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5(~~ ~~~ ~~~~~~~~~~~~~~~~ <br />• 2.04.6 Geoloqv Description <br />Doc Date:12/11/2001 <br />The coal mined at the Colowyo operation is located in the northern <br />extent of the Danforth Hills coal field of the Uinta Region. The <br />Danforth Hills field comprises the coal deposits on the northeast <br />flank of the Piceance Creek basin and is defined to the northeast <br />by the Axial Basin Anticline. <br />Stratiaraohy <br />The area is underlain by as much as 13,500 feet of sedimentary rock <br />consisting of approximately 4,500 feet of Paleozoic (570 million to <br />225 million years old) quartzite, limestone, shale, sandstone, and <br />gypsum and 9,000 feet of Mesozoic (225 million to 70 million years <br />old) limestone, shale, mudstone, sandstone, and conglomerate. The <br />coal seams mined at the Colowyo operation are of upper Cretaceous <br />Age. <br />The generalized geological structural setting is outlined in Figure <br />1 Geologic Structural Setting. The stratigraphic position of the <br />coals mined by Colowyo are graphically illustrated in Figure 2, <br />• Generalized stratigraphic Column. <br />The two most important geologic formations within the permit area <br />are the upper Cretaceous Iles and Williams Fork Formations of the <br />Mesa Verde Group. Details of each formation are set forth below. <br />Iles Formation - In general, the Iles Formation consists of light- <br />brown- to white, fine- to medium-grained, poorly-sorted calcareous <br />sandstone interbedded with red and dark maroon sandy and silty <br />carbonaceous shale. Thin lenticular coal beds are found in this <br />formation. The uppermost sandstone of the Iles Formation, the <br />Trout Creek Sandstone Member, appears to be a reliable marker <br />horizon at the base of the principal coal beds within the Williams <br />Fork Formation. The 75 foot thick Trout Creek member is a white, <br />fine-grained, well-sorted, massive sandstone that is fairly uniform <br />• <br />2.04.6-1 <br />