Chapter PA

ASHLAND COALFIELD, POWDER RIVER BASIN,

MONTANA: GEOLOGY, COAL QUALITY, AND COAL

RESOURCES

by S.B. Roberts,1 E.M. Wilde,2 G.S. Rossi,1 Dorsey Blake,1 M.S. Ellis,1

G.D. Stricker,1 A.M. Ochs,3 G.L. Gunther,3 J.H. Schuenemeyer,1 and

H.C. Power4

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

1

U.S. Geological Survey

2

Montana Bureau of Mines and Geology, Butte, Montana

3

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

4

University of Delaware, Newark, Delaware
PA-ii

Contents

Introduction...........................................................................................................................PA-1

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

Coal Quality..........................................................................................................................PA-6

Net Coal Thickness (Isopach) Map.................................................................................PA-7

Overburden Thickness (Isopach) Map...........................................................................PA-7

Coal Resources.....................................................................................................................PA-8

Sources of Data.................................................................................................................PA-11

Drill-hole Data.............................................................................................................PA-11

Geologic Map Data ....................................................................................................PA-12

Geographic Boundary Data......................................................................................PA-13

Acknowledgments............................................................................................................PA-14

References Cited................................................................................................................PA-15

Figures

PA-1. Index map showing the Ashland coalfield and vicinity, southeast 

 Montana.

PA-2. Index map showing 7.5’ and 30’ x 60’ quadrangle maps, Ashland coalfield, 

 southeast Montana.

PA-3. Map showing Federally owned coal, Ashland coalfield and vicinity, 

 southeast Montana.

PA-4. Index map showing the Knobloch coal resource unit assessment area, 

 Ashland coalfield, southeast Montana.

PA-5. Index map showing generalized bedrock geology, drill-hole locations, and 

 the Ashland coalfield boundary, southeast Montana.
PA-iii

Figures—continued

PA-6. Coal-bed terminology applied to the Knobloch coal resource unit and 

 associated coal beds, Ashland coalfield and vicinity, southeast Montana.

PA-7. Map showing the location of cross sections A-A’ and B-B’, Ashland 

 coalfield, southeast Montana.

PA-8. North-south cross section A-A’ showing proposed coal-bed correlations, 

 Ashland coalfield and vicinity, southeast Montana.

PA-9. West-east cross section B-B’ showing proposed coal-bed correlations, 

 Ashland coalfield and vicinity, southeast Montana.

PA-10. Net coal thickness (isopach) map of the Knobloch coal resource unit, 

 Ashland coalfield, southeast Montana.

PA-11. Overburden thickness (isopach) map for the Knobloch coal resource unit, 

 Ashland coalfield, southeast Montana.

Tables

PA-1. Net coal resources in the Knobloch coal resource unit in the Ashland 

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

 thickness, and reliability categories

PA-2. Net coal resources in the Knobloch coal resource unit reported for 7.5’ 

 quadrangles in the Ashland coalfield, Montana

PA-3. Net coal resources in the Knobloch coal resource unit, Ashland coalfield, 

 Montana, reported by county and Federal coal and surface ownership

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

 estimated total coal resources in the Knobloch coal resource unit in the 

 Ashland coalfield, Montana
PA-iv

Tables—continued

PA-5. Volumes and estimates of uncertainty for estimated total coal resources in 

 the Knobloch coal resource unit, Ashland coalfield, Montana, with 

 measurement error
PA-1

INTRODUCTION

The Ashland coalfield, as defined here, incorporates an area of some 520 square

miles in parts of Powder River and Rosebud counties in southeastern Montana (fig.

PA-1). The coalfield is named for the town of Ashland, which is on the banks of the

Tongue River just west and north of the main part of the coalfield. Access through

the area is provided by a limited network of U.S. highways and secondary or

unimproved roads.

The coalfield is defined by both geologic and geographic parameters. The western

boundary is delimited by the outcrop or burn line (clinker contact) of the Knobloch

coal bed, and the remaining boundary of the coalfield is defined primarily by

geographic limits of 7.5’ quadrangles within the study area (fig. PA-2). In the Cook

Creek Reservoir 7.5’ quadrangle, the boundary represents the geographic limit of

subsurface data of sufficient spacing for viable coal correlations. The Ashland

coalfield in this study is more areally restricted than the Ashland coalfield of Bass

(1932) (520 square miles versus 975 square miles), and encompasses all or part of

the Ashland, Beaver Creek-Liscom Creek, and Poker Jim-Odell Creek coal deposits

of Matson and Blumer (1973). Most of the coal in the Ashland coalfield is Federally

owned (fig. PA-3). Other than small wagon mines that produced coal locally for

ranchers in the early 1900’s (Bass, 1932), there has been no substantial coal mining

in the coalfield.

The Tongue River is west of the Ashland coalfield, and the central part of the

coalfield is dissected by the Otter Creek drainage (fig. PA-4). The majority of the

Ashland coalfield (approximately 66 percent) underlies the Custer National Forest.

Because section 522 (e) (2) (b) of the Surface Mining Control and Reclamation Act
PA-2

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, this coal

resource assessment addresses only those lands within the coalfield that fall outside

the Forest boundaries (fig. PA-4).

GEOLOGIC OVERVIEW AND COAL STRATIGRAPHY

The Fort Union Formation (Paleocene) outcrops throughout the Ashland coalfield;

younger rocks of the Wasatch Formation (Eocene) are exposed in areas just south

of the coalfield (fig. PA-5). Fort Union strata exposed in the coalfield are included in

the Tongue River Member; the underlying Lebo Member of the Fort Union

Formation is present in outcrops north of the coalfield (fig. PA-5). Rocks of the

Tongue River Member are essentially flat-lying, and are composed primarily of

shale, sandy shale, sandstone and thick coal beds. The thickness of the Tongue River

Member in this area ranges between 1,150 ft and 1,660 ft (Bass, 1932).

