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<br />• (Hunter Canyon or Williams Fork Formation). The base of the Tertiary contains red <br />to white sandstone, pebbly sandstone, and pebble conglomerate generally less than <br />100 feet thick (Donnell 1961, Donnell 1969). <br />7.2.1.2 Structural Geology <br />The Piceance Creek Basin is a broad, asymmetric, southeast to northwest trending <br />basin controlled by alternating anticlines and synclines. The deepest part of the basin <br />is associated with the Red Wash Syncline, which parallels the northern boundary of <br />the basin, and the South Rangely Syncline. These two synclines are separated by the <br />broad Rangely anticline (Murray and Haun 1974, Donnell 1961). The Piceance Site <br />occupies the eastern ascent of the South Rangely Synclinal trough (Figure 7-4). <br />Numerous lineations associated with joints, fractures, normal faults, and grabens <br />are present in the northern Piceance Creek Basin. The largest of the grabens is the <br />Dudley Bluffs fault system (Donnell 1961). The Dudley Bluffs Graben is located <br />approximately 1 mile south of the southern boundary of the Piceance Site. The <br />distribution of these lineations as derived from aerial photographic analysis of the <br />Piceance Creek Basin is presented in Figure 7-5 (Welder 1971). <br />The primary joint set in the Piceance Creek Basin strikes west by northwest, with a <br />secondary set striking north by northeast. Normal faults parallel the primary joint <br />• set, suggesting that jointing and faulting formed from similar stresses (Smith and <br />Whitney 1979). The primary joint set can be seen on topographic maps of the <br />project area, suggesting a high degree of structural control by jointing. <br />7.2.2 Project Geology <br />Piceance Site <br />The predominant surficial geologic formation that oLttcrops at the Piceance Site is <br />the Uinta Formation (Figure 7-6). There are two units, Unit 4 (TU,) and Unit 5 <br />(TUS), that outcrop in Sections 17 through 21, 28, and 29. Unit 4 occurs from the <br />Piceance Creek valley floor at elevation 6,100 feet up to 6,300 feet along Horse Draw <br />and up to 6,420 feet in the center of Section 20. Unit 4 is a buff to brown, cliff- <br />forming sandstone with a maximum thickness of 350 feet. Toward the southwest, <br />this unit thins and contains increasing proportions of silty marlstone and siltstone. <br />Unit 5 occurs in the western part of the Piceance Site. Unit 5 starts at about 6,380 feet <br />along Horse Draw and at about 6,460 feet in the center of Section 20 and extends up <br />to 6,650 feet. Unit 5 is a brown to buff, weathered sandstone with minor <br />concentrations of siltstone in its eastern portion. The sandstone has been described <br />as channel-formed, cross-stratified, medium- to fine-grained. The sequence contains <br />increasing proportions of gray and greenish-gray siltstone interbedded with ledge- <br />forming sandstone toward the west. Thickness varies between 280 and 500 feet. <br />American Soda, L.L.P. '7_8 <br />Commeraal Mine Plan <br />August 16, 1998 <br />