Tectonic and Sequence Stratigraphic Implications of the Morrison Formation-Buckhorn Conglomerate Transition, Cedar Mountain, East-Central Utah

TECTONIC AND SEQUENCE STRATIGRAPHIC IMPLICATIONS OF THE MORRISON FORMATION-BUCKHORN CONGLOMERATE TRANSITION, CEDAR MOUNTAIN, EAST-CENTRAL UTAH

A thesis presented to the faculty of the College of Arts and Sciences of Ohio University

In partial fulfillment of the requirements for the degree Master of Science

Xavier Roca

November 2003 This thesis entitled

TECTONIC AND SEQUENCE STRATIGRAPHIC IMPLICATIONS OF THE MORRISON FORMATION-BUCKHORN CONGLOMERATE TRANSITION, CEDAR MOUNTAIN, EAST-CENTRAL UTAH

BY

XAVIER ROCA

has been approved for

the Department of Geological Sciences

and the College of Arts and Sciences

Gregory Nadon

Assistant Professor of Geological Sciences

Leslie Flemming

Dean, College of Arts and Sciences ROCA, XAVIER M.S. November 2003. Geological Sciences Tectonic and Sequence Stratigraphic Implications of the Morrison Formation-Buckhorn Conglomerate Transition, Cedar Mountain, East-central Utah (222 pp.) Director of Thesis: Gregory Nadon

Abstract The contact between the Morrison Formation and the Buckhorn Conglomerate has been interpreted as a low order unconformity bridging the Jurassic and Cretaceous Periods in the northwestern Colorado Plateau. However, sedimentologic and stratigraphic data gathered at Cedar Mountain, the Buckhorn type section in east-central Utah, provide new evidence of the conformable transition of the two stratigraphic units. This evidence consists of gutter casts, mud injection, and dinosaur tracks at the base of the Buckhorn as well as interfingering of the Buckhorn with Morrison mudstones. The Buckhorn braided river deposit is interpreted to be the result of the exhumation, reworking, and eastern propagation of older synorogenic coarse materials of a late Jurassic flexural foredeep located in western Utah. This interpretation, when combined with accommodation rates inferred from Late Jurassic fluvial styles, defines a Morrison depositional sequence. Evidence of the Jurassic onset of Buckhorn deposition precludes its chronostratigraphic correlation with similar Lower Cretaceous conglomeratic deposits found throughout the North American Western Interior.

Approved: Gregory Nadon Assistant Professor of Geological Sciences To Carol Acknowledgements

I would like to thank Greg Nadon for the help offered during the preparation of this thesis, John Bird for the assistance received during the field work, and Mike Leschin for access to the Cleveland Lloyd Dinosaur Quarry. I also would like to express my gratitude towards the faculty of the Department of Geological Sciences for offering me the opportunity to continue my studies in geology. Finally, I would like to thank my people back home, for making me feel that I never left. Table of Contents

Page

Abstract ...... 3

Dedication ...... 4

Acknowledgements ...... 5

List of Tables ...... 8

List of Figures ...... 9

Chapter 1: Introduction ...... 13 1.1 Introduction ...... 13 1.2 Regional Tectonic Context ...... 13 1.3 Stratigraphic Overview ...... 18 1.4 This Study ...... 22 1.5 Study Area ...... 23

Chapter 2: Previous Work ...... 30 2.1 Introduction ...... 30 2.2 Regional Stratrigraphy ...... 30 2.3 Chronostratigraphy ...... 47 2.4 Jurassic – Cretaceous Compression ...... 49 2.5 Lower Cretaceous Conglomerate Tectonic Models ...... 57

Chapter 3: Methodology ...... 67 3.1 Introduction ...... 67 3.2 Stratigraphic Data ...... 67 3.3 Sedimentologic Data ...... 67 3.4 Petrographic Data...... 69

Chapter 4: Facies Analysis ...... 71 4.1 Introduction ...... 71 4.2 General Stratigraphy...... 71 4.3 Mudstone and Siltstone Facies ...... 81 4.4 Sandstone Facies ...... 88 4.5 Conglomerate Facies ...... 104 4.6 Carbonate Facies ...... 114 4.7 Depositional Environments ...... 116

6 Chapter 5: Stratigraphic Model ...... 121 5.1 Introduction ...... 121 5.2 Stratigraphic Contacts ...... 121 5.3 Source Area ...... 124 5.4 Timing of deposition ...... 128 5.5 Stratigraphic Model ...... 130

Chapter 6: Depositional Model ...... 134 6.1 Introduction ...... 134 6.2 Eustasy ...... 134 6.3 Climate ...... 136 6.4 Tectonics ...... 137 6.5 Depositional Model ...... 145

Chapter 7: Conclusions ...... 151

References ...... 153

Appendix A: Stratigraphic Sections ...... 165

Appendix B: Basin Member and Buckhorn Conglomerate Sandstone Point Counts.. 196

Appendix C: Buckhorn Conglomerate Pebble Lithologies ...... 197

Appendix D: Brushy Basin Member and Buckhorn Conglomerate Paleoflow Data .. 198

7 List of Tables

Page 4.1 Lithologies of the Brushy Basin Member and Buckhorn Conglomerate of the study area ...... 74

8 List of Figures

Page 1.1 Map of the North American Western Interior Basin during the Jurassic Period ...... 14

1.2 Paleogeographic maps of United States Western Interior from the Middle Jurassic to the Early Cretaceous ...... 15

1.3 Schematic block diagram of the Mesozoic subduction zone and related tectonic settings of the United States Western Interior ...... 16

1.4 Map Showing Tectonic features of the Western United States ...... 17

1.5 Paleoflow directions of the Lower Cretaceous conglomerate in the Western Interior Basin ...... 19

1.6 Chronostratigraphic diagram of the Jurassic-Cretaceous transition in western North America ...... 20

1.7Location map of the Study area ...... 24

1.8 Buckhorn type section ...... 25

1.9 Morrison and Cedar Mountain Formations exposures in central Utah ...... 26

1.10 Stratigraphic section of the Study area ...... 27

1.11 Geologic map of the Study area ...... 28

1.12 Map Showing Study area Morrison and Cedar Mountain Formation exposures. 29

2.1 Morrison and Cedar Mountain Formation stratigraphic section in the study area ...... 32

2.2 Isopach maps of the Morrison and Cedar Mountain Formations in Utah and western Colorado ...... 33

2.3 The Morrison Formation in the study area ...... 34

2.4 Brushy Basin Member paleogeographic map of the southern Western Interior ...... 37

9 2.5 Stratigraphic section of the Buckhorn Conglomerate at Cedar Mountain ...... 41

2.6 Buckhorn Conglomerate-Cloverly Formation correlation ...... 42

2.7 Cedar Mountain correlations from the Gunnison Plateau to Green River ...... 45

2.8 Morrison and Cedar Mountain cross section along central and eastern Utah ...... 46

2.9 Chronostratigraphic diagrams of the Morrison and Cedar Mountain in the San Rafael Swell ...... 50

2.10 The Sevier orogenic Belt ...... 52

2.11 Sevier orogenic belt of Utah ...... 53

2.12 Cross section and restored section of the Sevier orogenic belt in west-central Utah ...... 54

2.13 Location of Pre-Sevier thrusting in Utah, Nevada, and California ...... 56

2.14 Tectonic models of the Lower Cretaceous conglomerate deposition ...... 58

2.15 Extent of Lower Cretaceous conglomerate versus known foredeep area of modern and ancient foreland basins ...... 60

2.16 The foreland-basin system model ...... 61

2.17 Two-phase model of foreland basin stratigraphic infill ...... 63

2.18 Foreland depozone migration control on foreland basin stratigraphic record ...... 64

2.19 Chronostratigraphic diagram ...... 66

3.1 Location of measured stratigraphic sections and correlation lines ...... 68

4.1 Brushy Basin Member and Buckhorn Conglomerate of the study area ...... 72

4.2 Location map of pictures, diagnostic sedimentary structures, and petrographic point counting ...... 73

10 4.3 Lithostratigraphic correlation (Section A-A’) ...... 75

4.4 Lithostratigraphic correlation (Sections B-B’, C-C’, and D-D’) ...... 76

4.5 Buckhorn Conglomerate isopach map ...... 78

4.6 Buckhorn depositional pinchout ...... 79

4.7 Interfingering of the Brushy Basin and Buckhorn Conglomerate Members ...... 80

4.8 Brushy Basin Member mudstone facies ...... 82

4.9 Uppermost Brushy Basin Member laterite ...... 84

4.10 Uppermost injection at the Brushy Basin-Buckhorn Conglomerate contact ...... 85

4.11 Buckhorn Conglomerate mudstones ...... 87

4.12 Brushy Basin Member tabular sandstone ...... 89

4.13 Brushy Basin Member channel sandstone ...... 90

4.14 Brushy Basin Member paleoflow directions ...... 92

4.15 Brushy Basin and Buckhorn Conglomerate Members sandstone thin sections ...... 93

4.16 QmFLt ternary diagram of Brushy Basin and Buckhorn Conglomerate Members petrographic diagrams ...... 94

4.17 Buckhorn Conglomerate sandstone ...... 97

4.18 Buckhorn Conglomerate clastic dikes ...... 98

4.19 Buckhorn Conglomerate paleoflow directions ...... 99

4.20 Sandstone and conglomerate facies ...... 103

4.21 Brushy Basin Member conglomerate ...... 105

4.22 Well-indurated Buckhorn conglomerate ...... 107

11 4.23 Disaggregated Buckhorn conglomerate facies ...... 108

4.24 Buckhorn Conglomerate basal gutter casts ...... 111

4.25 Dinosaur track in the basal Buckhorn Conglomerate ...... 112

4.26 Brushy Basin and Unnamed Shale Members carbonate facies ...... 115

5.1 Morrison and Cedar Mountain source areas in the Western Interior pre-Late Jurassic stratigraphic succession ...... 125

5.2 Possible Location and extent of potential Buckhorn Conglomerate source areas 127

5.3 Chronostratigraphic diagram of sedimentary sucession ...... 129

5.4 Colorado Plateau Late Jurassic-Early Cretaceous chronostratigraphic diagram ...... 132

6.1 Comparison of Jurassic transgressive-regressive cycles of North America ...... 135

6.2 Jurassic cross-section and derived chronostratigraphic diagram from southwestern Utah to central Colorado ...... 138

6.3 Foreland-basin stratigraphic infill model ...... 140

6.4. Dynamic subsidence control on retroarc foreland stratigraphic configuration ...... 144

6.5. The Morrison depositional sequence ...... 146

12 CHAPTER 1: INTRODUCTION

1.1 Introduction The contact between the Morrison Formation and the Buckhorn Conglomerate Member of the Cedar Mountain Formation in the Colorado Plateau has been long believed to represent an erosional unconformity lasting more than 20 my representing the Jurassic – Cretaceous boundary. Nevertheless, some evidence of a conformable transition between both formations has been found, and reinterpretation of the regional stratigraphy bridging the Jurassic and Cretaceous periods has been proposed. Despite these reports, a final agreement on the temporal nature of the contact has not been reached. Consequently, detailed analysis of the transition between both formations is needed in order to bring more light into its temporal significance and to assess the possible genetic relationships between the Morrison Formation and the Buckhorn Conglomerate.

1.2 Regional Tectonic Context The Jurassic-Cretaceous (J/K) transition is recorded throughout the Western Interior of the American continent by a thin retroarc terrigenous sedimentary succession deposited after the retreat of the Sundance Sea (Middle Jurassic) and before the encroachment of the Middle Cretaceous Seaway (Fig. 1.1, 1.2). During this period, basin formation and sedimentation behavior in the Western Interior were controlled by the major tectonic events occurring along the subduction complex to the west (Fig.1.3). Subduction of oceanic lithosphere beneath the American plate had begun in the Early Triassic and continued throughout the Mesozoic and Early Tertiary, leaving a record of extensive episodic deformation and deposition in its retroarc basin (Elison, 1991; Lawton, 1994) (Fig.1.4).

13 Retroarc Thrust Belt

Subduction Zone Western Interior Basin

1,000 km

Fig.1.1: Map of North America indicating the approximate position of the subduction zone, related thrust belt, and extent of the Western Interior retroarc basin during the Jurassic Period (modified from Peterson, 1994).

14 Direction of Sundance Sundance Sea Sea retreat Accreted Arc (Nevadan Orogeny) Shallow marine

Tidal flats Marine ? ? Magmatic Plateau Arc Uplift ? Coastal Alluvial Plain ? Erg

Elevated Plateau Subductio

n Zone Magmatic Arc Erg Mogollon Highland Subduction Mogollon Highland

Zone

N N

0 500 km 0 500 km

Callovian-Oxfordian (~ 160 Ma) Oxfordian-Kimmeridgian (155 Ma) Latest Middle Jurassic Late Jurassic

? Pierre ? Seaway Forearc Foreland Basin Sevier Orogenic Sevier Belt Orogenic Magmatic Belt Arc ? Alluvial Plain

Su Alluvial Plain Subduction Zone b Magmatic d u Arc ctio

Mo n gollon Zone Mogoll Highl and on Burro Ma ssif Rift Basin

N N

0 500 km 0 500 km

Aptian-Albian (112 Ma) Late Campanian (~75 Ma) Late Early Cretaceous Late Cretaceous

Fig. 1.2: Paleogeographic maps showing the Middle Jurassic regression of the Sundance Sea, the Late Jurassic to Early Cretaceous sedimentary terrigenous conditions, and the subsequent encroachment of the Mancos Sea in the early Late Cretaceous (from Lawton, 1994).

15 Forearc Depositional Accreted Magmatic Retroarc foreland Onlap Terranes arc Retroarc Thrust Belt basin basin Hinterland Craton

Trench

N

Continental crust

Flow generated by Upper cold subducting slab Mantle Oceanic crust

Subducted lithospheric slab

Vertical exageration = 4

Fig. 1.3: Crust and upper mantle schematic block diagram showing the subduction complex during the Mesozoic of western North America subduction complex, and related tectonic settings. Plate convergence led to an extended period of compression regime responsible for the formation of forearc and retroarc foreland basins on both sides of the magmatic arc. It is in the latter where the transitional Jurassic-Cretaceous continental succession was deposited. A flexural foredeep depozone is located contiguous to the eastern limit of the thrust belt. East of the flexural forebulge, the backbulge depozone extends for several hundred kilometers onlaping onto the North American craton. Eastern limit of fold and thrust belt

49 o

Craton Pacific Ocean SRI=0.706

45 o

Cenozoic Cover

N 41 o

Sevier o 37 Hinterland

Western limit of Sevier Deformation 500 km Eastern limit of foreland deformation Sevier Thrust Belt

Area of Mesozoic metamorphism Pre-Jurassic suture Mesozoic plutons Terranes accreted in Jurassic and younger time Jurassic or younger suture

Terranes accreted in later Paleozoic / Early Triassic time Terranes accreted in middle Paleozoic time

Fig. 1.4: Map of the showing the most prominent tectonic, magmatic and metamorphic features of the western United States. The 87Sr/86Sr initial ratio (SRI=0.706) indicates the western limit of Precambrian crust. Accreted allocthon terranes accreted can be seen normally west of the pre-Paleozoic crustal margin. To the east, the Sevier hinterland presents different sectors that underwent localized metamorphism. Major plutonic bodies are found west and east to the Precambrian continental margin. The easternmost extent of the Sevier and Laramide orogenies are also indicated (modified from Elison, 1991).

17 Accretion of an island arc onto the western margin of the North American plate, recorded in the Late Jurassic Nevadan Orogeny, represented the end of a pre-Farallon plate subduction (Ingersol and Schweickert, 1986; Lawton, 1994). This orogeny was followed by a period of extremely low subsidence rates in the Western Interior and volcanic quiescence until the late Early Cretaceous (Currie, 1998a). The drop in sedimentation rates was probably caused by the absence of flexural and dynamic subsidence in the retroarc basin during a period without active oceanic lithosphere subduction under the American craton (Lawton, 1994; Currie, 1998a). Active sedimentation and volcanism did not return to the Western Interior until higher orthogonal convergence, and consequent shallower and more rapid subduction of the Farallon plate started magmatism, and the Sevier contractional event began creating flexural subsidence (Jordan, 1981; Page and Engebretson, 1984).

2.3 Stratigraphic Overview The terrigenous succession of the Western Interior is divided into two tectonostratigraphic units by conglomeratic deposits that are found discontinuously but systematically from southern Utah to western Alberta, covering an area of 1,600,000 km2 (Winslow, 1987) (Fig. 1.5). Although biostratigraphic control is poor, stratigraphic, paleocurrent, and petrographic data suggest that the deposition of the various conglomeratic units is roughly contemporaneous (Stokes, 1944; Armstrong, 1968; Kirkwood, 1976; Heller and Paola, 1989) (Fig. 1.6). Therefore, although comprising several formations along their extent, these conglomeratic deposits have received treatment as an informal stratigraphic unit, termed the Lower Cretaceous conglomerates of the Western Interior by Heller and Paola (1989). Debate still exists on the tectonic setting of conglomeratic deposition. Some workers have linked their occurrence to the early synorogenic stage of the late Early

18 ? N

? 500 km ?

?

? Eastern Limit of Mesozoic Thrusting ?

?

Inferred limit of Lower Cretaceous conglomerate

Fig. 1.5: Extent and paleoflow direction of Lower Cretaceous conglomerate throughout the Western Interior. Although there is no homogeneous density of measurements and paleocurrent dispersal is evident, a general eastern to slightly northeastern trend can be seen. The easternmost limit of the conglomerate is not well defined (from Heller and Paola, 1989).

19 AGE CENTRAL WESTERN CENTRAL COLORADO WESTERN CENTRAL BLACK HILLS WESTERN SOUTHERN (m.y) UTAH COLORADO COLORADO FRONT RANGE WYOMING WYOMING SOUTH DAKOTA MONTANA CANADA 99 Mowry ShaleAspen Sh. Mowry Shale Colorado Muddy Ss. Newcastle Ss. Mill Creek Fm. ALBIAN Glencairn Sh. Bear River Ss. South Thermopolis Sh. Skull Creek Sh. Shale Plate Plainview Ss. Rusty Beds Fall River Ss. Upper Fm. Beaver Smoot Sh. Mines Fm. Shale Mmb. sandstone Greybull Ss. 112 Burro Upper Draney Ls. Gladstone Fm. Canyon Purgatoire Lyte Fm. Bechler Fm. Cloverly Fm. Fuson Fm. Dakota Gp. Kootenai Fm. Blairmore Gp. APTIAN Buckhorn Lower Congl. Cadomin Fm. Fm. Fm. Purgatoire Peterson Ls.

Cedar Mountain Fm. Conglomerate 121 Up. Ephraim Conlg. Lakota Ss. BARR. Inyan Kara Gp. 127 HAUT. 132 VALAN. 137 Kootenay EARLY CRETACEOUS EARLY BERR. Group 144 Lower

TITHON. Gannett Group Ephraim 151 Morrison Fm. Morrison Fm. Fernie KIMM. Cong. 154 Group OXFORD. Summerville Fm. Ralston Fm. Stump Fm. Sundance Fm. Swift Fm. L.JURASSIC

Fig. 1.6: The Jurassic-Cretaceous transition in the Western Interior, showing the chronostratigraphic location of the Lower Cretaceous conglomerate and the underlying inferred Jurassic-Cretaceous unconformity. Although physical lithostratigraphic correlation is not possible, these coarse clastic chert deposits, limited in the south by the Buckhorn Conglomerate and in the north by the Cadomin Formation have been considered to be roughly contemporaneous. Note that the different formations used to describe the Lower Cretaceous conglomerate and overlying continental rocks, and the remarkable extent of the Morrison Formation underlying the inferred low order unconformity (modified from Heller and Paola, 1989). Cretaceous Sevier foreland basin (Armstrong and Oriel, 1965; Wiltschko and Dorr, 1983; Currie, 1998a), whereas others workers have interpreted their presence as a pre-Sevier event originating farther west (Heller and Paola, 1989; Yingling and Heller, 1992; Bjerrum and Dorsey, 1995). The unfossiliferous nature of the conglomerate has precluded a biostratigraphic approach to determine their genetic link to the embedded formations. Although timing and tectonic context are still debated, strong agreement exists on the roughly contemporaneous deposition, the braided to meandering river depositional environment and a Late Paleozoic western source (Heller and Paola, 1989). Fluvial and lacustrine environments preceded and followed conglomeratic deposition. The Late Jurassic Morrison in the U.S. and the Kootenay Formations in Canada are found throughout the whole retroarc basin underlying the conglomerate. Overlying the conglomeratic deposits are several late Early Cretaceous formations defined in different basins along the Western Interior (Fig. 1.6). The temporal gap between the upper and lower tectonostratigraphic units, which may extend as much as 40 my, has been long interpreted to represent a low order unconformity placed at the base of the Lower Cretaceous conglomerate (Stokes, 1944, 1952; Young, 1960; Currie, 1997, 1998a) (Fig. 1.6). This interpretation is consistent with the erosive nature of the base of the conglomerate into the underlying Late Jurassic sediments. However, studies in distant and widespread sites along the Western Interior have yielded different results. Stratigraphic evidence exists of a conformable transition between the fluvial systems of the Morrison Formation and the Lower Cretaceous conglomerate in the Colorado Plateau and the Big Horn Basin in northwestern Wyoming and southeastern Montana (Kirkwood, 1976; Winslow, 1987; Aubrey, 1998). Within the Colorado Plateau, west of the Colorado River, the J/K unconformity has been long placed between the Morrison Formation and the Buckhorn Conglomerate. The latter has been considered to represent the lower member of the Cedar Mountain

21 Formation (Stokes, 1944, 1952; Young, 1960; Currie, 1997, 1998a). Where the Buckhorn Conglomerate is not present, the J/K unconformity is placed at the contact between the Morrison Formation and the upper Unnamed Shale Member of the Cedar Mountain Formation. East of the Colorado River, the Morrison is overlain by the Burro Canyon Formation, which has been traditionally considered to be correlative with the Unnamed Shale Member of the Cedar Mountain Formation (Stokes, 1952; Young, 1960; Craig, 1981, Currie, 1998a). The contact between the two formations is placed at the base of the lowermost chert conglomerate, which has been interpreted to be the eastern expression of the Buckhorn Conglomerate (Stokes and Phoenix, 1948).

1.4 This Study Recent geochronologic work has refined when deposition of the Morrison and Cedar Mountain Formations ended (Cifelli et al., 1997; Kowallis et al., 1991, 1998), but the onset of Buckhorn sedimentation has yet to be confidently established. The chronostratigraphic interpretation of the Buckhorn Conglomerate is of vital importance in order to understand the tectonic history of the Colorado Plateau and the adjacent Cordillera magmatic arc at the J/K transition. The goal of this project is to assess the temporal nature of the Morrison Formation-Buckhorn Conglomerate contact. The results of this work will link the conglomeratic deposits of the Colorado Plateau to a specific tectonostratigraphic unit, and bring new insight into the tectonic behavior of the Western Interior reotroarc basin during the J/K transition. Consequently, new information regarding the style of retroarc basin fill during this period is presented. A sequence stratigraphic approach is applied to the alloformations within the continental succession to define the Morrison depositional sequence.

22 1.5 Study Area Cedar Mountain was chosen as the study area of this project, since it is in this locality that the type section of the Buckhorn Conglomerate was defined (Stokes, 1944) (Fig. 1.7, 1.8). Cedar Mountain trends to the southeast and gently dips to the northwest. Covering an area of 160 km2, it gradually rises from 1,750 m in its western bottom to an elevation of approximately 2,300 m in its eastern side. Cedar Mountain is located on the northern part of the western flank of the San Rafael Swell, a N-S trending Laramide anticline uplift, in Emery county, east-central Utah (Fig. 1.7). The Upper Jurassic - Lower Cretaceous succession is well-exposed along the periphery of the Swell, approximately 80 km east of the Sevier thrust front, as well as in other areas of east-central Utah (Fig. 1.9). The region’s semi-arid climate restricts vegetation coverage and provides excellent exposure of the sedimentary rocks. The succession exposed in the study area extends from the Lower Jurassic Navajo Sandstone to the Late Cretaceous Blue Gate Member of the Mancos Shale (Fig. 1.10, Fig. 1.11). Quaternary pediment, slope wash, and alluvium materials are present scattered in the western part of the area. The whole succession slightly dips about 4o to the northwest. Minor NE-SW reverse faulting is present. The Morrison and Cedar Mountain Formations are well-exposed within the study area following a NE-SW trend interrupted by the presence of Little Cedar and Cedar Mountains, the outline of which controls the outcrop belt of both formations (Fig. 1.12). The presence of these topographic features is due to the very resistant Buckhorn Conglomerate, which slows the erosional retreat of the underlying Jurassic succession.

23 Canada N H 10 H 6

US

Sevier Sevier Foreland Basin Foreland

Castle Dale

Cordillera Thrust belt Thrust Ferron San Rafael Swell

I-70

N Cedar Mountain Fm. Morrison Fm.

US 500 km 30 km B Mexico A

Cleveland Lloyd 5 km Dinosaur Quarry N

Cedar Mountain Fm. Morrison Fm. C

Fig. 1.7: Location map of the study area. A: Map of the western U.S showing location of the Mesozoic subduction zone, magmatic arc, Sevier orogenic belt, and contiguous foreland basin (modified from Currie, 1997). B: Morrison and Cedar Mountain Formation exposures along the margins of the San Rafael Swell in Emery County (modified from Crooks, 1986). C: Study area map showing the Morrison and Cedar Mountain Formation outcrop belt (modified from Picard, 1988).

24 A

B

Fig. 1.8: Buckhorn type section. A: Panoramic view of the Buckhorn Type section at the Buckhorn Wash. The detailed location is indicated in Figure 1.12. Picture looking northwest. B: Close-up of Fig. 1.8a. A detailed stratigraphic section of the Buckhorn type section is presented in Appendix A (site A).

25 o 112 110 o 40 o

Gunnison Plateau Price

Canyon 6 Range Fig. 2.7

Green

15 Wasatch Plateau River Salina San Rafael Swell

70

Pavant Range

Cedar Mountain Fm. Hanksville Morrison Fm. km

0 50 100 Sevier Henry UTAH Belt Mountains Escalante N Kaiparowits Plateau

Fig. 1.9: Map of central Utah showing the location of the Morrison and Cedar Mountain Formation exposures along the margins of the San Rafael Swell and Henry Mountains. The Morrison Formation is also present in the eastern flank of the Kaiparowits Plateau, but disappears west of the Wasatch Plateau. Deposits equivalent to the Cedar Mountain Formation have been identified in the Gunnison Plateau. Outcrop of the Canyon Range and Pavant thrusts of the Sevier orogenic belt are also shown (modified from Yingling and Heller, 1992). 26 Age FORMATION SYMBOL COLUMN

Alluvium Qal

Slope Qsw wash Pediment

QUATERNARY QTpm mantle U

Blue E Kmbg L Gate

S

A

H

U

S

O Ferron Ss. Kmf

S

E

O

C

C

N Tunun A k

A

T Shale

M Kmt

E

R

C Dakota U

U

R Shale

. A Kcd

T D Mbr.

