POWELL, CHARLES L., II
U. S. Geological Survey, Menlo Park, CA
Abstract: Biostratigraphy of the "Purisima" Formation in the San Francisco Bay Area, Central California
Three molluscan faunas, the La Honda, the Pillar Point, and the Santa Cruz, are recognized in the "Purisima" Formation in the greater San Francisco Bay area. The "Purisima" has thickness of more than 1.6 km which occur in scattered, discontinuous faultbounded sub-terranes, from Point Reyes to southeast of Santa Cruz. These subterranes demonstrate different stratigraphic histories and represent more than a single depositional basin. Lithologic divisions of the Formation are of local extent.
The lowermost fauna, the La Honda, suggests shallow-water depths and was deposited during the latest Miocene to early Pliocene. It is best expressed in the La Honda structural block and is contains the extinct mollusks Chione cf. securis, Shumard, Dendostrea? vespertina (Conrad), Mytilus coalingensis Arnold, and Swiftopecten parmeleei (Dail). These taxa correlate with the "Jacalitos-Etchegoin" provincial molluscan stage (late Miocene to early Pliocene) in California.
The Pillar Point fauna differs in representing deeper water (> 100 m). It contains only a single age-diagnostic taxon, Lituyapecten purisimaensis (Arnold), which only occurs only in the Purisima Formation, so its age is an example of circular reasoning. The Pillar Point fauna occurs only at Pillar Point west of the Seal Cove fault and contains Conchocele disjuncta Gabb, Lituyapecten purisimaensis (Arnold), Lucinisca annulata (Reeve) and Panope? sp.
The uppermost fauna, the Santa Cruz, suggests shallowwater depth and ranges in age from early to late Pliocene. Extinct taxa from this fauna suggest correlation with the "San Joaquin" provincial molluscan stage (early to late Pliocene) and include Macoma addicotti Nikas, Nanochlamys nutteri (Arnold), Searlesia portolaensis (Arnold), Nassarius grammatus (Dall), and Psephaea oregonensis (Arnold).
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California