Caracterização de Argilas e Análise Faciológica dos Cordões Arenosos na Planície Costeira da Baía de Sepetiba – Itaguaí/RJ

Abstract

This research was achieved on the coastal plain of Itaguaí county, at south Rio de Janeiro State. It’s outstanding is based in the fact that the sedimentation environment founded at Sepetiba bay is considered by some authors as an excellent field school to extra course formations of oil geologists, because of it’s great diversity in sedimentary facies that helps on the knowledge of several coastal environments. Furthermore, due to geological evolution complexity of the sedimentary basin of Sepetiba bay and to a great variety of sedimentary processes occurring during the Holocene, ending with the close of Marambaia inlet, after successives eustatics flunctuations. This geological site, initially under waves energy, was transformed, in a shallow energy site, like nowadays. All of these contributed for the registration of lithofacies, produced either by waves or by tides making this basin special in reference to others, located on a classic marine – parálico system. The present study had the main objective to investigate the clay fractions and the organic matter of subsurface samples at beach ridges founded in this area. It was also intended to characterize the local depositional environment, and the results were supported by geomechanic drills, mineralogical analyses (X-ray diffractions), geochemical analyses (TOC, total sulfur, organopalinology and pH) and granulometrical analyses. The data obtained indicated: i) the mineralogical fraction was mostly constituted by kaulinite, with presence of illite and trace of ghoetite and smectite; ii) the clay fractions presented low values of organic matter (TOC); iii) the pH in the fractions were predominantly acid; and iv) the organopalinology showed a great presence of cuticuls and spors, in most of samples. These results demonstrate a strong continental influence, as well, strong lixiviation processes, maybe caused by the paleo drainage action in this area. The lithofaciology association, determinated mainly by granulometrical and tactil-visual analyses, indicated periods of sea level changes/variations that helped in the constructions of these beach ridges, ending with the implementation of the actual flood plain, observed by the clay/pelits presence at superficial parts of the profiles. According to these, we concluded that the shore line of south Rio de Janeiro state had already be near of the studied area, and was associated with a fluvial-deltaic system.