DISTRIBUTION OF SEDIMENTATION IN THE UPPER PROTEROZOIC GULF OF THE SCHISTO-CALCAREOUS IN CONGO AND GABON - IMPLICATIONS FOR CENTRAL AFRICA

On the eastern border of the Central Africa Craton the NW-SE directed Nyanga-Niari Basin formed a gulf cross-cutting the Precambrian Chaillu Massif. This basin was linked to the NE-SW directed Comba Basin, the western extension of the Sangha Aulacogen. In these two basins the Late Proterozoic "Schisto-calcaire Cycle" comprises a transgressive zone with marls and conglomerates, a condensed dolomite section (pink dolomite) and a high sea level stand zone with a carbonate ramp and barrier prograding SW-NE towards the aulacogen. Between the Comba and Nyanga-Niari basins, submarine dunes and infratidal giant stromatolites, overlain by oncolitic dolomites, were formed. An internal shelf was isolated by an oolitic barrier. An evaporitic dolomite lagoon with algal mats appeared in the Nyanga-Niari Basin and in the western part of the Comba Basin. The lagoon was bordered by a shoal preceding a deepening of the internal shelf towards the aulacogen. The foreshore sedimentation started with a thin conglomeratic layer and continued with the "Oolithe de Kisantu" characterized by a high frequency of talc oncolites. In the nearshore zone, black pseudo-oolitic dolomite and algal mats were formed, with thinly laminated algal deposits near the tidal flat. In the northwestern part of the Nyanga-Niari Basin the continental "Serie de la Biloua" contains a continuous tidal breccia bar prograding towards the NE. In the southern part of the Comba Basin fluvial channel deposits occur consisting of alluvial levees' crevasse splays and tidal breccia located in the estuary. In the neighbourhood of the channel mouth the sedimentation is disturbed. Most of the facies described here are present in many other Proterozoic basins north and south of the Central African Craton. Everywhere, shelf carbonate ramps seem prograding towards the Sangha aulacogen. The continental slope deposits of the same age cannot be interpreted as the lower part of the Pan-african Molasse. In all these sedimentary environments, algal mats appear to be the main cause of organic matter productivity and of the primary carbonate precipitation. Finally, the lagoonal environment was probably very similar to present-day carbonate lagoons.