Stratigraphy of 

Mid-Cretaceous Formations at

GEOLOGICAL SURVEY P RO F E S S I ON A t P A P E R 1180-A
Stratigraphy of

Mid-Cretaceous Formations at 

Drilling Sites in 

Weston and Johnson Counties, 

Northeastern Wyoming

By E. A. MEREWETHER

PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKSNORTHEASTERN WYOMING

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1186-A

A description and comparison of the 

dissimilar formations of early 

Late Cretaceous age in core holes and 

outcrops on the eastern and western 

flanks of the Powder River Basin

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Merewether, Edward Alien, 1930-

Stratigraphy of mid-Cretaceous formations at drilling sites in Weston and Johnson Counties, northeastern Wyoming.

(Paleontology and stratigraphy of mid-Cretaceous rocks, northeastern Wyoming)

(Geological Survey Professional Paper 1186-A)

Bibliography: p. 25

Supt. of Docs, no.: I 19.16:1186-A

1. Geology, Stratigraphic Cretaceous. 2. Geology Wyoming Weston Co. 3. Geology Wyoming Johnson County.

I. Title II. Series. III. Series: United States Geological Survey Professional Paper 1186-A

QE687.M47 551.7'7'0978714 80-607109

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402
CONTENTS

Page

./\DS tract ----- — — ----.--.-—-——.., — ...... — ... — ........... —— .......... — .......... — .................. — ......................... j^

j.ntr Ouiiction TM-TMTM««— — ««-•——. — ............... — ................. — ._....«..................».._.......__..-...........»........ j^

o ^Itmf wi «. uigi lornnVixr ct^iijf — _ « _ .......... _ —.........— . ......... . .... .—.....— . ......... .... ...... ...... ..... .......... ... ......... . .. ^9

Belle Fourche Shale _______________________________________ 5

Greenhorn Formation -------------------»-----—— ——-«-——-———— -—--—— —-—«-»— -—»«—— 6

lp lt _ ____________ — — — — _ ........ .. - — . — .......... —— ..... ............ — ....... __ .......... - .... 7j

Pool Creek Member ——————————————————————————————————————— 10

Turner Sandy Member —————————————————————————————————— 10

od£>e Jzsreaj£s iviemoer ••—•——— — —————— ....-.-. — ..... — . — ......... — ......._................._....... — ... j^ ^

rkTTYin'l'irkTi Ul Hid L1U11 ................. " ....... ' .... *._...—— . - .......... .. ......... . ... ....... ...... ....... ...... .... 1 J^QQ

Belle Fourche Member ««—«——«———-————-———«-———«««««»—-——"—-——» 15

______ __ _ •--———— —— - - ........ _ ........ ___ ..... ____ ..... ____ ...» -1 j_ Q^y

Sage Breaks Member —————————————————————————————————— 19

Depositional environments and geologic history .————————————-——-—————— 20

ILLUSTRATIONS

Page

FIGURE 1. Index map showing the Powder River Basin and locations of outcrop sections, boreholes and selected oil and gas fields - 2

2. Correlation chart of the lower Upper Cretaceous formations at selected localities in northeastern Wyoming ——————— 3

3. Outcrop section of the lower Upper Cretaceous formations near Osage ———-————————-———————————————— 4

4. Lithologic and geophysical logs of lower Upper Cretaceous formations near Osage ————————————————............. g

5. Drawing showing crossbedding in the Turner Sandy Member near Osage —————————————————————————— 10

6. Photomicrograph showing authigenic crystal of feldspar on a corroded crystal of quartz, from sandstone in the Turner

	oanciy iviemoer ----------«-«—««...———.........................—..—.......—........-..........-...................................._.._......_.._..*.•............. j^^

7. Photomicrograph showing corroded grains of quartz, chert, and feldspar in a thin section of sandstone from the Turner

oanoy iviemoer -----—-•—....———...................—.........—.............—.......—...................—.......—._.._........................_..............._....*. j^^

8. Stratigraphic sections of the lower Upper Cretaceous Frontier Formation near Kaycee ————————————————---'— 14

9. Photomicrograph showing corroded grains of quartz, chert, and feldspar in a thin section of sandstone of the Belle

i7 ourcne iviemoer -----—...——.—.........._...._........_..........._....._.........—........._......_....._..........................._...........................*...... j^ ^

10. Photomicrograph showing angular to subrounded grains of quartz, chert, and feldspar in a thin section of sandstone of

fhd ullC Wall VV All V^/l Proolr CCXV 1V1C111L/C21 Momhor————————— ——„————„—. •-—— ____ —— - ___ 

... 

_________________________________________ 

. 

1Q_L ^

11. Map showing approximate thickness of a representative unit of sandstone in the Belle Fourche Member of the Frontier

s. ormacion --------—---—-.—-———-«.-————--—--———-.-——-————.._...._...._..__........................__.—....._.._.......__......................».............. ^ j^

12. Map showing approximate thickness of a representative unit of sandstone in the Wall Creek Member of the Frontier

Formation and the Turner Sandy Member of the Carlile Shale ——————————————————————————— 23

13. Outcrop sections and depositional environments of lower Upper Cretaceous formations in Johnson and Weston

v~/oun vies -------——«—«»«—-.—-—...........—.—.....................—....................—..........—.............................—.........—.....__.*...._......... ^4

TABLES

TABLE 1. Organic composition of lower Upper Cretaceous shale at boreholes 1 and 2, 

Powder River Basin, as determined by thermal-evolution analysis, 

combustion, and reflectance ——————————————————————————

in

PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKSNORTHEASTERN WYOMING

STRATIGRAPHY OF MID-CRETACEOUS FORMATIONS 

AT DRILLING SITES IN WESTON AND JOHNSON COUNTIES,

NORTHEASTERN WYOMING

By E. A. MEREWETHER

ABSTRACT

The sedimentary rocks of early Late Cretaceous age in Weston 

County, Wyo., on the east flank of the Powder River Basin, are 

assigned, in ascending order, to the Belle Fourche Shale, Greenhorn 

Formation, and Carlile Shale. In Johnson County, on the west flank 

of the basin, the lower Upper Cretaceous strata are included in the 

Frontier Formation and the overlying Cody Shale. The Frontier 

Formation and some of the laterally equivalent strata in the Rocky 

Mountain region contain major resources of oil and gas. These rocks 

also include commercial deposits of bentonite.

Outcrop sections, borehole logs, and core studies of the lower 

Upper Cretaceous rocks near Osage, in Weston County, and 

Kaycee, in Johnson County, supplement comparative studies of the 

fossils in the formations. Fossils of Cenomanian, Turonian, and 

Coniacian Age are abundant at these localities and form sequences 

of species which can be used for the zonation and correlation of 

strata throughout the region.

The Belle Fourche Shale near Osage is about 115 m (meters) thick 

and consists mainly of noncalcareous shale, which was deposited in 

offshore-marine environments during Cenomanian time. These 

strata are overlain by calcareous shale and limestone of the 

Greenhorn Formation. In this area, the Greenhorn is about 85 m 

thick and accumulated in offshore, open-marine environments 

during the Cenomanian and early Turonian. The Carlile Shale 

overlies the Greenhorn and is composed of, from oldest to youngest, 

the Pool Creek Member, Turner Sandy Member, and Sage Breaks 

Member. In boreholes, the Pool Creek Member is about 23 m thick 

and consists largely of shale. The member was deposited in offshoremarine environments in Turonian time. These rocks are disconformably overlain by the Turner Sandy Member, a sequence about 

50 m thick of interstratified shale, siltstone, and sandstone. The 

Turner accumulated during the Turonian in several shallow-marine 

environments. Conformably overlying the Turner is the slightly 

calcareous shale of the Sage Breaks Member, which is about 91 m 

thick. The Sage Breaks was deposited mostly during Coniacian time 

in offshore-marine environments.

In Johnson County, the Frontier Formation consists of the Belle 

Fourche Member and the overlying Wall Creek Member, and is 

overlain by the Sage Breaks Member of the Cody Shale. Near 

Kaycee, the Belle Fourche Member is about 225 m thick and is composed mostly of interstratified shale, siltstone, and sandstone. 

These strata are mainly of Cenomanian age and were deposited 

largely in shallow-marine environments. In this area, the Belle

Fourche Member is disconformably overlain by the Wall Creek 

Member, which is about 30 m thick and grades from interlaminated 

shale and siltstone at the base of the member to sandstone at the 

top. The Wall Creek accumulated during Turonian time in shallowmarine environments. These beds are overlain by the Sage Breaks 

Member of the Cody. Near Kaycee, the Sage Breaks is about 65 m 

thick and consists mainly of shale which was deposited in offshoremarine environments during Turonian and Coniacian time.

Lower Upper Cretaceous formations on the east side of the 

Powder River Basin can be compared with strata of the same age on 

the west side of the basin. The Belle Fourche Shale at Osage is 

represented near Kaycee by most of the Belle Fourche Member of 

the Frontier. The Greenhorn at Osage contrasts with beds of similar 

age in the Belle Fourche at Kaycee. An upper part of the Greenhorn 

Formation, the Pool Creek Member of the Carlile Shale, and the 

basal beds of the Turner Sandy Member of the Carlile, in Weston 

County, are represented by a disconformity at the base of the Wall 

Creek Member of the Frontier in southern Johnson County. A 

middle part of the Turner in the vicinity of Osage is the same age as 

the Wall Creek Member near Kaycee. A sequence of beds in the upper part of the Turner and in the overlying Sage Breaks in Weston 

County is the same age as most of the Sage Breaks Member of the 

Cody in southern Johnson County.

INTRODUCTION

This report describes the lower Upper Cretaceous 

strata at boreholes and nearby outcrops in Johnson 

and Weston Counties, Wyo. (fig. 1) and will supplement the paleontologic studies of W. A. Cobban (mollusca), W. G. E. Caldwell and B. R. North (foraminifera), F. E. May and B. L. Whitney (dinoflagellates), 

and D. J. Nichols (palynomorphs). Outcrops, cores, and 

geophysical logs of the formations were investigated 

during 1975-79. The cores and logs were obtained from 

boreholes in NE1/4SW1/4 sec. 6, T. 42 N., R. 81 W., 

near Kaycee (Kaycee 7 1/2 minute quadrangle) in Johnson County, and in SW1/4NW1/4, sec. 30, T. 46 N., R. 

63 W., near Osage (Osage 71/2 minute quadrangle) in 

Weston County (fig. 1). Molluscan fossils from outcrops
PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

109° 108° 107° 106° 105° 104°

45

44

43°

42°

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EXPLANATION

D

x Outcrop section

2 

• Borehole

®^ Oil and gas field,

production from lower 

Upper Cretaceous rocks

— — Approximate boundary of 

Powder River Basin

FIGURE 1.— Index map of Wyoming, showing the Powder River 

Basin and locations of outcrop sections A, B, and C, boreholes 1 

and 2, and selected oil and gas fields.

near the drilling sites are listed in the following pages 

and are represented by some of the species named on 

figure 2. Fossils obtained from the core are described 

in other chapters of this volume by the paleontologists 

named above.

The core of the lower Upper Cretaceous formations 

was supplied mainly by J. D. Tucker and S. J. Grant. 

Geophysical logs of the boreholes were furnished by 

R. A. McCullough. R. W. Brown and B. M. Madsen 

performed X-ray analyses of samples from the core. 

G. E. Claypool, V. E. Shaw, J. P. Baysinger, Nancy 

Conklin, and T. L. Yager determined the organic composition of shale samples. The grain-size distribution 

of samples of sandstone was provided by M. B. Sawyer 

and E. T. Cavanaugh, using an image-analyzing computer. Thin sections of sandstone were prepared by 

K. L. Gardner and examined by J. C. Webb. J. M. Nishi, 

M. J. Pinel, and E. T. Cavanaugh photographed 

samples of sandstone with a scanning electron 

microscope. Molluscan fossils from the region were 

prepared for study by R. E. Burkholder and identified 

by W. A. Cobban and N. F. Sohl. These contributions, 

from employees of the U.S. Geological Survey, are 

gratefully acknowledged.

