Chapter WS

FORT UNION COAL IN THE WILLISTON BASIN,

NORTH DAKOTA: A SYNTHESIS

By R.M. Flores and C.W. Keighin

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

Contents

Introduction.........................................................................................................................WS-1

History of Coal Mining....................................................................................................WS-3

Geological Setting..............................................................................................................WS-5

Depositional Setting..........................................................................................................WS-7

Coal Geology......................................................................................................................WS-9

Coal Resources and Coal Quality.................................................................................WS-12

Conclusions.......................................................................................................................WS-14

References.........................................................................................................................WS-16

Figures

WS-1. Generalized map showing the extent of the Williston Basin in North

Dakota and Montana. Also shown are county boundaries, geological

structures, and coal towns in North Dakota. The parts of the Williston

Basin in South Dakota and Saskatchewan Province, Canada are not

shown.

WS-2. Geologic map of the Williston Basin showing areal distribution of

Cretaceous, Tertiary, Quaternary, and Holocene rock.

WS-3. Generalized stratigraphic column showing the age, palynology biozones,

and nomenclature in the Williston Basin.

WS-4. The Ludlow Member of the Fort Union Formation consisting of

coarsening-upward deltaic sandstone, distributary channel sandstone,

interdistributary mudstone and siltstone, and thin coal beds formed in lowlying swamps exposed along the valley wall of the Little Missouri River,

North Dakota.
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Figures—Continued

WS-5. The Cretaceous Fox Hills Sandstone (white) overlain by the Hell Creek

Formation (gray brown) exposed in the Cedar Creek Anticline, Montana.

WS-6. The Cannonball Member of the Fort Union Formation consisting of

tabular, shoreface sandstone exposed along the Heart River, North Dakota.

WS-7. A tongue of the Cannonball Member consisting of oyster shell beds

(brackish water) in the Ludlow Member exposed along the valley wall of

the Little Missouri River, North Dakota.

WS-8. Tidal silty sandstone, siltstone, and mudstone deposits in the Ludlow

Member of the Fort Union Formation.

WS-9. Flaser-bedded silty sandstone and siltstone draped by wispy mudstone

representing tidal flat deposits in the Ludlow Member.

WS-10. Burrowed mudstone interbedded with burrowed lenticular siltstone

representing tidal flat deposits in the Ludlow Member.

WS-11. Barrier shoreface sandstone underlain by tidal flat deposits of the

Cannonball Member exposed in the Cave Hills, South Dakota. The contact

between the shoreface sandstone and the tidal deposits is a sequence

boundary.

WS-12. The Tongue River Member of the Fort Union Formation consisting of

fluvial channel sandstone, floodplain mudstone, siltstone, and silty

sandstone, and thin coal beds deposited in low-lying swamps exposed

along the valley wall of the Little Missouri River, North Dakota.

WS-13. The Sentinel Butte Member of the Fort Union Formation consisting of

fluvial channel sandstone and interfluvial mudstone and siltstone above the

valley wall of the Little Missouri River, North Dakota.
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Figures—Continued

WS-14. The Harmon and Hansen coal above fluvial channel sandstone

(immediately above the river on the right of the photos) and the Rhame bed

paleosol (light gray) below the coal exposed along the wall of the Little

Missouri River, North Dakota.

WS-15. The Harmon (upper bed) and Hansen (lower bed) coal separated by

floodplain mudstone and siltstone.

WS-16. Merged Harmon and Hansen coal interbedded with floodplain mudstone

and siltstone along the valley wall of the Little Missouri River, North

Dakota.

WS-17 Rhame bed paleosol is as much as 30 ft thick and consists of stacked,

bleached to varicolored, rooted mudstone and interbedded carbonaceous

mudstone.
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INTRODUCTION

Major coal resources of lignite rank (Wood and Bour, 1988; Britton and others,

1989) exist in the Paleocene Fort Union Formation in the Williston Basin in North

Dakota (figs. WS-1 and WS-2). The Fort Union strata make up the surface bedrock

over much of the Williston Basin. The strata (fig. WS-3) are conformably underlain

by the Upper Cretaceous Hell Creek Formation, conformably overlain by the

Paleocene/Eocene Golden Valley Formation, and unconformably overlain by

Quaternary glacial till in the northern half of the basin (fig. WS-2). The coaly nature

of the Fort Union strata was first documented in 1805 by Lewis and Clark (1893)

along the Missouri River where they observed “burnt earth” and “earth, which

seems to have been boiled and then hardened by exposure.” These rocks are now

known as clinker; they are red and orange, baked and fused rocks resulting from

burning of coal beds by spontaneous combustion, lightning, or range-fire ignitions.

