SILICEOUS MICROFOSSILS FROM NIAS ISLAND - THEIR SIGNIFICANCE FOR THE TERTIARY PALEOCEANOGRAPHY OF THE NORTHEAST INDIAN-OCEAN

Nias Island, located in the eastern Indian Ocean, is part of the emerg ent, non-volcanic outer-are ridge which marks the western margin of th e Sunda Forearc Basin of northwestern Sumatra. The island is structura lly divided into three main sub-basins and a basement high by three pr ominent SE-striking faults. Geologically ir consists of three principa l units; basement ophiolite complex, Oligocene to Recent sedimentary r ocks and diapiric mi lange. The recovery of indigenous radiolarians fr om a chert sample in the Mujoi Sub-basin constrains the oldest age of emplacement of the ophiolitic basement as Middle Eocene. The finding o f Miocene diatomite in the Lahewa Sub-basin is interpreted as the prod uct of upwelling effects and probably coincides with one of the coolin g intervals recognized from other pacts of the world. (C) 1998 Elsevie r Science Ltd. All rights reserved.