Lithostratigraphie et structure du Buem unité externe de la chaine panafricaine des Dahomeyides dans la region de Bassar (Togo)

Abstract

In the area of Bassar (Togo), the most external structural unit of the Pan-African (600 Ma) Dahomeyides fold belt, named the Buem, comprises two well defined zones: (1) A western zone, less affected by deformation and where the following lithostratigraphy can be established: (i) at the bottom, some sandstones, quartzites and mixtites are overlayed by microconglomeratic hematitites. The 1300 m-thick sequence is partially interpreted as a glaciogenic deposit; (ii) at the top, shales and siltstones with intercalations of sandstones and carbonates compound a thick sequence of 1500 m. This unmetamorphosed sequence, which can be correlated with the supergroup 2 in the Volta basin, shows a rather simple structure with a large-scale folding verging to the West. (2) An eastern zone, which displays nearly the same facies than the western one with, however, a finer grained mixtite and a strong cataclasis shown by hard rocks. The Pan-African orogeny in this zone is polyphased with an important thrusting and an anchizonal to epizonal metamorphism. The identification of those two zones leads to propose two possible acceptations of the Buem: (i) the Buem sensu lato, which includes those two zones in the previously defined Buem area; (ii) the Buem sensu stricto, represented only by the eastern zone with the characteristics of the Buem of the northern Benin. According to this second acceptation, the more logical one, the western zone must represent a para-autochtonous unit of the Voltaian Basin.