A Potential Paleotsunami Shell-Hash layer from the Los Penasquitos Marsh, San Diego County, California.
Authors:Nicole Bonuso, Jeremy Cordova, Mattew Kirby, Robert Leeper, Brady Rhodes
Mentor:Brady Rhodes, Professor of Geological Sciences, Cal State University, Fullerton
The Los Penasquitos Marsh is one of a series of coastal wetlands between San Diego and Orange County that formed within stream valleys that flooded and filled with sediment during Holocene sea-level rise. To test our hypothesis that these wetlands contain prehistoric tsunami deposits 21 reconnaissance cores between 48 and 321cm in length were collected and described. Nearly all the cores contained a peaty layer in the top 20-40cm, underlain by interbedded fine-medium gray sand and mud. The stratigraphy in the cores is consistent with the infilling of a lagoon behind a baymouth bar. Five of the cores, 1.0-1.5km inland from the present beach, intersect a 0.5–12.0cm-thick shell-hash layer between 233 and 280cm depth. To fully evaluate the shell-hash layer as a potential tsunami deposit, we collected a 285cm long 5cm-diameter core. In this core the 10 cm-thick shell hash consists of up to 3cm-diameter angular fragments of shells in a sandy matrix. The following genera were recovered: Mitrella, Venus, Spirotropis, Pecten, Nassarius, and an unidentified oyster. This assemblage suggests a quiet water marine source – from the lagoon and/or offshore. The core was analyzed for loss on ignition at 550° and 950°C and magnetic susceptibility. The LOI data are unremarkable, showing an expected spike within the shell-hash layer. The ms data show low values for lagoonal sediments, but a pronounced spike within the shell hash layer. We speculate the anomalously high ms value for the shell hash indicates a substantial component from offshore, where heavier minerals accumulated seaward of the baymouth bar. If correct, this layer may represent a large-wave event, either storm or tsunami. Three C-14 dates on shell fragments cluster between 1380-1420 yrs BP. To confirm our interpretations we plan a program of gouge coring and vibracoring to delineate the extent of the shell-hash layer.