Chapter PD DECKER COALFIELD, POWDER RIVER BASIN, MONTANA

Chapter PD

DECKER COALFIELD, POWDER RIVER BASIN, MONTANA: GEOLOGY, COAL QUALITY, AND COAL RESOURCES

By S.B. Roberts,1 G.L. Gunther,2 T.T. Taber,1 A.M. Ochs,2 Dorsey Blake,1 M.S. Ellis,1 G.D. Stricker,1 E.M. Wilde,3 J.H. Schuenemeyer,1 and H. C. Power4

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

1U.S. Geological Survey 2Consultant, U.S. Geological Survey, Denver, Colorado 3Montana Bureau of Mines and Geology, Butte, Montana 4University of Delaware, Newark, Delaware Contents

Introduction...... PD-1 Geologic Overview and Coal Stratigraphy...... PD-2 Coal Quality...... PD-10 Total, Net Coal Thickness (Isopach) Map ...... PD-11 Overburden Thickness (Isopach) Map...... PD-13 Coal Resources...... PD-13 Sources of Data...... PD-16 Drill-hole Data...... PD-16 Geologic Map Data ...... PD-20 Geographic Boundary Data...... PD-21 Acknowledgments...... PD-22 References Cited...... PD-23

Figures

PD-1. Index map showing the Decker coalfield and vicinity, southeast Montana. PD-2. Map showing the regional drainage pattern, Decker coalfield and vicinity, southeast Montana. PD-3. Index map showing 7.5’ and 30’ x 60’ quadrangle maps in the Decker coalfield, southeast Montana. PD-4. Map showing the distribution of Federally owned coal, Decker coalfield and vicinity, southeast Montana. PD-5. Map showing generalized bedrock geology, Decker coalfield and vicinity, southeast Montana.

PD-ii Figures—continued

PD-6. Chart showing coal-bed nomenclature for the Anderson-Canyon coal zone, Decker coalfield, Montana. PD-7. Index map showing the location of cross sections GP1-GP1’ and GP2-GP2’ (natural-gamma log profiles), Decker coalfield, southeast Montana. PD-8. Cross section GP1-GP1’ showing natural-gamma log profiles and coal-bed correlations in drill holes penetrating the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. PD-9. Cross section GP2-GP2’ showing natural-gamma log profiles and coal-bed correlations in drill holes penetrating the Anderson-Canyon coal zone Decker coalfield, southeast Montana. PD-10. Index map showing the location of restored stratigraphic/structural cross sections A-A’, B-B’, C-C’, and D-D’, Decker coalfield, southeast Montana. PD-11. Restored stratigraphic/structural cross section A-A’ showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. PD-12. Restored stratigraphic/structural cross section B-B’ showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. PD-13. Restored stratigraphic/structural cross section C-C’ showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana.

PD-iii Figures—continued

PD-14. Restored stratigraphic/structural cross section D-D’ showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. PD-15. Map showing the total, net coal thickness in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. PD-16. Map showing the thickness of overburden above the Anderson-Canyon coal zone, Decker coalfield, southeast Montana.

Tables

PD-1. Total, net coal resources in the Anderson-Canyon coal zone in the Decker coalfield, Montana, reported by county, overburden thickness, net coal thickness, and reliability categories PD-2. Total, net coal resources in the Anderson-Canyon coal zone reported for 7.5’ quadrangles in the Decker coalfield, Montana PD-3. Total, net coal resources in the Anderson-Canyon coal zone, Decker coalfield, Montana, reported by county and Federal coal and surface ownership PD-4. Computations of confidence intervals within reliability categories for estimated total, net coal resources in the Anderson-Canyon coal zone in the Decker coalfield, Montana PD-5. Volumes and estimates of uncertainty for estimated total, net coal resources in the Anderson-Canyon coal zone, Decker coalfield, Montana, with measurement error

PD-iv INTRODUCTION

The Decker coalfield is located about 16 mi northeast of Sheridan, Wyoming, and as defined here, covers about 1,100 square miles in parts of Big Horn, Powder River, and Rosebud Counties, Montana (fig. PD-1). Access through the coalfield is primarily restricted to one State highway and a network of secondary roads. The coalfield is between the Tongue and Powder Rivers, and most of the field is dissected by small, intermittent or perennial streams, including the Otter Creek and Hanging Woman Creek drainages (fig. PD-2). The Decker coalfield includes all or parts of fifty 7.5’ quadrangles (fig. PD-3) and is bounded on the south by the Montana state line, on the west and northwest by the Crow and Northern Cheyenne Indian Reservations, respectively, and on the north and east by the main outcrop line of the Canyon coal bed or the contact between unburned coal and clinker of the Canyon coal bed. The extent of the Canyon coal bed as used in this study is based on published maps of Baker (1929), Warren (1959), Bryson and Bass (1973), and Robinson and Culbertson (1984).

The Decker coalfield includes parts of the historically defined Northward Extension of the Sheridan coalfield (Baker, 1929), the Moorhead coalfield (Bryson and Bass, 1973), and the Birney-Broadus coalfield (Warren, 1959), as well as a more areally restricted Decker coalfield (Decker area) described in studies by Matson and Blumer (1973). Most of the coal in the Decker coalfield is federally owned (fig. PD-4). Coal mining has taken place in the Decker area since the early 1900’s, although historic production was primarily from small wagon mines producing coal locally for domestic use (Baker, 1929). Currently there are three operating surface mines in the coalfield: the Spring Creek, East Decker, and West Decker mines (fig. PD-1). From

PD-1 1995 to 1997, the combined production from these 3 mines has been about 20 million short tons per year (Resource Data International, Inc., 1998).

The Anderson-Canyon coal zone has been selected for resource assessment in the Decker coalfield primarily because there is continuing coal production from certain beds within this coal zone in Montana, as well as significant production from equivalent coal deposits in Wyoming. The Anderson-Canyon coal zone is considered to be stratigraphically equivalent to the Wyodak-Anderson coal zone as described by Flores and Bader (Chapter PS in this CD-ROM). For the purposes of this study, however, local coal-bed terminology as applied in Montana is adhered to in this chapter.

Although coal beds in the Anderson-Canyon coal zone extend into the Crow and Northern Cheyenne Indian Reservations, no assessment of the coal resources on tribal lands is included in this report. Also, part of the Decker coalfield extends into Custer National Forest (fig. PD-4), and Section 522 (e) (2) (b) of the Surface Mining Control and Reclamation Act of 1977 [SMCRA; Public Law 95-87 (30 U.S.C. 1201 et seq.)] strictly prohibits surface coal mining within the boundaries of Custer National Forest. Although we have included coal resource information for areas within the National Forest, the reader should take into account the limitations of coal resource development as constrained by SMCRA.

GEOLOGIC OVERVIEW AND COAL STRATIGRAPHY

Bedrock exposures in the Decker coalfield consist of Tertiary strata in the Fort Union Formation and the overlying Wasatch Formation (fig. PD-5). The contact between the formations is placed at the top of the Roland coal bed of Baker (1929).

PD-2 The structural dip of exposures in this area is generally less than 5 degrees with dip orientations varying from southeasterly in the western part of the coalfield to southwesterly in eastern areas of the coalfield. Although dips as steep as 72 degrees have been recorded (Galyardt and Murray, 1981), these steeper dips typically reflect structural attitudes associated with high-angle normal faults (Matson and Blumer, 1973), which are prevalent in the southern and western parts of the coalfield (fig. PD-5). These predominantly northeast-trending faults range in length from 1 to 6 miles, with offsets of as much as 300 ft; however, the average maximum offset is more on the order of 120-150 ft or less (see, for example, Matson and Blumer, 1973; Robinson and Culbertson, 1984).

The Fort Union Formation is Paleocene in age and the Wasatch Formation is Eocene in age; both formations are coal-bearing. The maximum thickness of the Wasatch Formation in the Decker coalfield is approximately 400 ft, whereas the maximum thickness of the coal-bearing section of the Fort Union alone is about 2,300 ft (Culbertson, 1987). The total Fort Union thickness may reach 3,500-3,600 ft in areas underlying the Wasatch Formation just east of the Tongue River Reservoir (Lewis and Roberts, 1978). Fort Union and Wasatch Formation rock types are similar to one another, and characterized by interbedded sandstone, siltstone, mudstone, carbonaceous shale and coal. Thin lenses of resistant limestone are present locally within the Fort Union Formation. Clinker deposits, formed by the natural burning of coal beds and the resultant baking or fusing of rocks overlying the burning coal, are present in both formations throughout much of the coalfield (see, for example, Heffern and others, 1993).

The Fort Union Formation is subdivided into three members (ascending order): the Tullock, Lebo, and Tongue River Members. The principal coal beds in this area are

PD-3 present in the Tongue River Member, which is the only member of the Fort Union Formation that crops out in the Decker coalfield (fig. PD-5). The Anderson-Canyon coal zone is in the upper part of the Tongue River Member, and the coal zone is defined as the interval of coal and associated clastic rocks between the top of the Anderson coal bed and the base of the Canyon coal bed. In areas where the Canyon coal bed splits, the base of the zone is placed at the base of the lower Canyon coal bed. Based on available drill-hole data, the coal zone ranges from 158 to 500 ft in thickness where the complete zone is preserved within the coalfield. Coal beds within the zone include the Anderson, Dietz 2, Dietz 3, Kirby, Cox, and Canyon coal beds (fig. PD-6) as well as local, unnamed coal beds that are distributed randomly throughout the coalfield. Our coal-bed terminology, particularly pertaining to the Anderson and Dietz 2 and Dietz 3 beds, is consistent with that of Culbertson (1987). However, alternative nomenclature is still widely used by other agencies and mine operators, and a brief summary of additional naming conventions for coal beds in this zone is included here (fig. PD-6). We’ve adopted Culbertson’s nomenclature primarily to maintain consistency with his other USGS reports pertaining to coal in this area (see, for example, Culbertson and others, 1979; Culbertson, 1980).

