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Stratigraphy, paleotectonics andpaleoenvironments of the Morrison Formation inthe Bighorn Basin of Wyoming and MontanaDibakar GoswamiIowa 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

U M I 300 N. ZeebRA Ann Aibor, MI 48106

stratigraphy, paleotectonics and paleoenvironments of the

Morrison Formation in the Bighorn Basin

of Wyoming and Montana

by

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

Charg^^f Major Work

For the Graduate College

Iowa State University Ames, Iowa

1993

Signature was redacted for privacy.

Signature was redacted for privacy.

Signature was redacted for privacy.

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

Figure 1.

Figure 2.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7.

Figure 8.

Figure 9.

LIST OF FIGURES

Page

GENERAL INTRODUCTION

Location map of the study area 2

Geological map of north-central Wyoming 3

PAPER I

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

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

Pre 1973 correlations and age inter­pretations 21

Identification of the Morrison-Cloverly contact from electrical logs 29

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

Quantitative log analysis of thin sandstone beds 35

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

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

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

V

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Figure 14.

Figure 15.

Figure 1.

Figure 2.

Figure 3.

Isopach map of the Morrison Formation in the Bighorn Basin of Wyoming 44

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

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

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

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

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

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 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

Photograph showing flat-bedded quartz­arenite subfacies of the lower Morrison in the southern part of the Bighorn Basin, Wyoming. Location: Thermopolis 118

vi

Figure 4. Photograph showing crossbedded quartz-arenite 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

1 0 9 1 0 8

* BILL INGS 1 0 7

'Montana

7b ËA^ TOOT H MTNS.

PAYOR MXNS

. 9 ».» ^ 4 » 9 4

» & » « « V

•̂ yrovx-.f

LOVELL

• CODY

• 4 » » V a « » 4 » » » «

IDAHO

SHELL

^%L WYOMING

45 —

SHERIDAN

GREYBULL

> ^ # » & » •••n <K

' * « ' '»»A ./.V tt'

BIGHORN BASIN

WORLAND'

# * \BIGHORN-vi

< u. IL

•S

'MTNS.

TENSLEEP

, ' » » « » 4 '

44 —

M'-i»', THERMOPOLIS w

RIVER MTNS.

1 0 9 WIND RIVER BASIN

108 I

1 0 7

_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

^ ~ \ oiim'PMOi THER110F0LIS

^0WL:.:;i2 I

Morrison-Cloverly outcrops

30 Km

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

I.

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

"USA FB A-A

H/Study Area' r-'o

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

Q. 3 O CK O

I-UJ z z §

TYGEE SS 2 3

J L

6 M U D D Y S S 7 I I

A' 9

1

LOWER BEAR RIVER FM

SMOOT FM

THERMOPOLIS FM.

RUSTY BEDS

SKULL CREEK SH.

CLOVERLY FM O^RRISO ̂

S U N D A N C

4

L 60Km

WYOMING IDAHO-WYOMING

THRUST BELT

INYAN KARA

GRR

M VO

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

s LU

CO

S W (U oc o

oc < m

0

1 Z) -3

AGE

.(MA)

ALBIAN

-113 APTIAN

119 BARAEMIAN

124 HAUTERIV.

131 VALANG.

138 BERRTASIAN

144

T1TH0N1AN 152

KIMMERIDQAN 156

OXFORDIAN

COLORADO PLATEAU

(TSCHUOV. 1884) (PETEnSOW.1972)

CEDAR MTN

BLKikWM»

MORRISON

ARAPIEN SAN

RAFAEL

N. FRONT RANGE

COLORADO (MACKENZIE.IS7I) (PETERS0N.iar2)

MOWRY MUDDY SKULL CREEK

PtAINVlEW

LYTLE

MORRISON

RALSTON

WESTERN WYOMING (FURERigrO) (EYEM969)

ASPEN BEAR RIVER SS.

3

SMOOT ORANEY IS.

BECHLER PETERSON LS.

B^AIM.

