Análise de ambientes deposicionais da bacia da Baía de Sepetiba/RJ com base em dados geológicos e geofísicos

Abstract

The study area is located at the basin of Sepetiba Bay continental shelf and inside the Sepetiba bay, where geological and geophysical techniques were used in order to investigate subsurface structures and thus perform a detailed geological modeling in order to reveal the formation process of this environment, if for eustatic variations or simply by tectonism. It was used 03 geophysical different tools - Ground Penetrating Radar, Airborne and Ground magnetic data (both on the continental shelf) and Seismic Profiling (inner part of the Bay of Sepetiba) and besides this, survey geologic and X-Ray diffraction data. In paleoenvironmental terms, the X-ray diffraction results showed that the sand ridge system A, the ridge and the interridge B is part of a fluvial-deltaic system, with marine influence and with performance of reworked sediments at the coastal plain, where there was drainage to flowed in Sepetiba Bay. The GPR results, together with survey data of the area, shown two well defined layers, delimited by depositional differences between them. The most superficial (until eight meters deep) appears with plane-parallel layers showing a low energy environment, and the layers below eight meters deep, showed irregular and chaotic reflectors evidencing have been deposited in an environment of high energy. The MAG results presented magnetic anomalies interpreted as intrusions of diabase dykes with NE/SW direction, the same direction of most structural lineaments mapped in the area and seen at the literature. The seismic profiling and high resolution frequency (3.5 kHz) results showed us some important depositional features. It was observed one paleochannel filled, located at west of the Sepetiba bay, in profile 01, as well as angular discordances and discontinuous progradational reflectors. In addition to these features, was observed the existence of basement, shaped like valleys and high in profile 01, and sandy terraces in all analyzed profiles. This fact demonstrates that the continental shelf and the coastal plain of Itaguaí passed through some small oscillations and stabilization variation in sea level during the last transgressive event in the Holocene. In conclusion, with all these data, the eustatic variations were the main environment depositional process, which collaborated with the sediments accumulation in the area, but also the tectonic processes of the Paleocene to the recent were important to formation of geological structures and to formation of depositional space.