Middle–Late Miocene Paleogeography of the Eastern Paratethys. Part VI. Sarmatian Basin: Stratigraphy, Facies, Paleogeography and Biota

The Sarmatian Basin was the largest of the Middle–Late Miocene Paratethys basins. At that time the transgression that began at the very beginning of the Sarmatian (Volhynian) time (12.65 Ma), resulted in the confluence of the Central and Eastern Paratethys and in the formation of a common rich endemic fauna in the conditions of a semi-isolated basin. As a result of the Early Sarmatian transgression, the coastal facies of the preceding Konkian basin were mostly reworked and eroded, and the Konkian fauna was partially redeposited in the basal Sarmatian beds. For the first time, vast areas on the northern and eastern shelves, from Ciscarpathia to the Transcaspian regions, were flooded. After a slight sea level drop in the second half of the Volhynian, the basin expanded and deepened again in the first half of the Bessarabian time when the transgression reached its maximum (about 11.5 Ma). The accumulation of “Cryptomactra clay” facies was widespread at that time. The degree of species endemism in the benthic fauna reached 85% of bivalves and 90% of gastropods. Although the Paratethys was largely hydrologically isolated during the Sarmatian, occasional occurrences of marine phytoplankton and benthic fauna in its western and Transcaucasian parts indicate that brief incursions from the global ocean may have occurred. From the second half of the Middle Sarmatian, climate aridization resulted in a reduction of water balance and instability of water level in the semi-isolated basin. The onset of the Greater Caucasus orogeny provided sharply increased input of coarse terrigenous material. A significant decrease in sea level occurred in the terminal Middle Sarmatian (‒150 m, about 10 Ma), resulting in the drying out of the Stavropol High in Ciscaucasia and the Dziruli Isthmus in Central Transcaucasia and in the complete separation of the Euxine and Caspian parts of the Paratethys. In the Late Sarmatian, the water level recovered but remained unstable. A catastrophic sea level drop occurred at the end of the Late Sarmatian (about 7.9 Ma).