In this investigation, samples from cores of shale and 

sandstone, and the clay fraction of the sandstone, were 

analyzed for constituent minerals by X-ray diffraction. 

The organic composition and thermal maturity of some 

of the rocks were determined by several methods 

(Merewether and Claypool, 1980). Organic-carbon 

content was obtained from the difference between total 

carbon, measured by combustion, and carbonate carbon, measured by acidification of a separate part of the 

sample. Hydrocarbon content and related characteristics were determined by thermal-evolution analysis 

(Claypool and Reed, 1976). The vitrinite reflectance 

reported herein is a mode of the reflectance values, in 

percent, for populations of vitrinite particles in oil.

The nomenclature used in this report for the size and 

sorting of the grains in sandstone and siltstone was 

described by Folk (1974). Micrographs, taken with a 

scanning electron microscope, and thin sections were 

used mainly to study the mineralogy of these rocks. 

Statistical descriptions of the grain sizes in samples of 

sandstone were obtained from an image-analyzing 

computer (Sawyer, 1977). For each sample, the 

diameters of 300 disaggregated grains were measured 

electronically, and the volume percentages of grains, in 

categories defined by grain diameter, were calculated. 

The computer program assumed that all grains are 

spherical and did not consider the variation in density 

of the grains in most rocks. As a consequence, data 

from the computer are not absolutely comparable with 

results from conventional procedures of sieving and 

weighing grains unless the grains are spherical and 

monomineralic.

STRATIGRAPHY

The strata of early Late Cretaceous age in Wyoming 

vary in lithology, thickness, and depositional environment, and commonly enclose one or more unconformities. They consist mainly of shale, siltstone, sandstone, 

conglomerate, and bentonite, but they locally include 

coal in western Wyoming and limestone in eastern 

Wyoming. At outcrops in Lincoln County, in southwestern Wyoming, the lower Upper Cretaceous Frontier Formation is about 670-790 m thick (Veatch, 

1907, p. 65-69) and was deposited in marine and non-
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING

0) 

O) 

CC

55

San

Coniacian

Turonian

Cenomanian

Informal 

substage

o 

tonian | |

Upper

Middle

Lower

o

Upper

Middle

Lower

Upper

Middle 

?

Lower

Western Interior molluscan fossil zones; 

numbers represent zones identified at 

outcrop sections (figs. 2, 9, and 1 4)

25 Clioscaphites saxitonianus 

24 Scaphites depressus

23 Scaphites ventricosus

22 Inoceramus deformis

21 Inoceramus erectus 

20 Inoceramus waltersdorfensis

19 Prionocydus quadratus 

18 Scaphites nigricollensis 

1 7 Scaphites whitfieldi 

16 Scaphites warreni 

15 Prionocydus macombi

14 Prionocydus hyatti 

13 Collignoniceras woollgari regulare 

12 Collignoniceras woollgari woollgari

11 Mammites nodosoides 

10 Watinoceras coloradoense

9 Sciponoceras gracile 

8 Dunveganoceras albertense 

7 Dunveganoceras pondi

6 Plesiacanthoceras wyomingense 

5 Acanthoceras amphibolum 

4 Acanthoceras alvaradoense 

3 Acanthoceras muldoonense 

2 Acanthoceras granerosense 

1 Calycoceras gilberti

No molluscan fossil record

K-Ar ages (million yrs.) 

(Obradovich and 

Cobban, 1975)

86.8

88.9

91.3 

92.1

Q/l

Osage area, 

Weston County1

Niobrara Formation 

(tower part)

0)

cc 

co

jl> 

CC

o

y 

Sage

Breaks 

Member

Turner 

Sandy 

Member

JjlLLL?liiUl

Pool 

Creek 

Member

«"i

Greenhorn 

Formation

Belle 

Fourche 

Shale

Kaycee area, 

Johnson 

County

r"

CC

a

o5 

£

^

0) 

CO

co 

>

T3

8

Formation Frontier 

Niobrara 

Member 

(lower part)

Sage 

Breaks 

Member

Wall 

Mem

^_—- -7

Be 

Four 

Men

Cre

IT

^->^

le 

che 

iber

ek

Central 

Natrona 

County

i 

1 i

|

\

Formation Frontier 

t) 

Niobrara 

Member 

ower part)

Sage 

Breaks 

Member

/Vail Creek 

Member

^, 

Unnam 

memb

ed 

er

Belle 

Fourche 

Member

1 Age of basal contact of Niobrara Formation from Evetts (1976, p.121).

FIGURE 2.—Correlation chart of the lower Upper Cretaceous formations at selected localities in northeastern Wyoming. Pattern

represents a hiatus in the sequence of beds.

marine environments during Cenomanian, Turonian, 

and Coniacian time. Near the Black Hills (fig. 1), a 

sequence of approximately the same age is about 370 m 

thick and was deposited in marine environments.

The sedimentary rocks of early Late Cretaceous age 

in Weston County, on the east side of the Powder River 

Basin (fig. 1), are included in, from older to younger, 

the Belle Fourche Shale, Greenhorn Formation, and 

Carlile Shale (fig. 2). They conformably overlie the 

Mowry Shale of Early Cretaceous age and are disconformably overlain by the Upper Cretaceous Niobrara 

Formation. Near the town of Osage, the lower Upper 

Cretaceous sequence is about 370 m thick and is composed of shale, siltstone, sandstone, limestone, and 

bentonite. These rocks were deposited in offshoremarine and nearshore-marine environments and contain invertebrate fossils of Cenomanian, Turonian, 

Coniacian, and Santonian age.

The lower Upper Cretaceous rocks in Johnson 

County, on the west side of the Powder River Basin 

(fig. 1), include the Frontier Formation and the overlying Sage Breaks Member of the Cody Shale (fig. 2). 

These strata conformably overlie the Lower Cretaceous Mowry Shale and are conformably overlain by 

the Niobrara Member of the Cody Shale. Near the 

town of Kaycee, the lower Upper Cretaceous sequence
PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

EXPLANATION

Siltstone

Shale

Limestone

Bentonite

Covered interval, probably shale

..... Conglomeratic

....... Sandy

*v kvA Silty

— — Clayey

-1- -•- Calcareous

o o Concretionary

|8 Beds containing Western Interior 

Zone fossil (fig. 2)

METERS FEET

O-r-0

25-

FIGURE 3.—Outcrop section A of the lower Upper Cretaceous formations near Osage, sees. 

16, 17, 19, 20, and 30, T. 46 N., R. 63 W., Weston County. Data included from Robinson 

and others (1964).
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING

is about 315 m thick and consists of shale, siltstone, 

sandstone, minor conglomerate, and bentonite. These 

rocks were deposited largely in nearshore-marine 

environments and contain molluscan fossils of 

Cenomanian, Turonian, and Coniacian Age.

BELLE FOURCHE SHALE

The Belle Fourche Shale was named by Collier (1922, 

p. 83) from outcrops near the Belle Fourche River in 

southwestern Crook County, on the west flank of the 

Black Hills (fig. 1). In Weston County, near Osage, the 

formation is about 115 m thick and is composed mainly 

of interstratified shale and bentonite. Most of the beds 

are soft. Consequently, the Belle Fourche seldom crops 

out and generally forms strike valleys. These strata 

conformably overlie the well-indurated Mowry Shale of 

Early Cretaceous age and are over lain by the ridgeforming Greenhorn Formation of Late Cretaceous age. 

The Belle Fourche Shale accumulated during Cenomanian time; the upper part of the formation near Osage 

contains invertebrate fossils of middle Cenomanian 

age (fig. 2). These rocks were deposited in offshoremarine environments.

The Belle Fourche Shale is about 118 m thick at the 

outcrop section near Osage (fig. 3), and 115-122 m 

thick in boreholes in the area. The formation is about 

113 m thick near Newcastle, southeast of Osage, and 

as much as 260 m thick in western Crook County 

(Robinson and others, 1964, p. 53-60), northwest of 

Osage. Robinson and others (1964, p. 53) concluded 

that this variation in thickness is caused by the lateral 

gradation of noncalcareous rocks in the upper part of 

the Belle Fourche into calcareous shale which is locally 

assigned to the overlying basal Greenhorn.

At outcrops in Weston County, the Belle Fourche is 

composed largely of noncalcareous shale, silty shale, 

and bentonite. Small amounts of calcareous shale and 

siltstone occur near the top of the formation. Units of 

dark-gray shale in the Belle Fourche are commonly 

more than 6 m thick. The units of siltstone are light 

gray to dusky yellow to pale brown, platy, and less 

than 1 m thick. Beds of very light gray to grayishorange bentonite, as much as 1 m thick, are abundant. 

Ferruginous concretions are common in the Belle 

Fourche, and sideritic concretions are especially 

plentiful in the basal 20 m of the formation.

Cores of the uppermost 25 m of the formation from 

borehole 1 in SW1/4 NW1/4 sec. 30, T. 46 N., R. 63 W. 

(fig. 1) include interlaminated shale and lesser siltstone (fig. 4). These rocks are very light gray to dark 

gray, largely noncalcareous, and contain small horizontal burrows and molluscan fossils. The laminae are 

commonly discontinuous, even, and parallel; discontinuous, wavy, and nonparallel; and microcrosslaminated. Some of the core has flaser bedding, lenticular bedding, and small slump structures. Samples 

from depths of about 258 m and 267 m were analyzed 

by X-ray diffraction. They contain quartz, sodic 

plagioclase, potassium feldspar, dolomite, pyrite, 

mica-illite, and chlorite. The laminae at a depth of 

258 m also contain montmorillonite and calcite. At 

267 m, the laminae include smectite or mixed-layer 

clay, and kaolinite. The organic composition of these 

samples (table 1) was described and interpreted by 

Merewether and Claypool (1980). In the borehole, the 

Belle Fourche includes potential source rocks for 

hydrocarbons, but the sampled beds are thermally immature (in an early stage of the hydrocarbongeneration process).

In the vicinity of Osage, the upper part of the Belle 

Fourche, which is about 40 m thick, contains invertebrate fossils of middle Cenomanian age (fig. 2). Species 

in selected collections from these beds are:

USGS D9911, SW1/4 sec. 17, T. 46 N., R. 63 W., from a ferruginous 

concretion layer 7 m below the top of the Belle Fourche Shale. 

Acanthoceras amphibolum Morrow 

Inoceramus rutherfordi Warren 

Borissiakoceras sp.

USGS D9920, SW1/4 sec. 17, T. 46 N., R. 63 W., from a ferruginous

concretion layer 13 m below the top of the Belle Fourche

Shale.

Acanthoceras amphibolum Morrow 

Inoceramus rutherfordi Warren 

USGS D9924, NW1/4 sec. 17, T. 46 N., R. 63 W., from a ferruginous

concretion layer 27 m below the top of the Belle Fourche

Shale.

Inoceramus arvanus Stephenson 

Ostrea beloiti Logan 

Acanthoceras alvaradoense Moreman?

USGS D9923, NW1/4 sec. 17, T. 46 N., R. 63 W., from a ferruginous

concretion layer 35 m below the top of the Belle Fourche

Shale.

Exogyra columbella Meek 

Borissiakoceras compressum Cobban 

USGS D9910, SW1/4 sec. 17, T. 46 N., R. 63 W., from a claystone

concretion layer 38 m below the top of the Belle Fourche

Shale.

Inoceramus eulessanus Stephenson 

Exogyra columbella Meek 

Acanthoceras sp. 

Borissiakoceras compressum Cobban

The Belle Fourche consists of clay-rich strata and 

contains fossils and burrows of marine origin. Apparently, these rocks were deposited in shallow-water, 

offshore-marine environments, where mild current 

action alternated with slack water. Interstratified 

sandstone and shale of the same age but of near-shoremarine origin crop out near Kaycee (fig. 1), about 

180 km west of the Osage area (Merewether and 

others, 1976).
6 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

TABLE 1. Organic composition of lower Upper Cretaceous shale at boreholes I and 2, Powder River Basin, as determined by thermalevolution analysis, combustion, and reflectance

[From Merewether and Claypool, 1980. N.d., not determined]

Borehole 

No.