Clinker, formed from the natural burning of coal beds (see, for example, Heffern

and Coates, 1997) is prevalent throughout the area, and clinker of the Knobloch coal

bed is a prominent feature of the coalfield (fig. PA-5). In places, clinker deposits

may attain a thickness of greater than 100 ft (Matson and Blumer, 1973), and clinker

cliffs and ridges typically support the extensive growth of Ponderosa pine.

Coal beds are present throughout the Fort Union Formation in the Ashland coalfield.

The Knobloch coal bed in the lower part of the Tongue River Member (fig. PA-6)

has been targeted as the priority coal resource assessment unit in the coalfield,

primarily because of its thickness and continuity. In this report, the single Knobloch

coal bed is designated as the Knobloch coal resource unit (fig. PA-6), following
PA-3

criteria used by Culbertson and Saperstone (1987). Within this framework, the term

“Knobloch coal resource unit” identifies the thick, single body of coal formed by the

merging of multiple beds in northern areas of the coalfield, as well as the uppermost

coal bed in the Knobloch coal zone in southern and eastern areas of the coalfield

where the thick coal bed has split (fig. PA-6). The Knobloch coal resource unit or its

interpreted equivalent has been identified in drill holes beyond the limits of the

Ashland coalfield; however, the paucity and distribution of data make correlations

difficult in the more peripheral areas, and for this reason, we have restricted our area

of Knobloch coal resource assessment to the Ashland coalfield as defined here (fig.

PA-4).

The net coal thickness (excluding partings) of the Knobloch coal resource unit

ranges from 3 to 71 ft, and the wide variability in thickness reflects the merging and

splitting of individual components comprising the coal resource unit. The coal beds

considered to comprise the Knobloch coal resource unit where several coal beds

have merged into one thick bed include the Knobloch (uppermost), Calvert, Nance,

Lay Creek, and Flowers-Goodale (lowermost) coal beds (Culbertson and

Saperstone, 1987; Sholes and Daniel, 1992; Wilde, in press) (fig. PA-6). The King

coal bed, which is present above the Knobloch coal bed in some areas, may merge

locally with the Knobloch coal resource unit (Culbertson and Saperstone, 1987;

Sholes and Daniel, 1992). The Lay Creek coal bed, which occurs stratigraphically

between the Knobloch coal bed and the underlying Flowers-Goodale coal bed in the

Foster Creek area (fig. PA-6) is interpreted to merge with the Knobloch coal

resource unit (Knobloch coal bed) in T. 2 S., R. 45 E., in the northern part of the

Ashland coalfield (see, for example, Sholes and Daniel, 1992). McKay (1976) and

Wilde (in press) have also identified a Lay Creek coal bed in the southern part of the

Willow Crossing 7.5’ quadrangle in T. 3 S., R. 45 E. In this area, the Lay Creek
PA-4

coal bed is about 100 ft below the Knobloch coal resource unit. The stratigraphic

relationship between the Lay Creek coal bed in the Willow Crossing 7.5’ quadrangle

and the Lay Creek coal bed as identified in the Foster Creek area is unclear.

However, the stratigraphic position of this coal bed in the Willow Crossing

quadrangle, which is well below the Knobloch coal resource unit, suggests that the

Lay Creek coal bed in the southern part of the Willow Crossing quadrangle may not

be equivalent to the Lay Creek coal bed in the Foster Creek area. Additionally, the

reader should note that we do not correlate the Flowers-Goodale bed in the Ashland

coalfield with the Flowers-Goodale coal bed in areas to the south and east of the

coalfield as has been done by previous workers (see, for example, Culbertson and

Saperstone, 1987; McClellan and Biewick, 1988). This is primarily because the

stratigraphic position of the “Flowers-Goodale” bed as identified in those previous

reports is as much as 100 to 150 ft below the Flowers-Goodale coal bed in the

Ashland coalfield, and we suggest that the Flowers-Goodale bed in the Ashland

coalfield pinches out in eastern and southern areas of the field (fig. PA-6). The

stratigraphic relationship between the Flowers-Goodale coal bed in the Foster Creek

area and the Flowers-Goodale coal bed as we identify it in the Ashland coalfield is

unclear, and no attempt at correlation is made in this study.

In areas east of the Ashland coalfield, the Knobloch coal resource unit (Knobloch

coal bed) is interpreted to be equivalent (in part) to the Sawyer coal bed (McClellan

and Biewick, 1988; Sholes and Daniel, 1992). Based on limited subsurface data, we

suggest that the Knobloch coal bed in the Foster Creek area may be stratigraphically

lower than the Knobloch coal bed in the Ashland coalfield. In the Greenleaf CreekMiller Creek area north of the coalfield, the Knobloch coal bed is identified below

the Sawyer coal bed and above the Rosebud coal bed (Matson and Blumer, 1973)

(fig. PA-6). Sholes and Daniel (1992) suggest that the Rosebud coal bed is
PA-5

equivalent to a lower split of the Knobloch coal bed, and based on available data, this

conclusion seems highly plausible.

Two lines of cross section (fig. PA-7) derived from drill-hole data, depict our

interpretations of Knobloch coal resource unit continuity within the Ashland

coalfield. Cross section A-A’ (fig. PA-8) depicts north-south correlations of the

Knobloch coal resource unit and associated coal beds in a 30 mile transect of the

coalfield. West-east cross section B-B’ (fig. PA-9) traverses some 22 miles of the

Ashland coalfield. The cross sections include natural-gamma (NG), spontaneous

potential (SP), and resistivity (RES) geophysical log profiles. The majority of the

natural-gamma logs are products of coal exploration drilling, whereas the

spontaneous potential and resistivity logs are from oil and gas test wells. The datum

is mean sea level (MSL) for both cross sections, and the horizontal distribution of

drill-hole control points along each line of section is scaled to reflect the relative map

distance between drill holes, although this horizontal scaling is approximate.

