Forced regression systems tracts on the Bengal Shelf

A vertical succession of five composite sequences has been identified within the upper 100 m of the outer Bengal Shelf by means of high-resolution multi-channel seismic data. Each sequence consists predominantly of up to 100 km long and some 10 m thick forced regression systems tracts. The internal reflection pattern of the regressive units show mainly prograding oblique clinoforms. Intervening transgressive systems tracts are represented by seismically transparent or chaotic layers. On the outer shelf three of the sequences cause shelf aggradation and retrogradation, and two of them cause mainly shelf progradation. Based on the hierarchy of systems tracts, their calibration by comparison with eustatic sea-level curves and reconstructed paleoshoreline positions the composite sequences are interpreted as eccentricity driven eustatic 4th order (Milankovitch) cycles with a periodicity of about 100 ky. Internal unconformities mark cycles of 5th or higher order. An average subsidence of the outer shelf is estimated to be less than 0.4 mm/year during the last 345 ky. The correlation between the shelf growth pattern and sea-level fluctuations is consistent with the enhanced deposition on the eastern Bengal submarine fan from 465 to 125 ky B.P., as was observed by other authors.