Mixed carbonate–siliciclastic sedimentation in forebulge grabens: An example from the Ediacaran Bambuí Group, São Francisco Basin, Brazil

Abstract

Mixed carbonate–siliciclastic successions have been described in multiple Phanerozoic sedimentary settings recording the dynamic interplay of tectonics, eustasy, climate, in situ carbonate production, and variations in siliciclastic sediment supply. The Ediacaran Bambuí 1st-order sequence (i.e., Bambuí Group) covers most of the intracratonic São Francisco basin (southeast Brazil) and encompasses thick packages of carbonate and fine- to coarse-grained siliciclastic strata. Recording a marine foreland basin stage that developed in the São Francisco plate during the Neoproterozoic–early Paleozoic West Gondwana assembly, the Ediacaran deposits unconformably overlie Archean–Paleoproterozoic metamorphic assemblages of the Sete Lagoas basement high and fill a series of kilometer-long grabens in the southern São Francisco basin. Seismic data reveal that these troughs developed through the extensional reactivation of ancient basement structures along with the tectonically driven forebulge uplift of the Sete Lagoas high, in the early evolutionary stages of the Bambuí basin cycle. Based on the detailed description of continuous drill cores of a well recently drilled during hydrocarbon exploration campaigns, we recognized two transgressive–regressive 2nd-order sequences preserved within one of the focused grabens: (i) Sequence 1 includes the glaciogenic deposits of the basal Carrancas Formation that grade upward into the carbonate ramp successions of the Sete Lagoas Formation; (ii) Sequence 2 contains the siliciclastic-dominated and deep water to deltaic strata of the Serra de Santa Helena Formation and passes upward into peritidal carbonates of the Lagoa do Jacaré Formation. These sedimentary successions encompass suites of retrogradational, aggradational, and progradational lower-rank cycles and are bounded by erosional surfaces. Regional seismic interpretation, well data, and the available literature indicate that most of these deposits and their correlatives are continuous over most of the Sete Lagoas basement high, showing remarkable variations within the studied forebulge grabens. Our analysis demonstrates that the depositional architecture of Sequence 1 and Sequence 2 marks the interplay of successive episodes of tectonically driven forebulge uplift and regional eustatic and climatic changes. The latter are mainly recorded by the lower second-order sequence (Sequence 1), which contains typical sedimentary features deposited in the aftermath of an important glaciation affecting lower latitudes during the late Neoproterozoic. An additional local accommodation control is restricted to the forebulge grabens and is associated with episodic and local subsidence pulses. As indicated by lower- to higher-rank cycles, it has been responsible for the deposition of thicker transgressive to early regressive successions, a few gravitational flow deposits, and the development of the deltaic to shallow marine carbonate cycles of Sequence 2. The overall stratigraphic architecture of the studied successions suggests that several basin mechanisms operating during the Phanerozoic were also present in the late Neoproterozoic. The close association between organic-rich post-glacial shales, potential reservoir facies, and rift-related migration paths and hydrocarbon traps indicate that forebulge grabens represent important elements to consider in hydrocarbon exploration campaigns.