The log profiles used in the cross sections were constructed from reduced

geophysical logs. Where natural-gamma logs were used, the data point displays a

“mirrored” image of the log profile to highlight the response in coal beds.

Spontaneous potential and resistivity logs are displayed in tandem as is typical of

these geophysical log records in oil and gas exploration. In this area, coal beds are

characterized by low natural-gamma and high resistivity responses. Spontaneous

potential logs were used primarily for differentiating high resistivity responses in

sandstone from similar high resistivity responses characteristic in coal beds.

The main coal-bed split area and split line B (fig. PA-7) approximate areas of coalbed splitting associated with the Knobloch coal resource unit. The location of these
PA-6

areas is highly inferred and is based on drill-hole data used in this assessment, as

well as some modification of interpretations of Culbertson and Saperstone (1987).

The area of the coalfield north of the main coal-bed split area is where the Knobloch

coal resource unit is formed from the merging of multiple beds, and the single bed

reaches a thickness in excess of 70 ft. South and southeast of this area, the Nance

and Flowers-Goodale coal beds are interpreted to split from the Knobloch coal

resource unit (fig. PA-8). Split line B delimits the point where the Calvert coal bed

splits from the Knobloch coal resource unit.

COAL QUALITY

Based on chemical analyses (as-received basis) from the Knobloch coal resource

unit in Tps. 3 and 4 S., R. 45 E. (Otter Creek EMRIA Site) (Culbertson and

Saperstone, 1987, after the U.S. Bureau of Land Management, 1975), the average

heat-of-combustion value for the Knobloch coal is 8,110 Btu/lb, the average ash

yield is 5.5 percent, and the average total sulfur content is 0.15 percent. The apparent

coal rank in this area is subbituminous C.

Based on as-received analyses, Matson and Blumer (1973) report calorific values

ranging from 7,761-9,070 Btu/lb, an ash yield ranging from 3.7-6.8 percent, and

total sulfur content ranging from 0.1-0.5 percent for the Knobloch coal bed in the

Ashland coal field. In the Foster Creek area (fig. PA-5), as-received analyses of the

Knobloch coal bed indicate heat-of-combustion values ranging from 7,380-7,840

Btu/lb, ash yield ranging from 6.7-8.7 percent, and total sulfur content from 0.3-1.6

percent (Matson and Blumer, 1973).
PA-7

In the Greenleaf Creek-Miller Creek area (fig. PA-5), Matson and Blumer (1973)

report as-received analyses of calorific value ranging from 8,209-8,935 Btu/lb, ash

yield ranging from 5.5-6.7 percent, and total sulfur content ranging from 0.42-0.44

percent for the Knobloch coal bed.

NET COAL THICKNESS (ISOPACH) MAP

The net coal thickness (isopach) map (fig. PA-10) is based on subsurface data from

202 drill holes, and is an interpretive map depicting the distribution and variation in

net coal thickness (excluding partings) of the Knobloch coal resource unit in areas of

the Ashland coalfield not included in Custer National Forest. Thicker net coal is

represented by warmer color shades (red and yellow), whereas cooler shades (blue)

represent areas of minimum net coal thickness.

Net coal thickness for the Knobloch coal resource unit varies from a minimum of 3

ft to a maximum of 71 ft. The thickest coal is found in areas north of the main coalbed split area (Tps. 2-4 S., Rs. 44-46 E.), where multiple beds have coalesced to

form a single thick coal body that typically ranges from 40 to 70 ft in thickness. The

Knobloch coal resource unit generally thins southward, with minimum net coal in

the southwestern part of the coalfield (T. 6 S., Rs. 42-46 E.) shrinking to 3 ft or less

(fig. PA-10).

OVERBURDEN THICKNESS (ISOPACH) MAP

The overburden thickness (isopach) map (fig. PA-11) is an interpretive map

showing the thickness of rock estimated to overlie the top of the Knobloch coal
PA-8

resource unit. In the Ashland coalfield, the overburden thickness in areas outside

Custer National Forest ranges from 0 at points along the Knobloch coal bed

outcrop/burn line (clinker contact) to more than 1,000 ft in the southwestern part of

the coalfield. Areas of thicker overburden are represented by shades of green and

yellow, while areas of less overburden are highlighted by shades of blue.

Comparison of the overburden and net coal thickness maps for the Knobloch coal

resource unit indicate that the thickest coal (north of the main coal-bed split area; fig.

PA-10) is present under overburden ranging typically from 0 to 500 ft thick (fig.

PA-11).

COAL RESOURCES

Coal resource estimates for the Knobloch coal resource unit are reported in tables

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

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

coal resources reported by 7.5’ quadrangle, and table PA-3 includes a summary of

coal resources reported by county and Federal surface and 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 Ashland 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)
PA-9

A is the area underlain by coal (in acres)

T is the 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 net coal-bed thickness greater than 2.5 ft 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. Coal resources are not tabulated for

areas within the Custer National Forest. However, based on data used in this study,

it is estimated that an additional 10-11 billion short tons of coal in the Kobloch coal

resource unit may underlie those areas of Custer National Forest included within the

Ashland coalfield as defined here.

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

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

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 (table PA-1) represent the thickness of overburden above the top of the

Knobloch coal resource unit.

