Quaternary Sedimentary Environments in Ganga Prodeltaic Region, Northern Part of Bay of Bengal

Sedimentological characteristics, fossil contents and geochemical behaviour of the seabed sediments of the inner continental shelf region off Ganga river mouth, Bay of Bengal reveal some aspects of macrotidal deltaic environment. Seabed sediments between 11 m and 45 m water depths, covering an area of about 1450 sq km, indicate the presence of three characteristic domains from West to East each having specific features. Relict calcareous/ferruginous concretions, oolites, pellets as well as older skeletal matters of bivalve, gastropod, scaphopod and benthic foraminifera occur in the sediments of western domain at about 20 to 35m water depths. The relict taxa generally show worn out tests and ferruginous coated surfaces. The central domain is made up of sand and sandy clay with common occurrence of foraminifera. Some carbonaceous material and minor disseminated grains of heavy minerals are commonly associated with these sediments. The eastern domain is covered by silt and silty sand, mainly composed of quartz, feldspar, muscovite and biotite, which represent recent sediments of Ganga delta.

Gastropod, bivalve, scaphopod, burrows, echinoid spines and benthic foraminifera are abundant in the western, poor in the central and rare in the eastern domain. However, Astrorotalia trispinosa and Astrorotalia-Ammonia are two dominant foraminiferal assemblages in this entire region. They are large and well ornamented in the western part compared to those in the east.

The subrounded ill sorted concretions, oolites and pellets along with the above insitu biota in the western domain suggest very shallow sedimentary facies, probably deposited in a littoral condition during the Late Pleistocene time. On the other hand, the composition and nature of sediments and distinct morphological changes of taxa in the eastern domain corroborates that the Ganga delta is under a process of continuous progradation during the Holocene time.