DJEYA Kouamé Leger^1,, KOFFI Chiaye Larissa Epouse Coulibaly², TOE-BI Kahou Katel Kizito³, KOFFI Djemele Akissi Tatiana⁴, YOUGONE BI Gervais¹, YAO N’goran Jean Paul¹

This study focuses on 13 samples from the SB well of Birisso, in the eastern onshore part of the Ivorian sedimentary basin, born from the opening of the South Atlantic during the Cretaceous. It combines lithology, binocular mineralogy and morphoscopy of quartz grains to reconstitute the depositional environments. The shaft shows four units with decreasing grain size upwards: purplish sands and clays at the base, fine sands with ferruginous sandstone, bitumen and marl in the middle, then a lateritic armour at the top. Quartz dominates (76.42%), followed by iron oxides (15.81%), while pyrite, glauconite, bitumen and carbonaceous debris are present in small quantities. The association glauconite-pyrite-organic matter indicates a shallow marine environment, first oxidizing then becoming reductive, sign of a marine transgression. Quartz grains are mostly blunt and shiny (60%), so they are transported by water over a long distance. The roundabouts (30%) reveal a wind input, and the angular grains (10%) a river transport. The sediments are thus of continental origin, resulting from aqueous and wind processes. The shallow marine environment, first oxidizing (glauconite) then becoming reductive and anoxic during a transgression (pyrite, bitumen), received continental sediments transported by water and wind, with variations in transport energy. Sedimentation is therefore complex and polyphasic, with alternations of transgression and regression, and the presence of zircons and phosphates opens up economic prospects.

Ivory Coast, morphoscopy, Birisso, Palaeoenvironments, Sedimentology