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Anticipated mining in the ground water general area includes <br />those mines listed above except for the Grassy Gap Mine, <br />Meadows No. 1 Mine, the Seneca II West Mine, and a portion of <br />the Seneca II Mine (Figure 31. <br />Description of the Hydrologic Environment <br />Regional Geology <br />Nine active or reclaimed surface and underground coal mines rim <br />the Twentymile Park Basin (Figure 31. This structural basin is <br />a subbasin of the larger Sand Wash Basin. The Twentymile Park <br />Basin consists of a deep central low flanked by two shallower <br />basins to the north and southwest. The southwestern basin is <br />structurally connected to the Sand Wash Basin by the doubly <br />plunging Hayden Syncline. The Hayden Syncline plunges to the <br />northwest into the Sand Wash Basin and provides a limited <br />outlet for the Twentymile Park Basin. The tight folding of the <br />sedimentary rock strata within the Twentymile Park Basin has <br />resulted in faulting within the basin. <br />The geologic units occurring within the Twentymile Park Basin <br />range in age from Late Cretaceous to Quaternary (Figure 41. <br />The oldest exposed sedimentary rocks are the Mancos Shale, <br />which was deposited in a deep marine to shallow marine <br />environment. After deposition of the Mancos Shale, sediments <br />of the Iles and Williams Fork Formations of the Mesaverde Group <br />were laid down mostly in terrestrial environments. Sediments <br />of the Lewis Shale which were deposited in a predominantly <br />marine environment (BLM. 1976) overlie the Mesa Verde Group. <br />The following description of the geologic units occurring <br />within the Twentymile Park Basin is modified from Warner and <br />Dale (1981) and Brogden and Giles (1977). <br />The Mancos Shale is a thick (approximately 5,000 feetl <br />homogeneous light-gray to dark-gray fossiliferous marine shale <br />with interbedded sandstone and limestone beds and is the oldest <br />unit exposed in the area (Figure 4). The sandstones are <br />generally thin bedded, fine grained, tan, and fossiliferous, <br />and form resistant ledges in the basal and upper parts of the <br />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 <br />conformably overlies the Mancos Shale (Figure 41. It consists <br />of the It es Formation and Williams Fork Formation. <br />- 17 - <br />