Chapter SB

A SUMMARY OF COAL IN THE FORT UNION

FORMATION (TERTIARY), BIGHORN BASIN, 

WYOMING AND MONTANA

By S.B. Roberts and G.S. Rossi

in U.S. Geological Survey Professional Paper 1625-A
SB-ii

Contents

Introduction...........................................................................................................................SB-1

Coal Production History.....................................................................................................SB-3

Coalfields...............................................................................................................................SB-4

Garland Coalfield (Wyoming)....................................................................................SB-5

Basin Coalfield (Wyoming)........................................................................................SB-5

Southeastern Coalfield (Wyoming)...........................................................................SB-6

Gebo Coalfield (Wyoming)........................................................................................SB-6

Meeteetse Coalfield (Wyoming)................................................................................SB-7

Oregon Basin Coalfield (Wyoming).........................................................................SB-7

Grass Creek Coalfield (Wyoming)............................................................................SB-8

Red Lodge-Bearcreek Coalfield (Montana).............................................................SB-9

Coal Quality........................................................................................................................SB-10

Coal-Bed Methane.............................................................................................................SB-12

References Cited.................................................................................................................SB-14

Figures

SB-1. Index map showing the Bighorn Basin and associated Tertiary coal basins in 

 the Northern Rocky Mountains and Great Plains region.

SB-2. Map showing generalized Upper Cretaceous (part), Tertiary, and Quaternary 

 geology, Bighorn Basin, Montana and Wyoming.

SB-3. Stratigraphic column showing Upper Cretaceous (part) and Tertiary 

 formations in the Bighorn Basin, Montana and Wyoming.

SB-4. Map showing locations of coalfields, abandoned coal mines, and measured 

 stratigraphic sections, Bighorn Basin, Montana and Wyoming.
SB-iii

Figures—Continued

SB-5. Lithologic and sedimentologic symbols used in measured stratigraphic 

 sections of the Fort Union Formation shown in figures SB-6 through SB-9 

 in this report.

SB-6. Measured stratigraphic section of the Fort Union Formation (part) at locality 

 A in the Basin coalfield, Tps. 51 and 52 N., R. 94 W., eastern Bighorn 

 Basin, Wyoming.

SB-7. Measured stratigraphic sections of the Fort Union Formation (part) at 

 localities B and C, Southeastern coalfield, Tps. 46 and 47 N., Rs. 89 through 

 91 W., southeastern Bighorn Basin, Wyoming.

SB-8. Measured stratigraphic section of the Fort Union Formation at locality D in 

 the Gebo coalfield, T. 45 N., Rs. 95 and 96 W., south-central Bighorn Basin, 

 Wyoming.

SB-9. Measured stratigraphic sections of the Fort Union Formation (part) at 

 localities E and F in the Grass Creek coalfield, Tps. 45 and 46 N., Rs. 97-99 

 W., southwestern Bighorn Basin, Wyoming.

SB-10. Schematic cross section A-A’ showing the lateral variation in Fort Union 

 Formation lithology and the stratigraphic position of coal zones in the Grass

 Creek coalfield, southwestern Bighorn Basin, Wyoming.

SB-11. Diagram comparing stratigraphic terminology for coal beds in the upper 

 coal zone of the Fort Union Formation, Grass Creek coalfield, southwestern

 Bighorn Bighorn Basin, Wyoming.

SB-12. Generalized stratigraphic column showing Fort Union Formation coal beds 

 in the area of Red Lodge, northern Bighorn Basin, Montana.
SB-1

INTRODUCTION

The Bighorn Basin of north-central Wyoming and south-central Montana (fig. SB1) occupies an area of about 7,500 mi2

 (Keefer and others, 1998) in parts of Park,

Hot Springs, Washakie, and Big Horn Counties, Wyoming, and in parts of

Stillwater, Sweetgrass, and Carbon Counties, Montana (fig. SB-2). The basin is

surrounded by mountainous uplifts including the Bighorn and Pryor Mountains to

the east and northeast, respectively, the Owl Creek Mountains to the south, the

Absaroka Range to the west, and the Beartooth Mountains to the northwest. Coalbearing strata in the basin are present within Cretaceous rocks of the Cloverly,

Frontier, Mesaverde, Meeteetse, and Lance Formations, and within Tertiary rocks of

the Paleocene Fort Union Formation and the Eocene Willwood and Tatman

Formations (fig. SB-3). Thicker coal beds and more extensive coal resources are

primarily restricted to the Upper Cretaceous Mesaverde and Meeteetse Formations

and the Paleocene Fort Union Formation (Glass and others, 1975). Based on the

distribution of coal-bearing outcrops and the interpreted extent of these rocks in the

subsurface, 11 coalfields have been designated within the Bighorn Basin: the

Bridger, Stillwater, Silvertip (part), and the combined Red Lodge-Bearcreek

coalfield (formerly the Red Lodge coalfield; Woodruff, 1909b) in Montana, and the

Garland, Basin, Silvertip (contiguous with the Silvertip coalfield in Montana),

Southeastern, Gebo, Grass Creek, Meeteetse, and Oregon Basin (Cody coalfield of

Woodruff, 1909a) coalfields in Wyoming (Luhr and Jones, 1985) (fig. SB-4).

