New insights into one of the oldest glacial deposits in the northern Alpine foreland (Höchsten, SW Germany)

Abstract

The present‐day landscape of the northern Alpine foreland is marked by the cumulated impact of weathering during interglacial, and of erosion and deposition during glacial periods of the Quaternary. Direct traces of the earliest phases of ice advance, as well as thorough studies thereof, exist only sporadically. Here, a succession of diamictic deposits, which has been interpreted as the infill of an Early Pleistocene overdeepened basin, is investigated with a combined sedimentological‐geotechnical approach including analysis via μCT scans, and standard tests of the water uptake, consistency, and compaction properties. The diamicts are exposed along a 4.5‐m‐deep profile, and are subdivided into a yellowish‐brown lower unit with a variable, carbonaceous, silty to sandy matrix, and a reddish‐brown upper unit that is free from carbonate and appears largely homogeneous. Although the lower unit is rather loose and surficially bioturbated, it contains microstructures indicative of subglacial deformation, which are lacking in the compact and clay‐rich upper unit. The lower part is interpreted as a secondary glacial deposit (i.e. it has been affected by limited sorting in water) that was overridden and sheared by a glacier briefly after deposition, and recently bioturbated. The upper part is less sorted, more massive and compact, and thus likely of a primary glacial nature. It is further characterized by a strong pedogenetic overprint typical of prolonged warm periods. This suggests that it is separated from the overlying glacifluvial gravel, which has an equivalent petrographic composition, by a full interglacial at least. Thus, the combination of geotechnical testing and CT‐based micromorphology offers a new, practical and cost‐effective approach to the characterization of glacially derived sediments.