M

E

C Buckhorn U

N

O Brushy Jmb

S

Basin

I

R

R

O

Salt

M Wash Jsw U U Summerville Js

Curtis Jcu

C

I U

S

S

A

Je

R

Entrada

U

J

Jc Carmel

U

Navajo JtRn Sandstone

Fig. 1.10: Stratigraphic section of the study area. The exposed succession extends from the Lower Jurassic to the Upper Cretaceous. A low order unconformity divides the Mesozoic rocks from Quaternary glacial and alluvial sediments. The contact between the Morrison and Cedar Mountain Formations is considered to represent the Jurassic- Cretaceous unconformity. Note that the Tidwell Member is not plotted (modified from Trimble and Doelling, 1978).

27 Kmf 2 SCALE 1:100,000 Kmt Cleveland Lloyd Dinosaur Quarry 6 0 5 km 10 km 4 Kmbg 2 N

2

QTpm QTpm Jms 2 Je Jcu

Js

3 1 Jmb

Kdc 2

2

2 4

Qsw Jmb Jmb

Qal Jc 3 2 4

inverse strike and dip road 4 1 fault symbol

Fig. 1.11: Geologic map of the study area. With a northwestern to southeastern trend, Cedar Mountain is the major geologic feature in the study area. It is capped by the resistant Buckhorn Conglomerate of the Cedar Mountain Formation. The whole Mesozoic succession slightly dips towards the northwest and shows minor faulting. Legend of stratigraphic units can be found in Figure 1.10 (modified from Witkind, 1988). Cleveland Lloyd 5 km Dinosaur Quarry

N

Buckhorn Conglomerate type section

Cedar Mountain

Little Cedar Mountain

Buckhorn Wash

Upper Shale Member Cedar Mountain Fm. Buckhorn Conglomerate Member

Brushy Basin Member Morrison Fm. Salt Wash Member

Fig. 1.12: Map showing exposures of the Morrison and Cedar Mountain Formations in the study area. The distribution of the Buckhorn Conglomerate controls the geomorphologic expression of Cedar and Little Cedar Mountains. Along their flanks the Morrison Formations is well-exposed, whereas the unnamed Shale Member of the Cedar Mountain Formation is restricted to the west along the strike of the dipping succession. The Tidwell Member of the Morrison Formation is not plotted (modified from Witkind, 1988). CHAPTER 2: PREVIOUS WORK

2.1 Introduction Numerous studies have investigated the stratigraphy and geochronology of the J/K transition in the Colorado Plateau as well as the tectonic events that controlled its formation. In this chapter, an overview of previous work is presented in order to establish the contextual framework in which the results of the present study can be evaluated. Four major aspects deserve special attention: the regional stratigraphy of the succession, its chronostratigraphy, the thrusting events controlling sediment accumulation, and the most accepted tectonic models of Lower Cretaceous conglomeratic deposition.

2.2 Regional Stratigraphy 2.2.1 Introduction The deposits of the Morrison and Cedar Mountain Formations of the Colorado Plateau have been extensively studied by a large number of workers and consequently its regional stratigraphy is fairly well constrained (Stokes, 1944, 1952; Craig et al., 1955; Young, 1960; Craig, 1967, Yingling and Heller, 1992; Currie, 1997, 1998a, 1998b). The succession is unconformably bounded by the Summerville Formation and Dakota Sandstone of late Middle Jurassic and early Late Cretaceous age respectively (Peterson, 1988, 1994). Mudstone is the dominant lithology, but sandstone and conglomeratic bodies, as well as carbonate beds, are present throughout the succession (Stokes, 1944). Bentonitic mudstones and volcanic ash layers are also present indicating a strong volcaniclastic input (Bilbey, 1992). With the exception of the basal strata, the rest of the succession is represented by fluvial deposits with sporadic lacustrine facies (Peterson, 1988, 1994).

30 2.2.2 Morrison Formation The Morrison Formation is a sheet-like unit extending for 1,500,000 km2 from north- central New Mexico to Montana covering almost the entire U.S. Western Interior retroarc basin (Peterson, 1972; McGookey et al., 1972; Hunt and Lucas, 1987) (Fig. 2.1). Further north, in Canadian territory, it is roughly correlative with the Kootenay Group (Winslow, 1987; Heller and Paola, 1989) (Fig. 1.6). The formation reaches its maximum thickness in central Utah, and gradually thins eastwards until it conformably onlaps the craton in the westernmost part of the Great Plains. West of the Wasatch Plateau, in western Utah the formation is absent (Fig. 2.2). Although the Morrison is characterized almost exclusively by fluvial and lacustrine deposits, its basal and northernmost parts show evidence of coastal deposition (Fig. 2.1). This rather gradual transition has been interpreted to record the last transgression and final northward retreat of late Middle Jurassic Sundance Sea (Peterson, 1988, 1994) (Fig. 1.2). Due to the vast extent and lithological changes of the formation, detailed correlations are difficult. In the Colorado Plateau region the Morrison shows several important facies variations. Detailed work of this lateral lithologic variability has led to the naming of nine different members. Despite this complexity, the formation generally shows a transition from a lower sand- dominated, cliff-forming interval to an upper mudstone-dominated, slope-forming interval enabling the formation to be divided into two informal units (Stokes, 1944; Peterson, 1988, 1994). Both informal subunits are represented in the study area and in most part of the Colorado Plateau by the Salt Wash and the Brushy Basin Members, respectively (Fig. 2.1, Fig. 2.3). The latter reaches the larger extent of all the Morrison members, and represents the top of the formation throughout the Colorado Plateau (Peterson, 1988, 1994). The lithologic correlation of the Morrison of the Colorado Plateau with the Morrison deposits at its type section in Colorado is based on a limited number of marker beds (Emmons et al., 1896; Peterson, 1988). The most extensive marker is the drastic change of the mudstone major clay mineralogy from non-smectitic to smectitic, which is interpreted as a result of a

31 Lithostratigraphic General Isotopic Lithologies Units Paleocurrent dates

Dakota Ss.

98.4 + 0.1

Unnamed Shale Mbr.

Buckhorn

CEDAR MOUNTAIN FM. CEDAR MOUNTAIN Conglomerate 148.1 + 0.1

Brushy Basin Mbr.

50 m 150.2 + 0.1

Salt Wash MORRISON FM. Mbr.

0 Tidwell Mbr. 154.7 + 0.2 Summerville Fm.

Conglomerate Mudstone Calcite nodule- bearing mudstone Gypsum Sandstone Limestone Calcrete

Fig. 2.1: Schematic stratigraphic section of the Morrison and Cedar Mountain Formations in the San Rafael Swell. Mean paleoflow directions of the different Members in the San Rafael and surrounding areas of the Colorado Plateau (from Craig, 1955, Cadigan, 1967; Harris, 1980; Osterwald et al., 1981; Conley, 1986; Crooks, 1986; Yingling and Heller, 1992, Currie, 1998a) and isotopic ages obtain in sites within the study area or localities of the San Rafael Swell (from Cifelli et al., 1997 and Kowallis et al., 1998) are plotted.

32 Morrison Fm.- Cedar Mountain - 100 150 Buckhorn Conglomerate Burro Canyon Fms. 50 200 Isopach Map Isopach Map 0 1000 500 250 250 200 150 300 100 40 20 N N

150

20 40 0

40 20 1000 250 400 200 60 0 40 20 0

200

150 0 50 100 100 km 100 km ABthickness in meters thickness in meters

Fig. 2.2: Isopach maps of the Morrison and Cedar Mountain Formation in Utah and western Colorado. A: The Morrison Formation presents a rapid depositional pinchout in central Utah, and a slight eastern thinning. B: Cedar Mountain isopach map includes the Burro Canyon Formation, but excludes the Buckhorn Conglomerate Member. A pronounsed westward thickening and a gradual eastern thinning can be observed. In the south the formation is truncated by pre-Dakota erosion as a result of the northern tilting of the southern Western Interior (after Currie, 1998). Buckhorn Conglomerate

Brushy Basin Member

Salt Wash Member Tidwell Member Summerville Formation

Fig. 2.3: The Morrison Formation in the study area. The Tidwell, Salt Wash, and Brushy Basin Members can be observed overlying the Summerville Formation. The contact between the Tidwell and Salt Wash Members is marked by the sharp change in mudstone coloration and is interpreted to be unconformable. The contact between the Salt Wash and Brushy Basin Member is typically considered at the upper resistant sandstone ledge. Where the ledge is not present the contact is not discernible. The formation is capped by the Buckhorn Conglomerate. sudden increase in volcanism (Bilbey, 1992; Peterson, 1994). This clay mineralogy transition is present throughout the Morrison of the Colorado Plateau a few meters above the base of the Brushy Basin Member (Peterson, 1994) (Fig. 2.1). In the study area the Morrison is represented by the Tidwell, Salt Wash, and Brushy Basin Members, with a total thickness of 123 m at the Cleveland Lloyd dinosaur quarry (Bilbey, 1992). In central Utah, the formation does not show any major variation in regional total thickness (Yingling, 1987).

2.2.2.1 Tidwell Member The Tidwell is the basal member of the Morrison Formation. It consists of red and greenish-gray mudstones with minor lenses of gray sandstone, limestone, and gypsum beds (Yingling, 1987; Peterson, 1988; Bilbey, 1992) (Fig. 2.1, Fig. 2.3). Near the Cleveland Lloyd dinosaur quarry, this member has a thickness of 14 m and consists of supratidal facies similar to those defined at the type section in the southeastern San Rafael Swell (Peterson, 1988; Bilbey, 1992). Chert and freshwater invertebrate and charophytes in micritic rock carbonate fragments similar to Tidwell limestones are present in basal Salt Wash sandstones, which suggest lithification of the Tidwell Member before the onset of fluvial deposition and the presence of at least a minor unconformity between both members (Bilbey, 1992).

2.2.2.2 Salt Wash Member The Salt Wash Member is predominantly a fluvial unit with minor lacustrine influence (Lupton, 1914) (Fig. 2.1). It is dominated by white to pale brown, massive to cross-bedded sheet sandstones and minor conglomerate interbedded with greenish-gray to dark reddish- brown siltstone and mudstone and lenses of gray lacustrine limestone (Stokes, 1944; Craig et al., 1955; Cardigan, 1967; Currie, 1998a). Numerous studies have concluded that the Salt Wash Member was deposited by a north- to east-trending fluvial system (Craig et al., 1955; Cadigan, 1967; Peterson, 1984; Yingling, 1987; Currie, 1998a). These paleoflow directions

35 point to the Mogollon Slope as the most probable source area (Bilodeau, 1986; Peterson, 1994) (Fig. 2.4). The Mogollon Slope is a rift shoulder extending along southern Arizona with a northwestern to southeastern trend, which started its gradual uplift during the very late Middle Jurassic (Lawton et al., 1993). Both braided and meandering fluvial facies are present, with a vertical and roughly north-south lateral gradational change from proximal, amalgamated, coarser braided to more distal, finer-grained lenticular meandering deposits (Peterson, 1984; Yingling, 1987). This change is consistent with the interpretation of a general western to southwestern source area. In the study area the Salt Wash is approximately 20 m thick and fines upward from amalgamated basal and lower trough-crossbedded sandstones to a more mudstone-rich interval with single-channel, medium-grained sandstones (Crooks, 1986; Yingling, 1987; Bilbey, 1992) (Fig. 2.2, Fig. 2.3). In southern and southeastern Emery County the Salt Wash Member is up to 79 m thick and towards northern Emery County it reaches up to 60 m in thickness (Osterwald et al., 1981). The decrease in thickness toward the study area led Bilbey (1992) to suggest a higher elevation of the Emery County region with respect to most of the other parts of Salt Wash alluvial plain. This interpretation is consistent with synsedimentary intrabasinal uplifts during deposition of this member (Peterson, 1984). The upper contact of the Salt Wash Member is arbitrarily placed at the uppermost resistant sandstone ledge, but where the sandstone pinches out the contact between both members is not discernible (Bilbey, 1992) (Fig. 2.3). Although in areas of the Colorado Plateau further east the contact has been reported to be marked by a paleosol and therefore to be unconformable, in the study area a paleosol is not present and the Salt Wash-Brushy Basin Members transition appears to be gradational (Crooks, 1986; Demko, 1991; Bilbey, 1992).

36 nds hla

o Hig

rn Elk

Weste ?

Lake Eastern Elko Highlands T'oo'dichi' (Saline-alkaline lake)

Mud flats, scarce streams, scattered small lakes and ponds Alluvial plain N Wind direction Mogollon Slope ? Contemporaneous igneous rocks

?

?

0 300 km

Fig. 2.4: Brushy Basin Member paleogeographic map. During deposition of the uppermost member of the Morrison Formation, a major part of the southern Western Interior consisted of mud flats, with scarce streams and short-lived small lakes. A major saline-alkaline lake was located in the four corners area extending up to 500 km in a northwest-southeast trend. Higher areas in the east and southeast supplied sediment to the alluvial plain, which coarsens towards these source areas (after Peterson, 1994).

37 2.2.2.3 Brushy Basin Member The Brushy Basin Member is the most extensive member of the Morrison Formation (Fig. 2.1). It is represented by overbank variegated green, reddish, brown, and purple mudstones with minor fluvial conglomeratic sandstones and lacustrine lenticular limestone (Gregory, 1938; Stokes, 1944; Craig et al., 1955; Cadigan, 1967; Peterson, 1984; Currie, 1998a). Previous workers agree that the Brushy Basin fluvial drainage system followed the same northern to eastern pattern as the underlying Salt Wash Member, but received a major volcaniclastic input and lacustrine influence (Stokes, 1944; Bilbey, 1992; Peterson, 1994). This is supported by the higher proportion of mudstone and higher content in smectitic clays. In the study area the member has an approximate thickness of 80 m, and a fairly constant northeasterly plaleoflow direction (Crooks, 1986; Yingling, 1987; Bilbey, 1992) (Fig. 2.1, Fig. 2.3). Channel conglomerate composed of chert have been observed in this member as well as in the Salt Wash (Crooks, 1986; Conley, 1986; Yingling, 1987; Bilbey, 1992; Currie, 1998a), but unlike the overlying Buckhorn the conglomeratic bodies show little lateral continuity. These data suggests deposition by anastomosed fluvial systems (Kantor, 1995). Yingling (1987) observed a transitional facies change in the Brushy Basin Member of central Utah. South of Interstate 70 (Fig. 1.9) the member is composed of laterally restricted conglomeratic sandstone channels carrying a coarse load interbedded with finer and more bentonitic mudstones, whereas further north the member shows broader and finer-grained sandstone channels interbedded with coarser and less bentonitic mudstones. This northward broadening of the Brushy Basin channels, together with the fining trend of its bedload and increase in reworked volcanoclastic fines is consistent with the northeastern paleocurrent direction proposed by other workers and (Stokes, 1944; Craig et al., 1955; Cadigan, 1967; Peterson, 1984; Currie, 1998a). Drainage from the Mogollon Slope and other western source areas led to the formation of a broad, low-gradient alluvial plain, with coarse facies restricted to the periphery of the

38 source areas, and an extensive area of mud flats where fluvial and short-lived lacustrine conditions coexisted (Peterson, 1994) (Fig.2.4). Clay mineral zonation in the uppermost Brushy Basin Member at the Four Corners area, indicates the existence of a large alkaline lake towards the end of the member deposition (Turner and Fisherman, 1991) (Fig. 2.4). Consequently, semi-arid conditions have been suggested during deposition of the Brushy Basin fluvio-lacustrine system (Turner and Fisherman, 1991).

2.2.3 Cedar Mountain Formation The Cedar Mountain formation is much more aerially restricted than the underlying Morrison Formation, being only present in the northwestern part of the Colorado Plateau and the surrounding areas (Fig. 2.1). It extends from central Utah to western Colorado showing a drastic western thickening towards the Sevier thrust belt (Fig. 2.2). Further south, the Cedar Mountain has been removed by pre-Dakota erosion due to Early Cretaceous northern tilting of the southern Western Interior (Aubrey, 1989). This interpretation is corroborated by the angular unconformity between both formations. To the east the Cedar Mountain Formation gradually thins until being erosionally truncated. Eustasy-induced valley incision prior to Dakota Sandstone deposition has been identified in its easternmost part explaining the irregular isopach pattern (Aubrey, 1989). The Cedar Mountain Formation is entirely represented by fluvial and minor lacustrine environments. Two major lithostratigraphic units can be easily recognized. A basal discontinuous thin conglomeratic deposit is sharply overlain by a much thicker mudstone-dominated interval. According to this abrupt change in lithologies, the formation has been divided into two different members: the lower Buckhorn Conglomerate Member and the upper Unnamed Shale Member (Stokes, 1944, 1952). Both members were first described at Cedar Mountain.

39 2.2.3.1 Buckhorn Conglomerate Member The Buckhorn Conglomerate is a sheet-like body composed of chert-pebble conglomerate and conglomeratic sandstones (Fig. 2.1, 2.5). Although first given formation status (Stokes, 1944), its discontinuity and difficult mapping led to its later redefinition as the lower member of the Cedar Mountain (Stokes, 1952). Similar conglomerates within the overlying Unnamed Shale Member and the erosive base of the Buckhorn were criteria used to consider both members as part of the same formation (Stokes, 1952). In the San Rafael Swell the member has been reported to reach a thickness of 25 m (Yingling, 1987), but in the study area the unit is thinner with a mean thickness of 9 m (Bilbey, 1992) and gradually pinches out towards the east and southeast (Crooks, 1986; Bilbey, 1992). In Cedar Mountain the Buckhorn reaches a maximum thickness of 17 m in the surrounding areas of its type section (Conley, 1986). Conley (1986) considered that similar deposits found at similar stratigraphic levels in the Henry Mountains represent the southern extension of the Buckhorn deposits present in the San Rafael Swell (Fig. 1.9). Yingling (1987) however, concluded that the Buckhorn facies are restricted to the northwestern part of the Swell. The discontinuous nature of the conglomerates precludes the physical correlation of both deposits, making the exact aerial delineation of the true Buckhorn Member impossible. Despite this uncertainty, previous workers have proposed the Buckhorn Conglomerate to be correlative with similar lower Cretaceous chert conglomerates in Uinta Mountains in northeastern Utah and northwestern Colorado, and considered the later part of the same member (Currie, 1997, 1998a). Further northeast, the Buckhorn has been interpreted to merge into the Cloverly Formation of central Wyoming (Heller and Paola, 1989; Currie, 1997, 1998a) (Fig. 2.6). Although there is general agreement on the braided river depositional environment of the Buckhorn Conglomerate (Stokes, 1952; Young, 1960; Conley, 1986; Crooks, 1986; Yingling, 1987; Bilbey, 1992; Currie, 1997, 1998a), there is little agreement on its paleoflow direction.

40 erosional surface at top of section

coarse conglomerate, massive (?) or faint foreset cross-stratification

clean, massive sandstone: well indurated

coarse massive conglomerate as basal unit

clean sandstone as below, lens of conglomerate present, scour contact at base

conglomerate, clasts are 1/2 inch max., horitzontally bedded

clean sandstone, parallel laminations

coarse conglomerate as basal unit, faint horizontal bedding, pebbles up to 4 inches

3 m foreset cross-stratified sandstone, occasional pebbles at the base of laminae

coarse conglomerate, pebbles to 6 inches max., massive 0

BRUSHY BASIN Mbr.

Fig. 2.5: Detailed stratigraphic section of the Buckhorn Conglomerate at Cedar Mountain. The unit is made up of stacked pebble-size, chert conglomeratic bodies. Sandstone lenses normally showing erosional bounding surfaces are found scattered along the unit (after Conley, 1986). 41 San Rafael SW Swell 190 km Dakota Ss. Unita Mountains 275 km Dakota Fm. Cedar Mountain Central Fm. Wyoming

Fall River Ss. Unnamed Shale Buckhorn Conglomerate Cloverly Conglomerate

Upper Lower Mudstone Cloverly Fm. Brushy Basin Mbr. Brushy Basin Mbr. e ton nds Sa e Morrison Fm. per ton Up uds le M idd Morrisson Fm. L. Brushy Basin Mbr. M ne sto and 50 m Sundance Fm. U. Salt Wash Mbr. er S Low

L. Salt Wash Mbr. Tidwell Mbr. Sallt Wash Mbr. Windy Hill Mbr. 0 Tidwell Mbr. Redwater Mbr. Lithologies Summerville Fm. Conglomerate Mudstone Calcrete Sandstone Limestone

Fig. 2.6: A Utah to Wyoming correlation of the Buckhorn Conglomerate and the Cloverly Formation. Due to its lithological similarity and stratigraphic position, the Buckhorn Conglomerate has been chronostratigraphically related to the Lower Cretaceous conglomerates of the Western Interior (from Currie, 1998a). Some workers have proposed a northeastern drainage (Yingling, 1987; Currie, 1998a), but paleocurrent directions measured at Cedar Mountain show an approximately eastward trend (Conley, 1986). The latter is corroborated by a similar eastern Buckhorn paleoflow direction on the eastern flank of the San Rafael Swell (Osterwald et al., 1981). The erosional base of the member has been long considered to represent a low order unconformity spanning as much as 40 my bridging the Jurassic and Cretaceous periods (Stokes, 1944, 1952; Imlay, 1952; Young, 1960; McGookey et al., 1972; Currie, 1998a). However, load casts of Buckhorn conglomerate injected into the uppermost Morrison mudstones near Dinosaur National Monument and Green River reported by Kirkwood (1976) and Yingling (1987), as well as the depositional pinchout of the Buckhorn Conglomerate into Brushy Basin mudstones near Cleveland Lloyd dinosaur quarry (Peterson and Turner personal communication in Aubrey, 1998) and Green River (Aubrey, 1998) contradict this long-held interpretation.

2.2.3.2 Unnamed Shale Member The Unnamed Shale Member is a mud-dominated variegated unit with minor lenses of conglomeratic sandstone and carbonate (Stokes, 1952; Young, 1960; Harris, 1980) (Fig. 2.1). Lithologically, this unit is very similar to the underlying Brushy Basin Member but with a paler color (Stokes, 1944). As a result, some authors have proposed the Unnamed Shale Member corresponds to reworked Morrison materials (Stokes, 1944; Osterwald et al., 1981). Due to the similarity of the two mud-dominated members, in those localities of the study area where the Buckhorn is not present, the contact between the Morrison and Cedar Mountain Formations is reported to be very difficult if not impossible to recognize with certainty (Crooks, 1986; Conley, 1986). In central Utah the Unnamed Shale Member is interpreted to be the product of a northeast trending meandering fluvial system (Katich, 1954; Harris, 1980; Osterwald et al., 1981). Carbonate nodules in the mudstones, which represent caliche deposits, are very

43 abundant and are used in the identification of the member (Stokes, 1944, 1952; Young, 1960). Locally, carbonate nodules bearing mudstones coalesce into micritic carbonate horizons (Crooks, 1986; Yingling, 1987; Currie, 1998a; Aubrey, 1998). They generally show red silica replacement (Aubrey, 1998). Although calcrete horizons are found in the whole formation, they tend to be more common towards the base of the unit (Aubrey, 1998). Recent work has identified a prominent basal calcrete near or directly above the Buckhorn Conglomerate, which suggests that the transition between the two members is unconformable (Currie, 1998a; Aubrey, 1998) (Fig. 2.1). Young (1960) identified three major conglomeratic intervals in the area of Cedar Mountain, the lowest one of which is the Buckhorn Conglomerate. He found that in the Colorado Plateau the middle conglomeratic interval is normally 30 m above the lower, but it immediately overlies the Buckhorn at its type section. The upper conglomeratic interval has been reinterpreted as the lowest informal member of the overlying Dakota Sandstone, resting unconformably on top of the Unnamed Shale Member (Yingling, 1987). Conglomeratic deposits interbedded with carbonate nodule-bearing mudstones several hundred meters thick are present in the Gunnison Plateau northwest of the study area (Schwans, 1988; Witkind et al., 1986; Sprinkel et al., 1992) (Fig. 1.9). These deposits have been correlated eastward with the upper Cedar Mountain of central and eastern Utah (Witkind et al., 1986; Yingling and Heller, 1992) (Fig. 2.7). This increase in thickness has been interpreted by some authors to be a response to the first flexural effects of the Sevier Belt (Heller and Paola, 1989; Yingling and Heller, 1992). However, the rapid westward thickening of the Unnamed Shale Member is not present in the underlying Buckhorn Member (Young, 1960; Craig; 1981; Yingling and Heller, 1992) (Fig. 2.8).

44 GUNNISON SAN RAFAEL GREEN RIVER PLATEAU SWELL 85 km 75 km Dakota Ss. EAST WEST 860 346 224 200 300 Cedar Sanpete 800 Mountain Fm. Fm.

100 200 700

Morrison Fm. 0 100 600

Cedar Mountain Fm. 0 500

400 ? Key

300 Conglomerate Sandstone

200 Interbedded mudstone and sandstone Mudstone Limestone 100 Nodular limestone

Bentonite meters 0

Fig. 2.7: Lithostratigraphic correlation of the Cedar Mountain Formations in Green River and San Rafael Swell with similar deposits of the Gunnison Plateau. Note the drastic western increase in thickness of the formation. Note also that the Buckhorn Conglomerate does not follow the same regional thickness pattern, and it is not present west of the San Rafael Swell. See Figure 1.9 for location of the stratigraphic correlation (after Yingling and Heller, 1992). 45 GUNNISON SAN RAFAEL GREEN RIVER PLATEAU SWELL E W

DAKOTA SANDSTONE UPPER CEDAR MOUNTAIN FM.

BUCKHORN CONGLOMERATE

LOWER SANPETE MORRISON FM. FM. ? ? CEDAR MOUNTAIN CANYON FM. RANGE THRUST 250 m

25 km

Fig. 2.8: Cross section of the Morrison and Cedar Mountain Formations from central to eastern Utah. Note that the Morrison Formation and the Buckhorn Conglomerate are not present in Central Utah. The overlying Dakota Formation is interpreted to have a western gradational contact with the Lower Sanpete Formation (after Yingling and Heller, 1992). 2.2.4 Burro Canyon Formation The Burro Canyon Formation (Stokes and Phoenix, 1948) is composed of mudstone- dominated deposits overlying the Morrison Formation east of the Colorado River (Stokes, 1952). The scour structures at the base of the first chert pebbly sandstone of the formation have been interpreted to indicate an unconformable contact with the underlying Morrison Formation (Stokes, 1952; Young, 1960). The similarity in lithology and stratigraphic position led to its correlation with the Cedar Mountain Formation further west (Stokes, 1952; Young, 1960; Craig, 1981; Currie, 1998a). However, several workers have reported that the basal conglomeratic sandstone pinches out into underlying Brushy Basin Member mudstones (Craig et al., 1961; Ekren and Houser, 1965; Aubrey, 1992). Aubrey (1998) suggested a conformable transition between the Morrison and Burro Canyon Formations, and a major unconformity within the upper portion of the latter, at the base of a major calcrete horizon or channel- sandstones with micritic carbonate rip-up clasts. Consequently, Aubrey (1998) divided the Burro Canyon Formation in a lower conglomeratic part correlative with the Buckhorn Conglomerate and conformable with the underlying Morrison Formation, and a calcrete bearing upper part equivalent to the Unnamed Shale Member of the Cedar Mountain Formation to the west.

