The Mid-Cretaceous Frontier 

Formation Near the Moxa Arch, 

Southwestern Wyoming

U.S. GEOLOGICAL SURiVEY PROFESSIONAL PAPER 1290!

The Mid-Cretaceous Frontier 

Formation Near the Moxa Arch, 

Southwestern Wyoming

By E. A. MEREWETHER, PAUL D. BLACKMON, and J. C. WEBB 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1290

A stratigraphic description, including some 

paleontologic and petrographic data, of lower 

Upper Cretaceous rocks in the Green River Basin

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1984
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data 

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

The mid-Cretaceous frontier formation near the Moxa arch, southwestern Wyoming. 

(Geological Survey Professional Paper; 1290) 

Bibliography: 29 p. 

Supt. of Docs. No.: 119.16:1290 

1. Geology, Stratigraphic Cretaceous. 2. Geology Green River Watershed (Wyo.-Utah) I. Blackmon, Paul

D. II. Webb, J. C. III. Title. IV. Series. 

QE687.M468 1984 551.7'7 83-600107

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Abstract

Introduction ..... 

Acknowledgments 

Previous work . .

Stratigraphy ...........

Overthrust belt .......

Uinta Mountains ......

Moxa arch ..........

Stratigraphic interpretations

Core-sample descriptions . . . . .

Borehole 1 ...........

Borehole 2 ...........

Borehole 3 ...........

Borehole 6 ............

Borehole 8 ...........

Borehole 11 ..........

Borehole 18 ..........

Summary and interpretations 

References cited ..........

Page

13

13

15

15

23

23

24

24

26

28

ILLUSTRATIONS

FIGURE 1. Map of southwestern Wyoming, northeastern Utah, and northwestern Colorado, showing outcrops of Frontier Formation, the crest of the Moxa arch, and location of selected gas fields, boreholes, outcrop sections, and fossil localities . 3

2. Correlation chart of the lower Upper Cretaceous formations at selected localities in Wyoming and Colorado ... 4

3. Stratigraphic diagram of the Frontier Formation and Mowry Shale at boreholes and outcrop in northwestern Colorado,

northeastern Utah, and southwestern Wyoming .................................... 10

4. Stratigraphic diagram of the Frontier Formation and Mowry Shale at boreholes and outcrop on the Moxa arch and

the overthrust belt, southwestern Wyoming ...................................... 11

5. Stratigraphic diagram of the Frontier Formation and Mowry Shale at boreholes on the Moxa arch, southwestern

Wyoming ............................................................ 12

6. Logs of cored and sampled strata in the Frontier Formation in borehole 1 on the Moxa arch, Wyoming ..... 14

7. Photomicrographs of thin sections of sandstone from boreholes 1, 2, and 3 in the Frontier Formation of southwestern

Wyoming .......................................................... 17

8. Logs of cored and sampled strata in the Frontier Formation and Hilliard Shale in borehole 2 on the Moxa arch, Wyoming 20

9. Logs of cored and sampled strata in the Frontier Formation in borehole 3 on the Moxa arch, Wyoming ..... 22

10. Photomicrographs of sandstone from boreholes 3 and 11 in the Frontier Formation of southwestern Wyoming, taken

with a scanning electron microscope ........................................... 25

11. Logs of cored and sampled strata in the Frontier Formation in borehole 8 on the Rock Springs uplift, Wyoming . . 26

12. Logs of cored and sampled strata in the Frontier Formation in borehole 11 on the Moxa arch, Wyoming .... 27

TABLES

TABLE 1. Location of selected boreholes and outcrop sections in the Green River Basin, Wyoming, Utah, and Colorado ... 9

2. Location, depth, lithology, and depositional environment of core samples from the Frontier Formation and the lower

part of the Hilliard Shale in southwestern Wyoming .................................. 14

3. Grain sizes of core samples of sandstone from the Frontier Formation of southwestern Wyoming .......... 15

4. Description of thin sections of sandstone from core samples of the Frontier Formation of southwestern Wyoming . 16

5. Composition (in percent) of thin sections of sandstone from core samples of the Frontier Formation of southwestern

Wyoming ..................*,.^.,,...,...............~.............. 18

6. Estimated mineral and organic-carbon content of core samples of sandstone from the Frontier Formation of southwestern

Wyoming ................................. ^ ......................... 18

7. Content of principal oxides (in weight percent) in core samples of sandstone from the Frontier Formation of southwestern

Wyoming ........................................................... 19

ni
IV CONTENTS

Page

TABLE 8. Approximate content of selected oxides and elements in core samples of sandstone from the Frontier Formation of southwestern Wyoming, determined by emission spectrographic analysis .......................... 21

9. Estimated mineral content by X-ray diffraction of core samples of shale from the Frontier Formation and Hilliard Shale

of southwestern Wyoming .................................................. 21

10. Organic composition, vitrinite reflectance, and hydrocarbon potential of core samples of shale from the Frontier Formation

and Hilliard Shale of southwestern Wyoming ....................................... 22

11. Correlation coefficients for amounts of sand, silt, clay, organic carbon, and selected elements in eleven samples of sandstone

of the Frontier Formation in southwestern Wyoming .................................. 28
THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE 

MOXA ARCH, SOUTHWESTERN WYOMING

By E. A. MEREWETHER, PAUL D. BLACKMON, and J. C. WEBB

ABSTRACT

The Frontier Formation in the Green River Basin of Wyoming, 

Utah, and Colorado, consists of sandstone, siltstone, and shale, and 

minor conglomerate, coal, and bentonite. These strata were deposited 

in several marine and nonmarine environments during early Late Cretaceous time. At north-trending outcrops along the eastern edge of 

the overthrust belt, the Frontier is of Cenomanian, Turonian, and 

early Coniacian age, and commonly is about 610 m (2,000 ft) thick. 

The formation in that area conformably overlies the Lower Cretaceous Aspen Shale and is divided into the following members, in ascending order: Chalk Creek, Coalville, Alien Hollow, Oyster Ridge 

Sandstone, and Dry Hollow. In west-trending outcrops on the northern flank of the Uinta Mountains in Utah, the Frontier is middle 

and late Turonian, and is about 60 m (200 ft) thick. These strata 

disconformably overlie the Lower Cretaceous Mowry Shale. In 

boreholes on the Moxa arch, the upper part of the Frontier is of 

middle Turonian to early Coniacian age and unconformably overlies 

the lower part of the formation, which is early Cenomanian at the 

south end and probably Cenomanian to early Turonian at the north 

end. The Frontier on the arch thickens northward from less than 

100 m (328 ft) to more than 300 m (984 ft) and conformably overlies 

the Mowry.

The marine and nonmarine Frontier near the Uinta Mountains, 

marine and nonmarine beds in the upper part of the formation on 

the Moxa arc ., and the largely nonmarine Dry Hollow Member at 

the top of the Frontier in the overthrust belt are similar in age. 

Older strata in the formation, which are represented by the disconformable basal contact of the Frontier near the Uinta Mountains, 

thicken northward along the Moxa arch and westward between the 

arch and the overthrust belt. The large changes in thickness of the 

Frontier in the Green River Basin were caused mainly by differential 

uplift and truncation of the lower part of the formation during the 

early to middle Turonian and by the shoreward addition of progressively younger sandstone units at the top of the formation during 

the late Turonian and early Coniacian.

The sandstone in cores of the Frontier, from boreholes on the Moxa 

arch and the northern plunge of the Rock Springs uplift, consists 

of very fine grained and fine-grained litharenites and sublitharenites 

that were deposited in deltaic and shallow-water marine environments. These rocks consist mainly of quartz, chert, rock fragments, 

mixed-layer illite-smectite, mica-illite, and chlorite. Samples of the 

sandstone have porosities of 4.7 to 23.0 percent and permeabilities 

of 0.14 to 6.80 millidarcies, and seem to represent poor to fair reservoir beds for oil and gas.

The shale in cores of the Frontier Formation and the overlying 

basal Billiard Shale, from the Moxa arch, Rock Springs uplift, and 

overthrust belt, was deposited in deltaic and offshore-marine environments. Samples of the shale are composed largely of quartz, micaillite, mixed-layer illite-smectite, kaolin, and chlorite. They also contain from 0.27 to 4.42 percent organic carbon, in humic and sapropelic

organic matter. Most of the sampled shale units are thermally mature, 

in terms of oil generation, and a few probably are source rocks for 

oil and gas.

INTRODUCTION

The Frontier Formation in the southern part of the 

Green River Basin (fig. 1), is composed largely of shale, 

siltstone, and sandstone of marine and nonmarine origin. Molluscan fossils in the marine beds indicate that 

the formation is of early Late Cretaceous age (Cenomanian, Turonian, and Coniacian) (fig. 2). The Frontier 

is about 60 m (200 ft) thick and disconformably overlies 

the Lower Cretaceous Mowry Shale at outcrops along 

the northern flank of the Uinta Mountains in Utah, and 

is about 610 m (2,000 ft) thick and conformably overlies 

the Lower Cretaceous Aspen Shale at outcrops on the 

east side of the overthrust belt in Wyoming (fig. 1). 

Sandstone units at the top of the Frontier are overlain 

comformably by the Upper Cretaceous Hilliard Shale 

in southwestern Wyoming and by the Upper Cretaceous Mancos Shale in northwestern Colorado (fig. 2).

The main purposes of this study of the Frontier Formation are to synthesize relevant published and unpublished stratigraphic data, to provide new paleontologic 

and petrographic information, and to present interpretations concerning the areal extent and depositional origin of some strata within the formation. Outcrops were 

examined and fossils were collected for this investigation by W. A. Cobban and E. A. Merewether in 1974 

and by E. A. Merewether in 1979. The Frontier in the 

subsurface was studied in 1980 by using borehole logs 

and cores. Molluscan fossils from the outcrops and cores 

were compared with W. A. Cobban's (written eommun., 

1981) sequence of Late Cretaceous index-fossils to establish the relative ages of beds in the formation. Samples of sandstone from cores were disaggregated and 

sieved to determine grain size and were analyzed by 

X-ray diffraction to establish mineral composition. Thin 

sections of samples from cores were examined to describe the mineralogy, texture, and porosity of some 

sandstone units. The elemental composition of sandstone samples from cores was determined by atomic
THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

absorption spectrophotometry (Shapiro, 1975), and 

semiquantitative, emission spectroscopy. Samples of 

shale from cores were analyzed by X-ray diffraction to 

establish their mineral composition and by chemical and 

pyrolytic procedures (Claypool and Reed, 1976; Espitalie' and others, 1977) to establish their organic composition. The hydrocarbon potential and thermal maturity of the sampled shale were determined from the 

pyrolysis and reflectance of constituent organic material.

ACKNOWLEDGMENTS

The U.S. Department of Energy, Western Gas Sands 

Project, provided most of the funds used for this investigation. Some cores and borehole records described in 

this study were contributed to the U.S. Geological Survey by C & K Petroleum, Inc. 1 Unpublished descriptions of outcrops on the north flank of the Uinta Mountains were supplied by C. M. Molenaar. W. A. Cobban 

and N. F. Sohl identified the fossil mollusks cited in 

the following pages. The fossils were prepared for study 

by R. E. Burkholder. R. M. Pollastro and J. W. Bader 

established the mineral composition of samples of shale. 

Vitrinite reflectance values for the shale samples were 

supplied by M. J. Pawlewicz. G. E. Claypool and T. 

A. Daws determined the organic composition of samples 

of shale and sandstone. The porosity and permeability 

of several sandstone samples were measured by J. W. 

Bader. L. R. Mahrt prepared samples of selected mineral grains, which were identified by D. L. Gautier. 

K. L. Gardner prepared thin sections of sandstone and 

sand grains. Samples of sandstone were examined and 

photographed with a scanning electron microscope 

(SEM) by C. W. Keighin. Rapid rock analyses (Shapiro, 

1975) of sandstone samples were provided by F. W. 

Brown and J. G. Reid. Semiquantitative spectrographic 

analyses of sandstone were provided by Leung Mei, W. 

B. Crandell, and D. W. Golightly. Correlation coefficients for analytical data were determined by J. J. Connor. The generous assistance of the U.S. Department 

of Energy, C & K Petroleum, Inc., and the abovenamed employees of the U.S. Geological Survey is 

gratefully acknowledged.

PREVIOUS WORK

The Frontier Formation and the overlying Hilliard 

Shale were named and described by Knight (1902) from 

outcrops in the vicinity of Kemmerer, Wyo., near the 

western edge of the Green River Basin (fig. 1). Veatch

'Company names are for descriptive purposes ordy and do not constitute endorsement 

by the U.S. Geological Survey.

(1907) reported that the Frontier on the west side of 

the Green River Basin is of Colorado age (mid-Cretaceous) and named a group of sandstone beds in the 

upper part of the formation the Oyster Ridge Sandstone 

Member. Veatch (1907) also named the Aspen Shale 

which underlies the Frontier in westernmost Wyoming. 

The Frontier at Cumberland Gap, south of Kemmerer 

(fig. 1), and at other localities near the Green River 

Basin was described and assigned to the Cenomanian, 

Turonian, and Coniacian Stages by Cobban and Reeside 

(1952). Reeside (1955) also described the Frontier, the 

associated fossils, and a hiatus at the base of the formation at outcrops along Vermillion Creek near the Uinta 

Mountains in northwestern Colorado (fig. 1). The outcropping Frontier at Vermillion Creek, Colo., Cumberland Gap, Wyo., and Coalville, Utah, was later investigated by Hale (1960, 1962). He recognized Reeside's 

(1955) unconformity at the Frontier-Mowry contact 

near the Uinta Mountains in the southern part of the 

Green River Basin, and proposed an unconformity at 

the top of the Oyster Ridge Sandstone Member of the 

Frontier at Cumberland Gap near the western edge of 

the basin. Hale (1962) also indicated that the formation 

at Vermillion Creek is represented at Cumberland Gap 

by a hiatus above the Oyster Ridge Sandstone Member. 

The unconformity at the base of the Frontier near the 

Uinta Mountains also was described by Weimer (1962), 

who suggested that the associated hiatus had been 

caused by an uplift in the Uinta Mountains area during 

the early Late Cretaceous. Hansen (1965) mapped and 

described outcrops of the Frontier on the northern flank 

of the Uinta Mountains near Flaming Gorge Reservoir. 

In southwestern Wyoming near the Moxa arch (fig. 1), 

DeChadenedes (1975) described and interpreted the 

Frontier Formation at outcrops and in the subsurface. 

