Thanks to the favourable position within the Middle Miocene marine succession of the Vienna Basin, a geochronological study of the St. Georgen tuff becomes essential for the understanding of the timing of the middle Badenian transgression. This contribution reports new data on separated zircon U-Pb ages/phase chemistry and the clay mineralogy of altered tuff used to infer on tephra provenance and paleoenvironmental conditions. The ages ranged 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, which was the strongest transgression in the entire Miocene record of the Vienna Basin. The compositional and temporal relationship between the tuff and neighbouring volcanism indicates the Harsány eruption in the central Pannonian Basin as the most plausible tephra source region. Eastern and NE directed tropospheric trade winds or easterlies are responsible for the transport of the Harsány tephra to its present location. Prevalence of halloysite, as well as post-depositional alteration reactions (glass-smectite-halloysite and kaolinite-halloysite) call for tephra fallout in the very shallow sea, which might have been affected by seasonal wetting and drying cycles at the onset of ash deposition.

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