Nearshore Sediments and Coastal Evolution of Paraiba do Sul River Delta, Rio de Janeiro, Brazil.

The Paraiba do Sul River delta is located on northern Rio de Janeiro State, on the coastal plain onshore Campos Sedimentary Basin, Brazil. Previous work Suggested the transference of sands from the shoreface to the beach as the main mechanism responsible for beach ridges progradation on both sides of the present river mouth. To investigate this hypothesis we conducted a geophysical sub-bottom survey and superficial grab sampling to map the nearshore sediment distribution and seabottom morphology adjacent to the coastal plain. The results revealed a continuous mud facies, related to the Paraiba do Sul River prodelta, isolating the offshore carbonate facies from the coastal, sandy, shoreface facies. The thickness of the mud facies decreases in the offshore direction, pinching out against the outer carbonate facies, accompanying a general rise of the sea-floor related to the occurrence of calcareous coralline algae and bryozoans bio-constructions. Our results favor an alternative explanation, indicating that the river is ultimately the main source for the coastal sands, which are probably reworked by alongshore currents on opposite directions apart from the river Mouth, resulting on the coastal progradation.