In his regional study, the Oyster Ridge Sandstone 

Member was called the "first Frontier sandstone", an 

underlying unit of marine shale was recognized, a 

marine sandstone underlying the shale was called the 

"second Frontier sandstone", and a sandstone in the 

lower part of the formation was called "third Frontier." 

DeChadenedes (1975) located nonmarine, deltaic, and 

marine facies of these rocks in the eastern part of the 

overthrust belt and the western part of the Green River 

Basin from the area of La Barge, Wyo., south to the 

Wyoming-Utah State line. The stratigraphy and structure of the oil- and gas-producing La Barge area, at 

the north end of the Moxa arch (fig. 1), were described 

by McDonald (1976). Names applied to members of the 

Frontier at Coalvffle, Utah, by Hale (I960) and Ryer 

(1976) were used in the <Jreen River Basin and overthrust belt of Wyoming by Myers (1977). The members 

of the Frontier at Cumberland Gap are, in ascending 

order, Chalk Creek, Coalville, Alien Hollow, Oyster
INTRODUCTION

111° 110° 109°

EXPLANATION

Outcrop of Frontier Formation

Contact

Approximate boundary of 

overthrust belt

Location of outcrop section

Crest of Moxa arch

Gas field, production from

Frontier Formation 

Line of cross section 

Borehole (numbered on tables

1 and 2 and figures 3 10) 

Fossil locality not in outcrop

section

Red Desert

108°

41

f

:tffiSfas

DUCHESNE

10 20 30 40 MILES

20 30 40 50 60 KILOMETERS

FIGURE 1. Map of southwestern Wyoming, northeastern Utah, and northwestern Colorado, showing outcrops of Frontier Formation, the 

crest of the Moxa arch, and location of selected gas fields, boreholes, outcrop sections, and fossil localities. Modified from Love and 

others (1955), Stokes and Madsen (1961), and Tweto (1979).

Ridge Sandstone, and Dry Hollow (fig. 2). Myers (1977) 

studied outcrops, cores, and borehole logs of the Frontier and described the depositional environments and 

areal distribution of units within the formation. Myers 

(1977, figs. 4 and 18) also located sediment-source areas 

and shorelines for his "second Frontier sandstone" 

(Chalk Creek and Coalville Members) and "first Frontier sandstone" (Oyster Ridge Sandstone and Dry Hollow Members). The outcrops at Cumberland Gap (Cobban and Reeside, 1952) also were analyzed in Ryer's 

(1977a, b) interpretations of the depositional history of 

the Frontier at Coalville, Utah. Wach (1977) described 

the stratigraphy, structure, and hydrocarbon potential

of sandstone units in the upper part of the Frontier 

along the Moxa arch. Outcropping units and members 

of the formation near the eastern edge of the overthrust 

belt in southwestern Wyoming have been mapped by 

Rubey and others (1975), and investigated by M'Gonigle 

(1979, 1980) and Schroeder (1978, 1980). Depositional 

environments and reservoir properties of upper parts 

of the Frontier on the Moxa arch, near the confluence 

of Lincoln, Sublette, and Sweetwater Counties, Wyo., 

were determined from cores and borehole logs by Hawkins (1980). The petrography and depositional environments of much of the Frontier on the Moxa arch were 

described and related by Winn and Smithwick (1980).
THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

Informal 

substage

Western Interior fossil zones;

numbers representing some zones are

shown on figures 3, 4, and 5 and noted

in text

Potassium-argon ages

(million years) (Lanphere

and Jones, 1978)

Cumberland 

Gap area. 

Wyoming

Vermillion

Creek area.

Colorado

Osage area,

northeastern

Wyoming1

Lower

Upper

Middle

Lower

 r 

Upper

Middle

Lower

Upper

Middle

Lower

26 Clioscaphites saxitonianus

25 Scaphites depressus

24 Scaphites ventricosus

23 Inoceramus deform is

22 Inoceramus erectus

21 Inoceramus waltersdorfensis

20 Prionocyclus quadratus

19 Scaphites nigricollensis

18 Scaphites whitfieldi

I 7 Scaphites warreni

16 Prionocyclus macombi

15 Prionocyclus hyatti

14 Subprionocyclus percarinatus

13 Collignoniceras woollgari regulare

12 Collignoniceras woollgari woollgari

II Mammites nodosoides

10 Watinoceras coloradoense

9 Sciponoceras gracile

8 Dunveganoceras albertense

7 Dunveganoceras pondi

6 Plesiacanthoceras wyomingense

5 Acanthoceras amphibolum

4 Acanthoceras alvaradoense

3 Acanthoceras muldoonense

2 Acanthoceras granerosense

1 Calycoceras gilberti

	No molluscan fossil record

88.9

Milliard

Shale

(lower

part)

91.1

93.5

94.4

96

Mancos

Shale

(lower

part)

Dry

Hollow 

Member

Frontier 

Formation

foyster Ridge\

Sandstone

Member

Alien Hollow 

Member

Coalville 

\ Member /

Chalk

Creek

Member

Niobrara Fm. 

(lower part)

(J

Sage

Breaks

Member

Turner 

Sandy 

Member

1. ...I.-MUIUJ

Pool Creek 

Member

Greenhorn 

Formation

Belle

Fourche

Shale

1 Age of basal contact of Niobrara 

Formation from Evetts (1976. p. 121)

FIGURE 2. Correlation of the lower Upper Cretaceous formations at selected localities in Wyoming and Colorado. Patterned area

represents hiatus in sequence of beds.

STRATIGRAPHY

The Frontier Formation in southwestern Wyoming 

and adjacent parts of Utah and Colorado is composed 

of shale, siltstone, sandstone, and minor conglomerate, 

bentonite, and coal. These rocks were deposited in 

offshore-marine, nearshore-marine, and nonmarine environments during early Late Cretaceous time. Molluscan fossils in the marine beds are of Turanian and early

Coniacian age (fig. 2). Strata in the formation crop out 

as a succession of north-trending ridges and valleys on 

the east side of the overthrust belt (fig. 1) and can 

be traced southeast and east beneath the surface of the 

Green River Basin to outcrops on the north flank of 

the Uinta Mountains in Utah and Colorado, and to outcrops in south-central Wyoming. The Frontier is locally 

more than 610 m (2,000 ft) thick in the overthrust belt 

and thins southeastward to less than 60 m (200 ft) near 

the Uinta Mountains.
STRATIGRAPHY

OVERTHRUST BELT

The outcrops of Frontier in the eastern part of the 

overthrust belt have been displaced eastward at least 

20 km (12 mi) by faulting (De Chadenedes, 1975; Royse 

and others, 1975). The section at Cumberland Gap (fig. 

1) on the easternmost thrust plate was designated 

(Ryer, 1977c) as the reference section for the Frontier 

and its Chalk Creek, Coalville, Alien Hollow, and Oyster Ridge Sandstone Members. The Frontier Formation 

is 596 m (1,955.4 ft) thick (Cobban and Reeside, 1952, 

p. 1924) and this thickness includes the Dry Hollow 

Member at the top (Myers, 1977, p. 274) (fig. 2). All 

of these members are adopted here by the U.S. Geological Survey except for the Dry Hollow, a geographic 

name that is preempted for a Tertiary latite in central 

Utah. This sequence conformably overlies the Lower 

Cretaceous Aspen Shale and is conformably overlain by 

the Upper Cretaceous Hilliard Shale. The sediments in 

the Frontier near Cumberland Gap accumulated during 

a period of about 7 million years (fig. 2) at a rate (compacted) of about 86 m/m.y. (meters per million years).

The Chalk Creek Member of the Frontier is 295 m 

(967.0 ft) thick at Cumberland Gap and about 427 m 

(1,400 ft) thick north of Kemmerer (Myers, 1977, p. 

276). The member is composed mainly of gray beds of 

nonmarine origin and consists of mudstone, siltstone, 

sandstone, and minor coal and bentonite (Cobban and 

Reeside, 1952, p. 1926-1929). Myers (1977, p. 276) proposed that the Chalk Creek had been deposited largely 

in fluvio-deltaic environments and that the member 

originated on an upper delta-plain at outcrops near 

Cumberland Gap and on a lower delta-plain at the outcrops north of Kemmerer. DeChadenedes (1975, fig. 1) 

and Myers (1977, fig. 4) depicted deltas in the Chalk 

Creek in the vicinity of Cumberland Gap and La Barge 

(fig. 1).

The Chalk Creek is largely of Cenomanian and earliest Turonian age and apparently is correlative with 

the Belle Fourche Shale and part of the Greenhorn Formation of northeastern Wyoming (fig. 2). The member 

conformably overlies the Lower Cretaceous Aspen 

Shale and is overlain conformably by the marine Coalville Member which contains molluscan fossils of early 

Turonian age. However, the basal part of the Chalk 

Creek in some areas could be Early Cretaceous, as proposed by Ryer (1977b) for the member in northeastern 

Utah. Strata typical of the lower part of the Chalk 

Creek seemingly intertongue with the upper part of the 

Aspen Shale at outcrops on the east side of the overthrust belt (J. W. M'Gonigle and T. A. Ryer, oral commun., 1980) and in the subsurface along the Moxa arch.

The Coalville Member at Cumberland Gap is 77 m 

(251.0 ft) thick and consists of three sequences of shale

and overlying fine-grained sandstone of marine origin 

(Cobban and Reeside, 1952, p. 1926). However, Myers 

(1977, p. 279) reported that the Coalville at other 

localities is about 30.6-45.7 m (100-150 ft) thick and 

includes coal beds and fossil oysters of fresh-water and 

brackish-water origin. Apparently, the member was deposited in nonmarine and shallow-marine environments 

on deltas and along the interdeltaic coast during a regional marine transgression. DeChadenedes (1975, fig. 

1) and Myers (1977, fig. 4) indicated depocenters for 

the member near La Barge and Cumberland Gap (fig.

1).

The Coalville Member at Cumberland Gap is of early 

Turonian age and is equivalent laterally to an upper 

part of the Greenhorn Formation in northeastern 

Wypming (fig. 2). Molluscan fossils in the member were 

reported by Cobban and Reeside (1952, p. 1926). A supplementary collection of fossils from south of Cumberland Gap includes the following species:

USGS D9302, SWV4 sec. 2, T. 17 N., R. 117 W., 

Uinta County, Wyo., from calcareous concretions in 

sandy shale between sandstone units in the upper part 

of the Coalville Member.

Idonearca sp.

Mytiloides mytiloides (Mantell)

Mytiloides aff. M. opalensis (Boese)

Ostrea sp.

Plicatula sp.

Camptonectes sp.

Pleuriocardia n. sp.

Cyprimerial sp.

Mammites nodosoides (Schlotheim)

Pugnellus fusiformis Meek

Rostellinda cf. R. plicata Dall

Rostellinda dalli (Stanton)

The Alien Hollow Member, which conformably overlies the Coalville Member, is 91.4 m (300.0 ft) thick 

and consists of dark-gray, calcareous and noncalcareous 

shale and bentonite at Cumberland Gap (Cobban and 

Reeside, 1952; Myers, 1977). Myers (1977, p. 280) reported that the Alien Hollow ranges in thickness from 

about 27.4 m (90.0 ft) at outcrops southwest of La 

Barge to about 115.8 m (380.0 ft) at outcrops near Kemmerer. The member was deposited in offshore-marine 

environments and, according to Myers (1977, fig. 8), 

intertongues with the overlying Oyster Ridge Sandstone Member north of Kemmerer (fig. 1).

Outcrops of the Alien Hollow yield mollusks and 

foraminifers of late early Turonian and early middle 

Turonian age (fig. 2, zones 11-12). Rocks of the same 

age in northeastern Wyoming are assigned to the upper 

part of the Greenhorn Formation and the basal part
THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

of the Carlile Shale (fig. 2). Fossils from the Alien Hollow, in addition to the collections of Cobban and 

Reeside (1952, p. 1925), include the following species:

USGS D9301, SWV4 sec. 34, T. 17 N., R. 117 W., 

Uinta County, Wyo., from calcareous siltstone in the 

upper part of the Alien Hollow Member.

Inoceramus sp.

Ostrea sp.

Cyprimeria sp.

Cymbophora emmonsi (Meek)

Corbulid

Collignoniceras woollgari (Mantell)

Placenticeras sp.

Cryptorhytisl utahensis Meek

Tritonideal huerfanensis Stanton

Rostellinda dalli (Stanton)

Rostellinda plicatula Ball

USGS D9304, SWV4 sec. 2, T. 17 N., R. 117 W., 

Uinta County, Wyo., from calcareous concretions in the 

lower part of the Alien Hollow Member.

Phelopteria gastrodes (Meek) 

Mytiloides mytiloides (Mantell) 

Ostrea sp.

Pleuriocardia n. sp. 

Cyprimeria sp.

USGS D9303, SWV4 sec. 2, T. 17 N., R. 117 W., 

Uinta County, Wyo., from calcareous concretions in the 

lower part of the Alien Hollow Member, below D9304.

Mytiloides mytiloides (Mantell) 

Ostrea sp.

Pleuriocardia n. sp. 

Cymbophora emmonsi (Meek)

USGS D9307, NEV4 sec. 31, T. 19 N., R. 116 W., 

Lincoln County, Wyo., from shaly sandstone and sandy 

shale at the base of the Alien Hollow Member.

Nemodon sp.

Mytiloides mytiloides (Mantell) 

Crassostrea soleniscus (Meek) 

Anomia subquadrata Stanton 

Brachidontes multilinigera (Meek) 

Cymbophora emmonsi (Meek) 

Pugnellus fusifomnis Meek

The Oyster Ridge Sandstone Member, which grades 

into the underlying shale of the Alien Hollow, consists 

of interstratified shale, siltstone and sandstone, and 

minor conglomerate and bentonite. These rocks crop 

out as a prominent north-trending hogback near the 

western edge of the Green River Basin and are 40.8 

m (134.0 ft) thick at Cumberland Gap. Myers (1977, 

p. 282) indicated that the sandstone of the Oyster Ridge

ranges in thickness from 15.2 m to more than 61.0 m 

(50.0 to more than 200.0 ft) in this region. The member 

commonly is composed of one or more upward-coarsening, very fine grained to medium-grained sandstone 

units overlain by shale and channel-filling sandstone, 

which were deposited mainly in progradational deltafront, delta-plain, shoreface, and foreshore environments (Myers, 1977). These strata, according to Myers 

(1977, fig. 18), were derived largely from a delta in 

the vicinity of Kemmerer and from another west of La 

Barge.

