Recognition of diagnostic criteria for recent- and paleo-tsunami sediments from Sri Lanka

Uncontaminated sediment from the December 26/2004 tsunami in southern Sri Lanka was collected from a previously empty arrack bottle stacked in a plastic case at an elevation of about 5 m above mean sea level and from several other localities. The sediments were analysed for grain size distribution, sediment petrography and microfossil assemblages and were compared with storm-surge sediment and nearshore sediment. The undiluted tsunami sediment is a brownish silty fine sand. It is characterized by a particular grain size distribution and an assemblage of planktonic/benthic microfossils consisting of foraminifera, radiolarians and spicules. Storm-surge and nearshore sediments are better sorted, have fewer microfossils, more rounded quartz grains, and a higher proportion of heavy minerals. Influenced by references in Sri Lankan ancient texts to past tsunamis we searched for paleo-tsunami sediments in potential locations such as the coastal depressions. In borehole samples and well cuttings from soil profiles in the coastal village of Peraliya, we were able to recognize two paleo-tsunami horizons, at 30 and 75 cm below the present ground surface, each consisting of brownish silty fine sand that remarkably resembled the recently deposited undiluted tsunami sediment.