Chapter SM

BULL MOUNTAIN BASIN, MONTANA

By G.D. Stricker

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

Contents

Introduction........................................................................................................ SM-1

Stratigraphy........................................................................................................ SM-1

Description of Coal............................................................................................ SM-2

Coal Quality....................................................................................................... SM-3

Resources........................................................................................................... SM-4

Production History............................................................................................. SM-5

Conclusions........................................................................................................ SM-5

References Cited ................................................................................................ SM-7

Figures

SM-1. Location of the Bull Mountain Basin, south-central Montana. Bull

Mountain Basin boundary drawn at the Lance-Fort Union boundary. “K”

denotes Cretaceous rocks and “T” denotes Tertiary rocks (from Connor,

1989).

SM-2. Generalized stratigraphy with principal coal beds in the Bull Mountain

Basin, southeastern Montana (modified from Woolsey and others, 1917;

Connor, 1989).

SM-3. Coal production for the years 1926-1960, Bull Mountain Basin, Montana.

Unpublished data from the Montana Coal Council (1998), 2301 Colonial

Drive, Helena, Montana 59801.

SM-4. Coal production for the years 1970-1997, Bull Mountain Basin, Montana.

Data from Resource Data International, Inc. (1998). Note change in

vertical scale from that in figure SM-3.
SM-iii

Table

SM-1. Summary of coal quality for all coal in the Bull Mountain Basin,

Montana. Calculated from the unpublished U.S. Geological Survey coal

quality database (USCHEM), February, 1992 and Connor (1989)
SM-1

INTRODUCTION

The Bull Mountain Basin (fig. SM-1) is located in south-central Montana and

contains coal of Tertiary age in the Fort Union Formation (fig. SM-2). The basin is

an asymmetrical syncline which trends east-west, contains minor anticlines and

synclines, and has beds that dip generally less than 5 degrees. Coal was produced

continuously between 1907 and 1960 and between 1970 and mid-1997. However,

the coal deposits in this basin have low importance in the current national coal

resource assessment (Flores and others, 1998). Even though the Bull Mountain

Basin contains coal of higher apparent rank than Powder River Basin coal, it is

unlikely to become a major coal producer in the Northern Rocky Mountains and

Great Plains Region within the next 20-30 years. This is because the basin is

small, about 750 square miles (Woolsey, 1909), and the coal is thinner, less than

17 ft thick, than the major coal producers in the Powder River Basin (Flores and

others, 1998).

STRATIGRAPHY

Because the Bull Mountain Basin and the Powder River Basin to the east were a

single depo-center during late Cretaceous and early Tertiary time, they have

similar stratigraphy. Post-deposition tectonics and erosion have isolated these

basins (Pierce and others, 1983). Figure SM-2 shows the stratigraphic position of

the major Tertiary coal beds in the Bull Mountain Basin (Woolsey and others,

1917; Connor, 1988). The Tertiary coal-bearing Fort Union Formation is of

Paleocene age and it overlies the Upper Cretaceous Lance Formation. The lower

200-300 ft of the Fort Union Formation contains the Lebo Shale Member (fig. SM-
SM-2

2). The Tullock Member of the Fort Union Formation, which underlies the Lebo

Shale Member to the east in the Powder River Basin, is not recognized in the Bull

Mountain Basin. Woolsey and others (1917) reported that the Lebo Shale Member

contains one coal bed called the Big Dirty, which is as thick as 17 ft near the

middle of the member. The “upper part” of the Fort Union Formation, as defined

by Woolsey and others (1917), is called the Tongue River Member (Connor,

1989), is a minimum of 1,650 ft thick, and contains 26 coal beds that range in

thickness from 1 to 16 ft (fig. SM-2). The Fort Union Formation is overlain, in

part, by Quaternary river terrace gravels and sandy alluvium. These deposits are as

thick as 20 ft. 

DESCRIPTION OF COAL

The Paleocene Fort Union Formation in the Bull Mountain Basin is considered a

minor coal-bearing formation. The 200-300-ft-thick Lebo Shale Member has been

described as the “somber-colored beds” by Richards (1910). Woolsey and others

(1917) report the Lebo Shale Member consists of olive-green, yellow, brown, and

dark-gray sandy-mudstone, sandstone, and coal. The only coal bed in the Lebo

Shale Member, the Big Dirty, is the thickest coal bed in the Bull Mountain Basin,

as much as 17 ft thick, but it is of no known economic interest because of the many

clastic partings and splits; hence its name. The Big Dirty coal bed was studied and

reported by Connor (1984). Connor notes that the numerous detrital layers in this

bed are crystal tuffs and altered volcanic ash, which indicate a period of intense

volcanism not recorded stratigraphically elsewhere in the Bull Mountain Basin. 

