Otolith-based age determination of mid-Burdigalian marine sediments in the North Alpine Foreland Basin CLAUDIA TESCHNER & BETTINA REICHENBACHER Precise age constraints are rare for the Oligocene to Miocene sediments in the North Alpine Foreland Basin, also referred to as Molasse Basin. This is mainly due to the lack of index fossils, absence of continuous outcrops and scarcity of radio- metrically datable rocks. In this study, we applied the Sr isotope dating technique to fossil teleost fish otoliths to deter- mine absolute ages for Lower Miocene (mid-Burdigalian, lower Ottnangian) marine sediments. The fossil otoliths from the marine genera Diaphus and Coelorinchus used in our analysis were recovered from the Neuhofen Beds in the SE German Molasse Basin. For the first time, leaching experiments were conducted on different otolith fractions to extract the ambient 87 Sr/ 86 Sr seawater signal. Based on the latest version of the evolution curve for the 87 Sr/ 86 Sr ratio in the global ocean, our new data suggest an age of 17.1 ± 0.3 Ma, which is at least 0.6 m.y. younger than hitherto assumed for the Neuhofen Beds based on biostratigraphy and lithostratigraphic correlations. We suggest that strong riverine inputs could account for this shift in the Sr isotope signal to younger ages. • Key words: Sr isotopes, fossil otoliths, biostratigraphy, chronostratigraphy, chemostratigraphy, Central Paratethys, Upper Marine Molasse. TESCHNER, C. & REICHENBACHER, B. 2017. Otolith-based age determination of mid-Burdigalian marine sediments in the North Alpine Foreland Basin. Bulletin of Geosciences 92(2), 143–152 (3 figures, 1 table). Czech Geological Survey, Prague. ISSN 1214-1119. Manuscript received January 1, 2017; accepted in revised form May 5, 2017; published online June 27, 2017; issued June 30, 2017. Claudia Teschner & Bettina Reichenbacher, Department of Earth and Environmental Sciences, Ludwig- Maximilians-Universität, D-80333 Munich, Germany; c.teschner@posteo.de, b.reichenbacher@lrz.uni-muenchen.de The establishment of high-resolution age models for se- dimentary successions is crucial for numerous research questions in the geosciences and related disciplines. Such models provide an absolute chronology that per- mits accurate dating of depositional episodes and related processes such as mountain uplift or climate change (see Gradstein 2012). Particularly in epicontinental settings, precise age estimates are often compromised by lateral and transverse changes in lithofacies and an absence of marine index fossils. The North Alpine Foreland Basin is a case in point. Here, the lack of continuous outcrops and the scarcity of volcanic ash or bentonites that pro- vide radiometric ages render precise correlation of the sediments with the Global Time Scale difficult (see Ab- dul Aziz et al. 2010; Grunert et al. 2013, 2015; Reichen- bacher et al. 2013; Roetzel et al. 2014; Pippèrr & Rei- chenbacher 2017; Sant et al. 2017). The North Alpine Foreland Basin (NAFB) developed during the late Eocene and persisted as an important sedi- mentary basin of Central Europe until the late Miocene (e.g. Lemcke 1988). It extended from Lake Geneva through Southern Germany to Lower Austria, where it merges with the Carpathian Foredeep. The mainly siliciclastic sediments of the NAFB, usually referred to as ‘Molasse’, derive largely from the Alps. Other important sediment suppliers were the Franconian Platform, com- posed of Jurassic limestone, and the Bohemian Massif with its granitic rocks (Kuhlemann & Kempf 2002) (Fig. 1). The thickness of the sediments in the NAFB ranges from a few tens of metres in the north to 4000 m close to the Alps (Lemcke 1988). Analysis of the Sr isotopic composition of fossil biogenic material is a well-established method for age de- termination. It is based on both the conservative behaviour of the 87 Sr/ 86 Sr ratio in the ocean and its long-term trend throughout the Phanerozoic (McArthur et al. 2012). Sr has a long ocean residence time on the order of several million years, which ensures its homogenous distribution in the oceans (Palmer & Edmond 1989). Over the course of the Phanerozoic, the 87 Sr/ 86 Sr ratio in seawater changed as a consequence of fluctuations in weathering intensities re- sulting from orogenies, climate change (Qing et al. 1998) and increased hydrothermal input during phases of conti- nental break-up (Palmer & Edmond 1989). In addition, variations in salinity levels – in marginal marine environ- ments, for example – can result in local offsets in the 87 Sr/ 86 Sr signal (e.g. Ingram & Sloan 1992, Bryant et al. 1995, Reinhardt et al. 2003). 143 DOI 10.3140/bull.geosci.1659