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Chapter PC
COLSTRIP 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 L.R. Bader,1
M.S. Ellis,1 G.D. Stricker,1 G. L. Gunther,3 A.M. Ochs,3 S. A. Kinney,1
J.H. Schuenemeyer,1 and H. C. Power4
in U.S. Geological Survey Professional Paper 1625-A
1U.S. Geological Survey2Montana Bureau of Mines and Geology, Butte, Montana3Consultant, U.S. Geological Survey, Denver, Colorado4University of Delaware, Newark, Delaware
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PC-ii
Contents
Introduction...........................................................................................................................PC-1
Geologic Overview and Coal Stratigraphy....................................................................PC-3
Coal Quality..........................................................................................................................PC-7
Total, Net Coal Thickness (Isopach) Map .....................................................................PC-8
Overburden Thickness (Isopach) Map.........................................................................PC-10
Coal Resources...................................................................................................................PC-11
Sources of Data..................................................................................................................PC-14
Drill-hole Data..............................................................................................................PC-14
Geologic Map Data .....................................................................................................PC-15
Geographic Boundary Data.......................................................................................PC-16
Acknowledgments.............................................................................................................PC-17
References Cited.................................................................................................................PC-18
Figures
PC-1. Index map showing the Colstrip coalfield and vicinity, south-central
Montana.
PC-2. Stratigraphic columns showing generalized stratigraphy and coal-bed
terminology in the Colstrip coalfield, south-central Montana.
PC-3. Index map showing 7.5’ and 30’x 60’ quadrangle maps in the Colstrip
coalfield and vicinity, south-central Montana.
PC-4. Map showing Federal coal ownership, Colstrip coalfield and vicinity, south-
central Montana.
PC-5. Generalized bedrock geologic map of the Colstrip coalfield area, south-
central Montana.
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Figures—continued
PC-6. Index map showing cross section locations, Colstrip coalfield, south-central
Montana.
PC-7. West-east cross section A-A’, Colstrip coalfield, south-central Montana.
PC-8. West-east cross section B-B’, Colstrip coalfield, south-central Montana.
PC-9. Southwest-northeast cross section C-C’, Colstrip coalfield, south-central
Montana.
PC-10. Southwest-northeast cross section D-D’, Colstrip coalfield, south-central
Montana.
PC-11. Total, net coal thickness (isopach) map of the Rosebud-Robinson coal zone,
Colstrip coalfield, south-central Montana.
PC-12. Map showing the estimated overburden thickness above the base of the
Rosebud-Robinson coal zone, Colstrip coalfield, south-central Montana.
Tables
PC-1. Total, net coal resources in the Rosebud-Robinson coal zone in the Colstrip
coalfield, Montana, reported by county, overburden thickness, net coal
thickness, and reliability categories
PC-2. Total, net coal resources in the Rosebud-Robinson coal zone reported for 7.5’
quadrangles in the Colstrip coalfield, Montana
PC-3. Total, net coal resources in the Rosebud-Robinson coal zone, Colstrip
coalfield, Montana, reported by county and Federal coal and surface
ownership
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PC-iv
Tables—continued
PC-4. Computations of confidence intervals within reliability categories for
estimated total coal resources in the Rosebud-Robinson coal zone in the
Colstrip coalfield, Montana
PC-5. Volumes and estimates of uncertainty for estimated total coal resources in
the Rosebud-Robinson coal zone, Colstrip coalfield, Montana, with
measurement error
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PC-1
INTRODUCTION
The Colstrip coalfield incorporates an area of some 395 square mi in parts of Big
Horn, Rosebud, and Treasure Counties in south-central Montana (fig. PC-1). The
coalfield is named for the town of Colstrip, which is adjacent to the Rosebud coal
mine in the northeastern part of the field. Access through the coalfield area is limited
to State Highways 315 and 384 and an associated network of improved and
unimproved gravel roads. Numerous, relatively small perennial or intermittent
streams transect the coalfield. Major drainages include Sarpy Creek and Rosebud
Creek (fig. PC-1).
In the Colstrip coalfield, the Rosebud-Robinson coal zone has been designated as a
priority coal assessment unit within the framework of the National Coal Resource
Assessment (NCRA) project of the U.S. Geological Survey. The coal zone includes
three primary coal beds (descending order): the Rosebud, McKay, and Robinson
coal beds (fig. PC-2). The coalfield extent is defined by both geologic and
geographic parameters. The western and northwestern boundary is delimited by the
outcrop or burn line (clinker contact) of the Robinson coal bed, whereas the northern
and eastern limits of the field are defined by the outcrop or burn line (clinker contact)
of the Rosebud coal bed (fig. PC-1). Ideally, the Robinson coal bed, which is the
stratigraphically lowest coal bed in the Rosebud-Robinson coal zone, would best
delineate the maximum geologic extent of the coalfield as we define it. However, the
absence of published geologic maps accurately depicting the continuity and position
of the Robinson bed in the central and eastern parts of the coalfield necessitated our
use of the stratigraphically higher Rosebud coal-bed horizon as the boundary for the
coalfield in these areas. This horizon is well established in published maps of
Matson and Blumer (1973) and Derkey (1986). The southern and southwestern
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PC-2
boundaries of the coalfield are defined by the Northern Cheyenne and Crow Indian
Reservation boundaries, respectively (fig. PC-1).
The Colstrip coalfield is mostly within the Lame Deer 30’ x 60’ quadrangle with
some extension into the eastern part of the Hardin 30’ x 60’ quadrangle, and the
field includes all or parts of nineteen 7.5’ quadrangles (fig. PC-3). The coalfield is
roughly equivalent to the southern half of the Forsyth coalfield as defined by Dobbin
(1929), and includes all of the Colstrip coal deposit of Matson and Blumer (1973).
Federally owned coal is interspersed with privately owned coal sections throughout
most of the coalfield (fig. PC-4).
Coal mining has been on-going in the Colstrip area since the early 1900’s (see, for
example, Dobbin, 1929), and currently three surface (strip) mines, the Rosebud, Big
Sky, and Absaloka (Sarpy Creek) coal mines, are producing coal in this area (fig.