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

Knobloch coal resource unit are shown in table PA-4 and table PA-5, respectively.
PA-10

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

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

measurements from 187 drill-hole locations. From these data, we computed 90-

percent confidence intervals on the volume of coal in the Knobloch coal resource

unit 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

nonparametric regression algorithm called loess (with span = 0.75), and using

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

spherical model. Parameter estimates were sill = 47.03 ft2, nugget = 10.37 ft2, and

range = 4.70 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 Knobloch coal resource unit 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.
PA-11

SOURCES OF DATA

DRILL-HOLE DATA

The following drilling reports contain much of the drill hole information used in this

study, although additional point data was collated from the U.S. Geological Survey

National Coal Resources Data System (NCRDS), unpublished field logs, and from

a limited set of proprietary records. Collectors and compilers of these data files

include W.C. Culbertson, E.J. McKay, M.W. McClellan, and L.R.H. Biewick of the

U.S. Geological Survey.

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.
PA-12

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

_____ 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.

GEOLOGIC MAP DATA

Derkey, P.M., 1986, Coal stratigraphy of the Lame Deer quadrangle, southeastern

Montana: Montana Bureau of Mines and Geology Geologic Map No. 43,

scale 1:100,000.

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 pages with plates.

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

1987.
PA-13

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, 1980, Surface and mineral management status

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

U.S. Bureau of Land Management, 1980, Surface and mineral management status

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

U.S. Bureau of Land Management, 1980, Surface and mineral management status

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

U.S. Bureau of Land Management, 1980, Surface and mineral management status

map, Powderville 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.
PA-14

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

from National Mapping Division, U.S. Geological Survey.

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) in 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 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), Craig Brunstein (USGS, Denver, CO), and

Katharine Varnes (USGS, Denver, CO).
PA-15

REFERENCES CITED

Bass, N.W., 1932, The Ashland coalfield, Rosebud, Powder River, and Custer

Counties, Montana: U.S. Geological Survey Bulletin 831, p. 19-105.

Culbertson, W.C., 1987, Diagrams showing proposed correlation and nomenclature

of Eocene and Paleocene coal beds in the Birney 30’ x 60’ quadrangle, Big

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

Survey Coal Investigations Map C-113, plates with text.

Culbertson, W.C., and Saperstone, H.I., 1987, Structure, coal thickness, and

overburden thickness of the Knobloch coal resource unit, Birney area, Big

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

Survey Coal Investigations Map C-112, scale 1:100,000.

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

98-789A, 49 p.

Gilmour, E.H., and Williams, L.A., 1969, Geology and coal resources of the Foster

Creek coal deposit, eastern Montana: Montana Bureau of Mines and Geology,

Bulletin 73, 9 p.
PA-16

Heffern, E.L., and Coates, D.A., 1997, Clinker--Its occurrence, uses, and effects on

coal mining in the Powder River Basin: Wyoming State Geological Survey,

Public Information Circular 38, p. 151-165.

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.

McClellan M.W., and Biewick, L.R.H., 1988, Stratigraphic framework of the

Paleocene coal beds in the Broadus 30’ x 60’ quadrangle, Powder River

Basin, Montana-Wyoming: U.S. Geological Survey Coal Investigations Map

C-119-A; scale 1:100,000.

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

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

Geological Survey Miscellaneous Field Studies Map MF-802, 1:24,000.

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.

Sholes, M.A., and Daniel J.A., 1992, The Knobloch coal bed, Powder River and

Rosebud Counties, Montana: Correlation and petrography, in Sholes, M.A.,

Coal Geology of Montana: Montana Bureau of Mines and Geology, Special

Publication 102, p. 105-135.
PA-17

U.S. Bureau of Land Management, 1975, Resource and potential reclamation

evaluation, Otter Creek study site: U.S. Bureau of Land Management

EMRIA Report No. 1, 200 p.

Wilde, E.M., (in press), Available coal resources of the Willow Crossing 7.5-

minute quadrangle, Rosebud and Powder River counties, Montana: to be

published as a Montana Bureau of Mines and Geology Open-File Report.

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.
106° 00' 106° 30'

45° 30'

45° 45'

45° 15'

Stacey

Birney

Otter

Quietus

Sayle Hall

Moorhead

V

Ashland

Brandenberg

 212

 212

Northern Cheyenne Indian

Reservation (part)

Powder Riv

Tongue River

Tongue River

 212 U.S. Highway

Secondary and unimproved roads

County line

0 5 10 Miles

Rosebud coal mine

(part)

Big Sky coal mine

(part)

Rosebud County

Powder Ri

ver County

Big Horn County

Rosebud County Custer County

Ashland coalfield

Ashland coalfield

Coal mine area

Northern Cheyenne Indian Reservation (part)

Explanation

Montana

Study Area

Knobloch coal bed

outcrop/burn line

Knobloch coal bed

outcrop/burn line

T.2N.

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

T.8S.

T.9S.

R.42E. R.43E. R.44E. R.45E. R.46E. R.47E. R.48E. R.49E.

Figure PA-1. Index map showing the Ashland coalfield and vicinity, southeast Mo
106° 00' 106° 30'

106° 30' 106° 00'

45° 30'

45° 45'

45° 15'

0 5 10 Miles

Stacey

Birney

Otter

Brandenberg

Ashland

North

Northern Cheyenne Indian

Reservation (part)

Powder Ri

Tongue River

Tongue River

Boundary separating adjacent

30' x 60' quadrangles

Big Sky coal mine

(part)

Ashland coalfield

Ashland coalfield

Coal mine area

Northern Cheyenne Indian Reservation (part)

Explanation

Montana

Study Area

Knobloch coal bed

outcrop/burn line

Figure PA-2. Index map showing 7.5' and 30' x 60' quadrangle maps, Ashland coal

southeast Montana.

