New 40Ar/39Ar, magnetostratigraphic and biostratigraphic constraints on the termination of the Badenian Salinity Crisis: Indications for tectonic improvement of basin interconnectivity in Southern Europe

Abstract

Widespread evaporites were deposited in large parts of the Central Paratethys during the so-called Badenian Salinity Crisis (BSC). The adverse environmental conditions that accompanied the BSC triggered a demise in the basin's marine fauna, inducing the so-called middle-Badenian-extinction-event. While tectonic activity preconditioned the Central Paratethys for isolation, it has recently been shown that the BSC was eventually triggered by the base-level drop accompanying the Mi3b global cooling event, which terminated the Middle Miocene Climatic Optimum. Here, we provide new constraints on the termination of the BSC by 40Ar/39Ar dating of a volcanic ash layer, located in a marl succession several meters above the Badenian evaporites of the Slănic Syncline in the Romanian East Carpathians. The results reveal that the BSC ended before 13.32 ± 0.07 Ma. Comparison with previously obtained geochronological results in Poland constrains the duration of the BSC to 500 kyr, assuming evaporite deposition in the Central Paratethys occurred as one event. The obtained 40Ar/39Ar results are complemented with paleomagnetic and micropaleontological analyses. These reveal that the investigated post-BSC marl interval in the Slănic Syncline was deposited in a period of reversed polarity corresponding to C5AAr. This is in agreement with calcareous nannoplankton from the same interval that belong to the NN6 zone. Ostracod and foraminifera marker species are indicative of the middle part of the regional Badenian stage, traditionally known as the Wielician. The foraminiferal assemblage is nevertheless very similar to Serravallian assemblages from the Mediterranean, which suggests that, in addition to a connection between the Central Paratethys and the Eastern Paratethys, there was a marine connection with the Mediterranean following the BSC. A comparison with post-BSC successions in Ukraine and Poland illustrates that the BSC was terminated by a transgression that re-installed normal marine conditions in the Carpathian foredeep. This basin-wide transgression resulted from reconnection of the Carpathian Foreland Basin with the Mediterranean and Eastern Paratethys, which improved the exchange of water and fauna. Global eustacy cannot explain re-connection of these basins, because global sea level on average remained just as low after the BSC as it had been during the crisis. The improved interconnectivity between the basins must therefore have been primarily caused by tectonic modification of the interconnecting gateways. Geodynamics thus played a crucial role in the re-establishment of a flourishing marine environment.