Stratigraphic Relationships Across the Triassic-Jurassic Boundary, Northwest Bighorn Basin; Park County, Wyoming

Home , Chugwater Formation, Curtis Formation, Ellis Group, Gypsum Springs Formation, Jelm Formation, Nugget Sandstone

STRATIGRAPHIC RELATIONSHIPS ACROSS THE TRIASSIC-JURASSIC BOUNDARY, NORTHWEST BIGHORN BASIN; PARK COUNTY, WYOMING

A Thesis By

Jonathan Logan Woods

Bachelor of Science, Oklahoma State University, 2016

Submitted to the Department of Geology and the faculty of the Graduate School of Wichita State University in partial fulfillment of the requirements for the degree of Master of Science

December 2018

STRATIGRAPHIC RELATIONSHIPS ACROSS THE TRIASSIC-JURASSIC BOUNDARY, NORTHWEST BIGHORN BASIN; PARK COUNTY, WYOMING

The following faculty members have examined the final copy of this thesis for form and content, and recommend that is be accepted in partial fulfillment of the requirements for the degree of Master of Science with a major in Earth, Environmental, and Physical Sciences.

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William Parcell, Committee Chair

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Bill Bischoff, Committee Member

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Collette Burke, Committee Member

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Donald Blakeslee, Committee Member

iii ACKNOWLEDGEMENTS

I would like to thank my advisor, William Parcell, for his invaluable guidance and support in both the field and laboratory stages of this project. I thank the members of my committee and the faculty of the Wichita State Department of Geology for the beneficial discussions and ideas along the way. I thank the Wichita State Department of Geology and the Sedimentary

Basin Analysis and Modeling Lab for accesses to software, data, equipment, and facilities utilized throughout this project. I would also like to thank Robert Baker and David Bruce for their assistance with field work.

iv ABSTRACT

Early Mesozoic stratigraphy in the foreland Rocky Mountains of Wyoming records deposition in a variety of depositional environments and tectonic settings which have led to contrasting interpretations of the stratal relationships of these units. In particular, the boundary between the Triassic and Jurassic represents a complex relationship consisting of multiple unconformities, formations, and lithologies. Since the 1970s, regional studies have relied on laterally extensive erosional surfaces or unconformities across the Western Interior as time constraining correlation tools. However, since the formal establishment of these unconformities, the recognition and proper placement of these surfaces within local areas has come under increasing scrutiny.

This study was undertaken to demonstrate the complexities of the Triassic-Jurassic boundary and to clarify simplifications and generalizations made during previous research. This study will allow future researchers to take into account the complexities of the Triassic-Jurassic boundary and avoid previous generalizations.

v ABBREVIATIONS

MD = Measured Depth

TVT = True Vertical Thickness

TST = True Stratigraphic Thickness

XRD = X-Ray Diffraction

WGS = Wyoming Geological Survey

SP = Spontaneous potential (well log curve)

API = American Petroleum Institute (unit of subsurface gamma ray measurement)

TCGR = Total count gamma ray (unit of surficial gamma ray measurement)

vi TABLE OF CONTENTS

Chapter Page

1. INTRODUCTION……………………………………………………………………………………………………………………1

1.1. Previous Studies……………………………………………………………………………………………………………1

2. GEOLOGIC SETTING & NOMENCLATURE.….………………………………………………………………………….7

2.1. Current Setting of the Bighorn Basin…………………………………………….……………………………….7 2.2. Geologic Setting of the Triassic…………………………….……………………………………………………….7 2.2.1. Triassic Nomenclature……….……………………………………………………………………………..10 2.3. Geologic Setting of the Jurassic………………………….……………………………………………………….11 2.3.1. Jurassic Nomenclature…………………………………………………….……………………………….12

3. GENERAL STRATIGRAPHY……………………………………….………………………………………………………….16

3.1. Red Peak Formation……………………..…………………………………………………………………………….16 3.2. Alcova Formation……….……………………………………………………………………………………………….19 3.3. Crow Mountain Formation………………………………………………………………………………………….20 3.4. Unnamed Red Beds…………………………………………………………………………………………………….22 3.5. Popo Agie Formation…………………………………………………………………………………………………..22 3.6. Gypsum Spring Formation………….……………………………………………………………………………….24

4. PURPOSE OF STUDY & METHODS….…………………………………………………………………………………..27

4.1. Purpose of Study…………………………………………………………………………………………………………27 4.2. Methods…....……………………………………………………………………………………………………………….27

5. LITHOFACIES DISTRIBUTION………………………..……………………………………………………………………..34

5.1. Lithofacies I – Red Peak siltstones………..….………….………………………………………………………34 5.2. Lithofacies II – Red Peak silty mudstones…..….……….……………………………………………………39 5.3. Lithofacies III – Red Peak sandstones……..….…………….…..…………………………………………….42 5.4. Lithofacies IV – Siltstone conglomerate….…….…………………………………………………………….47 5.5. Lithofacies V – Siltstone breccia….………………….…….…………………………………………………….49 5.6. Lithofacies VI – Basal gypsum…………………….……………………………………………………………….51

6. REGIONAL STRATIGRAPHIC CORRELATIONS..……………………………………………………………………..56

6.1. Red Peak Parasequences….…………………………………………………..…………………………………….57 6.1.1. Red Peak Parasequence 1…………………………………………………………………………………58 6.1.2. Red Peak Parasequence 2-4…………..…………………………………………………………………59

vii TABLE OF CONTENTS (continued)

Chapter Page

6.1.3. Red Peak Parasequence 5…………………………………………………………………………………60 6.1.4. Red Peak Parasequence 6…………………………………………………………………………………61 6.1.5. Red Peak Parasequence 7…………………………………………………………………………………62 6.2. Alcova Formation….…………………………………………………………………………………………………….63 6.3. Crow Mountain formation….……………………………..……………………………………………………….63 6.4. Gypsum Spring – Basal Gypsum….…………………..………………………………………………………….64

7. STRATIGRAPHIC MODEL….…………………………………………………..…………………………………………….67

8. CONCLUSIONS….……………………………………………………………………….……………………………………….79

9. FUTURE RESEARCH….…………………………………………..…………………………………………………………….81

REFERENCES….…………………………………………………………………….…………………………………………….83

APPENDICES….………………………………………………………….………………………………………………………..88 A: List of Wells….……………………………….……………………..………………………………………………….89 B: Type Log….………………………………….………………………………………..………………………………….96 C: Sample Descriptions….………………….………………………………………………………………………….98 D: Measured Sections….………………….…………………………………………….……………………………104

1. Clark’s Fork Canyon…………………………………………………………………………….105 2. Hogan Reservoir………………………………………………………………………………….109 3. Chief Joseph Highway “Dead Indian Hill”…………………………………………….111 4. South Cody “Raven Ridge”.………………………………………………………………….113

viii LIST OF FIGURES

Figure Page

1. Base map of study area……………………………………………………………………………………………………..2

2. Stratigraphic nomenclature of Triassic rocks……………………………………………………………………..5

3. Stratigraphic nomenclature of Middle Jurassic rocks………………………………………………….……..6

4. Generalized global plate distribution of the Triassic………………………………………………………….8

5. Triassic paleogeographic reconstruction of Western U.S. …………………………………………………9

6. Generalized global plate distribution of the Jurassic……………………………………..………………..13

7. Jurassic paleogeographic reconstruction of Western U.S. ………………………………………………14

8. Stratigraphic diagram of Triassic rocks in Wyoming and Idaho………………………………………..17

9. Map of study area displaying location of wells and outcrops…………………..………………………28

10. Annotated photograph of Clark’s Fork Canyon outcrop……………..……………………………………35

11. Thin section photograph of Lithofacies I………………………………………………………………………….37

12. Photograph of ripple marks in Lithofacies I……………………………………………………………………..38

13. Photograph of Lithofacies II in outcrop……………………………………………………………………………41

14. Photograph of anomalous coloration in Lithofacies III…………………………………………………….43

15. Thin section photograph of Lithofacies III………………………………………………………………………..45

16. Photograph of crossbedding in Lithofacies III………………………………………………………………….46

17. Thin section photograph of Lithofacies IV……………………………………………………………………….48

18. Photograph of Lithofacies V in outcrop……………………………………………………………………………50

19. Photograph of Lithofacies VI in outcrop…………………………………………………………………………..52

20. Thin section photograph of dolomitic interval of Lithofacies VI………………….……………………53

ix LIST OF FIGURES (continued)

Figure Page

21. Photograph near zone of truncation of Lithofacies VI …………………………………………………….65

22. Stratigraphic Model Stage 1………………………………………..…………………………………………………..71

23. Stratigraphic Model Stage 2………………………………………..…………………………………………………..72

24. Stratigraphic Model Stage 3………………………………………..…………………………………………………..73

25. Stratigraphic Model Stage 4………………………………………..…………………………………………………..74

26. Stratigraphic Model Stage 5………………………………………..…………………………………………………..75

27. Stratigraphic Model Stage 6………………………………………..…………………………………………………..76

28. Stratigraphic Model Stage 7………………………………………..…………………………………………………..77

29. Stratigraphic Model Stage 8………………………………………..…………………………………………………..78

x LIST OF PLATES

Plate Page

1. Chugwater Group isopach….…………………………………….………………………………………………………..115

2. Red Peak parasequence 1 isopach….………………………..………………………………………………………..116

3. Red Peak parasequence 2 isopach….………………………………..………………………………………………..117

4. Red Peak parasequence 3 isopach….………………………………..………………………………………………..118

5. Red Peak parasequence 4 isopach….…………………………………..……………………………………………..119

6. Red Peak parasequence 5 isopach….………………………………………..………………………………………..120

7. Red Peak parasequence 6 isopach….………………………………………..………………………………………..121

8. Red Peak parasequence 7 isopach….………………………………………..………………………………………..122

9. Gypsum Spring basal gypsum isopach…..……………………………………………….…………………………..123

10. S-N cross-section….………………………………………………………………………..………………………………..124 10.1 S-N cross-section (highlighting left (south) side)………………………………..……………….125 10.2 S-N cross-section (highlighting right (north) side)……….………………………………………126

CHAPTER ONE

INTRODUCTION

Stratigraphic correlation of the early Mesozoic in the foreland Rocky Mountains of

Wyoming has been hindered by a myriad of geologic discrepancies, including: lacking regional fossil control, incomplete type sections, and correlations based upon variable nomenclature.

These discrepancies have inhibited refined stratigraphic correlations and sound reconstruction of the regions Triassic and Jurassic paleogeography. In particular, the Triassic-Jurassic boundary represents a unique correlative problem due to varying patterns of erosion and deposition that exist across this boundary. The area that is now encompassed by the Bighorn Basin in Wyoming

(Fig. 1) represents particularly interesting stratigraphic relationships between the Triassic and

Jurassic strata. The Bighorn Basin has a successful history of hydrocarbon production and has been the focus of many structural, sedimentary, and stratigraphic investigations (Downs, 1952;

High and Picard, 1965, 1967, 1969; Kvale, et al., 2001; Lovelace and Lovelace, 2012; Parcell and

Williams, 2005; Pipiringos and O’Sullivan, 1978; Schmude, 2000; Thomas, 1965). The purpose of this study is to investigate lithologic variability, further clarify stratigraphic correlations, and to characterize the extent of Jurassic erosional events across the Triassic-Jurassic boundary within portions of Park County, Wyoming.

1.1 Previous Studies

Previous stratigraphic investigations have recognized discrepancies at the Triassic-

Jurassic boundary within the Bighorn Basin (Pipiringos, 1968; Pipiringos and O’Sullivan, 1978;

Schmude, 2000; Kvale et al., 2001; Parcell and Williams, 2005). The variable interpretations of the Triassic-Jurassic boundary include disagreements on the stratal relationships of underlying

Figure 1: Base map displaying the location of the Bighorn Basin and the units underlying the J-2 unconformity. Modified from Pipiringos and O’Sullivan (1978).

and overlying formations, lithofacies, as well as the nature of the boundary itself. These discrepancies are exacerbated by improper placement of the Triassic and Jurassic unconformities. Pipiringos & O’Sullivan (1978) formally outlined laterally extensive Triassic-

Jurassic unconformities across much of the Western Interior of the United States. The unconformities are named and formally recognized as the; Tr-1, Tr-2, Tr-3, J-0, J-1, J-2, J-3, & J-

4. These unconformities record episodes of uplift and subsequent erosion or non-deposition

(Pipiringos & O’Sullivan, 1978). As such, the study recognizes these erosional surfaces as representative and reliable correlation tools. However, since publication, the recognition and placement of these unconformities within local areas has come under more scrutiny. These localities include the Four Corners and Colorado Plateau region of the southwestern United

States (Lucas, et al., 1997), and portions of Wyoming and Montana (Schmude, 2000; Kvale et al., 2001; Parcell and Williams, 2005).

