Western Earth Surface Processes

U.S. Geological Survey
Professional Paper 1740

Age, Stratigraphy, and Correlations of the Late Neogene Purisima Formation, Central California Coast Ranges

By Charles L. Powell II, John A. Barron, Andrei M. Sarna-Wojcicki, Joseph C. Clark, Frank A. Perry, Earl E. Brabb, and Robert J. Fleck


This report is available online in pdf format (1.1 MB): USGS Professional Paper 1740 (Opens the PDF file in a new window. )

close-up photograph of bivalve shell fossils in sedimentary rock matrix


The Purisima Formation is an important upper Miocene and Pliocene stratigraphic unit in central California, cropping out from the coast at Point Reyes north of San Francisco to more extensive exposures in the Santa Cruz Mountains to the south. The fine-grained rocks in the lower parts of the Purisima Formation record a latest Miocene transgressive event, whereas the middle and upper parts of the formation consist of increasingly clastic-rich siltstones and sandstones resulting from uplift of adjacent coastal regions and the Sierra Nevada during Pliocene transgressive and regressive sea-level events. Exposures of the Purisima occur in three different, fault-bounded, structural blocks—the Santa Cruz, Pigeon Point, and Point Reyes tectonic blocks—that complicate correlations and regional age assignments. We summarize and compare published and new biostratigraphic and geochronologic data for various exposures of the Purisima Formation on the basis of mollusks, diatoms, radiometric dating, magnetostratigraphy, tephrochronology, and strontium isotope dating. On the basis of these data, we conclude that the Purisima Formation ranges in age from the latest Miocene (about 7 Ma) to the late Pliocene (about 2.6 Ma).

The Purisima Formation of Santa Cruz County, exposed in the sea cliffs from Santa Cruz to Rio del Mar, is here designated a supplementary reference section because it is the most complete and well studied Purisima section in central California.


This report is available online in pdf format (1.1 MB): USGS Professional Paper 1740 (Opens the PDF file in a new window. )

For more about the geology of the San Francisco Bay region, visit the San Francisco Bay Region Geology website

For questions about the content of this report, contact Chuck Powell

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