The Tongue River Member of the Fort Union Formation, which ranges in thickness

from at least 1,650 ft to more than 2,050 ft, is composed of massive-yellowish to

buff sandstone, light- to dark-gray mudstone, and coal. Aggregate thickness of
SM-3

coal in this member ranges from 20 to 85 ft. The Mammoth coal bed is near the

middle of the Tongue River Member and is the thickest coal bed, from 5 to 16

(Connor, 1989). Detailed cross sections (1988) and isopach maps (1989) of the

Mammoth coal bed were published by Connor. These cross sections and isopachs

indicate that the Mammoth coal bed is split into as many as three beds and reaches

maximum thickness only where there are no splits. Raw drill-hole and surfacesection data for the Bull Mountain coal field are available in Connor and Biewick

(1989). The Tongue River Member consists of fluvial-channel, overbank, and

paludal deposits. Major streams flowed south (Shurr, 1972) to southeast (Connor,

1988) into the ancestral Powder River Basin.

COAL QUALITY

Connor (1989) reported that the Mammoth coal in the Bull Mountain Basin ranges

in apparent rank from subbituminous A to high volatile bituminous C coal. Details

of Mammoth coal quality are available in Connor (1989). Pontolillo and Stanton

(1994) analyzed two samples from the basin. Those samples revealed a thermal

maturity level of Ro max = 0.62 percent, and Daniel and Cole (1982) report an Ro max

= 0.4-0.5 percent. These thermal maturity levels are consistent a with the apparent

rank of subbituminous A to high volatile bituminous C coal for the Bull Mountain

basin. Summary data for 18 coal-quality variables on table SM-1 were calculated

for the entire Bull Mountain Basin using all public available data in the U.S.

Geological Survey coal quality unpublished (USCHEM) database. A common

problem in statistical summaries of trace-element data arises when element values

are below the limits of detection. This results in a censored distribution. To

compute unbiased estimates of censored data for the elements in this table, we

adopted the protocol of reducing all “less than” values by 50 percent to generate a
SM-4

pseudo value for these data. Summary statistics of range (minimum and maximum

value) and arithmetic means were generated using the modified data.

Between 1974 and 1994, the U.S. Geological Survey analyzed samples of coal for

contents of major-, minor-, and trace-elements. Prior to performing the analyses,

most of the coal samples were dried at room temperature and humidity for as much

as 80 hours. Some samples, however, may have only been dried enough to allow

grinding (to less than 100 mesh), and moisture contents of the samples are

unknown, although moisture contents were probably similar to that which would

remain after air-dry loss determination (American Society for Testing and

Materials ASTM Standards, 1994, D3302-91). Since the actual moisture content

of the samples analyzed between 1974 and 1994 is unknown, the elemental

analysis of the samples cannot be converted to any other moisture basis. In

addition, these analyses can only provide an approximation of load factors (such

as, pounds of mercury per trillion Btu).

RESOURCES

Connor (1989) estimated the resources for the Mammoth coal bed, following the

methodology of Wood and others (1983). The Mammoth coal bed contains 1,100

million short tons. Pierce and others (1983) estimated the resources of the

Mammoth coal bed at 1,120 million short tons using the Kriging method (MoussetJones, 1980). Those are the only estimates available for the Bull Mountain Basin.
SM-5

The other 24 beds have not been considered a resource by previous workers,

although several of these beds have been mined.

PRODUCTION HISTORY

Coal has been mined commercially in the basin, but not continuously, since 1907

when the Republic Coal Company began operations on the Roundup coal bed (fig.

SM-2) near Roundup, Montana (fig. SM-1). Production prior to 1907 was limited

to a few tons per year, mined and utilized locally by the residents of the area for

heating and blacksmithing. Total production, from 1926 to 1960 (fig. SM-3), is

more than 23 million short tons (Montana Coal Council, written comun, 1998),

mainly from underground mines on the Roundup and McCleary coal beds. The

decision to mine these beds was probably influenced by proximity to rail

transportation. Production figures prior to 1926 are not available; there was no

commercial coal production during the 1960's. Coal production in the modern era

began in 1970 mainly from surface mines. Total production from 1970 to 1997

was 740 thousand short tons (fig. SM-4) (Resource Data International, Inc., 1998),

all from the Mammoth coal bed. The two production highs in 1990 and 1995 (fig.

SM-4) are due to the first underground mine producing coal from the Mammoth

coal bed.

