Stratigraphy, Paleotectonics and Paleoenvironments of the Morrison Formation in the Bighorn Basin of Wyoming and Montana Dibakar Goswami Iowa State University

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1993 Stratigraphy, paleotectonics and paleoenvironments of the Morrison Formation in the Bighorn Basin of Wyoming and Montana Dibakar Goswami Iowa State University

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Stratigraphy, paleotectonics and paleoenvironments of the Morrison Formation in the Bighorn Basin of Wyoming and Montana

Goswami, Dibakar, Ph.D.

Iowa State University, 1993

UMI 300 N. ZeebRA Ann Aibor, MI 48106 stratigraphy, paleotectonics and paleoenvironments of the

Morrison Formation in the Bighorn Basin

of Wyoming and Montana

Dibakar Goswami

A dissertation Submitted to the

Graduate Faculty in Partial Fulfillment of the

Requirements for the Degree of

DOCTOR OF PHILOSOPHY

Department: Geological and Atmospheric Sciences Major: Geology

ipprovi

Signature was redacted for privacy.

Charg^^f Major Work

Signature was redacted for privacy.

For the Graduate College

Iowa State University Ames, Iowa

1993 ii

TABLE OF CONTENTS

Page

GENERAL INTRODUCTION 1 Statement of the Problem 1

Research Objectives 6

Explanation of Format 7

Methods of Study 7

PAPER I. REGIONAL STRATIGRAPHY AND PALEOTECTONICS OF THE MORRISON FORMATION IN THE BIGHORN BASIN OF WYOMING AND MONTANA 10

ABSTRACT 11

INTRODUCTION 13

GEOLOGIC SETTING 15

REGIONAL STRATIGRAPHY OF THE MORRISON FORMATION 20

Upper Contact 2 0

Lower Contact 27

Subsurface Contact 28

Interpretation of Subsurface Geophysical Data 30

Stratigraphy 37

PALEOTECTONICS 61

CONCLUSIONS 69

REFERENCES 71

APPENDIX A. LOCATION OF STRATIGRAPHIC SECTIONS 83

APPENDIX B. GRAPHIC STRATIGRAPHIC SECTION 86

APPENDIX C. SUBSURFACE WELL LOG DATA OF THE MORRISON FORMATION IN THE BIGHORN BASIN OF WYOMING AND MONTANA 102 iii

PAPER II. FACIES AND DEPOSITIONAL ENVIRONMENTS OF THE MORRISON FORMATION IN THE BIGHORN BASIN OF WYOMING AND MONTANA 107

ABSTRACT 108

INTRODUCTION 110

LOWER MORRISON 112

Quartzarenite Lithofacies Association 112

Interbedded Silty Claystone and Sandstone Lithofacies Association 126

UPPER MORRISON 136

REGIONAL SYNTHESIS OF THE PALEOGEOGRAPHY AND DEPOSITIONAL ENVIRONMENTS 150

CONCLUSIONS 154

REFERENCES 156

GENERAL CONCLUSIONS 166

ACKNOWLEDGEMENTS 169 iv

LIST OF FIGURES

Page

GENERAL INTRODUCTION

Figure 1. Location map of the study area 2

Figure 2. Geological map of north-central Wyoming 3

PAPER I

Figure 1. Map showing generalized paleotectonic framework of the western United States during the Late Jurassic 16

Figure 2. East-west 1ithostratigraphic correlation of the Morrison, Cloverly, Ephraim Conglomerate of the Gannet Group, and the overlying formations across the state of Wyoming 18

Figure 3. Pre 1973 correlations and age inter pretations 21

Figure 4. Identification of the Morrison-Cloverly contact from electrical logs 29

Figure 5. Quantitative log analysis showing the use of both porosity and V-clay in the interpretation of thin sandstone beds 33

Figure 6. Quantitative log analysis of thin sandstone beds 35

Figure 7. Idealized section of the Morrison Formation showing the vertical distribution of environment, associated lithology, sandstone geometry, and architecture 38

Figure 8. North-south cross-section of the Morrison and Cloverly formations across the Bighorn Basin of Wyoming 39

Figure 9. East-west cross-section of the Morrison and Cloverly formations across the Bighorn Basin of Wyoming 41 V

Figure 10. Isopach map of the Morrison Formation in the Bighorn Basin of Wyoming 44

Figure 11. Sand-Shale ratio map of the Morrison Formation in the Bighorn basin of Wyoming 47

Figure 12. Photograph showing the quartzarenite lithofacies association of the lower Morrison in the southern part of the Bighorn Basin of Wyoming. Location: near Thermopolis, Wyoming 51

Figure 13. Isopach map of the eolian sandstone (quartzarenite lithofacies) of the Morrison Formation 53

Figure 14. Photograph of the channel sandstone lithofacies association of the Morrison Formation showing lateral accretion surfaces. Location: One mile south of Brock Ranch, Mayoworth 57

Figure 15. Photograph of the interbedded siItstone and claystone lithofacies association of the upper Morrison. The contact (person standing) between the Morrison and Cloverly formations is on top of the salmon colored unit below the white ash bed. Location: Coyote Basin 57

PAPER II

Figure 1. Graphs comparing cumulative probability curves of grain size analysis of the eolian sandstones (quartzarenite lithofacies) of the Morrison Formation with modern dune sand 114

Figure 2. Figure showing the paleowind directions for lower Morrison eolian sandstones (quartzarenite lithofacies) at various locations. Number of foreset dip directions measured is represented by n. The mean paleowind direction is represented by the arrow 116

Figure 3. Photograph showing flat-bedded quartz arenite subfacies of the lower Morrison in the southern part of the Bighorn Basin, Wyoming. Location: Thermopolis 118 vi

Figure 4. Photograph showing crossbedded quartzarenite subfacies of the lower Morrison in the western flank of the Powder River Basin. Location: Kaycee 118

Figure 5. Photograph showing silicified tree trunk in the crossbedded quartzarenite subfacies of eolian origin, lower Morrison. Location: SE Baker Cabin Road 125

Figure 6. A typical electric log response of the quartzarenite lithofacies interpreted to represent an eolian deposit. The high resistivity with sharp base is very typical of a clean eolian sandstone 127

Figure 7. Photograph showing burrows ("skolithos" trace fossil) in the interbedded claystone and sandstone lithofacies association of the lower Morrison Formation. Location: Coyote Basin 129

igure 8. Photograph showing escape burrows in the interbedded claystone and sandstone lithofacies association of the lower Morrison Formation. Location: Fox Mesa 129

Figure 9. Photograph showing lateral accretion surfaces in the channel sandstone lithofacies. Location: Coyote Basin 139

Figure 10. Channel sandstone lithofacies of the upper Morrison showing gradual decrease in the cross bed set thickness upward and multi-story channel sandstone deposits. Location: Fox Mesa 143

Figure 11. Figure showing the mean paleocurrent directions of the upper Morrison fluvial sandstones in the Bighorn Basin of Wyoming and Montana. The number of directions measured is represented by n. Mean paleocurrent direction is represented by the arrow 144

Figure 12. Electric log of a sandstone interpreted to represent a braided stream channel deposit in the upper part of the Morrison Formation. Note the cylindrical shape of the log 147 vii figure 13. Electric log of a sandstone interpreted to represent a meandering stream channel deposit in the Morrison Formation, Bighorn Basin, Wyoming. Note the fining upward sequence shown by the gradual decrease in resistivity curve 148 1

GENERAL INTRODUCTION

Statement of the Problem

The Morrison Formation is one of the most

widespread units in the Western Interior. Figure 1 shows the location of the present study area. Figure 2 shows

the outcrops of the Upper Jurassic and Lower Cretaceous

Morrison and Cloverly formations in the Bighorn Basin and

along the western flank of the Powder River Basin of

Wyoming and Montana. Geologic investigations of the Upper

Jurassic nonmarine Morrison Formation in the Western

Interior date back to the end of the last century. Most

studies of the Morrison Formation in these two basins were

in the form of broad geologic descriptions concerned with

establishing the regional stratigraphy and provenance.

The initial studies were by paleontologists and

stratigraphers interested in dinosaur remains preserved in

the mudstones and siltstones and in the lithology and

regional extent of the units. One of the reasons for

neglected study is due to geologists having concentrated

their studies in the more intriguing and comprehensible

fossiliferous marine deposits above and below the

Morrison. Further south, in the Colorado Plateau area,

the Morrison has received rather intensive study since 2

109 108 107 * BILLINGS 'Montana

IDAHO ^%L WYOMING

7bËA^ TOOTH MTNS. PAYOR MXNS

45 —

. 9 ».» ^ 4 » LOVELL 9 4 SHERIDAN » & »««V

•^yrovx-.f •CODY SHELL < u. 4 » •» 4» »« » V GREYBULL a « » IL •S BIGHORN BASIN #*\BIGHORN-vi'MTNS. > ^ # » & » •••n

44 — WORLAND' ' * «' '»»A TENSLEEP ./.V tt' ,' » » « » 4 '

THERMOPOLIS M'-i»', w

RIVER MTNS.

WIND RIVER BASIN 109 108 107 I _L

Figure 1. Location map of the study area Figure 2. Geological map of north-central Wyoming. The Morrison and Cloverly outcrops are shown in black. The arrows show the location of measured sections. 1. RED LODGE

MONTANA

WYOMING

• CODY

HYATTVILLE

^ ~ \ THER110F0LISoiim'PMOi ^0WL:.:;i2 30 KmI

Morrison-Cloverly outcrops 5 the middle of the last century when O.C. Marsh discovered the first of a prolific Morrison dinosaur fauna. Interest

in the formation was stimulated again following World War

II, when uranium was discovered in Morrison sandstones in

New Mexico and Utah. In Montana, away from the Bighorn

Basin, the Morrison has been the subject of economic

exploitation of coal and groundwater. However, this is

not so in the Bighorn Basin.

Recently, the Upper Jurassic-Lower Cretaceous

nonmarine interval in Wyoming, Idaho, and Utah has been

studied in an attempt to understand the tectonics of the

Sevier fold and thrust belt and its associated foreland

basin. Unfortunately, these studies have had to rely

entirely on the less detailed regional studies which

predate the mid 1960s. As a result, correlations between

the basins still remain confused because of the complexity

of the fluvial architectural elements within the nonmarine

interval. Depositional environments were often

interpreted with little emphasis placed on the importance

of primary structures. Great strides have since been made

in the interpretation of depositional environments through

analysis of facies defined by primary structures, bed

geometries, etc. Fluvial sediments are better understood

today and it is now known that many depositional

subenvironments may exist within the broad fluvial 6 setting. Extensive subsurface geologic information in the form of various geophysical logs is available. This information is extremely important not only as an input for detailed stratigraphie correlation but also for the delineation and distribution of different facies in the subsurface. No such study of the Morrison has been made so far in the Bighorn Basin area.

Research Objectives

The primary goal of this research is to interpret the stratigraphie relationships and paleoenvironments of the Morrison Formation in the Bighorn Basin in Wyoming and

Montana, through detailed field and subsurface investigations. Stratigraphie and sedimentologic analyses define the processes that are responsible for the transport, deposition, and preservation of various lithofacies of the Morrison Formation. They also offer explanations for the regional variations in the thickness, and lithology of each lithofacies. Detailed quantitative geophysical log interpretation was carried out for analysis of shaly sandstones and thin sand beds.

The area of study also covers the western flank of the Powder River Basin. However, this study is broadly categorized as the study of the Morrison Formation in the 7

Bighorn Basin of Wyoming and Montana.

Explanation of Format

The dissertation is divided into two self-contained papers which describe two unique approaches to this problem. Paper I broadly describes the stratigraphy of the Morrison Formation in the study area and its contact relationships and age. Paper I contains an account of the interpretation of the subsurface geophysical data. A detailed account of the paleotectonics based mainly on the stratigraphie evidence is also discussed.

Paper II describes an analysis of the lithofacies and their depositional environments. An interpretation of the electric log characteristics of the corresponding facies is presented. Each paper has its own conclusion, and following the second paper is a general conclusion made from the entire study.

Methods of Study

Surface Geology

The Morrison Formation is exposed along the margin of the Bighorn and Powder River Basins of Wyoming and

Montana. During this study 74 outcrops that spanned from 8 the top of the underlying Sundance Formation of Late

Jurassic-Oxfordian age to the base of the Cloverly

Formation of the Lower Cretaceous age were measured and described. A number of incomplete exposures were also measured and described. More than five months were spent in the field gathering data. The composition, texture, sedimentary structures, geometry, and contact relationships of the individual strata that comprise the

Morrison Formation were studied in detail. Lateral relationships, aerial extent and paleocurrent directions were determined and measured. The strata at each outcrop were subjected to a variety of examinations. Special attention was given to the size, shape, and current direction of cross-stratification. The type of stratification and the continuity of beds were noted. Key exposures of the Morrison Formation are described in Paper

Subsurface Geology

A major part of this research was a study of the available subsurface data, the identification of various lithofacies, the construction of various facies maps, etc. and finally the interpretation of facies in terms of tectonics and sedimentation. The subsurface data were correlated with surface data. Thus the study involved 9 both surface and subsurface geology on a regional basis involving-stratigraphy, facies analysis, and interpretation of depositional environments. About 300 well logs were examined and correlated. Facies analyses were carried out to understand the stratigraphy of the

Bighorn Basin. Special attention was given in the log interpretation of shaly sand and to "thin bed" analysis.

The latest techniques were used and computer programs were developed to suit the available subsurface information. 10

PAPER I. REGIONAL STRATIGRAPHY AND PALEOTECTONICS

OF THE MORRISON FORMATION IN THE BIGHORN

BASIN OF WYOMING AND MONTANA 11

ABSTRACT

The post-Oxfordian Morrison Formation is exposed along the margins of the Bighorn and Powder River basins of Wyoming and Montana and is present in the subsurface throughout both basins. In all areas studied, the

Morrison Formation lies conformably on the marine glauconitic litharenite of the uppermost Sundance (or

Swift) Formation. The upper contact of the Morrison

Formation is disconformable wherever it is overlain by the chert-pebble conglomerate unit of the basal Cloverly

Formation. The hiatus between the Morrison and Cloverly formations is of much shorter duration (<10 m.y) than earlier thought. The Morrison Formation consists mainly of sandstone, siltstone, and variegated claystone with occasional limestones.

Along the southern margin of the study area the lower Morrison is represented by isolated lenses of thick massive to crossbedded quartzarenite lithofacies of eolian origin representing the final phase of coastal deposition along the retreating margin of the Late

Jurassic Sundance sea. Equivalent facies in the north and in the north central part of the Wyoming are dark green silty mudstones and fine grained moderately well sorted, bioturbated sandstones of intertidal origin. The upper 12

Morrison is characterized by interbedded claystone and siltstone lithofacies and a channel sandstone lithofacies associations. These are of fluvial origin and comprises

65% of the entire Morrison Formation.

Field as well as subsurface evidence suggest that positive structural elements were active within the central and north central parts of Wyoming during the Late

Jurassic. The distribution of various facies show a process-response relation between the deposition and the orogeny. The Morrison Formation is thin and sandier along paleotopographic highs and sandier along paleotopographic lows, which suggests active structural control of facies distribution during deposition. The position of the these structures are coincident with similar documented Late

Paleozoic highs (Stevenson and Baars, 1977; Peterson,

1984) and with mountain ranges and arches of Laramide age. 13

INTRODUCTION

The Morrison Formation was first defined by

Eldridge (1896), who designated exposures near the town

Morrison, Colorado, as the type section. However, Cross

(1894) was the first to publish the name "Morrison" using

it to designate equivalent beds near the Pikes Peak

quadrangle, just south of the type section. Waldschmidt

and Leroy (1944) redefined the type section, selecting

exposures in road cuts along the west Alameda Parkway near the exposure originally described by Eldridge. They

subdivided the formation into six major parts, on the

basis of lithological characteristics.

