New interpretation of the basal Bambuí Group, Sete Lagoas High (Minas Gerais, E Brazil) by sedimentological studies and regional implications for the aftermath of the Marinoan glaciation: Correlations across Brazil and Central Africa

Abstract

Along the preserved southeast border (i.e., the Sete Lagoas High) of the Bambuí basin, the Pedro Leopoldo Member, basal succession of the Sete Lagoas Formation, unconformably overlies the Archean basement, and mostly includes carbonates with thin pelite intercalations and rare ruditic deposits. One of these, the so-called Carrancas conglomerate in its type-section, has been considered one of the lowermost rudite deposits of the Bambuí basin, being frequently ascribed to a Neoproterozoic glaciation. However, our detailed study, based on facies analysis, reveals that the Carrancas conglomerate was deposited by sediment gravity flow currents within the basal Pedro Leopoldo Member. Two outcrop sections in the São José de Lapa and Vespasiano areas, including thirteen abandoned quarry-cut and cliff outcrops, display eight distinct lithofacies (LF1 to LF8) forming a shallowing-upward carbonate ramp succession. It is composed, from the base to the top, by a mixed siliciclastic-carbonate outer ramp distally bounded by a slope-outer ramp system with sediment gravity flow deposits, a deep outer ramp developed below storm-wave base environments, an outer-middle ramp with aragonite pseudomorph crystal fans developed in a CaCO3 oversaturated below storm-wave base environment in suboxic/anoxic conditions. According to published isotope data correlating δ13C trends and values in basal carbonates of the Pedro Leopoldo Member of the Sete Lagoas and Januária highs, the relatively deep outer-slope ramp mixed siliciclastic-carbonate units of the study area appear to be coeval in age with the cap dolostone of shallow-water inner ramp of the Januária High. The absence of a cap dolostone in the Sete Lagoas High could be explained by lack of accommodation space or a regional erosion due to the tectonically driven forebulge uplift of the Sete Lagoas High. The Pedro Leopoldo Member in the Sete Lagoas High was accumulated in a brine-seawater bottom waters under suboxic/anoxic conditions in the relatively deeper portion of the carbonate ramp system, whilst the cap dolostone in the Januária High was developed in oxygenated mixed layer and intermediate water near the surface water in the shallower portion of the carbonate ramp system. Such lateral variations of facies and δ13C fluctuations are similar to those recorded by the cap carbonate sequences of the basal Schisto-Calcaire/Lukala (Sub)Group in the West Congo Belt and the basal Araras Group in the Paraguay Belt, which display a similar wide range of sub-environments like the studied part of the Pedro Leopoldo Member.