PC-1). Production is from the Rosebud coal bed in the Rosebud and Big Sky mines,
although some historic production of the McKay bed also took place in the Big Sky
mine. The Absaloka mine produces coal from the Robinson bed and the Rosebud-
McKay coal bed, which comprises a single bed at this location. From 1989 through
the early part of 1998, these three mines produced a combined 180+ million short
tons of coal from the Rosebud-Robinson coal zone (Resource Data International,
Inc., 1998); the majority of this total (about 60 percent) reflects production of the
Rosebud coal bed in the Rosebud mine alone. Some of the produced coal from the
Rosebud bed in the Rosebud mine feeds two 350 megawatt power plants adjacent to
the town of Colstrip.
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PC-3
GEOLOGIC OVERVIEW AND COAL STRATIGRAPHY
The Fort Union Formation (Paleocene) is the predominant bedrock unit in the area
of Colstrip, and the uppermost member of this formation, the Tongue River
Member, outcrops throughout the Colstrip coalfield (fig. PC-5). Older rocks in the
underlying Lebo Member and the basal Tullock Member of the Fort Union
Formation are exposed in areas just north and west of the coalfield, and deposits of
the Hell Formation Creek (Upper Cretaceous) are present to the west of the
Absaloka mine in T. 1 N., R. 36 E. In the coalfield, the thickness of the Tongue
River Member reaches a maximum of some 1,686 ft (Dobbin, 1929), although in
western areas of the coalfield near the Absaloka mine (Sarpy Creek area; fig. PC-1),
only about 750 ft of the Tongue River Member is preserved. In this same area, the
Lebo Member ranges from 75 to 125 ft in thickness, and the Tullock Member
attains a thickness of about 300 ft (Tudor, 1975).
Rocks of the Tongue River Member are composed primarily of shale, sandy shale,
sandstone and coal beds ranging in thickness from 3 to about 30 ft (Dobbin, 1929;
Chadwick and others, 1975). The underlying Lebo Member is composed mainly of
dark claystone and mudstone, and this member forms badlands in western parts of
the coalfield; the base of the Lebo Member is defined by the Big Dirty coal bed
(Dobbin, 1929). The Tullock Member, the lowermost member of the Fort Union
Formation, consists predominantly of interbedded sandstone (calcareous), gray
sandy shale, siltstone and carbonaceous shale.
In general, Fort Union Formation strata dip very gently (less than a few degrees) in
easterly and southerly orientations from west to east across the coalfield,
respectively. Locally, however, dips are steepened by high-angle normal faults that
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PC-4
are present throughout much of the Colstrip area (fig. PC-5). The faults in western
areas of the coalfield range from 0.5 to 3 mi in mapped length, with displacements
of as much as 140 ft in some cases (Robinson and Van Gosen, 1986). In the central
and eastern parts of the coalfield, faults exceeding 6 mi in mapped length are present
(Derkey, 1986). In most cases, fault displacement is slight (Matson and Blumer,
1973), although displacement in excess of 100 ft, based on drill-hole information
used in this study, is evident in central areas of the coalfield. Our depiction of fault
locations and orientations in this study is based on Robinson and Van Gosen
(1986), and Derkey (1986). Alternative interpretations can be found in Matson and
Blumer (1973).
Clinker, formed from the natural burning of coal beds, is a prominent feature of the
coalfield (fig. PC-5). In places where the Rosebud and McKay beds have merged to
a single bed, clinker deposits may attain a thickness of as much as 120 ft (Tudor,
1975).
The major coal beds in the Colstrip coalfield are present within the Tongue River
Member of the Fort Union Formation (fig. PC-2). Minor coal is present in the
lowermost Tullock Member, and one coal bed (the Big Dirty bed) at the base of the
Lebo Member is from 3 to 13 ft in thickness. The Rosebud-Robinson coal zone, the
priority assessment unit in the coalfield, is in the lower part of the Tongue River
Member. As mentioned previously, the coal zone comprises three main coal beds:
the lowermost Robinson coal bed, the base of which constitutes the base of the coal
zone, the overlying McKay bed, and the uppermost Rosebud coal bed, the top of
which defines the top of the coal zone. These coal bed names are consistent with
terminology of Dobbin (1929) and nomenclature adhered to in subsequent studies
of Matson and Blumer (1973), Tudor (1975), Robinson and Van Gosen (1986) and
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PC-5
Derkey (1986). An additional coal bed, named the Stocker Creek coal bed (Dobbin,
1929; fig. PC-2), is present stratigraphically above the Robinson coal bed in areas
just north of the coalfield boundary. Because we could not positively identify an
equivalent coal bed to the Stocker Creek bed within the assessment area boundaries,
we do not apply this terminology to the Rosebud-Robinson coal zone as defined in
this report. In the Absaloka mine area, the Rosebud and McKay beds merge to form
a single coal bed (Tudor, 1975; Robinson and Van Gosen, 1986). Each of these
main coal beds split in certain areas of the coalfield; where the Robinson coal bed
splits, the base of the lower split delimits the base of the coal zone. Where the
Rosebud coal bed splits, the top of the upper split defines the top of the coal zone.
Local thin and non-persistent coal beds are present within the zone in some parts of
the coalfield, and the number of coal beds within the Rosebud-Robinson zone varies
from one at points along the Robinson coal bed outcrop west of the Absaloka mine
to as many as five in the SW 1/4 of T. 2 N., R. 39 E. The maximum thickness of
the Rosebud-Robinson coal zone in the Colstrip coalfield is estimated to be about
365 ft.
Lines of cross section (fig. PC-6) depicting our coal-bed correlations in the coalfield
are shown in figures PC-7 through PC-10. These cross sections are based on
interpretations of geophysical logs and drill cuttings descriptions from coal
exploration projects conducted by the U.S. Geological Survey, Amax Coal
Company, Westmoreland Resources, Inc. (see, for example, Robinson and Van
Gosen, 1986), and the Western Energy Company. Most of the cross sections
traverse faulted areas, and an interpretation of fault displacement is shown where
applicable. In some areas, where the distance between control points (drill holes) is
significant, the estimated displacement along the faults is highly inferred. Fault
location and offset orientation is based mainly on Robinson and Van Gosen (1986)
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PC-6
and Derkey (1986), although some inferred faults are based on our observations of
the stratigraphic offset of correlative coal beds between drill holes along the lines of
section.
