A series of thin clayey beds topping shallow marine cyclothems was sampled from a Turonian succession in Israel and was analysed by XRD for the mineralogical composition of its clay fraction. The main goal was to search for potential relations between short-term shallowing and possible emergence events and clay mineralogy. The bulk (<2 μm), coarse (0.2–2 μm) and fine (–0.2 μm) clay fractions of a unique clay bed, dominated by R⩾1-type I-S were analysed for their chemical composition and dated by the K-Ar method. The fine clay fraction was dated at ~5 m.y. younger and the bulk clay to be slightly older than the known stratigraphic age, which means that the clay is mostly authigenic and formed at low-temperature, near-surface conditions. This is supported by the geological history of the region and by the variability of the clay assemblages and I-S composition within ~30 m vertical distance, neither of which indicates evidence for high temperatures. The sedimentological analysis suggests that the clay bed was deposited at marginal marine marshes where evaporated sea water turned into an alkaline solution of high K concentration. This is apparently the first study to provide convincing evidence for low-temperature, early diagenetic formation and preservation of I-S with R1-type, or similar ordering.