Evolução tectono-estratigráfica dos evaporitos Horizonte e Paripueira na porção alagoana da Bacia de Sergipe-Alagoas e suas implicações na abertura do Oceano Atlântico Sul

Abstract

Three evaporitic events occur in the SergipeAlagoas Basin, namely Horizonte, Paripueira and Ibura. The first two events, which are the focus of this work, belong to the Early Aptian synrift phase and are the oldest evaporites associated with the Gondwana breakup in the Brazilian margin. This sequence occurrs in restricted and localised structural lows in the onshore portion of the Alagoas subbasin, particularly in the Varela and Fazenda Guindaste Lows, forming layers with hundreds of meters in thickness. The upper sequence (Ibura event) is restricted to the onshore and offshore portions of the Sergipe SubBasin, and is equivalent to the main evaporite layer that was deposited during the South Atlantic opening. Core descriptions of the Horizonte evaporites in the Coqueiro Seco Formation revealed they consist only of halite. The Paripueira evaporites in the Maceió Formation are also formed only by halite, as indicated by well logs. Core samples of sediments interbedded with these salt layers show gravitational flux deposits, with immature texture and mineralogy, suggesting syntectonic deposition in a lacustrine environment. Seismic interpretation indicates the Varela and Paripueira Lows are half grabens controlled by synthetic and antithetic rift faults, as well as transversal release faults. Some of those faults show clear evidence of synsedimentary activity for both evaporitic events. This tectonism provided enough depositional space and accentuated the physiographic restriction, allowing the lakes desiccation and evaporite precipitation. Structural restorations revealed the spatial relationship between these two synrift evaporitic events. During deposition of the Coqueiro Seco Formation, the Varela Low registered the most significant amount of subsidence, with favorable conditions to form the Horizonte evaporites. Subsequently, during deposition of the Maceió Formation, the main tectonic activity migrated towards the Fazenda Guindaste Low, allowing the precipitation of the Paripueira evaporites. This theme cannot be explained by only one isolated discipline point of view, so a multidisciplinary basin analysis study has been conducted. Most of the evidences suggest the early evaporite events formed without contribution of marine incursions from the incipient Atlantic Ocean