Birney 7.5'

quadrangle

Browns Mountain

7.5' quadrangle

Poker Jim Butte

7.5' quadrangle Fort Howes

7.5' quadrangle

Yager Butte

7.5' quadrangle

Coleman Draw

7.5' quadrangle

Ashland 7.5'

quadrangle

Green Creek

7.5' quadrangle

Powderville 30' x 60'

quadrangle

Broadus 30' x 60'

quadrangle

Broadus 30' x 60'

quadrangle

Birney 30' x 60'

quadrangle

Lame Deer 30' x 60'

quadrangle

Cook Creek Reservoir

7.5' quadrangle

Willow Crossing

7.5' quadrangle

King Mountain

7.5' quadrangle

Birney Day School

7.5' quadrangle

Goodspeed Butte

7.5' quadrangle

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

R.42E. R.43E. R.44E. R.45E. R.46E. R.47E. R.48E. R.49
T.2N.

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

T.8S.

T.9S.

R.42E. R.43E. R.44E. R.45E. R.46E. R.47E. R.48E. R.49E.

106° 00' 106° 30'

106° 30' 106° 00'

0 5 10 Miles

Stacey

Ashland

Birney

Otter

Quietus

Sayle Hall

Moorhead

Brandenberg

Epsie

Volborg

Ashland coalfield

boundary

Custer National Forest

Northern Cheyenne Indian Reservation

(part)

Tongue River

Powder River

Tongue River

Rosebud Mine (part)

Big Sky Mine (part)

North

Montana

Study Area

Federally owned coal

Federally owned coal

Custer National Forest

Northern Cheyenne Indian Reservation

Coal mine

Knobloch coal bed

outcrop/burn line

Explanation

Figure PA-3. Map showing Federally owned coal, Ashland coalfield and vic

hM
106° 30'

45° 15'

Birney

T.4S.

T.5S.

T.6S.

T.7S.

R.42E. R.43E. 

Tongue River

Coal mine area 

Custer National Forest (part

Northern Cheyenne Indian R

Explanation 

Figure PA-4. Index 

southeast Montana.
T.2N.

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

T.8S.

T.9S.

R.42E. R.43E. R.44E. R.45E. R.46E. R.47E. R.48E. R.49E.

106° 00' 106° 30'

106° 30' 106° 00'

45° 30'

45° 45'

45° 15'

0 5 10 Miles

Stacey

Ashland

Birney

Otter

Quietus

Sayle Hall

Moorhead

Brandenberg

Epsie

Foster Creek Volborg

Area

Greenleaf CreekMiller Creek Area

Southern

Area

Eastern

Area

Ashland coalfield

boundary

Custer National Forest (part)

Northern Cheyenne Indian Reservation

(part)

Tongue River

Powder River

Tongue River

Rosebud mine (part)

Big Sky mine (part)

North

Montana

Study Area

Tw

Tftr

Tftr

Tftr

Tfl

Tfl

Tfl

Tfl

Tftr

Tw

Tftr

Tfl

Tw

Wasatch Formation

Fort Union Formation

Custer National Forest (part)

Northern Cheyenne Indian Reservation (part)

Coal mine area

Clinker (Knobloch coal bed)

Knobloch coal bed Drill hole penetrating Knobloch coal resource unit

or stratigraphically equivalent horizon

Explanation

outcrop

burn line

Figure PA-5. Index map showing generalized bedrock geology, drill-hole locations, and the

Ashland coalfield boundary, southeast Montana. Geology based on Matson and Blumer (1973),

Lewis and Roberts (1976), and Derkey (1986).

Tongue River Member. Also present in Custer National Forest and

in the Northern Cheyenne Indian Reservation as shown

Lebo Member
Flowers-Goodale * (lower split)

Flowers-Goodale * (upper split)

Nance

North South

West East

Southern Area

Greenleaf Creek-Miller Creek area Foster Creek area coal beds

coal beds

Eastern Area

* Based on coal bed terminology of

Culbertson and Saperstone (1987)

* Based on coal bed terminology and correlations

of Culbertson and Saperstone (1987)

Coal bed terminology of Matson and Blumer (1973) * Based on coal bed terminology of Gilmour and Williams (1969)

** Based on coal bed terminology and correlations

of McClellan and Biewick (1988)

Calvert

Nance

Flowers-Goodale

Knobloch

Knobloch

Knobloch

King (Sawyer)

Flowers-Goodale *

Flowers-Goodale **

Nance

Nance? (Knobloch **)

Flo

(alternati

wers-Goodale * 

ve correlation)

Flo

(alternati

wers-Goodale * 

ve correlation)

Knobloch (Sawyer **)

Flowers-Goodale

Flowers-Goodale

pinchout (this study)

Flowers-Goodale pinchout (this study)

Knobloch coal zone

X X'

Y Y'

Knobloch*

Knobloch

Sawyer

Rosebud

Lay Creek**

Flowers-Goodale*

Knobloch coal zone

Diagram not to scale

Diagram not to scale

?

(Knobloch coal resource unit)

(Knobloch coal resource unit) (Knobloch coal resource unit)

(Knobloch coal resource unit)

Merges with the Knobloch coal bed in the Knobloch coal

resource unit assessment area (Sholes and Daniel, 1992)

and

ddle

derson

tz 2

tz 3

nyon

ok

er

ll

er Jim (Pawnee)

ell (Cache)

obloch

wyer

g

lowers-Goodale 

ay Creek 

ret (Kendrick ?)

ham

g

ngue

al bed/zone

Epsie

Volborg

er Creek

Area

ern

a

d

River

North
106° 00' 106° 30'

106° 30' 106° 00'

45° 30'

45° 30'

45° 45'

45° 45'

45° 15'

45° 15'

0 5 10 Miles

Stacey

Birney

Otter

Brandenberg

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

R.42E. R.43E. R.44E. R.45E. R.46E. R.47E. R.48E. R.49E.

