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Stratal packages at the subsiding margin of the Cretaceous foreland basin, Utah. (Volumes I and H)

Schwans, Peter, Ph.D.

The Ohio State University, 1988

UMI SOON. Zeeb Rd Ann Arbor, MI 48106

STRATAL PACKAGES AT THE SUBSIDING MARGIN OF THE CRETACEOUS FORELAND BASIN, UTAH

VOLUME 1

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By-

Peter Schwans, M.S. (Dipl. Geol.)

The Ohio State University

1988

Dissertation Committee: Approved by

L. A. Krissek

G. D. McKenzie Advisor T. J. Wilson é Department of Geology and Mineralogy Copyright by Peter Schwans 1988 To My Parents ACKNOWLEDGEMENTS

I sincerely thank Dr. James W. Collinson for his guidance throughout the project, my wife Mary for her editorial expertise and patience, Dr. Peter Noel Webb for providing many opportunities, and Dr. Terry J. Wilson for her encouragement and numerous discussions. I am grateful to Drs. Steven R. Jacobson and Sheldon N. Nelson of Chevron Co. Inc, Denver, for the processing and classification of palynomorph samples, and Drs. Kirt Campion, Bill Devlin, and John H. Van Wagoner of EXXON Production Research Co., Houston, for invaluable discussions, much logistic support, and for gaining release of some materials presented herein. I also thank Dr. William A. Cobban of the U.S. Gelogoglcal Survey, Denver, for patiently identifying all invertebrate fossils. I am grateful to the field camp staff and the administration of Snow College at Ephraim, Utah, for room and board. Janice Gist of EXXON Production Research Company, Houston, helped with many diagrams and Beth Daye of Ohio State University, Columbus, processed many of the photos. This research wqs supported by the American Association of Petroleum Geologists, Atlantic Richfield Oil Company, the Chevron Field Studies Fund, Exxon Production Research Company, the Friends of Orton Hall Fund, Marathon Oil Company through Q.S..A. Grants #3491-85 and #3491-86, Sheil Western Exploration and Production Company, and SIGMA XI, The Scientific Research Society. ABSTRACT

The geologic history of the Cretaceous foreland basin in Utah has been interpreted in the past largely on the basis of lithostratigraphic correlations. This study uses sequence-stratigraphic concepts to identify unconformities and depositional sequences in the Cordilleran basin fill in Utah. The resulting sequence-stratigraphic framework is analyzed to test the concept of globally synchronous sea level changes in the Cretaceous foreland basin.

The Early Cretaceous history of the foreland basin in Utah is that of a differentially subsiding alluvial basin. Initial foreland basin subsidence near the thrust load was Airy isostatic in nature, following the thrust-related reactivation of basement structures, lineaments, and subsurface fault zones that originated during Proterozoic rifting. The Cretaceous foreland basin in Utah was narrow and elongate to the thrust front, while foreland areas distal to the thrust front (> 150 km) experienced little or no subsidence. Initial thrusting occurred along the Pavant 1 thrust and Charleston-Nebo thrust during the Barremian and early Aptian and resulted in anticlinal uplift above subsurface thrust ramps. The preservation of thick overbank mudstones with interspersed channel sandstones and numerous lacustrine limestones marks the onset of basin subsidence and erosion of fine-grained Mesozoic strata in the iv emergent source terrenes. Right-lateral faulting along the lineaments aided the structural and erosional dissection of the thrust terrenes, making Proterozoic strata and Paleozoic carbonates available to the fluvial systems during the Albian. As a result, eastward prograding conglomeratic sheet-fiood fan and braided stream systems formed at the western margin of the basin at the apex of the basinward termination of the lineaments, which acted as conduits for elastics from the hinterland.

Thrust sheet stacking, together with thrust sheet buttressing and transverse faulting along the reactivated basement structures during the Late Cretaceous, continued to result in maximum subsidence near the load. In addition, eastward propagation of the Charleston-Nebo allochthon and extensive overthrusting in northern Utah and southern Wyoming in the late Alblan caused broad flexural subsidence across Utah. Although flexural subsidence determined the large-scale configuration of the Late Cretaceous foreland basin, depositional systems architecture was primarily controlled by high frequency basin accommodation changes related to eustasy. Five eustatic highstands and three separate tectonic events are documented in central Utah. Tectonism partially overprinted the eustatic signature in the basin fill by further increasing basin accommodation near the load, while eustasy determined the frequent sediment accommodation changes and depositional geometries. VITA

January 2 ,1 9 5 5 ...... Born - Oldenburg, V/est Germany

1977 ...... B, S. (Vordipiom), Universitaet Muenchen, West Germany

1981 ...... M. S. (Dipiom) in Geology, Universitaet Kiel, West Germany

198 1 ...... Geological Assistant, BEB Erdgas & Erdoel GmbH, Hannover, West Germany

198 2 ...... Exploration Assistant, Uran- gesellshaft Canada Ltd., Toronto, Canada

1983-1988 ...... Graduate Student, Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio

198 6 ...... Geological Consultant, EXXON Production Research Company, Houston, Texas

198 7 ...... Summer Intern, EXXON Production Research Company, Houston, Texas

198 8 ...... Research Geologist, EXXON Production Research Company, Houston RESEARCH PUBLICATIONS

ARTICLE AND ABSTRACTS:

Schwans, p., 1087a, Initial foreland flexure and sedimentation. Early Cretaceous, central Utah [abs.]: Geological Society Abstracts with Programs, v. 19, p. 835.

Schwans, P., 1987b, Response of alluvial-marine deposition to episodic thrust-loading and sea-level changes in the Cretaceous foreland basin, Utah [abs.]: Geological Society Abstracts with Programs, v. 19, p. 835.

Schwans, P., 1987c, Sedimentological effects of the tectonic transition from fold-thrust deformation to thrust-cored uplift, proximal Sevier foreland (Six Mile Canyon-Price River-North Horn interval), Campanian-Paleogene of Utah [abs.]: Geological Society of America Abstracts with Programs, v. 19, p. 332.

Schwans, P., 1986a, Late Cretaceous depositional sequence evolution as a result of tectonism, basin subsidence, and sea level change, Turonian to Santonian, west-central Utah [abs.]: American Association of Petroleum Geologists Bulletin, v. 70, p. 1055.

Schwans, P. 1986b, Early Cretaceous depositional sequence evolution in the foreland of the Sevier overthrust belt in west-central Utah [abs.]: Geological Society of America Abstracts with Programs, v. 18, p. 411. Schwans, P., 1985a, Fanglomerate déposition in the foreland of the Sevier overthrust belt, control, facies, and timing of earliest syntectonic deposition in Utah, western U.S. [abs.]: Proceedings, International Fluvial Sedimentology Conference (IFSC), 3rd, Ft. Collins, Colorado, p. 34.

Schwans, P., 1985b, Depositional controls in the alluvial hinterland of the Cretaceous Interior Seaway: Tectonics, eustasy, and syntectonic sedimentation in central Utah, western U.S.A. [abs.]: Programme and Abstracts, Proceedings of International Symposium on Foreland Basins, Fribourg, Switzerland, p. 116.

Schwans, P ., and Stuecheli, P. J., 1984, Ruvial environments in an ephemeral stream. Pigeon Creek Canyon, central Utah: An example for alluvial fan-head trench deposits [abs.]: Geological Society of America Abstracts with Programs, v. 16, p. 189.

Schwans, P., and Zawiskie, J. .M., 1986, Terrestrial to nearshcr marine facies and trace fossils in the Indianola Group (Sanpete-Funk Valley Interval), proximal Sevier foreland, Turonian-Coniacian of centeral Utah [abs.]: Geological Society of America Abstracts with Programs, V. 18, p. 742.

Schwans, P., 1987, Depositional patterns and controls of major stratal packages at a foreland basin margin, Cretaceous, Utah: EXXON Production Research Company, 29p.

Schwans, P., 1986, Nonmarine to transitional alluvial-marine facies in the foreland of the Sevier Overthrust Belt, Cretaceous Pigeon Creek Fm. and Indianola Grp., west-central Utah: A field guide: The Ohio State University, Columbus, Ohio, 31 p.

Schwans, P. 1982, Geological report of Lone Tree Grid-Gus 8 claim, NTS 75 l-ll, Dubawnt Lake are, N. W. T., Canada: Urangesellschaft Canada Ltd., Toronto, Canada, 8p. TABLE OF CONTENTS

PAGE

DEDICATION ...... il

ACKNOWLEDGEMENTS ...... iii

ABSTRACT ...... iv

VITA ...... Vi

TABLE OF CONTENTS ...... ix

LIST OF TABLES ...... xiv

LIST OF FIGURES ...... xv

PREFACE ...... 1

INTRODUCTION ...... 2

OBJECTIVES ...... 2

EXPECTED SIGNIFICANCE OF PROJECT ...... 2

THE BASIN-ANALYSIS ASPECT ...... 2

SEQUENCE-STRATIGRAPHY APPUED TO FORELAND BASINS 7

GENERAL BACKGROUND ...... 10

Cretaceous plate-margin of North America - ...... 10

Ix Hinterland and fold-thrust belt relations ...... 12 Foreland basin evolution ...... 15 Foreland basin fill ...... 17

METHODS ...... 18

Seismic correlation ...... 18 Well-log correlation ...... 19 Imagery and outcrop correlation ...... 21

ONLAP MODEL AND CRITIQUE ...... 26

CHAPTER Î. EARLY CRETACEOUS SEQUENCE STRATIGRAPHY, LITHOFACIES, AND SUBSIDENCE MODEL ...... 30

SECTION 1.1 INTRODUCTION ...... 31

SECTION 1.2 DEPOSITIONAL RESPONSE OF PIGEON CREEK FORMATION, UTAH. TO INITIAL FOLD-THRUST BELT DEFORMATION IN A DIFFERENTIALLY SUBSIDING FORELAND BASIN (Schwans) ...... 33

Abstract ...... 33 Introduction and purpose ...... 35 Lower Cretaceous stratigraphy: Pigeon Creek Fm...... 38 Problem and history ...... 38 Definition and description ...... 43 Name and stratotype ...... 43 Composition and provenance ...... 51 Geographic distribution, dimensions ...... 53 Boundaries and age ...... 54 Boundaries ...... • 54 Age ...... 59 Regional relationships of Lower Cretaceous strata ...... 61 Bounding unconformities ...... 61 Strata forming the depositional sequence ...... 64 Pigeon Creek depositional architecture ...... 70 Lithofacies and facies assemblages ...... 70 Sheet conglomerate facies ...... 70 Description ...... 70 Interpretation ...... 76 Scour-base conglomerate and sheet sandstone facies ...... 79 Description ...... 79 Interpretation i ...... 83 Limestone to mudstone to sandstone facies . . . 85 Description ...... 85 Interpretation ...... 89 Foreland basin deformation and depositional response ... 92 The model ...... 92 Application to the Pigeon Creek Formation ...... 93 Conclusions ...... 96 Ackowledgements ...... 100

CHAPTER II. EARLY CRETACEOUS FORELAND BASIN SUBSIDENCE PATTERNS AND BASIN FILL ARCHITECTURE ...... 101

SECTION 11.1 INTRODUCTION ...... 102

SECTION 11.2 INITIAL FORELAND BASIN SUBSIDENCE AND SEQUENCE-STRATIGRAPHIC RESPONSE. BARREMIAN TO CENOMANIAN, UTAH (Schwans) ...... 105 Introduction ...... 105 Cordilleran belt and underlying basem ent • • 107 History 107 The tectonic elements ...... 110 Basement arches ...... 110 Cordilleran structures ...... 112 Lineaments and transverse faults ...... 114 Fold-thrust belt and basem ent interaction ...... 116 Thrust belt and foreland deformation ...... 117 Thrust succession ...... 117 Timing of thrusting ...... 121 Sequence stratigraphy of basin fill ...... 124 Age of sequence strata ...... 124 Hiatus of sequence boundaries ...... 129 implications for basin subsidence ...... 131 Basin fill architecture ...... 132 Previous work • • • 132 Proximal zone lithofacies ...... 134 Sheet flood-fan conglomerates . • ...... 134 Small-scale channel to sheet ...... sandstone ...... 137 Channel sandstone clusters ...... 137 Lacustrine deposits ...... 138 Overbank and floodplain strata ...... 139 Basin fill architecture in the proximal zone ...... 140 Distal zone lithofacies ...... 147 Braided stream and valley fill ...... 147 Low-sinuousity channel sandstones ...... 152 Overbank and floodplain strata ...... 153 Distal zone architecture ...... 154 Three-dimensional configuraUon of the basin ...... 159 Conclusions on foreland basin subsidence patterns ...... 166 CHAPTER ill. SEQUENCE STRATIGRAPHIC REVISION OF THE UPPER CRETATECOUS INDIANOLA GROUP, INDIANOLA GROUP UNDIFFERENTIATED, AND ASSOCIATED FORMATIONS ...... 169

SECTION III.1 INTRODUCTION ...... 170

SECTION III.2 SEQUENCE-STRATIGRAPHIC ANALYSIS OF AN UPPER CRETACEOUS FOLD-THRUST BELT AND FORELAND BASIN MARGIN SEQUENCE, INDIANOLA GROUP, UTAH (Peter Schwans) ...... 172

Introduction and purpose ...... 172 Current nomenclature ...... 176 Indianola Group ...... 176 Indianola Group (undifferentiated) ...... 184 Previous sequence-stratigraphic revision ...... 187 Proposed Late Cretaceous sequence-stratigraphy ...... 189 Lower unconformity: late Albian-early Turonian ...... 189 Upper unconformities: middle Campanian to early Paleocene ...... 197 Western Wasatch Plateau ...... 197 Gunnison Plateau and Cedar Hills • ...... 202 Definition of Indianola supersequence ...... 207 Subdividing the Indianola Group supersequence • • ...... 209 Middle to late Turonian unconformity (U3) ...... 209 Late? middle-early? late Conlacian unconformity (US) ...... 217 Latest? Santonlan-earliest Campanian and early Campanian unconformities (U6-8) 220 Discussion and conclusions ...... 225 LIST OF TABLES

TABLE PAGE

1. Summary chart of biostratlgraphic data-Western Wasatch Plateau and Gunnison Plateau. Table shows ranges (heavy black lines) of all fossil genera that have been reported for the Cretaceous formations in the area. Genera are listed by location, name, and by reference (e.g. were they were first reported) ...... 126

2. Summary chart of biostratigraphic data-Easiern Wasatch Plateau and Book Cliff area. Table shows ranges (heavy black lines) of all fossil genera that have been reported for the Cretaceous formations in the area. Genera are listed by location, name, and by reference (e.g. were they were first reported) ...... 127 LIST OF FIGURES

FIGURE PAGE

Figure 1 Map showing extent of Late Cretaceous (post-Turonian and pre-Campanian) Interior Seaway. Arrow points toward study area in inset box. Seaway (stippled) is outlined with solid line were certain and dashed were uncertain. After Steick (1975) and Kauffman (1977a, 1977b) ...... 5

Figure 2 Research results and summary , 6

Figure 3 History (a-d) of plate interaction showing position of subduction zone (thin line, barbs pointing east), magmatic arc (dashed pattern), frontal thrust (solid line with barbs), inferred position of subducted slab of Farallon plate (stippled), and area occupied by Laramide deformation (hachured). After Cross (1986, his figure 7) ••■■■•••••■••■itiessssssssccsssssE 13

Figure 4 Map of study area in central Utah showing plateau areas, aerial photo coverage (box pattern), and southern cross- section (wells 1 through 13) and northern cross-section (wells 1 through 27) (see appendix C for well data) . 20

Figure 5 Satellite photo mosaic of study area with section locations (dots) and names indicated ...... 22

Figure 6 Sequence-stratigraphic representation of the main depositional systems tracts and their relations to the various segments of the eustasy curve. (A) The systems tract related to depth. (B) Chronostratigraphic relations of systems tracts. From Haq, Hardenbol, and Vail (1987, their figure 1) ...... XV Figure 7 Map showing the Sevier (Cordilleran) fold-thrust belt and Late Cretaceous foreland basin area. Inset rectan gle is area covered by Figure 2. Based on McGookey (1972) 36

Figure 8 Generalized geologic map of study area. Pigeon Creek section locations: 1, type locality in Pigeon Creek Canyon; 2 and 3, southeastern Gunnison Plateau; 4, Salina Canyon. CRT, Canyon Range thrust; ETF, Eastern thrust fault limit during Albian and frontal zone of Pavant 1 thrust projected to surface; LSG, Little Salt Creek; PRT, Pavant Range thrust (Pavant 2 thrust); SMC, Six Mile Canyon; Th, Thistle. After Hintze (1980) and Villien and Kligfield (1986) 39

Figure 9 Stratigraphie chart for central Utah with correlations by various authors and revised stratigraphie correla tion of Lower Cretaceous strata as proposed by this study ...... 40

Figure 10 Type locality of Pigeon Creek Formation in Pigeon Creek Canyon, 6 km east of the town of Levan (section 1 in Fig. 8). Lower member crops out in foreground below white line and upper member above white line. View is to the east ...... 45

Figure 11 Pigeon Creek stratotype (Sec. 25, T.14 S., R.1 E.) (sec tion 1 in Fig. 8). Line connecting crosses (carbonate clast sizes) and full circles (quartzite clast sizes) shows clast size trend. Note the upward-coarsening trend in the average maximum clast sizes and the reverse stratigraphie order of the clast lithologies .. 46

Figure 12 Pigeon Creek section in the southeastern Gunnison Plateau (Sec. 32, T.18 S., R. 2 E.) (section 2 in Fig. 8). Boulder conglomerate at top is assigned to Upper Cretaceous Indianola Group and overlies Pigeon Creek strata disconformably ...... 48 Figure 13 Pigeon Creek section in the southeastern Gunnison Plateau (Sec. 7, T. 19 S., R. 2 E.) (section 3 in Fig. 8). Note Protelliptio in the lower member and angiosperm leaves in upper member. Palynomorphs of Weiss and Roche (in press) were recovered from the bivalve -bearing limestone ...... 49

Figure 14 Measured Pigeon Creek section in Salina Canyon (Sec. 33, T. 21 E., R. 1 E.) (section 4 in Fig. 8). Wiiiis (1986) reported late Albian fission-track ages aquired from zircons forsamples from upper mudstone interbeds • 50

Figure 15 Map showing isopachs (in meters), triangular plots of clast lithologies (normalized to 100 percent chert/sandstone, carbonate, and quartzite), average maximum particle sizes (MPS), and paleodispersal directions for Pigeon Greek Formation. Filled circles, MPS of upper member elastics; open circles, MPS of lower member elastics. Small arrow, mean orientation of trough cross-strata in lower member sandstones determined from 5-15 measurements of trough limbs per station; medium-sized arrow (lower member) and large arrow (upper member) represent paleocurrent directions obtained from clast imbrication (long axis) and orientation of gravel-furrows. Numbers indicate section locations shown in Figure 8; ELT, eastern limit of thrusting for Pavant 1 thrust ...... 52

Figure 16 Sequence-stratigraphic correlation chart of Lower Cret aceous of Utah with Pigeon Creek-Cedar Mountain deposi tional sequence and selected age data. After McGookey (1972), Fouch et al. (1983), and Tschudy et al. (1984) 56

Figure 17 Sequence-stratigraphic correlation chart for Lower Cretaceous of Utah and adjacent areas. Left side of diagram shows west to east relationships and right side north to south relationships of various formations. Compiled from McGookey (1972), Wiltschko and Dorr (1983), Fouch et al. (1983) 62 Kvii Figure 18 Neocomian to early Aptian paleogeography of Utah and adjacent areas. Paleotopographic highs are stippled. Adapted from Young (1960,1970), McGookey (1972), Wiltschko and Dorr (1983), Weimer (1984), DeCelles (1986), and Peterson (1986) ...... 63

Figure 19 Late Aptian and Albian paleogeography. Note upper Pigeon Greek conglomerates adjacent to emergent thrust terrane. Seaway is developing southward (dash pattern) Adapted from McGookey (1972), Wiltschko and Dorr (1983), Vuke (1984), Peterson (1986) 65

Figure 20 Schematic representation of Early Cretaceous paleo dispersal patterns and modal pebble/clast lithologies. Location 1, paleodispersal of lower (shorter arrow) and upper Pigeon Greek member (longer arrow); location 2, paleodispersal for Buckhcrn Conglomerate exposure (Vingling, 1987); location 3, regional paleodrainage direction for the Cedar Mountain Formation (Young, 1960,1970). Small arrows, paleovectors from sand stone channel bodies of upper Cedar Mountain Formation (Harris, 1980; Yingling, 1987). BC, Burro Canyon Formation; CM, Cedar Mountain Formation. Clast litholo gies: LCM, Lower Cedar Mountain; LPC, Lower Pigeon Creek; PC, Pigeon Creek Formation; UCM, Upper Cedar Mountain; UPC, Upper Pigeon Creek ...... 68

Figure 21 Sheet conglomerate facies in Little Salt Creek Canyon (LSC in Fig. 8) in the upper member with 2 m- to 5 m-thick sheets (between solid lines). Scour-based conglomerate channel-fills occur on top of sheets (channel bases dashed). View is to the south ...... 72 Figure 22 Pigeon Creek depositional sequence (section 2 in Fig. 8) (overturned to right). Lower member mudstones overlie Jurassic (J) strata along basal disconformity (solid line to left). Overlying upper member (right of dashed line) is truncated in turn by Tertiary (I) strata along angular unconformity (solid line to right). View is to the south ...... «...... 73

Figure 23 Exhumed base of upright-standing conglomerate sheet in the upper Pigeon Creek member (between section 2 and 3 in Fig. 8). View is to the south. Note arrow at right pointing to scale next to hammer (45 cm long) .. 74

Figure 24 Close-up of conglomerate sheet base shown in Figure 23. View is to the west. Scale Is 15 cm . • • ...... 75

Figure 25 Upward-fining conglomerate to mudstone sequence (between solid lines) (overturned to right). Sequence is 2 m to 4 m thick and comprises a basal conglomerate sheet (CS) overlain by massive mudstones (F). Sp, channel-form pebbly sandstones; Shr; horizontally bedded and rippled wings of sheet sandstone. Note scale of 1 m. Just north of section 2 in Figure 8. View is to the south ...... 80

Figure 26 Exhumed base of channel-form sandstone with pedogenic mottling around burrows of Skolithos sp. and Muensteria sp. Exposure is near section 2 in Figure 8. View is to the west ...... 82

Figure 27 Plugged calcrete horizon in lower Pigeon Creek mudstones (south of section 3 in Figure 2) with distinct macrofabric, intersecting cleavage planes with slickensided surfaces (small arrows) form wedge-shaped pads. Surfaces converge in a down ward direction (large arrow) with respect to former paleosurface. View is to the northeast ...... 88 Figure 28 A, Pigeon Creek depositional model for Early Cretaceous alluvial foreland basin areas proximal to fold-thrust belt. Laterally overlapping wet/dry fan bodies form a low-relief bajada adjacent to the thrust front. Basinal flood plains are dominated by overbank fines and playa lakes. B, Idealized Pigeon Creek depositional sequence with major lateral and vertical facies associations and environments ...... 95

Figure 29 Structural map with elements of the hingeline and the Mesozoic Cordilleran belt.1, basem ent highs; 2, Ephraim subsurface fault; 3, basem ent lineaments including: CFL = Cove Fort lineament, LL - Leamington lineament, SL = Scipio lineament, PL = Paragonah lineament; 4, Wasatch fault at Salt Lake City (SLC); 5, Mesozoic and Cenozoic thrust plates: C = Canyon Range, CN = Charlston-Nebo, G = Gunnison, F1 = Pavant 1, P2 = Pavant 2, S = Sheep- rock, TV = Tintic Valley, W = Wasatch. Modified after Morris (1983), Picha and Gibson (1985), and Villien and Kligfield (1986) 110

Figure 30 Map of study area with the major structural elements of the Cordilleran hinge zone shown in Figure 29. X-Y, structural cross-section of Figure 31; 1, basem ent lineaments (see Figure 29); 2, basem ent highs: FA - Fillmore Arch, SA » Juab-Sevier Anticline, SPA » Sanpete Anticline. Compiled from Sharp (1982), Morris (1983), Picha and Gibson (1985), Allmendinger et al. (1986), and Villien and Kligfield (1986) 112

Figure 31 Generalized structural and stratigraphie cross-section along X-Y (Fig. 30). Note that the Cretaceous section remains essentially unbroken above Cenozoic thrusts. Based on well and unpublished seimic- data, as well as Standlee (1982,1983), Royse (1983), Lawton (1985), Villien and Kligfield (1986) 117 Figure 32 Paleogeographic reconstruction of Pavant 1 thrust front (dashed line with sawteeth) and initial foreland basin (stippled). Note the relationship between location of basem ent highs, lineaments, and outline of thrust front. 1, Placid Oil WXC State; 2. Placid Oil WXC USA 1-2; A-B, C-D are cross-section shown in Fig. 48; black dots depict locations of measured Pigeon Creek sections . 120

Figure 33 Mosaic of high-altitude aerial photos showing Pigeon Creek rocks at stratotype (between arrows and dashed lines) in Pigeon Creek and Chicken Creek Canyons and easternmost extent of the Scipio lineament (complexly broked part of the plateau in the right half of photomosaic). View is to east (top) with outcrop facing west and strata dipping toward east. Indianola Group strata are above and Jurassic Arapien Shale below lower dashed line, respectively ...... 123

Figure 34 Sequence-stratigraphic time-space diagram and stratigraphie correlation chart for Lower Cretac eous foreland basin fill in Utah and western Colorado. Sequence with: BC, Burro Canyon Fm.; CM, Cedar Mountain Fm.; PC, Pigeon Creek Fm. Cretaceous strata above sequence: Kav, Allen Valley Sh.; Kd, Dakota Sst.; Ksp, Sanpete Fm.; Kt, Tununk Sh. Jurassic strata below sequence: Ja [undiff.], Arapien Sh. undifferentiated; Jm, Morrison Fm ...... 124

Figure 35 Depositional architecture of basin fill in the proximal zone. Thicknesses are based on measured sections (black dots), published geologic maps, and well data • 136 Figure 36 Pigeon Creek depositional sequence in enhanced section of high-altitude aerial photo of Pigeon Greek outcrops (between arrows, solid lines) in southern Gunnison Plateau area (central part of the basin). Beds are vertic al to overturned toward the west (up) and strike north- south (right to left). Tertiary strata are flat-lying and drape over the Cretaceous in upper half of photo. Juras sic rocks are visible in the lower half of the photo .... 139

Figure 37 Barremian to late Aptian paleogeography of proximal zone situated between the incipient thrust front (heavy dashes) and the Ephraim subsurface fault (thin long dashes with dots on downthrown side). Blank = flood basin areas. Dash pattern = lacustrine depressions in central part of basin. Dot pattern = sand sheet fans at thrust front and eastern terminus of lineaments (thin dashed lines). Elongate dot-patiem = sandy braided streams. Large arrows = main dispersal directions; small arrows = individual paleoflow vectors ...... 141

Figure 38 Late Aptian to late Albian paleogeography of proximal zone. Blank = overbank areas. Conglomerate pattern = coarse sheet flood fans at thrust front and lineament intersections. Dashed patterns = lacustrine depression. Dot pattern = fluvial sands along fan margins. Heavy dot pattern = braided streams. Large arrows - main dis persal directions; small arrows - individual paleoflow vectors ...... 143

Figure 39 Alluvial architecture of Pigeon Creek (KPC) depositio nal sequence (between wavy lines - unconformities) in central parts of the proximal zone (based on image in Fig. 36). I, lacustrine to fluvial sequence; ll-SV, braided stream to alluvial fan sequences. Sequences are sepa rated by bounding surfaces (heavy lines, dashed were uncertain), which are onlapped laterally by laterally continuous conglomerate to sandstone units (thin lines, dashed were uncertain) and intervening mudstones • • 146 Figure 40 Cedar Mountain depositional sequence (between arrows) in the distal zone. Ghannel-form pebble conglomerates (above black lines) and mudstones constitute the fill of a paleovalley in Jurassic Morrison Formation (below white line) ...... 148

Figure 41 Depositional architecture of Cedar Mountain paleovalley-fill sequence of Figure 40. Lower Cedar Mountain (Kcm, I) strata onlap the underlying Jurassic Morrison Formation (Jm) along an unconformity (hiatus from 136 Ma to 112 Ma). Upper Cedar Mountain (Kcm, u) mud stones are truncated along an unconformity (hiatus from 100 Mato 92 Ma) by nearshore marine Dakota Sandstone (Kd) ...... 149

Figure 42 Isopach map (in meters) showing total sediment thick ness distribution of Cedar Mountain-Burro Canyon Formations in southeastern Utah and southwestern Colorado. Zero line separates the basin from southern and western paleotopographic highs. Dashed line depicts boundary between Cedar Mountain (to left) and Burro Canyon (to right) Formations. GR, Green River. Compiled from Young (1960,1973a, 1973b), Munger (1965), Craig (1981), and Yingling (1987) 151

Figure 43 Generalized cross-section E-F in Figure 42 of Lower Cretaceous depositional sequence in southeastern Utah showing alluvial architecture. Kbc, Burro Canyon Forma tion; Kd, Dakota Sandstone; Jm, Morrison Formation; 1, incised paleovalley conglomerates; 2, laterally coalesced braided stream elastics, 3, fluvial to shoreface sandstones; 4, fluvial channel sandstones; 5, marine mudstones. Modified after Young (1960) .... 152 Figure 44 Generalized paleogeographic reconstruction of Barre mian to late Aptian depositional systems in the distal zone. Sediment accummulation (dotted pattern) and paleoflow directions (arrows) suggest sand dispersal along a south to north axis during lower Burro Canyon (LBC) deposition, while coeval lower Cedar Mountain (LCM) elastics were dispersed through seperate drainages to the northeast. GR, Green River. Compiled from Young (1960), Craig (1981), Yingling (1987), and Schwans (in press) ...... 155

Figure 45 Generalized paleogeographic reconstruction of late Aptian to late Albian depositional systems in the distal zone. Sediment accummulation (dotted pattern) and paleoflow directions (arrows) suggest dispersal to the north during upper Burro Canyon (UBC) deposition, w hereas coeval upper Cedar Mountain (UCM) elastics were transported by low-sinuous river (elongate dotted pattern) to the east and northeast. Coarser braided streams (heavy dot pattern) were active farther to the northwest, deriving their sediments from the fold-thrust belt. Compiled from Young (1960), Harris (1980), Craig (1981), Yingling (1987), and Schwans (in press) ...... 157

Figure 46 (A)-(B): Regional sediment thickness (isopachs In hundreds of meters) in the Cretaceous foreland basin in Utah. A, Barremian to late Aptian with restricted basin (3 to 1 sopach line) between thrust front (dashed line with sawteeth) and nonsubsiding craton (0 to 1 isopach line). B, late Aptian to late Albian basin (6 to 1 isopach). Dashed lines separate Pigeon Creek Fm. (lower = LPC; upper = UPC) from Cedar Mountain (lower = LCM; upper - UCM) and Burro Canyon (lower = LBC; upper » UBC) strata. Compiled from Young (1960), Craig (1981), Schwans (in press), and unpublished well data (wells are listed in appendix C) ...... 159 Figure 47 Total sediment thickness map (isopachs in meters) of the Barremian to late Albian Pigeon Creek Formation in the first-order basin. Black dots = location of m ea sured section; (for well listing see Figure 4 and appendice) ...... 162

Figure 48 Generalized cross-sections A-B and C-D (Figure 32) of the proximal basin fill. EF, Ephraim subsurface fault; PC, Pigeon Creek; 1, sheet-flood fans; 2, scour-based conglomerate; 3, channel-form sandstones; 4, lacus trine limestones; 5, sheet sandstones; 6 , cristalline basem ent rocks ...... 164

Figure 49 (A)-(B): Subsidence pattern maps. (A) Airy isostatic subsidence during the Barremian to late Aptian; (B) unrestricted flexural subsidence of the foreland basin in the Albian. Ephraim subsurface fault, dashed line with dots on downthrown side; dashed line with saw teeth, incipient fold-thrust belt; continuous line with sawteeth, emergent fold-thrust belt; heavy dot pattern, subsiding basin with sediment thickness >200 m (wells in appendix C) ...... 166

Figure 50 Summary of previous lithostratigraphic correlation schem es of Spieker (1946,1949a), Hunt (1950), Hays (1960), Thomas (1960), Fouch et al. (1982,1983), and Lawton (1984,1986). Unconformities are indicated with wavy lines, associated hiatuses in vertical line pattern ...... 174 Figure 51 Sequence stratigraphy of the Cretaceous of central Utah. Schematic time-space chart of upper parts of Arapien Shale (undifferentiated) (refers to Arapien Shale and Jurassic part of Twist Gulch Formation), Pigeon Creek Formation, Indianola Group, and North Horn Formation. First four columns show Indianola Group (undifferentiated) outcrop areas from west (left) to east (right). Next two columns show stratal units in the Wasatch Plateau. Unconformities are num bered (1 through 10). Tertiary unconformity (U11) between North Horn Formation and Flagstaff Limestone not shown. Extent of associated hiatuses by the verti cal line pattern, while lithofacies shifts between major environments and the main tectonic events are shown on right side of diagram ......

Figure 52 Map of measured sections and geographic areas intro duced in chapter. Piateaus are in dot pattern and inter vening valleys are blank. Sanpete Valley is located between the Wasatch and the Gunnison Plateaus, whereas the Juab-Sevier Valleys are to the west of the Gunnison Plateau ......

Figure 53 Measured section of Indianola Group stratotype in Six Mile Canyon, western Wasatch Plateau (Sec. 35, T. 18 S., R 2 E.) (see Figure 52 for location). Also shown are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols and abbreviations) ...... 179

Figure 54 Measured section of Pigeon Creek Formation and parts of Indianola Group in Salina Canyon (Sec. 33, T. 21 E., R. 1 E.). Also shown are fossils (appendix B), uncon formities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols and abbreviations) ...... 181 Figure 55 Measured section of upper parts of Pigeon Creek Formation, parts of Indianola Group, Unnamed Unit, and North Horn Formation in Lake Fork (SE 1/4 Sec. 34, T. 8 S., R. 4 E.). Shown also are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols an abbreviations) ...... 182

Figure 56 Photo of Pigeon Creek Formation (Kpc) overlying the Jurassic Arapien Shale (undifferentiated) (Ja) along U1 in Chicken Creek Canyon. Lower Cretaceous strata (Kpc) are overlain by rocks equivalent to the Allen valley Shale (Kav) along U2. View Is to the south with strata dipping to the east ...... • • • • 188

Figure 57 Photo of Pigeon Creek Formation (Kpc) truncated by U2 and overlain by basal channel-form sandstones of the Sanpete Formation (Ksp) in Salina Canyon. The Creta ceous strata are dipping to the east (right) and are unconformably (U3-11) overlain by Tertiary (T) rocks equivalent to the Flagstaff Limestone and Colton Formation. View is to the north ...... 190

Figure 58 Photo of Pigeon Creek conglomerates (Kpc) truncated by U2 and overlain by pebbly to cobbly channel-form conglomerates equivalent to the Alien Valley Shale (Kav). View is to the northwest with rocks dipping to the east ...... 191 Figure 59 Measured section (Sec. 12, T. 15 S., R 1 E.) of the here subdivided Indianola Group (undifferentiated), the first Unnamed Unit (Reddicks Canyon Conglomerate), the South Flat Formation, the Price River equivalent (Chris Canyon Conglomerate), second Unnamed Unit (Rees Valley Conglomerate), North Horn Formation, and the Flagstaff Limestone in Chicken Creek Canyon, north ern Gunnison Plateau (see Figure 52 for location). Also shown are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix D for symbols and abbre viations) ...... 193

Figure 60 Photomosaic of high-altitude aerial photos of the northern Gunnison Plateau and Cedar Hills. Indicated with black arrows are the top of the Pigeon Creek Formation in Chicken Creek Canyon (upper left), the base of the Funk Valley Formation and the Allen Valley Shale in the Cedar Hills, the Six Mile Canyon Formation, as well as the incised valley-fill unit and the angular unconformity (U9). The formations strike north to south (right-left) in the Gunnison Plateau and north east to southwest (lower right to upper left) in the Cedar Hills and are continuous throughout the area. View is to the west with strata vertical in the Cedar Hills and dipping to the east (bottom) in the Gunnison Plateau ...... 195

Figure 61 Photo shows truncation of the Allen Valley Shale (Kav) by US and fluvial conglomerates (to right) of the Lower Funk Valley Sandstone (Kfs, Iss) at Lake Fork (see Figure 52 for location). Marine mudstones and interbedded shoreface sandstones (below dashed lines) of the Lower Funk Valley Formation (Kfv, Ish) overlie the fluvial unit along a flooding surface (fs). North Horn deposits trunc ate east-dipping Indianola strata along U8-10. View is to the northeast ...... 196 Figure 62 Photo of U9 through U11 in Six Mile Canyon. Price River (Kpr) strata in lower left dip eastward and are onlapped by unnamed pebble sandstones (Unnamed = Sterling Sandstone) along an angular U9. Tertiary North Horn (KTnh and Flagstaff Limestone (Tf) onlap U10 and U11, respectively. View is to the south with strata dipping to east (below U10) and west (above U10) , , , 198

Figure 63 Measured section of Six Mile Canyon Formation, Price River Formation, the second Unnamed Unit, the North Horn Formation, and the Flagstaff Limestone in upper reaches of Six Mile Canyon (Sec. 3, T 18 S, R. 2 E.) (see Figure 52 for location). Also shown are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols and abbreviations) ...... 199

Figure 64 Photo of Price River conglomerates (Kpr = Chris Canyon Conglomerates) in Chicken Creek Canyon, which are onlapped by conglomerates of the second Unnamed Unit (Unnamed = Reese Valley Conglomerates) along the angular unconformity U9. Reese Valley Conglomerates are truncated by U10, which in turn is onlapped by Horth Horn (KTnh) mudstones. Note the pinch-out of the units over the short distance (< 200 m). View is to the north with strata below U9 dipping to the east, while those above are horizontal ...... 203

Figure 65 Measured section at Wales Gap (Sec. 2, T. 14 S., R 2 E.) with the Pigeon Creek Formation, strata equivalent to the Six Mile Canyon Formation(?), the second Unnamed Unit (e g., Reese Valley Conglomerates), and the North Horn Formation (see Figure 52 for location). Also shown are fossils, pollen, unconformities (wavy lines), major depositional systems, formations, and ages ...... 205

xxix Figure 66 Photo of lower parts of the Wales Gap section with Six Mile Canyon equivalent(?) (Ksx) truncated by Unnamed Unit (e.g., Reese Valley Conglomerates) along U8-9. View is to the south with rocks below U8-9 overturned to the east (left), whereas those above U8-9 are vertical to west-dipping ...... 206

Figure 67 Photo demonstrating the rapid pinch-out of various units (between U8-9 and U10 and U10 and U11) toward the west (left) at the western margin of the Gunnison Plateau in Mellor Canyon (see Figure 52 for location). View is to the north ...... 210

Figure 68 Photo of Sanpete Formation (Ksp) overlain by Allen Valley Shale (Kav) along flooding surface (fs) at the type locality at Six Mile Canyon. U3 truncates the Allen Valley Shale and is onlapped (dashed lines) by sand stones of the Lower Funk Valley Sandstone (Kfv, Iss) within a broadly incised valley, dis, downlap surface. View is to the east with strata vertical or steeply dipping toward the east ...... 211

Figure 69 Enhanced part of aerial photo showing the Indianola Group type section at Six Mile Canyon. The Sanpete Formation unconformably overlies (basal solid line) the Pigeon Creek Formation (bottom) and is in turn truncated by another unconformity (next white line), against which Lower Funk Valley sandstones lap out laterally (arrows). Tertiary Flagstaff Limestone is flat-lying and drapes over the vertical to steeply east-dipping Cretaceous section ...... 211 Figure 70 Photo with Upper (Kfv, umbr.) and Lower Funk Valley Sandstone/Shale member (Kfv, Imbr.) in Chicken Creek Canyon where they unconformably overlie rocks equival ent to the Allen Valley Shale (Kav) (see Figure 52 for location). The members comprise a basal conglomerate (cgl), a middle sandstone (sst), and an upper detrital carbonate (detrital carb.) part, and are bounded by U3. US, and U6 (not shown) View is to the southwest with strata dipping east (left) ...... 215

Figure 71 Photo of the Funk Valley type section at Six Mile Canyon. Lower Funk Valley Sandstone (Kfv, Iss) is overlain by Lower Funk Valley Shale (Kfv, Ish) along a flooding surface (fs), while Upper Funk Valley sandstones (Kfv, uss) overlie the unconformity US. U4 is found in the subsurface and in outcrops in the Book Cliffs. It is expressed here by its correlative conformity (ee) (see chapter 4). view is to the south with strata vertical or dipping steeply to the east ...... 218

Figure 72 Photo of the Upper Funk Valley Shale (Kfv, ush) in Six Mile Canyon. Marine mudstones overlie the Upper Funk Valley Sandstone (in foreground) along a flooding surface (fs) and are truncated by U 6 , against which fluvial- deltaic channel-forms of the Six Mile Canyon Formation (Ksx) lap out laterally. View is to the east with strata dipping between 90 to 4S degrees eastward • • 221

Figure 73 Photo of the quartzite-boulder conglomerate and incised valley fill (above U7) in Chicken Creek Canyon (here informally called Reddicks Canyon Conglomerate), which is dipping at 3S degrees to the east. Reese Valley Conglomerate (between US and U10) and Flag staff Limestone (above U 11) are visible in the back ground 223 PREFACE

The goal of the dissertation is to present an analysis of the history of of the Cretaceous foreland basin in Utah. The dissertation consists of a general introduction, four chapters, five appendices, and a list of references. The first chapter was accepted for publication by The Geological Society of America in January, 1988. The second chapter was written in article format in late 1987 and is at the moment being reworked by Bill Devlin, EXXON Production Research Company, and myself, to be submitted for publication at the end of 1988. To retain a certain degree of consistancy in format, chapters 3 and 4 were organized according to the format of chapter 1 and 2. As a result, some repetition of introductory information, however, could not be avoided.

Chapters 1 and 2 describe the Early Cretaceous and chapters III and IV the Late Cretaceous. The first section of each chapter consists of an introduction, which provides the link to the preceding chapter. The second section of each chapter consists of a specific introduction of the paper and the paper itself. Separate acknowledgements can be found in chapters 1 and 2 and are intended to highlight specific contributions by organizations and Individuals made at certain stages of the project. To provide a common approach to the concepts of sequence stratigraphy, important definitions are arranged in form of a glossary in appendix A. 1 INTRODUCTION

OBJECTIVES (1) To identify regional unconformities and subdivide the Cordilleran foreland basin fill in Utah into depositional sequences or units of chronostratigraphic significance. (2) To interpret the depositional sequences with respect to the stacking pattern of component depositional systems and systems tracts. (3) To establish a sequence-stratigraphic framework for the basin strata deposited at the subsiding margin of the Cordilleran foreland basin in Utah. (4) To document the spatial and temporal changes that occurred in the sediment availability and accommodation potential of the Cordilleran foreland basin during the Cretaceous.

EXPECTED SIGNIFICANCE OF PROJECT

JIdE BASINiANALYSI&ASEECI The study of fluvial to marine facies, sediment thicknesses, stratigraphies, dispersal directions, paleogeographies, and structure at a basin-wide scale, contributes much to the understanding of basin evolution. Two elements contribute to such an analysis: an understanding of depositional systems and an appreciation of stratigraphie 2 3 architecture. This study applies the concepts of sequence stratigraphy coupled with the Law of Superposition to answer a number of fundamental questions concerning the behavior of and the controlling factors on depositional systems in a foreland basin. In that context, the basin analysis documents the changes in sediment availability and basin accommodation that occurred in the Cordilleran foreland basin in Utah during the Cretaceous.

The study focuses on the sequence-stratigraphy and depositional architecture of sedimentary rocks at the western subsiding foreland basin margin. Diverse subsurface, outcrop, paléontologie, sedimentologic, and structural data, together with remote sensing, well-log, and unpublished seismic data, are integrated in the sequence-stratigraphic model. Emphasis is placed on delineating the major allocyclic depositional controls, such as tectonism, eustatic change, and climatic influence, and documenting the dynamic interaction of these factors with the fluvial to transitional marine depositional systems within a chronostratiographic framework. On that basis, the shifts in depositional systems, the character of the facies associations, and the unconformities subdividing the stratal succession, reflect the interplay of erosional and depositional regimes. Together these provide a clear record of the base level changes that occurred in response to tectonism and eustasy within the Cretaceous foreland basin. 4 The Cretaceous Western Interior Seaway of western North America is a classic example of foreland basin evolution and sedimentation (Fig. 1). The basin contains thousands of meters of sediment (5000 m) that accumulated over tens of millions of years (60 Ma). During that time the basin was carried by plate motions through climatic zones, was deformed by plate margin processes causing thin-skinned and thick-skinned deformation (retro-arc compression), was subjected to thermally or tectonically induced, large-scale downwarping (flexural subsidence), and influenced by globally synchronous sea level changes (eustasy). As a result, the basin evolved from a segmented nascent foreland basin characterized by mainly continental sedimentation to a transcontinental seaway, in which the whole spectrum of fluvial to offshore-marine depositional environments existed.

The analysis of the basin will show that the Early Cretaceous history in Utah is that of a differentially subsiding, alluvial foreland basin. Sediment accommodation was controlled by Airy isostatic subsidence following initial thrusting. As a result, a narrow basin formed between the nascent frontal zone and the non-subsiding craton to the east (Fig. 2). Conversely, the Late Cretaceous history is that of a marine basin, where sediment availability and basin accommodation was controlled by repeated thrust uplift, flexural, as well as Airy isostatic subsidence, and eustasy. Consequently, depositions; geometries at the subsiding foreland basin margin are mainly an expression of eustatic changes, although tectonic influences at times prevailed (Fig. 2). Figure 1. Map showing extent of Late Cretaceous (post-Turonian and pre-Campanian) Interior Seaway. Arrow points toward study area in inset box. Seaway (stippled) is outlined with solid line were certain and dashed were uncertain. After Steick (1975) and Kauffman (1977a, 1977b). SUMMARY

EARLY CRETACEOUS SEQUENCE STRATIGRAPHY:

* Tectonic uplift determined sediment availability

* Sediment accommodation is controlled by tectonic basin subsidence

* Basin subsidence is restricted to a narrow area between the thrust front and the non-subsiding craton. It is an expression of the inherent structural fabric of the cratonic margin.

LATE CRETACEOUS SEQUENCE STRATIGRAPHY:

* Sediment availability is controlled by uplift and/or basin subsidence

* Sediment accommodation is determined by tectonic flexure, basin subsidence and sealevel changes in the

Basinal subsidence patterns are large-scale and the result of the downward flexure of the lithospheric margin and craton

Figure 2. Research results and summary 7 SEQUEMOE-STRAUGRAPHY AP-PL1ED.IQ. FORELAND BASINS In this study, the Cretaceous foreland basin fill in Utah was used to test the sequence-stratigraphic concepts developed by Vail and co-workers in a tectonically active basin (Vail et al., 1977a, 1980; Vail and Todd, 1981 ; Vail, Hardenbol, and Todd, 1984; Haq, Hardenbol, and Vail, 1987; Vail, 1987; Van Wagoner et al., 1987). In that context, the following questions are addressed by this study. ( 1) What are the main factors determining basin subsidence and sequence development in a foreland basin ? (2) Are there differences in the style and subsidence pattern during early-stage and late-stage foreland basin evolution ? (3) now valid is the giobai correlation and assumed synchroneity of sea level changes in such a tectonically active basin ? and finally (4) to what extent do the depositional character and stacking pattern of stratal bodies in the basin fill reflect tectonic or eustatic change ?

Foreland basins develop through the downward flexure of the lithosphere in response to the vertical loads emplaced in an adjacent fold-thrust belt (Beaumont, 1981; Jordan, 1981). Although there is little debate on the subsidence-generating mechanism, much less is known about the subsidence and sedimentation history of foreland basins. Existing geodynamic models using decompacted geohistory plots commonly approximate the geometry of the Cordilleran basin fill as simple wedgs-shsped In cross-ssctlon and thinning toward the craton (Lawton, 1984; Heller et al., 1986; Cross, 1986; Paola, 1986). Numerous studies of the stratigraphies of the Cordilleran basin fill have shown. 8 however, that early-stage basin-fill geometries were controlled by what have been called intraforeland structures, which subdivided the nascent foreland basin into several subbasins of varied tectonic character (Schwartz, 1982; DeCelles, 1986; Schwans, 1987a, in press; Schwartz and DeCelles, in press). Sediment availability and basin accommodation in such forerunner basins were determined by uplift and tectonic subsidence. On that basis, it seems necessary to determine the correct basin subsidence rate profile and approximation of basin geometry, as well as the effect of differential subsidence on the stacking pattern of strata in the foreland basin.

The recognition of coastal onlap (see appendix A for definition) is fundamental to sequence stratigraphy and the recognition of sequence boundaries (Mitchum, Vail, and Thompson, 1977). The shifts in coastal onlap can be expressed as a function of the changes in sediment availability and the sediment accommodation potential of the basin (Vail, Hardenbol, and Todd, 1984). Sediment availability in a foreland basin is determined by the rate of thrusting and erosion, the thickness of the load, the resulting relief, the climate, source rock character, and the eustatic change in the basin (Schwans, 1985b, 1986a, 1987b; Tankard, 1986a, 1986b; Vail and Bowman, 1987). The basin accommodation potential Is determined by net tectonic uplift, net Airy isostatic and flexural subsidence (e.g., the response of the lithosphere under and near the load), and by the net eustatic changes in the marine basin. Long-term continuous flexural subsidence creates space (500 m to 1500 m) over several million years (5 to 10 M.Y.) (Jordan, 1981) and operates with a rate that is of the sam e low frequency. Conversely, more rapid changes in net accommodation may be caused by differential Airy isostatic subsidence under and near (within 50 km to 75 km) the load or by eustatic fluctuations (Schwartz and DeCelles, 1987; Schwans, 1987a). Together, these can substantially increase (>500 m) the accommodation potential of a foreland basin over geologically short periods (0.5 to 5 M.Y.). This is especially true. If a eustatic rise coincides with an episode of increased tectonic subsidence, thus dramatically increasing the net accommodation potential of the basin. Consequently, it is imperative to consider the interplay of such long-term and short-term changes and their effect on sediment distribution and systems tract architecture in the foreland basin.

Long-term sediment availability in the foreland basin is controlled by the changes in topographic relief at the basinal hinge, which is the area between the fold-thrust belt and the foreland basin proper and across which flexure occurs. Topographic relief in turn is determined by uplift, subsidence, and base level changes in the basin. Tectonic uplift in the fold-thrust belt enhances topographic relief, the rate of erosion, and ultimately the sediment flux into the basin. Base-level changes caused by eustatic fluctuations (50 m to 100 m) alter relief within the fold-thrust belt to basin profile (50 km to 100 km) on a short-îsmi basis (0.5 to 1.5 M.Y.) (Vail and Bowman, 1987). As a result, fluvial valley incision and coastal plain bypass may occur, significantly altering the 10 sediment flux at the basin margin. Furthermore, if tectonic uplift in the fold-thrust belt is coincident with a eustatic fall in the basin, the increase in relief at the basinal hinge will substantially add to the sediment flux, yet immediately decrease the basin accommodation potential. These and other effects must be reflected in the sedimentary record by an increase in the accummulation rates, changes in depositional architecture, and the character of the component depositional systems. General models of the development of sequences in foreland basins must incorporate above changes and controlling factors.

Tectonic subsidence patterns differ considerably within the separate segments of a fold-thrust belt and significantly overprint and obscure the coastal onlap and eustatic signal within a foreland basin. In order to answer the question of global synchroneity of coastal onlap events versus sea level fluctuations, it is necessary to qualify the individual effects of tectonism, subsidence, and eustasy. Some of the following questions will need to be answered in later chapters: How regional are the tectonic factors? To what extent do they control sedimentation and subsidence within the basin? And finally, do all shifts in coastal onlap that occur within the foreland basin mark global changes in sea level?

GENERAL BACKGROUND CRETACEOUS PLATE MARGiN OF NORTH AMERICA The North American plate margin during the Cretaceous is widely accepted as an example of an Andean-type tectonic and magmatic system 11 controlled by subduction of an oceanic plate beneath the continental margin (Dickinson, 1974,1976; Hamilton, 1978) (Fig 3). Models of absolute plate motions indicate that subduction rates along the margin varied in time and space. Varying modes of subduction can be related to states of stress, styles and positions of plate deformation, as well as to patterns of magma emplacement into the overriding continental plate (e.g. Coney, 1972; Sykes, 1978; Cross and Pilger, 1978a, 1978b; Engebretson, Cox, and Thompson, 1984, among others).

Atwater (1970) and Atwater and Molnar (1973) originally described the interaction between North America and oceanic Pacific plates (e.g. Kula, Farallon, and other Pacific plates) based on the rotations of magnetic signatures of oceanic crust, thus constraining the history for 80 Ma B. P. Later, Engebretson, Cox, and Gordon (1984) and Jurdy (1984), among others, refined and extented plate reconstructions to 150 Ma B.P., based on a hot spot and absolute motion reference frame. As a result of plate interactions, three structural complexes, now seen as synchronous responses to subduction of Pacific lithosphere beneath North America, characterize the North American plate margin: (1) the Sierra Nevada batholith as an analogue of a magamatic arc; a composite body of Late Triassic to Early Cretaceous plutons, where the bulk formed in the Late Jurassic and Early Cretaceous (140 M ato 80 Ma, after Hamilton, 1978); (2) the Great Valley sequence, sedimentary rocks deposited in a fore-arc basin that lap onto the Sierran basement to the east and are bounded to the west by the Franciscan assemblage; and (3) the Franciscan 1 2 assemblage, the remnant of a wedge of melange and imbricated rocks (latest Jurassic through Eocene), representing an accretionary complex with a complex subduction history (Dickinson, 1974,1976; Hamilton, 1978).

HINTERLAND AND FOLD-THRUST BELT RELATIONS Absolute motion of North America and the relative convergence between North America and the Pacific plates in the late Mesozoic caused North America to move with oblique absolute motion toward the trench and the Farallon plate, resulting in a moderate to steep angle of subduction and development of a voleano-plutonic arc along the western margin of North America (between 145 Ma and 85 Ma, after Cross, 1986) (Fig. 3a). Magmatism waned in the Sierra Nevadan province at 80 Ma and completely ceased by 70 Ma. During that time period, wedges of shelf-margin strata, thinning eastward onto the craton, were disrupted by folding and near-bedding plane thrusting, forming allochthons that were carried eastward over little-deformed basement (Jordan, 1981 ; Allmendinger and Jordan, 1981 ; Allmendinger et al., 1987) (Figs 3b, 3c, 3d).

This thin-skinned deformation of upper Proterozoic through Triassic continental-shelf and basin strata produced the Cordilleran fold-thrust belt, a classic example of a retro-arc fold-thrust belt (Dickinson, 1S74, 1978), with (1) west-dipping listric thrusts that sole out in major crustal detachments (Royse et al., 1975; Wiltschko and Dorr, 1983; Figure 3. History (a-d) of plate interaction showing position of subduction zone (thin line, barbs pointing east), magmatic arc (dashed pattern), frontal thrust (solid line with barbs), inferred position of subducted slab of Farallon plate (stippled), and area occupied by Laramide deformation (hachuredj. After Cross (1986, his figure 7). 1 4 Allmendinger et al., 1986,1987); (2) a minimum of 50 % shortening of supracrustal strata perpendicular to the thrust with miogeoclinal strata (Eocambrian to Triassic) thrust over thinner platform strata as young as Jurassic (Burchfiel and Davis, 1972; Burchfiel, 1975); and (3) the younging of major thrust faults from latest Jurassic in the west to earliest Eocene in the east (Armstrong and Oriel, 1965; Oriel and Armstrong, 1966; Oriel and Platt, 1979). Moderate-angle subduction beneath North America from 140 Ma to 80 Ma is thus expressed in Cordilleran-style (e.g. thin-skinned) shortening and foreland fold-thrust belt deformation located inboard of a well-defined volcano-plutonic arc.

Following the cessation of arc-magmatism in the Sierra Nevadan province at 70 Ma, a broad area devoid of magmatism occupied the southwestern part of North America (e.g. Arizona, Nevada, Utah, Wyoming) (Cross, 1986) (Figs. 3b, 3c). To the north and south of this magmatic gap, coastal arc-magmatism continued and expanded eastward between 80 Ma and 70 Ma. Magmatic activity ceased completely in Idaho and Montana at 70 Ma Coney (1972), Coney and Reynolds (1972), Snyder at al. (1976) and Cross and Pilger (1978a), among numerous other reports, relate the development of the magmatic gap to the reorientation and acceleration of the North America plate with respect to the Pacific plates within the absolute refence frame. As a result of accelerated motion, moderate-angle subduction shaliowed, high heat flow in arc areas began to dissipate, and lithosphere cooled and thickened, thus becoming increasingly resistant to deformation. Consequently, 15 compressive stresses at the western plate margin during the very latest Cretaceous and early Paleogene (70 Ma to 45 Ma) were relieved by buckling and shear in the brittle upper crust, affecting cratonic basem ent east of the Cretaceous fold-thrust belt and causing Laramide deformation (Cross, 1986) (Figs 3c, 3d). Laramide deformation was thick-skinned and caused vertical asymmetric uplifts of Proterozoic basement rocks or steep monoclines bordering deep, structural basins, which subsided and received sediments concurrent with uplift. As a result, the Colorado Plateau uplift and the Laramide uplifts and basins segmented foreland basin areas in Colorado, Wyoming, Idaho, and Montana (Fig. 3d).

