GEOPHYSICAL AND GEOLOGICAL CONSTRAINTS ON THE HYDROGEOLOGIC FRAMEWORK OF THE CALIFORNIA WASH REGION, SOUTHERN NEVADA

Gravity and seismic-reflection data provide insights into the subsurface stratigraphy and structure of the California Wash area of southern Nevada. This area is part of the lower Colorado flow system and stratigraphic and structural data are important in developing ground-water models of the system. Approximately 43 km of multi-channel seismic-reflection data along three lines in California Wash were reprocessed, and show strong, coherent reflections indicative of basin fill in the upper 1-3 km. Using picks on the top of pre-Cenozoic basement from the seismic data, we inverted the gravity data to determine basin thickness. The gravity inversion model indicates California Wash basin reaches depths of 2-3 km. The eastern margin of the basin coincides with a system of young (Quaternary and late Tertiary) faults, although both seismic and gravity data indicate that the major basin-bounding fault is 2-3 km west of the mapped young faults. The seismic-reflection data indicate a listric geometry to the basin-bounding fault. Based on the seismic data, we define two Cenozoic sedimentary packages in the California Wash basin; the interface between the two packages is at a depth of about 0.5 km. We interpret that the upper package consists of relatively undeformed Muddy Creek Formation and that the lower package is Horse Spring Formation.

An important hydrologic issue is the geometry of the Paleozoic carbonate aquifer across Caifornia Wash basin, and whether the aquifer is connected between the Arrow Canyon Range and the Muddy Mountains. Our preliminary geophysical and geological data suggest that at depth, there is an overlap of the aquifer across the basin, allowing for some degree of ground-water flow from the Arrow Canyon Range to the Muddy Mountains. Although the aquifer is likely connected across the basin, it is about 4 km thick in the Arrow Canyon range and only about 2 km in the Muddy Mountains.