U.S. GEOLOGICAL SURVEY BULLETIN 1917-E

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Chapter E

Red Beds of the Triassic Chugwater Group, 

Southwestern Powder River Basin, Wyoming

By V.V. CAVAROC and R.M. FLORES

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1917

EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Cavaroc, Victor V.

Red beds of the Triassic Chugwater Group, southwestern Powder River Basin, 

Wyoming / by V.V. Cavaroc and R.M. Flores.

p. cm. (Evolution of sedimentary basins Powder River Basin ; 

ch. E) (U.S. Geological Survey bulletin ; 1917-E)

"A multidisciplinary approach to research studies of sedimentary rocks and 

their constituents and the evolution of sedimentary basins, both ancient and 

modern."

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1917E

1. Geology, Stratigraphic Triassic. 2. Geology Powder River Watershed 

(Wyo. and Mont.) 3. Chugwater Group (Wyo.) 4. Rocks, Sedimentary 

Wyoming. I. Flores, Romeo M. II. Title. III. Series. IV. Series: U.S. 

Geological Survey bulletin ; 1917-E. 

QE75.B9 no. 1917-E 

[QE676]

557.3 s dc20 90-22634 

[551.7'62'0978] CIP
CONTENTS

Abstract El

Introduction El

Major stratigraphic units E2

Permian E2

Triassic E5 

The Red Wall valley E5

Morphology E5

Goose Egg Formation E5

Red Peak Formation E6 

Lithology E6 

Interpretation E9

Alcova Limestone E9

Crow Mountain Sandstone Ell

Unnamed red beds Ell

Jurassic strata E13 

Summary and conclusions E13 

References cited E15

FIGURES

1-2. Maps showing:

1. Mountains and basins in area of Powder River Basin, Wyoming E2

2. Areas of Early Triassic deformation and sedimentation, Utah and parts of 

Idaho, Nevada, Wyoming, and Montana E2

3. Chart showing Late Pennsylvanian to Middle Jurassic stratigraphy E3

4. Map and profile showing generalized geology and topography of Red Wall 

valley at headwaters of Middle Fork of Powder River E4

5. Stratigraphic cross section along Red Wall valley E7

6. Plane-light photomicrographs of Goose Egg and Red Peak Formations E8

7. Photographs of Red Peak Formation and Alcova Limestone E10

8. Plane-light photomicrographs of Crow Mountain Sandstone, unnamed red 

beds, and Sundance Formation E12

9. Photographs of Crow Mountain Sandstone, unnamed red beds, and Sundance 

Formation E14

10. Chart showing relationship between inferred depositional settings of Red 

Wall valley area and those for adjacent basins to west E15

TABLE

1. Microprobe and stable isotope values for selected carbonate beds E6

Contents III

EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN

Red Beds of the Triassic Chugwater Group, 

Southwestern Powder River Basin, Wyoming

By V.V. Cavaroc1 and R.M. Flores2

Abstract

Predominantly fine-grained detrital red sedimentary 

rocks deposited across eastern and central Wyoming during 

Early to Late Triassic time interfinger with a thick sequence of 

Cordilleran marine rocks in western Wyoming. These red beds 

of the Chugwater Group are about 985 ft (300 m) thick in the 

study area across the southwestern margin of the Powder River 

Basin and are upturned along the Bighorn Mountains. They 

represent marginal-marine to fluvial facies that overlie Permian 

to Lower Triassic gypsiferous red mudstones and marine 

dolomites of the Goose Egg Formation along about 40 mi (64 

km) of very well exposed outcrops. The Chugwater in this area 

is truncated and unconformably overlain by the Middle and 

Upper Jurassic Sundance Formation.

A complex sequence of facies is represented in the 

Chugwater Group. The lowermost 380 ft (116 m) on the 

southeastern side of the Bighorn Mountains (Red Peak Formation) is an internally heterogeneous facies that coarsens 

upward. It consists of interbedded red mudstones, lenticular 

braided channel sandstones, and widespread sheets of sandstones that suggest reworked delta-front facies. The detrital 

beds are overlain by the light-gray, brackish-marine, algal 

Alcova Limestone, which is 0-11.5 ft (0-3.5 m) thick. The 

study area may approximate the eastern margin of a regional 

marine transgression associated with increased rates of shelf 

subsidence. The Alcova Limestone is overlain by almost 100 ft 

(30 m) of intensely crossbedded, red sandstone (Crow Mountain Sandstone) that has properties indicating accretion of 

shallow-water coastal sands. Unnamed red beds of red to gray 

shales, red siltstones, and red to light-colored calcareous 

sandstones overlie the Crow Mountain. These strata represent 

a return to fluvial-deltaic and lacustrine environments. The 

uppermost unit of the Chugwater Group (Popo Agie Formation) was not observed in the study area.

Manuscript approved for publication August 30, 1990.

Department of Marine, Earth and Atmospheric Sciences, North 

Carolina State University, Raleigh, North Carolina 27695.

2U.S. Geological Survey, Box 25046, MS972, Denver, 

Colorado 80225.

Depositional environments of the Chugwater Group 

preserved in the study area probably represent near-shore 

equivalents of the shelf-dominated environments of central 

Wyoming.

INTRODUCTION

A major stratigraphic sequence of about 985 ft (300 

m) of the Triassic Chugwater Group crops out along the 

southwestern margin of the Powder River Basin in northeastern Wyoming (fig. 1). The uppermost 600 ft (183 m) 

is composed of sandy beds and forms an essentially 

continuous outcrop for about 40 mi (60 km). In this paper 

we describe the properties of these rocks and interpret their 

depositional environments. Geographically the rocks are 

between continental red beds to the east and marine red 

detritus of a paleoshelf to the west. These outcrops of the 

Chugwater are important in any regional reconstructions of 

Triassic sedimentation patterns in Wyoming, and the diversity of coastal environments represented in this red-bed 

sequence is important because red beds historically are 

believed to be associated with continental deposition.

