Tectonic cycles and sedimentary sequences in the Brazilian intracratonic basins

摘要

The geometry and petrology of sedimentary rocks preserved in the three intracratonic basins of the Brazilian craton (Paraná, Parnaiba, and Amazon) indicate the history and character of vertical movements of the cratonic area. Cyclic successions of erosional and depositional events are synchronous on the Brazilian craton and are correlated with cratons of other continents. The principal evolutionary stages of the Brazilian craton are interpreted as tectonic-sedimentary cycles, each represented by a stratigraphic record identified as a sequence, and each distinguished by its own special characteristics. Cambrian-Ordovician sedimentary rocks, representing deposition in paraplatform basins during a transitional stage at the close of the Brazilian orogenic cycle, are assigned to the Alpha sequence. The Beta (Ordovician and Silurian), Gamma (Devonian–early Carboniferous), and Delta (late Carboniferous–Late Permian sequences) and the Delta-A subsequence (Middle Triassic–Jurassic), corresponding to geotectonic cycles of the cratonic stabilization stage, indicate accumulation in large subsiding basins. The succession of facies in each of these sequences documents cyclical changes in the ratio of sediment supply to subsidence. Marine transgression and basin subsidence increased progressively through the Devonian phase of the Gamma sequence and declined thereafter, in a pattern similar to that of the North American craton and the Russian platform. The frequency of stratigraphic discordances in the three lower sequences suggests a higher degree of cratonic upwarping, progressively diminishing to a stage of maximum stabilization during Delta-A deposition, perhaps related to conditions immediately prior to rupture of the Gondwana plate. Breakup of the Gondwana plate, accompanied by volcanism and remobilization of cratonic areas, initiated the reactivation stage during which two geotectonic cycles are recognized. The first, represented by the Epsilon sequence (Cretaceous), began with local subsidence in isolated basins, followed by widespread continental sedimentation under platform conditions. The second reactivation cycle was responsible for accumulation of the Zeta sequence (Cenozoic), which was characterized by thin residual deposits on an extensive Tertiary peneplain and by accumulations in Quaternary basins, the latter of minor importance except where adjacent to the uplifted eastern continental margin.