FORELAND BASIN EVOLUTION The Cordilleran basin is a foreland basin situated between the fold-thrust belt and craton (Fig. 1). The basin developed by flexural depression of the lithosphere under and near the thrust load (Beaumont, 1981 ; Jordan, 1981). The flexural behavior of the lithosphere is dependent on ( 1) its thermal state or age at the time of loading (i.e., weak and stretched or strong and thick), ( 2) the rate at which thrust terranes are emplaced, (3) the residence time of the load (relaxation of the lithosphere), (4) the thrust-load thickness, and (5) the thermal state of lower crustal zones and the mantle (Coney and Reynolds, 1977; Beaumont, 1981; Quinian and Beaumont, 1985; Wiitschko and Dorr, 1983). As a result, flexural behavior of the lithosphere determines the rate and degree of subsidence, the basin outline, the cross-sectional shape, and 16 the three-dimensional configuration of the basin.

Jordan (1881) and Kominz and Bond (1982,1986) analyzed foreland basins of western North America and concluded that the magnitude of subsidence decreases with increasing distance to the erogenic belt. Following load emplacement, foreland subsidence begins gradually near the load and radiates outward (Quinlan and Beaumont, 1984), thus producing a convex-upward (accelerating) subsidence-rate curve. This subsidence behavior is not universal, however, as subsidence rate profiles for the Wyoming sector of the basin by Cross (1986) show a staircase character, with instantaneous subsidence instead of gradual acceleration. A final feature of foreland basins is the migration of depo-centers and their pinch-out toward the craton. Such migration may be gradual, as in the Wyoming sector of the Cordilleran belt (Jordan, 1981 ; Cross and Pilger, 1978a), or may occur in a stop-start fashion, as documented in the northern Appennines (Ricci-Luchi, 1986). In general, foreland basins are ( 1) deepest nearest the load, ( 2) thin asymmetrically toward the craton, and (3) have a basin axis that moves progressively with thrust-load emplacement.

In that context, this study will show that intial foreland basin subsidence in the Utah sector of the Cordilleran belt was Airy isostatic in character and restricted to areas near the thrust load. Subsidence occurred predominantly along pre-existing basem ent structures of the loaded Precambrian-rifted margin (see chapters 1 and 2). During the Late 17 Cretaceous an over-deepened moat adjacant to the frontal zone more or less captured most Cordilleran belt sediment (see chapter 4). The sedimentary strata in the foreland basin in Utah consequently record the transition from a period of initial Airy isostatic subsidence to a period of broad flexural subsidence.

fOBELANP_PASIN.f|LL A threefold, west to east subdivision of the Cordilleran foreland basin can be suggested on the basis of paleoenvironments and the character of the sedimentary basin fill (after Kauffman, 1977a, 1977b; Scott, 1977; Fouch et al., 1983; Weimer, 1984) (see also chapters 2 and 4):

(1) a western zone of maximum subsidence (5000 m in 80 M.Y.) and sedimentation with syntectonic, coarse fluvial to marine strata. Faunas reflect the freshwater to brackish-water and marine transition, high energy environments, and water depths no greater than 100 m.

(2) a central zone of limited subsidence (2500 m in 60 M.Y.) and rates of sedimentation, containing finer grained sediments. Faunas suggest deeper water (>100 m) and lower energy environments alternating with shallower conditions (< 100 m) and higher energy deposition as a result of transgressions and regressions. At least five periods of major sea level highstands can be identified, the initial late Aibian 18 highstand (Mowry phase: after Kauffman, 1977a), the early to early middle Turonian highstand (Greenhorn phase; after Kauffman, 1977a; Weimer, 1984), the earliest Coniacian highstand, the middle Coniacian highstand, and the late Santonian highstand (see chapter 4) (e.g. Niobrara to Judith River highstand; after Kauffman, 1977a; Weimer, 1984).

(3) a broad hinge zone between the central part of the basin and stable platform in Kansas, Nebraska, and Iowa. Subsidence is moderate to low (< 500 m in 60 M.Y.), while carbonate deposition is dominant. Suggested water depths range from 100 m to 200 m for the carbonate ramp environments (Kauffman, 1977a, 1977b).

METHODS SEISMIC CORRELATION The application of sequence-stratigraphic concepts to seismic sections, outcrop, and well log correlation studies provides the means to identify major surfaces and stratal packages of major, medium, and minor magnitude. In practice, major and medium sequences (see appendix A for definition) are discernable in seismic sections, and have been identified by this study (unpublished seismic data).

The basic principle of seismic stratigraphy is that major seismic reflections are generated at physical or stratal surfaces that have a velocity and/or density contrast, rather than gross lithostratigraphic 19 boundaries (Ramsayer, 1979). Only two types of sedimentary reflectors of chronostratigraphic significance are present in a sedimentary section at the time of deposition, stratal surfaces and unconformities. Consequently, no physical surface exists that represents a time-transgressive lithostratigraphic boundary and no seismic reflector will follow such a transition. Instead, seismic reflections tend to parallel the major stratal surfaces, rather than the uneven lithologie boundaries that cut across the stratal surfaces (Vail et al., 1977b).

Sequence boundaries can consequently be recognized on seismic sections by the termination patterns (see appendix A for definitions) of reflectors against a primary reflection surface, and can be traced on seismic sections for tens or hundreds of kilometers basinward and even farther along strike. The accuracy of stratigraphie correlations using seismic sections is dependent on data quality (Ramsayer, 1979). Seismic reflection correlation "approaches chronostratigraphic correlation and provides a level of synchroneity that is commensurate with the scale of strata represented" (Mitchum, Vail, and Thompson, 1977).

mkLQa.ÇgBBËLAnQM Minor surfaces, sequences beyond the limit of seismic resolution, as well as parasequences and parasequence sets (see appendix A for definitions) were mapped using well logs (see appendix C). A well log standard was defined by relating outcrop sections to nearby wells (for example. Six Mile Canyon and well 5 in Rg. 4). Following regional well 20

study Area

¥ \r

Figure 4. Map of study area in central Utah showing plateau areas, aerial photo coverage (box pattern), and southern cross-section (wells 1 through 13) and northern cross-section (wells 13 through 27) (see appendix C for well data). 21 log correlations, seismic sections (unpublished seismic data) were interpreted and tied into nearby wells using sonic and density logs. Well log and outcrop correlation in conjunction with seismic correlation is an excellent tool for detailed studies of stratigraphie relations in depositionally and structurally complex basins.

As well control was sufficiently dense (Fig. 4), correlations of gamma ray, self potential, density, porosity, or sonic logs allowed detailed mapping of flooding surfaces, resolution of parasequences, parasequence sets, and modeling of parasequence stacking patterns. Once the vertical and lateral succession of facies were recognized in well logs and outcrop sections, a three-dimensional model of deposition was created for stratal packages at the subsiding foreland basin margin in Utah. Together these data allowed detailed reconstruction of facies associations and their inherent stacking patterns, as well as palinspastic and paleoenvironmental reconstruction. Based on the above, sequence-stratigraphic modeling and mapping was used to create a three-dimensional impression of the depositional sequences that form the fill in the Cordilleran basin in Utah.

IMAGERY AND OUTCROP CORRELATION Satellite imagery (scale 1:125,000 and 1:250,000) and aerial photography (scale 1:52,000) proved useful In Identifying and tracing sequence boundaries and other large-scale surfaces in Cretaceous outcrops in Utah (Fig. 5). As the outcrop is continuous, boundaries were Figure 5. Landsat satellite photomosaic of study area with section locations (dots) and names indicated. 1 : 250.000 23 traced for tens to a hundred kilometers. Resolution of satellite imagery was sufficient to identify the lateral continuity and geometry of surfaces that bound sequences, to determine the vertical and lateral stacking patterns and the geometry of units within depositional sequences. Together with high-altitude and low-altitude aerial photography, satellite imagery was a powerful tool for identifying and correlating bounding surfaces identified in the outcrop on a scale that is commensurate with that of seismic sections.

The sequence-stratigraphic analysis of marine and alluvial outcrops (see chapters 1 and 3) subdivided the thick Cretaceous sedimentary succession into genetically significant stratal units (e.g. depositional sequences). On that basis, major shifts in deposition were identified that occurred during the evolution of the Cordilleran basin and are of chronostratigraphic significance (see chapters 2 and 4). According to Haq, Hardenbol, and Vail (1987) and Vail (1987), the most readily identifiable surface in marine outcrops is the transgressive surface (TS in Fig. 6a). It occurs above the lowstand deposits, which are characterized by sediments of the most regressive phase (see appendix A for definitions). Transgressive lags may thus lie on top of lowstand fan sediments, incised valley fills, or even on deltaic to estuarine sediments, which were deposited during the late phase of the lowstand wedge systems tract (see appendix A for definitions). Since associated transgressive deposits are very thin and become increasingly concordant to underlying deposits in sections landward from the incised siiîiili

r • S H Q /MAAGM WEOGE SYSTEMS T ÏU a

Figure 6. Sequence-stratigraphic representation of the main depositional systems tracts and their relations to the various segments of the eustasy curve. (A) The systems tract related to depth. (B) Chronostratigraphic relations of systems tracts. From Haq, Hardenbol, and Vail (1937, their figure 1). 25 valleys of the shelf «dge, transgressive surfaces are the most difficult surfaces to identify. Onlap patterns in subsurface sections landward of the shelf edge, for example, can be documented through detailed stratigraphie correlation of well logs.

The second most recognizable surface in marine outcrops is the surface of maximum flooding (mfs) or downlap surface (DLS) (see appendix A for definition) (Vail, 1987) (Figs. 6a, 6b). Downlap surfaces are associated with condensed sections and a marine hiatus (see appendix A for definitions). They indicate the level of maximum paleowater depth in stratigraphie sections. Due to the lack of terrigenous input, condensed sections are expressed as a zone of high pelagic fossil concentration or as hardgrounds (Vail, Hardenbol, and Todd, 1984). Downlap surfaces form during the maximum extent of a transgression (top of TST in Fig. 6a) and the start of a regression (see appendix A for definitions). The change from transgression to regression occurs as the rate of eustatic rise decreases and sediments begin to prograde over the surface. The equivalent surface in alluvial outcrops has not yet been described by Vail and co-workers and is also a subject of this study.

The third easily recognizable surface in marine and alluvial outcrops is the sequence boundary (SB) (Vaii, 1987) (Fig. 6a, 6b). The unconformity is principally recognized by the basinward (downward) shift of coastal onlap. The extent of the shift is expressed by the disparity that exists between facies that are above and below the unconformity. The less related the superposed facies are temporally, laterally, and genetically, the more significant the shift. In an outcrop situated near the alluvial to coastal plain boundary, for example, a substantial downward shift in coastal onlap can result in the superposition of alluvial over coastal plain and marine strata. At the same time in more basinward outcrops, deltaic sediments might be found resting unconformably on shelf mudstones within an incised valley. The degree of lateral offset between the facies above and below the unconformity in a down-depositional dip direction is thus a direct measure of the reiative sea level change that occurred in the basin.

ONLAP MODEL AND CRITIQUE Changes of relative sea level in a basin are the result of the interaction between eustasy, subsidence, and sediment availability. Together these factors control the accommodation potential of the basin and the distribution of the systems tracts. Integrating seismic data with lithologie, facies, biostratigraphic, radiometric, and well log data, Vail et al. (1977a, 1980) directly equated the relative changes of coastal onlap with relative changes in sea level (Subsidence/Eustasy plot in Fig. 6). Vail and Todd (1981) and Vail, Hardenbol, and Todd (1984) reexamined the coastal onlap model and approximated the relative s e a level at the facies change boundary between the coastal plain and the alluvial plain. By plotting the successive upper limits of the coastal plain through time, a chart of coastal onlap can be constructed that permits sediment 27 accummulation to continue while sea level begins to fall. The sawtoothed pattern of the global charts is interpreted as relative changes of coastal onlap.

Unfortunately, seismic resolution does not always allow exact identification of this relative sea level boundary. Consequently, the charts developed from seismic sections include data from both the alluvial plain with its slightly greater dips and the flatter coastal plain (Vail, Hardenbol, and Todd, 1984). Miall (1986) pointed out, however, that certainly not all sequences show such alluvial components. Thus even the adjusted or new model does not offer a solution in each case. This limit in resolution is the source of the differences between coastal onlap charts and the chart of relative change of sea level. Although both charts are very similar, charts of relative sea level versus distance show abrupt sawtooth shifts, whereas charts of relative sea level change versus distance show more gradual changes.

Since its first publication, the onlap model and the methodology of constructing the charts have been subject to a lively debate. Much of the criticism has centered on (1) the nonpublication of evidence, which is mostly proprietary; (2) the lack of refinement, updating, and calibration of the global coastal onlap charts with respect to new biostratigraphic, radiometric, and magnstostratiographic time scales; (3) the question of global correlation, the synchroneity of sea level changes, and their relation to event in abyssal basin areas; (4) the inadequate correction for 28 tectonic factors in basins of varied tectonic type; and (5) the question whether all coastal onlap is related to sea level fluctuations.

Following publication of the original global coastal onlap chart in Vail et al. (1977), several updated versions of the curves of sea ievel fluctuations have been published for the Jurassic and the Cenozoic (Vail and Hardenbol, 1979; Vail and Todd, 1981 ; Vail, Hardenbol, and Todd, 1984; Kerr, 1987). The most recent update on the methodology and the chronostratigraphic basis for global coastal onlap curves of the Jurassic, Cretaceous, and the Tertiary can be found in Haq, Hardenbol, and Vail (1987) and in Bally (1987).

Global unconformities mark the lows in the sawtooth pattern of the charts and subdivide the stratigraphie section into stratal packages related to global cycles of coastal onlap. Marine condensed sections within the vertically upward-deepening sequences are thought to be synchronous worldwide, yet may differ slightly in individual basins as a result of changes in the rates of deposition and subsidence (Vail, Hardenbol, and Todd, 1984) (see appendix A for definitions). Problems arise when this modei of globally synchronous sedimentation is applied to more recent deeper ocean basin environments. Tucholke and Embley (1984), for example, documented well-defined late Miocene and early Oligocènes unconformities in the South Atlantic at 4 km to 5 km water depth. The unconformities were interpreted to have formed in response to increased glaciation of West Antarctica with associated current 29 erosion during increased thermal abyssal circulation. Conversely, an underlying Eocene unconformity showed no effects of erosion as a result of deeper oceanic circulation. Contrary to the Vail model, which states that condensed sections form typically during times of maximum paleowater depths, the above unconformities "formed under conditions of reduced sediment supply, caused by elevated sea level and probably low productivity in surface waters" (Tucholke and Embley, 1984, p. 145). Consequently, as Miall (1986) pointed out, the possiblity needs to be examined, whether Cretaceous or older condensed sections and unconformities formed in response to thermal circulation.

Pitman (1978), Watts and Steckler (1979), and later Watts (1982), among others, have demonstrated that changes in coastal onlap on many subsiding divergent margins can solely be caused by subsidence-rate changes of the margin in response to post-rift cooling and flexure. In addition, Parkinson and Summerhayes (1985) showed that sequences within individual basins can develop by a combination of tectonic subsidence and eustasy. Subsequently, Watts and Thorne (1984) presented a series of Vail-type onlap curves based on Atlantic margin stratigraphies and well data that were synthesized using parameters of thermal contraction and sediment loading. The above discussion of the onlap model demonstrates the problems of correcting for tectonic factors in basins of varied type. CHAPTER !.

EARLY CRETACEOUS SEQUENCE STRATIGRAPHY, LITHOFACIES, AND SUBSIDENCE MODEL

30 SECTION 1.1

INTRODUCTION This chapter reevaluates the lithostratigraphic and sedimentologic characteristics of Lower Cretaceous strata in central Utah on the basis of the sequence-stratigraphic model of Vail and co-workers. New paléontologie, sedimentologic, and petrologic data introduced here allow stratigraphie subdivision of previously undifferentiated foreland basin fill strata in Utah with respect to occurrence, distribution, lithologie character, and the component depositional systems. A new stratotype and name. Pigeon Creek Formation, is introduced in the first part of the chapter for rocks deposited near the frontal zone of the fold-thrust belt. Pigeon Creek strata constitute the earliest syntectonic foreland basin fill, and document timing of inital Cretaceous thrusting in Utah. Pigeon Creek distribution accurately reflects the configuration of the nascent foreland basin near the thrust front.

In the second part of the chapter, proximal Pigeon Creek strata are linked to rocks of the chronostratigraphically correlative Cedar Mountain Formation within a sequence-stratigraphic framework. Cedar Mountain strata represent the earliest foreland basin deposits in areas distal to the frontal zone. The formations together constitute an unconformity-bounded depositional sequence of regional extent. Lower 32 Pigeon Creek faciès, lithology, and petrology are related to source terranes dominated by fine-grained Mesozoic strata. Upper Pigeon Creek strata define an unroofing sequence following erosive breaching of emergent thrust terranes. At the same time, however, the chronostratigraphically equivalent depositional systems of the lower and upper Cedar Mountain Formation in east-central Utah remained uninfluenced by thrust-loading and thrust-terrane erosion, and instead derived their detritus from continental sources to the south and southeast. Although chronostratigraphically equivalent, these two formations are very different in depositional and lithologie character. Changes in petrology and conglomerate clast distribution are described in the third part of the chapter and document the varied structural style of fold-thrust belt formation and foreland basin subsidence. These differences between the major depositional systems and their respective source areas reflect the non-integrated nature of the foreland basin during the Early Cretaceous. 33

SECTION 1.2

DEPOSITIONAL RESPONSE OF PIGEON CREEK FORMATION, UTAH,TO INITIAL FOLD-THRUST BELT DEFORMATION IN A DIFFERENTIALLY SUBSIDING FORELAND BASIN

(Peter Schwans)

ABSTRACT Unconformity-bounded depositional sequences represent primary units of chronostratigraphic significance. Two regional unconformities are recognized and employed to subdivide an alluvial foreland basin fill sequence, documenting earliest Cordilleran foreland basin subsidence and sedimentation in Utah. First, stratigraphie nomenclature of syntectonic Lower Cretaceous strata is redefined on a sequence-stratigraphic basis using new paléontologie, sedimentologic, and lithostratigraphic data. A new stratotype and name. Pigeon Creek Formation, is introduced for rocks that had previously been assigned to the Lower Cretaceous Morrison(?) Formation and Indianola Group Undifferentiated of central Utah. Two members are recognized. Rocks of the lower Pigeon Creek member were deposited east of the thrust front during Neocomian to middle Aptian time in an ephemeral braided stream system featuring a muddy flood basin with interspersed lacustrine depression. 34 Conglomerates and intercalated mudstones of the upper Pigeon Creek member were deposited in eastward-shedding, sheet flood-dominated fan systems during late Aptian to late Aibian time. Together, the braided stream and overbank elastics of the Pigeon Creek Formation document initial thrust uplift and sedimentation in a laterally restricted foreland basin that is only subsiding near the thrust load.

Second, the Pigeon Creek Formation, together with correlative strata in eastern Utah and western Colorado, is placed into a regional sequence-stratigraphic framework. By defining the bounding unconformities, the extent of the hiatuses, and the age of the sequence strata, the formations are shown to form a depositional sequence of regional extent bounded by globally recognized unconformities. Contrary to other workers, this study interprets inital thrusting and basin subsidence to have occurred as early as late Neocomian, as evidenced by the preservation of the fines of the lower Pigeon Creek member adjacent to the thrust front. Initial uplift occurred above subsurface thrust ramps, resulting in source terranes dominated by fine-grained lower Mesozoic strata. In contrast, conglomerates of the upper Pigeon Creek member are post-orogenic in nature and merely indicate hanging wall emergence during the late Aptian and Aibian. Clast distributions define an unroofing sequence following the erosive breaching of an emergent thrust terrane. Sedimentation in coeval, yet separate depositional systems in central Utah, remained uninfluenced by initial basin subsidence. Larger scale foreland basin subsidence in Utah did not occur 35 prior to the late Aibian. Finally, three phases of tectonic deformation and differential Cordilleran foreland basin subsidence are identified on the basis of the sequence-stratigraphic analysis of alluvial foreland basin strata in Utah.

INTRODUCTION AND PURPOSE The Late Jurassic and Early Cretaceous western margin of North America was dominated by the subduction of oceanic lithosphere beneath the continent. Coeval to continental margin deformation, eastward- directed thrusting and folding during the Early Cretaceous resulted in the formation of the Sevier (Cordilleran) overthrust belt (Royse et al., 1975; Burchfiel, 1980; Allmendinger et al., 1983; Wiitschko and Dorr, 1983) (Fig. 7). Initial foreland basin deformation in the impingent fold-thrust belt in Montana and Wyoming (Armstrong, 1968; Jordan, 1981), for example, was marked by a tectonic partitioning of the foreland basin by intraforeland structural elements and paleotopographic highs (Schwartz, 1982; Christopher, 1984; DeCelles, 1986). Little detail is known, however, about the character of initial uplift in the Utah segment of the Sevier belt and: (1) the depositional response in the incipient foreland basin; (2) the time-dependent rate of change in the capability of the incipient foreland basin to store sediment; and (3) the character of the earliest syntectonic clastic wedges.

Tectonics and sedimentation have been linked in the Montana-ldaho-Wyoming-northeastern Utah segment of the Sevier belt Sevier T-_j_beltx T - ~ / Study l^fah

flPQreiand v ^ = t “ V / ^

MEXICO

250 500

Rgure 7. Map showing the Sevier (Cordilleran) fold-thrust belt and Late Cretaceous foreland basin area. Inset rectangle is area covered by Figure 2. Based on McGookey (1972). 37 for the Late Cretaceous and Tertiary thrust systems and related synorogenic deposits (Royse et a!., 1975; Allmendinger and Jordan, 1981 ; DeCelles, 1986). In Utah, eastward-directed thrust movement emplaced Proterozoic, upper Paleozoic, as well as lower Mesozoic strata, in an imbricate fashion, over strata as young as Jurassic Navajo Sandstone (Christiansen, 1952; Hickcox, 1971; Burchfiel and Hickcox, 1972; Villien and Kligfield, 1986). Timing and style of earliest tectonism and foreland basin deformation in central Utah, however, remains controversial, as does the character of the synorogenic deposits.

Spieker (1946), Armstrong and Oriel (1965), and Armstrong (1968) suggested that deformation began during Early Cretaceous or possibly Late Jurassic time. Burchfiel and Hickcox (1972) considered Lower Cretaceous strata to be the first clear synorogenic sediments. Fouch et al. (1983) related the Cenomanian to Turonian Sanpete Formation of the Indianola Group to an early episode of thrust-fault activity in central Utah, whereas Lawton (1985), as well as Villien and Kligfield (1986), identified thrusting to have been continuous from the Aibian and until late Campanian. Most recently, Heller et al. (1986) suggested, based on subsidence analyses of sedimentary sequences, that initiation of thrust movement was recorded by basal Indianola Group conglomerates and was no older than Aibian time.

This chapter reevaluates the lithostratigraphic and sedimentologic characteristics of Lower Cretaceous clastic rocks in central Utah and 38 identifies and characterizes earliest foreland basin deposition in response to thrust initiation in the Utah segment of the Cordilleran fold-thrust belt. Lower Cretaceous strata, including rocks informally assigned by Spieker (1946, p.125) to the Lower Cretaceous Morrison(?) Formation, are redefined by introducing a new stratotype and name. Pigeon Creek Formation. Pigeon Creek depositional architecture is analyzed and the character of incipient fold-thrust deformation in central Utah is documented. Finally, this study introduces a tectono- stratigraphic framework for Lower Cretaceous strata in central Utah in order to provide an understanding of fold-thrust belt and foreland basin interaction.

LOWER CRETACEOUS STRATIGRAPHY: PIGEON CREEK FM. PROBLEM AND HISTORY Age assignments for Lower Cretaceous rocks in central Utah have been ambiguous since Spieker (1946,1949) first described the area (Fig. 8). A sequence of variegated shales, sandstones, conglomerates, and minor fresh-water limestones in the Wasatch Plateau, the Sanpete Valley area, and as far north as Thistle, was designated by Spieker (1946) as Morrison(?) Formation (Figs. 8,9). Spieker (1946, p.125) stated in his definition that "above the marine Jurassic and beneath the marine Coiorado" [e.g.Turonian Sanpete Formation] "in the Saiina Canyon section and in the section east of Thistle, there is a succession of variegated shales, sandstones, and conglomerates [that is] here designated Morrison(?) formation." The name was introduced by Spieker (1946) 39

{/ Range Thrusts j

I I Quaternary V//A Triassic-Jurassic . I Cretaceous & Tertiary |*.».*/| Precambrian-Paleozoic

Pigeon Creek exposures Thrust fault and section location

Figure 8. Generalized geologic map of study area. Pigeon Creek section locations: 1, type locality in Pigeon Creek Canyon; 2 and 3, southeastern Gunnison Plateau; 4, Salina Canyon. CRT, Canyon Range thrust; ETF, Eastern thrust fault limit during Albian and frontal zone of Pavant 1 thrust projected to surface; LSC, Little Salt Creek; PRT, Pavant Range thrust (Pavant 2 thrust); SMC, Six Mile Canyon; Th, Thistle. After Hintze (1980) and Villien and Kligfield (1986) Spiakai (1946) Wllklnd*ia).(i98e)

Motriion(?) Fm. Cadar Mtn.Fm.

Twiat Gulch Fi Twlit Gulch Fr Twial Gulch Fm.

Rgure 9. Stratigraphie chart for central Utah with correlations by various authors and revised stratigraphie correlation of Lower Cretaceous strata as proposed by this study. 41 based on stratigraphie position and lithologie similarities of the rocks with the Upper Jurassic Morrison Formation of Colorado.

According to Spieker (1946), the Morrison(?) Formation in the Salina district rests on Upper Jurassic Twist Gulch Formation of marine origin (Figs. 8, 9). Concerning the nature of the Morrison(?)/Arapien shale contact in Salina Canyon, Spieker (1946, p.125) stated [it] "is by no means clear-cut, but rather suggests intertonguing; above the first few feet of yellow sandstone, ochre shale, and conglomerate are thin-bedded red and gray shales just like those of the Arapien." Morrison(?) elastics are overlain by rocks of the Upper Cretaceous Indianola Group (Spieker, 1946) (Fig. 9). Elaborating on the nature of the contact, Spieker (1946, p.125) concluded that in "the Salina Canyon section not only is the basal boundary of the Morrison(?) indefinite, but its upper limit is likewise difficult to determine." The Morrison(?)/Sanpete Formation contact in Salina Canyon was drawn by Spieker (1946, p.125) at the "line of greatest apparent change between colored rocks of Morrison aspect and the dominantly buff and gray rocks of the type common in the regional Upper Cretaceous", [and, Spieker concluded], "but there is no clear-cut line of division." For the western Wasatch Plateau and areas to the west, Spieker (1946, p.125) stated that "no strata have ever been assigned to the Morrison formation, but it is possible that some of the conglomerates and red beds included in the Indianola Group (Undifferentiated) are the same as the unit here designated Morrison(?)" (Fig. 9). 42 This inability to separate the Lower Cretaceous Morrison(?) Formation from the Upper Cretaceous Indianola Group (undifferentiated) has resulted in repeated misinterpretations of these strata and has greatly hindered correlation of Lower Cretaceous rocks in central Utah. In the western part of the Gunnison Plateau, for example, Spieker (1949, p.105) identified red beds below the "typical red conglomerate of Indianola", and concluded that these beds "may belong in Morrison(?) formation”. Hunt (1950) however, mapped the same red beds as Jurassic Twist Gulch Formation. Hardy and Zeller (1953) mapped similar red conglomerates and shales to the south in Little Salt Creek (Fig. 8) as Indianola Group (undifferentiated), noting, however, that "the lower portions of the conglomerate may be Lower Cretaceous, or perhaps Upper Jurassic" (Hardy and Zeller, 1953, p.1277). Stokes (1972) examined Morrison(?) outcrops in the type area and concluded that the rocks belonged mostly, if not entirely, in the Lower Cretaceous.

More recently, Standlee (1982) suggested that all strata previously mapped as Morrison(?) in the Salina-Gunnison Plateau districts be assigned to the Lower Cretaceous Cedar Mountain Formation of east-central Utah. Lawton (1985), however, designated all Morrison(?) strata equivalent to the Jurassic Morrison Formation and included the upper parts of Standlee's Morrison(?) strata in the Upper Cretaceous Sanpete Formation of the lower indianola Group. Witkind et ai. (1986) assigned an Early Cretaceous age to the Morrison(?) strata. Witkind and Maley (in Witkind et al. [1986]) correlated lower Morrison(?) strata with 43 the Cedar Mountain Formation and tentatively assigned upper Morrison(?) conglomerates to Spieker* s (1946) Indianola Group (undifferentiated) (Fig. 9). Standlee in Witkind et al. (1986), however, assigned all Morrison (?) strata to the Cedar Mountain Formation (Fig. 9). In a recent correlation of these rocks, Weiss and Roche (in press) use the name Cedar Mountain for the beds formerly identified as part of the lower Morrison(?) and extended the base of the Upper Cretaceous Indianola Group to incorporate strata formerly recognized as part of the upper Morrison(7). Despite these varied attempts to correlate Lower Cretaceous strata of central Utah, the problem still remains whether Spieker*s Morrisonf?) beds above the Jurassic Twist Gulch correlate with the Upper Jurassic Morrison (Lawton, 1985), the Lower Cretaceous Cedar Mountain (Standlee, 1982), undifferentiated Indianola Group (Witkind et al., 1986), and/or the even younger part of the Indianola Group (Weiss and Roche, in press).

DEFINITION AND DESCRIPTION NAME AND STRATOTYPE Detailed stratigraphie analyses of Spieker’s Morrison(?) strata, and of conglomerates of the undifferentiated Indianola Group in the Salina-Gunnison Plateau district by Stuecheli (1984), Stuecheli and Collinson (1984), and Schwans (1985,1986), combined with the interpretation of new paiynomorph data and other age data, have allowed critical réévaluation of Lower Cretaceous rocks in central Utah. A new name. Pigeon Creek Formation, is introduced herewith to replace 44 Spieker's Morrison(?) Formation (Fig. 9). Rocks similar to the Morrison(?) that were erroneously placed by Spieker and his students (Schoff, 1951 ; Hardy and Zeller, 1953) into the Indianola Group (Undifferentiated) and/or the Twist Gulch Formation (Hunt, 1950), are also included in the Pigeon Creek Formation. This revision of the stratigraphie nomenclature of Lower Cretaceous rocks in central Utah is intended to resolve the problem of poorly defined Lower Cretaceous stratotypes, while refraining from unwarranted expansion of stratigraphie units. The name Morrison(?) is abandoned, and the new unit, Pigeon Creek Formation, which incorporates Spieker's Morrison(?) strata, his "typical red conglomerate of indianola [character]" (Spieker, 1949, p.105), and upper parts (e.g., Neocomian to Aptian part) of the Twist Gulch Formation, is established (Fig. 9).

The Pigeon Creek Formation is named after Pigeon Creek Canyon (Levan 7 1/2 Minute Quadrangle, Sec. 25, T. 14 S., R. 1E.) (Fig. 8, section 1 ), where it forms a 975 m-thick sequence of red mudstones, minor limestones, pebbly sandstones, and abundant conglomerates that dips moderately eastward (Fig. 10). Two lithologically distinct members are recognized at the type locality: a lower member that is 185 m thick and consists predominantly of red mudstone, intercalated pebbly channel-form and sheet sandstones, sheet conglomerates, and minor frssh-watsr limestones; and an upper member that is 790 m thick and comprises a succession of sheet conglomerates and thin beds of intercalated red mudstone (Figs. 10,11). The Pigeon Creek rocks in Figure 10. Type locality of Pigeon Creek Formation in Pigeon Creek Canyon, 6 km east of the town of Levan (section 1 in Fig. 8). Lower member crops out in foreground below white line and upper member above white line. View is to the east. 46

Pigeon Creek Fm.

^Upper unconformity ^

Pollen date

Membe

Lower unconformity

Clast size

Figure 11. Pigeon Creek stratotype (Sec. 25, T.14 S., R.1 E.) (section 1 in Fig. 8). Line connecting crosses (carbonate clast sizes) and full circles (quartzite clast sizes) shows clast size trend. Note the upward-coarsening trend in the average maximum clast sizes and the reverse stratigraphie order of the clast lithologies. 47 measured sections in the southeastern part of the Gunnison Plateau (Figs. 12,13) and in the Salina area (Fig. 14), however, are much thinner, with thicknesses of the lower and upper members ranging from 100 m to 200 m, and from 150 m to 250 m, respectively.

The upper member of the Pigeon Creek Formation is, at its type locality and along the west side of the Gunnison Plateau, a prominent cliff-former (Fig. 10). At the type section, the upper member is composed of parts of Spieker’s Indianola Group (Undifferentiated) (Spieker, 1946, p.129), or what Spieker (1949, p.105) called "typical red conglomerate of Indianola [character]", as well as Hunt's (1950, p.195-199) Twist Gulch units 125 through174. Hardy and Zeller (1953) mapped similar units to the south in Little Salt Creek as Indianola Group (Undifferentiated). The lower member is exposed at the type locality in slopes below the cliff-forming upper member (Fig. 10). The lower member consists partly of what Hunt (1950, p.199-202) identified as upper Twist Gulch, or his units 85 through 125. Spieker (1949) referred to these units only informally in his road log and stated that the "beds [below the typical red conglomerate of Indianola] may belong in [the] Morrison(?) formation" (Spieker, 1949, p.105). The contact between the upper and lower member of the Pigeon Creek Formation at the type locality is above the massive mudstone featuring a succession of a few thin ledge-forming sheet conglomerate beds, and below* the thick, cliff-forming conglomerate sequence with few thin mudstone interbeds (Figs. 10,11). The contact between the upper and lower member is easily 48

Upper Cretaceous fluvio marine seq u en ce Lower Cretaceous Pigeon Creek Fm.

Conglomerate

Upper member Lower member Fluvio-lacustrine d ep o sits II- ^ RippI* iirtlification Thin conglomtratt fh ^0}. Climbing ripplai Flamer bedding Flute and groove caut ® Oncolite (0».S to 5cm| A Carbonate nodule, eepianei

Bas* of Pigaon Cr*ak

Figure 12. Pigeon Creek section in the southeastern Gunnison Plateau (Sec. 32, T.18 S., R. 2 E.) (section 2 in Fig. 8). Boulder conglomerate at top is assigned to Upper Cretaceous Indianola Group and overlies Pigeon Creek strata disconformably. 49

Upper Cretaceous fluvio-marine sequence______Lower Cretaceous Pigeon Creek Fm.

Conglomerate

Upper member

Lower member

Fluvio-lacustrine deposits

Figure 13. Pigeon Creek section in the southeastern Gunnison Plateau (Sec. 7, T. 19 S., R. 2 E.) (section 3 In Fig. 8). Note Protelliptio in the lower member and angiosperm leaves in the upper member. Palynomorphs of Weiss and Roche (in press) were recovered from the bivalve-bearing limestone. 50

Upper Cretaceous fluvio-marine Upper unconformity sequence Lower Cretaceous fluvial sequence Carbonate nodules in mudstone > ÿff [— Ripple-stratified silutone Laterally continuous channel-form S a n d y pebble-conglomerate b r S l d e d «/ Stream

Pigeon Creek Fm. Basal unconformity Oxfordian Twist Gulch Formation Flaser-bedded siltstone Marginal marine to tidal flat Climbing-ripple stratification

Figure 14. Measured Pigeon Creek section in Salina Canyon (Sec. 33, 1 .21 E., R. 1 E.) (section 4 in Fig. 8). Willis (1986) reported late Albian fission-traek ages aquired from zircons for samples from upper mudstone interbeds. 51 identified in all the sections measured in the study area (Figs. 12,13).

COMPOSITION. PROVENANCE Conglomerates of the Pigeon Creek Formation are dominated to varying degrees by carbonate clasts and quartzite clasts (Fig. 11). Significant amounts of angular chert pebbles occur only in the lower member and in the lower part of the upper member (Fig. 11,15). Chert pebbles are brown-green to light brown and were most likely derived from Permian and other upper Paleozoic carbonates to the west. Conodonts from various types of carbonate clasts identified by J. Geitgey (persona! communication,1987) indicate that upper Paleozoic (Devonian and Missippian) carbonate clasts occur predominantly in the lower part of the upper member, whereas carbonate clasts of early Paleozoic age (Ordovician) are found throughout the formation (Fig. 11). The distribution of carbonate clasts is that of an inverse stratigraphy. Quartzite clasts reflect the entire range of Proterozoic and Cambrian quartzites, such as the red to purple quartzite and conglomerate of Proterozoic age, the pink to light gray Tintic Quartzite of Cambrian age, and the olive green quartzites belonging to the Cambrian Qphir Formation. Quartzite clasts, however, occur in varying amounts throughout the section (Figs. 11,15).

Sandstones of the Pigeon Creek Formation were determined by Stuecheli (1984) to be composed of reworked sedimentary detritus, such as reworked quartz with overgrowths, abundant carbonate lithic 52

• i*-j

Figure 15. Map showing isopachs (in meters), triangular plots of clast lithologies (normalized to 100 percent chert/sandstone, carbonate, and quartzite), average maximum particle sizes (MPS), and paleodispersal directions for Pigeon Creek Formation. Filled circles, MPS of upper member elastics; open circles, MPS of lower member elastics. Small arrow, mean orientation of trough cross-strata in lower member sandstones determined from 5-15 measurements of trough limbs per station; medium-sized arrow (lower member) and large arrow (upper member) represent paleocurrent directions obtained from clast imbrication (long axis) and orientation of gravel-furrows. Numbers indicate section locations shown in Figure 8; ELT, eastern limit of thrusting for Pavant 1 thrust. 53 detritus, minor feldspar and chert, and few stable to ultrastable heavy minerals including tourmaline, zircon, and sphene. The mudstones that dominate the lower member are mostly composed of smectitic clay (Stuecheli, 1984).

Eastward-directed thrust movement in Utah along the Pavant 1 thrust emplaced Proterozoic to Cambrian quartzites and Paleozoic carbonates over strata as young as the Jurassic (Hickcox, 1971 ; Villien and Kligfield, 1986). The composition of Pigeon Creek rocks suggests derivation of the elastics from a tectonically controlled western source during the Early Cretaceous. The presence of a large amount of fines in the lower member reflects a source dominated by Mesozoic fine-grained siliciclastics, as well as chert-bearing upper Paleozoic carbonates (Fig. 15). The significant amounts of smectitic clay in the mudstones of the lower member also suggest influx of airborne ash from a volcanic source during mudstone deposition. Upper member conglomerate distribution reflects the successive unroofing of an emergent thrust terrane dominated by lower Paleozoic carbonates and Proterozoic/Cambrian quartzites.

GEOGRAPHIC DISTRIBUTION. DIMENSIONS The Pigeon Creek strata crop out as far north as Thistle and as far south as Salina. Pigeon Creek rocks crop out continuously along the west side of the Gunnison Plateau as far south as Little Salt Creek. On the east side of the plateau, however, they occur only in a few isolated outcrops 54 (Fig. 8). Measured stratigraphie thicknesses range from 150 m in Salina Canyon (Fig. 14) to a maximum of 975 m at the type locality in Pigeon Creek Canyon (Fig. 11). At Thistle, Spieker (1946) reported approximately 600 m of Morrison(?) mudstone, which is identified in this study as the lower member of the Pigeon Creek Formation. Jefferson (1982) measured 400 m of red Morrison(?) strata in the Cedar Hills area, which are shown here as Pigeon Creek strata (Fig. 8).

The depositional strike of the Pigeon Creek Formation parallels the structural strike of the fold-thrust belt. Pigeon Creek strata are thickest nearest the thrust front and thin rapidly to the east, to the north, and to the south of the type locality (Fig. 15). The conglomerates are coarsest at the stratotype in the west and fine rapidly to the southeast and east (Fig. 15). The large-scale depositional geometry of the clastic Pigeon Creek Formation is wedge-shaped down-depositional dip and lens-shaped along depositional strike.

BOUNDARIES AND AGE BPUNPARIES At the type section, the alluvial elastics of the Pigeon Creek Formation rest disconformably on marine elastics of the Upper Jurassic Twist Gulch Formation, and are disconformably overlain by rocks of the Upper Cretaceous indianola Group (Undifferentiated) (Fig. 11). The bass of the Pigeon Creek Formation is below the lowest exposed channel-form or sheet sandstone that is characteristic of the Pigeon Creek, and is above 55 the uppermost exposed marine Twist Gulch strata (Jurassic) (Hunt, 1950, his unit 85, p.199). At the lower contact. Twist Gulch (Jurassic) is separated from Pigeon Creek strata by the following criteria: (1) red to ochre Pigeon Creek sandstones occur as channel-forms and sheets that are interbedded with massive red to gray mudstones, whereas the underlying beds of red and silty Twist Gulch mudstone are tabular and thin bedded, and exhibit abundant thinly laminated to rippled sandstones and siltstones; (2) lower member Pigeon Creek sandstones are coarse grained to pebbly, and consist of reworked sedimentary detritus, such as chert and carbonate lithic detritus and, to a very minor extent, of feldspar. In contrast. Twist Gulch sandstones are fine grained, contain traces of glauconite (Hardy, 1952; Standlee, 1982), but no chert pebbles, and are markedly feldspathic; and (3) the external geometry and the internal structure of Pigeon Creek beds indicate that deposition occurred under unidirectional flow conditions in a fluvial environments, whereas Twist Gulch strata were deposited under nonunidirectional flow conditions in marine to tidal flat environments. The above described differences in lithologie character and depositional environment that occur at the Pigeon Creek/Twist Gulch contact are consistent throughout the study area. These differences indicate a significant depositional hiatus, which separates the Jurassic Twist Gulch from the Lower Cretaceous Pigeon Creek Formation (Figs. 9,11,16). WEST-CENTRAL CENTRAL AGE DATA D«ko»a Sandtton i ! g).

Figure 18. Sequence-stratigraphic correlation chart for Lower Cretaceous of Utah with Pigeon Creek-Cedar Mountain depositional sequence and selected age data. Compiled from McGookey (1972), Fouch et al. (1983), and Tschudy et al. (1984). 57 The top of the Pigeon Creek stratotype is above the uppermost exposed unit of Pigeon Creek conglomerate and intercalated red m udstones of sheet geometry; it is at the base of the lowest white to yellowish-weathering channel-form sandstone and pebble-conglomerate that characterize strata of the Upper Cretaceous Indianola Group Undifferentiated. The Indianola Group contains quartzite-pebble to coarse-grained quartzose sandstones interbedded with thin-bedded pink clastic limestones. The conglomerates of the Pigeon Creek Formation, however, are much coarser and channel-fills are scarce to absent. The most prominent clasts in the Pigeon Creek rocks are gray to dark gray carbonates, and red to purple and olive green to brown quartzites. The beds overlying the Pigeon Creek stratotype rocks are what Hunt (1950, p.46) identified in the Gunnison Plateau as the "red zone" (i.e. all strata above Hunt's unit 174, p.195), and their base is herein defined as the base of the Upper Cretaceous Indianola Group Undifferentiated in the Gunnison Plateau area (Fig. 9).

The contact between the Indianola Group (undifferentiated) and the Pigeon Creek Formation in the study area is interpreted to represent a disconformity for the following reasons: (1 ) the abrupt disappearance of large and mineralogically unstable carbonate clasts at the contact coincides with the sudden appearance of mineralogically stable white to pink Quartzite pebbles in the overlying Indianola strata; (2) the clast sizes of all lithologies change at the contact from cobble size below the contact to pebble- to granule-, and sand-sized material above the 58 contact (Fig. 11); (3) the abrupt change in depositional character at the contact from sheet conglomerate and intercalated mudstone to channel-form sandstone and detrital carbonate, reflects significant changes in the style of transport and deposition, and in the character of the sediment load.

The abrupt change in the distribution of clast lithologies and the decrease in clast sizes at the top of the Pigeon Creek Formation are attributed to winnowing of Pigeon Creek strata during an extended period of nondeposition, erosion, and bypass prior to Indianola deposition. The differences in bedding, the energy conditions of deposition, together with changes in sediment sources across the contact, not only indicate a period during which a significant break in deposition occurred, but also reflect a change in the character of the depositional systems. The changes at the Indianola/Pigeon Creek contact occur throughout the study area and indicate a major depositional hiatus in the stratigraphie succession (Fig. 16).

In summary. Pigeon Creek strata in central Utah comprise an unconformity-bounded package of rocks (Figs. 9,18) that incorporates Spieker's Morrison(?) Formation and the "typical red conglomerate of Indianola [character]" (Spieker, 1946, p.105). The unconformities bounding the Pigeon Greek clastic wedge show evidence of a significant depositional hiatus. Such basic unconformity-bounded units of regional extent have been described by Vail et al. (1977) a s a separate kind of 59 stratigraphie unit called a depositional sequence (or synthem, according to ISS Classification, 1987).

AGE Palynomorphs of late Albian age were recovered by Standlee (1982, p.367) in Pigeon Creek Canyon (Fig. 8, section 1) 600 m above the base of what was called the Indianola Group (undifferentiated) (S. N. Nelson, personal communication, 1985) and what is here designated base of upper member of the Pigeon Creek Formation (Fig. 11). Pollen assem blages indicating various Late Cretaceous ages (Coniacian to Campanian) (S.N. Nelson, persona! communication, 1986) were recovered during the present study from Indianola Group rocks at various localities in the northern Gunnison Plateau. Nondistinct nonmarine palynomorphs of Late Cretaceous age (D. J. Nichols, written communication, 1981) were recovered in the southeastern part of the Gunnison Plateau (Fig. 8, sections 2,3) from Upper Cretaceous Indianola Group rocks that overlie conglomerates of the upper Pigeon Creek member (Figs. 12,13). The Sanpete Formation (Fig. 16) at its type locality in Six Mile Canyon (Fig. 8) is of Cenomanian(?) to Turonian age (Cobban, 1976; J. H. Hanley, personal communication,1982) and disconformably overlies lower member strata of the Pigeon Creek Formation.

Leaf impressions from sandstones of the upper Pigeon Creek member in the southeastern part of the Gunnison Plateau (Fig. 13) were identified, based on their veination patterns, as "likely examples of 60 primitive angiosperme of early Cenomanian to no older than late Albian age" (S.L. Wing, written communication, 1984). Bivalves belonging to the freshwater genus Protelliptio were recovered by Stuecheli (1984) from a sandy to oncolitic limestone near the top of the lower member in the southern part of the Gunnison Plateau (Fig. 13). Genus Protelliptio has been reported in western North America from rocks ranging in age from late Neocomian and Barremian to middle Cenomanian (J. H. Hanley, written communication, 1982). Palynomorphs found by Weiss and Roche (in press) in the bivalve-bearing limestone have yielded an Albian to Cenomanian age range. Witkind et al. (1986) recovered plant microfossils of Aptian to Albian age in the southeastern part of the Gunnison Plateau from mudstone lenses intercalated in conglomerate beds from a part of the section that is here called upper member of the Pigeon Creek Formation (Fig. 13).

Although some of the plant microfossils from both members are of longer range, evidence from the present study strongly suggests a late Neocomian to Aptian age range for the lower Pigeon Creek member and a late Aptian to predominantly Albian age range for the upper member of the Pigeon Creek Formation (Figs. 9,12,16) (see appendix B for paiynomorph samples). The Pigeon Creek Formation rests disconformably on Jurassic Twist Gulch strata and is separated from overlying Indianola Group (undifferentiated) by a major depositional hiatus that probably encompasses part of the latest Albian through early Cenomanian. Strata equivalent to the Sanpete Formation of Turonian age unconformably overlie the Pigeon Creek Formation.

REGIONAL RELATIONSHIPS OF LOWER CRETACEOUS STRATA BOUNDING UNCONFORMITIES The disconformity at the base of the Pigeon Creek wedge is analogous to the Neocomian to early Aptian unconformity (Figs. 16,17) that marks the base of the Cretaceous in the foreland basin (McGookey, 1972; Weimer, 1984). The formations overlying this regional unconformity include; (1) the Kootenai Formation in Montana (Suttner, 1969; DeCelles, 1986); (2) the Draney-Smoot-Smiths Formation interval, the Lakota and Wayan Formations, and the Gannett Group in Wyoming and Idaho (Ever, 1969; Rubey, 1973; Schmidt et al.,1981 ; Sippel et al.,1981); and (3) the Cedar Mountain and Burro Canyon formations of eastern Utah and western Colorado, respectively (Hale and Van de Graaf, 1964; Young, 1960,1970; Stokes, 1972; Tschudy et al.,1984; Vingling, 1987) (Fig. 18). In the study area, the Neocomian to early Aptian unconformity is interpreted to have resulted from erosion during earliest foreland basin deformation. During this time, depositional systems in east-central Utah remained largely unaffected by Cordilleran deformation and foreland basin subsidence.

The disconformity truncating the top of the Pigeon Creek Formation in west-central Utah is correlative with the depositional hiatus that resulted from the southwestward advance of the incipient seaway in Utah during the middle Albian to Cenomanian to early Turonian (Figs.16, 17) (Lessard, 1970,1973; Fouch et al., 1983; Vuke, 1984; Weimer, 1984; 62

Figure 17. Sequence-stratigraphic correlation chart for Lower Cretaceous of Utah and adjacent areas. Left side of diagram shows west to east relationships and right side north to south relationships of various formations. Compiled from McGookey (1972), Wiltschko and Dorr (1983), Fouch etal. (1983). 63

Canada

j \ Kootenai Fm. > I Gannett Group •'. Cedar ivitn. Fm. Nev. Tutah^g^

Pigeon | Creek

(Lower) Co]o.

Burro Canyon Fm.

Figure 18, Neocomian to early Aptian paleogeography of Utah and adjacent areas. Paleotopographic highs are stippled. Adapted from Young (1960,1970), McGookey (1972), Wiltschko and Dorr (1983), Weimer (1984), DeCelles (1986), and Peterson (1986). 64 Schwans, 1986a). Regional onlap against this surface is expressed, for example, by the Muddy Sandstone and the Dakota Sandstone that onlap the upper Cedar Mountain Formation along an erosional contact in central Utah (Young, 1960; Yingling, 1987), and by the marine Tununk Shale, which is correlative to the Sanpete Formation in the study area (Figs.17, 19). The transitional alluvial to marine strata overlying the Albian to Cenomanian unconformity reflect progressive marine deposition during widespread and unrestricted foreland basin subsidence in the Albian to Turonian. Widespread foreland basin subsidence was caused by thrust load emplacement in the fold-thrust belt of west-central Utah during the late Aptian to Aibian.

STRATA FORMING THE DEPOSITIONAL SEQUENCE The depositional sequence of regional extent is composed of the Neocomian to late Albian Pigeon Creek Formation of west-central Utah and the Neocomian to Aptian and Albian Cedar Mountain Formation of east-central Utah. Together, these formations record foreland basin deposition and deformation between the time of onset of earliest tectonic uplift in the Sevier belt and onset of marine deposition during the middle Albian to early Turonian.

The gross similarity in stratigraphie position and chronostratigraphic range of Pigeon Creek and Cedar Mountain rocks has led various workers to propose incorporating the strata here called Pigeon Creek Formation in the Cedar Mountain Formation (Fig. 9) (Standlee, 1982; Witkind et al.. 65

Canada < \ as-:| p -îzv ::c-r-:f:-> r_ - 1 I N Vt~_~_MOWRY ~ I I '> V - ~ - S E A - _ - -- r

? ! (Aspen - — — _ I Wayan Fm >J / S h a l e M ow ry-

I /li. Sandstone f Wyo. Pigeon I ^ D ik M a 's 'iS ’dswiTe"-

(Upper) '

Cedar Mtn.» irP I Fm.

Figure 19. Late Aptian and Albian paleogeography. Note upper Pigeon Creek conglomerates adjacent to emergent thrust terrane. Seaway is developing southward (dash pattern). Adapted from McGookey (1972), Wiltschko and Dorr (1983), Vuke (1984), Peterson (1986). 66 1986; Weiss and Roche, in press). The establishment of a composite Cedar Mountain/Pigeon Creek stratotype under the name, Cedar Mountain Formation, however, would result in the formation of a stratigraphie unit incorporating rocks that are distributed over a large area and display distinctly different lithologie, depositional, and stratigraphie characteristics. The grouping of these strata in the same formation would thus equate strata that are very different by exactly those characteristics that mark them as stratigraphie units of formational rank.

The following differences characterize the Cedar Mountain and Pigeon Creek Formations:

{1)Stratotype Subdivision : Pigeon Creek strata comprise a finer grained lower member and a coarse-grained upper member, whereas the coarse Buckhorn Conglomerate member of the Cedar Mountain Formation underlies the Cedar Mountain shales and sandstones (Fig. 16).

{2) Dimensions, Geometries : Pigeon Creek strata parallel the structural strike of the thrust-fold belt, are thickest nearest the thrust front, and thin to the east. They are absent east of the western base of the Wasatch Plateau and occur only within a restricted basina! area adjacent to the thrust front (Fig. 15). Conversely, Cedar Mountain rocks are of varying stratigraphie thickness and widely distributed throughout east-central Utah, and pinch out repeatedly against the intermittent 67 topographic highs (Fig. 20).

(3) Dispersal, Provenance : The provenance and dispersal of Pigeon Creek elastics are directly related to thrust initiation in the Cordilleran belt and subsequent denudation of the allochthonous complexes during the Neocomian to late Albian (Fig. 15). Dispersal directions and modal clast pebble data, as well as the erosional relief at the Buckhorn Conglomerate/Morrison contact, however, indicate that Buckhorn Conglomerate (lower Cedar Mountain Formation) deposition occurred in various superposed, east- to northeastward-shedding incised drainage systems. Chert is the most abundant constituent in the Buckhorn Conglomerate (Fig. 20). Yingling (1987) suggested derivation from Paleozoic sources to the southwest and south. In contrast to the conglomerate clast composition of the upper Pigeon Creek member, pebble compositions of sandstone channel bodies of the upper Cedar Mountain Formation show significant amounts of chert within the system, reflecting the influence of local source areas during the Aptian and Albian within Utah and south of Utah (Fig. 20) (Young, 1960,1970; Peterson, 1986). Associated paieocurrent vectors document an eastward to northeastward dispersal (Young, 1970; Yingling, 1987). At the sam e time, conglomerates of the upper Pigeon Creek member were shed eastward off the rising fold-thrust belt (Fig. 20).

{A)Stratigraphie Relationships Lateral : continuity between exposures of the Pigeon Creek and Cedar Mountain Formations has not 68

UCM

U ta h

LCM

!BC UPC

LPC

Figure 20. Schematic representation of Early Cretaceous paieodispsrsa! patterns and modal pebble/clast lithologies. Location 1, paleodispersal of lower (shorter arrow) and upper Pigeon Creek member (longer arrow); location 2, paleodispersal for Buckhorn Conglomerate exposure (Yingling, 1987); location 3, regional paleodrainage direction for the Cedar Mountain Formation (Young, 1980,1970). Small arrows, paleovectors from sandstone channel bodies of upper Cedar Mountain Formation (Harris, 1980; Yingling, 1987). BC, Burro Canyon Formation; CM, Cedar Mountain Formation. Clast lithologies: LCM. Lower Cedar Mountain; LPC, Lower Pigeon Creek; PC, Pigeon Creek Formation; UCM, Upper Cedar Mountain; UPC, Upper Pigeon Creek. 69 been documented. The Cedar Mountain Formation contains numerous, well documented intraformational disconformities of varying chronostrati graphic range (Young, 1960; Tschudy et a!., 1984), whereas the Pigeon Creek Formation comprises a 975 m-thick wedge of syntectonic elastics deposited adjacent to the thrust front. Consequently, the Cedar Mountain Formation already represents a composite stratotype where the total range of the unit has yet to be demonstrated. Pigeon Creek rocks, however, do not exhibit depositional breaks within their stratigraphie succession.