2.3 Chronostratigraphy 2.3.1 Morrison Formation The abundant dinosaur fauna in Morrison enabled early workers to date the formation as Jurassic (Schuchert, 1918; Simpson, 1926). However, the lack of diagnostic fauna precluded an accurate temporal constraint of the formation. Special interest was placed on determining the end of Morrison deposition, with both Late Jurassic and Early Cretaceous ages being proposed (Lee, 1915; Mook, 1916; Bakker, 1990; Miller et al., 1991).

47 Recent isotopic studies have shed more light on the timing of Morrison sedimentation. Sanidine 40Ar/39Ar ages from bentonite beds near the bottom and top of the formation show that Morrison deposition in the Colorado Plateau took place from approximately 155 to 148 Ma (Kowallis et al., 1991, 1998). Biostratigraphic (Schudack et al., 1998; Litwin et al., 1998), as well as paleomagnetic data (Steiner, 1994, 1998) support these isotopic ages. According to geologic time scale of Palmer and Geissman (1999) these dates translate to a period extending from the latest Oxfordian to the middle Tithonian. Altered volcanic beds 3.8 m above and 1.5 m below the lower and upper contacts of the Brushy Basin in Little Cedar Mountain, have yield ages of 150.2 ± 01 and 148.1 ± 2 Ma ages, respectively (Kowallis et al., 1998) (Fig. 2.1). According to these results deposition of the Brushy Basin Member in the study area took place in 2 my. However, the onset of Salt Wash Member deposition is still unconstrained.

2.3.2 Cedar Mountain and Burro Canyon Formations The unfossiliferous nature of the Buckhorn Conglomerate has precluded its biostratigraphic dating. However, the Lower Cretaceous conglomerates of the Cloverly Formation in central Wyoming have yielded a zircon fission track age of 129 ± 14 Ma (DeCelles and Burden, 1992). This age is supported by Valangenian palynomorph assemblages in the directly underlying mudstones previously considered to be part of the Morrison Formation (DeCelles and Burden, 1992). If the temporal correlation of the Cloverly Formationa and the Buckhorn Conglomerate is accurate as proposed by some authors (Heller and Paola, 1989; Currie, 1997, 1998a), these dates would indicate an Early Cretaceous age for the Buckhorn Conglomerate. Similarly, little is known about the age of the lower part of the Unnamed Shale Member. Although dinosaur fauna from this interval near Green River indicates a Barremian age (Kirkland, 1992), there are no unequivocal data to accurately constrain the timing of its deposition. Palynomorphs from the upper member of the Burro Canyon and Cedar Mountain

48 Formations have yielded Barremian to late Aptian ages, and Aptian to late Albian ages repectively (Tschudy et al., 1984). Angiosperm impressions from the equivalent deposits in the Gunnison Plateau indicate a late Albian or younger age (Wing in Schwans, 1988). Sanidine 40Ar/39 Ar dates from a bentonite 15 m below the top of the contact in the western flank of the San Rafael Swell have yielded 98.4 ± 0.1 Ma (Cifelli et al., 1997), indicating an early Cenomanian age for the culmination of Unnamed Shale Member deposition. In the San Rafael Swell several chronostratigraphic schemes for the Morrison and Cedar Mountain Formations have been proposed (Fig. 2.9). As mentioned above, most workers have interpreted both formations to be divided by an unconformity. However, based on evidence from the Swell and other nearby areas, Aubrey (1998) suggested a conformable contact. The presence of a calcrete horizon directly above the Buckhorn Member in this locality led the same author to place the unconformity at the top of the Buckhorn conglomeratic deposits.

2.4 Jurassic – Cretaceous Compression 2.4.1 Introduction Eastward to northeastward subduction of oceanic lithosphere beneath the North American plate as well as exotic terrane accretion resulted in the episodic contractile deformation of the western margin of the American continent during the Mesozoic and early Cenozoic (Elison, 1991) (Fig. 1.4). Location, extent, and timing of these tectonic events is fundamental in order to assess any control on the deposition of the Lower Cretaceous conglomerates. Therefore, the evolution of the Middle Jurassic to Early Cretaceous Western Interior retroarc foreland basin must be considered. The following overview is divided in two parts. The Sevier Orogenic belt is discussed first, since its younger development has resulted in better preserved structures and its more sound understanding. Evidence of earlier Middle to Late Jurassic thrusting in the Sevier hinterland is discussed later.

49 Yingling Stokes and Aubrey Currie AGE (1944) Heller (1998) (1998a) Ma (1992)

Dakota Ss. Dakota Ss. Dakota Ss. Dakota Ss. Cenomanian ? ? ? ? ? ? ? ? 100

Albian Unnamed Shale Unnamed C on Member Shale Unnamed 110 Member R n Formation Shale Member E n Formati ?

T Aptian Unnamed n Formation Shale Buckhorn A Conglomerate Member ? 120 Cedar Mountai C Member E N Barremian Cedar Mountai

e ? ? Cedar Mountai O o U Hauterivian ? 130 c Buckhorn S J/K Conglomerate o Cedar Mountain Formation Member m Valanginian J/K ? i ? J/K a ? 140 Berriasian ? Buckhorn Brushy Basin n Member Conglomerate Brushy Basin

? ation J Member ation Buckhorn Member

U Tithonian ? Congl. Mbr. .

Salt Wash Form on

J/K Form on Brushy Basin ris

R Member Fm Salt Wash Mbr. ris

150 r r

? Member o

A o

M ison ison

Kimmeridgian Brushy Basin M Tidwell Mbr. rr S Member Tidwell Mbr. o ? S M Lower Members

I Fm. orrison Lower Summerville Fm. ? Oxfordian Upper 160 C M Members Summerville Fm.

Fig. 2.9: Different chronstratigraphic diagrams proposed for the Morrison and Cedar Mountain Formations in the San Rafael Swell. Note that all the authors except Aubrey (1998) interpret the contact between the Morrison Formation and the Buckhorn Conglomerate to be unconformable.

50 2.4.2 Sevier Orogenic Belt The Sevier orogenic belt is a Late Mesozoic to Early Cenozoic fold and thrust belt that extends from southeastern Nevada to northern Montana and into Canada (Fig. 2.10). The belt represents thin-skinned retroarc deformation of the western margin of the North American continent (Armstrong, 1968; Burcherfield and Davis, 1975). It is characterized by eastward- younging thrust faults emplacing Precambrian and lower Paleozoic rocks over Mesozoic strata, with folding and thrusting affecting areas up to 100 km wide (Armstrong and Oriel, 1965). In Utah the thrust belt is divided into four segments: the Utah-Idaho-Wyoming segment, the Nebo salient, the central Utah segment, and the Nevada-southwestern Utah segment (Armstrong, 1968; Lageson and Schmitt, 1994; Currie, 2002) (Fig. 2.11). The segments are laterally decoupled from each other along transverse zones, but present similar structural patterns and amount of contraction (Lawton et al., 1994; Mitra and Sussman, 1997). In west-central Utah the Sevier belt is represented by the Central Utah segment, which holds the type section of the Sevier orogen (Armstrong, 1968). Structural studies conducted by Royse (1993) and DeCelles et al. (1995) have presented its subsurface configuration and structural reconstructions (Fig. 2.12). These authors show that the Central Utah segment involves deformation of Proterozoic to Tertiary rocks and it is composed of four major thrusts systems transported to the east. From older to younger, and from structurally upper to lower, the major thrusts are the Canyon Range, Pavant, Paxton, and Gunnison (Fig. 2.12a). The two older thrusts have a surface expression, whereas the two younger ones are blind structures. The structural reconstruction performed by DeCelles et al. (1995) shows that in west- central Utah Sevier activity started by the emplacement of the Canyon Range from the late Necomian through Albian time. This motion constrains the onset of the Sevier Orogeny in this region to the Early Cretaceous. According to Royse’s reconstruction, up to 6 km of sediment accumulated in front of the Canyon Range by the onset of the Late Cretaceous, but uplift of the initial thrust during the propagation of the orogenic front led to its subsequent erosion (Fig.

51 Eastern limit of the Sevier fold and thrust belt N

500 km

49o

Eastern limit 45 o of Jurassic and younger accretion

41o

37 o

Fig. 2.10: The Sevier orogenic belt. Extending from southeastern Nevada to northern Montana and into Canada, the late Mesozoic to early Cenozoic Sevier orogenic belt represents thin-skinned retroarc deformation of the western margin of the North American continent (after Elison, 1991).

52 o o 109 114 Utah- Idaho- Wyoming N 100 km Segment

41o

Uinta Mountains

Nebo Salient

Gunnison Canyon Range Plateau thrust Sevierfig.? Thrust Belt

San Rafael

Pavant Swell thrust Central Unco Utah mp ahgre Uplift Segment

Nevada SW Utah 37o Segment UT CO AZ NM

Sevier Belt Thrusts Laramide Thrusts Laramide Folds

Fig. 2.11: The Sevier orogenic belt of Utah (after Currie, 1998a). Four different thrust systems can be observed. From north to south they are: the Utah-Idaho Segment, the Provo Salient, the Central Utah Segment, and the Nevada-Southwestern Utah segment. The older Canyon Range and Pavant thrusts, as well as the easternmost limit of Sevier thrusting are indicated.

53 113 o 112 o W NW E SE Pavant Thrust Gunnison Thrust Canyon Range Thrust Paxton Thrust

S.L. S.L.

5 5 km

km 10 10 A 15 20 km. 15

o 112 o W NW 113 E SE Canyon Range Thrust Late Jurassic (?) - Early Cretaceous Foredeep

S.L. S.L.

5 5 98 Ma km

km 10 Gunnison Thrust 10

15 Paxton Thrust 15 B Pavant Thrust 20 km.

Precambrian Paleozoic Mesozoic Cenozoic

Precambrian crystalline rocks Lower Paleozoic Jurassic Tertiary

Proterozoic sedimentary units Upper Paleozoic and Triassic Jurassic intrusion

Cretaceous

Fig. 2.12: Cross section across the Central Utah segment of the Sevier orogenic belt. A: Unrestored cross section showing thrust deformation of Precambrian to Jurassic rocks. Four thrusts can be seen. From west to east and in order of emplacement they are the Canyon Range, Pavant, Paxton, and Gunnison thrusts. B: Restored cross section after Canyon Range thrust emplacement. A several km thick foredeep in front of the Canyon Range thrust is inferred. Location of the cross section is indicated in Figure 2.11 (after Royse, 1993). 2.12b). The late Early Cretaceous conglomeratic deposits interpreted as the westernmost part of the Cedar Mountain Formation of the Gunnison Plateau are possible remains of the exhumed foredeep succession (Royse, 1993).

2.4.3 Pre-Sevier thrusting Although Early Tertiary extensional faulting makes the tectonic restoration of the Sevier hinterland difficult, a broad region of Middle to Late Jurassic compressional deformation has been recognized in the area comprised by the Sevier thrust belt and the Cordilleran magmatic arc in the southern Western Interior (Allmendinger and Jordan, 1981, 1984; Oldow, 1984; Thorman et al., 1990, 1992; Smith et al., 1993) (Fig. 2.13). This extensive area can be divided in three major sectors. From east to west they are the Elko Orogenic Belt, the Central Nevada Thrust Belt, and the Luning-Fencemaker Thrust System. The Elko Orogeny is the name proposed for the Late Jurassic folding and thrusting in northern Nevada (Thorman et al., 1990, 1992; Taylor et al., 1993 in Bjerrum and Dorsey, 1995). The extent and age are constrained by the presence of Late Jurassic igneous rocks that intrude a thrusted succession in west-central and northwestern Utah (Allmendinger and Jordan, 1981, 1984). Previous work has pointed to this orogenic event as the agent responsible for the uplift of the western highlands, from which the Morrison Formation sediments where shed (Thorman et al., 1992; Peterson, 1994). The Central Nevada Thrust Belt consists of a large zone of deformation ranging from Permian to Early Cretaceous (Taylor et al., 1993 in Bjerrum and Dorsey, 1995). Up to 20 km of crustal thickening have been documented from the Ruby Mountains in the same region providing more evidence of a thrusting event in this region (Hodges et al., 1992). In western Nevada, the Middle to Late Jurassic Luning-Fencemaker Thrust System represents a wide zone of compressional deformation accounting for several hundred kilometers of crustal shortening (Oldow, 1984; Smith et al., 1993). This structural feature has been

55 Western edge Southern of Paleozoic Western continental shelf margin N Interior

Elko ? Elko Orogenic Belt Utah-Idaho Luning - Ancestral Fencemaker Trough Rocky Mts. Thrust System ? Uplift?

Central Nevada Thrust ? Belt (?) Fig. 6.2

Magmatic Monument Arc Bench Defiance Uplift? Eastern Sierra Thrust System 0 100 km

Fig. 2.13: Location of pre-Sevier thrusting in western Utah, Nevada, and southwestern California. Three major deformation zones have been identified. They are from east to west the Elko Orogenic Belt, the Central Nevada Thrust Belt, and the Luning- Fencemaker Thrust System (after Bjerrum and Dorsey, 1995).

56 inferred to represent crustal thickening along the Paleozoic continental shelf margin, as the result of the strike slip motion of the magmatic arc and the accretion of exotic terrains in the Jurassic to the American craton (Oldow, 1984). A southern extention of the Luning-Fencemaker thrust system has been documented in southern California (Boettcher and Walker, 1993). The interpretation of Middle to Late Jurassic orogenic belt is consistent with the existence of the Utah-Idaho Trough, a deep, elongated cratonal basin filled with a thick succession of nonmarine and marine sedimentary rocks that extends from southwestern Utah to eastern Idaho and western Wyoming (Fig. 2.13). Several workers considered this elongated sediment depozone to represent a Middle Jurassic retroarc foreland basin (Burchfiel and Davis, 1972; Oldow, 1984; Bjerrum and Dorsey, 1995). The data from both the thrust belt and associated foreland basin combine to form a compelling case for the presence of a Late Jurassic orogenic event. This has important implication for the source and timing of coarse clastic sediments shed into central Utah and the study area during this period.

2.5 Lower Cretaceous Conglomerate Tectonic Models 2.5.1 Introduction In this section the tectonic models that can explain the deposition of the Lower Cretaceous conglomerates found in the literature are discussed. These models are thermal doming, dynamic subsidence, isostatic rebound, foreland basin overfill, and foreland basin depozone migration (Fig. 2.14). Regardless of the specific tectonic mechanism responsible for the sedimentation of the Buckhorn Conglomerate, two main questions are to be answered. The first one is the syntectonic or post-tectonic nature of the conglomerate and the second one is its temporal relation with the Sevier Orogeny.

57 Thermal doming/dynamic plateau uplift

uplift erosional unroofing of Middle Jurassic thrust belt Isostatic rebound

Late Jurassic thrust belt Overfilled foreland basin

Late Jurassic Late Jurassic foreland-basin system thrust belt foredeep forebulge back-bulge

Paleozoic and Triassic location of present-day Lower/Middle Jurassic Wasatch Plateau Upper Jurassic 100 km

Fig. 2.14: Tectonic models of the Lower Cretaceous conglomerate deposition . Model A considers the deposition under effects of regional uplift unrelated to foreland basin dynamics. Models B, C, and D, consider different mechanisms through which the Lower Cretaceous conglomerate could have been deposited in a foreland basin system (after Currie, 1998a).

58 2.5.2 Tectonic models The absence of any thickening trend towards the Sevier thrust belt have led some workers to consider the Buckhorn Conglomerate to be a pre-Sevier depositional event (Yingling, 1987; Yingling and Heller, 1992; Lawton, 1994) (Fig. 2.8). A synorogenic nature of the conglomeratic deposits is not supported by the large extent of the Lower Cretaceous conglomerate, which is uncommon of flexural basin fill sediments (Heller and Paola, 1989) (Fig. 2.15). Alternatively, a western regional uplift of the eastern margin of the Cordilleran magmatic arc could better account for the thickness and extent of the gravels in the Western Interior. Two possible crustal events could explain the regional uplift: thermal doming (Heller, and Paola, 1989; Yingling and Heller, 1992) and dynamic plateau uplift (Lawton, 1994). The pre-Sevier thermal doming hypothesis is consistent with the Late Jurassic eastward shift and peak in magmatic activity in eastern Nevada and western Utah (Christiansen et al., 1994), and the abandonment of the Middle Jurassic magmatic arc in central California as the result of the Nevadan Orogeny (Lawton, 1994). However, accretion of an island arc to the continental margin would have induced a change to westward-directed subduction before the collision (Schweickert and Cowan, 1975; Ingersoll and Schweickert, 1986). Eastward subduction was later renewed with the onset of Farallon plate subduction (Lawton, 1994). The dynamic uplift hypothesis states that the end of pre-Farallon plate subduction ended dynamic subduction and triggered the regional uplift of the proximal setting of the retroarc basin in the Late Jurassic following the Nevadan Orogeny (Lawton, 1994). Uplift would not only have deposited the Lower Cretaceous conglomerates, but also the Morrison Formation (Lawton, 1994). This interpretation is based in lithospheric subduction models (Mitrovica et al., 1989; Gurnis, 1992) which show that the deepening or detachment of the subducting slab is compensated by a broad regional uplift. Some tectonic models consider the deposition of the Lower Cretaceous conglomerates to have taken place in a foreland basin setting (Fig.2.16). The isostatic rebound of the foredeep

59 600

500

400

300

200

100

TRANSPORT DISTANCE (km) DISTANCE TRANSPORT 0 Alps Andes Antler BeltApennin HimalayasPyrenees Sevier BeltCloverly Fm.

es LOCALITY

Fig. 2.15: Comparison of the extent of the Lower Cretaceous conglomerates in central Wyoming (Cloverly Formation) with the extent of materials deposited in foredeep depozone of modern and ancient foreland basins. Transport distance is considered to be the distance from the thrust front to the distal part of the foredeep (after Heller and Paola, 1989).

60 A.

FOLD-THRUST BELT

MAR OCEAN FORELAND BASIN GINAL BASIN MARGINAL OCEAN BASIN FOREBULGE

CRATON

B.

FORELAND BASIN SYSTEM OROGENIC WEDGE THRUST WEDGE-TOP FOREDEEP FOREBULGE BACK-BULGE TOPOGRAPHIC FRONT

FOLD-THRUST BELT FRONTAL TRIANGLE ZONE CRATON DUPLEX

Fig. 2.16: The foreland-basin system. A: Planar view of a schematic foreland basin. As the result of thrust loading a trough-shaped depression is formed between the fold- thrust belt and the contiguous craton. B: Cross section of a schematic foreland-basin system. Four different depozones are identified according to the differential flexure of the listhosphere. From proximal to distal settings they are: the wedge-top, foredeep, forebulge, and back-bulge (after DeCelles and Giles, 1996).

61 contiguous to a pre-Sevier thrust system during a period of tectonic quiescence could explain the large aerial extent and geometry of the conglomerates (Heller and Paola, 1989) (Fig. 2.17). During such a period, exhumation of the thrust loads induces the isostatic restabilization of the crust by uplifting the proximal settings of the flexural basin (Heller et al., 1988). Consequently, during post-orogenic periods old synorogenic materials are reworked and transported towards the distal settings of the foreland basin. Low accommodation conditions lead to the deposition of a thin and widespread coarse clastic unit, that onlaps the uplifted proximal materials. Similar extensive sheet-like clastic deposits could also have been deposited in the Western Interior as a consequence of the overfill of the Sevier foreland basin (DeCelles and Burden, 1992). This scenario takes place when the sedimentation rate resulting from the exhumation of the tectonic load is higher than the rate of flexural subsidence created by thrust emplacement (DeCelles and Giles, 1996). Negative accommodation in the flexural foredeep leads to the transport of synorogenic coarse detritus into the distal settings of the foreland basin. In contrast to the isostatic rebound model, the clastic unit in this case shows lateral continuity with the thrust belt sediments, merging into the thick foredeep deposits. Based on the similarity of the Middle Jurassic to Early Paleogene total subsidence curve of northeastern Utah and northwestern Colorado with the subsidence rate pattern expected from the temporal propagation of the different depozones of a foreland-basin system, some workers have proposed that the Sevier foreland basin was active since the Middle Jurassic (DeCelles and Currie, 1996; Currie, 1997, 1998) (Fig. 2.18). In this scenario, the Morrison Formation is interpreted as a back-bulge deposit and the Cedar Mountain Formation as a foredeep succession resulting from renewed loading and easterward migration of the foreland system. Existence of a Late Jurassic forebulge in central Utah has been reported to be indicated by the western onlap of the lower part of the Morrison Formation on Middle Jurassic strata (Currie, 1998b).

62 THRUST-DERIVED MOLASSE EXTRABASINAL STREAMS

ACTIVE THRUST BELT A

SUBSIDENCE

EROSION REWORKED DEPOSITS

INACTIVE THRUST BELT B

REBOUND

Fig. 2.17: Two-phase model of foreland-basin stratigarphic fill. A: Synorogenic sediments are trapped close to the thrust belt as the result of thrust loading-induced foredeep flexural subsidence. B: During post-orogenic stages, erosion of the thrust loads leads to the isostatic rebound of the foredeep. Previously deposited synorogenic materials are reworked and transported into distal parts of the foreland basin (after Heller et al., 1988).

63 Subsidence Rate Low High A Wedge-top depozone

Foredeep depozone

Forebulge depozone Back-bulge depozone

Basement

B JURASSIC CRETACEOUS PALEOGENE Middle LateNecomian Early 0

-1 Jtc -Jp

-2 Js-Jm Kk -3

-4 Tw Wasatch Fm. Kf -5 TKe Evanston Fm. Khf Hams Fork Conglomerate km Kh-Kw Henefer, Echo Canyon & Weber Canyon Fms. Kh- -6 Kf Frontier Fm. Kw Khf TKe Tw Kk Kelvin (or Cedar Mountain Fm.) -7 Js-Jm Stump & Morrison Fm. Jtc-Jp Twin Creek & Preuss Fms. -8 Depostion of Morrison Fm. in study area -9 Depostion of Cedar Mountain Fm. in study area -10 Unconformity -11 180 160 140 120 100 80 60 Ma

Fig. 2.18: Foreland-basin system subsidence curve. A: Idealized subsidence-curve produced by the temporal migration of the foreland-basin system depozones. B: Total subsidence curve for the Middle Jurassic to lower Eocene succession in northeastern Utah and northwestern Colorado (after DeCelles and Currie, 1996).

64 According to this model, deposition of the Buckhorn Conglomerate would have taken place during the migration of the flexural forebulge from central Utah to central Colorado during the Early Cretaceous, as the result of renewed thrusting and consequent eastern propagation of the foreland-basin system. Progressive migration of the foreland basin forebulge would have led to the incision of the Buckhorn fluvial system and therefore to the formation of a low order unconformity at its base (DeCelles and Currie, 1996; Currie, 1997, 1998a, 1998b) (Fig. 2.19).

65 North-central Northwestern Utah Colorado

50 . Eocene Wasatch Fm. Green River Fm. Wasatch Fm.

R

60 E Paleocene Evanston Fm. Ft. Union Fm. T Currant Cr. Fm. 70 Hams Fk. Cgl. Upper 80 W-E Cyn Cgl. Mesaverde Gp. Cretaceous

S Henefer Fm. Mancos Shale

90 U

O Frontier Fm.

E

100 Aspen Shale Dakota Fm Mowry Shale

C

A

110 T Burro Canyon Fm.

E Kelvin Fm. Cedar Mountain Fm.

R Lower ? 120 C Cretaceous ? Buckhorn 130 Jurassic-Cretaceous ? unconformity Conglomerate 140 Upper 150 Morrison Fm. Jurassic Stump Fm. Sundance Fm. 160 Entrada Ss. Preuss Fm.

C Carmel Fm.

Middle I

170 Twin Creek Fm.

S Jurassic

S Gypsum Springs Mbr. A 180 J2 Unconformity R

U

190 J Lower Jurassic Nugget Sandstone Glen Canyon Sandstone

200

. I

210 Upper Ankareh Fm. Chinle Fm R

Triassic T

Fig.2.19: Chronostratigraphic diagram of Middle Jurassic-lower Eocene of Utah and adjacent areas in Wyoming and Colorado. The authors consider the Buckhorn Conglomerate to correspond to a period of incised valley fill during the formation of the Jurassic-Cretaceous unconformity (after DeCelles and Currie, 1996).

66 CHAPTER 3: METHODOLOGY

3.1 Introduction Fieldwork for this project was performed during July and August of 2002. The field data together with literature sources are the basis for the conclusion derived in the present study. Stratigraphic, sedimentologic, and petrographic data were collected in order to better understand the genetic relationship between the Brushy Basin Member and the Buckhorn Conglomerate.

3.2 Stratigraphic Data Measurement and correlation of 30 stratigraphic sections that include the upper Brushy Basin Member, the Buckhorn Conglomerate Member, and locally the Unnamed Shale Member form the core of the field data (detailed sections are shown in Appendix A) (Fig. 3.1). Accessibility and outcrop conditions influenced the concentration of gathered data along the western part of the study area, where the succession is more accessible. Most the stratigraphic information is synthesized in a 24 km long southwest to northeast stratigraphic cross-section transverse to the orientation of Cedar Mountain. Three shorter cross-sections oriented roughly perpendicular to the longer section enhance the understanding of the third dimension of the succession. The measured sections and other field measurements were used to generate an isopach map of the Buckhorn Conglomerate.

3.3 Sedimentologic Data Sedimentologic data from the measured succession allowed the interpretation of the Brushy Basin and Buckhorn depositional environments. Although the contact between the two units is covered over most of the study area, where exposed, the contact was examined for sedimentary structures that could differentiate between a conformable and unconformable

67 A' SB RB Cleveland Lloyd Dinosaur Quarry UB N 5 km OB XB PB ZB CC WB JB IB D KB D' GB MB

J KA

EA IA C A

OA WA

Y S C' A P UA K ZA O M B F B'

Brushy Basin Member-Buckhorn Congolomerate contact KB Section location

Fig. 3.1: Location of the stratigraphic sections measured in this study. Four correlation lines were constructed. The longest one runs parallel to the strike of the succession. The other three are approximately parallel to the trend of Cedar Mountain. transition. Paleoflow data were collected in order to establish and compare the drainage direction of the fluvial paleosystems of both members. Measurements were taken from trough- crossbedded sandstones and pebbly conglomerates of both units. Two types of measurements were differentiated according to their outcrop conditions. Measurements taken on crossbeds with two-dimensional exposure were plotted separately from measurements taken on outcrops with complete exposure of the crossbeds. This differentiation was necessary since measurements obtained from crossbeds with three-dimensional exposure have a higher paleocurrent significance that those taken on two-dimensional structures. Paleoflow maps of both members were drawn.

