Icnologia, estratigrafia e paleoambiente da Formação Pastos Bons, Jurássico Superior -Cretáceo Inferior da Bacia do Parnaíba

Abstract

After the voluminous lava eruption related to the Central Atlantic Magmatic Province (CAMP), in the Triassic–Jurassic (~200 Ma), a large subsiding area formed in West Gondwana, allowing the Upper Jurassic–Lower Cretaceous lacustrine system of the Pastos Bons Formation, in the Parnaíba Basin. Despite recent studies on this lacustrine system, these deposits still need to be investigated in detail with palaeoenvironmental mapping based on the ichnological content, facies analysis and lacustrine sequence stratigraphy. Outcrop-based facies analysis allowed the identification of four palaeoenvironments: central lake, lakeshore, braided fluvial, and delta front. The ichnological content is concentrated in the lakeshore deposits and includes of eight ichnofossils, organized in three ichnofacies: Mermia (Cochlichnus anguineus and Lockeia siliquaria), Scoyenia (Agrichnium isp., Gyrochorte comosa, Palaeophycus tubularis, Planolites berveleyensis, and Rusophycus isp.), and Skolithos (Skolithos linearis). This lacustrine succession is organized in five millimeter- to meter-scale depositional cycles bounded by erosive surfaces or flooding surfaces. Four cycles define a retrogradational and aggradational stratal stacking pattern composing the transgressive systems tract, and one cycle indicates the progradational stratal stacking pattern constituting the highstand systems tract. The sequence stratigraphy suggests that post-CAMP thermal subsidence and responsible for creating the accommodation space, controlling the sedimentary supply and water inflow/outflow, and the proportional relationship between them. At the base of this system, an underfilled lacustrine basin characterized by fossil and ichnofossil-bearing layers and thick mudstones predominated, and after the maximum subsidence, an overfilled lacustrine basin characterized by cross-laminated sandstone of the fluvio-deltaic system was established.