Yucca Mountain, Nye County, Nevada, has been identified as apotential site for underground storage of high-levelradioactive waste. This geologic map compilation, includingall of Yucca Mountain and Crater Flat, most of the CalicoHills, western Jackass Flats, Little Skull Mountain, theStriped Hills, the Skeleton Hills, and the northeasternAmargosa Desert, portrays the geologic framework for asaturated-zone hydrologic flow model of the Yucca Mountainsite. Key geologic features shown on the geologic map andaccompanying cross sections include: (1) exposures ofProterozoic through Devonian strata inferred to have beendeformed by regional thrust faulting and folding, in theSkeleton Hills, Striped Hills, and Amargosa Desert near BigDune; (2) folded and thrust-faulted Devonian and Mississippianstrata, unconformably overlain by Miocene tuffs and lavas andcut by complex Neogene fault patterns, in the Calico Hills; (3)the Claim Canyon caldera, a segment of which is exposed northof Yucca Mountain and Crater Flat; (4) thick densely welded tononwelded ash-flow sheets of the Miocene southwest Nevadavolcanic field exposed in normal-fault-bounded blocks at YuccaMountain; (5) upper Tertiary and Quaternary basaltic cindercones and lava flows in Crater Flat and at southernmost YuccaMountain; and (6) broad basins covered by Quaternary and upperTertiary surficial deposits in Jackass Flats, Crater Flat, andthe northeastern Amargosa Desert, beneath which Neogene normaland strike-slip faults are inferred to be present on the basisof geophysical data and geologic map patterns.A regional thrust belt of late Paleozoic or Mesozoic ageaffected all pre-Tertiary rocks in the region; main thrustfaults, not exposed in the map area, are interpreted tounderlie the map area in an arcuate pattern, striking north,northeast, and east. The predominant vergence of thrust faultsexposed elsewhere in the region, including the Belted Range andSpecter Range thrusts, was to the east, southeast, and south.The vertical to overturned strata of the Striped Hills arehypothesized to result from successive stacking of three south-vergent thrust ramps, the lowest of which is the Specter Rangethrust. The CP thrust is interpreted as a north-vergentbackthrust that may have been roughly contemporaneous with theBelted Range and Specter Range thrusts.The southwest Nevada volcanic field consists predominantly of aseries of silicic tuffs and lava flows ranging in age from 15to 8 Ma. The map area is in the southwestern quadrant of thesouthwest Nevada volcanic field, just south of the TimberMountain caldera complex.The Claim Canyon caldera, exposed in the northern part of themap area, contains thick deposits of the 12.7-Ma Tiva CanyonTuff, along with widespread megabreccia deposits of similarage, and subordinate thick exposures of other 12.8- to 12.7-MaPaintbrush Group rocks. An irregular, blocky fault array,which affects parts of the caldera and much of the nearby area,includes several large-displacement, steeply dipping faultsthat strike radially to the caldera and bound south-dippingblocks of volcanic rock.South and southeast of the Claim Canyon caldera, in the areathat includes Yucca Mountain, the Neogene fault pattern isdominated by closely spaced, north-northwest- to north-northeast-striking normal faults that lie within a north-trending graben. This 20- to 25-km-wide graben includes CraterFlat, Yucca Mountain, and Fortymile Wash, and is bounded on theeast by the "gravity fault" and on the west by the BareMountain fault. Both of these faults separate Proterozoic andPaleozoic sedimentary rocks in their footwalls from Miocenevolcanic rocks in their hanging walls.Stratigraphic and structural relations at Yucca Mountaindemonstrate that block-bounding faults were active before andduring eruption of the 12.8- to 12.7-Ma Paintbrush Group, andsignificant motion on these faults continued until after the11.6-Ma Rainier Mesa Tuff was deposited. North of Crater Flat,in and near the Claim Canyon caldera, most of the tilting ofthe volcanic section predated the 11.6-Ma Rainier Mesa Tuff.In contrast, geologic relations in central and southern YuccaMountain indicate that much of the stratal tilting thereoccurred after 11.6 Ma, probably synchronous with the mainpulse of vertical-axis rotation that occurred between 11.6 and11.45 Ma.Beneath the broad basins, such as Crater Flat, Jackass Flats,and the Amargosa Desert, faults are inferred from geophysicaldata. Geologic and geophysical data imply the presence of thelarge-offset, east-west-striking Highway 95 fault beneathsurficial deposits along the northeast margin of the AmargosaDesert, directly south of Yucca Mountain and Crater Flat. TheHighway 95 fault is interpreted to be downthrown to the north,with a component of dextral displacement. It juxtaposes ablock of Paleozoic carbonate rock overlain by a minimalthickness of Tertiary rocks (to the south) against the Miocenevolcanic section of Yucca Mountain (to the north).Alluvial geomorphic surfaces compose the bulk of Quaternarysurficial units in the Yucca Mountain region. Depositsassociated with these surfaces include alluvium, colluvium, andminor eolian and debris-flow sediments. Photogeologic andfield studies locally have identified subtle fault scarps thatoffset these surfaces, and other evidence of Quaternary faultactivity.
