Midway Valley, located at the eastern base of Yucca Mountain in southwestern Nevada, is the preferred location of the surface facilities for the potential high-level nuclear waste repository at Yucca Mountain. One goal in siting these surface facilities is to avoid faults that could produce relative displacements in excess of 5 cm in the foundations of the waste-handling buildings. This study reviews existing geologic and geophysical data that can be used to assess the potential for surface fault rupture within Midway Valley. Dominant tectonic features in Midway Valley are north-trending, westward-dipping normal faults along the margins of the valley: the Bow Ridge fault to the west and the Paintbrush Canyon fault to the east. Both faults displace Quaternary sediments. Published estimates of average Quaternary slip rates for these faults are very low (=10 3 mm/yr), but the age of most recent displacement and the amount of displacement per event are largely unknown. Surtace mapping and interpretive cross sections, based on limited drillhole and geophysical data, suggest that additional normal faults, including the postulated Midway Valley fault, may exist beneath the Quaternary/Tertiary fill within the valley. Existing data, however, are inadequate to determine the location, recency, and geometry of this faulting.To confidently assess the potential for significant Quaternary faulting in Midway Valley, additional data are needed that define the stratigraphy and structure of the strata beneath the valley, characterize the Quaternary soils and surfaces, and establish the age of faulting. The use of new and improved geophysical techniques, combined with a drilling program, _tfers the greatest potentinl for resolving subsurface structure in the valley. Mapping of surfici_ll get,logic units and logging of soil pits and trenches within these units must be completed, using accepted state-of-the-art practices supported by multiple quantitative numeric_tl and r clat ivc age-tinting techniques." e! "_' ';':" " i_':_ Hoover et al. (1981) and Hoover (1989) 17 3.3 Soil-Geomorphology Studies in Crater Flat (Peterson, 1988) (iv)LIST OF TABLES Bull and Ku, 1975 and Ku et al., 1979) 16 Table 3-2 -Correlation characteristics of surficial deposits (from Hoover et al., 1981 andHoover, 1989) 19 Table 3-3 -Age and characteristics of stratigraphic units of Hoover et al. (1981) and Hoover (1989) 21 Valley, the proposed location of the prospective surface facilities. The data obtained during this study will be used in conjunction with other site characterization activities to support:(1) the siting of the surface facilities, and (2) an assessment of the potential effects of surface faulting on the design of the surface facilities.
SCOPE OF WORKThe scope of work for this task includes a review of available published and unpublished literature, maps, and data that are relevant to evaluation of the stratigraphy, structure, and tectonics of the Midway Valley area. Emphasis was placed on: (1) infbrmation pertaining to the nat...
