Establishment of a lithostratigraphic column in the Béli area (Northeastern Burkina Faso, West Africa) based on the occurrence of a glacial triad and a molassic sequences in Neoproterozoic sedimentary formations. Implications for the Pan-African orogeny

Studies conducted since the 1960s on the sedimentary formations of the Béli region have not permitted the establishment of a lithostratigraphic log for the area. Based on fieldwork involving drafting of cross-sections and lithologic, petrographic and structural studies, a triad and a molasse formation have been described for the first time in this portion of the basin. Therefore, as in the sedimentary formations of the Taoudeni and Volta Basins, the formations in the Béli area, located on the southern edge of the Gourma Basin (northeastern Burkina Faso), include an association of “Tillite-limestone-chert” that is correlated with the “triad” of the Taoudeni Basin. The lithostratigraphic section of the Béli area comprises eight formations. The triad overlays quartzitic sandstones and is covered by shales and dolomite. Above the triad is the metamorphic allochthonous unit composed of phyllites, which are covered by the molassic formation. As in the carbonate formations, some rocks of the cherty complex contain stromatolites as well as micro-organisms. The Pan-African deformation is characterized by folding, schistosity and a fault network. Compressive phases D1, D3 and D4 of the Pan-African orogeny are identified, and we suspect a reactivation of Pan-African fractures during the Cenozoic. The characteristics of the tectonic episodes are utilized to distinguish two structural units: the allochthonous unit of North Béli, characterized by a ductile deformation and an anchi-metamorphism thrust on the para-autochthonous unit of South Béli. The latter is marked by a dominant brittle deformation; it underwent non-metamorphism and is thrust on the Eburnean basement. The cherty formation of the triad contains a polymetalliferous mineralization that was probably enriched during the different tectonic events.