2008-09-29_PERMIT FILE - C1981019A (28)

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RULE 2 PERMITS 2.04.6 Geology Description The coal mined at the Colowyo operation is located in the northern extent of the Danforth Hills coal field of the Uinta Region. The Danforth Hills field comprises the coal deposits on the northeast flank of the Piceance Creek basin and is defined to the northeast by the Axial Basin Anticline. Stratig_raphy The area is underlain by as much as 13,500 feet of sedimentary rock consisting of approximately 4,500 feet of Paleozoic (570 million to 225 million years old) quartzite, limestone, shale, sandstone, and gypsum and 9,000 feet of Mesozoic (225 million to 70 million years old) limestone, shale, mudstone, sandstone, and conglomerate. The coal seams mined at the Colowyo operation are of upper Cretaceous Age. The generalized geological structural setting is outlined in Figure 2.04.6-1 Geologic Structural Setting. The stratigraphic position of the coals mined by Colowyo are graphically illustrated in Figure 2.04.6-2, Generalized Stratigraphic Column. The two most important geologic formations within the permit area are the upper Cretaceous Iles and Williams Fork Formations of the Mesa Verde Group. Details of each formation are set forth below. Iles Formation - In general, the Iles Formation consists of light brown- to white, fine- to medium-grained, poorly-sorted calcareous sandstone interbedded with red and dark maroon sandy and silty carbonaceous shale. Thin lenticular coal beds are found in this formation. The uppermost sandstone of the Iles Formation, the Trout Creek Sandstone Member, appears to be reliable marker horizon at the base of the principal coal beds within the Williams Fork Formation. The 75 foot thick Trout Creek member is a white, fine-grained, well-sorted, massive sandstone that is fairly uniform in thickness over a large area. The Trout Creek Sandstone Member, a common ridge-forming unit in the Danforth Hills, has been called the "White Rock" because of its characteristic exposures. The deposition of the Trout Creek Sandstone marked a major regression of the Late Cretaceous seaway over a large region. The resulting clean, well sorted blanket sand formed by this migrating beach and barrier island complex is an aquifer of regional extent in Northwest Colorado. Under the area to be mined within the Colowyo Permit area, the Trout Creek sandstone underlies the lowest surface recoverable seam to be mined by 800 feet. Considering the nature of the intervening strata which is comprised of primarily claystone, shales, and siltstones; it is very doubtful the Colowyo operation will impact the Trout Creek sandstone. The Iles Formation in the permit area is estimated to be 1,375 feet thick. The Iles Formation forms most of the cliffs that surround the Axial Basin. The Iles Formation lies approximately 1,700 feet beneath the actual mining area; however, minor folds or faults in the regional trend bring the formation nearer the surface. Williams Fork Formation - The Williams Fork Formation consists of alternating beds of sandstone, sandy shale and coal. It is distinguished from the Iles Formation by its thick zones of brick red sandstone and shale, which have been colored from adjacent naturally-burning coal beds, a common occurrence in the Axial area. The coal beds in the formation are uniformly distributed in the vicinity of the mine but generally vary greatly within the regional extent of the unit. The stratigraphic thickness of the Williams Fork Formation in the permit area has been estimated to be about 1,600 feet; its degree of areal extent in the Axial Basin is similar to that of the Iles Formation. Rule 2 Permits 2.04.6-1 Revision Date: 6/23/08 Revision No.: MR-91