Darton (1906) was the first to make a relatively

detailed geologic map of the Morrison Formation in the

Bighorn Mountains of Wyoming. He described the formation

and correlated it on the basis of its lithologie

similarity and stratigraphie position with the Morrison he

had studied earlier along the Colorado Front Range.

Several other workers have studied the Morrison in the

area, notably Carlson (1949), Richardson (1950), Weitz,

Love, and Harbinson (1954), Mirsky (1962), Ostrom (1970),

Winslow and Heller (1987), and Kvale (1986). These

studies were not comprehensive and did not incorporate the

available subsurface information. 14

Recently, the Jurassic and Cretaceous sediments were studied to better understand the tectonics of the early Sevier foreland basin and associated fold and thrust belt (Burchfield et a%., 1975; Dickinson, 1976; Jordan,

1981; Wiltschko and Dorr, 1983). Unfortunately, the study had to rely partly or entirely on the less detailed regional stratigraphie literature which predates the mid

1960s. Because of many difficulties such as the lack of fossil evidence, superficially similar lithologies, absence of datable horizons, etc. The Morrison and

Cloverly formations were mapped as "undifferentiated"

(Andrew et , 1947, Love and Christensen, 1985).

This study incorporates a detailed stratigraphie and sedimentologic analysis of the Morrison Formation in the Bighorn Basin of the Wyoming and Montana and the western flank of the Powder River Basin where excellent exposures allow detailed close examination of the Upper

Jurassic sequence. The location of the measured and described exposures is given in Appendix A and B respectively. In addition, detailed subsurface geophysical well log interpretations were also incorporated in the present study. 15

GEOLOGIC SETTING

During the Late Jurassic-Late Cretaceous time, the western margin of North America was occupied by subduction zone and its associated magmatic arc (Dickinson, 1976;

Burchfield and Davis, 1975). As the convergence progressed, an accretionary melange wedge, an outer arc basin, and a foreland thrust belt were formed. By the end of the Late Jurassic, a largely nonmarine retroarc foreland basin developed to the east of the belt which approximately paralleled the magmatic arc (Eisbacher, et al.. 1974, Dickinson, 1976). This foreland basin developed into the vast Western Interior Basin which extends from northern Canada to the Gulf of Mexico and is locally over 1600 km. wide from east to west (Figure 1).

The Upper Jurassic Morrison Formation and the overlying

Lower Cretaceous Cloverly and its equivalent deposits were deposited in this foreland basin as a laterally extensive sequence of fluvial and lacustrine sediments that marks a break in marine sedimentation in the Western Interior in

North America. It extends from southern New Mexico northward into Alberta, Canada and from the edge of craton well into the Cordilleran fold and thrust belt to the west. The term Morrison has been applied to the Upper

Jurassic nonmarine deposits in Colorado, Utah, New Mexico, 16

CANADA FB A-A "USA

H/Study Area' r-'o

SFTB

500 km

Figure 1. Map showing generalized paleotectonic framework of the western United States during the Late Jurassic (compiled from Dickinson, 1976 and Peterson, 1986; FB, forearc basin; SFTB, Sevier fold and thrust belt; black area represents Franciscan subduction complex) 17 and throughout much of Wyoming and South Dakota. However, in the westernmost Wyoming the lower part of the Ephraim

Conglomerate of the Gannet Group has been equated with the

Morrison Formation (Eyer, 1969; Purer, 1970; Waneless et al.. 1955). In western South Dakota along the southern flank of the Black Hills uplift, the Unkpapa Sandstone has been correlated with the lower part of the of the

Morrison (Imlay, 1947). Figure 2 shows the lithostratigraphic correlation of the Gannet Group and the equivalent Morrison and Cloverly formations in Wyoming.

The top of the Tygee and Muddy sandstones was used as the upper datum. The figure incorporates the data obtained from the present study in the Bighorn basin. In the

Bighorn Basin, these sediments accumulated as a clastic wedge thinning from eastern Idaho, western Wyoming, and central Montana to central Wyoming and then thickening again towards the Black Hill region. Figure 2. East-West lithostratigraphic correlation of the Morrison, Cloverly, Ephraim Conglomerate of the Gannet Group and the overlying formations across the state of Wyoming using the top of Tygee and Muddy sandstones as an upper datum (modified after Winslow, 1986).

Data sources: at A. (1) Section 26, Eyer, 1969; (2) Section 11, Blackstone, 1948; (3) Section 5, Blackstone, 1948; (4) Section GV-1, Purer, 1966; (5) Section 1, Blackstone, 1948; (6), (7), & (8) Sections from the present Study; (9) Section 19 and 21, Robinson et , 1964 and cross section B-B, Weimer et al., 1982 A' 9 2 3 6 MUDDYSS 7 TYGEE SS J L I I 1 LOWER BEAR RIVER FM THERMOPOLIS FM. SKULL CREEK SH. RUSTY BEDS INYAN SMOOT FM CLOVERLY FM KARA O^RRISO^ GRR

SUN DANC

Q. 4 3 O CK O M VO

I- UJ z z § L

60Km WYOMING IDAHO-WYOMING THRUST BELT 90 Km 20

REGIONAL STRATIGRAPHY OF THE MORRISON FORMATION

Upper Contact

Previous studies have suggested the presence of a major hiatus between the Morrison Formation, commonly interpreted as Late Jurassic in age, and the Lower

Cretaceous conglomeratic deposits of the Cloverly

Formation (Yen, 1951, 1952; Peck and Craig, 1962; Ostrom,

1970; Bell, 1956; Peck and Craig, 1962; Eyer, 1969;

Winslow and Heller, 1987). These deposits primarily represent widespread fluvial sedimentation that occurred throughout the Western Interior of the United States and

Canada. While the lower deposits retain the name of the

Morrison Formation throughout the region and the Colorado

Plateau of the United States (or the Kootenai Group in

Canada), the overlying conglomeratic sandstone is known by a variety of names in the United States and Canada (Figure

3). In Wyoming these conglomerates are included in the

Lower part of the Lakota Formation in the Black Hills, the upper member of the Ephraim Conglomerate in western

Wyoming, and the basal portion of the Cloverly Formation elsewhere.

A disconformity occurs at the basal channel scour surface which separates the Lower Cretaceous conglomeratic I COLORADO N. FRONT RANGE WESTERN CENTRAL EASTERN WESTERN WESTERN PLATEAU COLORADO WYOMING WYOMING WYOMING AGE MONTANA ALBERTA s (TSCHUOV. 1884) (MACKENZIE.IS7I) (FURERigrO) (QCHERlseO) (McGOOKEY,l972) (SUTTNEH. 1888) ( VARLEV,1984) LU .(MA) (PETEnSOW.1972) (PETERS0N.iar2) (EYEM969) (PETERSON, 1872) (PETERSOK1S72) (PETER30N.1972) (RU0KIN.1964) MOWRY ASPEN MOWRY MOWRY MUDDY BEAR RIVER SS. kWDDY NEWCASTLE COLORADO MILL CR. GROUP CO ALBIAN SKULL THERMOPOUS SCULL CREEK CREEK CEDAR RUSTY BEDS FALL RIVER BEAVER S PtAINVlEW SMOOT GREYBULL MINES -113 MTN ORANEY IS. •riflnlHIIIilllIln BECHLER FUSON GLADSTONE W LYTLE PETERSON LS. (U APTIAN BLKikWM» B^AIM. CLOVERLY KOOTENAI oc 119 .LAKOTA CADOMIN; o BARAEMIAN 124 HAUTERIV. 3 oc < 131 m VALANG. 138 BERRTASIAN 144 0 T1TH0N1AN LOWER MORRISON MORRISON MORRISON MORRISON MORRISON 1 152 EPHRAIM Z) KIMMERIDQAN -3 156 FERNIE SAN GROUP OXFORDIAN ARAPIEN RALSTON RAFAEL STUMP SUNDANCE SUNDANCE SWIFT

Figure 3. Pre 1973 correlation and age interpretation (adapted after Winslow and Heller, 1987) 22 sandstone (Pryor Conglomerate) from the underlying mudstone, siltstone, and sandstone of the Morrison

Formation. The contact was placed here primarily because of 1) significant differences in fossil faunae between the two sequences (Yen, 1951, 1952; Yen and Reeside, 1946;

Peck and Craig, 1962; Ostrom, 1970); and 2) similarities of faunae found above the Lower Cretaceous conglomerates with that of Aptian to Albian age elsewhere (Berry, 1929;

Bell, 1956; Peck and Craig, 1962; Eyer, 1969).

The duration of the hiatus between the Morrison and

Cloverly has been a subject of debate for over a century.

Most authors contend that the hiatus was of long duration

(>25 m.y.) and represents an extended period of nondeposition or erosion that lasted through most of

Neocomian time (Figure 3) (Cobban and Reeside, 1952; Peck and Craig, 1962; Ostrom, 1970; McGookey, 1972). The introduction of coarse grained sand and gravel following the hiatus is frequently interpreted as the progradation of syntectonic detritus derived from the uplift and erosion of the Sevier orogenic belt in the United States

(Mirsky, 1962; Kvale, 1985; DeCelles, 1985; Conley 1985) or the Columbian orogenic belt in western Canada (Price and Mountjoy, 1970; Eisbacher et ^., 1974; Virley, 1984).

In contrast, some recent observation suggest that the hiatus may be of much shorter duration (<10 m.y.), in 23 some areas. Absolute age dating and re-evaluation of relative age control has shown that: 1) the Morrison

Formation of the Colorado Plateau and New Mexico is at least partially Early Cretaceous in age (Kowallis, et al..

1986; Lee and Brookins, 1978; Brookins, 1979; Obradovich in Kowallis et al., 1986); and 2) the Lower Cretaceous conglomerate may be as old as Neocomian in age rather than

Aptian age (Anderson, 1973; Sohn 1979; Burden, 1984). In addition, absolute dating of the Morrison Formation shows that although the widespread Lower Cretaceous conglomerate forms a convenient mapping unit, it does not always mark the base of the Lower Cretaceous. Fission-track dating of bentonitic mudstones in the Colorado Plateau (Kowallis et al., 1986), Rb-Sr dating of bentonitic mudstones in New

Mexico (Lee and Brookins, 1978; Brookins, 1979) and K-Ar biotite dating from western Colorado (J.D. Obradovich, in

Kowalis et a%., 1986) have shown that the Morrison

Formation is at least partially Early Cretaceous in age.

Ages from fission-track dating of zircons in bentonitic mudstones (Kowallis et , 1986) range from 132 to 143

Ma, corresponding to the interval from the Tithonian to middle Neocomian periods (DNAG time scale of Palmer,

1983). Clay mineral investigations corroborate a gradual trend from uppermost Morrison Formation to the overlying

Cloverly Formation(Mantzios, 1986a; Winslow and Heller, 24

1987). While more studies are needed to understand the age relationship of the unconformities, conspicuous disconformities were found at the base of virtually every conglomerate body in the Morrison-Cloverly sequence

(Kvale, 1986, Winslow and Heller, 1987, Goswami and

Vondra, 1988). Many controversies exist with regards to the contact relationships between the Cloverly Formation (and

its lateral equivalents) and the underlying Morrison

Formation. The assumption that a major hiatus occurs at the base of the Lower Cretaceous conglomerate presents many difficulties in lithostratigraphic correlation. In

some areas no conglomerate is present in the entire Upper

Jurassic and Lower Cretaceous nonmarine sequence and the

section is dominated by mudstone deposits (Waage, 1955;

Young, 1960; Moberly, 1960; DeCelles, 1986, and the

present study).

Where conglomerate is absent, or where more than

one conglomerate lens occurs, lithostratigraphic

assignments are made by using the following criteria:

1) the appearance of different colored chert or other

pebble lithologies up-section; 2) a decrease in detrital

feldspar content up-section; 3) the increase in highly

polished pebbles and cobbles ("gastroliths") and

calcareous nodules in the Lower Cretaceous sequence: and 25

4) a difference in mudstone color and composition.

A detailed field investigation has allowed the writer to establish a well defined boundary between the

Morrison and Cloverly formations. The research reported on here investigated the possible physical criteria that could be used in defining the contact. While mudstone color is conspicuous, this alone cannot form a conclusive criterion since the color is caused by a variety of primary and secondary phenomena (Brown and Kraus, 1981;

Mantzios, 1986) and may not reflect clay mineral composition (Keller, 1962). A conspicuous horizon found in the study area is the presence of a moderately thick

(average 4.5 m) bed of white mudstone with ash above a salmon colored mudstone of the Morrison Formation. Field

investigations revealed that either a white mudstone with

ash bed with or a conglomeratic bed or, at places, both are present within the Bighorn basin area or elsewhere

(C.F Vondra, and S. Ambardar personal communication,

1992). The conglomerate and the white mudstone bed with ash were found to be excellent marker beds in the area of

study. In absence of the conglomerate the base of the white mudstone bed was used as the contact between the

Morrison and Cloverly formation. The white ash bed is

found to be more or less continuous in most of the eastern

part of the Bighorn Basin, while in the western part of 26 the Bighorn basin and along the western flank of the

Powder River basin the Pryor Conglomerate lies directly on top of the salmon colored Morrison mudstones. This field criterion can be applied throughout the study area and seems to provide a more logical approach in defining the contact between the two formations. Besides the stratigraphie and physical criteria of mudstone color, another physical aspect studied was the mineralogical variation in the sandstones of the Morrison Formation and the lower part of the Cloverly Formation. Petrological studies of 53 thin sections revealed a decrease in the feldspar content (K-feldspar + plagioclase) in the

Cloverly sandstones. The average feldspar content of the sandstones of the Morrison Formation was found to be 9%, while that of the Cloverly sandstones was 3%. Similar observations were reported from Montana and other neighboring areas (Suttner, 1969; Furer, 1966, Walker,

1965).

While more studies are needed to understand the age relationship of the unconformities, conspicuous disconformities were found at the base of virtually every conglomerate body in the Morrison-Cloverly sequence

(Kvale, 1986; Winslow and Heller, 1987, Goswami and

Vendra, 1988). 27

Lower Contact

Imprecise dating of the base of Morrison Formation may have incorrectly suggested that an episode of regional erosion occurred after the deposition of the Sundance

Formation. Imlay (1980) believed deposition of the

Morrison began during the Late Oxfordian in Wyoming.

Earlier workers (e.g., Imlay, 1947; Pipiringos, 1968;

Brenner and Davis, 1973, 1974; Pipiringos and O'Sullivan,

1978) placed the Morrison wholly within the Kimmeridgian.