Pipiringos & O’Sullivan (1978) recognize the J-0 – J-2 unconformities as representing the boundary between the Triassic Chugwater Group and overlying Middle Jurassic strata. This boundary has been interpreted in several different fashions across northern Wyoming. While

Pipiringos & O’Sullivan (1978) claim that within Park County, Wyoming, the top of the Early

Triassic Red Peak Formation (Fig. 2), is beveled by the J-2 unconformity, and overlain by the

Middle Jurassic Piper Formation. Picard (1978) also recognizes that in eastern Park County and throughout Big Horn County, Wyoming, the Chugwater Group is truncated below the Middle

Triassic Alcova Formation (Fig. 2). However, Picard (1978) does not address the overlying

Jurassic units, nor does he discuss the unconformity which represents the group’s upper boundary. A contrasting interpretation by Schmude (2000), recognizes the Middle Jurassic

Gypsum Spring Formation unconformably overlying the Late Triassic Popo Agie Formation throughout the Bighorn Basin. Schmude claims that this boundary represents the J-1 unconformity. Kvale (2001) also found the Triassic-Jurassic boundary to be represented by the

J-1 unconformity however, Kvale does not discuss the underlying Triassic formations. Cavaroc and Flores (1990) suggest that the entirety of the Triassic Chugwater Group is present throughout large portions of Wyoming. Contrary to other studies, Cavaroc and Flores (1990) place the Early Jurassic Nugget Sandstone (Fig. 3) unconformably above the Chugwater Group and claim that the base of the Nugget Sandstone represents the J-0 unconformity. However, at this point, the Nugget Sandstone has not been formally identified in the northern portions of the Bighorn Basin. This is either due to non-deposition or complete removal by later Jurassic erosional episodes.

Figure 2: Stratigraphic nomenclature of Triassic units within the Bighorn Basin. Modified from Cavaroc and Flores (1990).

Figure 3: Stratigraphic nomenclature of Jurassic units within the Bighorn Basin. Shaded region represents non-deposition within study area. Modified from Schmude (2000).

CHAPTER TWO

GEOLOGIC SETTING & NOMENCLATURE

2.1 Current Setting of the Bighorn Basin

This study is located within the interior boundaries of the Bighorn Basin, a Mesozoic structural basin which was formed in response to the Laramide Orogeny of the Cretaceous to early Cenozoic (Thomas, 1965; Dickinson, 2003). The Bighorn Basin (Fig. 1) is located in northwest Wyoming and stretches into portions of southwest Montana. The approximately

8,500 square mile structural basin is comprised of approximately 20,000 feet of sedimentary strata, which represents Cambrian through Miocene deposition. As aforementioned, the

Bighorn Basin began to develop into its current state during the Late Cretaceous as a response to the Laramide Orogeny. This tectonic event is responsible for the uplift of the bounding

Beartooth Mountains to the Northwest, the Bighorn Mountains to the East, and the Owl Creek and Bridger Mountains to the South. The Western boundary of the Bighorn Basin has been covered by the late Eocene Absaroka volcanics (Thomas, 1965; Dickinson, 2003). Exposure of vast amounts of the sedimentary strata within the Bighorn Basin has occurred within the last

100 million years. The Mesozoic strata are exposed along the interior flanks of the basin, often within monoclinal folds that were formed in response to compressive stresses of the Laramide

Orogeny.

2.2 Geologic Setting of the Triassic

Mesozoic strata of the Western Interior record sedimentation in a wide variety of depositional environments and tectonic settings. In the Early Triassic, the supercontinent

Pangea likely reached its largest size with landmass nearly equally distributed about the

Figure 4: Generalized global distribution of landmass during the Triassic. Red box indicates study area. Modified from Blakey (2003)

Figure 5: Paleogeography of the Western United States during the Early Triassic. Red box indicates study area. Modified from Blakey (2003)

equator (Muller, et. al., 2016) (Fig.4). It is postulated by Speed (1979) that during the late

Permian to Early Triassic that subduction was occurring along the southwestern margin of the modern United States and resulted in the accretion of island arc terranes to the North

American craton during Sonoma Orogeny. However, it is believed that throughout the Triassic, the area encompassing the Bighorn Basin region was only subjected to a degree of slow regional subsidence, increasing accommodation space and allowing the deposition of vast amounts of sediment (Johnson 1993). During this time the region was positioned along the western coast of Pangea, near the cratonic margin, between the 15° North to 23° North of the equator (Dubiel, 1994) (Fig. 5). During the Triassic this was a region of low topographic relief, residing at or just below sea level, gently dipping towards the Cordilleran “miogeocline” to the west and sourcing sediment from the stable craton to the east. Dubiel (1994) claims that the paleoclimate of the area was relatively humid with seasonal drying and monsoonal events, having a similar climate to modern day southwest India. However, it is proposed that no modern analog of comparable scale exists for this vast depositional plain (Johnson 1993).

2.2.1 Triassic Nomenclature

Within the Bighorn Basin the Triassic strata are comprised (in ascending order) of the

Permian-Triassic Goose Egg, or Dinwoody Formation, and the Chugwater Group (Fig. 2). The

Chugwater Group remains in contact with Middle Jurassic strata across the entire Bighorn

Basin, thus the earliest Triassic formations (Dinwoody/Goose Egg) will not be discussed in depth. Knight (1902) was the first to formally identify the Triassic red beds in the state of

Wyoming, when they were referred to as the “Laramie Plains red beds”. Soon after, Darton

(1904) formally named these strata the Chugwater Formation, which included all of the rock

units between the Pennsylvanian Tensleep Formation and the Jurassic Sundance Formation.

The Chugwater Formation was subdivided by Love (1939) into the Red Peak, Crow Mountain,

Popo Agie, and Gypsum Spring Members. However, the Gypsum Spring Member was later shown to be Middle Jurassic in age by Love, et al., (1945) based on the presence of Middle

Jurassic marine fauna. The Chugwater Formation was finally raised to group status and its members to formation status by Branson and Branson (1941). In modern usage, the Chugwater

Group is divided into five distinct formations/units throughout Wyoming (in ascending order); the Red Peak Formation, the Alcova Formation, the Crow Mountain Formation, Jelm Formation or “Unnamed” red beds, and the Popo Agie Formation (Fig. 2) (Picard, 1978; Cavaroc and

Flores, 1990). While Branson and Branson (1941) defined the Alcova Limestone as a unit within the Crow Mountain Formation, others (Cavaroc and Flores, 1990) define the Alcova as an individual formation due to its unique lithology as the only marine carbonate deposit within the

Chugwater Group. This nomenclature divides the four units of the Crow Mountain Formation into two separate formations. A basal sandstone unit, known as the “Sub-Alcova Sand”, and the overlying Alcova Limestone unit comprise the Alcova Formation, while the two upper

Sandstone units define the Crow Mountain Formation. For the sake of clarity in well log correlation throughout this study, the Alcova Formation will be referred to as a distinct unit in the fashion described by Cavaroc and Flores (1990).

2.3 Geologic Setting of the Jurassic

In contrast to the quiescent tectonic setting of the region during the Triassic, the Early

Jurassic time was marked by the earliest stages of Pangea’s breakup. This was caused by active spreading of the ancestral Atlantic Ocean, separating Africa and Europe from what is now the

North American Plate (Muller, et al., 2016) (Fig. 6). In response to the spreading occurring to the east, a complex active margin began to develop along the western edge of the North

American plate as it began to converge with ancestral Pacific plates. At this time the North

American Cordillera began to separate the Western Interior from the active plate margin to the west (Johnson, 1991). The absolute timing of the onset of development of the Cordilleran thrust belt and foreland basin system are still debated, ranging from Early to Late Jurassic

(Fuentes, DeCelles & Gehrels, 2009). However, the development of a foreland basin or dynamic subsidence across the region is likely to have occurred prior to the deposition of Middle Jurassic strata (DeCelles, 2004; Fuentes et al., 2009; Fuentes, DeCelles, Constenius and Gehrels, 2011;

Parcell and Williams, 2005). This subsidence allowed for the inundation of restricted shallow marine environments across the Western Interior during the Middle Jurassic (Parcell and

Williams, 2005). (Fig. 7)

2.3.1 Jurassic Nomenclature

Early Jurassic strata have not been formally recognized within the Bighorn Basin. This is either due to non-deposition or complete removal by later Jurassic erosional events. Thus,

Middle Jurassic strata lay directly over Triassic deposits throughout the Bighorn Basin (Kvale et al., 2001; Parcell and Williams, 2005; Pipiringos and O’Sullivan, 1978; Schmude, 2000). The

Middle Jurassic strata within the Bighorn Basin are represented by the (in ascending order);

Gypsum Spring Formation, Piper Formation, and Sundance Formation (Fig. 3) (Schmude, 2000;

Parcell & Williams, 2005). The Gypsum Spring Formation was originally included as the upper member of the Triassic Chugwater Formation (Love 1939). However, it was later shown to be

Middle

Figure 6: Generalized global distribution of landmass during the Jurassic. Red box indicates study area. Modified from Blakey (2003)

Figure 7: Paleogeography of western interior during Middle Jurassic. Approximate location of study area highlighted in red. Modified from Parcell and Williams (2005)

Jurassic in age by Love et al., (1945) based upon the presence of Middle Jurassic marine fauna.

The recognition and interpretation of unconformities in the Jurassic has also influenced local and regional correlations. In one interpretation the Piper Formation is considered age equivalent to the Gypsum Spring Formation because of their positioning within the stratigraphic column, similar lithologic units, and bounding unconformities (Imlay, 1945; Imlay,

1956; Parcell and Williams, 2005). An alternative and contrasting correlation supported by

Pipiringos and O’Sullivan (1978) and Schmude (2000) conclude that the Piper Formation is positioned above the J-2 unconformity and Gypsum Spring Formation within the Bighorn Basin.

The Sundance Formation was originally classified by (Darton, 1899) as all marine deposits between the Permian-Triassic red beds and the terrestrial deposits of the Upper Jurassic. The

Sundance was then subdivided into five distinct lithologic members, excluding the Gypsum

Spring and Piper Formations, by Imlay (1947). However, because the scope of this study is focused on the boundary between the Triassic and Jurassic, and the Sundance Formation has not been currently shown to directly overly Triassic strata within the study area, further discussion of the Sundance Formation will not ensue.

CHAPTER THREE

GENERAL STRATIGRAPHY

As previously discussed, this study encompasses the evaluation of Mesozoic stratigraphy ranging from the Early Triassic to the Middle Jurassic. The Triassic strata within the study area is comprised of formations of the Chugwater Group. The Chugwater Group lies unconformably above the Paleozoic strata and has been proposed as age equivalent to the Thaynes Formation of Western Wyoming and Eastern Idaho, representing the paralic or marginal marine deposits of the Triassic Wyoming platform (Storrs, 1991) (Fig. 8). The Middle Jurassic strata within the study area is represented by the Gypsum Spring and Piper Formations. The Gypsum Spring and or Piper Formations lie unconformably above the Chugwater Group throughout the study area.

This unconformity represents the J-1 or J-2 surfaces that were proposed by Pipiringos and

O’Sullivan (1978). In this chapter, lithologic variations and stratigraphic relationship of these formations will be discussed in depth.

3.1 Red Peak Formation

The Red Peak Formation is the oldest and the most laterally persistent unit within the

Chugwater Group. The Red Peak Formation lies unconformably atop the Dinwoody or Goose

Egg Formation throughout the state of Wyoming. This unconformity may represent the Tr-1 surface, however, the Tr-1 surface is difficult to identify in Wyoming due to a lack of fossil evidence and similarities in lithology between latest Permian and earliest Triassic rocks

(Pipiringos and O’Sullivan 1978). More recently, Lovelace and Lovelace (2012) suggested that this contact is actually conformable and represents continuous deposition.

Figure 8: Generalized stratigraphic diagram of inferred Triassic rock relations of Wyoming and Idaho. Red box indicates study area. Not drawn to scale. Modified from Storrs (1991).

Due to the extensive erosional events that occurred after the deposition of later Triassic strata within the study area, the Red Peak is the only formation of the Chugwater Group that was observed and measured during the field sessions of this study. While the Red Peak

Formation may appear rather homogenous in outcrop, it is, in fact, rather heterogeneous in lithological variations, with sediment ranging from mud and claystones to medium grained sandstones. The distinct lithofacies observed within the Red Peak Formation will be discussed in greater depth in Chapter Five of this report. The Red Peak Formation was originally subdivided into four distinct units by Picard (1978). The subdivisions established by Picard, in ascending order, are as follows; Silty Claystone Facies, Lower Platy Facies, Alternating Facies, and Upper

Platy Facies.

These subdivisions have been used by others to correlate the varying lithologic units of the Red Peak Formation across large portions of Wyoming. While this method of subdivision has proven to be applicable to outcrop correlation, and will be utilized when possible, it is less applicable in subsurface correlations due to the minute changes in lithology needed to divide the units. Picard (1978) attempted to correlate the four facies of the Red Peak Formation utilizing spontaneous potential (SP) and resistivity logs. However, this method is rather unreliable for subsurface correlation due to the homogenous nature of SP curves recording the

Red Peak Formation. For this reason, a new method of correlating the Red Peak Formation in the subsurface was developed for use in this study. This method relies on the use of gamma-ray curves and utilizes theories of sequence stratigraphy to divide the Red Peak Formation into seven distinct depositional cycles. The seven established depositional cycles have proven to be

a reliable means of correlation within the Red Peak Formation throughout Park County,

Wyoming (Appendix B).

3.2 Alcova Formation

The Alcova Limestone is a unique formation within the Chugwater Group as it is the only marine carbonate unit present within the group which represents a time of decreased clastic sedimentation and period of rapid marine transgression. The Alcova formation lies conformably above the Red Peak Formation throughout large portions of Wyoming, though its true depositional boundaries are still debated. As aforementioned, the Alcova Limestone was originally described by Lee (1927) and was then classified as a unit within the Crow Mountain

Formation by Love (1939). However, in more recent work (Cavaroc and Flores 1990) the four units of the Crow Mountain Formation have been divided into two separate formations. The two lower units; “Sub-Alcova Sand” as referred by Johnson (1993), and the Alcova Limestone comprise the Alcova Formation. The two upper Sandstone units are restricted to the Crow

Mountain Formation. This is the classification utilized in this study as it provides clarity and higher resolution of correlation within the subsurface.