1

1

1

1

1

1

1

1

1

1

1

1

2

2

2

2

2

2

Stratigraphic 

unit

Sage Breaks

Member of

Carlile Shale.

——— do ———

——— do ———

Turner

Sandy

Member of

Carlile Shale.

——— do ———

Pool Creek

Member of

Carlile Shale.

——— do ————

Greenhorn

Formation.

——— do ———

Belle Fourche

Shale.

——— do ———

Sage Breaks

Member of

Cody Shale.

Wall Creek

Member of

Frontier

Formation.

Belle Fourche

Member of

Frontier

Formation.

——— do ———

——— do ———

——— do ————

Sample 

depth 

(m)

29.0

59.4

80.8

112.2

130.2

146.3

157.0

163.4

199.7

244.5

257.6

266.7

48.8

82.6

86.6

137.2

222.8

277.1

Organic 

carbon 

(weight 

percent)

1.28

2.23

1.38

1.13

.97

1.06

4.29

2.17

2 on. OU

2.97

1.61

1.16

.7

.5

3.2

.4

1.2

2.4

Total 

pyrolytic 

hydrocarbon 

(weight 

percent)

0.17

.60

.25

.08

.14

.11

1.90

.76

1

f\Q • Uo

.99

.39

.19

.11

.071

.32

.058

.14

.35

Volatile 

hydrocarbon 

(ppm by 

weight)

33

35

30

9

44

24

200

56

96

65

32

23

65

38

79

44

69

71

Ratio of 

pyrolytic 

hydroarbon 

to organic 

carbon 

(percent)

13.6

26.9

17.9

7.5

14.7

10.6

44.3

35.1

1 Q "1 Jo. /

33.2

24.0

16.1

16.0

14.3

10.0

14.5

11.6

14.5

Temperature 

of maximum 

pyrolytic 

yield (°C)

459

455

469

467

464

461

446

451

451

454

458

472

477

487

466

471

475

476

Modal 

vitrinite 

reflectance 

at random 

orientation 

(percent)

N.d.

0.45

.50

.40

.65

N.d.

.50

N.d.

N.d.

.45

N.d.

.45

.45

.55

.45

.50

.50

.50

GREENHORN FORMATION

Darton (1909, p. 54-55) introduced the name 

Greenhorn Limestone into the northern Black Hills 

and Cobban (1951, p. 2183) changed the name to 

Greenhorn Formation in this area. In Weston County, 

near Osage, the formation is about 85 m thick and consists of interstratified calcareous and noncalcareous 

shale and siltstone, limestone, and bentonite. The indurated strata in the upper part of the formation form 

a conspicuous questa, which extends along the western 

flank of the Black Hills. In that region, the Greenhorn 

overlies and intertongues with the Belle Fourche Shale 

(Robinson and others, 1964, fig. 5) and is overlain by

the Carlile Shale. In the Osage area, the formation 

contains invertebrate fossils of middle and late Cenomanian and early Turonian age and was deposited in 

offshore, open-marine environments.

The Greenhorn Formation is about 86 m thick at 

borehole 1 and ranges in thickness from 80 to 90 m in 

other boreholes in the area. At outcrops near Osage, 

the formation is 75-84 m thick. Robinson and others 

(1964, p. 65-66) reported thicknesses of 90 m for the 

Greenhorn near Newcastle, and 27 m for the formation 

farther northwest in northwestern Crook County.

At outcrops in Weston County, limestone of the 

Greenhorn Formation is generally light gray to 

medium gray to grayish orange, slightly silty or sandy,
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING

laminated to thin bedded, and fossiliferous. Units of 

limestone are as much as 50 cm (centimeters) thick. 

The limestone at the top of the formation has abundant horizontal burrows which contain spreite and are 

about 1 cm in diameter and as much as 30 cm long. The 

shale is medium gray to grayish black, noncalcareous 

to very calcareous, and forms units as much as 10 m 

thick. Some of the shale encloses ferruginous and septarian limestone concretions and is fossiliferous.

A core of the Greenhorn from borehole 1 (figs. 1 

and 5) is composed mainly of interlaminated medium 

dark-gray to dark-gray shale and very light gray to 

light-gray siltstone. In general, these rocks grade from 

slightly calcareous near the base of the formation to 

very calcareous near the top. Most of the laminae are 

discontinuous, even, and parallel, but in the upper part 

of the formation some are discontinuous, wavy, and 

nonparallel. They commonly contain small horizontal 

burrows, fragments of molluscan fossils, and fish 

bones.

Samples of the core from depths of about 163 m, 

176 m, 200 m, and 244 m were analyzed by X-ray diffraction. All of these rocks contain quartz, calcite, 

clay, and pyrite, although the proportion of calcite to 

other minerals appears to be larger in the samples from 

163 m and 176 m. The laminae at depths of 200 m and 

244 m also contain mica-illite, chlorite, kaolinite, 

sodic plagioclase, potassium feldspar, and dolomite. 

At 163 m, the rock includes mica-illite, chlorite, smectite or mixed-layer clay, sodic plagioclase, potassium 

feldspar, and dolomite.

The organic composition of samples from depths of 

163 m, 200 m, and 245 m was reported by Merewether 

and Claypool (1980). Hydrogen-rich organic matter, 

which was derived largely from aquatic plants, is comparatively abundant in the sampled strata (table 1). 

These rocks could generate oil and gas if their organic 

components were not thermally immature.

The Greenhorn Formation in the Osage area contains 

abundant molluscan fossils of late Cenomanian and 

early Turonian age (fig. 2). Representative collections 

are:

USGS D5935, NE1/4 sec. 20, T. 46 N., R. 63 W., from calcareous

shale llm below the top of the Greenhorn Formation. 

Mytiloides mytiloides (Mantell) 

Pseudoperna bentonensis (Logan)

USGS D5934, Sl/2 sec. 17, T. 46 N., R. 63 W., from very thin, lightbrown calcarenite 27 m below the top of the Greenhorn Formation.

Inoceramus cf. /. mesabiensis Bergquist

Metoicoceras defordi Young

USGS D5932, Sl/2 sec. 17, T. 46 N., R. 63 W., from very thin, lightbrown calcarenite 34 m below the top of the Greenhorn Formation.

Phelopteria sp. 

Inoceramus pictus Sowerby 

Calycoceras sp. 

Metoicoceras defordi Young

USGS D5929, Sl/2 sec. 17, T. 46 N., R. 63 W., from soft, limonitic 

concretions 40 m below the top of the Greenhorn Formation. 

Dunveganoceras albertense (Warren)

USGS D5928, Sl/2 sec. 17, T. 46 N., R. 63 W., from soft, limonitic 

concretions 59 m below the top of the Greenhorn Formation. 

Inoceramus pictus Sowerby 

Calycoceras? canitaurinum (Haas) 

USGS D9898, SE1/4 sec. 7, T. 46 N., R. 63 W., from light-brown

limestone concretion near base of Greenhorn Formation. 

Inoceramus prefragilis Stephenson 

Calycoceras? canitaurinum (Haas) 

Dunveganoceras pondi Haas

In Weston County, the Greenhorn is composed 

largely of fossiliferous, calcareous, and argillaceous 

strata. Evidently, the formation was deposited in 

open-marine environments of low current energy, far 

from shore. Interstratified shale and sandstone of 

Greenhorn age but of nearshore-marine origin crop out 

near Kaycee and Casper on the southwest flank of the 

Powder River Basin (Merewether and others, 1979) 

(fig. 1).

CARLILE SHALE

The name Carlile Shale was first used in the Black 

Hills region by Darton (1909, p. 54-55). In the vicinity 

of Osage, the Carlile is about 165 m thick and includes, 

in ascending order, the Pool Creek Member, Turner 

Sandy Member, and Sage Breaks Member (fig. 2). 

Knechtel and Patterson (1962, p. 921) named the Pool 

Creek Member from outcrops along Pool Creek on the 

northeast flank of the Black Hills. The Pool Creek consists mostly of interlaminated shale and siltstone, and 

is calcareous at the base and noncalcareous at the top. 

These strata are disconformably overlain by the 

Turner Sandy Member, which is composed of interstratified shale, siltstone, and sandstone. The sandy 

beds of the Turner are conformably overlain by the 

slightly calcareous shale of the Sage Breaks. The 

Turner Sandy Member and Sage Breaks Member were 

named by Rubey (1930, p. 4) from outcrops near Osage.

Most of the Carlile is soft and forms a topography of 

little relief; however, beds of concretionary siltstone 

and sandstone in the Turner commonly crop out as 

small hogbacks and questas. In Weston County, the 

Carlile overlies the Greenhorn Formation and is 

disconformably overlain by the Niobrara Formation. 

The Carlile at Osage contains invertebrate fossils of 

Turonian, Coniacian, and Santonian Age and was
PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

SURFACE

ELEVATION

4,032 ft.

(1,229 m)

SAMPLE DEPTH,

IN METERS

Datum is depth

of 500 ft.

(152.4 m)

NUCLEAR AND

Gamma-ray curve 

17 CDS 40

I I I I I I I I I I 

30 ft /m

FIGURE 4.—Lithologic and geophysical logs of lower Upper Cretaceous formations near
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING

CALIPER LOGS NUCLEAR LOGS

Gamma-gamma curve Neutron curve 

85 cps 455 830 10 cps 135 265

EXPLANATION

Caliper curve I 

4 6 8 10 in.

I I \\

L ' ,. , .

FEET 

100—

200—,

300 —

—— 200 —

400—

500-

600—;

700 —

800—

.II ! I

I '

r.

1 •

1

• '

•

,

"i

^

\

l_

^

<

Jy

' \

^=*

-

' "

, .

600—

Calcareous

o o o

Concretionary

700—

800—

i 1 i 1 i i 1 a.

Osage, from borehole 1, SW1/4NW1/4 sec. 30, T. 46 N., R. 63 W., Weston County.
10 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

FIGURE 5.—Drawing showing crossbedding in the middle of the 

Turner Sandy Member near Osage. The hammer is 28 cm long. 

Direction of view is southeast.

deposited in offshore-marine and nearshore-marine 

environments.

POOL CREEK MEMBER

The Pool Creek Member is about 23 m thick at 

borehole 1 (fig. 4) and 20-28 m thick in nearby oil wells. 

At outcrops along the west side of the Black Hills, the 

member is about 30 m thick near Newcastle, about 

28 m thick near Osage, and about 15m thick near the 

border of Weston and Crook Counties (fig. 1) (Robinson 

and others, 1964, p. 72-74).

At outcrops in the vicinity of Osage (fig. 3), the Pool 

Creek consists of dark-gray, silty shale and minor 

light-gray bentonite, and has conspicuous layers of 

light-gray, calcareous concretions near the middle. The 

units of shale are as much as 12 m thick, and the beds 

of bentonite are as much as 34 cm thick. Locally, the 

lithology of the uppermost Pool Creek contrasts 

strongly with the lithology of the overlying basal sandstone of the Turner Sandy Member. The Pool Creek 

correlates with the unnamed member of the Frontier in 

central Natrona County (fig. 2). , Core of the Pool Creek Member from borehole 1 (fig. 4) 

consists largely of interlaminated dark-gray shale and 

light-gray siltstone. In the lower one-half of the 

member, most of the rocks are calcareous. The laminae 

in the core are generally discontinuous and are either 

wavy and nonparallel or even and parallel. Many 

laminae contain small horizontal burrows, although 

only a minor part of the Pool Creek is bioturbated. 

Molluscan fossils and fish bones are common.

