Composition and Depositional Environment of

Concretionary Strata of Early Cenomanian

(early Late Cretaceous) age,

Johnson County, Wyoming

U.S. Geological Survey Bulletin 1917–U

U.S. Department of the Interior

U.S. Geological Survey
Composition and Depositional Environment of

Concretionary Strata of Early Cenomanian

(early Late Cretaceous) age, 

Johnson County, Wyoming

By E.A. Merewether and D.L. Gautier

U.S. Geological Survey Bulletin 1917-U

U.S. Department of the Interior

U.S. Geological Survey

A multidisciplinary approach to research studies of

sedimentary rocks and their constituents and the evolution of

sedimentary basins, both ancient and modern

Evolution of Sedimentary Basins—Powder River Basin
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

This publication is also available online at:

http://greenwood.cr.usgs.gov/pub/bulletin/b1917-u/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Version 1.0

Published in the Central Region, Denver, Colorado

Manuscript approved for publication July 21, 2000

Graphics by E.A. Merewether and Gayle M. Dumonceaux

Photocomposition by Gayle M. Dumonceaux
III

Contents

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

Introduction ................................................................................................................................... 2

Stratigraphy ................................................................................................................................... 3

Description of Core.......................................................................................................... 6

Depositional Environments of Some Cenomanian Rocks ........................................ 7

Paleogeography of Some Cenomanian Rocks ........................................................... 9

Analytical Procedures.................................................................................................................. 9

Composition.................................................................................................................................... 16

Geochemistry of Carbon, Sulfur, and Iron and Authigenic Carbonates .............................. 19

Carbon, Sulfur, and Iron Geochemistry........................................................................ 20

Isotopic Values for Carbon and Oxygen ..................................................................... 22

Interpretations and Conclusions ................................................................................................ 24

Acknowledgments ........................................................................................................................ 26

References .................................................................................................................................... 26

Figures

1. Map of eastern Wyoming, eastern Montana, and adjoining areas, showing

the Powder River Basin ..................................................................................................... 2

2. Photograph showing concretionary strata of the lowermost Frontier Formation ... 3

3. Lithologic log of a lower part of the Frontier Formation ............................................... 4

4. Map showing approximate location of coastal sand during the early early

Cenomanian, late early Cenomanian, and early middle Cenomanian in the

Western Interior of the United States ............................................................................. 5

5–6. Photographs showing: 

5. Teichichnus (T) in a core of Unit 2 at a depth of 291.2 m (955.5 ft) ....................... 6

6. Zoophycos (Z) in a core of Unit 2 at a depth of 300.2 m (984.8 ft) .......................... 7

7. Chart showing age and molluscan-fossil sequence for some mid-Cretaceous

strata in Johnson County, Wyo., and sea-level curves for part of the

mid-Cretaceous .................................................................................................................. 8

8–9. Maps showing: 

8. Locations of speculative strandlines for early Cenomanian (lower

Belle Fourche Shale) epeiric seas in the mid-continent of North America,

the MacKenzie salient, and fossiliferous strata of early Cenomanian age

in southwestern New Mexico and western Texas ................................................. 10

9. Contours of thickness (m) of some lower Cenomanian strata in northeastern

Wyoming and an adjoining part of Montana ........................................................... 11

10. Depth profile for the phosphate (P2O5) content of 20 samples of core from

the Frontier Formation ....................................................................................................... 18
IV

11. Depth profile for the manganese content of 20 samples of core from the

Frontier Formation ............................................................................................................... 19

12. Depth profile for the sulfur content of 20 samples of core from the

Frontier Formation ............................................................................................................... 20

13. Plot showing comparison of the values for the amounts of total sulfur and total

iron for 20 samples of core from Units 1, 2, and 3 of the Frontier Formation ............ 21

14. Plot showing comparison of the amounts of total sulfur and total organic

carbon in 20 samples of core from Units 1, 2, and 3 of the Frontier Formation ........ 21

15. Ternary diagram showing comparison of the amounts of iron, organic

carbon, and total sulfur in 20 samples of core from Units 1, 2, and 3 of

the Frontier Formation ........................................................................................................ 22

16. Plot showing comparison of the values for hydrogen index and the amounts

of total sulfur for 20 samples of core from Units 1, 2, and 3 of

the Frontier Formation ........................................................................................................ 23

17. Plot showing organic carbon and sulfur contents of samples of the three units

compared with values from well-oxygenated, normal marine sediments

and freshwater lake sediments ......................................................................................... 25

18. Comparison of isotopic values for carbon and oxygen in 15 samples of siderite

from Unit 2 (Table 7) of the Frontier Formation with generalized fields of

published isotopic values for siderites from continental and marine 

depositional environments ................................................................................................. 25

1. Mineral constituents of shale and siltstone in Units 1, 2, and 3 from core of

the Frontier Formation ....................................................................................................... 12

2. Mineral constituents of concretions in Unit 2 from core of the

Frontier Formation .............................................................................................................. 13

3. Content, in weight percent, of major-element oxides in shale and siltstone

of Units 1, 2, and 3 from core of the Frontier Formation ............................................... 13

4. Elements in shale and siltstone (in parts per million) of Units 1, 2, and 3 

from core of the Frontier Formation ................................................................................ 14

5. Sulfur and carbon (in weight percent) in shale and siltstone of Units 1, 2, 

and 3 from core of the Frontier Formation ..................................................................... 15

6. Pyrolytic data for shale and siltstone of Units 1, 2, and 3 in core of the

Frontier Formation .............................................................................................................. 16

7. Isotopic values for carbon and oxygen in calcareous and sideritic concretions 

of Unit 2 in core of the Frontier Formation ..................................................................... 17

Tables
1

Composition and Depositional Environment of 

Concretionary Strata of Early Cenomanian 

(early Late Cretaceous) age,

Johnson County, Wyoming

By E.A. Merewether and D.L. Gautier

Abstract

Unusual, concretion-bearing mudrocks of early Late Cretaceous age, which were deposited in an early Cenomanian 

epeiric sea, have been recognized at outcrops in eastern Wyoming and in adjoining areas of Montana, South Dakota, 

Nebraska, and Colorado. In Johnson County, Wyo., on the 

western flank of the Powder River Basin, these strata are in the 

lower part of the Belle Fourche Member of the Frontier Formation. At a core hole in south-central Johnson County, they 

are informally named Unit 2. These strata are about 34 m 

(110 ft) thick and consist mainly of medium- to dark-gray, 

noncalcareous, silty shale and clayey or sandy siltstone; and 

light-gray to grayish-red bentonite. The shale and siltstone are 

either bioturbated or interlaminated; the laminae are discontinuous, parallel, and even or wavy. Several ichnogenera of 

deposit feeders are common in the unit but filter feeders are 

sparse. The unit also contains marine and continental palynomorphs and, near the top, a few arenaceous foraminifers. No 

invertebrate macrofossils have been found in these rocks. Unit 

2 conformably overlies lower Cenomanian shale in the lowermost Belle Fourche Member, informally named Unit 3, and is 

conformably overlain by lower and middle Cenomanian shale, 

siltstone, and sandstone within the member, which are informally named Unit 1.

 The mineral and chemical composition of the three Cenomanian units is comparable and similar to that of shale and 

siltstone in the Upper Cretaceous Pierre Shale, except that these 

units contain more SiO2 and less CaO, carbonate carbon, and 

manganese. Silica is generally more abundant and CaO is generally less abundant in river water than in seawater. The composition of Unit 2 contrasts significantly with that of the 

underlying and overlying units. Unit 2 contains no pyrite and 

dolomite and much less sulfur than Units 1 and 3. Sulfate is 

generally less abundant in river water than in seawater. Unit 2 

also includes sideritic and calcitic concretions, whereas Units 1 

and 3 contain neither concretions nor siderite and only sparse 

calcite. Carbon-sulfur-iron chemistry for the concretions suggests that sulfate availability was the limiting factor in pyrite 

formation and sulfide incorporation in Unit 2. Isotopic compositions of the carbon and oxygen in siderite and calcite from 

several concretions are variable and suggest cementation during 

early diagenesis in a variety of microenvironments. The 

isotopic composition of these carbonate minerals differs from 

that of Upper Cretaceous marine limestones. When considered 

in conjunction with the proportions of sulfur, organic carbon, 

and iron in Unit 2, major-element and micropaleontological 

data suggest that the composition of the original pore waters 

and of overlying waters in the late early Cenomanian sea was 

brackish to fresh. 

The mudrocks of Units 3 and 2, and a lower part of Unit 

1, accumulated on a shelf at low to moderate rates of sedimentation in association with variable but generally weak current action. In Unit 2 and laterally equivalent rocks of the 

region, the sideritic and calcitic concretions probably indicate 

the extent of a body of brackish to fresh and oxygen-deficient water. Rates of precipitation in this region during the 

mid-Cretaceous could have been unusually high and the precipitation probably was seasonal. The organic matter in Unit 

2 is humic-rich and would have been derived from continental environments. If the epeiric sea was brackish to fresh in 

the region of eastern Wyoming and contiguous areas, meteoric runoff from the adjoining lowlands must have been periodically large and the seaway north of the region probably 

was constricted. Seasonal changes in salinity might have 

been accompanied by changes in water temperature and 

oxygen content.

 The lower part of the Frontier Formation (Units 3, 2, and 

1) in Wyoming records an intermittently and easterly prograding shoreline during late early and early middle Cenomanian 

time. Laterally equivalent strata in Nebraska and adjoining 

areas seemingly reflect a westerly prograding shoreline during 

late early Cenomanian time and a retrograding shoreline during the latest early Cenomanian and the early middle Cenomanian. Presumably, in that region a marine regression was 

followed by a marine transgression. Tectonism in western 

Wyoming and adjoining areas during the late Albian and early 

Cenomanian, as well as a eustatic fall in the late early Cenomanian, could have caused marine regressions, whereas regional 

subsidence and a eustatic rise in the latest early Cenomanian 

and the early middle Cenomanian probably caused the transgression. The boreal epeiric sea of early Cenomanian time in 

eastern Wyoming and contiguous areas contracted during the 

late early Cenomanian; thereafter, in the early and middle 

Cenomanian, it expanded to join the Tethyan sea and became 

the Late Cretaceous epicontinental seaway. 
2 Evolution of Sedimentary Basins—Powder River Basin

104 102° 108° 106° ° 110°

48°

46°

44°

42°

40°

NORTH DAKOTA

SOUTH DAKOTA

SOUTH DAKOTA

NEBRASKA

NEBRASKA

UTAH COLORADO

WYOMING

WYOMING

MONTANA

Lewiston

Billings

Miles City

Sheridan

Rapid

City

Newcastle

Cheyenne

Casper

Rawlins

Lander

Craig

Denver

Powder

River

County

Johnson

County

USGS

Bailey

Flats

2

Introduction

Distinctive sedimentary rocks of early Cenomanian (early 

Late Cretaceous) age, characterized by dark-gray mudstone, 

abundant reddish and bluish concretions as thick as 0.6 m (2 

ft), and the absence of molluscan fossils, have been long 

recognized at outcrops along the edges of intermontane basins 

in eastern Wyoming and adjoining areas (fig. 1). In this report, 

we discuss the lithological and paleobiological contrast 

between these mudstones and other Cretaceous rocks in the 

region, and we suggest depositional environments for these 

contrasting rock types. 

Figure 1. Map of eastern Wyoming, eastern Montana, and adjoining areas, showing the Powder River 

basin (short dashed line) and selected counties, as well as outcrops (x), boreholes (o), and the 

approximate geographic extent (long dashed line) of concretionary strata of early Cenomanian age.
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 3

Ferruginous concretionary mudrocks comprise a lower part 

of the Frontier Formation and laterally equivalent lower parts of 

the Belle Fourche Shale, Benton Shale, and Graneros Shale in 

eastern Wyoming and in adjoining areas of Montana, South 

Dakota, Nebraska, and Colorado (fig. 1). Within that region, 

these concretionary rocks are a lithostratigraphic unit that overlies, with apparent conformity, lower Cenomanian beds, either 

the uppermost Mowry Shale or nonconcretionary mudrocks that 

overlie the Mowry. The unit is overlain, seemingly conformably, by nonconcretionary siliciclastic beds of early and middle 

Cenomanian age in the Frontier, Belle Fourche, Benton, and 

Graneros formations. 

The concretionary unit generally consists of noncalcareous, 

silty shale and clayey or sandy siltstone, minor sandstone, and 

bentonite as well as ferruginous concretions and concretionary 

beds (fig. 2). It was first investigated in northeastern Wyoming 

in 1923 by W.W. Ruby who sampled a concretion from a lower 

part of the Belle Fourche Shale, and subsequently determined 

that the concretion was composed of manganiferous siderite 

(Robinson and others, 1964, p. 54). 

At outcrops in the region (fig. 1), the concretion-bearing 

unit ranges in thickness from a featheredge to as much as 104 m 

(340 ft). At outcrops and a core hole in Johnson County, Wyo. 

