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SUNDAY MINE COMPLEX-HYDROGEOLOGIC REPORT <br /> The Mancos shale of the Upper Cretaceous period is homogeneous soft dark-gray shale. There <br /> is a downwarped mass of Mancos shale near the center of Big Gypsum Valley estimated to be <br /> 1,600 ft thick. <br /> The Upper Cretaceous Mesaverde Formation is thick interbedded yellowish-gray sandstone and <br /> light gray shale. The Mesaverde Formation outcrops near the center of Big Gypsum Valley <br /> within the downwarped mass of rocks and is only partially preserved. <br /> Thin Quaternary aged deposits in the area include landslide deposits, terraced gravels and <br /> alluvial deposits. <br /> 4.2 Local Geology <br /> The SMC lies within the Hamm Canyon quadrangle which covers most of the terrain that <br /> includes the northern portions of Big Gypsum Valley and Disappointment Valley. The Hamm <br /> Canyon quadrangle was first mapped by W.L. Stokes and D.A. Phoenix in 1944-45 and was <br /> later revised by F.W. Cater, Jr. in 1951 and published by the U.S. Geological Survey (USGS) in <br /> 1955. Geologic mapping of the Hamm Canyon quadrangle and 17 other quadrangles in the <br /> southwest part of Colorado was done to support an assessment of carnotite occurrences in the <br /> region by the USGS. A portion of Cater's geologic map showing the geology of the SMC is <br /> presented in Figure 4-5. <br /> As illustrated on Cater's geologic map, the geology of the SMC and surrounding area consists <br /> of sedimentary rocks ranging from Pennsylvanian to Quaternary in age, with most exposed <br /> rocks being of Mesozoic age including the Triassic, Jurassic, and Cretaceous periods. The <br /> sedimentary units are exposed in bands of rock trending northwest along the prominent ridges <br /> that border Big Gypsum and Disappointment Valleys, a result of the geologic structure. <br /> Other than the presence of a number of normal faults, the sediments occur in typical <br /> stratigraphic position for the region. The oldest rock is the Pennsylvanian age Hermosa <br /> Formation, the upper limestone member and the lower Paradox member. These rocks occur in <br /> discontinuous, broken outcrops in Big Gypsum Valley east and north of the SMC. The Paradox <br /> member consists mostly of gypsum salts but may contain some shales or sandstone; the <br /> outcrops are remnants of a much larger salt intrusion that formed an anticlinal structure whose <br /> axis trends northwest along Big Gypsum Valley. Cater(1955) reported that drilling in Paradox <br /> Valley penetrated the Paradox member of the Hermosa Formation to a depth of 10,800 ft and <br /> Western Water& Land, Inc. 9 <br />