139736eo.pdf (20MB) - Japan Oceanographic Data Center

part of the Seychelles-Mascarene block is of great antiquity, as much as 600 m. yrs. old (FISHER andBUNCE, 1974). It separated from the Indian Plate in early Paleocene and has subsided steadily sincethen (NORTON and SCLATER, 1979). Drilling in the middle of Mascarene Plateau has shown thatsince its separation it has been a site of pelagic carbonate sedimentation.The Madagascar Ridge has long been considered to be a submerged continuation ofMadagascar, but DSDP drilling on the ridge did not resolve the issue. The sedimentary record showsa drape of pelagic carbonates over early Eocene basaltic pyroclastic sediments, the latter pointing tovolcanic activity in the earliest Tertiary (SIMPSON et al., 1974).Drilling on the Mozambique Ridge revealed that it has an oceanic-type basaltic basement,somewhat older than Valanginian (ca 130-135 Ma). In the late Cretaceous it was a site of extensivevolcanism, but since then the ridge has been inactive (VALLIER, 1974). It was first the site ofdeposition of carbonate sands, and later pelagic carbonate oozes.The deeper basins of the Indian Ocean contain fairly well delineated seafloor magneticanomaly patterns that serve as important clues to the antiquity and spreading history of these basins.Major basins include, the Wharton Basin, east of the 9OE Ridge; the Central Indian Basin, sandwichedbetween the 90E and Chagos-Laccadive Ridges; the Crozet Basin, south of the inverted Y-shapedspreading center; Somali and Madagascar Basins, north and east of Madagascar, respectively; and theMozambique Basin, between the Madagascar and Mozambique Ridges. The oldest identifiablemagnetic anomalies on the Indian Ocean floor are of late Jurassic to early Cretaceous age.Other important features of the Indian Ocean include the Indus and Bengal Fans (Fig. l), bothof which comprise thick piles of terrigenous sediments, deposited since the collision of India withEurasia. The latter event is discussed in more detail in the next section.MODERN SEDIMENTARY PATTERNSThe sedimentary patterns of the Indian Ocean have been summarized on a regional basis inInitial Report volumes of the DSDP Legs 22 through 27, and on an oceanwide basis by KOLLA andKIDD (1982), KIDD and DAVIES (1978) and DAVIES and KIDD (1977).In general, land-derived (terrigenous) sediments (mainly muds and sands) accumulate alongthe continental margins, brought down by the rivers. Major areas of terrigenous input are the Indusand Bengal submarine fans, and to a lesser degree the Zambesi Fan, off central Africa coast. By farthe thickest and the most extensive pile of sediments is that of the Bengal Fan. Seismic data showsthis fan to be as much as 12 km thick (CURRAY and MOORE, 1974), extending from the mouth ofGanges River to the Ceylon Abyssal Plain in the northern part of the Central Indian Basin. The IndusFan is also of considerable extent, reaching up to the Carlsberg Ridge, with a maximum thickness ofover 10 km (NAINI and KOLLA, 1982). Both of these fans receive their great load of sedimentseroded from the relatively youthful mountain ranges of the Himalayas. The main locus ofvolcanogenic sediments is along the southern margin of the Indonesian Island Arc, which containssome 14% of the worlds active volcanos. The material from this island arc is silicic in composition(KIDD and DAVIES, 1978).The Indian Ocean carbonate compensation depth (CCD - below which all carbonate isdissolved and which separates the calcareous oozes from clays) lies at an average depth of about 4000meters. Thus carbonate accumulation is restricted to the relatively shallower parts of the deep basinsand on ridges and plateaus. Below the CCD, in deep basins, siliceous sediments and claysaccumulate. The biogenic siliceous sediments are, however, more characteristic of the equatorial andsubpolp high productivity areas. The isolated deeper basins, away from the continental influences orhigh productivity belts, receive negligible amounts of sediment. Examples of such areas are, thedeeper Wharton Basin, Crozet Basin and the southern Mascarene Basin. Where deep currents areactive and sedimentation is minimal, manganese nodule pavements may develop, e.g., between theBroken and Southeast Ridges (DAVIES and KIDD, 1977; KIDD and DAVIES, 1978).In the northern Indian Ocean the carbonate content of the sediments increases graduallytowards the south with the decreasing dilution by terrigenous input. In the Arabian Sea biogenicproductivity is highest along the Arabian Peninsula and in the Gulf of Aden, which is reflected in the57