Samples of the core from depths of 146 m and 157 m 

were analyzed to determine their mineral composition 

and organic-carbon content. Both samples contain 

quartz, mica-illite, smectite or mixed-layer clay, 

chlorite, sodic plagioclase, potassium feldspar,

dolomite, and pyrite. Kaolinite was recognized in the 

sample from a depth of 146 m. The laminae at about 

157 m contain minor calcite. Merewether and Claypool 

(1980) described the organic composition of some of 

these strata (table 1). The constituent organic matter is 

suitable for conversion to oil and gas. However, the 

sampled rocks have been only slightly altered by burial 

metamorphism and are, therefore, in an early stage of 

the petroleum-forming process.

In Weston County, the Pool Creek contains molluscan fossils of middle Turonian age. A representative 

collection of fossils from the vicinity of Osage is:

USGS D9896, NW1/4 sec. 35, T. 46 N., R. 63 W., from calcareous 

concretions about 14 m below the top of the Pool Creek 

Member.

Inoceramus cuvieri Sowerby

Pseudomelania hendricksoni Henderson

Scaphites larvaeformis Meek and Hayden

Collignoniceras woollgari regulars Haas

Tragodesmoceras carlilense Cobban

The Pool Creek Member near Osage consists mainly 

of interlaminated shale and siltstone, and contains 

small horizontal burrows and fossils of marine origin. 

The member probably accumulated in offshore-marine 

environments of comparatively low current energy. 

Sandstone and shale of approximately the same age 

but of nearshore-marine origin crops out west of 

Casper in southeastern Natrona County (fig. 1) and 

is assigned to the unnamed member of the Frontier 

Formation (Merewether and others, 1979).

TURNER SANDY MEMBER

The Turner Sandy Member is about 55 m thick in 

borehole 1 and ranges in thickness from 49 m to 59 m 

at other boreholes in the area. At outcrops, the 

member is about 50 m thick near Osage, approximately 57 m thick near the boundary of Weston and 

Crook Counties (Robinson and others, 1964, p. 72-73), 

and about 56 m thick in the vicinity of Newcastle.

In Weston County, the Turner is composed of interstratified shale, siltstone, and sandstone. These rocks 

disconformably overlie the Pool Creek Member and are 

conformably overlain by the Sage Breaks Member of 

the Carlile. At outcrops near Osage (fig. 3), the lower 

one-half of the Turner includes, from older to younger, 

a thin basal sandstone, a unit which grades from shale 

at the base to sandstone at the top, and another similar 

gradational unit. The upper one-half of the member 

consists of siltstone, shale, and minor sandstone. The 

shale in the Turner is mainly dark gray, silty, and noncalcareous, and the siltstone is largely medium gray, 

argillaceous or sandy, and noncalcareous. The units of
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 11

shale are as much as 8 m thick. Most of the sandstone 

in the member is light gray to grayish orange, very fine 

grained, silty, and calcareous; however, the basal sandstone is locally medium grained and contains sparse, 

coarse grains, granules, and pebbles of quartz and 

chert. The sand grains are dominantly angular to 

subrounded and generally are not well sorted. A few 

beds of sandstone appear to have good porosity. Units 

of sandstone are as much as 5 m thick. The siltstone 

and sandstone are commonly in sequences of very thin 

to thin, discontinuous beds and show scour and fill 

bedding, tabular crossbedding, trough crossbedding 

(fig. 5), and ripple marks. Paleocurrent directions, obtained from crossbeds and ripple marks, are 

southwest, northeast, and southeast, but the dominant 

direction seems to be southeast. Burrows, which are 

plentiful in the outcropping siltstone and sandstone, 

are parallel and perpendicular to the strata and include 

Ophiomorpha, Thalassinoides, Rhizocorallium and, 

tentatively, Rosselia, Cruziana, and Skolithos. 

Fragments of fossil wood in the basal sandstone are 

extensively bored. At outcrops, the Turner contains 

conspicuous calcareous siltstone and sandstone concretions, which are commonly fossiliferous.

The core of the Turner Sandy Member is composed 

of interstratified and intergradational light-gray sandstone, light-gray to medium-gray siltstone, and 

medium-gray to dark-gray shale (fig. 4). Shale is a 

minor component of the core and most of it is interlaminated with siltstone or sandstone. Generally, the 

laminae of shale and siltstone are discontinuous and 

are either wavy and nonparallel or even and parallel. 

Flaser and lenticular bedding are common near the 

base of the member. Some strata are cross-laminated 

and others are bioturbated. The shale and siltstone 

commonly enclose small, smooth burrows (including 

Siphonites ) which parallel the bedding planes. Most of 

the sandstone is very fine grained, thinly laminated to 

very thinly bedded, and horizontally stratified, and 

contains burrows. Some sandstone units are fine 

grained with sparse, medium to very coarse grains, 

some are slightly calcareous, and some are crossstratified or bioturbated. Many of the cored beds in the 

Turner contain molluscan fossils.

The organic composition of the shale at depths of 

about 112 m and 130 m in borehole 1 (table 1) was 

described by Merewether and Claypool (1980). Evidently, the organic matter in these strata was derived 

mainly from land plants and is thermally immature. 

Although the organic-carbon content of the sampled 

beds is comparatively low, the shale in the Turner 

could be a source rock for natural gas.

Samples of the core, which were analyzed by X-ray 

diffraction, consist of sandstone from depths of about

117 m, 125 m, and 138 m, and shale from depths of 

about 112 m and 130 m. Most of these rocks contain 

quartz, mica-illite, kaolinite, chlorite, potassium and 

sodium feldspars, and dolomite. The samples of sandstone also contain calcite, and the rocks from 112 m, 

117 m, 130 m, and 138 m contain pyrite. Micrographs 

of the sandstone, taken with a SEM, depict authigenic 

minerals, including quartz, feldspar (fig. 6), kaolinite, 

chlorite, and pyrite.

Thin sections of the sandstone from depths of about 

117m, 125 m, and 138 m show angular to subrounded 

grains of quartz, chert, and feldspar, and minor 

amounts of detrital chalcedony, biotite, and other 

minerals. Many of the grains of quartz, chert, and 

feldspar are angular because they are corroded and 

have been partially replaced by clay and calcite (fig. 7). 

Most of the sand grains appear to have been subrounded at the time of deposition.

Samples of sandstone from depths of about 117 m, 

125 m, and 138 m in borehole 1 were analyzed to determine the size and sorting of the constituent grains. In 

the sample from a depth of 117 m, the sand has a mean 

grain size of 1.91 phi (medium sand), a standard deviation of 0.47 phi (well sorted), and a skewness of +0.24 

(fine skewed), and a kurtosis of 2.62 (very leptokurtic). 

At a depth of 125 m, the sand has a mean grain size of 

2.76 phi (fine sand), a standard deviation of 0.54 phi 

(moderately well sorted), a skewness of —0.30 (coarse 

skewed); and a kurtosis of 2.22 (very leptokurtic). The 

sand in the sample from 138 m has a mean grain size of 

1.78 phi (medium sand), a standard deviation of 0.47 

phi (well sorted), a skewness of +0.66 (strongly fine 

skewed), and a kurtosis of 3.52 (extremely leptokurtic).

To summarize, these samples of Turner sandstone 

consist of clay, silt, and very fine to coarse sand grains. 

The amount of clay in the samples, estimated from 

X-ray analyses, is probably less than 10 percent. At 

depths of 117 m and 138 m, the sand is largely medium 

grained and is well sorted and angular to subrounded. 

The sand at 125 m is mostly fine grained, is moderately 

well sorted, and is angular to subrounded. The grainsize distribution for the sand in these three samples is 

asymmetric and very leptokurtic. Although the sand is 

moderately well sorted and well sorted, which is 

evidence of submature, mature, and supermature sedimentary textures, the rocks may contain a significant 

amount of clay, which characterizes immature textures. However, micrographs taken with a SEM show 

that much of the clay is authigenic. Furthermore, the 

roundness and presumably the sorting of the grains 

has been affected by post depositional dissolution. 

Therefore, these textures probably were mature or 

supermature before diagenesis.
12 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

FIGURE 6.—Authigenic crystal of feldspar (f) on a corroded crystal of quartz (q), from sandstone in the Turner Sandy Member at a depth of 125 m in borehole 1.

In the area of Osage, outcrops of the Turner contain 

invertebrate fossils of late Turonian age. Representative collections of these fossils are:

USGS D9915, NW1/4 sec. 20, T. 46 N., R. 63 W., from calcareous

sandstone concretions about 5 m below the top of the Turner

Sandy Member. 

Inoceramus incertus Jimbo 

Inoceramus lusatiae Andert 

Scaphites corvensis Cobban 

Bostrychoceras n. sp. 

Prionocyclus quadratus Cobban 

Eutrephoceras sp.

Perissoptera cf. P. prolabiata (White) 

USGS D9914, NE1/4 sec. 20, T. 46 N., R. 63 W., from calcareous

sandstone concretions about 17 m below the top of the Turner

Sandy Member. 

Inoceramus n. sp. 

Inoceramus lusatiae Andert 

Baculites yokoyamai Tokunaga and Shimizu 

Scaphites sp. 

Prionocyclus sp. 

Bellifusus wittistoni (Logan)

FIGURE 7.—Corroded grains (eg) of quartz (q), chert (ch), 

and feldspar (f), cemented by calcite (c) in a thin section 

of sandstone from the Turner Sandy Member, at a depth 

of 117 m in borehole 1.

USGS D9899, NW1/4 sec. 18, T. 46 N., R. 63 W., from sandstone

concretions about 24 m below the top of the Turner Sandy

Member.

Inoceramus perplexus Whitfield 

Pleuriocardia sp. 

Scaphites whitfieldi Cobban 

USGS D9912, NW1/4 sec. 20, T. 46 N., R. 63 W., from calcareous

sandstone concretions about 40 m below the top of the Turner

Sandy Member. 

Inoceramus dimidius White 

Scaphites warreni Meek and Hayden 

Prionocyclus wyomingensis Meek 

USGS D9917, NW1/4 sec. 20, T. 46 N., R. 63 W., from sandstone

concretions about 1.5 m above the base of the Turner Sandy

Member.

Inoceramus dimidius White 

Prionocyclus sp. 

USGS D10121, NW1/4 sec. 31, T. 47 N., R. 64 W., from sandstone

about 1 m above the base of the Turner Sandy Member. 

Inoceramus n. sp.

Scaphites warreni Meek and Hayden 

Prionocyclus macombi Meek
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 13

The Turner Sandy Member in the vicinity of Osage is 

composed of interstratified shale, siltstone, and sandstone, and contains horizontal and vertical burrows 

and invertebrate fossils of marine origin. Some shale 

units grade upward into sandstone units, which are 

overlain by more shale. Much of the sandstone is 

moderately well sorted to well sorted and consists of 

angular to subrounded, very fine and fine grains of 

quartz, chert, and feldspar. The sandstone is laminated 

to thin bedded and locally has scour-and-fill bedding 

and crossbedding. In Weston County, the Turner was 

deposited in shallow-marine environments (probably at 

depths of less than 60 m) in the distal part of a highdestructive, tide-dominated delta (Merewether and 

others, 1979). Some of these strata accumulated in the 

form of nearshore and offshore bars.

SAGE BREAKS MEMBER

In boreholes near Osage, the Sage Breaks is commonly 91-94 m thick. The part of the member 

penetrated at borehole 1 is about 85 m thick. Robinson 

and others (1964, p. 71-73) reported thicknesses of 

about 78 m at outcrops between Osage and Newcastle, 

and about 83 m at outcrops on the northwestern flank 

of the Black Hills.

In Weston County, outcrops of the Sage Breaks are 

composed largely of dark-gray, noncalcareous shale 

which contains several conspicuous layers of closely 

spaced concretions (fig. 3). The concretions are commonly light gray to grayish orange, calcareous, and 

septarian, and are as much as 45 cm thick and 1.8 m 

long. The Sage Breaks is conformable and gradational 

with the underlying Turner and is disconformably 

overlain by the Niobrara Formation.

