First evidence of marine influence in the Cretaceous of the Amazonas Basin, Brazil

An integrated investigation emphasizing sedimentological and ichnological studies of Cretaceous deposits of the Alter do Chão Formation, exposed in the western Amazonas Basin, was undertaken with the aim of determining depositional environments. Four facies associations attributed to upper shoreface, foreshore, delta mouth bar, and lower/middle shoreface-prodelta depositional environments were recognized. The upper shoreface deposits were deposited by storm flows. They are interbedded with highly bioturbated sandstones displaying Thalassinoides, Planolites and Diplocraterion traces. The foreshore deposits, which are coarser-grained than the shoreface strata, are characterized by tabular sandstones with planar or trough cross-lamination/stratification, wavy/flaser lamination, and parallel lamination. These strata also contain an abundance of trace fossils. The delta mouth bar deposits comprise upward-coarsening beds displaying a lobed geometry. The lower/middle shoreface-prodelta settings consist of well-stratified, very fine-grained sandstones and mudstones deposited mostly by storm wave action. A wave-dominated delta system that prograded into a marine-influenced basin is supported for the study area. Therefore, in contrast to previous interpretations, it seems that a widespread transgression during the Cretaceous would have resulted in the submergence of large continental areas in the north of Brazil, affecting sediment deposition even in the innermost portions of the intracratonic Amazonas Basin.