In summary, unconformity-bounded units of regional extent are useful for providing a framework for the stratigraphie analysis of sedimentary basins. Depositional sequences are especially useful when the unconformities and associated hiatuses allow deciphering the history of a complex basin and where genetically unrelated units together record the history of basin deformation. Based on the differences presented above, the incorporation of Pigeon Creek strata into the Cedar Mountain Formation, as proposed by various workers (Fig. 9), does not seem warranted. The Pigeon Creek and Cedar Mountain formations are genetically and lithostratigraphically unrelated unconformity-bounded units. Together, however, they record the deformation history of the evolving foreland basin during the Early Cretaceous. The Pigeon Creek and Cedar Mountain Formation can be regarded in that context as a depositional sequence (or synthem, ISS Classsification, 1987). 70 PIGEON CREEK DEPOSITIONAL ARCHITECTURE LITHOFACIES AND FACIES ASSEMBLAGES The lithofacies and the facies assem blages of the 975 m-thick, mudstone-rich and conglomeratic Pigeon Creek Formation document the changes in source area composition and in structural style that occurred with onset of fold-thrust belt deformation. The Pigeon Creek Formation is subdivided into a lower fine-grained member and an upper conglomeratic member (Fig. 16). The lower member consists of, in order of decreasing abundance, mudstone, coarse-grained pebbly sandstone, and limestone. The upper member consists of numerous small-scale, laterally coalesced and vertically superposed conglomerate sheets, associated sandstones, and thin intercalated mudstones (Figs. 11 through 14). Three major lithofacies are distinguished in the Pigeon Creek Formation: sheet conglomerate (60 %); scour-based conglomerate to sheet sandstone (10 %); and associated limestone, mudstone, and sandstone (30 %). Each lithofacies, and if present, each subfacies will be discussed with respect to its environment of deposition, lateral relationship to other subfacies, and its tectonic and/or climatic implications.

■SJjEEIiC.QN.gLOMER^E.BQl.ES Description Pigeon Creek conglomerates accumulated in sheet-like bodies that are a few hundreds of meters to a kilometer wide, and as much as 10 m thick. Each sheet consists of several 2 m- to 5 m-thick, upward-fining 71 conglomerate sequences with Intercalated mudstones and sandstones. Individual upward-fining sequences within the conglomerate sheets are interconnected and coalesce laterally, forming large-scale sheets. The large-scale conglomerate sheets either form a thick wedge of vertically and laterally coalesced sheets separated by 0.5 m- to 1 m-thick, laterally discontinuous mudstone and sandstone beds (Fig. 21), or are interbedded with mudstones tens of meters thick (Fig. 22).

The discontinuous beds of mudstone and sandstone are 0.1 m to 0.2 m thick and drape underlying conglomerates. The contacts between the sequences are sharp, but rarely erosional. Well-defined conglomeratic channel fills are scarce within sheets. The sheets have planar to slightly undulatory bases developed directly on underlying mudstones (Fig. 23). Basal surfaces exhibit small scale (10 cm to 50 cm deep) gravelly furrows, obstacle scours, and abundant flute, load, and drag marks (Fig. 24). Margins of individual large scale sheets are dominated by sandstone featuring small channel fills. These channel fills commonly have extensive wings of siltstone that interfinger laterally with the mudstones surrounding the conglomerate sheet. Sheet margin sandstones exhibit well developed cross-stratification and are extensively bioturbated. In sections more distal to the tectonic front, individual large-scale conglomerate sheets overlap laterally, and are offset vertically by thick mudstones. 72

Figure 21. Sheet conglomerate facies in Little Salt Creek Canyon (LSC in Fig. 8) in the upper member with 2 m- to 5 m-thick sheets (between solid lines). Scour-based conglomerate channel-fills occur on top of sheets (channel bases dashed). View is to the south. 73

Figure 22. Pigeon Creek depositional sequence (see section 2 in Fig. 8) (overturned to right). Lower member mudstones overly Jurassic (J) strata along basal disconformity (solid line to left). Overlying upper member (right of dashed line) is truncated in turn by Tertiary (T) strata along angular unconformity (solid line to right). View is to the south. 74

Figure 23. Exhumed base of upright-standing conglomerate sheet in the upper Pigeon Creek member (between section 2 and 3 in Fig. 8). View is to the south. Note arrow at right pointing to scale next to hammer (45 cm long). 75

Figure 24. Close-up of conglomerate sheet base shown In Figure 23. View Is to the west. Scale Is 15 cm. 76 The conglomerates commonly show crude normal grading, with clasts oriented subhorizontally to bedding. The conglomerates exhibit poor sorting, a polymodal grain-size distribution, an average maximum diameter of 5 cm to 50 cm, and are clast-supported. An upward-fining sequence ranges from unstratified and disorganized at the base, to crudely horizontally stratified toward the top. This upward increase in the degree of internal organization is accompanied by an overail grain size decrease from cobble-boulder sizes at the base to cobble-pebble sizes at the top. Fine-grained, horizontally laminated or rippled sandstones cap most of the conglomerate sheets.

Vector m eans of clast orientations, with a(t) b(i) imbrication and of elongation directions of gravelly furrows, yield a southeastward transport direction for lower member Pigeon Creek conglomerates, whereas vector means of upper member sheet conglomerates show eastward dispersal (Fig. 15). Furthermore, clast sizes in the conglomerates of the upper member decrease away from the stratotype, suggesting a dispersal direction similar to that obtained from associated imbrication data (Fig. 15).

Interpretation The Pigeon Creek sheet conglomerate facies was deposited in a sheet flood-dominated aliuvial fan system proximal to the thrust front. The paleoflow directions and the facies character reflect the supply of conglomerate and minor sand from adjacent tectonic uplands onto 77 gravelly fans of low relief oriented transverse to the thrust front. DIstally from the thrust front, conglomerates and to a greater degree sand and mud, formed composite sheet flood deposits, interfingering with and terminating in a muddy floodpiain. This association of conglomerate sheets encased in thick mudstones represents the interfingering of a sheet flood-dominated fan system with a basinal mud zone that lies downstream from, as well as transverse to, the gravelly fan systems.

The depositional geometry, coarseness, poor sorting, and internal organization of the conglomerate sheets is interpreted to have resulted from rapid deposition in shallow water by high discharge flows with relatively high sediment concentrations. Grain size distributions within the conglomerate sheets and the superposition of conglomerate beds and massive mudstones, suggests sedimentation under discontinuous flow conditions with variable flow competence and character of load. The absence of cross-stratification in the conglomerates indicates that significant bed surface topography was not developed to allow avalanching of gravel.

Transport and deposition of the elastics thus occurred under repeated flood stage conditions, in a shallow braidplain featuring extremely variable and discontinuous discharge. Minor floods affected only limited areas within the braidplain and reworked the deposits at various times. Lateral continuity of conglomerate sheets results from the 78 amalgamation and lateral overlap of Individual upward-fining sequences, which was caused by the migration of active fan segm ents and/or the avulsion-controlled switching of braidplain segments.

In his classification of alluvial systems, Friend (1983) described deposits of similar geometry and nature, as deposits of a coarse- to fine-grained, alternating bedload- and supension load-dominated sheetflood system. The lateral overlap and vertical offset of the large-scale conglomerate sheets by thick successions of basinal fines reflects the downstream termination and lateral migration of gravelly braidplains or iow-relief fans in the central mudzone of a continuously subsiding basin. Similar ancient mixed coarse- to fine-grained sheet flood sequences have been described by Stear (1983), Wells (1983, 1984), and Van Der Meulen (1986) as typical sedimentary fill of an actively subsiding basin in close proximity to tectonic source. Stratification similar to the Pigeon Creek type has been described, for example, by Rust (1978) and Hein and Walker (1977) from modern proximal-braided outwash fans. Ferguson and Werrity (1983) related modern mixed conglomerate and sand sheets, similar to the Pigeon Creek type, to repeated flood-stage deposition with minor episodes of reworking. 79 SCOUR-BASE CONGLOMERATE AND SHEET-SANDSTONE FACIES Description Pebbly to cobbly conglomerates and coarse to pebbly sandstones occur gradationally together and form sandy conglomeratic lenses, channel bodies, and sheet sandstones (Fig, 25). The conglomerate and sandstone facies form individual upward-fining sequences 2 m to 5 m thick. The facies commonly overlie the sheet conglomerate facies or occur as clusters of pebbly channel-form sandstone bodies within the basinal mudstones. Several subfacies are distinguished within the conglomerate and sandstone facies: scour-base conglomerates (50 %), channel-form trough crossbedded conglomerates to pebbly sandstones (30 %), and sheet sandstones and siltstones (20 %). The subfacies typically overly the sheet-conglomerate facies and comprise channel-form conglomerates above a scoured contact grading upward into trough crossbedded sandstones capped by extensive sheet sandstones and siltstones (Fig. 25).

The scour-base conglomerate subfacies is trough cross-stratified, texturally mature, and fine-grained, with average clast sizes of 2 cm to 5 cm in diameter. The subfacies exhibits well-developed imbrication and a polymodal grain-size distribution. Individual beds are lenticular, with distinctly erosional, sharp, and convex-upward bases. The channel-form subfacies commonly rests on the sheet conglomerate facies (Fig. 21).

The trough crossbedded, pebbly sandstone subfacies exhibits internal scours and reactivation horizons with gravelly lags. Trough 80

Figure 25, Upward-fining conglomerate to mudstone sequence (between solid lines) (overturned to right). Sequence is 2 m to 4 m thick and comprises a basal conglomerate sheet (CS) overlain by massive mudstones (F). Sp, channel-form pebbly sandstones: Shr; horizontally bedded and rippled wings of sheet sandstone. Note scale of 1 m. Just north of section 2 in Figure 8. View is to the south. 81 cross-stratification varies in scale, with large troughs found near the base and small-scale troughs and climbing-ripple stratification near the top of the facies. The crossbedded, pebbly sandstones constitute small symmetrical scour-base channel-fills that occur at the top of the scour-base conglomerates.

The channel-form sandstones show prominent red-white mottling and bioturbation along their bases and in the lateral wings (Fig. 26). Individual burrows are typically several millimeters to a centimeter in diameter, cylindrical in form, vertical to oblique to bedding in orientation, and in some cases exhibit meniscus-shaped backfilling. The burrows can be identified as Muensteria sp. based on the lack of wall structure, the ornamentation, and the cylindrical morphology. A second type of burrow without wall lining, ornamentation, and branching, but with a structureless fill, is interpreted as Skolithos sp.

The white mottles are found at the bases of channel-forms and appear as white halos around large individual burrows, or, if irregularly shaped, incorporate numerous burrows (Fig. 26). The mottled areas are bleached, possibly due to a depletion in iron. Underlying mudstones are iron-stained (e.g., no iron depletion) and feature calcite concretions, which are scattered along the base of the channel-form sandstones. The channel-form sandstones contain abundant macerated plant materia! with one horizon exhibiting a concentration of well-preserved impressions of primitive angiosperm leaves that were identified as late Figure 26. Exhumed base of channel-form sandstone with pedogenic mottling around burrows of Skolithos sp. and Muensteria sp. Exposure is near section 2 in Figure 8. View is to the west. 83 Albian to no older than Cenomanian (S. L Wing, written communication, 1984).

Channel-form sandstones grade upward and laterally into finer grained sheet sandstones and siltstones, capping the sheet conglomerate facies. The sheet sandstones to siltstones are commonly overlain by mudstones. The sandstones and siltstones may also form extensive wings to the channel-form sandstones, when these occur as clusters of channel-form bodies within thick mudstone. The sandstone sheets are horizontally stratified at their bases and exhibit climbing-ripple stratification near their tops. Paieocurrent data from trough cross-strata in sandstones of the lower Pigeon Creek member show a wider spread of vector means than the southeastward dispersal direction obtained from imbrication data of associated conglomerates (Fig. 15).

Interpretation The three subfacies are interpreted to represent the reworking of gravel, and the deposition of sand and minor gravel, in a predominantly sandy ephemeral bedload system along the lateral and downstream margins of the gravelly fan systems. The superposition and lateral association of the three subfacies represent waning flow conditions and the surficial reworking of the larger conglomerate sheets along sheet margins and on sheet tops. The Infilling of active lateral and cross-over channels that formed with post-flood bar em eigence produced the conglomeratic to sandy scour-based channel fills on top of the larger 84 conglomerate sheets. Sand and minor gravel deposition within these channels occurred in small transverse and lateral bars. Avulsion-controlled rapid migration of the predominantly sandy bedload channels caused minor reworking of the tops of the conglomerate sheet and deposition of sand-sized to silt-sized material in nearby overbank areas. Sheet sandstones, deposited as wings to the shallow symmetrical bedload channels, drape the conglomerates or were deposited along the margins of the larger conglomerate sheets. Minor floods in the various segments of the shallow-braided braidplain and the lateral, as well as downflow margins of the braidplain resulted in; (1) infilling of channels on bar platforms by sandy bar-top wash-over (crossbedded channel-form sandstones), (2) the deposition of horizontally stratified sands in overbank areas and on bar tops by sheet flow (sheet sandstones), and (3) the transport of sandy bed load into floodpiain areas adjacent to the braidplain via small symmetrical sandy bedload channels (channel-form clusters). The dispersion of the vector means of the three subfacies reflects the variations in flow directions with waning flood stage, braidplain margin deposition, and/or bar-top washover flow during minor flood events (Fig. 15).

The repeated modification of braided stream deposits by minor floods is common in modern braided streams (Williams and Rust, 1969; Costello and Walker, 1972). The transition from conglomeratic sheet flood deposition during flood conditions to conglomeratic incised and sandy shallow-braided channels during waning flow conditions is known 85 from modern sheet flood systems of distal alluvial fans. Blair (1987) and Ferguson and Werrity (1983) documented a variety of sedimentary structures similar to those found in Pigeon Creek rocks for sediments deposited under unconfined flow regimes on modern alluvial fans. The transition from sheet conglomerate to clusters of channel-form sandstones to mottled, flat sheets of sandstone and siltstone both downflow and laterally, as has been illustrated in the Pigeon Creek system (Figs. 12,13,14), is well documented from other ancient sheet-flood-dominated braided stream environments (Larsen and Steel, 1978; Hubert and Hyde, 1982; Wells, 1984).

LIMESTONE TO MUDSTONE TO SANDSTONE FACIES Description The lower member of the Pigeon Creek Formation consists of massive mudstones with carbonate nodules (50 %), interbedded thin siltstone beds (10%), channel-form limestone-pebble to oncolitic sandstones (30 percent), thick micritic limestones (7 %), and several thin sandy limestones (3 %). The micritic limestones form 0.5 m- to 10 m-thick units that are laterally continuous for several hundreds of meters to a kilometer. Several sandy and thin limestone units no thicker than 2 m are found at various stratigraphie levels in the lower member (Figs. 12,13). The thin sandy limestone units are laterally continuous over distances of 5 m to 10 m and occur adjacent to the ehannel-form sandstone subfacies. The micritic limestones are massive or thick-bedded to well-laminated, and contain sparse, sandy layers or laminar 8 6 concentrations of densely packed, ovoid to spheroidal grains of micrite and/or larger oncolites. In thin sections, lamination is typically disrupted or destroyed by nodular and banded calcrete crusts. In addition, micritic intraclasts and small pisolitic and oncolitic grains are coalesced in oolitically textured zones, which are laterally discontinuous and commonly capped by irregular calcrete laminae. Nuclei of the ooid grains consist of micritic fragments, or, in some cases, pebbles and/or bivalves. In the outcrop, the limestone subfacies grades laterally and vertically into ripple-stratified calcareous sandstones and channel-form sandstones.

Macrofossils in the limestones include molds of gastropods and bivalves, and abundant charophyte debris. Bivalves belonging to genus Protelliptio (J. H. Hanley, written communication, 1982) were found in one horizon in the upper part of the lower member of the Pigeon Creek Formation (Fig. 13).

Units of channel-form limestone-pebble to oncolitic sandstones form clusters of channel bodies (10 m to 50 m wide and 1 m to 5 m thick), which overlie limestones along convex-upward contacts, and occur along the depositional edges of the limestone beds. These calcarenites are dolomitic and commonly exhibit abundant internal scours and gravelly lags. Stratification consists of large- to smali-scaie trough cross-stratification. Thin-bedded and rippled siltstones and sandstones form wings to the small channel fills and are intercalated 87 in the massive mudstones.

The mudstones are 10 m to 50 m thick and are Intercalated sandstone channel bodies or clusters of sandstone channel bodies. The mudstones exhibit repeated intercalations of red to mottled mudstone horizons, or dark gray to gray to white mudstone horizons, as well as thin, cross-laminated sand and siltstone layers. The dark gray to gray mudstone horizons are characterized by a distinct pedogenic fabric featuring slickensided surfaces. The white mudstone horizons contain abundant burrowed zones, and stacked zones or beds of coalesced calcrete nodules. The gray mudstone units and calcrete zones are commonly found in lateral association with the channel-form sandstones and the siltstone wings. Individual calcrete nodules range from 1 cm to 5 cm in diameter (Fig. 27), whereas coalesced nodules are as much as 30 cm in diameter and show bands of laminated calcrete along their tops.

The pedogenic fabric in the dark gray mudstones consists of a blocky internal structure, individual blocks are prismatic and are bounded by well-developed cleavage planes with slickensided surfaces that generally meet at steep angles and form wedge-shaped units (Fig. 27). Thin section studies by Stuecheli (1984) of the prismatic mudstone blocks revealed a similar microfabric within the macroscopic peds; domains of finely layered, oriented clays bind prismatic fields of mors randomly oriented clay. Smectite and mixed-layer clays are the dominant clay minerals. Figure 27. Plugged calcrete horizon in lower Pigeon Creek mudstones (south of section 3 in Figure 2) with distinct macrofabric. Intersecting cleavage planes with slickensided surfaces (arrows) form wedge-shaped peds. Surfaces converge in a downward direction (arrows) with respect to former paleosurface. View is to the northeast. 89 Interpretation The limestone, mudstone and sandstone facies association is most prevalent in the lower member of the Pigeon Creek Formation and represents overbank deposition along the margins of the sheet-flood fan system and sandy to muddy flood basin deposition in a basinal mud zone. The facies association is interpreted to constitute the floodplain systems that existed in the axial part of the basin, which was oriented parallel to the tectonic uplands and periodically supplied with detritus by the ephemeral fan systems.

The channel-form sandstone facies represents a mixed-load ephemeral braided stream environment, while the mudstone and limestone facies represent playa lake and flood basin deposits that were repeatedly modified by fluvial processes during periods of inundation. Burrowing, plant growth, and postdepositional pedogenic processes reworked the mudstones and destroyed most sedimentary structures in the flood basin deposits. Sandy stream sedimentation in flood basin areas occurred with ephemeral inflow into overbank areas and the floodplain. Active stream deposition thus occurred in immediate proximity to flood basin suspension deposition.

Mudstone microfabrics similar to those in the Pigeon Creek mudstones are produced by modern pédologie processes characterizing the vertisols (Brewer.1976; Ahmad, 1983). According to Brewer (1976), modern vertisol structure is developed by the behavior of clay minerals 90 during seasonal wetting and drying cycles, which result In a sequence cf ped disruption, development cf shear planes, clay orientation, and disintegration into prismatic fields. The shrinkage and expansion of the clay during repeated wetting and drying forms a system of cross-cutting cracks and planes. On that basis, the Pigeon Creek mudstone fabric and the near-surface to surface accumulation of pedogenic calcrete Is Interpreted to have resulted from the repeated wetting and drying of ephemerally Innudated flood basins under a seasonal wet-dry climate. Sell formation probably occurred for extended periods of time, whereas other portions of the floodplain may have been subjected to relatively permanent saturation.

At the same time mudstones were undergoing pedogenesis, carbonates accumulated In shallow and unstratified playa lakes. The shallow water bodies shrank and expanded with ephemeral Inflow into the flood basins. Repeated subaerial exposure of the carbonates caused brecclation, mud cracking, bloturbation, and calcrete formation. Shoreline oscillations allowed the fluvial modification of sediments along the playa lake margins. Channel bodies with limestone-pebble and oncolltic channel-fllls, which either truncate the lacustrine deposits or occur along the deposltlonal edges of the playa deposits, are Interpreted to represent small bedload channels that discharged into the lacustrine depressions. The spatial distribution of the channel bodies suggests rapid lateral migration of channels by avulsion. Two distinct channel-margin environments are recognized: (1) laterally continuous. 91 horizontally stratified sheet sandstones that were deposited in crevasse splays extending into the surrounding floodplain; and (2) thin, rippled siltstones deposited immediately adjacent to the channel margin. Bed load transport in channels occurred near suspension deposition in the playa lakes.

The mechanism for the deposition of large amounts of fines close to gravelly fan systems is poorly documented. Regardless of channel pattern, however, all rivers experience overbank deposition via crevasse channels and sheet floods. The supposition that all braided rivers are so laterally unstable that fioodplalns do not exist is probably an expression of modern studies of braided alluvial fan systems (Bridge, 1985). It is thus erroneous to assum e that the association of large gravelly sheets and large amounts of fines Is incompatible with alluvial fan to braided stream and flood basin deposition models.

Consequently, the large amounts of fines and intercalated conglomerate sheets In the lower member of the Pigeon Creek Formation, are interpreted to represent a sheet flood system consisting of a wide braidplain terminating in a muddy floodplain. The Pigeon Creek sheet flood system lay fixed through time, forming a bajada and the thick basin-fill adjacent to the tectonic uplands. Ancient associations of large amounts of fines with gravelly fan systems, similar to that of the Pigeon Creek, have been described by Ramos and Sopena (1983), Stear (1983), Wells (1983), and others. Van Der Meulen (1986) described a gravelly. 92 sheet flood-dominated braidplain terminating in a muddy floodplain from the Buntsandstein of Spain. Graham (1983) and Tunbridge (1984) described mixed massive clay and sandy sheet flood deposits in the distal areas of ephemeral Devonian braided streams.

FORELAND BASIN DEFORMATION AND DEPOSITIONAL RESPONSE THE MODEL Tectonism and climatic variability strongly influence the deposltlonal style and lithologie composition of sediments in actively subsiding foreland basins (Miall, 1970; Heward, 1978a, 1978b). Climate controls weathering and the variability of discharge, thereby affecting the relief and sediment flux within a basin (Garner, 1959; Schumm, 1976,1981). Tectonism and basin subsidence affect the broader features of the basin architecture, such as the aggradation rates and channel densities of rivers (Allen, 1978; Smith and Smith, 1980), internal and external forms of sediment bodies (Leopold and Wolman,1957; Friend, 1978,1983), and the positions and orientations of rivers (Friend, 1983; Miall, 1983). The greatest thickness of foreland basin fill occurs adjacent to the thrust front, as a result of increased subsidence with the flexure of the lithosphere near the combined thrust and sediment load (Jordan, 1981). Rates of uplift and subsidence, the changes in climate, together with source rock character determine the rate of siiiciclastic influx to the basin, thus providing the necessary base-level control for the diversion of river patterns (Garner, 1979; Miall, 1983). 93 A key element of the foreland basin fill is its syntectonic character. Conglomeratic alluvial sequences in basin fills have commonly been interpreted to record major phases of basin alluviation caused by pulses of uplift (Miall, 1970; Steel et al., 1977; Steel and Aasheim, 1978). Less obvious and less often discussed, however, is the significance of finer grained sediments in tracking the timing of thrusting recorded by clastic wedges in a foreland basin. Furthermore, the impact of lithologically and structurally diverse source terranes on foreland basin fill patterns remains with few exceptions, largely unanswered (Graham et al., 1986; Lawton, 1986; Schwab, 1986).

Floodplain and alluvial fan systems adjacent to the thrust front of an incipient foreland basin are important components in the spectrum of sedimentary environments that are modified by basin subsidence. The age of the elastics and their relationship to the various subenvironments of the alluvial fan and floodplain system are a reflection of the time and the rate of uplift. An abrupt increase in the degree of basinal preservation of the finer grained, as well as the coarser grained, elastics indicates that increased rates of basinal subsidence coincide with pulses of uplift.

APPLICATION TO THE PIGEON CREEK FORMATION The lithologies of the Pigeon Creek Formation, their compositional changes, and the association of facies offer great interpretive potential for modeling both emergence and erosional stripping of thrust terranes 94 and foreland basin subsidence caused by thrust load emplacement. Pigeon Creek strata were deposited adjacent to the incipient Cordilleran belt at the western edge of a broader Rocky Mountain foreland. Accumulation of Pigeon Creek strata was independent of clastic deposition in east-central Utah, where erosional and remnant topographies acted as interfluves to the Early Cretacaceous drainages. Pigeon Creek provenance was directly related to the discrete tectonically controlled sources of the thrust-fold belt. Partial unroofing and dissection of incipient uplifts caused successive erosional exposure of the Mesozoic and Paleozoic and Proterozoic strata, resulting in an inverted stratigraphy in the Pigeon Greek elastics.

The thick mudstones of the lower Pigeon Creek member were derived from source terranes dominated by fine-grained Mesozoic strata. Uplift of source areas may have been anticlinal above subsurface thrust ramps or along the leading edge of the emergent thrust. Erosive breaching of the anticlinal uplifts provided upper to lower Paleozoic detritus to the drainages. Initial basin subsidence during early anticlinal uplift in the Neocomian to early Aptian is recorded by the preservation of large amounts of fines in a distinctly basin-shaped area adjacent to the tectonic uplands. Basin subsidence was restricted to areas proximal to the thrust front. Strata of the lower Pigeon Creek member were deposited in ephemeral conglomeratic to sandy sheet flood fan systems that were transverse to and terminated in a mud-dominated flood basin featuring playa lakes (Fig. 28a). Lower member facies associations and 95

Pigeon Creek Fm .- Deposltional Model

Idealized Oepositional Sequence AND FACIES

Figure 28. A, Pigeon Creek depositional model for Early Cretaceous alluvial foreland basin areas proximal to fold-thrust belt. Laterally overlapping wet/dry fan bodies form a low-relief bajada adjacent to the thrust front. Basinal flood plains are dominated by overbank fines and playa lakes. B, Idealized Pigeon Creek depositional sequence with major lateral and vertical facies associations and environments. 96 dispersal directions indicate that deposystems were deflected to parallel the axis of the basin and the incipient thrust front.

Subsequent hanging-wall emergence, thrust-nappe formation and thrust-loading changed base-level in the basin more significantly during the late Aptian and Albian. The erosional breaching of thrust-nappe complexes most likely occurred along fracture zones and structural lows of the allochthon (see chapter 2 for detailed discussion). Consequently, upper Paleozoic carbonates and Proterozoic/Cambrian quartzites became increasingly available to the Pigeon Creek drainages. The conglomeratic elastics of the upper member were deposited in ephemeral, sheet flood-dominated fan systems that prograded eastward over the mudstone-dominated lower member (Fig. 28a). The preservation of a 600 m-thick wedge of conglomerate in the basin adjacent to the thrust front indicates that the basinal response was rapid, with subsidence now less restrained and less restricted to discrete areas proximal to the thrust front.

CONCLUSIONS Based on lithostratigraphic, chronostratigraphic, and depositional data presented, the incorporation of Pigeon Creek strata into the Cedar Mountain Formation, as proposed by various workers, does not seem warranted. The Pigeon Creek and Cedar Mountain Formations together, however, form an unconformity-bounded unit of regional to interregional extent. The apparent differences in the depositional and formational 97 character of the two formations comprising the Pigeon Creek-Cedar Mountain depositional sequence are useful for developing an understanding of sedimentation response to earliest foreland basin evolution.

Earliest foreland basin deposition in Utah is marked by the thick sequences of syntectonic Pigeon Creek strata that were deposited proximal to the incipient thrust front in restricted areas of subsidence. Synchronous deposition of thin Cedar Mountain strata to the east, however, occurred throughout a vast and unrestricted area showing little or no subsidence. These apparent differences in early foreland basin subsidence between areas proximal and distal to the thrust front are also documented by the existence of multiple intraformational unconformities in the Cedar Mountain Formation, which are absent from the Pigeon Creek Formation.

Earliest foreland basin subsidence in Utah as shown by this study was initially restrained and restricted to areas most proximal to the thrust front. Contrary to other workers, the accumulation of thick flood-basin and lacustrine fines in a narrow and elongate basin during the Neocomian to early Aptian is interpreted in this study to document the onset of Sevier deformation and basin subsidence. At the same time, areas to the east featured drainages undergoing repeated degradation, rather than aggradation. The formation of a discrete conglomeratic wedge during the late Aptian and Albian is interpreted to reflect hanging-wall emergence 98 and thrust-terrane erosion, which thus postdates the time of initial uplift and foreland basin subsidence. Subsequent to thrust loading in the fold-thrust belt, large-scale foreland basin deformation was initiated in areas to the east. Similar depositional responses to foreland basin deformation have been described by SchedI and Wiltschko (1984) in the Idaho-Wyoming sector, and by Schwartz (1983) and DeCelles (1986) in the southwestern Montana sector of the thrust-fold belt. More work is necessary, however, to further comprehend the complex patterns of foreland basin deformation during the early stages of subsidence.

The tectono-sedimentologic évolution of the Lower Cretaceous Pigeon Creek Formation in central Utah can be summarized in several phases that reflect the changes in the style of tectonic deformation and basin subsidence.

Phase 1 Thrust-related: anticlinal uplift in the Utah segment of the fold-thrust belt may have developed as early as latest Neocomian and as late as early Aptian time. Erosion of predominantly Mesozoic strata resulted in a high influx of mainly fine-grained detritus into a narrow basin that formed parallel to the tectonic uplands. Uplift-related base-level changes caused basinal alluviation in areas proximal to the thrust front and formed, together with a seasonal wet/dry climate, the distinct ephemeral braided stream, flood basin and playa facies association of the lower Pigeon Creek member (Fig. 28b). Suoh climatic changes may have resulted from the topographic effects of the incipient 99 Cordilleran belt. Drainages synchronously active in east-central Utah, however, remained uninfluenced by fold-thrust belt activities. Buckhorn Conglomerate elastics were deposited in various superposed bed load to mixed-load drainage systems that incised and later filled erosional valleys and gullies in the Jurassic Morrison Formation.

Phase 2 : Thrusting along the Pavant 1 thrust as early as late Aptian and throughout the Albian, resulted in thrust emergence and thrust-nappe formation. The conglomeratic elastics of the upper Pigeon Creek member were shed eastward and prograded over the flood basin fines of the lower member in laterally overlapping sheet flood-dominated fan systems (Figs. 28a, 28b). The gradual thickening of chronostratigraphically equivalent strata of the upper Cedar Mountain Formation toward the northwest during Albian time is an indication that foreland basin subsidence had progressed farther to the east.

Phase 3 : Seaway advances into northern Utah and Colorado during the later part of widespread foreland basin subsidence in the late Albian and Cenomanian resulted in the onlap of the marine Dakota Sandstone against Cedar Mountain strata in east-central Utah. Equivalent Cenomanian(?) to Turonian sandstones of the Upper Cretaceous Indianola Group in the study area overlie the Lower Cretaceous Pigeon Creek wedge with apparent disconformity. 100 ACKNOWLEDGMENTS I am very grateful to Mary Dylewski for her support, unsurpassed editorial skills, and patience throughout this project. I thank Philip J. Stuecheli for his contributions in field mapping and aspects of the sedimentology, and Jam es W. Collinson for his comments and discussions. I am grateful to Chris Schmidt and other reviewers for their critical, but constructive reviews. I thank Steve Jacobsen and Sheldon Nelson of Chevron U.S.A., Inc., for providing the palynologie data, and Jim Geitgey of Atlantic Richfield Oil and Gas Company for identifying the conodonts. Karen Tyler did some of the drafting. This research is part of a Ph.D. dissertation and was supported by grants from The Geological Society of America (Grant #3491-85), the American Association of Petroleum Geologists, Atlantic Richfield Oil and Gas Company, Chevron Field Studies Fund, Shell Western Exploration and Production Company, and by Sigma XI, The Research Society.

[In press. Geological Society of America Memoir 171] CHAPTER ii

EARLY CRETACEOUS FORELAND BASIN SUBSIDENCE PATTERNS AND BASIN FILL ARCHITECTURE

101 10 2

SECTION K.1

INTRODUCTION This chapter presents the most recent research on Cordilleran foreland basin evolution during the Early Cretaceous in Utah, The chapter deals with the effect thrust loading in the Cordilleran hinge zone had on the basem ent that underlies the fold-thrust belt and western margin of the Cretaceous foreland basin in west-central Utah. Lithofacies character, depositional architecture, sediment dispersal, and the resulting sediment thickness distribution are analyzed to reconstruct the configuration of the Neocomian to Cenomanian history of the Cordilleran foredeep, as well as the three-dimensional patterns of subsidence therein.

Numerous two-dimensional modeling studies have recently attempted to qualitatively explain the effects of the various basin-forming factors on sedimentation patterns and basin-fill geometry in the subsiding Cordilleran foredeep (Paola,1986; Fleming and Jordan, 1987; Heller, Angevine, and Hill, 1987; Winslow and Heller,1987). The resulting two-dimensional subsidence profiles show the large-scale geometry of the foreland basin fill as wedge-shaped and thinning toward th'. craton. Detailed studies of sediment thickness distribution and sediment dispersal patterns in Lower Cretaceous Cordilleran basin fill have found, 103 however, that basin configurations do not conform to the standard subsidence profiles generated by the geodynamic models. Instead, Early Cretaceous Cordilleran foreland basin sedimentation occurred within several subbasins or was restricted to a small trough or forerunner basin separating the emergent thrust front from a broad non-subsiding craton. (Schwartz,1982; DeCelles,1986; Schwans.l 985,1987a,1987b, in press; Schwartz and DeCelles,1987; Schwartz and DeCelles, in press). Foredeep subsidence patterns and resulting basin configuration are thus more complex than geodynamic modeling has been able to predict.

This chapter wiii address this most evident contradiction between geodynamic prediction and stratigraphie evidence by reconstructing the Early Cretaceous basin-fill geometries in the Utah segment of the Cordilleran foreland basin within two time-stratigraphic intervals. The alluvial basin fill is analyzed with respect to (1) the syntectonic signature and timing of thrust uplift; (2) the sequence stratigraphy; (3) the depositional environments and resulting geometries; (4) total sediment thickness distribution and sediment accommodation; and (5) foreland basin subsidence patterns. Together these data will allow detailed reconstruction of the three-dimensional subsidence patterns of the Early Cretaceous foredeep.

The subsurface data presented here consist in part of unpublished seismic data that were processed during a research internship at EXXON Production Research Company, Houston, from June through December of 104 1988. The analysis of well log data, correlation of log patterns, and the resulting sequence-stratigraphic interpretations have been released by EXXON Production Research Company (through John H. Van Wagoner, December 1987) for publication. Interpretation of subsurface data is based on the calibration of well logs with outcrop and biostratigraphic data (see appendix 0) that were collected in the 1984 to 1986 field seasons. 105

SECTION 11.2

INITIAL FORELAND BASIN SUBSIDENCE AND SEQUENCE-STRATIGRAPHIC RESPONSE, BARREMIAN TO CENOMANIAN, UTAH

(Peter Schwans)

INTRODUCTION Foreland basin subsidence can be modeled as the downward flexure and regional isostatic adjustment of the lithosphere caused by vertical loads emplaced in the adjacent fold-thrust belt (Beaumont, 1978,1981 ; Jordan, 1981; SchedI and Wiltschko, 1984; Stockmal, Beaumont, and Boutilier, 1986). Regional variations in net subsidence are attributed to differences in the mass of the thrust load and its erosional products, the rates of load emplacement within thrust belt segments, and the inherent rheology of the loaded craton. Fundamental problems still exist, however, with such geodynamic modeling of ancient fold-thrust belts and associated foreland basins.

Estimates of the m ass of thrust sheets in the Cretaceous Cordillera of western North America are greatly complicated by late Mesozoic 106 uplift and erosion (SchedI and Wiltschko, 1984). In addition, subsequent Basin and Range extension (< 35 km) obscured the original structural record and boundaries of the Cordilleran hinterland in Nevada and Utah to a minor extent (Allmendinger, Jordan, and Miller, 1984). Furthermore, most geodynamic analyses model the loaded continental lithosphere as a laterally uniform plate (Beaumont, 1981; Jordan, 1981). Such extrapolation and application of perfectly elastic behavior theory to continental lithosphere, however, is questionable. Geophysical and geological evidence indicate to the contrary that basem ent structures underlying the Cretaceous Cordillera and foreland basin acted as two-dimensional, non-flexural components during Jurassic-Cretaceous compression and thrust-loading. The Peace River Arch in the West Alberta Basin (Steick, 1975; Cant, 1985) and the Sweetgrass Arch in northwestern Montana (Lorenz, 1982), for example, were repeatedly tectonically active during Cretaceous thrusting and clearly influenced sedimentation patterns in the foreland basin.

The purpose of this paper is to use new geologic data to constrain the factors determining Cordilleran foreland basin subsidence and basin geometry. Specifically, sequence-stratigraphic character, lithofacies architecture, and the thickness distribution of Lower Cretaceous sedimentary rocks in the Utah segment of the Cordilleran basin are analyzed to determine the three-dimensional aspects of foreland basin subsidence and the main influencing factors. Previous studies have described the depositional facies and the stratigraphie framework of 107 these initial foreland basin deposits in Utah (Schwans, 1985,1987a, in press). This paper expands on these findings and relates regional shifts in basinal sediment aggradation and accommodation in the basin to changes in foreland basin subsidence. On the basis of geologic and structural data, seismic data published by others, and unpublished drill hole data, this paper demonstrates that differential subsidence of fault-bounded basement blocks near the load had a significant influence on net subsidence, sediment accummulation pattern in the basin, and the configuration of the incipient foreland basin. Contrary to the concept of uniform and large-scale flexural foreland basin subsidence, this study will show that the basem ent first responded in more of an Airy isostatic manner adjacent to the thrust frOnt as a result of the inherent structural weakness of the crust. Flexural foreland subsidence in the rest of the Cordilleran foreland basin occurred only well after initial thrust-loading as large-scale loading in the overthrust belt of northeastern Utah and Wyoming encroached on more structurally coherent and flexurally competent continental crust.

CORDILLERAN BELT AND UNDERLYING BASEMENT HISTORY Compressional tectonism during the Cretaceous superposed supracrustal Cordilleran fold-thrust belt structures onto a structurally complex basement zone called the Cordilleran hingeline. In Utah, the hingeline consists of complexly faulted basem ent marked by structural highs and systems of faults related to the late Proterozoic to early 108 Cambrian rifting of western North America (Armstrong, 1968; Stokes, 1978; Picha and Gibson, 1985; Picha, 1986). In central Utah, the hingeline lies geographically between the Pavant and Canyon ranges to the west and the Wasatch Plateau to the east, and extends south of the Charlston-Nebo thrust far beyond the study area (Armstrong, 1968) (Fig. 29).

The Cordilleran hingeline is thus a diffuse boundary and major structural discontinuity that separates the thicker and unbroken crust of the craton from multiply deformed, thinned, and broken crust. During the Paleozoic the boundary between thinned and unthinned continentai crust separated a stable craton from the subsiding miogeocline to the west. Miogeoclinal strata in central Utah thicken substantially to the west of the hinge zone. For example, 1000 m of Paleozoic section beneath the Wasatch Plateau thicken to a maximum of 11,000 m in westernmost Utah (Standlee, 1982). The Cordilleran fold-thrust belt and the western margin of the foreland basin in Utah evolved within parts of this boundary zone. Fold-thrust belt emergence and foreland basin subsidence during the Mesozoic were closely linked to the tectonic fabric of the underlying basement. To characterize the interaction between thrust load, basin subsidence, and the basement structures at the hinge zone, it is first necessary to identify the major tectonic elements and their response to deformation. 109

UTAH

S tudy^ - A rea

Figure 29. Structural map with elements of the hingeline and the Mesozoic Cordilleran belt.1, basement highs (MA - Moxa Arch); 2, Ephraim subsurface fault (EF); 3, basem ent lineaments including: OFL - Cove Fort lineament, LL = Leamington lineament, SL ■ Scipio lineament, PL - Paragonah lineament; 4, Wasatch fault at Salt lake City (SLC); 5, Mesozoic and Cenozoic thrust plates: 0 - Canyon Range, CN - Charieston-Nebo, G ■ Gunnison, PI - Pavant 1. P2 - Pavant 2, S - Sheeprock, TV - Tintic Valley, W - Wasatch. Modified after Morris (1983), Picha and Gibson (1985), and Villien and Kligfield (1986). 110 THE TECTONIC ELEMENTS PASEMENIABCHES The cratonic basem ent underlying the fold-thrust belt and westernmost parts of the Cordilleran foreland basin features numerous structural highs and lows (Allmendinger et al., 1986; Allmendinger et al., 1987) (Fig. 30). Hansen et al. (1980) indentified a north-south trending uplift west of the Pavant and Canyon Range thrust sheets and defined it as the Fillmore-Maricopa arch (FA). Based on recent COCORP seismic data, Allmendinger et al. (1983) and Allmendinger et al. (1986) interpreted the arch to represent the western edge of the hinge zone and to have served as a major thrust ramp during Cretaceous compression. The arch thus separates the attenuated and weakened crust of western Utah from the structurally complex, thicker crust of the hinge zone (Picha, 1986). Overlying miogeoclinal Paleozoic rocks thicken substantially to the west of the arch.

Several smaller scale, north-south trending basem ent highs have been identified by Picha and Gibson (1985) based on unpublished seismic and well data east of the Fillmore-Maricopa Arch (FA) in the central part of the hinge zone. The arches lie in the subsurface of the Sevier-Juab Valley (SA), beneath the northern part of the Gunnison Plateau, in the southern and northern part of Sanpete Valley (SPA), and south of the Gunnison Plateau (southern extent of SPA) (Fig. 30). Standlee (1982) and Picha and Gibson (1985) defined the eastern margin of the hinge zone to be represented by the upthrown blocks of a prominent basem ent fault zone 111

UtahA I : :v;/ :/ ^ f

Figure 30. Map of study area with the major structural elements of the Cordilleran hinge zone shown in Figure 29. X-Y, structural cross-section of Figure 31; 1, basement lineaments (see Figure 29); 2 , basement highs: FA « Fillmore Arch, SA = Juab-Sevier Anticline, SPA = Sanpete Anticline. Compiled from Sharp (1982), Morris (1983), Picha and Gibson (1985), Allmendinger et al. (1986), and Villien and Kligfield (1986). 112 located beneath the Wasatch Plateau, called the "Ancient Ephraim subsurface fault" (EF) by Moulton (1975). The basem ent surface is offset across the fault zone by 1.4 km to 1 .8 km, with the west side downthrown to 9 km depth (Fig. 30). Hence, the basem ent fault zone also constitutes the western margin of the unbroken craton in central Utah.

CORDILLERAN STRUCTURES The miogeoclinal strata overlying the cratonic basem ent in the hinge zone remained largely undeformed until large-scale thrusting, erosion, local backthrusting and broad folding during the Cretaceous affected the area (Allmendinger and Jordan, 1981). The Cordilleran fold-thrust belt can be subdivided into a foreland and hinterland (Figs 29,30). The frontal zone of the thrust-fold belt in central Utah lies between the Wasatch Plateau and the Sevier-Juab Valley, whereas the hinterland includes the Canyon and Pavant Ranges and most of western Utah (see Fig. 5) (Allmendinger, Miller, and Jordan, 1984). The frontal zone consists of numerous aliochthonous structural assemblages that are detached at various structural levels and bounded by westward-dipping thrusts.

Parts of the Mesozoic structural framework have long been recognized and were described by Christiansen (1952), Black (1985), Brady (1985), Armstrong (1968), Burchfiel and Hickcox (1972), Morris (1983), and by Millard (1983), among others. Based on subsurface information from seismic profiling and well logs, Lawton (1985) and Royse (1983), Standlee (1982,1983), and Villien and Kligfield (1988) described the 113 structural geometry of the major thrust systems of the Sevier belt in central Utah. Individual thrusts are linked in an overall thrust network, which resembles the ramp-flat thrust geometry commonly found in fold-thrust belts.

The hinterland of the fold-thrust belt is characterized by three major Mesozoic thrust systems, which from west to east and in the order of descending structural level are the Canyon Range (0), the Pavant 2 (upper) (P2), and the Pavant 1 (lower) (PI) thrusts (Allmendinger et al., 1986; Villien and Kligfield, 1986 ) (Figs. 29, 30). On the basis of seismic and drill hole data, Standlee (1982,1983) and Royse (1983) showed that the foreland features the early Cenozoic Gunnison (G) and Wasatch (W) thrust systems, which extend as far east as the Wasatch Plateau, where they terminate as blind thrusts in Jurassic strata beneath the Wasatch Plateau (Figs. 29,30). A recent palinspastic reconstruction by Sharp (1984) shows that thrust-shortening in the hinterland in Utah (west of FA in Fig. 30) is on the order of 110 km to 200 km, while subsequent Tertiary Basin and Range faulting resulted in 35 km of overall extension.

The individual allochthons or plates consist of complexly broken and thrusted stratal assemblages. The major detachments act as boundaries to the thrusted assem blages and ultimately sole into constitute the root zones of the thrusts. The deeper Pavant plate (P1) is bounded below by the Pavant 1 and above by the Pavant 2 thrust, based on seismic and well data (Villien and Kligfield, 1986) (Figs. 29,30). The Pavant 1 plate 114 consists of Paleozoic carbonate rocks and Triassic to Jurassic clastic rocks. The lower Pavant 1 thrust is believed to have cut Middle to Upper Jurassic strata in a complex fashion. The higher Pavant 2 plate (P2) is bounded by the Pavant 2 thrust and the overlying Canyon Range thrusts. The plate is well-exposed in the Pavant and Canyon ranges and consists of Cambrian to Silurian carbonates (Millard, 1983). The overlying Canyon Range plate (C) consists of Upper Proterozoic to Cambrian elastics and carbonates. COCORP seimic data indicate that the Canyon Range thrust constitutes the structurally highest detachment surface in the hinterland in western Utah (Allmendinger et al., 1986). Finally, structural pieces of the complex Gunnison and W asatch thrust systems are found east of the frontal zone of the Mesozoic thrusts. The early Cenozoic thrusts resulted in structural duplication and complex folding of the Jurassic section. The overlying Cretaceous section, however, remains unbroken and has experienced only local folding and minor faulting (Allmendinger et al., 1986) (Figs. 30,31).

(ÎAULTS Four major northeast-trending basement lineaments (LL, SL, CFL, PL in Fig. 29) that originated during Proterozoic rifting are oriented perpendicular to the cratonic margin and the structural strike of the fold-thrust belt and transect the Cordilleran hinge zone in Utah (Figs. 29, 30) (Picha and Gibson, 1985, Picha, 1936). Two of these lineaments are recognized in the study area (Morris, 1983) and are easily identified on landsat imagery and high altitude aerial photos. The Leamington GEhCRALEED STRUQTURAL & STRATIGRAPHC CROSS-SECTON

Figure 31. Generalized structural and stratigraphie cross-section along X-Y (Fig. 30). Note that the Cretaceous section remains essentially unbroken above Cenozoic thrusts. Based on well and unpublished seimic data, and Standlee (1982,1983), Royse (1983), Lawton (1985), Villien and Kligfield (1986). 116 lineament (LL) terminates the Fillmore-Maricopa basem ent arch (FA) on the north and separates the Charlston-Nebo (CN) thrust salient to the north from the Canyon Range thrust (0) system to the south (Picha, 1986) (Fig. 30). The western and eastern limit of the Leamington lineament are, however, poorly known. Morris and Shepard (1964) and Morris (1983) assum ed that the nearly vertical fault plane of the Leamington lineament flattens and merges at its eastern extent with the Nebo thrust. The Scipio lineament (SL) in the south parallels the Leamington lineament to the north and constitutes the southern boundary of the Canyon Range allochthon (Picha and Gibson, 1985) (Figs. 29,30). The Scipio lineament seem s to terminate in the complexly broken central part of the Gunnison Plateau (see Fig. 5).

FOLD-THRUST BELT AND BASEMENT INTERACTION Based on surface, seismic, gravity, and drill hole data, Morris (1983) and Picha and Gibson (1985) interpreted the Leamington and Scipio lineaments as major basement structures that originated during Proterozoic rifting. The lineaments were reactivated during the Cretaceous as tear faults or lateral ramps of the eastward-propagating fold-thrust belt. Thrusting north of the Leamington lineament, for example, progressed far eastward and formed the prominent Charlston-Nebo (CN) thrust salient (Fig. 30).

To the south of the Leamington Lineament, the Fillmore-Maricopa (FA) arch acted as a major buttress to the Mesozoic and Cenozoic thrusts (Fig. 117 30). The Mesozoic plates are detached at upper Proterozoic and Cambrian levels and ramped over the westernmost basem ent high. The frontal zones of the thrusts lie just east of the Fillmore-Maricopa arch and consist of complex thrust slice assemblages. Displacement along the structurally complex frontal zones of the Pavant thrusts is thought to have been relatively small (Villien and Kligfied, 1986). The crustal block of the Fillmore arch is separated from the stable craton by a broken crustal zone (Picha and Gibson, 1985), which constitutes the central part of the hinge zone. Basement structures (SA, SPA) located in the central zone probably interacted repeatedly with the thrusts (Fig. 30). Thrust-loading warped the basement and reactivated structures, which in turn buttressed thrusts and determined thrust sheet geometry and stacking pattern (Allmendinger et al., 1986). Farther east, the upthrown block of the Ephraim subsurface fault (EF) acted as the easternmost ramp to the early Cenozoic Gunnison (G) and Wasatch thrusts (W) (Standlee, 1982) (Figs. 30, 31).

THRUST BELT AND FORELAND DEFORMATION THRUST SUCCESSION The previous discussion demonstrated that structural interaction between the Mesozoic fold-thrust belt and the underlying basem ent resulted in thrust sheet buttressing and stacking, and transverse faulting along reactivated basem ent lineaments. The frontal zone of the Cordilleran fold-thrust belt (e.g. Pavant 1) and the western margin of the foreland basin in Utah lie within the central weaker crustal zone of the 118 hingeline (Fig. 32). Therefore, the initial basem ent response near the eastward migrating load was most likely a funotion of the inherent structural weakness of the crust in the region. This, in turn, influenced sediment accommodation and net subsidence in the western part of the foreland basin (stippled pattern) (Fig. 32). In order to relate tectonic deformation to foreland subsidence, basin configuration, and sedimentation in the foreland, it is necessary to reconstruct the spatial relationships between the fold-thrust belt and basinal areas.

Morris (1983) identified the Charlston-Nebo and the Pavant 1 thrusts as sole thrusts based on geologic surface data. The Charlston-Nebo plate is exposed north of the Leamington lineament and comprises a thin sequence of Upper Devonian strata that unconformably overlie lower Paleozoic rocks (Hintze, 1980) (Fig. 32). South of the lineament, the correlative Pavant 1 plate is only known from subsurface data and consists of lower(?) Paleozoic carbonates and minor Mesozoic elastics (Standlee, 1982,1983). Middle to Upper Jurassic footwall strata of the Pavant 1 thrust were drilled, for example, in the Placid Oil WXC USA No. 1-2 well (well 2), which is located in the frontal zone of the Pavant 1 plate just east of the Gunnison Plateau (Villien and Kligfield, 1986) (Fig. 32). Rocks of Middle and Late Jurassic age (e.g. Callovian Twin Creek Limestone overlain by Kimmeridgian to Oxfordian Arapien Shale) were also drilled in the Placid Oil WXC Howard #1 A well (well 1), which is located just east of Chicken Creek Canyon (full circle at C) (Villien and Kligfield, 1986) (Fig. 32). In palinspastic reconstructions Villien and 119

Utah

l i i

Figure 32. Paleogeographic reconstruction of Pavant 1 thrust front (clashed line with sawteeth) and initial foreland basin (stippled). Note the relationship between location of basement highs, lineaments, and outline of thrust front. 1, Placid Oil WXC State; 2, Placid Oil WXC USA 1-2; A-B, C-D are cross-section shown in Fig. 48; black dots depict locations of measured Pigeon Creek sections. 120 Kligfield (1986) found that the Oxfordian strata in the Howard #1 A well were originally deposited in the Arapien basin to the west and were transported eastward by the Pavant 1 thrust, thus placing them in proximity to the nearshore Arapien rocks exposed in Chicken Creek Canyon to the east. In Chicken Creek Canyon they are overlain by nonmarine Aptian-Albian(?) strata, which Hunt (1950) mapped as Twist Gulch Formation, but Schwans (in press) recently identified as basal Pigeon Creek Formation (see Figs 10,11).

On the basis of unpublished seismic data (Villien and Kligfield, 1986) and the drill hole data, this report recognizes the Charlston-Nebo and Pavant 1 thrust episode as the initial and oldest event. The episode was succeeded by thrusting along the structurally higher P avant 2 and U ntie Valley (TV) thrusts (Standlee, 1982,1983; Villien and Kligfield, 1986) (Figs. 29, 30). Morris (1983) directly compared sedimentary strata of the Pavant 2 and Tintic Valley (TV) plates and showed that both consist of well-developed and stratigraphically identical Ordovician to Devonian carbonates. If the structural and sedimentary correlations between these plates are valid, it can be assumed that the Tintic Valley-Pavant 2 plate was thrust onto the Charlston-Nebo and Pavant 1 plates during a later Pavant episode. Renewed minor movements of the Tintic Valley plate may have resulted in eastward and vertical displacement along the Leamington lineament, causing subsequent breakage of the plate and resulting in the decoupling of the Charlston-Nebo plate from the Pavant 1 plate along the fault. 121 A final episode of thrusting emplaced the Canyon Range (0) and Sheeprock (S) plates eastward onto the Tintic Valley-Pavant 2 plate, leaving the Canyon Range and Sheeprock plates as the structurally highest plates (Figs. 29,30). The Canyon Range plate consists of Proterozoic to Ordovician siliciclastics and carbonates (Millard, 1983). The plate is terminated by the Leamington lineament to the north and by the Scipio lineament in the south. Significant east-directed and vertical displacement along the Leamington fault shortly after thrusting caused fragmentation and drag folding in the Canyon Range plate.

TIMING OF THRUSTING Schwans (1985a, 1986b, 1987a) identified a thick succession of Barremian to upper Albian conglomerates and mudstones in Pigeon Creek and Chicken Creek Canyon (Fig. 33) as the earliest syntectonic foreland basin deposits in central Utah, based on stratigraphie position, clast provenance, and structural considerations. These deposits are here referred to as Pigeon Creek Formation (PC) (Schwans, in press) (Fig. 34). The Pigeon Creek Formation is unconformably overlain throughout the region by Turonian alluvial to transitional marine deposits (Ks, Kav) belonging to the Upper Cretaceous Indianola Group (Fig. 34) (Schwans, in press). Recently, however, Cenomanian(?) to Turonian palynomorphs were reported by Villien and Kligfield (1986) from well cuttings of shales and sandstones (KS, Kav), which unconformably overlie hanging-wall strata of the Pavant 1 thrust in the Piacid Oil USA No.1-2 well (well 2 In Fig. 32). Together with alluvial to transitional marine strata and correlative 122

Western Gunnison Plateau/Chicken Creek Canyon

♦ 219- 176, ♦ 21 9 -1 7 7

Figure 33. Mosaic of high-altitude aerial photos showing Pigeon Creek rocks at stratotype (between arrows and dashed lines) in Pigeon Creek and Chicken Creek Canyons and the easternmost extent of the Scipio lineament (complexly broked part of the plateau in the right half of photomosaic). View is to east (top) with outcrop facing w est and strata dipping toward east (top). Indianola Group strata are above upper dashed line and Jurassic Arapien Shale below lower dashed line. 123

AeeCMY) Weat-^rtra. C«% /^(hem|Eas,em/S^.hem y^g^coKyado

QSamm#

Figure 34. Sequence-stratigraphic time-space diagram and stratigraphie correlation chart for Lower Cretaceous foreland basin fill in Utah and western Colorado. Sequence with: BC, Burro Canyon Fm.; CM, Cedar Mountain Fm.; PC, Pigeon Creek Fm. Cretaceous strata above sequence: Kav, Allen Valley Sh.; Kd, Dakota S st; Ksp, Sanpete Fm.; Kt, Tununk Sh. Jurassic strata below sequence: J a [undiff.], Arapien Sh. undifferentiated; Jm, Morrison Fm. 124 strata farther east (Kd, Kt), the Upper Cretaceous rocks in the well and in Pigeon Creek Canyon thus form the overlap assemblage to the Pavant 1 plate and associated erosional products (e.g. Pigeon Creek Formation, PC) (Fig. 34).

Earliest Cordilleran thrusting in central Utah occurred along the Charlston-Nebo-Pavant 1 thrust during Barremian to late Albian (116.5 Ma through 96 Ma, after Haq, Hardenbol, and Vail, 1987) (see chapter 1). The Cenomania (?) and Turonian age of the overlap strata places an upper limit on the final timing of Pavant 1 plate emplacement. Conversely, timing of earliest thrusting is given by the Barremian age of basal Pigeon Creek strata found in the proximal foreland. The Pavant 1 plate acted as the main source for the Pigeon Creek elastics and thus represents the initial Cordilleran allochthon in Utah. Later Mesozoic shortening in central Utah was accommodated by the development of a backward breaking succession of thrusts. This interpretation depends on the presented structural and stratigraphie evidence from well and outcrop data.