Where cored in borehole 1, the Sage Breaks Member 

consists mainly of medium-dark-gray, slightly 

calcareous, thinly laminated shale (fig. 4). Most of the 

shale in the basal 12 m of the member is slightly silty, 

and much of the shale in the uppermost 45 m of the 

member is pyritic. The core contains sparse concretions, a few small horizontal burrows, and many invertebrate fossils. A bed of bentonite about 8 cm thick 

was found about 18 m above the base of the Sage 

Breaks.

Core of the Sage Breaks, from depths of about 29 m, 

59 m, and 81 m in borehole 1, was sampled to determine the mineral composition and organic-carbon 

content of some of the shale. X-ray diffraction analysis 

indicates that the samples contain quartz, mica-illite, 

chlorite, kaolinite, calcite, dolomite, sodium and 

potassium feldspars, and pyrite. The organic matter in 

the samples (table 1) was described by Merewether and 

Claypool (1980). They concluded that the Sage Breaks

in the Osage area includes potential source rocks for oil 

and gas but that the member is thermally immature. 

Outcrops of the Sage Breaks in Weston County rarely contain megafossils, although molluscan fossils of 

middle Coniacian age were collected from the upper 

part of the member in the vicinity of Osage (figs. 2 

and 3). Representative collections of these fossils are:

USGS D10152, SE1/4 sec. 19, T. 46 N., R. 63 W., from small 

calcareous concretions about 26 m below the top of the 

Sage Breaks Member.

Calcareous worm tube

Inoceramus inconstans Woods

Inoceramus sp.

Baculites mariasensis Cobban

USGS D10150, SW1/4 sec. 12, T. 46 N., R. 64 W., from soft darkgray shale about 28 m below the top of the Sage Breaks 

Member.

Inoceramus deformis Meek

Inoceramus inconstans Woods

Pseudoperna congesta (Conrad)

The Sage Breaks Member near Osage is composed of 

thinly laminated, noncalcareous, and calcareous shale, 

and contains sparse horizontal burrows and many 

molluscan fossils. These rocks were deposited in 

offshore-marine environments of little current energy, 

which apparently were a great distance from the shore. 

Laterally equivalent units of nearshore-marine sandstone crop out in Fremont County (fig. 1) in central 

Wyoming.

FRONTIER FORMATION

The rocks of early Late Cretaceous age on the west 

flank of the Powder River Basin (fig. 1) are included in 

the Frontier Formation and the lower part of the overlying Cody Shale (fig. 2). They were first assigned to 

those formations by Hares (1916, p. 238) and Hares 

and others (1946). The Frontier conformably overlies 

the Lower Cretaceous Mowry Shale and is conformably overlain by the Cody Shale. In Johnson County, 

near Kaycee, the Frontier consists of the Belle Fourche 

Member, which was identified in this area by 

Merewether and Cobban (1973, p. 38), and the overlying Wall Creek Member, which was named by 

Wegemann (1911, p. 45). The Wall Creek in Johnson 

County rests disconformably on the Belle Fourche 

(Merewether and others, 1976, p. 35) and, at the top, interfingers with the lower part of the Cody Shale (Haun, 

1958, p. 84). Lithologic and geophysical logs of the 

Frontier Formation in borehole 2 (fig. 1) were depicted 

by Merewether and others (1976, fig. 4). In Natrona 

County, near Casper, the Frontier is composed of, from 

older to younger, the Belle Fourche Member, unnamed 

member, and Wall Creek Member (fig. 2). The lower
14 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

Cretaceous Upper 

Lower Cretaceous

| Cody Shale (prt)

Frontier Formation

co

a> 

co

.c 

0)

5

o

5

Wall Creek Member

|Belle Fourche Member 

Kaycee ou 

Sees. 2 a

1 co CD 

/ * <D ' o ~ —— •

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/vw V" *• Silty 

^_ —— Clayey

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8 Western Interior 

-j 1 —— ' Zone fossil (fig. 2)

METERS FEET 

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-100

FIGURE 8.—Stratigraphic sections of the lower Upper Cretaceous Frontier Formation near Kaycee, Johnson County.

and upper contacts of the unnamed member are disconformities.

The Frontier in the Kaycee area consists of about 

255 m of interstratified shale, siltstone, sandstone,

minor conglomerate, and bentonite (fig. 8) (Merewether 

and others, 1976). Outcrops of these rocks commonly 

form a series of questas and strike valleys. In the 

vicinity of Kaycee, the Frontier contains molluscan
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 15

fossils of Cenomanian and Turonian age, and evidently 

was deposited largely in nearshore-marine environments.

BELLE FOURCHE MEMBER

In holes drilled for oil and gas near Kaycee, the Belle 

Fourche Member is 211-227 m thick. The member is 

about 228 m thick at borehole 2 and is about 223 m 

thick at outcrops in the area.

In the southern part of Johnson County, the Belle 

Fourche locally consists of four units of mainly shale 

and siltstone, and three intervening units of mostly 

sandstone. At outcrops, the shale in the member is 

largely dark gray or brownish gray, silty, and noncalcareous. Most of the outcropping siltstone is 

medium dark gray, clayey or sandy, and noncalcareous. The shale and siltstone are generally soft but 

commonly contain calcareous and ferruginous concretions. Sideritic concretions are abundant in a basal 

unit about 30 m thick that consists of interstratified 

shale, siltstone, and bentonite. The sandstone in the 

Belle Fourche is mainly very light gray to medium 

gray, very fine and fine grained, silty, noncalcareous, 

and moderately porous. Most of the grains are angular 

to subrounded and are moderately well sorted to well 

sorted. Grains of quartz, chert, glauconite, mica, and 

pyrite, and fine fragments of coaly material have been 

observed in hand specimens. Most of the sandstone is 

laminated to thin bedded, and the strata are largely 

either discontinuous and nonparallel or bioturbated.

The oldest of the three informal units of sandstone in 

the Belle Fourche Member is about 145 m below the 

top of the member and was designated the Third Wall 

Creek sand by Richardson (1957). In ledges and cliffs 

3-4 km southwest of borehole 2 (fig. 1), this sandstone 

is as much as 40 m thick and is light gray, fine to 

coarse grained, and poorly sorted. Some beds near the 

top are conglomeratic and contain shale pebbles. The 

unit consists of tabular and trough crossbeds in sets as 

thick as 1.2 m, which are evidence of southeast-flowing 

paleocurrents. The lower part of the sequence includes 

wavy bedding and load structures. No burrows were 

observed in the sandstone. At outcrops about 15 km 

southeast of the borehole, this unit is represented by 

inter-laminated and interbedded shale, siltstone, and 

very fined grained sandstone. These rocks are horizontally bedded and crossbedded, and apparently contain 

no burrows.

The most conspicuous unit of sandstone in the Belle 

Fourche is about 57 m below the top of the member 

and was named informally the Second Wall Creek sand 

by Richardson (1957). It is also informally called the 

"Second Wall Creek sandstone" or "Second Frontier

sandstone" (Barlow and Haun, 1966, p. 2185). In the 

vicinity of Kaycee, the sandstone is as much as 63 m 

thick and forms a prominent light-gray questa. This 

unit of sandstone, which is overlain by the Soap Creek 

Bentonite Bed (fig. 8) of Richards and Rogers (1951), 

can be divided into two parts. The lower part is about 

48 m thick and grades from interlaminated shale, 

siltstone, very fine grained sandstone, and bentonite at 

the base, to fine-grained or medium-grained, crossbedded sandstone at the top. Cross-strata near the top of 

this sequence are in tabular sets as much as 30 cm 

thick. The lower part of the Second Wall Creek sand is 

mostly friable, but near the top it includes brownishgray, calcareous concretions as large as 3 m in 

diameter. These concretions contain scattered pebbles 

of chert, fossilized fragments of burrowed wood, 

Ophiomorpha, and molluscan fossils.

The upper part of the Second Wall Creek sand consists mainly of interstratified very fine grained and 

fine grained sandstone and less dark-gray and 

brownish-gray shale. A thin bed of conglomerate occurs near the top of the sequence. This conglomerate is 

as much as 30 cm thick and contains pebbles of chert 

and igneous rocks as much as 8 cm long. Sandstone 

beds near the base of the sequence contain burrows, 

and those in the upper part of the unit are crossstratified.

The uppermost unit of sandstone (not designated 

specifically by Richardson) in the Belle Fourche 

Member of the Kaycee area is about 14m below the top 

of the member (fig. 8) and ranges in thickness from 

about 6 m to 23 m. These rocks are poorly cemented, 

although they contain hard, calcareous concretions, 

and the unit generally crops out as a weak ledge. Most 

of the sandstone is medium gray, very fine grained to 

fine grained, and poorly sorted, and contains burrows 

and molluscan fossils. The upper part of the unit contains scattered coarser grains and pebbles as large as 

4.5 cm long and locally includes beds of conglomerate 

as much as 1 m thick. The pebbles consist largely of 

chert and quartz. At some outcrops, the upper part of 

the unit is cross-stratified and provides evidence of 

southward-flowing paleocurrents.

In core from borehole 2 (figs. 1 and 8), most of the 

shale in the Belle Fourche Member is dark gray, silty, 

non-calcareous, and interlaminated with siltstone, 

which is generally medium gray, argillaceous or sandy, 

and noncalcareous. However, near the top of the Belle 

Fourche Member, these rocks are mainly calcareous. 

The laminae of shale and siltstone are discontinuous 

and are either even and parallel or wavy and nonparallel. Flaser and lenticular bedding are common except at the base and top of the member. Most of the 

shale and siltstone contains small horizontal burrows,
16 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

and some units are bioturbated. Where interstratified 

with sandstone, the shale and siltstone commonly contain Siphonites.

Samples of shale from depths of 87 m, 137 m, 223 m, 

and 277 m in borehole 2 were analyzed by X-ray diffraction. They contain quartz, plagioclase, potassium 

felspar, mixed-layer clays, and mica-illite. The sample 

from a depth of 87 m also contains calcite, dolomite, 

sodium montmorillonite, and minor chlorite or 

kaolinite. At a depth of 137 m, the shale includes 

dolomite, kaolinite, and minor chlorite. The sample at 

223 m includes sodium montmorillonite and less 

chlorite and kaolinite.

The organic composition of the sampled shale was 

described by Merewether and Claypool (1980) and is 

summarized in table 1. These strata are in an early 

stage of the hydrocarbon-forming process and most of 

them are potential source rocks for gas.

The sandstone in the core from borehole 2 (fig. 8) is 

mainly light gray, very fine grained, silty, and noncalcareous. Richardson's (1957) Third Wall Creek sand, 

the lowest prominent unit of sandstone at outcrops in 

the area, is represented in the core by interbedded and 

inter-laminated siltstone, shale, bentonite, and very 

fine grained sandstone. These beds and laminae are 

largely discontinuous, wavy, and nonparallel, and commonly enclose small, horizontal burrows. They show 

flaser and lenticular bedding and slump structures.

The thick unit of sandstone (the Second Wall Creek 

sand) near the middle of the Belle Fourche Member can 

be divided into two parts in the core. The lower part, 

which is about 48 m thick, grades from interlaminated 

shale, siltstone, and very fine grained sandstone at the 

base, to thinly bedded, fine-grained and mediumgrained sandstone at the top. These rocks contain 

laminae and abundant fragments of coaly material. 

Strata in this sequence are discontinuous, wavy, and 

nonparallel at the base (some flaser bedding), bioturbated near the middle, and cross-stratified near the 

top. The lower half of the sequence contains small, 

horizontal-to-vertical burrows and Siphonites. Near 

the top of the sequence, small, horizontal-to-vertical 

burrows and Ophiomorpha are common. The upper 

part of the sandstone unit, which is about 15m thick, 

comprises even, parallel laminae and very thin beds of 

very fine grained sandstone. However, near the top of 

this sequence, some beds are fine grained and medium 

grained, and contain scattered coarse grains and small 

pebbles. Many of these strata are thinly laminated to 

very thinly bedded and are discontinuous, wavy, and 

nonparallel. Some of the upper part of the sandstone 

unit is bioturbated. Small, horizontal and vertical burrows are abundant. A Rhizocorallium was found near 

the base of this sequence, and sparse Ophiomorpha 

and Siphonites occur near the top.