Westward thickening of Triassic strata (fig. 1) in the 

Wyoming region reflects a gradual transition from the stable 

cratonic platform in the Dakotas, across the slowly subsiding cratonic margin in central Wyoming, to the rapidly 

subsiding Cordilleran trough near the border between 

Wyoming and Idaho (Carr and Paull, 1983). Shallow-water 

miogeosynclinal rocks extend from southwestern Wyoming 

into Nevada (fig. 2), and deeper water facies are present in 

Idaho and northern Utah (Collinson and Hasenmueller, 

1978). The deformed, Early Triassic volcanic belt that 

trends northward across central Nevada could be a remnant 

of the western margin of the Early Triassic trough (Hamilton, 1978). The transition from trough sediments in Idaho to 

sediments of the more stable platform edge in central 

Wyoming was recognized and described by Kummel

Red Beds of Triassic Chugwater Group, Wyoming E1
41'

GREEN RIVER 

I BASIN

0 50 100 MILES

0 50 100 KILOMETERS

Figure 1. Mountains and basins in the area of the Powder 

River Basin, Wyoming. Modified from King (1977). Contour 

lines show the approximate thickness (in feet) of Triassic strata 

(McKee and others, 1967a).

(1957). Black shale and limestone characterize the deeper 

water environments of the basin in eastern Idaho and grade 

eastward into carbonate and detrital rocks of the shallowwater shelf in Wyoming. Rocks of the shelf, in turn, are 

intercalated with about 1,000 ft (305 m) of red beds 

(Chugwater Group) of the inner shelf in central Wyoming. 

Outcropping red beds partly of the same age in the Black 

Hills area of northeastern Wyoming (Spearfish Formation) 

are continental in origin (Downey, 1986) and pinch out to 

the east.

Acknowledgments. The operators of most ranches 

along Middle Fork, Buffalo, and Beaver Creeks courteously 

provided access to their property during the 1 month of field 

work involved with this study. We would especially like to 

acknowledge the owners of the Norris Graves ranch, as well 

as the foreman of the Bar-C, for their assistance. Isotopic 

analyses of carbonate rocks were provided by Dr. William 

Showers at the regional stable isotope laboratory of North 

Carolina State University (NCSU). Microprobe analyses of 

rocks were also conducted at NCSU.

MAJOR STRATIGRAPHIC UNITS

Permian

Triassic strata of Wyoming record part of a 

depositional episode that extended from the Permian into 

the Early Jurassic. Rocks representing this episode are 

bounded by regional unconformities (Pipiringos and 

O'Sullivan, 1978) indicated as P-O and J-l on figure 3. The 

Permian transgression onto the older erosion surface (P-O) 

is reflected on the outer paleoshelf in western Wyoming by

EXPLANATION

Inner shelf facies 

Red beds dominant

Outer shelf facies 

Buff limestonesiltstone-sandstone

Basin facies 

Black limestoneshale common

 Deformation front

37°

0 50 100 MILES

[TTTT| |

0 50 100 KILOMETERS

Figure 2. Areas of Early Triassic deformation and sedimentation, Utah and parts of Idaho, Nevada, Wyoming, and 

Montana. Modified from Kummel (1957) and Hamilton (1978).

cherty, carbonaceous, phosphatic mudstones of the Phosphoria Formation (McKee and others, 1967a). These mudstones interfinger eastward with shallow-water marine 

carbonate and detrital rocks of the paleoshelf in the Park 

City Formation. Across central and eastern Wyoming, 

Permian rocks consist of the widespread, red, silty mudstone of the Goose Egg Formation. These rocks contain 

halite and white gypsum and include purple to white 

dolomite and limestone beds (Love and Christiansen, 1985). 

Similar Permian-Triassic rocks in the Black Hills area 

(fig. 1) are 250-700 ft (76-213 m) thick (J.E. Fox, U.S. 

Geological Survey, written commun., 1986). Boyd and 

Maughan (1973) differentiated Permian and Lower Triassic 

Goose Egg equivalent strata from the Upper Permian and 

Lower Triassic Spearfish Formation by the greater content 

of thin carbonate beds in underlying strata. Pipiringos and 

O'Sullivan (1978) also considered the lowest part of the 

Spearfish to be of Late Permian age.

The Permian-Triassic boundary is well documented 

as an erosional surface in parts of Utah, Arizona, New 

Mexico, and Colorado (Pipiringos and O'Sullivan, 1978), as 

well as southwestern Montana (Schock and others, 1981). 

Throughout Wyoming, however, the boundary is either 

marked by a lithologic change (western region) or cannot be 

readily identified (eastern region). Boyd and Maughan 

(1973) reviewed lithologic and paleontologic data

E2 Evolution of Sedimentary Basins Powder River Basin
GREEN RIVER 

BASIN

BLACK HILLS, 

WYOMING AND 

SOUTH DAKOTA

BIG HORN AND

WIND RIVER

BASINS

CASPER AREA OF

WIND RIVER

BASIN

Twin Creek

Limestone

(part)

Sundance Formation 

(part)

Gypsum Spring 

Formation

Gypsum Spring 

Member1

Gypsum Spring 

Formation

JURASSIC (?)

AND 

TRIASSIC (?)

Nugget Sandstone

Popo Agie Formation

Ankareh 

Formation Unnamed red beds

Crow Mountain 

Sandstone

Alcova Limestone

Tnaynes 

Limestone

Red Peak Formation

Woodside Spearfish Formation

Shale

Dinwoody Formation

Goose Egg Formation Minnekahta Limestone

Phosphona and Park City 

Formations

Minnelusa

Formation

(part) Tensleep Sandstone 

(part)

Member of Twin Creek Limestone

Figure 3. Late Pennsylvanian to Middle Jurassic stratigraphy, Wyoming. Stratigraphic nomenclature for Powder River Basin and 

adjacent areas is shown in figure 5. From Bolyard (1975), High and Picard (1967), Boyd and Maughan (1973), Pipiringos (1968), 

Pipiringos and O'Sullivan (1978), Picard (1978), and Keroher (1966). Reported unconformity positions (Pipiringos and O'Sullivan, 

1978) are indicated by the symbols Tr-1 through J-2; symbol P-O indicates Permian unconformity.

Red Beds of Triassic Chugwater Group, Wyoming E3
106°45'

VERTICAL EXAGGERATION X7

5 10 MILES

5 10 KILOMETERS

E4 Evolution of Sedimentary Basins Powder River Basin
associated with the boundary and concluded that sedimentation in the western region was continuous during the 

latest Permian and earliest Triassic and that the boundary is 

marked by olive-gray, dolomitic siltstone and limestone of 

the Dinwoody Formation. Lithologic changes from the 

Permian into the Triassic across western Wyoming probably 

represent the beginning of a westward shift of lithologic 

facies that is characterized by introduction of widespread 

red beds (Chugwater Group) into western Wyoming.

