Lithologic Evidence of the Jurassic/Cretaceous Boundary
LITHOLOGIC EVIDENCE OF THE JURASSIC/CRETACEOUS BOUNDARY
WITHIN THE NONMARINE CEDAR MOUNTAIN FORMATION,
SAN RAFAEL SWELL, UTAH
A thesis presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
James D. Ayers
August 2004 LITHOLOGIC EVIDENCE OF THE JURASSIC/CRETACEOUS BOUNDARY
WITHIN THE NONMARINE CEDAR MOUNTAIN FORMATION,
SAN RAFAEL SWELL, UTAH
BY
JAMES D. AYERS
has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences of Ohio University by
Gregory C. Nadon Assistant Professor
Leslie A. Flemming Dean, College of Arts and Sciences of Ohio University AYERS, JAMES D. M.S. August 2004. Geological Sciences
Lithologic Evidence of the Jurassic/Cretaceous Boundary Within the Nonmarine Cedar
Mountain Formation, San Rafael Swell, Utah (189 pp.)
Director of Thesis: Gregory C. Nadon
The Jurassic/Cretaceous boundary in central Utah occurs at a major unconformity within a foreland basin. Paleontological data are absent and the boundary is typically placed at the lower contact of the Buckhorn Conglomerate Member of the Cedar
Mountain Formation based on the assumption that this contact represents an erosional unconformity. Lithologic and petrographic evidence from this study indicate the presence of a Stage 6 calcrete and a groundwater silcrete, which both indicate long-term geomorphic stability. These data suggest that the Jurassic/Cretaceous boundary should be placed at the top of the calcrete within the basal Cedar Mountain Shale Member. The fluvial architecture of the strata below and above the boundary suggests that the lower
Cedar Mountain Formation records the subtle transition from deposition within the proximal back-bulge depozone during the Late Jurassic to deposition within the distal foredeep depozone during the Early Cretaceous.
Approved:
Gregory C. Nadon
Assistant Professor Acknowledgments
I greatly appreciate the patients and support of my committee, family and friends throughout the research and writing of this thesis. To my wonderful girlfriend Kristin, thank you for all the love, understanding, and encouragement. Your faith in my ability to complete this project is greatly appreciated. To Gregory C. Nadon, thank you for your guidance, inspiration, and for providing me the opportunity to work on this exciting project. You have been a wonderful mentor and a great friend. To Xavier Roca, thank you for your assistance and extended discussions in the field. You were a great inspiration. To Elizabeth Gierlowski-Kordesch and David L. Kidder, thank you for your assistance. I benefited greatly from our conversations.
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Table of Contents
Page
Abstract…………………………………………………………………………… 3
Acknowledgments…………………………………………………………….….. 4
List of Tables……………………………………………………………………... 7
List of Figures…………………………………………………………………….. 8
Chapter 1. Introduction…………………………………………………………… 11 1.1 Introduction…………………………………………………………… 11 1.2 Purpose....…………………………………………………………….. 17 1.3 Study Area……………………………………………………………. 17
Chapter 2. Previous Work………………………………………………………… 21 2.1 Introduction…………………………………………………………... 21 2.2 Stratigraphy…………………………………………………………… 21 2.2.1 Morrison Formation………………………………………… 21 2.2.2 Cedar Mountain Formation………………………………… 25 2.3 Late Jurassic-Early Cretaceous Foreland Basin Development………. 30 2.4 Nonmarine Sequence-Stratigraphy…………………………………… 33
Chapter 3. Methodology…………………………………………………………. 37 3.1 Fieldwork…………………………………………………………….. 37 3.2 Stratigraphic Data…………………………………………………….. 37 3.3 Sedimentologic Data………………………………………………….. 38 3.4 Petrographic Data……………………………………………………. 38
Chapter 4. Lithofacies Analysis………………………………………………….. 40 4.1 Introduction…………………………………………………………… 40 4.2 Facies A: Conglomerate Facies………………………………………. 40 4.2.1 Introduction………………………………………………… 40 4.2.2 Facies A1: Buckhorn………………………………………… 40 4.2.3 Facies A2: Buckhorn-equivalent……………………………. 51 4.2.3.1 Introduction………………………………………. 51 4.2.3.2 Indurated A2 Conglomerates……………………… 52 4.2.3.3 Poorly Indurated A2 Conglomerates……………… 54 4.2.4 Facies A3: Red Conglomerate………………………………. 57 4.3 Facies B: Sandstone Facies…………………………………………… 62 4.3.1 Introduction………………………………………………… 62 4.3.2 Facies B1: Buckhorn Sandstones…………………………… 62 4.3.3 Facies B2: Buckhorn-equivalent Sandstones……………….. 70 6
Page
4.3.4 Facies B3: White Sandstone………………………………… 71 4.3.5 Facies B4: Red Conglomerate Sandstones………………….. 78 4.3.6 Facies B5: Isolated Sandstone Bodies………………………. 79 4.4 Facies C: Siltstone Facies…………………………………………….. 80 4.5 Facies D: Mudstone Facies…………………………………………… 81 4.5.1 Facies D1: Mudstone……………………………………….. 81 4.5.2 Facies D2: Pebbly Mudstone……………………………….. 82 4.6 Facies E: Carbonate Facies…………………………………………… 85 4.6.1 Introduction………………………………………………… 85 4.6.2 Facies E1: Lacustrine Carbonate……………………………. 85 4.6.2.1 Lower Carbonate………………………………….. 85 4.6.2.2 Middle Carbonate………………………………… 103 4.6.2.3 Upper Carbonate…………………………………. 104 4.6.3 Facies E2: Calcrete…………………………………………. 106
Chapter 5. Lithofacies Associations and Fluvial Architecture…………………… 119 5.1 Introduction…………………………………………………………… 119 5.2 Lithofacies Associations……………………………………………… 123 5.2.1 Unit 1: The Buckhorn Conglomerate………………………. 123 5.2.2 Unit 2: The Buckhorn-equivalent…………………………… 127 5.2.3 Unit 3: The Lower Carbonate………………………………. 129 5.2.4 Unit 4: The Pedogenic Calcrete Profile…………………….. 130 5.2.5 Unit 5: The White Sandstone………………………………. 136 5.2.6 Unit 6: The Middle Carbonate……………………………… 137 5.2.7 Unit 7: The Red Conglomerate…………………………….. 138 5.2.8 Unit 8: The Upper Carbonate………………………………. 142 5.3 Fluvial Architecture…………………………………………………… 143
Chapter 6. Conclusions…………………………………………………………… 150
References………………………………………………………………………… 152
Appendix A: Measured Sections and Cross-sections…………………………….. 160
Appendix B: Paleocurrent Measurements………………………………………... 180
Appendix C: Point Count and Pebble Count Data……………………………….. 188
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List of Tables
Table Page
5.1 Lithofacies assemblage, lithology, and depositional environment of units…… 120
8
List of Figures
Figure Page
1.1 Generalized stratigraphic column…………………………………………….. 12 1.2 Diagrammatic cross-section of Late Jurassic-Late Cretaceous foreland basin.. 13 1.3 Generalized map of the Sevier orogenic belt and its foreland basin…………. 15 1.4 Chart of previous chronostratigraphic interpretations………………………... 16 1.5 Location map of the study area……………………………………………….. 18 1.6 Structural map of the Cedar Mountain region………………………………... 19
2.1 Generalized paleogeographic map of the Albian foreland basin system…….. 34 2.2 Nonmarine sequence-stratigraphic diagram………………………………….. 35
4.1 Diagrammatic cross-section of the facies within the study interval………….. 41 4.2 Map showing the location of measured sections……………………………... 43 4.3 a Outcrop photo of Facies A1 and B1…………………………………………. 44 b Close-up photo of Facies A1………………………………………………... 44 4.4 Paleocurrent map for Facies A1 and B1 as well as Facies A2 and B2………… 46 4.5 “Capping” megaquartz cement……………………………………………….. 48 4.6 Schematic section of a pedogenic silcrete……………………………………. 50 4.7 a Outcrop photo of the Indurated A2 Conglomerates and Facies B2…………. 53 b Close-up photo of the Indurated A2 Conglomerates………………………... 53 4.8 Outcrop photo of the Poorly Indurated A2 Conglomerates…………………... 55 4.9 a Outcrop photo of Facies A3………………………………………………… 58 b Outcrop photo of a well cemented portion of Facies A3…………………… 58 4.10 a Outcrop photo of oncoids within Facies A3……………………………….. 60 b Photo of a carbonate coated extraformational chert clast (oncoid)……….. 60 4.11 Paleocurrent map for Facies A3 and B4……………………………………... 61 4.12 a Ternary diagram of QmFL compositions…………………………………. 64 b Ternary diagram showing variations in lithic grain proportions………….. 64 4.13 a Outcrop photo of a highly indurated, silica-cemented area of Facies B1…. 66 b Slabbed sample from a highly indurated, silica-cemented area of Facies B1. 66 4.14 a Multiple generations of fracture-filling silica cements……………………. 67 b Multiple generations of fracture-filling silica cements……………………. 67 4.15 a GS-fabric silcrete………………………………………………………….. 69 b F-fabric silcrete……………………………………………………………. 69 4.16 a Outcrop photo of Facies B3……………………………………………….. 72 b Outcrop photo of Facies B3……………………………………………….. 72 4.17 a Outcrop photo of a conglomerate at the base of Facies B3………………... 74 b Outcrop photo of jasper within an intraformational carbonate clast……… 74 4.18 Paleocurrent map for Facies B3……………………………………………... 76 4.19 a Outcrop photo of Facies D2……………………………………………….. 83 b Outcrop photo of a centimeter-scale laminar calcic horizon……………… 83
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Figure Page
4.20 Isopach map of the Lower Carbonate……………………………………….. 86 4.21 a Outcrop photo of the Lower Carbonate…………………………………… 87 b Outcrop photo of the Lower Carbonate…………………………………… 87 4.22 a Outcrop photo of desiccation breccia……………………………………… 90 b Outcrop photo of desiccation breccia……………………………………… 90 4.23 a Outcrop photo of fault breccia…………………………………………….. 91 b Photo of a slabbed sample of fault breccia………………………………… 91 4.24 a Outcrop photo of a jasper-filled vugs……………………………………… 93 b Close-up photo of a jasper-filled vugs…………………………………….. 93 4.25 a Outcrop photo of a chert-filled vug………………………………………... 94 b Outcrop photo of isopachous jasper cement………………………………. 94 4.26 Outcrop photo of a massive jasper accumulation…………………………… 95 4.27 Outcrop photo of a laminar jasper horizon………………………………….. 95 4.28 a Ostracodes within the Lower Carbonate…………………………………... 96 b Ostracodes within the Lower Carbonate…………………………………... 96 4.29 Map of pedogenic features within the Lower Carbonate……………………. 98 4.30 Photo of a slabbed sample showing evidence of pedogenic modification…... 100 4.31 a Outcrop photo of mottling within the Upper Carbonate…………………... 105 b Photo of a slabbed sample showing red and yellow mottling……………... 105 4.32 Outcrop photo of Facies E2………………………………………………….. 108 4.33 a Outcrop photo of honeycomb calcrete…………………………………….. 109 b Outcrop photo showing carbonate nodules surrounded by Facies D2…….. 109 4.34 a Outcrop photo of hardpan calcrete………………………………………… 110 b Photo of a slabbed sample of boulder/cobble calcrete…………………….. 110 4.35 a Root mould lined with clay minerals……………………………………… 111 b Allorthic nodule within Facies E2…………………………………………. 111 4.36 a Outcrop photo of a V-shaped structure……………………………………. 113 b Outcrop photo of a vertically inclined, laminar jasper horizon…………… 113 4.37 a Allorthic nodules within a V-shaped structure…………………………….. 114 b Glaebular fabric within a vertically inclined, laminar jasper horizon…….. 114 4.38 a Outcrop photo of calcrete development within a sandstone/conglomerate.. 116 b Outcrop photo of circular, white discolorations on framework chert clasts. 116
5.1 Diagrammatic cross-section of the units within the study interval…………… 121 5.2 Panoramic photo showing the transition between Unit 1 and Unit 2………… 122 5.3 Generalized stratigraphic column of the study interval………………………. 124 5.4 Isopach map of Unit 1………………………………………………………… 125 5.5 Schematic section of Unit 4…………………………………………………... 131 5.6 Generalized diagram showing the distribution of groundwater silcrete……… 133 5.7 Isopach map of Unit 7………………………………………………………… 139
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Figure Page
5.8 a Outcrop photo of the contact between Unit 5 and Unit 7…………………… 140 b Outcrop photo showing a boulder of Unit 5 within Unit 7…………………. 140 5.9 Generalized map showing the flow direction of Units 1 and 2………………. 145 5.10 Generalized map showing the flow direction of Units 5 and 7……………… 147
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CHAPTER 1: INTRODUCTION
1.1 Introduction
The Cordilleran foreland basin of the Western Interior United States extends from western Canada to New Mexico and is comprised of a complex orogen that consists of thrust sheets, accreted terrains, and magmatic arcs (DeCelles and Burden, 1992). In east- central Utah, the Late Jurassic-Early Cretaceous stratigraphic interval within this basin system is represented by the strata of the Morrison and Cedar Mountain Formations (Fig.
1.1, 1.2), a mostly nonmarine succession shed eastward in response to structural disturbances along the western margin (Lawton, 1994).
The Morrison Formation is composed of fluvial, lacustrine, and some marginal marine deposits that extend from north-central New Mexico to southern Canada
(McGookey et al., 1972; Peterson, 1972; Lawton, 1994) while the Cedar Mountain
Formation is a fluvio-lacustrine succession that is present throughout east-central Utah between the Colorado River and the Gunnison Plateau, extending from south-central
Utah to northwestern Colorado (Witkind et al., 1986; Aubrey, 1998). The Cedar
Mountain Formation, as defined by Stokes (1944), consists of two members; a relatively thin basal conglomeratic member termed the Buckhorn Conglomerate and an upper mud- dominated member termed the Cedar Mountain Shale. The Morrison Formation and the two members of the Cedar Mountain Formation document the development and eastward migration of a succession of foreland basins (Currie, 1998, 2002), however, the task of relating these strata to specific orogenic or tectonic events has proven difficult resulting in several interpretations regarding the timing of initial thrust-related crustal loading and 12
Fig. 1.1 Generalized stratigraphic column of the strata between the Summerville Formation and Dakota Sandstone. Modified from Trimble and Doelling (1978) and Tschudy et al. (1984). The Tidwell Member is not present because the section is east of the study area.
13
Fig. 1.2 Diagrammatic cross-section of the Late Jurassic to Late Cretaceous foreland basin following deposition of the Dakota Sandstone. After Yingling and Heller (1992).
14
foreland basin development in the Western Interior. Some workers suggest that the
Morrison Formation and Buckhorn Conglomerate were deposited in the distal portions of a foreland basin that formed in association with the Middle-Late Jurassic Elko Orogen
(DeCelles and Burden, 1992; Currie, 1998) while others consider the onset of foreland basin development to be an Early Cretaceous event related to crustal loading within the
Sevier Orogen (Heller et al., 1986; Yingling, 1987; Heller and Paola, 1989; Yingling and
Heller, 1992), a narrow fold-thrust belt that stretches the length of the North American
Cordillera from southern Nevada to southern Canada (Heller et al., 1986) (Fig. 1.3).
The location of the J/K boundary within the Late Jurassic-Early Cretaceous stratigraphic interval of the Cordilleran foreland basin system further complicates accurately constraining the timing of foreland basin development in the Western Interior.
In east-central Utah, the location of the boundary has been the subject of considerable debate due to a lack of age diagnostic paleontological and palynological data (Fig. 1.4).
The boundary traditionally has been placed at the contact between the Morrison
Formation and Buckhorn Conglomerate Member based on the interpretation that the contact represents an erosional unconformity, which spans much of Early Cretaceous time (Stokes, 1944; Young, 1960; Ying ling and Heller, 1992; Currie, 1998). However, evidence of a conformable lower contact has also been reported (Aubrey, 1998; Roca,
2003) in which case the J/K boundary must occur above the Buckhorn Conglomerate.
Aubrey (1998) used the base of a calcrete within the lower Cedar Mountain Shale
Member as the boundary (Fig. 1.4). Understanding the Buckhorn Conglomerate/Cedar
Mountain Shale transition is crucial to understanding both the J/K unconformity in east- central Utah and the timing of initial foreland basin development in the Western Interior. 15
Fig. 1.3 Generalized map showing the Sevier orogenic belt and the extent of its foreland basin. Modified from Heller et al. (1986). Ruled pattern and heavy barbed line indicates the location and extent of the Sevier orogenic belt, dashed line indicates the approximate limit of its foreland basin.
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Fig. 1.4 Chart showing previous chronostratigraphic interpretations of the Late Jurassic to early Late Cretaceous stratigraphic interval in Utah. Ruled pattern denotes unconformities, question marks denote uncertainties in age. Time scale is from Palmer and Geissman (1999). Data is from Stokes (1944), Yingling and Heller (1992), Aubrey (1998), Currie (1998), and Kirkland et al. (1999).
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1.2 Purpose
The purpose of the current study was to evaluate the stratigraphic interval spanning the transition between the Buckhorn Conglomerate and Cedar Mountain Shale
Members at the type locality of the Cedar Mountain Formation and to provide a more detailed assessment of the J/K unconformity, which in east-central Utah represents a major sequence boundary within a developing foreland basin. The absence of paleontological and palynological data makes lithologic criteria the only means to determine the position of the boundary. The general disagreement as to the location of the boundary (Fig. 1.4) indicates that further evaluation of the temporal relationship between the Buckhorn Conglomerate and Cedar Mountain Shale Members as well as the
J/K transition in east-central Utah is necessary to understand the evolution of this portion of the foreland basin system.
1.3 Study Area
The Cedar Mountain region, which includes Cedar Mountain, Little Cedar
Mountain, and the Cleveland-Lloyd Dinosaur Quarry, was chosen as the study area for the current study because it is the type locality of the Buckhorn Conglomerate and Cedar
Mountain Shale Members (Stokes, 1944) (Fig. 1.5). Little Cedar and Cedar Mountains are oriented roughly perpendicular to the northwestern flank of the San Rafael Swell, a northeast-southwest trending, Laramide uplift present in Emery County, Utah (Stokes,
1987) (Fig. 1.5). The gently dipping western limb of the Swell provides the strata of the study interval with a northwest dip of between 2o and 4o (Witkind, 1995) (Fig. 1.6).
18
Fig. 1.5 Detailed map showing the location of Emery County, the San Rafael Swell, and the Cedar Mountain region. Bold red line demarcates the approximate limits of the study area.
19
Fig. 1.6 Detailed structural map of the Cedar Mountain region showing the location of measured sections in relation to structural disturbances. Modified from Witkind (1995). Note that one section was measured at each site and that each measured section is illustrated in Appendix A.
20
The semi-arid climate of the region limits vegetation providing excellent exposure of the study interval.
The Buckhorn Conglomerate Member is widespread throughout the Cedar
Mountain region where it forms the upper dip slope of both Little Cedar and Cedar
Mountains but complete exposures of the study interval from the base of the Buckhorn
Conglomerate through the lower Cedar Mountain Shale Member are typically restricted to the western margin of the study area or to the isolated knolls of Cedar Mountain.
Exposures along the western margin of the study area allow the Buckhorn
Conglomerate/Cedar Mountain Shale Member transition to be traced laterally for tens of kilometers, from the southwestern flank of Little Cedar Mountain to the Cleveland-Lloyd
Dinosaur Quarry (Fig. 1.5, Cross-section 1 of Appendix A). This combined with the limited vegetation of the Cedar Mountain region allows a detailed evaluation of the vertical and lateral arrangement of the strata of the study interval. 21
CHAPTER 2: PREVIOUS WORK
2.1 Introduction
Since the early 1900's, numerous workers have conducted studies on the
stratigraphy of the Morrison and Cedar Mountain Formations as well as the foreland
basins in which they were deposited and the tectonic events that controlled their
deposition. This chapter offers an overview of some of the most commonly cited studies
and gives a brief summary of the diverse interpretations related to the interval. The
chapter provides a historical background to which the results of the current study may be
compared.
2.2 Stratigraphy
2.2.1 Morrison Formation
The Morrison Formation extends from north-central New Mexico to southern
Canada as a broad, sheet deposit that thickens only slightly from east to west and covers
an area of approximately 1,500,000 km2 (McGookey et al., 1972; Peterson, 1972;
Lawton, 1994). In the San Rafael Swell, the Morrison consists of fluvial, lacustrine, and some marginal marine deposits that occur between the Middle Jurassic Summerville
Formation and the Early Cretaceous Cedar Mountain Formation (Fig. 1.1). The
Morrison, which was deposited during the regression of the Jurassic Sundance Sea, shows an overall transition from proximal fluvial sediments in the south to marginal marine sediments in the north (Peterson, 1988, 1994). 22
The Morrison Formation has long been considered Late Jurassic in age based on
the presence of abundant dinosaur fauna (Stokes, 1944, 1952; Lohman, 1965) but a lack
of age diagnostic fauna/flora has frustrated workers trying to constrain the timing of
deposition. Recent, isotopic analyses, such as single-crystal, laser-fusion and step
heating, plateau 40Ar/39Ar on sanidine, from bentonite beds near the bottom and top of the succession have yielded ages indicating that the Morrison ranges between 155 to 148 Ma in age (Kowallis et al., 1998), i.e., latest Oxfordian to middle Tithonian (Palmer and
Geissman, 1999).
The Morrison Formation has been subdivided into six members in the Four
Corners area of Arizona, Colorado, New Mexico, and Utah, which include the Tidwell,
Salt Wash, Brushy Basin, Westwater Canyon, Recapture, and Fiftymile Members
(Bilbey, 1992). Although there is significant lateral variability within the Morrison, it typically shows a transition from a lower sand-dominated, cliff-forming unit to an upper muddy, slope-forming unit (Stokes, 1944; Peterson, 1988, 1994). Both of these units are present within the study area where the Morrison Formation consists of the basal Tidwell
Member, the Salt Wash Member, and the upper Brushy Basin Member.
The Tidwell Member is interpreted as a marginal marine, tidal/estuary complex
(Currie, 1998) that consists of supratidal, sabkha-like facies within the study area (Bilbey,
1992). It is composed predominantly of red and greenish-gray mudstones interbedded with minor sandstones, limestones, and gypsum beds throughout the San Rafael Swell
(Yingling, 1987; Bilbey, 1992). Gypsum beds within the basal portion of the member are interpreted to record precipitation in an evaporative basin that may have been connected to a Jurassic seaway in Wyoming (Yingling, 1987). 23
The Salt Wash Member (Fig. 1.1), which records fluvial and minor lacustrine conditions (Lupton, 1914), thins toward the north-northeast from 90 to 30 m (Stokes,
1944). It is composed of mainly white to pale brown, massive to trough cross-stratified, cliff-forming sandstones and conglomerates, which are interbedded with red, gray, and green, slightly bentonitic mudstones, siltstones, and gray silty limestones (Stokes, 1944;
Cadigan, 1967; Yingling, 1987; Currie, 1998). The Salt Wash shows a gradation from braided fluvial deposits in the southwest to meandering fluvial deposits in the northeast
(Yingling, 1987) implying an east to northeast draining fluvial system with major source terrains in Arizona, southern Nevada, southern California, and western Utah (Stokes,
1944; Yingling, 1987; Bilbey, 1992; Currie, 1998).
The Brushy Basin Member is the most widespread and well-known member of the Morrison Formation (Fig. 1.1). The Brushy Basin is comprised of variegated red, purple, gray, green, and brown smectitic mudstones that are interbedded with sandstones, minor conglomerates, and thin limestones, as well as bentonite and authigenic chert beds
(Stokes, 1944; Conley, 1986; Yingling, 1987; Bilbey, 1992; Currie, 1998).
