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ii thz bto1o,BB .De:yaA1ttz:itt 1 lkt\Jiti11t,,y J 1dtr1:M$ofa1 1)u11itJt FLUVIAL EVOLUTION BETWEEN THE SALT WASH AND BRUSHY BASIN MEMBERS OF THE UPPER JURASSIC MORRISON FORMATION, SOUTH-CENTRAL UTAH
A THESIS ( SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY
Riyad Abdulrahim Ali-Adeeb
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
June 2007 Copyright© Riyad Abdulrahim Ali-Adeeb 2007 ABSTRACT
Abrupt changes in fluvial deposition between the Salt Wash.Member and Brushy Basin Member of the Upper Jurassic Morrison Formation, in western interior North America, have been documented across a seemingly basin-wide depositional unconformity that resulted from poorly understood base level changes. Detailed field observations, paleopedologic analysis, and petrographic analysis of proximal deposits in the Henry · Basin in south-central Utah, yield no depositional hiatus in the proximal basin during formation of the unconformity in the distal basin. Rather, the proximal stratal architecture suggest continuous deposition between the two members that likely resulted from a combination ofrelatively high basin subsidence rates and an increase in gravel fraction in the sediment source. Computed subsidence rates in the southern Henry Basin from recent radiometric dates document a time-averaged subsidence rate of 0.121 mm per year, while petrographic evidence suggests an up-section increase in chert-rich sediment source. Collectively these resulted in starving the distal basin of sediment and the formation of a distal depositional hiatus, while the proximal basin accumulated prograding sediments. A south-to-north transect near the western margin of the Morrison Basin documents a rapid pinch out of 150 meters of fluvial gravelly-sandstones of the Salt Wash Member between the southern Henry Basin to the south and the Emery High region 7 5 miles to the north. Mature paleosols at the base of the Morrison Formation in the north suggest that the lack of Salt Wash Member deposits were due to non-deposition rather than to post depositional erosion or incision. This suggests that syndepositional basin subsidence in the south channeled deposits there, while regions to the north underwent little to subsidence and deposition. The disparity in basin subsidence rates between closely spaced regions in proximal regions, combined with petrographic evidence of increased gravel-fraction in the sediment, suggests that tectonic thrusting to the west may have contributed an important role in producing continuous aggradation of sediment in the proximal basin, while contemporaneously starving the distal basin and forming a depositional unconformity. Incised valley-fill conglomerates at the contact between the two members also suggest that the change in sedimentation was influenced by a drop in base level that followed deposition of the Salt Wash Member.
1 ACKNOWLEDGMENTS
I dedicate this thesis to my wife, Jessica, who shares my passion for the Geological Sciences, who provided me with incredible support, and helped me begin, and complete this project.
I would like to thank my advisor, Dr. Timothy M. Demko, for sharing with me his passion for Mesozoic stratigraphy on the Colorado Plateau and the wonderful challenges in unraveling the mysteries of the dinosaur-rich Morrison Formation. I would also like to thank my thesis committee members, Dr. John Swenson for introducing me to the wonders of numerical basin modeling, Dr. Pat Farrell for her support in understanding the relevance of paleosols in the Morrison Formation, and Dr. Michael Jackson, for his insight and guidance of the paleomagnetic analyses in this project. I would also like to acknowledge the help and support of the faculty and staff of the Institute for Rock Magnetism at the campus of the University of Minnesota, Twin Cities.
I would also like to thank my colleagues Ryan Erickson, Nick Freiburger, and Joseph Beer for their insights, feedback and support for this project, and Eric Tharalson for his assistance in the field. In addition, many faculty members at the University of Minnesota Duluth provided much support and guidance throughout the duration of this project, including Dr. Richard W. Ojakangas for his help and feedback with petrographic analysis in this project.
Funding for this project was provided by the American Association of Petroleum Geologists, the Colorado Scientific Society, the University of Minnesota Graduate School, the College of Science and Engineering, and the Department of Geological Sciences at the University of Minnesota Duluth.
11 TABLE OF CONTENTS
ABSTRACT ...... i
ACKNOWLEDGMENTS ...... ii
TABLE OF CONTENTS ...... iii
LIST OF FIGURES ...... v
LIST OF TABLES ...... vii
1. INTRODUCTION ...... 1 1.1 Purpose ...... ·...... 1 1.2 Background ...... 1 1.3 Approach ...... 4
2. GEOLOGIC BACKGROUND ...... 5 2.1 Overview ...... ·...... , ...... 5 2.2 Tectonic Setting ...... 5 2.3 Paleogeography ...... 10 2.4 Paleoclimate ...... ; ...... 12 2.5 Stratigraphy of the Morrison Formation ...... : ...... 12 2.6 Age of the Morrison Formation ...... 17 2.7 Paleosols in the Morrison Formation ...... 18 2.8 Evolution of the Morrison Rivers ...... 21 2.9 Magnetostratigraphy of the Morrison Formation ...... 22 2.10 Base Level ...... 23
3. METHODS ...... 27 3.1 Measured Sections ...... 27 3.2 Paleocurrents ...... : ...... 29
111 3.3 Petrography ...... 30 3.4 Paleomagnetism ...... 30
4. RESULTS ...... 37 4.1 Lithology and Stratigraphy ...... 37 4.2 Facies Architecture ...... 65 4.3 Paleocurrents ...... 70 4.4 Petrography ...... 73 4.5 Paleomagnetism ...... 78
5. INTERPRETATIONS ...... 84 5.1 Depositional History Reconstruction ...... 84 5.2 Basin Subsidence ...... 90 5.3 Unconformity Paleosols ...... 91 5.4 Provenance ...... 93 5.5 Paleorn:agnetic Interpretation ...... 96
6. DISCUSSION ...... 101
7. CONCLUSION ...... : ...... 115 APPENDICES ... : ...... 116 I. Measured Sections and Paleocurrents ...... 116 II. Petrographic data ...... 143 III. Magnetostratigraphic data ...... 147
REFERENCES ...... 150
IV LIST OF FIGURES
Figure 1.1 Field study area and distribution of Morrison Formation outcrops ...... 2 Figure 2.1 Tectonic setting of western North America during the Late Jurassic Period ... 6 Figure 2.2 Subsidence in the Henry Basin ...... 9 Figure 2.3 Paleogeography of the Colorado Plateau during the Jurassic Period ...... 11 Figure 2.4 Stratigraphic nomenclatures of Upper Jurassic Strata on Colorado Plateau .. 14 Figure 2.5 Age of the Morrison Formation ...... 19 Figure 2.6 Magnetostratigraphic framework of the Morrison Formation ...... 24 Figure 2. 7 Basics of flu vial base level ...... 26 Figure 3 .1 Methods of stratigraphic measurements in the field ...... 28 Figure 3.2 Methods ofpaleomagnetic core sampling in the field...... 32 Figure 3.3 Demagnetization methods of core samples ...... 34 Figure 3.4 Natural Remnant Magnetization analyses methods ...... 36 Figure 4.1 Transect lines for stratigraphic cross-sections ...... 38 Figure 4.2 Fence diagram of the Morrison Formation in south-central Utah ...... 39 Figure 4.3 Stratigraphic architecture of the base of the Morrison Formation ...... 41 Figure 4.4 Types of cross-bedding in channel deposits of the Salt Wash Member ...... 43 Figilre 4.5 The Salt Wash Member in the southern Henry Basin ...... ' ...... 43 Figure 4.6 Paleosols in the Salt Wash Member ...... , ...... 45 I Figure 4.7 Stratal configuration of Morrison Formation in southern Henry Basin ...... 46 Figure 4.8 Stratal configuration of the Morrison Formation at Last Chance Wash ...... 48 Figure 4.9 Stratal configuration of the Morrison Formation near Moore, Utah ...... 50 Figure Stratal co_nfiguration of the Morrison Formation in Cathedral Valley ...... 52 Figure 4.11 Paleosols in the Tidwell Member in Cathedral Valley ...... 53 Figure 4.12 The Mid-Morrison conglomerate in Cathedral Valley ...... 55 Figure 4.13 Stratal configuration of Morrison Formation near Green River, Utah ...... 56 Figure 4.14 Stratal configuration of Morrison Formation in southern Henry Basin ...... 58 Figure 4.15 The Mid-Morrison conglomerate in south-central Utah ...... 61 Figure 4.16 Lateral accretion in the Brushy Basin Member ...... 62 Figure 4.17 Cross-sections of the Morrison Formation in south-central Utah ...... , . 67
v Figure 4.18 Stratal configuration of Morrison Formation in southern Henry Basin ...... 69 Figure 4.19 Paleocurrent directions in Morrison Formation in south-central Utah ...... 71 Figure 4.20 Petrographic ternary diagrams from the Morrison Formation deposits ...... 74 Figure 4.21 Photomicrographs of chert from the mid-Morrison conglomerate ...... 77 Figure 4.22 Photomicrograph of altered chert in the mid-Morrison conglomerate ...... 79 Figure 4.23 Natural Remnant Magnetization analysis of the Salt Wash Member ...... 81 Figure 4.24 Magnetic vector analyses of the Salt Wash Member ...... 83 Figure 5.1 Depositional history of the Morrison Formation in the Henry Basin ...... 85 Figure 5.2 Middle Morrison Formation Paleosols in the southern Henry Basin ...... 88 Figure 5.3 Petrographic ternary diagrams and provenance interpretations ...... 89 Figure 5.4 Virtual geomagnetic paleopole of the Salt Wash Member ...... 97 Figure 5.5 Iron oxide concretions as evidence of post depositional fluid flow ...... 98 Figure 5.6 Proximity of sampling sites to possible sources of fluid infiltration ...... 100 Figure 6.1 Numeric foreland basin models ...... 105 Figure 6.2 Foreland basin model ...... · ...... 106 Figure 6.3 Foreland basin model with a back-bulge basin ...... '...... 108 Figure 6.4 Mid-Morrison conglomerate in the lower Brushy Basin Member ...... 114
VI LIST OFTABLES
Table 4.1 Raw parameters used in petrographic analysis of thin sections ...... 75 Table 4.2 Calculated parameters used in petrographic analysis of thin sections ...... 75 Table 4.3 Percent mineral composition in the Morrison Formation ...... 75
vu 1. INTRODUCTION
1.1 Purpose
This study investigates the proximal deposits of the Upper Jurassic Morrison Basin in south-central Utah (Figure 1.1 ), with the aim of establishing evidence for a depositional hiatus in the proximal reaches of the basin. This is in light of ample work that documents a substantial depositional unconformity in the distal reaches of the Morrison Basin that has been attributed to a drop in base level and associated with a change in fluvial deposition style between the Salt Wash and Brushy Basin Members of the Upper Jurassic Morrison Formation. Furthermore, this study seeks to find a correlation between this change in fluvial deposition, a drop in base level, and the deposition of an incised valley- fill fluvial conglomeratic unit in between the Salt Wash and Brushy Basin Members. The study explores the relationship between the conglomerate and the regionally documented depositional hiatus separating the two members of the Morrison Formation, and attempts to place the deposit within the overall stratigraphic framework of the Morrison Formation. The aim is to investigate the significant factors that may have influenced this evolution of flu vial deposition within the tectonic and climatic setting of the Late Jurassic Period.
1.2 Background
The relationship between deposition of the Upper Jurassic Morrison Formation and regional tectonic events has long been disputed. Many workers have debated about the factors responsible for fluvial sedimentation in the late Jurassic and argued for deposition either in response to tectonic thrusting, subduction-related dynamic subsidence, or sedimentation during tectonically quiescent conditions. Much work has focused on the timing and initiation of deposition of the Morrison Formation and overlying Cretaceous Cedar Mountain Formation, however, little attention has been paid to the sedimentologic evolution within the Morrison Formation and its implications regarding the tectonic setting of the Late Jurassic Period.
1 B ·. UTAI-I +- •) '° " ....
'I \U L.\l\.E C: l I y • . - ., . ...,,,,, ______-. NOR.TH ·,,
so +miles NORTH Stud Area Inset B'/· .' h I - i s • 25 milesi . I .: - ,_( • •••
Hite
Figure 1.1. (A) Generaliied map of the approximate outline of the Morrison Basin and preserved outcrops of the Morrison Formation in western interior United States and in Utah. (B) Inset map of the study area in south-central Utah, showing exposed outcrops of the Morrison Formation in red, and locations of measured sections in black dots.
2 The stratigraphic transition from the Salt Wash Member to the Brushy Basin Member of the Morrison Formation has long puzzled workers, though it has rarely been studied in detail. This transition documents a marked shift in sedimentation style from amalgamated and braided sandy rivers of the Salt Wash Member to muddier meandering and anastomosing rivers of the Brushy Basin Member (Tyler and Ethridge, 1983; Peterson, 1984; Robinson and McCabe, 1998; Turner and Peterson 2004). The cause of this change has received little attention, but may be an allogenic response to known regional climatic fluctuations, or a response to increased mud influx from the source area associated with high volcanic ash air fall sedimentation (Owen et al., 1989). Although volcanic ash likely played a major role in determining the bulk volume of muddy lithofacies of the Brushy Basin Member, there is little evidence to suggest a sudden and unremitting volcanism that began in the hinterlands and would have abruptly altered fluvial sedimentation. Similarly, changing climatic patterns also may have played a role in the evolution of the Morrison Rivers; however, evidence suggests only a gradual climate change occurring predominantly during deposition of the Brushy Basin Member and not prior to it (Demko and Parrish, 1998), so climatic shifts alone cannot explain the abrupt change in depositional regime. Therefore, these fluvial changes were either in response to major and abrupt changes whose origins are yet unknown, or to more gradual changes whose record has been wiped out with landscape degradation following deposition of the Salt Wash Member, thus placing the two distinct fluvial systems seemingly conformable atop one another.
Paleomagnetic (Steiner et al., 1994), paleopedologic (Demko et al., 2004), and trace fossil evidence (Hasiotis, 2004) suggest that a regional depositional hiatus followed the deposition of the Salt Wash Member and might be related to a basin-wide base level change that could have drastically altered fluvial sedimentation within the Morrison Basin. A distinct conglomeratic deposit straddles the Salt Wash/Brushy Basin horizon that is deposited into paleovalleys incised into the upper Salt Wash Member. These small valleys represent a distinct phase of landscape degradation following the predominantly aggradational deposition recorded in the Salt Wash Member deposits. The unique fluvial style of this conglomerate and its distinct stratigraphic horizon may offer clues to the
3 evolution of fluvial sedimentation in the Morrison Formation and its relationship to regional or local tectonics.
1.3 Approach a_nd Study Area
This study employs sedimentologic, petrographic, paleopedologic, and magnetostratigraphic analyses within a stratigraphic :framework to reconstruct the depositional history of the middle part of the Morrison Formation. The study has two main objectives: I) to investigate the presence of a significant depositional hiatus between deposition of the Salt Wash and Brushy Basin Members by correlating key stratigraphic horizons from the center of the western depositional basin (southern Henry Basin) to fringing areas that received no Salt Wash Member deposits (Emery High region), and; 2) to investigate a discrete conglomeratic deposit incised into top Salt Wash deposits, and to resolve its relationship to the depositional hiatus. The study will reconstruct the stratigraphic architecture of the mid-Morrison conglomerate and related strata and investigate possible controls on sedimentation of the middle Morrison Formation.
The study examined outcrops along the western margins of the Morrison Basin in south- central Utah that represent some of the most proximal deposits of the Morrison Formation. Stratigraphic analysis was carried out from southern Henry Basin where the Salt Wash Member is thickest, northward to fringing areas in the Emery High where there are no Salt Wash deposits. A stratigraphic cross-section was constructed along this transect and perpendicular to paleo-flow direction in order to correlate key horizons from the center of the basin to its margin where deposits there record extended periods of sediment starvation and pedogenesis prior to deposition of the Brushy Basin Member. This allowed a comparison of strata across the Henry Basin and to determine whether fluvial evolution was isochronous throughout the basin.
4 2. BACKGROUND
2.1 Overview
The Morrison Formation was deposited during a time of rapid evolution of western North America as it was undergoing major tectonic, geographic, and climatic reorganization as it migrated from the tropics to the mid latitudes (Blakey, 1983, 1989; Hintze, 1988; Parrish and Peterson, 1988; Turner and Peterson, 2004). Exotic terranes were colliding into the western margin of the continent along a subduction margin (Figure 2.1) that drew the Farallon Plate beneath North America's western margin, and resulted in the formation of Cordilleran America (Blakey, 1983, 1989; Hintze, 1988). Several orogenic and crustal shortening thrust belts formed which may have played a key role in providing both the source and sink for deposition of the Morrison Formation, and include the Early to Middle Jurassic Nevadan and Elko Orogens, and the Late Jurassic(?) to Late Cretaceous Sevier Orogeny.
2.2 Tectonic Setting
The onset of deposition of the Morrison Formation has been the subject of debate and its relationship to fold and thrust orogenic events remains to be debated. Several models have been proposed to explain the tectonic setting of the Morrison Formation and include: 1) deposition occurred in a back-bulge basin in response to Sevier Orogenic tectonic thrusting (Currie, 1994, 1998); 2) deposition pre-dated thrusting and resulted from subduction-related hinterland doming and basin subsidence (Gurnis, 1992; Lawton, 1994); 3) a distal foreland setting associated with middle Jurassic thrusting and overfilling of the resulting foreland basin (DeCelles and Burden, 1992); and, 4) deposition from isostatic rebound of the middle Jurassic thrust belt under tectonically . quiescent conditions (Heller, 1986; Heller and Paola, 1989; Yingling and Heller, 1992).
Timing of thrusting and crustal shortening in Cordilleran North America is critical in order to understand the conditions responsible for deposition of the Morrison Formation
5 Plate Morrison TM Basin
Mogollan Highlands
Figure 2.1. Tectonic setting of western North America during the late Jurassic Period. Subduction of the Farallon Plate along the western margin of the continent likely played a role on the deposition of the Morrison Formation, however, the exact effects are not well understood. The main sources of sediment include the Nevadan/Sevier Hinterlands to the west, and the Mogollan Highlands to the south. From Blakey, 2003.
6 and its relationship to tectonic events, if any. Heller et al. (1986), Heller and Paola (1989), Yingling and Heller (1992); Lawton (1994) argue for a post-Jurassic timing of the Sevier fold-and-thrust belt, so that crustal thrusting began in the late-early Cretaceous Period based on stratigraphic thickening patterns within the depositional basin east of the Nevadan and Sevier hinterlands. Other workers have argued for an initiation of thrusting during the Late Jurassic beginning with the onset of deposition of continental units of the Morrison Formation (Armstrong, 1968; DeCelles and Burden 1992, DeCelles and Currie, 1996; Currie, 1998), and based on metamorphism of rocks in the hinterland (Allmendinger and Jordan, 1980, 984). Others still, have pushed back the timing of crustal shortening to the Middle Jurassic Period based upon structural and metamorphic 4eformation and granitic emplacements in the hinterland (1987; Hudec, 1992; Wells 1992), and on stratigraphic reconstructions of Middle Jurassic strata in the Utah-Idaho trough (Bjerrum and Dorsey, 1995). Isotope geochemistry of crustal emplacements and metamorphic core complexes place such compressional deformation between 160 and 150 Ma (Kistler, 1983; Miller et al.) and suggests that fold and thrust events in Nevada during the Late Jurassic was more suited to the Nevadan orogeny. This was later corroborated using structural work by Little (1987) and Schweichert et al. (1984) whom dated these structural features at 155 Ma. It is perhaps best to attribute thrust and fold tectonics in the Middle Jurassic to the Nevadan Orogeny, since there are no major coarse- grained alluvial deposits in Utah of Middle Jurassic age that may be attributed directly to thrust events.
Petrographic analyses document an increase in detrital chert grains with deposition of the Morrison Formation (Armstrong and Cressman, 1963), which Currie (1998) related this to thrusting of two terranes prior to the Paris-Willard thrust event that marks the onset of Sevier Orogenic fold and thrust belt (DeCelles and Burden 1992; Camilleri et al., 1997). These thrust sheets also contained Mesozoic and Paleozoic rocks and account for much of the quartz and chert in the Morrison Formation (Peterson 1987; Yingling 1987; DeCelles and Burden 1992).
7 Peterson (1983, 1984) attributed sedimentologic changes within the Salt Wash Member, including thickness variations, facies distribution, cross-bedding parameters and bedding ratios, to crustal "quivering" caused by activation of folds within the Morrison depositional basin. However, a lack of angular unconformities near positive structures such as in the Emery High suggested that subsidence of the basin was the main tectonic movement as opposed to uplift of the basin margins, although Peterson (1984) did not infer or suggest a mechanism for the subsidence. It is certainly possible that such fluctuations in cratonic crust may be attributed to pre-Sevier thrusting and subduction- related effects resulting from the subduction of the Farallon Plate beneath the western margin of North America at that time (Mitrovica et al., 1989; Gumis, 1992; Lawton, 1994, Burgess et al., 1997).
Robinson and McCabe (1998) called on syndepositional, differential subsidence (Figure 2.2) combined with a rise in base level from downstream expansion of lake systems as the mechanisms responsible for the strata! architecture and evolution of the Salt Wash Member. These mechanisms led to the change from low sinuosity, sandy river systems with thin but laterally extensive and amalgamated, sheeted sandy deposits, to more entrenched and meandering rivers with thicker multistory channel sandstones and thicker interfluve mudstones and paleosols.
It is evident that some tectonic movement played a role in the spatial and vertical distribution of sediments and on the strata! architecture of Morrison Formation deposits regardless of whether it was Sevier-related crustal thrusting, isostatically or thermally rebounding hinterlands, or subduction-related dynamic subsidence. It is therefore important to iriterpret how tectonic influences played a role in the evolution of the Morrison deposits and particularly how, if any effect tectonics played into the evolution between the low-sinuosity, sandy rivers of the Salt Wash Member and the higher sinuosity, muddy rivers of the Brushy Basin Member.
8 10km Vertical Exag. = 133x Differential Subsidence
Figure 2.2. Differential subsidence model for the Henry Basin during deposition of the Salt Wash Member, as proposed by Robinson and McCabe, 1998.
9 2.3 Paleogeography
The Morrison depositional basin was located within interior western North America at latitudes of 30-35° North during the Late Jurassic Period (Parrish and Peterson, 1988; Van Fossen and Kent, 1992; Peterson, 1994; Demko and Parrish, 1998). Following the break up of Pangaea, North America rapidly migrated out of the subtropics to the mid- latitudes that resulted in a significant shift in climatic patterns in the region. During the Middle Jurassic Period, an epicontinental seaway (Figure 2.3 A) transgressed into the interior west basins from the north through a narrow thoroughfare and reached as far south as southern Utah (Brenner, 1983; Kocurek and Dott, 1983; Hintze, 1988; Blakey, 1989, Peterson, This sea deposited thick successions of shallow marine deposits including mudstones, limestones, and sandstones that contained abundant shallow marine fauna. Fringing areas accumulated supratidal sabkha mudstones, evaporites, sandstones, and aeolian sandstones that collectively comprise the San Rafael Group (Hintze, 1988; Peterson, 1994; Anderson and Lucas, 1992).
By the Late Jurassic, and with the onset of Morrison Formation (Figures 2.3 A and B) deposition, the sea retreated rapidly northward (Turner and Peterson, 2004) despiti:: rising eustatic sea levels (Vail et al., 1977) and did not return back until the Cretaceous Period . . (Blakey, 1983; Hintze, 1988). The Nevadan/Sevier hinterlands to the west provideµ much of the sediment for the Morrison Formation, although to the south the Mogollan Highlands provided the source for the Recapture Member, another alluvial plain complex that is time-equivalent with the Salt Wash Member deposited further to the south and . southeast of the study area (Peterson, 1994). The Ancestral Rockies were by now low- lying hills that probably yielded little source or impediment to eastward traveling rivers (Dunagan and Turner, 2004). During the Latest Jurassic Period, during deposition of the Brushy Basin Member, lacustrine/palustrine complexes deposited thick accumulations of limestones, mudstone and paleosols in the Four Comers region that likely resulted from ground water influx as opposed to fluvial discharges (Turner and Fishman, 1991; Demko et al., 2004).
10 ------....1 .-.&a• ----r-----
NORTH
Figure 2.3. Paleogeographic reconstructions of the western interior United States during the Jurassic Period. The Jurassic Interior Seaway (A), also known as the Sundance-Stump Sea, deposited the San Rafael Group deposits that ended with deposition of the Summerville Formation in south- central Utah. This seaway retreated during the late Jurassic Period (B) and left behind an alluvial plain into which the Morrison Formation began deposition with the Tidwell Member into a subsiding basin, that was shortly followed by deposition or the Salt Wash Member . by predominantly low-sinuosity rivers and streams. These rivers abruptly changed character and deposition of the Brushy Basin Member (C) by anastomosing and meandering streams ensued during the latest Jurassic Period. Reconstructions from Blakey 2003.
11 2.4 Paleoclimate
The climate during the Middle Jurassic Period was arid-hyper arid and resulted in the deposition of playa and supratidal sabkha mudstones, aeolian sandstones and shallow marine deposits (Hintze, 1988; Demko et al., 2004). The Late Middle Jurassic saw the deposition of distinctive red to chocolate brown mudstones, siltstones and sandstones abundant in gypsum that characterize the Summerville Formation (Blakey, 1983). By the early Late Jurassic, the climate shifted to seasonal semi-arid conditions with the onset of deposition of the Morrison Formation (Hallam, 1982; Demko and Parrish, 2004).
Alternating bands of clay skins/clay-rich horizons and calcium carbonate horizons in paleosols of the Morrison Formation, suggest a semi-arid climate with seasonal wet-dry cycles (Demko et al., 2004). During relatively humid conditions, illuviation of clay-rich and iron oxide-rich minerals formed Bt and Bs horizons respectively, whereas during dry cycles, calcium carbonate accumulated as nodules or horizons in corresponding Bk or K horizons. Furthermore, vertic features such as slickensides indicate repeated shrinking and swelling of these units from cyclic wet-dry patterns. Thin section analysis of growth banding on freshwater unionid bivalve shells from the Morrison Formation suggests a change from a steady climate optimum for bivalve growth, to more cyclic annual precipitation during deposition of the Salt Wash Member, and more complex annual and pseudo annual patterns during deposition of the Brushy Basin Member (Good, 2004).
