Chapter 7

Detailed West to East Cross Section of 

Cretaceous and Lower Tertiary Rocks 

Across the South Part of the Bighorn Basin, 

Wyoming

By Ronald C. Johnson

Chapter 7 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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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:

Johnson, R.C., 2010, Detailed west to east cross section of Cretaceous and lower Tertiary rocks across the south 

part of the Bighorn Basin, Wyoming: U.S. Geological Survey Digital Data Series DDS–69–V, 3 p.

ISBN: 1–4113–2667–5
iii

Contents

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

References Cited............................................................................................................................................3

Plate

1. Detailed west to east cross section of Upper Cretaceous and Lower

Tertiary rocks, southern Bighorn Basin, Wyoming ............................................................ link
Introduction

This cross section presents the results of a detailed study 

of the depositional settings of Cretaceous and lower Tertiary 

rocks across the southern part of the Bighorn Basin, Wyoming, 

with a focus on the Upper Cretaceous marine shale interval. 

The wells are closely spaced in order to portray the detailed 

stratigraphy of these marine strata. The principal method of 

correlation was to trace distinctive patterns of the resistivity 

curves on electric logs. In low-resistivity shales such as 

the Cody Shale, these patterns can generally be correlated 

throughout the study area, particularly where well spacing is 

less than 3–4 mi (Johnson, 2003). Asquith (1970) attributed 

these patterns to variations in bentonite content from volcanic 

ash and used such variations to establish time-stratigraphic 

correlations in the Cretaceous shales of Wyoming. A large 

number of correlation lines are shown on the cross section, 

some of which could be recognized in nearly all wells, 

whereas others could be recognized only locally.

The cross section generally parallels a section presented 

by Asquith (1970) across the Bighorn Basin. The lower 550–

900 ft of the Cody Shale is distinctive in that the strata appear 

to prograde to the east. Asquith (1970) was the first to recognize the inclined character of these strata, and the numbers 

one through six labeled on the cross section presented here 

correspond to the stratigraphic markers labeled on Asquith’s 

Bighorn Basin cross section (his fig. 8). The inclined strata 

slope eastward at about one-half degree, and Asquith (1970) 

believed that these strata were deposited on the slope that 

occurred between the shelf and the deep part of the Cretaceous 

seaway. Asquith (1970) estimated the water depth at the time 

of deposition of these inclined strata at from 240 to 475 ft.

Ten different depositional settings for the Cretaceous 

and lower Tertiary rocks are recognized: 1) dark-gray to black 

organic-rich shales deposited in an offshore marine setting; 

2) dark-gray to black organic-rich, siliceous marine shales 

deposited in an offshore marine setting during a time of extensive regional volcanism; 3) gray shale and silty, sandy, and 

calcareous gray shale displaying large-scale clinoform features 

suggesting deposition on an offshore slope or ramp; 4) gray 

shale and silty, sandy, and calcareous gray shale displaying 

largely horizontal time lines suggesting deposition in a marine 

shelf setting; 5) complex, persistent, blanket-like intervals of 

mainly sandstone, conglomerate, and minor mudstone deposited in a sand-rich coastal plain and marginal marine setting; 

6) sandstone deposited in shoreface and nearshore-marine 

settings; 7) medium- to dark-gray shale, coal, lenticular fluvial 

sandstones, and persistent crevasse-splay sandstones deposited in a poorly drained coastal plain setting; 8) medium- to 

dark-gray shale, lenticular fluvial sandstones, and persistent 

crevasse-splay sandstones with only thin coal beds deposited 

in a well-drained coastal plain setting; 9) gray shale and thick, 

amalgamated fluvial sandstones deposited in a sandy coastal 

plain to continental basin setting; and 10) variegated maroon, 

purple, green, and gray mudstone and fluvial channel sandstones deposited in a well-drained continental basin setting.

The Cloverly Formation, the basal Cretaceous unit in 

the Bighorn Basin, is Early Cretaceous in age and consists 

mainly of sandstone and variegated mudstone of both marine 

and nonmarine origin (Keefer and others, 1998). The Prior 

Conglomerate, where present, is a readily recognizable basal 

unit, but where absent, the contact with the underlying Jurassic 

Morrison Formation is difficult to define because of lithologic 

similarities. Hence, the contact can only be approximately 

located along the line of cross section. The Cloverly Formation varies in thickness from about 140 to 325 ft along the line 

of section.

The Thermopolis Shale, as originally defined by Lupton 

(1916) from exposures near Thermopolis in the south-central 

part of the Bighorn Basin, included upper and lower shale 

units separated by muddy sandstone, which is now known as 

the Muddy Sandstone. The upper shale was referred to as the 

Shell Creek Shale by Eicher (1962). Present usage (Keefer 

and others, 1998) generally restricts the Thermopolis Shale to 

only the lower shale, treats the Muddy Sandstone as a separate 

formation, and includes the upper shale with the Mowry Shale. 