SEQUENCE STRATIGRAPHY OF BASIN FILL AGE OF SEQUENCE STRATA The earliest basin fill in the Utah segment of the Cordilleran foreland basin consists of the Pigeon Creek Formation (PC) and the chronostratigraphically correlative Cedar Mountain (CM) and Burro Canyon (BC) Formations of eastern Utah and western Colorado, 125 respectively (Fig. 34). These units form a series and conformable succession of genetically related alluvial strata, which is bounded by unconformities or correlative conformities (i.e., it is a depositional sequence) (Mitchum, et al., 1977; Schwans, in press) (Fig. 34). The unconformities or sequence boundaries are of regional extent and show evidence of varying degrees of subaerial erosion, indicative of a relatively significant depositional hiatus. To provide a first approximation of the changes that occurred in basin accommodation with subsidence, the distribution of stratal ages within the depositional sequence and the age range of the unconformities bounding the stratal package are presented below. Ages are based on several types of fossils, such as ganoid fish scales, freshwater ostracods, pelecypods, gastropods, silicified plant material, pollen assemblages, and a few fission track dates (Table 1, Table 2)

On the basis of available and new biostratigraphic data, Schwans (in press) assigned a Barremian(?) and Aptian to late Albian (116.5? MA through 96 Ma) age to Pigeon Creek strata of west-central Utah (Tablel). Stratigraphie position of specimens and pollen samples from the Pigeon Creek Formation are also discussed by Standlee (1982), Witkind et al. (1986), and by W eiss and Roche (in press). Chronostratigraphically equivalent Cedar Mountain strata in eastern Utah are middle Aptian to late Albian (111 Ma to 96 Ma) in age (Tschudy et al., 1984) (Fig. 34, Table 2). This age range is based on a few distinctive fossil specimens reported in Katich (1951), Stokes (1952; 1972), Yingling (1987), and Table 1. Summary chart of biostratigraphic data-Western Wasat Plateau and Gunnison Plateau. Table shows ranges (heavy bla( all fossil genera that have been reported for the Cretaceous fom in the area, genera are listed by location, name, and by referenc where first reported).

126

Summary chart of biostratigraphic data-Western Wasatch and Gunnison Plateau. Table shows ranges (heavy black lines) of genera that have been reported for the Cretaceous formations 3a. genera are listed by location, name, and by reference (e.g., St reported).

Y CHART OF BIOSTRATIGRAPHIC DATA - WESTERN WASATCH PLATEAU & G

A Hi« « llllW 4 ^ 1 : . # A | p f ‘ : 1 1 , i ! !P , H ! l l i H I M P H i ü ü ü l ^ l i III! l i m ü Z r '%!!! i " ; : m i [ H ! ! ' ' ■ i W i milk iO 1 H 1 I 1 H i 1 U H I . i l i l i I ' 1# ! j j H I j? 1 1 l i ^ i i ! ! ■ I ! ! ; ; IÜ I i i , i ,.i H 1 j H ; i i j 1 I III! i i i 1 W ' ! | | S T i ! ! M i i 'i i i . I n i l 1 i H i 1 11 i 1 ' 1 ! i

127

Table 2. Summary chart of the biostratigraphic data-Eastem Wasatch P lateau an d Book Cliff area . Table show s ranges (Heavy black lines) of all fossil genera that have been reported for the Cretaceous formations in the area. Genera are listed by location, name, and by reference (e.g., where first reported).

SUMMARY CHART OF BIOSTRATIGRAPHIC DATA - EASTERN WASATCH PLATEAU & BOOK CLIP Cedar Mountain f i l l 1 I I i i r i i ; ' m l ' l i m m .

1 1 :

i chart of the biostratigraphic data-Eastem Wasatch : Cliff area. Table shows ranges (Heavy black lines) of all have been reported for the Cretaceous formations in are listed by location, name, and by reference (e.g., îd).

SUMMARY CHART OF BIOSTRATIGRAPHIC DATA - EASTERN WASATCH PLATEAU & BOOK CLIFF AREA Cedar Mountain Datota M|suk St^point Blackhawk Cashegate Price RIvei Tununk Sh. Perron Sst. Bluegate Sh. S^Mbr. Emery Sst. illU!. f * i i '! ■- l i i i i j 1 , lf!T i1; l SÏ l II. ii -ii liirb. f PjJ U i i i i : l F: Ijiii ffl'iiifj;, 1 1! !! : " | i n Til!!': ■'i . ; I ' * ill:iii: i . ( 1 Mi l l ' :!!; .1 " ! . 111II; III 1 1 I ' D V r i‘ f t ' Ihi' I 11 i J' 1 t:i1 'Mi i ' l l 1II f ‘ ii Hi 1 M ilu! II i n i 1 i M 1 13 1 ! I i

128 especially In Young (1960) (Table 2). Recent pollen data by Tschudy et al. (1984) indicate a late or latest Albian age for the upper shales of the Cedar Mountain Formation. Fission-track data from basal parts of the formation indicate a Barremian to Aptian age (Kowallis and Heaton, 1985). The limited resolution of the varied age data is demonstrated in Table 1 and Table 2.

The Burro Canyon Formation of southeastern Colorado and easternmost Utah is the stratigraphie equivalent to the Barremian to latest Albian Cedar Mountain Formation (Craig et ai., 1955; 1961 ; Young, 1960). Based on a few characteristic fossil specimens and pollen, a Barremian? to middle Aptian and possibly early Albian (111 Ma through 102 Ma) age is assigned to upper parts of the Burro Canyon Formation (Simmons, 1957; Young, 1960,1973a; Tschudy et al., 1984) (Fig. 34). Hence, the upper Burro Canyon Formation is several million years older (approximately 6 Ma) than correlative Cedar Mountain rocks (Tschudy et al., 1984). Young (1960) and Craig (1981) summarize the fossils, which include few plants, calcareous algae, and few fresh-water gastropods and pelecypods, and Tschudy et al. (1984) present all previous and recent pollen data.

The above age data show that the strata of the Pigeon Creek-Cedar Mountain-Burron Canyon sequence range from the Barremian(?) to the latest Albian (116.5? Ma through 96 Ma). The total age range is represented most completely by strata of the Pigeon Creek Formation, 129 suggesting that sediment accommodation was most extensive and continuous in basinal areas near the thrust front. Conversely, age ranges for Cedar Mountain and Burro Canyon strata vary considerably throughout eastern Utah, documenting significant discontinuity in sedimentation and sediment accommodation in foreland areas further from to the thrust front.

HIATUS AT SEQUENCE BOUNDARIES Near the thrust front, Barremian(?) to middle Aptian Pigeon Creek (PC) deposits unconformably overlie Kimmeridgian and Oxfordian marine strata of the Arapien Shale (Ja [undiff.]) (Sprinkel, 1982; Villien and Kligfield, 1986) (Fig. 34). Conversely, in areas distal to the thrust front, in eastern Utah Aptian to Albian Cedar Mountain (CM) strata were deposited in paleovalleys cut into Jurassic mudstones of the Morrison Formation (Jm) (Young, 1960,1975; Conley and Nelson,1985; Yingling, 1987) (Fig. 34). Fission-track dates from correlative shales at the base of the Lower Cretaceous sequence indicate that intial foreland deposition in eastern Utah occurred no earlier than latest Barremian(?) to early Aptian (114 Ma to 107 Ma) (Kowallis and Heaton, 1985). The underlying Morrison Formation mudstones have yielded fission-track age peaks between 132 Ma and 143 Ma (Kowallis, Heaton, and Bringhurst, 1986). These dates correspond to a Late Jurassic (Tithonian to Berriaslan) to earliest Cretaceous age (Fig. 34). Finally, Barrsmian(?) to Aptian channel-form sandstones of the Burro Canyon Formation disconformably overlie Jurassic Morrison mudstones in southeastern 130 Utah and southwestern Colorado (Craig et al., 1955; Simmons, 1957; Ekren and Houser, 1959; Craig, 1981). Consequently, the extent of the depositional hiatus at the lower sequence boundary ranges from the Kimmeridgian-Oxfordian to the Barremian(?) through middle Aptian. The hiatus is greatest directly adjacent (within 10 km) to the Pavant 1 allochthon, smaller in the basinal area (10 km to 150 km) a bit further from the thrust front, and displays considerable variation throughout most of eastern Utah and western Colorado (>150 km).

The hiatus of the upper sequence boundary is determined by the age differences that exist between shoreline deposits of the Dakota Sandstone (Kd) and underlying alluvial Cedar Mountain-Burron Canyon and Pigeon Creek strata (Fig. 34). Scattered fossil occurrences in the shoreline sandstones and offshore mudstones indicate a middle Albian to Cenomanian and Turonian (100 Ma through 90 Ma) age range for the Dakota Sandstone (Kd) (Young, 1960; Peterson and Kirk, 1977; Peterson et al., 1980; Pouch et al., 1983; Vuke, 1984) (Fig. 34, Table 2). Dakota strata in northern Utah are as old as early to middle Albian (Young,1960; Vaughn and Picard, 1976), whereas Dakota deposits in central and southern Utah are of latest Albian to Cenomanian and Cenomanian to early Turonian age, respectively (May and Traverse, 1973; Zelt, 1985). Underlying Cedar Mountain strata in central Utah are of late to latest Albian age, whereas correlative Burro Canyon rocks in southeastern Utah are as old as Aptian to early Albian (Table 2). Finally, the Turonian Sanpete Formation (Ksp) of west-central Utah is correlative to the Dakota Sandstones and 131 unconformably overlies the Pigeon Creek Formation of Barremian(?) to latest Albian age, based on palynomorph samples and fission-track data (Willis, 1886; Schwans, in press) (Fig. 34, Table 1). Consequently, erosion preceding deposition of Dakota shoreline sandstone across Utah resulted in more pronounced and gradually increasing truncation of underlying strata in a southern and western direction. Erosion at the southern and eastern basin margins completely removed the Lower Cretaceous strata, resulting in the superposition of Upper Cretaceous Dakota Sandstone on Jurassic Morrison mudstone, or even older units (Craig, 1981; Regli, 1982).

IMPLICATIONS FOR BASIN SUBSIDENCE The ago distribution in the Pigeon Creek-Cedar Mountain-Burro Canyon sequence and the age range of the unconformities bounding the sequence are expressions of erosion, sedimentation, and the associated shifts in the sediment accommodation potential that occurred in the foreland basin in response to Early Cretaceous compression. The age relationships of strata above and below the basal sequence boundary indicate that inital Sevier thrust uplift in west-central Utah occurred no earlier than the Barremian. The age data together with sediment thickness data from the Pigeon Creek Formation show that the sediment accommodation potential increased significantly in areas near the thrust load (<150 km). Coaval strata in eastern Utah and western Colorado were deposited in laterally restricted paleolows that experienced only minor changes in the accommodation potential. This indicates that little or no 132 subsidence occurred in the foreland further (>150 km) from the thrust front. The depositional hiatus at the upper sequence boundary increases toward the tectonic front. This expansion in the hiatus is interpreted to reflect a significant shift in the sediment accommodation potential of the basin during the late Albian to Turonian. Basin accommodation increased from the northwest toward the southeast and formed a broad and marine basin.

The time lag between onset of thrusting in the Barremian(?) and widespread basin subsidence in the late Albian (Heller et al., 1986) indicates that early foreland basin subsidence was neither uniform, nor primarily controlled by the gradual flexural response of the lithosphere to thrust loading. Instead, basinal subsidence associated with the emplacement of the Pavant 1 thrust plate during the Aptian to early Albian was distinctly restricted to the central weaker zone of the hingeline and was non-uniform throughout Utah. Large-scaie basin subsidence and lithospheric flexure in Utah occurred after intial thrusting during the later part of the Albian, following the migration of thrusts across the hinge zone in northern Utah onto flexurally stronger and more competent continental crust.

BASIN FILL ARCHITECTURE PREVIOUS WORK Schwans (in press) identified and described the lithofacies of the Pigeon Creek Formation. Based on geologic field data (chapter 1) and well 133 log data (unpublished), as well as studies by Stokes (1952), Young (1960, 1973a, 1975), Craig (1981), and recently by Yingling (1987), seven facies associations are recognized in the Pigeon Creek-Cedar Mountain-Burro Canyon depositional sequence: (1) sheet flood-fan deposits; (2) braided stream and valley fill; (3) low-sinuousity channel sandstones; (4) small-scale channel to sheet sandstones; (5) channel sandstone clusters; (6) lacustrine deposits; (7) overbank and floodplain strata.

Two distinctly different zones of facies distribution and sedimentation are recognized in the foreland basin in Utah. The first zone is proximal to the thrust front (<150 km), where lacustrine and single-channel fluvial systems, and later sheet-flood dominated fans, aggraded to form a thick succession of stacked and amalgamated conglomerate and mudstone units of the Pigeon Creek Formation. The second zone is further from the thrust front (>150 km) in eastern Utah and western Colorado where braided to low-sinuousity channel sandstones and floodplain mudstones of the Cedar Mountain-Burro Canyon Formations were deposited by fluvial systems across a paleosurface of significant relief.

The geometry of fluvial strata is a sensitive indicator of variations that in the subsidence rates of fault-bounded basins (Allen, 1978; Bridge and Leeder, 1979; Alexander and Leeder, 1987). Little is known, however, about the aggradational behavior of depositional systems in foreland basins. On that basis, the spatial arrangement of the major facies 134 associations and the resulting stratal bodies within the Pigeon Creek-Cedar Mountain-Burro Canyon depositional sequence and reflects the first-order and second-order changes that occurred in the sediment accommodation potential of the foreland basin.

PROXIMAL ZONE LITHOFACIES ■ShEE r a OQRiEAN CONGLOMERATES Pigeon Creek conglomerates accummulated in thick and laterally continuous sheet-like bodies of tabular geometry. Individual conglomerate sheets are up to 10 m thick and 100 m to 1000 m wide in cross-section. Directly adjacent to the Pavant 1 thrust front, conglomerate sheets are stacked to form a stratal wedge that is 700 m thick (Fig. 35), whereas a little further down the depositional slope (10 km to 20 km), individual sheets are interbedded with mudstones (Fig. 36, 39). Well-defined channel fills within sheets are scarce, whereas pebble to cobble channel-form conglomerates and sandstones are common along the lateral margins of conglomerate sheets.

Schwans (in press) interpreted the conglomerate sheets and minor channel-form sandstones to have been deposited by laterally unconfined sheet flows flooding across a low-relief alluvial fan surface. Transport and deposition of conglomerate and sand took place during flood stage in high-gradient, shallow braidplain or sheet flood-fan systems proximal to the thrust front. Avulsion-controlled switching of active braidplain segments during flooding resulted in the lateral overlap and 135

Indianola Grp. Camp.

Pigeon Creek

Atoian

Araplen Sh. Klmm.

Figure 35. Depositional architecture of basin fill in the proximal zone. Thicknesses are based on measured sections (black dots), published geologic maps, and well data. 136

i r • ^Southern-Gunnison Plateau

Figure 36. Pigeon Creek depositional sequence (between arrows, solid lines) in high-altitude aerial photo in southern Gunnison Plateau area (central part of the basin). Beds are vertical to overturned toward the west (top) and strike north-south (right to left). Tertiary strata are flat-lying and drape over the Cretaceous In upper half of photo. Jurassic rocks are visible in the lower half of the photo. 137 amalgamation of individual conglomerate sequences. Down depositional slope, in more distal basinal areas, conglomerates intermixed to a greater degree with sand and mud, forming composite sheet flood-fan bodies that terminated in a muddy floodbasin.

SMALL-SCALE CHANNEL TO SHEET SANDSTONE Small channel-form sandstones occur along the tops and margins of conglomerate sheets, and are laterally gradational with finer-grained sheet sandstones, which drape the underlying conglomerates and/or form extensive wings to the channel bodies (Fig. 35). Channel fills are generally pebbly and trough crossbedded, whereas adjacent sheet sandstones are finer grained and mostly horizontally stratified.

Schwans (in press) interpreted this facies to have been deposited by . sandy to gravelly, ephemeral bedload streams along the edges and downstream margins of the gravelly sheet flood-fans. Minor floods and fluctuations in discharge caused sandy sheet deposition on bar tops, in overbank areas along the margins of the braidplain, and in the adjacent floodplain.

CHANNEL SANDSTONE CLUSTERS Multi-storey, stacked channel-form sandstones occur within thick floodplain mudstones in proximity to the lateral sheet flood-fan margins (Fig. 35). The facies are closely associated with the lacustrine deposits that dominate the central floodbasin. Amalgamation of channel-forms 138 was predominantly in a vertical direction, resulting in laterally restricted channel clusters. Individual channel-forms consist of pebbly sandstones that exhibit trough bedsets of varied scale and numerous internal scours with gravelly lags. Channel sandstones grade laterally into ripple-stratified siltstones that interfinger with the surrounding mudstones.

Schwans (in press) interpreted the facies to reflect repeated transport of sandy bedload from distal fan areas into adjacent overbank and mud-dominated floodplain areas via small symmetrical channels. The channels were primarily active during floods and were restricted in their lateral migration, resulting in vertical stacking and minor lateral overlap of individual channel bodies.

LACUSTRINE DEPOSITS A thick, lacustrine limestone forms the base to the Lower Cretaceous stratal package in the distal parts of the proximal zone (Fig. 35,36). The unit parallels the structural strike of the thrust front over several kilometers. Trough cross-stratified limestone-pebble to oncolitic channel sandstones are incised into the top of the limestone, while numerous thin, sandy limestones and small lensoid sandstones are found throughout the surrounding massive mudstones.

Schwans (in press) interpreted this facies association to represent lacustrine and minor fluvial, sandy to muddy overbank deposits on a 139 ponded floodplain. The laterally extensive nature of the basal lacustrine unit is conspicuous, suggesting that a lacustrine body of significant size occupied central areas of the proximal basin at the downstream margins of sandy sheet-flood fans. The sedlmentologlcal data, which Include brecclatlon, mudcracking, bloturbatlon, and calcrete formation, suggest that earliest Pigeon Creek deposition occurred In a large but shallow ephemeral lake of varied size and depth situated In a central flood basin (Schwans, in press). The extensive playa body underwent repeated modification by fluvial processes during seasonal floodplain inundation. Numerous small ephemeral ponds and shallow sandy bedload channels occurred throughout the central floodbasin.

OVERBANK AND FLOODPUIN STRATA Massive mudstones with minor Intercalated small channel-form sandstones and numerous calcrete horizons form the lower half of the basin fill In the proximal zone (Fig. 35, 36). The 10 m to 150 m-thick mudstones consist of stacked varied-colored white to mottled red mudstone horizons, featuring thin burrowed zones and stacked zones or horizons of coalesced calcrete nodules. Burrowing, plant growth, and pedogenesis reworked the mudstones and destroyed most primary sedimentary structures.

Schwans (in press) showed that Pigeon Creek mudstones and associated calcrete and palesol horizons were deposited in an extensive, ephemerally inundated floodbasin. The thickness (150 m) of the deposits 140 in central parts of proximal basin areas suggests that floodbasin environments were stable for considerable time (Neocomian through Aptian) and remained more or less uninfluenced by conglomerate fan deposition adjacent to the thrust front.

BASIN FILL ARCHITECTURE IN THE PROXIMAL ZONE Floodbasin mudstones and lacustrine limestones dominate the lower portion of the basin fill in the proximal zone (Fig. 34, 37). On the basis of paleocurrent data (Schwans, in press), the lacustrine and associated fluvial strata were deposited by a drainage system oriented longitudal to the thrust front (see appendix D). Paleoflow data in the lower Pigeon Creek formation show that initial sediment dispersal in the central and northern part of the area was to the southeast (Fig. 37). Thick conglomerate sequences overlie the lower mudstone and limestone package and interfinger to an increasing extent with the latter in upper parts of the basin fill (Fig. 34,37,38). Conglomeratic to sandy sheet fiood-fan system occur in a north-south elongate zone adjacent to the thrust front. Data from the latter strata show sediment dispersal was predominantly eastward with deposition occurring in systems oriented transverse to the basin axis (Fig. 38). Clast size and conglomerate thickness trends corroborate this dispersal data. Conglomerates are thickest and coarsest-grained along the Pavant 1 thrust front and in the northwestern part of the proximal zone, whereas Pigeon Creek fines are thickest to the east and south of the thrust front salient, in the more distal part of the basinal area. 141

Utah

Figure 37. Barremian to late Aptian paieogeography of proximal zone situated between the incipient thrust front (heavy dashes) and the Ephraim subsurface fault (thin long dashes with dots on downthrown side). Blank = flood basin areas. Dash pattern = lacustrine depressions in central part of basin. Dot pattern = sand sheet fans at thrust front and eastern terminus of lineaments (thin dashed lines). Elongate dot-pattern = sandy braided streams. Large arrows = main dispersal directions; small arrows = individual paleoflow vectors. Modified after Schwans (in press) 142

----1 Are R n Utah m:

Figure 38. Late Aptian to late Albian paieogeography of proximal zone. Blank . overbank areas. Conglomerate pattern « coarse sheet flood fans at thrust front and lineament intersections. Dashed patterns - lacustrine depression. Dot pattern - fluvial sands along fan margins. Heavy dot pattern « braided streams. Large arrows « main dispersal directions; small arrows » individual paleoflow vectors. Modified after Schwans (in press). 143 Earliest deposition in the proximal zone featured an extensive lacustrine body and minor longitudinal sandy bedload system in the axial part of a muddy and ponded floodbasin (Fig. 37). Laterally extensive sheet sandstones are found along the western margin, while a sandy to muddy ephemeral stream system interacted with and repeatedly modified the floodplain during periods. Lateral migration of bedload channels in the floodbasin was restricted, indicating that net subsidence and basin alluviation were rapid. Consequently, initial sediment accommodation in the proximal zone resulted in the deposition of thick lacustrine and muddy floodbasin sequences with minor channel to sheet sandstone bodies. The significant thickness of essentially similar facies indicates that net aggradation and net subsidence in the proximal zone were essentially balanced for a long period of time (i.e., throughout the Barremian to late Aptian). The association of a large lacustrine basin and basin fan systems in the proximal zone thus documents initial basin alluviation in response to earliest uplift and subsidence in an incipient foreland basin.

Later proximal zone deposition was marked by the formation of laterally overlapping, conglomeratic sheet flood-fan bodies that were located along the thrust front and western basin margin (Fig. 38). Conglomerate accummulation was most pronounced just east of the "triple junction" of the Charlston-Nsbo thrust, the Leamington lineament, and the Pavant 1 thrust, and at the supposed intersection between the Scipio Lineament and Pavant 1 thrust. Erosive breaching of the 144 thrust-nappe complex most likely occurred along these major fracture zones, which acted as conduits for hinterland elastics entering the foreland. The pronounced vertical stacking of conglomerate sheets and fan segments records the rapid aggradation of fan systems near the area of maximum relief in response to hanging wall emergence during the Albian. Increased basin subsidence allowed accummulation of large amounts of basinal fines in the central part of the basin.

The documented coexistence of gravelly elastics and thick mudstones is in contrast to traditional suppositions that associations of braided stream elastics and overbank fines is incompatible. This study, however, interprets the spatial distribution of conglomerate and mudstone bodies in the basin fill as an important indicator of the rapid alluviation and subsidence that occurred in the basin. According to Friend (1983), similar coarse to fine, alternating large-scale conglomerate sheet and thick mudstone successions reflect the lateral sweeping of a gravelly braidplain or alluvial fan system in a continuously subsiding basin. Similar mixed coarse braided stream to fine floodbasin sequences have been described, for example, in the thrust fault-bounded Late Cretaceous Fossil (Hurst and Steidtmann,1986) and Green River Basins of Wyoming (Shuster and Steidmann,1987), and from the Miocene Ebro foreland basin in Spain (Nichols,1987).

One lacustrine fluvial sequence (I) and three distal alluvial fan sequences (II, II, IV) comprise the basin fill in central parts of the 145 proximal basin area (Fig. 36,39). Although detailed geologic mapping has shown that the stratal package has been faulted in places, the major relationships between individual sequences remain easily identifiable. The lacustrine sequence is 50 m- to 150 m-thick and laterally continuous over a distance of 7 km (I in Fig. 39). Although the basal package pinches to the north, similar fine-grained lacustrine to floodplain sequences are found in numerous outcrops along the Pavant 1 and Charlston-Nebo thrust front within a 300 km north-south elongate basinal area. Consequently, initial basin deposition in response to subsidence was marked by the formation of lacustrine basins along the thrust front.

The three overlying alluvial fan sequences range in thickness from 100 m to 200 m (ll-IV in Fig. 39) and are laterally offset against each other. The sequence represent the proximal to distal expressions of separate fan bodies, which are mainly stacked vertically and thicken considerably toward the thrust front. No other conglomeratic bodies of similar age have been reported in areas farther north, although thin sequences of sheet-like to large-scale channel-form conglomerates are present in the northernmost part of the study area, just east of the Charlston-Nebo thrust front. There, Lower Cretaceous conglomerates are increasingly truncated, however, by Upper Cretaceous fluvial to nearshore strata, indicating that movement along the Charlston-Nebo salient was continuous, even though movement along the Pavant 1 thrust 146

Southern Gunnison Plateau

I — Kpc

Figure 39. Alluvial architecture of Pigeon Creek (KPC) depositional sequence (between wavy lines « unconformities) in central parts of the proximal zone (based on image in Fig. 36). I, lacustrine to fluvial sequence; ll-IV, alluvial kin sequences. Sequences are seperated by bounding surfaces (heavy lines, dashed were uncertain), which are onlapped laterally by laterally continuous conglomerate to sandstone units (thin lines, dashed were uncertain) and intervening mudstones. North is to right. 147 south of the Leamington lineament ceased in the late Albian.

DISTAL ZONE LITHOFACIES BRAIDED STREAM AND VALLEY FILL Cedar Mountain-Burro Canyon conglomerates and sandstones of the distal zone occur as laterally restricted channel bodies that form the basal fill within paleovalleys eroded into underlying Jurassic and older strata (Fig. 40). These pebble to cobble conglomerates and sandstones are trough to planar cross-bedded and occur in small, 2 m to 10 m-wide, lenticular channel-forms. Numerous lenticular channel bodies are commonly laterally amalgamated to form tabular sandstone bodies. 50 m to 100 m to a kilometer wide (Fig. 41). Together with intermittent 5 m to 10 m-thick mudstones, the tabular sandstone bodies onlap the Jurassic paleosurface, forming 25 m to 50 m-thick paleovalley fills that are hundreds of meters to a few kilometers wide. In some paleovalleys the intercalated mudstone units are absent and two conglomerate bodies are superposed to form one coarse unit (Fig. 42,43).

The conglomerates and sandstones were deposited by small bedload to mixed load-dominated, braided and low-sinuous stream channels situated in shallow and broad paleovalleys. Valleys were seperated by paleohighs that acted as interfluves to the drainages. The presence of innumerous gravelly lags and lensoid channei-fiils within the tabular sandstones, as well as the superposition and lateral association of conglomeratic channel-forms and mudstones in the valley-fill, suggests that 148

Figure 40. Cedar Mountain depositional sequence (between arrows) in the distal zone. Channel-form pebble conglomerates (above black lines) and mudstones constitute the fill of a paleovalley in Jurassic Morrison Formation (below white line). 149

; .71 '" Y*"./''

Figure 41. Paleovalley-fill sequence of Figure 40. Lower Cedar Mountain (Korn, I) strata onlap the underlying Jurassic Morrison Formation (Jm) along an unconformity (hiatus from 136 Ma to 112 Ma). Upper Cedar Mountain (Kcm, u) mudstones are truncated along an unconformity (hiatus from 100 Ma to 92 Ma) by nearshore marine Dakota Sandstone (Kd). 150

Figure 42. Total sediment thickness distribution (isopachs in meters) for Cedar Mountain-Burro Canyon Formations. Zero line separates the basin from southern and western paleotopographic highs. Dashed line depicts boundary between Cedar Mountain (to left) and Burro Canyon (to right) Formations. GR. Green River. Compiled from Young (1960,1973a, 1973b), Munger (1965), Craig (1981), and Yingling (1987). 151

Figure 43. Generalized cross-section E-F in Figure 42 of Lower Cretaceous depositional sequence. Lower Cretaceous (Kbc - Burro Canyon Fm.) depositional sequence (between wavy lines « unconformities) occurs as valley fills within the Jurassic Morrison Formation (Jm). comprising numerous stacked, erosionally based (thin wavy lines) sandstones and thin intercalated mudstones. Cretaceous sequence is unconformably overlain by Dakota Sandstone (Kd). Modified after Young (1960). 152 sedimentation was marked by varying discharge. The lateral overlap and amalgamation of channel-fills within the confinement of the paleovalley produced the tabular gravelly bodies. Times of higher discharge resulted in downcutting and broadening of paleovalleys, while periods of lower discharge were marked by minor upbuilding. Streams thus alternated between cutting and gradually infilling paleolows, creating laterally discontinuous gravelly paleovailey-fills and a paleosurface of varying relief within the broad foreland of eastern Utah.

LOW-SINUOUSITY CHANNEL SANDSTONES Large channel sandstones (several 100 m to 1 km wide) comprising several channel segments are found in the upper Cedar Mountain Formation. According to Harris (1980) individual segments may consist of pebbly to cobbly channel sandstone bodies that intersect and truncate each other to a varying degree, Indicating their erosive nature. Channel fills have convex-upward bases and are 5 m to 200 m wide. Aerial photographs show channel segments with point bar morphology and arcuate ridges produced by lateral accretion; these alternate with mid-channel sandstones. In cross-section, point bar deposits consist of finer-grained, epsilon cross-bedded sandstones with climbing ripples and trough crossbeds, whereas mid-channel deposits consist of coarse sandstones with numerous pebbly to cobbly scour-fills (Harris, 1980).

Deposition of this facies occurred in mixed load- to suspended load-dominated streams discharging across a muddy floodplain of low 153 relief. The presence of well-developed point bars, set in muddy overbank deposits of adjacent floodplain environments, indicates that stream-flow conditions in the drainage system of the upper Cedar Mountain Formation were continuous and uniform, rather than ephemeral. Lateral and downstream channel migration was insufficient, however, to produce large-scale depositional bodies of meander belt geometry. Instead, meander bends of the channel sandstones appear flattened and are of low sinuosity. Based on channel width, depth, and wavelength, Harris (1980) classified the paleochannels in the upper Cedar Mountain Formation as low-sinuousity to transitional straight.

OVERBANK AND FLOODPLAIN STRATA Thick mudstones with thin intercalated siltstone to sandstone units and lacustrine limestones surround the large-scale, low-sinuousity channel sandstone bodies. This facies association constitutes the bulk of the upper part of the Cedar Mountain-Burro Canyon stratal package (Fig. 41). Mudstones are massive to thickly bedded, consist of stacked horizons of varied-colored, red to gray mudstone, and exhibit numerous zones of coalesced calcrete nodules and laminated calcrete. Numerous thin and laterally discontinuous micritic limestones, together with small channel-form sandstones featuring planar crossbedding and minor ripple-stratification, are common in the mudstones.

This facies association accummulated in ponded floodplains throughout eastern Utah. Few low-sinuousity channels dissected the 154 floodplain, exhibiting little tendency for lateral and downstream channel migration. The presence of numerous thin and laterally restricted limestones suggests that floodplains were most likely poorly drained. Conersely, the absence of extensive limestone bodies suggests that floodplain inundation (e.g., subsidence) was insufficient to produce and support larger lacustrine bodies.

DISTAL ZONE ARCHITECTURE Basin subsidence and subsequent alluviation in the distal zone during the Early Cretaceous was insignificant when compared to coeval basinal aggradation rates in the proximal zone. Total sediment accummulation in southeastern Utah and southwestern Colorado, for example, did not exceed 60 m (Fig. 42). Earliest Cedar Mountain-Burro Canyon deposition (Barremian to iate Aptian) was largely restricted to shallow paleo-depressions separated by intraforeland topography (Figs. 4 4 ,46a). Paleoflow data from the lower Burro Canyon (LBC) and lower Cedar Mountain (LCM) Formations corroborate the isopach distribution, suggesting north-directed sediment dispersal along an axis linking sources in southeastern Utah and southwestern Colorado with depositional centers in eastern Utah and western Colorado (Young, 1960; Craig, 1981; Yingling, 1987; Schwans, in press) (Figs. 4 4 ,46a). Correlative basal Cedar Mountain conglomerates and mudstones accummulated within wide and shallow paleovalleys that are up to 3-5 km wide and 40 m deep (Young, 1960,1975). Barremian to late Aptian foreiand basin deposition throughout the distal zone is thus marked by 155

LCM

/ LB C

Figure 44. Paleogeographic reconstruction of Barremian to late Aptian depositional systems in the distal zone. Sediment accummulation (dotted pattern) and paleoflow directions (arrows) suggest sand dispersal along a south to north axis during lower Burro Canyon (LBC) deposition, while coeval lower Cedar Mountain (LCM) elastics were dispersed through separate drainages to the northeast. GR. Green River. Compiled from Young (1960), Craig (1981), Yingling (1987), and Schwans (in press). 156 laterally discontinuous sedimentation and minor aggradation.

The accommodation of Cedar Mountain (UCM) and Burro Canyon (UBC) sediments increased significantly in central Utah and parts of western Colorado during the late Aptian through late Albian. Total sediment thicknesses range from 60 m in southeastern Utah and western Colorado to a maximum of 300 m in northern and northwestern Utah (Fig. 46b). Paleoflow in central Utah was to the northeast and east, while sediments in eastern Utah and western Colorado were dispersed in a separate and predominantly northward draining system (Harris, 1980; Yingling, 1987) (Fig. 45). The fanning of paleodispersal directions at the northern end of the depositional axis has been attributed by Craig (1981) to the merging of two major transport systems in eastern Utah.

Net aggradation and net subsidence in the distal zone increased substantially following the late Aptian, but essentially kept pace with each other throughout the Albian. This change in the accommodation potential of the distal foreland during the late Aptian and Albian is interpreted to document the shift from laterally restricted (<150 km) subsidence near the thrust front to larger scale subsidence (>150 km), thus incorporating central Utah as well. The response of the Late Cedar Mountain-Burro Canyon alluvial systems to this change in basin accommodation was marked by (1) the alluvial infilling of paleolows; (2) the onlap and subsequent burial of the eastern, erosionally dissected margin of the basin by an apron of coarse elastics deposited by 157

UCM

UBC

Figure 45. Paleogeographic reconstruction of late Aptian to late Albian depositional systems in the distal zone. Sediment accummulation (dotted pattern) and paleoflow directions (arrows) suggest dispersal to the north during upper Burro Canyon (UBC) deposition, whereas coeval upper Cedar Mountain (UCM) elastics were transported by low-sinuous river (elongate dotted pattern) to the east and northeast. Coarser braided streams (heavy dot pattern) were active farther to the northwest, deriving their sediments from the fold-thrust belt. Compiled from Young (1960), Harris (1980), Craig (1981), Yingling (1987), and Schwans (in press). 158

(A)

Utah :h è w LPC Y ' 4 V

(B)

Figure 46. (A)-(B); Regional sediment thickness map (isopachs in hundreds of meters) in Utah. (A) Barremian to late Aptian with restricted basin (3 to 1 isopach line) between thrust front (dashed line with saw teeth) and nonsubsiding craton (0 to 1 isopach line). (B) late Aptian to late Albian basin (6 to 1 isopach line). Dashes separate Pigeon Creek Fm. (LPC - lower; UPC - upper) from Cedar Mountain (LCM ■ lower; UCM - upper) and Burro Canyon (LBC » lower; UBC « upper) strata. Compiled from Young (1960), Craig (1981), Schwans (in press) (wells in appendix C). 159 distributary systems; and (3) the upbuilding of thick floodplain mudstone sequences featuring interspersed low-sinuousity channel sandstones in the central part of the basin. Furthermore, large-scale subsidence during the late Albian through Turonian resulted in the southwestward as well as southeastward advance of Dakota shoreline systems, gradually onlapping the alluvial deposits of the upper Cedar Mountain-Burro Canyon Formations. As a result, early Turonian shoreline and offshore deposits of the Dakota Sandstone rest unconformably on alluvial strata of the lower Burro Canyon Formation of Barremian age, on the Jurassic Morrison Formation, and on older strata along the eastern edge of the foreland basin, (Young, 1960; Craig, 1981) (Figs. 34,43). Shoreline strata of the Sanpete Formation of Turonian age unconformably overlie thrusted Jurassic strata of the Pavant 1 allochthon at the thrust front and western margin of the basin (Schwans, in press) (Fig. 34).

THREE-DIMENSIONAL CONFIGURATION OF THE BASIN The hiatus represented by the sequence boundaries, the age of the sequence strata, and the depositional architecture of the basin fill accurately reflect the changes that occurred in the sediment accommodation potential of the foreland basin during the Early Cretaceous. In order to produce a complete three-dimensional reconstruction of the basin fill, sediment thickness maps were constructed on the basis of geologic surface and subsurface data within the chronostratigraphic framework. Initial Sevier compression and thrust-shortening in west-central Utah occurred during the Barremian. 160 Subsequent thrust-shortening during the Late Cretaceous, as well as later Cenozoic extension, was mainly confined, with few local exception, to the hinterland of the Pavant 1 frontal zone. These findings are corroborated by recent palinspastic restorations and reconstructions of the thrusted geologic section in Utah by Sharp (1984), Villien and Kligfield (1986), and Allmendinger et al. (1986). Consequently, sediment thicknesses of the Early Cretaceous basin fill in Utah need no or little palinspastic correction.

Isopach maps of the Lower Cretaceous basin fill in Utah were constructed within the constraints of the sequence stratigraphie framework presented here, using measured surface sections and published sediment thickness data (Figs. 46a, 46b). On the basis of these data, a distinct sediment distribution pattern and basin geometry emerge for the Early Cretaceous foreland basin of Utah; (1) a restricted elongate trough of maximum sediment accommodation adjacent to the thrust front with gently sloping eastern margin; and (2) a wide area to the east with little or no sediment accommodation. The western margin of the foreland basin parallels the structural strike of the thrust-fold belt, whereas the eastern and southern margins are largely determined by depositional pinch-out of Cedar Mountain-Burro Canyon strata against intraforeland topography and the irregular paleotopographic margin in southern and eastern Utah. 161 The two isopach maps (Figs. 46a, 46 b) constructed for the proximal and the distal zone of the foreland basin confirm that earliest deposition and sediment accummulation in Utah during the Barremian to late Albian were laterally discontinuous throughout the foreland basin. Initial maximum sediment accommodation was restricted to a north-trendi.ig, 100 km-wide, basinal zone that was elongate to the emergent thrust front over a distance of approximately 350 km (Fig. 46a). Within the confines of this first-order basin, total sediment thicknesses of Pigeon Creek strata range from 1000 m at the thrust front to a minimum of 200 to 100 m in the south and east (Fig. 47). Isopachs show that the area of maximum sediment accummulation lies adjacent to the "triple-junction" (Figs. 3 2 ,46a, 46b). Farther northward. Pigeon Creek isopach trends are progressivley truncated by the eastward-projecting salient of the Charlston-Nebo thrust, suggesting decoupling of the Pavant 1 plate and the Charlston-Nebo plate following the Albian. Along the eastern margin of the Pigeon Creek basin, isopachs are deflected from a north-south elongate axis around a northwest-southeast striking feature with little or no sediment accummulation (Figs. 46 b, 47).

Regional cross-sections based on well log data show that the three-dimensional depositional geometry of the basin fill is wedge-shaped in the northwest to southeast and west to east direction (Fig. 32,48). The eastern limit of the basin coincides with the Ephraim subsurface fault zone, which is located in the subsurface of the Wasatch Plateau (Figs. 48,49a). Pronounced basement subsidence occurred west Utah

Figure 47. Total sediment thickness map (isopachs in meters) of the Barremian to late Albian Pigeon Creek Formation. Black dots - location of measured section; (for well listing see Figure 4 and appendic 0). 163

Paleozoic - L. Mesozoic

Palebzoic-L Mesozoic

Rgure 48. Cross-sections A-B and C-D (Figure 32) of the proximal basin (PC, Pigeon Creek), between thrust front (left margin) and the Ephraim subsurface fault (EF). 164

(A)

(B)

Figure 49. (A)-(B); Subsidence pattern maps. (A) Airy isostatic subsidence during the Barremian to late Aptian; (B) unrestricted flexural subsidence of the foreland basin in the Albian. Ephraim subsurface fault, dashed line with dots on downthrown side; dashed line with sawteeth, incipient fold-thrust belt; continuous line with sawteeth, emergent fold-thrust belt; heavy dotted pattern, subsiding basin with sediment thickness >200 m (wells in appendix C). 165 of the Ephraim fault zone In response to thrust loading The structural boundary thus served as a fundamental feature controlling the three-dimensional configuration of the basin during the Neocomlan to late Aptian (Fig. 46a, 49a). Subsidence was primarily restricted to areas west of the Ephraim fault zone, because the fault zone decoupled the more competent crust to the east from the Inherently, weaker zone In the west. As a result, flexural stress In the continental during thrust-loading was not transmitted across the boundary and did not result In a broad foreland basin as was formed later during the Late Cretaceous.

Total sediment thickness of Cedar Mountain-Burro Canyon strata In the distal foreland range from 0 m to 60 m at the eastern basin margin (Fig. 42) to a maximum of 100 m to 300 m In northwestern and northern Utah (Fig. 46a, 46b). Sediment accummulation during the Neocomlan to late Aptian mainly occurred in a northwest-trending and elongate paleolow that transected the eastern margin of the foreland, linking secondary sediment accummulation maxima In southeastern and eastern Utah with those In northern Utah. Sediment accummulation In the distal foreland and along the eastern basin margin were clearly controlled by repeated denudation of the eastern and southern source areas. Sediment accummulation Increased considerably during the Albian, resulting in the eastward-directed widening and coalescence of the secondary accummulation centers within the broader foreland. The eastern and southern basin margin In Utah at this time were determined by the 166 depositional pinch-out of Cedar Mountain-Burro Canyon strata against source areas and the subsequent erosion that occurred at the base of the overlying Dakota Sandstone.

CONCLUSIONS ON FORELAND BASIN SUBSIDENCE PATTERNS The two regional isopach maps constructed for the Neocomian to late Aptian (116.5 Ma through 110 Ma) and the late Aptian to late Albian (108 Ma through 96 Ma) document the mode of basin subsidence in response to thrust loading (Figs. 46a, 46b ). Initial loading of the broken crust in the hinge zone during the Barremian to late Aptian resulted in non-flexural and differential subsidence of basem ent near the load (<150 km), whereas the foreland area distal (>150 km) to the thrust front experienced little or no changes in its accommodation potential (Fig. 49a). The initial formation of a large lacustrine body, as well as the subsequent accummulation of 300 m of fine-grained and coarse-grained elastics within the Early Cretaceous basin document rapid and restricted subsidence. Basement response was thus more Airy isostatic in nature than flexural. Coeval strata in eastern Utah reached thicknesses no greater than 50 m (Fig. 46a). Within the confines of the basin, maximum sediment accummulation occurred adjacent to the thrust-front and at the thrust front-lineament intersections, which constituted a site of preferred erosion. The three-dimensional configuration of the foreland basin during the Barremian to late Aptian was controlled by thrust uplift to the west, the right-lateral offset of the Charlston-Nebo thrust salient against the Pavant thrust along the Leamington lineament in the north. 167 and by movement along the Ephraim basement fault zone. The latter thus separates a basin from a non-subsiding craton (Fig. 49a). The Barremian to late Aptian subsidence patterns in Utah thus do not coincide with those of a foreland basin formed by uniform lithospheric flexure.

During the late Albian (90 Ma through 96 Ma) the Cordilleran foreland basin experienced a profound acceleration in subsidence. Consequently, a significant increase in its sediment accommodation potential occurred throughout Utah from northwest to southeast and east (Fig. 46b). At this time, the areally-restricted basin was overprinted or replaced by a much larger flexural basin (Figs. 49b). Within the broadly subsiding basin (>150 km), drainage in eastern Utah and western Colorado adjusted to the new base level by changing from a bedload-dominated, braided stream system to a mixed to suspended load-dominated stream system. The opening of the foreland basin in northern Utah during the Albian and the subsequent southwestward migration of the locus of subsidence during the Late Albian to Turonian is interpreted to reflect the shift from Airy isostatic to large-scale flexural foreland basin subsidence. Initiation of predominantly flexural subsidence in the foreland basin was most likely driven by extensive overthrusting and loading that occurred during that time in the thrust belt in northern Utah and in Wyoming (Jordan, 1981 ; Wiltschko and Dorr, 1983; Heller et al., 1986). 168 ACKNOWLEDGEMENTS I am grateful to Kiri Campion and John H. Van Wagoner of EXXON Production Research Company, Houston, for making the data accessible to me and for the helpful discussions. I thank Terry Wilson for her encouragment early on and especially Bill Devlin of EXXON Production Research Company, Houston, for his unrelenting interest and contributions through innumerous discussions. This research is part of a Ph.D. dissertation and was made possible by grants from the American Association of Petroleum Geologists, Atlantic Richfield Oil Company, Chevron Field Studies Fund, Shell Western Exploration and Production Company, Sigma XI, The Research Society, and by G.S.A. Grant #3491-86. CHAPTER lif

SEQUENCE-STRATIGRAPHIC REVISION OF THE UPPER CRETACEOUS INDIANOLA GROUP, INDIANOLA GROUP UNDIFFERENTIATED, AND ASSOCIATED FORMATIONS

169 170

SECTION III.1

INTRODUCTION This chapter revises the existing Upper Cretaceous lithostratigraphy for rocks found between the western thrust front and the foreland basin proper in Utah, In the absence of sufficient paleontological data, previous correlations were based primarily on facies successions and lithologie affinities (Spieker, 1946; Fouch etal., 1983; Lawton, 1982,1984). The deposits of the fluvial and transitional marine indianola Group are characterized, however, by numerous lateral and vertical facies changes. Consequently, few obvious lithologie marker beds exist, suggesting that lithostratigraphic correlations are at best general in character.

On the basis of previous studies by Schwans (1985b, 1986a, 1987b, in press), this chapter uses sequence-stratigraphic concepts to identify unconformities and depositional sequences in the Indianola Group, Indianola Group (undifferentiated), and related formations. Unconformities allow establishment of a chronostratigraphic framework independent of rock lithology, fossils, lithofacies affinities, or even more subjective criteria (Mitchum, et al., 1977). The first part of the chapter discusses the problems of the existing stratigraphie nomenclature of the Indianola Group and Indianola Group (undifferentiated). Problems of correlating Indianola Group rocks arise, for example, from the numerous abrupt facies transitions that occur in 171 the thrust front to basin deposits, an overali lack of paleontological data, and the disconnected outcrop pattern. The second part of the chapter identifies and describes the unconformities bounding the Indianola strata in the region. Existing paleontological data are reevaluated and new data introduced to determine the extent of the hiatus at each unconformity, thus placing Indianola group deposits into a chronostratigraphic framework. The third part of the chapter defines and describes the unconformities that subdivide the Indianola Group into several depositional sequences. Such groups of depositional sequences are designated a supersequence and are of interregional extent. In conclusion, five types of unconformities that commonly occur in alluvia! to marine strata! successions deposited at subsiding foreland basin margins are identified. SECTION III.2

SEQUENCE-STRATIGRAPHIC ANALYSIS OF AN UPPER CRETACEOUS FOLD-THRUST BELT AND FORELAND BASIN MARGIN SEQUENCE, INDIANOLA GROUP, UTAH

(Peter Schwans)

INTRODUCTION AND PURPOSE The 4000 m-thick, alluvial to transitional marine strata of the Indianola Group and Indianola Group (undifferentiated) were deposited in central Utah during the Late Cretaceous as several basinward-thinning clastic wedges, extending from the Cordilleran thrust front eastward into the foreland basin. Together with correlative deposits of the basin proper in eastern Utah and westernmost Colorado, the Indianola Group constitutes the Late Cretaceous foreland basin fill in Utah. Numerous workers have recognized the importance of Indianola strata as sensitive indicator of thrust deformation and basin evolution in Utah (Spieker, 1946,1949a, 1949b, 1954; Christiansen, 1952; Lautenschlager, 1952; Young, 1952,1955; Armstrong, 1958, Burchfiei and Hickcox, 1972). In the absence of extensive paleontological data, stratigraphie correlations between thrust front strata and marine Indianola deposits were established on the basis of formational character, lithologie affinities. 173 and facies successions (Hale and Van De Graaff, 1964; Lawton,1982, 1983,1984; Fouch etal.,1983; Ryer, 1981; Ryer and McPhillips, 1983) (Fig. 50). As a result of the numerous lateral and vertical facies changes and minor erosional surfaces existing in such clastic successions, few obvious marker beds are available, and present lithostratigraphic correlations of Indianola strata are at best general in character.

Unconformities (for definition see appendix A) serve as useful stratigraphie marker horizons in thick sedimentary successions (Wheeler, 1958,1959; Krumbein and Sloss, 1951,1963; Weimer, 1984). They are solely determined by the relations of the adjacent strata (discordance or concordance), the nature of the surface (erosive or nonerosive), and its lateral continuity and discontinuity (Vail et al., 1977; Van Wagoner et al., 1987). On the basis of previous work by Schwans (1985b, 1986a, 1987b, in press), this study introduces a sequence-stratigraphic analysis of the alluvial and marine Indianola Group strata deposited at the western margin of the subsiding Cordilleran foreland basin in Utah (Fig. 51). The resulting stratigraphie framework is independent of rock types, fossils, depositional processes, lithofacies affinities, or even more subjective criteria. Measurement of the extent of the hiatus places the surfaces in a time-stratigraphic framework.

First, the study discusses the problems that exist in the present stratigraphie nomenclature of Upper Cretaceous through Lower Tertiary rocks in central Utah. Previous nomenclature is revised based on the 174

SUMMARY OF LITHOSTRATIGRAPHIC CORRELATIONS

Spieker (1946, Hunt (1950) Hays (1960) Lawton (1984, 1949a) Thomas (1960) (1983)

Flagstaff Ls. Ragstaff Ls. Ragstaff Ls. Ragstaff Ls. Ragstaff Ls.

Castlegate

Canyon Fm. South Rat Fm.

Funk Valley Funk Valley Funk Valley Funk Valley

Allen Valley Allen Valley Allen Valley Allen Valley (undlff.)

Sanpete Fm. (Mbrs. I-IV) Sanpete Fm. Sanpete Fm. Sanpete

Morrison(?)

Araplen Shale Arapien Sh.

Figure 50. Summary of previous lithostratigraphic correlation schem es from Spieker (1946,1949a), Hunt (1950), Hays (1960), Thomas (1960), Fouch etal. (1982,1983), and Lawton (1984,1986). Unconformities are indicated with wavy lines, associated hiatus in vertical line pattern. 175

Figure 51. Sequence stratigraphy of the Cretaceous of central Utah. Schematic time-space chart with Arapien Shale (undifferentiated) (refers to Arapien Shale and Jurassic part of Twist Gulch Formation), Pigeon Creek Formation, Indianola Group formations, and the North Horn Formation. Rrst four columns show Indianola Group (undifferentiated) outcrop areas from west (left) to east (right). Last two columns of right show stratal unite in the Wasatch Plateau. Unconformities (U) are numbered (1 through 10). Tertiary unconformity (U11) between North Horn Formation and Flagstaff Limestone is not shown. Extent of associated hiatuses is indicated by the vertical line pattern, while lithofacies shifts between major environments and the main tectonic events are shown on right side of diagram. 176 unconformities bounding the Indianola Group (unconformities 2 and 8 in Fig. 51). Secondly, the individual unconformities subdividing the Indianola Group are identified and characterized. As a result, the Indianola Group is shown to consist of several superposed sequences, which together form a supersequence. Indianola sequences are several tens to hundreds of meters thick and hundreds to thousands of square kilometers in extent. In conclusion, five types of unconformities are presented as characteristically occurring in thick alluvial to transitional marine foreland basin successions.

CURRENT NOMENCLATURE INDIANOLA GROUP Indianola strata were first described from scattered outcrops on the western flank of the Wasatch Plateau by Spieker and Reeside (1925, 1926) and Spieker (1930,1946,1949a, 1949b) (Fig. 52). The seemingly incongruous assemblage of clastic rocks was grouped by Spieker (1946, p. 126) into a composite stratigraphie unit called the Indianola Group. The Indianola Group was subdivided into formations where possible and undifferentiated where not. Four formation were recognized at the stratotype in the Six Mile Canyon area and are, in ascending order, the Sanpete Formation, Allen Valley Shale, Funk Valley Formation, and Six Mile Canyon Formation. Throughout the region, the Indianola Group unconformably overlies the Pigeon Creek Formation and correlative deposits (Schwans, in press) and is truncated by an angular unconformity (Figs. 51,52). p T / Lake Fork*

1 U tah 1

Log Hollow e Chidian Cr. ^ «Wales Gap I g M0iiorC.|)t ^ ^ Six Mile C .# ^

1 . /

SallnaC .^ a 32 KHometers::*

Figure 52. Map of measured sections and geographic areas introduced in chapter. Plateaus are in dot pattern and intervening valleys are blank. Sanpete Valley located between Wasatch Plateau and Gunnison Plateau, whereas the Juab-Sevier Valleys are to the west of the Gunnison Plateau. 178 The lower Turonian Sanpete Formation in Six Mile Canyon contains basal braided stream conglomerates grading upward into shoreline sandstones and intercalated marine mudstones (Spieker and Reeside, p. 432 in Table 1). Sanpete rocks rest on mudstones, which Spieker (1946, p. 125) tentatively designated as Morrison(?) Formation of presumed Early Cretaceous age. Schwans (in press), however, recently redefined Lower Cretaceous stratigraphy and designated the mudstones as part of the Pigeon Creek Formation of Barremian through late Albian age (Figs. 51, 52, 53). Fouch et al. (1982) and Lawton (1984,1986) further reported Sanpete-equivalent rocks of supposed Cenomanian to early Turonian age from Lake Fork and Salina Canyon, solely based on lithostratigraphic considerations (Figs. 50, 52).

Marine mudstones of the Allen Valley Shale in Allen Valley at Six Mile Canyon (Figs. 52,53) overlie Sanpete strata along a sharp contact. The marine unit is middle Turonian in age, as indicated by the occurrence of the ammonite CoHignoniceras woollgari (Mantell) (Spieker and Reeside, 1926, p. 432, in Table 1). Ryer and McPhillips (1983) also reported CoHignoniceras woollgari from mudstones overlying the Sanpete-equivalent in Salina Canyon. Based on palynomorphs, Lawton (1982,1984) and Fouch et al. (1983) also identified Allen Valley Shale-equivalent in Lake Fork. Recent réévaluation of the palynomorph sample, however, only provided an indeterminate Albian through Campanian age (J. Shane, 1987, pers, com., in Table 1), thus casting doubt on previous lithostratigraphic correlations. 179

Figure 53. Western Wasatch Plateau - Six Mile Canyon. Measured section of Indianola Group strataotype in Six Mile Canyon (Sec. 35. T. 18 S., R 2 E.) (see Figure 52). Shown are fossils (see appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols and abbreviations). 180 The Funk Valley Formation in Funk Valley at Six Mile Canyon overlies the Allen Valley Shale and comprises three members: a lower fluvial to transitional marine shoreline sandstone and lagoonal mudstone unit, a middle offshore-marine mudstone unit, and an upper shoreline sandstone unit (Spieker, 1946, p. 128). All units are Coniacian in age, as indicated by Inoceramus erectus (Meek) and Inoceramus deformis (Meek), which have been retrieved from the lower and middle, and the upper unit, respectively (Spieker and Reeside, 1926, p. 432; Katich, 1953, p. 860, in Table 1). Coniacian palynomorphs were recently recovered from the middle marine unit, whereas the Late? Coniacian ammonite Placenticeras tntschi (Grossouvre) occurs in the upper sandstone unit (S. N. Nelson, 1987, pers.com.; Cobban, 1985, pers. com., in Table 1). This study further recognizes an additional upper offshore-marine mudstone unit at Six Mile Canyon, which overlies the upper shoreline sandstones and is here designated upper Funk Valley Shale member (Fig. 53). Funk Valley strata are also present in Salina Canyon to the south and at Lake Fork (Figs. 52, 54, 55).

The Six Mile Canyon Formation constitutes the uppermost Indianola formation at the stratotype. The formation occurs below the angular unconformity decribed by Spieker (1946) and above the upper Funk Valiey Shale member recognized herein. Spieker (1946, p. 128) identified a lower fluvial conglomerate and sandstone member, a middle coal-bearing Campanian-age member, as indicated by plant fossils and mollusks (Table 2), and an upper fluvial cobble-conglomerate and sandstone member of supposed Late Campanian age beneath the angular Funk Valley

Rgure 34. Measured section of Pigeon Creek Formation and parts of Indianola Group in Salina Canyon (Sec. 33, T. 21 E., R. 1 E.). Also shown are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages, (see appendix E for symbols and abbreviations). 182

Figure 55. Measured section of upper parts of Pigeon Greek Formation, parts of Indianola Group, Unnamed Unit, and North Horn Formation in Lake Fork (SE 1/4 Sec. 34, T. 8 S., R. 4 E.). Also shown are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages, (see appendix E for symbols and abbreviations). 183 unconformity. Spieker (1946, p. 128-130) regarded fluvial sandstones above the surface as equivalent to the Price River Formation of late Campanian age, based on bivalve and gastropod assem blages from the central and eastern Wasatch Plateau (Figs. 50). Fouch et al. (1982,1983) retrieved indeterminate Campanian pollen from the sam e unit in Six Mile Canyon, which led Lawton (1984, p. 48) to designate a new upper member of the Six Mile Canyon Formation. Lawton (1984,1986) did not recognize the angular unconformity and instead argued for the presence of a minor disconformity separating the new member from the underlying Six Mile Canyon Formation of Spieker (1946) (Fig. 50). At the sam e time, however, Lawton designated Paleocence strata located above an angular unconformity he recognized at Lake Fork as equivalent to the upper Six Mile Canyon Formation (Figs. 50,52).

