Palaeoenvironmental changes during the late Badenian - earliest Sarmatian (Middle Miocene) in Central Paratethys inferred from foraminiferal and palynological data

The Badenian/Sarmatian boundary in the Central Paratethys has been traditionally identified by the faunal turnover recording an important environmental change possibly controlled by the change from marine to brackish conditions. The strata below the Badenian/Sarmatian boundary in the northern Carpathian Foredeep in Poland are included into the Pecten beds, and those above it into the Syndesmya beds. Foraminiferal study of the Babczyn 2 borehole which is one of the crucial sections in the northern Carpathian Foredeep, well-known for the depositional age of rhyolite tuff within the Pecten beds dated by Śliwiński et al. (2012) at 13.06 ±0.11 Ma, indicated that in fact the boundary occurs within the Syndesmya beds. This conclusion is based upon the rapid change from a stenohaline foraminiferal fauna to a euryhaline one.

Benthic foraminifera and palynofacies from the Upper Badenian  Neobulimina longa, Hanzawaia crassiseptata and lower Sarmatian Elphidium angulatum and Anomalinoides dividens zones have been studied (Peryt et al., 2021).  Benthic assemblages are moderately to highly diversified. The benthic foraminiferal successions in the studied interval suggest normal marine salinity, middle shelf depth basin, with relatively small oxygenation and productivity changes during the late Badenian. The rapid change in the taxonomic composition between the H. crassiseptata and E. angulatum zones reflected by extinction/disappearance of stenohaline taxa from the foraminiferal assemblages and replace them by euryhaline forms resulted from shallowing and decrease in salinity of the Polish Carpathian Foredeep Basin.

Palynofacies is dominated by terrestrial elements represented by palynodebris (black and dark brown phytoclasts and cuticles) and pollen grains. The proportion of marine elements (dinoflagellate cysts, prasinophytes, acritarchs, and rare zooclasts) rarely exceeds 10%. Taxonomic diversity of the dinoflagellate cyst assemblages is low. Majority of samples yielded assemblages dominated by 3–4 species with remaining taxa represented by rare or even single specimens. Qualitative and quantitative analysis of aquatic palynomorphs show that the borehole interval studied accumulated during variable, unstable sedimentary conditions. A short interval that occurs just above the last occurrence of stenohaline foraminifers is characterized by lack of dinoflagellate cysts and flowering of Leiosphaeridiaceae. Lack of dinoflagellate cysts points to conditions disastrous for dinoflagellate cysts. But the most likely reason was salinity increase above the level tolerable even for hypersaline forms (e.g., Polysphaeridium) but still favourable for Leiosphaeridia. These possibly hypersaline conditions were associated with stagnant, possibly stratified waters that led to anoxic conditions in the bottom waters manifested by amorphous organic matter. A cease of these conditions was caused by a possible sea level rise and a gradual return of a less saline water regime. The latter interpretation can be supported by high frequency of Polysphaeridium (a genus known from hypersaline environments).

The study was financed by the National Science Centre, Poland, grant No. UMO-2017/27/B/ST10/01129.

Peryt, D., Garecka, M., Peryt, T.M., 2021: Geological Quarterly, 65: 18, doi: 10.7306/gq.1584; Śliwiński, M., Bąbel, M., Nejbert, K., Olszewska-Nejbert, D., Gąsiewicz, A., Schreiber, B.C., Be-Nowitz, J.A., Layer P., 2012: Palaeogeography Palaeoclimatology Palaeoecology, 326–328: 12–29.