Surface and subsurface geology of the Simi Valley area, Ventura County, California

Graduation date: 1982Surface and subsurface mapping are combined to determine the structure and geologic history of the eastern\ud half of the Simi fault in the Simi Valley area. Upper Cretaceous rocks exposed south and east of the Simi Valley are overlain unconformably by the nonmarine Simi Conglomerate of Paleocene age. The Marine Paleocene unit conformably overlies and is locally interbedded with the Simi Conglomerate; it is composed of Simi Conglomerate clasts that were reworked during a marine transgression. Siltstone of the Santa Susana Formation conformably overlies the Marine Paleocene unit, disconformably overlies the Simi Conglomerate, and was deposited in a deepening marine basin. The overlying Llajas Formation was deposited during a marine regressive-transgressive-regressive cycle. The latter regression continued in late Eocene time with deposition of the overlying nonmarine Sespe Formation. Extension and normal faulting followed deposition of the Sespe, with the early Miocene Vaqueros Formation being deposited unconformably over the Sespe. Formation of the Simi anticline and Simi fault occurred after deposition of the Vaqueros and prior to the deposition of the Conejo Volcanics, and resulted in 3850 feet (1173 m) of separation along the Simi fault in the western part of the study area. The Conejo Volcanics were erupted into a structurally controlled marine basin in the Santa Monica Mountains-Conejo Hills area during the middle Miocene. The flows extended northward over the Simi fault to the southwest flank of Big Mountain. Sedimentary rocks of the Topanga Formation were deposited prior to, during, and after deposition of the Conejo Volcanics. The Neogene Topanga, Modelo, and Pico Formations in the Simi Valley area are thin, discontinuous, and bounded by unconformities, with the late Miocene to early Pliocene Towsley Formation being entirely absent. This implies that the Neogene in the Simi Valley area was characterized by shallow-water sedimentation interrupted by periods of nondeposition and erosion, and is in contrast to the conformable, predominately deep-water Neogene sequence present in the East Ventura basin. The Neogene sequence contains evidence that the Simi Valley became a structural high during the early Miocene and remained elevated through the Pliocene.\ud Formation of the south-dipping Marr, Brugher, Llajas, Ybarra Canyon, and possibly the Corredo faults is assumed to have occurred during Pico deposition, coincident with the formation of the south-dipping Frew fault in the Aliso Canyon area. Movement along the present trace of the Simi fault truncated the Marr fault and is assumed to have occurred subsequent to Pico deposition. The northdipping C.D.L.B., Strathearn, and Joughin faults are inferred to have formed at this time, and ceased movement prior to the deposition of the Saugus Formation. Folding of the Happy Camp syncline occurred after deposition of the Saugus, possibly in response to the same northeast-southwest-oriented stressfield that formed the Santa Susana fault. The Simi fault cuts older alluvium east of Tapo Canyon, but does not cut slump blocks of Conejo Volcanic that overlie the fault north of Tierra Rejada Valley