The Search for a Paleotsunami Record in the Coastal Wetlands of Southern California: A Progress Report

Coastal wetlands provide the most promising environment in southern California for the deposition, preservation, and recognition of sandy/gravely tsunami deposits. To begin a search for a tsunami record, we chose to focus on, from south to north, Oneonta Slough (OS) south of San Diego, Los Penasquitos Marsh (LPM) within the Torrey Pines Reserve, San Elijo Lagoon (SEL) just south of Encinitas, and the Seal Beach Wetlands (SBW) within the Seal Beach Naval Weapons Station. These sites were chosen based on the relative lack of anthropogenic disturbance and the ability of obtaining a research permit. Fieldwork in all of these sites is limited to 3-4 months in the fall and winter due to the long breeding seasons of endemic endangered species. Reconnaissance fieldwork has included the description of over 60 gouge cores of between 1 and 5 m depth. Cores from OS, LPM, and SEL generally contain a single organic rich (peaty) zone extending 10-30 cm from the surface, overlying interbedded mud and fine-medium sand layers consistent with the infilling of tidal lagoons that formed behind baymouth bars following early Holocene sea-level rise. Cores from SBW contain a much more complicated stratigraphy that includes multiple peaty zones interlayered with mud, suggesting episodic subsidence of the marsh. We are currently investigating the hypothesis that this stratigraphy originated due to coseismic subsidence related to seismicity along the Newport-Inglewood fault zone. In LPM and SBW, reconnaissance cores intersected distinctive shell-hash layers up to 10 cm thick. Larger diameter cores, obtained using a Livingstone piston corer and a vibracorer, were collected to analyze these layers. Preliminary data suggest that the shells were derived at least in part from quiet water areas of the lagoon, or seaward from the beach. These layers at both sites show a significantly higher magnetic susceptibility than the enclosing lagoonal mud and sands, thus favoring an offshore source. Shell samples from this layer at LPM yielded C-14 dates of 1795, 1835, and 1895 cal yrs BP (not corrected for the marine reservoir). Future work will involve additional coring at LPM and SBW to delineate the extent of the fossil hash layers, and new work at Bolsa Chica Marsh, south of SBW to help delineate the extent of subsidence and its relation to the Newport-Inglewood fault zone.