3.4 Petrographic Data Several sandstones and conglomerates of both members were sampled to compare their petrographic characteristics in order to determine if a transition in source area across the was present. Fifteen sandstone thin sections were obtained and studied on the microscope. No feldspar staining was performed. The proportion of the clast lithologies of six medium-grained sandstones was calculated. In each sample the lithology of a minimum of 400 sand grains was studied in a strip of the rock perpendicular to the bedding direction. Digital pictures of the thin section area were taken and assembled into a photomosaic, and the area of the counted grains was calculated using Canvas. The results are summarized in Appendix B. Petrographic data from the conglomeratic facies of the two members were also collected. At least 360 pebbles were counted in an area of 1 m2. Chips of the counted pebbles were collected and studied under a binocular microscope to determine the lithology. Eight Buckhorn Conglomerates pebble counts were used to see if any petrographic trend throughout the Buckhorn deposits was present. Pebble counts of Brushy Basin Conglomerates were precluded by their much smaller clast size. However, they were sampled and studied under the

69 binocular microscope. Lithoclast types between both members were compared in order to assess any difference in source area between the members.

70 CHAPTER 4: FACIES ANALYSIS

4.1 Introduction This chapter discusses in detail the stratigraphy and different lithofacies present in the upper Brushy Basin Member, Buckhorn Conglomerate, and lowermost Upper Shale Members in the study area. The succession has been divided in four different lithofacies assemblages: mudstones and siltstones, sandstone, conglomerate, and carbonate. A description of each stratigraphic unit within the study area is followed by a detailed description and interpretation of the different lithofacies grouped by stratigraphic unit. Finally, the facies information is summarized interpreted in terms of the depositional environment of each stratigraphic member.

4.2 General Stratigraphy 4.2.1 Brushy Basin Member (Morrison Formation) The Brushy Basin Member is present throughout the whole study area with a mean thickness of 75 m and very little variation (Fig. 4.1a, 4.1b, and 4.2). Variegated, smectitic claystone (Keller, 1962) is the most diagnostic feature of this member (Table 4.1). Sandstone and conglomeratic bodies, as well as thin siltstone and freshwater limestone beds are also present. Although they are found throughout the member, the coarse clastic and carbonate lithologies are more common toward the base of the unit; the upper part of the member is normally represented exclusively by smectitic mudstone (Fig. 4.3 and 4.4). One bentonite bed close to the upper contact of the unit was also identified. The predominance of claystone results in the formation of gentle slopes with channel sandstones and conglomerates exposed as more resistant ledges. Locally, sandstone bodies show intense weathering. Erosional retreat of the Brushy Basin Member mudstones causes the collapse of the edges of the overlying Buckhorn Conglomerate resulting in the large covered areas near the upper contact. 71 A B

C D

Fig. 4.1: Brushy Basin Member (Morrison Formation) and Buckhorn Member (Cedar Mountain Formation). A and B: Variegated, mud-dominated deposits of the Brushy Basin Member (sites KB and MB). C: Buckhorn well-cemented facies (stick is 1.5 m long, site J). D: Intensive weathering and disaggregation of the Buckhorn in the northern part of the study (site GB).

72 SB 5 km RB Cleveland Lloyd Dinosaur Quarry N ZB PB BA WB JB IB KB

HB MB J T Buckhorn Conglomerate KA type section EA IA

Cedar T A Mountain A CC

WA OA AA S R Little Cedar ZA Mountain P UA Q Buckhorn Wash F

EC

Cedar Mountain Overlook

Morrison Formation Cedar Mountain Formation Contact Sedimentary Structures Site Sandstone Upper Shale Member point count Brushy Basin Member gutter casts ZA Pebble point Salt Wash Member Buckhorn Conglomerate Member mud injection dinosaur track count

Fig. 4.2: Location map of the pictures shown in this chapter. Diagnostic sedimentary structures at the Brushy Basin-Buckhorn Conglomerate, as well as location of sandstone and conglomerate framework counts are also indicated. Brushy Basin Member Buckhorn Conglomerate Conglomerates 2.3% 72.1% Sandstone 15.3% 27.9% Siltstone 4.5% 0% Mudstone 77.2% <0.1% Limestone 0.7% 0% Total 100% 100%

Table 4.1: Lithologies of the Brushy Basin Member and Buckhorn Conglomerate in the study area. The percentages have been obtained from the measured stratigraphic sections (Appendix A).

74 A SW NE

A O 0.1km P 3.6 km Y 2.3 km WA 1.3 km OA 4.5 km IA 0.5 km EA 0.4 km KA 0.6 km J 1.6 kmMB 1.2 km JB 1.2 km CC 0.3 km XB 0.4 km WB 0.2 km ZB 0.2 km PB 0.6 km OB 2.4 km UB 1.1km RB 0.2 km SB A' usm usm usm usm bc bc bc bc bc

bbm bbm

bbm bbm

10 m 24 km

Brushy Basin Member (bbm) and Upper Shale Member (usm) Buckhorn Conglomerate (bc) (Cedar Mountain Fm.)

Conglomerate Siltstone Limestone Conglomerate Mudstone

Sandstone Mudstone Bentonite Sandstone Calcrete

Fig. 4.3: Stratigraphic correlation measured in this study (location map in Figure 3.1). A: SW-NE section transverse to the orientation of Cedar Mountain. Note the depositional pinchouts of the Buckhorn Conglomerate and its local interfingering with the underlying Brushy Basin Member (boxes). Note also the presence of calcrete horizons at both margins of the section. Detailed sections are shown in Appendix A. Datum is the top of the Buckhorn Conglomerate. BCD

WENW SE W E

GB1.2 km IB 1.5 km KB O 0.1kmP 1.2 kmUA 0.5 km M 1.3 kmK 1.7 km F A 1.8 kmWA 1.1 kmSZA 7.4 km Buckhorn Conglomerate Type Section B B ' CC 'DD ' usm usm usm

bc bc bc

bbm

bbm

bbm

10 m 10 m 10 m 5 km 10 km 2.5 km

Brushy Basin Member (bbm) and Upper Shale Member (usm) Buckhorn Conglomerate (bc) (Cedar Mountain Fm.)

Conglomerate Siltstone Limestone Conglomerate Mudstone

Sandstone Mudstone Bentonite Sandstone Calcrete

Fig. 4.4: Stratigraphic correlations measured in this study (location map in Figure 3.1). B: W-E section along the southern flank of Little Cedar Mountain. C: NW-SE section along the axis of Cedar Mountain. D: W-E section along the area of Buckhorn induration change. Detailed sections are shown in Appendix A. Datum is the top of the Buckhorn Conglomerate. 4.2.2 Buckhorn Conglomerate (Cedar Mountain Formation) The Buckhorn Conglomerate is a coarse clastic unit comprising clast-supported pebble and cobble conglomerates and interbedded conglomeratic sandstones (Table 4.1). Although intramember lens correlation is precluded by rapid lateral facies change, the unit shows an overall fining-upward trend (Fig.4.1c, 4.1d). Blocky mudstone lenses are very rare, but locally present. The unit displays a broad sheet geometry (following classification of Friend, 1983), ranging in thickness between less than 1 and more than 16 m, and presenting a width : thickness ratio of 3,000 : 1 (Fig. 4.5). The Buckhorn has depositional pinchouts south of Little Cedar Mountain (Fig. 4.6), and in the area southwest of the Cleveland Lloyd Quarry road. In the latter, the Brushy Basin Member and the Buckhorn Conglomerate show local interfingering (Fig. 4.7). Although Buckhorn thickness variations are common throughout the study area, there is a general trend of maximum thickness along the approximately east-west axis of Cedar Mountain (Fig. 4.5). Grain size has a fairly homogeneous distribution throughout the Buckhorn exposures. The cementation of this member also shows significant variations. Two main types can be easily distinguished, which affect strongly the geomorphology of the outcrops. In the areas surrounding the type section, the Buckhorn Conglomerate is well-cemented (Fig. 4.1c), and this accounts for the positive relief of both Cedar and Little Cedar Mountains. North of the northern flank of Cedar Mountain, the Buckhorn becomes rapidly less indurated, and tends to erode very easily forming a semi-covered level of loose pebble conglomerates and disaggregated sandstones (Fig. 4.1d). Minor northeastern to northwestern inverse faulting affects the units locally, with a few meters of offset in some localized sites, and slickensides of decimeter-scale movement being fairly abundant. When the overlying member has not been removed by erosion, the Buckhorn is overlain by light purple to pink mudstones with carbonate nodules, and locally by colored chert and quartzite conglomerate and conglomeratic sandstones, as well as one or several micritic carbonate horizons of the Unnamed Shale of the Cedar Mountain Formation. 77 2.1Cleveland Lloyd 1.6 Dinosaur Quarry 2.8 3.5

0 6.8 0 3.1 0 7.2 2.5 6.2 2.3 3.8 3.8 3.5 11.9 4.3 4.5 3 7.8 10.5 8.5 6 7.3

>16 9.1 8.1 7.5 8.0

10.5 9 16.3 11.3 10.5 9.1 7.9 9.4 8.4 6.9 13.25 0.9 10.5 7.7 >6.9 12.1 10.1 10.6 0.8 4.6 4.6 8.2 5.0 8.3 9 0 2.4 10.2

> 16 m 6 12-15 m 9-12 m 3 6-9 m 0 3-6 m Buckhorn Conglomerate Isopach Map 0-3 m Contour interval = 3m N 5 km

Brushy Basin Member-Buckhorn Conglomerate contact

Fig. 4.5: Buckhorn Conglomerate isopach map of the western half of the study area. Note that the main thickness axis coincides with the E-W orientation of the Cedar Mountain. Also note the depositional pinchouts in the south and the north. The unit becomes progressively thicker towards the west suggesting an eastern flow across an unconfined Buckhorn alluvial plain.

78 SW NE

Fig. 4.6: Buckhorn depositional pinchout in the southwestern part of the study area. The presence of weathered gray sandstone indicating the outcrop of Buckhorn distal facies (arrow inside the circle on the right). The resistant beds in the outcrop across the road correspond to a calcrete horizon stratigraphically above the Buckhorn (arrow inside the circle on the left). The distance between outcrops is approximately 100 m. The field of view is 180 degrees (sites O and P). Note the car for scale (box). SW NE

A 50 m

Upper Buckhorn Conglomerate lens Lower Buckhorn Conglomeratic lens SW NE

Brushy Basin Member mudstones interfingering between Buckhorn lenses

50 m

B Fig. 4.7: Brushy Basin Member and Buckhorn Conglomerate interfingering. The Buckhorn is represented by two channel lenses of chert pebbly sandstones and conglomerates, separated by Brushy Basin Member uppermost mudstones (sites WB, ZB, and PB). A: Picture of the outcrop. B: Outline of the two Buckhorn Conglomerate lenses interfingering with the Brushy Basin Member uppermost mudstones. 4.3 Mudstone and Siltstone Facies 4.3.1 Brushy Basin Member Mudstones and Siltstones Description: Blocky swelling claystone, weathering to a characteristic frothy surface texture is the predominant lithology of the Brushy Basin Member. The most outstanding feature is its variegated coloring, comprising a wide range with several hues of green, gray, blue, purple, pink, and red (Fig. 4.8a). Meter- and decimeter-scale banding and millimeter-scale mottling are constant outcrop features of Brushy Basin mudstones throughout the study area. The mudstone facies are commonly smectitic (Keller, 1962; Crooks, 1986; Bilbey, 1992) and comprise a wide variety of grain sizes, ranging from claystones to sandy siltstones (Fig. 4.8b). Nevertheless, clay with low percentage of silt is the most common grain size. The smectitic nature of the mudstones indicates input of fine-grained volcanigenic material. A bentonite bed with a thickness of 64 cm was observed 25 m below the upper contact near the Cleveland Lloyd Dinosaur Quarry (Bilbey, 1992). It shows dark purple flame-like microstructures floating in a light purple matrix. Calcite is the main mudstone cement. When abundant in siltstones, the beds form more resistant ledges within mudstone successions with thickness varying from less than 5 cm to 30 cm. Calcite nodules and veins are not very common but present in some outcrops. Root traces are commonly found associated with these diagenetic features. The sedimentary structures of the uppermost member mudstones, immediately below the contact between both formations are of especial interest. Centimeter-scale banding is the most common feature (Fig. 4.8b). It involves a series of laterally transitional purple and greenish horizons showing different oxidation-reduction stages. Near the Cleveland Lloyd Dinosaur Quarry, irregular decimeter-scale bodies of platy brown mudstones are common within an interval of two meters below the overlying Buckhorn conglomeratic facies. South of the southern Buckhorn depositional pinchout in the study area, very similar platy mudstones have been observed forming a horizontal 7 cm thick layer above a white disaggregated, discontinuous 81 A

B

Fig. 4.8: Brushy Basin Member mudstones. A: Variegated, meter-scale banded mudstone beds (site IB). B: Banded claystone and a lower siltstone below the contact with a Buckhorn basal sandstone (site WB).

82 sandstone lens (Fig. 4.9). Similar structures have been observed by the author in outcrops on Interstate 70 southwest of Green River, where the lowermost conglomeratic facies of the Cedar Mountain Formation are present a few meters above the contact with the underlying Brushy Basin Member. Flame structures are locally present at the contact between the Brushy Basin mudstones and the overlying Buckhorn conglomerate deforming primary lamination in the mudstones. The best example is found at the easternmost tip of the Little Cedar Mountain (Fig. 4.10). Although the lithological change between both formations is consistently sharp along the southern part of the study area, pebbles and granules of Buckhorn lithologies are found in the uppermost Brushy Basin mudstones in some localities in the northern part.

Interpretation: I interpret Brushy Basin Member mudstone facies to represent overbank fluvial deposits with lacustrine influence and some air-borne volcanigenic input. Well-cemented sandy siltstone beds are present throughout the succession representing distal facies of crevasse splay deposits. Calcite nodules are inferred to be the result of dissolution and reprecipitation of the calcite cement of the surrounding mudstones. These features, as well as the associated root traces, indicate horizons of pedogenic alteration (Currie, 1998b). The flame structures are interpreted to have resulted from the injection of the uppermost Brushy Basin Member mudstones into the overlying Buckhorn Conglomerate, since the original lamination of the Morrison fine-grained materials is disrupted. Therefore, I consider flame structures to be the result of soft sediment deformation (Prothero and Schwab, 1996). Due to lateral gradual color transitions, a diagenetic origin is suggested for the purple and green banded mudstones directly below the overlying Buckhorn Conglomerate, probably as a result of variations in the paleo-groundwater level. However, pedogenic processes may have played a secondary role on the characteristic banding of the mudstones. Branching platy structures of the uppermost blocky purple Brushy Basin Member mudstones at Cleveland Lloyd 83 A 2.5 m

B 10 cm

Fig. 4.9: Uppermost Brushy Basin Member laterite. A: Extremely weathered, white, discontinuous sandstone. B: Close-up of A. Platy claystone layer in the uppermost Brushy Basin Member mudstones (site O).

84 A

B

Fig. 4.10: Uppermost Brushy Basin Member flame structures. A: Outcrop showing the contact between the Brushy Basin and Buckhorn Members (stick is 1.5 m long). B: Close-up of Figure 4.10a. Brushy Basin Member mudstone injected into overlying Buckhorn Conglomerate. Note that the structure disrupts the primary lamination of the mudstones (site F).

85 dinosaur quarry are interpreted to represent pedogenic features of a lateritic soil. These mudstone structures coincide with previous description of kaolinite recrystallization of the same interval shown in X-ray diffraction (Bilbey, 1992). The presence of similar structures where the Buckhorn Conglomerate is absent in other localities of the study area and east-central Utah supports their pedogenic nature. At Cleveland Lloyd dinosaur quarry, this mudstone interval was considered to represent a B horizon of a lateritic paleosol, however, it was reported to belong to the lowermost Cedar Mountain Shale Member (Bilbey, 1992). Recognition of the Buckhorn above the mudstone deposits at this locality, leads me to place these deposits in the uppermost Brushy Basin Member.

4.3.2 Buckhorn Conglomerate Mudstones Description: Mudstones are the least abundant lithologic facies in the Buckhorn Conglomerate. However, scarce lenses are present at different stratigraphic levels within the member, always bounded by erosive contacts. They are very similar in color and texture to the uppermost Brushy Basin Member mudstones, having blocky textures and diagenetic purple and green banding. Several mudstone lenses were sampled for palynological dating of the unit, but all specimens were barren (R. Litwin, personal communication, 2003). In the study area the best preserved mudstone lens is found 1.5 m below the top of the member below the power line on the western flank of the Buckhorn Wash (Fig. 4.11). It has a maximum thickness of 1.2 m in the center part and rapidly pinches out on both sizes, over less than 5 m. The lens is composed of smectitic blocky claystone with intensive discoloration, combining transitional changes between dark purple and light green. Mudstone lenses close to the Buckhorn base tend to be thinner, never reaching more than 20 cm, and contain a much wider range of grain sizes, with fine sand sizes being frequent in most of the samples.

86 A

B

Fig. 4.11: Buckhorn Conglomerate mudstones. A: A Buckhorn Conglomerate mudstone lens below the power line on the western flank of the Buckhorn Wash. B: Close-up of Figure 4.11a, showing discoloration and pedogenic alteration of the mudstone (site AA).

87 Interpretation: Buckhorn mudstone lenses are inferred to represent overbank facies or abandoned channel fill. Pedogenic alteration represented by root discoloration of these deposits supports this interpretation. The small proportion of this facies in Buckhorn deposits, as well as their erosion-bounded lens shape can be explained by intensive reworking of the floodplain sediments in the Buckhorn fluvial system.

4.4 Sandstone Facies 4.4.1 Brushy Basin Member Sandstones Description: Brushy Basin Member sandstones contain the whole range of sand grain sizes, but fine-grained sandstones are the most common. They tend to be light to tan in color, but locally sandstone bodies have the same coloration as the surrounding greenish or reddish mudstones. Sandstones close to the top of the member have a characteristic white color, and appear to be intensively weathered. Tabular and lens-shaped bodies are the two end-member sandstone geometries present (following classification of Friend, 1983). Tabular sandstones are typically less than 25 m, but locally extend up to 200 m in width. They rarely exceed 50 cm in thickness, are commonly very fine-grained, and slightly fine upward (Fig. 4.12a). Horizontal lamination and millimeter-scale rippling and bioturbation at the top are the common sedimentary structures. Locally, trough- crossbedded sandstones with limited lateral continuity are also present (Fig. 4.12b). Lens geometries typically exceed 2 m in thickness, but locally reach up to 5 m thick, and rarely exceed 30 m in lateral extent (Fig. 4.13a). An overall fining-upward grain size trend is common in all these bodies. Whereas the base is always erosive, the upper part of some lenses shows a transitional gradation into the overlying mudstones. Trough-crossbedding is the most common sedimentary structure (Fig. 4.13b). Reactivation surfaces are abundant forming complex trough-crossbedded cosets. Planar-crossbedding and horizontally-bedded sandstone lenses are also present, but are much less common. Bioturbation is found at the top of some of 88 A

B

Fig. 4.12: Tabular Brushy Basin Member sandstones. A: Thin, massive to slightly rippled, fine-grained sandstone representing overbank crevasse splay deposits (site KA). B: Tabular trough-crossbedded sandstone representing channel-fill deposits of short-lived secondary streams (scale is 1.5 m long, site UA).

89 A

B

Fig. 4.13: Brushy Basin Member channel sandstones. A: Fine-grained sandstone channel (scale is 1.5 m long, site ZA). B: Trough-crossbedded channel-fill sandstone (site J).

90 these bodies. Wood and dinosaur bone fragments are also found in some of the sandstones, normally as a lag deposit. Measurements from crossbedded sandstones show a fairly constant northeastern paleoflow direction (Fig. 4.14). This paleocurrent direction is consistent with previously reported measurements (Craig et al., 1955; Cadigan, 1967; Peterson, 1984; Yingling, 1987; Currie, 1998a). Texturally, Brushy Basin Member sandstones show varying degrees of maturity depending on grain size; finer-grained samples show a higher maturity. In both coarser and fine- grained sandstones mud-size matrix is almost absent, but sorting is commonly poor in the former and increases with decreasing grain size. A large proportion of the grains, especially the finer grain fraction, show high angularity. However, the full spectrum of grain roundness can be found in the same sample, with some grains presenting extremely high roundness. Grain sphericity is moderate to low, decreasing with increasing grain size. Mudstone intraclasts and muscovite are sometimes present. Mudstone, carbonate, and fine-grained sandstone intraclasts from cobble to pebble size are commonly present in the coarse channel sandstones. In thin section monocrystalline undulatory and non-undulatory quartz, and a variety of chert types including microcrystalline, macrocrystalline, and chalcedony are the most common grain lithologies (Fig. 4.15a, b, and c). K-feldspar, plagioclase, plutonic polycrystalline quartz, quartzite, and tuff fragments are accessory lithologies. Zircon and other heavy minerals are common. Grain size appears to be a strong control on grain lithologies, with chert being progressively more abundant in coarser sandstones. Nevertheless, the sandstones have an extremely high compositional maturity. The point count results of three medium-grained sandstones are shown in figure 4.16a and in Appendix B. They show a mean QmFLt relation of 54.4 : 2.7 : 42.9. According to these results, Brushy Basin Member sandstones can be classified as litharenites (Pettijohn et al., 1987), falling in the recycled orogen field of Dickinson (1985) (Fig. 4.16b). 91 SB

Cleveland Lloyd UB RB Dinosaur Quarry

OB

ZB XB WB CC

JB IB KB

MB

n = 528

J KA

EA

IA

FA

N B OA WA

ZA

O UA

5 km Brushy Basin Member Paleoflow Map 3D n>20 20>n>10 10>n>5 5>n>3 n<3 2D Brushy Basin Member-Buckhorn Conglomerate contact

Fig. 4.14: Paleoflow directions from channel trough-crossbedded sandstones of the Brushy Basin Member. Note the general NW paleoflow direction of this member with relatively low local paleocurrent dispersion. Detailed locations of measured sandstone beds are in Appendix D.

92 A D

ADBBM-UA-17 BC-A-14

B E E

BBM-ZA-13 BC-KB-9

C F F

BBM-OA-17 BC-WB-2

Fig. 4.15: Brushy Basin Member (A, B, and C) and Buckhorn Conglomerate (D, E, and F) thin sections. Detailed locations of the sampled beds can be found in Appendix A.

93 Qm

F Lt

Qm Upper Shale Member Brushy Basin Member Undifferentiated Morrison Fm. Buckhorn Conglomerate 11 Undifferentiated Cedar Mountain Fm.

Recycled Orogen

Continental Block Magmatic Arc

F 23 13 Lt

Fig 4.16: QmFLt ternary diagram showing Brushy Basin Member and Buckhorn Conglomerate sandstone clast composition. Note the petrographic similarity of the channel sandstones of both units. Data from Crooks, 1986, San Rafael Swell, (squares), Currie, 1998a, Uinta Mountains, (circles), and present study (triangles).

94 Cement phases include quartz, chert, calcite, and clays. Quartz cements consist of overgrowths, some of which show abrasion. Chert (Fig. 4.15b), calcite (Fig. 4.15c), and clay minerals are void-filling cements, whose relative proportions vary among samples. Regardless of proportions, cement represents a very small volume of rock. Long contacts and triple junctions are fairly common. Broken grains and pressure-dissolution contacts are also found. These normally affect chert grains, which locally also show significant deformation.

Interpretation: Considering geometry, thickness, internal structures, and abundant bioturbation, the tabular sandstone bodies are interpreted as crevasse splay deposits. Thin planar-bedded, and ripple and horizontal lamination represent proximal and distal facies, respectively. Thicker planar laminated, tabular bodies with low continuity may represent small channels viewed in a near longitudinal channel section. Lenticular cross-bedded sandstones represent channel-fill deposits seen in transverse section. The white sandstones close to the top of the member are interpreted to have undergone intensive leaching and consequent discoloration, as a result of penetrative weathering soon after their deposition. The fact that the Brushy Basin Member sandstones present the full spectrum of grain roundness suggests a variety of source areas. Long contacts, triple junctions, broken grains, and pressure dissolution contacts indicate compaction of the sandstone framework during diagenesis.

4.4.2 Buckhorn Conglomerate Member Sandstones Description: Buckhorn Conglomerate sandstones occur in a spectrum from relatively well-sorted to conglomeratic sandstones. However, within even the most well-sorted, fine- grained sandstone beds scattered granules and pebbles are found. Pebbly sandstones sometimes grade laterally to conglomeratic units. Although a wide variety of grain sizes are present, fine-grained sandstones similar to those of the underlying Brushy Basin Member are more common. They are typically light brown to tan in color, becoming darker with increasing 95 coarse fraction. The sandstone facies tend to be more abundant toward the upper portion of the member (Fig. 4.17a) forming an overall fining-upward tendency, but they are present throughout the unit. Sandstone is the only lithology present near the depositional pinchouts. Sandstone interbedded with conglomeratic facies show both erosive and transitional contacts and very low lateral continuity, rarely exceeding 10 m in lateral extent (Fig. 4.17a). This is especially true for sandstones near the base. Towards the top, where the unit becomes finer and sandstone lenses thicker, some beds are more laterally extensive. Some sandstone lenses are made up of a single bed, whereas others contain several reactivation surfaces (Fig. 4.17b). These reactivation surfaces can be recognized as bounding successive fining upward cycles and different internal structures. At site I (Fig. 4.2), a 15 cm wide fracture in a trough-crossbedded sandstone lens with multiple reactivation surfaces close to the top of the unit is filled with overlying pebbly conglomerate (Fig. 4.18a). The contact between the clastic dike and the host rock is sharp and the former dies out gradationally 1.5 m below the overlying conglomeratic lens. At site F there are clay-filled fractures in conglomerate facies (Fig. 4.18b). Three major sedimentary structures can be identified. With decreasing abundance they are: trough-crossbedding, planar-crossbedding, and massive to crudely horizontal bedding. Climbing ripples and rippled-laminated beds are found associated with the first two types. Scour surfaces are very common and are normally associated with channel-shaped lens facies. Paleoflow measurements from Buckhorn trough-crossbedded sandstones were taken mostly from the top of the unit in the southeastern part of the study area, since the weak cementation of the Buckhorn further north precluded measurements in several sites. The data show a strong eastern to southeastern paleoflow direction coinciding with the thickness axis of the Buckhorn Conglomerate and orientation of the Cedar Mountain (Fig. 4.19). Buckhorn sandstones show very similar variation in textural maturity to those in the Brushy Basin Member. Sorting tends to be moderate to poor. Commonly, fine-grained

96 A

B

Fig. 4.17: A: Buckhorn Conglomerate sandstones. Large-scale trough-crossbedded sandstone with lower gradational contacts into pebble conglomerate (site S). B: Channel-fill, trough-crossbedded sandstone close to the top of the Buckhorn (site EA).