CONCLUSIONS

Fort Union coal in the Bull Mountain Basin will not be a major source of coal in

the next two decades. Although the Mammoth coal bed has been mined in the past

and the quality (apparent rank, ash yield, and sulfur content, see table SM-1) is
SM-6

good, however, the small basinal area and thin beds indicate a low resource

potential. On a positive note, infrastructure for rail transport of coal is in place,

and if circumstances were good, minor amounts (minor when compared to

projected Powder River production; Flores and others, 1998) of coal could be

produced from the Bull Mountain Basin.
SM-7

REFERENCES CITED

American Society for Testing and Materials, 1994, Annual book of ASTM

Standards, Section 5, Petroleum products, lubricants and fossil fuels, Vol

05.05 Gaseous fuels; coal and coke; section D3302-91: American Society

for Testing and Materials, Philadelphia, Pennsylvania, p. 360-366.

Choate, R., Johnson, C.A., and McCord, J.P., 1984, Geologic overview, coal, and

potential for methane recovery from coalbeds–Powder River Basin, in

Rightmire, C.T., Eddy, G.E., and Kirr, J.N., eds., Coalbed methane resources

of the United States: American Association of Petroleum Geologists, Studies

in Geology v. 17., p. 135-162.

Connor, C.W., 1984, Ash-fall sequences in a Paleocene coal–potential indicator of

synchronicity between Montana and Wyoming basins, in Houghton, R.L.,

and Clausen, E.N., eds., 1984 Symposium on the geology of Rocky

Mountain coal: Bismark, N.D., North Dakota Geological Society,

Publication 84-1, p. 137-151.

 1988, Maps showing outcrop, structure contours, cross sections, and isopachs

of partings–Mammoth coal bed, Paleocene Tongue River Member of the

Fort Union Formation, Bull Mountain coal field, south-central Montana:

U.S. Geological Survey Coal Investigations Map C-126-A, 2 sheets, scale

1:50,000.

 1989, Maps showing coal-split boundaries, isopachs of coal splits, coal
SM-8

resources, and coal quality_Mammoth coal bed, Paleocene Tongue River

Member of the Fort Union Formation, Bull Mountain coal field, SouthCentral Montana: U.S. Geological Survey Coal Investigations Map C-126-B,

2 sheets, scale 1:50,000.

Connor, C.W., and Biewick, L.R.H., 1989, Drill-hole and surface-section data, Fort

Union Formation, Bull Mountain coal field, south-central Montana–209

records as entered in the National Coal Resources Data System: U.S.

Geological Survey Open-File Report 89-6, 305 pages.

Daniel, J.A., and Cole, G.A., 1982, Petrography of the Mammoth coal seam, Bull

Mountain Basin, Musselshell County, Montana, in Gurgel, K.D., ed., Fifth

Symposium on the Geology of Rocky Mountain Coal, 1982: Utah

Geological and Mineral Survey Bulletin 118, p. 168-178.

Flores, R.M., Bader, L.R., Cavaroc, V.V., Ellis, M.S., Gunter, G.L., Johnson, L.C.,

Keighin, C.W., Likarish, D., Nichols, D.J., Ochs, A.M., Roberts, S.B.,

Stricker, G.D., Takahashi, K.I.,Taber, T.Y., Warwick, P.D., Govern, G., and

Murphy, E.C., 1998, Compilation of data on strippable Fort Union coals in

the Northern Rocky Mountains and Great Plains Region: Proceedings of the

23rd International technical conference on coal utilization and fuel systems,

Clearwater, Florida, March 9-13, 1998, p. 839-848.

Mousset-Jones, P.F., 1980, ed., Geostatistics, McGraw Hill, New York, 166 p.

Pierce, F.W., Grundy, W.D., Connor, C.W., and Marbeau, J.P., 1983,

Geostatistical study of three coal deposits in the western United States, in
SM-9

Wang, Y.J., and Sanford, R.L., eds., Proceedings, 1st Conference on use of

computers in the coal industry, Morgantown, West Virginia, August 1-3,

1983: New York, Society of Mining Engineers, of American Institute of

Mining, Metallurgical, and Petroleum Engineers, Inc., p. 359-373.

Pontolillo, James, and Stanton, R.W., 1994, Vitrinite reflectance variations in

Paleocene and Eocene coals of the Powder River, Williston, Hanna,

Bighorn, and Bull Mountain Basins, U.S.A., in Pontolillo, James, ed.,

Abstracts: Eleventh Annual Meeting of The Society for Organic Petrology,

Sept. 25-30, 1994, Jackson, Wyoming, p. 82-84.

