Chapter 6

Subsurface Stratigraphic Cross Sections 

Showing Correlation of Cretaceous and 

Lower Tertiary Rocks in the Bighorn Basin, 

Wyoming and Montana

By Thomas M. Finn

Chapter 6 of 

Petroleum Systems and Geologic Assessment of Oil and Gas in the 

Bighorn Basin Province, Wyoming and Montana

By U.S. Geological Survey Bighorn Basin Province Assessment Team

U.S. Geological Survey Digital Data Series DDS–69–V

U.S. Department of the Interior

U.S. Geological Survey

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Volume Title Page
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Finn, T.M., 2010, Subsurface stratigraphic cross sections showing correlation of Cretaceous and lower Tertiary rocks 

in the Bighorn Basin, Wyoming and Montana: U.S. Geological Survey Digital Data Series DDS–69–V, 

14 p.

ISBN: 1–4113–2667–5
iii

Contents

Introduction.....................................................................................................................................................1

Stratigraphy ....................................................................................................................................................6

Lower Cretaceous Rocks ....................................................................................................................6

Cloverly Formation.......................................................................................................................6

Thermopolis Shale.......................................................................................................................6

Muddy Sandstone........................................................................................................................6

Upper Cretaceous Rocks.....................................................................................................................6

Mowry Shale.................................................................................................................................6

Frontier Formation........................................................................................................................8

Cody Shale ....................................................................................................................................8

Mesaverde Formation.................................................................................................................8

Meeteetse Formation, and Lewis and Bearpaw Shales .......................................................9

Lance Formation...........................................................................................................................9

Paleocene Rocks ................................................................................................................................10

Fort Union Formation.................................................................................................................10

Lower Eocene Rocks..........................................................................................................................10

Willwood and Tatman Formations, undivided.......................................................................10

Acknowledgments.......................................................................................................................................10

References....................................................................................................................................................10

Figures

1. Map of the Rocky Mountain region showing Laramide basins ............................................2

2. Index map of the Bighorn Basin.................................................................................................3

 3. Stratigraphic chart of Cretaceous and lower Tertiary rocks ................................................4

4. Index map of the Bighorn Basin showing the network of cross sections ..........................5

5. Regional stratigraphic cross section of Cretaceous 

and Paleocene rocks ...................................................................................................................7

Plates

1–6. Stratigraphic sections showing correlation of Cretaceous through lower Tertiary rocks

 1. Cross section A–A’ along the Nye-Bowler lineament ................................................... link

 2. Cross section B–B’ from the Beartooth Mountain front to the 

Whistle Creek anticline .................................................................................................. link

 3. Cross section C–C’ from the Heart Mountain gas field to Emblem gas field ............ link

 4. Cross section D–D’ from the Oregon Basin anticline to Greybull ............................... link

 5. Cross section E–E’ from the Grass Creek anticline to the Five Mile trend................. link

 6. Cross section F–F’ from Montana to the southeastern margin of the

 Bighorn Basin

 Sheet 1 of 2 ........................................................................................................................ link

 Sheet 2 of 2 ........................................................................................................................ link
Introduction

The stratigraphic cross sections presented in this report 

were constructed as part of a project conducted by the U.S. 

Geological Survey (USGS) to characterize and evaluate the 

undiscovered oil and gas resources of the Bighorn Basin 

(BHB) in north-central Wyoming and south-central Montana 

(fig. 1). The primary purpose of the cross sections is to show 

the stratigraphic framework and facies relations of Cretaceous 

and lower Tertiary rocks in this intermontane sedimentary 

and structural basin, which formed in the Rocky Mountain 

foreland during the Laramide orogeny (Late Cretaceous 

through early Eocene time). The BHB is nearly 180 mi long, 

100 mi wide, and encompasses about 10,400 mi2

. The basin is 

structurally bounded on the northeast by the Pryor Mountains, 

on the east by the Bighorn Mountains, and on the south by the 

Owl Creek Mountains. The northern margin includes a zone 

of faulting and folding referred to as the Nye-Bowler lineament (Wilson, 1936). The northwestern and western margins 

are formed by the Beartooth Mountains and Absaroka Range, 

respectively (fig. 2).

The cross sections were constructed from borehole 

geophysical logs of 62 wells drilled for oil and gas exploration and production. The stratigraphic interval extends from 

the base of the Cretaceous into the lower Eocene (fig. 3). 

The datum for all sections is the top of the Teapot Sandstone 

Member of the Mesaverde Formation or equivalent strata. This 

datum was selected because it is easily identified on most well 

logs and is present throughout the basin.