(Merewether and others, 1976), the unit is about 34 m (110 ft) 

thick (fig. 3). The rocks in Johnson County are herein assigned 

to the late early Cenomanian, although they could possibly be 

early middle Cenomanian. The Mowry Shale, which underlies 

the Frontier and laterally equivalent formations, contains molluscan fossils of early Cenomanian age (Cobban and Kennedy, 

1989). Nonconcretionary beds, which overlie the concretionbearing unit in lower parts of the Frontier and laterally 

equivalent formations, contain molluscan fossils of earliest 

middle Cenomanian age. In Johnson County, early middle 

Cenomanian mollusks were collected from beds about 50 m 

(165 ft) above the concretionary unit. Throughout the region 

(fig. 1), the concretionary strata contain few macrofossils but 

they contain scattered foraminifers, which indicate deposition in 

marine environments of less than normal salinity (Eicher, 1965, 

1967; Eicher and Diner, 1985). Palynomorphs of marine and 

continental origins were identified by Okumura (1994) in core 

of the lower part of the Frontier from Johnson County. 

Dinocysts of the Litosphaeridium siphonophorum biozone were 

recognized in samples of core from the concretionary unit. In 

Wyoming and adjoining areas (McGookey and others, 1972, 

figs. 17, 26), the locations of shorelines for an upper part of the 

Mowry Shale and for the concretion-bearing strata in the overlying formations seemingly indicate a reduction in the areal extent 

of the mid-Cretaceous epeiric sea (fig. 4). 

Stratigraphy

Concretionary strata of late early Cenomanian age, in the 

lower part of the Belle Fourche Member of the Frontier Formation, crop out along the western margin of the Powder River 

Basin (fig. 1). They were cored near the western margin of the 

basin at a borehole, USGS Bailey Flats 2, in south-central 

Johnson County, Wyo. (NE1

/4SW1/4 sec. 6, T. 42 N., R. 81 W.). 

The core is stored by the U.S. Geological Survey in Lakewood, 

Colo. At outcrops in Johnson County (NW1/4NE1/4 sec. 3, T. 41 

N., R. 81 W.), these strata are about 34 m (110 ft) thick and are 

Figure 2. Outcropping concretionary strata of the lowermost Frontier Formation in NE1

/4 sec. 5, T. 

38 N., R. 85 W., Natrona County, Wyo. Arrows indicate conspicuous, ledge-forming, concretionary 

beds. Strata between underlying light-gray bentonite (uppermost Mowry Shale) at far-right center 

and overlying sandstone (Frontier Formation) at center skyline are about 54 m (177 ft) thick. View is 

west-northwest.
4 Evolution of Sedimentary Basins—Powder River Basin

. . . . . .

. . . . . .

. . . . . .

. . . .

. . . . . . . . .

. . . . . . . . . . . .

. . . . . . . . . .

x x x x x x

F

D9853

bt

tf

EXPLANATION

Sandstone

Sandstone, silty

Sandstone, calcareous 

Siltstone

Siltstone, sandy

Siltstone, clayey

Shale

Shale, silty

Bentonite

Concretion, sideritic

Concretion, calcareous

Fossil bivalve

Fossil collection-USGS

 Denver catalog number

Bioturbation

Scattered trace fossils

. . . . . . . . x x x x x x x x x x x

x x x x x x x x x x x . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . .

x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x x x x

. . . . . . .

. . . . . . . .

. . . . 

. . . . . . . .. . . . . . . . . . . . . . . .

. . . . . . . .

. . . . . . . . . 

. . . . . . . . . 

700

750

800

850

. . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . 

. . . . . . . . . 

x x x x x x x x x x x

x x x x x x x x x x x

. . . . . . . . . 

. . . .

. . . . . .

. . . . . .

900

950

1000

(250)

(300)

. . . . . . . . x x x x x x x x x x x

. . . .

x x x x x x x x x x x

. . . .

. . . .

. . . . x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x x x x

x x x x x x x x

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

tf

bt

bt

bt

bt

bt

bt

bt

bt

bt

bt

bt

bt

bt

bt

731.2, 731.6

748.7

781.0

803.4

823.7

845.1

864.5

873.3

889.5

900.2

909.0

911.2

926.6

946.6

972.3

988.7

1009.5

1015.6

1026.7

1031.7

1041.7

F

F

F

F D9853

SAMPLE DEPTH,

IN FEET

DEPTH BELOW SURFACE, 

IN FEET AND (METERS) Unit 2 Unit 3 Unit 1

Belle Fourche Member (part)

Upper Cretaceous (part)

Frontier Formation (part)

USGS BAILEY FLATS 2

Figure 3. Lithologic log of a lower part of the Frontier Formation, from USGS, Bailey Flats 2. NE1

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

Johnson County, Wyo. Location of core hole shown in figure 1.
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 5

CANADA

0 100 200 300 KILOMETERS

0 100 200 MILES

Helena

Billings

Bismarck

Cody

Lander

Rawlins

Casper

Cheyenne

Rapid City

Pierre

Lincoln

Salt

Lake

City

Denver

Pueblo

Montrose

Dodge City

Boise City

Amarillo Albuquerque

Roswell

Fort Worth

Austin

30°

35°

40°

45°

115° 110° 105° 100° 95°

COLORADO

UTAH

ARIZONA

NEW MEXICO

OKLAHOMA

TEXAS

MEXICO

IDAHO

MONTANA NORTH DAKOTA

MINNESOTA

SOUTH DAKOTA

IOWA

NEBRASKA

KANSAS

MISSOURI

WYOMING

Figure 4. Approximate location of coastal sand during the early early Cenomanian (short-dashed lines derived from lithofacies in the upper part of the Mowry Shale), late early Cenomanian (long-dashed lines derived from lithofacies laterally equivalent to the concretionary, lower part of the Belle Fourche Member of the Frontier Formation ), and early middle Cenomanian 

(solid lines derived from lithofacies in the middle part of the Belle Fourche Member) in the Western Interior of the United 

States. Modified after McGookey and others (1972) and Cobban (1993). Location of corehole in northeastern Wyoming shown 

by open circle. 
6 Evolution of Sedimentary Basins—Powder River Basin

T

composed of moderately indurated, medium- to dark-gray, silty 

shale and clayey or sandy siltstone, and soft, light-gray to grayish-red bentonite. The shale and siltstone are noncalcareous and 

enclose scattered, lenticular, dusky-red and dusky-blue 

ferruginous concretions and concretionary beds which are as 

thick as 20 cm (7.9 in.).

Description of Core 

Core of most of the lowest part of the Belle Fourche Member (Unit 3 in this report) from depths of 312-318 m (1024-1042 

ft), which underlies the concretionary strata, consists of interlaminated, medium- to dark-gray silty shale and siltstone and 

laminae and beds of light-gray bentonite (fig. 3). Some of these 

mudrocks are slightly pyritic. The laminae of shale and siltstone 

are discontinuous, wavy, and nonparallel and enclose small, 

mainly horizontal burrows, mostly Planolites, and a few 

Teichichnus, and Asterosoma. Samples of the core also contain 

the following arenaceous foraminifers, in low diversity assemblages (as many as 3 species per sample): Verneuilinoides perplexus (Loeblich) is moderately abundant; Haplophragmoides 

sp., Trochammina sp., Psamminopelta sp., and Ammobaculoides sp. are sparse (B.R. North and W.G.E. Caldwell, retired, 

written commun., 1977). 

Where cored, the concretionary strata (Unit 2 in this report) 

are at depths between 277 and 312 m (910–1,024 ft; fig. 3). The 

strata consist mainly of bioturbated mudrock and interlaminated, 

medium- to dark-gray, silty shale; clayey siltstone; minor sandy 

siltstone; and very light to light gray, bentonitic shale and 

bentonite (fig. 3). The rocks are generally noncalcareous and, 

where not bioturbated, are thinly laminated to very thinly bedded. The laminae and beds, which differ in grain size and composition, are discontinuous, even parallel and discontinuous, 

wavy nonparallel. Trace fossils are common in the shale and 

siltstone, and consist mainly of Planolites. A few specimens of

Chondrites, Teichichnus (fig. 5), Zoophycos (fig. 6), Asterosoma, Terebellina, and Skolithos were recognized. Most of the 

trace fossils are small (less than 5 mm in diameter) and filled 

with mud, silt, or sand; and nearly all represent deposit feeders. 

No foraminifers were found in the core of Unit 2 from depths of 

282 to 312 m (925–1024 ft); one foraminifer (?Gavelinella sp. 

A) was found in Unit 2 from depths of 277 to 282 m (910–925 

ft) (B.R. North and W.G.E. Caldwell, written commun., 1977). 

Palynomorphs in the unit include the species Ovoidinium verrucosum of the dinocyst biozone Litosphaeridium siphonophorum

(Okumura, 1994). Scattered intervals of sideritic to calcareous 

cement in the core are grayish red to light brown, grade into the 

surrounding strata, and are as thick as 0.6 m (2 ft). In outcrops, 

these cemented intervals are concretions. 

Unit 2 of the core can be divided into three parts. The 

basal part is 4.9 m (16 ft) thick and composed mainly of discontinuous, even, parallel laminae of silty shale that encloses a few 

small burrows. The middle part is 18.3 m (60 ft) thick and consists mostly of bioturbated clayey siltstone. The upper part of 

Unit 2 is 11.6 m (38 ft) thick and composed dominantly of discontinuous, even, parallel laminae of silty shale and clayey siltstone that contain many small burrows. 

The rocks overlying the concretionary strata, at depths from 

222.5 m (730 ft) to 277 m (910 ft) in the core hole (Unit 1 in this 

report), consist of bioturbated mudrock and interlaminated, 

Figure 5. Teichichnus (T) in a core of Unit 2 at a depth of 291.2 m (955.5 ft) in USGS Bailey Flats 2, 

south-central Johnson County, Wyo. Location of corehole shown in figs. 1 and 4; stratigraphy of 

core shown in fig. 3. Scale in millimeters and centimeters.

6 Evolution of Sedimentary Basins—Powder River Basin
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 7

Z

Z

medium-gray to dark-gray shale, siltstone, and very fine to 

medium grained sandstone, and light-gray bentonite (fig. 3). 

Some of these rocks are slightly calcareous, and some contain 

pyrite. The laminae of Unit 1 generally contain trace fossils, 

mostly Planolites but a few Teichichnus, Terebellina, and Zoophycos. Low diversity assemblages of mainly arenaceous foraminifers (as many as 4 species per sample) in Unit 1 include 

Trochammina sp. A, Trochammina sp., Trochammina rainwateri, Miliammina ischnia, Ammobaculites impexus, "polymorphinid", Rhizammina sp., Hormosina sp., Psamminopelta sp., 

Haplophragmoides impensus, Haplophragmoides sp. A, and 

Verneuilinoides perplexus (B.R. North and W.G.E. Caldwell, 

written commun., 1977). Molluscan fossils of earliest middle 

Cenomanian age, including the ammonite Borissiakoceras compressum Cobban (USGS Mesozoic locality D9853, identified by 

W. A. Cobban in 1976), were found in the core of this sequence 

at a depth of about 227 m (745 ft). These species belong to a 

Tethyan fauna (fig. 7; fossil zone 6) that characterizes deposits of 

the warm, southern part of the Late Cretaceous epeiric sea, near 

the present-day Gulf of Mexico (Cobban, 1993).

Unit 1 of the core is also divisible into three parts. The 

basal part is 16.5 m (54 ft) thick and consists mainly of bioturbated, clayey to sandy siltstone that is very slightly pyritic. The 

middle part of Unit 1 is 31.4 m (103 ft) thick and composed 

mostly of interlaminated shale, siltstone, and very fine to 

medium-grained sandstone. These laminae are discontinuous, 

wavy, and nonparallel; flaser and contorted bedding and small 

burrows are also present. In the upper part of Unit 1, the rocks 

are mainly interlaminated shale and siltstone and 7.0 m (23 ft) 

thick. The laminations in the upper part are discontinuous, 

wavy, and nonparallel; minor flaser bedding is present. Small 

burrows are abundant in these rocks and marine molluscan 

fossils of middle Cenomanian age (fig. 7; fossil zone 6) were 

found in the uppermost 4.9 m (16 ft) of the unit. 

Depositional Environments of Some

Cenomanian Rocks 

According to Byers and Larson (1979), the uppermost 

Mowry, which underlies Unit 3 of the Frontier in Johnson 

County, was deposited in quiet water in a marginally aerobic 

marine environment, at water depths between 15 and 150 m 

(49-492 ft). The locations of two early Cenomanian strandlines, a middle Cenomanian strandline, and the core hole in 

south-central Johnson County (fig. 4) indicate that Units 3 and 2 

in the core were deposited in offshore environments, probably 

during a marine regression, and that Unit 1 was deposited in 

nearshore environments. Okumura (1994) found that the relative abundance of nonmarine palynomorphs in the core 

increases gradually upward through the three units, which 

reflects the increasing proximity of the western shoreline of the 

seaway to the site of the core hole. Unit 1 was interpreted by 

Tillman (Tillman and Merewether, 1994) as distal valley fill as 

well as tidal and storm deposits in bays and estuaries. At outcrops about 10 km (6 mi) south of the core hole, Unit 1 is represented mainly by the sandstone of Frewens Castle which 

accumulated, according to Willis and others (1999), on the 

shoreface of a tide-influenced river delta. 

The grain sizes of Units 3, 2, and 1 generally increase 

upward. Unit 2 and most of Unit 1 comprise a sequence much 

Figure 6. Zoophycos (Z) in a core of Unit 2 at a depth of 300.2 m (984.8 ft) in USGS Bailey Flats 2, 

south-central Johnson County, Wyo. Location of corehole shown in figs. 1 and 4; stratigraphy of 

core shown in fig. 3. Scale in millimeters and centimeters.

Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 7
8 Evolution of Sedimentary Basins—Powder River Basin Relative change of coastal onlap (Haq and others, 1987) Landward Basinward H

Units

Mowr

y Shale

1

23

Belle Fourche 

Member (part)

Frontier Formation

(part)

South-central

Johnson County, 

Wyo.

94

98

*10 Acanthoceras amphibolum

8 Aca

nthoceras m

uldoo

n

e

nse

7 Acanthoceras 

granerosens

e

* 6 Conlinoceras tarra

nte

nse

(Gap in biostratigraphic record)

5 Neogastroplites maclearni

4 Neo

gastroplites americanus

3 Neo

gastroplites muelleri

* 2 Neogastroplites cornutus

*12 Dunve

ganoceras pondi

11 Plesiacanthoceras wyomingense A h hib l 11 Plesiaca

nth

oceras w

yo

min

g

e

nse

A h hib l

Fossil zones (modified

from Cobban, in Cobban

and others, 1994)

Late 

Cenomanian (part)

Middle

Cenomanian

Early 

Cenomanian

AGE

Late Cretaceous (part) EPOCH 969597 Ma

13 Dunve

ganoceras problematicu

m

9 Aca

nthoceras belle

nse

p

9

Aca

nth

oceras b

elle

nse

p

1 Neo

gastroplites haasi

99 

*

* Radiometrically dated by Obradovich (1993)

Figure 7. Age and molluscan-fossil sequence for some mid-Cretaceous strata in Johnson County, Wyo., and sea-level curves for part of the mid-Cretaceou
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 9

like the progradational sequence of shale described by Rice and 

Gautier (1983). The laminae of mudrocks in Unit 3 and the laminae and thin beds of mudrocks and sandstone in Units 2 and 1 

are discontinuous and are either even and parallel or wavy and 

nonparallel; they apparently record variable, mainly weak currents near the seafloor and a prograding western shoreline. All 

three units contain burrows and Units 2 and 1 are partly bioturbated which suggests intermittent low rates of sedimentation 

(for this epeiric sea) in oxygen-bearing water. The estimated 

rate of vertical sedimentation for Units 3, 2, and 1 is about 13 

cm (5.1 in.) per thousand years. This value is derived from the 

total thickness of the units, an estimated 50 percent compaction 

of the former sediments, and the radiometric ages of bounding 

beds of bentonite (Obradovich and Cobban, 1975; Fouch and 

others, 1983; Obradovich, 1993). However, Tillman proposed 

(Tillman and Merewether, 1994) that Unit 1 includes sequence 

boundaries and flooding surfaces which probably represent 

missing thicknesses of strata. 

The diversity of trace fossils in the three units is low, typical 

of either dysaerobic or brackish-water environments (Ekdale and 

others, 1984, p. 75; Ekdale and Mason, 1988). “Low-diversity 

assemblages of Planolites, Trichichnus and/or Chondrites are 

commonly associated with mudrocks” that were deposited in 

oxygen-deficient, offshore paleoenvironments (Mieras and others, 1993). Most of the trace fossils in the units, Planolites, 

Chondrites, Teichichnus, and Zoophycos, reportedly are abundant in shales that accumulated in poorly oxygenated waters. 

Several of the benthic foraminiferal genera in the low 

diversity assemblages from Units 3 and 1 (B.R. North and 

W.G.E. Caldwell, written commun., 1977) have been assigned 

to shallow-water environments (less than 30 m (100 ft) deep) of 

the shelf (M.A. Carey, oral commun., 1993). Bones of marine 

reptiles, from an ichthyosaur and an elasmosaur, were found in 

strata overlying the Mowry Shale in northeastern Wyoming 

(Nace, 1941; Massare and Dain, 1989). 

 No molluscan fossils of marine, brackish-water, or freshwater origin have been found in Units 3 and 2 in Johnson 

County or in laterally equivalent rocks elsewhere in the region 

(fig. 1). However, if the water in the epeiric sea was less alkaline than normal seawater or was very slightly acidic, as Gill and 

Cobban (1966) suggested, the paucity of calcareous fossils in 

these rocks might indicate only low rates of sedimentation. 

Nevertheless, marine molluscan fossils of late early Cenomanian age have been collected from strata of the boreal sea in 

west-central Canada (Warren and Stelck, 1958) and from beds 

of the Tethyan sea in southwestern New Mexico (Cobban, 1987) 

and western Texas (Böse, 1910; Young, 1958; Cobban and 

Kennedy, 1989) (fig.8). Marine molluscan fossils of early middle Cenomanian age were collected from the upper part of Unit 

1 of the core as well as from many outcrops in the Rocky Mountains (fig. 4; Cobban, 1993). 

A sequence of lower Cenomanian strata that includes Unit 

2 and a lower part of Unit 1 of the core, between depths of 254 

and 312 m (833–1,024 ft) (fig. 3), is confined by two conspicuous beds of bentonite. These bentonites were identified on the 

geophysical logs of about 300 boreholes scattered throughout 

northeastern Wyoming and an adjacent area of Montana, and 

thicknesses of the intervening strata were calculated. The 

thicknesses of the sequence between the bentonites (fig. 9) range 

from less than 40 m (130 ft) in Sheridan County, Wyo., and 

adjoining areas to more than 60 m (200 ft) in the vicinity of 

Crook County, Wyo., and in parts of Natrona, Converse, and 

Niobrara Counties, Wyo. The gradual changes in the thickness 

resemble those of a depositional shelf or possibly a basin. However, in the core hole the strata between depths of 254 m (833 ft) 

and 261 m (857 ft) were interpreted as tidally influenced valleyfill and those between depths of 261 m (857 ft) and 281 m (923 

ft) were interpreted as distal valley-fill and open-bay deposits by 

R.W. Tillman (Tillman and Merewether, 1994). 

Asquith (1970) estimated that the depth of water at the 

outer edge of a Late Cretaceous shelf in northeastern Wyoming, 

where the sediments were dominantly clay, is about 60 m (200 

ft). Weimer (1984) reported that estimates of the water-depths 

for Cretaceous shelves generally range from 30 to 90 m (100 to 

300 ft). The upper lower Cenomanian rocks in southern 

Johnson County, Wyo., were deposited near the inner edge of a 

shelf in shallow water, possibly less than 30 m (100 ft) deep. 

Paleogeography of Some Cenomanian Rocks 

The Mowry Shale was deposited in early Cenomanian time 

(Stelck and Armstrong, 1981; Cobban and Kennedy, 1989) in a 

boreal epeiric sea that extended from the Arctic Ocean to southcentral Colorado (fig. 4; Haun and Weimer, 1960). In the Frontier Formation of Johnson County, Wyo., the upper part of Unit 1 

accumulated during middle Cenomanian time in an epicontinental sea that extended northward from the Gulf of Mexico to the 

Arctic Ocean (fig. 4). This part of Unit 1 contains Tethyan molluscan fossils of earliest middle Cenomanian age (zone 6, fig. 7). 

The strata between the top of the Mowry and the upper part of 

Unit 1, assigned to Units 3, 2, and the lower part of Unit 1, record 

deposition generally during a marine regression and a subsequent widespread marine transgression that contributed to the 

opening of the Late Cretaceous transcontinental seaway (fig.4). 

 Studies by Eicher (1965, 1967) and Eicher and Diner 

(1985) of foraminifera in the outcropping lower Cenomanian 

beds of Colorado, Wyoming, and Montana include two fossil 

localities in northeastern Wyoming and one in south-central 

Montana. Eicher (1967) reported that about 90 percent of his 

samples contained only arenaceous foraminifers, and that foraminifera in northeastern Wyoming are sparse in the lowermost 

Belle Fourche but increase in abundance and variety upward in 

the sequence. From their data, Eicher (1967) and Eicher and 

Diner (1985) concluded that this early Cenomanian assemblage 

lived in a restricted seaway, probably in poorly oxygenated water 

of less than normal marine salinity, and that an open sea of normal salinity lay to the south. The data collected during the current investigation are consistent with most of their conclusions. 

Analytical Procedures

The mineral constituents of 22 whole-rock samples of core 

from the lower part of the Belle Fourche Member of the Frontier 
10 Evolution of Sedimentary Basins—Powder River Basin

0

0

1000 KILOMETERS

600 MILES

MEXICO

MacKenzie

 Salient

CANADA

N

UNITED STATES

Formation (Units 3, 2, and 1; figs. 1, 3) were determined by Xray diffraction (XRD; tables 1, 2). Amounts of the oxides of 

major elements in 20 samples of the core (table 3) were measured by X-ray spectroscopy (Taggart and others, 1987). The 20 

samples were also analyzed for many elements (table 4) by 

induction-coupled plasma spectroscopy (Crock and others, 

1987). Amounts of sulfur and total carbon (table 5) were determined by combustion with infrared detection; the amount of carbonate carbon was determined by acidification and coulometric 

titration; and the organic-carbon content was calculated from the 

amounts of total carbon and carbonate carbon (Jackson and 

others, 1987). 

Organic matter (table 6) was analyzed by using the RockEval procedure of Espitalie and others (1977). In this procedure, amounts of hydrocarbons in the samples are estimated 

from pyrolysis assays and are reported as S1 (free or absorbed 

hydrocarbons in the rock), S2 (hydrocarbons produced mainly 

by pyrolysis of solid organic matter), S3 (carbon dioxide produced by pyrolysis of organic matter), and T(S2) (temperature 

of maximum pyrolytic yield) (G.E. Claypool, written commun., 

Figure 8. Locations of speculative strandlines (dotted lines) for early Cenomanian (lower Belle Fourche Shale) epeiric 

seas in the mid-continent of North America, of the MacKenzie salient, and of fossiliferous strata (indicated by ) of early 

Cenomanian age in southwestern New Mexico and western Texas. Modified from McGookey and others (1972), Williams 

and Stelck (1975), and Stelck and Armstrong (1981). 
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 11

104°

0 20 40 60 KILOMETERS

0 10 20 30 40 MILES

BIG HORN

ROSEBUD

ο ο ο

ο

ο

ο ο ο ο

ο ο ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο ο

ο

ο ο

ο

ο ο

ο

ο ο

ο ο

ο

ο

ο

οο ο

ο

ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο

ο ο ο

ο

ο ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο ο

ο

ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο

ο

οο

ο ο

ο

ο ο ο ο ο

ο

ο ο

ο

ο

ο ο ο

ο

ο

ο

ο

ο

ο

ο

ο ο

ο

ο

οο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο ο ο ο ο

ο

ο ο

οο

ο

ο ο

ο

οο ο

ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο ο

ο

ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο οο

ο

ο

ο

ο

ο

ο ο

ο

ο

ο

ο

ο ο

ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο ο

ο

ο

ο

ο

ο ο

ο

ο ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο

ο οο ο

ο

ο

ο

ο

ο ο

ο ο

ο

ο

ο ο οο

ο

ο

ο

ο ο

ο

ο

ο

ο ο

ο

ο

ο

οο

ο

ο

ο

ο

ο

ο

ο

POWDER RIVER CARTER

CROOK

CAMPBELL

ο

WESTON

NIOBRARA

CONVERSE

PLATTE GOSHEN

ALBANY

CARBON

FREMONT

NATRONA

WASHAKIE

BIG HORN

SHERIDAN

JOHNSON

Sheridan

ο

MONTANA

WYOMING

WYOMING

SOUTH DAKOTA NEBRASKA

40

50

60

60

50

50

40

40

50

50

60

50

60

USGS

BAILEY 

FLATS 2 

108° 107° 106° 105°

45°

44°

43°

Buffalo

Gillette

Newcastle

Casper

Figure 9. Contours of thickness (m) of some lower Cenomanian strata in northeastern Wyoming and an adjoining part of Montana. Thicknesses determined from geophysical logs of boreholes (o). Location of corehole, USGS Baily Flats 2, in southern Johnson County, Wyo., 

shown by black dot.
12 Evolution of Sedimentary Basins—Powder River Basin Stratigraphic unit Depth in m Depth in ft Quartz Potassium feldspar Plagioclase Kaolinite- chlorite Illite- montmorillonite Sodium- montmorillonite Mixed laye

clay 

Unit 1 222.9 731.2 X X X X X X 

223 731.6 X X X X X X 228.2 748.7 X X X X X X 238.1 781 X X X X X X 244.9 803.4 X X X X X X 251.1 823.7 X X X X X X X 257.6 845.1 X X X X X X 263.5 864.5 X X X X X X 266.2 873.3 X X X X X X 271.1 889.5 X X X X X X 274.4 900.2 X X X X X X 277.1 909 X X X X X X Unit 2 277.7 911.2 X X X X X X 282.4 926.6 X X X X X X288.5 946.6 X X X X X X296.4 972.3 X X X X X X301.4 988.7 X X X X X X307.7 1009.5 X X X X X X309.6 1015.6 X X X X X X X

Unit 3 312.9 1026.7 X X X X X X 

314.5 1031.7 X X X X X X 317.5 1041.7 X X X X X 

Table 1. Mineral constituents of shale and siltstone in Units 1, 2, and 3 from core of the Frontier Formation, Johnson County, Wyo.