2.5 Stratigraphy of the Morrison Formation
The Upper Jurassic Morrison Formation was deposited in the interior west of North America in a region encompassed by the modem Colorado Plateau, Rocky Mountains, and western plains of the United States and Canada (Blakey, 1983; Hintze, 1988; Turner and Peterson, 2004). The formation is composed of heterolithic units of sandstone, siltstone, mudstone, and limestone with thin interfingering volcanic ash beds, deposited under dominantly terrestrial environments. Rivers systematically drained the Nevadan/Sevier Hinterlands to the west and southwest, and the Mogollan Highlands to
12 the south and southwest, and eventually reached the Jurassic interior Seaway to the north and northeast (Walker, 1974; Blakey, 1983; Tyler and Ethridge, 1983). Sedimentary packages record deposition by fluvial, shallow lacustrine, palustrine, small deltaic systems, and in the extreme north and the lowermost deposits, show marine influences including shallow marine and marginal marine deposits (Craig et al., 1955; Mullens and Freeman, 1957; Brenner, 1983; Tyler and Ethridge 1983; Poulton 1984; Peterson 1984, 1986; Robinson and McCabe, 1997, 1998; Anderson and Lucas 1995). Although the Morrison Formation is dominated by fluvial deposits and related strata, in some areas,' especially in the Four Comers region, it contains thick deposits of wetland/lacustrine and small deltaic systems that prograded into those lake systems with fringing aeolian sandstones (Turner and Fishman, 2004).
The Morrison Formation is stratigraphically bounded below by the J-5 unconformity and above by the K-1 unconformity of Pipiringos and O'Sullivan (1978) (Figure 2.4). Dating and correlating strata within the Morrison depositional basin has been challenging because of the complex interfingering between alluvial lenses of deposits within the basin that lack palpable, distinct, and correlatable horizons (Turner and Peterson, 2004). In addition, at no one locality within the basin is the formation "complete" and includes all of the named stratigraphic horizons, thus making basin-wide correlations particularly difficult.
Within the study area, the Morrison Formation is divided into three formal stratigraphic . units based on lithologic and facies differences. Starting at the base they are the Tidwell, Salt Wash and Brushy Basin Members (Peterson, 1984; Hintze, 1988). Although several other member designations throughout the Morrison Basin have been recognized and formally designated (Turner and Peterson, 2004), only their time-equivalent facies exist in this area. The base of the Morrison is well established with the onset of deposition of the Tidwell Member (Peterson, 1984; Turner and Peterson, 1984; Robinson and McCabe; 1997. 1998; Currie, 1998; Bernier, 2003), and the top is defined either by a well- developed paleosol that marks the contact between the top of the Morrison Formation and the base of the Cedar Mountain Formation, or by truncation of the top Morrison deposits
13 Southeast NW New Mexico Westem Central Northeaslem San Rafael Soulhwest Area: Utah SW San Juan Wyoming Wyoming Utah Swell, Utah Utah (Four Comers) Basin
Ovorlyingstnit.1 L011¥er Crtrtaceous Lowor Cretaceous Lower Cretaceous Lower Cretaceous Upper(?) Cre
Rodwolcr Redwater Member Oxlordian Aodwoter a Member Membef Sur.dance Fonnation u I 111 l I J ... Summerville Fm a. 1111111 111111 l 0. I I 11111 11 I 1111111 I 1'! E COOis Pino Butl:o Ss "' a ! Curtisfotmatlon Cl.lrtisf0f111atlon ..., Callovian IAon>bcr Wan.>l
Figure 2.4. Regional correlation of Jurassic Stratigraphy and nomenclature. Adopted from Hasiotis (2004).
14 by the Buckhorn Conglomerate of the Cedar Mountain Formation (Currie, 1998; Demko et al., 2004; Skipp, 1997).
Some workers have argued against a formal Tidwell Member designation (Anderson and Lucas 1992, 1994, 1995) and proposed to lump its deposits with the underlying Middle Jurassic Summerville Formation, indicating that the Morrison Formation begins at the base of the fluvial strata of the Salt Wash Member and not at the base of the Tidwell Member. This challenging view was based primarily on perceived lithological similarities between the Tidwell Member and underlying Summerville Formation units, and ignores any chronostratigraphic implications or consideration of the presence of a major unconformity that separates the two units within much of the basin (Pipiringos · and O'Sullivan, 1978; Peterson, 1994; Turner and Peterson, 2004). This reorganization was based on field observations within distal deposits of the Morrison Formation in strata that appeared to these workers as lithologically similar and conformable.
Despite this apparent conformity of the units in the distal parts of the basin, the unconformity separating the Tidwell Member from the Summerville Formation in the proximal basin demonstrates that the two units are indeed separated by a major unconformity that represents a vacuity between the two units and thus the distal conformity is a correlative conformity to the proximal unconformity, thereby giving the appearance of continuous sedimentation in the distal basin (Blakey, 1996).
Although in some localities, the Summerville Formation and the Tidwell Member of the Morrison Formation are superficially similar, the base of the Tidwell Member represents a distinct basin-ward shift in facies and its base is marked by the deposition of a coarse- to fine-grained fluvial sandstone that Turner and Peterson (2004) were able to correlate . across the basin from near Denver, Colorado, to the western Colorado .Plateau in southern Utah. In addition, within the Colorado Plateau and in the southern Henry Basin area, angular relationships between the Summerville Formation and the Tidwell Member of the Morrison Formation show evidence of post Summerville erosion and truncation by the Tidwell Member deposits, suggesting a significant period of time elapsed in between
15 deposition of the two units and the formation of the J-5 unconformity. The Tidwell Member designation is therefore, justified and used in this study.
The Tidwell Member was deposited along a flat-lying alluvial and coastal plain complex, although marine influences on Tidwell deposits were short-lived and only recorded north of the study area in the vicinity of Dinosaur National Monument (Currie, 1998; Turner and Peterson, 2004). The unit consists of interbedded sandstone and mudstone, and locally the basal sandstone bed (correlative to "bed A" of O'Sullivan, (1980a)) can be several meters thick. Sandstones are characterized by trough and ripple cross-beds, as well as symmetrical ripples suggesting both fluvial as well as wave influences (Currie, 1998). Locally green-colored mudstones and limestones record the presence of small lakes in the area, and gravelly matrix-supported deposits suggest debris flows at the Fremont River east of Capitol Reef National Park, Utah.
The Salt Wash Member consists of dominantly fluvial sandstones interbedded with correlative flood plain mudstones and poor to well-developed paleosols. Salt Wash sandstones consist of sandy to pebbly channel fills deposited by migrating subaqueous dunes and two-dimensional bars in low- to moderate-sinuosity rivers and preserved as trough cross-beds and planar-tabular cross-beds in fine to coarse- grained sandstones (Peterson, 1984; Robinson and McCabe, 1998). Channel deposits include migrating point bars, longitudinal bars and thalweg conglomerates. Overall, the Salt Wash channel sandstOnes coarsen up-section suggesting that the unit prograded.
Robinson and McCabe (1997) carried out a detailed Jacies description of the Salt Wash Member and divided the lithology into eight main facies: 1) trough cross-bedded sandstone, 2) planar-tabular cross-bedded sandstone, 3) horizontally laminated sandstone, 4) massive sandstone, 5) granule-pebble sandstone, 6) mudstone intraclast conglomerate, 7) ripple cross-laminated sandstone, and 8) siltstone and mudstone. These facies are adopted for describing Salt Wash channel deposits, but are modified to include the various representative paleosols that are abundant within the member and particularly near the top of the Salt Wash Member. In that regards, paleosol descriptions and degrees
16 of development in this study have been adopted from the methods of Retallack (1988) and Mack (1993).
Within the study area, the Brushy Basin Member was deposited by fluvial, lacustrine, and palustrine environments under slightly more humid conditions than the Salt Wash Member as suggested by plant taphofacies (Parrish et al., 2004; Demko et al., 2004). The Brushy Basin Member is informally separated into two parts. A thin lower half is dominated by non-swelling illitic clays and brown to white sandstone beds (Owen et al., 1989) deposited by fluvial and lacustrine systems on an alluvial plain (Turner and Fishman, 1991). The thicker upper part of the member was also deposited on an alluvial plain but is dominated by mudstones and characteristic swelling smectitic clays (Owen et al., 1989), that resulted from the alteration of ash beds deposited from a volcanic arc to the west and southwest (Turner and Fishman, 1991). Lacustrine green and greenish-grey mudstones are scattered throughout the member, and sandstones and conglomerates are found higher up in the member, although most of the upper member is dominated by mudstone deposits.
2.6 Age of the Morrison Formation
The Morrison Formation was deposited in a little over 6.7 million years during the Late Jurassic Period, within the Kimmeridgian and Tithonian stages between 154.8 Ma to 148.1 Ma (Kowallis et al., 1998) (Figure 2.5).The age of the formation continues to be fine-tuned by several workers U$ing radiogenic dating of ash beds, and fission. tracks of detrital zircons (Dawson, 1971; Bilbey, 1993; Kowallis et al., 1987, 1998; 2004), palynological evidence (Litwin et al., 1998), and biostratigraphy of charophytes and ostracodes (Schudack et al., 2004).
Laser fusion of plagioclase crystals from ash beds within the Morrison Formation yield 40Ar;39Ar ages near the top and base of the Brushy Basin Member of 148.1 Ma and 150.3 Ma respectively, and 40 Ar/3 9Ar dates of ash beds from the near the base of the Tidwell Member give ages of 154.8 Ma (Kowallis et al., 1998). Dawson (1971) reported an
17 40Ar!3 9Ar age of 153.0 Ma from a horizon just above a green glauconitic shale near the base of the Morrison Formation in the Dinosaur National Monument area in northeastern Utah. The green glauconitic nature of the shale is likely of marine origin, which the Tidwell Member is known to contain near its base (Peterson, 1994; Currie, 1998). Therefore, the age reported must be either from near the top of the Tidwell Member, or near the base of the Salt Wash Member. These dates peg the duration of deposition of the three members to the following: Tidwell Member at 1.8 million years; Salt Wash Member at 2. 7 million years, and; Brushy Basin Member at 2.2 million years. Palynological ages, as well as charophyte and ostracode ages, place the Morrison Formation within the Kimmeridgian and Tithonian stages of the Upper Jurassic (Schudack et al., 1998), in agreement with the radioisotopic dates.
2. 7 Paleosols
Paleopedological and ichnological analyses within the distal Morrison Basin suggests that landscape degradation and sediment starvation followed deposition of the Salt Wash Member and resulted in well-developed paleosols in these parts of the Morrison depositional basin (Demko at al., 2004; Hasiotis, 2004). At specific localities within the study area, there are especially well-developed paleosols at horizons between the members of the Morrison Formation, that suggest periods of little to no sedimentation in between deposition of the units. These paleosols and the stratal architectures of related strata are central to the depositional interpretation of the Morrison Formation.
Paleosols are abundant within the Morrison Formation, with evidence of various degrees of maturity and development. Evidence of the paleosols includes stacked horizons, root structures (including halos, casts, and rhizocretions), a multitude of terrestrial arthropod burrows, nests and dwelling structures, dinosaur trampling, sub-social mammalian burrows (Demko et al., 2004; Hasiotis, 2004), clay and calcium accumulation horizons, brecciation, and pedogenic structures including slickensides, and prismatic peds (Demko et al., 2004). Paleosols in the Morrison Formation very commonly show evidence of clay and/or calcite illuviation that resulted in clay covered grains, and clasts, or calcium
18 Figure 2.5. Chronology of the Morrison Formation in south-central Utah. The entire Morrison Formation was deposited over a period under 7 million years that spanned the Kirnrneridgian and Tithonian stages of the late Jurassic Period. Radiometric ages from Dawson, 1971 and Kowallis et al., 1998.
19 carbonate accumulation as concretions, distinct layers, or accumulations around biogenic structures such as around roots as rhizocretions (Demko et al., 2004).
Paleosols are often developed in fine-grained floodplain deposits and particularly in mudstones, however, some paleosols are developed in channel sandstones and siltstones as a result of channel abandonment and exposure to pedogenesis. In some locations where pedogenesis in these sandstones was extensive, ganister (a silicified sandy soil) formed (sensu Retallack, 1988). Some of the most mature paleosols in the Morrison Formation are of this type and occur at major depositional boundaries which may represent very long extended periods of exposure and pedogenesis (Demko et al., 2004).
Although paleosol identification and classification can be quite complex, only a few of the major types of paleosols are recognized in the study area and so analyses are relegated to these types. Retallack (1988) developed a system to identify paleosol horizons similar to the U.S. Department of Agriculture (Soil Survey Staff 1951, 1962, 1975) classification of modem soils with some modifications and additions that are unique to·paleosols including commonly used soil horizons such as 0, A, E, B, C, and R. The presence of all horizons at any one locality is rare and more typically one or two are present at any one location or region. Additionally a K horizon designation has been added to paleosols that is not used by the USDA classification, and designates thick, continuous layers of calcite. Only the B and K horizons are identified within the study area. Retallack (1988) offers a detailed description of paleosol horizons, and the criteria .for their designation.
Clay accumulating layers are referred to as Bt horizons, calcium carbonate accumulating layers are Bk, gypsum-rich layers are By, and layers with structured forms such as well- developed peds are referred to as Bw (Retallack, 1988). K horizons are similar to the . calcium carbonate accumulating Bk horizons but ones where accumulation levels are so concentrated that it forms massive calcrete layers, also known as calcrete hardpans, indicative of later stages of paleosol development, corresponding to stage III and higher of the six stages of carbonate accumulation in paleosols (Machette (1985).
20 Unconformity paleosols are especially well-developed, mature paleosols that denote extensive pedogenesis, extended periods of low to no and subaerial exposure, and water table-related interactions and processes (Joeckel, 1991; Blodgett, 1998). Within and related to deposition of the Momson Formation, three unconformity paleosols have been identified at key horizons and are designated the basal, middle, and upper Morrison paleosols (Demko et al., 2004). The basal Morrison paleosol is developed in upper Summerville Formation that records an extended hiatus and possible erosion prior to deposition of the Morrison Formation. The mid-Morrison paleosol separates the Salt Wash Member from the Brushy Basin Member, and the upper Morrison paleosol that is developed in the upper Brushy Basin deposits, denotes an extended period of non- deposition and exposure prior to deposition of the overlying Cretaceous Cedar Mountain Formation. If the mid-Morrison paleosol can be traced into the southern Henry Basin it would suggested that the proximal Morrison Basin underwent a depositional hiatus similar to that recorded in the distal parts of the basin and that a mid-Morrison unconformity might be basin-wide.
2.8 Evolution of the Morrison Rivers
Although the Morrison Formation was deposited under a diverse range of environments, fluvial deposits dominate the study area (Craig et al., 1955; Peterson, 1994; Robinson and McCabe, 1998). The upper two members of the formation, however, represent two drastically different fluvial systems (Robinson, and McCabe, 1998; Turner and Peterson, 2004). The lower Salt Wash Member was deposited by small, amalgamated, low sinuosity sandy to gravelly rivers, under a flashy discharge regime (Mullens and Freeman, 1957; Peterson 1984; Robinson and McCabe1997, 1998). In contrast, the Brushy Basin Member was deposited by moderate to high sinuosity, muddy rivers that developed stronger channel margins and levees and that caused the rivers to meander rather than avulse as the major method of channel migration with lower rates of avulsion (Turner and Fishman, 1991 ), and in tum allowed floodplains to develop more mature paleosols.
21 Robinson and McCabe ( 1998) suggested that relatively high coarse sediment ratios in the Salt Wash Member resulted in the deposition of unstable channel margins and levees. This resulted in frequent breaching and avulsion which was the main mode of channel migration. The end result is amalgamated channel-fill deposits with numerous pebble- lags at the base that fine upwards into dominantly trough cross-beds, and some planar- tabular cross-beds. Poorly to moderately developed paleosols in floodplain deposits of the Salt Wash Member also document rapid channel migration and sedimentation rates that did not expose floodplain deposits for long periods of time that resulted in poorly to moderately developed paleosols (Robinson and McCabe, 1998; Demko et al., 2004).
Furthermore, Salt Wash Member deposits record an evolution of river systems from almost strictly sandy, braided nature that deposited sheeted sand bodies, to ones that exhibited more stable banks with some ribbon sand body deposits and thicker channel storeys with lower width to thickness ratios (Peterson, 1984; Robinson and McCabe, 1997, 1998). These deposits also document a wider range of fluvial facies that include levees, crevasse splays, channel thalwegs, point bars, longitudinal bars, abandoned channel fill, floodplain mudstones and paleosols that are significantly thicker than lower Salt Wash mudstones. The evolution of the Salt Wash fluvial regime was attributed predominantly to: 1) dynamic subsidence, with the southern part of the basin subsided at faster rates than the northern basin, and; 2) downstream influences from expanding lake systems (Robinson and McCabe, 1998). However, evidence for lake expansion was not confirmed by field observations.
2.9 Magnetostratigraphy of the Morrison Formation
Magnetostratigraphic analysis of the Morrison Formation in northeast New Mexico and southwest Colorado (Figure 2.6) showed that the formation has two distinct paleopoles: one for the lower Morrison Formation and one for the upper Morrison Formation, that correspond to the Salt Wash and Brushy Basin Members in south-central Utah (Steiner et al., 1994). The two paleopoles indicate a considerable missing time between the two units
22 which is interpreted as a depositional hiatus that coincides with the change in depositional regime between the two members. The paleomagnetic data suggest that the Colorado Plateau rotated clockwise by as much as 7 .5 degrees in between deposition of the two units. Therefore, the change in fluvial sedimentation may have occurred over a long duration of time that is condensed into the unconformity separating the sand- dominated Salt Wash Member from the mud-dominated Brushy Basin Member. Construction of a similar magnetostratigraphic framework was attempted for the Morrison Formation in south-central Utah in order to determine whether the mid- Morrison unconformity exists in the proximal basin as well.
2.10 Base Level
The Morrison Formation in south-central Utah was deposited in an alluvial plain (Turner and Peterson, 2004) which, unlike a coastal plain, is far from marine sea level fluctuations and its effects (Posamentier and Vail, 1988). Although fluvial deposition is controlled by a multitude of variables including sediment source and type, basin vegetation density, and biologic activities, four main factors exert a dominant effect on large-scale stratal architectures, and these are: 1) accommodation space 2) water discharge; 3) sediment supply, and; 4) gravel-to-sand fractions (Posamentier and Allen, 1999; Marr et al., 2000). All four variables can be . influenced by either climatic or tectonic perturbations in varying degrees depending on the amplitude of forcing and the rate of change.
Climatic changes can increase water discharge levels and sediment supply through increased precipitation patters, and ensuing differential weathering can alter gravel-sand fractions. Increased sediment supply can reduce accommodation space although basin subsidence rates are far more influential on accommodation space. Tectonic perturbation can affect all four variables with various degrees depending on the rates of change. Tectonism in the hinterland can increase fluvial discharge and sediment supply through uplift, while subsidence in the depositional basin can increase accommodation space (Shanley and .McCabe, 1994). Source rock can also affect gravel-sand ratios, but it can
23 MORRISON TRUJILLO NORWOOD BRIDGEPORT SLICKROC K COMPOSITE
7 5
s o Upper · Morrison Formation 2 5
s o Lower 25 Morrison Formation
Magnetostratigraphy of the Morrison Formation in New Mexico and Colorado
Lower Morrison Formation
Upper Morrison Formation
Two paleopoles for the Lower and Upper Members of the Morrison Formation in .New Mexico and Colorado
Paleopoles from distal Morrison Basin, From Steiner et al., 1994 New Mexico and Colorado
Figure 2.6. Magnetostratigraphy of the Morrison Formation in northeast New Mexico and southwest Colorado developed by Steiner et al., (1994). Calculated paleopoles for the lower and upper Morrison (equivalent to the Salt Wash and Brushy Basin Members in south-central Utah) show two distinct paleopoles that suggest an extended hiatus between deposition of the two members in this region.
24 also be affected by differential weathering brought on by changing precipitation patterns in the hinterland
Accommodation space is often thought of as bounded above by sea level in sequence stratigraphic analyses of basins proximal to, or within reach of, fluctuating marine influences. However, in alluvial basins, accommodation space is governed by a more complex and conceptual surface known as base level. Base level denotes an imaginary surface, analogous to a potential energy surface, to which fluvial systems strive to attain and maintain, where neither erosion nor sedimentation takes place (Barrell,1917; Mackin, 1948; Sloss 1962; and Wheeler,1964). Base level therefore, is a dynamic surface whose position is governed by complex interactions of a multitude of factors including sediment supply, discharge rates, subsidence rates, and other factors that control the position base level relative to the fluvial equilibrium profile (Figure 2.7 A).
Rates of basin subsidence and sediment supply control the position of this surface both in space and time. Increased accommodation through increased basin subsidence (Figure 2. 7 B) raises base level and increases accommodation space, the volume of space between the current fluvial profile and the raised base level. If basin uplift occurs or fluvial discharge rates increase, base level is lowered (Figure 2. 7 C) and the system begins to degrade the landscape trough fluvial incision and erosi0n. Base level however, is very complex and may not be at the same levels and in all parts of a depositional basin. Such factors including variability in subsidence rates, variability in water flux, slope angles, vegetation density, and other factors all. influence base level to various levels and in varying degrees throughout the basin. Furthermore, a single forcing factor may affect different parts of the basin at different levels, thus creating an uneven base level position within the same basin (Ethridge et al., 1998).
25 A Equilibrium After Posamenller and Allen, 1999 j Source j ------.
Grain size c a •
Fluvial Profile = Base level
Raise Base level: Positive accommodation { •. •. .. Bas . Basin Subsidence Potential depositJon .': ...... j ·Aggradat ion Subsidence t"Fluvial profile Base level: Negative accommodation { Uplift Potential 2!!!!il!!! .• .. profile . 1 ...... t . Degradation Base level drops • • • • • • • • • ! Uplift
Figure 2.7 Base level is an imaginary surface unto which fluvial systems strive to attain after which (at equilibrium) neither net erosion nor deposition occurs (A). The elevation of this surface above or below the current fluvial profile determines whether rivers aggrade (accumulate sediment in the alluvial plain) such as if basin subsidence rates exceed sedimentation rates (B) or degrade the landscape through net erosion of the alluvial plain if basin uplift occurs (C). This surface is controlled by subsidence rates, sedimentation rates, and fluvial discharge rates. In tum these factors are controlled by tectonics and climate.
26 3.METHODS
3.1 Measured Sections
Thirty three stratigraphic sections .were measured within the study area and observations were focused heavily on the interval between the Salt Wash and Brushy Basin Members of the Morrison Formation. However, because the Salt Wash Member pinches out to zero thickness in the north, and because there are no distinct correlative horizons within the member, the strategy was to measure from the base of the Salt Wash Member up into the lower units of the overlying Brushy Basin Member. A few measured sections were made of the entire Morrison, including the lower Tidwell Member and the entire Brushy Basin Member. These sections were made in order to document the thickness of the Morrison Formation, and to place the mid-Morrison conglomerate within the framework of the overall Morrison Formation. Locations of measured sections were chosen to follow thickness changes in the Salt Wash Member from a maximum thickness in the Henry Basin to zero thickness in the West San Rafael area. In addition, specific locations were chosen to coincide with exposures of the mid-Morrison conglomerate, or with good exposures of paleosols of interest below the Brushy Basin Member, with a few locations having both.
The stratigraphy was measured using traditional methods with a Jacob staff and Brunton Transit compass with an inclinometer (Compton, 1985) (Figure 3.1 A). Observations of . following properties were made: lithology type, bed thickness, rock color, grain size, grain composition, sedimentary structures, trace fossil abundance and type where identifiable, paleocurrent directions, and paleosol type, thickness, and degree of development.
Measured sections were drafted digitally and correlated using marker horizons at the base and top of the Salt Wash Member. The base of the Salt Wash Member was easily correlated and almost ubiquitously present as a large-scale basin-ward shift in facies as a distinct fluvial sandstone that overlies the thin-bedded fluvial, lacustrine and deltaic
27 Figure 3.1. Undergraduate student Erik Tharalson demonstrates technique of measuring sections (A) using a Jacob Staff and Brunton Compass with an inclinometer. Inclinometer is set to the dip angle of the strata and measurements are sighted along dip directed ascents. Paleocurrent directions (B) were measured from only trough cross-beds in channel deposits of the Morrison Formation. Planar-tabular cross beds were avoided.
28 deposits ofthe underlying Tidwell Member. The top of the Salt Wash Member frequently consists of a laterally continuous pebbly sandstone that is often stained a bright yellow color due to the mineralization and alteration of Uranium-Vanadium deposits (Craig, 1955).
The top of the Salt Wash Member is in some cases defined by a moderately to well developed paleosol that was used as the datum to hang the stratigraphic columns. The top of the Middle Jurassic Summerville Formation is pedogenically modified where there is little to no truncation by overlying strata and is used as a correlative horizon especially in the north end of the study area where the Salt Wash Member is thinnest to non-existent.
It should be noted that obtaining accurate measured sections at the Salt Wash/Brushy Basin Member transition at some localities was especially challenging since Brushy Basin mudstones erode much more rapidly than underlying sandstone-rich Salt Wash strata, and often erode back from the top of the Salt Wash Member by as much as 300 meters or more. Sighting to the base of the Brushy Basin units was at times challenging, if not impossible, using a traditional Jacob staff and inclinometer, and some error in these thickness measurements should be expected, but the overall integrity of the stratigraphy and stacking patterns should be consistent. This problem was not encountered particularly where strata are dipping steeply near the Waterpocket Fold and the Brushy Basin strata lie directly on top of Salt Wash strata, making measurement easier and more accurate.
3.2 .Paleocurrents
Five hundred thirty four total paleocurrent directions (276 from the upper Salt Wash, 186 from the mid-Morrison conglomerate, and 72 from the lower Brushy Basin) were collected from trough.cross-beds according to methods of Potter and Pettijohn (1963) and High and Picard (1974) using a Brunton Transit compass. Planar-tabular cross-beds were avoided altogether for paleocurrent measurements since these can be produced by migrating transverse bars with dip directions that are highly divergent from mean current flow (Smith, 1979; Cant and Walker, 1978). Interestingly, Robinson and McCabe (1998)
29 noted a low degree of dip direction spread in planar-tabular cross-beds of the Salt Wash Member in south-central Utah, and showed a strong correlation in paleocurrent directions between those indicators and ones obtained from trough cross-beds.
Paleocurrent data for the Morrison Formation were plotted on an equal area stereonet and separated according to their members: the Salt Wash Member, mid-Morrison conglomerate, and Brushy Basin Member. Paleocurrent analysis was further accomplished by separating the current directions into three distinct regions: the Henry Basin, the Emery High, and Green River, Utah (Figure 3.1 B).
3.3 Petrography
Forty three petrographic thin sections were prepared from representative specimens collected from the upper Salt Wash, mid-Morrison conglomerate, and lower Brushy Basin, and analyzed petrographically under transmitted light. 400 grains were counted per thin section, and every attempt was made to reconstruct original detrital composition of the grains in a manner similar to the Gazzi-Dickinson method (Ingersoll, 1984). The hand specimens collected in the field were chosen from the finest grain-size population possible, although some conglomerates only yielded very coarse sandstone as the finest grain-size population. Point counting parameters and results are discussed in section 4.
