Petrology, Diagenesis, and Sedimentology 

of Oil Reservoirs in Upper Cretaceous 

Shannon Sandstone Beds, Powder 

River Basin, Wyoming

U.S. GEOLOGICAL SURVEY BULLETIN 1917-C

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Chapter C

Petrology, Diagenesis, and Sedimentology 

of Oil Reservoirs in Upper Cretaceous 

Shannon Sandstone Beds, Powder 

River Basin, Wyoming

By PAULA L HANSLEY and C. G. WHITNEY

A multidisciplinary approach to research studies of sedimentary 

rocks and their constituents and the evolution of sedimentary 

basins both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1917

EVOLUTION OF SEDIMENTARY BASINS POWDER RIVER BASIN
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

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For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Hansley, Paula L.

Petrology, diagenesis, and sedimentology of oil reservoirs in Upper 

Cretaceous Shannon Sandstone Beds, Powder River Basin, Wyoming / by 

Paula L. Hansley and C.G. Whitney.

p. cm. (Evolution of sedimentary basins Powder River 

Basin) (U.S. Geological Survey bulletin ; 1917-C)

"A multidisciplinary approach to research studies of sedimentary rocks and 

their constituents and the evolution of sedimentary basins both ancient and 

modern."

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

1. Petroleum Geology Wyoming Steele Shale. 2. Geology, 

Stratigraphic Cretaceous. 3. Steele Shale (Wyo.) I. Whitney, Carroll Gene, 

1949- .II. Title III. Series. IV. Series: U.S. Geological Survey bulletin ; 

1917-C.

QE75.B9 no. 1917-C 

[TN870.5] 

557.3 s-dc20

[553.2'8'09787] 90-3812

CIP
CONTENTS

Abstract Cl 

Introduction Cl 

Research design C3 

Depositional environment C5 

Sedimentology C6 

Petrology C7

Framework mineralogy C8 

Diagenetic alterations C9 

Siderite CIO 

Pyrite CIO 

Quartz CIO 

Chlorite CIO 

Calcite Cll 

Glauconite-smectite Cll 

Albite C14

Minor authigenic phases C14 

Secondary porosity C14 

Oil C17 

Isotopy C17

Procedure C17 

Calcite C17 

Dolomite C19 

Siderite C20 

Discussion C20

Provenance C20

Origin of glauconite C20 

Origin of dolomite C21

Petrology of outcrop and core sandstones C23 

Diagenesis and paragenesis C24 

Depositional environment C27 

Mid-shelf model C27 

Alternative model C28 

Conclusions C28 

References cited C29

Appendix. Core descriptions of the Shannon Sandstone Beds of the Steele 

Member of the Cody Shale, Powder River Basin, Wyoming C32

FIGURES

1. Map showing locations of outcrops and drillholes from which samples of 

the Shannon Sandstone Beds of the Steele Member of the Cody Shale and 

the Groat Sandstone Bed of the Gammon Ferruginous Member of the 

Pierre Shale were collected in the Powder River Basin C2

2. Photograph showing outcrop of Shannon Sandstone Beds at the Salt Creek 

anticline C3

Contents
3. Chart showing stratigraphy of Upper Cretaceous rocks in the subsurface 

of the western part of the Powder River Basin C3

4. Map showing inferred paleogeography at the time of deposition of the

Shannon Sandstone Beds C5 

5-10. Photographs of core of the Shannon Sandstone Beds showing:

5. Facies la C7

6. Facies 2 C7

7. Facies 3 C8

8. Facies 4 C8

9. Bedded siderite in facies 4 C9

10. Coal partings at top of facies 2 CIO

11. Diagram showing composition of sandstones from the Shannon and Groat 

Sandstone Beds Cll

12. Plot of plagioclase versus potassium feldspar abundance, Shannon and

Groat Sandstone Beds C14 

13-15. Photomicrographs of:

13. Detrital glouconite grains and quartz CIS

14. Dolomite grain and overgrowth CIS

15. Detrital polycrystalline dolomite clast and quartz C16

16. Cathodoluminescence micrograph of detrital dolomite grains and 

overgrowths C16

17. Scanning electron micrograph of detrital dolomite grain and clay 

rim C17

18. Photograph of core of the Shannon Sandstone Beds showing detrital

siderite clasts in facies la C17 

19-20. Scanning electron X-ray fluorescence micrographs showing:

19. Siderite and calcite C18

20. Siderite overgrowth on dolomite C18 

21-26. Scanning electron micrographs of:

21. Framboidal pyrite and quartz overgrowths C19

22. Authigenic pyrite and chlorite C19

23. Iron oxide pseudomorph after pyrite C19

24. Quartz overgrowth coated with oil C19

25. Etch pits in quartz overgrowth C20

26. Intergrowth of authigenic quartz and chlorite C20

27. Photomicrograph of authigenic chlorite rim under quartz 

overgrowth C21

28. Color photomicrograph and Cathodoluminescence micrograph showing 

diagenesis of potassium feldspar C22

29. Scanning electron micrograph of highly diagenetically evolved 

glauconite C23

30. X-ray diffraction profiles of glauconite from deep and shallow sandstones 

of Shannon Sandstone Beds C23

31. Photomicrograph of chessboard albite C23

32. Scanning electron X-ray fluorescence micrographs of ferroan dolomite 

overgrowth on detrital dolomite grain C23

33. Scanning electron micrograph of skeletal plagioclase grain C24

34. Photomicrograph of oil between detrital quartz grain and quartz 

overgrowth C25

35. Diagram showing paragenesis of diagenetic alterations in the Shannon 

Sandstone Beds C26

IV Contents
TABLES

1. Outcrops and cores of the Shannon Sandstone Beds of the Steele Member of 

the Cody Shale and of the Groat Sandstone Bed of the Gammon Ferruginous 

Member of the Pierre Shale, Wyoming and Montana C4

2. Correlation of depositional facies in Shannon Sandstone Beds as described 

herein with facies as defined by Tillman and Martinsen (1984) C6

3. Point counts of selected thin sections of the Shannon Sandstone Beds and 

Groat Sandstone Bed, Powder River Basin C12

4. Identification criteria for detrital dolomite C17

5. Stable isotope values (5 13C and 5 18 O) for calcite, dolomite, and siderite in 

Shannon Sandstone Beds C25

Contents

EVOLUTION OF SEDIMENTARY BASINS-POWDER RIVER BASIN

Petrology, Diagenesis, and Sedimentology of 

Oil Reservoirs in Upper Cretaceous Shannon 

Sandstone Beds, Powder River Basin, Wyoming

By Paula L Hansley andC.G. Whitney

Abstract

The Shannon Sandstone Beds, which are contained 

within the Steele Member of the Upper Cretaceous Cody 

Shale, comprise linear, marine shelf sandstone ridges that 

contain productive oil reservoirs in the west-central Powder 

River Basin. Sandstones of the Shannon are mature litharenites dominated by chert fragments, glauconite pellets, and 

detrital dolomite grains and clasts. Detrital dolomite is a 

significant (as much as 15 percent) component of finegrained sandstones. Diagenetic alterations in near-surface 

(< 100 m) sandstones are markedly different than alterations 

in deeply buried (> 2,500m) sandstones. Shallow 

sandstones contain unaltered framework grains that have 

floating and point contacts surrounded by calcite cement. In 

contrast, deeply buried sandstones are characterized by 

chlorite grain coatings, quartz and albite overgrowths, and 

extensive framework grain alteration notably, detrital 

potassium feldspar grains partly to completely replaced by 

calcite. Interstratified glauconite-smectite, the major 

authigenic clay mineral in the Shannon, becomes less 

expandable with burial depth. Secondary porosity is locally 

abundant in shallow reservoirs, but secondary pores are 

commonly filled with authigenic minerals in deeper reservoir 

sandstones. Secondary porosity developed late in diagenesis and was followed by the late (?) Tertiary migration of 

oil into Shannon reservoirs.

The Shannon has been interpreted to be a mid-shelf 

marine sandstone that was deposited in at least 100 m of 

water. However, many characteristics of Shannon sand 

ridges are not satisfactorily explained by a mid-shelf model: 

abrupt changes of facies, abundant siderrte and detrital 

dolomite, coal fragments and partings, juxtaposition of siderite and glauconite in crossbedded sandstones, and 

sideritic shale intervals barren of foraminifera. A possible 

alternative interpretation is that Shannon sand ridges were

Manuscript approved for publication February 19, 1990.

originally part of a linear shoreline sequence that was 

submerged during a transgression and then reshaped by 

longshore currents and marine organisms on the middle

shelf.

INTRODUCTION

Upper Cretaceous Shannon Sandstone Beds of the 

Steele Member of the Cody Shale form linear sandstone 

ridges that trend north-south in the west-central part of 

the Powder River Basin (fig. 1). The age of the Shannon 

as determined by potsssium-argon (K-Ar) dating of 

associated Upper Cretaceous bentonite beds is early 

Campanian, about 81 Ma (J.D. Obradovich, U.S. 

Geological Survey, Denver, oral commun., 1989). The 

Shannon crops out around the Salt Creek anticline in the 

southwestern part of the basin, where it consists of two 

stacked sand ridges (fig. 2), and at a few localities in 

Johnson County (Hose, 1955; Mapel, 1959; Gill and 

Burkholder, 1979) north of the Salt Creek area. In the 

subsurface, Shannon sand bodies are encased in shale 

(fig. 3), which provides a seal for stratigraphic traps from 

which oil is produced in many oil fields including Heldt 

Draw, Hartzog Draw, and Teapot Dome. Shannon 

reservoirs are less than 2 m to more than 20 m thick 

(Tillman and Martinsen, 1987).

The major goals of this study were to characterize 

the mineralogy and diagenesis of the Shannon in the 

Powder River Basin so that provenance, effects of 

organic acids on mineral diagenesis, and a paragenetic 

sequence of diagenetic alterations, including the relative 

timing of oil migration into Shannon reservoirs, could be 

determined. In addition, knowledge of the petrology and 

sedimentology of sand ridges such as the Shannon should 

lead to a better understanding of the genesis of sand 

ridges. Comparisons and contrasts between diagenetic

Upper Cretaceous Shannon Sandstone Beds, Wyoming C1
106'

Kf

Kmv

Kn

KpCu

EXPLANATION

Wasatch Formation

Fort Union Formation

43' 

Lance Formation

Lance Formation, Fox 

Hills Sandstone and 

Lewis Shale, undivided

Fox Hills Sandstone 

and Lewis Shale

Mesaverde Formation 

Cody Shale 

Rerre Shale 

Niobrara Formation

Cretaceous-Precambrian 

rocks, undivided

10 20 30 KILOMETERS

Basin axis

Outcrop locality 

HD Hammond 

HC Horse Creek 

OC Oak Creek 

SC Salt Creek

Core locality: numbers 

refer to table 1

Oil field

WYOMING

Figure 1. Locations of outcrops and drillholes from which samples of the Shannon Sandstone Beds of the Steele Member 

of the Cody Shale and the Groat Sandstone Bed of the Gammon Ferruginous Member of the Pierre Shale were collected in 

the Powder River Basin. Geology modified from Keefer (1974); geology of Black Hills not represented because of complexity 

and irrelevance to report.

C2 Evolution of Sedimentary Basins Powder River Basin
v^^ * 2 Facies 2 and V> '-,- . ** *'-- *" <;

 ' ..is- -<;

Figure 2. Outcrop of Shannon Sandstone Beds showing facies 1,2 and 3 at the Salt Creek 

anticline. Looking west.

