A section at Aktulagay (west Kazakhstan), in the Peri-Tethys realm, exposes mid-neritic to upper bathyal Ypresian marls, clays and silts. These range from Zone NP10 to early Zone NP14, with abundant and diverse microfossil assemblages. Multidisciplinary analysis has identified dinoflagellate cyst, calcareous nannofossil, planktonic and benthic foraminiferid and pteropod zones and events. Calibration of a key interval in the evolution of the shark Otodus has been possible for the first time. Episodic low-oxygen facies, including sapropelic clays widely distributed in Peri-Tethys, are represented here and can be placed within a detailed biostratigraphic framework. The current lithostratigraphic terminology is modified, with the introduction of the Aktulagay Formation. Paleoenvironmental aspects are discussed; five depositional sequences are tentatively identified. This section can be correlated in detail with the succession in the North Sea Basin, with implications for paleogeographic reconstructions. High-resolution biostratigraphic calibration between disparate fossil groups makes this a key reference section for northern mid-latitude Ypresian biostratigraphy.
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