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<br />the Green River Formation: The Dry Rock (at base), Thirteen mile Creek, and <br />Black Sulfer (at top). <br />Within the immediate lease area, only three stratigraphic units crop out <br />(Figure G-3). Most of the area is underlain by Tu5 Unit of the Uinta <br />Formation. This unit ranges in thickness from 150 to 500 ft and is <br />comprised of brown- to buff-colored sandstone and siltstone. Sandstone <br />bodies tend to be channel form and make prominent outcrops in some of the <br />shallow drainages that transect the lease area. The Tu5 is underlain <br />conformably by the Thirteenmile Creek tongue (Thtu) of Green River <br />Formation. This unit is confined to outcrops along the northern boundary of <br />the lease area near Yellow and Duck Creeks and in Horse Draw. The <br />Thirteenmile Creek tongue ranges in thickness from 10 to 100 ft and consists <br />mainly of light-colored marlstone and marly siltstone and lesser amounts of <br />oil shale and fossiliferous lacustrine limestone. The Thirteenmile Creek <br />tongue is underlain conformably by the Tu4 tongue of the Uinta. This tongue <br />also crops out only along the northern edge of the lease, and consists of <br />• buff to brown cliff-forming channel-form sandstone and some siltstone and <br />silty marlstone; it ranges in thickness from 0 to 350 feet. <br />The valley bottoms of Yellow and Piceance Creeks and many of their <br />tributaries contain Quarternary alluvium. These deposits range in thickness <br />from 0 to 50 feet, and consist of gray to buff-colored sand, silt, and <br />gravel. <br />Structural orientation of surface outcrops in the lease area is difficult to <br />determine because of the poor quality of most exposures. Measured dips <br />rarely exceed 10 degrees. Dip directions are generally towards the <br />synclinal axis of the South Rangely syncline, a feature that transects the <br />northern part of the Lease before splitting into two branches (Figure G-3). <br />Faults are apparently not abundant on the Lease. The Dudley Bluffs graben, <br />which is a major structural feature on the Piceance Creek dome, approaches <br />the lease area from the southeast (Figure G-3), but dies out after crossing <br />• Ryan Gulch. Smith and Whitney (1979) indicate that a fracture system may <br />~~ extend along strike onto and across the lease area towards Yellow Creek. <br />G-4 <br />