AGE1 

(Ma)

 71 

ft A

STAGE

Maastrichtian

Campanian

Santonian 

(part)

, *

BJ-S'0

IsE

£

0)

.c

T3 

O 

O

STRATIGRAPHIC UNIT

Lance Formation

Fox Hills Sandstone

Lewis Shale

Teapot Sandstone Member

I I II I I I I I I I I II I Unnamed shale member

Parkman Sandstone Member

<$ussex Sandstone Bed

<Shannon Sandstone Bed

IV/l Ko

McmDcr <FJshtooth sandstone

Niobrara Member 

(part)

J.D. Obradovich (written commun., 1989).

Figure 3. Stratigraphy of Upper Cretaceous rocks in the 

subsurface of the western part of the Powder River Basin.

alterations of outcrop and deeply buried sandstones were 

made by examination of both surface and subsurface 

samples.

Acknowledgments. This report is a product of the 

U.S. Geological Survey Evolution of Sedimentary Basins 

Program. Gary L. Skipp was responsible for sample 

preparation and heavy-mineral separations. KJ. 

Esposito prepared specimens and generated X-ray diffractograms. April K. Vuletich determined the isotopic 

values for carbonate samples. E.A. Merewether 

graciously took the senior author to outcrops of the 

Shannon in the Salt Creek area and helped search for 

outcrops elsewhere in the basin. Greg Anderson of 

Home Petroleum Company, Denver, provided core 

(D568) from a producing reservoir sandstone in the 

Shannon for petrologic analyses.

RESEARCH DESIGN

The petrology of the Shannon Sandstone Beds was 

examined in samples taken from 10 cores (table 1) 

collected from drillholes along strike of the Shannon 

from Teapot Dome to the Wyoming-Montana State line 

(fig. 1). The Shannon in the cores is 12.3-39.0 m thick. 

Two cores (C221, D568) contain oil-producing intervals. 

Samples were also collected from outcrops of the 

Shannon at Salt Creek (fig. 1, locality SC) and from 

outcrops of the Shannon equivalent, the Groat Sandstone 

Bed of the Gammon Ferruginous Member of the Pierre 

Shale, in the eastern part of the basin (fig. 1, localities 

OC, HC, and HD). Samples of the Steele Member were 

collected from the cores for vitrinite reflectance studies 

so that the burial history of the Shannon could be 

assessed (Nuccio, 1990).

Petrographic thin sections were prepared from all 

sandstone samples collected from core and outcrop. The 

mineralogy of the Shannon was quantified (by volume) 

by counting 300 points each of selected thin sections.

Upper Cretaceous Shannon Sandstone Beds, Wyoming C3
ary Basins PowderRiver Basin

Table 1. Outcrops and cores of the Shannon Sandstone Beds of the Steele 

Member of the Pierre Shale, Wyoming and Montana 

[Locations shown by USGS designation (cores) or by two-letter code (outcrops) on fig. 1.

Name

Felix and Scisson, Inc. 63 5 x 10 Woods Petroleum Corp. 13-9 Pine Tree Unit Woods Petroleum Corp. No. 7-22 Diamond Shamrock Corp. No. 14-4 

Southland Royalty Co. Van Irvine #3

Woods Petroleum Corp. Knight State 16-1 Davis Oil Company No. 10 

Southland Royalty Co. Beaver Creek Federal No. 11 

Woods Petroleum Corp. Marton-Federal No. 20-1 

Home Petroleum Co. H-83 Culp Draw Unit

Oil field

Teapot Dome 

Wildcat Pine Tree Hartzog Draw 

Wildcat

Heldt Draw Heldt Draw 

Wildcat 

Wildcat 

Hartzog Draw

Member of the Cody Shale and of the Groat Sandstone Bed of the Gammon Ferruginous 

Leaders (--) indicate not applicable]

USGS 

designation

C221 C652 C651 

A915 A817

A771 B626 

A729 A232 

D568

Deoth

Location

Sec. 10, T. 38 N., R. 78 W. Sec. 13, T. 41 N., R. 76 W. Sec. 7, T. 42 N., R. 75 W. Sec. 4, T. 43 N., R. 74 W. Sec. 22, T. 44 N., R. 77 W.

Sec. 16, T. 45 N., R. 76 W. Sec. 36, T. 46 N., R. 76 W Sec. 1, T. 48 N., R. 78 W. 

Sec. 20, T. 50 N., R. 80 W. Sec. 5, T. 45 N.,R. 76 W.

(feet)

245-366 

10,790-10,840 10,193-10,252 

9,325-9,384 9,885-9,9479,445-9,480 9,365-9,418 

8,580-8,618 

9,526-9,554 

9,495-9,544

(meters)

74.7-111.6 

3,289.6-3,304.9 3,107.6-3,125.6 2,843-2,861 3,013.7-3,032.6

1,879.6-1,890.2 

2,855.2-2,871.3 

1,615.9-2,627.4 

2,904.3-2,912.8 2,894.8-2,909.8

Salt Creek (SC) 

Owl Creek (OC)* Horse Creek (HC)* 

Hammond (HP)*

Sec. 15, T. 39 N., R. 79 W. Sec. 26, T. 57 N., R. 2 E. Sec. 24, T. 9 S., R. 61 E. Sec. 26. T. 6 S.. R. 56 E.

*Groat Sandstone Bed.
Each thin section was stained with alizarin red-S for 

calcite identification and with potassium cobaltinitrate 

for potassium feldspar identification. Polished thin 

sections of selected samples were also prepared for 

reflected light and scanning electron microscope (SEM) 

studies. Qualitative chemical analysis and elemental 

mapping were conducted on selected polished sections 

using the SEM energy-dispersive X-ray system (SEMEDX). Heavy minerals (sp gr>2.87) were separated 

from light minerals by gravity settling in bromoform; 

magnetic heavy minerals were separated from nonmagnetic heavy minerals using a hand magnet, and nonmagnetic heavy fractions were mounted on petrographic 

slides using canada balsam. Clay mineralogy was 

determined on the < 2-fita. fraction of sandstones using 

X-ray powder diffraction after air drying, saturation with 

ethylene glycol, and heat treatment at 550 °C. 

Examination of the cathodoluminescence of minerals 

under a Luminoscope enabled discrimination between 

detrital and authigenic phases that appeared 

homogeneous in transmitted light. A few typical 

sandstones containing one major carbonate phase were 

selected for isotope analyses. Stable isotope values (818 O 

and 813C) were determined for siderite clasts and beds, 

detrital dolomite, and calcite.

DEPOSITIONS. ENVIRONMENT

The Shannon Sandstone Beds are considered to 

be a classic example of a mid-shelf sandstone (Tillman 

and Martinsen, 1984, 1987). Shannon sand ridges are 

interpreted to have formed at least 100-160 km from 

shore on a wide shelf along the western margin of the 

Western Interior Cretaceous seaway (Tillman and Martinsen, 1984,1987). This interpretation is based primarily 

on a lack of shoreline facies and associated diagnostic 

trace fossils in the Shannon and on intertonguing of the 

Shannon both landward and seaward with marine shale. 

The position of the shoreline during Shannon deposition 

has been defined by ammonite zonation, mapping of 

strandlines, and K/Ar dating of bentonites (fig. 4) (Gill 

and Cobban, 1973). Shannon sand ridges are thought to 

have been deposited and shaped by storm currents 

flowing south-southwest (essentially parallel with the 

shoreline) across the shelf, transporting sediment from 

shoreline sands of the Eagle Sandstone about 320 km to 

the north in present-day southern Montana (Tillman and 

Martinsen, 1984). Gaynor and Swift (1988), however, 

proposed that the source for Shannon sand was 

"submarine erosion of the Montana shelf by currents 

that produced a southward-extending sheet of muddy 

sand on the shelf. Shannon sand ridges were then built 

vertically on this thin layer of sediment by a complex 

interaction of storm, tidal, and longshore currents. The 

greatest accumulation of sand was in the Salt Creek

anticline area (drillhole location C221, fig. 1), which may 

have been an actively growing paleostructural high 

around which sedimentation focused during Shannon 

deposition (Tillman and Martinsen, 1985).

According to the sea-level curves of Haq and 

others (1987), a eustatic rise occurred from about 83 to 

81 Ma. Strandline reconstruction shows that at least one 

minor regression also occurred during this interval (Gill 

and Cobban, 1973, fig. 13). The Shannon was apparently 

deposited during this transgressive interval, about 

81.5-81 Ma, by intermittent storm currents. The 

mechanism for concentrating sand into vertically accreting, coarsening-upward, linear ridges by storm-driven 

currents has been obscure, but recently Gaynor and Swift 

(1988) presented a complex model that explains the 

dynamics of ridge formation. They proposed that the 

Shannon is composed of beds and groups of beds that 

were deposited by storm-surge events; beds of a single 

event are separated by disconformities representing 

varying lengths of time. According to their model, the 

base of the Shannon, which is commonly marked by a 

zone of phosphatic pebbles, disconformably overlies the 

lower part of the Steele Member.

112° 110° 108° 106° 104° 102° 100°

45° MONTANA 

WYOMING

IDAHO

_J

L:

UTAH

Rock 

Springs

NORTH DAKOTA

Rapid City

SOUTHjOAKqjA_ 

NEBRASKA

11 COLORADO

0 50 100 KILOMETERS

EXPLANATION

V Strandline defined by Baculities sp. (smooth)

^! Approximate position of Shannon sand ridges

- - - - Proposed shelf edge

 Boundary of Powder River Basin

Figure 4. Inferred location of Strandline defined by Baculites sp. (smooth) at the time of deposition of the Shannon 

Sandstone Beds. Generalized position of Shannon sand 

ridges and the edge of continental shelf are also shown. 

Compiled from Gill and Cobban (1973), Spearing (1976), and 

Seeling (1978).

Upper Cretaceous Shannon Sandstone Beds, Wyoming C5
The closest modern analogue to Shannon sand 

ridges on the Campanian shelf are mid-shelf sand bodies 

on the present-day Atlantic continental shelf off the east 

coast of the United States (Swift, 1985). Sizes of modern 

and ancient sand-ridge fields are similar. A typical cluster 

of Shannon sand ridges is 2.4 by 17.6 km at the Heldt 

Draw oil field, whereas a typical cluster of ridges on the 

modern continental shelf is 1.6-3.2 km by 16 km. Like 

Shannon sand ridges, modern ridges also rest on a 

transgressive surface. The formation of modern (and 

ancient) sand ridges, however, is a subject of debate. 

Some workers have proposed that modern sand ridges 

are storm- or tidal-built complexes that are presently 

forming on the shelf (Huthnance, 1982; Swift and others, 

1986; Gaynor and Swift, 1988). Others contend that 

mid-shelf sand ridges are reworked barrier island or 

shoreface sequences. They argue for a multiple-stage 

origin for these linear sand bodies beginning with 

deposition as part of a shoreline sequence (accounting 

for their linearity), subsequent degrading and reworking 

during a transgression, and then, finally, reworking and 

aggrading by shelf currents into their present forms 

(Beaumont, 1984; Kiteley and Field, 1984; Stubblefield 

and others, 1984).

