In the Cadomian belt (Upper Proterozoic) some sedimentary formations contain interbedded graphitic cherts. The sedimentological study carried out on the Lamballe Formation (Armorican Massif) shows that the associated terrigenous rocks were laid down between the shoreface zone and the inner shelf and were associated with storm wave action. These results question hypotheses about the origin of the black! cherts and the validity of using chert as a lithostratigraphic marker. (C) Academie des Sciences/Elsevier, Paris.