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Twentymile Coal, LLC (TC) <br />Geologic and Engineering Data Report <br />Lease by Modification Application — Wolf Creek Reserve April 2014 <br />1 for interburden and local lithology, and Plate 5 for coal thickness. <br />b. Structure <br />The coal to be mined by the planned underground operations is located at the southeast tip of the <br />Yampa Coal Field of the Green River Region. The Yampa Coal Field is a synclinal extension of the <br />Washakie -Sand -Wash structural basin situated in northwestern Colorado and south-central <br />Wyoming. The main axis of the basin is highly modified by numerous smaller anticlines, synclines, <br />and faults. The boundary of the Yampa Coal Field is defined to the southwest by the Axial Basin <br />Anticline and is defined on the east by the Park Range Uplift. <br />The Permit Area is within Twentymile Park, a structural and topographic basin located immediately <br />north of the reclaimed Colorado Yampa Coal Company's surface mines in Routt County. The basin <br />is underlain by a large, high-quality coal reserve which is mineable in many areas by underground <br />methods. The geology and prominent structural features are illustrated on Figure 3. <br />Strata in Twentymile Park have been subjected to progressive structural deformation which has <br />formed large folds and faults. Several large, north -trending asymmetrical folds are the most <br />prominent structural features associated with the Twentymile Park Basin. These folds cause the <br />strata to dip in excess of 15 percent near the periphery of the Basin and form the outer boundaries of <br />the mine area. The coal seams are also disturbed by northwest -striking normal faults. <br />Detailed site-specific lithology of the area is shown on Figures 1 and 2. The coal to be mined by the <br />planned underground operations is the Wolf Creek Seam. The Wolf Creek Seam lies within a <br />sequence of sedimentary rocks characterized by shales, claystones, mudstones, siltstones, <br />sandstones, and minor coal stringers in the Upper Cretaceous Mesaverde Group. This sedimentary <br />sequence is about 75 million years old and is composed of terrigenous elastics deposited in offshore, <br />shallow, and near -marine environments at the western edge of an epeiric seaway. This seaway was <br />located in interior western North America during the latter part of the Early Cretaceous and most of <br />Late Cretaceous time. <br />The sandstones were formed in beach environments, and are generally very fine to fine-grained, of <br />low porosity, and are well -cemented. The very thick shale formations, such as the 590 -foot shale <br />member below the Twentymile Sandstone, are offshore marine deposits and are characterized by <br />their lateral continuity, fine-grained texture, and extremely low horizontal and vertical permeability. <br />The youngest rocks exposed in the planned area belong to the massive, marine -deposited Lewis <br />Shale. Near the base of the Lewis Shale is the Fish Creek Seam, which is only approximately four <br />feet thick. Approximately 165 feet of interbedded shale, siltstone, and fine-grained sandstone <br />separate the Fish Creek Seam from the underlying massive Twentymile Sandstone. The <br />Twentymile Sandstone is a key mapping unit and can be identified and traced throughout most of <br />the Yampa Coal Field. The rock immediately below the Twentymile Sandstone is another massive <br />marine shale member, a "tongue" of the Lewis Shale, which averages approximately 590 feet in <br />thickness. <br />The Fish Creek Seam, the Twentymile Sandstone, and the thick marine shale make up the majority <br />of the Williams Fork Formation. The lower portion of the Williams Fork Formation consists of the <br />coal -bearing sequence. This sequence is the "Middle Coal Group" of the MesaVerde Group. The <br />"Middle Coal Group" contains the Lennox Seam, Wadge Seam, and the Wolf Creek Seams. <br />The Lennox Seam is located 10 to 15 feet below the marine shale member and ranges in thickness <br />from zero to four feet. The low seam height, poor quality, and poor lateral continuity of the seam <br />render it impractical to mine. In the proposed mine area, the strata below the Lennox Seam consist <br />mainly of stacked deltaic sequences. Four distinct sequences have been mapped throughout the <br />mine area. They are characterized by a coarsening of mean grain size upwards within each <br />WCR BLM Lease App Geologic & Engineeering Data Report 0414.docx Page 3 <br />