The Anderson, Dietz 2, and Dietz 3 coal beds are the targets of coal production in the Decker coalfield. Although coal is not currently produced from the Canyon coal bed in this area, there has been production from an equivalent coal bed (Monarch bed) at the Big Horn mine in the Sheridan coalfield of Wyoming. There is no reported production of the other coal beds (Kirby and Cox beds) in the coal zone. Coal-bed thickness in the Anderson-Canyon coal zone is quite variable, and this variability is often the result of merging and splitting of individual beds within the zone, particularly in regard to the Anderson and Dietz 2 and Dietz 3 coal beds. For

PD-4 example, in the Spring Creek mine (fig. PD-1) and areas just south of the mine, the Anderson, Dietz 2, and Dietz 3 beds have merged to form a single bed as much as 88 ft thick at some locations. North and west of the Spring Creek mine area, this thick coal bed splits into two beds comprised of a single Anderson coal bed (as much as 34 ft thick), and a coalesced Dietz 2 and Dietz 3 coal bed as much as 54 ft thick (Cole and Sholes, 1980). However, southeast of the Spring Creek mine, in and near the Decker mines (fig. PD-1), the Anderson and Dietz 2 coal beds form a single bed (from 42 to 57 ft in thickness) while the Dietz 3 coal bed has split into a single bed from 15 to 21 ft in thickness (Cole and Sholes, 1980). Additionally, in eastern parts of the East Decker coal mine, the Anderson, Dietz 2 and Dietz 3 coal beds form three individual beds, and just east of the mine, the Dietz 2 and Dietz 3 coal beds split rapidly into 4 individual coal beds. This splitting and merging of coal beds is characteristic of all coal beds within the Anderson-Canyon zone and, for that matter, most or all the coal beds within the Tongue River Member of the Fort Union Formation.

Maximum net coal thickness (excluding partings) for individual coal beds in the Anderson-Canyon zone, where not coalesced with one another, is about 35-37 ft for the Anderson coal bed, 20-25 ft for both the Dietz 2 and Dietz 3 coal beds, and about 30-33 ft for the Canyon coal bed. The Kirby coal bed, which is only present in the northwestern part of the coalfield, is generally less than 10 ft thick, while the Cox coal bed in eastern areas of the coalfield is usually less than 5 ft thick. Each of these coal beds thicken and thin locally within the coalfield.

A series of 6 cross sections, derived from drill-hole data, is used to depict our interpretations of coal-bed continuity in the subsurface throughout the Decker coalfield. Two cross sections (fig. PD-7) show interpretations of coal-bed continuity

PD-5 using natural-gamma (NG) log profiles (figs. PD-8 and PD-9), whereas the remaining four lines of cross section (fig. PD-10) represent more “idealized” stratigraphic and structural reconstructions (figs. PD-11 through PD-14) based on drill-hole lithologic records. In all cross sections, the datum is mean sea level, and the horizontal distribution of drill-hole control points along each line of section is scaled to approximate the relative map distance between drill holes.

Many of the cross sections traverse faulted areas, and interpretations of fault displacement are shown where applicable. Faults shown in the cross sections are based on Robinson and Culbertson (1984), or they are inferred from observations of stratigraphic offset between drill holes along the lines of section. Coal-bed offset as depicted in the cross sections is very diagrammatic in that the actual structural attitude of the beds in the vicinity of the faults is not indicated. In some areas, where the distance between control points (drill holes) is significant, the displacement along the faults is highly inferred.

Natural-gamma log profiles (figs. PD-8 and PD-9) were constructed using reduced natural-gamma geophysical logs. Each control point displays “mirrored” images of the natural-gamma log recorded at that drill hole. This type of geophysical log measures the natural radioactivity emitted by all rocks in a manner similar to a very sensitive Geiger counter. Typically, coal beds in the study area are very low in natural radioactivity, and mirror imaging NG logs at each control point is used to highlight this characteristic. This type of geophysical log is extremely useful in determining the presence and thickness of coal beds penetrated by drill holes. Coal- bed names for coal above and below the Anderson-Canyon coal zone as shown on the natural-gamma log profiles are modified from data of W.C. Culbertson in the U.S. Geological Survey National Coal Resources Data System (NCRDS).

PD-6 Cross section GP1-GP1’ (fig. PD-8) traverses approximately 35 mi of the Decker coalfield, from an area near the Decker mines to just south of Quietus (fig. PD-7). Just to the west of the Decker coal mine, and within the Spring Creek coal mine, the Anderson, Dietz 2 and Dietz 3 beds merge into a single bed. However, these coal beds split rapidly to the west and east from this area. Additionally, in areas east of the East Decker mine, the Dietz 2, and Dietz 3 beds split into multiple beds, and in this report, are included within the broader Dietz subzone. Some of the coal beds within the Dietz subzone pinchout farther east. Near the town of Quietus, a single Dietz coal bed is all that remains of the subzone. In contrast to the Dietz beds, the Anderson and Canyon coal beds are essentially continuous along the cross section.

Cross section GP2-GP2’ (fig. PD-9) traverses approximately 30 mi of the Decker coalfield, beginning south of the Spring Creek mine near the Montana-Wyoming state line, and extending north to an area just south of Kirby, near the boundary separating the Crow and Northern Cheyenne Indian Reservations (fig. PD-7). In this line of section the Anderson, Dietz 2, and Dietz 3 coal beds comprise a single coal bed in the area extending from the south end of the cross section (near the state line) to just north of the Spring Creek mine, where the bed splits into an Anderson bed and a merged Dietz 2 and 3 coal bed. Farther north, the Dietz 2 and Dietz 3 coal beds split into individual beds, and the Kirby coal bed becomes a constituent of the coal zone as well. Faults locally offset the overall northward structural rise of the coal zone, and faulting is particularly prevalent in the northern parts of the cross section. Additionally, the Anderson-Canyon zone thickens toward the north.

The restored stratigraphic/structural panels (figs. PD-11 through PD-14) were also constructed using drill-hole information, and they include interpretations of the lateral distribution of coal beds and associated rocks (for example, sandstone and

PD-7 mudstone) along each line of section. Lithologic data are from NCRDS, and in certain instances, no differentiation of rock types between coal beds was available. In these cases, the coal-bed correlations are maintained, and all other rock types are included in the general category of “rock.”

Cross section A-A’ (fig. PD-11) covers about 54 mi from west to east across the southern part of the Decker coalfield, extending from the Decker coal mine areas to the eastern limit of the Anderson-Canyon coal zone (fig. PD-10). Interpretations of sandstone and mudstone highlight the build up of ancient river deposits (sandstone) within the Anderson-Canyon coal zone in the west-central and eastern parts of the study area. Interbedded mudstone represents ancient floodplain deposits, which were sites of fine-grained sediment deposition and coal-bed mire development. Note that in areas where the Anderson, Dietz 2, and Dietz 3 beds merge (western part of the cross section) the predominant lithology associated with the coal beds is mudstone. There is an increase in sandstone in areas where the Dietz 2 and Dietz 3 coal beds have split to form the Dietz subzone, suggesting that the course of ancient rivers influenced the geometry and distribution of coal deposits. Fault offsets along the line of section locally disrupt the overall eastward structural rise of the Anderson-Canyon coal zone. Also note the interpreted continuity of primary coal beds across the width of the coalfield.

Cross section B-B’ (fig. PD-12) traverses about 23 mi in the Decker coalfield, and extends from near Kirby to areas south and west of the Spring Creek coal mine (fig. PD-10). Note the overall northward structural rise of the Anderson-Canyon coal zone, which is disrupted by fault offset in the northern part of the cross section. Although lithologic information on rocks associated with the coal beds is sparse, there is an apparent buildup of ancient river deposits (sandstone) in areas where the

PD-8 Anderson has split from the Dietz 2 and Dietz 3 coal beds; intervening sandstone deposits reflect disruption of peat accumulation by ancient rivers, resulting in the development of two coal beds. This is in contrast to areas near the Spring Creek mine where all three coal beds have merged into a single bed, and the surrounding rock is characterized by mudstone.

Cross section C-C’ (fig. PD-13) covers approximately 27 miles in the central part of the Decker coalfield, east of the areas of active coal mining (fig. PD-10). In this area, a single Dietz coal bed is all that remains of the Dietz 2 and Dietz 3 beds and their correlative splits included in the Dietz subzone. At the northern end of the cross section, the Dietz coal bed merges with the Anderson to form a single bed that is 58 ft thick. Also note that the Canyon coal bed splits into upper and lower Canyon beds in the south-central part of the cross section. Along this line of section, much of the coal interburden is composed of mudstone (floodplain deposits), and only minor buildup of ancient river sandstones is observed in localities near the north, central, and southern limits of the cross section.

Cross section D-D’ (fig. PD-14) extends for 26 miles from north to south near the eastern limit of the Decker coalfield (fig. PD-10). Much of the cross section includes areas where only the Dietz and Canyon coal beds are present. Through most of this area, the Dietz coal beds are included within the Dietz subzone, which merges locally with the Anderson coal bed near the southern end of the cross section. Along this cross section, coal beds dip gently southward to a point of abrupt structural rise at the extreme south end of the cross section. Both the Anderson and Dietz subzone coal beds outcrop intermittently along the section. Extensive river deposits of sandstone are well developed in southern areas of the traverse, with floodplain mudstone deposits becoming increasingly abundant northward along the line of

PD-9 section. The Anderson coal bed and Dietz subzone coal beds converge in areas where interburden is composed primarily of floodplain mudstone, and split northward where ancient river courses (sandstone) disrupted continuous peat accumulation.

COAL QUALITY

The principal coal beds in the Anderson-Canyon coal zone (Anderson, Dietz 2, Dietz 3, and Canyon coal beds) are primarily subbituminous in apparent rank (see, for example, Matson and Blumer, 1973; Fine and others, 1980), although sample analyses from these coal beds in the central part of the Decker coalfield indicate that the apparent rank may vary locally from lignite A to subbituminous C (Affolter and others, 1979). For purposes of resource calculations, we considered the coal beds to be subbituminous.

Matson and Blumer (1973) report that heat-of-combustion values (on an as-received basis) for the Anderson coal bed range from 6,594 to 9,850 Btu/lb, typical ash yields from 3.0 to 9.1 percent (locally exceeding 27 percent), and total sulfur content from 0.1 to 0.9 percent. Their reported as-received analyses for the Dietz coal beds, based on data from the individual beds or where the two beds have merged, indicate heat-of-combustion values ranging from 6,019 to 9,561 Btu/lb, ash yields from 2.5 to 14.1 percent, and total sulfur content from 0.1 to 2.4 percent, with most sulfur values being less than 1.0 percent. As-received analyses for the Canyon bed indicate heat-of-combustion values ranging from 6,646 to 9,113 Btu/lb, ash yields ranging from 3.2 to 10.7 percent (locally exceeding 30 percent), and total sulfur content varying from 0.1 to 2.5 percent, although more typical total sulfur values are less than 1.0 percent.

