The Boipeba member in the Jatobá Rift (northeastern Brazil): Occurrence, facies characterization and palaeoenvironmental implications

Abstract

Fluvio-aeolian and lacustrine depositional systems are commonly associated with the initial sedimentary processes in rift-type basins, typically indicative of relatively calm tectonic settings. Fluvio-aeolian sandstones from the Boipeba Member of the Aliança Formation have been documented in well-drilled samples from the Recôncavo and Tucano basins, with thicknesses decreasing progressively from south to north. These units constitute the Brotas Group, encompassing deposits from the Jurassic period (Dom João local Brazilian stage). However, due to the absence of any records within the Jatobá Basin up until now, its occurrence in this basin remains uncertain. Based on facies analysis and depositional architectural elements, the fluvio-aeolian system of the Boipeba Member can be categorized into four associations (1) medium to low energy fluvial channels, (2) floodplains, (3) dunes and (4) sand sheets. Palaeocurrent data have indicated a variable paleoflow direction ranging from SW and NW, which is expected for the rift-onset phase fluvial system. Within the broader context of the Recôncavo-Tucano-Jatobá Rift System (RTJ), five distinct fluvial and/or aeolian units can be identified: the Tacaratu Formation (Syneclisis), the Boipeba Member (the basal unit of the Aliança Formation), the Sergi Formation (Early Rift), the São Sebastião Formation (Rift), and the Marizal Formation (Post-Rift). The Boipeba Member is the first unit of the Early Rift Sequence in the RTJ, deposited in a fluvio-aeolian environment until it evolves to the exclusively lacustrine deposition of the Capianga Member (the upper unit of the Aliança Formation). The subsequent recurrence of the fluvio-aeolian cycle is evident in the Sergi Formation. Faciological and petrographic analyses of outcrops in drainage channels in the Tacaratu municipality, located in the southeastern part of the Jatobá Basin, were performed. The rock formations appear as whitish-gray sandstones, typically thin to medium, subrounded, and consistently sorted. Some samples exhibit clay matrix and contain feldspars. Notably, the presence of planar cross-stratifications and a distinct upper contact with sediments of lacustrine origin provide diagnostic features indicating the Boipeba Member. Based on the facies, depositional system and palaeocurrent analyses, the deposits of the Boipeba Member in the Jatobá Basin were interpreted as resulting from fluvial channel sedimentation, followed by subsequent aeolian reworking. This process transitioned gradually to a lacustrine infilling, exemplified by the Capianga Member. Therefore, the Aliança Formation within the Jatobá Basin signifies the complete deposition of a fluvio-aeolian and lacustrine system, exhibiting both of its members, as observed in the other basins of the RTJ.