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Three closely-spaced sites (Sites 717, 718, and 719) were drilled on the distal Bengal Fan in the central Indian Ocean at a water depth of 4735 m and to a maximum penetration of 962 mbsf. This paper presents a synthesis of results from both shipboard and shore-based studies, with particular emphasis on the lithofacies, their depositional processes, and sediment source. The sediment record at all three sites can be divided into five distinct lithologic units: Units I, III, and IV are mud-rich with relatively low accumulation rates (<70 m/m.y.), whereas Units II and V are silt-rich with high rates of sedimentation, in parts in excess of 350 m/m.y. The oldest sediments recovered are early Miocene in age, approximately 17 Ma. Seven sediment facies are recognized. Light-gray silt and mud turbidites (Facies 1 and 2) are the most dominant throughout, and were derived from rapid erosion of the Himalayas. Transport was to the Ganges delta/shelf region and then resedimentation occurred downslope, in some cases in very large powerful turbidity currents that traveled over 2500 km and deposited beds up to 2.5 m thick. Dark-gray, organic-rich, mud turbidites (Facies 3) are interbedded with the other facies, particularly in the mud-rich units. They have a distinctive clay and silt mineral composition that indicates derivation from the continental margin off southeast India and Sri Lanka. Biogenic turbidites (Facies 4), comprising nannofossils, foraminifers, and clay, are much less common. Two types are identified, one from the outer shelf/upper slope off Sri Lanka and the other from a local seamount source. Pelagic clays and calcareous pelagic clays (Facies 5 and 6) are developed as thin caps over some of the mud and biogenic turbidites. They mostly have a low to absent biogenic content, indicating deposition below the carbonate compensation depth for all but the last 2 m.y., as well as significant early diagenetic dissolution of siliceous microfossils. Continuously bioturbated, structureless muds of part turbidite and part pelagite aspect (Facies 7) are interpreted to have been deposited from a thick suspension cloud above and beyond the distal feather edge of the "true" turbidity current. These sediments are here termed "hemiturbidites." The effects of Himalayan uplift, eustatic and local sea-level fluctuations, local tectonics related to intraplate deformation, and fan channel/lobe processes have closely interacted to produce the observed sedimentary record of the past 17 m.y. on the Bengal Fan. Different stages of Himalayan uplift and erosion can be recognized from the composition of distal fan turbidites.

Sediment Facies and Processes on the Distal Bengal Fan, Leg 116