Summary:
The mine extracts thermal coal from the Anderson and Canyon Seams, with up to 44 and 36 feet, respectively, in thickness.
General Geology and Coal Resources
Stratigraphic units include, in descending order, recent (Holocene age) alluvial and eolian deposits, the Eocene age Wasatch Formation (the overburden), and the Paleocene age Fort Union Formation (which contains the target coal beds). [Dec 2008, Final Environmental Impact Statement, p. 118-120]
Surficial deposits within the general analysis area include alluvial and eolian deposits, clinker, and weathered Wasatch and Fort Union Formations. Although clinker is present in the general analysis area, the tract has no appreciable amounts of clinker. There are thin alluvial deposits along ephemeral streams. These alluvial deposits typically consist primarily of poorly to well-sorted, irregularly bedded to laminated, unconsolidated sand, silt, and clay with minor intervals of fine gravel. The valley floors of Horse Creek, Spring Creek and Antelope Creek contain appreciable amounts of alluvium both in width and depth. The alluvial deposits in Horse Creek, Spring Creek and Antelope Creek contain much more coarse-grained material (sands and gravels) than the ephemeral tributaries that drain most of the general analysis area. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
The Eocene Wasatch Formation forms most of the overburden overlying the mineable coal seams in the general analysis area. It consists of interbedded lenticular sandstones, siltstones, shales, and thin discontinuous coals. There is no distinct boundary between the Wasatch Formation and the underlying Paleocene Fort Union Formation. From a practical standpoint, however, the top of the mineable coal zone is considered as the contact between the two formations. Overburden thickness averages 260 feet in the Bureau of Land Management (BLM) study area and ranges from around 20 ft to more than 460 ft. The overburden is relatively thin in the vicinity of the major channels within the tract and increases in thickness away from the channel bottoms. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
The Fort Union Formation consists primarily of shales, mudstones, siltstones, lenticular sandstones, and coal. It is divided into three members: Tongue River (which contains the mineable coal seams), Lebo, and Tullock, in descending order. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
The Tongue River Member of the Fort Union Formation consists of interbedded claystone, silty shale, carbonaceous shale, and coal, with lesser amounts of finegrained sandstone and siltstone. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
The nomenclature of the mineable coal seams in the Fort Union Formation varies from mine operator to mine operator. The U.S. Geological Survey (Flores et al. 1999) refers to the thick mineable coals in the Gillette coal field as the WyodakAnderson coal zone of the Tongue River Member of the Fort Union Formation. Locally these beds are referred to as Wyodak, Wyodak-Anderson, Anderson, and Canyon. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
There are four mineable seams in the West Antelope II Lease By Application tract (referred to by the operator as the Anderson, Lower Anderson, Canyon/Upper Canyon, and Lower Canyon). The total coal thickness ranges from 15 to 86 ft. Interburden between the coal seams varies from 5 to around 115 ft. The total overburden thickness (including interburden where present) ranges from about 20 ft to approximately 550 ft. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
The Fort Union coal seams are subbituminous and are generally low-sulfur, lowash coals. Typically, the coal being mined has a higher heating value and lower sulfur content south of Gillette than north of Gillette. The ash content in the coal seams is expected to vary from 3.5 to 8 percent, the sulfur content from 0.15 to 0.4 percent, and the moisture content from 23 to 28 percent. [Dec 2008, Final Environmental Impact Statement, p. 118-120]
The Lebo and Tullock Members of the Fort Union Formation underlie the Tongue River Member. They consist primarily of sandstone, siltstone, mudstone, shale, and coal. In general, the Tullock Member contains more sand than the Lebo Shale Member. [Dec 2008, Final Environmental Impact Statement, p. 118-120]