The Thermopolis Shale is Early Cretaceous in age and is about 

100 to 190 ft thick along the line of section.

The Mowry Shale is Early to Late Cretaceous in age 

(Obradovich and others, 1996) and can be divided into a 

lower unit of soft, fissile shale similar lithologically to the 

Detailed West to East Cross Section of Cretaceous and 

Lower Tertiary Rocks Across the South Part of the Bighorn 

Basin, Wyoming

By Ronald C. Johnson
2 Detailed West to East Cross Section of Cretaceous and Lower Tertiary Rocks, South Part of the Bighorn Basin, Wyo.

Thermopolis Shale, and an upper unit of hard, brittle siliceous 

shale (Keefer and others, 1998). Thin beds of bentonite occur 

throughout, but are most numerous in the upper part. The contact with the overlying Frontier Formation is placed at the base 

of a prominent bentonite bed that generally marks a lithologic 

change from hard siliceous shale below to soft fissile shale 

above. The Mowry Shale is about 450–500 ft thick along the 

line of section.

The Upper Cretaceous Frontier Formation overlies the 

Mowry Shale and consists of two to four nearshore marine, 

marine shelf, and estuarine sandstones interbedded with 

marine shale. In outcrop in the southeastern part of the basin, 

Merewether and others (1998) recognized marine sandstones 

deposited in a variety of marine environments. The Frontier 

ranges in thickness from about 450 to 550 ft along the line of 

cross section. 

The Upper Cretaceous Cody Shale overlies the Frontier 

Formation and ranges in thickness from about 2,700–3,700 

ft. It interfingers with the underlying Frontier Formation and 

the overlying Mesaverde Formation. The Cody Shale consists 

of a main part that forms the bulk of the formation, and the 

Claggett Member that is a westward projecting shale tongue 

within the Mesaverde Formation. The inclined strata in the 

lower part of the Cody studied by Asquith (1970) appears to 

be “unique” in the Cody, as marker beds for the remainder of 

the Cody Shale are not inclined and largely horizontal. The 

Claggett Member is about 100–250 ft thick and generally 

thickens to the east before merging with the main part of the 

Cody Shale, which is the eastern limit of the lower member of 

the Mesaverde Formation. 

The Upper Cretaceous Mesaverde Formation is a variable 

sequence of sandstone, mudstone, coal, and carbonaceous 

shale deposited in a variety of marginal marine and coastal 

plain settings (Johnson and others, 1998) along the west margin of the Cretaceous seaway. It overlies and interfingers with 

the Cody Shale and consists of: 1) the lower member, 2) the 

middle part, and 3) the Teapot Sandstone Member (Keefer and 

others, 1998; Johnson and others, 1998). The lower member 

contains sandy intervals deposited in shoreface, nearshore 

marine and marine shelf environments at its top and base. The 

rest of the unit consists of medium- to dark-gray shale and 

carbonaceous shale, coal, lenticular fluvial channel sandstones, 

and thin persistent crevasse-splay sandstones deposited in a 

poorly-drained, lower coastal plain setting. Most of the coal in 

the Mesaverde Formation is in the lower member. Maximum 

thickness of the lower member is about 650 ft at the western 

end of the cross section. It thins and pinches out into the Cody 

Shale near the middle of the cross section. 

The middle part of the Mesaverde Formation is separated 

from the lower member by the Claggett Member of the Cody 

Shale, and ranges in thickness from about 450 to 750 ft. Along 

the western part of the cross section, the middle part consists 

of a basal marginal marine sandstone that is overlain by a 

nonmarine sequence deposited in a coastal plain setting. The 

unit grades to the east into mainly marginal marine sandstone, 

and then into mostly marine shale. 

The Teapot Sandstone Member varies in thickness from 

about 100 to 200 ft and is a persistent blanket-like complex 

interval of mainly sandstones that are locally conglomeratic 

deposited in nonmarine and marginal marine environments. 

It is mainly nonmarine along the western part of the cross 

section, where it is overlain and underlain by rocks deposited 

in a coastal plain setting, and has characteristics of marginal 

marine deposits along the eastern part of the cross section, 

where it is overlain and underlain by marine shale (Johnson 

and others, 1998).

The Upper Cretaceous Meeteetse Formation is a distinctive unit consisting of banded dark-gray shale, sandstone, 

carbonaceous shale, and coal deposited in a marginal marine 

and coastal plain setting west of the Lewis Seaway. The 

formation generally contains less sandstone than the underlying Mesaverde Formation, although locally there is abundant 

sandstone (Johnson and others, 1998). The Meeteetse Formation, which is about 600–900 ft thick along the line of section, 

intertongues with the Lewis Shale in the eastern part of the 

cross section. 

