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• ReoionalGeologkSouthwestColoradoexhibitsawideandunusualvarietyofphysiography,mineral <br />resources, soils, groundwater conditions, and surface water conditions which are, to a great extent, <br />influenced by the region's geology. In this part of the San Juan Coal Region (Nucla-Naturita Coal <br />Field), a series of major northwest trending structural features and complex intrafomiational <br />stratigraphic relationships control the overall occurrence and quality of coal reserves. <br />Most of the principal structures in the Nucla Area have undergone a complex tectonic history of <br />recurring differential uplift, subsidence, deposition, erosion, folding, and faulting beginning in early <br />Pennsylvanian time and lasting to the present (Lohman, 1965). The largest of these structures is <br />the 100-mile long Uncompahgre Arch, a northwest trending upwarp whose axis follows the crest of <br />the Uncompahgre Plateau roughly 10 to 15 miles directly north and northeast of the mine site. Other <br />well developed anticlines in the area include the Paradox Valley Complex, the Gypsum Valley <br />Anticline, and the Sinbad Valley Anticline. These anticlines, in contrast to the metamorphiGigneous <br />core complex found in the Uncompahgre Arch, have intrusive cores of salt and gypsum derived from <br />the Paradox Member of the Hermosa Formation. Far to the west in Utah, Tertiary laccoliths and <br />stocks are evident in the LaSal mountains. <br />• Lying between anticlines are several northwest trending synclines which contain the greater part of <br />coal reserves found in the region. Characteristically, these synclines are shallow, broad, structurally <br />simple folds with gently dipping flanks. The Nucla Syncline (San Miguel Syncline), is the largest of <br />these structures occurring in the area. It is approximately 65 miles long. <br />A generalized geologic cross-section showing the mine's setting to adjacent structural features is <br />presented in Figures 2.04.5-1. <br />In general, the present structural relief of antiGines and syndines found in the region is due primarily <br />to compressive and isostatic forces which produced folding in the late Pennsylvania, late Cretaceous, <br />Eocene and late Pliocene or early Pleistocene times (Cater, 1970). All of the structures show <br />various degrees of defom~ation along their outer margins in the form of monoclines and high angle <br />faults trending parallel or subparallel to the direction of the axis. A major fault scarp of unknown <br />displacement marks the abrupt southwest edge of the Uncompahgre Uplift, and is shown in Figure <br />2.04.5-1. Small folds and faults in Quaternary deposits on the crest and salt anticlines may indicate <br />• that structural readjustments are still occurring (Cater, 1055). <br />2.04.5 - 2 <br />