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4.2.3. Stratigraphy <br />4.2.3.1. Depositional History <br />The Piceance Creek Basin contains a thick assemblage of sedimentary rocks, <br />ranging in age from Cambrain through middle-Tertiary. Numerous stratigraphic units <br />have been identified in the basin and adjacent areas. Table 4-1 summarizes the <br />stratigraphic nomenclature used in northwest Colorado (Tweto, 1977). <br />The oldest rocks in northwest Colorado are Proterozoic (1.4 to 1.8 billion years) <br />schists, gneisses and granites, such as those exposed in Glenwood Canyon and on <br />the flanks of the White River Uplift. In the northern part of the Piceance Creek <br />Basin, similar Proterozoic rocks occur approximately 24,000 feet below the surface <br />(Murray and Haun, 1974). This depth provides insight into the magnitude of the <br />sedimentary sequence in the basin. <br />Cambrian through Mississippian rocks in the White River Uplift area are <br />predominately marine limestones and dolomitic limestones with some sandstones <br />and siltstones; description of these units is given in Campbell (1981). This <br />sequence, which includes the Sawatch Quartzite, Peerless, Dotsero, Manitou, <br />Parting and Dyer Formations, Harding and Gilman Sandstones, and Leadville <br />Limestone (Table 4-1), thins progressively across the Piceance Creek Basin in a <br />northeast to southwest direction (i.e., towards the Uncompahgre Uplift). <br />Pennsylvanian and Permian stratigraphic units (Molas, Belden, Minturn, Eagle <br />Valley, Maroon, State Bridge Formations and Weber Sandstone) are represented by <br />marine black shales and evaporites and nonmarine detrital rocks deposited in the <br />ancient Eagle Basin. Like older Paleozoic rocks, the Permo-Pennsylvanian <br />sequence thins from east to west across the basin. <br />Daub & Associates, Inc. Page 4-11 NSI Mine Plan 2010 Rev. <br />Printed: 7/5/2010 Section 4 Geology