Estimates of total in-place Bighorn Basin coal resources (bituminous and

subbituminous rank coal) in both Upper Cretaceous and Paleocene strata in the basin

are on the order of 1.8 billion short tons (R.W. Jones, written communication

reported in Love, 1988). Berryhill and others (1950) estimated that 17.9 million tons

of Upper Cretaceous bituminous coal resources are present to a depth of 3,000 ft in
SB-2

the Silvertip coalfield (Wyoming), and of this total, Glass and others (1975) suggest

that only 4.1 million tons should be considered as a recoverable resource within

constraints of overburden, coal thickness, and production and mining loss. Original

estimates of subbituminous coal resources (Upper Cretaceous and Paleocene) in

marginal areas of the Wyoming part of the Bighorn Basin are on the order of 564

million tons (Berryhill and others, 1950). However, Glass and others (1975)

indicate that only 41.7 million tons of subbituminous-rank coal can be considered as

a recoverable resource. In the Red Lodge-Bearcreek coalfield of Montana, Rawlins

(1986) estimates that the remaining Paleocene coal reserves (subbituminous rank) in

the Bearcreek district alone may exceed 700 million tons.

This report presents a brief summary of the geology, coal production history, and

coal quality of Tertiary (Paleocene) coal deposits in the Fort Union Formation of the

Bighorn Basin as part of the assessment of Tertiary coal resources in the Northern

Rocky Mountains and Great Plains region (fig. SB-1). Within the framework of the

National Coal Resources Assessment (NCRA) project of the U.S. Geological

Survey, Fort Union coal in the Bighorn Basin is considered as “low priority” in

terms of its viability as a significant production target in the near future. This low

prioritization is not intended to preclude all possibility of future coal production

within the basin. In fact, it should be noted that Fort Union coal in the Red LodgeBearcreek coalfield of Montana (Bearcreek district) is currently being assessed by

private concerns as a possible fuel source for mine-site power generation, although

this is in a very early stage of evaluation (James Gruber, U.S. Bureau of Land

Management, oral communication, 1999). In the context of the NCRA project,

however, the concept of “low priority” suggests a degree of skepticism with regard

to any large-scale upswing or significant rejuvenation of coal mining in the area. In

reference to Paleocene coal in the Bighorn Basin, this skepticism is supported by the
SB-3

restricted areal extent of thicker coal deposits (see, for example, Woodruff, 1909b;

Rawlins, 1986; Roberts and Stanton, 1994), the lack of thick or persistent coal beds

in Fort Union Formation outcrops throughout most of the basin (see, for example,

Roberts, 1998), and the apparent lack of significant Fort Union coal in the

subsurface (Johnson, 1998). Additionally, although Cretaceous coal resources were

a large part of the historic coal production, particularly in the Wyoming part of the

Bighorn Basin, Cretaceous coal is not addressed in detail in this report.

COAL PRODUCTION HISTORY

A total of 53 million tons of Cretaceous and Paleocene coal is estimated to have

been produced in the Bighorn Basin as of 1983 (Love, 1988). Coal mining in the

basin began in the late 1800’s, with larger, more commercial mines opening in the

early 1900’s (Glass and others, 1975). Between 1919 and 1929, coal production in

the Wyoming part of the basin ranged from 450,000 to 550,000 tons per year, and

then decreased to between 90,000 and 200,000 tons per year from 1930 to 1956

(Glass and others, 1975). A total of 12,271,664 tons is reported to have been

produced in Wyoming, and about 90 percent of this production (11,032,114 short

tons) came from 13 or more mines working Upper Cretaceous coal deposits

(Mesaverde Formation ) in the Gebo coalfield (Glass and others, 1975). The

remaining historic coal production accounting for the 1983 total (Love, 1988) came

primarily from Montana, where as much as 30-40 million tons of coal is estimated

to have been produced from the Fort Union Formation in the Bearcreek district of

the Red Lodge-Bearcreek coalfield (Rawlins, 1986). Commercial mining in the Red

Lodge-Bearcreek coalfield began around 1890, and annual production reached as

much as 1,000,000 tons in the 1920’s. Development of surface coal mines in

Colstrip, Montana, during the 1920’s, as well as the eventual conversion of
SB-4

locomotives to diesel fuel, greatly reduced the demand for Red Lodge-Bearcreek

coal. Following a brief revival of coal production during World War II, most of the

mines in the Red Lodge-Bearcreek area closed by the early 1950’s (Rawlins, 1986).

The most recent coal production in the Bighorn basin took place in the Grass Creek

coalfield in Wyoming (fig. SB-4), where some 300,500 tons of Fort Union coal

was produced between 1989 and 1993; maximum coal production occurred in 1991

when 108,720 tons of coal was mined (Resource Data International, 1998).

Operations there ceased by 1994 and currently, there are no active coal mines in the

Bighorn Basin.

