U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5256

Outcrops, Fossils, Geophysical Logs, and Tectonic 

Interpretations of the Upper Cretaceous Frontier Formation 

and Contiguous Strata in the Bighorn Basin, Wyoming and 

Montana

Outcrops, Fossils, Geophysical Logs, and 

Tectonic Interpretations of the Upper 

Cretaceous Frontier Formation and 

Contiguous Strata in the Bighorn Basin, 

Wyoming and Montana

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

Scientific Investigations Report 2009–5256

U.S. Department of the Interior

U.S. Geological Survey
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:

Merewether, E.A., Cobban, W.A., and Tillman, R.W., 2010, Outcrops, fossils, geophysical logs, and tectonic 

interpretations of the Upper Cretaceous Frontier Formation and contiguous strata in the Bighorn Basin, Wyoming and 

Montana: U.S. Geological Survey Scientific Investigations Report 2009–5256, 49 p.

Library of Congress Cataloging-in-Publication Data

Merewether, E. A. (Edward Allen), 1930-

Outcrops, fossils, geophysical logs, and tectonic interpretations of the upper Cretaceous : frontier formation and 

contiguous strata in the Bighorn Basin, Wyoming and Montana / by E.A. Merewether, W.A. Cobban, and R.W. Tillman.

p. cm. -- (Scientific investigations report ; 2009-5256)

ISBN 978-1-4113-2785-6 (pbk. : alk. paper)

1. Frontier Formation. 2. Geology, Stratigraphic--Cretaceous. 3. Geology--Bighorn Basin (Mont. and Wyo.) I. Cobban, 

William Aubrey, 1916- II. Tillman, Roderick W. III. Title. 

QE688.M47 2010

51.7’709787--dc22

 2010015213
iii

Contents

Abstract...........................................................................................................................................................1

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

Outcrops near Deep Lake, Wyoming .........................................................................................................3

Outcrops near Cody, Wyoming....................................................................................................................3

Outcrops near Elk Butte, Wyoming...........................................................................................................14

Outcrops in the Pitchfork Oil Field, Wyoming .........................................................................................14

Outcrops at Hamilton Dome, Coal Draw, and near Thermopolis, Wyoming ......................................14

Outcrops at Black Mountain, Wyoming...................................................................................................14

Outcrops near Shanty Ridge and Big Trails, Wyoming..........................................................................25

Outcrops at Mud Gulch and Joe Emge Creek, Wyoming......................................................................25

Outcrops near Ten Sleep, Wyoming .........................................................................................................25

Outcrops at Bonanza Oil Field, Wyoming.................................................................................................34

Outcrops near Greybull, Wyoming ...........................................................................................................34

Outcrops at Lovell Draw and North Emblem, Wyoming........................................................................34

Outcrops at Crooked Creek, Wyoming.....................................................................................................34

Outcrops near Warren, Montana..............................................................................................................45

Interpretations and Conclusions...............................................................................................................45

Summary........................................................................................................................................................48

Acknowledgments.......................................................................................................................................49

Reference Cited............................................................................................................................................49

Figures

1. Map of north-central Wyoming and south-central Montana showing areas of 

outcrops of the Upper Cretaceous Frontier Formation in the Bighorn Basin.....................2

2. Late Cretaceous molluscan fossil zones in the Western Interior.........................................8

3. Map of northern Wyoming showing locations of selected outcrops of the Frontier 

Formation and regional boundaries of biostratigraphic units...............................................9

4. Formations and members of early Late Cretaceous ages and corresponding 

fossil zones for selected localities in northern Wyoming....................................................10

5. Description of Frontier Formation in outcrops near Deep Lake, 

northwestern Bighorn Basin.....................................................................................................11

6. Description of Frontier Formation in outcrops near Cody, 

western Bighorn Basin ..............................................................................................................12

7. Fossil localities near Cody, Park County, Wyoming ..............................................................13

8. Description of Frontier Formation in outcrops near Elk Butte, 

western Bighorn Basin ..............................................................................................................15

9. Fossil localities near Elk Butte, Park County, Wyoming.......................................................16

10. Middle Turonian sandstone disconformably overlain by a Lower Coniacian 

sandstone near Elk Butte, western Bighorn Basin...............................................................17

11. Description of Frontier Formation in outcrops near Pitchfork oil field, 

western Bighorn Basin ..............................................................................................................18
iv

12. Fossil localities near Pitchfork oil field, Park County, Wyoming.........................................19

13. Descriptions of Frontier Formation in outcrops near Hamilton Dome, 

Coal Draw, and Thermopolis, southern Bighorn Basin ........................................................20

14. Description of Frontier Formation in outcrops at Black Mountain, 

southern Bighorn Basin.............................................................................................................22

15. Fossil localities near Black Mountain, Hot Springs County, Wyoming..............................23

16. Fossil localities near Black Mountain, Hot Springs County, Wyoming..............................24

17. Descriptions of Frontier Formation in outcrops near Big Trails and 

Shanty Ridge, southeastern Bighorn Basin ...........................................................................26

18. Descriptions of Frontier Formation in outcrops near Joe Emge Creek and 

Mud Gulch, southeastern Bighorn Basin ...............................................................................28

19. Descriptions of Frontier Formation in outcrops near Ten Sleep, 

southeastern Bighorn Basin .....................................................................................................30

20. Fossil localities near Ten Sleep, Washakie County, Wyoming............................................32

21. Fossil localities near Ten Sleep, Washakie County, Wyoming............................................33

22. Descriptions of Frontier Formation in outcrops in Bonanza oil field, 

eastern Bighorn Basin ...............................................................................................................35

23. Fossil localities near Bonanza oil field, Big Horn County, Wyoming..................................36

24. Fossil localities near Bonanza oil field, Big Horn County, Wyoming..................................37

25. Descriptions of Frontier Formation in outcrops near Greybull, 

eastern Bighorn Basin ...............................................................................................................38

26. Fossil localities near Greybull, Big Horn County, Wyoming.................................................40

27. Fossil localities near Greybull, Big Horn County, Wyoming.................................................41

28. Fossil localities near Greybull, Big Horn County, Wyoming.................................................42

29. Descriptions of Frontier Formation in outcrops near North Emblem and 

Lovell Draw, eastern Bighorn Basin........................................................................................43

30. Description of Frontier Formation in outcrops near Crooked Creek,

eastern Bighorn Basin ...............................................................................................................44

31. Description of Frontier Formation at outcrops near Warren, 

eastern Bighorn Basin ...............................................................................................................46

32. Stratigraphic cross section of Frontier Formation and lower beds of the 

overlying Cody Shale, northwestern Wyoming .....................................................................47

Table

1. Late Cretaceous molluscan fossils from the Bighorn Basin, north-central 

Wyoming and south-central Montana ......................................................................................4
Abstract

In the Bighorn Basin of north-central Wyoming and 

south-central Montana, the Frontier Formation of early 

Late Cretaceous age consists of siliciclastic, bentonitic, and 

carbonaceous beds that were deposited in marine, brackishwater, and continental environments. Most lithologic units are 

laterally discontinuous. The Frontier Formation conformably 

overlies the Mowry Shale and is conformably overlain by 

the Cody Shale. Molluscan fossils collected from outcrops of 

these formations and listed in this report are mainly of marine 

origin and of Cenomanian, Turonian, and Coniacian ages.

The lower and thicker part of the Frontier in the Bighorn 

Basin is of Cenomanian age and laterally equivalent to the 

Belle Fourche Member of the Frontier in central Wyoming. 

Near the west edge of the basin, these basal strata are disconformably overlain by middle Turonian beds that are the age 

equivalent of the Emigrant Gap Member of the Frontier in 

central Wyoming. The middle Turonian beds are disconformably overlain by lower Coniacian strata. Cenomanian strata 

along the south and east margins of the basin are disconformably overlain by upper Turonian beds in the upper part of the 

Frontier, as well as in the lower part of the Cody; these are, in 

turn, conformably overlain by lower Coniacian strata.

Thicknesses and ages of Cenomanian strata in the 

Bighorn Basin and adjoining regions are evidence of regional 

differential erosion and the presence of an uplift during the 

early Turonian centered in northwestern Wyoming, west 

of the basin, probably associated with a eustatic event. The 

truncated Cenomanian strata were buried by lower middle 

Turonian beds during a marine transgression and possibly 

during regional subsidence and a eustatic rise. An uplift in the 

late middle Turonian, centered in north-central Wyoming and 

possibly associated with a eustatic fall, caused the erosion of 

lower middle Turonian beds in southern and eastern areas of 

the basin as well as in an adjoining region of north-central 

Wyoming. Similarly, in east-central Wyoming and an adjacent 

area to the south, Cenomanian strata are disconformably 

overlain by upper middle and lower upper Turonian strata that 

Outcrops, Fossils, Geophysical Logs, and Tectonic 

Interpretations of the Upper Cretaceous Frontier 

Formation and Contiguous Strata in the Bighorn Basin, 

Wyoming and Montana

probably reflect uplift and erosion in that region during the 

interim period of middle Turonian time.

During later subsidence and a marine transgression, upper 

Turonian deposits buried Cenomanian beds in areas along 

the south and east margins of the Bighorn Basin and buried 

lower middle Turonian beds in much of northern Wyoming. 

Upper Turonian and lower Coniacian strata are apparently 

conformable in eastern and southern areas of the basin as well 

as near Riverton, Kaycee, and Casper in central Wyoming. 

Upper Turonian strata are absent on the west flank of the 

Bighorn Basin and in outcrops west of the basin, where middle 

Turonian beds are disconformably overlain by lower Coniacian beds . The conformable upper Turonian and lower Coniacian beds apparently transgressed an eroded middle Turonian 

surface in the region, but only Coniacian strata overlie middle 

Turonian beds on the west side of the basin and areas farther west. Coniacian strata onlap the truncated lower middle 

Turonian surface west of the basin, indicating a region that had 

higher elevation possibly resulting from tectonic uplift. 

In east-central Wyoming and an adjoining region to the 

south, upper middle Turonian and lower upper Turonian strata 

are disconformably overlain by lower and middle Coniacian 

beds. That region apparently was uplifted and eroded during 

the latest Turonian. 

Introduction

The Frontier Formation in the Bighorn Basin (fig. 1) of 

Wyoming and Montana is composed of shale, siltstone, sandstone, conglomerate, and bentonite of early Late Cretaceous 

age (Cenomanian, Turonian, and Coniacian Stages) that were 

deposited in marine, brackish-water, and continental environments. Most of the lithologic units are laterally discontinuous. 

The formation includes flooding surfaces and unconformities. 

Thicknesses are as much as 640 ft at outcrops on the west 

flank of the basin and as much as 860 ft at outcrops on the east 

flank. Isopach and overburden maps for the formation in the 

basin have been prepared by M.A. Kirschbaum (written commun., 2008). The Frontier Formation conformably overlies the 
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Red Lodge

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Cody

Greybull

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Ten Sleep

Thermopolis

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Cody, 1974

Elk Butte, 1973

Pitchfork

oil field,

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Hamilton Dome, 1973

Coal Draw, 1999

Thermopolis, 1973 Black 

Mountain, 

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Black Mountain, 

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Ridge,

1997

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Ridge,

1999

Big Trails,

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Mud Gulch,

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Creek, 

1997

McClellan Gulch,

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Ten Sleep, 

1996, 1998

Bonanza oil field, 1974

Bonanza oil field, 1996

Potato Ridge, 

 1998,1999

Greybull,

1998,1999 

Lovell Draw,

1996, 1998

North Emblem,

1974

Crooked Creek,

1974

MONTANA

WYOMING

Warren, 

1974

0 10 20 30 40 50 60 MILES

0 20 40 60 80 90 KILOMETERS

,

Figure 1. North-central Wyoming and south-central Montana showing locations of towns in the region (black 

squares), outline of the Bighorn Basin (red), locations of outcrops of the Frontier Formation (green), and outcrop sites 

examined for this report (red diamonds), with names and dates of original description (blue) as referenced in text.

2 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana
Outcrops near Cody, Wyoming 3

Mowry Shale and is conformably overlain by the Cody Shale, 

both of which are marine formations. At the top of the Mowry 

in most of this region is a bed of bentonite that is informally 

named the Clay Spur. 

Outcrops at 20 localities near the margins of the Bighorn 

Basin were examined and described by the authors of this 

report during the years 1973–1974 and 1996–1999. The dates 

of those periods of field work are not necessarily the same as 

the dates of related publications. Several of those outcrops are 

herein correlated with the geophysical logs of nearby boreholes. Horizons designated on the geophysical logs in this 

report were traced throughout the subsurface of the Bighorn 

Basin by M.A. Kirschbaum (written commun., 2008). 

Several beds in the Mowry, Frontier, and Cody of this 

region contain molluscan fossils of Cenomanian, Turonian, 

and Coniacian ages, which were collected by personnel of the 

U.S. Geological Survey (USGS) and identified by W.A. Cobban (table 1). The species named in this report are from fossil 

faunas designated by USGS collection numbers, for example: 

D9259. Important fossils are listed and assigned informal zone 

numbers in figure 2. A zone includes the listed species and 

can include several other species. Most of the Frontier in the 

Bighorn Basin is Cenomanian (fig. 2, Zone 6 through Zones 

14, 15, 16, 17, or 18) and is approximately equivalent in age 

to the Belle Fourche Member (BFM) of the Frontier in central 

Wyoming (figs. 3, 4). 

At outcrops of the Frontier within the town limits of 

Cody, Wyo., near the west edge of the basin, Cenomanian 

strata are disconformably overlain by middle Turonian 

strata (fig. 2, Zones 29 and 30) that are equivalent in age to 

the upper part of the Emigrant Gap Member of the Frontier 

(Merewether, 1996) near Casper in east-central Wyoming. 

Near the south and east margins of the basin, Cenomanian 

strata are disconformably overlain by upper Turonian strata 

(fig. 2, Zones 36 and 37) that are the age-equivalent of part of 

the Wall Creek Member of the Frontier at outcrops a few miles 

west of Casper. 

Middle Turonian beds (equivalent in age to an upper part 

of the Emigrant Gap Member) on the west side of the Bighorn 

Basin are disconformably overlain by lower Coniacian beds. 

Near the south and east edges of the basin, where middle 

Turonian beds are absent, upper Turonian beds (equivalent 

in age to parts of the Wall Creek Member) are conformably 

overlain by lower Coniacian beds. 

Outcrops near Deep Lake, Wyoming 

Outcrops of the Frontier in the vicinity of Deep Lake 

(figs. 1, 5), as described by Merewether and others (1975), 

consist of strata that originated in marine and nonmarine depositional environments. Sandstones range in grain size from 

very fine to very coarse and commonly enclose thin beds of 

conglomerate. At this locality, the formation is 630 ft thick and 

comprises, in ascending order, 338 ft of marine strata, about 

200 ft of nonmarine strata, and 92 ft of marine strata. The nonmarine sequence is much thicker here than at other outcrops 

examined for this report. 

Near Deep Lake, molluscan fossils of early Coniacian 

age — D9259, Cremnoceramus deformis dobrogensis (fig. 2, 

Zone 40), and D9258, Cremnoceramus crassus inconstans

(fig. 2, Zone 41) (table 1)—were collected from the uppermost 

sandstone of the Frontier and from the overlying basal Cody 

Shale, respectively. The age of the lower marine sequence and 

the overlying nonmarine sequence is probably Cenomanian. 

Most of the marine sequence in the upper part of the Frontier 

could be of middle Turonian age. The contact between the 

nonmarine strata and the overlying marine strata could be a 

disconformity that represents the latest Cenomanian and the 

early Turonian. The basal contact of the uppermost sandstone 

of the Frontier could be a disconformity that represents late 

Turonian time. 

Outcrops near Cody, Wyoming

The outcropping Frontier in northern Cody (fig. 1) was 

described by Merewether and others (1975) and Siemers 

(1975). It is recorded on geophysical logs and by cores from 

boreholes in that area (fig. 6). The Frontier at these outcrops 

consists of siliciclastic and bentonitic strata and minor conglomerate, carbonaceous beds, and lignite deposited in marine 

and continental environments. Most sandstones are very fine 

or fine grained. The marine strata commonly contain burrows 

and a few contain molluscan fossils (fig. 7). In this area, the 

formation is about 600 ft thick and includes two disconformities as well as several flooding surfaces. 

At the outcrops in Cody (figs. 1, 6), the Frontier comprises three depositional units: (1) a lower 450-ft-thick 

sequence of mostly marine siliciclastic and bentonitic strata of 

apparent Cenomanian age; (2) a middle 30-ft-thick sequence 

of nonmarine sandstone, carbonaceous shale, and lignite probably also of Cenomanian age; and (3) an upper 115-ft-thick 

sequence of marine siliciclastic and bentonitic strata that lies 

disconformably on the underlying unit (fig. 6). Fossils (fig. 7) 

collected from near the middle of this upper sequence include 

Inoceramus howelli (D8997) of early middle Turonian age 

(fig. 2, Zones 29 and 30). The lacuna (a gap in the stratigraphic 

record) between the nonmarine unit and the overlying marine 

sequence could encompass much of late Cenomanian and all 

of early Turonian time.