The variegated, smectitic, swelling mudstones represent overbank deposits that are largely the product of altered ash or bentonite that has been reworked with sand and silt (Stokes, 1944; Currie, 1998). Variegation typically occurs as bands less than 1 m thick but may reach a thickness of more than 10 m (Stokes, 1944). The mudstones become less smectitic toward the north, although bentonite beds are still present, which has been interpreted to represent a larger volcanogenic input in the south (Yingling,
1987). 24
The sandstones and conglomerates of the Brushy Basin Member are interpreted as the deposits of northeasterly draining meandering or anastomosed fluvial systems that followed the same drainage pattern as the underlying Salt Wash Member (Stokes, 1944;
Yingling, 1987, Bilbey, 1992; Currie, 1997, 1998; Roca, 2003). These deposits consist of laterally restricted, sandstone and conglomeratic bodies south of Interstate 70, which grade northward into broader, finer-grained, sandstone bodies (Yingling, 1987).
The thin limestones are commonly blue or gray on fractured surfaces but weather a yellow color (Stokes, 1944). Ostracodes and charophytes present within the limestones indicate that they are shallow lacustrine or pond deposits, which in some cases were later subjected to pedogenic alteration (Yingling, 1987; Currie, 1998). Some of these lacustrine deposits were extensive as indicated by the presence of alkaline/saline lake deposits in the east-central Colorado Plateau that measure 500 km in length and 300 km in width (Turner and Fisherman, 1991).
Kowallis et al. (1998) obtained ages of 150.18 + 0.11 and 148.07 + 0.17 Ma from bentonite beds at the bottom and top of the Brushy Basin Member at Little Cedar
Mountain indicating that deposition within the study area lasted approximately 2 million years. The lower contact of the Brushy Basin is gradational (Stokes, 1944) but there is debate as to the nature of the upper contact. Some workers suggest that the upper contact is an erosional unconformity (Stokes, 1944; Young, 1960; Conley, 1986; Currie, 1998) that represents a large part of Early Cretaceous time (Young, 1960) while other workers interpret the contact to be conformable (Aubrey, 1998; Roca, 2003) (Fig. 1.4).
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2.2.2 Cedar Mountain Formation
The Cedar Mountain Formation is a fluvio-lacustrine succession that occupies the stratigraphic position between the Late Jurassic Brushy Basin Member and the Late
Cretaceous Dakota Sandstone (Fig. 1.1). It is present throughout east-central Utah between the Colorado River and the Gunnison Plateau, extending from south-central
Utah to northwestern Colorado, but is absent further to the south due to pre-Dakota erosion (Witkind et al., 1986; Aubrey, 1998).
The Cedar Mountain Formation is divided into the basal Buckhorn Conglomerate
Member and the upper informal shale member, which were originally described along the southwest flank of Cedar Mountain, Emery County, Utah (Stokes, 1944). The informal shale member is often referred to as the Upper Shale or Unnamed Shale Member but is referred to here as the Cedar Mountain Shale Member following the original nomenclature proposed by Stokes (1944). More recently, Kirkland et al. (1999) proposed a classification further subdividing the Cedar Mountain Shale Member into four informal units, the Yellow Cat, Ruby Ranch, and Mussentuchit Members, as well as the Poison
Strip Sandstone. This classification is not used here because it is based on exposures in eastern Utah that have yet to be correlated to the type locality at Cedar Mountain.
The age of the Buckhorn Conglomerate is poorly constrained due to a lack of age diagnostic paleontological and palynological data, which has thus far only yielded bone fragments, silicified logs, and non-diagnostic mollusks (Stokes, 1950). The Buckhorn is commonly considered Latest Jurassic to Early Cretaceous in age based on its close relation to the well-dated upper portion of the underlying Brushy Basin Member (early to middle Tithonian) and the overlying Cedar Mountain Shale Member. 26
Age diagnostic fauna/flora as well as isotopic data retrieved from the Cedar
Mountain Shale Member suggest that deposition lasted from Barremian to early
Cenomanian time. A dinosaur fauna from the base of the Cedar Mountain Shale near
Green River indicates a Barremian age (Kirkland, 1992) while palynomorphs taken from
the upper part of the interval (~ 14 m below Dakota contact) in Emery County contain
angiosperm pollen that are late Albian age (Tschudy et al., 1984). Similarly, a bentonite
bed located about 15 m below the Dakota Sandstone contact in the western flank of the
San Rafael Swell yielded an age of 98.4 + 0.1 Ma, i.e., earliest Cenomanian, for the end of Cedar Mountain Shale deposition (Cifelli et al., 1997).
Paleontological and palynological data from strata interpreted to be the Cedar
Mountain Formation equivalents at the Gunnison Plateau suggest that Cedar Mountain deposition could have extended from the Barremian to the Albian. Palynomorphs from near the base of these strata in the southeastern Gunnison Plateau are Aptian to middle
Albian age (Sprinkel et al., 1992) or Barremian to Albian age (Witkind et al., 1986), while angiosperm leaf impressions from near the top of the interval are late Albian or younger (Schwans, 1988).
The Buckhorn Conglomerate Member occurs as a discontinuous sheet of irregularly bedded, granule to cobble conglomerates composed mainly of chert interbedded with conglomeratic sandstones (Fig. 1.1). The unit achieves a maximum reported thickness of approximately 25 m (Yingling, 1987). The Buckhorn has been interpreted as the result of deposition by an easterly to northeasterly draining braided fluvial system (Stokes, 1952; Young, 1960; Conley, 1986; Yingling, 1987; Bilbey, 1992; 27
Yingling and Heller, 1992; Currie, 1998) that has source terrains to the west and southwest (Stokes, 1944, 1960).
The Buckhorn Conglomerate is one of several similar conglomeratic deposits that occur at the same stratigraphic position throughout the Western Interior from southern
Nevada to Canada, including the Burro Canyon, Cloverly, Kelvin, Kootenai, Lakota,
Lytle, and Pryor Formations (Heller and Paola, 1989; Lawton, 1994; Aubrey, 1998).
These deposits, which Heller and Paola (1989) referred to as the Lower Cretaceous gravels, are commonly correlated to the Buckhorn Conglomerate. However, Stokes
(personal communication in Conley, 1986) stated that the Buckhorn as he originally defined it (Stokes, 1944) is confined to the Cedar Mountain area and that even nearby conglomeratic deposits, such as those in southern Utah, which occur at a similar stratigraphic position, are only broadly correlative.
The base of the Buckhorn Conglomerate Member is sharp and locally erosive.
Some workers suggest that the Buckhorn Conglomerate/Brushy Basin Member contact represents a major unconformity spanning much of Early Cretaceous time (Young, 1960), in which case the contact is considered to be the J/K boundary (Stokes, 1944; Young,
1960; Currie, 1998) (Fig. 1.4). However, Stokes (1950) suggested that this basal unconformity might not represent a significant hiatus and that deposition of the Buckhorn may have spanned a considerable amount of time and contain the J/K boundary. Other workers suggest that the lower contact of the Buckhorn is conformable (Aubrey, 1998;
Roca, 2003) based on the presence of load casts (Kirkwood, 1976; Yingling, 1987; Roca,
2003) as well as evidence of interfingering between the two members (Peterson and
Turner personal communication in Aubrey, 1998) (Fig. 1.4). 28
The Cedar Mountain Shale Member is a mud-dominated, slope-forming deposit that occupies a stratigraphic position between the Buckhorn Conglomerate Member and the Dakota Sandstone (Fig. 1.1). It consists of variegated, gray to greenish-gray, purple, brown, or red, slightly bentonitic to moderately calcareous mudstones and minor interbedded siltstones, sandstones, and conglomerates as well as a discontinuous, chert bed at or near the base (Stokes, 1944; Young, 1960; Peterson et al., 1980; Bilbey, 1992).
These sediments are approximately 100 m thick at the type locality (Stokes, 1944), possess an average thickness of 49 m within the San Rafael mining district (Trimble and
Doelling, 1978), and thicken from east to west (Stokes, 1952; Craig, 1981). The colors of the Cedar Mountain Shale are typically more dull and subdued than the colors of the
Brushy Basin Member (Stokes, 1944) but the two units are similar enough that distinction between them is difficult where the Buckhorn Conglomerate is absent (Stokes, 1944;
Young, 1960; Peterson et al., 1980; Conley, 1986).
The Cedar Mountain Shale Member is interpreted as the deposit of a meandering fluvial system (Katich, 1954; Harris, 1980). Harris (1980) reported that southwest of
Green River, Utah four of the Cedar Mountain Shale sandstones are exposed as exhumed paleochannels, which shown an overall eastward flow direction. The mudstones of the member represent overbank deposits that were affected by pedogenic processes as indicated by the presence of abundant carbonate nodules.
The carbonate nodules, which indicate calcrete formation, are characteristic of the
Cedar Mountain Shale (Stokes, 1944; Conley, 1986; Yingling, 1987; Aubrey, 1998;
Currie; 1998) and may locally constitute as much as 30% of the mudstones (Young,
1960). The nodules are present throughout the member but are typically more common 29
near the base (Aubrey, 1998) where they may form a nodular horizon located approximately 3 to 7 m above the Cedar Mountain Formation/Morrison Formation contact (Trimble and Doelling, 1978). This nodular horizon is often considered an important index marker that can be used to approximate the base of the Cedar Mountain
Shale Member when the Buckhorn Conglomerate is absent (Trimble and Doelling, 1978).
Locally, the carbonate nodules coalesce into laterally continuous beds up to 10 m thick forming a nodular to hardpan calcrete southeast of Green River, Utah (Aubrey, 1998) as well as at locations throughout northeastern Utah and northwestern Colorado (Currie,
1997, 1998). Aubrey (1998) argued that the base of this calcrete or the base of the lowest lithologically distinct bed containing calcrete marks the J/K boundary (Fig. 1.4).
The first occurrence of well-rounded, highly polished, quartz, quartzite and chert pebbles, which are common within the nodular mudstones of the basal Cedar Mountain
Shale but rare within the Brushy Basin Member, is another important index marker for the base of the Cedar Mountain Shale (Young, 1960, Bilbey, 1992). The pebbles are similar to the clasts of the Buckhorn and are interpreted to be derived from Paleozoic rocks but there is no agreed means of deposition (Young, 1960, Bilbey, 1992). These clasts have been interpreted as gastroliths (Stokes, 1944), as clasts transported by a volcanic ash/debris flow or in an ash choked stream (Bilbey, 1992), and as a residual or lag gravel polished by wind (Stokes, 1950). Young (1960) refuted the latter interpretation based on the erratic distribution of the pebbles and a general lack of faceted surfaces.
30
2.3 Late Jurassic-Early Cretaceous Foreland Basin Development
The Late Jurassic-Early Cretaceous stratigraphic interval of the Western Interior
United States documents the development and eastward migration of a succession of foreland basins. Relating the strata of these basins to specific orogenic or tectonic events has proven difficult because the thrust sheets that comprise the orogenic belts, as well as the most proximal foreland basin strata, typically have been uplifted and eroded or buried beneath basins during Tertiary extension and structural inversion (Currie, 2002). Thus, the timing of initial thrust-related crustal loading and foreland basin development in the
Western Interior is the subject of considerable debate (Currie, 1998).
Early workers suggested that the Late Jurassic to Early Cretaceous conglomerates of the Western Interior foreland basin recorded the initial thrust sheet emplacement within the Sevier orogenic belt (Armstrong, 1968; Wiltschko and Dorr, 1983), a narrow fold-thrust belt that stretches the length of the U.S. Cordillera from southern Nevada to southern Canada and consists of the large-scale, east-vergent thrusts and associated folds found between the Sevier "hinterland" to the west and the Western Interior foreland basin to the east (Heller et al., 1986) (Fig. 1.3). Based on the dating of these conglomerates, shortening within the Sevier is inferred to have occurred as early as Late Jurassic time and continued through earliest Tertiary time (Armstrong, 1968; Wiltschko and Dorr,
1983). Although the timing of initial thrusting has yet to be well constrained (Heller et al., 1986).
In contrast, other workers have suggested that the Morrison Formation and
Buckhorn Conglomerate Member are pre-Sevier deposits (Heller et al., 1986; Yingling,
1987; Heller and Paola, 1989; Yingling and Heller, 1992) that were shed eastward from a 31
Late Jurassic-Early Cretaceous thermally generated uplift in the area of Sevier hinterland
(Heller and Paola, 1989; Yingling and Heller, 1992) or were deposited in the distal portion of a foreland basin that formed in association with the Late Jurassic Elko Orogen
(Yingling and Heller, 1992). These models imply a late Early Cretaceous age for the initiation of thrusting within the Sevier based on recent paleontological data from synorogenic conglomerates (Heller et al., 1986) and the observation that the first strong asymmetric westward thickening of Late Jurassic-Early Cretaceous strata began with the deposition of the Cedar Mountain Shale Member (Yingling, 1987; Yingling and Heller,
1992).
Lawton (1994) suggested that the Middle-Late Jurassic strata of the Western
Interior were deposited in a retroarc foreland basin system during a period of subduction- related subsidence. Lawton agreed that the Morrison Formation and Buckhorn
Conglomerate Member were pre-Sevier deposits but suggested that they were shed eastward from a regional uplift in the proximal retroarc foreland basin during the Late
Jurassic following the Elko Orogen (Lawton, 1994). Roca (2003) agreed with the subduction-related subsidence model proposed by Lawton (1994) and suggested that a period of shallow plate subduction may have extended the effects of dynamic subsidence into the back-bulge region providing the increased accommodation needed to deposit the
Morrison depositional sequence, a relatively thick back-bulge deposit (DeCelles and
Giles, 1996). Roca (2003) argued that the Buckhorn Conglomerate was the result of the erosion of the regionally uplifted Late Jurassic foredeep in western Utah following a decrease in subsidence related to the end of subduction-related subsidence. 32
DeCelles and Burden (1992) suggested that the Morrison Formation and Cedar
Mountain Formation-equivalents in central Wyoming were deposited in an overfilled
Late Jurassic-Early Cretaceous foreland basin that formed in association with the Elko
Orogen. Later, DeCelles and Currie (1996) and Currie (1997, 1998) argued that the
Morrison and Cedar Mountain Formations are the products of an eastward migrating Late
Jurassic-Early Cretaceous foreland basin system. Based on this latter model, deposition of the Morrison Formation and Buckhorn Conglomerate occurred in the back-bulge depozone adjacent to an overfilled foredeep (Currie, 1998). The continued eastward migration of the foreland basin system during the Early Cretaceous is interpreted to be the result of the cratonward propagation of thrust loads within the Sevier orogenic belt
(Currie, 1998). This renewed crustal loading resulted in the formation of a foredeep depozone in parts of central Utah that were previously occupied by the Late Jurassic forebulge and subsequent migration of the forebulge into eastern Utah and western
Colorado (Currie, 1997, 1998). During this time underfilled conditions within the foredeep restricted sediment supply to areas uplifted by the forebulge resulting in an unconformity marked by the calcrete at the base of the Cedar Mountain Shale Member
(Currie, 1997, 1998). Formation of the calcrete was followed by overfilled conditions within the foredeep, which resulted in the depositional onlap of the Cedar Mountain
Shale Member onto the forebulge in eastern Utah and western Colorado (Currie, 1998).
Thickness variations of the Cedar Mountain Shale Member have been used to infer the geometry of the Albian foreland basin system (Currie, 1997), which demarcate a foredeep depozone in central Utah, a northeast trending forebulge that stretched from the southeastern San Rafael Swell to northwestern Colorado, and a back-bulge depozone that 33
extended into Colorado (Fig. 2.1). Although the foredeep, forebulge, and back-bulge depozones of the Early Cretaceous foreland basin system are documented by the Early
Cretaceous stratigraphic interval, only the back-bulge depozone of the Late Jurassic foreland basin is preserved due to uplift and erosion during the cratonward propagation of the Sevier thrust belt (Royse, 1993; Currie, 1997). However, the foredeep and forebulge depozones of the Late Jurassic foreland basin system are inferred to have been located in west-central and central Utah, respectively (Royse, 1993; Currie, 1997). Royse (1993) used structural restoration to show that as much as 6 km of sediment may have been deposited in a foredeep in west-central Utah, conceivably during Late Jurassic time while
Currie (1998) suggests that the forebulge of the Late Jurassic foreland basin was located beneath the present day Wasatch Plateau.
2.4 Nonmarine Sequence-Stratigraphy
Currie (1997) proposed a general nonmarine sequence-stratigraphic model for the
Morrison and Cedar Mountain Formations based primarily on fluctuations in basin accommodation in order to show the relationship between accommodation development and evolution of the Late Jurassic-Early Cretaceous foreland basin system depozones.
This model subdivided the interval into four depositional sequences termed the UJ-1, UJ-
2, LK-1, and LK-2 sequences, each of which was separated by a sequence-bounding unconformity (Fig. 2.2). The UJ-1 and UJ-2 depositional sequences represent the strata of the Morrison Formation while the LK-1 and LK-2 sequences represent the strata of the
Buckhorn Conglomerate and Cedar Mountain Shale Members.
34
Fig. 2.1 Generalized paleogeographic map showing the structural configuration of the Albian foreland basin system in Utah and Colorado. Modified from DeCelles and Currie (1996) and Currie (2002). Ruled pattern and heavy black barbed line shows location and extent of the Sevier orogenic belt, hachured pattern and heavy red barbed line shows eastern extent of Middle to Late Jurassic thrusting and igneous activity. Note the location of the San Rafael Swell in relation to the foredeep, forebulge, and back-bulge.
35
Fig. 2.2 Regional correlation and nonmarine sequence-stratigraphic diagram of the Upper Jurassic to Lower Cretaceous strata between central Wyoming and the San Rafael Swell. Modified from Currie (1997). The diagram depicts the depositional sequences and sequence-bounding unconformities referred to in the text.
36
The LK-1 sequence embodies the sediments between the base of the Buckhorn
Conglomerate and the calcrete near the base of the Cedar Mountain Shale or the sediments between the Brushy Basin Member and the calcrete where the Buckhorn is absent. Based on this sequence-stratigraphic framework the sequence-bounding unconformity at the base of the LK-1 sequence is interpreted as a low order unconformity that is designated the J/K boundary (Fig. 2.2). The LK-2 sequence includes the sediments between the calcrete at the base of the Cedar Mountain Shale and the Dakota
Formation. The unconformity marked by the calcrete at the base of the LK-2 sequence represents a regional hiatus in deposition that resulted from the eastward migration of the flexural forebulge from central Utah to eastern Utah and western Colorado (Fig. 2.2).
Reports of a conformable transition between the Morrison Formation and the
Buckhorn Conglomerate (Aubrey, 1998; Roca, 2003) suggest that there is no sequence bounding unconformity at the base of the Buckhorn Conglomerate and that the position of the J/K boundary as well as the LK-1 depositional sequence warrants revision (Fig.
2.2). The accurate placement of the J/K boundary within this sequence-stratigraphic framework is vital to understanding the evolution of the Late Jurassic-Early Cretaceous foreland basin system. 37
CHAPTER 3: METHODOLOGY
3.1 Fieldwork
Field research was conducted during July and August 2002. The field work involved the measurement of 28 detailed measured sections of the Buckhorn
Conglomerate, basal Cedar Mountain Shale Member, and locally the uppermost Brushy
Basin Member. Each measured section was described in terms of thickness, sedimentary structures, grain-size variations, lithological contacts, and presence or absence of diagenetic and pedogenic alteration. In addition to the description of each lithology in outcrop a total of 122 hand samples were collected for further laboratory analysis and photographs from all measured sections were taken to document field observations.
3.2 Stratigraphic Data
The locations of measured sections (Appendix A) were chosen initially based on completeness of the section from the base of the Buckhorn Conglomerate through the basal Cedar Mountain Shale Member. However, outcrop conditions concentrated most sections along the western flank of Little Cedar and Cedar Mountains where the base of the Buckhorn Conglomerate is not exposed. Lithostratigraphic correlation of the data permitted construction of a detailed 21 km cross-section (Cross-section 1 of Appendix
A), synthesized from 19 measured sections, that is oriented roughly perpendicular to the trend of Cedar Mountain. A shorter cross-section (Cross-section 2 of Appendix A), consisting of 5 measured sections, was constructed from data gathered near Bob Hill
Knoll. Cross-section 2 and Cross-section 1 have common points at Site A and Site V to 38
show the spatial distribution of lithologies within the succession. The measured sections as well as other field measurements were used to produce isopach maps.
3.3 Sedimentologic Data
Paleoflow measurements were obtained from trough cross-stratified sandstones and conglomerates of both the Buckhorn Conglomerate and basal Cedar Mountain Shale
Members following the methods of DeCelles et al. (1983) (Appendix B). The measurements were taken primarily from cross-stratified beds that exhibited three- dimensional exposures of trough limbs but some data were collected from exposures with mainly vertical faces. In the laboratory, the data were grouped according to lithostratigraphic unit and location and the vector mean for each unit at each location was determined. The vector means from all locations were then grouped according to lithostratigraphic unit to determine the cumulative flow direction for each unit.
3.4 Petrographic Data
All 122 hand samples retrieved from the field were slabbed and then twenty-three of the samples were chosen for petrographic analysis based on the presence of internal structures and fabrics as well as stratigraphic location; 1 from the conglomeratic facies, 9 from the sandstone facies, 12 from the carbonate facies, and 1 from the mudstone facies.
These twenty-three samples were sent out for preparation where the samples from the carbonate and mudstone facies were stained with Alizarin Red S and potassium ferricyanide to distinguish calcite from dolomite while those from the conglomerate and sandstone facies were stained with sodium cobaltinitrite to distinguish orthoclase from 39
quartz. The thin-sections were analyzed by petrographic microscopy using standard petrographic techniques (Tucker, 2001).
Five point counts were conducted on thin-sections of the sandstone facies. Digital photographs of the thin-sections were taken and compiled into a photomosaic. Next, a grid of equal dimensions was placed over each mosaic and each intersection of the grid was designated a node. Using these nodes in conjunction with the petrographic microscope, a minimum of 400 points was counted within each sample. The results of each point count are summarized in Appendix C.
Five pebble counts were conducted on pebbles collected and retrieved from the field. At each pebble count location, approximately 400 pebbles were collected from a vertical face within an area measuring 1m2. Chips of the collected pebbles were examined using a binocular microscope in the laboratory to determine lithology and clast color. The results of each pebble count are summarized in Appendix C. 40
CHAPTER 4: LITHOFACIES ANALYSIS
4.1 Introduction
The lower Cedar Mountain Formation consists of interbedded conglomerates, sandstones, mudstones, and carbonates. The study interval contains five lithofacies: conglomerates, sandstones, siltstones, mudstones, and carbonates. Several of the lithofacies are divided into subfacies on the basis of petrologic, petrographic, and lithostratigraphic data. Each lithofacies is described and interpreted in this chapter and then compiled into facies associations in Chapter 5.
4.2 Facies A: Conglomerate Facies
4.2.1 Introduction
Conglomerate is the dominant lithology within the lower Cedar Mountain
Formation and is present at various stratigraphic levels in the study interval. These coarse clastic deposits are subdivided into three conglomerate subfacies based on framework clast composition and color, weathering characteristics, and stratigraphic position.
4.2.2 Facies A1: Buckhorn
Description:
Facies A1 is a conglomerate facies present at the base of the lower Cedar
Mountain Formation (Fig. 4.1). It is continuously exposed throughout the Little Cedar and Cedar Mountain area from just southwest of the Nipple between Site V and Site U to 41
Fig. 4.1 Generalized diagrammatic cross-section showing the vertical and lateral distribution of facies within the study interval. Facies B5 is not represented because its isolated sandstone bodies cannot be confidently linked to other sandstone deposits. The diagram is based on Cross-section 1 of Appendix A but is vertically exaggerated (V.E. = 600X). The amount of vertical exaggeration varies between individual facies. 42
the southwest flank of Little Cedar Mountain between Site E and Site BB (Fig. 4.2). The
facies is composed of extraformational, clast-supported, granule to cobble conglomerates
containing 90% chert and 10% quartzite with rare sandstone, siltstone, and mudstone
clasts (Fig. 4.3a, b; Appendix C). The conglomerates are moderately sorted (Tucker,
2001) and contain rounded to sub-rounded (Tucker, 2001) clasts, but sub-angular clasts
are also present. Roundness decreases with decreasing clast-size, so sections containing
larger amounts of granules tend to be sub-angular. Framework clasts are most commonly
black or shades of gray, but brown, white, and red clasts also occur. The presence of
crinoid stem fragments shows that a portion of the chert fraction is silicified Paleozoic
limestone (Stokes, 1944; Conley, 1986). Scattered bone fragments and petrified wood
are present within the upper horizons. The dark color typical of the framework clasts
combined with a manganese oxide staining imparts a very dark brown to black
appearance to Facies A1 throughout the Little Cedar and Cedar Mountain area.