Some differences exist between modern and 

ancient ridges. For example, ancient sand ridges are 

generally farther from shore than modern ridges (Rice 

and Shurr, 1983). Some modern ridges (those closer to 

shore), unlike those of the Shannon, are oriented 

obliquely (5°-10°) to the coastline, an apparent anomaly 

that may be related to geotrophic storm current flow 

(Huthnance, 1982). Mid-shelf ridges, however, are 

oriented parallel with the coastline; thus, not all shelf 

ridges may form by the same process (Stubblefield and 

others, 1984). The Shannon sand ridges, unlike modern 

ridges on the Atlantic shelf, are restricted to a narrow 

band on the shelf. Modern sand ridges rest on a sandy 

shelf, whereas Shannon sand ridges extend upward from 

a muddy shelf and have a higher angle of repose. The 

angle of repose of a sediment has been interpreted to be 

directly related to grain size of shelf sediments; thus, 

deposition of the Shannon on a muddy rather than a 

sandy shelf may have resulted in the higher angle of 

repose for Shannon sand ridges (Swift and Rice, 1984).

Inferred water depth of 20-90 m during Shannon 

deposition is based on foraminiferal evidence from the 

Steele Member: most foraminifera are those 

characteristic of the inner shelf to shallow mid-shelf, and 

planktonic varieties are rare (Tillman and Martinsen, 

1984); however, the Steele has an impoverished fauna 

characterized by low diversity of species. Lack of diversity 

may be due to factors such as low pH or abnormal 

salinity rather than to water depth. Interestingly, no 

foraminifera have been found in sideritic shale interbedded with sandstone of the Shannon.

Table 2. Correlation of depositional facies in Shannon 

Sandstone Beds as described herein with facies of the 

Shannon as defined by Tillman and Martinsen (1984)

This paper Tillman and Martinsen (1984)

la 

Ib

2

3

4

Ridge-margin sandstone 

Central ridge sandstone 

Inter-ridge sandstone 

Bioturbated shelf sandstone 

Shelf silty shale_______

Petrologic and sedimentologic data gathered 

during the present study indicate some characteristics of 

the Shannon that need to be addressed by the sand-ridge 

model: (1) abundant detrital dolomite grains and clasts; 

(2) large siderite clasts at the top and bottom of crossbedded sandstone facies; (3) coal fragments and partings; 

(4) sideritic shales barren of foraminifera within the 

Shannon; (5) siderite and glauconite in the same bed; and 

(6) abrupt facies changes.

SEDIMENTOLOGY

The studied cores were divided into four 

descriptive facies: (1) fine- to medium-grained 

glauconitic sandstone containing locally common rip-up 

clasts and minimal burrowing; (2) very fine to fine 

grained, burrowed (<75 percent burrowing) sandstone; 

(3) very fine to fine grained, bioturbated (> 75 percent 

burrowing) sandstone; and (4) dark-gray silty shale 

containing thin discontinuous sandstone laminae and 

rare to moderate burrowing. Correlation of these 

descriptive facies with genetic facies, as defined by Tillman and Martinsen (1984,1985,1987), is shown on table 

2; facies in each core are described in the appendix.

Facies 1 has the best reservoir potential because of 

its relatively high porosity and permeability and larger 

grain size. It can be divided into two sub-facies: (la) lowto medium-angle, trough crossbedded, glauconitic 

sandstone containing locally abundant shale and siderite 

rip-up clasts (fig. 5), and (Ib) moderate-angle trough 

crossbedded to planar-laminated sandstone in which 

glauconite is less common and siderite clasts are rare. 

Abundant glauconite in facies la may produce a distinct 

green color. Facies 2 is characterized by discontinuous, 

horizontal, rippled sandstone and shale laminae that are 

truncated by horizontal, inclined, and (rarely) vertical 

burrows (fig. 6). Facies 3 lacks bedding and locally is 

mottled due to extensive burrowing. The only identified 

burrows are Terebellina or "donut" burrows (fig. 7), 

Zoophycos, and Chondrites. The diversity of burrowers is 

probably greater inasmuch as Tillman and Martinsen 

(1987) found 6-10 types of burrows, including Terebellina, Teichichnus, Zoophycos, Chondrites, and Planolites, 

in a detailed sedimentological study of the Shannon at the

C6 Evolution of Sedimentary Basins Powder River Basin
Hartzog Draw field. Facies 4 is made up of laminated or 

massive shale and rare sandstone lenses containing minor 

burrowing and current ripples (fig. 8). The shale may be 

dolomitic, and bedded siderite is common (fig. 9). This 

facies is barren of foraminifera (Tillman and Martinsen, 

1984).

The extensive burrowing and dominance of 

horizontal burrows in facies 2, 3, and 4 indicate a low to 

moderate sedimentation rate below wave base. Terebellina is thought to have lived in water at least 20 m deep 

(Chamberlain, 1978), but it, like Chondrites, can exist in 

a wide variety of depths from nearshore (about 10 m 

depth) to offshore (>60 m water depth) (Seilacher, 

1978). The occurrence of organisms is controlled by 

many factors, only one of which is water depth; therefore, 

strict bathymetric interpretations based on trace fossil 

data are tenuous. Trace fossils give only a general 

indication of water depth in the case of the Shannon, as 

has been interpreted as a mid-shelf setting. Other criteria 

must be used to better define the environment of 

deposition.

Figure 5. Abundant shale rip-up clasts characteristic of 

facies 1a in the Shannon Sandstone Beds; core A915 at 

9,348 ft. Core is 4 in. in diameter.

Figure 6. Burrowed sandstone characteristic of facies 2 of 

the Shannon Sandstone Beds; core D568. Core is 4 in. in 

diameter.

Coal fragments were observed in several cores, and 

apparent coal partings were found at the top of facies 2 in 

core A817 (fig. 10).

PETROLOGY

Sandstones from deeply buried cores ( > 2,500 m) 

are litharenites, whereas those from the outcrops and 

shallow cores (<100 m) are feldspathic litharenites 

according to Folk's (1974) classification (fig. 11). 

Shallowly buried and outcropping sandstones are plotted 

together, separate from deeply buried sandstones. 

Deeply buried sandstones contain less feldspar (fig. 11), 

mostly because of a loss of potassium feldspar (table 3). 

In Folk's classification, monocrystalline and polycrystalline quartz are grouped together, granitic and gneissic 

fragments are combined with feldspar, and chert is 

considered to be a rock fragment. Grain size of 

sandstones in the study area is from very fine to medium

Upper Cretaceous Shannon Sandstone Beds, Wyoming C7
Figure 7. Terebellina ("donut") burrows in bioturbated 

sandstone of facies 3 of the Shannon Sandstone Beds; core 

A232. Core is 4 in. in diameter.

Figure 8. Horizontally laminated black shale and interbedded sandstone with minor burrowing in facies 4 of the 

Shannon Sandstone Beds; core D568. Core is 4 in. in 

diameter.

grained, and the coarser sandstones are in the crossbedded and planar-laminated sandstone facies (facies la 

and Ib). Because these sandstones contain many rock 

fragments, grain size must be taken into account when 

interpreting sandstone classification.

Framework Mineralogy

Point-count analyses for 57 petrographic thin 

sections are shown in table 3. Major framework 

constituents are subangular to subrounded quartz, feldspar grains, and rock fragments. Monocrystalline quartz 

grains are generally free of inclusions and exhibit straight 

extinction. Polycrystalline quartz grains are probably 

plutonic in origin because they do not have sutured grain 

contacts. Plagioclase grains are untwinned or display 

simple twins and are of albite to andesine composition. 

The amount of plagioclase in shallow and deep samples is 

highly variable due to differing degrees of diagenesis. 

Potassium feldspar (microcline, orthoclase, perthite) is

most abundant in outcrops and shallowly buried 

sandstones from the Teapot Dome oil field (fig. 1, 

locality C221) but has been replaced almost totally by 

calcite in sandstones from greater depths (fig. 12). Very 

fine grained sandstones generally contain more feldspar 

than coarser grained sandstones.

Major lithic fragments are chert, glauconite, 

dolomite, and siderite; minor varieties are igneous (plutonic and volcanic), sedimentary (argillaceous, 

phosphatic, and siltstone), and metamorphic (schist and 

phyllite). Angular to subangular chert is the most 

abundant rock fragment. Glauconite pellets (fig. 13) 

make up as much as 45 percent of rock fragments and 21 

percent of total framework grains in some sandstones, 

especially in the crossbedded sandstone facies (1). In 

some beds they are so abundant as to give the sandstone 

a green color. Compaction has transformed some of the 

soft glauconite grains into pseudomatrix. Because 

glauconite pellets are composed mostly of interstratified 

glauconite-smectite, a diagenetic phase, their mineralogy 

is discussed later in the section on diagenetic alterations.

C8 Evolution of Sedimentary Basins Powder River Basin
Figure 9. Bedded siderite (gold) and abundant glauconite 

(dark grains) in facies 4 of the Shannon Sandstone Beds 

overlying sandstone containing shale rip-up clasts in facies 

1a; core A915 at 9,352.7 ft. Core is 4 in. in diameter.

Detrital monocrystalline (fig. 14) and polycrystalline (fig. 15) dolomite grains make up as much as 42 

percent of rock fragments in fine-grained sandstones. 

The paucity of dolomite in fine- to medium-grained 

sandstones of the Shannon reflects the susceptibility of 

dolomite to abrasion in strong currents. Several criteria 

were used to determine whether single dolomite grains 

are detrital or authigenic: abraded surfaces on grains, 

positive size correlation with detrital quartz, clay rims on 

grains, composite grains, and round cores under euhedral overgrowths (table 4). A key test was examination of 

dolomite grains under a Luminoscope: detrital and 

authigenic parts of grains exhibited different degrees of 

cathodoluminescence so that some grains, which were 

apparently homogeneous under transmitted light, were 

seen to consist of rounded (detrital) cores within euhedral (authigenic) dolomite, ankerite, siderite, or other 

carbonate overgrowths (fig. 16). In addition, many 

dolomite grains have subrounded (interpreted to be

abraded) surfaces coated by authigenic clay (fig. 17) and 

are sorted by size in the sandstones that is, dolomite 

grains are just slightly smaller than associated quartz 

grains because of the higher density of dolomite.

Detrital siderite clasts (as much as 10 cm in 

diameter in the cores) make up as much as 22 percent of 

rock fragments in some sandstones of the Shannon 

(Ranganathan and Tye, 1986). Clasts are rounded to 

oblate and commonly are oriented parallel with bedding. 

They are associated with shale rip-up clasts and are most 

abundant at the top or base of facies la (fig. 18). The 

clasts are considered to be detrital because (1) they 

contain silt-size quartz and feldspar grains embedded in 

a micritic siderite matrix that is considerably finer 

grained than the sandstone in which the siderite clasts are 

found, and (2) bedding does not extend through the 

clasts. Siderite is present as lenses in facies 4 and as small 

rhombic crystals in facies 1 (see section on diagenetic 

alterations).

Minor framework constituents include muscovite, 

various heavy minerals (sp gr > 2.87), and organic matter 

such as fossil fragments and plant material. The most 

common heavy minerals noted in grain mounts are 

biotite, chlorite, dolomite, zircon, tourmaline, and apatite. Minor rutile, garnet, and hornblende were also 

observed. Biotite and chlorite compose more than half of 

most heavy-mineral fractions. Dolomite, which has a 

specific gravity of about 2.87, is discussed above. Zircons 

are colorless (rarely light pink) and well rounded; only a 

few euhedral grains were noted. Tourmaline is well 

rounded and gray brown to pink. Garnet is colorless or 

light pink and exhibits minor etch features. One or two 

rounded, dark-red rutile grains were observed in each 

sample. Elongate hornblende grains exhibiting distinct 

pleochroism (Z, yellow brown or olive green; X, green) 

and small extinction angles (< 15°) are locally common.

Opaque heavy minerals, mainly iron-titanium 

oxide minerals, are present in each sample.

