Chapter PC COLSTRIP COALFIELD, POWDER RIVER BASIN, MONTANA

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 Survey 2Montana Bureau of Mines and Geology, Butte, Montana 3Consultant, U.S. Geological Survey, Denver, Colorado 4University of Delaware, Newark, Delaware 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.

PC-ii 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

PC-iii 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

PC-iv 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

PC-1 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.

PC-2 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

PC-3 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

PC-4 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)

PC-5 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).

PC-6 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

PC-7 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

PC-8 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.

PC-9 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

PC-10 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)

PC-11 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.

PC-12 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

PC-13 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.

PC-14 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.

PC-15 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.

PC-16 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).

PC-17 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.

PC-18 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.

PC-19 46¡ 00' Robinson coal bed outcrop/burn line Rosebud coal bed outcrop/burn line 39 T.3N. eek

Sarpy Cr

T.2N.

Rosebud coal bed Colstrip Robinson coal bed outcrop/burn line outcrop/burn line Robinson coal bed Treasure outcrop/burn line County

Rosebud coal bed Rosebud outcrop/burn line Big Horn mine County

T.1N. Colstrip coalfield Big Sky 384 Absaloka eek To Hardin mine ud Cr mine Roseb

Mapped limit of Rosebud coal bed Rosebud (Derkey, 1986) 45¡ 45' County T.1S. Crow Indian Reservation (part)

To Lame Deer

T.2S. Northern Cheyenne Indian Reservation (part)

R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E. 107¡ 15' 107¡ 00' 106¡ 45' Explanation 106¡ 30' 0 3 6 Miles Coal mine area Colstrip coal field— Indian Reservation/Tribal lands Resource assessment area for the Montana Rosebud-Robinson coal zone 384 State Highway North Perennial or intermittent stream County line Secondary or unimproved road Study Area

Figure PC-1. Index map showing the Colstrip coalfield and vicinity, south-central Montana. Central and eastern Colstrip coalfield (Rosebud and Big Sky coal mine areas)

Coal beds

Richard L Western Colstrip

coalfield Proctor (Absaloka coal mine area)

Coal beds

Crum Sawyer L L Popham Porcupine

Walborn Lee L Tertiary (part) Tertiary Hammond Fort Union Formation (part) Union Formation Fort

Tongue River Member (part) River Tongue L Stray

Rosebud Rosebud Rosebud-McKay McKay McKay Stray Stocker Creek* Robinson Robinson

Burley

Lebo Member (part) Rosebud-Robinson coal zone L, local coal bed * Present in outcrops north of the coalfield boundary.

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. Hardin 30' x 60' Lame Deer 30' x 60' quadrangle quadrangle 46¡ 00' 46¡ 00' Minnehaha Creek South Iron Spring McClure Creek

Trail Creek School Colstrip West Colstrip East Hammond Draw NW

Colstrip Rosebud mine Treasure County

Rosebud Big Horn mine Rosebud County County Colstrip SW Wolf School Padlock Hammond Draw SW Ranch Rough Draw Colstrip SE Sarpy School Absaloka Big Sky mine mine Iron Spring SW

45¡ 45' 45¡ 45' Crow Indian Reservation (part)

Chalky Point Jean's Fork NW Jean's Fork NE Black Spring Jimtown Badger Peak Garfield Peak

Northern Cheyenne Indian Reservation (part)

107¡ 15' 107¡ 00' 106¡ 45' 106¡ 30' Explanation 0 3 6 Miles

Indian Reservation/Tribal lands Colstrip coalfield Montana North Coal mine area Boundary between adjacent 30' x 60' quadrangles County line Study Area

Figure PC-3. Index map showing 7.5' and 30' x 60' quadrangle maps in the Colstrip coalfield and vicinity, south-central Montana. 46¡ 00' Rosebud T.3N. County

T.2N. Colstrip Rosebud mine Treasure County Rosebud mine

T.1N.

