New aspects of Neoproterozoic-Cambrian transition in the Corumbá region (state of Mato Grosso do Sul, Brazil)

Abstract

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.