Some of the marine beds in the Oyster Ridge Sandstone Member contain molluscan fossils of middle Turonian age. As a consequence, this member can be correlated with part of the Pool Creek Member of the Carlile 

Shale in northeastern Wyoming (fig. 2). Fossils in the 

Oyster Ridge Sandstone Member reported by Cobban 

and Reeside (1952, p. 1924), include Ostrea soleniscus 

Meek at the top of the member and Collignoniceras 

woollgari (Mantell) and Ostrea aff. 0. anomioides Meek 

near the middle of the member.

The Dry Hollow Member, at the top of the Frontier, 

overlies the Oyster Ridge Sandstone and is composed 

of mudstone, siltstone, sandstone, conglomerate and 

coal. At Cumberland Gap, the Dry Hollow is 92.0 m 

(303.0 ft) thick (Cobban and Reeside, 1952, p. 1923- 

1924). Most of the member was deposited in delta-front 

and delta-plain environments, but the uppermost sandstone and shale are of shallow-water marine origin. This 

marine unit, which generally overlies a coal bed, is 

about 13.0 m (43.0 ft) thick at Cumberland Gap and 

as much as 36.6 m (120.0 ft) thick elsewhere in the 

region (Myers, 1977, p. 285). McDonald (1976, fig. 20) 

indicated that the "first Frontier sandstone," at the top 

of the formation in the La Barge area, was derived 

from a delta near the Frontier outcrops west of La 

Barge. Myers (1977, fig. 18) proposed that the two main 

sources of the shallow-marine and deltaic sediments in 

the Oyster Ridge and Dry Hollow Members were at 

Kemmerer and near outcrops of the Frontier west of 

La Barge.

At Cumberland Gap, the marine Oyster Ridge Sandstone Member of middle Turonian age is overlain by 

units of, from oldest to youngest, brackish-water shale, 

nonmarine strata, and marine beds (Cobban and 

Reeside, 1952, p. 1923-1924). Myers (1977, figs. 2 and 

19) assigned the units to the Dry Hollow Member and 

indicated that the Oyster Ridge Sandstone and Dry 

Hollow Members in the region are conformable. However, Hale (1962, p. 219) had proposed that the contact 

of the members is a disconformity, which represents 

some of middle and late Turonian time. Beds in the 

upper part of the Dry Hollow, about 30 m (98.4 ft) 

below the top, contain mixed assemblages of marine
STRATIGRAPHY

and nonmarine dinoflagellates (J. W. M'Gonigle and D. 

J. Nichols, written commun., 1981). The lowest of these 

beds is of Turonian age; a bed about 4 m (13 ft) higher 

in the sequence is of Coniacian age. Furthermore, early 

Coniacian mollusks of marine origin had been collected 

by Cobban and Reeside (1952, p. 1923) from the top 

of the Dry Hollow. Presumably, most of the member 

is Turonian but about the upper one-third of the member is early Coniacian. The Oyster Ridge Sandstone 

Member and about the lower two-thirds of the Dry Hollow are of Turonian age and, according to Myers (1977, 

p. 275), seem to be gradational and conformable. However, if the strata in the upper part of the Oyster Ridge 

Sandstone Member and in the lower part of the Dry 

Hollow are of middle Turonian age and are overlain 

by late Turonian to early Coniacian beds of the Dry 

Hollow, the disconformity and hiatus suggested by Hale 

(1962) could be within the lower two-thirds of the Dry 

Hollow. J. W. M'Gonigle (written commun., 1981) 

mapped a chert-bearing conglomeratic unit in the lower 

part of the Dry Hollow south of Cumberland Gap.

The Dry Hollow Member corresponds in age to part 

of the Carlile Shale of northeastern Wyoming (fig. 2). 

Collections of molluscan fossils from marine strata in 

the member at several localities are as follows:

USGS D11053, SEV4 sec. 13, T. 16 N., R. 118 W., 

Uinta County, Wyo., from crossbedded, fine-grained 

sandstone at the top of the Dry Hollow Member.

Inoceramus erectus Meek

USGS D11056, SWV4 sec. 1, T. 26 N., R. 116 W., 

Lincoln County, Wyo., from sandstone near the top of 

the Dry Hollow Member.

Lopha sannionis (White) 

Crassostrea sp. 

Gyrodes depressa Meek

USGS D11058, NWV4 sec. 12, T. 22 N., R. 116 W., 

Lincoln County, Wyo., from sandstone near the top of 

the Dry Hollow Member.

Inoceramus erectus Meek 

Crassostrea sp. 

Pleuriocardia curium (Meek)

Marine sandstone of the Dry Hollow Member, in the 

uppermost Frontier, is overlain comformably by marine 

shale of the Hilliard. Cobban and Reeside (1952) reported that a sandstone about 60 m (about 200 ft) above 

the base of the Hilliard near Cumberland Gap contains 

the early Coniacian fossil Inoceramus erectus Meek. 

These marine rocks of the uppermost Frontier and 

basal Hilliard are equivalent in age to part of the Sage 

Breaks Member of the Carlile Shale in northeastern 

Wyoming (fig. 2).

UINTA MOUNTAINS

The outcropping Frontier at the southern edge of the 

Green River Basin, on the north flank of the Uinta 

Mountains (fig. 1), is generally about 60 m (200 ft) thick 

and is composed of marine and nonmarine shale, siltstone, sandstone, and minor coal and bentonite (C. M. 

Molenaar, written commun., 1980). Molenaar (written 

commun., 1980) suggested that these rocks were deposited in deltaic and shallow-marine environments. Molluscan fossils in the formation are of middle and late 

Turonian age (fig. 2, zones 15-18). Reeside (1955) recognized a major hiatus at the contact of the Frontier and 

the underlying Lower Cretaceous Mowry Shale near 

the Uinta Mountains in northwestern Colorado. The 

thickness and age of the Frontier indicate that the disconformable basal contact of the formation on the southern margin of the Green River Basin represents strata 

of Cenomanian and early Turonian age in the lower part 

of the Frontier at Cumberland Gap. Marine sandstone 

units at the top of the Frontier near the Uinta Mountains intertongue with the overlying marine shale (the 

Hilliard Shale in Wyoming and the Mancos Shale in Colorado). The outcropping Frontier in these areas is about 

the same age as the middle of the Carlile Shale in northeastern Wyoming (fig. 2).

Selected collections of fossils from the Frontier and 

the overlying shale near the Uinta Mountains are as 

follows:

USGS D9282, NWV4 sec. 30, T. 10 N., R. 100 W., 

Moffat County, Colo., from calcareous concretions in 

the Mancos Shale, about 4 m (13 ft) above the Frontier 

Formation.

Inoceramus perplexus Whitfield 

Scaphites whitfieldi Cobban

USGS D9284, SWV4 sec. 30, T. 3 N., R. 22 E., 

Daggett County, Utah, from concretions in the lower 

part of the uppermost sandstone in the Frontier Formation.

Inoceramus perplexus Whitfield

Baculites yokoyamai Tokunaga and Shimizu

Prionocyclus novimexicanus (Marcou)

USGS D9288, NEV4 sec. 34, T. 3 N., R. 21 E., 

Daggett County, Utah, from siltstone about 12 m (39 

ft) below the top of the Frontier Formation.

Inoceramus dimidius White 

Cymbophora sp.

USGS D9281, NWV4 sec. 30, T. 10 N., R. 100 W., 

Moffat County, Colo., from siltstone 12 m (39 ft) below 

the top of the Frontier Formation.
8 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

Inoceramus aff. I. cuvieri Sowerby 

Corbula sp. 

Gyrodes sp. 

Rosellindal sp.

USGS D9280 NWVi sec. 30, T. 10 N., R. 100 W., 

Moffat County, Colo., from interlaminated shale and 

siltstone in the lowermost 3 m (10 ft) of the Frontier 

Formation.

Inoceramus sp. 

Prionocyclus hyatti (Stanton)

MOXA ARCH

In the subsurface along the Moxa arch (fig. 1), the 

Frontier Formation consists mainly of marine and nonmarine shale, siltstone, and sandstone. DeChadenedes 

(1975) determined from borehole logs that the formation 

ranges in thickness from less than 150 m (500 ft) near 

the Wyoming-Utah State line, to more than 335 m 

(1,100 ft) in northeastern Lincoln County, Wyo. The 

disconformity at the base of the Frontier at outcrops 

on the northern flank of the Uinta Mountains, extends 

northward and divides the formation in the Green River 

Basin into two stratigraphic units. On the Moxa arch, 

the lower unit seems to be largely marine siltstone and 

shale, and it thickens irregularly northward from less 

than 10 m (32.8 ft) at the Wyoming-Utah State line 

to at least 170 m (557.8 ft) near La Barge. The thinning 

to the south was caused mostly by regional mid-Cretaceous tectonism and erosion. Truncated strata at the 

top of this unit presumably are successively younger 

from south to north; from an early Cenomanian age 

near the Uinta Mountains to a probable early Turonian 

age near La Barge. Some of the thickening apparently 

is caused by intertonguing of the lowermost Frontier 

and uppermost Mowry.

The part of the Frontier above the disconformity, between the Uinta Mountains and La Barge, is mostly 

sandstone that was informally named the "second Frontier sandstone" by DeChadenedes (1975) and Myers 

(1977). Wach (1977, figs. 5 and 9) indicated that the 

thickness of his "2nd Frontier sandstone," which includes units of sandstone, siltstone, and shale, increases 

by onlap northward along the arch. He suggested that 

some of these rocks were derived from deltas near 

Kemmerer and La Barge. Cored beds in the "second 

Frontier sandstone" of McDonald (1973), DeChadenedes 

(1975), and Myers (1977) reportedly were deposited in 

deltaic and shallow-marine environments. Hawkins 

(1980) proposed that some of these strata accumulated 

in barrier island, back-barrier, and coastal-plain environments. According to Winn and Smithwick (1980), 

sandstone units in the upper part of the Frontier, along

the axis of the arch, were deposited as offshore storm 

bars and on a wave-dominated delta as distributarymouth bars, fluvial point-bars and related sediments. 

A younger sandstone unit, which is the uppermost 

Frontier in the vicinity of La Barge, was informally 

named the "first Frontier sandstone" (DeChadenedes, 

1975; McDonald, 1976; Myers, 1977).

Cores of the upper part of the Frontier and the lower 

part of the Billiard Shale, from the east flank of the 

Moxa arch in northwestern Sweetwater County, Wyo., 

contain molluscan fossils of middle Turonian to early 

Coniacian age. Strata of the same age in northeastern 

Wyoming are assigned to the Carlile Shale (fig. 2). The 

species in the cores are named as follows:

USGS D11363, SEVi sec. 5, T. 24 N., R. Ill W., 

Sweetwater County, Wyo. In borehole 2 (table 1) at 

a depth of 2,836.9-2,837.2 m (9,307.0-9,308.0 ft) from 

shale in the lower part of the Billiard Shale, about 10.4 

m (34 ft) above the top of the Frontier Formation.

Scaphites uintensis Cobban? 

Inoceramus sp.

Remarks: The age of S. uintensis is early Coniacian 

(fig. 2, probably zone 22).

USGS D11207, SEV4 sec. 9, T. 24 N., R. Ill W., 

Sweetwater County, Wyo. In borehole 4 (table 1) at 

a depth of 2,917.6 m (9,571.75 ft) from siltstone in the 

Frontier, about 6.1m (20 ft) below the top of the formation.

Lopha sannionis (White) 

Pleuriocardia pauperculum (Meek)

Remarks: The range of L. sannionis is late Turonian 

to middle Coniacian (fig. 2, zones 18-23).

NEx/4 sec. 3, T. 24 N., R. Ill W., Sweetwater 

County, Wyo. In borehole 3 (table 1) at depths of 

2,948.1 m (9,672 ft) and 2,949.8 m (9,677.5 ft), from 

sandstone in the Frontier, about 63.1-64.8 m (207-212.5 

ft) below the top of the formation.

Ostrea cf. 0. anomioides Meek

Remarks: The range of this fossil probably is middle 

Turonian to early Coniacian (fig. 2, zones 13-22).

STRATIGRAPHIC INTERPRETATIONS

Strata in the Frontier have been traced, with 

borehole logs and cores (table 1), from outcrops on the 

north side of the Uinta Mountains through the Green 

River Basin and across thrust faults to outcrops in the 

overthrust belt. Stratigraphic cross sections indicate 

striking increases in the thickness of the formation from 

east to west, along the north flank of the Uinta Mountains (fig. 3) and from the Moxa arch to the overthrust
STRATIGRAPHY 

TABLE 1. Location of selected boreholes and outcrop sections in the Green River Basin, Wyoming, Utah, and Colorado (fig. 1)

Borehole 

No. Operator

1 

2 

3 

4 

5

6 

7 

8 

9 

10

11 

12 

13 

14 

15

16 

17 

18 

19

20

21 

22 

23 

24 

25 

26

Southland Royalty Co. 

C & K Petroleum, Inc. 

j _. _

Davis Oil Co. & SouthlandRoyalty Co. 

K. D. Luff, Inc. 

Carter Oil Co. 

K. D. Luff, Inc. 

Amoco Production Co. j ,- QO 

Marathon Oil Co. 

Mountain Fuel Supply Co. 

Max Pray Co. "~ - - 

Brinkerhoff Drilling Co. 

Sundance Oil Co. 

Sun Oil Co. 

Oil Development Co. 

of Texas. 

Pure Oil Co. 

Lone Star Producing Co. 

 Ar^

PIT ff* r^-i 1 /"V» _

Location

Lease name

Stead Canyon East 1 

Lincoln Road 1 

Storm Shelter 1-A 

Amoco 2-27 

Dry Muddy Creek 1 

Hams Fork 2 

P-.-H-F-F 1

Albert Creek 1 

Church Buttes 16 

Big Piney Federal 1 

Black's Fork 1 

Leavitt Creek 1 

Butcher Knife Springs 1 

Currant Creek Federal 1 

TIC A 1 9Q __

/"»T»/"» 1 T? J 1

31-19 (11-102).

sec

18 

5 

3 

9 

29

27 

34 

23 

16 

25

18 

30 

22 

36 

14

19

7 

24 

34 

8

21 

20 

3 

19 

29 

19

. T.

26 

24 

24 

24 

23

23 

22 

22 

20 

20

20 

19 

19 

18 

17

15 

15 

14 

14 

14

14 

14 

12 

12 

3 

11

N. 

N. 

N. 

N. 

N.

N.

