Geophysical Studies Based on Gravity and Seismic Data of Tule Desert, Meadow Valley Wash, and California Wash Basins, Southern Nevada

Gravity and seismic data from Tule Desert, Meadow Valley Wash, and California
Wash, Nevada, provide insight into the subsurface geometry of these three
basins that lie adjacent to rapidly developing areas of Clark County, Nevada.
Each of the basins is the product of Tertiary extension accommodated with the
general form of north-south oriented, asymmetrically-faulted half-grabens.
Geophysical inversion of gravity observations indicates that Tule Desert and
Meadow Valley Wash basins are segmented into subbasins by shallow, buried
basement highs. In this study, basement refers to pre-Cenozoic bedrock units
that underlie basins filled with Cenozoic sedimentary and volcanic units. In
Tule Desert, a small, buried basement high inferred from gravity data appears
to be a horst whose placement is consistent with seismic reflection and
magnetotelluric observations. Meadow Valley Wash consists of three subbasins
separated by basement highs at structural zones that accommodated different
styles of extension of the adjacent subbasins, an interpretation consistent
with geologic mapping of fault traces oblique to the predominant north-south
fault orientation of Tertiary extension in this area. California Wash is a
single structural basin. The three seismic reflection lines analyzed in this
study image the sedimentary basin fill, and they allow identification of faults
that offset basin deposits and underlying basement. The degree of faulting and
folding of the basin-fill deposits increases with depth. Pre-Cenozoic units are
observed in some of the seismic reflection lines, but their reflections are
generally of poor quality or are absent. Factors that degrade seismic reflector
quality in this area are rough land topography due to erosion, deformed
sedimentary units at the land surface, rock layers that dip out of the plane of
the seismic profile, and the presence of volcanic units that obscure underlying
reflectors. Geophysical methods illustrate that basin geometry is more
complicated than would be inferred from extrapolation of surface topography and
geology, and these methods aid in defining a three-dimensional framework to
understand groundwater storage and flow in southern Nevada.

[Summary provided by the USGS.]