The J-5 unconformity was used to explain the supposed lack of Upper Oxfordian sediments in the Western Interior

(Pipiringos and O'Sullivan, 1978). Imlay's (1980) Late

Oxfordian placement of the lower Morrison Formation brought the existence of the J-5 unconformity into question. Imlay's (1980) findings also suggested that the tentative assignment of a Kimmeridgian age to the Windy

Hill Sandstone Member of the Sundance Formation

(Pipiringos and O'Sullivan, 1978) should be revised to

Late Oxfordian. During this study, no evidence of a surface of regional erosion was encountered. The tabular coquina facies in the Upper Sundance is often traceable for tens of miles in outcrop. The contact of the Morrison

Formation with underlying Sundance Formation is in most localities at the base of a lenticular green mudstone 28

immediately above the highest coquina of the Sundance

Formation. Where the upper Sundance unit is shale or sandstone, the contact is less obvious, but is still recognizable by observing details. The conformable nature

of the contact between the Morrison Formation and the underlying Sundance Formation at several localities has also been reported by other workers (Kvale, 1986; Mirsky,

1962; Uhlir, 1987a, and b).

Subsurface Contact

In the subsurface, the Sundance-Morrison contact is

picked above the highest occurrence of glauconite on

sample logs. It generally corresponds to a high

resistivity "kik" on electric logs that is associated with

upper Sundance/Swift beds. Drill sample logs also help in

picking the contact between the Sundance and Morrison

formations where the evidence of lithology is distinct and

can be correlated with the electrologs. Where the Pryor

Conglomerate is present its identification on electrologs

is easy. It is recorded by a high resistivity and most

often shows the pattern of a fining upward inverted cone

shape with a sharp base that is typical of a fluvial

channel deposit (Figure 4). Several shale/claystone beds were recognized which can be traced throughout the basin 29

6 - 57N - 96w Federal # 1.

CLOVERLY FORMATION

HÎYOR CONGLOMERATE) MORRISON FORMATION

^^Res 1st ivity s.p.

1600-

JilORRISON FORMATION 1700 SUNDANCE FORMATION

Figure 4. Identification of Morrison-Cloverly contact from electrical logs. Note the fining upward inverted cone shape of electrolog represent the Pryor Conglomerate at the base of the Cloverly Formation. 30 and a few are good "marker" horizons. Detailed resistivity correlation helped to establish the various lithology contacts for both the Morrison and Cloverly formations.

Interpretation of Subsurface Geophysical Data

Surface exposures of the Morrison Formation occur only along the margins of the Bighorn basin. The entire central part of the basin is underlain by younger strata.

Therefore, a complete stratigraphie study of the Morrison

Formation necessitates a detailed analysis of subsurface data.

Detailed stratigraphie correlation of the Morrison and Cloverly formations in the Bighorn and Powder River basins of Wyoming based on geophysical logs and outcrop studies was carried out. Proper subsurface log interpretation is possible only when all logs required for interpretation are available. Since the Morrison Formation is not important from the point of view of oil and gas exploration, all the geophysical logs are not always available for the Morrison Formation.

The Morrison is dominantly fine grained with thin interbeds of sand. The Smaller thickness of the individual sand bodies compounds the problem of sand/shale 31 interpretation. Conventional dispersed shaly sand processing becomes inaccurate when the thickness of the bed reaches the vertical resolution limit of the logging tool used to quantify it. The accuracy varies with both sensor design, logging methods and post logging signal processing. When a logging tool passes through a thin bed it incorporates the properties of the thin bed with the properties of the surrounding beds. Effectively the

"picture" taken of the formation becomes "blurred" (Doll et al.. 1960; Poupon et al., 1971, Schlumberger, 1975).

In the case of the Morrison Formation similar effects can be seen. Most of the subsurface logs are old and their resolution is poor and difficult to identify the thin sand beds directly from a close examination of the resistivity,

S.P., and other geophysical logs.

The presence of shale in shaly sand further complicated the issue. Whenever a shale is present within a formation, all of the porosity tools eg. sonic, and neutron will record excessive porosity (Poupon et al.,

1971, Poupon et al., 1954). The only exception to this is the density log. It will not record an excessively high porosity if the density of the shale is equal to or greater than the matrix density (Gondouin et al., 1957;

Doll, et , I960; Asquith, 1982; Tittman et ^., 1965).

However, a combination of the information of V-clay 32

(volume of clay) and porosity data gives a good evaluation of the thin sand present in a formation (Poupon et ; 1971, Smits, 1968; Asquith, 1982).

A method for the petrophysical analysis of laminated sand/shale sequence of "thin beds" was developed to interpret the subsurface geophysical logs available for the Morrison Formation of the study area. The first step in thin bed analysis is to determine the volume of the shale (V-Clay) either from the gamma ray log or from the spontaneous potential (S.P.) along with the resistivity. The porosity was determined from the resistivity log. Resistivity was obtained from several available logs recording the resistivity such as the proximity log, micro-laterolog, short normal log, or shallow laterolog.

A number of logs were digitized and computer programs to calculate both porosity and V-clay were run to obtain the sand-shale ratio of the Morrison Formation.

Figure 5 shows the use of quantitative log interpretation data of both porosity and volume of clay (V-clay) in the interpretation of thin sand beds. The low percencentages of clay content and porosity of about 27% helped to identify these sand beds although the resistivity of the sandstone interval is lower than that of a typical sandstone interval (Figure 6). Figure 5 Quantitative log analysis showing the use of both porosity and V-clay in the interpretation of thin sandstone beds. The low V-clay content and the porosity helped to identify these thin sand beds. 30 - -QW Fed ^ 1 —30 1.00

0.90 — 0.45 O.fiO

0.70- -0.35 0.60-

-0.50-^ LO.25

0.40-1

0.30 4 4"- "0.15

0.20

0.10- dli -0.05 0.00 ffl 14.1" 14.14 14.16 14.18 14.2 14.22 14.24 14.26 14.28 14.3 Depth,ft (Thousands)

Vcl Porosity Figure 6. Quantitative log analysis of thin sandstone beds. Although the resistivities of these sandstone intervals are low, the quantitative log analysis shows low clay content and porosity (see figure 9) of 27% for these thin intervals suggesting the lithology as sandstone. 30 - 82W Fed #1-30 250

0.90- 0.80 1 -200

0.70- 0.50 - f -150 "d 0.50- > 0.40- -100

0.30- 0.20- 0.10-

14.12 14.14 14.16 14.18 14.2 14.22 14.24 14.26 14.28 14.3 Depth,it (Thousands)

Vd Resistivity 37

Stratigraphy

The Upper Jurassic Morrison Formation in the north central Wyoming consists mainly of sandstone, siltstone, and variegated claystone of fluvial, eolian, and intertidal origin. Figure 7 is a highly idealized graphic section of the Morrison Formation showing the vertical distribution of environmental facies and sandstone geometry and architecture.

The thickness of the Morrison Formation in the study area ranges up to 107 m (350 ft.) along the western flank of the Bighorn Basin while along the east flank it decreases to less than 27 m (90 ft.). The thickness increases again eastward in the Powder River Basin.

Figure 4 shows the lithostratigraphic correlation of the

Morrison and Cloverly formations across the state of

Wyoming from the Idaho-Wyoming border and eastward to the

South Dakota - Wyoming border. The Tygee and equivalent

Muddy sandstone are used as the upper datum in the Figure

4 which incorporates sections from the present study.

Correlation of the Morrison and Cloverly formations in the

Bighorn basin is shown in Figures 8 and 9. Here also, a resistivity horizon (M-3 in figures) corresponding to the

Muddy Sandstone is used as a datum. The basic data of contact, correlation of resistivity horizons, sand SOUTH NORTH ENVIRONMENT & LITHOFACIES CLOVERLY FM. PRYOR CONGLOMERATE

FLUVIAL (MEANDERING AND BRIDED STREAMS) Interbedded siltsone and clystone lithofacies & • Channel sandstone lithofacies MORRISON FM. EOLIAN & TRANSITIONAL MARINE Quartzarenite Lithofacies w 09 Interbedded olive green claystone& bioturbated sandstone lithofacies SHALLOW MARINE SUNDANCE FM. Sandstones!Umestones!Shale

LACUSTRINE LIMESTONE

SCOUR SURFACE OR DISCONFORMITY

Figure 7. An idealized section of the Morrison Formation showing the vertical distribution of depositional environment, associated lithofacies, sandstone geometry and architecture Figure 8. North-south cross-section of the Morrison and Cloverly formations across the Bighorn Basin of Wyoming 40

SB—' Gloyérly Formation. Morrison Formation

Sundance Formation

Ml- - Ml M2 - Mi H3 - Mi GR - Gi SP - S; Res- Ri Horizoj Vertici

':"'r;....

-J ? Ciovei^

NORTH LEGEND KMS Ml - Marker Horizon no.l t M2 - Marker Horizon no.2 MONTANA M3 - Marker Horizon no.3 WYOAONG CxR - Gamma Ray Y SP - Spontaneous Potential Res- Resistivity IV % Horizontal Scale: 1cm = 5.25 KM %)j % Vertical Scale ; 1cm = 70 M \l % \ \

THERMOPOUS

MORRISON-CLOVERLY OUTCROPS Figure 9. East-west cross-section of the Morrison and Cloverly formations across the Bighorn Basin of Wyoming 42

-Ml -M2 _M2 SP-J SR-i Res SP4 es SP SP-' GR. es

Cloverly Formation

Morrison Formation

es Sundance Formation

LEGEND Ml - Marker Horizon r M2 - Marker Horizon r M3 - Marker Horizon r GR - Gamma Ray SP - Spontaneous Pote Res- Resistivity Horizontal Scale; 1er Vertical Scale s Ici

BIGHORN MTNS. w

Formation

NORTH

MONTANA LEGEND WYOMING larker Horizon no.l {arker Horizon no.2 larker Horizon no.3 % jamma Ray SECTION LINE spontaneous Potential Resistivity antal Scale: 1cm = 5-25 KM pal Scale i 1cm = 70 M

niERMOFOLIS

MORRISON-CtOVERLY OUTCROPS 43 thickness, etc. are given in Appendix C.

An isopach map of the Morrison Formation (Figure

10) was prepared incorporating both surface and subsurface information. As revealed from the study of the sections, and isopach map of the Morrison Formation , there seems to be a broad pattern of deposition of the Morrison Formation within the study area. The isopach of the Morrison

Formation shows a northwesterly increase in thickness in the Bighorn Basin and on a smaller scale several downwarps are present. In general, increased thickness of the

Morrison Formation towards the east and west side of the

Bighorn Mountain and local accumulation in the Rose Dome area are apparent. The thick areas tend to occur in present day structurally low areas and thin areas tend to be in the present-day structurally high areas.

In general, within the study area, the Morrison

Formation is composed dominantly of non-marine mudstone with minor amounts of sandstone, siltstone and limestone.

Sandstones are often thick and resistant enough to form ridges. Morrison is typically poorly exposed in a long strike valley with a few resistant sandstone layers protruding for short distances. The valley is bounded below by the dipslope of a hogback or cuesta upheld by the uppermost Sundance, and by the scarp face of a ridge formed by the Cloverly Formation. Although mudstone is Figure 10. Isopach map of the Morrison Formation in the Bighorn Basin of Wyoming LOVELL r-i/.Vv

S \9 U1 BIGHORN BXSIN

KAtCEE

THERMOPOLIS

WIND RI\'ER BASIN^ ^ARMINTO 25 km 200—- = 200 feet contour line 108 00* 46 the dominant rock type in most of the area, sandstone constitutes the dominant lithology at several localities - to the south near Thermopolis as well as to the east. Thick lenses of sandstones occur along the western flank of the basin near Cody and at few localities in Montana. On the sand/shale ratio map (Figure 11) these areas are marked by a higher ratio (>.75). Mudstone may occur in any part of the section, but it is the most prominent in the upper half. Sandstone is restricted essentially to the lower and middle parts, while siltstone occurs throughout. Limestone is very local and occurs only in the lower half.

It is almost superfluous to note that the Morrison and the overlying Cloverly formations vary considerably from one place to another. Variation consists of changes in rock type and color, topographic expression, and differences in sequence and thickness. Regardless of the rock type, the lower half of the Morrison Formation characteristically is calcareous and the upper half is non or less calcareous. The limestone is gray, finely crystalline, silty to sandy, and resistant. It occurs locally in the lower half of the Morrison as a single ledge less than .70 m thick. It thins laterally or grades into calcareous siltstone and sandstone within a few tens or hundreds of feet. Figure 11. Sand-Shale ratio map of the Morrison Formation in the Bighorn basin of Wyoming MONTANA WYOMING

«HEBIDAN NORTH • m SHELL îS BIGHORN BASIN

m# ERMOPOLIS I####

WIND RIVER BASIN ARMINTO

109.0Cr 108.00 107.00 SCAI£ ( KM I 49

Relationships of the sandstones to adjacent lithologie units are quite varied within the formation, and the unit contacts may be either gradational or sharp. Some contacts are marked by obvious disconformities e.g. where the Pryor Conglomerate of the Cloverly Formation is present. Laterally, the sandstones terminate abruptly or grade into other lithologie types. In general, the following lateral changes were observed;

(1) Fine to medium grained brown or gray quartzarenites are of variable thickness and generally lenticular, but some interfinger or terminate abruptly against claystones.

Contacts above and below of these sandstone lenses are often abrupt and locally display minor disconformity

(2) Fine grained quartzwackes commonly grade into siltstones or terminate abruptly. At several locations the argillaceous sandstones grade into the more coarse, brown sandstones described in (1).

Lower Morrison

In the southern part of the Bighorn Basin near

Thermopolis and to the east along the western flank of the

Powder River B&sin, the lower Morrison is represented by 50 isolated lenses of thick massive to cross-stratified quartzarenite lithofacies (Figure 12). The sandstone ranges in thickness from 7 m. (24 ft.) to 36 m. (118 ft.)

It is a conspicuous sandstone unit that weathers to vertical cliffs, rounded benches or poorly exposed slopes

The sandstones are fine to very fine grained, moderately well to well sorted. They are dominantly off-white to pale yellowish gray and mostly friable. Bedding is medium to very thick with large scale cross- stratification, beds of horizontal lamination, and rare massive beds. Sets of cross stratification are both tabular planar and wedge planar with thickness ranging from 1 to 13.8 m.. Cosets are rarely interbedded with flat bedded intervals. Individual foresets may show reverse grading making boundaries between them very distinct.

While the contact with the uppermost Sundance is sharp, the top of the unit is gradational with fluvial mudstone. The quartzarenite lithofacies pinches out into olive green mudstone after continuation for about .5 to 1 km.. The unit is confined to the southern part of the

Bighorn Basin and the southwestern part of the of the

Powder River Basin.

The quartzarenite lithofacies association of the southern part of the basin is interpreted to be of eolian 51

Figure 12. Photograph showing quartzarenite lithofacies assocaition of lower Morrison in the southern part of Bighorn Basin. Location: near Thermopolis, Wyoming 52 origin. The faciès possesses distinctive characteristics that indicate that it was deposited in an eolian environment. Evidence for this includes: thick sets of steeply dipping, tabular-planar, and wedge planar cross stratification; distinct internal stratification that conforms with both ancient and modern day eolian deposits; contorted stratification, nonmarine trace fossils on exposed bedding planes; and well sorted texture. Although the quartzarenite lithofacies of eolian origin is present as isolated sand bodies in the southern part of the study area, Figure 13 depicts the general distribution of the unit in the area. Details concerning sandstone geometries, structures and sub-facies are discussed in

Goswami, 1993 that deals with the facies and depositional environments.

In the northern part of the Bighorn basin the lower

Morrison is represented by interbedded olive green silty claystone and sandstone lithofacies association. The sandstones are very fine grained, calcareous, quartzarenites and quartzwackes. They overlie the well sorted glauconitic litharenite of the upper Sundance

Formation.