Absaloka Big Sky mine mine

Big Horn 45¡ 45' County T.1S. Colstrip coalfield Crow Indian Reservation (part)

T.2S. Northern Cheyenne Indian Reservation (part)

R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E. 107¡ 15' 107¡ 00' 106¡ 45' Explanation 106¡ 30' 0 3 6 Miles

Coal mine area Federally owned coal Montana

Indian Reservation/Tribal Lands North

County line Privately owned coal (includes State-owned coal) Study Area

Coalfield boundary

Figure PC-4. Map showing Federal coal ownership, Colstrip coalfield and vicinity, south-central Montana. 46¡ 00' Rosebud Treasure Tflt Tflt County County Tflt T.3N. Tflt Robinson coal bed outcrop/burn line

D U Rosebud coal bed outcrop/burn line Tftr Tftr Colstrip T.2N. Tftr Rosebud mine Robinson coal bed U outcrop/burn line D D U U D

Rosebud U D mine D U D U D Tftr U U D T.1N. Tflt Big Sky AbsalokaU D U mine Khc mineD U Tftr D U U D D D U U Rosebud coal bed U D U D U D Big Horn Rosebud D outcrop/burn line County County U D Mapped limit of Rosebud coal bed 45¡ 45' (Derkey, 1986) T.1S. Tftr Crow Indian Reservation (part)

T.2S. Northern Cheyenne Indian Reservation (part)

R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.

107¡ 15' 107¡ 00' 106¡ 45' 106¡ 30' 0 3 6 Miles Clinker (Rosebud, McKay, Rosebud-McKay, Bedrock geologic units and Robinson coal beds) Fault Coal mine area Fort Union Formation (Paleocene) Identified Inferred Montana Indian Reservation/Tribal lands Tftr Tongue River Member Tflt D Lebo and Tullock Members (undifferentiated) U D U, upthrown block North County line U D, downthrown block Study Area Khc Hell Creek Formation (Upper Cretaceous)

Figure PC-5. Generalized bedrock geologic map of the Colstrip coalfield area, south-central Montana. 46¡ 00'

T.3N.

D U C' WEC-1956 4-2LD T.2N. WEC 4-1LD D' 5861 WEC 1-24LD 6064 U D D U U U D 9-1LD D 4-3LD

9-2LD 10-1LD 4-61OCH 16-83H Rosebud Mine U 4-21H D U U D D 15-4H 9-3LD U D 10-100LD 11-32LD 11-35LD A' A 15-32H 10-2LD 12-25LD 5-11(B) 12-27LD 11-36LD OCH 9-7LD D T.1N. Absaloka 9-11LD Big Sky Mine 11-48LD D Mine U D 11-49LD U 9-18LD 9-17LD 9-16LD D U U 16-130H D D U 9-20LD C U D 16-45H 11-51LD U D 16-128H D U 11-52LD 16-46H 16-43H D U 16-44H 9-21LD 10-31LD B D 16-10H 11-54LD 45¡ 45' 9-23LD 18-2LD B' 20-2LD T.1S. 20-1LD 17-6LD North Crow Indian Reservation D (part)

T.2S. Northern Cheyenne Indian Reservation (part)

R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E. 107¡ 15' 107¡ 00' 106¡ 45' 106¡ 30' 0 3 6 Miles Explanation Montana Clinker (Rosebud, McKay, Rosebud-McKay, Identified fault Colstrip coalfield Inferred fault and Robinson coal beds) Indian Reservation/Tribal Lands 9-23LD U U Drill hole or outcrop data point D U, upthrown block Coal mine area D Study Area D, downthrown block

Figure PC-6. Index map showing cross section locations, Colstrip coalfield, south-central Montana. West East A A'

31.9 Miles

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 Elevation (in feet) above mean sea level 3800 9-2LD El. 3750 ft 10-100LD El. 3725 ft 10-1LD El. 3685 ft 3700 15-32H 9-3LD El. 3645 ft 11-32LD El. 3642 ft El. 3633 ft 11-35LD 10-2LD 5-11(B)OCH El. 3615 ft El. 3588 ft Areas of erosion El. 3605 ft 4-61OCH 3600 (location approximate) El. 3570 ft Rbn Area of erosion 9-1LD 15-4H (location approximate) El. 3540 ft El. 3515 ft 4-21H El. 3508 ft 16-83H 3500 El. 3474 ft ormation (part) Rsb

Rsb- ertiary (part)

aleocene (part) Rsb T Rbn Mck

P 3400 Mck Rsb Rsb 12-27LD

ort Union F El. 3322 ft F 12-25LD Mck 3300 Clinker Rbn Rsb- El. 3280 ft Mck Mck Mck Rock (undifferentiated) Rbn Rbn Rbn Rsb Rsb Mck 3200 Rosebud-Robinson Mck coal zone Rbn Rsb, Rosebud coal bed/zone Inferred 3100 Mck, McKay coal bed/zone fault Area of erosion Rbn, Robinson coal bed/zone (location approximate)

El., elevation in feet above Upthrown 3000 mean sea level Downthrown block Cross section traverses area near block the intersection of two faults Datum is mean sea level

