Paleogeography and depositional environments of the Vaqueros and Rincon Formations, Topatopa Mountains, Ventura County, California

The early Miocene in southern California was a period of tectonic transformation of the tectonic plate boundary setting from a subduction complex to a transtensional dextral transform setting. The deposition of the Vaqueros and Rincon Formations in what is presently the central Top atop a Mountains occurred during the commencement of basin development and rotation. This retrogradational sequence of strata consists of nonmarine sandstone to marine sandstone and siltstone. Two areas were studied within the Topatopa Mountain region, the Piru Creek area and the Sespe Condor Sanctuaty area near Fillmore. Initial deposition of the basal Vaqueros was within a nonmarine interdistributary marsh in an upper delta plain environment, possibly fluctuating with a foreshore beach. The strata in the Piru area were closer to the source, indicating that drainage was towards the south or present-day west. Studies in the surrounding region have indicated that the Ventura basin was a large southeast-opening basin that formed an embayment, protected from the open ocean by a north-south-trending highland. Nonmarine deposition continued until transgression of the sea introduced marine deposits in the Sespe Condor Sanctuary. In the central Topatopa Mountains, the Vaqueros was predominantly deposited in a wave-dominated environment. Near the close of Vaqueros deposition, the shoreline was straightened and became less irregular and the Condor Sanctua1y and Piru region was transformed into a predominantly wave-dominated coastline with some tidal influences. Deposition of the Rincon Formation began in an inner shelf environment. Silt and mud accumulated in this region until medial Rincon time when an influx of unfossiliferous sand occurred. Submergence of the Topatopa Mountain area allowed for increased oceanic current circulation on the inner shelf due to the opening of a strait west-northwest of the study area. Rincon mud deposition continued as the basin deepened to outer shelf and open ocean depths. The culmination of detritus deposition in the Rincon Formation is marked by an ash-fall tuff and the initiation and accumulation of starved chemical-organic basin deposits typical of the Monterey Formation in an open ocean environment.