Integrated approach of assessing sedimentary characteristics of onshore sand deposits on the Velankanni coast, Tamil Nadu, India: sheds light on extreme wave event signatures

The refraction of the 2004 Indian Ocean tsunami waves caused drastic devastation along East coast of India, mainly in the area of investigation. Here, we appraise and integrate the sedimentary characteristics and microfossil studies of the area. The gigantic tsunami waves caused the landward fining of sediments that were carried as suspended load. Tsunami sediments have distinctive characteristics, like fine-to-medium grained sand, moderately to poorly sorted sediments which indicating sudden winnowing followed by tranquil flood. Positively skewed grain size distribution indicating unidirectional transport, and mesokurtic to platykurtic character implying poorly sorted single source origin. The species of benthonic foraminifers and ostracods reside in marine environment indicating shallow water origin of sediments. The onshore deposits are vertically divided into three depositional units interpreted from Ground Penetrating Radar (GPR) signatures-Unit 1 is a relatively continuous parallel layer indicative of calm environment; Unit 2 has paleochannels and burial scarps, seen as oblique reflections that might be indicative of an intense erosional environment; Unit 3 is interpreted as 2004 tsunami layer, has three subunits. Each main units have been separated by Heavy Mineral Concentrated (HMC) layers, deposited by continuous wave action (similar to 20 cm) and by the tsunami (> 30 cm) activity, evidenced by low magnetic susceptibility values at the bottom compared to the top of the HMC layers. GPR has been effectively utilized in this paper as subsurface imaging tool for the interpretation and reconstruction of stratigraphy, and also helped to unearth the erosional and depositional environments.