Clay and glauconite fractions separated from sedimentary rocks in Israel were analysed for their mineralogical composition by X-ray diffraction and dated by the K-Ar method. The main goal was to evaluate early diagenetic processes which might have affected the K-Ar system. One series of samples was taken from two contemporaneous mid-Albian to mid-Turonian (~105–91 Ma) shallow marine successions in which early diagenetic K-feldspar was found to be widespread. Another series was of samples of large spatial distribution associated with a mid-Turonian emergence event. None of the outcrops had been subjected to high temperatures due to deep burial or other thermal events. The abundant clay minerals in the first series are randomly ordered illite-smectite (I-S) phases of various discrete and mixed compositions. Illite, kaolinite, ordered I-S and palygorskite are sometimes present. The variability in clay assemblages and I-S composition within short vertical distances and the proximity to the provenance implies mostly a detrital origin for the clay minerals. The K-Ar ages of clay fractions in the first series are no older than 30% of the stratigraphic ages, much younger than the known detritus provenance ages. A few have known stratigraphic ages and one is younger by 10%. The K-Ar ages of associated glauconite fractions are younger by 5–13% than the stratigraphic ages. The ages of the clay fractions of the second series, which are on average more illitic, are mostly younger and no older than 22% more than the stratigraphic age. The age distribution and the clay mineralogy in the two series suggests that illitization of detrital I-S phases may start right after deposition under specific conditions, namely, contact of the sediments with K-rich brines, produced by evaporation in restricted marine environments. The process may continue from a few to many millions of years after burial, as long as K-enriched solutions are available. However, in most cases the near-surface temperatures did not enable a completion of the K-Ar reset process. It seems that low-temperature illitization is better explained as being dependent on solution composition and not on wetting-drying cycles.