While neither the Sub-Alcova Sand or the Alcova Limestone were observed in outcrop during the field sessions, likely due to removal by erosion within the study area, the Alcova

Formation was identified in subsurface well logs within southern portions of Park County. The

Sub-Alcova Sandstone, as reported by Johnson (1993), is a fine-medium grained sandstone that coarsens upward into the overlying Alcova Limestone. It is postulated that this unit may represent that early stages of marine transgression that would later deposit the Alcova

Limestone. This hypothesis is due to the abundance of calcite cement, lack of micaceous

sediment, and overall increased rounding and sorting of grains in comparison to the underlying

Red Peak Formation. As reported by (Storrs 1991, Picard 1978) the Alcova Limestone ranges in thickness throughout west-central Wyoming from approximately 0-5 meters (0-15 ft.) and is described as millimeter-laminated, thinly bedded, finely crystalline, limestone or dolomitic limestone. It can be dived into a lower, stromatolitic algal zone and an upper zone that often contains fossil fragments or trace fossils. Fossils of Corosaurus alcovensis described by Storrs

(1991) as well as an analysis of strontium ratios by Lovelace (2015) allow for enhanced age constraints of the Alcova Limestone to the early portions of the Middle Triassic.

Within the subsurface, the Alcova Limestone is readily identified due to its unique log curve signatures. The Alcova has a very low or “clean” gamma ray signature that causes it to stand out against the underlying Red Peak Formation. The Limestone unit within the Alcova also displays a very high spike in resistivity that is often several orders of magnitude higher than that of the surrounding units (Appendix B).

3.3 Crow Mountain Formation

The Crow Mountain Formation, also known as the Crow Mountain Sandstone, lies unconformably above the Alcova Limestone throughout much of the State of Wyoming. This unconformity represents the Tr-2 surface from Pipiringos and O’Sullivan (1978). Like other formations within the Chugwater Group, the Crow Mountain Formation was first formally defined by Love (1939) from outcrop in the northern Powder River Basin. The formation was originally subdivided into two units; the Basal Sandstone and the Upper Siltstone. High and

Picard (1967) also included the two underlying units of the Alcova Formation within the Crow

Mountain Formation.

As with the Alcova Formation, the Crow Mountain Formation was not observed during field sessions of this study, but was observed in subsurface well logs within southern portions of

Park County. This is likely due to complete removal by erosional events that had a greater effect in the northern portions of the study area. As aforementioned the Crow Mountain Formation can be divided into two members. As described by Tohill and Picard (1966) the basal sandstone of the Crow Mountain is composed of well sorted, calcareous sandstone, grain sizes range from fine to coarse, and over all the unit coarsens upward. The unit also contains evidence of bioturbation but no fossils have been observed. Tohill and Picard (1966) and Johnson (1993) also make mention of some “frosted” grains within the upper portions of the Basal Sandstone that would allude to the possibility of aeolian influence upon deposition. However, the frosted grains have only been observed within the Powder River Basin. The upper member of the Crow

Mountain formation is known as the Upper Sand and Siltstone facies. The sandstones within this unit are very fine grained, well sorted and rounded, and cemented by a calcareous cement.

The siltstones are well sorted and calcareous and display sedimentary structures such as cross stratification and asymmetrical ripple marks (Tohill and Picard 1966).

Within the subsurface, the Crow Mountain Formation is easily identifiable above the

Alcova Formation. This is due to the sharp increase in gamma ray signature in comparison to the Alcova Limestone, as well as two fining upward sequences that are present within the gamma ray pattern. However, in places where the Alcova Formation was not deposited, distinguishing the Crow Mountain Formation from the sandy units within the upper Red Peak

Formation can be more difficult. The Basal sandstone of the Crow Mountain generally displays a coarsening upward pattern in gamma ray logs, while the upper sand/siltstone displays a fining

upward pattern. These can also be difficult to distinguish in the subsurface because a sporadic facies change between the sand and silt exists within the upper member (Appendix B).

3.4 Jelm Formation or “Unnamed Red Beds”

The Unnamed Red Beds, also referred to as the Jelm Formation, lay unconformably above the Crow Mountain Formation throughout Wyoming (Cavaroc and Flores 1990). This disconformity represents a period of non-deposition during the Middle Triassic and the Tr-3 erosional surface as described by Pipiringos and O’Sullivan (1978). The Unnamed Red Beds consist of mostly of siltstones, however some massive to cross-stratified, fine grained sandstones have been reported. While the overall lithology of the Unnamed Red Beds is similar to that of the Red Peak Formation of the Early Triassic, High and Picard (1965) claim that the depositional environment and source areas shifted during this time based on the presence terrestrial sedimentary structures, such as dunes and fluvial channels. It is reported that the

Unnamed Red Beds may represent deposition during a transitional phase with a shift from marine depositional environments from the Early to Middle Triassic strata to a terrestrial depositional environment in the Late Triassic.

The Unnamed Red Beds were not observed in outcrop or in subsurface investigation during this project. For a more in depth study of the Unnamed Red Beds the reader is referred to the collection of studies by High, Picard and Tohill, as well as the work of Johnson (1993).

3.5 Popo Agie Formation

The youngest depositional sequence of the Triassic within the Bighorn Basin is known as the Popo Agie Formation. The Popo Agie Formation lies conformably above the Unnamed Red

Beds and represents the final phase of Triassic deposition. Unlike the aforementioned

formations within the Chugwater Group, the Popo Agie Formation is characterized as the only unit within the group that was deposited in a strictly terrestrial environment. The Popo Agie

Formation was first formally identified by Williston (1904) in his study of Late Triassic fossil fragments within the upper portions of Triassic strata. High and Picard (1965) formally subdivided the Popo Agie Formation into four distinct lithofacies throughout south-central

Wyoming. The lithofacies (in ascending order) are as follows; Lower Carbonate unit, Purple unit,

Ocher unit, and Upper Carbonate unit.

According to High and Picard (1965) the Lower Carbonate unit of the Popo Agie

Formation lies conformably above the Unnamed Red Beds where the strata gradually grade upward from red siltstone into limestone conglomerates, silty limestones and dolomites.

Overlying the Lower Carbonate unit is the Purple and Ocher units. These units are characterized by laterally variable fine-grained sandstones and siltstones with a unique abundance of the mineral analcime. High and Picard (1965) denote the presence of lenticular sandstone bodies with erosional bases and sedimentary structures that represent an upward decrease in depositional energy which is common in fluvial channel deposits. They support this interpretation with the presence of tabular sandstone bodes which represent crevasse-splays or sheet flood deposits, which are also common in terrestrial fluvial systems. The Upper

Carbonate unit of the Popo Agie Formation is fairly similar to that of the Lower Carbonate unit.

The upper Carbonate unit is characterized by silty limestones and dolomites, but unlike the

Lower Carbonate unit, there are also reports of green-grey silty claystones and shales within this unit. While there are slight variations between the Upper and Lower Carbonate units of the

Popo Agie Formation, they are believed to share a common lacustrine depositional environment.

Johnson (1993) claims that these freshwater carbonates may have been deposited in shallow lakes that existed on a broad, flat, westward-prograding coastal plain.

As with the Unnamed Red Beds, the Popo Agie Formation was not observed in outcrop or in the subsurface during this project. For a more in-depth study of the Popo Agie Formation the reader is referred to the collection of studies by High, Picard and Tohill, as well as the work of Johnson (1993).

3.6 Gypsum Spring Formation

The Gypsum Spring Formation is the oldest Jurassic unit present within the area of study. Throughout Park County the Gypsum Spring Formation has been shown to unconformably overly heavily eroded beds of the Triassic Chugwater Group. This unconformity represents the J-1 erosional surface as described by Pipiringos and O’Sullivan (1978). As previously discussed, the Gypsum Spring Formation was originally grouped as a member within the Chugwater Formation, however it was shown to be Middle Jurassic in age by (Love et al.,1945) based upon the presence of Middle Jurassic marine fauna. The deposition of the

Gypsum Spring Formation is significant in that it represents the earliest developmental stages of a foreland basin system or a large degree of dynamic subsidence that occurred throughout the study area prior to the deposition of the Middle Jurassic strata. As previously discussed, this shift in tectonic activity allowed for the inundation of shallow, restricted, marine environments across the study area which led to the deposition of the Gypsum Spring Formation.

Before further discussion of the lithofacies distribution within Gypsum Spring

Formation can ensue, it is important to discuss the discrepancies that exist amongst the Middle

Jurassic strata of Northern Wyoming and Southern Montana. Love (1939) originally divided the

Gypsum Spring Formation into three distinct lithofacies based on a type section from the Owl

Creek Mountains. In ascending order, they are as follows: basal gypsum member, middle red claystone member, and an upper grey limestone member. This lithofacies distribution is almost identical to that of the Piper Formation of Montana except for the Piper Formation also contains an upper red claystone member above the grey limestone member. Several studies of the Middle Jurassic strata within the Bighorn Basin (Gilbert, 2012; Imlay, 1945; Parcell and

Williams, 2005) have shown that the upper red claystone member is present within northern portions of the Bighorn Basin. This would suggest that the type section of the Gypsum Spring

Formation established by Love (1939) is incomplete and that the Gypsum Spring and the Piper

Formations are actually age equivalent to one another and only vary in state to state nomenclature. However, other workers (Pipiringos and O’Sullivan 1978, Schmude 2000) claim that the upper red claystone member is actually observable above a chert pebble conglomerate at the top of the Gypsum Spring Formation which would represent the J-2 surface established by Pipiringos and O’Sullivan (1978). This would suggest that the Piper Formation may have actually been deposited after the Gypsum Spring Formation and the upper red claystone was the only member that was deposited into northern portions of Wyoming. While it is agreed upon that both the Gypsum Spring and Piper Formations are of Middle Jurassic in age, their true stratigraphic relationship is still debated. However, because the scope of this study focuses

the basal units of the Middle Jurassic strata, the discrepancies of the upper units will not be covered further in depth and future work is suggested to resolve this debate.

This report will focus on the basal gypsum unit of the Middle Jurassic strata as it was observed in both outcrop and in the subsurface. For the sake of clarity will be referred to as the basal unit of the Gypsum Spring Formation unless otherwise denoted. In outcrop the massive white gypsum of the basal gypsum unit is readily identifiable as it sits unconformably above the red beds of the Chugwater Group. Within the subsurface the basal gypsum unit is also readily indefinable because of the low gamma ray signature that is in contrast to the underlying

Chugwater Group. The gypsum unit also has a very unique resistivity response that is often several orders of magnitude higher than that of the surrounding units. This allows for the contact between the Chugwater Group and the Gypsum Spring Formation to be readily identifiable in both outcrop and within the subsurface.

CHAPTER FOUR

PURPOSE OF STUDY & METHODS

4.1 Purpose of Study

As previously discussed, early Mesozoic stratigraphy of the Bighorn Basin records sedimentation in a wide variety of depositional environments and tectonic settings. Numerous difficulties surrounding stratigraphic correlation such as, lacking regional fossil control, incomplete type sections, and correlations based upon variable nomenclature, have led to contrasting interpretations of the stratal relationships of these units. In particular, the boundary between the Triassic and Jurassic represents a complex relationship consisting of variable unconformities, formations, and lithologies. The difficulties surrounding correlation along the Triassic-Jurassic boundary are further compounded by unquantified amounts of erosion and the lack of regional marker beds within the Triassic strata. The purpose of this study is to investigate lithologic variability, further clarify stratigraphic correlations, and to characterize the extent of Jurassic erosional events across the Triassic-Jurassic boundary within portions of Park County, Wyoming. To accomplish these goals this study utilized outcrop analysis coupled with subsurface well log evaluation which will be discussed in further depth within forthcoming sections of this chapter.

4.2 Methods & Procedures

Outcrop field descriptions, well log analyses, outcrop to subsurface gamma ray correlation, and petrographic work form the foundation of this study. Outcrop data was collected over two field sessions (June-October 2017) along the western flank of the Bighorn

Basin in Park County, Wyoming. A total of four outcrop localities (Fig. 9 & Appendix C) were

Figure 9: Map displaying the location of outcrops and wells utilized in this study. Modified from Healer et. al., (1996)

observed within a 50-mile radius of Cody, Wyoming. They are as follows: (1.) Clark’s Fork

Canyon (2.) Hogan Reservoir - “Carnivore Corner” (3.) Chief Joseph Highway – “Dead Indian Hill”

(4.) South of Cody - “Raven Ridge” (Fig. 9). These outcrops were selected by noting Triassic outcrop on a surficial geologic map of the Bighorn Basin (provided by the Wyoming Geologic

Survey open source GIS database). The outcrops were then checked for viability and accessibility using Google Earth Pro. Data collected from outcrop included lithologic and sedimentary descriptions, gamma ray measurements, as well as the collection of samples for petrographic and mineral analysis. Subsurface data was provided from subsurface wireline logs

(well logs) from approximately 130 wells throughout Park County, within the interior of the

Bighorn Basin. This well log data is currently available from the Sedimentary Basin Analysis &

Modeling Lab in the Department of Geology at Wichita State University.

Outcrop measurement and description played a pivotal role in the execution of this project. Field descriptions included but were not limited to measurements of stratigraphic thickness with a Jacob’s staff, total count gamma-ray measurements, lithology (including; color, size and shape of grains), sedimentary structures and bedding, the presence of fossils and or trace fossils, and structural attitude of outcrop. The basal gypsum unit of the Gypsum Spring

Formation was the target for the upper boundary of measured section. When available, up to or greater than 33 meters (100 ft.) of the upper portions of the Triassic strata were measured, below the basal gypsum unit of the Gypsum Springs Formation.

