Laserfiche WebLink
predominantly marine environment (USBLM, 1976). Younger sedimentary <br />rocks of the Late Cretaceous and Cenozoic periods do not occur in the <br />Twentymile Park Basin due to erosion or non-deposition. The following <br />description of the geologic units occurring within the Twentymile Park <br />Basin is modified from Warner and Dale (1982) and Brogden and Giles <br />(1977). <br />The Mancos Shale is a thick (approximately 5,000 ft.) homogeneous light <br />gray to dark-gray fossiliferous marine shale with interbedded sandstone <br />and limestone beds and is the oldest unit exposed in the area <br />(Figure 4). The sandstones are generally thin bedded, fine grained, tan, <br />and fossiliferous; and form resistant ledges in the basal and upper parts <br />of the formation. The overall area occupied by the Mancos Shale is <br />characterized by rolling, hummocky topography: <br />The Mesaverde Group is approximately 3,000 feet thick and conformably <br />overlies the Mancos Shale (Figure 4). It consists of the Iles Formation <br />and Williams Fork Formation. <br />The Iles Formation is the lower unit and is approximately 1,500 ft. <br />thick. It consists of interbedded light brown to white, massive, <br />fine-grained, ledge-forming sandstones, brown to black carbonaceous <br />shale, sandy shale and coal beds. The coal beds are assigned to the <br />lower coal group (Figure 4) of the Mesaverde, and are distributed <br />throughout the middle and upper parts of the formation. Four persistent <br />sandstone beds occur within the Iles Formation. They are: 1) the Tow <br />Creek sandstone member at the base, 2) a double ledge-forming sandstone <br />sequence 400 feet or more above the base, 3) a light gray sandstone <br />sequence of variable composition associated with the upper (No. 3) coals <br />of the lower group situated about 900 to 1,000 feet above the base, and <br />4) the Trout Creek sandstone member which caps the formation (Figure 4). <br />The Trout Creek sandstone is a 50 to 100 foot thick, light brown to light <br />gray, fine-grained, massive sandstone. <br />The upper unit of the Mesaverde Group is the Williams Fork Formation <br />which is approximately 1,000 to 2,000 feet thick and includes all beds <br />between the top of the Trout Creek Sandstone Member and the base of the <br />overlying Lewis Shale (Figure 4). The formation is conformable to its <br />base and top, and it is divided into three units, the lower, middle and <br />upper units. <br />The lower unit is about 1,000 feet thick, consisting of shale, thin <br />sandstone beds, sandy shale and several coal beds of the middle coal <br />group. The middle coal group contains the coal seams of economic <br />importance in this area. In ascending order they are the Wolf Creek, <br />Wadge and Lennox coals. The extremely poor lateral continuity of the <br />Wolf Creek and Lennox coals in the area of the proposed Foidel Creek Mine <br />make them unsuitable for development. The middle unit of the Williams <br />Fork Formation includes a massive, white, cross-bedded, cliff-forming <br />sandstone about 100 to 200 feet thick, called the Twentymile Sandstone <br />Member (Figure 4). It is used as a key marker bed throughout the <br />region. The upper units of the Williams Fork Formation consist of <br />interbedded sandstone, sandy shale, shale, sandstone and several thin <br />coal beds of the upper coal group. <br />-33- <br />