Additional accounts of coal deposits in the journals of the Lewis and Clark

expedition are the observation of numerous outcrops of coal in river banks and

nearby hills. During the expedition’s first winter, the party used Fort Union lignite in

a blacksmith’s forge (Budge, 1954).

Meek and Hayden (1862) defined these coal-bearing strata as “Fort Union Coal

Group.” Succeeding U.S. Geological Survey (USGS) appraisals of federallyowned coal lands resulted in subdivision of this “group” into “members,” which

formed the basis for early coal resource estimates (Leonard and others, 1925;

Campbell, 1929). Currently, these coal-bearing strata are recognized by the USGS

as the Fort Union Formation; it is subdivided, from bottom to top, into the Ludlow,

Cannonball, Tongue River, and Sentinel Butte Members (see fig. WS-3). The North

Dakota Geological Survey (NDGS) recognized the same strata as Fort Union
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Group; it is subdivided, from bottom to top, into the Ludlow, Cannonball, Slope,

Bullion Creek, and Sentinel Butte Formations (see fig. WS-3). This report uses the

USGS nomenclature; however, in this report we will attempt to correlate the

members of the formation with formations of the group recognized by NDGS.

The Fort Union Formation exists over 32,000 square miles in the Williston Basin

in North Dakota. In addition, the formation extends into the westernmost part of the

basin in eastern Montana, southernmost part of the basin in northwestern South

Dakota, and northernmost part of the basin in the southern part of the Saskatchewan

Province, Canada, where the strata are called the Ravenscrag Formation (Whitaker

and others, 1978; see fig. WS-3). This study investigates only the Fort Union

Formation in the Williston Basin in North Dakota because associated coal (for

example, Harmon, Beulah, and Hagel coal) is mined here and has contributed more

than 30 million short tons or about 3 percent of the U.S. total coal production in

1998 (Resource Data International, Inc., 1998). Also, during the next few decades

the coal is expected to fuel four mine-mouth power plants and three other power

plants that transmit electricity to Iowa, Minnesota, South Dakota, and Wisconsin.

Drill-hole data of the minable coal are available for analysis only in North Dakota.

In the 1950’s a number of subsurface data points from petroleum exploration,

exploration for uranium-enriched strata (including lignite beds), and drilling

associated with lignite mining and utilization became available for the Fort Union

Formation in North Dakota. In the 1970’s, increased demand for low sulfur Fort

Union lignite led to shallow drilling by coal companies, USGS-NDGS cooperative

projects, North Dakota State Water Commission, North Dakota Subsurface

Minerals program, and Montana Bureau of Mines and Geology. The shallow

drilling (as deep as 500 ft) from the joint USGS-NDGS cooperation (U.S.
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Geological Survey and North Dakota Geological Survey, 1976; 1977; 1978; 1979;

1980), coal companies, the water commission, and subsurface minerals program

provided surface and subsurface stratigraphic information on the Fort Union

Formation in North Dakota for our coal assessment. Log reports of these drilling

activities are archived in the North Dakota Geological Survey, and more than 6,000

drill-hole records were converted into digital files, which were used in this

assessment. (For a discussion concerning database creation and resource evaluation

methodology see Chapter DB in this CD-ROM.). The non-proprietary digital files

of drill holes are available through the U.S. Geological Survey’s National Coal

Resources Data System in Reston, Virginia (email: mdcarter@usgs.gov).

HISTORY OF COAL MINING

Mining of Fort Union coal in the Williston Basin in North Dakota began in 1873

near Sims about 35 miles west of Bismarck. The railroad played an important role

in the development of the first recorded underground mine. The mine was

abandoned after several attacks by the Sioux despite military escorts provided by Lt.

