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State Reg. <br />• to the metamorphic/igneous core complex found in the Uncanpahgre <br />Arch, have intrusive cores of salt and gypsum derived from the <br />Paradox Member of the Hermosa Formation. Far to the west in <br />Utah, Tertiary laccoliths and stocks are evident in the LaSal <br />tdountains. <br />Lying between anticlines are several northwest trending syn- <br />clines which contain the greater Fart of coal reserves found <br />in the region. Characteristically, these synclines are shallow, <br />broad, structurally simple folds with gently dipping flanks. <br />The Nucla Syncline (San Miguel Syncline), as shown in Exhibit <br />6-1, is the largest of these structures occurring in the area. <br />It is approximately 65 miles long. <br />Geologic maps showing outcrop aril structural relationships of <br />the area are presented in Exhibits 6-1 and 6-2. Structure con- <br />tours drawn on the base of the Dakota Sandstone are illustrated <br />in Exhibit 6-2. A generalized geologic cross-section showing <br />the mine's setting to adjacent structural features is presented <br />in Figure 6-1. <br />In general, the present structural relief of anticlines and sny- <br />clines found in the region is due primarily to compressive an3 <br />isostatic forces which produced folding in the late Pennsylvania, <br />late Cretaceous, Eocene and late Pliocene or early Pleistocene <br />times (Cater, 1970). All of the structures shoo various de- <br />grees of defazmation along their outer margins in the form of <br />monoclines and high angle faults trending parallel or subparal- <br />lel to the direction of the axis. A major fault scarp of un- <br />known displacement marks the abrupt soutwest edge of the <br />Uncanpahgre Uplift, and is shown in Figure G-1. Small folds <br />and faults in Quaternary deposits on the crest and salt anti- <br />clines may indicate that structural readjustments are still <br />occurring (Cater, 1955). <br /> <br />6-3 <br />