Syn-depositional, low temperature illite formation at the Jurassic-Cretaceous boundary (Purbeckian) in the Jura mountains (Switzerland and France) : K/Ar and delta O-18 evidence

K/Ar ages and oxygen isotope compositions were determined on illitic clay f ractions of Purbeckian marls Swiss and French Jura Mountains in order to ex amine their syn-depositional origin which had previously been suggested bas ed on sedimentological and mineralogical data. Measured ages range between 122.3 +/- 2.3 Ma and 136.5 +/- 2.5 Ma, thus being either coeval or slightly younger other than the accepted age of sedimentation (Jurassic/Cretaceous boundary). Because total burial depth was less than 1000 m, delta O-18 valu es around 20 parts per thousand are compatible with a formation of illite b y replacement of smrctitr in an evaporated sea-water. Therefore, in additio n to the two common types of illite resulting either from the fragmentation of pre-existing micas or diagenetic formation in deeply-buried sediments, illitic clay minerals formed at low temperature do exist and may be useful for age determination.