The Santonian-Campanian boundary and the end of the Long Cretaceous Normal Polarity-Chron: Isotope and plankton stratigraphy of a pelagic reference section in the NW Tethys (Austria)

The base of the Campanian still lacks a designated primary marker and a defined GSSP (Global Boundary Stratotype Section and Point). We present a Santonian-Campanian boundary section from the Postalm in the Northern Calcareous Alps of Austria as a possible reference section for the northwestern Tethys. Including nearby complementary Gosau Group sections, the stratigraphy for this time interval is based on palaeomagnetic and stable isotope data, planktonic foraminifera and calcareous nannoplankton biostratigraphy, and strontium isotope stratigraphy, together with published ammonite, crinoid and inoceramid data. The Postalm section shows a deepening trend from upper Santonian conglomerates and grey shelf marls to pelagic bathyal red marly limestones of mainly Campanian age. Palaeomagnetic data allow identifying the top of the Long Cretaceous Normal Polarity-Chron C34n and the following reversal in the lower part of the red marly limestones. A 1 m-thick interval of high magnetic susceptibility is present at the end of C34n. Two of the main suggested biomarkers to pinpoint the Santonian -Campanian boundary, i.e. the last occurrence of the planktonic foraminifer Dicarinella asymetrica and the first occurrence of the nannofossil Broinsonia parca parca, occur in close proximity above the reversal, which is suggested herein as the main marker event for the base of the Campanian. Strontium isotope stratigraphy indicates a value of 0.707534 (mean of four measurements at the boundary interval) for the base of the Campanian in the Postalm section. Both carbon and oxygen isotope values show a negative excursion just below the boundary. The positive Santonian-Campanian carbon isotope event starts probably just below the reversal but seems to be diagenetically distorted. This interval is considered to correspond to a short sea-level high in the late Santonian followed by a distinct lowstand at the Santonian-Campanian boundary.