The upper marine sequence at the top of the Frontier is 

capped by an 8-ft-thick pebble-bearing marine sandstone that 

might rest disconformably on middle Turonian strata. This unit 

is overlain by marine shale of the basal Cody Shale, about 15 

ft of which is exposed at this outcrop and which contains the 

fossil Cremnoceramus deformis (D8996) of early Coniacian 

age (fig. 2, Zone 40). A hiatus at either the base or the top of 

the uppermost Frontier sandstone might represent late middle 

and late Turonian time. 
Table 1. Late Cretaceous molluscan fossils collected by the U.S. Geological Survey in the Bighorn Basin, north-central Wyoming and south-central in Wyoming unless indicated otherwise. USGS number County Locality Section Township Range Representative fossil Age D13651 Big Horn Bonanza oil field NW 1/4 31 49 N. 90 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD9253 Big Horn Bonanza oil field NW 1/4 15 49 N. 91 W. Cremnoceramus deformis erectus Coniacian, Early D9254 Big Horn Bonanza oil field W 1/2 15 49 N. 91 W. Gastropod Coniacian, Early D8992 Big Horn Bonanza oil field SW 1/4 15 49 N. 91 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD8993 Big Horn Bonanza oil field SW 1/4 15 49 N. 91 W. Mytiloides cf. M. incertus Turonian, Late D9267 Big Horn Bonanza oil field SE 1/4 25 49 N. 91 W. Dunveganoceras pondi Cenomanian, Late D9268 Big Horn Bonanza oil field SE 1/4 25 49 N. 91 W. Mytiloides incertus Turonian, Late D9269 Big Horn Bonanza oil field SE 1/4 25 49 N. 91 W. Mytiloides incertus Turonian, Late D8994 Big Horn Bonanza oil field SE 1/4 16 49 N. 91 W. Cremnoceramus deformis dobrogensis Coniacian, Early D8847 Big Horn Torchlight Dome SW 1/4 19 51 N. 92 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD8848 Big Horn Torchlight Dome SW 1/4 19 51 N. 92 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD8849 Big Horn Torchlight Dome SW 1/4 19 51 N. 92 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD13731 Big Horn Potato Ridge NE 1/4 25 52 N. 92 W. Acanthoceras amphibolum Cenomanian, MiddlD13919 Big Horn Potato Ridge NE 1/4 25 52 N. 92 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD13918 Big Horn Potato Ridge NE 1/4 25 52 N. 92 W. Dunveganoceras pondi Cenomanian, Late D13920 Big Horn Potato Ridge NW 1/4 25 52 N. 92 W. Dunveganoceras pondi Cenomanian, Late D13921 Big Horn Potato Ridge NW 1/4 25 52 N. 92 W. Metoicoceras frontierense Cenomanian, Late D9754 Big Horn Devils Kitchen SW 1/4 1 52 N. 93 W. Baculites yokoyamai Turonian, Late D13915 Big Horn Devils Kitchen SW 1/4 1 52 N. 93 W. Pycnodonte aff. P. kellumi Cenomanian, Late D9752 Big Horn Devils Kitchen SE 1/4 2 52 N. 93 W. Dunveganoceras pondi Cenomanian, Late D9753 Big Horn Devils Kitchen SE 1/4 2 52 N. 93 W. Pycnodonte aff. P. kellumi Cenomanian, Late D13916 Big Horn Devils Kitchen SE 1/4 2 52 N. 93 W. Dunveganoceras pondi Cenomanian, Late D13729 Big Horn Greybull NW 1/4 14 52 N. 93 W. Inoceramus sp. Cenomanian D13730 Big Horn Greybull NW 1/4 14 52 N. 93 W. Dunveganoceras pondi Cenomanian, Late D13652 Big Horn Greybull NE 1/4 16 52 N. 93 W. Lingula subspatulata Cenomanian D9278 Big Horn Greybull SE 1/4 22 52 N. 93 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD13728 Big Horn Greybull SE 1/4 34 52 N. 93 W. Dunveganoceras pondi Cenomanian, Late D13229 Big Horn Greybull SE 1/4 24 53 N. 93 W. Neogastroplites haasi Cenomanian, Early D13230 Big Horn Greybull NE 1/4 24 53 N. 93 W. Neogastroplites haasi Cenomanian, Early D13917 Big Horn Lovell Draw SE 1/4 10 53 N. 94 W. Dunveganoceras conditum Cenomanian, Late D9276 Big Horn North Emblem 3 54 N. 95 W. Inoceramus ginterensis Cenomanian, Late D9277 Big Horn North Emblem 3 54 N. 95 W. Baculites sp. Turonian ? 
Table 1. Late Cretaceous molluscan fossils collected by the U.S. Geological Survey in the Bighorn Basin, north-central Wyoming and south-central 

in Wyoming unless indicated otherwise.—ContinuedUSGSnumber County Locality Section Township Range Representative fossil Age D9270 Big Horn North Emblem SW 1/4 10 54 N. 95 W. Pycnodonte n. sp. Cenomanian, Late D9271 Big Horn North Emblem NE 1/4 15 54 N. 95 W. Dunveganoceras albertense Cenomanian, Late D1430 Big Horn Crooked Creek SW 1/4 32 56 N. 94 W. Dunveganoceras albertense Cenomanian, LateD9272 Big Horn Crooked Creek N 1/2 5 57 N. 95 W. Metoicoceras muelleri Cenomanian, Late D9273 Big Horn Crooked Creek N 1/2 5 57 N. 95 W. Cremnoceramus cf. C. deformis Coniacian, Early D1727 Hot Springs Black Mountain NE 1/4 17 42 N. 90 W. Dunveganoceras ? Cenomanian, Late D1728 Hot Springs Black Mountain NW 1/4 17 42 N. 90 W. Acanthoceras ? wyomingense Cenomanian, MiddlD1728 Hot Springs Black Mountain NW 1/4 17 42 N. 90 W. Inoceramus ginterensis Cenomanian, LateD1730 Hot Springs Blue Springs NE 1/4 16 42 N. 92 W. Dunveganoceras pondi Cenomanian, Late D1726 Hot Springs Warm Spring Creek SE 1/4 3 42 N. 94 W. Neogastroplites haasi Cenomanian, Early D8953 Hot Springs Black Mountain SW 1/4 25 43 N. 91 W. Mytiloides scupini Turonian, Late D8954 Hot Springs Black Mountain SW 1/4 25 43 N. 91 W. Mytiloides scupini Turonian, Late D8955 Hot Springs Black Mountain SW 1/4 25 43 N. 91 W. Scaphites cf. S. nigricollensis Turonian, Late D13914 Hot Springs Black Mountain SW 1/4 25 43 N. 91 W. Cremnoceramus cf. C. deformis Coniacian, Early D13648 Hot Springs Black Mountain NE 1/4 26 43 N. 91 W. Unio sp. ? D13649 Hot Springs Black Mountain SE 1/4 26 43 N. 91 W. Brachidontes arlingtonanus Cenomanian, MiddlD1679 Hot Springs Black Mountain SW 1/4 26 43 N. 91 W. Pteria (Oxytoma) sp. CenomanianD8843 Hot Springs Black Mountain NE 1/4 27 43 N. 91 W. Conlinoceras gilberti Cenomanian, MiddlD8844 Hot Springs Black Mountain NE 1/4 27 43 N. 91 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD8846 Hot Springs Black Mountain NE 1/4 27 43 N. 91 W. Mytiloides scupini Turonian, Late D8842 Hot Springs Black Mountain NE 1/4 27 43 N. 91 W. Forresteria sp. Coniacian, Early D8843 Hot Springs Black Mountain NW 1/4 28 43 N. 91 W. Conlinoceras tarrantense Cenomanian, MiddlD13822 Hot Springs Black Mountain NW 1/4 28 43 N. 91 W. Baculites mariasensis Coniacian, Early D8845 Hot Springs Lake Creek oil field NW 1/4 14 43 N. 92 W. Mytiloides scupini Turonian, Late D9314 Hot Springs Lake Creek oil field NW 1/4 14 43 N. 92 W. Cremnoceramus deformis dobrogensis Coniacian, Early D9315 Hot Springs Blue Springs NE 1/4 33 43 N. 92 W. Dunveganoceras pondi Cenomanian, Late D9316 Hot Springs Blue Springs NE 1/4 33 43 N. 92 W. Inoceramus perplexus ? Turonian, Late D9317 Hot Springs Blue Springs NE 1/4 33 43 N. 92 W. Inoceramus perplexus Turonian, Late D9318 Hot Springs Blue Springs NE 1/4 33 43 N. 92 W. Cremnoceramus cf. C. deformis erectus Coniacian, Early D1729 Hot Springs Blue Springs NE 1/4 33 43 N. 92 W. Calycoceras canitaurinum Cenomanian, Late D9750 Hot Springs Thermopolis NE 1/4 9 43 N. 94 W. Cremnoceramus deformis dobrogensis Coniacian, Early 
Table 1. Late Cretaceous molluscan fossils collected by the U.S. Geological Survey in the Bighorn Basin, north-central Wyoming and south-central in Wyoming unless indicated otherwise.—Continued USGS number County Locality Section Township Range Representative fossil Age D8951 Hot Springs Thermopolis SE 1/4 9 43 N. 94 W. Cremnoceramus crassus inconstans Coniacian, Early D9770 Hot Springs Hamilton Dome 18 44 N. 97 W. Cremnoceramus deformis dobrogensis Coniacian, Early D9771 Hot Springs Hamilton Dome 18 44 N. 97 W. Cremnoceramus crassus inconstans Coniacian, Early D9262 Park Wood River NE 1/4 15 47 N. 101 W. Cremnoceramus deformis dobrogensis Cenomanian, Early D9265 Park Pitchfork SE 1/4 2 48 N. 102 W. Cremnoceramus deformis dobrogensis Coniacian, Early D9266 Park Pitchfork SW 1/4 11 48 N. 102 W. Neogastroplites americanus Cenomanian, Early D9263 Park Pitchfork 14 48 N. 102 W. Neogastroplites americanus Cenomanian, Early D9264 Park Pitchfork 14 48 N. 102 W. Lingula sp. Cenomanian D9818 Park Pitchfork 14 48 N. 102 W. Metengonoceras sp. Cenomanian D11916 Park Elk Butte NW 1/4 32 51 N. 100 W. Inoceramus howelli Turonian, Middle D8998 Park Elk Butte SE 1/4 31 51 N. 100 W. Inoceramus howelli Turonian, Middle D8999 Park Elk Butte NE 1/4 31 51 N. 100 W. Cremnoceramus deformis erectus Coniacian, Early D11917 Park Elk Butte SW 1/4 29 51 N. 100 W. Cremnoceramus inconstans Coniacian, Early D8997 Park Cody N 1/2 31 53 N. 101 W. Inoceramus howelli Turonian, Middle D8996 Park Cody N 1/2 31 53 N. 101 W. Cremnoceramus deformis dobrogensis Coniacian, Early D9259 Park Deep Lake W 1/2 21 56 N. 103 W. Cremnoceramus deformis dobrogensis Coniacian, Early D9258 Park Deep Lake W 1/2 21 56 N. 103 W. Cremnoceramus deformis deformis Coniacian, Early D9002 Park Frannie oil field SE 1/4 23 58 N. 98 W. Cremnoceramus deformis dobrogensis Coniacian, Early D9001 Park Frannie oil field SE 1/4 23 58 N. 98 W. Metoicoceras mesabiensis Cenomanian, Late D9000 Park Frannie oil field SE 1/4 23 58 N. 98 W. Metoicoceras sp. Cenomanian, Late D9006 Washakie No Wood Ranch 3 42 N. 89 W. Cremnoceramus waltersdorfensis Turonian, Late D9251 Washakie No Wood Ranch NE 1/4 10 42 N. 89 W. Inoceramus perplexus Turonian, Late D13828 Washakie Shanty Ridge NW 1/4 26 43 N. 89 W. Dunveganoceras pondi Cenomanian, Late D13827 Washakie Shanty Ridge NW 1/4 26 43 N. 89 W. Dunveganoceras pondi Cenomanian, Late D9252 Washakie Big Trails SW 1/4 24 44 N. 88 W. Cremnoceramus deformis erectus Coniacian, Early D13823 Washakie Mud Gulch SW 1/4 4 45 N. 88 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD13824 Washakie Mud Gulch SW 1/4 4 45 N. 88 W. Scaphites impendicostatus Coniacian, Early D13825 Washakie Joe Emge Creek SW 1/4 13 46 N. 89 W. Metoicoceras mosbyense Cenomanian, Late D13228 Washakie Joe Emge Creek SW 1/4 13 46 N. 89 W. Scaphites preventricosus Coniacian, Early D13650 Washakie Ten Sleep area SE 1/4 22 47 N. 89 W. Inoceramus eulessanus Cenomanian, MiddlD9009 Washakie Ten Sleep area SE 1/4 22 47 N. 89 W. Plesiacanthoceras wyomingense Cenomanian, Middl
Table 1. Late Cretaceous molluscan fossils collected by the U.S. Geological Survey in the Bighorn Basin, north-central Wyoming and south-central 

in Wyoming unless indicated otherwise.—ContinuedUSGSnumber County Locality Section Township Range Representative fossil Age D13732 Washakie Ten Sleep area SE 1/4 22 47 N. 89 W. Dunveganoceras sp. Cenomanian, Late D13910 Washakie McClellan Gulch NW 1/4 25 47 N. 89 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD13911 Washakie McClellan Gulch NW 1/4 25 47 N. 89 W. Inoceramus sp. D13908 Washakie McClellan Gulch NW 1/4 25 47 N. 89 W. Placenticeras pseudoplacenta Turonian, Middle oD13909 Washakie McClellan Gulch SW 1/4 25 47 N. 89 W. Plesiacanthoceras wyomingense Cenomanian, MiddlD13734 Washakie Ten Sleep area NE 1/4 27 47 N. 89 W. Cremnoceramus deformis dobrogensis Coniacian, Early D8956 Washakie Ten Sleep area NE 1/4 27 47 N. 89 W. Mytiloides scupini Turonian, Late D8957 Washakie Ten Sleep area NE 1/4 27 47 N. 89 W. Mytiloides scupini Turonian, Late D13242 Washakie Ten Sleep area NE 1/4 27 47 N. 89 W. Inoceramus sp. Turonian, Late D13913 Washakie Ten Sleep area NW 1/4 27 47 N. 89 W. Placenticeras pseudoplacenta Turonian, Middle oD13733 Washakie Ten Sleep area NW 1/4 27 47 N. 89 W. Inoceramus sp. Coniacian D9751 Washakie Ten Sleep area NE 1/4 27 47 N. 89 W. Placenticeras sp. Turonian, Middle oD13912 Washakie McClellan Gulch NW 1/4 36 47 N. 89 W. Mytiloides scupini Turonian, Late D9274 Carbon (MT) Warren 25 8 S. 24 E. Metoicoceras defordi Cenomanian, Late D9275 Carbon (MT) Warren 25 8 S. 24 E. Cremnoceramus cf. C. deformis Coniacian, Early 
8 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana UPPER CRETACEOUS (part) Turonian SERIES Stages Informal substages

Western Interior ammonite age

spans and radiometric ages (MY)

Santonian (part) Coniacian

lower upper middle lower upper middle

Zone nos.