The Robinson coal bed is estimated to be about 150-225 ft above the base of the
Tongue River Member (Dobbin, 1929). This coal bed ranges from 14-23 ft in
thickness (Dobbin, 1929) with its greater thickness attained in the Absaloka mine
area. In this same area, west of the mine, the coal bed splits into a generally thin
upper bench (2-3 ft thick) and a thicker lower bench as much as 16 ft thick
(Robinson and Van Gosen, 1986). The overlying McKay coal bed is estimated in
this study to be only 40-50 ft above the Robinson coal bed in the central part of the
coalfield (T. 1 N., R. 39 E.), and may be separated from the Robinson bed by as
much as 120 ft of interburden where the Rosebud and McKay beds have merged
near the Absaloka mine (T. 1 N., Rs. 37 and 38 E.) (Tudor, 1975). In the extreme
southern and southeastern part of the coalfield, we suggest that the McKay coal bed
is not present (fig. PC-8). Where not merged with the Rosebud bed, the McKay coal
bed reaches a maximum thickness of about 17 ft in western areas of the coalfield
(see, for example, Robinson and Van Gosen, 1986), and averages about 8 ft in
thickness throughout much of the central and eastern part of the coalfield (Matson
and Blumer, 1973). The Rosebud coal bed averages about 25 ft in thickness
throughout much of the central coalfield area (Matson and Blumer, 1973), and
reaches a maximum of about 29 ft within the Rosebud coal mine (Chadwick and
others, 1975). Near the Absaloka mine (fig. PC-1), the Rosebud coal bed is as much
as 19 ft thick where not merged with the McKay coal bed (Robinson and Van
Gosen, 1986), whereas the merged Rosebud-McKay coal bed is locally as much as
35 ft thick (Tudor, 1975).
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PC-7
COAL QUALITY
The apparent rank of coal in the Rosebud-Robinson coal zone ranges from
subbituminous C to low-range subbituminous B (Robinson and Van Gosen, 1986).
In western coalfield areas surrounding the Absaloka mine, as-received analyses of
57 samples from the Robinson coal bed indicate an average moisture of 25.31
percent, average ash yield of 7.88 percent, average total sulfur content of 0.79
percent, and a mean heat-of-combustion value of 8,665 Btu/lb. As-received analyses
of 37 samples from the McKay coal bed indicate average values for moisture, ash
yield, total sulfur content, and heat-of-combustion are 24.34 percent, 8.83 percent,
0.89 percent, and 8,642 Btu/lb, respectively. In 28 samples of the Rosebud coal bed
in the Absaloka mine area, the average as-received values for moisture, ash yield,
total sulfur content, and heat-of-combustion were 24.22 percent, 8.24 percent, 0.65
percent, and 8,767 Btu/lb, respectively (Robinson and Van Gosen, 1986). For
comparison, the average values for ash yield, total sulfur content, and heat-of-
combustion of coal shipped from the Absaloka mine between 1989 and the first
quarter of 1998 are 8.9 percent, 0.6 percent, and 8,745 Btu/lb, respectively
(Resource Data International, Inc., 1998).
Chemical analyses (on an as-received basis) based on 16 samples of the Rosebud
coal bed from coal exploratory drill holes throughout the Colstrip coalfield indicate
moisture ranging from 20.24 percent to 23.88 percent (arithmetic mean = 22.50
percent), ash yields ranging from 7.86 percent to 12.58 percent (arithmetic mean =
9.54 percent), total sulfur content ranging from 0.53 percent to 7.20 percent
(arithmetic mean = 1.22 percent), and heat-of-combustion values ranging from
7,810 Btu/lb to 9,090 Btu/lb (arithmetic mean = 8,835 Btu/lb) (see, for example,
Matson and Blumer, 1973). In a study of total sulfur content in the Rosebud and
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PC-8
McKay coal beds in the Rosebud mine area, Chadwick and others (1975) indicate
that sulfur concentrations exceeding 1.2 percent are present in discreet areas within
the coal mine, and these higher sulfur concentrations are typically associated with
pyrite occurrences in the lower and upper 1 ft or so of the Rosebud and McKay coal
beds. Although total sulfur content in the Rosebud coal bed can be quite high in
areas of these pyritic concentrations, selective mining that excludes recovery of the
higher sulfur uppermost and lowermost intervals of the Rosebud bed has mitigated
the sulfur issue to a certain degree. This is perhaps reflected in the 0.67 percent
average total sulfur content for produced coal from the Rosebud bed shipped
between 1989 and the first quarter of 1998 (Resource Data International, Inc., 1998).
Additionally, Rosebud coal bed product shipped from the Rosebud mine during this
same time had an average ash yield of 8.4 percent, and an average heat-of-
combustion of 8,637 Btu/lb. Coal shipped from the Big Sky mine (primarily the
Rosebud bed) from 1989 to early 1998 had an average total sulfur content of 0.69
percent, average ash yield of 8.7 percent, and an average heat-of-combustion of
8,781 Btu/lb (Resource Data International, Inc., 1998).
TOTAL, NET COAL THICKNESS (ISOPACH) MAP
The total, net coal isopach map (fig. PC-11) is an interpretation of total (cumulative),
net coal thickness for coal beds within the Rosebud-Robinson coal zone in the
Colstrip 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 coal thickness. It is important to bear in mind, however, that
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PC-9
the isopach map reflects the distribution of the cumulative (summed) coal thickness
for all coal beds within the Rosebud-Robinson zone rather than the thickness of any
individual coal bed except in areas stratigraphically below the McKay (or Rosebud-
McKay) coal bed where thickness values reflect the thickness of the Robinson (or
lower Robinson) coal bed alone.
About 300 drill-hole and outcrop data points were used to develop the total coal
isopach map. Only the public data points are shown (fig. PC-11), and proprietary
data from mine operators and the Cheyenne and Crow Tribes that were also used in
the calculations are omitted. Based on all of these data, the total, net coal in the
Rosebud-Robinson coal zone ranges in thickness from 2.6 ft (outcrop of the lower
Robinson bed) to about 60 ft. Because of the paucity of data in central and southern
parts of the coalfield (for example, T. 1 N., R. 39 E., and T. 1 S., Rs. 40 and 41 E.;
fig. PC-11), we consider our interpretations ot total, net coal thickness to be
somewhat speculative in these areas.