Diagenetic Alterations

Diagenetic alterations are most pronounced in 

coarser sandstones of facies 1 in deep cores. Major 

authigenic minerals are siderite, pyrite, quartz, chlorite, 

glauconite-smectite, kaolinite, calcite, dolomite, ferroan 

carbonates, and albite. Minor authigenic minerals 

include apatite, hematite, barite, anhydrite, and 

potassium feldspar. Both intragranular and intergranular 

secondary porosity are well developed locally in the 

cross- and planar-bedded sandstones of facies 1. 

Framework grain alteration is mostly confined to feldspars. Potassium feldspar grains are relatively unaltered 

in shallow samples of all facies but have been replaced by 

calcite or albite in the deep samples. In very fine grained

Upper Cretaceous Shannon Sandstone Beds, Wyoming C9
Figure 10. Coal partings at the top of facies 2 of the Shannon Sandstone Beds; 

core A817 at 9,920.6 ft. Core is 4 in. in diameter.

sandstones of deep cores, however, potassium feldspars 

are unaltered, possibly because of the low permeability of 

the sandstones. Plagioclase exhibits varying degrees and 

types of alteration, from skeletalization in shallow 

samples to albitization in deeper samples.

Siderite

Small (<5 fim) rhombic siderite crystals 

commonly form a cement in sandstones containing siderite clasts. The siderite crystals formed early in the 

depositional history of the Shannon as indicated by their 

precipitation directly on the margins of framework grains 

and by their occurrence at grain contacts. They have been 

replaced locally by calcite cement (fig. 19). Qualitative 

chemical analyses made using the SEM-EDX system 

show that the siderite contains more calcium than most 

reported siderites. Some overgrowths on detrital 

dolomite grains have the composition of siderite (fig. 20).

Pyrite

Pyrite was most commonly found in samples that 

also contain organic matter and oil, notably in samples 

from core C221 (fig. 1). Different pyrite morphology of 

and textural relations between pyrite and other 

authigenic phases indicate that more than one generation 

of pyrite is present. Apparently late, framboidal pyrite 

(formed by bacterially mediated sulfate reduction after 

uplift) fills pores rimmed by quartz overgrowths (fig. 21),

pyrite euhedra are intergrown with authigenic chlorite 

(fig. 22), and micron-size (early?) euhedral pyrite is in 

glauconite grains and within patches of calcite cement. 

SEM-EDX spectra of pyrite euhedra from samples from 

the Teapot Dome oil field (fig. 1, locality C221) indicate 

that the pyrite lacks sulfur; the crystals are probably iron 

oxide pseudomorphs after pyrite (fig. 23).

Quartz

Detrital quartz grains in facies 1 from deep cores 

are commonly cemented by syntaxial quartz overgrowths 

that are coated with oil (fig. 24) and locally etched (fig. 

25) in reservoir sandstones. Quartz overgrowths 

generally overlie chlorite rims, but authigenic chlorite 

and quartz are intergrown in some samples (fig. 26).

Chlorite

Chlorite was found in relatively deep cores 

( > 2,500 m) but not in outcrop or shallow core samples. 

It is most commonly a coating or alteration product (?) of 

glauconite grains, but it also rims framework grains. 

Grain coatings of chlorite lie underneath albite and 

quartz overgrowths (fig. 27); chlorite-lined pores are 

commonly filled with (later) calcite cement. In some 

samples, chlorite and quartz overgrowths appear to have 

precipitated at the same time (see fig. 26).

C10 Evolution of Sedimentary Basins Powder River Basin
QUARTZ

QUARTZARENITE

SUBARKOSE SUBLITHARENITE

FELDSPAR AND GRANITIC 

ROCK FRAGMENTS

ROCK FRAGMENTS 

AND CHERT

Figure 11. Composition (as determined from point counts) of sandstones from the Shannon and Groat Sandstone Beds. 

Circles indicate outcrop and shallow samples (< 150 m depth); triangles indicate deep samples (> 2,500 m depth). Sandstone 

classification diagram from Folk (1974).

Calcite

Calcite that precipitated before significant 

compaction is found between muscovite plates and as a 

poikilotopic cement in outcrop and near-surface 

sandstones. In sandstones that were cemented by early 

calcite, chlorite and quartz overgrowths are rare, grain 

contacts are tangential, and skeletal plagioclase grains 

are within calcite-cemented areas (as, for example, in 

samples C221-253, -256, -262). Ferroan calcite cement 

(possibly a replacement of earlier carbonate cement) is 

found in the Groat Sandstone Bed. In deeply buried 

sandstones, calcite cementation followed chlorite and 

quartz overgrowth precipitation. This later calcite 

cement partly to completely replaces and (or) infills most 

potassium feldspar in fine- and medium-grained 

sandstones (fig. 28). Complete replacement is inferred by

a spectrum of replacement textures ranging from 

potassium feldspar partly replaced by calcite to grainsized patches of calcite devoid of potassium feldspar. In 

many samples, patches of calcite cement in optical 

continuity are evidence of later calcite dissolution.

Glauconite-Smectite

The term "glauconite" has had a dual meaning for 

many years. Glauconite is the mineralogical name for a 

specific iron-rich mica that is distinguished from celadonite and other micas by chemical composition, d(060) 

value, and infrared spectral characteristics (Bailey, 1980; 

Bailey and others, 1984). A naked-layer mineral 

composed of interstratified glauconite and smectite 

layers is termed interstratified glauconite-smectite. The 

identification of glauconite as a mineral is independent of

Upper Cretaceous Shannon Sandstone Beds, Wyoming C11
C12

Table 3. Point counts of selected thin sections of the Shannon Sandstone Beds and Groat Sandstone Bed, Powder River Basin

[Core and outcrop localities shown on fig. 1 and given in table 1. Abbreviations: Rk Frag: rock fragments; og, overgrowth; Other: anhydrite, barite

overgrowths]

Sample <t> Qtz K-feldspar Plag Rk Frag Glauconite Dolomite Opaques Muse Biotite Clay Calcite Qtzog A

A771

9,449.5

9,453

9,456

9,457

9,459.5

9,467.5

9,475

2.10

2.10

2.00

2.00

2.30

115

148

168

137

135

115

113

0

0

0

0

0

0

1

6

13

3

4

9

7

11

64

54

53

50

55

41

41

24

27

9

22

13

7

6

0

4

0

3

0

22

12

0

0

0

0

0

1

0

0

1

0

0

0

2

0

0

1

0

0

0

2

2

23

25

39

34

25

87

101

4

2

4

3

14

5

5

63

18

17

26

30

11

7

A232

9,527

9,540

9,548

3.20

3.30

3.25

112

110

128

2

6

4

21

4

8

34

30

54

31

19

29

12

18

3

0

0

2

0

0

1

1

0

1

75

105

42

5

3

23

5

0

5

C652

10,800

10,807

10,818

10,828

3.00

3.00

3.25

3.75

118

102

118

95

1

0

2

2

20

21

16

15

50

51

55

48

12

17

7

1

24

18

12

36

1

1

0

1

0

0

1

2

2

0

0

8

32

23

40

73

4

0

7

1

27

62

33

15

A729

8,580

8,587.5

8,588

8,599.5

8,614

2.50

2.25

2.20

2.50

1.90

87

139

143

142

138

0

0

0

0

0

24

22

10

25

16

56

55

68

32

56

46

7

3

7

15

3

0

0

10

0

0

0

0

1

0

0

1

1

0

0

5

0

0

1

0

56

22

15

16

22

1

7

7

0

6

20

43

47

47

45

C221

253.8

269.2

300

334

337.2

353

358

362

365

2.00

2.50

3.00

2.75

3.00

2.75

3.00

3.00

3.00

123

127

117

141

131

106

109

109

97

4

12

13

8

15

10

17

9

13

2

18

8

26

8

14

8

9

9

59

45

38

64

46

53

43

42

47

35

16

14

7

15

14

13

7

6

0

0

15

4

6

15

22

22

25

0

0

1

2

0

1

0

0

0

0

1

1

0

1

2

0

0

2

0

0

1

1

2

1

2

3

7

2

11

77

4

32

46

63

94

90

75

0

1

4

0

4

2

0

1

0

4

0

0

0

4

0

0

0
Table 3. Continued

O

5

a

Sample <j> Qtz K-feldspar Flag Rk Frag Clauconite Dolomite Opaques Muse Biotite Clay Calcite Qtzog A

D568

9,507 2.00 

9,509.8 2.25 

9,512 2.25 

9,513.5 0.75 

9,515.5 1.75 

9,526.5 3.50 

9,535 3.25 

9,542 3.25

110 

121 

114 

87 

100 

83 

117 

125

1 

0 

0 

0 

0 

6 

3 

3

4 

12 

9 

1 

12 

7 

12 

4

74 

60 

56 

128 

52 

4 

63 

40

20 

18 

20 

5 

28 

3 

7 

7

1 

3 

0 

0 

1 

23 

18 

20

0 0 

0 0 

0 2 

0 0 

1 0 

0 1 

0 0 

0 0

11 

1 

0 

0 

1 

1 

1 

0

80 

44 

26 

57 

73 

93 

44 

83

0

2 

1 

4 

0 

43 

2 

2

0 

26 

44 

0 

30 

0 

26 

9

C651

10,198 3.00 

10,202 2.50 

10,210 2.00 

10,227 1.60 

10,230 2.25 

10,234 2.25 

10,248 3.25

108 

117 

125 

113 

111 

136 

120

3 

0 

0 

0 

1 

0 

0

10 

16 

12 

3 

13 

17 

22

52 

69 

53 

75 

32 

49 

59

20 

42 

13 

23 

5 

11 

15

21 

1 

5 

0 

4 

2 

11

0 0 

0 0 

0 0 

0 0 

0 0 

1 0 

0 0

0 

1 

0 

0 

0 

0 

3

57 

19 

42 

38 

103 

48 

40

6 

6

3 

2 

3 

0 

7

14 

18 

37 

35 

26 

28 

21

A817

9,899.6 2.75 

9,907 2.50 

9,911.4 2.50 

9,917.6 3.00 

9,931 3.50

126 

118 

121 

139 

113

0 

0 

0 

0

15

31 

25 

23 

20 

16

36 

77 

23 

59 

60

12 

16 

23 

16 

5

4 

2 

1 

3 

31

0 0 

0 1 

0 0 

0 0 

1 0

1 

1 

1 

2 

0

16 

5 

41 

25 

43

3 

8 

2 

10 

7

30 

31 

26 

11 

10

B626

9,391 2.50 

9,404.5 2.50

128 

105

0

2

24 

21

52 

60

28 

12

13 

0

0 1

1 1

0 

0

15 

1

7 

93

26 

2

*HORSE CREEK (HQ

3 3.00 

5 1.50 

6 2.75

103 

52 

89

20 

3 

10

3 

0

4

64 

104 

61

18 

51

24

11 

2 

8

1 0 

0 0 

1 0

1

0

1

5 

7 

2

41 

79 

97

0 

0 

0

*HAMMOND (HD)

1 2.75 97 13 7 52 4 6 0 0 1 3 116 0 

SALT CREEK (SC)

2 2.75 

4 2.50 

6 2.75

133 

110 

124

12 

18 

16

16 

3 

26

49 

46 

77

2 

3 

10

8 

0 

0

0 2 

0 0 

0 0

0

1 

1

9

5 

8

19 

114 

1

0 

0 

0

(O

*Groat Sandstone Bed.

O 

o>
40

30

UJ

O 

QC 

UJ 

Q.