Triassic

Picard and his co-workers described stratigraphic 

relationships and sedimentological facies of Triassic red 

beds in central and southern Wyoming in a series of papers 

(High and Picard, 1965, 1967; Picard, 1966, 1967, 1975a, b, 

1978; Picard and High, 1968, 1972; Picard and others, 

1969). High and Picard (1967) integrated their field units in 

the Big Horn and Wind River Basins with published 

stratigraphic nomenclature (for example, Love, 1939, 1957; 

Branson and Branson, 1941; McKee and others, 1959). In 

their revision of the nomenclature, the Chugwater Formation was raised to group status, and its three named 

members, in ascending order, the Red Peak, Crow 

Mountain, and Popo Agie (fig. 3), were raised to formation 

rank. Picard (1975a) correlated the widespread marine 

Alcova Limestone Member of the Crow Mountain Formation with the uppermost part of the Thaynes Limestone. 

Pipiringos and O'Sullivan (1978) maintained the Alcova 

Limestone as a formation that separates the Red Peak 

Formation from the younger Crow Mountain Sandstone and 

Jelm Formation. Picard (1978) later presented a synthesis of 

the lithologic types and environments of the Chugwater 

Group based on his earlier studies within each formation. 

The fine-grained detritus of the Crow Mountain and. Red 

Peak in central Wyoming were interpreted as shallow-water 

shelf to tidal-current deposits, whereas the Popo Agie was 

interpreted as delta-plain and lacustrine deposits.

The Jelm Formation of southeastern Wyoming 

(Knight, 1917) is a distinctive fluvial unit of conglomeratic 

red sandstone. Pipiringos and O'Sullivan (1976) showed 

that the unit is unconformably overlain by Jurassic strata 

along the Laramie Range (fig. 1) and the Front Range of 

Colorado. The Jelm pinches out to the south in Colorado.

Figure 4 (facing page). Generalized geology and topography 

of the Red Wall valley at the headwaters of the Middle Fork of 

the Powder River. The dotted pattern shows the outcrop area of 

Permian and Triassic strata (Love and Christiansen, 1985). The 

heavy black line connects the locations of outcrop sections 

(sites 1-9, fig. 5) and delineates the Red Wall hogback. The 

two arrows designate the line of the west-east topographic 

profile. C-M, undifferentiated Cambrian through Mississippian rocks; PM, Pennsylvanian and Mississippian rocks; "&P, 

Triassic and Permian rocks; J, Jurassic rocks; K, Cretaceous 

rocks.

Near Laramie, it overlies the Red Peak Formation and 

pinches out northward. Picard (1978) correlated the Jelm 

with light-tinted, fine-grained sandstones and micrites of 

deltaic origin that lie between the Crow Mountain 

Sandstone and Popo Agie Formation in western Wyoming. 

The youngest Triassic strata in western and central 

Wyoming (fig. 3) are part of the Triassic (?) and Jurassic (?) 

Nugget Sandstone. They are unconformably overlain by 

younger Jurassic strata and are absent east of the Bighorn 

Mountains. In a regional study of Upper Triassic to Lower 

Jurassic sandstones that includes the equivalent Nugget, 

Navajo, and Aztec Sandstones, Stanley and others (1971) 

interpreted these rocks to be mainly of shallow marine 

origin in western and central Wyoming. Picard (1975b) and 

Knapp (1978) re-evaluated the markedly crossbedded, arkosic to subarkosic Nugget Sandstone and concluded that it is 

primarily an eolian deposit. Love (1957) presented evidence 

that suggests the lower part of the Nugget interfingers with 

the upper part of the underlying Popo Agie Formation, 

whereas High and Picard (1965) showed an unconformity 

between the formations.

THE RED WALL VALLEY 

Morphology

During the Paleocene, Mesozoic strata were folded to 

form the western margin of the Powder River Basin by 

uplift of the Bighorn Mountains (Flores and Ethridge, 1985; 

Whipkey, 1988). Differential erosion by the headwaters of 

the Middle Fork of the Powder River along the southeastern 

flank of the uplift produced the northeasterly to northerly 

trending valley shown in figure 4. More resistant Pennsylvanian limestones and indurated sandstones in the 

southern part of the Bighorn Mountains form the western 

margin of this valley. Less resistant red shales and mudstones of the Goose Egg Formation, which unconformably 

overlie Pennsylvanian strata, form most of the valley floor. 

The eastern side of the valley consists of almost continuous 

outcrops of sandy Triassic beds (upper part of the Red Peak 

Formation and Crow Mountain Sandstone) that form a "red 

wall" along the valley. This hogback, which dips about 16° 

eastward to southeastward, is shown in figure 4 by a heavy 

line and is known as Red Wall. The less sandy, unnamed red 

beds form a sloping terrace atop the hogback and, in turn, 

are overlain by Jurassic (?) sandstones of the Sundance 

Formation that form the less continuous cliffs informally 

named the "gray wall." Despite excellent outcrops, Carlson 

(1949) has provided one of the few detailed descriptions of 

the strata in this area.

Goose Egg Formation

The Goose Egg Formation is poorly exposed 

throughout the floor of the Red Wall valley and was not

Red Beds of Triassic Chugwater Group, Wyoming E5
Table 1. Microprobe and stable isotope values for 

selected carbonate beds Red Peak Formation

Fe Ca Mg_

Alcova Limestone (algal facies) 

Site 4 0.016 53.674 

Site? 0.008 53.619

Goose Egg Formation (dolomite bed) 

Near site 7 0.048 29.291

0.556

0.740

20.913

813C Std. dev. 818O Std. dev.

Sundance Formation (sandy limestone) 

Site 4 +0.537 0.006 -9.620 0.020

Alcova Limestone (algal facies)

Site 4 top -1.032 0.002 -5.245 0.023 

Site 4 bottom -1.131 0.009 -5.574 0.011 

Site 7 -0.877 0.007 -4.842 0.016

Goose Egg Formation (dolomite bed) 

Near site 7 +5.055 0.006 +2.210 0

systematically investigated. It crops out best as local hills in 

the vicinity of site 7 and at site 1 where the valley narrows 

(fig. 4). Outcrops are composed dominantly of red mudstones and thin, rippled siltstones. Beds (less than 3 ft or 

1 m thick) of white gypsum separated by red mudstone are 

locally present. The gypsum beds are thinly laminated but 

contorted. Rarely, very thin dolomite interbeds overlie the 

gypsiferous interval. One such dolomite near site 7 is finely 

crystalline, contains indistinct outlines of marine foraminifera (fig. 6A), and has a Ca:Mg ratio of approximately 1 

(table 1). The carbon-13 isotopic value for the dolomite 

suggests marine deposition (Arthur and others, 1983, figs. 

