Relative sea-level change in Bangladesh during the Holocene.

Abstract

The thesis aims to reconstruct the Holocene sea-level history in Bangladesh. Detailed litho-, bio-, and chrono-stratigraphic techniques have been applied to elucidate the nature of sedimentary sequences in association with the events of the Holocene marine transgressions and regressions. Samples have been collected from two separate sites, one at Panigati near Khulna and another at Matuail near Dhaka. The study shows evidence of five periods of marine transgression, each followed by a regression, during the Holocene. Each minerogenic sediment layer indicates a marine episode and these sediments were deposited under intertidal to estuarine conditions; each peat layer is in situ and indicates a retreat of the sea. It is difficult to separate the eustatic components contributing to these relative sea-level movements, although processes operating locally and regionally are clearly evident. Two separate sea-level curves, together with possible error ranges, have been proposed for Bangladesh; since the early mid-Holocene, an average relative sea-level rise of 1.07 mm. yr -1 has been estimated. The reconstructed sea-level curves show that during the early and mid-Holocene both sedimentation and subsidence rates were much lower than during the last millennium. Differential spatio-temporal progradation and coastline movements have also been evident. The Ganges and Brahmaputra rivers have provided a continuous sediment supply but their convergence is only of recent origin. A possible hypothesis of two separate estuarine systems for these two rivers has been put forward.