Abstract
The shelf and upper slope off southern Brazil are underlain by two broad sedimentary basins, the Santos in the north and the Pelotas in the south. The landward edge of maximum sediment thickness in the basins is delineated by the Rio Grande–Santos fault, which apparently formed during (or subsequent to) early rifting of the South Atlantic. Both basins also appear to be divided into several north-south sections by the intersection of east-west fracture zones.
Although depth to basement in both basins exceeds 7 km, the lower basin fill consists of Cretaceous lava (Serra Geral). In addition, the Santos basin contains Aptian salt overlying the Serra Geral. Thus, total post-Aptian terrigenous accumulation in the Santos basin is generally less than 4 km, mostly derived from erosion of the adjacent Serra do Mar; at least some of the Pelotas sediments were transported northward from the Rio de La Plata. Unconsolidated sediment, assumed to be post-Miocene in age, is markedly thinner in the Santos basin than the Pelotas, possibly because of continued retreat of the Serra do Mar.
Shallow seismic profiling shows that the upper strata in the Santos basin accumulated as several cycles of foreset beds, the greatest thickness being in the northern part. Comparison with available drilling and eustatic data suggests that these strata were deposited in the middle Tertiary, prograding over an Eocene unconformity. In contrast, sediments in the Pelotas basin show no sign of foreset deposition but, rather, suggest accumulation in near-shore environments (notably lagoons and estuaries) similar to the coastal environments along the present shoreline. Uplift in the Pelotas basin may have occurred as recently as the late Tertiary–early Quarternary (as inferred from seismic profiles over the Patos arch), followed by extensive erosion and subsequent sedimentation.