LOWER EPHRAIM

STUMP

CENTRAL WYOMING (QCHERlseO)

(PETERSON, 1872)

MOWRY kWDDY

THERMOPOUS RUSTY BEDS GREYBULL

CLOVERLY

MORRISON

SUNDANCE

EASTERN WYOMING

(McGOOKEY,l972) (PETERSOK1S72)

MOWRY NEWCASTLE SCULL CREEK

FALL RIVER

FUSON .LAKOTA

MORRISON

SUNDANCE

WESTERN MONTANA

(SUTTNEH. 1888) (PETER30N.1972)

COLORADO GROUP

•riflnlHIIIilllIln

KOOTENAI

MORRISON

SWIFT

WESTERN ALBERTA

( VARLEV, 1984) (RU0KIN.1964)

MILL CR.

BEAVER MINES

GLADSTONE

CADOMIN;

FERNIE GROUP

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 —

O.fiO

0.70-

0.60-

-0.50-^

0.40-1

0.30 4 4"-

0.20

0.10- dli

0.00

0.45

-0.35

LO.25

ffl

"0.15

-0.05

14.1" 14.14 14.16 14.18 14.2 14.22 14.24 14.26 14.28 Depth,ft (Thousands)

14.3

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

"d >

0.40-

0.30-

0.20-

0.10-

250

-200

-150

0.90-

0.80 1

0.70-

f 0.50 -

0.50-

-100

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

14.3

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 CLOVERLY FM.

ENVIRONMENT & LITHOFACIES CLOVERLY FM. PRYOR CONGLOMERATE

MORRISON FM.

FLUVIAL (MEANDERING AND BRIDED STREAMS)

Interbedded siltsone and clystone lithofacies & •

Channel sandstone lithofacies

EOLIAN & TRANSITIONAL MARINE

Quartzarenite Lithofacies

Interbedded olive green claystone& bioturbated sandstone lithofacies

SUNDANCE FM. SHALLOW MARINE

Sandstones!Umestones!Shale

LACUSTRINE LIMESTONE

SCOUR SURFACE OR DISCONFORMITY

w 09

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

Gloyérly Formation.

Morrison Formation

Sundance Formation

SB—'

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

Horizoj Vertici

' : " ' r ; . . . .

- J

Ciovei^ ?

L E G E N D

Ml - Marker Horizon no.l M2 - Marker Horizon no.2 M3 - Marker Horizon no.3 CxR - Gamma Ray SP - Spontaneous Potential Res- Resistivity

Horizontal Scale: 1cm = 5.25 KM Vertical Scale ; 1cm = 70 M

NORTH

t

Y

IV %

%)j % \l %

\ \

KMS

MONTANA

WYOAONG

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 Res SR-i SP-' es SP4 SP GR. es

Cloverly Formation

Morrison Formation

Sundance Formation es

L E G E N D

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

L E G E N D

larker Horizon no.l {arker Horizon no.2 larker Horizon no.3 jamma Ray spontaneous Potential Resistivity

antal Scale: pal Scale i

1cm = 5-25 KM 1cm = 70 M

NORTH

MONTANA

WYOMING

% SECTION LINE

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

BIGHORN BXSIN

KAtCEE

THERMOPOLIS

WIND RI\'ER BASIN^ ^ARMINTO

108 00*

U1

200—- = 200 feet contour line 25 km

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

SCAI£ ( KM 108.00 109.0Cr 107.00 I

00

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

THERMOPOLia^^

WIND RIVER BASIN

SCAIE (Km) Contours are In feet 109' 108

tJi

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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Weed, D. D., and C. F. Vondra. 1987. Implications of an eolian sandstone unit of the basal Morrison Formation, central Wyoming. Am. Assoc. Petrol. Geol. Bull. 71:626.

Weed, D. D. 1988. Implications of an eolian unit of the basal Morrison Formation, central Wyoming. M.S. Thesis. Iowa State University, Ames, Iowa.

Weimer, R. J., J. J., Emme, C. L. Farmer, L. O. Anna, T. L. Davis and R. L. Kidney. 1982. Tectonic influences on sedimentation of Early Cretaceous east flank Powder River Basin, Wyoming and South Dakota. Colorado School of Mines Qtr. 77:61.

Weitz, J. L., Love, J.D., and Harbison, M. 1954. Geologic map of Natrona county, Wyoming. 9th Ann. Field Conf. Guidebook. Wyoming Geol. Assoc., Laramie, Wyoming.

Wiltschko, D. v . , and Dorr, J. A. 1983. Timing of deformation in overthrust belt and foreland of Idaho, Wyoming and Utah. Am. Assoc. Pet. Geol. Bull. 67:1304-1322.

