Geologic map of the Yucca Mountain region, Nye County, Nevada

Yucca Mountain, Nye County, Nevada, has been identified as a
potential site for underground storage of high-level
radioactive waste. This geologic map compilation, including
all of Yucca Mountain and Crater Flat, most of the Calico
Hills, western Jackass Flats, Little Skull Mountain, the
Striped Hills, the Skeleton Hills, and the northeastern
Amargosa Desert, portrays the geologic framework for a
saturated-zone hydrologic flow model of the Yucca Mountain
site. Key geologic features shown on the geologic map and
accompanying cross sections include: (1) exposures of
Proterozoic through Devonian strata inferred to have been
deformed by regional thrust faulting and folding, in the
Skeleton Hills, Striped Hills, and Amargosa Desert near Big
Dune; (2) folded and thrust-faulted Devonian and Mississippian
strata, unconformably overlain by Miocene tuffs and lavas and
cut by complex Neogene fault patterns, in the Calico Hills; (3)
the Claim Canyon caldera, a segment of which is exposed north
of Yucca Mountain and Crater Flat; (4) thick densely welded to
nonwelded ash-flow sheets of the Miocene southwest Nevada
volcanic field exposed in normal-fault-bounded blocks at Yucca
Mountain; (5) upper Tertiary and Quaternary basaltic cinder
cones and lava flows in Crater Flat and at southernmost Yucca
Mountain; and (6) broad basins covered by Quaternary and upper
Tertiary surficial deposits in Jackass Flats, Crater Flat, and
the northeastern Amargosa Desert, beneath which Neogene normal
and strike-slip faults are inferred to be present on the basis
of geophysical data and geologic map patterns.
A regional thrust belt of late Paleozoic or Mesozoic age
affected all pre-Tertiary rocks in the region; main thrust
faults, not exposed in the map area, are interpreted to
underlie the map area in an arcuate pattern, striking north,
northeast, and east. The predominant vergence of thrust faults
exposed elsewhere in the region, including the Belted Range and
Specter Range thrusts, was to the east, southeast, and south.
The vertical to overturned strata of the Striped Hills are
hypothesized to result from successive stacking of three south-
vergent thrust ramps, the lowest of which is the Specter Range
thrust. The CP thrust is interpreted as a north-vergent
backthrust that may have been roughly contemporaneous with the
Belted Range and Specter Range thrusts.
The southwest Nevada volcanic field consists predominantly of a
series of silicic tuffs and lava flows ranging in age from 15
to 8 Ma. The map area is in the southwestern quadrant of the
southwest Nevada volcanic field, just south of the Timber
Mountain caldera complex.
The Claim Canyon caldera, exposed in the northern part of the
map area, contains thick deposits of the 12.7-Ma Tiva Canyon
Tuff, along with widespread megabreccia deposits of similar
age, and subordinate thick exposures of other 12.8- to 12.7-Ma
Paintbrush 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 faults
that strike radially to the caldera and bound south-dipping
blocks of volcanic rock.
South and southeast of the Claim Canyon caldera, in the area
that includes Yucca Mountain, the Neogene fault pattern is
dominated 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 Crater
Flat, Yucca Mountain, and Fortymile Wash, and is bounded on the
east by the "gravity fault" and on the west by the Bare
Mountain fault. Both of these faults separate Proterozoic and
Paleozoic sedimentary rocks in their footwalls from Miocene
volcanic rocks in their hanging walls.
Stratigraphic and structural relations at Yucca Mountain
demonstrate that block-bounding faults were active before and
during eruption of the 12.8- to 12.7-Ma Paintbrush Group, and
significant motion on these faults continued until after the
11.6-Ma Rainier Mesa Tuff was deposited. North of Crater Flat,
in and near the Claim Canyon caldera, most of the tilting of
the volcanic section predated the 11.6-Ma Rainier Mesa Tuff.
In contrast, geologic relations in central and southern Yucca
Mountain indicate that much of the stratal tilting there
occurred after 11.6 Ma, probably synchronous with the main
pulse of vertical-axis rotation that occurred between 11.6 and
11.45 Ma.
Beneath the broad basins, such as Crater Flat, Jackass Flats,
and the Amargosa Desert, faults are inferred from geophysical
data. Geologic and geophysical data imply the presence of the
large-offset, east-west-striking Highway 95 fault beneath
surficial deposits along the northeast margin of the Amargosa
Desert, directly south of Yucca Mountain and Crater Flat. The
Highway 95 fault is interpreted to be downthrown to the north,
with a component of dextral displacement. It juxtaposes a
block of Paleozoic carbonate rock overlain by a minimal
thickness of Tertiary rocks (to the south) against the Miocene
volcanic section of Yucca Mountain (to the north).
Alluvial geomorphic surfaces compose the bulk of Quaternary
surficial units in the Yucca Mountain region. Deposits
associated with these surfaces include alluvium, colluvium, and
minor eolian and debris-flow sediments. Photogeologic and
field studies locally have identified subtle fault scarps that
offset these surfaces, and other evidence of Quaternary fault
activity.