Sediment distribution in western Atlantic off northern Brazil; structural controls and evolution

A sediment-isopach map of the western equatorial Atlantic off northern Brazil reveals major structural trends and significant sediment accumulations. The main features of the region beyond the continental slope are: the Amazon Cone, the aseismic Ceara Rise, the Demerara and Ceara Abyssal Plains, the North Brazilian Ridge, and the Fernando de Noronha and Para-Maranhao basins which are semi-isolated basins located on the continental margin seaward of the coastal basins of the Brazilian shelf. The maximum thickness of sediments in the area (> 10 km) exists under the shallowest (upper) part of the Amazon Cone. Sediment thickness in the center of Fernando de Noronha and Para-Maranhao basins exceeds 3 km. Beyond the Amazon Cone and these basins, the sediments are generally about 1 km thick in the abyssal plains and on the higher parts of Ceara Rise. Starting approximately 100 m.y. ago, sediments on the outer margin were derived from segments of the shelf between 37 and 39 degrees W long. and between 43 and 45 degrees W long. The northern Brazilian shelf between 39 and 43 degrees W, and west of 45 degrees W, off the Amazon River, contained shelf-edge structural highs which did not allow sediments to be carried to the outer margin until Cenozoic time. The shelf-edge highs are believed to be landward extensions of the Romanche and St. Paul's fracture zones. Beginning in Miocene time, a major phase of terrigenous transport to the outer margin resulted in the Amazon Cone. At least 2 km of sediment was deposited on the rest of the outer margin during the same interval. Sediments have been carried to the abyssal plains through gaps in the North Brazilian Ridge only since middle to late Miocene. The Ceara Rise is covered primarily by pelagic sediments. However, its western part has been buried under approximately 3 km of Amazon Cone sediments during the last 6 m.y.