97 A

B

Fig. 4.18: Buckhorn Conglomerate clastic dikes. A: A 25-cm wide and approximately 1 m long conglomeratic clastic dike in a trough-crossbedded Buckhorn Conglomerate sandstone lens (site I). B: 2 m long claystone clastic dike a few centimeters wide in the uppermost Buckhorn conglomeratic lens (site F).

98 RB Cleveland Lloyd Dinosaur Quarry

ZB PB

WB

NB

GB KB

MB RA VA LB QA EB n = 584 MA TA J KA JA EA

CB IA

HA LA D IAbis N BB A

YB OA

WA U QB R

Q Y S

XA

I I ZA K UA M

G G F

5 km Buckhorn Conglomerate 3D Paleoflow Map n>20 20>n>10 10>n>5 5>n>3 n<3 2D Brushy Basin Member-Buckhorn Conglomerate contact

Fig. 4.19: Paleoflow directions from channel trough-crossbedded sandstones of the Buckhorn Conglomerate. Most measurements were taken from the discontinuous, trough-crossbedded sandstone at the top of the unit. Note the general E to SE paleoflow direction of this member, with relatively high local dispersion. Detailed location of measured sandstone beds are in Appendix A.

99 sandstones appear to show higher textural maturities. Roundness and sphericity vary substantially within the same specimen regardless of grain size. Grain lithologies are very similar to the ones observed in the underlying Bruhsy Basin Member, with undulatory and non-undulatory monocrystalline quartz, and chert representing the major lithologic clast varieties (Fig. 4.15 d, e, and f). K-feldspar and plagioclase are also present but in much small amounts similar to the underlying Brushy Basin Member. However, plutonic polycrystalline quartz, quartzite and tuff fragments show a small increase. Zircon and other heavy minerals are abundant. Chert grain varieties are also very similar to those present in Brushy Basin Member sandstones. Crinoid stem fragments can be seen in some of the microcrystalline grains, showing the original carbonate origin of the source area. The results of point-counting of three medium- grained sandstones are shown in Figure 4.16a and Appendix B. A mean Qm,F,Lt ratio of 46.9 : 1.5 : 51.6 lead the Buckhorn sandstones to be classified as lithic arenites following the classification of Pettijohn et al., (1987). Although a slight increase in chert and decrease in quartz and feldspar is noted compared to the examples of the underlying member, they fall in the recycled orogen field of Dickinson (1985) like the Brushy Basin Member sandstones (Fig. 4.16b). In contrast to the surrounding units, the Brushy Basin and Buckhorn Members show a pronounced increase in lithic grains due to a peak in chert content. Data from this study are plotted together with data from the literature to show this petrographic trend (Fig. 4.16a). Chert, calcite, and clay minerals are the main cement phases. The latter is almost only present in samples from the northern part of the study area (Fig. 4.15 e, and f). Quartz overgrowths and calcite cements are less abundant than in Brushy Basin sandstones. Nevertheless, sample A-14, which corresponds to the uppermost sandstone right at the top of the Buckhorn at its type section, shows poikilotopic calcite cementation replacing between 30 and 50% of the clastic framework (Fig. 4.15d). This is an exception to the rest of the sandstones sampled, and can be explained by the close vertical proximity to a micritic limestone 100 bed 2 m above (section A, Appendix A). Except for this sample, the others examined show a very low percentage of cement. However, in several outcrops sandstones at the top of the Buckhorn are intensely silica cementated. Although fabric is absent, long, triple point, and pressure dissolution contacts are fairly common. Sutured grains are also being present. This indicates relevant compression during burial.

Interpretation: Trough-crossbedded sandstone lenses are interpreted to be the product of dune migration in channels under a relatively low flow regime. Both decreasing and increasing flow regime vertical grain size changes are found, with a tendency for the former to have transitional contacts and the latter to show bounding erosion surfaces. Thin-bed, planar- crossbedded sandstones are considered to be sand waves deposited during low flow regime within channels, whereas the thicker and more laterally extensive beds may represent linguoid channel bars deposited under a slightly higher flow regime (Miall, 1977). Horizontally laminated facies showing parting lineation and crudely horizontally laminated and rippled sandstones are considered to be channel-fill facies deposited during higher and lower energy flow, respectively. Comparison of the Buckhorn Conglomerate sandstone point-count results to those of Brushy Basin sandstones, together with the mentioned clast lithologies affinity shows that the source area of both members was the same. The conglomeratic clastic dike found at site I is interpreted as a seismite. Formation and infill of the fracture in the host sandstone took place after deposition of the overlying conglomerate. As a result of seismic activity the fracture was opened in the underlying, consolidated sandstone, and was instantaneously filled by the overlying still unlithified conglomerate.

4.4.3 Unnamed Shale Member Sandstones Description: A coarse clastic unit with a thicknesses varying from 1 to 5 m sitting directly on top or above near the Buckhorn Conglomerate is present in several outcrops throughout the 101 study area. This unit corresponds to the middle conglomeratic interval of Young (1960). It is composed of a varying number of lenses of pebble and cobble conglomerate and locally carbonate nodule-bearing mudstone with lateral transitions to trough-crossbedded, granular and pebbly sandstone beds. Rapid, but gradational transitions among different grain sizes are common. This facies is typically very well cemented by calcite accounting for its characteristic high resistance to erosion. At the Buckhorn Conglomerate type section (section A, Appendix A), this unit is 4.4 m thick and overlies a purple calcareous mudstone sitting on top of the Buckhorn (Fig. 4.20a). It consists of three beds of pebbly to clean, coarse to fine-grained, trough-crossbedded sandstone. The higher proportion of bright color chert and quartzite pebbles, abundant carbonate intraclasts, and dinosaur bones makes this facies readily identifiable from similar Buckhorn grain sizes in most sites. Locally, these sandstone bodies show a lateral transition to micritic calcite horizons, as described by Aubrey (1998). Massive, red to clear silica forming discontinuous horizons up to 1 meter thick, and thinner irregular vein structures are present in some outcrops at this stratigraphic level throughout the study area, normally found replacing micritic carbonate, including the Upper Shale Member sandstones at the Buckhorn type section

Interpretation: Lowermost Upper Shale Member sandstones are interpreted to represent unconfined channel fill facies. Penetrative carbonate cementation is interpreted to be early in origin and therefore a pedogenic nature is inferred. Massive red silica horizons and veins are considered to be the result of soil development. Red chert granules and pebbles are commonly found in this unit. Although they appear to be the same as similar red clasts found in the underlying Buckhorn and Brushy Basin Member, their larger size and the presence of authigenic jasper veins in the same interval may indicate reworking of pedogenic silcrete

102 A

B

Fig. 4.20: Sandstone and conglomeratic facies of the lower Upper Shale Member. A: Multistory, well-cemented, trough-crossbedded sandstone, overlying banded mudstones with calcite nodules of the same member less than a meter above the Buckhorn Conglomerate at its type section (stick is 1.5 m long, site A). B: Massive cobble-size conglomerate with abundant recystallized, micritic matrix, laterally grading into trough cross-bedded sandstones facies similar to A. (site XB).

103 horizons found in the same unit. Long, triple point, and pressure dissolution contacts, and sutured grains indicate compression during burial.

4.5 Conglomerate Facies 4.5.1 Brushy Basin Member Conglomerate Description: Brushy Basin Member conglomerate are found in several outcrops within the study area. They consist of dark-colored, fine pebble to granular conglomerate with varying amounts of sandy matrix (Fig. 4.21a). The largest clast found has an A-axis of 4 cm, but commonly grains do not exceed granule size. Light-colored conglomerates are found at Cleveland Lloyd Dinosaur Quarry and three nearby localities, a few meters below the upper contact of the member (Fig. 4.21b). Conglomerate thickness varies between 10 cm and 3.2 m. Invariably, they are trough- crossbedded and occur at the base of sandstones lenses grading upward into finer sandstone facies. Laterally, conglomerates are constrained by the geometry of the lenses and rarely reach more than 25 in width. Granular conglomerate is the most characteristic, showing moderate to low textural maturities. Roundness and sphericity tend to be constantly high. Black, gray, brown, white, and red chert are the major clast in the lower Brushy Basin Member conglomerates, although quartzite and sandstone clasts are also present. Intraclasts are very common, with carbonate and mudstone being the most abundant. Wood and dinosaur bone fragments are present in some conglomeratic deposits. Conglomerate located a few meters below the upper contact of the member, contain chert lithologies similar to the lower conglomerate, but with a strong predominance of light gray and white grains. The upper conglomerate is normally associated with white, trough- crossbedded sandstones. Although not as abundant as the lower ones, they have also been identified near Green River in the same stratigraphic position (Yingling, 1987).

104 A

B

Fig. 4.21: Brushy Basin Member conglomerates. A: Lower Brushy Basin Member dark, chert granular conglomerate, associated with channel-fill trough-crossbedded sandstones (site IA). B: Light gray, chert conglomerate at the base of a sandstone capped-channel, 6 m below the Buckhorn Conglomerate at Cleveland Lloyd Dinosaur Quarry (scale is 1.5 m long, site RB). 105 Interpretation: Brushy Basin Member conglomerate is interpreted as channel-fill deposits. Their occurrence indicates the presence of the major sediment conduits of the Brushy Basin Member alluvial plain. In spite of their low percentage compared to the rest of clastic facies in the Brushy Basin paleosystem, the clast varieties are indicative of the lithological composition of the source area.

4.5.2 Buckhorn Conglomerate Description: Clast-supported cobble, pebble, and granular conglomerate is the most characteristic facies of the Buckhorn Member. They are normally found in laterally discontinuous lenses ranging in thickness between 35 cm and 4 m (Fig. 4.22a). Coarser clasts reach cobble sizes, with the biggest clast found having an A-axis 35 cm long. However, the mean grain size ranges from 2 to 4 cm in diameter. Some centimeter-scale conglomeratic lenses close to the base have very abundant clay matrix, locally becoming mud-supported. The color of the conglomerate varies with cementation. In the areas surrounding the Buckhorn type section, where the member is well-indurated, the general color of the conglomerate is invariably dark (Fig. 4.22a). This is due to manganese oxide staining of the framework clasts (Conley, 1986). Whitish, brown, and black are the most abundant colors, but pink, red, orange, blue and green clasts are also present. In the conglomerate textural maturity tends to be relatively high, although coarse and medium sandy is always present as a matrix. Framework clasts are always very-well rounded, but generally with low sphericity. Towards the north, the color of the conglomerate tends to be lighter. A bluish metallic staining is present at several localities where the unconsolidated conglomerate crop out. In these areas, Buckhorn conglomerate tend to have a variegated muddy and sandy matrix (Fig. 4.23a). The presence of interstitial mud seems to be the cause of the weathering and disaggregation of the conglomerate. Variation in cementation, however, varies strongly over just a few meters (Fig. 4.23 b and c). 106 A

B

Fig. 4.22: Well-indurated Buckhorn Conglomerate A: Buckhorn Conglomerate showing the stacking of several conglomeratic lenses. Erosive contacts are common. Planar- and trough-crossbedded gravels, as well as massive lenses are present (site EA). B: Trough- crossbedded Buckhorn conglomerate (site T).

107 A

B

C

Fig. 4.23: Different outcrop aspects of the disaggregated, mud-rich matrix Buckhorn conglomeratic facies north of Cedar Mountain. A: Semi-consolidated, chert conglomerate with brightly colored variegated muddy matrix (site HB). B: Semi-consolidated Buckhorn conglomerate at Cleveland Lloyd Dinosaur Quarry (site RB). C: 20m E from B, the Buckhorn becomes partly cemented and crops out as a blanket of chert pebbles with bright purple mud matrix (site RB).

108 The transition between the main two types of Buckhorn conglomerate is found in the northwestern margin of Cedar Mountain Formation (between site MB and JB in figure 4.3). Although intensive weathering of the muddy conglomerate gives the appearance of an abrupt contact locally, in other sites a more gradational change can be seen between the two Buckhorn subfacies. Several types of bedding structures are found in Buckhorn Conglomerate. Massive to crudely-bedded layers are associated with lenses never exceeding 10 m in width and varying between 35 cm and 2.5 m in thickness. These lenses are always bounded at the base by scour surfaces and sometimes grade into a more organized structure. The upper contacts of these beds are commonly erosional surfaces. At the top of the Little Cedar Mountain (site F) a 2 m long fracture in the uppermost massive conglomeratic lens is infilled with clay material (Fig. 4.18b). This clastic dike is located at a similar stratigraphic level within the Buckhorn and approximately 1 km west from the one example within a sandstone at site I. Planar-crossbedded conglomerate tends to be more laterally continuous, sometimes reaching 25 m in width. Their thickness varies between 50 cm and 2 m. Although vertical gradational transitions to other structures are present, these beds tend to be bounded by erosion surfaces. Trough-crossbedding is very common, especially toward the top of the unit in lenses of variable lateral extent. Two types of trough-crossbedded conglomeratic deposits can be distinguished according to the nature of their basal contact. Trough-crossbedded lenses with erosional bases tend to have low continuity, normally between 3 and 15 m, and thicknesses that vary between 40 and 2.5 m. The other type of trough cross-bedding is found in lenses with relatively uniform thicknesses ranging between 50 cm and 2 m, and extending laterally up to 150 m. Rapid transitions from sandy conglomerate to pebbly sandstones are especially common in this type of lens.

109 Gutter casts are very common at the base of the Buckhorn Conglomerate (Fig. 4.24). They can be seen in most of the outcrops where the contact between both members is well- exposed, as well as on the base of large fallen blocks of the Buckhorn. These structures consist of grooves incised into Brushy Basin Member mudstones and filled with pebbly Buckhorn conglomerate. They have decimeter-scale thickness, and extend laterally for a few meters. Although they show local paleoflow variations, gutter casts commonly have an east-west direction. Approximately 75 m west of the Cedar Mountain overlook, a rounded depression in the uppermost mudstones of the Brushy Basin Member is filled with Buckhorn pebbly sandstone (Fig. 4.25). Black, gray, brown, and white silicified limestone, with a minor contribution of red and orange chert and quartzite are the most abundant clast lithologies within the Buckhorn (Appendix C). Chert varieties are very similar to those found in the underlying Brushy Basin Member conglomerate. Crinoid stem fragments, colonial corals, bryozoans, and fussilinids were observed in some of the chert clasts. Sandstone clasts were found in some pebble counts. Red petrified wood with well-preserved cell structure and dinosaur bone fragments are also found at the top of the conglomerates. Lithoclast varieties do not show any geographic pattern. Diagenetic processes show different degrees of alteration throughout the study area. Quartz and calcite are the most common cement but its abundance varies significantly. This is especially true for quartz, which shows an increase towards the top of the member. In some localities the top of the unit is very well-cemented with extremely indurated silicified pebble conglomerate and sandstones.

Interpretation: Massive to crudely-bedded conglomeratic beds are interpreted as channel-lag deposits and longitudinal channel bars. The planar-crossbedded conglomeratic lenses are inferred to be the product of linguoid bars, as well as deltaic growth from older channel bar remnants (Miall, 1977). Laterally constrained, trough-crossbedded lenses with 110 A

B B

C

Fig. 4.24: Gutter casts at the base of the Buckhorn Conglomerate. A, B, C: Gutter cast several centimeters thick, and a few meters long are found in almost every outcrop where the contact between the two members is well-exposed. B shows downstream ramifications of the casts (sites J, A, and CC).

111 A

B C

Fig. 4.25: Dinosaur track at the base of the Buckhorn Conglomerate. A: Round, downward cast filled with Buckhorn pebbly sandstone incising into uppermost Brushy Basin Member banded claystone. erosional bases represent channel-fill deposits accreting from the channel walls. Laterally extensive, trough-crossbedded conglomeratic bodies showing rapid transition into pebbly sandstones are considered to represent channel dunes (Miall, 1977). The fossil content indicates that chert clasts correspond to silicified Upper Paleozoic materials (Stokes, 1944; Conley, 1986). Different degrees of framework cementation, probably controlled by the amount of smectitic clay in the matrix, may explain the transition between well- and weakly-indurated conglomerate. The claystone clastic dike at the top of the unit in site F is thought to be a seismite deposit, due to its formation as the infill of a fracture in the uppermost Buckhorn conglomeratic lens. Due to its similar stratigraphic position and geographic location, its synchronicity with the seismite deposit found in site I is suggested. The depressions in the uppermost mudstones of the Brushy Basin Member filled with Buckhorn pebbly sandstone are interpreted as dinosaur tracks. Dinosaur foot prints have been found in the Brushy Basin Member, but they have not been reported previously from the base of the Buckhorn Conglomerate.

4.5.3 Unnamed Shale Member Conglomerate Description: Conglomeratic deposits reaching up to cobble size associated with well- cemented, trough-crossbedded, bright colored to white, pebbly sandstones of the Upper Shale Member are present in scattered outcrops in the northern part of the study area (Fig. 4.20b). These conglomerates rarely exceed 3 m in thickness and tend to have abundant sandy matrix with intensive calcite recrystallization. Bright colored chert and quartzite pebbles are abundant in some sites, whereas in other areas the clasts tend to be lighter in color. Dinosaur bone fragments, and especially micritic carbonate intraclasts are characteristic features of conglomeratic deposits at the base of the Upper Shale Member.

113 Interpretation: The conglomeratic deposits of the lowermost Unnamed Shale Member are interpreted as channel lag deposits and longitudinal bards according to their massive structure. The latter is consistent with the rapid lateral gradation of this facies with finer clastic deposits. Their scattered occurrence and low thickness indicate their deposition as a result of sporadic high discharge events.

4.5 Carbonate Facies 4.5.1 Brushy Basin Member Limestones Gray micritic limestone beds are common in the lower part of the Brushy Basin Member (Fig. 4.26a). They rarely exceed 20 cm thick, but can reach up to 64 cm. They commonly extend for a few tens of meters before pinching out into Brushy Basin Member mudstones and weather as thin resistant tan ledges. Mollusks and charophytes are the most common fauna, showing the lacutrine nature of these beds (Masura, 1998).

4.6.3 Unnamed Shale Member Calcrete Micritic carbonate horizons are commonly present at or near the base of the Upper Shale Member, when this unit is present. Red and black chert pebbles and granules are found floating scattered in the micritic matrix. Carbonate intraclasts and jasper veins are very common. The lower contact is sharp where the unit overlies mudstones, and gradational where it sits on top of an Upper Shale Member sandstone. In the southernmost measured section, the carbonate horizon forms a massive 2 m thick resistant ledge (Fig. 4.26b), whereas in the northernmost one the unit is composed of several horizons interbedded with pebbly mudstone. This unit coincides in description and stratigraphic position with the calcrete horizons described by Aubrey (1996) and Currie (1998a). Therefore, it is reasonable to infer the existence of a pedogenic horizon above the Buckhorn Conglomerate, affecting the lowermost strata of the Upper Shale Member. 114 A

B

Fig. 4.26: Carbonate facies of the Brushy Basin Member and Upper Shale Member. A: Micritic, gray freshwater limestone lens in lower Brushy Basin Member mudstones. B: Two meter thick dark gray, calcrete horizon one meter above the Buckhorn near its southwestern depositional pinchout (scale is 1.5 m long, site P).

115 4.7 Depositional Environments 4.7.1 Brushy Basin Member The absence of point bar structures in the channels of the Brushy Basin Member indicates the low sinuosity of its drainage network, precluding a meandering fluvial system (Miall, 1992). At the same time, the low lateral continuity of the channels and the high proportion of overbank mudstones are not characteristic of a braided river deposit (Miall, 1992). However, an anastomosed fluvial environment is consistent with the predominance of fine-grained sandstone facies in the channel lenses and the dominance of overbank mudstones in the alluvial succession (Nadon, 1994). Previous work on the depositional environment of the upper Brushy Basin Member at Cleveland Lloyd dinosaur quarry suggested a similar anastomosed fluvial environment (Kantor, 1995; Richmond and Morris, 1996). The anastomosed river style is an intrinsic response of the alluvial system to a rapid base level rise (Zhang et al., 1997). In such cases, the fluvial system undergoes fast vertical accretion and anastomosed fluvial conditions tend to be attained in order to maintain the depositional equilibrium. Due to the complexity of alluvial systems, and the large number of parameters that control its dynamics and fluvial style, other factors probably contributed to the formation of the Brushy Basin anastomosed fluvial channels. Two especially important factors had to be a continuous discharge and the supply of mud-dominated sediment (Ritter et al., 1995). Both conditions were attained during deposition of the Brushy Basin Member. A base level rise at the onset of Brushy Basin Member deposition has been suggested by Crooks (1986), Yingling (1987), and Currie (1997) most probably as a result of an increase in subsidence (Lawton, 1994). The predominance of mudstone deposits with abundant but not intensive pedogenic alteration, laterally constrained channel sandstones and conglomerate grading upwards into overbank fine-grained materials, well-preserved crevasse splay deposits, and fresh-water limestone beds, all point to the rapid deposition of the Brushy Basin Member in the study area as the fluvial response to a rapid increase in accommodation space. This 116 conclusion is consistent with isotopic ages obtained from altered bentonitic beds close to the base and top of the member at Little Cedar Mountain (Kowallis et al., 1998). These ages constrain the period of Brushy Basin Member deposition to approximately 2 my (Kowallis et al., 1998). During rapid base level rise, both the floodplain and channel alluvial subsystems respond to the increase in accommodation space with a specific stratigraphic signature (Wright and Marriott, 1993). Thick mudstone overbank materials indicate rapid vertical accretion of the fluvial system, with rapid and widespread storage of fine-grained sediment on the floodplain (Wright and Marriott, 1993). The minimum lateral migration of the channel system across the floodplain can lead to the preservation of shallow lacustrine deposits if the water level leads to partial flooding of the alluvial plain. Rapid floodplain accretion will also inhibit strong pedogenic development in a single horizon. Instead, immature pedogenic horizons will be common (Wright and Marriott, 1993). These features are all present in the Brushy Basin Member succession in the study area. Rapid vertical accretion tends to produce low sinuosity and lateral continuity of the fluvial channels. The channel shifting takes place by avulsion with multiple small channels being active at the same time (Nadon, 1994; Farrell, 2001). The presence of several channels at the same stratigraphic level in the area of Bull Hollow (sites J, KA, EA, IA, Appendix A, Fig. 1.11) could be explained as a result of rapid channel infill and avulsion with possible reactivation during floods. I interpret the Brushy Basin Member to be an anastomosed deposit based on these data.

4.7.3 Buckhorn Conglomerate The Buckhorn Conglomerate displays a sheet-like geometry, low width/thickness ratio, rapid facies transitions from cobble to clay grain sizes, abundant cut-and-fill structures, and irregular bedding contacts. All this characteristics are considered to indicate a braided river 117 depositional environment (Smith, 1970). This interpretation is supported by the identification of longitudinal and linguoid bars. Therefore, I agree on the braided nature of the Buckhorn alluvial system as previously suggested (Young, 1960; Crooks, 1986; Conley, 1986; Yingling, 1987; Bilbey, 1992; Currie, 1997, 1998a). The large lateral extent of the member indicates an unconfined braided depositional environment, probably as a response to a decrease in subsidence (Fig. 4.5). The maximum thickness of the Buckhorn measured in this study was 16 m, however Yingling (1987) states that the Buckhorn reaches a maximum thickness of 25 m. The geometry of the deposit suggests a low subsidence rate during the full development of the braided fluvial system. Such low accommodation rates are consistent with braiding in an alluvial system (Wright and Marriott, 1993). The exposure of clast-supported, well-cemented Buckhorn, which is essentially mud- free, is very similar to the Scott-type braided river model of Miall (1977). This braided fluvial style is characteristic of proximal settings, with fluvial sedimentation consisting of the superposition of small-scale cycles of gravely longitudinal-bar deposits and cross-laminated sandstone lenses corresponding to periods of flooding and waning flow regime respectively. Both types of structure are found in the study area where the Buckhorn is well cemented, and are well developed at the type section. The poorly cemented Buckhorn subfacies, with muddy matrix locally shows the same lithofacies characteristics, but more commonly is represented by massive conglomeratic deposits with little or no interbedded sandstone lenses. Where this subfacies is not totally disaggregated, horizontal conglomerate lenses and banding of the muddy matrix are locally present, features also reported by Conley (1986). According to these lithological characteristics, as well as the relatively low thickness of this facies, I interpret these deposits to be the result of flooding events overfilling the main channels of the Buckhorn system and spreading of the poorly sorted bedload into the alluvial plain. 118 The low percentage of mud and interbedded sandstones in these conglomerates precludes deposition by debris flows. However, hyperconcentrated flows are a possible mechanism of deposition (Prothero and Schwab, 1996). I find it more probable that the mud and clay fraction of the matrix was deposited between flooding events, with these regions of the alluvial plain receiving mainly mud-dominated overbank sedimentation. Consequently, I consider the mud matrix to be mostly secondary in origin. Towards the top the Buckhorn is commonly a sand-dominated, trough-crossbedded facies with thin conglomeratic beds found only as channel lag deposits. This upper Buckhorn lithofacies resemble the Donjek braided river type of Miall (1977). A Scott-Donjek gradational facies assemblage transition was also observed by Crooks (1986) in what she considered the laterally general fining tendency of the unit towards its margins in the northern and southern parts of the western flank of the San Rafael Swell. The contact between both Buckhorn lithofacies assemblages seems to be gradational, with the upper sandstone lenses being discontinuous and interbedded with the lower conglomeratic deposits. Consequently, a gradational temporal change from the lower gravel- dominated Scott-type to the upper sand-dominated, trough-crossbedded Donjek-type interval is inferred. Scott-type deposits are characterized by continuous discharge with sporadic high stage peaks typical of proximal setting in the alluvial depositional system, whereas Donjek-type braided rivers tend to receive discontinuous discharge events responsible for the deposition of conglomeratic based, fining upward sandstone bodies in more distal fluvial setting receiving finer grained sediment load (Miall, 1977). Therefore, I find possible the existence of a downflow gradational transition between both types of braided alluvial plains, and interpret the vertical lithofacies assemblage change as the retrogradation of the Buckhorn alluvial system towards the end of its deposition. If similar temporal river type oscillations took place during the sedimentation of the lower part of the member, subsequent erosion during deposition of the

119 conglomeratic units mostly in form of longitudinal bars destroyed any evidence of these variations.

