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Mineralogical and geochemical characteristics of Early Sarmatian bentonite and marls from the Hrvatsko Zagorje Basin, Croatia

A bentonite layer deposited between horizontally laminated marls was documented in the Sutla-II column recovered from northwestern part of the Hrvatsko Zagorje Basin (i.e. southwestern marginal belt of the Pannonian Basin System). Mineralogical analyses were performed by XRD and thermal analyses, while geochemical methods on the bulk sample included ICP-OES and ICP-MS. Additional chemical analyses were carried out on the coeval bentonite from northeastern Bosnia and Herzegovina (Ugljevik) to establish a potential regional correlation. Calcite and clay minerals (smectite, illite, kaolinite and in some samples chlorite) are the most abundant minerals in marls. The bentonite is dominated by montmorillonite (60- 70%). Besides them, it contains opal-CT, calcite, and small content of quartz. CEC of marls varied between 16.5 and 24.5 meq/100 g and in bentonite it revolved around 83 meq/100g. Provenance analysis indicates that bentonite was formed by intensive weathering of felsic to intermediate type volcanic ash. Smectite in marls, apart from its terrigenous origin, was likely sourced from the alteration of volcanic ash as well. The described occurrences of bentonite are associated with a distant volcanic eruption. Coeval deposition times and similar trace element abundances of bentonite analyzed herein and that from Ugljevik (Badurina et al., 2021) suggest that both likely originated from the Apuseni Mts or Eastern Carpathians via Early Sarmatian volcanism.

Sarmatian volcanism, bentonite, marls, clay minerals, provenance, Pannonian Basin System

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