N. 

N. 

N. 

N.

N. 

N. 

N. 

N. 

N.

N. 

N. 

N. 

N. 

N.

N. 

N. 

N. 

N. 

N. 

N.

R,

111 

111 

111 

111 

111

103 

112 

103 

114 

113

112 

115 

112 

117 

112

112 

110 

119 

116 

114

113 

108 

107 

105 

25 

102

w. 

w. 

w. 

w. 

w.

w. 

w. 

w. 

w. 

w.

w. 

w. 

w. 

w. 

w.

w. 

w. 

w. 

w. 

w.

w. 

w. 

w. 

w.

E.

w.

County

Lincoln 

Sweetwater 

 do 

 " d o 

Sweetwater 

Lincoln 

 do 

j

 do 

Uinta 

Sweetwater 

Uinta 

Sweetwater 

Uinta 

 do 

 do 

Sweetwater 

DaggettMoffat 

State

Wyoming. 

Do. 

Do. 

Do. 

Do.

Do. 

Do. 

Do. 

Do. 

Do.

Do. 

Do. 

Do. 

Do. 

Do.

Do. 

Do. 

Do. 

Do. 

Do.

Do. 

Do. 

Do. 

Do. 

Utah. 

Colorado.

Outcrop section

Cumberland Gap 

Chokecherry Draw 

Vermillion

& 32 19 N. 116 W.

3 N. 21 E.

10 N. 101 W.

Lincoln 

Daggett 

Moffat 

Wyoming.

Utah.

Colorado.

belt (fig. 4), and from south to north along the axis 

of the Moxa arch (fig. 5). Near the Uinta Mountains, 

the thickness increases westward from 43.0 m (141.0 

ft) at Vermillion Creek in Colorado to about 570.0 m 

(1,870.0 ft) at borehole 18 in Wyoming (fig. 3). Similarly, the thickness of the Frontier increases southwestward from about 135.6 m (445.0 ft) at borehole 11 on 

the Moxa arch to 596.0 m (1,955.4 ft) at Cumberland 

Gap (fig. 4). Along the Moxa arch, the thickness of the 

formation increases irregularly northward from about 

19.8 m (65.0 ft) at borehole 16 to about 224.3 m (736.0 

ft) at borehole 1 (fig. 5).

The Frontier in borehole 18 and at Cumberland Gap 

is part of a thrust plate that was displaced eastward 

at least 20 km (12 mi; DeChadenedes, 1975; Royse and 

others, 1975) and thrust over strata in the Green River 

Basin. This foreshortening of the region caused much

of the apparent increase in the rate of westerly thickening for the Frontier (figs. 3 and 4). Furthermore, the 

outcropping Frontier in the thrust plate is locally as 

much as 4.5 km (about 14,800 ft) higher than the formation in the subsurface beneath the plate. Consequently, 

the Frontier is not continuous between the Moxa arch 

and the overthrust belt as shown on figures 3 and 4.

Hale (1962, fig. 2) depicted the thickening of the 

Frontier and the truncation of progressively younger 

strata in the lower part of the formation, from the Moxa 

arch westward through the eastern part of the overthrust belt. Thomaidis (1973, pi. 1) depicted northward 

thickening of the lower part of the Frontier along the 

Moxa arch. Northward thickening in most of the Cretaceous strata on the arch was reported by Stearns and 

others (1975, p. 11 and pi. 1).

The disconformity at the base of the Frontier on the
/

WE

1

Milliard 

Shale 

(part)

Frontier Formation

Aspen 

Shale 

(part)

\ -ST8 19 2

Foreshortened ^ 

_ _ by thrusting

l^^^SBS

; !<:: ;..&-??* r-: r-T- ~:~-

",' ' ''»; is* ''.'V"'"''"1 ';:'?-~ "~ "-/

P^

___.____..

£HI-Z-Z-Z-Z-ZH:rZ;Z~2^-_-_

tr.-~j~-r-r.^f

pF

?

0 2

*1 2 km*:i~z-i-i-: _____ s^S£fe£

w^

1 1 

*1 1 km*

HrirZ-Z

~_ -T _ _ zriririr"-.

:': ''.-" .' Mostly

Z-Z-Z-Z-I Mostlj

 15 Fossil 

ind 

fos: 

zon

? Forma 

wh

? Lithol* 

wh<

617 2

-i~i-i-i-i~i-i~:

:_______.^Z-Z-Z-ZT^S

 ^^_-_3_-_r;-

-I-I-I^I-I-I-Z-I-^: 

_ _ 

r*. .- 

_ _ _ - R^-I^-I-I^^

z-z-z-z-z-z-z-z-z-z1:

EXPLANATION

f marine and 

marine sandstone . . Vermillu

i marine shale Creek 

siltstone 1 7

collection Number | 

cates occurrence of a i 

;il that represents fossil l 

e listed on, figure 2

tion boundary Queried 3 re extent unknown

agic boundary Queried ;re extent unknown

2 2

* 19 km * 

z. - _______ ^i-z-^i-i-i^:

3 2

*M5 km-*

:-Z-Z-Z-Z-Z-Z4 2 Z-Z-C-Z-Z-E iili^fpjrnfifl :z=i^-i~i~:

-^ Unconformity Queried 

where extent unknown

3n Outcrop section 

BoreholeArbitrary stratigraphic nomenclature boundary

A'

EAST 

Vermillion 

5 26 Creek

. *>n Irmji ~ - "" !.«. ^

L ... .-* *" '^ .-* >*,'.* '*''..**:*» :'!<:*,!": ' ^W* i "* ''*" l-:-usag9£^v^; jiiiii^^li:^^

rij7-~i.~i-~zjrLriri; 'i_~i.~Lr^rijri,r^rir^-~

METERS FEET 

0--0

100 -~ 

-500

Mancos 

Shale 

(part)

Frqntler 

Formatipn

Mowry 

Shale

LO THE MID-CRETACEOUS FRONTIER FORMATION NEAR MOXA ARCH,WYOMING

FIGURE 3. Stratigraphic diagram of the Frontier Formation and Mowry Shale at boreholes and outcrop in northwestern Colorado, northeastern Utah, and southwestern Wyoming along line of section A-A'. Location of boreholes and outcrop shown on figure 1 and table 

1. Datum is bed in Hilliard Shale and laterally equivalent rocks. Thrust faulting has displaced the mid-Cretaceous strata in borehole 18 eastward at least 20 km, to depicted present location about 26 km west of borehole 19.
STRATIGRAPHY 11

B

SOUTHWEST 

Cumberland 

Gap 14

«r 1 n l/-m ». ^

Milliard 

Shale 

(part)

' '**"'" "' Frontier 1 Formation ;f !, t 

Dry 

Hollow 

Member

Oyster ^ 

Ridge 

Sandstone 

Member

Alien 

Hollow 

Member

CoalvMIe - Member

Chalk 

Creek 

Member

Aspen 

Shale 

(part)

Foreshortened by 

.thrusting. _

B'

NORTHEAST

12 9 10 11

^y^s^m^-

 .,"'_ ?*,$* " ** r'.JV^

*"*** :. /

tJz-^l

II

I

&<sst%-f*~ '^ipfi - 

t*Xfi& -/

W

./t _ 

~^^^F'

 _ _ _ ^s

l^ies^^o^^

Ii^"? Sscontf Fwji

>jjal,irtforn ''.-f ".usage*

*. _ i - i» ,z _ ^ . .-,

,;$t^^^£r-_- rtr^./

^^f5i-_-_-_-^.- ^

--T-X

^ 1 

EXPLANATION

;;:i.: ;;;;;;;; Mostly nonmarine

::::uu;:::: ; Strata

' '- .' .' ' " ." .' ' Most 

 '." - ' " ' sa

_i_~_~_ - Most

 11 Fossi 

Nu 

oc 

fo; 

th 

on

ndstone

ly marine shale 

d siltstone ^ __ t-^^^

\ collection 

mber indicates Cumberland 

currence of a p_ 

;sil that represents p 

e fossil zone listed 1 2 

figure 2 (

I

I

km

~^r

' \tier 

&i\

S&,

1

=^7:"

TERS

0 -i

00 -

Forr 

Q 

u

Lith 

Q 

u

Unc 

w

Out 

Bon

Arb

n

Milliard 

Shale 

(part)

Frontier 

Formation

Mowry 

Shale

FEET 

- 0

- 500

nation boundary 

ueried where extent 

nknown

ologic boundary 

ueried where extent 

nknown

onformity Queried 

/here extent unknown

crop section 

shole

trary stratigraphic 

omenclature boundary

FIGURE 4. Stratigraphic diagram of the Frontier Formation and Mowry Shale at boreholes and outcrop 

on the Moxa arch and the overthrust belt, southwestern Wyoming along line of section B-B'. Location 

of boreholes and outcrop shown on figure 1 and table 1. Thrust faulting has displaced the mid-Cretaceous 

strata at Cumberland Gap eastward at least 20 km, to the present location about 10 km north of and 

about 4.5 km higher than the mid-Cretaceous rocks in borehole 14.
12 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

c

SOUTH 

16 15

Hilliard 

Shale 

(part)

Frontier 

Formation

Mowry 

Shale

13 11

C'

NORTH 

1

-21 km- -18 km- -12 km- -14 km

:.______.. rDatum. _ _ -. ._ ____

EXPLANATION

-13 km- -16 km- -17 km13-22

Mostly marine and 

nonmarine sandstone

Mostly marine shale 

and siltstone

Fossil collection Numbers 

indicate occurrence of a 

fossil that represents fossil 

zones listed on figure 2

 ? Formation boundary Queried 

where extent unknown

 ? Lithologic boundary Queried 

where extent unknown

 ~ ? ~ Unconformity Queried where 

extent known

5 Borehole

100-

Frontier 

Formation

Mowry 

Shale

FIGURE 5. Stratigraphic diagram of the Frontier Formation and Mowry Shale at boreholes on the Moxa arch, southwestern Wyoming 

along line of section C-C'. Location of boreholes shown on figure 1 and table 1. Datum is bed in Hilliard Shale.

north flank of the Uinta Mountains extends northward 

and is within the formation in the Green River Basin 

(fig. 5). Below the disconformity, the lower part of the 

Frontier in the southern part of the basin is probably 

of early Cenomanian age and locally is less than 10 m 

(32.8 ft) thick. On the Moxa arch near La Barge, the 

lower part of the formation is of Cenomanian and probably early Turonian age, and is at least 170 m (557.8 

ft) thick. Laterally equivalent strata at Cumberland 

Gap in the overthrust belt may be of Cenomanian to 

middle Turonian age and are at least 500 m (1,640.5 

ft) thick (fig. 4). Evidently, the truncation of the lower 

part of the Frontier along the disconformity decreases 

northward along the Moxa arch and westward between 

the arch and the overthrust belt, reflecting a mid-Cretaceous uplift in the vicinity of the Uinta Mountains 

(Weimer, 1962, p. 129).

The upper part of the Frontier, above the disconformity, thickens irregularly northward and westward 

in the Green River Basin, mainly by the accretion of 

sandstone units at the top of the formation (Wach, 

1977). This part ranges in thickness from less than 60 

m (196.9 ft) at the south edge of the basin, to at least 

80 m (262.5 ft) near La Barge, and probably to about 

90 m (295.3 ft) at Cumberland Gap.

Fossiliferous rocks of middle and late Turonian age 

(fig. 2, zones 15-18), which overlie the disconformity 

near the Uinta Mountains, can be followed in the subsurface northward to boreholes 2, 3, and 4 near the 

crest of the Moxa arch (fig. 1). Cores of some of these 

strata and overlying beds, from the upper part of the 

Frontier and the lower part of the Hilliard Shale in 

boreholes 2, 3, and 4, contain mollusks of middle Turonian to early Coniacian age (fig. 2, zones 13-22). The
CORE-SAMPLE DESCRIPTIONS 13

fossiliferous rocks near the Uinta Mountains and on the 

Moxa arch are about the same age and are of deltaic 

and marine origin. They are probably also laterally 

equivalent to part of the Dry Hollow Member of the 

Frontier, which is of Turonian to early Coniacian age 

and of nonmarine and shallow-marine origin, at Cumberland Gap in the overthrust belt (fig. 2).

Sandstone units at the top of the Frontier, near the 

Uinta Mountains and in boreholes on the Moxa arch 

south of La Barge, were assigned to the "second Frontier sandstone" and correlated with strata of the Chalk 

Creek and Coalville Members in the overthrust belt by 

DeChadenedes (1975, p. 152-156) and Myers (1977, fig. 

4). Similarly, Winn and Smithwick (1980, p. 139-140) 

indicated that the Frontier on the arch is of Cenomanian age and is laterally equivalent to the Chalk Creek 

and Coalville. The Chalk Creek is mainly of Cenomanian age and the Coalville is of early Turonian age at 

Cumberland Gap (fig. 2). However, the "second Frontier sandstone" (Myers, 1977, fig. 4) in boreholes 2, 3, 

and 4 on the Moxa arch can be traced southwestward 

in the subsurface to outcrops of the Dry Hollow Member (Turonian through early Coniacian age) at Cumberland Gap. Although the boreholes between the Moxa 

arch and the overthrust belt are widely spaced and the 

Frontier at Cumberland Gap has been displaced eastward, the "second Frontier sandstone" apparently is an 

eastern extension of part of the Dry Hollow Member. 

Furthermore, the fossils in the "second Frontier sandstone" (Myers, 1977, fig. 4) in boreholes 2, 3, and 4 

are middle Turonian to early Coniacian in age and correspond in age to fossils in the Oyster Ridge Sandstone 

and Dry Hollow Members at Cumberland Gap. From 

this evidence, the shallow-marine and deltaic strata of 

the "second Frontier sandstone" (Myers, 1977, fig. 4) 

on the Moxa arch are interpreted to be seaward facies 

of nonmarine beds in the Dry Hollow Member.

Deltas that supplied the sediments in the lower part 

of the Frontier, in the Chalk Creek and overlying Coalville Members and in laterally equivalent strata, may 

have been located near Cumberland Gap and west of 

La Barge on the east side of the overthrust belt (DeChadenedes, 1975, fig. 1; Myers, 1977, fig. 4). 

