Summary:
The strata in the Dry Fork Mine (DFM) permit area generally show northwest to north-northwest strikes and gentle westward dips. The synclinal axis trends north to northwest near the western edge of the PRB, adjacent to the Casper Arch and the Big Horn Mountains. Gentle folds occur on the eastern and northern flanks, while faults and locally overturned beds are characteristic of the much steeper western and southern flanks. The northern and eastern areas of the basin have gentle dips that seldom exceed a few degrees. Regional dips in the Gillette area are from 0.5 degrees in the surface sediments to 3 degrees in the deeper sediments toward the southwest. No major faults have been mapped in the Gillette area (Hodson et al. 1973).
The general analysis area (northern group of mines - Dry Fork, Rawhide, Buckskin, Eagle Butte, Synthetic Fuels, and Wyodak, in the part of the Northern Great Plains that includes most of northeastern Wyoming) contains the following stratigraphic units or layers (in descending order from the surface): Quaternary (most recent) deposits, the Eocene Wasatch Formation, and the Paleocene Fort Union Formation.
Surficial Geology – Quaternary deposits in the general analysis area consist of alluvium, sheetwash, colluvium, and lacustrine deposits. Quaternary deposits represent a minor component of the surficial geology within the DFM area. Exposed and weathered Wasatch Formation residuum comprises the majority of the surficial geology within the Project Area. The Wasatch Formation residuum flanks a ridge of exposed Wasatch bedrock that occupies the central portion of the general analysis area.
Wasatch Formation - The Eocene Wasatch Formation comprises the majority of the overburden, which consists mainly of interfingering lenses of claystone, siltstone, and sandstone. The overburden contains clinker (variously called Wyoming porcelanite or scoria), coal stringers and carbonaceous shales.
Fort Union Formation - The Paleocene Fort Union Formation consists primarily of siltstones, mudstones, claystones, shales, lenticular sands/sandstones, and coal seams. The Fort Union Formation is divided into three members, in descending order, the Tongue River (which contains the mineable coal seams), the Lebo, and the Tullock (Flores et al. 1999). The coal to be mined in this area is within the Wyodak-Anderson coal seam, which is the uppermost unit of the Tongue River Member of the Paleocene Fort Union Formation. In the mine permit area, a claystone and carbonaceous coal lens separate the Wyodak-Anderson seam into the upper Anderson coal seam and the lower Canyon coal seam. For this EA, the coal to be mined will be referred to as the Wyodak-Anderson seam. The coal is consistently underlain by shale, claystone, or occasionally siltstone throughout the Dry Fork mine. The Wyodak coal seam is nearly flat-lying with minor variations and dip reversals, which very likely reflect the depositional environment (structural highs and lows) and/or differential compaction, rather than major tectonic deformation. Underlying the Fort Union Formation are the Cretaceous age Lance Formation, Fox Hills Sandstone, and Pierre Shale. Occasionally, mine water supply wells are constructed in the Lance-Fox Hills aquifer; otherwise, no mine related disturbance extends below the Fox Hills Sandstone.