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The origin of tunnel valleys

English title The origin of tunnel valleys
Applicant Schlunegger Fritz
Number 175555
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geologie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Geology
Start/End 01.03.2018 - 28.02.2022
Approved amount 882'027.00
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All Disciplines (2)

Discipline
Geology
Other disciplines of Earth Sciences

Keywords (6)

Quaternary chronology; Erosion; Subglacial Erosion; Sedimentology; Tunnel valleys; Quaternary landscape evolution

Lay Summary (German)

Lead
Verborgene Tunnel Täler im Mittelland
Lay summary

Der geologische Untergrund des Mittellandes wird von bis zu 200 Meter tiefen Furchen durchzogen, den sogenannten Tunnel Valleys. Weil diese Vertiefungen von Tonsteinen und Moränen während der letzten Vereisungen zugeschüttet wurden, sind Details über Verlauf und Geometrie dieser Erosionsfurchen weitgehend unbekannt. Zudem gibt es nur wenige Informationen über die Zeit, wann diese Furchen in den Untergrund des Mittellandes gelegt wurden. Erste Datierungen implizieren, dass sie vor etwa 270'000 bis 180'000 Jahren vor heute gebildet wurden. Analoge, bis zu 200 Meter tiefe Erosionsfurchen queren ebenfalls die Stadt Bern und führen zu Problemen beim Bauen. Ziel des Projektes ist es, die Ursache dieser Vertiefungen zu erforschen. Hier geht es insbesondere um die Klärung der Fragen, ob diese Vertiefungen durch Gletschererosion oder durch Schmelzwasser unter dem Gletscher zu Stande gekommen sind. Allenfalls könnte eine Tieferlegung des globalen Meeresspiegels zur Bildung dieser Schluchten rund um das Schweizer Mittelland geführt haben. Wir planen, diese Vertiefungen mit Hilfe gravimetrischer Untersuchungen zu kartieren, da diese zu einer Anomalie im Schwerefeld führen sollten. Des Weiteren werden wir die Verfüllungen dieser Vertiefungen durch zwei ungefähr 200 Meter tiefe Bohrungen abteufen, so dass wir die Verfüllungsgeschichte rekonstruieren können. In diesem Zusammenhang werden wir auch Datierungen durchführen. Bereits bestehende geologische Informationen früherer Untersuchungen deuten darauf hin, dass es sich bei den Verfüllungen um Seeablagerungen handeln könnte. Damit glich die Landschaft Berns vor ein paar Hunderttausenden von Jahren den Fjorden rund um Norwegen. Diese Hypothese gilt es zu testen und ihre Ursachen zu erkunden.

Direct link to Lay Summary Last update: 02.01.2018

Responsible applicant and co-applicants

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Project partner

Abstract

This project aims at understanding the origin of tunnel valleys that are c. 250 meters deeply carved in the Swiss Plateau and other regions surrounding the Alps. I plan to focus my research to the Bern area situated in Central Switzerland where the landscape has evolved under the influence of two paleo-glaciers with moisture sources on the northern and southern sides of the Alpine crest. The research plan is organized to address two major objectives that center on the timing and the mechanisms of tunnel valley formation. In particular, I plan to explore whether these erosional features were formed through fluvial processes, or by a combination of glacial carving and incision by overpressurized subglacial meltwater. As a closely related effort, I plan to answer the question of whether or not these valleys were formed in response to one or multiple glaciations, and when this phase of bedrock carving occurred. Since the tunnel valleys are all buried by hundreds of meters-thick lacustrine mudstones, glacial till and fluvial gravels, the search for the origin and the timing of tunnel valley formation can only be accomplished through indirect approaches. Here, I plan to infer the erosional mechanisms through reconstructions of the geometry of these tunnels because glaciers tend to form U-shaped erosional features, while fluvial erosion returns V-shaped incisions. Erosion by subglacial meltwalter is anticipated to yield up-sloping thalwegs and pothole bedrock geometries. I plan to disclose these morphometric properties through a detailed gravity survey (PhD project A), thereby benefitting from the large density contrast between the Molasse bedrock and the unconsolidated Quaternary deposits overlying the erosional features. In the same sense, the timing when the tunnel valleys formed can only be determined in an indirect way. I plan to address this point through reconstructions of the sedimentary architecture of the tunnel valley fill in a detailed temporal framework. While this approach yields minimum constraints on the timing when the tunnel valleys were formed, it will reasonably solve the question of whether bedrock erosion occurred during one or multiple glaciations. Reconstructions of the sedimentological architecture and the provenance of the tunnel valley fill (PhD project B) will be accomplished on the basis of two (and possibly three) new deep core drillings where the sedimentary fabric will be studied in detail. This analysis will be complemented by information from tens of shallow destructive drillings per year that are carried out in the region for geothermal and groundwater exploration purposes. As a third major task, I plan to date the tunnel valley fill through the analysis of the palynomorphic records encountered in the drilled cores (PhD project C) complemented with 230Th/234U dating of calcareous fossils (gastropods, and possibly charophytes) that will mainly be embedded in the lacustrine sediments. This project will address a yet unresolved problem because similar modern examples are not available. In particular I expect scientific progress in the fields of (i) geomorphology, leading to an improved understanding of erosional mechanisms possibly related to glacial conditions; (ii) chronology, enhancing our understanding of the timing of landscape evolution, and (iii) biostratigraphy and ecology, where the planned tasks will improve our understanding of past paleo-circulation patterns over the Central European Alps and the vegetation history surrounding the Bern area and beyond. As a broader implication, a better understanding of the origin and the timing of tunnel valleys will have implications on site assignments for future nuclear waste disposals, and it will offer a basis for a better prediction of possible wave amplifications during future earthquakes.
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