[No entry indicates mineral was not found in sample. Minerals identified using X-ray diffraction. Location of core hole shown in fig. 1; stratigraphic units shown in fig. 3]
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 13

1 281.8 924.5 XX X X X XX

1 282.1 925.5 XX X X X XX

5 287.2- 942.2-

287.3 942.7 X X X XX X

6 291.7- 957.0-

291.9 957.8 XX X X X XX XX

5 293.1- 961.6-

293.2 962 XX XX X X XX

5 306.7- 1006.2-

306.9 1006.8 XX X X X XX XX 

Table 2. Mineral constituents of concretions in Unit 2 from core of the Frontier 

Formation, Johnson County, Wyo.

[More abundant minerals (XX), less abundant minerals (X). No entry indicates mineral was not found in 

sample. Minerals identified using X-ray diffraction. Location of core hole shown in fig. 1]

No. of Depth Depth Quartz Potassium Plagioclase Clay Calcite Siderite

samples in m in ft feldspar 

Table 3. Content, in weight percent, of major element oxides in shale and siltstone of Units 1, 2, and 3 from core of the Frontier 

Formation, Johnson County, Wyo. 

[All oxides determined by X-ray fluorescence spectroscopy. Location of core hole shown in fig. 1; stratigraphic units shown in fig. 3] 

Stratigraphic unit Depth, Depth, SiO2 Al2O3 FeTO3 MgO CaO Na2O K2O TiO2 P2O5

in m in ft

Unit 1 223 731.6 69.3 12.6 3.11 1.49 0.98 1.53 2.27 0.5 0.16

228.2 748.7 63.5 14.7 3.95 1.62 0.67 1.26 3.23 0.59 0.18

238.1 781 63.8 16.2 3.81 1.25 0.64 1.41 2.88 0.55 0.14

244.9 803.4 66.4 14.4 3.9 1.2 0.72 1.38 2.57 0.62 0.15

251.1 823.7 64.8 14.9 4.11 1.4 0.64 1.33 2.68 0.6 0.16

257.6 845.1 71.7 12.5 3 0.88 0.37 0.94 2.97 0.57 0.17

263.5 864.5 72.7 11.6 3.71 1.01 0.52 1.23 2.19 0.47 0.17

266.2 873.3 69.6 12.9 3.73 1.17 0.51 1.33 2.31 0.5 0.18

271.1 889.5 68.7 14 3.69 1.02 0.42 1.08 3.16 0.55 0.16

274.4 900.2 69.4 13.7 3.47 1.04 0.34 0.97 3.57 0.56 0.16

Unit2 277.7 911.2 67.1 14.8 3.77 1.2 0.46 1.21 2.96 0.52 0.14

282.4 926.6 68.8 14.2 3.29 1.11 0.53 1.31 2.44 0.5 0.14

288.5 946.6 68 14.7 2.92 1.01 0.44 1.11 3.56 0.58 0.13

296.4 972.3 71.1 13.3 3.15 0.91 0.59 1.26 2.65 0.48 0.13

301.4 988.7 70.9 13.3 3.03 0.95 0.62 1.59 1.97 0.42 0.11

307.7 1009.5 66.8 15.4 3.36 1.03 0.38 1.18 3.1 0.52 0.1

309.6 1015.6 64.2 16.4 4.04 1.14 0.39 1.25 2.43 0.47 0.1

Unit3 312.9 1026.7 68.8 13.9 3.61 0.97 0.47 1.1 2.84 0.49 0.12

314.5 1031.7 66.9 14.8 3.6 1.02 0.41 1.17 2.83 0.49 0.11

317.5 1041.7 72.8 11.9 2.21 0.94 0.45 1.89 0.84 0.18 0.07

Unit 1, mean 68 13.8 3.65 1.21 0.58 1.25 2.78 0.55 0.16

std. dev. 3.2 1.4 0.36 0.24 0.19 0.19 0.46 0.05 0.01

Unit 2, mean 68.1 14.6 3.37 1.05 0.49 1.27 2.73 0.5 0.12

std. dev. 2.4 1.1 0.41 0.11 0.1 0.15 0.52 0.05 0.02

Unit 3, mean 69.5 13.5 3.14 0.98 0.44 1.39 2.17 0.39 0.1

std. dev. 3 1.5 0.81 0.04 0.03 0.44 1.15 0.18 0.03
14 Evolution of Sedimentary Basins—Powder River Basin Table 4. Elements in shale and siltstone (in parts per million) of Units 1, 2, and 3 from core of the Frontier Formation, Johnson County, W

[Elements identified by induction-coupled plasma spectroscopy. Location of core hole shown in fig. 1; stratigraphic units shown in fig. 3] Stratigraphic Depth, Depth, Mn, Ba, Be, Ce, Co, Cr, Cu, Ga, La, Li, Nb, Nd, Ni, Pb, Sc, Sr, 

unit in m in ft in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm in ppm 

Unit 1 223 731.6 110 690 2 69 9 77 25 19 41 41 10 30 33 24 11 210 

228.2 748.7 130 630 3 62 7 75 41 18 36 41 10 29 30 20 10 200 238.1 781 130 650 3 110 9 70 32 21 62 46 17 58 29 26 13 230 244.9 803.4 150 640 2 79 8 56 55 18 46 36 13 36 22 21 9 220 251.1 823.7 90 600 2 76 7 53 26 16 44 33 9 35 22 20 9 210 257.6 845.1 100 590 2 65 9 69 25 17 39 36 7 31 28 23 10 130 263.5 864.5 76 600 2 61 7 64 19 15 36 30 4 31 21 16 8 170 266.2 873.3 72 620 2 65 7 68 20 17 38 30 30 23 17 8 180 271.1 889.5 83 700 3 72 12 82 27 19 42 40 9 37 34 22 11 170 274.4 900.2 78 640 2 66 13 78 29 19 39 37 5 32 37 23 11 150 

Unit 2 277.7 911.2 77 680 3 75 12 93 30 21 44 41 12 37 44 24 12 190 

282.4 926.6 170 680 3 72 7 68 25 19 43 33 4 35 25 21 9 200 288.5 946.6 250 620 3 69 10 86 32 20 41 37 8 34 34 22 12 170 296.4 972.3 150 660 3 72 10 64 25 18 42 33 8 34 31 23 9 180 301.4 988.7 160 680 3 77 7 52 20 19 45 27 13 37 25 23 8 220 307.7 1009.5 160 580 3 73 13 84 28 22 44 40 12 36 35 20 12 180 309.6 1015.6 280 560 3 72 10 71 35 22 43 44 11 34 28 22 11 180 

Unit 3 312.9 1026.7 76 550 3 67 7 66 25 18 40 36 8 32 26 19 10 170 

314.5 1031.7 88 540 3 70 8 62 32 19 41 36 9 33 24 22 10 160 317.5 1041.7 51 1400 4 68 4 19 11 17 38 33 9 32 10 26 5 260 

Unit 1, mean 102 636 2 72 9 69 30 18 42 37 9 35 28 21 10 187 

std. dev. 27 37 0 14 2 9 11 2 8 5 4 9 6 3 2 32 

Unit 2, mean 178 637 3 73 10 74 28 20 43 36 10 35 32 22 10 189 

std. dev. 67 51 0 3 2 14 5 2 1 6 3 1 7 1 2 17 

Unit 3, mean 72 830 3 68 6 49 23 18 40 35 9 32 20 22 8 197 

std. dev. 19 494 1 2 2 26 11 1 2 2 1 1 9 4 3 55 
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 15

1984). The genetic potential or total hydrocarbon yield (S1 + 

S2) is a semiquantitative determination of the original hydrocarbon-generating capacity of the sampled rock. The hydrogen 

index (HI), which is the pyrolytic hydrocarbon yield (S2) normalized by the organic carbon content, is used to determine the 

type of organic matter in the sample. Similarly, the oxygen 

index (OI) is the pyrolytic organic CO2 yield (S3) normalized by 

the organic carbon content, which also helps to evaluate the type 

of organic matter. The transformation ratio or production index 

(S1/S1 + S2), as well as the temperature of maximum pyrolytic 

yield [T(S2)], reflect the thermal maturity of the sampled strata. 

The core of Unit 2 contains many concretions, most of 

which are cemented by siderite and calcite. Samples were collected from vertical profiles through most of the concretions; 

several samples were collected from each concretion. The 

samples and their depths within the core are listed in tables 2 

and 7. Each sample was crushed and ground in a porcelain mill 

for standard mineral analysis by X-ray diffraction and for stable 

carbon and oxygen isotopic analysis. Amounts of carbon and 

oxygen isotopes were measured using a double-collecting, 

isotope-ratio, Finnigan MAT mass spectrometer on CO2 generated by reacting about 50 mg of crushed sample with an excess 

of 100 percent phosphoric acid (McCrea, 1950). The samples of 

siderite were reacted for six days at 25°C and no fractionation 

factor was applied (as would be necessary at higher temperatures of reaction) to the δ18 O data; samples of calcite were 

treated similarly. All carbon and oxygen isotope data are 

reported with reference to the PDB-University of Chicago 

standard. For most samples, stable carbon and oxygen isotopic 

compositions of either calcite or siderite were measured; in 

several samples however, significant amounts of both calcite and 

siderite were recognized. In such samples, separation and 

collection of CO2 gas from both carbonates was attempted by 

means of a differential dissolution procedure whereby CO2

Table 5. Sulfur and carbon (in weight-percent) in shale and siltstone of Units 1, 2, and 3 from core of 

the Frontier Formation, Johnson County, Wyo. 

[Total sulfur and total carbon by combustion with infrared detection; carbonate carbon by acidification and coulometric 

titration; organic carbon, calculated amounts of total and carbonate carbon using method of Jackson and others (1987). 

Location of core hole shown in fig. 1; stratigraphic units shown in fig. 3] 

Stratigraphic Depth, Depth, Total S Total C Organic C Carbonate C

unit in m in ft

Unit 1 223 731.6 0.6 1.36 1.17 0.19

228.2 748.7 1 2.14 2.04 0.1

238.1 781 0.84 1.82 1.8 0.02

244.9 803.4 0.77 1.66 1.58 0.08

251.1 823.7 0.97 1.9 1.82 0.08

257.6 845.1 0.9 1.18 1.18 0

263.5 864.5 1.01 1.08 1.06 0.02

266.2 873.3 0.83 1.44 1.42 0.02

271.1 889.5 0.64 1.26 1.24 0.02

274.4 900.2 0.36 1.41 1.39 0.02

Unit 2 277.7 911.2 0.09 1.58 1.55 0.03

282.4 926.6 0.06 1.43 1.38 0.05

288.5 946.6 0.06 1.55 1.49 0.06

296.4 972.3 0.1 1.23 1.18 0.05

301.4 988.7 0.11 1.02 0.96 0.06

307.7 1009.5 0.08 1.45 1.41 0.04

309.6 1015.6 0.08 1.52 1.46 0.06

Unit 3 312.9 1026.7 0.49 1.34 1.32 0.02

314.5 1031.7 0.44 1.68 1.66 0.02

317.5 1041.7 0.21 0.92 0.87 0.05

Unit 1, mean 0.79 1.53 1.47 0.06

std. dev. 0.21 0.34 0.33 0.06

Unit 2, mean 0.08 1.4 1.35 0.05

std. dev. 0.02 0.2 0.21 0.01

Unit 3, mean 0.38 1.31 1.28 0.03

std. dev. 0.15 0.38 0.4 0.02
16 Evolution of Sedimentary Basins—Powder River Basin

released during the first hour of phosphoric-acid digestion is 

regarded as being derived entirely from calcite. Any CO2

collected between one and two hours is discarded and CO2

collected after two hours is regarded as derived entirely from 

siderite. 

Composition

The composition of the 20 samples of the Belle Fourche 

Member from Johnson County, Wyo., was compared to the composition of 134 samples of marine rocks and 58 samples of nonmarine rocks from the Upper Cretaceous Pierre Shale and 

equivalent strata in Montana, North Dakota, and South Dakota 

described by Schultz and others (1980, table 14). The shale and 

siltstone of the Pierre and equivalent beds were assigned to 

several categories of marine and nonmarine rocks. The compositions of the three units of the Belle Fourche Member are similar to those of marine rocks of the Pierre except for the amounts 

(arithmetic means) of SiO2, CaO, carbonate carbon, and manganese. The mean content of SiO2 is larger in the three units 

(about 68–70 percent) than in the Pierre (about 61 percent), 

although the SiO2 content of the units is similar to that of 

marine siliceous shale (67 percent, 7 samples) in the Pierre. 

Average amounts of CaO in the three units are 0.44–0.58 percent; significantly less than the 1.9 percent in most marine shale 

and siltstone of the Pierre; however, marine shale overlying bentonite in the Pierre (6 samples) contains 0.72 percent CaO. 

 Silica, iron, phosphorus, and dissolved organic matter are 

more abundant and calcium and sulfate are less abundant in 

river water than in seawater, according to Liss (1976) and 

Berner and Berner (1996). Mean amounts of carbonate carbon 

in the units (0.03–0.06 percent) are also much lower than the 

Table 6. Pyrolitic data from shale and siltstone of Units 1, 2, and 3 in core of the Frontier Formation, Johnson County, 

Wyo. 