Colonel George A. Custer, who was stationed in Fort Lincoln, southwest of

Bismarck (Oihus, 1983). The Northern Pacific Railroad, which created the Northern

Pacific Coal Company, purchased and developed the abandoned mine. Five lignite

beds totaling about 22 ft in thickness yielded the first recorded production in the

basin of 100 tons daily, which was used for locomotive fuel and by area consumers

for heat (Leonard, 1926). The Northern Pacific Railroad played a major part in

further development of lignite in the Williston Basin due to a Congressional land

grant of about 10 million acres that included the coal resources beneath that land

grant (Brekke, 1992).
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Other underground mining followed prior to 1900 near Minot. This underground

mine was the first to use compressed air tools, fan ventilation, and cable cars

(Oihus, 1983). The mine produced about 600 tons of coal, which was shipped via

rail to eastern North Dakota. A mine near Dickinson used the room and pillar

method of mining for a coal bed that was as much as 26 ft thick. Small open-pit

mines worked by either a few miners or one person also opened prior to 1900.

Some of these underground and open-pit mines operated mainly during the winter,

supplying heating fuel for nearby towns (Wilder and Wood, 1903).

The Williston Basin lignite industry blossomed from 1900 to 1920 with new

interests in the coal quality and quantity, by-products, briquetting, and gasification of

the coal deposits (Brekke, 1992). Commercial mine operations were established in

Beulah, Bowman, Center, Coalbank, Columbus, Glen Ullin, Hanks, Hazen,

Hebron, Medora, New Salem, Noonan, Scranton, Underwood, and Zap. There was

an emergence of strip mining in these areas, from 1920 to 1940 with the

introduction of the steam shovels. The shovel simplified mining operation and

increased production capacity to as much as 700,000 tons per year (Oihus, 1983).

The period of 1940-1966 was marked by a decline in mining activity in the

Williston Basin (Brekke, 1992). Three hundred twenty mines were working in 1940

but that number had declined to 38 mines by 1965. Production peaked during this

period to 3,280,000 tons in 1951 (Carlson and Laird, 1964). Most of this production

went to fueling coal-fired electric generation plants. The decline in coal mining was

directly related to a shift from coal to reliance on oil and natural gas and installation

of dams for generation of electric power.
WS-5

New and expanded coal-fired power plants required the rapid growth of the lignite

mining industry from 1966 to the present. Currently, seven coal-fired power plants

supply electricity for in-state and out-of-state customers. Production peaked to as

much as 32,000,000 short tons in 1996 (E.C. Murphy, NDGS, oral commun.,

1998). Currently the number of active coal mines has dwindled to six mines, four

lignite and two leonardite (oxidized lignite). The four coal mines are large surface

mines. No underground mining is being done.

GEOLOGICAL SETTING

The Williston Basin is a structural and sedimentary basin located on the western

edge of the craton in central North America. It contains more than 15,000 ft of

Cambrian to Quaternary age rocks (Carlson and Anderson, 1966). These rocks dip

on the average 20 ft/mi toward the basin center in northwestern North Dakota. The

Miles City arch separates the southwestern margin of Williston Basin from the

Powder River Basin in Montana and Wyoming (see fig. WS-1). Late Laramide or

middle to late Eocene positive structures representing basement-block movements

(Anna, 1986; Clement, 1987) include the Nesson and Cedar Creek anticlines and the

Poplar dome along the western and northern parts of the basin (see fig. WS-1).

These movements were followed by a regional epeirogenic uplift leading to the

current topographic expression (Gerhard and others, 1982). The relatively thin Fort

Union Formation, as much as 1,868 ft thick (Carlson, 1985), suggests that regional

subsidence of the Williston Basin may not have strongly influenced sediment

accumulation.

Although the basin is structural in origin and formed during periodic tectonic

movements, regional Fort Union sediment accumulations were mainly controlled by
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depositional settings. However, local accumulation of Fort Union sediments may

have been partly influenced by local tectonic movements along both the Cedar Creek

anticline, which may have diverted streams flowing from the west and southeast

from the Powder River Basin, and the Nesson anticline, which may have controlled

the orientation of stream channels (Daly and others, 1985; Flores, 1986; Diemer and

others, 1992). Downwarping of intervening areas between the Cedar Creek and

Nesson anticlines may have ponded and concentrated accumulations of thick peat

deposits as exemplified by the Harmon and Hansen coal zones. Timing of these

tectonic movements and corresponding overthickening of chronostratigraphically

contemporaneous Fort Union sediments are not well documented.

