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RULE 2 PERMITS <br />environment, with the sedimentation influenced by sea -level changes, indicated by the presence of both <br />marine and non- marine rocks. The group consists of the Iles and the Williams Fork Formations and <br />overlies the Mancos Shale. Carbonaceous rocks are present in both formations; however, in the area of <br />the Colowyo Mine, thick coal beds are found only in the Williams Fork Formation. <br />Iles Formation - The Iles Formation conformably overlies the Mancos Shale. It is exposed on the edges <br />of the Axial Basin and south and west of the Colowyo revised permit area in the Wilson Dome area. It <br />has a thickness of approximately 1,500 feet. It is composed of littoral sands deposited along a <br />regressional ocean margin. <br />The Iles is generally comprised of light brown- to white, fine- to medium - grained, poorly sorted <br />calcareous sandstone interbedded with red and dark maroon sandy and silty carbonaceous shale. Thin <br />lenticular coal beds are found in the formation. A thick sandstone at the top of the Iles Formation, the <br />Trout Creek Sandstone Member, is a reliable marker horizon for drill holes in the Danforth Hills and <br />surrounding areas. <br />The Trout Creek Sandstone is light - colored, fine- grained, well - sorted, massive sandstone that was <br />deposited in a marginal- marine or littoral environment. The thickness of the Trout Creek Sandstone <br />varies across the area but is generally 60 to 80 feet. The deposition of the Trout Creek Sandstone marked <br />a major regression of the Late Cretaceous seaway in this region. The resulting clean blanket sand formed <br />by this migrating beach and barrier island complex is an aquifer of regional extent in northwestern <br />Colorado. The Trout Creek Sandstone Member, a common ridge - forming unit in the Danforth Hills, has <br />been called the "White Rock" because of its characteristic white sandstone exposures. <br />The Trout Creek sandstone underlies the lowest surface recoverable seam to being mined at South Taylor <br />(the G789 seams) by approximately 590 feet. <br />Williams Fork Formation - The Williams Fork Formation is the predominant coal- bearing unit in the <br />Colowyo revised permit area. The formation conformably overlies the Iles Formation. The Williams <br />Fork consists of a typical lagoonal sequence of interbedded tan to light gray sandstones, light gray to gray <br />siltstones, sandy, silty, or carbonaceous gray mudstones and coals. This sedimentary sequence is an <br />example of cyclothems deposited along a linear clastic shoreline located at the edge of an epicontinental <br />seaway. The formation ranges from beach sands grading into lowermost deltaic sediments deposited by <br />sluggish brackish water at the base, to middle and upper deltaic deposits deposited in a bayou setting in <br />the upper portions of the formation. This sequence was formed as the shore transgressed seaward, <br />resulting in the gradation from marine sediments below the formation to terrestrial sediments above the <br />formation. <br />The Williams Fork Formation is the predominant coal- bearing unit in the Mesaverde Group. The coal <br />beds in the formation are uniformly distributed near the mine but generally vary in thickness and extent <br />away from the current Colowyo mine area. The coals are part of the Fairfield Coal Group. The natural, <br />pre- historic burning of underlying coal beds has baked areas of the Williams Fork Formation within the <br />Colowyo revised permit area and the adjacent area. These surface areas are easily distinguished by their <br />red -brown to orange -brown color on the surface. These coal burns are known to extend into the rock for <br />up to several hundred feet. <br />The stratigraphic thickness of the Williams Fork Formation in the Colowyo revised permit area is about <br />1,700 feet. Although the formation contains some thick sandstone layers, the Twenty Mile Sandstone <br />Member (within the Williams Fork Formation in the Craig, Hayden, and Steamboat Springs areas) is not <br />present in the Collom permit expansion area. A facies change within the Williams Fork Formation has <br />eliminated the Twenty Mile Sandstone in the Danforth Hills Area. <br />Collom — Rule 2, Page 28 Revision Date: 9/28/11 <br />Revision No.: PR -03 <br />