PD-10 For comparison, chemical analyses of shipped coal from the Spring Creek (merged Anderson, Dietz 2 and Dietz 3 beds) and the Decker coal mines (merged Anderson and Dietz 2 beds and the Dietz 3 bed) covering the period from January 1989 to January 1998, indicate heat-of-combustion values ranging from 8,382 to 11,600 Btu/lb (unweighted mean = 9,420 Btu/lb), ash yields ranging from 3.20 to 41.0 percent (unweighted mean = 4.37 percent), and total sulfur content ranging from 0.21 to 1.18 percent (unweighted mean = 0.36 percent). The total tonnage of coal shipped in this time frame was about 142.5 million tons (Resource Data International, Inc., 1998).

TOTAL, NET COAL THICKNESS (ISOPACH) MAP

The total, net coal isopach map (fig. PD-15) is an interpretation of cumulative, net coal thickness for coal beds within the Anderson-Canyon coal zone in the Decker coalfield. At each drill hole or outcrop data point, net coal thickness (excluding partings) is measured for each coal bed within the zone, and then these individual coal thicknesses are summed to derive a cumulative value of total net coal within the coal zone at that data point. These total coal thickness values for all data points are then contoured by a computer modeling program to develop maps showing lines of equal total, net coal thickness. The isopach map reflects the distribution of the cumulative (summed) coal thickness for all coal beds within the Anderson-Canyon zone rather than the thickness of any individual coal bed except in areas stratigraphically below the Dietz 3 coal bed (or merged Dietz 2 and Dietz 3 coal beds), where thickness values reflect the thickness of the Canyon (or lower Canyon) coal bed alone.

PD-11 Approximately 400 data points (drill-hole and outcrop locations) were used in the calculation of the total, net coal isopach map of the Anderson-Canyon coal zone. This total includes limited confidential data from mine operators and data from wells in northern Wyoming (near the Montana state line) which helped to define isopach trends along the southern boundary of the coalfield. Only public (non-proprietary) data points are shown in the total coal isopach map (fig. PD-15).

Because the isopach map for the Anderson-Canyon coal zone is based on cumulative coal thickness values, the following factors, either individually or collectively, can account for variations in the total, net coal thickness as seen on the map: (1) thickening or thinning of individual coal beds within the zone, (2) reduction in the number of coal beds because of lateral pinchout, and (3) reduction in the number of coal beds because of erosion in stratigraphically lower horizons nearing the outcrop limits of the coal zone, and (4) reduction of the total coal in clinker areas where coal has been lost because of natural burning. Therefore, thickness variations exhibited by apparent “thinning or thickening” trends in the total coal isopach map actually reflect variations in the total coal accumulation influenced by all or some of the factors listed above.

Areas of thicker coal accumulation are shown by warmer (yellow and red) color shades, whereas minimal coal accumulation areas are highlighted by cool (blue) shades (fig. PD-15). Based on the total, net coal isopach map, the thickest total coal accumulation in the Anderson-Canyon coal zone is concentrated in Tps. 8 and 9 S., Rs. 38 to 40 E., adjacent to active mines and in the area where the Anderson, Dietz 2, and Dietz 3 coal beds have typically merged into a single coal bed (figs. PD-8 and PD-9).

PD-12 OVERBURDEN THICKNESS (ISOPACH) MAP

The overburden isopach map (fig. PD-16) represents the thickness of rock overlying the top of the Anderson-Canyon coal zone. The areal extent of overburden is constrained by the Anderson coal-bed outcrop (or the contact between the Anderson coal bed and adjacent clinker deposits), the Montana-Wyoming state line, and tribal lands along the western boundary of the coalfield. Areas between the Anderson and Canyon coal-bed outcrops (or clinker contacts), which define the top and base of the coal zone, respectively, contain exposed rocks of the Anderson-Canyon coal zone. These areas of “exposed coal zone” are shaded gray on the overburden map (fig. PD-16). The overburden interpretation was derived by subtracting an elevation grid (with faults) representing the top of the Anderson-Canyon coal zone (that is, the top of the Anderson coal bed) from a Digital Elevation Model (DEM) grid of topographic elevation, resulting in the calculation of a grid of thickness values that could then be contoured as overburden thickness isopachs. Because strata included in the Anderson-Canyon coal zone may extend from tens to hundreds of feet stratigraphically below the top of the zone (Anderson coal bed), the overburden as shown (fig. PD-16) actually represents a minimum overburden thickness. Thicker overburden is highlighted by green and yellow colors, whereas minimal overburden thickness is represented by blue shades.

COAL RESOURCES

Coal resource estimates for the Anderson-Canyon coal zone are reported in tables PD-1 through PD-3. In table PD-1, resources are tabulated by county, total, net coal thickness categories, and overburden categories. Table PD-2 includes a summary of coal resources reported by 7.5’ quadrangle and overburden category, and table PD-3 includes a summary of coal resources reported by county and Federal surface and

PD-13 subsurface ownership criteria. It is important to note that these resource estimates are not comparable to ‘reserve’ estimates, and therefore, do not reflect in any way the amount of coal that can be economically produced at the present time.

The quantification of coal resources in the Decker coalfield follows a scheme of resource calculation reported in Wood and others (1983). The basic quantification formula is as follows: R = A x T x C, where R is the coal resource estimate (in short tons) A is the area underlain by coal (in acres) T is the cumulative (total) net coal thickness (in feet; excluding partings) based on isopach grids, and C is a weight/unit volume (density) conversion factor (in short tons/acre-foot).

All resources are reported in millions of short tons and are reported in accordance with criteria established by Wood and others (1983). Coal tonnages were calculated for coal beds greater than 2.5 ft in thickness using an average density (C) for subbituminous coal of 1,770 short tons per acre-foot. Tonnage values are rounded to two significant figures and the tabulated totals may not equal the sum of the components because of independent rounding. Areas for which resource estimates are not reported include the Spring Creek and East and West Decker coal mines, the CX Ranch coal lease, tribal lands (Crow and Northern Cheyenne Indian Reservations), areas beneath the Tongue River Reservoir, and areas underlying clinker of the Anderson, Dietz 2, and Dietz 3 (or Dietz) coal beds.

Reported reliability categories (table PD-1) include measured, indicated, inferred, and hypothetical coal resources. Measured resources occur within a 0.25-mile radius

PD-14 of a data point, indicated resources within the area between 0.25 miles and 0.75 miles from a data point, and inferred resources are present within a radius from 0.75 miles to 3 miles surrounding a data point. Hypothetical resources are present in areas more than 3 miles beyond a data point (Wood and others, 1983).

Overburden categories, excluding the exposed zone category (see, for example, table PD-1), represent the thickness of overburden above the top of the Anderson-Canyon coal zone. Because the coal zone may extend from tens to hundreds of feet stratigraphically below the top of the zone, the reported overburden categories represent only a minimum overburden thickness; overburden to deeper horizons in the zone may actually be greater than the minimum overburden category as reported. The exposed zone category in the Anderson-Canyon coal zone resource tables represents areas of the coalfield where the complete coal zone is not present because of erosion or natural burning of the coal beds.

Confidence limits and estimates of uncertainty for total coal resources in the Anderson-Canyon coal zone are shown in table PD-4 and table PD-5, respectively. A confidence interval is a statistic designed to capture uncertainty associated with a point estimate. Confidence limits reported here are based on total, net coal thickness measurements from 366 drill-hole and outcrop locations. From these data, we computed 90-percent confidence intervals on the volume of coal in the Anderson- Canyon coal zone in the measured, indicated, inferred, and hypothetical categories. In this case, volume refers to the calculated resource in millions of short tons (MST).

The confidence limits were derived through a complex series of steps. These steps included modeling coal thickness trends and removing the trends using a

PD-15 nonparametric regression algorithm called loess (with span = 0.5), and using residual thickness to compute a semivariogram and fitting the semivariogram to a spherical model. Parameter estimates were sill = 239.85 ft2, nugget = 18.99 ft2, and range = 6.59 miles. Standard deviations of coal thickness were obtained from the semivariogram model. Differences in point densities were compensated for by calculating sample size, called pseudo n, within each reliability category and calculating the variability of volume for each of the reliability categories. The volume of coal in the Anderson-Canyon coal zone was then calculated at a 90-percent confidence interval with measurement error. A description of the methodology used is given in Schuenemeyer and Power (1998) and Ellis and others (1999).

The three main potential sources of error that might bias the confidence intervals are preferential sampling, measurement errors, and model fitting. The probabilistic interpretation of a confidence interval is based upon a random sample, which does not exist in this situation, because there is preferential sampling in those areas deemed to be minable. Measurement error can be caused by an error in recording the coal-bed thickness or in the definition of coverage areas. Modeling fitting variability and bias result from the choice of models and fitting procedures.

SOURCES OF DATA

DRILL-HOLE DATA

The following is a list of published drilling reports that contain most of the drill-hole information used in this study, although additional point data were collated from the

PD-16 U.S. Geological Survey National Coal Resources Data System (NCRDS), unpublished field logs, and from a limited set of proprietary records.

Culbertson, W.C., Gaffke, T.M., and Correia, G., 1980, Coal exploratory holes drilled in 1978-79 in the Birney 1¡ x 1/2¡ quadrangle, Big Horn and Rosebud Counties, Montana, for coal beds in the Tongue River Member of the Paleocene Fort Union Formation: U.S. Geological Survey Open-File Report 80-55, 68 p.

Hansen, W.B., and Culbertson, W.C., 1985, Correlated lithologic logs and analyses from 1982 coal drilling in Big Horn, Prairie, Rosebud, and Treasure Counties, Montana: U.S. Geological Survey Open-File Report 85-738, 125 p.

Kirschbaum, M.A., Correia, G.A., and McPhillips, M., 1982, Coal exploratory drilling during 1979 in western Powder River County, Montana: U.S. Geological Survey Open-File Report 82-748, 141 p.

Matson, R.E., 1971, Strippable coal in the Moorhead coal field, Montana: Montana Bureau of Mines and Geology Bulletin 83, 18 pages with plates.

Matson, R.E., and Blumer, J.W., 1973, Quality and reserves of strippable coal, selected deposits, southeastern Montana: Montana Bureau of Mines and Geology Bulletin 91, 135 pages with plates.

U.S. Bureau of Land Management and U.S. Geological Survey, 1977, Hanging Woman Creek Study Area; Resource and potential reclamation evaluation: EMRIA Report No. 12, 309 p.

