Clay mineralogy and sedimentation in the eastern Indian Ocean

Abstract

The distributions of clay minerals in the surface sediments based on about 100 new samples from the eastern Indian Ocean reveal several distinctive sediment sources which differ from those reported previously by other workers. We observe the following: (1) large zones around the periphery of the study area are characterized by abundant smectite. These largely volcanogenic sediments are derived from the Indonesian Archipelago in the northeast, from submarine volcanics of the Mid-Indian Ocean Ridge and nearby areas in the southwest, and from the Deccan Traps of India along the western side of the Bay of Bengal. The effect of Indonesian volcanism is also observed in the Cocos-Roo Rise area and on the Ninety East Ridge between 05°N and 15°S. The chief agents of transport in the case of Deccan smectite are turbidity currents and counter-clockwise surface water circulation, and, in the case of Indonesian volcanics, wind. (2) The Ganges Cone and the Java Trench are characterized by relatively more abundant illite and chlorite dispersed primarily by turbidity currents from the Ganges and Brahmaputra rivers. The influence of this sediment source extends well south of the equator on both sides of the Ninety East Ridge. The southwestern part of the Ganges Cone, is, however, characterized by smectite-rich sediments derived from the Deccan Traps. (3) A zone extending west of Australia to the Ninety East Ridge, contains relatively abundant kaolinite which is probably wind-borne from that continent. Comparison of clay mineralogy in several of these regions between Quaternary and pre-Quaternary sediments indicates changes in sediment sources and/or transport mechanisms in the geological past. For example, west of Australia, abundant volcanogenic smectite was deposited during the Cretaceous-Eocene and the relatively high kaolinite characteristics of the Quaternary sediments has been supplied only since the Miocene.