COALFIELDS

The following coalfield summary of Fort Union Formation coal in the Bighorn

Basin is compiled primarily from Glass and others (1975) and Glass (1981). In

these previous studies the authors adopted nomenclature of Jepsen (1940) and refer

to the Fort Union Formation as the Polecat Bench Formation. Additional references

are cited where applicable. Measured sections for specific coalfields or areas adjacent

to coalfields are modified from Woodruff (1909b) and Roberts (1998). An

explanation of symbols used in the measured sections of Roberts (1998) is shown

in figure SB-5. Carbonaceous shale as identified in these measured sections

generally contains thin beds or, more typically, laminations of coal. No summaries

are included for the Silvertip, Stillwater, and Bridger coalfields because pertinent

data on Fort Union coal in these areas is lacking, and to a lesser degree, because

Cretaceous coal was the primary production target. Figure SB-4 shows the locations

of all the coalfields.
SB-5

GARLAND COALFIELD (WYOMING)

Although the apparent lenticular nature of Fort Union coal beds in this field

restricted any significant production, five small mines (fig. SB-4) were opened on a

basal Fort Union coal bed and at least two small prospects (locations unknown) may

have been opened on a stratigraphically higher Fort Union coal bed. Mined coal-bed

thickness ranged from 4.4 to 7 ft for the basal coal horizon and from 2.5 to 3.5 ft for

the higher coal horizon. At least one small mine produced coal from the Cretaceous

Mesaverde Formation. No production figures were recorded for this coalfield.

BASIN COALFIELD (WYOMING)

Workable Fort Union coal beds are present in this coalfield, although their lateral

extent is limited and the coal beds tend to grade rapidly to carbonaceous shale (see,

for example, Washburne, 1909). A total of 17,542 tons of Paleocene coal was

produced in three mines where the mined coal-bed thickness ranged from 3 to 8.7 ft

and averaged 6.5 ft. A measured stratigraphic section (fig. SB-6) of about 2,300 ft

of Fort Union Formation strata in the northern part of the Basin coalfield near

Greybull (fig. SB-4; locality A) indicates an abundance of carbonaceous shale

horizons. Carbonaceous shale and coal beds in the measured section are typically on

the order of a few ft or less in thickness. In the southern part of the coalfield near

Manderson (fig. SB-4), coaly carbonaceous shale horizons of as much as 14.7 ft in

thickness have been observed in outcrop.
SB-6

SOUTHEASTERN COALFIELD (WYOMING)

The Southeastern coalfield encompasses a remote area of the Bighorn Basin, and

thus, has played only a very minor role in coal production. There are no coal

production figures from this field. Glass and others (1975) report that two small

mines operated in Fort Union strata. However, Luhr and Jones (1985) indicate that

one of these mines (Kimball Draw mine) was opened in Cretaceous strata of the

undifferentiated Lance-Meeteetse Formations rather than in the Fort Union

Formation. The coal mined from the Fort Union Formation was 3.5 ft thick.

Elsewhere in the coalfield, Fort Union coal beds locally (albeit rarely) attain a

thickness ranging from 5.6 to 10 ft in outcrops. Measured stratigraphic sections (fig.

SB-7) of parts of the Fort Union Formation in the Honeycombs area and along U.S.

Highway 16 about 12 miles east of Worland (fig. SB-4, localities B and C,

respectively) show abundant carbonaceous shale and some coal beds. At the

Honeycombs, coal beds are locally as much as 4 ft in thickness, although these coal

beds typically include layers of bone (high ash) coal and carbonaceous shale.

Carbonaceous shale beds are generally less than 3 ft thick. Placement of the contact

separating the Paleocene Fort Union Formation from the underlying Upper

Cretaceous Lance and Meeteetse Formations (undifferentiated) in the Honeycombs

section is based on palynological analyses of Nichols (1998).

GEBO COALFIELD (WYOMING)

Most of the historic coal production (90 percent) in the Wyoming part of the

Bighorn Basin was from the Gebo coalfield. However, this production came entirely

from mines working Upper Cretaceous coal in the Mesaverde Formation. Only one
SB-7

small mine in the Gebo coalfield (fig. SB-4) has been identified as working Fort

Union coal. A measured stratigraphic section (fig. SB-8) representative of the Fort

Union Formation in the Gebo area (fig. SB-4, locality D) shows an abundance of

carbonaceous shale and less abundant coal beds. Similar to previously described

areas, carbonaceous shale beds are generally a few ft or less in thickness, as are the

coal beds. However, these coal and carbonaceous shale beds thicken and thin within

short distances, and it is likely that the Fort Union coal mined in the Gebo field is

laterally equivalent to carbonaceous shale and thin coal beds in the middle or upper

part of the locality D measured section. The lowermost coal bed in the Fort Union

section at locality D (fig. SB-8) actually consists of a 5-ft-thick interval of

interbedded coal, bone coal and carbonaceous shale.

MEETEETSE COALFIELD (WYOMING)

As much as 8,000 tons of coal was produced in the Meeteetse coalfield. One mine

produced coal from a 4-ft-thick coal bed near the base of the Fort Union Formation.

Additional coal prospects and mines in the coalfield worked Cretaceous coal in the

Mesaverde Formation and to a lesser degree, coal in the Meeteetse Formation.

OREGON BASIN COALFIELD (WYOMING)

The Oregon Basin coalfield was another minor player in terms of Bighorn Basin

coal production. Total production for the field is estimated to have been less than

3,000 tons. The coalfield, which was formerly known as the Cody coalfield

(Woodruff, 1909a), included 13 operating coal mines. Glass and others (1975)

report that two mines produced coal from Fort Union Formation in the Oregon
SB-8

Basin field. However, subsequent work by Luhr and Jones (1985) indicate that these

mines opened in the Cretaceous Mesaverde Formation. In addition to coal

production from the Mesaverde Formation, some coal production also came from

the Cretaceous Meeteetse Formation.