4.7.4 Lowermost Unnamed Shale Member Deposition of the lowermost sediments of the Unnamed Shale Member took place under a more arid climate than present in the Buckhorn Conglomerate, as indicated by the presence of intensive calcretization. This pedogenic feature indicates a poorly drained alluvial plain, with evaporation exceeding precipitation rates (Currie, 2002). Laterally continuous, erosionally bounded lenses gradual transitions from massive conglomerate to trough- crossbedded sandstones and finally merging into a calcrete are interpreted as the result of episodic braided river conditions with highly seasonal discharge. The discontinuous high magnitude discharge events indicated by the stratigraphy of these deposits is consistent with an alluvial system deposited under an arid climatic regime (Ritter et al., 1995).

120 CHAPTER 5: STRATIGRAPHIC MODEL

5.1 Introduction In this chapter a regional stratigraphic model is presented for the uppermost Jurassic and lowermost Cretaceous strata of the Colorado Plateau. In order to construct this model the nature of the transitions between the Buckhorn Conglomerate and the surrounding stratigraphic units must be considered. The source areas of the Buckhorn and Brushy Basin Members as well as their chronostratigraphic framework need also to be evaluated. The nature of the transitions indicates where unconformities, if present, exist. An understanding of the Buckhorn and Brushy Basin Member source area is important in order to detect any major change in the area of alluvial drainage supplying sediment. Finally, the evaluation of the timing of deposition of the Buckhorn is necessary in order to understand the temporal link of this member with respect to the units in which it is embedded.

5.2 Stratigraphic Contacts

5.2.1 Brushy Basin Member - Buckhorn Conglomerate Contact

Currie (1997, 1998a, 1998b) interpreted the Buckhorn Conglomerate as an incised valley fill deposit. However, the interfingering between the upper Brushy Basin Member and the Buckhorn Conglomerate (Fig. 4.7), as well as the depositional pinchouts of the Buckhorn (Fig. 4.6) identified in this study preclude valley incision and later infill as a genetic model for the Buckhorn Conglomerate in the study area. Instead, the stratigraphic relationships between members point to a conformable transition from an anastomosed to a braided river depositional system. This interpretation is further supported by sedimentary structures found at base of the Buckhorn and clast and grain composition.

121 Three types of structures found at the contact between the Morrison and Cedar Mountain Formations point to a still plastic rheology of the uppermost Morrison Formation in the study area at the onset of Buckhorn deposition. These structures are mud injection involving disruption of the primary lamination of the uppermost Brushy Basin (Fig. 4.10), abundant gutter casts at the base of the Buckhorn, locally showing radial flow patterns (Fig. 4.24), and dinosaur tracks in the uppermost Brushy Basin Member mudstones infilled with Buckhorn conglomeratic materials (Fig. 4.25). I interpret the widespread distribution and abundance of these structures to indicate minimum compaction of the uppermost Brushy Basin Member prior to the onset of Buckhorn braided river conditions. The rheology of the Brushy Basin Member sediments implies that the uppermost Morrison materials could not have been buried prior to Buckhorn deposition and, therefore, the scouring at the base of the Buckhorn Conglomerate does not indicate significant erosion of the Morrison Formation as previously suggested (Stokes, 1944, 1952; Imlay, 1952; Young, 1960; McGookey et al., 1972; Crooks, 1986; Conley, 1986; Currie, 1998a). Formation of an erosive lower surface is a common characteristic of braided river dynamics (Miall, 1977, 1992). Fluvial incision in response to sea level fall does not exceed a few meters (Leeder and Stewart, 1996). However, accommodation decrease as a result of tectonic processes may lead to higher alluvial incision (Currie, 1997). Moreover, the degree of basal erosion is related to the river discharge and competence, and therefore there is a wide range of erosive capacities, reaching up to 25 m in large modern braided rivers such as the Brahmaputra (Miall, 1977). Therefore, based on modern analogs, the erosive relief at the base of the Buckhorn Conglomerate in the study area can be explained by braided river dynamics and does not require an external base level fall. The petrographic data show a very strong similarity between the conglomerate and sandstone composition of both members. This similarity indicates that the Brushy Basin anastomosed and Buckhorn braided river systems were shed from a compositionally very 122 similar, and potentially the same source area. This fact does not necessarily imply a conformable fluvial transition, but nevertheless, it is useful in order to support consistent with this interpretation. A conformable transition is more likely to take place when a common source area is responsible for shedding detritus to the two consecutive alluvial systems.

5.2.2 Buckhorn Conglomerate – Unnamed Shale Member Contact In the study area the contact between the two members of the Cedar Mountain Formation is locally erosional, with the relief of this local disconformity not exceeding 3 m. However, the preservation of crossbedding at the top of the uppermost sandstone of the Buckhorn Conglomerate at its type section and other localities, and intensive poikilotopic calcite cementation as the result of the formation of a calcrete horizon directly overlying it suggests that an erosion surface does not separate the Buckhorn Member from the lowermost Upper Shale Member rocks in those areas. The presence of a calcrete horizon is indicated by the local occurrence of a massive to nodular micritic carbonate bed on top of a laterititc paleosol at the top of the Brushy Basin Member or the Buckhorn Conglomerate affecting Unnamed Shale Member deposits. This pedogenic horizon is interpreted to indicate a long period of negative accommodation in the study area following the end of Buckhorn deposition. Therefore, a major paraconformity is inferred at the base of the Unnamed Shale Member. This interpretation is consistent with similar calcrete intervals found at the base of the Unnamed Shale Member in other areas of the Colorado Plateau and Uinta Mountains (Aubrey, 1998; Currie, 1997, 1998a). In some localities of the study area, carbonate pedogenic alteration seems to have been more or less continuous resulting in a hardpan calcrete up to 2 m thick, whereas in other areas there was deposition of contemporaneous conglomerates, sandstones, and mudstones of the lowermost Unnamed Shale Member (site SB, Appendix A). Calcrete fragments found as intraclasts in these clastic beds indicate the early origin of the pedogenic horizon, and may 123 account for the erosion of the basal calcrete present in other areas. The penetrative carbonate recrystallization is interpreted as calcretization of channel facies deposited during periods of sporadic river discharge events. The abundance of carbonate nodules in the lower mudstone succession of the Unnamed Shale Member, which have been reported to represent up to 30% of the rock in the study area (Bilbey, 1992) indicates that conditions in which evaporation exceeded precipitation rates and extremely low subsidence remained after initial formation of the hardpan calcrete.

5.3 Source Area The passive tectonic behavior of the southwestern North America margin during most of the Late Neoproterozoic and Paleozoic led to the deposition of a thick and extensive passive margin succession in western Utah and Nevada with a shallow siliciclastic-dominated lower interval and a shallow carbonate-dominated upper part (Currie, 1998a) (Fig. 5.1). Silicification of the upper Paleozoic limestones took place in the source area prior their erosion and deposition in the J-K transition retroarc succession (Conley, 1986). This interpretation is consistent with the abundant magmatic activity in Nevada during the Triassic and Jurassic periods, which could have supplied silica-rich fluids for an in place hydrothermal silificication of the shallow marine carbonates. The presence of silicified Late Paleozoic fauna in the chert clasts and quartzite grains shows that denudation of the Brushy Basin and Buckhorn Members source area involved erosion of nearly the whole Paleozoic continental margin succession and potentially Late Precambrian rocks as well (Fig. 5.1). The presence of extremely well-rounded, spherical grains in the sandstones of both members, but especially within the Brushy Basin Member, may be evidence of Mesozoic reworked materials. These grains show textural features of an eolian depositional environment, which are abundant in the Jurassic system of the western Colorado Plateau (Peterson, 1988, 1994). Conley (1986) found breccia lithoclasts in the Buckhorn 124 J volcaniclastics TR sandstone P mudstone P carlcareous mudstone

limestone PZ quartzite

PC

Fig. 5.1: Late Precambrian to Middle Jurassic schematic section of the Western Interior. Later thrusting resulted in this succession becoming the source area for the younger retroarc foreland basin. Clast and gravel lithologies found in the Morrison Formation and Buckhorn Conglomerate can be related to the older succession of the western margin of the American continent (modified from Currie, 1998a).

125 deposits, which he interpreted as Triassic in origin based on lithologic characteristics. Overlying the Upper Shale Member there is a substantial increase in Lower Paleozoic quartzite clasts occurs in the Dakota Sandstone and Sanpete Formations (Yinbling, 1987; Currie, 1998a). The petrographic data of the Morrison and Cedar Mountain Formations in the western San Rafael Swell (Crooks, 1986) and the Uinta Mountains in northeastern Utah and northwestern Colorado (Currie, 1998b), as well as observations from the present study, indicate a peak in chert detritus and a low in feldspar grains in the upper Brushy Basin and Buckhorn Conglomerate Members (Fig. 4.16a). This petrographic trend differentiates these two stratigraphic units from the rest of the Upper Jurassic-Lower Cretaceous succession in the southern part of the Western Interior. Consequently, this temporally constrained sandstone composition variation probably records a major tectonic event further west in the source area of the retroarc basin. The fact that Brushy Basin and Buckhorn Conglomerate sandstones plot in the reworked origin field of Dickinson (1985), supports this hypothesis (Fig. 4.16b). Yingling (1987) suggested four possible Buckhorn Conglomerate source areas indicated by an unconformity separating Pennsylvanian and older rocks from overlying Late Cretaceous or younger materials (Fig. 5.2). The three main regions are located following a northeastern to northwestern trend paralleling the Sevier thrust belt in western Utah and southeastern Nevada. This location is consistent with the east to southeast paleoflow direction of the Buckhorn river system (Fig. 4.14). The northeast directed paleoflow of the Brushy Basin Member (Fig. 4.19) could have also been partly supplied by a similar southwestern source area. A smaller potential source area is found further south in east-central Arizona. However, this location in not consistent with paleoflow data and probably was out of the influence of the Brushy Basin and Buckhorn fluvial drainage basins in the study area. Nevertheless, this region could have supplied detritus similar to the western source areas in other portions of the Colorado Plateau at the same time.

126 km. 0 200 ?

?

Sevier Belt

Fig. 5.2: Possible location of the Buckhorn source area. The areas in the map represent zones where Pennsylvanian or older rocks are unconformably overlain by Cretaceous or younger materials. The westernmost areas are consistent with the paleoflow data from the Brushy Basin and Buckhorn Conglomerate (after Yingling, 1987).

127 5.4 Timing of deposition Due to its lateral extent, a diachronous onset of Buckhorn sedimentation is expected. Although the Buckhorn fluvial paleosystem is at least 25 km wide in the study area, it is not clear that this indicates a long period of Buckhorn processes, since braided rivers are known to undergo rapid lateral migration (Best and Bristow, 1993). However, its local presence above the lateritic interval in Cleveland Lloyd dinosaur quarry suggests a younger age of the Buckhorn, or a different conglomeratic body in the northern part of the study area. Its rapid thickness variations within the study area can be explained by the lateral migration of the active channels of the braided flood plain as well as syn- and post-depositional erosion. The temporal gap present at the contact between the Brushy Basin and Buckhorn Conglomerate Members may be no more than a diastem in some sectors, whereas it may represent a high order unconformity in others. Nevertheless, when deposition of the Brushy Basin and Buckhorn Members is considered on a basin scale, their contact within the study area represents a nearly conformable transition. Therefore, I consider that the onset of Buckhorn Conglomerate deposition took place during the late Tithonian age of the Late Jurassic (Fig. 5.3), approximately 148 Ma, according to isotopic data from the top of the Brushy Basin Member (Kowallis et al., 1998). Sampling of mudstone lenses at different stratigraphic levels within the Buckhorn failed to produce any palynologic data to constrain the timing of Buckhorn sedimentation. Therefore, there is still no paleontologic evidence to conclude that the Buckhorn deposits bridge the Jurassic and Cretaceous Periods (Stokes, 1944). However, if the boundary between the two Periods is placed at 144 Ma (Palmer and Geissman, 1999), and if deposition of the Buckhorn braided alluvial plain took more than 4 my, the unit would extend into the Early Cretaceous. The occurrence of a lateritic soil at the uppermost Brushy Basin Member is interpreted to represent interfluve pedogenesis during Buckhorn deposition. Similarly, the presence of seismite deposits within the member indicates lithification of some intervals of the unit before 128 Cedar Mountain AGE Isotopic (San Rafael Swell) Age Ma Dakota ? Sandstone Cenomanian ? 100 98

Albian

110 Unnamed Shale Member

Aptian 120

N Barremian Cedar Mountain Fm.

Cretaceous (part) e ? o 130 c Hauterivian o m Valanginian i 140 a Berriasian n ? Tithonian Buckhorn Conglomerate 148 150 Brushy Basin Member 150 Kimmeridgian Salt Wash Member Morrison Fm. Tidwell Member 155

Jurassic (part) Oxfordian 160 Summerville Fm.

Fig. 5.3: Chronostratigraphic diagram of the sedimentary succession recording the Jurassic-Cretaceous transition in Cedar Mountain (San Rafael Swell). The Buckhorn is considered to be the uppermost member of the Morrison Formation because of evidence of the conformable transition between the Brushy Basin Member and the Buckhorn Conglomerate. A low order unconformity is placed at the top of the Buckhorn. The amount of time within the unconformity is not known.

129 deposition of the overlying Buckhorn materials. Therefore, although no cryptic sequence boundaries have been observed in the Buckhorn deposits, I consider it possible that a prolonged time gap is present within the Buckhorn cementation to account for the cementation of the host bed, prior to seismite formation. The presence of both features suggests that Buckhorn deposition took place over a relatively long span of time, and could have extended well into the Early Cretaceous. Following the deposition of the Buckhorn Conglomerate a prolonged period of negative accommodation and carbonate pedogenic activity took place as indicated by a calcrete horizon found directly above the Buckhorn in the San Rafael Swell (Crooks, 1986; Conley, 1986; Aubrey, 1998) and northeastern Utah (Currie, 1998a). If the onset of Buckhorn deposition is latest Jurassic (Kowallis et al., 1998) and the top of the Unnamed Shale Member is earliest Late Cretaceous (Cifelli et al., 1997), the unconformity within the lower part of the Unnamed Shale Member may span as much as 40 my. Unfortunately, the lack of chronostratigraphic control from the lower part of the Upper Shale Member precludes the quantification of the temporal gap represented in its basal unconformity. However, given the fact that no major unconformity has been reported to be embedded in this member, and the broad time span that it seems to represent, I conclude that a low order unconformity is present at the basal calcrete. This interpretation agrees with previous work that point to a major depositional break directly above the Buckhorn Conglomerate, rather than at the base (Kirkwood, 1976; Aubrey, 1998).

5.5 Stratigraphic Model Considering stratigraphic, sedimentologic, and petrographic data, the present study agrees with recent work that have suggested a reinterpretation of the regional stratigraphy of the uppermost Jurassic and lowermost Cretaceous of the Colorado Plateau (Aubrey, 1998). Two major tectonosedimentary units (TS) are identified divided by a low order unconformity marked by a 130 calcrete or correlatable erosional surfaces (Fig. 5.4). TS-1 includes both the Morrison Formation and Buckhorn Conglomerate in central Utah, as well as the lower part of the Burro Canyon Formation in southeastern Utah and western Colorado. TS-2 includes the Upper Shale Member of the Cedar Mountain Formation in eastern Utah, equivalent conglomeratic deposits in the Gunnison Plateau in central Utah, and the uppermost carbonate nodule-bearing interval of the Burro Canyon Formation in western Colorado. The top of TS-1 is placed at the top of the Buckhorn Conglomerate or at the top of the lateritic horizon where the former is not present. Lateritization could explain the presence of manganese staining of the Buckhorn Conglomerate framework and the regional lack of smectitic clays in the uppermost Brushy Basin Member, as they were weathered to illitic and kaolinitic clays (Peterson, 1988). Syndepositional oxidation of clay minerals under conditions of extremely active leaching by meteoric waters also provides an explanation for the discoloration of the conglomerate and sandstone bodies of the uppermost Brushy Basin Member in Cleveland Lloyd dinosaur quarry (Bilbey, 1992), the rest of the study area, and in Interstate 70 southwest of Green River (Yingling, 1987). Similarly, it could also account for the strong red coloration of the upper Brushy Basin Member in those areas where the overlying Buckhorn Conglomerate is very thin or absent. According to the structural restoration of the Central Utah sector by DeCelles et al. (1995), movement of the first Sevier thrust system did not start until Neocomian time (approximately 130 Ma). Since TS-1 records the end of Jurassic and potentially the very beginning of Cretaceous sedimentation in the southern Western Interior, it had to be deposited under tectonic conditions that preceded the onset of Sevier thrusting. This is corroborated by the absence of westward thickening in TS-1. In contrast to TS-1, TS-2 rapidly thickens towards the Sevier thrust belt, indicating its flexural origin (Heller and Paola, 1989; Yingling and Heller, 1992). Middle to Late Early Cretaceous palynological ages yielded by TS-2 (Tschudy et al., 1984) agree with the onset Sevier thrust 131 West-central Utah East-centralUtah East Utah SouthWestern AGE (Gunnison Plateau) (San Rafael Swell) (Green River) Colorado Ma

Coniacian Mancos Shale (part) Mancos Shale Mancos Shale Mancos Shale (part) (part) (part) 90 Turonian Sanpete Formation Dakota Sandstone ? Dakota Sandstone Cenomanian ? Dakota Sandstone ? ? 100 ? ? Albian

110 Unnamed Shale Unnamed Shale Upper Member Member unit of the Aptian Burro Canyon Formation Cretaceous (part) 120 Cedar Mountain Fm. Cedar Mountain Fm.

N Barremian Cedar Mountain Fm. e ? ? ? o 130 c Hauterivian o m Valanginian i 140 a Berriasian n ? ? ? Lower unit of the Lower unit of the Buckhorn Conglomerate Tithonian Burro Canyon Fm Burro Canyon Fm 150 Kimmeridgian Morrison Formation Morrison Formation Morrison Formation Jurassic (part)

Fig.5.4: Regional chronostratigraphic diagram of the sedimentary succession recording the Jurassic- Cretaceous transition in the western part of the Colorado Plateau and adjacent areas. Two main sedimentary packages are divided by a low order unconformity. The Buckhorn Conglomerate and the correlative lower part of the Burro Canyon Formation are considered to be part of the lower stratigraphic unit (modified from Aubrey, 1998). emplacement (DeCelles et al., 1995) supporting this interpretation. The base of TS-2 is placed at the top of the basal calcrete, at the erosional base of the lowermost calcrete intraclast-bearing conglomerate or sandstones (Aubrey, 1998), or at the drastic mudstone color change of the Brushy Basin Member and Unnamed Shale Member as seen near Green River and Arches National Park. The revision of the regional stratigraphy presented here removes the Buckhorn Conglomerate from the Cedar Mountain Formation and considers it to be part of the underlying Morrison Formation. This lithostratigraphic reinterpretation is based on the conformable transition between the Brushy Basin Member and the Buckhorn Conglomerate as indicated by their local interfingering, similarity in sandstone and conglomerate compositions and the sedimentary structures at their contact. Although their lithostratigraphic affinity is not obvious, evidence of their genetic relationship and the identification of a low order unconformity within the Cedar Mountain Formation support the redefinition of the Buckhorn Conglomerate as the uppermost member of the Morrison Formation.

133 CHAPTER 6: DEPOSITIONAL MODEL

6.1 Introduction In order to test the hypothesis of the conformable transition between the Brushy Basin and Buckhorn Conglomerate Members, a basin-scale depositional model able to account for their change in fluvial style and their different paleoflow directions must be constructed. Based in well-understood stratigraphic configurations of retroarc foreland basins, as well as foreland- basin and dynamic subsidence stratigraphic infill numerical models, an allostratigraphic approach is used to build a conceptual depositional model. This model incorporates the geologic processes that could have potentially controlled the unconformity distribution and stratigraphic configuration of the succession in the Colorado Plateau during the Middle - Late Jurassic. As in most stratigraphic basin infill models eustatic, climatic, and tectonic controls must be considered.

6.2 Eustasy In order to infer an eustatic control on the deposition of a sedimentary succession, the transgressive and regressive cycles must be chronostratigraphically correlative with similar cycles present in the sedimentary packages of other widely spaced basins, as well as with the global sea level curve. Comparison of sea level cycles present in the Middle and Late Jurassic stratigraphic record of the Colorado Plateau with their analogues in the northern part of the Western Interior, those present in other areas of the American craton, and the global eustatic curve of Hallam (1988) shows their lack of agreement (Peterson, 1994) (Fig. 6.1). Consequently global sea level oscillations can not be identified in the Jurassic succession of the Colorado Plateau. However, it is probable that relative sea level fluctuations had an influence on the temporal distribution of depositional environments of the Western Interior during the invasion of the Utah-Idaho Trough by the Sundance Sea in the Middle Jurassic. During the Late Jurassic 134 Global Western Gulf of SW. Northern Arctic Time Scale Unconformities Sea-Level Interior Mexico Canada Yukon Islands Curve Ma Reg Trans Reg Trans Reg Trans Reg Trans Reg Trans Low High 141 K-1 K-1

T T T T T Tithonian

150 Late Kimmeridgian K K K K K Oxfordian J-5 O O O O J-5 J-4 O J-4 C C Callovian J-3 C C C J-3 160 J-2c J-2c B B B Bathonian J-2b B B J-2b

Middle B B 170 Bajocian B B B J-2 J-2 A A Aalenian A A ? A J-1 J-1

180 JURASSIC Toarcian T T T T T

190 Pliensbachian P P P P P J-0b J-0b Early

Sinemurian S S S S S 200

Hettangian H H ? H H H 208 J-0 J-0 after after after after after after Peterson Salvador Poulton Poulton Poulton Hallam (1994) (1991) (1984) (1984) (1984) (1988)

Arctic Islands

Northern Yukon SW. Canada

Western Interior Basin

Gulf of Mexico

1,000 km

Fig. 6.1: Comparison of the transgressive-regressive cycles of the Jurassic succession of the southern Western Interior with other basins of the American craton and with the global sea level curve (after Hallam, 1988 in Peterson, 1994).

135 however, marine conditions had retreated to the north and had little impact on base level oscillations of the Colorado Plateau region (Fig. 1.2). Therefore, in this model neither eustasy nor relative sea level are considered to have affected stratigraphic patterns during deposition of the Salt Wash, Brushy Basin, and Buckhorn Members.

6.3 Climate River discharge is a direct response to precipitation rates. Therefore, in alluvial systems, climate has a strong control on the volume and size of their sediment supply. Changes in both parameters are present in the studied succession. The transition from Scott to Donjek braided river style in the upper interval of the Buckhorn Conglomerate is essentially a result of a decrease in bedload grain size, which can be a consequence of decrease in discharge. Similarly, the unconformity represented by the calcrete horizon at the base of the Unnamed Shale Member indicates very low sedimentation rates after deposition of the Buckhorn Conglomerate. The climatic transition from an arid climate during most of the Jurassic to more humid towards the end of the period is inferred from the presence of a lateritic soil at the top of the Brushy Basin Member, as well as the abundant dinosaur fauna in the upper Morrison Formation (Bilbey, 1992; Richmond and Morris, 1996). It is difficult to conceive the existence of such a large population of these vertebrates under the previously suggested arid conditions (Turner and Fisherman, 1991). Rapid polar wander of the American craton indicated by paleomagnetic studies of oceanic and continental rocks suggests a rapid northward drift of the American plate during this period (Steiner, 1983). This led to the abandonment of the zone of subtropical atmospheric cells that characterized the Lower and Middle Jurassic and placed the southern Western Interior in the zone of westerly winds (Parrish and Peterson, 1988). The more humid climate present during Brushy Basin Member and Buckhorn Conglomerate deposition could account for a more continuous supply of alluvial detritus from the source area and the

136 progradation of Buckhorn fluvial paleosystem. Therefore, climatic conditions may have contributed to the Upper Jurassic Brushy Basin -Buckhorn alluvial style transition. A later gradual transition from a humid climate to an arid one may be indicated by the gradual facies assemblage change observed in the uppermost Buckhorn. A switch from Scott to Donjek braided river types could indicate a gradational decrease in continuity and magnitude of the river discharge, leading to a decrease in grain size of the materials supplied from the source area. Therefore, climatic oscillations could also have played a role during deposition of the Buckhorn – Upper Shale Members transition.

6.4 Tectonics Documented intensive thrusting in western Utah and central Nevada during the Middle and Late Jurassic suggests that during this period sedimentation in the Western Interior took place in a retroarc foreland basin (Oldow, 1984; Thorman et al., 1990, 1992; Bjerrum and Donsley, 1995). This hypothesis is corroborated by the fact that the unconformity bounded sedimentary packages of the Middle and Late Jurassic succession in the southern western interior show a similar stratigraphic configuration to the well-understood Upper Cretaceous of Alberta (Plint et al., 1993) and Cenozoic Alpine (Sinclair et al., 1991) retroarc foreland-basin successions. All three present characteristic coarsening and shallowing upward packages with a progressive distal onlap on the craton (Fig. 6.2). Whereas the high order unconformities that separate successive sequences are thought to be the result of episodic thrusting, the low order composite unconformity on which the sequences progressively onlap is regarded as the result of flexural forebulge uplift and migration. The large aerial extent of the succession, however, requires the addition of dynamic subsidence to the formation and amplification of the retroarc basin (Lawton, 1994). Rapid shallow eastern subduction of the pre-Farallon plate under North America could have triggered a large-scale (hundreds of km) westward tilting of the American craton and the subsequent 137 Fluvial Sequence NE Paleo- & current Unconformity SW K-1 Morrison Fm.

J-5 CSR J-3 J-2 Entrada Ss. J-2 W Seq. 5,6 300 m Ve. 120 x U. Carmel 0 0 80 km J-S up Eolian L. Carmel Seq. 4 Fluvial Navajo Ss. J-2 Sabkha Seq. 3 TC J-1 Kayenta Fm. CSR = Curtis and Summerville Fm. M M = Moenave Seq. 1,2 TC = Temple Cap Ss. J-sub-Kay W = Wingate Ss. J-0

150 CSR K-1 E W J-3 Morrison Fm. 6 ? K-1 Entrada Ss. ? J-5 160 5 Upper Carmel J-S-up 4 Lower Carmel 170 3 TC ? J-2, Composite Unconformity 180 ? ?

Jurassic Navajo Sandstone 190 2 ? LowerKayenta Middle Fm. Upper ? 200 J-sub-Kay 1 Moenave Fm. Wingate Fm. J-0

Fig. 6.2: Stratigraphy of the Utah-Idaho Trough. A: Cross section of the Jurassic succession from southwestern Utah to central Colorado. Depositional sequences and bounding unconformities are indicated. Location of the section can be seen in Figure 2.13. B: Chronostratigraphic diagram of A. Note the progressive onlap of the Middle to Jurassic depositional sequences on the J-2 composite unconformity can be observed (after Bjerrum and Dorsey, 1995). 138 amplification of subsidence in the retroarc basin. In the proximal settings of the foreland basin both flexural (Jordan, 1981) and dynamic subsidence could have contributed to the deposition of the several kilometer-thick Middle Jurassic Utah-Idaho Trough succession (Lawton, 1994; Currie, 1998b). Therefore, tectonic processes were likely the most important geological mechanism responsible for sediment accumulation in the southern Western Interior during Middle and Late Jurassic times. In order to assess the specific contribution of flexural and subduction-induced processes on the stratigraphic configurations of a retroarc foreland basin, available numerical models are taken into account. The effects of thrust loading during orogenic compressional events (Flemings and Jordan, 1990; Jordan and Flemings, 1991) are first presented, followed by a discussion of the effects of larger scale subduction processes (Mitrovica et al., 1989; Gurnis, 1992).

