The Paleobeach Ridges of Digha Coastal Tract, West Bengal, India: Observation and Implication for Sea Regression from 500 YBP to 200 YBP

Abstract

Paleobeach ridges in and around the Digha coast, West Bengal, India give significant information about the sea regression in eastern Indian coast during Holocene. Beach ridges paralleling the Bay of Bengal coast in West Bengal and a few remnants beach ridges further landward in Kanthi (West Bengal) and Jaleswarpur (Odisha), give a tentative boundary of the north eastern most palaeo-shoreline. From optically stimulated luminescence (OSL) dates of the Paleobeach ridges, it is suggested that around 500 YBP, the sea was near Kanthi and around 378 YBP the sea was near Jaleswarpur (35 km inland). After that, the sea sequentially regressed in phases towards south and attained the present position at Digha. The Photogeology & Remote Sensing (PGRS) Study of different satellite imageries (2006 onwards) along with the Survey of India Toposheets (1930, 1972) of Digha area indicates that the coast line to be steadily regressed in the east of Digha, whereas, it shows fluctuation in the west of it. This fluctuation of strand line position may be due to the differential sediment influx through the Subarnarekha river mouth with time. Regional neotectonic activities also play a role in shaping the coast. The concave orientation of the beach ridge plains to the present shoreline reflects the supply of sediments in the direction of palaeolongshore currents. The shape of the inner beaches shows that the deposition prograded north-west wards. Major type of cross-beddings present in the Paleobeach ridges represented by low angle laminated sand, indicates the wide seaward dipping part of longshore bars. The grain size analysis of the sediment samples of the beach ridges indicate positively skewed and mesokurtic. The mean size of resultant sediments is finer, indicating that the energy level or turbulence of the eroding processes was incapable of transporting sediments coarser than the mean size of the source sediments.