This study utilized a handheld gamma-ray scintillometer (geometrics GR-310) to collect total count gamma-ray measurements from the four measured sections within the study area.

These measurements were used to correlate outcrop data to subsurface gamma ray well logs,

and also to aid in stratigraphic correlation by detecting minute changes in lithology, such as variable quantities of clay bearing mineral, that may be overlooked in traditional lithologic field descriptions. The measurements were taken on a 1.5-meter (5 foot) interval, at a one hundred second accumulation, from either a fresh surface on outcrop or a trench dug (approximately 30 cm or 1 foot) into the unit (depending on lithology and orientation of beds). Gamma ray curves were then created from the collected outcrop measurements in Microsoft Excel and imported into IHS Kingdom as raster images.

The subsurface well logs were provided by the Sedimentary Basin Analysis and Modeling

Lab within the Department of Geology at Wichita State University. The raster images were loaded into IHS Kingdom Suite and depth registered to allow for digital correlations and creation of isopach maps. Approximately 130 wells were selected based on the presence and quality of gamma ray and resistivity or laterolog curves, as these curves were the primary tool used in correlation of both subsurface and outcrop data. The well logs were evaluated and utilized to correlate varying formations, lithologies, and depositional cycles of Triassic and

Middle Jurassic strata. Defined cycles were then used to create nine isopach maps and a supporting cross-section (Plates 1-10). The cross-sections and isopach maps allow for the visualization and evaluation of depositional/erosional geometries and paleotopographic features as they occur within the subsurface along the Triassic-Jurassic boundary.

There are a few important factors of note in evaluation of the subsurface data in this study. The first and most obvious is that the well data from the raster images is registered in measured depth (MD). This means that the units shown on well logs are actually displaying true vertical thickness (TVT), instead of true stratigraphic thickness (TST). This can cause units to

appear thicker in well logs depending on the dip orientation of the beds. Originally, isopach maps of the correlated units were generated without regard to the discrepancies of measurement. However, this caused anomalous zones of thickening to occur, especially in wells that were taken from established oil and gas fields within the study area. This is likely because the wells were targeting subsurface structures which cause the TVT to read higher than the actual TST value. To overcome these anomalies, dip meter logs were observed, when available, and extrapolated to nearby wells where the anomalous zones of thickening were occurring.

From the dip meter logs an estimated TST was calculated and a percentage of error was established and applied to surrounding wells that demonstrated the anomalous thickening. For example; If a well containing a dip meter log was observed to show a ten percent increase in thickness of a unit, ten percent of the unit’s TVT would then be subtracted to display an estimated TST in the gridded isopach map. It is also important to note that this method was not applied to all wells utilized in this study. Because of the drastic variations in subsurface structures throughout the study area (Walton 1947; McCabe 1948), this method was only applied to wells that occurred within a reasonable distance (near or within the same established oil and gas field) from the wells containing dip meter logs. Thus, the isopach maps referred to in this study are actually an amalgamation of isochore and isopach maps, as this displays the TST of the units by diminishing the influence of Laramide structures (Walton 1947;

McCabe 1948) within the subsurface. This method proved to be efficient in eliminating the anomalies that occurred in or near oil and gas fields within the study area.

Another important factor to discuss in regard to the correlation of well data was the establishment of seven coarsening upward cycles within the Red Peak Formation (Appendix B).

The Red Peak Formation constitutes the vast majority of strata within the Chugwater Group within Park County, ranging in thickness from approximately 274 meters to less than 152 meters (~900-500 ft.) thick. The establishment of the seven Red Peak coarsening upward cycles allowed for correlation of much higher resolution than would have been possible by only correlating the Chugwater Group based on formations alone. The establishment and correlation of these depositional cycles also allowed for estimation of the amount of erosion that truncates the upper portions of the Chugwater Group throughout the study area. The utilization of depositional cycle correlation will be discussed in further detail in forthcoming chapters.

Along with field descriptions, petrographic analysis aided in the definition of lithologic units in the measured sections. Descriptions of thin sections allowed for an enhanced assessment of grain size and mineralogical content of samples. Twenty-seven samples were selected, prepared, and sent to the National Petrographic Lab in Houston, Texas to be made into thin sections. The thin sections were analyzed under a Meiji MX 9300 polarizing petrographic microscope to determine texture, grain size, and mineral compositions. Photos of thin section samples were taken with a SPOT Insight QE camera.

Select samples containing minerals that were unidentifiable in thin section were prepared and analyzed using a Rigaku Miniflex II X-ray Diffraction (XRD) Machine. This XRD

Machine is housed within the Department of Geology at Wichita State University. Jade

Software version 9 was utilized to evaluate and clarify the mineral abundances of the of the sample(s) in question.

Utilizing the aforementioned techniques, a stratigraphic model was constructed to illustrate the extent and differential patterns of Jurassic erosional events upon the underlying

Triassic strata. This stratigraphic model demonstrates the variability of formations, lithologies, and unconformities that exist along the Triassic-Jurassic boundary within Park County,

Wyoming.

CHAPTER FIVE

LITHOFACIES DESCRIPTIONS

The Red Peak Formation of the Chugwater Group and the basal gypsum unit of the

Gypsum Spring Formation are the units exposed in outcrop within the study area. The exposed portions of the Red Peak Formation within Park County are interpreted to correlate with the

Lower Platy facies, Alternating Facies, and Upper Platy Facies established by High and Piccard

(1967) (Figure 10). This chapter will describe and define varying lithologies that are exposed in outcrop and their subsurface gamma ray signatures along the Triassic-Jurassic boundary within northern portions of Park County Wyoming.

5.1 Lithofacies I – Red Peak Siltstone

Lithology

The most common and abundant lithology within the Red Peak Formation are red siltstones. The siltstones are present in the Lower Platy, Alternating, and Upper Platy Facies of

High and Piccard (1967) that were observed in outcrop. However, they are most common within the Alternating Facies. The siltstones are generally arkosic to sub-arkosic, ranging from poorly to well sorted, and comprised of sub-rounded grains. As observed in thin section (Fig.

11) the grains within the siltstone facies range from approximately 0.02 - 0.05 mm in diameter.

The siltstones contain moderate quantities of opaque iron bearing minerals. These minerals are assumed to be hematite based on the findings of Tohill and Picard (1966). However, as discussed by Johnson (1993) the original iron bearing minerals may have been magnetite, ilmenite, biotite, and hornblende. After deposition in a warm, wet, oxygen rich environment the minerals were broken down into immature iron oxides and hydroxides which

Figure 10: Annotated photograph of the Clark’s Fork Canyon Outcrop of this study. Geologists in foreground for scale.

later converted into the hematite. These iron bearing minerals are the primary cement, although calcareous cement is also common within the siltstones. In outcrop, the siltstone facies display bedding thicknesses ranging from thin, mm-scale beds to massively bedded intervals. The thinly bedded intervals often display wavy structures and locally contain green- grey iron-reduction spots. The massively bedded units of the siltstone facies contain small, mm- scale, cross beds in some localities and asymmetrical ripple marks are often visible (Fig. 12).

Massively bedded intervals are more resistant than the thinly bedded intervals and often form ledges or “fins” in outcrop. It is common for the siltstone facies to grade upwards from the thinly bedded intervals into the more massively bedded intervals.

Outcrop Gamma Ray and Subsurface Geophysical Response

Within the subsurface and in outcrop the gamma ray signature of the siltstone facies is often “intermediate” when compared to the sandstone and silty mudstone facies. In subsurface well logs, the gamma ray reading for this facies are approximately 75 API (American Petroleum

Institute) units, ranging from approximately 60-90 API. In outcrop the gamma ray readings from this facies range from approximately 100-200 TCGR (total count gamma ray). The fluctuations in the gamma ray signature are attributed to variable concentrations of silt and clay bearing minerals. The siltstones often dominate large intervals of coarsening upwards sequences that were observed in both outcrop and in subsurface well logs.

Environment of Deposition

Interpretations of depositional environments within the Red Peak Formation vary widely from fluvial flood plains to tidal flat complexes. The siltstone facies observed in this report are interpreted to have been deposited in a shallow, nearshore marine environment along a shelf

Figure 11: Photograph of thin section displaying Lithofacies I (siltstone). Field of view is approximately 5 mm wide. PPL, 10X magnification.

Figure 12: Photograph displaying ripple marks present within Lithofacies I (siltstone). Scale bar in lower left corner is 2 ft.

or passive margin. This is supported by the presence of calcareous cements, cross bedding, and asymmetrical ripple marks. The siltstone facies are interpreted to represent intermediate depositional energy between the silty mudstones and fine-medium grained sandstones. While not contained to the siltstone facies, Lovelace and Lovelace (2012) demonstrate the presence of tracks (walking and swimming) from a wide array of vertebrate and invertebrate fauna that further support the interpretation that the Red Peak Formation was deposited in a shallow marine environment. This environment was likely heavily influenced by small scale fluctuations in water depth and sedimentation rates that can be attributed the cyclical depositional stacking patterns observed in both outcrop and the subsurface.

5.2 Lithofacies II – Red Peak Silty Mudstone

Lithology

A less common lithology observed within the Red Peak Formation are intervals of silty mudstones and claystones. The mudstones are present in the Lower Platy, Alternating, and

Upper Platy Facies of High and Piccard (1967) that were observed in outcrop. However, they are most common within the Lower and Upper Platy facies. The mudstones are generally observed as a darker shade of red when compared to the siltstones and sandstones, and green-grey reduction spots are very common within the mudstone intervals (Fig. 13). In outcrop the mudstones are generally highly weathered, making identification of bedding difficult. When observable, the mudstones are thinly bedded, appearing blocky or platy, with occurrence of reduction spots increasing along the bedding planes. Thinly bedded intervals of gypsum and small gypsum nodules are also common within the mudstone facies. The presence of the gypsum nodules within these intervals may be secondary, occurring after deposition of the

overlying Gypsum Spring Formation. However, the thinly bedded intervals of gypsum may suggest that the mudstone facies were deposited under highly evaporitic conditions, at or just below water level. As observed in thin section the silt grains within the mudstone facies range in size from 0.01 – 0.025 mm in diameter and vary from rounded to sub rounded. The mudstones are generally well sorted while minor intervals of poor sorting are interbedded throughout. The poorly sorted portions of these mudstones generally show an increase in silt sized sediment, however, silt is moderately common in most of the mudstones observed. As observed within the other lithofacies of the Red Peak Formation the mudstones contain a notable amount of opaque iron bearing minerals that coat the grains and cements in a characteristic rust color. As discussed by Tohill and Picard (1966) the most common clay minerals found in the mudstone intervals is illite, though other micaceous minerals are likely present. The mudstones often make up the lower portions of coarsening upwards sequences and grade upward into siltstones in observed outcrops and subsurface sections of the Red Peak

Formation.

Outcrop Gamma Ray and Subsurface Geophysical Response

Within the subsurface and in outcrop, the gamma ray signature of the silty mudstone facies is often “high” when compared to the sandstone and siltstone facies. In subsurface well logs, the gamma ray reading for this facies are generally greater than 90 API, ranging from approximately 90-150 API. In outcrop the gamma ray readings from this facies range from approximately 150-300 TCGR. The fluctuations in the gamma ray signature are attributed to variable concentrations of silt and clay bearing minerals. As previously discussed, the

Figure 13: Photograph displaying bedding and iron reduction spots present within Lithofacies II (silty mudstone). Handle of rock hammer is 1 ft. long.

mudstones often represent the lowest intervals of coarsening upwards sequences that were observed in both outcrop and in subsurface well logs.

Environment of Deposition

The silty mudstone facies are interpreted to have been deposited in a shallow, nearshore marine environment. The presence of thinly laminated bedding and increased concentrations of clay bearing minerals suggest deposition of these intervals occurred in a low energy environment, such as a tidal flat complex. The presence of thinly bedded intervals of gypsum support this interpretation as periods of subaerial exposure and evaporitic conditions would be needed for the precipitation of gypsum. Intervals with increased silt content likely represent influxes of sediment supply or an increase in depositional energy due to cyclical fluctuations in water levels.

5.3 Lithofacies III – Red Peak Sandstone

Lithology

The least common lithofacies within the Red Peak Formation are fine to medium grained sandstones. The sandstones are present in the Lower Platy and Alternating Facies of

High and Piccard (1967) that were observed in outcrop. However, they are most common and abundant within the Alternating Facies. These sandstones generally portray the characteristic

“brick” red coloring of the Red Peak Formation. However, at the top of Triassic section at both

Clarks Fork Canyon and Hogan Reservoir outcrops of this study, the sandstone facies are actually golden to light brown in color (Fig. 14). The coloration of these beds suggests some degree of exposure may have occurred, allowing for surficial and or groundwater to percolate through these intervals and leech out the iron bearing material. At the Hogan Reservoir outcrop

Figure 14: Photograph displaying gold-brown coloration of Lithofacies III (sandstone) at the top of Triassic section of the Hogan Reservoir outcrop. Handle of rock hammer is 1 ft. long.

this variation in color is sporadic and often areas near bedding planes and fractures display the lighter coloration, this further suggests that this is due to interaction with groundwater after deposition. The sandstone facies within the Red Peak Formation are generally arkosic, and like the siltstones, are well sorted and grains are rounded to sub-rounded. As observed in thin section (Fig. 15), the grains within the sandstones range in size from approximately 0.125 – 0.35 mm in diameter. Along with quartz grains the sandstone facies also contain abundant opaque iron bearing minerals. The sandstones are generally well cemented however some intervals of friable sandstones were observed in outcrop. As was stated within the siltstone facies these iron bearing minerals are interpreted to be responsible for the perception of iron oxide cement that is common within the sandstones, however, calcareous cement is also moderately common. In outcrop, the sandstones are generally massively bedded but some intervals of mm- scale cross bedding were observed (Fig. 16). Unlike the siltstones, the sandstones were not observed to display ripple marks, even in the upper portions of exposed bedding plains.