3.4 Paleomagnetism
Two hundred thirty eight cores of mostly fine-grained deposits were collected for paleomagnetic analysis using a portable gas powered drill with a 12" stainless steel drill bit attachment that has a 1" inner diameter (Figure 3.2 A). 2.5 gallon water jugs fitted with manual pumps were used to supply water to the drill bit for cleansing the .bit of debris during drilling, and to keep the bit head from overheating. Drilling was followed · by orienting the cores while still intact in the rock face using an orienting tool and a Brunton Transit compass (Figure 3.2 B) by measuring the azimuth (direction of axis of core into rock face) and plunge (core axis plunge from horizontal) of the core. The core
30 was marked with a scratch orientation mark along the length of the top of the core for later orienting in the lab. The cores were then broken off with a screwdriver and sampled individually.
The sampling strategy was to collect cores from all the fine.-grained floodplain mudstones and paleosols of the Salt Wash Member, and one or two stratigraphic horizons from the overlying Brushy Basin Member for stratigraphic reference. Channel sandstone deposits were avoided almost completely since these units were deposited over much shorter time periods than the floodplain mudstones that constitute minimal depositional time compared to overbank mudstones and paleosols. In addition, coring of the sandstones often proved futile due to poor cementation, and they crumbled easily into sand during drilling. Channel sandstones were however, attempted sparingly when they were the only well exposed units at critical horizons, but with limited success.
Within the overbank units sampling was attempted at every half a meter of stratigraphic interval where allowable, in order to collect samples that record all magnetic polarity intervals. Three core samples were targeted for each stratigraphic horizon however, due to the fragility of the mudstones and siltstones, it was often not viable to obtain that many cores for a particular site, and sometimes meant that only one core had to represent this stratigraphic horizon. 238 cores were recovered for the field from five distinct sites along from the southern Henry Basin northward to the Emery High. Several problems were encountered with the drilling and orienting processes that included very soft rock that crumbled during drilling, cores that shrunk and disintegrated after drying, and soft mudstones that resulted in uneven cores with high degrees of orientation error. Paleomagnetic lab work was performed at a facility in the Institute for Rock Magnetism at the University of Minnesota, Twin Cities Campus.
Core samples were cut down to 1" length, weighed, and entered into a database with relevant data including stratigraphic formation, stratigraphic interval, lithology, color,
31 Figure 3.2. Core samples for paleomagnetic analyses were obtained by drilling (A) in the Morrison Formation using a portable gas-powered drill to collect 1-inch diameter and about 3 to 4 inch long cores. Water is pumped into the bit for cleanse the drill free of debris and to keep the bit head from overheating. After drying, the cores were oriented using a special tool and a Brunton compass (B). Core axis azimuth and dip were collected while the cores were still attached to the rock. The cores were then broken off and tagged.
32 core orientation (azimuth of core, and dip from horizontal). The cores were then analyzed for natural remnant magnetization.
Natural Remanent Magnetization (NRM)
Two demagnetization methods were employed: Thermal Demagnetization (Figure 3.3 A), and Alternating Field Demagnetization (Figure 3.3 B). Prior studies by Steiner et al., (1994) showed that thermal demagnetization proved to be far more successful. However, because of the quicker alternating field demagnetization, both processes were attempted.
Thermal Demagnetization
A suite of samples were thermally demagnetized in 15 steps usmg a magnetically shielded furnace at 100, 200, 300, 350, 400, 450, 500, 525, 550, 575, 600, 620, 640, 660, and 670 degrees Celsius. Natural remanent magnetization was measured after each heating/cooling step using a 2-G Superconducting Magnetometer with a sensitivity of 2e-11 Am2 (Figure 3.4) and the data examined using orthogonal vector component Zijderveld plots (Zijderveld, 1967). In between every third or fo.urth heating step, magnetic susceptibilities for each sample were measured using a KappaBridge AC susceptibility bridge, in order to monitor thermal alteration of magnetic minerals such as alteration of smectites into magnetite that can drastically alter the magnetic character of the specimens (Jovane et al., 2007). Fortunately, no samples measured were found to have any appreciable gain or loss above an order of magnitude change in magnetic susceptibility from heating.
Alternating Field Demagnetization
Another suite of samples were demagnetized in 16 steps usmg an alternating field demagnetizer to 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 75, 80, 100, 150, and 200 milliTesla (mT). The demagnetization process subjected the specimens to alternating magnetic fields in three orthogonal directions m three separate steps in order to
33 Figure 3.3. Demagnetization of the core specimens was performed by two methods. Thermal demagnetization (A) was accomplished by step-wise heating a batch of specimens using a magnetically shielded furnace. Alternating Fields (B) demagnetizes specimens along three orthogonal directions at each step using an alternating field generator, signal amplifier, and specialized computer software.
34 demagnetize them. The magnitude of the field intensity was increased incrementally at each step from 0 to 200 mT. Remanent magnetizations were measured after each demagnetization step-using the Superconducting Magnetometer.
Natural Rema11e11ce Analysis
All samples were analyzed for remanent magnetization usmg a Superconducting Magnetometer (Figure 3.4). Remanent magnetic intensity was analyzed versus demagnetization temperature or alternating field intensity depending on the demagnetization process used. From the orientation of the specimen in the field and the resultant magnetic field vector after demagnetization each step, the magnetic polarity of the specimen was plotted for each step simultaneously on an equal area stereonet and a Zijderveld Plot. Both plots display the magnetic polarity history of each specimen as "layers" of magnetization superimposed onto the original magnetic signal acquired during or just post deposition of the sample. Each demagnetization step removed a layer of magnetization until the final and presumed original magnetization of the sample is revealed.
(
35 Figure 3.4. Natural Remnant Magnetization analysis is performed using a 2-G Superconducting Magnetometer at the Institute for Rock Magnetism at the University of Minnesota, Twin Cities Campus. Magnetic polarities of each specimen are measured separately and after every demagnetization step. Samples are oriented using markings made in the field to reconstruct the orientation of the magnetization vector. in space. These vectors are plotted after each demagnetization step on a Zijderveld plot and equal area stereonets (See Figure 4.22).
36 4. DATA AND RESULTS
4.1 Lithology and Stratigraphy
Within the study area, the thickness of the Morrison Formation ranges between 256 meters in the south (Saleratus Point), 45 meters in the west (Last Chance Wash), 165 meters in the north (Little Cedar Mountain), and 155 meters in the east (Missile Range) (Figures 4.1 and 4.2). All three members of the Morrison Formation have been identified within the study area, with the exception of the Emery High region west of the San Rafael Swell. The base of the Morrison Formation is represented by the lower parts of the Brushy Basin Member that rests unconformably atop the Middle Jurassic Summerville Formation along the J-5 unconformity. This stratal configuration shows that either the basal Morrison deposits were never deposited here, or that later erosion removed those units prior to deposition of the Brushy Basin Member. No evidence of scouring or major erosion exists. Instead a mature paleosol is ·developed in upper most Summerville Formation deposits and directly below Brushy Basin Member deposits, suggesting that no deposition of the lower Morrison Formation strata occurred here and that the Summerville deposits were exposed to pedogenesis and paleosol formation during deposition of the lower Morrison deposits to the south. This means that the entire lower Morrison Formation is missing here and some process must account for the rapid thinning of these units from the south where approximately 170 meters of lower Morrison strata (including Tidwell, Salt Wash and lower Brushy Basin deposits) pinch out in a relatively short distance of approximately 75 miles . .This stratal relationship between the Morrison Formation and the Summerville Formation plays into the understanding in the evolution of deposition between the Salt Wash and Brushy Basin Members. The following 1s a description of the members and the results of field mapping and observations.
37 MAP LEGEND
• Towns e Measured Sections
SO miles
NORTH
Bu llfrog
Figure 4.1. Stratal relationships in the Morrison Formation were analyzed along two transects. One transect (A-B-A') follows exposures of the formation along the western margin of the Henry Basin perpendicular to paleoflow directions, and the other (A-B-B') skirts along the eastern margin to the Emery High and from the Emery High downstream towards the Green River area (Figures 4.2 and 4.17 for cross-sections). The Salt Wash Member is thickest within the southern Henry Basin and gradually thins northwards. At Last Chance Wash in the Emery High region, both the Tidwell and Salt Wash Members are entirely absent. General paleocurrent flow directions in the Morrison Formation are to the east-northeast.
38 0\ A' M B I Henry Basin J Little Cedar Hartnet Last Chance Mountain Wash Bitter Draw Halls Creek 1-70 The Creek Overlook Sandy Post Divide Freemont A Ranch River l " IEmery High I
River I
Cainville West B' Reef t--- North Bottoms 15altWashMerTiber! Hanksville East
ISO meters •' l'l!!I 25 miles Genera! Paleocurrent \ Missile Pyserts Directions Vertical Exa ggeration = l SOx Range Shootaring Hole Canyon
Figure 4.2. Fence diagram of the Morrison Formation within the study area. The Henry Basin (south) is on the left and the Emery High is to the right. Most Tidwell Member thicknesses are approximate. Some of the Brushy Basin Member thicknesses are from Currie (1998), Peterson (1994), and Turner and Peterson (2004). Tidwell Member
The Tidwell Member is the most heterogeneous of all the units within the study area despite being the thinnest and generally under15 meters thick. To the south and in the Shootaring Canyon area, the member consists of lacustrine and deltaic mudstones and limestones, marginal-lacustrine mudstones and paleosols, and fluvial sandstones and conglomerates (Demko et al., 2005). Superficially, Tidwell deposits resemble those of the underlying Summerville Formation however, a sequence boundary separates the two and marks a significant basin-ward shift in facies from marine influenced sabkha sediments to continental fluvial deposits within the study area.
The Tidwell Member consists predominantly of thin-bedded green-grey limestones, beige-brown, to red-brown mudstones, siltstones, and sandstones deposited by shallow playa lakes, evaporite basins marginal to a retreating sea, fluvial sandstones, deltaic sediment gravity flows, and very thin and poorly developed paleosols. Few Tidwell deposits contain gypsum-rich mudstones and sandstone though a few distinct gypsum beds occur mainly in the northern parts of the study area near Tidwell Bottoms.
The base of the Tidwell Member (Figure 4.3) is often marked by a thick fluvial sandstone in the southern part of the study area (correlative to "bed A" of O'Sullivan (1980a)), and by greenish-grey limestone and mudstones and reddish-brown siltstones and mudstones deposited by shallow lakes in the north and east. However, much of the Tidwell Member is distinctly reddish-brown and likely represents prolonged weathering . between deposition of beds. With the exception of Last Chance Wash, where the entire lower Morrison package is missing, this member was identified at every location within the study area.
40 Figure 4.3. The base of the Morrison Formation in Shootaring Canyon in the southern Henry Basin, Utah is marked by a facies change at the base of "Bed A" of the Tidwell Member on top of the middle Jurassic Summerville Formation, which represents an erosional truncation surface and the J-5 unconformity. The Tidwell Member represents a basin-ward shift in facies, from marginal marine sabkha mudstones of the underlying Summerville Formation, to fluvial sandstones and lacustrine mudstones.
41 Salt Waslt Member
The Salt Wash Member is thickest in the southern Henry Basin and averages about 150 meters thick and gradually thins and pinches out completely to the north at Last Chance Wash in the Emery High Region. Further north, at Little Cedar Mountain, Salt Wash Member deposits crop out again and thicken northward to 32 meters thick, and continue to thicken further northwards. Salt Wash deposits also thin eastwards out of the southern Henry Basin, to about 52 meters thick at Horse Bench, south of Green River, Utah.
The Salt Wash Member consists dominantly of fluvial conglomerates, sandstones, siltstones, mudstones, and paleosols, with a minor component of thin lacustrine mudstones, siltstones and carbonates, and small isolated outcrops of fine-grained aeolian sandstones. Channel sandstones vary in color both laterally and vertically but are dominantly white, cream-white, cream-yellow, light-grey, pink-orange, orange-red, reddish-brown and dark brown. Mudstones and paleosols are also multicolored, but are dominated by various shades of red and brown, though reddish-brown, and burgundy-red are the most common. The top 10 or 20 centimeters of most mudstone intervals . and immediately below overlying channel deposits are gleyed, and likely represent interaction from ground waters seeping down from overlying channel sandstones and conglomerates.
Channel deposits consist of well-rounded to subangular, pebbly to ·fine-grained, quartz and chert-rich sandstones and siltstones. These deposits often exhibit trough cross-beds, and planar-tabular cross-beds (Figure 4.4), although a few horizons near the base of channel deposits are massive. Many basal channel deposits also contain cobble-sized mudstone rip-up clasts and pebble-lags of limited lateral and vertical extent that represent the highest flow velocities along the channel thalweg, and grade up quickly into sand- sized deposits. Lower Salt Wash Member sandstones are characterized by thin, .single and multistorey channel deposits that are dominated by amalgamated channel deposits under low-flow regimes (Southard and Boguchwal, 1990). Upper Salt Wash Member sandstones (Figure 4.5), consist of thicker and wider, single and multistorey channel sandstones, and display a wider range of sedimentary structures, including larger
42 Figure 4.4. Various types of cross-bedding is found within the Salt Wash Member that document different depositional flow regimes and discharge velocities. Plane-bedded cross-beds (A) are found mostly in the upper part of the member and denote upper flow regime (Jacob staff for scale is 2 meters long).Planar-tabular cross-beds (B) were deposited by migrating longitudinal bars on top of high amplitude trough cross-beds suggest high discharges values (Pencil is approximately 15 cm long). Amalgamated trough cross-beds (C) are the most common deposits and are dominant in the lower part of the member suggest deposition by migrating subaqueous 3-dimensional dunes in a low- flow regime.
Bullfrog Creek
Figure 4.5. Canyon wall exposure in Bullfrog Creek in the southern Henry Basin. The lower Salt Wash Member consists of highly . amalgamated channel sandstone deposits, with thin floodplain mudstones and immature paleosols deposited dominantly under a low flow regime. The upper Salt Wash Member exhibits thicker multistorey deposits separated by thicker floodplain mudstones and better developed paleosols, with deposition under both low and upper flow regimes. Total Salt Wash Member thickness here is approximately 150 meters thick, and the Brushy Basin Member is approximately 75 meters thick.
43 amplitude trough cross-beds and planar-tabular cross-beds also deposited in a low flow regime, but contain higher amounts of plane-bedded laminations that are interpreted to be formed under upper flow regimes and represent higher discharge fluxes (sensu Southard and Boguchwal, 1990). In some locations especially in the upper strata, horizontally laminated sandstones are very common, very thick (over 2 meters), and dominate the channel sandstones of the Salt Wash Member. Abandoned channel fill including fine- grained sandstone, ripple-trough cross-beds and laminated fine-grained sandstones, siltstones, and laminated mudstones are present above most channel storeys and multistorey deposits. These are much more prevalent in the upper part of the Salt Wash Member, and represent abandoned channel deposits resulting from migration channel deposition into another area. Floodplain deposits in the upper Salt Wash Member are · thicker, more laterally extensive, than the lower part of the member. Additionally, paleosols within document a relatively higher degree of development than the thin, laterally limited, and poorly developed protosols in the lower member (Figure 4.6). Paleosol horizons stack vertically to form thick, multi-generation paleosols separated by thin layers of laminated mudstones and siltstones. These stacking patterns suggest multiple episodes of alternating fine-grained sediment accumulation and pedogenesis.
Within southern Henry Basin total Salt Wash Member deposits are approximately 150 meters thick, and are the thickest, most multistoried, and overall coarser than any other area of the basin (Figures 4.2 and 4.7). Collectively these document the highest water and sediment discharge rates within the basin. However, about 20 miles to the northwest at Bitter Creek Divide and along the eastern edge of Capitol Reef National Park, an isolated outcrop of the Salt Wash Member contains a thick succession of sandstones and conglomerates and is the coarsest and contains most voluminous accumulation of conglomerates anywhere in the study area. This might have been an area that received continuous sedimentation with higher discharge values that directly sourced the hinterlands, possibly because of its more westward proximity to the source area.
The thickness of the Salt Wash Member, as well as the Tidwell Member, gradually decrease northward towards the Emery High in the western San Rafael Swell area, were
44 Figure 4.6. Lower Salt Wash Member paleosols in Shootaring Canyon (A) are thin (B) and poorly developed argillic paleosols. Upper Salt Wash Member paleosols (C and D) are much thicker and better developed argillic calcisols and vertisols. Jacob staff colored segments in (D) are 20 cm long. Note that bedding is still preserved despite rooting and burrowing (arrows) by crayfish in (C) and by mammals in (D). These also exhibit development of sparse and small calcite nodules. Dinoturbated mudstones next to Jacob staff(C) are evidence of dinosaur trampling grounds near fertile streams (Hasiotis, 2004).
45 \0 ""'"
Hansen Canyon Southern Henry Basin
Figure 4.7. Photo mosaic of the Morrison Formation at Hansen Canyon in the southern Henry Basin, where the Salt Wash Member is approximately 150 meters thick. and is thickest within the study area. Both the Tidwell and Salt Wash Members thin northward towards the Emery High where the Brushy Basin Member rests. directly on top of the Summerville Forniation. The Tidwell Member however. does not pinch out until well within close proximity of the Eme1y High suggesting that the Emery High may not have been a topographic feature until after deposition of the Tidwell Member. they are completely absent at Last Chance Wash (Figure 4.8). This absence continues northward to Interstate 70 west of the San Rafael Swell where the Tidwell Member, and a sliver of Salt Wash Member sandstone crop out (Figure 4.2) ..Salt Wash Member deposits are however absent just to the north near the town of Moore, Utah, but shortly north of that they crop out and thicken northwards to about 35 meters at Little Cedar Mountain .
. On the east side of the Henry Mountains near Pyserts Hole (Figure 4.1 ), 110 meters of Salt Wash Member deposits are recorded, and just south of Green River this unit measures 52 meters thick. The Morrison Formation is absent between the eastern Henry Mountains and Green River area because of Cenozoic uplift of the Laramide Monument Upwarp and ensuing erosion by the Colorado River drainage.
Unconformity paleosols
Paleosols in the Morrison Formation are dominated by argillic calcisols that contain abundant calcium carbonate accumulation, and vertisols rich in smectitic mudstones. Calcium carbonate accumulated as either individual concretions, or in especially well developed paleosols, they accumulated in relatively thick (0.5 - 1 meter) horizons of nodules or as continuous layers of calcrete hard pans. Vertisols contain abundant vertic expansions features such as slickensides from shrinking and expanding of the clays with variation in soil moistures (Demko et al., 2004).
In fringing areas of the Henry Basin, such as at Last Chance Wash in the Emery High region, are thick, strongly to very strongly developed, silicified calcisols consisting of highly brecciated and recemented channel sandstones and mudstones (sensu Retallack, 1988, and Mack, 1993 for paleosol recognition criteria). These paleosols are developed in the upper Summerville Formation and directly below the Brushy Basin Member, and are the most mature paleosols within the study area (Figure 4.8). This suggests a long period of sediment starvation and depositional hiatus prior to deposition of the Brushy Basin in this region. These paleosols represent perhaps several hundreds of thousand years to millions of years of exposure and pedogenesis (Demko et al., 2004), with minimal
47 Last Chance Wash Emery High
Figure 4.8. The Morrison Formation at Last Chance Wash in the Emery High, is represented by the Brushy Basin Member only. The Brushy Basin Member rests directly on a very mature silicified "supersol'', developed on top of the Summerville Formation in tidal . inlet sandstones and mudstones. Between the Emery High and the southern Henry Basin (75 miles to the south), around 170 meters of Salt Wash and Tidwell Members deposits pinched out., but more likely during deposition of the Salt Wash Member that may have eroded the Tidwell Member here before/during development of the paleosol here. The paleosol represents a hiatus in deposition at Last Chance Wash during deposition of the Tidwell and Salt Wash Members to the south.
48 sedimentation or erosion of the underlying units prior to deposition of the Brushy Basin Member. These paleosols represents the amalgamation of the lower Morrison paleosol and the mid-Morrison paleosol that formed in this region where little to no Tidwell or Salt wash Member units were deposited here. It is possible, and probably likely, that some Tidwell Member deposits, and perhaps some Salt Wash Member units were deposited in these fringing regions that were later either eroded or pedogenically modified prior to deposition of the Brushy Basin J\1ember.
Just north of Last Chance Wash near the town of Moore, Utah, a strongly developed paleosol is observed below a channel sandstone and conglomerate belonging to the Brushy Basin Member (Figure 4.9). The paleosols near Moore, are approximately 4 meters thick, and well exposed and are characterized by multiple stacked B-horizons capped by a thick calcrete K-horizon. The B-horizons consist of multiple pedogenic features including different types, size, and episodes of rooting, calcareous rhizocretions, vertic · slickensides, thick, tabular water-table related calcareous accumulations, mammalian burrows, and well developed prismatic peds. These paleosols are developed in mudstone deposits that resemble those of the Tidwell Member.
Below this paleosols is a poorly exposed sandstone outcrop with trough cross-beds, which is interpreted to belong to the underlying Summerville Formation and contains marine red-brown mudstones and siltstones. The sandstone deposit therefore, could be a tidal inlet like those found in the Summerville Formation at Last Chance Wash. Because the paleosol overprints the original character of the mudstones deposits, their origins are also hard to decipher. However they are clearly developed in terrestrial setting with abundant root structures, and mammalian burrows. Their stratal position atop the Summerville Formation and a distinct similarity in color and texture to the Tidwell Member at Hartnet Draw, suggests that they belong to the Tidwell Member. This stratal configuration, therefore, implies that the paleosol is developed prior to deposition of the Brushy Basin and developed in Tidwell Member deposits, with no Salt Wash Member deposits here.
49 Figure 4.9. East of Moore, Utah (A), A thick and mature paleosol (B) is developed in the Tidwell Member and capped by a channel deposit of the Brushy Basin Member. This paleosol also demon- strates that the Tidwell Member was deposited in the region that was left stranded after Salt Wash Member deposition commenced to the south. Deposition returned back north to the Emery 1-1 igh after a very mature paleosol developed in the Tidwell Member deposits. This paleosol consists of multiple stacked paleosols (Bt, Bk, Bw, and K-horizons). Pedogenic features include vertic slickensides (C), mammalian burrows with claw markings (D), well-developed prismatic peds (E), extensive pedogenic calcium carbonate nodules and rhizocretions (F), and water table-related calcium carbonate accumulation forming a I to 2 meter thick . calcrete hardpan (F and G).The thickness and level of develop- ment of this paleosol suggests a long duration of exposure and pedogenesis.
50 The strata! architecture near the town of Moore, is similar to that at Last Chance Wash, except that the duration of a hiatus prior to deposition of the Brushy Basin Member near Moore, is slightly shorter because of the time consumed by deposition of the Tidwell Member, where it's absent at Last Chance Wash. This is why the paleosol near Moore, is not as mature as the one observed at Last Chance Wash. This duration however, represents a long period of exposure of the Tidwell Member while the Salt Wash Member was being deposited to the south, until enough deposits accumulated and the Brushy Basin Member overlapped the Emery High and the paleosols developed there. This also supports the interpretation that the Salt Wash Member onlaps onto the Emery High, and so was an active structure after deposition of the Tidwell Member and during deposition of the Salt Wash Member.
Further south, at Hartnet Draw in the Cathedral Valley, the base of the Morrison Formation is marked by a strongly developed paleosol in upper Summerville Formation sandstones (Figure 4.10). Overlying the paleosol are 7 meters of lacustrine mudstones, siltstones and fluvial sandstones with a few thin layers of gypsum and make up the Tidwell Member. At the top of the Tidwell Member is another strongly developed silicified calcisol that also suggests exposure and pedogenesis of the Tidwell Member prior to deposition of the Salt Wash Member (Figures 4.10 and 4.11 ). The presence of the Tidwell Member here with a strongly developed paleosol in the top, further suggests that the Tidwell Member was deposited prior to activation of the structural Emery High feature with a depositional hiatus that followed. This configuration suggests deposition by playa-lakes along a flat plain prior to, or during, initial basin subsidence. The onset of deposition of the Salt Wash Member was likely channeled to the south with the onset of basin subsidence, leaving this area exposed for prolonged periods of time. It is however, conceivable that some fluvial Salt Wash Member deposition occurred here before being ' completely channeled south to the rapidly subsiding basin.
Fluvial deposition did not return north to the Hartnet Draw area until after deposition of the Salt Wash Member filled up the Henry Basin in the latter stages, and after the paleosol in the Tidwell Member was allowed to develop its strong character. This also
51 Hartnet Draw, Cathedral Valley
Figure 4.10. (A) Mature silicified calcisol (right red arrow) is developed in tidal sandstones separates the Summerville Formation from the Tidwell Member of the Morrison Formation. Another very mature paleosol in the upper Tidwell Member (left red arrow) separates it from the overlying Salt Wash Member and formed during deposition of thick successions of Salt Wash Member deposits in the southern Henry Basin, while this region underwent little to no sedimentation. After the basin the southern Henry Basin filled up, fluvial sedimentation returned with the deposition of the upper part of the Salt Wash Member. This is further substantiated with the similarity in facies of the Salt Wash Member deposits at Hartnet Draw, to the upper Salt Wash deposits in the southern Henry Basin (see Figure 4.5), and suggest that the Salt Wash Member onlapped onto the Emery High. (B) Photomicrograph of the paleosol developed in the upper Summerville Formation shows extensive alteration ofterrigenous grains to microspar calcium carbonate with calcite reaction rims (white arrows). (C) Similarly, the paleosol in the Tidwell Member illustrates a high degree of pedogenesis with almost complete alteration to calcite; with a few remaining halos (black arrows and circled clast). Both photomicrograph$ were taken in cross-polarized light.
52 Figure 4.11. The Tidwell Member at Hartnet Draw has a moderately to strongly developed paleosol consisting of multiple generations of pedogenic calcium carbonate accumulations (A) and a mature silicified calcisol (B) suggesting a significant depositional hiatus prior to deposition of the Salt Wash Member. After deposition of the Tidwell Member the onset of subsidence in the southern Henry Basin likely channeled most deposition to the southern regions while the Tidwell Member lay exposed to pedogenic processes until deposition returned.
53 supports the idea that Salt Wash Member deposits onlap onto underlying strata of the Summerville Formation and Tidwell Member deposits as it filled the basin from the south.· Therefore, the base of the Salt Wash Member in the Emery High represents a sequence boundary similar to the base of the Morrison Formation between the Tidwell Member and the Summerville Formation. Fluvial incision at the top of the Salt Wash Member shows evidence of erosion that was followed by deposition of the mid-Morrison conglomerate. If there was a mid-Morrison paleosol at the top of the Salt Wash Member, erosion prior to deposition of the mid-Morrison conglomerate and the Brushy Basin Member, would have removed it (Figure 4.12).
