The Pintail Coal Bed and Barrier Bar G 

A Model for Coal of Barrier Bar-Lagoon Origin, 

Upper Cretaceous Almond Formation, 

Rock Springs Coal Field, Wyoming

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1398
The Pintail Coal Bed and Barrier Bar G 

A Model for Coal of Barrier Bar-Lagoon Origin, 

Upper Cretaceous Almond Formation, 

Rock Springs Coal Field, Wyoming

By HENRY W. ROEHLER

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 139

Description of a paralic coal-forming 

depositional environment from studies 

of outcrops on the southeast flank of 

the Rock Springs uplift

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Roehler, Henry W. 

The Pintail coal bed and barrier bar G.

(U.S. Geological Survey professional paper ; 1398)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1398

1. Coal Geology Wyoming Rock Springs Region. 2. Geology, Stratigraphic Cretaceous. 

3. Geology Wyoming Rock Springs Region. 4. Sedimentation and deposition. I. Title. 

II. Series: Geological Survey professional paper ; 1398. 

TN805.W8R64 1988 553.2'4'0978785 85-600252

For sale by the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225
CONTENTS

Page

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

Introduction ....................................... 1

Location and accessibility of the study area ......... 1

Physiography ................................... 2

Field methods ................................... 2

Rock Springs coal field ........................... 2

Stratigraphy of the Almond Formation in southwest Wyoming

and adjacent areas ............................. 7

Nomenclature, lithology, and thickness .............. 7

Outcrops of the Almond Formation in the Rock Springs

uplift ........................................ 8

Paleoclimate and paleogeography ................... 8

Paleontology and age ............................. 9

Evidence for barrier deposition based on previous investigations ..................................... 9

Causes for barrier formation ....................... 12

Paleoenvironments ............................... 12

Barrier-lagoon definition and classification .............. 15

Terminology used in the paper ..................... 15

Summary of barrier morphology and nomenclature ... 15

Description of barrier bar G and its lagoon ............. 16

General characteristics ........................... 16

Cross sections constructed from measured sections .... 19

Evidence for barrier bar G progradation ............. 19

Color changes in shoreface sandstones .............. 19

Lithofacies at selected localities: ................... 21

Locality A Lower shoreface ................... 21

Locality B Ebb-tidal delta .................... 21

Locality C Dunes and upper shoreface .......... 22

Locality D Dunes and upper and middle shoreface 23 

Locality E Upper and middle shoreface and tidal

channel ................................... 25

Locality F Tidal channel and middle shoreface ... 26

Description Continued 

Lithofacies Continued

Locality G Recurved spit, tidal inlet, and flood-tidal

delta ..................................... 29

Locality H Tidal flat and flood-tidal delta ....... 29

Locality I Flood-tidal delta and lagoon ......... 33

Locality J Washover fan and lagoon ........... 34

Sandstone petrology ............................. 34

Depositions! history and paleogeography ............ 36

Youth ...................................... 36

Maturity .................................... 38

Old age ..................................... 46

Abandonment ............................... 46

Marine transgression ......................... 46

Paleoecology .................................... 46

Classification of plant habitats ................. 47

Evidence of reed swamp and forest swamp habitats 47

Holocene analog ............................. 50

Coal composition and rank ........................... 50

Proximate analyses of core samples ................ 50

Ultimate analyses of core samples .................. 50

Weathering of outcrop coal samples ................ 53

Vitrinite reflectance .............................. 53

Trace element analyses ........................... 54

Coal development potential ........................... 54

Graphic sections and description of coal outcrops ..... 54

Area! distribution of the coal as a result of the depositions!

environment .................................. 54

Estimated coal resources ...................... 57

Mining methods and problems ................. 57

Conclusions ........................................ 57

References cited .................................... 58

ILLUSTRATIONS

PLATE 1. Measured sections and restored cross section of barrier bar G and the Pintail coal bed .................. In pocket

2. Graphic sections of the Pintail coal bed .......................................................... In pocket

Page

FIGURE 1. Index map showing the location of the study area ..................................................... 3

2. Geologic map of the Almond Formation in the study area showing the location of barrier and coal outcrops, measured

sections, and drill holes ........................................................................ 4

3. Map showing localities A to J for which depositions! environments and lithofacies are described in detail ..... 5

4. Physiographic map showing relationships of Late Cretaceous coastlines .................................. 6

5. Restored cross section of Upper Cretaceous rocks in southwest Wyoming and northwest Colorado ............ 8

6. Location map of the study area on the western shore of the Late Cretaceous epeiric sea .................... 10

7. Paleogeographic map of part of the western shoreline of the epeiric sea showing the location of barrier bar G and

the Pintail coal bed ........................................................................... 11

8. Location map and cross section of measured sections L to S of the Almond Formation on the east flank of the Rock

Springs uplift .................................................................................. 13

9. Measured section showing fossils, coal thicknesses, and depositions! environments at section Q in the Almond

Formation ..................................................................................... 14

10. Aerial photograph of barrier bar G .................................................................. 15

11. Physiographic map showing nomenclature of mesotidal barrier islands, tidal inlets, and tidal creeks in plan view 17

m
IV CONTENTS

Page

FIGURE 12. Cross section showing the morphology and lithofacies nomenclature of barrrier bars and associated rocks in the Almond

Formation ................................................................................... 18

13. Restored cross section along the line of barrier-coal outcrops illustrating depositional environments and lithofacies 20

14. Columnar section at locality A (lower shoreface) ....................................................... 21

15. Photograph of a lower shoreface sandstone outcrop at locality A. ........................................ 21

16. Columnar section at locality B (ebb-tidal delta) ....................................................... 22

17. Rose diagram showing sediment transport directions of sandstone beds in an ebb-tidal channel at locality B . . . 22

18. Photograph of a possible channel margin linear bar at locality B ........................................ 22

19. Photograph of a possible swash bar at locality B ...................................................... 23

20. Rose diagram showing sediment transport directions of a possible swash bar at locality B ................... 23

21. Columnar section at locality C (dunes and upper shoreface) ............................................. 24

22. Photograph of barrier outcrops at locality C .....!.................................................... 24

23. Photograph of dune, forebeach, and surf sandstone lithofacies at locality C ................................ 24

24. Photograph of root casts and organic staining on the top of dune sandstone at locality C .................... 25

25. Columnar section at locality D (dunes and upper and middle shoreface) ................................... 25

26. Photograph of hematite replaced Ophiomorpha sp. at locality D ......................................... 26

27. Photograph of a beach profile at locality D ........................................................... 26

28. Photograph of the Pintail coal bed exposed in a trench dug at locality D .................................. 26

29. Columnar sections at locality E (upper and middle shoreface and tidal channel) ............................ 27

30. Photograph of interbedded sandstone, siltstone, and shale in a tidal channel at locality E ................... 28

31. Rose diagram showing sediment transport directions of sandstone beds in a tidal channel at locality E ........ 28

32. Columnar section at locality F (tidal channel and middle shoreface) ...................................... 28

33. Photograph of outcrops at locality F ............................................................. 29

34. Columnar section at locality G (recurved spit, tidal inlet, and flood-tidal delta) ............................. 30

35. Photograph of outcrops on the north side of Highway 430 at locality G ................................... 31

36. Rose diagram showing sediment transport directions in a tidal inlet at locality G .......................... 31

37. Photograph of flood ramp or flood channel sandstones at locality G ...................................... 31

38. Rose diagram showing sediment transport directions of sandstone beds in a flood-tidal delta at locality G ..... 31

39. Columnar section at locality H (tidal flat and flood-tidal delta) .......................................... 32

40. Photograph of a sand wave in a flood-tidal delta of barrier bar G at locality H ............................. 32

41. Rose diagram showing sediment transport directions of sandstone beds in a flood-tidal delta at locality H ..... 32 

42-46. Photographs:

42. Outcrops at locality H ..................................................................... 33

43. A tidal flat sequence at locality H ........................................................... 33

44. Skolithosl burrows at locality H ............................................................ 33

45. Diplocrateriortf burrow at locality H ......................................................... 33

46. Crassostrea sp. shells at locality H .......................................................... 33

47. Columnar section at locality I (flood-tidal delta and lagoon) ............................................. 34

48. Photograph of outcrops at locality I ................................................................. 34

49. Columnar sections at locality J (washover fan and lagoon) .............................................. 35

50. Photograph of parallel, tabular-bedded sandstone in a washover fan at locality J ........................... 36

51. Photomicrographs of sandstones composing barrier bar G and its lateral equivalents ........................ 37

52. Graph of grain sizes of selected sandstone samples from outcrops of barrier bar G and its lateral equivalents ... 40 

53-57. Paleogeographic maps:

53. Barrier bar G and its lagoon in the stage of youth ............................................. 41

54. Barrier bar G and its lagoon in the stage of maturity. .......................................... 42

55. Barrier bar G and its lagoon in the stage of old age ............................................ 43

56. Barrier bar G and its lagoon at the time the barrier was abandoned .............................. 44

57. The study area following marine transgression ................................................. 45

58. Cross section showing forest swamp and reed swamp habitats envisioned for barrier bar G and its lagoon ...... 47

59. Photograph of a fossil tree stump at the top of the Pintail coal bed at locality I ........................... 48

60. Diagram showing percentages of six palynomorph categories in the Pintail coal bed and in a carbonaceous shale parting 48

61. Photomicrographs of palynomorphs from the Pintail coal bed at locality E ................................ 49

62. Maceral analyses of samples of the Pintail coal bed at locality D ........................................ 50

63. Photomicrographs of macerals in the Pintail coal bed at locality D ....................................... 51

64. Diagrammatic reconstruction of part of barrier bar G showing the location of reed swamp and forest swamp habitats 52

65. Proximate analyses, in percent (as-received), of core samples of the Pintail coal bed from core holes MSR-10C and

MSR-14C. ................................................................................... 53

66. Ultimate analyses, in percent (as-received), of core samples of the Pintail coal bed from core holes MSR-10C and MSR-14C 53

67. Proximate analyses, in percent (as-received), of weathered outcrop channel samples of the Pintail coal bed and its

carbonaceous shale split at locality D. ............................................................ 54

68. Average amounts of major, minor, and trace elements in the Pintail coal bed as compared with coal of all ranks in

the United States. ............................................................................. 55

69. Generalized isopachous map of the Pintail coal bed. ................................................... 56

70. Cross sections showing a model for a barrier-lagoonal, coal-forming environment ........................... 59
THE PINTAIL COAL BED AND BARRIER BAR G 

A MODEL FOR COAL OF BARRIER BAR-LAGOON ORIGIN,

UPPER CRETACEOUS ALMOND FORMATION,

ROCK SPRINGS COAL FIELD, WYOMING

By HENRY W. ROEHLER

ABSTRACT

Barrier bar G and the overlying Pintail coal bed are in the upper 

part of the Almond Formation; they were deposited near the head of 

a shallow marine embayment that was located along the western 

shores of the Late Cretaceous (early Maestrichtian) epeiric sea. The 

barrier is composed of a lens of dominantly fine grained, quartzose 

sandstone more than 60 miles long and about 4 miles wide that 

separates gray marine shale to the east from continental rocks containing coal to the west. Eighteen miles of outcrops of the Almond 

Formation in the study area on the southeast flank of the Rock Springs 

uplift expose a long cross-sectional profile of the barrier and its lagoon 

and marine equivalents.

Barrier bar G was deposited along a mesotidal coastline that had 

numerous tidal inlets, large flood-tidal deltas, and small ebb-tidal 

deltas. Lithofacies of the barrier sandstone include lower, middle, and 

upper shoreface, dune, tidal channel, and washover sequences that 

are easily identified by their sedimentary structures and fossils.

The Pintail coal bed was deposited in a lagoon, landward of barrier 

bar G. As the barrier prograded, the back-barrier flat was progressively 

covered by swamp deposits that include the seaward wedge-out of the 

coal bed. The coal bed is massive and locally more than 6 feet thick. 

It contains splits and partings of carbonaceous shale. The splits and 

partings are interpreted to be deposits of tidal inlet and tidal creek 

origin that suggest direct relationships exist between water salinity, 

plant habitats, and lithologies. Analytical data indicate that the Pintail coal bed was deposited in a freshwater, forest swamp habitat that 

contained thick-stalked, woody vegetation, including trees and shrubs. 

Similar data show that the carbonaceous shale was deposited in a tidally influenced, brackish-water, reed swamp (salt marsh) habitat that 

contained herbaceous, thin-stalked, aquatic, grasslike vegetation, 

without appreciable peat accumulation.

The Pintail coal bed is a low-sulfur (<1 percent) bituminous coal with 

heating values that range between 11,780 and 12,240 British thermal units per pound on an as-received basis. Total in-place resources 

of the bed within the study area are estimated to be 77 million short 

tons. Underground methods appear to be the most favorable manner 

for recovering the Pintail coal along with other overlying and underlying coal beds from a single mine containing several mine levels.

A barrier-lagoon, coal-forming model has been developed from the 

depositional history of barrier bar G and the Pintail coal bed. The 

model demonstrates that the barrier and its lagoon evolved through 

saltwater to brackish-water to freshwater stages, with the times of 

maximum peat accumulation occurring in the freshwater stage.

INTRODUCTION

The investigation is part of the ongoing research by 

the U.S. Geological Survey into the occurrence, quality, and quantity of the coal deposits of the United 

States. The research provides economic and engineering data for coal leasing and sale, safe mine design, profitable mine operation, and adequate reclamation of 

lands disturbed by mining.

The paper presents the results of a study of the origin, 

area! distribution, composition, and genetic relations of 

the Pintail coal bed, barrier bar G, and associated rocks 

in a small segment of the Rock Springs coal field in 

Wyoming. A model was developed for a bayhead barrier bar-lagoon coal-forming depositional environment 

from the paleogeography, sedimentology, and paleontology of the upper part of the Almond Formation during part of the Late Cretaceous. The methodology used 

can be applied to studies of coal beds of similar origin 

in other parts of the Rock Springs coal field and in other 

coal fields in the western United States.

The author has borrowed freely from the geologic literature in writing the paper, but the conclusions reached 

are exclusively his and are based mostly on field observations. Peat beds associated with barrier bar sands in 

Holocene deposits are useful in interpreting the overall 

depositional setting for the Pintail coal bed and barrier 

bar G, but the author does not believe that good analogs 

of the Upper Cretaceous deposits investigated in this 

study are present anywhere in the world today.

LOCATION AND ACCESSIBILITY OF THE 

STUDY AREA

The Pintail coal bed, barrier bar G, and associated 

rocks were studied along a belt of northeast-trending 

outcrops within the Almond Formation on the southeast

1
THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

flank of the Rock Springs uplift in southwest Wyoming. 

The outcrops are about 18 mi long, and are located in 

T. 15-17 N., R. 101-102 W., 21-28 mi southeast of Rock 

Springs, Wyo. (fig. 1).

The northern part of the study area is accessible by 

an improved gravel road that trends east from a junction with Wyoming Highway 430, 19 mi southeast of 

Rock Springs, Wyo. The southern part of the study area 

is accessible by an unimproved road that crosses 

Highway 430 in a northeast direction 31 mi southeast 

of Rock Springs and follows a valley formed along the 

outcrops of the Lewis Shale, the stratigraphic unit 

which overlies the Almond Formation. The accessibility of local outcrops is generally limited to foot travel 

or to four-wheel-drive vehicles on abandoned seismograph roads or on trails used seasonally by hunters and 

sheep ranchers.

PHYSIOGRAPHY

The desert terrain of the study area is predominantly barren northeast-trending sandstone ridges that are 

separated by soil- and sage-covered shale valleys. Annual precipitation is less than 8 in. (Root and others, 

1973). The area is uninhabited except for a few people 

at the Mud Springs Ranch on Wyoming Highway 430, 

30 mi southeast of Rock Springs. Altitudes of outcrops 

of barrier bar G and adjacent rocks vary between 7,000 

and 7,300 ft above sea level, but nearby escarpments 

may rise a few hundred feet higher. The rocks are well 

exposed, except for a few dry, alluvium-filled drainages 

that cross the line of outcrops. Sand dunes, soil, and 

vegetation cover outcrops locally.

FIELD METHODS

The Pintail coal bed, barrier bar G, and other coal 

beds and stratigraphic units discussed in the paper were 

named by the author during field investigations between 1974 and 1979. The units are shown on geologic 

maps of the Camel Rock, Cooper Ridge NE, and Mud 

Springs Ranch 7 1/2-minute topographic quadrangles 

(Roehler, 1977b, 1978a, 1979a). Resources of the Pintail coal bed are listed in U.S. Geological Survey Professional Papers 1065-B and 1065-C (Roehler, 1979b, 

1979c). An abbreviated version of the present investigation was included in a 1979 field trip guidebook for 

Cretaceous rocks of the Rock Springs uplift (Roehler, 

1979d).

The outcrops in the study area offer an excellent 

opportunity for the geological investigation of a wellexposed, ancient barrier sandstone and an associated 

coal bed. Outcrops of the barrier and coal trend about 

N. 25 ° E. at an angle of about 30 degrees to the N. 5 ° W. 

strike of the ancient barrier strandline. Hence, a long, 

oblique, cross-sectional profile is visible through the barrier and its marine and lagoon equivalents along the 

18 mi of outcrops studied. During the investigation, 104 

stratigraphic sections were measured along these outcrops with Jacob's staff and Abney level (fig. 2). Fourteen additional sections were measured at 10 localites 

(localities A to J, fig. 3) to provide supplementary 

details concerning lithofacies and primary sedimentary 

structures. The entire length of the outcrops was walked 

while measuring the sections, which allowed continuous 

observation and recording of details of thickness and 

facies changes.

Twenty-seven samples of sandstone from outcrops 

were collected for petrographic study; four samples of 

coal were collected for palynomorph identifications; 

three samples of coal and carbonaceous shale were 

collected for maceral, as well as proximate and ultimate, analyses. The Pintail coal bed was trenched 94 

times with pick and shovel to reveal the thickness and 

characteristics of freshly exposed coal, and the composition of splits, partings, seat rock, and roof rock. 

Four widely spaced holes were drilled by the U.S. 

Geological Survey in 1977 as part of a coal-resource 

assessment program. No cores were taken in the drill 

holes, but geophysical logs were run and these provide 

limited data concerning the thickness of the barrier 

sandstone and coal bed for distances downdip from 

outcrops.