In summary, reasonably v;ell-dated Indianola rocks have been recognized in three localities along the western flank of the Wasatch Plateau (Figs. 53,54,55). Since existing stratigraphie correlations are based primarily on lithostratigraphic criteria, some problems arise with the lateral equivalence of units. The tentatively revised Six Mile Canyon Formation of Lawton (1984), for example, now straddles two unconformities of indeterminate character, thus raising some questions about the actual stratigraphie extent of the formation. 184 INDIANOLA GROUP (UNDIFFERENTIATED) Stratigraphically undivided Indianola strata crop out in the Cedar Hills and in the Gunnison Plateau between Chicken Creek and Log Hollow Canyon (Fig. 52) and are referred to as Indianola Group (undifferentiated) (Spieker, 1946, p. 128). Hunt (1950, p. 26-27) identified two distinct units in the Indianola (undifferentiated): a lower conglomerate unit featuring a mixed carbonate- and quartzite-clast assemblage, and an upper quartzite-conglomerate unit (Fig. 50). The lower unit rests on the Jurassic Twist Gulch Formation, whereas the upper unit is overlain in the northern Gunnison Plateau by coal-bearing strata, which Hunt (1950, p. 54) designated South Flat Formation of Late Campanian age (Figs. 50, Table 1). South Flat strata overlie the Indianola Group (undifferentiated) along an angular unconformity and are truncated by yet another angular unconformity. Quartzite conglomerates located above the second angular unconformity were identified by Thomas (1960, p. 25) as Price River Formation (Fig. 50).

Standlee (1982, p. 367) recently reported on late Albian pollen from the lower Indianola Group (undifferentiated) unit of Hunt (1950). Based on the latter date, Fouch etal. (1982,1983) and Lawton (1982,1984) suggested that the Albian unit be included in the Indianola Group Undifferentiated. They also favored including the South Flat Formation in the Six Mile Canyon Formation (Fig. 50). As a result of this proposed revision, however, the Albian through Campanian Indianola Group (undifferentiated) would be stratigraphically equivalent to the Turonian through Upper Campanian Indianola Group at the type section. 185 Schoff (1951, p.624 ), in describing the Indianola Group (undifferentiated) in the Cedar Hills, recognized three major units: (1) a lower fluvial conglomerate and mudstone unit of presumed Early Cretaceous age; (2) a middle fluvial to marine sandstone and marine mudstone unit of indeterminate early Late Cretaceous age, based on bivalves; and (3) an upper fluvial conglomerate and sandstone unit (Figs. 50,52). Hunt (1950, p. 52) later reported the middle Turonian Collignoniceras woollgari from lower marine beds of the middle unit (Fig. Table 2). Schoff (1951, p. 625) also retrieved specimens of the Coniacian marine bivalve Inoceramus deformis from another marine mudstone unit near the top of the second unit (Fig. 50).

Recently, Jefferson (1982), together with Lawton (1982,1984) reexamined the Cedar Hills stratigraphy (Fig. 3). Basal parts of the middle unit were correlated with the Sanpete Formaton and designated Sanpete Formation-equivalent, based on the bivalves Camptonectes sp. and Veniella sp. of possible Turonian age. The Collignoniceras wooligari -bearing lower marine mudstones were correlated with the Allen Valley Shale and designated Allen Valley Shale-equivalent (Jefferson, 1982, p. 70; Lawton, 1984, p. 48). Two superposed, upward-fining, fluvial to marine packages comprise the upper part of the middle unit and were correlated entirely with the upper member of the Funk Valley Formation (Jefferson, 1982, p.71), based on inoceramus deformis from marine strata in the upper package. Strata equivalent to the lower Funk Valley member were not recognized (Fig. 50). Finally, Jefferson (1982, p. 72) reevaluated the third unit of Schoff (1951) on lithostratigraphic grounds 186 only. Basal fluvial pebble-conglomerates were compared to the lower Six Mile Canyon Formation member, whereas massive conglomerates above, previously mapped by Schoff (1951) as Price River Formation, were thought to be within the Six Mile Canyon Formation. The latter are overlain by a thick fluvial sandstone to siltstone succession, which Jefferson (1982, p. 72) considered the counterpart of the middle member of the Six Mile Canyon Formation at the type locality. A thin succession of coarse conglomerates at the very top of the Cedar Hills section, just below the angular unconformity and the North Horn Formation, was correlated with the upper conglomerate unit of the Six Mile Canyon Formation (Jefferson, 1982, p. 72).

In conclusions, the previous discussion clearly shows that although Indianola rocks are reasonably well dated at the type section, little or no paleontological data exist for other outcrops in the western Wasatch Plateau and the Indianola Group Undifferentiated to the west. Previous studies thus correlate Indianola exposures on the basis of lithology, lateral continuity of facies, and the general stratigraphie position of strata. Correlations are hindered, however, by disconnected outcrops, as well as the abrupt facies transitions that occur between outcrops. As a result, no regionally recognized stratigraphie marker horizons exist, indicating that present lithostratigraphic correlations are at best general in character. A sequence-stratigraphic framework established independent of rock lithology, formational characteristics, fossil assemblages, lithofacies, or even more subjective criteria is desirable. 187 PREVIOUS SEQUENCE-STRATIGRAPHIC REVISION In a series af abstracts and a recent paper, Schwans (1985a, 1986b, 1987a, in press) defined a sequence-stratigraphic framework for Lower Cretaceous strata In Utah. Two regional unconformities are Identified bounding a sedimentary package of Barremlan to late Albian age, which Is designated the Pigeon Greek Formation. The lower Jurasslc-Cretaceous unconformity (U1) separates Oxfordian through KImmerldglan rocks from overlying Barremlan through Aptian Pigeon Greek (Kpc) strata (Fig. 56). The rocks beneath the unconformity are marine deposits of Jurassic age, which Spieker (1946) and Hunt (1950) has Identified as basal Twist Gulch Formation overlying Arapien Shale. These rocks are here referred to as Jurassic Arapien Shale (Ja). On that basis, the hiatus of the lower sequence boundary (U1) comprises 19.5 M.Y. (136 Ma to 116.5 Ma after Haq, Hardenbol, and Vail, 1987, their figures 3 ,4 ). Upper Albian Pigeon Greek rocks are separated from overlying Indianola Group (undifferentiated) deposits (Kav) by an unconformity (U2) and hiatus with a maximum extent of 6 M.Y. (96 Ma to 90 Ma after Haq, Hardenbol, and Vail, 1987, their figure 3). As a result of this sequence-stratigraphic revision of Lower Cretaceous stratigraphy, remaining Indianola Group (undifferentiated) rocks are demonstrated to be of Late Cretaceous age. Figure 56. Photo showing Pigeon Creek Formation (Kpc) overlying the Jurassic Arapien Shale (undifferentiated) (Ja) along U1 in Chicken Creek Canyon. Lower Cretaceous strata (Kpc) are overlain by rocks equivalent to the Allen valley Shale (Kav) along a U2. View is to the south with strata dipping to the east. 189 PROPOSED LATE CRETACEOUS SEQUENCE-STRATIGRAPHY LOWER UNCONFORMITY: LATE ALBIAN-EARLY TURONIAN The type-lndianola Group and the revised Indianola Group (undifferentiated) together overlie the late Albian through early Turonian unconformity in the region (U2) (Fig. 51). In Six Mile Canyon, for example, basal Sanpete rocks of early to middle Turonian age disconformably overlie Aptian to Albian? Pigeon Creek mudstones (Fig. 53). Sanpete strata are of early to middle Turonian age, as indicated by calcareous nannofossils (Villien and Kligfield, 1986),Inoceramus labiatus (Schlotheim) (Katich, 1953, p. 860), and other bivalves and gastropods (Spieker and Reeside, 1926, p. 424) (Table 1). To the south in Salina Canyon (Fig. 54), fluvial to transitional marine shoreline sandstones of the Sanpete Formation are of early Turonian age, based on Inoceramus labiatus and Mytiloides sp. of. M. columbianus (Heinz) (Katich, 1953, p. 860; W. A. Cobban, 1987, pers. com.) (Table 1). Here, the basal Sanpete (Ksp) channel truncates sheet-sandstones and mudstones of the Pigeon Creek Formation (Kpc) (Fig. 57), which are of late Albian age, as suggested by fission-track ages acquired by Willis (1986) from zircons. In contrast, basal Indianola shoreline sandstones and intercalated marine mudstones at Lake Fork (Fig. 55) are middle Turonian in age, as indicated by an assemblage of marine bivalves (W. A. Cobban, 1987, pers. com., in Table 1).

Farther west in Chicken Creek Canyon in the Gunnison Plateau, upper Albian Pigeon Creek conglomerates (Kpc) (Fig. 58) are truncated by 190

Figure 57. Photo of Pigeon Creek Formation (Kpc) truncated by U2 and overlain by basal channel-form sandstones of the Sanpete Formation (Ksp) in Salina Canyon. The Cretaceous strata are dipping to the east (right) and are unconformably (U3-11) overlain by Tertiary (T) rocks equivalent to the Flagstaff Limestone and Colton Formation. View is to the north. 191

Figure 58. Photo of Pigeon Creek conglomerates (Kpc) truncated by U2 and overlain by pebbly to cobbly channel-form conglomerates equivalent to the Allen Valley Shale (Kav) in Chicken Creek Canyon. View is to the northwest with rocks dipping to the east. 192 channel-form sandstones of the Indianola Group Undifferentiated along an uneven erosional surface. Coniacian pollen (S. N. Nelson, 1987, pers. com) occur higher up in the Indianola Group (undifferentiated) section (Fig. 59). Similar relationships are present in the Cedar Hills to the north, where Turonian fluvial conglomerates cut into underlying Pigeon Creek-equivalent strata, forming an erosional surface (U2) that can be traced on enhanced images of high-altitude aerial photographs southwestward to the Pigeon Creek stratotype and as far as Chicken Creek Canyon (Fig. 60), where it constitutes the top of the Pigeon Creek Formation (Fig. 56,57).

In summary, Indianola rocks in the western Wasatch Plateau are early and middle Turonian in age and disconformably overlie Aptian to Albian fluvial conglomerates and mudstones of the Pigeon Creek Formation (Fig. 51). Contrary to Lawton (1982,1984) and Fouch et al. (1982,1983), basal Indianola strata at Lake Fork are middle Turonian in age and thus correlate with the Allen Valley Shale (Kav in Fig. 61). The unconformable character of the surface separating Lower from Upper Cretaceous rocks is suggested by its erosional nature and the lateral lap out of overlying Indianola strata against the surface. On a regional scale, Indianola strata onlap underlying Pigeon Creek strata toward the north and west, with the hiatus increasing in the direction of onlap and encompassing parts of the Aptian and late Albian, as well as the Cenomanian through early Turonian stages (Fig. 51). 193

Rgure 59. Measured section in Chicken Creek Canyon (Sec. 12. T. 15 S., R 1 E.) of the herein subdivided Indianola Group (undifferentiated), the incised conglomerate unit (Reddicks Canyon Conglomerate), the South Rat Formation, Price River equivalent (Chris Canyon Conglomerate), the second Unnamed Unit (Reeds Valleys Conglomerate), the North Horn Formation, and Flagstaff Limestone (see Rgure 52 for location). Also shown are fossils (appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols and abbrieviations). Figure 60. Photomosaic of high-altitude aerial photos of the northern Gunnison Plateau and the Cedar Hills. Indicated with black arrows are the top of the Pigeon Creek Formation in Chicken Creek Canyon (upper left), the base of the Funk Valley Formation and the Allen Valley Shale (in Cedar Hills), the Six Mile Canyon Formation, as well as the incised valley-fill unit and the angular unconformity (U9). The formations strike north to south (right to left) In the Gunnison Plateau and northeast to southwest (lower right to upper left) in the Cedar Hills and are continuous throughout the area. View is to the west with strata vertical in the Cedar Hills and dipping to the east (bottom) in the Gunnison Plateau.

194 195 Figure 60.

-^^-SSZIESSlIZZ^SljÏ—^^ ^5525Z^^^SZZZZZ5n_ZZ M^TiWWïPrr- m m m »

m m M

y j . iL Angular t#Bonfbrmity \ > - d ::^ .. r s ^ k . - c

196

Figure 61. Photo shows truncation of the Allen Valley Shale (Kav) by U3 and fluvial conglomerates (to right) of the Lower Funk Valley Sandstone (Kfs, Iss) at Lake Fork (see Figure 52 for location). Marine mudstones and interbedded shoreface sandstones (below dashed lines) of the Lower Funk Valley Formation (Kfv, Ish) overlie the fluvial unit along a flooding surface (fs). North Horn deposits truncate east-dipping Indianola strata along U8-10. View is to the northeast. 197 UPPER UNCONFORMITIES; MIDDLE CAMPANIAN TO EARLY PALEOCENE WESTERN WASATCH PLATEAU Throughout the region, rocks of the Indianola Group and Indianola Group Undifferentiated are bounded at their top by four different unconformities, which in places may merge to form a single surface (U8 through 11 in Fig. 51). East-dipping Indianola strata in Six Mile Canyon are overlain by four units separated by unconformities. Quartzite conglomerates, quartzose sandstones, and intercalated mudstones of the upper Six Mile Canyon member of Spieker (1946) have cut into strata of the underlying middle coal-bearing member. Pollen indicate an early Campanian age for the coal-bearing strata and a late Campanian age for the quartzite-conglomerate and sandstone interval above (S. N. Nelson, 1986,1987, pers. com., in Table 1). The latter interval is truncated by an initially inclined erosional surface, against which the horizontal strata of the Price River Formation of Spieker (1946) lap out laterally. The horizontal strata consist of quartzite-pebble sandstones and here referred to as unnamed unit (Unnamed) (Fig. 62). The unnamed unit is onlapped by North Horn mudstones (KTnh) along another erosional and initially inclined surface, and North Horn strata are in turn truncated by the now west-dipping, yet intially horizontal base of the Flagstaff Limestone (Tf) (Fig. 62). Pollen indicate latest Campanian through Maestrichtian? and Paleocene ages for the unnamed unit and the North Horn Formation, respectively (Figs. 82, 63 ) (S. N. Nelson, 1986,1987, pers. com.; J. Shane, 1987, pers. com.; J. M. Zawiskie, 1987, pers. com.), while Early Eocene pollen (Fouch et al.,1983), Paleocene bivalves and gastropodes (LaRocque, 1960), and Eocene vertebrate fossils have been Figure 62. Photo of U9 through U11 In Six Mile Canyon. Price River (Kpr) strata in lower left dip eastward and are onlapped by unnamed pebble sandstone (Unnamed) along the angular U9. Tertiary North Horn (KTnh and Flagstaff Limestone (Tf) onlap U10 and U11, respectively. View is to the south with strata dipping to east (belov/ U10) and west (above U10). 199

rth Horn

Price River

lUpp. Funkl Valley Sh.1

Figure 63. Measured section of the Six Mile Canyon Formation, Price River Formation, the unnamed pebbly sandstone unit (Sterling Sandstone), the North Horn Formation, and the Flagstaff Limestone in Six Mile Canyon (Sec. 3, T 18 S, R. 2 E.) (see Figure 52 for location). Also shown are fossils (see appendix B), unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for .symbols and abbreviations) 2 0 0 reported from the Flagstaff Limestone (Rich and Collinson, 1973; Stanley and Collinson, 1979).

Based on the above data, the middle and upper Six Mile Canyon members of Spieker (1946) are here recognized to be separated by a unconformity (U8), whereas angular unconformities (U9 through U11) separate the upper Six Mile Canyon member of Spieker (1946) and the unnamed pebbly sandstone unit (e.g., Spiekers’s [1949] Price River Formation), the unnamed sandstone unit and the North Horn Formation, and the North Horn Formation and the Flagstaff Limestone (U11 not shown in Fig. 51). The unconformities (U8 through U11) merge toward the west over a short distance to form a single surface (U11 ) at the base of the Flagstaff Limestone, which truncates all underlying type-lndianola strata. Individual hiatuses encompass, in ascending order, the early late Campanian (U8), the early? Maestrichtian (U9), the latest Maestrichtian through earliest Paleocene (LI10); and (4) the late Paleocene? through earliest Eocene? (U11 not shown) (Fig. 51).

The measurement of the hiatus places the unconformities at the top of the Indianola Group into a time-stratigraphic framework. On that basis, the upper Six Mile Canyon Formation member of Spieker (1946) below the first angular unconformity (U10) is chroncstratigraphically equivalent to the upper Campanian Price River Formation and consequently cannot be part of the Six Mile Canyon Formation, as previously suggested by Lawton (1984,1986) and Fouch et al. (1982, 1983). Instead, the unit is here designated equivalent to the Price River 201 Formation (Figs. 62,63). As a result, the underlying lower to middle Campanian Six Mile Canyon Formation now comprises a lower braided stream and an upper coal-bearing member (Fig. 63).

At Lake Fork, an unnamed and horizontal boulder-conglomerate unit laps out laterally against a surface and angular unconformity (U9), which truncates steeply-dipping marine mudstones and sandstones of the Indianola Group(Figs. 51,55). Indianola rocks beneath the unconformity are early Coniacian in age, as indicated by the plicate oyster Lopha sannionis (White) (W. A. Cobban, 1986, pers. com., in Table 1), and are here regarded as equivalent to the Lower Funk Valley Shale (Kav, Ish) (Fig. 61). The boulder conglomerates above the angular unconformity are themselves onlapped along an erosional surface (U10) by Paleocene fluvial mudstones and intercalated channel forms of the North Horn Formation. Consequently, the angular unconformity at Lake Fork corresponds to that in Six Mile Canyon, while the conglomerates above the surface and below the North Horn are correlative to the unnamed pebbly sandstone unit in Six Mile Canyon of Maestrichtian age.

To the south in Salina Canyon (Fig. 54), an eastward dipping Indianola sequence is truncated at its top by an initially inclined erosional surface (U3-11), which is onlapped by fluvial and lacustrine finer elastics of the Lower Eocene Flagstaff Limestone and Colton Formation (T) (Stanley, and Collinson, 1979; Zawiskie, Chapman, and Alley, 1982) (Fig. 57). Marine mudstones located beneath the angular unconformity and above the uppermost shoreline sandstone unit are of Coniacian age (Spieker and 2 0 2 Reeside, 1926, p. 433) and consequently age-equivalent with the lower Funk Valley Shale member (Fig. 51).

GUNNISQNfjLATEAU A!^D.£FP.AB ÜIUS The coal-bearing South Flat Formation is early Campanian in age, based on pollen assemblages initially described by Hays (1960) and reinterpreted by J. Shane from EXXON Production Research Company (J. Shane,1987, pers. communication) (Fig. 59), and thus is correlative to the upper member of the type-Six Mile Canyon Formation. The South Flat Formation represents a mappable body of distinct lithology and character and hence treated here as an independent formation within the revised Indianola Group (undifferentiated) (Fig. 51).

The South Flat Formation in Chicken Creek Canyon is truncated at the top by unnamed quartzite conglomerates along an uneven erosional surface (US) (Fig. 59). The first unnamed unit is truncated by a second unnamed quartzite-conglomerate unit, which overlies and laps out against an initially inclined erosional surface (U9) (Unnamed in Fig. 64). Paleocene to Maestrichtian North Horn (KTnh) mudstones (J. M. Zawiskie, 1987, pers. com.) onlap the second unnamed conglomerate unit along another, yet less inclined erosional surface in a western direction (U10) (Fig. 51,64). Throughout the plateau. North Horn strata are truncated by the Lower Eocene Flagstaff Limestone (U11, not shown in Fig. 51).

On the basis of described stratal relations, an unconformity (U8) is recognized separating the South Flat Formation and the first unnamed Figure 64. Photo of Price River conglomerates (Kpr) in Chicken Creek Canyon (here informally called Chris Canyon Conglomerate), which are onlapped by conglomerates of the second Unnamed Unit (Unnamed or here informally called Rees Valley Conglomerate ) along the angular unconformity U9. The Unnamed unit is truncated by U 10, which in turn is onlapped by Horth Horn (KTnh) mudstones. Note the pinch-out of all units over the short distance (< 200 m). View is to the north with strata below U9 dipping to the east, while those above are horizontal. 204 conglomerate unit (Unnamed In Fig. 59), while angular unconformities separate the first and the second unnamed conglomerate unit, the second unnamed conglomerate unit and the Horth Horn Formation, and the North Horn and the Flagstaff Limestone (U9 through U11). Maestrichtian pollen were retrieved from the second unnamed conglomerate unit in Wales Gap (Unnamed in Figs. 65, 66), indicating its age-equivalence with the unnamed sandstone unit and the boulder-conglomerate unit located above the angular unconformity (U9) in Six Mile Canyon and Lake Fork, respectively (Fig. 51). Consequently, the first unnamed conglomerate unit below the angular unconformity (U9) is equivalent to the upper Campanian Price River Formation (Kpr) in Six Mile Canyon (Fig. 64) and can be called Price River-equivalent (Fig. 59). To avoid any miscorrelation in the future, the first unnamed conglomerate unit in the Gunnison Plateau is here informally named Chris Canyon Conglomerate, for its type locality in Chris Canyon (Sec. 7, T. 15 S., R. 1 E.). The second unnamed conglomerate unit above the angular unconformity in the Gunnison Plateau is here informally called Rees Valley Conglomerate, for its type locality in Rees Valley (Sec. 17, T. 15 S., R. 1 E.). The equivalent conglomerate unit in Lake Fork should be called Red Narrows Conglomerate, while the unnamed pebbly sandstone unit above the angular unconformity in Six Mile Canyon (Sec. 25, T. 18 S., R. 2 E. ) is here informally named Sterling Sandstone, for its location near the town of Sterling.

Similar relationships exist in the Cedar Hills, where a fine-grained interval is overlain by an unamed quartzite conglomerate unit along an 205

North

Six Mile Canyon

Sand Mud/Silt

Figure 65. Measured section at Wales Gap (Sec. 2, T. 14 S., R 2 E.) with the Pigeon Creek Formation, strata equivalent to the Six Mils Canyon Formation (?), the second Unnamed Unit (Rees Valley Conglomerate), and the North Horn Formation (see Figure 52 for location). Also shown are fossils (appendix B), pollen, unconformities (wavy lines), major depositional systems, formations, and ages (see appendix E for symbols and abbreviations). 206

Figure 66. Photo of lower parts of the Wales Gap section with Six Mile Canyon equlvalent(?) (Ksx) truncated by second Unnamed Unit (Rees valley Conglomerate) along U8-9. View is to the south with rocks below U8-9 overturned to the east (left), whereas those above US-S are vertical to west-dipping. 2 0 7 erosional surface. The latter is in turn truncated by an intinally inclined and erosional surface, against which North Horn strata lap cut. Based on relationships observed in the region, the angular unconformity In the Cedar Hills is correlative with the one in the Wasatch and Gunnison Plateaus (U9). Consequently, the conglomerates below the surface in the Cedar Hills are equivalent to the Price River Formation or Chris Canyon Conglomerate, while the fine-grained interval below is correlative to the South Flat Formation and the upper Six Mile Canyon member. The second unnamed conglomerate unit, which is located above the angular unconformity (U9) and below the unconformity (U10) in the Wasatch and Gunnison Plateau (e.g., Rees Valley Conglomerate and Red Narrows Conglomerate and Sterling Sandstone, respectively) and which is overlain by the Horn Horn Formation, is clearly absent in the Cedar Hills, indicating that both unconformities (U9 and U10) have merged to form a composite unconformity (Fig. 51).

DEFINITION OF INDIANOLA SUPERSEQUENCE The Indianola Group and Indianola Group (undifferentiated) together comprise an unconformity-bounded (U2 and U8) depositional sequence that ranges from the Turonian through the lower Campanian (Fig. 51). The sequence unconformably overlies the upper Albian Pigeon Creek Formation and is bounded at the top by four unconformities, which separate four units of latest Cretaceous through early Tertiary age. These unconformities are, in ascending order: (1) the late Campanian unconformity (U8); (2) the latest Campanian? through early? Maestrichtian unconformity (U9), (3) the latest Maestrichtian through 208 early? Paleocene unconformity (U10), and (4) the early? Eocence unconformity (not shown) (Fig. 51). The four units (e.g., Price River Formation, Sterling Sandstone, North Horn Formation, and Flagstaff Limestone) separated by the unconformities are exposed at the Indianola stratotype in Six Mile Canyon. The units are absent in the subsurface in Sanpete Valley to the west, but correlative strata are found in the Gunnison Plateau (e.g., Chris Canyon Conglomerate, Rees Valley Conglomerate, North Horn Formation, and Flagstaff Limestone), and in the Cedar Hills to the west and northwest.

Much of the problem of correlating the four units throughout the region has resulted from the fact that intervening unconformities merge and separate to a varied extent throughout the area. A surface may thus form a single angular unconformity or correlative disconformity. Hence, the intermittent pinch-out of the units or the merging of unconformities determines the distribution of strata in the region, as well as the extent of the resulting composite hiatus. The hiatus at the top of the Indianola Group consequently encompasses the early late Campanian at Six Mile Canyon, the late Coniacian through early Eocene at Lake Fori< and Salina Canyon, the early late Campanian at Chicken Creek Canyon, the early through late Campanian at Wales Gap, and the late Campanian through Maestrichtian at the Cedar Hills. This concept of merging unconformities and additive hiatuses is exceptionaiiy well demonstrated at Mellor Canyon at the southwestern margin of the Gunnison Plateau, where three separate units pinch out to the west within a few hundred meters, creating a composite hiatus that spans the late Campanian through early 209 Eocene (Fig. 67).

SUBDiVEDiNG THE INDIANOLA GROUP SUPERSEQUENCE MIDDLE TO LATE TURONIAN UNCONFORMIT/ (U3) In Six Mile Canyon, Sanpete sandstones are overlain by the Allen Valley Shale along a sharp surface (fs) (Figs. 53,68). The Allen Valley Shale is truncated at the top by shoreline sandstones of the Lower Funk Valley Sandstone member along an intially curved and laterally extensive erosional surface. Lower Funk Valley sandstones lap out laterally against the erosional surface in a north-south direction along depositional strike, forming a large-scale lenticular stratal body lying within a paleovalley (Fig. 68). The relationships between strata located above and below the surface are easily observed on high-altitude aerial photos (Fig. 69). In addition, the presence of the early middle Turonian ammonite Collignoniceras woollgari in the Allen Valley Shale and of the early Coniacian bivalve Inoceramus erectus in the lower Funk Valley Sandstone member above (Spieker and Reeside, 1926, p. 433, in Table 1) suggests that this surface represents a significant hiatus, which spans the late middle Turonian through early Coniacian. On that basis, the erosional surface separating the Allen Valley Shale and the Lower Funk Valley Sandstone member is idientified as an unconformity (U3) (Figs. 51,53). The unconformity Is characterized by substantial erosion, palsâvâiley incision, and a basinward shift of facies (e.g. shoreline sandstone) above the surface with respect to facies below (e.g. offshore marine mudstones) (see Lithofacies in Fig. 51). Figure 67. Photo demonstrating the rapid pinch-out of various units (between U8-9 and U10, U10 and U11) toward the west (left) at the western margin of the Gunnison Plateau in Mellor Canyon (see Figure 52 for location). View is to the north. 211

Figure 68. Photo of Sanpete Formation (Ksp) overlain by Allen Valley Shale (Kav) along flooding surface (fs) at the type locality at Six Mile Canyon. U3 truncates the Allen Valley Shale and is onlapped (dashed lines) by sandstones of the Lower Funk Valley Sandstone (Kfv, Iss) within a broadly incised valley, dis is downlap surface. View is to the east with strata vertical or steeply dipping toward the east. (»wm «9F ''«/

Infrared/HIgh Altitude Six Mile Canyon

Figure 69. Enhanced part of aerial photo showing the Indianola Group type section at Six Miie Canyon. The Sanpete Formation unconformably overlies (basai solid line) the Pigeon Creek Formation (bottom) and is in turn truncated by another unconformity (next white line), against which Lower Funk Valley sandstones lap out laterally (arrows). Tertiary Flagstaff Limestone Is flat-lying and drapes over the vertical to steeply east-dipping Cretaceous section. 213 Similar relationships exist at Lake Fork (Fig. 61), where coarse fluvial conglomerates overlie an uneven erosional surface, which truncates underlying interbedded shoreline sandstones and marine mudstones equivalent to the Allen Valley Shale (Kav) of middle Turonian age. The age of the Allen Valley Shale is based on a bivalve assemblage featuring Phelopteria gastrodes (Meek), and the bivalves Ven/e/Za mortoni (Meek and Hayden), Rynchostren cf. R. suborbiculata (Lamarck), and Cyprimeria sp.., among others (W. A. Cobban, 1988, pers. com.) (Figs. 55,61, Table 1). The conglomerates above lap out along depositional strike against the erosional surface and are thus large-scale lenticular in cross-section. No fossils were recovered from the fluvial conglomerates, yet marine strata immediately above the fluvial conglomerates yielded the plicate oyster Lopha sannonionis (White), which is commonly found in nearshore marine rocks of latest Turonian to early Coniacian age (W. A. Cobban, 1986, pers. com., in Fig. Table 1). On that basis, the surface separating the fluvial strata from those equivalent to the Allen Valley Shale is identified as an unconformity (US) (Figs. 51,55,61). The fluvial conglomerates constitute a fluvial paleovalley fill (to right of US) (Figs. 55,61), which is abruptly overlain by offshore marine mudstones along a sharp surface (fs) (Figs. 51,55, 61). The unconformity is characterized by erosion, paleovalley incision, a significant basinward shift in facies above the surface with respect to those below, and in addition is marked by hiatus spanning the late? Turonian through early? Coniacian. The fluvial paleovalley-fill unit and overlying marine sandstones together are thus equivalent to the lower Funk Valley Sandstone member (Kf, Iss), whereas the marine mudstones 214 above are correlative to the lower Funk Valley Shale (Kfv, Ish) (Fig. 61).

Similar relationships exist in Salina Canyon, where shoreline sandstones of the Coniacian Lower Funk Valley Sandstone member overlie offshore-marine mudstones along an erosional surface. The mudstones below the erosional surface conformably overlie the Sanpete Formation with sharp contact (Flooding surface in Fig. 54), and contain the early middle Turonian ammonite Collignoniceras woollgari (Katich, 1953; Ryer and McPhillips, 1983) (Fig. 54). The hiatus at the erosional surface thus spans the late Turonian through earliest? Coniacian and is herewith identified as an unconformity (U3), which separates the Allen Valley Shale from the overlying Lower Funk Valley Sandstone member (Fig. 51).

A corresponding surface is found in the Gunnison Plateau at Chicken Creek Canyon (Figs. 51, 59,70). Interbedded pebbly sandstones and minor mudstone equivalent to the Allen Valley Shale are truncated at the top by channel-form bodies of cobble-conglomerate. The latter lap out along depositional strike against an uneven erosional surface and form the coarse-clastic base of a 300 m-thick, upward-fining conglomerate, sandstone, and detrital carbonate sequence. The surface is characterized by erosion, lateral lap out of strata above, and is here identified as an unconformity (U3) (Fig. 51). The strata above the unconformity are of Coniacian age, based on pollen from the top of the interval (S. N. Nelson, pers. com., 1987), and thus equivalent to the Lower Funk Valley Sandstone/Shale member (Kfv,l) (Figs. 59,70). Similar relations occur in the Cedar Hills to the north, where basal conglomerates of a Coniacian 215

Kfv.Imbr Kfv.umbr.

Figure 70. Photo with Upper (Kfv, umbr.) and Lower Funk Valley Sandstone/Shale member (Kfv, Imbr.) in Chicken Creek Canyon where they unconformably overlie rocks equivalent to the Allen Valley Shale (Kav) (see Figure 52 for location). The members comprise a basal conglomerate (cgl), a middle sandstone (sst), and an upper detrital carbonate (detrital carb.) part, and are bounded by U3, U5, and U6 (not shown). View is to the southwest with strata dipping east (left). 216 alluvial to marine interval truncate offshore-marine mudstones of middle Turonian age, based on occurrences of Collignoniceras woollgari (Hunt, 1950) and Inoceramus deformis (Schoff, 1951, p. 623) from the lower mudstones and the upper marine unit of the alluvial to marine interval, respectively.

In summary, a late middle and late Turonian through earliest Conician hiatus and unconformity (US) can be identified along the western Wasatch Plateau on the basis of the relations of strata above and below the surface. In this area, the surface is characterized by erosion, paleovalley formation, and a basinward shift in facies. The correlative unconformity in the Cedar Hills and the Gunnison Plateau separates strata equivalent to the Allen Valley Shale located below the surface from rocks equivalent to the Lower Funk Valley member and above the surface. The lateral continuity of both units Is conspicuous throughout the northern Gunnison Plateau and the Cedar Hills, a s demonstrated by high-altitude aerial photographs (Fig. 60). In western areas, the unconformity is characterized by erosion, lateral lap out of strata above the surface, and a minor basinward shift in facies above the unconformity (e.g., conglomerates) with respect to those below (e.g., overbank strata). On a regional scale. Lower Funk Valley strata onlap the Allen Valley Shale regionally toward the northwest and west, indicating that the hiatus in the west is greater than in the east. 217 LATE? MIDDLE-EARLY? LATE CONIACIAN UNCONFORMITY (US) Middle Coniacian marine mudstones of the Lower Funk Valley Shale member (Kfv, Ish) abruptly overlie upper shoreline sandstone of the Lower Coniacian Lower Funk Valley Sandstone member (Kfv, Iss) along a sharp and laterally continuous surface (fs) at Six Mile Canyon (Figs. 63, 71). The middle Coniacian marine mudstones are in turn truncated at the top by the Upper Funk Valley Sandstone member (Kfv, uss) along a laterally continuous erosional surface (US) (Figs. S I, 71). The relations of the strata above and below the surface, as well as the continuity of the surface are documented by a high-altitude aerial photographs (Fig. 69). Detailed outcrop investigations (Fig. 59) show the surface to be characterized by minor erosion, a basinward shift of facies above the surface (e.g. shoreline sandstones) with respect to the facies below (e.g. offshore marine mudstones), and a late? middle through early? late Coniacian hiatus (see Lithofacies in Fig. SI). On that basis, the surface is here identified as an unconformity (US) (Fig. 71). The Six Mile Canyon section constitutes the only exposure of Upper Funk Valley member strata at the western margin of the Wasatch Plateau.

In Chicken Creek Canyon detrital carbonates at the top of the Coniacian interval and equivalent to the Lower Funk Valley Sandstone/Shale (Kfv, Iss/sh) are truncated by laterally extensive eobble-congiomerates, forming the coarse-clastic base of a second, upward-fining succession of alluvial conglomerates, sandstones, and detrital carbonates (Figs. S9, 70). The conglomerates lap out along depositional strike against the erosional surface. Relations between the 2 1 8

Figure 71. Photo of the Funk Valley type section at Six Mile Canyon. Lower Funk Valley Sandstone (Kfv. Iss) is overlain by Lower Funk Valley Shale (Kfv, Ish) along a flooding surface (fs), while Upper Funk Valley sandstones (Kfv, uss) overlie the unconformably US. U4 is found in the subsurface to the east and in outcrops in the Book Cliffs. It is expressed here by its correlative conformity (cc) (see chapter 4). View is to the south with strata vertical or dipping steeply to the east. 219 conglomerates and underlying detrital carbonates are thus identical to those observed between the base of the first upward-fining interval and the underlying deposits equivalent to the Allen Valley Shale (Fig. 59,70). Both intervals are conspicuous on high-altitude photographs of the northern Gunnison Plateau (Fig. 60). In addition, identical relations exist in the Cedar Hills, where deposits that are equivalent to the Lower Funk Valley member underlie conglomerates of a second upward-fining, alluvial to marine interval. Lateral correspondence between the stratal succession in the Cedar Hills and that in the Gunnison Plateau is easily demonstrated on high-altitude aerial photographs (Fig. 60).

In summary, the surface is identified as an unconformity (US) of regional extent that separates rocks equivalent to the Lower Funk Valley member from overlying basal coarse-clastic rocks at the base of another upward-fining interval (Figs. 51,53, 60). The unconformity in the Gunnison Plateau and the Cedar Hills is correlative to the late middle to early late Coniacian unconformity (US) and characterized by erosion, as well as a minor basinward shift in facies above the surface with respect to those below (see Lithofacies in Fig. 51). Furthermore, the upward-fining interval located above the unconformity in the Cedar Hills and in the Gunnison Plateau is equivalent to the Upper Funk Valley Sandstone/Shale member and thus of late? Coniacian through late Santonian age. 2 2 0 LATEST? SANTONIAN-EARLIEST CAMPANIAN AND EARLY CAMPANIAN UNCONFORMITIES (U6-U8) At the type section in Six Mile Canyon (Figs. 53,61), shoreline sandstones of the Upper Funk Valley Sandstone member of late? Coniacian through late Santonian age are abruptly overlain by mudstones of the newly recognized Upper Funk Valley Shale member (Kfv, ush) along a sharp surface (fs) (Figs. 53, 63, 72). The marine mudstones are late Santonian in age, based on marine mudstones of equivalent age that were reported by Villien and Kligfield (1986, p. 290) from the subsurface in nearby Sanpete Valley. The upper Funk Valley Shale member is in turn truncated by braided stream conglomerates of the lower Six Mile Canyon Formation (Ksx) along an erosional surface (U6) (Fig. 53, 63, 72). The surface is erosional and marked by a significant basinward shift in facies located above the surface (e.g. braided stream conglomerates) with respect to those below (e.g. offshore-marine mudstones) (see Lithofacies in Fig. 51). On that basis, the erosional surface is here Identified as an unconformity (U6) separating the lower braided stream member of the Six Mile Canyon Formation from the upper Santonian Upper Funk Valley Shale member (Rgs. 51,63). The lower Six Mile Canyon member is probably of early Campanian age, as it overlies the upper Santonian Upper Funk Valley Shale and underlies the middle Campanian Upper Six Mile Canyon Formation member (Fig. 63). The hiatus of the unconformity most likely spans the latest Santonian through earliest Campanian. 221

Figure 72. Photo of the Upper Funk Valley Shale (Kfv, ush) In Six Mile Canyon. Marine mudstones overlie the Upper Funk Valley Sandstone (in foreground) along a flooding surface (fs) and are truncated by U6, against which fluvial-deltaic channel-forms of the Six Mile Canyon Formation (Ksx) lap out laterally. View is to the east with strata dipping between 90 to 45 degrees eastward. 2 2 2 The South Flat Formation in the northern Gunnison Plateau is early to middle Campanian in age and correlative with the Upper Six Mile Canyon member. In both Log Hollow Canyon and Chicken Creek Canyon, South Flat rocks overlie unnamed alluvial quartzite-boulder conglomerates (Fig. 59). The boulder conglomerates overlie in turn an initially curved and laterally restricted erosional surface, which truncates upper parts of an carbonate-quartzite cobble-conglomerate and sandstone interval (Fig. 59,73). The cobble conglomerate and sandstone interval forms a 150 m-thick, upward-fining sequence, whose basal, laterally continuous, cobble conglomerates have cut into detrital carbonates of the Upper Funk Valley member of late? Coniacian and Santonian age (Figs. 59,70).

A massive quartzite-boulder conglomerate unit higher in the section (Figs. 59,73) pinches out parallel to depositional strike against its lower erosional bounding surface, which is interpreted as a laterally restricted paleovalley. Several such 5 km- to 10 km-wide, paleovalley fills are clearly visible on high-altitude aerial photos of the northern Gunnison Plateau, where they occur intermittently (Fig. 60). Where the paleovalley fills are absent. South Flat strata rest unconformably on the underlying cobble conglomerate and sandstone intenral. On the basis of these relations, the cobble conglomerate and sandstone interval is recognized as an unconformity-bounded sequence (between U6 and U7), which in western areas overlies rocks equivalent to the Coniacian Upper Funk Valley Sandstone/Shale member and is overlain by the lower to middle Campanian South Flat Formation (Figs. 51,59). The interval in question is consequently correlative to the Lower Six Mile Canyon 223

Figure 73. Photo of the quartzite-boulder conglomerate and incised valley fill above U7 (here informally called Reddicks Canyon Conglomerate) in Chicken Creek Canyon, which is dipping at 35 degrees to the east. Rees Valley Conglomerate visible in background between U9 and U10, while Flagstaff Limestone located in background above U11. 224 member and designated accordingly (below U7 in Fig. 73). Its lower bounding surface in turn correlates with the latest Santonian through earliest Campanian unconformity (U6) recognized at the Indianola type section, while the upper unconformity occurs at the type section between the Lower and Upper Six Mile Canyon member. Although the quartzite-conglomerate unit is absent to the east, its associated hiatus most likely encompasses part of the early Campanian and is expressed at the Indianola type section by the correlative conformity.

Finally, similar relationships exist between identical stratal packages in the Cedar Hills. Jefferson (1982, p. 71) reported an 850 m-thick, upward-fining interval of conglomerates and sandstones above the upper marine sandstone to mudstone interval, previously identified as equivalent to the upper Funk Valley member of late Coniacian age. The conglomerate and sandstone interval in the Cedar Hills is truncated by an unnamed quartzite-boulder conglomerate unit, which forms a laterally restricted paleovalley fill identical to that found in Chicken Creek Canyon. The paleovalley-fill conglomerates are best exposed in Reddicks Canyon (Sec. 7, T. 15 S., R. 1 E) near Chicken Creek Canyon and are here informally designated Reddicks Canyon Conglomerate. The paleovalley fill in the Cedar Hills is again succeeded by a thick interval of alluvial sandstones and siltstones previously identified as equivalent to the South Flat Formation.

On the basis of above stratal relations and succession of surfaces, the paleovalley-fill unit in the Cedar Hills below the strata equivalent to 2 25 the South Flat Formation is here identified as correlative to the Reddicks Canyon Conglomerate in the Gunnison Plateau. Accordingly, the thick coarse interval below the paleovalley-fill unit in the Cedar Hills is equivalent to the lower Six Mile Canyon. The three units are separated by two unconformities (U 6 and U7), which merge along strike and down-depositional dip (e.g. eastward) to form the correlative conformity between the Lower and Upper Six Mile Canyon Formation members at the type section. The greater thickness of the strata in the Cedar Hills than in the Gunnison Plateau is interpreted to demonstrate the differential degree of erosion occurring along the unconformities. The strata equivalent to the South Flat Formation in the Cedar Hills are in turn truncated by the sam e late Campanian unconformity (Ü8), which constitutes the upper bounding surface for the Indianola Group throughout the region (Fig. 51).

DISCUSSION AND CONCLUSIONS Four unconformities subdivide the alluvial to transitional marine deposits of the Indianola Group supersequence. The unconformities are located in the foreland of the Cordilleran fold-thrust belt and identified by the relations of the strata to the surface, varying degrees of erosion, paleovalley incision, significant shifts between facies located above and below the surface, and more or less significant hiatuses. The unconformities document changes in sediment availablity and basin accommodation in the foreland basin during the Late Cretaceous. Many of the correlation problems in previous lithostratigraphies are a result of the abrupt facies changes that occur between thrust front and basinal 226 deposits, an overall absence of paleontological data in the syntectonic thrust front successions, the disconnected outcrop pattern between the thrust front and marine basin margin sections of the Indianola Group, and the pinch-out of the unconformity-bounded units from east to east, toward the area of uplift. Instead of emphasizing the common characteristics of the various strata and lithofacies as a means of correlation (such as in lithostratigraphic correlations), this sequence-stratigraphic analysis utilizes a single objective criterion, the unconformity-bounded depositional sequence. The resulting sequence-stratigraphic framework is independent of the rock types, fossils, depositional processes, lithofacies succession, or even more subjective criteria.

The interplay of erosional and depositional regimes at the Cordilleran foreland basin margin is reflected in the nature of the unconformities subdividing the Indianola Group. Five general unconformity types are identified in the Indianola supersequence:

Type 1 : Subaerial exposure, depositional bypass, and minor erosion of alluvial to transitional marine foreland strata generally results in an erosional surface that is concordant within a rock succession, yet subtly truncates underling horizontal strata on a regional scale. Strata above will lap out against the uneven erosional surface, onlapping the surface in a up-depositional dip direction. Where onlap is not visible, because of the the scale of the outcrop, for example, other criteria for identification can be used, such as drastic changes in grain size, the 22 7 presence or absence of mineralogically unstable or stable lithic components, changes In depositional style, paleosol formation, or changes in provenance, among others.

The late Albian through early Turonian unconformity at the base of the Indianola Group supersequence serves as a good example, as it is marked, among other changes, by a conspicuous absence of carbonate clasts when compared to the underlying Pigeon Creek strata, an abrupt and significant relative increase in mineralogically stable quartzite clasts, and a significant reduction in grain size of the latter. These changes were the result of winnowing and weathering of Pigeon Creek carbonate clasts during exposure, regional denudation, redeposition, and depositional bypass under a humid climate, which removed of most of the mineralogically unstable lithic components. Final burial of the unconformity occurred during the early to middle Turonian, with onlap parallel in the direction of increasing hiatus.

Type 2: Angular unconformities in foreland areas are expressed by lateral lap out of strata against an initially inclined erosional surface that truncates older tilted, folded, or arched strata. The angular unconformity is marked by a hiatus that increases toward the fold-thrust belt and decreases basinward. The late Campanian through Tertiary unconformities at the top of the Indianola Group supersequence are excellent examples for this unconformity type. Erosion and angularity were accentuated along the western margin of the Wasatch Plateau and both margins of the Gunnison Plateau, for example, by repeated or 2 2 8 continuous uplift along the Gunnison and Wasatch thrust systems during the Late Cretaceous. Blind thrusting during the early Cenozoic, together with sait flowage (Stanley and Collinson, 1979), thus caused uplift in the Sanpete and Sevier Valleys, producing increasing angularity in the successive synorogenic to postorogenic strata packages (e.g.. Prive River Formation, Chris Canyon and Rees Valley Conglomerate, Sterling Sandstone, North Horn Formation, Flagstaff Limestone), which filled in the paleolows and onlapped the paleohighs. Hence, the degree of angularity along individual surfaces, together with the hiatus, increases toward the areas of uplift (e.g. the Sevier and Sanpete Valleys) and decreases toward the center of the intervening paleolows (e.g. Gunnison Plateau and Wasatch Plateau). As a result, the basinward extensions of the angular unconformities at the foreland basin margin are conformities in the foreland basin proper.

Type 3: Regional truncation and paleovalley incision can be identified by the lateral termination of initially horizontal strata by erosion and the subsequent lateral lap out and up-dip onlap of horizontal strata against the resulting surface. Within the thrust front to foreland basin profile, fluvial downcutting following an uplift event in the fold-thrust belt produces deeply incised, laterally restricted paleovalleys near the fold-thrust belt. These valleys are then filled with coarse alluvial elastics, as is the case with the Reddicks Canyon Conglomerate unit in the Gunnison Plateau of early Campanian age (U7). In contrast, erosional truncation in response to changes in the accommodation potential of the foreland basin (e.g. subsidence or 2 29 eustatic change) will result from subaerial exposure of offshore areas, thus causing the lap-out of alluvial strata against an erosional surface truncating marine deposits, as is the case with the late Turonian to earliest Coniacian unconformity (U3). In both cases, the hiatus increases and decreases along depositional strike and increases up-depositional dip, because of back-filling against the surface.

Type 4: Shifts of depositional systems commonly occur in association with regional truncation and paleovalley incision (e.g., type 3 unconformity) and can be identified in vertical sections of outcrop by the superposition of spatialiy, temporally, and genetically unrelated facies and depositional systems along a surface of regional extent. Depending on the position of the outcrop in the thrust front to foreland basin profile, unconformities in the outcrop are of obvious or subtle nature. In outcrops at the alluvial to coastal plain boundary, for example, a downward shift in coastal onlap will place alluvial over coastal plain or offshore strata. There, the degree of lateral offset between the facies above and below the unconformity in a down-depositional dip direction is a direct measure of the relative sea level changes occurring in the basin.

The shift in the depositional systems above and below the unconformity (U6) that separates the Lower Funk Valley Shale member and the Upper Funk valley Sandstone member, for example, serves as an excellent example. In contrast, in outcrops near the thrust front, the sam e downward shift would be less evident, because alluvial deposits are emplaced on top of alluvial deposits. However, even here minor 2 3 0 downcutting together with a shift in facies (e.g. braided system over overbank system) would occur on a regional scale (depending on uplift and subsidence). On a regional scale, the hiatus increases from the foreland basin toward the thrust front, as is the case in the up-depositional dip extension of the middle to late Coniacian unconformity (U6).

Type 5: Significant changes in the sedimentary, faunal, or floral record of alluvial foreland basin fills may occur in response to a major break in the climatic and depositional conditions, thus indicating a significant hiatus. Climate controls the amount of precipitation, variability of discharge and weathering characteristics, and, hence the relief and sediment flux. Climatic changes can thus be a first-order sedimentary control in non-marine environments on a basin-wide scale. Under certain conditions the presence of distinct paleosol horizons or zones, together with changes in the floral or faunal record, and in the depositional style, may indicate an unconformity in alluvial successions. Again the late Albian through Turonian unconformity at the base of the Indianola Group may senre as a possible example.

STRATAL PACKAGES AT THE SUBSIDING MARGIN OF THE CRETACEOUS FORELAND BASIN, UTAH

VOLUME 2

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Peter Schwans, M.S. (Dipl. Geol.)

The Ohio State University

1988

Dissertation Committee: Approved by

L. A. Krissek

G. D. McKenzie Advisor T. J. Wilson Department of Geology and Mineralogy TABLE OF CONTENTS

PAGE

TABLE OF CONTENTS ......

CHAPTER IV LATE CRETACEOUS DEPOSITIONAL SYSTEMS, BASIN ARCHITECTURE, AND FORELAND BASIN SUBSIDENCE ...... 231

SECTION IV.1 INTRODUCTION ...... 232

SECTION IV.2 STRATAL PACKAGES AT THE SUBSIDING MARGIN OF A FORELAND BASIN, TURONIAN TO MAESTRICHTIAN. UTAH (Peter Schwans) ...... 234

Introduction ...... 234 Previous work ...... 235 Thrust belt and foreland basin relations ...... 237 Depositional systems ...... 243 Background ...... 243 Proximal bedload system ...... 244 Block and boulder conglomerate facies ...... 244 Scour-based boulder- to cobble-conglomerate facies ...... 248 Matrix-supported cobble-conglomerate to sandstone facies ...... 254 Facies associations ...... 258 Bedload to mixed load braided stream system ...... 260 Coalesced scour-fill conglomerate>sandstone facies • • 260 Coarse-based scour-fill sandstone facies ,.,.. 287 Incised channel sandstone and detrital carbonate facies ...... 272 Mudstone, coal, and channel sandstone facies . 278 Facies association ...... 281 Fluvial-dominated shoreline system ...... 283 Distributary and interchannel facies ...... 284 inner-outer channel mouth bar facies ...... • 293 Facies associations ...... 300 Wave-dominated shoreline system ...... 302 Aggradational shoreline facies ...... 303 Transgressive shoreline facies ...... 309 Facies Associations • ■ 318 Offshore marine system 318 Facies associations ...... 320

SECTION IV.3 BASIN FILL ARCHITECTURE ...... 323

Systems tract and sequence model ...... 323 Early through middle Turonian sequence ...... 327 Latest? Turonian through early-late Coniacian sequences 341 Late Coniacian through late Santonian sequence ...... 353 Foreland basin subsidence patterns ...... 381

APPENDICES

A. HISTORY AND KEY DEFINITIONS OF SEQUENCE STRATIGRAPHY ......

B. MEASURED SECTIONS AND DESCRIPTIONS OF LOCALITIES OF CALCAREOUS MICROFOSSIL AND PALYNOMORPH SAMPLES ...... 385 C. WELL NAMES AND LOCATIONS ...... 393

D. PALEOFLOW DATA ...... 397

E. SYMBOLS AND ABBREVIATIONS ...... 404

LIST OF REFERENCES ...... 408 LIST OF FIGURES

FIGURE PAGE

Figure 74 Photo showing Pavant 2 plate (S-D) below Canyon Range plate (PC-Ca) separated by Canyon range thrust, which lies above folded part of U2-5. The synorogenic conglomerates between U2-5 and U6-7 are folded. U6-7 is onlapped by conglomerates is a western direction (left). Quartzite conglomerates above U9 constitute the overlap assemblage to the Canyon Range thrust. View is to the north with strata unfolding to the east (right) ...... 240

Figure 75 Measured Oak Creek Canyon section with unconformi ties, depositional environments, formations, and inferred ages indicated ......

Figure 76 Photo of basal block and boulder conglomerates from the folded part of Figure 74. View is to the east. Scale is 1.5 m ......

Figure 77 Channel-form conglomerates (incised valley fill) in Lake Fork (to right of white left line) overlying a marine interval with nearshore sandstones (sst.) and mudstones (mdst ), which in turn overlie hummocky crossbedded (hcs) sandstones. Strata are vertical and unfolding to the southeast (right). View is to the northeast • ■ . - ......

