Stratigraphic re-evaluation of the stratotype for the regional Ottnangian stage (Central Paratethys, middle Burdigalian)

The Ottnangian stage represents the middle Burdigalian (c. 18.1-17.2 Ma) within the regional stratigraphic concept for the Central Paratethys. The section Ottnang-Schanze in the North Alpine Foreland Basin of Upper Austria has been defined as its stratotype by Rogl et al. (1973). We present an updated stratigraphic evaluation of the section based on biostratigraphy of foraminifers, dinoflagellate cysts and calcareous nannoplankton as well as magnetostratigraphy. In agreement with earlier studies, assemblages of benthic foraminifers (co-occurrence of Amphicoryna ottnangensis and Sigmoilopsis ottnangensis, mass-occurrences of Lenticulina inornata) document a late early Ottnangian age. Dinoflagellate cyst Exochosphaeridium insigne is recorded for the first time in the early Ottnangian and its occurrence together with Apteodinium spiridoides, Cordosphaeridium cantharellus and Glaphyrocysta reticulosa s. l. extends the regional dinoflagellate zone Ein from the middle to the early Ottnangian. On a global scale, the revealed marker species indicate zone D17a (middle-late Burdigalian). Calcareous nannoplankton assemblages with the very rare occurrence of Sphenolithus cf. belemnos and S. aff. heteromorphus show remarkable affinities to Mediterranean nannoplankton zone MNN3b. Together with the frequent occurrence of Helicosphaera ampliaperta and the absence of Triquetrorhabdulus carinatus an assignment to standard nannoplankton zone NN3 (early-middle Burdigalian) is indicated. Magnetostratigraphy revealed an inverse polarisation for the outcrop. In combination with the biostratigraphic age constraints and the present correlation of the Ottnangian to the Bur3 sea-level cycle the section belongs to polarity chron C5Dr.2r. For the first time, an absolute age between 17.95-18.056 Ma for the stratotype can be inferred.