No abstract

Leg 26 of the Deep Sea Drilling Project in the southern Indian Ocean was planned to investigate: (1) the history of the Southwest Branch of the Indian Ocean Ridge and the history of the crust and oceanic sedimentation in that general area; (2) the history of the Ninetyeast Ridge; (3) the origin and history of Broken Ridge-Naturaliste Plateau; and (4) the oldest sediment in the Indian Ocean and the date of the breakup of Gondwanaland. Most of these objectives were met.Crustal ages in the southwest Indian Ocean span the Cretaceous. Pelagic sedimentation has occurred intermittently here until the Miocene when a western boundary undercurrent constrained by the Southwest Branch brought terrigenous materials south into the Mozambique Basin and prevented sedimentation in the Madagascar Basin. The Southwest Branch is spreading at 0.9 cm/yr north northeast and is no older than middle Eocene based on extrapolation of this rate. The carbonate compensation depth in the southwest Indian Ocean has been shallower than its present depth during the Late Cretaceous and Miocene.Sites on the Ninetyeast Ridge range in age from Eocene-Oligocene to Late Cretaceous and become progressively older to the north. This suggests that the ridge belongs to the Indian plate. The stratigraphy consists of pelagic calcareous sediments overlying shallow-water and littoral volcanic sequences. Because sites to the north can be shown to be the same age as the Indian plate, some of the ridge formed near or on the crest of the ancient Southeast Branch of the Indian Ocean Ridge. The origin of the ridge can be modeled as a combination point volcanic source construction and a leaky transform fault.Broken Ridge was a shallow marine carbonate platform in the Late Cretaceous. It was uplifted above sea level in the Eocene as a result of rifting and uplift of the Southeast Branch which split Broken Ridge from Kerguelen Plateau. Since then Broken Ridge has subsided and accumulated pelagic carbonates. The Naturaliste Plateau is older than middle Albian. It received detrital sediments until the Late Cretaceous and pelagic carbonates since then. Erosion has removed large sections of the Tertiary from the plateau. Basement under both features is unknown, but the drilling results suggest that these structures are marine and volcanic (oceanic) in origin.The Wharton Basin is oldest in the south, and formed during north-south spreading, at about 6 cm/yr, which began about 130 m.y.B.P. These observations indicate that India separated from Antarctica rather than from Western Australia. Potassium-argon dating of basalt from Site 257 in the southeastern Wharton Basin indicates that there is an intrabasalt unconformity and that the lower basalt unit is possibly as old as Early Jurassic.Regional unconformities centered on the Oligocene, early Tertiary, and Late Cretaceous can be recognized in the Indian Ocean Basin. These unconformities are expressed either as a true disconformity or are implied by the existence of dissolution facies which 'Woods Hole Oceanographic Institution...