Abstract: Regional Relations of Paleocene and Eocene Strata of the Gualala Block

BRABB, EARL E., DAVID L. JONES, and CARL M. WENTWORTH, U.S. Geological Survey, Menlo Park, CA

The strata of German Rancho, of late Paleocene to middle Eocene age, comprise a thick (+20,000 ft.) submarine fan assemblage of arkosic sandstone, felsic-clast conglomerate, and thick interbeds of thin-bedded sandstone, siltstone, and mudstone. These strata conformably overlie the clastic strata of Anchor Bay characterized by predominantly mafic clasts in conglomerate and by tropical rudistids of Cretaceous age. A distinctive red mudstone with abundant foraminifers of early Eocene age is exposed at Havens Neck and at several other localities around the town of Gualala near the base of the German Rancho sequence. Molluscan fossils, including Campanilegreenellum, indicate deposition under tropic marine conditions.

Rocks in central California that are similar in age and lithic character to the German Rancho are known only on the San Francisco Peninsula at Point San Pedro, and a rudistid-bearing mafic conglomerate like the Anchor Bay conglomerate and red mudstone like that in the lower part of the German Rancho strata have recently been found nearby. Other Paleocene and Eocene strata in the Santa Cruz Mountains, the Santa Lucia Mountains and Western Transverse Ranges rest on different basement rocks, are different in Ethology, and are overlain by units not present in the Gualala area.

Based on these attributes, we conclude that German Rancho stratra have been transported northward from an original source region located at least as far south as the Transverse Ranges. Because postulated Neogene displacements of the San Andreas Fault system are insufficient to accomplish this amount of northward transport, additional post-Eocene, pre-San Andreas transport along the continental margin must be considered.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California