Integrated bio- and chemostratigraphy of the Cretaceous – Paleogene boundary interval in the Zagros Basin (Iran, central Tethys)

Abstract

An expanded Upper Cretaceous – Lower Paleocene succession, which encompasses the Cretaceous – Paleogene boundary (K – Pg), was studied in detail from the Zagros Basin (southwest Iran, central Tethys). In order to characterize the K – Pg transition, high resolution sampling was performed in the upper part of the Gurpi Formation. The studied interval consists of dark grey marly shales and marls with interbedded limestones. Based on calcareous nannofossils, the studied section is attributed to calcareous nannofossil zones CC26b to NP3 (latest Maastrichtian – early Paleogene). High relative abundances of the nannofossil species Micula staurophora (= Micula decussata) in the Maastrichtian part (CC26) might delineate cool surface water conditions, which are followed by an increase in abundance of Micula murus about 2 m below the K – Pg boundary. Reworked Cretaceous taxa were found at the K – Pg boundary and in the overlying Danian (NP 1–3). These findings hamper a reliable determination of the last occurrence of Cretaceous calcareous nannofossil taxa at the K – Pg boundary. The K – Pg boundary is located in the uppermost part of a positive carbon isotope excursion. Diversity and absolute abundances of calcareous nannofossils show a significant decrease at the K – Pg boundary. A bloom of the dinoflagellate cyst Cervisiella operculata (formerly Thoracosphaera operculata), which is indicating environmental stress, is recorded slightly below the boundary. Throughout the lowermost Danian (NP1), the assemblages are dominated by a bloom of small species like Neobiscutum romeinii, Neobiscutum parvulum, Futyania petalosa and small Cruciplacolithus primus. These taxa indicate stressful, and eutrophic surface water conditions throughout the initial recovery phase. The Dan-C2 and Lower C29n events, recorded in the lower Danian NP1 and lower NP2 biozones, are marked by negative carbon isotope excursions. High relative abundances of Prinsius dimorphosus followed by a Coccolithus pelagicus bloom in the full recovery phase (middle Danian, NP 3), are attributed to a short term cooling phase followed by warm surface water environments recorded along with the highest absolute abundance of calcareous nannofossils.