Chapter PD

DECKER COALFIELD, POWDER RIVER BASIN,

MONTANA: GEOLOGY, COAL QUALITY, AND COAL

RESOURCES

By S.B. Roberts,1 G.L. Gunther,2 T.T. Taber,1 A.M. Ochs,2 Dorsey

Blake,1 M.S. Ellis,1 G.D. Stricker,1 E.M. Wilde,3 J.H. Schuenemeyer,1 and

H. C. Power4

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

1

U.S. Geological Survey

2

Consultant, U.S. Geological Survey, Denver, Colorado

3

Montana Bureau of Mines and Geology, Butte, Montana

4

University of Delaware, Newark, Delaware
PD-ii

Contents

Introduction...........................................................................................................................PD-1

Geologic Overview and Coal Stratigraphy....................................................................PD-2

Coal Quality.......................................................................................................................PD-10

Total, Net Coal Thickness (Isopach) Map ..................................................................PD-11

Overburden Thickness (Isopach) Map........................................................................PD-13

Coal Resources..................................................................................................................PD-13

Sources of Data.................................................................................................................PD-16

Drill-hole Data.............................................................................................................PD-16

Geologic Map Data ....................................................................................................PD-20

Geographic Boundary Data......................................................................................PD-21

Acknowledgments............................................................................................................PD-22

References Cited................................................................................................................PD-23

Figures

PD-1. Index map showing the Decker coalfield and vicinity, southeast Montana.

PD-2. Map showing the regional drainage pattern, Decker coalfield and vicinity, 

 southeast Montana.

PD-3. Index map showing 7.5’ and 30’ x 60’ quadrangle maps in the Decker 

 coalfield, southeast Montana.

PD-4. Map showing the distribution of Federally owned coal, Decker coalfield and 

 vicinity, southeast Montana.

PD-5. Map showing generalized bedrock geology, Decker coalfield and vicinity, 

 southeast Montana.
PD-iii

Figures—continued

PD-6. Chart showing coal-bed nomenclature for the Anderson-Canyon coal zone, 

 Decker coalfield, Montana.

PD-7. Index map showing the location of cross sections GP1-GP1’ and GP2-GP2’

 (natural-gamma log profiles), Decker coalfield, southeast Montana.

PD-8. Cross section GP1-GP1’ showing natural-gamma log profiles and coal-bed 

 correlations in drill holes penetrating the Anderson-Canyon coal zone, 

 Decker coalfield, southeast Montana.

PD-9. Cross section GP2-GP2’ showing natural-gamma log profiles and coal-bed 

 correlations in drill holes penetrating the Anderson-Canyon coal zone 

 Decker coalfield, southeast Montana.

PD-10. Index map showing the location of restored stratigraphic/structural cross 

 sections A-A’, B-B’, C-C’, and D-D’, Decker coalfield, southeast 

 Montana.

PD-11. Restored stratigraphic/structural cross section A-A’ showing coal-bed 

 correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast 

 Montana.

PD-12. Restored stratigraphic/structural cross section B-B’ showing coal-bed 

 correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast 

 Montana.

PD-13. Restored stratigraphic/structural cross section C-C’ showing coal-bed 

 correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast 

 Montana.
PD-iv

Figures—continued

PD-14. Restored stratigraphic/structural cross section D-D’ showing coal-bed 

 correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast 

 Montana.

PD-15. Map showing the total, net coal thickness in the Anderson-Canyon coal 

 zone, Decker coalfield, southeast Montana.

PD-16. Map showing the thickness of overburden above the Anderson-Canyon 

 coal zone, Decker coalfield, southeast Montana.

Tables

PD-1. Total, net coal resources in the Anderson-Canyon coal zone in the Decker 

 coalfield, Montana, reported by county, overburden thickness, net coal 

 thickness, and reliability categories

PD-2. Total, net coal resources in the Anderson-Canyon coal zone reported for 7.5’ 

 quadrangles in the Decker coalfield, Montana

PD-3. Total, net coal resources in the Anderson-Canyon coal zone, Decker 

 coalfield, Montana, reported by county and Federal coal and surface 

 ownership

PD-4. Computations of confidence intervals within reliability categories for 

 estimated total, net coal resources in the Anderson-Canyon coal zone in the 

 Decker coalfield, Montana

PD-5. Volumes and estimates of uncertainty for estimated total, net coal resources 

 in the Anderson-Canyon coal zone, Decker coalfield, Montana, with 

 measurement error
PD-1

INTRODUCTION

The Decker coalfield is located about 16 mi northeast of Sheridan, Wyoming, and as

defined here, covers about 1,100 square miles in parts of Big Horn, Powder River,

and Rosebud Counties, Montana (fig. PD-1). Access through the coalfield is

primarily restricted to one State highway and a network of secondary roads. The

coalfield is between the Tongue and Powder Rivers, and most of the field is

dissected by small, intermittent or perennial streams, including the Otter Creek and

Hanging Woman Creek drainages (fig. PD-2). The Decker coalfield includes all or

parts of fifty 7.5’ quadrangles (fig. PD-3) and is bounded on the south by the

Montana state line, on the west and northwest by the Crow and Northern Cheyenne

Indian Reservations, respectively, and on the north and east by the main outcrop line

of the Canyon coal bed or the contact between unburned coal and clinker of the

Canyon coal bed. The extent of the Canyon coal bed as used in this study is based

on published maps of Baker (1929), Warren (1959), Bryson and Bass (1973), and

Robinson and Culbertson (1984).

The Decker coalfield includes parts of the historically defined Northward Extension

of the Sheridan coalfield (Baker, 1929), the Moorhead coalfield (Bryson and Bass,

1973), and the Birney-Broadus coalfield (Warren, 1959), as well as a more areally

restricted Decker coalfield (Decker area) described in studies by Matson and Blumer

(1973). Most of the coal in the Decker coalfield is federally owned (fig. PD-4). Coal

mining has taken place in the Decker area since the early 1900’s, although historic

production was primarily from small wagon mines producing coal locally for

domestic use (Baker, 1929). Currently there are three operating surface mines in the

coalfield: the Spring Creek, East Decker, and West Decker mines (fig. PD-1). From
PD-2

1995 to 1997, the combined production from these 3 mines has been about 20

million short tons per year (Resource Data International, Inc., 1998).

The Anderson-Canyon coal zone has been selected for resource assessment in the

Decker coalfield primarily because there is continuing coal production from certain

beds within this coal zone in Montana, as well as significant production from

equivalent coal deposits in Wyoming. The Anderson-Canyon coal zone is

considered to be stratigraphically equivalent to the Wyodak-Anderson coal zone as

described by Flores and Bader (Chapter PS in this CD-ROM). For the purposes of

this study, however, local coal-bed terminology as applied in Montana is adhered to

in this chapter.

Although coal beds in the Anderson-Canyon coal zone extend into the Crow and

Northern Cheyenne Indian Reservations, no assessment of the coal resources on

tribal lands is included in this report. Also, part of the Decker coalfield extends into

Custer National Forest (fig. PD-4), and Section 522 (e) (2) (b) of the Surface

Mining Control and Reclamation Act of 1977 [SMCRA; Public Law 95-87 (30

U.S.C. 1201 et seq.)] strictly prohibits surface coal mining within the boundaries of

Custer National Forest. Although we have included coal resource information for

areas within the National Forest, the reader should take into account the limitations

of coal resource development as constrained by SMCRA.

GEOLOGIC OVERVIEW AND COAL STRATIGRAPHY

Bedrock exposures in the Decker coalfield consist of Tertiary strata in the Fort

Union Formation and the overlying Wasatch Formation (fig. PD-5). The contact

between the formations is placed at the top of the Roland coal bed of Baker (1929).
PD-3

The structural dip of exposures in this area is generally less than 5 degrees with dip

orientations varying from southeasterly in the western part of the coalfield to

southwesterly in eastern areas of the coalfield. Although dips as steep as 72 degrees

have been recorded (Galyardt and Murray, 1981), these steeper dips typically reflect

structural attitudes associated with high-angle normal faults (Matson and Blumer,

1973), which are prevalent in the southern and western parts of the coalfield (fig.

PD-5). These predominantly northeast-trending faults range in length from 1 to 6

miles, with offsets of as much as 300 ft; however, the average maximum offset is

more on the order of 120-150 ft or less (see, for example, Matson and Blumer,

1973; Robinson and Culbertson, 1984).

The Fort Union Formation is Paleocene in age and the Wasatch Formation is

Eocene in age; both formations are coal-bearing. The maximum thickness of the

Wasatch Formation in the Decker coalfield is approximately 400 ft, whereas the

maximum thickness of the coal-bearing section of the Fort Union alone is about

2,300 ft (Culbertson, 1987). The total Fort Union thickness may reach 3,500-3,600

ft in areas underlying the Wasatch Formation just east of the Tongue River

Reservoir (Lewis and Roberts, 1978). Fort Union and Wasatch Formation rock

types are similar to one another, and characterized by interbedded sandstone,

siltstone, mudstone, carbonaceous shale and coal. Thin lenses of resistant limestone

are present locally within the Fort Union Formation. Clinker deposits, formed by the

natural burning of coal beds and the resultant baking or fusing of rocks overlying the

burning coal, are present in both formations throughout much of the coalfield (see,

for example, Heffern and others, 1993).

The Fort Union Formation is subdivided into three members (ascending order): the

Tullock, Lebo, and Tongue River Members. The principal coal beds in this area are
PD-4

present in the Tongue River Member, which is the only member of the Fort Union

Formation that crops out in the Decker coalfield (fig. PD-5). The Anderson-Canyon

coal zone is in the upper part of the Tongue River Member, and the coal zone is

defined as the interval of coal and associated clastic rocks between the top of the

Anderson coal bed and the base of the Canyon coal bed. In areas where the Canyon

coal bed splits, the base of the zone is placed at the base of the lower Canyon coal

bed. Based on available drill-hole data, the coal zone ranges from 158 to 500 ft in

thickness where the complete zone is preserved within the coalfield. Coal beds

within the zone include the Anderson, Dietz 2, Dietz 3, Kirby, Cox, and Canyon

coal beds (fig. PD-6) as well as local, unnamed coal beds that are distributed

randomly throughout the coalfield. Our coal-bed terminology, particularly pertaining

to the Anderson and Dietz 2 and Dietz 3 beds, is consistent with that of Culbertson

(1987). However, alternative nomenclature is still widely used by other agencies

and mine operators, and a brief summary of additional naming conventions for coal

beds in this zone is included here (fig. PD-6). We’ve adopted Culbertson’s

nomenclature primarily to maintain consistency with his other USGS reports

pertaining to coal in this area (see, for example, Culbertson and others, 1979;

Culbertson, 1980).