PD-17 U.S. Bureau of Land Management, U.S. Bureau of Reclamation, and U.S. Geological Survey, 1977, Bear Creek Study Area; West Moorhead coalfield; Resource and potential reclamation evaluation: U.S. Department of the Interior EMRIA Report No. 8, 148 pages with appendices.

U.S. Geological Survey and Montana Bureau of Mines and Geology, 1976, Preliminary report of coal drill-hole data and chemical analyses of coal beds in Campbell, Converse, and Sheridan Counties, Wyoming; and Big Horn, Richland, and Dawson Counties, Montana: U.S. Geological Survey Open- File Report 76-450, 382 p.

_____ 1977, Geophysical logs for Big Horn, Fallon, and Powder River Counties, Montana; Chapter A of preliminary report of 1977 coal drilling in eastern Montana and northeastern Wyoming: U.S. Geological Survey Open File Report 77-721-A, 77 p.

_____ 1977, Geophysical logs for Big Horn County, Montana; Chapter B of preliminary report of 1977 coal drilling in eastern Montana and northeastern Wyoming: U.S. Geological Survey Open File Report 77-721-B, 126 p.

_____ 1977, Geophysical logs for Powder River and Dawson Counties, Montana; Chapter C of preliminary report of 1977 coal drilling in eastern Montana and northeastern Wyoming: U.S. Geological Survey Open File Report 77-721-C, 79 p.

PD-18 _____ 1977, Preliminary report on 1976 drilling of coals in Campbell and Sheridan Counties, Wyoming; and Big Horn, Dawson, McCone, Richland, Roosevelt, Rosebud, Sheridan, and Wibaux Counties, Montana: U.S. Geological Survey Open-File Report 77-283, 403 p.

_____ 1980, Coal drilling during 1978 in Big Horn, Daniels, Dawson, Musselshell, Richland, Roosevelt, Rosebud, Valley, and Yellowstone Counties, Montana: U.S. Geological Survey Open-File Report 80-267, 271 p.

_____ 1981, Coal drilling during 1979 in Beaverhead, Big Horn, Dawson, Fallon, Garfield, McCone, Musselshell, Powder River, Prairie, Roosevelt, Rosebud, and Wibaux Counties, Montana: U.S. Geological Survey Open-File Report 81-314, 622 p.

_____ 1982, Coal drilling during 1980 in Big Horn, Dawson, McCone, Powder River, Prairie, Richland, Rosebud, and Treasure Counties, Montana: U.S. Geological Survey Open-File Report 82-026, 256 p.

PD-19 GEOLOGIC MAP DATA

Baker, A.A., 1929, The northward extension of the Sheridan coal field, Big Horn and Rosebud counties, Montana: U.S. Geological Survey Bulletin 806, pp. 15-67, with plates.

Bryson, R.P., and Bass, N. Wood, 1973, Geology of the Moorhead coal field, Powder River, Big Horn, and Rosebud counties, Montana: U.S. Geological Survey Bulletin 1338, 116 p. with plates.

Culbertson, W.C., U.S. Geological Survey, unpublished map compilation, 1970- 1987.

McKay, E.J., 1976, Preliminary geologic map and coal sections of the King Mountain quadrangle, Rosebud and Powder River counties, Montana: U.S. Geological Survey Miscellaneous Field Investigations Map MF-817, 2 plates with text.

Robinson, L.N., and Culbertson, W.C., 1984, Map showing isopachs of coal and overburden of the Canyon coal bed, Birney 30’ x 60’ quadrangle, Big Horn, Rosebud, and Powder River Counties, Montana: U.S. Geological Survey Coal Investigations Map C-96-B, 2 plates with text.

Warren, W.C., 1959, Reconnaissance geology of the Birney-Broadus coal field, Rosebud and Powder River counties, Montana: U.S. Geological Survey Bulletin 1072-J, pp. 561-585, with plates.

PD-20 GEOGRAPHIC BOUNDARY DATA

Federal lands and mineral status from:

Biewick, L.R.H., Urbanowski, S.R., Cain, Sheila, and Neasloney, Larry, 1998, Land status and Federal Mineral ownership in the Powder River Basin, Wyoming and Montana: a digital data set for geographic information systems: U.S. Geological Survey Open File Report 98-108, coverage scale 1:100,000.

U.S. Bureau of Land Management, 1981, Surface and mineral management status map, Birney 30’ x 60’ quadrangle, Montana, scale 1:100,000.

U.S. Bureau of Land Management, 1982, Surface and mineral management status map, Lodge Grass 30’ x 60’ quadrangle, Montana, scale 1:100,000.

U.S. Bureau of Land Management, 1990, Surface and mineral management status map, Broadus 30’ x 60’ quadrangle, Montana, scale 1:100,000.

State and County boundary data from U.S. Geological Survey National Mapping Division and based on 1990 TIGER files digitally compiled by the Water Resources Division of the U.S. Geological Survey.

Digital coverages of 1:100,000- and 1:24,000-scale quadrangle map index acquired from National Mapping Division, U.S. Geological Survey.

PD-21 Public Land Survey System (PLSS) data from Montana Natural Resource Information System (NRIS), Montana State Library, scale 1:100,000.

Digital hydrologic coverage from U.S. Environmental Protection Agency, Reach File Version 3, scale 1:100,000.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the diligent and exhaustive efforts of W.C. Culbertson (Emeritus, USGS) for his past field studies and compilation of pertinent stratigraphic and geologic information that was critical to the completion of this study. His foundation of published and unpublished work facilitated our study in many ways. We also thank Laura Roberts for her expertise in resource calculation methodology, Laura Biewick, and Shane Urbanowski for their diligence in creating accurate and available digital coverages of Federal Lands and Minerals ownership status, David Ferderer for locating, obtaining, and formatting public domain files of road and drainage networks in Montana, Gerald Sullivan for his GIS coverage development in the initial stages of the project, and Jim Hunsicker for his compilation of coal quality information. This report also benefited greatly from the thoughtful comments and reviews of Mitch Henry (USGS, Denver, CO), Tim Rohrbacher (USGS, Denver, CO), Mark Kirschbaum (USGS, Denver, CO), John SanFilipo (USGS, Reston, VA), John Spencer (BLM, Billings, MT), Ed Heffern (BLM, Cheyenne, WY), Craig Brunstein (USGS, Denver, CO), and Katherine Varnes (USGS, Denver, CO).

PD-22 REFERENCES CITED

Affolter, R.H., Hatch, J.R., and Culbertson, W.C., 1979, Chemical analyses of coal from the Tongue River Member, Fort Union Formation, Moorhead and southeastern part of the northward extension of the Sheridan coal fields, Powder River and Big Horn counties, Montana: U.S. Geological Survey Open-File Report 79-538, 68 p.

Baker, A.A., 1929, The northward extension of the Sheridan coal field, Big Horn and Rosebud counties, Montana: U.S. Geological Survey Bulletin 806, p. 15- 67, with plates.

Bryson, R.P., and Bass, N. Wood, 1973, Geology of the Moorhead coal field, Powder River, Big Horn, and Rosebud counties, Montana: U.S. Geological Survey Bulletin 1338, 116 p. with plates.

Cole, G.A., and Sholes, M.A., 1980, Geology of the Anderson and Dietz coal beds, Big Horn county, Montana: Montana Bureau of Mines and Geology, Geologic Map 14, 23 p. with plates.

Culbertson, W.C., 1980, Diagrams showing correlation of Paleocene coal beds underlying the Birney 1¡ x 1/2¡ quadrangle, Big Horn, Rosebud, and Powder River Counties, Montana: U.S. Geological Survey Open-File Report 80-666, 6 p.

PD-23 _____ 1987, Diagrams showing proposed correlation and nomenclature of Eocene and Paleocene coal beds in the Birney 30’x60’ quadrangle, Big Horn, Rosebud, and Powder River counties, Montana: U.S. Geological Survey Coal Investigations Map C-113, 2 plates with text.

Culbertson, W.C., Kent, B.H., and Maple, W.J., 1979, Preliminary diagrams showing correlation of coal beds in the Fort Union and Wasatch Formations across the northern Powder River Basin, northeastern Wyoming and southeastern Montana: U.S. Geological Survey Open-File Report OF-79- 1201, 11 p.

Ellis, M.S., Gunther, G.L., Flores, R.M., Ochs, A.M., Stricker, G.D., Roberts, S.B., Taber, T.T., Bader, L.R., and Schuenemeyer, J.H., 1999, Preliminary report on coal resources of the Wyodak-Anderson coal zone, Powder River Basin, Wyoming and Montana: U.S. Geological Survey Open-File Report 98-789B, 49 p.

Fine, D.E., Matson, R.E., and Daniel, J.A., 1980, A section on coal resources of the Anderson and Dietz coal beds, in Cole, G.A., and Sholes, M.A., Geology of the Anderson and Dietz coal beds, Big Horn county, Montana: Montana Bureau of Mines and Geology, Geologic Map 14, 23 p. with plates.

Galyardt, G.L., and Murray, F.N., 1981, Geologic map and coal sections of the Pearl School Quadrangle and the easternmost part of the Bar V Ranch Quadrangle, Big Horn County, Montana: U.S. Geological Survey Open-File Report 81-870, 3 plates with text, scale 1:24,000.

PD-24 Heffern, E.L., Coates, D.A., Whiteman, Jason, and Ellis, M.S., 1993, Geologic map showing the distribution of clinker in the Tertiary Fort Union and Wasatch Formations, northern Powder River Basin, Montana: U.S. Geological Survey Coal Investigations Map C-142, scale 1:175,000.

Lewis, B.D., and Roberts, R.S., 1978, Geology and water-yielding characteristics of rocks of the northern Powder River Basin, southeastern Montana: U.S. Geological Survey Miscellaneous Investigations Series, Map I-847-D, 1 sheet with text, scale 1:250,000.

Matson, R.E., and Blumer, J.W., 1973, Quality and reserves of strippable coal, selected deposits, southeastern Montana: Montana Bureau of Mines and Geology Bulletin 91, 135 p. with plates.

Resource Data International, Inc., 1998, COALdat: Coal database, 1320 Pearl Street, Suite 300, Boulder, Colorado, 80302.

Roberts, L.N.R., Mercier, T.J., Biewick, L.R.H., and Blake, D., 1998, A procedure for producing maps and resource tables of coals assessed during the U.S. Geological Survey’s National Coal Assessment [abs.]: Proceedings of the 1998 Pittsburgh Coal Conference, Abstracts with Programs; non-paginated CD-ROM publication.

