Evolução sedimentar do cone de deposição do rio Guandú, Baía de Sepetiba – RJ

Abstract

This study aimed to understand the sedimentary evolution of Guandu River depositional fan, in Sepetiba Bay, RJ, as a function of Holocenic sea level changes. Integration of geological and geophysical data, afforded to trace local variability of sea level , with a possible neotectonic component. The resultant curve is close to the Sea Level Variation Curve as defined by Suguio, 2005. Three different moments in the evolution of the sedimentary fan were identified, and the three corresponding scenarios were discussed. Initially, with a lower sea level than present (-11 m), the bay was smaller in area with its opening facing south and thus more influenced by storm oceanic waves. In this scenario, the fan was deposited in front of Madeira Island, with a sandy character and forming a wave-dominated delta. In the second scenario with a higher sea level (+ 5 m), a broad bay was formed, with a big southwest-facing mouth, and, consequently, even more prone to strong wave action. Depositional fan in this condition was dislocated to a position correspondent today to Itaguai municipality and Santa Cruz district, in Rio de Janeiro. As related to another depositional fan previously described in this bay, this second scenario probably had an even more sandy delta, dominated by waves. At last, the third, present, scenario with the current sea level, shows a well sheltered bay, protected from oceanic wave action in its east portion, and a river dominated delta (bay head delta) developing with coarse sediments deposition restricted to the river’s mouth and a silty depositional fan produced by the settling of the fine sediments plume.