The Anderson, Dietz 2, and Dietz 3 coal beds are the targets of coal production in

the Decker coalfield. Although coal is not currently produced from the Canyon coal

bed in this area, there has been production from an equivalent coal bed (Monarch

bed) at the Big Horn mine in the Sheridan coalfield of Wyoming. There is no

reported production of the other coal beds (Kirby and Cox beds) in the coal zone.

Coal-bed thickness in the Anderson-Canyon coal zone is quite variable, and this

variability is often the result of merging and splitting of individual beds within the

zone, particularly in regard to the Anderson and Dietz 2 and Dietz 3 coal beds. For
PD-5

example, in the Spring Creek mine (fig. PD-1) and areas just south of the mine, the

Anderson, Dietz 2, and Dietz 3 beds have merged to form a single bed as much as

88 ft thick at some locations. North and west of the Spring Creek mine area, this

thick coal bed splits into two beds comprised of a single Anderson coal bed (as

much as 34 ft thick), and a coalesced Dietz 2 and Dietz 3 coal bed as much as 54 ft

thick (Cole and Sholes, 1980). However, southeast of the Spring Creek mine, in and

near the Decker mines (fig. PD-1), the Anderson and Dietz 2 coal beds form a

single bed (from 42 to 57 ft in thickness) while the Dietz 3 coal bed has split into a

single bed from 15 to 21 ft in thickness (Cole and Sholes, 1980). Additionally, in

eastern parts of the East Decker coal mine, the Anderson, Dietz 2 and Dietz 3 coal

beds form three individual beds, and just east of the mine, the Dietz 2 and Dietz 3

coal beds split rapidly into 4 individual coal beds. This splitting and merging of coal

beds is characteristic of all coal beds within the Anderson-Canyon zone and, for that

matter, most or all the coal beds within the Tongue River Member of the Fort Union

Formation.

Maximum net coal thickness (excluding partings) for individual coal beds in the

Anderson-Canyon zone, where not coalesced with one another, is about 35-37 ft for

the Anderson coal bed, 20-25 ft for both the Dietz 2 and Dietz 3 coal beds, and

about 30-33 ft for the Canyon coal bed. The Kirby coal bed, which is only present in

the northwestern part of the coalfield, is generally less than 10 ft thick, while the Cox

coal bed in eastern areas of the coalfield is usually less than 5 ft thick. Each of these

coal beds thicken and thin locally within the coalfield.

A series of 6 cross sections, derived from drill-hole data, is used to depict our

interpretations of coal-bed continuity in the subsurface throughout the Decker

coalfield. Two cross sections (fig. PD-7) show interpretations of coal-bed continuity
PD-6

using natural-gamma (NG) log profiles (figs. PD-8 and PD-9), whereas the

remaining four lines of cross section (fig. PD-10) represent more “idealized”

stratigraphic and structural reconstructions (figs. PD-11 through PD-14) based on

drill-hole lithologic records. In all cross sections, the datum is mean sea level, and

the horizontal distribution of drill-hole control points along each line of section is

scaled to approximate the relative map distance between drill holes.

Many of the cross sections traverse faulted areas, and interpretations of fault

displacement are shown where applicable. Faults shown in the cross sections are

based on Robinson and Culbertson (1984), or they are inferred from observations of

stratigraphic offset between drill holes along the lines of section. Coal-bed offset as

depicted in the cross sections is very diagrammatic in that the actual structural

attitude of the beds in the vicinity of the faults is not indicated. In some areas, where

the distance between control points (drill holes) is significant, the displacement along

the faults is highly inferred.

Natural-gamma log profiles (figs. PD-8 and PD-9) were constructed using reduced

natural-gamma geophysical logs. Each control point displays “mirrored” images of

the natural-gamma log recorded at that drill hole. This type of geophysical log

measures the natural radioactivity emitted by all rocks in a manner similar to a very

sensitive Geiger counter. Typically, coal beds in the study area are very low in

natural radioactivity, and mirror imaging NG logs at each control point is used to

highlight this characteristic. This type of geophysical log is extremely useful in

determining the presence and thickness of coal beds penetrated by drill holes. Coalbed names for coal above and below the Anderson-Canyon coal zone as shown on

the natural-gamma log profiles are modified from data of W.C. Culbertson in the

U.S. Geological Survey National Coal Resources Data System (NCRDS).
PD-7

Cross section GP1-GP1’ (fig. PD-8) traverses approximately 35 mi of the Decker

coalfield, from an area near the Decker mines to just south of Quietus (fig. PD-7).

Just to the west of the Decker coal mine, and within the Spring Creek coal mine, the

Anderson, Dietz 2 and Dietz 3 beds merge into a single bed. However, these coal

beds split rapidly to the west and east from this area. Additionally, in areas east of

the East Decker mine, the Dietz 2, and Dietz 3 beds split into multiple beds, and in

this report, are included within the broader Dietz subzone. Some of the coal beds

within the Dietz subzone pinchout farther east. Near the town of Quietus, a single

Dietz coal bed is all that remains of the subzone. In contrast to the Dietz beds, the

Anderson and Canyon coal beds are essentially continuous along the cross section.

Cross section GP2-GP2’ (fig. PD-9) traverses approximately 30 mi of the Decker

coalfield, beginning south of the Spring Creek mine near the Montana-Wyoming

state line, and extending north to an area just south of Kirby, near the boundary

separating the Crow and Northern Cheyenne Indian Reservations (fig. PD-7). In this

line of section the Anderson, Dietz 2, and Dietz 3 coal beds comprise a single coal

bed in the area extending from the south end of the cross section (near the state line)

to just north of the Spring Creek mine, where the bed splits into an Anderson bed

and a merged Dietz 2 and 3 coal bed. Farther north, the Dietz 2 and Dietz 3 coal

beds split into individual beds, and the Kirby coal bed becomes a constituent of the

coal zone as well. Faults locally offset the overall northward structural rise of the

coal zone, and faulting is particularly prevalent in the northern parts of the cross

section. Additionally, the Anderson-Canyon zone thickens toward the north.

The restored stratigraphic/structural panels (figs. PD-11 through PD-14) were also

constructed using drill-hole information, and they include interpretations of the

lateral distribution of coal beds and associated rocks (for example, sandstone and
PD-8

mudstone) along each line of section. Lithologic data are from NCRDS, and in

certain instances, no differentiation of rock types between coal beds was available. In

these cases, the coal-bed correlations are maintained, and all other rock types are

included in the general category of “rock.”

Cross section A-A’ (fig. PD-11) covers about 54 mi from west to east across the

southern part of the Decker coalfield, extending from the Decker coal mine areas to

the eastern limit of the Anderson-Canyon coal zone (fig. PD-10). Interpretations of

sandstone and mudstone highlight the build up of ancient river deposits (sandstone)

within the Anderson-Canyon coal zone in the west-central and eastern parts of the

study area. Interbedded mudstone represents ancient floodplain deposits, which

were sites of fine-grained sediment deposition and coal-bed mire development. Note

that in areas where the Anderson, Dietz 2, and Dietz 3 beds merge (western part of

the cross section) the predominant lithology associated with the coal beds is

mudstone. There is an increase in sandstone in areas where the Dietz 2 and Dietz 3

coal beds have split to form the Dietz subzone, suggesting that the course of ancient

rivers influenced the geometry and distribution of coal deposits. Fault offsets along

the line of section locally disrupt the overall eastward structural rise of the

Anderson-Canyon coal zone. Also note the interpreted continuity of primary coal

beds across the width of the coalfield.

Cross section B-B’ (fig. PD-12) traverses about 23 mi in the Decker coalfield, and

extends from near Kirby to areas south and west of the Spring Creek coal mine (fig.

PD-10). Note the overall northward structural rise of the Anderson-Canyon coal

zone, which is disrupted by fault offset in the northern part of the cross section.

Although lithologic information on rocks associated with the coal beds is sparse,

there is an apparent buildup of ancient river deposits (sandstone) in areas where the
PD-9

Anderson has split from the Dietz 2 and Dietz 3 coal beds; intervening sandstone

deposits reflect disruption of peat accumulation by ancient rivers, resulting in the

development of two coal beds. This is in contrast to areas near the Spring Creek

mine where all three coal beds have merged into a single bed, and the surrounding

rock is characterized by mudstone.

Cross section C-C’ (fig. PD-13) covers approximately 27 miles in the central part of

the Decker coalfield, east of the areas of active coal mining (fig. PD-10). In this area,

a single Dietz coal bed is all that remains of the Dietz 2 and Dietz 3 beds and their

correlative splits included in the Dietz subzone. At the northern end of the cross

section, the Dietz coal bed merges with the Anderson to form a single bed that is 58

ft thick. Also note that the Canyon coal bed splits into upper and lower Canyon beds

in the south-central part of the cross section. Along this line of section, much of the

coal interburden is composed of mudstone (floodplain deposits), and only minor

buildup of ancient river sandstones is observed in localities near the north, central,

and southern limits of the cross section.

Cross section D-D’ (fig. PD-14) extends for 26 miles from north to south near the

eastern limit of the Decker coalfield (fig. PD-10). Much of the cross section includes

areas where only the Dietz and Canyon coal beds are present. Through most of this

area, the Dietz coal beds are included within the Dietz subzone, which merges

locally with the Anderson coal bed near the southern end of the cross section. Along

this cross section, coal beds dip gently southward to a point of abrupt structural rise

at the extreme south end of the cross section. Both the Anderson and Dietz subzone

coal beds outcrop intermittently along the section. Extensive river deposits of

sandstone are well developed in southern areas of the traverse, with floodplain

mudstone deposits becoming increasingly abundant northward along the line of
PD-10

section. The Anderson coal bed and Dietz subzone coal beds converge in areas

where interburden is composed primarily of floodplain mudstone, and split

northward where ancient river courses (sandstone) disrupted continuous peat

accumulation.