GRASS CREEK COALFIELD (WYOMING)

Mines in the Grass Creek coalfield produced the second largest amount of coal in

the Wyoming part of the basin. As of 1975, some 67,337 tons of coal had been

mined in this field, 66,422 tons of which came from a single underground mine that

closed in 1973. Surface mining continued into the early 1990’s until production

finally ceased in the Grass Creek coalfield just prior to 1994. Although minor

amounts of coal were produced from the Upper Cretaceous Mesaverde Formation,

Paleocene coal production predominates in this area, and the overwhelming majority

of coal production came from the Paleocene Fort Union Formation. Only the lower

1,030 ft or less of the Fort Union Formation is preserved in this coalfield (Hewett,

1926; Roberts and Stanton, 1994). Measured stratigraphic sections (fig. SB-9) of the

Fort Union Formation near the Grass Creek strip mine and east of the mine on Ilo

Ridge (fig. SB-4, localities E and F, respectively) indicate that coal and

carbonaceous shale is present in two zones (lower and upper) in this area (Roberts

and Bossiroy, 1995). The lateral extent of both coal zones is limited (fig. SB-10),

and the lower coal zone is better developed in southeastern areas of the coalfield

whereas the upper coal zone is best developed in the western part of the coalfield.

The lower coal zone ranges from 45 to 130 ft thick, and includes coal beds as much

as 3.5 ft thick. However, most coal and carbonaceous shale beds in this zone are 1.5

ft thick or less. The upper coal zone is between 425 ft and 655 ft above the basal bed
SB-9

of the Fort Union Formation (Hewett, 1926), and includes the 36.8-ft-thick

Mayfield coal bed, which is the thickest reported coal bed in the Fort Union

Formation anywhere in the Bighorn Basin. Figure SB-11 shows the varying

terminology applied to coal beds in this part of the formation. The Mayfield coal bed

was the primary target for production in this zone, and in the Grass Creek strip mine

this coal bed is about 26 ft thick including 4.2 ft (cumulative thickness) of claystone

partings (Roberts and others, 1994). The Rider 1 and 2 coal beds (figs. SB-9 and

SB-11), which are restricted to the Grass Creek coal mine area, attain a maximum

thickness of 9 ft and 19 ft, respectively. Prior to mining in the Grass Creek strip

mine, Stewart (1975) estimated coal reserves in the Mayfield coal bed to be on the

order of 15 million tons underlying an area of about 468 acres.

RED LODGE-BEARCREEK COALFIELD (MONTANA)

The Red Lodge-Bearcreek coalfield (formerly the Red Lodge coalfield)

encompasses about 32 mi2 between the communities of Red Lodge and Bearcreek in

south-central Montana (Woodruff, 1909b; fig. SB-4). More coal was produced

from this area than from any other coalfield in the Bighorn Basin, and production

totals were on the order of 30-40 million tons between 1870 and the middle 1960’s

(see, for example, Rawlins, 1986). The coalfield includes the Red Lodge and

Bearcreek mining districts, which are geographically separated by a high

topographic ridge. All coal production in this field came from Fort Union Formation

coal. In this area, the Fort Union Formation consists of a lower, non-coal-bearing

interval that is about 5,700 ft thick, a middle coal-bearing interval about 825 ft thick,

and an upper, non-coal-bearing interval about 1,975 ft thick (Woodruff, 1909b). In

the Red Lodge district, at least seven coal beds, originally designated as coal beds 1

through 7 (descending order), were identified in the coal-bearing interval of the Fort
SB-10

Union (Woodruff, 1909b). Additional beds within the coal zone were later

discovered, and subsequently designated as coal beds 1 1/2 and 4 1/2 (fig. SB-12).

Note that coal bed 7 typically includes a zone of multiple thin coal beds. A similar, if

not identical coal-bearing succession is present in the Bearcreek district (Woodruff,

1909b), although Rawlins (1986) designates eight “major” coal beds in this area of

the field. The coal beds are as much as 11.5 ft thick, and may locally include

numerous partings of carbonaceous shale. At least 16 underground mines operated

in the Red Lodge-Bearcreek coalfield during its history of production (Luhr and

Jones, 1985). In an evaluation of six coal beds, each greater than 4 ft thick in the

Bearcreek district, Rawlins (1986) estimates that total Fort Union coal reserves in

this area may exceed 700 million tons.

COAL QUALITY

Quality analyses (heat-of-combustion, ash yield, moisture, and total sulfur content)

for Fort Union coal in many parts of the basin are lacking, and analytical data

typically emphasize major production targets such as the Mayfield coal bed and coal

beds in the Red Lodge-Bearcreek coalfield. Glass and others (1975) report the

following summary analyses (on an as-received basis) for Fort Union Formation

coal in the Wyoming part of the Bighorn Basin, based on 12 samples (locations

unknown): moisture ranging from 10.7 to 17.8 percent (arithmetic mean of 14.3

percent), ash yield ranging from 5.0 to 15.3 percent (arithmetic mean of 9.8

percent), total sulfur content ranging from 0.2 to 2.1 percent (arithmetic mean of 0.6

percent), and heat-of-combustion values ranging from 8,858 to 11,246 Btu/lb

(arithmetic mean of 10,150 Btu/lb). The apparent rank of Fort Union Formation coal

beds ranges from subbituminous to bituminous.
SB-11

As-received analyses based on 10 face channel samples from highwall exposures of

the Mayfield coal bed in the Grass Creek strip mine (Roberts and others, 1994)

