Terminal glacial overdeepenings: Patterns of erosion, infilling and new constraints on the glaciation history of Northern Switzerland

Abstract

Glacially overdeepened basins are a common landform of subglacial erosion. However, the controlling erosional–depositional processes and their age remain poorly understood on a global scale. Terminal overdeepenings near the former glacier margins are critical for the understanding of subglacial processes and their development over time. This study examines the geomorphology and sedimentology of buried terminal overdeepenings eroded below the Rhein Glacier and adjacent lobes in the distal northern foreland of the European Alps. The evolution of erosion and infilling in the overdeepened troughs over time is investigated using high-quality drill cores (∼1463 m of core in total) that were logged for lithofacies, petrophysical, geotechnical and compositional properties. The drill data is integrated with 2D-reflection seismics (∼41 km in total) and supplementary subsurface data. This extensive dataset reveals that the studied overdeepened basins include twelve typical facies associations (partially emplaced in characteristic sequences) and characteristic architectural elements. These categories serve as effective tools to reduce the high facies variability and facilitate easier comparison and correlation of the valley fill. Our analysis shows that the formation of terminal overdeepenings on soft sedimentary bedrock (Molasse) is the result of a combination of erosional processes. Subglacial water erosion and evacuation are the dominant processes and active during periods of glacier–bed decoupling and flushing. Direct subglacial erosion occurs during glacier–bed coupling and is documented by bedrock glacitectonites. The valley fill architecture shows that the studied overdeepenings typically undergo a multiphase evolution, with several phases of overdeepening erosion, deposition and partial re-erosion (or re-activation). The combined dataset (including geochronological data) suggests that the overdeepenings were eroded during many, if not all, extensive glaciations during the Middle–Late Pleistocene that reached the distal foreland. These are findings relevant for the large number of overdeepenings known from the Northern Alpine foreland and overdeepened features worldwide. They corroborate the importance of overdeepenings as archives for the paleoenvironmental change and landscape evolution during the Quaternary.