THE GLACIAL MAMBERE FORMATION (CENTRAL-AFRICAN-REPUBLIC) - PALEOGEOGRAPHIC RECONSTRUCTION AND SIGNIFICANCE FOR THE AFRICAN PALEOZOIC

The Mambere Formation constitutes a horizontal unit located in the western and southwestern part of the Central African Republic. Stratigraphical and sedimentological study provides strong argument to attribute these deposits a glacial origin. A palaeogeographical reconstruction has been outlined in order to precise the age of the formation. Two main categories of glacial deposits have been recognized: - glaciogenic deposits made of basal tills (with facetted pebbles) and flow tills (with flattened blunt pebbles); - reworked glacial deposits formed of sandstones and conglomeratic sandstones, in continuous beds, lenses or isolated blocks, together with siltstones and bedded sandstones. The southerly provenance of the detrital material is demonstrated by quartz grain surface analysis and heavy-mineral study. This material results essentially from the dismantling of the Precambrian Schistoquartzitic Complex and secondarily from the Granitogneissic Complex. According to the palaeomagnetic polar paths and the migration of the glacial centers over the African continent during the Palaeozoic, the Mambere Formation may be attributed a Lower Silurian age by reference to similar formations observed in Cameroon or a Lower Carboniferous age by comparison with the glacial formations reported from Niger and Egypt.