indicate moisture ranging from 6.6 to 14.2 percent (arithmetic mean of 9.8 percent),

ash yield ranging from 6.9 to 26.5 percent (arithmetic mean of 13.8 percent), total

sulfur content ranging from 0.27 to 0.72 percent (arithmetic mean of 0.36 percent),

and heat-of-combustion values ranging from 9,023 to 11,003 Btu/lb (arithmetic

mean of 10,124 Btu/lb). Based on six samples from this same coal bed, Glass and

others (1975) report the following analyses (on an as-received basis): moisture

ranging from 10.7 to 12.8 percent (arithmetic mean of 12.3 percent), ash yield

ranging from 5.0 to 9.4 percent (arithmetic mean of 7.4 percent), total sulfur content

ranging from 0.3 to 0.6 percent (arithmetic mean of 0.4 percent), and heat-ofcombustion values ranging from 10,730 to 11,246 Btu/lb (arithmetic mean of

10,970 Btu/lb). The apparent rank of the Mayfield coal bed varies from

subbituminous B to high volatile A bituminous (see, for example, Berryhill, 1950;

Glass, 1991).

Analyses (on an as-received basis) from core samples of five coal beds in the

Bearcreek district of the Red Lodge-Bearcreek coalfield indicate moisture of 10-11

percent, ash yield ranging from 10-12 percent, total sulfur content of 1.2-3.0 percent,

and heat-of-combustion values ranging from 10,500-10,700 Btu/lb (Rawlins, 1986).

The high sulfur values likely reflect the presence of pyrite which is concentrated at

the top and base of the coal beds (Rawlins, 1986). In this same report, analytical

results (on an as-received basis) based on a fresh channel sample from the Brophy

mine indicate moisture of 11.04 percent, ash yield of 10.68 percent, total sulfur

content of 1.33 percent, and a heat-of-combustion value of 10,539 Btu/lb (Rawlins,

1986).
SB-12

COAL-BED METHANE

Little information is published regarding the coal-bed methane potential of the Fort

Union Formation in the Bighorn Basin. Indirectly, the presence of coal-bed gas

associated with Fort Union coal is evidenced by the 1943 gas explosion (methane ?)

in the Smith Mine in the Bearcreek mining district of Montana. Additionally, Nuccio

and Finn (1998) suggest that thermal maturity parameters observed in Fort Union

Formation strata allow for a “fair to good” potential for biogenic gas generation

from coal and carbonaceous shale around the periphery of the basin and in the deep

trough of the basin. Perhaps the single, most limiting factor reducing the potential

for coal-bed methane resources in the Fort Union Formation is the apparent lack of

thick, persistent coal in much of the basin. Stewart (1975) and Roberts and Stanton

(1994) demonstrate that the Mayfield coal bed, which is the thickest Fort Union coal

bed, and additional thick coal beds associated with this bed are restricted to the areas

within a mile or so of the Grass Creek strip mine. The restricted areal extent of these

thick coal deposits and their relatively shallow depth of occurrence (maximum of

200 ft of overburden; Stewart, 1975) greatly diminish the potential for significant

coal-bed methane resources in this area. Additional stratigraphic studies of Roberts

(1998) in other areas of the Wyoming part of the Bighorn Basin indicate that only

relatively thin (less than 5 ft thick) coal and carbonaceous shale beds are

predominant in most other Fort Union outcrops. This apparent lack of thick coal

beds in Fort Union outcrops in most of the marginal areas of the southern Bighorn

Basin (Wyoming) decreases the likelihood of coal-bed methane resource

development in these areas as well.

Woodruff (1909b) identified an area underlain by “workable” coal in the Red

Lodge-Bearcreek coalfield that incorporates about 32 mi2. Within this area,
SB-13

cumulative coal thickness can reach in excess of 80 ft (Woodruff, 1909b). This,

coupled with the indirect evidence of coal-bed gas, based on the 1943 mine

explosion, make the Red Lodge-Bearcreek coalfield a potential area for coal-bed

methane exploration. As of this writing, however, we could not locate any published

information regarding coal-bed methane exploration or assessment in this area.

The potential for coal-bed methane associated with deep, subsurface coal in the basin

seems low, based on studies of Johnson (1998). Using interpretations of resistivity,

sonic, density, and gamma-ray logs from oil and gas tests, Johnson demonstrated

that cumulative coal thickness in the Fort Union Formation in deeper parts of the

basin rarely exceeds 10 ft. The thickest cumulative coal (26 ft) was found in a well

penetrating the Fort Union Formation in the southern part of the Bearcreek mining

district. Overall, however, the potential for coal-bed methane accumulations derived

from deeper Fort Union coal seems hampered by the mere lack of laterally

extensive, thick coal beds in the formation throughout most of the Bighorn Basin.
SB-14

REFERENCES CITED

Berryhill, H.L., Jr., Brown, D.M., Brown, A., and Taylor, D.A., 1950, Coal

resources of Wyoming: U.S. Geological Survey Circular 81, 78 p.

Glass, G.B., 1981, Coal deposits of Wyoming, in Reid, S.G., and Miller, D.D.,

eds., Energy resources of Wyoming: Wyoming Geological Association 32nd

Annual Field Conference Guidebook, p. 181-236.

