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. TAB 6 GEOLOGY AND OVERBURDEN ASSESSMENT Geologic Descriotion Regional Structure. The Seneca II-W Mine is situated in the rugged Williams Fork Mountains which are within the southeastern synclinal portion of the Sand Wash structural basin of northern Colorado and central Wyoming (Bass et al., 1955). In northern Colorado, the synclinal portion of the Sand Wash basin is bordered on the southwest by the Axial Basin anticline, and on the east by the Park Range anticline. Numerous folds modify the synclinal structure of the region, the largest of which is the Tow Creek anticline located approximately 10 miles northeast of the mine permit area (Figure 6-1). The Tow Creek anticline as well as the area that lies approximately 11 miles to the south of it are heavily faulted. Approximately two miles southeast of the Seneca 11-W Mine site is a fault trending northeast to southwest. • Regional Stratigra phy. The exposed rock sequence in the region consists primarily of about 13,500 feet of Late Cretaceous , Tertiary, and Quaternary age sedimentary deposits. Some intrusive and extrusive igneous rocks that are probably Miocene in age (Bass et al., 1955) occur region ally. the largest regional occurrence of extrusive rocks occurs in the Flat Tops about 20 miles south of the Seneca II-W Mine permit area. The oldest rock formation outcropping in the region is the Mancos shale of Late Cretaceous age. The nearest outcrop to the Seneca II-W Mine is immediately east of the permit area along Sage Creek. The Mancos shale is about 4,900 feet thick and is conformable with the overlying Mesaverde group and the underlying Dakota sandstone (Bass et al., 1955). The Mancos shale is predominantly a dark-gray marine shale with interbedded layers of sandstone near the top and interbedded layers of sandstone and limestone near the base. There is a sandstone layer about 50 feet in thickness situated approximately 300 feet above the base of the formation which is thought to be equivalent to part of the Frontier sandstone (Bass et al., 1955). Above the Mancos Shale lies the Mesaverde Group, also of late Cretaceous age. This group consists of two formations, the oldest being the Iles Formation and the youngest being the Williams Fork Formation (Figure 6-1). Together these • two formations represent about 2,550 to 3,550 feet of sandstone interbedded with shales, sandy shales, and coal (Bass et ai., 1955).