QUATERNARY GEOLOGY OF THE AMBOY 1:100,000-SCALE QUADRANGLE, CALIFORNIA

A USGS project focused on the arid Southwest USA (http://deserts.wr.usgs.gov/) conducted surficial geologic mapping and process studies as critical elements in the study of landscape dynamics. The Amboy 1:100,000-scale Quadrangle is located in the eastern Mojave Desert, between Los Angeles, CA and Laughlin, NV along the Interstate 40 corridor. Landscape features include high-relief mountains, low pediments and intervening alluvial valleys. The area spans the transition from the tectonically inactive eastern Mojave Desert to the active central Mojave and as such each area has distinct geomorphic expressions. Surficial map units are broken out by field and remote sensing interpretations into depositional process and age classifications based on inset relationships, pedogenesis, pavement development, source lithology, elevation, and other geomorphic characteristics. The timing of deposition, geomorphic expression, and nature of alluvial, groundwater discharge, wash, and eolian systems were analyzed to determine landscape development patterns. These patterns include timing and rates of erosion and deposition, mountain front and stream profiles, and spatial and temporal relationships between mapped units. These characteristics are compared based on the influence of late Quaternary tectonics, and bedrock source lithology. Preliminary results show that the tectonically quiescent eastern Mojave portion of the map area developed symmetric valley networks at near consistent base levels through the late Pleistocene, followed by incision or burial in the Holocene. The tectonically active central Mojave portion of the map area consists of asymmetric northwest trending valleys and indeterminate timing of base level changes. Ongoing studies involve investigating the relationships between surficial geology, hydrologic and biologic systems.