AbstractsEarth & Environmental Science

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.