GEOLOGIC MAP OF THE SLEEPY VALLEY 7.5' QUADRANGLE, LOS ANGELES COUNTY, CALIFORNIA

The California Geological Survey (CGS) recently completed geologic mapping of the Sleepy Valley 7.5' quadrangle in northeastern Los Angeles County as part of STATEMAP, an ongoing cooperative effort with the USGS to produce seamless geologic maps of 7.5' and 30'x60' quadrangles in California. This map presents a digital compilation of the best available geologic mapping augmented by new 1:24,000-scale bedrock field mapping and petrographic analysis by CGS, resulting in better lithologic descriptions and differentiation of the igneous and metamorphic units. CGS also utilized available LiDAR data, aerial photography, and field reconnaissance to complete new detailed mapping of Quaternary units and to more accurately locate major pre-Holocene faults. This detailed geologic mapping allows for an improved understanding of the geologic framework in this tectonically active area.

The San Andreas Fault Zone crosses obliquely through the northeast corner of the Sleepy Valley quadrangle, with distinctive rock units found north of, within, and south of the fault zone. North of the fault, the area is primarily underlain by Upper Cretaceous to Lower Tertiary Portal Schist. Breccia, arkosic sandstone, and clay shale of the Pliocene Anaverde Formation are exposed on a ridge within the fault zone. South of the fault, the dominant bedrock rock unit is the Upper Cretaceous to Lower Tertiary Pelona Schist, which is exposed in the Sierra Pelona, a westward-plunging antiform in the center of the map area. Other basement rocks include a Proterozoic quartzo-feldspathic and amphibolite gneiss complex, the San Gabriel Mountains anorthosite-gabbro-syenite complex, and Cretaceous gneissic granite and tonalite units. Mylonitic rocks mapped at the contact between the Pelona Schist and the Cretaceous gneissic granite formed as a result of movement along the south-dipping Vincent Thrust. The intrusive and metamorphic rocks are locally overlain by fluvial and volcaniclastic rocks of the Upper Oligocene Vasquez Formation. This formation marks the earliest sedimentation in the Soledad basin, which developed as a result of crustal extension associated with incipient development of the San Andreas transform margin. This basin was transrotated and tectonically inverted along preexisting normal faults during the Miocene.