Foraminiferal Assemblages of Inferred Onshore Paleotsunami Deposits in Southwestern Andaman Islands, India

Quantitative and qualitative characteristics of foraminiferal species within the inferred paleotsunami deposits of southwestern Andaman Island were examined using sediment samples from six geoslice sections up to a depth of ∼2.5m. In this study, it is considered that the foraminiferal assemblages of the 2004 tsunami deposit (Unit b) as a modern analogue to trace similar events in the geological past. Paleontological and quantitative data analysis suggests 2 biofacies in the stratigraphy. Biofacies I is indicative of a subtidal sediment provenance and its mixing with intertidal sediments due to the tsunami, whereas, Biofacies II is an intertidal/marshy sediment provenance/environment. Foraminiferal tests >180µm in dimension in the tsunami facies (Biofacies I) show abrasion marks, which otherwise show high preservation potential than the pre-tsunami (Biofacies II) that indicates high dissolution activity and low preservation potential. Based on these inferences, 4 tsunami zones, 4 marsh zones, and 1 basin filling zone was elucidated in the stratigraphical column. In order to refine the chronostratigraphy of this area, the contemporaneous foraminifera recovered from a 15cm thick paleotsunami deposit (Unit g) were dated using 14 C accelerator mass spectrometry (AMS). The AMS age suggests that AD 1005–1190 was the predecessor of 2004 Sumatra-Andaman earthquake (Mw 9.3) and associated Indian Ocean tsunami.