Lithofaciological analysis of the exposed rocks of the Itapecuru Formation, northeastern Parnaiba Basin, Brazil: paleoenvironmental implications

摘要

Lithofaciological analysis was performed in 21 exposures of the Upper Aptian Itapecuru Formation along the Itapecuru river, and in an outcrop on the MA-020 state highway, in the northeastern side of the Parna?ba Basin, Maranha?o State, Brazil. The analysis aimed to contribute to the paleoenvironmental understanding of the siliciclastic sedimentation in continental (terrestrial and aquatic) and marine environments of the formation. The stratigraphic and sedimentological interrelationships and paleoenvironmental knowledge of the deposit?s origin are hampered due to the lack of adequate outcrops and of integrated multidisciplinary studies, as well as to the few accurate biostratigraphic data. The Itapecuru Formation sediments reach 700 m of thickness in the subsurface of the Parna?ba Basin and were deposited near to the paleoequator at the end of the rift stage, a tectonic event responsible for the Gondwana breakup in the Early Cretaceous. In the analyzed sedimentary succession (totalizing 89.3 m thickness) it was defined 12 lithofacies, grouped in two architectural elements ? mixed flat and tidal channel. These elements integrate two estuarine systems dominated by tidal currents, with the older system corresponding to a high-energy marine dominated zone of the estuary, and the younger one representing a lowenergy central zone of the estuary. The marine conditions identified by the architectural elements in deposits where there are no recovered marine palynomorphs were important for tracking marine action in the Late Aptian. The results described here help understand the evolutionary scenario of the Equatorial Atlantic Segment in Brazil in relation to temporally distinct depositional sequences.