Resource Data International, Inc., 1998, COALDAT database, version 4.1

(includes data current as of January, 1998): comprehensive dataset. 

Software and database available from Resource Data International, Inc.,

1320 Pearl Street, Suite 300, Boulder, Colorado 80303.

Richards, R.W., 1910, The central part of the Bull Mountain coal field, Montana:

U.S. Geological Survey Bulletin 381, p. 60-81, 2 map plates.

Shurr, G.W., 1972, Paleocurrent indicators in Tongue River sandstones of the Bull

Mountain syncline, Montana, in Lynn, J., Balster, C.A., and Warne, J.R.,

eds., Crazy Mountain Basin, 21st Annual Geological Conference: Montana

Geological Society, p. 107-112.

Wood, G.H., Jr., Kehn, T.M., Carter, M.D., and Culbertson, W.C., 1983, Coal

resource classification system of the U.S. Geological Survey: U.S.

Geological Survey Circular 891, 65 p.
SM-10

Woolsey, L.H., 1909, The Bull Mountain coal field, Montana, in Cambell, M.R.,

ed., Contributions to Economic Geology, 1907, Part II--Coal and Lignite:

U.S. Geological Survey Bulletin 341, p. 62-77.

Woolsey, L.H., Richards, R.W., and Lupton, C.T., 1917, The Bull Mountain coal

field, Musselshell and Yellowstone Counties, Montana: U.S. Geological

Survey Bulletin 647, 218 p., 1 sheet, scale 1:125,000.
o o o

o

o

o

MONTANA

WYOMING

Lewistown

Havre

Miles City

Jordan

Roundup Forsyth

Billings

BULL MOUNTAIN

BASIN

WILLISTON

BASIN

POWDER

RIVER

BASIN

K

K

K

K

K

T

T

T

T

T

110 

48 

46 

108 106 104 

0 50 100 MILES

0 100 KILOMETERS

Figure SM-1. Location of the Bull Mountain Basin, south-central Montana.

Bull Mountain Basin boundary drawn at the Lance-Fort Union boundary. "K" denotes Cretaceous rocks and "T" denotes Tertiary rocks (from Connor, 1989).
SYSTEM

SERIES Holocene

Paleocene

FORMATION

Fort Union

MEMBER

Tongue River Lebo Shale Lance (part)

Q

U

ATERNAR

Y TERTIAR

Y CRETACEOUS

(part)

COAL BED

THICKNESS

OF COAL

BEDS (in feet)

THICKNESS

(in feet)

0-20

890-1,210

200-300

700-800

765 - 845 + +

Unnamed coal

Bull Mountain

Rock Mesa

Mammoth

Dougherty

Buckley

Wildhorse

Roundup

McCleary

Carpenter

Big Dirty

Rehder split

Summit

Fatig

3-7

3

3-4

2.5-6.5

2-7.5

0-5.5

5-16

1.5-5

0-6

0-6

0-8

0-8

2-17

0-3

Figure SM-2. Generalized stratigraphy with principal coal beds in the 

Bull Mountain Basin, southeastern Montana (modified from Woolsey and 

others, 1917; Connor, 1989).


Table SM-1. Summary of coal quality for all coal in the Bull Mountain Basin,

Montana. Calculated from the unpublished U.S. Geological Survey coal quality

database (USCHEM), February, 1992 and Connor (1989)

Variable Number Range Mean

of samples Minimum Maximum

Moisture 112 9.6 32.00 17.21

Ash 112 5.60 10.40 8.32

Total sulfur 112 0.50 1.20 0.82

Calorific value 212 5,760 10,990 9,760

lb SO2

 3 12 1.09 2.46 1.70

MMMFBtu 412 6,400 12,160 10,730

Antimony 59 0.37 9.1 1.7

Arsenic 59 1.0 35 7.0

Beryllium 511 0.42 1.9 0.73

Cadmium 512 0.066L 0.18 0.077

Chromium 5 9 2.6 6.5 3.6

Cobalt 5 9 0.65 5.5 1.4

Lead 512 2.3L 34 6.7

Manganese 512 8.3 130 41

Mercury 512 0.040 0.28 0.11

Nickel 5 12 0.66 9.0 2.7

Selenium 5 7 0.48 0.98 0.75

Uranium 5 12 1.1 6.8 1.8

 

1

 Value is in percent and on an as-received basis.

2

 Value is in British thermal units (Btu).

3

 Value is in pounds per million Btu and on an as-received basis.

4

 Value is in British thermal units on a moist, mineral-matter-free basis.

5

 Value is in parts per million (ppm) on a whole-coal and as-received basis; AL@

 denotes less than value shown.