PALEOLINHAS DE COSTA NA PLATAFORMA CONTINENTAL DAS REGIÕES SUL E NORTE BRASILEIRA

Abstract

An analysis of surface sediments on the broad northern southern Brazilian shelf has revealed the presence of three zones rich in terrigenous sand sized material. These coarse componentes were deposited during regressions and were sorted during following transgressions. Thus, they are markers of sealevel stillstands that interrupted the late Quaternary eustatic fluctuations. The outer zone is located along the 170 m depth contour, the intermediate zone along the 100-110 m contour, and the inner zone extends from the 60 m contour line to the shore. Two age interpretations are given for the two outer zones. One calls for a rate of subsidence of 3m/l,000 years for the basins due to sediment loading or compaction, to explain the difference observed between the - 170 m zone and the - 130 m sealevel average for the beginning of the Holocene transgression 18,000 years ago. In this case, the sea would reach the - 110 m level at 16,000 years BP. An alternate hypothesis places the - 170 m sealevel at the beginning of the Mid-Wisconsin transgression 40,000 years ago, and the - 110 m sealevel at 18,000 years BP. Evidence favoring subsidence is more convincing, hut C 14 dates determinations are needed to resolve the issue. The innermost zone in the Amazon area is predominantly mod em and derives from river discharge. The corresponding zone in the south is of relict nature and is presently being reworked.