TECTONIC AND STRATIGRAPHIC EVOLUTION OF THE SYN-RIFT EVAPORITES IN THE SERGIPE - ALAGOAS BASIN, NE BRAZIL, AND IMPLICATIONS FOR THE SOUTH ATLANTIC OCEAN OPENING

Three evaporitic events occur in the Sergipe-Alagoas Basin, namely Horizonte, Paripueira and Ibura. The first two events, which are the main focus of this work, belong to the Early Aptian syn-rift phase and are the oldest evaporites associated with the Gondwana breakup in the Brazilian margin. This sequence occurrence is restricted to local structural lows in the onshore portion of the Alagoas sub-basin, 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 Sub-Basin, 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 Fm. revealed they consist only of halite. The Paripueira evaporites in the Maceió Fm. 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 syn-tectonic 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 syn-sedimentary 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 syn-rift evaporitic events. During the Coqueiro Seco Fm. deposition, the Varela Low registered the most significant amount of subsidence, with favorable conditions to form the Horizonte evaporites. Subsequently, during the Maceió Fm. deposition, 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 early evaporite events without contribution of marine incursions from the incipient Atlantic Ocean.