Pleistocene tunnel valleys in northern Germany - Influence of the underlying geology on tunnel-valley formation and reactivation
Country / Region: Germany
Begin of project: January 1, 2021
End of project: December 31, 2025
Status of project: March 4, 2025
Glacigenic erosional processes are considered as highly relevant for the long-term safety of a repository. During the ice ages, the meltwater incised deep subglacial channels (“tunnel valleys”) beneath the ice sheets. In northern Germany, such tunnel valleys attain great depth, but are hardly visible today. The formation and evolution of tunnel valleys is an import research topic for the long-term safety for radioactive waste disposal. The aim of our research is to improve the potential to predict future tunnel-valley formation. One focus is the impact of the subsurface geology on tunnel-valley formation and the reactivation of infilled tunnel valleys.
During the ice ages, subglacial tunnel valleys were incised in many areas of northern Germany. It has to be expected that tunnel-valley formation will recur during potential future ice ages. The deep erosion during the formation of such tunnel valleys can theoretically have an impact on the integrity of the geological barrier of a repository. The Site Selection Act (StandAG) defines a minimum depth of an isolation rock zone below the largest expected depth of exogenic processes, which could have a direct or indirect negative impact on the integrity of the isolation rock zone. Tunnel valleys are specified here indirectly as “intensive erosion caused by ice ages” (StandAG Section 23 (5) No. 3).
The formation of tunnel valleys from meltwater discharge under high pressure is one of the deepest erosive processes. In the zones in northern Germany affected by Pleistocene glaciations, tunnel valleys with depths of 100 to 300 m are present over wide areas, and some extend to a maximum depth of almost 600 m. Despite many years of research, there are still many open questions concerning the formation and development of tunnel valleys and their infills. Tunnel valleys frequently have complex histories with repeated phases of erosion and deposition, which can extend over several ice ages.
Objectives
This project looks into the formation and distribution of tunnel valleys in northern Germany. The aim of the project is to improve our understanding of the controlling factors for the formation and re-activation of tunnel valleys, with the ultimate aim of improving the predictability of future tunnel-valley formation.
The following key issues will be studied in this project:
- What is the impact of the geological structure (e.g. lithology, faults, salt structures) on the formation of tunnel valleys, and their properties (e.g. distribution, geometry, dimensions)?
- Which factors control the incision of tunnel valleys and their reactivation during repeated glaciations?
- How did the tunnel valleys in northern Germany and their infills develop over time?
- Is it possible to predict the formation of tunnel valleys during future ice ages?
Results and publications
To improve the understanding of the impact of the subsurface on tunnel-valley formation, we compared the distribution, orientation and depth of the tunnel valleys to the geological structural elements of the North German Basin. Especially faults and salt structures have previously been related to enhanced erosion. However, our analysis reveals that faults and salt structures are no consistent indicators for tunnel-valley incision. The results are published in the international scientific journal “Boreas”:
https://onlinelibrary.wiley.com/doi/10.1111/bor.12694
In the project „SUCHTIEFE“, which was assigned by the Federal Company for Radioactive Waste Disposal (BGE), we prepared a new overview map on the distribution of tunnel valleys in northern Germany. The regional variation of the maximum depth of erosion was mapped as a “depth-zone map”. These depth zones may be used to deduce an additional regional safety depth for the minimum depth of a repository. The results are available in a final report:
Tiefe, Verbreitung und Bedeutung für die Mindesttiefe eines Endlagers (2023) (in German only)
Left: Overview map, showing the distribution of the subglacial tunnel valleys in northern Germany. Today, most tunnel valleys are completely infilled by sediment and no longer visible at the surface. Right: Simplified map of the maximum tunnel-valley depths. Tunnel-valley courses are restricted to their thalwegs clustered into five depth zones based on their maximum depths of erosion. The depth zones can aid to estimate potential future depths of erosion
Source: BGR
We also published the overview map on tunnel valley distribution, the depth-zone map and their connection to the regional geology of northern Germany in “E&G Quaternary Science Journal” the journal of the German Quaternary Association (DEUQUA):
Besides tunnel-valley formation, further glacigenic processes should be considered in the site-selection process. We published a summary on the advance and decay of ice sheets and glaciers, tunnel-valley formation and glacitectonic deformation in a short report:
Conference contributions
The results of the project are regularly presented at German and international conferences. In December 2021 BGR organised a workshop on subglacial erosion and its relevance for the long-term safety of radioactive waste repositories in cooperation with the Federal Company for Radioactive Waste Disposal (BGE). As a follow-up on the workshop, a special issue of „E&G Quaternary Science Journal“ was published, comprising several contributions from the workshop:
Rinnen-Workshop 2021 – Resümee (in German only)