive Flood Plain and Stacked Broad Fluvial Channels Db, Dd and Dl (central-eastern part of basin) Y ellow ms Mottled, often relict bedding Moderately-drained flood plain paleosol FA 2 Stacked broad channelized ss Trough cross-stratified, occasionally lateral accretion Braided and sinuous fluvial channels 50/50 (MS33) > 100 m (MS33) Isolated and narrow channelized ss Trough cross-stratified Isolated small stream Thin-layered ss and sits Massive or trough cross-stratified (sometimes indistinct) Overbank deposit, often pedogenically-altered Extensive Flood Plain Red ms/silty ms Massive or mottled, usually in continuous shape Well-drained flood plain paleosol FA Db (eastern part of basin) Y ellow ms Mottled, often relict bedding Moderately-drained flood plain / levee paleosol 15/85 < 100 m 3 and Isolated Small Steams Isolated and narrow channelized ss Trough cross-stratified Isolated small stream (MS14) (MS14) Thin-layered ss and slts Massive or trough cross-stratified (sometimes indistinct) Overbank deposit, often pedogenically-altered All members (northern Upward-fining package of cgl-ss Structureless or imbrication (cgl), often trough cross-stratified (ss) Alluvial fan channel/lobe 45/35/20 FA (cgl/ss/ms) 4 Alluvial Fan Complex Red ms/silty ms Massive or mottled Well-drained flood plain (interchannel) paleosol n/a margin of basin) Green ms/silty ms Mottled, thin-layered/veined gypsums, carbonaceous materials Playa or w etland deposit in the distal fan (MS01) Red ms/silty ms Mottled, usually in continuous shape Well-drained flood plain paleosol Dry and Wet Flood Plains Dd (western Green ms/silty ms Mottled, thin-layered/veined gypsums, carbonaceous materials Poorly-drained wetland or shallow lacustrine deposit > 100 m (stacked FA Stacked broad channelized ss fluvial Trough cross-stratified, occasionally lateral accretion Braided and sinuous fluvial channels 27/73 5 and Fluvial Channels part of basin) Tabularly and continuously bedded ss Massive or wave rippled, carbonaceous materials Marginal lacustrine deltaic deposit (MS15) channels), > 1,000m Yellow ms Mottled, often relict bedding Moderately-drained flood plain (deltaic ss) / levee paleosol Thin-layered ss and slts Massive or trough cross-stratified (sometimes indistinct) Overbank deposit, often pedogenically-altered Green, gray, and dark g ray ms Massive or laminated Lacustrine deposit Tuff and tuffaceous bed Massive or trough cross-stratified Ash fall and reworked deposit FA Extensive Lacustrine Deposits Dl (western part of basin) Channelized ss Trough cross-stratified Relatively small and isolated fluvial channel 5/95 n/a 6 Tabularly bedded ss Massive or wave rippled, carbonaceous materials Marginal lacustrine deltaic deposit (MS06) Red and maroon ms Massive, mottled or thin-layered Well-drained paleosol Ls Fossiliferous Lacustrine deposit Abbreviations: Cgl = Conglomerate, Ss = Sandstone, Slts = Siltstone, Ms = Mudstone, Ls = Limestone, FA = Facies Association, MS = Measured Section 45 46 and Picard 1972). It consists of three fluvial facies associations of FA1 (amalgamated braided fluvial channels), FA2 (extensive flood plain and stacked broad fluvial channels), and FA3 (extensive flood plain and isolated small steams) in the E-W cross section of Fig. 2.3 (see the detailed descriptions and interpretations of facies associations in Table 2.1). This member has relatively distinct contacts with the underlying Uinta Formation, particularly at several locations (e.g., MS13, MS24) in the mid-western part of the basin. In these locations, dominant green mudstones and conspicuous stromatolitic limestones of the Uinta Formation that clearly indicate a lacustrine environment are overlain by amalgamated channelized sandstones of Db (FA1). Thus, the base of Db marks a sequence boundary that represents an abrupt basinward shift of facies. This sequence boundary becomes gradually obscure to the west (MS28) where both Db and the Uinta Formation are dominated by sandstones, and to the east (MS03, MS10, and MS23) where both Db and the Uinta Formation are dominated by mudstones (Fig. 2.3). This sequence boundary forms an angular unconformity in the northern margin of the basin, where Db overlies the older rocks (Anderson and Picard 1972), and