Upper Campanian bentonite layers in the Scaglia-type limestone of the northern Dinarides (SE Slovenia)

Abstract

The research area is located in south-eastern Slovenia and belongs to the eastern extensions of the Slovenian Basin adjacent to the Adriatic Carbonate Platform. The studied section is 25 m thick and consists of grey, brownish and reddish pelagic limestones and calciturbidites, and two bentonite layers 110 and 10 cm thick. Both volcanoclastic horizons consist of greenish, fine-grained muddy material. To date, no other bentonites of similar age are known in the wider Southern Alpine and Dinaridic region. The late Campanian to Campanian/Maastrichtian age of the succession was determined from planktonic foraminifera and calcareous nanoplankton assemblages. XRD analysis revealed the presence of calcite, quartz, muscovite/illite and smectite, which prevails in the clay fraction and is indicative of bentonite clays. We interpret these bentonite horizons as resulting from direct deposition of volcanic ash in a marine environment with a concomitant admixture of detrital material (quartz, muscovite) and later diagenetic carbonatization. The geochemical composition suggests a felsic volcanic origin. The most likely source is the Late Cretaceous–Paleogene bimodal rhyolitic/basaltic volcanism documented along the Sava Suture Zone in present-day Croatia, Bosnia and Herzegovina, Serbia, and North Macedonia. In the Central Apennines three locations with bentonites have been documented. Further spatial and temporal analysis of similar volcanogenic horizons could yield important data on the paleogeographic and tectonic evolution of the Adria/Apulia continental margin.