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~_~ The Williams Fork formation has been divided into three units, an upper and <br />lower unit separated by the Twenty Pfile Sandstone. The lower and upper <br />units have similar lithologies; however, only the upper unit will be <br />surface mined. The Twenty Mile Sandstone unit is comprised of a thick <br />main sandstone bed, interbedded sandy shale, shale, coal, another massive <br />sandstone bed, a series of gray and sandy shales and an oyster coquina bed. <br />The Upper Williams Fork overlies the Twenty Mile Sandstone member and is <br />composed primarily of complex interbedded sandstone, siltstone, sandy shale, <br />and shale strata with some coal seams. These strata were deposited in a <br />deltaic environment, The unit thickens from east to west. The thickness <br />reaches about 850 feet in the vicinity of the Trapper Mine. <br />The Lewis Shale rests conformably upon the Williams Fork formation to the <br />north of the mining limits. .This formation will not be disturbed by mining. <br />The Yampa River alluvium varies in width from 1.5 miles in non-resistant <br />material to approximately the width of the river when crossing resistant <br />strata. The widest area associated with the river crossing the Lewis Shale <br />is in the area of the Big Bottom syncline. Drill cuttings in this area <br />indicate the thickness of the alluvium to be 10 to 12 feet and consist of <br />gravel and sands. The alluvial systems in the permit area are shallow and <br />narrow where their gradients are steep (greater than five per cent), Signif- <br />scant alluvium does not appear until after the gradients become fairly small. <br /> Figure 2.7-2 shows a simplified geologic map of the Trapper Mine area. Amore <br />il~ detailed presentation of [he geology of Moffat County is presented on Map M53. <br />2-353 <br /> <br />