Abstract Mineralogical, morphological and isotopic (RbSr and KAr) determinations were made on some detrital smectites of Palaeocene and Cenomanian ages from D.S.D.P. drillings in the Atlantic Ocean. These minerals are not inert in their depositional environment; authigenic laths grow on detrital sheets with sharp borders. This authigenesis could occur slightly after deposition in a closed system, for some of these smectites. It has been tentatively quantified by the RbSr and KAr isotopic methods, which seem also well suited to evaluate the chemical extent of this authigenesis. At least, no preferential loss of 40Ar vs. 87Sr could be detected in the minerals, even in those which are smaller than 0.2 μm.