At Horse Bench locality south of Green River, Utah, a moderately mature paleosol separates the Salt Wash and Brushy Basin Members, well downstream from the Henry Basin (Figures 4.13 A, B). This argillic calcisol contains abundant . calcium carbonate nodules and shows a moderate level of brecciation in interbedded siltstone deposits, and is moderately to strongly developed on top of 54 meters of Salt Wash Member deposits. This level of pedogenesis is weaker than those in the E·mery High but distinctly stronger · than any within the southern Henry Basin. Within the Horse Bench locality, fluvial incision scoured out the mid-Morrison paleosol, and the mid-Morrison conglomerate is deposited on top of this major truncation surface in the upper Salt Wash Member (Figure 4.13 C). In the northeast Henry Basin, near Hanksville, Utah, the mid-Morrison paleosol is moderately to strongly developed on top of 40 meters of the Salt Wash Member. This region is closer to the Emery High and may been influenced by this structural feature and explain why only 40 meters of Salt Wash Member deposits are recorded here .despite being closer. to the center of the basin that Horse Bench locality where 54 meters were deposited.
Within the southern Henry Basin, paleosols in the upper Salt Wash Member do not show the level of development, thickness, or maturity that is observed in the Emery High, the Green River, or Hanksville regions where the member is not as thick. Although some paleosols in this region show some levels of development that increase up-section, none
54 Figure 4.12. The mid-Morrison conglomerate at Hartnet Draw was deposited into incised scour fill in the upper Salt Wash Member sandstones. Although no well-developed paleosols are observed in the upper Salt Wash Member, fluvial incision prior to deposition of the conglomerate may have eroded away these deposits.
55 Horse Bench Green River, Utah
Figure 4.13. (A) Strata of the Middle Morrison Formation at Horse Bench south of Green River, Utah, show laterally continuous deposits of Salt Wash and Brushy Basin sandsto_nes, separated by a moderately to strongly developed paleosol (B) This paleosol contains abundant" calcium carbonate nodules (arrow), vertic expansion features, and mild to moderate brecciation of abandoned channel fill. Note that calcium carbonate nodules increase in density to the top where they form a near continuous layer of calcite nodules. This paleosol suggests a long duration of exposure, though not as long as indicated by the paleosols in the upper Summerville Formation in the Emery High. At the Horse Bench locality, the mid-Morrison conglomerate was deposited into incised valley fill on top of the Salt Wash Member (C). Fluvial incision eroded the mid-Morrison paleosol just prior to deposition of the conglomerate erasing its evidence there.
56 exhibit horizons of dense carbonate nodules, continuous layers of calcium carbonate accumulation, strongly developed prismatic peds, dense rooting, burrowing or other features that denote extended pedogenesis (Figure 4.14). Instead the paleosols there represent poor to moderate levels of development, and do not indicate a prolonged depositional hiatus between deposition of the Salt Wash and Brushy Basin Members like that expressed in the Emery High, or further downstream in Colorado and New Mexico.
Within south-central Utah, the mid-Morrison paleosols show a strong correlation between decreasing maturity levels and increased thickness of the Salt Wash Member. The paleosols also show decreasing maturity along a transect form the Emery High southwards into the southern Henry Basin. This suggests that either the final stages of deposition of the Salt Wash Member, or the initial stages of deposition of the Brushy Basin Member, represent a retrogradational stage and retreat of deposition southwestwards and upstream into the Henry Basin. This also suggests that sedimentation in the southern Henry Basin did not cease after deposition of the Salt Wash Member, or to the length of duration that other parts of the basin experienced.
Mid-Morrison Co11glomerate
Incised into upper Salt Wash Member deposits are channel-form bodies of distinct conglomeratic nature with limited lateral continuity, and contain the first appearance of abundant red and green chert pebbles. These deposits have been designated the mid- Morrison conglomerate in this study and they represent .a distinct stage of fluvial deposition, different from either the underlying Salt Wash Member or overlying Brushy Basin Member deposits (Figure 4.15). The stratal position of this unit relative to the mid- Morrison paleosols plays a key role in the interpretation of base level fluctuations after deposition of the Salt Wash Member and prior to the Brushy Basin Member. The unit is exposed at only a few localities including Swap Mesa, Freemont River, Hartnet Draw, interstate 70, and Horse Bench south of Green River, Utah. This conglomerate is similar to lower Brushy Basin Member conglomerates based on its lithologic character, and to upper Salt Wash Member conglomerates based on its stratigraphic position. However, the
57 58 Figure 4.14. Paleosols in the upper Salt Wash Member in the southern Henry Basin are poorly developed compared to their correlatives to the north and south of Green River, Utah. (A) Uninterrupted, laminated beds with sparse root structures, burrows, and almost no pedogenic calcium carbonate accumulations in this paleosol in Shootaring Canyon, suggest rapid sedimentation rates, minimal exposure before burial, and no evidence of a prolonged depositional hiatus. (B) Moderately developed paleosol peds in the top of this paleosol are the only indication of some soil maturity in an otherwise weakly developed paleosol near the top of the Salt Wash Member in Hansen Canyon. Large, massive silt blocks above scale in photograph are likely mammalian burrows. (C) Anemic calcium carbonate accumulations and relatively unbroken stratigraphy in this paleosol attest to a moderately developed paleosol in Shootaring Canyon. (D) Brecciation in this silicified calcisol in Hansen Canyon may be the best developed paleosol in this upper Salt Wash sandstone in Hansen Canyon, though it is isolated, and does not represent very high levels ofpaleosol maturity.
59 umque incised nature of the conglomerate into upper Salt Wash Member deposits represents a distinct fluvial stage following deposition of the Salt Wash Member. Deposition of this unit and its relationship to flu vial base level, may lend insights into our understanding of fluvial evolution in the Morrison Formation.
Three characteristics identify this unit and distinguishes it from either the Salt Wash or Brushy Basin Members: 1) The unit is deeply incised into underlying deposits and is of limited lateral extent to a single channel width: i.e. no evidence of avulsion or meander; 2) The conglomerate represents the first occurrence of abundant red and green chert pebbles in the Morrison Formation; 3) The conglomerate rests stratigraphically on top of, or within, well-developed paleosols established into upper Salt Wash Member units, and; 4) Where the mid-Morrison conglomerate is exposed for long distances, aerial photographs and paleocurrent data (Figure 4.15) show the unit to have narrow and well defined margins, with a low channel thalweg to valley length ratio that suggests deposition by relatively low-sinuosity rivers (sensu Miall, 1977). This is in contrast to lower Brushy Basin Member conglomerates that show evidence of deposition by moderate- to high-sinuosity rivers (Figure 4.16).
Although similarly cqlored red and green chert grains are found in upper Salt Wash Member conglomerates, they are not nearly as abundant as in the mid-Morrison · conglomerate and constitute only a minor part of the clast population of the Salt Wash conglomerates which are dominated by white, grey, beige and brown clasts. This unit represents a basin-ward shift in facies following fluvial incision of the paleovalleys into which it is deposited. This incision likely reflects a geomorphic degradational stage that resulted from a drop in base level, after deposition of the dominantly aggradational Salt Wash Member under high base level conditions. The drop in base level is concomitant with the change in fluvial deposition between the Salt Wash and Brushy Basin Member, and whose origins will be explored in following sections.
60 Swap Mesa, Capitol Reef National Park
Mid-Morrison congl.
---> Paleoflow , , Direction -- , -- " \ ·-- ,.------.... / \ 1l Paleocurrents (_ n=81 ' ' ·, \ l ,,....
Interstate 70 c Emery High
Figure 4.15. (A) Aerial view of the Mid-Morrison conglomerate at Horse Bench south of Green River, Utah, comnbined with paleocurrent directions show that the mid-Morrison conglomerate was deposited by a single-thread channel with a relatively low sinuosity. This was likely due to containment of the river to the narrow paleovalley that it incised into, in the upper Salt Wash Member deposits (Image from Google Earth, 2007). (B) The Morrison Formation at Swap Mesa in the southwest Henry Basin, also docu- ments a narrow profile of the mid-Morrison conglomerate incised into upper Salt Wash deposits. (C) The mid-Morrison conglomerate is deposited into incised Salt Wash Member deposits. Strongly developed paleosols in the upper Summerville Formation and the Tidwell Member suggest that the Salt Wash Member onlapped onto the Emery High with the uppermost deposits overlapping the region. Incised valley-fill deposits of the mid-Morrison conglomerate rest atop the Salt Wash Member, and capped by the Brushy Basin mudstones (not seen in picture). The mid-Morrison conglomerate represents a significant basin-ward shift in facies that is attributed to a drop in base level and responsible for the incision of underlying units. The shift in facies was contemporaneous with the first appearance of red and green chert pebbles, suggesting an associated uplift in the source regions responsible for both drop in base level, shift in faceis, and changing source.
61 Figure 4.16. (A) Lateral accretion sets in a lower Brushy Basin channel deposit at Tidwell Bottoms documents the onset of channel meandering as opposed to avulsion in the Salt Wash Member. (B) Well-defined lateral accretions in a channel sandstone deposit immediately above the mid- Morrison conglomerate at Horse Bench demonstrates the onset of deposition of the Brushy Basin Member directly on top of the incised conglomerate fill and well-developed mid-Morrison paleosol (see Figure 4.13). Arrows point to the of channel migration with paleoflow directions roughly into or out of the plane of the page.
62 Brushy Basin Member
Only the lower units of the Brushy Basin Member were investigated and described in this study. At certain locations the entire Brushy Basin Member was measured for thickness only in order to place the mid-Morrison conglomerate within an overall frame of reference of the Morrison Formation framework (Figure 4.2).
In the southern Henry Basin, the Brushy Basin Member is approximately 80 meters thick. Deposits thin slightly to the north along the western margin of the basin, and along the Waterpocket Fold of Capitol Reef National Park. Brushy Basin Member deposits thicken rapidly and considerably in the Cathedral Valley area. This thickening is accompanied by an increase in the number and thickness of conglomeratic deposits within. However, the thickness of the Brushy Basin Member is also controlled by the amount of post depositional incision at the Brushy Basin Member/Cedar Mountain Formation contact, although exact thicknesses were not measured in this region. The Brushy Basin Member at Last Chance Wash thins back down to 45 meters. There, the lower horizons of the Brushy Basin Member are absent, and rests directly on top of Middle Jurassic Summerville Formation. The Tidwell and Salt Wash Members are entirely absent there. Traced northwards from there, the Brushy Basin thickens back considerably, so that at Little Cedar Mountain northwest of the San Rafael Swell, the unit reaches its maximum thickness within the study area at approximately 125 meters. Total Brushy Basin Member thicknesses were not measured in the Green River area, however, Currie (1998) reported approximately 95 meters thickness near the Green River Cutoff that shows a moderate thickening with downstream progression.
The lower Brushy Basin Member consists of multicolored mudstones, siltstones, sandstones, conglomerates and a minor amount of limestone and silicified limestone. The . lower most units of the member consist of dominantly of fluvial deposits and a minor component of lacustrine deposits, and are dominated by illitic clays similar to those in the underlying mudstone deposits of the Salt Wash Member. The remainder of the Brushy Basin is overwhelmingly dominated by expanding smectitic clays that swell and contract
63 from rain and snow precipitation resulting in "popcorn" texture and badland topography with little to no vegetation cover. At some localities the illite-to-smectite transition is concomitant with the Salt Wash/Brushy Basin contact, so that the entire Brushy Basin Member deposits there are dominated by expanding smectites. In other areas, the lower few meters of Brushy Basin Member mudstones consist of non-expanding illitic clays. Despite the abundance of exposures and near total absence of vegetation cover, the badland nature of the member forms a masking veil up to several inches thick that covers almost the entire unit. With the exception of a few well exposed conglomerates, pits were frequently dug through the cover to expose the bedrock for observation.
In the southern part of the study area, the Brushy Basin is approximately 54 to 64 meters thick, and consists of mudstone, sandstone, and conglomeratic lenses. Channel sandstones and conglomerates are white, grey, light-beige, yellow-brown, and red-brown colors. Much of the member however, is dominated by red, reddish-brown, burgundy, purple-red, grey, white, and grey mudstones. Green-grey and bright pistachio-green limestones and mudstones are attributed to lacustrine/palustrine sediments deposited under reducing conditions. Those deposits are however, found higher up within the member and were not examined closely.
The lower Brushy Basin Member contains numerous orange-brown to yellow-brown · conglomeratic units that in some places look very similar to underlying Salt Wash sandstones and conglomerates. However, these units often exhibit moderately to well- defined lateral accretion sets that suggest deposition by meandering stream channels which differentiates them from the amalgamated Salt Wash channel deposits (Figure 4.16).
The top of the Brushy Basin Member is often marked by.a very mature and thick paleosol that marks the contact with similar looking _deposits of the Cretaceous Cedar Mountain Formation (Figure 4.8). This paleosol suggests that sedimentation rates dropped considerably or ceased entirely after deposition of the Brushy Basin Member at least in the areas of the paleosol formation, and possibly associated with wide-spread landscape
64 degradation. In other areas the top of the Brushy Basin Member is marked by a truncation (?) surface at the base of the Buckhorn Conglomerate of the Cedar Mountain Formation. In some parts of the southern Henry Basin, the upper contact of the Brushy Basin Member is truncated at the base of marine oyster beds of the Dakota Sandstone. See Appendix I for complete measured sections.
4.2 Facies Architecture
Even though this study. focuses ()n the Salt Wash/Brushy Basin Members transition, the Tidwell Member is relevant. This is because the entire lower Morrison Formation deposits pinch northwards from the southern Henry Basin to the Emery High, where the Tidwell and Salt Wash Members are absent. There, Middle Jurassic Summerville Formation is overlain directly by the Brushy Basin Member. Therefore, the entire lower Morrison Formation units were examined and analyzed in order to evaluate the strata} relationships between the southern and northern regions of the basin within the study area. Salt Wash Member deposition in the southern Henry Basin evolved through time from thin, single and multi-storey channel deposits, to thick, amalgamated multistorey deposits. However, such changes are not observed in areas to the north near the Emery High and northeast near Green River. Instead there only the upper, thicker facies of the Salt Wash are present with a distinct lack of the lower Salt Wash fluvial facies that are only observed in the southern Henry Basin. This suggests that lower Salt Wash deposits may not have . been deposited in these fringing areas of the basin and only after sedimentation rates exceeded basin subsidence rates in the southern Henry Basin the rivers were migrated northward and outward and deposited the upper facies of the Salt Wash Member. This also suggests that Salt Wash strata onlapped onto underlying units as deposits accumulated and filled the basin (Figure 4.17).
The contact between the Salt Wash and Brushy Basin Members in some areas is ambiguous at best. To aid in assessing the contact, five criteria were used to define it as follows: 1) the top of the last laterally continuous sandstone/conglomerate and associated
65 floodplain deposits; 2) the top of a very mature paleosol in the upper Salt Wash Member 3) the first appearance of well-defined lateral accretion sets in fluvial channel deposits that mark the onset of stream channel meander; 4) the first appearance of smectite-rich mudstones atop unmistakable Salt Wash Member deposits, or; 5) the first appearance of thick, mudstone-dominated deposits. When present, the first four characteristics are easily recognizable in the field, however, the fifth characteristic can be subjective, especially when deposits appear to grade into one another as they appear to in some areas of the southern Henry Basin. There, mid-Morrison paleosols either never developed strongly, or ensuing landscape degradation from fluvial incision related to lowered base level may have eroded away such paleosols. Although the two members appear to be mutually exclusive and likely do not interfinger (Figure 4.18), it is conceivable that this region may have undergone near continuous deposition that might better explain the lack of well-developed paleosols in the upper Salt Wash Member. Perhaps only a minor hiatus in deposition occurred there while the remainder of the Basin underwent a prolonged period of sediment starvation and depositional hiatus.
Where present, the mid-Morrison conglomerate rests at the Salt Wash/Brushy Basin Members contact (Figure 4.15). The conglomerate was deposited into mcised paleovalleys that likely resulted from landscape degradation following deposition of the Salt Wash Member. This incision was most likely in response to a drop in fluvial base level after being high for probably the entire duration of deposition of the Salt Wash Member. At Horse Bench, south of Green River, Utah, the strata! relationship between the mid-Morrison conglomerate and the Salt Wash/Brushy Basin contact is clear. The. conglomerate was deposited into an incised valley in the upper Salt Wash Member, and the Brushy Basin deposits rest directly on top of the conglomerate. Along the margins of the conglomerate, a mature paleosol separates the Salt Wash Member from the Brushy Basin Member, and shows that the conglomerate was deposited after a depositional hiatus that likely resulted form a drop in base level as reflected by geomorphic degradation of the upper Salt Wash deposits into paleovalleys.
66 r:- A A B A' \0 Saleratus Halls Creek The Bitter Sandy Freemont Jail House Hartnet Last Chance 1-70 Moore Little Cedar Point Overlook Post Creek Ranch River Rock Draw Wash Mountain Divide
Cedar Mountain Formation
Brushy Basin Mt>mbcr -t -- z 0 ·.:; Ill E !/; bUJ UJ V) LL.0 I V) c: i $: V) 01 SaltWash V'I Member Emery High 0 "E I I Vertical Exaggeration= 1BOx a: 0 u 2s·miles TldwellMbr. Summerville ------"""" NORTH Stomgly developed paleosol Formation IHenry Basin I Moderately de'ttlopf'd paleoso\ Weak to moderately paleoM>f
B A B B' Saleratus Shootaring Pysets Hanksville Salt Wash Last Chance Tidwell Horse Missile Point Canyon Hole Wash Bottoms Bench Range Cedar Mounulin Formation r-t- l=------l------
z c: 0 .Q
!/; bUJ .... V) .£ Summerville Formation ·15 : tu• IHenry Basin I Figure 4.17. (A) Cross-section of the Morrison Formation from south to north along the western margin of the Henry Basin. (B) Similar cross-section along the eastern margin of the basin, and towards the Green River area (see Figures 4.1 and 4.2 for locations). The mid-Morrison paleosol is illustrated by three colors according to the level of maturity: Red= mature, Yellow= moderate, and Green= immature. The level of maturity correlates with decreasing thickness of the Salt Wash Member. Regions that accumulated thick Salt Wash deposits have immature to moderately mature paleosols. These paleosols increase in maturity northward and both, the basal and mid-Morrison paleosols, show increased levels of maturity along this trend with the most mature paleosols in the Emery High region. 68 Bullfrog Creek, Southern Henry Basin (north view) Hansen Canyon, Southern Henry Basin (west view) Figure 4.18. Photo mosaics of the middle Morrison stratal architectures in the southern Henry Basin, at Bullfrog Creek (A), and Hansen Canyon (B). No major landscape degradation of upper Salt Wash deposits is observed that would correlate to fluvial incision from a drop in base level. This suggests that little erosion occured after depsotion of the Salt Wash Member and possibly depsotion never ceased in the southern Henry Basin between the two members. Furthermore, low levels ofpaleosol development at these respective horizons, do not suggest a prolonged period of exposure that could correspond to a depositional hiatus. 69 4.3 Paleocurrents Paleocurrent data (Figure 4.19) from trough cross-beds show that the upper Salt Wash Member was deposited by northeast rending rivers with a mean direction of 043 degrees with a relatively low spread in directions (Figure 4.19 A). A somewhat higher divergence in directions in the upper Salt Wash Member is also evidence of deposition by slightly higher sinuosity rivers than the lower Salt Wash Member observed by Peterson (1994) who focused on the lower Salt Wash Member, and Robinson and McCabe (1998) who studied the entire Salt Wash Member in the Henry Basin. This is supported by evidence of more strongly developed channel margins and thicker channel storeys with a wider- variety of fluvial facies that include a basal conglomeratic lag, trough cross-beds, abandoned channel fill, and thick overbank/floodplain mudstones with relatively well- developed paleosols. Current directions are also more consistent with a fluvial system deposited along an alluvial plain as opposed to an alluvial fan as first proposed by (Craig et al., 1955). Such alluvial fans would otherwise show a fanning out of current directions had they emanated from a single point source. Salt Wash flow directions document northeast trending rivers throughout the study area that eventually reached the retreating Jurassic Interior Seaway to the northeast (Hintze, 1988; Blakey, 2003). In the Green River area, the rivers appear to have taken on more northerly route, possibly following a more direct path towards the sea. The Brushy Basin Member shows a much higher degree of variation m overall paleocurrent directions. Mean flow is to the east-southeast at 099 degrees (Figure 4.19 C). The spread in directions might be related to the higher sinuosities of the rivers that deposited the Brushy Basin Member. Interestingly paleocurrent flow directions in the Cathedral Valley region are dominantly to the south-southeast, whereas paleocurrents in both the Green River area and the Henry Basin record northerly directions. Within the southern Henry Basin paleocurrent directions are more varied and show a general north- northwest flow. However, because of the poor exposures of channel sandstones and 70 c / Gree n Rive r Mea n Olrectlon:099 N=72 B Brushy Basin 25mlles . Composite J NORlll A i / L ii... . + I . b. ' I I NClllJI I i I 25 mlles I Composite 71 Figure 4.19. Paleocurrent directions in the Morrison Formation obtained from 534 measurements from trough cross-beds in channel deposits. (A) Salt Wash Member paleocurrent directions document deposition by low-sinuosity rivers with low-divergence in overall current directions. Furthermore, directions are unidirectional throughout the study area and argue against deposition on an alluvial fan as suggested by Craig et al., (1955), Mullens and Freeman (1957) and Tyler and Ethridge (1983). (B) Current directions in the mid-Morrison conglomerate also exhibit low-divergence in directions, and combined with its incised nature, suggests deposition along narrow incised valleys into which the rivers were confined to. (C) Brushy Basin Member deposits document current directions that are much more heterogeneous with high divergence. This is likely a reflection of the higher-sinuosities of anastomosing and meandering streams of the Brushy Basin (see Figure 4.16). Poor exposures of the Brushy Basin channel deposits in the Henry Basin resulted in the collection of only a few paleocurrent indicators. 