Winslow, N. 1986. Timing and tectonic significance of upper Jurassic and lower Cretaceous nonmarine sediments. Bighorn Basin, Wyoming and Montana. M.S. Thesis. University of Wyoming, Laramie, Wyoming.

Winslow, N., and P. Heller. 1987. Evaluation of unconformities in Upper Jurassic and Lower Cretaceous nonmarine deposits. Bighorn Basin, Wyoming and Montana, U.S.A. Sed. Geol. 53: 181-202.

Wengerd, S. A. and M. L. Matheny. 1958. Pennsylvanian system of Four Corners region. Am. Assoc. Pet. Geol. Bull. 42:2048-2106.

82

Wright, J. C., and D. D. Dickey. 1963. Relations of the Navaho and Carmel formations in southwest Utah and adjoining Arizona. U.S. Geol. Surv. Prof. Paper 450E;63-67.

Yen, T. C. 1951. Fresh-water mollusks of Cretaceous age from Montana and Wyoming. U.S. Geol. Survey Prof. Pap. 233A:l-20.

Yen, T. C. 1952. Molluscan fauna of the Morrison Formation. U.S. Geol. Survey Prof. Pap. 2338:1-51.

Yen, T. C., and Reeside, J. B. Jr. 1946. Freshwater mollusks from the Morrison Formation (Jurassic) of Sublette, County, Wyoming. J. Paleont. 20:52-58.

Young, R. G. 1973. Dakota Group of Colorado Plateau. Am. Assoc. Pet. Geol. Bull. 44:156-194.

83

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 I 536 T44N R96W

NEl/4 S35 T43N R96W

NEl/4 S16 T43N R94W

BIG TRAIL Wl/2 i S25 T43N R88W *

SWl/4 S30 T45N R87W *

El/2 S25 T45N R88W

84

TENSLEEP

HYATTVILLE

SHELL /GREBULL

LOVELL

Sl/2 S24 T47N R88W

NEl/4 S26 T47N R89W

El/2 S15 T47N R89W

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

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

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

30 m

15 m

0 m

5 m

///y///

Sedimentary Structure & Paleocurrent Data

Large scale planar cross stratification

Small scale planar cross stratification

trough cross stratification

ripple lamination

wavy "bedded sediments

plane/parallel bedded sediments

concretions

interclast

coal

conglomerate

mean paleocurrent direction

87

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

Y

CURRENT AZIMUTH

REMARKS

CLOVERLY FM Pryor Conglomerate

MORRISON FM Variegated-pink, reddish "brown, gray green calcareous mudstone with thin fine grained discontinuous massive sandstone. Medium to coarse grained, white to "buff, 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 AZIMUTH

REMARKS

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 inter-beds 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 AZIMUTH

REMARKS

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

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

90

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

CURRENT AZIMUTH

REMARKS

. White ash with mudstone CLOVERLY FM

1 / MORRISON FM

•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 AZIMUTH

REMARKS

_ White ash with mudstone CLOVERLY FM

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

Variegated, greenish-yellow to purplish mudstone.

Pale brown to tan colored, cross bedded fine to medium grained sandstone and silt-stones. 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 AZIMUTH

REMARKS

_ ^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. -Olive green calcareous mudstone. MORRISON FM 1

93

LOCATION: HYATTVILLE wi/2 S2, r91w I

CURRENT AZIMUTH

REMARKS

• Pryor Conglomerate

/ I mOVEBlï PK

/ • «

"buff colored fine grained sandstone. - White to buff colored, cross "bedded medium

grained sandstone.

- Olive green calcareous mudstone with thin lense of ripple laminated medium grained sandstone.

- Tan colored "bioturbated fine grained sand­stone. Wavy bedding is common.

MORRISON FM

r / • «

"buff colored fine grained sandstone. - White to buff colored, cross "bedded medium

grained sandstone.

- Olive green calcareous mudstone with thin lense of ripple laminated medium grained sandstone.

- Tan colored "bioturbated fine grained sand­stone. Wavy bedding is common.

MORRISON FM

/ • «

"buff colored fine grained sandstone. - White to buff colored, cross "bedded medium

grained sandstone.

- Olive green calcareous mudstone with thin lense of ripple laminated medium grained sandstone.

- Tan colored "bioturbated fine grained sand­stone. 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 AZIMUTH

remarks

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

Î

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

Pale green, cross bedded, glauconitic fine grained sandstone.