clearly marks the uplift of Uinta Mountains at the beginning of deposition of the Duchesne River Formation. There is a significant contrast in fluvial styles between the west and east in this basal member (Fig. 2.3). The west is characterized by FA1 of a braided river system with a high net-sandstone- to-gross-thickness ratio (NTG), while the east has FA2 and FA3 of a braided, meandering and isolated river system with a low NTG. Correspondingly, there is a change in the total thickness of Db between the east and west; the western part of the basin is a few hundred meters thinner than the east (Fig. 2.3). This thickness change indicates the western part of the basin had repetitive channel cut-and-fills resulting in less aggradation, while the east had more stable aggradation. 2.5.1.2 Dry Gulch Creek Member (Dd) The second Dry Gulch Creek Member (Dd) of the Duchesne River Formation is characterized by red and green/gray fine-grained rocks with interbedded sandstones (Andersen and Picard 1972). It consists of two fluvial-lacustrine facies associations of FA5 (dry and wet 47 flood plains and fluvial channels) and FA2 in the E-W cross section of Fig. 2.3 (detailed descriptions and interpretations of facies associations in Table 2.1). This member has a conformable contact with the underlying Db, and the basal beds interfinger upsection to the east of Roosevelt (Bryant 1989) (Fig. 2.3). The contacts are nearly isochronous to the west of MS15 near Roosevelt as the basal green/gray mudstones are widely traceable. Although there is a significant contrast in fluvial-lacustrine styles between the west (FA5) and east (FA2) in this second member, the basin-wide thickness change is subtle for this member, compared with the underlying Db and overlying Dl (Fig. 2.3). 2.5.1.3 Lapoint Member (Dl) The third Lapoint Member (Dl) of the Duchesne River Formation is characterized by dominant green/gray mudstones and minor red fine-grained and coarse-grained rocks (Andersen and Picard 1972). It consists of two fluvial-lacustrine facies associations of FA6 (extensive lacustrine deposits) and FA2 in the E-W cross section of Fig. 2.3 (detailed descriptions and interpretations of facies associations in Table 2.1). The basal contacts are characterized by the near isochronous occurrence of tuff/tuffaceous beds (K-Ar ages of ~40 Ma reported by several researchers as mentioned above) and therefore, the base of this member is used for a time-marker/datum for regional correlations (Fig. 2.3). There is a significant contrast in fluvial-lacustrine styles between the west (FA6) and east (FA2) in this third member. Correspondingly, there is a huge basin-wide thickness change recognized in this member (the west is several hundred meters thicker than the east). The thickness change was likely caused by a differential subsidence during the deposition of Dl, as an extensive and thick lacustrine facies in the west indicates accommodation space was created at higher rates than in the east. 2.5.1.4 Starr Flat Member (Ds) The uppermost Starr Flat Member (Ds) of the Duchesne River Formation is characterized by dominant conglomerates and sandstones with lesser amounts of fine-grained rocks (Andersen and Picard 1972). It consists of two fluvial facies associations of FA1 and FA4 48 in the E-W cross section of Fig. 2.3. The basal contacts are sharp at its type locality (MS09 John Starr Flat), where amalgamated channelized sandstones of FA1 overlie green/gray mudstones (FA6) of Dl, indicating an abrupt basinward shift of facies (i.e., sequence boundary). In some other areas, unconformable relationships have been reported (Rowley et al. 1985; Bryant et al. 1989) where conglomeratic facies of Ds overlie mudstone-dominated Dl or older deposits. This member has patchy distributions in the northern margin of the basin, and is sometimes hard to distinguish confidently from the overlying Oligocene Bishop conglomerates. 2.5.2 Sandstone Compositions and Provenances This section focuses on the sandstone compositions and textures of the fluvial-dominated basal member (Db) in order to understand provenances and drainage systems/patterns. As described in the previous section, Db has significant differences in fluvial styles and net-sandstone-to-gross-thickness ratio (NTG): high NTG in the western part and low NTG in the eastern part of the basin as shown in Fig. 2.4. The sandstone compositions and textures in combination with paleocurrent data provide clues for deciphering mechanisms of these contrasting facies architectures. 