Facies A1 consists of irregularly bedded, laterally discontinuous, amalgamated lenses (Friend, 1983) ranging in thickness between 20 cm and 4 m. Individual lenses commonly possess erosional contacts at both the upper and lower boundaries. When found in contact with the underlying Brushy Basin Member, the lower boundaries are sharp and locally erosive but also show examples of soft-sediment deformation, such as gutter casts, flame structures, and load casts (Roca, 2003). Gutter casts, which are sinuous, elongate ridges with U-shaped cross-sections, are very common where the contact between Facies A1 and the Brushy Basin Member is exposed. Flame structures and load casts are rare with only one example of each reported.
43
Fig. 4.2. Map of the Cedar Mountain region showing the locations of measured sections referred to in the text.
44
a
b
Fig. 4.3 (a) Outcrop photo of Facies A1 and B1 from the type locality of the Buckhorn Conglomerate, Site A (bushes for scale). (b) Close-up photo of Facies A1, Site A (lens cap for scale).
45
Other sedimentary structures include massive to horizontal bedding, pebble
imbrication, trough cross-stratification and, less commonly, planar cross-stratification
and large-scale bar forms. Massive to horizontally bedded conglomerates are bounded at
the base by scour surfaces and commonly at the top by erosional surfaces. Planar and
trough cross-stratified beds are typically bounded by erosional surfaces and may show a
vertical gradation in clast size within individual sets. Paleocurrent data obtained
predominantly from the upper horizons of the facies show a combined paleoflow
direction of 081o (Fig. 4.4, Appendix B).
Secondary silica and calcite cements are abundant and typically provide
conglomerates that are very resistant to erosion. The degree of cementation varies
significantly throughout the study area especially with respect to silica. At some
locations in the upper horizon of Facies A1, silica cementation in the form of jasper gives the conglomerates metaconglomerate-like characteristics (Conley, 1986). These jasper- cemented conglomerates are highly indurated and fracture through rather than between clasts.
Petrographic examination of a highly indurated, jasper-cemented sample of Facies
A1 reveals a clast-supported framework of rounded to well-rounded fossiliferous chert
and minor amounts of polycrystalline quartz clasts. The matrix is composed of fine-
grained, well sorted, sub-rounded to rounded grains that are predominantly
monocrystalline quartz with lesser amounts of orthoclase feldspar and chert lithic
fragments as well as minor detrital zircon and glauconite. Framework and matrix are
well cemented by both microcrystalline quartz and chalcedony providing an extremely
dense rock with minimal porosity. A complex assemblage of silica phases fills both 46
Fig. 4.4 Map showing the distribution of paleocurrent orientations for the combined data of Facies A1 and B1 as well as Facies A2 and B2. The arrows indicate the mean flow direction at each station while the associated numbers indicate the number of measurements collected per station. The Rose Diagrams show the cumulative flow direction of each facies. 47
fractures and voids. The most common inward succession is microcrystalline quartz and
disordered chalcedony - coalescing spherulites or overlays of chalcedony - quartzine
(length-slow chalcedony), though other successions may be found. One example of
"capping" megaquartz cement (e.g., Milnes and Thiry, 1992) was found on a framework chert clast (Fig. 4.5).
Interpretation:
Facies A1 is interpreted to be the product of gravel bar and gravel bedform migration within an easterly draining braided fluvial system (Miall, 1992). The combined paleocurrent data of the facies exhibits a higher directional variability than was anticipated since braided fluvial systems are generally of low sinuosity and should show a low directional variance (Miall, 1976, 1977). This variability may reflect changing current directions caused by local variations in the orientation of individual channel reaches as the fluvial system migrated laterally across the channel belt.
Massive to horizontally bedded strata and pebble imbrication are interpreted to be the result of deposition as longitudinal bars or channel lag (Miall, 1977). Trough cross- strata represent bedforms deposited in channels whereas planar cross-stratification suggests the migration of linguoid bar forms (Miall, 1977). Large-scale bar forms probably represent preserved longitudinal or transverse bars. The gutter casts are interpreted as features that were scoured into the Brushy Basin Member mudstones and subsequently infilled by Facies A1. The flame structure was formed when the pliable
Brushy Basin Member mudstones were squeezed upward into overlying beds of Facies
A1 while the load cast resulted from the passive infilling of a dinosaur track within the
mudstones of the Brushy Basin Member (Roca, 2003). The distribution and abundance 48
Fig. 4.5 ''Capping" megaquartz cement on a framework chert clast, Sample 22, Site E. Scale bar is 0.5 mm.
49
of the gutter casts, flame structure, and load cast indicate minimum compaction of the
uppermost Brushy Basin Member and suggests that these sediments could not have been
buried prior to the deposition of Facies A1 (Roca, 2003). Lack of fine-grained
framework, degree of sorting, and abundant erosional surfaces indicate a high degree of
reworking within the depositional system.
The indurated jasper-cemented, metaconglomerate-like portions of Facies A1 are interpreted to be a groundwater silcrete based on micromorphology and a lack of macromorphological organization commonly attributed to pedogenic silcretes.
Pedogenic silcretes commonly display structures and fabrics that are organized into characteristic horizons such as nodular, columnar, massive, and pseudo-breccia (Fig. 4.6)
(Milnes and Thiry, 1992). The silcrete within Facies A1 shows no such organization but
does exhibit microscopic evidence of groundwater silcrete formation.
The silcrete displays complex successions of void- and fracture-filling silica cements that may contain quartzine (length-slow chalcedony). This is typical of groundwater silcretes, which commonly possess vug- and fracture-fillings that consist of complex successions of microcrystalline quartz, chalcedony, quartzine, megaquartz, and less frequently calcite (Summerfield, 1983a, b). Vug- and fracture-fillings are comparatively rare in pedogenic silcretes and are usually restricted to a thin lining of microcrystalline quartz (Summerfield, 1983a). Likewise, quartzine is reported to be a silica phase present in groundwater silcretes (Summerfield, 1983a; Thiry and Milnes,
1991; Milnes and Thiry, 1992) but absent in pedogenic silcretes (Summerfield, 1983a).
Summerfield (1983a) stated that complex fillings in groundwater silcretes are most
50
Fig. 4.6 Schematic section showing the generalized macromorphological organization of a pedogenic silcrete. Modified from Milnes and Thiry (1992).
51
commonly associated with calcrete and that the silcrete in such instances is invariably
closely adjacent to a calcrete unit.
A lack of colloform features also supports the interpretation that the indurated
jasper-cemented, metaconglomerate-like portions of Facies A1 are not a pedogenic silcrete (Summerfield, 1983a). Colloform features are typically vertically stacked, concave-up accumulations that suggest the rhythmic precipitation of SiO2 and TiO2 within pedogenic silcretes. The example of "capping" megaquartz cement found on a framework chert clast of Facies A1 is similar to cappings of microcrystalline quartz
described in relation to a pedogenic silcrete formed within conglomerates of the Paris
Basin (Milnes and Thiry, 1992) but since Facies A1 does not exhibit pedogenic silcrete characteristics, the "capping" megaquartz cements are interpreted to record the downward percolation of silica-rich groundwater and subsequent precipitation of silica cements as the water migrated toward the groundwater silcrete.
4.2.3 Facies A2: Buckhorn-equivalent
4.2.3.1 Introduction:
Facies A2 is a conglomerate facies that is the homotaxial equivalent of Facies A1
(Fig. 4.1). It is exposed from just southwest of the Nipple between Site U and Site V and extends at least as far north as the Cleveland-Lloyd Dinosaur Quarry, at Site X (Fig. 4.2).
Facies A2 is subdivided into two subfacies on the basis of weathering characteristics:
Indurated Conglomerates and Poorly Indurated Conglomerates.
Both the indurated and poorly indurated conglomerates consist of
extraformational, clast-supported, granule to cobble conglomerates composed of 98% 52
chert and 2% quartzite with rare sandstone clasts (Appendix C). A portion of the chert
clasts are silicified Paleozoic limestone (Stokes, 1944; Conley, 1986), which contain
bryozoans, crinoid stem fragments, and corals. Clast colors include white, cream, red,
brown, and an abundance of green and tan, with lesser amounts of gray and black. The
conglomerates are composed of rounded to sub-rounded clasts with a sub-angular to sub-
rounded granular fraction.
4.2.3.2 Indurated A2 Conglomerates
Description:
The indurated conglomerates are rare within Facies A2 and were measured at only
one location, Site W (Fig. 4.2). These conglomerates are well cemented, possess a sand-
sized matrix, and consist of irregularly bedded, laterally discontinuous lenses that range
between 45 cm and 3 m in thickness (Fig. 4.7a, b). The sand-sized matrix may be weakly
variegated displaying pale colors of gray, yellow, and purple within the uppermost beds.
The lower beds locally incise the Brushy Basin Member but there are no signs of soft
sediment deformation.
The indurated conglomerates exhibit massive bedding, possess planar and trough
cross-stratification, and may show fining-upward trends within individual beds. Massive
beds tend to be bounded at the base by scour surfaces and at the top by erosional surfaces
while planar and trough cross-stratified beds are commonly bounded by erosional
surfaces. The combined paleocurrent data indicate a paleoflow direction of 037o (Fig.
4.4, Appendix B).
53
a
b
Fig. 4.7 (a) Outcrop photo of the Indurated A2 Conglomerates and Facies B2, Site W (person for scale). (b) Close-up photo of the Indurated A2 Conglomerates, Site W (lens cap for scale).
54
Interpretation:
The indurated conglomerates, which are similar in morphology and sedimentary
structures to Facies A1, are interpreted to represent gravel bar and gravel bedform migration (Miall, 1992) within a northeasterly draining braided fluvial system. The lack of soft-sediment deformation at the lower contact of the conglomerates suggests erosion into a more compacted Brushy Basin Member than was the case with Facies A1.
4.2.3.3 Poorly Indurated A2 Conglomerates
Description:
The poorly indurated conglomerates dominate Facies A2 and are found at most
locations. These conglomerates are poorly cemented, poorly sorted, massive, rarely
exceed 3 m in thickness, and weather to form a cover of gravel on underlying slopes (Fig.
4.8). The poorly indurated nature of the conglomerates is due in part to the presence of a
substantial silt to mud matrix. The matrix is commonly light gray, purple, or maroon,
and may be banded (variegated) in outcrop. Framework chert clasts are locally altered to
a maroon color due to hematite staining, when found in association with the variegated
matrix. This alteration forms a rind that decreases in intensity toward the center of clasts.
Interpretation:
Braided fluvial conglomeratic deposits typically contain little in the way of matrix
(Miall, 1977, 1992, Rust and Koster, 1984). Where such beds are present they are
typically interpreted as debris flow deposits associated with proximal alluvial fans (Rust
and Koster, 1984). The presence of the matrix eliminates a typical braided fluvial
interpretation for the poorly indurated conglomerates and the distance from any mountain 55
Fig. 4.8 Outcrop photo of the Poorly Indurated A2 Conglomerates, Site U (lens cap for scale).
56
front makes a debris flow interpretation unlikely. The poorly indurated conglomerates are therefore interpreted to be hyperconcentrated flow deposits. Hyperconcentrated flows are described as high-discharge flows of low water-to-sediment ratios (40-70 wt.% sediment), which result in deposits that are massive or exhibit crude horizontal stratification, display no scour and fill structures, and range in grain/clast-size from sand to boulder grade (Manville and White, 2003; Svendsen et al., 2003). The term hyperconcentrated flow was originally applied to sediment-laden flows with a substantial clay mineral content (Manville and White, 2003). Trask (1959) stated that the strength of these flows is dependent upon the composition of the clay mineral content and that the strength increases significantly if smectite, rather than illite or kaolinite dominates. A flow containing 2% smectite possess the same strength as a flow containing 10% illite or kaolinite (Trask, 1959; Rodine and Johnson, 1976).
The Brushy Basin Member is composed of smectitic mudstones and abundant bentonite beds (Stokes, 1944; Bilbey, 1992; Currie, 1998), while the Cedar Mountain
Shale Member consists of slightly smectitic mudstones (Young, 1960; Bilbey, 1992).
This suggests a significant volcanic input (volcanic ash) during deposition of the Brushy
Basin (Stokes, 1944; Currie, 1998) that most likely persisted during deposition of Facies
A2. It is possible that the remobilization of the volcanic ash and smectitic muds resulted in the development of hyperconcentrated flows. Incorporation of the ash as a suspended component would have increased shear strength, viscosity, and effective density, providing the necessary competence to transport granule- to cobble-size clasts (Manville and White, 2003; Svendsen et al., 2003). Detailed mineralogical studies of the silt to 57
mud matrix that were beyond the scope of this project are needed to evaluate the validity
of this interpretation.
4.2.4 Facies A3: Red Conglomerate
Description:
Facies A3 is a conglomerate facies that is laterally continuous throughout the central portion of the study area from Site AA to between Site N and Site V (Fig. 4.1,
4.2). The facies consists of poorly sorted, clast-supported, granule to boulder conglomerates composed of 93% extraformational clasts and 7 % intraformational clasts
(Appendix C). Scattered bone fragments are locally present. The extraformational clasts are composed of 91% chert and 2% quartzite, tend to be rounded to sub-rounded, and exhibit black, gray, brown, tan, and red colors. Facies A3 contains a higher percentage of red chert clasts than any other conglomerate facies in the study area. Intraformational clasts are composed of fine-grained sandstone, siltstone, and carbonate lithologies that vary from reddish-brown to light gray giving the deposit an overall reddish color in outcrop (Fig. 4.9a). The intraformational clasts are angular to sub-rounded and generally no more than 5 to 10 cm long, but can reach as much as 35 cm in length. These larger cobble- to boulder-sized clasts are typically composed of fine-grained, light gray sandstone and light gray carbonate.
Facies A3 displays varying amounts of cementation and is locally well cemented.
The well cemented areas contain such a high percentage of carbonate as framework clasts, matrix, and cement that the facies has an overall gray color in outcrop and weathers like a carbonate (Fig. 4.9b). At isolated localities both intraformational and 58
a
b
Fig. 4.9 (a) Outcrop photo showing the typical reddish color of Facies A3, Site N (lens cap for scale). (b) Outcrop photo showing a well cemented portion of Facies A3 that is gray in color and weathers like a carbonate due to abundant carbonate framework clasts, matrix, and cement, Site J (lens cap for scale). 59
extraformational clasts are symmetrically coated by carbonate with some coatings
possessing concentric laminations (Fig 4.10a, b).
Facies A3 possesses a sharp, erosive lower boundary and incise several underlying lithologies throughout the central portion of the study area. Massive beds dominate the facies at most localities but pebble imbrication as well as scattered trough and rare planar cross-stratified beds are present. The massive beds may fill troughs or scoops that possess an erosive lower contact while planar and trough cross-stratified beds are commonly bounded by erosional surfaces. The facies has a combined paleocurrent direction of 211o (Fig. 4.11, Appendix B).
Interpretation:
Facies A3 is interpreted to be the result of gravel bar and gravel bedform migration within a braided fluvial system (Miall, 1992). Trough cross-strata represent bedforms deposited in channels whereas massive beds and pebble imbrication are the result of deposition as longitudinal bars or channel lag (Miall, 1977). Planar cross-strata are the result of linguoid bar form migration (Miall, 1977).
The abundance of intraformational clasts within the facies indicates significant erosion and incorporation of floodplain and underlying lithologies. The cobble- to boulder-sized, fine-grained, light gray sandstone clasts present within Facies A3 are interpreted to be the result of the erosion and incorporation of Facies B3 whereas the cobble- to boulder-sized, light gray carbonate clasts are interpreted to be the result of the erosion and incorporation of the middle carbonate of Facies E1 based on lithological similarities, the proximity of Facies A3 to Facies B3 and the middle carbonate, as well as the erosive nature of Facies A3 throughout the central portion of the study area. 60
a
b
Fig. 4.10 (a) Outcrop photo showing oncoids within a well cemented portion of Facies A3, near Site N (lens cap for scale). (b) Photo of a carbonate coated extraformational chert clast (oncoid) collected from Facies A3, near Site N (lens cap for scale).
61
Fig. 4.11 Map showing the distribution of paleocurrent orientations for the combined data of Facies A3 and B4. The arrows indicate the mean flow direction at each station while the associated numbers indicate the number of measurements collected per station. The Rose Diagram shows the cumulative flow direction of the facies. 62
This suggests that both Facies B3 and the middle carbonate of Facies E1 were lithified prior to the incision and deposition of Facies A3. The carbonate coated intraformational and extraformational clasts are interpreted as oncoids, a common feature of calcretes
(Tucker, 2001), which have two possible origins. They were either inherited from Facies
E2 or represent the erosion and incorporation of calcrete unit that is not preserved within the study area. This interpretation is substantiated by Gibling and Rust (1990) who showed that even low energy fluvial systems are capable of incising indurated duricrusts.
4.3 Facies B: Sandstone Facies
4.3.1 Introduction
Sandstones are present throughout the study area at various stratigraphic levels within the lower Cedar Mountain Formation. The sandstones are quite variable in terms of composition, cementation, grain size, and sorting. They typically occur as lenses that are interbedded with the conglomerate facies but also comprise a laterally continuous sheet (Friend, 1983) throughout the central portion of the study area. The facies is subdivided into five subfacies on the basis of petrologic and petrographic data as well as stratigraphic position.
4.3.2 Facies B1: Buckhorn Sandstones
Description:
Facies B1 occurs as lenses that are interbedded with Facies A1 throughout the
Little Cedar and Cedar Mountain area from just southwest of the Nipple between Site V and Site U to the southwest flank of Little Cedar Mountain between Site E and Site BB 63
(Fig. 4.1, 4.2). The sandstones range from very fine-grained to pebbly, are very well- to
moderately well sorted (Tucker, 2001), and are light gray but weather to a light brown to
tan color. The facies contains scattered granule- to pebble-size clasts within even the
most well sorted, fine-grained beds, commonly grades either vertically or laterally into
Facies A1, and may contain small lenses of conglomeratic material. Point count data from the type section of the Buckhorn indicate that Facies B1 is a sublitharenite, which consists of 81% monocrystalline quartz and 19% lithic fragments (Fig. 4.12a, Appendix
C). The lithic fraction contains 11% chert and 8% polycrystalline quartz (Fig. 4.12b,
Appendix C).
Facies B1 occurs as lenses that range from less than 10 cm to nearly 3 m in
thickness and rarely exceed 10 m in lateral extent. Sedimentary structures include
massive to horizontal bedding, trough cross-stratification, and planar cross-stratification.
Massive to horizontally bedded lenses are composed of fine- to coarse-grained
sandstones that contain varying amounts of granule- to pebble-size clasts and may fill
shallow troughs or scoops. Trough and planar cross-stratified beds occur as both solitary
and grouped cosets that are composed of medium- to coarse-grained sand sizes and often
have granule- to pebble-sized clasts at the base of individual sets. Trough cross-stratified
beds may contain cosets of mutually cross-cutting troughs. The combined paleocurrent
data indicate a paleoflow direction of 081o (Fig. 4.4, Appendix B).
The degree of cementation within Facies B1 varies significantly especially with respect to silica. At isolated locations along the southwest flank of Little Cedar
Mountain, the facies is well cemented by silica providing a highly indurated, dense,
64
Fig. 4.12 Ternary diagrams showing framework-grain compositions of Facies B1, B3, and B4. (a) QmFL compositions of Facies B1 (circles), Facies B3 (triangles), and Facies B4 (squares). (b) Diagram showing variations in the lithic grain proportions of Facies B1 (circles), Facies B3 (triangles), and Facies B4 (squares). The diagram depicts the percentage of polycrystalline quartz to carbonate to chert within the lithic fraction.
65
glassy, purple to gray, rock that is disrupted by intersecting jasper-filled veinlets (Fig.
4.13a, b).
The thin section examination of one highly indurated, well cemented sample
reveals a very fine-grained sandstone that has a significant silt fraction. Framework
grains are predominantly composed of monocrystalline quartz with lesser amounts of
orthoclase feldspar and detrital zircon grains. Minor plagioclase feldspar and chert lithic
grains are also present. The grains are sub-rounded to rounded, well sorted, and display
no preferential orientation. Framework grains are well cemented by microcrystalline
quartz and lesser amounts of chalcedony, resulting in an extremely dense rock with
minimal porosity. Inclusions of calcite are present within the microcrystalline quartz and
chalcedony cement. Fracture-fillings are present that show a complex inward succession
of silica phases, which include microcrystalline quartz, chalcedony, quartzine (length-
slow chalcedony), and megaquartz (Fig. 4.14a, b). Although other successions may be
present, the two most common are microcrystalline quartz - coalescing spherulites of
chalcedony - chalcedony overlays - a final void-filling by chalcedony or megaquartz, and
microcrystalline quartz - coalescing spherulites of quartzine or chalcedony - a final void-
filling by quartzine or chalcedony.
Interpretation:
Facies B1 is interpreted as the product of sand bedform migration (Miall, 1992)
within an easterly draining braided fluvial system. The directional variability of the
combined paleocurrent data are interpreted to record the same local variations in channel
orientation as Facies A1 since both facies were deposited by the same fluvial system.
Massive to horizontally bedded lenses, which commonly fill shallow troughs or scoops, 66
a
b
Fig. 4.13 (a) Outcrop photo of a highly indurated, silica cemented portion of Facies B1, SW flank of Little Cedar Mountain near Site BB (lens cap for scale). (b) Slabbed sample from the highly indurated, silica cemented portion of Facies B1 showing a varicolored purple and gray rock that is disrupted by intersecting jasper-filled veinlets, Sample 30, SW flank of Little Cedar Mountain near Site BB (dime for scale).
67
a
b
Fig. 4.14 (a) Multiple generations of fracture-filling silica cements within a highly indurated, silica cemented sample of Facies B1, Sample 30, SW flank of Little Cedar Mountain near Site BB. Crossed nicols. Scale bar is 0.5 mm. (b) Same as previous photo but with quartz plate inserted. Scale bar is 0.5 mm.
68
represent a channel-fill facies deposited under either lower or upper flow regime
conditions. The trough cross-stratified beds are considered to be the product of dune
migration during lower flow regimes (Miall, 1977). Smaller-scale, planar cross-stratified
beds are interpreted to be the result of sand wave migration while larger-scale planar
cross-stratified beds record the migration of linguoid bar forms, both of which were
deposited under lower flow regime conditions (Miall, 1977).
The highly indurated sandstones of Facies B1, which are well cemented by
microcrystalline quartz and chalcedony, are interpreted to be a groundwater silcrete based
on both macro- and micromorphology. This silcrete lacks the macromorphological
organization that is typically associated with pedogenic silcretes (Fig. 4.6) and is similar
to hard, glassy, well-cemented, sandstones with low residual porosity that were described
in relation to groundwater silcretes within the Fontainbleau Sand of the Paris basin
(Milnes and Thiry, 1992).