Moreover, an upper part of the Chalk Creek Member, 

the Coalville Member, and a lower part of the overlying 

Alien Hollow Member were deposited during a regional 

marine transgression in the Cenomanian and early 

Turonian (Merewether, 1982). The upper part of the 

Alien Hollow and the overlying Oyster Ridge Sandstone 

and Dry Hollow Members in the overthrust belt and 

laterally equivalent beds in the western part of the 

Green River Basin were derived mainly from deltas in 

the vicinity of Kemmerer and La Barge (Wach, 1977, 

fig. 9; Myers, 1977, fig. 18). These rocks, excepting an

upper part of the Dry Hollow, were deposited during 

a regional marine regression, mainly in the middle 

Turonian (Merewether, 1982). The hiatus at the base 

of the Frontier on the northern flank of the Uinta 

Mountains and within the formation along the Moxa 

arch, represents uplift and truncation of the Mo wry 

near the Uinta Mountains and the lower part of the 

Frontier in the Green River Basin during early to middle Turonian time. Subsequently, shallow-marine and 

deltaic beds in the upper part of the Frontier on the 

arch and in the Frontier along the Uinta Mountains 

were deposited over the erosional surface during a middle Turonian transgression, prograded seaward in a late 

middle and earliest late Turonian regression, and accumulated thereafter during a late Turonian transgression. This latter transgression, which was regional and 

generally continued into the Coniacian, is represented 

by an upper part of the Dry Hollow Member of the 

Frontier and a lower part of the overlying Hilliard 

Shale.

CORE-SAMPLE DESCRIPTIONS

Cores of beds in the Frontier and basal Hilliard from 

four boreholes along the Moxa arch, two boreholes on 

the north flank of the Rock Springs uplift, and one 

borehole in the overthrust belt (fig. 1, table 2), were 

examined and sampled for analyses. The cored strata 

in the Frontier are in the middle and upper parts of 

the formation and were deposited in offshore, 

shoreface, delta-front, and delta-plain environments 

(table 2) during Turonian time. The cored beds in the 

Frontier on the Moxa arch are within the "second Frontier sandstone" of DeChadenedes (1975) and Myers 

(1977) and probably grade westward into parts of the 

Dry Hollow Member of the Frontier at Cumberland 

Gap.

BOREHOLE 1

Core sample S-l was obtained from a cross-stratified, 

fine-grained sandstone in the Frontier at a depth of 

2,600.5 m (8,531.5 ft) in borehole 1 (figs. 1 and 6, table 

2). The sandstone is of marine origin and was deposited 

in a shoreface or delta-front environment. Sample S-l 

is composed of about 82 percent sand, 11 percent silt, 

and 7 percent clay by weight (table 3). The sand has 

a modal grain size of 160 micrometers (|xm) and a 

maximum grain size of 340 |xm (table 4). Porosity and 

permeability of the sample are about 10.4 percent and 

0.97 millidarcies (mD), respectively. In thin section (fig. 

7A), the rock has clay-filled, intergranular microporosity, minor intergranular mesoporosity, and consists of 

detrital grains of quartz, chert, rock fragments, and 

feldspars that are cemented by quartz, calcite, and clay
14 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

TABLE 2. Location, depth, lithology, and depositional environment of core samples from the Frontier Formation 

and the lower part of the Hilliard Shale in southwestern Wyoming

Core 

sample 

No.

S-l 

L4 

L4-1 

L4-2 

L2-1

L2-1A 

L2-2 

L2-2A 

L2-3 

L2-4

L2-5 

A2-1 

A-1A 

A-l 

C-l

C-1A 

C-2 

C-2A 

F-l

^ ' 

^="v,Gamm

Borehole 

No. 

(fig. 1)

1 

2 

2 

2 

3

3 

3 

3 

3 

3

3 

6 

8 

8 

11

11 

11 

11 

18

Boreh 

Southland 1 

Stead Can^ 

Sec. 1 8, T. 26

FEET 

8500 -,

 ->

bi .

<y curve b 7 .

b~ ?: : .

^-_ __

8600- ~^r

Meters

2,600.5 

2,837.0 

2,890.1 

2,898.8 

2,914.7

2,916.7 

2,922.8 

2,928.3 

2,933.8 

2,939.9

2,948.1 

2,408.6 

2,130.9 

2,133.1 

3,383.9

3,386.7 

3,394.5 

3,405.8 

1,609.4

Depth

Feet

8,531.5 

9,307.5 

9,481.5 

9,510.3 

9,562.3

9,569.0 

9,589.0 

9,607.0 

9,625.0 

9,645.0

9,672.0 

7,902.0 

6,991.0 

6,998.0 

11,101.7

11,110.8 

11,136.3 

11,173.5 

5,280.0

ole 1 

loyalty Co. 

fon East 1 

N., R. 1 1 1 W.

. .--f-S-1

: 55M 

^ CFGPD

^s:^z

Sonic \ 

curve "^ 

~~^s?

Depositional 

Lithology environment

Sandstone S

Shale C 

Sandstone D 

 do 

 do E

Shale ----- 

Sandstone 

Shale - 

Sandstone D 

 do 

 do 

Shale C 

 do D 

Sandstone 

 do D

Shale - 

Sandstone E

O11CL-LC

 do C

Depositional 

environment

 r-

£

Shoreface?

: - 

METERS FEET 

0--0

-50

horeface? 

ffshore marine, 

elta-front? 

Do. 

telta-plain.

Do. 

Do. 

Do. 

elta-front. 

Do.

Do; 

ff shore-marine? 

elta-front? 

Do. 

elta-front or 

shoreface. 

Do. 

elta-plain. 

Do. 

ffshore marine.

EXPLANATION

.-.. .-..: . .: Sandstone

No core

Clayey 

-1- - Calcareous 

b Burrows 

bi Bioturbated 

S 1 Sample locality 

55 M Location and amount of 

CFGPD natural gas 55,000 

cubic feet (1,557.6 cubic 

meters) per day

20-

30 -L 100 

FIGURE 6. Logs of cored and sampled strata in the Frontier Formation in borehole 1 on the Moxa arch, Wyoming (fig. 1).

(table 5). Choquette and Pray (1970) defined microporosity and mesoporosity as size terms for pores having diameters of less than 1/16 mm and 1/16-4 mm, 

respectively. In Folk's (1974) classification, the sandstone sample is a litharenite. Analysis of the sample 

by X-ray diffraction indicates the presence of mixedlayer illite-smectite, mica-illite, kaolin, and 7-A 

(angstrom) trioctahedral clay (table 6). The sample also 

contains 0.03 percent organic carbon. Analysis by 

atomic absorption, spectrophotometry (rapid rock-analysis; Shapiro, 1975) indicates smaller amounts of magnesium oxide (MgO) and titanium oxide (Ti02) in sample
CORE-SAMPLE DESCRIPTIONS 15

TABLE 3. Grain sizes of core samples of sandstone from the Frontier 

Formation of southwestern Wyoming

Core sample 

No.

Sand 

(>62 mm)

Weight-percentage composition (mm = micrometers) 

Silt Clay Total

(2-62 mm) «2 mm)

S-l

L4-1

L4-2

L2-1

L2-2

L2-3

L2-4

L2-5

A-l

C-l

C-2

82.3

73.0

90.8

82.7

79.7

85.1

76.9

74.9

84.7

78.9

77.4

11.4

9.7

5.3

11.1

12.8

7.9

10.5

12.3

10.8

13.3

15.6

6.8

17.8

4.8

6.2

7.7

8.2

13.0

12.5

4.7

8.2

6.9

100.5

100.5

100.9

100.0

100.2

101.2

100.4

99.7

100.2

100.4

99.9

S-l than in most of the other samples (table 7). Analysis by emission spectrography shows smaller amounts 

of Ti02 , phosphate (P205), cobalt (Co), copper (Cu), 

chromium (Cr), nickel (Ni), scandium (Sc), and zinc 

(Zn), and a larger amount of manganese (Mn) in sample 

S-l than in most of the samples (table 8). Sample S-l 

was obtained from a depth of 2,600.5 m (8,531.5 ft) in 

the upper part of a sandstone that contains natural gas 

at depths of 2,600.0-2,612.2 m (8,530.0-8,570.0 ft.) (fig. 

6). The reported initial potential flow of gas from this 

unit is 1,557.6 m3 (cubic meters; 55,000 ft3) per day. 

In the gas-bearing beds, the porosity ranges from 5.5 

to 20.5 percent and the permeability ranges from 0.03 

to3.1mD.

BOREHOLE 2

Core samples L4-1 and L4-2 were obtained from 

crossbedded, fine-grained sandstone in the Frontier 

from depths of 2,890.1 m (9,481.5 ft) and 2,898.8 m 

(9,510.3 ft), respectively, in borehole 2 (figs. 1 and 8, 

table 2). The sandstone beds apparently were deposited 

in delta-front environments. Sample L4-1 contains 

about 18 percent clay, 10 percent silt, and 73 percent 

sand (table 3), and has a modal grain size of 130 (xm 

(table 4). This sandstone sample has a porosity of as 

much as 16.8 percent and a permeability of as much 

as 6.8 mD. The rock has clay-filled, intergranular microporosity (table 4) and consists mostly of detrital quartz 

and chert, which are cemented by clay and quartz (table 

5). In Folk's (1974) classification, this sandstone is a 

sublitharenite. The clay is composed of mica-illite and 

a 7-A trioctahedral clay and lesser amounts of mixedlayer illite-smectite and chlorite (table 6). Sample L4-1 

contains 0.04 percent organic carbon (table 6). It contains larger amounts of Fe203 , FeO, and Cu, and 

smaller amounts of P205 than most of the samples (tables 7 and 8).

Sample L4-2 has a modal grain size of 160 (xm (table 

4) and consists of about 91 percent sand, 5 percent silt, 

and 5 percent clay (table 3). The sample of sandstone

has a porosity of 13.8-15.7 percent and a permeability 

of as much as 5.0 mD. In thin section, the rock has 

intergranular and intragranular mesoporosity and clayfilled, intergranular microporosity, (table 4) and is composed largely of detrital quartz, chert, and rock fragments, which are cemented by clay and quartz (fig. IB; 

table 5). This sandstone is a sublitharenite (Folk, 1974). 

Clays in the sample include mixed-layer illite-smectite 

and mica-illite (table 6). The organic carbon content is 

0.06 percent. Rapid rock analysis of L4-2 shows small 

amounts of alumina (A1203), calcium oxide (CaO), Ti02, 

and P205 , in comparison with the other samples (table 

7). Spectrographic analysis of sample L4-2 indicates relatively small amounts of all the oxides and elements 

reported on table 8.

Samples L4-1 and L4-2 were collected from the top 

and base, respectively, of a sandstone that contains natural gas at depths of 2,890.8-2,897.8 m (9,484-9,507 ft) 

in borehole 2 (fig. 8). In the gas-bearing sequence, 

porosity is 6.3-18.4 percent and permeability is 0.07-5.8 

mD. The reported initial potential flow of gas from this 

rock is 11,526.2 m3 (407,000 ft3) per day.

Core of a lower part of the Hilliard Shale in borehole 

2 (figs. 1 and 8) consists of fossiliferous shale that was 

deposited in an offshore-marine environment (table 2). 

Sample L4, from a depth of 2,837.0 m (9,307.5 ft), is 

composed mainly of quartz, dolomite, calcite, mica-illite, 

mixed-layer illite-smectite, kaolin, and chlorite (table 

9). Clays comprise about 47 percent of the sample. The 

mean of vitrinite reflectance values for sample L4 is 

0.67 percent, which probably indicates a thermally mature shale in an early stage of oil and gas generation 

(table 10).

BOREHOLE 3

Core samples L2-1 and L2-2 were collected from 

crossbedded and contorted-bedded, fine-grained sandstone at depths of 2,914.7 m (9,562.3 ft) and 2,922.8 

m (9,589.0 ft), respectively, in borehole 3 (figs. 1 and 

9, table 2). These rocks seem to be of nonmarine origin 

and probably were deposited on a delta plain. The samples consist of about 80-83 percent sand, 11-13 percent 

silt, and 6-8 percent clay (table 3). Modal grain size 

of the sand is 140 |xm and maximum grain size is 452 

|xm in sample L2-1 and 301 |xm in sample L2-2 (table 

4). The porosity and permeability of sample L2-1 are 

about 12.4 percent and 0.14 mD, respectively. Sample 

L2-2 has a porosity value of about 17.0 percent and 

a permeability value of about 0.27 mD. In thin sections 

(fig. 7C), the rocks have clay-filled, intergranular microporosity and rare moldic and intergranular mesoporosity (table 4). They are composed mainly of rock fragments and detrital grains of quartz, chert, feldspar, and 

mica, which are cemented largely by clay, quartz, and
16 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

TABLE 4. Description of thin sections of sandstone from core samples of the Frontier Formation of southwestern Wyoming

[jtm, micrometers; d, porosity; K, permeability; mD, millidarcies]

Core

sample

No.

Grain size (urn)

Modal Maximum

160 340

Estimated

sorting

Well - 

Estimated

angularity

to 

well rounded.

Fabric

Porosity and

permeability

clay-filled intergranular 

and secondary imtragranular

L4-1

L4-2

L2-1

L2-2

L2-3

L2-4

,2-5

A-l

C-l

C-2

130

160

140

140

140

80

90

100

120

130

300

390

452

301

600

Extremely Subangular 

well. to

rounded.

Bimodal, raatrixand frameworksupported.

Wel] Subrounded

to 

well rounded.

Granular mosaic

Moderately Subangular 

to well. to

subrounded.

Well- -do301

241

301

331

 do Subangular

to 

rounded.

 do Subangular

to 

subrounded.

Moderately do 

to well.

Moderately Rounded 

well. to

subrounded.

Grain-supported-

 do --- 

Granular mosaicMa trix-suppor ted 

Frameworksupported.

Granular mosaicWell Subangular

to 

subrounded.

Moderately do ---- 

well.

 do 

Cement-supported 

mesoporosity. <j>=10.4 

percent, K=0.97 mD.

Microporosity; clay-filled 

intergranular. <j>=as much as 

16.8 percent, K=as much as 

6.8 mD.

Mesoporosity and microporosity; 

intergranular, intragranular, 

and clay-filled intergranular. 

<t>=13.8-15.7 percent, K=as much 

as 5.0 mD.

Mostly microporosity; clayfilled intergranular, rare 

moldic and intergranular. 

<|>=12.4 percent, K=0.14 mD 1 .

Mostly microporosity; clayfilled intergranular, rare 

moldic and intergranular. 

<1)=17.0 percent, K=0.27 mD 1 .

Mostly microporosity; clayfilled intergranular, clayfilled moldic and rare 

intergranular. <j>=12.3 percent, 

K=0.29-0.55 mD 1 .

