LATE QUATERNARY TECTONIC AND PALEOFLUVIAL SIGNIFICANCE OF STRATH TERRACE GRAVEL DEPOSITS IN THE CHILENO CREEK DRAINAGE SYSTEM, NORTHWESTERN MARIN COUNTY, CALIFORNIA

Understanding tectonic influences on fluvial dynamics and soil development in a given area can dramatically increase the ability to assess regional seismic potential. Lithostratigraphic and pedostratigraphic profiling was utilized for geological and paleoenvironmental reconstructions within the Chileno Creek drainage region in Marin County, California. Forty-seven terrace gravel clasts were characterized beneath a flat geomorphic saddle-ridge located ~40 m above the modern floodplain of Chileno Creek. Clasts range in size from cobble to boulder, are subrounded to well rounded, and oblate (disk) to equant. Many clasts exhibit shear fractures that have been subsequently re-annealed. Matrix colors range from 10YR2/2M (surface) to 5YR4/6M (subsurface). Normalized clast composition consists of 47% Franciscan metabasalt, graywacke sandstone, and chert, and 53% mafic to felsic continental volcanic rocks. Weathering rinds on andesitic and basaltic cobbles vary from 3 to 5 mm in thickness. Initial estimates based on subsurface soil rubification and weathering rind indices, as well as correlation with other published data, suggest an age of ~300 ka for the geomorphic surface and its underlying deposits.

The elevated gravels differ considerably from the younger sediments, soils, and paleosols that comprise the terrestrial deposits of the floodplain. The >2 mm coarse fraction of the more recent sediments consists primarily of very angular to subrounded, low sphericity pebbles that were derived from Franciscan sources. The exotic volcanic clasts under the terrace surface originated from Cenozoic volcanic centers located a considerable distance east of the modern headwaters of Chileno Creek. Comparison of textural characteristics of the fluvial deposits also indicates a significant local change in flow velocity, paleostream gradient, and drainage basin development during the late Quaternary. The elevated gravels are thus interpreted to be tectonically uplifted strath terrace deposits associated with a Pleistocene drainage system that has itself been bifurcated due to tectonic activity near the modern drainage divide.