NEW GEOLOGIC MAPPING IN NORTHEASTERN LOS ANGELES COUNTY, CALIFORNIA

The California Geological Survey (CGS) recently completed geologic mapping of the Ritter Ridge 7.5’ quadrangle in northeastern Los Angeles County as part of STATEMAP. An ongoing cooperative effort, with the U.S. Geological Survey, has been underway to produce seamless 1:100,000-scale geologic maps in southern California. Where warranted, new geologic mapping at 1:24,000-scale is being conducted. This detailed geologic map information is regularly used by decision-makers on variety of local and regional issues that include geologic-hazard mitigation, land-use planning, mineral resource evaluation, and watershed-basin analysis. In addition, this mapping is used extensively by the CGS Seismic Hazard Zonation Program in developing hazard zone maps for liquefaction and seismically induced landsliding.

Initial work included digitizing and compiling existing geologic maps, ranging in scale from 1:12,000 to 1:62,500. Additions and modifications were made to these data based on new field mapping and supported by interpretations using digital orthophoto layers and stereo-pair aerial photographs. The area is traversed by the northwest-trending San Andreas Fault Zone, with distinctive rock units found north and south of the fault, as well as within the fault zone. North of the fault, the area is primarily underlain by Mesozoic age Portal Schist, Cretaceous Holcomb Quartz Monzonite, and old and young alluvium. Exposed within the fault zone are Tertiary Ritter Formation overlain by fluvial deposits of the Pliocene Anaverde Formation. South of the fault, basement rocks include a Proterozoic quartzo-feldspathic and amphibolite gneiss complex, the San Gabriel Mountains anorthosite-gabbro-syenite complex, and Triassic Mt. Lowe Intrusive Suite. Also exposed are Mesozoic rocks that include diorite, Pelona Schist, and mylonite. Draped over the intrusive and basement rocks are volcanic rocks of the Oligocene to Miocene Vasquez Formation. Structural components include Sierra Pelona, a westward-plunging antiform comprised of Pelona Schist. Mylonitic rocks mapped at the contact between the Pelona Schist and the basement complex, have been interpreted as a segment of the south-dipping Vincent Thrust Fault, described as the most important structural feature in the basement rocks of the San Gabriel Mountains.