NEW GEOLOGIC MAPPING OF THE SIERRA NEVADA FOOTHILLS IN AMADOR AND CALAVERAS COUNTIES, CALIFORNIA

The California Geological Survey (CGS) is engaged in a multi-year project to prepare new 1:24,000-scale geologic maps for selected 7.5’ quadrangles in the central Sierra Nevada foothills of California. The project is supported with federal matching funds through the STATEMAP component of the National Cooperative Geologic Mapping Program administered by the USGS. So far, preliminary geologic maps of the Ione, Wallace and Valley Springs 7.5-minute quadrangles have been made available for download from the CGS Preliminary Geologic Maps web page. New detailed mapping of the Irish Hill 7.5-minute quadrangle is on track for release in June 2016, and new mapping of the San Andreas and/or Jenny Lind 7.5-minute quadrangles are planned for 2017. The long-range goal is to digitally compile a new geologic map for the San Andreas 30’ x 60’ quadrangle based on CGS’s detailed mapping in the more developed areas, and best available mapping for remaining areas.

The western foothills of the Sierra Nevada Mountains is a region of geologic complexity primarily as a result of accretionary tectonics and interactions between the western margin of the North American Plate and adjacent plates during Late Paleozoic and Mesozoic time. Several accreted terranes have been identified by previous workers and are part of the mapping area. These include, from west to east, the Jurassic Foothills (Western) terrane, a serpentinite belt, a mélange belt, and the Mother Lode belt. Separating the Foothills terrane and the serpentinite belt is the Bear Mountains Fault Zone. Reactivation of the Bear Mountains Fault Zone during the Quaternary has manifested along the Waters Peak, Ione, and Haupt Creek faults. CGS mapping has better characterized the location of and movement along these reactivated faults where they have displaced Cenozoic units.

Overlying the highly deformed tectonically emplaced Paleozoic and Mesozoic rocks is a sequence of mostly non-marine sedimentary and volcaniclastic rocks of Eocene to Miocene age that occur discontinuously in the higher elevations. These units were typically deposited in paleochannels and now form inverted topography. The Oligocene to Miocene Valley Springs Formation is in part smectitic and poses a potential slope stability hazard.