Abstract

A large collection of USNS Eltanin deep-sea sedimentary cores and bottom photographs from the southeast Indian Ocean between long 70°E and 120°E, and between Antarctica and lat 30°S, were analyzed. Cores from the crest and flanks of the mid-ocean ridge are mostly late Quaternary in age, with only rare breaks in sedimentation. In great contrast, flanking this zone in deep basins immediately to the south of the ridge in the South Indian Basin and in a broad zone in the western sector of South Australian Basin, there are areas where bottom currents have systematically eroded or inhibited deposition of sediments ranging in age from Quaternary to Pliocene, and occasionally middle Tertiary. This regional deep-basin erosion extends northward between Broken Ridge and the Naturaliste Plateau to the Wharton Basin where sediments as old as Late Cretaceous are exposed. As indicated by disconformities, ocean-floor characteristics, and seismic-profile data, much of the shallower, north-trending Kerguelen Plateau has also undergone widespread erosion by bottom currents.

The erosional disconformities in the deep basins have been created by general increase in velocities of Antarctic Bottom Water during the last 2.5 m.y., apparently with major separate pulses during the Brunhes epoch (t = 0.69 m.y. to present) and part of the Matuyama epoch (t = 2.43 to 0.69 m.y.). Extensive areas of manganese nodules have developed in conjunction with this bottom-current activity, most spectacularly as a vast pavement in the northwestern sector of the South Australian Basin. This feature, which we name the “Southeast Indian Ocean Manganese Pavement,” is approximately 106 km2 in area.

The available evidence indicates long-term major erosion by the eastward-flowing Circumpolar Current across the central and southern parts of the Kerguelen Plateau. In the deep basins, high-velocity Antarctic Bottom Water flows eastward through the northern sector of the South Indian Basin, with important northward flow crossing the mid-ocean ridge at 110°E and 120°E into the South Australian Basin. This northward branch traverses the western sector of the South Australian Basin, the Southeast Indian Ocean Manganese Pavement, and then flows between Broken Ridge and Naturaliste Plateau into the Wharton Basin. Major Cenozoic to Late Cretaceous hiatuses in the Wharton Basin revealed by deep-sea drilling suggest that northward-flowing bottom water through this conduit has been a very long term feature.

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