THE DEEP-SEA BENGAL FAN

 

The deep-sea Bengal fan is one of the largest deep-sea cones in the world: 3,000 kms long and 1,000 wide. It makes most of the Gulf of Bengal. It is formed by sediment redeposition from the Gange-Brahmaputa rivers which supply most (70%) of the material drained from the Himalayas: nearly 2 billions tons per year of sediment are redeposited, suggesting a denudation rate of more than 70 cm per 1000 years for the Himalayan chain. Rates of sedimentation of 35 cm per 1000 years are registered in the abyssal plain 1000 kms off India. Thick sedimentary deposits (the inner part of the cone can range in thickness up to 12 km) reflect the successive phases of the Himalayan orogenesis that began after the India plate collided with Asia, and the recent climatic changes.

Numerous piston-cores of the "Marion Dufresne 1" were collected along North/South transects on the Bengal Fan. A detailed study of the distal hemipelagic sediments of the Bengal Fan (between the Equator and 10 S) resulted in a chronology of the main turbiditic episodes that can be correlated to the main uplift and erosion events of the Himalayan chain. The same dilated sequences offered a rare opportunity to recognize the high-resolution cycles that characterized the biosiliceous inputs of the tropical oceanic belt during the last 15 million years.
The longest piston-core of the core collection (54 m of a 4 My. continuous sediment record) was collected in this area (10S) at a depth of 5360 m.