Numerous burrows ("skolithos" trace fossil) and escape structures are common in siltstone and quarzwackes.

Thicker sandstone bodies show pronounced lenticularity. Figure 13. isopach map of the eolian sandstone (quartzarenite lithofacies) of the lower Morrison Formation # CODY

BIGHORN BASIN

KAYCEE 75

tJi

THERMOPOLia^^

WIND RIVER BASIN

SCAIE (Km) 109' 108 Contours are In feet 55

The siltstones are 30 to 60 cm thick and laterally continuous over several meters. The claystones of this lithofacies association contain organic remains including coaly material and rare, poorly preserved casts of gastropods (Kvale, 1986). Microfaunal remains include dinoflagellates indicative of brackish water conditions

(reported by E. Kvale, Indiana Geol. Survey, pers. comm.,

1988). North of Sheep Mountain this unit contains discontinuous lenses of poorly sorted arenaceous fossiliferous siltstone. The fossils consists of highly fractured bivalves (Kvale, 1986)

The sandstones and siltstones of the upper part of this lithofacies are mostly massive, but undulatory varve- like laminae of regularly or irregularly spaced finer grained siltstones and silty claystones are locally preserved. Dish structures and small scale slump structures often cut the laminae, indicating dewatering of rapidly deposited sediments (Lowe and Lopiccolo, 1974;

Lowe, 1975).

The upper few centimeters of the siltstones are brecciated and display more oxidized (rust colored) coloration. On geophysical logs, the interbedded silty claystone and sandstone lithofacies association is characterized by low resistivity with occasional small bands of high resistivity features. 56

Upper Morrison The upper part of the Morrison Formation is characterized by fine to medium grained channel sandstone, variegated mudstones, and siltstones. The entire assemblage of lithologies can be divided into two lithofacies associations: 1) a fine grained quartz arenite lithofacies of fluvial channel origin and 2) an interbedded siltstone and claystone lithofacies association of overbank fluvial deposits. The fluvial deposits constitute about 75% of the entire formation observed both in outcrops as well as in the subsurface.

Channel Sandstone Lithofacies

The channel sequences are typically broad, erosive based 1.5 to 5 m thick lenticular bodies. The dominant primary structures include ripple laminations, parallel laminations with primary current lineations on bedding plane surfaces, and also solitary large scale troughs.

Both ribbon and sheet sand bodies are present within the channel sequence. Sheet sandstones are multistory type.

In exposures oriented perpendicular to paleoflow lateral accretion surfaces are evident (Figure 14). These beds contain well defined fining upward grain size. Both the set thickness of the cross-strata and grain size diminish upward and laterally toward the channel margin. Sand Figure 14. Photograph of the channel sandstone lithofacies association of the Morrison Formation showing lateral accretion surfaces. Location: One mile south of Brock Ranch, Mayoworth

Figure 15. Photograph of the interbedded siltstone and claystone lithofacies association of the upper Morrison. The contact (person standing) between the Morrison and Cloverly formations is on top of the salmon colored unit below the white ash bed. Location; Coyote Basin 58 59 bodies showing sheet morphology, low paleocurrent variance, abundant erosional surfaces, high sand/shale ratio with lack of in situ mud rocks are commonly observed in the channel sandstone assemblage. The massive sandstones commonly contain large claystone intraclasts that are attributed to failure of channel bank during flood.

Interbedded claystone and siltstone lithofacies

association

This facies consists of interbedded reddish brown and grayish olive siltstones and claystones and typically represents the uppermost part of the Morrison Formation

(figure 15). Of all the sediments of the Morrison and

Cloverly formations in the Bighorn Basin this lithofacies association contains the most abundant faunal remains. The famous Howe Dinosaur Quarry (Brown, 1935; 1937), located 2 km east of the confluence of Beaver Creek and Cedar Creek occurs in this facies. Most of the siltstones and claystones are calcareous. The fine grained nature of the unit is reflected by a badlands type of topography that results from weathering and erosion. The reddish color of the claystones is probably the result of in situ weathering of non-red sediments which contained iron- bearing minerals such as hornblende or biotite (Walker, 60

1967). In very fresh exposures, many of the reddish units exhibit pale olive mottles which are localized around small carbonate glaebules (<3cm diameter) (Mantzios, 1986a and 1986b). In fresh samples, carbonate content is much higher in the pale olive mottles than the red matrix. The remnants of downward branching rootlets were noted in the red horizons (Mantzios, 1986a).

Interbedded with claystones are thin sandstones which are generally only a few centimeters (2.5 to 20 cm) thick, laterally discontinuous and very fine grained. The sandstones are ripple cross-laminated or locally massive. 61

PALEOTECTONICS

The Jurassic is commonly regarded as a time of tectonic quiescence (Gilluly and Reeside, 1920,). Stokes

(1954a, b) probably was the first to document the association of crustal movement during Jurassic time. He and several subsequent workers (Wright and Dickey, 1963;

Peterson, 1980, 1984) also noticed the relationship of sedimentologic features in some of the Jurassic rocks of the Colorado Plateau to crustal movements. Although the structural activity certainly was subdued, sufficient evidence is now available to document that deformation occurred in a large part of the region during the Jurassic including the Bighorn Basin (Suttner, 1969; Suttner et al.. 1981, Schwartz, et al., 1983; Uhlir et ^., 1988,

Kvale, 1986. Walcott (1980), Stokes (1954a, b), Heylmun

(1958), Wegerd and Matheny (1958), Fetzner (1960),

Kirkland (1963), McKee (1963), Wright and Dickey (1963),

Lessentine (1965), Baars and Stevenson (1977), Stevenson and Baars (1977), Peterson (1980, 1984), and Doelling

(1981) have shown that many segments of the Colorado

Plateau cratonic block were sites of slight but repeated movements throughout much of Precambrian, Paleozoic,

Triassic, Jurassic time. Present day structural attitudes generally have less than 2 degrees of dip throughout most 62 of the region, this small amount of deformation reflects the combined effects of Laramide tectonism as well as that which occurred in earlier times. Similar studies by

Suttner (1969); Kvale and Vondra, 1985a; Uhlir, 1987,

Schwartz, 1983) have recorded the effect of Jurassic tectonic activity in various other parts of the foreland basin in Wyoming and Montana and all the way up to

Alberta, Canada. Thus the amount of structural deformation can be so slight that it will appear insignificant, but none the less, it can produce anomalous thickness variations and can produce distribution of sedimentary facies.

Suttner et al., (1981), Schwartz (1983), and

DeCelles (1984, 1986) have demonstrated the existence of such structures in southwest Montana. And Kvale (1986) presented preliminary data suggesting Early Cretaceous uplift of ancestral Bighorn and Beartooth structures in north-central Wyoming. In addition, Lorenz (1982) postulated uplift of the Sweetgrass Arch in southwest

Wyoming. Recent work has shown that pre-Laramide deformation within the eastern and central Powder River

Basin has occurred intermittently during the Phanerozoic, influencing depositional patterns of the Permian Minnelusa

Formation; the Lower Cretaceous Lakota (Cloverly)

Formation, Fall River Sandstone and Muddy Formation; and 63 the Upper Cretaceous Turner, Sussex, Shannon and Parkman

sandstones (Gott et , 1974; Rasmussen and Bean, 1984;

Slack, 1981; Weimer et al., 1982). The onset of Jurassic

thrusting in the Cordillera, initial intraforeland uplift,

and Sundance/Ellis-Morrison sedimentation in the foreland

basin roughly coincided with the accretion of Stikinia

(Suttner et al., 1985). In addition widespread arc

magmatism (e.g.. Sierra Nevada batholith) occurred in the

Cordillera during the Middle and Late Jurassic (Armstrong

and Suppe, 1973; Armstrong et ^., 1977; Hamilton, 1978;

Allmendinger et ^., 1984) suggests that Middle-Late

Jurassic eastward thrusting on the Manning Canyon

detachment of northern Utah and Southern Idaho may have

been the first episode of Mesozoic Cordilleran thrusting

in the western U.S. Although direct correlation between

Manning Canyon thrusting and foreland sedimentation has

not been demonstrated, analysis of Middle Jurassic strata

in Wyoming by Heller et al., (1986) suggests that a major

episode of foreland subsidence occurred simultaneously in

eastern Idaho and/or western Wyoming. This episode of

subsidence may have been related to distal lithospheric

flexure owing to loading by the Nansel thrust plate.

Schwartz (1983) and Decelles (1984, 1986) suggest that

Middle to Late Jurassic Intraforeland reactivation and

deposition of the Ellis Group and Morrison Formation in 64 southeastern Montana were related to coeval Cordilleran thrusting farther north in Idaho.

The most helpful data for use in paleotectonic analysis of the region are thickness variation and facies distribution. Thicknesses of the various stratigraphie units were measured at outcrops or obtained from oil and gas well logs. The data are depicted on a isopach map

(Figure 14). Although the Morrison is a relatively thin formation for analysis of subsidence history several interpretations can be made from the isopach map.

Thicknesses of the Morrison Formation range from less than 27 m (90 feet) to greater than 107 m (350 feet) in the study area. The isopach map (Figure 14) shows the presence of a positive feature more or less along the existing Bighorn Mountains trending N-S. The trough axis trends approximately NW-SE. On a smaller scale several downwarps are apparent from the isopach map. In general, increased thickness of the Morrison Formation towards the east and west side of the Bighorn Mountain and local accumulation in the Rose Dome area are apparent. In general thick areas tend to occur in the present day structurally low areas and thin areas tend to be in present-day structurally high areas. This suggests that basins and uplifts that exist today were structurally active during Late Jurassic time and that Laramide 65 tectonic forces rejuvenated pre-existing structural

elements. Although subsurface as well as surface data are

not available for the Bighorn Mountains an analysis of the

isopach map and the sand/shale ratio map of the Morrison

Formation shows a rough correlation between the thickness

and the sand/shale ratio. The Morrison Formation is less

sandy where the formation is thin and gradually becomes

coarser as the thickness increases. Alternatively, it can

be interpreted that the Morrison Formation is thin and

less sandy along paleotopographic highs and sandier along

paleotopographic lows, which suggests active structural

control of facies distribution during deposition.

Although absence of data in the region limits present

interpretation to some extent, the overall information can

be used along with other positive criteria in the

interpretation of paleotectonics and sedimentation. The

general isopach pattern (Figure 14) suggests that the area

west of the Bighorn Basin subsided as a northwest plunging

structural trough on a smaller scale. The important

positive elements can be easily interpretated from the

isopach map (Figure 14) and stratigraphie correlation

(Figure 13) where thinning of the Morrison Formation in a

N-S trend along the existing Bighorn Mountains is apparent

not only from surface outcrop studies but also from the

data obtained from the subsurface. The isopach map of the 66

Morrison Formation shows several areas suggestive of an irregular basin in the study area (McKee et , 1956;

Mirsky, 1960; Thompson, 1950, Goswami and Vondra, 1992).

Paucity of eolian deposits in northern Wyoming may reflect an earlier onset of fluvial sedimentation in northern Wyoming. Burial of potential Sundance eolian source areas in north-central Wyoming by Morrison fluvial sediments may have occurred before dune development could take place. Tops of barrier coast line deposits of the uppermost Sundance Formation throughout the northern

Bighorn Basin generally contain subtle, small scale dune deposits (Uhlir, 1987a). Small dunes are common features of modern barrier beach deposits (Reading, 1986). Dune deposits atop Sundance barrier sands in central Wyoming are not present (Uhlir, 1987a). This may suggest that efficient eolian reworking of uppermost Sundance sediments did not take place in northern Wyoming as it did in central and south-central Wyoming. Early stages of dune construction of the Morrison coastal plain in north- central Wyoming may have been reworked by a broad fluvial environment before any stabilization took place. Effective fluvial reworking of coastal sediments in the area of this investigation either did not take place, was dynamically low, or did not occur until dunes had become stabilized.

The reason for the paucity of Morrison eolian 67 patches in the north-central Wyoming may have its answer in a hypothesis different from the one just presented.

Positive structural elements have been suggested to have occurred in central and eastern Wyoming during the Late

Jurassic (Curtis et al., 1958; Thompson, 1958; Mirsky,

1962). Evidence for this was primarily recognized from isopach maps of Wyoming. Mirsky (1962) found evidence of local thinning of the Morrison Formation near the town of

Barnum in the southeastern portion of the Bighorn

Mountains. Mirsky (1962, p. 1675) suggests that this area of thinning "coincides with the southern part of the

Bighorn uplift, suggesting that a positive element was developing at that time."

Szigeti and Fox (1981) found evidence of thinning in Upper Jurassic Sundance rocks of western Nebraska from isopach maps compiled by McKee et al., (1956). They proposed that this was due to the subtle presence of the

Casper Arch during the Late Jurassic. Szigeti and Fox

(1981) suggested that wind activity from the south, across the structural high, was responsible for the downwind deposition of the eolian Unkpapa Sandstone Member after withdrawal of the Sundance Sea from eastern Wyoming. The

Unkpapa Sandstone Member is confined to the lower portion of the Morrison Formation in the Black Hills of South

Dakota. Limited northward extension of the Unkpapa was 68 controlled by availability of source sands that were confined to the structural, topographic high (Szigeti and

Fox, 1981). The Unkpapa has been correlated with the eolian units of this study (Uhlir, 1987a; Weed and Vondra,

1987, Weed, 1988).

Wind activity across positive escarpments tends to induce dune deposition downward from these features in low lying areas where wind velocity is reduced (Ahlbrandt,

1974; Ahlbrandt and Fryberger, 1982). Positive elements in the central and south-central Wyoming may have also controlled dune accumulation in the area of this report.

Uhlir (1987a) has suggested that structurally related topographic highs existed during the Late Jurassic in central Wyoming. He suggests that locations of these positive elements coincide with the present day Laramide uplifts (e.g.. Owl Creek, Granite, Laramie and Casper uplifts). Paucity of dunes in south-central Wyoming may be due to the fact that this area was too far downwind from the structural highs to receive any eolian sediments derived from them (Uhlir, 1987a, 1987b). 69

CONCLUSIONS

Field and subsurface studies of the Upper Jurassic

Morrison Formation in the Bighorn Basin have resulted in the following conclusions ;

1. The contact between the underlying Sundance /

Swift Formation and the Morrison is conformable while that with the overlying Cloverly Formation may be disconformable. The erosional disconformities may be seen at the base of the Pryor Conglomerate.

2. Recent studies have shown the hiatus between the Morrison and Cloverly formations is of a much shorter duration (<10 m.y) than earlier thought.

3. Within the Bighorn Basin area, the Morrison

Formation is composed predominantly of claystone

(approximately 75%) with interbedded sandstone (Approximately 10%), siltstone (5%) and thin carbonates

(<1%) and attains a maximum thickness of about 107 m (350 ft). Studies have shown that the Morrison Formation consists mainly of eolian transitional marine and fluvial sediments. Along the southern margin of the Bighorn

Basin, the lower part of the Morrison is represented by 70 quartz arenite lithofacies of eolian origin. Equivalent facies in the north and in the north central part of

Wyoming are dark green silty mudstone and fine grained bioturbated intertidal sandstones. The upper part of the

Morrison formation is represented by an interbedded claystone and siltstone lithofacies and a channel sandstone lithofacies associations of fluvial origin.

4. Regional variation of isopach map, sand/shale ratio map and the distribution of facies indicate that positive structural elements were active in the central and north central parts of Wyoming during the Jurassic.

