Geology of the Yucca Mountain site area, southwestern Nevada
Yucca Mountain in southwestern Nevada is a prominent, irregularly shaped upland formed by a thick apron of Miocene pyroclastic-flow and fallout tephra deposits, with minor lava flows, that was segmented by through-going, large-displacement normal faults into a series of north-trending, eastwardly tilted structural blocks. The principal volcanic-rock units are the Tiva Canyon and Topopah Spring Tuffs of the Paintbrush Group, which consist of volumetrically large eruptive sequences derived from compositionally distinct magma bodies in the nearby southwestern Nevada volcanic field, and are classic examples of a magmatic zonation characterized by an upper crystal-rich (>10% crystal fragments) member, a more voluminous lower crystal-poor (<5% crystal fragments) member, and an intervening thin transition zone. Rocks within the crystal-poor member of the Topopah Spring Tuff, lying some 280 m below the crest of Yucca Mountain, constitute the proposed host rock to be excavated for the storage of high-level radioactive wastes. Separation of the tuffaceous rock formations into subunits that allow for detailed mapping and structural interpretations is based on macroscopic features, most importantly the relative abundance of lithophysae and the degree of welding. The latter feature, varying from nonwelded through partly and moderately welded to densely welded, exerts a strong control on matrix porosities and other rock properties that provide essential criteria for distinguishing hydrogeologic and thermal mechanical units, which are of major interest in evaluating the suitability of Yucca Mountain to host a safe and permanent geologic repository for waste storage. A thick and varied sequence of surficial deposits mantle large parts of the Yucca Mountain site area. Mapping of these deposits and associated soils in exposures and in the walls of trenches excavated across buried faults provides evidence for multiple surface-rupturing events along all of the major faults during Pleistocene and Holocene times; these paleoseismic studies form the basis for evaluating the potential for future earthquakes and fault displacements. Thermoluminescence and U-series analyses were used to date the surficial materials involved in the Quaternary faulting events. The rate of erosional downcutting of bedrock on the ridge crests and hillslopes of Yucca Mountain, being of particular concern with respect to the potential for breaching of the proposed underground storage facility, was studied by using rock varnish cation-ratio and 10 Be and 36 Cl cosmogenic dating methods to determine the length of time bedrock outcrops and hillslope boulder deposits were exposed to cosmic rays, which then served as a basis for calculating long-term erosion rates. The results indicate rates ranging from 0.04 to 0.27 cm/k.y., which represent the maximum downcutting along the summit of Yucca Mountain under all climatic conditions that existed there during most of Quaternary time. Associated studies include the stratigraphy of surficial deposits in Fortymile Wash, the major drainage course in the area, which record a complex history of four to five cut-and-fill cycles within the channel during middle to late Quaternary time. The last 2-4 m of incision probably occurred during the last pluvial climatic period, 22-18 ka, followed by aggradation to the present time. Major faults at Yucca Mountain-from east to west, the Paintbrush Canyon, Bow Ridge, Stagecoach Road, Solitario Canyon, Fatigue Wash, Windy Wash, and Northern and Southern Crater Flat Faults-trend predominantly north, are spaced 1-5 km apart, have bedrock displacements ranging from 125 m to as much as 500 m, and exhibit Quaternary movements of several centimeters to a few meters. Displacements are predominantly down to the west, and bedrock/alluvium contacts commonly are marked by fault-line scarps. The predominant northerly fault trend changes to a more northeasterly trend in adjacent areas south