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State Reg. <br /> glomerate of the Dakota separates the Dakota Sandstone frcn the <br /> underlying Burro Canyon Formation (Lohman, 1965) . The Burro <br /> Canyon ccrrprises a varied assemblage of white, gray, and light <br /> brown sandstcne and conglomerates interbedded with green and <br /> purplish shales, siltstones, and mudstones, and thin beds of <br /> imrpure limestdne. It may attain a maximum thickness of 200 feet <br /> in the area (Williams, 1964) . <br /> In the Nucla Syncline, exposures of the Burro Canyon are re- <br /> stricted to the rims of mesas and deep canyons located to the <br /> west and northwest of the mine. Small map scale and lithologic <br /> similarity with the Dakota Sandstone cause these formations to <br /> be mapped together as undifferentiated in the southern part of <br /> the syncline (see Exhibit 6-1) . <br /> Pre-Burro Canyon Formations (U. Jurassic to Precambrian) . <br /> Table 6-1 summarizes the general lithologic characteristics and <br /> thicknesses for Pre-Burro Canyon Formations. As mentioned be- <br /> fore, these formations show slight to extreme variations in <br /> thickness between anticlines and synclines due to past tectonic <br /> activity and syntectonic sedimentation. Most, if not all, of <br /> these formations probably underly the Nucla Syncline. However, <br /> the lateral extent and thickness of most formations cannot be <br /> predicted accurately over a few miles distance. <br /> Exposures of these formations are generally restricted to the <br /> steeply, dipping flanks and cores of anticlines, and the walls <br /> and floors of deep canyons. Excellent exposures of all for- <br /> mations, except the Pennsylvanian Hermosa Formation, are seen <br /> in the Unaweep, Dolores, and San Miguel River Canyons along <br /> Route 141 between Whitewater, Colorado and Naturita, Colorado. <br /> 6-8 <br />