Depositional environments and paleogeographic setting of the Santa Margarita formation, Ventura County, California

The Santa Margarita Formation in the upper Sespe Creek-Pine Mountain area consists of' a 225- to 300-m-thick stratigraphic sequence of marine sedimentary rock which grades vertically to nonmarine sedimentary rock. Exposures occur in the central portion of' an east-west trending synclinical drag fold on the footwall side of' Pine Mountain fault. The formation is assigned to the "Delmontian" Stage, upper Miocene, on the basis of Astrdapsis antiselli Conrad. The Santa Margarita Formation overlies the Monterey Formation with a slight angular uncomformity. Within the study area the Santa Margarita Formation is divided into five facies, each representing a specific depositional environment. 1. Facies A, the lowest unit, consists of white, thick to very thick beds of burrowed and bioturbated, massive, fine-grained sandstone and coarse-grained siltstone, with a basal, pebble to cobble conglomerate. Locally it is a medium- to coarse-grained, large-scale, cross-bedded sandstone. In addition scattered oyster beds and massive, medium- to coarse-grained sandstone beds occur. Facies A is interpreted as a transgressive lag deposit overlain by lower shoreface and transition zone deposits. The cross-bedded sandstone was probably a pro-grading, storm-generated sand sheet. Oyster reefs and sandstone beds indicate shoaling conditions in the northwestern part of the area. (See more in text.)