Depositional environments of the Vaqueros Formation in the Big Mountain area, Ventura County, California

The Vaqueros Formation (Oligocene- lower Miocene), exposed in a northward-dipping homocline in the Big Mountain area, Ventura County, California, is a transgressive-regressive sequence which conformably overlies the Sespe Formation and is unconformably overlain by the Conejo Volcanics in the west and the Calabasas Formation in the east. Three members are distinguished: a lower sandstone, siltstone, and limestone sequence; a middle sandy siltstone; and an upper sandstone, siltstone, and limestone sequence. In the lower member, the lowest sandstone and siltstone beds are beach and marsh deposits. Overlying these deposits are unfossiliferous sandstone and interbedded fossiliferous sandstone and siltstone which represent deposition in shoreface and transition zone environments. Limestone beds in the transition zone environment which are composed of shell debris and interbedded siltstone are storm-lag deposits and those predominantly containing whole Anomia, Terebra, and Pecten shells are shoal deposits. Fossiliferous siltstone beds near the top of the lower member represent deposition in a sha1low offshore shelf environment in water no deeper than 60 m. (See more in text)