ESTRUTURA, ESTRATIGRAFIA E EVOLUÇÃO DAS BACIAS MARGINAIS BRASILEIRAS

Abstract

The structural framework of the Brazilian marginal basing is basically defined by the following features: synthetic and antithetic tilted step-fault blocks, structural inversion zones, hinges with compensation grabens, shallow platforms, homoclinal structures, growth faults with rollovers, salt and shale diapirs, and igneous structures. The two tilted step-fault blocks are interpreted as complementary synchronous systems, separated by the inversion zone. They developed in the Early Cretaceous (Neocomian-Barremian) in rift valleys adjacent to a central graben and related to the rift phase of the Gondwana continent. The hinges, shallow platforms, homoclinal structures and diapirs were preferentially formed in the Late Cretaceous-Recent, during the transitional and drift phases of the South American and African plates. Two Aptian evaporitic cycles: Paripueira and Ibura were distinguish in the Sergipe-Alagoas type-basin, and tentatively correlated with the counter parts of the other Brasilian marginal basins. Based on paleogeographic, paleoenvironmental, tectonic-sedimentary, lithological, paleoontological and geochemical considerations, the following points are admitted: a) the evaporitic basins were controlled by assymmetric taphrogenic structures of the rift phase and were supplied by marine water from a central graben, by passing an external barrier; b) the Paripueira evaporites rest on the continental crust and consequently the continentaloceanic crust contact will be located away from the external saliferous limit; and c) the Brazilian marginal evaporites would have not been continous with their African counterparts. Four tectonic-sedimentary evolutive phases were differentiated: intumescence phase (Late Jurassic-Early Cretaceous), when crustal uplift, formation of peripheric basins and deposition of continental sequences took place; rift phase (Neocomian-Barremian), with development of intense taphrogenic activity and formation of a symmetric central graben, along the intumescence axis, adjacent rift valley systems where syn-sedimentation of fluvial-deltaic-lacustrine sequences occurred; transition phase (Aptian), when two evaporitic cycles were deposited in some Brazilian marginal basins; and drift phase (Albian-Recent), when the hinges, shallow platforms, extense homoclinal were developed; thick clastic-carbonatic sequences were deposited; and salt and shale diapirism took place. Two epochs of major igneous activity occurred: in the Early Cretaceous associated to the rift phase; and in the Oligocene-Miocene, during the drift phase.