ROCK SPRINGS COAL FIELD

The Rock Springs coal field is roughly 72 mi long 

(north-south) and 60 mi wide (east-west). It includes all 

the Rock Springs anticlinal uplift and parts of the adjacent Green River, Great Divide, and Washakie basins 

in southwestern Wyoming, in the area immediately 

north of the common boundary of Wyoming, Utah, and 

Colorado.

Coal has been mined continuously in the Rock Springs 

coal field since 1867, when the Union Pacific Railroad 

was constructed across southern Wyoming. Since 1867, 

nearly 35,000 acres of the coal field have been worked 

through more than 65 openings. Total production from 

the field as of 1980 was estimated by the author to be
INTRODUCTION

Cretaceous-Tertiary boundary

UTAH I COLORADO 

I

FIGURE 1. Index map showing the location of the study area in the Rock Springs uplift, Wyoming.

about 250 million short tons. These figures represent 

about 3 percent of the geographical area and less than 

1.5 percent of the total resources of the field.

The Rock Springs coal field contains nearly 6,000 ft 

of coal-bearing rocks, if the thicknesses of the Rock 

Springs, Almond, and Lance Formations of Cretaceous
THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

R103W 10B°5S' R102W R101 W

41°25'

MEASURED

SECTJONS ON !

BARRIER BAR G} 41°20'

2 KILOMETERS

FIGURE 2. Geologic map of the Almond Formation (shaded) in the study area showing the location of outcrops of barrier bar G (heavy line), measured sections (104) across barrier bar G and the Pintail coal 

bed, and U.S. Geological Survey drill holes.

age, and the Fort Union and Wasatch Formations of 

Tertiary age, are combined. The five formations contain 

more than 150 minable beds of low-sulfur (<1 percent)

sub-bituminous to bituminous (9,000 to 13,000 Btu/lb) 

coal within seven distinctly different coal-forming 

depositional environments identified by the author in
INTRODUCTION

108" 55' R10?W

2 KILOMETERS

FIGURE 3. Map of the study area showing localities A to J for which depositional environments and 

lithofacies of barrier bar G, the Pintail coal bed, and associated rocks are described in detail. The 

stratigraphic positions of the localities are shown by large dots in cross section in figure 13.

the field. They are alluvial plain, coastal plain, strand 

plain, delta plain, and barrier-lagoon in Cretaceous rocks 

(fig. 4), and intermontane fluvial-paludal and paludallacustrine in Tertiary rocks. This paper examines one 

coal bed and the underlying rocks of barrier-lagoon 

origin.
THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

FIGURE 4 (above and facing page). Physiographic map showing hypothetical relationships of depositional environments 

associated with marine coastlines in the western interior of the United States in the Late Cretaceous.
STRATIGRAPHY OF THE ALMOND FORMATION IN SOUTHWEST WYOMING AND ADJACENT AREAS

EXPLANATION

Map 

No. Morphologic Feature Example

1 Transgressive sandstone

2 Arcuate delta

3 Delta front barrier islands 

	and interdistributary bays

4 Longshore-offshore bars

5 Barrier plain depositional

	envoirnment of barrier bar G

6 Submarine fan

7 Abondoned delta lobe

8 Delta front accretionary bar

9 Strand plain ridge and swale

10 Strand plain with bay

1 1 Distributary mouth bar

1 2 Strand plain barrier island

1 3 Strand plain bay mouth bar

14 Sound with accretionary bar

15 Shelf channel

1 6 Offshore marine

1 7 Deep marine

1 8 Coastal plain

19 Alluvial plain

Upper part of Almond Formation barrier bar AA 

NW % NE % sec. 22, T. 1 6 N., R. 102 W. (Wyo.)

Lower part of Rock Springs Formation sec. 20, 

T. 1 9 N., R. 104 W. and sees. 21, 28, 

T. 20 N., R. 102 W. (Wyo.)

Lower part of Rock Springs Formation N Vi SE !4 sec. 24,

T. 1 9 N., R. 1 02 W. (Wyo.) 

Dad Sandstone Member of the Lewis Shale 

T. 16 N., R. 91 W. (Wyo.)

Upper part of Almond Formation T. 15-16 N.,

R. 102 W. (Wyo.) 

Lower part of Blair Formation Highway I-80 roadcut 

SW V4 NE Y* sec. 32, T. 20 N., R. 102 W. (Wyo.) 

Lower part of Rock Springs Formation sec. 20,

T. 1 9 N., R. 104 W. (Wyo.) 

Lower part of Rock Springs Formation S V* sec. 23,

T. 1 2 N., R. 107 W. (Wyo.) 

Upper part of Rock Springs Formation NC sec. 1 5,

T. 3N., R. 21 E. (Utah) 

McCourt Sandstone Tongue of Rock Springs Formation 

S '/2 sec. 1 9, T. 12 N., R. 107 W. (Wyo.) 

Rock Springs Formation E Vi NE V4 SE % sec. 1 2,

T. 1 8 N., R. 102 W. (Wyo.) 

McCourt Sandstone Tongue of Rock Springs Formation 

S /2 sees. 20, 21 and 22, T. 12 N., R. 107 W. (Wyo.) 

Sego Sandstone Cottonwood Canyon T. 1 9 S.,

R. 23-24 E. (Utah)

Fox Hills-Lance Formations NE % and S n/2 

sec. 21, T. 1 9 N., R. 100 W. (Wyo.)

Middle part of Blair Formation SW V4 SE V* sec. 36,

T. 1 7 N., R. 103 W. (Wyo.)

Baxter Shale sec. 1 9, T. 1 8 N., R. 103 W. (Wyo.) 

Niobrara Limestone Eastern Wyoming; Eastern Colorado 

Lower part of Almond Formation SE % SE % sec. 29,

T. 1 6 N., R. 102 W. (Wyo.) 

Ericson Sandstone NW14 NE % sec. 20, T. 1 6 N.,

R. 102 W. (Wyo.)

STRATIGRAPHY OF THE

ALMOND FORMATION IN

SOUTHWEST WYOMING AND

ADJACENT AREAS

NOMENCLATURE, LITHOLOGY, AND THICKNESS

The name "Almond coal group" was originally used 

by Schultz (1920) for 700-950 ft of white and brown 

sandstone, shale, clay, and coal that crop out at the top 

of the Mesaverde Formation near the Almond Stage 

Station at Point of Rocks, Wyo. Hale (1950) later 

elevated the Mesaverde Formation in the Rock Springs 

uplift to a group, and then he subdivided the group in 

descending order into four formations: (1) the Almond 

Formation composed of 400-800 ft of gray and brown 

sandstone, siltstone, shale, and coal; (2) the Ericson Formation composed of 900-1,200 ft of gray sandstone;

(3) the Rock Springs Formation composed of 

1,200-1,800 ft of gray and brown sandstone, shale, siltstone, and coal; and (4) the Blair Formation composed 

of 1,200-1,700 ft of gray shale and siltstone. The Almond Formation conformably overlies the Ericson Formation, most commonly named the Ericson Sandstone; 

their contact is gradational. The Almond Formation 

normally intertongues with and is overlain by the Lewis 

Shale. The relations of the Almond to overlying and 

underlying Cretaceous formations across the study area 

are shown on a generalized regional northwestsoutheast cross section (fig. 5).

Chronostratigraphic and lithostratigraphic equivalents of the Almond Formation are present in the upper 

part of the Williams Fork Formation in northern Colorado (Izett and others, 1971), the Meeteetse Formation 

in central Wyoming (Gill and Cobban, 1973), and the 

Adaville Formation in western Wyoming (Smith, 1965).
THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

70 M.Y.

UNDIVIDED BLAIR AND BAXTER FORMATIONS

DEPOSITIONAL ENVIRONMENT

H 80 M.Y.

Marine Shoreline Swamp- Fluvial

lagoon

FIGURE 5. Generalized northwest-southeast cross section A-A' of Upper Cretaceous formations in southwest Wyoming and northwest 

Colorado, showing their age (in millions of years), thickness, stratigraphic relations, and basic depositional environments.

OUTCROPS OF THE ALMOND FORMATION 

IN THE ROCK SPRINGS UPLIFT

The Almond Formation is exposed as a band of outcrops from 1 to 3 mi wide that nearly encircles the Rock 

Springs uplift. The formation has a maximum thickness 

of nearly 800 ft along parts of the east flank of the 

uplift, where it is conformably overlain by the Lewis 

Shale; it thins and is missing in places along the west 

flank because of Laramide erosion across the crest of 

the uplift in late Late Cretaceous time (Roehler, 1961). 

The formation is unconformably overlain in most places 

on the west flank by the Paleocene Fort Union Formation. The Pintail coal bed has been identified in outcrops 

on both the east and west flanks, but barrier bar G is 

restricted to the east flank. The locations of outcrops 

of the Almond Formation in the study area are shown 

in figure 1.

PALEOCLIMATE AND PALEOGEOGRAPHY

The Almond Formation was deposited in a subtropical to warm-temperate climate (Kaufman, 1977, 

p. 91) along the western shores of the epeiric sea (fig. 

6) that extended from northern Alaska and Canada 

southward to the Gulf of Mexico across central North 

America during the Late Cretaceous (Gill and Cobban, 

1966). The epeiric sea was thousands of miles long and 

hundreds of miles wide. The depositional basin of the 

epeiric sea was asymmetric and the zone of maximum 

subsidence and sediment accumulation was along the 

western margin of the seaway (King, 1959). When barrier bar G and the Pintail coal bed were deposited, the 

western shores of the sea were slowly transgressing 

westward across what is now the southern part of the 

State of Wyoming and had reached the area of the 

east flank of the Rock Springs uplift, as shown on a
STRATIGRAPHY OF THE ALMOND FORMATION IN SOUTHWEST WYOMING AND ADJACENT AREAS

paleogeographic map (fig. 7). At this time the shoreline 

consisted of a chain of barrier islands that trended 

north-south across the east flank of the uplift. The barrier islands were situated at the head of an embayment 

in an interdeltaic area between the Red Desert delta 

(Asquith, 1970) to the north and an unnamed delta west 

of Craig in northwestern Colorado. The embayment is 

commonly called the Rock Springs embayment by 

petroleum geologists working in the area.

Moderately high tides that affected the origin and 

development of barrier bar G (discussed later) probably resulted from a focusing of tidal currents as they 

flowed westward and became constricted toward the 

head of the Rock Springs embayment. The sands that 

accumulated along the shores of barrier bar G were 

constantly reworked by waves and drifted southward 

as the result of pronounced longshore currents.

PALEONTOLOGY AND AGE

Few age-diagnostic megafossils from the Almond Formation have been collected in the Rock Springs uplift 

area. J. R. Gill (USGS Locality D6870) found the ammonite index fossil Baculites baculus near the top of the 

Almond Formation about 20 mi north of the study area 

in NEV4 sec. 10, T. 20 N., R. 101 W., at a collecting site 

about 0.5 mi south of the Jim Bridger Powerplant. The 

specimen was collected from a stratigraphic interval 

about 300 ft above barrier bar G and the Pintail coal 

bed. Gill and others (1970) reported the presence of the 

ammonite index fossil Baculites eliasi in the basal part 

of the Lewis Shale northwest of Rawlins, Wyo., in rocks 

that are stratigraphic equivalents of the middle part of 

the Almond Formation, barrier bar G, and the Pintail 

coal bed in the Rock Springs uplift. The overall paleontological relations suggest that the upper part of the 

Almond Formation is 70-72 million years old and 

earliest Maestrichtian in age (Gill and others, 1970, 

P-6).

Fossil collections from the Almond Formation in 

the Rock Springs uplift area include a large assemblage of freshwater, brackish-water and saltwater 

mollusks, and a few vertebrates and ammonites. The 

fauna listed below were collected by J. R. Gill and the 

author at U.S. Geological Survey Localities D6401, 

D6402, D6404, D6411, D6413, D6416, D6869, D6870, 

D6871, D9185, D9186, D9187, D9190, D9191, D9193, 

D9398, D9399, D9400, D9401, and D9402. (Data concerning the location of collection sites, and the identification and paleoecology of the taxa, are available at 

the Branch of Paleontology and Stratigraphy, U.S. 

Geological Survey, P. O. Box 25046, Denver, Colorado 

80225.)

Modiolus cf. M. meekii 

Crenella sp. 

Pholadomya sp. 

Astarte sp.

Dentalium pauperculum 

Micrabacia americana

Lucina sp. 

Tellina sp.

Corbicula cytheriformis 

Tulotomops sp. 

Nuculana sp. 

Pecten (CMamys) sp.

Euspira obliquata

Cryptorhytis n. sp. 

Pyropsis cf. P. bairdi 

Oligotycha concinna 

Cylichna sp.

Dimorphoptychia cf. D.

rutherfordi

Anguispira cf. A. russeli 

Dircella cf. D. insculpta 

Ancilla n. sp. 

Eoacteon cf. E. linteus 

Haminea sp.

Mollusks:

Nerita n. sp. 

Oreohelix sp. 

Holospira cf. H. dyeri 

Anomia gryphorhynchus 

Anemia micronema 

Leptesthes sp.

Ethmocardium n. sp. 

Crassostrea subtrigonalis 

Crassostrea wyomingensis 

Veloritina occidentalis 

Veloritina cleburni 

Glycymeris wyomingensis

Glycymeris holmesiana 

Pachymelania wyomingensis 

Rhombopsis n. sp. 

Banis cf. B siniformis 

Buccinidae n. genus and sp. 

Melampus n. sp. 

Corbula perundata

Terebrimya sp.

Teredina sp. (tubes only) 

Cymbophora holmesi 

Protodonax sp. 

Inoceramus balchii 

Perrisonota sp.

Vertebrates:

Shark teeth 

Dinosaur bones 

Turtle bones 

Fish bones and scales

Ammonites:

Hoploscaphites quadrangularis 

Baculites baculus 

Sphenodiscus sp.

EVIDENCE FOR BARRIER DEPOSITION 

BASED ON PREVIOUS INVESTIGATIONS

The author believes that the sandstones in the upper 

part of the Almond Formation are of barrier origin, and 

this agrees with the interpretations of other geologists 

who have worked in these rocks during the past 20 years. 

The sandstone morphology fits the critera for barrier 

islands that were described by Bernard and others (1962, 

p. 185), and their sedimentary characteristics agree with 

those used by Davies and others (1971) in constructing 

a model for ancient barrier environments.

A marine shoreline origin for the upper part of the 

Almond Formation was first recognized by Hale (1950, 

p. 54), who stated that the sandstones in that part of
10 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

FIGURE 6. Map of North America showing the location of the study area on the western shore of the Late Cretaceous

epeiric sea. From Gill and Cobban (1966).

the section "pass imperceptibly into marine shales." In 

1960 Weimer speculated that the upper part of the Almond Formation was deposited as barrier islands. A few 

years later, Jacka (1965) described primary sedimentary 

structures and lithofacies that demonstrated that Almond sandstones were unmistakably of barrier origin. 

Investigations by Flores in the Almond Formation 

(1978) included a cursory sedimentologic examination of 

barrier bar F, a sandstone unit that overlies and is similar in most respects to barrier bar G in the study area.
STRATIGRAPHY OF THE ALMOND FORMATION IN SOUTHWEST WYOMING AND ADJACENT AREAS

112° 110° 108° 106° 104°

11

CHEYENNE I

_°__J_

FIGURE 7. Paleogeographic map of part of the western shoreline of the Late Cretaceous epeiric sea showing the location of barrier bar

G and the Pintail coal bed.
12 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

CAUSES FOR BARRIER FORMATION

Barriers commonly form along coasts where there 

are low-angle seaward slopes and abundant, renewable 

supplies of sediments that are reworked by waves 

and longshore currents (Busch, 1974; Hayes and 

Kana, 1976; Hayes, 1979). Tidal influence is sometimes necessary to sustain inlets, open-water bays, and 

lagoons. Three theories of barrier origin were proposed by Hoyt (1967): (1) evolution from offshore 

submerged bars, which emerge and develop into barrier 

islands by lowering of sea level; (2) longshore sediment 

transported from a spit, headland, or other coastal projection that eventually entraps part of a coastline 

behind it to form a lagoon; and (3) beach ridges that 

develop into islands by submergence and the flooding 

of landward areas.

The barrier bars in the Almond Formation probably 

formed by processes that combine the first two theories 

of Hoyt. Their origin and location in the Rock Springs 

coal field is specifically attributed to the formation of 

the Rock Springs embayment, which resulted from a 

major marine transgression and the submergence of a 

coastal plain, and to the southward longshore drift of 

sediments from the Red Desert delta to the north, which 

tended to cut off the head of the Rock Springs embayment to form a barrier chain. The superpositioning of 

four other barrier systems in the Almond Formation 

above barrier bar G in the study area may have resulted 

from slow but continuous coastal subsidence and contemporaneous minor marine transgression, and from 

the differential compaction of coastal sediments. The 

sands that composed the barrier shorelines were essentially noncompactible, whereas the adjacent soft, finertextured marine and lagoonal sediments, mostly muds, 

were highly compactible. In this depositional setting, 

abandoned lenses of barrier sandstone remained as 

submerged moundlike ridges that provided the locus for 

renewed barrier bar sedimentation. In the vicinity of 

the mounds, waves began to drag the bottom, the wave 

energy was dissipated, and a submarine bar began to 

grow vertically, similar to a process described by 

Johnson (1919). The barren sand banks that subsequently emerged evolved first into isolated, vegetated 

barrier islands, and then into a barrier coastline attached to the mainland. The self-perpetuating superpositioning (allocyclism) of barrier sandstones in the 

upper part of the Almond Formation was terminated 

by an external event a eustatic rise of sea level that 

resulted in a major marine transgression accompanied 

by deep coastal submergence, deposition of marine

muds, complete abandonment of former shorelines, and 

subsequent formation of shorelines 20-25 mi west of the 

study area.

PALEOENVIRONMENTS

The stratigraphic position of barrier bar G and its 

relation to other barrier bars and other depositional environments in the Rock Springs uplift are shown on a 

generalized restored stratigraphic cross section (fig. 8). 

Figure 8 was constructed from eight measured sections 

and adapted from a more detailed cross section (Roehler, 

1978b).

