C153705 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I I Beaver Reservoir Company Attention: Mr. Neil Thompson February 4, 1993 Page 7 The sedimentary rocks were intruded by granodioritic rocks during Middle to Late Tertiary Time. These intrusions were primarily in the form of stocks, dikes, sills, and laccoliths. Although the region is generally floored by much older Precambrian Age rocks, the granodioritic intrusions general occurred at or near the boundary between the mesaverde and underlying Mancos shale. In addition there are volcanic deposits in the form of ash flow tuff, basalt flows and volcanic breccia (Ellis et aI. 1987). The major structural feature in the project region is an anticline which forms the down Wlllp of the Piceance Basin and the intrusive rocks ,in the form of stocks, dikes, sills, and laccoliths. The axis of the anticline is located northeast of the project area and has a northwest-southeast trend. There is a thrust fault along the west edge of the Elk Mountains approximately 25 miles from the Project (Gaskill et al. 1977). This fault has been inteIpreted to be Laramide in age, (Kirkham and Rogers 1981). Another fault has been inteIpreted to be about 30 milessoutllwesLof the Project (Gaskill et al. 1977). This fault is referred to as the Curecanti (Gaskill-eLaI.1977)-or the Cimarron Fault (Kirkham and Rogers 1981). This fault has been inteIpreted as being Precambrian in age. Neither of the faults are classified as active or potentially active (Kirkham and Rogers 1981). SITE GEOLOGY The site geology was developed based on data collected by Reclamation for their dam study in 1940 and the data collected from the current test hole drilling program and geophysical investigation performed for this report. The bedrock underlying Beaver Reservoir generally consists of the Mesaverde Formation, inteIpreted to be either the lower portion of the Ohio Creek Member and/or the overlying Barren Member. Laccolithic intrusions which formed peaks such as Coal Mountain located southwest .for the Project. These laccolithic intrusions into the Mancos shale and the overlying .Mesaverde Formation resulted in a tilting of the overlying bedrock. At the Project the' bedrock has a strike of approximately N50W and a dip towards the east at approximately 20 to. 25 degrees. Faulting likely accompanied the tilting of the beds. Several faults have been inteIpreted as present within the project area based on test hole information, mine workings, and outcrop data (Ellis et al. 1987 and Gaskill et al. 1977). The bedrock is intermittently exposed in the project area downstream of the dam, limited areas within the reservoir, and around the western margin of the reservoir. The existing dam embankment and reservoir blanketing mantles the bedrock in the immediate vicinity of the dam. Bedrock underlying the dam consists primarily of sandstone and shale with 23067/LEtO 02-04-93(4:24pm)/RPT