The location of these positive elements coincides with present day Laramide uplifts. 71

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APPENDIX A. LOCATION OF STRATIGRAPHIC SECTIONS

The locations of the major stratigraphie sections measured during the research for this dissertation are presented in this appendix using the section, township and range system of the United States Public Land Survey. Asterisks (*) following range names denote measured sections that are depicted graphically in APPENDIX B.

Location/Area Section Township Range

CODY NEl/4 Sll T52N R102W

NWl/4 S18 T52N RIOIW

NEl/4 S20 T52N RIOIW

SWl/4 S16 T53N R102W

THERMOPOLIS NEl/2 S24 T43N R95W *

SEl/4 Sll T43N R95W

SEl/4 S19 T43N R94W

Nl/2 I536 T44N R96W

NEl/4 S35 T43N R96W

NEl/4 S16 T43N R94W

BIG TRAIL Wl/2 iS25 T43N R88W *

SWl/4 S30 T45N R87W *

El/2 S25 T45N R88W 84

TENSLEEP Sl/2 S24 T47N R88W

NEl/4 S26 T47N R89W

El/2 S15 T47N R89W

HYATTVILLE Wl/2 S2 T49N R91W

Sl/2 S13 T49N R91W

Sl/4 S20 T49N R90W

SEl/4 S35 T50N R91W

NWl/4 S7 T49N R89W

SHELL /GREBULL El/2 S26 T26N R91W

El/2 SIO T51N R91W

El/2 S9 T55N R95W

NEl/4 S5 T55N R92W

SEl/4 S7 T55N R92W

Wl/2 S12 T54N R92W

NEl/4 SU T52N R92W

El/2 S7 T54N R91W

El/2 S5 T52N R90W

NEl/4 S21 T52N R90W

El/2 S13 T54N R94W

El/2 S5 T54N R93W

LOVELL NEl/4 S9 T55N R95W

NEl/4 S21 T55N R95W

Nl/2 S30 T58N R95W

Wl/2 S28 T58N R95W 85

BRIDGER SEl/4 S5 T6S R24E

NEl/4 S8 T6S R24E

Wl/2 S17 T7S R24E

MAYOWORTH Nl/2 S21 T45N R83W

SWl/4 S5 T45N R83W

El/2 S8 T45N R83W

Sl/2 SIO T44N R83W

KAYCEE Wl/2 S12 T43N R83W

Sl/2 S23 T41N R83W

Sl/2 SU T40N R83W

El/2 S4 T39N R83W

NEl/4 S4 T39N R83W

ARMINTO NEl/2 S19 T38N R86W

Nl/2 S25 T39N R86W

El/2 S4 T38N R87W 86

APPENDIX B. GRAPHIC STRATIGRAPHIC SECTIONS

EXPLANATION OF SYMBOLOGY Sediment Texture»

0^0 mudstone I slltstone fine grained sandstone medium to coarse grained sandstone

Sedimentary Structure & Paleocurrent Data 30 m Large scale planar cross stratification

///y/// Small scale planar cross stratification

15 m trough cross stratification

ripple lamination wavy "bedded sediments 0 m plane/parallel bedded sediments

5 m concretions

interclast

coal

conglomerate

mean paleocurrent direction 87

LOCATION: cody? nei/u, sec. ii, t52n., r.io2w. CURRENT REMARKS AZIMUTH

Pryor Conglomerate CLOVERLY FM MORRISON FM Variegated-pink, reddish "brown, gray green calcareous mudstone with thin fine grained discontinuous massive sandstone. Medium to coarse grained, white to "buff, Y large scale planar cross "bedded and plane "bedded sandstones. Iron staining and frequent ironstone concretions are present in the upper 6.3 meters. -Medium to fine grained, white to buff, cross stratified sandstones and siltstones. The sandstones are mostly fria"ble.

-Planar "bedded to cross "bedded "brownish gray to carbonaceous siltstone.

Gray-green to light gray mudstones with rare thin "bands of limestone.

Medium to fine grained cross stratified and ripple laminated sandstones ajid siltstones. Set thickness range from 22 cm to 65 cm.

' Covered - apparently nonresistant mudstone. MORRISON FM Interbedded greenish sandstone and shale SUNDANCE FM 88

LOCATION: CODY SWl/4 Sl6, T53N, R102W CURRENT REMARKS AZIMUTH

Pryor Conglomerate CLOVERLY FM

MORRISON FM Variegated, dark to light gray, purplish mudstone. Rusty to salmon colored zone in the upper half. Fresh surface light to dark gray.

White to buff, small scale cross bedded fine - medium grained sandstones with interbeds of laminated sandstone.

Olive green to greenish-yellow unstratified mudstone.

Concealed by rock boulders and soil cover. Apparently nonresistant mudstone and may contain sandstone lenses.

Green colored cross bedded glauconitic sandstone. No marine shells at this level but present approximately 8m. below. 89

LOCATION: THERMOFOLIS mi/z s 24, T^3N, R95W CURRENT REMARKS AZIMUTH

Pryor Conglomerate CLOVERLY FM MORRISON FM Greenish tan and gray mudstone Brownish gray laminated siltstone

Greenish tan and gray clored mudstone. Nonresistant

• Deep green to dark fine grained laminated siltstone

Fine grained "brilliant white, large scale cross "bedded sandstone. The set thickness range ftom 50cm. to 10m. The cross "beds dip at a high angle (22-32 ) . The sandstones are generally fria"ble weathering into smooth rounded, sloping surfaces.

Parallel "bedded "brilliant white fine grained calcareous sandstones with inter"beds of olive ^ green mudstone. MORRISON FM

Glauconitic, medium grained SUNDANCE FM sandstone weathering from green to light "brown. Coquina limestone 2.23m below. 90

LOCATION: MAHOGONY BUTTE Wl/2 S25, T^^N, R88W CURRENT REMARKS AZIMUTH

. White ash with mudstone CLOVERLY FM MORRISON FM 1 •Variegated, purple to salmon colored mudstone. / •Tan to "buff colored, fine grained sandstone. •Tan to "buff colored wavy bedded siltstone. Brownish white, fine grained laminated )> sandstone.

Gray-green, silty mudstone. MORRISON FM Greenish, cross bedded siltstone. SUNDANCE FM 'coquinoid limestone. 91

LOCATION: BIGTRAIL swi/4 S30, T^5N, H87W CURRENT REMARKS AZIMUTH

_ White ash with mudstone CLOVERLY FM

"White to buff colored, cross MORRISON FM "bedded, fine grained sandstone

Variegated, greenish-yellow to purplish mudstone. Pale brown to tan colored, cross bedded fine to medium grained sandstone and siltstones. Upper part contains laminations of coal. The sandstones form prominant ledge. The lowermost part contain mudstone interJ- clast.

Drab gray or gray green and olive green mudstone. Massive to poorly stratified.

MORRISON FM Coarse grained biiff nnlnrmil sandm+.mne. -Green fissile shales. SUNDANCE FM Green, cross bedded coarse grained sandstone. 92

LOCATION: ÎMSLEEP si/2 824, T^TN, R88M CURRENT REMARKS AZIMUTH

_ ^hlte ash with mudstone CLOVERLY FM

-Reddish brown to yellowish gray MORRISON FM mùdstone-upper part salmon colored. -Small scale cross stratified medium grained \ _ huff colored sandstone. -Purplish to reddish "brown mudstone. LRipple laminated, "bioturbated, fine grained taB colored sandstone.

1 -Olive green calcareous mudstone. MORRISON FM 93

LOCATION: HYATTVILLE wi/2 S2, r91w I CURRENT REMARKS AZIMUTH

• Pryor Conglomerate

I mOVEBlï PK

• « / "buff colored fine grained sandstone. - White to buff colored, cross "bedded medium grained sandstone. r - Olive green calcareous mudstone with thin lense of ripple laminated medium grained sandstone. - Tan colored "bioturbated fine grained sand stone. Wavy bedding is common. MORRISON FM 1 1 'Greenish-gray fine grained cross "bedded sand- j stone. SUNDANCE FM ^ Pale whitish-gray coarse grained limestone. 94

location: SHELL/GREYBULL (Little sheep Mi.) El/2 S9, T55Ni R95W CURRENT remarks AZIMUTH

White mudstone CLOVERLY FM

Reddish-brown to salmon MORRISON FM colored mudstone with thin Intervals of brown to tuff colored fine grained ripple laminated and parallel tedded sandstones. ' The small sandstone lenses often form minor ridges.

- Variegated mudstone

Brown to tan colored, tioturbated fine grained sandstone. "Skolithos" trace fossils and escape structures are prominent. The sand lenses are often wavy "bedded to ripple laminated. Mudstones are olive green in color. MORRISON FM ÎPale green, cross bedded, SUNDANCE FM glauconitic fine grained sandstone. 95 location: sheu, NEl/4 su, T52N, R92W CURRENT remarks AZIMUTH

- White mudstone CLOVERLY FM MORRISON FM Variegated, gray, red-trown, salmon colored mudstone. Fresh surfaces purple-gray. Pale "brownish, cross bedded fine grained sandstone.

• Greenish-gray to pale yellow mudstone.

Brown to tan colored, cross stratified fine to / medium grained calcareous sandstone.

Greenish-gray to pale yellow mudstone.

Soil cover-apparently nonresistant mudstone. -, , MORRISON FM SUNDANCE ffr V7777X Greenish-gray, glauconitic fine grained sandstone. 96

location: crystal ceeek ei/2 s5, t5^, r93w CURRENT remarks AZIMUTH

White mudstone with occasional ash.CLOVERLY FM

-Variegated, yellow-gray, to rose- ^ X pink mudstone. Upper 3.6m salmon colored. Mudstones are calcareous and massive. - Brown, light brown cross bedded to parallel 1 stratified, fine to medium grained sandstone. ^Li^t gray to maroon colored mudstone. -White to tan colored, cross bedded, fine to medium grained sandstone.

-Partly concealed, predominantly lighy-gray to olive green silty mudstone. MORRISON FM Gray green massive sandstone. SUNDANCE FM ^Greenish shale, thinly stratified. h ^Coarse grained coquinoid limestone. 97

location: BRIDGER SEIA 35, T6S, R2^ CURRENT remarks AZIMUTH

— Pryor Conglomerate CLOVERLY FM Calcareous mudstone - MORRISON FM Variegated, pale-green and gray in the upper 25-5 m, gray rose and reddish-tarown in the lower part. The mudstones are silty and prominent thin light-tan, massive sandstone lenses are scattered throughout.

/ White to "buff colored fine grained ripple Ilaminated and cross stratified to massive sandstones. •Pale brownish yellow plane "bedded medium grained sandstone. Greenish-gray calcareous mudstone. Light tan colored wavy to ripple laminated fine grained sandstone and siltstone.

Greenish-gray calcareous mudstone.

Light gray ripple laminated fine grained MORRISON FM greenish-yellow cross "bedded SUNDANCE FM I sandstone «•Dark green fissile shales. -Coarse grained coquinoid limestone. 98

location: BRIDGER SEIA. S8. T6S, R24E CURRENT remarks AZIMUTH Pryor Conglomerate CLOVERLY FM MORRISON FM Variegated, pale green and yellowish-tflrown calcareous muds tone. Prominent thin tan massive sandstone lenses are scattered throughout.

/ White to light tan cross "bedded to massive

//. fine to coarse grained sandstone. Scour faces are common with interclast of calcareous \ mudstone. The set thickness of the cross "beds range from 7cm to 1.2m. Gradual decrease in set thickness is observed.

Olive green calacareous mudstone.

•Light brown plane "bedded fine to medium grained sandstone. MORRISON FM

Light green cross "bedded fine grained sandstone. SUDANCE FM 99

location: MAYOWORTF (Gordon Ranch) El/2 S8, T45N, R83W CURRENT remarks AZIMUTH

_Pryor Conglomerate CLOVERLY FM

1 - Light-gray to tan colored MORRISON FM cross iDedde to laminated fine grained sand stones with inter"bed of cross "bedded siltstones.

- Olive green (lower half) to red-"hrown (upper part) massive muds tone. The mudstones are calcareous and silty. Light-gray ripple laminated sandstone. - Light "brown, cross "bedded, fine grained sand i stone. The "basal part contains rip up clast of mudstone. MORRISON FM "GreeiUsh gray, planar "bedded SUNDANCE FM to cross "bedded sandstone. 100

location: MAYOWORTH SI/2 sic., T¥IN, R83W CURRENT remarks AZIMUTH

Pryor Conglomerate CLOVERLY FM

MORRISON FM • Pale yellowish-green to "brown mudstone. Calcareous and locally silty. Unstratified, fresh rock "breaks into irregular waxy chips.

Dark tan colored ripple laminated fine grained sandstone.

Concealed by grass cover. Apparently non- res istent mudstone.

-Buff colored, ripple laminated sandstone Brilliant white, hig angle cross stratified //// / fine to medium grained sandstone. The lower part (1.35®) is parallel "bedded. Light greenish-gray cross bedded sandstone Light green cross bedded siltstone. Coarse grained coquinoid limestone. Gryphaea shells are abundant. SUNDANCE FM 101 location: kaycee wi/2 s12, t39n, r83w CURRENT remarks AZIMUTH

•6>< Pryor Conglomerate CLOVERLY FM 1reddish brown mudstone MORRISON FM

-Ccncealed-grass covered, apparently nonresistant mudstones.

