Biogenic Sedimentary Structures in Outer Bengal Fan Deposits Drilled during Leg 116

Abstract

Bioturbation in the outer Bengal Fan is influenced by two major factors, bottom-water oxygenation and turbidite deposition. Biogenic traces reflect changing bottom-water oxygenation over the last 17 m.y.; in the Miocene clear oxygenation cycles are developed in the hemipelagically deposited sections. Since the Pliocene these cycles are less pronounced and an intermediate level of oxygenation is documented by the trace fossils. As only three closely spaced sections were studied, it is uncertain whether the change in oxygenation is a regional phenomenon or just a local effect resulting from changing bottom topography due to intraplate deformation. Turbidite deposition also strongly influences bioturbation at the sites studied. In the outer Bengal Fan, fine-grained parts of turbidites are often burrowed. In contrast, biogenic reworking is less intense or nonexistent in the sandy parts of turbidites. A considerable number of the fine-grained sections are bioturbated by Phycospiphon, and the thickness of the reworked section can exceed the penetration depth of this trace. In this case, we infer that bioturbation and sediment accumulation were in equilibrium and that sedimentation was relatively slow for normal deposition by a turbidity current of the order of 1 cm per day during an event.