The Mio-Pliocene transition in the Dacian Basin (Eastern Paratethys): Paleomagnetism, mollusks, microfauna and sedimentary facies of the Pontian regional stage

Well-documented, high-resolution sedimentary sections are critical to our understanding of the often eventful evolution of semi-isolated basins that form during the final stages of continent-continent con-vergence, as exemplified by the Mediterranean-Paratethys system. Due to its nearly land-locked position in the Late Miocene, the Mediterranean experienced the renowned Messinian Salinity Crisis. At the same time, the equally restricted Eastern Paratethys to the north-east evolved in a potentially related, but very distinctive way. The events of this period are fully recorded in the sediments deposited during the Pontian regional stage in the Dacian Basin, part of the Eastern Paratethys, which we here investigate in detail in the best available section. The studied interval of the Slanicul de Buzau section is more than 2 km thick and almost continuously exposed. It is analyzed for paleomagnetism, mollusks, microfauna and sedimentary facies. This allows us to refine previous results from nearby but less complete sections, with particular improvements concerning resolution, biostratigraphy and sedimentology. The marine incursion just below the base of the Pontian (6.1 Ma) is shown to significantly predate the early Pontian highstand. The biostratigraphically defined onset of the middle Pontian (6.0 Ma) coincides with the highstand and slightly predates the major regression (5.9-5.6 Ma) for which the middle Pontian is best known. In the here-investigated section, the transgression at the beginning of the late Pontian (5.6 Ma) is followed by a regressive trend culminating between 5.4 and 5.2 Ma around the Mio-Pliocene boundary (5.33 Ma). The late Pontian sedimentation then becomes relatively stable and the fauna gradually transitions (4.8 Ma) into assemblages characteristic for the Dacian stage of the Dacian Basin, while overall significantly diverging from the rest of the Eastern Paratethys. Finally, we discuss several factors that could affect accuracy and applicability of the updated chronostratigraphy, warranting some caution.& COPY; 2023 Published by Elsevier Masson SAS. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

Automated summary

The study investigates a section in the Eastern Paratethys, which is important for understanding the Late Miocene stratigraphy in the Mediterranean-Paratethys region. Findings show that there was a significant early marine incursion during the Late Zekepian, starting at approximately 6.1 Ma, while the onset of the early Middle Zekepian occurred at 6.0 Ma, coinciding with the major regression period (5.9-5.6 Ma). Late Zekepian sedimentation became relatively stable, with a gradual transition in fauna to assemblages characteristic of the Dacian stage, significantly differing from other areas in the Eastern Paratethys.