COAL QUALITY

The principal coal beds in the Anderson-Canyon coal zone (Anderson, Dietz 2,

Dietz 3, and Canyon coal beds) are primarily subbituminous in apparent rank (see,

for example, Matson and Blumer, 1973; Fine and others, 1980), although sample

analyses from these coal beds in the central part of the Decker coalfield indicate that

the apparent rank may vary locally from lignite A to subbituminous C (Affolter and

others, 1979). For purposes of resource calculations, we considered the coal beds to

be subbituminous. 

Matson and Blumer (1973) report that heat-of-combustion values (on an as-received

basis) for the Anderson coal bed range from 6,594 to 9,850 Btu/lb, typical ash

yields from 3.0 to 9.1 percent (locally exceeding 27 percent), and total sulfur content

from 0.1 to 0.9 percent. Their reported as-received analyses for the Dietz coal beds,

based on data from the individual beds or where the two beds have merged, indicate

heat-of-combustion values ranging from 6,019 to 9,561 Btu/lb, ash yields from 2.5

to 14.1 percent, and total sulfur content from 0.1 to 2.4 percent, with most sulfur

values being less than 1.0 percent. As-received analyses for the Canyon bed indicate

heat-of-combustion values ranging from 6,646 to 9,113 Btu/lb, ash yields ranging

from 3.2 to 10.7 percent (locally exceeding 30 percent), and total sulfur content

varying from 0.1 to 2.5 percent, although more typical total sulfur values are less

than 1.0 percent.
PD-11

For comparison, chemical analyses of shipped coal from the Spring Creek (merged

Anderson, Dietz 2 and Dietz 3 beds) and the Decker coal mines (merged Anderson

and Dietz 2 beds and the Dietz 3 bed) covering the period from January 1989 to

January 1998, indicate heat-of-combustion values ranging from 8,382 to 11,600

Btu/lb (unweighted mean = 9,420 Btu/lb), ash yields ranging from 3.20 to 41.0

percent (unweighted mean = 4.37 percent), and total sulfur content ranging from

0.21 to 1.18 percent (unweighted mean = 0.36 percent). The total tonnage of coal

shipped in this time frame was about 142.5 million tons (Resource Data

International, Inc., 1998).

TOTAL, NET COAL THICKNESS (ISOPACH) MAP

 The total, net coal isopach map (fig. PD-15) is an interpretation of cumulative, net

coal thickness for coal beds within the Anderson-Canyon coal zone in the Decker

coalfield. At each drill hole or outcrop data point, net coal thickness (excluding

partings) is measured for each coal bed within the zone, and then these individual

coal thicknesses are summed to derive a cumulative value of total net coal within the

coal zone at that data point. These total coal thickness values for all data points are

then contoured by a computer modeling program to develop maps showing lines of

equal total, net coal thickness. The isopach map reflects the distribution of the

cumulative (summed) coal thickness for all coal beds within the Anderson-Canyon

zone rather than the thickness of any individual coal bed except in areas

stratigraphically below the Dietz 3 coal bed (or merged Dietz 2 and Dietz 3 coal

beds), where thickness values reflect the thickness of the Canyon (or lower Canyon)

coal bed alone.
PD-12

Approximately 400 data points (drill-hole and outcrop locations) were used in the

calculation of the total, net coal isopach map of the Anderson-Canyon coal zone.

This total includes limited confidential data from mine operators and data from wells

in northern Wyoming (near the Montana state line) which helped to define isopach

trends along the southern boundary of the coalfield. Only public (non-proprietary)

data points are shown in the total coal isopach map (fig. PD-15).

Because the isopach map for the Anderson-Canyon coal zone is based on

cumulative coal thickness values, the following factors, either individually or

collectively, can account for variations in the total, net coal thickness as seen on the

map: (1) thickening or thinning of individual coal beds within the zone, (2) reduction

in the number of coal beds because of lateral pinchout, and (3) reduction in the

number of coal beds because of erosion in stratigraphically lower horizons nearing

the outcrop limits of the coal zone, and (4) reduction of the total coal in clinker areas

where coal has been lost because of natural burning. Therefore, thickness variations

exhibited by apparent “thinning or thickening” trends in the total coal isopach map

actually reflect variations in the total coal accumulation influenced by all or some of

the factors listed above.

Areas of thicker coal accumulation are shown by warmer (yellow and red) color

shades, whereas minimal coal accumulation areas are highlighted by cool (blue)

shades (fig. PD-15). Based on the total, net coal isopach map, the thickest total coal

accumulation in the Anderson-Canyon coal zone is concentrated in Tps. 8 and 9 S.,

Rs. 38 to 40 E., adjacent to active mines and in the area where the Anderson, Dietz

2, and Dietz 3 coal beds have typically merged into a single coal bed (figs. PD-8 and

PD-9).
PD-13

OVERBURDEN THICKNESS (ISOPACH) MAP

The overburden isopach map (fig. PD-16) represents the thickness of rock overlying

the top of the Anderson-Canyon coal zone. The areal extent of overburden is

constrained by the Anderson coal-bed outcrop (or the contact between the Anderson

coal bed and adjacent clinker deposits), the Montana-Wyoming state line, and tribal

lands along the western boundary of the coalfield. Areas between the Anderson and

Canyon coal-bed outcrops (or clinker contacts), which define the top and base of the

coal zone, respectively, contain exposed rocks of the Anderson-Canyon coal zone.

These areas of “exposed coal zone” are shaded gray on the overburden map (fig.

PD-16). The overburden interpretation was derived by subtracting an elevation grid

(with faults) representing the top of the Anderson-Canyon coal zone (that is, the top

of the Anderson coal bed) from a Digital Elevation Model (DEM) grid of

topographic elevation, resulting in the calculation of a grid of thickness values that

could then be contoured as overburden thickness isopachs. Because strata included

in the Anderson-Canyon coal zone may extend from tens to hundreds of feet

stratigraphically below the top of the zone (Anderson coal bed), the overburden as

shown (fig. PD-16) actually represents a minimum overburden thickness. Thicker

overburden is highlighted by green and yellow colors, whereas minimal overburden

thickness is represented by blue shades.

COAL RESOURCES

Coal resource estimates for the Anderson-Canyon coal zone are reported in tables

PD-1 through PD-3. In table PD-1, resources are tabulated by county, total, net coal

thickness categories, and overburden categories. Table PD-2 includes a summary of

coal resources reported by 7.5’ quadrangle and overburden category, and table PD-3

includes a summary of coal resources reported by county and Federal surface and
PD-14

subsurface ownership criteria. It is important to note that these resource estimates

are not comparable to ‘reserve’ estimates, and therefore, do not reflect in any way

the amount of coal that can be economically produced at the present time.

The quantification of coal resources in the Decker coalfield follows a scheme of

resource calculation reported in Wood and others (1983). The basic quantification

formula is as follows: R = A x T x C,

where

R is the coal resource estimate (in short tons)

A is the area underlain by coal (in acres)

T is the cumulative (total) net coal thickness (in feet; excluding partings) based

on isopach grids, and

C is a weight/unit volume (density) conversion factor (in short tons/acre-foot).

All resources are reported in millions of short tons and are reported in accordance

with criteria established by Wood and others (1983). Coal tonnages were calculated

for coal beds greater than 2.5 ft in thickness using an average density (C) for

subbituminous coal of 1,770 short tons per acre-foot. Tonnage values are rounded to

two significant figures and the tabulated totals may not equal the sum of the

components because of independent rounding. Areas for which resource estimates

are not reported include the Spring Creek and East and West Decker coal mines, the

CX Ranch coal lease, tribal lands (Crow and Northern Cheyenne Indian

Reservations), areas beneath the Tongue River Reservoir, and areas underlying

clinker of the Anderson, Dietz 2, and Dietz 3 (or Dietz) coal beds.

Reported reliability categories (table PD-1) include measured, indicated, inferred,

and hypothetical coal resources. Measured resources occur within a 0.25-mile radius
PD-15

of a data point, indicated resources within the area between 0.25 miles and 0.75

miles from a data point, and inferred resources are present within a radius from 0.75

miles to 3 miles surrounding a data point. Hypothetical resources are present in

areas more than 3 miles beyond a data point (Wood and others, 1983).

Overburden categories, excluding the exposed zone category (see, for example, table

PD-1), represent the thickness of overburden above the top of the Anderson-Canyon

coal zone. Because the coal zone may extend from tens to hundreds of feet

stratigraphically below the top of the zone, the reported overburden categories

represent only a minimum overburden thickness; overburden to deeper horizons in

the zone may actually be greater than the minimum overburden category as reported.

The exposed zone category in the Anderson-Canyon coal zone resource tables

represents areas of the coalfield where the complete coal zone is not present because

of erosion or natural burning of the coal beds.

Confidence limits and estimates of uncertainty for total coal resources in the

Anderson-Canyon coal zone are shown in table PD-4 and table PD-5, respectively.

A confidence interval is a statistic designed to capture uncertainty associated with a

point estimate. Confidence limits reported here are based on total, net coal thickness

measurements from 366 drill-hole and outcrop locations. From these data, we

computed 90-percent confidence intervals on the volume of coal in the AndersonCanyon coal zone in the measured, indicated, inferred, and hypothetical categories.

In this case, volume refers to the calculated resource in millions of short tons

(MST).

The confidence limits were derived through a complex series of steps. These steps

included modeling coal thickness trends and removing the trends using a
PD-16

nonparametric regression algorithm called loess (with span = 0.5), and using

residual thickness to compute a semivariogram and fitting the semivariogram to a

spherical model. Parameter estimates were sill = 239.85 ft2, nugget = 18.99 ft2, and

range = 6.59 miles. Standard deviations of coal thickness were obtained from the

semivariogram model. Differences in point densities were compensated for by

calculating sample size, called pseudo n, within each reliability category and

calculating the variability of volume for each of the reliability categories. The volume

of coal in the Anderson-Canyon coal zone was then calculated at a 90-percent

confidence interval with measurement error. A description of the methodology used

is given in Schuenemeyer and Power (1998) and Ellis and others (1999).