The microcrystalline quartz and chalcedony cements impart the well cemented portions of Facies B1 with a GS-fabric to massive F-fabric (Summerfield, 1983a, b),
which may be transitional within a single field of view (Fig. 4.15a, b). Calcite inclusions
within these fabrics are the remnants of an earlier generation of calcite cement that has
been subsequently replaced. GS- and F-fabrics are members of a genetic classification of
silcretes that is based on general fabric characteristics, with an emphasis on the
proportion of matrix (< 30 µ) to skeletal grains (> 30 µ) and the silica phases present
(Summerfield, 1983a, b). These fabrics are characteristic of silcrete formation within a
grain-supported host sediment that has intergranular voids filled by nonsiliceous cements
(Summerfield, 1983a, b). 69
a
b
Fig. 4.15 (a) GS-fabric silcrete composed of predominantly skeletal quartz grains cemented by microcrystalline quartz, Sample 30, SW flank of Little Cedar Mountain near Site BB. Crossed nicols. Scale bar is 0.5 mm. (b) F-fabric massive silcrete composed of predominantly skeletal quartz grains floating in a microcrystalline quartz matrix, Sample 30. Crossed nicols. Scale bar is 0.5 mm.
70
The highly indurated sandstones contain fracture-fillings that show a complex
inward succession of silica phases including microcrystalline quartz, chalcedony,
quartzine, and megaquartz similar to those present in the highly indurated conglomerates
of Facies A1. The occurrence of these complex fracture-fillings combined with the presence of quartzine and the lack of colloform features supports the interpretation that the highly indurated sandstones of Facies B1 are part of a groundwater silcrete profile.
4.3.3 Facies B2: Buckhorn-equivalent Sandstones
Description:
Facies B2 is a sandstone facies that is interbedded with the resistant conglomerates of Facies A2 at isolated locations from just southwest of the Nipple between Site U and
Site V to as far north as the Cleveland-Lloyd Dinosaur Quarry, Site X (Fig. 4.1, 4.2).
The sandstones range from fine-grained to pebbly and are well- to moderately well-sorted
(Tucker, 2001). The facies commonly grades either vertically or laterally into the resistant conglomerates of Facies A2 and contains scattered granule- to pebble-size clasts
as well as small lenses of conglomeratic material. The sandstones are white to light gray
but weather to a light brown to tan color.
Facies B2 occurs as beds that range from less than 15 cm to nearly 1 m in
thickness and display massive to horizontal bedding as well as trough and planar cross-
stratification. The massive to horizontally bedded sandstones range from medium-
grained to pebbly, commonly have erosive bases, and may show a fining-upward trend.
Trough and planar cross-stratified beds occur as solitary cosets that are composed of fine- 71
to coarse-grained sand sizes and often show a fining-upward trend within individual sets.
o Facies B2 shows a combined paleocurrent direction of 037 (Fig. 4.4, Appendix B).
Interpretation:
Facies B2 is interpreted as the product of sand bedform migration (Miall, 1992) in
a northeasterly draining braided fluvial system. The massive to horizontally bedded
sandstones with erosional bases represent a channel-fill facies deposited during either
lower or upper flow regime conditions (Miall, 1977). The trough cross-stratified beds are
the product of dune migration while planar cross-stratified beds are the product of sand
wave migration both of which were deposited under lower flow regimes conditions
(Miall, 1977).
4.3.4 Facies B3: White Sandstone
Description:
Facies B3 occurs throughout the central portion of the study area as a laterally
continuous sheet that is characterized by large-scale planar and trough cross-stratified
cosets (Fig. 4.1; 4.16a, b). The facies consists of white to light gray, well cemented
sandstones, which are well sorted overall. The sandstones are typically fine-grained but
also contain coarser grained lenses that commonly include chert granules and pebbles.
The colors of these chert clasts include black, gray, brown, orange, tan, white, pink,
purple, green, yellow, and red. Scattered petrified wood and bone fragments are also
present. Facies B3 is a litharenite with an average composition of 70% monocrystalline quartz, 2% feldspar, and 28% lithic fragments (Fig. 4.12a, Appendix C). The lithic
72
a
b
Fig. 4.16 (a) Outcrop photo of Facies B3 showing large-scale cross-stratification, Site B (hammer for scale). (b) Outcrop photo of Facies B3 showing massive bed capped by cross-stratified bedforms, Site M (shovel handle for scale). 73
fraction consists of 3% polycrystalline quartz, 16% chert, and 9% detrital carbonate (Fig
4.12b, Appendix C).
The base of Facies B3 is sharp, irregular, and contains varying amounts of intraformational clasts, which are gray to reddish-brown in color and predominantly consist of carbonate and less commonly of siltstone. Locally, where the base of Facies
B3 intersects an underlying carbonate unit (the Lower Carbonate of Facies E1) an
abundance of intraformational clasts results in Facies B3 becoming locally conglomeratic.
These conglomerates are massive, clast-supported, and contain extraformational granule- to pebble-sized chert clasts and intraformational carbonate clasts (Fig 4.17a). The conglomerates are typically less than 1 m thick and fine upward into fine-grained sandstones of Facies B3. Extraformational clasts are sub-rounded to rounded and display colors of black, gray, brown, orange, tan, white, pink, purple, and red. The intraformational clasts are composed of light gray carbonate, are sub-angular to sub- rounded, and may reach lengths of 20 cm. Some intraformational carbonate clasts contain inclusions of jasper and, less commonly, crystals of calcite spar (Fig. 4.17b). The conglomerates are overall poorly sorted and well cemented. The matrix is composed of the light gray, fine-grained sandstone typical of Facies B3 and weathers in a manner
similar to the intraformational carbonate clasts, causing the more resistant chert granules
and pebbles to protrude from weathered surfaces. A strong reaction to dilute
hydrochloric acid indicates an abundance of calcite cement and possibly significant fine-
grained detrital carbonate.
Sedimentary structures within the sandstones of Facies B3 include massive bedding, trough and planar cross-stratification, low-angle crossbeds, and planar 74
a
b
Fig. 4.17 (a) Outcrop photo of a massive conglomerate containing abundant intraformational carbonate clasts, base of Facies B3, Site P (lens cap for scale). (b) Jasper inclusion within an intraformational carbonate clast, base of Facies B3, Site P (lens cap for scale). 75
lamination. The massive beds often contain intraformational clasts at their base, may be
laterally continuous for tens of meters, can approach 3 m in thickness, typically fill
troughs or scoops, and are often capped by other sedimentary structures such as planar
and trough cross-strata or low-angle cross-beds. Trough and planar cross-stratified beds
are found as solitary or superimposed sets and commonly show fining-upward trends
within individual sets, which grade from coarse-grained or pebbly sandstone to fine-
grained sandstone. Trough cross-strata commonly occur as cosets of mutually cross-
cutting troughs. Individual planar cross-stratified sets may reach 50 cm in thickness.
Planar laminated strata are composed of very fine- to fine-grained sandstone and have
lamina of approximately 5 mm in thickness. The combined paleocurrent data obtained
o from Facies B3 indicate a paleoflow direction of 132 (Fig. 4.18, Appendix B).
Interpretation:
Facies B3 is interpreted to be the result of sand bedform migration (Miall, 1992)
within a southeasterly draining braided fluvial system. The high directional variability of
the combined paleocurrent data suggest that this braided fluvial system was characterized
by significant channel avulsion and lateral migration similar to the braided system that
deposited Facies A1 and B1. Massive beds that occupy troughs or scoops and containing
intraformational clasts along their basal contacts are interpreted to be scour fills (Miall,
1977). The presence of intraformational clasts and relation to underlying lithologies
suggest that these massive beds represent a primary channel fill on which subsequent bed
and bar forms would migrate. Evidence of these subsequent bed and bar forms are trough
cross-stratification, planar cross-stratification, and low angle crossbeds. Trough cross-
stratification is the product of dune migration under lower flow conditions while low 76
Fig. 4.18 Map showing the distribution of paleocurrent orientations for Facies B3. The arrows indicate the mean flow direction at each station while the associated numbers indicate the number of measurements collected per station. The Rose Diagram shows the cumulative flow direction of the facies. 77
angle crossbeds represent the deposits of washed out dunes (Miall, 1977). Small-scale,
planar cross-stratification represents sand wave migration while larger-scale, planar
cross-stratification is the result of linguoid bar form migration, both of which were
deposited under lower flow conditions (Miall, 1977). The planar laminated strata suggest
planar bed flow deposited during upper flow regime conditions (Miall, 1977).
Facies B3 incises several underlying facies throughout the central portion of the
study area as indicated by lithostratigraphic data (Cross-sections 1 and 2 of Appendix A)
and the presence of a sharp, irregular base that contains intraformational clasts. These
facies include Facies D2, E2, and locally the lower carbonate of Facies E1. The ability of
a fluvial system to incise deposits such as Facies E2 was demonstrated by Gibling and
Rust (1990). The massive, clast-supported conglomerates found at the base of Facies B3 where it intersects the lower carbonate are interpreted as a channel lag (Miall, 1977). The size and roundness of the soft, easily weathered, intraformational carbonate clasts indicates minimal transport and relatively rapid deposition (Prothero & Schwab, 1996).
Jasper inclusions in these clasts are interpreted to be the result of groundwater silcrete development within the original carbonate host (the Lower Carbonate of Facies E1). This implies that the lower carbonate was lithified and overprinted by the groundwater silcrete prior to the deposition of Facies B3. Therefore an unknown but significant amount of time elapsed between deposition of the lower carbonate and Facies B3.
78
4.3.5 Facies B4: Red Conglomerate Sandstones
Description:
Facies B4 is present throughout the central portion of the study area as lenses that
are interbedded with Facies A3 (Fig. 4.1). The sandstones are litharenites composed of
37% monocrystalline quartz, 2% feldspar, and 61% lithic fragments (Fig. 4.12a,
Appendix C). The lithic component consists of 5% polycrystalline quartz, 19% chert, and 36% detrital carbonate (Fig. 4.12b, Appendix C). Scattered bone fragments and petrified wood are also present. An abundance of reddish-brown intraformational grains and clasts gives the rocks an overall tan to reddish-brown color. The sandstones are moderately well cemented, poorly sorted, typically medium- to very coarse-grained, contain varying amounts of granule and pebble clasts, and may grade either vertically or laterally into Facies A3. The granule and pebble clasts are similar in color to those found
in Facies A3. Sedimentary structures within the facies include massive bedding, pebble imbrication, as well as planar and trough cross-stratification.
Interpretation:
Facies B4 is interpreted to be the product of sand deposition within a braided
fluvial system (Miall, 1992). The structures suggest that the depositional environment
was similar to that of Facies B1. Point count data show that Facies B4 contains a higher percentage of lithic fragments than any other sandstone facies within the study interval with as much as 36% of the fragments being composed of detrital carbonate. The abundance of detrital carbonate within the facies is interpreted to be a result of the incorporation of a significant amount of underlying carbonate or adjacent floodplain lithologies. 79
4.3.6 Facies B5: Isolated Sandstone Bodies
Description:
Facies B1, B2, B3, and B4 are widespread, recognizable deposits within the study
area. Facies B5 represents isolated sandstone bodies found at the top of Sites F, W, and
X, which cannot be confidently linked to the other sandstone deposits (Cross-section 1 of
Appendix A). The three sandstone bodies of Facies B5 are present at different stratigraphic horizons and are separated by significant distances. A distance of nearly 16 km separates the deposits of Sites F and W while those of Sites W and X are separated by approximately 5.5 km (Fig. 4.2).
Facies B5 is composed of light gray sandstones that are variable in terms of grain
size and degree of sorting. The sandstones range from poorly- to well-sorted and from
fine- to very coarse-grained. Individual beds are typically massive or trough cross-
stratified and commonly show fining upward trends.
At Site X (Fig. 4.2), Facies B5 is composed of superimposed beds that show fining upward trends from granule and pebble conglomerates to fine-grained sandstones.
At this location the facies contains intraformational carbonate clasts, detrital carbonate grains, and calcite cement as indicated by both macro- and microscopic analysis.
Interpretation:
All three sandstone bodies of Facies B5 are interpreted to be the result of deposition within a fluvial system but distinction of the variety of fluvial system is precluded by lack of exposures. Massive beds, some of which possess granules and pebbles at their lower boundaries, represent scour fills while trough cross-stratified beds are the result of dune migration within channels under a relatively low flow regime 80
(Miall, 1977). The spatial distribution and stratigraphic position of the sandstone bodies
suggest that they were likely non-contemporaneous and were deposited by different
fluvial systems. Abundant intraformational carbonate clasts and detrital carbonate grains
within the sandstone body at the top of Site X in conjunction with its stratigraphic
position indicates that the sandstone body is locally responsible for the removal of Facies
E2 (Cross-section 1 of Appendix A).
4.4 Facies C: Siltstone Facies
Description:
Facies C occurs at scattered locations within different stratigraphic levels of the lower Cedar Mountain Formation but was present within a measured section only at
Site F (Fig. 4.2). The Site F siltstones are present as two individual beds, which both display a tabular geometry, possess sharp upper and lower contacts, and range in thickness from 30 cm to 50 cm. These siltstones are very well sorted, reddish-brown in color, and contain planar lamination consisting of lamina that are typically thinner than
2 mm. Facies C is not portrayed in Fig. 4.1 because the siltstones comprise a minimal proportion of the study interval.
Interpretation:
The grain size, tabular geometry, and planar lamination suggest that Facies C is best interpreted as overbank fines (Miall, 1992). These overbank fines were probably deposited under flood conditions as a result of floodwaters inundating floodplain areas.
81
4.5 Facies D: Mudstone Facies
4.5.1 Facies D1: Mudstone
Description:
Facies D1 is composed of blocky mudstones (Retallack, 1988) that commonly display gradational colors of purple, red, gray, and green. Though the colors are variable, typically showing vertical gradations, they are almost always more dull than the colors associated with the Brushy Basin Member mudstones. The facies is easily eroded to form slopes unless sheltered by more resistant overlying lithologies. Weathered slopes do not display the characteristic "popcorn texture" associated with the smectitic swelling clays of the Brushy Basin Member mudstones.
Facies D1 occurs within the study area as lenses with typically gradational lower contacts and sharp, erosive upper contacts. These lenses are scattered among Facies A1 and B1 at various stratigraphic levels as well as above at isolated locations (Fig. 4.1). The
mudstones above Facies A1 and B1 are in contact with the upper boundary of these facies, approach 3 m in thickness, and show no evidence of erosional upper contacts.
Interpretation:
Facies D1 is interpreted as abandoned channel-fill deposits (Miall, 1992) based on relationship to Facies A1 and B1 as well as the presence of the upper erosional surfaces.
The erosion-bounded lenses probably record deposition in abandoned channels that were subsequently reactivated while the mudstones in contact with the upper boundary of
Facies A1 or B1 were deposited in abandoned channels that were not reactivated. The
blocky texture of the mudstones suggests that they were subjected to pedogenic
modification (Retallack, 1988). The presence of erosional surfaces as well as the 82
minimal amount of Facies D1 preserved within the study area implies significant reworking of the fine-grained deposits within the fluvial depositional system.
4.5.2 Facies D2: Pebbly Mudstone
Description:
Facies D2 occurs throughout the study area from Site X to Site F as a laterally continuous stratum that is absent only when eroded by Facies B3 (Fig. 4.1, 4.2). The facies is similar to Facies D1 in terms of composition, texture, and erodability but
contains scattered granule- to pebble-size chert clasts that reach lengths of 5 to 6 cm and
are white, gray, black, brown, tan, orange, or red in color. The mudstones are dominantly
purple in color but shades of gray and red are also present. The chert clasts are typically
scattered but may be locally abundant. They are generally most abundant above the
poorly indurated conglomerates of Facies A2 but are also present above Facies A1.
Unweathered exposures display a hackly appearance (Retallack, 1988), as well as
calcareous nodules and laminar calcic horizons (Fig. 4.19a, b). The nodules are white in
color, rarely exceed a few centimeters in diameter, are relatively soft, and have a chalky
appearance. The calcic horizons occur as disrupted to continuous, centimeter-scale
laminae. Jasper is rare but locally present. At Site BB along the southwest flank of Little
Cedar Mountain (Fig. 4.2), jasper occurs as a 5 cm thick layer.
Petrographic analysis of a calcareous nodule reveals a micritic groundmass and
approximately 10% detrital components. The detrital grains consist of silt- to very fine-
grained sand sizes of monocrystalline quartz, with lesser amounts of chert,
polycrystalline quartz, and minor plagioclase feldspar. The sample also contained 83
a
b
Fig. 4.19 (a) Outcrop photo of Facies D2 showing hackly texture, calcareous nodules, and laminar calcic horizons, Site J (lens cap for scale). (b) Centimeter-scale laminar calcic horizon within Facies D2, Site J (lens cap for scale). 84
abundant cavities and scattered fractures. The cavities commonly have walls lined with a
thin rim of clay minerals or iron oxide staining and tend to be filled by drusy calcite
cements. The fractures are filled by material similar to the groundmass as well as fibrous
calcite cements that are elongated perpendicular to the orientation of the fracture.
Interpretation:
Facies D2 is interpreted to be floodplain deposits that were modified into a calcic paleosol. The hackly appearance and presence of calcareous nodules and laminar calcic horizons support this interpretation. The hackly appearance is interpreted to be the result of fracturing along cutans bounding soil peds (Retallack, 1988) while the calcareous nodules and laminar calcic horizons are the result of Stage 1-2 calcic soil development
(Machette, 1985). Cavities lined with clay or iron oxide staining are interpreted to be root casts, which is consistent with pedogenic modification and paleosol formation. The scattered granule- to pebble-size chert clasts are probably the "gastroliths" of Stokes
(1944). The origin of these clasts is historically not well understood but their abundance, occurrence, and similarity to Facies A1 and A2 suggests that the large percentage were reworked from Facies A1 and A2 possibly as a result of dinoturbation or they may be hyperconcentrated flow deposits similar to the poorly indurated conglomerates of Facies
A2. The laminar jasper accumulation along the southwest flank of Little Cedar Mountain
is interpreted to be a groundwater silcrete based on stratigraphic position as well as
proximity and lateral relationship to the groundwater silcrete present within the upper
horizon of Facies A1 and Facies B1.
85
4.6 Facies E: Carbonate Facies
4.6.1 Introduction
The carbonate facies is subdivided into two subfacies based on petrologic and
petrographic data: Lacustrine Carbonate Facies (E1) and Calcrete Facies (E2). The
Lacustrine Carbonate Facies occurs as three distinguishable carbonate deposits based on lithostratigraphic data, which are termed the Lower, Middle, and Upper Carbonate. Each carbonate bed within Facies E1 is discussed below separately because of differences in stratigraphic position and silica concentration.
4.6.2 Facies E1: Lacustrine Carbonate
4.6.2.1 Lower Carbonate:
Description:
The lower carbonate is the most widespread of the three carbonate deposits within
Facies E1. It is present throughout most of the Little Cedar and Cedar Mountain area as a
laterally continuous stratum that ranges in thickness from less than 0.5 m to more than 9
m (Fig. 4.1). The carbonate achieves a maximum thickness near Bob Hill Knoll and
shows a trend of decreasing thickness away from the Bob Hill Knoll area (Fig. 4.20). It
has a depositional pinch-out at the northwest flank of Cedar Mountain near Site V as well
as at the southwest flank of Little Cedar Mountain between Sites G and H (Fig. 4.2).
The lower carbonate is composed of a dense, resistant, massive, light gray mudstone to granular or pebbly floatstone (Dunham, 1962) that typically weathers yellowish-brown (Fig. 4.21a, b). The carbonate exhibits intense diagenetic alteration, which causes the macromorphology and color to vary considerably. The detrital 86
Fig. 4.20 Isopach map showing thickness variations and lateral distribution of the Lower Carbonate of Facies E1. Contours are in meters. The * indicates that the total carbonate thickness includes a covered interval between the Lower Carbonate and the top of Facies A1, B1, or D1 (see Appendix A). Dashed lines indicate uncertainties.
87
a
b
Fig. 4.21 (a, b) Outcrop photos showing the typical light gray mudstone to granular or pebbly floatstone of the Lower Carbonate of Facies E1, Site R (shovel handle for scale).
88
component varies in regards to both concentration and size. Allochems range from sand- to pebble-size and may reach 6 cm in diameter. Scattered granule- to pebble-size chert clasts are present that locally may be abundant. Chert clast colors include black, gray, white, brown, and red. In addition to detrital siliciclastic grains and clasts, the carbonate locally contains intraclasts, cavities that have walls lined with a thin rim of clay minerals or iron oxide, rare ostracode bioclasts, and purple-red mottling. A network of intersecting subvertical and subhorizontal fractures is present in some areas that give exposures a blocky to columnar appearance.
The petrographic analysis of three samples obtained from the Bob Hill Knoll area reveals that the carbonate is a muddy micrite (Mount, 1985). The micrite contains 1-5% detrital grains, which are dominated by silt to very fine-grained sand. The matrix consists of finely crystalline, massive, patches that are surrounded by more coarsely crystalline micrite, microspar, or spar. The matrix is disrupted by large, irregularly shaped, millimeter-scale cavities that are filled by coarsely crystalline spar or drusy calcite cements. Numerous centimeter-scale stylolites crosscut the matrix as well as the spar and drusy-cemented cavities.
Petrographic analysis of two samples obtained from portions of the lower carbonate found farther from the Bob Hill Knoll area shows that the carbonate is a sandy micrite (Mount, 1985) composed of a typically uniform matrix that includes 10-60% detrital grains. These detrital grains are predominantly very fine-grained sand but significant amounts of medium-grained sand to granule-size clasts are also present. The micritic matrix is disrupted by subcylindrical to circular or elliptical cavities, which have no preferred orientation, and are filled by microspar, spar, and drusy calcite cements. 89
Some cavities have walls lined with a thin rim of clay minerals or iron oxide. The size of subcylindrical cavities approaches 1 mm in width and several millimeters in length while circular to elliptical cavities have diameters up to 2 mm.
The lower carbonate locally contains significant amounts of carbonate intraclasts providing rudstone to floatstone textures, which may or may not occur in association with localized faults. The rudstones and floatstones that are not associated with faulting consist of angular to sub-angular intraclasts, which rarely exceed 5 cm (Fig. 4.22a, b).
These intraclasts show minimal signs of displacement and show virtually no evidence of mechanical reworking. The rudstones to floatstones that occur in association with a fault contain very angular to sub-angular intraclasts, which may approach 20 cm in length
(Fig. 4.23a, b). These intraclasts are larger overall, typically more angular, and contain larger amounts of matrix between individual intraclasts suggesting greater displacement.
The petrographic analysis of two rudstones to floatstones found in association with a fault near Bob Hill Knoll shows that they are a muddy allochem limestone (Mount,
1985) composed of two generations of intraclasts. The first generation is represented by angular to sub-rounded fragments that are composed of a relatively uniform micritic matrix and approximately 5% silt to very fine-grained detrital component with rare stylolites. The second generation is present as angular to subangular fragments that contain intraclasts of the first generation and up to 20% detrital grains. These second generation intraclasts have a microspar to spar matrix and overall have a coarser-grained clastic component than the first generation. The matrix that cements both generations of intraclasts is composed of microspar to spar cements, contains 30% detrital grains, and is similar in appearance to the second generation intraclasts. 90
a
b
Fig. 4.22 (a, b) Outcrop photos of desiccation breccias developed within marginal lacustrine deposits of the Lower Carbonate of Facies E1, Site H as well as between Sites AA and H respectively (lens cap for scale).
91
a b
Fig. 4.23 (a) Outcrop photo of carbonate breccia associated with a fault in the Lower Carbonate of Facies E1, Site S (lens cap for scale). Note the angularity and dispersion of the intraclasts in comparison to the intraclasts of the desiccation breccia (Fig. 4.22a, b). (b) Slabbed sample from the fault breccia of the Lower Carbonate of Facies E1, Sample 83, Site S (dime for scale).
92
Silica within the lower carbonate is common and shows a vertical increase in abundance. The silica is present as veinlets, vug fillings, massive accumulations, and laminar horizons of jasper with lesser amounts of white, cream, light gray, blue, or purple chert. The jasper-filled veinlets occur as millimeter-scale accumulations that often form a complex intersecting network. Vug-fillings are present as patchily distributed to coalesced accumulations that range from millimeter- to centimeter-scale and have no discernable preferred shape or orientation (Fig. 4.24a, b). The vug-fillings contain no relict structures and have margins that show no gradation with the surrounding carbonate
(Fig 4.25a). The vugs are most commonly filled by jasper or chert but may show an inward succession from isopachous jasper and chert to calcite spar or quartz crystals
(Fig 4.25b). The silica also occurs as millimeter- to centimeter-scale massive accumulations, which may reach 1.2 m in thickness (Fig 4.26). These massive areas possess gradational contacts with the surrounding carbonate and contain granule- to pebble-size chert clasts similar to those present in the lower carbonate. Laminar horizons occur as continuous to discontinuous accumulations of variable lateral extent that reach a thickness of 48 cm. These horizons may possess a wavy habit that exhibits wavelengths of approximately 1m. (Fig. 4.27).
