Landscape evolution of the northern Alpine Foreland: constructing a temporal framework for early to middle Pleistocene glaciations

Abstract

The Deckenschotter deposits are believed to represent the oldest Quaternary sediments in the Alpine Foreland and are thus a geoarchive, documenting paleoenvironmental changes during the Quaternary. Lithostratigraphic positions of the Deckenschotter deposits in Switzerland have been extensively studied. However, compared to late Quaternary glaciations, the timing of these accumulations is poorly understood. The investigations related to the dissertation shed light on this timing and hence on the landscape evolution of the northern Alpine Foreland. The study area is located in the northern Alpine Foreland of Switzerland where seven sites of the Höhere (“higher”; HDS) Deckenschotter, Tiefere (“lower”; TDS) Deckenschotter and Hochterrasse (“higher terrace”) were investigated; from east to west, these sites are Irchel (three sites: Wilemer, Steig and Hütz), Stadlerberg, Siglistorf, Rechberg, Ängi, Mandach and Pratteln (Hohle Gasse) (Fig. 1). From each site either sediment samples were collected for dating with cosmogenic 10Be and/or 36Cl depth-profile dating or clasts with quartz-rich lithologies were sampled for isochron burial dating with 10Be and 26Al. At Irchel Steig, the same outcrop was dated using both methods. In addition, detailed investigations of clast fabrics, petrographic compositions and clast morphometries enable the identification of sediment source areas and the interpretation of their transport mechanisms and depositional environments. Finally, coupling the reconstructed chronologies with interpolated vertical height differences between the bedrock underlying the Deckenschotter deposits and the bedrock beneath the modern Rhine River allows for the estimation of post-depositional bedrock incision rates. Analyses show that the HDS at Wilemer Irchel (Claude et al., 2017c), Stadlerberg (Claude et al., 2017a) and Siglistorf (Akçar et al., 2017) accumulated approximately around 2 Ma ago. Clasts were eroded from the northern Central Alps and brought to the foreland by paleoglaciers (Fig. 2a). In the foreland, glaciers also eroded conglomerates of the Miocene Molasse and transport was furthermore provided by glacial outwash. The influence of mainly the Linth paleoglacier is recognized in the provenance of the sediments. Clasts at the sites Wilemer Irchel, Stadlerberg and Siglistorf were deposited in a glacier-proximal environment. At that time, the Alpine Rhine, originating in the central Eastern Alps, was draining through Lake Constance into the Danube River and finally eastwards into the Black Sea (Fig. 2a). Estimated longterm bedrock incision rates are on the order of 130 ± 60– 150 ± 40 m Ma−1 for the time interval from 2 Ma until the