Geologic structure of the Upper Ojai Valley and Chaffee Canyon areas, Ventura County, California

Abstract

The Upper Ojai Valley is a tectonic depression between opposing reverse faults, Its northern border is formed by the active, north-dipping San Cayetano fault with 6.0 km of dip-slip displacement in the Silverthread oil field and 2.6 km displacement west of Sisar Creek; the fault dies out farther west in Ojai Valley. The southern border is formed by the late Quaternary Sisar-Big Canyon-Lion fault set which dips south and merges into the Sisar decollement within the south-dipping, ductile Rincon Formation. Folds with north-dipping axial planes, including the Lion Mountain anticline and Reeves syncline, are middle Pleistocene or older and are related to movement on a frontal strand of the San Cayetano fault. In late Quaternary time, the Sulphur Mountain anticlinorium and the Big Canyon syncline began forming as fault-propagation folds, followed closely by the ramping of the south-dipping faults to the surface over the Saugus Formation. To the east, the San Cayetano fault locally overrides and folds the south-dipping faults. Cross-section balancing shows that the Miocene and younger rocks above the Sisar decollement are shortened 6.7km more than the more, competent rocks below. A solution to this bed-length problem is that the decollement becomes a ramp and merges at depth with the steeply south-dipping Oak Ridge fault. This implies that the Sisar, Big Canyon, and Lion faults are frontal thrusts to the Oak Ridge fault. The total horizontal shortening since Pliocene time is 14.5km. Recently-drilled wells in the Chaffee Canyon oil field, Ventura County, California, reveal that the Wiley Canyon producing anticline formed in the Pleistocene prior to much of the displacement on the Oak Ridge fault. The east and west plunge in part predates deposition of the Vaqueros Formation of early Miocene age. The dip on the south strand of the Oak Ridge fault increases eastward across the field from 70-75° to 83-85°; farther east, the fault plane is overturned and dips north. The Torrey fault can be traced northwest under a landslide east of Wiley Canyon anticline but not farther northwest.