Paleoenvironments, δ13C and δ18O signatures in the Neoproterozoic carbonates of the Comba Basin, Republic of Congo: Implications for regional correlations and Marinoan event

The Ediacaran Schisto-Calcaire Group is a similar to 1300 m-thick succession belonging to the West Congo Supergroup in Central Africa. In the Comba Basin, it consists of three carbonate-dominated units defined as formations (SCI to SCIII) that are unconformably overlain by clastic deposits (Mpioka Group) interpreted as a molassic formation associated with the Panafrican Orogen. The underlying Upper Tillite and Cap Carbonate (SCIa) units, considered as markers of the Snowball Earth event were studied in three sections. We investigated the carbonates of the Schisto-Calcaire Group by defining new microfacies (MF1-MF7) and we performed C and O isotopic analyses in order to constraint the depositional and diagenetic events directly after the Marinoan interval. Stratigraphic variations of the stable isotopes are important in the series with lighter delta O-18 values (>1.5 parts per thousand) than those of the Neoproterozoic ocean in the SCIc unit. According to regional stratigraphy a temperature effect can be dismissed and a freshwater surface layer is the origin of such negative delta O-18 values in this unit. The negative delta C-13 anomaly (-3.5 parts per thousand on average) of the Cap Carbonate is similarly to the delta O-18 values (-6.4 parts per thousand on average) in the range of the marine domain during postglacial sea level rise. The sample suite as a whole (SCII and SCIII formations) displays heavier delta O-18 and delta C-13 than those of the lower part (SCI unit) of the Schisto-Calcaire Group. The comparison with the Lower Congo (Democratic Republic of Congo) and Nyanga (Gabon) basins shows that the meteoric flushing in SCIc unit of the Schisto-Calcaire Group was regional and not local, and could be derived from a climatic evolution. Although an overall overprint is present, our isotopic relationships argue against overall diagenetic resetting of primary compositions and suggest that with careful examination combined with detailed petrographic analysis general depositional and diagenetic controls can be discerned in oxygen and carbon isotopic data in the Schisto-Calcaire Group

Molecular identification of fungi microfossils in a Neoproterozoic shale rock

Precambrian fossils of fungi are sparse, and the knowledge of their early evolution and the role they played in the colonization of land surface are limited. Here, we report the discovery of fungi fossils in a 810 to 715 million year old dolomitic shale from the Mbuji-Mayi Supergroup, Democratic Republic of Congo. Syngenetically preserved in a transitional, subaerially exposed paleoenvironment, these carbonaceous filaments of ~5 μm in width exhibit low-frequency septation (pseudosepta) and high-angle branching that can form dense interconnected mycelium-like structures. Using an array of microscopic (SEM, TEM, and confocal laser scanning fluorescence microscopy) and spectroscopic techniques (Raman, FTIR, and XANES), we demonstrated the presence of vestigial chitin in these fossil filaments and document the eukaryotic nature of their precursor. Based on those combined evidences, these fossil filaments and mycelium-like structures are identified as remnants of fungal networks and represent the oldest, molecularly identified remains of Fungi

Neoproterozoic uppermost Haut-Shiloango Subgroup (West Congo Supergroup, Democratic Republic of Congo): Misinterpreted stromatolites and implications for sea-level fluctuations before the onset of the Marinoan glaciation

The middle Neoproterozoic carbonate-dominated uppermost Haut-Shiloango Subgroup (Sh8h and Sh8i members) in the Lower-Congo Province of the Democratic Republic of Congo is considered as recording pre-glacial shallow-marine sedimentation with stromatolitic reefs overlain by the Upper Diamictite Formation. We investigated these stromatolitic carbonates in order to highlight their biogenicity. Newly defined lithofacies and geochemical analyses (stable isotopes, major, trace and REE+Y elements) are used to provide insights into the origins of the depositional events that occurred immediately before Marinoan global glaciation. These insights should in turn provide constraints on the models developed for this glaciation event.The series consists of three shaly and carbonate lithofacies: (i) alternating limestones and claystones (lithofacies 1); (ii) nodular wackestones (lithofacies 2); and (iii) clast-supported conglomerates and breccias (lithofacies 3). Lithofacies 1 is an open marine low-energy mid/outer ramp system with hummocky cross-laminations and distal tempestites; lithofacies 2 is a distal slope facies with synsedimentary contorted structures, slided and slumped semi-consolidated limestone beds; lithofacies 3 consists of debris flows deposited in a basinal setting controlled by synsedimentary faults. None of the facies exhibits petrographic evidence of biogenicity such as stromatolitic laminar-reticulate fabrics and/or associated sediments (e.g. peloids, oncoids, ooids) or typical features such as mudcracks or clotted fabrics. The uppermost Haut-Shiloango Subgroup is made up from the stratigraphic succession of the three lithofacies and corresponds to a deepening-upward evolution from storm-influenced lithofacies in mid- and outer-ramp to deep-water environments, with emplacement of mass flow deposits in toe-of-slope settings. These processes occurred along tectonically active continental margins locally influenced by altitude glaciers, developed after a rift-drift transition.Uniform flat non-marine shale-normalized REE+Y patterns indicate freshwater-influenced signatures in the Sh8h carbonates. Moderate Y, Zr and Rb values reflect continental detrital inputs in nearshore environments rather than in deep-water environments. These nearshore sediments have been reworked from shallow inner- to mid-ramp settings into deeper outer-ramp and deep-water slope environments as a consequence of the tilting and uplifting of blocks. The blocks belonged to a graben-like basin related to the 750-670. Ma oceanic spreading in the central-southern Macaúbas Basin. © 2013 Elsevier Ltd.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

The Neoproterozoic record in Gabon and Democratic Republic of Congo: A stable isotope sudy.

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Sedimentology and sequence stratigraphy of the late Precambrian carbonates of the Mbuji-Mayi supergroup in the Sankuru-Mbuji-Mayi-Lomami-Lovoy basin (Democratic Republic of the Congo)

The late Mesoproterozoic-middle Neoproterozoic carbonate succession (1155 Ma-800 Ma) of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) represents a classic late Precambrian carbonate sequence whose architecture is poorly known. Here we present new data and synthesis of microfacies analysis, sequence stratigraphy, Fischer plots coupled with C and O isotopes, to evaluate the paleoecology and sea level variations of the carbonate series of the Mbuji-Mayi Supergroup, and to establish hierarchical approach stratigraphic framework from which to resolve the evolution of the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Our microfacies and sequence stratigraphy analyses show that the carbonate succession consists of strata accumulated on a ramp, during cyclic sedimentation across the inner ramp. Here plurimetric ‘thin’ peritidal cycles (±4 m-thick on average) record a relative maximum sea level of ca. 4 m, with fluctuations in the range around 1-4 m. This shallow-water depth and the abundance of cyanobacteria suggest that water column was oxygenated. By contrast the subtidal cyclic facies at the outer/middle ramp, preserve ‘thick’ subtidal sequences characterized by an average thickness of ±17 m. Accurate relative sea level fluctuations are difficult to assess in this ‘deeper’ environment since the facies could have been deposited in a wide range of shallow water that did not completely fill the accomodation space or available space. A probable magnitude for sea-level fluctuations here is around 10-20 m. These data are the first to place a quantitative constraint on the late Mesoproterozoic to middle Neoproterozoic carbonate deposits that have lively covered much of the Congo Shield at the end of the Precambrian, and is therefore an important type section for Central Africa.SCOPUS: ch.binfo:eu-repo/semantics/publishe