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Planktonic Foraminiferal Biozonation of the Oligocene in the Northern Adriatic Sea

Samples for detailed micropalaeontological research were selected from two deep exploration wells (Istra more-3 and Istra more-4) in the Adriatic Sea. On the basis of index taxa and selected species of planktonic foraminifera, the standard planktonic foraminiferal zonation for Oligocene sediments after Wade et al. (2011) was applied (zones O1–O7). The investigated deposits overlie the upper Eocene (E16) sediments, and are themselves overlain by lower Miocene rocks (Zone M1). Distinct changes in the composition of the planktonic foraminiferal assemblage are observed near the Eocene–Oligocene boundary. In the uppermost Eocene, just below the Eocene–Oligocene boundary, genera Cribrohantkenina and Globigerinatheka as well as most species of the genus Turborotalia (T. cerroezulensis, T. cunialensis, and T. cocoaensis), disappear. Chiloguembelina cubensis appears just below the E–O boundary, and Cassigerinella chipolensis, Chiloguembelina sp., Dentoglobigerina tapuriensis, D. tripartita, D. venezuelana, Dentoglobigerina sp., and Subbotina sp. appear above the boundary. The transition between the Eocene and Oligocene is also characterized by a reduction in size of the specimens of genus Pseudohastigerina as well as of species Chiloguembelina cubensis and Ch. ototara. The boundary between the lower and upper Oligocene is defined by the highest common occurrence (HCO) of Ch. cubensis ; that is, the proportion of this species in the planktonic foraminiferal community in the small size fraction (125–63 lm) is greater than 5 %. The Oligocene–Miocene boundary is marked by the first appearance of the species Paragloborotalia kugleri.

Biostratigraphy,  Oligocene, Planktonic foraminifera, Northern Adriatic Sea

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