A bentonite clay layer is documented in the Sutla-II column in the Hrvatsko Zagorje Basin, which is a part of the south-western marginal belt of the Pannonian Basin System. The origin of the clay is attributed to the alteration of felsic to intermediate volcanic ash, which had been deposited between horizontally-laminated marls in a marine environment. Provenance analysis indicates that the marls were sourced from mixed, dominantly-felsic source rocks. Smectite present in the marls is therefore not solely of terrigenous origin and may also be related to volcanic ash weathering. Based on the fossil content, an inference has been made suggesting Early Sarmatian age of the sediment hosting the bentonite clay intercalation. The sedimentological and palaeontological data are in favour of the sedimentation at an inner shelf area marked by unstable palaeoenvironmental conditions. The upper part of the Sutla-II column was deposited in the high-energy environment consisting of impure biocalcarenite and biocalcrudite coupled with fossiliferous litharenite, which all mark an intensive redeposition of older rocks and fossiliferous formations. The bentonite clay likely originated from distant tephra sourced from volcanic eruptions, presumably located in the north-eastern part of the Carpathian–Pannonian Region during the post-rift stage of the back-arc Pannonian Basin System development.