The Fort Union Formation is thick along the basin axis in the northwestern part of

North Dakota (Carlson, 1985; Cherven and Jacob, 1985; Daly and others, 1985,

1992). Beds dip at low angles (1-2 degrees) toward the basin axis. Based on the

NDGS classification of Fort Union Group, Daly and others (1992) informally

divided it into lower and upper parts. The lower part is as much as 656 ft thick and

is composed of the Cannonball Formation, which is stratigraphically equivalent to

the Ludlow, Slope, and lower Ravenscrag Formations (stratigraphically equivalent

to the Ludlow Member of the USGS). The upper part is as much as 1,312 ft thick

and is composed of the Bullion Creek and Sentinel Butte Formations

(stratigraphically equivalent to the Tongue River and Sentinel Butte Members of the

USGS), and upper Ravenscrag Formation (fig. WS-3). The coal beds in the upper

part of the Fort Union are thicker than those in the lower part (Daly and others,

1992). Subdivision of Fort Union strata is based on color and marker beds. For

example, drab-gray rock units include the Slope, Ludlow, and Sentinel Butte, and

bright yellow rock units include the Bullion Creek/Tongue River. In addition, coal

beds (for example, Harmon coal bed), thick sandstone beds (for example, basal
WS-7

sandstone), and white siliceous beds (for example, Rhame bed; see fig. WF-2 in

Chapter WF of this CD-ROM) mark boundaries of these rock units (Royse, 1970;

Jacob, 1975; Rehbein, 1977, 1978; Wehrfritz, 1978; Groenewold and others, 1979).

DEPOSITIONAL SETTING

The physical variations of the Fort Union strata are mainly controlled by their

depositional environments (Royse, 1970; Jacob, 1973; Warwick, 1982; Belt and

others, 1984; Cherven and Jacob, 1985; Warwick and others, 1997). The strata of

the lower part of the Fort Union (Ludlow and Cannonball) were deposited mainly in

deltaic and marine environments, respectively (Belt and others, 1984; Cherven and

Jacob, 1985). Ludlow coal beds (fig. WS-4) were interpreted as deposits that

accumulated in swamps in interdistributary and abandoned deltaic areas. However,

the work by Flores (1992) on stratigraphically equivalent rocks in the northwestern

part of the Williston Basin in Montana suggests alluvial deposition and that the

underlying Upper Cretaceous Fox Hills Sandstone and Hell Creek Formation (fig.

WS-5) were deposited in delta front and delta plain environments, respectively. This

Upper Cretaceous to Lower Paleocene rock package represents a regressive event

trending towards the east into North Dakota. Recent work by Warwick and others

(1996; 1997) on the Ludlow and Cannonball strata (figs. WS-6 and WS-7) suggest

their deposition in tide-dominated (figs. WS-8, WS-9, and WS-10) and marine

environments and that accumulation of Ludlow coal beds was directly influenced by

eustatic sea-level rise and fall. These eustatic sea-level changes were expressed as the

transgressions (toward the west) and regressions (toward the east) of the Cannonball

Sea (situated mainly in the east-central part of the basin), in which the marine

Cannonball strata were deposited (Cvancara, 1976). During transgressions, deltaic

deposits from previous regressions were reworked by waves and tidal processes
WS-8

forming stacked, coarsening-upward parasequence sets of barrier bars (fig. WS-11;

Warwick and others, 1997). These authors suggested that although thin coal beds

accumulated in tidal-intertidal and back-barrier swamps in North Dakota and South

Dakota, thicker coal beds developed in swamps associated with contemporaneous

fluvial environments farther landward or to the west in Montana.

The upper part of the Fort Union strata (Tongue River and Sentinel Butte; see figs.

WS-12 and WS-13) was interpreted as fluvial and deltaic deposits (Royse, 1970;

Warwick, 1982; Winczewski, 1982; Jacob, 1976; Cherven and Jacob, 1985; Daly

and others, 1985). Although Jacob (1976) and Winczewski (1982) interpreted the

environments of deposition of the upper Fort Union sediments as deltaic and fluvial,

respectively, no compelling arguments were presented to explain the accumulations

of the thick and widespread coal beds and zones (for example, Harmon and Hansen,

Hagel, Beulah-Zap). These coals (for example, Harmon coal) and associated rocks

probably accumulated in swamps on abandoned deposits of fluvial-channel belts

(fig. WS-14) that migrated into nearby interfluvial areas (fig. WS-15). Merging and

splitting of these coal zones (fig. WS-16) attest to autocyclic processes associated

with the fluvio-deltaic environments. This fluvio-deltaic deposition, which

accompanied a widespread regression of the Cannonball Sea, developed an

extensive platform from eastern Montana to western North Dakota on which the

thick, white, siliceous (silcrete) Rhame Bed paleosol or ancestral soil (fig. WS-17)

