TEMPORAL SIGNIFICANCE OF VOLCANIC ASH, PETRIFIED WOOD, AND UPLIFTED FLUVIAL TERRACE DEPOSITS IN THE CHILENO AND SAN ANTONIO CREEK DRAINAGES, MARIN COUNTY, CALIFORNIA

Uplifted Quaternary cut-and-fill terraces can provide crucial information in terms of the temporal, tectonic, and fluvial history of a given region. Whereas it is not particularly different to obtain relative dates from fluvial terrace deposits, there is a paucity of actual absolute dates available for such deposits in Marin County, California. Several cut-and-fill terraces as well as one strath terrace were investigated by the primary author during previous research. Although a relative age-dating scheme was devised, no absolute dates were obtained. The purpose of this study is to provide a preliminary characterization of a recently examined cut-and-fill terraces which might provide important information about the timing and rate of uplift in the southern North Coast Ranges.

An uplifted cut-and-fill terrace was studied by the authors near the headwaters of the Chileno and San Antonio Creek drainages in northwestern Marin County, California. The terrace is located ~90m above the modern floodplane. Clasts range in size from cobble to boulder and are primarily sub-rounded to well-rounded, and oblate (disk) to equant. Clasts are almost entirely Franciscan and Sonoma Volcanics cobbles with lesser amounts of Tertiary sedimentary rocks (Wilson Grove FM.?) and petrified wood fragments. Surface soil color is 5YR 5/6D. Initial relative age estimates based on soil rubification, geomorphic position, clast composition, and correlation with other published data, suggest an age of >300 ka for the cut-and-fill terrace. Ash beds as well as reworked, ashy, fluvial deposits were also found in the terrace deposits by the authors. It is possible that the deposits in this study might be correlative with the Glen Ellen Formation located to the northeast. Chemical correlation of the ash deposits is currently underway and will hopefully provide crucial information in terms of the temporal significance of the terrace deposits.