72 conglomerates, only a few data points were collected in this region (n=9), and so confidence in the mean direction of the paleocurrents in this region is low. Even though the mid-Morrison conglomerate would traditionally be overlooked and most likely lumped with basal Brushy Basin deposits, paleocurrent data show it to be much more similar to underlying Salt Wash Member deposits (Figure 4.19 B). Mean flow of these rivers was to the northeast at 065 degrees with a narrow spread in directions, suggesting deposition by low-sinuosity rivers, similar to those that deposited the underlying Salt Wash Member. The narrow divergence in directions however, supports the idea that these units were deposited by rivers confined to the valleys in which they incised into, possibly reoccupied relict/abandoned Salt Wash channel and river beds to preserve a similar direction. Within the Henry Basin, paleocurrent directions in the mid- Morrison conglomerate are to the east-southeast, while they exhibit northeasterly directions in the Emery High areas. At the Horse Bench locality, south of Green River, Utah, the unit is exposed for long distances along its flow axis, and combined with aerial photographs (Figure 4.15) show that the rivers flowed east-northeastward with a relatively low-sinuosity. Appendix I contains all paleocurrent data and corresponding stratigraphic horizons. 4.4 Petrography Petrographic results from analyses of 43 thin sections are summarized in Tables 4.1-4.3 and plotted on six different ternary diagrams: QFL, QrnFLt, QrnFC, QpLvLs, and QmQtC (Figures 4.20 A-E). Raw parameters are presented in Table 4.1 and computed parameters for the ternary diagrams are presented in Table 4.2. Summary of the data is presented in Table 4.3. In general, Morrison Formation sandstones are mature, quartz- rich sandstones with minor amounts of feldspar and are classified as quartzarenite, subarkose, or sublitharenite using the McBride (1963) classification. However, if the abundant chert grains are counted as lithics rather than quartz, the sandstones would be classified as lithicsubarkose, sublitharenite, or litharenite using the same classification 73 A) B) Q Om Magmatic Arc Om Op C) D) Collision Orogen Sources F E) Om Legend (Plots A-El + Brushy Basin Member • Mid-Morrison conglomerate e Salt Wash Member Total number of samples, N =43 c Figure 4.20. Petrographic results from 43 thin section analyses of the middle Morrison Formation channel deposits. See Table 4.2 for ternary end member classifications. 74 Table 4.1 . Raw point count parameters for petrographic analysis of the Morrison Formation. y-..... A _..... DH Qm Monocrystalline quartz Qp Polycrystalline (recrystallized) quartz Qt Tectonized (sutured) quartz Qr Recycled quartz Fp Plagioclase feldspar Fk Potassium feldspar Lv Volcanic lithic fragment Lm Metamorphic lithic fragment Ls Sedimentary lithic fragment Ccf Fine-grained chert (non-recognizable origins) Ccc Coarse-grained chert (non-recognizable origins) Crc Chert of carbonate origins Crs Chert of other terrigenous origins Cp Pedogenically altered chert Ca Chalcedony/Agate Co Other unidentifiable chert Table 4.2. ·Recalculated point count parameters. lrillrYJJI- -- .f!ll • 0 Q I i I I.fr- uer; u Q Total quartz, including monocrystalline, polycrystalline and chert F Total Feldspar L Lithic rock fragments, excluding chert and polycrystalline quartz Qm Monocrystalline quartz, including recycled monocrystalline quartz grains Lt Total lithic rock fragments, including chert and polycrystalline quartz c Total chert, excluding polycrystalline quartz Qp Polycrystalline quartz, excluding chert Lv Volcanic lithic rock fragments Ls Sedimentary lithic rock fragments including all chert Qt Tectonized chert (with sutured boundaries) T abl e 4 ..3 T emary d"1agram component percentages see ta bl e 4 2 tior component descnptlons ) . Member %QFL % QmFLt %QmFC %QpLvLs Upper Salt Wash 90,4,6 55, 4, 41 61,5,34 21,5, 74 Mid-Morrison Congl. 88,2, 10 22,2, 76 25,3, 72 5,4,92 Lower Brushy Basin 90,5,6 44,5,51 49,5,46 10 ;3, 87 75 scheme. Since the chert is a diagenetic alteration of marine carbonates, it is best to characterize these as lithics, thus classifying the sandstones as lithic to sublitharenites. Salt Wash sandstones consist of very-fine to very-coarse sand grains and conglomerates composed of very-coarse sand to pebble-sized clasts, and generally coarsening upward. The majority of Brushy Basin sandstones in the study area are coarse-grained, though some finer-grained varieties are also observed. Morrison Formation sandstone grains are sub-rounded to well-rounded, but a few specimens consist of sub-angular and angular grams. The majority of the quartz is monocrystalline and non-undulatory with lesser amounts of undulatory, recrystallized, or tectonized (sutured) varieties. Specimens from near the northern Waterpocket Monocline, a Laramide-aged tectonic fold, contain anomalously high content of quartz grains that exhibit post depositional brittle fractures that might be related to Cenozoic structural folding of the monocline. Chert is the overwhelmingly dominant lithic component and consists of six main varieties: fossiliferous, spicular, pure, chalcedonic, agate, and other unidentifiable fine-grained chert (Figure 4.21). However, a large percentage of the chert grains contain either direct evidence of a fossiliferous carbonate or oolitic packstone protolith, or relict features that are undoubtedly of similar origins despite extensive diagenesis. Other lithics include fine-grained volcanic rock fragments rich in sanidine grains from altered rhyolites, few mudstone grains, and some pedogenically modified chert and quartz grams. Accessory minerals include biotite, hornblende, and zircon. Some grains show extensive, post-depositional alteration to clays, while others display a concomitant precipitation of dolomite and hematite as multigeneration hematite coated · rhombohedral overgrowths. Common cementing agent is recrystallized and networked calcite with "floating" terrigenous grains, although many sandstones are poorly cemented with high primary porosities. A few samples from the mid-Morrison conglomerate and Brushy Basin Member are cemented purely by secondary chalcedony matrix with no 76 Figure 4.21. Photomicrographs of channel deposits in the mid-Morrison conglomerate showing various types of chert clasts. Virtually all the chert is of diagenetic alteration of Paleozoic carbonates. (A) Some clasts clearly display an oolitic/fossiliferous packstone origins of these rocks. (B) Sponge spicules are visible in this spicular variety, and (C) unidentifiable fine-grained varieties are also common. (D) Brightly colored chert grains are or' similar origins as the other non-colored chert with no distinguishable difference that may serve in provenance analyses. The red color may however be a secondary coloration acquired further upstream from pedogenic processes and exposure to the atmosphere. Photomicrographs A and D were taken in plane polarized light, and B and C in cross-polarized light. Scale bar applies to all four photomicrographs. 77 calcite or clay or porosity and displays ubiquitous and distinct interference patterns under cross-polarized light. Two specific specimens were collected from mature paleosols developed in sandstone deposits in order to analyze pedogenic effects on these sandstones. One such specimen was collected from a paleosol at the contact between the Summerville Formation and the Tidwell Member at Hartnet Draw in Cathedral Valley, The other specimen was collected from a paleosol in a Tidwell Member sandstone at I-70 in the Emery High. Both specimens show extensive, and complex alteration of chert and quartz grains to micrite and microspar calcite, with only a few identifiable intact grains, while the remainder of the grains were only identified by a relict halo (Figure 4.10 B and C). Although these were not included in the petrographic analysis, owing to a low count of identifiable grains, they were however, used to aid in documenting the level of pedogenesis at these horizons. The nature of the developed paleosol in the upper Summerville Formation at Hartnet Draw, further attests to the presence of the J-5 unconformity between the Summerville Formation and the Tidwell Member of the Morrison Formation. Interestingly, the mid-Morrison conglomerate contains a disproportionate number of chert grains with extensive alteration to microspar calcite (Figure 4.22). Comparison of these altered chert grains to specimens from pedogenically modified channel sandstones (Figure 4.10) shows a strong similarity, and suggests that the alteration in the mid- Morrison conglomerate may be of pedogenic origins. See Appendix II for complete data of petrographic thin section data. 4.5 Paleomagnetism Demagnetization Results Thermal demagnetization proved to be more . successful than alternating fields demagnetization at separating magnetic components. This is most likely due to a high contribution of the magnetic signal from iron oxides and hydroxides with high stabilities 78 Figure 4.22. Photomicrograph of a mid-Morrison conglomerate deposit under cross-polarized light. The mid-Morrison conglomerate displays a disproportionate quantity of "dirty" chert clasts that have undergone extensive alteration (pedogenesis?) to microspar calcite. The direct association of unaltered chert clasts (A) with highly altered chert clasts (B) suggests possible recycling of previously deposited chert clasts in an upstream basin. Combined with the abundance ofred chert that may also be of pedogenic nature, this suggests that these clasts were previously deposited, exposed to pedogenic processes during a depositional hiatus after deposition of the Salt Wash Member and ensuing fluvial scouring by the mid-Morrison conglomerate. These were later remobilized and deposited further downstream along with fresh new sediment. 79 to alternating fields (Steiner and Helsely, 1975, Steiner et al., 1994). However a few specimens were demagnetized successfully using alternating fields. At low temperatures most specimens displayed various degrees of random demagnetizations suggesting complex episodes of magnetic overprinting of the original magnetic signal since acquisition. Further demagnetization removes this overprint and the original signal is presumed to be revealed at higher steps of demagnetization. Three main categories of magnetic polarity were observed. The first category of samples exhibit a univectorial decay trajectory to the origin suggesting a single demagnetization signal with small errors that yield fairly accurate paleopoles (Figure 4.23 A). Natural Remnant Magnetization (NRM) values also decay to zero or trajectories that approach zero values. These samples are ideal for magnetostratigraphic reconstruction of the Morrison Formation and for _paleopole calculations. A second category of specimens show univectorial decay trajectories towards a non-zero value suggesting some remnant magnetization that remains locked in the sample (Figure 4.23 B). While these did not converge to zero, the specimens that converged to small enough values were used in the magnetic analyse_s. A third category of samples show very complex and seemingly random demagnetization · decay trajectories even above the demagnetization steps (thermal or alternating fields) that would normally remove the complex overprint (Figure 4.23 C). This suggests a complex magnetic acquisition history that may have b.een acquired by various magnetic minerals including, hematite, magnetite, and goethite. These minerals are the likely source of the richly varied colors of the Morrison Formation including red, brown, and yellow. These specimens did not yield demagnetization trajectories that decayed to zero or a singularity suggesting a remnant magnetization not removed by the demagnetization process. These specimens were not used in magnetostratigraphic analyses or paleopole calculations. 80 NRM Decay Stereonet Plots Zijderveld Plots A Single mllJnetk compont>nt, trajectories dccil'j to the origin '1 •• :p---; .._ .. ,1 "" .. B Single- magnetic compcncnt. tr.ajt?Clorle\ do no! dP.Cd'f' to the cuigln " ' • Ill! ···· tlRIJ ' I i r .. -,,. ;• 0 ·- c Multiple m.ignclic components Mean pole could not obtained ... Ii'" I\/ / /-' . lln 100 'IOI! • ., Figure 4.23 Natural Remnant Magnetization (NRM) intensity plots, stereonet diagrams, and Zijderveld plots from examples of three specimens with three different categories of demagnetization behavior. (A) Category one samples show a gradual magnetic intensity decay with corresponding demagnetization and univectorial decay to the origin, from which original magnetic polarities can be obtained with confidence. (B) Category two samples display intensity decreases (NRM plot) and univectorial decay to non-zero values (Zijderveld plot). (C) Some specimens yielded a third demagnetization category with NRM intensities that do not decay or converge to zero and exhibit seemingly random demagnetization trajectories (stereonet and Zijderveld plots) that suggest complex magnetization histories acquired over long periods of time. Only specimens with categories one and specimens of category two with low errors, were used in paleomagnetic analyses of the Morrison Formation. 81 Paleomag11etic Analysis From the Zijderveld plots, least-square lines were fit to the linear segments of the demagnetization trajectories and used to calculate magnetic vector declination and inclination. These plots (Figure 4.24 A and B), show that all the specimens have roughly north-directed declinations with inclinations below horizontal. Equal area stereonet plot of the individual specimens (Figure 4.24 C), shows that the resultant magnetic vector has an orientation of 353 degrees from true north and 54 degrees below horizontal, with a 95 percent confidence circle of radius 7.4 degrees. The unidirectional nature of the magnetic poles over the entire thickness of the Salt Wash Member suggests a nearly isochronous acquisition of magnetic signal that was not acquired over the course of deposition of the unit. This unfortunately makes magnetostratigraphic analysis and correlation of the Salt Wash Member not suitable within the Henry Basin. Reasons for this anomalous result are discussed later in section 5.5. For the complete paleomagnetic data see Appendix III. 82 NRM Declination NRM Inclination 100 100 . 90 VI 90 l (jj QJ •• QJ QJ .§.- l .§.- • (jj 80 (jj 80 .c .c • E E QJ QJ :2 70 •• :2 70 ..r::. •• ..r::. .. VI VI • Iii 60 Iii 60 .....Vl- • .....Vl- • 0 0 • • QJ QJ Vl 50 . • VI so .0"' .c"' • • E E 0 .g 40 40 . Iii Iii i:: > ....QJ (jj .!: 30 • 30 u 1: ·=-u .. :.c :.c a. a. i• 20 20 ·;:::;Cl t• • •• • • • Vl • • Vl • 10 . .. - 10 .. . 0 • 0 • -180 0 180 -90 0 90 Declination from north Inclination from horizontal A (degrees) B (degrees) Site-mean pole data: Declination = 353.8 .. Inclination= 54.3 • Number of samples (N) = 34 • Resultant vector length (R) = 31 .29 Precision parameter (k) = 12.16 • Radius of95% confidence circle (a95) = 7.4 c Figure 4.24. Magnetic vector orientations for all stratigraphic horizons of the Salt Wash Member specimens in the Henry Basin have north-oriented declinations (A) and downward-oriented indinations (B).This is also shown on an equal area stereonet (C) as unidirectional sample set with a 95 percent confidence circle of radius 7.4 degrees. This suggests that magnetization of these samples occurred simultaneously, and therefore, after deposition of the entire sedimentary package yielding the data useless for magnetostratigraphic analyses. 83 5. INTERPRETATIONS 5.1 Depositional History Figure 5.1 shows a schematic time-stepped depositional model for the Morrison Formation in the Henry Basin and surrounding regions. The model shows a cross-section perpendicular to paleoflow from the southern Henry Basin (left) through the Emery High (center), to Little Cedar Mountain (right). Paleosol development level is portrayed in three stages: weakly developed (green), moderately developed (yellow), and ·strongly developed (red). With northward retreat of the Jurassic Interior Seaway during the latter stages of, or after, deposition of the Summerville Formation, a subsequent drop in base level related to dropping in sea level results in degradation of the Summerville Formation in the southern areas and in the southern Henry Basin. After sea level dropped significantly and base level was no longer tied to sea level, and with the creation of basin from the void left behind the sea, base level rebounded back up with the creation of accommodation space in the alluvial plain. This created a terrestrial equivalent to a forced regression with Tidwell Member deposition down-stepping eastwards and following the retreating sea level, eventually aggrading after base level rose. This explains why the boundary between the Summerville and Morrison Formations in the southern Henry Basin is a truncation surface (Figure 4.3) whereas in areas to the north in the Emery High, the Tidwell Member rests atop well developed paleosols in the upper Summerville Formation (Figure 4.10). Onset of basin subsidence in the southern Henry Basin likely began with deposition of the Salt Wash Member. Initial basin subsidence was relatively slow (Robinson and McCabe, 1998), and combined with high rates of sedimentation of the lower Salt Wash Member suggests that deposition of the lower Salt Wash Member was basin-wide including in the Emery High region. An increase in subsidence rates channeled all deposition southward to the southern Henry Basin. This leaves the lower Salt Wash 84 "Tl lCi" 1 Saleratus Halls Creek The Bitter Sandy Freemont Jail House Hartnet Last Chance 1-70 Moore Little Cedar c Point Overlook Post Creek Ranch River Rock Draw Wash Mountain iil Divide Y1 Base Level c.. rt> ,...... :---" Ill x 0 "O "O Ill Ill Modera1e I Low 0 0-0"0 x; CiJ -· c.. :J -· 0 - Ill d". Depsotion of the Summerville Formation and pedogenesis across the J-5 unconformity. :J 0 0 0 0 ...... , :J """t\O>r+°' 21 Saleratus Halls Creek The Bitter Sandy Freemont Jail House Hartnet Last Chance 1-70 Moore Little Cedar rt - - 0 Ill 3 Point Overlook Post Creek Ranch River Rock Draw Wash Mountain lll:J-oo Divide 0 l.C QJ c.. Base Level <-01.C Ill Ill Ill Ill - ::::!.. Moderate 'S. 0 -0 .... V'I Q) @' -c ,...... -·- 81 A B A' -· :J 3 0 - Base Level Saleratus Halls Creek The Bitter Sandy Freemont Jail House Hart net Last Chance 1-70 Moore Little Cedar 3 :J "O Point Overlook Post Creek Ranch River Rock Draw Wash Ill QJ Ill Mountain "'"' ,.... i3 Very High Divide - "' :J Ced.ar Mouut.lln Ill rt c.. Fo•ma!loo - :!! n ;:;· -- - c - - .... QJ - - - - Ill :J 8ru\11y 8.1 ,Jn ;=;-@ 8 Membt-f --- c: :E: "O '< QJ QJ Ill.... "' - 3 -· 0 :J g,0 ;§ :T :!! Emery Hig-h c.. ro c.. I I V\ m ::;· 3 ro NORTH o ro n Vt'fllc.ilE.t.J9C)ef.>lkln• 180x: -'< 0 '< ::r: :J Somcte-r1 "O - · ::!. l.C 0 Rise in base level results in rapid deposition of Brushy Basin JS mile\ 0 c Summ'1v1Tlc IHenry Ba?OJ fotm.ltlor1 .... :!! rt Member followed with truncation by Cretaceous deposits - 8 8( Wt'.lk to modtrot tely developed pa!coM>I 00 Vi Member and underlying Tidwell Member deposits exposed to erosion in the fringing areas to the north, while deposition resumed in the south (Figure 4.8). Virtually all of the Salt Wash Member was deposited under aggradational regime with a relatively high base level and accommodation space. This implies that basin subsidence rates were higher than sedimentation rates to produce high accommodation space for sediments to prograde without incision or landscape degradation. This produced amalgamated sheeted-sandstone deposits with high lateral continuity, and thin and poorly developed floodplain mudstones and paleosols (Figures 4.5 and 4.6). Increased subsidence rates (Robinson and McCabe, 1998) and subsequent increase in base level during deposition of the upper Salt Wash Member, allowed the rivers to build thicker, more isolated sandstone bodies with corresponding thicker and wider floodplain mudstones. This is also resulted in slightly longer durations of exposure of floodplain deposits and development of more mature paleosols than in the lower member. Fluvial incision was limited to channel-scales incision with only one to two meters of scour. Amalgamation of multistorey channel deposits in the upper half of the Salt Wash Member is attributed to increased subsidence rates, and possibly to downstream expansion of lake systems that increased accommodation space (Robinson and McCabe 1998). However, without evidence of expanding lacustrine deposits, it is exceedingly hard to imagine how such small lakes could expand enough to affect base level at appreciable levels, especially when subsidence rates were high enough to raise base level and provide ample accommodation space. Furthermore, lacustrine deposits within the Salt Wash Member document small, sparse, isolated, and short-lived alluvial "ponds" that rapidly filled with sediments and likely had little effect on upstream river deposition. Additionally, pedogenic carbonate nodules within the upper Salt Wash paleosols in downstream reaches of the Henry Basin, document continued semi-arid conditions that may not support large lake systems (Figure 4.13). Basin subsidenc:e was likely the dominant factor controlling the strata! architecture. 86 During the later stages of deposition, isolated fluvial incision in the upper Salt Wash Member may be related to late fluctuating base levels. Whether these small incision features can be related to dropping base level or allogenic behavior of fluvial deposition is difficult to interpret. However, there is little evidence of widespread landscape degradation after deposition of the Salt Wash Member in the southern Henry Basin. If subsidence rates were high enough in the southern Henry Basin, then it is possible that this part of the basin did not experience the depositional hiatus similar to areas to the north or downstream (Figure 5.2). Careful examination of deposits in upper Shootaring Canyon turns up no correlative unconformity paleosol in this area (Figure 5.2 B, D, E). Instead, aggradational stacking of Salt Wash deposits suggests continuous deposition in this region (Figure 5.2 C). After deposition of the Salt Wash Member, base level dropped and resulted in fluvial incision, followed by deposition of the mid-Morrison conglomerate. A lack of incision and deposition of the mid-Morrison conglomerate in the southern Henry Basin, suggests that base level may not have dropped below. fluvial equilibrium in that part of the basin where subsidence rates likely were too high, and kept base level from dropping below fluvial equilibrium. In other regions where subsidence rates were not as rapid, base level dropped after deposition of the Salt Wash Member, and fluvial incision ensued. Following incision, deposition of the mid-Morrison conglomerate represented a basin- ward shift in facies. Evidence of a depositional hiatus between deposition of the Salt Wash and Brushy Basin Members within the southern Henry Basin is lacking due to a . distinct lack of mature paleosols in the upper Salt Wash Member.. Reasons for this are discussed in the paleosol section (section 5.3). Another puzzling question that arises is the discrepancy in depositional hiatus and aggradation between the distal and proximal. This will be addressed in the discussion section (section 6), after basin subsidence is addressed (Section 5.2). The mid-Morrison conglomerate represents deposition by low-sinuosity single-thread channels that re-occupied low-lying areas relict Salt Wash channels or incised 87 Shootaring Canyon Southern Henry Basin Poorly developed Mid-Morrison Paleosol ? Figure 5.2. {A) Stratigraphy of the middle Morrison Formation in upper Shootaring Canyon in the southern Henry Basin. (B) Close up of the Salt Wash - Brushy Basin contact showing the paleosols (insets) near the contact. Low paleosols maturities do not suggest a prolonged depositional hiatus in deposition between the two units that correlates to the unconformity in the distal Morrison Basin. (C) The top of the Salt Wash Member in the northern part of the canyon shows a fluvial channel deposit with well- developed lateral accretion sets that suggests deposition by meandering rivers, and thus a change in fluvial deposition style and the base of the Brushy Basin Member. (E) Close inspection of the contact between these two units also shows a very poorly developed paleosol. Amalgamation of channel deposits (F) suggests continuous deposition and aggradation oftluvial deposits instead. Furthermore, vertical stacking patterns of these deposits suggest that the strata( architecture was influenced by high rates of subsidence in the southern Henry Basin that likely drove continuous deposition in at least this region. 88 Qm Qm A Qm B I ...... ' ." ...... \ If. "1 \ legend (Plots A. B) Clfain .sl 1c (Plot C) • e v... • Mkl·Monlson e e Uppffs.dt W.1\hY.errber • Medium Lt e fine c Figure 5.3. (A) Data from Currie (1998) re-plotted shows that the Salt Wash Member evolves from quartz and feldspar rich source (lower Salt Wash in orange) to a chert-rich source (upper Salt Wash in red). Brushy Basin Member deposits are in blue. This compares well with the chert-rich specimens (B) examined in this study from predominantly the upper Salt Wash Member. (C) The same data in (B) is plotted showing the grain size of each specimen as proportional to circle diameter. This clearly shows a direct correlation between increased lithic chert content and increased grain size. This is the basis for the pulsed chert source that is a reflection of the gradual yet episodic gravel deposits in the Salt Wash Member. 89 paleovalleys after deposition of the Salt Wash Member. The conglomerate represents the first occurrence of abundant red and green chert pebbles and may represent sourcing new source material from the hinterland. Upstream, the mid-Morrison conglomerate is thicker than at downstream localities (8 meters at Swap Mesa, versus 4 meters at Horse Bench). This suggests that incision was in response to an upstream influence on base level that may be tied to upstream crustal uplift, rather than to downstream fluctuating lake levels Deposition of the mid-Morrison conglomerate and the Brushy Basin Member represents rejuvenated rise in base level. Increased isolation of channel deposits within the Brushy Basin Member, suggests even higher base levels that may reflect higher rates of basin subsidence than during deposition of the Salt Wash Member. 