Abstract: Inner and Middle Submarine Fan Sequences in the German Rancho Formation (Paleocene-Eocene) on the Gualala Block, Sonoma and Mendocino Counties, California

ANDERSON, THOMAS B., Department of Geology, Sonoma State University, Rohnert Park, CA 94928

Turbidite sequences of the German Rancho Formation are well-exposed along the Sonoma and Mendocino County coastline. The unit was deposited in a submarine fan system in the southeast-northwest trending Gualala Basin. The sandstones are primarily feldsarenites and contain characteristic turbidite structures and sequences including reworked macrofossils and deep-water trace fossils. Inner fan deposits occur in the central part of the area with adjacent middle fan facies cropping out to the northwest and southeast.

Inner fan facies are present in the Stillwater Cove-Salt Point area. Turbidite facies A and B are present in depositional units that average 2-4 m thick and channel into underlying units. An exceptional channel just north of Stump Beach Cove is approximately 50 m across and 8 m thick. Channel-fill conglomerates are composed of granitoid (40%), quartzite (14%), and volcanic (7%) clasts with minor chert, sandstone, and siltstone. Locally limestone (19%) and detrital gastropod shells (24%) are present. The class compositions vary between individual units which are interpreted to have formed in an inner fan channel system which was fed by a submarine canyon which delivered sediments from different sources at different times.

Middle fan deposits are present in the Fort Ross area to the south and along the Sea Ranch coast in extreme northwestern Sonoma County. Turbidite facies B, C, and D are present in sections in these areas. A representative section at Gualala Regional Park contains deposits of both high-and low concentration turbidity currents. High concentration deposits are thick (4m) sandstones with prominent dish and pillar structures and shale ripup clasts concentrated at the top of individual flow units. These rocks are interbedded with finer-grained, thinner bedded (<1 m) strata containing Bouma sequences of varying completeness. Thinning and fining up sequences are present.

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