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lJ <br />GHAPTERTHREE <br />Affected Environment , <br />south, the Uncompahgre Uplift to the southwest, the Douglas Creek Arch to the: west, and the <br />Axis] Basin Anticline to the north (Newman 1980) (Figure 3.1-1). <br />Surface elevations range from about 5,000 feet above mean sea level (msl) in the Colorado River <br />valley to over 8,000 feet ms] on the Roan Plateau north of the Colorado River and Cathedral <br />Bluffs northwest of the Colorado River. The major feature within the basin is a dissected plateau <br />standing as much as 3,000 feet above adjacent lowlands. The northem portion ~~f the structural <br />basin has been eroded into a topographic basin by the drainages of the Yellow fmd Piceance <br />Creeks, tributaries of the White River. The topography consists of ridges and valleys with local <br />relief ranging from 200 to 600 feet (Weeks et al. 1974). <br />3.1.1.2 Structural Geology <br />The Piceance Creek Basin is a broad, asymmetric, southeast-northwest trending basin controlled <br />by alternating anticlines and synclines. The deepest part of the basin is associated with the Red <br />Wash Syncline, which parallels the northern boundary of the basin, and the South Rangely <br />Syncline. These two synclines are separated by the broad Rangely Anticline. <br />Deposition of sediments into this region began with downwarping of the basin floor in the <br />Cretaceous and continued through Eocene time. Low stream gradients and moderate uplift of the <br />mazginal mountains prevented significant erosion of the basin's perimeter. This sequence of <br />events resulted in the creation of the Wasatch, Crreen River, and Uinta Formations in and azound <br />a series of landlocked lakes (Bradley 1964, Tweto 1980). The surFcial sedimentary rocks from <br />this depositional sequence are shown in Figure 3.1-1. <br />Numerous lineations associated with joints, fractures, normal faults, and graben:> aze present in <br />the northern Piceance Creek Basin. The lazgest of the grabens is the Dudley Bluffs fault system. <br />The Dudley Bluffs Graben is located approximately one mile south of the southf;m boundary of <br />the Piceance Site. The primary joint set in the Piceance Creek Basin strikes west by northwest, <br />with a secondary set striking north by northeast. <br />3.1.1.3 Stratigraphy <br />The Piceance Creek Basin contains stratified sediments ranging in age from Cambrian through <br />middle Tertiary. The northern half of the basin is deepest and has the thickest stratigraphic <br />sequence of sediments. Stratigraphically there are approximately 28,000 feet of rock units <br />between the highest point on the White River Uplift to the east and the lowest depth of the basin, <br />the Precambrian crystalline basement. The Precambrian crystalline basement rock is estimated <br />to be 24,000 feet below ground surface (bgs) in the central portion of the northern Piceance <br />Creek Basin (Murray and Haun 1974). <br />In general, a thin veneer of unconsolidated Quaternary alluvium, valley fill, and terrace deposits <br />occupies low-lying azeas once occupied by ancient river channels. Below these tmconsolidated <br />sediments lie approximately 8,000 feet of Tertiary sedimentary deposits. The Tertiary section <br />consists of three major formations, the Uinta (Eocene), Green River, and Wasatch (Paleocene- <br />Eocene) Formations. These sediments grade into the unconformable top of the Cretaceous Mesa <br />3-2 Geology <br />