PD-25 Roberts, S., Gunther, G., Taber, T., Ochs, A., Blake, D., Bader, L., and Wilde, E.,1998, The Anderson-Canyon coal zone, Decker coalfield, northern Powder River Basin, Montana: Considerations for future strippable coal resource development [abs.]: Proceedings of the 1998 Pittsburgh Coal Conference, Abstracts with Programs; non-paginated CD-ROM publication.

Schuenemeyer, J.H., and Power, H., 1998, An uncertainty estimation procedure for U.S. coal resources, [abs.], in Pittsburgh Coal Conference, 15th Annual Meeting, Abstracts with Programs (CD-ROM): Pittsburgh, Pennsylvania, 13 p.

Warren, W.C., 1959, Reconnaissance geology of the Birney-Broadus coal field, Rosebud and Powder River counties, Montana: U.S. Geological Survey Bulletin 1072-J, p. 561-585, with plates.

Wood, G.H., Jr., Kehn, T.M., Carter, M.D., and Culbertson, W.C., 1983, Coal Resource Classification System of the U.S. Geological Survey: U.S. Geological Survey Circular 891, 65 p.

PD-26 Big Horn Rosebud Powder River 212 212 T. 3 S. County County County Busby 45¡ 30' 45¡ 30'

T. 4 S. er Northern Cheyenne Indian Reservation (part)

ongue Riv T

314

T. 5 S. Canyon coal bed outcrop

Kirby Birney

T. 6 S. Crow Indian Reservation (part) Indian Reservation Crow T. 7 S. er

314 wder Riv o P T. 71/2 S.

T. 8 S.

Spring Creek Decker coalfield mine Quietus TR Moorhead

East Decker West mine T. 9 S. Decker mine CXR Big Horn Montana Decker T. 10 S. County 45¡ 00' T. 10 S. R. 48 E. R. 49 E. R. 50 E. 45¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. 107¡ 00' 106¡ 00' Wyoming 0 5 10 Miles Explanation Decker coalfield—Assessment area for Montana Indian Reservations the Anderson-Canyon coal zone Mine or coal lease 212 U.S. highway Anderson-Canyon coal zone outliers North (CXR, CX Ranch lease area) (Canyon coal bed—unassessed areas) Study Area 314 State highway TR Tongue River Reservoir Canyon coal bed outcrop (lake and wetlands) Secondary and unimproved roads River County line

Figure PD-1. Index map showing the Decker coalfield and vicinity, southeast Montana. Big Horn Rosebud Powder River T. 3 S. County County County Busby 45¡ 30' 45¡ 30' er

T. 4 S. Otter Cr Northern Cheyenne Indian Reservation (part) ongue Riv T

eek

T. 5 S. ation (part)

Kirby Birney

T. 6 S. w Indian Reserv Cro T. 7 S. er

wder Riv o P T. 71/2 S.

Decker coalfield T. 8 S.

Hanging

Spring Creek Quietus mine

TR W Moorhead

oman Cr East Decker West mine T. 9 S. Decker

mine eek CXR Big Horn Montana Decker T. 10 S. County 45¡ 00' T. 10 S. R. 48 E. R. 49 E. R. 50 E. 45¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. 107¡ 00' R. 42 E. R. 43 E. 106¡ 00' Wyoming Explanation 0 5 10 Miles Decker coalfield—Assessment area for Montana Indian Reservations the Anderson-Canyon coal zone Mine or coal lease Anderson-Canyon coal zone outliers North (CXR, CX Ranch lease area) (Canyon coal bed—unassessed areas) Study Area

TR Tongue River Reservoir Canyon coal bed outcrop (lake and wetlands)

River Perennial or intermittent stream County line

Figure PD-2. Map showing regional drainage pattern, Decker coalfield and vicinity, southeast Montana. Big Horn Rosebud Olive T. 3 S. County County Willow Crossing Coleman Draw Home Creek Butte Samuelson Ranch Leslie Creek Hardin Busby 30' x 60' Ashland Lame Deer 30' x 60' Powderville 30' x 60' 45¡ 30' 45¡ 30' er Powder River Lodge Grass County 30' x 60' Epsie Northern Cheyenne Indian Reservation (part) Sonnette ongue Riv Epsie NE T. 4 S. T Green Creek Yager Butte Threemile Buttes Birney Day King Mountain T. 5 S. Cook Creek School Clubfoot Creek Butte ation (part)

Kirby Birney Birney Poker Jim Yarger Butte Goodspeed Butte Phillips Butte T. 6 S. Taintor Desert Butte Fort Howes Hodsdon Flats eek Ranch 7.5' Birney SW Browns Mountain Kirby Lonesome Peak w Indian Reserv

Spring Cr er

Cro Spring wder Riv T. 7 S. Gulch o Reanus Cone P Hamilton Bloom Creek Huckins School Half Moon Hill Lacey Gulch Baldy Peak Tongue River Stroud Creek Draw Otter Sayle Dam

V Ranch NE T. 71/2 S. Bar

T. 8 S.

Spring Creek Decker Moorhead Holmes Ranch Quietus Bradshaw Creek mine Decker coalfield

TR Moorhead Pine Butte School Quietus Bear Creek

V Ranch East Decker Sayle Hall Bay Horse West School mine Forks Ranch T. 9 S. Pearl School Decker Three Bar Ranch Bar mine Lodge Grass Big Horn Montana Birney 30' x 60' Broadus 30' x 60' 30' x 60' T. 10 S. CXR Decker County T. 10 S. 45¡ 00' Dead Horse Lake Corral Creek Homestead Draw 45¡ 00' Box Elder Draw Cabin Creek NE Black Draw R. 49 E. R. 50 E. Monarch Acme Bar N Draw Cedar Canyon OTO Ranch Roundup Draw Cabin Creek NW R. 48 E. R. 46 E. R. 47 E. Wyoming R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. Burgess Junction R. 39 E. R. 40 E. Sheridan 30' x 60' Recluse 30' x 60' 30' x 60' 107¡ 00' 0 5 10 Miles 106¡ 00' Explanation Decker coalfield-—Assessment area for Indian Reservations Montana the Anderson-Canyon coal zone County line Mine or coal lease Andersdon-Canyon coal zone outliers North (CXR, CX Ranch lease area) (Canyon coal bed—unassessed areas) River Study Area Hardin TR Tongue River Reservoir 30' x 60' (lake and wetlands) 30' x 60 ' quadrangle maps (scale 1:100,000) Lodge Grass 30' x 60'

Figure PD-3. Index map showing 7.5' and 30' x 60' quadrangle maps in the Decker coalfield, southeast Montana. Big Horn Rosebud 212 212 T. 3 S. County County Powder River County Busby 45¡ 30' 45¡ 30' er T. 4 S. Northern Cheyenne Indian Reservation (part) Custer National Forest ongue Riv T (part)

314

T. 5 S. ation (part)

Kirby Birney

T. 6 S. w Indian Reserv Cro T. 7 S. er

wder Riv 314 o P T. 71/2 S.

T. 8 S.

Spring Creek mine Quietus TR Moorhead Decker coalfield East Decker West mine T. 9 S. Decker mine Montana Big Horn CXR T. 10 S. Decker County 45¡ 00' T. 10 S. R. 48 E. R. 49 E. R. 50 E. 45¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. 107¡ 00' R. 42 E. R. 43 E. 106¡ 00' Wyoming 0 5 10 Miles Explanation Montana Indian Reservations Federally owned coal 212 U.S. highway Mine or coal lease Federally owned coal (CXR, CX Ranch lease area) (Custer National Forest) Study Area North 314 State highway

TR Tongue River Reservoir Canyon coal bed outcrop (lake and wetlands) Secondary and unimproved roads

River County line

Figure PD-4. Map showing the distribution of Federally owned coal, Decker coalfield and vicinity, southeast Montana. Big Horn T. 3 S. County Busby Rosebud Powder River

County County C Ttr 45¡ 30' 45¡ 30'

T. 4 S.

ongue River Northern Cheyenne Indian Reservation (part) T

T. 5 S. C A North ation (part) Ttr

C C Kirby Birney C A C Ttr C T. 6 S. Ttr C A

w Indian Reserv C C A ongue River C A T Cro C T. 7 S.

owder River C P

T. 71/2 S. C Big Horn T. 8 S. County A Tw A A Spring Creek A A mine Ttr Quietus Moorhead A East Decker A West mine T. 9 S. Decker mine Tw Tw C Montana Decker Tw C CXR T. 10 S. 45¡ 00' A Tw T. 10 S. R. 48 E. R. 49 E. R. 50 E. 45¡ 00' 106¡ 00' 107¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. Wyoming Explanation 0 5 10 Miles Clinker (Anderson, Dietz 2 and Dietz 3 coal beds)—Also present within the Montana Crow and Northern Cheyenne Indian Reservations Indian Reservations County line Bedrock geologic units Mine or coal lease Tw Wasatch Formation—Also present within the Crow and (CXR, CX Ranch lease area) Northern Cheyenne Indian Reservations Tongue River Reservoir (lake) Study Area Ttr Fort Union Formation (Tongue River Member)—Also present within the Tongue River Reservoir (wetlands) Crow and Northern Cheyenne Indian Reservations

A Anderson coal bed—Outcrop or contact with clinker (burn line) Identified fault (bar and ball on downthrown side) Inferred fault (bar and ball on downthrown side) C Canyon coal bed—Outcrop or contact with clinker (burn line)

Figure PD-5. Map showing generalized bedrock geology, Decker coalfield and vicinity, southeast Montana. Geology based on Baker (1929), Warren (1959), Bryson and Bass (1973), McKay (1976), and W.C. Culbertson (USGS, unpublished mapping, 1970-1987). Fault locations and displacement modified from Robinson and Culbertson (1984). Montana Bureau of Mines USGS (this report) and Geology Decker Coal Company Spring Creek Coal Company Coal beds Coal beds Coal beds Coal beds

West East West East

Anderson Anderson Dietz 1 (upper) Anderson

Kirby (present locally) Merge locally to form one coal bed (Decker Mine) Merge to form one coal bed (Spring Creek Mine) Dietz 2 Dietz 1 Dietz 1 (lower) Dietz 1 Dietz Dietz Dietz coal zone subzone Dietz 3 Multiple Single Dietz 2 Dietz 2 Dietz 2 ( beds )( bed ) Tertiary (part) Tertiary Anderson-Canyon coal zone Anderson-Canyon Wyodak-Anderson coal zone* Wyodak-Anderson Fort Union Formation (upper part) Union Formation Fort