Glass. G.B., (compiler), 1991, Wyoming description of seams, in Lick, R.,

publisher, 1991 Keystone Coal Industry Manual: Maclean Hunter, Chicago,

p. s190-s216.

Glass, G.B., Westervelt, K., and Oviatt, C.G., 1975, Coal mining in the Bighorn

Coal Basin of Wyoming, in Exum, F.A., and George, G.R., eds., Geology

and mineral resources of the Bighorn Basin: Wyoming: Geological

Association 27th Annual Field Conference Guidebook, p. 221-228.

Green, G.N., and Drouillard, P.H., 1994, The digital geologic map of Wyoming in

ARC/INFO Format: U.S. Geological Survey Open-File Report 94-0425, 10

p.

Hewett, D.F., 1926, Geology and oil and coal resources of the Oregon Basin,

Meeteetse, and Grass Creek Basin quadrangles, Wyoming: U.S. Geological

Survey Professional Paper 145, 107 p.
SB-15

Jepsen, J.L., 1940, Paleocene faunas of the Polecat Bench Formation, Park County,

Wyoming: Proceedings of the American Philosophical Society, v. 83, p. 217-

340.

Johnson, R.C., 1998, Coal in the deep subsurface of the Bighorn Basin, Wyoming

and Montana, in Keefer, W.R., and Goolsby, J.E., eds., Cretaceous and lower

Tertiary rocks of the Bighorn Basin, Wyoming and Montana: Wyoming

Geological Association 49th Guidebook, p. 199-210.

Johnson, R.C., Keefer, W.R., Keighin, W.C., and Finn, T.M., 1998, Detailed

measured sections of the Meeteetse and Lance Formations and the lower part

of the Fort Union Formation, in Keefer, W.R., and Goolsby, J.E., eds.,

Cretaceous and lower Tertiary rocks of the Bighorn Basin, Wyoming and

Montana: Wyoming Geological Association 49th Guidebook, Plate II (Map

file).

Keefer, W.R., Finn, T.M., Johnson, R.C., and Keighin, W.C., 1998, Regional

stratigraphy and correlation of Cretaceous and Paleocene rocks, Bighorn

Basin, Wyoming and Montana, in Keefer, W.R., and Goolsby, J.E., eds.,

Cretaceous and lower Tertiary rocks of the Bighorn Basin, Wyoming and

Montana: Wyoming Geological Association 49th Guidebook, p. 1-30.

Love, J.D., 1988, Geology of the Bighorn Basin, northern Wyoming and southern

Montana in Chapter 8 -- Basins of the Rocky Mountain Region, in Sloss,

L.L., ed., Sedimentary cover-North American Craton, U.S.: The Geology of

North America, v. D-2, p. 201-204.
SB-16

Luhr, S.C., and Jones, R.W., (compilers), 1985, Extent of coal-bearing rocks and

locations of coal mines in the Bighorn Coal Basin, Montana and Wyoming:

Geological Survey of Wyoming Open-File Report 85-4, scale 1:250,000.

Nichols, D.J., 1998, Palynological age determinations of selected outcrop samples

from the Lance and Fort Union Formations, Bighorn Basin, Montana and

Wyoming, in Keefer, W.R., and Goolsby, J.E., eds., Cretaceous and lower

Tertiary rocks of the Bighorn Basin, Wyoming and Montana: Wyoming

Geological Association 49th Guidebook, p. 117-130.

Nuccio, V.F., and Finn, T.M., 1998, Thermal maturity and petroleum generation

history of Cretaceous and Tertiary source rocks, Bighorn Basin, Wyoming

and Montana, in Keefer, W.R., and Goolsby, J.E., eds., Cretaceous and lower

Tertiary rocks of the Bighorn Basin, Wyoming and Montana: Wyoming

Geological Association 49th Guidebook, p. 211-232.

Raines, G.L., and Johnson, B.R., 1996, Digital representation of the Montana State

geologic map: a contribution to the Interior Columbia River Basin Ecosystem

Management Project: U.S. Geological Survey Open-File Report 95-691, 19

p.

Rawlins, J.H., 1986, Bearcreek coalfield, in Geology of the Beartooth Uplift and

adjacent basins: Montana Geological Society and Yellowstone Bighorn

Research Association Joint Field Conference and Symposium, p. 253-255.

Resource Data International, Inc., 1998, COALdat: Coal database, 1320 Pearl Street,

Suite 300, Boulder, Colorado 80302.
SB-17

Roberts, S.B., 1998, An overview of the stratigraphic and sedimentologic

characteristics of the Paleocene Fort Union Formation, southern Bighorn

Basin, Wyoming, in Keefer, W.R., and Goolsby, J.E., eds., Cretaceous and

lower Tertiary rocks of the Bighorn Basin, Wyoming and Montana:

Wyoming Geological Association 49th Guidebook, p. 91-116.

Roberts, S.B., and Bossiroy, Dominique, 1995, Stratigraphy and coal geology of the

lower part of the Fort Union Formation in the Grass Creek coal mine area,

southwestern Bighorn Basin, Wyoming: U.S. Geological Survey Coal

Investigations Map C-146, 1 plate with text.

Roberts, S.B., and Stanton, R.W., 1994, Stratigraphy and depositional setting of

thick coal beds in the Grass Creek coal mine, southwestern Bighorn Basin,

Wyoming, in Flores, R.M., Mehring, K.T., Jones, R.W., and Beck, T.L.,

eds., Organics and the Rockies Field Guide, Wyoming State Geological

Survey, Public Information Circular No. 33, p. 125-138.

