Event beds in Andaman beach sediments – records of 2004 Tsunami

The coastal zones of the Andaman Islands in the Bay of Bengal experienced the severe impact of an earthquake-generated tsunami on 26 December 2004. This study, conducted in 2023 investigates the sedimentary attributes of tsunami deposits found in trench and pit sections across three beaches: Bada Balu, Collinpur, and Mayabunder. The identified tsunami deposits include matrix-supported gravelly sand, massive sand, matrix-supported gravelly organic mud, normally graded to massive sand, and large bouldery gravel facies types. Bipartite flows, characterized by lower debris flows and upper turbidity currents generated after flow separation are responsible for the formation of gravelly mud or gravelly sand and sand couplets. Massive sand drapes over erosional surfaces are attributed to the waning phases of sand-laden tsunami run-up, while large bouldery gravel spreads, featuring rounded boulders, represent traction carpet deposits sourced from emerged coral reefs. The extreme variation in sediment composition and texture reveals that the strength of the tsunami waves and their propagation directions varied significantly across different coastal zones, largely influenced by coastal morphology. The distinct sedimentary structures of the studied 2004 earthquake-generated tsunami will serve as a valuable tool for interpreting tsunami deposits in the ancient record. Furthermore, the diagnostic criteria established in this study can be applied to distinguish between tsunami and cyclonic storm deposits.