Depositional model of the Ipubi Member (Araripe Basin, northeastern Brazil) from facies analysis, petrographic and isotopic characterization of evaporItes

Abstract

Evaporitic rocks are paramount in sedimentological and stratigraphic studies due to their frequent association with petroleum source rocks as caprocks and significance as paleoclimatic and paleoenvironmental indicators. Facies analysis and quantitative petrography allowed the definition of facies and habits of the Ipubi evaporites in the Araripe Basin, collected in outcrops located mainly in the Araripina region, Pernambuco. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to complement the compositional and textural analysis of the constituents. The dominant sulfates are gypsum and anhydrite and locally celestite. Macroscopically three facies were identified (Laminated evaporite, Massive evaporite and Fractured evaporite) and microscopically thirteen sulfate habits. Among these, primary habits and textures are nodular anhydrite, palisade and chevron gypsum, the latter two constituting the laminated texture. Other sulfates were precipitated in shallow post-depositional conditions. S and O isotopic analyses in sulfates yielded δ³⁴S values between +10.27‰ and +17.99‰ and δ¹⁸O between +7.72‰ and +13.30‰, attesting to the marine composition of the source brine. The results indicate that the Ipubi Member evaporites were deposited in subaqueous (salinas) and intrasedimentary settings in a coastal sabkha.