?

North

Northern Cheyenne Indian

Reservation (part)

Powder River

Tongue River

Tongue River

Knobloch coal resource unit assessment area

Custer National Forest (part)

Northern Cheyenne Indian Reservation (part)

Explanation Montana

Study Area

Ashland coalfield

boundary

Ashland

Drill hole

Coal-bed split line

(inferred location)

Coal-bed split area

(inferred location)

Point ID

Figure PA-7. Map showing the location of cross sections A-A' and B-B', Ashland

coalfield and vicinity, southeast Montana. The locations of the main coal-bed split

area and coal-bed split line B are highly inferred and modified from Culbertson and

Saperstone (1987).

Split line B

31-111LD

31-109LD

7-3B 

7-36B 

7-33B 

24-2B 

15-6B 

15-2B 

15-1B 

24-6B 

Main coal-bed

split area

8-1B 

32-15LD 

32-145LD

32-144LD

31-34LD 

31-33LD 

1-21BR 

2-5BR 

A

B

B'

15-2B

A'
Flowers-Goodale pinchout

(this study)

GL 3255 ft

31-111LD

GL 3217 ft

31-109LD

GL 3095 ft

7-3B

GL 3140 ft

7-33B

GL 3290 ft

7-36B

15-6B

GL 3789 ft

Total depth

7825 ft

15-1B

GL 4131 ft

15-2B

GL 3420 ft

Lebo Member

Tullock Member

(part)

24-2B

GL 3811 ft

24-6B

GL 3884 ft

3100

3000

2900

2800

2700

2600

2500

2400

2300

2200

2100

2000

1900

3200

3300

3400

3500

3600

3700

3800

3900

4000

4100

4200

North South

Otter

Wall ?

Poker Jim

Poker Jim Odell

Odell

Otter Otter

Brewster-Arnold

Ferry

Cook

Cook Cook

Flowers-Goodale (lower split)**

Flowers-Goodale (upper split)**

Terret

Stag

No Record

No Record

Total depth

7247 ft

Total depth

7584 ft

Total depth

7466 ft

A A'

Elevation (in feet) above MSL

Cross section datum is MSL

Witham

Kendrick

Split line B

Main coal-bed

split area

Nance

Nance

Nance

King (Sawyer)

Calvert

FlowersGoodale 

King (Sawyer)

King (Sawyer)

?

?

?

?

Flowers-Goodale*

Tertiary (part)

Fort Union Formation (part)

Tongue Ri

ver Member (part)

NG

NG

NG

NG

NG

NG

NG

NG

NG

NG

SP RES

SP RES

1.2

mi

2.2

mi

1.3

mi

2.5

mi

8.0

mi

1.3

mi

4.5

mi

5.4

mi

3.7

mi

Nance ?

Knobloch coal bed

Knobloch coal bed

2-5BR

GL 3791 ft 

"Mirrored" natural-gamma

log traces (arrows indicate

decreasing natural-gamma) Spontaneous potential (SP) and

resistivity (RES) log traces (arrows

indicate negative (-) SP deflection 

and increasing resistivity.

Coal bed with parting 

Coal bed 0

100 feet

Drill hole point ID

Surface elevation (feet above MSL)

Drill-hole records

GL, Ground level (in feet)

MSL, Mean sea level

Knobloch coal resource unit

Knobloch coal zone

Knobloch

King (Sawyer)

Coal bed name (alternate nomenclature)

Coal bed name

KCRU

KCRU

KCRU

KCRU

Knz

Knz

Knz

Knz

** Alternative Flowers-Goodale correlation and terminology of McClellan and Biewick (1988)

*Alternative Flowers-Goodale correlation and terminology of Culbertson and Saperstone (1987)

Explanation

Figure PA-8. North-south cross section A-A' showing proposed coal bed correlations, Ashland coalfield and vicinity,

southeast MontanaCoalbed and coalzone terminology modified from unpublished drillhole information (1996)

No Record
Flowers-Goodale pinchout

(this study)

Sawyer

Sawyer

Odell

Cook

Wall coal zone

(or equivalent)

Otter

Odell (Cache)

Brewster-Arnold

Witham

Witham

Terret

Knobloch coal bed 

 (Sawyer )

Nance ? 

Flowers-Goodale

Flowers-Goodale** (Broadus ?)

Kendrick (?)

31-34LD

GL 3304 ft

31-33LD

GL 3330 ft

32-145LD

GL 3148 ft

32-15LD

GL 3305 ft

32-144LD

GL 3231 ft

8-1B

GL 3558 ft

GL 3983 ft

GL 3791 ft

2-5BR

3000

3100

3200

3300

3400

3500

2500

2400

2600

2700

2800

2900

3600

3700

3800

3900

4000

West East No Record No Record No Record

Total depth

6400 ft

Total depth

6836 ft

Total depth

7128 ft

1-21BR

Poker Jim (Pawnee)

Poker Jim

B B'

Elevation (in feet) above MSL

Cross section datum is MSL

Tertiary (part)

Fort Union Formation (part)

Tongue Ri

ver Member (part)

Nance

NG

NG

NG

NG

NG

NG

SP RES NG

2.8

mi

3.1

mi

1.7

mi

3.0

mi

3.8

mi

3.4

mi

4.2

mi

Knobloch coal bed KCRU

KCRU

KCRU

NG

SP RES

2-5BR

GL 3791 ft 

"Mirrored" natural-gamma

log traces (arrows indicate

decreasing natural-gamma) Spontaneous potential (SP) and

resistivity (RES) log traces (arrows

indicate negative (-) SP deflection 

and increasing resistivity.