Figure PC-7. West-east cross section A-A', Colstrip coalfield, south-central Montana. West East B B'

29.9 Miles 0.4 0.9 0.3 0.7 0.7 1.1 0.8 0.8 0.9 1.2 mi mi mi mi mi 1.5 mi 1.3 mi 1.8 mi mi mi 6.7 mi mi 2.3 mi mi mi 5.3 mi 3.2 mi

16-128H El. 3801 Elevation (in feet) above mean sea level

16-10H El. 3555 16-46H 16-45H 9-20LD 3700 El. 3660 El. 3665 16-43H El. 3655 El. 3560 9-18LD 11-54LD 16-130H 9-16LD Rsb- El. 3605 9-17LD 10-31LD El. 3600 El. 3590 El. 3590 Mck 16-44H El. 3580 El. 3580 3600 El. 3560 Rbn 18-2LD 3500 El. 3460 Rsb- Rsb Mck Rsb- Mck 20-2LD 3400 Mck 11-51LD El. 3350 Rsb El. 3325 Rsb Tertiary (part) Tertiary Rsb Knobloch ? Paleocene (part) Paleocene Rbn Rbn 11-52LD 3300 El. 3265 Mck 20-1LD Mck El. 3230 Rbn Rsb Fort Union Formation (part) Union Formation Fort Area of erosion Rsb (location approximate) 3200 Rbn Rbn Rsb Rsb 3100 Rsb Rbn Rbn Rbn Rbn 3000 Clinker Faults Rsb, Rosebud coal bed/zone Rbn Rock (undifferentiated) Mck, McKay coal bed/zone Identified Inferred Rbn, Robinson coal bed/zone 2900 Rosebud-Robinson coal zone El., elevation in feet above Upthrown mean sea level Downthrown block Downthrown 2800 block block

Datum is mean sea level

Figure PC-8. West-east cross section B-B', Colstrip coalfield, south-central Montana. Figu Tertiary (part) Paleocene (part) Fort Union Formation (part) r e PC-9 Southwest Rbn Rsb El. 3325ft 11-51LD C . Southwest- 1.1 mi Area of erosion El. 3340ft 11-49LD Area of erosion Mck 1.0 mi Rbn Rsb El. 3490ft 11-48LD Mck Rsb n ortheast crosssection C-C', Colstrip coal Area of erosion 3.2 mi Rock (undifferentiated) Clinker (Rosebud bed) coal zone Rosebud-Robinson El. 3545ft 11-36LD Mck Rsb 0.8 mi El. 3615ft 11-35LD Downthrown ? ? block 13.5 Miles Inferred fault 3.3 mi Upthrown ? ? block Mck Rbn Rsb El. 3288ft 4-3LD mean sealevel El., elevation infeetabove Rbn, Robinsoncoalbed/zone Mck, McKaycoalbed/zone Rsb, Rosebud coalbed/zone 1.3 mi WEC-5861 El. 3377ft 0.7 mi f ield, south-central Montana. WEC-6064 El. 3352ft 0.9 mi El. 3480ft 4-2LD 0.6 mi Northeast El. 3470ft 4-1LD 0.6 mi WEC-1956 El. 3435ft C' Mck Rsb Elevation (infeet)above Datum ismeansealevel mean sealevel 2900 3000 3100 3200 3300 3400 3500 3600 Figu

Tertiary (part)

r Paleocene (part) e PC-10 Fort Union Formation (part) Southwest Mck Rsb El. 3805ft 17-6LD D . Southwest- 1.4 mi El. 3670ft 9-23LD Rock (undifferentiated) coal zone Rosebud-Robinson 1.3 mi El. 3630ft 9-21LD n ortheast cross section D-D', Colstrip coal 2.2 mi mean sealevel El., elevation infeetabove Rbn, Robinsoncoalbed/zone Mck, McKaycoalbed/zone Rsb, Rosebud coalbed/zone El. 3590ft 9-16LD Mck Rsb 1.4 mi El. 3710ft 9-11LD 13.5 Miles 1.6 mi El. 3750ft 9-7LD Rbn 3.6 mi f ield, south-central Montana. Mck Rsb El. 3750ft 9-2LD 2.0 mi Northeast El. 3420ft 1-24LD D' Mck Rbn Elevation (infeet)above Datum ismeansealevel mean sealevel 3200 3300 3400 3500 3600 3700 3800 3900 46¡ 00' Robinson coal bed outcrop/burn line Rosebud coal bed outcrop/burn line T.3N.