[Determined with Rock-Eval procedures of Espitalie and others (1977). TOC, total organic carbon in percent; Tmax, temperature of maximum 

yield of hydrocarbons during pyrolysis in degrees C; HI, hydrogen index (mg HC/g TOC); OI, oxygen index (mg CO2/g TOC); PI, production 

index (S1/S1+S2); S1, S2, S3, mg of HC or CO2/g rock]

Stratigraphic Depth, Depth, TOC Tmax HI OI PI S1mg/g S2mg/g S3mg/g

unit in m in ft

Unit 1 223 731.6 1.29 426 51 27 0 0 0.67 0.35

228.2 748.7 1.33 427 87 24 0.04 0.05 1.17 0.32

238.1 781 1.47 431 68 23 0.01 0.01 1 0.35

244.9 803.4 1.23 430 83 35 0.03 0.03 1.03 0.44

251.1 823.7 1.34 439 59 29 0.06 0.05 0.8 0.4

257.6 845.1 1.2 430 103 27 0.03 0.04 1.24 0.33

263.5 864.5 1.26 425 78 28 0.03 0.03 0.99 0.36

266.2 873.3 1.24 429 79 33 0.01 0.01 0.99 0.41

271.1 889.5 1.45 430 60 26 0 0 0.88 0.38

274.4 900.2 1.43 433 91 27 0.01 0.01 1.31 0.39

Unit 2 277.7 911.2 1.64 434 117 25 0.01 0.01 1.93 0.41

282.4 926.6 1.41 432 109 28 0 0 1.55 0.4

288.5 946.6 1.71 432 119 26 0 0.01 2.05 0.45

296.4 972.3 1.55 431 129 27 0 0.01 2.01 0.42

301.4 988.7 0.81 428 86 43 0.01 0.01 0.7 0.35

307.7 1009.5 1.46 431 89 28 0.02 0.03 1.3 0.41

309.6 1015.6 1.64 431 75 28 0.01 0.01 1.24 0.46

Unit 3 312.9 1026.7 1.09 430 112 30 0.01 0.01 1.23 0.33

314.5 1031.7 1.42 433 179 26 0.01 0.02 2.55 0.37

317.5 1041.7 0.76 422 88 40 0.03 0.02 0.67 0.31

Unit 1, mean 1.32 430 76 28 0.02 0.02 1.01 0.37

std. dev. 0.1 4 16 4 0.02 0.02 0.2 0.04

Unit 2, mean 1.46 431 103 29 0.01 0.01 1.54 0.41

std. dev. 0.31 2 20 6 0.01 0.01 0.5 0.04

Unit 3, mean 1.09 428 126 32 0.02 0.02 1.48 0.34

std. dev. 0.33 6 47 7 0.01 0.01 0.97 0.03
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 17

mean amount in marine shale and siltstone of the Pierre (0.45 

percent, 134 samples), except for marine shale overlying bentonite (0.07 percent, 6 samples). The low values for CaO and carbonate carbon in the units probably reflect the dearth of calcite 

in the sequence and the exclusion of data from the concretions in 

Unit 2. The mean content of manganese in the units is 72–178 

ppm. In marine shale and siltstone of the Pierre (134 samples), 

the average manganese content is 750 ppm, although the average 

manganese content of marine shale overlying bentonite (6 

samples) is 135 ppm. 

The lithologic differences between the concretionary strata 

(Unit 2 of the Belle Fourche Member) and the underlying and 

overlying rocks (Unit 3 and Unit 1 of the Belle Fourche, respectively), which are noticeable at outcrops, correspond to differences in the mineralogical and chemical composition of core 

from the three units. Mineral constituents of 22 samples of the 

core are listed in table 1. Samples of the laminae and beds generally contain quartz, potassium feldspar, plagioclase, kaolinitechlorite, illite-montmorillonite, and other mixed-layer clays. 

Several of these samples include sodium montmorillonite. Most 

of the clays probably formed during early diagenesis. The three 

samples from Unit 3 also contain pyrite and possibly minor 

dolomite. Pyrite and dolomite were found in most of the samples from Unit 1 but were not confirmed in the seven samples 

from Unit 2. Most of the concretions in Unit 2 contain quartz, 

plagioclase, clay, calcite, and siderite; several of them also 

contain potassium feldspar (table 2).

Amounts of the oxides of major elements in 20 samples of 

the core are shown in table 3. The major-element compositions of 

samples from the three units, excluding the concretions in Unit 2, 

are very similar, although the content of P2O5 seems to decrease 

slightly with depth (fig. 10). The P2O5 in these rocks could be in 

constituent clay minerals, volcanogenic apatite, and biogenic 

apatite, as suggested by Schultz and others (1980, p. B59). 

The 20 samples of core, excluding those from concretions, 

were also analyzed for chemical elements. Mean amounts of the 

elements in Units 3, 2, and 1 vary slightly, although not significantly, with the depth and age of the units (table 4). Nevertheless, the mean content of manganese is greater in Unit 2 than in 

Units 1 and 3 (fig. 11). Amounts of boron and vanadium in sedimentary rocks have been used to indicate the salinity of basin 

waters and to separate marine and nonmarine depositional environments (Potter and others, 1963). For these samples, the 

boron content was not determined and the vanadium content 

ranges from 24 ppm to 120 ppm, which is significantly less than 

the values (226–353 ppm) reported by Leckie and others (1990) 

for estuarine, restricted marginal marine, and open-marine 

shales of the Cretaceous Shaftesbury Formation in Alberta. 

Table 7. Isotopic values for carbon and oxygen in calcareous and sideritic concretions 

of unit 2 in core of the Frontier Formation, Johnson County, Wyo.

[Values determined by mass spectroscopy. Standard NBS19 relative to PDB: ∆13 C+1.95/mil; ∆18 0-2.20/mil] 

Depth, ∆13 C- ∆18 O- ∆13 C- ∆18 Oin m (ft) CALCAREOUS CALCAREOUS SIDERITIC SIDERITIC

281.8 (924.5) -3.82 -6.85

282.1 (925.5) -1.42 -6

287.2-287.3 0.48 -9.57

(942.2-942.7) 0.29 -9.71

0.38 -9.76

1.49 -9.63

1.17 -9.59

291.7-291.9 -13.62 -11.6 -10.99 -10.61

(957-957.8) -12.5 -11.27 -7.7 -7.74

-1.77 -6.76

-2.03 -6.32

-6.29 -8.01

-12.49 -11.5

293.2 (962.0) 7.93 -9.1

8.25 -7.97

4.33 -8.92

306.7-306.9 -15.25 -12.09 -5.59 -9.52

(1006.2-1006.8)

-5.91 -7.98

-4.53 -7.67

-6.59 -8.95

-18.67 -12.5 -2.13 -10.53
18 Evolution of Sedimentary Basins—Powder River Basin

-220

-240

-260

-280

-300

-320

0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

P205, IN PERCENT

DEPTH, IN METERS

Unit 1

Unit 2

Unit 3

The amounts of sulfur and carbon in the 20 samples are 

listed in table 5. Mean values for total carbon, organic carbon, 

and carbonate carbon in each of the three units, not including 

data from the concretions in Unit 2, are only slightly different. 

The sulfur content, however, is much lower in Unit 2 than in 

Units 1 and 3 (fig. 12).

The amount and character of the organic matter in the three 

cored units are represented in table 6. The content of total organic 

carbon (TOC) ranges from 1.20 to 1.47 percent (mean is 1.32 percent) for Unit 1, from 0.81 to 1.71 percent (mean is 1.46 percent) 

for Unit 2, and from 0.76 to 1.42 percent (mean is 1.09 percent) 

for Unit 3. For the samples from each unit, the mean temperature 

of maximum pyrolytic yield (Tmax) ranges from 428°C to 432 

°C, and is slightly less than the temperature of 435°C that generally represents the initial generation of oil in source rocks. 

Average amounts of free or absorbed hydrocarbons in samples of the units (S1) range from about 0.01 to 0.02 mg/g. The 

mean amounts of hydrocarbons produced mainly by pyrolysis of 

solid organic matter in the units (S2) range from 1.01 to 1.54 mg/

g. Mean amounts of carbon dioxide produced by pyrolysis of 

organic matter (S3) in the three units range from 0.34 to 0.41 mg/

g. The production index (PI) is S1/S1+S2 and, where averaged 

for each unit, ranges from 0.01 to 0.02. These values are less 

than the PI of about 0.1 that indicates the transition from thermal immaturity to thermal maturity and the beginning of oil 

generation. Means of the hydrogen index (HI; S2/organic C 

content) and the oxygen index (OI; S3/organic C content) for the 

three units range from 76 to 126 and 28 to 32, respectively, and 

both indices increase with depth. The organic material in the 

units, as indicated by the values for HI and OI, is hydrogen poor 

and could be dominantly humic (Type III kerogen). The pyrolysis data suggest that the sampled beds are humic rich and gas 

prone but poor source rocks and thermally immature. Humic, 

continentally derived, organic matter is less reactive than marine 

organic matter which can limit the formation of pyrite in the 

host sediments (Davis and others, 1988). The organic carbon 

contents (TOC) of all samples of the three units are less than 2 

percent and the hydrogen indices (HI) of nearly all of the samples from Unit 1 are less than 100 which could indicate that the 

organic matter in Unit 1 is refractory (Dean and Arthur, 1989). 

However, the nature of the organic carbon did not prevent the 

formation of pyrite in Unit 1. 

Figure 10. Depth profile for the phosphate (P2O5) content of 20 samples of core from the 

Frontier Formation of Johnson County, Wyo. Location of core hole shown in fig. 1; stratigraphy of core shown in fig. 3.
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 19

-220

-240

-260

-280

-300

-320

0

MANGANESE, IN PARTS PER MILLION

DEPTH, IN METERS

Unit 1

Unit 2

Unit 3

100 200 300

The three samples of Unit 3 in the Belle Fourche, from the 

core hole in Johnson County, Wyo., contain the mean amounts 

of 2.23 percent Fe, 0.38 percent total S, and 1.28 percent organic 

carbon, and have a mean value for HI of 126 (tables 4-6). Analytical data representative of the underlying Mowry, from twenty 

samples of core from depths of 1,540–1,558 m (5,052–5,110 ft) 

in Powder River County, Mont. (fig. 1), include mean values of 

2.56 percent Fe, 1.50 percent total S, and 2.80 percent organic 

carbon, and have a mean value for HI of 271 (Dean and Arthur, 

1989, table 4). In the two groups of samples, the amount of iron 

is similar but the amounts of sulfur and organic carbon are generally larger in the samples from Powder River County. The HI 

value from Powder River County is also larger than the value 

from Johnson County; it suggests more hydrogen-rich, reactive, 

organic matter, probably of marine origin. 

The chemical composition of the three units varies slightly 

but significantly. The amounts of iron (3.14–3.65 percent 

Fe2O3) and organic carbon (TOC, 1.09–1.46 percent) in the 

units are similar, but the mean values for sulfur content (0.08–

0.79 percent ), manganese content (72–178 ppm), and HI (76–

126) are different. Sulfur is much less abundant in Unit 2 (0.08 

percent) than in Unit 1 (0.79 percent) and Unit 3 (0.38 percent). 

Sulfur values of less than 0.2 percent are found in rocks from 

continental fresh-water environments (Schultz and others, 1980, 

p. B53, table 14). Manganese is slightly more abundant in Unit 

2 (178 ppm), which contains sideritic and calcitic concretions, 

than in Unit 1 (102 ppm) and Unit 3 (72 ppm). The mean HI 

values are 76 for Unit 1, 103 for Unit 2, and 126 for Unit 3 (table 

6). In mudrocks that have an organic carbon content of less than 

2 percent and an HI of less than 100, the organic matter is highly 

refractory (Dean and Arthur, 1989). Consequently, the organic 

matter in at least Unit 1 apparently is highly refractory and probably not suitable for bacterial metabolism. 

Geochemistry of Carbon, Sulfur, and Iron

and Authigenic Carbonates

The wide geographic occurrence of the distinctive unit of 

nonfossiliferous, sideritic mudrocks in the lower part of the 

Belle Fourche Member of the Frontier Formation and in 

Figure 11. Depth profile for the manganese content of 20 samples of core from the 

Frontier Formation of Johnson County, Wyo. Location of core hole shown in fig. 1; 

stratigraphy of core shown in fig. 3.
20 Evolution of Sedimentary Basins—Powder River Basin

laterally equivalent formations indicates that the diagenetic features of these rocks reflect critical stratigraphic and paleoenvironmental changes that occurred during the transition between 

deposition of the marine Mowry Shale and deposition of marine 

beds near the middle of the Belle Fourche. In the following 

paragraphs, the origin of the sideritic rocks in the lower part of 

the Belle Fourche Member is described, discussed, and interpreted in terms of the geochemistry of carbon, sulfur, and iron in 

the mudstone and the mineralogic and stable isotopic compositions of the carbonate minerals.

 The core of Unit 2 includes six concretions that commonly 

contain quartz, plagioclase, potassium feldspar, siderite, calcite, 

and clay (table 2). However, the mineral composition varies, 

both between and within the concretions. In the concretions at 

depths of 281.8 m (924.5 ft) and 282.1 m (925.5 ft), siderite and 

quartz are the most abundant minerals. At depths of 287.2–

287.3 m (942.2–942.7 ft), the concretion contains a large amount 

of calcite but includes important amounts of siderite and quartz 

in the upper part. At depths of 291.7–291.9 m (957.0–957.8 ft), 

the lowermost and uppermost parts of the concretion contain 

abundant quartz, calcite, and feldspar; the middle parts contain 

abundant quartz, siderite, and feldspar. The concretion at depths 

of 293.1–293.2 m (961.6–962.0 ft) contains much quartz, 

siderite, and feldspar in the lower part and much quartz, feldspar, 

and clay in the upper part. At depths of 306.7–306.9 m (1006.2–

1006.8 ft), the outermost parts of the concretion contain 

abundant quartz, siderite, feldspar, and calcite; the interior of the 

concretion contains much quartz, siderite, and feldspar. 