Scaphites ferronensis 33

Prionocyclus macombi 31

Collignoniceras praecox 29

Collignoniceras woollgari 28

Clioscaphites saxitonianus Cladoceramus undulatoplicatus

Scaphites depressus 87.14 44 Magadiceramus crenelatus

Scaphites ventricosus 43 Volviceramus involutus

Western Interior inoceramid 

age spans

Cremnoceramus crassus inconstans

39 Cremnoceramus deformis erectus

Scaphites mariasensis 38 Cremnoceramus waltersdorfensis

42 Volviceramus koeneni

 88.55

41

Cremnoceramus deformis dobrogensis

Prionocyclus germari 37 Mytiloides scupini

Scaphites nigricollensis 36 Mytiloides incertus

35 Inoceramus dakotensis

Scaphites whitfieldi 34 Inoceramus perplexus

Scaphites warreni 32 Inoceramus dimidius

Prionocyclus hyatti 92.46 30

Inoceramus howelli

Mytiloides hercynicus

Mammites nodosoides 27

lowerVascoceras birchbyi 93.48 26

25 Pseudaspidoceras 

 flexuosum

Mytiloides subhercynicus

Mytiloides mytiloides

Mytiloides kossmati

Watinoceras devonense 24 Mytiloides puebloensis

Nigericeras scotti 23 Mytiloides hattini

upper

Cenomanian 

Neocardioceras juddii 22

21

20

19

18

17

16

15

14

13

12

11 

10 

 9

Burroceras clydense

Euomphaloceras 

 septemseriatum

Inoceramus pictus

Vascoceras diartianum 

Dunveganoceras conditum

Dunveganoceras albertense

Dunveganoceras problematicum

Dunveganoceras pondi 94.71

Inoceramus ginterensis

Plesiacanthoceras wyomingense

Acanthoceras amphibolum 94.96

Acanthoceras bellense

Plesiacanthoceras muldoonense

Acanthoceras granerosense 

Conlinoceras tarrantense 95.73

Inoceramus prefragilis

Inoceramus rutherfordi

Inoceramus eulessanus

middle

90.60

0.39

0.59

0.46

0.58

0.58

93.19 0.42

93.99 0.57

0.49

0.50

0.51

Magadiceramus subquadratus

Cremnoceramus crassus crassus

40

Inoceramus arvanus

lower

(Gap in biostratigraphic record)

Neogastroplites maclearni 

 8

 7

 6

Neogastroplites americanus 

Neogastroplites muelleri 

Scaphites preventricosus 

93.32 0.38

93.82 0.30

Neogastroplites cornutus 

Neogastroplites haasi

99.6 0.9 

Inoceramus maclearni

98.88 

 0.35 

1

2

3

4

5

45

Figure 2. Molluscan fossil zones, 

informal zone numbers, fossil-ranges, 

and radiometric ages for marine strata 

for Cenomanian, Turonian, Coniacian, 

and Santonian Stages (Upper 

Cretaceous) in the Western Interior, 

United States (modified after Cobban 

and others, 2006).
Park 

Fremont 

Teton 

Natrona 

Sublette 

Campbell 

Johnson 

Converse 

Big Horn 

Sheridan 

Washakie 

Hot Springs 

T. 5 N.

R. 5 E.

T. 5 N.

R. 5 W.

T. 30 N.

R. 75 W.

T. 50 N.

R. 85 W.

T. 50 N.

R. 75 W.

T. 55 N.

R. 70.W.

T. 30 N.

R. 95 W.

T. 50 N.

R. 95 W.

T. 35 N.

R. 70 W.

T. 55 N.

R. 85 W.

T. 45 N.

R. 70 W.

T. 45 N.

R. 75 W.

T. 55 N.

R. 95 W.

T. 50 N.

R. 70 W.

T. 45 N.

R. 90 W.

T. 50 N.

R. 90 W.

T. 50 N.

R. 80 W.

T. 35 N.

R. 75 W.

T. 35 N.

R. 80 W.

T. 30 N.

R. 80 W.

T. 40 N.

R. 85 W.

T. 55 N.

R. 75 W.

T. 30 N.

R. 70 W. T. 30 N.

R. 90 W.

T. 45 N.

R. 85 W.

T. 55 N.

R. 90 W.

T. 40 N.

R. 90 W.

T. 45 N.

R. 80 W.

T. 35 N.

R. 90 W.

T. 55 N.

R. 80 W.

T. 35 N.

R. 115 W.

T. 40 N.

R. 75 W.

T. 45 N.

R. 95 W.

T. 40 N.

R. 80 W.

T. 30 N.

R. 85 W.

T. 40 N.

R. 70 W.

T. 35 .N

R. 85 W.

T. 45 N.

R. 115 W.

T. 55. N

R.105 W.

T. 35 N.

R. 110 W.

T. 45 N.

R. 100 W.

T. 55 N.

R. 100 W.

T. 30 N.

R. 110 W. T. 30 N. R.105 W. T. 30 N. R. 100 W.

T. 50 N.

R. 100 W.

T. 40 N.

R. 115 W.

T. 30 N.

R. 115 W.

T. 50 N.

R. 105 W.

T. 40 N.

R. 110 W.

T. 40 N.

R. 105 W.

T. 1 N.

R. 1 E.

0 25 50 100 MILES

0 25 50 100

Casper

x 

Emigrant Gap

Riverton

x 

Conant Creek

Cody

x 

Elk Butte

Ten Sleep

x 

Meeteetse

x 

Pitchfork

x 

Black Mountain

Greybull x 

x 

Potato Ridge

x 

Crooked Creek

Thermopolis x 

x 

Deep Lake

x 

Kaycee

Buffalo

x 

Mount Everts 

x 

Upper Slide Lake ? ? 

? 

x 

A

A'

x 

La Bonte Creek

105° 109° 108° 107° 106° 110° 111°

111°

106° 109° 108° 107° 110°

 45°

 44°

 43°

150 KILOMETERS

Figure 3. (facing page) Northern Wyoming showing locations of selected outcrops of the Frontier Formation (x) and equivalent strata. Long dashed boundary of a region in north-central Wyoming where lower Middle Turonian (fig. 2, Zones 28–31) strata are absent. Short dashed lines are the approof regions in northwestern and east-central Wyoming where Lower or Middle Coniacian strata disconformably overlie Turonian strata. Dotted lines aboundaries enclosing a region in northwestern and central Wyoming where lower Upper Turonian (fig. 2, Zones 34–35) strata are absent, except for tMountain bounded by the dotted line with question marks, in which these rocks are present. A—A’, location of regional cross-section (fig. 32) of the lower beds of the overlying Cody Shale. Brown dots mark towns in the region.
UPPER CRETACEOUS (part) Conia

(part

Cenomanian Turonian 

lower middle upper lower middle upper low

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

123456789

CS(

Formation

FrontierMowry Shale Frontier Formation Frontier Formation hiatus hiatus

Mowry Shale Frontier Formation Cody Sha

 (part)

Formation

Frontierhiatus

Mowry Shale 

Frontier Formation(Belle Fourche Member)

hiatus

FrontierFormation(Emigrant Gap Member)

Member)

(Wall Creek

Formation

Frontierhiatus

Mowry Shale Belle Fourche Shale Greenhorn Formation

(part)

Shale

Carlilehiatus Carlile Shale (part) hiatus

Mowry Shale 

Frontier Formation hiatus Cody Shale (part)

Mowry Shale Frontier Formation

Formation

Frontier

hiatus

Mowry Shale

Formation

Frontier

Formation

FrontierFrontier Formation hiatus hiatus

hiatus

Formation

FrontierMowry Shale ? hiatus

ation

FormFrontier

Formation

FrontierMowry Shale ?

Formation

Frontierhiatus

ation

FormFrontier 

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

123456789

Cody Sh (part)

Cody S(part)

?

?hiatus

Figure 4. Formations and members of early Late Cretaceous ages and corresponding fossil zones (fig. 2) for selected localities in northern Wyomin
0

100

200

300

400

500

Sandstone, v.f. gr., silty–burrows 

Shale, well indurated 

OUTCROP SECTION–DEEP LAKE AREA

W1/2 Sec. 21, T.56N., R.103 W.

PARK COUNTY, WYOMING

DESCRIBED IN 1974 BY E.A. MEREWETHER

AND WILLIAM STANTON

FEET

600

700

FRONTIER

FORMATION

Concealed, probably shale 

Bentonite

Sandstone, v.f. gr., silty–

 conglomerate at top–burrows

Siltstone and sandstone 

Siltstone–abundant plant fossils

Sandstone, v.f. gr., silty–

 conglomerate at top of unit

CODY 

SHALE

(part)

Interlaminated siltstone and 

 less shale–burrows

Siltstone

Shale, bentonitic 

Sandstone, v.f. and f. gr.–v.c. gr. 

 at top of unit 

Shale–fossils at 710 ft; D9258, 

 Cremnoceramus crassus inconstans

 (fig. 2, Zone 41) 

Shale, carbonaceous, and 

 claystone–septarian 

 concretions at top of unit 

Shale, carbonaceous, 

 and siltstone 

750

Bentonite

Interlaminated shale and siltstone–fish scales–burrows

Interlaminated siltstone and shale–bioturbated

Bentonite

Siltstone–abundant burrows

Bentonite

Interlaminated siltstone, sandy, 

 and shale

Interlaminated siltstone, sandy, 

 and less shale

Bentonite

Sandstone, v.f. gr., silty–

 conglomerate at top

Interlaminated siltstone and 

 less shale–burrows

Shale 

Interlaminated siltstone 

 and less shale

Sandstone, v.f. and f. gr.–

 conglomerate at top of unit

Siltstone, sandy–minor 

 carbonaceous shale and lignite

Siltstone–lignite at top of unit

Interbedded claystone and 

 siltstone 

Sandstone, v.c. gr.–pebbles

Interbedded siltstone, sandy, 

 and sandstone, v.f. and f. gr.

Sandstone, v.f. to v.c. gr.–

 conglomeratic at 595 ft– 

 carbonaceous siltstone at top 

Interbedded sandstone, v.f. to 

 m. gr., and siltstone, sandy

 Sandstone, v.f. to v.c. gr.–

 pebbles and cobbles–burrows

Siltstone; sandy in upper part 

 of unit–burrows

Sandstone, v.f. gr., silty

Siltstone, sandy–abundant 

 burrows

Sandstone, v.f. gr., silty–fossils 

 at top; D9259, Cremnoceramus 

 deformis dobrogensis 

Siltstone

Bentonite

MOWRY 

SHALE ?

(part)

 (fig. 2, Zone 40) Sequence includes carbonaceous beds

? C

C

Bivalves

EXPLANATION

Possible contact of formations ?

Contact of formations

Probable correlation of strata

C concealed

Fossils

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

 

 

v.f. gr. Very fine grained

f. gr. Fine grained

m. gr. Medium grained

c. gr. Coarse grained

sandstone siltstone/mudstone

shale bentonite

lignite

Figure 5. Columnar section and lithologic description of Frontier Formation in outcrops near Deep Lake, northwestern 

Bighorn Basin (fig. 1).

Outcrops near Cody, Wyoming 11
12 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Siltstone with shale in middle of unit

0

100

200

Interlaminated siltstone and shale–burrows

Interbedded sandstone, v.f. and f. gr., and 

 less conglomerate

Shale–silty in 

 upper part

OUTCROP SECTION–CODY 

N1/2 Sec. 31, T.53N., R.101W.

PARK COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER AND 

WILLIAM STANTON

FEET

Sandstone–f. gr. in lower part of unit,

 v.f. gr. in upper part of unit–burrows

300

Siltstone 

400

500

600

Bentonite 

Sandstone, v.f. gr.–burrows

Sandstone, v.f. to v.c. gr.–pebbles–burrows

Lignite

Interlaminated siltstone and less 

 mudstone

Interbedded sandstone, v.f. gr., 

 and siltstone

Sandstone–v.f. gr. in lower part, f. gr. in 

 upper part–burrows

Sandstone, v.f. gr.–pebbles at top–burrows

Sandstone, v.f. gr., silty–burrows

CODY 

SHALE

(part)

FRONTIER 

FORMATION

?

Siltstone 

Bentonite 

Siltstone in lower part of unit,

 silty shale in upper part of unit–burrows 

Interbedded sandstone, v.f. and f. gr., and 

 less conglomerate

Interbedded siltstone, sandy, and 

 less shale–burrows

Interbedded sandstone, v.f. gr., silty,

 and less shale 

Sandstone, v.f. and f. gr.–conglomeratic 

 at top

Siltstone, sandy 

Sandstone; v.f. gr., silty near base of unit;

 v.f. and f. gr. near top of unit; pebbles at top–

 burrows

Siltstone with interlaminated mudstone

 near base of unit–burrows 

Interlaminated siltstone and shale–burrows

Shale–burrows 

Siltstone, carbonaceous, and lignite 

Siltstone and sandstone–root casts 

Bentonite 

Siltstone, sandy–burrows–fossils; D8997, 

 Inoceramus howelli (fig. 2, Zones 29 and 30)

Siltstone–burrows 

Siltstone; sandy at base; interbedded 

 shale at top–burrows

Sandstone; v.f. gr., silty, at base; f. gr. at 

 top–deltaic

Interstratified sandstone and shale

Sandstone, v.f. and f. gr.–pebbles at 

 base–burrows

Shale–fossils; D8996, Inoceramus 

 deformis (fig. 2, Zone 40) 

?

GENERAL AMERICAN OIL CO.

DACE-FEDERAL 1-30

1900 FT. F.S.L./2150 FT. F.E.L.

Sec. 30, T. 52 N., R. 100 W.

PARK COUNTY, WYOMING

FEET

F200

F150

F100

F300

F400

F500

F600

M100

 

Bivalves

EXPLANATION

Plant fragments

? Approximate position of disconformity

Contact of formations

v.f. gr. Very fine grained

f. gr. Fine grained

m. gr. Medium grained

c. gr. Coarse grained

Probable correlation of strata

sandstone siltstone/mudstone

shale bentonite

lignite

Fossils

F400 Example of a correlative stratigraphic horizon 

 for the subsurface of the Bighorn Basin

 (M.A. Kirschbaum, written commun., 2008)

 

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

Resistivity log Gamma Ray 

log Caliperlog Spontaneous Potential log

Figure 6. Columnar section and lithologic description of Frontier Formation in outcrops near Cody, western Bighorn 

Basin (fig. 1). Also shown are surface-to-subsurface correlations with a nearby borehole. This section is modified from 

the version published in Kirschbaum and others (2009). 
x x D8996

D8997

44°30’

44°35’

109°05’ 109°00’

0 1 2 MILES

Figure 7. Fossil localities near Cody in the Cody quadrangle (15-minute, 1949), Park County, Wyoming. 

D numbers refer to USGS fossil collections (table 1).

Outcrops near Cody, Wyoming 13
14 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Outcrops near Elk Butte, Wyoming 

In outcrops near Elk Butte, about 14 mi southeast of 

Cody and about 13 mi north of Meeteetse (fig. 1), only an 

upper part of the Frontier is well exposed. It was not described 

in detail (Merewether and Cobban made an unpublished partial description in 1973), but the entire formation is recorded 

on geophysical logs from nearby boreholes (fig. 8). A slightly 

carbonaceous mudstone in these outcrops is overlain by very 

fine and fine grained sandstones at least 30 ft thick. The lower 

part of this sandstone sequence contains the fossil bivalve 

Inoceramus howelli (D11916; fig. 2, Zones 29 and 30; fig. 9) 

of early middle Turonian age. 

Near the top of the Frontier, the Turonian sandstones are 

disconformably overlain by about 20 ft of a darker, coarse 

grained sandstone that is conglomeratic at the top (figs. 8, 10). 

Fossils from the upper part of this darker sandstone include 

Cremnoceramus deformis erectus (D8999; fig. 2, Zone 39) of 

earliest Coniacian age. The lacuna between the two sandstones (fig. 10) probably spans late middle and late Turonian 

time. Conformably overlying the Frontier is the Cody Shale, 

which contains the fossil Cremnoceramus crassus inconstans

(D11917; fig. 2, Zones 40 and 41; fig. 9) of late early Coniacian age, 22 ft above the base.

Outcrops in the Pitchfork Oil Field, 

Wyoming 

The Frontier is exposed in the Pitchfork oil field, about 

10 mi west of Meeteetse (fig. 1), where it was described by 

Merewether and others (1975). It is recorded on geophysical logs from boreholes in the area (fig. 11). The formation 

consists of siliciclastic and bentonitic beds that were deposited mainly in shallow-marine environments. Many of these 

strata contain burrows and a few contain molluscan fossils. 

The Frontier in this area is at least 520 ft thick, comprises 

several stratigraphic sequences, and includes one or two 

disconformities. 

In the oil field, the Frontier overlies the Mowry Shale, the 

upper 95 ft of which comprises sandstone, siltstone, shale, and 

bentonite. The sandstones and siltstones commonly contain 

burrows but Ophiomorpha are rare. An ammonite of early 

Cenomanian age (Neogastroplites americanus, D9263; fig. 2, 

Zone 4) was collected from sandstone near the middle of this 

sequence (figs. 11, 12). 

The overlying Frontier is composed of very fine and 

fine grained sandstone, siltstone, shale, and bentonite. Most 

of these strata were deposited in marine environments, 

although fossil bivalves (D9264) that lived in brackish water 

were found in one of the lower beds (fig. 11). A long-ranging 

ammonite, Metengonoceras sp. (Albian and early Cenomanian, D9818), was also found in a lower part of the formation. 

The Frontier in these outcrops commonly contains marine 

burrows. However, a few beds near the top of the formation, 

including carbonaceous shale, are nonmarine. The ages of 

these strata are Cenomanian and probably Turonian. Strata of 

these ages might be separated by a disconformity. 

The main part of the Frontier is disconformably overlain 

by a 20-ft-thick sandstone at the top of the formation that is 

very fine to very coarse grained and contains pebbles. It also 

contains marine burrows, including Ophiomorpha, and fossil 

bivalves, including Cremnoceramus deformis (D9265; fig. 2, 

Zone 40). The age of this uppermost Frontier and of a basal 

part of the overlying Cody Shale is early Coniacian. At this 

disconformity, the lacuna might include the uppermost middle 

and upper Turonian and the lowest Coniacian. 

Outcrops at Hamilton Dome, Coal 

Draw, and near Thermopolis, Wyoming 

The Frontier in outcrops near the southwestern margin of 

the basin (figs. 1, 13), at Hamilton Dome and near Thermopolis, Wyo., were described by Merewether and others (1975). 

Outcrops at Coal Draw were described by R.W. Tillman and 

R. Bottjer (written commun., 1999). Sandstones at these 

localities are mainly very fine grained; rarely are they coarse 

grained or contain pebbles. The formation is at least 560 ft 

thick, and the strata are mostly of marine origin but contain a 

few carbonaceous beds at Coal Draw and near Thermopolis. 

Burrows are common in the marine beds. 

The age of the lower part of the Frontier probably is 

Cenomanian. Sandstones at the top at Hamilton Dome and 

near Thermopolis contain molluscan fossils (D9770 and 

D8951, respectively) of early Coniacian ages (fig. 2, Zones 40 

and 41). Regional correlations in the Bighorn Basin indicate 

that these outcrops enclose several flooding surfaces as well 

as a disconformable contact that separates Cenomanian from 

Turonian beds. 