Outcrop Gamma Ray and Subsurface Geophysical Response

Within the subsurface and in outcrop the gamma ray signature of the sandstone facies is often “low” or “clean” when compared to the siltstone and silty mudstone facies. In subsurface well logs, the gamma ray reading for this facies are generally less than 60 API, ranging from approximately 15-75 API. In outcrop the gamma ray readings from this facies range from approximately 25-150 TCGR. The fluctuations in the gamma ray signature are attributed to variable concentrations of sand, silt and clay bearing minerals. The sandstone facies often represents the upper portions of the coarsening upwards cycles that were observed in both outcrop and the subsurface.

Figure 15: Photograph of thin section displaying Lithofacies III (sandstone). Field of view is approximately 5 mm wide. PPL, 10X magnification.

Figure 16: Photograph displaying small scale cross bedding present within a fine grained interval of Lithofacies III (sandstone).

Environment of Deposition

The sandstone facies is interpreted to have been deposited in a shallow, nearshore marine environment. This is supported by the presence of calcareous cements and cross bedding observed in outcrop. The sandstone facies is interpreted to represent periods of high depositional energy in comparison the silty mudstones and siltstone facies. As previously discussed, Lovelace and Lovelace (2012) demonstrate the presence of tracks (walking and swimming) from a wide array of vertebrate and invertebrate fauna that further support the interpretation that the Red Peak Formation was deposited in a shallow marine environment.

This environment was likely heavily influenced by small scale fluctuations in water depth and sedimentation rates that can be attributed the cyclical depositional stacking patterns observed in both outcrop and the subsurface.

5.4 Lithofacies IV –Siltstone Conglomerate

Lithology

At the top of the Chugwater Group, below the basal gypsum unit, at the “South Cody” outcrop a siltstone conglomerate (Fig. 17) was observed that is not present at the three other outcrops of this study. This siltstone is poorly sorted with lithic fragments ranging from 0.1 to

1.3 mm in diameter. The majority of clasts appear rounded to sub rounded with few clasts appearing angular in hand sample. In thin section the clasts appear to be mainly quartz, potentially a moderate amount of chert, and a small amount of dolomitic (?) clasts. These clasts may have been derived from dolomitic intervals of the Alcova Limestone, but further research

Figure 17: Photograph of thin section displaying Lithofacies IV (siltstone conglomerate). Field of view is approximately 5 mm wide. PPL, 10X magnification.

would be needed to support this hypothesis. The siltstone itself is the characteristic “brick” red in color and the interval is very thin (>2 ft.) and can be easily missed in outcrop.

The siltstone conglomerate facies bears a striking resemblance to the siltstone conglomerate that is described by Schmude (2000). Schmude claims that this siltstone conglomerate represents the J-1 unconformity, which is agreeable in terms of stratigraphic location where the siltstone conglomerate was observed during this study. However, because this interval was only observed at one locality, it may suggest that these deposits are localized to paleotopographic features that were not wide spread across the study area.

5.5 Lithofacies V – Siltstone Breccia

Lithology

As will be discussed in forthcoming chapters of this report, an anomalous interval was observed at the “Clark’s Fork Canyon” outcrop of this study. At this locality the basal gypsum unit of the Gypsum Spring Formation is missing altogether, either due to no deposition or later removal by erosion. In place of the basal gypsum is a siltstone breccia (Fig. 18) that was also observed and discussed by Parcell and Williams (2005). The siltstone breccia is a thin interval, less than 2 m (6 ft.) thick, and contains angular clasts of chert and limestone. The limestone clasts were not observed in the sample taken for this study but were noted by Parcell and

Williams (2005). The siltstone breccia is light grey to light purple in color, poorly sorted, and calcareous. It is important to make note of the siltstone breccia as it is remarkably similar to the chert bearing beds that are discussed by Pipiringos and O’Sullivan (1968, 1978) and Schmude

(2000). Both studies claim that the J-2 unconformity is marked by a chert bearing horizon that represents an erosional lag deposit, and that the lithic clasts within this interval are derived

Figure 18: Photograph displaying anomalous chert bearing siltstone breccia (Lithofacies V) at the top of the Triassic section of the Clark’s Fork Canyon outcrop. Scale in lower left corner is approximately 1 ft.

from the Gypsum Spring Formation. This may suggest that the Triassic-Jurassic boundary is marked by the J-2 unconformity at this location and the middle Jurassic section is represented by the Piper Formation. Schmude (2000) claims that the Piper Formation is observable above the Gypsum Spring Formation and the J-2 surface. However, this is speculative, as previously discussed there is debate as to the stratal relationships between the Gypsum Spring and Piper

Formations.

5.6 Lithofacies VI – Basal Gypsum Unit

Lithology

Unconformably overlying the red beds of the Triassic Chugwater Group throughout the majority of the study area is the basal gypsum unit of the Gypsum Spring Formation (Fig. 19). In outcrop this unit is a massive to brecciated bed of white gypsum that ranges in thickness for 0-

25 meters (0-75 feet) throughout the study area. Within the gypsum interval there are alternating beds of red claystones, and dolomites as observed by (Gilbert 2012) during his field studies in Big Horn County. Within Park County the red claystone intervals were not noted in outcrop, however, a thin bed of dolomite (Fig. 20) was observed at the base of the gypsum interval at the “Dead Indian Hill” outcrop of this study (Fig. 9). This dolomitic bed was sampled and evaluated in both thin section and XRD to confirm the lithology and agrees with the results of Gilbert (2012). It is suggested by Schmude (2000) that the beds of red claystone and dolomite represent high-order shallowing upward cycles within the basal gypsum unit. Within the subsurface the gypsum has been altered to anhydrite which is concluded from lithology logs from wells observed within the study area (e.g., E.R Dyer #1) and discussed by Parcell and

Williams (2005). While it is assumed that the basal gypsum unit was originally deposited as

Figure 19: Photograph displaying the basal gypsum unit (Lithofacies VI) above the Triassic section of the South Cody outcrop. Handle of rock hammer is 1 ft. long.

Figure 20: Photograph of thin section displaying dolomitic interval within Lithofacies VI (Basal Gypsum unit). Field of view is approximately 5 mm wide. PPL, 10X magnification.

gypsum it is likely that is was altered into anhydrite during dehydration that occurred during diagenesis. The brecciated texture of the unit that is observed at some outcrop localities has been attributed to a recent dissolution-collapse feature that occurred at the unit was uplifted and rehydrated via interaction with groundwater after burial (Schmude 2000).

Outcrop Gamma Ray and Subsurface Geophysical Response

Variations in lithology from the red beds of the Chugwater Group to the basal gypsum unit of the Gypsum Spring Formation is readily identifiable in both outcrop and the subsurface.

The basal gypsum unit is represented by a very “low” or “clean” gamma ray response due to lacking amounts of radioactive material. Within the subsurface, the contact between the basal gypsum unit and the underlying red beds of the Chugwater Group is easily recognizable on geophysical well logs, with a gamma ray signature ranging from approximately 5-15 API. In outcrop, the gamma ray signature of the basal gypsum unit ranges from approximately 5-50

TCGR. While the dolomitic beds observed in outcrop are not distinguishable in well logs, the claystone intervals are generally represented by spikes within the overall low gamma ray signature of the basal gypsum unit (Appendix B).

Environment of Deposition

The basal gypsum unit of the Gypsum Spring Formation represent the first inundation of the Middle Jurassic seaway into a broad, shallow (>10 meters) basin covering large portions of what is now the state of Wyoming (Parcell and Williams 2005). The gypsum is interpreted to have been deposited in a restricted marine environment. Schmude (2000) suggests sabkha conditions were dominant during the deposition of the gypsum unit and that the alterations of dolomite and claystone beds represent cyclical shifts in sea level. As discussed by Gilbert

(2012), Parcell and Williams (2005), and Schmude (2000), several positive tectonic features such as the Belt Island complex of Montana and the Sheridan Arch of Wyoming were uplifted prior to or during the inundation of the Middle Jurassic Seaway and were the mechanisms which were restricting the influx and circulation of fresh marine water into the region and controlling variations in depositional patterns of the basal gypsum unit (Fig. 7).

CHAPTER SIX

REGIONAL STRATIGRAPHIC CORRELATIONS

Approximately 130 geophysical well logs and 4 measured stratigraphic sections were correlated across Park County Wyoming. As previously discussed, outcrop gamma ray measurements were collected from the four measured sections and well logs were selected based on the presence and quality of gamma ray logs. This allows for the measured sections in outcrop to be correlated to the subsurface well logs. From these data, nine isopach maps and a supporting south-north cross section were generated to visualize the erosional and depositional variations that were observed along the Triassic-Jurassic boundary.

The tops of the Dinwoody Formation and the Chugwater Group were defined, correlated, and a gridded isopach map was generated of the entire Chugwater Group (plate 1).

This allowed for the visualization of the apparent thinning of the Chugwater Group in a south to north orientation. However, utilizing this method alone does not allow for the differentiation between depositional thinning and erosional truncation. If depositional thinning was the driving mechanism for the thinning of the Chugwater Group one would expect to see the units within the group pinching out in a predictable manor towards the basin margins. However, if erosional truncation was the driving mechanism, one would expect to see a relatively uniform thickness of the lower units and the upper units would begin to pinch out due to truncation via uplift or tilting of the beds in a given orientation.

To accomplish this differentiation, the Red Peak Formation, which represents the majority of the Chugwater Group within the study area, was divided into seven coarsening upward parasequences. A parasequence, as defined by Van Wagoner et al., (1990), is a

“relatively conformable succession of genetically related beds or bedsets bounded by marine- flooding surfaces or their correlative surfaces. In special positions within the sequence, parasequences may be bounded either above or below by sequence boundaries.”. While parasequences may not be applicable for correlation of units within varying depositional environments, shallow marine environments such as coastal plains, deltas, and beaches generate repetitive sediment stacking patterns which allow for the use of parasequences as a correlative tool. According to Van Wagoner et al., (1990), parasequences range in thickness from 10 to more than 1000 ft., can be correlated over lateral distances of 10 to 10s of thousands of miles, generally represent deposition over 102-104 years, and are most readily identifiable in outcrop and well logs. In a typical coarsening upward parasequence, bedsets thicken, sandstones coarsen, and the sandstone/mudstone ratio increases in an upward fashion. The vertical-facies associations within coarsening upward parasequences are interpreted to represent gradual shifts in water depth and depositional energy.

The use of parasequence correlations was not needed for the Alcova and Crow

Mountain formations as they make up a small portion of the Chugwater Group within the study area and are only observable within subsurface well logs in the southernmost portions of the study area. Thus correlations based on formation tops were adequate for the correlative resolution needed in this study.

6.1 Red Peak Parasequences

To allow for higher resolution in correlation and to differentiate between depositional pinch-outs and erosional truncation, the Red Peak Formation was subdivided into seven coarsening upward parasequences (RP-1 to RP-7) for use in this study. Within these

parasequences, the coarsening upward pattern has been interpreted as representing a cyclical increase in depositional energy and water depth that was likely driven by small scale eustatic shifts that transgressed and regressed across the broad depositional plain of the Wyoming shelf. The intervals (or bedsets) of high gamma ray signatures have been interpreted as representing silty mudstones with increased presence of clay bearing minerals, such as described in section 5.2 (Lithofacies II). The intervals (or bedsets) of “intermediate” gamma ray signatures have been interpreted as representing siltstones with decreased presence of clay bearing material, such as described in section 5.1 (Lithofacies I). The intervals (or bedsets) with a relatively low or “clean” gamma ray signatures have been interpreted as fine to medium sandstones with the largest grain size and lowest concentration of clay bearing material, such as described in section 5.3 (Lithofacies III).

6.1.1 Red Peak Parasequence 1 (RP-1)

Red Peak parasequence 1 (RP-1) represents the base of the Red Peak Formation throughout the study area, and likely correlates to the silty claystone facies of High and Piccard

(1967). The parasequence begins at the top of the Dinwoody (or Goose Egg) Formation and is interpreted as the first stage of Early Triassic progradation as large volumes of clastic material began to prograde from the East to the West across the Wyoming shelf. While not observed in outcrop the contact between RP-1 and the underlying Dinwoody is readily identifiable in gamma ray curves of subsurface well logs (Appendix B). The contact is represented by a sharp increase in gamma ray signature which is interpreted as decrease in grain size and increase in clay bearing sediment found at the base of RP-1. In subsurface well logs RP-1 is comprised mainly of mudstones and siltstones (Lithofacies I & II) and gradually transitions into sandstones

(Lithofacies III) near the top of the sequence (Appendix B). Throughout the study area RP-1 maintains a relative thickness ranging from 44-29 meters (145-95 ft.) and does not show signs of depositional pinch outs within the study area (Plate 2 and 10).

6.1.2 Red Peak Parasequences 2-4 (RP-2 - RP-4)

Red Peak parasequences 2 through 4 (RP-2 – RP-4) were correlated (Plate 10) and a isopach map of each of these parasequences can be seen in plates 3-5 (isopachs). As observed with RP-1, these parasequences show lateral persistence throughout the study area.

RP-2 ranges in thickness from 40-26 meters (130-85 ft.) throughout the study area, and likely correlates to portions of the lower platy facies of High and Piccard (1967). The base of RP-

2 can be recognized on well logs as a sharp increase in gamma ray signature when compared to the top of RP-1. The basal interval of RP-2 is dominated by mudstones and siltstone (Lithofacies

I & II) which is capped by three distinct and characteristic decreases in gamma ray reading.

