Summary and evaluation of existing geological and geophysical data near prospective surface facilities in Midway Valley, Yucca Mountain Project, Nye County, Nevada; Yucca Mountain Site Characterization Project

Abstract

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...

Petrography, mineralogy, and chemistry of calcite-silica deposits at Exile Hill, Nevada, compared with local spring deposits

“…However, the vein calcretes adjacent to or within the Rainier Mesa Tuff (882,pl and 901,pl) have a flatter chondrite-normalized REE slope and a more pronounced negative Eu anomaly than the calcretes at Trench 14. This different pattern at Trench 14A has similarities to two of the different tuffs found at Trench 14A (the Rainier Mesa Tuff, sample 431,pl, that forms the hanging wall of the fault, and a n unnamed vitric nonwelded tuff within the fault, sample 88O,pl, that is part of the unit designated tb by Gibson et al 1992).…”

Near-surface velocity modeling at Yucca Mountain using borehole and surface records from underground nuclear explosions

“…YuccaMountah station 29 is located east of the mountain block near the proposed location for the repository surface facilities in hole UE-25 RF#4 Figure 2). The site geology is relatively simple, with 46m of alluvium overlying 35m of nonwelded Unit 'X' tuff over the Tiva Canyon member of the Paintbrush tuff (Gibson et al, 1992). The downhole accelerometer was located 82m below the surface, just below the boundary between Unit 'Y and the Tiva Canyon.…”

“…The Tiva Canyon tuff (TCw thermal stratigraphic unit) is 1.5 W s e c ; the Yucca Mountain and Pah Canyon members (PTn thermal stratigraphic unit) are assigned 2.3 W s e c , and the two thermal stratigraphic units corresponding to the Topopah Springs member, TSwl and TSw2, are assigned velocities of 3.1 and 3.9 km/sec, respectively. The Tiva Canyon member of the Paintbrush T e which tops the hole, is quite slow where measured in Midway Valley (see Gibson et al, 1992); this is consistent with the value of 1.5 km/sec found here, The velocity of the Topopah Springs member is variable, depending on the degree of welding and lithophysal content (Spengler, Chornack Muller and Kibler, 1984), but the modeling process revealed it to be relatively homogeneous at the wavelengths sampled here, thus we differentiate only between the two major thermal stratigraphic units. The model for station 28 appears in Figure 15 25,27), however the overall level of radial ground motion predicted by the transfer function is quite consistent with the observations.…”

Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

“…The rock magnetic properties of the TC Tuff are documented by abundant published data (Egli & Lowrie, ; Jackson et al, ; Rosenbaum, , ; Rosenbaum & Rivers, ; Rosenbaum & Snyder, ; Schlinger et al, , ; Till et al, ; Worm & Jackson, ), and the geologic setting of the TC Tuff has been extensively studied due to its proximity to the potential nuclear waste repository at Yucca Mountain (Buesch et al, ; Flint et al, ; Frizzell & Shulters, ; Gibson et al, ; Istok et al, ; Levy et al, ; Moyer et al, ; Rautman & Engstrom, ). In this paper, we present the first paleointensity results from the basal vitric zone TC Tuff, as well as previously unpublished rock magnetic results from two new sections.…”