The Almond Formation in the Rock Springs uplift 

was divided by Jacka (1965) into two parts: an upper 

part consisting of marsh, mudflat, lagoonal-bay, barrier 

beach, surf zone, infra-surf zone, and marine facies; and 

a lower part consisting of alluvial facies. The author 

(Roehler, 1977a) has expanded Jacka's two-part division 

into three parts upper, middle, and lower that more 

clearly define and separate the key lithologic, geographic, and paleontologic units of the formation. The 

upper part (100-400 ft thick) is light-gray, very fine 

grained to medium-grained sandstone and inter bedded 

gray shale, gray and brown carbonaceous shale, and 

coal, that was deposited in shallow marine, barrier bar, 

and lagoonal environments. The middle part (125-250 ft 

thick) is dark-gray and brown carbonaceous shale, coal, 

and some thin interbedded gray shale and gray, finegrained, crossbedded sandstone, that was deposited in 

a lagoonal environment. The lower part (100-600 ft 

thick) is mostly interbedded gray, very fine grained to 

medium-grained, crossbedded sandstone, dark-gray and 

brown carbonaceous shale, and sparse, very thin beds 

of coal, that was deposited in a swampy coastal plain 

environment. Fossils collected from the upper, middle, 

and lower parts of the formation consist of saltwater, 

brackish-water and freshwater fauna and flora, respectively. The upper part contains marine mollusks, shark 

teeth, ammonites, and numerous trace fossils such as 

burrows, borings, tracks, and trails. The middle part has 

abundant oysters and other brackish-water mollusks. 

The lower part contains sparse vertebrate remains that 

include dinosaur, turtle, crocodile, and fish bones.

One of the sections measured in the study area, section Q (fig. 9), shows that barrier bar G is located at 

the base of a sequence of sandstones that were first 

mapped as stratigraphic marker beds AA, A, D, F, and 

G (Roehler, 1978a) and later identified as barrier bars 

(Roehler, 1979c). Barrier E is present at the north edge
STRATIGRAPHY OF THE ALMOND FORMATION IN SOUTHWEST WYOMING AND ADJACENT AREAS 13

OUTCROP OF 

ALMOND FORMATION

Rock Springs, 

Wyoming

0 5 10 MILES

I I I

M N 0 P Q R

BARRIER. ~~BARG

FIGURE 8. Location map and cross section of measured sections L to S showing depositional environments of the Ahnond Formation on the east flank of the Rock Springs uplift.

of the study area, but is missing in section Q. Three of 

the barriers, A, F and G, are visible on an aerial 

photograph (fig. 10), taken about 1.0 mi north of section Q. The lagoonal section that underlies barrier bar 

G on fig. 9 is the landward expression of barrier bars 

that do not crop out in the Rock Springs uplift, but are

present in subsurface rocks east of the uplift in the 

Washakie basin. The section labeled swamp at the base 

of the Almond Formation in figure 9 is part of a broad 

coastal plain that was present before the advance of the 

Late Cretaceous epeiric sea westward across the Rock 

Springs uplift area.
OUTCROP OF

ALMOND 

FORMATION

Rock Springs

N

j,

i-lOO FEET

SECTION Q SHOWING

COAL THICKNESS (IN FEET)

AND FOSSILS

6.2

4.8 

0.6

1.2 

4.2

1.4

SECTION Q 

DEPOSIT

ENVIRON

=E=H

BURROWS 

=Lr=:-l=r-^ BURROWS, CEPHALOPODS

^V.^;X\V;jy BURROWS, SHARK TEETH 

feJTJJF^g OYSTERS

PINTAIL COAL BED

v/v-r-'::^'-:-?! OYSTERS

OYSTERS, SNAILS

-T^Z zr r OYSTERS

DINOSAUR, TURTLE, 

CROCODILE AND 

FISH BONES

5 10 MILES 

J_____I

LITHOLOGY

SANDSTONE SHALE CAR

_ ___ _

BONACEC 

SHALE

3US 

FIGURE 9. Fossils, coal thicknesses, and depositional environments at section Q in the Almond Formation in sec. 33, T. 16 N., R. 102 W

to other rocks in the Almond Formation on the east flank of the Rock Springs uplift is shown in figure 8.
BARRIER-LAGOON DEFINITION AND CLASSIFICATION 15

FIGURE 10. Oblique aerial view of barrier bar G and overlying barrier bars A and F. View is to the northeast from about 500 feet above 

ground level in NEW sec. 28, T. 16 N., R. 102 W. Width of 

photograph is approximately 3.5 mi.

BARRIER-LAGOON DEFINITION 

AND CLASSIFICATION

TERMINOLOGY USED IN THE PAPER

The American Geological Institute Glossary of 

Geology (Bates and Jackson, 1980) defines a barrier 

island as a long, low, narrow wave-built sandy island 

that is sufficiently above high tide and parallel to the 

shore, and that commonly has dunes, vegetated zones, 

and swampy terranes extending lagoonward from the 

beach. A barrier chain is defined as a series of barrier 

islands, barrier spits, and barrier beaches extending 

along a coast for a considerable distance. The glossary 

states that the term "barrier bar," although synonymous with "barrier island," is not acceptable because 

a bar is ordinarily depicted as a submerged ridge. The 

terms "barrier island" and "barrier bar" are nonetheless 

used interchangeably in this report because: (1) barrier 

bar G during its formation evolved from a bar to an 

island to a coastal beach; and (2) the terms as applied 

in this paper refer to a linear shoreline sandstone composing a barrier coastline, and not to specific morphological features, such as beaches, bars, or islands

along that coastline. Barriers are characteristically 

unstable because they are mostly unconsolidated sand 

of relatively uniform composition and texture that is 

continuously reworked by winds, tides, longshore currents, and storms. As a result of this reworking, tidal 

inlets and bays migrate laterally along the length of the 

barrier chains, constantly changing the shape and 

geographic location of the inlets and islands that make 

up the chains. Geologists studying ancient barriers find 

that the geometries of specific morphological features 

are difficult to delineate from studies of outcrops, and 

they are nearly impossible to delineate in subsurface 

rocks unless drill holes are closely spaced. On the other 

hand, the sandstone lens composing the overall barrier 

chain is easily identified and correlated, and it is this 

linear unit to which the terms bar and island are here 

applied.

The glossary defines a coastal lagoon as a shallow 

stretch of seawater, such as a sound, channel, bay, or 

saltwater lake, near or communicating with the sea and 

partly or completely separated from it by a low, narrow elongate strip of land, such as a reef, barrier island, 

sand bank, or spit. The term lagoon in this paper is applied to the areas of incursion of saltwater or brackish 

water between a barrier island chain and a coastal plain.

SUMMARY OF BARRIER MORPHOLOGY 

AND NOMENCLATURE

The physiography of coastal areas is strongly affected 

by tides. According to Hayes (1979) coastal types for 

medium wave energy (Height=2-5 ft) are as follows:

Class

Microtidal

Low-mesotidal

High-mesotidal

Low-macrotidal

Macrotidal

Tidal Range 

0-3 ft 

3-6 ft 

6-12 ft 

12-16 ft 

16ft

Example

Gulf of St. Lawrence

New Jersey

Plum Island, Mass.

German Bight

Bristol Bay, Alaska

Barrier bars will form along coasts affected by 

microtides and mesotides, but the extreme hydrologic 

energy of macrotides prevents barrier formation. Hayes 

and Kana (1976) describe the morphological differences 

between microtidal and mesotidal barriers as follows:

Barrier 

type

Microtidal 

Mesotidal

Length

Long (18-62 mi) 

Stunted (2-12 mi)

Shape

Elongate; 

hot dog

Drumstick

Washover features

Abundant washover 

terraces and washover 

fans 

Minor beach ridges or 

washover terraces; 

washover fans are rare

Tidal 

inlets

Infrequent 

Many

Flood-tidal 

deltas

Large, commonly 

coupled with 

washovers 

Moderate size 

to absent

Ebb-tidal deltas

Small to absent

Large with strong 

wave-refraction 

effects
16 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

A mesotidal system is suggested for barrier bar G by 

comparing the microtidal-mesotidal differences with the 

data shown on plate 1. The islands of barrier bar G are 

5-7 mi long; the islands are roughly drumstick-shaped; 

washover fans are present, but not common; tidal inlets are numerous; flood-tidal deltas are large as wide 

as several miles; and ebb-tidal deltas are moderately 

large to small. The estimated tidal range of barrier bar 

G is between 4.5 and 8 ft.

The morphologic nomenclature of modern mesotidal 

barrier islands, tidal inlets, and tidal deltas is described 

on a planimetric sketch (fig. 11). Nearly all the features 

identified in figure 11 are recognizable in the barrier sequences in the Almond Formation; the features specifically associated with barrier bar G are discussed at 

length later in the paper.

The morphologic terms applied to barrier islands and 

associated rocks in the Almond Formation, which include barrier bar G, are identified and illustrated on a 

schematic cross section (fig. 12). The sands that were 

deposited by waves and currents along the shores and 

in nearshore areas of the seaward parts of the barriers 

comprise the berm, forebeach, upper shoreface (surf), 

middle shoreface, and lower shoreface lithofacies. The 

lower shoreface sands graded seaward into shallow 

marine shales. The overall shoreface sequence coarsened 

upward, and it coarsened laterally from a seaward to 

landward direction. Dune sands were deposited by wind 

in subaerial parts of the barriers, above the level of high 

tides. Sands were deposited by tidal currents in ebbtidal deltas, in flood-tidal deltas, and on tidal flats. 

Washover fans resulted from severe storms that washed 

sand from seaward parts of the barriers onto the backbarrier flats and into the lagoons. Lenticular sand 

bodies, such as fluvial channel deposits and tidal channel deposits, were present in parts of the lagoons. The 

sediments of lagoonal origin were mostly bay-fill muds, 

paludal-carbonaceous mud and peat, and oyster reefs 

composed of sand and shell material. Landward of the 

lagoons were coastal swamps.

Barriers normally expand, or grow, by continuous accretion of sediments in seaward directions (progradation), in landward directions (retrogradation), or in 

lateral directions parallel to the shoreline. Barriers prograde because the sediment supply exceeds the rate of 

subsidence of the marine shoreline (Busch, 1974). Why 

the superposed barriers in the study area (that is, barriers AA, A, D, E, and F, fig. 9) developed separately 

and not as a single, thick, long-lived, retrograding and 

prograding barrier sandstone system is a subject for 

speculation. A logical explanation is that differential 

subsidence along the marine shorelines caused fluvial 

avulsion that resulted in sediment starvation and barrier abandonment.

Peats that form in association with prograding barrier systems, such as barrier bar G, frequently become 

economically important deposits of coal. On the other 

hand, peats associated with retrograding barrier 

systems probably have little economic importance. The 

sands that compose retrograding barriers are derived 

from beach and shoreface parts of the barrier, usually 

during storms. These sands are constantly reworked 

and washed landward over emergent parts of the barrier into the lagoon, eroding and contaminating peat 

deposits that may be present in the lagoon. The 

retrograding process is probably too rapid for the formation of large, stable lagoons and for the origin and 

preservation of thick peat deposits. The upper part of 

barrier bar A A in the study area is a retrograding sandstone sequence; it is not overlain by coal (see fig. 9). 

Retrograding sandstones in Upper Cretaceous rocks in 

the western United States are seldom more than a few 

feet thick; they are usually poorly sorted, iron stained, 

bioturbated, and commonly contain chert pebbles, clay 

clasts, shark teeth, and shell material.

DESCRIPTION OF BARRIER BAR G 

AND ITS LAGOON

GENERAL CHARACTERISTICS

The sandstone unit called barrier bar G in this report 

is a segment of a north-trending marine shoreline. The 

sandstone is lenticular in cross section, is more than 

60 mi long, has a maximum vertical thickness of 95 ft, 

and is about 3.5 mi wide. It separates an interval of 

marine rocks (which contain marine fossils) to the east, 

from continental rocks (which contain freshwater and 

brackish-water fossils) to the west. The marine rocks 

are mostly gray shale. The continental rocks are mostly gray mudstone, lenticular sandstone, carbonaceous 

shale, and coal. The continental rocks are partly laterally equivalent to and partly lap eastward (seaward) onto 

the barrier sandstone. The evidence indicates that the 

continental rocks are of lagoon origin, but only the 

seaward parts of the lagoon are represented within the 

area studied.

The barrier sandstone coarsens upward and landward; 

it is composed dominantly of gray, very fine grained to 

medium-grained, angular to subangular, clear to milky 

quartz grains with minor amounts of other variously colored rock fragments. The sandstone is readily divisible 

into tidal channel, tidal inlet, tidal delta, dune, washover 

fan, and shoreface lithofacies. The paleogeography, 

paleontology, and sedimentology provide overwhelming 

evidence that the sandstone studied is of barrier origin.
1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

14. 

15. 

e 

 - 

FIGURE 11. Nomenclature of mesotidal barrier islands, tidal inlets, and tidal deltas. (From photographs and sketches by Hayes 
EAST

/Dunes ,Washover Jidal flat

/Epeiric sea Ebb-tidal delta

.Lagoon Oyster bed

Shallow marine 

FIGURE 12. Cross section showing the morphology and lithofacies terminology of barrier islands and associated rocks in the 
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 19

CROSS SECTIONS CONSTRUCTED FROM 

MEASURED SECTIONS

Measured sections were used to construct two cross 

sections (plate 1) that correlate stratigraphic units and 

identify depositional environments and lithofacies, 

respectively. The measured sections illustrated in the 

columns on plate IA use two remarkably persistent 

oyster beds as data lines to correlate stratigraphic units. 

The uppermost of these beds is called the oyster marker 

bed (OMB), and is present from less than 10 to more 

than 80 ft above the Pintail coal bed in sections 40 to 

104. A second persistent oyster bed is called the oyster 

ridge marker (ORM), and is situated at a lower stratigraphic position; it is in contact with or overlies the Pintail coal bed by as much as 38 ft in sections 7 to 35. 

Plate IB interprets the depositional environments and 

identifies specific lithofacies from the lithologies and 

sedimentary structures that are illustrated in the measured sections on plate IA. The pronounced distortion 

of bedding planes above and below the Pintail coal bed 

on plate IB results partly from the lenticular shape, 

thickness differences, and differential compaction of the 

various lithologic units, and partly from the fact that 

the base of the Pintail coal bed was used as a datum 

plane when constructing the cross section. For convenience, a simplified page-size reduction of plate IB is 

included in the text as figure 13.

Marine deposits in the open-water parts of the Late 

Cretaceous epeiric sea, seaward of barrier bar G, are 

shown along the left margin of plates IA and IB, near 

section 1. At section 1, barrier bar G is composed of less 

than 10 ft of lower shoreface (LSF) sandstone and 

siltstone within thick nearshore marine shale (MS). The 

barrier sandstone thickens southwestward (landward) 

from left to right across plates IA and IB, through 

mostly ebb-tidal delta (ED), dune (DU), forebeach (FB), 

surf (SU), middle shoreface (MSF), and tidal channel 

(TC) sandstone lithofacies, between sections 2 and 55. 

The barrier reaches its optimum thickness in section 47, 

where it is composed of mostly forebeach (FB) and middle shoreface (MSF) sandstone lithofacies. Between sections 53 and 57 a tidal inlet (TI) is present that marks 

the boundary between barrier bar G and a flood-tidal 

delta (FD) situated in the lagoon landward of the barrier. Sections 58-104 are composed of flood-tidal delta 

(FD), tidal flat (TF), bay-fill (BF), oyster bed (OB), salt 

marsh (SW), tidal channel (TC), coal (PS), and washover 

fan (WO) lithofacies of lagoonal origin that are lateral 

landward equivalents of barrier bar G. Barrier bar G 

is overlain in most of the sections by peat swamps (PS) 

that formed the Pintail coal bed and by adjacent salt 

marsh (SW) lithofacies. The oyster ridge marker (ORM), 

its lateral bay-fill (BF) equivalents, and the overlying

beds, including the Waxwing coal bed and the oyster 

marker bed (OMB), are probably not related to Carrier 

bar G, but are part of an overlying barrier sequence, 

which is not shown. Barrier bar G is nearly everywhere 

underlain by nearshore-marine shale (MS), which is 

slightly carbonaceous in places. Most or all of the Finch 

coal bed, sections 64-99, was deposited in a lagoon that 

predates barrier bar G.

EVIDENCE FOR BARRIER BAR G PROGRADATION

Repetitive vertical successions of lithofacies in barrier bar G and in associated rocks in the study area indicate that the barrier prograded during its depositional 

history. For example, the vertical successions between 

sections 10 and 55 on plates IA and IB can be consistently numbered from bottom to top (marine to nonmarine) as follows:

6. Lagoon-fill material composed of mud, oyster 

beds, and tidal channel sand.

5. Lagoon and back-barrier flat carbonaceous mud 

and peat.

4. Dune, washover, tidal channel, and forebeach 

sand.

3. Upper shoreface surf zone sand.

2. Middle shoreface sand.

1. Marine mud.

If Walther's Law (Walther, 1893-94) is applied, which 

by paraphrasing states that a vertical lithofacies can 

be used to predict the lateral distribution of the same 

lithofacies, then the vertical succession numbered 1-6 

is simply a reflection of a horizontal succession. Therefore, barrier bar G is the result of progradation. Obviously, if the barrier had retrograded, the number 

sequence would be reversed (1 at the top; 6 at the 

bottom).