SUNDANCE FM

95

location: sheu, NEl/4 su, T52N, R92W

CURRENT AZIMUTH

- White mudstone

remarks

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

V7777X SUNDANCE ffr

Greenish-gray, glauconitic fine grained sandstone.

96

location: crystal ceeek ei/2 s5, t5 ,̂ r93w

CURRENT AZIMUTH

remarks

White mudstone with occasional ash.CLOVERLY FM

h X -Variegated, yellow-gray, to rose- ^

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. ^Coarse grained coquinoid limestone.

97

location: BRIDGER SEIA 35, T6S, R2^

CURRENT AZIMUTH

— Pryor Conglomerate

remarks

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 AZIMUTH

remarks

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 AZIMUTH

remarks

_Pryor Conglomerate CLOVERLY FM

i 1 - Light-gray to tan colored MORRISON FM

cross iDedde to laminated fine grained sand­stones 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 sand­stone. 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 AZIMUTH

/

remarks

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 s 12, t39n, r83w

•6><

W7B

CURRENT AZIMUTH

1reddish brown mudstone

-Ccncealed-grass covered, apparently nonresis-tant mudstones.

/

remarks

Pryor Conglomerate CLOVERLY FM

MORRISON FM

Small scale trough cross stratified buff colo­red 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 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

28 44 94 4150 3970 28 44 91 5565 5430 23 44 95 3060 2878 30 44 91 5050 4880 27 44 100 3320 3140 27 44 75 13780 13510 28 44 91 5542 5382 5 45 91 9350 9170 1 45 104 2970 2680 a 45 89 6650 6480 1 45 79 13530 13420 26 45 82 6900 6680 9 45 90 7611 7492 19 45 92 8730 8533 22 46 95 12280 12130 9 46 76 12100 11910 9 46 81 11020 10830 25 46 100 4540 4360 2 46 94 11580 11450 18 46 98 2780 2675 6 46 89 6480 6280

10 46 99 4685 4460 11 46 89 5041 4900 7 47 89 1015 770

18 47 102 2070 1810 21 47 81 12150 12000 12 47 102 2720 2390 23 47 90 3040 2820 13 47 91 6122 5850 20 47 91 9002 8710 28 47 93 10885 10703 36 47 93 2410 2230

Morrison Thickness Sand Thickness Sand/Shale Ratio [

180 11 0.0651 135 29 0.2736 182 25 0.1592 170 22 0.1486 180 4 0.0227 270 101 0.5976 160 24 0.1765 85 40 0.8889 290 250 6.25 85 20 0.3077 110 2 0.0185 220 101 0.8487 119 7 0.0625 197 44 0.2876 ISO 87 1.381 190 61 0.4729 190 42 0.2838 180 33 0.2245 130 33 0.3402 105 2 0.0194 200 15 0.0811 225 25 0.125 141 19 0.1577 245 24 0.1086 2̂ 33 0.1454 150 60 0.6667 330 30 0.1 220 37 0.2022 272 140 1.06 292 45 0.1822 182 4 0.0225 180 75 0.7143

I Section Township Range Sundance Top Morrison Top

11 47 77 11980 11850 5 48 102 3700 3430 1 48 99 6770 6600 23 48 103 2850 2600 34 48 100 3350 3120 20 48 76 11560 11440 23 48 101 4400 4140 7 48 90 1250 990 33 49 103 4700 4458 13 49 93 9980 9820 34 49 76 11290 11060 2 49 102 2200 1960

17 49 99 9760 9470 17 50 91 2452 1960 1 50 92 2311 2191 31 50 94 13050 12890 23 50 93 8020 7840 19 50 103 5305 5090 17 50 101 6200 5950 31 50 94 13055 12900 12 50 93 7000 6710 2 50 100 6810 6510 30 50 82 14250 14110 1 51 101 4120 3910 18 51 92 2070 1850 2 51 100 21250 21030 3 51 93 2040 1750 6 51 91 2360 2070 7 51 77 10940 10730

14 51 93 1893 1518 26 51 91 1480 1250 32 52 103 2540 2315

Morrison Thickness Sand Thickness Sand/Shale Ratio |

130 82 1.7083 270 30 0.125 160 75 0.4286 250 43 0.2077 230 70 0.4375 120 45 0.6 260 75 0.4054 260 35 0.1555 242 135 1.2617 160 4 0.0256 230 48 0.2637 240 12 0.0526 290 29 0.1111 261 15 0.061 183 32 0.2119 160 27 0.203 180 19 0.118 215 21 0.1082 250 20 0.087 155 17 0.1232 290 33 0.1284 300 80 0.3636 140 23 0.1966 210 38 0.2209 220 18 0.0891