5.2 Basin Subsidence Robinson and McCabe (1998) characterized the deposition of the Salt Wash Member in a differentially subsiding basin, whereby, the basin was "hinged" at the north and subsided in the southern regions. Therefore, subsidence rates increased in a southerly tract, with maximum subsidence rates in the southern Henry Basin. They computed time-averaged basin subsidence rates of 0.0062 mm/yr in the north and extrapolated it to the southern Henry Basin to 0.022 mm/yr, based on decompacted sediment thicknesses. While this maximum subsidence rate is too low for a typical foreland basin that average between 0.12 and 0.18 mm/yr (Allen and Allen, 2005), it is still a relatively high subsidence rates for an intracratonic basin not associated with rifting. These subsidence rates were calculated using fission track isotopic dates (Kowallis, 1987) obtained from the underlying Tidwell Member and near the base of the Brushy Basin Member and used , them as bounding dates. These dates were, however, later recanted (Kowallis, oral communication, 2007). More precise 40 Ar/3 9Ar Laser-fusion ages from 3 meters above the base of the Brushy · Basin yielded a date of 150.3 Ma (Kowallis et al., 1998) (Figure 2.5). Similarly 40Ar/39Ar from a stratigraphic horizon just above a green, glauconitic shale near the base of the Morrison Formation in the Dinosaur National Monument was dated at 153 Ma (Dawson, 90 1971). Since only the Tidwell Member is known to contain marine influenced deposits in this region, the shale can be assumed to belong to the upper Tidwell Member. These two dates pin the duration of sedimentation of the Salt Wash Member to a maximum of 2.7 million years, not counting lost the time at the bounding unconformities below and above the member, as well as the time required to deposit the lower 3 meters of the Brushy Basin Member. Therefore, this duration was be used to calculate a minimum, time- averaged subsidence rate during deposition of the Salt Wash Member at 0.039 mm/yr in the Freemont River area, and extrapolated to 0.121 mm/yr in the southern Henry Basin. These newly calculated subsidence rates are almost one order magnitude faster than the previously calculated rates, and have rather large implications regarding fluctuating base levels. The high rate of subsidence in the southern Henry Basin may have been responsible for having kept base level in that region high despite dropping elsewhere in the Morrison Basin. This would allow for continuous aggradation in the southern Henry Basin, while downstream areas where subsidence rates were lower, may have undergone fluvial incision due to lowered base level. 5.3 Unconformity Paleosols At vanous locations, the base and top of the Morrison Formation are marked by especially well-developed paleosols that indicate prolonged periods of little to no sedimentation and exposure of the underlying deposits to pedogenic processes. These processes include ·thick accumulations of calcium carbonate nodules . and horizons, illuviation of clay and iron oxides, extensive burrowing by arthropods and mammals, dense root structures and formation of prismatic peds. Additionally, in areas fringing the Henry Basin, the contact between the Salt Wash and Brushy Basin Members is also characterized by similarly mature paleosols such as in the Emery High west of the San Rafael Swell, Utah, and further downstream near Fort Wingate National Monument in Colorado (Demko et al., 2004). The maturity of these paleosols, combined with magnetostratigraphic evidence of distinct paleopoles for each member in downstream reaches of the basin, has led to the interpretation that the Morrison Basin underwent a 91 prolonged depositional hiatus in between deposition of the Salt Wash and Brushy Basin Member (Steiner et al., 1994). Unfortunately, paleomagnetic data could not be used to validate a distinction in paleopoles between the two units in the southern Henry Basin. However, similarly mature unconformity paleosols, are distinctly absent in the southern Henry Basin where the Salt Wash Member is thickest. The absence of these paleosols in the southern Henry Basin leads to one of three conclusions: 1) the paleosols are present but were not encountered, or are manifested differently in this part of the basin and not recognized in the field; 2) following deposition of the Salt Wash Member, widespread geomorphologic landscape degradation erased the record of an unconformity-related paleosols, and ensuing deposition would have placed the Brushy Bain Member on top of an erosional truncation surface as opposed to unconformity paleosols, or; 3) the paleosols in this region were never developed to the levels seen in fringing areas that would suggest that deposition there was continuous while the rest of the basin clearly underwent sediment starvation and an extended depositional hiatus. Extensive field investigation in the southern Henry Basin, including 13 measured sections in this region alone, did not yield an unconformity paleosol with similar levels of maturity to those in the Emery High region (Figures 4.14, 5.2). Therefore, it can be assumed with a good level of confidence that such paleosols do not exist in the southern Henry Basin and that a depositional hiatus of significant duration did not occur in this part of the basin. Furthermore, evidence of landscape degradation in the Salt Wash Member is sparse and of limited nature suggesting that erosion of Salt Wash deposits, was perhaps limited to local fluvial incision and not widespread geomorphic degradation (Figure 4.18). The strata! architecture in the southern Henry Basin suggests that sedimentation was continuous between the Salt Wash and Brushy Basin Members (Figure 5.2) and further suggests continuous sedimentation in this part of the basin. Therefore, the lack of unconformity paleosols, and lack of a major erosional truncation surface(s), suggests that sedimentation in the southern Henry Basin was continuous, and that the changes in 92 sedimentologic regime between the tWo members is a reflection of allogenic, or extra- basinal changes, that may be related to changes in the hinterland and source area. If the Henry Basin did ultimately receive continuous sedimentation, then some reason must account for the disparity in the depositional hiatus observed in fringing areas and regions downstream an the continuous sedimentation in the proximal southern Henry Basin. Elevated subsidence rates in the southern Henry Basin may have lead to retraction of sedimentation to the proximal regions and continuous accumulation of thick deposits, while starving distal downstream basin from sediments. This scenario can produce so- called distal unconformities while proximal regions accumulate sediment at high rates (Marr et al., 2000; Allen and Allen, 2005). The fact that this region received the thickest Salt Wash deposits anywhere may also suggest that sedimentation never ceased in the southern Henry Basin, even when other regions were clearly receiving little to no sediment input. 5.4 Provenance · Recalculated petrographic data (Tables 4.1 and 4.2) were plotted on various ternary diagrams (Figure 4.20) for analyses. Middle Morrison Formation sandstones plot within the continental block/recycled orogen regions of the provenance ternary diagrams (Dickinson et al., 1983), with subtle differences between the members of the Morrison Formation (Figure 4.20 A). Salt Wash sandstones are spread over both provenance fields, whereas the mid-Morrison conglomerate and Brushy Basin sandstones are concentrated in the recycled orogen field. Analysis of total lithic rock fragment, feldspar, and monocrystalline qµartz, shows that the mid-Morrison conglomerate is especially rich in lithic content, and. virtually all the Morrison Formation sandstones are poor in feldspar content (Figure 4.20 B). When chert was accounted as the only lithic fragment, the data illustrates a shift from quart-rich sources for most of the Salt Wash Member, to sources exceedingly rich in diagenetic chert for the mid-Morrison conglomerate, while the Brushy Basin Member sourced rocks 93 with roughly equal amounts of quartz and chert (Figure 4.20 C). In addition, the sandstones rich in quartz also contain higher abundances of feldspar, whereas deposits that are rich in chert are much more depleted in feldspars. This mutual exclusion of the two mineral groups (quart + feldspar, versus chert) suggests two sediment source rocks for the Morrison Formation that shifted from quartz and feldspar sources to a chert-rich source. The chert was likely derived from Paleozoic chert-bearing limestones and dolostones that are abundant and exposed in the Sevier thrust belt in western Utah and Nevada today (Stokes, 1944; Peterson, 1987; Yingling and Heller, 1992). Upper Salt Wash sandstones contain slightly higher amounts of volcanic rock fragments (Figure 4.20 D} and are relatively depleted in tectonized (sutured) quartz (Figure 4.20 E); Interestingly, the mid-Morrison conglomerate and lower Brushy Basin Member are almost devoid of volcanic rock fragments in light of the fact that deposition of the Brushy Basin Member is often attributed to the onset of volcanism in the hinterland, based on the numerous volcanic ash beds found within. These volcanoes must have been limited to the far reaches of the hinterland, or that rivers that sourced these volcanoes likely drained westward into the proto-Pacific Ocean, and not into the Morrison foreland basin. Salt Wash Member conglqmerates and the mid-Morrison conglomerate are also notably richer in tectonized quartz. Re-plotting petrographic data of the Morrison Formation from Currie (1998), shows that the chert component increases up-section in the Salt Wash Member, while the quartz and feldspar components correspondingly decrease (Figure 5.3 A). This increase continues with deposition of the mid-Morrison conglomerate (Figure 5.3 B). Analysis of the lithic fragment content, predominantly chert in this case, with relative grain-size, shows an interesting correlation, one of increased chert content with increased grain size (Figure5.3 C). Despite having employed the Gazzi-Dickinson method in the petrographic point counts, which .is supposed to eliminate grain-size related changes in composition, it is evident that coarse-grained specimens are more abundant in chert. Several scenarios may be able produce such a relationship. 94 Preferential weathering of chert-rich source rocks could have been associated with an increase in fluvial discharge that could have transported the coarser, chert-rich sediments. However, the Paleozoic · chert-bearing carbonate source rocks are extremely well consolidated deposits owing to near ubiquitous certification. These units are much harder, and more resistant to weathering than the Proterozoic, Paleozoic and Lower Mesozoic quartz-rich and arkosic sandstones of the Sevier thrust belt in Utah and Nevada, that are the likely sources of quartz and feldspar for the Morrison Formation deposits (Armstrong and Cressman, 1963; Sippel, 1982; Allmendinger, 1983; DeCelles and Burden, 1992). An increase in precipitation in the hinterland, and ensuing higher fluvial discharges are, therefore, unlikely to preferentially erode out these chert-bearing units and deposits coarser chert-rich deposits in the Morrison Formation. Therefore, the shift towards chert- rich deposits in the Morrison Formation could not have been a climatic response to changing precipitation patterns in the hinterland. Since the chert increase is only seen in pulses of gravelly deposits that increase up- section, this is also unlikely to produced by a gradual unroofing of a chert-rich source rock by a thermally, or isostatically rebounding source region to the west. This would more likely produce a more gradual and increasing chert-signal and regardless of grain size, contrary to what is observed. The pulsed nature of the chert is more in line with thrusting rather than gradual unroofing of the hinterland. Another explanation for the chert-rich conglomerates comes from temporary upstream storage of coarse-grained chert-rich sediments in a proximal foredeep basin. This could have provided a ready sediment source that was remobilized during high discharge events and carried deeper into the basin as conglomeratic deposits. Crustal thrusting in the hinterland may have brought up these Paleozoic chert-rich source rocks to the surface, and also provided the high subsidence rates required. to trap these deposits in a rapidly subsiding foredeep. This scenario would argue for require tectonic thrusting in the hinterland to provide the chert source, and to trap sediments proximally. The abundance of pedogenically altered chert grains/clasts in the mid-Morrison conglomerate, strongly suggest that these sediments may have been previously deposited upstream and exposed 95 to pedogenic alteration, such as observed in paleosols developed in channel sandstones (Figure 4.22). With ensuing high discharge events associated with dropping base level, these upstream deposits would have been remobilized, and deposited along with freshly sourced and unaltered chert grains in the mid-Morrison conglomerate. Therefore, a drop in base level after deposition of the Salt Wash Member would have been concomitant with fluvial incision, scouring of upstream deposits, and deposition of the mid-Morrison conglomerate downstream. See Appendix III for complete petrography data. 5.5 Paleomagnetic Interpretation: Why is the Salt Wash Member magnetically mono-polar? From the magnetic polarity results (Figure 4.24) a virtual geomagnetic paleopole is calculated for the study area and compared to previously computed Late Jurassic paleopole positions from the Colorado Plateau. This produces a paleopole at a location of 84.24 degrees north latitude, and 131.31 degrees longitude with a 95% confidence ellipse of 7.32 degrees site-to-pole semi-axis, and 10.41 degrees semi-axis perpendicular to the great circle (Figure 5.4). This geomagnetic pole coincides with a middle Cenozoic paleopole, between the Eocene and Miocene Epochs (Butler, 1998), and suggests that the magnetic polarity of the Salt Wash Member deposits was reset during the middle Cenozoic. Throughout the Salt Wash Member, and especially near the top of the member, numerous channel sandstone deposits are bleached white, and contain . an abundance of spherical hematite concretions, also known as Moqui Marbles (Figure 5.5). These diagenetic concretions formed long after deposition and lithification of the sediments, via fluid infiltrations that dissolved and mobilized iron oxide minerals, predominantly hematite, from certain regions, and concentrated the minerals at other locations in the form of these concretions (Chan, oral communication, 2006). Beitler et al., (2005) performed detailed geochemical analyses of bleached sandstone units in the lower Jurassic Navajo Sandstone in south-central Utah, and found strong evidence for ground water/fluid infiltrations combined with reducing conditions that were responsible for dissolving the iron oxides 96 North American Apparent Pole Wander Path Lower and Upper Morrison Formation paleopoles, from Steiner et al., 1994 Calculated site-mean virtual geomagnetic pole for Salt Wash Member: Latitude: 84.24 Longitude: 131 .31 Site-to-pole great circle semi-axis= 7.32 degrees semi-axis perpendicular to great circle = 10.41 degrees Figure 5.4. Calculated virtual geomagnetic _pole from analyzed cores of the Salt Wash Member in the Henry Basin, Utah, plots in a region that correlates to a mid-Cenozoic paleopole (Eocene- Miocene ). Morrison paleopoles from Steiner et al., (1994). Figure adopted from Butler (1998). This suggests that the Salt Wash Member in the Henry Basin was reset during the middle Cenozoic between the Eocene and Oligocene Epochs. 97 Figure 5.5. Bleaching of Salt Wash sandstones and occurrence of diagenetic hematite concretions suggest post depositional fluid infiltrations that dissolved, migrated and re-precipitated hematite, the presumed magnetic signal carrier, during the middle Cenozoic and was ultimately responsible for resetting the magnetization of the sandstone-rich Salt Wash Member, · thus yielding a mid- Cenozoic paleopole. 98 and hydroxides from one region and concentrating them in other areas where oxidizing conditions prevailed. This resulted in the precipitation of iron oxide concretions in dense clusters. Furthermore, bleaching of the sandstones was found to occur primarily in areas proximal to Laramide-aged structural folds and/or Mid-Cenozoic igneous intrusions associated with emplacement of the Oligocene-Miocene Henry Mountains Coincidentally, a Laramide feature; the Waterpocket Fold, and the Henry Mountain laccoliths are within very close proximity of the study area (Figure 5.6). Therefore, the Salt Wash Member of the Morrison Formation may have undergone similar diagenetic alterations as the underlying lower Jurassic Navajo Sandstone. The dissociation of the iron oxides, the primary magnetic signal carriers, may have erased the Late Jurassic magnetic .polarity of the host Salt Wash Member deposits. During subsequent re- precipitation of these minerals, magnetic polarities would have been reacquired as chemical remnant magnetization with mid-Cenozoic pole directions. Although the loss of the paleomagnetic information from these deposits deems magnetostratigraphic analyses useless within the study area, the Cenozoic polarity of the Salt Wash Member may be used as an independent verification of the timing of the migration of these fluids through the Salt Wash Member, and precipitation of the hematite concretions between the Eocene and Miocene Epochs. 99 Oblique View of the Henry Basin Waterpocket Satellite Image: Google Earth Fold Henry Moutains Bleached sandstones Morrison in proximty of fold Formation Figure 5.6. Proximity of sampling sites to the Henry Mountains and Waterpocket Fold might have contributed to abundance of fluid infiltrations (Beitler, 2003) and resetting of the Salt Wash Member magnetic remanence during folding of the Laramide Waterpocket Fold, or during emplacement of the Oligocene-Miocene Henry Mountain laccoliths. 100 6. DISCUSSION The following summarizes the main observations in the Morrison Formation in south- central Utah: 1) 160 meters of Salt Wash Member deposits accumulated in the southern Henry Basin and correlate to zero meters in the Emery High where extensive pedogenesis of underlying middle Jurassic strata is observed; 2) Salt Wash Member deposition prograded northeastward into the Henry Basin; 3) increase in upper flow regime structures up-section in the Salt Wash Member documents either an increase in total water discharge flux, or possibly an increase in flashy discharge episodes; 4) computed subsidence rates of 0.121 mm per year in the southern Henry Basin documents that subsidence was over five times the previously reported subsidence rates; 5) petrographic data show an up-section increase in chert that is directly associated with an increase in grain size, between the lower Salt Wash Member and the mid-Morrison conglomerate; 6) the Salt Wash Member onlaps northwestwardly onto a structural high in the Emery High region; 7) aggradation of the Salt Wash Member deposits signals deposition under dominantly high base level conditions, 8) paleocurrent data in the Salt Wash Member suggests dominant northeast directed fluvial transport, and a low divergence of lenticular mid-Morrison conglomeratic deposits suggests deposition by along narrow incised valleys; 9) there is no paleopedologic evidence of a depositional hiatus in between deposition of the Salt Wash and Brushy Basin Members in the southern Henry Basin that correlates to an unconformity in the distal Morrison Basin, and; 10) changes in fluvial deposition between the Salt Wash and Brushy Basin Members were accompanied by fluvial incision and deposition of the mid-Morrison conglomerate in response to a drop in base level. In order to better understand the evolution of deposition in the Morrison Formation, a holistic approach has to be attempted that can account for all the observations above. The main question that arises from these observations refers to the discrepancy in the depositional hiatus between the proximal basin and distal basin. That is, how can sediments accumulate continuously in the proximal reaches of the Morrison Basin while the distal reaches experiences a prolonged depositional hiatus, long enough to record a 101 change in paleopole position? The evolution of fluvial deposits between these two members of the Morrison Formation within the study area, has to be reflected in all the observations, and is likely the result of more than one factor, as opposed to a single cause. Unidirectional paleocurrent data throughout the Salt Wash Member and within the entire study area (Figure 4.19) combined with extensive pedogenesis of Middle Jurassic deposits in the Emery High, strongly suggest the presence of a structural barrier to river flow and deposition in that region. The presence of the Tidwell Member in virtually all localities within the Emery High, arid the near total absence of the Salt Wash Member there, suggests that this region could not have been a barrier to flow until after deposition of the Tidwell Member. The Emery High, therefore, either did not subside at all, or subside fast enough, relative to the southern Henry Basin where thick deposits of the Salt Wash Member deposits accumulated. Prolonged exposure of the Summerville Formation and Tidwell Member deposits in the Emery High resulted in moderately to strongly developed paleosols at these horizons during deposition of the Salt Wash Member in the southern Henry Basin (Figure 5.1). Differential subsidence in the Henry Basin channeled the majority of deposition of the Salt Wash Member to the southern Henry Basin, where approximately 150 meters of deposits accumulated. Changes in stratal architectures and channel body geometries within the Salt Wash Member also suggest that overall subsidence rates likely increased with time . (Robinson and McCabe, 1998). Time-averaged subsidence rates calculated from radiometric dates from ash beds in the Morrison Formation (Kowalis et al., 1998), show that the southern Henry Basin was subsiding at a minimum of 0.121 mm per year, and over five times the rate previously calculated by Robinson and McCabe (1998) using older.fission tracks dates that were later recanted (Kowalis, oral communication, 2007). This higher rate of subsidence compares with foreland basin subsidence rates that average between 0.050- 0.18 mm/yr (Robinson and McCabe, 1998; Allen and Allen, 2005). This rate may have corresponded to early to middle stages of foreland basin subsidence, that suggest possible tectonic thrusting and crustal loading in the hinterland to the west. The 102 . subsidence may have been in addition to regional depression of the crust from dynamic subsidence from subduction-related interactions of the margin of the continent (Lawton, 1994). Cant (1998) attributed subsidence rates of 0.040 mm per year to active thrusting in the Cordillera, west of a similar depositional system of the lower Cretaceous Manville Group in Alberta, Canada. The higher subsidence rates in the Morrison Basin, suggests that thrusting may have been active in the Morrison hinterlands. Camilleri et al., (1997) and Currie (1998), calculated over 40 kilometers of displacement in the Canyon Range thrust in central Utah that pre-dated Aptian times. Such displacement and timing of thrusting could have provided the crustal loading to locally, and rapidly subside the southern Henry Basin in the Late Jurassic, while dynamic subsidence may be called on to explain the wide-spread, regional subsidence that allowed accumulation of deposits over such a large basin within interior western North America. Tectonic thrusting also better explains why the southern Henry Basin was rapidly subsiding, while a relatively short 75 miles to the north, the Emery High clearly was not. Deposition of the Salt Wash Member in a back bulge basin was proposed by Currie and DeCelles (1996), and Currie (1998). However, some observations in the Morrison Formation do not support such a scenario. Paleocurrent data do not suggest any topographic influence on river flow directions due to the presence of the Emery High. Although the topographic relief of this feature may have kept it above fluvial deposition, it is unlikely to have had a significant effect on river flow directions had it been present. .It would have had to . remain stranded just above the fluvial profile and exposed to pedogenic processes. Although, crustal kinematics and flexure do not support the formation of a significantly large forebulge without the deposition of some 3 to 5 kilometers of sediments in the foredeep basin to the west (Heller and Paola, 1989; Heller et al, 2003), onlap of the Salt Wash Member onto the Emery High strongly argues for syntectonic basin subsidence with relative uplift of the Emery High. The foredeep required to produce a forebulge over 100 meters high may be explained with the deposition of over 2000 meters of preserved shale and mudstone of the Middle Jurassic Arapien Shale and Twist Gulch Formation in the Utah-Idaho trough basin, approximately 103 30 miles to the west (Hintze, 1988; Bjerrum and Dorsey, 1995). These were deposits were undoubtedly thicker prior to erosion along the K-1 unconformity with ensuing uplift during the Cretaceous Period. The missing deposits would have contributed further to the depression and flexure of the crust and in the formation of the Emery High forebulge. Petrographic analyses of channel deposits of the Morrison Formation show steadily increasing chert content up-section, while quartz and feldspar content declines (Figure 5.3 A, B). Furthermore, analysis of lithic chert content with corresponding grain size shows a direct correlation between increased lithic chert content and increased grain size, with both increasing up-section (Figure 5.3 C). Introduction of new, coarser, chert-rich sediment source suggests an increase in the sediment gravel fraction that would have major effects on deposition of the Salt Wash Member. The rapid change in sediment source also suggests that thrusting in the hinterland may have exhumed Paleozoic chert- bearing limestones and dolostones, and changed sediment influx content from quart-rich to chert-rich. Numerical modeling of foreland basin deposition by Marr et al. (2000) generated various cross-sections of depositional packages and stratal architectures in foreland basins in response to varying certain major forcing variables. These models also show dramatic differences between depositional cycles and erosion/sediment starvation between the proximal and distal reaches of the basin (Figure 6.1 ). The models produced deposition in response to sinusoidally varying amplitudes of four dominating variables: sediment flux; water flux; subsidence rates; and gravel fraction, and examined the different responses in sedimentation and erosion in different parts of the basin. These models were used to compare deposition in the Morrison Basin. and examining variables that may lead to the production of a stratal architecture similar to that observed in the Morrison Basin (Figures 6.2 and 6.3). Field collection of data required for sediment flux analyses in the Morrison Formation is far more involved than the scope of this study allows, and so comparing changes in sediment flux changes in the Morrison Formation was not attempted. Furthermore, owing to an alluvial nature and little age control in the Salt Wash Member, presents a challenge in determining whether sedimentation cycles within the 104 Cross-sections 0.1 [8] 0.0!5 ! 