Carbon, Sulfur, and Iron Geochemistry 

All of the core samples from Johnson County contain iron 

far in excess of that in stoichiometric pyrite (S/Fe=1.15); iron 

therefore probably was not a limiting component in the diagenetic evolution of these rocks (fig. 13). Sulfur, however, exhibits significant variation, in that it is highly depleted in samples 

from Unit 2 of the Belle Fourche. The samples from Units 3 

and 1 of the Belle Fourche contain carbon and sulfur in abundances similar to those in normal marine rocks, although as a 

group these samples show a large amount of scatter resulting 

from variations in sulfur content. Samples from Unit 2, however, are uniformly depleted in sulfur and plot on a sulfur/carbon diagram (fig. 14) and on a sulfur-iron-carbon ternary 

diagram (fig. 15) where values from nonmarine rocks generally 

occur (Berner and Raiswell, 1984; Dean and Arthur, 1989). 

Both the relative abundance of sulfur compared to iron and carbon and the absolute abundance of sulfur are very low in the 

rocks of Unit 2. 

Sulfur in argillaceous rocks such as those of this study is 

contained mainly in pyrite that formed in the sulfate-reduction 

zone (Berner, 1984) during early diagenesis. Low amounts of 

sulfide sulfur in shale can result from several causes. (1) The 

organic matter in the original sediment at the onset of sulfate 

reduction was of such low abundance or so metabolically refractory as to be an unsuitable substrate for bacterial activity. (2) 

The amount of aqueous sulfate available to the sulfate-reducing 

-220

-240

-260

-280

-300

-320

0.0 0.2 0.4 0.6 0.8 1.0 1.2

TOTAL SULFUR, IN PERCENT

DEPTH, IN METERS

Unit 1

Unit 2

Unit 3

Figure 12. Depth profile for the sulfur content of 20 samples of core from the Frontier 

Formation of Johnson County, Wyo. Location of core hole shown in fig. 1; stratigraphy of 

core shown in fig. 3.
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 21

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0 123 4

TOTAL IRON, IN PERCENT

TOTAL SULFUR, IN PERCENT

3

3

3

2 2

2 2 2 2 2

1

1

1

1

1

1

1

1 1

1

1.2

1.0

0.8

0.6

0.4

0.2

0.0

0.6 0.8 1.0 1.2 1.4 1.6 1.8

TOTAL ORGANIC CARBON, IN PERCENT

TOTAL SULFUR, IN PERCENT

3

3

3

2

2

2

2 2 2

2

1

1

1

1

1 1

1

1

1 1

S/C = 0.4

Figure 13. Comparison of the values for the amounts of total sulfur and total iron for 20 

samples of core from Units 1, 2 (shaded), and 3 of the Frontier Formation, Johnson 

County, Wyo. The dashed line is the stoichiometric line for pyrite (FeS2). Location of 

core hole shown in fig. 1; stratigraphy of core shown in fig. 3.

Figure 14. Comparison of the amounts of total sulfur and total organic carbon in 20 

samples of core from Units 1, 2 (shaded), and 3 of the Frontier Formation, Johnson County, 

Wyo. S/C = 0.4, an average value for Holocene sediments that contain appreciable 

organic matter and were deposited in oxic marine environments (Sweeney, 1972). 

Location of core hole shown in fig. 1; stratigraphy of core shown in fig. 3.
22 Evolution of Sedimentary Basins—Powder River Basin

x

x x

x

x

+ +

+

Fe

OC S

FeS2; S/Fe = 1.15

Two samples

Three samples

bacteria was so low as to provide a limit on their activity and 

thus on the formation of pyrite. (3) Reactive iron was present in 

such limited amounts as to preclude the formation of significant 

amounts of solid iron sulfides. Cause 2 is clearly evident in the 

composition of the samples of Unit 2 that contain abundant iron, 

as indicated by the presence of sideritic cements. The hydrogen 

indices (HI) of organic matter in Unit 2 are, on average, higher 

than those of Unit 1, which contains amounts of pyrite normal 

for marine rocks. A plot of HI against sulfur (fig. 16) shows that 

variation in the quality of the organic matter probably was not 

the limiting factor in the formation of pyrite in these samples. 

Rather, the opposite seems to have been the case; that is, samples 

having very low sulfur contents actually have comparatively 

high hydrogen indices.

The carbon, sulfur, and iron chemistry strongly suggests 

that sulfate availability was the limiting factor in pyrite formation and sulfide incorporation in Unit 2 of the Belle Fourche. 

Such sulfate limitation is consistent with a water column that 

was brackish to fresh from top to bottom, as opposed to a stratified water column with more brackish waters at the top, as proposed for other units of the Cretaceous system in North America 

(Pratt, 1984). Analyses of diagenetic carbonates in the cores of 

Unit 2, described in the following section, further support this 

interpretation. 

Isotopic Values for Carbon and Oxygen 

In samples of the core, both siderite and calcite display a 

wide variation in carbon isotopic composition and a somewhat 

smaller variation in oxygen isotopic values (table 7). In contrast to many concretionary, fine-grained rocks, for which large 

variations in isotopic composition (especially for carbon) have 

been found within single concretions, the range in the isotopic 

composition of calcite carbon in these samples (from +1.49 to 

-18.67) includes values from several concretions rather than 

one. Isotopic values for oxygen in calcite similarly have a 

greater range between concretions than within them, with 

values from -9.57 to -12.50. Siderites likewise display a wide 

variation in the isotopic composition of carbon, ranging from 

+8.25 to -10.99, and most variation occurs between concretions rather than within them. The oxygen isotopic composition of siderites ranges from -6.00 to -10.61.

Mozley and Burns (1993) compiled stable isotope data that 

had been obtained from many concretions of marine origin. 

From their calcite concretions, most of the δ13C values range 

from +3 to -22‰ PDB and most of the δ18O values range 

between 0 and -10‰ PDB. From their siderite concretions, 

most of the δ13C values range between 7 and -15‰ PDB and 

Figure 15. Comparison of the amounts of iron, organic carbon, and total sulfur in 20 samples of core 

rom Unit 1 ( ), Unit 2 (x), and Unit 3 (+) of the Frontier Formation, Johnson County, Wyo. Location of 

core hole shown in fig. 1; stratigraphy of core shown in fig. 3.
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 23

200

100

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

TOTAL SULFUR, IN PERCENT

HYDROGEN INDEX (HI)

2

2 2

2

2 2

2

3

3

3 1

1

1

1

1 1

1

1

1

1

most of the δ18O values range from 1 to -12‰ PDB. The initials 

PDB represent Peedee belemnite. Carbon dioxide obtained 

from belemnites from the Peedee Formation in South Carolina is 

used as a standard of comparison in determining the isotopic 

composition of carbon and oxygen. The carbon isotopic values 

from the calcite and siderite in the core resemble the values from 

marine concretions reported by Mozley and Burns (1993). 

However, the oxygen isotopic values from the calcite and siderite in the core contrast with those from the marine concretions 

described by Mozley and Burns (1993). 

The oxygen isotopic composition of calcite and siderite is a 

function of the precipitation temperature and the 18O/16O ratio 

of the water from which the carbonate minerals precipitated. 

The range of oxygen isotope values from calcites suggests precipitation within a range of temperatures but at a temperature of 

no more than about 22°C. It seems more likely, however, that 

the variation in δ18 O values for both siderite and calcite reflects 

changes in the isotopic composition of waters from which siderite and calcite were precipitated. If the shale of the lower Belle 

Fourche was deposited in waters of normal marine salinity, the 

oxygen isotope values indicate either that precipitation occurred 

at temperatures of 70-92°C or that the oxygen isotopic composition of pore waters changed significantly prior to carbonate precipitation. Neither of these possibilities seems likely inasmuch 

as organic-matter maturation data indicate that temperatures this 

high were never present during the burial history of the Belle 

Fourche and that many of these cements are probably of early 

diagenetic origin. More likely, the shales were deposited from 

waters at much lower temperatures but having an isotopic 

composition strongly influenced by meteoric runoff, an explanation consistent with the carbon-sulfur-iron geochemical data 

from enclosing shales of Unit 2. Meteoric waters in Wyoming 

in Cenomanian time are believed to have had δ18 O isotopic 

ratios of -7 to -10‰ SMOW (Standard Mean Ocean Water; 

dPDB=0.97006 dSMOW-29.94) (Dickinson, 1984). If the 

cements of the lower Belle Fourche in the core from Johnson 

County were precipitated directly from such meteoric waters, 

these values suggest precipitation of calcite at temperatures of 

15-28°C; temperatures reasonable for near-surface conditions of 

early diagenetic concretion formation in the Cretaceous.

Carbon isotopic data for the carbonates of Unit 2 of the 

Belle Fourche display a wide range that evidently results from 

multiple sources and (or) varying processes, which is typical of 

conditions during the formation of early diagenetic carbonates 

in organic-carbon-rich nonmarine rocks. Each discrete carbonate displays a comparatively narrow range of values, but differences between carbonates are large. For example, calcite from 

samples at a depth of about 306.6 m (1,006 ft) contains carbonate carbon that is highly depleted in the heavy carbon isotope 

13C, with values (-15 and -19‰) approaching those of organic 

carbon fixed through photosynthesis (-20 to -30‰). In contrast, 

calcite at a depth of 287.1 m (942 ft) contains carbonate carbon 

that is indistinguishable from that of marine carbonate carbon 

(+0.3 to +1.17). Siderites show a smaller but nevertheless disparate range of carbon isotopic compositions. The lightest 

(most 13C-depleted) carbon isotopic values suggest the input of 

oxidized organic carbon, whereas the heaviest values (as much 

as +8.25) indicate a strong depletion in the light isotope 12C, as 

Figure 16. Comparison of the values for hydrogen index and the amounts of total sulfur 

for 20 samples of core from Units 1, 2 (shaded area), and 3 of the Frontier Formation, 

Johnson County, Wyo. Location of core hole shown in fig. 1; stratigraphy of core shown in 

fig. 3.
24 Evolution of Sedimentary Basins—Powder River Basin

might be expected in the presence of methanogenic microorganisms (see, for example, Gautier and Claypool, 1984). No particularly significant carbon isotopic fractionation is to be expected 

between calcite and siderite. Consequently, the significant differences in carbon isotopic composition between siderites and 

calcites in the Johnson County core strongly suggest that the calcite and siderite did not precipitate simultaneously but formed 

under chemically different conditions. The siderites are systematically less depleted in 13C or, conversely, are more depleted in 

the light isotope 12C than are the corresponding calcites. This 

trend suggests, but does not prove, that the siderites formed in 

conjunction with a methanic environment of early diagenesis. 

Interpretations and Conclusions

Most of the sediment in Units 3, 2, and 1 in southern 

Johnson County (fig. 3) and in correlative rocks nearby (fig. 9) 

were deposited slowly in quiet-water environments on a subaqueous shelf. This interpretation is based on poor sorting, 

intense burrowing, the paucity of sand, the apparently low rate of 

sedimentation [as little as 13 cm (5.1 in.) per 1,000 years], and 

the gradual changes in thickness for age-equivalent rocks (fig. 

9). However, the organic matter in the rocks apparently is 

mostly terrestrial and is thermally immature. Data from the 

pyrolysis assays and the variable abundance and diversity of the 

deposit-feeding ichnogenera (Ekdale and Mason, 1988) in these 

strata typify aerobic to dysaerobic environments in sediments on 

the sea floor. The high degree of bioturbation, especially in Unit 

2 and the lower part of Unit 1 (fig. 3), might have reduced the 

magnitude of the HI values for those rocks. Foraminifers in the 

mudrocks of Units 3, 2, and the lower part of Unit 1 lived in 

water probably less than 30 m (100 ft) deep and in association 

with generally weak current action. Eicher and Diner (1985) 

proposed that the paucity of foraminifers in laterally equivalent 

rocks elsewhere suggests poorly oxygenated bottom water. 

From samples of outcrops in eastern Wyoming and southeastern 

Montana, Eicher (1967) determined that the abundance and variety of arenaceous foraminifera in lower Cenomanian strata generally increase upward. The low diversity of arenaceous 

foraminifers, the scarcity of filter-feeding ichnogenera, the varying diversity and abundance of deposit-feeding ichnogenera, and 

the absence of macrofossils in Units 3, 2, and the lower part of 

Unit 1 indicate an adverse benthic habitat, probably water of seasonally variable temperature, oxygen content, and salinity, with 

generally less than normal marine salinity. 

