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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

par Delpomdor, Franck ;Kant, F.;Préat, Alain
Référence Journal of African earth sciences, 90, page (49-63)
Publication Publié, 2014-02

Article révisé par les pairs

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Résumé :

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.