Are Tsunamis Always Sedimentologically Important Events? A Case Study of Sedimentological and Geochemical Analyses of Coastal Lagoons, Sri Lanka

Jackson, Kelly L.¹, Eugene C. Rankey¹, Gregor P. Eberli¹, Falk Amelung¹, Miriam S. Andres¹, Larry C. Peterson¹, Peter K. Swart¹, H.A.H. Jayasena², K.V. Wilbert Kehelpannala³ (1) University of Miami, RSMAS, CSL, Miami, FL (2) University of Peradeniya, Peradeniya, Sri Lanka (3) Institute of Fundamental Studies, Kandy, Sri Lanka

Tsunamis are termed ‘waves of destruction.' But are they always sedimentologically important events? Sediment cores from Sri Lanka provide the unique opportunity to compare 2004 tsunami-related coastal lagoon sedimentation with older lagoon sediments to test this question. The 2004 tsunami transported material into lagoons, but the overall geomorphic and hydrologic conditions remained unchanged. These tsunami deposits include limited lateral and vertical continuity.

Short cores from five lagoons feature mud-dominated background sedimentation, interrupted by sand layers with sharp erosive bases, and that fine and/or coarsen upwards, have variable thicknesses up to 15 cm, and include grain sizes from fine granule to fine sand. These layers are interpreted to represent paleotsunami deposits and at least 7 such deposits are preserved within the upper 4 meters of sediment from Karagan Lagoon, Hambantota. AMS C-14 dating of molluscs from these intervals suggest ages from 4180 to 6120 radiocarbon ybp. These dates indicate that, the molluscs within the lagoon are older than the sediment, that lagoon sediments have been eroded, or that any tsunami deposits from the last 4000 years have not been preserved.

Ongoing sedimentological and geochemical analyses provide observations that differentiate paleotsunami deposits from background lagoonal sedimentation including variations in grain size, mineralogy, carbon and oxygen isotopes, and organic carbon. Although paleotsunami deposits can be preserved, results show tsunamis are not always pronounced sedimentological events that leave a widespread sedimentary record. They also do not cause net alterations to the overall geomorphic and hydrodynamic conditions.