Tongue River Member (upper part) River Tongue Cox (present locally)

Canyon Canyon Canyon Canyon

Diagram not to scale *Terminology of Flores and Bader, Chapter PS, this CD-ROM

Figure PD-6. Chart showing coal-bed nomenclature for the Anderson-Canyon coal zone, Decker coalfield, Montana. Coal-bed designations based on Sholes and Cole (1981), Culbertson (1987) and terminology of local mine operators. Big Horn Rosebud T. 3 S. County County Busby Rosebud Powder River County County 45¡ 30' 45¡ 30'

T. 4 S. ongue River Northern Cheyenne Indian Reservation (part) T

T. 5 S. North ation (part)

10-4B Kirby GP2' Birney

T. 6 S. 9-30B

9-32B

w Indian Reserv 9-25B ongue River T Cro

T. 7 S. 17-30B

owder River P 17-25B T. 71/2 S. 17-22B Big Horn County 17-16B T. 8 S. 25-1B Decker coalfield Powder River 25-7B County 27-21B 28-7aB 28-10B Quietus 27-50B 26-7B Moorhead 29-22B

25-13B 29-30B 30-13B 29-51B 30-11B 26-68B 26-73B 26-44B T. 9 S. 25-76B 25-56B GP1' GP1 26-81B 25-71B Montana Decker T. 10 S. 45¡ 00' T. 10 S. GP2 25-72B R. 48 E. R. 49 E. R. 50 E. 45¡ 00' 106¡ 00' 107¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. Wyoming Explanation 0 5 10 Miles Montana Decker coalfield Identified fault (bar and ball on downthrown side)

Inferred fault (bar and ball on downthrown side) Indian Reservations Study Area Drill hole Mine or coal lease County line 28-7aB Point ID Tongue River Reservoir (lake) Tongue River Reservoir (wetlands)

Figure PD-7. Index map showing the location of cross sections GP1-GP1' and GP2-GP2' (natural-gamma log profiles), Decker coalfield, southeast Montana. West East 35 Miles GP1 GP1' Elevation (in feet) above MSL

4300

27-21B Wasatch Formation (part) 25-76B G.L. 4123' 4200 G.L. 4150' (partial record) No 4100 Data 30-11B No G.L. 3983' Data Wasatch Formation (partial record) 4000 Roland 30-13B 25-56B 28-7aB G.L. 3870' G.L. 3870' G.L. 3890' (partial record) 3900

Squirrel Creek Fort Union Formation Decker coal mine area No Data Anderson coal bed A No 3800 Data 27-50B Smith 28-10B G.L. 3655' 29-30B 29-51B 3700 26-68B Smith G.L. 3650' G.L. 3621' G.L. 3630' Dietz 2 and 3 G.L. 3621' Cl D2, D3 29-22B

coal zone coal beds A No G.L. 3562' 3600 Estimated limit of 26-44B Data A Anderson-Canyon merged Anderson and G.L. 3528' A Dietz 2 and 3 coal beds 26-73B A Canyon coal bed Cyn A Dtz G.L. 3486' 26-81B A 3500 G.L. 3436' Dtz A A A, D2, D3 Dsz Dsz Dtz Cyn Tertiary (part) Tertiary ? D2 Dsz 3400 D2 Dsz Cook D3 D3 Cox Cyn 3300 Otter (part) A Cyn D2 White Cyn Cyn NG Cyn Cyn Cyn Cyn 3200 NG NG White Cook-Otter Fort Union Formation (part) Union Formation Fort

Tongue River Member (part) River Tongue NG NG D3 Cook NG Cook Cook Otter Otter 3100 ? NG Upper Otter White Cyn 100 Feet ? Cook-Otter Zone Wall Zone ? Wall Zone Lower Otter Wall Zone Cook 3000 Cook ? NG Otter Wall Zone NG 0 Otter Wall Zone NG 2900 all Zone NG W

NG Zone Wall NG 2800 Explanation TD = 11,750' NG ? Cl Clinker NG Cross section datum is mean sea level (MSL) Fault Anderson-Canyon coal zone ? Drill hole A, Anderson coal bed ? Inferred Identified "Mirrored" natural-gamma (NG) A,D2,D3: Anderson, Dietz 2 and Dietz 3 coal beds (merged) 26-81B Drill-hole point ID log traces (arrows indicate AD2: Anderson and Dietz coal beds (merged) TD = 8434' TD = 8150' G.L. 4200' Surface elevation decreasing natural-gamma) D2, Dietz 2 coal bed (in feet above mean sea level) Upthrown Upthrown D3, Dietz 3 coal bed block block Dsz, Dietz subzone (coal beds undifferentiated) TD, Total depth (in feet) Downthrown Dtz, Dietz coal bed G.L., Ground level (in feet) block Cox, Cox coal bed Cyn, Canyon coal bed Coal bed with parting

Coal bed

Figure PD-8. Cross section GP1-GP1' showing natural-gamma log profiles and coal-bed correlations in drill holes penetrating the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Fault displacement as shown is diagrammatic. Coal-bed terminology modified from Culbertson (1987). Tertiary (part) modif Anderson-Can Figur Fort Union Formation (part) Tongue River Member (part) G.L. 4200' ied fromCulbertson (1987). e PD-9 G.L., Groundle 26-81B Drill hole 0.8 milesa coal bedestimatedfromadrillhole Depth toandthicknessoftheCan Drill-hole pointID (feet abo Surf . CrosssectionGP2-GP2' sho w ace ele Anderson-Canyon ay v yon coalzone,Deck coal zone . el (infeet) v Anderson coalbed e meanseale Can v Dietz 2and3 ation South coal beds GP2 yon coalbed v el) G.L. 3534' yon 25-72B NG Cyn D2, Explanation D3 A ? G.L. 3550' 25-71B NG 1.4 milesa coal bedestimatedfromadrillhole Depth toandthicknessoftheCan

Upthro Squirrel Creek D3 D2 A

block Smith wn G.L. 3870' 25-56B Inferred NG w ay er coalf . Do A, D2,D3 ? wnthro Cyn block F Cook ault G.L. 3895' wn Identif 25-13B NG ied G.L. 3644' ield, southeastMontana. F Upthro yon 26-7B wing natural-g NG block Spring Creekcoalminearea wn G.L. 3687' 25-7B NG A, D2, Cook Cyn D3 decreasing naturalg log traces(arro "Mirrored" natural-g G.L. 3798' 25-1B NG ? G.L. 4010' amma logprof 17-16B NG NG ws indicate amma) Cyn Coal bed Coal bedwithparting amma (NG)

A, D2, Smith D3

Carlson W Cook

all 30 Miles D2, D3 A G.L. 4085' 17-22B ault displacementas sho NG ? iles andcoal-bedcorrelations indrillholespenetratingthe G.L. 4528' 17-25B NG Smith D2, D3 Cyn, Can D3, Dietz3coalbed D2, Dietz2coalbed Anderson-Cany D23: Dietz2and3coalbeds(mer A, D2,D3: Kby A, Cyn 100 Feet 0 A Anderson coalbed , Kirbycoalbed ? yon coalbed D2, D3 Smith Anderson, Dietz2and3coalbeds(mer Cyn Cook A G.L. 4478' 17-30B on coalzone NG ? G.L. 4625' 9-25B NG Smith Cyn D2, D3 wn isdiagrammatic. Coal-bedterminology ged) A ? D2, D3 A Kby G.L. 4492' 9-32B NG Kby ged) Cyn A D2, D3 G.L. 4522' 9-30B NG A Kby D3 D2 Cyn North GP2' G.L. 4580' 10-4B NG Cross sectiondatumismeanseale Ele v ation (infeet)abo 3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200 4300 4400 4500 4600 4700 v e MSL v el (MSL) Big Horn Rosebud T. 3 S. County County Busby Rosebud Powder River County County 45¡ 30' 45¡ 30' D D1 T. 4 S. D2 ongue River ation (part) Northern Cheyenne Indian Reservation (part) T D3

D4 T. 5 S.

D5 North D6 w Indian Reserv

Kirby Birney C D7 C1 Cro D8 C2 T. 6 S. C3 B D9

B1 D10 B2 C4 B3 D11

ongue River T D12 T. 7 S. B4

C5 owder River D13 P B5 B6 C6 B7 T. 71/2 S. D14 B8 Big Horn County C7 T. 8 S. D15 A' Decker coal field A11 B9 C8 Quietus C9 B11 B10 C10 D16 Moorhead C11 A10 D'

A5 C12 T. 9 S. B12 A4 B13 A6 C' B14 A7 A8 Montana A2 A3 A Decker T. 10 S. A9 45¡ 00' T. 10 S. A1 R. 48 E. R. 49 E. R. 50 E. 45¡ 00' B' 106¡ 00' 107¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. Wyoming Explanation 0 5 10 miles Montana Decker coal field Identified fault (bar and ball on downthrown side)

Indian Reservations Inferred fault (bar and ball on downthrown side) Study Area Mine or coal lease A3 County line Tongue River Reservoir (lake) Drill hole Tongue River Reservoir (wetlands)

Figure PD-10. Index map showing the location of restored stratigraphic/structural cross sections A-A', B-B', C-C', and D-D', Decker coalfield, southeast Montana. West East A 54 Miles A' Elevation (in feet) A9 above MSL A11 A8 4200 4100 A5 A6 A10 4000 And A1 3900 Dtz A3 A7 And 3800 A4 Cyn Dtz 3700 And A2 ? And Cyn 3600 D23 Dtz Cyn 3500 And Cyn 3400 ormation (part)

er Member (part) And/D23 And Dsz

v And 3300 ertiary (part) D23 And Cyn T Dsz Cox Cyn D2 3200 D2 Cyn Cyn D3 D3 Cyn ort Union F

ongue Ri 3100 F T Cyn 3000 0 2900 100 2800 200 Feet Top of coal zone Explanation Anderson-Canyon coal zone Cross section datum is And, Anderson coal bed mean sea level (MSL) Wasatch Formation D23, Dietz 2 and 3 coal beds D2, Dietz 2 coal bed Lithology A4 Identified faults D3, Dietz 3 coal bed Dsz, Dietz subzone (coal beds undifferentiated) Sandstone Coal Downthrown Dtz, Dietz coal bed Upthrown Cox, Cox coal bed Mudstone block block Cyn, Canyon coal bed Drill Hole Rock (undifferentiated) Base of coal zone

