2018-11-05_REVISION - C1981044 - Laserfiche WebLink

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Williams Fork Mines Prepared by: R. Reilley M.S. GISP C1981044 5 November 2018 5 of which over one-third is snowfall (averaging 66.5 inches/year). Average annual precipitation at the Trapper Mine, next to the Williams Fork Mines, is 16.7 inches. The mean annual temperature in Craig is 43°F, with recorded extremes of -45°F and +100°F. Winds are predominantly from the west, but are locally modified by topographic features. The growing season for the area around Craig averages 77 days. Physiographic Setting. The No. 5 and No. 5A portals of the Williams Fork Mines are driven into the steep- sided walls of the valley that the Williams Fork River has cut through at the west end of the Williams Fork Mountains. The mines’ facilities area is in the bottom of the valley. The elevation of the valley bottom is 6,150 ft. From the valley bottom, the permit area extends up to the crestal ridge of the Williams Fork Mountains. The crest of the Williams Fork Mountains forms a ridge extending about 30 miles east-west at elevations between 7,400 and 7,800 feet. The Williams Fork River crosses through the mine permit area and flows into the Yampa River in the northwest corner of the permit area. The Yampa River is one of the largest tributaries of the upper Colorado River system. Stratigraphic Setting. Bedrock at the ground surface in the Williams Fork Mines permit area is a sequence of sandstones, siltstones, shales, and coals that are part of the Cretaceous-age Williams Fork Formation. The Williams Fork Formation is part of the regionally extensive Mesa Verde Group. The Williams Fork Formation crops out along a six-mile wide belt that extends along the entire length of the Williams Fork Mountains. The Williams Fork Formation is subdivided into the following three units (in ascending stratigraphic order): the lower Williams Fork, the Twentymile sandstone, and the upper Williams Fork. Approximate thicknesses are: lower Williams Fork, 840 ft.; Twentymile sandstone, 120 ft.; and upper Williams Fork, 850 ft. Unconsolidated alluvial deposits of Quaternary age fill stream drainages in the permit area and surrounding areas. The alluvium is thickest in the Yampa and Williams Fork River valleys. Coal Seam Stratigraphy. The coal seams formally mined at the Williams Fork Mines are in the lower and upper units of the Williams Fork Formation. Each seam is less than 10 feet thick. The No. 9 Mine removed coal from the P seam in the upper Williams Fork Formation. The No. 5 Mine removed coal from the F seam in the lower Williams Fork. The No. 6 Mine portal was used to access and mine coal from the E seam in the lower Williams Fork. Geologic Structural Setting. The Williams Fork Mines are situated on the northeast flank of the northwest plunging Moffat Anticline. The Moffat Anticline is part of the larger-scale Axial uplift that extends northwestward across northwest Colorado to the Uinta Mountains in northeast Utah. The northeast limb of the Moffat Anticline dips northward into the Big Bottom syncline. Although only small-displacement gravity faults have been found in the permit area, the large displacement Moffat thrust fault probably lies several thousand feet beneath the ground surface of the permit area. The surface trace of the Moffat thrust is two miles southwest of the permit area. Local Coal Mining History. Previous mining within the Williams Fork Mines permit area included: the Fields Mine which operated in the E seam during the 1930s; the Wise Mine and the Wise Hill No. 2 which operated in the C seam (below the E seam) in the 1940s and 1950s; the Leo White and Baker Mines which operated in the Hart seam (between the F seam and Twentymile sandstone); and the Wise Hill Nos. 3 and 4, which operated in the Hart seam during the 1970s. Surface Water Features. Drainages within and adjacent to the permit area (on the north-facing slope) are ephemeral and generally extend south to north down the slope in a dendritic pattern. These drainages flow primarily in response to snowmelt or heavy rains. Flows in these drainages that do not infiltrate into the ground or evaporate eventually enter the Williams Fork River, and then flow the short distance to the confluence with the Yampa River in the northwest corner of the permit area.