0 Los> -I -0.1 c .. >< sediment flux E" ·0.15 'i;;:.. so 75 100 0.5 1.5 distance (km) lllVlr [g 0.1 0.05 0 !c t0!5.. GF -0.1 FT -0.15 water flux so 75 100 0.5 1.5 .dlltanc:e(km) 0.1 .§-0.os j • -0.1 subsidence .0.15 75 100 1 1.5 dl&ta.:. (km) [8] Distal 11Tv1r::::::: =-.- Unconformity . .., •• - .. , ·., o'\.. ··· ·-··,·., !? ,·' ··- ·- · .':..- ...... ,.,. ··lo·-·-·-·- ..... I!! ...... _,_ ...... '. . _,.·' ...... '. .. ..'.. - '··...... · ·'· '··...... -' gravel fraction 0 75 100 o.s 1 1.5 lllVlr Figure 6.1 Numerically generated cross-sections of basin stratigraphy from fast forcing periods from Marr et al. (2000). Forcing variables are (A) sediment flux, (C) water flux, (E) subsidence and (G) gravel fraction. Also shown are the phase relationships of the gravel front position (GF), fluvial toe position (FT), proximal accumulation (PA) and distal accumulation (DA) relative to forcing for each boundary conditions with vertical variation about its mean value. 105 Foreland Basin Model WEST EAST Proximal Distal Hinterland Foreland Basin Foreland Basin Progradation of Salt Wash Deposition A ) Low reg ioanl dynamic subsidence rates creats anough accomodation space for depsotion of ndwell and Dynamic Subsidence of crust lower Salt Wash Members of Morrison Formation. from subduction to west l Progradation of proximal Retrogradation Salt Wash deposition offluvial toe results in » a depositional hiatus B ( Tectonic loading increases basin subsidence l during depsotion of upper Salt Wash Member. Combined with an increase in gravel fraction causes progradation of proximal depsotion, Dynamic Subsidence and retrog radation of fluvial toe. Depostional hiatus ensues in distal basin. and Tectonic Thrusting l Incision of upper Salt Wash Member followed by depostionof the c mid-Morrison conglomerate Isostatic rebound of crust following hiatus in thrusting lowers base level and ensuing degradation of upper Salt Wash Member. Progradation of Brushy Basin » D Renewed subsidence from onset of subduction to the west and regional dynamic subsidence \ accomodates deposition of Brushy Basin Member. Dynamic Subsidence l 106 Figure 6.2. Depositional model for the Morrison Formation in south-central Utah. This model predicts continuous sedimentation in the proximal basin, while the distal basin experiences a prolonged depositional hiatus. It also explains the incision of the upper Salt Wash Member with halting in thrusting in the hinterland. The model however cannot explain the westward onlap of the Salt Wash strata onto the Emery High. 107 Foreland Basin Model with a Back Bulge Basin WEST EAST Hinterland Foreland Basin Forebulge Backbulge Basin Thrusting in Hinterland A Deposition of Salt Wash Member in foreland basin, and back bulge basin. Rate of forebulge uplift= agradation of deposits. i l Uplift of forebulge from crustal deformation ! Subsidence of foreland basin from crustal loading 1 Depositional hiatus in Morrison Basin and drop in base level from isostatic rebound of crust after hiatus in thrusting. B Degradatioj of Upper Salt Wash Member. l Downwarp rfbound i of forebulge Isostatic rebound of crust after hiatus in thrusting l Deposition of Mid-Morrison conglomerate and Brushy Basin Member ( after onset of dynamic subsidence l l Regional dynamic subsidence of crust l from onset of subduction to west 108 Figure 6.3 Depositional model of the Morrison Formation in south-central Utah with a back-bulge basin. This model explains the westward onlap of the Salt Wash Member onto the Emery High forebulge, but does not explain the difference between proximal deposition in the southern Henry Basin, and the depositional hiatus in the distal basin. 109 Salt Wash Member correspond to rapid or slow forcing of the four variables in question. However, given that only 150 meters of Salt Wash Member deposits accumulated in the southern Henry Basin, the forcing parameters are assumed to correlate to rapid forcing periods. Of the four forcing factors, changes in subsidence rates and sediment gravel fractions produced results that are very similar to those observed in the Morrison Formation. The models show that an increase in basin subsidence results in accumulation of proximal deposits, while starving the distal basin of sediments. This can produces a sequence boundary-like unconformity that may be analogues to the distal unconformity observed in the Morrison Formation, while the proximal southern Henry Basin continuously accumulates sediments without a hiatus (Figure 6.1 E, F). Furthermore, increased sediment gravel fractions also showed a strong response by accumulating, and prograding, proximal deposition, while forming a distal unconformity similar to that in the basin subsidence response cycle (Figure 6.1 G, H). Modest increases in water discharge in the Salt Wash Member are reflected by an increase in upper flow regime sedimentary structures in the upper Salt Wash Member. The models show that an increase in water discharge has the effect of prograding the gravel front, although proximal accumulation rates drop, while distal accumulation rates increased (Figure 6.1 B, C). This is in contrast to what is observed in the Salt Wash Member, where contemporaneous proximal accumulation and distal starvation are recorded. This discrepancy may be due to the low amplitude of change in water discharge in the Morrison Formation, or that episodic short-lived flashy discharge events resulted in the progradation of only isolated gravely deposits in the distal upper Salt Wash Member and not widespread progradation of the Salt Wash Member. The increase in upper flow regime flow observed in the Salt Wash Member may most likely be the result of funneling overall water discharges into better developed river channels. This is reflected . in an up-section increase in the number of ribbon channel deposits and a decrease in sheeted sandstone deposits in the Salt Wash Member with more developed floodplain paleosols. Therefore, the increase in water discharge observed in the Salt Wash Member, 110 may be a local artifact, while overall fluvial discharge may not have changed much, and thus may not be a major contributor to the strata! architecture of the Salt Wash Member. Elevated subsidence rates in the proximal Morrison Basin coupled with an increase in gravel fraction of the sediments may have played large roles in continuously accumulating and prograding deposits into the proximal basin, while starving the distal Morrison Basin and forming a depositional hiatus. The mature paleosols in the upper Salt Wash Member deposits of the distal basin may have formed while the southern Henry Basin continued to accumulate sediments. Therefore, sedimentation in the proximal basin, may have been continuous during the evolution between the Salt Wash and Brushy Basin Members, while the distal basin recorded a prolonged hiatus, during which mature paleosols formed Ethridge et al. (1998) suggested that downstream base level changes have less of an impact on the strata! architecture of the proximal basin than they do on the distal basin. The proximal basin is more strongly influenced by upstream controls than to downstream ones whose may not be felt upstream, or to the magnitude that they would have on downstream reaches. Therefore, thicker localized incision of the upper Salt Wash Member in the more proximal areas may reflect an upstream control on landscape degradation that was followed by deposition of the mid-Morrison conglomerate. Within the proximal southern Henry Basin, a few conglomeratic deposits in the lower Brushy Basin . Member deposits strongly resemble those of the mid-Morrison conglomerate in composition, morphology, and sinuosity patterns (Figure 6.3). These conglomerates may have been deposited contemporaneously with the mid-Morrison conglomerates downstream. Therefore, while the distal basin was starved of sediment and the mid-Morrison paleosols where developing in these regions, the proximal basin was still aggrading fluvial deposits while evolving into deposition of the muddier deposits of the Brushy Basin Member. The onset of deposition of the mid-Morrison conglomerate therefore, may not have occurred until after the change in fluvial style from the Salt Wash Member to the Brushy Basin Member in upstream areas. Initial phases of deposition of 111 the Brushy Basin Member may not have prograded far into the basin, so that only the proximal reaches accumulated sediments while the distal reaches continued to experience sediment starvation. This suggests that the transition between the two members was not contemporaneous. Laboratory experiments by Tai and Paola (2007) showed that increased floodplain vegetation had a dramatic effect on the evolution of the rivers. Increased vegetation density channeled water flow from shallow, braided streams into sinuous single-thread channels. Within the Morrison Formation, there is evidence of climatic wetting and decreased seasonality between the Salt Wash and Brushy Basin Members that may have played a role in the change in fluvial sedimentation style. The wetter climate would have allowed for increased perennial floodplain vegetation.density that increased bank stability and affect river flow patterns. It is probably unlikely that the gradual change in climate observed could have solely influenced this major change in fluvial sedimentation, though it could have played a role. Another probable cause for the evolution of fluvial sedimentation may have also been tied to accommodation space. Subsidence of the southern Henry Basin would have channeled river flow to low-lying areas that contributed to the braided nature of the rivers. As the basins filled up with deposition of the Salt Wash Member, lateral accommodation increased restricting flow to fewer channels and forcing them to meander. This exposed floodplain deposits to longer durations of pedogenesis, and formed more mature paleosols. The change in sediment type from the introduction of chert-bearing Paleozoic carbonates, with the introduction of ash falls from the volcanic arc to the west, likely contributed to the muddier sediments of the Brushy Basin. The onset of volcanism, also suggests that subduction processes began which would have acted to induce dynamic subsidence of the·basin, thus elevating overall basin subsidence rates, that resulted in the isolated channel deposits of the Brushy Basin Member. The complexity and heterogeneity of the transition between the Salt Wash and Brushy Basin Members is probably accentuated by the large size of the basin where other 112 allogenic signals may have been amplified, or had a larger role in the evolution of the Morrison Rivers including the poorly known effects of dynamic subsidence. Multiple factors likely played a role in the evolution of fluvial deposition in the Morrison Formation, including fluctuating subsidence rates, sediment gravel fractions, decreased aridity and seasonality, increased density of floodplain vegetation, increased accommodation space, introduction of dynamic subsidence, and also likely to increased mud content that accompanied volcanic ash fall from the arc to the west. 113 Mid-Morrison conglomerate? ...... """...... Paleoflow, oblique out of page Figure 6.3 . Within the proximal reaches of the Morrison Basin (A), some conglomeratic deposits in the lower Brushy Basin Member very closely resemble those of the mid-Morrison conglomerate (Band C). These units may be upstream correlatives of the incised conglomeratic units at Horse Bench, south of Green River, Utah. The conglomerates in the southern Henry Basin are, however, not incised into upper Salt Wash deposits. This may be because fluvial deposition in this area did not cease after deposition of the Salt Wash Member, and onset of deposition of the mid-Morrison conglomerate ensued after deposition of the lower most Brushy Basin Member deposits to the south. 7. CONCLUSION The evolution of the fluvial deposition in the Morrison Formation from amalgamated, sandy, braided rivers of the Salt Wash Member to muddy, sinuous rivers, of the Brushy Basin Member was not synchronous throughout the basin as once thought. Thorough field investigation in the proximal basin in south-central Utah, shows a lack of well- developed paleosols separating the two members that might signal a depositional hiatus. Furthermore, minimal fluvial incision in the upper Salt Wash Member in the southern Henry Basin suggests that little geomorphic landscape degradation followed deposition of the Salt Wash Member that would have eroded away evidence of a depositional hiatus in the proximal basin. The depositional hiatus expressed in distal unconformity of the Morrison Basin, therefore, may not correlate to an unequivocal proximal unconformity in the southern Henry Basin, where sedimentation may have been continuous instead, with accumulation of thick Salt Wash Member deposits. High subsidence rates, and increased gravel fraction of the sediment during deposition may have contributed to aggrading proximal deposits while contemporaneously starving the distal basin. This evidence, . combined with the evolution of sediment sources from quart-rich to chert-rich sources suggests that syntectonic thrusting in the hinterland likely played a major role in the evolution of sedimentation in the Morrison Formation. The mid-Morrison conglomerate, . . is an incised, valley-fill fluvial unit deposited on top of the Salt Wash Member, and suggests a drop in base level and basin-ward shift in facies that followed deposition of the Salt Wash Member. Downstream thinning of the conglomerates suggests an upstream control on base level and was likely induced by isostatic uplift of the.hinterland following thrusting during deposition of the Salt Wash Member and prior to deposition of the Brushy Basin Member. 115 APPENDIX I, MEASURED SECTIONS AND PALEOCURRENT DATA 116 .. B UTAH + l \K l. Cl I Y lfi· • Inset B NORTH ·:r . NORTH -..-v-.: I ,I ' t '<• MS-16• 25 miles ..,··J ,..J MS-lie' SO +miles MS-Ile MS -Jo• Hile 110°44'55" W 110°43'25" W WGS84 110°40'40" W z in.... 0 0 *"M *"M z z 0.... 0.... 0 0 "M "M z z in in 0 0 0 0 "M "M z z 0 0 0 0 0 0 "M "M z in !'l.... .;l- o ___-JM " 110°44'55" W 110°43'25" W WGS84 110°40'40" W (A) Field study area in south-central Utah showing locations of measured sections (B) and (C). Locations denote the base of the sections. See also Figure 1.1 for locations. 117 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #1 Shootaring Canyon, Southern Henry Basin Dakota Sandstone 214 '$. Brushy Basin Member 150 Plnlc·brown l 140 • Ycllow·bclgc -"V= Cre.1m-yc ll ow ') 130 'V Orange·brtr.•m • Orange-brown 'V ') = YY Ye llow-beige -'<-- Orange·brown 120 'V CrcJm·ycllow • _-v Orange-Pink .' ) 'V 110 'V Cream-pink Red·brown 'V Crcam-whftc 100 'V --v :;: OrJngc-brovm 'V Q ::::! . Cre.1m -yellow "'0 Beige :J 90 Orange -brown 1l "T1 Yellcw-he!l)e 0 ) 3 .,.;;:;: Cream-white !!! 80 o· :J 'V Cred111 -whlte 'l 'V 70 -:t...._, Beige-brown ...._, 60 'V BcilCJt'·brown Salt Wash Member ) 50 e 4i Red ·brown l Beige-brown 40 . ') 30 ') Q;uk txown A- Grt.oe n-Plnk 20 =-v 'V 'V l 'V= 10 'V 'V 'V 'V Red-brown Tidwell Member 0 VI .., VI VI VI Summerville Formation Vi VI VI VI Ol E I I u ,.!.. E u 118 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #2 Hansen Canyon, Dakota Sandstone -vi Gray-brown 255 I Southern Henry Basin Beij)e·btown 0<0,017. 330, 353, 352, ' 'V 100. 100.095,087,076, Brushy Basin Member 170 Cream-white • JOS. 025. 036, 322, 3211, 343, 00S, 142, 148 160 ' 'll tfcige-brown 00 Beige-brown 'll• 'V 150 ' ....• _.... Cream-yellow 1 140 ' l:l clgc-brown 'V Cream-white 'V 130 ' _-v Cream-white • 'V 'V 1 Crc.im-whltc 120 1l llrown ' 'V Grd'j-whlle 110 ' s: Gray-white Light beige v;· --v 0 'V Pink-brown :J 100 ' -n 'll O.:ukbrown 0 'V 'V 3 'V Gray-white !!:. 90 Ci ' Gr •1 "' 'V 1l - 40 'V Plnk-b1own 'V Pink-brown 1 .!-1l O.ukbrown 30 'V Pink-brown Or.1nge-brcwn Plnk-bro-.vn 1 1l .t Oa1kbrown 20 'V Red -brown _-v Orange-brown 'V P!nk-bmwn 'V 'V Pink-brown 10 ' 'V R 1l - '' Red-brown ' 119 Lithology & Structures Color Paleocurrents Unit Location Measured Section #3 (Thickness in meters) Shootaring Canyon, Southern Henry Basin 120 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #4 Shootaring Canyon, Southern Henry Basin 140 ' Top of exposure I 1l Red·bro'Ntl 130 1l..c. 290 • iso= Pink-whit'! Brushy Basin Member J2 ..... Grl'Cn·yellow "<,,1l Cre.\m·yellow 130 "<,, Greeo·yellow Yt>llow·brown Red ·brown Red-brown -is -"<,, Cre.lm-yellow J.10 "<,, -"<,, Cream-white I Gray-while 90 s: ...... C1 e;\m·whlt1? "<,,- ;;;· 80 _-<,, 0 -"<,, Cream-white :::J "Tl "<,, 0 'I "<,, 3 70 "<,, Gray-white 5· :::J 11-.!- Red-brown 'I 60 -""'lS A· Or.anqc·brawn Orange- brown Salt Wash Member l 50 Red-brown 'l 40 Pink·belge 30 Red-brown lOS 'I 20 Gray -brown 'I - 10 Hcd-brown 0 Vl .... Vl Vl Vl Tidwell Member Vi Vl Vl Vl Ol E .,!. I I v E v 121 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #5 Shootaring Canyon, Southern Henry Basin 130 •6 Top of exposure I 'V Ycl/ow·brcwn 120 • ' .I 'V Gray-white 110 'V Gr01y·whltc • • CreAm·whlte l 90 • Gray-white lJi-v Gray·white 355 s;: ...... lJift> Brushy Basin Member •11= Pink·'.'thlle g 70 _ 030 v;· Cre<1m -9ray 353 0 :::J 134 ,, ilil 0 3 i5' 280, 105 :::J Gr.1y-whitt' so 100, 130 130,325 Gray -white 40 ..... CrNm·ycll aw •l Gr.ly·whltc 30 347 Crc.l m-yc/low ·325.005,355 095 l 20 308 Gray- white Burgundy Purple 10 060 Yellow·beJ9e Salt Wash Member tMS Brown 3SO 0 Ill .... Ill Ill Ill Iii Ill Ill Ill Cl E I I u ,.!. E u 122 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #6 Shootaring Canyon, Southern Henry Basin Dakota Sandstone 180 120 • '<,, 132 '<,, 130. 150 Oeiae·brown 030 •I '<,, 110 Brushy Basin Member •I 100 90 '<,, o-11 Cre.:tm-whitc 097 1 80 Iii Red -brcwn • l s:: 70 -"''<,, Q . Ii!! Buruundy "'0 Red -brown :J II 1l Cream -white 'Tl 60 0 138, 159, lC.O QI3 '<,, Gray-white .... I 5· 50 132 :J 160 13 7, 128 I Gray-white 40 Crcam11ray Salt Wash Member I Orange-Pink 30 Red -brown 'V'\l '<,, 20 306 Orange-Pink l 10 Orange- Pinle. Rl!'d·brown 0 Ill .!::! Ill Ill Ill Ill Ill Ill Ol E Ill I I u ..!. E u 123 Lithology & Structures Color Pafeocurrents Unit Location (Thickness in meters) Measured Section #7 50 - 178, 132 Brushy Basin Member • Beige-brown Shootaring Canyon, ' O'lJ '] Southern Henry Basin 40 - • -ll • '<.J'.=-. -11 Cream -yellow 090 AAA Chclllltlon ripples '] Orangc·Pink 007,097 l Ripple X·lleds 30 010,053 Q 'V Trough •·bed\ "Ila> ::::!. • OrJnge-brown 091, 116 Sw.Jlcy trough x·bcds 0 • - "' Planar-tabular x·bcds :J ..... 'I Salt Wash Member 'Tl Parall el ldmina1lons 20 - Gray-white 100, 124 0 ,,..., l..ltcral sets • 172,097 3 Pebble IJgs Soft sediment deformation • C1e.1m-gr;1y 020,087 J2. ' 15· 'VY u ack!io ] 'V ll6 10 :J II Muslv-et>eeh Orange-Pink 050.0J7, 0S l,0 10 llem.11i1e concre1lon\: •«.-1 0 "' • 'V ll9 a. bparsc, lntcnsc) 'V Cream-gray ' 065,]44 « MammJ6an burrows , _-v-vl Orange-Pink Arthropod/Annelid burrO".vs: 0 {sparS<", abund.1nt, lntensc) Ill Ill Ill Root struc tures: Ill Ill Cl .q._. {sp.Jrse. abundant.. Intense) I l E u u ,!. Root halos E .iii Wood fragments "'4i e<02> Carbonate nodulc'i: @ hJrdfJtln Prlsm.ltk pcds c& Brcccla!t'd bt'ds 'X' GypHm1 Veins 1!.\\11 Vertie !Jlc.kemldes P.ilcosol maturily: nl!l lilllil Immature, moderdte, nMlure Measured Section #8 Shootaring Canyon, Southern Henry Basin 124 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #9 70 'V Bcigc--l:rown 200, I 76, 191 Shootaring Canyon, ' '\! ... Southern Henry Basin 60 ' Grccn'")'cllow 020 Brushy Basin Member Osclllatlon ripples Green-yellow Ripple x-beds 'V Troogh x-beds 50 ' Ycllow-bra:m 031 Swaley trough x-bC'ds ...... Ploln.u-tabular w-b 80 - Measured Section #10, ' (1ea1n-yellow • Bullfrog Creek, • '\!'V Bci9c·brown 0;11kbrown 70 -' 'V Southern Henry Basin .•' 60 -' 137 088,070 (ream-yt!'llow ' 130 •,' Brushy Basin Member 50 - Light beige 'lll While 'i ----C'lli'V Cream -yellow ., \'/hire 40 'lli'lli Bel9e-brown ' '\!'VI _LhJht ' 30 -' Yellow·bro·.vn ' Cream-yellow .' 20 ' • 'V Salt Wash Member •' Yello '.v-brown 10 -' ' • Or.mge-Plnk • '\!-"" 0 ' Ill Ill Ill Ill Ill Ol E I u E u 125 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #11 Saleratus Point, 160 Southern Henry Basin 'V Gtay-\vhlte 'V l 'V &>i<]c-brown 150 Brushy Basin Member •co eei,(je-b1own l 140 AAA Osc llldtlon ripples •ll DDark brcwn Ri pple X·beds Cream·ycllow 'V Troug h x-bOOs Swalcy trough x-bcds P1anar-tabular x-bcds ' Cream-yellow ..... 130 Pa r.die! t..mllldtiom • 007 /// L.lteral accretion 5els Pebble lags .l light beige 3<-0 .12. Soft sediment dcformalion 120 yy Oes\lriltioncrac ks • ..... Ught bclgc AllA\Slve beds • Hem.Hite concretion(: 0 "' a. moder ' Gray-white 60 ' Yellow-brown l Plnk-belge 50 'V ll'V lig ht b Ughtbeigl' 30 Light b<'igc Orange-brown Beige Rcd-btO'.Yn 0.Jrk brown Cream-g ray Pin k Cre.lm·plnk 0 Ill Ill Ill Ill Tidwell Member Ill Ill Ill Cl E Ill .,!. I I u E u 126 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Dakota Sandstone Measured Section #12 198 Hansen Canyon, l Southern Henry Basin 160 ...... , White 080,055 Whit(' Light beige Brushy Basin Member l White 140, 140 150 light beige 081, ISO Oscll lallon ripples White Rlpplex·beds ...... , Trough x·heds light beige 101,090,080 ' Sin.J!cy !rough x-bcds Gr.1Y·White 140 ' A. Pl.mar-t.ibuLu x-bcds !Bl Bc!gc-brown P.HdHel t."!mln.. Hlons Crc.1m-whltc L.1t<'fill acc retion sets 31 5,3 18 ' 355,J24 --- ' Beige -brown .n. Soft sediment dcformatk>n 130 358, 352 yy Oenk.nlon cra<.lu 335 • Beige-brown 067,0.f5, 0 10 095,001 0 CJ) 8i Hen 1alite concretions: l Beige-brown " modcrntc, lnlcnsc) 120 MJmmaUan burrows Pink-beige 6 • !Bl Arthrcpod/Annclid burrows: Pink-beige 'll "% (sp;us.t?, .1bund .lnt, intense) l light beige Roo t srructums: (\pdrMO,.abundant. 110 Gr."l)' ·whlte .i. Root hd!os rragmcn ts 6clgc·brown • D!nos.iw I s:: (sp.irsc,abund.mt) 100 C.irbonJte nodu!C>s.: g abundant) White ;;;· 0 Cl> Cillcrcte hardp.Jn ...... , Beige -brown ::J Prlsmatlcp..."ds 'll' 70 ...... , Gray-white ...... , Bclgc -hra."n llelgc·brown 50 White Purple Gray·whltc 40 Pink-white 'll Red 30 ,l. 'll Brown Beige 20 .t n Dark brown Crc.im ·white I 10 Pink·whlte Pink·white 'll ...... , Plnk-wl1lte 0 Ill Ill Ill Ill Tidwell Member .== Ill Ill Ill Cl E Ill u ,.!. E' u' 127 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Dakota Sandstone ? Measured Section #13 186 "VI I Halls Creek Overlook, '$- Capitol Reef National Park, 140 Southern Henry Basin • Brushy Basin Member 'I ,A.A.A Os<:Ul3tlon ripples 130 "V Light beige -"V Rlppl e x·beds • Beige-brown "V lroU Brown 50 - "V Pink 40 ' "V light bclgt' "V Pink O>l! "V Light beige White "V Pink "V'lli Pink·brown "V Pink "V1l Or.i ngc·Pink 10 "V light beige "V "V Pln k 0 "V Pink Ill .... Ill Ill Ill Tidwell Member Ui Ill Ill Ill C\ E ,.!. I I u ·e u 128 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) 180 - • Dakota Sandstone Measured Section #14 •' - , Pyserts Hole, 170 - ' East Henry Basin ', 160,_ I ' AAA Oscillation 150 -' Crc.Jm-yel!ow ntpple x-bcd!o 'V Trough x-beds • = ' Cre.un-yellow Sw.dEy trough 'V Pl.lMr-r.,bul.ar x-beds ' Brus hy Basin Member ..... '- P.u.1 llcl famin.:itions 140 U<;htl>!lge • 'V" ...... l ateral acuelkm sets 'V ' S2. Soft sediment d<-fotm.ition ' 'V Beige-brown yy Dcsslcoulon cr;icks 130 -' 'V 1'.fass!vc beds • om "& Hematfleconcre llons: In tense) ' « MammaU;m burrows 120 '- Arthropod/Annelld burrows: • 11 • Root structures.: . (sp.usc,;ibund;mt, intense) , .+ .+ Root h,JJfC\ 110 Wood fragments Gray· brown 357,000,H4 " • Dinosaur • (spa rse,abund.lntl • Beige-brawn ' 11-V (dl l)Ofldll!' nodule\: 'V White (sp.Jrsc,abundant) 100 -' J<-0 s: • 'V Gray·whlle g 0 C.:ilcrete hardpan 11.j. Pink v;· Prlsrua Uc ' 0 ' :::J 91eccl..ited beds Orange-Pink ,, "8-• 90 -' Poll!lge • 'V ' 'V Delge·hrown ' .1111 Purple 10 ' -- ;$ • 'V ' 'V . --v D.irkbrown 0 ' --Ill Tidwell Member Ill I Ci u u 129 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #15 Freemont River, Capitol Reef National Park, Northern Henry Basin Osc illation x-OO ] 80 Gray-w hlle Cre.sm-whl te Red-lirown Sal t Wash Member Crc.im·whitc Red -brown Gr.1y-whllc 065,MS,HS Red-brown White 0 Gray-whllc Ill .... Ill Ill Ill Tidwell Member Ill Ill Ill Ol E 'f I u E u 130 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) 150 - • 'V BehJ e-- brown Dakota Sa ndstone ' 'I Measured Section #16 140 - I Sandy Ranch, • Capitol Reef National Park, 'I 130 - Western Henry Basin • .