The three main potential sources of error that might bias the confidence intervals are

preferential sampling, measurement errors, and model fitting. The probabilistic

interpretation of a confidence interval is based upon a random sample, which does

not exist in this situation, because there is preferential sampling in those areas

deemed to be minable. Measurement error can be caused by an error in recording

the coal-bed thickness or in the definition of coverage areas. Modeling fitting

variability and bias result from the choice of models and fitting procedures.

SOURCES OF DATA

DRILL-HOLE DATA

The following is a list of published drilling reports that contain most of the drill-hole

information used in this study, although additional point data were collated from the
PD-17

U.S. Geological Survey National Coal Resources Data System (NCRDS),

unpublished field logs, and from a limited set of proprietary records.

Culbertson, W.C., Gaffke, T.M., and Correia, G., 1980, Coal exploratory holes

drilled in 1978-79 in the Birney 1° x 1/2° quadrangle, Big Horn and Rosebud

Counties, Montana, for coal beds in the Tongue River Member of the

Paleocene Fort Union Formation: U.S. Geological Survey Open-File Report

80-55, 68 p.

Hansen, W.B., and Culbertson, W.C., 1985, Correlated lithologic logs and analyses

from 1982 coal drilling in Big Horn, Prairie, Rosebud, and Treasure

Counties, Montana: U.S. Geological Survey Open-File Report 85-738, 125 p.

Kirschbaum, M.A., Correia, G.A., and McPhillips, M., 1982, Coal exploratory

drilling during 1979 in western Powder River County, Montana: U.S.

Geological Survey Open-File Report 82-748, 141 p.

Matson, R.E., 1971, Strippable coal in the Moorhead coal field, Montana: Montana

Bureau of Mines and Geology Bulletin 83, 18 pages with plates.

Matson, R.E., and Blumer, J.W., 1973, Quality and reserves of strippable coal,

selected deposits, southeastern Montana: Montana Bureau of Mines and

Geology Bulletin 91, 135 pages with plates.

U.S. Bureau of Land Management and U.S. Geological Survey, 1977, Hanging

Woman Creek Study Area; Resource and potential reclamation evaluation:

EMRIA Report No. 12, 309 p.
PD-18

U.S. Bureau of Land Management, U.S. Bureau of Reclamation, and U.S.

Geological Survey, 1977, Bear Creek Study Area; West Moorhead coalfield;

Resource and potential reclamation evaluation: U.S. Department of the

Interior EMRIA Report No. 8, 148 pages with appendices.

U.S. Geological Survey and Montana Bureau of Mines and Geology, 1976,

Preliminary report of coal drill-hole data and chemical analyses of coal beds

in Campbell, Converse, and Sheridan Counties, Wyoming; and Big Horn,

Richland, and Dawson Counties, Montana: U.S. Geological Survey OpenFile Report 76-450, 382 p.

_____ 1977, Geophysical logs for Big Horn, Fallon, and Powder River Counties,

Montana; Chapter A of preliminary report of 1977 coal drilling in eastern

Montana and northeastern Wyoming: U.S. Geological Survey Open File

Report 77-721-A, 77 p.

_____ 1977, Geophysical logs for Big Horn County, Montana; Chapter B of

preliminary report of 1977 coal drilling in eastern Montana and northeastern

Wyoming: U.S. Geological Survey Open File Report 77-721-B, 126 p.

_____ 1977, Geophysical logs for Powder River and Dawson Counties, Montana;

Chapter C of preliminary report of 1977 coal drilling in eastern Montana and

northeastern Wyoming: U.S. Geological Survey Open File Report 77-721-C,

79 p.
PD-19

_____ 1977, Preliminary report on 1976 drilling of coals in Campbell and Sheridan

Counties, Wyoming; and Big Horn, Dawson, McCone, Richland, Roosevelt,

Rosebud, Sheridan, and Wibaux Counties, Montana: U.S. Geological Survey

Open-File Report 77-283, 403 p.

_____ 1980, Coal drilling during 1978 in Big Horn, Daniels, Dawson, Musselshell,

Richland, Roosevelt, Rosebud, Valley, and Yellowstone Counties, Montana:

U.S. Geological Survey Open-File Report 80-267, 271 p.

_____ 1981, Coal drilling during 1979 in Beaverhead, Big Horn, Dawson, Fallon,

Garfield, McCone, Musselshell, Powder River, Prairie, Roosevelt, Rosebud,

and Wibaux Counties, Montana: U.S. Geological Survey Open-File Report

81-314, 622 p.

_____ 1982, Coal drilling during 1980 in Big Horn, Dawson, McCone, Powder

River, Prairie, Richland, Rosebud, and Treasure Counties, Montana: U.S.

Geological Survey Open-File Report 82-026, 256 p.
PD-20

GEOLOGIC MAP DATA

Baker, A.A., 1929, The northward extension of the Sheridan coal field, Big Horn

and Rosebud counties, Montana: U.S. Geological Survey Bulletin 806, pp.

15-67, with plates.

Bryson, R.P., and Bass, N. Wood, 1973, Geology of the Moorhead coal field,

Powder River, Big Horn, and Rosebud counties, Montana: U.S. Geological

Survey Bulletin 1338, 116 p. with plates.

Culbertson, W.C., U.S. Geological Survey, unpublished map compilation, 1970-

1987.

McKay, E.J., 1976, Preliminary geologic map and coal sections of the King

Mountain quadrangle, Rosebud and Powder River counties, Montana: U.S.

Geological Survey Miscellaneous Field Investigations Map MF-817, 2 plates

with text.

Robinson, L.N., and Culbertson, W.C., 1984, Map showing isopachs of coal and

overburden of the Canyon coal bed, Birney 30’ x 60’ quadrangle, Big Horn,

Rosebud, and Powder River Counties, Montana: U.S. Geological Survey

Coal Investigations Map C-96-B, 2 plates with text.

Warren, W.C., 1959, Reconnaissance geology of the Birney-Broadus coal field,

Rosebud and Powder River counties, Montana: U.S. Geological Survey

Bulletin 1072-J, pp. 561-585, with plates.
PD-21

GEOGRAPHIC BOUNDARY DATA

Federal lands and mineral status from:

Biewick, L.R.H., Urbanowski, S.R., Cain, Sheila, and Neasloney, Larry, 1998,

Land status and Federal Mineral ownership in the Powder River Basin,

Wyoming and Montana: a digital data set for geographic information

systems: U.S. Geological Survey Open File Report 98-108, coverage scale

1:100,000.

U.S. Bureau of Land Management, 1981, Surface and mineral management status

map, Birney 30’ x 60’ quadrangle, Montana, scale 1:100,000.

U.S. Bureau of Land Management, 1982, Surface and mineral management status

map, Lodge Grass 30’ x 60’ quadrangle, Montana, scale 1:100,000.

U.S. Bureau of Land Management, 1990, Surface and mineral management status

map, Broadus 30’ x 60’ quadrangle, Montana, scale 1:100,000.

State and County boundary data from U.S. Geological Survey National Mapping

Division and based on 1990 TIGER files digitally compiled by the Water Resources

Division of the U.S. Geological Survey.

Digital coverages of 1:100,000- and 1:24,000-scale quadrangle map index acquired

from National Mapping Division, U.S. Geological Survey.
PD-22

Public Land Survey System (PLSS) data from Montana Natural Resource

Information System (NRIS), Montana State Library, scale 1:100,000.

Digital hydrologic coverage from U.S. Environmental Protection Agency, Reach

File Version 3, scale 1:100,000.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the diligent and exhaustive efforts of W.C.

Culbertson (Emeritus, USGS) for his past field studies and compilation of pertinent

stratigraphic and geologic information that was critical to the completion of this

study. His foundation of published and unpublished work facilitated our study in

many ways. We also thank Laura Roberts for her expertise in resource calculation

methodology, Laura Biewick, and Shane Urbanowski for their diligence in creating

accurate and available digital coverages of Federal Lands and Minerals ownership

status, David Ferderer for locating, obtaining, and formatting public domain files of

road and drainage networks in Montana, Gerald Sullivan for his GIS coverage

development in the initial stages of the project, and Jim Hunsicker for his

compilation of coal quality information. This report also benefited greatly from the

thoughtful comments and reviews of Mitch Henry (USGS, Denver, CO), Tim

Rohrbacher (USGS, Denver, CO), Mark Kirschbaum (USGS, Denver, CO), John

SanFilipo (USGS, Reston, VA), John Spencer (BLM, Billings, MT), Ed Heffern

(BLM, Cheyenne, WY), Craig Brunstein (USGS, Denver, CO), and Katherine

Varnes (USGS, Denver, CO).
PD-23

REFERENCES CITED

Affolter, R.H., Hatch, J.R., and Culbertson, W.C., 1979, Chemical analyses of coal

from the Tongue River Member, Fort Union Formation, Moorhead and

southeastern part of the northward extension of the Sheridan coal fields,

Powder River and Big Horn counties, Montana: U.S. Geological Survey

Open-File Report 79-538, 68 p.

Baker, A.A., 1929, The northward extension of the Sheridan coal field, Big Horn

and Rosebud counties, Montana: U.S. Geological Survey Bulletin 806, p. 15-

67, with plates.

Bryson, R.P., and Bass, N. Wood, 1973, Geology of the Moorhead coal field,

Powder River, Big Horn, and Rosebud counties, Montana: U.S. Geological

Survey Bulletin 1338, 116 p. with plates.

Cole, G.A., and Sholes, M.A., 1980, Geology of the Anderson and Dietz coal beds,

Big Horn county, Montana: Montana Bureau of Mines and Geology,

Geologic Map 14, 23 p. with plates.

Culbertson, W.C., 1980, Diagrams showing correlation of Paleocene coal beds

underlying the Birney 1° x 1/2° quadrangle, Big Horn, Rosebud, and Powder

River Counties, Montana: U.S. Geological Survey Open-File Report 80-666,

6 p.
PD-24

_____ 1987, Diagrams showing proposed correlation and nomenclature of Eocene

and Paleocene coal beds in the Birney 30’x60’ quadrangle, Big Horn,

Rosebud, and Powder River counties, Montana: U.S. Geological Survey Coal

Investigations Map C-113, 2 plates with text.

