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<br />This assessment is divided into the following three major subsections: <br />Description of the Existing Environment; Probable Hydrologic Consequences of <br />of Mining; and Cumulative Hydrologic Impacts of Mining. Surface and ground <br />water are discussed separately in each subsection. <br />Description of the Hydrologic Environment <br />A. Regional Geology <br />The upper Yampa River Basin is comprised of sedimentary rock strata which has <br />been structurally deformed. The area covered by this CHIS lies within the <br />Sand Wash Basin, a large structural basin covering northwestern Colorado and <br />south central Wyoming (Figure No. 4). Many smaller structural features <br />further define this basin. <br />Six of the mines considered in this CHIS are located in the Twentymile Park <br />Basin. This structural basin is a sub-basin of the larger Sand Wash Basin. <br />The Twentymile Park Basin consists of a deep central basin flanked by two <br />shallower basins to the north and southwest. The southwestern basin is <br />structurally connected to the Sand Wash Basin by the doubly plunging <br />Hayden Syncline. The Hayden Syncline plunges to the northwest into the Sand <br />Wash Basin and provides a Timited outlet for the Twentymile Park Basin. The <br />tight folding of the sedimentary rock strata within the Twentymile Park Basin <br />has resulted in faulting within the basin. <br />The three remaining mines lie in structurally separated areas. Numerous small <br />anticlines and synclines such as the Sage Creek Anticline and Hayden Syncline <br />isolate these areas. <br />The geologic units range in age from Late Cretaceous to Quaternary <br />(Figure No. 1). The oldest exposed sedimentary rocks are that of the <br />Mancos Shale, which was deposited in a deep marine to shallow marine <br />environment. After deposition of the Mancos Shale, sediments of the Iles and <br />Williams Fork Formations of the Mesaverde Group were deposited mostly in <br />terrestrial environments. Following deposition of the Mesaverde Group, <br />sediments of the Lewis Shale were deposited in a predominantly marine <br />environment (BLM, 1976). The following description of the geologic units is <br />modified from Warner and Dale (1981) and Brogden and Giles (1977). <br />The Mancos Shale is a thick (approximately 5,000 ft.) homogeneous light-gray <br />to dark-gray fossiliferous marine shale with interbedded sandstone and <br />limestone beds and is the oldest unit exposed in the area. The sandstones are <br />generally thin bedded, fine grained, tan, and fossiliferous, and form <br />resistant ledges in the basal and upper parts of the formation. The overall <br />area occupied by the Mancos Shale is characterized by rolling hummocky <br />topography . <br />The Mesaverde Group is approximately 3,000 feet thick and conformably overlies <br />the Mancos Shale. It consists of the Iles Formation and Williams Fork <br />Formation. <br />-24- <br />