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Abstract: A sedimentological and C–O isotopic study has been carried out in nine sections of the Sete Lagoas Formation at its classical outcropping area, in the southern tip of the São Francisco craton (central Brazil), with the objective of refining its stratigraphic position within the Neoproterozoic. At the study area, the Neoproterozoic Sete Lagoas Formation comprises two shallowing-upward megacycles, corresponding to more than 200m in thickness. Each cycle is limited by a flooding surface amalgamated with a third-order sequence boundary. The first megacycle presents deep-platform deposits with abundance of crystal fans (aragonite pseudomorphs). These deposits are characterized by negative C-isotope values (–4.5‰). They grade upward to storm-wave and tide-influenced layers with δ 13C values around 0‰. In the second megacycle, a new transgression drowned the platform, depositing a thick, mixed sub-storm wave-base succession. This megacycle comprises deposits of lime mudstone-pelite rhythmite, which grade to crystalline limestone rich in organic matter, both with unusually positive δ 13C values (up to+14‰). Regional correlation of Sete Lagoas deposits indicate that they rest atop glaciomarine rocks of the Macaúbas Group and basal strata show seafloor precipitates with negative δ 13C values. Therefore, it is possible to characterize the Sete Lagoas carbonate as a cap carbonate sequence. The very high δ 13C in the second megacycle together with geochronologic data suggest that this unit correlates better with post-Sturtian sequences. Some differences in the depositional record are observed between Sete Lagoas and the other post-Sturtian units previously described in North America, Australia, and Namibia. Those differences may in part be due to deposition in shallower settings of the Sete Lagoas carbonates, thus preserving a thick record of storm- and wave-influenced sedimentation not found elsewhere. Alternatively, they may also be attributed to diachronic deposition of the so-called post-Sturtian cap carbonate sequences. [Copyright &y& Elsevier]

This study aims to update the knowledge about the importance of the late Ediacaran fossil record and geochemical data from Corumbá, State of Mato Grosso do Sul, at the Brazilian-Bolivian border. The Corumbá graben system is located near a triple junction developed above a hot spot of two young (545–480 Ma) Brasiliano provinces: the mostly Bolivian Chiquitos-Tucavaca aulacogen which cuts across the Amazon Craton/Rio Apa Block, and Paraguay Fold Belt. The Neoproterozoic sedimentary cover of South Paraguay Belt starts with the metasediments, diamictites and iron formation of the Jacadigo Group, now related to an end-Cryogenian age. Most remarkable geochemical and paleontological data come from the overlying Corumbá group, mainly from the dolostones with stromatolites of the Bocaina Formation, and limestones with shale and silty intercalations at this group's upper part, in the Tamengo Formation. This last unit contains a fossil assemblage correlated to late Ediacaran fauna. This fauna contains originally substrate-emergent tube-like Corumbella werneri, Cloudina lucianoi and microfossils. Furthermore, the fossils from the Corumbá Group in Brazil and Paraguay represent the most important witnesses for the occurrence of late Ediacaran fossils close to the basal Cambrian boundary in South America. Therefore, the Corumbá region is significant for paleogeographical reasons and, on the other hand, allows insights into the evolution of the oldest skeletonized metazoans. After new research results, the high degree of similarity of the geological facies evolution with other parts of the world (e.g. Yangtze Platform/Southern China, Siberia, Spain and Namíbia) can be demonstrated, where the fragmentation of the Rodinia super-continent and Neoproterozoic glaciations are also well-documented. The sharp top contact of the shallow marine Tamengo Formation with the laminated black shales (containing rare angular dropstones) of the discordantly overlying Guaicurus Formation indicates that the latter represents a new transgressive glacially influenced marine onlap succession. A Cambrian age of the Guaicurus Shales is not (yet) biostratigraphically verified, however, the underlying fossil record of cloudinids indicates a terminal Ediacaran age for the top of the Tamengo Formation. The microtubular cloudinids are interpreted as dysoxic analogues of recent tubeworms and are suggested to serve as first skeletonized worldwide “index fossils” to delineate the onset of a Phanerozoic-type body fossil vectorial evolutionary pathway. Based on the FAD of cloudinids as marker fossils, a revision of the Precambrian/Cambrian boundary is here advocated. This would avoid placing this important GSSP into the virtually worldwide Nomtsas-Baykonurian glacial hiatus. [ABSTRACT FROM AUTHOR]