Facies and paleoenvironments in Paraíba do Sul Deltaic Complex area, North of Rio de Janeiro state, Brazil

Abstract

The Paraíba do Sul Deltaic Complex (PSDC) is a large Quaternary coastal plain located along Rio de Janeiro's northern coast and forms part of the onshore portion of Campos Basin. The main studies of the PSDC focused on modeling the paleoenvironmental evolution and were based on paleogeographic reconstructions, surface sampling, radiocarbon datings (14C), drillcores from shallow wells (<40 m depth) and results of ground penetrating radar (GPR) surveys. The paucity of deep subsurface data, in particular continuous drillcores, has led to divergent interpretations of not only the age, paleoenvironmental conditions and stratigraphic position of the PSDC deposits, but also of the underlying sedimentary units. In order to improve the knowledge about the evolution of PSDC, this study aims the stratigraphic characterization and paleoenvironmental interpretation by facies analyzes of a deep drillcore (410 m) recovered at the central area of PSDC. Fourteen sedimentary facies, grouped in six facies associations related to a wide variety of depositional paleoenvironments have been identified. The depositional history of the PSDC began with a transgressive event that led to the drowning of an incised valley cutting the continental deposits of the São Tomé Member, which is represented by the distal alluvial fan (DAF) and fluvial (F-I) facies associations in the analyzed drillcore. The valley infill is marked by two Pleistocene depositional sequences representing estuarine (FE-I facies association) and estuarine to open marine (FE-II facies association) paleoenvironments deposited in response to high-frequency base-level oscillations within a major transgressive sequence after the Marine Isotope Stage 6 (Penultimate Marine Transgression - 123.000 years B.P.). This transgression was followed by a regressive phase (normal to forced) that promoted the development of extensive Pleistocene beach-ridge terraces (BR facies association), which are still partially preserved at the surface of the study area. Marine and lagoonal deposits associated with the transgressive period occurred after the Marine Isotope Stage 2 (Last Marine Transgression - 5.100 years B.P.) documented in the previous studies, were not found in the analyzed drillcore, suggesting that these deposits may have discontinuously preserved within the study area or that they were eroded by Holocene fluvial systems represented by F-II facies association.