Because the isopach map for the Rosebud-Robinson 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, (3) reduction in the number
of coal beds 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 some or all of the factors
listed above.
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PC-10
Areas of thicker coal accumulation are shown by warmer (yellow) color shades,
whereas minimal coal accumulation areas are highlighted by cool (blue) shades.
Based on the total, net coal isopach map (fig. PC-11), thicker total coal
accumulations are present in T. 1 N., Rs. 38 and 39 E., and in isolated areas west of
the Absaloka mine in T. 1 N., Rs. 36 and 37 E. Total coal accumulation apparently
decreases markedly in southern and southeastern areas of the coalfield, although data
scarcity deems we tread lightly on this interpretation.
OVERBURDEN THICKNESS (ISOPACH) MAP
The overburden isopach map (fig. PC-12) is an interpretation of the varying
thickness of rock that overlies the base of the Rosebud-Robinson coal zone within
the Colstrip coalfield, and for this reason, defines the maximum overburden for the
coal zone. The overburden in this case also includes all coal beds and interburden in
the Rosebud-Robinson coal zone. Based on drill-hole and outcrop data used in this
study, the Rosebud-Robinson coal zone ranges in thickness from 3.6 ft where the
coal zone comprises outcrops of the lower Robinson coal bed alone, to a maximum
of about 365 ft where the entire coal zone is present. Consequently, the minimum
overburden (that is, the thickness of rock overlying the top of the Rosebud-
Robinson coal zone) can vary from the maximum overburden by as much as 365 ft.
Use of maximum overburden criteria in this study follows analogous methodology
used in coal resource studies in the Colorado Plateau region of Colorado and Utah
(see, for example, Hettinger and others, 1996; Brownfield and others, 1998). These
interpretations of overburden are derived by subtracting a computer-generated grid
of values representing the bottom surface (elevation) of the Rosebud-Robinson coal
zone (base of the Robinson or lower Robinson coal bed) from a Digital Elevation
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PC-11
Model (DEM) grid of topographic elevation, resulting in a calculated grid of
thickness values that can be contoured.
Areas of thicker overburden (fig. PC-12) are represented by shades of green and
yellow, while areas of less overburden are highlighted by shades of blue.
Comparison of the total coal isopach map and the overburden thickness map
indicates that areas of thickest cumulative coal in the Rosebud-Robinson coal zone
are coincident (in part) with areas of maximum overburden thickness ranging from
less than 100 ft to more than 1,000 ft.
COAL RESOURCES
Coal resource estimates for the Rosebud-Robinson coal zone are reported in tables
PC-1 through PC-3. In table PC-1, resources are tabulated by county, net coal
thickness categories, and overburden categories. Table PC-2 includes a summary of
coal resources reported by 7.5’ quadrangle, and table PC-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 Colstrip 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)
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PC-12
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 Rosebud, Big Sky, and Absaloka coal mines, tribal
lands (including parts of the Crow and Northern Cheyenne Indian Reservations),
and areas underlying clinker of the Rosebud, McKay, and Rosebud-McKay coal
beds.
Reported reliability categories (table PC-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 PC-1) represent the thickness of overburden above the
base of the Rosebud-Robinson coal zone. Therefore, the overburden in this case
represents a maximum overburden estimate, and the overburden includes all coal
beds and interburden in the Rosebud-Robinson coal zone as well as rocks overlying
the coal zone.
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PC-13
Confidence limits and estimates of uncertainty for total coal resources in the
Rosebud-Robinson coal zone are shown in table PC-4 and table PC-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 160 drill-hole and outcrop locations. From these data, we
computed 90-percent confidence intervals on the volume of coal in the Rosebud-
Robinson 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
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 = 118.64 ft2, nugget = 55.52 ft2, and
range = 2.63 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 Rosebud-Robinson 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
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PC-14
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 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.
U.S. Geological Survey and Montana Bureau of Mines and Geology, 1977,
Geophysical logs for Dawson, McCone, Richland, and Rosebud Counties,
Montana; Chapter F of Preliminary report of 1977 coal drilling in eastern
Montana and northeastern Wyoming: U.S. Geological Survey Open File
Report 77-721-F, 74 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.
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PC-15
GEOLOGIC MAP DATA
Derkey, P.D., 1986, Coal stratigraphy of the Lame Deer 30 x 60 Minute
Quadrangle, southeastern Montana: Montana Bureau of Mines and Geology
Geologic Map GM-43, scale 1:100,000.
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 Map I-847-D, 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 pages with plates.
Robinson, L.N., and Van Gosen, B.S., 1985, Maps showing geology and coal
resources of the northeastern part of the Crow Indian Reservation, Big Horn
County, Montana: U.S. Geological Survey Miscellaneous Field Studies Map
MF-1796, scale 1:24,000.
Robinson, L.N., and Van Gosen, B.S., 1986, Maps showing the coal geology of the
Sarpy Creek area, Big Horn and Treasure Counties, Montana: U.S.
Geological Survey Miscellaneous Field Studies Map MF-1859, scale
1:24,000.
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PC-16
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, 1981, Surface and mineral management status
map, Hardin 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.
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.
Page 21
PC-17
ACKNOWLEDGMENTS
The authors gratefully acknowledge Mr. Mike Shea (Western Energy Company) for
his assistance and cooperation with regard to this study. We also 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 Katherine Varnes (USGS, Denver, CO).
Page 22
PC-18
REFERENCES CITED
Brownfield, M.E., Roberts, L.N.R., Johnson, E.A., and Mercier, T.J., 1998,
Assessment of the distribution and resources of coal in the Deserado coal
area, Lower White River coal field, northern Piceance basin, Colorado: U.S.
Geological Survey Open-File Report 98-352, 28 p.
Chadwick, R.A., Rice, R.C., Bennett, C.M., and Woodriff, R.A., 1975, Sulfur and
trace elements in the Rosebud and McKay coal seams, Colstrip field,
Montana, in Doroshenko, J., Miller, W.R., Thompson, Jr., E.E., and
Rawlins, J.H., eds., Energy Resources of Montana: Montana Geological
Society 22nd Annual Publication, p. 167-176.
