Paleomagnetic properties of the ignimbrites from the famous fossil footprints site, Ipolytarnóc (close to the Hungarian- Slovak frontier) and their age assignment

On geological grounds, the ignimbrites/tuffs of the Ipolytarnóc area were regarded as belonging to the oldest of the three Miocene “tuff complexes” of the Pannonian Basin. From the paleomagnetic point of view, the three complexes are significantly different in the area which comprises North Hungary and South Slovakia, since the oldest is characterized by about 80º, the middle by about 30º counterclockwise, while the youngest about 10º clockwise rotation with respect to the present North. The significant differences in declinations are attributed to block rotations, which affected the area in the time intervals of 18.5–17.5 Ma and 16–14.5 Ma. Earlier, a pilot study on three ignimbrite sites from Ipolytarnóc indicated only 30º of counterclockwise rotation, which suggested that the volcanism was younger than assumed, while the underlying Eggenburgian sediments showed the expected large rotation. Following up indications, we carried out a detailed paleomagnetic study at this locality on a large number of samples from the terrestrial sequence which covers the Eggenburgian sandstone. These were the “footprint sandstone” and the overlying unaltered, ignimbrites which represent three volcanic cycles separated by clastic sediments. Ignimbrites from the surrounding area were also tested. The results obtained are of high quality, statistically well defined on the site level and are highly consistent on the between-site level. The polarity is normal for the first two cycles and reversed for the youngest. The rotation angle indicated is about 30º. Thus, we conclude that the ignimbrites at the Ipolytarnóc area are indeed younger than 17.5 Ma, so they belong to the “Middle Tuff Complex” of the Pannonian Basin.