O

20

10

ir- A

O

5 10 15 

POTASSIUM FELDSPAR, IN PERCENT

20

Figure 12. Plot of plagioclase versus potassium feldspar 

abundance (percent of normalized framework grains) as 

derived from point counts of petrographic thin sections of 

the Shannon and Groat Sandstone Beds. Solid circles indicate 

outcrop samples; open circles indicate shallow core samples 

(<120 m depth); solid circles indicate deep core samples 

(>2,500m depth).

its mode of origin. Glauconite also has historically been 

used as a morphological term for sand-size greenish 

grains that are commonly thought to be fossil fecal pellets 

(McRae, 1972). When used as a rock term in this 

manner, it does not imply a particular mineralogical 

composition, though glauconite pellets commonly are 

rich in the mineral glauconite. Odin and Fullagar (1988) 

attempted to clarify the terminology by introducing the 

term "glaucony" as a facies term for sediments 

containing the green pellets, and "glauconitic mineral" 

for the minerals that make up such sediments. We use 

the term "glauconite" as a morphological or rock term, 

and we use the specific mineralogical term "glauconitesmectite" for the interstratified mineral.

Glauconite pellets in the Shannon are composed 

dominantly of interstratified glauconite-smectite having a 

high proportion of smectite layers in shallow samples and 

a low proportion of smectite layers in the deeper samples. 

A d(060) of 1.52 A confirms that the phase is glauconitesmectite. The proportion of smectite layers in the

glauconite-smectite from the shallow samples (75-112 

m) is from 40 to 60 percent. Deeper samples 

(2,640-3,333 m) have expandabilities of 10-20 percent. 

Well-developed lamellae of glauconite in the deeper 

samples, as seen under the SEM (fig. 29), are 

characteristic of older, highly diagenetically evolved 

glauconite (Odin and Morton, 1988). Both the shallow 

and deep samples commonly contain discrete mica and 

discrete chlorite (fig. 30).

Albite

Albitized plagioclase and potassium feldspar grains 

and albite overgrowths on plagioclase and on albitized 

potassium feldspar grains were found in deep core 

samples ( > 2,500 m). Albitized potassium feldspar grains 

are similar in appearance to "chessboard" albite (fig. 31) 

as described by Walker (1984); however, SEM-EDX 

analysis did not detect any residual potassium in the 

albite, but perhaps the sensitivity of the instrument was 

not high enough or replacement of the potassium feldspar by albite was complete. Some potassium feldspar 

grains have been both partly albitized and partly calcitized.

Minor Authigenic Phases

Apatite cement surrounds framework grains in 

sample D568-9,513.5, which is a poorly sorted sandstone 

that contains large (as much as 10 jim) foraminiferal, 

plant, and phosphatic rock fragments. Apatite cement 

around sedimentary fragments that have an apatite 

matrix suggests that apatite in the detrital fragments was 

dissolved and reprecipitated in the Shannon. Euhedral 

potassium feldspar overgrowths are rare in the Shannon 

but were noted in samples C221-340 and C221-353 and 

in the Groat Sandstone Bed underlying ferroan calcite 

cement. Small patches of barite and anhydrite cement 

were observed in only a few thin sections.

Small (< 5 jun) euhedral dolomite grains in very 

fine grained sandstones may be authigenic. Ferroan 

dolomite overgrowths are locally common on detrital 

dolomite grains (fig. 32), particularly in cores C652 and 

A771.

Secondary Porosity

In near-surface and surface sandstones, moldic 

porosity has resulted from dissolution of plagioclase (fig. 

33). In places, ragged edges of framework grains and 

patches of calcite cement in voids imply the dissolution of 

calcite cement. Intragranular secondary microporosity 

has resulted from partial dissolution of chert grains.

C14 Evolution of Sedimentary Basins Powder River Basin
.,.,/*$" ''

, ^*'

Q

* .

<*--

Figure 13. Photomicrograph of detrital glauconite grains (G) and quartz (Q). V, void filled 

with blue-dyed epoxy. Core A771 at 9,457 ft. Plane-polarized light; field length 1.2 mm.

Figure 14. Photomicrograph of rounded detrital dolomite grain (D) with dolomite 

overgrowth (Do). V, void. Core C221 at 334 ft. Plane-polarized light; field length 0.34 mm.

Upper Cretaceous Shannon Sandstone Beds, Wyoming C15
Figure 15. Photomicrograph of detrital polycrystalline dolomite clast (D) and quartz 

(Q). Core C221 at 276 ft. Crossed nicols; field length 0.43 mm.

Figure 16. Cathodoluminescence micrograph showing (1) detrital dolomite grain (red) 

and calcite overgrowth (orange); (2) calcite grain (orange) with dolomite overgrowth 

(red); and (3) dolomite grain (red) with dolomite overgrowth (red). Core A771 at 9,475 ft. 

Field length 1.5 mm.

C16 Evolution of Sedimentary Basins Powder River Basin
Table 4. Identification criteria for detrital dolomite

Grains have abraded surfaces

Positive size correlation with detrital quartz

Clay rims on grains

Composite grains

Round cores under euhedral overgrowths as revealed by:

Cathodo luminescence

X-ray fluorescence elemental mapping_______

Figure 17. Scanning electron micrograph of detrital 

dolomite grain (D) and clay rim (C). Core C652 at 10,807 ft. 

Field length 0.15 mm.

Figure 18. Golden-brown detrital siderite clasts (S) lying 

parallel with bedding at base of facies 1a of the Shannon 

Sandstone Beds; dark grains are glauconite. Core C221 at 

253.8 ft. Core is 4 in. in diameter.

Minor etching of quartz grains was noted in some oilbearing sandstones (see fig. 25).

Oil

During SEM analysis, oil droplets were noted on 

the surfaces of quartz overgrowths that project into 

secondary pores (see fig. 24). Oil was also observed on 

quartz overgrowths in thin section (fig. 34).

ISOTOPY

isotopes were fractionated. Samples containing a high 

percentage of one carbonate phase (such as calcite, 

siderite, or dolomite) were selected for 613C and 818O 

analyses. Samples were pulverized, isolated under a 

vacuum, and then reacted with 100 percent phosphoric 

acid resulting in the evolution of CO2 (McCrea, 1950). In 

samples containing calcite and siderite, the CO2 evolved 

during the first 3 hours was assumed to be from calcite; 

the CO2 evolved over the next 93 hours was assumed to 

be from siderite. Two samples containing only dolomite 

as a carbonate phase were also analyzed. Isotopic data 

are shown on table 5.

Procedure

The stable isotope geochemistry of minerals can 

give valuable information regarding the chemistry and 

temperature of the pore waters from which the minerals 

precipitated and the chemical process by which the

Calcite

Two typical calcite-bearing sandstone samples of 

the Shannon were selected: one from Salt Creek (sample 

SC-4) contains 38 percent calcite by volume, and the

Upper Cretaceous Shannon Sandstone Beds, Wyoming C17
Figure 19. Scanning electron X-ray fluorescence micrograph showing the relation 

between authigenic siderite (gray) and calcite (pink) in a pore; quartz grains are purple. 

Note correlation between elemental composition and color. Core A771 at 9,461.5 ft.

VIP «-3 »WV 1C CHIP

COMPOSITION 

NO ELEMENTS 4.66 X O

0.48 X 

37.42 X 

1.32 X '-

MO cn

51 CR

MO si cn

FE

MO FE

51 FE

MB SI FE

OR FE

MO CM FE 

SI OR FE 

MO SI C* FE 

ALL PHASES

C 42 X 

0.0S X 

2.43 X T 

0.20 X 

0.C7 X 

7 .01 X 

22* X 

0.42 X 

0

AMI *^ 

.DO / 

.34 X <

Figure 20. Scanning electron X-ray fluorescence micrograph showing siderite 

overgrowth (red) on detrital dolomite grain (blue); siderite also fills fracture in dolomite 

grain. V, void (pink). Core A771 at 9,461.5 ft.

C18 Evolution of Sedimentary Basins Powder River Basin
Figure 21. Scanning electron micrograph of framboidal 

pyrite (P) in pore lined by quartz overgrowths (Qo). Core 

C652 at 10,800 ft. Field length 90 mm.

Figure 22. Scanning electron micrograph of authigenic 

pyrite euhedra (P) and authigenic chlorite (Ch). Core B626 at 

9,391 ft. Field length 30 mm.

Figure 23. Scanning electron micrograph of iron oxide 

pseudomorph (octahedral crystal contains iron but no sulfur) 

after pyrite (P); vermicular kaolin'rte (K) surrounds the pseudomorph. Core C221 at 334 ft. Field length 25 mm.

Figure 24. Scanning electron micrograph of quartz 

overgrowth coated with oil (raised protuberances are oil 

droplets). Core D568 at 9,509.6 ft. Field length 28 mm.

other from the Teapot Dome oil field (sample 

C221-253.8) contains 25 percent calcite by volume. In 

both sandstones, framework grains are solidly cemented 

with poikilotopic calcite. Neither sandstone displays 

significant diagenetic alteration, and high minus-cement 

porosity values for both indicate that calcite was 

precipitated during early diagenesis. The light 613C 

values (-10.61 and -12.72 per mil) of calcite (inferred to 

have precipitated at ambient temperatures) probably 

resulted from the mixing of CO2 derived from the 

metabolism of organic matter (about -25 per mil), CO2 

derived from the atmosphere (about -7 per mil), and (or) 

CO2 derived from dissolution of marine carbonate ( 613 C 

of about 0 per mil) (Carothers and Kharaka, 1980). The 

relatively light 618 O values (-12.00 and -12.25 per mil) 

suggest meteoric water influence and (or) precipitation

of calcite in a closed system in which the 618O of the 

water became increasingly lighter as oxygen-bearing 

minerals precipitated (Longstaffe, 1988).

Dolomite

Two sandstone samples, C221-300 (about 5 

percent dolomite by volume) and C652-10,828 (12 

percent dolomite by volume), that contain detrital 

dolomite and no other carbonate minerals were chosen 

for isotopic analyses. The dolomite is present as monocrystalline and polycrystalline grains. Dolomite overgrowths compose a very small percentage of the total 

volume of dolomite. The isotopic values ( 618O, -6.10 and 

-4.64 per mil; S13C, -2.65 and -4.20 per mil) are slightly

Upper Cretaceous Shannon Sandstone Beds, Wyoming C19
Figure 25. Scanning electron micrograph of etch pits in 

quartz overgrowth from reservoir sandstone. Core D568 at 

9,509.8 ft. Field length 26 mm.

Figure 26. Scanning electron micrograph of intergrowth of 

authigenic quartz (Qa) and chlorite (Ch). Core A729 at 8,580 

ft. Field length 60 mm.

lower than values typical of carbonate-platform 

dolomites that formed by the replacement of a preexisting calcium carbonate phase and, in the process, 

preserved isotopic values of the early carbonate (Land, 

1980). Lower isotope values may be due to incorporation 

of light carbon derived from oxidation of organic matter.

Siderite

Isotopic analysis of detrital siderite clasts collected 

from cores yields relatively light 813C values (-12.95 to 

-14.45 per mil) that suggest the concretions formed 

during sulfate reduction rather than methanogenesis, a 

process characterized by heavy 813C from about -4 to 

+ 11 per mil (Curtis and others, 1986). 618O values

(-2.61 and -2.49 per mil) are slighter lighter than seawater values, possibly due to diagenetic processes. The 

613C (-7.05 and -5.92 per mil) and 618O (-2.25 and -0.85 

per mil) values for two samples of bedded siderite are 

somewhat heavier than comparable values for siderite 

clasts. The 618O values are fairly typical of seawater the 

lower value may be slightly evolved owing to diagenesis 

and the 613C values may indicate minor input of some 

heavy CO2 derived from methanogenesis. More 

definitive interpretations cannot be made using isotopic 

values for only two samples.

