U–Pb zircon age and mineralogy of the St Georgen halloysite tuff shed light on the timing of the middle Badenian (mid-Langhian) transgression, ash dispersal and palaeoenvironmental conditions in the southern Vienna Basin, Austria

Abstract

A geochronological study of the St Georgen tuff, favourably positioned within the Middle Miocene marine succession of the Vienna Basin, is essential to our understanding of the timing of the middle Badenian transgression. We report here new data on the separated zircon U–Pb ages/phase chemistry and the clay mineralogy of altered tuffs and then use these data to infer the provenance of the tephra and the palaeoenvironmental conditions. The ages of the tuff range between 15.78 ± 0.27 and 14.36 ± 0.31 Ma, with a weighted mean age of 14.59 ± 0.2 Ma. This defines the onset of the second Badenian transgression in the Central Paratethys region, which was the strongest transgression in the entire Miocene record of the Vienna Basin. The compositional and temporal relationships between the tuff and the neighbouring volcanism suggest that the Harsány eruption in the central Pannonian Basin is the most plausible source region for the tephra. West- and SW-directed tropospheric trade winds or easterlies were responsible for the transport of the Harsány tephra to its present location. The prevalence of halloysite, in addition to post-depositional alteration reactions (glass–smectite–halloysite and kaolinite–halloysite) suggest the fallout of tephra in a very shallow sea, which may have been affected by seasonal wetting and drying cycles at the onset of ash deposition. Supplementary material: Halloysite mineralogy, global geochemistry and U-Pb zircon age data are available at https://doi.org/10.6084/m9.figshare.c.6294944