Culbertson, W.C., Kent, B.H., and Maple, W.J., 1979, Preliminary diagrams

showing correlation of coal beds in the Fort Union and Wasatch Formations

across the northern Powder River Basin, northeastern Wyoming and

southeastern Montana: U.S. Geological Survey Open-File Report OF-79-

1201, 11 p.

Ellis, M.S., Gunther, G.L., Flores, R.M., Ochs, A.M., Stricker, G.D., Roberts,

S.B., Taber, T.T., Bader, L.R., and Schuenemeyer, J.H., 1999, Preliminary

report on coal resources of the Wyodak-Anderson coal zone, Powder River

Basin, Wyoming and Montana: U.S. Geological Survey Open-File Report

98-789B, 49 p.

Fine, D.E., Matson, R.E., and Daniel, J.A., 1980, A section on coal resources of the

Anderson and Dietz coal beds, in Cole, G.A., and Sholes, M.A., Geology of

the Anderson and Dietz coal beds, Big Horn county, Montana: Montana

Bureau of Mines and Geology, Geologic Map 14, 23 p. with plates.

Galyardt, G.L., and Murray, F.N., 1981, Geologic map and coal sections of the

Pearl School Quadrangle and the easternmost part of the Bar V Ranch

Quadrangle, Big Horn County, Montana: U.S. Geological Survey Open-File

Report 81-870, 3 plates with text, scale 1:24,000.
PD-25

Heffern, E.L., Coates, D.A., Whiteman, Jason, and Ellis, M.S., 1993, Geologic

map showing the distribution of clinker in the Tertiary Fort Union and

Wasatch Formations, northern Powder River Basin, Montana: U.S.

Geological Survey Coal Investigations Map C-142, scale 1:175,000.

Lewis, B.D., and Roberts, R.S., 1978, Geology and water-yielding characteristics of

rocks of the northern Powder River Basin, southeastern Montana: U.S.

Geological Survey Miscellaneous Investigations Series, Map I-847-D, 1 sheet

with text, scale 1:250,000.

Matson, R.E., and Blumer, J.W., 1973, Quality and reserves of strippable coal,

selected deposits, southeastern Montana: Montana Bureau of Mines and

Geology Bulletin 91, 135 p. with plates.

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

Suite 300, Boulder, Colorado, 80302.

Roberts, L.N.R., Mercier, T.J., Biewick, L.R.H., and Blake, D., 1998, A procedure

for producing maps and resource tables of coals assessed during the U.S.

Geological Survey’s National Coal Assessment [abs.]: Proceedings of the

1998 Pittsburgh Coal Conference, Abstracts with Programs; non-paginated

CD-ROM publication.
PD-26

Roberts, S., Gunther, G., Taber, T., Ochs, A., Blake, D., Bader, L., and Wilde,

E.,1998, The Anderson-Canyon coal zone, Decker coalfield, northern Powder

River Basin, Montana: Considerations for future strippable coal resource

development [abs.]: Proceedings of the 1998 Pittsburgh Coal Conference,

Abstracts with Programs; non-paginated CD-ROM publication.

Robinson, L.N., and Culbertson, W.C., 1984, Map showing isopachs of coal and

overburden of the Canyon coal bed, Birney 30’ x 60’ quadrangle, Big Horn,

Rosebud, and Powder River Counties, Montana: U.S. Geological Survey

Coal Investigations Map C-96-B, 2 plates with text.

Schuenemeyer, J.H., and Power, H., 1998, An uncertainty estimation procedure for

U.S. coal resources, [abs.], in Pittsburgh Coal Conference, 15th Annual

Meeting, Abstracts with Programs (CD-ROM): Pittsburgh, Pennsylvania, 13

p.

Warren, W.C., 1959, Reconnaissance geology of the Birney-Broadus coal field,

Rosebud and Powder River counties, Montana: U.S. Geological Survey

Bulletin 1072-J, p. 561-585, with plates.

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.
45° 00'

107° 00' 106° 00'

Indian Reservations

Anderson-Canyon coal zone outliers

(Canyon coal bed—unassessed areas)

Canyon coal bed outcrop

Mine or coal lease

Tongue River Reservoir

(lake and wetlands)

Explanation

0 5 10 Miles

Crow Indian R

Kirby Birney

Quietus

Moorhead

Decker

Powder Riv

Spring Creek

mine

West

Decker

mine

East Decker

mine

CXR

(CXR, CX Ranch lease area)

TR

TR

Decker coalfield—Assessment area for

the Anderson-Canyon coal zone

River

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E. R. 49 E. 

T. 9 S.

T. 10 S.

T. 8 S.

T. 7 S.

T. 6 S.

T. 10 S.

T. 71/2 S.

Secondary and unimproved roads

County line

314

212

314

U.S. highway

State highway

Decker coalfield

Montana

Study Area

Figure PD-1. Index map showing the Decker coalfield and vicinity, southeast Montana.

Big Horn

County
45° 00'

107° 00' 106° 00'

Indian Reservations

Anderson-Canyon coal zone outliers

(Canyon coal bed—unassessed areas)

Canyon coal bed outcrop

Mine or coal lease

Tongue River Reservoir

(lake and wetlands)

Explanation 0 5 10 Miles

Montana

Wyomi

Crow Indian Re

Kirby Birney

Quietus

Moorhead

Decker

North

Powder River

Spring Creek

mine

West

Decker

mine

East Decker

mine

CXR

(CXR, CX Ranch lease area)

TR

TR

Decker coalfield—Assessment area for

the Anderson-Canyon coal zone

River Perennial or intermittent stream

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E. R. 49 E. R. 50 E.

T. 9 S.

T. 10 S.

T. 8 S.

T. 7 S.

T. 6 S.

T. 10 S.

T. 71/2 S.

County line

Decker coalfield

Montana

Study Area

Figure PD-2. Map showing regional drainage pattern, Decker coalfield and vicinity, southeast Montana.

Big Horn

County

Hanging 

Woman Creek
Birney 30' x 60' Broadus 30' x 60' 

Hardin

30' x 60' 

Lodge Grass

30' x 60' 

Sheridan 30' x 60' Recluse 30' x 60' 

Lodge Grass

30' x 60' 

Burgess Junction

 30' x 60'

30' x 60 ' quadrangle maps (scale 1:100,000)

45° 00'

107° 00'

106° 00'

Indian Reservations

Andersdon-Canyon coal zone outliers

(Canyon coal bed—unassessed areas)

Mine or coal lease

Tongue River Reservoir

(lake and wetlands)

Explanation

0 5 10 Miles

Crow Indian R

y

Quietus

Moorhead

Decker

North

Powder River

Spring Creek

mine

West

Decker

mine

East Decker

mine

CXR

(CXR, CX Ranch lease area)

TR

TR

Decker coalfield-—Assessment area for

the Anderson-Canyon coal zone

River

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E. R. 49 E. R. 50 

T. 9 S.

T. 10 S.

T. 8 S.

T. 7 S.

T. 6 S.

T. 10 S.

T. 71/2 S.

County line

Decker coalfield

Montana

Study Area

Figure PD-3. Index map showing 7.5' and 30' x 60' quadrangle maps in the Decker coalfield, southeast Montana

Big Horn

County

Pearl School

Decker

Holmes Ranch

Pine Butte School

Forks Ranch

Quietus Bear Creek

School Sayle Hall

Half Moon Hill Tongue River

Dam

Spring

Gulch

Lacey Gulch Stroud Creek

Hamilton

Draw Otter

Reanus Cone

Kirby

Taintor Desert Birney SW

Birney

Browns Mountain

Poker Jim

Butte Fort Howes

Goodspeed Butte

Yarger Butte

Bradshaw Creek

Sayle

Phillips Butte

Hodsdon Flats

Bloom Creek

Moorhead

Three Bar Ranch

Bay H

Bar V Ranch Bar V Ranch NE 

Spring Creek Ra

Monarch Acme Bar N Draw Cedar Canyon OTO Ranch Roundup Draw Box Elder Draw Cabin Creek NW Cabin Creek NE Black Draw

Dead Horse Lake Corral Creek Homestea

Huckins School

Lonesom

Peak

Baldy Pe
45° 00'

107° 00' 106° 00'

Indian Reservations

Canyon coal bed outcrop

Mine or coal lease

Tongue River Reservoir

(lake and wetlands)

Explanation

0 5 10 Miles

Mon

Wyo

Crow Indian 

Quietus

Moorhead

Decker

North

Powder River

Spring Creek

mine

West

Decker

mine

East Decker

mine

CXR

(CXR, CX Ranch lease area)

TR

TR

River

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E. R. 49 E. R. 5

T. 9 S.

T. 10 S.

T. 8 S.

T. 7 S.

T. 6 S.

T. 10 S.

T. 71/2 S.

212

Secondary and unimproved roads

County line

314

314

U.S. highway

State highway

Decker coalfield

Montana

Study Area

Figure PD-4. Map showing the distribution of Federally owned coal, Decker coalfield and vicinity, southeast Montana.

Federally owned coal

Federally owned coal

(Custer National Forest)

Big Horn

County
Figure PD-5. Map showing generalized bedrock geology, Decker coalfield and vicinity, southeast Montana. Geology based on Baker (1929), Warren (

Bryson and Bass (1973), McKay (1976), and W.C. Culbertson (USGS, unpublished mapping, 1970-1987). Fault locations and displacement modified fr

Robinson and Culbertson (1984).

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 71/2 S.

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E.

