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EVIDENCE OF AN EARLY HOLOCENE PRECURSOR TO THE 2004 SUMATRA-ANDAMAN EARTHQUAKE
At both study sites, the organic-rich buried soil is dominated by a Rhizophora and Ceriops pollen assemblage, which suggests a mangrove environment. The soil is overlain by a thin (5-10 cm) coarse grained clastic unit identified by a reduction in organic carbon and an increase in grain size. The pollen assemblages of the sand unit reveal an increase in species diversity with a mixture of freshwater and mangrove taxa. Foraminifera are dominated by estuarine species such as Ammonia parkinsoniana with a notable contribution of deep water taxa including planktonics. The chaotic pollen and foraminifera assemblages of the sand unit are indicative of a tsunami because these high energy events inundate coastal and inland areas transporting, and redepositing deep water, estuarine and freshwater sediment. The microfossils of the overlying silty-sand clastic unit indicate a transition into a subtidal/estuarine environment with the introduction of foraminiferal taxa that occupy shallow water depths (e.g. Elphidium advenum). The pollen assemblage consists of freshwater spores and mangrove taxa but in low concentrations.
The buried soils hosted a mangrove environment that formed at approximately mean high high water during a period of interseismic deformation. The soil was abruptly overlain by a thin clastic unit of tsunami origin. Radiocarbon dates from the sharp upper contact of the buried soil suggest this rapid sea-level rise event, associated with coseimic subsidence, occurred at c. 7 cal kyr BP. The tsunami deposit is overlain by subtidal/estuarine deposits forming below mean tide level, suggesting a coseismic subsidence of at least 0.85 m. Further interseismic strain and gradual sea-level fall over a period of c. 1000 years returned the depositional environment to a mangrove.