Dobbin, C.E., 1929, The Forsyth coal field, Rosebud, Treasure, and Big Horn
Counties, Montana: U.S. Geological Survey Bulletin 812, p. 1-55.
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.
Hettinger, R.D., Roberts, L.N.R., Biewick, L.R.H., and Kirschbaum, M.A., 1996,
Preliminary investigations of the distribution and resources of coal in the
Kaiparowits Plateau, southern Utah: U.S. Geological Survey Open-File
Report 96-539, 72 p.
Page 23
PC-19
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.
Resource Data International, Inc., 1998, Coaldat: coal database, 1320 Pearl Street,
Suite 300, Boulder, Colorado, 80302.
Robinson, L.N., and Van Gosen, B.S., 1986, Maps showing the coal geology of the
Sarpy Creek area, Big Horn and Treasure Counties, Montana: U.S.
Geological Survey Miscellaneous Field Studies Map MF-1859, scale
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 publication), 13 p.
Tudor, M.S., 1975, Geologic exploration and development of coal in the Sarpy
Creek area, Big Horn County, Montana, in Doroshenko, J., Miller, W.R.,
Thompson, Jr., E.E., and Rawlins, J.H., eds., Energy Resources of Montana:
Montana Geological Society 22nd Annual Publication, p. 159-164.
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.
Page 24
Coal mine area Colstrip coal field—
Indian Reservation/Tribal lands
County line
State Highway
Secondary or unimproved roadPerennial or intermittent stream
Explanation
39
384
384
Resource assessment area for theRosebud-Robinson coal zone
Big HornCounty
RosebudCounty
TreasureCounty
T.2N.
T.3N.
T.1N.
T.1S.
T.2S.
R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.
106° 30'106° 45'107° 00'107° 15'
46° 00'
45° 45'
Colstrip
Rosebud coal bedoutcrop/burn line
Mapped limit ofRosebud coal bed
(Derkey, 1986)
Rosebud coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Colstrip coalfield
Rosebud coal bedoutcrop/burn line
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation (part)
0 3 6 Miles
Absalokamine
Big Skymine
North
Rosebudmine
Montana
Study Area
Figure PC-1. Index map showing the Colstrip coalfield and vicinity, south-central Montana.
To Hardin
To Lame Deer
Robinson coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Rosebud Creek
Sarp
y C
reek
Page 25
Crum
Porcupine
Walborn
Hammond
Stray
Stray
L
L
L
L
L
Rosebud-McKayRosebud Rosebud
McKay
Robinson
McKay
RobinsonStocker Creek*
Richard
Sawyer
Proctor
Popham
Lee
Burley
Tert
iary
(pa
rt)
Fort
Uni
on F
orm
atio
n (p
art)
Tong
ue R
iver
Mem
ber
(par
t)
Lebo Member (part)
Western Colstripcoalfield
Central and eastern Colstripcoalfield
Coal beds
Coal beds
(Absaloka coal mine area)
(Rosebud and Big Sky coal mine areas)
Figure PC-2. Stratigraphic columns showing generalized coal stratigraphy and coal-bed terminology in the Colstrip coalfield, south-central Montana. Coal bed names based on Robinson and Van Gosen (1985) and Dobbin (1929). Diagram not to scale.
Rosebud-Robinson coal zone
* Present in outcrops north of the coalfield boundary.
L, local coal bed
Page 26
Rough Draw
Colstrip SW
Colstrip West Colstrip East
Colstrip SE
Trail Creek School
McClureCreek
Sarpy School
Chalky Point Black Spring Jimtown Badger Peak GarfieldPeak
Minnehaha Creek South
Wolf School
Jean's Fork NEJean's Fork NW
Iron Spring SW
PadlockRanch
Iron Spring
Hammond Draw NW
Hammond Draw SW
Hardin 30' x 60'quadrangle
Lame Deer 30' x 60'quadrangle
Big HornCounty
RosebudCounty
TreasureCounty
106° 30'106° 45'107° 00'107° 15'
45° 45'
46° 00'46° 00'
45° 45'
Colstrip
0 3 6 Miles
North
County line
Montana
Study Area
Indian Reservation/Tribal lands Colstrip coalfield
Coal mine areaBoundary between adjacent 30' x 60' quadrangles
Figure PC-3. Index map showing 7.5' and 30' x 60' quadrangle maps in the Colstrip coalfield and vicinity,south-central Montana.
Explanation
Absalokamine
Rosebudmine
Rosebudmine
Big Skymine
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation (part)
Page 27
Colstrip coalfield
Coal mine area
Privately owned coal(includes State-owned coal)
Federally owned coal
Indian Reservation/Tribal Lands
County line
Coalfield boundary
Big HornCounty
RosebudCounty
TreasureCounty
T.2N.
T.3N.
T.1N.
T.1S.
T.2S.
R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.
106° 30'106° 45'107° 00'107° 15'
46° 00'
45° 45'
Colstrip
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation (part)
0 3 6 Miles
Absalokamine
Big Skymine
North
Rosebudmine
Rosebudmine
Montana
Study Area
Figure PC-4. Map showing Federal coal ownership, Colstrip coalfield and vicinity, south-central Montana.
Explanation
Page 28
Figure PC-5. Generalized bedrock geologic map of the Colstrip coalfield area, south-central Montana.
Tftr
Tftr Tftr
UU
U U
U
U
U
U
U
U
U
U
U
U
U
UU
UUD
D D
D
D
UD
D
DD
D
D
U
U
DD
D
D
DD
DDD
D
D
Fault
U, upthrown block
D, downthrown block
Identified Inferred
T.2N.
T.3N.
T.1N.
T.1S.
T.2S.
R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.