T. 9 S.

T. 10 S.

T. 10 S.

R. 49 E. R. 50 E.

45° 00'

107° 00' 106° 00'

Clinker (Anderson, Dietz 2 and Dietz 3 coal beds)—Also present within the 

Crow and Northern Cheyenne Indian Reservations Indian Reservations

County line

Mine or coal lease

Fort Union Formation (Tongue River Member)—Also present within the

Crow and Northern Cheyenne Indian Reservations

Wasatch Formation—Also present within the Crow and

Northern Cheyenne Indian Reservations Tongue River Reservoir (lake)

Tongue River Reservoir (wetlands) 

Anderson coal bed—Outcrop or contact with clinker (burn line)

Canyon coal bed—Outcrop or contact with clinker (burn line)

Explanation

Montana

Wyoming

Big Horn

County

Kirby Birney

Quietus

Moorhead

Decker

East Decker

West mine

Decker

mine

Spring Creek

mine

North

Identified fault (bar and ball on downthrown side)

Tw

Tw

Tw

Tw

Tw

Tw

Ttr

Ttr

Ttr

Ttr

Ttr

A

C

C

Inferred fault (bar and ball on downthrown side)

Tongue River

Powder River

Bedrock geologic units

C

C

C

C

C

C

C

C

C

C C

C

C

C

A

A

A

A

A

A

A

A

A

A

A

C

A

A

CXR

(CXR, CX Ranch lease area)

0 5 10 Miles

Crow Indian Reservation (par

Montana

Study Area
Anderson Anderson Anderson

Kirby (present locally)

Dietz 2 Dietz 1 Dietz 1 (lower) Dietz 1

Dietz 2

Dietz 1 (upper)

Dietz 2 Dietz 2

Cox (present locally)

Canyon Canyon Canyon Canyon

USGS (this report)

Tertiary (part)

Fort Union Formation (upper part)

Tongue River Member (upper part)

Diagram not to scale

*Terminology of Flores and Bader, Chapter PS, this CD-ROM

Montana Bureau of Mines 

and Geology Decker Coal Company Spring Creek Coal Company

Dietz coal zone

Dietz 3

Dietz

subzone 

Dietz

Merge to form one coal bed

(Spring Creek Mine)

Merge locally to form one

coal bed (Decker Mine)

Wyodak-Anderson coal zone*

Anderson-Canyon coal zone

Multiple

beds

Single

bed

West East West East

( )( )

Coal beds Coal beds Coal beds Coal beds

Figure PD-6. Chart showing coal-bed nomenclature for the Anderson-Canyon coal zone, Decker coalfield,

Montana. Coal-bed designations based on Sholes and Cole (1981), Culbertson (1987) and terminology of
45° 30'

Big Horn

County

Powder River

County

Powder River

County

Rosebud

County

Quietus

Moorhead

North

ver

Powder River

29-51B

30-13B

30-11B
No

Data Estimated limit of 

merged Anderson and 

Dietz 2 and 3 coal beds

3400

3300

3200

3100

3000

2900

2800

3500

3600

3700

3800

3900

4000

4100

4200

4300

Elevation (in feet) above MSL

Anderson-Canyon

coal zone

Anderson coal bed

Dietz 2 and 3

coal beds

Canyon coal bed

A

D2, D3

Cyn

Cyn

Cyn

A, D2, D3

D3

D3

D3

A

D2

A

D2

A

A

A

A

A

A

A

D2

Cyn

Cyn

Cyn Cyn

Cyn

Cyn

Cyn

Cyn

Cox

Dtz

Dtz

Dtz

25-76B

G.L. 4150'

25-56B

G.L. 3870'

26-68B

G.L. 3621'

26-73B

G.L. 3486' 26-81B

G.L. 3436'

26-44B

G.L. 3528'

27-50B

G.L. 3655'

27-21B

G.L. 4123'

28-7aB

G.L. 3890'

28-10B

G.L. 3650'

29-22B

G.L. 3562'

29-30B

G.L. 3621'

29-51B

G.L. 3630'

30-13B

G.L. 3870'

30-11B

G.L. 3983'

26-81B

G.L. 4200'

Cl

West East

GP1 GP1'

?

"Mirrored" natural-gamma (NG)

log traces (arrows indicate

decreasing natural-gamma)

Coal bed with parting 

0

100 Feet

Drill-hole point ID

Surface elevation

(in feet above mean sea level)

Cross section datum is mean sea level (MSL)

Fault

Upthrown

block

Upthrown

block

Downthrown

block

Clinker Cl

(partial record)

(partial record)

(partial record)

TD = 8434' 

TD = 11,750'

TD = 8150' 

Wasatch Formation

Fort Union Formation

Tertiary (part)

Fort Union Formation (part)

Wasatch Formation (part) Tongue River Member (part)

Decker coal mine area

Cook

Otter (part)

Cook

Cook

Otter

Wall Zone

Cook

Wall Zone

Cook-Otter Zone Wall Zone

Wall Zone

Otter

Cook

Cook

Otter

Otter

Wall Zone

Wall Zone

White

White

White

Cook-Otter

Wall Zone

Smith

Squirrel Creek

Roland

Smith

Upper Otter

Lower Otter

?

?

Dsz Dsz

Dsz Dsz

?

?

?

?

?

Drill hole

TD, Total depth (in feet)

G.L., Ground level (in feet)

35 Miles

No

No Data

Data

No

Data

No

Data

Explanation

Inferred Identified

Figure PD-8. Cross section GP1-GP1' showing natural-gamma log profiles and coal-bed correlations in drill holes penetrating the Anderson-Canyon coal

zone, Decker coalfield, southeast Montana. Fault displacement as shown is diagrammatic. Coal-bed terminology modified from Culbertson (1987). 

A, Anderson coal bed

AD2: Anderson and Dietz coal beds (merged)

A,D2,D3: Anderson, Dietz 2 and Dietz 3 coal beds (merged)

D2, Dietz 2 coal bed

D3, Dietz 3 coal bed

Dsz, Dietz subzone (coal beds undifferentiated)

Dtz, Dietz coal bed

Cox, Cox coal bed

Cyn, Canyon coal bed

Anderson-Canyon coal zone

Coal bed 

NG

NG

NG NG

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG
Anderson-Canyon

coal zone

Anderson coal bed

Dietz 2 and 3

coal beds

Canyon coal bed

A

A, D2, D3

A

D2

D3

A, D2,

D3

A, D2,

D3

A

A

A

A

A A

A

D2, D3

D2, D3

D2, D3 D2,

D3

D2,

D3 D2,

D3

D3

D2

D2, 

D3

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

South

GP2

North

GP2'

3400

3300

3200

3100

3000

3500

3600

3700

3800

3900

4000

4100

4200

4300

4400

4500

4600

4700

Elevation (in feet) above MSL

Cross section datum is mean sea level (MSL)

Tertiary (part)

Fort Union Formation (part)

Tongue Ri

ver Member (part)

Spring Creek coal mine area

25-72B

G.L. 3534'

25-71B

G.L. 3550'

25-56B

G.L. 3870'

25-13B

G.L. 3895'

26-7B

G.L. 3644'

25-7B

G.L. 3687'

25-1B

G.L. 3798'

17-16B

G.L. 4010'

17-22B

G.L. 4085'

17-25B

G.L. 4528' 17-30B

G.L. 4478'

9-25B

G.L. 4625'

9-32B

G.L. 4492'

9-30B

G.L. 4522'

10-4B

G.L. 4580'

Depth to and thickness of the Canyon

coal bed estimated from a drill hole 

0.8 miles away. 

Depth to and thickness of the Canyon

coal bed estimated from a drill hole 

1.4 miles away. 

Kby Kby

Kby

Smith

Smith

Smith

Squirrel Creek

Smith

Cook

Cook

Cook

Wall

Carlson

Cook

?

?

?

?

?

?

30 Miles

?

26-81B

G.L. 4200'

NG

"Mirrored" natural-gamma (NG)

log traces (arrows indicate

decreasing natural gamma)

Coal bed with parting 

Coal bed 

0

100 Feet

Drill-hole point ID

Surface elevation

(feet above mean sea level)

Drill hole

G.L., Ground level (in feet)

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG NG

NG

NG

Smith

Explanation

Anderson-Canyon coal zone

A, Anderson coal bed

D23: Dietz 2 and Dietz 3 coal beds (merged)

A, D2, D3: Anderson, Dietz 2 and Dietz 3 coal beds (merged)

Kby, Kirby coal bed

D2, Dietz 2 coal bed

D3, Dietz 3 coal bed

Cyn, Canyon coal bed

Figure PD-9. Cross section GP2-GP2' showing natural-gamma log profiles and coal-bed correlations in drill holes penetrating the

Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Fault displacement as shown is diagrammatic. Coal-bed terminology

modified from Culbertson (1987). 

Fault

Upthrown

block

Upthrown

block

Downthrown

block

Inferred Identified
T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 4 S.

T. 71/2 S.

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E.

T. 9 S.

T. 10 S.

T. 10 S.

R. 49 E. R. 50 E.

45° 00'

45° 00'

107° 00' 106° 00'

Indian Reservations

Decker coal field

County line

Mine or coal lease

Tongue River Reservoir (lake)

Tongue River Reservoir (wetlands) 

Explanation

Montana

Wyoming

Northern Cheyenne Indian Reservation (part)

Big Horn

County

Kirby Birney

Quietus

Moorhead

Decker

North

Identified fault (bar and ball on downthrown side)

Inferred fault (bar and ball on downthrown side)

Drill hole

Tongue River

Tongue R

Powder River

0 5 10 miles

Crow Indian Reservatio

Decker coal field

Montana

Study Area

Figure PD-10. Index map showing the location of restored stratigraphic/structural cross sections A-A', B-B', C-C', and D-D',

Decker coalfield, southeast Montana.

A

D'

A'

C'

C

B'

B

A1

B14

B1

B3

B4

B5 B6

B7

B8

B9

B11 B10

B12

B13

A2 A3

A3

A4

A5

A6

A7 A8

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10 C11

C12

A9

A10

A11

D2

D3

D4

D5

D6

D7

D8

D9

D10

D11

D12

D13

D14

D15

D16 

B2
And

And

And

And

And

And

And

And

D23

And/D23

D23

D2

D2

Cyn D3 D3

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

Cyn

Dsz

Dsz

Dtz

Dtz

Dtz

Cox 

Cyn

?

Identified faults

Drill Hole

Lithology

Upthrown

block

Downthrown

block

Sandstone

Wasatch Formation

Mudstone

Rock (undifferentiated)

Coal

Explanation

0

100

200 Feet

Anderson-Canyon coal zone

And, Anderson coal bed

D23, Dietz 2 and 3 coal beds

D2, Dietz 2 coal bed

D3, Dietz 3 coal bed

Dsz, Dietz subzone (coal beds undifferentiated)

Dtz, Dietz coal bed

Cox, Cox coal bed

Cyn, Canyon coal bed

East

A'

West

A 54 Miles

Tertiary (part) 

Fort Union Formation (part)

Tongue Ri

ver Member (part)

Top of coal zone

Base of coal zone

4200

4100

4000

3900

3800

3700

3600

3500

3400

3300

3200

3100

3000

2900

2800

Elevation (in feet) 

above MSL

Cross section datum is

mean sea level (MSL)

Figure PD-11. Restored stratigraphic/structural cross section A-A' showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield,

southeast Montana. Displacement along faults as shown is diagrammatic. Fault location and relative displacement modified from Robinson and

Culbertson (1984). 