6.4.1 Foreland Flexural Basin Stratigraphic Configuration Numerical models indicate that the alternation of periods of thrust loading and subsequent tectonic quiescence (tectonic cycles) in orogenic belts leads to the formation of unconformity bounded stratigraphic sequences in the contiguous foreland basin (Flemings and Jordan, 1990; Jordan and Flemings, 1991) (Fig. 6.3d). The model discussed here considers a viscoelastic lithosphere rheology, fully alluvial conditions during the whole tectonic cycle, and no influence of relative sea level oscillations. These parameters are considered to be suitable for the modeling of the Late Jurassic succession of the Western Interior. The models show that crustal thrusting has a different flexural response throughout the foreland-basin system. In proximal areas thrust loading leads to the rapid flexural subsidence of a laterally constrained foredeep, whereas in the distal part of the basin it triggers the tectonic uplift of a forebulge (Fig. 6.3a, 6.3c). Therefore, during periods of thrusting, sedimentation takes place in proximal settings while erosion and formation of an unconformity occurs in the distal 139 Flexural Bulge A uplift Thrusting

Foredeep flexural subsidence Flexural Bulge B attenuation Quiescence

Isostatic rebound

Flexural Bulge C uplift Renewed thrusting

Foredeep flexural subsidence

1 Km

10km D Time Deformation front (my) 12 10 Unconformity Sequence 2 8 C 6 4 B Sequence 1 2 A 0 0 100 200 Distance (km) from initial thrust front

Fig. 6.3: Foreland-basin stratigraphic infill model. Computer simulation of the stratigraphic configuration of a depositional sequence resulting from a thrust cycle. No sea level effects are considered. Note the uplift of the flexural forebulge during thrusting and its subsequent attenuation during tectonic quiescence (modified from Jordan and Flemings, 1991).

140 parts of the basin. Rapid foredeep subsidence leads to a scenario where the accommodation rate exceeds the sedimentation rate. Consequently, all the sediment supplied to the foreland basin is rapidly trapped in its proximal setting, the basin is underfilled, and the fluvial drainage system adopts an axial direction, this is it runs parallel to the thrust belt (Heller et al., 1988). Since flexural subsidence is considered to be almost synchronous to thrusting (Turcotte and Schubert, 1982), the rapid shift of the coarse facies towards the thrust belt and the minimum aerial extend of the foreland basin stratigraphic fill are interpreted to represent the onset of the depositional sequence. During the following period of tectonic quiescence, exhumation of the tectonic load leads to a switch in flexural and depositional conditions in the foreland basin. The lack of renewed thrust loading in the thrust belt leads to the absence of flexural subsidence in the foredeep, where the accommodation rate is no longer higher than the sedimentation rate allowing the overfilling of the basin. As a consequence the fluvial drainage system adopts a direction perpendicular to the thrust front transporting detritus from the trust belt into more distal regions (Heller et al., 1988). The release of compressional stresses leads to the attenuation or slow subsidence of the forebulge, enabling the progradation of the foreland clastic wedge into more distal areas (Fig. 6.3b). As the thrust belt undergoes progressive denudation, the rebound of the proximal foredeep as a result of crustal isostatic compensation enhances the reworking and transportation of the poorly consolidated, coarse-grained materials of the synorogenic clastic wedge into more distal settings (Heller et al., 1988). Therefore, the results of these models agree with the idea originally proposed by Heller et al. (1988), which suggests that the maximum aerial extent of coarse fluvial facies in a foreland basin does not correspond to synorogenic sedimentation, but rather to post-tectonic deposition (Fig. 2. 17). Therefore, the superposition of coarse-grained facies on top of finer-grained sediments takes place in a conformable manner as the clastic wedge rapidly advances towards the distal foreland basin in the late stage of the depositional sequence. Although high order 141 unconformities with an autocyclic fluvial origin can occur as a result of lateral migration of the main locus of fluvial sedimentation during the progradation of the clastic wedge, when seen at the depositional sequence temporal scale, the vertical transition from fine- to coarse-grain fluvial deposits in distal areas has a conformable origin. In its distal part, the depositional sequence is capped by the unconformity generated during the subsequent period of thrust emplacement, as a forebulge is uplifted (Fig. 6.3d). In the proximal areas another unconformity may occur if the period of tectonic quiescence is long enough for isostatically-driven foredeep exhumation to take place. However, if isostatic rebound is rapidly interrupted by renewed thrust loading, successive depositional sequence may have a conformable transition in the proximal areas. In this scenario the distal unconformable surface merges into an interval of increased channel clustering in the proximal settings (Posamentier and Allen, 1993). This model predicts a conformable base in the proximal areas laterally emerging into an unconformable lower surface with increasing stratigraphic hiatus towards the distal areas. This unconformity configuration is the opposite of that found on passive margins, where unconformities merge into conformable surfaces towards the distal parts of the basin and increase their duration toward the proximal ones (Posamentier et al., 1988).

6.4.2 Dynamic Subsidence Stratigraphic Configuration Subduction of oceanic lithosphere has important subsidence and uplift effects on the retroarc basin of the overriding continental plate influencing long-term regional sedimentation and erosional events (Mitrovica et al., 1989; Gurnis, 1992). Dynamic subsidence is the result of long wavelength tilting of overriding lithospheric plates as convective flow takes place in the upper mantle in order to compensate the presence of a higher density lithospheric mass beneath the overriding plate (Fig. 6.4a). Several parameters control the lateral extent and vertical magnitude of the dynamic subsidence of a craton. These factors are temperature contrasts between the down-going 142 lithospheric slab and the overthrusting mantle, continental flexural rigidity, and the angle of subduction (Mitrovica et al. 1989; Gurnis, 1992). Whereas the magnitude of vertical dynamic subsidence is susceptible to all the above parameters, the wave-length is mostly controlled by the angle of subduction (Mitrovica et al., 1989). Shallower subduction induces a higher dynamic subsidence effect, with subduction angles lower than 45o being able to produce up to 1000 km of horizontal deflection in the overriding continental crust (Mitrovica et al., 1989). However, when the angle of subduction is deepened again or the subducting slab is detached by tectonic mechanisms, the withdrawal of the sinking oceanic lithosphere is isostatically re-equilibrated by the dynamic uplift of the region previously undergoing subduction. Consequently, in a similar manner to flexural basins, periods of quiescence of this mechanism lead to the uplift and consequent denudation of the succession previously accumulated. Gurnis (1992) presented a general model in which the dynamic cumulative stratigraphy in the retroarc basin is linked to the different stages of plate and slab configuration during the evolution of a subduction zone (Fig. 6.4b). The model considers the angle of subduction in an active continental margin to be primarily controlled by the age and consequent density of the subducting oceanic lithosphere. After subduction begins, the angle of the down-going slab tends to shallow progressively as younger, less dense lithospheric material is subducted. This leads to an increase of the magnitude of vertical subsidence, as well as an inland migration of the dynamic depocenter. During this stage the numerical model predicts subsidence rates of the order of 100 m / my. The latest stage is characterized by dynamic rebound of the previous dynamic basin as the oceanic basin approaches its closure. Although in the model the depth and dip of the sinking slab are maintained constant, the final period of oceanic subduction leads to a gradual formation of a plateau with uplift rates of the order of 10 m / my.

143 A Retroarc foreland basin

Flexural Subsidence

Dynamic Subsidence Craton

Upper Oceanic lithosphere Mantle

Flow generated by Subducted Slab cold subducting slab

B

Ocean-continent evolution Dynamically controlled stratigraphy

Subsidence 200 Forebulge

-200

-600 Initiation of subduction 2000

Subsidence 0

-2000 Elevation (m) -6000 Dynamic shallowing of slab 2000

1000 Uplift 0 0 -1000 Dynamic deeping -2000 or detachment 0 400 800 1200 of the slab Distance (km)

Fig. 6.4: The dynamic subsidence model. A: Comparison of the aerial extent of dynamic and flexural subsidence (modified from Currie, 1998b). B: Plate and slab configuration during three stages of the evolution of a subduction zone and the resulting cumulative stratigraphy. It can be seen that the deepening of the subducting slab leads to the regional uplift of the materials previously accumulated during dynamic subsidence in the overriding plate (after Gurnis, 1992). 144 6.5 Depositional Model Based on the stratigraphic configuration of foreland basins depicted by numerical models I find evidence to suggest that the Brushy Basin Member and Buckhorn Conglomerate transition took place in a retroarc foreland basin during a period of tectonic quiescence. The Buckhorn Member represents the progradation of proximal facies of the same fluvial system within the overfilled back-bulge depozone of a later Jurassic foreland-basin system. Possible modern analogues are found in alluvial valleys near Banff in Alberta, were an abrupt transition from braided to anastomosed rivers occurs (Smith and Smith, 1980). Therefore, the Buckhorn Conglomerate is included in the Morrison depositional sequence, defined in this study to comprise the materials bounded by the unconformities at the base of the Salt Wash Member and at the top of the Buckhorn Conglomerate (Fig.6.5). Onset of the sequence deposition started with Late Jurassic thrusting in western Utah and eastern Nevada associated with the Nevadan Orogeny. This compressional event has been dated to be 155 ± 3 Ma in central California (Schweickert et al., 1984) coinciding with the onset of Morrison deposition around 155 ± 0.5 Ma (Kowallis et al., 1998). The Nevadan compressional event, driven by exotic terrane accretion farther west in the subduction zone, would account for the tectonic loading in the orogenic belt. Paleomagnetic data indicates a 10 ±

3o rotation of the Colorado Plateau in mid-Morrison time (Steiner, 1998), suggesting that the effects of the continental accretion were felt further inland in the retroarc basin. Lack of information on the onset of Salt Wash deposition does not allow the present study to constrain more precisely the onset of the Morrison depositional sequence. The location of the Late Jurassic orogenic front is recorded by the remains of the Elko orogenic belt of northwestern Utah and northeastern Nevada and maybe the Central Nevada thrust belt in the Basin and Range area (Fig. 2.13). However, Cenozoic extension has overprinted the actual location of the thrust belt. The Nevadan Orogeny has been considered to have been a short-lived event (Schweickert et al., 1984), and Late Jurassic extensional faulting

145 s overfilled basin underfilled basin Accomm. rate versus Sed. rate braided braided Fluvial Style anastomosed meandering (transitional) _ 0 Total Total Subsidence + _ 0 + Dynamic Subsidence _ 0

+ Flexural Subsidence

subducting slab subducting subduction uplift detachment

Shallowing of Shallowing Constant shallow Constant Dynamic Slab

rebound

isostatic Thrusting Quiescence

Tectonic Activity Foredeep General Paleo- current

Lithologies .

Salt Wash Salt Wash

Mbr.

Mbr

Brushy Basin Basin Brushy

Conglomerate Buckhorn Buckhorn Lithostratigraphic Units Sequence Depositional Morrison

0

the result of the changing accommodation rates. accommodation changing the of result the

ratios, indicating the change from a underfilled to overfill foreland basin conditions. Note the transition of fluvial styles a styles fluvial of transition the Note conditions. basin foreland overfill to underfilled a from change the indicating ratios,

during deposition of the sequence are presented. Paleoflow directions are related to the accommodation: sediment supply sediment accommodation: the to related are directions Paleoflow presented. are sequence the of deposition during Fig.6.5: The Morrison depositional sequence. Flexural, dynamic, and total subsidence in the proximal back-bulge area back-bulge proximal the in subsidence total and dynamic, Flexural, sequence. depositional Morrison The Fig.6.5: 50 m 50 in northwestern Utah (Allmendinger and Jordan, 1984) is interpreted as the rapid collapse of the orogenic belt (Bjerrum and Dorsey, 1995). Tectonic loading was followed by a period of regional uplift and rapid denudation of the Upper Jurassic flexural foredeep. This foredeep was likely located approximately along the Utah-Idaho Trough, but later Sevier orogenic uplift has left behind only a low order unconformity. Isostatic rebound, thermal doming, and dynamic uplift may have all contributed to the unroofing of the Middle and Late Jurassic foredeep. According to the depositional pinchout of the lower Morrison Formation onto a forebulge in central Utah (Currie, 1998a), deposition of the formation in the study area is interpreted to have taken place in the most proximal part of the back-bulge of the retroarc foreland basin (Fig. 2.16). The accumulation of a more than 100 m thick fluvial succession represented by the Salt Wash and Brushy Basin Members in no more than 6 my, suggests that during most of the deposition of the Morrison depositional sequence, the back-bulge depozone was experiencing rapid subsidence (Fig. 2.3). Considering the large aerial extent of the Morrison Formation, dynamic subsidence seems to have been the major factor controlling subsidence, with a secondary imprint of thrust-driven flexural events experienced by the back- bulge depozone during the Nevadan tectonic cycle. Dynamic subsidence of the Western Interior retroarc could have been the result of a period of shallow plate subduction during the accretion of the island arc continental block. The attempted subduction of more buoyant crust would have forced the down-going slab to decrease the subduction angle and therefore extend the effects of dynamic subsidence into the back-bulge region. This interpretation is consistent with the thrusting in the foreland orogenic belt synchronous with dynamic subsidence, since shallow subduction may lead to both phenomena occurring simultaneously. Moreover, Late Jurassic eastern migration of the magmatic region (Christiansen et al., 1994) could be also explained by a shallowing of the subducting slab. 147 The base level oscillation expected from foreland basin numerical models in the distal parts of a foreland basin during a thrusting-quiescence cycle is seen in the Morrison depositional sequence as a second-order subsidence control (Fig. 6.5). Initial thrusting is recorded by forebulge uplift, and the formation of an unconformity between the Tidwell and Salt Wash Members (Bilbey, 1992). Sequence deposition started during the late thrusting stage when dynamic subsidence exceeded flexural uplift. The low thickness of the Salt Wash Member in the study area compared to those further east, as well as synsedimentray intrabasin uplifts during the deposition of the member support active tectonism during its deposition (Peterson, 1984). Low accommodation rate resulting from the superposition of a flexural uplift and dynamic subsidence led to the formation of a braided fluvial system in the proximal settings of the back-bulge zone in southern Colorado Plateau (Fig. 6.5). Due to the presence of a forebulge in central Utah, and the increasing dynamic subsidence as a result of the shallowing of the subducting slab. the back-bulge depozone became underfilled. Consequently, the Salt Wash fluvial system flowed approximately parallel to the thrust belt, draining the Mogollon highland and following the retreat of the Sundance Sea (Fig. 1.2). Following the thrust loading period, relaxation of compression stresses led to the subsidence of the forebulge region. The consequent slight increase of the accommodation rate is seen in the stratigraphic record as a gradational transition from a braided to a more meandering fluvial system in the southern Western Interior (Fig. 6.5). As dynamic subsidence reached its maximum rate, the Brushy Basin Member alluvial system became anastomosed in order to keep up with the high accommodation rates. The base level rise and intensification of volcanism seen in the Brushy Basin Member are consistent with a period of active shallow subduction beneath the American craton. Coinciding with the change in fluvial facies, the rotation of the Colorado Plateau took place (Steiner, 1998), and shortly afterward volcanism in the Cordilleran orogen experienced a drastic intensification as seen in the abrupt increase in smectitic, volcaniclastic-

148 derived mudstones a few meters above the base of the Brushy Basin Member (Peterson, 1994). Attenuation of the flexural forebulge in central Utah allowed the conformable progradation of reworked coarse materials from the foredeep into central and eastern Utah. It is important to point out that no western depositional onlap has been reported for the upper part of the Morrison Formation. Therefore, there is no evidence for the existence of a structural barrier separating the foredeep and back-bulge depozones during the deposition of the upper part of the Morrison Formation. Furthermore, the supply of Upper Paleozoic chert and Lower Paleozoic quartzite from a western area (Yingling and Heller, 1992) as indicated by the presence of these lithologies in the Brushy Basin Member, suggests that the early Morrison forebulge if still present did not completely isolate the area from the proximal foredeep. The detachment and end of pre-Farallon plate subduction produced a drastic subsidence decrease and a subsequent dynamic uplift in the Western Interior retroarc basin. In the study area and east-central Utah, the top of the Morrison depositional sequence records the decrease in subsidence by the deposition of the Buckhorn Conglomerate unconfined braided river system (Fig. 6.5). This drastic drop in accommodation superimposed the proximal Buckhorn river facies on top of the Brushy Basin Member on the distal fluvial facies in the study area. I believe that the isostatic rebound of the retroarc forebulge after a period of tectonic quiescence and the first stages of dynamic uplift combined to produce eastern progradation of the proximal facies of the Morrison alluvial system. Climatic oscillation could have contributed to higher and more continuous river discharge during this period enhancing the eastern progradation of the proximal alluvial environments draining the thrust belt. However, I consider that the drastic increase in volume and grain size of the sediment supply could be better explained as the regional uplift of the Late Jurassic foredeep in western Utah. The rapid change from a northeastern paleoflow direction of the Brushy Basin Member fluvial system to an east-to-southeasterly paleocurrent of the Buckhorn can also be linked to this 149 change in the rate of formation of accommodation space. During active dynamic subsidence, the high accommodation rate led to an underfilled, rapidly sinking back-bulge area and consequently the fluvial system ran parallel to the thrust front, this is the axial drainage of the back-bulge region (Heller et al., 1988; DeCelles and Giles, 1996). The subsequent accommodation decrease led to the overfilling of the flexural basin and the progradation of the Buckhorn alluvial system perpendicular to the orogenic belt (Heller et al., 1988; DeCelles and Giles, 1996) (Fig. 6.5). The Morrison depositional sequence is capped with a paracomformity represented by a calcrete horizon that I interpret to be a result of dynamic uplift of the Utah-Idaho Trough (Fig. 6.5). Continued regional uplift could have produced the reworking of the calcrete horizon, the deposition of the conglomerate bodies in the lowermost Upper Shale Member of the Cedar Mountain Formation, and the sharp contact between the latter and Brushy Basin Member where the Buckhorn Conglomerate and the overlying calcrete are not present. Therefore, the data from this study support the previous interpretations of a regional uplift in the hinterland between the Sevier belt and the Cordilleran magmatic arc (Heller and Paola, 1989; Yingling and Heller, 1992; Lawton, 1994) following Late Jurassic thrusting to explain the deposition of the Buckhorn Conglomerate. However, in the study area the location of the related unconformity is found directly above the Buckhorn Conglomerate and not at its base as previously suggested. I also agree that the Morrison Formation was deposited in a foreland basin setting (DeCelles and Currie, 1996; Currie, 1997, 1998a, 1998b). Nevertheless, structural studies indicate that the thrusting events responsible for the formation of the Morrison flexural depozone systems were unrelated to the younger Sevier Orogenic belt (DeCelles et al., 1995).

150 CHAPTER 7: CONCLUSIONS

The results of the present study shed more light into the understanding of the deposition of the terrigenous succession recording the Jurassic-Cretaceous transition in the Colorado Plateau. Stratigraphic, sedimentologic, and petrographic data offer consistent evidence of the conformable transition between the Brushy Basin Member of the Morrison Formation and the Buckhorn Conglomerate Member of the overlying Cedar Mountain Formation. These data include the interfingering of the two members, gutter casts, mud injection, and dinosaur tracks at the base of the Buckhorn Conglomerate, and finally similarity in sandstone composition. However, an unconformity, locally represented by a hardpan calcrete, was identified at the base of the Unnamed Shale Member of the Cedar Mountain Formation. The genetic relationship between the Buckhorn Conglomerate and the underlying Brushy Basin Member and the presence of a low order unconformity separating the two current members of the Cedar Mountain Formation, leads me to propose that the Buckhorn be reinterpreted as the uppermost member of the Morrison Formation. The low order unconformity long believed to occur at the base of the Buckhorn Conglomerate is placed at its top. Insufficient chronostratigraphic control precludes confirming if the unconformity bridges the Jurassic and Cretaceous Periods. This unconformity divides the succession in two different tectonostratigraphic (TS) units deposited in temporally unrelated foreland basin systems. The lower TS unit was deposited by pre-Sevier orogenic events in Central and western Utah and includes the Morrison Formation, Buckhorn Conglomerate, and lower part of the Burro Canyon Formation. The upper TS unit corresponds to the initial stages of the Sevier orogeny and consists of the Unnamed Shale Member of the Cedar Mountain Formation and the upper part of the Burro Canyon Formation.

151 The lower TS unit is named the Morrison depositional sequence. This alloformation is bounded by a basal high order unconformity between the Tidwell and the Salt Wash Members of the Morrison Formation and a low order unconformity at the top of the Buckhorn Conglomerate, and includes the Salt Wash and Brushy Basin Members of the Morrison Formation and the Buckhorn Conglomerate Member formerly assigned to the Cedar Mountain Formation. Deposition of the Morrison sequences took place during a complete foreland basin terrigenous depositional cycle. The basal unconformity is interpreted as the result of forebulge uplift as a crustal response to the Nevadan Orogeny. The Brushy Basin Member-Buckhorn Conglomerate transition is believed to record the conformable eastern propagation of Late Jurassic flexural foredeep reworked materials, as the result of isostatic rebound of the proximal depozone due to prolonged thrust tectonics quiescence and dynamic uplift following the detachment of the pre- Farallon plate. Both mechanisms coexisted to produce the negative accommodation rates responsible for the formation of the upper sequence boundary. Shallow subduction of the same plate induced dynamic subsidence of the Colorado Plateau and acceleration of accommodation rates during the deposition of the sequence. The late Jurassic onset of Buckhorn deposition precludes its chronostratigraphic correlation with the Valinginan Cloverly Formation and questions its contemporaneous deposition with the rest of the Lower Cretaceous conglomerate as proposed by previous workers. Since the tectonic mechanisms of Buckhorn deposition are interpreted to have had plate-scale effects, the same tectonic events could be related to the deposition of the lithostratigraphically correlative conglomerate deposits in the rest of the Western Interior. However, this study suggests that the Lower Cretaceous conglomerate is time transgressive in nature.

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164 Appendix A: Stratigraphic Sections

(the location of the sections is presented in figure 3.1)

Legend of lithologies

mudstone massive sandstone carbonate nodule- cross-bedded sandstone bearing mudstone siltstone massive conglomerate

bentonite planar-crossbedded conglomerate

carbonate trough-crossbedded conglomerate

165 Section A (Buckhorn Conglomerate Type Section)

GRAIN SIZE cobble (Fig. 1.8b) pebble granule sand silt v c mfv clay REMARKS METERS

28

calcrete replacing sandstone, abundant jasper veins

26

bright colored chert and quartzite granules and pebbles

24 (Fig. 4.20a)

purple mudstone with carbonate nodules 22

USM calcrete, micritic carbonate, scattered chert granules, carbonate intraclasts, jasper and calcite veins present 20

Mean = 167 Sandstone point-count n = 4 BC-A-14 (Fig.4.15d) 18 (3D)

Mean = 153 n = 2

16 (2D)

14

Mean = 119 BC n = 3 12 (2D)

10

Mean = 31 n = 1 8 (2D)

6 Mean = 96 n = 1 (2D) 4 BBM?

2

0

166 SECTION CC

GRAIN SIZE

cobble pebble granule sand sil v c mfv clayt REMARKS METERS

bright colored pebbly sandtone, 10 USM rapid lateral grain size variation

BBM blocky gray mudstone

8

white sandstone

6 Mean = 16 n = 26 (3D)

4

2

0

167 SECTION EA

GRAIN SIZE GRAIN SIZE cobble cobble pebble pebble granule granule sand sand sil sil v c mfv clayt REMARKS v c mfv clayt REMARKS METERS METERS

62 large cobble size carbonate intraclasts 26 USM Mean = 151 60 n = 7 Mean = 14 (3D) 24 n = 2 (3D)

58

22

Mean = 185 BC 56 n = 2 (2D) 20 (Fig. 4. 17b) 54 Mean = 62 n = 16 (2D) 18 mudstone and fine sandstone intraclasts 52 Mean = 180 (Fig. 4.21a) n = 1 16 (2D) BBM 50 10.5 m 14

40 12

38

10

36

8

34 6

32

4 Mean = 57.8 n = 3 (2D) 30

2

Mean = 104.0 n = 1 28 (2D) 0 168 SECTION F

GRAIN SIZE cobble pebble granule sand silt v c mfv clay REMARKS METERS

34 mud-filled Mean = 89 clastic dikes, (pebble count, Fig. 4.18b) n = 2 (3D) 32

Mean = 150 n = 1 30 (2D)

28

26 (Fig. 4.10a)

BC 24 gutter casts and mud-injection contact structures (pebble count, Fig. 4.10b) BBM

14 9.5 m

12

10

8

6

4

2

0

169 SECTION GB

GRAIN SIZE cobble pebble granule sand silt v c mfv clay REMARKS METERS

calcrete, micritic carbonate, gradational base, scattered bright color chert and quartzite pebbles and granules,carbonate intraclasts

bright color chert and quartzite pebble conglomerate, 14 sandy lenses, locally trough cross-bedded, intensively USM calcite recrystallized sandy matrix, very abundant cobble size carbonate intraclasts and dinosaur bones pebbly purple mudstone 12 disaggregated cobble and pebble conglomerate, muddy and sandy matrix, sharp erosional base, gradational top

10 (Fig. 4.1d) BC

8 laterally disaggragated

6

BBM

4

2

0

170 SECTION IA

GRAIN SIZE GRAIN SIZE cobble cobble pebble pebble granule granule sand sand silt silt v c mfv v c mfv clay REMARKS clay REMARKS METERS BIOTURBATION METERS

24 5.4 m 66 cobble-size carbonate intraclasts USM 20

mudstone intraclasts 64

18 Mean= 108 n = 4 62 (2D) BC Mean = 53 16 n= 2 (3D) 60

14

58 BBM 12 9.5 m 50

10

48

8

18.0 m

6 30

4 28

Mean = 10 Mean = 323 2 n = 26 n=1 26 (2D) (3D)

mudstone intraclasts

0

171 SECTION IB

GRAIN SIZE cobble pebble granule sand silt v c mfv clay METERS REMARKS

USM 22 calcrete, micritic limestone appearance floating chert granules and pebbles jasper veins

20 BC completely disaggregated cobble and pebble Buckhorn Congomerate, abundant mudstone surrounding non-consolidated pebble-supported framework 18

BBM

16

14

purple mudstone (Fig. 4.8a)

