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

As part of a USGS project to map Quaternary deposits in the arid southwestern United States, the Amboy 1:100,000-scale quadrangle, located along Interstate 40 in the eastern Mojave Desert, roughly between Barstow, CA and Laughlin, NV is being mapped and developed as a digital geologic database. Since the area spans the transition from the tectonically inactive eastern Mojave Desert to the active central Mojave, it includes high-relief mountains, low relief pediments, and intervening alluvial valleys that have distinct geomorphic expressions. Surficial map units are identified by field and remote sensing interpretations and classified by age and depositional process based on sedimentologic features, inset relationships, pedogenesis, pavement development, source lithology, elevation, and other geomorphic characteristics. Surficial geologic maps in a spatial database are combined with field-based surficial process and ecosystem studies to analyze landscape development patterns and relationships between physical properties and ecological function.

We studied patterns of landscape development using timing and rates of erosion and deposition, mountain front and stream profiles, and spatial and temporal relationships between mapped units. Preliminary results show the tectonically quiescent eastern Mojave area developed symmetric valley networks at similar base level 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, though the timing of base level changes are indeterminate.

Preliminary results from studies of dynamics between surficial geologic units (physical properties and morphology), soil moisture, climate, and biologic systems show correlations between depositional process and pedogenesis (expressed as soil structure and texture), and distribution and characteristics of common Mojave Desert vegetation. Derivative maps from the surficial geology database along with these correlations are used as the basis for modeling vulnerability and recoverability of the landscape to natural and anthropogenic changes.