Figure PD-11. Restored stratigraphic/structural cross section A-A' showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Displacement along faults as shown is diagrammatic. Fault location and relative displacement modified from Robinson and Culbertson (1984). North South B 23 Miles B' Elevation (in feet) above MSL B5 Eroded area 4700 (approximate) B3 B4 B6 4600 B1 B2 4500 D23 And Kby B7 4400 And D23 D23 4300 Cyn D23 B10 B13 4200 B8 B12 Cyn And B14 4100 B9 0 Cyn D23 B11 4000

ormation (part) And

er Member (part) 100 3900 v ertiary (part)

T 200 Feet D23 And 3800 3700 ort Union F ongue Ri A2 F T ? Cyn D23 3600 3500 ? Cyn And D23 3400 And/D23 Explanation Cyn 3300 Cyn Faults 3200 Top of coal zone Sandstone Coal Identified Inferred B9 3100 Mudstone Anderson-Canyon coal zone 3000 Downthrown Upthrown Downthrown And, Anderson coal bed Rock (undifferentiated) block block block Kby, Kirby coal bed Cross section datum is ? D23, Dietz 2 and 3 coal beds mean sea level (MSL) No record Drill Hole Cyn, Canyon coal bed Base of coal zone

Figure PD-12. Restored stratigraphic/structural cross section B-B' showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Displacement along faults as shown is diagrammatic. Fault location and relative displacement modified from Robinson and Culbertson (1984). The eroded area as shown is diagrammatic and approximately located. North South C 27 Miles C' Elevation (in feet) above MSL 4300 C1 C3 A8 4200 C2 C12 C4 4100 And/Dtz C5 C7 4000 And C6 UC C11 3900 Dtz C9 C10 UC And C8 3800 LC And ormation (part) LC Dtz And 3700 er Member (part) Dtz v Cyn Dtz ertiary (part) 3600 T Cyn Cyn Cyn 3500 ort Union F ongue Ri F T 3400 0 3300 100 Top of coal zone 3200 Explanation 200 Feet Anderson-Canyon coal zone Cross section datum is Wasatch Formation And, Anderson coal bed mean sea level (MSL) Dtz, Dietz coal bed Lithology Cyn, Canyon coal bed C4 UC, Upper Canyon coal bed Sandstone Coal LC, Lower Canyon coal bed Mudstone Base of coal zone Rock (undifferentiated) Drill Hole

Figure PD-13. Restored stratigraphic/structural cross section C-C' showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. North South D 26 Miles D' Elevation (in feet) above MSL D6 4300 Eroded area Eroded area D7 D12 A11 D1 (approximate) 4200 D2 (approximate) D5 D13 D14 D16 D3 Eroded area D8 D15 (approximate) Dsz D10 4100 D4 D9 D11 Cyn And And 4000 Cyn Dsz Dsz Cyn 3900 Cyn 3800 ormation (part)

er Member (part) 3700 v Eroded area ertiary (part)

T (approximate) 3600 3500 ort Union F ongue Ri F T Explanation 0 3400 Top of coal zone 100 Clinker Coal Anderson-Canyon coal zone 3300 Sandstone 200 Feet D8 And, Anderson coal bed Mudstone Dsz, Dietz coal subzone Cyn, Canyon coal bed Cross section datum is Rock (undifferentiated) mean sea level (MSL) Drill Hole Base of coal zone

Figure PD-14. Restored stratigraphic/structural cross section D-D' showing coal-bed correlations in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Eroded areas as shown are diagrammatic and approximately located. Big Horn Custer National Forest T. 3 S. County boundary (part) Busby Rosebud Powder River County County 45¡ 30' 45¡ 30'

T. 4 S. ongue River T Decker coalfield Northern Cheyenne Indian Reservation (part) boundary

Decker coalfield T. 5 S. boundary North ation (part)

Kirby Birney

T. 6 S.

w Indian Reserv Custer National Forest boundary (part) Cro ongue River T. 7 S. T

owder River P

T. 71/2 S.

T. 8 S. Big Horn

Spring Creek County mine Quietus Moorhead

East Decker West mine T. 9 S. Decker mine Montana Decker CXR T. 10 S. 45¡ 00' 45¡ 00' T. 10 S. R. 48 E. R. 49 E. R. 50 E. 106¡ 00' 107¡ 00' R. 39 E. R. 40 E. R. 41 E. R. 42 E. R. 43 E. R. 44 E. R. 45 E. R. 46 E. R. 47 E. Wyoming 0 5 10 Miles Explanation Montana Total, net coal thickness Clinker (Anderson, Dietz 2, Dietz 3, and/or Dietz coal beds)—Also present County line within the Crow and Northern Cheyenne Indian Reservations 2.5-10 ft 60-80 ft Indian Reservations 10-20 ft 80-100 ft Study Area Mine or coal lease Custer National Forest boundary 20-40 ft > 100 ft (maximum = 111 ft) (CXR, CX Ranch lease area) 40-60 ft Tongue River Reservoir and wetlands

Anderson-Canyon coal zone outliers Canyon coal bed--Outcrop or contact with clinker (burn line) (Canyon coal bed—unassessed areas)

Drill hole or outcrop data point

Figure PD-15. Map showing the total, net coal thickness in the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Custer National Forest Big Horn T. 3 S. boundary (part) County Busby Rosebud Powder River County County 45¡ 30' 45¡ 30' Canyon coal bed T. 4 S. ongue River Northern Cheyenne Indian Reservation (part) T (outcrop/clinker contact)

T. 5 S. North ation (part)

Kirby Birney

T. 6 S.

w Indian Reserv Custer National Forest boundary (part) Cro ongue River T. 7 S. T

owder River P

T. 71/2 S. Anderson coal bed outcrop/clinker contact T. 8 S.

Spring Creek mine Quietus Moorhead

Overburden thickness Indian Reservations Exposed coal zone—Includes Mine or coal lease Study Area areas of clinker (CXR, CX Ranch lease area) Fault (bar and ball on downthrown side) County line 0-100 ft Tongue River Reservoir and wetlands Identified Inferred 100-200 ft Anderson-Canyon coal zone outliers 200-500 ft (Canyon coal bed—unassessed areas) Custer National Forest boundary

500-1000 ft Anderson coal bed—Outcrop or contact with clinker (burn line) > 1,000 ft Canyon coal bed—Outcrop or contact with clinker

Figure PD-16. Map showing the thickness of overburden above the Anderson-Canyon coal zone, Decker coalfield, southeast Montana. Fault location and relative displacement modified from Robinson and Culbertson (1984). Table PD-1. Total, net coal resources in the Anderson-Canyon coal zone in the Decker coalfield, Montana, reported by county, overburden thickness, total, net coal thickness, and reliability categories. Resources are shown in millions of short tons (MST) and with two significant figures. Zeros (0) indicate that no coal resources were calculated within those categories. The table does not include resources in mine or coal lease areas, areas underlying clinker of the Anderson and/or Dietz coal beds, and areas underlying the Tongue River Reservoir and associated wetlands. Columns may not sum because of independent rounding

County Minimum Total, net Reliability categories (distance from data point) overburden 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 Exposed zone 2.5-5 ft. 0 1.3 0.86 0 2.2 5 Ð10 ft. 2.9 15 20 0 38 10-20 ft 20 47 140 0 210 20-30 ft 55 170 260 1.8 490 30-40 ft 40 230 310 4.6 590 40-50 ft 55 260 400 13 730 50-100 ft 92 630 1,100 1.2 1,800 Exposed zone total 260 1,400 2,200 21 3,900 0-100 ft. 10-20 ft 2.4 2.0 26 0 30 20-30 ft 11 50 41 0 100 30-40 ft 6.7 72 130 0 210 40-50 ft 10 210 360 0 570 50.100 ft 320 1,300 1,900 0 3,500 100-150 ft 0 3.0 6.7 0 9.7 0-100 ft. total 350 1600 2500 0 4,400 100-200 ft 10-20 ft 0 0 2.3 0 2.3 20-30 ft 0 14 16 0 29 30-40 ft 0 26 53 0 79 40-50 ft 5.9 120 290 0 420 50-100 ft 330 1,800 2,000 0 4,200 100.0-150 ft 16 92 46 0 150 100-200 ft total 360 2,100 2,500 0 4,900 Table PD-1. Total, net coal resources, Anderson-Canyon coal zone—continued

County Minimum Total, net Reliability categories (distance from data point) overburden 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 200-300 ft 30-40 ft 0 5.0 32 0 37 40-50 ft 7.3 59 290 3.3 360 50-100 ft 400 1,900 3,100 0 5,400 100-150ft 68 160 81 0 310 200-300 ft total 470 2,100 3,500 3.3 6,100 300-400 ft 30-40 ft 0 2.7 12 0 14 40-50 ft 2.0 29 210 32 270 50-100 ft 280 1,700 2,600 0 4,600 100-150 ft 17 130 120 0 270 300-400 ft total 300 1,900 2,900 32 5,100 400-500 ft 40-50 ft 0 0 40 50 89 50-100 ft 120 880 1,600 0 2,600 100-150 ft 1.3 180 170 0 360 400-500 ft total 120 1,100 1,800 50 3,100 500-1000 ft 40-50 ft 0 0 0.74 0 0.74 50-100 ft 54 270 1,800 71 2,200 100-150 ft 22 100 86 0 210 500-1000 ft total 76 380 1,900 71 2,400 1000-1500 ft 50-100 ft 0 0 2.3 10 12 1000-1500 ft total 0 0 2.3 10 12 BIG HORN total 1,900 11,000 17,000 190 30,000 POWDER RIVER Exposed zone 2.5-5 ft 2.2 11 39 0.23 53 5-10 ft 11 54 130 4.4 200 10-20 ft 65 330 530 23 950 20-30 ft 100 420 480 86 1,100 30-40 ft 64 320 500 100 980 40-50 ft 20 140 430 55 640 50-100 ft 14 110 530 24 680 Exposed zone total 280 1400 2,600 290 4,600 Table PD-1. Total, net coal resources, Anderson-Canyon coal zone—continued

