Surface and subsurface mapping are combined to determine the geologic history along the San Gabriel fault near the town of Castaic. Palomas Gneiss, Whitaker Granodiorite, and Pelona Schist are basement terranes encountered in the subsurface. West of the San Gabriel fault, basement is unconformably overlain by marine middle to late Miocene Modelo Formation. The late Miocene to early Pliocene Towsley Formation overlies the Modelo and was deposited in a submarine fan environment.
East of the San Gabriel fault, marine Paleocene San Francisquito Formation accumulated while Pelona Schist was undergoing regional metamorphism at depths of 20 to 27 kilometers. Nonmarine Oligocene(?) Vasquez Formation is faulted against both the Pelona Schist and San Francisquito Formation. Charlie Canyon Megabreccia accumulated in late Oligocene(?) time as a large landslide deposit. The source is controversial but may have been from the LaFanza Range. Pelona Schist-bearing San Francisquito Canyon Breccia accumulated in late Miocene(?) (Barstovian) time as the Pelona Schist first became subject to erosion in northern Soledad basin.
Nonmarine alluvial fan and lacustrine deposits of the middle to late Miocene Mint Canyon Formation unconformably overlie older units east of the San Gabriel fault and apparently intertongue with San Francisquito Canyon Breccia. A late Miocene (Mohnian) marine transgression resulted in deposition of the Castaic Formation in Soledad basin, while the Modelo and possibly Towsley Formations accumulated in Ventura basin. Violin Breccia intertongues with Castaic Formation and accumulated at the base of a Miocene San Gabriel fault scarp. Marine Pliocene Pico Formation unconformably overlies the Castaic Formation east of the San Gabriel fault and conformably overlies the Towsley Formation west of the San Gabriel fault; it is overlain conformably by marine to nonmarine Plio-Pleistocene Saugus Formation. Quaternary landslides, and older, intermediate, and younger alluvium overlie older rocks throughout the study area.
The St. Francis fault may have been active in late Oligocene and early Miocene time, removing Charlie Canyon Megabreccia by right-slip from a possible source in the La Panza Range. The low-angle San Francisquito fault and associated Bee Canyon fault and San Francisquito syncline were active after deposition of the Charlie Canyon Megabreccia but prior to deposition of the Mint Canyon Formation.
The San Gabriel fault became active after deposition of the Mint Canyon Formation in late Miocene time. The San Gabriel fault exhibits right-lateral separation of approximately 60 kilometers on the Mint Canyon and older formations, approximately 30 kilometers on Violin Breccia and Modelo Formation, and about two kilometers on the Pico Formation. San Gabriel fault "C"(?) and Castaic Hills reverse fault moved in late Miocene time and became inactive prior to deposition of the Pico Formation.
The Charlie Canyon anticline, Charlie Canyon syncline, Castaic anticline, and Ridge Basin syncline were formed in the Castaic Formation prior to deposition of the Saugus Formation. The Saugus is deformed by: (1) folding which produced the Dry Canyon syncline, Dry Canyon anticline, Townsend syncline, Loma Verde anticline, North and South Hasley Canyon synclines, and Oak Canyon anticline; (2) reverse faulting on the south-dipping Hasley fault; (3) normal faulting in eastern Castaic Hills oil field; (4) normal faulting on the San Gabriel fault. The San Gabriel fault may also offset Quaternary alluvium by normal separation in Castaic Creek. Geomorphic evidence suggests continuing movement on the San Gabriel fault, but the nature of seismic activity on the fault is still unresolved.