Sequence stratigraphy and chemostratigraphy of an Ediacaran-Cambrian foreland-related carbonate ramp (Bambuí Group, Brazil).

Abstract

In the terminal Neoproterozoic, drastic climate changes associated with biological innovations are coupled to isotope and elemental geochemical anomalies. However, lateral variability and local depositional controls may affect global geochemical signals, which can only be tracked through a proper stratigraphic/paleogeographic assessment. Here, we investigate the sequence stratigraphy and chemostratigraphy of the basal units of the Bambuí Group, central-east Brazil. This stratigraphic unit records a foreland basin system developed during the Ediacaran-Cambrian West Gondwana assembly and represents a 1st-order sequence, in which the two lowermost 2nd-order sequences record major geochemical disturbances. The first 2nd-order sequence started with the deposition of a transgresive systems tract, possibly in a postglacial scenario, which accompanies a negative-topositive δ13Ccarb excursion. The early highstand systems tract represents the establishment of a marine carbonate ramp throughout the basin. In terms of chemostratigraphy, it corresponds to a δ13Ccarb plateau close to 0‰ and Sr/Ca ratios around 0.001. The late highstand stage coincides with a remarkable increase in Sr content and Sr/Ca ratios at basinal scale. Occurrences of the Cloudina sp. late Ediacaran index fossil were reported in this stage. An erosional unconformity associated with a dolomitic interval, locally including subaerial exposure features, marks the top of the first 2nd-order sequence. This sequence boundary heralds an abrupt increase in δ13Ccarb values, up to +14‰. These extremely high δ13Ccarb values and high Sr/Ca ratios persist throughout the overlying sequence, as a result of progressive and enhanced restriction of the foreland basin system. Basin restriction at this stage has implications for the paleontological and chemostratigraphic record of epicontinental basins of the West Gondwana in the terminal Ediacaran. Late Ediacaran Sr-rich intervals in these basins show unusually nonradiogenic 87Sr/86Sr ratios, which may represent local depositional controls and deviations from the modern oceanographic models. Physiographic barriers and stressful conditions likely represented extreme environments for metazoan colonization.