DISCUSSION

Provenance

Sand deposited as the Shannon may have been 

reworked from shoreline sands (see fig. 4) of the Eagle 

Sandstone (Gautier, 1981a). Paleogeographic reconstructions indicate that the source area for the Eagle and 

therefore the Shannon was in western Wyoming and 

Montana (Shurr, 1984). The abundance of chert suggests 

that source rocks were predominantly sedimentary and 

included minor plutonic, metamorphic, and volcanic 

components; however, some labile minerals and rock 

fragments may have been lost by attrition resulting from 

the great distance (>300 km) of the Shannon from 

source areas and extensive reworking by shelf currents.

The relatively light carbon isotopic values for detrital siderite clasts in the Shannon indicate that siderite did 

not form during methanogenesis (Curtis and others, 

1986). In contrast, heavy values, averaging + 5 per mil, 

for siderite in the Shannon Sandstone Member of the 

Gammon Shale in Montana and southwestern North 

Dakota are characteristic of fractionation during methanogenesis (Gautier, 1981b). Light S13C values suggest 

that the siderite clasts formed in lagoonal or estuarine 

environments, both of which are characterized by iron 

concretions and where isotopicalh/ light CO2 is available. 

The siderite was then ripped up, transported, and redeposited by currents on the shelf. The large size of many 

clasts precludes long-distance transport.

Origin of Glauconite

Studies of glauconite in Holocene sediments 

suggest that glauconite formation involves the alteration 

of a substrate, such as fecal pellets or foraminiferal tests, 

in contact with seawater at depths between 60 and 500 m 

(Odin and Fullagar, 1988; Odin and Morton, 1988). 

Although glauconite cannot be used as a precise waterdepth indicator, its presence constrains the depth of

C20 Evolution of Sedimentary Basins Powder River Basin
Figure 27. Photomicrograph of authigenic chlorite rim (Ch) under quartz overgrowth 

(Qo). Core A771 at 9,453 ft. Crossed nicols; field length 0.55 mm.

formation to below wave base on the continental shelf. 

Glauconite forms in an environment where winnowing 

exceeds detrital input. Thus, transgression produces an 

environment that is highly favorable for glauconite 

formation, and glauconite is commonly found at the base 

of a transgressive sequence (Odin and Fullagar, 1988).

Origin of Dolomite

Detrital dolomite is a common constituent of many 

Western Interior Cretaceous sandstones (Sabins, 1962; 

Young and Doig, 1986); however, in a previous 

petrologic study (Ranganathan and Tye, 1986), all 

dolomite in the Shannon was considered to be authigenic. 

The most probable sources for the detrital dolomite are 

erosion of syndepositional dolomite from shallow tidal 

flats by transgression of the Cretaceous sea and 

weathering of dolomite-bearing rocks in the source area.

Supratidal crusts may have provided some 

dolomite to the Shannon. Sabins (1962) suggested that 

monocrystalline dolomite grains ("primary" dolomite) in 

marine Cretaceous sandstones formed nearby within the 

depositional basin and were reworked locally into the 

sandstones. He did not, however, suggest a mechanism 

for dolomite precipitation, an important point because 

dolomite is not known to precipitate directly from normal 

marine seawater. The only modern environment in which 

dolomite precipitation at ambient temperatures has been 

documented is a supratidal setting in which the Mg/Ca

ratio is elevated due to evaporation of seawater (Hardie, 

1987); however, dolomite that forms in a tidal 

environment tends to be micron size and not sand size, as 

are most grains found in the Shannon.

Several Paleozoic formations, such as the Ordovician Bighorn Dolomite, in the source area of the 

Shannon may have provided sand-size dolomite grains. 

Modern stream sediments in central Texas contain 

abundant detrital dolomite that has weathered from the 

Ellenburger Group (Ordovician); however, dolomite 

decreases greatly in abundance downstream when 

streams reach more abrasive marginal- and open-marine 

environments (Amsbury, 1962). Likewise, carbonate 

fragments may have been more abundant in the source 

area than present-day amounts in the Eagle and Shannon 

indicate.

To minimize abrasion, dolomite grains were 

probably transported in fast-moving streams and moved 

rapidly to the shoreline and out onto the shelf. The 

Campanian shoreline along the western margin of the 

seaway was formed by the Eagle Sandstone, which 

contains 5-10 percent (by volume) detrital dolomite as 

polycrystalline and mostly monocrystalline sand-size 

grains (Gautier, 1981a). The sizes and textures of 

dolomite grains in the two formations are similar; thus, 

some detrital dolomite was probably transported by 

longshore and storm currents from the Eagle shoreline 

onto the shelf where it was incorporated along with other 

clastic grains into Shannon sand ridges. In this study, the

Upper Cretaceous Shannon Sandstone Beds, Wyoming C21
Figure 28. Diagenesis of potassium feldspar. A Photomicrograph showing partial 

replacement of potassium feldspar (K) by calcite (Ca). Core A232 at 9,527 ft. Crossed 

nicols; field length 0.5 mm. B, Cathodoluminescence micrograph showing replacement of 

potassium feldspar (blue) by calcite (orange). Core A771 at 9,475 ft. Field length 0.4 mm. 

Exposure time 33 seconds; 20 kV.

C22 Evolution of Sedimentary Basins Powder River Basin
Figure 29. Scanning electron micrograph showing lamellar 

morphology characteristic of highly diagenetically evolved 

glauconite. Core B626 at 9,391 ft. Field length 26 mm.

16.8

3.34

10 15 20 25 30 35

°26

Figure 30. Typical X-ray diffraction profiles of glycolsaturated, oriented specimens of the <2-/um fraction of 

interstratified glauconite-smectite of representative samples 

from shallow and deep sandstones in the Shannon 

Sandstone Beds. The shallow samples (upper profile, 

present depth 75 m, 55 percent smectite layers) exhibit a 

higher proportion of smectite layers in the interstratified 

glauconite than do the more deeply buried samples (lower 

profile, present depth 2,642 m, 10 percent smectite layers). 

Cu Ka radiation; peak positions in angstroms. Chl, chlorite; Q, 

quartz; unlabeled peaks are glauconite-smectite.

Figure 31. Photomicrograph of chessboard albite (arrow). 

Core A771 at 9,457 ft. Crossed nicols; field length 0.5 mm.

Figure 32. Scanning electron X-ray fluorescence 

micrographs (Ca, Mg, Fe) of ferroan dolomite overgrowth 

(Fe) on detrital dolomite grain. Core A771 at 9,475 ft. Field 

length 0.12 mm.

Shannon was found to contain significantly more detrital 

dolomite than the Eagle (as much as 15 percent). 

Another source, therefore, is required to have provided 

dolomite to the Shannon, or, alternatively, reworking by 

currents may have concentrated dolomite in Shannon 

sand ridges.

Petrology of Outcrop and 

Core Sandstones

The framework mineralogy of outcropping 

sandstones of the Shannon (such as at Salt Creek, 

Hammond, Horse Creek, and Oak Creek, fig. 1) is

Upper Cretaceous Shannon Sandstone Beds, Wyoming C23
Figure 33. Scanning electron micrograph of skeletal 

plagioclase grain. Core C221 at 269.2 ft. Field length 0.23 

mm.

similar to that of deeply buried core sandstones, but the 

diagenetic history is in general much simpler. 

Outcropping sandstones tend to be tightly cemented with 

early poikilotopic calcite cement that makes up as much 

as 30 percent by volume of the rock, and framework 

grains generally have point contacts. In samples from the 

Groat Sandstone Bed, the calcite cement is ferroan. 

Porosity is rare in the Groat except for minor dissolution 

of framework grains, especially plagioclase. Dissolution 

of plagioclase postdated precipitation of calcite cement 

because skeletal plagioclase grains are not crushed and 

are not filled with calcite. At Hammond, the Groat is 

characterized by potassium feldspar overgrowths that 

were precipitated before ferroan calcite cement and by a 

later generation of pore-filling, nonferroan calcite 

cement. At Salt Creek, secondary porosity is best 

developed in the upper part of the Shannon, hematitic 

grain coatings are common, and late, coarsely crystalline 

kaolinite locally fills vugs formed by dissolution of grains 

or cement.

Authigenic chlorite, less expandable glauconitesmectite, and secondary overgrowths of quartz, albite, 

dolomite, and siderite compose a suite of minerals 

characteristic of sandstones that have been deeply 

buried. Precipitation of these minerals is interpreted to 

have occurred just prior to or at about the same time as 

the migration of organic acids and oil into these 

sandstones. Sandstones of the Shannon that are now at or 

near the surface in the Salt Creek area were never buried 

deeply enough for these minerals characteristic of deeper 

burial to precipitate (see following section on diagenesis 

and paragenesis).

In a study to determine whether reservoir quality 

(porosity and permeability) of a particular depositional 

facies in the subsurface could be assessed by examining 

surface sandstones in the Shannon, Jackson and others

(1987) found that facies in outcrop sandstones from the 

Salt Creek locality have sedimentological properties (and 

thus reservoir properties) similar to the same facies in 

shallowly buried sandstones from the Teapot Dome oil 

field; however, they did not examine deeply buried 

( > 2,500 m) sandstones. As shown by data in our study, 

shallowly buried (< 100 m) and outcropping sandstones 

have similar diagenetic histories; thus, extrapolation of 

reservoir properties from outcropping to shallowly 

buried sandstones is valid. Extrapolations from 

outcropping to deeply buried sandstones is invalid, 

however, because of the vastly different diagenetic 

history of deeply buried sandstones.

Diagenesis and Paragenesis

The paragenesis of authigenic minerals as inferred 

by their textural relations is shown on figure 35. Early 

diagenesis was highlighted by the precipitation of 

carbonate minerals calcite in most sands and siderite 

where conditions were extremely reducing and alkaline 

and [SC>4~ ] was low and [Fe2 + ] was high. The two samples 

containing apparently early calcite cement have very 

negative 618O values typical of calcite precipitated from 

meteoric water. Perhaps the calcite that precipitated 

early and filled primary pores recrystallized during later 

diagenesis in waters that were fresh, rather than brackish 

or saline. The only other reasonable process by which 

light oxygen isotopes could have been produced is by 

meteoric waters during early diagenesis. It is difficult, 

however, to envision meteoric waters migrating into (far) 

offshore sands of the Shannon unless sea level dropped 

after Shannon deposition, and there is no evidence for 

such a drop. Some calcite precipitated later because it 

fills pores that are rimmed by chlorite and by quartz and 

albite overgrowths. Radial extinction of ferroan calcite 

cement in the Groat Sandstone Bed suggests that the 

calcite replaced early aragonite or magnesium calcite 

cement.

In marine sediments, siderite commonly forms 

below the zone of sulfate reduction during methanogenesis after sulfate has been depleted in the formation of 

pyrite (Curtis and others, 1986). The 613C values of 

carbon in bedded siderite in this study are in the lower 

part of the range (-4 to +11 per mil) of methanogenic 

siderite. The siderite may contain a mixture of CO2 

derived from sulfate reduction or from some other 

process (that is, oxidation of methane). Alternatively, the 

siderite could have been deposited in a fresh- or 

brackish-water environment and redistributed into the 

Shannon. A probable source of iron for authigenic siderite cement was the early alteration of abundant detrital 

biotite and chlorite in the Shannon. Most pyrite probably 

formed during sulfate reduction early in diagenesis

C24 Evolution of Sedimentary Basins Powder River Basin
Figure 34. Photomicrograph of oil (o) between detrital quartz grain (Q) and quartz 

overgrowth (Qo) in secondary pore (blue); brown material under quartz overgrowth is 

authigenic chlorite. Core D568 at 9,512 ft. Plane-polarized light; field length 0.2 mm.

