1988-04-11_PERMIT FILE - C1981008A (4)

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State Reg. 2.04.6(1) (a) C. STRATIGRAPHY. with the exception of unconsolidated Quaternary deposits, the rocks in the Nucla area range in age from Precambrian to Late Cretaceous. There is a profound ero- sional uniformity separating Precambrian igneous and metamorphic basement complex rocks fran the overlying sedimentary rocks of Mesozoic Age in the Uncompahgre Uplift. This represents the absence of part of the Precambrian, all of the Pennsylvanian and Permian, and much of the Triassic, including the basal part of the Chinle Formation (Cater, 1970) . This unconformity is the result of late Pennsylvania tectonic activity and erosion pre- viously mentioned in the section on structural geology. In sharp contrast, the missing sequence of rocks represented by the hiatus in the Uncompahgre Uplift is present in thicknesses ranging from 13,000 to 15,000 feet in the Paradox Valley Anti- cline, only 15 miles to the southwest. The probable relation- ships of these two structures are shown in the geologic cross section, Figure 6-1. A geologic columnar section showing the stratigraphic position, lithologic characteristics, thicknesses, and formations exposed in the region is presented in Table 6-1 and Exhibit 6-1. The Cretaceous rocks exposed in the Nucla area were formed by a transgressive sea which slowly advanced across the Colorado Plateau area in response to its gentle subsidence (Young, 1973) . As a result, several belts of transitional depositional environ- ments migrated westward across the Plateau to form the many dif- ferent lithologies of Cretaceous Age described in the stra- tigraphic column (Table 6-1) . From landward to seaward, they include the piedmont, floodplain, paludal, deltaic, littoral, and neritic environments. Quaternary Deposits. These deposits are composed of thin layers of soil and unconsolidated eolian terrace silts and sands located 6-5