County Minimum Total, net Reliability categories (distance from data point) overburden coal thickness Measured Indicated Inferred Hypothetical Total thickness (<1/4 mi) (1/4-3/4 mi) (3/4-3 mi) (>3 mi) (MST) POWDER RIVER 0-100 ft 2.5-5 ft 0 0.22 1.0 0 1.3 5-10 ft 0 0.091 3.9 0 4.0 10-20 ft 0 24 33 0 57 20-30 ft 0 21 100 4.3 130 30-40 ft 10 82 220 36 340 40-50 ft 8.7 83 370 34 490 50-100 ft 70 370 710 45 1,200 0-100 ft total 88 580 1,400 120 2,200 100-200 ft 2.5-5 ft 0 0 0.16 0 0.16 5-10 ft 0 0 2.8 0 2.8 10-20 ft 0 3.2 4.1 0 7.3 20-30 ft 0 1.5 11 2.4 15 30-40 ft 0 3.4 88 6.9 99 40-50 ft 0.81 43 270 1.5 320 50-100 ft 53 230 770 27 1,100 100-200 ft total 54 290 1,100 38 1,500 200-300 ft 5-10 ft 0 0 0.33 0 0.33 10-20 ft 0 0 0.24 0 0.24 30-40 ft 0 0 49 0.68 49 40-50 ft 4.5 55 250 0 310 50-100 ft 28 170 760 23 980 200-300 ft total 33 230 1,100 24 1,300 300-400 ft 30-40 ft 0 0 11 0 11 40-50 ft 18 77 180 0 280 50-100 ft 7.5 110 700 0 810 300-400 ft total 26 190 890 0 1,100 400-500 ft 40-50 ft 9.7 21 24 0 55 50-100 ft 14 100 370 0 490 400-500 ft total 24 130 400 0 540 Table PD-1. Total, net coal resources, Anderson-Canyon coal zone—continued

County Minimum Total, net Reliability categories (distance from data point) overburden coal thickness Measured Indicated Inferred Hypothetical Total thickness (<1/4 mi) (1/4-3/4 mi) (3/4-3 mi) (>3 mi) (MST) POWDER RIVER 500-1000 ft 50-100 ft 1.0 9.7 3.9 0 15 500-1000 ft total 1.0 9.7 3.9 0 15 POWDER RIVER total 510 2,800 7,600 470 11,000 ROSEBUD Exposed zone 2.5-5 ft. 3.6 11 17 0 32 5-10 ft. 29 99 90 0 220 10-20 ft 41 210 340 14 610 20-30 ft 23 170 210 0 410 30-40 ft 16 55 130 0 200 40-50 ft 1.9 58 90 0 150 50-100 ft 0.81 57 290 0 350 Exposed zone total 110 670 1,200 14 2,000 0-100 ft 10-20 ft 0 2.6 8.9 0 11 20-30.0 ft 2.5 2.8 20 0 25 30-40 ft 2.2 7.7 24 0 34 40-50 ft 0 31 99 0 130 50-100 ft 46 260 640 0 950 0-100 ft total 51 300 790 0 1,100 100-200 ft 10-20 ft 0 0 0.36 0 0.36 30-40 ft 0 0 0.78 0 0.78 40-50 ft 0 0 12 0 12 50-100 ft 12 84 150 0 250 100-200 ft total 12 84 170 0 260 200-300 ft 40-50 ft 0 0 0.87 0 0.87 50-100 ft 2.3 18 13 0 33 200-300 ft total 2.3 18 13 0 34 ROSEBUD total 180 1,100 2,100 14 3,400 Grand total (MST) 2,600 14,000 27,000 680 45,000*

* Of this total, approximately 3,500 MST are estimated to be present within Custer National Forest Table PD-2. Total, net coal resources in the Anderson-Canyon coal zone reported for 7.5’ quadrangles in the Decker coalfield, Montana. Coal resources are reported in millions of short tons (MST) rounded to two significant figures. Zeros (0) indicate that no coal resources were calculated within those categories. Overburden categories based on Wood and others (1983), and overburden is measured from the ground surface to the top of the uppermost coal bed (minimum overburden). Reported resources do not include areas within active coal mines or coal leases, areas underlying clinker of the Anderson and/or Dietz coal beds, or areas overlain by the Tongue River Reservoir and associated wetlands. Columns may not sum because of independent rounding

7.5-minute quadrangle map Minimum overburden Total Exposed zone 0.0-100 ft 100-200 ft 200-500 ft 500-1000 ft > 1000 ft (MST) Acme 18 10 13 140 1.8 0 180 Bar N Draw 0 0 7.9 170 0 0 170 Bar V Ranch 43 76 44 170 0 0 340 Bar V Ranch NE 10 200 210 280 0 0 710 Bear Creek School 320 500 420 1,500 11 0 2,800 Birney 91 0 0 0 0 0 91 Birney Day School 4.2 0 0 0 0 0 4.2 Birney SW 160 3.2 0.36 0 0 0 170 Black Draw 20 42 9 0 0 0 71 Bloom Creek 130 0 0 0 0 0 130 Box Elder Draw 0 0 1.8 130 0 0 130 Bradshaw Creek 580 340 170 62 0 0 1,200 Browns Mountain 81 0.81 0 0 0 0 82 Cabin Creek NE 38 37 53 47 0 0 180 Cabin Creek NW 0 0 2.5 150 3.2 0 160 Cedar Canyon 0 0 0 7.7 120 12 140 Clubfoot Creek 2.3 0 0 0 0 0 2.3 Cook Creek Butte 97 0 0 0 0.0 0 97 Decker 140 66 160 1,400 30 0 1,800 Forks Ranch 250 720 680 1,200 13 0 2,900 Fort Howes 160 0 0 0 0 0 160 Goodspeed Butte 55 0 0 0 0 0 55 Green Creek 74 0 0 0 0 0 74 Table PD-2. Total, net coal resources by 7.5’ quadrangle, Anderson-Canyon coal zone—continued

7.5-minute Quadrangle map Minimum Overburden Total Exposed 0.0-100 ft. 100-200 ft. 200-500 ft. 500-1000 ft. > 1000 ft. (MST) Zone Half Moon Hill 670 670 400 470 0 0 2,200 Hamilton Draw 630 720 600 600 0 0 2,600 Hodsdon Flats 220 0 0 0 0 0 220 Holmes Ranch 38 170 250 1,700 720 0 2,900 King Mountain 29 0 0 0 0 0 29 Kirby 730 510 350 190 0 0 1,800 Lacey Gulch 480 410 260 240 2.2 0 1,400 Monarch 5.8 0 0 0 0 0 5.8 Moorhead 4.3 0 0 0 0 0 4.3 OTO Ranch 0 0 0 16 120 0 140 Otter 480 71 47 25 0 0 620 Pearl School 58 260 600 3,400 310 0 4,600 Phillips Butte 540 78 11 0.57 0 0 630 Pine Butte School 26 200 270 1,800 1,100 0 3,400 Poker Jim Butte 480 600 230 34 0 0 1,300 Quietus 57 360 890 2,300 0 0 3,600 Reanus Cone 260 28 3.9 0 0 0 290 Roundup Draw 0 0 0 132.44 0.74 0 130 Sayle 770 330 100 6.3 0 0 1,200 Sayle Hall 550 650 590 870 0 0 2,700 Sonnette 78 0 0 0 0 0 78 Spring Creek Ranch 32 0 0 0 0 0 32 Spring Gulch 620 160 91 62 0 0 930 Stroud Creek 330 340 160 130 0 0 970 Taintor Desert 670 240 64 7.6 0 0 980 Threemile Buttes 34 0 0 0 0 0 34 Tongue River Dam 340 13 0 0 0 0 350 Grand Total (MST) 10,000 7,800 6,700 17,000 2,500 12 45,000*

* Of this total, approximately 3,500 MST are estimated to be present within Custer National Forest Table PD-3. Total, net coal resources in the Anderson-Canyon coal zone, Decker coalfield, Montana, reported by county and by Federal coal and surface ownership. Coal resources in areas underlying clinker of the Anderson and/or Dietz coal beds, areas within active coal mines or leases, and areas underlying the Tongue River Reservoir and associated wetlands are not included in the resource total. Coal resources are reported in millions of short tons (MST) with two significant figures. Column will not sum due to independent rounding

County Federal Ownership Total (MST) BIG HORN No Federal coal or Federal surface ownership 3,100 No Federal coal, but Federal surface ownership 5.8 Federal coal, but no Federal surface ownership 25,000 Federal coal and Federal surface ownership 2,000 BIG HORN Total 30,000 POWDER RIVER No Federal coal or Federal surface ownership 700 No Federal coal, but Federal surface ownership 1.5 Federal coal, but no Federal surface ownership 7,200 Federal coal and Federal surface ownership 3,400 POWDER RIVER total 11,000 ROSEBUD No Federal coal or Federal surface ownership 150 No Federal coal, but Federal surface ownership 1.1 Federal coal, but no Federal surface ownership 1,500 Federal coal and Federal surface ownership 1,800 ROSEBUD total 3,400 Grand total (MST) 45,000*

* Of this total, approximately 3,500 MST are estimated to be present within Custer National Forest Table PD-4. Computations of confidence intervals within reliability categories for estimated total coal resources in the Anderson-Canyon coal zone in the Decker coalfield, Montana. Volume refers to the calculated resource in millions of short tons (MST). NA, not applicable

Parameter Reliability Category Entire Measured Indicated Inferred Hypothetical Area Area (in square meters) 133,426,743 704,761,677 1,340,332,461 42,411,593 2,220,932,473 Percent of area 6 32 60 2 100 Acres 32,970 174,150 331,203 10,480 548,804 SD (Standard deviation (in ft) from 5.712 7.730 13.093 16.089 NA semivariogram model) Acre feet (Acres x SD) 188,333 1,346,167 4,336,582 168,610 NA Volume standard deviation (MST) 21 192 1,794 298 2,305 Pseudo n (Minimum number of points in the area) 262 154 18 1 NA

Table PD-5. Volume and estimates of uncertainty for estimated total coal resources in the Anderson-Canyon coal zone, Decker coalfield, Montana, with measurement error. Resource calculations are in millions of short tons (MST) with four significant figures. Volume refers to the calculated resource in millions of short tons (MST).

Parameter Reliability Category Entire Measured Indicated Inferred Hypothetical Area Volume (MST) 2,621 14,400 27,030 675 44,730 Lower 90% confidence bound (MST) 2,587 14,090 24,080 184 40,940 Upper 90% confidence bound (MST) 2,655 14,720 29,980 1,166 48,520