Coal bed with parting 

Coal bed 0

100 feet

Drill hole point ID

Surface elevation (feet above MSL)

Drill-hole records

GL, Ground level (in feet)

MSL, Mean sea level

Knobloch coal resource unit

Knobloch coal zone

Knobloch

King (Sawyer)

Coal bed name (alternate nomenclature)

Coal bed name

KCRU

Knz

Knz

Knz

** Alternative Flowers-Goodale correlation and terminology of McClellan and Biewick (1988)

Explanation

Figure PA-9. West-east cross section B-B' showing proposed coal bed correlations, Ashland coalfield and

vicinity, southeast Montana. Coal-bed and coal-zone terminology modified from unpublished drill-hole

information (1996) of W.C. Culbertson and L.R.H. Biewick in the National Coal Resources Data System

Main coal-bed

split area
2.5-10 ft

30-40 ft

50-60 ft

40-50 ft

60-70 ft

> 70 ft (maximum = 71 ft)

10-20 ft

20-30 ft

Explanation

Net coal thickness (in feet)

Custer National Forest (part)

Northern Cheyenne Indian Reservation (part)

Main coal-bed split area

Split line B

106° 00'

106° 30'

45° 30'

45° 45'

45° 15'

0 5 10 Miles

Stacey

Birney

Otter

Brandenberg

Ashland

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

R.42E. R.44E. R.46E. 

Rosebud

County

Powder River

County

Big Horn County

Main coal-bed

split area

Split line B

Main coal-bed

split area

Tongue River

Tongue River

Mo

Ashland 

boun

Northern Cheyenne Indian

Reservation (part)

Figure PA-10. Net coal thickness (isopach) map of the Knobloc

Ashland coalfield, southeast Montana.

Custer National Forest

(part)

Custer National Forest

(part)

?

Drill hole (public data)

County line
Explanation

Overburden thickness (in feet) 

0-100 ft

100-200 ft

200-500 ft

500-1000 ft

> 1000 ft

106° 00'

106° 30'

106° 30' 106° 00'

45° 30'

45° 45'

45° 15'

0 5 10 Miles

Stacey

Birney

Otter

Brandenberg

Ashland

T.1N.

T.1S.

T.2S.

T.3S.

T.4S.

T.5S.

T.6S.

T.7S.

R.42E. R.44E. R.46E. R.48E.

Rosebud

County

Rosebud

County

Custer

County

Powder River

County

Big Horn County

Tongue River

Tongue River

Montana

Study Area

Ashland coalfield

boundary

Northern Cheyenne Indian

Reservation (part)

County line

Custer National Forest (part)

Northern Cheyenne Indian Reservation (part)

Figure PA-11. Overburden thickness (isopach) map for the Knobloch coal 

Ashland coalfield, southeast Montana.

Custer National Forest

(part)

Custer National Forest

(part)
Table PA-1. Estimated net coal resources for the Knobloch coal resource unit reported by county, overburden, net coal thickness, and

reliability categories. Resources are reported in millions of short tons (MST) and with two significant figures. Zeros (0) indicate that

no coal resources were calculated within those categories. Reported resources do not include areas within Custer National Forest.

Columns may not sum correctly because of independent rounding

County Overburden Net Reliability categories (distance from data point)

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 10-20 ft 1.8 7.8 10 0 20

20-30 ft 33 88 100 0 220

30-40 ft 23 21 17 0 61

40-50 ft 20 44 8.4 0 73

50-100 ft 110 360 140 0 610

0-100 ft total 190 520 280 0 980

100-200 ft 10-20 ft 0.27 0 34 2.0 36

20-30 ft 17 46 52 0 110

30-40 ft 14 20 18 0 52

40-50 ft 43 56 49 0 150

50-100 ft 310 720 260 0.91 1,300

100-200 ft total 380 840 410 2.9 1,600

200-300 ft 10-20 ft 0 0 35 0.11 35

20-30 ft 6.7 14 39 0 59

30-40 ft 0 11 12 0 23

40-50 ft 6.7 23 72 0 100

50-100 ft 170 460 210 0 840

200-300 ft total 180 510 370 0.11 1,100

300-400 ft 10-20 ft 0 1.4 37 0 38

20-30 ft 1.7 5.9 15 1.5 24

30-40 ft 0 2.2 4.9 0 7.1

40-50 ft 0 0.67 11 0 12

50-100 ft 9.3 64 140 0 220

300-400 ft total 11 75 210 1.5 300
Table PA-1. Net coal resources, Knobloch coal resource unit—continued.

County Overburden Net Reliability categories (distance from data point)

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 400-500 ft 10-20 ft 0 0 16 0 16

20-30 ft 1.6 0.41 6.0 0.082 8.1

30-40 ft 0 0 0.29 0 0.29

40-50 ft 0 0 2.9 0 2.9

50-100 ft 0 6.0 66 0 72

400-500 ft total 1.6 6.4 92 0.082 100

500-1000 ft 10-20 ft 0 2.1 24 0 26

20-30 ft 0 0 0.74 0 0.74

40-50 ft 0 0 12 0 12

50-100 ft 0 0.36 16 0 17

500-1000 ft total 0 2.4 53 0 55

POWDER RIVER total 770 1,900 1,400 4.6 4,100

ROSEBUD 0-100 ft 5-10 ft 0.19 0.30 0 0 0.48

10-20 ft 12 56 35 0 100

20-30 ft 26 61 0 0 87

30-40 ft 5.7 22 9.8 0 38

40-50 ft 13 29 12 0 53

50-100 ft 11 81 130 0 230

0-100 ft total 68 250 190 0 510

100-200 ft 2.5-5 ft 0 0.30 0 0 0.3

5-10 ft 1.1 8.3 7.8 0 17

10-20 ft 21 53 24 0 98

20-30 ft 32 39 4.4 0 76

30-40 ft 2.7 14 0.18 0 17

40-50 ft 5.2 5.9 0.58 0 12

50-100 ft 31 82 28 0 140

100-200 ft total 93 200 64 0 360
Table PA-1. Net coal resources, Knobloch coal resource unit—continued.