The uppermost of the three prominent units of sandstone (not named by Richardson) in the Belle Fourche 

Member is about 23 m thick in the core (fig. 8). This 

unit, near its base, is composed of interlaminated 

shale, siltstone, and very fine grained sandstone. These 

rocks contain comparatively abundant, very fine to 

coarse fragments of coaly matter. Most of the laminae 

are discontinuous and are either wavy and nonparallel 

or even and parallel. Flaser bedding occurs rarely. 

Overlying the basal strata is a sequence of laminae and 

very thin beds of bioturbated, very fine grained and 

fine-grained sandstone. At the top of the unit, the 

sandstone is fine grained and coarse grained, and contains a few very coarse grains of chert and quartz. 

These strata are laminated to thin bedded and are 

discontinuous, wavy, and non-parallel. In this uppermost unit of sandstone, small horizontal burrows are 

common, and some beds are bioturbated. The unit contains sparse molluscan fossils.

Samples of sandstone from the core, from depths of 

99 m, 143 m, and 150 m, were analyzed by X-ray diffraction and viewed with a SEM. All of the samples 

contain quartz, sodic plagioclase, potassium feldspar, 

clay minerals, and dolomite. The clay, which is mostly 

authigenic, is dominantly kaolinite at a depth of 99 m 

in the uppermost unit of sandstone in the Belle 

Fourche and largely sodium montmorillonite at depths 

of 143 m and 150 m, in the thick unit of sandstone near 

the middle of the member.

The petrology of some of the sandstone in the 

Frontier of Wyoming was summarized by GoodelJ 

(1962, p. 206-208). In thin sections, the sandstone in 

the Belle Fourche Member, at depths of 99 m, 104 m, 

150 m, 164 m, and 242 m, is composed mainly of 

angular to subrounded grains of quartz, chert, and 

feldspar, and sparse biotite and chalcedony. Many 

grains are corroded (fig. 9) and have been partly replaced by clay at depths of 150 m and 242 m, and by 

dolomite and calcite at depths of 99 m, 104 m, and 

164 m. The sample from a depth of 99 m contains a few 

euhedral and subhedral crystals of quartz which are 

probably sand grains with overgrowths. It can be inferred from thin sections that the grains were 

subangular to subrounded prior to diagenesis.

Sandstone in the core, from depths of about 99 m, 

104 m, 150 m, 164 m, and 242 m, was analyzed to 

determine grain sizes. The thin sandstone at 242 m, 

which grades laterally into Richardson's (1957) Third 

Wall Creek sand, consists mostly of fine sand. For the 

sand grains in this sample, the mean size is 2.79 phi 

(fine sand), the standard deviation is 0.51 phi 

(moderately well sorted), skewness is —1.00 (strongly 

coarse skewed), and kurtosis is 3.88 (extremely leptokurtic). The samples from depths of 150 m and 164 m
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 17

100/im

FIGURE 9.—Corroded grains (eg) of quartz (q), chert (ch), and 

feldspar (f) cemented by calcite (c) in a thin section of 

sandstone of the Belle Fourche Member, from a depth of 

164.4 m in borehole 2.

are from the upper part of Richardson's (1957) Second 

Wall Creek sand, which is near the middle of the Belle 

Fourche Member. For the sand grains at 164 m, the 

mean size is 2.31 phi (fine sand), the standard deviation 

is 0.52 phi (moderately well sorted), skewness is +0.49 

(strongly fine skewed), and kurtosis is 2.57 (very leptokurtic). The sand in the sample from 150 m has a mean 

diameter of 2.62 phi (fine sand), a standard deviation of 

0.42 phi (well sorted), a skewness of —0.02 (nearly symmetrical), and a kurtosis of 2.45 (very leptokurtic). 

Samples of the uppermost unit of sandstone in the 

Belle Fourche were obtained from the core at depths of 

99 m and 104 m. They were collected from the upper 

part of the unit. At 104 m, the sand grains have a mean 

diameter of 2.26 phi (fine sand), a standard deviation 

of 0.48 phi (well sorted), a skewness of +0.25 (fine 

skewed), and a kurtosis of 3.40 (extremely leptokurtic). 

At 99 m, the sand grains have a mean diameter of 1.87 

phi (medium sand), a standard deviation of 0.64 phi 

(moderately well sorted), a skewness of +0.22 (fine 

skewed), and a kurtosis of 2.63 (very leptokurtic).

In summary, these samples include clay, silt, and 

very fine to medium grains of sand, but they are 

moderately well sorted to well sorted. Furthermore, 

four of the samples are dominantly fine grained and 

one is dominantly medium grained. The skewness 

varies between strongly coarse skewed and strongly 

fine skewed, but the kurtosis is more consistent, from 

very leptokurtic to extremely leptokurtic. Before post 

depositional alteration, these sandstone units probably contained little clay, were generally well sorted, 

and consisted mainly of subrounded grains. These 

rocks are interpreted to be texturally mature.

Outcrops of the Belle Fourche Member in southern 

Johnson County contain molluscan fossils of middle 

and late Cenomanian and earliest Turonian age. 

Representative collections of these fossils are:

USGS D9829, SW1/4 sec. 7, T. 42 N., R. 81 W., from small limestone 

concretions about 1 m below the top of the Belle Fourche 

Member.

Mytiloides subhercynicus (Seitz)?

Placenticeras? sp.

USGS D5725, SW1/4 sec. 2, T. 43 N., R. 82 W., from top of sandstone unit about 10 m below the top of the Belle Fourche 

Member.

Phelopteria sp.

Inoceramus pictus Sowerby

Ringicula? sp.

Sciponoceras gracile (Shumard)

Worthoceras vermiculum (Shumard)

Scaphites sp.

Neocardioceras? sp.

Placenticeras sp.

Metoicoceras sp.

USGS D5723, SW1/4 sec. 2, T. 43 N., R. 82 W., from septarian 

concretions in a sandstone unit, about 19 m below the top of 

the Belle Fourche Member. 

Inoceramus pictus Sowerby 

Sciponoceras sp.

Dunveganoceras albertense (Warren) 

Metoicoceras muelleri Cobban

USGS D9789, SW1/4 sec. 15, T. 42 N., R. 81 W., from septarian 

concretions in sandstone unit, about 25 m below the top of the 

Belle Fourche Member. 

Inoceramus ginterensis Pergament 

Tarrantoceras sp.

Dunveganoceras albertense (Warren) 

Metoicoceras sp.

USGS D9814, NW1/4 sec. 22, T. 42 N., R. 81 W., from sandstone 

concretions about 42 m below the top of the Belle Fourche 

Member.

Plesiacanthoceras wyomingense (Reagan)

USGS D9812, NW1/4 sec. 22, T. 42 N., R. 81 W., from limestone 

concretions in shale about 52 m below the top of the Belle 

Fourche Member. 

Stomohamites sp. 

Tarrantoceras sp.

Plesiacanthoceras wyomingense (Reagan)

USGS D6947, NE1/4 sec. 27, T. 42 N., R. 81 W., from sandy 

limestone concretions in siltstone, about 54 m below the top 

of the Belle Fourche Member.
18 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

Inoceramus aruanus Stephenson 

Ostrea beloiti Logan 

Acanthoceras amphibolum Morrow 

USGS D8465, SW1/4 sec. 2, T. 43 N., R. 82 W., from calcareous

sandstone concretions in sandstone, about 71m below the top

of the Belle Fourche Member. 

Inoceramus pictus Sowerby 

Acanthoceras pepperense Moreman 

USGS D9817, SE1/4 sec. 35, T. 42 N., R. 81 W., from ferruginous,

silty concretions about 105 m below the top of the Belle

Fourche Member.

Inoceramus cf. /. arvanus Stephenson 

Ostrea beloiti Logan

Acanthoceras cf. A. alvaradoense Moreman 

USGS D9801, Wl/2 sec. 13, T. 42 N., R. 82 W., from ferruginous,

silty concretions about 132 m below the top of the Belle

Fourche Member. 

Ostrea sp.

Turrilites (Euturrilites) scheuchzerianus Bosc 

Acanthoceras muldoonense Cobban and Scott 

USGS D9807, NEl/4 sec. 10, T. 42 N., R. 82 W., from calcareous

silty concretion about 133 m below the top of the Belle

Fourche Member.

Inoceramus eulessanus Stephenson 

Arrhoges? sp.

Acanthoceras muldoonense Cobban and Scott 

Borissiakoceras compressum Cobban 

USGS D9850, SW1/4 sec. 13, T. 42 N., R. 82 W., from calcareous

concretion about 135 m below the top of the Belle Fourche

Member.

Inoceramus macconnelli Warren 

Exogyra sp.

Calycoceras (Conlinoceras) tarrantense (Adkins) 

USGS D9805, SW1/4 sec. 12, T. 42 N., R. 82 W., from calcareous

concretion about 142 m below the top of the Belle Fourche

Member.

Inoceramus eulessanus Stephenson 

Borissiakoceras compressum Cobban 

Johnsonites sulcatus Cobban

Apparently, the Belle Fourche Member in southern 

Johnson County was deposited mainly in shallowwater, nearshore-marine environments near a wavedominated, high-destructive delta (Merewether and 

others, 1979)» Some of the sandstone bodies represent 

nearshore bars (Merewether and others, 1979, p. 80), 

which accumulated in response to southeast- and 

southwest-flowing currents (Towse, 1952, figs. 6 

and 8). The uppermost beds of most sandstone units 

are locally composed of conglomeratic sandstone or 

conglomerate which probably was deposited on 

beaches and in tidal channels. However, Richardson's 

Third Wall Creek sand (1957), in the lower part of the 

Belle Fourche, is a thick, linear, southeast-trending 

body that appears to be a distributary channel deposit."

WALL CREEK MEMBER

The Wall Creek Member, which disconformably 

overlies the Belle Fourche Member near Kaycee, is 

29-38 m thick at most outcrops and boreholes in the

area. However, where cored in borehole 2, the Wall 

Creek is about 27 m thick. In southern Natrona 

County (fig. 1), the Wall Creek disconformably overlies 

the unnamed member of the Frontier (fig. 2).

At outcrops in southern Johnson County (fig. 8), the 

Wall Creek grades from interlaminated shale, siltstone, and very fine grained sandstone at the base of 

the member to fine-grained, cross-stratified sandstone 

at the top of the member. The shale is mainly dark 

gray to olive gray, silty, and noncalcareous. Most of 

the siltstone is medium gray, clayey or sandy, and noncalcareous. In the lower part of,the Wall Creek, these 

rocks generally contain small septarian concretions 

which are sparsely fossiliferous. The sandstone in the 

basal part of the member is medium gray to light 

brownish gray, horizontally laminated, and contains 

horizontal burrows. Near the top of the member, the 

sandstone is light gray to yellowish gray, irregularbedded and planar crossbedded, and contains horizontal and vertical Ophiomorpha and other burrows. The 

crossbeds indicate southeast-flowing paleocurrents 

(Towse, 1952, fig. 10).

Where cored in borehole 2 (fig. 8), the Wall Creek 

Member consists of, in ascending order: interlaminated, dark-gray, silty shale and medium-gray, sandy 

siltstone; medium-gray, argillaceous and sandy 

siltstone; medium-gray, silty, very fine grained sandstone; and interstratified light-gray, very fine grained, 

slightly calcareous sandstone and minor medium-gray, 

silty shale. The laminae of shale and siltstone are 

mostly discontinuous, wavy, and nonparallel, and enclose small horizontal burrows. Some interlaminated 

shale, siltstone, and sandstone in the lower part of the 

member show flaser bedding. Much of the sandstone in 

the Wall Creek at borehole 2 consists of well-sorted, 

angular to subrounded grains and appears to have 

good porosity. Most of the sandstone contains fine 

fragments of coaly matter, and some contains rip-up 

clasts of shale. The silty sandstone is commonly 

bioturbated. In the upper part of the member, the 

slightly calcareous sandstone has microcrosslaminations and discontinuous, wavy, nonparallel 

laminations, and it contains small horizontal burrows.

