Geology of Oak Ridge Uplift, Ventura and Los Angeles Counties, California

Thomas W. Dibblee, Jr., Robert S. Yeats

The low mountain range south of the Santa Clara River valley, that includes, from east to west, the Santa Susana and Oak Ridge Mountains and South Mountain, is herein referred to as the Oak Ridge uplift. It is a continuous east-trending range about 36 mi (58 km) long. This uplift is separated from the east-west-trending San Gabriel Mountains by the Newhall Pass area.

The Oak Ridge uplift is one of the youngest features of the Transverse Ranges. It evolved during Quaternary time from compressive anticlinal uplift of the Cenozoic sediments on the south flank of the Ventura basin along the Oak Ridge and Santa Susana faults. Terrestrial and marine Pliocene-Pleistocene detrital sediments are exposed on the flanks along most of the Oak Ridge uplift. This series is extremely thick on the north flank but very thin on the south flank.

Miocene marine formations, the Oligocene nonmarine Sespe Formation, and Paleogene to Upper Cretaceous strata form the core of this uplift. The Miocene units include siliceous shale of the Monterey (Modelo) Formation that forms its grassy crest. The Sespe Formation is underlain by the marine Llajas Formation (middle Eocene), which together with the underlying Paleocene and Upper Cretaceous marine units crop out along the Simi Hills uplift.

The anticlinal Oak Ridge uplift, composed of several domed segments, apparently evolved from an elongate seamount that developed during the Pliocene, south of the deep Ventura basin depositional trough. The western part of the uplift was thrust northward along the Oak Ridge fault against the elongate east-west-trending depocenter. The eastern part was thrust southward along the Santa Susana fault, in part against the preexisting Simi Hills uplift.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.