Figure 78 Vaiiey-fill conglomerates in Chicken Creek Canyon (see Figure 52 for location) organized into 10 m- to 30 m-thick, upward-fining sequences. View is to the north with rocks dipping to the east (right) at 35 degrees ...... 251 Figure 79 Detail section of incised valley fill unit in Chicken Greek Canyon (see Figure 59) with maximum particle sizes (MPS) and facies abbreviations indicated. NG. normal graded or upward-fining sequence; RG, reverse graded or upward- coarsening sequence • • • ...... 252

Figure 80 Photo of individual quartzite-conglomerate sheet with boulder-conglomerate channel-form (5 m wide and 2 m deep) in foreground (left) and crossbedded cobble- conglomerates and interchannel areas in the background. Note mudstone plug in upper parts of channel. View is up the depositional dip to the west with strata dipping east at 30 to 35 degrees • • 253

Figure 81 Photo of north-facing slope in Mellor Canyon where conglomerates equivalent to the Six Mile Canyon Formation are unconformably (white line) overlain by a thick interval of deformed debris-fiow facies (Deformed Interval). These in turn are onlapped by Tertiary North Horn deposits along an angular un conformity. View is to the southeast with strata dipping eastward (left). Section to base of Tertiary is approximately 350 m thick ...... 256

Figure 82 Photo showing deformed debris-flow conglomerates and interbedded sandstones exposed in the deformed Interval in Figure 81. View is to the east. Hammer to left is 45 cm long ...... 257

Figure 83 Measured section in Mellor Canyon (Sec. 17, T. 18 S., R. 1 E.) in the southwestern Gunnison Plateau (see Figure 52 for location). No fossils were found in the coarse- clastic section. Also shown are depositional environ ments. formations, and inferred age ...... 261 Figure 84 Detail section of mid-fan fôcies from lower part of Mellor Canyon section in Figure 83 with maximum particle sizes (MPS), facies abbreviations and Inter pretations indicated. Matrix-supported debris-flow deposits are shown in white on black background .... 262

Figure 85 Photo of stacked, upward-coarsening sequences of sheet and wedge geometry in Mellor Canyon where they are up to 1 km wide and up to 60 m thick (mid-fan interval). Bases of individual sheets are flat or concave downward. The mid-fan interval is overlain by channel- form sandstones (sst.) and minor mudstones (mdst.) along a sharp surface (lower white line). View is to north with strata dipping eastward ...... 263

Figure 86 Photo of lower-fan interval located south of Mellor Canyon, offset along depositional strike from the mid-fan section shown in Figures 85. Shown are laterally linked, thin sandstone and pebble-conglomer ate channel-forms separated by thick, fine-grained intervals. View is to the northeast with strata dipping east (right) ...... 265

Figure 87 Detail section of lower-fan interval from Mellor Canyon in Figure 83. Shown are the mid-fan to lower-fan transition, as defined by the maximum particle sizes (MPS) and the facies (symbols and abbreviations in appendix E) ...... 266

Figure 88 Photo of uniform coarse-grained, trough crossbedded sandstones in Chicken Creek Canyon from the Upper Funk Valley Sandstone/Shale interval. View is to the east with strata dipping east at 35 degrees. Hammer to left is 45 cm long ...... 269 Figure 89 Photo of the coarse-clastic base of a 350 m-thick, upward-fining interval in Chicken Creek Canyon (base of Upper Funk Valley Sandstone/Shale) Basal conglomerates above bounding surface (solid to dashed line) have eroded into the fine-clastic top of another underlying, upward-fining. View is to the southeast with strata dipping east (left) ...... 270

Figure 90 Photo of coarse-clastic base of Lower Funk Valley Sandstone/Shale in Chicken Creek Canyon. Quartzite-carbonate conglomerates overlie bounding surface and unconformity (US) that has cut into the fine-grained interval comprising stacked channel-form sandstones set in well-stratified, detrital carbonates. View Is to the north with strata dipping east (right) . , , 273

Figure 91 Photo of ripple forms (large one underlain by small ones) in detrital carbonates in Chicken Creek Canyon 274

Figure 92 Photo of an impression of Phoenicites sp. in uniform coarse sandstones in Chicken Creek Canyon (base of Upper Funk Valley Sandstone/Shale) (see Figure 53 for location) ...... 276

Figure 93 Photo of the ichnofossil Anchorichnus coronus from the upper detrital carbonate interval of the Lower Funk Valley Sandstone/Shale member in Chicken Creek Canyon (see Figure 53) 277

Figure 94 Photo of plant compressions from siltstones in the uppermember of the Six Mile Canyon Formation at the type locality (see Figure 53 for location). A specimen of Ficus sp. is seen just to the left of the scale ...... 280 Figure 95 Photo showing stacked, upward-fining sequences of the Sanpete Formation (right of left line, which is U2) in Salina Canyon. Sequences are interpreted to represent distributary channels that shifted into mudstone-domin ated interdistributary deposits. View is to the north. Strata below flatlying Tertiary unfolding to the east ...... 285

Rgure 96 Photo of large-scale, low-amplitude, trough crossbed ding in Six Mile Canyon at the base ofS- to 20 m-thick, upward-fining sequences of sheet geometry (base of Lower Funk Valley Sandstone). View is to the north with strata vertical to overturned to the west ...... 287

Figure 97 Photo of stacked upward-fining sequences (approx. 10 m thick) of sheet geometry at the base of the Lower Funk Valley Sandstone. Basal coarse sandstones feature large-scale, low-amplitude, trough crossbedding shown in Figure 96 and fine upward into finer grained, current-rippled sandstones (poorly exposed part between sandstone sheets). View is to the south with strata dipping east (left) ...... 288

Figure 98 Photo of vertical to subhorizontal burrows of Planolites sp, and vertical shafts of ?Skolithos sp. from finer grained sandstones in upper fine part of the upward-fining sequences of Figure 97. Scale is in centimeters 289

Figure 99 Flaser- to lenticular-bedded fine sandstone in the upper fine interval of Figure 97 ...... 291

Figure 100 Upward-coarsening mudstone (in slope to left) to sandstone sequence In Salina Canyon (see Figure 54) here Interpreted to represent outer channel mouth bar deposits set in interdistributary mudstones and overlain by inner mouth bar channels and shelly bar depos its. View Is to the north with strata dipping to the east (right). Lower part of sandstone body Is approxi mately 15m thick ...... 294 xxxx Rgure 101 Photo of a lower fine interval in Salina Canyon (see Figure 54) interpreted to represent distal mouth bar to interdistributary bay deposits. Flaser- to lenticular- bedded fine sandstones are intercalated in mud stones ...... 295

Figure 102 Photo of shell material in shelly bar of Figure 101 (central part of sandstone body). Shell fragments are mainly of the brackish-water oyster Crassostrea soleniscus ...... 297

Figure 103 Photo showing symmetrical scour in Salina Canyon here interpreted as minor distributary channel super posed onto outer channel mouth bar sandstones. View is up the depositional dip and to the west ...... 298

Figure 104 Photo showing sets of hummocky cross-stratified sandstone (at coin in center) overlain by thin ripple- stratified and bioturbated intervals (top of set) in wave-dominated, lower shoreface sandstones. Photo is from the Upper Ferron Sandstone and exemplifies the aggradational shoreline facies ...... 305

Figure 105 Photo of Teichichnus sp. maze in upper shoreface sandstones in the Upper Funk Valley Sandstone in Six Mile Canyon ...... 306

Figure 106 Photo showing well-defined network of Thalassinoides of. T. suevicus in upper shoreface sandstones, upper Sanpete Formation, Six Mile Canyon ...... • 307

Figure 107 Photo showing spreiten-conneoted shafts of Rhizo- corallium sp. in upper shoreface sandstone, upper Sanpete Formation, Six Mile Canyon.Soale is in centimeters •. • ...... 308 Figure 108 Photo of low-angle trough cross-stratified sets of flat to gently undulating morphology in upper part of Sanpete Formation at Six Mile Canyon (see Figure 53). Sets are part of large, sheet-like sandstone bodies that form the top of upward-coarsening sequences here interpreted as transgressive shoreline deposits resting on a transgressive surface. View is to the south with strata dipping steeply to east (left) ......

Figure 109 Photo of pebbly channel form (pebbly scour-fill subfacies) at top of a hummocky cross-stratified, shoaling-upward sequence at the top of the upper Funk Valley Sandstone member in Six Mile Canyon (see Figure 53). Here interpreted as top of a transgressive sequence, above which abrupt deepening occurs. View is to the north with strata dipping steeply to the east (right). Staff is 1.5 m long ...... 313

Figure 110 Photo of stacked sequence of amalgamated, hummocky cross-stratified, fine-grained sandstone interval in Lake Fork (see Figure 55). Hummocky sets are up to 0.5 m high, overlie shelly lags at hammer (45 cm long), and are capped by ripple-stratified sets (furrows). Viev/ is to the southwest with strata dipping to the southeast (left) • ...... 314

Figure 111 Photo of sole marks and indeterminate burrows at base of thin sandstone intercalated in marine mudstone of the Allen Valley Shale in Six Mile Canyon. Scale is in centimeters ...... 319

Figure 112 Photo of Allen Valley Shale at Six Mile Canyon uncon formably overlain (above white line) by Lower Funk Valley Sandstone. Middle part of Allen Valley Shale (between dashed black lines) features an increase in the frequency of rippled sandstones, which is here interpreted to represent the shift from a distal off shore to a more proximal pro-delta environment. View is to the southeast with strata vertical or dipping to the east (left) ...... 321 Rgure 113. Souther cross-section of Cretaceous basin fill in the southern Wasatch Plateau. Note vertical exaggeration in scale. Well 13 is tie-well with northern cross- section (see Rgure 4 and appendix 0) 329

Rgure 114 Northern cross-section of Cretaceous basin fill in the southern Wasatch Plateau. Note vertical exaggeration in scale. Well 13 is tie-well with northern cross- section (see Rgure 4 and appendix C) ...... 331

Figure 115 Sequence-stratigraphic interpretation of a part of the Indianola Group supersequence at the type section, dn, downlap; on, onlap; te, erosional truncation; U, uncon formity ...... 333

Rgure 116 Isopach (in meters) map of Sanpete-Dakota Sand stone interval in cross-sections (between basal unconformity and first flooding surface). Paleocurrent roses (in 30 degree sections) based on measurements (see numbers) in Salina Canyon (28) and Six Mile Canyon (36) (see Rgure 129) 336

Rgure 117 Isopach (in meters) of lower part of Allen Valley- Tununk Shale interval in cross-sections (between first flooding surface and first downlap surface). Paleo current roses (in 30 degree sections) based on measure ments (see numbers) in Salina Canyon (28) and Lake Fork (21 ) (see Figure 129) 336

Figure 118 Isopach (in meters) map of upper part of Allen Valley- Tununk Shale interval in cross-sections (between first downlap surface and U3). Interval features two down lap wedges ...... 338 Figure 119 Isopach (in meters) map of Lower Funk Vaiiey-Ferron Sandstone interval (Lower Ferron) in cross-sections (between U3 and U4). Paleocurrent roses (in 30 degree sections) based on measurements (see numbers) in Six Mile Canyon (63), Lake Fork (25), Chicken Creek Canyon (73), northern Canyon Range (14), and Oak Creek Canyon ( 11), central Canyon Range (see Figure 129) 343

Figure 120 Photo of Upper Ferron Sandstone in the eastern Wasatch Plateau (near well 9 in cross-section). The Upper Ferron Sandstone overlies U4 and consists of stacked upward-shallowing parasequences (three are shown) separated by flooding surfaces (fs). U3 is ex pressed here by its correlative conformity ...... 345

Figure 121 Isopach (in meters) of Upper Ferron Sandstone in the cross-sections (between U4 and the first flooding surface). Arrow depicts major paleoflow directions as reported by Cotter (1971,1975a, 1975b) ...... 346

Figure 122 Isopach (in meters) map of Lower Funk Valley- Bluegate Shale (Coniacian) interval in cross-sections (between datum and downlap surface) • • • ...... 349

Figure 123 isopach (in meters) map of downlap wedges at top of Lower Funk Valley-Bluegate Shale (Coniacian) interval in cross-sections (between downlap surface and U5) • • 350

Figure 124 Photo of aggrading shoreline complexes of the Emery Sandstone (located between wells 9 through 13). fs, major flooding surface at top of Emery Sandstone in cross-sections ...... 355 Figure 125 Isopach (in meters) of Upper Funk Valley-Emery Sand stone (Coniacian) interval in cross-sections (between U5 and flooding surface). Paleocurrent roses (in 30 degree sections) are based on measurements (see numbers) in correlative strata in Log Hollow Canyon (42), Chicken Creek Canyon (100), Six Mile Canyon (35), and from conglomerates in the hinterland in the Canyon Range « : « « « « « : : : : « « « « ...... s e e t s 5 » s

Figure 126 Isopach (in meters) of Upper Funk Valley-Emery Sand stone (Santonian) interval in cross-sections (between flooding surface and US in Figure 122). Paleocurrent roses (in 30 degree sections) are based on measure ments (see numbers) in correlative strata in Log Hollow Canyon (42), Chicken Creek Canyon (100), and from conglomerates in the hinterland in the Canyon Range ...... 359

Figure 127 (A) Stratigraphie relations of three idealized depo sitional sequences ( 1b through 3b) and depositional systems to downlap surfaces (DLS), condensed sections (CS), and unconformity types ([ 1], [2]). (B) Chronostratigraphic chart with stratigraphie relations in (A) plotted in time (ordinate) against distance graph. From Vail, Hardenbol, and Todd (their Figure 8) • • • • 372

Figure 128 Schematic representation of parasequence stacking patterns and related well-log responses (GR-gamma ray; DT-sonic). (A) Progradational; (B) Aggradational; (C) Retrogradational. Modified after Van Wagoner at al. (1987) 375

Figure 129 Map of study area (inset rectangle in box) showing the locations of all measured sections (full circles) and wells used in cross sections (1 through 27) ^87 CHAPTER IV

LATE CRETACEOUS DEPOSITIONAL SYSTEMS, BASIN ARCHITECTURE, AND FORELAND BASIN SUBSIDENCE

231 2 32

SECTION IV.1

INTRODUCTION This chapter analyzes the lithologie character, depositional geometries, and architecture of the Late Cretaceous foreland basin fill in Utah. The facies associations, the shifts of the depositional systems, and the unconformities subdividing the Indianola Group supersequence document the interplay of erosional and depositional regimes at the foreland basin margin. Together with sediment distribution and isopach data, these provide a clear record of changes in the sediment accommodation potential of the basin in response to broad flexural subsidence, tectonic uplift, and eustasy.

The previous chapters showed that the Early Cretaceous history of the Cordilleran basin in Utah is that of a differentially subsiding alluvial basin where sediment accommodation was primarily controlled by Airy isostatic subsidence, resulting in a narrow and restricted basin adjacent to the thrust front. The Late Cretaceous history of sedimentation and basin formation addressed in this chapter is that of a marine basin where sediment accommodation becomes increasingly controlled by eustatic changes in the foreland basin. Tectonic subsidence and flexural rebound of the foreland basin following erosion of the thrust load, as recently suggested by Heller et al. (1988), is not a major limiting factor on the depositional geometries and unconformities. Sediment 233 availability, however, is significantly influenced by uplift, the post-orogenic sediment flux, and changes in relief across the basinal hinge.

The first section of the chapter (section IV.2) characterizes the major depositional systems that were active within the fold-thrust belt to basin depositional profile. In the second section of the chapter (section IV.3), the lateral association and vertical association of genetically related depositional systems within the existing chronostratigraphic framework is placed into the systems tract model (terminology by Brown and Fisher, 1977). Systems tracts architecture is interpreted to reflect the degree of influence tectonic episodes in the fold-thrust belt, basin subsidence, and eustatic events in the foreland basin had on basin accommodation. The resultant model of foreland basin architecture and formation is that of a basin controlled by broad flexural and Airy isostatic subsidence subsidence, and eustasy. 23 4

SECTION IV.2

STRATAL PACKAGES AT THE SUBSIDING MARGIN OF A FORELAND BASIN, TURONIAN TO MAESTRICHTIAN, UTAH

(Peter Schwans)

INTRODUCTION The application of sequence-stratigraphic concepts to seismic section, outcrop, and well log correlation studies provides the means to identify major surfaces and strata! packages of primary chronostratigraphic significance and varying magnitude (Mitchum, Vail, and Thompson, 1977). The apparent global synchroneity of onlap events in passive continental margin sequences was used by Vail and his colleagues to conclude that global sea level changes controlled the spatial distribution and facies of depositional sequences in sedimentary successions (Vail and Mitchum, 1979; Vail and Todd, 1981 ; Vail, Hardenbol, and Todd, 1984). The stacking and termination patterns of stratal units and sequences were directly equated with different phases of global sea level cycles, such as lowstand, transgressive, and highstand, and allowed construction of a global coastal onlap chart and eustatic curve (Haq, Hardenbol, and Vail, 1987). Since Vail's group inferred a cause and effect relationship between eustasy and stratal patterns in sequences, a lively debate has centered on the factors 235 controlling sequence development in individual basins, as well a s on the the so-called third-order cycles (Pittman, 1978; Watts, 1982; Watts et al., 1982; Kominz, 1984; Parkinson and Summerhayes, 1985).

To consider the problem of global synchroneity and identify the major factors determining stratal architecture in a basins, this study examines sedimentary packages deposited at the subsiding margin of a foreland basin. The Upper Cretaceous Indianola Group in Utah is analyzed to test the sequence-stratigraphic concept and its application to a tectonically active basin. The objectives of the analysis are to document the stratal distribution patterns at the western basin margin, to establish a sequence-stratigraphic and depositional model, and to determine the major controls on sequence development in the Late Cretaceous foreland basin of Utah. The effects of eustasy and foreland basin subsidence on Indianola sedimentation patterns are addressed by (1) describing the major depositional facies and systems; ( 2) documenting the succession of system tracts (Brown and Fisher, 1977, p. 215); and (3) identifying the major landward and basinward shifts of the latter in the stratal succession.

PREVIOUS WORK Indianola Group outcrops are scattered throughout west-central Utah and occur between Lake Forkto the north, Salina Canyon in the south, and the Wasatch Plateau in the east, and the Canyon and Pavant Ranges in the west (Fig 52). Although numerous abstracts and papers have been published on the lithostratigraphy and syntectonic character of Indianola 236 Group rocks (Young, 1955; Van De Graaff, 1964; Campbell, 1971; Fouch et al., 1982; 1983; Ryer and McPhillips, 1983; Dickinson, Lawton, and Inman, 1985; Lawton, 1983,1985,1986), few studies have addressed the relations of thrusting, foreland deformation, sedimentation, and eustasy (Ryer, 1982; Schwans, 1985b, 1986a, 1987b; Vail and Bowman, 1987). Schwans (1987a, in press) characterized Early Cretaceous foreland basin subsidence patterns in Utah and showed that sediment accommodation was mainly controlled by the differential response of structural basem ent elements to initial compression during the Barremian to Aptian.

During the late Albian, extensive overthrusting in southern Wyoming and northeastern Utah caused large-scale flexural basin subsidence in northern Utah, resulting in the southward advance of the interior seaway into central Utah (Jordan, 1981; Wiltschko and Dorr, 1983; Vuke, 1984; Heller et al., 1986; Heller et al., 1988) (Figs. 46a, 46b, 49a, 49b). Consequently, any analysis of the sediment distributions patterns in the Cordilleran foreland basin needs to incorporate the possibility of eustasy influencing accommodation patterns. As Indianola strata comprise the Later Cretaceous foreland basin fill in Utah, the spatial arrangement of major facies associations and the resulting stratal bodies should thus accurately reflect any changes that occurred in the sediment accommodation potential of the basin, be these tectonic or eustatic in origin (Schwans, 1985b, 1986a, 1987b). 237 In order to provide a reasonable stratigraphie basis for such an analysis, a sequence-stratigraphic framework was established for the Indianola Group in the chapter three. Unconformities objectively subdivide the sedimentary package into seven depositional sequences, which constitute the Indianola Group supersequence (Fig. 51). Individual sequences represent episodes of genetically related sedimentation, while assembled as supersequence they document the sedimentation history of the Late Cretaceous foreland basin.

THRUST BELT AND FORELAND BASIN RELATIONS Since the strata of the indianola Group supersequence are intimately related to fold-thrust belt and foreland deformation, it is first necessary to reconstruct spatial relationships between the tectonic belt and the foreland basin. The Cordilleran fold-thrust belt can be subdivided into foreland and hinterland areas (Fig.s. 29,30). The foreland lies between the Wasatch Plateau and the Valley Mountains, whereas the hinterland comprises areas to the west (Allmendinger, Miller, and Jordan, 1984). The frontal zone of the thrust belt consists of allochthonous structural assemblages detached at various levels and bounded by west-dipping thrusts (Standlee, 1982,1983; Royse, 1983). Based on information from unpublished seismic profiles, as well as well logs and outcrop studies, the Cretaceous thrusts in Utah have a ramp-flat geometry.

The hinterland contains three major Mesozoic thrust systems, which from east to west and in the order of ascending structural level are the 23 8 Pavant 1 (lower), Pavant 2 (upper), and the Canyon Range thrusts. The foreland in contrast only contains the Gunnison-Wasatch thrust system, whose faults terminate as blind thrusts in Jurassic strata west of the Gunnison Plateau and beneath the Wasatch Plateau (Standlee, 1982; Lawton, 1985; Villien and Kligfield, 1986) (Fig. 31). The individual allochthons or plates in the region consist of complexly broken stratal assemblages. In addition, both the hinterland and foreland are transected by two major, west to northeast-striking lineaments, which acted as tear-faults or lateral ramps to the eastward propagating tectonic belt (Figs. 29,30). Cretaceous thrust-shortening in the hinterland in western Utah was on the order of 110 km to 200 km (Sharp, 1984).

The Early Cretaceous tectonic evolution of central Utah was described in detail in chapters one (Schwans, in press) and two. The Pavant 1 and Charlston-Nebo thrusts are identified as sole thrusts in the region, based on structural and geologic surface data by Morris (1983) and seismic and drill hole data by Villien and Kligfield (1986). Schwans (in press) provided extensive stratigraphie evidence to demonstrate that initial uplift in the region commenced during the Barremian along the Pavant 1 thrust, while hanging-wall emergence did not occur prior to the Albian.

Controversy exists, however, concerning the mode of thrusting during the Late Cretaceous (Lawton, 1335,1388; Villien and Kligfield, 1986; Schwans, 1987b, 1987c). On the basis of geologic surface data discussed in chapter two and regional correlations of electric well log-pattems presented herein, this study finds that the basal sequence of the 239 Indianola Group supersequence (early-middle Turonian sequence in Fig. 51) disconformably overlies both the Lower Cretaceous basin fill and hanging wall strata of the Pavant 1 thrust. The basal hiatus of the Indianola supersequence is of late Albian through Turonian age and provides an upper limit on latest Pavant 1 thrusting (Fig. 51, Tectonic Event 1).

The Pavant 1 thrust is cut by the structurally higher Pavant 2 thrust in the west (Figs. 29,30). The Pavant 2 thrust is exposed in parts of the Pavant Range and superposes Cambrian carbonates over strata as young as Jurassic (Hickcox, 1971; Baer eta!., 1982; Davis, 1983). Indianola conglomerates of the late? Coniacian to Santonian sequence (between U2-5 and U6-7) are exposed at the western flank of the Canyon Range and disconformably overlie Devonian Simonson Dolomite (S-D) (Millard, 1983) (Figs. 51, 74). The deformed Upper Paleozoic strata are part of the Pavant 2 plate and are completely buried by Indianola conglomerates toward the northern termination of the range. The sam e Indianola sequence is also exposed along the eastern flank of the Pavant Range, where conglomerates onlap Pavant 2 hanging-wall rocks in a southwestward direction.

The Canyon Range is bordered to the north by the west-east trending Leamington lineament, while the Sclpio lineament separates the Cariyon from the Pavant Range in the south (Figs. 29, 30). North of the Leamington lineament the Tintic Valley thrust cuts the Charlston-Nebo thrust in the west and comprises hanging-wall strata identical in age to Figure 74. Photo showing Pavant 2 plate (S-D) below Canyon Range plate (PC-Ca) and separated by Canyon range thrust, which lies concealed beneath folded Cretaceous conglomerates. The synorogenic conglomerates between U2-5 and U6-7 are folded and onlapped by conglomerates above U6-7. Quartzite conglomerates above U9 constitute the overlap assemblage to the Canyon Range thrust. View Is to the north with strata unfolding to the east 241 those exposed south of the Leamington fault in the Pavant 2 plate (Morris,1983). Both lineaments acted as right-lateral ramps during eastward-directed compression and transect the fold-thrust belt in central Utah (Picha, 1986). If the latter relations are valid, then the Tintic Valley plate is correlative with the Pavant 2 plate and that both plates were thrust onto the Charlston-Nebo-Pavant 1 plates during the latest Turonian through Coniacian.

The Proterozoic to Ordovician siliciclastics and carbonates (Millard, 1983) of the Canyon Range plate form the structurally highest ailochthon in the region. Together with the Sheeproek plate north of the Leamington lineament, the Canyon Range plate overrides the structurally lower Tintic Valley and Pavant 2 plates (Morris, 1983; Holladay, 1984; Villen and Kligfield, 1986). Canyon Range thrusting resulted in local folding of the underlying Pavant 2 plate (Millard, 1983), as well as overthrusting and partial folding of conglomerates of the late Coniacian through Santonian Indianola sequence (Fig. 74, between U2-5 and U6-7). The folded conglomerates are in turn onlapped along a disconformity and angular unconformity by conglomerates of the early to late Campanian Indianola sequence (Fig. 74, between U6-7 and U 8), which itself is onlapped along a disconformity by strata of the late Campanian Indianola sequence (Fig. 74, between U 8 and U9). The Canyon Range thrust cuts the Campanian sequence and is burled by quartzite-boulder conglomerates of the Maestrichtian to early Paleocene sequence (Fig. 74, above U9). The latest Cretaceous to Tertiary sequence constitutes the overlap assemblage to the Canyon Range thrust and places an upper age limit on 2 42 thrust timing (Fig. 51).

in summary, individual sequences of the Indianola Group supersequence successively onlapped the eastward-dipping Albian paleosurface in the foreland and hinterland during the Late Cretaceous. Indianola deposits successively bury the structurally higher and younger thrust plates located west of the frontal zone. The Tintic Valley-Pavant 2 thrust surfaced during the latest Turonian through late? Coniacian (Fig. 51, Tectonic Event 2). Unpublished seismic data show, for example, subtle regional angularity between the top of the Turonian sequence (Allen Valley Shale) and the base of overlying latest Turonian through early-middle Coniacian sequence (Lower Funk Valley Sandstone) (Fig. 31). Movement along the Pavant 2 plate continued well into the Santonian. Initial Canyon Range thrusting during the eariy Campanian emplaced the Canyon Range and Sheeproek plates over the Tintic and Pavant 2 plates (Fig. 51, early part of Tectonic Event 3). Renewed thrusting during the late Campanian, together with significant eastward-directed and vertical displacement along the Leamington fault and lateral ramp, caused plate breakage, tilting of the southern block, and drag folding in the Canyon Range plate (Millard, 1983; Morris, 1983; Holladay, 1984; Picha and Gibson, 1985; Picha, 1986) (Fig. 51, late part of Tectonic Event 3).

Blind thrusting along the Gunnison-Wasatch thrust systems throughout the latest Campanian, Maastrichtian, and Paleocene segmented the foreland east of the Pavant 1 thrust front with a series of 243 north-south elongate, anticlinal and synclinal structures (Standlee, 1982,1983; Royse, 1983; Lawton, 1985) (Fig. 51, Tectonic Event 4). Uplift of Mesozoic strata and erosive beveling of the anticlines during the Maestrichtian to early Paleocene produced a significant Cretaceous-Tertiary unconformity (Fig. 51). Upper Campanian strata correlative to the Price River Formation (e.g., Chris Canyon Conglomerate) and exposed at the margins of the uplifts were folded and in turn onlapped along a prominent angular unconformity by Maastrichtian conglomerates and sandstones (e.g., Reese Valley Conglomerate, Sterling Sandstone, Red Narrows Conglomerate). Later, Paleocence through lower Eocene North Horn Formation and Flagstaff Limestone (Fig. 51) onlapped these syn- to post-orogenic units. The North Horn and Flagstaff elastics and limestones eventually filled the intervening basins. This succession of tectonic and depositional events represents a combination of backstopping thrust emergence followed by basinward-directed bedding plane thrusting. The proposed deformational sequence is thus contrary to that of Lawton (1986), who described a forward-breaking thrust sequence on the basis of compositional trends in Indianola Group sandstones.

DEPOSITIONAL SYSTEMS BACKGROUND Five distinct facies associations called depositional systems (terminology of Brown and Fisher, 1977) are identified in the Indianola Group: (A) proximal bedload system; (B) bedload to mixed load braided stream system; (C) fluvial-dominated shoreline system; 244 (D) wave-dominated shoreline system; and (E) offshore marine system. The inferred paieoenvironments represented by the various facies associations were determined through detailed studies of unit geometries, sedimentary structures, textural variations, bedding characteristics, body fossil assemblages, ichnofossils, and lateral associations.

PROXIMAL BEDLOAD SYSTEM Three partially intergradational coarse-clastic facies are recognized: (1 ) block and boulder conglomerate facies; ( 2) scour-based, boulder-cobbie conglomerate facies; and (3) matrix-supported, cobble conglomerate to pebbly sandstone facies.

BLOCK AND BOULDER CONGLOMERATE FACIES Block and boulder conglomerates are only found adjacent to the Canyon Range thrust in the Canyon Range (Figs. 52,7 4 ,7 5 ). The coarse-clastic rocks disconformably overlie Devonian Simonson Dolomite of the upper Pavant 2 plate (S-D) and have been folded at the frontal zone of the Canyon Range plate (PC-Ca) (Figs. 51,74). The facies consists of chaotic to crudely horizontally stratified, quartzite-block and quartzite-carbonate boulder deposits organized into 20 m- to 30 m-thick, crude upward-fining sequences (Fig. 75). The term wedge or sheet is used here to describe bodies that are 500 m to a 1000 m wide along depositional strike and extend 300 to 600 m down dip. In the outcrop, individual sheets overlie irregular scour surfaces, whose 245 BwilnUfp. Formation Aga

Unnamed H Unit eq.? p Ii?

cgtVpmxmm: alluviaJ (an & i ' i . ? ! 5

South Flat œ ï 1 Unnamed Ï S

I rZ? 1

Simonson ' a-unooiitotrnity rJ-,l7 I®* Dolomite Dev.

Figure 75. Measured Oak Creek Canyon section with unconformities, depositional environments, formations, and inferred ages indicated. 2 46 margins are obscured by the extreme coarse character of the conglomerates. On a macro scale, sheets and wedges appear flat-based with respect to underlying sheets.

Block and boulder conglomerates exhibit poor sorting, a polymodal clast-slze distribution, an open to partially Infilled framework, and are exclusively clast supported (Fig. 76). Average maximum clast diameters range from 30 cm to 100 cm, although blocks In excess of 3 m width have been found. Isolated carbonate boulders are also present, yet do not exceed 1.5 m In diameter. Quartzlte-boulder llthologles In the lower folded part of the section (below U6-7 In Fig. 74) are Identical to those of the Precambrlan Caddy Canyon Quartzite and the Mutual and Inkom Formations, all of which are present In the Canyon Range and the burled Pavant 2 plate, whereas upper quartzlte-blocks and boulder llthologles (above US In Fig. 74) Indicate derivation from the Cambrian Tintic Quartzite, which constitutes the dominant unit In the Canyon Range plate (Millard, 1983; Holladay, 1984). In contrast, the thin and up to 10 m-wide, conglomeratic sandstones separating block and boulder sequences consist almost exclusively of carbonate pebbles and granules. The sandstones drape underlying conglomerates and form bedsets of planar, wavy, horizontal, and curved Inclined laminations, which are apparent due to the segregation of pebbles or granules from sand-sized particles.

Single blocks of Immense size were produced and emplaced In single events, such as rock avalanches or rockfalls. Blair (1987a), for example. 247

Figure 76. Photo of basal block and boulder conglomerates from the folded part of Figure 74. View is to the east. Note scale. 248 reported 4 m by 6 m blocks transported by catastrophic floodwaters. In contrast, chaotic to poorly stratified Indianola boulder-conglomerates were transported as bedload-concentrated flows during flood discharges. The dominance of large clasts, lack of stratification, and the complete absence of clay and silt in the interstices indicate that flow behavior was non-cohesive and dominated by frictional and inertial effects (Fisher, 1971). Such non-cohesive sediment-gravity flows are generated by high-water discharge in steep-gradient channels containing abundant sediment (Church and Desloges, 1984; Blair, 1987a). The stratified nature and the presence of isolated channel forms in overlying boulder deposits point toward a fluvial transport mode. The stratified boulder conglomerates are here interpreted to reflect transport in surge-type flows similar to those described by Baker (1973,1976), among others. Major reworking removed most of the original channel and bar perimeter surfaces. Within the deposits, quartzite boulders were most likely transported as bedload, while carbonate cobbles, pebbles, and granules are though to have formed the saltation and suspended load.

SCOUR-BASED BOULDER- TO COBBLE-CONGLOMERATE FACIES Scour-based, carbonate- to minor quartzite-boulder and -cobble conglomerates, together with intercalated pebble- to granule-sized sandstones and thin mudstones overlie the coarse quartzite-boulder conglomerates in the Canyon Range (between U6-7 and US in Fig. 74). Conglomerates of identical character, yet with different clast lithology are found in the Cedar Hills, the Gunnison Plateau, and at Lake Fork in 2 km to 5 km wide and up to 400 m deep, curved-downward scours here 249 called paleovalleys. Incised channel-form conglomerates or paleovalley-fills overlie fine-clastic fluvial strata in the Gunnison Plateau and the Cedar Hills, while scours in Lake Fork have cut into marine nearshore and offshore fine-clastic strata (Figs. 61,77).

The valley-fill deposits are comprised of quartzite clasts organized into fining-upward, 10 m- to 30 m-thic, and 300 m-long sheets and wedges (Figs. 78, 79). Individual sheets consist of numerous well-defined, interconnected boulder-conglomerate channel-forms and cobble-conglomerate interchannel areas (Fig. 80). At the base of sheets, unstratified and crudely to horizontally stratified boulder conglomerates (CU and OS: cr-h in Fig. 79) overlie curved and laterally restricted channel scours, which are up to 25 m wide and 5 m deep (Fig. 78,80). In contrast, horizontal, trough, and low-angle planar crossbedded sets of cobble- and pebble-sized conglomerates (OS; h-t-p in Fig. 79) occur in interchannol areas and in the upper portions of boulder-conglomerate channel fills. Boulder conglomerates are poorly sorted, exhibit a polymodal grain size distribution, and feature average maximum clast-sizes between 25 cm and 50 cm. On the other hand, cobble conglomerates are sorted, show clast imbrication, and feature average maximum clast sizes between 10 cm and 15 cm. Both coarse-conglomerate types are overlain by horizontally bedded to trough crossbedded sets of pebbly, granular to very coarse-grained sandstones (Shpb or St or Sh-t in Fig. 79) deposited over 2 m- to 5 m-wide, shallow, yet downward curved scours (sc in Fig. 79), which are located in interchannel areas and at the top of large channel-fill sequences (Figs. 250

Figure 77. Channel-form conglomerates or Incised valley fill strata in Lake Fork (to right of left white line) overlying a marine interval with nearshore sandstones (sst.) and mudstones (mdst.), which in turn overlies hummocky crossbedded (hcs) sandstones. Strata are vertical and unfolding to the southeast right). View is to the northeast. 251

Figure 78. Valley-fill conglomerates (Reddicks Canyon Conglomerates at type locality) in Chicken Creek Canyon (see Figure 52 for location) organized into 10 m- to 30 m-thick upward-fining sequences. View is to the north with rocks dipping to the east at 35 degrees. 252

tS;cr-h,p-sc

; i i

Rgure 79. Detail section of incised valley fill unit in Chicken Creek Canyon (see Figure 59) with maximum particle sizes (MPS) and facies abbreviations indicated (appendix E). NG, normal graded or upward-fining sequence; Rg. reverse graded or upward-coarsening sequence. 253

Figure 80. Photo of individual quartzite-congiomerate sheet with boulder-conglomerate channel-form (5 m wide and 2 m deep) in foreground (left) and crossbedded cobble-conglomerates and interchannel areas in the background. Note mudstone plug in upper parts of channel. View is up the depositional dip to the west with strata dipping east at 30 to 35 degrees. 2 5 4 78, 80). Completely homogenized, coarse to granule-sized sandstones (Sds-h in Fig. 79) and with silty mudstones drape well-imbricated cobble- to pebble-conglomerates in interchannel areas.

In summary, the boulder- to cobble-conglomerates, together with the pebbly to granular sandstones, and minor mudstones were transported by high-gradient streams following seasonal flood-surges under wet-tropical climatic conditions. Peak discharges affected the whole valley, caused large-scale scouring, and transported the coarsest clasts in bedload-concentrated flows within well-defined deep channels. Boulder were either deposited in longituda! bars as normally graded and crudely to horizontally stratified, openwork gravels, or as poorly stratified basal fills in the channels. In contrast, cobbles and pebbles migrated and accreted in gravelly transverse bars, gravelly dunes, or waves. Waning flow conditions caused bar emergence and flow diversion into interbar areas, the sand- to pebble-floored bar cross-over channels, and especially into gravel-based lateral channels. Decreasing flow allowed infilling of smaller channels and deposition of sand and minor mud ontop of bars, thus partially infilling the upper portions of the othenvise openwork gravels. The base of the Funk Valley Formation and the Reddicks Canyon Conglomerate both constitute Incised valley fills.

MATRIX-SUPPORTED CONGLOKŒRATE TO SANDSTONE FACIES Thin units featuring poorly sorted, matrix-supported cobble conglomerates and homogenized, pebbly to granular sandstones (MS in Fig. 79) occur in various measured sections. On the average, however. 255 such deposits are of minor importance, since these units are no thicker than 0.5 m to 1 m. Clasts up to 20 cm in diameter and float in a sand-sized matrix.

In Mellor Canyon, along the western margin of the southern Gunnison Plateau, deformed matrix-supported conglomerates and homogenized sandstones have accummulated in a 200 m- to 500 m-wide and up 50 m-deep, hollowed scour here called paleovalley (Figs. 67,81). The deformed conglomerates overlie conglomerates that are equivalent to the lower Six Mile Canyon member along a sharp surface, and consist of polymodal, matrix-supported, quartzite-conglomerates and interbedded quartzose sandstone, which together are arranged in complex folds (Fig. 82). The deformed valley-fill sequence is capped by Paleocene and Eocene trough cross-stratified cobble conglomerates overlying steep-walled scours that are 2 m to 10 m wide and up to 4 m deep.

The thin, matrix-supported conglomerate and sandstone units, and the thick and complexly folded interval in Mellor Canyon are interpreted to represent polymodal sorted, debris-flow conglomerates. The segregation of cobble-conglomerate and sandstone horizons within individual folds in Mellor Canyon shows that the apparent viscosity of the flow was sufficient to retain original bedding. The rheological behavior of the flow was thus most likely dominated by the interacting frictional effects of the non-cohesive particles (Fisher, 1971). This is also indicated by the absence of clay and inverse grading. Because no satisfactory depositional process and model is available in the literature, the deformed 256

Figure S i. Photo of north-facing slope in Mellor Canyon where conglomerates equivalent to the Six Mile Canyon Formation are unconformably (white line) overlain by a thick interval of deformed debris-flow facies (Deformed Interval). These in turn are onlapped by Tertiary North Horn deposits along an angular unconformity. View is to the southeast with strata dipping eastward. Section to base of Tertiary is approximately 350 m thick. 257

Figure 82. Photo showing deformed debris-flow conglomerates and interbedded sandstones exposed in the deformed interval in Figure 81. View is to the east. Hammer to left is 45 cm long. 258 conglomerates and sandstones are here interpreted as "frozen" deposits of a heavily-loaded, semi-fluid or partially liquified, coarse-clastic debris flow, which may be similar to the density-modified grain flows of Lowe (1976) or the heavily-freighted debris flows described by Rodine and Johnson (1976).

£AQ.Ii §-ASgOQiATlQNS The coarsest clastic alluvial facies of the Indianola supersequence accummulated as flood-related stream and debris-flow deposits within 2 km- to 5 km-wide and 350 m- to 400 m-deep paleotopographic depressions here identified as incised paleovalleys (Fig. 51). Deposition occurred in a coarse bedload-dominated, anastamosed-braided (terminology by Schumm, 1981) to braided stream system under a perennial, yet probably seasonal wet-tropical climate. Numerous high-discharge floods produced bedload-concentrated flows within a network of active channels or first-order channels, as well as subchannels. The channel pattern over the entire area resembles the anastamosing pattern of Schumm (1981), with more or less fixed-braided rather than mobile-braided channels.

First-order channels carried major portions of the load during flood conditions and avuised episodically. Second-order channels were only reactivated during flood periods, when waters inundated major bars and higher alluvial areas adjacent to major channels. The repeated interaction of channel and interchannel areas is suggested by the erosional truncation of the lateral margins of interchannel areas by 259 major channels, by the presence of multi-storey, small-scale scours, and by sandstone-mudstone drapes in interchannel areas. Where a mixed quartzite to carbonate source was present, as in the basal block and boulder deposits of the Canyon Range, quartzite boulders clearly constituted the bedload portion of the fluvial surge flows, whereas carbonate cobbles and pebbles were deposited under waning flow conditions. The various depositional environments together form the proximal bedload system of the Indianola Group.

To explain the coarse clastic nature of other ancient braided stream or alluvial-fan sequences, numerous workers have assumed sheetflood deposition or other proximal fan processes (Bluck, 1965; Steel, 1974; Reward, 1978a, 1978b; Mack and Rasmussen, 1984; Blair, 1987b). On that basis, it is common belief that wet and humid fans are characterized by perennial flow and steady erosional processes, as well as an explicit lack of m ass movement (Bull, 1972; Schumm, 1977; Miall, 1978). The character of Indianola strata, however, is incompatible with the existing humid fan model, as surge-flow deposits occur intergradationally with stream-flow and debris-flow deposits. Contrary to existing hunid fan models, this study interprets the above described combination of conglomeratic deposits to reflect flood-transport under a wet-tropical climate characterized by seasonal flow fluctuations and storm related floods. 260 BEDLOAD TO MIXED LOAD BRAIDED STREAM SYSTEM Four facies constitute the bedload to mixed-load braided stream system: (1) coalesced scour-fill conglomerates and sandstones: ( 2) coarse-based scour-fill sandstones; (3) incised channel-form sandstones in stratified detrital carbonates; and (4) massive mudstone intervals with isolated channel sandstones, siltstones, and coals. The facies are most common in the medial to distal foreland of the fold-thrust belt and together comprise the bulk of the fluvial deposits in the region.

COALESCED SCOUR-FILL CONGLOMERATE-SANDSTONE FACIES The facies forms 10 m- to 60 m-thick, upward-fining (NG) and upward-coarsening (RG) sequences (Figs. 83,84) of sheet or wedge geometry stacked in a book-like fashion. On a macro scale, sheets or wedges are up to 1 km wide with erosional flat and concave-downward bases and irregular, hollowed or convex-upward tops (Fig. 85). In the outcrop, sheet bases consist of laterally linked, irregular to curve-shaped scours overlain by numerous vertically stacked, 2 m to 5 m-thick, upward-fining sets of cobble-pebble conglomerates and sandstones (Fig. 83). Scours and coarse-clastic sets are arranged in a complex multi-storey pattern, forming sheets of varied thickness and conglomerate content.

Scour sizes at the base of sheets vary greatly in extent, are up to 200 m wide and 15m deep (Fig. 85), and are overlain by crudely to horizontally stratified cobble conglomerates (CS; or, h in Fig. 84) and unstratified remnants of fluvially reworked debris-flow Bed Inlerp. Environment Formation Age

Lacustrine Flagstaff Ls. ui g Pebbly to cobbly. eefb.toquanz. Mixed load cÿ-channeHorm» channels with North Horn submerged Angular unconformity (loodplain Cvb.'^uirU.CQl. ! Mass flow UnnametJ slope creep •Unit 1

Mixed load braided stream trough croiftwddtA ehiniuljotm. sandy overbank, interchannel

rnuMplowddd. Bedload braided Six tilile mixed load Canyon 1 Mixed load stream system h : ■ ^with^andy^

Figure 83. Measured section in Mellor Canyon (Sec, 17, T. 18 S., R. 1 E.) in the southwestern Gunnison Plateau (see Figure 52 for location). No fossils were found in the coarse-clastic section. Also shown are depositional environments, formations, and inferred age. Figure 84. Detailed section of mid-fan facies from lower part of Mellor Canyon section in Figure 83 with maximum particle sizes (MRS), facies abbreviations and interpretations indicated. Matrix-supported debris-flow deposits are shown in white on black background. Figure 85. Photo of stacked, upward-coarsening sequences of sheet and wedge geometry in Mellor Canyon where they are up to 1 km wide and up to 60 m thick (mid-fan interval). Bases of individual sheets are flat or concave-downward. The mid-fan interval is overlain by channel-form sandstones (sst.) and minor mudstones (mdst.) along a sharp surface (lower white line). View is to north with strata dipping eastward. 264 conglomerates (MS or CU in Fig. 84). The basal coarse-clastic strata are overlain by horizontal, trough, and planar cross-stratified, cobble- to pebble-sized, scour and fill conglomerates (CS,h-t-p in Fig. 84). The small-scale, upward-coarsening or upward-fining scour and fill sequences are arranged in a multi-storey pattern and make up the bulk of individual sheets.

The up to 60 m-thick conglomerate sheets commonly thin, however, along depositional strike over distances of several hundred meters to two kilometers to form 5 m- to 10 m-thick sheets (Fig. 85) intercalated with 10 m- to 20 m-thick, rippled and horizontal to wavy laminated sandstone intervals (Sr, Sh, SwI in Fig. 86 ). Basal scours are overlain by diversely stratified, scour-fill, cobble-pebble conglomerates (CS; h-p-t or CS; t-h-hs in Fig. 87). Upper portions of thin sheets comprise pebbly scour-fill sets of varied-sized trough cross-strata (Ssc, Stsc-pb, Shpb, SItpb in Fig. 87) overlain by wavy-bedded, ripple-stratified, finer-grained sandstone (SwI or Sr in Fig. 87). Smaller scour surfaces are complexly coalesced and typically outlined by pebbles.

The highly intergradational subfacies together represent transport and deposition in shallow, lateral migrating braid channels on a convex-upward, low relief alluvial fan. Constant migration and switching of interconnected channels resulted In the reworking of channel perimeters and bar macro-forms. The pattern is that of a complex shallow-braided channel network with no dominant hierarchy of channels. Flow conditions were perennial, yet marked by fluctuations in 265

Figure 86 . Photo of lower-fan interval located south of Mellor Canyon, offset along depositional strike from the mid-fan section shown in Figure 85. Shown are laterally linked channel-form sandstones and pebble-conglomerates separated by thick, fine-grained sandstone intervals. View is to the northeast with strata dipping east. 266

Figure 87. Détail section of lower-fan faciès interval from the Meior Canyon section in Figure 83. Snown are the mid fan to lower fan transition, as defined by the maximum particle sizes (MPS) and the facies (symbols and abbreviations in appendix E). 267 runoff, as indicated by the complex coalescence of the varied-grained and diverse stratified scour and fill deposits.

On a macro scale, gravelly mid-fan strata accummulated in up to 60 m-thick sheets that pinch out laterally and interfinger with lower-fan and inter-fan deposits (Figs. 85, 86 ). The strata together form a substantial part of the Six Mile Canyon Formation and its equivalents in the Gunnison Plateau, the Cedar Hills, and the Pavant Range (Fig. 51) Source areas were dominated by Paleozoic carbonates and to a minor extent by Preoambrian-Cambrian quartzites, as demonstrated by the clast iithology distribution within the fan bodies. Based on diagnostic conodonts recovered from different carbonate clast types, it can be shown that clasts of this facies were solely derived from Ordovician, Devonian, and Mississippian carbonates (J. Geitgey, 1987, pers. com.), which constitute the Pavant 2 plate in the Pavant and Canyon Ranges (Millard, 1983; Holladay, 1984). In general, the facies are similar to ancient gravel-dominated proximal to medial braided stream deposits described McGowen and Groat (1971), Steel et al. (1977), Reward (1978a, 1978b), Rust (1978), and by Ramos and Sopena (1983), among others.

COARSE-BASED SCOUR-FILL SANDSTONE FACIES The coarse-based, scour-fill sandstone facies comprise the lower portion of large-scale, upward-fining fluvial sequences that are up to 350 m-thick and best exposed in the Gunnison Plateau (Figs. 59,70). Basal bounding surfaces of these sequences consist of numerous linked, curved-downward scour surfaces overlain by broadly lenticular pebble 268 sandstones and minor cobble conglomerates, which grade upward into uniform coarse, trough crossbedded sandstones (Fig. 88). Individual scour surfaces curve downward and are 25 m to 50 m wide and up to 1.5 m deep. Overlying gravelly strata are clast-supported, polymodally sorted, and exhibit numerous groove and load casts, as well as elongate scours on basal surfaces. The cobble- and pebble-sized clasts line scours and form stacked, trough cross-stratified lenticular sets of pebbly to minor cobbly sandstone. Maximum clast sizes do not exceed 10 cm and average around 2 cm to 5 cm. The lenticular, conglomeratic sandstone channel-forms are arranged in a multi-storey pattern and coalesce laterally to form tabular bodies several tens of kilometers wide and up to 150 m thick (Fig. 60).

Basal coarse-clastic strata are overlain by monotonous and uniform coarse-grained sandstone sequences that are up to 75 m thick and continuous along strike for several tens of kilometers. Sandstone intervals consist of 5 m-to 10 m-thick, repetitive sequences of poorly defined, pebble-lined scours overlain by trough cross-strata of varied scale (Figs. 59,89). Basal broad-lenticularly bedded, coarse sandstones form sets up to 50 cm- to 100 cm-high and up to 5 m long, which grade upward into 50 cm high and 1 m long trough crossbedded sets of finer sandstone (Fig. 88). Less common are solitary sets of planar sross-strate. Thin intervals of rippled and horizontally iamlnsted fine-grained sandstone or siltstone at the top the sequences are truncated by the overlying pebble-lined scour. 269

Figure 88. Photo of uniform coarse-grained, trough crossbedded sandstones in Chicken Creek Canyon from the Upper Funk Valley Sandstone/Shale intenral. View is to the east with strata dipping east at 35 degrees. Hammer to left is 45 cm long. Figure 89. Photo of the coarse-clastic base of a 350 m-thick, upward-fining interval in Chicken Creek Canyon (base of Upper Funk Valley Sandstone/Shale). Basal conglomerates above the bounding surface (solid to dashed line) have eroded into the fine-clastic top of another underlying, upward-fining interval. View is to the southeast with strata dipping east. 271 On a macro scale, basal conglomeratic channel-forms and overlying uniform trough-crossbedded sandstones form the basal coarse-clastic portion of the up to 350 m-thIck sequences of the Funk Valley Formation equivalent In the Gunnison Plateau. The sheets are strike-aligned over distances greater than 50 km and thin baslnward over several tens of kilometers. The lowermost sequence overlies flat to Irregular surfaces that formed by erosion Into transitional alluvial to marine deposits equivalent to the Allen Valley Shale In the Gunnison Plateau and the Cedar Hills. Clast Iithology distributions at the base of the overlying alluvial sequences Indicate a mixed carbonate-quartzlte source. The basal pebble to cobble conglomerates of the up to 350 m-thIck and laterally continuous sequences are Interpreted to represent deposition by mixed gravel-sand braided rivers situated In a low-rellef alluvial plain. The elastics were transported In rapidly switching, shallow brald-channels. The uniform, scour-fill sandstones overlying the basal conglomerates represent scouring and dune/megarlpple migration and deposition In sinuous to braided channels of sand-dominated rivers. The conspicuous absence of bar-form stratification In the pebble- to cobble-conglomerates and the lack of channel perimeters In the sandstones Is Interpreted to mark an unstable channel behavior. The alternation of coarse-stratlfled and fine elastics In the upper sandstone Interval reflects deposition under waning flow conditions. The facies are similar to ancient distal gravel- to sand-dominated braided stream deposits described by Smith (1970,1971,1974), Cant and Walker (1976, 1978), Walker (1976), Cant (1978), and Rust (1978), among others. 272 INCISED CHANNEL SANDSTONE AND DETRITAL CARBONATE FACIES Incised channel-sandstones and detrital carbonates constitute the fine-clastic upper portions of the 350 m-thick, coarse to fine sequences of the Funk Valley equivalent in the Gunnison Plateau (Fig. 90). The fine clastic facies forms 50 m- to 100 m-thick stratal packages, consisting of numerous stacked, 0.5 m- to 1 m-thick, diverse stratified, tabular units and intercalated broad lenticular channel forms. The fine-clastic interval overlies the uniform-coarse sandstones with sharp contact and is in turn truncated at its top by the lower bounding surface of another upward-fining, coarse-clastic unit.

Intercalated broad lenticular channel forms consist of trough cross-stratified, lenticular sets of carbonate-pebble sandstones, which overlie curved-downward scour surfaces up to 50 m wide and 1.5 m to 2 m deep (Fig. 90). The varied-sized and shallow-based channel forms are flat-topped and exhibit well-defined lateral margins, indicating that above size estimates represent actual maximum scour sizes. Pebbly fills are clast-supported, polymodally sorted, and exhibit maximum clast sizes that average around 2 cm (Fig. 59). Pebbly channel-forms are set in thick sequences of fluvially deposited detrital carbonates that consist of fine to coarse limestone and dolostone rock fragments and show an abundance of ripple cross-stratification (Fig. 91), including elimbing-rippie laminae sefe, wedge-planar cross-stratification, trough cross-stratification, contorted cross-laminated sets, and microcross-stratified in sets no greater than 1 cm to 2 cm high. An abundance of preserved structures, such as ripple forms, load and flute 273

Figure 90. Photo of coarse-clastic base of Lower Funk Valley Sandstone/Shale in Chicken Creek Canyon. Quartzite-carbonate conglomerates over lie bounding surface and unconformity (US) that has cut into the fine-grained interval comprising stacked channel-form sandstones set in well-stratified, detrital carbonates. View is to the north with strata dipping east. 274

Figure 91. Photo of ripple forms (large one underlain by small ones) in detrital carbonates in Chicken Creek Canyon. 275 casts, rhizoliths, macerated plant material, whole leaf compressions, as well as biogenic traces, are present throughout the strata. The 4 cm high and 20 cm long trough cross-sets, together with the sub-horizontal to wavy laminated sets comprise more than 60 percent of the detrital carbonate facies.

Plant fossils are tentatively identified to include leaves of the genera Phoenicites (Fig. 92), Ulmus ?, Ficus, and Populus (after Tidwell, 1975). Feeding and foraging traces of arthropods and worms on bedding surfaces are dominated by meniscate forms referrable to the Scoyenia ichnofacies and include Anchorichnus ooronus (Fig. 93) as dominant species, various Planolites sp., Edaphichnium, "Type 5" burrows (Sown and Kraus, 1983), and indeterminate meniscate forms that are 1 mm to 3 mm in diameter (Schwans and Zawiski, 1986).

The incised channel-sandstone and detrital carbonate facies are interpreted to represent deposition in a low-sinuosity, mixed to suspended load, fixed-channel system set within a wide, low-relief floodplain. Although single-channel perimeters are reasonably well defined, width-to-depth ratios range from 10 to 30, thus placing the channels between the ribbon and sheet classes of Friend, Slater, and Williams (1979). The dominance of trough crossbedded pebbly channel fills indicates that bedioad transport occurred in sinuous-crested dunes and ripples under steady flow conditions. Episodic shallow flooding of interchannel areas resulted in traction-deposition of fine-clastic carbonate fragments, which othenvise constituted the suspended load. 276

Figure 92. Photo of an impression of Phoenicites in uniform coarse sandstones in Chicken Creek Canyon (base of Upper Funk Valley Sandstone/Shale) (see Figure 53 for location). 277

Figure 93. Photo of the ichrscfcssi! A nuharichnus coronus from detrital carbonate Interval in the Lower Funk Valley Sandstone/Shale member in Chicken Creek Canyon (see Figure 53). 278 The wetting of vegetated floodplain areas not only provided the appropriate wet-sediment environment for several deposit-feeding and foraging arthropods and worms, but also encouraged the formation of stable channel margins.

MUDSTONE. COAL. AND CHANNEL SANDSTONE FACIES Massive mudstones with numerous intercalated, finely laminated siltstones, few channel-sandstones, and coals together accummulated in intervals up to 300 m-thick stratal that commonly overlie the coarse member of the bedload to mixed load braided stream system along a sharp surface. The facies interval is represented by the upper Six Mile Canyon Formation member in the Wasatch Plateau and by the South Flat Formation and its equivalent in the Gunnison Plateau and Cedar Hills, respectively (Figs. 51, 59,63). In general, fine-clastic facies are poorly exposed, making detailed studies and interpretation difficult.

In the western Wasatch Plateau, the facies features massive carbonaceous mudstones with few intercalated coals, numerous thin interbeds of laterally discontinuous, ripple-stratified sandstones and siltstones, and broad lenticular, trough-crossbedded, coarse-grained sandstones (Fig. 63). Lenticular coarse-grained sandstones overlie flat to irregular and, in some cases, pebble-lined scours that are up to 50 m wide and less than 0.5 m deep. Conversely, ripple-stratified fine sandstones are sharply based in mudstone, are commonly sole-marked, and fine upward into horizontal to wavy laminated, ripple-stratified to micro-crossbedded, fine to very fine sandstone and siltstone. Wisps of 2 79 mudstone encase the small-scale sets, thus Impressing a wafer-like appearance or structure upon the very fine-grained rock. The latter is brownish-grey in color, because of disseminated organic matter, and displays an abundance of leaf compressions and other plant remains.

Where mudstone intervals are thin, as in the Gunnison Plateau and the Cedar Hills, coarse, granule- to pebble-based, lenticular sandstones grade upward into trough-crossbedded and ripple-stratified, fine-grained sandstones (South Flat Fm. in Fig. 59). Several such upward-fining sequences are arranged into a multi-storey pattern. Less common are sets of planar cross-strata and sots of wavy lamination or in-phase current ripples, while coals are present in thin beds and as channel fills (Hunt, 1950; Hays, 1960).

The fine-clastic intervals in both the Gunnison Plateau and in the Wasatch Plateau yielded diverse megafloras of similar composition. Hunt (1950, p. 63) reported Ficus planiœstata Lesquereux, F. praelatifolia (Knolwton), Cinnamomum, and Sabalites montanus (Lesquereux) Dorf from the Gunnison Plateau, while this study recovered plant compressions from both areas and tentatively identified them as Phoenicites, Ginkgo, Jugians ?, Dryophyiium and Ficus (after Tidwell, 1975) (Fig. 94).

In summary, the coal-bearing mudstone-siltstone and channel-sandstone facies accummulated in a wide and ponded muddy floodplain dissected by shallow, low-sinuousity, sand-floored channels. 280

Figure 94. Photo of plant compressions from siltstones in the upper member of the Six Mile Canyon Formation at the type locality (see Figure 63 for location). A specimen of Ficus is seen just to the left of the scale. 2 8 1 Fine sands and silts were introduced into vegetated flood basin areas through sheet-like crevasse-splays during high-water stage. Laterally discontinuous sheets of pebble-based, ripple-stratified sandstones and siltstones are interpreted to represent such distal crevasse splays, where rapidly waning, but slightly pulsating flow across the splay surface formed varied-sized sets of small and mud-lined ripples. Broad lenticular, trough crossbedded, coarse-grained sandstones represent the fills of shallow channels with stable margins. Laterally extensive coals formed in inundated parts of the floodplain in areas distal to the thrust front (i.e. Wasatch Plateau), while channel coals accummulated in abandoned and ponded channels in the more proximal areas (i.e. Gunnison Plateau). Floras similar to the assemblage reported here have been described from the stratigraphically correlative Blackhawk Formation (Fig. 51), where they have been interpreted to indicate wet and humid lowland conditions (Tidwell, 1975).