In most wells, a gamma-ray or spontaneous-potential 

log was used in combination with a resistivity or conductivity 

log to identify and correlate units. The gamma-ray and 

spontaneous-potential logs are typically used to differentiate 

between sandstone and shale; however, in the Bighorn Basin 

the spontaneous-potential response is typically subdued in the 

sandstone intervals showing little curve deflection. In areas 

of greater drilling density, logs from wells located between 

control wells on the cross sections were used to aid in making 

correlations. Coal beds were identified from gamma-ray 

logs in combination with density and (or) sonic logs where 

Subsurface Stratigraphic Cross Sections Showing 

Correlation of Cretaceous and Lower Tertiary Rocks in the 

Bighorn Basin, Wyoming and Montana

By Thomas M. Finn

available, and are shown as a long, heavy black bar on the 

depth track of each log. The heavy black bars representing 

coal beds only show the position of the coal bed(s) and are not 

proportional to true thickness. In addition to the stratigraphic 

information, oil and gas shows, oil- and gas-producing 

intervals, perforated intervals, and drillstem test intervals 

are also shown on the cross sections. This information was 

compiled from IHS Energy Group (2008) well data, the 

Wyoming Oil and Gas Conservation Commission Web site 

(2008), and drilling reports in the USGS well log files.

The locations of the six stratigraphic cross sections are as 

follows (fig. 4):

• Section A–A ́ extends for about 26 mi along the NyeBowler lineament (pl. 1).

• Section B–B ́ extends for 30 mi southeast from the 

Beartooth Mountain front to the southwest flank 

of Whistle Creek anticline (fig. 2) on the northeast 

margin of the basin (pl. 2).

• Section C–C ́ extends nearly east-west about 40 mi 

from Heart Mountain gas field to Emblem gas field 

(fig.2) (pl. 3).

• Section D–D ́ extends from near the southern end of 

the Oregon Basin anticline for about 50 mi to near 

the Greybull oil field (fig. 2) on the eastern margin of 

the basin (pl. 4).

• Section E–E ́ extends from near the southeast end of 

Grass Creek anticline northeast about 40 mi to near 

the southern end of the Five Mile trend (fig.2) (pl. 5).

• Section F–F ́ extends approximately 130 mi southeast from the Nye-Bowler lineament, through the 

Clark’s Fork sub-basin and along the Five Mile trend 

to the Sand Creek anticline (fig. 2) near the southeastern margin of the basin (pl. 6, 2 sheets).

For sections A–A ́ to F–F ́ (pls. 1–6) the horizontal scale 

is about 1 in. = 1.25 mi and the vertical scale is about 1 in. = 

500 ft.
2 Subsurface Stratigraphic Cross Sections Showing Correlation of Cretaceous and Lower Tertiary Rocks, Bighorn Basin

0 50 100 MILES

MT

ID

MT

WY

ND

SD

WY

CO

NE

ID

UT

UT CO

AZ NM

CO

NM

Black Hills uplift

uplift

Casper

arch

Owl Creek uplift

Powder

River

Basin

Crazy

Mountains

Basin

Bull

Mountains

Basin

Absaroka

volcanics

San

Juan

volcanic

eld

San Juan

Basin

Raton

Basin

Denver

Uinta Basin

Piceance

Basin

Greater Green

River Basin

Wind River Basin

Basin

Hanna

Basin

Hartville uplift

Rawlins

uplift

Uinta uplift

Granite Mountains

uplift

Wind

River uplift

White

River

uplift

Gunnison uplift

Nacimiento

uplift

Rio

Grande rift

Sangre

de

Cristo uplift

Sangre

de

Cristo

uplift

Park

Range

uplift

Sierra Madre

uplift

Uncompahgre

uplift

Bridger

uplift

Beartooth

uplift

Miles City arch

San Rafael

Swell

Medicine

Bow

uplift

Shirley

Basin

Laramie

Basin

North and Middle

Park Basins

South Park

Basin

Sawatch

uplift

Front

Range

uplift

San Juan

uplift

Axial Basin uplift

Teton uplift

Basin

Thrust or

 reverse fault

Cordilleran

orogenic

belt

(Neogene)

NE

SD

WY

Bighorn

Bighorn

Reed Point

syncline

Pryor

Mtns.

Nye-Bowler lineament

112° 110° 108° 106° 104°

36°

38°

40°

42°

44°

46°

Figure 1. Map of the Rocky Mountain region extending from southern Montana to northern 

New Mexico showing the locations of Laramide sedimentary and structural basins (in brown) 

and intervening uplifts. Modified from Dickinson and others (1988).
Introduction 3