Small scale trough cross stratified buff colo red fine grained sandstone. Fine grained, brilliant white, large scale / cross stratified sandstone. Set thickness of the cross strata range from 30 cm to 70 cm. Generally friable weathering into smooth W7B rounded sloping surface. MORRISON FM Glauconitic sandstone, SUNDANCE FM weathering from green to light brown. Coquina limestone 2m below. 102

APPENDIX C. SUBSURFACE WELL LOG DATA OF THE MORRISON FORMATION IN THE BIGHORN BASIN OF WYOMING

The appendix includes only those wells that were selected for detailed log interpretation and analysis. It does not include those wells that were used for correlation purposes. I Section Township Range Sundance Top Morrison top Morrison Thickness Sand Thickness Sand/Shale Ratio [

28 44 94 4150 3970 180 11 0.0651 28 44 91 5565 5430 135 29 0.2736 23 44 95 3060 2878 182 25 0.1592 30 44 91 5050 4880 170 22 0.1486 27 44 100 3320 3140 180 4 0.0227 27 44 75 13780 13510 270 101 0.5976 28 44 91 5542 5382 160 24 0.1765 5 45 91 9350 9170 85 40 0.8889 1 45 104 2970 2680 290 250 6.25 a 45 89 6650 6480 85 20 0.3077 1 45 79 13530 13420 110 2 0.0185 26 45 82 6900 6680 220 101 0.8487 9 45 90 7611 7492 119 7 0.0625 19 45 92 8730 8533 197 44 0.2876 22 46 95 12280 12130 ISO 87 1.381 9 46 76 12100 11910 190 61 0.4729 9 46 81 11020 10830 190 42 0.2838 25 46 100 4540 4360 180 33 0.2245 2 46 94 11580 11450 130 33 0.3402 18 46 98 2780 2675 105 2 0.0194 6 46 89 6480 6280 200 15 0.0811 10 46 99 4685 4460 225 25 0.125 11 46 89 5041 4900 141 19 0.1577 7 47 89 1015 770 245 24 0.1086 18 47 102 2070 1810 2^ 33 0.1454 21 47 81 12150 12000 150 60 0.6667 12 47 102 2720 2390 330 30 0.1 23 47 90 3040 2820 220 37 0.2022 13 47 91 6122 5850 272 140 1.06 20 47 91 9002 8710 292 45 0.1822 28 47 93 10885 10703 182 4 0.0225 36 47 93 2410 2230 180 75 0.7143 I Section Township Range Sundance Top Morrison Top Morrison Thickness Sand Thickness Sand/Shale Ratio |

11 47 77 11980 11850 130 82 1.7083 5 48 102 3700 3430 270 30 0.125 1 48 99 6770 6600 160 75 0.4286 23 48 103 2850 2600 250 43 0.2077 34 48 100 3350 3120 230 70 0.4375 20 48 76 11560 11440 120 45 0.6 23 48 101 4400 4140 260 75 0.4054 7 48 90 1250 990 260 35 0.1555 33 49 103 4700 4458 242 135 1.2617 13 49 93 9980 9820 160 4 0.0256 34 49 76 11290 11060 230 48 0.2637 2 49 102 2200 1960 240 12 0.0526 17 49 99 9760 9470 290 29 0.1111 17 50 91 2452 1960 261 15 0.061 1 50 92 2311 2191 183 32 0.2119 31 50 94 13050 12890 160 27 0.203 23 50 93 8020 7840 180 19 0.118 19 50 103 5305 5090 215 21 0.1082 17 50 101 6200 5950 250 20 0.087 31 50 94 13055 12900 155 17 0.1232 12 50 93 7000 6710 290 33 0.1284 2 50 100 6810 6510 300 80 0.3636 30 50 82 14250 14110 140 23 0.1966 1 51 101 4120 3910 210 38 0.2209 18 51 92 2070 1850 220 18 0.0891 2 51 100 21250 21030 95 0.76 3 51 93 2040 1750 290 20 0.0741 6 51 91 2360 2070 290 53 0.231 7 51 77 10940 10730 210 72 0.5217 14 51 93 1893 1518 375 33 0.0965 26 51 91 1480 1250 230 22 0.1058 32 52 103 2540 2315 225 44 0.2431 [Section Township Range Sundance Top Morrison Top Morrison Thicl

20 52 100 2770 2560 210 30 0.2016 29 52 93 1830 1690 140 2 0.0151 28 52 92 2740 2530 210 7 0.0294 32 52 100 2100 1870 230 26 0.1275 24 52 94 10060 9870 190 70 0.5833 9 52 82 12830 12500 330 60 0.2222 29 52 101 6320 6160 160 70 0.7765 9 52 82 12660 12400 260 51 0.244 24 52 94 10050 9820 230 58 0.3372 21 53 92 1540 1300 240 21 0.09 21 53 101 2075 1760 315 88 0.3876 9 53 84 13580 13420 160 30 0.2407 15 53 100 13300 13090 210 50 0.3125 13 53 102 2250 2000 250 12 0.054 a 54 102 4900 4655 245 75 0.4412 10 54 81 11260 11070 190 38 0.25 11 54 95 950 690 260 14 0.0561 30 54 92 1590 1360 230 2 0.01 8 54 102 4850 4570 280 10 0.037 11 55 74 8250 8000 250 60 0.3158 19 55 95 6340 6080 260 24 0.1017 33 55 98 11690 11469 221 30 0.1571 18 55 99 11170 10930 240 81 0.5094 12 56 100 2275 2205 70 20 0.4 28 57 96 2400 2225 175 26 0.1745 5 57 98 4717 4360 357 28 0.0851 13 57 83 9780 9560 220 55 0.3333 23 57 80 9540 9410 130 20 0.1818 23 57 79 9560 9430 130 2 0.001 6 57 96 1690 1470 220 83 0.6058. 6 57 97 1740 1495 245 16 0.0699 7 57 101 9940 9590 350 68 0.2411 [Section Township Range Sundance Top Morrison Top Morrison Thickness Sand Thickness Sand/Shale Ratio |

12 57 77 8720 8390 330 8 0.0248 4 57 100 7680 7450 230 22 0.1058 11 57 103 10090 9810 280 10 0.037 5 57 98 4890 4650 240 17 0.0762 6 57 96 1690 1480 210 82 0.6406 23 57 80 9540 9395 145 18 0.1417 13 40 82 1750 1560 190 105 1.2353 29 40 81 1820 1650 170 2 0.001 34 40 79 2840 2620 220 90 0.6923 22 40 82 1860 1680 180 80 0.8 31 40 80 1695 1510 185 55 0.4213 31 40 89 1190 1000 190 57 0.4286 13 40 81 1710 1470 240 18 0.0811 26 42 89 745 579 202 18 0.0978 3 42 92 1611 1409 193 110 1.3253 5 43 94 1803 1610 193 110 1.3253 4 43 75 1335 13335 225 100 0.855 18 43 81 3270 3080 190 80 0.7273 8 43 90 4400 4150 250 4 0.0121 13 43 83 750 530 220 180 4.5 3 43 93 2640 2500 140 42 0.4286 7 43 89 3320 3160 160 5 0.0323 8 43 94 1380 1100 280 4 0.01 1 43 92 2950 2750 200 7 0.0753 S 43 99 3720 3500 220 4 0.031 107

PAPER II. DEPOSITIONAL ENVIRONMENTS OF THE MORRISON

FORMATION IN THE BIGHORN BASIN OF

WYOMING AND MONTANA 108

ABSTRACT

The Morrison Formation has long been recognized as

being fluvial in origin. Recent studies have demonstrated that it consists of a complex of eolian, transitional marine, lacustrine, and fluvial deposits. Sedimentologic features including composition, sorting, sedimentary structures, biogenic structures, and subsurface geophysical data have helped to identify various subfacies

within the Morrison Formation.

In the southern part, the lower Morrison is characterized by eolian sediments that were deposited by

unimodal winds from the south and southeast in a semi-arid

environment. They represent the final phase of coastal

deposition along the retreating margin of the Late

Jurassic Sundance sea. The equivalent facies in the

north central Bighorn Basin is represented by intertidal

deposits comprising the interbedded olive green silty

claystone and sandstone lithofacies association. The rest

of the Formation is represented by fluvial deposits and

constitutes about 65% of the entire Morrison Formation.

They represent both vertical and lateral accretionary

deposits of stream channel and flood basin environments.

Channel sandstones comprise only 20% of the fluvial

sequence and are characterized by both ribbon and sheet 109 sandstones. Both meandering and braiding were the key fluvial processes of sedimentation. Sheet sandstones are multistory clearly demonstrating a series of vertical building episodes. In exposures oriented perpendicular to the paleoflow lateral accretion surfaces are evident.

This feature plus significant differences in paleocurrents between adjacent stories indicate that sheet sandstones were deposited by sinuous streams that were laterally mobile. The predominance of overbank mudstones may have been related to prolonged stream confinement within a meander belt. Sandbodies showing sheet morphology, low paleocurrent variance, abundant erosional surfaces, high sandstone/shale ratio, a paucity of in situ mudrock point to a braided fluvial pattern. Ribbon sandbodies are significantly smaller than sheets and possess a generally scoop-shaped base. Scour surfaces are marked by olive green mudclasts. 110

INTRODUCTION

Earlier workers have postulated simple models of fluvial and lacustrine deposition for the Morrison

Formation in Wyoming and Montana (Mook, 1916; Colbert,

1961, 1980; Dodson et al./ 1980; Moberly, 1960; Hallam,

1975). Mook (1916) proposed a normal alluvial floodplain under humid climatic conditions for the Morrison, and most subsequent workers have accepted this interpretation for both the Morrison and the Cretaceous deposits immediately overlying it. Stokes (1944), however, noting a general lack of channel deposits and the absence of plant material, objected to the humid floodplain theory and instead suggested an arid climate origin. Moberly (1960) presented evidence for a more humid climate for the

Cloverly Formation of Wyoming than that postulated by

Stokes for the Morrison of the Colorado Plateau. He compared the sediments to modern soils described by Mohr and Van Buren (1947) in Indonesia, that developed by subaerial and subaquatic weathering of volcanic ash under tropical climatic conditions.

Early interpretations of the depositional environments placed little emphasis on the importance of primary structures. In recent years great strides have been made in the interpretation of depositional Ill environments through analysis of facies defined by primary structures, bed geometries, etc. Fluvial sediments are better understood today and it is now known that many depositional sub-environments may exist within the broad fluvial setting. Today extensive subsurface information in the form of various geophysical logs is available.

This information is crucial not only for detailed stratigraphie correlation but also, for the delineation of different facies, and their distribution throughout the basin particularly where the Morrison is overlain by younger formations. Subsurface information was used in this study for detailed stratigraphie correlation, facies variations, and "thin bed" analysis of the mudstone dominated Morrison Formation. Various subenvironments were delineated on the basis of primary structures, trace fossils, lithology, and paleogeographic analysis.

Paleocurrent analysis of the individual sand bodies were obtained to find the mean paleocurrent direction of the sandstone units. 112

LOWER MORRISON

In the southern part of the Bighorn Basin near

Thermopolis and to the east along the western flank of the

Powder River Basin, the lower Morrison is represented by massive to crossbedded quartzarenite lithofacies association. The equivalent lithofacies in the northern part of the Basin is represented by interbedded silty claystone and sandstone. The depositional history of these lower Morrison lithofacies can be explained as follows.

Quartzarenite Lithofacies Association

This unit is composed of very fine to fine grained well sorted sandstones. Bedding is medium to very thick with large scale crossbedding. Individual forsets often show reverse grading making boundaries between them very distinct. The unit is chracterized by a single, dominant paleocurrent direction. From the analysis of the structure, composition, and texture, it is evident that the unit possesses distinctive characteristics that suggest that the sandstones were deposited in an eolian environment. Evidence for this includes: well developed thick sets of steeply dipping, tabular planar, and wedge 113 planar crossbedding; distinct internal stratification that conforms with previous descriptions of both ancient and modern day eolian deposits; flat-bedded and occasionally contorted beds and well sorted texture. Also the cumulative probability curves of the grain size distribution of the unit are similar in shape to curves presented by Visher (1969) for coastal dunes (Figure 1).

The paleocurrent analysis suggests deposition by unimodal wind (Figure 2).

On the basis of structure, the quartzarenite lithofacies can be divided into two subfacies. They are flat-bedded quartzarenite subfacies and b) crossbedded quartzarenite subfacies. The detailed characteristics of these two subfacies and the interpretation of their depositional environments are described below.

Flat-bedded quartzarenite subfacies

This subfacies commonly overlies the marine

Sundance Formation and underlies the crossbedded sandstone lithofacies. The unit has a tabular geometry, and occurs in most locations. It is usually separated from the underlying fossiliferous coquinoidal limestone or green sandstone of the Sundance Formation by a thin greenish mudstone (Figure 3). Rocks of this facies tend to split more or less perfectly along the bedding plane. Figure 1. Graphs comparing cumulative probability curves of grain size analysis of the eolian sandstones (quartzarenite lithofacies) of the Morrison Formation with modern dune sand 115

A. BEACH DUNES (VISHER, 1969)

2 3 4 0 DIAMETER

B. MORRISON-EOLIAN SANDSTONES

.-99 9

••99 t

••1

0 1 2 3 4 5 0 DIAMETER Figure 2. Figure showing the paleowind directions for lower Morrison eolian sandstones (quartzarenite lithofacies) at various locations. Number of foreset dip directions measured is represented by n. The mean paleowind direction is represented by the arrow CODY Figure 3. Photograph showing flat-bedded quartzarenite subfacies of the lower Morrison in the southern part of the Bighorn Basin, Wyoming. Location: Thermopolis

Figure 4. Photograph showing crossbedded quartzarenite subfacies of the lower Morrison in the western flank of the Powder River Basin. Location: Kaycee 119 120

Neither channelling nor ripple lamination were observed in vertical exposures. According to Hunter (1981), these represent initial sand deposits on a flat surface under limited sand supply conditions. Migrating dunes with steeply dipping foresets eventually covered these deposits and enhanced preservability.

The flat-bedded sequence has been reported from many desert floor deposits, both modern as well as ancient

(Mckee and Tibbits, 1964; Walker and Harms, 1972);

Steidmann, 1974). Fryberger et (1979) have documented the widespread occurrence of modern flat-bedded eolian sands. They suggest that ancient counterparts have been overlooked. Sheets of flat-bedded sand are typically well developed around the margins of dune dominated sand seas

(Fryberger et al, 1979). In the rocks of this study area, the commonly observed upward change from flat-bedded to large scale cross-bedded sandstone may be related to a progradational shift from up wind to down wind conditions.

Crossbedded quartzarenite subfacies

The crossbedded sandstone facies overlies either the flat-bedded sandstone facies or lies directly on the uppermost Sundance Formation with a thin layer of greenish mudstone in between the two (Figure 4). Approximately 90% of the eolian unit consists of fine to very fine grained 121 cross-bedded quartzarenite (Goswaml and Vondra, 1989).

The cross bedding is medium to large scale. The thickest cross bed set measured in the study area is 14 m thick.

Sets in excess of 5 m thick are common. The cross bed sets are both tabular planar and wedge planar and contain

individual foresets that are tangential to the basal

boundary of the set. At many localities, inverse grading within each foreset makes boundaries between foresets distinct. Individual foresets are highly tabular, and re traceable for several meters.

In outcrop, the cross bed sets are composed of two distinct types of foresets, here designated as type A and type B. Type A foresets are thicker, less laterally continuous, more steeply dipping than type B strata. The dip of the type B foresets range from 12® to 23° (gentler than type A) and average 18°. Type A foresets average 2-3 cm in thickness and taper out at their toes. The dip of the forests averages 24° but may be as steep as 30°. The type B foresets average 2 mm in thickness. Many crossbed sets of a meter or more are composed of type B foresets of uniform thickness.

Several lines of evidence strongly suggest an eolian origin for this facies. Hunter (1977a, 1980) has demonstrated the distinctive nature of strata produced by climbing wind ripples. Rather than producing foreset- 122 laminated strata similar to climbing subaqueous ripples, migrating wind ripples produce a single tabular, inversely graded stratum. The grading is apparently related to the concentration of fines in the ripple troughs and coarser particles on the crests. Strata produced contribute to top set, bottom set, and gently dipping foreset deposits of small eolian dunes (Hunter, 1977a). Type B foresets are analogous to Hunter's grainflow deposits which are formed by avalanching on the dune slipface.

Stratification identical to type B of this study is widespread in the Mesozoic Navaho, Entrada, and Wingate sandstones and in the Late Paleozoic DeChelly and Coconino

Sandstones (Peterson and Pipiringos, 1976; Jordan and

Douglas, 1980; Loope, 1984; Fryberger, 1984). The abundance of stratification of wind ripple origin in the cross bedded sandstone facies helps to explain the scarcity of vertebrate trackways. Since climbing ripples produce diachronous strata, the bedding planes between these strata never show evidence of phenomena produced at the sediment/air (or water) interface such as tracks, raindrop imprints, and wind ripple topography (Kocurek and

Dott, 1981, 1983; Kocureck and Nielson, 1986).

Occasional convolute bedding is found in few localities. It is mostly restricted to the type A strata.

The restriction of convolute bedding to type A strata is 123 related to high porosity and relative instability of grainflow (avalanche) deposits. Hunter (1977a) reported porosities as high as 50% in grainflow deposits of small

Oregon dunes compared to only 39% in wind ripple deposits.

