RECONSTRUCTION OF HOLOCENE SEA LEVEL CHANGE AND PALEOCOASTLINE FOR THE EASTERN COASTAL MARGIN OF BANGLADESH USING MICROPALEONTOLOGICAL EVIDENCES

Because of environmental sensitivity, resistance to chemical weathering from ambient water and narrow ecological distribution of individual species, foraminifera remain at the center of microfossil based paleoenvironmental and paleoecological investigations. The present research has assessed the indicative values of salt-marsh foraminifera to reconstruct paleo-coastline and Mid-Holocene sea level change based on some taxonomic, statistical and morphological parameters derived from the recorded species from Cox’s Bazar to Bardeil area. The entire study area was divided into three segments; namely, the Himchari region, the Inani region and the Bardeil region and samples were collected from 15 closely spaced locations (with 1.5 km spacing). A total of 51 species representing 40 genera and 35 families was recorded. Most of the recorded species are benthic in origin. Among them Elphidium, Cibicides, Trochamminita, Eponoides, Massilina, Robulus and Cyclammina are prominent in terms of frequency of occurrence and relative abundance. The Himchari region has been deemarcated as Trochammina-Elphidium assemblage zone, Inani region as Elphidium-Eponoides assemblage zone and the Bardeil region as Cibicides-Elphidium assemblage zone. Both the correlation of the relative abundance chart with Saraswati’s schematic diagram and the plot of shell type ratios on Murray’s ternary diagram marks the marginal marine to inner-shelf zone as the paleo-environment of the recorded species. A depth of about 20m has been estimated as the paleodepth of the recorded species from Van der Zwaan equation that also represents the depth of inner-shelf. The age of the recorded species was estimated as 3490 to 3880 years B.P. by carbon dating. A paleo-coastline has been drawn along the plain of intersection of the sampling points representing marginal marine and those that representing inner-shelf as paleo environment. On an average, a rate of 8mm/year of coastal shifting has been estimated in GIS by using 250m transects for each of the segment. Based on the paleoenvironmental significance of recorded species, age dating and coastline shifting, this study justifies a regression of sea in the study area after the Mid-Holocene sea level rise.