A paleo-Brahmaputra? Subsurface lithofacies analysis of Miocene deltaic sediments in the Himalayan-Bengal system, Bangladesh

The Bengal foreland basin contains a succession of up to 16+ km of dominant ly deltaic deposits, eroded from the eastern Himalayas and the Indo-Burman ranges and carried by major river systems similar to the present-day Ganges and Brahmaputra. Analysis of electric logs and core descriptions acquired during oil and gas exploration in Bangladesh allows construction of lithofa cies maps, which constrain depositional patterns of Miocene strata. Compila tions of sand thickness and sand/shale ratio of the Miocene Surma Group sho w that Lower to Middle Miocene strata of the Bhuban Formation accumulated i n a large, elongate trough. Sand thickness and percentage both decrease mar kedly away from this depocenter, which describes a large-scale bend, runnin g initially westward from Rashidpur (northeast Bengal basin) and curving so uthward toward the Bengal fan. Middle to Upper Miocene strata of the Boka B il Formation show a similar geographic trend in deposition of coarsest and thickest sediment, but the major depocenter had shifted northward relative to that of the Bhuban Formation by some 30 km, passing near Beani Bazar. Th ese trends suggest that deltaic deposits of the Surma Group filled the Sylh et trough of the northeast Bengal basin from the east. Published seismic da ta from western Bangladesh show that additional large channels also contrib uted materials to the Bengal basin from the northwest during the Late Mioce ne, but these channels resulted in very little accumulation in the northwes tern part of the basin, probably due to restricted subsidence of underlying continental crust. This study suggests that there was a major drainage sys tem similar to the modern Brahmaputra River during Miocene time, which carr ied orogenic sediments eroded from the uplifted terranes of the eastern Him alayas and Indo-Burman ranges to the eastern Bengal delta. (C) 1999 Elsevie r Science B.V. All rights reserved.