Late Quaternary changes in depositional processes along the western margin of the Indus Fan

Detailed sedimentological, geochemical and isotopic analyses were carried out on sediment samples from ODP Site 720A on the Indus Fan, Arabian Sea. High values of calcium carbonate associated with low values of Al and Ti from 0 to 375 ka, and low values of calcium carbonate along with high values of Al and Ti from 375 to 525 ka represent two distinct sedimentary sequences. The sediments deposited from 525 to 375 ka correspond to a turbidite sequence, characterized by a high terrigenous input of coarse-grained sediments composed mostly of sand and silt. The sediments deposited from 375 ka to the present day comprise a pelagic sequence, consisting of pelagic material and clay. The major turbidity flow between 375 and 525 ka resulted in the greatest development of the Indus Fan during the late Quaternary. Most of the active channels were buried by 375 ka, followed by deposition of mainly pelagic sequences since then. Enrichment of an Indus-derived Himalayan clay mineral assemblage (illite and chlorite) in both the turbiditic and pelagic sequences reveals that the source and supply of clay minerals to the Indus Fan were the same during pre- and post-turbidite deposition. At ODP Site 720A, Al, Ti and terrigenous material do not show any systematic changes with respect to glacial and interglacial periods, suggesting that sea-level changes are not directly responsible for the terrigenous material supply to this site. Rather, a major switch in distributary channels away from the western margin of the Indus Fan is suggested.