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Location: 110 km E from Billings, Montana, United States
698 Upper Sarpy Creek RoadP.O. Box 449HardinMontana, United States59034
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The proposed development area is located in the Powder River Basin (PRB), a part of the Northern Great Plains that includes most of northeastern Wyoming and a smaller portion of southeastern Montana. The Absaloka Mine and the South Extension are located near the northwestern edge of the PRB, in an area consisting primarily of dissected rolling hills, plateaus, and ridges of moderate to low relief that formed in the near-flat lying sedimentary strata. Resistant sandstone and clinker beds cap most of the upland areas and form steep cliff escarpments and isolated knobs. Elevations range from about 3,500 to 3,790 feet above sea level, slopes range from nearly flat on the valley bottoms and ridge tops to around 40 and 50 percent on the flanks of the surrounding ridges and hilltops, and approximately 61 percent of the surface has a slope of 10 percent or less.The three lowermost coal seams of the Tongue River Member of the Fort Union Formation are the Rosebud, McKay, and the Robinson. In the Absaloka Mine area, all younger, stratigraphically higher coal seams have been removed by erosion. In parts of the current mine area, the Rosebud and McKay seams are joined into a single seam referred to as the Rosebud-McKay, which averages 32 feet in thickness. Mining within the proposed development area would be limited to the Rosebud and McKay coal seams. Where not affected by erosion or oxidation, the Rosebud and McKay seams are relatively consistent in thickness throughout the proposed development area. The Rosebud coal seam thickness ranges up to 22.3 feet and averages 17.9 feet. The McKay coal seam thickness ranges up to 16.6 feet and averages 12.5 feet. All or parts of the Rosebud and McKay coal seams have been removed by erosion in the Middle Fork Sarpy Creek drainage bottom (Figure ES-3). Recent alluvial and/or colluvial deposits have replaced the coal in these areas. This feature effectively separates the proposed development area into western and eastern coal reserve blocks. A claystone parting, ranging from less than 1 foot to more than 30 feet and averaging 11.7 feet thick, separates the Rosebud and McKay seams throughout the proposed development area. Mining would remove an average of approximately 70 feet of overburden under the Proposed Action. The Robinson seam, which averages just over 20 feet in thickness, would not be mined in the proposed development area. The Robinson seam lies below and is separated from the McKay seam by approximately 80 to 100 feet of interburden. The Robinson seam was mined in the early years of the mine’s operation, but is no longer mined primarily due to customer concerns regarding poor combustion characteristics.
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