106° 30'106° 45'107° 00'107° 15'
46° 00'
45° 45'
Colstrip
Tflt
Tflt
Tftr
Tftr
Tftr
Khc
TfltTflt
Tflt
Rosebudmine
Rosebudmine
Big Skymine
0 3 6 Miles
Absalokamine
Coal mine area
Clinker (Rosebud, McKay, Rosebud-McKay,and Robinson coal beds)
Indian Reservation/Tribal lands
County line
Montana
Study Area
Bedrock geologic units
North
Fort Union Formation (Paleocene)
Tongue River Member
Lebo and Tullock Members (undifferentiated)
Hell Creek Formation (Upper Cretaceous)
Big HornCounty
RosebudCounty
TreasureCounty
Rosebud coal bedoutcrop/burn line
Mapped limitof Rosebud coal bed
(Derkey, 1986)
Robinson coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
RosebudCounty
Rosebud coal bedoutcrop/burn line
Tflt
Tftr
Khc
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation (part)
Page 29
Identified faultInferred fault
Big Sky Mine
Rosebud Mine
AbsalokaMine
5-11(B)OCH
15-32H
15-4H
4-61OCH 16-83H 4-21H
9-3LD
9-2LD
9-1LD
10-1LD
10-100LD 11-32LD 10-2LD 11-35LD
12-27LD
12-25LD
16-46H 16-10H
16-45H16-43H
16-44H
16-130H
16-128H
9-18LD 9-17LD 9-16LD
9-20LD
18-2LD
10-31LD 11-54LD
11-51LD
20-1LD 20-2LD
11-49LD
11-48LD
11-36LD
4-3LD
4-2LDWEC-1956
WEC6064
WEC5861
17-6LD
9-23LD
9-23LD
9-21LD
9-11LD
9-7LD
A
D D
D
D
U
DD
U U
D
DD
D D
U
U
U
U
D
D
D
U
U
U
DU D
D
D
D
U
U
U
U
U
U
DD
DU
U
U
A'
C'
B
D
D'
B'
C
1-24LD
11-52LD
4-1LDT.2N.
T.3N.
T.1N.
T.1S.
T.2S.
R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.
106° 30'106° 45'107° 00'107° 15'
46° 00'
45° 45'
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation(part)
North
Colstrip coalfield
Indian Reservation/Tribal Lands
Clinker (Rosebud, McKay, Rosebud-McKay,and Robinson coal beds)
Drill hole or outcrop data point U, upthrown blockD, downthrown block
Coal mine area
0 3 6 Miles
Montana
Study Area
Figure PC-6. Index map showing cross section locations, Colstrip coalfield, south-central Montana.
Explanation
Page 30
Rbn
Rbn
RbnRbn
Rbn
Rbn
Rsb
Rsb
Mck
Rsb
Mck
Mck
Mck
Rbn
Rsb-Mck
Rsb-Mck
Rsb
Rsb
Rsb
Mck
Mck
3700
3600
3500
3400
3300
3200
3100
3000
3800
Elevation (in feet) abovemean sea level
Datum is mean sea level
Tert
iary
(pa
rt)
Pale
ocen
e (p
art)
Fort
Uni
on F
orm
atio
n (p
art)
A A'
West East
1.5 mi 2.3 mi 2.2 mi 2.3 mi 2.0 mi 0.9 mi 3.7 mi 2.0 mi 1.9 mi 2.0 mi 2.9 mi 3.6 mi 1.6 mi 1.4 mi 1.6 mi
31.9 Miles
5-11(B)OCH
15-32H
15-4H
4-61OCH
16-83H
4-21H
9-3LD
9-2LD
9-1LD
10-1LD
10-100LD
10-2LD
11-32LD11-35LD
12-27LD
12-25LD
Rock (undifferentiated)
Clinker
Rosebud-Robinsoncoal zone
Rsb, Rosebud coal bed/zoneMck, McKay coal bed/zoneRbn, Robinson coal bed/zone
El., elevation in feet abovemean sea level
El. 3588 ft
El. 3645 ft
El. 3515 ft
El. 3570 ft
El. 3474 ft
El. 3508 ft
El. 3642 ft
El. 3750 ft
El. 3540 ft
El. 3685 ft
El. 3725 ft
El. 3605 ft
El. 3633 ft
El. 3615 ft
El. 3322 ft
El. 3280 ft
Inferredfault
UpthrownblockDownthrown
block
Areas of erosion(location approximate)
Area of erosion(location approximate)
Area of erosion(location approximate)
Cross section traverses area nearthe intersection of two faults
Figure PC-7. West-east cross section A-A', Colstrip coalfield, south-central Montana.
Page 31
Rbn
Rbn Rbn
Rsb
Rsb
Rsb
Mck
RbnRsb
Rbn
Rbn
Rsb
Rsb
Knobloch ?
Rsb
Rsb
Rbn
Rbn
Rbn
Rsb-Mck
Rsb-Mck
Rbn
Mck
Rsb
Mck
16-46HEl. 3660
16-10HEl. 3555
16-45HEl. 3665
16-44HEl. 3560
16-43HEl. 3560
16-130HEl. 3590
16-128HEl. 3801
9-18LDEl. 3605 9-17LD
El. 3580
9-16LDEl. 3590
9-20LDEl. 3655
18-2LDEl. 3460
10-31LDEl. 3580
11-54LDEl. 3600
11-51LDEl. 3325
11-52LDEl. 3265
20-1LDEl. 3230
20-2LDEl. 3350
Elevation (in feet) abovemean sea level
Datum is mean sea level
Tert
iary
(pa
rt)
Pale
ocen
e (p
art)
Fort
Uni
on F
orm
atio
n (p
art)
B B'
West East
29.9 Miles
Rock (undifferentiated)
Clinker
Rosebud-Robinsoncoal zone
Rsb, Rosebud coal bed/zoneMck, McKay coal bed/zoneRbn, Robinson coal bed/zone
El., elevation in feet abovemean sea level
3700
3600
3500
3400
3300
3200
3100
3000
2900
2800
0.4mi
0.9mi
0.3mi
0.7mi 1.5 mi
0.7mi 1.3 mi 1.8 mi
1.1mi 6.7 mi 2.3 mi 5.3 mi 3.2 mi
0.8mi
0.9mi
1.2mi
0.8mi
Rbn
Rsb-Mck
Faults
Identified Inferred
UpthrownblockDownthrown
block
Downthrownblock
Area of erosion(location approximate)
Figure PC-8. West-east cross section B-B', Colstrip coalfield, south-central Montana.