Mesoporosity and microporosity; 

secondary intergranular, 

intragranular, clay-filled and 

clay-lined intergranular. 

(t>=20-23 percent, K=0.43-0.48 mD

Microporosity; clay-filled 

intergranular. <t>=14.5- 

14.8 percent, K=0.51-0.58 mD 1 .

Microporosity and rare

mesoporosity; clay-filled 

intergranular and moldic. 

<t>=14.6 percent, K=as much as 

0.63 mD.

Mostly microporosity, some 

mesoporosity; clay-filled 

intergranular, clay-filled 

moldic, intergranular and 

moldic. <|>=12.8-14.8 percent, 

K=as much as 0.91 mD.

Microporosity and rare mesoporosity; clay-filled intergranular and clay-filled moldic 

porosity, rare moldic. 

<|>=4.7-6.7 percent, K=as much 

as 0.95 mD.

'Porosity and permeability values provided by C & K Petroleum, Inc.

FIGURE 7 (facing page). Photomicrographs of thin sections of sandstone in the Frontier Formation of southwestern Wyoming. A, Angular 

to subrounded grains of quartz (q) and chert (ch), cemented by calcite (cc) and authigenic clay (c), in core sample S-l of borehole 

1. B, Quartz overgrowths (qo) and subangular to well-rounded quartz grains (q) in core sample L4-2 of borehole 2. C, Subangular 

grains of quartz (q) and chert (ch), and intergranular calcite (cc), authigenic clay (c), and pores (p), in core sample L2-1 of borehole 

3. D, Subangular grains of quartz (q) and chert (ch) and intergranular, authigenic clay (c) in core sample L2-3 of borehole 3.
CORE-SAMPLE DESCRIPTIONS 17
18 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

TABLE 5. Composition (in percent) of thin sections of sandstone from core samples of the Frontier Formation of southwestern

Wyoming

[tr, less than 1 percent; leaders ( ) indicate none reported]

Detrital grains:

Potassium feldspar 

Chert 

Biotite -

Muscovite 

Glauconi t e 

Calcareous fossils 

Detrital matrix 

Cements:

Calcite 

Pyrite -

Kaolin 

Clays other than 

kaolin, undivided 

S-l L4-1

10 fct;

1

2 

19 5

5 

tr 

1 

9 2

11 tr

 tr

14 26

L4-2 L2-1

f.Q 1 1

1

tr tr 

12 35

1 6

 tr

 

8

C

7

1

9 13

Core samples

L2-2 L2-3

39 57

tr tr

3 

21 18

7 5

3 

tr 

tr 

4 6

__

3

19 13

L2-4

48

3

tr

10

11

tr

tr

 -

6

tr

tr

22

L2-5

59

6

2

tr

_ _

_ 

1 

1

5

tr

6

tr

20

A-l

50

2

tr 

12

tr

tr

__

 

 

11

 

 

tr

1

23

C-l

42

2

tr 

13

8

tr

tr

 

tr

9

 

 

 

tr

25

C-2

30

2

1 

20

12

tr

tr

 

 

1

26

 

 

 

7

TABLE 6. Estimated mineral and organic-carbon content of core samples of sandstone from the Frontier Formation of southwestern

Wyoming

[Mineral content determined by X-ray diffraction given as parts in ten; <1 = 5-9 percent; tr = <5 percent. Organic-carbon content (in weight percent) determined by

combustion and acidification. Leaders ( ) indicate none reported]

S-l

Quartz 7+

Calcite 1

Goetnite 

Siderite 

Smectite - 

Illite-smectite,

mixed-layered <1

Mica-illite tr

Chlorite 

7-angstrom

trioctahedral clay tr

Kaolin tr 

Organic carbon 0.03

L4-1

7

tr

tr

<l

1

tr

1

0.04

Core samples

L4-2 L2-1 L2-2

9+ 7+ 7+

tr tr <1

 tr -

 tr

 

tr ~~* " ~

<i <i <i

tr tr tr

 tr tr

 tr tr

0.06 0.14- 0.01

.16

L2-3 L2-4

9 7+

tr 

1 1+

tr tr

 tr

 tr

0.06 0.03

L2-5

6+

tr 

1+

tr

tr

1+

tr

tr

tr

tr 

0.13-

.16

A-l

8+

tr

tr

tr

tr

tr

tr

tr

tr

0.04

C-l

8

tr

tr

1

tr

tr

tr

0.06-

.07

C-2

6

1+

tr

1

tr

tr

<l

0.10
CORE-SAMPLE DESCRIPTIONS 19

TABLE 7. Content of principal oxides (in weight percent) in core samples of sandstone from the Frontier Formation of southwestern

Wyoming

[Determined by atomic absorption and apeetrophotometry (rapid rock-analysis, Shapiro, 1975)]

Core samples

Oxide S-l L4-1 L4-2 L2-1 L2-2 L2-3 L2-4 L2-5 A-l C-l C-2

Si02 

A1 2°3 

Fe 9 0o 

FeO 

MgO 

T n

KaR ___

Ti02

p n . __ , _ P 2°5

co2 

H

rii _

IT H2f\u- -

Total 

86.1

3.7

0.28

<0.01 

0.13

4.6

0.21

0.42

0.10

0.03

0.05 

3.6

0.64

0.46

100

87.3

4.8

1.9

1.4

O

Q £.

.JO

0

7Q . / O

0.22

O

Q'J

.O J

0.20

0.05

<0.01 

0.04

1.4

0.58

100

96.2

2.1

0.05

0.08 

0.42

<0.01 

0.16

n U. ^J J

0.07

0.02

<0.01 

0.03

0.49

0.37

101

85.2

5.2

0.41

0.80 

0.48

-a -j j. j

0.20

1.5

0.17

0.11

0.10 

2.2

1.5

0.59

101

84.4

7.2

0.51

1.0

0

1Q .JO

O

C Q

 JO 1.1

0.75

0

1 Q lo

0

1 Q lo

0.05 

0.41

1.2

0.55

99

95.2

3.2

<0.01

0.16 

0.33

<0.01

0.13

0.54

0.11

0.02

0.05 

0.03

0.06

1.0

101

90.2

5.9

0.07

0.76

0

1 Q . lo

<0.01

O QO« J£ 

1 ^

1 * J

0.16

0.05

0.03 

0.11

1.4

0.67

101

82.1

4.3

0.35

0.40

0

1 Q io

4.9

0

1 Q

 lo

0.93 

0.20

0.13

0.03 

3.9

0.94

0.52

99

95.4

2.5

0.04

0.56 

0.07

0.35

0.15

0.31 

0.16

0.12

<0.01 

0.03

0.46

0.26

100

91.0

6.2

0.20

0.36

0

1 Q .17

0.09

O O Q ./O

O

Q f

.OO

U . JJ

0.17

<0.01 

0.03

1.2

0.54

101

71.9

6.5

0.36

0.84 

0.25

8

0. o

0.66

1.0

0

1 Q lo

0.11

0.10

6.6

0.92

0.63

99

calcite (table 5). Photomicrographs of the samples (fig. 

10A, 5), taken with a scanning electron microscope 

(SEM), show pore-lining authigenic quartz and clays. 

The sampled rocks are litharenites (Folk, 1974). X-ray 

diffraction analyses indicate minor amounts of mixedlayer illite-smectite, mica-illite, chlorite, kaolin, and 7- 

A trioctahedral clay in the sandstones (table 6). Organic 

carbon content of these rocks is 0.14-0.16 percent (L2- 

1) and 0.01 percent (L2-2). In comparison with the 

other samples, L2-1 contains large amounts of MgO, 

potassium oxide (K20), and manganese oxide (MnO). 

Sample L2-2 contains relatively large amounts of A1203 , 

sodium oxide (Na20), and P205 (table 7). Emission spectrographic analyses indicate comparatively large 

amounts of MgO, Co, Cr, lanthanum (La), Ni, vanadium 

(V), and Zn in both samples and relatively large 

amounts of iron oxide (Fe203), Na20, P205, Cu, and 

Sc in L2-2 (table 8).

Core samples L2-3, L2-4, and L2-5 are also from 

borehole 3 (figs. 1 and 9, table 2), but were obtained 

from very fine grained and fine-grained sandstone at 

depths of 2,933.8-2,948.1 m (9,625.0-9,672.0 ft). These 

marine rocks probably were deposited in delta-front environments (table 2) and are composed of about 75-85 

percent sand, 8-12 percent silt, and 8-13 percent clay 

(table 3). Modal grain sizes are 140 |xm (L2-3), 80 |xm 

(L2-4), and 90 |xm (L2^5) (table 4).

In sample L2-3, the porosity is about 12.3 percent 

and the permeability is 0.29-0.55 mD. This sandstone 

in thin section (fig. ID) has clay-filled, intergranular

and clay-filled, moldic microporosity, rare mesoporosity 

(table 4), and consists largely of detrital quartz, chert, 

and rock fragments, which are cemented by clay and 

quartz (table 5). The rock is a litharenite (Folk, 1974). 

Clay minerals in the sandstone, as determined by X-ray 

diffraction analysis, include mixed-layer illite-smectite 

and minor mica-illite (table 6). The sample contains 0.06 

percent organic carbon (table 6) and, in comparison with 

the other samples, smaller amounts of Fe203 , CaO, 

Na20, P205 , Co, La, Mn, Ni, Sc, ytterbium (Yb), and 

Zn (tables 7 and 8).

A sequence of mainly sandstone beds, represented 

by samples L2-2 and L2-3 (fig. 8), contains natural gas 

at depths of 2,923.1-2,938.4 m (9,590-9,640 ft). In these 

rocks, the porosity is 1.5-19.9 percent and the permeability is 0.07-1.2 mD. The reported initial potential flow 

of gas is 28,320 m3 (1 million ft3) per day. C & K Petroleum, Inc. reported slightly above-normal reservoir 

pressure and a pressure gradient of 12.04 kilopascal/m 

(0.533 psi/ft) for a sandstone in this sequence.

The porosity and permeability of sample L2-4 are 

20-23 percent and 0.43-0.48 mD, respectively. In thin 

section, the sandstone has intergranular and intragranular mesoporosity, clay-filled and clay-lined, intergranular microporosity (table 4), and is composed 

mainly of detrital quartz, chert, rock fragments, and 

feldspar, which are cemented by clay and quartz (table 

5). Minor amounts of zircon, leucoxene, and apatite also 

are present in the sample (D. L. Gautier, written commun., 1981). In Folk's (1974) classification, the rock is
20 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

Borehole 2 

C & K Petroleum, Inc.

Lincoln Road 4 

Sec. 5, T. 24 N., R. 111 W.

FEET 

9200 -i

Depositional 

environment

Spontaneous 

potential ( 

curve

9300 -Z--=TTJ 

I

bi 

bi 

b

HI

cc

9500- 

b

-Resistivity 

(spherically 

focused, 

average)

EXPLANATION

Sandstone

Siltstone 

Shale 

No core

Offshore 

marine

Delta-front and 

delta-plain

-L4-1 

407 M 

CFGPD -i

L4-2

° ° ° ° Conglomeratic

 - Sandy

~ ~ ~ Silty

 Clayey 

^ ^ Calcareous

Carbonaceous 

Clay clasts 

Burrows 

Bioturbated 

Inoceramus 

Sample locality 

407 M Location and amount of 

CFGPD natural gas 407,000

cubic feet (11,526.2 cubic 

meters) per day

METERS FEET 

fl-T-0

C

cc

b

bi

I

L4-1

Delta-front?

10-

20-

-50

30-L 10o

FIGURE 8. Logs of cored and sampled strata in the Frontier Formation and Hilliard Shale in borehole 2 on the Moxa

arch, Wyoming (fig. 1).

a litharenite. The clay consists of mixed-layer illitesmectite and lesser amounts of mica-illite, chlorite, 

kaolin, and 7-A trioctahedral clay (table 6). Organiccarbon content is 0.03 percent. The sample contains relatively large amounts, compared with the other samples, of K20, barium (Ba), and boron (B) (tables 7 and 

8).

Sample L2-5 has a porosity of 14.5-14.8 percent and 

a permeability of 0.51-0.58 mD. In thin section, the 

sandstone has clay-filled, intergranular microporosity 

(table 4) and consists largely of detrital quartz, chert, 

and rock fragments, cemented by ,clay, calcite, and 

quartz (table 5). The rock is a sublitharenite (Folk, 

1974). Mixed-layer illite-smectite is the most abundant

clay mineral, although mica-illite, chlorite, kaolin, and 

7-A trioctahedral clay are present also (table 6). The 

organic carbon content is 0.13-0.16 percent. Spectregraphic analysis indicates a comparatively large amount 

of Ba, strontium (Sr), yttrium (Y), Yb, and zirconium 

(Zr) in this sample (table 8).