Outcrops at Black Mountain, Wyoming

At Black Mountain (fig. 1), which is about 24 mi east of 

Thermopolis, Wyo., outcrops of the Frontier were described 

by W.A. Cobban, (written commun., 1973), Merewether and 

others (1975, 1998), and R.W. Tillman and R. Bottjer (written commun., 1999). In this area, the formation is 665 ft thick 

and consists of siliciclastic and bentonitic strata deposited in 

a variety of marine environments. The lower and main part, 

620-ft thick, is separated from the upper 45 ft by a disconformity (fig. 14). Sandstones in the lower part are generally very 

fine or fine grained, although several are medium grained and 

contain pebbles. 

Burrows are common in these strata and molluscan 

fossils have been collected from beds near the top (figs. 15, 

16). Ammonites of earliest middle Cenomanian age, 
Outcrops at Black Mountain, Wyoming 15

0

100

Mudstone, slightly carbonaceous

Shale–fossils; D11917, 

 Cremnoceramus crassus 

 inconstans (fig. 2, Zones 40 and 41) 

OUTCROP SECTION–ELK BUTTE 

SW 1/4 SE 1/4 Sec. 31, T.51N., R.100W.

PARK COUNTY, WYOMING

DESCRIBED IN 1973 BY 

E.A. MEREWETHER AND W.A. COBBAN

FEET

Sandstone, very fine and fine 

 grained, silty–fossils; D11916, 

 Inoceramus howelli (fig. 2, 

 Zones 29 and 30)

CODY SHALE

(part)

Sandstone, coarse grained–fossils 

 at top; D8999, Cremnoceramus 

 deformis erectus (fig. 2, Zone 39)

FRONTIER

FORMATION

TEXACO INCORPORATED

GOVT. GWEN KNAPP 2

675 FT. F.S.L., 3110 FT. F.W.L.

Sec. 26, T.51N., R.101N.

PARK COUNTY, WYOMING

Concealed

FEET

M100

F600

F500

F400

F300

F200

F100

Gamma Ray log Resistivity log

MOWRY 

SHALE (part) 

C concealed

Bivalves

EXPLANATION

? Approximate position of disconformity

Contact of formations

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

Probable correlation of strata

sandstone siltstone/mudstone

shale

Fossils

F400 Example of a correlative stratigraphic horizon 

 for the subsurface of the Bighorn Basin

 (M.A. Kirschbaum, written commun., 2008)

 

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

?

?

C

C

C

Very fine grained

Fine grained

Medium grained

Coarse grained

Figure 8. Columnar section and lithologic description of Frontier Formation in outcrops near Elk Butte, 

western Bighorn Basin (fig. 1). Also shown are surface-to-subsurface correlations with a nearby borehole. 
16 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

D11916

D11917

D8998

D8999

0 1 2 MILES

T.

51 

N.

T.

50

N.

R. 101 W. R. 100 W.

Figure 9. Fossil localities near Elk Butte in the Oregon Basin quadrangle (15-minute, 1913), Park County, Wyoming.

D numbers refer to USGS fossil collections (table 1).
Figure 10. Middle Turonian sandstone disconformably overlain by a lower Coniacian sandstone near Elk Butte, 

western Bighorn Basin (fig. 1). Pointer indicates location of disconformity. Fossils collected from ledges near O included 

Inoceramus howelli (fig. 2, Zones 29 and 30).

Outcrops at Black Mountain, Wyoming 17
18 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Siltstone, sandy–burrows 0

100

200

Interbedded shale and siltstone

Shale, silty 

OUTCROP SECTION–PITCHFORK 

Sec. 14, T.48N., R.102W.

PARK COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER

FEET

Shale 

Sandstone, v.f. gr., silty–burrows–

 fossils; D9263, Neogastroplites americanus (fig. 2, Zone 4)

300

Siltstone–burrows 

400

500

600

700

Bentonite 

Bentonite and siltstone 

Sandstone; v.f. gr., silty–burrows–

 Ophiomorpha at top of unit

Interbedded sandstone, siltstone, 

 and shale–burrows 

Sandstone, v.f. gr., silty

Interbedded siltstone and shale

Siltstone–sandy at top of unit

Interbedded siltstone and shale

Interbedded bentonite and shale

Sandstone–v.f. gr., silty–burrows

Sandstone, v.f. gr., silty–at top,

 siltstone, sandy at base

Sandstone, v.f. gr., silty

FRONTIER 

FORMATION

?

Shale 

Siltstone–burrows 

Shale, silty–burrows–fossils;

 D9264, Lingula sp. and brackish-water bivalves 

Shale, silty 

Siltstone, sandy–burrows–fossils;

 D9818, Metengonoceras sp. (fig. 2, early Cenomanian or older)

Siltstone–burrows 

Siltstone 

Sandstone, v.f. gr.

Siltstone 

Sandstone, v.f. and f. gr., silty–

 burrows

Siltstone, sandy

Interbedded sandstone and shale–

 burrows

Sandstone, v.f. and f. gr.–sharp base

Interbedded sandstone, v.f. and f. gr., and less shale–nonmarine? 

Shale, carbonaceous 

Interbedded sandstone, v.f. and f. gr., 

 and siltstone–marine

Sandstone, f. to v.c. gr.–pebbles at 

 top–burrows–Ophiomorpha–fossils

 at top; D9265, Cremnoceramus 

 deformis 

?

MOWRY

SHALE 

(part)

Bentonite 

(fig. 2, Zone 40)

LOUISIANA LAND & EXPLORATION COMPANY

PITCHFORK 42-12

2156 FT. F.N.L./780 FT. F.E.L. 

Sec. 12, T.48N., R.102W.

PARK COUNTY, WYOMING

FEET

M200

M100

F600

F500

F400

F300

F200

F100

Gamma Resistivity logs

Ray log

Spontaneous 

Potential log

Caliper log

C

 

Bivalves

EXPLANATION

Cephalopods

? Approximate position of disconformity

Contact of formations

v.f. gr. Very fine grained

f. gr. Fine grained

m. gr. Medium grained

c. gr. Coarse grained

Probable correlation of strata

sandstone

siltstone/mudstone

shale

bentonite Fossils

F400

 

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

C concealed

Example of a correlative stratigraphic horizon 

 for the subsurface of the Bighorn Basin

 (M.A. Kirschbaum, written commun., 2008)

Figure 11. Columnar section and lithologic description of Frontier Formation in outcrops near Pitchfork, western 

Bighorn Basin (fig. 1). Also shown are surface-to-subsurface correlations with a nearby borehole. 
Figure 12. Fossil localities near Pitchfork oil field in the Sunshine Reservoir quadrangle (15-minute, 1957), 

Park County, Wyoming. D numbers refer to USGS fossil collections (table 1).

xxx

x

x

D9265

D9266

D9263

D9264

D9818

0 1 2 MILES

44 ̊05’

44 ̊10’

T. 

48 

N.

T. 

49 

N.

R. 102 W. R. 101 W.

Outcrops at Black Mountain, Wyoming 19
20 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 13. Columnar sections and descriptions of Frontier Formation in outcrops at Hamilton Dome, Coal Draw, and near Thermopolis, 

southern Bighorn Basin (fig. 1).

Siltstone–burrows

0

100

200

Interbedded siltstone, sandy, and less 

 shale, silty

Sandstone, v.f. gr., silty

Shale–small bivalves at base of unit 

OUTCROP SECTION–HAMILTON DOME 

Sec. 18, T.44N., R.97W.

HOT SPRINGS COUNTY, WYOMING

DESCRIBED IN 1973 BY 

E.A. MEREWETHER

FEET

Shale, silty–burrows at top of unit 

Sandstone, v.f. gr., very silty

300

Siltstone, slightly sandy–burrows 

400

500

600

700

Bentonite (Clay Spur?)

Bentonite 

Sandstone; v.f. gr., silty, at base; v.f. and 

 f. gr. with pebbles at top–burrows–

Interbedded shale and less siltstone 

Sandstone, v.f. gr., silty–burrows

Interbedded sandstone, v.f. gr., and less 

 shale–burrows–Ophiomorpha

Mudstone, sandy

Concealed, probably siltstone

Interbedded sandstone, v.f. gr., and 

 less shale

Interbedded sandstone, v.f. and f. gr., and minor shale–encloses bentonite– burrows near top of unit

Sandstone–v.f. gr.–burrows–sharp basal contact

Sandstone, f. and m. gr.

Shale–septarian concretions in 

 upper part of unit

 Cremnoceramus deformis (fig. 2, Zone 40)

Sandstone, v.f. gr., silty– Ophiomorpha– fossiliferous; D9770–

CODY 

SHALE

(part)

FRONTIER 

FORMATION

? ?

Shale and less sandstone, v.f. gr., silty–burrows

Shale 

Siltstone, slightly sandy 

Bentonite 

Shale 

Ophiomorpha

Concealed, probably sandstone

Concealed–fossiliferous concretions 

 near base of unit; D9771–

Cremnoceramus crassus inconstans 

 (fig. 2, Zone 41)

0

100

200

Interbedded siltstone, shale, and bentonitic shale

Sandstone, f. gr.–partly 

 bioturbated–middle shoreface

Shale–marine 

OUTCROP SECTION–COAL DRAW 

SE 1/4 Sec. 23 and NW Sec. 24, T.43N., R.97W.

HOT SPRINGS COUNTY, WYOMING

DESCRIBED IN 1999 BY R.W. TILLMAN 

AND R. BOTTJER

FEET

Shale, bentonitic 

Concealed 

Sandstone, v.f. gr.–offshore marine

Sandstone, v.f. gr.–burrows– middle shoreface

300

400

Bentonite (Clay Spur?)

Sandstone, v.f. gr., slightly carbonaceous–burrows in basal 

 beds–

Interbedded shale and less 

 siltstone and bentonite 

Sandstone, v.f. gr.–bioturbated–

 lower shoreface

Sandstone, f. gr., bioturbated–

 middle shorface–transgressive- marine at top

Sandstone–f. and m. gr. –pebbles– 

 burrows–transgressive marine at top

Sandstone, v.f. gr., partly bioturbated–lower shoreface

Shale–marine

FRONTIER 

FORMATION

MOWRY 

SHALE (part)

distributary-mouth bar

Bentonite 

Interbedded siltstone and less 

 sandstone and shale–estuarine 

Sandstone–v.f. gr. at base, f. gr. at 

 top–carbonaceous–estuarine at 

 base–tidal accretion-bar at top

Sandstone, f. gr.–tidal or fluvial

 channel in lower part

Sandstone, f. gr.–burrows– shoreface

Concealed 

Sandstone–v.f. gr. at base; mostly f. gr.–burrows–lower shoreface at 

 base–mostly middle shoreface

Sandstone, m. gr.–burrows– shoreface

(part)

? ?

? ? 

MOWRY 

SHALE (part)

C

C

C

C

C

C
ce

basal 

d–

–

sive- 

bles– 

ine 

ce

ar

s 

rine 

r. at 

ne at 

 top

al

stly 

ace at 

ace

Siltstone, sandy

0

100

200

Interbedded siltstone, clayey, and less 

 shale, silty–burrows

Sandstone, v.f. gr., silty

Shale, silty 

OUTCROP SECTION–THERMOPOLIS 

SE 1/4 Sec. 9, T.43N., R.94W. HOT SPRINGS COUNTY, WYOMING

DESCRIBED IN 1973 BY 

E.A. MEREWETHER

FEET

Shale, silty 

Siltstone 

Sandstone, v.f. gr., silty–pebbles at top of unit–Ophiomorpha in upper part 

 of unit

Sandstone, v.f. gr.–silty in middle of unit

300 Siltstone; clayey at base of unit;

 sandy at top of unit 

Siltstone, sandy–bentonitic in 

 lower part of unit 

400

500

600

700

Bentonite (Clay Spur?)

Bentonite 

Sandstone, v.f. gr., silty

Interbedded siltstone and shale 

Siltstone, slightly sandy, carbonaceous

Sandstone, v.f. and f. gr.–burrows

Interstratified siltstone and less 

 shale, bentonitic

Siltstone, sandy

Siltstone, sandy, slightly carbonaceous

Siltstone–thin sandstone in 

 middle of unit

Interbedded sandstone, v.f. and 

 f. gr., and less shale

Interstratified sandstone, v.f. gr., silty, and less siltstone–concretions at base 

Sandstone–v.f. gr. at base of unit, pebbles near top of unit

Siltstone, sandy, and less shale

Sandstone, v.f. and f. gr., silty

Siltstone, sandy and clayey

Shale–silty in upper part of unit

Interbedded sandstone, v.f. gr., v. silty, and less shale–abundant burrows– Ophiomorpha–fossiliferous; D8951–

 Cremnoceramus crassus inconstans 

 (fig. 2, Zone 41)

Siltstone, sandy 

Interbedded shale, silty, and less 

 sandstone, v.f. gr., silty

Siltstone, slightly sandy and clayey

Interlaminated shale and siltstone – burrows at base of unit–scattered 

 pebbles 

CODY 

SHALE

(part)

FRONTIER 

FORMATION

? ?

? ? 

MOWRY 

SHALE (part)

Bivalves

EXPLANATION

Possible contact of formations ?

Contact of formations

C concealed

Fossils

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

sandstone

siltstone/mudstone

shale

bentonite

Very fine grained

Fine grained

Medium grained

Coarse grained

Outcrops at Black Mountain, Wyoming 21
0 100 200 300 400 500 Bentonite Sandstone, v.f. and f.gr.– ripples–abundant burrows Sandstone, v.f. and f. gr.– coarsens upward– pebbles at top–burrows Sandstone, v.f.gr. Bentonite Shale OUTCROP SECTION–BLACK MOUNTAIN SW 1/4 Sec. 25, NW 1/4 Sec. 36, T.43N., R.91W. HOT SPRINGS COUNTY, WYOMING DESCRIBED IN 1973 BY E.A. MEREWETHER FEET Sandstone, v.f.gr.–pebbles at top–slump structures at base Sandstone; v.f. gr. at base, conglomeratic at top; Sandstone, v.f. gr.–burrows–fossils, D8955, Scaphites cf. S. nigricollensis (fig. 2, Zone 36?) Sandstone, v.f. to v.c. gr.–granules and pebbles–fossils, D8953, Mytiloides scupini (fig. 2, Zone 37) 600 Sandstone, v.f.gr.–abundant burrows–fossil bivalves 700 Silty shale and clayey siltstone Shale and siltstone–burrows Shale Siltstone, sandy Shale and siltstone, interlaminated Siltstone Siltstone Siltstone, sandy Siltstone and shale Siltstone and shale Shale Sandstone, v.f.gr. Shale Bentonite Siltstone, sandy Siltstone, sandy Siltstone–fossils, D8844, Plesiacanthoceras wyomingense (fig. 2, Zone 14) Siltstone, sandy, and shale–fossils, D8954, Mytiloides scupini (fig. 2, Zone 37) Siltstone, sandy, and shale Shale, silty CODY SHALE (part) FRONTIER FORMATION ANR PRODUCTION COMPANY MUD CREEK-STATE 2-16 745 FT. F.S.L., 1200 FT. F.E.L. Sec. 16, T. 43 N., R. 91 W. HOT SPRINGS COUNTY, WYOMING WILLIAMS PRODUCTION RMT CO. FEDERAL W-52477 43-1 NE 1/4, SE 1/4 Sec. 1, T.42N., R.91W. HOT SPRINGS COUNTY, WYOMING FEET FEET Shale Bentonite Bentonite F300 F400 F500 F600 F600 F500 F400? F300? Caliper log Gamma Ray log Spontaneous Potential log Resistivity logs Resistivity logs Caliper log Gamma Ray log Spontaneous Potential log MOWRY SHALE (part) Bivalv

Cepha

Disco

Cont

v.f. gr. Very fine grain

f. gr. Fine grained m. gr. Medium graine

c. gr. Coarse grained Prob

sandstone 

shale 

Fossils Zones refer to numbe

D numbers refer to US

EXPLA

F400 Exam

for t

 (M.A

Figure 14. Columnar section and lithologic description of Frontier Formation in outcrops near Black Mountain, southern Bighorn Basin (fig. 1). Also shown are surface-to-subsurface correlations with nearby boreholes.
x

x

D8843

D13822

0 1 2 MILES

40°

T. 

43 

N.

R. 91 W.

T. 

42 

N.

Figure 15. Fossil localities near Black Mountain in the Packsaddle Creek quadrangle (7.5-minute, 1960), Hot 

Springs County, Wyoming. D numbers refer to USGS fossil collections (table 1).

Outcrops at Black Mountain, Wyoming 23
24 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

x 

x 

x x D13914

D8953

D8954

D8955

D13648

D13649

40’00”

40’30”

107°45’ 107°42’30”

T. 

43 

N.

T. 

42 

N.

R. 91 W. R. 90 W.

0 1 2 MILES

Figure 16. Fossil localities near Black Mountain in the Bader Draw quadrangle (7.5-minute, 1960), Hot 

Springs County, Wyoming. D numbers refer to USGS fossil collections (table 1).
Outcrops near Ten Sleep, Wyoming 25

Conlinoceras tarrantense (D8843; fig. 2, Zone 9; table 1), 

were found from 100 to 120 ft below the top. Ammonites 

of latest middle Cenomanian age, Plesiacanthoceras 

wyomingense (D8844; fig. 2, Zone 14), are in the uppermost 

20 ft of this lower sequence. 

The upper sequence consists of a basal fine to very 

coarse grained sandstone that contains granules and pebbles, 

a sandy siltstone, and an upper very fine grained sandstone. 

