Paleotsunami Deposits in Coastal Lagoons, Sri Lanka: Are Tsunamis Always Sedimentologically Important Events?

Tsunamis are often termed `waves of destruction', however, are they always sedimentologically important events? Though they are substantial events, they leave an ambiguous sedimentary record and do not significantly modify the coastal morphology. Coastal lagoons of Sri Lanka provide a means for a case study for understanding the sedimentologic importance of tsunamis. Sediment cores from Sri Lankan coastal lagoons provide the unique opportunity to compare tsunami-related coastal lagoon sedimentation associated with the 2004 Indian Ocean tsunami to older lagoon sediments. In this case, the tsunami transported material into the lagoons but their geomorphology and hydrological conditions remained unchanged. The 2004 tsunami left a sedimentological footprint in the lagoons and our study thus far suggests that at least 7 paleotsunami deposits are preserved within the upper 4 meters of sediment. Short cores collected from five lagoons feature mud-dominated background sedimentation, interrupted by sand layers that are distinguished by sharp erosive bases, fine and/or coarsen upwards, have variable thicknesses from 1 to 15 cm, and include grain sizes ranging from fine granule to fine sand. These distinct layers are interpreted to represent paleotsunami deposits. Dates calculated using AMS C-14 dating on molluscs recovered within sediment cores, range 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 the sediments from the last 4000 years have not been preserved. The 2004 tsunami deposited sediments of variable thickness in Sri Lankan coastal lagoons but the coastal morphology was not significantly affected. We conclude that tsunamis leave an ambiguous sedimentological record and are not coastal forming processes.