1-39), whereas the positive oxygen-18 values are compatible with an oxidizing, red-bed setting. Other prominent 

gypsiferous zones are along Buffalo Creek (about 80 ft or 

24 m below the base of site 5, fig. 5) and along the road 

following Middle Fork. Overall, the characteristics of the 

formation suggest accumulation in shallow, evaporitic 

marine waters. The lack of well-developed beds of displacement gypsum nodules discounts coastal sabkha flats as the 

dominant depositional setting.

The top of the Goose Egg in the Red Wall valley most 

likely is gradational into the overlying Red Peak Formation. 

At site 1, it is interpreted to lie at the top of the youngest 

gypsum bed. Newell (1967), however, considered the 

boundary to be a regional paraconformity, whereas Maughan 

(m McKee and others, 1967b) attributed subsurface 

thinning to localized tectonic uplift and erosion prior to Red 

Peak deposition.

Lithology

The Red Peak Formation is dominated by red mudstone and siltstone; sandstone is common in its upper half. 

At site 1 (fig. 5), a complete section of the Red Peak crops 

out along the valley wall and is 380 ft (116 m) thick. Sheets 

of rippled siltstone, commonly about 3 ft (1 m) thick, are 

common within mudstones of the lower part of the 

formation (fig. 74). The siltstones grade laterally into 

mudstones. Thin laminations and isolated nodules of white 

gypsum are commonly in the mudstones. The flaggy nature 

of many of the siltstone beds is caused by thin beds or 

laminae of silty mud. Sedimentary structures in the siltstone 

vary from visually massive, to wavy laminated, to almost 

symmetrical, vertically stacked bedforms. Somewhat 

thicker lenses of siltstone to very fine grained sandstone that 

fine upward are also present. These lenses have a scoured 

base overlain by well-developed, tabular ripple sets and 

abundant ripple-drift laminations. Small rip-clasts of mudstone have been found.

A few widespread sheets of generally fine to very fine 

grained sandstone as thick as 40 ft (12 m) characterize the 

middle and upper parts of the Red Peak Formation. These 

sandstone sheets dip gently within the formation in an 

apparent southwesterly direction (see fig. 5). The crude en 

echelon stacking of the sheets forms a very low angle 

accretionary sequence of interbedded sandstone and finer 

grained rocks. The sheets typically have sharp bases and 

rapidly grade into siltstone at their tops. The dominant 

bedform is current ripples. Where the sandstone is thick, 

subspherical weathering cavities are common on vertical 

outcrop faces. The updip ends of the accretionary sheets 

commonly are marked by high-competency current features 

including internal, deep (as much as 10 ft or 3 m) trough 

scours and sets of lateral accretion scours. Many scour 

surfaces are overlain by contorted rip-up clasts of laminated 

mudstone as long as 6 in. (15 cm). Small-scale sets of 

tabular cross-laminated sandstone (fig. IB), commonly with 

ripple drift sets, typically fill the scours. The sheet sandstones are feldspathic, generally poorly sorted, and very fine 

to fine grained. Iron-stained grains and minor iron-stained 

matrix are typical (fig. 6B). The uppermost parts of the sand 

bodies, however, tend to contain better sorted grains in 

crude laminae that segregate the very fine grains from the 

fine and medium grains (fig. 6Q. The sandstone sheets 

laterally break up into thinner sandstone units in their 

downslope distal direction. Near this breakup point, the 

sandstone coarsens to fine grained and probably attains its 

best sorting and porosity (fig. 6£>). These areas of increased 

grain size and sorting imply more prolonged reworking of 

the distal parts of the sheet sandstone beds.

Very thick (more than 40 ft or 12 m) lenticular 

sandstone bodies, as wide as 3 mi (5 km), are the other type

E6 Evolution of Sedimentary Basins Powder River Basin
NORTHEAST SOUTHWEST

s.

I

o

I

X

Crow Mounta ni 

andstone '

METERS 

, 125

100

75

 50

25

UJ 

____ LLJ s

oo

 25

' BO

400^

[HT] Sandstone, fine to medium Tr Calcareous interval

||jjj Sandstone, very fine Limestone (datum)

| | Mudstone-siltstone vy Bedded gypsum

v^l Clay shale ^ Trough scours

and (or) crossbeds

Rip-up clasts

5 MILES

0 5 KILOMETERS 

VERTICAL EXAGGERATION X125

Goose Egg Formation

T

Figure 5. Stratigraphic cross section along the Red Wall valley. Datum is the base of the Alcova Limestone. The locations of the outcrop sections (1-9) are shown in 

figure 4.
E8 Evolution of Sedimentary Basins Powder River Basin
of sandstone common in the middle and upper parts of the 

Red Peak Formation. Visually massive and planarlaminated units, suggestive of episodes of higher flow 

regime conditions, are commonly interbedded with current 

rippled units. Internal scours and vertically repeated 

alternations between upper and lower flow regimes indicate 

episodic fluctuations in the velocity of depositing currents. 

The alternations are sometimes separated by thin, sandy 

mudstone beds.

The uppermost part of the Red Peak Formation (lower 

unit of the Crow Mountain as used by Picard, 1978) is a 

variegated, massive to turbated, silty to very fine grained 

sandstone as thick as 10 ft (3 m) in the Red Wall valley. The 

sandstone is discontinuous (fig. 5) and locally contains 

bioturbated areas (fig. 1C} that are suggestive of rooting by 

bushes or small trees. Similar mottled variegated colors and 

turbation are present in laterally equivalent, finer grained 

rocks. The mottling and rootlike turbation are evidence that 

the top of the Red Peak is, at least locally, a paleosol. The 

absence of reworked sediment between the mottled debris 

and the base of the overlying Alcova Limestone indicates 

fairly rapid subsidence and transgression in the area.

Interpretation

Descriptions and interpretations of the Red Peak 

Formation of the Red Wall valley are generally compatible 

with those for rocks in west-central Wyoming (Picard, 

1978). The mudstone-dominated lower part of the Red Peak 

is roughly analogous to the lower two facies of Picard. It is 

essentially a continuation of rock types typical of the Goose 

Egg except for the absence of gypsum beds and the greater 

abundance of beds of sandy siltstone, the latter having 

structures suggestive of episodes of rapid sedimentation. 

