• Argha Sarkar
• Prasanta Kumar Sen

The present study aims to understand the trend of sea level and coast line change in Lower Bengal basin during Holocene through the evidence of mangrove peat, mangrove wood, wood fragments, estuarine clay and organic rich mud. The biological remains of distinct ecofacies recovered from the measured sections and chronologically dated sediments in nine locations viz., Kumirmari, Dakshin Harishpur, Taldi, Canning, Namkhana, Bakkhali, Diamond Harbour, Raidighi and Gangasagar are used as evidences for the reconstruction of Holocene sea-level curve. The sections are studied up to maximum depth of 41 m from surface. The sea level curve reveals that during 17726±485 cal yr BP to 10720±330 cal yr BP very slow sea level rise was occurred at the depth of -26 m to -21.5 m below present mean sea level (MSL). After 10720±330 cal yr BP onwards up to 10452±70 cal yr BP rapid rise of sea level takes place (transgressional phase) at the depth of -21.5 m to -3.65 m below present MSL. The environment of deposition of the area during that period was mixed brackish water and fresh water tidal mangrove vegetation (Phase-I). The very next phase of deposition in the study area is tidal mangrove vegetation (Phase-II) prevalent during 10452±70 cal yr BP to 8481±44 cal yr BP when there was more or less no sea level change since 1971 yrs. at the depth of -3.65 m to -3.25 m below present MSL. The rapid fall of sea level (-3.25 m to -39 m below present MSL) occurred during 8481±44 cal yr BP - 7034±223 cal yr BP resulting regression in the study area that causes Phoenix dominated mangrove upland (Phase-III). Rapid rise of sea level (transgression) was also initiated during this phase and rose at maximum height from the depth of -39 m to -4.3 m below present MSL in the next phase. The bioassemblage of the next phase of deposition (Phase-IV) with abundance of mangrove pollen grains reveals swampy mangrove vegetation in the study area during 5099±214 cal yr BP – c. 3779 cal yr BP. In this phase the sea level rise at the depth of -4.3 m to -0.75 m below present MSL. It appears from the sea level curve that the sediment of brackish water mixed fresh water Heritiera forest phase (Phase-V) was deposited within a relatively short time span compared to other phases during c. 3779 – c. 3198 cal yr BP when the sea level existed adjacent to present MSL. The fresh water grey clay with sand deposits dated between c. 3200 cal yr BP to present occur at the depth of present MSL to +1 m (surface) and the assemblage indicate existence of a fresh water grassland (Phase-VI) in and around the area of deposition.

Sea level change; Holocene; Present Mean Sea Level; Lower Bengal basin; Environment of deposition

"Lower Bengal basin, India witnessing Sea Level change and change of Coastline during Holocene", International Journal of Emerging Technologies and Innovative Research (www.jetir.org), ISSN:2349-5162, Vol.6, Issue 2, page no.396-423, February-2019, Available :http://www.jetir.org/papers/JETIR1902761.pdf

"Lower Bengal basin, India witnessing Sea Level change and change of Coastline during Holocene", International Journal of Emerging Technologies and Innovative Research (www.jetir.org | UGC and issn Approved), ISSN:2349-5162, Vol.6, Issue 2, page no. pp396-423, February-2019, Available at : http://www.jetir.org/papers/JETIR1902761.pdf