Surface and subsurface geology of the Camarillo and Las Posas Hills area, Ventura County, California

by Jakes, Mary Clare

Surface and subsurface data are combined to determine the

structure of the western half of the Simi fault system in the Las Posas

and Camarillo Hills area. Cretaceous (?) to Eocene sedimentary

rocks, present only in the subsurface, are overlain by late Eocene

to early Miocene nonmarine stata (Sespe Formation) and middle

Miocene volcanics and sedimentary rocks (Conejo Volcanics and

Topanga Formation undifferentiated), in part exposed in the Las Posas

Hills. Late Miocene marine beds (Modelo Formation) are present in

the subsurface in the Camarillo Hills and may crop out in the eastern

Santa Rosa Valley. These rocks are overlain unconformably by

marine Pliocene-Pleistocene beds (Saugus Formation), older and

younger Quaternary alluvium, and alluvial fan deposits. Normal

faults cause the Sespe to subside towards a thick volcanic pile, built

up in the Conejo Hills in the middle Miocene. Volcanic rocks buttressed against and later overtopped these Sespe subsidence structures.

Reverse faults in the Oxnard plain and the left-lateral Somis

fault are truncated by the unconformity at the base of the Saugus.

Miocene and older strata were broadly folded in the Las Posas anti-cline

and Santa Rosa syncline prior to deposition of the Saugus Formation

and displacement on the Simi fault zone. The Bailey fault, a

northwest-trending range-front fault, shows reverse separation, commonly

follows Sespe subsidence structures and north-dipping normal

faults which cut the Sespe. The Camarillo Hills anticline, Springville

dome and post-Saugus Las Posas anticline appear to be pressure

ridges adjacent to the Simi fault system on the north. Older alluvium

deposits are uplifted and warped. The Camarillo fault cut and warped

older alluvium.