QUATERNARY GEOLOGY OF THE MESQUITE LAKE 1:100,000-SCALE QUADRANGLE, CALIFORNIA AND NEVADA

As part of a USGS project to map Quaternary deposits in the arid Southwest USA (http://deserts.wr.usgs.gov), this map and associated database depict process and age characteristics of surficial deposits and generalized bedrock units. The 100,000-scale Mesquite Lake Quadrangle, located south of Las Vegas, NV encompasses characteristic Basin and Range topography with northwest-trending, high-relief mountains separated by arid internally and externally drained alluvial valleys. Geomorphic position, deposit surface roughness, pavement maturity, development of pedogenesis, source lithology, and elevation were used to differentiate deposit ages. Geologic mapping, conducted using field methods and interpretation of remote sensing images, identified units offset by Quaternary tectonics, debris-flow prone areas, and active aeolian features. Evidence of tectonics is primarily limited to late Pleistocene and older deposits. Faults are oriented northwest and located in the Spring, Bird Spring, Lucy Gray, Highland, and McCullough Mountains. Some Quaternary faults were linked to previously recognized bedrock structures with late Tertiary offset. In the Kingston Range, we identified five regionally extensive, Quaternary debris-flow sequences. As a proxy for rock strength, Schmidt Hammer measurements of debris-flow boulders depict decreasing values with increasing deposit age. Mid-Holocene deposits exhibit similar values to unweathered material whereas boulders on mid-Pliestocene deposits express substantially lower strength as the varnished surfaces spall and the granitic rock weathers to grus. Mass wasting is also an active process during the present arid climate, as evidenced by the intermingling of asphalt and a 1930's vintage automobile with debris-flow deposits. Lastly, valley axis floodplains, salt flat deposits, and playas were determined to be important sources for eolian transport. In Pahrump, Mesquite, and Ivanpah Valleys, sand sheets, dunes, and vegetation mounds are well developed on the eastern and southeastern edges of valley axis and playas. Continuing research efforts are investigating the relationships between surficial geology, relative erodibility, hydrologic properties, and plant distribution in the context of land use and climate change.