Rosebud coal bed outcrop/burn line T.2N. Colstrip Rosebud Robinson coal bed outcrop/burn line mine

Robinson coal bed outcrop/burn line Treasure County Rosebud mine

T.1N. Big Horn County Rosebud Absaloka County Big Sky mine mine

Mapped limit of Rosebud coal bed Rosebud coal bed outcrop/burn line 45¡ 45' (Derkey, 1986) T.1S. Crow Indian Reservation (part)

T.2S. Northern Cheyenne Indian Reservation (part)

R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E. 107¡ 15' 107¡ 00' 106¡ 45' 106¡ 30' Explanation 0 3 6 Miles Montana Total, net coal thickness

2.5-10 ft 30-40 ft Clinker (Rosebud, McKay, Rosebud-McKay, North and Robinson coal beds) County line Study Area 10-20 ft 40-50 ft Coal mine area 20-30 ft 50-60 ft (maximum = 60 ft) Indian Reservation/Tribal lands Drill hole or outcrop data point (public data)

Figure PC-11. Total, net coal thickness (isopach) map of the Rosebud-Robinson coal zone, Colstrip coalfield, south-central Montana. 46¡ 00' Robinson coal bed outcrop/burn line Treasure T.3N. County

Colstrip T.2N. U Rosebud coal bed Rosebud Robinson coal bed D outcrop/burn line outcrop/burn line mine D U Robinson coal bed D outcrop/burn line U U D

Rosebud U D D U mine U D U D U D

T.1N. Rosebud coal bed outcrop/burn line Absaloka Big Sky D mine mine U U D U D D U D U D U U U D D D U Rosebud coal bed Mapped limit of D outcrop/burn line Rosebud coal bed 45¡ 45' (Derkey, 1986) T.1S. Big Horn Crow Indian Reservation (part) County Rosebud County

T.2S. Northern Cheyenne Indian Reservation (part)

R.37E. R.38E. R.39E. R.40E. R.41E. R.42E. R.43E.

107¡ 15' 107¡ 00' 106¡ 45' 106¡ 30' Maximum overburden thickness (in feet) 0 3 6 Miles Montana 0-100 ft Fault Identified Inferred Coal mine area 100-200 ft North U D D Indian Reservation/Tribal lands U 200-500 ft Study Area 500-1000 ft County line D, downthrown block > 1000 ft U, upthrown block

Figure PC-12. Map showing the estimated overburden thickness above the base of the Rosebud-Robinson coal zone, Colstrip coalfield, south-central Montana. Table PC-1. Total, net coal resources in the Rosebud-Robinson coal zone reported by county, maximum overburden, total, net coal thickness, 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 two significant figures. Columns may not sum because of independent rounding

County Maximum Total, net Reliability categories (distance from data point) overburden coal thickness Measured Indicated Inferred Hypothetical Total thickness (<1/4 mi) (1/4-3/4 mi) (3/4-3 mi) (>3 mi) (MST) BIG HORN 0-100 ft 2.5-5 ft 1.3 2.9 0.023 0 4.1 5-10 ft 14 36 6.9 0 57 10-20 ft 11 110 91 0 210 20-40 ft 13 150 100 0 260 0-100 ft total 39 290 200 0 530 100-200 ft 2.5-5 ft 0.093 0 0 0 0.093 5-10 ft 4.4 5.7 0.32 0 10 10-20 ft 9.9 39 5.3 0 54 20-40 ft 70 310 92 0 470 100-200 ft total 85 350 97 0 540 200-500 ft 5-10 ft 0.68 0.34 0 0 1 10-20 ft 21 61 4.5 0 87 20-40 ft 250 890 110 0 1,300 200-500 ft total 270 950 120 0 1,300 500-1000 ft 5-10 ft 2.3 13 99 17 130 10-20 ft 34 200 290 0 520 20-40 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 Table PC-1. Total, net coal resources, Rosebud-Robinson coal zone—continued