These strata, 15- to 65-ft thick, enclose marine bivalves of late 

Turonian age (D8845, D8846, D8953, and D8954, Mytiloides

scupini, fig. 2, Zone 37; table 1). The lacuna between the 

lower and upper sequences of the Frontier in this area spans 

late Cenomanian and early and middle Turonian time. In 

contrast, an outcrop of the upper part of the Frontier about 

6 mi west of Black Mountain, between Black Mountain and 

Thermopolis (fig. 1), contains an early late Turonian bivalve, 

Inoceramus perplexus (D9317; fig. 2, Zone 34). 

Overlying the Frontier is marine shale of the Cody Shale, 

the lower beds of which contain molluscan fossils of early 

Coniacian age: D8842, Forresteria sp., and D13822, Baculites 

mariasensis (fig. 2, Zones 39–41). The contact of the Frontier 

and the Cody in this area could be either conformable or a 

disconformity that represents a minor lacuna. 

Outcrops near Shanty Ridge and Big 

Trails, Wyoming

Outcrops near the southeastern margin of the basin 

(figs. 1, 17) were described by Merewether and others (1975, 

1998). In this area, several parts of the Frontier are concealed 

and the stratigraphy is probably complicated by faulting. 

Thickness a few miles east of Shanty Ridge could be as much 

as 855 ft. Sandstones in these outcrops range from very fine 

to medium grained and a few contain pebbles. Marine beds 

containing burrows are common; however, several beds were 

deposited probably in estuarine environments. Fossils collected from the upper part of the Frontier near Shanty Ridge 

(D13827 and D13828) include Dunveganoceras pondi of 

earliest late Cenomanian age (fig. 2, Zone 15). 

At outcrops about 3 mi south of Shanty Ridge, fossils 

from the upper part of the Frontier include the early late 

Turonian bivalve, Inoceramus perplexus (D9251; fig. 2, Zone 

34). Outcrops of the lower part of the Cody Shale, about 2 mi 

south of Shanty Ridge, contain the latest Turonian bivalve, 

Cremnoceramus waltersdorfensis (D9006; fig. 2, Zone 38). 

Outcrops at Mud Gulch and Joe Emge 

Creek, Wyoming

South of Ten Sleep (fig. 1), near the southeastern margin 

of the Bighorn Basin, the Frontier at two localities (fig. 18) 

was described by Merewether and others (1998) and was 

examined further by Tillman and Merewether in 1999. Sandstones in these sequences are mostly very fine and fine grained 

and rarely medium grained and pebble-bearing. The formation 

includes flooding surfaces and transgressive beds and was 

deposited in environments that ranged from offshore marine to 

estuarine and possibly fluvial. It is as much as 860 ft thick. 

The Frontier at these outcrops probably is entirely of 

Cenomanian age and is disconformably overlain by upper 

Turonian strata of the basal Cody Shale. Ammonites in the 

upper part of the Frontier are Plesiacanthoceras wyomingense

(D13823) of the latest middle Cenomanian (fig. 2, Zone 14) 

and Metoicoceras mosbyense (D13825) of the early late Cenomanian (fig. 2, Zone 17). Molluscan fossils of early Coniacian 

age in the lower part of the Cody Shale include Scaphites

preventricosus (D13228; fig. 2, Zones 39–41; table 1) and 

Scaphites impendicostatus (D13824; fig. 2, Zone 40; table 1). 

The disconformable contact of the Cenomanian and Coniacian 

strata apparently represents latest Cenomanian and most of 

Turonian time. 

Outcrops near Ten Sleep, Wyoming

About 3 mi west of Ten Sleep (fig. 1), outcrops of the 

Frontier were examined by Merewether and others (1975, 

1998) and additionally by Tillman and Merewether in 1996. 

The Frontier is faulted in parts of that area. Sequences of 

the exposed strata can be correlated with those penetrated in 

nearby boreholes (fig. 19). Where described, the formation is 

as much as 860 ft thick and consists of siliciclastic and bentonitic beds that were deposited in marine and estuarine environments. A disconformity in the upper part separates strata of 

Cenomanian and Turonian ages. 

The lower part of the Frontier, of Cenomanian age, is at 

least 620 ft thick. Sandstones in these strata are mostly very 

fine or fine grained and rarely medium to very coarse grained 

and pebble-bearing. Thin beds of conglomerate are associated 

with a few of the sandstone beds. This sequence was deposited 

mainly in nearshore and estuarine environments but rarely in 

channels. Many of the beds contain burrows and those near 

the top of the sequence contain fossil bivalves and ammonites 

(figs. 19, 20, 21): D13911, Inoceramus sp., and D9009 and 

D13910, Plesiacanthoceras wyomingense of latest middle 

Cenomanian age (fig. 2, Zone 14). 

The upper part of the Frontier, 140–170 ft thick, is mostly 

siliciclastic strata that accumulated in marine and estuarine 

depositional environments. This sequence includes a disconformity that separates middle Cenomanian strata from upper 

Turonian strata. A sandstone at the top of the Frontier apparently is of late Turonian age and is conformably overlain by 

the Cody Shale. Marine shale composing the lowermost Cody 

is about 40 ft thick and contains fossiliferous concretions. 

Collections of fossil bivalves from the basal Cody, D8956 and 

D8957, include Mytiloides scupini of late Turonian age (fig. 2, 

Zone 37). Another collection, D13734, from overlying beds, 
26 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 17. Columnar sections and lithologic descriptions of Frontier Formation in outcrops near Big Trails and 

Shanty Ridge, southeastern Bighorn Basin (fig. 1).

100

200

300

400

500

O

S

W

D

R

E

FEE

600

700

0

100

200

300

400

500

Bentonite (Clay Spur)

Mudstone–estuarine

Sandstone, v.f. gr., partly 

 bioturbated–middle shoreface

Sandstone, v.f. gr., and siltstone–

 ironstone concretions–burrows–

 marine

Sandstone, v.f. gr.–root 

 traces–flooding surface

 at top of unit–

 beach-backshore

Shale, silty, bentonitic–marine

Concealed

OUTCROP SECTION–LIGHTENING RIDGE

SW 1/4 Sec. 22, NW 1/4 Sec. 27, T.43N., R.88W.

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1999 BY 

R.W. TILLMAN AND R. BOTTJER

FEET

Interbedded sandstone and 

 less shale–lower shoreface 

Shale, siliceous–silty and slightly 

 sandy in upper 30 ft–marine

Shale, bentonitic–ironstone

 concretions–marine 

Interbedded siltstone and less 

 sandstone, v.f. gr.–marine

Sandstone, v.f. gr.–bioturbated in 

 lower part–middle shoreface

Sandstone; f. gr. in lower part, 

 m. gr. in upper part–pebbles–

 transgressive marine

Sandstone, v.f. and f. gr.–

 burrows–Ophiomorpha– middle shoreface

600

700

Sandstone, v.f. gr.–middle shoreface 

Sandstone, m. gr.–pebbles at 

 base of unit–transgressive

Sandstone, v.f. and f. gr.–burrows– 

 lower shoreface

Shale, bentonitic–ironstone 

 concretions–marine 

Shale, bentonitic–marine

Concealed–includes shale and bentonite

Concealed

Bentonite

Shale–marine 

Shale, silty–estuarine 

800

Concealed–includes a thin 

 sandstone–pebbles

900

Concealed

Siltstone, sandy–marine

Sandstone, v.f. and f. gr.–

 middle shoreface

Interbedded sandstone and less 

 shale–lower shoreface

Sandstone, f. gr., and less siltstone 

 and shale–lower shoreface

Concealed

Concealed

Sandstone, v.f. and f. gr.–middle 

 shoreface

Sandstone, v.f. gr.–ironstone 

 concretions–marine

Shale, silty–marine 

0

100

200

300

400

500

Bentonite (Clay Spur?)

Mudstone–estuarine

Sandstone, f. gr.

Sandstone, f. gr.–inner shelf

Sandstone, v.f. gr.–burrows–

 inner shelf

Bentonite

Shale–partly concealed

Concealed

OUTCROP SECTION–BIG TRAILS

Sec. 13, T.44N., R.88W.

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER AND IN 

1997 BY R.W. TILLMAN

FEET

Interlaminated shale, siltstone, 

 and sandstone, v.f. gr.–estuarine 

Shale–marine

Shale–marine 

Sandstone, m. gr.–pebbles

Sandstone, v.f. gr.–scattered

 concretions–inner shelf

Sandstone, m. gr.–pebbles–

 transgressive marine

Sandstone, v.f. and f. gr.–

 fossilized wood and bone –

 abundant concretions–

 burrows

600

700

Sandstone, v.f. and f. gr.–

 coarsens upward–middle 

 shoreface

Sandstone, v.f.gr.–marine

Sandstone, v.f. and f. gr.–burrows– 

 shoreface?

Shale–marine 

Shale, bentonitic

Bentonite

Concealed

Concealed

Bentonite

Shale–marine 

Shale–marine 

800

Concealed–concretions 

 at 652 ft

900

Concealed

Frontier

Formation Frontier

Formation

Cody 

Shale

(part)

Frontier

Formation

(part)

C

C

C

C

C 

C

C

C

C

C

C
0

100

200

300

400

500

Bentonite (Clay Spur?)

Sandstone, f. and m. gr.–

 transgressive

Sandstone, f. gr.–bioturbated– middle shoreface

Sandstone, v.f. gr., bentonitic–

 lower shoreface

Sandstone, v.f. and f. gr.–

 burrows–middle shoreface

Bentonite

Shale, bentonitic 

Shale–marine

D13827–Dunveganoceras pondi

Concealed

OUTCROP SECTION–SHANTY RIDGE DRAW

SE 1/4, NW 1/4 Sec. 26, T.43N., R.89W.

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1997 BY 

R.W. TILLMAN AND 

E.A. MEREWETHER

FEET

Interbedded sandstone, v.f. gr., and 

 shale, sandy–inner-shelf 

Shale, silty–marine

Shale, silty–estuarine 

Sandstone, v.f.gr.

Shale, bentonitic 

Shale, bentonitic–marine 

Sandstone, f.gr.–burrows–

 middle and upper shoreface

Sandstone, v.f. gr., bentonitic– inner shelf

Sandstone, v.f. gr., silty–estuary?

Sandstone, m.gr., pebbly–transgressive

Fossiliferous concretion–

Sandstone, pebbly–estuarine

600

700

Concealed

Shale, silty

Sandstone, v.f.gr., shaley–inner-shelf

Sandstone, v.f.gr.–inner-shelf

Shale

(fig. 2, Zone 15)

Sandstone, v.f. and f. gr.–middle 

 and lower shoreface

Sandstone, v.f. and f. gr.–burrows–

 lower shoreface

Shale, sandy, bentonitic–

 inner shelf 

Concealed

Sandstone, f. gr.–foreshore

Sandstone, v.f. gr.–shoreface

Sandstone, m. gr.–transgressive

Mudstone–estuarine

Mudstone–estuarine 

Bentonite

Concealed

Fossiliferous concretion–

Concealed

D13828–Dunveganoceras pondi 

(fig. 2, Zone 15)

tly 

shoreface

 siltstone–

s–burrows–

t 

ace

–marine

RIDGE

3N., R.88W.

e and 

reface 

and slightly 

marine

stone

and less 

arine

turbated in 

horeface

wer part, 

pebbles–

e

r.–

ha– 

dle shoreface 

bles at 

essive

r.–burrows– 

stone 

ne

hale and bentonite

 thin 

ne

r.–

e and less 

ce

ess siltstone 

reface

r.–middle 

nstone 

Frontier

Formation

(part)

C

C

C

EXPLANATION

Contact of formations

C concealed

Fossils

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

sandstone

siltstone/mudstone

shale

bentonite

Plant fragments

Cephalopods

Very fine grained

Fine grained

Medium grained

Coarse grained

Outcrops near Ten Sleep, Wyoming 27
28 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 18. Columnar sections and lithologic descriptions of Frontier Formation in outcrops at Joe Emge Creek and Mud Gulch, 

southeastern Bighorn Basin (fig. 1).

0

100

200

300

400

500

Bentonite (Clay Spur?)

Sandstone–mainly concealed

Sandstone, f.gr.

Sandstone, m.gr.–transgressive

Shale, siliceous–marine 

Shale–marine

Concealed

OUTCROP SECTION–JOE EMGE CREEK

NE 1/4, SW 1/4 Sec. 13, T.46N., R.89W.

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1997 BY 

R.W. TILLMAN AND 

E.A. MEREWETHER

FEET

Interbedded shale, siltstone, 

 and sandstone–estuarine 

Shale, silty–estuarine

Shale–marine 

Sandstone, f.gr.

Shale, bentonitic 

Shale, bentonitic, and sandstone 

Sandstone, v.f.gr.

Sandstone, v.f. gr., shaley, 

 bentonitic

Sandstone, m.gr.–tidal-bar

Sandstone, f. and m. gr.–

 transgressive

Mudstone–estuarine

Sandstone, f. gr.

600

Sandstone, v.f.gr.–inner shelf

700

Shale

Sandstone, v.f. and f. gr., silty–

 marine shelf

800

900

Sandstone, m.gr.–burrows–transgressive

Shale, silty–estuarine

Mudstone–estuarine

Sandstone, v.f.gr.–burrows

Siltstone

Sandstone, f.gr.

Sandstone, f.gr.–marine

Interbedded shale and siltstone–

 estuarine 

Interbedded shale, silty, and sandstone–

 estuarine 

Siltstone–estuarine

Interbedded shale, silty and 

 bentonitic, and siltstone 

Interbedded shale, bentonitic, 

 and sandstone 

Shale, bentonitic 

Concealed–at 639, D13825, 

 Metoicoceras mosbyense 

 (fig. 2, Zone 17)

Shale, bentonitic 

Bentonite and shale, bentonitic

Shale, bentonitic–marine 

Shale, silty–estuarine

Siltstone

Shale–marine

Cody 

Shale

(part)

Frontier

Formation

Shale, bentonitic 

Mudstone–estuarine

Bentonite and shale, bentonitic

Sandstone, v.f.gr.–bioturbated

Mudstone–estuarine

Siltstone

Sandstone, v.f.gr.–estuarine

Fossils, D13228, Scaphites preventricosus

 (fig. 2, Zones 39-41)

0

100

200

300

400

500

Bentonite (Clay Spur?)

Sandstone, f.gr., thin bedded–

 shelf-ridge (?) 

Sandstone, f.gr., laminated–partly 

 bioturbated–upper shelf-ridge 

Sandstone, m.gr. ; sharp top and base–

 transgressive

Sandstone, v.f.gr.–transgressive marine

Bentonite

Shale, slightly silty–marine 

Shale, silty–grades to siltstone at top–

 estuarine

D13823–Plesiacanthoceras wyomingense

Concealed

OUTCROP SECTION–MUD GULCH 

SW 1/4, SW 1/4 Sec. 4, T.45N., R.88W.; 

NW 1/4, NW 1/4 Sec. 9, T.45N., R.88W.

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1997 BY R.W. TILLMAN 

AND E.A. MEREWETHER

FEET

Interbedded mudstone, sandy or 

 bentonitic, with sandstone, v.f.gr., in 

 lenses–ridge-margin facies

Shale, silty–estuarine

Shale, bentonitic–marine 

Sandstone, v.f.gr.

Shale, bentonitic and minor sandstone 

Shale, bentonitic 

Sandstone, v.f.gr.–lower shoreface

Sandstone, f.gr., bentonitic

Sandstone, m.gr., cross bedded

Sandstone, m.gr., pebbly–transgressive

Mudstone–estuarine

Sandstone, v.f.gr.

Mudstone–estuarine

600

Sandstone, v.f.gr.

700

Shale–marine

Concealed

Shale, silty–estuarine

Sandstone, v.f.gr.–burrows, tidal creek

Sandstone, v.f.gr.

Mudstone–estuarine

800

900

Sandstone, f. and m.gr., poorly sorted,

 cross bedded; sharp lower and upper 

 contacts–valley-fill, tidal bar–

 transgressive base

Shale–marine

Sandstone, v.f.gr.–estuarine

Sandstone, v.f.gr.–marine

Siltstone–marine

D13824–Scaphites impendicostatus

 (Zone of Cremnoceramus deformis–

 fig. 2, Zone 40)

(fig. 2, Zone 14)

Concealed

Concealed

Concealed

Bentonite

?

Concealed

Concealed

Concealed

Concealed

?

Frontier

Formation

Cody 

Shale

(part)

C

C

C

C

C

C

C

C C

C

C

C

 

 

 
Cephalopods

? Approximate position of disconformity

Contact of formations

Probable correlation of strata

C concealed

sandstone

siltstone/mudstone

shale

bentonite

Fossils

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

Very fine grained

Fine grained

Medium grained

Coarse grained

Outcrops near Ten Sleep, Wyoming 29
30 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 19. Columnar sections and lithologic descriptions of Frontier Formation in outcrops near Ten 

Sleep, southeastern Bighorn Basin (fig. 1). Also shown are surface-to-subsurface correlations with 

nearby boreholes.

0

100

200

300

400

500

OUTCR

Secs. 2

WASHA

DESCR

E.A. M

FEET

600

700

800

900

CODY SHALE 

(part)

FRONTIER 

FORMATION

BASS ENTERPRISES PRODUCTION COMPANY

BASS MEYERS GULCH-FEDERAL

711 FT. FSL, 845 FT. FWL

Sec. 28, T. 47 N., R. 90 W.