Shale in the Wall Creek at a depth of 82.5 m in 

borehole 2 was analyzed by X-ray diffraction. It consists mainly of quartz, plagioclase, potassium feldspar, 

dolomite, mica-illite, chlorite, and mixed-layer clays. 

The organic composition of the shale at a depth of 

82.6 m is shown in table 1. Merewether and Claypool 

(1980) concluded that these strata are thermally immature but may be potential source rocks for natural 

gas.

Sandstone in the member, at a depth of 58 m in 

borehole 2, is composed largely of quartz, calcite,
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 19

FIGURE 10.—Angular to subrounded grains of quartz (q), 

chert (ch), and feldspar (f), cemented by calcite (c), in a 

thin section of sandstone of the Wall Creek Member, 

from a depth of 57.9 m in borehole 2.

potassium feldspar, kaolinite, and mica-illite. Clay 

minerals make up 5-10 percent of the sampled bed and 

are mainly authigenic. SEM micrographs of the rock 

show subangular sand grains, interlaminated crystals 

of kaolinite, and moderate porosity (Merewether and 

others, 1976, fig. 8).

Thin sections of the sandstone at depths of 58 m and 

63 m show angular to subrounded grains of quartz, 

chert, feldspar, and minor biotite. The grains of quartz 

and feldspar are commonly corroded and have been 

partly replaced by calcite (fig. 10). Most of the sand 

was probably subrounded when deposited.

Samples of sandstone from depths of 58 m and 63 m 

were analyzed to determine the size and sorting of the 

constituent grains. The sand at a depth of 63 m is 

mainly fine grained (mean size 2.54 phi) and well sorted 

(standard deviation 0.48 phi). For this sample, the 

skewness is —0.03 (nearly symmetrical) and the kurtosis is 2.86 (very leptokurtic). A sandstone near the

top of the Wall Creek, at a depth of 58 m, is also largely 

fine grained (mean size 2.37 phi) and well sorted (standard deviation 0.43 phi). However, the skewness is 0.34 

(strongly fine skewed) and the kurtosis is 3.03 (extremely leptokurtic). When deposited, the sand contained 

little clay, was well sorted, and consisted mostly of 

subrounded grains. The sampled sandstone of the Wall 

Creek may be either texturally mature or texturally 

supermature.

The Wall Creek Member in the vicinity of Kaycee 

contains invertebrate fossils of late Turonian age 

which are typified by Scaphites whitfieldi Cobban. 

Representative collections of the fossils are:

USGS D9792, NWl/4 sec. 1, T. 42 N., R. 82 W., from a sandstone

concretion about 7 m below the top of the Wall Creek

Member.

Inoceramus perplexus Whitfield 

Scaphites whitfieldi Cobban 

Prionocyclus novimexicanus (Marcou) 

USGS D8489, SWl/4 sec. 7, T. 41 N., R. 80 W., from small septarian

concretions about 10 m below the top of the Wall Creek

Member.

Inoceramus perplexus Whitfield 

Prionocyclus novimexicanus (Marcou)

In the southern part of Johnson County, the Wall 

Creek Member grades from interlaminated siltstone 

and shale at the base of the member to cross-stratified, 

well-sorted sandstone at the top of the member. These 

rocks contain burrows and molluscan fossils of marine 

origin and are interpreted to have been deposited in 

shallow water near the western margin of a highdestructive, tide-dominated delta (Merewether and 

others, 1979, p.89-91). The sandstone of the member 

near Kaycee probably accumulated as a submarine offshore bar.

CODY SHALE

Hares and others (1946) applied the name Cody 

Shale to the thick body of gray shale that conformably overlies the Frontier Formation on the west flank 

of the Powder River Basin. In the lower part of the 

Cody, the basal unit of noncalcareous shale and the 

overlying unit of calcareous shale were called the Sage 

Breaks Member and Niobrara Member, respectively, 

by Merewether and others (1977).

SAGE BREAKS MEMBER

In the southwestern part of the Powder River Basin, 

the lower part of the Sage Breaks Member of the Cody 

interfingers with the upper part of the Wall Creek 

Member of the Frontier (Haun, 1958, p. 84). On the 

western flank of the basin in Johnson County, the Sage 

Breaks is conformably overlain by the Niobrara.
20 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

The Sage Breaks Member, which consists mainly of 

soft, gray shale and many septarian concretions, rarely 

crops out and generally forms a topography of little 

relief. In boreholes in the vicinity of Kaycee, the 

member is generally 65-70 m thick. The lower part of 

the Sage Breaks, where cored in borehole 2, is about 

57 m thick. Outcrops of the member near Kaycee are 

composed of dark-gray, silty shale and lesser mediumgray to dark-gray, clayey or slightly sandy siltstone 

(fig. 8). These rocks enclose many layers of septarian 

calcareous concretions and limestone concretions, 

which are as much as 1 m in diameter and are sparsely 

fossiliferous.

Core of the Sage Breaks from borehole 2 is composed 

largely of silty shale and argillaceous siltstone, which 

are medium gray to dark gray and locally slightly 

calcareous. Scattered grains of mica and pyrite were 

identified in hand specimens of the core.

Most of the Sage Breaks is thinly laminated to 

laminated. The laminae are mainly discontinuous and 

either wavy and parallel or even and parallel; however, 

some are wavy and nonparallel. Core from the lower 

part of the member has minor flaser and lenticular bedding. Many laminae are bioturbated or enclose small 

horizontal burrows and Siphonites. They also contain 

fossil mollusks and foraminifera.

A sample of core from a depth of about 49 m, in the 

lower part of the Sage Breaks, was analyzed by X-ray 

diffraction and by thermal evolution and combustion 

procedures. The sampled shale contains quartz, plagioclase, potassium feldspar, dolomite, mica-illite, 

chlorite, and mixed-layer clays. The organic composition of the shale (Merewether and Claypool, 1980) 

(table 1) indicates that the member has some potential 

as a source rock for natural gas.

Outcrops of the Sage Breaks Member near Kaycee 

contain sparse invertebrate fossils of latest Turonian 

through middle Coniacian age. Representative collections of these fossils are:

USGS D9795, NW1/4 sec. 6, T. 42 N., R. 81 W., from shale about

8 m below the top of the Sage Breaks Member. 

Inoceramus deformis Meek

USGS D9826, SW1/4 sec. 6, T. 42 N., R. 81 W., from large septarian 

concretions about 10 m below the top of the Sage Breaks 

Member.

Inoceramus cf. /. lusatiae Andert 

Anisomyon sp. 

Eutrephoceras sp.

USGS D8492, SWl/4 sec. 6, T. 42 N., R. 81 W., from small septarian 

concretions about 19 m below the top of the Sage Breaks 

Member.

Inoceramus erectus Meek (early form)

USGS D8468, NE1/4 sec. 11, T. 43 N., R. 82 W., from septarian 

concretions about 44 m below the top of the Sage Breaks 

Member.

Eutrephoceras sp. 

Proplacenticeras stantoni (Hyatt)

USGS D8467, NE1/4 sec. 11, T. 43 N., R. 82 W., from limestone

concretions about 53 m below the top of the Sage Breaks

Member.

Inoceramus perplexus Whitfield 

Baculites yokoyamai Tokunaga and Shimizu 

Scaphites corvensis Cobban 

Prionocyclus quadratus Cobban 

USGS D9804, SWl/4 sec. 6, T. 42 N., R. 81 W., from septarian

concretions about 56 m below the top of the Sage Breaks

Member. 

Inoceramus sp. 

Prionocyclus quadratus Cobban

In southern Johnson County, near Kaycee, the Sage 

Breaks Member consists mainly of silty shale and contains burrows and molluscan fossils of marine origin. 

These strata accumulated in offshore-marine to openmarine environments of comparatively low current 

energy during a local marine transgression. Units of 

shallow-marine sandstone, which are laterally 

equivalent to parts of the Sage Breaks Member, crop 

out near Osage, Casper, Rawlins, and Riverton (fig. 1).

DEPOSITIONAL ENVIRONMENTS AND 

GEOLOGIC HISTORY

The lower Upper Cretaceous formations in northeastern Wyoming are distinctive and represent a variety of depositional environments. Isopach maps of 

these strata (Merewether and others, 1979) provide 

evidence that the region resembled the wide-shelf 

sedimentary model of Asquith (1974, fig. 4) during 

much of early Late Cretaceous time. His model consists of fluvial, paludal, barrier-island, shelf, 

slope, and basin environments which parallel the 

shoreline and have a collective width of 322-644 km.

The Belle Fourche Shale near Osage, on the eastern 

flank of the Powder River Basin, is about 115 m thick 

and consists mainly of shale and siltstone. These rocks 

accumulated in offshore-marine environments, probably on the slope between the shelf and the basin, 

during early and middle Cenomanian time (fig. 2, 

zones 1-5). They were deposited during a period of 

about 2.0 million years (fig. 2) at a vertical sedimentation rate (after compaction) of about 57.5 m per million 

years. On the western flank of the basin near Kaycee, 

about 180 km west of Osage, strata of early and middle 

Cenomanian age make up most of the Belle Fourche 

Member of the Frontier Formation. This sequence is 

about 170 m thick and includes shale, siltstone, and 

sandstone. These rocks were deposited in shallowmarine and nonmarine environments of the shelf, in 

the distal part of a high-destructive, wave-dominated 

delta (fig. 11). The rate of vertical sedimentation (after 

compaction) for this part of the Belle Fourche Member 

was about 85 m per million years.
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING

107° 106°

21

I C x -——I JOHNSON COUNTY ________• _

40 MILES

I

I I I I I 

10 0 10 20 30 40 50 KILOMETERS

EXPLANATION 

e

—— 15 —— Isopach line—Showing thickness in meters

*2 Borehole—Number designates borehole 

shown on tig 1 and table 2

* Outcrop section or fossil locality—Letter 

designates outcrop section shown on 

figs 1 and 9

FIGURE 11.—Map showing approximate thickness of a representative unit of sandstone (fossil-zones 5 and 6) in the Belle Fourche Member of the Frontier Formation.
22 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

The Greenhorn Formation of the Osage area is about 

85 m thick and is composed largely of calcareous shale 

and limestone. These beds accumulated mainly in the 

open-marine environment of the basin during the middle and late Cenomanian and early Turonian (fig. 2, 

zones 5-11). In the Kaycee area, strata of middle and 

late Cenomanian and earliest Turonian age (fig. 2, 

zones 5-10) form the upper part of the Belle Fourche 

Member of the Frontier. These strata are about 55 m 

thick. Younger beds of Turonian Age (fig.2, zones 

11-16) are missing from the sequence at Kaycee and 

are represented by the disconformity at the base of 

the Wall Creek Member. The upper part of the Belle 

Fourche Member was deposited on the shelf in shallowmarine environments, probably near the seaward terminus of a high-destructive, wave-dominated delta.

In Weston County, the Pool Creek Member of the 

Carlile Shale consists of about 23 m of shale and 

siltstone. These strata accumulated mostly in offshore 

environments on the slope during middle Turonian 

time (fig. 2, zones 12-14). No rocks of middle Turonian 

age occur near Kaycee, in southern Johnson County.

The Turner Sandy Member of the Carlile near Osage 

is about 50 m thick and is composed largely of shale, 

siltstone, and sandstone. It was deposited on a submarine shelf, largely in offshore and shoreface environments, during late Turonian time (fig. 2, zones 16-19). 

The sandstone probably accumulated as channel fillings, nearshore bars, and offshore bars, in the 

distal part of a tide-dominated, high-destructive delta 

(fig. 12).