COLOR CHANGES IN SHOREFACE SANDSTONES

The color change of very light gray or white in dune 

and upper shoreface sandstones to tan and brown in 

middle and lower shoreface sandstones is abrupt and 

striking in most places in barrier bar G in the study 

area. Several possibilities exist to explain these and 

similar color changes in sandstones along the western 

shorelines of the Late Cretaceous epeiric sea (fig. 6): (1) 

the presence of hematite and other iron minerals in the 

middle and lower shoreface sandstone that are present 

in only minor amounts or are missing in the upper 

shoreface and dune sandstone (the presence and absence 

of these minerals could have resulted from a mechanical 

segregation of light and heavy mineral sand grains by
NORTHEAST 

EXPLANATION

OMB-Oyster marker bed 

BF-Bay fill 

TC-Tidal channel 

DU-Dune 

FB-Forebeach 

FD-Flood-tidal delta 

LSF-Lower shoreface 

WO-Washover fan 

(ORM)-Oyster ridge marker 

MS-Marine shale 

MSF-Middle shoreface

SU-Surf 

SW-Salt marsh 

ED-Ebb-tidal delta 

TF-Tidal flat 

RS-Recurved spit 

TI-Tidal inlet 

PS-Peat swamp

J Bu rrows

~^~x Oyster bed

 Locality discussed in text

FIGURE 13. Restored cross section along barrier-coal outcrops in the Almond Formation illustrating depositional environments a
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 21

waves and wind before the lithification of the sands); 

(2) sun bleaching of exposed beach sands prior to 

lithification; (3) organic leaching of dune and upper 

shoreface sandstones by acid ground waters associated 

with overlying coal beds either during or after lithification; and (4) detrital dolomite or biotite that is present 

only in the middle and lower shoreface. No. 1 most easily 

explains the color change in barrier bar G.

LITHOFACIES AT SELECTED LOCALITIES

Ten localities, lettered A to J (plate 1), were selected 

for detailed examination because the rocks are well exposed and reflect the principal barrier-lagoon lithofacies 

in the study area. Each of the localities is described and 

illustrated by columnar sections and photographs. The 

stratigraphic positions of sandstones that were sampled 

for petrographic analysis at the localities are identified 

at the left margins of the columnar sections by a letter 

and number (for instance D3 is locality D, sample 

number 3). The same system of letters and numbers is 

used to identify horizons sampled for coal and microfossil studies.

LOCALITY A LOWER SHOREFACE

Barrier bar G at locality A is composed of 1- and 

1.5-ft-thick benches of lower shoreface sandstone that 

is interbedded with 3.7 ft of gray, soft shale of nearshore 

marine origin (figs. 14 and 15). The sandstones are very 

fine grained, silty, hematitic, and weather brown and 

tan. The bedding in the upper bench consists of even, 

parallel laminae that suggest quiet-water deposition. 

The lower bench is massive and bioturbated, and contains abundant trace fossils of Ophiomorpha sp., a long, 

rough-textured crustacean burrow Vz-\ in. in diameter. 

Barrier E is present at the top of the columnar section 

in figure 14; it wedges out in outcrops about 2 mi to 

the south.

LOCALITY B EBB TIDAL DELTA

Exposures of barrier bar G at locality B consist of 

brown and tan sandstone capping a south-trending 

ridge. The sandstone is mostly gray, very fine grained 

to fine grained and poorly sorted. It contains a few 

small, rounded hematite concretions, and sparse, thin 

lenses of small, gray, clay pebbles with some interbedded gray shale in the lower part. The structures in 

the sandstone consist of small-scale, narrow, elongate, 

steeply dipping trough crossbeds; convolute bedding is 

present locally (fig. 16). The long axes of the troughs

Middle and lower 

shoreface 

barrier bar E

Nearshore marine

Lower shoreface 

barrier bar G

A1 

Nearshore marine

Gray and white fine-grained sandstone containing Crassostrea sp. 

and Ophiomorpha sp.

Gray shale

Gray, very fine grained sandstone 

containing Oph/omorphia sp. 

snd interbedded gray shale

Gray shale

 _ - Gray shale -ID FEET

-5

-0

A1 Sample collected for petrographic 

analysis, (see fig. 51)

FIGURE 14. Columnar section of part of the Almond Formation at 

locality A, NWV4SWW sec. 15, T. 17 N., R. 101 W. Depositional 

environments and lithof acies are identified to the left of the column 

and lithologies are described to the right of the column.

FIGURE 15. Lower shoreface sandstone (LSF) of barrier bar G at 

locality A in NWViSW1/* sec. 19, T. 17 N., R. 101 W. The lower 

shoreface sandstone is interbedded with and underlain by nearshore 

marine shale (MS).

and the dips of bed laminae indicate an east to northeast 

sediment transport direction, as shown in a rose diagram (fig. 17). At the place where the section was measured the sandstone is only 18.5 ft thick, but in exposures 

500 ft to the north it is more than 40 ft thick. Where 

the section thickens, there are abundant gray-clay pebble lenses and some disseminated plant debris.

The paleogeographic location within thick nearshore 

marine shale, the irregular thickening, and the sedimentary structures of the sandstone at locality B suggest
22 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Middle shoreface 

barrier bar E

Nearshore marine

Bl

Ebb deltabarrier bar G

Nearshore marine

;-'.: ;-.-.::-:.: .:;;-.-ih

 U^-;^::-V.

Tan and brown very fine grained 

sandstone containing 

Ophiomorpha sp.

Gray shale

Gray, very fine grained, crossbedded sandstone and interbedded gray shale

~ Gray shale

-ID FEET

-5

Bl Sample collected for petrographic 

analysis, (see fig. 51)

FIGURE 16. Columnar section of part of the Almond Formation at 

locality B located in C NWV* sec. 25, T. 17 N., R. 102 W. Depositional environments and lithofacies are identified to the left of the 

column and lithologies are described to the right of the column.

that it was deposited as part of an ebb-tidal delta, 

possibly the terminal lobe (fig. 11, no. 14). An ebb-tidal 

delta hypothesis for the origin of this sandstone is supported by the close association and bedding characteristics of two other laterally equivalent sandstones that 

crop out south and southwest of locality B. The first 

sandstone, about 6 ft thick, crops out in a small area 

of a dry wash about 1,500 ft south of locality B (fig. 18). 

It consists of white sandstone containing very low-angle, 

hummocky crossbeds as much as 0.1 ft thick that dip 

uniformly from S. 63° E. to N. 75° W. in seaward directions. The sandstone is probably part of a channel margin linear bar (fig. 11, no. 15). A second sandstone 

outcrop forms an isolated knob that caps a ridge about 

2,500 ft southwest of locality B (fig. 19). This 37-ft-thick 

sandstone is light gray and fine grained. It consists of 

small-scale, low-angle, planar crossbeds, 0.7-1.0 ft thick, 

that have multidirectional dips (see rose diagram, fig. 

20). The variability in the directions of dip suggests that 

this sandstone was deposited as a swash bar near a tidal 

inlet (fig. 11, no. 13), where there was a complex 

intermingling of longshore and tidal currents.

20

30 -- 

40

50 -LPERCENT

FIGURE 17. Rose diagram showing sediment transport directions 

of sandstone beds in an ebb-tidal 

channel at locality B; 50

measurements.

FIGURE 18. White sandstone (arrow) believed to have been deposited 

as a channel margin linear bar. Outcrop is in NWV4SEV4 sec. 25, T. 

17 N, R. 102 W

LOCALITY C DUNES AND UPPER SHOREFACE

A measured section at locality C (fig. 21) is accessible by a straight road that trends west-southwest from 

near the southeast corner of sec. 36, T. 17 N., R. 102 W. 

This road was originally bulldozed for a seismograph
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 23

FIGURE 19. Sandstone interpreted as part of a swash bar near a tidal 

inlet of barrier bar G. Outcrop is in NEV4SWV4 sec. 25, T. 17 N., 

R. 102 W.

FIGURE 20. Rose diagram 

showing sediment transport 

directions of sandstone beds in 

a swash bar at locality B; 50 

measurements.

line during oil and gas explorations, but it has been 

subsequently used and improved by ranchers and 

hunters.

The barrier crops out as a prominent, white, ledgeforming escarpment at locality C and for several miles 

south of the locality as shown in figure 22. Dune and 

upper shoreface (forebeach and surf) sandstone lithofacies are well exposed. A thick middle shoreface sandstone is also present, but it is largely covered by soil 

and vegetation on a long dip slope that rises topographically (westward) from the upper parts of the barrier 

that are shown in figure 22.

The dune lithofacies is composed of light-gray, fineto medium-grained sandstone that has landward-dipping, low-angle, small-scale, parallel crossbeds. The dune 

sandstone is 8 ft thick and is welded to the underlying 

forebeach sandstone (fig. 23). The top of the dune sandstone was eroded and weathered prior to the deposition 

of the overlying Pintail coal bed; the upper 1 ft of the 

sandstone has abundant root casts and is dark gray 

because of organic staining (fig. 24). Underlying the 

dune sandstone are 5 ft of light-gray, fine- to mediumgrained forebeach sandstone in even, parallel beds 

0.2-0.4 ft thick. The surf zone that underlies the 

forebeach is composed of 25 ft of light-gray, fine- to 

medium-grained sandstone in small-scale, low-angle, 

trough crossbeds and some small-scale, high-angle, 

planar crossbeds.

The Pintail coal bed at locality C consists of 3.2 ft of 

clean, bright coal; the bed wedges out and is missing 

in sections that were measured north of locality C. The 

tidal channel that overlies the Pintail coal bed in figure 

21 is a local lens of fining-upward brown sandstone with 

small-scale, trough crossbeds. The channel is poorly exposed and is not visible in the photograph (fig. 20). The 

oyster ridge marker (ORM) crops out as a small brown 

ledge about 20 ft above the Pintail coal bed. The oyster 

ridge marker is composed of 5 ft of fine- to mediumgrained brown sandstone with no distinct bedding; it 

contains weathered and partly transported oyster shell 

fragments.

LOCALITY D DUNES AND UPPER AND MIDDLE SHOREFACE

Locality D is in a remote part of the study area. An 

abandoned, west-trending seismograph road provides 

access by four-wheel drive vehicles across the southwest 

part of sec. 11 and the southeast part of sec. 10, T. 16 

N., R. 102 W. The road branches near the center of sec. 

11 from a well-used, north-trending road situated on the 

Lewis Shale. In the southcenter of the NW^SE1/^ sec. 

10, T. 16 N., R. 102 W., the west-trending seismograph 

road enters an area of active sand dunes and becomes 

impassable. Locality D is located about 0.5 mi north of 

the sand dunes and is accessible only on foot.

The outcrops of barrier bar G at locality D are 82 ft 

thick (fig. 25). The dune sandstones at the top of the 

barrier are light-gray, very fine grained to medium 

grained and are about 4 ft thick. The bedding is mostly 

small-scale, low-angle, planar, and trough crossbeds.
24 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Lower shoreface

Bay fill

Lower shoreface

Marine

Lower shoreface

Bay fill

Lower shoreface 

Marine

Oyster reef(ORM) 

Bay fill

Tidal channel

Back barrier 

flat swamp

Gray, very fine grained hematitic 

sandstone containing 

Ophiomorpha sp.

Gray shale

Gray, very fine grained hematitic

sandstone 

Gray shale

Brown siltstone containing 

Ophiomorpha sp.

 Gray shale

Surf

'C2

Brown hematitic sandstone 

containing Ophiomorpha sp.

Gray shale

Gray sandstone containing 

Crassostrea sp.

Gray shale

Brown, fine-grained, crossbedded, 

lenticular sandstone

Pintail coal bed. 3.2 ft

Light-gray, very fine to mediumgrained, crossbedded sandstone, rooted at the top

-10 FEET

-5

-0

Sample collected for petrographic 

analysis, (see fig. 51)

FIGURE 21. Columnar section of part of the Almond Formation at 

locality C in NEV4NEV4 sec. 3, T. 16 N., R. 102 W. Depositional environments and lithof acies are identified to the left of the column and 

lithologies are described to the right of the column.

The forebeach lithof acies consists of 18 ft of light-gray, 

very fine grained to medium-grained sandstone in parallel to subparallel beds. The surf zone is 15 ft thick and 

is composed of light-gray, very fine grained to mediumgrained sandstone in small-scale, low-angle, trough 

crossbeds and some small-scale, fairly high-angle,

FIGURE 22. Outcrops of barrier sandstone at locality C in 

NEV4NEV* sec. 3, T. 16 N., R. 102 W. DU, dune sandstone; FB, 

forebeach sandstone; SU, surf sandstone. View is southward.

FIGURE 23. Barrier bar G at locality C in NEViNE1/* sec. 3, T. 16 

N., R. 102 W., showing dune (DU), forebeach (FB), and surf (SU) 

sandstone lithofacies. Scale is indicated by person standing in the 

upper right part of the photograph.

planar crossbeds. The basal 43 ft of the barrier are composed of tan and brown, very fine grained to finegrained, middle shoreface sandstone. The middle 

shoreface sandstone is bioturbated and contains abundant Ophiomorpha sp. (fig. 26); the upper part has two 

thin dark-brown hematitic layers that are resistant to 

weathering and cap dip slopes.

A cross-sectional profile of a beach is preserved at the 

top of the barrier sequence along the north side of a dry 

wash that trends east-west and crosses the outcrops at 

locality D. The dune sandstone (part of the foredune 

ridge) at the top of the barrier can be seen to grade 

laterally into a berm sandstone across a swale that is 

interpreted as the back shore or back beach. This relationship is visible in figure 27, where a person is standing in the swale; the dunes cap the outcrops upslope to
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 25

FIGURE 24. Root casts and organic staining on the upper surface of 

dune sandstone in barrier bar G at locality C in NEViNEVi sec. 3, 

T. 16 N., R. 102 W.

the left, and the berm forms a low rise about 30 ft to 

the right. Remnants of planar crossbedding in the berm 

include foreset laminae, an avalanche crest, and tabularlike bedding that suggests laminar flow from a marineward direction up the forebeach (right to left). The 

eroded remnants of a ridge-and-runnel system are present as a small mound with low relief on the top of the 

outcrops of the forebeach 140 ft to the right (seaward) 

of the crest of the berm. The ridge-and-runnel system 

has planar crossbedding with distinct foresets that dip 

westward (landward) at 20°-35°, in a direction normal 

to the strike of the strandline. The position of the ridgeand-runnel system suggests that the beach had a broad 

swash zone and a low angle of slope.

The Pintail coal bed at locality D is 5.5 ft thick, but 

is split into a lower coal bench 2.2 ft thick and an upper 

coal bench 3.2 ft thick by a 3.1 ft of carbonaceous shale. 

The lower coal bench rests directly upon rooted barrier 

sandstone. The upper coal bench is overlain by a 

brackish-water unit, the oyster ridge marker bed (ORM). 

The stratigraphic relations of the Pintail coal bed and 

adjacent rocks are shown in figure 28, a photograph 

taken of outcrops above the swale mentioned in the 

preceding paragraph.

LOCALITY E UPPER AND MIDDLE SHOREFACE 

AND TIDAL CHANNEL

The locality is adjacent to a Mountain Fuel Supply 

Company natural gas pipeline that trends westward for 

about 13 mi from Brady field to where it crosses the 

barrier-coal outcrops in the study area in sec. 16, T. 16 

N., R. 102 W. It is accessible by a road that parallels 

the pipeline.

Marine - '

Swamp =^^=-=T

Bay fill - . Dark-gray silty shale

Gray shale

Tan, very fine grained, crossbedded

sandstone 

Coal, 0.4 ft 

Gray carbonaceous shale

Forebeach

Surf

 

D3

D4

Bay fill r-^^Z

Gray and brown sandstone containing 

Crassostrea sp.

Upper Pintail coal bed, 3.2 ft 

Gray carbonaceous shale 

Lower Pintail coal bed, 2.2 ft

Light gray, very fine grained to mediumgrained sandstone

Light gray, fine- to medium-grained 

sandstone

Light gray, fine-to medium-grained, 

crossbedded sandstone

lllii

Gray and brown, very fine grained to 

fine-grained partly hematitic sandstone 

containing Ophiomorpha sp. *

-10 FEET

-5

-0 

Gray, slightly carbonaceous shale

03

Samples collected for maceral analysis 

Samples collected for palynomorph analysis 

Samples collected for proximate-ultimate analysis 

Samples collected for petrographic analysis, 

(see fig. 51)

FIGURE 25. Columnar section of part of the Almond Formation at 

locality D in NEV4 sec. 10, T. 16 N., R. 102 W. Depositional environments and lithofacies are identified to the left of the column 

and lithologies are described to the right of the column.

Barrier bar G has a thickness of 70 ft and is composed 

of forebeach, upper shoreface (surf) and middle shoreface
26 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

FIGURE 26. Hematite replaced Ophiomorpha sp. in middle shoreface 

sandstone of barrier bar G at locality D in NE1/^ sec. 10, T. 16 N., 

R. 102 W.

FIGURE 27. Barrier bar G at locality D in NEJ4 sec. 10, T. 16 N., 

R. 102 W., showing the oyster ridge marker (ORM), the Pintail coal 

bed, and dune (DU), berm (BM), forebeach (FB), surf (SU), and middle 

shoreface (MSF) sandstone lithofacies. Scale is indicated by a person (circled) standing in the swale between the dune-washover and 

berm sandstones. View is northward parallel to the f acies strike of 

the barrier; the sea was to the right.

sandstone in the northern part of the locality. The 

sedimentary structures of these sandstones are similar 

to those described previously. The upper 57 ft of the 

upper and middle shoreface barrier sandstones is replaced southward across the locality by rocks deposited 

in a tidal channel, as shown in figure 29. The tidal channel is filled with small, lenticular lenses of finingupward, tan, small-scale, low-angle, trough crossbedded, 

very fine grained to medium-grained sandstone that is 

interbedded with thin beds of gray shale and siltstone 

(fig. 30). The directions of the dips of the bedding in the 

channel sandstones indicate that the sediment 

transport direction was eastward (seaward) (fig. 31). The

FIGURE 28. The Pintail coal bed exposed in a trench dug at locality 

D in NEV4 sec. 10, T. 16 N., R. 102 W. The oyster ridge marker 

- (ORM) overlies the coal bed and the barrier sandstone underlies the 

coal bed.

unidirectional eastward transport indicates that the 

channel did not act as a conduit for flood tides. The 

south margin of the channel is covered by Quaternary 

sand dunes, soil, and vegetation for several hundred feet 

south of locality E, but on aerial photographs the channel appears to have an overall width of about 2,100 ft. 

The Pintail bed is split into three thin, widely separated coals, each of which is less than 1.5 ft thick at 

locality E; southward for a short distance along outcrops, the two lower coal beds wedge out and are missing. The oyster ridge marker bed here is 5-8 ft thick. 

Many of the Crassostrea sp. shell fragments within the 

marker bed are water-worn, pitted, and crisscrossed by 

numerous borings. The borings are sinuous, less than 

0.04 in. wide, and probably are caused by the sponge, 

Clinona sp. (W. A. Cobban, oral commun., 1979).

