LATE QUATERNARY SEDIMENTARY ENVIRONMENTS AND LANDFORMS IN THE GANGES DELTA

Late Quaternary sediments, deposited since the lowest stand of sea-level during the last glacial maximum, in the Ganges Delta and its surrounding region were classified into five units according to their sedimentary facies. These are the lowest unit of sandy gravels, the lower unit of sand with a few gravels, and the middle, upper and uppermost units consisting mainly of sand and silt with occasional peat layers. The contact between the lower and middle units is fairly sharp, and the upper part of the lower unit is oxidized in some places. This suggests a period of subaerial exposure after the deposition of the lower unit. During the low stand of sea-level, about the time of the last glacial maximum, rivers in the Ganges Delta and its surrounding region dissected the surfaces of the region, and deposited the lowest unit on the valley floors. The lower unit was deposited over the lowest unit during an early stage of the post-glacial transgression. Between ca. 12,000 yr BP and 10,000 yr BP, the surface of the delta was slightly dissected and the top of the lower unit was weathered and oxidized. Following this period, the middle and upper units of alluvial or deltaic sand and silt with occasional peat were deposited. The coastline in the early Holocene retreated towards the central part of the present Ganges Delta. In the middle and late Holocene, the silt and clay with occasional peat layers of the uppermost unit indicate that the lowland gradually became marshy and poorly drained as the rate of transgression became slower.