Roberts, S.B., Stanton, R.W., and Flores, R.M., 1994, A debris flow deposit in

alluvial, coal-bearing facies, Bighorn Basin, Wyoming, USA: Evidence for

catastrophic termination of a mire: International Journal of Coal Geology, v.

25, p. 213-241.

Stewart, W.W., 1975, Grass Creek coalfield, Hot Springs County, Wyoming, in

Exum, F.A., and George, G.R., eds., Geology and mineral resources of the

Bighorn Basin: Wyoming Geological Association 27th Annual Field

Conference Guidebook, p. 229-233.
SB-18

Washburne, C.W., 1909, Coalfields of the northeast side of the Bighorn Basin,

Wyoming, and of Bridger, Montana: U.S. Geological Survey Bulletin 341, p.

165-199.

Woodruff, E.G., 1909a, Coalfields of the southwest side of the Bighorn Basin,

Wyoming: U.S. Geological Survey Bulletin 341, p. 200-219.

_____ 1909b, The Red Lodge coalfield, Montana: U.S. Geological Survey Bulletin

341, p. 92-107.
112° 108° 104° 100°

48°

44°

40°

Powder 

River

Basin

Bighorn 

Basin

Wind River 

Basin

Hanna

Basin

Williston

Basin

Denver

Basin

Greater Green 

River Basin

Raton

Basin

0 50 100 150 Miles

Canada

United States

North North-Middle

Park Basin

Bull Mountain

Basin

Tertiary coal basins

Bighorn Basin

Northern Rocky Mountains

and Great Plains region

Figure SB-1. Index map showing the Bighorn Basin and associated

Tertiary coal basins in the Northern Rocky Mountains and Great Plains

region.

Montana

North

Dakota

South

Dakota

Nebraska

Wyoming

Colorado

New

Mexico
entiated)

ated)

Bighorn Mountains

th

s

44° 00'

109° 108° 00' 00' 

45° 00'

ka Range

Pryor Mountains

Billings

Laurel

Red Lodge

Powell

Cody

Meeteetse

Manderson

Basin

Greybull

Carbon North

County

llwater

ounty

Washakie

County

Big Horn

County 

Big Horn

County 

Park

County
Frontier Formation

Cody Shale

Mesaverde Formation*

Lower Member

Claggett Member

Teapot Sandstone Member

Meeteetse

Formation *

Bearpaw Shale

Lewis Shale

Lance Formation *

Fort Union Formation*

Willwood Formation*

Tatman Formation*

Eocene

Paleocene

Cretaceous (part)

Upper Cretaceous (part)

Tertiary (part)

* Coal-bearing formation

Erosion or non-deposition

Bighorn Basin

Figure SB-3. Stratigraphic column showing Upper Cretaceous (part)

and Tertiary formations in the Bighorn Basin, Montana and Wyoming.

Chart modified from Keefer and others (1998).
Big Horn River

Big Horn River

Greybull River

Shoshone River

Clarks Fork of the

Yellowstone River

Yellowstone River

Billings

Laurel

X

X

X

X

X

X

X

X X

X

Y

Y

Y

Y

Y

Y

Fort Union Formation outcrops

County line

Coalfield boundary

Open-pit or surface coal mine

Measured section 

Underground coal mine, location known

Underground coal mine, location questionable

Coal prospect, location questionable

Coal prospect, location known

Cross section location

Interstate Highway

Town

U.S. highway

State highway

Explanation

Honeycombs

area

Grass Creek

strip mine

Ilo

Ridge

T.46N.

T.48N.

T.50N.

T.52N.

T.54N.

T.56N.

T.58N.

T.9S.

T.7S.

T.5S.

T.3S.

T.1S.

R.18E. R.20E.

R.99W.

R.102W.

R.97W. R.95W.

R.93W. R.91W. R.89W.

Grass Creek

coalfield

Meeteetse

coalfield

Oregon Basin

coalfield

Garland

coalfield

Silvertip

coalfield

Stillwater

coalfield

Bridger

coalfield

Red LodgeBearcreek

coalfield

Gebo

coalfield

Basin

coalfield

Southeastern

coalfield

A

A'

E

F

D

B

A

A C

A A'

0 6 12 Miles

Scale 1:500,000

Figure SB-4. Map showing locations of coalfields, abandoned coal mines, and measured stratigraphic sections,

Bighorn Basin, Montana and Wyoming. Abandoned mine locations and coalfield boundaries modified from

Luhr and Jones (1985).

44° 00'

109° 00' 108° 00'

45° 00'

44° 30'

45° 30'

North

Montana

Wyoming

I-90

I-90

Carbon

County

Stillwater

County

Sweetgrass

County

Washakie

County

Big Horn

County 

Big Horn

County 

Hot Springs

County

Park

County

Red Lodge

Bearcreek

Cody

Meeteetse

Worland

Thermopolis

Greybull

Basin

Manderson

Powell

Alt.

14

310

310

310

212

14

20

16

20

20

16

120

120

431

171

120

Mayfield's

Ranch site
Diamictite

Conglomerate

Carbonaceous shale (CS)

Coal (C)

Conglomeratic sandstone

Sandstone

Mudrock (siltstone, mudstone)

Partially covered interval

(lithology inferred)

Trough crossbeds

Planar-tabular crossbeds

Parallel or subparallel lamination

Low-angle bedding or lamination

Ripple lamination

Contorted bedding or lamination

Figure SB-5. Lithologic and sedimentologic symbols used in measured stratigraphic

sections of the Fort Union Formation shown in figures SB-6 through SB-9 in this report.