formed during the sea-level low stand (Wehrfritz, 1978; Christensen, 1984). This

generally eastward regression was interrupted by minor transgressions (Cherven

and Jacob, 1985).
WS-9

COAL GEOLOGY

The coal geology of the Fort Union Formation in the Williston Basin in North

Dakota may be subdivided into that of the lower part (Ludlow through Cannonball

rocks) and that of upper part (Tongue River and Sentinel Butte rocks). This

dichotomy follows differences in the biostratigraphy, stratigraphy, thickness,

sedimentology or depositional settings, and areal distribution of coal in these parts of

the Fort Union Formation. This dichotomy is best shown by Warwick (1982) and

Belt and others (1984) and by Cherven and Jacob (1985) across the west-central part

of the basin.

The biostratigraphy based on palynology (analysis of spores and pollen) of the Fort

Union Formation in the northern Rocky Mountains and Great Plains region was

developed by Nichols and Ott (1978); see also Nichols (1994; 1996). Based on this

zonation, Nichols subdivided the Fort Union Formation in the Williston Basin in

North Dakota into six Zones, P1-P6 (see fig. WS-3, and Chapter WB in this CDROM). The Ludlow Member of the Fort Union Formation is within Zones P1-P3

(Warwick and others, 1996; 1997). The Tongue River and Sentinel Butte Members

of the Fort Union Formation are within zones P4-P6 (Warwick and others, 1996;

1997).

The coal stratigraphy in the lower and upper parts of the Fort Union Formation

differs in the vertical and lateral patterns of coal beds and zones and associated

clastic rocks. The vertical stratigraphic pattern in the lower part of the formation

consists of coal-bed-zone “bundles.” These coal “bundles” are interbedded with

thick clastic intervals (as much as 240 ft thick) of sandstone, siltstone, and mudstone
WS-10

beds. The coaly “bundles” are in turn interbedded with thin clastic intervals (as

much as 80 ft thick) consisting mainly of siltstone, mudstone, and subordinate

sandstone. Laterally, the coal beds split and merge within a 2-7 mi distance, forming

a coal zone (Belt and others, 1984). The individual coal beds of this zone vary from

a few inches to 13 ft thick.

In the upper part of the formation (Groenewold and others 1979), the vertical

stratigraphic pattern consists of coal-bed-zone “bundles” that are interbedded with

thick clastic rock intervals (as much as 175 ft thick) of sandstone, siltstone, and

mudstone. These coal “bundles” are in turn interbedded with thinner clastic intervals

(as much as 125 ft thick) consisting mainly of mudstone, siltstone, and limestone,

and subordinate sandstone. Laterally, the coal beds split and merge within a 9-20 mi

distance, forming a coal zone (Groenewold and others, 1979). Coal beds of this

zone vary from a few inches to 40 ft thick. The coal-bed-zone “bundling” is

represented by Harmon and Hansen, Hagel, and Beulah-Zap beds. Coal beds and

zones targeted by our assessment and their associated stratigraphy are demonstrated

in Chapter WF of this CD-ROM.

The vertical and lateral patterns of the coal stratigraphy in the lower and upper parts

of the Fort Union are controlled by the sedimentology or depositional environments

of these rocks. In the lower part of the formation the short distance (within a 2-7 mi

distance) of splitting and merging of coal beds forming a zone is controlled by the

areal size and rate of sedimentation followed by avulsion or shift of the deltaic

depositional systems that formed these rocks. Thus, a rapid rate of sedimentation in

small depositional systems, such as in subdeltas (for example, crevasse splays),

produced thin clastic intervals of mainly siltstone, mudstone, and subordinate

sandstone. These sedimentary deposits formed short-lived platforms on which
WS-11

swamps accumulated thin peat deposits. Frequent autocyclic (lateral) shifts of these

environments influenced the splitting and merging of the peat deposits and resulting

thickness of peat formed on these platforms. Stacking of the peat deposits and

sediments related to autocyclic deltaic systems caused coal-bed-zone “bundling” or

stacked thin coal beds. The intervening thick intervals of sandstone, siltstone, and

mudstone represent the major distributary channel-overbank deposits of the delta

systems. These deposits in turn were reworked and modified by tidal incursions as a

result of regional and local sea-level transgressions. This tidal reworking as well as

restricted platforms of peat deposition may have contributed to the limited areal

distribution of the peat deposits. The peat deposits formed in these environments are

exemplified by coal beds in the Tongue River and Sentinel Butte Members of the

Fort Union Formation.