• Drown l "" AA.Ao Os. 131 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) ,_ 170 • Dakota Sandstone Measured Section #17 • Swap Mesa, 'l Capitol Reef National Park, 160 - • • Western Henry Basin ' l 150 - • AAA Oscillation ripples • TroU(}h x·beds ' Sw.llcy trough x·bcdi 140 -' A. PL.'ln.lt·lolbuL1rx-bcds Brushy Basin Member ' P.1ra !Jcl laminatlcns • / N' Laceral accretla n sets ' Pebble bgS 130 -' .n Scft sediment dcform.1tion ' yy Ocnk'atlon cracks ' • Massive beds ' 0 CD 8' Hema tile concrttl::im: l (sp.1fK", modcr.1tc, inte>n sc) 120 - • « burrow s /\rthropod/Annclld bmrows: ' (Sptllse, Jl1 undant, intense) Root struc t urei: (sp.. m .c,abum.l 3nt,intcnw) 110 -' 110, 116 Beige-brown Hoot hillos Mid-Morrison ..A- ' Criea m·white Wea d fr119mcnts • Conglomerate ' • Dinosaur trampling Beige-brown il14i abundant) 100 -' Yellow-brown 115 •' (spctr\e,dbundtint) ' 0 h i'rdpan ' 90 11.4- Yellow·bmwn • 43'• Rreccl."lted beds JSJ maturity: ' Yellow-brown n iii! mm Imm.nu re, moderate, mature ' u.] ().15 80 -' Ol!l9e-brown • ' 70 ' Befge·brown 08) • 1l Pink ..wh itc ' 60 ' Yellow-brown 040 • Salt Wash Member • 'l 50 - ' ' ' 40 ' • St>ige-brown Sl"iye--brown ' ..-· Cream-white 'I 30 - Beige-brown Plnk-whitc • 'l - 20 Cream-yellow • Crea m-yellow CrNm·whitc ' -u White 'l 10 - 0000 '-0 . Cream-yellow Crc.lm·whltc ' Crcam·whi!e ' Gray-white 'I 0 Tidwell Member C'I "' "'I E u"' u 132 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) 150 - Measured Section #19 • Salt Wash, --'-(.,11 ·:1 Brown 045, I 76, OSS Buckhorn Conglomerate . ' '-(., Cainville Reef, 140 -' Emery High ' .' 130 '- .AA-'. Osdlation ripples I Ripple x·bed\ ' '-(., Trough •·bed\ . Sw.:dcy trough x·beds 120 '- ..... Pl.inar·t..l.bu!.u x·bcds • Pil rAllel laminations • ,.,.,.,.,.,. LiltN.11.tccrctlcnscts ' Pebble "'9• '-(.,11 136 Sl. Sc(L deform.ttion 110 -' VY Oeu!caticn cracks • • Mas!.fvebeds • 0 Q) a. (spMsc, roodcr.uc, ' '-(., 11 Y'11lr e 116, 15 1 100 ' '-(., Pink·white « Mammall.m burrows Arthropod/Annclld burrows: • (sp.J rsc,Jbund;mt,intt'nsc) ' Hoot structures: ' (spar\e,abund anl. lntenioe) 90 '- ·H·"*-),. Hoot • Brushy Basin Member _, Wood Oinos.;,ur lrdmpllnq ' t.i 4i (sp.:me,abundant) 80 -' C.:i rbonate nodules: • @ c.1lcrctc h.:udp;m ' Prismatic peds 70 -' -4"• Hrcccl.1tcd bros P.Jleoso l maturity: ' a Ii! m lmm.1lurc, moderate, mature ' 60 -' • • 020, 044, 030 Beige-brown 092, 0 10 'l 50 - • '-(., 11 090,WS, 128 '-(., Q) light beige J)66,092 ' 40 -' • .'-(., '-(., 054, 098 ' Bclgc·brown . '-(., 30 -' '-(., 348, 040, 03S • '-(., ( ream-'.vhlte • '-(.,11 010,0"'5 ' -- Cream-pink Salt Wash Member 20 -' '!..J-.. Cream·pink • ' YcUow·bcigc 060, 040,030 . Pfnk·bclgc 062, 0S2 10 ' • Light beige ' Tidwell Member . _ _,_,,ii 0 ' LI htbcl c VI VI Summerville Formation VI VI I Ci f: u u 133 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) 60 Measured Section #20 Top of exposure Brushy Basin Member Hartnet Draw, ' on(i;6 ______"MTd-"Moriison -- Grol)'·whllc Cathedral Valley, 'V Conglomerate 020.058 Emery High Yel low·beige 1 121,007 40 Light beige tto Salt Was h Member Oo;citl;.ticn ripples 190, 1% Ripple x-bcds Gray-white 016,061, 110. 125 Trough x-IJeds Be/ge-brnvm 129 Sw.iley trough x·beds ffilBl Rcd ·brown Or.ln<;c·brawn Tidwell Member Pa ra llel bmln.ltlons /Al>" lotter.ii ;iccrP.tlon M!ts ISO Pellble lago; J2. Soft sediment derorm.\l!on YY Oesslc.:ition cracks M.usivcbro-. ' 0 CD "lb Hcmdt itc concrctk>ru: 10 ' Summerville Formation !\parse, modeMte,lnren\eJ M.1mmitli.ln Arthropod/Annelid burrows: Root st1uctures: 0 (sparw,abund."lnt, lnlcns<') VI VI VI Rooth.1\os VI VI O'l Wood hitgments E E u u C11honate nodules: C;ik:rete hilrdp.'n Prisn'l.ltlc µeds 8r1? 50 ' Brushy Basin Member 1 40 Whlle 1 Pink-beige 'V Yellow-beige Red -broo.'.'n &:-igc·brown 1------CreanH1o·hlt(> klge-brO'.•m Summerville Formation - Beige-brown .-yy Beige-brown 0 - Red -brown 112. 140 VI VI VI VI Ci E u u 134 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) 70 Measured Section #22 Top of exposure Brushy Basin Member Tidwell Bottoms, Gray·brown 110, 120,095 East San Rafael Swell, Green River Beige-brown 110, 135 Or,rnge·brown 195 light beige 135.265.295, 290.310 Oscil lation ripples Ocigc·brown s:: Salt Wash Member I Ripple x-l>eds Rcd·brown g fro ugh x-bOOs Bcd·brcwn v;· Swalcy trough x-bt'-d\\lve beds 110 130, 160 oc;gr:bfown Hem.1titeconcre11ons: Red-brown 0 CD "lb hparse, moder 135 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) , P.i!eocurrentdlfe Measured Section #23 70 'V West Horse Bench, 105,092. 144, 120 Green River 60 ' AAA Osc ll l.lllon ripples Ripple x-bcds • Brushy Basin Member 'V Trough x-bcch ' Swaley trough x-beds so ..... PLm.u-t.1b ul4' r x-be1ls P;irallcl lamlna1Jons Bel 70 Measured Section #24 Top of exposure East Horse Bench, 60 'V 'V Gr 0 'V'\I Bel e--brown Ill .... Ill Ill Ill Tidwell Member Ill Ill Ill C'I E Vi I I u ..!. E u 136 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #25 90 White Sands Missile Range, Green River 1 006,342 80 300,342 C11Mm·white Brushy Basin Member .....,. 010,011 Beige· brown ...... Osdlatlon ripples .l r«'n-g roiy Rlpple x·bed\ 70 Ii! Plnlr; ·brov.,, "V l ro U 137 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) 60 - • Measured Section #26 ' Top of exposure Brushy Basin Member .I 16S,2<:0, 265, 231 Interstate 70, 50 "' Gray Red-brCYNO West San Rafael Swell, • Mid-Morrison ' e Or;ingt?-P!nk '"V llS, 143. 145,089, 171. 175 Conglomerate Emery High ' '"VJ Gray OlS.040 40 ' • Salt Wash Member • Orange-Pin ' II "X 1S 00 Red-brown 30 ' II ..e-e.- Tidwell Member Osclll.11lon ripples II Red-brown • ..e> Ripple x-beds II ' '"V TromJh •·bt?d\ ' II Red-brown 20 ' SwJlcy troug h x·bcds '"V [0 ..... PIJn.n·ttlbufJr x-bcds "'.i.-x II • Orange-Pink Parallcl Limlnations /// L.1tcral .xcrction K"IS ' -""'ll Gray·brown Summerville Formation .I Pebble l•gs 10 - 12. Soft SC'dlmcnt dcformoltlcn yy Dcsslc.1tlon cracks • Red-brown • II M.:usl\'cbcds '> 0 a> l:b 0 P.cd·brown burrO'm Ill a "' Ill Ol Arthropod/Annelid burrQ".•1s: I u 1l "Ii 1% {sp..,rS{', abuMant, lntens.?) "' u Root \tructures.: E (sparse, Jbundant. lntcn!c) ,!. Root hJ/os Wood fr.lgmcnu S4!• Carbon.m• bp 20 Measured Section #27 Brushy Basin Member Moore, West San Rafael Swell, Tidwell Member Emery High '"V'"V 0 Gr;iy 094, 108 Summerville Formation In Ill .., Ill Ill Ill Ill Ol E Ill "' I I u "' E u 138 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Red ·hrown Measured Section #28 50 Green- e C-lkrete h.udp."ln Prl'ill'littkped'i 41'• Br1?Ccl.Med bed'i 'X Gypsum Ve tns l!l\11 Wnk sllcken.ddes m."lturity: D IBI lilllil Immature. moderate. mature 60 Measured Section #29 Red·llrown Brushy Basin Member Jailhouse Rock, 'V !!!I Gray 50 C1e.:1111-gr.ay I Cathedral Valley, Belge·hrown Emery High llcd·brown g ... iii Purple v;· 0 40 :;, 1%e>m Grccn·ycllow Salt Wash Member ..,, CrNm-gray 0 OIJ,OSS,040,0]}. 30 11"' Grd)'·Whlte 3 335, 32S,Ol), l S0,040 !'! ""'V Crc.-im ·whltc Ci' 'V Beige :;, ""• Beige 20 iii Tidwell Member 'X 'II iii Purple l 10 Purple llghl beige Summerville f ormation = 1!"' Red·brown light beige Credm·whlte 0 Gray ·brow n Ill .!: Ill Ill Ill. "" Ill Ill Ill Ci E Ill I I u ,.!. E u 139 Lithology & Structures Color Paleocurrents Unit Location 150 Measured Section #30 The Post, 'l Buckhorn conglomerate Capitol Reef National Park, 140 Yellow·beice • Western Henry Basin • 1 130 • l AAA Osdlatlnn ripples 120 Rfpplcx-bc-ds • Brushy Basin Member '-(,, Tro09h x·beds Sw.llt'>)I trough x·lleds l ..... Pl.1Mr·1Abul.1r x·beds 110 PJt.lllcl IJminaticns /.# Lnerill .1cc.retlo n sets Pebble logs l t======:l'-<.,,ll (I) Plnk·belge .12. Solt 100 yy Dcsskatlon cr-'tks (I) Plnk·bra,om Red-brow n ... Milnlvcbrds 0 (I) lb Hem.itlteconcrctlora: Yel low-beige (sp.:us.c, modcratc, lnt<'nse) Red-brown Mamm.11/;in burrow-. 90 021 ,027 « Arthropod/Annelid burrows: Yellow-beige 125,097 hp.me, abundcmt. Rool stn.1<.tures: .j. hp.irsc, abund;int, rntcns.c) 1 ois 80 A· Root h.llo.. 4N Wood fragmenH Or.1nge-bcown I======'-<,,& Dinosaur trampnng v;· 61 4i (SJl<'lie,.lbundaOI) CtP..lm-yel low 0 'l :;, nodules: 70 Red-brown bp.:1 rs.c,abund;,nt) O' @ C.dc rete httrdp;m Cream-yellow 070.094 3 Pris mati c 'l !!! Rr e((latOO 60 Yellow-beige 5· .'6' :;, • "X. Gypsum Veins !!.Ill! Vertie CrcJm-white l P11reos.ol m;itu1ity: 0 llil liiffi lmmJturc, mocforJtc, mature 50 .j. U9htbei9e 031. 063,080 Ye llow-beige Salt Wash Member m Bcl9c-brown l fl Orang I!· brown 40 O Yellow·bek]e 30 Yellow·hekJe Orange-Pink Y"llow-tx!kjc Ye-llow-bekje Yel!ow-bejge 129, 145 D<'i9e 070 light beige Orange-brown Orangt--brown Orange-brown Red-brown Grol)'·hrown 0 '-<.,,-ll Ughtb<>I c Tid well Member "' "' Ci E ..,!. "'I "'I u "' "' E u 140 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #31 Bitter Creek Divide, Capitol Reef National Park, West Henry Basin 160 Dakota Sandstone ' OS Carbonate nodules: '<.- o.11kb1own (Spd rM;>, .tbumlanl) 100 ' 0 Clkrctc hardp.in Prlsm,111c peds ' 4"• Urcccl.1teod bros 90 ' "<..11S Cream-yellow s: "'X. GypsmnVeim 11.1!11 Vcrllt s!ickcmltlcs '<.- Oelge g ;;;· 1111 lllml P.lleosol m.lturity: '<.- Crcam-ycllow 0 n hmna ture, 1nodefdte, ma lwe Cream-white :i 80 "'T1 '<.- Cream-yellow 0 '<.- C1eam·yellow 3 70 ' o· '<.- 016,001,017,036,0JS :i '<.- Cream-yellow ' 053,007 60 ' ...... Cream-gray '<.- '<.- Ycllow-bclgc '<.- Gr.Jy·brown so Red ·brown 11--...... 124 Salt Wash Member - ...... GM)' '<.- Gr.ly '<.-11 Crcilm-yrllow (ream-yellow 30 099. 101.82 ...... Gr.l)' - '<.- light bel9e 20 ., 0 Crcam·yc-ltow -'<.- Gr,ly 044,0JO '<.- 081.DaS light lle!ge 10 Crcain·yC'l low ...... light beige e Purple 4" n Plnk-bra.vn Crcam·whitc 0 6'-l c·brown Ill .... Ill Ill Ill Tidwell Member iii Ill Ill Ill Cl E ..!,. I I u E u 141 Lithology & Structures Color Paleocurrents Unit Location (Thickness in meters) Measured Section #32 70 Factory Butte, I light helye 6 Cainville Reef, ' Cre.lm·whlte C65 l Emery High 60 - I Brushy Basin Member ' 0 Killallon ripplc!i Ripple X·bed\ 50 -' Trough x-bcd'i I Swalcy trouqhx-bcds Gro1y-whlte 257, 156,179, 269,337 • light beige s:: ..... Plandr·Llbu!Jr x-bcds .l ,j. Burgundy g P.n.1 1lel lamlt1.1tions light beige 002.353.350 40 - ...., v;· /A' Lateral J<.crctlcn scti 11 Grccn- 80 • Measured Section #33 l South Pinto Hills, 70 Brcwn Hanksville, North Henry Basin 60 ' Brushy Basin Member Sciqe·brown Bei9e-brown Bcigc·brcrwn l 089 50 Beige-brown 006 Bcic:fe 318,284,032,052 llghl beige 002 s:: l light bei2e 347,016.005 g 40 Red-brown v;· Iii Red-brown 0 :J Red-brown ..,, !Bl !3cf.gc·bra.vn 0 30 3 Gr;iy ....QI Salt Wash Member a· • Beige .. ::J 20 ' Plnk·bclgc B<>l 142 APPENDIX II, PETROGRAPHIC DATA 143 "<:f" "<:f" ...... c Strat Modal RAW PARAMETERS CALCULATEDPARAMTERS • Meas. Unit Horizon Grain Section Quartz Feldspar Fragments Chert Qp Qm+ (cm) Size Qm Q F l i.,, Qp c u Qpc Lac omJonl otl Or Fn I Fk · lv I lm I La Ccf I Ccc I Crc I Cra I Co I Ca I Co 45 'MS.15 BB 4900 ml 162 8 3 0 20 9 4 0 4 22 1 0 3 1 12 0 0 249 162 11 173 39 212 29 8 58 50 43 48·1 MS.15 MM 4900 . vcU 17 2 2 0 4 0 4 0 25 58 13 89 32 0 10 67 0 323 17 4 21 269 290 4 29 302 273 294 lf1 MS.15 BB 7340 ml 85 9 7 0 14 1 3 0 10 58 24 10 24 0 14 49 0 308 85 16 101 179 280 15 13 208 195 189 82 MS.20 SW 5900 ml 182 18 12 0 38 4 1 0 9 17 7 11 2 0 5 16 2 320 182 28 210 60 268 40 10 98 88 69 63 MS.20 SW 6800 fU 243 11 19 2 18 4 8 0 32 0 16 0 0 0 7 1 0 381 245 30 275 24 299 22 40 94 54 56 75 MS.26 MM 5500 mU 78 8 2 0 1 33 5 0 8 37 35 11 8 48 18 18 43 347 78 10 88 214 259 34 11 . 235 224 220 n MS.26 MM 8300 ml 107 8 25 1 1 24 6 0 35 40 42 2 11 3 30 42 0 377 108 33 141 170 311 25 41 244 203 20!5 78 MS.27 MM 1000 cl 78 8 3 40 0 3 1 0 15 42 40 15 4 10 18 87 15 379 118 11 129 231 345 3 16 258 242 246 70 MS.23 SW 800 ml 218 9 7 0 3 6 2 0 6 2 12 7 1 3 3 10 4 293 218 18 234 42 272 9 8 86 58 48 71 MS.23 MM 3400 vcL 42 1 10 0 0 5 6 0 104 29 43 20 1 27 7 34 10 339 42 11 !53 171 214 5 110 292 182 275 72 MS.24 BB 6000 cl 201 3 19 1 3 12 2 0 27 19 20 0 0 11 3 5 18 344 202 22 224 76 282 15 29 127 98 103 73 MS.23 BB 6800 cU 133 2 5 0 4 12 0 0 27 65 48 12 9 7 50 3 0 377 133 7 140 194 334 16 27 228 201 221 1 MS.2 SW 11400 n.. 252 12 10 14 27 20 2 0 7 17 17 1 0 0 0 1 0 380 288 22 288 36 324 47 9 67 58 43 44 MS.15 SW !500 n.. 299 9 6 0 19 41 3 0 3 1 19 0 0 0 0 0 0 400 299 15 314 20 334 60 6 41 35 23 5 MS.2 SW 17960 fU 290 3 5 28 5 5 7 0 12 0 8 0 0 0 1 0 0 382 318 8 326 7 333 10 19 34 15 19 28 MS.11 SW 7660 vcl 57 1 6 0 3 4 10 0 35 30 60 42 74 12 14 38 1 387 !57 7 64 271 334 7 45 323 278 306 29 MS.11 SW 12400 vcL 42 3 5 0 2 2 9 0 31 25 45 86 64 0 20 46 1 381 42 8 50 287 336 4 40 335 295 318 4 MS-2 MM 19400 cl 95 2 9 0 1 5 16 1 12 45 56 44 52 3 11 24 3 379 95 11 106 238 341 6 29 278 249 250 48 MS.15 BB 10900 vcL 140 8 10 0 2 2 1 0 16 24 63 18 64 0 16 12 0 378 140 18 158 197 355 4 17 232 215 213 64 MS.17 SW 8200 cl 132 3 10 0 3 1 6 0 19 9 35 57 63 0 5 19 1 363 132 13 146 189 333 4 25 227 202 208 55 MS-17 MM 10400 ml 213 7 13 0 8 0 20 0 25 17 23 15 22 1 2 21 0 387 213 20 233 101 334 8 45 . 166 121 126 56 MS.17 MM 10800 cl 82 5 9 0 4 1 18 0 44 31 47 35 112 3 e 23 0 400 82 14 76 257 333 6 62 333 271 301 79 MS-31 SW 2400 vcU 104 2 1 0 1 0 2 0 30 20 17 35 102 31 8 15 6 374 104 3 107 234 335 1 32 269 237 264 80 MS-31 SW 6300 vcL 17 1 4 0 0 0 e 0 32 34 66 72 137 1 8 14 0 392 17 5 22 332 364 0 38 375 337 364 81 MS-31 MM 12200 cu 13 0 7 0 0 0 11 0 18 27 63 42 187 1 16 15 0 400 13 7 20 351 371 0 29 387 358 369 11 MS.2 SW 16800 fU 296 8 36 7 14 10 2 0 14 0 12 0 1 0 0 0 0 400 303 44 347 13 360 24 16 73 !57 27 6 MS-2 SW 18xxx ml 248 8 32 1 3 9 15 0 5 2 47 9 11 2 3 3 2 400 249 40 289 79 366 12 20 139 119 84 8 MS.2 SW 18200 ml 252 8 1e 9 2 11 7 0 7 4 54 1 19 8 1 2 1 400 261 24 285 88 372 13 14 126 112 95 36 MS-13 SW 8900 cl 194 12 11 8 2 8 10 0 7 4 70 8 57 3 4 3 1 400 200 23 223 150 372 10 17 190 173 167 52 MS.15 SW 3900 cU 57 3 7 0 1 4 12 0 37 21 57 79 107 1 4 10 0 400 !57 10 67 279 346 5 49 338 289 316 51 MS.15 SW 4000 cu 173 5 4 0 5 10 21 0 15 14 29 18 71 11 4 3 17 400 173 9 182 167 332 16 36 212 176 182 !50·1 MS.15 MM 3900 cU 103 4 7 1 2 5 14 0 13 31 42 32 134 0 10 2 0 400 104 11 116 251 366 7 27 289 262 264 63 MS.15 SW 4300 n.. 293 12 7 O · 20 16 6 0 8 0 21 0 3 9 5 0 0 400 293 19 312 38 350 36 14 71 67 46 13 MS-5 BB 12100 cU 162 8 2 0 1 7 14 0 12 17 53 20 91 0 11 2 0 400 162 10 172 194 366 8 26 230 204 206 18 MS-6 SW 5600 ml 265 24 6 22 6 7 4 0 9 7 27 0 22 0 1 0 0 400 287 30 317 57 374 13 13 100 87 66 25 MS-8 SW 2400 ml 311 11 13 0 9 12 9 0 5 3 19 1 2 1 1 0 3 400 311 24 335 30 362 21 14 . 68 64 35 26 MS-8 SW 2600 n.. 294 8 12 0 23 14 3 0 6 3 27 0 4 2 1 2 1 400 294 20 314 40 353 37 9 69 60 46 40 MS.14 SW 10400 fU 269 18 18 3 . 3 8 0 0 25 1 36 2 7 B 2 0 4 400 272 36 308 68 362 9 25 119 94 83 30 MS.12 SW 12300 cl 184 14 ·4 41 5 1 5 0 14 4 30 38 31 13 6 2 8 400 225 18 243 132 367 6 19 169 150 146 33 MS.12 SW 13500 cl 121 18 5 12 1 0 11 0 8 23 77 41 81 1 2 1 0 400 133 23 166 226 382 1 17 266 249 232 34 MS.12 MM 14600 cu 95 6 1 2 1 0 7 0 4 35 103 48 72 15 3 4 4 . 400 97 7 104 284 384 1 11 302 291 288 37 MS.13 SW 11600 mu 289 20 14 5 8 5 3 0 0 0 35 8 10 0 2 0 1 400 294 34 328 66 383 13 3 93 90 66 38 MM 1">AM n ., ..,, ttQ MS.13 vcU 64 6 0 0 5 0 1 12 103 39 109 46 10 0 3 4nn 64 6 70 2 6 ...... '"'...... c in TERNARY DIAGRAM PARAMETERS '<;f" •E Meas. Strat Modal ...... u Unit Horizon Grain !. Section 1:1) (cm) Size Q F L Qm F Lt Qm F c Qp Lv Lac Qm Qt c 45 MS-15 BB 4900 ml 85 12 3 65 12 23 70 13 17 19 7 74 79 2 19 46-1 MS-15 MM 4900 vcU 90 1 9 5 1 93 6 1 93 1 1 . 97. 6 9 93 47 MS-15 BB 7340 ml 91 5 4 28 5 68 30 5 64 8 1 91 31 7 66 62 MS-20 SW 5900 ml 84 13 3 57 13 31 65 14 21 29 1 70 72 6 24 63 MS-20 SW 6800 fU 83 6 11 68 6 26 84 8 8 32 9 60 85 7 8 75 MS-26 MM 5500 mu 85 11 4 22 10 68 24 10 66 4 2 94 27 2 73 77 MS-26 MM 6300 ml 82 7 11 29 7 65 36 8 56 14 2 84 36 19 56 78 MS-27 MM 1000 cl 95 1 4 31 1 68 34 1 66 4 0 95 34 2 66 70 MS-23 SW 800 ml 94 3 3 74 3 23 81 3 16 24 3 73 82 3 16 71 MS-23 MM 3400 vcL 65 2 33 12 1 86 19 2 78 4 2 94 19 19 77 72 MS-24 BB 6000 cl 87 5 9 59 4 37 69 5 26 17 2 81 68 9 26 73 MS-23 BB 6800 cu 89 4 7 35 4 60 39 5 57 3 0 97 40 4 58 1 MS-2 SW 11400 fL 85 12 2 70 12 18 76 13 10 33 3 64 85 4 12 44 MS-15 SW 500 fL 84 15 2 75 15 10 79 16 5 37 7 56 92 2 6 5 MS-2 SW 17960 fU 92 3 5 88 3 9 95 3 2 24 21 . 56 96 2 2 28 MS-11 SW 7860 vcL 87 2 12 15 2 83 17 2 81 2 3 95 17 9 81 29 MS-11 SW 12400 vcL 88 1 11 11 1 88 13 1 86 2 3 95 13 10 86 4 MS-2 MM 19400 cl 91 2 8 25 2 73 28 2 70 4 6 90 28 8 70 48 MS-15 BB 10900 vcL 94 1 5 37 1 62 41 1 58 8 0 92 40 7 57 54 MS-17 SW 8200 cl 92 1 7 36 1 63 41 1 58 6 3 92 40 7 57 55 MS-17 MM 10400 mL 86 2 12 55 2 43 66 2 31 12 12 76 65 6 31 56 MS-17 MM 10800 cl 83 1 16 16 1 83 19 2 79 4 5 90 19 12 78 79 MS-31 SW 2400 vcU 91 0 9 28 0 72 31 0 69 1 1 98 31 1 69 80 MS-31 SW 6300 vcL 90 0 10 4 0 96 5 0 95 1 2 97 5 16 94 81 MS-31 MM 12200 cu 93 0 7 3 0 97 4 0 96 2 3 95 4 30 95 11 MS-2 SW 16800 fU 90 6 4 76 6 18 89 7 4 60 3 37 86 11 4 6 MS-2 SW 18xxx mL 92 3 5 62 3 35 73 4 23 29 11 60 69 11 22 8 MS-2 SW 18200 mL 93 3 4 65 3 32 72 4 24 19 6 75 72 6 24 36 MS-13 SW 8900 CL 93 3 4 50 3 48 56 3 42 12 5 83 55 5 42 52 MS-15 SW 3900 cu 87 1 12 14 1 85 17 1 82 3 4 93 17 10 81 51 MS-15 SW 4000 cu 87 4 9 43 4 53 49 . 4 47 4 10 86 so 2 49 50-1 MS-15 MM 3900 cu 92 2 7 28 2 72 29 2 69 4 5 91 29 8 69 53 MS-15 SW 4300 fL 88 9 4 73 9 18 80 10 10 27 8 65 87 2 11 13 MS-5 BB 12100 cu 92 2 7 41 2 58 45 2 53 4 6 90 45 1 54 18 MS-6 SW 5600 mL 94 3 3 72 3 25 80 4 18 30 4 66 82 2 16 25 MS-8 SW 2400 ml 91 5 4 78 5 17 86 6 8 35 13 51 88 4 8 26 MS-8 SW 2600 fL 88 9 2 74 9 17 79 10 11 29 4 67 85 4 12 40 MS-14 SW 10400 fU 91 2 6 88 2 30 80 3 17 30 0 70 78 6 17 30 MS-12 SW 12300 cl 94 2 5 56 2 42 . 62 2 36 11 3 86 62 2 37 33 MS-12 SW 13500 CL 96 0 4 33 0 67 37 0 63 9 4 87 37 4 62 34 MS-12 MM 14500 cu 97 0 3 24 0 76 25 0 74 2 2 95 25 1 74 37 MS-13 SW 11500 mu 96 3 1 74 3 23 81 4 15 37 3 60 81 5 15 38 MS-13 MM 12400 vcU 98 1. 2 16 1 84 16 1 83 2 1 97 17 0 83 \0 '<:f' c ...... GI Strat Modal E Meas. Unit Horizon Grain ROUNDING Matrix, porosity, cement Section ¥a. (cm) Size UJ 46 MS-16 BB 4900 ml Subrounded to angular Recryatallzed and networked carbonate 46-1 MS-15 MM 4900 vcU Subrounded to subangular Mostly porosity 47 MS-15 BB 7340 ml Subrounded to subangular Mostly porosity, some recrystallized carbonate 62 MS-20 SW 6900 ml Rounded to subrounded Recrystallzed and networked carbonate 83 MS-20 SW 8800 fU Rounded to aubangular Recrystallzed and networked carbonate 75 MS-26 MM 5500 mu Subrounded to subangular Recrystallzed and pedogenicaily formed carbonate matrix 77 MS-26 MM 6300 ml Subrounded to angular Recrvstalized and networked carbonate, more porosity than cement 78 MS-27 MM 1000 cl Subrounded to subangular Mostly rounded carbonate rhombs, little recrystallzed carbonate 70 MS-23 SW 800 ml Rounded to subrounded Mostly rounded carbonate rhombs, some recrystallzed and networked carbonate 71 MS-23 MM 3400 vcL Subrounded to aubangular Recryatallzed and networked carbonate 72 MS-24 BB 6000 cl Rounded to subangular Most cement consists of thick chalcedony rims around grains, some calcite 73 MS-23 BB 6800 cu Subrounded to angular Most cement consists of thick chalcedony rims around grains, no calcite 1 MS·2 SW 11400 fl Subroundlid to angular Poorly cemented, mostly clay, minor calcite, well compacted, low porosity 44 MS-16 SW 500 fl Subrounded to angular Recrystallzed calcite, minor clay 6 MS-2 SW 17960 fU Subrounded to angular Granular calcite, clay and hematite 28 MS-11 SW 7660 vcL Rounded to subangular Poorly cemented, weathered calcite, clay and hematite 29 MS-11 SW 12400 vcL Rounded to subangular Poorly cemented, weathered calcite, clay and hematite 4 MS·2 MM 19400 CL Subangular to rounded Well cemented, hematite, clay, weathered and recrystallzed calcite 48 MS-16 BB 10900 vcL Angular to subrounded Poorly cemented, mostly porosity, minor clay cement mostly around rims of grains 64 MS-17 SW 8200 cl Rounded to subangular Poorly cemented, mostly porosity,. minor hematite and clay 55 MS-17 MM 10400 ml Well rounded to subangular Mostly hematite, clay and weathered recrystallzed calcite 56 MS-17 MM 10800 cl Rounded to subangular Clay, weathered and recrystal.lzed calcite and opaques, tote of porosity 79 Ms-31 SW 2400 vcU Well rounded to subangular Recrystallzed and networked carbonate, some weathered calcite too, minor opaques, soine porosity 80 Ms-31 SW 6300 vcL Rounded to subangular Lots of porosity, some clay, minor hematite, no calcite 81 MS-31 MM 12200 cu subrounded to aubangular Almost entirely agate/chalcedony/coarse chert Interlocking rims, little clay 11 MS·2 SW 16800 fU Subrounded to angular Very poorly cemented, no calcite at all, little clay cement 6 MS·2 SW 18xxx mL Rounded to subangular Hematite stained weathered and recrystallzed calcite; llttle secondary porosity 8 MS·2 SW 18200 mL Subrounded to subangular Hematite stained weathered and reerystallzed calcite; some caly, Low secondary porosity 36 MS-13 SW 8900 cl Subrounded to subangular Hematite stained weathered and recrystallzed calcite; some caly, Low secondary porosity 62 MS-15 SW 3900 cu Subrounded to subangular A lot of porosity, some recrvstalized calcite, little clay 51 MS-15 SW 4000 cu Well rounded to subangular Almost entirely recrvstallzed and networked calcite, some weathered calcite 50-1 MS-15 MM 3900 cu Rounded to subangular No calcite, ·mostly clay and hematite 53 MS-15 SW 4300 fl Rounded to subangular Recrystallzed and networked carbonate, almost matrix supported 13 MS-6 BB 12100 cu Well rounded to subangular Crystallzed agate/chalcedony and clay, almost no secondary porosity 18 MS-6 SW 6600 ml Subangular to angular Low porosity, little clay cement 25 MS-8 SW 2400 ml Subrounded to subangular Recrvstallzed detrltal? calcite, little networking, little secondary prosity, some hematite and little clay 26 MS-8 SW 2600 fl Angular to subrounded Recrystallzed calcite, hematite and clay, little secondary porosity 40 MS-14 SW 10400 fU Angular to subangular Recrystallzed and altered calcite, hematite, clay, little secondary porosity 30 MS-12 SW 12300 cl Rounded to subangular Recrystallzed dolomite (few rhombs show rounding), Intense hematite staining of rhombs 33 MS-12 SW 13600 cl Rounded to subangular Poorly cemented, high porosity (secondary) almost all cement Is clay, but tittle exists. 34 MS-12 MM 14600 cu Well rounded to subangular Recrvstallzed dolomite (few rhombs show rounding), Intense hematite staining of rhombs 37 MS-13 SW 11500 mu Angular to subrounded Recrystallzed calcite and dolomite, Intense hematite staining of rhombs, some clay, little porosity 38 MS-13 MM 12400 vcU Rounttad to subrounded mostlv chert and aaate. same calicte and clav. althouah most calcite is altered or renlA"ed bv ci.v. APPENDIX III, PALEOMAGNETISM DATA 147 00 Magnetic Vector Orientation ...... Sample Stratigraphic ""' Unit Lithology Color No. Horizon (cm) Declination Inclination 43 4500 Salt Wash Siltstone Beige 355 52 44 4500 Salt Wash Siltstone Beige 2 41 47 5460 Salt Wash Siltstone Beige 358 52 48 5460 Salt Wash Siltstone Beige 311 55 49 5480 Salt Wash Siltstone Beige 1 54 50 5480 Salt Wash Siltstone Beige 355 56 51 5520 Salt Wash Siltstone White 41 46 55 7260 Salt Wash Siltstone Red-brown 291 21 56 7260 Salt Wash Siltstone Beige 5 59 57 7400 Salt Wash Mudstone Red-brown 356 56 58 7480 Salt Wash Mudstone Red-brown 346 67 59 7490 Salt Wash Siltstone Pink 37 58 60 7700 Salt Wash Mud stone Red-brown 10 45 61 7700 Salt Wash Mudstone Red-brown 4 55 66 8000 Salt Wash Mudstone Red-brown 327 41 68 21540 Brushy Basin Mud stone Red-brown 55 72 69 21500 Brushy Basin Mudstone Red-brown 3 59 71 20800 Brushy Basin Mud stone Red-brown 50 -7 73 9140 Salt Wash Mud stone Red-brown 332 43 74 9140 Salt Wash Mud stone Red-brown 349 44 75 9360 Salt Wash Mud stone Red-brown 30 50 76 9360 Salt Wash Mud stone Red-brown 15 44 77 10250 Salt Wash Mud stone Red-brown 7 55 78 10250 SaltWasJ1 Mud stone Red-brown 19 56 81 10640 Salt Wash Siltstone Red-brown 6 58 83 11300 Salt Wash Siltstone Red-brown 357 69 84 11300 Salt Wash Siltstone Pink 10 54 173 100 Salt Wash Sandstone White 251 12 177 1340 Salt Wash Mudstone Pink-brown 291 63 180 1600 Salt Wash Mudstone Pink-brown 337 57 171 1700 Salt Wash Siltstone Light-beige 17 37 182 1900 Salt Wash Mudstone Orange-brown 37 56 183 2000 Salt Wash Siltstone White 359 45 184 2160 Salt Wash Sandstone Pink 17 41 0\ -.::t" ...... Demagnetization Field Strength (mllllTesla) Demagnetization Temperature (Celsius) Simple No. 0 5 10 15 20 25 30 35 "° 45 50 60 75 80 100 150 200 20 100 200 300 350 400 450 500 525 550 575 BOO 620 640 660 670 43 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x 47 x x x x x x x x x x x x x x x x x x x x x x x x x x x ""48 x x x x x x x x x x x x x x x x 49 x x x x x x x x x x x x x x x x 50 x x x x x x x x x x x x x x x x 51 x x X . x x x x x x x x x x x x x 55 x x x x x x x x x x x x x x x x 56 x x x x x x x x x x x x x x x x 57 x x x x x x x x x x x x x x x x 58 x x x x x x x x x x x x x x x x 59 x x x x x x x x x x x x x x x x 60 x x x x x x x x x x x x x x x x 61 x x x x x x x x x x x x x x x x 66 x x x x x x X . x x x x x x x x x 68 x x x x x x x x x x x x x x x x 69 x x x x x x x x x x x x x x x x 71 x x x x x x x x x x x x x x x x 73 x x x x x x x x x x x x x x x x 74 x x x x x x x x x x x x x x x x x x x x x x x x x x x 75 x x x x x x x x x x x x x x x x x x x x x x x x x x x 76 x x x x x x x x x x x x x x x x x x x x x x x x x x x T7 x x x x x x x x x x x x x x x x x x x x x x x x x x x 78 x x x x x x x x x x x x x x x x x x x x x x x x x x x 81 x x x x x x x x x x x x x x x x x x x x x x x x x x 83 x x x x x x x x x x x x x x x x x x x x x x x x x x 84 x x x x x x x x x x x x x x x x x x x x x x x x x x 173 x x x x x x x x x x x x x x x x x 177 x x x x x x x x x x x x x x x x x 180 x x x x x x x x x x x x x x x x x 171 x x x x x x x x x x x x x x x x x 182 x x x x x x x x x x x x x x x x x 183 x x x x x x x x x x x x x x x x x 184 x x x x x x x x x x x x x x x x x REFERENCES CITED Allen, P. 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