Page 32
Tert
iary
(pa
rt)
Pale
ocen
e (p
art)
Fort
Uni
on F
orm
atio
n (p
art)
Rock (undifferentiated)
Clinker (Rosebud bed)
Rosebud-Robinsoncoal zone
Rsb, Rosebud coal bed/zoneMck, McKay coal bed/zoneRbn, Robinson coal bed/zone
El., elevation in feet abovemean sea level
Inferredfault
UpthrownblockDownthrown
block
Rbn
Rbn
Mck
Mck
Rsb
Rsb
Rsb
Rsb
Rsb
Mck
Mck
11-51LDEl. 3325 ft
11-49LDEl. 3340 ft
11-48LDEl. 3490 ft
11-36LDEl. 3545 ft
11-35LDEl. 3615 ft
4-3LDEl. 3288 ft
WEC-5861El. 3377 ft WEC-6064
El. 3352 ft
4-2LDEl. 3480 ft
4-1LDEl. 3470 ft
WEC-1956El. 3435 ft
3600
3500
3400
3300
3200
3100
3000
2900
Elevation (in feet) abovemean sea level
Datum is mean sea level
?
?
?
?
C C'
Southwest Northeast
1.1 mi 1.0 mi 3.2 mi 3.3 mi 1.3 mi 0.7 mi 0.9 mi 0.6mi
0.6mi
0.8 mi
13.5 Miles
Area oferosion
Area oferosion
Rsb
Mck
Rbn
Area oferosion
Figure PC-9. Southwest-northeast cross section C-C', Colstrip coalfield, south-central Montana.
Page 33
Tert
iary
(pa
rt)
Pale
ocen
e (p
art)
Fort
Uni
on F
orm
atio
n (p
art)
Rock (undifferentiated)
Rosebud-Robinsoncoal zone
Rsb, Rosebud coal bed/zoneMck, McKay coal bed/zoneRbn, Robinson coal bed/zone
El., elevation in feet abovemean sea level
3900
3800
3700
3600
3500
3400
3300
3200
Elevation (in feet) abovemean sea level
Datum is mean sea level
D D'
Southwest Northeast
1.4 mi 1.3 mi 2.2 mi 1.4 mi 1.6 mi 3.6 mi 2.0 mi
13.5 Miles
Mck
Rsb
Rsb
Mck
RsbMck
MckRbn
Rbn
17-6LDEl. 3805 ft
9-23LDEl. 3670 ft
9-21LDEl. 3630 ft
9-16LDEl. 3590 ft
9-11LDEl. 3710 ft
9-7LDEl. 3750 ft
9-2LDEl. 3750 ft
1-24LDEl. 3420 ft
Figure PC-10. Southwest-northeast cross section D-D', Colstrip coalfield, south-central Montana.
Page 34
2.5-10 ft
10-20 ft
20-30 ft
30-40 ft
40-50 ft
50-60 ft (maximum = 60 ft)
Total, net coal thickness
Figure PC-11. Total, net coal thickness (isopach) map of the Rosebud-Robinson coal zone, Colstrip coalfield, south-central Montana.
Coal mine area
Clinker (Rosebud, McKay, Rosebud-McKay,and Robinson coal beds)
Indian Reservation/Tribal lands
County line
Drill hole or outcrop data point (public data)
Big HornCounty Rosebud
County
TreasureCounty
Explanation
T.2N.
T.3N.
T.1N.
T.1S.
T.2S.
R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.
106° 30'106° 45'107° 00'107° 15'
46° 00'
45° 45'
Colstrip
Rosebud coal bedoutcrop/burn line
Mapped limit ofRosebud coal bed
(Derkey, 1986)
Rosebud coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Rosebud coal bedoutcrop/burn line
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation (part)
0 3 6 Miles
Absalokamine
Big Skymine
North
Rosebudmine
Rosebudmine
Montana
Study Area
Page 35
0-100 ft
100-200 ft
200-500 ft
500-1000 ft
> 1000 ft
Maximum overburden thickness (in feet)
FaultIdentified Inferred
Figure PC-12. Map showing the estimated overburden thickness above the base of the Rosebud-Robinson coal zone,Colstrip coalfield, south-central Montana.
Big HornCounty
RosebudCounty
TreasureCounty
Coal mine area
Indian Reservation/Tribal lands
County line
T.2N.
T.3N.
T.1N.
T.1S.
T.2S.
R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.
106° 30'106° 45'107° 00'107° 15'
46° 00'
45° 45'
Colstrip
Rosebud coal bedoutcrop/burn line
Mapped limit ofRosebud coal bed
(Derkey, 1986)
Robinson coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Robinson coal bedoutcrop/burn line
Rosebud coal bedoutcrop/burn line
Rosebud coal bedoutcrop/burn line
Northern Cheyenne Indian Reservation (part)
Crow Indian Reservation (part)
0 3 6 Miles
Absalokamine
Big Skymine
North
Rosebudmine
Rosebudmine
Montana
Study Area
D D
DD
D
D
D
D
D
D
D
D
DD
D
D
D
D
D
U
U
U U
U
U
D DUU
U
U
U
U
U
U
U
U UU
U
U
D, downthrown blockU, upthrown block
Page 36
Table PC-1. Total, net coal resources in the Rosebud-Robinson coal zone reported by county, maximum overburden, total, net coalthickness, and reliability categories. Resources do not include coal in tribal lands, areas underlying clinker, or coal mine areas. Zeros (0)indicate categories where no coal resources were calculated. Resources are reported in millions of short tons (MST) with twosignificant figures. Columns may not sum because of independent rounding
County Maximum Total, net Reliability categories (distance from data point)overburdenthickness
coal thickness Measured(<1/4 mi)
Indicated(1/4-3/4 mi)
Inferred(3/4-3 mi)
Hypothetical(>3 mi)
Total(MST)
BIG HORN 0-100 ft 2.5-5 ft 1.3 2.9 0.023 0 4.15-10 ft 14 36 6.9 0 5710-20 ft 11 110 91 0 21020-40 ft 13 150 100 0 260
0-100 ft total 39 290 200 0 530100-200 ft 2.5-5 ft 0.093 0 0 0 0.093
5-10 ft 4.4 5.7 0.32 0 1010-20 ft 9.9 39 5.3 0 5420-40 ft 70 310 92 0 470
100-200 ft total 85 350 97 0 540200-500 ft 5-10 ft 0.68 0.34 0 0 1
10-20 ft 21 61 4.5 0 8720-40 ft 250 890 110 0 1,300
200-500 ft total 270 950 120 0 1,300500-1000 ft 5-10 ft 2.3 13 99 17 130
10-20 ft 34 200 290 0 52020-40 ft 28 340 560 0 930
500-1000 ft total 64 560 950 17 1,600>1000 ft 5-10 ft 0 0 9.4 0 9.4