Explanation
Unconformity

0

100 Feet

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

C

CS

Base

Top

 SB-6. Measured stratigraphic section of the Fort Union Formation (part) at locality A in the Basin

ld, Tps. 51 and 52 N., R. 94 W., eastern Bighorn Basin, Wyoming. Measured section location shown

re SB-4 (this report). Explanation for lithologic symbols shown in figure SB-5 (this report). Tfu, Fort

Formation (Tertiary); Kl, Lance Formation (Cretaceous). Modified from Roberts (1998).
CS

Figure SB-7. Measured stratigraphic sections of the Fort Union Form

coalfield, Tps. 46 and 47 N., Rs. 89 through 91 W., southeastern Bigh

shown in figure SB-4 (this report). Explanation for lithologic symbols 

Union Formation (Tertiary); Klm, Lance and Meeteetse Formations, u

Roberts (1998).

Base
0

100 Feet

(part)

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

CS

C

CS

CS

CS

CS

SB-8. Measured stratigraphic section of the Fort Union Formation at locality D in the Gebo coalfield,

, Rs. 95 and 96 W., south-central Bighorn Basin, Wyoming. Measured section location shown in figure

his report). Explanation for lithologic symbols shown in figure SB-5 (this report). Tfu, Fort Union

on (Tertiary); Kl, Lance Formation (Cretaceous). Modified from Roberts (1998).

Base
Basal bed of the Fort Union

Formation (Hewett, 1926)

100 Feet

0

Mayfield coal bed

Rider 1 coal bed

Rider 2 coal bed

Locality E

Locality F

CS

CS

CS

CS

CS

CS

CS

C

C

C

8 miles

Mayfield coal

zone

Upper coal

zone

Lower coal

zone

Figure SB-9. Measured stratigraphic sections of the Fort Union Formation (part) at

localities E and F in the Grass Creek coalfield, Tps. 45 and 46 N., Rs. 97-99 W.,

southwestern Bighorn Basin, Wyoming. Measured section locations shown in figure

SB-4 (this report). Explanation for lithologic symbols shown in figure SB-5 (this report).

Modified from Roberts (1998).
No data

No data

?

Tertiary (Paleocene)(part)

Fort Union Formation (lower part)

Lance Formation (upper part)

Upper Cretaceous (part)

Conglomerate No Data

Lance Formation

(Cretaceous)

Measured section

(approximate location) Sandstone

Mudrock

Coal and/or

carbonaceous shale

Fort Union Formation lithology

1 2 Miles

100 Feet

Explanation

Upper coal

zone

Lower coal

zone

 SB-10. Schematic cross section A-A' showing the lateral variation in Fort Union Formation lithology and the

aphic position of coal zones in the Grass Creek coalfield, Tps. 45 and 46 N., Rs. 97-99 W., southwestern Bighorn

Wyoming. Cross section location shown in figure SB-4 (this report). Modified from Roberts and Bossiroy (1995).
Section 21

T. 46 N., R. 99 W.

Rider 2 coal bed (Roberts and others, 1994)

Rider 1 coal bed (Roberts and others, 1994)

Mayfield coal bed (Roberts and others, 1994)

Mayfield coal bed (Roberts and Bossiroy, 1995)

Grass Creek coal bed (Roberts and Bossiroy, 1995)

Mayfield coal bed of Hewett (1926), Stewart (1975),

Mayfield coal bed of Stewart (1975) Glass and others (1975), and Glass (1981)

Gwynn coal bed of Stewart (1975)

Mayfield coal bed of Hewett (1926)

Unnamed coal bed of Hewett (1926)

30 Feet

0

(y, )

Inferred upper coal zone boundary

(Roberts and Bossiroy, 1995)

Mayfield coal zone

(Roberts and others, 1994)

Diamictite

Conglomerate

Sandstone

Mudrock

Coal

gure SB-11. Diagram comparing stratigraphic terminology for coal beds in the upper coal zone of the Fort Union

ormation, Grass Creek coalfield, southwestern Bighorn Basin, Wyoming. Location of measured sections shown in

gure SB-4 (this report). Modified from Roberts and Bossiroy (1995).
Coal bed 1 (7 ft)

Coal bed 1 1/2 (5 ft )

Coal bed 2 (8 ft with partings)

Coal bed 3 (10 ft) (ranges from 4 to 11.5 ft*) 

Coal bed 4 (10 ft with partings)

Coal bed 4 1/2 (3.5 ft with partings)

Coal bed 5 (12 ft with partings)

Coal bed 6 (4.9 ft)

Coal bed 7 (multiple thin beds) 

100 Feet

0

*Range of coal thickness from Rawlins (1986)

Sandstone

Shale

Carbonaceous shale

Coal

Explanation

Approximate scale

Tertiary (part)

Paleocene (part)

Fort Union Formation (part)

Figure SB-12. Generalized stratigraphic column showing Fort Union Formation coal beds

in the area of Red Lodge, northern Bighorn Basin, Montana. Stratigraphy adapted from

Woodruff (1909b) and Rawlins (1986).