Arquitetura Sedimentar e Evolução Deposicional da Baía de Sepetiba (RJ) durante o Pleistoceno Superior-Holoceno (últimos ~130 ka)

Abstract

The Sepetiba Bay, located on the western portion of Rio de Janeiro State coastal area, consists of an embayment where estuarine conditions prevail, because of limited contact with the open ocean due to the existence of Restinga da Marambaia - an extensive barrier island of some 40 km extent. The present study aims at investigating the sedimentary and stratigrahic evolution of Sepetiba bay’s infilling units, based on seismic units seismofacies anlyses of circa 800 km of high-resolution reflection seismic profiles (100 J boomer seismic source). Resultos of sesimic analyses were coupled with recently-dated sedimentary and paleogeographic evolutional stages of the Marambaia barrier island, as well as with previous studies carried out across the Rio de Janeiro continental shelf. Integration of all seismic analysess results with those from previous works in the area reveal compelling stratigraphic and paleogeographic informations about the estuarine system developed in the Sepetiba bay are during the latest Pleistocene-Holocene transgression that drowned the area (last ~20 ky), such as: (i) the existence of a rather developed paleo-fluvial network formed by four main fluvial channels (channels C1 to C4) sculptered during the widespead erosion that occurred between MIS5e-MIS2. This paleo fluvial system crosses the area of the present-day Marambaia barrier island, and is correlated with the downstream paleo fluvial system preserved on the adjacent shelf. This burried paleo fluvial system also lays in direct space connection with the upstream present-day catchment basin of Sepetiba bay; (ii) the existence of a transgressive-highstand sedimentary sucession, up to 30 m thick, composed of units U2 to U5. This succession testifies the onset of estuarine conditions in the Sepetiba area, which evolved from an open ocean estuarine system (depostion of unit U2) to progressively more isolated estuarine conditions (deposition of units U3 and U4 between ~8,0-7,5 ky B.P.) until attaining the configuration of a low energy estuarine environment no longer in connection with the open sea (deposition of Unit U5 since ~ 5,8 ky B.P.), similar to the present-day depositional system within the bay. All these distinct enviromental and paleogeopraphic evolutional stages are directely connected to the constructional and closure phases of the Marambaia barrier island itself.