WASHAKIE COUNTY, WYOMING

0

100

200

300

400

500

Bentonite 

Sandstone, v.f. gr.

Sandstone, f. gr.–m. gr. at top of unit

Sandstone, f. gr.–middle shoreface

Shale–marine

OUTCROP SECTION –TEN SLEEP AREA 

SW 1/4, SE 1/4 Sec. 22, T.47N., R.89W. 

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1996 AND 1998 BY 

R.W. TILLMAN AND 

E.A. MEREWETHER

FEET

Sandstone, f.gr., bentonitic–

 lower shoreface

Sandstone, f. gr. –pebbles nearby–

 channel and shoreface

Sandstone, m. and c. gr.–pebbles–tidally

 reworked fluvial

600

Shale, bentonitic

Shale–marine

Shale, siltstone, and sandstone, f. gr.

Sandstone and mudstone, interbedded–estuarine

Shale and less sandstone, f.gr.–

 estuarine

Mudstone–estuarine

Shale and less sandstone, v.f. and f. gr. 

Sandstone, f. and m. gr.–pebbles at top 

 of unit–middle shoreface

Mudstone, siltstone, shale, and 

 sandstone–estuarine

Sandstone, v.f. and f. gr. –middle 

 and upper shoreface

Sandstone; c. gr. –tidally 

 reworked fluvial

Concealed

?

Bentonite

Shale, bentonitic

Sandstone, f. gr.–lower shoreface

Sandstone, f. gr.–upper shoreface–

 transgressive lag at top of unit

Bentonite

Shale–marine

Mudstone, siltstone, shale, and 

 sandstone–estuarine

Mudstone, siltstone, sandstone, and 

 bentonite–pebbles–nearshore 

Siltstone and bentonite, 

 interbedded–nearshore

Sandstone, f.gr., bentonitic

Sandstone, f.gr.–middle shoreface

Fossils, D8956, Mytiloides scupini (fig. 2, Zone 37) at 525 ft

FRONTIER 

FORMATION 

(part)

FEET

Bentonite 

M100

F600

F500

F400

F300 

Resistivity 

log

Gamma Ray 

log

Spontaneous 

Potential log

C

Bivalves

Plant fragments

Cephalopods

? Approximate position of disconformity

Possible contact of formations ?

Fossils

Disconformity

Contact of formations

Probable correlation of strata

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

F400

 

Example of a correlative stratigraphic 

 horizon for the subsurface of the 

 Bighorn Basin (M.A. Kirschbaum, 

 written commun., 2008)

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

C concealed

sandstone

siltstone/mudstone

shale

bentonite

Very fine grained

Fine grained

Medium grained

Coarse grained
0

100

200

300

400

500

Bentonite (Clay Spur?)

Sandstone

Sandstone, v.f. to v.c. gr.–pebbles at 

 base of unit

Sandstone, v.f. gr., silty–burrows

Bentonite

Shale, silty

OUTCROP SECTION –TEN SLEEP AREA 

Secs. 22 and 27, T.47N., R.89W. 

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER

FEET

Sandstone, v.f.gr., and siltstone

Interbedded sandstone; v.f. gr., 

 and shale 

Sandstone, v.f. gr., silty–bed of 

 conglomerate at top of unit

600

Sandstone, v.f. and f. gr., silty–c. gr. 

 with pebbles at top of unit

700

Interbedded silty shale and 

 siltstone–burrows 

Siltstone and minor shale–burrows

Shale–bentonitic, with plant fragments

 at base of unit 

Siltstone

Siltstone in lower part of unit; sandstone, 

 v.f. gr., and shale in upper part of unit

Sandstone, v.f. and f. gr.–pebbles at top

Shale, silty–burrows

Bentonite

Siltstone, sandy

Shale

Fossils, D8957, Mytiloides scupini,

 (fig. 2, Zone 37)

Bentonite

Interbedded shale and less 

 sandstone, v.f. gr., 

 silty–burrows 

Sandstone, v.f. gr. at base of unit, v. c. gr. 

 with pebbles at top of unit

Sandstone, v.c. gr. with pebbles at base 

 of unit, v. f. gr. with pebbles at top of unit

800

900

Poorly exposed interbedded 

 shale, silty, and siltstone, 

 clayey

Fossils, D9009, Plesiacanthoceras 

 wyomingense (fig. 2, Zone 14),

 20 ft above base of unit

Sandstone, 

 v.f. gr., silty–siltstone, 

 sandy, at base of unit

Fossils, D13734, Cremnoceramus deformis (fig. 2, Zone 40)

CODY SHALE 

(part)

FRONTIER 

FORMATION

CHORNEY RAYMOND 

SHOAL UNIT 2

2180 FT. FSL, 616 FT. FWL

Sec. 6, T. 46 N., R. 89 W.

WASHAKIE COUNTY, WYOMING

300

400

500

Bentonite 

Sandstone, silty

Sandstone, c. gr.–pebbles–

 tidal channel-fill

Sandstone, f. gr.–middle 

 shoreface

OUTCROP SECTION –McCLELLAN GULCH 

SE 1/4, NW 1/4 Sec. 25, T.47N., R.89W. 

WASHAKIE COUNTY, WYOMING

DESCRIBED IN 1998 AND 1999 

BY R.W. TILLMAN 

FEET

Sandstone, f.gr.

Sandstone, 

 v.f. gr.–estuarine

600

Shale, bentonitic

Shale, silty, bentonitic–

 marine or estuarine

Shale, bentonitic, and sandstone

Sandstone, f. and m. gr.–conglomerate 

 at top of unit–transgressive at top

Sandstone, v.f. gr. – channel-fill

?

Sandstone, f. gr., and shale

Sandstone, f. and m. gr.–

 upper shoreface

Mudstone, carbonaceous–

 estuarine

Siltstone–marine or estuarine

Sandstone, f.gr.–middle 

 and upper shoreface

Fossils, D13912, Mytiloides scupini (fig. 2, Zone 37) at 945 ft

700

800

900

Shale, 

bentonitic

Sandstone, silty

Sandstone, f. and m. gr.–

 pebbles at top–transgressive

Shale, bentonitic–marine

Shale, bentonitic–marine

Bentonite 

Shale–marine

Poorly exposed, siltstone?

Fossils, D13911, 

 Inoceramus sp., at 555-560 ft

Fossils, D13910, Plesiacanthoceras wyomingense (fig. 2, Zone 14) at 585 ft

Fossils, D13908, Placenticeras pseudoplacenta (fig. 2, Zones 28-38) at 810 ft

repeated section

lly

. 

p 

Bentonite 

Bentonite 

Shale–marine

FRONTIER 

FORMATION 

(part)

Fossils, D13732, Dunveganoceras sp. (fig. 2, Zones 15-18 ?) at 504 ft or higher

Fossils, D13650, Inoceramus eulessanus (fig. 2, Zones 9-11)

FEET

F300

F400

F500

F600

M100

Caliper log Gamma 

Ray log

Sonic log

C

Bivalves

Plant fragments

Cephalopods

Fossils

Outcrops near Ten Sleep, Wyoming 31
32 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

x

x x

x

x

x xx

x

D13650 D13732

D9009

D13242

D9751

D13733

D13734 D8956

D13913

x D8957

107°32’30” 107°30’

44°

02’

30”

0 1 2 MILES

Figure 20. Fossil localities near Ten Sleep in the Wild Horse quadrangle (7.5-minute, 1967), Washakie County, 

Wyoming. D numbers refer to USGS fossil collection (table 1).
x

x

x

x

x

107°30’ 107°27’30”

44°

44°

02’

30”

0 1 2 MILES

D13911

D13910

D13908

D13909

D13912

Figure 21. Fossil localities near McClellan Gulch in the Ten Sleep quadrangle (7.5-minute, 1967), Washakie 

County, Wyoming. D numbers refer to USGS fossil collections (table 1).

Outcrops near Ten Sleep, Wyoming 33
34 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

contains Cremnoceramus deformis of early Coniacian age 

(fig. 2, Zone 40). 

The time span of the lacuna represented by the disconformity in the Frontier in this area probably includes the late 

Cenomanian, the early and middle Turonian, and an early part 

of the late Turonian. 

Outcrops at Bonanza Oil Field, 

Wyoming

The Bonanza oil field is about 18 mi northwest of Ten 

Sleep, Wyo., and about 25 mi southeast of Greybull, Wyo. 

(fig. 1). Outcrops of the Frontier in this area (fig. 22) were 

discussed by Merewether and others (1975), and examined 

further by Tillman and Merewether in 1996. The Frontier is at 

least 745 ft thick and consists mainly of sandstone, siltstone, 

and shale deposited in marine and estuarine environments. The 

sandstones are mainly very fine to fine grained and are rarely 

conglomeratic. 

Many beds in the Frontier at the Bonanza locality contain 

burrows, and several beds in the upper part contain molluscan 

fossils (figs. 22, 23, 24) including Plesiacanthoceras wyomingense of latest middle Cenomanian age (D8992; fig. 2, 

Zone 14) and Dunveganoceras pondi of earliest late Cenomanian age (D9267; fig. 2, Zone 15). Fossils in basal strata 

of the overlying Cody Shale include Mytiloides incertus of 

late Turonian age (D9268 and D9269; fig. 2, Zone 36) and 

Cremnoceramus deformis dobrogensis of early Coniacian age 

(D8994; fig. 2, Zone 40). 

Outcrops near Greybull, Wyoming 

Outcrops of the Frontier about 3 mi northeast of Greybull, Wyo. (figs.1, 25), were described by Merewether and others (1975) and examined further by Tillman and Merewether 

in 1998 and 1999. Outcrops about 10 mi southeast of Greybull, near Potato Ridge (fig. 1), were also described by the 

authors in 1998 and 1999. The strata were deposited in 

offshore, nearshore, and brackish-water environments. The 

sandstones are mostly very fine and fine-grained; a few are 

medium-grained and contain pebbles. In this area, the Frontier 

is about 715 ft thick, although in a few places thickness measurements might reflect some faulting. As shown in figure 25, 

correlations can be made with equivalent sequences in nearby 

boreholes. 

Collections of fossils from localities northeast of Greybull (figs. 26, 27) include D13652, Lingula subspatulata

(possibly a brackish-water species), from the top of the thick 

sandstone in the lower part of the Frontier (the Peay Sandstone 

Member). Collections of ammonites from the upper part of the 

Frontier in that area are: D9278, Plesiacanthoceras wyomingense of latest middle Cenomanian age (fig. 2, Zone 14), and 

D9752 and D13916, Dunveganoceras pondi of earliest late 

Cenomanian age (fig. 2, Zone 15). In that area, the Frontier is 

disconformably overlain by marine shale of the Cody Shale. 

A collection of fossils from the basal Cody, D9754, contains 

Baculites yokoyamai of late Turonian age (fig. 2, Zones 34–39; 

table 1). The lacuna at or near the Frontier-Cody contact spans 

most of the upper Cenomanian as well as the lower and middle 

Turonian. 

At the outcrops southeast of Greybull near Potato Ridge 

(fig. 28), the lower part of the Frontier is at least 535 ft thick 

and is of Cenomanian age. Collections of molluscan fossils 

from that area include: D13731, Acanthoceras amphibolum

of late middle Cenomanian age (fig. 2, Zone 13); D13919, 

Plesiacanthoceras wyomingense of latest middle Cenomanian 

age (fig. 2, Zone 14); D13920, Dunveganoceras pondi of 

earliest late Cenomanian age (fig. 2, Zone 15); and D13921, 

Metoicoceras frontierense of early late Cenomanian age 

(fig. 2, Zone 16; table 1). 

Outcrops at Lovell Draw and North Emblem, 

Wyoming

Outcrops of the Frontier Formation between Greybull and 

Lovell, Wyo. (figs. 1, 29), were examined at Lovell Draw in 

1996 and 1998 and at North Emblem by Merewether and others (1975). The formation, where exposed in this area, is 670 

ft thick and of Cenomanian age. Sandstones in these exposures 

are generally very fine or fine grained and are rarely medium 

grained and pebble-bearing. Most of the strata at these localities were deposited in nearshore-marine environments. Many 

beds contain burrows and a few near the top of the formation 

enclose molluscan fossils. 

Fossils include the ammonite Dunveganoceras conditum

(D13917) of middle late Cenomanian age (fig. 2, Zone 18). 

Mollusks from near the top of the formation at North Emblem 

include the bivalve Inoceramus ginterensis (D9276) of early 

late Cenomanian age (fig. 2, Zones 16–18). Outcrops about 

2 mi south of North Emblem contain the early late Cenomanian ammonite Dunveganoceras albertense (D9271; fig. 2, 

Zone 17). At North Emblem, concretions about 100 ft above 

the base of the overlying Cody Shale contain Baculites sp. 

of Turonian age (D9277). A disconformity separating beds 

of middle late Cenomanian age in the Frontier from beds of 

Turonian age in the Cody probably spans the uppermost Cenomanian and the lower and middle Turonian. 

Outcrops at Crooked Creek, Wyoming

Outcrops of the Frontier and the lower part of the Cody 

at Crooked Creek, Wyo. (figs. 1, 30), about 8 mi north of 

Lovell, Wyoming, were described by Merewether and others 

(1975). These strata, about 760 ft thick, can be traced into 
Outcrops at Crooked Creek, Wyoming 35

Figure 22. Columnar sections and lithologic descriptions of Frontier Formation in outcrops in the 

Bonanza oil field, eastern Bighorn Basin (fig. 1).

0

100

200

300

400

500

Bentonite

Sandstone–v.f. and f. gr., pebbles at top–

 crossbedded and ripples near top–

 prograding–mainly shoreface

Sandstone–v.f. gr., silty–pebble-bearing 

 concretions at 390 ft 

Shale and siltstone at base–silty 

 sandstone and shale at top

Shale, bentonitic 

Shale

Concealed–probably siltstone

OUTCROP SECTION–BONANZA OIL FIELD

S 1/2 Sec. 25, T.49N., R.91W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER

FEET

600

700

800

Shale and less siltstone

Siltstone and less shale

Siltstone

Siltstone, sandy, and shale, silty–burrows

Shale and less siltstone

Shale and less siltstone–ironstone 

 concretions

Siltstone, clayey

Sandstone–v.f. gr., silty

Siltstone and less shale

Siltstone and less shale–sandy at top

Siltstone–fossiliferous concretion at 

 base–D9267, Dunveganoceras pondi (fig. 2, Zone 15)

Siltstone and less shale–

 poorly exposed

Concealed–probably siltstone and 

 shale–fossiliferous concretions at 

 820 ft–D9269, Mytiloides incertus (fig. 2, Zone 36)

Fossiliferous concretions at base–

 D9268, Mytiloides incertus (fig. 2, Zone 36)

Fossiliferous concretions at base–

 D8992, Plesiacanthoceras wyomingense (fig. 2, Zone 14)

Siltstone–poorly exposed–

 fossiliferous concretions–D8994, 

 Cremnoceramus deformis

Cody Shale dobrogensis (fig. 2, Zone 40) (part)

Frontier

Formation

(part)

?

0

100

200

300

400

Bentonite

Sandstone–v.f. gr.

Sandstone–f. gr.–lower shoreface 

Shale, bentonitic–marine 

Shale, bentonitic–marine

Shale, silty–estuarine 

Sandstone–f. gr.–burrows–

 middle shoreface

Sandstone–f. gr.–burrows–

 middle shoreface

Sandstone–v.f. gr.–burrows

Sandstone–f. gr., crossbedded

OUTCROP SECTION–BONANZA OIL FIELD

NW 1/4, NW 1/4 Sec. 23, T.49N., R.91W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1996 BY 

R.W. TILLMAN AND 

E.A. MEREWETHER

Sandstone–f. gr.–shoreface

Sandstone–f. gr.–burrows–

 upper shoreface

Shale, bentonitic 

Bentonite

Sandstone–c. gr. at base, v.f.gr. at top

Shale, bentonitic–marine

Bentonite

Mudstone, bentonitic–nonmarine 

Mudstone, sandy–estuarine 

Shale, bentonitic–ironstone 

 concretions near top 

Bentonite

Mudstone 

Shale, bentonitic

Bentonite

Sandstone–f. gr.–pebbles near 

 base–middle shoreface 

FEET

Frontier

Formation 

(part)

C

C

C

Bivalves

Cephalopods

? Approximate position of disconformity

Fossils

Probable correlation of strata

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

C concealed

sandstone

siltstone/mudstone

shale

bentonite

Very fine grained

Fine grained

Medium grained

Coarse grained
36 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

x

x

x

x

x

D8994

D8992

D9253

D9254

D8993

107°47’30” 107°45’ 44°

15’

44°

12’

30”

0 1 2 MILES

Figure 23. Fossil localities near the Bonanza oil field in the McDermotts Butte quadrangle (7.5-minute, 1967), Big Horn 

County, Wyoming. D numbers refer to USGS fossil collections (table 1).
D13651

D9267

D9269

D9268

0 1 2 MILES

44°

10’

44°

12’

30”

Figure 24. Fossil localities near the Bonanza oil field in the Weintz Draw quadrangle (7.5-minute, 1967), Big Horn 

County, Wyoming. D numbers refer to USGS fossil collections (table 1).