These decreases are likely caused by an increase in the deposition of sandstones and siltstones

(lithofacies I & III). Following the three decreasing “spikes” the sequence gradually coarsens upward where it is likely dominated by fine grained sandstones (Lithofacies III) (Appendix B).

RP-3 ranges in thickness from 26-15 meters (85-50 ft.) throughout the study area, and likely correlates to potions of the lower platy facies of High and Piccard (1967). The base of RP-3 can be recognized on well logs as a sharp increase in gamma ray signature when compared to the top of RP-2. As observed in RP-2, the basal interval of RP-3 is dominated by varying intervals of mudstones and siltstone (Lithofacies I & II). Throughout the remainder of RP-3 the presence of sandstone and siltstone (Lithofacies I & III) increase and the presence of mudstones

(Lithofacies II) decreases in an upward fashion (Appendix B).

RP-4 ranges in thickness from 47-21 meters (155-70ft.) throughout the study area, and likely correlates to portions of the alternating facies of High and Piccard (1967). The base of RP-

4 can be recognized on well logs as a sharp increase in gamma ray signature when compared to the top of RP-3. As observed in RP-2 and RP-3, the basal interval of RP-4 is dominated by varying intervals of mudstones and siltstone (Lithofacies I & II). In some instances, a sharp decrease in gamma ray was observed within the middle of RP-4 which is attributed to a short period of increased deposition of sandstones (Lithofacies III). However, this was not observed in all wells and it is more common for RP-4 to grade upward in a more gradual fashion. The common pattern of RP-4 demonstrates a gradual increase in grainsize from the basal interval of mudstones (Lithofacies II) to the middle interval of siltstones (Lithofacies I) which is thin capped by an interval of sandstones (Lithofacies III) (Appendix B).

As aforementioned RP-2 through RP-4 are persistent throughout the study area, while they do appear to thin slightly to the north, they do not show signs of large scale depositional influenced thinning as the patterns within the gamma ray signatures are identical to that of the parasequences within the thicker sections throughout southern portions of Park County.

6.1.3 Red Peak Parasequence 5 (RP-5)

Red Peak parasequence 5 was correlated throughout the study area which can be seen in plate 6 (RP-5 isopach) as well as in plate 10 (cross section).RP-5 ranges in thickness from 53-

17 meters (175-55 ft.) throughout the study area, and likely correlates to portions of the alternating facies of High and Piccard (1967). The contact between the basal interval of RP-5 and the top of RP-4 can be identified in well logs as a distinct increase in gamma ray signature.

In most instances, RP-5 demonstrates the most gradual and consistent coarsening upward

pattern when compared to the other parasequences. The basal unit is dominated by mudstones

(Lithofacies II) which grade upward into siltstones (Lithofacies I) and then continues to gently grade upward into sandstones (Lithofacies III) (Appendix B).

While RP-5 does not display complete erosional truncation, it does thin drastically towards the northern portions of the study area where it is directly overlain by Middle Jurassic sediment. It is interpreted that RP-5 once displayed the same lateral consistency as the older

Red Peak parasequences, but was eroded to its current thickness due to erosional events that also completely removed younger Triassic strata. This is supported by the disappearance of the upper portions of RP-5 gamma ray pattern in the northern most portions of the study area.

6.1.4 Red Peak Parasequence 6 (RP-6)

Red Peak parasequence 6 was correlated throughout the study area which can be seen in plate 7 (RP-6 isopach) as well as in plate 10 (cross section). RP-6 ranges in thickness from 73-

0 meters (240-0 ft.) throughout the stud area, and likely correlates to portions of the alternating facies of High and Piccard (1967). Much like RP-5, RP-6 demonstrates a very gradual and consistent coarsening upward pattern when compared to the other parasequences. The basal unit is dominated by mudstones (Lithofacies II) which grade upward into siltstones

(Lithofacies I) and then continues to gently grade upward into sandstones (Lithofacies III)

(Appendix B). Unlike the underlying parasequences, RP-6 is the first parasequence to display complete erosional truncation within the study area. While the thickness of RP-6 is relatively consistent throughout southern portions of Park County it can be observed pinching out towards the Wyoming-Montana Border. It is interpreted that RP-6 once displayed the same lateral consistency as the older Red Peak parasequences. However, due to increased erosion in

the northern portions of the study area, RP-6 is truncated approximately 40 kilometers (25 mi.) north of Cody, Wyoming.

It is also important to note that within subsurface well logs and drilling reports, the top of RP-6 can be denoted as the top of the Curtis Formation, which is an informal name given to the Crow Mountain Formation (Tohill and Picard 1966). This is problematic and may cause confusion in future work. In places where the Alcova Formation is present, the gamma ray pattern of RP-6 is easily recognizable below the Alcova and RP-7. However, where the overlying intervals have been eroded away, RP-6 may occur as the uppermost parasequence within the

Triassic strata. This may be the origin of the miscorrelation that occurs in a few observed historical well logs.

6.1.5 Red Peak Parasequence 7 (RP-7)

Red Peak parasequence 7 was correlated throughout the study area which can be seen in plate 8 (RP-7 isopach) as well as in plate 10 (cross section). RP-7 ranges in thickness from 87-

0 meters (285-0 ft.) throughout the study area, and likely correlates to the upper platy facies of

High and Piccard (1967). The contact between the basal interval of RP-7 and the top of RP-6 can be identified in well logs as a pronounced increase in gamma ray signature (Appendix B). The middle portions of RP-7 are dominated by siltstones (Lithofacies I) which grade upward into sandstones of (Lithofacies III) or into the basal sandstone of the Alcova Formation. Unlike the underlying parasequences, RP-7 displays the greatest amount of erosional truncation. While the thickness of RP-7 is relatively consistent throughout southern portions of Park County it can be observed pinching out approximately 9.5 kilometers (6 mi.) north of Cody Wyoming. As discussed with RP-6, It is interpreted that RP-7 once displayed the same lateral consistency as

the older Red Peak parasequences. However, due to increased erosion in the northern portions of the study area RP-7 displays the largest amount of erosional truncation.

6.2 Alcova Formation

The Alcova Formation, while not observed in outcrop, is observed and correlated within the subsurface throughout southern portions of the study area (Plate 10). While it has been suggested (Storrs, 1991) that the depositional limits of the Alcova formation exist near the southern portions of Park County, Wyoming, this would have to be speculative as the Tr-2 erosional surface of Pipiringos and O’Sullivan (1978) truncates the upper limestone unit of the

Alcova Formation. The J-1 surface is also superimposed on the Alcova limestone in the study area, which would suggest that it was subjected to further erosional events after the development of the Tr-2 surface. Within the study area, the Alcova Formation is truncated by the J-1 surface 24 kilometers (15 mi.) south of Cody, Wyoming and the basal gypsum unit of the

Gypsum Spring Formation lies unconformably above this surface (Plate 10).

6.3 Crow Mountain Formation

The Crow Mountain Formation was not observed in outcrop but was observed and correlated within the subsurface throughout southern and southeastern portions of the study area (Plate 10). As with the Alcova Formation, the true lateral extent of the Crow Mountain can only be speculated as it has been truncated by the Tr-3 unconformity of Pipiringos and

O’Sullivan (1978). The J-1 surface also truncated the top of the Crow Mountain within the study area, which suggests that it was subjected to further erosional events after the development of the Tr-3 surface. Within the study area the Crow Mountain Formation is truncated by the J-1

surface approximately 29 kilometers (18 mi.) south of Cody, Wyoming and the basal gypsum unit of the Gypsum Spring formation lies unconformably above this surface (Plate 10).

6.4 Gypsum Spring - Basal Gypsum Unit

Deposition and presence of the basal gypsum unit is laterally extensive throughout the study area and was observed in all wells throughout the study area. In the subsurface, the basal gypsum unit ranges in thickness from 3-25 meters (9-75 ft.) and thins in a south to north orientation. While persistent within the subsurface, there are two anomalous zones that were observed in outcrop (Plate 9). The first anomaly was observed at Clark’s Fork Canyon (Fig. 9), in the northern most outcrop observed in this study. At the top of the red beds of the Triassic

Chugwater Group, the basal gypsum unit of the Gypsum Spring Formation is missing altogether.

In the stratigraphic location of the basal gypsum unit lies a chert bearing siltstone breccia as discussed in section 5.5 of this report. The absence of the gypsum unit at this locality may suggest that this area represented a topographic high during deposition which inhibited the deposition of the characteristic basal gypsum unit. The second anomalous zone occurs approximately 8 kilometers (5 mi.) south of Clark’s Fork Canyon at the Hogan Reservoir outcrop.

At the northern boundary of this approximately 180 meter (600 ft.) wide outcrop the basal gypsum unit is missing but the siltstone chert conglomerate is not present. However, approximately 25 meters (75 feet) south of the northern boundary of this outcrop the basal gypsum unit reappears and can be observed thickening to the south (Fig. 21). The area near the

Clark’s Fork Canyon and Hogan Reservoir outcrops may represent a topographic high during the time of deposition of the basal gypsum unit and inhibited deposition at these localities. A contrasting interpretation to this is that this anomalous zone actually represents the J-2

Figure 21: Photograph displaying reappearance of the basal gypsum unit above the Triassic section of the Hogan Reservoir outcrop. Handle of rock hammer is 1 ft. long.

unconformity, which would imply that the Gypsum Spring Formation was deposited at these localities, but was subsequently eroded and the overlying Piper Formation was then deposited atop of this erosional surface. This interpretation would have to assume that the Gypsum

Spring and Piper Formations are not age equivalent, and that some degree of uplift and erosional events occurred between their respective periods of deposition. This would explain the chert bearing siltstone breccia observed at Clark’s Fork Canyon, if it is in fact the same unit described to represent the J-2 erosional surface by Pipiringos and O’Sullivan (1968, 1978).

However, as aforementioned, the stratal relationships between these two formations has been debated in literature (Pipiringos and O’Sullivan, 1978; Schmude, 2000; Parcell and Williams,

2005; Gilbert, 2012) and continued research would be needed to support this hypothesis.

CHAPTER SEVEN

STRATIGRAPHIC MODEL

The Triassic-Jurassic boundary within the Bighorn Basin represents a complex relationship consisting of multiple unconformities, formations, and lithologies. These stratal relationships developed over millions of years in response to variable periods of deposition, uplift and subsequent erosion. As discussed in Chapter Six, where not truncated by younger erosional events, the units of the Chugwater Group are laterally persistent throughout the study area. While some units do appear to thin slightly to the north, they do not show signs of large scale stratigraphic thinning as the patterns and intervals of the gamma ray signatures are identical to that of the units within the thicker sections throughout southern portions of Park

County. The subsurface data utilized in this study suggest that the Chugwater Group was likely deposited in a uniform fashion across the study area. However, erosional events of the Jurassic removed the Late Triassic units and truncated the Chugwater Group below the middle Triassic strata throughout the majority of the study area. Utilizing the outcrop and subsurface investigations of this study, a stratigraphic model was constructed to illustrate the extent and differential patterns of Jurassic erosional events upon the underlying Triassic strata. This stratigraphic model demonstrates the variability of formations, and unconformities that exist along the Triassic-Jurassic boundary within Park County, Wyoming.

Stage one of this model (Fig. 22) represents deposition of the seven parasequences of the Red Peak Formation. The Red Peak Formation represents Early Triassic progradation as large volumes of clastic material were deposited from the East to the West across the Wyoming shelf. As previously discussed, the seven parasequences within the Red Peak Formation have

been interpreted to demonstrate a response to cyclical fluctuations in sedimentation rates, depositional energy and small scale eustatic shifts.

Stage two of this model (Fig. 23) represents the deposition of the Alcova Formation. As discussed by Storrs (1991) Johnson (1993) the deposition of the Alcova Limestone represents a period of rapid marine transgression from the Cordilleran “Miogeocline” to the west, across the

“Wyoming Platform” to the east. Due to the thin nature and lateral extensiveness of this unit, it is postulated that a small eustatic shift may have been responsible for flooding the coastal plain in which the Red Peak Formation was deposited, and allowing for the deposition of the Alcova

Formation.

Stage three of this model (Fig. 24) occurs during a period of time following the deposition of the Alcova Formation. This stage of the model represents a period of rapid marine regression followed by a period of non-deposition and erosion which marks the Tr-2 surface of

Pipiringos and O’Sullivan (1978).

Stage four of this model (Fig. 25) represents deposition of the Crow Mountain

Formation. Following the deposition of the Alcova Formation, and the period of non-deposition which generated the Tr-2 surface, progradation of clastic sediment resumed across the

Wyoming shelf. Interpretations of the depositional environments of the Crow Mountain

Formation are similar to that of the Red Peak Formation. However, Cavaroc and Flores (1990) suggest that the Crow Mountain was likely exposed to prolonged periods of higher energy waters which is supported by sedimentary structures and the lack of clay bearing minerals in comparison to the Red Peak Formation.

Stage five of this model (Fig. 26) occurs a period of non-deposition and erosion following the deposition of the Crow Mountain Formation. This stage of the model represents the end of deposition during the Middle Triassic and is represented by the Tr-3 surface of Pipiringos and

O’Sullivan (1978).

