Depositional setup and characteristics of the storm deposit by the 2007 Cyclone Sidr on Kuakata Coast, Bangladesh

Abstract

This study examines the sedimentary characteristics, provenance, and depositional processes of the storm deposits from 2007 Cyclone Sidr that have been identified on the southern coast of Bangladesh. Three sedimentary units up to 70 cm thick have been identified in fifteen geological core collected at a distance of 135–277 m from the coast. Laboratory analyses of grain size, TOC, TN, δ13C, and diatom assemblages were carried out to characterize the deposits. Massive to parallel laminated bluish gray mud underlies the storm overwash deposits. White to light gray, massive to parallel laminated, normal graded 1–17 cm thick sand overlies the mud unit with sharp to erosional contact. The mean grain size and thickness of this sand unit decrease landward with increasing sorting value, where grain size distribution of sand is comparable with modern beach sand. The unimodal sand that dominated the base of the storm deposits grades into bimodal olive-gray sandy silt in the upper part of the deposits. The grain size implies that the sand carried from the beaches and mud likely sourced from the suspended, nearshore sediment of the bay and adjacent rivers. TOC/TN, δ13C values and the presence of brackish mudflat origin diatoms with silty sand laminae in bluish gray mud indicate a tide-influenced environment. Post-storm sand and sandy silt sediments unit contain freshwater and marine-brackish diatoms that have likely been deposited from flooding caused by storm surge water from the bay and overbank flooded river water due to subsequent heavy rainfall. These sediments were sorted according to storm waves and settled over the study area. This study indicates that the low-lying delta coast gets the influences of high water levels both from the bay and the river during a storm. The modern storm deposit will help to characterize the paleo/prehistoric tropical cyclone in geological time for future studies in this area.