Paleobathymetry and sediments of the Indian Ocean (in Indian Ocean geology and biostratigraphy; studies following Deep-Sea Drilling legs 22-29 Heirtzler, Bolli, Davies, Saunders, Sclater, J. R., H. M., T. A., J. B., J. G.

Am. Geophys. Union, Washington, D.C., United States( 1977)pp. 25-59

Index Terms/Descriptors: algae; assemblages; biostratigraphy; calcite compensation depth; Cenozoic; Cretaceous; Deep Sea Drilling Project; Eocene; Foraminifera; Indian Ocean; Invertebrata; lithosphere; lithostratigraphy; Mesozoic; microfossils; mid-ocean ridges; ocean circulation; oceanography; Oligocene; paleo-oceanography; Paleogene; Plantae; plate tectonics; Protista; sea-floor spreading; sedimentation; sediments; stratigraphy; subsidence; tectonophysics; Tertiary

Latitude & LongitudeS60°00'00'' - N30°00'00'' and E30°00'00'' - E120°00'00''

Summary

We establish a simple relation between subsidence and age for both normal ocean floor and the aseismic ridges in the Indian Ocean. This subsidence is accounted for by the cooling and contraction of the lithospheric plate as it moves away from a center of spreading. We use the relation between subsidence and age to construct paleobathymetric charts of the ocean for the early Oligocene (36 m.y.b.p.), the early Eocene (53 m.y.b.p.) and the late Cretaceous (70 m.y.b.p.). We conclude from these charts that the Indian Ocean between the middle Cretaceous and the Oligocene may have been separated by the Ninetyeast Ridge/Kerguelen Plateau complex and the Madagascar, Amirantes, Mascarene, Chagos complex into three basins which were not connected at depths below 2,000 m. We discuss the implications these complexes and the active mid-ocean ridge axis may have had for deep water circulation patterns in the Indian Ocean.

As an example of the application of these charts we use 19 drill sites to reconstruct the past history of the Calcite Compensation Depth (CCD) in the Indian Ocean. The average depth of this boundary shallows from more than 4,500 m at present to 4,000 m in the Oligocène and remains approximately constant till the Campanian. In the Wharton Basin it shallows in the Albian and Aptian. Major differences from the average in the Northern Arabian Sea and the Australia and Antarctic Basins can be accounted for by proximity to the continental shelf and the presence of bottom water in the Antarctic Circumpolar Current. We assume a constant CCD of 4,000 m for the rest of the Indian Ocean and compute the surface distribution of carbonate and clay sediments in the Oligocene. The shallowing of the CCD results in a marked reduction in the surface distribution of calcareous sediments between the present and the Oligocene. The reason for such a dramatic diminution of carbonate sediments is not known.