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The K‐Ar system in clay fractions from shallow marine carbonate shelf environments was investigated on silicate fractions (clay minerals, feldspar) separated from 20 Lower Jurassic to Upper Cretaceous sedimentary rocks, deposited in the southern Tethys ocean. The range of lithologies investigated included dolomite and chalk [IR (insoluble residue)<10%], marl, shale (IR= 70–85%) and sandstone (IR>90%). The results show that K‐bearing clay fractions often have K‐Ar ages similar to the suggested age of deposition, which means either supply of land‐derived authigenic K‐bearing clays or synsedimentary diagenetic authigenesis, or both. This K‐Ar synsedimentary signal is recorded in clay fractions from the whole range of studied lithologies and stratigraphic units. Among the clay minerals, the synsedimentary K‐Ar signature was recorded and retained in illite/smectite of the <2‐um and <0.2‐um fractions. A prominent synsedimentary signature is found in K‐feldspars, from shaly and especially from calcareous rocks, which is substantiated by their authigenic origin based on idiomorphic crystal morphology and their limited size distribution (4–10 um). Post‐depositional closure of the K‐Ar system is indicated by ages up to 15 Ma younger than the stratigraphic age in different lithologies from dispersed localities. A distinct late diagenetic (20–25 Ma younger) event is recorded in the formation of authigenic K‐feldspar within Upper Cretaceous chalk and shale. In the IR and >10‐um fractions the K‐Ar ages reflect the contribution of detrital mica and feldspar which accompanies the kaolinite‐dominated samples. The overall results differ considerably from K‐Ar age patterns observed in deep‐sea sediments, a difference which may be connected with the occurrence of brines in these shelf deposits. The findings indicate the potential in the K‐Ar dating of fine IR fractions of marine shelf sediments in terms of geochronological‐stratigraphic and palaeogeographical aspects as well as in the petrology of clay minerals themselves.