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The project area is in the northwest portion of a large northwest-southeast trending <br />structural downwarp known as the Piceance Basin. Subsidence of the Basin probably began <br />some 70 million years ago, with the onset of the Laramide Orogeny during Late Cretaceous <br />times, and continued until the Late Eocene (Young and Young 1977: 46). During this <br />period of roughly 25 million years, the Basin received as much as 9000 feet of Tertiary <br />stream and lake deposits, all of which gently dip toward the center of the downwarp. <br />Subsequent erosion has exposed the Uinta Formation, formerly known as the Evacuation <br />Creek member of the Green River Formation, which underlies the study area. Of middle-late <br />Eocene age, the Uinta consists of grey to yellow-brown marlstone, siltstone, sandstone, and <br />tuff (Cashion 1973). <br />Bedrock in the area is the Uinta Formation (formerly the Evacuation Creek Member <br />of the Green River Formation; Eocene; Tweto 1979). The formation consists of fluviatile <br />sand- and siltstone and interbedded petroliferous lacustrine shale. The most prominent and <br />resistant beds are dark yellow to brown, calcareous siltstone and sandstone originally <br />deposited on flats bordering a shrinking Eocene lake and eventually filling the lake. More <br />resistant to erosion, silt- and sandstone cap higher elevations within the area. Many of the <br />sandstones represent channels crossing the flats to the lake, and vary from fine to coarse <br />sand in thin to massive beds; laminar foreset beds are easily distinguishable in the channel <br />sands. Other thin-bedded sandstones represent littoral facies in relatively quiet water. <br />Thicker beds (most channel sandstone) have concretions varying in size from a few <br />centimeters to half a meter or more, either round or elongated, formed by iron migration and <br />accumulation. Incompetent bedding formed by de-watering of saturated sediment by <br />overloading are common. Volcanic provenance of the original sediment is expressed by the <br />secondary mineral assemblage which includes secondary mica, clay, and zeolites. <br />The Uinta Formation conformably overlies the Green River Formation. Murphey <br />and Daitch (2007) state that the Uinta Formation formed by fluvial processes after basin <br />filling slowed the lacustrine deposition of the Green River Formation. It is mainly <br />comprised of very-fine to medium grained light brown, tan and gray sandstones and light <br />gray siltstones and marlstones. Fluvial deposition with interbedded overbank deposits <br />dominates in the basins as streams flow in from the north and east. Exposures of the Uinta <br />Formation within the Piceance Basin are restricted mainly to the upper reaches of the <br />plateaus. Vertebrate fossils of the Uinta Formation are not as numerous in the Piceance <br />Creek Basin as in the Uinta Basin, possibly due to the local lack of exposure. Plant fossils <br />are relatively uncommon in all localities of the Uinta Formation, however the Piceance <br />Basin has yielded plant specimens from every stratigraphic unit of the Uinta Formation, <br />making them scientifically significant. <br />Lacustrine facies rock are largely fissile, petroliferous shale, dark gray in fresh <br />breaks, but weathers white. Surface exposures on slopes give the appearance of a <br />“racetrack,” with “lanes” delineated by slightly more resistant shale beds that form minor <br />breaks in the slope that collect colluvium and provide purchase for plants to take root. The <br />more resistant beds appear to represent deeper water facies; intervening beds have more silt, <br />although remaining fissile. Thin, discontinuous beds of limestone are present in the shale <br />3