Outcrops at Crooked Creek, Wyoming 37
38 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 25. Columnar sections and lithologic descriptions of Frontier Formation in outcrops near Greybull and 

Potato Ridge, eastern Bighorn Basin (fig. 1). Also shown are surface-to-subsurface correlations with nearby 

boreholes. This section is modified from the version published in Kirschbaum and others (2009).

0

100

200

300

400

500

Bentonite 

Sandstone, f.gr.–burrows–

 shoreface

Sandstone, v.f. and f.gr.–burrows–

 distal distributary-mouth bar

Mudstone, sandy, carbonaceous–

 distal distributary-mouth bar

Sandstone, v.f. and f..gr. –

 few burrows–distributary-

 mouth bar–(Peay Sandstone)

Sandstone, m.gr.–pebbles–

 sharp base

Bentonite

Shale, silty–includes thin sandstone and 

 siltstone beds–marine 

Shale, bentonitic–marine

FEET

Shale–marine 

Shale–marine

Siltstone–nearshore 

Shale–siltstone in basal 2 ft–

 marine

Shale, bentonitic–marine

Shale–marine

Shale, bentonitic–marine

Bentonite

Shale–marine

Sandstone, v.f. to f.gr.–nearshore 

Shale, bentonitic–nearshore 

Sandstone, v.f. gr.–abundant chert 

 pebbles–nearshore 

Siltstone–marine

Sandstone, v.f. to f..gr., shaley–marine

Shale–marine

Siltstone, bentonitic–offshore

600

700

Siltstone–offshore 

Shale, bentonitic–marine

Sandstone, v.f. to m.gr.–offshore 

Siltstone, bentonitic–offshore 

Sandstone, silty–offshore 

Sandstone, v.f. to f. gr–shoreface 

Sandstone, f. gr.–upper shoreface 

Sandstone, m. gr.–pebbles–shoreface

Siltstone–marine 

Siltstone–marine 

Fossils, D13916, Dunveganoceras pondi

 (fig. 2, Zone 15)

800

Shale, silty–marine

Fossils, D13652, Lingula subspatulata 

 (brackish water?)

Fossils, D9754, Baculites yokoyamai (fig. 2, Zones 34-39)

?

Frontier

Formation

Cody 

Shale

(part)

faulted outcrop ? 

Fossils, D9753, Pycnodonte aff. P. kellumi (fig. 2, Zones 17-18)

Fossils, D9278, Plesiacanthoceras wyomingense 

 (fig. 2, Zone 14) 

FEET

M100

M200

F600

F500

F400

F300

F200

Gamma Ray log Neutron log

?

?

C

 

Bivalves

Cephalopods

? Approximate position of disconformity

Fossils

Probable correlation of strata

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

F400

 

Example of a correlative stratigraphic horizon 

 for the subsurface of the Bighorn Basin

 (M.A. Kirschbaum, written commun., 2008)

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

Possible contact of formations

C concealed ?

sandstone

siltstone/mudstone

shale

bentonite

OUTCROP SECTION–GREYBULL 

SE 1/4 Sec. 2, T.52N., R.93W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1998 AND 1999 

BY R.W. TILLMAN AND 

E.A. MEREWETHER

GULF OIL CORP. 

EMMETT-FEDERAL 1

660 FT. F.S.l., 660 FT. F.W.L.

Sec. 13, T. 52 N., R.94 W.

BIG HORN COUNTY, WYOMING

Very fine grained

Fine grained

Medium grained

Coarse grained
FEET

DATA BIG HORN BASIN 

DATA 43-6

NE 1/4, SE 1/4 Sec. 6, T.51N., R.91W.

BIG HORN COUNTY, WYOMING

Gamma 

Ray log Resistivity 

logs

Spontaneous

Potential log

0

100

200

300

400

500

Concealed

Bentonite 

Sandstone, v.f. gr.–pebbles at top

Siltstone, sandy–distributary-mouth bar

Sandstone, v.f. gr.–distributary-mouth bar

Sandstone, v.f. gr.–distributary-mouth bar

Shale, bentonitic–marine

FEET

Concealed

Sandstone, f. gr.–distributary-mouth bar

Sandstone, f. to m. gr.–few pebbles

Siltstone, shaley–lagoonal or marine

Shale, bentonitic–marine

Bentonite

Bentonite

Siltstone, bentonitic–marine

Bentonite

Sandstone, v.f. gr.–burrows

Bentonite

Sandstone, v.f. and f. gr.–pebbles at 437 ft–

 sharp lower contact

Siltstone, bentonitic–fossils at 445 ft, D13919

 Plesiacanthoceras wyomingense

 (fig. 2, Zone 14)

Sandstone, v.f. gr.–f. gr. at base–pebbles 

 at base and top

Siltstone, shaley

Fossils at 485 ft, D13920, Dunveganoceras pondi (fig. 2, Zone 15)

Frontier

Formation

(part)

Fossils at 390 to 400 ft, D13731, Acanthoceras amphibolum (fig. 2, Zone 13)

Fossils at 500 ft, D13921, Metoicoceras frontierense (fig. 2, Zone 16)

C

M100

F600

F500

F400

F300

Resistivity

logs

Spontaneous 

Potential log

Gamma 

Ray log FEET

DATA BIG HORN BASIN

DATA 43-6

NE 1/4, SE 1/4 Sec. 6, T.51N., R.91W.

BIG HORN COUNTY, WYOMING 

OUTCROP SECTION–POTATO RIDGE

NW 1/4, NW 1/4 Sec. 25, T.52N., R.92W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1998 AND 1999 BY 

R.W. TILLMAN AND 

E.A. MEREWETHER

Outcrops at Crooked Creek, Wyoming 39
40 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

D9278

D13730

D13729

D13652

0 1 2 MILES

Greybull

30’

108°02’30” 108°00’ 44°

30’

44°

15’

Figure 26. Fossil localities near Greybull in the Greybull South quadrangle (7.5-minute, 1966), Big Horn County, 

Wyoming. D numbers refer to USGS fossil collections (table 1).
Figure 27. Fossil localities near Greybull in the Devils Kitchen quadrangle (7.5-minute, 1960), Big Horn County, 

Wyoming. D numbers refer to USGS fossil collections (table 1).

D13916

D9754 D13915

D9752

D9753

108°00’ 108°57’30”

44°

30’

44°

32’

30”

0 1 2 MILES

Outcrops at Crooked Creek, Wyoming 41
42 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

D13921

D13920

D13918

D13919

D13731

R I D G E

44°

27’

30”’

R. 92 W.

T. 

52 

N.

T. 

51 

N.

R. 91 W.

0 1 2 MILES

Figure 28. Fossil localities near Potato Ridge in the Manderson NE quadrangle (7.5-minute, 1960), 

Big Horn County, Wyoming. D numbers refer to USGS fossil collections (table 1).
Figure 29. Columnar sections and lithologic descriptions of Frontier Formation in outcrops 

near North Emblem and Lovell Draw, eastern Bighorn Basin (fig. 1).

0

100

200

300

400

500

Sandstone, v.f. gr., silty–coarsens 

 upward–trough crossbeds

Sandstone, v.f. to m. gr.–

 pebbles–burrows 

Shale and interlaminated

 siltstone–burrows 

OUTCROP SECTION–NORTH EMBLEM

Secs. 3 and 4, T.54N., R.95W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER

FEET

600

700

Sandstone, v.f. gr.–wood 

 fragments and bentonite 

 near base

Interlaminated shale and siltstone, 

 overlain by siltstone, sandy

Shale–concretions near base of unit–

 fossiliferous concretions near top 

 of unit–D9277, Baculites sp.

Frontier

Formation

(part)

Sandstone, v.f. and f. gr.–large 

 brown concretions–burrows

 include Ophiomorpha

Interbedded shale and sandstone, 

 v.f. gr.–burrows

Sandstone, f. to m. gr.–pebbles– 

 siltstone and shale in middle 

Concealed 

Bentonite

Siltstone, bentonitic, and

 sandstone, v.f. gr.–burrows

Shale overlain by conglomerate

Sandstone, v.f. gr., silty, 

 concretionary–burrows, 

 Ophiomorpha

Sandstone, v.f. to f. gr.–

 conglomerate at top of unit–

 burrows

Shale and interlaminated

 siltstone 

Siltstone

Interlaminated sandstone, v.f. gr., 

 and less siltstone and shale–

 concretions and burrows 

Interlaminated siltstone and shale

 overlain by sandstone, v.f. to m. gr.–

 pebbles–burrows

Siltstone–burrows–overlain by 

 sandstone, f. to m. gr.–pebbles

Shale–concretions at top of 

 unit

Shale overlain by siltstone, 

 concretionary–fossiliferous near

 top of unit–D9276, Inoceramus ginterensis–(fig. 2, Zones 16-18)

750

Cody 

Shale

(part)

0

100

200

300

400

500

Bentonite

Sandstone, v.f. and f. gr.–fines 

 upward–interbedded shale–

 distal distributary-mouth bar

Sandstone, m. gr.–few pebbles 

Shale, silty, bentonitic–marine 

Shale, silty, bentonitic–pebbles 

 at base

Concealed

OUTCROP SECTION–LOVELL DRAW 

NW1/4, SE1/4 Sec. 10, T.53N., R.94W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1996 AND 1998 BY 

R.W. TILLMAN AND 

E.A. MEREWETHER

FEET

600

700

Shale, siliceous–marine

Sandstone, v.f. gr.–nearshore marine

Interbedded sandstone, f. gr., and 

 siltstone

Siltstone–fossiliferous concretion 

 at 666 ft–D13917, Dunveganoceras conditum (fig. 2, Zone 18)

Frontier

Formation

(part)

Concealed, probably silty shale–

 marine

Sandstone, v.f. and f. gr.–distributary–

 mouth bar

Interbedded shale and sandstone–

 delta-mouth bar

Sandstone, m. gr.–base is flooding 

 surface

Shale, silty–marine 

Bentonite

Shale, bentonitic

Shale, silty–marine 

Shale, bentonitic–marine

Sandstone, f. gr.–marine

Bentonite

Sandstone, f. gr.–marine

Sandstone, v.f. gr.–shoreface

Sandstone, m. gr.–c. gr. with pebbles

 in middle of unit–upper shoreface

?

Concealed

C

C

C

C

 

Cephalopods

? Approximate position of disconformity

Contact of formations

Fossils

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

Bivalves

Plant fragments

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

C concealed

sandstone

siltstone/mudstone

shale

bentonite

Very fine grained

Fine grained

Medium grained

Coarse grained

Outcrops at Crooked Creek, Wyoming 43
44 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 30. Columnar section and lithologic description of Frontier Formation in outcrops near Crooked Creek, 

eastern Bighorn Basin (fig. 1). Also shown are surface-to-subsurface correlations with a nearby borehole.

OUTCROP SECTION–CROOKED CREEK

N1/2 Sec. 5, T.57N., R.95W.

BIG HORN COUNTY, WYOMING

DESCRIBED IN 1974 BY 

E.A. MEREWETHER

FOREST OIL CORPORATION

STEVENS 1

1300 FT. FSL, 1300 FT. FWL

Sec. 27, T.57N., R.96W.

BIG HORN COUNTY, WYOMING

FEET

Caliper 

log

Gamma 

Ray log

Density 

log

Neutron 

log

Cephalopods

Fossils

Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

Bivalves

Probable correlation of strata

Contact of formations

0

100

200

300

400

500

Sandstone, v.f. gr., silty–

 burrows 

Shale, silty, bentonitic near 

 base and top–burrows 

FEET

600

700

Interlaminated shale, silty, 

 and siltstone–burrows

Shale, silty–fossiliferous 

 septarian concretions at 

 828-833 ft, D9273, 

 Cremnoceramus cf. C. deformis

 (fig. 2, Zones 39-40)

Frontier

Formation

Shale, silty 

Concealed 

Bentonite and shale–burrows

Siltstone, sandy, and 

 sandstone, near top of 

 unit–fossiliferous 

 concretions at 618 ft, 

 D9272, Metoicoceras 

 muelleri (fig. 2, Zone 17)

Sandstone, v.f. gr., silty; v.f. to 

 f. gr. and concretionary near 

 top of unit–burrows

Siltstone–burrows 

Siltstone, clayey–burrows

Interlaminated shale, silty, 

 and siltstone–mainly siltstone

 at top–burrows

Interbedded siltstone, partly 

 sandy, and less shale–includes

 small concretions with pebbles. 

800

Cody 

Shale

(part)

Bentonite and siltstone

Interlaminated siltstone, sandy, 

 and less shale–burrows

Interbedded shale and less 

 siltstone–burrows

Bentonite

Bentonite

?

 

? Approximate position of disconformity

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

C concealed

sandstone

siltstone/mudstone

shale

bentonite

F500

F600?

M100?

C

Very fine grained

Fine grained

Medium grained

Coarse grained
Interpretations and Conclusions 45

the subsurface on geophysical logs of boreholes in the area. 

The Frontier sandstones in these outcrops are mainly very 

fine grained, and accumulated mostly in nearshore-marine 

environments. 

Many beds contain burrows, and a siltstone in the upper 

part of the formation encloses the ammonite Metoicoceras

muelleri (D9272) of early late Cenomanian age (fig. 2, Zone 

17). Concretions in the overlying Cody, 65 to 70 ft above the 

Frontier, contain the bivalve Cremnoceramus cf. C. deformis (D9273) of early Coniacian age (fig. 2, Zones 39 or 40). 

Strata between a fossiliferous siltstone in the Frontier and the 

fossiliferous shale in the Cody in this area enclose a disconformity that probably represents latest Cenomanian and early and 

middle Turonian time. 

Outcrops near Warren, Montana

About 25 mi northwest of Lovell near Warren, Mont. 

(figs. 1, 31), outcrops of the Frontier and basal strata of the 

overlying Cody were described by Merewether and others (1975). Sandstones in the Frontier are mostly very fine 

grained; they rarely include medium- or coarse-grained and 

pebble-bearing beds. The strata in the Frontier commonly display burrows and contain fossils near the middle and at the top 

of the formation. They were deposited probably in nearshoremarine environments. Formation thickness is about 620 ft. 

Fossils collected from near the middle of the Frontier 

include the ammonite Metoicoceras defordi (D9274) of early 

late Cenomanian age (fig. 2, Zone 17). Fossil bivalves from 

a sandstone at the top of the Frontier consist of Cremnoceramus cf. C. deformis (D9275; fig. 2, Zones 39–40) of the lower 

Coniacian. The Cenomanian strata in these outcrops are disconformably overlain by beds of Turonian age, as at outcrops 

to the southeast. The lacuna at the disconformable contact 

spans most of the upper Cenomanian, the lower and middle 

Turonian, and most of the upper Turonian.

Interpretations and Conclusions 

Outcropping beds of the Frontier Formation near the margins of the Bighorn Basin (fig. 1) are mainly siliciclastic and 

bentonitic and most were deposited in marine and brackishwater environments. However, outcropping strata along 

the northwest flank of the basin near Deep Lake and Cody 

include carbonaceous beds that accumulated in continental 

environments. 

The lower and thickest part of the Frontier Formation in 

the Bighorn Basin, including the nonmarine beds, is herein 

assigned a Cenomanian age. Those strata are approximately 

correlative with outcropping Cenomanian beds at Conant 

Creek near Riverton (fig. 3; Merewether and Cobban, 2007) 

and with the Belle Fourche Member of the Frontier near 

Kaycee and Casper (figs. 3, 4; Merewether and others, 1976, 

2007). The Cenomanian strata are 480 ft thick at Cody and 

680–690 ft thick near Ten Sleep. Detailed thicknesses of the 

Frontier in the Bighorn Basin have been presented by Kirschbaum (written commun., 2008).

Molluscan fossils of Cenomanian age have been collected 

from outcrops at Pitchfork, Black Mountain, and near Shanty 

Ridge, and from other outcrops on the east flank of the basin 

(fig. 1, table 1). The oldest Cenomanian fossils (fig. 2, Zone 4) 

were collected in the Mowry Shale at Pitchfork. The youngest 

(fig. 2, Zone 18) were collected in the Frontier at Lovell Draw; 

outcrops northwest of that locality might also be of that age. 

However, to the southeast, the youngest Cenomanian outcrops 

are somewhat older—Zone 17 at Joe Emge Creek, Zone 16 at 

Potato Ridge, possibly Zone 15 near Shanty Ridge, and Zone 

14 at Black Mountain. Along the east flank of the basin, the 

top of the Cenomanian strata apparently is an irregular truncated surface that increases in age toward the south. 

In north-central Wyoming, east of the Bighorn Basin 

(fig. 3), the age of the top of the Belle Fourche Member of 

the Frontier Formation increases southward, ranging from 

Zones 21 or 22 of the late late Cenomanian near Buffalo to 

Zones 16 or 17 of the early late Cenomanian in the vicinity of 

Casper. The irregular but progressive southward truncation 

of Cenomanian strata between Buffalo and Casper and on the 

east flank of the Bighorn Basin indicates differential uplift 

and erosion of parts of that region during the early Turonian. 