In the upper part of the formation coal beds split and merge over a long distance

(within a 9-20 mi distance) forming a coal zone. Such zones are controlled by the

rapid rate of sedimentation in wide alluvial plains that were drained by meandering

and braided streams and accompanying crevasse splays. Avulsion or shift and

prolonged abandonment of these stream deposits formed extensive alluvial

platforms on which swamps developed and very thick peat deposits accumulated.

However, rapid successions of avulsion or shift and abandonment of the fluvial

depositional systems along the width of the alluvial plain provided coal-bed-zone

“bundling.” In addition, the vast expanse of abandoned alluvial platforms

contributed to the extensive areal distribution of the peat deposits. This accounts for

the widespread coal deposits of the Tongue River and Sentinel Butte Members of

the Fort Union Formation.
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COAL RESOURCES AND COAL QUALITY

Early estimates of the coal resources in the Williston Basin in North Dakota were

based mainly on outcrop data that were stratigraphically limited. Leonard and others

(1925) evaluated only the coal beds equal to or greater than 4 ft thick in the upper

300-400 ft of the Fort Union Formation, which resulted in a coal resource totaling

516 billion short tons. Campbell (1929) was the first to present comprehensive coal

resource estimates in the Williston Basin in North Dakota. Utilizing coal beds 3 ft

thick or greater, Campbell (1929) estimated the coal resource in North Dakota as

600 billion short tons. Using subsurface data from 0-3,000 ft below the surface and

coal beds 2.5 ft thick or greater, Brant (1953) estimated the remaining identified coal

resource in the Williston Basin in North Dakota to be more than 350 billion short

tons. Pollard and others (1972) identified and mapped strippable lignite resources in

North Dakota. Averitt (1975), using coal beds 2.5 ft thick or greater, estimated the

total identified and hypothetical coal resources 0-6,000 ft below the surface in North

Dakota to be more than 530 billion short tons. In addition, Averitt (1975) estimated

the surface coal reserve base determined by potential mining methods (for example,

strip mining) to be more than 16 billion short tons.

Most of the coal resources of the Williston Basin in North Dakota are contained in

the Harmon and Hansen coal beds and zones in the southwestern part of the area,

and in the Hagel and Beulah-Zap coal beds and zones in the east-central part. These

coal beds and zones (for example, Harmon and Hansen, Hagel, and Beulah-Zap)

were assessed in the Bowman-Dickinson, Center-Falkirk, and Beulah-Zap

coalfields; assessment is limited by the extent and distribution of available drill-hole

data. The coal beds and zones may exist beyond the coalfield limits. Estimates of

coal resources of the Harmon and Hansen, Hagel, and Beulah-Zap coal beds and
WS-13

zones are presented in Chapter WN in this CD-ROM. Estimates of coal resources in

beds thicker than 2.5 ft are: Harmon and Hansen coal beds and zones—67 billion

short tons; Hagel coal beds and zones—4.4 billion short tons; and Beulah-Zap coal

beds and zones—4.8 billion short tons; a total for the North Dakota part of the

basin—76.2 billion short tons (see Chapter WN of this CD-ROM). These coal

resource estimates are broken down by thickness, overburden, ownership, and

county (see Chapter WN). The methodology of calculating the resources of these

coal beds and zones is discussed in Chapter DB and Chapter WN).

The Fort Union lignite in the Williston Basin in North Dakota is classified according

to the percent fixed carbon and calorific value calculated on a mineral-matter-free

basis. Fort Union lignite in North Dakota is low in percent (arithmetic mean) fixed

carbon and calorific value, which ranges from 2,625 to 7,980 Btu/lb (n= 216

samples; as-received basis; Tewalt and others, 1992). Flores and others (1998)

reported calorific value (arithmetic mean) of 6,504 Btu/lb (lignite A; n=310; asreceived basis) for the Fort Union lignite in North Dakota. Sulfur and ash contents

of the Fort Union lignite range from 0.2 to 4.0 and 3.5 to 30.7 percent, respectively

(arithmetic means; n=216 samples; as received basis; Tewalt and others, 1992).

Flores and others (1998) reported sulfur and ash contents of 0.86 and 7.99 percent

(arithmetic mean), respectively (n=310; as-received basis). The coal quality and

geochemistry of the Fort Union lignite in the Williston Basin in North Dakota are

presented in Chapter WQ of this CD-ROM.

