2013-04-18_PERMIT FILE - C2009087 (6)

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Steep cliffs are formed from resistant sandstone members such as the Twentymile Sandstone Member of the Mesaverde Group (Robson & Stewart, 1990). The geologic and structural setting of • the regional and general area is outlined in Map 2.04.5 -M1, Regional Geology and Hydrology. The geologic history of the Sand Wash Basin is characterized by periods of deposition followed by deformation related to tectonic activity (EPA, 2004). The tectonic activity in the region has affected depositional patterns, coal occurrence and maturity, and hydrology (Tyler and Tremain, 1994). Basement rock formations in the basin can be as deep as 17,000 feet (Tyler and Tremain, 1994). Coal in the Sand Wash Basin was formed from peat deposited in swamps along a broad coastal plain during the late Cretaceous when the Western Interior Seaway that was spread across what is now North America retreated to the northeast. Intermontane basins developed and coal- bearing fluvial - lacustrine sediments were deposited during the Laramide Orogeny (Quarterly Review, 1993). Sediments that eroded from nearby uplifts covered peat beds that developed in the basins. These alternating periods of deposition of organic material and sands repeated many times creating layers of coal interbedded with layers of sandstone and other sedimentary rocks that filled the basin (EPA, 2004). The geologic sequence in the Sand Wash Basin and adjacent areas is shown on Figure 2.04.5 -F2, Regional Stratigraphic Column. The coal- bearing formations in the basin and the region include the Iles and Williams Fork of the Mesaverde Group, the Fort Union Formation, and the Wasatch Formation. These formations were deposited during the Upper Cretaceous, Paleocene, and Upper Paleocene, respectively. The Fort Union and Wasatch Formations are not present within the PSCM permit area. The Upper Cretaceous Williams Fork Formation contains the thickest and most extensive coalbeds (EPA, • 2004). The Wadge Seam, which is the target coal seam for the PSCM, is contained within a sedimentary sequence of interbedded sandstone, sandy shale, shale, and coal in the Williams Fork Formation. Figure 2.04.5 -F3, Generalized Northwest- southeast Cross Section of the Mesaverde Group in the Yampa Coal Field, presents a depiction of the depositional and stratigraphic relationships in the regional area. The mineable coal seams present in the Lower (Mount Harris) Member of the Williams Fork Formation are referred to as the Middle Coal Group (Kirschbaum and Biewick, 2000) and include, in descending order, the Lennox, Wadge and Wolf Creek coal beds. The Lennox bed is found approximately 300 feet above the base of the Williams Fork Formation and is generally about four feet thick. The Wadge coal bed, which is proposed to be mined at the PSCM, ranges from six to 14 feet in thickness and is found approximately 230 feet above the base of the Williams Fork Formation. The Wolf Creek bed, which ranges from zero to 18 feet thick, is found between 40 and 100 feet above the base of the Williams Fork Formation. Regional Surface Water Hydrology The PSCM permit area is drained principally by Grassy Creek and its tributaries. Minor tributaries of Fish Creek drain the southeast corner of the permit area. See Map 2.04.5 -M1, Regional Geology and Hydrology, for a delineation of the major basins in the region. Grassy Creek is a perennial tributary of the Yampa River, which it joins approximately five miles downstream of the PSCM facilities. Fish Creek is also perennial, and joins Trout Creek approximately five miles upstream of • Trout Creek's confluence with the Yampa River. PSCM Permit App. 2.04 -17 6/15/09