HISTORICAL POLLEN AS AN INDICATOR FOR CONSTRAINING EARTHQUAKE STRATIGRAPHY ALONG THE SAN ANDREAS FAULT, VEDANTA MARSH, OLEMA CA

Approximately 2500 years of accumulated sediments were exposed in a 4-m deep trench excavated across the northern San Andreas fault at Vedanta marsh 70 km north of San Francisco. Detailed pollen sampling and extraction was conducted in order to help constrain the age of paleoearthquakes and to reconstruct associated coseismic subsidence. West of the fault the stratigraphic sequence consists of six peat and organic soil horizons (labeled from top P10-P60) that alternate with clay layers. Coarse clastic sediments including alluvium and colluvium are interbedded with the marsh sediments at the eastern margin of the marsh and within the fault zone. Above P10 is a 1-m thick highly bioturbated section. It consists of a lower 0.35-m thick weathered silty clay capped by a silty gravel and a thin layer of clay. These layers are erosionally overlain by a channel gravel sequence with cross-bedded fine-grained sand and iron-stained pebbles and modern fill. Evidence for the 1906 and penultimate earthquake are contained in the section above P20. AMS radiocarbon dating of P10, P20 and P60 yielded ages of ca. 300 yr B.P., 700 yr B.P. and 2100 yr B.P. In order to date the upper bioturbated layers samples were collected for pollen identification. No non-native species, including Rumex acetosella (Sheep Sorrel), Plantago lanceolata (Ribwort), and Eucalyptus, were found in the upper .5-m of fine-grained sediments above P10. This suggests that the thin clay layer pre-dates the mid 1800’s when the region was first settled by Europeans. The potential age of this layer suggests there was a change in sedimentation style prior to historical ranching and logging possibly related to coseismic subsidence. The appearance of the clastic sediments in the upper section probably represents historical landuse changes in the watershed. Refinement of the marsh stratigraphy is expected with additional evidence from three other trench exposures and a 21-m sediment core obtained from the edge of the marsh, with a basal AMS radiocarbon date of ca. 11,500 yr B.P. The sedimentological evidence from the Vedanta marsh site has the potential of providing one of the longest paleosiesmic records along the northern segment of the SAF.