Samples L2-1A and L2-2A from depths of 2,916.7 

m (9,569.0 ft) and 2,928.3 m (9,607.0 ft), respectively, 

in borehole 3 consist of shale that was deposited in 

delta-plain environments (table 2). Sample L2-1A is 

composed mainly of mica-illite, mixed-layer illite-smectite, and quartz, but includes small amounts of kaolin, 

chlorite, and pyrite (table 9). The sample also contains 

0.64 percent organic carbon, 0.04 mg/g (milligrams per
CORE-SAMPLE DESCRIPTIONS 21

TABLE 8. Approximate content of selected oxides and elements in core samples of sandstone from the Frontier Formation of 

southwestern Wyoming, determined by emission spectrographic analysis

S-l L4-1 L4-2

Core samples 

L2-1 L2-2 L2-3 L2-4 L2-5 A-l C-l C-2

Oxides (in weight percent) recalculated from elements

AL2°3

Fe 2°3

MgO

CaO

Na2 0

K2°

Ti02

P 2°5

MnO

2.8

.29

.11

1.7

.071

.18

.017

<.16

.048

3.6

2.6

.27

.097

.066

.30

.092

<.16

.004

1.6

.11

.043

.025

.024

.16

.016

<.16

.001

4.7

1.3

.38

1.5

.13

.41

.11

.19

.028

6.4

1.7

.33

.29

.69

.34

.095

.28

.011

2.3

.13

.061

.045

.036

.23

.025

<.16

.001

6.8

1.2

.28

.14

.31

.71

.11

.18

.006

4.2

.99

.20

2.5

.12

.40

.12

.25

.027

1.4

.54

.10

.17

.045

.088

.050

.23

.002

4.2

.96

.23

.29

.19

.37

.15

.21

.003

5.3

1.2

.27

4.1

.32

.45

.07

.19

.085

Selected elements (in parts per million)

Ba

B

Co

Cu

Cr

La

Mn

Ni

Sc

Sr

V

Y

Yb

Zn

Zr

200

20

<1.0

3.1

1.8

14

370

<1.5

<1.0

170

8.1

3.9

.17

<15

31

300

38

2.2

8.6

5.6

14

28

4.5

2.1

37

19

4.5

.57

31

82

160

13

<1.0

3.0

2.2

11

6.1

<1.5

<1.0

24

5.9

<1.5

<.15

<15

16

410

39

5.0

6.0

11

28

220

5.9

2.1

170

31

6.5

.48

50

64

270

42

4.1

47

9.6

32

88

4.4

2.5

120

27

8.8

.56

55

48

270

21

<1.0

5.0

3.9

<10

10

<1.5

<1.0

28

9.7

3.1

<.15

<15

22

460

57

1.0

5.2

6.6

17

46

3.4

2.2

110

23

7.2

.68

20

61

430

45

<1.0

4.7

5.5

27

210

2.3

2.1

310

18

9.8

.68

20

91

92

23

1.1

7.8

3.2

11

16

1.6

<1.0

13

5.6

3.9

.22

<15

89

350

52

1.7

7.3

12

15

26

4.0

2.1

78

21

6.7

.49

27

77

310

45

2.5

4.9

7.7

19

660

3.5

3.7

290

28

11

1.1

35

100

TABLE 9. Estimated mineral content by X-ray diffraction of core samples of shale from the Frontier Formation and

Milliard Shale of southwestern Wyoming

[Leaders ( ) indicate none detected]

L4 L2-1A L2-2A

Core samples

A2-1 A-1A C-1A C-2A F-l

Mineral content (approximate weight percent)

Pyrite -- 3

- 3

- 6

- 7

- 47

33 51 

2 2

65 47

33 51

7 

5 

56 49

40

1

59

35 

4 

2 

2 

4 

53

46 

4 

4

46

Clay minerals (estimated percent of total clay)

Mica-illite and mixedlayer illite-smectite 85 95 

Kaolin and chlorite 15 5

70

30

90

10

75

25

100 95

5

90

10
22 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

TABLE 10. Organic composition, vitrinite reflectance, and hydrocarbon potential of core samples of shale from the Frontier Formation

and Milliard Shale of southwestern Wyoming

[C, carbon; mg/g, milligrams per gram; COg, carbon dioxide;°C, degrees Celsius; leaders ( ) indicate not analyzed]

_______________________Core samples__________________________

L4 L2-1A L2-2A A2-1 A-1A C-1A C-2A

Transformation ratio (Sj/S^+S-) 

Mean vitrinite reflectance (percent) 

Estimated thermal maturity

0.67 

Mature

0.14

0.80

Mature

0.02

0.72

Mature

0.09

0.74

Mature

0.14

0.75

Mature

0.06

1.04

Mature

0.24

1.14

Mature

F-l

Organic C (weight percc 

Volatile hydrocarbon, 5 

Pyrolytic hydrocarbon, 

Estimated capacity for 

Pyrolytic C0 2 , S 3 ( mg/$ 

Temperature of maximum 

Hydrogen index (mg hydi 

Oxygen index (mg C02/g

mt) 

5 1 (mg/g) 

_\ _ _

I) 

pyrolytic yield ( C) 

r\ w 

0.64 

0.04 

0.25 

Nil 

0.20 

462 

40 

32

trial 

humic.

2.12 

0.04 

1.84 

Slight 

0.24 

466 

87 

11

Marine 

sapropelic?

1.27 

0.08 

0.83 

Nil 

0.16 

455 

65 

13

Marine 

sapropelic.

0.27 

0.01 

0.07 

Nil 

0.09 

456 

24 

36

Terrestrial 

humic.

4.42 

0.28 

4.48 

Large 

0.49 

474 

101 

11

Marine 

sapropelic.

0.92 

0.12 

0.37 

Nil 

0.45 

462 

40 

49

Terrestrial 

humic.

1.71 

0.25 

5.84 

Large. 

0.33 

430 

341 

19

Marine 

sapropelic.

0.04

0.48

Immature.

Spontaneous

potential

curve.

Borehole 3 

C & K Petroleum, Inc.

Lincoln Road 2 

Sec. 3, T. 24 N., R. 111 W.

FEET 

9500 H

9600- 

c

b

b

bi-o 

bi-o 

bi o

9700 H

L2-1

L2-2

-L2-2/ , 

1000M 

CFGPD -L2-3

-L2-4

-L2-5

-Resistivity

(spherically

focused,

average)

curve

Depositional 

environment

Delta- plain

Delta-front

EXPLANATION

Sandstone 

Siltstone

Shale

No core METERS FEET 

0-rO

10-

-50

 > > ° ° Conglomeratic 

............ Sandy

~ -~ ~ Silty

 Clayey

-"- '- Calcareous

Carbonaceous

Burrows

Bioturbated

Oysters

Sample locality

Location and amount of 

natural gas 1,000,000 

cubic feet (28,320 cubic 

meters) per day

c 

b 

bi 

o 

L2-1

1000M 

CFGPD

20-I

30-

100

FIGURE 9. Logs of cored and sampled strata in the Frontier Formation in borehole 3 on the Moxa arch, Wyoming

(fig. 1).
CORE-SAMPLE DESCRIPTIONS 23

gram) volatile hydrocarbon (SO, and 0.25 mg/g pyrolytic hydrocarbon (S2, table 10). Total hydrocarbon yield 

or the original hydrocarbon-generating capacity (volatile hydrocarbon, S^ plus pyrolytic hydrocarbon, S2) 

is 0.29 mg/g, which indicates that L2-1A is a poor 

source rock for oil and gas (G. E. Claypool, written 

commun., 1980). The hydrogen index (40) and oxygen 

index (32) reflect terrestrial humic organic matter in 

the sample (table 10). Thermal alteration of the shale, 

indicated (table 10) by the temperature of maximum 

pyrolytic yield (462°C), the transformation ratio or production index (S1/(S1 + S2)=0.14), and vitrinite reflectance (0.80 percent), is sufficient for oil generation, although this shale is a poor source rock for hydrocarbons.

Sample L2-2A consists largely of mica-illite, mixedlayer illite-smectite, kaolin, chlorite, and quartz (table 

9). The sample also contains 2.12 percent organic carbon, 0.04 mg/g volatile hydrocarbon, and 1.84 mg/g 

pyrolytic hydrocarbon (table 9). For L2-2A, the total 

hydrocarbon yield is 1.88 mg/g, which represents a marginal source rock for oil (G. E. Claypool, written commun., 1980). From the hydrogen index (87) and oxygen 

index (11), the type of organic matter in the sample 

is interpreted to be dominantly marine sapropelic. The 

temperature of maximum pyrolytic yield (466°C) and 

the vitrinite reflectance (0.72 percent) indicate that the 

sampled bed is thermally mature with regard to oil generation.

BOREHOLE 6

Sample A2-1 consists of marine shale in the Frontier 

Formation from a depth of 2,408.6 m (7,902.0 ft) in 

borehole 6, on the north flank of the Rock Springs uplift 

(fig. 1, table 1). The shale is composed mostly of mica-illite, mixed-layer illite-smectite, quartz, and lesser 

amounts of pyrite, feldspar, kaolin, and chlorite (table 

9). Organic carbon comprises 1.27 percent of the sample 

(table 10). The total hydrocarbon yield of A2-1 is 0.91 

mg/g, of which 0.08 mg/g is volatile hydrocarbon and 

0.83 mg/g is pyrolytic hydrocarbon. These values indicate a poor source rock for oil (G. E. Claypool, written 

commun., 1980). Marine sapropelic organic matter in 

the shale is indicated (table 10) by the hydrogen index 

(65) and the oxygen index (13). The temperature of 

maximum pyrolytic yield (455°C) and the vitrinite reflectance (0.74 percent) for sample A2-1 are evidence 

of an early stage of thermal alteration and oil generation (table 10).

The Frontier Formation and overlying Cretaceous 

and Tertiary strata in the Superior Oil Corp. Pacific 

Creek-Federal 1 well (sec. 27, T. 27 N., R. 103 W.), 

about 39 km (24.2 mi) north of borehole 6, were investigated by Law and others (1980). A core sample of 

marine shale, from the upper part of the Frontier at 

a depth of 6,249 m (20,503 ft), contains about 0.8 percent organic carbon, largely in the form of humic-kerogen (Law and others, 1980, p. 29 and fig. 7). A vitrinite 

reflectance value of 1.0-1.1 percent for the sample indicates sufficient thermal alteration of the humic material 

for the generation of gas in the Frontier.

BOREHOLE 8

Core sample A-l was obtained from a very fine 

grained sandstone containing sparse burrows, at a 

depth of 2,133.1 m (6,998.0 ft) in borehole 8 on the 

north flank of the Rock Springs uplift (figs. 1 and 11, 

table 2). The sandstone probably was deposited in a 

delta-front environment. Sample A-l has a modal grain 

size of 100 |xm and consists of about 85 percent sand, 

11 percent silt, and 5 percent clay (table 3). The sample 

has a porosity of about 14.6 percent and a permeability 

of as much as 0.63 mD. Thin sections of the rock show 

clay-filled, intergranular microporosity and rarely show 

moldic porosity. This sandstone sample is composed 

mainly of quartz, chert, and feldspar, which are 

cemented by clay and quartz (tables 4 and 5). According 

to Folk's (1974) classification, the rock is a sublitharenite. In sample A-l, the clay includes mixed-layer 

illite-smectite, mica-illite, chlorite, kaolin, and 7-A trioctahedral clay, and the organic-carbon content is 0.04 

percent (table 6). Rapid rock analysis shows comparatively small amounts of MgO, Na20, K20, and MnO 

in the sample (table 7). Spectrographic analysis indicates relatively small amounts of A1203 , K20, Ba, La, 

Sc, Sr, V, and Zn (table 8). The sandstone units in 

the Frontier at borehole 8 apparently do not contain 

natural gas, even though the porosity ranges from 0.6 

to 15.0 percent and the permeability ranges from less 

than 0.01 to 11.0 mD in the cored sandstone.

Sample A-1A from a depth of 2,130.9 m (6,991.0 ft) 

in borehole 8 consists of shale of shallow-marine or 

brackish-water origin (table 2). The shale is composed 

largely of mica-illite, mixed-layer illite-smectite, kaolin, 

chlorite, and quartz (table 9). The sample also contains 

(table 10) organic carbon (0.27 percent), and yields volatile hydrocarbon (0.01 mg/g) and pyrolytic hydrocarbon (0.07 mg/g). Total hydrocarbon content is 0.08 mg/ 

g, which is insufficient for a source rock for oil or gas 

(G. E. Claypool, written commun., 1980). The hydrogen 

index (24) and oxygen index (36) for A-1A seem to represent terrestrial humic organic matter in the rock 

(table 10). With regard to oil generation, the shale is 

thermally mature, as indicated by the temperature of 

maximum pyrolytic yield (456°C), the transformation 

ratio (0.14), and vitrinite reflectance (0.75 percent).
24 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

BOREHOLE 11

Samples C-1 and C-2 were collected from cores of 

cross-stratified, very fine grained to fine-grained sandstone at depths of 3,383.9 m (11,101.7 ft) and 3,394.5 

m (11,136.3 ft), respectively, in borehole 11 (figs. 1 and 

12, table 2). These rocks probably were deposited in 

deltaic environments. Core sample C-1 has a modal 

grain size of 120 |xm and is composed of about 79 percent sand, 13 percent silt, and 8 percent clay (tables 

3 and 4). Porosity is 12.8-14.8 percent and permeability 

is as much as 0.91 mD. The sandstone has mostly clayfilled, intergranular microporosity and some intergranular and moldic mesoporosity, and consists mainly of detrital quartz, chert, rock fragments, and feldspar, 

cemented by clay and quartz (table 5). The thin sections 

also include minor amounts of tourmaline, zircon, 

leucoxene, and magnetite or ilmenite (D. L. Gautier, 

written commun., 1981). In Folk's (1974) classification, 

the sandstone is a litharenite. Viewed with a scanning 

electron microscope, pores in sample C-1 contain (fig. 

10C) mixed-layer illite-smectite. Clays in the sample include mixed-layer illite-smectite and lesser amounts of 

mica-illite, chlorite, and 7-A trioctahedral clay (table 

6). Organic-carbon content is 0.06-0.07 percent. The 

sample also contains relatively large amounts of Ti02 , 

P205 , B, and Cr (tables 7 and 8). Gas was not found 

in this sandstone unit.

Sample C-2 is composed of about 77 percent sand, 

16 percent silt, and 7 percent clay, and has a modal 

grain size of 130 n-m (tables 3 and 4). Porosity and permeability of the sample are about 4.7-6.7 percent and 

as much as 0.95 mD, respectively. In thin section, the 

rock shows secondary moldic and intergranular 

mesoporosity and clay-filled, intergranular microporosity, and consists mostly of detrital quartz, chert, rock 

fragments, and feldspar. Authigenic minerals include 

calcite, clay, and quartz (tables 4 and 5). This sandstone 

is also a litharenite (Folk, 1974). Micrographs of sample 

C-2 (fig. 10.D), taken with a scanning electron microscope, show corroded, angular grains of quartz 

cemented by* clay. The clay in the sample is largely 

mixed-layer illite-smectite, but includes mica-illite, chlorite, and 7-A trioctahedral clay (table 6). Organic carbon content is 0.10 percent. The sample contains comparatively large amounts of CaO, Na20, K20, MnO, B, 

Sc, Sr, V, Y, Yb, and Zr (tables 7 and 8). Sample C-2 

was obtained from the upper part of a sandstone that

produces natural gas at depths of 3,397.7-3,403.8 m 

(11,147-11,167 ft) in borehole 11 (fig. 12). These reservoir rocks reportedly yield gas at an initial potential 

flow rate of 70,885.0 m3 (2.503 million ft3) per day.

Sample C-1A, from a depth of 3,386.7 m (11,110.8 

ft) in borehole 11 (fig. 12), is a carbonaceous shale, 

which was deposited in a deltaic environment (table 2). 