These differences are not unexpected, given the more 

eastward location of the Red Wall valley. The lower 

mudstones probably represent a very distal subfacies of the 

sheet sandstone intervals; the sheet-sandstone environments 

probably episodically supplied an increased influx of finegrained detritus (fig. 5). Fresh water associated with these 

episodes may be the reason for the relative absence of 

gypsum in these rocks.

Figure 6 (facing page). Plane-light photomicrographs of the 

Goose Egg and Red Peak Formations. Site locations shown in 

figure 4; sheet sandstpnes shown in figure 5. A, Dolomite of 

the Goose Egg Formation contains remnants of a rotaloid 

benthonic foraminifera (near site 7). B, Sandstone in the central 

part of the proximal end of sheet sandstone 1 of the Red Peak 

Formation (site 3) shows the poor sorting and iron staining 

typical of the sheet sandstones. C, Sandstone near the top of 

sheet sandstone 1 of the Red Peak Formation (site 3} has good 

porosity, negligible matrix, and crudely developed laminae, all 

of which suggest reworking. D, Sandstone from the center of 

the distal part of sheet sandstone 2 of the Red Peak Formation 

is coarser grained and better sorted and has well-developed 

porosity as compared to B.

The sheet-sandstone part of the Red Peak is equivalent lithologically to Picard's (1978) alternating facies, 

which contain siltstone ledges traceable for more than 15 mi 

(24 km) in the southern Big Horn Basin. Unlike the 

sandstone sheets of the Red Wall valley, the ledges described by Picard are made up of siltstones that are mostly 

massive, contain subordinate ripples, and commonly are 

disturbed or burrowed. The interval containing the siltstones 

has been interpreted as shelf or possibly tidal flat in origin. 

The sheet sandstones in the Red Wall valley probably 

represent the shallow-water accretion of reworked deltafront sandstones, possibly associated with autocyclic destructive periods. Their upslope or proximal ends exhibit 

high-competency scour structures and commonly interfinger into lenticular, channel-filling sandstones. The most 

pronounced effects of extended reworking and size winnowing are the segregation of laminae of very fine grained 

and fine-grained sandstone near the tops of the sheets and 

the removal of most of the finer size fractions in the more 

distal parts of the sandstone sheets. The dominance of 

ripples rather than larger scale, lower regime crossbedding 

expected in such a setting is due to the very fine grain size.

Scattered thick, lenticular channel-filling sandstones 

represent the fluvial systems that supplied large quantities 

of detritus to the delta front. These sandstone units and their 

associated well-developed, fine-grained (floodplain) 

deposits correspond to the upper platy facies of Picard 

(1978). The sandstones are characterized by vertical repetition of structures of the upper and lower flow regimes in 

cosets that commonly are separated by thin beds of sandy 

silt. Concave-upward scour surfaces are common within the 

channel sandstones. These features all suggest intermittent 

discharge in sandy, braided channels. The very fine grained 

to fine-grained sand and the obvious competency to 

transport much larger clasts (scour and rip clasts) imply 

long distances of transport. Paleosols at the top of the 

formation indicate subaerial exposure at the end of this 

cycle of detrital deposition.

Alcova Limestone

The Alcova Limestone, as thick as 11 ft (3.5 m), 

records an episode of marine transgression. The Red Wall 

valley is near the eastern limit of this widespread limestone 

tongue that crops out over an area of about 31,000 mi2 

(80,000 km2) in central Wyoming (Picard, 1978). The 

Alcova was deposited as algal mats that grade upward to 

closely packed and laterally linked algal stromatolite 

mounds as much as 2 ft (60 cm) in radius. None of the 

marine invertebrates, calcarenite, or pelmicrite reported 

from other areas was found in the Red Wall valley. The 

CarMg ratio of greater than 50 and the mildly negative 

oxygen-18 values (table 1) suggest minimal diagenetic 

dissolution and cementation. Relatively low carbon-13 

values imply water more brackish than that of either the

Red Beds of Triassic Chugwater Group, Wyoming E9
I

Q. re SIoo

£

Figure 7. Photographs of the Red Peak Formation and Alcova Limestone. Site locations shown in figure 4. A, Mudstone and siltstone of the basal part of the Red Peak Formation at site 1. The cliffs in the distance to the south are formed by the more sandy, central and upper parts of the unit. B, Small-scale sets of tabular cross laminations 

and ripple drift laminae within internal scours of sandstone sheet 3 of the Red Peak Formation at site 6; rope shown for scale. C, Intensely root-turbated sandstone 

immediately underlying the Alcova Limestone (site 6) is evidence against extensive subaerial erosion after deposition of the mounds. Hammer shown for scale. D, Relief 

on upper surface of algal mounds at top of the Alcova Limestone at site 6 is evidence against extensive subaerial erosion after deposition of the mounds.
dolomite in the Goose Egg or the sandy limestone in the 

Sundance.

Relatively rapid subsidence resulted in carbonate 

accumulation directly on top of the older, partly subaerial 

deltaic surface. Algal mats that change upward into laterally 

linked mounds record deepening waters, possibly brackish 

because of mixing with terrestrial runoff. The environment 

apparently never was a suitable habitat for marine shelf 

invertebrates before it was inundated by well-winnowed and 

sorted, relatively coarse and rounded sands of the overlying 

Crow Mountain Sandstone. The eastern pinchout limit of 

the Alcova may therefore represent the actual landward 

extent of marine transgression in this area rather than later 

erosional truncation. The apparent rapidity and broad extent 

of the transgression that indicate widespread tectonic 

subsidence was superimposed on the earlier coastal-deltaic 

setting.

Crow Mountain Sandstone

The thick (as much as 100 ft or 30 m), predominantly 

light red to orange-red sandstone of the Crow Mountain 

Sandstone directly overlies the Alcova Limestone. This 

sandstone forms the upper part of the steep red walls along 

most of the Red Wall valley. The basal part of the sandstone 

locally contains scattered granules of limestone, and the 

basal contact may have scoured through the Alcova where it 

is thinnest (in the north between sites 1 and 2, figs. 4 and 5). 

Maughan (1972a, b) used similar small limestone clasts to 

define the base of the Crow Mountain in areas where the 

Alcova is absent; he assigned an erosional unconformity to 

the base of the formation. Although some scour is 

associated with the introduction of sand across freshly 

deposited algal beds, the common preservation of algal head 

mound relief on the top of the limestone (fig. ID} argues 

against extensive subaerial erosion of the limestone in this 

valley prior to accumulation of the sandstone.