Bagnold (1941) noted the instability of avalanched sand when subjected to vertically applied stress. Doe and Dott

(1980) believe that the deformation in the eolian Webber

Sandstone occurred after it was buried and subsided below the groundwater table. They argue that liquefaction is not likely in unsaturated sediments because pore pressure can rapidly dissipate. Although they declined to call this style of deformation a diagnostic criterion of eolian sandstones, they point out that it is common in such deposits.

Interdune deposits are represented by thin intervals of flat-bedded sandstone. Interdune deposits associated with dunes that were formed by unimodal winds, generally are thin, lenticular and diachronous reflecting a pattern of continual encroachment and burial by adjacent dunes as demonstrated by McKee and Miola (1975) in the

White Sand Dunes National Monument. Unlike modern dune environments where interdune deposits are abundant and regularly spaced (McKee and Miola, 1975; Ahlbrandt and

Fryberger, 1980; Simpson and Loope, 1985), the dune deposits in study area are very widely spaced and 124 interdune deposits are rare. A large area between dune forms coupled with very slow dune migration rates probably restricted the production of interdune deposit to local areas.

The wavy nature of the thin interdune deposits in the area of study is due to differential loading of wet sands (Hunter, 1981). Periodic flooding of the interdune area may have resulted in saturated layers that became contorted during burial and compaction.

The absence of raindrop impressions (Walker and

Harms, 1972, Fryberger, 1979; McKee, 1979) and deformation structures produced by slumping damp sand or damp sand crests on slip faces (Bigarella et al., 1969; McKee et al.. 1971; McKee and Bigarella, 1979) suggests that the paleoclimate was indeed arid. However, the presence of silicified tree trunks (Figure 5) and vertebrate tracks

(Weed, 1988) at dune exposures suggests that enough moisture was available to sustain sparse floral and faunal population suggesting climate to be at least semi-arid.

Electrical log characteristics of eolian deposits

Electrolog characteristics of clean dune sand are very conspicuous and easy to interpret. In general the radioactivity of dune sand is low and the porosity often 125

Figure 5. Photograph showing silicified tree trunk in the crossbedded quartzarenite subfacies of eolian origin, lower Morrison. Location: SE Baker Cabin Road 126 high, ranging up to 30%. On density vs neutron cross-plot the sand points of the eolian part fall very close to the sandstone line indicating clearly that quartz is the main component. The increase in radioactivity corresponds to the interdune wadi or sabkha deposits which are also reflected by density and neutron responses. As mentioned by Ahlbrandt et , (1982) " the dipmeter is an extremely useful device for recognizing and interpreting eolinites in the subsurface". Unfortunately dipmeter logs were not available in the area of study. The high resistivity with a sharp base combined with low gamma ray radioactivity and the information obtained from both the density and neutron log helped to identify the eolian unit of the Morrison

Formation. The interdune deposits are characterized by low resistivity, high radioactivity and show relatively low density and higher neutron porosity. A typical eolian clean sand body in the area of study is shown on Figure 6.

Interbedded Silty Claystone and Sandstone Lithofacies

Association

The lower part of the Morrison Formation in the northern part of the Bighorn Basin is represented by the interbedded olive green silty claystone and sandstone lithofacies association. The sandstone are massive to 127

32-40N-80W SWELL OIL CO GOVT. UNIT NO. 1

MORRISON FORMATION s-

SUNDANCE FORMATION o

SP Resistivity

Figure 7. A typical electric log response of the quartzarenite lithofacies interpreted to represent an eolian deposit. The high resistivity with sharp base is very typical of a clean eolian sandstone. 128 flaser or wavy bedded. Bioturbation is very common in the sandstones and siltstones. The massive sandstones are 0.5 to 1.2 m thick, consist of fine grained quartz arenites and quartzwackes and usually form flat bottomed lenticular channel deposits. Occasionally varve like laminae of

irregularly spaced fined grained siltstones are locally

preserved. Dish structure and small slump faults often cut across the laminae, indicating dewatering of rapidly

deposited sediments (Lowe and Lopiccolo, 1974; Lowe,

1975). The bioturbated sandstones and siltstones contain

abundant vertical burrows of the "skolithos" trace fossils

(Figure 7) and other borings. The siltstones are mostly

calcareous with ripple laminations, irregularly bedded

sandstones and contain abundant vertical domichnia of

pelecypods (Figure 8). Discontinuous lenses of poorly

sorted arenaceous siltstones with abundant highly

fragmented bivalves were also reported (Kvale, 1986). At

places the population densities are sufficiently high as to preclude the development of primary sedimentary

structures. The biogenic structures are a product of

locomotion. Trace fossils displaying shallow depressions

with trough like relief occur abundantly on bedding

planes of siltstones. These are interpreted to be resting traces (cubichnia) made by a animal that temporarily

settles onto, or digs into the substrate surface. Figure 8. Photograph showing burrows ("skolithos" trace fossils) in the interbedded claystone and sandstone lithofacles association of the lower Morrison Formation. Location: Coyote Basin

Figure 9. Photograph showing escape burrows in the interbedded claystone and sandstone lithofacles association of the lower Morrison Formation. Location: Fox Mesa 130 131

Discontinuous lenses of poorly sorted arenaceous fossiliferous siltstones were also reported north of Sheep

Mountain (Kvale, 1986).

Earlier studies have established that the uppermost

Sundance Formation in the Bighorn Basin of Wyoming was deposited in a setting dominated by tidal processes

(Goswami and Vondra, 1988; Kvale and Vondra, 1985a, b;

Uhlir, 1987). Existence of a transitional marine environment is not only evident from a regional analysis of paleogeography and paleotectonics of the foreland basin

but also suggested by the lithology of the lower Morrison

in the northern part of Bighorn Basin. From the available evidence, this facies is comparable to modern tidal flat deposits of North Sea (Boersma and Terwindt, 1981;

Terwindt 1971; Visser 1980) and intertidal flat deposits

of Baffin Island, Canada (Aitken et al,. 1988). The transition from the uppermost Sundance to lower Morrison

in the north central part of this Bighorn Basin records a

late Jurassic marine regression.

The interpretation of an intertidal or supratidal

environment is based on the following evidence recorded in the stratigraphie sequence:

1) The presence of trace fossils (eg. "skolithos" and

borings of pelecypods) typical of an intertidal to

supratidal community and microfaunae (dinoflagellates) 132 indicative of brackish water conditions. 2) A low energy environment is suggested by the paucity of sedimentary structures typical of waves or currents and by the dominance of clay size material. 3) The high organic content including the occurrence of storm deposited shell fragments (Kvale, 1986) indicates nearshore condition. 4)

The presence of sedimentary structures which characterize the mixed bedding or mixed lithologies - such as flaser, wavy, and lenticular bedding typical of an intertidal environments are present in the interbedded claystone and sandstone lithofacies association.

The alternating sand-mud layers of the interbedded claystone and sandstone lithofacies consist of slightly undulating, parallel bedded sand and mud layers. Such structures are mainly observed in shallow sub-tidal and intertidal environments (van Straaten, 1954; Terwindt and

Breusers, 1972; Reineck and Wunderlich, 1968). They indicate relatively low-energy conditions, with short periods of alternating sand and silt transport by tidal currents and waves with mud deposition occurring from suspension (Reineck and Wunderlich, 1968). Wavy bedding and flaser bedding may form in other environment as well.

Flaser bedding "implies sand and mud, as well as current activity and pauses in this activity" (Reineck, 1967).

Sand is transported and deposited in the form of ripples 133

while the mud is held in suspension during periods of

current activity. The mud which is held in suspension is

deposited in the ripple troughs or completely covers the

ripples during low energy slack water conditions. At the

beginning of the next current cycle the mud is eroded from the crest of the sand ripples and is covered by newly

deposited sand (Reineck, 1967). In wavy bedding, the mud

layers fill the ripple troughs and cover the ripple

crests.

Biogenic structures provide one of the most

reliable data for environmental reconstruction. The

ichnofauna present in rocks records a variety of organic

behavior. Dwelling structures (domichnia) of bivalves and

polychaetes constitute the most abundant traces found in

the lower Morrison Formation of the North Central Bighorn

Basin of Wyoming. The assemblage of organisms inhabits

substrates ranging from muddy sand through fine grained

sand. Some biogenic structures contain all the elements

of the "skolithos" trace fossils. "Skolithos" trace

fossils are generally 10-15 mm in diameter with an average

length of 30 cms (Figure 7). They show full reliefs with

no internal structure. These are dwelling burrows

inhabited by gregarious suspension feeding wormlike

animals (Frey et , 1984; Seilacher, 1964). Skolithos

association occur in shallow marine environments (eg. 134 littoral) where sedimentation rates are relatively high and particulate food is kept in suspension. Modern examples of the long vertical burrows in intertidal environments commonly reflect the adaptations by trace markers to combat fluctuations of both temperature and salinity. Long vertical, more substantial burrows of suspension feeders tend to dominate in turbulent water deposits, where organic matter is held in suspension

(Frey, 1978, 1967; Seilacher, 1967; Warme, 1975).

The occurrence of burrows/borings is locally dependent on the presence of a desirable substrate. The greatest organism densities are found in the intertidal zone and at shallow subtidal depths (Frey, 1978; Warme,

1975). Studies have shown that borers produce a variety of borings, from simple pits and blind tubes to branching complexes of interconnected voids. The shape of even some of the most distinctive borings is variable, changing proportions or even basic geometry under circumstances of different, substrate hardness restriction and other factors (Sanders, 1958; Purdy, 1964; Bromley, 1970; Warme,

1975). Specific characteristics of the substrate have been identified from the animal-sediment relationships.

This has been clearly stated by Purdy (1964) in his follow-up to studies by Sanders (1958) of marine bottom communities in Long Island Sound and Buzzard Bay. Purdy 135 emphasized that substrate in marine depositional environments is characterized by a continuous gradation or

"textural continuum," from sand size to clay- size material. This gradation occurs in response to changes in physical energy. Referring to Morrison sequence discussed above, similar "textural continuum" can be inferred in which silty and muddy substrate is dominated by specialized suspension feeders representing relatively few species individuals. The other end of the continuum - the high-energy end - is characterized by clean, well sorted sediments containing almost no trace fossils.

The tan and light brown color of the sandstone and siltstones suggest an oxidizing environment. The upper parts of the siltstone units are calcareous and commonly brecciated with 0.5 to 1 cm fragments surrounded by an in filling of overlying siltstones. The brecciation and in filling by the overlying material, the more oxidized tan color, and the presence of rootlets suggests subaerial exposure and pedogenic activity (Mantzios, 1986a, 1986b;

Kvale, 1986). 136

UPPER MORRISON

The upper part of the Morrison Formation consists of Interbedded reddish brown and grayish olive claystones and siltstones llthofacles association and a channel sandstone llthofacles associations. The channel sandstones comprise only about 20% of the sequence.

The Interbedded reddish brown and grayish olive slltstone and claystone llthofacles association typically represent the uppermost part of the Morrison Formation.

Of all the sediments of the Morrison and Cloverly formations In the Bighorn Basin, this llthofacles association contains the most abundant faunal remains.

Ostrom (1970) Identified fossil remains of Steaosaurus.

Camptosaurus. Allosaurus and Apatosaurus. The famous Howe

Dinosaur Quarry, located just east of the Beaver

Creek/Cedar creek confluence, occurs at this Interval where over 4000 dinosaur bones were collected (Brown,

1935; 1937). Evidence of pedogenlc features such as plant roots, burrows, glaebules, pedotubules, cutans, color mottling and clay-rich zones are present (Mantzlos,

1986a). The coloration, presence of carbonate glaebules

(caliche) and downward branching rootlets suggest that the reddish siltstones and claystones are the remains of better drained soils formed on a more distal part of the 137 floodplain, where as the grayish olive siltstones represent a more proximal floodplain setting where oxidizing conditions were not as intense perhaps because of higher groundwater table (Brown, 1985; Mantzios, 1986a;

Kvale, 1986).

The channel sandstones are typically broad, erosive based 1.5 to 5 m thick, multichannel deposits which fine upwards to siltstones and claystones. Both ribbon and sheet sandbodies are present within this channel sequence.

Allen (1965) shows models of deposits formed by braided, strongly meandering and low sinuosity streams.

The sandstones of the study area show variability in extent and quantity from one measured section to the next. Most sandstones appear to be discontinuous and not widespread, contrary to the braided and low sinuosity stream models of Allen. A criterion suggesting the channel sandstones are of an alluvial origin is the disconformable contact of the sandstones with underlying sediments. These are incised into, and at places interfinger with interbedded siltstone and claystone.

Alluvial channels cut downward into the lower sediments

(Reineck and Singh, 1980). However, predominance of claystones indicates that vertical accretion by overbank flow is the dominant form of Morrison deposition. In exposures oriented perpendicular to paleoflow, lateral 138 accretion surfaces are evident (Figure 9). This features plus significant differences in paleocurrents between the adjacent stories of the multistorey sheet sandstones are strong indicative of highly sinuous streams rather than either low sinuosity or braided streams. Sinuous streams are formed in flood plain with cohesive and vegetated overbanks (Collinson, 1978; Reineck and Singh, 1980). The abundance of large sauropod remains at this level suggests that vegetation was at least seasonally abundant, but lack of coal or coaly beds indicates that extensive bogs or swamps did not exist. Presence of numerous small channel sandstones, and lack of thick sandstone units or deep clay plug fillings, indicates that the streams were small.

The absence of persistent sandstone bed at several localities indicates stabilization of the streams for a long period of time (Collinson, 1978; Reineck and Singh,

1980).

The channel sandstone complex exhibits a variety of sedimentary structures mainly large-scale cross- stratification, small-scale cross-stratification, and horizontal stratification. These sedimentary structures provide valuable information concerning both the various hydrodynamic factors and sediment supply factors. Large- scale trough cross-bedding (pi-cross-stratification of

Allen, 1963a), consist of elongate scour channels infilled 139

Figure 9. Photograph showing lateral accretion surfaces in the channel sandstone lithofacies. Location: one mile south of Brock Ranch, Mayoworth 140

with curved foreset bedding. The scoop-shaped laminae

comprise a coset and are either symmetrical or

asymmetrical normal to the scour channel axis. Allen

(1936a) explains large scale trough cross-stratification

as resulting from the migration of large-scale,

asymmetrical dunes with curved crests, where as Harms and

Fahnestock (1965) postulate the filling of elongate

depressions by irregularly shaped migrating dunes in the

upper part of the lower-flow regime. Large-scale tabular-

cross stratification (omikron class of Allen) is

represented by tabular-shaped, cross-laminated, single and

coset units bounded by relatively planar surfaces.

Maximum inclinations average 23 degrees. Allen (1963a)

relates omikron-cross-stratification to migrating trains

of large-scale symmetrical ripples with essentially

straight crests. Harms and Fahnestock (1965) propose such

stratification from bars with sinuous, relatively long,

avalanche slopes and generally flat upstream surfaces that

commonly develop on the downstream margin of point bar

deposits during the meandering of a thalweg. They

observed that flow regime downstream of the avalanche

face, where the loci of tabular cross laminae deposition

occurs, is in the lower part of the low-flow regime.

Allen (1963b) states small-scale structures could

arise from migrating trains of small-scale asymmetrical 141 ripples with essentially straight crests. Such structures are generally associated with environments involving large and rapid sand accumulation (McKee, 1985).

Horizontal stratification in upper Morrison channel sandstones consists of tabular sets of horizontal laminae.

