Isochron-burial dating of glaciofluvial deposits: First results from the Swiss Alps

In this study, we use isochron-burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic Be-10 and Al-26 in individual clasts from a single stratigraphic horizon can be used to calculate an isochron-burial age based on an assumed initial ratio and the measured Al-26/Be-10 ratio. We suggest that, owing to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20 000 Be-10 atoms/g and <150 000 Al-26 atoms/g. Using an Al-26/Be-10 ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron-burial age of 1.5 +/- 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 and 1.3 Ma. Our results indicate that isochron-burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations. Copyright (c) 2017 John Wiley & Sons, Ltd.