3D morphology of a glacially overdeepened trough controlled by underlying bedrock geology

Lukas Gegg,Gaudenz Deplazes,Lorenz Keller,Herfried Madritsch,Thomas Spillmann,Flavio S Anselmetti,Marius W Buechi

doi:10.1016/j.geomorph.2021.107950

Abstract

Subglacial overdeepenings are common elements of mountain forelands and have considerable implications for human infrastructure. Yet, the processes of overdeepening by subglacial erosion and especially the role of bedrock geology are poorly understood. We present a case study of the Gebenstorf-Stilli Trough in northern Switzerland, a foreland overdeepening with a regionally unique, complex underlying bedrock geology: in contrast to other Swiss foreland overdeepenings, it is incised not only into Cenozoic Molasse deposits, but also into the underlying Mesozoic bedrock. In order to constrain the trough morphology in 3D, it was targeted with scientific boreholes as well as with seismic measurements acquired through analysis of surface waves. Our results reveal an unexpectedly complex trough morphology that appears to be closely related to the bedrock geology. Two sub-basins are incised into calcareous marls and Molasse deposits, and are separated by a distinct ridge of Jurassic limestones, indicating strong lithological control on erosional efficiency. We infer generally relatively low glacial erosion efficiency sensu stricto (i.e. quarrying and abrasion) and suggest that the glacier's basal drainage system may have been the main driver of subglacial erosion of the Gebenstorf-Stilli Trough.

Highlights

Subglacial overdeepenings, i.e. closed troughs eroded below the fluvial base level, are common elements of formerly glaciated mountain forelands (Cook and Swift, 2012)
The southern section is embedded in the Folded Jura (FJ), whereas the middle section is mostly incised into Molasse deposits, and the northern section into the Mesozoic of the Tabular Jura (TJ, Fig. 2A)
These morphologies are interpreted as a result of geological as well as paleo-glaciological differences: while Alpine-type overdeepenings are carved into zones of weakness by thick ice streams in the mountain valleys, foreland-type overdeepenings occur in the generally rather poorly lithified Molasse deposits independently from structural weak zones (Preusser et al, 2010; Dürst Stucki and Schlunegger, 2013)

Summary

Introduction

Subglacial overdeepenings, i.e. closed troughs eroded below the fluvial base level, are common elements of formerly glaciated mountain forelands (Cook and Swift, 2012). In the northern European Alpine example, they underlie ~10% of the land surface (Dürst Stucki and Schlunegger, 2013) Despite their significance for, among others, construction projects, groundwater and radioactive waste disposal (Preusser et al, 2010; Stumm, 2010), the understanding of overdeepenings in terms of formation processes and controlling factors is limited and subject to debate (Cook and Swift, 2012; Alley et al, 2019). This applies especially to the influence of bedrock geology on the efficiency of subglacial erosion (Goudie, 2016). Most of these investigations focused on glacial erosion in nonoverdeepened settings

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Conclusion