The composition of 20 samples of core from the Belle 

Fourche Member has been compared to the composition of 134 

samples of marine rocks and 58 samples of nonmarine rocks 

from the Upper Cretaceous Pierre Shale and equivalent strata in 

Montana, North Dakota, and South Dakota described by Schultz 

and others (1980, table 14). The compositions of the three units 

of the Belle Fourche Member are similar to those of marine 

rocks of the Pierre except for the amounts of SiO2, CaO, carbonate carbon, and manganese. The mean content of SiO2 is 

larger in the three units (about 68–70 percent) than in the Pierre 

(about 61 percent). Average amounts of CaO in the three units 

are 0.44–0.58 percent; significantly less than the 1.9 percent in 

most marine shale and siltstone of the Pierre. Mean amounts of 

carbonate carbon in the units (0.03–0.06 percent) are also much 

lower than the mean amount in marine shale and siltstone of the 

Pierre (0.45 percent). The low values for CaO and carbonate 

carbon in the units probably reflect the dearth of calcite in the 

sequence and the exclusion of data from the concretions in Unit 

2. The mean content of manganese in the units is 72–178 ppm. 

In marine shale and siltstone of the Pierre, the average manganese content is 750 ppm.

 For Units 3, 2, and 1, the respective mean contents are: 

sulfur (weight percent), 0.38, 0.08, 0.79; MgO (weight percent), 

0.98, 1.05, 1.21; CaO (weight percent), 0.44, 0.49, 0.58; manganese (ppm), 72, 178, 102; and FeTO3 (weight percent), 3.14, 

3.37, 3.65. The MgO/CaO ratios for Units 3, 2, and 1 are 2.2, 

2.1, and 2.1, respectively. These data do not indicate clear 

differences in the composition and origin of the three units, 

although a comparison of the amounts of sulfur and carbon in 

the three units (fig. 17) suggests that sediments in Units 3 and 1 

could have been deposited in normal marine environments and 

sediments in Unit 2 might have been deposited in fresher water, 

sulfate-limited environments. However, the ternary diagram for 

iron, sulfur, and organic carbon in the samples (fig. 15) shows a 

concentration of values near Fe and between Fe and OC which 

resembles the concentration of values in a ternary diagram for 

samples of Pleistocene and Holocene silty clay from the continental slope and rise of northern California (Dean and Arthur, 

1989, fig. 10). 

Mozley and Wersin (1992) compiled isotopic values for 

carbon and oxygen in siderites that had originated in continental 

and marine depositional environments (fig. 18). Most of the 

comparable values from the siderites in Unit 2 resemble those 

from siderites that are either continental or marine. 

All three units contain sulfur, although the percentage in 

Unit 2 is significantly less than the percentages in Units 3 and 1 

(table 5). Nevertheless, pyrite is absent in Unit 2 and present in 

Units 3 and 1 (table 1). The absence of pyrite can be caused by 

nonreactive iron, refractory organic matter, or a reduction in the 

salinity of the sea (Davis and others, 1988). Mean HI values 

for all samples of the units are less than 130 (table 6), indicating that the organic material now in the units is nonreactive and 

would have limited the formation of pyrite. However, although 

the organic material in all of the samples is refractory only Unit 

2 is without pyrite. Mean percentages of FeTO3 in Units 3, 2, 

and 1 (table 3) are 3.14, 3.37, and 3.65, respectively. Mean 

prcentages of iron in Units 3, 2, and 1 are 2.2, 2.5, and 2.7, 

respectively. 

 If the differences in the amounts of P2O5 in the three units 

(table 2) represent differences in the amounts of biogenic apatite, which seems likely, then the changes in the content of P2O5 

(fig. 10) might reflect changes in the amount of Cenomanian 

biotic activity. Presumably, the number of organisms, including 

foraminifers, palynomorphs, and burrowers, was relatively low 

when Unit 3 and the lowest part of Unit 2 were deposited but the 

number increased during deposition of the upper part of Unit 2 

and the lower part of Unit 1 (fig. 10). Biotic activity in the sediments and water column apparently increased slowly during late 

early Cenomanian time although a corresponding increase in the 

abundance of fossils in the units is not evident. 
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 25

-20 -15 -10 -5 5 10 15 20

-10

-20

-30

-40

10

20

30

40

0

-2

δ13C

δ18O

Siderites from

continental

environments

Siderites from

marine

environments

4

3

2

1

Unit 1

Unit 2

Unit 3

X

+

0 1 2 3 4 5 6 7

0

Normal marine values

SULFUR, IN PERCENT

ORGANIC CARBON, IN PERCENT

S:C = 0.4

Freshwater lake values +

+

X X

+

Figure 17. Organic carbon and sulfur contents of samples of the three units compared with 

values from well-oxygenated, normal marine sediments (Berner and Raiswell, 1983) and from 

fresh water lake sediments (Berner and Raiswell, 1984).

Figure 18. Comparison of isotopic 

values for carbon and oxygen in 15 

samples of siderite from Unit 2 (table 

7) of the Frontier Formation, Johnson 

County, Wyo., with generalized fields 

of published isotopic values for 

siderites from continental and marine 

depositional environments (W.E. 

Dean, written commun.,1995; Mozley 

and Wersin, 1992). 
26 Evolution of Sedimentary Basins—Powder River Basin

The data for carbon, sulfur, and iron, together with the 

mineralogic and isotopic composition of authigenic mineral 

species in mudrocks of Unit 2, suggest cementation during early 

diagenesis in a variety of microenvironments. The interstitial 

waters from which the minerals precipitated were far removed in 

elemental and isotopic composition from normal marine waters; 

the original pore-water composition probably was brackish to 

fresh, similar to conditions of the overlying waters in the Belle 

Fourche sea and supporting the contention of Eicher and Diner 

(1985) who proposed that the seaway contained poorly oxygenated water of less than normal marine salinity. Presumably, the 

distinctive episode of concretion formation that characterized 

deposition of Unit 2 was a widespread occurrence of strong 

dilution of the boreal epeiric sea, presumably by waters derived 

from uplands near the seaway. Barron (1986) concluded that the 

seasonal precipitation rates on mid-latitude continents of the 

northern hemisphere during the mid-Cretaceous were unusually 

high. His estimates of mid-Cretaceous precipitation rates for 

July in Wyoming and adjoining areas are 0.5–1.5 cm per day 

(72–216 in. per year). This seasonal addition of a large quantity 

of fresh water to the southern part of the boreal sea might also 

temporarily affect the temperature of the sea. For the mudrocks 

of Unit 2, the diagenetic features are evidently indicators of 

oceanographic conditions. 

The widespread sideritic concretions of late early Cenomanian age (fig. 1) probably reflect the approximate lateral extent 

of the body of comparatively fresh water; however, saltwater is 

indicated in the early Cenomanian seaway of west-central 

Canada by the occurrence of marine molluscan fossils. If a 

southern part of the epeiric sea (fig. 1) was brackish to fresh, the 

inflow of fresh water was intermittently large, as Barron (1986) 

indicated, and the influx of saltwater was comparatively small. 

A reduction in the proportion of saltwater to fresh water could 

have been caused partly by a constriction of the seaway to the 

north, perhaps near the Mackenzie salient of northwestern 

Canada (Williams and Stelck, 1975) (fig. 8). 

 The foregoing stratigraphic, paleontologic, and geochemical data indicate that Units 3, 2, and a lower part of Unit 1, a 

gradational, coarsening-upward sequence, were deposited in 

shallow brackish to fresh water, mostly during a marine regression. This regression apparently is represented by the reduction in the area of the epeiric sea in Wyoming and Colorado 

during late early Cenomanian time, as depicted by McGookey 

and others [1972, figs. 17 (then assigned to the latest Albian) 

and 26]. The upper part of Unit 1 reflects the subsequent 

marine transgression, which is also represented by the enlargement of the epicontinental sea and the introduction of Tethyan 

mollusks during the earliest middle Cenomanian (fig. 4; Cobban, 1993). The boreal epeiric sea of mid-Cretaceous time in 

eastern Wyoming and contiguous areas contracted in the late 

early Cenomanian and expanded in the late early(?) and middle 

Cenomanian, to join the Tethyan Sea and form a north-trending transcontinental seaway. Tectonism and associated erosion 

in western Wyoming and adjoining areas during the late Albian 

and early Cenomanian and a eustatic fall during the late early 

Cenomanian could have caused regressions, whereas a eustatic 

rise in the late early(?) and middle Cenomanian probably 

caused the transgression. 

 Comparison of the ages of the units with the sea level 

curves of Haq and others (1987) suggests that Unit 3 could have 

accumulated during a gradual eustatic rise, that Unit 2 was 

deposited thereafter partly during a eustatic fall, and that Unit 1 

subsequently accumulated mainly during a eustatic rise (fig. 7). 

Nevertheless, the western shore of the epeiric sea in Wyoming 

was evidently prograding eastward in late early to middle Cenomanian time (fig. 4), while sea level was both falling and rising. This marine regression must have been caused mostly by 

the Albian and Cenomanian tectonism and erosion in Montana, 

Idaho, and Utah reported by Wiltschko and Dorr (1983), Heller 

and others (1986), and Dyman and others (1988). 

Acknowledgments

W.A. Cobban supplied important stratigraphic information 

and identified molluscan fossils of mid-Cretaceous age from 

outcrops and core of the region. B.R. North and W.G.E. Caldwell, while at the University of Western Ontario, London, 

Ontario, Canada, identified and interpreted foraminifers from 

core of the Frontier Formation in Johnson County, Wyoming. 

Additional interpretations of the foraminifers were provided by 

M.A. Carey. Trace fossils were identified by B.L. Mieras, a 

stratigraphic consultant, while at the University of Colorado, 

Boulder. Samples of strata from the core hole in Johnson 

County were analyzed for minerals by G.L. Skipp; major element and trace element compositions were determined by J.N. 

Taggart, A.J. Bartel, D.F. Siems, and D.L. Fey; analyses for percentages of carbon and sulfur were conducted by L.L. Jackson 

and E.L. Brandt. The organic composition and thermal maturity 

of the rocks were determined by T.A. Daws and J.G. Palacas. 

Samples of concretions from the core hole were analyzed for 

minerals by R.M. Pollastro and T.M. Finn. Isotopic analyses 

were performed by C.N. Threlkeld and A.K. Vuletich. Thoughtful and constructive reviews of this report were contributed by 

Walter E. Dean, Karen J. Franczyk, and Michele L. Tuttle. The 

essential and generous contributions of effort and information 

by these scientists and technicians of the U.S. Geological 

Survey, the University of Western Ontario, and the University of 

Colorado are gratefully acknowledged.

References 

Arthur, M.A., Dean, W.E., Pollastro, R.M., Claypool, G.E., and Scholle, 

P.A., 1985, Comparative geochemical and mineralogical studies of 

two cyclic transgressive pelagic limestone units, Cretaceous 

Western Interior Basin, U.S. in Pratt, L.M., Kauffman, E.G., and Zelt, 

F.B., eds., Fine-grained deposits and biofacies of the Cretaceous 

Western Interior seaway—Evidence of cyclic sedimentary processes: Society of Economic Paleontologists and Mineralogists, 

Field Trip Guidebook 4, 1985 Midyear Meeting, Golden, Colorado, p. 

16–27.

Asquith, D.O., 1970, Depositional topography and major marine environments, Late Cretaceous, Wyoming: American Association of Petroleum Geologists Bulletin, v. 54, no. 7, p. 1184–1224.
Composition and Depositional Environment of Concretionary Strata, Johnson County, Wyoming 27

Barron, E.J., 1986, Mathematical climate models--Insights into the relationship between climate and economic sedimentary deposits, in 

Parrish, J.T., and Barron, E.J., eds., Paleoclimates and economic 

geology: Society of Economic Paleontologists and Mineralogists 

Lecture Notes for Short Course 18, p. 31–83. 

Berner, E.K., and Berner, R.A., 1996, Global environment—Water, air, 

and geochemical cycles: Prentice Hall, 376 p. 

Berner, R.A., 1984, Sedimentary pyrite formation—An update: Geochimica et Cosmochimica Acta, v. 48, no. 4, p. 605–615. 

Berner, R.A., and Raiswell, R., 1983, Burial of organic carbon and pyrite 

sulfur in sediments over Phanerozoic time—A new theory: 

Geochimica et Cosmochimica Acta, v. 47, p. 855–862. 

Berner, R.A., and Raiswell, R., 1984, C/S method for distinguishing freshwater from marine sedimentary rocks: Geology, v. 12, p. 365-368. 

Böse, Emile, 1910, Monografía geológica y paleontológica del Cerro de 

Muleros cerca de Ciudad Juárez, Estado de Chihuahua, y 

descripción de la fauna cretácea de La Encantada, Placer de Guadalupe, estado de Chihuahua [Geologic and paleontologic monograph on the Cerro de Muleros near the city of Juarez, state of 

Chihuahua, and a description of the Cretaceous fauna of La Encantada, Placer de Guadalupe, state of Chihuahua]: Instituto Geológico 

de México Boletín 25, 193 p. 

Byers, C.W., and Larson, D.W., 1979, Paleoenvironments of Mowry Shale 

(Lower Cretaceous), western and central Wyoming: American 

Association of Petroleum Geologists Bulletin, v. 63, no. 3, p. 354–361.

Cobban, W.A., 1987, Ammonite faunas of the Sarten Sandstone (Cretaceous), Luna County, New Mexico: U.S. Geological Survey Bulletin 

1641-B, p. B1–B17.

———1993, Diversity and distribution of Late Cretaceous ammonites, 

Western Interior, United States, in Evolution of the Western Interior 

Basin: Geological Association of Canada Special Paper 39, p. 435–

451.

Cobban, W.A., and Kennedy, W.J., 1989, Acompsoceras inconstans 

zone, a lower Cenomanian marker horizon in Trans-Pecos Texas, 

U.S.A.: Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, v. 178, p. 133–145. 

