Neogene sedimentation and erosion in the Amirante Passage, western Indian Ocean

Abstract

Twenty piston cores from the northern Mascarene Basin and Amirante Passage reflect the effects of the Deep Western Boundary Current (DWBC) upon the lithologic and stratigraphic record of the late Cenozoic. The cores span a depth interval of 3350 to 5200 m, representing the transition zone between modern North Atlantic Deep Water (NADW)-Circumpolar Water (CPW) and the underlying Antarctic Bottom Water (AABW). During the late Cretaceous and for much of the Paleogene, pelagic sedimentation occurred in the absence of significant bottom current activity. The formation of the global psychrosphere near the Eocene-Oligocene boundary initiated the DWBC, part of which could enter the Madagascar Basin via deep fractures in the Southwest Indian Ridge. The DWBC was well developed before the early Miocene, transporting course detrital sands northward into the passage from turbidite deposits along the continental margin of Madagascar. The DWBC was confined to depths below ∼ 4 km until the middle Miocene, when the flow strengthened and shoaled to depths <3300 m. Strong DWBC flow continued intermittently until the latest Pleistocene, producing extensive erosional surfaces. Today the flow of the DWBC is relatively weak, with strong only below ∼ 3850 m in the western channels. Pleistocene and late Tertiary erosion at intermediate depths (3 to 4 km) in the Indian Ocean contrasts with depositional continuity at the same depths farther ‘upstream’ in NADW. Fluctuations in the intensity of circumpolar flow rather than in the rate of production of NADW may have been the major controlling factor in the late Tertiary erosional history of the Amirante Passage.