Geologic Map of the Mound Spring Quadrangle, Nye and Clark Counties, Nevada, and Inyo County, California

The Mound Spring quadrangle, the southwestern-most 7.5'
quadrangle of the area of the Las Vegas 1:100,000-scale
quadrangle, is entirely within the Pahrump Valley, spanning the
Nevada/California State line. New geologic mapping of the
predominantly Quaternary materials is combined with new studies
of gravity and geochronology in this quadrangle. Eleven
predominantly fine-grained units are delineated, including playa
sediment, dune sand, and deposits associated with several cycles
of past groundwater discharge and distal fan sedimentation.
These units are intercalated with 5 predominantly coarse-grained
alluvial-fan and wash gravel units mainly derived from the Spring
Mountains. The gravel units are distinguished on the basis of
soil development and associated surficial characteristics.
Thermoluminescence and U-series geochronology constrain most of
the units to the Holocene and late and middle Pleistocene.
Deposits of late Pleistocene groundwater discharge in the
northeast part of the quadrangle are associated with a down-to-
the-southwest fault zone that is expressed by surface fault
scarps and a steep gravity gradient. The gravity field also
defines a northwest-trending uplift along the State line, in
which the oldest sediments are poorly exposed. About 2 km to
the northeast a prominent southwest-facing erosional escarpment
is formed by resistant beds in middle Pleistocene fine-grained
sediments that dip northeast away from the uplift. These
sediments include cycles of groundwater discharge that were
probably caused by upwelling of southwesterly groundwater flow
that encountered the horst.