Proximal carbonated ramp segment from the Upper Proterozoic north of the Central African craton (southeastern Central African Republic)

Near Bakouma (southwest Central African Republic), the ante Pan-African carbonated formations are deeply karstified and overlaid by uranium and phosphorus bearing sediments, probably Eocene in age. The sedimentological study of drilling cores allows the proposal of a carbonated ramp model for the lithological pile. This ramp was backed on to a large argilaceous continental rise with feldspathic sandy deposits. In the marine domain, pelitic sediments of the coastal plain progressively change into dolomites deposited on a dissipative beach limited by a discontinuous and partially emerged stromatolic bioherm. In the south, the stromatolic bioherms of Kassa-Limassa are interbedded in lagoonal silicified dolomites bearing evidence of evaporitic processes. Native copper is locally present in shore deposits. The progradation of the facies is southwards. This model is compared to the beach of the carbonated ramp evidenced for the Schisto-calcaire Group in Congo. The carbonated ramps of Central Africa, established before the Pan-African orogeny, can be Linked to the same tectono-eustatic regressive period as the Schisto-calcaire ramp (West-Congolian Supergroup). By comparaison with the results obtained in south Congo, the age of the carbonated ramp in the north of Central Africa could be near the Precambrian-Cambrian boundary.