Sequences and stratigraphtc hierarchy of the Paraná basin (Ordovician to Cretaceous), southern Brazil

Abstract

The Parana Basin, a vast sedimentation area during Paleozoic and Mesozoic times, holds a stratigraphic record ranging in age from late Ordovician to late Cretaceous and comprising six supersequences or unconformity-bounded units (Milani, 1997): Rio Ivai (Ordovician-Silurian), Parana (Devonian), Gondwana I (Carboniferous-early Triassic), Gondwana II (middle-late triassic), Gondwana III (late Jurassic-early Cretaceous), and Bauru (late Cretaceous). Three of them coincide with major Paleozoic transgressive-regressive cycles, and the others are Mesozoic continental sedimentary packages with associated igneous rocks. These supersequences are the remnant record of successive phases of sediment accumulation alternating with times of erosion. The evolution of each supersequence was constrained by a particular tectonic and climatic setting. The Rio Ivai supersequence is closely associated with basin inception and its geometry suggests that deposition was to some extent controlled by normal faulting. The Parana supersequence deposited during a time of widespread marine flooding over the cratonic area of southwester Gondwana. From the deposition of the Gondwana I supersequence onward tru intracration conditions were established. Sharing Gondwana's dessication trend the Parana Basin sedimentation history culminated with extensive desertic conditions during the Jurassic. The Lower Cretaceous Serra Geral continental flood basalts are related to the initial moments of South Atlantic rifting and the upper Cretaceous Bauru continental cover ended the history of the basin. The hydrocarbon potential of the Parana Basin is related to two well defined source beds: the Devonian shales (Ponta Grossa Formation) and the upper Permian bituminous shales and limestones (Irati Formation). Sandy reservoirs can be found in the lower Devonian Furnas Formation, in the upper Carboniferous/lower Permian Iatarare Group and in the lower Permian Rio Bonito Formation. The role of intrusive bodies in the maturation of source rocks and in the trapping of hydrocarbons seems to be crucial and deserves more investigation. (author)