Ridge Basin and San Gabriel fault in the Castaic Lowland, southern California

Robert S. Yeats

Published 2003, SCEC Contribution #641

In the Castaic Lowland, the Ridge Basin is overlain unconformably at its southeastern end by Fernando and Saugus formations, which were deposited after folding of the Ridge Basin syncline. The area southwest of the San Gabriel fault is underlain by a subsurface ridge of basement rocks overlain by lower Mohnian and younger strata. The southwest boundary of the basement ridge is the faulted margin of a Miocene rift in the east Ventura Basin. This fault is mapped as the Canton fault, a precursor of the San Gabriel fault that underwent about 30 km of right slip mostly in the middle Miocene, displacing basement rocks and Paleogene strata from their outcrop in Piru and Canton creeks to the subsurface near Placerita Oil Field. The Devil Canyon fault, mainly lower Mohnian, crosses the basement ridge northwest of Castaic Hills Oil Field.
The Castaic Formation of the Ridge Basin continues into the subsurface, where a Mohnian submarine turbidite fan is truncated obliquely by the San Gabriel fault. This fan, equivalent in age to the upper Mohnian part of the Modelo Formation, may have been ponded against the San Gabriel fault. Early displacement on the San Gabriel fault is documented in wells near Honor Rancho Oil Field containing sedimentary breccia near the top of the Mint Canyon Formation, with the age estimated as 10-12 Ma. The lower Mohnian Devil Canyon Conglomerate fan crosses the Canton fault and has its apex northwest of the Honor Rancho Oil Field; it was derived from the San Gabriel Mountains, now offset at least 35 km. The Hasley Conglomerate at the base of the Towsley Formation, equivalent in age to nonmarine strata of the Ridge Basin and to the upper part of the Castaic Formation in the Castaic Lowland, has its source in the western San Gabriel Mountains and Tertiary volcanics in the Soledad Basin, offset 15-25 km. Offsets of the Fernando and Saugus formations are no more than a few kilometers. The youngest beds of the Castaic Formation are equivalent in age to part of the Fernando Formation; these strata, too, may not have large offsets across the fault. The San Gabriel fault dips northeast, with normal separation north of Honor Rancho Oil Field and reverse separation farther southeast. When Quaternary strata are rotated back to horizontal, the fault dip is nearly vertical. The change in separation corresponds to a change in fault strike and represents a boundary between seismogenic segments of the fault. Northwest of the segment boundary, the fault crosses mountainous terrain; during the late Miocene, a high-standing block on the southwest shed debris to form the Violin Breccia and breccia in the Mint Canyon Formation. The San Gabriel fault is largely inactive in this region. Southeast of the boundary, the fault crosses the Castaic Lowland and has little topographic expression now and apparently did not during deposition of the Castaic Formation. In this region, the San Gabriel fault is active, displacing Holocene deposits and creating tectonic landforms. The transfer of present-day activity may be by way of the Holser fault, although that fault is not known to connect with the San Gabriel fault at or near the surface.

Citation
Yeats, R. S. (2003). Ridge Basin and San Gabriel fault in the Castaic Lowland, southern California. , : Geological Society of America. doi: 10.1130/0-8137-2367-1.131.