2.5.2.1 Sandstone Compositions Sandstones of the Duchesne River Formation are quartzose, sublithic, and lithic (Folk 1968 classification) (Fig. 2.5a), indicating feldspar is a very minor component of sandstones over the basin. In a QFL diagram with field dimensions (Dickinson et al. 1983), sandstones are mostly plotted in the petrofacies area of recycled orogenic (Fig. 2.5b). The breakdown of lithic grains from sublithic and lithic sandstones is shown in Figure 2.5c. Although the data in the plots are very scattered, carbonate grains are the most common and clastics are relatively minor. 49 Figure 2.4. Net-sandstone-to-gross-thickness ratio (NTG) map and schematic fluvial styles of Db. Points of control data for NTG map are highlighted by yellow circles (accompanied with red text numbers of NTG used for contouring). The western part of the basin (from Tabiona to Roosevelt) exhibits a high net-to-gross ratio (over 60%), whereas the eastern part of the basin (Roosevelt to the eastern margin of Db distribution) shows a gradual decrease in NTG (60 to 14%). 2.5.2.2 Regional Trends and Provenances In order to examine the geographical difference of sandstone compositions, a cross plot of longitude versus percent rock fragments of grains was created and juxtaposed along with a paleocurrent map (Fig. 2.6). Paleocurrents of the Duchesne River Formation indicate overall southerly sediment transport from the Uinta Mountains, as noticed by Andersen and Picard (1974). However, by closely looking at flow patterns of the basal member, the western part of the basin shows more eastward and southeastward flows. In contrast, the central-eastern and eastern parts of the basin have respectively south-southwestward flows and randomly directed flows. The plot of percent rock fragments indicates relative richness of quartz grains because feldspar is a minor component for all sandstones. Newly acquired data on this plot (Fig. 2.6) show the richness in rock fragments in the east (6-37%) relative to the west (1-7%). 50 Figure 2.5. Ternary QFL(R) plots showing sandstone compositions of the Duchesne River Formation (data from Andersen and Picard (1974) and this study). Plots a) Folk's (1968) classification and b) Dickinson et al. (1983) classification indicates that feldspar is a very minor component of sandstones over the basin. Plot c) shows the breakdown of lithic grains that indicates that carbonate grains are the most common and clastics are relatively minor. Thin section petrography, which provides visual information on sandstone textures and properties (i.e., porosity), also supports a distinct compositional difference between the eastern and western portions of the basin. The sandstones from the west (samples 1, 2, and 4) are richer in quartz (over 90% of total normalized grains), more porous (point count porosities ranging from 14.7% to 17.6%), and less matrix and/or cement materials (1.9% to 5.3% of total counts) than those from the east (samples 5, 6, 7, 8, and 9) (Fig. 2.7). It is likely that lithic-sublithic sandstones of the eastern part of the basin have lower porosity (6.1% to 12.3%) and a higher percentage of matrix and/or cement materials (10.4% to 25.0%), because some lithic grains were deformed or dissolved and migrated or precipitated into the original pore space during diagenesis. Compositionally mature quartz-rich sandstones in the west were favorable to keep the original pore space from destructions by cementations or grain deformations. 51 Figure 2.6. Paleocurrent data from Db (plot a) and longitude versus percent rock fragments of grains (plot b). a) Paleocurrents basically show overall southerly transports from the Uinta Mountains. Nevertheless, by closely looking at flow patterns, the western part of the basin shows more eastward and southeastward flows, whereas the eastern part of the basin has respectively south-southwestward or randomly directed flows. Correspondingly, the plot of percent r