Petrographic data from one massive silica accumulation identify the rock as an intensely silicified muddy micrite (Mount, 1985), which consists of detrital grains and bioclasts surrounded by a micritic to sparitic matrix. Detrital grains are monocrystalline quartz that vary in size from silt to fine-grained sand. The bioclasts were identified as ostracode and ostracode fragments (Lisa Parks, personal communication, 2003)
(Fig. 4.28a, b). The silica zone consists of a relatively uniform microcrystalline quartz 93
a
b
Fig. 4.24 (a) Outcrop photo of millimeter- to centimeter-scale jasper-filled vugs within the Lower Carbonate of Facies E1, between Sites AA and H (shovel handle for scale). (b) Close-up photo of jasper-filled vugs within the Lower Carbonate of Facies E1, between Sites AA and H (lens cap for scale).
94
a
b
Fig. 4.25 (a) Outcrop photo of a chert-filled vug within the Lower Carbonate of Facies E1, near Site P (lens cap for scale). Note that the margins show no gradation with the surrounding carbonate indicating the void-filling nature of the silica. (b) Outcrop photo of a chert-filled vug within the Lower Carbonate of Facies E1 that shows an inward succession from isopachous jasper to calcite spar, Site T (lens cap for scale). 95
Fig. 4.26 Outcrop photo of a massive jasper accumulation within the Lower Carbonate of Facies E1, Site B (lens cap for scale).
Fig. 4.27 Outcrop photo of a laminar jasper horizon within the Lower Carbonate of Facies E1, Site AA (lens cap for scale). Note the wavy habit of the laminar horizon. Wavelength is approximately 1 m.
96
a
b
Fig. 4.28 (a, b) Ostracodes within the Lower Carbonate of Facies E1, Sample 87, between Sites AA and H. Crossed nicols. Scale bar is 0.5 mm and 0.25 mm respectively.
97
matrix that shares a gradational contact with the original carbonate. This microcrystalline quartz matrix is disrupted by numerous silica-cemented fractures and cavities and contains scattered silt to medium-grained monocrystalline quartz grains. Small fractures and cavities are filled by coarsely crystalline microcrystalline quartz or megaquartz while larger fractures and cavities tend to be filled by a more complex assemblage of silica phases. The most complex assemblage shows an inward succession from a microcrystalline quartz matrix - chalcedony overlays - coalescing spherulites of chalcedony - a final void-filling by quartzine. In addition to the silica-cemented fractures and cavities, the sample is disrupted by a well-developed network of randomly oriented, intersecting, spar-filled fractures, which crosscut all other cement phases.
Interpretation:
The lower carbonate is interpreted as a lake deposit. The lacustrine interpretation is favored because the macro- and microscopic morphology is not indicative of a calcrete
(Machette, 1985, Wright and Tucker, 1991) and the lateral extent, approximately 11 km, is considered too large to be a spring deposit (Gierlowski-Kordesch, 1998; Pedley et al.,
2003).
Petrologic and petrographic data allow characterization of both marginal lacustrine and basinal lacustrine deposits within the lower carbonate (Freytet and Plaziat,
1982; Platt and Wright, 1991; Freytet and Verrecchia, 2002). The basinal lacustrine deposits are centered near Bob Hill Knoll and are characterized by thicker carbonate accumulations and a silt dominated detrital component that constitutes only 1 to 5% of the carbonate (Fig. 4.29).
98
Fig. 4.29 Map of the Lower Carbonate of Facies E1 showing the distribution of pedogenic features and percentage of detrital siliciclatic grains relative to thickness variations. Contours are in meters.
99
The marginal lacustrine deposits are characterized by thinner accumulations, a coarser-grained detrital component that ranges from 10 to 60%, purple-red mottling, rudstones to floatstones containing intraclasts that show minimal signs of displacement or mechanical reworking, and subcylindrical to circular or elliptical cavities that may have walls lined with a thin rim of clay minerals or iron oxide (Fig. 4.29). The rudstones and floatstones are interpreted to be desiccation breccias that were formed during periods of subaerial exposure (Freytet and Plaziat, 1982) and the subcylindrical to circular or elliptical cavities are interpreted as root casts. The root casts suggests the presence of rooted aquatic plants and a marshy environment or rooted terrestrial flora that prevailed during periods of subaerial exposure. The association of root casts, desiccation breccias, and mottling is consistent with deposition in a marginal lacustrine setting that has been affected by pedogenic modification (Platt and Wright, 1991; Freytet and Verrecchia,
2002) (Fig 4.30).
Petrographic data indicate an overall decrease in the size and concentration of the detrital component laterally toward the basinal lacustrine deposits. This decrease is consistent with lacustrine systems, which are known to baffle and trap clastic material in marginal lacustrine marsh zones (Platt and Wright, 1991). The term basinal lacustrine deposits is not meant to imply deposition within a deep-water lake setting. On the contrary, the presence of rare breccias near the basinal lacustrine deposits suggests that the lake system was probably shallow and periodically subjected to desiccation.
However, the absence of mottling and root structures in the basinal lacustrine deposits suggests only occasional subaerial exposure assuming that these features were not destroyed by diagenetic alteration. 100
Fig. 4.30 Slabbed sample from the Lower Carbonate of Facies E1 showing evidence of pedogenic modification, Sample 104, between Sites AA and H (dime for scale).
101
Macroscopic evidence of diagenetic alteration is present throughout the lower carbonate as calcite filled veinlets and silica accumulations. Microscopic evidence of diagenetic alteration in basinal lacustrine deposits occurs as patches possessing different crystal sizes and large, irregularly shaped, millimeter-scale cavities that are filled by coarsely crystalline spar or drusy calcite cement. These cavities are interpreted as dissolution features while the areas of different crystal sizes are due to recrystallization of the original micritic fabric. The presence of stylolites, which crosscut both the patches and the vug-filling cements, suggests that the diagenetic alteration occurred prior to deep burial.
The two rudstones to floatstones found in association with a fault near Bob Hill
Knoll were formed as a breccia due to successive fracturing and recementation. This means that the first generation of intraclasts represents the original basinal lacustrine carbonate while the second generation and matrix are calcite cements, precipitated subsequent to the initial brecciation. A stylolite found within a first generation intraclast suggests that deep burial preceded faulting.
The jasper and chert present within the lower carbonate resulted from the infilling of karst dissolution features (vugs and veinlets) as well as replacement of the carbonate host (massive accumulations and laminar horizons). The void-filling nature of the silica is indicated by the presence margins that show no gradation with the surrounding carbonate, the absence of relict structures, and the inward succession from isopachous jasper and chert to quartz crystals or calcite spar. Replacement of the lower carbonate is indicated both macro- and microscopically by accumulations that share a gradational 102
contact with the surrounding carbonate and contain grains or clasts inherited from the
carbonate host.
The silica is interpreted to be a groundwater silcrete that replaced and filled karst
dissolution features within the lower carbonate. Milnes and Thiry (1992) described
similar silicified limestones within the Paris Basin, which they interpreted to be a silcrete.
Like the silcrete of the lower carbonate, the silicified limestones of the Paris Basin
display karst dissolution features, abundant millimeter- to centimeter-scale silica-filled
joints that form a complex network, and massive silicified areas (Milnes and Thiry,
1992).
The groundwater origin of the silcrete is indicated by the lack of
macromorphological organization commonly associated with pedogenic silcretes (Fig.
4.6) as well as the presence of silica-filled fractures and cavities similar to those of Facies
A1 and B1, which show a complex inward succession of silica phases and contain
quartzine. The silcrete does not display the characteristic silcrete fabrics of Summerfield
(1983a, b) because this silcrete fabric classification was devised to describe silcretes that
form within a grain-supported host sediment. Further evidence of a groundwater origin is
based on the stratigraphic position of the silcrete. The silcrete within the lower carbonate
is laterally equivalent to the silcrete within the upper horizons of Facies A1 and B1 as well as the silcrete of Facies D2, which are also interpreted to be a groundwater silcrete.
The presence of intraformational carbonate clasts, which were derived from the
lower carbonate, within the basal channel lag of Facies B3 (Fig. 4.17a) indicates that the
lower carbonate was lithified prior to deposition of Facies B3. Likewise, local jasper inclusions within these clasts (Fig. 4.17b) suggest that the groundwater silcrete 103
overprinted the lower carbonate prior to the incision and deposition of Facies B3.
Therefore, it is inferred that the silica formed in the near surface after lithification and karstification but prior to deep burial of the lower carbonate, which is consistent with the interpretation of a groundwater silcrete origin.
4.6.2.2 Middle Carbonate:
Description:
The middle carbonate is not as prominent or as widespread as the lower carbonate, but does occur in several measured sections throughout the central portion of the study area (Fig. 4.1). It is similar in appearance to the lower carbonate and is composed of a dense, resistant, massive, light gray mudstone to granular floatstone that commonly weathers a yellowish-brown color. Detrital grains and clasts are less abundant here than in the lower carbonate. The allochems vary in both concentration and size, ranging from sand- to granule-size. The color of granule chert clasts includes white, black, gray, brown, green, and red. The middle carbonate contains less silica than the lower carbonate with the jasper occurring most commonly as veinlets; there is one example of laminar jasper.
Interpretation:
Similarities in macroscopic appearance and occurrence between the middle and lower carbonates suggest that the middle carbonate was also deposited within a lacustrine system. Like the lower carbonate, the macroscopic morphology of the middle carbonate is not indicative of a calcrete and the deposit is considered to be too great in lateral extent, approximately 9 km, to be a spring deposit. 104
4.6.2.3 Upper Carbonate:
Description:
The upper carbonate intersects a measured section at only one location, Site M
(Fig. 4.1, 4.2). The carbonate is present as two distinct beds that will be referred to as the lower and upper beds. These beds are lithologically similar to the lower carbonate but contain no silica accumulations.
The lower bed is approximately 4 m thick and is composed of a dense, resistant, massive, light gray mudstone that contains an abundant fine-grained detrital component as well as scattered granule-size chert clasts. The top 64 cm of the bed is characterized by a chaotic pattern of intense red and yellow mottling as well as abundant spar-filled veinlets and vugs (Fig. 4.31a, b). The microscopic analysis of one mottled sample shows that the carbonate is a muddy micrite (Mount, 1985), which contains 20-25% detrital grains. These grains are predominantly silt to very fine-grained sand and, less commonly, medium-grained sand. Millimeter-scale subcylindrical as well as circular to elliptical cavities filled by drusy calcite cement are present. The subcylindrical cavities are vertically oriented, may reach 1.5 mm in width, have lengths of more than a centimeter, and taper downward in width. The circular to elliptical cavities may reach
1.25 mm in diameter. In plain light, concentric bands of discoloration, which show an outward progression from yellowish-brown to light brown to red, ring both types of cavities. The red color is seen as dark red haloes that encircle detrital grains. These haloes coalesce to form the red discoloration but are also scattered throughout portions of the carbonate that are yellowish- or light brown.
105
a
b
Fig. 4.31 (a) Outcrop photo from the Upper Carbonate of Facies E1 showing a chaotic pattern of red and yellow mottling as well as abundant spar-filled veinlets and vugs, Site M (lens cap for scale). (b) Slabbed sample from a mottled portion of the Upper Carbonate of Facies E1, Sample 49, Site M (dime for scale). Note that the sample typically shows an outward progression from root casts to yellow mottling to red mottling.
106
The upper bed is a 2 m thick accumulation of interbedded light gray mudstone
and light gray granular to pebbly floatstone. Chert clasts are rare within the mudstone but
represent a significant fraction of the floatstone. The upper bed shares a sharp contact
with the underlying lower bed.
Interpretation:
The upper carbonate is an interpreted lacustrine deposit. The lower and upper
beds are believed to record local fluctuations in lake depth and detrital input within the
lacustrine system. The subcylindrical and circular to elliptical cavities are interpreted as
root casts. These root casts in conjunction with the red to yellow mottling imply
alteration by pedogenic processes, which is a common feature of marginal lacustrine
settings (Freytet and Verrecchia, 2002). The presence of this pedogenic alteration at the
top of the lower bed suggests a decrease in lake level and possibly subaerial exposure.
The upper bed is inferred to represent an increase in lake depth and a return to
lacustrine sedimentation. It contains a significantly higher concentration of detrital grains and clasts than the lower bed, which suggests that the upper bed was deposited in shallower water or that the deposit was closer to a source of clastic sediments than the lower bed.
4.6.3 Facies E2: Calcrete
Description:
Facies E2 is a discontinuous but prominent calcic horizon that occurs near the
northern and southern margins of the study area (Fig. 4.1). This facies is composed of
granular and pebbly mudstone to floatstone that may reach a thickness of nearly 5 m 107
(Fig. 4.32). Chert clasts and less commonly carbonate intraclasts are observed floating in
a carbonate groundmass. Chert clast colors include black, gray, brown, white, and an
abundance of red. The micritic groundmass is gray on fresh surfaces, but weathers
yellowish- to reddish-brown.
The concentration of calcite increases up-section from a horizon of dispersed or
partially coalesced nodules to a horizon of coalesced nodules, both of which are
surrounded by a blocky purple mudstone matrix (Facies D2) (Fig. 4.33a, b). The nodular horizons are capped by a highly indurated sheet-like calcic horizon (Fig. 4.34a), which grades upward into a horizon containing multiple generations of brecciated carbonate fragments (Fig. 4.34b). Jasper is common within the sheet-like and brecciated calcic horizons as veinlets or centimeter-scale laminar accumulations but is typically less common within the nodular horizons.
Petrographic analysis of two samples from the nodular horizon reveals an alpha fabric (Wright and Tucker, 1991) consisting of detrital siliciclastic grains, many of which possess etched margins; crystallaria; circum-granular cracks; circular to elliptical cavities, some of which have walls lined with a thin rim of clay minerals (Fig. 4.35a); allorthic nodules (Fig. 4.35b); and a micritic to microsparitic groundmass. The detrital component ranges from silt to pebble size; is angular to well-rounded; and consists of monocrystalline quartz, polycrystalline quartz, and detrital chert. Allorthic nodules range from 100 µ to more than 1 mm in diameter, are most commonly sub-rounded to rounded,
possess distinct margins, and contain detrital grains similar to those found within the
groundmass (Fig. 4.35b). These nodules also contain previous generations of allorthic
nodules with similar compositions. 108
Fig. 4.32 Outcrop photo of Facies E2 showing vertical gradation from honeycomb to hardpan calcrete, Site G (shovel handle for scale).
109
a
b
Fig. 4.33 (a) Outcrop photo of honeycomb calcrete within Facies E2, Site F (lens cap for scale). (b) Outcrop photo showing carbonate nodules of Facies E2 surrounded by the blocky purple mudstones of Facies D2, Site J (lens cap for scale).
110
a
b
Fig. 4.34 (a) Outcrop photo of hardpan calcrete within Facies E2, Site G (lens cap for scale). Note that the jasper typically follows laminations within the calcrete. (b) Slabbed sample of boulder/cobble calcrete from Facies E2 showing multiple generations of brecciated carbonate fragments, Sample 115, Site Y (nickel for scale).
111
a
b
Fig. 4.35 (a) Clay lined root mould from a carbonate nodule of Facies E2, Sample 25, Site F. Crossed nicols. Scale bar is 0.5 mm. (b) Allorthic nodule within a carbonate nodule of Facies E2, Sample 113, Site Y. Crossed nicols. Scale bar is 0.5 mm.
112
Near the Cleveland-Lloyd Dinosaur Quarry at Site Z (Fig. 4.2), the granular and
pebbly mudstone to floatstone is present as a group of five vertically inclined, downward-
pointing, V-shaped structures that are flanked on either side and below by the poorly
indurated conglomerates of Facies A2. In map view, these V-shaped structures are linear features that have an average trend of 135o (Fig. 4.36a). The structures occur stratigraphically below the nodular, sheet-like, and brecciated calcic horizons and consist of light gray carbonate with scattered granule- to pebble-size clasts that decrease in abundance toward the center. One prominent structure measures 1.2 m wide at the top and 2.6 m in height and contains an abundance of jasper in the form of veinlets and a centimeter-scale, vertically inclined, laminar horizon (Fig. 4.36b).
The micromorphology of the vertically inclined, V-shaped structures is nearly identical to that of samples from the nodular horizon. The sample contains detrital siliciclastic grains, allorthic nodules, circum-granular cracks, crystallaria, and a micritic to microsparitic groundmass. Allorthic nodules resemble those of nodular horizon but are found in higher concentrations (Fig. 4.37a). They range from 150 µ to nearly 3 mm in diameter, are sub-angular to rounded, and are most commonly more coarsely crystalline than the micritic to microsparitic groundmass. The nodules possess distinct margins and contain detrital grains similar to those of the groundmass. Larger allorthic nodules contain previous generations of allorthic nodules with a similar micromorphology. Portions of the vertically inclined, V-shaped structure are heavily overprint by microcrystalline quartz. Within these areas, both matrix and nodules are silicified, preserving the original fabric and providing a glaebular appearance (Fig. 4.37b)
(Summerfield, 1983a, b). These silicified zones host an abundance of vertically oriented 113
a b
Fig. 4.36 (a) Outcrop photo of a vertically inclined, downward-pointing, V-shaped structure near the Cleveland-Lloyd Dinosaur Quarry, Site Z (hammer for scale). (b) Outcrop photo showing a centimeter-scale, vertically inclined, laminar horizon within a V-shaped structure, Site Z (lens cap for scale).
114
a
b
Fig. 4.37 (a) Allorthic nodules within a V-shaped structure near the Cleveland-Lloyd Dinosaur Quarry, Sample 118, Site Z. Crossed nicols. Scale bar is 0.5 mm. Note the presence of silicified and partially silicified nodules. (b) Glaebular fabric within a vertically inclined, laminar horizon of a V-shaped structure, Sample 118, Site Z. Crossed nicols. Scale bar is 0.5 mm.
115
fractures that have been filled by silica cements. The most common fracture-filling sequence is microcrystalline quartz to chalcedony overlays or coalescing spherulites of chalcedony and in large fractures a final void-filling by coarsely crystalline calcite spar.
Site U (Fig. 4.2) is capped by light gray carbonate that shows an overall upward decrease in allochem concentration. This carbonate is composed of interbedded lenses that cumulatively comprise a large-scale bar form (Fig. 4.38a). Individual lenses grade upward from a pebbly rudstone to a sandy to granular wackestone, both of which contain significant sand- to pebble-size allochems. Chert pebbles within the pebbly rudstone possess circular, white discolorations on their surface (Fig. 4.38b). The carbonate contains jasper veinlets as well as a laminar horizon, which may reach a thickness of nearly 10 cm.
Interpretation:
Facies E2 is interpreted to represent floodplain deposits that were modified to form a pedogenic calcrete. The pedogenic calcrete grades up section from a Stage 3-4 nodular and honeycomb calcrete to a Stage 5-6 hardpan and boulder/cobble calcrete
(Netterberg and Caiger, 1983; Machette, 1985). The allorthic nodules are interpreted as reworked calcrete intraclasts, while the circular to elliptical cavities represent root casts and moulds. Multiple generations of the reworked calcrete intraclasts and brecciation within the boulder/cobble calcrete are consistent with a well-developed calcrete profile
(Machette, 1985). Machette (1985) stated that calcic soils of Stage 6 morphology characterized by multiple generations of brecciation and recementation are the climax products of relatively continuous carbonate accumulation over perhaps millions of years.
116
a
b
Fig. 4.38 (a) Outcrop photo of Facies E2 showing calcrete development within a sandstone to conglomerate host, Site U (hammer for scale). Note the presence of a large- scale bar form. (b) Close-up photo showing circular, white discolorations on the surface of framework clasts, Site U (lens cap for scale). The discolorations indicate the clast- supported nature of the deposit.
117
The vertically inclined, downward-pointing, V-shaped structures represent large-
scale desiccation features that were infilled by reworked calcrete clasts from the
pedogenic calcrete. The presence of these structures within the poorly indurated
conglomerates of Facies A2 indicates that the structures extend several meters beneath the
upper boundary of Facies E2. Paik and Lee (1998) described similar meter-scale, downward-tapering structures within paleosols, which they interpret to be desiccation cracks that formed due to repeated wetting and drying cycles. Paik and Lee stated that some of these desiccation cracks were filled with pedogenic calcite that contains nodules, circum-granular cracks, and crystallaria similar to the downward-pointing, V-shaped structures near the Cleveland-Lloyd Dinosaur Quarry.
The carbonate that caps Site U is believed to represent calcrete development within a sandstone to conglomerate host. The circular, white discolorations associated with chert pebbles of the pebbly rudstone are interpreted to be the result of clast contacts
(Tucker, 2001). This indicates that the rudstone and probably wackestone is clast or grain-supported. The clast to grain-supported fabric suggests that the upward gradation from rudstone to wackestone represents preserved fining-upward trends within individual lenses of the original sandstone to conglomerate host. The presence of a bar form suggests that the host rock was deposited within some type of fluvial system, though a lack of sedimentary structures and exposures prevents distinction of the variety. The stratigraphic location of the deposit indicates deposition contemporaneous with or subsequent to deposition of Facies D2 but prior to formation of Facies E2.
The jasper within the hardpan and boulder/cobble horizons may be explained by the common occurrence of secondary silica within many older calcretes (Wright and 118
Tucker, 1991). Both Aubrey (1998) and Currie (1997, 1998) described similar silica
accumulations in a stratigraphically equivalent, mature calcrete at locations southeast of
Green River, Utah as well as in northeastern Utah and northwestern Colorado, which
indicates that the silicification of the calcrete is not a localized event. The above
explanation provides an alternative to the interpretation proposed by Bilbey (1992), who
suggested that silica within a micritic limestone at the Cleveland-Lloyd Dinosaur Quarry,
which is equivalent to Facies E2 (see Site Y of Appendix A), is a Magadi-type chert.
Conversely, the jasper present within the V-shaped structure near the Cleveland-
Lloyd Dinosaur Quarry is interpreted to be a groundwater silcrete based on relation to
similar stratigraphically equivalent silica accumulations and presence of a glaebular
micromorphology. The groundwater silcrete within the V-shaped structure occurs at
approximately the same stratigraphic level as the groundwater silcrete of Facies A1, B1,
D2, and the lower carbonate of Facies E1 (Cross-sections 1 and 2 of Appendix A). The
glaebular micromorphology of the silica is interpreted to have resulted from the
silicification and preservation of the original reworked calcrete clast fabric. Glaebular
fabrics have been reported in silcretes as features that are inherited from calcretes
(Summerfield, 1983a). 119
CHAPTER 5: LITHOFACIES ASSOCIATIONS AND FLUVIAL
ARCHITECTURE
5.1 Introduction
The facies and subfacies of the lower Cedar Mountain Formation form eight recognizable stratigraphic units (Table 5.1). Units 1, 2, 4, and 7 are composed of two or more facies while Units 3, 5, 6, and 8 are made up of a single facies each. The coarse clastic deposit present at the base of the Cedar Mountain Formation appears to occur as a single laterally continuous stratum within the study area, but detailed investigation shows that it can be separated into two distinct units, Unit 1 (Buckhorn) and Unit 2 (Buckhorn- equivalent), based on petrographic and paleocurrent data as well as resistance to weathering (Fig. 5.1). Unit 1 is the conglomeratic unit described by Stokes (1944) at
Buckhorn Flat along the southwest flank of Cedar Mountain while Unit 2 is homotaxial but possesses different lithological characteristics. The contact between Unit 1 and Unit
2 is southwest of the Nipple between Site V and Site U (Fig. 4.2). Although the contact is obscured, rapid changes in the color of framework clasts and proportion of framework clast lithologies as well as resistance to weathering indicates an abrupt change in facies
(Fig. 5.2). Units 3, 6, and 8 are separate lacustrine carbonates of Facies E1 (Fig. 5.1).
