Mineralogical, morphological and isotopic (Rb-Sr and K-Ar) determinations were made on some detrital smectites of Paleocene and Cenomanian ages. 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 th Rb-Sr and K-Ar isotopic methods, which seem also well suited to evaluate the chemical extent of this authigenesis. At least, no preferential loss of (super 40) Ar vs. (super 87) Sr could be detected in the minerals, even in those which are smaller than 0.2mu m.