Cobban, W.A., Merewether, E.A., Fouch, T.D., and Obradovich, J.D., 1994, 

Some Cretaceous shorelines in the Western Interior of the United 

States, in Caputo, M.V., Peterson, J.A., and Franczyk, K.J., eds., 

Mesozoic systems of the Rocky Mountain region, USA: Society of 

Economic Paleontologists and Mineralogists (Society for Sedimentary Geology), Rocky Mountain Section, p. 393–413. 

Couch, E.L., 1971, Calculation of paleosalinities from boron and clay mineral data: American Association of Petroleum Geologists Bulletin, v. 

55, no. 10, p. 1829–1837. 

 Crock, J.G., Briggs, P.H., Jackson, L.L., and Lichte, F.E., 1987, Analytical 

methods for the analysis of stream sediments and rocks from wilderness study areas: U.S. Geological Survey Open-File Report 87-

84, p. 22–28.

Davis, H.R., Byers, C.W., and Dean, W.E., 1988, Pyrite formation in the 

Lower Cretaceous Mowry Shale: effect of organic matter type and 

reactive iron content: American Journal of Science, v. 288, p. 873–

890. 

Dean, W.E., and Arthur, M.A., 1989, Iron-sulfur-carbon relationships in 

organic-carbon-rich sequences, I-Cretaceous Western Interior 

seaway: American Journal of Science, v. 289, p. 708–743. 

Dickinson, W.W., 1984, Isotope geochemistry of carbonate minerals in 

nonmarine rocks—Northern Green River Basin, Wyoming: Boulder, 

University of Colorado, Ph.D. thesis, 150 p. 

Dyman, T.S., Perry, W.J., Jr., and Nichols, D.J., 1988, Stratigraphy, petrology, and provenance of the Albian Blackleaf Formation and the Cenomanian to Turonian lower part of the Frontier Formation in parts of 

Beaverhead and Madison Counties, Montana: The Mountain Geologist, v. 25, no. 3, p. 113–127. 

Eicher, D.L., 1965, Foraminifera and biostratigraphy of the Graneros 

Shale: Journal of Paleontology, v. 39, no. 5, p. 875–909.

———1967, Foraminifera from Belle Fourche Shale and equivalents, 

Wyoming and Montana: Journal of Paleontology, v. 41, no. 1, p. 167–

188. 

Eicher, D.L., and Diner, Richard, 1985, Foraminifera as indicators of water 

mass in the Cretaceous Greenhorn sea, Western Interior, in Pratt, 

L.M., Kauffman, E.G., and Zelt, F.B., eds., Fine-grained deposits and 

biofacies of the Cretaceous Western Interior Seaway—Evidence of 

cyclic sedimentary processes: Society of Economic Paleontologists 

and Mineralogists Midyear Meeting, Golden, Colo., 1985, Field Trip 

Guidebook 4, p. 60–71.

Ekdale, A.A., Bromley, R.G., and Pemberton, S.G., 1984, Ichnology, the 

use of trace fossils in sedimentology and stratigraphy: Society of 

Economic Paleontologists and Mineralogists Short Course 15, 317 p.

Ekdale, A. A., and Mason, T. R., 1988, Characteristic trace-fossil associations in oxygen-poor sedimentary environments: Geology, v. 16, 

no. 8, p. 720–723. 

Espitalie, J., Laporte, J.L., Madec, M., Marquis, F., Leplat, P., Paulet, J., 

and Boutefeu, A., 1977, Rapid method for source rock characterization and for evaluating their petroleum potential and their degree 

of evolution (English Trans.): Institut Français du Petrole, v. 32, p. 

23–42.

Fouch, T.D., Lawton, T.F., Nichols, D.J., Cashion, W.B., and Cobban, W.A., 

1983, Patterns and timing of synorogenic sedimentation in Upper 

Cretaceous rocks of central and northeast Utah, in Reynolds, M.W., 

and Dolly, E.D., eds., Mesozoic paleogeography of the west-central 

United States: Society of Economic Paleontologists and Mineralogists, Rocky Mountain Section, Rocky Mountain Paleogeography 

Symposium 2, p. 305–336. 

Gautier, D.L., and Claypool, G.E., 1984, Interpretation of methanic diagenesis by analogy with processes in modern diagenetic environments,

in McDonald, D.A., and Surdam, R.C., eds., Clastic diagenesis: 

American Association of Petroleum Geologists Memoir 37, p. 111–

123. 

Gill, J.R., and Cobban, W.A., l966, The Red Bird section of the Upper Cretaceous Pierre Shale in Wyoming, with a section on A new Echinoid 

from the Cretaceous Pierre Shale of eastern Wyoming, by P.M. Kier: 

U.S. Geological Survey Professional Paper 393-A, p.A1–A73. 

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

sea levels since the Triassic: Science, v. 235, p. 1156–1167.

Harms, J.C., Southard, J.B., and Walker, R.G., 1982, Structures and 

sequences in clastic rocks: Society of Economic Paleontologists 

and Mineralogists Short Course 9, 249 p.

Haun, J.D., and Weimer, R.J., 1960, Cretaceous stratigraphy of Colorado, 

in Weimer, R.J., and Haun, J.D., eds., Guide to the geology of Colorado: Rocky Mountain Association of Geologists, p. 58–65.

Heller, P.L., Bowdler, S.S., Chambers, H.P., Coogan, J.C., Hagen, E.S., 

Shuster, M.W., Winslow, N.S., and Lawton, T.F., 1986, Time of initial 

thrusting in the Sevier orogenic belt, Idaho-Wyoming and Utah: 

Geology, v. 14, no. 5, p. 388–391. 

Jackson, L.L., Brown, F.W., and Neil, S.P., 1987, Major and minor elements requiring individual determination, classical whole rock analysis, and rapid rock analysis, in Baedecker, P. A., ed., Methods for 
28 Evolution of Sedimentary Basins—Powder River Basin

geochemical analysis: U.S. Geological Survey Bulletin 1770-G, p. 

G1–G23. 

Leckie, D.A., Singh, Chaitanya, Goodarzi, Fariborz, and Wall, J.H., 1990, 

Organic-rich, radioactive marine shale—A case study of a shallowwater condensed section, Cretaceous Shaftesbury Formation, 

Alberta, Canada: Journal of Sedimentary Petrology, v. 60, no. 1, p. 

101–117. 

Liss, P.S., 1976, Conservative and non-conservative behavior of dissolved 

constituents during estuarine mixing, in Burton, J.D., and Liss, P.S., 

eds., Estuarine chemistry: Academic Press, 229 p. 

Massare, J.A., and Dain, L.E., 1989, The marine reptiles of the Mowry 

Shale (Albian) of northeastern Wyoming [abs.]: Journal of Vertebrate Paleontology, v. 9, supplement to no. 3, p. 32A. 

McCrea, J.M., 1950, On the isotope chemistry of carbonates and a paleotemperature scale: Journal of Chemical Physics, v. 18, p. 849–854. 

McGookey, D.P., Haun, J.D., Hale, L.A., Goodell, H.G., McCubbin, D.G., 

Weimer, R.J., and Wulf, G.R., 1972, Cretaceous System, in Mallory, 

W.W., ed., Geologic atlas of the Rocky Mountain region, United 

States of America: Denver, Colo., Rocky Mountain Association of 

Geologists, p. 190–228.

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 Laudon, R.B., Curry, W.H., III, 

and Runge, J.S., eds., Geology and energy resources of the Powder 

River: Wyoming Geological Association, Annual Field Conference, 

28th, 1976, Casper, Wyo., Guidebook, p. 33–44. 

Mieras, B.L., Sageman, B.B., and Kauffman, E.G., 1993, Trace fossil distribution patterns in Cretaceous facies of the Western Interior Basin, 

North America, in Caldwell, W.G.E., and Kauffman, E.G., eds., Evolution of the Western Interior Basin: Geological Association of Canada, Special Paper 39, p. 585–620. 

Mozley, P.S., and Burns, S.J., 1993, Oxygen and carbon isotopic composition of marine carbonate concretions: an overview: Journal of 

Sedimentary Petrology, v. 63, no. 1, p. 73–83. 

Mozley, P.S., and Wersin, Paul, 1992, Isotopic composition of siderite as 

an indicator of depositional environment: Geology, v. 20, p. 817–820. 

Nace, R.L., 1941, A new ichthyosaur from the Late Cretaceous, northeast 

Wyoming: American Journal of Science, v. 239, p. 908–914. 

Obradovich, J.D., 1993, A Cretaceous time scale, in Caldwell, W.G.E., and 

Kauffman, E.G., eds., Evolution of the Western Interior Basin: Geological Association of Canada Special Paper 39, p. 379–396. 

Obradovich, J.D., and Cobban, W.A., 1975, A time-scale for the Late Cretaceous of the Western Interior of North America, in Caldwell, 

W.G.E., ed., The Cretaceous System in the Western Interior of North 

America: Geological Association of Canada Special Paper 13, p. 31–

54. 

Okumura, T.A., 1994, Palynostratigraphy and paleoecology of mid-Cretaceous formations at drilling sites in Weston and Johnson Counties, 

Powder River Basin, Wyoming: Boulder, University of Colorado, 

Ph.D. thesis, 289 p.

Potter, P.E., Shimp, N.F., and Witters, B., 1963, Trace elements in marine 

and freshwater argillaceous sediments: Geochimica et Cosmochimica Acta, v. 27, p. 669–694. 

Pratt, L.M., 1984, Influence of paleoenvironmental factors on preservation of organic matter in middle Cretaceous Greenhorn Formation, 

Pueblo, Colorado: American Association of Petroleum Geologists 

Bulletin, v. 68, p. 1146–1159. 

———1985, Isotopic studies of organic matter and carbonate in rocks of 

the Greenhorn marine cycle, in Pratt, L.M., Kauffman, E.G., and Zelt, 

F.B., eds., Fine-grained deposits and biofacies of the Cretaceous 

Western Interior seaway—Evidence of cyclic sedimentary processes: Society of Economic Paleontologists and Mineralogists, 

Field Trip Guidebook no. 4, 1985 Midyear Meeting, Golden, Colorado, 

p. 38–48. 

Rice, D.D., and Gautier, D.L., 1983, Patterns of sedimentation, diagenesis, 

and hydrocarbon accumulation in Cretaceous rocks of the Rocky 

Mountains: Society of Economic Paleontologists and Mineralogists 

Short Course 11, 31 p.

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.

Schultz, L.G., Tourtelot, H.A., Gill, J.R., and Boerngen, J.G., 1980, Composition and properties of the Pierre Shale and equivalent rocks, 

northern Great Plains region: U.S. Geological Survey Professional 

Paper 1064-B, 114 p.

Stelck, C.R., and Armstrong, J., 1981, Neogastroplites from southern 

Alberta: Bulletin of Canadian Petroleum Geology, v. 29, no. 3, p. 399–

407. 

Sweeney, R.E., 1972, Pyritization during diagenesis of marine sediments: 

University of California in Los Angeles, Ph.D. thesis, 184 p. 

Taggart, J.E., Jr., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, A.J., and 

Stewart, K.C., 1987, Analysis of geologic materials by wave lengthdispersive x-ray fluorescence spectrometry, in Baedecker, P.A., 

ed., Methods for geochemical analysis: U.S. Geological Survey 

Bulletin 1770-E, p. E1–E19. 

Tillman, R.W., and Merewether, E.A., 1994, Field guide for valley-fill, 

estuarine and shelf ridge sandstones, mid-Cretaceous Frontier Formation, central Wyoming: American Association of Petroleum 

Geologists Annual Convention, Denver, Colorado, June 12-15, 1994: 

Society of Economic Paleontologists and Mineralogists (Society for 

Sedimentary Geology), Rocky Mountain Section, field trip guidebook, 90 p., with appendix, 68 p. 

Warren, P.S., and Stelck, C.R., 1958, Lower Cenomanian Ammonoidea 

and Pelecypoda from Peace River area, western Canada: 

Research Council of Alberta, Geologic Division, Bulletin 2, part II, 

p. 36–51.

Weimer, R.J., 1984, Relation of unconformities, tectonics, and sea-level changes, Cretaceous of Western Interior, U.S.A., in Schlee, 

J.S., ed., Interregional unconformities and hydrocarbon accumulation: American Association of Petroleum Geologists Memoir 36, 

p. 7–35. 

Williams, G.D., and Stelck, C.R., 1975, Speculations on the Cretaceous 

palaeogeography of North America, in Caldwell, W.G.E., ed., The 

Cretaceous System in the Western Interior of North America: Geological Association of Canada Special Paper 13, p. 1–20. 

Willis, B.J., Bhattacharya, J.P., Gabel, S.L., and White, C.D., 1999, Architecture of a tide-influenced river delta in the Frontier Formation of 

central Wyoming, USA: Sedimentology, v. 46., p. 667–688. 

Wiltschko, D.V., and Dorr, J.A., Jr., 1983, Timing of deformation in overthrust belt and foreland of Idaho, Wyoming, and Utah: American 

Association of Petroleum Geologists Bulletin, v. 67, no. 8, p. 1304–

1322. 

Young, Keith, 1958, Cenomanian (Cretaceous) ammonites from TransPecos Texas: Journal of Paleontology, v. 32, no. 2, p. 286–294.