Unit 4 consists of a complex assemblage of Facies D2 and E2 that together form a mature pedogenic calcrete profile (Fig. 5.1). Unit 5, which is a single clastic facies composed of
Facies B3, and Unit 7, which contains both Facies A3 and B4, are the result of deposition by different fluvial systems (Fig. 5.1).
120
Lithofacies Depositional Unit Association Lithology Environment
Unit 1 Facies A1, B1, D1 Granule-cobble conglomerates Braided Fluvial with interbedded sandstones and rare mudstones
Unit 2 Facies A2, B2 Granule-cobble conglomerates Braided Fluvial with interbedded sandstones
Unit 3 Facies E1 Mudstone to Lacustrine (Lower Carbonate) granular-pebbly floatstone
Unit 4 Facies D2, E2 Pebbly mudstones containing Pedogenic Calcrete nodular, honeycomb, hardpan, and boulder/cobble calcrete
Unit 5 Facies B3 Fine- to medium-grained Braided Fluvial sandstones
Unit 6 Facies E1 Mudstone to granular floatstone Lacustrine (Middle Carbonate)
Unit 7 Facies A3, B4 Granule-boulder conglomerates Braided Fluvial with interbedded sandstones
Unit 8 Facies E1 Mudstone to Lacustrine (Upper Carbonate) granular-pebbly floatstone
Table 5.1 Table indicating the lithofacies association, lithology, and depositional environment of each unit. Note that the units are numbered based on lithostratigraphic relationships not sequence of depositional events.
121
Fig. 5.1 Generalized diagrammatic cross-section showing the vertical and lateral distribution of units within the study interval. The diagram is based on Cross-section 1 of Appendix A but is vertically exaggerated (V.E. = 600X). The amount of vertical exaggeration varies between individual units. 122
Fig. 5.2 Panoramic photo showing the transition between Unit 1 and Unit 2, between Sites U and V. Scale bar is .25 km. Note that the contact between the two units is covered but a rapid change in resistance to weathering indicates an abrupt change in facies. Unit 1 (Site V) weathers to form large boulders, which litter underling slopes, while Unit 2 (Site U) is easily weathered to form a cover of gravel. 123
The vertical and lateral arrangement of these units makes up the fluvial
architecture of the study interval. This architecture allows a more detailed evaluation of
the characteristics of the J/K unconformity in the study area. The documented variations
in provenance for the fluvial deposits combined with the presence of a calcrete and an
associated groundwater silcrete indicate a long-term hiatus in sedimentation occurred
between the deposition of Units 4 and 5. This hiatus is proposed as the J/K unconformity
in the study area (Fig. 5.3).
5.2 Lithofacies Associations:
5.2.1 Unit 1: The Buckhorn Conglomerate
Description:
Unit 1 consists of the granule to cobble conglomerates of Facies A1 with
interbedded sandstone lenses of Facies B1 and rare abandoned channel fill deposits of
Facies D1 (Fig. 4.3a, b; 5.1). At the type locality, Facies A1 comprises as much as 71% of the total vertical section. The unit is well cemented and resistant to erosion creating a persistent cliff former that forms the upper dip slope of both Little Cedar and Cedar
Mountains. It is continuously exposed throughout the Little Cedar and Cedar Mountain area, but pinches out just southwest of the Nipple between Sites V and U as well as the southwest flank of Little Cedar Mountain near Site BB (Fig. 4.2).
Unit 1 has a sheet geometry with a width/thickness ratio of approximately 550:1
(Friend, 1983) and a thickness that ranges from less than 1 m to more than 16 m (Fig
5.4). Though thickness is variable, there is a trend of maximum thickness paralleling the
east-west axis of Cedar Mountain. The combined paleocurrent direction from all 124
Fig. 5.3 Generalized stratigraphic column of the study interval showing the stratigraphic relationship of the units and the depozones in which the units were deposited. Jagged red line indicates the location of the J/K unconformity. Jagged black line indicates the unconformity at the base of Unit 7.
125
Fig. 5.4 Isopach map showing thickness variations and lateral distribution of Unit 1. Contours are in meters. Dashed lines indicate uncertainties.
126
cross-beds is 081o, which is also roughly parallel to the east-west axis of Cedar Mountain
(Fig. 4.4).
Discussion:
Unit 1 is interpreted as the deposit of a braided fluvial system based on the predominance of coarse grain sizes, geometry, lack of lateral accretion surfaces, presence of planar cross-stratification, and minimal overbank deposits (Miall, 1977). Several previous workers also reached this interpretation (Stokes, 1952; Young, 1960, Conley,
1986; Yingling, 1987; Bilbey, 1992; Yingling and Heller, 1992; Currie, 1998). The lack of lateral accretion surfaces within Unit 1 is consistent with this interpretation (Allen,
1970; Miall, 1992). The abundance of planar cross-stratification is significant since it is common within braided fluvial systems but is less common than trough cross- stratification in other fluvial settings (Miall, 1977). Overbank deposits are very rare within Unit 1, which is another characteristic of braided systems (Miall, 1977). Finally, the unit displays a sheet geometry, which is indicative of deposition within mobile channel belts (Friend, 1983). Mobile channel belts are the result of steady channel migration or channel switching and are attributed to deposition by high sinuosity meandering or braided fluvial systems.
Both easterly (Craig, 1981; Conley, 1986) and northeasterly (Yingling, 1987;
Yingling and Heller, 1992; Currie, 1998) flow directions have been proposed for the
Buckhorn. The data of this study show that Unit 1 was deposited by a fluvial system that drained toward the east. The depositional pinch-outs at both the northwest flank of Cedar
Mountain as well as the southwest flank of Little Cedar Mountain in conjunction with paleoflow direction and a trend of maximum thickness paralleling the east-west axis of 127
Cedar Mountain suggest that the fluvial system which deposited Unit 1 was confined to
the Little Cedar/Cedar Mountain area. This orientation, combined with the resistant
nature of the unit, explains the positive relief of both Little Cedar and Cedar Mountains
and accounts for their protrusion into the San Rafael Swell while other Cedar Mountain
Formation deposits are confined to a thin rim along the margin of the Swell.
The presence of gutter casts, flame structures, and load casts indicate minimal
compaction of the uppermost Brushy Basin Member prior to deposition of Unit 1. This
suggests that the contact between Unit 1 and the Brushy Basin Member is conformable
and does not represent a significant hiatus in deposition (Roca, 2003).
5.2.2 Unit 2: The Buckhorn-equivalent
Description:
Unit 2 is dominated by the poorly indurated conglomerates of Facies A2 (Fig. 4.8) but locally occurs as resistant beds consisting of the indurated conglomerates of Facies
A2 interbedded with sandstone lenses of Facies B2 (Fig. 4.7a, b; 5.1). In sharp contrast to
Unit 1, Unit 2 typically weathers to form a cover of gravel on underlying slopes and contains a significant amount of green framework clasts. It is present from just southwest of the Nipple between Sites U and V and extends north at least as far as
Cleveland-Lloyd Dinosaur Quarry, at Site X (Fig. 4.2).
The unit has a sheet geometry (Friend, 1983), ranging in thickness from less than
2 m to more than 12 m, and possesses a width/thickness ratio of no less than 120:1. At locations where resistant beds are present, the lower contact is sharp and locally incises the Brushy Basin Member, but unlike Unit 1, there are no signs of soft sediment 128
deformation at the base. The typical slope forming habit of Unit 2 restricts preservation
of paleoflow indicators but limited paleocurrent data obtained from resistant beds
indicate a paleoflow direction of 037o (Fig. 4.4).
Discussion:
Unit 2 is interpreted to be the deposit of a braided fluvial system based on an
abundance of coarse grain sizes, geometry, lack of lateral accretion surfaces and presence
of planar cross-stratification. The presence of sand and gravel barforms and bedforms
indicates that the resistant beds, consisting of the indurated conglomerates of Facies A2 and Facies B2, were deposited within channelized portions of the fluvial system under
conditions similar to those of Unit 1. The lack of these features and the presence of
interstitial silt and mud within the easily weathered, poorly indurated conglomerates of
Facies A2 are interpreted to be the result of deposition by hyperconcentrated flows during high magnitude flow events in unchannelized portions of the fluvial system that were unaffected by reworking as the stream returned to normal flow conditions. Pebble count data show that Unit 2 contains an abundance of light colored framework clasts, which include a significant amount of green clasts. This color variation combined with differences in framework clast lithologies and paleoflow direction indicates that Unit 2 had a southwestern source terrain.
Stokes (1944) stated that conglomerates, which differ from the Buckhorn (Unit 1) in details of composition and weathering, are present at approximately the same stratigraphic horizon over wide areas of the Rocky Mountains and that a facies present over wide areas of southern Utah is composed of colorful clasts, which include green clasts as important index markers. Stokes referred to these conglomerates as Buckhorn- 129
equivalents. The limited paleocurrent measurements obtained from Unit 2 and the presence of green framework clasts indicate that Unit 2 is more closely related to the facies described by Stokes in southern Utah (Buckhorn-equivalents) than to Unit 1
(Buckhorn).
Conley (1986) described conglomerates northeast of the study area in the Flattop
Mountain area (Fig. 4.2) that had a strongly banded, varicolored red, brown, and purple matrix. Conley interpreted these conglomerates to be the Buckhorn Conglomerate but the presence of a variegated matrix suggests that they are Unit 2 (Buckhorn-equivalent), which is consistent with the northeast paleoflow direction of the Unit 2.
The lower contact of Unit 2 is sharp, locally erosive, and shows no signs of soft sediment deformation indicating an unconformable contact with the Brushy Basin
Member. The crosscutting relationship of the paleoflow indicators combined with the conformable contact at the base of Unit 1 and the unconformable contact at the base of
Unit 2 suggests that deposition of Unit 1 (Buckhorn) preceded deposition of Unit 2
(Buckhorn-equivalent).
5.2.3 Unit 3: The Lower Carbonate
Description:
Unit 3 (the Lower Carbonate of Facies E1) is a lacustrine carbonate that has a maximum thickness near Bob Hill Knoll and shows a trend of decreasing thickness away from that area (Fig. 4.20; 4.21a, b; 5.1). Unit 3 typically overlies Unit 1 and occurs over the same area pinching out at the northwest flank of Cedar Mountain and near the southwestern flank of Little Cedar Mountain (Fig. 4.2). 130
Discussion:
Unit 3 is interpreted to represent the deposits of a lacustrine system that occupied
a topographic low resulting from the abandoned channel belt of Unit 1. The northern and
southern boundaries of the lacustrine system coincide with the natural limits of the Unit 1
channel belt. The western boundary of Unit 3 was not located because the rocks are not
exposed at the surface while the eastern boundary was not located because the unit
extends out of the study area to the east. Deposition of Unit 3 is interpreted to have
occurred prior to deposition of Unit 2 because Unit 3 is typically found in direct contact
with the upper boundary of Unit 1. If deposition of Unit 2 had proceeded Unit 3, there
should be evidence of Unit 2 between Units 1 and 3 because the fluvial system of Unit 2
would have occupied the topographic low of the abandoned Unit 1 channel belt had it not
been previously filled by Unit 3.
5.2.4 Unit 4: The Pedogenic Calcrete Profile
Description:
Unit 4 is composed of Facies D2 and E2 to form a pedogenic calcrete profile that shows a vertical increase in development from the Stage 1-2 calcic paleosol (Facies D2)
(Fig. 4.19a, b) to the Stage 6 pedogenic calcrete (Facies E2) (Fig. 4.32, 5.1, 5.5).
Complete sections of Unit 4 are confined to the extreme northern and southern margins of
the study area near the Cleveland-Lloyd Dinosaur Quarry, at Site Y, as well as the
southwestern flank of Little Cedar Mountain, at Sites BB, F, and G (Fig. 4.2). The
pedogenic calcrete of Facies E2 is typically absent throughout the central portion of the study area due to erosion by Unit 5 but nodules of the nodular calcrete horizon are locally 131
Fig. 5.5 Schematic section showing the generalized macromorphological organization of Unit 4. Note the vertical increase in stage of development and maturity. Complete sections of Unit 4 were only recorded near the northern and southern margins of the study area near the Cleveland-Lloyd Dinosaur Quarry, at Site Y, as well as the southwestern flank of Little Cedar Mountain, at Sites BB, F, and G (see Appendix A).
132
present within the upper horizon of Facies D2 at isolated locations. The calcic paleosol of
Facies D2 is laterally continuous throughout the study area and is typically present where sections of the basal Cedar Mountain Shale are preserved.
The Stage 1-2 calcic paleosol of Facies D2 (Fig. 4.19a, b) grades vertically into the
Stage 3-4 nodular to honeycomb calcrete horizons of Facies E2 (i.e., Netterberg & Caiger,
1983; Machette, 1985). These nodular to honeycomb calcrete horizons consists of
dispersed or partially coalesced nodules that grade up-section to coalesced nodules, all of
which are surrounded by the calcic paleosol of Facies D2 (Fig. 4.33a, b). The nodular horizons are capped by the Stage 5-6 hardpan and boulder/cobble calcretes of Facies E2
(i.e., Netterberg & Caiger, 1983; Machette, 1985). The hardpan and boulder/cobble
calcretes consist of a highly indurated sheet-like calcic horizon (Fig. 4.34a), which grades
vertically into a horizon containing multiple generations of brecciated carbonate
fragments (Fig. 4.34b).
The groundwater silcrete overprints a wide range of lithofacies within the lower
units of the Cedar Mountain Formation (Units 1, 2, 3, and 4) but occurs most commonly
between 5 and 10 m beneath the upper boundary of Unit 4 (Fig. 5.6). It consists of
mainly jasper and less commonly white, cream, light gray, blue, or purple chert that
occurs as a non-displacive cement within clastic lithofacies and a void-filling or replacive
cement in carbonate lithofacies. The highest concentrations of silica are associated with
Unit 3 (the Lower Carbonate of Facies E1) but the silcrete is also present at similar
stratigraphic levels within the laterally equivalent portions of Unit 1 (Facies A1 and B1),
Unit 2 (the V-shaped structures of Facies E2), and Unit 4 (Facies D2) (Fig. 5.6).
133
Fig. 5.6 Generalized diagram showing the vertical and lateral distribution of the groundwater silcrete in relation to Unit 4. Note that the highest concentrations of silica occur at approximately the same stratigraphic horizon regardless of host lithology. Jagged lines indicate erosion. Dashed lines indicate uncertainties.
134
Discussion:
Unit 4 is a mature Stage 6 pedogenic calcrete that contains multiple generations of
brecciated intraclasts, which suggests long-term carbonate accumulation over millions of
years (Machette, 1985). This implies that the top of Unit 4 represents a major
unconformity and that a long-term hiatus in sedimentation occurred between the
deposition of Units 4 and 5. Since Facies E2 is typically absent due to erosion by Unit 5, the upper boundary of Facies D2 marks the location of the unconformity throughout much of the study area.
The groundwater silcrete formed at depths extending up to 15 m beneath the
unconformity marked by the top of Unit 4 (Fig. 5.6). This proximity and the distribution
of the highest concentrations of silica 5 to 10 m beneath Unit 4, regardless of host
lithology, indicate that the groundwater silcrete and pedogenic calcrete (Unit 4) formed in
association with the same paleoweathering surface. Smith et al. (1997) described
silcretes associated with the Cambro-Ordovician, mid Lower Ordovician, and Lower to
Middle Ordovician unconformities of the Midwestern United States that are similar to the
silcrete of the study area. Smith et al. found that the silcrete was patchily distributed up
to 30 m below the unconformities but was most abundant within 5 m. Leith (1925) stated
that silica is most abundant immediately below an unconformity but that silicified
material typically extends deep beneath unconformities along fractures and paleokarst
and that the highest concentrations are typically associated with the carbonate facies,
which is consistent with observations of this study.
The temporal relationship between the silcrete and calcrete is not well constrained
but formation of the two could have been contemporaneous since groundwater silcretes 135
(Summerfield, 1983a) and pedogenic calcretes (Tucker, 2001) form in similar
environments under very stable geomorphic and geologic conditions over significant
periods of time (Machette, 1985; Milnes and Thiry, 1992). Likewise, Summerfield
(1983a) stated that complex void-fillings in groundwater silcretes, such as those in Facies
A1 and B1 as well as the lower carbonate of Facies E1, are most commonly associated
with calcretes and that the silcrete in such instances is invariably closely adjacent to a
calcrete unit.
The silica is not considered to be a Magadi-type chert as suggested by Bilbey
(1992), even though it is most abundant within a lacustrine carbonate (Unit 3), because it does not exhibit features that are characteristic of a Magadi-type chert such as soft- sediment deformation, surface reticulation, or evidence of evaporite minerals. Soft- sediment deformation within Magadi-type cherts is a common feature inherited from the magadiite precursor prior to being converted to chert (Surdam et al., 1972; Sheppard and
Gude, 1986; Krainer and Spotl, 1998). Deformation in such cases occurs as contorted bedding, folding, lobate protrusions, casts of underlying mud cracks, or crystal casts
(Surdam et al., 1972; Sheppard and Gude, 1986). Surface reticulations and cracks are formed in Magadi-type cherts as a response to volume reduction when the magadiite is dehydrated to chert (Surdam et al., 1972; Sheppard and Gude, 1986; Krainer and Spotl,
1998). Surdam et al. (1972) stated that these features, which indicate volume change and soft-sediment deformation, are a unique characteristic of Magadi-type cherts. Magadi- type cherts are formed in saline, alkaline environments so are commonly found in association with evaporite minerals (Sheppard and Gude, 1986; Krainer and Spotl, 1998). 136
The stratigraphic position and macromorphology of Unit 4 suggest that it is
laterally equivalent to the calcretes described by Aubrey (1998) and Currie (1997, 1998),
which indicates that the unconformity at the top of the unit is regionally extensive. This
regional unconformity is proposed as the J/K unconformity in the study area. Placement
of the J/K unconformity at the top of Unit 4 differs from Aubrey (1998) who placed the
J/K boundary at the base of the lowest calcrete within the Cedar Mountain Shale
Member.
5.2.5 Unit 5: The White Sandstone
Description:
Unit 5 consists of the white to light gray, fine-grained sandstones of Facies B3
(Fig. 4.16a, b; 5.1). The unit is present throughout the central portion of the study area, from Site H to as far north as Site W, as a laterally continuous stratum that ranges in thickness from less than 1 m to nearly 5 m and has a sheet geometry (Friend, 1983) with a width/thickness ratio of 2,650:1 (Fig. 4.2).
Unit 5 possesses an irregular lower contact that is sharp, erosive, and contains varying amounts of intraformational clasts that are primarily carbonate. The base of the unit locally becomes conglomeratic where it intersects Unit 3, providing conglomerates that are typically less than 1 m thick and fine upward into fine-grained sandstones. These conglomerates are massive, clast-supported, and include intraformational carbonate clasts that may contain inclusions of jasper (Fig 4.17a, b). The combined paleocurrent data indicate a paleoflow direction of 132o (Fig. 4.18).
137
Discussion:
Unit 5 is interpreted as the deposit of a braided fluvial system based on geometry and the presence of planar cross-stratification as well as a lack of lateral accretion surfaces and overbank deposits. Paleocurrent data show that this braided fluvial system drained toward the southeast. The presence of detrital carbonate and intraformational carbonate clasts within Unit 5 indicates significant erosion and incorporation of underlying carbonate strata. Evidence of this is seen throughout the central portion of the study area where the unit incises Unit 4 and localized portions of Unit 3.
The massive, clast-supported conglomerates at the base of Unit 5 are interpreted to represent the erosion and incorporation of Unit 3 as a channel lag. The jasper inclusions within the intraformational carbonate clasts of these conglomerates are the result of groundwater silcrete development in Unit 3, which implies that Unit 3 was lithified and overprinted by the groundwater silcrete prior to deposition of Unit 5.
Therefore, an unknown but significant amount of time elapsed between deposition of
Units 3 and 5. This is further evidence for a long-term hiatus in sedimentation between the deposition of Units 4 and 5, since the groundwater silcrete within Unit 3 formed in association with Unit 4.
5.2.6 Unit 6: The Middle Carbonate
Description:
Unit 6 is a lacustrine carbonate composed of the middle carbonate of Facies E1
(Fig. 5.1). It is not as prominent or as widespread as Unit 3 but it does occur throughout 138
the central portion of the study area. Where present, Unit 6 is locally gradational with the upper boundary of Unit 5 suggesting a conformable contact between the two units.
Discussion:
Similarities between Unit 6 and Unit 3 in terms of lithology and relation to an underlying fluvial deposit suggest that the units were deposited in a similar lacustrine depositional setting. Unit 6 may represent the deposits of a lacustrine system that occupied a topographic low resulting from the abandonment of the channel belt that deposited Unit 5. This interpretation is difficult to confirm because of the restricted ability to map the lateral extent of Unit 6 in relation to Unit 5 due to limited exposures but the conformable nature of their contact suggests that a significant amount of time did not elapse between deposition of the two units.
5.2.7 Unit 7: The Red Conglomerate
Description:
Unit 7 consists of the conglomerates of Facies A3 interbedded with sandstone lenses of Facies B4 (Fig. 4.9a, b; 5.1). It is laterally continuous throughout the central portion of the study area from Site AA to between Site N and Site V as a coarse clastic sheet that ranges in thickness from 0.5 m to more than 7 m and has a width/thickness ratio of 1,400:1 (Friend, 1983) (Fig. 4.2, 5.7).
The base of Unit 7 incises underlying lithologies providing a sharp, irregular lower boundary that is typically found in contact with Unit 5 and less commonly Unit 6 throughout the central portion of the study area (Fig. 5.8a). At such locations, the unit may contain cobble- to boulder-size clasts of Unit 5 and Unit 6 (Fig. 5.8b). At isolated 139
Fig. 5.7 Isopach map showing thickness variations and lateral distribution of Unit 7. Contours are in meters. Dashed lines indicate uncertainties.
140
a
b
Fig. 5.8 (a) Outcrop photo showing the sharp, irregular contact between Unit 5 and Unit 7, Site N (hammer for scale). (b) Outcrop photo showing a boulder of Unit 5 within a conglomerate of Unit 7, Site J (lens cap for scale).
141
locations, Unit 7 contains oncoids that are present as both intraformational and
extraformational clasts that are symmetrically coated by carbonate, some of which
possess concentric laminations (Fig 4.10a, b). The combined paleocurrent direction for
Unit 7 is 211o (Fig. 4.11).
Discussion:
Unit 7 is interpreted as the deposit of a braided fluvial system based on a predominance of coarse grain sizes, geometry, presence of planar cross-stratification, and a lack of lateral accretion surfaces and overbank deposits. The paleocurrent data suggest that this fluvial system drained toward the south-southwest whereas the isopach data suggest that the fluvial system had a northwest-southeast trend. This discrepancy is probably a function of the limited number of paleocurrent measurements obtained from
Unit 7 due to the typically massive nature of Facies A3 and the limited occurrence of
Facies B4. By using the paleocurrent data in conjunction with the isopach data it is
possible to obtain a more accurate paleoflow direction. Unit 7 shows a trend of
maximum thickness in the central portion of the study area near Site J and decreases in
thickness toward the northeast and southwest, which together with the southerly
paleoflow direction suggests a southeasterly draining fluvial system
The oncoids within Unit 7 are interpreted to have two possible origins. They
were either inherited from Unit 4 or represent the erosion of a second calcrete unit that is
not preserved within the study area. The presence of the cobble- to boulder-sized clasts
of Units 5 and 6 suggest that both units were lithified prior to the incision and deposition
of Unit 7. This implies that the lower contact of Unit 7 represents a second significant 142
unconformity within the study area, which based on the data collected, is considered to be
shorter than that of the unconformity associated with Unit 4 (Fig. 5.3).