The main part of the red sandstone (fig. 8A) is 

moderately well sorted and commonly contains subrounded 

fine to medium sand grains. Porosity is high and the grain 

rims are stained by hematite. Toward the southwest, the 

sandstone commonly is white, sorting and roundness are 

less developed, grains lack hematitic rims, and carbonate 

cement is common (fig. 8£).

The most striking characteristic of the upper 

sandstone unit is the abundance and diversity of current 

crossbedding. This fine- to medium-grained sandstone has 

characteristic large-scale, lower flow regime bedforms. In 

general, the lowest part of the sandstone is dominated by 

trough crossbeds. Crudely tabular crossbed sets, some of 

which exhibit bimodal foreset dip directions and reactivation surfaces (fig. 9A), are common in the middle part of

the sandstone. These grade upward through localized units 

of small trough sets into rippled flags (fig. 9E) of very fine 

grained to fine grained sandstone.

The Crow Mountain Sandstone represents a return of 

detrital influx into the area following the rapid subsidence 

and resulting deposition of the Alcova Limestone. The 

paucity of pore-filling clay in this thick sandstone interval 

implies a prolonged period of water turbulence sufficient to 

prevent clay deposition. The vertical sequence of sedimentary structures suggests that sand migrated in shallow 

water as the result of wave- and (or) tidal-driven current 

action. Initially, deposition was from discontinuous-crested 

dunes. Continued deposition of the sand offset subsidence, 

and successive tidal cycles began to rework accumulated 

sandbars. Equivalent units to the west (Tohill and Picard, 

1966) represent more seaward facies in that they are 

somewhat finer grained and more calcareous and commonly 

show vertical, inclined, and horizontal burrows. Paleocurrent directions from crossbeds in the more seaward 

facies are strongly bimodal (northeast and southwest), 

evidence for dominance of tidal control over wave control. 

Tohill and Picard interpreted the more western facies as 

representing an upward change from shallow marine shelf 

to tidal flat and beach remnants. Similar shoaling is 

reflected in the Red Wall valley area by the transition at the 

top of the formation into a lower energy, very fine grained 

sand flat dominated by ripples (fig. 5).

Unnamed Red Beds

The unnamed red beds (probably equivalent to the 

Sips Creek Member of the Jelm Formation) are composed 

of interbedded lenticular red to pale-red to locally white 

sandstone, red siltstone and mudstone and intervals of 

gray-green clay shale. They most likely grade downward 

into and locally interfinger with the Crow Mountain 

Sandstone. The red beds form much of the gently sloping 

bench that separates the Red Wall from the base of the 

younger gray wall along the valley.

The sandstones are thick (as much as 35 ft or 10 m), 

lenticular, fine-grained, poorly sorted channel fills and 

tabular splays (generally less than 10 ft or 3 m thick). The 

basal scoured contact of the channel fills are commonly 

overlain by lag rip-up clasts of siltstone. Crossbedding in 

the channels includes interbeds of lower flow regime 

large-scale trough sets and upper flow regime planar laminae. The tops of the channel-filling sandstone units mostly 

are gradational with overlying units and ripple dominated. 

Splay sands are thinner and generally can be differentiated 

by their very fine sand to silty grains, dominance of internal 

ripples, and common gradational base.

Field observations of the upper contact of the red beds 

are too few to clearly define the contact (fig. 5). Near the

Red Beds of Triassic Chugwater Group, Wyoming Ell
E12 Evolution of Sedimentary Basins Powder River Basin
top of the red beds, however, light-yellow to white, calcareous sandstone is associated with gray-green clay shale 

(sites 4 and 6, fig. 5) and red mudstone. Some of the 

sandstone is intensely burrowed (fig. 9C). Locally, the 

upper part of the red beds includes sandy to silty intrasparite 

(fig. 8C) containing intraclasts of silty micrite. Elsewhere 

(top of site 7, fig. 5), units of sandstone are flaser bedded. At 

one location (site 2), this part of the interval contains a white 

gypsum bed. These upper units are included with the red 

beds of the Red Wall valley based on the descriptions of 

silty micrites and light-gray-green finer detritus that are 

present in the Jelm of the adjacent Wind River Basin 

(Picard, 1978).

The characteristics of the red beds are consistent with 

the interpretation of Picard (1978) that equivalent strata of 

the Jelm Formation are part of a generally westward 

prograding fluvial deltaic plain. The red beds are equivalent 

to and probably were supplied by the high-gradient, pebbly, 

alluvial channel facies of the Jelm Formation in southeastern Wyoming (Knight, 1917; Pipiringos and O'Sullivan, 

1976).

The calcareous sandstones, silty micrites, and red to 

gray-green finer detritus of the upper part of the red beds 

probably represent extensive lake deposits that accumulated 

in passive areas of a well-integrated alluvial floodplain. 

Large lake deposits have been described as a major 

component of the Popo Agie Formation (fig. 3), but the rock 

types of the Red Wall valley are not very similar to those 

described by Picard. The lower part of the Popo Agie in the 

Wind River Basin is a carbonate unit that is 7-33 ft (2- 

10 m) thick. The basal bed of the Popo Agie in the Wind 

River Basin is a limestone microconglomerate that is locally 

gradational with the underlying Jelm. Silty to sandy (quartz) 

microcrystalline limestone and dolomite also are present. 

Picard (1978) reported that the lower limestone is overlain 

by gray-red to red-brown, smectite-rich detritus of the 

purple unit. The dominantly lacustrine deposits of the Popo 

Agie (Picard and High, 1972) are characterized by fossilized 

reptiles, amphibians, and freshwater invertebrates.

Figure 8 (facing page). Plane-light photomicrographs of the 

Crow Mountain Sandstone, unnamed red beds, and Sundance 

Formation. Site locations shown in figure 4. A, Typically red, 

upper part of the Crow Mountain Sandstone (site 3) contains 

coarser, more rounded, and better sorted grains than the sheet 

sandstones of the Red Peak Formation (fig. 66). The sandstone 

has open pores and iron-stained grains. B, Part of the Crow 

Mountain Sandstone is almost white (site 9). Grains are not 

iron stained and pores are filled with carbonate cement. C, 

Silty intrasparite sandstone from the lake (?) deposits in the 

unnamed red beds (top of measured section 6, fig. 5) contains 

intraclasts composed of silty micrite. D, Large trough crossbedded sandstone of the Sundance Formation consists of 

alternating laminae of quartz sand and pseudo-oolitic (oncolite?) carbonate sand. The large grain is a small chert pebble.

