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Recent reconstructions of the Mesozoic and Cenozoic spreading history of the Indian Ocean (McKenzie and Sclater, 1971) have provided a hypothetical framework within which geostructural and faunal events may be examined. Drilling during DSDP Leg 23A (March-April, 1972) was conducted with the aim, not only of determining the sedimentary and biostratigraphic history of selected regions in the Arabian Sea, but of comparing the ages determined for igneous basement with the predictions of McKenzie and Sclater's model. The degree of corroboration and the implications of observed sea-floor ages for the geotectonic interpretation of Indian Ocean history are discussed elsewhere in this volume. Sediments were recovered from seven holes at six sites (219 through 224) in the Arabian Sea. Location and recovery data are presented below (Figure 1, Table 1). In terms of their objectives, and to a lesser extent their geographic positions, the sites may be divided into three groups. Sites 219 and 220 are associated with the ChagosLaccadive Ridge, a north-south-trending structure extending southward from the eastern margin of the Arabian Sea. The former is located near the crest of the ridge, in a relatively deep gap between shoal areas represented geographically by the Laccadive and Maldive Island groups. Prior to the drilling of Site 219, coral rock and related debris represented the only sedimentary rock recovered from this ridge. As a result, its Tertiary sedimentary history was unknown, as well as the nature of the underlying basement. Site 220 is situated slightly to the southwest, in deeper water along the western flank of the ChagosLaccadive Ridge within a quiet magnetic zone (McKenzie and Sclater, 1971). It was anticipated that igneous basement at this location would be no older than 38 m.y. (Late Eocene). The primary objectives of drilling at Site 220 included the sampling of this basement and the age determination of a shallow unconformity observed in seismic records. The extensive sedimentary sequences at these sites contained the only abundant and well-preserved planktonic foraminiferal faunas recovered during Leg 23A.

Cenozoic Planktonic Foraminifera and Biostratigraphy, Arabian Sea Deep Sea Drilling Project, Leg 23A