Bacia pré-siluriana na porção centro-oeste da Província Parnaíba, NE do Brasil

Abstract

Pre-Silurian grabens have been interpreted in the substrate of Parnaíba Basin, with implications in the subsidence control and consequent evolution of the syneclise at the Phanerozoic. This research presents the results from the interpretation of a 2-D seismic grid at the mid-western region of Parnaíba Basin, focusing on the pre-Silurian interval and the underlying crystalline basement. The aim was to achieve a clearer understanding about the pre-Silurian basin-forming mechanisms and, consequently, the geotectonic context in which they were inserted. The pre-Silurian basin at Balsas (MA) is compartmentalized by a high structure, in two main depocenters. In the central sector of the study area, the pre-Silurian strata have an external wedge geometry, tilted towards an important N-S trending normal fault, with a minimum length of 100 km, which delimits this depocenter to the east. In turn, the depocenter to the west of the structural high has a saucer-like external geometry, tapering both toward the internal high and toward the westernmost regions of the study area. It was possible to individualize at least two seismic sequences within the pre-Silurian interval. Unconformably lying over the crystalline basement, we have SEQ 1, characterized by a set of high-amplitude parallel reflectors, which form a gentle arch above the structural high. SEQ 2 is characterized by low-frequency and low-amplitude reflectors, presenting as east-dipping parallel seismic facies, in the central region, that progressively changes to a chaotic configuration to the west. It was observed that both seismic sequences are deformed by folds and thrusts in the western sector of the study area, in contrast to the extensional character at the central sector. The structural analysis of the basement allowed to observe that the pre-Silurian section overlie a substrate tectonically imbricated in a contractional regime, marked by folded reflectors associated with east-verging ductile shear zones. These tectonic sheets are thrusted over a seismically transparent basement unit, which characterize the easternmost regions of the study area. It is understood that the pre-Silurian interval corresponds to the wedge-top depozone of a foreland system developed with the advance of Araguaia Belt’s retro-wedge (tectonically imbricated basement) over the Parnaíba Block (acoustically featureless basement), during the amalgamation of West Gondwana in the Neoproterozoic. The development of new thrust ramps in the frontal portion of the orogenic wedge promoted the uplift of the structural high, dividing the basin in different depocenters. At the end of the collision, the thrust faults became preferential planes for normal reactivation during orogenic collapse, resulting in the half-graben geometry, observed in the central sector. In this context, it is interpreted that SEQ 1 represents carbonate accumulations associated with the early stages of the foreland system, posteriorly buried by the advance of SEQ 2 molassic siliciclastic sediments. Finally, it should be noted that the truncation of sills intercalated in SEQ 2, by the Pre-Silurian Unconformity, points to the occurrence of a late-collisional magmatic event, prior to Mosquito Magmatism.