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Lithostratigraphic and Pedostratigraphic Profile—Jacobsen Ranch, Marin County, California: Buried Soils and the Implications for Cultural Resources Management Along Chileno Creek

J. E. Wilen, Jr.
Department of Geology, Univ of California Davis, Davis, CA

Very few geological studies attempt to identify late Quaternary soils and sediments in Marin County, California. Preliminary lithologic and pedologic stratigraphic profiles were described in this report in order to locate buried soils on the Jacobsen Ranch in northwest Marin County. Approximately two meters of late Quaternary deposits are exposed along the banks of Chileno Creek and consist of several alternating beds of gravel, sand, and silt. In addition to the overprinting of modern soil development on the uppermost deposits, two buried soils exist locally as well. The deepest paleosol is herein designated the Jacobsen Ranch Soil. Two obsidian fragments were found at the upper contact of the Jacobsen Ranch Soil with an overlying fluvial granule gravel bed. Bedrock in the area consists of Mesozoic Franciscan Complex chert, metabasalt, and greywacke sandstone, none of which are sources of volcanic glass. Based upon characteristics determined by visual analysis, the provenance of one of the lithic fragments is the Annadel area (Plio-Pleistocene Sonoma Volcanics) while the other shard is from the Clear Lake area (Pleisto- Holocene Clear Lake Volcanics). The flakes have definitive percussion patterns and are interpreted to be cultural resources associated with native Californians.

The obsidian shards have slightly rounded edges and minor flake scar ridges indicative of a certain degree of transport and redeposition. However, the fragments only show a minimal amount of fluvial wear and, as such, were not removed far from their original site of primary deposition on the buried soil surface. A buried prehistoric site is thus more than likely located a short distance upstream or upslope from the soil profile location on the Jacobsen Ranch. The artifacts point to the potential for locating and utilizing buried soils for cultural resources management in the Chileno Creek drainage area.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California