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Tsunami Sediments in Coastal Lagoons, Sri Lanka: Implications for Paleotsunamis

Kelly L. Jackson1, Falk Amelung1, Miriam S. Andres1, Gregor Eberli1, H.A.H. Jayasena2, K.V. Wilbert Kehelpannala3, Larry Peterson1, and Eugene C. Rankey1
1 University of Miami, RSMAS, Miami, FL
2 University of Peradeniya, Peradeniya, Sri Lanka
3 Institute of Fundamental Studies, Kandy, Sri Lanka

Tsunamis represent a potentially important control on coastal sedimentary systems. Sediment cores from Sri Lankan coastal lagoons provide a unique opportunity to compare tsunami-related coastal lagoon sedimentation associated with the 2004 Indian Ocean tsunami to older lagoon sediments. Information on the recurrence probability of tsunami events in Sri Lanka is important to understanding how tsunamis can influence coastal systems. The subduction zone that initiated this event is capable of producing similar events on the average of every 350 years due to its 6 cm/yr convergence rate. This average, calibrated with Sri Lankan historical records and implications for paleotsunamis in the sediment cores, may decipher the tsunami record in Sri Lanka.

Southeastern Sri Lanka has abundant coastal lagoons, many of which were inundated by the 2004 tsunami. Numerous one to three meter cores collected from five different lagoons feature mud-dominated layers interbedded with sand-rich layers. These sand-rich layers commonly have sharp bases, fine upwards, vary in thickness from 1 to 15 cm, and include grain sizes ranging from fine granule to fine sand. These distinct layers potentially are related to either paleotsunamis or storms. Their origin, as well as their chronology, determined by radiocarbon date constraints, will be a key to unlocking the tsunami sediment record in Sri Lanka.

Ongoing work includes detailed sedimentological analyses, radiocarbon dating, and mollusc identification. These data will provide chronology and environmental constraints of the interbedded mud and sand layers in order to determine whether these sand-rich layers are in fact paleotsunami or, rather, storm deposits.