Abstract:

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 Soğucak 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 Gökçetepe 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 Gümbel, 1870, Nemkovella Less, 1987, Orbitoclypeus Silvestri, 1907, Asterocyclina Gümbel, 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 Gökçetepe and the rareness of Nummulites and Assilina in both localities. The cooccurrence of Heterostegina reticulata and H. gracilis in Gökçetepe 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 Gökçetepe 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.

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