Ediacaran forebulge grabens of the southern São Francisco basin, SE Brazil: Craton interior dynamics during West Gondwana assembly

Abstract

The Ediacaran Bambuí Group corresponds to the main sedimentary succession exposed in the intracratonic São Fransico basin, E Brazil. Comprising an unconformity-bounded 1st-order sequence, it has been widely interpreted as the record of a marine foreland system developed on the São Francisco plate during the Brasiliano/PanAfrican West Gondwana assembly. Nevertheless, the lack of well characterized foreland system depocenters and the paucity of precise geochronological data have hampered the understanding of its basinal evolution. Based on new subsurface data (2D seismic and well), aerogeophysical information, field work and literature compilation, we present the first tectono-stratigraphic analysis of a series of kilometer-long subsurface grabens preserved in the southern São Francisco basin and discuss their significance in the context of the Bambuí foreland system. Occupying mostly the crest of the Sete Lagoas basement high, they formed through the extensional reactivation of ancient cratonic structures and evolved partially contemporaneously to the deposition of two mixed carbonate-siliciclastic, transgressive–regressive 2nd-order sequences of the basal to middle Bambuí Group. These troughs developed in consequence of successive pulses of forebulge uplift and migration of the Sete Lagoas basement high, apparently in the late Ediacaran. Together with an important post-glacial eustatic rise that affected lower latitudes, this forebulge dynamics and its distinctive structures controlled the tectono-stratigraphic architecture and dispersal of the studied Ediacaran strata. The recognition of this foreland depocenter and its tectono-stratigraphic evolution reveals important aspects of one of the most expressive orogenic basins developed during the West Gondwana assembly. Along with available paleotectonic reconstructions, it may contribute to explain the apparent mismatch between the evolution of the southern Brasília belt and the Ediacaran basin. Furthermore, flexural subsidence interference patterns associated with the evolution of the neighboring Brasiliano orogenic systems seem to have played an important role on the evolution of the Sete Lagoas forebulge, as well as on the development of the entire Bambuí foreland system. Our study offers elements to comprehend the mechanisms operating on stress and sedimentary partitioning within foreland systems, as well as to better evaluate the potential for Precambrian mineral and unconventional hydrocarbon resources of the São Francisco basin and elsewhere.