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D.6 Regional and Site Geology The project area is located within the Piceance Creek Basin. The Piceance Creek Basin is abroad, asymmetric, southeast-northwest structural and topographic basin located in northwest Colorado, which has an areal extent of approximately 7,225 square miles. Deposition of sediments into the region began with the downwarping of the basin floor in the Creataceous and continued through Eocene time. Sediments in the Piceance Creek Basin range in age from Cambrian through middle Tertiary. The basin surface generally consists of a thin veneer of unconsolidated Quaternary alluvium and other deposits, below which lies approximately 8,000 feet of Tertiary sedimentary deposits. The three major formations that make up these Tertiary sediments are the Uinta, Green River, and Wasatch Formations. The stratigraphic sequence of interest to the Yankee Gulch Project is the Green River Formation. General stratigraphy is presented in Figure D-6 and a stratigraphic column for the Piceance Site is provided in Figure D-7. The Green River Formation is divided into four members: tt~e Parachute Creek (upper member), Garden Gulch (intermediate member), Douglas Creek (lowest member), and Anvil Points (lateral correlative of the Douglas Creek and Garden Gulch Members, and part of the lower Parachute Creek Member). The Parachute Creek Member is the uppermost unit of the Formation and contains virtually all of the oil shale, nahcolite, and dawsonite resources of the Piceance Creek Basin. It is approximately 1,500 feet thick at the Piceance Site, but is as thick as 2,000 feet in the depositional center of the basin. The Parachute Creek Member is generally considered to comprise three distinct zones which are referred to as the Mahogany, Leached, and Saline Zone. The Mahogany Zone is a rich oil shale interval that behaves as a leaky semi-confining layer. It is about 180 feet thick and is located between two thin layers of lean oil shale known as the A- Groove and the B-Groove. The Leached Zone is a zone in which all of the soluble saline minerals such as halite and nahcolite have been removed by the percolation of groundwater. It contains badly degraded oil shale. The Saline Zone is separated from the Leached Zone by awell-defined contact known as the Dissolution Surface, which occurs at depths ranging from 1,400 to 1,800 feet below ground surface. The Saline Zone contains the nahcolite deposits that are of interest to the Yankee Gulch Project. In addition to significant concentrations of nahcolite, dawsonite, and halite, the Saline Zone also contains various oil shale zones. The remaining three members of the Green River Formation, the Garden Gulch, Douglas Creek, and Anvil Points Members, contain mainly sandstone, siltstone, marlstone, algal limestone, and some lean, clay-rich oil shale. The clay-rich oil shale is abundant in the Garden Gulch Member, which has a mineralogical composition that includes quartz, illite, dolomite, and alite. D-15