The shale consists mostly of mica-illite, mixed-layer illite-smectite, and quartz (table 9) but also contains 4.42 

percent organic carbon (table 10). In this sample, the 

volatile hydrocarbon (0.28 mg/g), pyrolytic hydrocarbon 

(4.48 mg/g), and total hydrocarbon yield (4.76 mg/g) indicate a rich source rock for oil and gas (G. E. Claypool, 

written commun., 1980). Evidence of marine, sapropelic, organic matter in the sample (table 10) is provided by the hydrogen index (101) and the oxygen index 

(11). Oil and gas generation in this thermally mature 

shale is indicated (table 10) by the temperature of 

maximum pyrolytic yield (474°C) and vitrinite reflectance (1.04 percent).

Sample C-2A is shale of deltaic origin from a depth 

of 3,405.8 m (11,173.5 ft) in borehole 11 (table 2). It 

is composed mainly of mica-illite, mixed-layer illitesmectite, kaolin, chlorite, and quartz (table 9). The sample also contains 0.92 percent organic carbon, and yields 

0.12 mg/g volatile hydrocarbon and 0.37 mg/g pyrolytic 

hydrocarbon (table 10). The shale evidently is a poor 

source rock for oil and gas (G. E. Claypool, written 

commun., 1980), as indicated by the total hydrocarbon 

yield (0.49 mg/g). Terrestrial humic matter in the sample is represented by the hydrogen index (40) and oxygen index (49). Thermal alteration, as represented by 

the temperature of maximum pyrolytic yield (462°C), 

the transformation ratio (0.24), and vitrinite reflectance 

(1.14 percent) for the shale, is suitable for the generation of oil and gas.

BOREHOLE 18

Core sample F-l is from a depth of 1,609.4 m (5,280 

ft) in borehole 18 in the eastern part of the overthrust 

belt (fig. 1, tables 1 and 2). The sample is shale of 

offshore-marine origin and is composed mostly of micaillite, mixed-layer illite-smectite, kaolin, chlorite, 

quartz, feldspar, and pyrite (table 9). Analytical results 

for the organic material in the shale (table 10) include 

amounts of organic carbon (1.71 percent), volatile hydrocarbon (0.25 mg/g), and pyrolytic hydrocarbon (5.84

FIGURE 10 (facing page). Photomicrographs of sandstone in the Frontier Formation of southwestern Wyoming, taken with a scanning 

electron microscope. A, Quartz overgrowths (qo) and pore-lining authigenic quartz (aq), mica-illite (i), and chlorite (cl) in core sample 

L2-1 of borehole 3. B, Grains of quartz (q) cemented by clays (c), and pore-lining authigenic quartz (aq) in core sample L2-2 of 

borehole 3. C, Mixed-layer illite-smectite (ic) lining a pore in core sample C-1 of borehole 11. D, Corroded grains of quartz (q) cemented 

largely by mixed-layer illite-smectite (ic) in core sample C-2 of borehole 11.
CORE-SAMPLE DESCRIPTIONS 25
26 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

Spontaneous 

potential 

curve i

Borehole 8

K. D. Luff, Inc.

Amoco Champlin 1-23

Sec. 23, T. 22 N.f R. 103 W.

FEET 

6900-1

Depositional 

environment

Resistivity 

(spherically 

focused, 

average)

7000 Delta-front? b 

A-1

EXPLANATION

Sandstone

Shale

No core

Sandy

Silty

Clayey

Calcareous

Burrows

Sample locality

METERS FEET 

O-r-0

ID30-

50

-100

7100-

FiGURE 11. Logs of cored and sampled strata in the Frontier Formation in borehole 8 on the Rock Springs uplift,

Wyoming (fig. 1).

mg/g). The total hydrocarbon yield of 6.09 mg/g indicates that the sampled bed is a rich, potential source 

rock for oil (G. E. Claypool, written commun., 1980). 

Marine sapropelic matter in sample F-l is represented 

by the hydrogen index (341) and oxygen index (19). The 

temperature of maximum pyrolytic yield (430°C), transformation ratio (0.04), and vitrinite reflectance (0.48 

percent) are evidence that the shale is thermally immature with regard to oil generation.

SUMMARY AND INTERPRETATIONS

The samples of sandstone from cores of the Frontier 

Formation are composed of 73.0-90.8 percent sand, 5.3- 

15.6 percent silt, and 4.7-17.8 percent clay by weight 

(table 3). Modal grain sizes (table 4) range from 80 jjum 

(very fine sand) to 160 (xm (fine sand). The grains are 

moderately well sorted to extremely well sorted and 

are subangular to well rounded. These rocks consist 

largely of quartz, chert, feldspar, rock fragments, micaillite, mixed-layer illite-smectite, chlorite, and 7-A 

trioctahedral clay (tables 5 and 6). The sampled units 

of sandstone are litharenites and sublitharenites (Folk, 

1974) and most of them were deposited in deltaic environments. Chert is most abundant in the sandstone of 

delta-plain origin.

The only samples that do not contain chlorite are S-l, 

L4-2, and L2-3 (table 6). These also contain less Fe203 ,

Ti02, B, Ni, Yb, and Zr than the other samples (table 

8), which may indicate the lack of chlorite and relative 

deficiencies of heavy minerals. Furthermore, samples 

S-l and L2-3 were obtained from gas-bearing strata 

(figs. 6 and 9) and sample L4-2 was obtained from 

slightly below gas-bearing beds (fig. 8). None of the 

samples of chloritic sandstone are from strata that contain gas, although samples L2-2 and C-2 are from 

slightly above gas-bearing beds.

Porosities of the sandstone samples range from 4.7 

to 23.0 percent and permeabilities are from 0.14 to 6.80 

mD. The samples generally have clay-filled, intergranular microporosity, although some have clay-filled, moldic microporosity and intergranular and moldic 

mesoporosity. Porosity has been reduced by compaction, including the mechanical deformation of ductile 

grains, and by precipitation of quartz, calcite, and authigenic clays, including mixed-layer illite-smectite, 

illite, chlorite, kaolin, and 7-A trioctahedral clay. Authigenic clays commonly line and fill intergranular pores 

and block pore throats, thereby reducing porosity and 

permeability. Because of their characteristic microporosity, these sandstone units of the Frontier Formation are poor to fair reservoir beds for oil and gas. However, the permeability of the sandstone may be improved by fracturing treatments.

The composition of the sandstone samples, expressed 

as oxides of important rock-forming elements, was de-
CORE-SAMPLE DESCRIPTIONS 27

Spontaneous/ 

potential) 

curvt

Borehole 11 

C & K Petroleum, Inc.

Chrisman 1 

Sec. 18,T. 2O N., R. 112 W.

FEET 

11.000-1

Depositional 

environment

11,100

11,200

Resistivity 

(spherically 

focused, 

average) 

urve

EXPLANATION

Sandstone

Siltstone

Shale

No core

Conglomeratic

Silty

METERS FEET

0

10-

Delta-front?

Delta-plain

 Clayey 

_i_ _i_ Calcareous 

cc Clay clasts 

b Burrows 

C-1 Sample locality

12503M Location and amount of 

JCFGPD natural gas 2,503,000 

cubic feet (70,885 cubic 

meters) per day

20-

30

0

100

FIGURE 12. Logs of cored and sampled strata in the Frontier Formation in borehole 11 on the Moxa arch, Wyoming

(fig. 1).

termined by atomic absorption, spectrophotometry 

(rapid rock-analysis), and by semiquantitative emission 

spectroscopy (tables 7 and 8). These rocks, which are 

mainly of deltaic origin, contain about 72-96 percent 

Si02, 2-7 percent A12O3, 0.1-3 percent Fe2O3 + FeO, 

0.1-0.5 percent MgO, as much as 9 percent CaO, 0.1-1 

percent Na«>0, and 0.3-1.5 percent K2O. The samples 

of sandstone from the delta plain (L2-1, L2-2, and C2), in comparison with the other sandstone samples, 

contain larger amounts of iron oxide, Co, Ni, and Zn.

Grain sizes and chemical <lata were compared statistically by means of correlation coefficients (r) by J. J. 

Connor (table 11). For the 11 samples of this study, 

r values of 0.52 and more are significant (the probability 

that these values do not arise by chance is 95 percent). 

Many elements are significantly and negatively correlated with the amounts of sand (r values of -0.52 to

-0.83). The correlations indicate that most of the elements are concentrated in the silt and clay fractions 

of the samples. However, there is a positive correlation 

(r=0.64) between sand and Zn. Significant positive r 

values relate the silt fraction to Fe, Mg, Ca, Ti, Mn, 

B, Cr, Sc, Sr, V, Y, Zr, Al, and Na. The clay fraction 

has a significant positive correlation with Fe and Ba. 

Organic carbon is significantly related only to Gu (r=

-0.62). Of the major rock-forming elements, the closely 

associated Fe and Mg (r=0.94), and Mg and Al (r=0.87) 

probably represent mica-illite, chtorite, mixed-layer

illite-smectite, and biotite, mainly in the silt fraction 

of the samples. Calcium is correlative with Sr (r=0.85) 

and with Mn (r=0,95), which seems to indicate carbonate minerals in the silt fraction. Sodium correlates most 

strongly with Al (r=0.91) and probably represents sodic 

feldspar in the silt fraction. The correlation of K and 

Al (r=0.92) in the silt and clay fractions of these rocks 

largely represents potassium feldspar and mica-illite. 

Potassium and barium also are closely associated 

(r=0.95), perhaps indicating barium-bearing mica-illite 

in the silt and clay. Titanium is most directly related 

to B (r=0.94), Cr (r=0.89), and Fe (r=0.87), which 

probably are associated in heavy minerals such as tourmaline and ilmenite in the silt fraction. Phosphorus (P) 

correlates significantly only with Cu (r=0. 8).

Shale samples from cores of the Frontier are composed mainly of quartz, mica-illite, mixed-layer illitesmeetite, kaolin, and chtorite (table 9). However, they 

also contain 0.27-4.42 percent organic carbon, including 

0.01-0.28 mg/g volatile hydrocarbon and 0.07-5.84 mg/g 

pyrolytic hydrocarbon (table 10). The organic matter 

in the samples was derived from terrestrial humic and 

marine sapropelic material.

Shale samples were obtained from boreholes 2, 3, and 

11 (fig. 1 and table 1), which produce gas from the 

Frontier, and from boreholes 6, 8, and 18, in which 

the Frontier is nonproductive. Samples A2-1 and A-1A 

from boreholes 6 and 8, respectively, on the Rock
28 THE MID-CRETACEOUS FRONTIER FORMATION NEAR THE MOXA ARCH, WYOMING

TABLE 11. Correlation coefficients for amounts of sand, silt, clay, organic carbon, and selected elements in eleven samples of sandstone 

of the Frontier Formation in southwestern Wyoming (statistically significant values are 0.52 and more)

Silt Clay Organic C Al Fe Mg Ca Na K Tl P Mn Ba B Co Cu Cr La Ni Sc Sr V 1 1b Zn Zr

-0.62 -0.80 -0.01 -0.71 -0.83 -0.76 -0.44 -0.58 -0.66 -0.74 -0.05 -0.47 -0.63 -0.83 0.07 -0.27 -0.53 -0.40 -0.1

-.05 .42 .54 .41 -.07 .16 .50 47 .00 01 .55 .50 -.20 .10 .22 

-.10 -.08 -.02 .37 -.28 .16 14 -.43 21 .30 .06 .14 -.62 .14

.49 .35 35 -.09 95 .36 .45 .54 -.06 .29

.72 68 .09 57 .56 .82 .32 .60 .73

.42 .45 .61 .00 .26

.42 .62

.54

12 .1

08 -.0

56 .0

71 .4

60 .2

59 5

83 .7

50 .2

65 .8

m 4

0.05 -0.53 -0.69 -0.56 -0.69 0.64 -0.

-

-

43 

00

50

52

67

11

01

02

04

10

14 

25

85

67

90

54

00

42

73

37

47

39

12

91

48

82

55

63

42

89

77

85

48

69

07 

19

61

84

60

40

25

66

72

20

64

76

.78

40 

29

16

64

22

27

15

72

50

42

84

47

84

85

-

56 

26

41

11

31

5

8

4

8

0

7

6

03

75

02

24

- i

Sand

Clay 

Org. C

Fe

Ca

Na

Mn

Co

Cu

Cr

La

Nl

Sc

Sr

V

Y

Yb

Zn

Springs uplift (fig. 1) contain inadequate hydrocarbons 

for the generation of oil and their organic constituents 

are insufficiently altered for the generation of gas (table 

10). Similarly, the organic matter of sample F-l from 

borehole 18 in the overthrust belt is thermally immature and has not generated oil or gas. Samples L4, L2- 

1A, L2-2A, from gas-producing boreholes on the Moxa 

arch, have marginal thermal maturity as source rocks 

for gas and samples L2-1A and L2-2A do not contain 

enough hydrocarbons to be source rocks for oil. Evidently, the gas in the reservoir rocks of boreholes 2 

and 3 was derived from beds below the sampled shale 

units. The thermal maturation of samples C-1A and C2A, which represent beds above and below a gas-bearing sandstone in borehole 11 on the Moxa arch (fig. 

10), is suitable for the generation of oil and gas. Furthermore, the total hydrocarbon yield (4.76 mg/g) and 

type of organic matter (marine sapropelic) for C-1A are 

typical of a source rock for oil and gas. For sample 

C-2A, the total hydrocarbon yield (0.49 mg/g) is inadequate and the humic organic matter is inappropriate 

for the generation of oil, although this shale may have 

yielded gas. Presumably, some of the gas in the reservoir rocks of borehole 11 was generated in the sampled 

shale units (fig. 10).

In conclusion, interbedded sandstone, siltstone, and 

shale of deltaic and nearshore-marine origin and of middle Turonian to early Coniacian age comprise the upper 

part of the Frontier Formation, the "second Frontier 

sandstone" of DeChadenedes (1975) and Myers (1977), 

in much of southwestern Wyoming. On the Moxa arch 

and on the Rock Springs uplift, these strata commonly 

include lenticular reservoir beds and thermally mature 

source rocks for gas at depths of about 2,591-3,871 m 

(8,500-12,700 ft). The broad geographic distribution of 

these strata and the large variation in depths of the

mature source rocks and gas-bearing sandstone reservoirs may indicate that the hydrocarbon resources in 

the upper part of the Frontier in the Green River Basin 

are extremely large. The lower part of the formation 

in the basin is a truncated unit of shale, siltstone, and 

sandstone that thickens to the north and west. Because 

outcrops and cores of these rocks are sparse and relevant petrologic data are lacking, conclusions concerning 

their composition, age, and depositional environment 

are tentative and interpretations regarding their oil and 

gas potential would be speculation.

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