A1

A2

A3 A4

A4

A5 A6

A7

A8

A9

A10

A11
Cyn

D23 D23

D23

And

And

And

Kby

Cyn

Cyn

Cyn

Cyn

Cyn

Faults

Explanation 0

100

200 Feet

North South B B'

D23

D23

D23

And

And

And

And/D23

23 Miles

Tertiary (part)

Fort Union Formation (part)

Tongue River Member (part)

Eroded area

(approximate)

D23

?

?

4200

4300

4400

4500

4600

4100

4000

3900

3800

3700

3600

3500

3400

3300

3200

Elevation (in feet) 

above MSL

4700

Cyn

D23

B1 B2

B3 B4

B5

B6

B7

B8

B9

B10

B11

B12 B13

B14

A2
Sandstone

Mudstone

Rock (undifferentiated)

Coal

Explanation

0

100

200 Feet

North

C

South

C'

Anderson-Canyon coal zone

And, Anderson coal bed

Dtz, Dietz coal bed

Cyn, Canyon coal bed

UC, Upper Canyon coal bed

LC, Lower Canyon coal bed

And/Dtz

And

And And And

Dtz

Dtz Dtz Cyn Dtz

Cyn

Cyn Cyn

UC

UC

LC

LC

27 Miles

Tertiary (part)

Fort Union Formation (part)

Tongue Ri

ver Member (part)

Drill Hole

Top of coal zone

Base of coal zone

4200

4300

4100

4000

3900

3800

3700

3600

3500

3400

3300

3200

Elevation (in feet) 

above MSL

Cross section datum is

mean sea level (MSL)

Figure PD-13. Restored stratigraphic/structural cross section C-C' showing coal-bed correlations in the Anderson-Canyon coal zone,

Decker coalfield, southeast Montana. 

C1 C2 C3

C4

C4

C5

C6 C7

C8 C9 C10

C12

A8

Wasatch Formation

C11

Lithology
Cyn Cyn And And Dsz Dsz

Dsz

Cyn

Cyn

Sandstone

Mudstone

Rock (undifferentiated)

Clinker Coal

Explanation 0

100

200 Feet Anderson-Canyon coal zone

And, Anderson coal bed

Dsz, Dietz coal subzone

Cyn, Canyon coal bed

North South

D 26 Miles D'

y 

p

Fort Union Formation (part)

Tongue Ri

ver Member (part)

Drill Hole

Top of coal zone

Base of coal zone

4200

4300

4100

4000

3900

3800

3700

3600

3500

3400

3300

Elevation (in feet) 

above MSL

Cross section datum is

mean sea level (MSL)

Figure PD-14. Restored stratigraphic/structural cross section D-D' showing coal-bed correlations in the Anderson-Canyon coal

zone, Decker coalfield, southeast Montana. Eroded areas as shown are diagrammatic and approximately located. 

D1

D2 D3

D4

D5

D6 D7

D8

D8

D9 D10 D11

D12

D13 D14 D15

A11

D16

Eroded area

(approximate)

Eroded area

(approximate)

Eroded area

(approximate)

Eroded area

(approximate)
Decker coalfield

boundary

T. 71/2 S.

T. 10 S.

T. 10 S.

45° 00'

45° 00'

107° 00' 106° 00'

Clinker (Anderson, Dietz 2, Dietz 3, and/or Dietz coal beds)—Also present

within the Crow and Northern Cheyenne Indian Reservations

Indian Reservations

2.5-10 ft

Total, net coal thickness

10-20 ft

20-40 ft

40-60 ft

60-80 ft

80-100 ft

> 100 ft (maximum = 111 ft)

County line

Mine or coal lease Custer National Forest boundary

Tongue River Reservoir and wetlands

Anderson-Canyon coal zone outliers

(Canyon coal bed—unassessed areas)

Drill hole or outcrop data point

Canyon coal bed--Outcrop or contact with clinker (burn line)

Explanation

Montana

Wyoming

Northern Cheyenne Indian Reservation (part)

Big Horn

County

Kirby Birney

Quietus

Decker

East Decker

W mine est

Decker

mine

Spring Creek

mine

North

Tongue River

Tongu

Powder River

CXR

(CXR, CX Ranch lease area)

0 5 10 Miles

Crow Indian Reservation (part)

Montana

Study Area

Moorhead

Figure PD-15. Map showing the total, net coal thickness in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana.

Decker coalfield

boundary

Custer National Forest

boundary (part)

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

T. 4 S.

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E.

T. 9 S.

R. 49 E. R. 50 E.
T. 71/2 S.

T. 10 S.

T. 10 S.

45° 00'

45° 00'

107° 00' 106° 00'

Indian Reservations

0-100 ft

Exposed coal zone—Includes

areas of clinker

Overburden thickness

100-200 ft

200-500 ft

500-1000 ft

> 1,000 ft

County line

Mine or coal lease

Tongue River Reservoir and wetlands

Anderson-Canyon coal zone outliers

(Canyon coal bed—unassessed areas)

Anderson coal bed—Outcrop or contact with clinker (burn line)

Explanation

Montana

Wyoming

Northern Cheyenne Indian Reservation (part)

Kirby Birney

Quietus

Decker

East Decker

West mine

Decker

mine

Spring Creek

mine

North

Tongue River

To

Powder River

CXR

(CXR, CX Ranch lease area)

0 5 10 Miles

Crow Indian Reservation (part)

Montana

Study Area

Canyon coal bed—Outcrop or contact with clinker

Moorhead

Figure PD-16. Map showing the thickness of overburden above the Anderson-Canyon coal zone, Decker coalfield, southeast Montana.

Fault location and relative displacement modified from Robinson and Culbertson (1984).

Anderson coal bed

outcrop/clinker contact

(outcrop/clinker contact)

Custer National Forest

boundary (part)

Custer National Forest boundary

Fault (bar and ball on downthrown side)

Identified Inferred

T. 8 S.

T. 7 S.

T. 6 S.

T. 5 S.

R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E.

R. 48 E.

T. 9 S.

R. 49 E. R. 50 E.
Table PD-1. Total, net coal resources in the Anderson-Canyon coal zone in the Decker coalfield, Montana, reported by county,

overburden thickness, total, net coal thickness, and reliability categories. Resources are shown in millions of short tons (MST) and with

two significant figures. Zeros (0) indicate that no coal resources were calculated within those categories. The table does not include

resources in mine or coal lease areas, areas underlying clinker of the Anderson and/or Dietz coal beds, and areas underlying the Tongue

River Reservoir and associated wetlands. Columns may not sum because of independent rounding

County Minimum Total, net Reliability categories (distance from data point)

overburden

thickness

coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

BIG HORN Exposed zone 2.5-5 ft. 0 1.3 0.86 0 2.2

5 –10 ft. 2.9 15 20 0 38

10-20 ft 20 47 140 0 210

20-30 ft 55 170 260 1.8 490

30-40 ft 40 230 310 4.6 590

40-50 ft 55 260 400 13 730

50-100 ft 92 630 1,100 1.2 1,800

Exposed zone total 260 1,400 2,200 21 3,900

0-100 ft. 10-20 ft 2.4 2.0 26 0 30

20-30 ft 11 50 41 0 100

30-40 ft 6.7 72 130 0 210

40-50 ft 10 210 360 0 570

50.100 ft 320 1,300 1,900 0 3,500

100-150 ft 0 3.0 6.7 0 9.7

0-100 ft. total 350 1600 2500 0 4,400

100-200 ft 10-20 ft 0 0 2.3 0 2.3

20-30 ft 0 14 16 0 29

30-40 ft 0 26 53 0 79

40-50 ft 5.9 120 290 0 420

50-100 ft 330 1,800 2,000 0 4,200

100.0-150 ft 16 92 46 0 150

100-200 ft total 360 2,100 2,500 0 4,900
Table PD-1. Total, net coal resources, Anderson-Canyon coal zone—continued

County Minimum Total, net Reliability categories (distance from data point)

overburden

thickness

coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

BIG HORN 200-300 ft 30-40 ft 0 5.0 32 0 37

40-50 ft 7.3 59 290 3.3 360

50-100 ft 400 1,900 3,100 0 5,400

100-150ft 68 160 81 0 310

200-300 ft total 470 2,100 3,500 3.3 6,100

300-400 ft 30-40 ft 0 2.7 12 0 14

40-50 ft 2.0 29 210 32 270

50-100 ft 280 1,700 2,600 0 4,600

100-150 ft 17 130 120 0 270

300-400 ft total 300 1,900 2,900 32 5,100

400-500 ft 40-50 ft 0 0 40 50 89

50-100 ft 120 880 1,600 0 2,600

100-150 ft 1.3 180 170 0 360

400-500 ft total 120 1,100 1,800 50 3,100

500-1000 ft 40-50 ft 0 0 0.74 0 0.74

50-100 ft 54 270 1,800 71 2,200

100-150 ft 22 100 86 0 210

500-1000 ft total 76 380 1,900 71 2,400

1000-1500 ft 50-100 ft 0 0 2.3 10 12

1000-1500 ft total 0 0 2.3 10 12

BIG HORN total 1,900 11,000 17,000 190 30,000

POWDER RIVER Exposed zone 2.5-5 ft 2.2 11 39 0.23 53

5-10 ft 11 54 130 4.4 200

10-20 ft 65 330 530 23 950

20-30 ft 100 420 480 86 1,100

30-40 ft 64 320 500 100 980

40-50 ft 20 140 430 55 640

50-100 ft 14 110 530 24 680

Exposed zone total 280 1400 2,600 290 4,600
Table PD-1. Total, net coal resources, Anderson-Canyon coal zone—continued