Table 5. Stable isotope values (5 18O and 5 13C) for 

calcite, dolomite, and siderite in Shannon Sandstone Beds 

[Sample number is core designation and depth. Samples shown by 

location in fig. 1 and described in table 1. 8^C in PDB, 8^°O in 

SMOW. Leaders (--) indicate not determined]

Sample number

A77 1-9,457

D-9,507

D-9,513.5

C651-10J95 1

A915-9.352.7 1

A729-8,617.8 1

A915-9.355 1

C652-10.828

C221-300

SC-4

C221-253.8

Lithology

Siderite pebble

Siderite pebble

Siderite pebble

Bedded siderite

Siderite pebble

Siderite pebble

Bedded siderite

Detrital dolomite

Detrilal dolomite

Calcite cement

Calcite cement

813C 

(per mil)

-14.45

-12.39

-12.95

-7.05

-9.04

-9.59

-5.92

-2.65

-4.20

-12.72

-10.61

8180 

(per mil)

-2.25

-2.61

-2.49

-0.85

-6.10

-4.64

-12.25

-12.00

Isotope values measured by A.K. Vuletich, U.S. Geological 

Survey; all other values measured by Global Geochemistry 

Corporation.

(Berner, 1971), but some (as evidenced by pyrite 

overlying chlorite) apparently precipitated later in 

reducing conditions that accompanied oil migration in 

shallowly buried sandstones.

Textural relations suggest that chlorite 

precipitation began before and continued during quartz 

overgrowth formation. Chlorite precipitated at about the 

same time as dolomite and albite overgrowths formed.

Because chlorite is not found in outcropping or nearsurface sandstones, which have not been deeply buried, it 

evidently formed during burial diagenesis. Fluids related 

to petroleum generation and migration may have 

provided some ions required for the formation of chlorite.

Precipitation of albite is inferred to have preceded 

calcite cementation because most plagioclase grains are 

albitized rather than replaced by calcite. If the plagioclase had not been albitized, it would have preferentially 

been replaced by calcite before potassium feldspar 

because plagioclase is more soluble than potassium feldspar under normal burial diagenetic conditions (Garrels 

and Christ, 1965). Growth boundaries between quartz 

and albite overgrowths indicate that these minerals were 

cogenetic. Potassium feldspar overgrowths in the Groat 

Sandstone Bed underlie ferroan carbonate cement and 

thus precipitated somewhat earlier than the carbonate 

cement.

The initial mineral composition of the glauconite 

was smectitic interstratified glauconite-smectite. The 

generally inhomogeneous and poorly crystalline mineral 

composition explains the commonly diffuse nature of 

glauconite X-ray powder diffraction patterns. As 

temperature and pressure increased during burial diagenesis, glauconite-smectite became more micaceous, in 

much the same way as the illite-smectite series (Ireland 

and others, 1983). Thus, the decreased expandability of 

glauconite-smectite in deeper samples of the Shannon is

Upper Cretaceous Shannon Sandstone Beds, Wyoming C25
Pyrite 

Siderite/ankerite

GI a u c o n ite/s m ectite

Chlorite 

Quartz

Albite

Calcite 

Dolomite

Potassium feldspar 

Secondary porosity 

Petroleum

Kaolinite

Iron oxide minerals

Sulfate reduction

Methanogenesis Overgrowths

Decreasing expandability 

Ferroan

Organic acids(?) Meteoric K^O

Figure 35. Paragenesis of diagenetic alterations in the Shannon Sandstone Beds in the west-central part of 

the Powder River Basin.

probably a result of burial diagenesis. The exact 

mechanism of this reaction is not well understood, 

however, and the extent of reaction cannot be used as a 

precise thermal indicator. In an iron-rich environment, 

soluble organic species related to petroleum generation 

and migration may play an important role in the 

glauconite-smectite reaction.

Textural relations indicate that secondary porosity 

developed after precipitation of calcite, chlorite, and 

various types of overgrowths but before migration of oil

into sandstone reservoirs. Ions (for example, Mg2 * and 

Ca2 + ) released during the conversion of expandable 

glauconite-smectite to less expandable glauconitesmectite may have facilitated formation of dolomite. Iron 

incorporated into ferroan cements may have been 

derived from degradation of iron-rich biotite and chlorite. Grains and overgrowths may have been etched by 

organic acid- and carbonic acid-bearing pore fluids just 

prior to the migration of oil (MacGowan and Surdam, 

1988). Concentrations of organic acid anions as high as

C26 Evolution of Sedimentary Basins Powder River Basin
10,000 ppm have been measured in oil-field brines 

(Carothers and Kharaka, 1978). Dissolution of aluminosilicate framework grains and carbonate cements by 

organic acids in the subsurface has been shown to occur 

at burial diagenetic temperatures (80-100 °C; Surdam 

and others, 1984). Dissolution of quartz grains by organic 

acid-bearing fluids of neutral pH has also been shown to 

occur at ambient temperatures (Bennett and Siegel, 

1987).

The presence of oil in large secondary pores 

indicates that oil migrated into Shannon reservoirs 

relatively late in the diagenetic history. Some 

investigators have suggested that the Cody Shale 

contained the source beds for oil in Shannon reservoirs, 

whereas others have contended that Lower Cretaceous 

shales were sources for most Upper Cretaceous oil 

(Momper and Williams, 1984). Unfortunately, burial 

history reconstructions do not constrain the timing of oil 

generation for the Cody: kinetic and time-temperature 

modeling indicates that oil generation occurred between 

47 and 30 Ma, whereas vitrinite reflectance data suggest 

that the Cody was not capable of generating oil until 

maximum burial, about 10 Ma, and is still within the oil 

window (Nuccio, 1990). In the Steele Member just below 

shallow sandstone reservoirs in the Shannon at Teapot 

Dome, relatively low vitrinite reflectance values (about 

0.4; Nuccio, 1990) indicate thct maximum temperatures 

were less than 100 °C.

The paragenesis of minor phases is difficult to 

determine. In sample A232-9,540, however, barite 

cement apparently postdates calcite cement. In shallow 

samples late euhedral pyrite and kaolinite are present 

together in large secondary pores. Both pyrite and kaolinite form in acidic conditions (Garrels and Christ, 

1965), and pore water may have been rendered acidic by 

the presence of organic acids and dissolved CO2 

associated with the migration of oil. The incursion of 

meteoric water into these shallow samples probably 

oxidized many pyrite euhedra to iron-oxide pseudomorphs of pyrite.

Depositional Environment

Mid-Shelf Model

Aspects of Shannon sand ridges (and other midshelf sand ridges) that are not satisfactorily explained by 

the mid-shelf model include siderite clasts and glauconite 

in the same facies, large siderite clasts, coal fragments 

and partings, abrupt facies changes, general upward 

coarsening of grain size, abundant detrital dolomite, and 

sideritic shale barren of foraminifera within the Shannon. 

Other aspects of mid-shelf sand ridges, such as the

dynamics of ridge formation on the shelf, have been 

questioned by many authors (see Stubblefield and others, 

1984).

The apparently anomalous occurrence of glauconite (oxidizing/reducing conditions) and siderite 

(extremely reducing) in the coarsest crossbedded 

sandstones has been explained by multiple periods of 

erosion and redeposition of sand-ridge facies by shelf 

currents (Gaynor and Swift, 1988). Siderite and shale 

rip-up clasts formed when storm currents eroded bedded 

siderite (formed during early diagenesis) and shale in 

previously formed sand-ridge facies. These clasts were 

then redeposited in newly formed sand-ridge facies (Tillman and Martinsen, 1984; Gaynor and Swift, 1988). 

Some siderite and coal fragments may have been rafted 

onto the shelf by longshore and storm currents. Gaynor 

and Swift (1988) described sand ridges as being formed 

by a series of erosional and depositional events. They 

interpreted the common abrupt facies changes between 

sideritic and glauconitic medium-grained sandstone and 

shale as marking erosion surfaces that were created on 

the up-current sides of sand ridges by the strong action of 

shelf currents.

Some proponents of the mid-shelf model have 

proposed that upward-coarsening grain size in the 

Shannon is related to episodic tectonic activity along the 

Sevier orogenic belt far to the west (Swift, 1985). During 

each pulse of thrust faulting, increased sediment supply 

and decreased water depths in the Campanian seaway 

resulted in upward-coarsening sequences. Gaynor and 

Swift (1988) described coarsening in upper shoreward 

facies as a product of current action.

The abundance of slightly abraded dolomite in 

Shannon sand ridges, which are inferred to have been 

deposited more than 100 km from land, requires a 

particular set of circumstances because dolomite clasts 

can tolerate only a slight reworking before being 

significantly abraded. In the mid-shelf model, storm 

currents would have to transport dolomite rapidly onto 

the shelf with a minimum of abrasion to avoid 

destruction of grains.

Sideritic shale barren of foraminifera within the 

Shannon is difficult to account for in the context of the 

present mid-shelf model. Perhaps abnormal salinities 

and (or) depleted oxygen levels resulting from the 

restriction of the Cretaceous seaway limited the diversity 

of fauna.

For the mid-shelf model to be viable, the points 

discussed in this section must be satisfactorily explained. 

Although many dynamic sedimentological aspects of the 

formation of sand ridges have been elucidated carefully 

by the model presented by Gaynor and Swift (1988), an 

alternative model for sand-ridge formation should also 

be considered.

Upper Cretaceous Shannon Sandstone Beds, Wyoming C27
Alternative Model

Interpretations of the depositional environment of 

linear marine Cretaceous sandstones and modern midshelf sand bodies have been modified in light of new 

concepts in event stratigraphy and eustatic fluctuations 

(Flint and others, 1986, 1987; Downing and Walker, 

1988). Some geologists advocate that mid-shelf sand 

ridges were once part of a shoreline sequence (for 

example, shoreface or barrier island) that was degraded 

during a transgression and then reworked during a 

post-transgressive period on the shelf (Kiteley and Field, 

1984; Stubblefield and others, 1984). In this model, 

modern linear sand ridges on the Atlantic shelf are 

interpreted to be submerged barrier islands or shoreface 

remnants rather than actively forming shelf ridges (Rampino and Sanders, 1980; Stubblefield and others, 1984). 

Likewise, linear sand ridges of the upper Albian Viking 

Formation in Alberta, formerly thought to be mid-shelf 

bars, have been reinterpreted as relict shoreface sand 

bodies that were reworked by tidal currents and wave 

action during a sea-level transgression and redeposited as 

ridges in 5-15 m of water (Downing and Walker, 1988). 

In northeastern Colorado, the Hygiene Sandstone 

Member, which is enclosed in the marine Pierre Shale, 

was originally deposited in a nearshore setting and then 

reworked by southerly storm currents (Kiteley and Field, 

1984).

Most of these sand ridges, including the Viking 

Formation and Turonian Cardium Formation of Alberta, 

are marked by numerous erosional unconformities 

(Bergman and Walker, 1986; Flint and others, 1986; 

Downing and Walker, 1988). These sand bodies are 

commonly overlain and underlain by erosion 

(transgression) surfaces (Flint and others, 1986) that are 

made obvious in some areas by rapid facies changes, 

deep scouring (for example, 20 m of relief), and pebble 

beds several meters thick. Subsurface correlations by 

Flint and others (1986) show additional subtle erosion 

surfaces in the Cardium that are only recognizable by 

"sideritic horizons with a scattering of granules or coarse 

to very coarse sand grains."