5000

-5000

-10000

-5000

0 -5000

0

5000

0

0

0

-5000

0

0

F

F

F

F

F

F

F

F

F

T

50

N

T

55

N

T

9

S

T

45

N

T

40

N

R94W R90W R85W

R95W

R100W

R104W

T

50

N

T

55

N

T

57

N

T

9

S

T

5

S

R15E R20E R25E R35E

R101W R95W

R90W R85W

R35E R40E

R105W

T

40

N

T

45

N

T

5

N

R5W R1W R1E R5E

T

8

N

R4W

T

8

N

R3E

Montana

Wyoming

110°

45°

44°

109° 108° 107°

0 10 20 30 40

MILES

Precambrian rocks

Tertiary volcanic rocks

Oil field

Gas field

Fault

Bighorn Basin Province

Absaroka

Range

Beartooth

Mountains

Creek Mountains

Owl

Mountains

Bighorn 

Physiographic or structural

 feature mentioned in text Mountains

Pryor

Nye-Bowler lineament

Big Trails fault

Rattlesnake Mountain

anticline

Thermopolis anticline

Sheep 

Mountain

anticline

7

Heart

Mountain

Emblem

Greybull

Worland

Greybull

Manderson

Byron

Elk Basin

Dry

Creek Golden

Dome

Badger

Basin

Little

Bualo

Basin

Grass Creek

Worland

Five Mile

Oregon

Basin

Dobie Ceek

Garland

Meeteetse

3

4

5

6

7

8

2

1

Figure 2. Index map of the Bighorn Basin showing major structural elements, and oil and gas fields that produce from 

Cretaceous and Tertiary reservoirs. Physiographic features referred to in the text: (1) Grass Creek anticline, (2) Oregon 

Basin, (3) Five Mile trend, (4) Whistle Creek anticline, (5) Oregon Basin fault, (6) Greybull, (7) Sand Creek anticline, 

and (8) Clark’s Fork sub-basin. Structure contours drawn on top of the Teapot Sandstone Member of the Mesaverde 

Formation. Contour interval = 1,000 ft. Modified from Johnson and Finn (1998), and Ver Ploeg (1985).
PALEOCENE UPPER CRETACEOUS

CRETACEOUS TERTIARY (part)

EOCENE (part)

JURASSIC

(part) LOWER CRETACEOUS

?

?

System Series

Cody Shale

"chalk kick" "chalk kick" "chalk kick"

Cody Shale Cody Shale

Telegraph Creek Fm.

Mowry Shale

Meeteetse Formation

Lance

Formation

Fort Union

Formation

Muddy Sandstone

Thermopolis Shale

Morrison Formation

Tatman Formation

Claggett Mbr.

Lower

member

Mesaverde Formation

Teapot Ss. Member

Frontier Formation Frontier Formation Frontier Formation

Lewis Sh.

Willwood Formation

Northern Bighorn Basin Northern Bighorn Basin Southern Bighorn Basin

Bearpaw Shale Bearpaw Shale

Lennep Sandstone

Willwood Formation

Wasatch Formation

Mowry Shale Mowry Shale

Muddy Sandstone Muddy Sandstone

Thermopolis Shale Thermopolis Shale

Morrison Formation Morrison Formation

Cloverly

Formation

Cloverly

Formation

Cloverly

Formation

Lance

Formation

Hell Creek

Formation

Meeteetse

Fm. Lewis Sh.

Fort Union

Formation

Fort Union

Formation

Mesaverde Formation Mesaverde

Formation

(main part)

Mesaverde

Formation

(main part)

Lower member

Claggett Shale Claggett Mbr.

Eagle Sandstone

Virgelle Ss. Mbr.

Parkman Sandstone

Judith River

Formation

Teapot Ss. Member

Montana Wyoming

Pryor Conglomerate

Mbr.

Pryor Conglomerate

Mbr.

Pryor Conglomerate

Mbr.

Greybull Ss. Mbr. Greybull Ss. Mbr. Greybull Ss. Mbr.

"rusty beds" "rusty beds" "rusty beds"

middle

part

middle

part

Maastrichtian Campanian Santonian

Coniacian Turonian Cenomanian Albian Aptian

Neocomian

Stage

Figure 3. Stratigraphic chart of Cretaceous and lower Tertiary rocks in the Bighorn 

Basin, Wyoming and Montana. Modified from Keefer and others (1998), and Johnson 

and Finn (2004).

4 Subsurface Stratigraphic Cross Sections Showing Correlation of Cretaceous and Lower Tertiary Rocks, Bighorn Basin
Introduction 5

T

50

N

T

55

N

T

9

S

T

45

N

T

40

N

R94W R90W R85W

R95W

R100W

R104W

T

50

N

T

55

N

T

57

N

T

9

S

T

5

S

R15E R20E R25E R35E

R101W R95W

R90W R85W

R35E R40E

R105W

T

40

N

T

45

N

T

5

N

R5W R1W R1E R5E

T

8

N

R4W

T

8

N

R3E

Montana

Wyoming

110°

45°

44°

109° 108° 107°

0 10 20 30 40

MILES

Precambrian rocks

Tertiary volcanic rocks

Bighorn Basin Province

Absaroka

Range

Beartooth

Mountains

Creek Mountains

Owl

Mountains

Bighorn 

Mesaverde outcrop

Well location Mountains

Pryor

Nye-Bowler lineament

Big Trails fault

Rattlesnake Mountain

anticline

Thermopolis anticline

Sheep 

Mountain

anticline

E

E'

D

D'

C

C'

B

B'

A

A'

F

F'

Plate 6 (south half)

Plate 5

Plate 4

Plate 6 (north half)

Plate 1

Plate 2

Plate 3

Figure 4. Index map of the Bighorn Basin showing the network of cross sections presented in this report. 

