FLOOD EVENTS AND SEDIMENTATION IN SOUTH SAN FRANCISCO BAY, CALIFORNIA

Cores containing tidal flat sediments record episodic pulses of fine- to coarse-grained sand and gravel extending over 300 m onto the muddy tidal flats adjacent to the mouth of San Francisquito Creek in south San Francisco Bay, California. The coarse sediment pulses identify past flood events that have occurred in San Francisquito Creek since the stream channel was moved 1.6 km to the north of its historic location in 1930. The flood deposits consist of "clean" sand and gravel in beds ranging from 8 to 44 cm thick that essentially lack micro- and macrofossils. The sand beds have sharp basal contacts overlying intensely bioturbated shell-rich mud. Horizontal planar or low angle laminations form much of the flood strata, changing up to small- or large-scale crossbeds that terminate in bioturbated muddy fine-grain sand. The flood-generated strata form distinctive fining-up depositional cycles.

Interbedded with the flood deposits are intensely bioturbated, black sandy mud beds ranging from 1 to 22 cm thick containing thin, horizontal, 1 to 2 cm thick organic-rich strata and rare small-scale crossbeds. Abundant wood fragments, plant debris, benthic foraminifers and ostracodes along with gastropods, pelecypods, and diatoms are found within the non-flood related tidal flat strata.

To date five flood-related sand and gravel beds have been identified in the intertidal strata and are correlated to the peak flow flood events of over 4,000 cubic feet per second within San Francisquito Creek. The clastic deposits are correlated with the 1950, 1955, 1958, 1982 and 1998 flood events on San Francisquito Creek. With the exception of the 1955 flood event, the flood deposits are related to periods of increased rainfall during El Niño periods.