Tewalt and others (1992) reported that the trace elements of environmental concern

in Fort Union coal, which can impact coal utilization and reclamation, include

arsenic and selenium. Arsenic and selenium range from 1 to 63 ppm and 0.15 to 3.3

ppm, respectively (arithmetic mean; on a whole coal; as-received basis; Swanson
WS-14

and others, 1976; Tewalt and others, 1989). Stricker and others (1998) reported the

arithmetic mean for arsenic as 9.1 ppm (on a whole coal, as-received basis) and for

selenium as 0.74 ppm (on a whole-coal, as-received basis). In addition, Stricker and

others (1998) compared concentrations of these trace elements and others

(antimony, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury,

nickel, and uranium) with coal from other coal regions in the conterminous U.S.

(for example, Colorado Plateau, Illinois Basin, and Appalachian Basin). They

concluded that the Fort Union coal in the Williston Basin in North Dakota has low

concentrations of trace elements of environmental concern.

Inorganic composition of the Fort Union lignite includes mainly pyrite, quartz, and

clay minerals (Benson and others, 1992). Other minerals present in lesser amounts

include gypsum, other sulfides, carbonates, barite, rutile, and hematite. The pyrite,

gypsum, barite, and other sulfides contribute to the sulfur content of the coal. The

clay minerals and quartz contribute to the ash content of the coal. The distribution of

these minerals varies with location in the coal beds (Zygarlicke, 1987). A significant

proportion of the inorganic components is organically related; that is, most of the

elemental components of these minerals exist as organic coordination complexes

(for example, macerals) in the coal (Benson and others, 1992).

CONCLUSIONS

The Fort Union lignite represents a significant energy resource in the Williston

Basin in North Dakota. Its mining history and development spans about 125 years

and continues to support local and regional consumers. The most important use of

this coal resource is as fuel for mine-mouth power plants and other electric power

generating plants. Presently, the Harmon and Hansen, Hagel, and Beulah-Zap coal
WS-15

beds and zones are mined in open pits. Production from these coal mines mainly

supports seven in-state electric power plants and one degasification plant. The

electricity generated from the power plants is transmitted to utilities in nearby midwestern states.

In order to continue supporting the electric power and degasification plants during

the next century, assessment of the Fort Union coal in the Williston Basin in North

Dakota focused on the Harmon and Hansen, Hagel, and Beulah-Zap coal beds and

zones. These coal deposits are as much as 60 ft thick, lignite A in rank, and have

low-sulfur, ash, and trace-element contents. Coal production in 1996 from six coal

mines was more than 32 million short tons and was reduced to more than 30

million short tons from five mines in 1998. The Harmon and Hansen, Hagel, and

Beulah-Zap coal beds and zones contain resources of as much as 76.2 billion short

tons in coal beds more than 2.5 ft thick. The thick coal beds were deposited mainly

in swamps related to fluvial and deltaic environments. The extensive areal

distribution of these coal beds and zones reflects accumulation in raised swamps on

abandoned alluvial platforms. The thickness, areal distribution, high quality, and

chemistry of this coal make it a clean and compliant energy resource.
WS-16

REFERENCES

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WS-17

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WS-1. Generalized map showing the extent of the Williston Basin in North Dakota and Montana. Also shown are county boundaries,

al structures, and coal towns in North Dakota. The parts of the Williston Basin in South Dakota and Saskatchewan Province, Canada,

hown.

illiston Basin 

North Dakota

South Dakota

Montana 

POPLAR DOME

● ●

●

●

●

●

●

● ● ●

●

●

●

● ●

●

●

Columbus

Hanks

Minot

Dickinson

Coalbank

Zap

Beulah

Hazen

Center

Hebron New 

Salem

Sims

Glen Ullin Bismarck

Bowman

Scranton CEDAR CREEK ANTICLINE

NESSON

ANTICLINE

BASIN

AXIS

MILES CITY ARCH

N

0 50 100 Miles
North Dakota

South Dakota

Montana 

POPLAR DOME

0 50 100 Miles

N

MILES CITY ARCH

48°

46°

47°

lluvium

Glacial till

laxville Gravel

ndifferentiated Miocene rocks

White River Group

Golden Valley Formation

entinel Butte Formation

ullion Creek Formation

lope Formation

annonball Formation

udlow Formation

ndifferentiated Fort Union

ell Creek Formation

ox Hills Sandstone

earpaw Shale

CEDAR CREEK ANTICLINE

Figure WS-2. Geologic map of the Williston Basin showing areal distribution of Cretaceous, Tertiary,

Quaternary, and Holocene rocks (adapted from Schruben and others, 1974; Raines and Johnson, 1996).