LOCALITY F TIDAL CHANNEL AND MIDDLE SHOREFACE

The locality is about 0.5 mi north of Wyoming 

Highway 430 and is accessible by an unimproved road 

that branches northward from the highway in SWV4 sec. 

33, T. 16 N., R. 102 W., 0.5 mi east of the Mud Springs 

Ranch.

The upper 10-15 ft of barrier bar G is covered by soil 

and vegetation at locality F. The exposed part consists 

of 12 ft of tan and brown, very fine grained to finegrained, bioturbated, middle shoreface sandstone containing abundant Ophiomorpha sp. (fig. 32). The barrier sandstone is abnormally thin at the locality, and 

a considerable part of it here may have been eroded prior 

to the deposition of overlying beds.

Overlying barrier bar G at locality F is about 10 ft 

of white, lenticular tidal channel sandstone (fig. 33). This
NORTH

Mountain Fuel Supply Company 

natural gas pipeline

Tidal channelOyster reef (0MB)

Swamp

barrier 

barG

Bay fill

Oyster reef (0MB) 

Swamp

Gray sandstone containing

Crassostrea sp. 

Pintail coal bed

Carbonaceous shale and coal

Light-gray sandstone, 

rooted at the top

Gray and tan, fine-grained, 

crossbedded sandstone

''&~.!&.?&7;<-3i Rust, fine-grained sandstone

'''''' containing Ophiomorphd Sp

Gray, very fine grained 

bioturbated sandstone

Rust, very fine grained sandstone

Middle shoreface

Light-gray and tan, very fine grained 

bioturbated sandstone

~ 7 Gray Shale

SOUTH

- Gray shale

' rr-*.' VrrsV.'^iJ^X

S^V''.'.'^:';-/".*^-^ : -~*a Tan sandstone containing .-..'iX.'-v£i*'.''V''^»v.''; >J*' *THJ .-> , Crassostrea sp. 

Carbonaceous shale and coal (Pintail)

Tan, very fine grained to 

medium-grained, lenticular, 

crossbedded sandstone and 

interbedded gray shale and siltstone

Brown sandstone containing 

lenses of clay pebbles

Gray, very fine grained sandstone 

containing Op/ii'omorpha sp.

Nearshore 

marine

 - Gray shale

0.1 MILE

I

Sample collected for petrographic 

analysis (see fig. 51)

Sample collected for palynomorph 

analysis

FIGURE 29. Columnar sections of part of the Almond Formation at locality E in SEVt sec. 16, T. 16 N., R. 102 W. Depositional environments and lithofacies are identified

to the left of the columns and lithologies are described to the right of the columns.

M 5S 

tB 

> &O

> *

ai i

H

to
28 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

FIGURE 30. Thin, lenticular sandstone interbedded with siltstone 

and shale in a tidal channel sequence in barrier bar G in the southern 

part of locality E in SEV4 sec. 16, T. 16 N., R. 102 W. Scale is indicated by a person standing near the center of the photograph.

FIGURE 31. Rose diagram 

showing sediment transport 

directions of sandstone beds in 

a tidal channel at locality E; 

100 measurements.

fining-upward, very fine grained to medium-grained 

sandstone is poorly sorted and has small-scale trough 

crossbeds. It is much lighter colored, coarser, and 

cleaner than most of the lagoonal sandstones studied, 

which suggests that it was derived from reworked dune 

and beach sands from the upper part of the barrier

Bay fill

Oyster reef (OMB)

Swamp 

Tidal channel

Bayfm

Tidal channel

Middle shoreface J 

barrier bar G

F2 

Swamp

Tidal channel (?)

Bay fill

Gray shale

Gray sandstone containing

Crassostrea sp. 

Gray carbonaceous shale 

Waxwing coal bed, 1.0 ft

Brown, very fine grained, 

crossbedded sandstone

Gray shale, sandy at the top

Gray carbonaceous shale

Pintail coal bed, 4.3 ft 

Gray carbonaceous shale

White, very fine grained to medium- 

"'--<-< -£ grained, lenticular, crossbedded 

sandstone

Covered

$&&&

-svtf

Gray and brown, very fine grained 

to fine-grained sandstone 

containing Ophiomorpha sp.

Gray carbonaceous shale

Tan, very fine grained sandstone

Gray shale

-10 FEET

-5

-0

 Samples collected for petrographic analysis 

F2 (see fig. 51)

FIGURE 32. Columnar section of part of the Almond Formation at 

locality F in SVzNW^ sec. 33, T. 16 N., R. 102 W. Depositional environments and lithofacies are identified to the left of the column and 

lithologies are described to the right of the column.

located to the east. The sands were probably transported by meteoric and tidal waters in a small stream 

that flowed landward (westward) off the back-barrier 

flat through a swampy area at the seaward edge of the 

lagoon.

The Pintail coal bed is composed of 4.3 ft of clean, 

bright coal in a trench that was dug above the tidal 

channel at the locality. Twenty-one feet above the top 

of the Pintail coal bed is a 1-ft-thick bed of coal that was 

named the Waxwing coal bed by the author (Roehler, 

1978a). The Waxwing coal bed is lenticular and is missing in outcrops north of locality F, but it persists for 

more than 6 miles to the south of the locality as a bed 

having a persistent thickness between 1 and 2 ft.

The oyster marker bed (OMB) that overlies the Waxwing coal bed at locality F has an appearance and
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 29

0MB

FIGURE 33. Tidal channel (TC) situated above a middle shoreface 

sandstone lithofacies (MSF) of barrier bar G at locality F in 

SV4NWV4 sec. 33, T. 16 N., R. 102 W. The oyster marker bed (OMB) 

forms a small ridge overlying the tidal channel. The Pintail coal bed 

overlies the tidal channel here.

composition similar to the oyster ridge marker (ORM) 

at localities C, D, and E. However, the two marker beds 

are clearly at different stratigraphic positions, as shown 

on plate 1, and, as explained later in a discussion of the 

geologic history of barrier bar G, they have different 

origins.

LOCALITY G RECURVED SPIT, 

TIDAL INLET, AND FLOOD-TIDAL DELTA

Locality G straddles Wyoming Highway 430 in SWl/4 

sec. 33, T. 16 N., R. 102 W. and NW1^ sec. 4, T. 15 N., 

R. 102 W., 0.7 mi east of the Mud Springs Ranch. The 

barrier outcrops 200 ft north of the highway (section 

3, fig. 34) consist of middle shoreface, surf, and recurved 

spit sandstone. Only the upper 10 ft of the middle 

shoreface sandstone is exposed there; it consists of tan, 

bioturbated, fine- to medium-grained sandstone with 

scattered Ophiomorpha sp. The overlying surf sandstone is about 6 ft thick and is composed of white, fineto medium-grained sandstone in small-scale, trough 

crossbeds. The recurved spit at the top of the barrier 

sandstone sequence is 10 ft thick and is composed of 

white, fine- to medium-grained sandstone in thin, even, 

parallel beds that dip southward (into an adjacent tidal 

inlet) and contain a few, scattered Ophiomorpha sp. 

Southward along the barrier outcrops, but still north 

of Highway 430 (sections 4 and 5, fig. 34 and fig. 35), 

the surf and recurved spit sandstones are replaced by 

a tidal inlet that cuts through the barrier sandstones. 

The inlet has a scoured base and is filled with tan and 

gray, very fine grained to medium-grained sandstone 

that contains coal fragments and clay pebbles in largescale, low-angle, trough crossbeds. The crossbeds dip 

mostly in northwest and southeast directions (fig. 36), 

which correspond to the transport directions for the 

sediments that moved landward through the inlet on 

flood tides and seaward through the inlet on ebb tides, 

respectively. Mineralized wood in the lower parts of the 

inlet sandstone sometimes contains Teredo tubes, the 

burrows of a small pelecypod, commonly called a ship 

worm. The inlet sandstone thins south of Highway 430 

across the locality, as shown in figure 34.

Quaternary alluvium fills the valley of Salt Wells 

Creek and covers outcrops for several hundred feet 

south of Highway 430 at locality G. Near the south edge 

of the locality the barrier section is again well exposed 

(section 6, fig. 34). There, the tidal inlet has thinned to 

6 ft of tan, crossbedded sandstone; it is underlain by 

10 ft of tan, very fine grained to medium-grained, 

tabular, crossbedded sandstone, that comprises the 

northern edge of a large flood-tidal delta. The flood-tidal 

delta was deposited mostly by migrating megaripples 

(fig. 37) in a sandstone sequence that has distinctly 

northwest (landward) sediment transport directions (fig. 

38). The direction of dip of the bedding indicates a 

strong northwest flow of tidal currents across part of 

a flood ramp and through a flood channel. The geographic relations of the flood ramp and flood channel 

lithofacies are illustrated in figure 11, nos. 1 and 2. 

Below the flood-tidal delta sandstone are 8 ft of lightgray, even, parallel, thin-bedded sandstone that is tentatively designated a recurved spit. Normal surf and 

middle shoreface sandstone units are present at the base 

of the section.

The Pintail coal bed at locality G is composed of 

3.6-4.2 ft of clean coal, except for outcrops immediately north of Highway 430, where it is burned to red 

powdery ash and clinkers. A tidal-channel sandstone of 

lagoonal origin and the oyster marker bed overlie the 

Pintail coal bed and are easily identified in the outcrops 

at the locality illustrated in figure 35.

LOCALITY H TIDAL FLAT AND FLOOD-TIDAL DELTA

Flood-tidal delta equivalents of barrier bar G at locality H are composed of 36 ft of tan, very fine grained 

to medium-grained sandstone (fig. 39). The sandstone 

here comprises a segment of the southern part of the 

flood-tidal delta that was described at locality G. The 

flood-tidal delta sandstone at locality H has large-scale, 

planar crossbeds with mostly steeply southwest-dipping 

foreset laminae (fig. 40). These beds were deposited by 

southwest-migrating sand waves along the southern 

part of the flood ramp (fig. 41). The flood-tidal delta is 

overlain by 13 ft of tan and gray, very fine grained, irregularly lenticular, rippled sandstone in beds as thick
NORTH

Barrier bar G 

and lateral 

equivalents

SOUTH

1.0ft

3.6ft

TC Tidal channel

0MB Oyster marker bed

BF-Bayfill

SW Salt marsh

RS Recurved spit

SU-Surf

TF-Tidal Flat

FD-Flood tidal delta

MSF Middle shoreface

Tl-Tidal inlet

U Ophiomorpha sp.

^ Crassostrea sp.

G4

Sample collected for petrographic analysis, 

(see fig. 51)

H Tl

h-1 

2$h-1 

f

Oo 

> fa

w

S3

W > S3O 

I

O

FIGURE 34. Columnar sections of part of the Almond Formation at locality G in SWVi sec. 33, T. 16 N., R. 102 W and NW sec. 4, T. 15 N., R. 102 W. Lithologies

are identified on plate 1.
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 31

FIGURE 35. Barrier bar G on the north side of Highway 430 in SWVi 

sec. 33, T. 16 N., R. 102 W., showing the oyster marker bed (OMB), 

tidal channel (TC), tidal inlet (TI), recurved spit (RS), surf (SU), and 

middle shoreface (MSF) sandstone lithofacies. Person (circled) in the 

left center of photograph provides scale.

FIGURE 36. Rose diagram showing sediment transport directions 

of sandstone beds in a tidal inlet 

at locality G; 100 measurements.

as a few inches that are interbedded with thin, gray, 

sandy shale. These rocks are of tidal-flat origin (figs. 42 

and 43). The tidal flat (see fig. 11, no. 3) has numerous 

trace-fossil burrows tentatively identified by the author 

as Skolithos (fig. 44) and Diplocraterion (fig. 45). The 

Skolithos burrows resemble the burrows of the Holocene 

fiddler crab, Uca sp., that have been observed on tidal

FIGURE 37. Parallel sets of northwest-dipping (landward) laminae 

that are part of a flood ramp or flood channel in a flood-tidal delta 

sandstone lithofacies at locality G in SWViNWVi sec. 4, T. 15 N..R102 W. Camera lens cap is used for scale.

FIGURE 38. Rose diagram 

showing sediment transport 

directions of sandstone beds 

in a flood-tidal delta at locality G; 50 measurements.

flats along the coast of South Carolina. Shells of 

Crassostrea sp. are present in a 2-ft-thick sandstone at 

the base of the tidal flat sequence in measured sections 

about 200 ft north of the locality (sections 71 and 72, 

plate 1), and these shells are believed to be located in 

an oyster flat resting on the landward part of the flood 

delta (fig. 11, no. 3).
32 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Oyster reef (0MB)

Hi'

Bay fill 

Swamp

Tidal channel 

Bay fill

Swamp

Tidal flat

H2

H3

Flood-tidal deltabarrier bar G

Swamp

Gray and brown, very fine grained 

to medium-grained sandstone 

containing Crassostrea sp.

Gray shale; ironstone concretions 

Waxwing coal bed, 1.2 ft 

Tan, very fine grained sandstone 

Gray shale

Pintail coal bed, 5.3 ft 

Carbonaceous shale

Tan, very fine grained to mediumgrained sandstone and finely 

interbedded gray.shale; 

abundunt burrows

Tan, very fine grained to mediumgrained, sandstone, crossbedded

r 10 FEET 

-5

Gray carbonaceous shale

H3

Samples collected for petrographic analysis, 

(see fig. 51)

FIGURE 39. Columnar section of part of the Almond Formation at 

locality H in NEViSWW sec. 8, T. 15 N., R. 102 W. Depositional environments are identified to the left of the column and lithologies are 

described to the right of the column.

The Pintail coal bed is 5.3 ft thick at locality H. The 

oyster marker bed is 24 ft thick and has dark-brown 

layers that contain shell concentrations and interbedded 

tan layers that contain only scattered shells, all in sandstone matrices. Parts of the oyster marker bed have lowangle foreset laminae with southerly dip components,

FIGURE 40. Sand wave in a flood-tidal delta (FD) of barrier bar G 

at locality H in NE1/4 SW1/4 sec. 8, T. 16 N., R. 102 W. The foreset 

laminae dip strongly to the southwest. Handle of the pickmattock 

used for scale is 17 in. long.

FIGURE 41. Rose diagram 

showing sediment transport directions of sandstone beds in the southern 

part of a flood-tidal delta 

at locality H; 100 measurements.

which suggests a southward accretion direction for the 

oyster bed. Crassostrea valves in the bed are locally 

large and well preserved (fig. 46).
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 33

FIGURE 42.-Locality H in NEV4SEV4 sec. 8, T. 15 N., R. 102 W., 

showing the stratigraphic positions of the oyster marker bed (OMB), 

the Pintail coal bed, a tidal flat sequence (TF), and a flood-tidal delta 

sequence (FD).

FIGURE 43. Tidal flat sequence of thin interbedded sandstone and 

shale at locality H in NEV4SWV4 sec. 8, T. 15 N., R. 102 W.

FIGURE 44. Skolithosl burrows in a tidal flat sequence at locality 

H in NEV4SWV4 sec. 8, T. 15 N., R. 102 W. Photograph shows vertical orientation. Knife at top of photograph is about 0.3 ft long.

FIGURE 45, Diplocraterion'} burrow in tidal flat sequence at locality H in NEV4SWV4 sec. 8, T. 15 N., R. 102 W. Photograph shows 

vertical orientation.

FIGURE 46. Crassostrea sp. in the oyster marker bed (OMB) at locality H in NEV4SWV4 sec. 8, T. 15 N., R. 102 W. Pencil is about 0.5 ft 

long.

LOCALITY I FLOOD-TIDAL DELTA AND LAGOON

The lateral equivalents of barrier bar G at locality I 

consist of 7 ft of white, crossbedded sandstone that was 

deposited at the southern edge of the flood-tidal delta discussed at localities G and H. The flood-tidal delta sandstone here is thinner and is rapidly wedging out southward in outcrops into lagoonal rocks that are composed 

mostly of gray shale, siltstone, carbonaceous shale, and 

coal (figs. 47 and 48). Overlying the flood-tidal delta sandstone are thin beds of shale and sandstone that contain 

Crassostrea sp.; these are part of an oyster flat (fig. 11, 

no. 3). Between the oyster flat and the overlying Pintail 

coal bed are 4.7 ft of thin-bedded, gray, current-rippled, 

extensively burrowed sandstone, siltstone, and thin, interbedded gray shale that correlate to the tidal flat sequence containing the Skolithos burrows at locality H.
34 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Oyster reef (OMB)

Bay fill 

Swamp 

Bay fill

Swamp A

Tidal flat ^

Oyster reef 

Bay fill

Flood-tidal deltabarrier bar G

Swamp 

Bay fill 

Swamp

Bay fill 

Swamp

12

Bay fill

r 10 FEET

-5

Tan and gray sandstone containing Crassostrea sp.

Gray shale

Waxwing coal bed, 2.0 ft

Gray, partly carbonaceous shale 

and siltstone

Pintail coal bed, 4.4ft

Gray, very fine grained sandstone 

and siltstone; burrows

Gray sandstone containing 

Crassostrea sp.

Gray shale containing

v££ ^-^li^li^^y^White. very fine grained to

medium-grained, crossbedded 

sandstone

Carbonaceous shale and coal 

Gray shale and siltstone 

Gray carbonaceous shale 

Gray shale and sandstone

Gray shale and siltstone

Coal, 2.4ft

Gray carbonaceous shale

Finch coal bed, 4.0ft 

Gray shale and sandstone

A Sample collected for palynomorph analysis

0 Sample collected for petrographic analysis

12 (see fig. 51)

FIGURE 47. Columnar section of part of the Almond Formation at 

locality I in SWWNWV4 sec. 17, T. 15 N., R. 102 W. Depositional 

environments are identified to the left of the column and lithologies 

are described to the right of the column.

The Pintail coal bed at the locality is composed of 

4.4 ft of clean, bright coal. Overlying the Pintail coal 

bed are 18 ft of bay-fill and swamp deposits composed 

of shale and siltstone with 2.0 ft of coal, the Waxwing 

coal bed, near the middle. The oyster marker bed is 19 

ft thick and contains layered shell concentrations.

