Rapid, complex back barrier pedestal formation preconditions washover deposition on the southeast Indian coast

Washover deposits formed by overwash are important deposits for evaluating the stratigraphy and evolution of coastal environments. Examination of preserved washover fans can provide a palaeotempestological record that inform past and recent coastal risk with a view to predicting future coastal risk. The recognition of past washover deposits in coastal systems requires detailed knowledge of the internal structure of recent deposits including washover fans. We used very high frequency Ground Penetrating Radar and satellite imagery to examine the internal architecture of the 31st December 2011 Cyclone Thane surge-generated washover deposit on the beach that blocked the Thenpennai River at Thazhamkuda, near Cuddalore in Tamil Nadu Province, southeastern India. Cyclone Thane overwash overtopped the beach and deposited sediments on the contemporary beach and behind the beach within the former channel of the blocked river. The modern washover fan thus contains sediments that are preserved subaqueously and subaerially. We demonstrate that the internal architecture of the fan at Thazhamkuda is largely controlled by the pre-existing topography, and erosional and depositional processes as the cyclone washed sediment inland. At the landward margin of the fan, terminal foreset bedding is preserved and is likely to be one of the only features that can discriminate storm over tsunami deposits. • The architecture of washover fans is controlled by pre-existing topography. • Complex internal washover fan stratigraphy preserves erosional and depositional features. • Terminal foresets are unique to storm fans and absent in tsunami deposits.