Stage six of this model (Fig. 27) represents the deposition of the Jelm and Popo Agie

Formations. The deposition of these units marks the beginning of the Late Triassic and also a drastic shift in sedimentation and depositional environments. While the Early-Middle Triassic strata record deposition in marine dominated environments the Late Triassic strata is dominated by terrestrial deposition. According to High and Piccard (1965) the “Unnamed Red

Beds” or Jelm Formation represent the transitional period between marine and terrestrial deposition. While the Jelm Formation is interpreted to have been deposited in environments similar to that of the Red Peak and Crow Mountain Formations, the Popo Agie Formation is interpreted to have been deposited in large terrestrial lakes and fluvial systems (High & Piccard,

1965). This is supported by the presence of lacustrine carbonates and fluvial sedimentary structures described by High and Piccard (1965) and Johnson (1993). This stage also represents the final depositional period of the Triassic.

Stage seven of this model (Fig. 28) represents a period of uplift which is responsible for large quantities of erosion that occurred prior to the deposition of the Middle Jurassic Gypsum

Spring Formation. While the exact timing and tectonic process for this event are unresolved,

Fuentes et. al., (2011) suggest that pre-existing weaknesses within basement rocks were likely reactivated in response to the early stages of development of the Cordilleran foreland basin system. In response to the compressive stresses to the west, reactivation of weaknesses in the

basement rock likely allowed for tilting of Triassic (and older) strata in a north to south orientation which caused the erosional patterns that were observed in this study. This interpretation is supported by non-deposition of the Early Jurassic Nugget Sandstone and the observed truncation of hundreds of feet of Triassic strata within the study area. This period is interpreted to represented the J-0/J-1 surfaces of Pipiringos and O’Sullivan (1978).

The eighth and final stage one of this model (Fig. 29) represents the first depositional period of the Middle Jurassic. This period is marked by the deposition of the basal gypsum unit of the Gypsum Spring Formation. This stage of the model is significant in that it represents the developmental stages of a foreland basin system or a large degree of dynamic subsidence that occurred throughout the study area. As previously discussed, this shift in tectonic activity allowed for the inundation of shallow, restricted, marine environments across the study area which led to the deposition of the Gypsum Spring Formation (Parcell and Williams 2005).

SOUTH NORTH

Figure 22: Diagram of stage one; deposition of the Red Peak Formation. Not drawn to scale.

SOUTH NORTH

Figure 23: Diagram of stage two; marine transgression and deposition of the Alcova Formation. Not drawn to scale.

SOUTH NORTH

Figure 24: Diagram of stage three; marine regression followed by erosion and development of the Tr-2 surface. Not drawn to scale.

SOUTH NORTH

Figure 25: Diagram of stage four; deposition of the Crow Mountain Formation. Not drawn to scale.

SOUTH NORTH

Figure 26: Diagram of stage five; erosional period resulting in the development of the Tr-3 surface. Not drawn to scale.

SOUTH NORTH

Figure 27: Diagram of stage six; deposition of the Jelm and Popo Agie Formations. Marking the end of Triassic deposition. Not drawn to scale.

SOUTH NORTH

Figure 28: Diagram of stage seven; period of uplift and erosion resulting in truncation of Triassic strata and the development of the J-0 and J-1 surfaces. Not drawn to scale.

SOUTH NORTH

Figure 29: Diagram of stage eight; deposition of the basal gypsum unit of the Gypsum Spring Formation. Not drawn to scale.

CHAPTER EIGHT

CONCLUSIONS

Mesozoic stratigraphy of the Bighorn Basin records deposition in a variety of depositional environments and tectonic settings which have led to contrasting interpretations of the stratal relationships of these units. In particular, the boundary between the Triassic and

Jurassic represents a complex relationship consisting of multiple unconformities, formations, and lithologies. This study was undertaken to demonstrate the complexities of this boundary and to expand upon simplifications and generalizations that have been made during previous works. This will allow for future research to take into account the complexities of the Triassic-

Jurassic boundary and avoid previous over generalizations.

As observed in plate 10, the Triassic-Jurassic boundary within Park County, Wyoming is represented by stacked unconformities between parasequences of the Early Triassic Red Peak

Formation, Middle Triassic Alcova and Crow Mountain Formations, and the basal gypsum unit of the Middle Jurassic Gypsum Spring Formation. The exposed portions of the Red Peak

Formation within Park County are interpreted to correlate with the Lower Platy facies,

Alternating Facies, and Upper Platy Facies established by High and Piccard (1967). In the southern portions of the study area the Triassic-Jurassic boundary is underlain by early Middle

Triassic Alcova Formation and the late Middle Triassic Crow Mountain Formation. This pattern of progressively older (from south to north) Triassic strata below the Triassic-Jurassic boundary suggests that erosion was more drastic in northern portions of the study area and that the

Triassic strata was uplifted prior to Middle Jurassic deposition. This event may coincide with the earliest stages of onset of the Cordilleran thrust belt and foreland basin system (Fuentes et. al.,

2011), possibly coinciding with uplift of the Belt Island complex of south-central Montana. This uplift event was responsible for the regional configuration upon which the Middle Jurassic seaway was inundated.

To summarize these conclusions, the Triassic-Jurassic boundary within Park County,

Wyoming has been found to be represented by the contact between three varying formations of the Triassic Chugwater Group (in descending order); the late Middle Triassic Crow Mountain

Formation, the Middle Triassic Alcova Formation, and the Early Triassic Red Peak Formation.

The Red Peak Formation is represented at the Triassic-Jurassic boundary by portions of the

Upper Platy and Alternating facies of High and Piccard (1967), which coincide with RP-7 through

RP-5 of this study. Throughout the majority of the study area the variable Triassic strata are overlain by basal gypsum unit of the Middle Jurassic Gypsum Spring Formation. As previously discussed this contact represents the J-1 surface as described by Pipiringos and O’Sullivan

(1978). This is supported by the presence of the siltstone conglomerate that was observed at the South Cody outcrop of this study and discussed by Schmude (2000). The siltstone breccia, and the lack of the characteristic basal gypsum unit of the Gypsum Spring Formation at the

Clark’s Fork Canyon outcrop, may suggest that the Triassic-Jurassic boundary at this locality is represented by the J-2 surface of Pipiringos and O’Sullivan (1978). However, it is likely more plausible that this boundary is represents the J-1 surface and the basal gypsum unit was either not deposited due to a local paleotopographic high, or a potential facies change in the gypsum interval. Further research on this topic is recommended as it is needed to support a sound reconstruction of the stratal relationships between the Gypsum Spring and Piper Formations and the Triassic-Jurassic boundary.

CHAPTER NINE

FUTURE RESEARCH

As previously discussed, the Mesozoic stratigraphy of the Bighorn Basin continues to be a perplexing topic due to several factors, including: poor faunal diversity, incomplete type sections, and variable nomenclature. As discussed in this study and by Gilbert (2012), disagreement surrounding the stratal relationships between the Gypsum Spring and Piper

Formations continues to hinder the sound reconstruction of the Triassic-Jurassic boundary in portions of northern Wyoming and Southern Montana. Future research focused on the chemostratigraphic signatures of these units is suggested as means to affirm or dispute their equivalencies and to better establish their stratigraphic relationship. This would also aid in the delineation of the proper placement of the J-2 surface which may act as a correlatable time constraint within these Middle Jurassic intervals with lacking faunal diversity.

While studies of the Chugwater Group are abundant, they have decreased in frequency since the works of High and Piccard during the 1960’s-1970’s. Lovelace and Lovelace have led the resurgence of research on this interval with their work in 2012 which has brought forth new information concerning the paleoecology and associated paleoenvironments of the Red Peak

Formation. However, the Chugwater Group records a vast span of time and continued research with modern techniques and approaches is needed to continue to unravel the complexities of these units. A chemostratigraphic analysis of the Chugwater Group could aid in the differentiation of conflicting formations boundaries by potentially identifying shifts in sediment source and depositional environments. In particular, the formation boundaries of the Red Peak,

Alcova, and Crow Mountain Formation should be targeted so that an agreement could be

reached on their true divisions which would aid in the utilization of correct nomenclature in future work.

Further studies utilizing correlation methods established in this report are also suggested, as it allows for a higher resolution of correlation of the Red Peak Formation. Outcrop gamma ray measurements from complete sections of the Red Peak Formation should be targeted to allow for sound reconstruction of outcrop to subsurface correlation. In other areas across northern Wyoming where the Triassic-Jurassic boundary is represented by the contact between the Red Peak Formation and overlying Middle Jurassic units, this method could be utilized to form a more complete understanding of the differential patterns of erosion on

Triassic sediment and how these patterns effected the deposition of Middle Jurassic strata.

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Fuentes, F., DeCelles P. G., Constenius, K. N., & Gehrels, G. E., (2011). Evolution of the Cordilleran foreland basin system in northwestern Montana, U.S.A. Geological Society of America Bulliten. 123(3/4): 507-533

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APPENDICES

APPENDIX A LIST OF WELLS

Well Name UWI Operator S-T-R Field Marathon Oil Federal 34-13 49029214060000 Co. 34-52N-106W N/A General Crude Carl R Krueger 49029200240000 Oil Co. 8-52N-105W N/A Equity Res Krueger 2 49029201830000 Corp. 17-52N-105W N/A Sohio Petroleum Govt-Jenkins 1 49029201710000 Co. 31-52N-104W N/A Phillips Buffalo Bill A 49029214870000 Petroleum Co. 26-53N-104W N/A Atlantic Buffalo Bill Unit 49029201720000 Richfield Co. 29-52N-103W N/A Federal 29 49029204790000 Federal Gov. 29-52N-103W N/A Quintana Hunt-Fee 49029209850000 Production Co. 32-52N-103W N/A State - Slam Badger Oil Dunk 49029213420000 Corp. 20-51N-103W N/A Sam Gary Jr. & ROB 34-12 49029216660000 Associates 34-51N-104W N/A Marathon Oil Govt-1 49029202930000 Co. 5-49N-105W N/A soco-ishawooa- feder 49029214680000 Snyder Oil Co. 7-49N-105W N/A Marathon Oil Full moon 1-52 49029215740000 Co. 22-52N-102W N/A General Sabine Nielson American Oil 1-50B 49029212220000 Co. 22-52N-102W N/A Coronado Oil AMAX-govt 49029068750000 Co. 24-52N-102W N/A Diamond-1 49029068600000 Husky Oil Ltd. 28-52N-102W N/A Odessa Natural Carper Federal 49029202910000 Corp. 10-51N-102W N/A Sabine Sabine-State 49029213460000 Production Co. 36-52N-102W N/A Diamond Creek 49029211430000 Davis Oil Co. 29-52N-101W N/A Coronado Oil Arnold -Govt 49029321480000 Co. 7-51N-101W N/A Goldmark Federal 1-14 49029213130000 Energy Co. 14-51N-102W Half Moon Marathon Oil Morrison 41 49029213900000 Co. 23-51N-102W Half Moon Morrison 32 49029211490000 Husky Oil Ltd. 23-51N-102W Half Moon Morrison 25 49029210390000 Husky Oil Ltd. 23-51N-102W Half Moon

Well Name UWI Operator S-T-R Field Goldmark Federal 4-22 49029211860000 Energy Co. 22-51N-102W Half Moon Federal 5-22 49029211570000 Husky Oil Ltd. 22-51N-102W Half Moon Goldmark Federal 5-27 49029211420000 Energy Co. 27-51N-102W Half Moon Morrison 11 49029055490000 Husky Oil Ltd. 26-51N-102W Half Moon Morrison 9 49029055830000 Husky Oil Ltd. 26-51N-102W Half Moon Morrison 23 49029210440000 Husky Oil Ltd. 23-51N-102W Half Moon Morrison 12 49029055960000 Husky Oil Ltd. 23-51N-102W Half Moon Morrison 31 49029211500000 Husky Oil Ltd. 23-51N-102W Half Moon Morrison 40 49029212180000 Husky Oil Ltd. 26-51N-102W Half Moon Federal 13-10 49029213000000 Apache Corp. 13-51N-102W N/A Sabine Federal Sabine 1-34 49029213470000 Production Co. 34-51N-101W N/A Jones L. E. Hunt 1-8 49029213920000 Production Co. 8-50N-102W N/A Terra State-9 49029213490000 Reseources Inc. 26-50N-102W Ferguson Ranch Terra Hunt 10-26 49029213500000 Reseources Inc. 26-50N-102W Ferguson Ranch Sabine Federal 1-28 49029214070000 Production Co. 28-50N-101W N/A Phelps 13 49029207000000 Texaco Inc. 2-49N-102W Spring Creek Spring Creek 40 49029213350000 Texaco Inc. 2-49N-102W Spring Creek Unit-33 49029211920000 Texaco Inc. 13-49N-102W Spring Creek Spring Creek Unit 45 49029214220000 Texaco Inc. 13-49N-102W Spring Creek Samedan Oil Ranch Thomas 49029205040000 Corp. 9-49N-102W N/A Excel Energy Excel State 11-3 49029211470000 Corp. 3-48N-103W Rose Creek Federal Breene Royal Resources 1-6 49029201520000 Corp. 6-48N-102W N/A Rawhide Federal The National Oil 33-30 49029210900000 Co. 30-49N-101W N/A Rawhide 5 49029210430000 Husky Oil Ltd. 5-48N-101W N/A Pitchfork 65 49029209510000 Husky Oil Ltd. 2-48N-102W Pitchfork Pitchfork 70 49029209400000 Husky Oil Ltd. 11-48N-102W Pitchfork Louisiana Land N/A (East of Pitchfork 42-12 49029212240000 & Exploration 12-48N-102W Pitchfork) Unit 75 49029209660000 Husky Oil Ltd. 11-48N-102W Pitchfork

