Summary:
The Trapper Mine is located in the Yampa coal field that is composed of coal-bearing rocks that are Upper Cretaceous in age. The coal-bearing rocks mainly outcrop south of the Yampa River in northwest Colorado. The Yampa coal field extends east to west about 50 miles (80.5 km) and is about 40 miles (64.4 km) north to south.
The surface bedrock in the Trapper Mine Project Area is mainly the Upper Cretaceous Williams Fork Formation, which is part of the Mesaverde Group, a regional unit that contains a large coal resource in northwest Colorado (Brownfield and Johnson 2008). The Williams Fork Formation consists of interbedded sandstones, siltstones, shale, and coals and crops out along a six-mile-wide (9.7-km) belt that extends along the entire length of the Williams Fork Mountains (Tweto 1976). The Williams Fork Formation is underlain by the Illes Formation, also part of the Mesaverde Group. The Illes Formation is lithologically similar to the Williams Fork Formation consisting of sandstones, mudstones, carbonaceous shale, and coal (Brownfield and Johnson 2008). The Lewis Shale, which is stratigraphically higher than the Mesaverde Group, outcrops in the northern extremity of the SMCRA permit area. The Lewis Shale also is Upper Cretaceous in age and is composed of uniform dark-gray marine shale, but also contains thin sandstones (Johnson 1987; Tweto 1976). Surficial deposits consist of alluvium, residuum derived from the Williams Fork Formation and the Lewis Shale, and landslide deposits (Madole 1989). Quaternary alluvial deposits composed of sand, silt, clay, and gravel are present in the stream drainages in the permit area and surrounding areas, but alluvium is thickest in the Yampa River and Williams Fork River valleys (CDRMS 2013). Residuum is composed of sand, silt, clay, gravel, and boulders derived from the underlying bedrock.
The landslide deposits that are part of the surficial deposits are either rotational slides or earthflows and involve the Williams Fork Formation (Madole 1989). A major landslide occurred at the mine in October 2006 (CDRMS 2013). Prior to the slide, there was heavy precipitation that facilitated the movement of an estimated 35 million cubic yards of material with original in-place dimensions of 250 acres and 100 feet (30.5 meters) deep (Buchsbaum 2011). The material was largely contained in an active pit, but the slide made it necessary for the mine to alter its mine plan.
Structurally, the mine is bounded on the west, north, and northeast by three major folds. From west to east, the Williams Fork Anticline, Big Bottom Syncline, and Breeze Mountain or Buck Peak Anticline (Brownfield and Johnson 2008). At the mine, the beds of the Williams Fork Formation dip generally to the north at an altitude of 14 to 16 degrees. The Project Area lies on the south flank of the Sand Wash Basin, a structural basin which is the southeast extension of the Greater Green River Basin (Horn and Richardson 1959; TMI 1981 et seq.).
A large unnamed down-to-the-north normal fault lies about two miles (3.2 km) south of the SMCRA permit area in T9N, R90W (Brownfield and Johnson 2008; Tweto 1976). The fault as mapped by Tweto (1976) is about four miles (6.4 km) long and is not classified as active. No active faults have been identified in the area (USGS and Colorado Geological Survey 2006). There is a low probability of strong ground motion if a maximum credible earthquake were to occur in the vicinity. Horizontal ground motions are expected to be 18 to 20 percent of the acceleration gravity with a 2 percent probability of exceedance in 50 years (Petersen et al. 2015).
The upper Williams Fork member contains the coals that are mined at the Trapper Mine and are designated (from lowest to highest): R, Q, M, L, K, I, and H. The coal seams that are mined at the Trapper Mine have consisted of a mixture of subbituminous A, B, and C, and high-volatile bituminous C coals. Average, as-received, values of ash yield are 7.05 percent and sulfur content is 0.40 percent, with a heat value of 9,931 British thermal units per pound (Johnson et al. 2000).