LOCALITY J WASHOVER FAN AND LAGOON

The lateral equivalents of barrier bar G at locality 

J, near the southern boundary of the study area, are 

mostly gray, parallel, tabular, very fine grained to 

medium-grained sandstone beds with landward-dipping 

laminae and some small-scale, trough crossbeds that

FIGURE 48. Locality I showing the oyster marker bed (OMB), the 

Pintail coal bed, a tidal flat sequence (TF), a flood-tidal delta sequence (FD), and the Finch coal bed. Person (circled) is about 5.5 ft 

tall.

are part of a large washover fan of probable storm 

origin. The washover fan is exposed along a line of outcrops that cross the northeast part of sec. 20, T. 15 N., 

R. 102 W, but it is covered south of there in the 

southeast part of sec. 20 (south of section 7, figure 49). 

The intertonguing relations of the washover fan and adjacent lagoonal rocks to the north are illustrated in sections 1, 2, and 3, in figure 49. Thin, tabular beds that 

compose part of the washover fan are shown in figure 

50. The section underlying the fan is mostly gray sandstone, siltstone, shale, carbonaceous shale, and coal that 

are bay-fill and swamp deposits of lagoonal origin.

The Pintail coal bed is present above the washover 

fan at locality J, where it consists of 3.1-4.3 ft of clean 

coal. A swamp sequence composed mostly of carbonaceous shale separates the Pintail coal bed from the 

overlying Waxwing coal bed. Overlying the Waxwing 

coal bed is a bay-fill sequence of mostly soft, dark-gray 

shale capped by the ledge-forming oyster marker bed.

SANDSTONE PETROLOGY

The sandstone lithofacies of barrier bar G and its 

lateral equivalents are composed 65-85 percent of clear 

to milky quartz grains; 15-35 percent of varicolored 

rock fragments, mica, feldspar, and brown, tan, or white 

cement; and 1-2 percent of heavy mineral grains. 

Photomicrographs of the various sandstone lithofacies 

from localities A to J are shown in figure 51. The sand 

grains are angular to subangular and the longest dimension ranges from less than 1/16 mm to more than 

1/4 mm; the mean grain diameter commonly i& 

1/8-1/4 mm (fig. 52). The sandstones are predominantly
NORTH

Finch coal bed (with ^l(;Si:^ 

tonstein parting]

.2 ft

TC Tidal channel 

0MB Oyster marker bed 

BF-Bayfill 

SW Swamp 

TF-Tidal flat 

WO Washover

U Ophiomorpha sp. 

"V^ Crassostrea sp. 

&7 Fossil wood

d 

o

I I3 i i

O Oww > 

» 

o

> 2 do o o

0 100 200 FEET 

I I I

J1, Sample collected for petrographic analysis, 

(see fig. 51)

FIGURE 49. Columnar sections (1, 3, 4, 5, 6, 7) of the Almond Formation at locality J in E J/2 sec. 20, T. 15 N., R. 102 W. Lithologies are identified on plate 1.

COen
36 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

FIGURE 50. Parallel, tabular-bedded sandstone composing part of a 

washover fan at locality J. By removing the dip component related 

to postdepositional deformations, it is obvious that these eastwarddipping sandstones originally dipped gently westward. The Jacob's 

staff used for scale is 5.0 ft long.

fine grained, and barrier bar G can be characterized as 

a fine-grained barrier system. The coarsest sand grains 

are predictably found in dune, forebeach, tidal-inlet, and 

flood-delta lithofacies, because of the high energy level 

of the environments; the sand grains become progressively finer textured seaward from the forebeach 

to the middle shoreface to the lower shoreface. The 

grains are poorly to moderately well sorted. The surf, 

forebeach, berm, and dune sandstones are nearly always 

cleaner and better sorted than other lithofacies.

Three kinds of interstitial cement have been observed 

visually in outcrop sandstone samples. Sand grains in 

the middle shoreface are cemented by hematite and an 

unidentified white clay, which is probably illite. The upper shoreface and dune sandstones are also cemented 

by white clay. The tidal-channel, flood-tidal delta, and 

tidal-flat sandstones are cemented by an undetermined 

tan carbonate, which is probably calcite.

The heavy mineral grains are 90-95 percent nonopaque. The nonopaque fraction is generally composed 

of 10-25 percent garnet, 25-50 percent zircon, 5-15 

percent rutile, and 25-45 percent tourmaline. The heavy 

minerals indicate plutonic source beds.

DEPOSITIONAL HISTORY 

AND PALEOGEOGRAPHY

Barrier bar G evolved through stages of youth, 

maturity, and old age before the barrier system was 

abandoned and later partly destroyed by marine transgression and erosion. Five stages of evolution are

portrayed on paleogeographic maps (figs. 53 to 57). The 

stratigraphic positions of the five stages are indicated 

on plate 1. The paleogeographic maps are diagrammatic 

reconstructions that depict the locations of the depositional environments and lithofacies that were identified 

in measured sections along the line of barrier outcrops; 

the author has used artistic license to depict hypothetical conditions away from the outcrops conditions that 

are, nonetheless, consistent with those of Holocene 

barriers.

A close relationship of the Pintail coal bed and barrier bar G is demonstrated from the fact that the coal 

and barrier sandstone are either in contact or are 

situated within a few feet of each other across the area 

studied. The relationship is not coincidental. The prograding barrier sandstone provided a stable platform 

for peat accumulation and a reservoir of freshwater for 

utilization by plants growing upon the barrier. Conversely, the fibrous peat mat that formed the Pintail 

coal bed provided a blanket of extremely tough, resilient 

material that shielded the underlying soft barrier sand 

from fluvial and marine erosion prior to burial and 

diagenesis.

YOUTH

The earliest period of deposition of barrier bar G is 

not well understood because it is poorly preserved in 

the rock record. The barrier appears to have originated 

as a chain of narrow, barren, or sparsely vegetated sand 

banks, or sand islands, situated 1.0 to 2.0 mi offshore, 

whose sedimentation was controlled by southward 

longshore currents (fig. 53). The subaerial parts of the 

islands were probably formed by linear dune ridges 

(DU). Back-barrier flats were small or missing. A forebeach (FB) sand deposit was present on the seaward 

side of the barrier and shallow marine parts included 

surf (SU), middle shoreface (MSF), and lower shoreface 

(LSF) sands. Mud (MS) was deposited in deeper marine 

waters for unknown distances offshore.

Early in the barrier history a lagoon (LA) evolved from 

a saltwater arm of the sea into a narrow, brackish, openwater estuary or bay. The resulting lagoon was thus 

situated between small, barren, barrier islands to the 

east and a freshwater coastal plain to the west. The 

lagoon contained numerous linear sand bars and oyster 

reefs (OB). Landward of the lagoon were broad areas of 

tidal creeks and salt marshes (SW). Tidal inlets (TI) 

through the barriers were located opposite the mouths 

of major drainages entering the lagoon from the west. 

This relationship suggests that the barrier itself formed 

by the submergence of a beach ridge during marine 

transgression.
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 37

Loc. Al, lower shoreface Loc. Bl, ebb tidal delta Loc. Cl, dune

Loc. C2, forebeach Loc. C3, surf Loc. Dl, dune

Loc. D1A, berm Loc. D2, forebeach Loc. D3, surf

FIGURE 51 (above and following pages). Photomicrographs of sandstones composing barrier bar G and its lateral equivalents. 

Sample collection sites are indicated on columnar sections in figures that accompany descriptions of localities A to J. Magnification X 50; under slightly polarized light to enhance grain images.

Sparse vegetation along the shores of the lagoon was 

adapted to saltwater and brackish water. The vegetation probably included emergent hydrophytes similar 

to the cordgrass, Spartina, and the mangrove, Rhizophora, genera that are common in similar Holocene

environments. Sediments deposited at the margins of 

the lagoon were mostly carbonaceous muds, whereas 

dark-gray noncarbonaceous muds were deposited in 

subtidal open-water parts of the lagoon. The chemistry 

of the lagoon substrate probably is alkaline and aerobic
38 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Loc. El, forebeach Loc. E2, surf

Loc. E3, upper tidal channel Loc. E4, lower tidal channel Loc. Fl, tidal channel

Loc. F2, middle shoreface Loc. Gl, tidal flat Loc. G2, recurved spit

nearly everywhere, and therefore was not conducive to 

the formation and preservation of peat.

MATURITY

The barrier reached its optimum size and shape during the stage of maturity (fig. 54). During this period 

of development it consisted of a distinct chain of 

drumstick-shaped islands, the ends of which were

terminated by recurved spits at tidal inlets. The 

barrier had well developed foredune ridges (DU) and 

freshwater peat swamps situated across back-barrier 

flats (PS). The influence of tides upon sedimentation is evident by the presence of flood-tidal deltas 

(FD), ebb-tidal deltas (ED), tidal inlets (TI), and tidal 

channels (TC). Oyster beds (OB) were present on parts 

of the flood-tidal deltas. The presence of a large 

washover fan (WO) suggests that the barrier was
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 39

Loc. G3, middle shoreface Loc. G4, tidal inlet Loc. G5, flood tidal delta

Loc. HI, oyster marker bed Loc. H2, tidal flat Loc. H3, flood tidal delta

Loc. II, tidal flat Loc. 12, flood delta

occasionally subjected to severe hurricane-type storms. 

Forebeach (FB), surf (SU), middle shoreface (MSF), and 

lower shoreface (LSF) sand lithofacies actively prograded seaward, and marine mud (MS) was deposited 

offshore.

The geographic location and geometry of the large 

flood-tidal delta in the northwest part of T. 15 N., R. 

102 W. is readily discernible in sandstone outcrops. 

Sand waves on the flood ramp in sec. 4, T. 15 N., R.

102 W. (locality G, fig. 37) have foreset laminae that dip 

northwest. Similar northwest dips are present southward for several hundred feet across outcrops of the 

flood-tidal delta in sec. 5, T. 15 N., R. 102 W., but at 

a point in the northwest part of sec. 8, T. 15 N., R. 102 

W., the northwest dips in the foresets abruptly change 

direction to the southwest. The southwest dips persist 

from there southward across the west part of sec. 8, T. 

15 N., R. 102 W. (fig. 40). The change in dip directions
40 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Locality and sample

A1

B1

C1

C2

C3

D1A

D1

D2

D3

D4

E1

E2

E3

E4

F1

F2

G1

G2

G3

G4

G5

H1

H2

H3

11

12

J1

Depositional 1environment

LSF

FD

DU

FB

SU

BM

DU

FB

SU

MSF

FB

SU

TC

TC

TC

MSF

TC

RS

MSF

Tl

FD

OMB

TF

FD

TF

FD

WO

GRAIN SIZE

Silt

Vfei

Very fine Fine Medium 

nm Va mm V4 mm Vi mm

I ' '

i

1 1

1 1 

1 i 1

1 1

1 1

1 1

1 I 

1 1

1 1

1 1

| . |

1 I 

1 i 1

 1

 1

 . 1

1 - 1

| . |

i 1

FIGURE 52. Grain sizes of selected sandstone samples from outcrops of barrier bar G and its lateral equivalents. Abbreviations for depositional environments are explained on plate 1.

is construed as reflecting a planimetric axis, which confirms that the flood-tidal currents fanned outward into 

the lagoon after leaving the tidal inlet (fig. 11). Further 

evidence concerning the size and shape of the flood-tidal 

delta is provided by the location of tidal flats and oyster 

flats in shallow, brackish-water, lagoonward parts of the 

flood-tidal delta at localities H and K, discussed 

previously.

As a result of barrier progradation the lagoon (LA) 

behind the barrier began to expand in a seaward direction. The expansion was primarily over the back-barrier 

flat, where it was accompanied by the weathering and

erosion of the exposed upper parts of the underlying 

barrier sand. Saltwater-saturated sands in the upper, 

older parts of the barrier were flushed by freshwaters 

of meteoric origin, and dense concentrations of shrubs 

and trees appeared. Large freshwater rivers entered the 

lagoon from the coastal plain to the west, which introduced freshwater vegetation and initiated peat (PS) 

deposition in areas that were partly interspersed with 

or rested upon older brackish-water bay-fill and salt 

marsh (SW) deposits. The bay-fill and salt marsh sediments were composed of gray, silty mud and gray and 

brown carbonaceous mud, respectively.
R. 103W.

DESCRIPTION OF BARRIER BAR G AND ITS LAGOON

R. 102 W. R. 101 W.

41

5 KILOMETERS

FIGURE 53. Paleogeographic map of barrier bar G and its lagoon in the stage of youth. SW, Tidal creeks 

and salt marshes; LA, lagoon; OB, oyster reef; FB, forebeach; TI, tidal inlet; DU, dune ridges; SU, 

surf; MSF, middle shoreface; LSF, lower shoreface; MS, offshore mud. Heavy line shows the geographic 

location of outcrops of the barrier in the studied area.
42 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

R. 103 W. R. 102 W. R. 101 W.

5 KILOMETERS

FIGURE 54. Paleogeographic map of barrier bar G and its lagoon in the stage of maturity. PS, Peat 

swamp; SW, tidal creeks and salt marshes; LA, lagoon; WO, washover; FD, flood tidal delta; ED, 

ebb tidal delta; TI, tidal inlet; OB, oyster reef; DU, dune ridges; FB, forebeach; SU, surf; MSF, middle shoreface; LSF, lower shoreface; MS, offshore mud. Heavy line shows the geographic location 

of outcrops of the barrier in the studied area.
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 43

R. 103W. R. 102W. R. 101 W.

5 KILOMETERS

FIGURE 55. Paleogeographic map of barrier bar G and its lagoon in the stage of old age. PS, Peat swamp; 

SW, tidal creeks and salt marshes; DU, dune ridges; FB, forebeach; SU, surf; MSF, middle shoreface; 

LSF, lower shoreface; MS, offshore mud. Heavy line shows the geographic location of outcrops of 

the barrier in the studied area.
44 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

R. 103 W. R. 102 W. R. 101 W.

5 KILOMETERS

FIGURE 56. Paleogeographic map of barrier bar G and its lagoon at the time the barrier was abandoned. 

PS, Peat swamp; TI, tidal inlet; SW, tidal creeks and salt marshes; DU, dune ridges; SU, surf; MSF, 

middle shoref ace; LSF, lower shoreface; MS, offshore mud. Heavy line shows the geographic location 

of outcrops of the barrier in the area studied.
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON

R. 103 W. R.102W. R. 101 W.

45

5 KILOMETERS

FIGURE 57. Paleogeographic map of the study area following marine transgression over barrier bar G. 

PS, Peat swamp; SW, tidal creeks and salt marshes; LA, lagoon; ORM, oyster reef; TC, tidal channel; The heavy line shows the geographic location of outcrops of the barrier.
46 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

OLD AGE

The lagoon behind barrier bar G began to fill in, and 

the barrier system ceased to exist as a well-defined 

island chain. It acquired many of the physiographical 

characteristics of a strand plain or foreland, as is apparent by the disappearance of bay-fill muds (BF) that 

were replaced by salt marsh deposits (SW) in the rocks 

directly underlying the Pintail coal bed (see fig. 55 and 

plate 1). The barrier sands continued to prograde seaward by uninterrupted deposition of forebeach (FB), surf 

(SU), middle shoreface (MSF), and lower shoreface (LSF) 

facies; marine mud (MS) deposition continued offshore.

The lagoon continued to expand seaward over the 

back-barrier flat. Former open-water areas of the lagoon 

were replaced by tidal channels and salt marshes (SW). 

The waters freshened and islands and platforms of thick 

vegetation were introduced where lenticular peat 

deposits (PS) formed. Freshwater swamps covered parts 

of the dune field (DU) and all of the back-barrier flat, 

and they formed point-like projections landward from 

the back-barrier flats into brackish-water salt marshes 

(SW) in the lagoon. Inlets (TI) were maintained by tides, 

but flood- and ebb-tidal deltas were much reduced in size 

or were absent.

ABANDONMENT

The author speculates that the rate of progradation 

of the barrier sandstone slowed, then ceased, and the 

shoreline began to subside. Figure 56 depicts the study 

area at the approximate time that shoreline submergence and marine transgression started following barrier 

abandonment. At that time the shoreline was nearly 

straight. The shoreline was located nearly 3 mi east of 

where the deposition of barrier bar G began, and few 

vestiges of barrier-lagoon physiography remained. The 

barrier was reduced to a sandy beach with remnants of 

a dune field behind it. The shoreline continued to be 

washed by tides, and salt marshes (SW) were present in 

small areas inland from tidal inlets (TI). Almost all the 

former lagoonal area inland from the dune fields (DU) 

was occupied by freshwater peat swamps (PS), which 

subsequently formed a blanket coal deposit (the upper 

part of the Pintail coal bed). Surf (SU), middle shoreface (MSF), and lower shoreface (LSF) sand facies were 

present but were much attenuated seaward of the forebeach (FB); marine mud (MS) deposition continued 

unabated offshore.

MARINE TRANSGRESSION

Barrier bar G was partly destroyed by marine erosion

that accompanied minor transgression of the epeiric sea, 

probably only a few miles westward across the 

top of the abandoned barrier system. A minor marine 

transgression is suggested by the fact that only a thin 

interval of marine shale separates barrier bar G from 

the overlying superposed barrier bar F. The record of 

the marine transgression is obscure, but the shorelines 

at that time consisted of tidal channels and salt 

marshes; no transgressive beach sandstones have been 

identified.

As the coastline submerged, the freshwater peat 

swamps (PS) overlying barrier bar G were first replaced 

by brackish-water salt marshes and then by a large openwater lagoon (LA) behind a new barrier complex. The 

evidence for this interpretation is based on the presence 

of tidal channels (TC), oyster beds (ORM), and carbonaceous shale (SW) that rest directly upon the Pintail coal bed, and by the fact that all these lithologies 

are in turn overlain by gray, bay-fill (BF) mudstone or 

shale (plate 1).

The erosion that accompanied the marine transgression obliterated nearly all the topographic expression 

of barrier bar G. Some of the irregular thinning of the 

Pintail coal bed in sections 10 to 32 (plate 1) probably 

resulted from this erosion. The erosion surface is not 

readily apparent in outcrops of barrier bar G, but similar 

marine erosion is visible where it has removed large 

segments of barrier bar D in the study area in the WVz 

sec. 22, T. 16 N., R. 102 W (Roehler, 1979c). Channellike 

depressions in barrier bar D that are filled with marine 

shale are as much as 30 ft deep.