5.2.8 Unit 8: The Upper Carbonate
Description:
Unit 8 is a lacustrine carbonate composed of the upper carbonate of Facies E1
(Fig. 5.1). It is the least well preserved of the eight units and was measured at only one location, Site M, due to a lack of exposures (Fig. 4.2, Cross-section 1 of Appendix A).
The unit is present as a 6 m thick accumulation that consists of two beds that are lithologically similar to Unit 3 and share a conformable contact with Unit 7.
Discussion:
The vertical section of Unit 8 measured at Site M is interpreted to record local fluctuations in lake depth and detrital input into a lacustrine system but the lateral distribution of the system and its genetic relationship with underlying lithologies could not be determined due to a lack of exposures. The contact between Unit 8 and Unit 7 is conformable, which suggests that deposition of Unit 8 followed soon after that of Unit 7.
This temporal relationship might account for the abundance of calcite cement in localized portions of Unit 7.
143
5.3 Fluvial Architecture
Foreland basin systems contain complex stratigraphic architectures (Flemings and
Jordan, 1990; DeCelles and Giles, 1996) that respond to multiple governing variables including tectonics, subsidence rates, sediment supply, climate, and eustasy. In recent years, an array of studies (Jordan, 1981; Heller et al., 1988; Flemings and Jordan, 1990;
Paola and Heller, 1992) have proposed both qualitative and quantitative models in an attempt to better understand the interaction between these variables. One qualitative approach is to examine the fluvial architecture of the deposits. The fluvial architecture of the study interval shows that the western flank of the San Rafael Swell experienced changes in the amount and rate of accommodation development in response to thrust loads within the orogen to the west.
The fluvial architecture of the study interval indicates that the strata of the Brushy
Basin Member and the lower Cedar Mountain Formation comprise two depositional sequences that are separated by the regional unconformity at the top of Unit 4 (Fig. 5.3).
The depositional sequence below the unconformity consists of the Brushy Basin Member as well as Units 1, 2, 3, and 4 while the sequence above the unconformity consists of
Units 5, 6, 7, and 8. These sequences are inferred to document the cratonward migration of a Late Jurassic-Early Cretaceous foreland basin system (e.g., Currie, 1997, 1998) and indicate the subtle transition from deposition within the proximal back-bulge depozone during Late Jurassic time to deposition within the distal foredeep depozone during Early
Cretaceous time.
The data of the current study support the interpretation of Roca (2003) that the coarsening upward trend from the mud-dominated Brushy Basin Member to the gravel- 144
dominated Buckhorn Conglomerate (Unit 1) records the cratonward progradation of coarse clastic sediments and that these sediments were deposited during a period of tectonic quiescence in the back-bulge depozone of a Late Jurassic foreland basin that formed adjacent to the Elko Orogen (Fig. 5.9). However, the upper units of the lower depositional sequence (Unit 2, Unit 3, Unit 4) (Fig. 5.3) record a previously unrecognized period of sedimentation that occurred within the back-bulge depozone. This phase of sedimentation is inferred to have occurred in the proximal back-bulge depozone adjacent to an underfilled foredeep depozone. The underfilled state of the foredeep is interpreted as a flexural response (DeCelles and Giles, 1996) to thrust-induced subsidence (Flemings and Jordan, 1990) in the proximal portion of the basin that may represent the initial thrust-related crustal loading of the Sevier Orogen.
The lacustrine strata of Unit 3 record the impoundment of the Unit 1 drainage systems in relation to tectonic uplift (Pietras et al., 2003) that is interpreted as the initial flexural response to tectonic deformation within the Sevier Orogen. Subsidence within the back-bulge depozone (DeCelles and Giles, 1996) increased accommodation and permitted the impoundment of the Unit 1 (Buckhorn) fluvial system. Underfilled conditions within the foredeep depozone during this time trapped coarse sediments adjacent to the thrust front (Heller et al., 1988) restricting further transport of the typical coarse-grained Unit 1 clastic sediments to the distal portions of the basin, explaining the decreased clastic supply that permitted carbonate accumulation within the impounded
Unit 1 fluvial channel belt. During deposition of Unit 2, Unit 3, and Unit 4, the forebulge of the Late Jurassic foreland basin system was located west of the study area and trended roughly northeast-southwest across central Utah (Currie, 1997). The location and 145
Fig. 5.9 Generalized paleogeographic map showing the flow direction of Unit1 and Unit 2 in relation to structural disturbances during the Late Jurassic. Location of structural disturbances modified from Lawton (1994) and DeCelles and Currie (1996). Red arrow indicates flow direction of Unit 1, blue arrow indicates flow direction of Unit 2. Hachured pattern and heavy barbed line shows the eastern extent of Middle to Late Jurassic thrusting and igneous activity in eastern Nevada and western Utah, ruled pattern shows the location of the Mogollon Highland. 146
orientation of the forebulge in conjunction with the northeasterly flow direction of the
Unit 2 suggest that Unit 2 was deposited by a fluvial system that flowed parallel to the forebulge along the axial drainage of the back-bulge depozone (DeCelles and Giles,
1996). The southwestern source terrain of Unit 2 suggests that the fluvial system may have transported sediments from the Mogollon Highlands, which was active until late
Early Cretaceous time (Lawton, 1994) (Fig. 5.9). The fluvial system responsible for deposition of the Unit 4 overbank deposits probably followed a similar axial-drainage system as the Unit 2.
The model of forebulge-controlled sedimentation is consistent with the development of the calcrete profile at the top of Unit 4 (Currie, 1997, 1998). The underfilled state of the foredeep present in central Utah prohibited sediment supply to areas uplifted by the forebulge (Currie, 1998) and provided the stable geologic and geomorphic conditions necessary for the development of a mature calcrete profile
(Machette, 1985). The model of a migrating forebulge also explains the thickness variations between the calcrete profile of Unit 4 and the calcretes described by Currie
(1997, 1998) and Aubrey (1998). The thicker calcrete described by Currie and Aubrey suggests that strata of the study area were uplifted as the forebulge migrated eastward but that the forebulge was stable in eastern Utah and northwestern Colorado for a longer duration of time (Fig. 2.1).
The fluvio-lacustrine sediments of Units 5, 6, 7, and, 8 document the first strata of the study interval to be deposited within the distal foredeep depozone of the Early
Cretaceous foreland basin system, based on the placement of the J/K unconformity at the top of Unit 4. The southeasterly flow direction of Units 5 and 7 (Fig. 5.10) indicate that 147
Fig. 5.10 Generalized paleogeographic map showing the flow direction of Unit5 and Unit 7 in relation to the Sevier orogenic belt. Location of structural disturbances modified from Lawton (1994) and DeCelles and Currie (1996). Red arrow indicates flow direction of Unit 5, blue arrow indicates flow direction of Unit 7. Diagonal ruled pattern and heavy barbed line shows the location and extent of the Sevier orogenic belt in Utah, horizontal ruled pattern shows the location of the Mogollon Highland.
148
both units flowed roughly perpendicular to the Sevier Orogen suggesting that they were
deposited during overfilled conditions within the foredeep depozone (DeCelles and
Burden, 1992). The lacustrine deposits of Units 6 and 8 are interpreted to record the
impoundment of the Unit 5 and 7 drainage systems respectively (Pietras et al., 2003).
This impoundment occurred as a response to thrust-induced subsidence (Flemings and
Jordan, 1990) related to episodic thrusting events within the Sevier Orogen. During these
episodes, clastic sediments were likely trapped adjacent to the thrust front (Heller et al.,
1988) decreasing the clastic supply to the distal portions of the foredeep and permitting
carbonate accumulation within the impounded Unit 5 and 7 channel belts. The fluvial
architecture of the depositional sequence above the J/K unconformity implies that Units
5, 6, 7, and, 8 document alternating periods of overfilled and underfilled conditions that
could be related to phases of tectonic quiescence and activity in the Sevier Orogen
(DeCelles and Burden, 1992) or to variations in climate.
The data of this study are consistent with the interpretation that the contact
between the Brushy Basin Member and the Buckhorn Conglomerate does not represent a
significant hiatus in deposition within the study area and that the Buckhorn Conglomerate
occurs within the UJ-2 depositional sequence (Roca, 2003) (Fig. 2.2). The current study
also supports the interpretation that the calcrete at the top of Unit 4 (the base of the LK-2
depositional sequence) represents a regionally extensive unconformity that marks a
significant hiatus in deposition (Currie, 1997, 1998; Aubrey, 1998) (Fig. 2.2, 5.3). These data and the fluvial architecture of the study interval suggest that the top of Unit 4 marks the J/K unconformity. The placement of the J/K unconformity at the top of Unit 4 indicates that the initial thrust-related crustal loading of the Sevier Orogen occurred 149
during latest Late Jurassic time rather than late Early Cretaceous time as postulated by other workers (Heller et al., 1986; Yingling, 1987; Heller and Paola, 1989; Yingling and
Heller, 1992). However, it is important to note that the placement of the J/K unconformity within the study interval is based on petrologic, petrographic, and lithostratigraphic criteria so additional chronostratigraphic analyses of the study interval are needed to further constrain the placement of the J/K unconformity within the study area. 150
CHAPTER 6: CONCLUSIONS
1. The Buckhorn Conglomerate Member consists of two homotaxial units, Unit 1
(Buckhorn) and Unit 2 (Buckhorn-equivalent). Unit 1 is the conglomeratic unit
described by Stokes (1944) at Buckhorn Flat along the southwest flank of Cedar
Mountain while Unit 2 is more closely related to a conglomeratic facies described
by Stokes in southern Utah. The crosscutting relationship of the paleoflow
directions combined with the conformable contact at the base of Unit 1 and the
unconformable contact at the base of Unit 2 suggests that deposition of Unit 1
preceded deposition of Unit 2.
2. Petrographic and petrologic data provide support for the previous interpretations
of Currie (1997, 1998) and Aubrey (1998) that the calcrete near the base of the
Cedar Mountain Shale Member (Unit 4) is a mature Stage 6 pedogenic calcrete
profile.
3. Multiple generations of brecciation and recementation within the upper horizons
of the calcrete indicate a long-term hiatus in sedimentation that lasted perhaps
millions of years.
4. The top of the calcrete marks the location of the Jurassic/Cretaceous boundary
within the study interval.
151
5. Petrologic, petrographic, and lithostratigraphic data reveal a previously
unrecognized groundwater silcrete within the lower Cedar Mountain Formation.
The silcrete formed at depths extending up to 15 m beneath the unconformity
marked by the top of the calcrete (Unit 4) but occurs most commonly within
5 to 10 m of the unconformity.
6. The proximity of the calcrete and groundwater silcrete indicates that both formed
in association with the same paleoweathering surface.
7. The placement of the J/K unconformity at the top of Unit 4 combined with the
interpretation the there is no significant hiatus in deposition at the base of the
Unit 1 (Buckhorn) indicates that Units 1, 2, 3, and 4 should be considered part of
the UJ-2 depositional sequence and that Units 5, 6, 7, and 8 should be considered
part of the LK-1 depositional sequence.
8. The sediments of the study interval record the subtle transition from deposition
within the proximal back-bulge during the Late Jurassic to deposition within the
distal foredeep during the Early Cretaceous. 152
REFERENCES
Allen, J.R.L., 1970, Studies in fluvialtile sedimentation: A comparison of fining-upwards cyclothems, with special references to coarse-member composition and interaction, Journal of Sedimentary Petrology, 40, p. 298-323.
Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah, Geological Society of America Bulletin, 79, p. 429-458.
Aubrey, W.M., 1998, A newly discovered, widespread fluvial facies and unconformity marking the Upper Jurassic/Lower Cretaceous boundary, Colorado Plateau, Modern Geology, 22, p. 209-233.
Bilbey, S.A., 1992, Stratigraphy and sedimentary petrology of the Upper Jurassic- Lower Cretaceous rocks at Cleveland-Lloyd Dinosaur Quarry with a comparison to the Dinosaur National Monument Quarry, Utah, [PhD. Thesis], University of Utah, 295 p.
Cadigan, R.A., 1967, Petrology of the Morrison Formation in the Colorado Plateau region, U. S. Geological Survey Professional Paper 556, 113 p.
Cifelli, R.L., Kirkland, J.I., Weil, A., Deino, A.L., and Kowallis, B.J., 1997, High precision 40Ar/39Ar geochronology and the advent of North America's Late Cretaceous terrestrial fauna, Proceedings of the National Academy of Sciences of the U.S.A., 94, p. 11163-11167.
Conley, S.J., 1986, Statigraphy and depositional environment of the Buckhorn Conglomerate Member of the Cedar Mountain Formation (Lower Cretaceous), central Utah, [MS Thesis], Fort Hays State University, 127 p.
Craig, L.C., 1981, Lower Cretaceous rocks, southwestern Colorado and southeastern Utah, 1981 Field Conference, Rocky Mountain Association of Geologists, p. 195-200.
Currie, B.S., 1997, Sequence Stratigraphy of nonmarine Jurassic-Cretaceous rocks, central Cordilleran foreland basin system, Geological Society of America Bulletin, 109, p. 1206-1222.
Currie, B.S., 1998, Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain Formations, NE Utah-NW Colorado: relationships between nonmarine deposition and early cordilleran foreland basin development, Journal of Sedimentary Research, 68, p. 632-652.
153
Currie, B.S., 2002, Structural configuration of the Early Cretaceous Cordilleran foreland- basin system and Sevier thrust belt, Utah and Colorado, The Journal of Geology, 110, p. 697-718.
DeCelles, P.G., and Burden, E.T., 1992, Non-marine sedimentation in the overfilled part of the Jurassic-Cretaceous Cordelleran foreland basin: Morrison and Cloverly Formations, central Wyoming, USA, Basin Research, 4, p. 291-313.
DeCelles, P.G., and Currie, B.S., 1996, Long-term sediment accumulation in the Middle Jurassic-Early Eocene Cordilleran retroarc foreland basin system, Geology, 24, p. 591-594.
DeCelles, P.G., and Giles, K.A., 1996, Foreland basin systems, Basin Research, 8, p. 105-123.
DeCelles, P.G., Langford, R.P, and Schwartz, R.K., 1983, Two new methods of paleocurrent determination from trough cross-stratification, Journal of Sedimentary Petrology, 53, p. 629-642.
Dunham, R.J., 1962, Classification of carbonate rocks according to depositional texture, American Association of Petroleum Geologists Memoir, 1, p. 108-121.
Flemings, P.B., and Jordan, T.E., 1990, Stratigraphic modeling of foreland basins: Interpreting thrust deformation and lithosphere rheology, Geology, 18, p. 430-434.
Freytet, P. and Plazizt, J.C., 1982, Continental carbonate sedimentation and pedogenesis- Late Cretaceous and Early Tertiary of southern France, Contributions to Sedimentology 12, 213 p.
Freytet, P., and Verrechia, E.P, 2002, Lacustrine and palustrine carbonate petrography: an overview, Journal of Paleolimnology, 27, p. 221-237.
Friend, P.F., 1983, Towards the field classification of alluvial architecture or sequence, Special Publication of the International Association of Sedimentologists, 6, p. 345-354.
Gibling, M.R., and Rust, B.R., 1990, Ribbon sandstones in the Pennsylvanian Waddens Cove Formation, Sydney Basin, Atlantic Canada: the influence of siliceous duricrusts on channel-body geometry, Sedimentology, 37, p. 45-65.
Gierlowski-Kordesch, E.H., 1998, Carbonate deposition in an ephemeral siliciclastic alluvial system: Jurassic Shuttle Meadow Formation, Newark Supergroup, Hartford Basin, USA, Paleogeography, Paleoclimatology, Paleoecology, 140, p. 161-184. 154
Harris, D.R., 1980, Exhumed paleochannels in the Lower Cretaceous Cedar Mountain Formation near Green River, Utah, Brigham Young University, Geology Studies, 27, part 1, p. 51-66.
Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., Hagen, E.S., Shuster, M.W., and Winslow, N.S., 1986, Time of initial thrusting in the Sevier orogenic belt, Idaho-Wyoming and Utah, Geology, 14, p. 388-391.
Heller, P.L, Angevine, C.L., and Winslow, N.S., 1988, Two-phase stratigraphic model of foreland-basin sequences, Geology, 16, p. 501-504.
Heller, P.L., and Paoloa, C., 1989, The paradox of Lower Cretaceous gravels and the initiation of thrusting in the Sevier orogenic belt, Geologic Society of America Bulletin, 101, p. 864-875.
Jordan, T.E., 1981, Thrust loads and foreland basin evolution, Cretaceous, western United States, American Association of Petroleum Geologists Bulletin, 65, p. 2506-2520.
Katich, P.J., 1954, Cretaceous and early Tertiary Stratigraphy of central Utah with emphasis on the Wasatch Plateau area and adjacent canyonlands, central and south-central Utah: In 5th Annual Field Conference Guidebook, Intermountain Association of Petroleum Geologists, p. 42-54.
Kirkwood, S.G., 1976, Stratigraphy and petroleum potential of the Cedar Mountain and Dakota Formations, northwestern Colorado, [MS Thesis], Colorado School of Mines, 193 p.
Kirkland, J.I., 1992, Dinosaurs define a two-fold Lower Cretaceous zonation of the Cedar Mountain Formation, central Utah, Geologic Society of America Abstracts with Programs, 24, p. 22.
Kirkland, J.I., Cifelli, R.L., Britt, B.B., Burge, D.L., DeCourten, F.L., Eaton, J.G., and Parrish, J.M., 1999, Distribution of vertebrate faunas in the Cedar Mountain Formation, east-central Utah: In Vertebrate Paleontology in Utah (Ed. by Gillette, D.D.), Salt Lake City: Utah Geologic Survey, p. 201-217.
Kowallis, B.J., Christiansen, E.H., Deino, A.L., Peterson, F., Turner, C.E., Kunk, M.J., and Obradovich, J.D., 1998, The age of the Morrison Formation, Modern Geology, 22, p. 235-260.
Krainer, K., and Spotl, C., 1998, Abiogenic silica layers within a fluvio-lacustrine succession, Bolzano Volcanic Complex, northern Italy: a Permian analogue for Magadi-type cherts?, Sedimentology, 45, p. 489-506.
155
Lawton, T.F., 1994, Tectonic setting of Mesozoic sedimentary basins, Rocky Mountain region, United States: In Mesozoic Systems of the Rocky Mountain Region, USA, (Ed. by Caputo, M.V., Peterson, J.A., and Franczyk, K.J.), Denver: Rocky Mountain Section (SEPM), p. 1-25.
Leith, C.K., 1925, Silicification of erosion surfaces, Economic Geology, 20, p. 513-523.
Lohman, S.W., 1965, Geology and artesian water supply, Grand Junction Area Colorado, U.S. Geologic Survey Professional Paper 451, 149 p.
Lupton, C.T., 1914, Oil and gas near Green River, Grand County, Utah, U .S. Geological Survey Bulletin, 541, p. 135-140.
Machette, M.N., 1985, Calcic soils of the southwestern United States: In Soils and Quaternary geology of the southwestern United States (Ed. by Weide, D.L.), Geological Survey of America Special Paper 203, p. 1-21.
Manville, V. and White, J.D.L., 2003, Incipient granular mass flows at the base of sediment-laden floods, and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands, Sedimentary Geology, 155, p. 157-173.
McGookey, D.P., Haun, J.D., Hale, L.A., Goodell, H.G., McCubbin, D.G., Weimer, R.J., and Wulf, G.R., 1972, Cretaceous System: In Geologic Atlas of the Rocky Mountains Region (Ed. by Mallory, W.M.), Denver: Rocky Mountains Association Geologists, p. 190-228.
Miall, A.D., 1976, Paleocurrent and paleohydrological analysis of some vertical profiles through a Cretaceous braided stream deposit, Banks Island, Arctic Canada, Sedimentology, 23, p. 459-483.
Miall, A.D., 1977, A review of the braided-river depositional environment, Earth Science Reviews, 13, p. 1-62.
Miall, A.D., 1992, Alluvial deposits: In Facies models: Response to sea level change (Ed. by Walker, R.G., and James, N.P.), Geological Association of Canada, p. 119-142.
Milnes, A.R., and Thiry, M., 1992, Silcretes: In Weathering, soils and paleosols (Ed. Martini, I.P., and Chesworth, W.), Amsterdam: Elsevier Scientific Publishing, p. 349-377.
Mount, J., 1985, Mixed siliciclastic and carbonate sediments: a proposed first-order textural and compositional classification, Sedimentology, 32, p. 435-442.
156
Netterberg, F., and Caiger, J.H., 1983, A geotechnical classification of calcretes and other pedocretes: In Residual deposits: Surface related weathering processes and materials (Ed. by Wilson, R.C.L.), The Geological Society of London, Oxford: Blackwell Scientific Publishing, p. 235-243.
Paik, I.S., and Lee, Y.I., 1998, Desiccation cracks in vertic paleosols of the Cretaceous Hasandong Formation, Korea: genesis and paleoenvironmental implications, Sedimentary Geology, 119, p. 161-179.
Palmer, A.R., Geissman, J., 1999, 1999 Geologic Time Scale, The Geologic Society of America.
Paola, C., Heller, P.L., and Angevine, C.L., 1992, The large-scale dynamics of grain-size variation in alluvial basins, 1: Theory, Basin Research, 4, p. 73-90.
Parks, L., 2003, personal communication.
Pedley, M., Martin, J., Delgado, S., and Garcia Del Curas, M., 2003, Sedimentology of Quaternary perched springline and paludal tufas: criteria for recognition, with examples from Guadalajara Province, Spain, Sedimentology, 50, p. 23-44.
Peterson, F., 1988, Stratigraphy and nomenclature of Middle and Upper Jurassic rocks, western Colorado Plateau, Utah and Arizona, U.S. Geological Survey Bulletin, 1633-B, p. 17-56.
Peterson, F., 1994, Sand dunes, sabkhas, streams, and shallow streams: Jurassic paleogeography in the southern part of the Western Interior basin: In Mesozoic Systems of the Rocky Mountain Region, USA, (Ed. by Caputo, M.V., Peterson, J.A., and Franczyk, K.J.), Denver: Rocky Mountain Section (SEPM), p. 233-268.
Peterson, F., Ryder, R.T., and Law, B.E., 1980, Stratigraphy, sedimentology, and regional relationships of the Cretaceous system in the Henry Mountains region, Utah: In Henry Mountains Symposium (Ed. by Picard, M.D.), Utah Geological Association, p. 151-170.
Peterson, J.A., 1972, Jurassic System: In Geologic Atlas of the Rocky Mountains Region (Ed. by Mallory, W.M.), Denver: Rocky Mountains Association Geologists, p. 177-189.
Pietras, J.T, Carroll, A.R., and Rhodes, M.K, 2003, Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming, Journal of Paleolimnology, 30, p. 115-125.
157
Platt, N.H., and Wright, V.P., 1991, Lacustrine carbonates: facies models, facies distributions, and hydrocarbon aspects, International Association of Sedimentologists Special Publication, 13, p. 57-74.
Prothero, D.R., and Schwab, F., 1996, Sedimentary Geology: An introduction to sedimentary rocks and stratigraphy, New York: W. H. Freeman and Company, 575 p.
Retallack, G.J., 1988, Field recognition of paleosols, Geological Society of America Special Paper 216, 20 p.
Roca, X., 2003, Tectonic and sequence stratigraphic implications of the Morrison Formation-Buckhorn Conglomerate transition, Cedar Mountain, east-central, Utah, [MS Thesis], Athens: Ohio University, 222 p.
Rodine, J.D., and Johnson, A.M., 1976, The ability of debris, heavily freighted with coarse clastic material, to flow on gentle slopes, Sedimentology, 23, p. 213-234.
Royse, F., 1993, Case of the phantom foredeep: Early Cretaceous in west-central Utah, Geology, 21, p. 133-136.
Rust, B.R., and Koster, E.H., 1984, Coarse alluvial deposits: In Facies Models, Second Edition (Ed. By Walker, R.G.), Geological Association of Canada, p. 53-69.
