Sedimentary rocks more than 1.6 kilometers thick are attributed to the upper Miocene
to upper Pliocene Purisima Formation in the greater San Francisco Bay area. These
rocks occur as scattered, discontinuous outcrops from Point Reyes National Seashore in
the north to south of Santa Cruz. Lithologic divisions of the Formation appear to be of local
extent and are of limited use in correlating over this broad area. The Purisima Formation
occurs in several fault-bounded terranes which demonstrate different stratigraphic histories
and may be found to represent more than a single depositional basin. The precise age and
stratigraphic relationship of these scattered outcrops are unresolved and until they are put
into a stratigraphic and paleogeographic context the tectonic significance of the Purisima
Foramtion can only be surmised. This paper will attempt to resolve some of these problems.
Mollusks and echinoderms are recorded from the literature and more than 70 USGS
collections that have not previously been reported. With the exception of one locality, the
faunas suggest deposition in normal marine conditions at water depths of less than 50 m
and with water temperatures the same or slightly cooler than exist along the present coast
of central California. The single exception is a fauna from outcrops between Seal Cove
and Pillar Point, where both mollusks and foraminifers suggest water depths greater than
Three molluscan faunas, the La Honda, the Pillar Point, and the Santa Cruz, are
recognized based on USGS collections and published literature for the Purisima Formation.
These biostratigraphically distinct faunas aid in the correlation of the scattered Purisima
Formation outcrops. The lowermost La Honda fauna suggests shallow-water depths and
an age of late Miocene to early Pliocene. This age is at odds with a younger age determination
from an ash bed in the lower Purisima Formation along the central San Mateo
County coast. The Pillar Point fauna contains only a single age diagnostic taxon,
Lituyapecten purisimaensis (Arnold), which is reported as Pliocene in age, but it only occurs
in the Purisima Formation, so its age here is an example of circular reasoning. However,
based on tentative lithologic correlations this fauna may represent the same period of
time as the upper part of the La Honda fauna. This fauna differs from either the La Honda
or Santa Cruz faunas in that it represent significantly deeper water. The uppermost Santa
Cruz fauna also suggests shallow-water depths and a possible age range of early to late
The bivalve molluscan taxon Lyonsia, and gastropod taxon Rictaxis sp., cf. R.
punctocaelatus (Carpenter) are reported here for the first time from the Purisima Formation.