Sarmatian biostratigraphy of a marginal sea in northern Croatia based on calcareous nannofossils

Abstract

Calcareous nannofossils of the Serravallian/Tortonian transition, known as the Sarmatian in the Paratethys were obtained from northern Croatia, Central Paratethys. The nannofossil biostratigraphy of the studied area included early Sarmatian biohorizons; the reliability of these and previous biohorizons was tested, and the known biostratigraphic and magnetostratigraphic datums of the Paratethys were compared with those from the adjacent Mediterranean and beyond. Six out of sixteen bioevents were selected to define five subzones (PNN6d-PNN8b) and crossover for the Badenian/Sarmatian boundary. Variations in the index fossil datums between the Paratethys and Tethys provide a broad perspective on local versus regional and global impacts on the nannofloras. The Paratethys/Tethys gateways (PTGs), and their progressive geodynamical closure, exerted a fundamental control on the salinity, circulation patterns and climate of the area. The global regression at the end of the Badenian, and the tectonic events associated with the Badenian/Sarmatian transition, also restricted the PTGs, initiating acmes of Calcidiscus pataecus and Reticulofenestra pseudoumbilicus, causing a small, early Sarmatian extinction event that occurred in coastal environments, which are the most affected by short-term environmental fluctuations. Stronger connections through the PTGs during the early/mid-Sarmatian transition induced a transgression that brought deep-dwelling Discoaster species into the area. The final closure of the PTG with the Mediterranean, which had been placed at the Badenian/Sarmatian boundary, actually occurred in the late Sarmatian as a result of tectonic movements that caused shallowing and a drastic reduction in nannofossil biodiversity. The PTG with the Indian Ocean existed until the early Tortonian, which allowed an early, but brief, occurrence of small, warm-water ceratoliths.