Section A–A’, plate 1; section B–B’, plate 2; section C–C’, plate 3; section D–D’, plate 4; section E–E’, plate 5; and 

section F–F’, plate 6.
Stratigraphy

The stratigraphic nomenclature used on these cross sections shows the change in nomenclature from the Wyoming 

part of the basin to the Montana part of the basin and is shown 

in figures 3 and 5. The Wyoming nomenclature is modified 

from Keefer and others (1998), the Montana nomenclature 

is modified from Johnson and Finn (2004) for the area in the 

vicinity of the Nye-Bowler lineament.

Lower Cretaceous Rocks

Cloverly Formation

The basal Cretaceous rocks in the Bighorn Basin are represented by the Cloverly Formation consisting of 210 to 385 

ft of interbedded sandstone, variegated shale, claystone, and 

minor amounts of conglomerate (Keefer and others, 1998). 

The basal unit, where present, is referred to as the Pryor 

Conglomerate Member, has an irregular distribution, and is not 

distinguished separately from the overlying strata on the cross 

sections presented here. The Pryor is comprised of sandstone, 

conglomeratic sandstone, and black and gray chert pebble 

conglomerate. The middle part of the formation, referred to as 

the Little Sheep Mudstone and Himes Members by Moberly 

(1960) is comprised of variegated shale, and claystone, interbedded with thin sandstone beds that accumulated in floodplain, fluvial, and lacustrine environments. These units are 

overlain by very fine to medium-grained sandstone referred 

to as the Greybull Sandstone Member (Hintze, 1914; Mills, 

1956; Keefer and others, 1998). The thickness of the Greybull 

is highly variable, ranging from 5 to 70 ft (Keefer and others, 

1998), and is absent locally. The Greybull is latest Aptian to 

early Albian in age (Kvale and Vondra, 1993; May and others, 1995), and is interpreted to be a fluvial/estuarine channel 

deposit that accumulated in paleovalleys formed on a lowstand 

surface that developed on the nonmarine part of the Cloverly 

Formation during the initial Cretaceous marine transgression 

(Kvale and Vondra, 1993; Mitchell, 1997; Furer and others, 

1997). The uppermost part of the Cloverly Formation, referred 

to as the “rusty beds” (Love and others, 1945), consists of 

finely laminated siltstone and shale with minor thin sandstone 

beds that accumulated in tidal flats during the continued transgression of the Cretaceous sea during Albian time (Moberly, 

1960). The Greybull Sandstone Member and “rusty beds” have 

a combined thickness of about 100 ft throughout most of the 

basin but range up to around 160 ft. Based on fission track 

dating, the age of the Cloverly Formation is Early Cretaceous 

(Neocomian to Albian) (Heady, 1992; May and others, 1995; 

Zaleha, 2006).

Thermopolis Shale

The Thermopolis Shale (known as the Skull Creek Shale 

in some other Rocky Mountain basins), as used in this report, 

refers to the lower shale tongue of the Thermopolis Shale of 

Lupton (1916), Mills (1956), and Haun and Barlow (1962) 

underlying the Muddy Sandstone, but excludes the “rusty 

beds” that are included with the underlying Cloverly Formation. The Thermopolis consists of 125 to 230 ft of marine 

shales and siltstones and represents continued deposition during sea-level rise in Albian (Early Cretaceous) time (Burtner 

and Warner, 1984; Hagen and Surdam, 1984). The Thermopolis consists of dark-gray to black shale, interbedded with thin 

layers of siltstone, sandy claystone, and bentonite. The basal 

contact is gradational with the underlying “rusty beds;” the 

upper contact may be sharp and unconformable or gradational 

with the overlying Muddy Sandstone.

Muddy Sandstone

The Muddy Sandstone is composed of very fine to 

medium-grained sandstone interbedded with minor amounts of 

shale, siltstone, carbonaceous shale, and coal of latest Albian 

(Early Cretaceous) age (Paull, 1962). The formation was 

deposited in fluvial, marginal marine, and estuarine environments and ranges in thickness from 7 to 125 ft (Finn and others, Chapter 3, this CD–ROM). The thickest accumulations are 

associated with an incised valley-fill complex that developed 

on the exposed surface of the Thermopolis Shale during sealevel lowstand (Dolson and others, 1991).

