Quaternary paleo-geography and geohazard scenario of the Bengal delta of Bangladesh

The Ganges-Brahmaputra-Meghna (GBM) delta is an extreme example of a deltaic coastal region vulnerable to natural disasters like earthquake, differential subsidence and shortening, arsenic contamination in groundwater, salt-water intrusion, storms surges, and flooding. The sea level changes during the Quaternary period have major bearing on the geohazard scenario of the OBM delta. Inbred tectonism of the Indian plate has been playing a significant role in the development of the GBM delta. The faults and lineaments are of seismogenic potentiality and are active in terminating by block upliftment and subsidence in the delta. The subsidence rates in Jamuna depression, Sylhet basin and Meghna delta plain are 4.3 mm/yr, 7.5 mm/yr and 3.9 mm/yr respectively. It is clearly envisaged that the Sylhet basin is relatively faster subsiding basin in the GBM delta. The subsided sub-basins enhance the marine transgression in the LateQuaternary period. The neotectonic depressions and most of the lower delta plain are the most vulnerable sites for earthquake hazards because the zone within the upper 200 ft of the ground level in these depressions is mostly composed of unconsolidated clay and silt materials. During the Holocene marine transgression land-ocean interaction occurred along two main flow domains of the river system. As a result, numbers of estuarine lobes have been developed within the transgressive domain. These lobes are the regions of complex interaction between fluvial and marine processes that might have acted as geochemical traps for the transitional metals bonded in the finer grained sediments. During the Holocene regressive phase there was frequent oscillatory changes in the sea-level in the GBM delta straddles with the monsoon climate and tectonic instability that promoted channel instabilities, migration and reconfiguration i.e. chute cutoff, neck cutoff, channel avulsion. The arsenic contaminated aquifers of Holocene land-ocean interface lobes in these palaeo- meandering belts occur below the chute cut-off, chute bar and back swamp deposits. Shallow sandy aquifers might be reducing enough below 40-50 ft to release arsenic into solution. Deep aquifers below the 'maximum marine flooding deposits' are low in arsenic and iron because most of the FeOOH and MnOOH coatings have been dissolved due to the prevJilence of depth dependent reducing condition in the depositional horizon. r The paleogeographic status of the GBM delta during Quaternary such as marine transgression- regression, neotectonic upliftment and subsidence, duration of sub-aerial exposure, and land-sea water interaction is associated with various geological environments.