The Upper Cretaceous Lance Formation overlies the 

Meeteetse Formation with apparent conformity along the line 

of section. It varies in thickness from 400 to 1,200 ft with 

the thinnest section being in the extreme eastern part of the 

cross section. The lower part of the Lance consists of thick 

amalgamated fluvial sandstones and gray shales that are easily 

distinguished from the underlying, much less sandy, Meeteetse 

strata. This interval of amalgamated fluvial sandstones can 

be as much as 900 ft thick along the line of section. It grades 

up into a much less sandy sequence in the upper part of the 

formation.

The Paleocene Fort Union Formation overlies the Lance 

Formation. The upper part of the formation is eroded along 

most of the line of section. Only in the British Petroleum no. 

1 Gillies Draw well (well no. 2 on cross section) is the Fort 

Union section complete. The Fort Union Formation is 5,750 

ft thick in that well. A pronounced angular unconformity 

between the Lance and overlying Fort Union Formation can be 

recognized along both the east and west margins of the basin 

(Keefer and others, 1998). However, this angularity diminishes rapidly toward the center of the basin and throughout 

the central part of the basin, including along the line of cross 

section presented here, and the contact between the Lance 

and Fort Union Formation is difficult to locate. The contact 

is somewhat arbitrarily placed at the base of a thick fluvial 

sandstone, because a sandstone occurs at the base of the Fort 

Union Formation at many localities around the margins of the 

basin (Roberts, 1998: Keefer and others, 1998; Johnson and 

others, 1998).
References Cited 3

References Cited

Asquith, D.O., 1970, Depositional topography and major 

marine environments, Late Cretaceous, Wyoming: American Association of Petroleum Geologists Bulletin, v. 54, no. 

1, p. 1184–1224.

Eicher, D.L., 1962, Biostratigraphy of the Thermopolis, 

Muddy, and Shell Creek Formations, in Enyert, R.L., and 

Curry, W.H., eds., Symposium on early Cretaceous rocks 

of Wyoming and adjacent areas: Wyoming Geological 

Association,16th Guidebook, p. 72–93.

Finn, T.M., and Johnson, R.C., 2005, Subsurface stratigraphic 

cross sections of Cretaceous and lower Tertiary rocks in the 

Southwestern Wyoming Province, Wyoming, Colorado, and 

Utah: U.S. Geological Survey Digital Data Series DDS69-D, chp. 14, 12 p.

Johnson, R.C., 2003, Depositional framework of the upper 

Cretaceous Mancos Shale and lower part of the upper 

Cretaceous Mesaverde Group, western Colorado and 

eastern Utah: U.S. Geological Survey Digital Data Series 

DDS-69-B, chp. 10, 24 p.

Johnson, R.C., Keefer, W.R., Keighin, C.W., and Finn, T.M., 

1998, Detailed outcrop studies of the upper part of the 

upper Cretaceous Cody Shale and the upper Cretaceous 

Mesaverde, Meeteetse, and Lance Formations, Bighorn 

Basin, Wyoming, in Keefer, W.R., and Goolsby, J.E., eds., 

Cretaceous and lower Tertiary rocks of the Bighorn Basin, 

Wyoming and Montana: Wyoming Geological Association 

49th Guidebook, p. 59–78. 

Keefer, W.R., Finn, T.M., Johnson, R.C., and Keighin, 

C.W., 1998, Regional stratigraphy and correlation of 

Cretaceous and Paleocene rocks, Bighorn Basin, Wyoming 

and Montana, in Keefer, W.R., and Goolsby, J.E., eds., 

Cretaceous and lower Tertiary rocks of the Bighorn Basin, 

Wyoming and Montana: Wyoming Geological Association 

49th Guidebook, p. 1–30.

Lupton, C.T., 1916, Oil and gas near basin, Big Horn County, 

Wyoming: U.S. Geological Survey Bulletin 621, 

p. 157–190.

Merewether, E.A., Tillman, R.W., Cobban, W.A., and 

Obradovich, J.D., 1998, Outcrop-sections of the upper 

Cretaceous Frontier Formation, southeastern Bighorn 

Basin, Wyoming, in Keefer, W.R., and Goolsby, J.E., eds., 

Cretaceous and lower Tertiary rocks of the Bighorn Basin, 

Wyoming and Montana: Wyoming Geological Association 

49th Guidebook, p. 31–42.

Obradovich, J.D., Cobban, W.A., Merewether, E.A., and 

Weimer, R.J., 1996, A time framework for the late Albanian 

and early Cenomanian strata of northern Wyoming and 

Montana: Geological Society of America Abstacts with Programs, 1996 Annual Meeting, Denver, Colorado, p. A-66.

Roberts, S.B., 1988, An overview of the stratigraphic and 

sedimentologic characteristics of the Paleocene Fort Union 

Formation, southern Bighorn Basin, Wyoming, in Keefer, 

W.R., and Goolsby, J.E., eds., Cretaceous and lower Tertiary 

rocks of the Bighorn Basin, Wyoming and Montana: Wyoming Geological Association 49th Guidebook, 

p. 91–116.

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