Dakota

South

Dakota

g

ary

e/Miocene

e

ne

ne

ous

e
Zones

(Nichols and Ott, 1978) Saskatchewan,

Canada

North Dakota

Geological Survey

Williston Basin

U.S. Geological

Survey

Golden Valley

Formation

Sentinel Butte

Member

Tongue River

Member

Fort Union Formation

Ludlow

Member

Cannonball

Member

Cannonball

Formation

Ravenscrag Formation

Fort Union Group

Golden Valley

Formation

Hell Creek

Formation

Fox Hills

Sandstone

Frenchman

Formation

Eastend

Sandstone

och

ernary

ne

cene

r

ceous

P6

P5

P4

P3

P2

P1

GLACIAL TILL DEPOSITS

Sentinel Butte

Formation

Bullion Creek

Formation

Ludlow

Formation

Slope

Formation
Figure WS-4. The Ludlow Member of the Fort Union Formation consisting of coarsening-upward deltaic sandstone, 

distributary channel sandstone, interdistributary mudstone and siltstone, and thin coal beds formed in low-lying swamps

exposed along the valley of the Little Missouri River, North Dakota. Photograph by R.M. Flores.
Figure WS-5. The Cretaceous Fox Hills Sandstone (white) overlain by the Hell Creek Formation (gray Brown) exposed in 

the Cedar Creek Anticline, Montana. Photograph by R.M. Flores.
Figure WS-6. The Cannonball Member of the Fort Union Formation consisting of tabular, shoreface sandstone exposed 

along the Heart River, North Dakota. Photograph by R.M. Flores.
Figure WS-7. A tongue of the Cannonball Member consisting of oyster shell beds (brackish water) in the Ludlow 

Member exposed along the valley wall of the Little Missouri River, North Dakota. Photogragh by R.M. Flores.
Figure WS-8. Tidal silty sandstone, siltstone, and mudstone deposits in the Ludlow Member of the Fort Union Formation. 

Photograph by R.M. Flores.
Figure WS-9. Flaser-bedded silty sandstone and siltstone draped by wispy mudstone representing tidal flat deposits in the 

Ludlow Member. Photograph by R.M. Flores
Figure WS-10. Burrowed mudstone interbedded with burrowed lenticular siltstone representing tidal flat deposits in the 

Ludlow Member. Photograph by R.M. Flores.
Figure WS-11. Barrier shoreface sandstone underlain by tidal flat deposits of the Cannonball Member exposed in the 

Cave Hills, South Dakota. The contact between the shoreface sandstone and the tidal deposits is a sequence boundary. 

Photograph by R.M. Flores.
Figure WS-12. The Tongue River Member of the Fort Union Formation consisting of fluvial channel sandstone, 

floodplain mudstone, siltstone, and silty sandstone, and thin coal beds deposited in low-lying swamps exposed along the 

valley wall of the Little Missouri River, North Dakota. Photograph by R.M. Flores.
Figure WS-13. The Sentinel Butte Member of the Fort Union Formation consisting of fluvial channel sandstone and 

interfluvial mudstone and siltstone above the valley wall of the Little Missouri River, North Dakota. Photogragh by 

R.M. Flores.
Figure WS-14. The Harmon and Hansen coal above fluvial channel sandstone (immediately above the river on the right 

of the photo) and the Rhame bed paleosol (light gray) below the coal exposed along the wall of the Little Missouri River,

North Dakota. Photograph by R.M. Flores.
Figure WS-15. The Harmon (upper bed) and Hansen (lower bed) coal separated by floodplain mudstone and siltstone. 

Photograph by R.M. Flores. 
Figure WS-16. Merged Harmon and Hansen coal interbedded with floodplain mudstone and siltstone along the valley 

wall of the Little Missouri River, North Dakota. Photograph by R.M. Flores.
Figure WS-17. Rhame bed paleosol is as much as 30 ft thick and consists of stacked, bleached to varicolored, rooted 

mudstone and interbedded carbonaceous mudstone. Photograph by R.M. Flores.