Latest Priabonian larger benthic foraminiferal assemblages at the demise of the Sogucak Carbonate Platform (Thrace Basin and Black Sea shelf, NW Turkey): implications for the shallow marine biostratigraphy

The Eocene-Oligocene transition corresponds to a significant biotic turnover globally. In shallow marine realms, the larger benthic foraminifera (LBF) belonging to most nummulitids and all orthophragminids became extinct at or near the Eocene/Oligocene boundary, reflecting the environmental deterioration. We here record the advanced developmental stages of midlatitude LBF from the Sogucak Carbonate Platform in NW Turkey on the brink of their extinction in the Tethys. The LBF assemblages in the uppermost part of the platform, exposed in Gokcetepe and Karaburun in the Thrace Basin and Black Sea shelf, are represented by the dominant occurrence of Heterostegina d'Orbigny, 1826, Spiroclypeus Douville, 1905, Operculina d'Orbigny, 1826, Discocyclina Gumbel, 1870, Nemkovella Less, 1987, Orbitoclypeus Silvestri, 1907, Asterocyclina Gumbel, 1870, and subordinate Assilina d'Orbigny, 1826, Nummulites Lamarck, 1801, and Pellatispira Boussac, 1906, characterizing shallow benthic zone (SBZ) 20. In general, the occurrence of Heterostegina, Spiroclypeus, Orbitoclypeus, and Asterocyclina in both sections suggests a deep environmental setting in the shelf. This interpretation is also supported by the presence of abundant planktonic foraminifera in Gokcetepe and the rareness of Nummulites and Assilina in both localities. The cooccurrence of Heterostegina reticulata and H. gracilis in Gokcetepe suggests a stratigraphic position in the lower part of SBZ 20, while in the absence of H. reticulata, Karaburun samples are referred to the upper part of the same biostratigraphic zone. Spiroclypeus carpaticus specimens in Gokcetepe possess more primitive features than those in Karaburun as recorded in the number of operculinid chambers (parameter X), further supporting the age relations of both sections. The occurrence of D. samantai, N. daguini, A. alticostata, and O. zitteli in association with H. gracilis and S. carpaticus in the latest Priabonian is reported here for the first time. Our findings prompt the revision of the upper stratigraphic ranges of these taxa in the Tethys.