Lower Paleogene geology of the Simi Valley area, Ventura County, California

The Simi Conglomerate, Las Virgenes Sandstone, and Santa Susana formation, of early Paleocene through early Eocene age, represent nonmarine through deep-marine depositional environments. The distribution of types of environments is, in part, confined by the Runkle Canyon fault. Strata west of the fault are referred to as “western” deposits. The Simi Conglomerate, present at the base of the sequence, consists mostly of poorly bedded, clast-supported conglomerate and subordinate sandstone. These rocks are interpreted as gravelly and sandy braided-river deposits on an alluvial plain. The overlying Las Virgenes Sandstone, present only west of the Runkle Canyon fault, consists of variable amounts of sandstone, mudrock, and granule conglomerate, and is interpreted as representing sandy braided-river, meandering-stream, and nearshore-marine deposits. The western Santa Susana formation reflects deposition in a deepening marine environment. The lower portion of the formation consists of sandstone and siltstone and represents transition zone to offshore and shelf deposits. The upper portion consists mostly of mudrock with subordinate amounts of interbedded sandstone and lenses of conglomerate and represents slope deposition. The uppermost part of the formation may represent shelf deposition. (See more in text.)