The basal strata of the Turner Member at Osage 

(fig. 2, zone 16) have no age equivalents in the sequence 

of beds near Kaycee. However, strata near the middle 

of the Turner Member, which have an estimated total 

thickness of as much as 18 m, are approximately 

equivalent in age (fig. 2, zone 17) to the Wall Creek 

Member of the Frontier at Kaycee. The Wall Creek 

Member near Kaycee is about 30 m thick and consists 

of shale, siltstone, and sandstone of shallow-marine 

origin which accumulated on the shelf in offshore and 

shoreface environments. The sandstone was deposited 

as an offshore bar near a tide-dominated, highdestructive delta (fig. 12).

In the vicinity of borehole 1, in Weston County, the 

Sage Breaks Member of the Carlile is mostly slightly 

calcareous shale and is about 91 m thick. The member 

was deposited on the slope and in the basin, in offshore and open-marine environments, during latest 

Turonian, Coniacian, and early Santonian time 

(Evetts, 1976).

Near Osage, strata of latest Turonian through middle Coniacian age (fig. 2, zones 19-22), in the Turner 

and Sage Breaks Members, are as much as 85 m thick. 

This sequence of beds, which was deposited mainly on

the slope and in the basin, is about the same age as the 

Sage Breaks Member of the Cody Shale, where penetrated by borehole 2 in Johnson County. In borehole 2, 

the Sage Breaks is about 57 m thick and consists of silty shale. These laminae probably accumulated in 

offshore-marine environments on the shelf and slope.

In conclusion, the slope sediments of the Belle 

Fourche Shale and the basin sediments of the overlying Greenhorn Formation, in Weston County, are 

about the same age as the shelf sediments of deltaic 

origin in the Belle Fourche Member of the Frontier, in 

Johnson County (fig. 13). Furthermore, the two sequences have similar thicknesses. However, when 

some constituent stratigraphic units of the same age 

are compared, the thickness and rate of vertical 

sedimentation at the two localities are significantly 

different. The main source of these Cenomanian and 

lower Turonian sediments was a large delta which 

probably was in northwestern Wyoming and southwestern Montana (Goodell, 1962, p. 204; Merewether 

and others, 1979).

On the east flank of the Powder River Basin, the 

shale of the Belle Fourche and the overlying calcareous 

rocks of the Greenhorn are interpreted as evidence of a 

marine transgression during the Cenomanian and early 

Turonian. This transgression is also indicated by 

the areal distributions of successive sandstone units 

in the Belle Fourche Member of the Frontier, on the 

west flank of the basin (Merewether and others, 1979, 

figs. 7-9).

A marine regression in the early middle Turonian 

was accompanied by deposition of the basal beds of the 

Pool Creek Member and by the truncation of some 

lower Turonian and upper Cenomanian beds at the top 

of the Belle Fourche Member in the southwestern part 

of the Powder River Basin. During this regression, a 

large delta was developing in western Wyoming which, 

subsequently, prograded east and northeast to the 

southern part of the Powder River Basin (Towse, 1952, 

p. 2001; Cobban and Hook, 1979, fig. 3). Deposition of 

the middle Turonian Pool Creek and the unnamed 

member of the Frontier was followed by a regional 

uplift in the early late Turonian and the erosion of 

some Turonian and Cenomanian beds in northeastern 

Wyoming. During the subsequent marine transgression, this truncated surface was buried by shelf 

sediments, first by the basal Turner near Osage, later 

by the Wall Creek Member near Kaycee (fig. 12), and 

still later by the Wall Creek near Casper. The accumulation of the deltaic sediments of the Turner and 

Wall Creek continued through the late Turonian in this 

region while the delta front retreated toward the 

southwest. This transgression, which began in the 

early late Turonian and continued into the Coniacian in 

Wyoming, is also represented at Osage and Kaycee by
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 23

WESTON COUNTY 

NIOBRARA COUNTY

NATRONA COUNTY 

CARBON COUNTY"

42-

•75-

EXPLANATION

Isopach linein meters

-Showing thickness

30 40 MILES

10 0 10 20 30 40 50 KILOMETERS

Borehole—Number designates 

borehole shown on fig. 1 and 

table 2

Outcrop section or fossil locality — 

Letter designates outcrop section 

shown on figs 1, 2 and 9

Boundary of sandstone body where 

removed by erosion during the 

Turonian

FIGURE 12.—Map showing approximate thickness of a representative unit of sandstone (fossil-zone 17) in the Wall Creek Member of the

Frontier Formation and the Turner Sandy Member of the Carlile Shale.
24 PALEONTOLOGY AND STRATIGRAPHY OF MID-CRETACEOUS ROCKS-NORTHEASTERN WYOMING

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EXPLANATION

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METERS FEET 

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FIGURE 13.—Outcrop sections and depositionaL environments of lower Upper Cretaceous formations in Johnson

and Weston Counties.
MID-CRETACEOUS FORMATIONS, WESTON AND JOHNSON COUNTIES, NORTHEASTERN WYOMING 25

the slope deposits of the Sage Breaks. Another marine 

regression, during the early late Coniacian, caused the 

disconformity at the base of the Niobrara in the 

eastern and southern parts of the Powder River Basin. 

It was followed by another marine transgression and 

deposition of the Niobrara.

REFERENCES CITED

Asquith, D. O., 1974, Sedimentary models, cycles, and deltas, 

Upper Cretaceous, Wyoming: American Association of 

Petroleum Geologists Bulletin, v. 58, no. 11, p. 2274-2283.

Barlow, J. A., Jr., and Haun, J. D., 1966, Regional stratigraphy of 

Frontier Formation and relation to Salt Creek field, Wyoming: 

American Association of Petroleum Geologists Bulletin, v. 50, 

no. 10, p. 2185-2196.

Claypool, G. E., and Reed, P. R., 1976, Thermal-analysis technique 

for source-rock evaluation—quantitative estimate of organic 

richness and effects of lithologic variation: American Association of Petroleum Geologists Bulletin, v. 60, no. 4, p. 608-611.

Cobban, W. A., 1951, Colorado shale of central and northwestern 

Montana and equivalent rocks of Black Hills: American 

Association of Petroleum Geologists Bulletin, v. 35, no. 10, p. 

2170-2198.

Cobban, W. A., and Hook, S. C., 1979, Collignoniceras woollgari 

woollgari (Mantell) ammonite fauna from Upper Cretaceous of 

Western Interior, United States: New Mexico Bureau of Mines 

and Mineral Resources Memoir 37, 51 p.

Cobban, W. A., and Reeside, J. B., Jr., 1952, Frontier formation, 

Wyoming and adjacent areas: American Association of 

Petroleum Geologists Bulletin, v. 36, no. 10, p. 1913-1961.

Collier, A. J., 1922, The Osage oil field, Weston County, Wyoming: 

U.S. Geological Survey Bulletin 736-D, p. 71-110.

Darton, N. H., 1909, Geology and water resources of the northern 

portion of the Black Hills and adjoining regions in South 

Dakota and Wyoming: U.S. Geological Survey Professional 

Paper 65,105 p.

Evetts, M. J., 1976, Microfossil biostratigraphy of the Sage Breaks 

Shale (Upper Cretaceous) in northeastern Wyoming: The 

Mountain Geologist, v. 13, no. 4, p. 115-134.

Folk, R. L., 1974, Petrology of sedimentary rocks (2nd ed.): Austin, 

Texas, Hemphill Publishing Co., 182 p.

Gill, J. R., and Cobban, W. A., 1973, Stratigraphy and geologic 

history of the Montana Group and equivalent rocks, Montana, 

Wyoming, and North and South Dakota: U.S. Geological 

Survey Professional Paper 776, 37 p.

Goodell, H. G., 1962, The stratigraphy and petrology of the Frontier 

formation of Wyoming, in Symposium on Early Cretaceous 

rocks of Wyoming and adjacent areas: Wyoming Geological 

Association, 17th Annual Field Conference, 1962, Guidebook, 

p. 173-210.

Hares, C. J., 1916, Anticlines in central Wyoming: U.S. Geological

Survey Bulletin 641-1, p. 233-279. 

Hares, C. J., and others, 1946, Geologic map of the southeastern

part of the Wind River Basin and adjacent areas in central

Wyoming: U.S. Geological Survey Oil and Gas Investigations

Preliminary Map 51, scale 1:126,720.

Haun, J. D., 1958, Early Upper Cretaceous stratigraphy, Powder 

River Basin, Wyoming, in Wyoming Geological Association, 

13th Annual Field Conference, 1958 Powder River Basin, 

Guidebook, p.84-89.

Knechtel, M. M., and Patterson, S. H., 1962, Bentonite deposits of 

the northern Black Hills district, Wyoming, Montana, and 

South Dakota: U.S. Geological Survey Bulletin 1082-M, 

p. 893-1030.

Merewether, E. A., and Claypool, G. E., 1980, Organic composition 

of some Upper Cretaceous shale, Powder River Basin, Wyoming: American Association of Petroleum Geologists Bulletin (in 

press).

Merewether, E. A., and Cobban, W. A., 1973, Stratigraphic sections 

of the Upper Cretaceous Frontier Formation near Casper and 

Douglas, Wyoming: Wyoming Geological Association Earth 

Science Bulletin (Guidebook Issue), v. 6, no. 4, p. 38-39.

Merewether, E. A., Cobban, W. A., and Cavanaugh, E. T., 1979, 

Frontier Formation and equivalent rocks in eastern Wyoming: 

The Mountain Geologist, v. 6, no. 3, p.67-102.

Merewether, E. A., Cobban, W. A., Matson, R. M., and Magathan, 

W. J., 1977, Stratigraphic diagrams with electric logs of Upper 

Cretaceous rocks, Powder River Basin, Johnson, Campbell, 

and Crook Counties, Wyoming, section A-A': U.S. Geological 

Survey Oil and Gas Investigations Map OC-73.

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

Upper Cretaceous Frontier Formation in the Kaycee-Tisdale 

Mountain area, Johnson County, Wyoming, in Geology and 

energy resources of the Powder River: Wyoming Geological 

Association, 28th Annual Field Conference, September 1976, 

Casper, Wyoming, Guidebook, p. 33-44.

Obradovich, J. D., and Cobban, W. A., 1975, A time-scale for the 

Late Cretaceous of the Western Interior of North America, in 

W. G. E. Caldwell, ed., The Cretaceous System in the Western 

Interior of North America: Geological Association of Canada 

Special Paper 13, p. 31-54.

Richards, P. W., and Rogers, C. P., Jr., 1951, Geology of the Hardin 

area, Big Horn and Yellowstone Counties, Montana: U.S. 

Geological Survey Oil and Gas Investigations Map OM-111, 

scale 1:63,360, 2 sheets.

Richardson, E. E., 1957, Geologic and structure contour map of the 

Tisdale anticline and vicinity, Johnson and Natrona Counties, 

Wyoming: U.S. Geological Survey Oil and Gas Investigations 

Map OM-194, scale 1:31,680.

Robinson, C. S., Mapel, W. J., and Bergendahl, M. H., 1964, Structure and stratigraphy of the northern and western flanks of the 

Black Hills uplift, Wyoming, Montana, and South Dakota: 

U.S. Geological Survey Professional Paper 404,134 p.

Rubey, W. W., 1930, Lithologic studies of fine-grained Upper

Cretaceous sedimentary rocks of the Black Hills region: U.S.

Geological Survey Professional Paper 165-A, p. 1-54. 

Sawyer, M. B., 1977, Image analysis as a method for determining

grain-size distributions in sandstones: U.S. Geological Survey

Open-File Report 77-280,13 p.

Towse, Donald, 1952, Frontier Formation, southwest Powder River 

Basin, Wyoming: American Association of Petroleum 

Geologists Bulletin, v. 36, no. 10, p. 1962-2010.

Veatch, A. C., 1907, Geography and geology of a portion of 

southwestern Wyoming, with special reference to coal and oil: 

U.S. Geological Survey Professional Paper 56,178 p.

Wegemann, C. H., 1911, The Salt Creek oil field, Wyoming: U.S. 

Geological Survey Bulletin 452, p. 37-83.

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