Summary:
Four active mine areas at the Rosebud Mine operate: Area A; Area B; Area C; and Area F.
The Rosebud Mine is located in the northwestern portion of the Powder River structural basin, a broad northeast-trending synclinal structural basin in eastern Wyoming and southeastern Montana bound on three sides by mountain uplifts. The Paleocene Fort Union Formation is the predominant bedrock unit in the vicinity of the Rosebud Mine and consists of gently dipping (less than a few degrees) sedimentary rocks. Coal is within the Tongue River Member of the Fort Union Formation. The highest coal bed in the analysis area stratigraphic sequence is the Rosebud Coal bed, which averages 22.6 feet thick over the permit area. The next coal seam located below the Rosebud Coal bed is the McKay Coal bed, which averages 7.9 feet thick.
Rosebud Mine Area B lies within the East Fork Armells Creek and Middle Rosebud Creek drainage basins. In the Colstrip area, the Fort Union Formation is 445 feet thick and thickens to the south to a maximum of 2,125 feet. Geologic formations that are exposed at the surface are limited to the Tongue River Member of the Paleocene Fort Union Formation, and Quaternary Alluvium. Quaternary-age alluvium and colluvium overlie the Tongue River Member locally, mostly along drainageways. Within the analysis area, relatively thin deposits of silty clay and gravelly sand comprise the Quaternary alluvial fill occurring within portions of Richard Coulee. Thin unmapped unconsolidated alluvial deposits are also present in the Lee and Rape Coulee drainages and associated tributaries. Sandstone, claystone, interbedded claystone and sandstone, and subbituminous coal beds of the Tongue River Member comprise the remainder of the stratigraphic sequence in the analysis area. Faults in the area typically have small offsets.
The highest coal bed in the analysis area stratigraphic sequence is the Rosebud Coal bed. This bed averages 22.6 feet thick over the permit area. Typically, the first 1-foot layer of the Rosebud deposit is high in sulfur content, generally represented by pyrite and marcasite. The lowest 0.8-foot portion of the Rosebud Coal bed also has a high sulfur content represented by the occurrence of pyrite.
Within the analysis area, the average overburden thickness ranges from a few feet to more than 100 feet.
The next coal seam located below the Rosebud Coal bed is the McKay Coal bed, which averages 7.9 feet thick. Between the two major coal beds is the interburden, which is composed of similar lithologies to the overburden, except that it does not contain clinker. The interburden material consists of sandstone and claystone and ranges in thickness from a few feet to more than 100 feet. The sandstone and claystone located below the McKay Coal is referred to as the underburden or Sub-McKay and includes the remainder of the Tongue River Member below the McKay Coal. The lithologies of this group are similar to the overburden and interburden, except what may be more laterally continuous sandstones.
The weighted-average proximate coal quality (as-received basis) of the Rosebud Coal bed in the Project area is 8,378 British thermal units (Btus) per pound, 0.91 percent sulfur, 25.44 percent moisture, a volatility of 28.95 percent, a fixed carbon of 35.10 percent, and 10.41 percent ash. The coal quality of the McKay Coal bed is inferior to that of the Rosebud Coal bed due to a higher sulfur content and higher iron and sodium content in the ash.