Geologic History and Origin of Sao Paulo Plateau (Southeastern Brazilian Margin), Comparison with the Angolan Margin, and the Early Evolution of the Northern South Atlantic

Abstract

The Sào Paulo Plateau is a prominent marginal plateau in the southern Brazilian margin. It is underlain by oceanic crust and is bounded to the north and south by fracture zones. Seismic profiles and drilling data from Site 356 (located on its southeastern corner) indicate that the region occupied by the plateau has acted as a depocenter and has evolved as a marginal plateau since the Aptian time. The Oceanographic conditions and depositional environments prevailing over the plateau during its early evolution were almost identical to those suggested in the Angolan margin by the drilling data at Site 364 (DSDP). Reconstruction of the evolution of Sào Paulo Plateau and comparison between the drilled geologic sequences on the opposing margins suggest the following history for the northern South Atlantic. Coarse terrigenous sediments were deposited in coastal basins at both the margins following the opening of the South Atlantic in the Neocomian. By Aptian time, a linear narrow basin, with barriers to the south and north, was created. The Walvis and Sào Paulo ridges formed the southern barrier whereas the equatorial fracture zones appear to have formed the northern barrier. An evaporite layer, which continuously extended from Brazil to Africa, was deposited in the basin during middle to late Aptian. At the end of Aptian, the evaporite deposition ceased and the evaporite layer was split along the present-day seaward boundaries of the diapir zones located in the Brazilian and West African margins. As evidenced by limestones with sapropels of late Aptian to early Albian age at Site 364, stagnant conditions prevailed in the basin at the end of evaporite deposi-tion. Open-marine limestones, drilled at Site 356 as well as at Site 364, suggest that the basin became well oxygenated by the Albian time. Another basin-wide period of stagnation is suggested by black shales and mudstones of Turonian-Coniacian age at Site 356 and of Turonian-Santonian age at Site 364. The terrigenous nature of these sediments suggests supply from the adjacent shelves in the two margins.