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Biostratigraphy of Marine Pliocene-Pleistocene Deposits, Simi Valley, California

Lindsey T. Groves, Richard L. Squires

Richly fossiliferous, marine deltaic deposits exposed in the northern Simi Valley, southern California, are important biostratigraphically because they straddle the Pliocene-Pleistocene boundary.

These deposits consist of fine to coarse-grained sandstone with interbedded conglomerate. The best exposure is in Gilibrand Quarry, where 250 m are exposed in a continuous section with four main fossiliferous zones rich in mollusks. Most of these mollusks are complete and display growth series that represent a life assemblage with minimal postmortem transport.

An erosional surface is present between the second and third fossiliferous beds in the quarry. Beds below the erosional surface contain mollusks indicative of Pliocene age. These mollusks are the pectinid Patinopecten healeyi and the epitoniid gastropod Opalia varicostata. Beds above the erosional surface contain a calcareous nannofossil assemblage that suggests a Pleistocene age. These species are Coccolithus pelagicus, Helicosphaera carteri, and Dictyococcites productus. (An absence of Discoaster species and Emiliana huxleyi would place this assemblage in the early or medial Pleistocene.)

The name lower Saugus Formation is assigned to all of these marine Pliocene-Pleistocene deposits because W. S. W. Kew in 1924 originally named and mapped these strata as Saugus Formation. Confusion will be avoided by discontinuing usage of the names Fernando Formation, Pico Formation, and Sunshine Ranch Member of the Saugus Formation in this area for these strata. The lower Saugus Formation in the Simi Valley is faunally similar and time-correlative with the San Diego Formation in San Diego County.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.