Jurassic Strata

The base of the gray wall was encountered in several 

of the measured sections (fig. 5). Southwestward (site 9), 

rocks in the lower part of the wall consist of a thick, locally 

calcareous, texturally submature to mature white quartz 

sandstone. Structures in the lower 50 ft (15 m) of the 

vertical cliff consist of alternating zones of platy, upper flow 

regime planar laminations and large-scale trough crossbeds. 

At the northeastern end of the valley (sites 3, 4), laterally 

equivalent strata include white sandstone, the basal part of 

which contains very gently dipping, platy beds of planar 

laminations that underlie trough crossbed sets more than 8 ft 

(2.5 m) high (fig. 9D). This sandstone consists of laminae of 

medium- to coarse-grained, well-sorted pseudo-oolites 

(algal?) and medium-grained quartz sand that contains some 

rounded chert granules (fig. 8D). Carbon-13 values (table 1) 

indicate that the sand accumulated in marine waters.

The character of the crossbeds and their oolitic nature 

and suggested marine affinities indicate that the gray wall is 

most similar to beds of oolitic sandstone to sandy oolitic 

limestone of the Canyon Springs Sandstone Member of the 

Sundance Formation. This unit crops out along the western 

side of the Bighorn Mountains (Imlay, 1957). It is 

characterized by large-scale trough crossbedding and forms 

a relatively continuous blanket of nearshore sand-wave 

deposits (Rautman, 1978) in central Wyoming (Kocurek 

and Dott, 1983). It was deposited during the second Jurassic 

marine transgression that extended eastward beyond the 

limits of the Nugget Sandstone (Imlay, 1957). If our 

interpretation is correct, the gypsum bed at the top of site 2 

(figs. 4, 5) could represent a southern outlier of the Gypsum 

Spring Formation of the northwestern Black Hills (Mapel 

and Bergendahl, 1956) and north-central Wyoming (Imlay, 

1957). Picard (1978), however, has reported minor amounts 

of gypsum within the dominantly lacustrine rocks of the 

Popo Agie Formation.

SUMMARY AND CONCLUSIONS

The Chugwater Group in the Red Wall valley is a 

complex interval of predominantly fine grained, detrital 

rocks that were deposited in a coastal environment. The 

facies represent parts of a generally westward PermianTriassic progradation of fine-grained terrigeneous detritus 

associated with regional subsidence of the shallow-water 

Cordilleran shelf. The fine grain sizes and mineralogical 

maturity of the sandstones suggest long transport distances. 

This pattern of detrital influx at rates somewhat greater than 

that of subsidence was interrupted only once by the abrupt 

eastward marine transgression recorded by the Alcova 

Limestone. Deposition of pebbly fluvial sandstones in 

southeastern Wyoming and northeastern Colorado after this 

transgression indicates the development of less distant 

source terrains for the succeeding detritus.

Red Beds of Triassic Chugwater Group, Wyoming E13
I

I

Figure 9. Photographs of the Crow Mountain Sandstone, unnamed red beds, and Sundance Formation. Site locations shown in figure 4. A, Crudely tabular, bimodal sets 

ofcrossbeds in the central part of the Crow Mountain Sandstone (site 3) and the reactivation surfaces between the sets indicate tidal currents. B, Upper part of Crow Mountain 

Sandstone displays abrupt transition from trough crossbeds to overlying platy, ripple laminae that accompanies the decrease in grain size at the top of the unit (site 3). 

Hammer shown for scale. C, Intensely burrowed, lenticular, lake (?) sandstone within the gray-green claystone near the top of the unnamed red beds at site 7. D, Large trough 

crossbeds in the Sundance Formation (site 3). Staff is 5 ft long.
Depositional facies of formations in western and 

central Wyoming (Picard, 1978) are compared with the 

depositional facies of the rocks in the Red Wall valley in 

figure 10. Evaporitic marine mudstones and siltstones of the 

Goose Egg Formation in the Red Wall valley grade upward 

into shelf and distal delta mudstones and silty sandstones of 

the basal part of the Red Peak Formation. Reworked red 

delta-front sheet sandstones, characteristic of the central 

part of the Red Peak, separate distal delta mudstones from 

fluvial-deltaic red facies that supplied detritus to the system. 

The sheet sandstones thus probably represent approximately 

the shoreline position for equivalent shelf-dominated red 

beds to the west.

The Alcova Limestone and Crow Mountain Sand 

stone record a perturbation in the overall net imbalance that

FORMATIONS 

OF THE 

CHUGWATER 

GROUP 

(this report)

Popo Agie 

Formation

Unnamed 

red beds

Crow 

Mountain 

Sandstone

Alcova 

Limestone

Red Peak 

Formation

SUBDIVISIONS OF PICARD (1978)

FOI

Agie Formation Popo 

Crow Sandstone Mountain 

c

0

Red PeakFormati

NATION! 

LITHOFACIES

Upper 

carbonate 

(0-13 ft)

Ocher 

(0-99 ft)

Yellow 

(33-66 ft)

Lower 

carbonate 

(6-33 ft)

Jelm 

Formation 

(35-99 ft)

Upper 

sandstone, 

siltstone 

(0-50 ft)

Basal 

sandstone 

(16-1 30 ft)

Alcova 

Limestone 

Member 

(0-30 ft)

Variegated 

sandy 

(3-1 6 ft)

Upper 

platy 

(17-83 ft)

Alternating 

(200-430 ft)

Lower 

platy 

(11 5-380 ft)

Silty 

claystone 

(150-280 ft)

ENVIRONMENT

Fluvial, 

lacustrine

Lacustrine, 

fluvial

Lacustrine, 

fluvial

Fluvial, 

lacustrine

Deltaic plain

Tidal flat

Marine shelf, 

nearshore

Marine shelf

Nearshore 

marine

Tidal flat

Shelf, 

tidal flat

Tidal flat

Transitional 

paralic

DEPOSITIONAL 

SETTINGS OF 

RED WALL VALLEY

Formation 

absent (?)

Lacustrine, fluvial 

~-___^^ (200ft)

Fluvial, ~^~~~ - 

deltaic (61 m)

(<15ft) 

Tidal flat,

beach (?) 