The oyster marker bed (ORM) that immediately 

overlies tidal channel sandstones in sections 11, 21, 22, 

and 31 (plate 1) originated by the draping of oyster beds 

across a small delta system where salt and fresh waters 

mixed, as depicted in figure 57.

PALEOECOLOGY

The peat beds from which the Pintail coal bed 

formed are of lagoonal origin and were deposited under 

freshwater conditions. Peat deposition under saltwater 

or brackish-water conditions did not take place for the 

following reasons: (1) the water salinity restricted the 

growth of many types of plants; (2) the oxidation of 

alkaline substrates; and (3) the extensive degradation 

of organic matter by micro-organisms. This implies 

that most of the peat accumulated during the late stages 

of barrier-lagoon development when the lagoon had 

largely filled in and had become a large freshwater 

swamp.
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 47

CLASSIFICATION OF PLANT HABITATS

Marshes, swamps and bogs have been classified under 

the general term "wetlands" by the U.S. Fish and 

Wildlife Service (Cowardin and others, 1979) and the 

U.S. Army Corps of Engineers Institute for Water 

Resources (Reppert and others, 1979). The wetland 

classification explains the distinguishing characteristics 

of basic wet environments, such as marine, estuarine, 

riverine, lacustrine, and palustrine, and it works well 

in modern-day ecosystems, where sediments and living 

organisms can be observed in three dimensions. The 

classification system is difficult to use by geologists 

studying ancient rocks and fossils in two dimensions 

along outcrops that divide the eroded and covered parts 

of formations. For this reason, a more geologically 

oriented classification system of "forest swamp" and 

"reed swamp", which was applied by Stach and others 

(1982), is used in this paper. The terms "forest swamp" 

and "reed swamp" correspond to tree swamp and salt 

marsh, respectively.

The forest swamp habitat consisted of freshwater, 

poorly drained swamps and bogs dominated by profuse, 

thick-stalked, woody vegetation that included trees and 

shrubs and produced extensive peat accumulations. The 

characteristic resulting lithology was coal. The reed 

swamp habitat consisted of brackish-water vegetation 

that was intermittently or permanently water-covered, 

and that was dominated by sparse, herbaceous, thinstalked, grasslike, salt-tolerant vegetation, and no appreciable peat accumulations. The characteristic 

resulting lithology was carbonaceous shale.

The forest swamp and reed swamp habitats envisioned by the author for barrier bar G and its lagoon 

are illustrated diagrammatically in figure 58. The surface of the barrier was the site of peat accumulations

in a forest swamp habitat under freshwater conditions. 

The lower, buried parts of the barrier sandstone were 

probably saturated by salt water, as are modern barrier islands along the coast of South Carolina and 

Georgia, but the upper parts contained freshwater. The 

interface of freshwater and saltwater within the sandstone was maintained by the density differences of the 

two waters and by the continuous recharge of 

freshwater of meteoric origin. The lagoon behind the 

barrier had areas of both forest swamp and reed swamp 

habitats. Areas adjacent to tidal inlets and tidal creeks 

were occupied by brackish-water vegetation as a result 

of the influx of seawater during periods of high tides. 

Between areas of tidal influence were islands and platforms of freshwater occupied by forest swamp 

vegetation.

EVIDENCE OF REED SWAMP AND FOREST SWAMP HABITATS

Fossil-wood (limb and trunk) fragments characteristic 

of the forest swamp habitat are present at a number 

of places along the outcrops of the Pintail coal bed, 

usually in the upper 1-2 ft of the bed (fig. 59). Most of 

the wood has been identified as coniferous, possibly 

pinaceous, and shows little evidence of growth rings 

(R. A. Scott, oral commun., 1979). The lack of growth 

rings probably indicates that the Late Cretaceous 

climate at the time of the coal formation was warm and 

wet and had little seasonal variation. The carbonaceous 

shale splits and partings in the Pintail coal bed generally 

do not contain fossil wood, but they do contain abundant impressions of unidentified aquatic, herbaceous, 

ribbonlike plants that characterize the reed swamp 

habitat.

Palynomorphs in the Pintail coal bed and in carbonaceous shale associated with the coal bed are presently

LAGOON

Reed swamp (brackish water)

with islands of forest swamp

(freshwater)

BARRIER BAR G 

Forest swamp (freshwater)

EPEIRIC SEA 

(saltwater)

 . Peat __ __ _ 

 Carbonaceous shale 

FIGURE 58. Cross section showing hypothetical forest swamp and reed swamp habitats envisioned for barrier bar G and its lagoon.

LTH, low tide height; HTH, high tide height. Not to scale
48 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

FIGURE 59. Fossil tree stump at the top of the Pintail coal bed at 

locality I in SWWNWW sec. 17, T. 15 N., R. 102 W. The stump has 

crumbled into blocks below the jackknife (0.3 ft long).

being studied by Parley Fleming at the University of 

Colorado. Preliminary results of his investigations indicate that the relative abundance of certain categories 

of palynomorphs varies widely in the coal and shale 

samples. The percentages of palynomorphs in six 

categories are shown in figure 60 (Parley Fleming, written commun., 1981). Although the data will require 

substantiation by the study of many more coal and

shale samples, the coal assemblage appears to be composed largely of angiosperm pollen, whereas the carbonaceous shale assemblage is composed largely of 

spores and algal cysts. The differing distribution of 

these categories indicates that there is a direct relationship between the palynomorphs and the lithology, which 

suggests the existence of distinct habitats. Some of the 

most common palynomorphs identified by Fleming are 

illustrated in figure 61.

The Pintail coal bed was sampled for maceral analysis 

at locality D (fig. 24). The bed at locality D consists of 

a lower coal bench, 2.2 ft thick, a middle carbonaceous 

shale parting, 3.1 ft thick, and an upper coal bench, 

3.2 ft thick. Significant maceral matter was obtained 

from both coal benches, but the carbonaceous shale parting is brown and oxidized and contains very little relict 

carbonaceous material that could be observed 

microscopically in maceral terms. The macerals from the 

upper and lower coal benches, figures 62 and 63, were 

identified by R. W. Stanton (written commun., 1981). 

Stanton concluded that the coal benches contain significant amounts of macerals derived from woody material, 

whereas the carbonaceous shale visually contained 

negligible woody material (less than 1 percent). The 

woody material was mostly derived from large trees, a 

fact that suggests it originated in a forest swamp 

habitat. The only significant difference in the maceral

PALYNOMORPH CATEGORY, IN PERCENT

SAMPLE LITHOLOGY AND LOCALITY

Pintail coal bed

Carbonaceous 

shale parting

Loc. D (fig. 24)

Loc. E (fig. 28)

Loc. 1 (fig. 42)

Loc. D (fig. 24)

18

12

17

23

15

28

17

20

7

5

2

28

0

1

0

10

14

16

17

16

46

38

47

3

FIGURE 60. Percentages of six palynomorph categories in the Pintail coal bed and in a carbonaceous shale parting in the coal (Parley Fleming, written commun., 1981).
DESCRIPTION OF BARRIER BAR G AND ITS LAGOON 49

B

D

«^K**>.

FIGURE 61. Palynomorphs from the Pintail coal bed at locality E (A, 5, C, D, E) and from a carbonaceous shale parting 

in the coal at locality E (E, G). A,Erdtmanipottispachysandroides; B, LiKacidites sp.; C, Tricolpites sp.; D, Proteacidites 

cf, P. thalmanni; E, Aquilapollenites quadrilobus; F, Schizosporis parvus; G, Algal cyst? X 1,000.

composition of the two coal benches is that the coal in 

the upper bench shows a greater degree of diagenetic 

degradation and evidence of more oxidation (higher inertinite content).

The most revealing evidence for the presence of reed 

swamp (noncoal-forming) swamp and forest swamp 

(coal-forming) habitats is the thickness and location of

carbonaceous shale splits and partings in the Pintail 

coal bed. As mentioned previously, the peat in the lower 

half of the coal bed accumulated during the stages of 

maturity and old age in lenses on freshwater islands and 

platforms that formed in the lagoon on and between 

areas of brackish water where carbonaceous mud was 

deposited (figs. 54 and 55). The splits and partings in
50 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

Total Vitrinite

Total Exinite

Total Inertinite

Pintail

Lower Bench 

(2.2 ft)

Q*3

...... 1

...... 1

85 

...... 1

...... 2

3 

___ -a

...... 1

12

coal bed

Upper Bench 

(3.2 ft)

71 

2 

5 

Trace 

78 

1 

1 

Trace 

2

9 

1 

10 

Not detected 

20

FIGURE 62. Maceral analyses (in percent) of samples of the Pintail 

coal bed at locality D. For location of sampled intervals see figure 25.

the lower half of the coal bed are thus directly related 

to water salinity and indirectly to the geographic positions of tidal creeks and tidal inlets. During the 

abandonment stage (fig. 56), freshwater peat nearly 

blanketed the lagoon and back-barrier flat as a feature 

of a discrete forest swamp, and these beds formed the 

laterally persistent, massive bench of coal that is present in the upper half of the Pintail bed.

The thick carbonaceous shale split in the Pintail coal 

bed in measured sections 19-34 (plate 1), adjacent to 

locality E in the west-central part of T. 16 N., R. 102 W., 

exemplifies the reed swamp-forest swamp habitats. 

Figure 64 is a block diagram of the area that interprets 

the location of the habitats from an elevated, oblique 

angle looking east across locality E during the stage of 

old age (compare with fig. 55). In figure 64 note that reed 

swamp habitats occupied low topographic areas in juxtaposition to tidal creeks and tidal inlets. The substrate 

in these areas was composed of the remains of brackishwater, herbaceous plants that were deposited as carbonaceous mud (SW). At slightly higher topographic 

elevations, possibly as little as 3-6 ft, were islands and 

platforms of freshwater-woody vegetation representing 

the forest swamp habitat (PS). The substrate there was 

primarily peat, the precursor of the Pintail coal bed.

HOLOCENE ANALOG

The coal-forming depositional environment of the Pintail coal bed is somewhat analogous to Quaternary peat 

accumulations in the Snuggedy Swamp in South 

Carolina. Snuggedy Swamp was investigated by Staub 

and Cohen (1977), who believed that it began as a lagoon

composed of salt marshes and open-water bays behind 

a Pleistocene barrier island. In time, freshwater vegetation encroached upon the salt marshes and formed 

islands of peat in areas between tidal creeks. The peat 

islands eventually enlarged and coalesced to form a 

blanket of peat that covered most of the swamp and 

lapped onto the barrier island. The freshwater-peat deposits in Snuggedy Swamp are as much as 15 ft thick. 

The salt marsh deposits in the swamp are mainly clays, 

silts, and sands, but with a few thin (less than 12 in 

thick) discontinuous peats of probable brackish-water 

origin.

The Snuggedy Swamp is dissimilar to the Pintail coal 

bed in several important ways. The swamp is located 

several miles inland from active barrier-lagoon sedimentation. The quality of the peat deposited in Snuggedy 

Swamp is only low enough in ash within the central 

parts of the swamp to yield a medium-high ash coal 

following the effects of coalification. Some of the 

thickest peat deposits occupy former inlets of the 

Pleistocene barrier island.

COAL COMPOSITION AND RANK

PROXIMATE ANALYSES OF CORE SAMPLES

Analyses of core samples from holes drilled in the 

study area in 1982 by the U.S. Department of Energy, 

Laramie Energy Technical Center, indicate that the Pintail coal bed is High Volatile C Bituminous in rank and 

has heating values that range between 11,780 and 

12,240 Btu/lb on a moist, mineral-matter-free basin as 

determined by the Parr Formula (ASTM Standard 

D-388-77).

As received, proximate analyses of core samples from 

the Pintail coal bed are shown in figure 65. The U.S. 

Department of Energy MSR-10C hole was drilled in the 

NEV4SEV4 sec. 28, T. 16 N., R. 102 W. The Pintail coal 

bed was cored at depths between 163 and 179 ft. The 

hole is situated near measured section 43 (plate 1). Core 

hole MSR-14C was drilled in the SEttSEtt sec. 8, T. 15 

N., R. 102 W. and cored the Pintail coal bed at depths 

between 351 and 372 ft. The hole is located near 

measured section 61 (plate 1). As shown, the analyses 

of the coal from the two holes are remarkably similar, 

although they are 2V& miles apart.

ULTIMATE ANALYSES OF CORE SAMPLES

Ultimate analyses of coal samples from U.S. Department of Energy core holes and from outcrops in the area 

studied indicate that the Pintail coal bed consistently
COAL COMPOSITION AND RANK 51

D

FIGURE 63. Coal macerals in the Pintail coal bed (lower bench, A, B; upper bench, C, D) at locality D. tc, telecollinite; 

rs, resinite; id, inerto-detrinite; sf, semifusinite. For location of sampled intervals see figure 25. X 350.
Ul

Open sea

(saltwater)

OS Tidal inlet Tl

barrier bar G

Reed swamp

(brackish water)

SW

Forest swamp

(freshwater)

PS

Sandstones composing abondoned 

of barrier bar G

~-f iiuaiuiiaimei .« ' .. ^.'" * -" ' ' **"« 

 ^/» * ""«*""* * ' *' * * "" ' '*^'^\ * *. " * ". 

Peat 

" 

swamp

(Pintail coal bed)

PS

FIGURE 64. Diagrammatic reconstruction of part of barrier bar G showing the location of reed swamp and forest swamp habitats. The block diagram is an oblique aerial view 

from sec. 18, T. 16 N., R. 102 W. eastward toward the tidal inlet located in sec. 13, T. 16 N., R. 102 W. The tidal channel in the center-foreground is at locality E (see plate 1).
COAL COMPOSITION AND RANK 53

U.S. Department of Energy hole

PROXIMATE 

ANALYSIS 

(PERCENT)

Ash 

Heating value (Btu/1 b)

MSR-10C; 

NEV4 sec. 28, 

T. 16 N., R. 102 W.

_ 1 q O

__ A Q

___ 01 Q

__ en 9

11.140.0

MSR-14C; 

SEV4 sec. 8, 

T. 15 N., R. 102 W.

1 0 Q

?1 A.

R1 ?

11,380.0

FIGURE 65. Proximate analyses, in percent (as-received), of core 

samples of the Pintail coal bed from core holes MSR-10C and 

MSR-14C. Data from McClurg and others (1983).

contains less than 1 percent sulfur. These analyses, in 

figure 66, reveal that hydrogen ranges between 5 and 

6 percent and nitrogen between 0.8 and 1.1 percent. The 

amount of carbon is variable and appears to reflect 

changes in coal composition that take place between 

fresh, unweathered coal samples and weathered coal. 

The analyses of outcrop samples show a decrease of carbon by nearly 50 percent and an increase of oxygen by 

nearly 50 percent, when they are compared with the 

analyses of the core samples (fig. 66).

WEATHERING OF OUTCROP COAL SAMPLES

The amount of surface weathering of coal beds in the 

Rock Springs coal field was studied by Schultz (1910). 

Schultz collected samples from several mines at various 

distances from entry ways and then calculated the 

amount of overburden at the sample sites. His work 

indicates that the heating value of the coal was reduced 

by at least 35 percent by the affects of weathering along 

outcrops.

The upper and lower benches of the Pintail coal bed 

and its carbonaceous shale split were sampled in a 

trench that was dug in outcrops at locality D in NE*/4 

sec. 10, T. 16 N., R. 102 W. The three samples were submitted to Geochemical Testing, Somerset, Pa., for proximate analysis. Surface weathering makes the analytical 

results unreliable, but they are nevertheless listed in 

figure 67. The analyses indicate that the coal has very 

low heating value (4,427 and 5,519 Btu/lb). Using the 

Parr formula, the lower coal bench on a moist, mineralmatter-free basis has a heating value of 6,440 Btu/lb and 

is classified lignite A; the upper bench has a heating 

value of 4,880 Btu/lb and is classified lignite B.

If the heating values of the outcrop channel samples 

of the Pintail coal bed are increased by 35 percent, the 

resulting values are 8,900 Btu/lb (subbituminous C) for 

the lower bench and 6,600 Btu/lb (lignite A) for the upper bench. These values suggest possibly higher ranks 

for the two coal benches than is indicated by the 

analyses of the outcrop samples, but the values are still 

considerably lower than the bituminous C rank indicated by the core samples taken in the test holes 

drilled by the U.S. Department of Energy.

VITRINITE REFLECTANCE

The average vitrinite reflectance (Ro) of several 

coal samples from the upper and lower coal benches 

of the Pintail coal bed at locality D (fig. 25) was 

0.42 (R. W. Stanton, written commun., 1981). This 

amount of reflectance places the Pintail coal bed in 

a subbituminous rank, probably subbituminous B, 

on a scale of maturation that was published by Dow 

(1978). The data, however, are from samples of nearsurface, weathered coal, and they too may not be 

reliable.

U.S. Department of Energy hole Outcrop channel samples locality D;

ULTIMATE 

ANALYSIS 

(PERCENT)

Ash _____ ,

Sulfur 

Corehole MSR-10C; 

NE% SEVi sec. 28, 

T. 16 N., R. 102 W.

4 90

C RA

64.00

O O C 1

Corehole MSR-14C; 

SEVi SE1/4 sec. 8, 

T. 15 N., R. 102 W.

6.40 

5.66 

64.67 

0.75 

21.60

Lower coal 

bench

5.92 

29.92 

1.04 

0.36 

49.44 

13.32

Carbonaceous 

shale split

1.72 

2.74 

0.11 

0.04 

12.41 

82.98

Upper coal 

bench

5.89 

37.43 

1.06 

0.38 

46.41 

8.83

FIGURE 66. Ultimate analyses, in percent (as received), of core samples of the Pintail coal bed from core holes 

MSR-10C and MSR-14C and from outcrops at locality D.
54 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

PROXIMATE 

ANALYSIS 

(PERCENT)

Volatile matter ----

Heating value --

Locality D;

Lower coal 

bench

O J .\J£-

24.93

5,519.00

NE^sec. 10, T. 16 N

Carbonaceous 

shale split

1 VX. &*J

CO OP

6.05 

0.04

298.00

., R. 102W.