Upper Cretaceous Rocks

Mowry Shale

According to Keefer and others (1998), the Mowry Shale 

in the Bighorn Basin consists of two distinct units (pls. 1–6). 

The lower part is about 160 to 400 ft of soft fissile clay-rich 

shale similar to the Thermopolis Shale and is referred to as 

the upper Thermopolis Shale by several authors including 

Mills (1956), and Haun and Barlow (1962), and as the Shell 

Creek Shale by Eicher (1962). The upper part is about 240 

to 400 ft of hard brittle siliceous shale. Numerous gray to tan 

bentonite beds are common throughout the unit and range in 

thickness from a fraction of an inch to about 7 ft (Byers and 

Larson, 1979). The siliceous shales are dark-brown to black, 

organic-rich, and contain an abundance of fish scales (Burtner and Warner, 1984). Locally, thin fine-grained sandstones 

occur in the middle to upper part of the siliceous part of the 

Mowry. These units, referred to informally as the “Kimball” 

and “Octh Louie” sands have produced oil from fields on the 

6 Subsurface Stratigraphic Cross Sections Showing Correlation of Cretaceous and Lower Tertiary Rocks, Bighorn Basin
Figure 5. Regional northwest-southeast stratigraphic cross section of Cretaceous and Paleocene (part) rocks, Bighorn Basin, Wyoming and Montandifferent lithologies and environments of deposition: red, fluvial sandstone and conglomerate; yellow, marine and marginal marine sandstone and siltsestuarine sandstone; tan, tidal sandstone, siltstone, and shale; various shades of gray are marine shales; various shades of green are coastal plain anshales, and coal; brown, sandstone, siltstone, shale, coal and conglomerate of continental origin.

0

0

5 10 15 20 MILES

500

FEET

1,000

Lewis Sha

Meeteetse Formation

Lower member Claggett Member of Cody Shale

Teapot Sandstone Member

Mesaverde Formation (middle part)

Cody Shale

Cody S

Cloverly Formation Jurassic rocks undivided

(part)

Thermopolis Shale

Frontier Formation

Muddy Sandstone

Mesaverde Formation

?

?

?

Bearpaw Shale

Lennep Sandstone

Eagle Sandstone

Parkman Sandstone

Telegraph Creek Formation

Judith River Formation

Claggett Shale

Virgelle Sandstone Member

Lance Formation

Lance Formatio

Hell Creek Formation

Fort Union Formation

(part)

Fort Union Formation

(part)

North Montana Wyoming 

Nye-Bowler

lineament

Badger

Basin

McCulloch

Peak

Cotto

C

"chalk kick"

Emblem

Bench

Red

Point Fritz Dobie Creek Five Mile Worland

Mowry Shale

Cody Shale

Montana

Wyoming
east side of the basin (Pierce, 1948; Mills, 1956; Cardinal and 

others, 1989). Rubey (1931) defined the top of the Mowry 

Shale as the Clay Spur Bentonite Bed in the northwestern part 

of the Powder River Basin. However, in the Bighorn Basin, no 

definitive correlation can be made to the Clay Spur but, based 

on correlations with well logs in the northern Wind River 

Basin and the Powder River Basin, the top of the Mowry was 

placed at a distinctive high-gamma marker that is believed to 

be equivalent to the Clay Spur. The upper part of the Mowry 

Shale was deposited in marine environments ranging from 

prodelta deposits along the western margin of the seaway to 

fine-grained deposits that accumulated in an oxygen-starved 

offshore marine basin (Davis and others, 1989). The combined 

thickness for the lower and upper parts of the Mowry range 

from about 400 ft in the southeastern part of the basin to more 

than 800 ft near the Beartooth uplift (Finn and others, Chapter 

3, this CD–ROM). Paleontologic evidence and radiometric 

dating indicate that the upper siliceous part of the Mowry is 

largely early Cenomanian (Late Cretaceous) in age (Cobban 

and Kennedy, 1989; Obradovich and others, 1996); however, 

radiometric dates for bentonite beds in the basal Mowry (Shell 

Creek equivalent) indicate an Albian (Early Cretaceous) age 

(Obradovich and others, 1996).

Frontier Formation

The Frontier Formation consists of sandstone, siltstone, 

shale, and bentonite that accumulated in a marine or marginal 

marine setting. In the western and northwestern parts of the 

basin, the Frontier also includes some nonmarine strata including minor amounts of carbonaceous shale and coal (Siemers, 

1975; Merewether and others, 1975). The sandstones are 

generally believed to be deltaic or shoreface in origin (Hunter, 

1952; Van Houten, 1962; Goodell, 1962; Siemers, 1975; 

Merewether and others, 1975; Merewether and others, 1998). 