20-40 ft 0 0 200 0 200>1000 ft total 0 0 210 0 210
BIG HORN total 460 2,200 1,600 17 4,200
Page 37
Table PC-1. Total, net coal resources, Rosebud-Robinson coal zone—continued
County Maximum Total, net Reliability categories (distance from data point)overburdenthickness
coal thickness Measured(<1/4 mi)
Indicated(1/4-3/4 mi)
Inferred(3/4-3 mi)
Hypothetical(>3 mi)
Total(MST)
ROSEBUD 0-100 ft 5-10 ft 2.0 3.6 0.26 0 5.810-20 ft 11 7.5 16 1.0 3620-40 ft 0.056 0.23 0 0 0.29
0-100 ft total 13 11 16 1.0 42100-200 ft 5-10 ft 3.9 2.6 25 22 54
10-20 ft 12 33 150 27 22020-40 ft 1.1 0.46 0 0 1.6
100-200 ft total 17 36 180 49 280200-500 ft 5-10 ft 9.4 77 320 130 540
10-20 ft 67 430 1,600 230 2,30020-40 ft 88 410 300 0 800
200-500 ft total 160 920 2,200 360 3,600500-1000 ft 5-10 ft 6.2 30 79 42 160
10-20 ft 22 78 950 310 1,40020-40 ft 16 190 980 35 1,200
500-1000 ft total 44 300 2,000 380 2,700>1000 ft 10-20 ft 0 0 39 5.8 45
20-40 ft 0 0 410 7.7 420>1000 ft total 0 0 450 14 460
ROSEBUD Total 240 1,300 4,800 810 7,100TREASURE 0-100 ft 5-10 ft 2.0 30 19 0 51
10-20 ft 0 5.1 1.2 0 6.420-40 ft 2.7 15 1.7 0 19
0-100 ft total 4.7 50 22 0 76
Page 38
Table PC-1. Total, net coal resources, Rosebud-Robinson coal zone—continued
County Maximum Total, net Reliability categories (distance from data point)overburdenthickness
coal thickness Measured(<1/4 mi)
Indicated(1/4-3/4 mi)
Inferred(3/4-3 mi)
Hypothetical(>3 mi)
Total(MST)
TREASURE 100-200 ft 5-10 ft 13 34 9.6 0 5610-20 ft 24 46 7.2 0 7720-40 ft 22 88 5.6 0 120
100-200 ft total 58 170 22 0 250200-500 ft 5-10 ft 0.36 3.6 0 0 4.0
10-20 ft 15 150 94 0 26020-40 ft 55 210 210 0 470
200-500 ft total 70 370 300 0 740500-1000 ft 10-20 ft 0.26 3.2 9.4 0 13
20-40 ft 0.81 7.2 210 0 210500-1000 ft total 1.10 10 220 0 230
TREASURE total 130 600 560 0 1,300Grand total (MST) 830 4,000 6,900 830 13,000
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Table PC-2. Total, net coal resources in the Rosebud-Robinsoncoal zone reported by 7.5-minute quadrangle. Coal resource totalsdo not include resources in tribal lands, areas underlying clinker, orin coal mine areas. Coal resources are reported in millions of shorttons (MST) with two significant figures. Column may not sum dueto independent rounding
7.5-minute Quadrangle Total (MST)BADGER PEAK 520BLACK SPRING 810CHALKY POINT 730COLSTRIP EAST 67COLSTRIP SE 340COLSTRIP SW 1,000COLSTRIP WEST 6.8GARFIELD PEAK 200HAMMOND DRAW SW 51IRON SPRING 8.0IRON SPRING SW 320JEANS FORK NE 92JIMTOWN 690McCLURE CREEK 980MINNEHAHA CREEK SOUTH 540ROUGH DRAW 2,100SARPY SCHOOL 3,100TRAIL CREEK SCHOOL 40WOLF SCHOOL 1,100Grand total (MST) 13,000
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Table PC-3. Total, net coal resources in the Rosebud-Robinson coal zone reported by county and by Federal coal andsurface ownership. Coal resource totals do not include resources in tribal lands, areas underlying clinker, or in coal mineareas. Coal resources are reported in millions of short tons (MST) with two significant figures. Column may not sumdue to independent rounding
County Federal ownership Total (MST)BIG HORN No Federal coal or surface ownership 2,800
Federal coal, but no Federal surface ownership 1,300Federal coal and surface ownership 97
BIG HORN total 4,200ROSEBUD No Federal coal or surface ownership 4,000
Federal coal, but no Federal surface ownership 3,000Federal coal and surface ownership 190
ROSEBUD total 7,100TREASURE No Federal coal or surface ownership 910
Federal coal, but no Federal surface ownership 370Federal coal and surface ownership 2.0
TREASURE total 1,300Grand total (MST) 13,000
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Table PC-4. Computations of confidence intervals within reliability categories for estimated total coal resources in the Rosebud-Robinsoncoal zone in the Colstrip coalfield, Montana. Volume refers to the calculated resource in millions of short tons (MST). NA, not applicable
Parameter Reliability category EntireMeasured Indicated Inferred Hypothetical Area
Area (in square meters) 53,972,398 249,745,998 493,881,231 87,696,026 885,295,653Percent of area 6 28 56 10 100Acres 13,337 61,714 122,041 21,670 218,761SD (Standard deviation (in ft) from variogram model) 9.216 11.122 13.049 13.197 NAAcre feet (Acres x SD) 122,916 689,387 1,592,500 285,976 NAVolume standard deviation (MST) 21 164 1,085 506 1,777Pseudo n (Minimum number of points in the area) 106 55 7 1 NA
Table PC-5. Volume and estimates of uncertainty for estimated total coal resources in the Rosebud-Robinson coal zone, Colstripcoalfield, 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 EntireMeasured Indicated Inferred Hypothetical area
Estimated total coal resources (MST)) 832 4,017 6,948 826 12,620Lower 90% confidence bound (MST) 797 3,746 5,162 0 9,699Upper 90% confidence bound (MST) 867 4,287 8,733 1,659 15,550