Upper coal 

bench

8

O"3

27.90

no m

4,427.00

FIGURE 67. Proximate analyses, in percent (as-received), of 

weathered outcrop channel samples of the Pintail coal bed and 

its carbonaceous shale split at locality D. The sampled intervals 

are indicated in figure 25.

TRACE ELEMENT ANALYSES

Outcrop samples of the Pintail coal bed at locality D 

were analyzed for major, minor, and trace element content by emmission spectrographic analysis (J.L. Harris and others, written commun., 1983). The data are 

listed in figure 68, where they are compared to average 

abundance of elements in coal of all ranks in the United 

States (Swanson and others, 1976).

COAL DEVELOPMENT POTENTIAL

GRAPHIC SECTIONS AND 

DESCRIPTION OF COAL OUTCROPS

The Pintail coal bed is fairly well exposed in the study 

area but it usually has sparse, thin vegetation, such as 

sage and grass, and a few inches of soil and rock veneer 

covering it. The coal interval weathers drab gray and 

is commonly seen in outcrop as a dark-gray, poorly 

vegetated band within lighter drab-gray and drabbrown, more vegetated slopes. In only a few places is 

bare coal visible. Where the line of coal outcrops crosses 

large drainages, such as Salt Wells Creek, the coal may 

be covered by thick intervals of Quaternary alluvium. 

The bed most commonly crops out near the base of 

slopes formed by the weathering of thick, soft shale and 

thin sandstone that are present between more resistant, 

topographically higher ridges formed by thick barrier 

sandstones. The angles of the surface slopes seldom exceed 35 °. In only a few places is the coal exposed along 

the banks or near the bottom of minor drainages or dry 

washes. Large vegetation, such as the juniper trees 

common to the region, never grow directly upon the 

coal.

The Pintail coal bed is generally clean and bright in 

trenches or channels that are dug along outcrops. 

Within the study area, the bed thickens and thins, 

splits, and is burned locally, as indicated on plate 1. The 

coal is massive (5 ft or more thick) in sections 18, 56-57, 

69, 71, 73, 75-80, and 100 (plate 1). It thins to less than 

1.3 ft in sections 24-30. Splits in the coal range in 

thickness from less than 0.2 ft to more than 12 ft and 

are usually composed of carbonaceous shale. The coal 

is partly or totally burned in sections 31 and 54 (plate 

1), where it weathers dull red and consists of clinkers 

or black, powdery ash. The coal has primary and secondary sets of nearly vertical cleats that trend in N. 65 E. 

and N. 20 W.

The rocks that directly overlie the Pintail coal bed are 

mostly soft, gray or brown carbonaceous shale. The carbonaceous shale in places changes laterally to firmly 

cemented gray and tan, fine- to medium-grained sandstone, and gray siltstone that may contain fragments 

of oysters and other mollusk shells. The sandstone and 

siltstone rarely show evidence at their bases of scouring and removal of the upper part of the underlying coal 

bed.

The coal bed either rests directly upon the sandstones 

composing barrier bar G, or is underlain by gray or 

brown carbonaceous shale. No underclays were identified anywhere in the study area. When the coal rests 

upon the barrier sandstone, the upper 1-2 ft of the sandstone commonly contains small root casts and molds 

and is discolored brown to nearly black. The discoloration was caused by humic acids that were present in the 

Pintail peat beds that migrated downward during 

coalification into the underlying seat rock composed of 

highly porous and permeable-barrier sand.

AREAL DISTRIBUTION OF THE COAL AS A RESULT 

OF THE DEPOSITIONAL ENVIRONMENT

The Pintail coal bed was deposited entirely within the 

lagoon that formed landward of and lapped seaward 

onto barrier bar G. The areal distribution of the coal 

is illustrated by an isopach map that shows net coal 

thicknesses, excluding splits and partings (fig. 69). The 

isopach map was prepared from coal measurements 

along outcrops, from geophysical logs of drill holes, and 

from interpretations of the geographic distribution of 

forest swamp (coal-forming) and reed swamp (noncoalforming) habitats within the lagoon.

A major north-south trend of coal thickening is readily apparent on the isopach map. This trend conforms 

to the direction of the trend of the shoreline of the Late 

Cretaceous epeiric sea across the study area, to the 

direction of facies strike of barrier bar G, and to the
COAL DEVELOPMENT POTENTIAL 55

Outcrop samples from locality D

Element Lower 

coal 

bench

Carbonaceous 

shale 

split

Upper 

coal 

bench

Average for coal in 

the United States 

(Swanson and 

others, 1976)

Percent

Si

Al

Fe

Mg

Ca

Na

K

Ti

3.0

2.7

1.4

0.027

0.32

0.012

0.059

0.035

33.0

6.5

1.1

0.51

0.055

0.12

1.8

0.37

2.8

0.77

0.48

0.13

1.1

0.002

0.038

0.05

2.6

1.4

1.6

0.12

0.54

0.06

0.18

0.08

Parts per million

As

B

Ba

Cd

Ce

Co

Cr

Cu

Ga

Hg

Li

Mn

Mo

Nb

Ni

Pb

Sb

Sc

Se

Sr

Th

U

V

Y

Yb

Zn

Zr

3.8

15.0

410.0

0.2

37.0

21.0

9.7

6.8

5.4

0.17

3.0

13.0

5.1

2.5

52.0

5.1

3.0

6.8

1.42

570.0

1.4

2.5

17.0

32.0

1.9

34.0

32.0

6.2

50.0

330.0

0.09L

85L

2.8

72.0

35.0

7.9

0.13

19.0

36.0

1.8L

11.0

7.8

13.0

7.0

8.4

130.0

10.0

4.4

81.0

19.0

1.3

22.0

100.0

0.4

8.5

120.0

0.0 1L

11L

3.0

6.7

9.4

7.0

0.07

3.6

1 1.0

0.4

1.2

4.6

1.9

0.1

1.5

...

330.0

1.2

0.7

5.7

6.2

0.3

3.1

17.0

15.0

50.0

150.0

1.3

...

7.0

15.0

19.0

19.0

0.18

20.0

100.0

3.0

3.0

15.0

16.0

1.1

3.0

4.1

100.0

4.7

1.8

20.0

10.0

1.0

39.0

30.0

FIGURE 68. Average amounts of major, minor, and trace elements in the Pintail coal bed 

as compared with an average for coal of all ranks in the United States. Leaders ( ) 

indicate no data; L indicates less than value shown.

configuration of the seaward parts of the lagoon landward of the barrier.

An eastward trend of thinning from the line of outcrops of barrier bar G to the zero isopach line for the 

coal is also indicated in figure 69. This trend is the 

result of thinning of the Pintail peat beds in a seaward direction across the back-barrier flats. The zero 

isopach line marks the location of the wedge-out of 

the peat beds behind the shorelines of the barrier at 

the time that the barrier was abandoned (compare figs. 

56 and 69).

Several narrow trends of east-west coal thinning cross 

the major north-south trend of coal thickening. The locations of these trends coincide with the locations of tidal 

inlets and tidal channels. Saltwater tides entered the

lagoon through these inlets and channels and introduced noncoal-forming, reed swamp habitats in which 

carbonaceous muds were deposited. The carbonaceous 

muds rested upon or laterally replaced intervals of peat 

that were forming in forest swamp habitats, and in this 

way carbonaceous shale splits and partings were introduced into the coal bed. The net thickness of the coal 

is thinner where the splits and partings are present. An 

example of a coal thinning at a tidal inlet and tidal channel is present in sec. 14 and 15, T. 16 N., R. 102 W. (fig. 

69). In that area the coal consists of thin upper and 

lower benches that are separated by a thick carbonaceous shale split. The split is lenticular along outcrops 

and is visible between measured sections 18 and 35 on 

plates LA and IB (see also fig. 64).
56 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

R. 103 W. R. 102 W. R. 101 W.

Outcrop of 

Pintail coal bed

FIGUEE 69. Generalized isopachous map of the Pintail coal bed showing coal thickness in feet, excluding 

splits and partings. Isopach interval is 1 ft.

The irregular thickening and thinning of the Pintail 

coal bed in outcrops from sec. 5, T. 15 N., R. 102 W. 

southward to sec. 28, T. 15 N., R. 102 W. (fig. 69) probably is caused partly by in-filling by peat in channels 

and in swales that formed across the flood-tidal delta 

sandstone depicted in figure 54, and partly by later

decomposition of the upper part of the peat bed as a 

result of seawater entering the lagoon through a nearby tidal inlet. Some coal thinning along outcrops in the 

study area can be attributed to the erosion that was 

associated with the marine transgression that took 

place after the barrier system was abandoned (fig. 57)
COAL DEVELOPMENT POTENTIAL 57

and to the consequent biogenic degradation of the peat 

beds in those places by saltwater and brackish-water 

organisms living in the overlying lagoon (note the position of the brackish-water oyster bed (ORM) in sections 

10 to 35, on plate 1).

ESTIMATED COAL RESOURCES

The amount of coal in place in the Pintail coal bed in 

the study area is estimated to be about 77 million short 

tons. About 30 percent of these resources are located 

where the bed is more than 2.5 ft thick, and where it 

is considered minable.

Measured and indicated coal resources for the Pintail 

coal bed, and for other coal beds in the Almond Formation, in the study area, are listed in Roehler (1979b; 

1979c). These resource estimates were derived mostly 

from outcrop data. Isopach maps of individual coal beds 

are shown, and the resources are listed by section and 

township for intervals of coal thickness and overburden. 

More accurate resources for the Pintail coal bed cannot be computed without additional exploratory drilling and coring to determine the rank and thickness of 

the coal in areas downdip of outcrops.

MINING METHODS AND PROBLEMS

The Pintail coal bed dips from 5° to 8° eastward in 

a desert terrain of ridges and valleys; the thickness of 

the overburden increases rapidly downdip, and the 

amount of recoverable coal is small. These facts generally preclude surface mining methods, because of 

the large initial capital investments required for these 

types of mines, and because of the high costs of overburden removal and site reclamation. The underground 

mining method that appears most acceptable is a drift 

mine with a mine opening directly into the coal bed, 

where the coal can be transported to the surface by 

track haulage or conveyor. Room and pillar, longwall, 

or shortwall mining methods could be used, but because 

of the high mining costs in relation to the amount of 

recoverable coal, the room and pillar method is probably 

the best method. One favorable area for mining the coal 

is downdip of outcrops between measured sections 65 

and 97 (fig. 2 and plate 1) in sees. 8, 17, and 20, T. 15 

N., R. 102 W. The coal in this area is generally more 

than 4 ft thick and has no partings. The recoverable coal 

probably is less than 10 million short tons, but several 

other minable coal beds situated above and below the 

Pintail coal bed in the same area (Roehler, 1979c) 

substantially increase the amount of coal that could be 

recovered from a single mine having several levels.

Roof conditions have been studied and evaluated by 

Petranoff and others (1980) in the Stansbury mine in 

the northern part of the Rock Springs coal field. The 

rocks overlying the Pintail coal bed closely resemble 

those investigated in the Stansbury mine, and the roof 

conditions are expected to be similar. The roof rock of 

the Pintail coal bed across the northern part of the 

study area, in outcrops in T. 16-17 N., R. 102 W., is composed of limy, hard, fine-grained sandstone containing 

oyster shell fragments, or firmly cemented, fine- to 

medium-grained sandstone in broad, lenticular beds. 

The roof conditions in this area are rated excellent, 

based on the studies in the Stansbury Mine. In the 

southern part of the study area, in outcrops in T. 15 N., 

R. 102 W., the roof rock is composed of 1-5 ft of carbonaceous shale, which is overlain by 3-6 ft of mudstone. The roof conditions here are rated only good to 

fair. Areas of poor roof conditions susceptible to roof 

falls have been identified in a few places along the outcrops of the coal. For instance, if conventional rock bolts 

are used as roof supports, unsafe conditions apear to 

be present between measured sections 83 and 94 in sees. 

17 and 20, T. 15 N., R. 102 W. In that area the Pintail 

coal bed is overlain by incompetent soft, rooted, carbonaceous shale and coal in which roof bolts could not 

be anchored securely.

A more serious problem in underground mining will 

be possible water flooding because of the proximity of 

sandstone aquifiers. Barrier bar G and its lateral sandstone equivalents are porous and permeable and 

underlie the coal nearly everywhere. The confined water 

in these sandstones is constantly recharged by meteoric 

water along outcrops. If the aquifiers were penetrated 

during normal underground mining operations, increased hydrostatic pressures downdip of outcrops 

could cause mine flooding that would result in property 

damage and financial loss. The thickest aquifer in the 

study area is the Ericson Sandstone that underlies the 

Pintail coal bed at intervals ranging from 400 to 550 ft. 

The Ericson Sandstone is nearly 1,000 ft thick and is 

a well known artesian system locally. It provides the 

freshwater used at the Black Butte coal mine, located 

12 mi northeast of the study area.

CONCLUSIONS

Paleogeographic, paleontologic, and sedimentologic 

studies of the Pintail coal bed and barrier bar G reveal 

that the plant debris that formed the coal and the barrier sands were deposited at the head of a marine embayment located along the western shores of the Late 

Cretaceous epeiric sea. The investigations further reveal 

that the quartzose sandstones composing the barrier
58 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

coarsen upward and landward, but are dominantly fine 

grained; that the barrier sandstones prograded along 

a mesotidal coastline; that associated coal is of lagoonal 

origin; and that the lagoon had forest swamp and reed 

swamp habitats. The morphologic characteristics of the 

barrier and its lagoon are identified by their sedimentary structures and lithofacies.

The geological data acquired in the investigation permit the construction of a model for a barrier-lagoon, 

coal-forming environment based on the depositional 

history. The model is illustrated by cross sections in 

figure 70 that show stages of youth, maturity, old age, 

abandonment, and marine transgression. The earliest 

record of barrier-coal deposition, during the stage of 

youth (timeline A), was marked by the appearance of 

a narrow, open, saltwater estuary or bay (LA) behind 

barren sandbanks that emerged from the sea floor 

1-2 mi off the Late Cretaceous shoreline. The sandbanks rapidly enlarged to form a chain of barrier islands 

by the accretion of sediments swept southward by 

longshore currents. A brackish-water lagoon (LA) subsequently evolved between barrier islands (BB) to the east 

and a freshwater-coastal plain (CS) to the west. Sparse 

saltwater and brackish-water vegetation spread across 

the shores of the lagoon, but the mud substrate was 

alkaline and aerobic and therefore not conducive to the 

formation and preservation of peat. The barrier shorelines began to prograde during the stage of maturity 

(timeline B), and the lagoon (LA) behind the barrier 

began to expand in landward and seaward directions. 

The expansion seaward was across the back-barrier flat, 

and this was accompanied by the weathering and erosion (LU) of the upper, older parts of underlying barrier sands. The saltwater-saturated sands in the older, 

eroded parts of the back-barrier flat were soon flushed 

by freshwater of meteoric origin, and dense concentrations of shrubs and trees entered this sand platform to 

form peat swamps (PS). Similar vegetation and peat 

swamps encroached upon shallow, freshwater, landward 

parts of the lagoon. The lagoon continued to expand in 

landward and seaward directions during the stage of 

old age (timeline C). Large rivers entered the lagoon 

from the coastal plain to the west, and this freshwater 

incursion, accompanied by sediment in-filling of the 

lagoon, introduced forest swamps and peat deposition 

in scattered patches across the lagoon. Platforms and 

islands of peat (PS) formed in this manner, and these 

were interspersed with and rested upon nonpeatforming, brackish-water reed swamps and bay-fill muds 

(BF). Sedimentation slowed and then ceased during the 

abandonment stage (timeline D), and the barrier was 

reduced to a narrow, straight, sandy, coastal beach with 

remnants of a dune field behind it. The lagoon filled in 

and almost disappeared; it was replaced by extensive

freshwater forest swamps and blanket-like peat deposits 

(PS). Following the barrier-lagoon abandonment the 

coastline submerged as a result of subsidence and 

marine transgression. Erosion (MU) accompanied the 

marine transgression and obliterated nearly all the surface expression of the barrier and removed parts of adjacent peat beds.

The thickness and geometry of the Pintail coal bed 

are directly related to the lagoonal environment in 

which it was deposited. The thickest parts of the coal 

bed trend north-south, in the direction of the facies 

strikes of the barrier shoreline and the seaward edge 

of the lagoon. An overall trend of eastward coal thinning and wedge-out is present in the. study area along 

the north-south trend of thickening, where the peat beds 

lapped eastward from the lagoon onto the back-barrier 

flat and wedged out near the barrier shoreline. Several 

minor trends of east-west thinning cross the north-south 

trend of thickening at nearly right angles, and the 

trends of thinning coincided with the locations of tidal 

inlets, tidal channels, and flood-tidal deltas. Petrographic and palynological studies indicate that peat was 

deposited only in areas of freshwater where forest 

swamps developed; reed swamps developed and carbonaceous mud was deposited in areas of saltwater or 

brackish-water influence near tidal inlets, tidal channels, 

and flood-tidal deltas. The carbonaceous mud that was 

deposited in the tidal areas (where the coal thins) forms 

carbonaceous shale splits and partings that are widespread in the lower half of the Pintail coal bed. The splits 

and partings in the lower part of the bed that change 

to a massive, laterally persistent bench of coal at the 

top of the bed are the fundamental identifying 

characteristics of the coal deposited in the barrierlagoon environment.

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EAST

Youth

CONCLUSIONS

,BB /LA

59

WEST

Maturity

Old age PS

Abandonment

Marine transgression

EXPLANATION r100 FEET

-50

2 MILES 

J

A, B, C, D Timelines depicting stages of barrier growth 

OS Open sea (saltwater) 

MS Marine mud 

BB Barrier bar 

LA Lagoon (brackish water) 

BF-Bay fill mud 

PS Peat swamp (freshwater) 

CS Coastal plain (freshwater)

MU Unconformity caused by marine transgression and erosion 

LU Unconformity caused by expansion and progradation of 

the lagoon over the back-barrier flat

FIGURE 70. Cross sections showing a model for a barrier-lagoonal, coal-forming environment based on the depositional

history of barrier bar G and the Pintail coal bed.
60 THE PINTAIL COAL BED AND BARRIER BAR G-WYOMING

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U.S. GOVERNMENT PRINTING OFFICE: 1988 573-047/66,060 REGION NO 8