“FOSSIL HASH:” A LATE PLEISTOCENE TSUNAMI DEPOSIT AT 2ND STREET, SAN PEDRO, CALIFORNIA

The exposure on the north side of 2nd Street, near Mesa Street, San Pedro, California, is one of the last and best exposures of early Pleistocene Lomita Marl, Timms Point Silt, and San Pedro Sand in the Palos Verdes Peninsula. The Lomita Marl dips 13º more than the younger San Pedro Sand, which implies that tilting was taking place before and after deposition of the early Pleistocene San Pedro Sand at this location. Overlying and truncating the early Pleistocene marine sediments is a horizontal wave-cut bench; therefore tilting had ceased prior to formation of the wave-cut bench.

Late Pleistocene “fossil hash,” mapped by Woodring et al. (1946) as Palos Verdes Sand, rests unconformably on the wave-cut bench. We analyzed the sediments and fossils of the “fossil hash” bed, with particular emphasis on determining its mode of deposition. The “fossil hash” bed, approximately 1-2 feet thick, fills irregularities on the surface of the wave-cut bench and thins in a westerly direction. This bed is overlain by a relatively thin orange sand with sparse foraminifers, which indicates a marine origin, and should probably be considered part of the Palos Verdes Sand. Nonmarine older alluvial terrace deposits overlie the marine deposits. The “fossil hash” bed contains an admixture of littoral to outer neritic (0-200 m water depth), including megafossils (0-27 m) and microfossils (0-200 m), with pelagic (Neogloboquadrina pachyderma) and benthic (miliolid, 0-20 m, bolivinid, 20-100 m, and cassidulinid, 100-200 m, foraminifer) forms. This megafossil assemblage includes small and large, whole and broken shells. The sediments include sand and silt with some schist pebbles. The “fossil hash” bed exhibits a lack of sorting, grading, and stratification.

The chaotic sedimentary fabric plus the mixed faunal assemblage of the “fossil hash” derived from water depths of littoral to outer neritic (0-200 m), and landward thinning argue for deposition by a tsunami that scoured the ocean bottom from a depth of 200 m.