Sedimentation on Distal Bengal Fan, ODP Leg 116

Dorrik A. V. Stow

Three sites were drilled on the distal Bengal fan in the central Indian Ocean. The first of these is located in the center of a late Miocene flexure, where the total sediment thickness is greatest (1.5-2.0 km). The second is located on the flank of the flexure, where sediment is thinner, and the third is on the flank of an adjacent fault block, where the post-deformation sediment cover is thinnest. This last site bottomed in early Miocene (17 Ma) sediments at a total depth of 962 m below sea floor.

The entire section at all three sites is dominated by fan sediments. Grey micaceous silt and mud turbidites are most abundant, and were probably derived from the Ganges delta/delta front. Dark, organic-rich mud turbidites were more likely derived from large slumps on the upper slope of the northern and western Bay of Bengal. These are interbedded with biogenic turbidites from shelf-slope and local seamount sources, and with thin pelagic clays. Sediment accumulation rates were highest for the silty turbidite sections of Miocene and late Pleistocene age, and lower for the mud turbidites that dominated in the latest Miocene and Pliocene. A 1-m.y. hiatus (or extremely reduced section) during the early Pleistocene marks a major change in turbidity current supply and, possibly, increased bo tom circulation in the central Indian Ocean.

Four main factors controlled sedimentation on the distal fan during this period: (a) Himalayan uplift supplied much detritus during the Miocene and late Pleistocene; (b) sea level fluctuation partly but not wholly controlled the resedimentation of this material to the deep fan; (c) intraplate deformation; and (d) channel/lobe switching have had more local effects.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.