A level of silicified limestones with nummulites and daucines has been discovered in Toubab Dialaw, located in the horst of Ndiass. Interbedded between the diatreme and the ferruginous lateritic crust, this level is characterized by sheath folds. The biostratigraphic analysis revealed the presence of nummulites, daucina and a planktonic association with Globigerinatheka kugleri (Bolli, Loeblich et Tappan, 1957) which characterizes P11-P13 zones of late Lutetian to early Bartonian age. This indicates that the Tertiary volcanic activity in the Cap Vert peninsula, here represented by the Toubab Dialaw diatreme, till now considered late Eocene to late Pliocene, began earlier, during the Middle Eocene and more precisely between 48 and 45 Ma. Moreover, the presence of sheath folds indicates that the sedimentary material has been subject to a submarine slide along a low slope. Finally, all the petrographical, palaeontological and structural data allow to precise the set up history of the volcanic and sedimentary formations constituting the littoral cliffs of Toubab Dialaw.