Miocene Evolution of the Central Paratethys: Insights from Chronostratigraphic, Paleomagnetic and Numerical Modelling Studies

Recent paleomagnetic and geochronologic investigations have provided new age constraints on the Middle Miocene stratigraphy of the Central Paratethys. The causes and consequences of a number of events apparent in the stratigraphic record can now be more adequately evaluated. The beginning and end of the Badenian Salinity Crisis (BSC) were dated on the basis of ⁴⁰Ar/³⁹Ar measurements on tuff samples from below, within, and above evaporites, collected in the Polish Carpathian Foredeep and the Romanian Southern Carpathians. According to the results, the BSC lasted for approximately 400 kyr. This implies that the frequently observed small scale sedimentary cyclicity in the Ca-sulfate and halite deposits is not annual but in the order of ~10 years. This implies a driving mechanism other than seasonality. The onset of BSC evaporite deposition is pinpointed at 13.81 ± 0.08 Ma. It is thus time-equivalent with the Langhian-Serravallian boundary. Correlation to oxygen isotope records shows that the BSC evaporites were just preceded by glacial event Mi-3b, suggesting a causal relationship. The corresponding sea-level fall most likely restricted the open-marine gateway to the Mediterranean, thereby trapping the salt in the deep Paratethys basins. The unmistakable importance of gateways in the Miocene Mediterranean-Paratethys system is also reflected in the results of numerical box modelling studies. These demonstrate that the Paratethys became more sensitive to climatic influences when water exchange with the open ocean became restricted. They furthermore highlight that severing of the Indian Ocean gateway could have lead to significant cooling of the land-locked sea. Temporary closure of the Indian Ocean gateway due to the formation of the so called ‘Gomphoterium Land Bridge’ during the Burdigalian (Ottnangian) might thus serve as an explanation for the enigmatic cooling event observed in the Central Paratethys. Chronostratigraphic research in the Transylvanian Basin sheds additional light on the Paratethys Miocene evolution. Nine radio-isotopically dated tuff layers in the basin’s Middle Miocene infill and several paleomagnetically investigated sections were traced along seismic lines into a synthetic stratigraphic column in the basin centre. The onset of deposition of the 50 m thick Dej Tuff Complex, traceable from Romania up to the German Molasse, is constrained between the first occurrence of Orbulina suturalis at 14.56 Ma and the 14.38±0.06 Ma ⁴⁰Ar/³⁹Ar age for one of the basal tuff layers. The Badenian-Sarmatian boundary is dated at 12.80±0.05 Ma. At present, no major concurrent climatic event is known from the global isotope record. A nearby tectonic cause also appears to be absent. This suggests the change in the Paratethys’ environment that characterizes the Badenian-Sarmatian transition might relate to tectonic activity in a more distant part of the system. The 11.3 Ma Sarmatian-Pannonian boundary is, on the contrary, shown to coincide with an approximately 20° CW tectonic rotation of the Transylvanian Basin. This corroborates the idea that enhanced tectonically induced uplift in the Eastern and Southern Carpathians lead to the final isolation of the Central Paratethys and triggered the transition from marine to freshwater conditions.