Schwans, P., 1988, Depositional response of Pigeon Creek Formation, Utah, to initial fold-thrust belt deformation in a differentially subsiding foreland basin: In Interaction of the Rocky Mountain foreland and Cordilleran thrust belt (Ed. by Schmidt, C.J., and Perry, W.J.), Geological Society of America Memoir 171, p. 531-556.
Sheppard, R.A., and Gude, A.J., 1986, Magadi-type chert-A distinctive diagenetic variety from lacustrine deposits, U.S. Geological Survey Bulletin 1578, p. 335-345.
Smith, G.L., Dott, R.H., and Byers, C.W., 1997, Authigenic silica fabrics associated with Cambro-Ordovician unconformities in the upper Midwest, Geoscience Wisconsin, 16, p. 25-36.
Sprinkel, D.A., Weiss, M.P., and Fleming, R.W., 1992, Stratigraphic reinterpretation of a synorogenic unit of late Early Cretaceous age, Sevier orogenic belt, central Utah, Geological Society of America Abstracts with Programs, 24, p. 63.
Stokes, W.L., 1944, Morrison Formation and related deposits in and adjacent to the Colorado Plateau, Geological Society of America Bulletin, 55, p. 951-968.
158
Stokes, W.L., 1950, Pediments concept applied to Shinarump and similar conglomerates, American Association of Petroleum Geologists Bulletin, 61, p. 91-98.
Stokes, W.L., 1952, Lower Cretaceous in the Colorado Plateau, American Association of Petroleum Geologists Bulletin, 36, p. 1766-1776.
Stokes, W.L., 1960, Inferred Mesozoic history of east-central Nevada and vicinity, Intermontain Association of Petroleum Geologists 11th Annual Field Conference, p 117-121.
Stokes, W.L., 1987, Geology of Utah, Utah Museum of Natural History Occasional Paper Number 6, 280 p.
Summerfield, M.A., 1983a, Petrography and diagenesis of silcrete from the Kalahari Basin and Cape Costal Zone, Southern Africa, Journal of Sedimentary Petrology, 53, p. 895-909.
Summerfield, M.A., 1983b, Silcrete: In Chemical sediments and geomorphology: precipitates and residua in the near-surface environment (Ed. by Goudie A.S.), London: Academic Press, p. 59-91.
Surdam, R.C., Eugster, H.P., and Mariner, R.H., 1972, Magadi-type chert in Jurassic and Eocene to Pleistocene rocks, Wyoming, Geologic Society of America Bulletin, 83, p. 2261-2266.
Svendsen, J., Stollofen, H., Krapf, C.B.E., and Stanistreet, I.G., 2003, Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia, Sedimentary Geology, 160, p. 7-31.
Thiry, M., and Milnes, A.R., 1991, Pedogenic and groundwater silcretes at Stuart Opal Field, south Australia, Journal of Sedimentary Petrology, 61, p. 111-127.
Trask, P.D., 1959, Effect of grain size on strength of mixtures of clay, sand, and water, Geological Society of America Bulletin, 70, p. 569-579.
Trimble, D.L., and Doelling, H.H., 1978, Geology and uranium-vanadium deposits of the San Rafael River mining area, Emery County, Utah, Utah Geological and Mineralogical Survey Bulletin, 113, 112 p.
Tschudy, R.H., Tschudy, B.D., and Craig, L.C., 1984, Palynological evaluation of Cedar Mountain and Burro Canyon Formations, Colorado Plateau, U.S. Geological Survey Professional Paper, 1281, 24 p.
159
Tucker, M.E., 2001, Sedimentary Petrology: An introduction to the origin of sedimentary rocks (3rd ed.), Oxford: Blackwell Scientific Publishing, 262 p.
Turner, C.E., and Fisherman, N.S., 1991, Jurassic Lake T'oo'dichi': A large alkaline, saline lake, Morrison Formation, eastern Colorado Plateau, Geologic Society of America Bulletin, 103, p. 538-558.
Wiltschko D.V., and Dorr J.A., 1983, Timing of deformation in overthrust belt and foreland of Idaho, Wyoming, and Utah, American Association of Petroleum Geologists Bulletin, 67, p. 1304-1322.
Witkind, I.J., 1995, Geologic map of the Price 1o x 2o Quadrangle, Utah, U.S. Geological Survey.
Witkind, I.J., Standlee, L.A., and Maley, K.F., 1986, Age and correlation of Cretaceous rocks previously assigned to the Morrison (?) Formation, Sanpete- Sevier Valley area, central Utah, U. S. Geological Survey Bulletin, 1584, 9 p.
Wright, V.P., and Tucker, M.E., 1991, Calcretes: An introduction: In Calcretes (Ed. by Wright, V.P., and Tucker, M.E.), International Association of Sedimentologists Reprint Series 2, Oxford: Blackwell Scientific Publishing, p. 1-22.
Yingling, V.L., 1987, Timing of initiation of the Sevier orogeny: Morrison and Cedar Mountain Formations and Dakota Sandstone, east-central Utah, [MS Thesis], Laramie: University of Wyoming, 169 p.
Yingling, V.A., and Heller, P.L., 1992, Timing and record of foreland sedimentation during the initiation of the Sevier orogenic belt in central Utah, Basin Research, 4, p. 279-290.
Young, R.G., 1960, Dakota Group of Colorado Plateau, American Association of Petroleum Geologists Bulletin, 44, p. 156-194.
APPENDIX A:
MEASURED SECTIONS AND CROSS-SECTIONS
Map showing the location of measured sections and trend of cross-sections
A' Site M Site F CROSS-SECTION 1 A GPS Location: GPS Location: Site J 0519254 E 0512425 E 4346956 N GPS Location: 1. 4338510 N GRAIN SIZE 3 0517661 E 0.30 Km Km 0 8 cobble K GRAIN SIZE 1 m 4344995 N 1. pebble cobble granule pebble 0.36 Km GRAIN SIZE 0.58 Km sand granule cobble silt sand pebble v c mf v clay Site W silt granule Site K v c mf v 1.09 clay sand K GPS Location: silt GPS Location: m Type Section v c mf v clay Site N 0523167 E Site Y Site AA 0517676 E 4348663 N 1.0 Site I Site L 4.2 0.21 6 Km 4345292 N 1.27 m GPS Location: K 0.61 Km GPS Location: GRAIN SIZE 7 K m GPS Location: GRAIN SIZE Km 9 K 1.18 Km GPS Location: GPS Location: 0520506 E .3 cobble m 0526705 E 0515968 E Site A cobble 1 pebble Km Site B 0517174 E 0518373 E 4347440 N 00 4352610 N 4342048 N pebble granule 1. Site X 0.63 Km granule sand GRAIN SIZE GPS Location: 4344649 N 4346139 N GRAIN SIZE Site BB Site G GPS Location: sand Unit 8 Unit 8 Site V silt 2.24 Km GRAIN SIZE cobble 0516261 E GRAIN SIZE GRAIN SIZE cobble v c mf v GPS Location: 0516940 E silt 0.39 Km clay pebble 2 cobble v c mf v clay pebble GPS Location: GPS Location: .06 K pebble 4343131 N cobble cobble GPS Location: granule 0526937 E m 4344028 N pebble pebble granule 0512469 E 0512875 E granule sand 0521658 E sand 4352644 N sand GRAIN SIZE granule granule silt GRAIN SIZE silt Site U Site Z v c mf v 4338779 N 4339071 N silt cobble sand sand v c mf v 4347979 N clay GRAIN SIZE cobble clay v c mf v clay pebble silt silt GRAIN SIZE Site H pebble v c mf v v c mf v cobble GRAIN SIZE granule clay clay GRAIN SIZE GPS Location: GPS Location: granule pebble cobble cobble sand cobble sand 0521787 E 0525934 E granule pebble pebble GPS Location: silt pebble silt sand granule granule v c mf v clay granule 4348243 N 4352020 N 0514243 E v c mf v clay silt sand sand sand v c mf v clay silt silt 4340469 N v c mf v silt v c mf v clay clay Unit 7 v c mf v GRAIN SIZE clay Unit 6 Unit 7 GRAIN SIZE cobble pebble cobble granule pebble sand granule silt sand v c mf v clay silt v c mf v clay Unit 5 GRAIN SIZE Unit 4 cobble Unit 4 pebble Unit 5 granule Unit 4 ? Calrete Developed ? sand within Sandstone silt v c mf v clay Unit 3
Unit 2
Scale LEGEND
10 Conglomerate Carbonte Nodules in Mudstone
Conglomerate with Abundant Limestone Intraformational Clasts Cross-bedded Sandstone Limestone Breccia Unit 1 Sandstone Calcrete 5 Siltstone Chert (Jasper) Low
Mudstone Silica Concentration Desiccation Crack Pebbly Mudstone High
0 Meters Cleveland-Lloyd Dinosaur Quarry B' 9.67 Km Site T B Site A GPS Location: CROSS-SECTION 2 GPS Location: 0525290 E 1.75 Km Site Q 0.18 Site V 0516261 E 4339264 N Km GPS Location: 4343131 N GRAIN SIZE 0.15 Km GPS Location: cobble 0525622 E 0521658 E GRAIN SIZE pebble 4340651 N Site R Km cobble granule 9 4347979 N pebble sand GRAIN SIZE 1.06 Km 7.7 silt Site P cobble GPS Location: GRAIN SIZE granule v c mf v sand clay pebble 0526758 E cobble silt granule pebble v c mf v GPS Location: 4340741 N clay sand granule 0526546 E silt sand v c mf v GRAIN SIZE clay Site S silt 4340519 N cobble v c mf v pebble clay GRAIN SIZE granule GPS Location: cobble sand pebble silt 0526327 E v c mf v granule Unit 7 clay 4341689 N sand silt GRAIN SIZE v c mf v Unit 5 clay cobble Unit 6 pebble granule Unit 6 sand Unit 5 silt Unit 4 v c mf v clay Unit 4 Unit 3 Unit 3
Scale
10
5
0 Meters Unit 1 APPENDIX B:
PALEOCURRENT MEASUREMENTS Unit 1 (Buckhorn)
SITE A SITE J Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 18 26 S 108 22 20 S 112 121 13 N 31 36 8 S 126 110 9 S 200 90 18 S 180 75 13 S 165 41 23 S 131 49 17 S 139 152 24 S 242 85 29 S 175 154 17 S 244 88 25 S 178 129 29 S 219 25 10 S 115 115 19 S 205 135 15 N 45 70 18 S 160 55 22 N 325 85 14 S 175 73 21 N 343 120 17 S 210 140 32 N 50 75 20 S 165 135 13 N 45 21 20 S 111 125 20 N 35 157 23 N 67 110 8 N 20 155 18 N 65 113 12 N 23 122 20 N 32
SITE B SITE P Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 64 20 S 154 86 17 S 176 140 14 S 230 50 25 S 140 SITE S 23 26 S 113 Dip Flow 65 5 N 335 Strike Dip Direction Direction 95 6 S 185 105 20 S 195 35 15 S 125 14 14 N 284 160 18 N 70 27 35 S 117 0525522 E / 4341672 N 87 23 S 177 Dip Flow 110 23 N 20 Strike Dip Direction Direction 51 12 S 141 43 34 S 133 123 13 N 33 150 20 N 60 157 23 N 67 142 18 N 52 48 25 S 138 75 15 N 345 5 23 S 95 50 18 N 320 5 15 N 275 74 14 N 344 5 20 N 275 85 30 N 355 160 15 S 250 105 27 N 15 85 15 N 355 46 25 N 316 146 15 S 236 96 8 N 6 75 23 N 345 105 21 N 15 90 10 N 0 102 19 N 12 96 18 N 6 SITE D 136 20 N 46 Dip Flow 105 17 N 15 Strike Dip Direction Direction 144 16 N 54 41 11 N 311 157 10 N 67 60 25 S 150 87 21 N 357 80 17 N 350 125 10 N 35 111 23 N 21 Unit 1 (Buckhorn)
0525954 E / 4341807 N 0515246 E / 4341275 N Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 3 18 N 273 82 15 N 352 75 22 N 345 44 26 N 314 126 9 N 36 85 22 N 355 162 21 N 72 119 13 N 29 136 20 N 46 71 12 N 341 150 12 N 60 128 17 N 38 79 23 N 349 104 16 N 14 96 21 N 6 80 24 N 350 138 29 N 48 77 21 N 347 121 17 N 31 162 24 N 72 133 19 N 43 167 27 N 77 107 21 N 17 141 25 N 51 170 18 N 80 131 29 N 41 69 24 N 339 135 15 N 45 1 21 S 91 153 21 N 63 72 13 N 342 162 29 N 72 19 17 S 109 158 34 N 68 150 17 N 60 14 22 S 104 127 16 N 37 144 14 N 54 147 19 N 57 155 27 N 65 57 15 N 327 31 17 S 121 25 15 S 115 104 15 N 14 103 17 N 13 156 26 N 66 35 13 S 125 165 23 N 75 9 22 S 99 143 24 N 53 14 33 S 104 142 35 N 52 99 29 N 9 141 28 N 51 92 18 N 2 146 24 N 56 0521409 E / 4345116 N 174 15 N 84 Dip Flow Strike Dip Direction Direction 0525216 E / 4344154 N 118 27 N 28 Dip Flow 10 20 S 100 Strike Dip Direction Direction 50 25 N 320 131 32 N 41 125 20 N 35 123 21 N 33 0 7 E 90 165 23 N 75 5 15 S 95 65 15 S 155 55 17 S 145 20 25 S 110 155 16 S 245 161 20 N 71 130 18 N 40 153 27 N 63 165 25 N 75 10 14 S 100 10 15 N 280 45 19 S 135 160 23 N 70 168 22 N 78 157 24 N 67 4 32 S 94 143 15 N 53 73 8 N 343 102 19 N 12 53 21 N 323 34 25 N 304 145 33 N 55 121 36 N 31 Unit 1 (Buckhorn) Unit 2 (Buckhorn-equivalent)
0521161 E / 4346961 N SITE W Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 165 25 N 75 155 28 N 65 165 24 N 75 17 30 S 107 140 21 N 50 140 26 N 50 171 16 N 81 134 20 N 44 167 18 N 77 19 24 N 289 40 27 S 130 75 11 N 345 0 32 E 90 120 32 N 30 15 36 S 105 27 15 S 117 15 25 S 105 71 40 S 161 151 21 N 61 55 32 S 145 97 17 S 187 23 19 S 113 29 24 S 119 15 25 S 105 142 17 N 52 74 15 S 164 37 18 S 127 126S 91 31 16 S 121
0526543 E / 4339717 N Dip Flow Strike Dip Direction Direction 57 13 S 147 73 18 S 163 89 33 S 179 66 18 S 156 81 15 S 171 85 22 S 175 130 24 S 220 120 9 S 210 52 13 S 142 102 12 S 192 712S 97 120 15 S 210
0516805 E / 4344063 N Dip Flow Strike Dip Direction Direction 59 6 N 329 107 24 N 17 22 9 N 292 86 21 N 356 5 9 N 275 33 11 N 303 35 19 N 305 42 3 N 312 28 18 N 298 49 20 N 319 12 23 N 282 168 13 S 258 Unit 5 (White Sandstone)
SITE A SITE AA (Continued) Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 59 16 S 149 168 14 S 258 80 26 S 170 165 10 N 75 122 17 N 32 67 24 S 157 0 27 W 270 180 23 E 90 80 25 S 170 1 12 S 91 97 6 S 187 69 10 S 159 119 14 S 209 10 11 S 100 68 26 N 338 0 15 W 270 140 29 S 230 171 21 N 81 74 15 S 164 160 25 N 70 61 10 S 151 162 8 S 252 75 24 S 165 140 15 S 230 0 9 W 270 146 26 N 56 41 17 S 131 17 15 S 107 SITE B 170 28 N 80 Dip Flow 165 20 N 75 Strike Dip Direction Direction 84 25 S 174 55 22 S 145 58 28 S 148 106 6 S 196 41 24 S 131 25 17 N 295 76 25 S 166 116 10 S 206 138 15 S 228 90 10 S 180 153 22 S 243 SITE AA 64 21 S 154 Dip Flow 99 25 S 189 Strike Dip Direction Direction 44 39 S 134 150 28 N 60 144 32 S 234 100 8 N 10 22 24 S 112 155 25 N 65 29 11 S 119 105 10 S 195 77 17 S 167 105 18 S 195 65 28 S 155 154 17 N 64 130 30 S 220 175 20 N 85 117 16 S 207 22 25 S 112 165 35 S 255 55 18 N 325 127 24 S 217 117 14 S 207 133 77 S 223 103 7 N 13 169 26 S 259 2 27 S 92 142 27 S 232 172 34 N 82 144 24 S 234 165 25 S 255 15 23 S 105 48 30 N 318 146 38 S 236 115 24 S 205 143 15 S 233 17 18 N 287 125 34 S 215 127 28 S 217 119 14 S 209 114 22 S 204 91 32 S 181 158 19 N 68 127 26 S 217 95 9 N 5 70 29 S 160 117 19 S 207 53 31 S 143 67 30 N 337 34 5 N 304 129 23 N 39 10 15 N 280 139 15 N 49 175 33 N 85 8 40 S 98 105 21 S 195 15 34 S 105 35 14 S 125 Unit 5 (White Sandstone)
SITE B (Continued) SITE J Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 120 10 S 210 55 15 S 145 18 29 N 288 150 10 N 60 99 22 S 189 142 18 N 52 5 14 S 95 38 22 N 308 23 25 S 113 49 18 S 139 140 15 S 230 75 14 S 165 34 17 S 124 SITE I 45 18 S 135 Dip Flow 18 20 S 108 Strike Dip Direction Direction 78 23 N 348 127 14 N 37 25 11 N 295 10 9 S 100 65 12 N 335 140 21 N 50 55 18 N 325 0 27 W 270 176 15 S 266 132 32 S 222 127 16 N 37 65 21 N 335 37 22 N 307 145 18 S 235 147 12 N 57 135 21 S 225 151 26 N 61 120 28 S 210 104 14 N 14 55 19 S 145 61 11 N 331 100 14 S 190 87 25 S 177 159 20 N 69 173 32 N 83 121 6 N 31 160 5 N 70 72 26 S 162 132 18 S 222 122 14 S 212 108 27 N 18 170 19 S 260 95 27 N 5 102 24 N 12 45 19 S 135 159 22 S 249 138 34 N 48 60 16 S 150 159 21 N 69 25 26 S 115 55 23 N 325 150 18 N 60 146 15 N 56 12 32 S 102 178 20 N 88 118 13 S 208 55 10 N 325 135 18 N 45 59 20 N 329 175 15 N 85 25 19 N 295 135 27 N 45 28 20 S 118 177 25 N 87 71 5 N 341 80 17 N 350 124 32 N 34 125 19 N 35 103 27 S 193 15 25 N 285 82 21 N 352 174 15 S 264 69 17 N 339 160 27 S 250 77 15 S 347 107 28 S 197 107 12 N 17 169 32 S 259 172 12 N 82 4 38 S 94 126 18 N 36 115 31 N 25 71 11 N 341 109 16 N 19 91 27 N 1 5 26 S 95 25 27 N 295 85 7 S 175 38 30 N 308 124 30 S 214 153 26 N 63 62 28 N 332 134 35 S 224 Unit 5 (White Sandstone)
SITE J (Continued) SITE Q Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 83 15 N 353 31 18 N 301 35 40 N 305 147 30 N 57 102 22 N 12 75 19 N 345 87 9 N 357 173 29 N 83 128 23 N 38 48 27 N 318 34 7 S 124 41 25 N 311 21 22 S 111 84 14 N 354 123 20 S 213 132 20 N 42 119 19 N 29 SITE K 16 12 N 286 Dip Flow 74 19 S 164 Strike Dip Direction Direction 110 26 N 20 167 17 N 77 130 15 N 40 155 13 N 65 111 23 N 21 135 11 N 45 15 24 S 105 SITE T 105 8 N 15 Dip Flow Strike Dip Direction Direction SITE M 150 50 S 240 Dip Flow 26 24 S 116 Strike Dip Direction Direction 55 14 S 145 86 20 S 176 9 5 S 99 75 17 S 165 61 20 S 151 76 18 S 166 47 18 S 137 70 20 N 340 144 22 S 234 170 33 S 260 36 19 S 126 22 20 S 112 SITE N 105 26 N 15 Dip Flow 9 17 S 99 Strike Dip Direction Direction 10 30 S 100 20 8 S 110 20 23 S 110 68 20 S 158 10 17 S 100 175 25 N 85 83 26 S 173 82 13 S 172 57 25 S 147 62 12 S 152 65 24 S 155 15 17 S 105 2 26 S 92 39 20 N 309 50 40 S 140 65 14 N 335 91 20 N 1 20 25 N 290 28 24 N 298
SITE P Dip Flow Strike Dip Direction Direction 21 15 S 111 82 19 N 352 87 21 N 357 112 15 N 22 Unit 5 (White Sandstone) Unit 7 (Red Conglomerate)
0516398 E / 4343602 N SITE AA Dip Flow Dip Flow Strike Dip Direction Direction Strike Dip Direction Direction 160 24 S 250 27 24 S 117 150 8 S 240 35 20 S 125 105 13 S 195 50 10 S 140 86 17 S 176 50 15 S 140 167 23 S 257 115 20 N 25 160 21 S 250 30 22 S 120 125 17 S 215 73 16 S 163 160 31 S 250 36 24 S 126 109 19 S 199 141 19 N 51 125 15 S 215 172 20 N 82 151 20 S 241 170 33 S 260 35 25 S 125 119 10 S 209 155 23 S 245 12 13 S 102 SITE B 44 7 N 314 Dip Flow 10 29 S 100 Strike Dip Direction Direction 153 11 S 243 31 15 S 121 140 14 S 230 178 21 N 88 179 19 N 89 26 30 N 296 130 21 S 220 15 34 N 285 151 19 S 241 45 20 S 135 155 16 S 245 178 50 S 268 143 19 S 233 48 23 S 138 98 27 S 188 5 24 N 275 57 3 S 147 10 36 N 280 145 13 S 235 135 24 S 225 130 20 S 220 0 21 W 270 90 14 S 180 120 21 S 210 33 13 S 123 0 31 E 90 105 16 S 195 3 36 N 273 69 27 S 159 SITE I 80 17 S 170 Dip Flow 149 17 S 239 Strike Dip Direction Direction 108 20 S 198 122 23 N 32 87 18 S 177 152 23 N 62 42 17 S 132 42 13 N 312 22 14 S 112 95 18 S 185 6 14 S 96 32 5 N 302 148 27 N 58 SITE J 163 25 N 73 Dip Flow 15 27 S 105 Strike Dip Direction Direction 41 15 S 131 8 23 N 278 32 25 S 122 130 23 N 40 43 17 S 133 92 36 N 2 85 32 S 175 0516964 E / 4344500 N 130 19 S 220 Dip Flow 139 14 S 229 Strike Dip Direction Direction 175 17 S 265 147 30 S 237 45 11 N 315 132 26 S 222 61 23 S 151 27 16 S 117 149 14 S 239 101 38 S 191 26 15 S 116 50 17 S 140 95 16 S 185
APPENDIX C:
POINT COUNT AND PEBBLE COUNT DATA
Point Count Data UNIT 1 UNIT 5 UNIT 7
Sample 6 Sample 54 Sample 75 Sample 86 Sample 52 QUARTZ Monocrystalline Quartz 261 202 175 132 93
FELDSPAR Orthoclase 0 10 4 0 5 Plagioclase 0 0 0 0 0
LITHICS Polycrystalline Quartz 26 9 8 6 13 Chert 35 35 41 42 48 Carbonate 0 16 27 18 90
OTHER 0 1 1 0 1 CEMENT 78 137 144 202 150 TOTAL 400 410 400 400 400
Pebble Count Data UNIT 1 UNIT 2 UNIT 7
Site A Site B Site U Site W Site X Site K Chert 352 361 410 402 396 376
Quartzite 46 38 7 7 9 10
Sandstone 0 0 2 0 0 7
Carbonate 0 0 0 0 0 20
Total 398 399 419 409 405 413