K/Ar ages and oxygen isotope compositions were determined on illitic clay fractions of Purbeckian marls from the Swiss and French Jura Mountains in order to examine their syn-depositional origin which had previously been suggested based on sedimentological and mineralogical data. Measured ages range between 122.3+ or -2.3 Ma and 136.5+ or -2.5 Ma, thus being either coeval or slightly younger than the accepted age of sedimentation (Jurassic/Cretaceous boundary). Because total burial depth was less than 1000 m, delta 18 O values around 20 per mil are compatible with a formation of illite by replacement of smectite in an evaporated sea-water. Therefore, in addition 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.

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