County Minimum Total, net Reliability categories (distance from data point)

overburden

thickness

coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

POWDER RIVER 0-100 ft 2.5-5 ft 0 0.22 1.0 0 1.3

5-10 ft 0 0.091 3.9 0 4.0

10-20 ft 0 24 33 0 57

20-30 ft 0 21 100 4.3 130

30-40 ft 10 82 220 36 340

40-50 ft 8.7 83 370 34 490

50-100 ft 70 370 710 45 1,200

0-100 ft total 88 580 1,400 120 2,200

100-200 ft 2.5-5 ft 0 0 0.16 0 0.16

5-10 ft 0 0 2.8 0 2.8

10-20 ft 0 3.2 4.1 0 7.3

20-30 ft 0 1.5 11 2.4 15

30-40 ft 0 3.4 88 6.9 99

40-50 ft 0.81 43 270 1.5 320

50-100 ft 53 230 770 27 1,100

100-200 ft total 54 290 1,100 38 1,500

200-300 ft 5-10 ft 0 0 0.33 0 0.33

10-20 ft 0 0 0.24 0 0.24

30-40 ft 0 0 49 0.68 49

40-50 ft 4.5 55 250 0 310

50-100 ft 28 170 760 23 980

200-300 ft total 33 230 1,100 24 1,300

300-400 ft 30-40 ft 0 0 11 0 11

40-50 ft 18 77 180 0 280

50-100 ft 7.5 110 700 0 810

300-400 ft total 26 190 890 0 1,100

400-500 ft 40-50 ft 9.7 21 24 0 55

50-100 ft 14 100 370 0 490

400-500 ft total 24 130 400 0 540
Table PD-1. Total, net coal resources, Anderson-Canyon coal zone—continued

County Minimum Total, net Reliability categories (distance from data point)

overburden

thickness

coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

POWDER RIVER 500-1000 ft 50-100 ft 1.0 9.7 3.9 0 15

500-1000 ft total 1.0 9.7 3.9 0 15

POWDER RIVER total 510 2,800 7,600 470 11,000

ROSEBUD Exposed zone 2.5-5 ft. 3.6 11 17 0 32

5-10 ft. 29 99 90 0 220

10-20 ft 41 210 340 14 610

20-30 ft 23 170 210 0 410

30-40 ft 16 55 130 0 200

40-50 ft 1.9 58 90 0 150

50-100 ft 0.81 57 290 0 350

Exposed zone total 110 670 1,200 14 2,000

0-100 ft 10-20 ft 0 2.6 8.9 0 11

20-30.0 ft 2.5 2.8 20 0 25

30-40 ft 2.2 7.7 24 0 34

40-50 ft 0 31 99 0 130

50-100 ft 46 260 640 0 950

0-100 ft total 51 300 790 0 1,100

100-200 ft 10-20 ft 0 0 0.36 0 0.36

30-40 ft 0 0 0.78 0 0.78

40-50 ft 0 0 12 0 12

50-100 ft 12 84 150 0 250

100-200 ft total 12 84 170 0 260

200-300 ft 40-50 ft 0 0 0.87 0 0.87

50-100 ft 2.3 18 13 0 33

200-300 ft total 2.3 18 13 0 34

ROSEBUD total 180 1,100 2,100 14 3,400

Grand total (MST) 2,600 14,000 27,000 680 45,000*

* Of this total, approximately 3,500 MST are estimated to be present within Custer National Forest
Table PD-2. Total, net coal resources in the Anderson-Canyon coal zone reported for 7.5’ quadrangles in the Decker coalfield, Montana.

Coal resources are reported in millions of short tons (MST) rounded to two significant figures. Zeros (0) indicate that no coal resources

were calculated within those categories. Overburden categories based on Wood and others (1983), and overburden is measured from the

ground surface to the top of the uppermost coal bed (minimum overburden). Reported resources do not include areas within active coal

mines or coal leases, areas underlying clinker of the Anderson and/or Dietz coal beds, or areas overlain by the Tongue River Reservoir

and associated wetlands. Columns may not sum because of independent rounding

7.5-minute quadrangle map Minimum overburden Total

Exposed zone 0.0-100 ft 100-200 ft 200-500 ft 500-1000 ft > 1000 ft (MST)

Acme 18 10 13 140 1.8 0 180

Bar N Draw 0 0 7.9 170 0 0 170

Bar V Ranch 43 76 44 170 0 0 340

Bar V Ranch NE 10 200 210 280 0 0 710

Bear Creek School 320 500 420 1,500 11 0 2,800

Birney 91 0 0 0 0 0 91

Birney Day School 4.2 0 0 0 0 0 4.2

Birney SW 160 3.2 0.36 0 0 0 170

Black Draw 20 42 9 0 0 0 71

Bloom Creek 130 0 0 0 0 0 130

Box Elder Draw 0 0 1.8 130 0 0 130

Bradshaw Creek 580 340 170 62 0 0 1,200

Browns Mountain 81 0.81 0 0 0 0 82

Cabin Creek NE 38 37 53 47 0 0 180

Cabin Creek NW 0 0 2.5 150 3.2 0 160

Cedar Canyon 0 0 0 7.7 120 12 140

Clubfoot Creek 2.3 0 0 0 0 0 2.3

Cook Creek Butte 97 0 0 0 0.0 0 97

Decker 140 66 160 1,400 30 0 1,800

Forks Ranch 250 720 680 1,200 13 0 2,900

Fort Howes 160 0 0 0 0 0 160

Goodspeed Butte 55 0 0 0 0 0 55

Green Creek 74 0 0 0 0 0 74
Table PD-2. Total, net coal resources by 7.5’ quadrangle, Anderson-Canyon coal zone—continued

7.5-minute Quadrangle map Minimum Overburden Total

Exposed

Zone

0.0-100 ft. 100-200 ft. 200-500 ft. 500-1000 ft. > 1000 ft. (MST)

Half Moon Hill 670 670 400 470 0 0 2,200

Hamilton Draw 630 720 600 600 0 0 2,600

Hodsdon Flats 220 0 0 0 0 0 220

Holmes Ranch 38 170 250 1,700 720 0 2,900

King Mountain 29 0 0 0 0 0 29

Kirby 730 510 350 190 0 0 1,800

Lacey Gulch 480 410 260 240 2.2 0 1,400

Monarch 5.8 0 0 0 0 0 5.8

Moorhead 4.3 0 0 0 0 0 4.3

OTO Ranch 0 0 0 16 120 0 140

Otter 480 71 47 25 0 0 620

Pearl School 58 260 600 3,400 310 0 4,600

Phillips Butte 540 78 11 0.57 0 0 630

Pine Butte School 26 200 270 1,800 1,100 0 3,400

Poker Jim Butte 480 600 230 34 0 0 1,300

Quietus 57 360 890 2,300 0 0 3,600

Reanus Cone 260 28 3.9 0 0 0 290

Roundup Draw 0 0 0 132.44 0.74 0 130

Sayle 770 330 100 6.3 0 0 1,200

Sayle Hall 550 650 590 870 0 0 2,700

Sonnette 78 0 0 0 0 0 78

Spring Creek Ranch 32 0 0 0 0 0 32

Spring Gulch 620 160 91 62 0 0 930

Stroud Creek 330 340 160 130 0 0 970

Taintor Desert 670 240 64 7.6 0 0 980

Threemile Buttes 34 0 0 0 0 0 34

Tongue River Dam 340 13 0 0 0 0 350

Grand Total (MST) 10,000 7,800 6,700 17,000 2,500 12 45,000*

* Of this total, approximately 3,500 MST are estimated to be present within Custer National Forest
Table PD-3. Total, net coal resources in the Anderson-Canyon coal zone, Decker coalfield, Montana, reported by county and

by Federal coal and surface ownership. Coal resources in areas underlying clinker of the Anderson and/or Dietz coal beds, areas

within active coal mines or leases, and areas underlying the Tongue River Reservoir and associated wetlands are not included

in the resource total. Coal resources are reported in millions of short tons (MST) with two significant figures. Column will not

sum due to independent rounding

County Federal Ownership Total (MST)

BIG HORN No Federal coal or Federal surface ownership 3,100

No Federal coal, but Federal surface ownership 5.8

Federal coal, but no Federal surface ownership 25,000

Federal coal and Federal surface ownership 2,000

BIG HORN Total 30,000

POWDER RIVER No Federal coal or Federal surface ownership 700

No Federal coal, but Federal surface ownership 1.5

Federal coal, but no Federal surface ownership 7,200

Federal coal and Federal surface ownership 3,400

POWDER RIVER total 11,000

ROSEBUD No Federal coal or Federal surface ownership 150

No Federal coal, but Federal surface ownership 1.1

Federal coal, but no Federal surface ownership 1,500

Federal coal and Federal surface ownership 1,800

ROSEBUD total 3,400

Grand total (MST) 45,000*

* Of this total, approximately 3,500 MST are estimated to be present within Custer National Forest
Table PD-4. Computations of confidence intervals within reliability categories for estimated total coal resources in the Anderson-Canyon

coal zone in the Decker coalfield, Montana. Volume refers to the calculated resource in millions of short tons (MST). NA, not applicable

Parameter Reliability Category Entire

Measured Indicated Inferred Hypothetical Area

Area (in square meters) 133,426,743 704,761,677 1,340,332,461 42,411,593 2,220,932,473

Percent of area 6 32 60 2 100

Acres 32,970 174,150 331,203 10,480 548,804

SD (Standard deviation (in ft) from

semivariogram model)

5.712 7.730 13.093 16.089 NA

Acre feet (Acres x SD) 188,333 1,346,167 4,336,582 168,610 NA

Volume standard deviation (MST) 21 192 1,794 298 2,305

Pseudo n (Minimum number of points in the area) 262 154 18 1 NA

Table PD-5. Volume and estimates of uncertainty for estimated total coal resources in the Anderson-Canyon coal zone, Decker

coalfield, Montana, with measurement error. Resource calculations are in millions of short tons (MST) with four significant figures.

Volume refers to the calculated resource in millions of short tons (MST).

Parameter Reliability Category Entire

Measured Indicated Inferred Hypothetical Area

Volume (MST) 2,621 14,400 27,030 675 44,730

Lower 90% confidence bound (MST) 2,587 14,090 24,080 184 40,940

Upper 90% confidence bound (MST) 2,655 14,720 29,980 1,166 48,520