Many of the sandstones are continuous for tens of miles, 

but are truncated due to erosion, mainly by marine flooding 

surfaces and appear to pinch out in all directions into marine 

shale (M.A. Kirschbaum, USGS, oral commun., 2008). The 

Frontier is early Cenomanian to early Coniacian (Late Cretaceous) in age with a major unconformity in the upper part of 

the unit (Merewether and others, 1975). The unconformity is 

based on the study of ammonite zones in surface sections and 

was not identified in the subsurface in this study.

Cody Shale

The Cody Shale in the Bighorn Basin consists of marine 

shale, sandstone, and siltstone deposited during a major 

marine transgressive-regressive cycle that extended from 

Coniacian to Campanian (Late Cretaceous) time (Kauffman, 

1977). The Cody ranges in thickness from about 1,700 ft in the 

northern part of the basin to nearly 3,800 ft in the southeastern 

part of the basin (Finn and others, Chapter 3, this CD-ROM). 

The lower and upper contacts of the Cody are conformable 

and interfinger extensively with the underlying Frontier and 

overlying Mesaverde Formations. The Cody Shale consists of 

two main units, the lower main body and the upper Claggett 

Member (Keefer and others, 1998).

The main part interfingers with and extends from the 

top of the Frontier Formation to the base of the Mesaverde 

Formation and is composed of marine shale with increasing 

amounts of interbedded sandstone in the upper 500 to 1,200 

ft (pls. 1–6). The upper sandy part in the Bighorn Basin is 

similar to but is somewhat less sandy than the upper sandy 

member of the Cody Shale in the Wind River Basin (see Finn, 

2007; and Johnson, 2007, for correlation and descriptions of 

the Cody Shale in the Wind River Basin). Many of the sandstones in the upper part of the Cody are blanket-like, can be 

traced over many miles, and appear to pinch out into marine 

shale in all directions. The uppermost several hundred feet 

of the sandy part of the Cody is commonly referred to as the 

Telegraph Creek Formation in Montana (figs. 3, 5, and pl. 1). 

The Cody is poorly exposed in the Bighorn Basin, however 

limited outcrop descriptions by Johnson and others (1998) of 

uppermost Cody Shale indicate that the sandstones are very 

fine to medium-grained, exhibit a variety of bedding features 

including hummocky cross-bedding that indicates a nearshore 

marine origin, and in many cases are laterally persistent.

According to Keefer and others (1998) the Claggett 

Member (known as the Claggett Shale in Montana) is a westward thinning tongue of marine shale and fine-grained sandstone that is split from the main part of the Cody Shale by the 

lower member of the Mesaverde Formation (Eagle Sandstone 

in Montana) (figs. 3, 5). The Claggett Member extends across 

the eastern and central parts of the basin, and thickens eastward from zero to nearly 500 ft thick at its eastern limit where 

it merges with the main part of the Cody Shale (fig. 5) (Finn, 

Chapter 3, this CD–ROM).

Mesaverde Formation

The Mesaverde Formation consists of basal regressive 

marginal marine sandstone, overlain by interbedded nonmarine sandstone, siltstone, shale, carbonaceous shale, and 

coal deposited in coastal plain and marginal marine environments as the western shoreline of the Cretaceous sea retreated 

eastward across the Bighorn Basin (Severn, 1961; Mackenzie, 

1975; Klug, 1993; Johnson and others, 1998). The Mesaverde 

is Campanian (Late Cretaceous) in age and can be subdivided 

into a lower member, a middle or main part, and the Teapot 

Sandstone Member (figs. 3, 5) (Keefer and others, 1998; Johnson and others, 1998). 

The lower member of the Mesaverde Formation, referred 

to as the Eagle Sandstone in Montana by Johnson and Finn 

(2004), is an eastward thinning wedge of marginal marine and 

nonmarine strata. It conformably overlies and interfingers with 

the upper sandy part of the Cody Shale and is separated from 

the middle member or main body of the Mesaverde by the 

8 Subsurface Stratigraphic Cross Sections Showing Correlation of Cretaceous and Lower Tertiary Rocks, Bighorn Basin
Stratigraphy 9

Claggett Member of the Cody Shale (fig. 5). The lower member is present in the central and eastern parts of the basin, and 

it consists of very fine to medium-grained sandstone, siltstone, 

shale, carbonaceous shale, and coal deposited as an eastwardprograding deltaic complex (Mackenzie, 1975; Johnson and 

others, 1998). The basal beds of the lower member consist 

of very fine to fine-grained sandstone that were deposited 

in a marginal marine setting and have been referred to as 

the Virgelle Sandstone Member in Montana by Johnson and 

Finn (2004) or as the Virgelle sandstones by Severn (1961). 