County Overburden Net Reliability categories (distance from data point)

thickness coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

ROSEBUD 200-300 ft 2.5-5 ft 0.24 0.06 0 0 0.30

5-10 ft 0.36 8.8 17 0 26

10-20 ft 9.4 46 59 0 110

20-30 ft 3.2 17 2.1 0 22

30-40 ft 2.1 17 0.61 0 20

40-50 ft 0 0.35 0 0 0.35

50-100 ft 17 78 15 0 110

200-300 ft total 32 170 94 0 290

300-400 ft 2.5-5 ft 0.078 0.56 0.12 0 0.75

5-10 ft 2.8 9.6 38 0 50

10-20 ft 4.1 29 61 0 94

20-30 ft 0.98 11 1.7 0 14

30-40 ft 0 3.7 2.5 0 6.2

50-100 ft 4.4 24 3.3 0 32

300-400 ft total 12 78 110 0 200

400-500 ft 2.5-5 ft 0.42 1.7 0.74 0 2.9

5-10 ft 1.9 11 26 0 39

10-20 ft 1.3 17 60 0 78

20-30 ft 0 2.9 1.9 0 4.8

30-40 ft 0 0.44 2.2 0 2.6

50-100 ft 1.2 9.3 0 0 11

400-500 ft total 4.8 42 91 0 140

500-1000 ft 2.5-5 ft 0.10 0.79 0.076 0 0.97

5-10 ft 1.0 20 120 0.042 140

10-20 ft 2.3 22 180 4.2 210

20-30 ft 0.37 2.0 3.0 0 5.4

30-40 ft 0 0 0.18 0 0.18

50-100 ft 0 0.66 0 0 0.66

500-1000 ft total 3.8 45 300 4.2 360
Table PA-1. Net coal resources, Knobloch coal resource unit—continued.

County Overburden Net Reliability categories (distance from data point)

thickness coal thickness Measured

(<1/4 mi)

Indicated

(1/4-3/4 mi)

Inferred

(3/4-3 mi)

Hypothetical

(>3 mi)

Total

(MST)

ROSEBUD 1000-1500 ft 5-10 ft 0 0 20 0 20

10-20 ft 0 0.16 4.4 0 4.5

20-30 ft 0 0 2.2 0 2.2

1000-1500 ft total 0 0.16 26 0 27

ROSEBUD total 210 780 880 4.2 1900

Grand total (MST) 980 2,700 2,300 8.8 6,000
Table PA-2. Net coal resources in the Knobloch coal resource unit reported by 7.5-minute quadrangle. Coal resources are reported in

millions of short tons (MST) and with two significant figures. Reported resources do not include areas within Custer National Forest.

Column may not sum correctly because of independent rounding

7.5-minute quadrangle Total (MST)

ASHLAND 330

BIRNEY 620

BIRNEY DAY SCHOOL 200

BROWNS MOUNTAIN 280

COLEMAN DRAW 1,000

COOK CREEK RESERVOIR 300

FORT HOWES 180

GOODSPEED BUTTE 130

GREEN CREEK 220

KING MOUNTAIN 1,000

POKER JIM BUTTE 56

WILLOW CROSSING 1,400

YAGER BUTTE 330

Grand total (MST) 6,000
Table PA-3. Net coal resources in the Knobloch coal resource unit reported by county and by Federal coal and surface ownership.

Resources within Custer National Forest are not included in the resource tabulation. Coal resources are reported in millions of short tons

(MST) and with two significant figures. Column may not sum because of independent rounding

County Federal ownership Total (MST)

POWDER RIVER No Federal coal or surface ownership 1,800

Federal coal, but no Federal surface ownership 2,100

Federal coal and Federal surface ownership 200

POWDER RIVER total Federal coal 2,300

POWDER RIVER total 4,100

ROSEBUD No Federal coal or surface ownership 730

Federal coal, but no Federal surface ownership 790

Federal coal and Federal surface ownership 360

ROSEBUD total Federal Coal 1,100

ROSEBUD total 1,900

Total Federal coal 3,400

Grand total (MST) 6,000
Table PA-4. Computations of confidence intervals within reliability categories for the Knobloch coal resource unit, Ashland coalfield,

Powder River Basin, Montana. Areas within Custer National Forest are not included in the table. 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) 55,222,034 168,110,743 225,695,422 1,060,821 450,089,020

Percent of area 12 37 50 0 100

Acres 13,646 41,541 55,771 262 111,219

SD (Standard deviation (in ft) from variogram

model)

3.757 4.640 7.020 7.576 NA

Acre feet (Acres x SD) 51,264 192,732 391,488 1,986 NA

Volume standard deviation (in millions of short tons

(MST))

9 56 395 4 463

Pseudo n (Minimum number of points in the area) 109 37 3 1 NA

Table PA-5. Volume and estimates of uncertainty for resources in the Knobloch coal resource unit, Ashland coalfield, Powder River

Basin, Montana with measurement error. Coal resources in the Custer National Forest are not included in the resource tabulation.

Volume refers to the calculated resource in millions of short tons (MST). Resources are reported in millions of short tons (MST) with

four significant figures

Parameter Reliability Category Entire

Measured Indicated Inferred Hypothetical Area

Volume (MST) 979.5 2,729 2,285 8.800 6,003

Lower 90% confidence bound (MST) 965.0 2,636 1,636 3.000 5,240

Upper 90% confidence bound (MST) 994.0 2,822 2,934 14.60 6,764