FACIES ASSOCIATIONS The coarse- to fine-clastic alluvial facies of the Indianola supersequence were deposited by bedload- to mixed load-dominated, mobile-braided as well as fixed-single channel streams in the foreland of the fold-thrust belt under a perennial, yet markedly seasonal, wet-tropical climate. Rapid lateral migration of cobble-dominated braided stream stretches proximal to the thrust-fold belt produced sheets several tens of meters thick and up to two kilometers wide. Several sheets together form a depositional body several kilometers wide and over 350 m thick, which is here called an alluvial-fan 282 mega-sequence (terminology of Howard, 1978a). Every sheet within the mega-sequence represents a depositional cycle on a large compound alluvial fan.

In Indianola sections, upward coarsening or reverse grading (RG) documents reactivation of major fan segments and is characterized by decreasing sandstone percentages, and increasing conglomerate bedset thickness and average maximum clast size (Figs. 84,87). Conversely, upward-fining or normal grading (NG) documents abandonment of a major fan segment and is represented by an increase in sandstone percentage associated with decreases in bedset thickness and clast size (Figs. 84, 87). Consequently, small-scale sequences represent autocyclic depositional episodes within the alluvial fan system.

In contrast, the pebble-cobble sandstones, uniform coarse sandstones, and the incised channel forms set in thick overbank sequences in the Gunnison Plateau are interpreted to have accummulated in a low-relief foreland environment. Rapid and extensive lateral movement of distal braided streams deposited a facies succession dominated by 350 m-thick, upward-fining and strike-aligned sheets several tens of kilometers wide and tens of kilometers long. Lower coarse-clastic portions reflect initial high-gradient deposition by mixed-load braided streams, while overlying sandstones and fine-ciastic intervals document low-gradient deposition, first by sandy-braided and later by sandy and single-channel low-sinuousity streams. Farther down-depositional dip, ribbon-like channel sandstones set within thick overbank sequences 283 accummulated in low-sinuousity, stable and single-channel streams in wet and humid lowland areas.

Such large-scale cyclicity of alluvial fan and floodplain sequences has commonly been interpreted as sedimentation episodes driven by tectonic uplift or basin-floor subsidence (Steel et al., 1977; Steel and Aasheim, 1978; Mack and Rasmussen, 1984; Arguden and Rodolfo; 1986; Blair, 1987a, 1987b; Heller et al., 1988). These mechanisms are difficult to employ, however, in a foreland basin, where short-term increases in the rate of tectonic uplift (during thrust terrane emergence) and basin subsidence are of insufficient amplitude to produce the necessary relief for the post-orogenic basinal penetration of fluvial gravels, as envisioned by Heller et al. (1988), for example. By constructing a simple mass-balance model that relates sediment size to transport distance within a foreland basin, Paola (1986) showed that extensive thin grave! sheets are an indicator of low and uniform subsidence, implying reduced tectonic activity. On that basis, this study attributes the relative variability in the proportions of coarse and fine elastics within major stratigraphie intervals (i.e. lower Funk Valley Sandstone/Shale) to the aggradational behavior of laterally migrating depositional systems in response to changes in the accommodation potential of the foreland basin.

FLUVIAL-DOMINATED SHORELINE SYSTEM Two major facies form the fluvial-dominated shoreline system: (1) tabular-lenticular, pebbly, minor cobbly, granular distributary 28 4 sandstones set in interchannel mudstone-siltstone intenrals; and ( 2) lenticular inner-outer channel mouth bar sandstones and shelly banks set in mudstones. The facies crop out along the western margin of the Wasatch Plateau and together constitute approximately 50 % of the transitional alluvial to marine deposits of the Indianola supersequence (Figs. 53, 53, 55, 69). The strata are organized into numerous 5 m- to 20-thick, fining-upward sequences, as well as few 2 m- to 5 m-thick, coarsening-upward sequences. On a macro scale, the latter are stacked into 150 m- to 250 m-thick stratal packages that overlie large-scale, curved-downward scour surfaces, which are interpreted as shallow incised paleovalleys. The valleys are up to 7 km wide and 100 m deep, as indicated by high-altitude aerial photos (base of Funk Valley Fm. in Fig. 69). The stratal packages parallel the thrust front and thin basinward over several tens of kilometers.

DJSTBIBUTABy AND INTERCHANNEL FACIES Individual upward-fining sequences overlie erosional scours, while coarsening upward sequences are sharp-based (Figs. 54,95). Both sequences are overlain by massive mudstone-siltstone facies along a sharp surface. Coarse-grained deposits are well exposed, but finer-grained deposits are poorly exposed, making detailed studies and interpretation of the lateral facies relationships difficult. 285

Figure 95. Photo showing stacked, upward-fining sequences of the Sanpete Formation (right of left line, which is U2) in Salina Canyon. Sequences are interpreted to represent distributary channels that shifted into mudstone-dominated interdistributary deposits. View is to the north with strata below flatlying Tertiary unfolding to the east. 286 Tabular-lenticular, pebbly, minor cobbly, granular to coarse sandstones form the base of 5 m- to 20 m-thick, upward-fining sequences. Scour surfaces curve downward or undulate irregularly, thus maintaining a more or less constant position with respect to the strata beneath, and can be traced laterally for 100 m to several kilometers (Figs. 68,69). Basal granular, pebbly, coarse- and medium-grained sandstones overlying the large-scale- scour surfaces are arranged in up to 1 m high and 2.5 m long sets of very low-amplitude, trough crossbedding (Fig. 96). Subordinate scours and reactivation surfaces are found throughout the basal units and are outlined by pockets or strings of pebbles, rip-up clasts, and shell fragments.

The basal coarse-grained and low amplitude trough-crossbedded sandstones are overlain by coarse- to medium-grained sandstones, which exhibit smaller trough-crossbedded sets capped by non-parallel wavy to horizontal laminated, and/or current-rippled, finer sandstones. The finner grained sandstones together form the upper fine portion. Upward-fining sequences are separated by mudstone-siltstone intervals, which overlie sequences along a sharp surface (Fig. 97).. The latter is markedly bioturbated and features vertical to subhorizontal cylindrical burrows of Planolites sp., ?Skolithos sp.,as well as of several medium to small meniscate forms (Schwans and Zawiskie, 1986) (Fig. 98). In addition, log molds, macerated plant materials, and diverse assem blages of brackish-water and marine mollusks are common throughout the upward fining rock intervals. Assemblages include such marine bivalves 287

Figure 96. Photo of large-scale, low-amplltude, trough crossbedding in Six Mile Canyon at the base of 5 m- to 20 m-thick, upward-fining sequences of sheet geometry (base of Lower Funk Valley Sandstone). View is to the north with strata vertical to overturned to the west (left). Figure 97. Photo of stacked upward-fining sequences (approx. 10 m thick) of sheet geometry at the base of the Lower Funk Valley Sandstone. Basal coarse sandstones feature large-scale, low-amplitude trough crossbedding shown in Figure 36 and fine upward into finer grained, current-rippled sandstones (poorly exposed part between sandstone sheets). View is to the south with strata dipping east (left). 289

Figure 98. Photo of vertical to subhorizontal burrows of Planolites sp. and bertical shafts of 7Skolithos sp. from finer grained sandstones in upper fine part of the upward-fining sequences of Figure 97. Scale in centimeters. 290 as Inoceramus or Mytiloides sp., Aphrodina sp., Pleuriocardia of. P. pauperculum (Meek), Pseudoptera propleura (Meek), Tellina ? isonemla (Meek), T. white! (Stanton), Corbufa sp., the scaphopod Er\taUs sp., and the brackish-water gastropod Xenophora sp. (W. A. Cobban, 1987, pers. com.) (see lowermost bivalve assemblage in Fig. 55).

The siltstone-mudstone intervals separating the coarse tabular-lenticular sandstone bodies are up to 50 m-thick and contain intercalated, laterally discontinuous, thin horizons of horizontal laminated sandstone, lenticular- to flaser-bedded fine sandstone, as well as homogenous or soft-sediment deformed coarser sandstone. Numerous small-scale, discrete upward-coarsening sequences occur within the mudstone-siltstone intervals. Basal finer portions of the sequences consist of 5 m- to 8 m-thick, flaser- to lenticular-bedded mudstones (Fig. 99), fine sandstones, and siltstones. This finer interval is in turn overlain by the upper coarse part of the sequence, which consists of ripple-laminated, medium to coarse sandstones and medium-sized sets of trough crossbedding, which are capped by wave-oscillation ripples and sets of wavy non-parallel laminae. The upper coarser portion is commonly intensely bioturbated and overlain by yet another massive mudstone-siltstone interval along a sharp surface. Inoceramid debris and other shell fragments are found throughout the unit. Figure 99. Flaser- to lenticular-bedded, fine sandstone in the upper fine interval of the upward-fining sequence of Figure 97. 292 Large-scale, eroslvely-based and planar-topped, upward-fining sandstone-mudstone sequences are interpreted to represent major distributary channels that shifted into mudstone-dominated interdistributary deposits by avulsion from a pre-existing distributary. Avulsion was rapid and resulted in basal scouring of interdistributary bay deposits. The presence of coarse lags, rip-up clasts, discordant erosion surfaces, and fragmented biogenic material indicates that distributaries experienced repeated flooding. The infilling and final abandonment of distributaries is denoted in the upward fining trend and the overall decrease in bedset sizes. Individual channel bodies are limited to a few hundred meters in width and reach maximum thicknesses of 20 m. Following abandonment, biogenic reworking affected the upper parts of channel fills.

Thick mudstone-siltstone intervals document fine-grained interchannel bay deposition and infilling following repeated sediment incursions from adjacent distributary channels via crevasse splays or overbank flow. The presence of thin lenticular- to flaser-bedded strata, together with homogenous and soft-sediment deformed units implies repeated, yet minor sediment influx into areas otherwise characterized by quiet-water sedimentation. Interdistributary bays were most likely dominated by brackish-water conditions, as indicated by the fauna. Sharp-based, upward-coarsening sandstone sequences within bay intervals are here interpreted as small-scale crevasse splays. Minor crevasse splays formed during sudden and discrete incursions of sediment into interdistributary bay areas, and were subsequently 293 reworked by waves into subordinate sand spits. According to Elliot (1974), such wave reworking of crevasse-sand units ultimately produces intensely bioturbated coarsening-upward sequences with trough cross beds, symmetrical ripples, and horizontal lamination. Similar distributary channel to interdistributary bay sequences have been described by Elliot (1974,1976a, 1976b), Morton and Donaldson (1978), Fielding (1984,1986), and Hopkins (1984) among others.

[NNER-OUTER CHANNEL MOUTH BAR FACIES The facies forms upward-coarsening, mudstone-sandstone packages that are up to 30 m thick and over 100 m wide (Figs. 54,55,100). Individual upward-coarsening sequences consist of a finer lower portion sharply overlain by a coarser, sandstone-prone portion. In the outcrop, the basal finer interval features horizontal-laminated mudstones and intercalated ripple-laminated, flaser to lenticular bedded sandstones (Fig. 101). The low-amplitude ripple forms are well preserved, lined by mudstone laminae, and arranged in a non-parallel fashion. Basal surfaces of rippled sandstones feature an abundance of varied-sized sole marks, such as flute and groove cast, as well as subhorizontal burrows of Planolites sp. (Schwans and Zawiskie, 1986).

The lower mudstone-siltstone interval is abruptly overlain by massive, fine to msdlum sandstones, which are either well-stratified or feature a distinct bioturbate texture and an abundance of poorly 294

Figure 100. Upward-coarsening mudstone (in slope to left) and sandstone sequence in Salina Canyon (see Figure 54) here interpreted to represent outer channel mouth bar deposits set in interdistributary mudstones and overlain by inner mouth bar channels and shelly bar deposits. View is to the north with strata dipping to the east (right). Sandstone body is approximately 15 m thick. 295

Figure 101. Photo of a lower fine interval in Salina Canyon (see Figure 54) interpreted to represent distal mouth bar to interdistributary bay deposits. Flaser- to lenticular-bedded fine sandstones are intercalated in mudstones. 296 horizontally stratified shell material. Bioturbated fine-medium sandstones contain abundant poorly-horizontally stratified and layered to tightly packed shell fragments and whole pelecypod valves, including large shells of the marine bivalve Aphrodina sp., ?Cympophora, the brackish-water oyster Crassostrea soleniscus (Meek), and the inoceramid Mytiloides cf. M. columbianus (Heinz) (W. A. Cobban, 1987, pers. com.) (Fig. 102). In contrast, well-stratified sandstone bodies show large-scale horizontal and inclined laminae sets at their base grading into large sets of low-amplitude, trough cross-strata. Inclined laminae sets are in some cases lined by clay drapes and truncated by subtly inclined, irregular scour surfaces. These scour surfaces would be indiscernable from inclined laminae sets, if they were not lined by coarse sand and to a minor extent by shell hash. Wavy non-parallel ripple sets and asymmetrical ripple forms cap the upward-coarsening sequence and are ultimately overlain by another massive mudstone-sandstone interval along a sharp nonerosive surface.

In a few cases, 2 m- to 4 m-wide and up to 2 m deep, curved-downward, symmetrical scours are superposed onto and have eroded into upper sandstones of the 30 m-thick sequences (Fig. 103). Scours are broadly u-shaped, lined with shell debris, and overlain by vertically stacked sets of convex-downward laminae sets that line the scour surface and are more or less symmetrical about the axis of the channel scour. The degree of set curvature decreases upward within the symmetrical channel scours. 297

Figure 102, Photo of shell material in shelly bar of Figure 101 (central part of sandstone body). Shell framents are mainly of the brackish-oyster Crassostrea soleniscus. 298

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■' 'fi* '

Figure 103. Photo showing symmetrical scour in Salina Canyon here interpreted as minor distributary channel superposed onto outer channel mouth bar sandstones. View is up the depositional dip and to the west. 299 In summary, finer lower mudstone-sandstone parts of upward-coarsening sequences are interpreted to represent distal mouth bar to interdistributary bay or lagoonal fine-clastic sedimentation in a minor deltaic-distributary channel. Asymmetrical ripple-form sandstones were deposited following sediment incursions from flooded distributary channels, while non-parallel wavy ripples and laminated mudstones represent intermittent wave reworking and suspension deposition, respectiveiy. Together, the facies record the interaction of channel discharge, wave rewod(ing, and fluvial to brackish-water bay sedimentation in distal mouth bar areas.

The lower finer interval is abruptly overlain by massive and large-scale trough cross-stratified sandstones here interpreted as outer channel mouth bar deposits. The presence of inclined laminae sets and subordinate truncation surfaces lined with shell debris implies wave reworking of outer mouth-bar surfaces, while sparse clay drapes on set surfaces indicate minor tidal influence and flow fluctuation. The regime in front of the mouth bar is characterized by the complex interaction of constructive and destructive processes. Finally, the avulsion of minor distributary channels or crevasse channels during delta progradation may have resulted in symmetrical channel incision into preexisting bay and mouth bar deposits, thus superposing inner stream mouth bar channels onto outer stream mouth bar cross-strata. Such superposing has been described, for example, by Fisk (1955), Brown (1979), and Hopkins (1984). 300 EACJE§ A g ?Q g (ATIONS Various criteria can be used to classify the response of depositional regimes to the interaction of fluvial and marine processes. As a result, river-dominated deltas, such as the Mississippi Delta, may be contrasted with the tide-influenced Ganges-Brahmaputra Delta (Coleman, 1969; Coleman and Wright, 1973; Wright and Coleman, 1973; Fisk et al., 1954; Fisk, 1955,1961). The tabular to lenticular channel sandstones, channel mouth bar deposits, shelly bars, and marine bay mudstones located within shallow-incised valleys are common indicators of a fluvial-dominated shoreline and delta system. The system is represented to a varying extent in all Indianola Group formations and documents the aggradational and progradational behavior of fluvial-dominated delta systems located at the margin of the Cordilleran foreland basin.

The stacking of several coarse, pebbly to minor cobbly, tabular-lenticular distributary channel bodies within shallow-incised paleovalleys (basal part of Lower Funk Valley Sst. in Figs. 53,68) is interpreted to reflect the progradation of a coarse-clastic shoreline and delta into a shallow marine basin. On a general scale, Indianola delta progradation caused the large-scale erosion of older offshore marine strata (Allen Valley Shale in Figs. 53,68). Major broad and shallow distributary rivers subsequently infilled paleolows by depositing their gravelly to coarse sandy bedioad and minor muddy suspended load in shallow marine to brackish-water lagoons and interdistributary bay areas. Infilling of the paleolows, as well as individual distributaries was flood-induced and multi-episodic, as evidenced by the stacked tabular 301 bodies and the multiple scour and fill patterns within them. Bays sequences are thin, indicating that most fine-clastic sediment was transported farther offshore and not stored in nearshore areas.

In contrast, delta aggradation is marked by interdistributary bay intervals up to 60 m-thick, well preserved inner-outer river mouth bar bodies, and numerous stacked, small-scale coarsening upward crevasse-spit sequences (mid part of Lower Funk Valley Fm. in Figs. 53, 71). Shallow marine to brackish-water bays were filled by crevasse-supplied sediment incursions from river distributaries. Wave reworked crevasse splays In outer parts of bays to form sand spits or shelly beach-bar sequences. Submerged outer river mouth bars were occasionally incised by small u-shaped channels with distinct aggradational fills. Fisk (1960) postulated that such fills form in response to diminishing flow upstream, causing stream mouth aggradation and back filling. In contrast, Hopkins (1984) pointed out that u-shaped channels can also be infilled incrementally under bankfull flow in a tidally influenced regime. Periods of delta aggradation are thus especially evidenced by increased storage of fines in bay areas and to a minor extent by wave- and tidally-influenced deposits, possibly due to the increasingly submerged state of bay areas and distributaries.

Two types of coarse-grained deltas are recognized in general: fan deltas and braid deltas. Fan-deltas are formed by the progradation of an alluvial fan into a standing body of water, while braid deltas form by the progradation of a braided river into a standing body of water (McPherson 302 et al, 1987). McPherson et al. (1987) point out correctly that the distinction is important, as numerous workers unnecessarily associate coarse-grained, bedload-dominated deltaic sedimentation with fan-deltas (McGowen, 1971; Erxleben, 1975; Galloway, 1976; Schumm, 1981 ; Postma and Roep, 1985). The distinction, however, provides the valuable diagnostic element that fan-deltas are principally fan shaped, while braid deltas are not (Nilsen, 1985). In that context, Indianola deltaic deposits are here interpreted to represent sedimentation in a sandy to minor gravelly braid-delta setting. As in braided rivers, Indianola fluvial-deltaic strata are coarse sandy to gravelly and feature an abundance of trough cross-stratification and normal grading of gravel beds. Indianola delta progradation first produced conspicuously extensive tabular bodies, which are marked by lateral internal continuity of lithofacies. Upper parts of progradational delta sequences, however, are marked by less tabular and more lensoid delta bodies. Together, these characteristics mark the progradation and later aggradation of a sandy to gravelly shoreline and delta into a shallow, but deepening marine basin.

WAVE-DOMINATED SHORELINE SYSTEM Two major facies constitute the wave-dominated shoreline system or high-energy shoreline complex: (1) aggradational shoreline facies featuring rippled mudstone-siltstone intervals, hummocky cross-stratified and low-amplitude trough crcssbeddsd fine sandstones, and bioturbated smaller trough cross-strata; and (2) transgressive shoreline facies consisting of low-amplitude trough crossbedded sandstones, hummocky cross-strata, and, to a minor extent, pebbly 303 channel forms and mudstone-siltstone intervals. Both shoreline facies are well-exposed along the western margin of the Wasatch Plateau and consist of numerous stacked, 5 m- to 100 m-thick, coarsening- and fining-upward sequences.

AGGRADATIONAL SHORELINE FACIES The aggradational shoreline facies can be subdivided into three subfacies, which together form 10 m- to 30 m-thick, upward-coarsening sequences (Upper Funk Valley Sandstone in Fig. 53). The lowermost subfacies consists of interbedded mudstones and fine sandstones, which accummulated in intervals up to 15 m-thick. The fine sandstones are. In some cases, convoluted, mostly planar laminated, trough crossbedded, and/or exhibit small-scale hummocky cross-stratified sets that are 10 cm to 30 cm high. Numerous well-preserved symmetrical and asymmetrical ripple forms are found throughout the basal fine-clastic part of the subfacies. Set sizes and sandstone frequency increase toward the top of the subfacies, which is abruptly overlain by the massive, uniform fine-grained sandstones of the second subfacies along an irregular, yet sharp surface.

Basal sandstones of the second subfacies are partially homogenized to convoluted, parallel wavy-laminated, and horizontally laminated. The basal sandstone is up to 50 cm thick interval and overlain by fine sandstones, featuring either large-scale, low-amplitude trough cross-stratification or hummocky cross-stratification (HCS in Figs. 53, 54,55). The low-amplitude trough cross-strata exhibit wavelengths of 2 304 m to 5 m and are organized into sets up to 50 cm high. Set surfaces are flat to inclined curved-downward and truncate underlying gentlu inclined laminae at a low angle. In contrast, sets of hummocky cross-strata overlie irregular erosional surfaces, are up to 50 cm high with wavelengths in excess of 1.5 m (Fig. 104), and are capped by 10 cm- to 20 cm-thick, ripple-stratified and bioturbated intervals. Hummocky cross-stratified fine sandstones and iow-angle trough cross-strata are stacked into 5 m- to 10 m-thick sequences.

The third subfacies comprises the top of the coarsening upward sequence and consists of trough cross-bedded, ripple-stratified, and intensely bioturbated fine to medium sandstones (Fig. 105). Bioturbate textures have commonly completely destroyed primary stratification and include single burrows and complex mazes of Teichichnus sp., well-defined networks of Thallassinoides cf. T. suevicus (Fig. 106), spreiten-connected shafts of Rhizocorallium sp. (Fig. 107), and shafts of Ophiomorpha nodosa (Schwans and Zawiskie, 1986). The bioturbated and trough cross-stratified subfacies is up to 5 m-thick, lies at the top of the upward-coarsening sequence, and is abruptly overlain by another fine-clastic interval along a sharp surface. Figure 104, Photo showing sets of hummocky cross-stratified sandstone (at coin in center) overlain by thin ripple-stratified and bioturbated intervals (top of set) in wave-dominated, lower shoreface sandstones. Photo is from the Upper Perron Sandstone and exemplifies aggradational shoreline facies. 306

Rgure 105. Photo of Teichichnus sp. maze m upper shoreface sandstones in the Upper Funk Valley Sandstone in Six Mile Canyon. 307

Figure 106. Photo showing well-defined network of Thalassinoides cf. T. suevicus in upper shoreface sandstones, upper Sanpete Formation, Six Mile Canyon. Figure 107. Photo showing spreiten-connected shafts of Rhizoœrallium sp. in upper shoreface sandstone, upper Sanpete Formation, Six Mile Canyon. Scale is in centimeters. 3 09 The sequence of subfacies outlined here is interpreted to represent deposition in a wave-dominated, lower to upper shoreface environment under variable wave-energy conditions and sediment supply (LSF to USF in Fig, 53). Low-angle, low-amplitude trough cross-stratified sets and hummocky cross-strata were both most likely produced by storm waves, which combined with unidirectional flow to act on the bed in offshore areas below fairweather wave base (Swift et al., 1983). Subsequent fainweather conditions resulted in burrowing activity and minor wave-rippling. Smaller trough crossbedded sets and bioturbated sandstones at the top of individual coarsening-upward sequences most likely represent upper shoreface deposition. Although flow and sediment transport processes during storm surges are not well understood and subject to much discussion (Hunter and Clifton 1982; Swift et al., 1983; Allen, 1985), the here discussed low-angle trough-crossbedding and hummocky sets clearly do not represent sedimentation under constant fainveather conditions. Nottvedt and Kreisa (1987) recently postulated that what is now recognized as hummocky cross-stratification is indeed related to low-relief, hummocky bedforms of combined-flow origin, while low-angle trough cross-stratification is thought to represent the fundamental geometry that is obtained when these migrate.

IBAbtS.g BESglVE SHOREUNE FACIES Four subfacies are arranged to form three distinct types of upward-fining and upward-coarsening sequences. The subfacies and their sequential arrangemenst are described separately below. 310 The first sequence (Type 1) coarsens upward and is 5 m to 10 m thick. The lowermost subfacies is a thin pebble conglomerate up to 30 cm-thick that occurs as lags and in shallow scours disconformably above alluvial strata or the wave ripple-stratified bay deposits. The low-angle trough cross-stratified subfacies described earlier overlies the pebbly scour facies and forms intervals that are 4 m to 8 m thick. Individual trough cross-stratified sets are flat to gently undulating and up to 50 cm high with wavelengths exceeding 1.5 m (Fig. 108). As was previously the case, large-scale low-angle sets grade into the trough to ripple cross-stratified and bioturbated subfacies featuring a diverse assemblage of ichnogenera, including Ophiomorpha nodosa, various Planolites sp. and Paleophycus sp., Rhizocorallium sp., Teichichnus sp., and Thallaslnoldes of. T. suevicus (top of Sanpete Fm. in Fig. 53). In places, the thin packstones and shelly fine-grained sandstones of the fourth subfacies cap the sequence and consist of broken and jumbled marine to brackish-water bivalves including Cymbophora utahensis (Meek), Crassostrea soleniscus (Meek), Lopha sannionis (White), Cyphmeria sp., Pleurocardia sp., Aphrodina? andsp., the brackish-water gastropod Carota dalll (Stanton) (W.A. Cobban, 1986, pers. com.).

The second sequence (Type 2) also coarsens upward and forms intervals that are up to 23 m thick (top of Upper Funk Valley Sandstone In Fig. 53). The lowermost subfacies is between 4 m and 8 m thick and consists of the ripple-stratified, interbedded mudstone and fine sandstone subfacies. As is the case with the aggradational shoreline Figure 108. Photo of low-angle trough cross-stratlfled sets of fiat to gently undulating morphology in upper part of Sanpete Formation at Six Mile Canyon (see Figure 53). Sets are part of large, sheet-like sandstone bodies that form the top of upward-coarsening sequences here Interpreted as transgressive shoreline deposits resting on a transgressive surface. View Is to the south with strata dipping steeply to east (left). 312 faciès, fine-clastic basal intervals are abruptly overlain by the hummocky cross-stratified subfacies along a sharp, yet irregular surface. The hummocky cross-stratified sets, together with the thin ripple-stratified to now minor burrowed subfacies form 5 m- to 10 m-thick stratal units, which in turn are truncated by shallow scour surface 2 m to 5 m wide. Scour surfaces are curved downward and overlain by inoceramid debris-bearing, poorly stratified pebble conglomerates (Fig. 109). The pebbly scour fill subfacies is abruptly overlain by marina ripple-stratified, interbedded mudstones and fine sandstones.

The third sequence type (Type 3) fines upward and forms up to 100 m-thick, fine-clastic intervals (upper parts of Lower Funk Valley Sandstone in Fig. 55). The basal subfacies is a 5 cm- to 10 cm-thick fine-grained sandstone with well-preserved bivalves including Cyprimeria sp., Crassatella andrewsi Henderson, and Pleuriocardia sp. (W. A. Cobban, 1986, pars. com.). The basal shelly subfacies is overlain by stacked sequences of amalgamated, hummocky cross-stratified, medium- to fine-grained sandstones (0.5 m to 1m thick) separated by thin, wave-rippled to non-parallel wavy laminated units (10 cm to 30 cm thick), which sometimes contain shelly lags that decrease upward in frequency. The lower hummocky cross-stratified subfacies is exposed in high cliffs up to 80 m at Lake Fork (Figs. 61,110), where it grades upward into a burrowed, flaser- to lenticular-bedded mudstone-siltstone subfacies overlain by massive offshore marine mudstones. 313

Figure 109. Photo of pebbly channel form (pebbly scour-flll subfacies) at top of a hummocky cross-stratified, shoaling-upward sequence at the top of the upper Funk Valley Sandstone member in Six Mile Canyon (see Figure 53). Here interpreted as top of a transgressive sequence, above which abrupt deepening occurs. View is to the north with strata dipping steeply to the east (right). Staff is 1.5 m long. 314

Figure 110. Photo of stacked sequence of amalgamated, hummocky cross-stratified fine-grained sandstone interval in Lake Fork (see Figure 55). Hummocky sets are up to 0.5 m high, overlie shelly lags at hammer (45 cm long), and are capped by ripple-stratified sets (furrows). View is to the southwest with strata dipping to the southeast (left). 315 In summary, these three sequences are interpreted as transgressive shoreline deposits, which together record the landward-directed backstepping or retrogradational behavior of a wave-dominated shoreline across subaerial braid-delta, interdeltaic bay, and submerged delta front environments. Type 1 sequences represent earliest marine strata resting on a transgressive surface, which has cut and reworked alluvial as well as interdeltaic strata. This transgressive facies is rare and occurs at the base of the marine portion of the Sanpete Formation (Figs. 53,64) and in the middle part of the lower Funk Valley Sandstone member (890 m above base in Fig. 53). Balsley (1982) described similar subfacies associations from the Campanian Blackhawk Formation.

Type 2 sequences occur at the top of the upper Funk Valley Sandstone member in Six Mile Canyon and reflect repeated and punctuated flooding of wave-dominated lower shoreface environments and the landward located sandy to gravelly braid-delta platform (Fig. 53). Initial deepening occurred across lower shoreface environments and resulted in shoaling of waves on an increasingly submerged delta platform. Subsequent minor distributary progradation superposed pebble-filled channels onto lower shoreface strata. The three superimposed upward-coarsening sequences are here identified as shallowing-upward sequences bounded by surfaces across which abrupt deepening occurred and are thus similar to the punctuated aggradationai cycles or PAC’s of Goodwin and Anderson (1985). 316 Finally, type 3 upward-fining sequences represent drowning of wave-dominated lower shoreface environments due to the gradual establishment of a deeper open marine setting. The upward-fining trend suggests an upward decrease in wave energy and sediment supply, which is accompanied by an increase in burrowing activity and symmetrical ripple occurrence, implying an increasingly seaward position below wave base. Consequently, uppermost parts of this sequence consist of flaser to lenticular bedded mudstones, which were most likely deposited in deeper water. The type 3 Indianoia sequence represents a excellent example of classic hummocky sequences, which lack the mudstone zone. Similar sequences have been described by Cotter (1975) from the Perron Sandstone in Utah and by Dott and Bourgois (1982) and Hunter and Clifton (1982), among others, from transgressive Upper Cretaceous rocks in Oregon.

Aggradationai shoreline strata comprise 80 % of the high-energy shoreline complex and produce stratal intervals that are 100 m to 250 m thick. In contrast, transgressive facies comprise 20 % of the high-energy shoreline deposits, are thus much thinner and exhibit thicknesses varying from 30 m to 70 m. Two contrasting delta models can be constructed for both wave-dominated shoreline sequences: one implies that deposition occurred in a single depositionai and deitaic episode, whereas the second model identifies the delta as a series of stacked strike-aligned tabular bodies of strike-elongated cuspate to arcuate plan geometry. 317 Transgressive shoreline or delta facies occur in several shallowing-upward sequences (PAC's), which are bounded by surfaces across which abrupt deepening occurred. Several such PAC's form a thin interval of stacked tabular stratal bodies, which were offset in a landward direction in a retrogradational pattern. At any particular position on the delta front to basin profile, the pattern is marked by an up-section increase in water depth, which is accompanied by a decrease in the depositionai thickness of the shallowing-upward packages. This arrangement forms a compound sand body that disconformably overlies older strata along a transgressive surface and is itself overlain by offshore strata along a sharp surface. Consequently, the compound body reflects a single, almost uninterrupted episode of backstepping shorelines. Examples are the upper part of the Sanpete Formation (below fs in Fig. 68), as well as uppermost units in the lower and upper Funk Valley Sandstone members (above cc and below fs in Fig. 71).

Although aggradationai shoreline or delta facies are also arranged in shallowing-upward sequences (LSF to USF in Figs. 53,54, 55) bounded by surfaces across which abrupt deepening occurred (i.e. PAC's), individual upward-fining packages are much thicker and water depths remain more or less consistant for any particular point in the delta front to basin profile. The shallowing-upward packages together form thick intervals of tabular strata!, which are stacked in an aggradationai pattern (above U5 in Fig. 71). This arrangement forms a compound sand and shale body that overlies older offshore strata along an unconformity surface and is itself overlain by offshore strata along a sharp surface. As a result, 318 deposition reflects an extended, repeatedly interrupted episode of aggrading shorelines.

OFFSHORE MARINE SYSTEM Major facies of the offshore marine system accumulated as 60 m- to 200 m-thick, fine-clastic intervals that separate the major coarse-clastic sandstone bodies of the Indianoia supersequence (Figs. 53, 54, 55). Typically the facies consists of carbonaceous massive mudstones, and intercalated siltstones to very fine sandstones (below and above dis in Fig. 68). Less common are thin micritic, dark gray limestones, which are laterally discontinuous and contain calcareous microfossils.

The dominant primary structure in mudstones is horizontal parallel lamination. Common are intercalated stringers of sandstone ripple forms of symmetrical and asymmetrical morphology, which feature trough cross-laminae sets, as well as an abundance of sole marks and bioturbate textures on basal surfaces (Fig. 111). Although bioturbation is pervasive, ichnofossil assem blages are generally restricted to Teichichnus sp. and Planolites sp. Lenticular to flaser-bedded siltstones and very fine sandstones are common in the othenwise massive mudstones and either form isolated stringers or occur in discrete intervals. 319

Figure 111. Photo of soie marks and indeterminate burrows at base of thin sandstone intercalated in marine mudstone of the Allen Valley Shale in Six Mile Canyon. Scale is in centimeters. 320 One such interval is found in the middle part of the Allen Valley Shale at Six Mile Canyon, where interbedded mudstones and rippled sandstones increase in frequency toward the base of the overlying deltaic-distributary, coarse-clastic sandstones of the basal Funk Valley Formation (Fig. 112). Mudstones of the lower Allen Valley Shale contain the ammonite Colligninoceras wollgari. According to Lessard (1970, 1973), planktonic foraminifers are abundant in lower parts of the Allen Valley Shale, while upper parts show low planktonic foraminiferal numbers to total exclusion and high percentages of benthonic foraminifers (see Allen Valley Shale in Fig. 53).

FACIES ASSOCIATIONS The facies are interpreted to represent the suspended load of rivers, which bypassed delta front regions and were deposited from suspension or as sediment flows in distal pro-delta and offshore regions in water depths below effective wave base (Coleman, 1976). Upper parts of individual marine mudstone intervals are characterized by increasing percentages of siltstone and sandstone ripples, whereas micritic limestones occur in lower or middle portions of the fine-clastic marine intervals. This trend is here interpreted to reflect a gradual change in sedimentation from distal offshore areas to proximal pro-delta environments.

On a macro scale, offshore mudstones overlie the transgressive shoreline facies and are in turn truncated by deltaic progradational 321

M Tabular/lenllc. sst./mdst.,

Figure 112. Photo of Allen Valley Shale at Six Mile Canyon unconformably overlain (above white line) by Lower Funk Valley Sandstone. Middle part of Allen Valley Shale (between black dashed line) features an increase in the frequency of rippled sandstones, which is here interpreted to represent the shift from a distal offshore to a more proximal pro-delta environment. View is to the southeast with strata vertical or dipping to the east (left). 322 strata. On the basis of planktonic versus benthonic foraminifer distributions, Lessard (1970,1973) showed that the greater water depths present in the lower parts of the Allen Valley Shale, for example, are significantly reduced in its upper parts. The initial upward-deepening at the base of the Allen Valley Shale corroborates the upward-deepening trend described in underlying transgressive facies. In contrast, the gradual shallowing trend in the upper part of the Allen Valley Shale is interrupted by incision and subsequent delta progradation. SECTION IV. 3

BASIN FILL ARCHITECTURE SYSTEMS TRACT AND SEQUENCE MODEL Three-dimensional assemblages of genetically related and contemporaneous depositionai systems are called system tracts (Brown and Fisher, 1977). The fundamental building blocks of systems tracts are parasequences or parasequence sets, which comprise genetically related beds or bedsets bounded by surfaces across which there is an abrupt increase in water depth (Vail, 1987; Vail, 1987; Van Wagoner, 1985; Van Wagoner et al., 1987). The bounding surfaces are called flooding surfaces and are characterized by minor relief, submarine erosion, and nondeposition (no subaerial erosion or shift in facies), thus indicating a minor hiatus or diastem (Van Wagoner, 1985; Van Wagoner et al., 1987). Individual parasequences therefore represent an upward-shallowing series of facies and are progradational (Goodwin and Anderson, 1985; Van Wagoner, 1985).

Individual systems tracts are characterized by the stacking patterns of parasequences or parasequence sets, which can be progradational, aggradationai, or retrogradational, depending on the rate of sediment availability versus accommodation in the basin (Van Wagoner et al., 1987) (see appendix A for definitions). Because the boundaries of parasequence sets represent major flooding surfaces 324 attributed to basinwide deepening events, they may coincide with sequence boundaries (unconformities), downlap surfaces, or transgressive surfaces (Van Wagoner et a!., 1987). On that basis, a regional regression is defined as the seaward displacement or the aggradation and progradation of parasequences and parasequence sets across the basin. In the outcrop, shoreline and associated coastal plain strata migrate basinward over older offshore deposits along an unconformity or downlap surface, and form what Van Wagoner et al. (1987) called the highstand systems tract. Conversely, a transgression is defined as the landward displacement of parasequences and parasequence sets. As a result, shoreline deposits backstep over or onlap onto older coastal plain strata along a sharply defined transgressive surface with the hiatus increasing in the direction of onlap, thus producing what Van Wagoner et al. (1987) called the transgressive systems tract. An aggradationai parasequence stacking pattern is indicative of a stationary sea level, such as during maximum flooding, and results in constant waterdepths for any point in the shoreline to offshore profile.

If the above model is correct, then the stacking patterns of parasequences, parasequence sets, and systems tracts within a depositionai sequence can be related to specific segments of the curve of relative coastal onlap, such as highstand and lowstand (Haq, Hardenbol, and Vail, 1987). It is important to note, however, that highstand and lowstand, together with the intervening transitional segments on the curve of relative coastal onlap are function of the 325 interaction between eustatic change, sediment availability, and accommodation in the basin. Haq, Hardenbol, and Vail (1987), Vail (1987), and Van Wagoner et al. (1987) subdivided depositionai sequences into three systems tract types (see appendix A):

Type 1 ; The lowstand wedge systems tract comprises aggradationai and progradational parasequence sets, which onlap the sequence boundary in a landward direction. The lower part of the systems tract is characterized by stream incision and coastal plain bypass and forms during the early part of a relative sea level lowstand until the time when the relative sea level fall stabilizes. The upper part forms in the late part of the relative sea level lowstand, a period when sediment availability begins to gradually exceed the rate of fall, resulting in the infilling of incised valleys. The top of this systems tract is the transgressive surface.

Type 2; The transgressive systems tract comprises one or more retrogradational parasequence sets and forms during a rapid rise in relative sea level or the early part of a relative sea level highstand. Parasequences onlap the transgressive surface in a landward direction where it may coincide with the sequence boundary. The top of the systems tract is the downlap surface where parasequence rétrogradation changes to aggradation, thus marking the surface of maximum flooding. 326 Type 3: The highstand systems tract comprises aggradationai parasequence sets overlain by progradational sets and forms during a relative sea level highstand, a time where the rate of relative sea level rise is gradually outstripped by sediment availability. The highstand systems tract onlaps the sequence boundary or the surface correlative to the transgressive surface in a landward direction and down laps onto the surface of maximum flooding in a basinward direction. The highstand systems tract is bounded at the top by another sequence boundary.

Together the systems tracts form a depositionai sequence. The analysis of the stacking patterns of parasequences, parasequence sets, and depositionai systems provides a powerful tool for subdividing thick sedimentary packages into chronostratigraphically significant and genetically related units. The three lower sequences of the Indianoia Group supersequence are analyzed using extensive subsurface and geologic surface data. The three sequences will provide a standard for (1) documenting the changes in sediment availability and basin accommodation that occurred during the Turonian through Early Campanian; and (2) relating these changes to the tectonism, eustasy, and foreland basin subsidence. Because only limited regional subsurface and geologic outcrop data exists for the early to middle Campanian sequence (Six Mile Canyon Formation, South Flat Formation, Starpoint Sandstone, and Blackhawk Formation) and the late Campanian sequence (Price River Formation and equivalent), they are not included in the detailed analysis. The isopach 327 maps presented herein are constructed on the basis of measured sections and thickness data from regional well-log cross-sections, which together are correlated within the sequence-stratigraphic framework presented in the previous chapter.

EARLY THROUGH MIDDLE TURONIAN SEQUENCE The sequence consists of the lower Turonian Sanpete Formation, the middle Turonian Allen Valley Shale, the upper Albian through Cenomanian Dakota Sandstone, and the upper Cenomanian through upper Turonian Tununk Shale (Table 1, Table 2,53). Basal parts of the Turonian sequence consist of fluvial channel sandstones and overbank mudstones of the bedload to mixed-load braided stream depositionai system (lower fluvial Dakota Sandstone and the Sanpete Formation) (Figs. 113,114). The fluvial deposits are abruptly overlain by a thin sequence of interbedded shoreline sandstones and marine mudstones (upper marine Dakota Sandstone and Sanpete Formation) of the wave-dominated depositionai system. This thin sequence of transitional nonmarine to marine deposits comprises two retrogradational parasequence sets, which together with the fluvial strata, onlap (on) onto the unconformity (U2) and lower sequence boundary (Fig. 115). Outcrops at the western basin margin consequently show fluvial deposits overlain by nearshore marine strata, such as lower-snoreface sandstones and marine mudstones (top of Sanpete Fm.) (Fig. 115). Figure 113. Southern cross-section of Cretaceous basin fill in the southern Wasatch Plateau. Note vertical exaggeration in scale. Well 13 is tie-well with northern cross -section (see Rgure 4 and appendix 0). Figure 113. 329 T e rtia ry

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Figure 114. Norüiern cross-section of Cretaceous basin fill in the northern Wasatch Plateau. Note vertical exaggeration in scale. Well 13 is tie-well with southern cross-section (see Figure 4 and appendix 0).

330 Figure 114. 331 2 5 2 6 2 4

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Figure 115. Sequence-stratigraphic Interpretation of a part of the Indianola Group supersequence at the type section, dn, downlap; on, onlap; te. eroslonal truncation; U, unconformity.

332 Figure 115.

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3 34 Paleoflow vectors from the lower fluvial deposits of the Dakota Sandstone in central and northern Utah show that dispersal was to the northeastward and north, while nearshore deposits in the upper Dakota Sandstone were transported toward the west and southwest (Vaugh and Picard, 1976; Yingling, 1987). Paleoflow data from lower shoreface sandstones in upper parts of the Sanpete Formation show north-south and northeast-southeast bimodality in Salina Canyon and Six Mile Canyon (at the western basin margin), respectively (Fig.116). The Sanpete-Dakota interval of the Turonian sequence thins basinward over a distance of 80 km to 100 km, from 300 m near the thrust front to 50 m in the basin proper, indicating that sediment accommodation was highest adjacent to the thrust front (Figs. 113, 114,116)

In the regional cross sections, deeper-water marine Allen Valley-Tununk mudstones overlie the nearshore deposits of the upper Sanpete-Dakota interval along a sharp flooding surface. The stacking pattern of parasequence sets in the lower part of the Allen Valley-Tununk interval is retrogradational, indicating establishment first of distal lower-shoreface environments and later deeper-offshore marine environments of the offshore-marine depositional system (Figs., 113,114). Together these strata comprise the upper-marine part (early middle Turonian) of the Turonian sequence and accummulated to thicknesses exceeding 150 m in a 80 km- to 100 km-wide and north-south elongate zone located just east of the thrust front (Fig. 117). Two distinctly aggradational 335

Utah

Sanpete-Dakota Set. EarSy Turonian

Figure 116. Isopach (in meters) map of Sanpete-Dakota Sandstone interval in cross-sections (between basal unconformity and first flooding surface). Paleocurrent roses (in 30 degree sections) based on measurements (see numbers) in Salina Canyon and Six Mile Canyon (see Figure 127). 336

Utah

Allen Valley-Tununk Shale Eariy-Mid Turonian

Figure 117. Isopach (in meters) of lower part of Allen Valley-Tununk Shale interval in cross-sections (between first flooding surface and first downiap surface). Paleocurrent roses (in 30 degree sections) based on measurements (see numbers) in Salina Canyon and Lake Fork (see Figure 127). 337 parasequence sets of clinoform geometry seperated by a retrogradational set characterize the upper-marine part (late? middle Turonian) of the Allen Valley-Tununk Interval. The clinoform bodies thin eastward over a distance of 60 km to 80 km from 90 m at the basin margin to 10 m in the basin proper (Fig. 118) and downiap eastward against two major flooding surfaces (Figs. 113,114). On a regional scale, the surfaces of maximum flooding merge basinward to form a condensed section in the basin proper.

Identical stacking pattern of lithofacies, facies associations, and depositional systems are found in nearby outcrops at the western margin of the foreland basin. Lower parts of the Allen Valley Shale above the main flooding surface (fs) in Six Mile Canyon, for example, consist of offshore-marine mudstones overlying nearshore sandstones (Figs. 68,115). Lessard (1970,1973) found that high percentages of pelagic foraminifers only occur in the lower parts of the Allen Valley Shale (and correlative Tununk Shale), indicating waterdepths greater than 100 m. Conversely, upper parts of the Allen Valley Shale at Six Mile Canyon consist of offshore-marine mudstones and intercalated wave-rippled sandstones, indicating greater proximity to a siliciclastic source and shallower conditions (Fig. 112). There, planktonic foraminifers are greatly reduced in number, if not absent, while benthonic foraminifers are present in high percentages, thus indicating significantly shallower conditions (Lessard 1970,1973). Paleoflow data from wave-influenced deltaic strata of the Allen Valley Shale-equivalent at Lake Fork exhibit some degree of 338

Allen Valley-Tununk Sh. Downiap Wedge Mid Turonian?

Figure 118. Isopach (in meters) map of upper part of Alien Valley-Tununk Shale interval in cross-sections (between first downiap surface and U3). Interval features two downiap wedges 339 bimodality, with a northeastern vector and an opposing southwestern one (Fig. 117). Middle Turonian marine-offshore sedimentation extended as far west as the Valley Mountains and was at its maximum within a basinal area elongate to the shoreline. During the late? middle Turonian, two downlap-wedges extended northward from an area southeast of Salina Canyon and east of Six Mile Canyon (Fig. 118). The orientation of the downiap wedges corroborates previous paleoflow data obtained in outcrops from lower to middle Turonian coarse-clastic, deltaic deposits.

The lower-fluvial part of the Sanpete-Dakota interval is here interpreted to comprise the basal fluvial parts of a transgressive systems tract. The strata were deposited during the early part of a relative rise in sea level and onlap onto the lower sequence boundary. An accelerated rate of relative sea level rise during the early middle Turonian resulted in the backstepping of shorelines across Utah toward the thrust belt and the deposition of the upper nearshore-marine part of the Dakota Sandstone-Sanpete interval, as the accommodation potential of the basin outstripped sediment supply (see Lithofacies in Fig. 51). Full offshore-marine conditions were established during the early part of a sea level highstand in the early middle Turonian (basal Allen Valley Shale). The change from shoreline rétrogradation to aggradation initiated formation of the highstand systems tract. The highstand systems tract overlies the transgressive systems tract along a downiap (dn) surface or condensed section and was deposited during the late Middle Turonian, 3 40 as sediment availabilty began to gradually exceed basin accommodation (Fig. 115). A more rapid decline in basin accommodation (probably without substantial increases in the sediment supply) during the early? late Turonian resulted in truncation (te) of previously deposited strata and the formation of the upper sequence boundary (US), above which the bulk of the prograding highstand systems tract is found (Fig. 115).

The sediment thickness data presented here indicate that a maximum of 450 m of sediment accummulated during the early and middle Turonian (4 M. Y. after Haq, Hardenbol, and Vail. 1987) within a narrow zone adjacent to the thrust front. Relative sea level changed from a lowstand to a highstand in the early through middle Turonian back to a lowstand in the late Turonian. The resulting shifts in coastal onlap are expressed in the foreland basin as a function of changes in sediment availability and basin accommodation. Both are controlled by net subsidence and net eustatic changes within the basin. Although large-scale flexural subsidence occurred in northern Utah during the Albian through Turonian, little indication exists that significant fluctuations occurred in the net subsidence rate of the foreland basin during that time. Furthermore, previous discussion has shown that thin-skinned deformation and thrust-loading in central Utah had ceased as early as late Albian (Pavant 1 event), if these relations are correct, then the described shifts in coastal onlap during the early and middle Turonian were mainly controlled by eustasy, although continuous flexural subsidence augmented the 341 eustatic change. The here identified Turonian shift in coastal onlap in the Cordiileran foreland basin coincides with the middle-late ^ Turonian fall (91 Ma) documented by Haq, Hardenbol, and Vail (1987) for continental margin areas.

LATEST? TURONIAN AND EARLY-LATE CONIACIAN SEQUENCES Two superposed depositional sequences overlie the Allen Valley-Tununk Shale in centrai Utah; (1) a latest? Turonian through earliest? Coniacian sequence, consisting of lower parts of the Lower Funk Valley Sandstone, the Perron Sandstone (equivalent to the Lower Perron, terminology by Davis, 1954), and correlative basinal mudstones (wells 8-10, above correlative conformity in Pigs. 113, 114); and (2) a late early? through middle Coniacian sequence, comprising the Perron Sandstone (equivalent to Upper Perron, terminology by Davis, 1954) and the Bluegate Shale (Pigs. 113,114, Table 1, Table 2). As stated earlier, the latest? Turonian through earliest? Coniacian sequence (first sequence) constitutes the bulk of the progradational highstand systems tract, which overlies the upper sequence boundary (U3) of the Turonian sequence (Pig. 115).

In the regional cross sections, aggradational and progradational parasequence sets, composed of fluvial channel sandstones, intercalated delta plain mudstones, and coals, onlap the unconformity (Pigs. 113,114). The fluvial deposits record the upper delta plain environments of a fluvially dominated depositional system and grade basinward into aggradational shoreline sandstones and offshore 342 marine mudstones. Paleoflow data obtained from crossbedded, fluvially-incised channel-conglomerates and shoreline sandstones of the Lower Funk Valley Sandstone in western outcrops (Lake Fork and Six Mile Canyon) indicate a predominantly southern dispersal (Fig. 119). Davis (1954) and Hale (1972) reported similar paleoflow directions from the Lower Ferron Sandstone (equivalent to Ferron Sandstone in the northern Wasatch Plateau).

On the basis of well log-pattern interpretations and measured sections, the Lower Funk Valley-Ferron Sandstone sequence (latest? Turonian through earliest? Coniacian) constitutes a wedge-like body that thins eastward over a distance of 80 km to 100 km from 200 m at the thrust front to 50 m in the basin proper (Fig.119). Dispersal occurred in fluvially dominated shoreline complexes (also called the Vernal Delta) to the southeast and resulted in the progradation and aggradation of shorelines. Distributaries supporting the systems were fed by bedload to mixed-load braided stream systems located in the Gunnison Plateau, Lake Fork, Cedar Hills, and the Canyon Range. Paleocurrent data from braided stream strata deposited in a wide alluvial plain (Gunnison Plateau) show dispersal toward the southwest to southeast, while dispersal in areas farther north (Lake Fork) w as due south and occurred through north-south elongate, incised fluvial valleys (Fig. 119). Proximal bedload and alluvial fan systems in hinterland areas (Canyon Range) transported their coarse boulder-conglomerates northward toward the Leamington lineament. This zone of structural weakness acted as a conduit for 343

Utah

Lower Funk Valley-Ferron Set. Late Turonian Vernal Delta

Figure 119. Isopach (in meters) map of Lower Funk Valley-Ferron Sandstone interval (Lower Ferron) in cross-sections (between U3 and U4). Paleocurrent roses (in 30 degree sections) based on measurements (see numbers) in Six Mile Canyon, Lake Fork, Chicken Creek Canyon, northern Canyon Range, and Oak Creek Canyon, central Canyon Range (see Figure 127). 344 transporting erosional debris from hinterland areas into the foreland of the fold-thrust belt.

The highstand systems tract of the latest Turonian through earliest? Conician sequence is truncated abruptly by Conician strata (second sequence) along an unconformity (U4) and upper sequence boundary (Figs. 113,114). As a result, Conician fluvial-deltaic sandstones (Upper Ferron Sandstone, terminology by Davis, 1954) are superposed onto Middle Turonian offshore-marine mudstones (uppermost Tununk Shale above correlative conformity), indicating a significant hiatus (Fig. 120). On a regional scale, Coniacian channel sandstones onlap (on) the sequence boundary and unconformity (U4) from east to west, grading landward into interbedded lower to upper delta-plain mudstones, coals, and channel sandstones ( Figs. 113, 114). The unconformity changes landward into its correlative conformity. As a result, only the correlative conformity (cc) is represented in western outcrops (Six Mile Canyon) (Fig. 71), where it is located in coastal plain strata and only marked by very minor truncation (te) accompanied by a minor shift in facies (Fig. 115).

The deltaic strata (Upper Ferron, also called the Last Chance Delta) above the unconformity and lower sequence boundary occur within Incised valleys and are interpreted to constitute the lowstand wedge of a lowstand systems tract. Paleocurrent and paleohydrologic data by Cotter (1971,1975a, 1975b) show that Coniacian distributaries prograded northeastward (Fig. 121) and were fed by a 345

Figure 120. Photo of Upper Ferron Sandstone in the eastern Wasatch Plateau (near well 9 in cross-section). The Upper Ferron Sandstone overlies U4 and consists of stacked upward-shallowing parasequences (two are shown) separated by flooding surfaces (fs). US is expressed here by its correlative conformity. Utah

Ferron S st Coniacian Last Chance Delta

Figure 121. Isopach (In meters) of Upper Ferron Sandstone in the cross-sections (between U4 and the first flooding surface). Arrow depicts major paleoflow directions as reported by Cotter (1971,1975a, 1975b). 347 mixed load-dominated, meandering stream system that drained source regions in southwestern Utah. The Coniacian deltaic strata accummulated to form several radially arranged, elongate stratal bodies that are up to 60 km long and 30 km wide (Fig. 121). Individual bodies exhibit an average thickness ranging between 50 m and 80 m and are here interpreted as bar finger sands of the Upper Ferron distributary channel-mouth delta system (Last Chance Delta).

The lowstand wedge is abruptly overlain by marine mudstones of the Coniacian Bluegate Shale (Table 1, Table 2) along a major flooding surface here interpreted as a trangressive surface. Parasequences above the surface onlap (on) onto the sequence boundary (U4) and correlative conformity (cc) in a landward direction and are here identified as basal parts of a Coniacian transgressive systems tract (Figs. 113,114). The above relationships are reflected in western outcrops, such as Six Mile Canyon, by the superposition of retrogradational shoreline sequences (transgressive system) over fluvial-dominated deposits (lower part of Lower Funk Valley Sandstone) along the correlative conformity (cc) (Figs. 71,115). The transgressive systems tract thus rests on the lowstand wedge in the east, backsteps toward the west, and is in turn overlain by several stacked retrogradational parasequence sets, which constitute the Lower Funk Valley Shale to the west and the Bluegate Shale in the 348 The Bluegate Shale comprises the upper offshore-marine part of the transgressive systems tract and is represented in western outcrops by offshore-marine mudstones and wave-dominated shoreline sandstones (Fig. 61). The shoreline systems accummulated in two discrete depo-centers near the thrust front (Fig. 122) and were supplied by sandy-braided to low-sinuous single-channel systems located in the Gunnison Plateau and Cedar Hills. As was previously the case, deterioration of highstand conditions during the late middle Coniacian is demonstrated by the change from a retrogradational parasequence stacking pattern to an aggradational and progradational one (middle part of Bluegate Sh. in Figs. 113,114). Two geographically separate downiap wedges were formed as a response and more or less overlie the two depositional centers, indicating that sediment supply was indeed related to two major hinterland sources located to the west and northwest (Fig. 123).

In summary, declining accommodation conditions in the foreland basin during the late part of the early to middle Turonian eustatic rise resulted in minor valley incision (U3) (Fig. 68, 69). Deltaic aggradation and progradation (Lower Funk Valley-Lower Ferron Sandstone) occurred during the latest? Turonian, as sediment availability exceeded basin accommodation, isopach distribution shows that basin accommodation was most pronounced near the thrust front (200 m in 1.5 M.Y.) within a zone up to 150 km wide here identified as a first-order basin, which corresponds in outline to the restricted basin that existed during the Barremian through Aptian 349

Utah

Lower Funk VaüeyHBÎuegat© Shaie Coniacian

Figure 122. Isopach (in meters) map of Lower Funk Valley-Bluegate Shale (Coniacian) interval in cross-sections (between datum and downiap surface). 350

Utah

Low er Funk Valley-1 Downiap Wedge C oniacian

Figure 123. Isopach (In meters) map of downiap wedges at top of Lower Funk Valley-Bluegate Shale (Coniacian) Interval In cross-sections (between downiap surface and US). 351 (Fig. 49). Sediment accommodation in the foreland basin proper to the east, however, occurred at a mucc lower rate (50 m in 1.5 M.Y.). Subsequent acceleration in the rate of relative sealevel fall (probably without substantial increases in sediment supply and subsidence rate) during the latest? Turonian and earliest? Coniacian caused prominent valley incision (U4) along the eastern margin of the zone of greatest subsidence or first-order basin, it is important to note that the unconformity (U4) is only present at the eastern margin of this zone of maximum subsidence. Overlying lowstand strata (Upper Ferron Sandstone or Last Chance Delta) lie within laterally restricted valleys with maximum thicknesses of 100 m, while overlying transgressive (250m) and highstand deposits (250m) accummulated in the first-order basin .