(<5m)

(100ft) 

Nearshore 

coastal 

(40m)

(<8ft) 

Brackish-water 

nearshore 

(<2.5m)

(350ft)

Distal 

fluvialdeltaic ^ "~

<^ Reworked 

\. nearshore 

> to 

./shallow shelf

(107 m)

Figure 10. Relationship between inferred depositional 

settings of the Red Wall valley area and those reported by 

Picard (1978) for adjacent basins to the west. Note that the 

lower two units ("variegated sandy unit" and "Alcova 

Limestone Member") of the Crow Mountain Formation of 

Picard coincide with the uppermost Red Peak Formation and 

the Alcova Limestone, respectively, in this paper (see fig. 3).

had favored terrigenous influx and progradation over shelf 

subsidence. Rapid transgression of marine waters resulted 

in deposition of the Alcova Limestone across the partly 

leached, subaerial sediments that form the top of the Red 

Peak Formation. This widespread episode of increased 

subsidence rate implies regional tectonic controls possibly 

related to compressional tectonic events along the western 

margin of the Cordillera. The Alcova Limestone, in turn, 

was buried beneath a thick, reworked Crow Mountain 

coastal sandstone that was introduced by some combination 

of wave and (or) tidal currents. The source for this coastal 

sand was likely the thick, pebbly alluvial sand that dominates the Jelm Formation in southeastern Wyoming (fig. 3). 

Deposition of very fine sand and silt as tidal (?) flats, 

recorded by the uppermost part of the Crow Mountain, 

indicates the return of active detrital progradation.

The unnamed red beds in the Red Wall valley mark 

the re-establishment of prograding fluvial-deltaic facies in 

the area. The youngest micritic units record the completion 

of terrigenous progradation as deposits of widespread floodplain lakes accumulated. Rocks of the Chugwater Group 

most likely terminate against an erosional unconformity that 

marks the base of the Triassic (?) and Jurassic (?) transgressive Nugget Sandstone.

The remarkable outcrops of Chugwater in the Red 

Wall valley of the southwestern Powder River Basin 

provide valuable information for regional paleoenvironmental studies in an area strategically located between 

carefully studied areas to the west, southeast, and east. The 

outcrops display an unusual group of fine-grained, generally 

low gradient red-bed facies that accumulated across the 

shoreline zone of a shallow coastal shelf.

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Formation (Triassic), west-central Wyoming: Journal of 

Sedimentary Petrology, v. 36, p. 904-926.

____ 1967, Stratigraphy and depositional environments of the 

Red Peak Member of the Chugwater Formation (Triassic), 

west-central Wyoming: Contributions to Geology, University of Wyoming, Laramie, v. 6, p. 39-67.

____ 1975a, Shelf edge carbonate-redbed transitions, Red Peak 

and Thaynes Formations (Triassic), western Wyoming and 

adjacent Idaho, in Bolyard, D.W., ed., Symposium on Deep 

Drilling Frontiers of the Central Rocky Mountains: Rocky 

Mountain Association of Geologists, p. 99-107.

____ 1975b, Facies, petrography and petroleum potential of the 

Nuggett Sandstone (Jurassic), southwestern Wyoming and 

northeastern Utah, in Bolyard, D.W., ed., Symposium on 

Deep Drilling Frontiers of the Central Rocky Mountains: 

Rocky Mountain Association of Geologists, p. 109-127.

____ 1978, Stratigraphy of Triassic rocks in west-central

Wyoming, in Resources of the Wind River Basin, 30th 

Annual Guidebook: Wyoming Geological Association, 

p. 101-130.

Picard, M.D., Aadland, R., and High, L.R., Jr., 1969, Correlation 

and stratigraphy of Triassic Red Peak and Thaynes

E16 Evolution of Sedimentary Basins Powder River Basin
Formations, western Wyoming and adjacent Idaho: American Association of Petroleum Geologists Bulletin, v. 53, 

p. 2274-2289.

Picard, M.D., and High, R.L., Jr., 1968, Shallow marine currents 

on the early(?) Triassic Wyoming shelf: Journal of 

Sedimentary Petrology, v. 38, p. 411^23.

____ 1972, Critera for recognizing lacustrine rocks, in Rigby, 

J.K., and Hamblin, W.K., eds., Recognition of ancient 

sedimentary environments: Society of Economic Paleontologists and Mineralogists Special Publication 16, 

p. 108-145.

Pipiringos, G.N., 1968, Correlation and nomenclature of some 

Triassic and Jurassic rocks in the south-central part of 

Wyoming: U.S. Geological Survey Professional Paper 

594-D, 26 p.

Pipiringos, G.N., and O'Sullivan, R.B., 1976, Stratigraphic 

sections of some Triassic and Jurassic rocks from Douglas, 

Wyoming, to Boulder, Colorado: U.S. Geological Survey 

Oil and Gas Investigations Chart OC-69, 1 sheet.

____ 1978, Principal unconformities in Triassic and Jurassic 

rocks, western interior United States Preliminary survey: 

U.S. Geological Survey Professional Paper 1035-A, 29 p.

Rautman, C.A., 1978, Sedimentology of a Late Jurassic barrierisland complex; lower Sundance Formation of the Black 

Hills: American Association of Petroleum Geologists 

Bulletin, v. 62, p. 2275-2289.

Schock, W.A., Maughan, E.K., and Wardlaw, B.R., 1981, 

Permian-Triassic boundary in southwestern Montana and 

western Wyoming, in Tucker, T.E., ed., Field conference 

and symposium guidebook to southwest Montana: Montana 

Geological Society, p. 59-69.

Stanley, K.O., Jordan, W.M., and Dott, R.H., 1971, Early Jurassic 

paleogeography and sediment dispersal for the Western 

United States: American Association of Petroleum 

Geologists Bulletin, v. 55, p. 10-19.

Tohill, B., and Picard, M.D., 1966, Stratigraphy and petrology 

of the Crow Mountain Sandstone Member (Triassic), 

Chugwater Formation, northwestern Wyoming: American 

Association of Petroleum Geologists Bulletin, v. 50, 

p. 2547-2565.

Whipkey, C.E., 1988, Provenance of lower Tertiary sandstones of 

the Powder River Basin, Wyoming: Raleigh, North Carolina 

State University, M.S. thesis, 145 p.

*U.S. GOVERNMENT PRINTING OFFICE: 1991-573-049/26039 Red Beds of Triassic Chugwater Group, Wyoming E17



SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

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Open-File Reports include unpublished manuscript reports, maps, 

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depositories. They are a nonpermanent form of publication that may be 

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Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

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Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

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present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

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Oil and Gas Investigations Charts show stratigraphic information 

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in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

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