County Maximum Total, net Reliability categories (distance from data point) overburden coal thickness Measured Indicated Inferred Hypothetical Total thickness (<1/4 mi) (1/4-3/4 mi) (3/4-3 mi) (>3 mi) (MST) ROSEBUD 0-100 ft 5-10 ft 2.0 3.6 0.26 0 5.8 10-20 ft 11 7.5 16 1.0 36 20-40 ft 0.056 0.23 0 0 0.29 0-100 ft total 13 11 16 1.0 42 100-200 ft 5-10 ft 3.9 2.6 25 22 54 10-20 ft 12 33 150 27 220 20-40 ft 1.1 0.46 0 0 1.6 100-200 ft total 17 36 180 49 280 200-500 ft 5-10 ft 9.4 77 320 130 540 10-20 ft 67 430 1,600 230 2,300 20-40 ft 88 410 300 0 800 200-500 ft total 160 920 2,200 360 3,600 500-1000 ft 5-10 ft 6.2 30 79 42 160 10-20 ft 22 78 950 310 1,400 20-40 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 7,100 TREASURE 0-100 ft 5-10 ft 2.0 30 19 0 51 10-20 ft 0 5.1 1.2 0 6.4 20-40 ft 2.7 15 1.7 0 19 0-100 ft total 4.7 50 22 0 76 Table PC-1. Total, net coal resources, Rosebud-Robinson coal zone—continued

County Maximum Total, net Reliability categories (distance from data point) overburden coal thickness Measured Indicated Inferred Hypothetical Total thickness (<1/4 mi) (1/4-3/4 mi) (3/4-3 mi) (>3 mi) (MST) TREASURE 100-200 ft 5-10 ft 13 34 9.6 0 56 10-20 ft 24 46 7.2 0 77 20-40 ft 22 88 5.6 0 120 100-200 ft total 58 170 22 0 250 200-500 ft 5-10 ft 0.36 3.6 0 0 4.0 10-20 ft 15 150 94 0 260 20-40 ft 55 210 210 0 470 200-500 ft total 70 370 300 0 740 500-1000 ft 10-20 ft 0.26 3.2 9.4 0 13 20-40 ft 0.81 7.2 210 0 210 500-1000 ft total 1.10 10 220 0 230 TREASURE total 130 600 560 0 1,300 Grand total (MST) 830 4,000 6,900 830 13,000 Table PC-2. Total, net coal resources in the Rosebud-Robinson coal zone reported by 7.5-minute quadrangle. Coal resource totals do not include resources in tribal lands, areas underlying clinker, or in coal mine areas. Coal resources are reported in millions of short tons (MST) with two significant figures. Column may not sum due to independent rounding

7.5-minute Quadrangle Total (MST) BADGER PEAK 520 BLACK SPRING 810 CHALKY POINT 730 COLSTRIP EAST 67 COLSTRIP SE 340 COLSTRIP SW 1,000 COLSTRIP WEST 6.8 GARFIELD PEAK 200 HAMMOND DRAW SW 51 IRON SPRING 8.0 IRON SPRING SW 320 JEANS FORK NE 92 JIMTOWN 690 McCLURE CREEK 980 MINNEHAHA CREEK SOUTH 540 ROUGH DRAW 2,100 SARPY SCHOOL 3,100 TRAIL CREEK SCHOOL 40 WOLF SCHOOL 1,100 Grand total (MST) 13,000 Table PC-3. Total, net coal resources in the Rosebud-Robinson coal zone reported by county and by Federal coal and surface ownership. Coal resource totals do not include resources in tribal lands, areas underlying clinker, or in coal mine areas. Coal resources are reported in millions of short tons (MST) with two significant figures. Column may not sum due 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 1,300 Federal coal and surface ownership 97 BIG HORN total 4,200 ROSEBUD No Federal coal or surface ownership 4,000 Federal coal, but no Federal surface ownership 3,000 Federal coal and surface ownership 190 ROSEBUD total 7,100 TREASURE No Federal coal or surface ownership 910 Federal coal, but no Federal surface ownership 370 Federal coal and surface ownership 2.0 TREASURE total 1,300 Grand total (MST) 13,000 Table 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. 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) 53,972,398 249,745,998 493,881,231 87,696,026 885,295,653 Percent of area 6 28 56 10 100 Acres 13,337 61,714 122,041 21,670 218,761 SD (Standard deviation (in ft) from variogram model) 9.216 11.122 13.049 13.197 NA Acre feet (Acres x SD) 122,916 689,387 1,592,500 285,976 NA Volume standard deviation (MST) 21 164 1,085 506 1,777 Pseudo 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, Colstrip coalfield, Montana, with measurement error. Resource calculations are in millions of short tons (MST) with four significant figures. Volume refers to the calculated resource in millions of short tons (MST).

Parameter Reliability category Entire Measured Indicated Inferred Hypothetical area Estimated total coal resources (MST)) 832 4,017 6,948 826 12,620 Lower 90% confidence bound (MST) 797 3,746 5,162 0 9,699 Upper 90% confidence bound (MST) 867 4,287 8,733 1,659 15,550