Miocene sedimentation and subsidence during continent–continent collision, Bengal basin, Bangladesh

The Bengal basin, a complex foreland basin south of the eastern Himalayas, exhibits dramatic variability in Neogene sediment thickness that reflects a complicated depositional and tectonic history. This basin originally formed as a trailing margin SE of the Indian continental crust, complicated by convergence with Asia to the north and oblique convergence with Burma to the east. Newly compiled isopach data and previously reported seismic data show evidence of thickening of basin fill toward the south, opposite of the pattern typically seen in foreland basins. This is presumably due to sedimentary loading of voluminous deltaic sediments near the continent–ocean boundary and basinward downfaulting analogous to that in the Gulf of Mexico. Isopach data show that there is considerable vertical relief along the base of the Miocene stratigraphic sequence, probably due to down-to-the-basin faulting caused by focused deltaic sedimentation and associated crustal flexure. In contrast, when viewed in east–west profile, basin shape is more typical of a foreland basin, with strata thickening eastward toward the Indo–Burman ranges, which reflects east–west convergence with Southeast Asia. Comparison of the lateral and vertical extent of the Bhuban and Boka Bil Formations with the Bouguer anomaly map of Bangladesh suggests that considerable subsidence of the Sylhet trough (in the northeastern part of the Bengal basin), which has the lowest gravity value of the region, had not taken place by the end of the Miocene. This post-Miocene subsidence is attributed to tectonic loading from southward thrusting of the Shillong Plateau along the Dauki fault. Relatively uniform Miocene isopachs across the Sylhet trough confirm that this began in the Pliocene, consistent with results of recent research on sediment provenance. In the northwest, in the region south of the Siwalik foreland basin, continental crust has not as yet been loaded, allowing relatively little accommodation space for sediment accumulation. The Miocene here is very thin. Deltaic progradation across most of Bangladesh during the Miocene followed earlier, more proximal progradation across Assam, immediately northeast of the Bengal basin, and has been followed by continued progradation into the southern Bangladesh coastal and offshore region.