Marathon Oil Pitchfork 85 49029214320000 Co. 14-48N-102W Pitchfork

Well Name UWI Operator S-T-R Field Pitchfork N/A (south of Federal 1-24 49029210220000 Beard Oil Co. 24-48N-102W Pitchfork) Wyoming Gould-1 49029217730000 Wildcat Oil Co. 17-48N-100W N/A Sabine Schultheis 1-4 49029213530000 Production Co. 4-47N-101W N/A Federal 9-1 49029213860000 Apache Corp. 9-47N-101W N/A May #9 49029213710000 Conoco Inc. 22-47N-101W Sunshine North Quintana Aspen Creek 1 49029210040000 Production Co. 26-46N-102W N/A State 36-4 49029213630000 Apache Corp. 36-48N-103W N/A Deuel State 12- The National Oil 31 49029210840000 Co. 31-48N-102W N/A State 36-11 49029213230000 Apache Corp. 36-47N-102W N/A Amoco Little Buffalo Unit 21 49029207720000 Production Co. 34-48N-100W Basin Marathon Oil Little Buffalo Buffalo 20-1 49029210260000 Co. 2-47N-100W Basin US NWD LBB- Amoco Little Buffalo 4 49029202740000 Production Co. 10-47N-100W Basin Amoco Little Buffalo Unit 231 49029209340000 Production Co. 2-47N-100W Basin Buffalo Creek Fed-1 49029206010000 Gulf Oil Corp. 32-48N-99W N/A Amoco Little Buffalo Unit 233 49029209710000 Production Co. 1-47N-100W Basin Little Buffalo Amoco Little Buffalo BN-225 49029209430000 Production Co. 1-47N-100W Basin Little Buffalo Amoco Little Buffalo BN-224 49029209460000 Production Co. 1-47N-100W Basin Amoco Little Buffalo Unit 348 49029213480000 Production Co. 12-47N-100W Basin Amoco Little Buffalo Unit 193 49029208220000 Production Co. 13-47N-100W Basin Amoco Little Buffalo Unit 189 49029208440000 Production Co. 13-47N-100W Basin General American Oil Federal 1-20 49029209020000 Co. 20-52N-100W Wiley General Dace-Federal 1- American Oil 30 49029210160000 Co. 30-52N-100W Oregon Basin Northwest Wiley Canal 1 49029208860000 Exploration Co. 1-51N-101W Oregon Basin

Well Name UWI Operator S-T-R Field Marathon Oil Owens A-11 49029211320000 Co. 29-52N-100W Oregon Basin Marathon Oil Owens A-9 49029208110000 Co. 29-52N-100W Oregon Basin Marathon Oil Frisby A-21 49029214830000 Co. 35-52N-100W Oregon Basin Marathon Oil Atherly 10 49029211330000 Co. 33-52N-100W Oregon Basin Marathon Oil Atherly 9 49029211360000 Co. 33-52N-100W Oregon Basin Marathon Oil Custer 46 49029217200000 Co. 31-52N-100W Oregon Basin Marathon Oil Custer 35 49029214140000 Co. 5-51N-100W Oregon Basin Marathon Oil Sidney 14 49029208830000 Co. 5-51N-100W Oregon Basin Marathon Oil Pauline 13 49029212080000 Co. 5-51N-100W Oregon Basin Marathon Oil Sonners A/C 6 49029215780000 Co. 17-51N-100W Oregon Basin Marathon Oil Freeman 10 49029210700000 Co. 20-51N-100W Oregon Basin Marathon Oil Ostland 2 49029210240000 Co. 19-51N-100W Oregon Basin Marathon Oil Orchard 18 49029211600000 Co. 28-51N-100W Oregon Basin Marathon Oil Sarah 58 49029211230000 Co. 32-51N-100W Oregon Basin Marathon Oil Baston-A 20 49029209150000 Co. 6-50N-100W Oregon Basin Marathon Oil Baston-B 23 49029214030000 Co. 5-50N-100W Oregon Basin Marathon- Marathon Oil Texaco-Fed 1 49029208050000 Co. 10-50N-100W Oregon Basin Sohio-Federal 1- Santa Fe Energy 13 49029209080000 Corp. 13-50N-100W N/A USA 9613 16-24 49029210670000 Anschutz Corp. 24-50N-100W N/A Terra Federal 1-33 49029208910000 Reseources Inc. 33-49N-99W Meeteetse Catfish unit 1 49029210890000 Davis Oil Co. 33-53N-100W N/A McWilliam 7- 58A 49029213390000 Husky Oil Ltd. 4-52N-101W Cody Marathon Oil Cody unit 19-69 49029214180000 Co. 35-53N-101W Cody Reitz 6-71 49029209160000 Husky Oil Ltd. 35-53N-101W Cody

Well Name UWI Operator S-T-R Field Marathon Oil Cody unit 20-69 49029214240000 Co. 35-53N-101W Cody Walliker Etal 6- 69 49029206830000 Husky Oil Ltd. 34-53N-101W Cody Marathon Oil Tomilson 16-38 49029214090000 Co. 27-53N-101W Cody Marathon Oil Cody unit 23-69 49029215080000 Co. 35-53N-101W Cody Cricket 7 49029211520000 Husky Oil Ltd. 28-53N-101W Shoshone Cricket 9 49029211730000 Husky Oil Ltd. 21-53N-101W Shoshone Merit Energy Cricket 16 49029309470000 Co. 21-53N-101W Shoshone Shoshone State- 6 49029211440000 Husky Oil Ltd. 17-53N-101W Shoshone Federal 3-17 49029210330000 Husky Oil Ltd. 17-53N-101W Shoshone Diamond Mola Federal Shamrock 12-13 49029214260000 Exploration Co. 13-53N-102W N/A Marathon Oil Cody unit 22-69 49029214460000 Co. 3-52N-101W Cody West Heart Mountain State KGH Operating West Heart 1 49029217300000 Co. 12-54N-103W Mountain North American Two Dot 2-27 49029211990000 Resources Co. 27-55N-103W N/A Coulee unit 1 49029207190000 Davis Oil Co. 5-55N-100W N/A Impel federal 1- 23 49029206180000 Impel Corp. 23-58N-103W N/A Dow federal 1- Natomas N. 34 49029210850000 America 34-58N-100W Silvertip east Two Dot 2 49029206340000 Karogen Corp. 12-56N-100W Bearcat Continental Oil SEB unit 45 49029206820000 Co. 17-57N-99W South Elk Basin Amoco Elk Basin 333 49029211130000 Production Co. 36-58N-100W Elk Basin Sun Exploration & Production E R Dyer 1 49029215450000 Co. 22-58N-98W North Frannie Frannie PT unit Continental Oil 140 49029206590000 Co. 23-58N-98W Frannie Frannie unit 146 49029213870000 Conoco Inc. 25-58N-98W Frannie Sage Creek Cabot Petroleum federal 2-1 49029215730000 Corp. 1-57N-98W Sage Creek Federal 42-10 49029212000000 National Oil Co. 10-57N-98W Sage Creek

Well Name UWI Operator S-T-R Field Badger Oil Lauren federal 1 49029209780000 Corp. 13-57N-98W Sage Creek Goldmark Federal 11-14 49029213360000 Energy Co. 14-57N-98W Sage Creek Evergreen- Hilliard Oil & Wagner 1 49029212710000 Gas Inc. 13-57N-98W Sage Creek PC federal 6 49029215960000 Sierra Energy 31-57N-98W Little Polecat Unit 24-16 49029206350000 Texaco Inc. 16-56N-98W Whistle Creek Mantua Draw 49029211170000 Forest Oil Corp. 35-56N-98W Garland Kinney Costal Marathon Oil 82 49029210050000 Co. 14-56N-98W Garland Kinney Costal Marathon Oil 88 49029212120000 Co. 13-56N-98W Garland Kinney Costal Marathon Oil 81 49029207770000 Co. 24-56N-98W Garland Kinney Costal Marathon Oil 72 49029206870000 Co. 25-56N-98W Garland

APPENDIX B TYPE LOG

APPENDIX C LIST OF SAMPLES

Clarks Fork Canyon, Wyoming ( Lat: 44.864218° Long: -109.298791° )

Sample I.D. Unit Thickness (ft.) Thin Section Description Carbonate bench within middle Jurassic section. Marine fossil CF-1 approx.: 5.00 fragments present. Grey-light purple, fine grained silt/sandstone. Chert? clasts CF-2 6.00 x (brecciated) present Yellow/golden, fine grained silt/mudstone. Fissile bedding. CF-3 10.20 x Grades downward to maroon, fissile, siltstone Gold-maroon, very fine grained sandstone/siltstone. Thin/fissile CF-4 5.00 x bedding CF-5 5.00 x light purple-grey, siltstone. 1-3" beds "Brick" red, fine grained sandstone. Grades upward from CF-6 20.00 x massive bedding to thin bedding Red-orange, fine-medium grained sandstone. Wavy bedding CF-7 5.00 x with small "cm" scale cross beds. Fe reduction spots. CF-8 3.50 "Brick" red, fine grained sand/siltstone. Fissile bedding red-orange, fine grained sandstone. Massively bedded. Lacking CF-9 1.50 x sedimentary structures Red, very fine grained sandstone. Thinly bedded. "grey" Fe CF-10 6.00 reduction spots present Maroon-orange, siltstone, thinly bedded. Coarsens upward to CF-11 3.00 fine grained sandstone, massively bedded Maroon-red, fine-medium grained sandstone. Well cemented. CF-12 25.00 x Forms "fin". Ripple marks present at top. "Brick" red, very fine grained sandstone. Thin/wavy bedding. CF-13 5.00 x Color leaching present at base

Red, fine grained sandstone/siltstone. Large ripples marks CF-14 8.00 x present at top. Small scale cross bedding visible. Red-Maroon siltstone/ very fine grained sandstone. Thinly CF-15 4.00 bedded at top-massive bedding at base Maroon-red, medium grained sandstone. Massively bedded CF-16 19.00 with intervals of thin bedding. Fe reduction spots. Red-orange, very fine grained siltstone/mudstone. Very small CF-17 3.50 "mm scale" crossbedding Maroon-red, siltstone at base, coarsens upward to fine grained CF-18 15.00 sandstone. Ripples marks present at top Red, fine grained sandstone, massively bedded at top with thin CF-19 11.00 bedding at base. Ripple marks and cross bedding. Red/irregular streaks of grey (Fe reduction), fine grained CF-20 12.00 sandstone/siltstone. Massively bedded Red-Maroon, siltstone/mudstone. Very thin/wavy bedding. CF-21 5.00 Fissile and friable "Brick" red, siltstone at base coarsening upward to fine grained CF-22 15.00 sandstone at top. Wavy bedding at base Red with local grey spots (Fe reduction), very fine CF-23 11.00 siltstone/mudstone. Cross bedding visible CF-24 10.00 Maroon-red siltstone, thinly bedded (mostly covered) CF-25 17.00 Red, fine grained sandstone/siltstone. Massively bedded Red, interbedded mudstone/siltstone. Thin bedding at base/ CF-26 16.00 massive bedding at top.

100

Hogan Reservoir "Carnivore Corner", Wyoming ( Lat: 44.794696°Long: -109.281303°) Sample I.D. Unit Thickness (ft.) Thin Section Description HR-1 10.00 x red, fine grained sandstone. Massively bedded, poorly consolidated "Brick" red, very fine grained sandstone. Highly cemented (sampling HR-2 5.00 difficult) HR-3 8.00 Maroon-red, fine grained sandstone/siltstone. No sample taken. HR-4 5.00 x Red, fine grained sandstone. Massively bedded, gypsum viens (?) HR-5 6.00 x Maroon-red, fine grained sandstone/siltstone. Massively bedded Pale red, very fine grained sandstone/siltstone. Small (cm) scale HR-6 4.00 x cross bedding present HR-7 5.00 x Yellow-tan, very fine grained sandstone/siltstone. Massively bedded yellow-tan with sporadic zones of maroon, very fine grained HR-8 10.00 x sandstone/siltstone. Wavy bedding yellow-tan with sporadic zones of maroon, very fine grained HR-9 9.00 sandstone/siltstone. Wavy bedding yellow-white, very fine grained sandstone/siltstone. Wavy bedding. HR-10 12.00 x Irregular contact with overlying gypsum White, massively bedded gypsum. Pinches out at northern end of HR-11 0-6.00 x outcrop. Approx. 5' at southern end of outcrop.

101

Chief Joseph Highway "Dead Indian Hill", Wyoming ( Lat: 44.712350° Long: -109.295454° ) Sample I.D. Unit Thickness (ft.) Thin Section Description DIH-1(a) 5.25 x Red, fine grained sandstone. Massively bedded DIH-2(b) 80.00 x Highly covered* Red, siltstone/mudstone. DIH-3(d) 7.00 x Red, fine grained sandstone/siltstone DIH-4(e) 1.00 x light grey, white gypsum / anhydrite (?)

102

South Cody “Raven Ridge”, Wyoming ( Lat: 44.480628°Long: -109.090988°) Sample I.D. Unit Thickness (ft.) Thin Section Description RR-1A 4.00 x Maroon-red, silty mudstone. Fe reduction spots present RR-1B 5.00 x Red, siltstone. Fe reduction spots present Pale red, conglomeratic/brecciated siltstone. Contains irregular RR-2 3.00 x clasts of chert(?) RR-3 8.00 white, gypsum. Massively bedded/brecciated

103

APPENDIX D MEASURED SECTIONS

104

D-1: Clark’s Fork Canyon

105

106

107

108

D-2: Hogan Reservoir

109

110

D-3: Chief Joseph Highway “Dead Indian Hill”

111

112

D-4: South Cody “Raven Ridge”

113

PLATES

114

115

116

117

118

119

120

121

122

123

Plate 10: South – North Cross-section

124

Plate 10-1: South – North Cross-section (Left-South)

125

Plate 10-2: South – North Cross-section (Right-North)

126