EVIDENCE FOR A NEOPROTEROZOIC CARBONATE RAMP ON THE NORTHERN EDGE OF THE CENTRAL AFRICAN CRATON - RELATIONS WITH LATE PROTEROZOIC INTRACRATONIC TROUGHS

During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, th e Sangha aulacogen being the main structure of this type in the studie d area. This transverse basin connects with other basins on the northe rn and south-western borders of the craton. During the Cryogenian, thi s network of basins was filled with fluvio-deltaic and lacustrine peri glacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the no rthern edge of the craton, leading to the development of carbonate sed imentation in a broad outer shelf environment associated with nearshor e barriers and evaporitic lagoons. These facies are similar to those d eveloped in the West Congolian Schisto-calcaire (shale-limestone) ramp succession. The North-Central African ramp succession (sediment slope ) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Math ematical analysis of the bedding pattern yields a period of 29-30 days for the lunar month, a result which is in agreement with astrophysica l evidence for this epoch (i.e. 650 Ma ago). Major subsidence and seis mic activity on this gently sloping platform, associated with the prox imity of the Sangha aulacogen, caused the triggering of carbonate turb idites and mass flow deposits. The proliferation of microbial mats und er euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Co ngolian and North-Central African ramps prograded northwards and south wards, respectively, into the Sangha aulacogen. The sea at that time w as restricted to a long graben-like basin, while a remaining area of m arine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in terms of an overall gently slopi ng ramp model with progradation converging towards the Sangha aulacoge n.