The basal sandstones grade upward into interbedded shale, 

carbonaceous shale, coal, and sandstone that were deposited 

in a delta plain setting (Mackenzie, 1975). The lower member 

is thickest at its western limit where it is generally around 400 

to 500 ft thick but locally exceeds 800 feet (Finn and others, Chapter 3, this CD–ROM). It merges with the main part 

of the Mesaverde Formation to the west, and thins to zero to 

the east where it grades into the upper part of the Cody Shale 

(fig. 5). West of the Claggett pinchout, the lower and middle 

Mesaverde members merge as one unit simply referred to as 

Mesaverde Formation. 

Where the Claggett is present, that part of the Mesaverde 

Formation overlying it is referred to as the main body of the 

Mesaverde Formation by Keefer and others (1998), whereas 

Johnson and others (1998) referred to it as the middle part. 

According to Johnson and Finn (2004) it is, in part, equivalent 

to the Parkman Sandstone and the lower part of the Judith 

River Formation in Montana (figs. 3, 5). It consists of interbedded sandstone, siltstone, shale, carbonaceous shale, and 

coal deposited in marginal marine, coastal plain, and fluvial 

environments (Severn, 1961; Mackenzie, 1975; Johnson and 

others, 1998). The basal strata of the middle member are very 

fine to fine-grained and have been interpreted as progradational units that accumulated as shoreface sandstones as the 

western shoreline of the seaway retreated eastward across the 

basin (Keefer and others, 1998; Johnson and others, 1998). 

The sandstones in the nonmarine part of the Mesaverde are 

very fine to medium-grained, lenticular, and originated as 

stream channel and crevasse splay deposits (Johnson and others, 1998). The Mesaverde generally thins from west to east 

where a combined thickness of the main part of the Mesaverde 

and Teapot Sandstone Member, excluding the lower member 

or Eagle Sandstone, ranges from greater than 1,200 ft in the 

southwestern part of the basin to less than 500 ft along the 

eastern margin of the basin (Finn and others, Chapter 3, this 

CD–ROM). 

The Teapot Sandstone Member, the uppermost member 

of the Mesaverde Formation, consists of light-gray to white, 

fine- to coarse-grained sandstone, and minor amounts of gray 

mudstone (Johnson and others, 1998). Based on outcrop studies by Mackenzie (1975) and Johnson and others (1998), it is 

considered to be fluvial in origin throughout most of the basin; 

however, in the eastern part of the basin, Johnson and others 

(1998) described hummocky cross-bedding in the lower part 

of the unit indicating that it is in part marginal marine in origin. The thickness of the Teapot is highly variable and ranges 

from a few tens of feet to more than 300 ft (Keefer and others, 

1998; Johnson and others, 1998).

Meeteetse Formation, and Lewis and Bearpaw 

Shales 

The Meeteetse Formation, and Lewis and Bearpaw 

Shales are intertonguing marine, marginal marine, and nonmarine strata of latest Campanian and early Maastrichtian 

(Late Cretaceous) age (Gill and Cobban, 1966; Merewether, 

1996) (figs. 3, 5). Johnson and Finn (2004) considered the 

Meeteetse/Lewis/Bearpaw interval in the Bighorn Basin to be 

equivalent to the upper part of the Judith River Formation and 

overlying Bearpaw Shale in Montana (figs. 3, 5). The marine 

strata in this interval are included with the Lewis and Bearpaw 

Shales, and the marginal marine and nonmarine strata with the 

Meeteetse Formation. The maximum combined thickness for 

the Meeteetse, Lewis, and Bearpaw interval ranges from more 

than 1,100 ft in the northern and southwestern parts of the 

basin to about 500 ft in the southeastern part (Finn and others, 

Chapter 3, this CD–ROM).

The Meeteetse Formation is comprised of alternating 

thin beds of very fine to medium-grained sandstone, siltstone, 

shale, carbonaceous shale, and coal that accumulated in poorly 

drained coastal environments along the western shore of the 

Cretaceous seaway (Keefer and others, 1998). The Lewis 

Shale is a westward thinning tongue of marine shale and sandy 

shale interbedded with thin sandstone beds. It is present in the 

eastern part of the basin, ranges in thickness from more than 

300 ft on the east side of the basin, and thins westward to zero 

where it grades into the Meeteetse Formation (Keefer and others, 1998). In the northern part of the basin the Bearpaw Shale 

overlies, and in part intertongues with, the Meeteetse Formation and represents the last stages of marine sedimentation in 

the Bighorn Basin. Where present, the Bearpaw is about 80 

to 200 ft thick, and according to Johnson and others (1998) 

is comprised of fissile gray shale and thin beds of very fine 

grained sandstone.