The individual laminae is about 2 cm. in thickness.

Horizontal stratification is a product of plane-bed or low standing wave transport (Harm and Fahnestock, 1965) marking the lower portion of the upper-flow regime. Such an environment would be expected in restricted alluvial channels during high discharge or on point bar surfaces as accumulating sand creates shoaling conditions. Thus the sinuous channel complexes which are associated with the overbank deposits are dominated by climbing ripple laminations and horizontal bedding indicates high depositional rates and high flow in flashy streams (Miall,

1978; Picard and High, 1977; Turnbridge, 1981)

Width/thickness ratio for lenticular bedded sheets are greater than 200, suggesting deposition in broad, shallow, laterally migrating stream systems. Most of the massive sandstones commonly contain large mudstone intraclast and is attributed to failure of channel bank.

Thin, lenticular very fine grained sandstones above and adjacent to the channels are interpreted as crevasse splays (Reineck and Singh, 1980; 142

Picard and High, 1973).

Grain size variation from silt to sandstone within cosets indicates fluctuating flow. Presence of massive beds at the base of the channels may represent longitudinal bedforms in deeper portions of the channel

(Hein and Walker, 1977) or thalweg channel deposits (Eynon and Walker, 1974). The overwhelming volumetric dominance of overbank mudstones to channel sandstones may suggests the presence of additional processes such as wind or sheet wash (Wells, 1983).

Vertical decrease in set thickness suggests shallowing of water through time (Figure 10). Where set thickness of planar crossbeds increases upward, increase in flow strength and water depth are postulated. Sand bodies showing sheet morphology low paleocurrent variance, abundant erosional surfaces, high sand-shale ratio and the paucity of in situ mud rocks point to a braided fluvial system (Condon and Peterson, 1986; Turner-Peterson, 1986).

Paleocurrent directions were determined by measuring the azimuth of the crossbed dip direction of the planar cross- stratification and the bearing of the trough axes of the trough cross-stratification. Figure 11 shows the mean paleocurrent directions of upper Morrison sandstones obtained at several localities. Along the western flank of the Bighorn Basin, near Cody, the upper 143

Figure 10. Channel sandstone lithofacies of the Upper Morrison showing gradual decrease in the cross bed set thickness upward and multi-story channel sandstone deposits. Location: Fox Mesa Figure 11. Figure showing the mean paleocurrent directions of the upper Morrison fluvial sandstones in the Bighorn Basin of Wyoming and Montana. The number of directions measured is represented by n. Mean paleocurrent direction is represented by the arrow. n=27

RED LODGE MONTANA WYOMINO

n=474 SHELL .

M =se A. UI HYATTVILLE

KAYCEE 146

Morrison sandstones show easterly paleocurrent direction while in the rest of the Basin, the trend ranges from NW-

NE.

Electrical log characteristics

Typical electrical log characteristics of fluvial deposits have been identified by many workers (Galloway and Hobday, 1983; Asquith, 1978; Schlumberger, 1989).

Galloway and Hobday (1983) proposed a generalized depositional model for a braided channel with theoretical

S.P. and resistivity log response. The S.P. shape of a braided channel sandstone is typically a smooth cylinder.

Each sand starts with an abrupt lower contact. The resistivity curves of the upper and lower boundaries are also abrupt giving a generally cylindrical shape. Similar log characteristics are found in several well logs of the upper Morrison Formation (Figure 12).

Galloway and Hobday (1983) also proposed an idealized S.P. log response for point bar deposits of a meandering stream systems. The S.P (or Gamma ray) and the resistivity curves for a point bar deposit form a bell shape pattern typically showing a fining upward sequence with a sand/shale ratio lower than 1. Figure 13 depicts a similar pattern observed in the upper part of the

Morrison Formation. In absence of a good S.P. log, a 147

3-54N-77W TRIGOOD OIL CO

CLOVERLY FORMATION

MORRISON FORMATION

SUNDANCE FORMATION

SP Resistivity

Figure 12. Electric log of a sandstone interpreted to represent a braided stream channel deposit in the upper part of the Morrison Formation. Note the cylindrical shape of the log. 148

18-47N-102W FOURBEAR NO. 19-A

i\) MORRISON o o FORMATION

SP Resistivity

Figure 13. Electric log of a sandstone interpreted to represent a meandering stream channel deposit in the Morrison Formation, Bighorn Basin, Wyoming. Note the fining upward sequence shown by the gradual decrease in resistivity curve 149 gamma-ray log or good resistivity response aids in the interpretation of a sedimentary sequence of a point bar in the Morrison Formation. Generally one can observe a sharp contact at the bottom of the sand with high resistivity and S.P deflection. 150

REGIONAL SYNTHESIS OF THE PALEOGEOGRAPHY AND DEPOSITIONAL

ENVIRONMMENT

A consideration of the regional paleogeography of the Western Interior helps explain the eolian, intertidal

(transitional marine) and the fluvial depositional environments of the Morrison Formation in the area of study. During the late Oxfordian to early Kimmeridgian time, central Wyoming was dominated by a marine environment (Vuke, 1984; Imlay, 1947). Tabular, laterally extensive, glauconitic sandstones and coquinas of the uppermost Sundance Formation are the deposits of a prograding, east - west trending, barrier coastline

(Uhlir, 1987a).

The barrier coastline possessed laterally migrating tidal inlets (Uhlir, 1987a, Uhlir and Vondra, 1984).

Periodic flooding of a coastal mud-flat landward from the

barrier coastline by the Sundance Sea resulted in a 2.5 to

7m sequence of interbedded sandstones and mudstones. This sequence lies above the sandstone and coquina facies in central Wyoming. Barrier beach deposits of the coastline were left behind as topographic highs on the broad coastal

plain as the Sundance Sea withdrew northward from central

Wyoming.

Wind activity that swept across the barrier beach 151 highs deposited loose sand on the muddy Morrison coastal plain in the form of isolated dune bodies. Dune patches were generally only a short distance (a few kilometers) from source sands. Dune patches consisted primarily of simple barchan dunes spaced at irregular intervals on the coastal plains. The geometry and spacing of the barchan dunes were controlled by the sand supply which was very low. The dune lengths were no greater than 1 km, and the original dune heights were probably 5 to 20 m (Weed, 1988;

Weed and Vondra, 1987). Sediment supply was greatest along the southern margin of the study area where the thickest dune accumulations took place. Here dune spacing was reduced and dune heights were increased. Local interdune areas formed along this margin.

The lower Morrison dune activity represents the culmination of extensive sand deserts that once were so prolific in the southern and central Western Interior during the Jurassic (e.g. the Nugget, Navajo, and Entrada sandstones) (Peterson and Pipiringos, 1976; Jordan and

Douglas, 1980; Loope, 1984b; Fryberger, 1984). Extensive

eolian deposition probably could not develop after the

North American Plate moved out of the hot, arid to semi-

arid climate of the Trade Wind Belt during the Late

Jurassic (Steiner, 1975, 1978). Sand sea developments in the central portion of the Western Interior ceased after 152 final withdrawal of the Late Jurassic Sundance Sea

(Mirsky, 1962; Kvale, 1986).

Dune construction ceased as sand source areas were, exhausted and buried. Burial of the dunes took place as

Morrison fluvial environments encroached from the northwest. The early Morrison coastal setting was eventually replaced by a fluvial depositional regime.

A consideration of the regional paleogeography of the Western Interior helps to explain the extensive low energy fluvial environment of the Morrison Formation. A

vast regional lowland, comprised of recently exposed sea floor or beach deposits remained following the rapid withdrawal of the Sundance sea. The topography must have

been of exceptionally low relief, probably close to sea

level, and in all likelihood did not immediately possess a definite regional slope. Thus there was probably no

integrated drainage system, and ephemeral lakes and swamps

must have developed (Walker, 1974, Moberly, 1960).

Although the existence of lakes implies that the

overall efficiency of the surface drainage was poor, the

similarity of the lower Morrison mudstone to the sediments

of the well-drained swamp environment described by Coleman

(1966) on the Mississippi Delta shows that subsurface

drainage was efficient enough to maintain an oxidizing

environment in the sediments (Coleman, 1966). In the 153

Mississippi Delta example, the well-drained swamp is inundated only during times of high flooding, and for a large part of the year the surface sediments are exposed and the water table is low, insuring subsurface oxidation.

Despite the great amount of vegetation and abundance of plant material in the surficial soil layer, there is very little organic material in the subsurface sediments of the well- drained swamp, due to extended exposure to oxidation

(Coleman, 1966, Collinson, 1978). This may provide an explanation for the lack of plant material in the

Morrison, although the scarcity of rootlet tubules implies that vegetation was probably sparse to begin with. The preservation of calcareous ostracode carapaces and charophyte oogonia, as well as reptilian bones, suggests that soils were alkaline rather than acidic (Walker, 1974;

Peck and Craig, 1962). The presence of limestone is also

indicative of alkaline conditions. The beds of fine grained, dense, fresh - water limestone were most likely formed by consolidation of a limy mush of disaggregated charophyte material. According to Sigurd Olson (fide

Curry, 1962), abundant modern growths of blue green algae are favored by shallow alkaline lakes. 154

CONCLUSIONS

Contrary to earlier belief of uniform fluvial origin of the Morrison Formation in the Bighorn Basin of

Wyoming, recent studies demonstrated that it consists of a complex of eolian, transitional marine, and fluvial deposits. Distribution of these environments and related lithofacies facies have been identified both in the surface and subsurface.

1. Thick, parallel bedded to crossbedded sandstone bodies at the base of the Morrison Formation in the southern part of the Bighorn Basin are of eolian origin.

Source of these eolian deposits was most likely the barrier beach deposits of the uppermost Sundance

Formation. The lower Morrison dunes were patchy, deposited by unimodal winds from south-southeast in a semi-arid environment. They represent the final phases of coastal deposition along the retreating margin of the Late

Sundance epicontinental sea and culmination of extensive sand deserts that once were so prolific in the southern and central Western Interior. The deposits are characterized by well developed high angle large scale cross stratification, and inverse grading. Based on the structure three main sub facies namely parallel bedded. 155 cross-bedded and interdune faciès were identified.

2. The intertidal deposits of lower Morrison

Formation are present mostly in the north central part of the Bighorn Basin. These sediments are transitional with upper Sundance tidal deposits. They are interpreted to have been deposited on a coastal tidal flat.

3. A consideration of regional paleogeography of the Western Interior helps explain the extensive low energy fluvial environments of the Morrison Formation. A low energy fluvial environments with regional lowland probably close to a sea level remained following the rapid withdrawal of Sundance sea. The mudstones were deposited as overbank material from flooding of high sinuosity, fluvial channels. Predominance of claystones and siltstones indicate that vertical accretion by overbank flow is the dominant form of fluvial deposition. The channel sandstone of sheet type were deposited in broad, shallow, laterally migrating streams. In most areas especially in the upper Morrison, the meander belt of small streams must have been essentially stabilized for long periods of time. 156

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GENERAL CONCLUSIONS

Surface and subsurface geological data bring to light several significant conclusions on stratigraphie, paleotectonic, and depositional environments of the

Morrison Formation in the Bighorn Basin of Wyoming and Montana. Approximately 300 geophysical logs and 70 measured section provided data for various types of map of the Upper Jurassic Morrison Formation. These data

illustrate depositional unit geometry, sandstone

depocenter distribution, and large scale lithofacies variation within the Morrison Formation. Studies have revealed different environments of deposition contrary to the earlier belief of a uniform fluvial environment.

Strata deposited in transitional marine, eolian, and

fluvial environments have been identified within the

Morrison Formation and their distribution show a process

response relation between the deposition and the orogeny.

The Morrison Formation lies conformably on the marine

Sundance (or Swift) Formation. The upper contact is

disconformable wherever the Morrison Formation is directly

overlain by the chert-pebble conglomerate unit of the

basal Cloverly Formation. In most places the contact

between the Morrison and the overlying Cleverly formations

is a paraconformity. 167

In the Bighorn Basin, the Morrison Formation consists of sandstone, siltstone, variegated mudstone/claystone with occasional limestone. The thickness of the formation range from 107 m (350 feet) to

21 m (70 feet). The formation is thin and less sandy along paleotopographic highs and sandier along paleotopographic lows, which suggests active structural control of facies distribution during deposition. Several other stratigraphie evidence also show existence of positive features in the central and eastern Wyoming during Late Jurassic. These highs were in position that are now occupied by Laramide structures of much greater magnitudes. Thus the regression of the Middle and Late

Jurassic sea and the onset of the extensive nonmarine deposits of the Morrison Formation regionwide reflect a change in the tectonic and sedimentologic framework of the northern Rocky Mountain during the late Jurassic.

In the southern part of the Bighorn Basin, the lower Morrison is represented by quartzarenite lithofacies of eolian origin. Regional paleogeography may have had a significant influence on the wind pattern in Wyoming at the onset of the Morrison deposition. The Oxfordian seaway lay immediately adjacent to the early Morrison coastal plain. The eolian sandstone deposition occurred by the activity of a dominant northward paleowind. 168

Equivalent faciès in the northern part are olive green silty mudstones and fine grained well sorted, bioturbated quartzwackes of intertidal origin. The upper part of the

Morrison Formation consists of sediments of fluvial origin represented by channel sandstones and an interbedded claystone and siltstone lithofacies association. The interbedded siltstones and claystones are of overbank origin and dominates the fluvial section of the Morrison

Formation. 169

ACKNOWLEDGMENTS

This research was funded in part by grants from

National Science Foundation to Dr. Carl F. Vondra and

Sigma Xi, the Scientific Society of America.

Special thanks are extended to Dr. Carl F. Vondra of Iowa State University, under whose guidance the study was completed. His interest, criticism and support throughout the course of the research has been valuable.

His valuable suggestions and criticisms helped to refine the ideas presented in this manuscript. Thanks are also due to Drs. John Lemish, Robert Cody of the Iowa State

University Department of Geological and Atmospheric

Sciences and Thomas E. Fenton of the Iowa State

University Department of Agronomy and T. A1 Austin of the

Iowa State University Department of Civil Engineering, who served as committee members and offered useful comments and suggestions. The author is thankful to Shiv Ambardar for his help as a partner in the field project.

A very special word of thanks and appreciation must be addressed to Dr. Bijon Sharma, Senior Geophysicist,

National Institute of Petroleum and Energy Research,

Bartlesville, Oklahoma, who helped in geophysical well log 170 interpretation. It is a pleasure to acknowledge his help.

It is with the most heartfelt thanks that I mention here the one person - Sangeeta Goswami, my wife who suffered through endless hours of being ignored, having to deal with an irritable husband and having to manage financial needs and occasionally type and proofread. She has been patient with me when procrastination resulted in unreasonable pressure on her. In sort, without Sangeeta I

wouldn't have been able to proceed and complete my Ph.D.

Once more I wish to express my gratitude to my wife for

her endless patience, sacrifice, interest and help during the years of my graduate studies. I am also thankful to

my little son, Rahul who behaved like an understanding

adult.

I am very thankful to my elder brothers Dr. Anil

Goswami and Dr. Umesh Goswami and the two sisters-in-law

Dr. Alaka Goswami and Dr. Rita Goswami for their continued

encouragement for higher education. My late parents

Bhubaneswar Dev Goswami, father and mother Jajneswari

Devi, were always in my heart and their memories gave me

continued moral support. Finally I dedicate this thesis to my elder brothers and late parents.