HR: 17:20h
AN: T24A-05    [Abstracts]
TI: Mangroves, Coral, and the Search for a Paleotsunami Deposit Along the Andaman Coast of Thailand.
AU: * Rhodes, B
EM: brhodes@fullerton.edu
AF: California State University, Fullerton, Department of Geological Sciences, Fullerton, CA 92834, United States
AU: Jankaew, K
EM: kjankaew@yahoo.co.uk
AF: Chulalongkorn University, Department of Geology, Bangkok, 10330, Thailand
AU: Kirby, M
EM: mkirby@fullerton.edu
AF: California State University, Fullerton, Department of Geological Sciences, Fullerton, CA 92834, United States
AB: Finding a preserved paleotsunami deposit along Thailand's Andaman coast has proven to be a major challenge. The coastline is tectonically stable precluding the preservation of tsunami deposits due to sudden sea level changes. Much of the coast is fringed by wide beach plains that have been thoroughly disturbed during a long history of placer tin mining. In the rare undisturbed areas, distinguishing potential paleotsunami sand layers from interlayered beach deposits is frustratingly difficult. In the central part of Phangnga Province, several large areas of mangrove-fringed coastal lagoons formed during Holocene sea level rise. The fine sand, mud and peat that accumulated in these quiet lagoons provide a suitably contrasting host for coarser tsunami deposits. The December 2004 tsunami entered these tidal channels destroying swaths of mangroves up to 200 m wide. Satellite images showing the patterns of destroyed mangroves suggest that the energy of the tsunami varied greatly along the lagoons' shores. Sampling in shallow trenches along transects across these swaths suggests that the sand layer marking this tsunami is thickest where the mangroves are destroyed, and thins rapidly landward into the standing mangroves. Intense bioturbation within the mangroves is already mixing the sand with the underlying peat and mud. Thus, deposition of any paleotsunami sand layer would be as discontinuous lenses along the paleoshoreline, and mixing with the enclosing sediment may mask it. Perhaps the best opportunity for discovering a paleotsunami deposit is to locate an unusually coarse deposit that could endure bioturbation. We found one such deposit in the Thap Lamu lagoon where the 2004 tsunami deposited a litter of angular coral-gravel blanketing an unusually large zone of destroyed mangroves. Cores collected along two transects across this zone penetrated a similar layer of coral gravel at 1-2 m depth. This coral-gravel layer thins landward, and cross cuts a facies boundary between peat and lagoonal mud formed as mangroves encroached across the lagoon, suggesting that the coral-gravel layer represents an event horizon and not an unusual facies. Preliminary AMS 14C dates are equivocal but suggest an age of around 3000 cy BP for the coral gravel layer. We hypothesize that both the modern and buried coral gravel layer represent tsunami deposits derived from a similar offshore point source. Discovery of other unusually coarse, paired deposits in this region would support our interpretation.
DE: 3020 Littoral processes
DE: 4558 Sediment transport (1862)
DE: 4564 Tsunamis and storm surges
SC: Tectonophysics [T]
MN: 2007 Joint Assembly