Palaeogeographic significance of stromatolitic buildups on late proterozoic platforms: The example of the West Congo basin
Abstract The West Congo Basin, equatorial Africa, is filled with a Late Proterozoic sedimentary succession. Its middle part is made up of the “Schisto-calcaire” (shale-limestone) Group. A study of the lower part of this group has resulted in a model of carbonate sedimentation. A stromatolite buildup, very similar to the present barrier reefs and cut by tidal and/or swell channels, separates an intertidal and supratidal inner platform from a subtidal outer one. The inner platform is characterized by deposition of dolomitic limestones containing gypsum crystals epigenetically replaced by calcite and chalcedony. The outer platform is made up of stromatolite biostromes arranged in elongate domes parallel to marine currents alternating with limestones with thin shale stringers. In Proterozoic basins, stromatolitic barrier reefs mark transitions either between inner and outer platforms, or between platform and deep basin. Subtidal stromatolite biostromes, with an elongation of ten to hundreds of meters, locally encroached upon some of the slowly subsiding, flat-bottomed epicontinental basins.