The asymmetry in basin accommodation (in west to east cross-sections) is here interpreted to reflect increased subsidence near the thrust front forming a first-order basin, while subsidence distal to the load (>150 km eastward) occurred at a much reduced rate. The latest Turonian sequence (0.5 M.Y. to 1 M.Y. after Haq, Hardenbol, and Vail, 1987) comprises 200 m of strata, while the overlying Coniacian sequence (1 M.Y.) consists of 650 m of strata. Sediment accommodation changes in the basin are a direct reflection of the changes in basin subsidence rate and eustasy, both of which may result in relative changes in sea level and the formation of unconformities. Although tectonic subsidence was undoubtedly the major driving force for long-term basin subsidence starting in the 352 late Albian, as Lawton (1984 ) and Heller et al. (1986) have pointed out, the above documented and apparent high-frequency changes in the accommodation potential of the foreland basin could not have been controlled by tectonically-induced subsidence-rate changes. Consequently, eustatic changes, rather than basin subsidence and sediment supply, determined the formation of the unconformities (US and U4) in the foreland basin. At the sam e time, however, differential subsidence near the thrust front controlled the regional character of the unconformities.

On that basis, the peculiar limited areal extent of the latest? Turonian-earliest? Coniacian unconformity (U4) is easily explained. Basin subsidence in the first-order basin was sufficiently high to com pensate for the eustatic drop, causing continuous sedimentation in the first-order basin, whereas subsidence along the eastern margin of the first-order basin was outstripped by the eustatic fall. Subsequent infilling of incised valleys (lowstand wedge) occurred as basin subsidence began to outpace the slowing rate of eustatic fall during the earliest? Coniacian. The renewed eustatic rise in the early Coniacian caused shoreline transgression (upper part of Lower Funk Valley Sst. and Sh. in Plate 1, Plate 2). and maximum flooding during the middle Coniacian. The subsequent decline in the accommodation potential of the basin characterizes the intiation of another aggradational to progradational highstand systems tract (Emery Sst.), which overlies the transgressive systems tract along a downiap surface and condensed section (Figs. 113,114). The eustatic 353 lowstands documented in the Cretaceous foreland basin coincide with two lowstands (90 M.Y. and 88.5 M.Y.) proposed by Haq, Hardenbol, and Vail (1987) for continental shelf areas.

The sequence of events described produced the complex superposition of two deltaic bodies, which has led to much confusion in the literature (Van De Graaff, 1964; Maione, 1971; McGookey, 1972; Fouch et al., 1983; Ryer and Lovekin, 1986). Hale (1972) and Ryer and McPhillips (1983), for example, identified the Lower and Upper Ferron delta (i.e., the Vernal and Last Chance Delta, respectively) as contemporaneous and evoked the presence of a semi-enclosed lagoon or what Hale (1972) called Sanpete embayment. Cobban (1976) and Merewether and Cobban (1986) erroneously placed the Ferron Sandstone into the upper Middle Turonian, based on fossils recovered from mudstones above the Ferron Sandstone in the northeastern Wasatch Plateau (Plate 2). The latter mudstones, however, represent pro-delta deposits of the Upper Ferron delta system, which overlie the Lower Ferron.

LATEflOimCIAN THROUGH LATE SANTONIAN SEQUENCE This sequence consists of the upper Coniacian through Santonian? Upper Funk Valley Sandstone, the upper? Santonian through lower? Campanian Emery Sandstone, the upper Santonian Upper Funk Valley Shale, and the upper Santonian through lowermost? Campanian Masuk Shale (Table 1, Table 2). As with previous sequences, a more rapid decline in basin accommodation during the late part of the eustatic 354 highstand (early through middle Coniacian) caused partial truncation (te) of progradational shoreline sandstones deposited earlier (Figs. 71,115). The uncornfomity (U5) and lower boundary to the Coniacian-Santonian sequence becomes a correlative conformity in the basin proper, and is thus located in eastern outcrops in upper parts of the Bluegate Shale (Figs. 113,114).

Progradational and aggradational parasequence sets located above the sequence boundary form the highstand systems tract (Upper Funk Valley-Emery Sst.) of the late Coniacian through late Santonian sequence, comprising fluvial to wave-dominated shoreline systems grading downslope into the offshore-marine depositional system (Figs. 124). Basal sequence strata onlap (on) the lower sequence boundary (U5) landward and downiap (dn) onto the correlative conformity in the basin (Fig. 115). Parasequence stacking patterns indicate that regional coastal plain and shoreline upbuilding was punctuated by major flooding events, producing retrogradational parasequence sets (Figs. 113,114).

Paleodispersal vectors from contemporaneous wave-dominated shoreline sandstones in Lake Fork and Six Mile Canyon show a bimodal southwest-northeast and southwest-northwest distribution, respectiveiy (Fig. 125). The shoreline was oriented north-south in the south and paralleled the northeastward projecting Charlston-Nebo salient in the north (Fig. 125). Fluvial strata in the alluvial foreland, such as the Gunnison Plateau, were dispersed to the south. Isopachs 355

Figure 124. Photo of aggrading shoreline complexes of the Emery Sandstone (between wells 9 through 13). fs, major flooding surface above Emery Sandstone in cross-sections. 356

Utah

Upper Funk Vafiey-tmery sst. Oonlacian? Hitarval

Figure 125. Isopach (in meters) of Upper Funk Valley-Emery Sandstone (Coniacian) interval in cross-sections (between US and flooding surface). Paleocurrsnt roses (in 30 degree sections) are based on measurements in correlative strata in Log Hollow Canyon (42). Chicken Creek Canyon (100), Six Mile Canyon (35), and from c^nglomsraîss in the hinterland in the Canyon F 357 for the Coniacian part of the sequence (below the major flooding surface in Figs. 113,114) show that 300 m to 400 m of elastics accummulated in basinal areas just east of the frontal zone. Maximum basin accommodation again lies more or less within the first-order basin, which has been well-defined since the Turonian, although the onset of an eastward-directed widening trend is now apparent (Fig. 125).

Offshore-marine mudstones of the Upper Funk Valley Shale and the Masuk Shale overlie the highstand systems tract along a major flooding surface and transgressive surface (Figs. 113,114,115). On a regional scale, one or two retrogradational parasequence sets, which include lower-shoreface sandstones and offshore-marine mudstones, overlie the transgressive surface, forming the middle? Santonian transgressive systems tract (Figs. 113,114). In western outcrops, stacked upward-coarsening sequences of marine mudstone overlain by hummocky-crossbedded sandstone and capped by pebbly channel-form bodies represent the shoaling-upward parasequences or shoreline portion of the Santonian transgressive systems tract (Figs. 108,115). Retrogradational parasequence sets representing offshore-marine environments abruptly overlie the shoreline deposits in the subsurface and the outcrop (Fig. 72).

On a regional scale, parasequence stacking patterns change from retrogradational to aggradational, marking the onset of the predominantly aggradational and minor progradational highstand 358 systems tract (above Upper Funk Valley Sh. In Figs. 113,114). No distinctive downlap surface is observable, because of the overall proximity of the cross-sections to the basin margin and their parallel orientation to the paleoslope. Nevertheless, downlapping did occur within the basin proper to the southeast. Isopachs depicting the transgressive systems tract and the aggradational part of the highstand systems tract (Santonian part of sequence) show a maximum of 500 m of nearshore to transitional offshore-marine strata in northeastern central Utah (Fig. 126). The widening trend of the basin first observed in the late Conician continued, forming a wide marine basin opening toward the northeast (Fig. 126).

In summary, as in previous depositional sequences the lower sequence boundary (U5) for the late Conician to late Santonian sequence is marked by subaerial exposure, a basinward shift in facies, a downward shift in coastal onlap, and onlap of overlying strata. As a result of the shift in facies, nonmarine or very shallow marine rocks above the sequence boundary, such as lower-shoreface sandstones (Upper Funk Valley Sandstone), directly overlie deeper water marine deposits deposited in the shoreface-offshore transition zone (Lower Funk Valley Shale). No rocks are present that represent deposition in intermediate environments. The strata above the sequence boundary constitute the progradational and aggradational highstand systems tract, and are thickest east and southeast of the frontal zone of the fold-thrust belt. Deposition occurred during the late part of relative sealevel fall and early relative sea level rise 359,

Utah

Upper Funk Valley-Emery Sst. SantonianY interval

Figure 126. Isopach (in meters) of Upper Funk Valley-Emery Sandstone (Santonian) inten/al in cross-sections (between flooding surface and US). Paleocurrent roses (in 30 degree sections) are based on measurements in correlative strata in Log Hollow Canyon (42), Chicken Creek Canyon (100), and from conglomerates in the hinterland in the Canyon Range. 360 (Middle to Late Coniacian). Consequently, sediment accommodation in the foreland basin w as first outstripped by sediment availability, and later exceeded the latter. As the renewed sea level rise began to outpace sediment availability during the late Santonian, shorelines were pushed as far west as the western Wasatch Plateau and Sanpete Valley area. As in previously described depositional sequences, shoreline aggradation and progradation during the late part of the highstand (latest? Santonian to earliest? Campanian) resulted in the truncation (te) of previously deposited shoreline sandstones and incision (US) on a regional scale (Figs. 72,115).

An earlier discussion of the tectonic history of central Utah showed that Pavant 2 thrusting commenced during the late Coniacian and was continuous until the late Santonian. Uplift and erosion of the Pavant 2 orogen increased sediment availability and sediment flux for the fluvial and deltaic depositional systems in foreland areas. In addition to increasing relief across the fold-thrust belt to basin margin profile, thrusting also caused increased flexural subsidence and the marked widening of the first-order basin during the late Coniacian through late Santonian with the area of maximum accommodation migrating eastward. Up to 900 m of nearshore to transitional offshore-marine sediments accummulated during that period (3.5 M.Y. to 4 M.Y. after Haq, Hardenboi, and Vail, 1987) within the foreland basin. 361 Jordan (1981) showed that long-amplitude flexural subsidence can only create space of similar magnitude (500 m to 1500 m) over several million years (5 M.Y. to 10 M.Y.). The high-frequency changes documented in the late Coniacian through Santonian basin accommodation show that only parts of the net accommodation can be related to tectonic subsidence, indicating that eustatic change must have been another active control and most likely augmented tectonic net-subsidence. On that basis, systems tract architecture in the Upper Funk Valley-Emery Sandstone interval is interpreted to reflect the change from a eustatic lowstand in the Middle Coniacianto eustatic highstand in the latest Santonian to earliest Campanian. These data correspond to those of Haq, Hardenboi, and Vail (1987), who documented identical eustatic changes for continental shelf areas (at 88.5 Ma and 86 Ma, respectively).

FORELAND BASIN SUBSIDENCE PATTERNS The sediment thickness maps of the Turonian to earliest Campanian basin fill, together with the described vertical and lateral systems tract associations, provide a clear record of the major controls determining foreland basin sediment accummulation and distribution. Three areas of distinct sediment distribution and basin geometry are identified in Utah; a first-order area of maximum sediment accommodation, or deep moat, east of and adjacent to the frontal zone; a distal foreland basin area featuring significantly reduced sediment thicknesses; and a tectonic hinterland west of the frontal zone, displaying isolated secondary sediment accummulation 362 maxima. Within the confines of the first-order basin, total Upper Cretaceous sediment thicknesses range from a maximum of 4000 m at the thrust front to a minimum of 1350 m in the east and southeast. In the foreland basin proper of eastern Utah, total sediment thicknesses are significantly reduced, and range between 650 m and 1000 m, documenting the sediment-starved conditions and reduced rates of basin subsidence for the more central parts of the basin.

The Late Cretaceous first-order basin parallels the thrust belt over a distance of 350 km, and evolved from a basin that was 80 km to 150 km wide in the Turonian through middle Coniacian to one 200 km wide in the late Conician through Santonian. Long-term basin accommodation results from gradual flexural subsidence, which creates space (> 500 m) over several million years (> 5 M.Y. to 10 M.Y.). In that context, the northeastward migration of the zone of maximum basin accommodation during the Santonian following Pavant 2 thrust-loading serves as a good example. Shorter term (0.5 M.Y. to 4 M.Y.) changes in the accommodation potential of the foreland basin, however, are controlled by eustasy. A eustatic rise, for example, can augment net-tectonic subsidence, as was the case during the late Coniacian through Santonian, resulting in significant sediment accummulation (> 500 m). On the other hand, eustasy may override and exceed tectonic change, as was the case in the middle Coniacian. There, the rate of eustatic fall outpaced tectonic subsidence and resulted in pronounced valley incision in one area (U4, unconformity at 88.5 Ma yrs.), while eustasy was outstripped in another area. 363 forming only a correlative conformity.

Despite the overriding effect of eustasy, tectonic uplift determines, although to a limited extent, the topographic relief at the basin margin, and thus erosion and the sediment flux into the basin. If tectonic uplift at the basin margin, for example, coincides with a period of decreasing rates of eustatic rise, the increase in relief will substantially increase the sediment flux. The initial rapid progradation of wave-dominated shoreline systems during the late Coniacian (Upper Funk Valley-Emery Sandstone interval) serves as a good example. Subsequent acceleration in the eustatic rise during the Santonian, however, counteracted the tectonic influence and caused aggradation. Nevertheless, tectonism can determine the formation of major depositional centers, such as during the middle Coniacian, when two major depo-centers formed just east of the frontal zone and the triple junction of the Charlston-Nebo thrust salient, the Pavant 2 thrust, and the Leamington lineament (Lower Funk Valley-Bluegate Shale interval). Furthermore, the flexural response to thrust-loading can control isopach trends and basin outline, as was the case during the Santonian along the southeastern margin of the eastward-projecting Charleston-Nebo salient (Upper Funk Valley-Emery Sandstone interval).

Sediment distribution in the hinterland of the fold-thrust belt was controlled by a combination of backward-breaking thrust uplift and transverse faulting along the Leamington and Scipio lineaments 364 during the late Coniacian through Santonian (Pavant 2) and in the early Campanian (early Canyon Range) as well as late Campanian (Late Canyon Range) (see Tectonic events (2) and (3) in Fig. 51). On the basis of measured sections, maximum sediment accummulation occurred south of the intersection of the Sheeprock-Canyon Range thrust and the Leamington lineament. Sediment thicknesses change from a maximum of 1800 m at the fault intersection to 350 m farther south. The southern extent of Indianola strata is determined by the depositional pinch-out of the conglomerates against northeast-dipping, Devonian Simonson Dolomite of the Pavant 2 plate. A secondary accummulation center lies south of the intersection of the Pavant 2 thrust and the Scipio fault. A minimum of 400 m of Indianola strata is exposed just to the south of the intersection.

The distribution of stratal thicknesses of the Indianola Group supersequence described above defines a distinct pattern of sediment accommodation and subsidence for the Late Cretaceous foreland basin. Within the confines of the first-order basin, maximum sediment accummulation occurred near the thrust front-Leamington lineament intersection and east of the leading edge of the Early Cretaceous thrust front. Secondary accummulation centers, located in the hinterland of the fold-thrust belt, formed on top of the broken Tintic valley and Pavant 2 plates as a result of right-iaterai movement along the lineaments, accompanied by vertical rotation and down-dropping of southern blocks. Sediment accummulation and distribution in the hinterland thus depicts a piggy-back scenario. In 365 contrast, sediment accummulation and distribution in the foreland is predominantly related to flexural subsidence and eustasy, although Airy isostatic subsidence near the load (within 150 km of the load) most likely controlled in parts the outline of the Turonian through Coniacian first-order basin. APPENDIX A

HISTORY AND KEY DEFINITIONS OF SEQUENCE-STRATIGRAPHY

366 367

INTRODUCTION The appendix presents an overview of the history and key definitions of sequence stratigraphy, thus establishing a standard in terminology, which is critical for the communication and the understanding of the concept and its applications. Many of the original concepts were presented in a series of articles on seismic stratigraphy by Vail et al. (1977). In the following years, numerous workers have debated and reapplied these early concepts, thus adding to the original terminology in a number of ways (Pitman, 1978; Vail and Todd, 1981 ; Watts, 1982; Parkinson and Summerhayes, 1985). In order to utilize the most recent concepts, it is necessary to understand the historical background of sequence stratigraphy, define the terminology, and the stratigraphie application, thus providing a more common approach to sequence-stratigraphic analysis.

HISTORICAL BACKGROUND The recognition and naming of unconformity-bounded units or sequences is not a new concept, but instead has a long history. Earliest stratigraphie units recognized in Europe, for example, were identified on the basis of bounding unconformities. Many of the systems of the standard global stratigraphie scale were thus originally unconformity-bounded units. Renewed interest in unconformities and associated units arose in the first half of the twentieth century. 3 68 during a time where tectonic episodes were considered essentially synchronous world-wide. Much of the interest waned, however, once geologists found that the main assumption of tectonic synchroneity was not true and the resulting stratigraphie resolution was only sufficient for correlations on the largest scale.

Unconformity-bounded units were first recognized as independent stratigraphie units of distinct character during the 1950s and 1960s (Wheeler, 1958; Sloss, Krumbein, and Dapples, 1949). Sloss, Krumbein, and Dapples (1949) defined sequences as "operational units" that consisted of stratal assemblages and were separated by "marked discontinuities in the stratal record of the craton, which may be traced and correlated for great distances on the objective bases of lithologie and faunal breaks, and continuity in distribution and facies of the transgressive strata found above the discontinuities" (Sloss, Krumbein, and Dapples, 1949, p. 109-111). Later, Krumbein and Sloss (1951,1963) added that sequences are "the rock record of a major tectonic cycle" and are "characterized by basal rock units which lie unconformably on earlier rock over much of the North American craton. The unconformities extend across virtually all tectonic elements, and may be traced across shelves, through intracratonic basins, to the margins of the eugeosyncline." (Krumbein and Sloss, 1951, p. 380-381). Finally, Wheeler (1959) called attention to the chronostratigraphic significance of sequences and cited the need for the definition of these "space-time variable, time-stratigraphic units" (Wheeler, 1959, p. 1977).

The recognition of the chronostratigraphic significance of sequences came to a breakthrough in the 1960s andl 970s with the increasing desire to analyze and model basins on a global scale. Sedimentary sequences seperated from each other by major breaks in sedimentation were recognized as the most obvious features of most sedimentary succession, whether observed in the outcrop, on seismic sections, or in separate basins of varied type. Advances in reflection seismology led to the recognition that primary seismic reflections parallel stratal surfaces and unconformities. On this basis, Vail and co-workers proposed that depositional sequences (defined below) bounded by unconformities and their correlative conformities represented primary units of chronostratigraphic significance (Mitchum, Vail, and Thompson, 1977; Mitchum, Vail, and Sangree, 1977; Vail, Mitchum, and Thompson, 1977; Vail and Mitchum, 1977). Stratal geometries and patterns of stratal onlap, downlap, truncation (defined below), as well as basinward or landward shifts of onlap were used to reconstruct and interpret the sea level histories of various continental margins. The resulting coastal onlap curves were largely based on interpretations of seismic sections with paleontological age control from well data. The apparent worldwide synchroneity of onlap events in different basins led to the generation of charts by EXXON geologists showing global cycles of relative sea level changes (Vail, Mitchum, and Thompson, 1977; Vail and Mitchum, 1979; Vail, 370 Hardenboi, and Todd, 1984; Haq, Hardenboi, Vail, 1987; Vail, 1987). Since the early works by Vail and his co-workers, it has become commonplace to analyze sedimentary stratigraphies of modern and ancient continental margins using the concept of sequence stratigraphy (Cooper, 1977; Heckel, 1977; Heckel, 1980; Kiteley, 1983; Johnson, Rong, and Yang, 1985; Poag and Ward, 1987).

GLOSSARY OF TERMS FUNDAMENTALS AND KEY DEFINITIONS Sequence stratigraphy the -study of stratal packages within a time-stratigraphic framework that consist of a relatively conformable succession of genetically related strata bounded by surfaces of erosion or nondeposition, or their correlative conformities (first stated by Mitchum, Vail, and Thompson, 1977).

Depositional sequence - a relatively conformable succession of genetically related strata bounded by unconformities and their correlative conformities; this is basic unit of sequence stratigraphy (Mitchum, Vail, and Thompson, 1977). Sequences are generally tens to several hundreds of meters thick, with scales possibly varying between a few millimeters to a kilometer. The limits of the magnitude of sequences are set by the resolution limit of the various tools, such as seismic sections, satellite imagery, high-altitude to low-altltude aerial photos, outcrops, well log correlations, and core sampling. Distinct groups of superposed sequences are called supersequences. 371 which are on the same order of magnitude as Sloss' original sequences. The depositional sequences of Vail and his co-workers are, however, more than unconformity-bounded units, since they are, contrary to Sloss' sequence, recognized even where their boundaries are conformities (Mitchum, Vail, and Sangree, 1977; Vail, Mitchum, and Thompson, 1977; Haq, Hardenboi, Vail, 1987).

Unconformity - a surface of subaerial erosion or subaerial exposure that separates younger from older strata, with a significant hiatus (defined below) indicated (Van Wagoner et al., 1987). The original definition by Mitchum, Vail, and Thompson (1977) was broader and included submarine surfaces, whereas the above definition restricts the term to surfaces marked by subaerial erosion on their landward portions. Vail, Hardenboi, and Todd (1984, p. 130) identified two types of unconformities: Type 1 unconformities and Type 2 unconformities (defined below) (1 and 2 in Fig. 127).

Hiatus - total interval of geologic time not represented in the rock record along a stratigraphie surface (Mitchum, Vail, and Thompson, 1977) (Fig. 127). Hiatuses may be attributed to erosion or to the nondeposition of strata, or both. Measurement of the magnitude of the hiatus at the sequence boundaries places the strata of the sequence into a time-stratigraphic framework (Haq, Hardenboi, Vail, 1987). The identification of the magnitude is generally limited by the degree of resolution that is offered by the tool. Hiatuses may expand along the 372

TOPIAPCIPI

IB) CHRONOSTRATIGRAPHIC CHART

SUBAERIAL HIATUS SECTION ICSI

MA}OMUM SUBAERIAL EXPOSURE

E23AU1IV1M .PIAW COASTAL PUIN Q NEARSHORE EÔ Ô ) Rt (ÏISBllIHIŒ BOUNDARY TYPE OR AGE to SBUBICE OBITIRCATION DIS DOVYNIAP SURFACE

Figure 127. (A) Stratigraphie relations of three idealized depositional sequenoes (1b through 3b) and depositional systems to downlap surfaoes (DLS), condensed sections (CS), and unconformity types ([1], [2]). (B) Chronstratigraphio chart with stratigraphie relations in (A) plotted in time (ordinate) against distance graph. From Vail, Hardenboi, and Todd (their Figure S) 373 bounding surfaces, or decrease below of what is considered a significant or resolvable hiatus. As a result, the extent of a hiatus may change considerably on a regional scale, marking an unconformity in one area and a conformity in another. The concept of the magnitude of tl ie hiatus is thus similar to the concept of the magnitude of a sequence (Mitchum, Vail, and Thompson, 1977).

Type 1 unconformity - subaerial and submarine, determined by the downward shift (basinward) of coastal onlap (defined below) commonly below the shelf edge (Vail, Hardenboi, and Todd, 1984, p. 130) (1 in Fig. 127). Type 1 unconformities are formed "when the rate of eustatic sea level fall exceeds the rate of subsidence at the shelf edge" (Vail, Hardenboi, and Todd, 1984, p. 131). They result in the subaerial exposure and erosion of the shelf, and in the initiation of submarine valley entrenchment at the shelf edge.

Type 2 unconformities - subaerial, characterized by downward shifts of coastal onlap (defined below) to a position at or landward of the shelf edge (Vail, Hardenboi, and Todd, 1984, p. 130) (2 in Fig. 127). Type 2 unconformities are formed "when the rate of fall of eustatic sea level is less that the rate of subsidence at the shelf edge, but exceeds the rate of subsidence on the inner portion of the shelf" (Vail, Hardenboi, and Todd, 1984, p. 131). 374 Conformity - a surface separating older from younger strata, along which there is there is no evidence of erosion or nondeposition, and no significant hiatus indicated (Mitchum, Vail, and Thompson, 1977).

Marine flooding surfaces - planar surfaces that separate older from younger strata and bound parasequences, parasequence sets (defined below) (Van Wagoner, 1985; Van Wagoner et al., 1987), or stratal packages, which Goodwin and Anderson (1985) called Punctuated Aggradational Cycles (or PACs). The flooding surfaces have correlative surfaces in the coastal plain and downdip on the shelf.

Parasequence or Parasequence sets (PACs) - are restricted to the basinward marine portion of stratal packages and are bounded by surfaces across which there is evidence of an abrupt deepening possibly accompanied by minor submarine erosion. Indicating a small diastem. The upward-shallowing stratal packages are marked by the absence of gradationally upward-deepening cycles, and the bounding surfaces are attributed to instantaneous, basin-wide deepening events. The fundamental pattern within parasequences or PACs is thus an upward-shallowing series of facies above a sharply defined surface of upward deepening (Goodwin and Anderson,1985) (Fig. 128).

Condensed section - thin marine stratigraphie interval or surface that separates older from younger strata, in/upon which little or no sediment has accummulated over long periods of geologic time (less 375

Figure 128. Schematic representation of parasequence stacking patterns and related well-log responses (GR«gamma ray; DT-sonic). (A) Progradational; (B) Aggradational; (C) Retrogradational. Modified after Van Wagoner et al. (1987). 376 than 1 cm/1000 yrs.), possibly as a result of sediment bypass or lack of sediment availability (Fig. 127b). A condensed sections develops when "the rate of relative rise of sea level (eustatic rise plus subsidence) is significantly greater than the rate of accummulation, causing the depositional site to shift landward and resulting in low sedimentation rates (starved conditions) seaward of the depositional site" (Vail, Hardenboi, and Todd, 1984, p. 135).

Basinward extensions of unconformities - conformities, marine flooding surface, surfaces of nondeposition, or condensed sections. Together the surfaces are an expression of the variations in sediment supply and the changes that occurred in the accommodation potential of the basin through eustatic and basin subsidence-rate changes.

Landward extensions of unconformities surfaces - exhibiting evidence of erosion or prolonged subaerial exposure, such as regional truncation, valley incision, channeled surfaces, paleokarst horizons, calcrete-cemented hardgrounds, leached zones, or zones featuring stacked paleosol horizons. The landward surfaces commonly display significant relief as a result of erosion in response to base level changes in the basin or through tectonic uplift, which accentuates erosion and ultimately the relief. As a result, the degree of angularity of subaerial unconfomity can change with increasing distance from the area of deformation. What is thus an unconformity of pronounced angularity in one area becomes a surface marked by subtle truncation 377 only discernable on a regional scale in another. Finally, this subtle truncation may turn farther basinward into a surface marked only by a significant hiatus.

STRATAL COMPONENTS OF DEPOSITIONAL SEQUENCES Sequence stratigraphie classification places emphasis on the relation of the strata to the bounding surfaces. Discordance is the main physical criterion in determining sequence boundaries. On seismic sections, for example, discordant reflection termination patterns indicate unconformities and can be carried into areas where the reflector becomes concordant (Ramsayer, 1979). The patterns are based on the physical relations of strata, as well as on the parallelism between the strata and the surface, or the lack of it. The presence of concordance or the absence of discordance do not preclude, however, the presence of an unconformity. Especially in seismic or outcrop sections that parallel the depositional strike of stratal packages within a sequence, strata may show apparent concordance. Sections oriented down depositional dip, however, would show the true discordance between the strata. Accordingly, what may appear as apparent onlap in one direction may turn out to be true downlap in the other. Consequently, intersecting outcrop, seismic, as well as well log-correlation sections are needed to determine the true relations between strata and surface. 378 In seismic sections, reflectors that onlap (defined below) an underlying inclined surface seem to terminate abruptly against the surface at an high angle, if viewed in a regional well log correlation or especially in the outcop, the same onlap relationship could easily be mistaken for depositional thinning or simple concordance. In both cases, the degree of resolution that is offered by the tool clearly determined the apparent stratal relationships, as well as the size of the depositional bodies. An acute awareness of the scale and the varying degrees of tool resolution (i.e. seismic vs. well log vs. outcrop) is consequently critical for the correct identification of stratal relationships. Vail and co-workers defined two types of discordance: lapout and truncation.

Lapout -lateral depositional termination of a stratum. These are represented by three basic termination patterns of reflectors and strata: onlap, downlap, toplap (defined below) (Fig. 127a). The termination patterns can be identified on seismic sections, in laterally extensive outcrop sections, and in regional correlations of well log-patterns.

Truncation - the lateral termination of a stratum by erosion (Mitchum, Vail, and Thompson, 1977, p. 59) (Fig. 127b). The pattern occurs at the top of a depositional sequence. Erosion may result, for example, from large-scale channeling in response to the lowering of base level, or the regional denudation of strata following uplift 379 and/or subsidence. Resulting unconformities thus mark those exceptional periods "when the rate of eustatic se a level fall is greater than that of subsidence, producing relative falls in sea level" (Vail, Hardenboi, and Todd, 1984, p. 136). Contrary to Mitchum, Vail, and Thompson (1977, p. 59), who state that structural truncation may produce discordant relations that are only of "minor, if any, regional chronostratigraphic significance with respect to unconformities or hiatuses", this study has found that the distinction between erosional and structural truncation is especially critical in tectonically active basins.

Onlap - an initially horizontal stratum laps out against an intiaiiy inclined surface of greater inclination (Mitchum, Vail, and Thompson, 1977, p. 57). Refers to the progressive landward encroachment (by coastal or alluvial plain) of the coastal deposits of a given sequence (Vail, Hardenboi, and Todd, 1984). As a result, the hiatus increases into the direction of onlap.

Downlap - an initially inclined stratum terminates downdip against an initially inclined surface (Mitchum, Vail, and Thompson, 1977, p. p. 58). Commonly associated with a marine hiatus (DLS in Fig. 127). The figure shows the stratigraphie and the chronostratigraphic relations. The strata lap out laterally against a preexisting and intiaiiy inclined surface, signifying a nondepositional hiatus of increasing duration in the direction of downlap (Ramsayer, 1979). 380 The pattern is typically associated with those periods when sediment availability is greater than sediment accommodation within the basin, resulting in downlapping, for example, of the toesets of prodelta deposits during progradation. According to Vail, Hardenbol, and Todd (1984), downlap associated with condensed sections marks the change from the end of a transgression (defined below) to onset of a regression (defined below). This decrease in the rate of eustatic rise results in the progradation of sediments over the starved surface.

Toplap - depicts a pattern where initially inclined strata terminate against an overlying surface mainly as a result of nondeposition (Mitchum, Vail, Thompson, 1977, p. 58) (Fig. 127). Typically an expression of deposition controlled by base level, where the latter is too low to allow sedimentation further updip (Ramsayer, 1979). Toplap is commonly associated with sedimentary bypassing and minor erosion occuring above sealevel in shallow marine environments, such as in the topsets of deltaic clinoforms. Note that the hiatus is of erosional nature and decreases in the direction of the younger strata.

SYSTEMS TRACTS COMPONENTS AND THEIR PATTERNS Sysîsm îmoîs - three-dimensional facies associations of discrete character and geometry derived from several genetically linked, contemporaneous depositional systems (Brown and Fisher, 1977). 381 System tracts are defined by their position within a sequence and the stacking pattern of parasequences or parasequence sets (Haq, Hardenbol, and Vail, 1987). A vertical succession of systems tracts can in turn be related to the different phases of sea level cycles and segments on the sea level curve (Van Wagoner, 1985; Haq, Hardenbol, and Vail, 1987; Van Wagoner et al., 1987). The prograding highstand and the lowstand fan systems tracts (defined below) are related to eustatic highstand and lowstand. Conversely, the lowstand fan wedge, and the transgressive systems tracts (defined below) are related to slow eustatic falls and increased rates of eustatic rise, respectively.

Regression - seaward displacement or the progradation of parasequences and parasequence sets (Fig. 128a). Shoreline and associated coastal plain deposits migrate basinward, with coastal plain strata prograding over older offshore deposits.

Stationary shoreline aggradation - of parasequences and parasequence sets (Fig. 128b). Occurs when sediment availability and the basin accommodation are essentially balanced.

Transgression - landward displacement or retrogradational pattern of parasequences and parasequence sets (Fig. 128c). As a result, coastal onlap occurs and shoreline deposits shift successively landward, backstopping over older coastal plain strata. 38 2 Lowstand fan systems tract - deposited during a period when the rate of eustatic rise decreases so rapidly that the relative sea level fall is greater than the subsidence at the shelf edge (FAN in Fig. 127). The total withdrawal of the sea below the shelf edge causes the subaerial exposure and ravinement of the shelf, resulting in sedimentary bypass of shelf areas (Haq, Hardenbol, and Vail, 1987). As a result, elastics are transported across the shelf and deposited directly onto the slope in fan complexes. The systems tract is bounded by two unconformities (Vail, 1987; Van Wagoner et al., 1987).

Lowstand wedge systems tract - deposited as soon as regional subsidence rates fall below the decreasing rates of eustatic fall. The resulting rise in relative sea level causes the backfilling of the earlier incised valleys, as well as the progradation of lowstand deposits over leveed channel complexes and fan deposits on the fan slope. The progressive flooding of estuarine embayments and incised valleys may culminate in their complete infilling by deltaic deposits (Haq, Hardenbol, and Vail, 1987). Depending on the interaction of eustasy, subsidence, and sediment supply, the relative rise in sea level may ultimately cause backstepping of wave-dominated shoreline systems across the shelf. The systems tract overlies an unconformity and may grade into overlying transgressive facies (2a in Fig. 127). 383 Transgressive systems tract - deposited when the rate of eustatic rise is greater that sediment supply (Haq, Hardenbol, and Vail, 1987). Once sediment availability begins to outstrip the rate of sediment accommodation on the shelf upbuilding or even regression may occur. The formation of transgressive systems tracts is thus especially dependent on the interaction between the rate of eustatic rise and sediment availability. Transgression across the shelf commences with the formation of a transgressive surface, on top of which landward backstepping, lagoonal to back-barrier, and shoreline facies are deposited. During rapid transgression elastics and fines are increasingly stored in nearshore sediment sinks, which causes starved or condensed sections to form on the shelf. The transgressive systems tract may rest directly on lowstand fan deposits or on the lowstand wedge systems tract (Top of 2a in Fig. 127a) and is overlain by the progradational highstand system tract (2b in Fig. 127a).

Prograding highstand systems tract - deposited in the highstand phase and characterized by an upward-shallowing succession of facies, grading into nonmarine strata near the top of the systems tract (Haq. Hardenbol, and Vail, 1987) (2b in Fig. 127). The resulting decline in the rate of accommodation causes the progressive thinning of iopset beds, the thickening of foresets, and ultimately shoreline regression. The prograding highstand systems tract 384 overlies a condensed section and is bounded at the top by an unconformity (Vail, 1987; Van Wagoner et al., 1987). APPENDIX 8

MEASURED SECTIONS AND DESCRIPTIONS OF LOCALITIES OF CALCAREOUS MICROFOSSIL AND PALYNOMORPH SAMPLES

385 KEY TO MEASURED SECTIONS WITH LOCATIONS Sections are listed below according to numbers and abbreviations (capital letters). The abbreviations are also used in micro-fossil samples listed below. Section locations are shown in Figure 129 and described below by section (S), township (T), and range (R). Numbers refer to those shown on drafted sections in text, while abbreviations below are the first letter of the location names in Figure 129.

MEASURED SECTION & NUMBER S T R

MCI- Mellor Canyon 17 18 S. 1 E.

TC2-Timber Canyon 12 17 S. 1 W.

TC 3 “ Timber Canyon 18 17 S. 1 E.

SG 4 - Southeastern Gunnison 7 19 S. 2E . (upper part of SG 9 below)

CCS-Chicken Creek Canyon 12 15 S. 1 E.

HC 6 - Hop Creek Ridge 20 12 S. 3 E

LF 7 - Lake Fork 34 8 S. 4E .

CC8-Chicken Creek Canyon 7 19 S. 2E . Plateau (lower part of section location 1 in chapter 1)

SG 9 - Southeastern Gunnison 7 19 S. 2E . (section location 3 in chapter 1) 387

Utah

Chicken Creek #

Salt Creek #

C # Gunnison R .^ S. Gunnison A

Figure 129. Map of study area (inset rectangle in box) showing the locations of all measured sections (full circles) and wells (well symbols with 1 through 27) used in cross-sections. 388

(continued)

MEASURED SECTION & NUMBER S I R

SG 10 -- Southeastern Gunnison 32 18S. 2E . (section location 2 in chapter 1)

SG 11 - Southeastern Gunnison 7 19 S. 2E . (detailed SG 9)

SM 12 - Six Mile Canyon 35-36 18 8 . 2E .

SM 13-Six Mile Canyon 36-25 18S . 2E . (upper part of SM 12)

SG 14 -- Southeastern Gunnison 7 19 S. 2E . (south of SG 9)

SA 1 5 - Salina Canyon 33 21 E. 1 E.

RC16-Red Canyon 29 18 S. 1 E.

WG 17 - Wales Gap 25 15S. 2E .

CC18-Chicken Creek Canyon 12 15 8 . 1 E. (detail-section of upper part of CC 5)

OC 19 - Oak Creek Canyon 10 17 8. 2W.

MG 20 - Maple Grove Canyon 6 21 8. 2 VV.

LF 21 - Lake Fork 2 9 8. 4 E .

LH 22 - Log Hollow Canyon 27 13 8. 2E . 3 89

MICROFOSSIL AND PALYNOMORPH LOCALITIES The location, stratigraphie position, and depositional environment of samples are given, together with the collectors name and that of the paleontologist interpreting the sample. If sampled by this study, then the first sample numbers refers to the numbering system used herein, whereas the second number in parenthesis refers to the number assigned by the interpreting paleontologist or institution. The samples are grouped by age and sample number, with reference to the formation name.

NONDISTINGT LATE CRETACEOUS PMC 200/1 (P5268-1)-Sample from Mellor Canyon, southwestern Gunnison Plateau, NE 1/4 Sec. 17, T. 18 S., R. 1 E. Collected by P. Schwans from siltstcne in lower Six Mile Canyon Formation, 200 m above base of measured section MC 1, braided stream. Identified as terrestrial by S. N. Nelson from Chevron Co., Denver, no age (6/87).

(D6367)--Sample from southeastern end of Gunnison Plateau, NW 1/4 Sec. 7, T. 19 S., R. 2 E. Collected by J. W. Collinson from 130 m above base of SG 11, braided stream sandstones and interbedded mudstones. Identified by D. J. Nichols (7/26/81). Presumed to be basal Sanpete Formation.

TURONIAN (D6294)--Sampled by T. D. Fouch and T. F. Lawton from basal part of Indianola Group in Lake Fork drainage, NE 1/4 NW 1/4 Sec. 11, T. 10 S., R. 4. E. Identified by D. J. Nichols as Turonian? or Allen Valley Shale age (Fouch et al., 1983, p. 334). Reinterpreted by J. Shane, EPR Co., Houston, as Albian to Campanian (9/87). Here designated Allen Valley Shale.

CONIACIAN PABG (P5259-2)-Sample from Six Mile Canyon, 2.5 mi east of Sterling, S W 1/4, Sec. 26, T. 18 S., R. 2 E. Collected by P. Schwans from 850 m atxîve base of measured section SM 12, from dark-grey limestone in Lower Funk Valley Shale member. Identified by S. N. Nelson, Chevron Co., Denver, as Coniacian (6/87). 390 PCC155Q/1 (P5262-1)-Sample from Chicken Creek Canyon, 4 mi up-canyon from Levan, SE 1/4, Sec. 12, T. 15 S., R. 1 E. Collected by P. Schwans from detrital carbonates located below massive conglomerate ridge, braided stream environment. Here designated upper fine part of Upper Funk Valley Sandstone/Shale. Identified by S. N. Nelson, Chevron Co., Denver (6/87), and J. Shane, EPR Co., Houston (9/87) as Coniacian.

PLF 360/1 (P5264-1)-Sample from Lake Fork, 3 miles east of US 89 and US 29 intersection, SE 1/4 Sec. 34, T. 8 S., R. 4 E. Collected by P. Schwans from what is here designated base of Lower Funk Valley Shale or 360 m above base of measured section LF 7/21, nearshore marine depositional environment. Identified by S. N. Nelson, Chevron Co., Denver, as Campanian (6/87), which is impossible. Sample was reevaluated by J. Shane, EPR Co., Houston, and dated as Coniacian and younger (9/87). The latter age is confirmed by bivalve ages identified by W. A. Cobban (Lopha sannionis [White]) (7/86). See also D6294.

PLF 390/2 (P5264-2)~Sample from Lake Fork, 3 miles east of US 89 and US 29 intersection, SE 1/4 Sec. 34, T. 8 S., R. 4 E. Collected by P. Schwans from what is here designated base of Lower Funk Valley Shale or 390 m above base of measured section LF 7/21, nearshore marine depositional environment. Identified by S. N. Nelson, Chevron Co., Denver, as Campanian (6/87), which is impossible. Sample reevaluated by J. Shane, EPR Co., Houston, and dated as Coniacian and younger (9/87). The latter age is confirmed by bivaive ages identified by W. A. Cobban {Lopha sannionis [White]) (7/86). See also P5264-1.

CAMPANIAN (D6303)--Sampled by D. J. Nichols and T. F. Lawton from Six Mile Canyon, 2 mi east of Sterling, SW 1/4 SW 1/4 Sec. 30, T. 18 S., R. 3 E. Sample is from what is here designated second Unnamed Unit located above angular unconformity (their Price River Formation), approximately 2330 m above base of measured section SM 12/13 and 10 m below North Horn, braided stream depositional environment. Identified by D. J. Nichols as Campanian, confirmed by J. Shane, EPR Co., Houston (9/87).

PBH-Base (P5261-1)-Sampled along 1-70, approximately 14 miles east of Salina (road cut at mile 69), SW 1/4 Sec. 18, T. 22 S., R. 3 E. Collected by P. Schwans from base of Blackhawk Formation and first coal 391 above road level, lower delta plain depositional environment. Identified by S. N,. Nelson, Chevron Co., Denver as Campanian (7/87) and by J. Shane, EPR Co., Houston as indeterminate Late Cretaceous (9/87). Here regarded as time-correlative to upper Six Mile Canyon Formation.

PSM 1900/4 (P5274-1)--Sample from Six Mile Canyon, 2.5 miles east of Sterling, NE 1/4 Sec. 36, T. 18 8., R. 2 E. Collected by P. Schwans from upper member of Six Mile Canyon Formation coal at 1950 m above base of measured section SM 12/13 and 350 m below angular unconformity, lower alluvial plain to upper delta plain depositional environments. Identified by S. N. Nelson, Chevron Co., Denver, as Campanian (7/87).

PSM 2174/1 (P4964-1)-Sample from Six Mile Canyon, 3 miles east of Sterling, NW 1/4 Sec. 36, T. 18 S., R. 2 E. Collected by P. Schwans from upper part of what is here designated top of upper member of the Six Mile Canyon Formation or 2275 m above base of measured section SM 12/13, sandy to pebbly, low-sinuosiiy stream environment with abundant overbank. Identified by S. N. Nelson, Chevron Co., Denver, as Campanian (1/86).

PSM 2275/7 (P5275-3)“ Sample from Six Mile Canyon, 3 miles east of Sterling, SE 1/4 Sec. 25, T. 18 S., R. 2 E. Collected by P. Schwans from upper part of what is here designated Price River Formation or 2275 m above base of measured section SM 12/13 and 25 m below angular unconformity, braided stream depositonal environment. Identified by S. N. Nelson, Chevron Co., Denver, as late Campanian (7/87). J. Shane, EPR Co., Houston, reevaluated the sample and reported a late Campanian or younger age (9/87).

MAESTRICHTIAN (P3722-1)--Sampled by J. M. Zawiskie (his sample KPr-2) from Six Mile Canyon, 3 miles east of Sterling, SW 1/4 Sec. 30, T. 18 S., R. 3 E. Collected from channel coal in what is here designated second Unnamed Unit, at approximately 2330 m above base of measured section 12/13 of P.Schwans, sandy, pebbly braided stream environment. Identified by S. N. Nelson, Chevron Co., Denver, as Maestrichtian (J. M. Zawiskie, pers. com. 1987). 3 92 (P3731-9,11,12)--Sampled by J. M. Zawiskie (his samples Petes Canyon coal, NHW-148.2,160) at Petes Canyon, NW 1/4 See. 36, T. 15 S., R. 2 E., just south of Wales Gap.

(P4677-2,3 ,4 ,5 ,6 ,7 ,8 ,13)-Sampled by J. M. Zawiskie (his samples CC-11,14,18,19, 22, 24, 25,35) at Wales Gap, NW 1/4 Sec. 25, T. 15 S., R. 2 E., and Coal Canyon. Collected from upper part (mudstones) of what is here designated second Unnamed Unit, open lacustrine to swampy floodplain environments. Identified by S. N. Nelson, Chevron, Denver, as Maestrichtian (J. M. Zawiskie, pers. com., 1987).

PAWG-3 (P4963-3)--Sample from Wales Gap, 1 mile west of Wales, NW 1/4 Sec. 25, T. 15 S., R. 2 E. Collected by P. Schwans from upper part (mudstones) of what is here designated second Unnamed Unit or 205 m above base of measured section WG 17. Identified by S. N. Nelson, Chevron Co., Denver, as Late Cretaceous, Coniacian through Maestrichtian (1/86). Resampled by P. Schwans and identified by J. Shane of EPR Co., Houston, as Maestrichtian (9/87).

PSM 2330/2 (P4966-2)--Sample from Six Mile Canyon, 3 miles east of Sterling, SW 1/4 Sec. 30, T. 18 S., R. 3 E. Collected by P. Schwans from middle of what is here designated second Unnamed Unit or 2330 m above base of measured section 12/13, sandy, pebbly braided stream environment. Identified by S. N. Nelson, Chevron Co., Denver, as late Campanian through Maestrichtian (1/86), confirmed by J. Shane, EPR Co., Houston (9/87). APPENDIX C WELL NAMES AND LOCATIONS

393 394

WELLS IN CROSS-SECTIONS Wells below are organized first by well number (Fig. 129) as shown in Plate 1 and Plate 2. Secondly, well are listed below by township (T), then range (R). and finally section (8). Also shown are company, well name, and log-service performed. Log-types include: GAL = caliber; DEN = density; GR = gamma ray; IL = induction log; NEU = neutron; POR = porosity; RES = resistivity; SON = sonic.

S T COMPANY WELL SERVICE

1 24 21 S 1 E Phillips Petrol. Wasatch Plateau #1 GR-SON

2 25 21 S 1 E Pacific Trans. Wasatch Plat. #1-25 GR-POR

3 30 2 2 S 3 E Pan Am. Petrol. Porcupine Ridge #1 GR-SON

4 20 2 2 S 3 E Phillips Oil U.S. "D" #1 GR-SON

5 27 1 9 8 3 E Phillips Petrol. U.S. "E" #1 GR-SON

6 20 20 8 4 E California Oil Musinia Unit#1 GR-SON

7 16 22 8 5 E Willard Pease State of Utah #1-0 GR-POR

8 23 21 8 5 E Vortt Explor. Link Canyon #1 GR-IL

9 9 21 8 7 E Pacific Nat. Gas Ferron Unit #42-9 GR-NEU

10 21 2 0 S 7 E Pan American Ferron Unit #4 GR-SON

11 4 20 8 7 E Fossil Petrol. Federal #4-1 GR-RES

12 28 1 9 8 7 E Northwest #1 Federal GR-NEU 395 (continued)

# S T R COMPANY WELL SERVICE

13 2 1 9 S 7 E Chandler Assoc. #10-2 Unit State GR-POR

14 33 IB S 7 E Texas internat. Federal 41-33 GR-SON

15 15 IB S 7 E Tiger Oil Curtis 41-15 GR-NEU-DEN

16 25 1 7 S 7E Utah Southern #1 State GR-NEU

17 17 15S 7 E Energy Res. #1 Pole Canyon GR-NEU-DEN

IS 4 17 3 6 E Vortt Explor. Indian Creek #14-17 GR-POR

19 10 1 7 8 6 E Vortt Explor. Indian Creek #5 GR-NEU-DEN

20 2 1 7 8 6 E Vortt Explor. Indian Creek #2 GR-SON

21 27 168 6E Three States #2 M.D. Kearns GR-NEU

22 16 16 8 5E Tenneco Oil Black Canyon #1 GR-SON

23 14 1 5 8 3 E Hanson Oil Co. Moroni #1-AXA GR-SON

24 16 1 5 8 3 E Tennessee Gas #1 J. W. Irons SP-RES

25 29 1 5 8 3 E Phillips Oil #1 Price "N" SP

26 15 1 6 8 1 E Dixel Res. Gunnison State #1 GR-NEU

27 24 19 S 2W Placid 01! WXCUSA#1-2 GR-NEU-DEN 39 6

WELLS FOR PIGEON CREEK-CEDAR MOUNTAIN ISOPACHS Wells below are organized first by township (T), then by range (R), and finally section (8). Also shown are company and well name.

S T R COMPANY WELL

14 1 5 8 3 E Hanson Oil Moroni #1-AX

13 1 5 8 7 E ARCO Oil Hiawatha

27 15 8 9 E 8hell Oil #1 North 8prings

33 15 8 12E Equity Oil #1 Mounds

15 168 1 E Dixel Resources Gunnison 8tate #1

15 1 7 8 8 E Phillips Oil #1 Unit

15 1 8 8 7 E Tiger Oil Curtis #41-15

33 18 8 7E Texas International Federal #41-33

27 19 8 3 E Phillips Petroleum U.8. "E"#1

24 21 8 1 E Phillips Petroleum Wasatch Plateau #1

9 21 8 7 E Pacific Gas Ferron Unit #42-9

20 22 8 3 E Phillips Oil U.8. "D"#1

16 22 8 5 E Willard Pease 8tate of Utah #1-0

34 22 8 5 E 8kelly Oil #1 Emery Unit A r r c N u iA u

PALEOFLOW DATA 398 Paleocurrent data were corrected for tectonic tilt using a standard equal-angle stereonet, and were plotted in current roses at 30 degree intervals (e.g., 1-30 degrees, 31-60 degrees 331-360 degrees). No statistical reductions were performed, since data distribution and sampling is not sufficient to warrant such calculations. Orientation of paleoflow vectors are listed by locality for each stratigraphie interval. Italic numbers refer to imbrication data where orientation of a(t) axis was converted (+ 90 degrees) to dip direction of b (i) axis. Note that data are randomly listed.

Lake Fork 4 22 40 46 52 57 198 214 216 233 248 253 254 254 293 297 327 327 348 357 359

Salina Canyon 116 128 138 139 148 147 154 153 163 168 173 182 182 188 203 203 206 272 220 228 275 319 341 343 346 346 346 346

Six Mile Canyon 10 12 12 15 17 42 47 54 62 63 64 104 106 108 111 117 123 131 185 191 228 259 259 313 335 341 399 LOWER FUNK VALLEY SANDSTONE Lake Fork 13 15 79 88 113 119 122 123 139 144 171 173 178 178 179 183 191 192 192 203 222 253 353 355 303

Salina Canyon 260 260 261 262 269 270 276 279 280 282 284 288 294 294 297 303 303 317

Six Mile Canyon 5 61 102 115 159 177 188 193 193 195 199 201 201 202 202 204 217 227 237 240 247 297 304 307 309 311 311 312 315 317 320 322 335 343 29 88 111 115 161 252 252 285 342 55 77 85 89 97 105 111 175 193 199 206 208 31 55 85 104 202 233 351 352 357

LOWER FUNK VALLEY EQUIVALENT Chicken Creek Canyon 57 71 108 114 169 171 179 187 273 347 313 120 78 96 112 119 124 135 141 142 143 143 154 161 400 LOWER FUNK VALLEY EQUIVALENT fCONT.) Chicken Creek Canyon 150 128 203 184 186 214 138 117 167 133 146 174 214 186 163 250 213 213 88 128 180 165 91 133 118 136 212 112 100 150 110 214 218 181 188 221 208 142 167 187 195 125 110 92 148 142 204 102 104 75 58 81

FUNK VALLEY FORMATION fINDETERMINATB Log Hollow Canyon 34 36 38 41 48 56 199 203 208 272 268 263 256 242 4 16 27 38 52 135 137 165 225 326

46 44 42 352 22 24 17 354 11 270 25 69 274 38 67 66 248 302 336

Wide Canyon (Canyon Range) 13 195 210 265 277 278 296 302 302 315 319 328 331 353

Oak Creek Canyon (Canyon Range) 18 21 26 33 351 32 41 46 49 78 286 401 UPPER FUNK VALLEY FORMATION Chicken Greek Canyon 1 6 9 12 18 19 41 51 54 62 65 90 96 98 100 101 106 113 116 126 132 133 139 144 144 145 146 146 147 153 152 153 154 155 163 162 168 169 170 175 181 359 195 225 247 295 37 41 43 45 53 54 57 63 70 75 80 87 105 110 112 121 122 122 130 133 135 138 141 143 146 153 153 155 159 164 165 167 172 172 175 176 177 177 177 178 179 181 181 182 189 189 213 264 271 280 283 304 321

Six Mile Canyon 152 175 191 202 216 222 252 293 313 315 3 277 10 76 193 204 213 224 224 228 234 248 259 259 276 295 321 321 27 325 331 343 350

REDDICKS CANYON CONGLOMERATE Chicken Creek Canyon 338 356 4 334 305 350 2 330 360 355 334 334 7 332 340 355 353 20 78 56 28 334 50 20 50 4 0 2 SIX MILE CANYON FORMATION Mellor Canyon 1 5 7 11 65 68 73 76 86 92 91 92 93 96 98 163 261 265 268 270 305 315 317 326 333 341 342 344 354 356 13 23 31 91 189 194 204 303

85 85 70 25 76 78 32 359 6 39 29 46 106 39 14 114 82 2 339 107 9 107 8 110 339 144 94 129 114 42 49 24 83 351 44 24 14 338 64 47 7 356 23 126 109 349 124 118 96 102 334 105 114 342 126

Six Mile Canyon 137 146 170 212 229 232 235 256 288 26 61 133 137 149 151 157 180 180 189 345 358 13 26 29 41 73 75 171

Timber Canyon 21 22 25 27 39 43 47 48 50 61 62 76 83 95 99 105 113 121 125 127 134 148 153 155 154 157 159 159 1 S3 167 167 171 173 181 189 201 243 248 256 303 335 341 343 346 349 353 357 403 SIX MILE CANYON EQUIVALENT Pavant Range 39 54 64 89 113 121 143 150 153 159 169 173 177 179 177 185 189 195 209 211 215 218 222 227 229 243 281 351

SOUTH FLAT FORMATION Chicken Creek Canyon 14 32 34 47 57 288 APPENDIX E

SYMBOLS AND ABBRVIATIONS 405

SYMBOLS FOR SEDIMENTARY TEXTURES AND STRUCTURES

H Homogenized bed a iz v? Wave oscillation ripples

Even, parallel Wavy, parallel

Wavy, nonparallel Trough cross beds

//// Planar cross beds Current ripples

Climbing ripples Hummocky cross beds, HCS

jTLiT" Planar bed Flaser bed

Contorted bedding ■■y Groove or flute cast

W Load cast Scour

S.Y.MBQLS-EC)RFQ.SSIIS

Foram Plant remains, general

0 Pollen V Gastropods

Cephalopods ^ Pelecypods 40 6

SYMBOLS FQR FOSSILS (continued)

Stems Leaves

^ Roots Y Churned

U Burrowed

FACIES ABBREVIAI lQNa

SYMBOL DESCRIPTION

G Gravel

S Sand

F Mudstone

OS Gravel, clast supported

MS Gravel, matrix supported or crudely stratified

m massive

P planar cross beds

h crude horizontal stratification, sheet-like t trough cross-stratification, associated with multiple scours ts like t and with sandy scour fills 40 7 EACIES ABBREVIATIONS (continued)

SYMBOL DESCRIPTION

sc scoured and lag surfaces

r ripple stratification, small-scale trough crossbedding

b burrowed

It large trough cross-stratification

pb pebbly wr wavy ripple stratification LIST OF REFERENCES

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INFORMATION to USERS the Most Advanced Technology Has Been Used to Photo­ Graph and Reproduce This Manuscript from the Microfilm Master

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