Within the Bighorn Basin, vertical movement evidently was 

largest at the south end, near Black Mountain, but uplift apparently was most pronounced in the region of Mount Everts 

and Upper Slide Lake in northwestern Wyoming, west of the 

basin (fig. 3). At those localities, strata overlying the Mowry 

Shale and underlying middle Turonian beds are interpreted 

to be middle Cenomanian or older and laterally equivalent 

to a lower part of the Cenomanian sequences in the Bighorn 

Basin. These strata at Mount Everts and Upper Slide Lake are 

also appreciably thinner (180–260 ft thick) than those in the 

Bighorn Basin (fig. 32). The early Turonian uplift might have 

been accompanied by a eustatic sea-level fall (Haq and others, 

1987). 

Deposition on the eroded surfaces of Cenomanian 

sequences in the Bighorn Basin and elsewhere in northern 

Wyoming (figs. 3, 4) evidently began with a marine transgression during early middle Turonian time (fig. 2, Zones 28–30). 

The disconformable contact between Cenomanian and middle 

Turonian strata at Cody (figs. 1, 6), near the western margin of 

the basin, could represent most of the late Cenomanian and all 

of early Turonian time. Middle Turonian strata were identified 

in the basin only at Cody, where they are 115 ft thick, and at 

Elk Butte (fig. 2, Zones 29 and 30), although they probably 

extend north at least to Deep Lake and south to Pitchfork. 

They are correlative with middle Turonian beds in the Frontier 

south of the Bighorn Basin, at Conant Creek (Merewether and 

Cobban, 2007), and with an upper part of the middle Turonian 

Emigrant Gap Member of the Frontier southeast of the basin 

near Casper (fig. 4). Slightly older middle Turonian fossils, 

representing Zone 28 (fig. 2), were collected from outcrops 
46 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

Figure 31. Columnar section and lithologic description of Frontier Formation in outcrops near 

Warren, eastern Bighorn Basin (fig. 1).

0

100

200

300

400

500

Sandstone, v.f. gr., silty; f. to m. gr. 

 at top of unit–pebbles in middle 

 and at top of unit–burrows 

Shale, silty, bentonitic 

OUTCROP SECTION–WARREN

Sec. 25, T.8S., R.24E.

CARBON COUNTY, MONTANA

DESCRIBED IN 1974 BY 

E.A. MEREWETHER

FEET

600

700

 Cremnoceramus cf. C. deformis 

 (fig. 2, Zones 39-40)

Frontier

Formation

(part)

Shale–silty in upper part–

 fossiliferous concretions–D9274, 

Concealed 

Bentonite

 Metoicoceras defordi

 (fig. 2, Zone 17)

Sandstone, f. to v.c. gr.–

 pebbles at base of unit

Interbedded siltstone, sandy, 

 and less shale and sandstone–

 burrows 

Siltstone, poorly exposed–

 septarian concretions near top 

 of unit

Interbedded sandstone, v.f. gr., 

 silty, and less siltstone and shale–

 pebbles at base of unit–

 ironstone concretions

Cody 

Shale

(part)

Interbedded shale and less 

 siltstone–scattered ironstone 

 lenses–burrows

Interbedded siltstone, sandy, 

 and less shale–sandstone

 near base

Concealed–possibly siltstone

 and bentonite 

Siltstone–clayey at base–sandy

 at top–concretions in upper 

 part of unit

Shale, silty, at base; siltstone, sandy, 

 at top 

Sandstone, v.f. gr., silty–concretions

 at 497 ft–burrows 

Shale–ironstone concretions 

Shale–concretions–gravel at 

 top of unit 

Concealed–probably shale 

Shale 

Concealed–concretions 

Siltstone, sandy, at base of unit–

 sandstone, v.f. gr., silty, at top–

 burrows–fossiliferous concretions 

 at 614 ft–D9275, 

Shale–concretions 

?

C

C

C

C

Bivalves

Cephalopods

? Approximate position of disconformity

Fossils Zones refer to numbered fossil zones shown in figure 2. 

D numbers refer to USGS fossil collections (table 1). 

EXPLANATION

Contact of formations

v.f. gr. 

f. gr. 

m. gr. 

c. gr. 

C concealed

sandstone

siltstone/mudstone

shale

bentonite

Very fine grained

Fine grained

Medium grained

Coarse grained
Interpretations and Conclusions 47

Cody x

Mount Everts

A

Ten Sleep Kaycee

A‘ x x x

U

U

U U

U

U

U

U

Lower middle Turonian strata

Lower middle 

Turonian strata

Lower Coniacian and 

upper Turonian strata

Lower and middle 

Cenomanian strata

Lower and middle 

Cenomanian strata

Horizon of 

(fig. 2, Zone 40)

Horizon of

(fig. 2, Zone 14)

? Upper Cenomanian strata 

U

0

500 FEET

vertical scale

Approximate extent of the Bighorn Basin

Cremnoceramus

deformis

dobrogensis

Plesiacanthoceras

wyomingense

Figure 32. Stratigraphic cross section of Frontier Formation and lower beds of the overlying Cody Shale in 

northwestern Wyoming (line A–A’, fig. 3). Dashed line marks contact with the underlying Mowry Shale. Dotted 

lines are horizons of two molluscan index-fossils. U indicates disconformable contact.

east of the basin in the vicinity of Kaycee and from the lower 

part of the Emigrant Gap Member near Casper, as well as from 

outcrops at Mount Everts and Upper Slide Lake in northwestern Wyoming (fig. 3). 

Early middle Turonian fossils of Zones 28–31 (fig. 2) are 

absent in part of north-central Wyoming (fig. 3; Merewether 

and Cobban, 1986, their figs. 11 and 12), which indicates an 

uplift in that region and possibly another eustatic fall (Haq and 

others, 1987) during the late middle or early late Turonian. 

Similarly, in east-central Wyoming and in an adjacent area to 

the south, Cenomanian strata are disconformably overlain by 

upper middle and lower upper Turonian strata that probably 

reflect uplift and erosion in that region (Weimer, 1984) during 

the middle Turonian. 

A conformable sequence of upper middle Turonian, upper 

Turonian, and lower Coniacian strata disconformably overlies beds as old as middle Cenomanian and as young as early 

middle Turonian in much of northern Wyoming. Outcropping 

Cenomanian sequences in southern and eastern areas of the 

Bighorn Basin are disconformably overlain by strata containing late Turonian fossils that represent Zone 34 between Thermopolis and Black Mountain, Zone 37 on Black Mountain, 

Zone 34 near Shanty Ridge, Zone 37 near Ten Sleep, and Zone 

36 at the Bonanza oil field. These Turonian beds are 140–170 

ft thick near Ten Sleep and as much as 65 ft thick at Black 

Mountain. 

At Black Mountain, this mid-Frontier disconformity 

spans all of the late Cenomanian and the early and middle 

Turonian. At the Bonanza oil field, it represents most of the 

late Cenomanian, the early and middle Turonian, and an early 

part of the late Turonian. The lacuna at the disconformity 

between the Cenomanian and upper Turonian sequences near 

the south and east margins of the Bighorn Basin represents 

two periods of erosion that correspond to two lacuna elsewhere in the Frontier, notably at the Conant Creek and Emigrant Gap localities in central Wyoming (fig. 4). 

 Middle Turonian strata in the upper part of the Frontier, which are present at Cody and Elk Butte (fig. 1) and 

probably north and south of those localities, are disconformably overlain by beds of early Coniacian age (fig. 2, Zones 

39–41). At most outcrops near the west edge of the basin, the 

uppermost Frontier sandstone is early Coniacian in age, but 

at outcrops near the east margin, sandstone of Coniacian age 

was identified only near Warren (fig. 1). Middle Turonian beds 
48 Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the Bighorn Basin, Wyoming and Montana

and overlying lower Coniacian beds at Cody are comparable 

in age to strata at Conant Creek in central Wyoming and at 

Mount Everts and Upper Slide Lake in northwestern Wyoming 

(fig. 4). Correlative beds near Casper are within the Emigrant 

Gap Member of the Frontier and a lower part of the Cody 

Shale. At this disconformity near Cody, the lacuna apparently includes the late middle Turonian and the late Turonian 

indicating a single erosional episode. The tectonic history of 

northern Wyoming during the early Late Cretaceous, which 

contains middle Turonian strata disconformably overlain by 

Coniacian beds, was described and interpreted by White and 

others (2002).

At outcrops of the uppermost Frontier Formation and the 

basal Cody Shale near the east and south edges of the Bighorn 

Basin, beds of late Turonian age (fig. 2, Zones 37 and 38) 

appear to be conformably overlain by beds of early Coniacian age (fig. 2, Zones 39 and 40); if there is a disconformity, 

it is insignificant. Those beds are correlative with conformable strata of the Wall Creek Member of the Frontier and a 

lower part of the overlying Cody Shale near Casper. Because 

they are probably conformable, an earlier interpretation by 

Merewether and others (2007, their figs. 20 and 21) that a 

lacuna exists at the base of lower Coniacian beds in nearly all 

of northern Wyoming seemingly is incorrect. 

The collections of fossils from northern Wyoming that 

were compiled and studied for this report are more numerous than those used for the publication by Merewether and 

others (2007). Consequently, a few of the sedimentologic and 

tectonic interpretations in that publication (particularly those 

illustrated by the maps in their figs. 20 and 21) should be modified. The disconformable contact of the Turonian and Coniacian strata in north-central Wyoming indicated by those maps 

is not confirmed by data in the present report. That contact 

along the eastern flank of the Bighorn Basin and in adjoining 

areas south and east of the basin apparently is conformable. 

The biostratigraphy of the Frontier Formation and the 

lower part of the Cody Shale in the Bighorn Basin and surrounding areas is evidence of the tectonic history of the region 

(White and others, 2002) and of possible eustatic events (Haq 

and others, 1987) during the Turonian (fig. 32). Truncated 

sequences of Cenomanian strata overlain by middle Turonian 

beds on the west flank of the basin and elsewhere in northern Wyoming reflect a withdrawal of the sea and associated 

erosion that were caused partly by regional uplift of most of 

northern Wyoming during the early Turonian (Merewether 

and Cobban, 1986). The amount of this uplift and erosion 

apparently was greatest in northwestern Wyoming, west of 

the Bighorn Basin. Later in the early Turonian, subsidence 

and probably a eustatic rise allowed a marine transgression 

and the deposition of lower middle Turonian (fig. 2, Zones 

28–30) strata throughout the basin and elsewhere in northern 

Wyoming. 

The absence of middle Turonian strata near the south 

and east margins of the Bighorn Basin, as well as in adjoining areas to the east (fig. 3; see also Merewether and Cobban, 

1986, their figs. 11 and 12), indicates uplift and the erosion of 

those strata possibly in late middle Turonian time. Thereafter, 

upper Turonian strata accumulated on Cenomanian strata in 

the southern and eastern parts of the basin during a marine 

transgression. These upper Turonian strata are overlain with 

apparent conformity by lower Coniacian beds along the southern and eastern flanks of the basin and elsewhere in northern 

Wyoming. 

However, lower Coniacian beds were deposited on 

middle Turonian (EGM) beds near the west edge of the basin 

and farther to the west. The ages and thicknesses of the middle 

Turonian, upper Turonian, and lower Coniacian strata in 

northwestern Wyoming are interpreted to indicate that higher 

elevations existed on the west side of the basin and west of 

the basin during a late Turonian and early Coniacian marine 

transgression. 

In east-central Wyoming (fig. 3) and in an adjoining area 

to the south, lower Coniacian beds disconformably overlie 

either upper middle Turonian or middle upper Turonian beds. 

This area was uplifted and eroded for a second time (Weimer, 

1984), probably in the late Turonian. 

Summary

 The areal and temporal distributions of molluscan 

index fossils of Cenomanian, Turonian, and Coniacian 

ages in outcrops of the Bighorn Basin and adjoining areas 

provide evidence of several depositional and tectonic events. 

Significant lacunae are proposed for two horizons, at the 

contact of Cenomanian and lower middle Turonian strata and 

at the contact of lower middle Turonian with upper Turonian 

or lower Coniacian strata. The extensive and variable lacuna 

between Cenomanian and middle Turonian strata indicates 

uplift and erosion centered in northwestern Wyoming during 

the early Turonian. In that region and elsewhere in northern 

Wyoming, the erosion was followed by deposition during a 

marine transgression in the early middle Turonian. Thereafter, 

the lacunae between Cenomanian and upper Turonian strata as 

well as between middle Turonian and lower Coniacian strata 

in the Bighorn Basin indicate that topographic relief, resulting 

from differential uplift, induced erosion in the region during 

the latest middle Turonian. This uplift and the higher terrain 

appears to have been centered in north-central Wyoming, east 

of the basin, and was followed by a marine transgression in 

the late Turonian. In north-central and northwestern Wyoming, 

marine strata transgressed northwestward during the late 

Turonian and early Coniacian, onlapping a truncated and 

irregular surface composed of Cenomanian beds on the east 

flank of the Bighorn Basin and of middle Turonian beds on the 

west flank of the basin.

In east-central Wyoming and an adjoining region to the 

south, upper middle Turonian and lower upper Turonian strata 

are disconformably overlain by lower and middle Coniacian 

beds. That region apparently rose and was eroded during the 

latest Turonian. 
References Cited 49

Acknowledgments

In 2007, a regional study of the Frontier Formation by 

M.A. Kirschbaum of the U.S. Geological Survey (USGS) provided an opportunity for a comprehensive investigation of the 

biostratigraphy of the formation and associated strata in the 

Bighorn Basin. Molluscan fossils collected in the basin during 

1957 and 1958 by George H. Horn, Robert L. Rioux, and John 

R. Dyni of the USGS are named and included in table 1 of this 

report. Assistance with descriptions of outcrops in the region 

was provided by William Stanton in 1974 and R. Bottjer in 

1999. In 2008, S.M. Condon prepared the map showing outcrops of the Frontier Formation in the Bighorn Basin as well 

as the locations of outcrops described in this report (fig. 1). 

Comprehensive reviews of the manuscript were provided in 

2009 by R.B. O’Sullivan, W.R. Keefer, D.J. Nichols, and M.S. 

Ellis of the USGS. The administrative and technical support 

provided by the preceding geologists were important contributions and are gratefully acknowledged. 

References Cited

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McKinney, K.C., 2006, A USGS zonal table for the Upper 

Cretaceous middle Cenomanian-Maastrichtian of the 

Western Interior of the United States based on ammonites, 

inoceramids, and radiometric ages: U.S. Geological Survey 

Open-File Report 2006-1250, 45 p. 

Haq, B.U., Hardenbol, J., and Vail, P.R., 1987, Chronology of 

fluctuating sea levels since the Triassic: Science, v. 235, 

p. 1156-1167. 

Kirschbaum, M.A., Merewether, E.A., and Condon, S.M., 

2009, Stratigraphy and age of the Frontier Formation and 

associated rocks, central and southern Bighorn Basin, 

Wyoming—surface to subsurface correlation: The Mountain 

Geologist, v. 46, no. 4, p. 125-147. 

Merewether, E.A., 1996, Stratigraphy and tectonic implications of Upper Cretaceous rocks in the Powder River Basin, 

northeastern Wyoming and southeastern Montana: U.S. 

Geological Survey Bulletin 1917-T, 92 p. 

Merewether, E.A., and Cobban, W.A., 1986, Biostratigraphic 

units and tectonism in the mid-Cretaceous foreland of 

Wyoming, Colorado, and adjoining areas, in Peterson, 

J.A., ed., Paleotectonics and sedimentation in the Rocky 

Mountain region, U.S.: American Association of Petroleum 

Geologists Memoir 41, p. 443–468. 

Merewether, E.A., and Cobban, W.A., 2007, Outcrop descriptions and fossils from the Upper Cretaceous Frontier 

Formation, Wind River Basin and adjacent areas, Wyoming, 

in Petroleum systems and geologic assessment of oil and 

gas resources in the Wind River Basin Province, Wyoming: 

U.S. Geological Survey Digital Data Series DDS-69-J, 

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Merewether, E.A., Cobban, W.A., and Obradovich, J.D., 2007, 

Regional disconformities in Turonian and Coniacian (Upper 

Cretaceous) strata in Colorado, Wyoming, and adjoining 

states—Biochronological evidence: Rocky Mountain 

Geology, v. 42, no. 2, p. 95–122. 

Merewether, E.A., Cobban, W.A., and Ryder, R.T., 1975, 

Lower Upper Cretaceous strata, Bighorn Basin, Wyoming 

and Montana, in Wyoming Geological Association Guidebook, 27th Annual Field Conference, Bighorn Basin: 

p. 73–84. 

Merewether, E.A., Cobban, W.A., and Spencer, C.W., 1976, 

The Upper Cretaceous Frontier Formation in the KayceeTisdale Mountain area, Johnson County, Wyoming, in

Wyoming Geological Association Guidebook, 1976, 28th 

Annual Field Conference: p. 33–44. 

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 Field Conference Guidebook, p. 31–42. 

Siemers, C.T., 1975, Paleoenvironmental analysis of the Upper 

Cretaceous Frontier Formation, northwestern Bighorn 

Basin, Wyoming, in Exum, F.A., and George, G.R., eds., 

Geology and mineral resources of the Bighorn Basin: 

Wyoming Geological Association 27th Field Conference 

Guidebook, p. 85–100. 

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migration in the Cretaceous Western Interior basin of the 

central United States: Basin Research, v. 14, p. 43–54. 
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Box 25046, Mail Stop 939

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(303) 236-1647

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Merewether and others—Outcrops, Fossils, Geophysical Logs, and Tectonic Interpretations in the—Scientific Investigations Report 2009–5256

Bighorn Basin, WY

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