Weimer (1983) stated that two major criteria for 

the recognition of unconformities caused by eustatic 

changes are (1) missing facies in a normal regressive 

sequence and (2) phosphatic nodules and glauconite on 

scour surfaces. Phosphatic pebbles are near the base of 

the Shannon at several localities (Spearing, 1976) and 

were noted in one of the cores of the present study. This 

phosphatic zone at the base of the Shannon has been 

interpreted to be a regional transgressive disconformity 

(Gaynor and Swift, 1988).

The most favorable environment of formation for 

siderite is fresh to brackish water that has high alkalinity, 

low sulfate activity, and low oxygen content (Garrels and 

Christ, 1965) in swampy, coal-bearing lagoonal or

deltaic-estuarine settings (Curtis and others, 1986). Siderite is not common in normal marine sedimentary rocks 

(Ellwood and others, 1988) and where present may be a 

product of methanogenesis; however, stable isotope 

values on carbon in siderite clasts of this study are not 

characteristic of methanogenic carbonate. If Shannon 

sand bodies were originally deposited on the shelf as 

shoreline facies during a regression, then the siderite 

could indeed be a product of lagoonal, estuarine, or 

deltaic deposition, affording an explanation for the large 

size and abundance of the clasts and light 613C values. 

Likewise, coal fragments could have been derived from 

reworking of a nonmarine facies and coal partings, and 

sideritic shales within the Shannon could be remnants of 

backbeach or deltaic facies. Stubblefield and others 

(1984) proposed that interlayered mud and fine sand in 

modern mid-shelf sand ridges was deposited in backbeach or shoreface facies.

Many characteristics of Shannon sand ridges can be 

explained by this alternative depositional model. The 

absence of typical shoreface trace fossils such as Ophiomorphia is not proof that the Shannon was not originally 

part of a shoreline sequence because trace fossils could 

have been obliterated in the reworking process. In 

addition, the abundance of glauconite may not indicate 

original deposition on the mid- to outer shelf because 

glauconite could have been picked up by the 

transgressing sea and incorporated into reworked 

shoreline sand bodies. Some dolomite may have been 

derived from nearby tidal flats alleviating the problem of 

long-distance transport. The impoverished fauna in 

shales of the Shannon may be, in part, an artifact of 

intense reworking by currents. The near absence of plant 

fragments, which are common in a lagoonal or estuarine 

setting, may be due to current winnowing.

CONCLUSIONS

The Shannon Sandstone Beds in the Powder River 

Basin are chert- and glauconite-rich litharenites. The 

abundance of sedimentary rock fragments (chert, 

dolomite, and fine-grained clastic fragments) and the 

paucity of labile heavy-mineral species suggest that 

source rocks were primarily sedimentary and (or) that 

source areas were a long distance to the west. Major 

diagenetic differences exist between sandstones now on 

the surface or shallowly buried and deeply buried 

sandstones. Authigenic chlorite, quartz, and albite did 

not form in shallow reservoir sandstones because, as 

vitrinite reflectance values indicate, the sandstones have 

never been deeply buried.

Pervasive secondary porosity, dissolved potassium 

feldspars, and etched quartz grains in reservoir 

sandstones may have developed in response to the 

migration of organic acid-bearing fluids through deeply

C28 Evolution of Sedimentary Basins Powder River Basin
buried sandstones just prior to the emplacement of oil. 

Dissolution of aluminosilicate minerals provided ions for 

the formation of secondary minerals such as quartz and 

albite that grew into secondary pores. Oil migrated from 

Cretaceous source beds (possibly the Cody Shale) into 

Shannon reservoirs late in the postdepositional history of 

the Shannon as indicated by textural relations between 

oil and other diagenetic phases and by burial history 

modeling.

A synthesis of petrologic, sedimentologic, and 

paleontologic data from this and other studies suggests 

that rather than having been originally deposited on the 

middle shelf, sand ridges may be a product of a sequence 

of sedimentological processes: deposition of regressive 

(linear) shoreline facies on the shelf; degradation and 

reworking of sand bodies during a transgression; and, 

finally, aggradation and reworking by shelf currents and 

marine organisms in deeper water on the middle shelf.

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Upper Cretaceous Shannon Sandstone Beds, Wyoming C31
Appendix. Core descriptions of the Shannon Sandstone Beds of the Steele Member of the Cody Shale, Powder River Basin,

Wyoming

[Drillhole locations shown in fig. 1. Interval given in feet. Facies described in table 2 and in text]

Interval

9,485-9,505.8

9,505.8-9,506.8 

9,506.8-9,508.8

9,508.8-9,512.8 

9,512.8-9,519.2 

9,519.2-9,543.5

9,543.5-9,544

Facies

4a

2 

la

la 

la 

3

4b

Description

CORE D568

Laminated, rippled shale and very fine grained sandstone; sparse burrows; maximum bed 

thickness, 1.5 in. 

Rippled sandstone/shale; <15 percent shale; moderate burrowing. 

Moderate-angle trough crossbedded sandstone; glauconitic; shale ripups; 4 6-in.-thick zone of 

siderite pebbles (4-8 in. diameter) at 9,508.8 ft. 

Oil-stained interval; few shale ripups; one siderite clast (1.5 in.). 

Glauconitic, rippled, crossbedded sandstone; round siderite clasts (1 in.) at top; shale ripups. 

Bioturbated sandstone; compacted circular burrows; locally mottled; discontinuous shale 

laminae (<20 percent); lenses of glauconitic fine-grained sandstone. 

Dark-gray shale; some very fine grained sandstone.

CORE A232

9,526-9,554 3 Bioturbated, very fine grained, glauconitic sandstone/siltstone; clusters of Terebellina 

burrows; glauconitic planar laminae at 9,532 ft; rippled intervals; thin, wispy black laminae; 

horizontal to inclined burrows.

CORE A729

8,580-8,582 

8,582-8,582.5 

8,582.5-8,587.3

8,587.3-8,590 

8,590-8,592.6 

8,592.6-8,594 

8,594-8,607 

8,607-8,618

245-253.2 

253.2-254.8 

254.8-366

2 

la 

2

la 

2 

la 

2 

la

4b 

la 

la

Burrowed sandstone; Terebellina burrows; shale ripups. 

Moderate-angle crossbedded, glauconitic sandstone; shale ripups. 

Burrowed, rippled sandstone/shale; long vertical burrows (2 in.); horizontal burrows; 3-in.- 

thick sandstone intervals. 

Crossbedded sandstone; low-angle troughs; shale ripups; one siderite pebble (1 in.). 

Burrowed sandstone/shale. 

Rippled sandstone; sparse shale ripups. 

Burrowed sandstone/shale; rippled. 

Low to moderate-angle crossbedded, glauconitic sandstone; rippled; abundant shale ripups; 

sparse horizontal burrows; siderite zone at 8,617.8 ft.

CORE C221

Massive silty shale/sandstone. 

Moderate- to high-angle crossbedded sandstone; very glauconitic. 

Oil-stained, rippled sandstone; abundant ripups; local bioturbated sandstone and wavy shale 

laminae; siderite at 346.5 ft; local burrows; 3-in.-thick mottled layer at 263.5 ft; shalier toward 

base.

CORE B626

9,365-9,379 

9,379-9,389.3 

9,389.3-9,395.2

9,395.2-9,419

3 

4a 

la

3

Bioturbated sandstone/shale; shalier towards base. 

Laminated silty shale; sparse to moderate burrows. 

Low-angle crossbedded, rippled sandstone; siderite pebbles at top flattened along bedding; 

coarse glauconite pellets; shale ripups; sparse pebbles; thin burrowed intervals. 

Burrowed, rippled sandstone/shale; long, vertical burrows; some intervals are bioturbated.

COREA771

9,445-9,449 

9,449-9,462

9,462-9,480

4a 

la

3

Laminated silty shale; sparse burrows. 

Medium-grained, glauconitic, oil-stained sandstone; siderite pebbles and clay ripups; zones of 

siderite pebbles (as large as 4 in.) at top and base. 

Bioturbated sandstone/shale; sharp facies break between this unit and unit above; abundant 

horizontal and vertical burrows.

COREC651

10,193-10,198 

10,198-10,208 

10,208-10,216 

10,216-10,225 

10,225-10,242

10,242-10,253

10,790-10,829 

10,829-10,840

4a 

2 

la 

2 

la

2

2 

4a

Laminated, rippled, dark-gray shale; sparse burrows; 1.5-in.-thick bedded siderite at 10,195 ft. 

Burrowed sandstone/shale; thin intervals of crossbedded sandstone; vertical burrows. 

Low-angle crossbedded sandstone; some planar laminae; sparse siderite pebbles. 

Burrowed sandstone/shale; rippled. 

Low- to high-angle crossbedded sandstone; abundant shale ripups locally; siderite at 10,227 

and 10,235 ft; a few intervals of burrowed sandstone. 

Bioturbated/burrowed sandstone/shale; long vertical burrows; some horizontal burrows.

CORE C652

Burrowed, rippled sandstone/shale; U-shaped burrow at 10,796 ft; sand lenses (1-3 in. thick); 

long (>1 in.) vertical burrows common. 

Black, laminated shale; rare burrows; bedded siderite at 10,831.8 ft; dolomitic from 10,830 to 

10,832 ft (X-ray identification).

C32 Evolution of Sedimentary Basins Powder River Basin
Appendix. Continued

Interval

9,885-9,892 

9,892-9,897.8 

9,897.8-9,918.4

9,918.4-9,947

9,325-9,346.5

9,346.5-9,348.5 

9,348.5-9,351.5 

9,351.5-9,353.2 

9,353.2-9,354

9,354-9,358 

9,358-9,384.5

Facies

2 

3 

Ib

2

4a

la

2 

la 

2

4a 

4b

Description

CORE A8 17

Burrowed sandstone/shale; sparse vertical burrows; rippled. 

Bioturbated sandstone; abundant vertical burrows. 

Very fine grained sandstone; few ripups, pebbles, or crossbeds; 2 l-in.-diameter siderite 

pebbles at 9,913.5 ft; glauconitic, planar laminae at 9,918 ft. 

Burrowed sandstone/shale; shalier at base; rippled; sparse burrows in some intervals; 

horizontal and inclined burrows; coal partings from 9,920.6 to 9,920.8 ft.

CORE A9 15

Laminated black shale and minor sandstone; rare inclined burrows; 5-in.-thick, very fine 

grained sandstone at 9,339.2 ft. 

Fine-grained sandstone; abundant shale ripups; l-in.-thick siderite bed at top. 

Burrowed sandstone/shale; vertical and inclined burrows. 

Glauconitic sandstone; low-angle crossbeds; siderite at base; coal fragment at 9,352 ft. 

Burrowed sandstone/shale; vertical and inclined burrows; brown-green clay clast (nonmarine?) 

at 9,353. 8 ft. 

Laminated shale/sandstone; minor inclined burrows; 4-in. -thick siderite bed at 9,355 ft. 

Massive silty gray shale; no burrows; rippled.

 fr u.s. GOVERNMENT PRINTING OFFICE: 1991 573-866 Upper Cretaceous Shannon Sandstone Beds, Wyoming C33

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Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

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

topographic, hydrologic, and geologic investigations. They also include 

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

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

geographic coverage than Professional Papers. They include the results 

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

as collections of short papers related to a specific topic.

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

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

investigations in all phases of hydrology, including hydrogeology, 

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

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

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

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

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

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

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

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

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

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

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

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

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

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

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

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

environmental studies or wilderness mineral investigations.

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

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

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

Catalogs

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

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

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List.)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971- 1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.