95 0.76 290 20 0.0741 290 53 0.231 210 72 0.5217 375 33 0.0965 230 22 0.1058 225 44 0.2431

[Section Township Range Sundance Top Morrison Top

20 52 100 2770 2560 29 52 93 1830 1690 28 52 92 2740 2530 32 52 100 2100 1870 24 52 94 10060 9870 9 52 82 12830 12500 29 52 101 6320 6160 9 52 82 12660 12400 24 52 94 10050 9820 21 53 92 1540 1300 21 53 101 2075 1760 9 53 84 13580 13420 15 53 100 13300 13090 13 53 102 2250 2000 a 54 102 4900 4655 10 54 81 11260 11070 11 54 95 950 690 30 54 92 1590 1360 8 54 102 4850 4570 11 55 74 8250 8000 19 55 95 6340 6080 33 55 98 11690 11469 18 55 99 11170 10930 12 56 100 2275 2205 28 57 96 2400 2225 5 57 98 4717 4360 13 57 83 9780 9560 23 57 80 9540 9410 23 57 79 9560 9430 6 57 96 1690 1470 6 57 97 1740 1495 7 57 101 9940 9590

Morrison Thicl<ness Sand Thickness Sand/Shale Ratio |

210 30 0.2016 140 2 0.0151 210 7 0.0294 230 26 0.1275 190 70 0.5833 330 60 0.2222 160 70 0.7765 260 51 0.244 230 58 0.3372 240 21 0.09 315 88 0.3876 160 30 0.2407 210 50 0.3125 250 12 0.054 245 75 0.4412 190 38 0.25 260 14 0.0561 230 2 0.01 280 10 0.037 250 60 0.3158 260 24 0.1017 221 30 0.1571 240 81 0.5094 70 20 0.4 175 26 0.1745 357 28 0.0851 220 55 0.3333 130 20 0.1818 130 2 0.001 220 83 0.6058. 245 16 0.0699 350 68 0.2411

[Section Township Range Sundance Top Morrison Top

12 57 77 8720 8390 4 57 100 7680 7450

11 57 103 10090 9810 5 57 98 4890 4650 6 57 96 1690 1480 23 57 80 9540 9395 13 40 82 1750 1560 29 40 81 1820 1650 34 40 79 2840 2620 22 40 82 1860 1680 31 40 80 1695 1510 31 40 89 1190 1000 13 40 81 1710 1470 26 42 89 745 579 3 42 92 1611 1409 5 43 94 1803 1610 4 43 75 1335 13335

18 43 81 3270 3080 8 43 90 4400 4150 13 43 83 750 530 3 43 93 2640 2500 7 43 89 3320 3160 8 43 94 1380 1100 1 43 92 2950 2750 S 43 99 3720 3500

Morrison Thickness Sand Thickness Sand/Shale Ratio |

330 8 0.0248 230 22 0.1058 280 10 0.037 240 17 0.0762 210 82 0.6406 145 18 0.1417 190 105 1.2353 170 2 0.001 220 90 0.6923 180 80 0.8 185 55 0.4213 190 57 0.4286 240 18 0.0811 202 18 0.0978 193 110 1.3253 193 110 1.3253 225 100 0.855 190 80 0.7273 250 4 0.0121 220 180 4.5 140 42 0.4286 160 5 0.0323 280 4 0.01 200 7 0.0753 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

MORRISON-EOLIAN SANDSTONES B.

.-99 9

• • 9 9 t

• • 1

0 1 2 4 3 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 quartz-arenite subfacies of the lower Morrison in the southern part of the Bighorn Basin, Wyoming. Location: Thermopolis

Figure 4. Photograph showing crossbedded quartz-arenite 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 .

=se

HYATTVILLE

KAYCEE

M A. UI

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 - 5 4 N - 7 7 W

TRIGOOD OIL CO

CLOVERLY FORMATION

MORRISON FORMATION

o

SUNDANCE FORMATION

Resistivity SP

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

SP

i\) o o

MORRISON

FORMATION

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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166

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.