Lance Formation

The Lance Formation, referred to as the Hell Creek 

Formation in Montana by Johnson and Finn (2004), consists 

of interbedded sandstone, shale, and some conglomerate; it 

represents the uppermost Cretaceous rocks in the Bighorn 

Basin. Minor amounts of coal are present locally, which Johnson (1998) concluded to be of minor importance. Sandstone is 

the predominant lithology throughout much of the basin with 

sandstone generally occurring in the lower part of the formation, whereas the upper part is interbedded sandstone and 

shale (Keefer and others, 1998; Johnson and others, 1998). 
The sandstones are fine- to coarse-grained, locally conglomeratic, and were deposited in fluvial systems (Johnson and 

others, 1998). In the northern part of the basin, a sandstone 

interval in the basal part of the Lance, overlying the Bearpaw 

Shale, was interpreted as a shoreface deposit by Keefer and 

others (1998), and was believed to correlate with the Fox Hills 

Sandstone. Johnson and Finn (2004) referred to this marginal 

marine interval as the Lennep Sandstone in Montana (figs. 3, 

5). The Lance is generally considered to be Maastrichtian 

(Late Cretaceous) in age; however, palynogical age determinations by Nichols (1998) indicate that locally it is, in part, 

earliest Paleocene in age. The onset of the Laramide orogeny 

and initial partitioning of the Rocky Mountain foreland basin 

into the smaller Laramide basins is indicated by the Lance 

thickness patterns in the Bighorn Basin, which range from 

less than 500 ft in the southeastern part of the basin to greater 

than 1,500 to 1,800 ft near the basin trough adjacent to and 

east of the Oregon Basin fault (Finn and others, Chapter 3, this 

CD–ROM).

Paleocene Rocks

Fort Union Formation

The Fort Union Formation, referred to as the Polecat 

Bench Formation by Bown (1975) is comprised of sandstone, 

siltstone, conglomeratic sandstone, conglomerate, carbonaceous shale, and coal (Bown, 1975; Johnson, 1998; Keefer and 

others, 1998; Roberts, 1998). The conglomerates accumulated 

mainly as alluvial fans along the northwestern margin of the 

basin (Hickey, 1980; Johnson and Middleton, 1990), and as 

a braidplain system that flowed northeast into the basin from 

surrounding highlands in the southwestern part of the basin 

(Kraus, 1985; Roberts and Stanton, 1994). The sandstones are 

very fine to coarse-grained and represent a variety of fluvial 

deposits, whereas the finer grained and carbonaceous deposits represent lacustrine and swamp deposits (Hickey, 1980; 

Yuretich and others, 1984; Roberts and Stanton, 1994; Hickey 

and Yuretich, 1997; Roberts, 1998). The thickness of the Fort 

Union indicates continued subsidence of the basin trough 

during the Laramide orogeny and reflects the present-day 

structural configuration of the basin. The Fort Union is generally less than 1,000 ft thick around the margins of the basin, 

thinning locally to about 300 ft in the north, and thickening 

to more than 7,500 feet in the structurally deepest central part 

of the basin. In outcrops around the margins of the basin, the 

Fort Union unconformably overlies the Lance and locally 

other older Upper Cretaceous rocks (Bown, 1975; Love and 

Christiansen, 1985; Keefer and others, 1998; Roberts, 1998). 

The Fort Union is Paleocene in age based on fossil pollen dating by Nichols (1998) and Roberts (1998), and by mammalian 

faunas reported by Gingerich (1983).

Lower Eocene Rocks

Willwood and Tatman Formations, undivided

Strata that represent the lowermost Eocene in the Bighorn 

Basin are assigned to the Willwood Formation and consist of 

variegated shales, sandstones, and conglomerate (Van Houten, 

1944; Bown, 1980). Bown (1980) reported a maximum thickness of nearly 2,400 ft from exposures in the southern part of 

the basin. The lower Eocene Tatman Formation overlies and 

in part interfingers with the Willwood and is present only as 

isolated erosional remnants in the south-central part of the 

basin (Love and Christiansen, 1985; Bown, 1980). The Tatman ranges up to nearly 900 ft thick, and is composed of finegrained sandstone and carbonaceous shale that accumulated 

in lakes and swamps (Van Houten, 1944). Keefer and others 

(1998) reported a combined thickness for the Willwood and 

Tatman Formations of up to about 5,000 ft in the subsurface 

along the deep axis of the basin. The combined thickness of 

the Fort Union and lower Eocene rocks ranges from less than 

1,000 ft around the margins of the basin to greater than 12,000 

ft near the basin axis and reflects the present-day structure of 

the basin (Finn and others, Chapter 3, this CD–ROM).

Acknowledgments

The author thanks Ron Johnson, Mark Kirschbaum, and

Tom Judkins for numerous helpful discussions regarding the 

Cretaceous and Tertiary geology and stratigraphic nomenclature of the Bighorn Basin. The manuscript and plates benefited 

from reviews by Ron Johnson, Laura Biewick, Dick Keefer, 

Doug Nichols, and Gayle Dumonceaux, and their suggestions 

and comments are greatly appreciated.

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