QUATERNARY GEOLOGY OF THE CALICO MOUNTAINS PIEDMONT AND COYOTE LAKE, MOJAVE DESERT, CALIFORNIA

The 1:24,000 scale detailed geologic map and digital database of the Calico Mountains piedmont and Coyote (dry) Lake depicts surficial deposits and generalized bedrock units. Mapping includes parts of the Coyote Lake, Yermo, Alvord Mountain and Harvard Hill 7.5 minute quadrangles. The mapping is part of a USGS project to investigate the spatial distribution of deposits linked to changes in climate and to provide framework geology for land use management (http://deserts.wr.usgs.gov) and to understand the Quaternary tectonic history of the Mojave Desert. The map and database was produced using field methods, analyses of remote sensing images, and previous mapping. Geologic deposits were delineated by process of deposition, geomorphic position, surface roughness, pavement maturity, pedogenesis, and inset relations.

The study area lies at the boundary between fault domains composed of northwest-striking faults and east-striking faults in the northeast region of the Mojave Desert. Mapping revealed a previously unmapped north-striking Quaternary fault, named the Tin Can Alley (TCA) fault, in the Calico piedmont, which may link to the Southwest Coyote Basin (SWCB) fault. The SWCB fault, first mapped by Meek(1994), is a northwest-striking strike-slip fault with a component of compression. The fault cuts Pleistocene alluvial units and is associated with Pleistocene groundwater-discharge deposits. South of the SWCB fault lies the north striking TCA fault, a 7+ km long complex set of faults that run the length of the piedmont. They cut middle Pleistocene alluvial fan deposits and make several right steps to produce grabens. Fan deposits exhibit ~1 m high scarps, truncations and warps.

Mapping has also revealed extensive alluvial fans deposited under past climatic conditions. Fans consist mainly of Pleistocene, early and middle Holocene deposits. GIS analysis shows that 25% of the area is covered by Pleistocene alluvial deposits, 17% is covered by Pleistocene to early Holocene alluvial deposits, 45% is covered by mid Holocene deposits, and only 10% is covered by late Holocene alluvial deposits. The spatial distribution suggest most of these fans appear to be relicts of an aggradational cycle during past climates, which apparently were unlike the modern climate.