12

10 Mean = 17 Mean = 303 n = 3 n = 13 (2D) (3D) 8 abundant mudstone intraclasts

6

4

2

0

172 SECTION J

GRAIN SIZE GRAIN SIZE cobble cobble pebble pebble granule granule sand sand silt silt v c mfv clay REMARKS v c mfv clay REMARKS METERS METERS

20

4.5 m 46 Mean = 323 n = 2 16 (2D)

(Fig. 4.13b) 44

14

Mean = 202 42 n = 3 (2D) 12 Mean = 53 Mean = 34 n = 61 n = 9 (2D) (3D) 40

10 wood and bone fragments, Mean = 260 and mudstone intraclasts n = 2 (2D) BC 38 8 3.5 m BBM Mean = 118 6 n = 1 micritic limstone, 15.5 m mollusc fossils (2D) (Fig. 4.26a) 5.2 m 22

0

173 SECTION JB

GRAIN SIZE

cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

26 USM carbonate intraclasts, dinosaur bones

semi-consolidated, abundant varigated sandy mudstone matrix BC 24

disaggregated conglomerate, abundant mudstone matrix pebbles show a bluish metallic hue

BBM 22 Mean = 48 n = 1 (3D)

20 purple blocky mudstone

18 Mean=39 Mean = 84 n = 8 n = 5 (2D) (3D)

16 mudstone intraclasts, locally purple banding present

14

12

10

8 Mean=77 n = 17 (3D) 6 abundant mudstone intraclasts, purple banding, silckensides

4

2

0 174 SECTION K

GRAIN SIZE

cobble pebble granule sand silt REMARKS v c mfv clay METERS

26 Mean = 292 n = 1 (3D) 24

BC 22

BBM

20

18

16

14

12

10

8

6

4

2

0 175 SECTION KA

GRAIN SIZE GRAIN SIZE

cobble cobble pebble pebble granule granule sand sand silt silt v c mfv v c mfv clay REMARKS clay REMARKS METERS METERS BIOTURBATION BIOTURBATION

22

20 56

18 54

Mean = 270 Mean = 146 n = 1 n = 1 16 (2D) 52 (2D)

largest Buckhorn Conglomerate 14 clast in the study area (25 cm) 50

BC

12 BBM 48 16.5 m micritic limestone, 10 moldic texture, molluscs 32

8 mudstone intraclasts

30

6

Mean=31 Mean=52 28 n = 10 n = 6 (2D) (3D) 4

26

2

24

0 micritic limestone

176 SECTION KB GRAIN SIZE cobble (Fig. 4.1a) cobble pebble pebble granule granule sand sand silt silt v c mfv clay REMARKS v c mfv clay REMARKS METERS METERS BIOTURBATION BIOTURBATION

calcrete, micritic carbonate, floating red and black chert granules and fine pebbles 68 jasper, and white quartz 28 veins present

granular to cobble bright color chert and quartzite Mean = 57 66 26 conglomerate, fine pebbly n = 1 sandstone interbedded, (2D) increasing upward carbonate recrystallization, gradational top 64 24 purple mudstone with carbonate nodules USM

62 22 Mean = 349 n = 2 (2D) Sandstone point-count 60 BC-KB-9 (Fig.4.15e) 20 Mean = 139 Mean = 30 n = 3 n = 1 (2D) (3D) 18 58 Mean = 44 n = 1 (2D)

BC Mean = 228 56 n = 1 (2D) 16 dinosaur bone fragments, mudstone intraclasts Mean = 50 n = 1 (2D) 54 14 Mean = 50 (2D) n = 1 Mean = 107 12 52 n = 1 (2D) Mean = 120 n = 4 Mean = 65 (2D) BBM n = 1 (2D) 10 40 7.5 m

8 38

6 36 8.5 m

4 34

2 32

0 177 SECTION M

GRAIN SIZE

cobble pebble granule sand silt v c mfv clay REMARKS METERS

24 Mean = 213 n = 1 (3D) 22 general channel geometry

BC 20 BBM

18

16

14

12

10

8

6

4

2

0

178 SECTION MB (Fig. 4.1b)

GRAIN SIZE cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

Mean = 79 30 n = 1 (2D)

Mean = 262.0 28 n = 1 Mean = 91 (2D) BC n = 1

BBM 26 (2D)

80 cm long siltstone lens 9.5 m 50 cm long pebbly claystone 18

Mean = 170

16 n = 1 (2D)

locally purple banding present 14

12

10

8

6

4

2

0

179 Section O (Fig. 4.6 left)

GRAIN SIZE

cobble pebble granule sand silt REMARKS v c mfv clay METERS BIOTURBATION

12

calcrete, micritic carbonate, floating red and black chert pebbles and granules, 10 USM jasper veins

BBM

8 (Fig. 4.9b) purple platy mudstone, laterite

laterally discontineous, disaggregated sandstone body 6 at approximately the same position as the Buckhorn Conglomerate distal sandstones in section P (Fig. 4.9a)

4

2 Mean = 290 n = 7 (2D) 0

180 SECTION OA

GRAIN SIZE GRAIN SIZE cobble cobble pebble pebble granule granule sand sand silt silt v c mfv v c mfv clay REMARKS clay REMARKS BIOTURBATION BIOTURBATION METERS METERS

Sandstone point-count 66 BBM-OA-17 22 (Fig.4.15c)

mudstone and fine-grained Mean = 100 64 sandstone intraclasts n = 1 20 (2D)

62

18 Mean = 288 n = 1 Mean = 270 Mean = 341 60 (2D) n = 12 n = 1 (2D) (3D) 16 Mean = 312 58 n = 1 (2D) 14 BC

micritic limestone BBM 56

12

mudstone intraclasts 54

10

52

8

50

6 micritic limestone micritic limestone 48 micritic limestone 4 micritic limestone

46

2 28.5 m

22 0

181 SECTION OB

GRAIN SIZE cobble pebble granule sand sil v c mfv clayt REMARKS METERS

22 calcrete, micritic carbonate floating chert granules and pebbles abundant chert granules and fine pebbles 20

bright color pebbly chert conglomerate USM

Mean = 351 18 Mean = 338 BBM n = 20 n = 8 (2D) (3D)

thin cross-stratified 16 white granular sandstone

14

12

10

8

6

4

2

0

182 SECTION P (Fig. 4.6)

GRAIN SIZE

cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

14 calcrete, micritic carbonate, floating red and black chert pebbles and granules, USM jasper veins 12 (Fig. 4.26b) BBM purple blocky mudstone

10 most distal Buckhorn conglomerate, pinches out 20 m to the W, intensively recrystallized

8

6

mudstone intraclasts

4

2

0

183 SECTION PB (Fig. 4.7 right)

GRAIN SIZE cobble pebble granule sand sil REMARKS v c mfv clayt METERS BIOTURBATION

28

scattered chert pebbles, decerasing in abundance towards the top of the unit, intensively recrystallizated by quartz 26 BC disaggregated granular conglomerate BBM blocky purple mudstone 24

BC Mean = 170 Mean = 179 n = 32 22 n = 13 (2D) (3D)

20 BBM

18

16

14

12

mudstone intraclasts

10

8

6

4

abundant mudstone intraclasts

2

0

184 SECTION RB

GRAIN SIZE GRAIN SIZE cobble cobble pebble pebble granule granule sand sand silt REMARKS silt v c mfv REMARKS v c mfv clay clay METERS METERS BIOTURBATION

calcrete, micritic carbonate, floating red and black chert granules and pebbles, jasper 52 veins Mean = 45 calcrete replacing fine-grained n = 22 sandtone with abundant red and black chert granules and fine (2D) 50 pebbles, jasper veins present 24 Mean = 2 chert pebbly purple mudstone n = 12 USM (3D) disaggregated chert pebble 48 conglomerate with muddy abundant mudstone BC 22 matrix intraclasts (Fig. 4.23b)

BBM 46 irregular bands of brown platy 20 claystone within purple Mean = 121 mudstone, laterite, scattered chert n = 1 pebbles below the BC contact (2D) 44 18

(Fig. 4. 21b) 42 16

40 Mean = 156 14 Mean = 106 bentonite n = 9 n = 17 (2D) (3D) 38 basal gray chert conglomerate 12

36 10

34 8

32 6

30 4

28 2

26 0 185 SECTION S

GRAIN SIZE

cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

26

24 Mean = 186 n = 1 (2D)

22 Mean = 215 n = 1 (2D) 20

Mean = 72

18 n = 2 (2D) Mean = 352 n = 1

16 (2D)

Mean = 142 n = 1 14 (2D)

BC gutter casts 12 BBM

10

8

6

4

2

0

186 SECTION SB

GRAIN SIZE cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

calcrete replacing fine-grained sandtone with abundant bright color chert a and quartzite 18 granules and fine pebbles bright color pebbly chert conglomerate poorly developed calcrete replacing fine-grained sandstone with aboundant bright color chert granules and fine pebbles 16 bright color pebbly chert conglomerate chert pebbly mudstone calcrete replacing fine grained sandstone 14 with aboundant colorful chert granules and fine pebbles 4 cm thick horizontal jasper veins USM chert pebbly mudstone

12 disaggregated chert pebble conglomerate with sandy muddy matrix BC (pebble count, Fig. 4.23c)

BBM 10

8 irregular structures of brown platy claystone within purple mudstone, laterite

6

4

Mean = 97 n = 13 2 (2D)

irregular structures of brown platy mudstone 0 within purple mudstone

187 SECTION UA

GRAIN SIZE GRAIN SIZE

cobble cobble pebble pebble granule granule sand sand silt silt v c mfv v c mfv REMARKS clay REMARKS clay METERS BIOTURBATION METERS BIOTURBATION Mean = 156 n = 1 (3D) 50 Mean = 241 24 n = 1 (2D) BC 48 Mean = 179 BBM n = 6 (2D) 22 Mean = 15 n = 1 46 (3D)

20

Mean = 51 Mean = 120 n = 2 44 n = 1 (2D) 18 (2D)

mudstone intraclasts 42

16 Mean = 352 n = 4 40 (3D)

14 mudstnone intraclasts Sandstone point-count BBM-17 38 (Fig.4.15a) 12

36

10 Mean = 52 n = 1 (3D) 34

8 Mean = 64 n = 1 32 (3D) 6

30

4 Mean = 31 micritic limestone, n = 13 moldic texture, gastropods 28 (3D)

2 Mean = 50 n = 31 (2D) 26 basal chert conglomerate, 0 mudstone intraclasts

188 SECTION UB

GRAIN SIZE

cobble pebble granule sand sil v c mfv clayt REMARKS METERS BIOTURBATION

disaggregated pebbly to cobble conglomerate, sandy muddy matrix, 10 vertical funnel-shaped structures up to 110 cm of lateral and 245 cm vertical extent, where massive quartz cement replaces the matrix vertical and horizontal jasper and white quartz veins present within these structures 8

Mean = 280 Mean = 355 6 n = 1 n = 1 (2D) (3D)

irregular bands of brown platy claystone 4 within purple mudstone, laterite

2

0

189 SECTION WA

GRAIN SIZE cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

Mean = 143 n = 1 (3D)

46

Mean = 107 n = 2 44 (3D)

Mean = 231 n = 1 (3D) 42

Mean = 10 n = 1 40 (3D)

BC 38 BBM

13.2 m

26

24

Mean = 39 Mean = 120 3.5 m n = 2 n = 1 (2D) (3D)

20

mudstone intraclasts

14 6.0 m

12 mudstone intraclasts

10

Mean = 0 8 n = 29 8.7 m (2D) Mean = 88 n = 4

0 (3D)

190 SECTION WB

GRAIN SIZE (Fig. 4.7 left) cobble pebble granule sand silt v c mfv clay REMARKS METERS BIOTURBATION

28

26

24 Mean = 193 Sandstone point-count BC-WB-2 BC n = 3 (2D) (Fig.4.15f) BBM 22 Mean = 201 Mean = 200 n = 2 n = 1 (2D) (3D)

20 banded mudstone (Fig. 4.8b)

18

16

mudstone intraclasts

14

12 abundant mudstone intraclasts Mean = 30 n = 1 (2D) 10

8

6

4

2

0

191 SECTION XB

GRAIN SIZE cobble pebble granule sand silt v c mfv clay REMARKS METERS

bright color pebbly sandtone, USM rapid lateral grain size variation 6 BBM blocky gray mudstone

4

white chert granules beds in white sandstone

2

0

192 SECTION Y

GRAIN SIZE

cobble pebble granule sand silt v c mfv clay REMARKS METERS

12 Mean = 84 n = 2 (3D) 10

8 Mean = 205 n = 1 (2D)

6 BC Mean = 165 n = 1 BBM? (2D) 4

2

0

193 SECTION ZA

GRAIN SIZE GRAIN SIZE cobble cobble pebble pebble granule granule sand sand silt silt v c mfv v c mfv clay REMARKS clay REMARKS METERS METERS BIOTURBATION

62 Mean = 312 (2D) n = 1 60

Mean = 340 58 (2D) n = 1

24

56

Mean = 287 22 (2D) BC 54 n = 1

BBM 20 52 11.5 m

18

42

16

40

14

38

12 micritic limestone 36 micritic limestone 10 micritic limestone 34

micritic limestone, 8 moldic texture 32 molluscs

6 Mean = 72 Mean = 40 mudstone intraclasts 30 n = 18 n = 10 (Fig. 4.13a) (2D) (3D) 4

Mean = 25 28 n = 3 2 (2D) Sandstone point-count 26 BBM-ZA-13 (Fig.4.15b) mudstone intraclasts 0

194 SECTION ZB

(Fig. 4.7 center) GRAIN SIZE cobble pebble granule sand sil v c mfv tclay REMARKS METERS BIOTURBATION

28

scattered chert pebbles, decerasing in abundance towards the top of the unit, 26 intensively recrystallizated by quartz

BC Mean = 156 n = 1 BBM blocky purple mudstone 24 (2D)

BC Mean = 170 Mean = 179 n = 32 BBM n = 13 22 (2D) (3D)

20 (fig.)

18

16

14

12

Mean = 340 n = 1 10 (2D)

8

6

4

2

0

195 Appendix B: Brushy Basin Member and Buckhorn Conglomerate Sandstone Point Counts

Brushy Basin Member Buckhorn Conglomerate Sample UA-17 ZA-13 OA-17 KB-9 A-14 WB-2

N 500 400 400 500 400 400 monocrystalline Quartz % 59.5 50.3 53.3 45.9 50.8 43.8

Feldspar % K-feldspar 1.6 2.6 2.1 1.5 0.9 1.2 Plagioclase 0.5 0.8 0.5 0.3 0.4 0.3 total 2.1 3.4 2.6 1.8 1.3 1.5

Lithics % chert 36.3 42.5 40.3 49.2 43.3 49.3 pluricrystalline Quartz plutonic 0.3 0.6 0.5 0.7 1.1 1.4 quartzite 1.4 2.1 1.9 2.1 2.3 2.9 tuff 0.4 1.1 0.5 0.3 1.2 1.1 mudstone (intraclast) 0 0 0.9 0 0 0 total 38.4 46.3 44.1 52.3 47.9 54.7

TOTAL % 100 100 100 100 100 100

196 Appendix C: Buckhorn Conglomerate Pebble Counts

Sample F-baseF-top A-base A-top KB-base KB-top PB SB

N 400 400 363 400 400 400 400 400

Chert % black 18.5 8.25 13.87 10.75 12 1.75 14 1.75 dark gray 25 26.5 19.3 18.5 15.75 8.25 24.5 10.25 light gray 26.5 31 21.77 27.5 18.5 18.5 29.25 12.5 brown 4.5 4.75 12.9 6.5 7.5 26 9.5 39 red 0.25 5 1.38 2 0.75 2.25 0.75 1.25 orange 00000000 white 2 1 4.13 6.75 4.25 2 3.75 0 total 76.75 76.5 73.35 72 58.75 58.75 81.75 64.75 Quartzite % gray 12.75 12.5 14.25 15.5 26.5 28.25 4.75 28.5 red 0 1.75 1.9 2 1.75 2.5 0.25 0.5 orange 0.25 1.5 0.3 0.75 2.5 3 0.25 0.25 white 3.5 3.25 3.85 2.25 5 1.75 4 1.5 total 16.5 19 20.5 35.75 35.5 9.25 30.75 Sandstone % light gray 1.5 0.5 1.4 0 1.5 0 1 0

Unknown % 5.25 4 4.95 7.5 4 5.75 8 4.5

Total % 100 100 100 100 100 100 100 100

197 Appendix D: Brushy Basin Member and Buckhorn Conglomerate paleoflow data

198 BRUSHY BASIN MEMBER PALEOFLOW DATA

Sites=23 n=528

2D Sites=24 n=347 3D Sites=26 n=181

n

Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) L R L+R Mean L R L+R Mean

Bx 303 272 288 20 22 ~ 5

J3 84 55 29 76 113 68 85 282 196 45 40 1 27 63 12 19 47 3 315 299 308 352 334 315 21 40 346 52 14 325 51 339 320 340 308 5 199 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) L R L+R Mean L R L+R Mean

344 352 305 331 342 45 81 43 65 132 136 105 111 20 13 102 129 123 24 116 113 104 114 119 91 94 133 112 122 111 137 132 106 100 53 34 61 9 70 70 32.4

O3266 232 269 288 272 NO BC 26 4.1

200 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean

31 290 70 77(to O-7)

EA 2 104 104 10 1 59.6 4 332 70 186 58 30 3 58.1 9 350 166 90 11 93 299 319 105 295 95 15 166 42 143 324 352 54 1 62 15 16 2 18 22 40.6

FA x 346 338 314 326 279 292 311 338 51 66 ~ 50

IA 17 188 191 27 189 108 40 4 48.2 19 339 65 326 26 321 12 26 330 3 31 309 26 43 52 26 342 39

201 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean

10 306 20 19 24 50 349 309 34 22 308 11 324 26 2 28 32 41.1

KA 23 270 270 10 1 39.6 33 126 346 99 2 111 310 96 356 88 322 14 66 325 13 72 350 32 52 10 6 16 17 26

OA 17 341 194 341 325 191 345 210 350 182 356 291 345 144 270 341 12 1 13 13 43.5

UA 7 64 64 01 1 42.3 11 52 52 01 1 41.5 17 139 236 46 324 52 300 79 342 145 272 55 321 101 311 101 22 124 41 120 140 11 7 18 35.4 20 0 102 51 20 2 30.2 23 138 303 60 14 119 346 74 305

202 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean 74 40 65 345 190 19 91 203 308 55 136 341 99 118 335 114 85 335 75 44 290 102 129 26 75 65 37 89 31 105 95 101 105 96 104 110 50 31 31 13 44 20.5 31 166 326 310 148 331 7 55 5 50 4 352 54 979 7.1

WA 3 216 348 120 25 176 24 208 58 191 5 211 9 180 6 195 9 105 350 211 349 4 357 0 29 21 4 12 28 5 5 22 305

203 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean 321 94 88 29 4 33 34.5 7 49 120 30 40 120 21 340 22.4

ZA 3 122 288 289 25 30 3 55.3 13 50 355 120 14 28 81 35 98 77 86 84 22 91 76 30 25 56 34 304 289 66 112 176 164 0 120 89 41 72 41 18 10 28 31 31.7

IB 4 326 71 234 320 64 199 21 232 325 251 10 37 40 7 51 17 53 303 313 16 16 11.1

JB 2 114 25 108 41 107 30 101 56 63 70 71

204 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean 34 22 20 45 52 67 77 017 17 18.5 7 125 306 120 131 249 106 153 305 85 144 65 134 39 42 84 85 13 6.7 9 48 48 01 131 3.2

KB 25 38 205 107 180 100 164 140 120 140 43 7 45.3 31 57 57 10 18 36.4

MB 4 170 170 10 11 14.3

OB 11 71 290 15 265 60 308 320 105 236 351 58 230 300 96 222 306 64 272 44 82 314 18 109 290 86 306 280 332 351 338 22 8 30 30 NO BC

RB 30 121 121 10 1 27.6 36 46 49 334 104 71 35 35 30 326 320 42 58 55 13 16 310 66 171

205 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean 75 244 44 298 40 315 42 328 76 70 234 49 145 279 145 65 45 2 22 12 36 23.3 53 146 104 76 144 53 160 120 156 130 125 184 119 218 81 221 174 217 154 77 16 352 63 29 350 169 156 111 106 917 28 65 5.1

SB 2 136 76 0 72 84 85 125 134 132 122 83

206 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean 87 92 97 13 0 13 13 9.8

UB 4 280 280 355 355 11 22 5.1

WB 13 30 30 10 11 18.7

XB 3 29 60 35 65 102 5 64 346 78 35 292 NO BC 298 (1.7 to 44 cherty 33 106 014 14 14 sand.)

ZB 5 340 340 01 11 16.4

CC 3 26 348 15 45 349 52 16 23 1 15 34 83 64 132 173 154 149 306 295

207 n Distance Total Total top BC S U 2 D 3 D 2 D 3 D Unit Site (m) LRL+RMeanLRL+RMean 291 289 303 312 NO BC 35 (1.4 from 80 cherty 359 16 026 26 26 sand.)

208 BUCKHORN CONGLOMERATE PALEOFLOW DATA

2D Sites=16 n=133 3D Sites=32 n=451

n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top

A1 96 96 10 1 13.9 2 31 31 10 1 12.4 3 219 200 3 0 3 119 119 11 7 158 2 0 2 149 154 7.3 12 139 139 10 1 3 14 (top) 159 164 167 178 167 04 412 0

B ~base 212 299 96 20 22 ~8

Dtop 200 108 166 53 76 166 78 35 85 36 39 89 133 142 94 149

209 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 90 49 30 93 144 51 276 32 238 132 112 148 185 72 113 86 120 93 100 03434 34 0

F2(2)150 150 2.4 top 76 102 89 12 33 0

Gtop 139 82 105 150 129 113 144 75 118 08 88 0

I ~middle 170 170 ~5 top 248 256 281 262 13 44 0

J10118 118 10 1 9.2 13 94 305 104 202 30 3 8.8 14 239 281 260 20 2 6.3 15 243 44 324 20 2 4.9 17 219 219 10 1 3.4 21(top) 156 156 10 110 0

210 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top

K9 292 292 10 11 0.5

M 3(top) 213 213 1 0

Qtop 190 331 102 354 87 40 44 0

Rtop 36 129 90 35 94 83 211 234 96 08 88 0

S5142 142 10 1 10.5 7 352 352 10 1 7.7 9 70 75 73 20 2 7 11 215 215 10 1 1.9 17(top) 186 186 10 16 0

U ~middle 344 344 10 11 ~3

Y2 165 165 10 1 5.6 5 205 205 10 1 4.2 13(top) 89 80 85 02 24 0

EA 15 180 180 10 1 9.2 19 194 176 285 2 2 4 185 164 225 5 21 (top) 164 96 190 U= 179 154 38 175

211 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 151 07 713 0 HA top 50 20 40 51 59 37 43 06 66 0

IA 26 46 60 53 20 22 2.8

IA bis ~top 274 274 10 11 0

JA ~middle 150 150 10 11 ~5

KA 41 146 146 10 11 6.1

LA top 78 149 39 100 105 138 94 176 91 30 126 137 108 138 111 121 108 80 120 61319 19 0

MA top 94 100 63 96 100 61 172 152 165 143 141 114

212 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 141 132 92 121 161 81 111 134 98 129 144 107 122 102 66 133 115 185 143 81 65 132 140 90 117 03636 36 0

OA 25 312 312 10 1 4.7 28 288 288 10 1 2.8 31 100 100 10 13 0.6

QA top 36 15 58 12 44 17 19 22 59 55 65 18 343 90 44 36 01515 15 0

213 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top RA top 295 272 316 318 288 297 314 57 285 269 279 263 243 299 275 329 350 316 310 303 292 340 271 300 331 292 289 290 268 298 02929 29 0

TA top 155 123 120 181 105 91 176 209 105 139 09 99 0

UA 33 120 120 10 1 2.3 35 15 15 01 1 1.9 36 303 115

214 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 218 126 218 211 179 60 6 1.3 37 241 241 10 1 1 39 156 156 01 110 0

VA top 129 199 172 308 300 171 71 82 144 111 122 189 97 120 160 165 121 94 266 31 235 174 135 170 115 167 155 142 66 66 83 135 03131 31 0

WA 12 10 10 10 1 4.2 13 231 231 01 1 3.6 14 60 154 107 02 2 1.7 16 (top) 143 143 01 15 0

215 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top XA top 167 119 129 118 241 180 214 190 168 188 246 245 148 155 135 137 25 227 291 260 271 226 265 268 230 216 240 204 02727 27 0

ZA 23 304 304 10 1 5.7 24 287 287 10 1 5.4 28 340 340 10 1 0.5 29 312 312 10 14 0

BB top 75 84 355 87 104 145 150 131 139 99 37

216 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 231 101 42 39 39 134 130 160 71 63 142 70 215 71 85 99 75 96 02828 28 0

CB top 69 125 139 125 151 162 129 137 124 181 155 147 96 161 117 133 103 156 110 154 134 02020 20 0

EB top 56 101 130 71 45 45

217 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 5 103 99 29 54 35 30 60 57 21 171 48 82 10 46 82 75 64 30 75 135 60 60 47 107 10 57 91 24 29 27 45 57 69 71 21 83 76 85 90 65 190 78 107 25

218 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 60 38 04949 49 0

GB 3 259 40 54 240 25 321 54 41 55 6.1

KB 45 65 65 10 1 10.2 46 107 107 10 1 10 48 50 50 10 1 9.4 49 50 50 10 1 9 50 228 228 10 1 8.8 51 44 44 10 1 8.5 53 30 30 10 6.7 56 21 318 350 66 66 21 39 0

LB top 3 24 22 52 50 27 30 33 30 59 34 28 20 25 37 56 45 80 21 60 58 49 84 345 90 40 02525 25 0

MB 12 91 91 10 1 3.3 13 262 262 10 1 2.6

219 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 14 79 79 10 13 0.5

NB 323 323 1 0.6

PB 16 12 221 192 345 163 160 350 157 172 4 193 190 359 175 196 190 211 149 155 163 169 175 197 165 164 171 164 201 249 170 73 184 138 104 146 158 174 191 212 188 157 154 160 181 183 170 179 36 13 49 49 0

QB top 49 120 65 117 56 69 50 74 119 354 10 1

220 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 30 21 110 306 4 61 143 85 48 5 55 120 51 354 100 38 77 58 47 74 118 102 69 61 03535 35 0

RB 0-1 249 249 10 1 0.8 0-2 246 246 10 1 0.8 0-3 239 239 10 13 0.8

WB 2 198 200 205 202 200 21 3 6.8 3 221 169 189 193 30 36 6.1

YB top 144 86 101 57 82 129 87 124 120 150

221 n 2D 3 D Total Total Distance S U L R Mean (top) Mean 2 D 3 D Unit Site to top 118 108 103 87 112 134 94 89 97 95 121 137 141 117 135 116 140 125 95 112 02929 29 0

ZB 13 156 156 10 11 1.2

222