An Ancient Giant Post-Glacial Landslide Hidden in a Danish Tunnel Valley

An Ancient Giant Post-Glacial Landslide in a Danish Tunnel Valley
Williams, J.M.1*, Winther, M.1,2, Høyer, A.-S.1, Møller, I.1, Damsgaard, A.1, and Svennevig, K.1
*[email protected]
1Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark
2Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
Denmark is a low-relief landscape generally considered stable since deglaciation despite increasing national focus on modern slope instability (Svennevig et al., 2020). Here, we present the first geomorphic and subsurface characterization of a giant paraglacial paleo-landslide on the southern flank of the Vejle Ådal tunnel valley, informally named the Nørre Vilstrup Landslide, in eastern Jutland. Geomorphic mapping using high-resolution (0.4 m) national LiDAR reveals a ~11.1 km² lobate deformation complex that protrudes ~0.5 km northward into the valley and appears to deform the valley floor. The feature is defined by (i) a ~1 km² proximal depression, (ii) a main lobate body with ~12 valley-parallel deformation fabrics expressed as ~east-west ridges and troughs, and (iii) a distal band of convex “stands” that are buttressed against the northern valley wall and mirror the landslide morphology across the valley. This morphology is consistent with large-scale translation and frontal toe thrusting.
Transient Electromagnetic (TEM) surveys combined with borehole stratigraphy from the national Danish borehole database provide subsurface characterization. Preliminary results indicate that the subsurface clay and sand units appear disrupted in a pattern that mirrors the lobate surface morphology. Ongoing interpretation evaluates whether the subsurface structures support the landslide hypothesis, specifically examining evidence for thrusting and potential upward intrusion of clay during landsliding beneath the proximal depression.
Remnant curvilinear lineations preserved on the plateau and within the complex indicate inherited subglacial morphology, possibly formed by meltwater erosion (Adamczyk et al., 2022) or subglacial landsliding (Clark & Livingstone, 2018). The landslide likely developed within a dynamically evolving post-/paraglacial landscape shaped by deglacial unloading, groundwater reorganization, and fluvial incision along the tunnel valley margin.
We reconstruct the geomorphic evolution in four relative time steps: (1) subglacial excavation of the Vejle Ådal tunnel valley beneath the Scandinavian Ice Sheet. Curvilinear lineations preserved on the plateau predate the landslide complex; (2) postglacial drainage reorganization and gully incision along the valley flanks; (3) large-scale translational failure that displaced the southern valley flank northward, forming the lobate deformation complex, proximal depression, and distal toe thrust; and (4) post-failure fluvial reworking modified the landslide toe and reorganized valley-floor drainage.
Possible alternate formative processes include subglacial meltwater erosion capable of producing complex valley-side morphologies governed by hydraulic potential gradients beneath ice sheets (Hooke & Jennings, 2006; Kehew et al., 2012) and paraglacial fluvial incision and sediment remobilization during early post-glacial landscape adjustment that can modify tunnel valley margins and generate lobate depositional complexes along valley sides (Ballantyne, 2002). However, none of these hypotheses explain the overall morphology described in the study area.
Our working hypothesis is that the Nørre Vilstrup structure is a landslide and represents one of the largest paleo-landslides in Denmark and suggests that deep (~100 m scale) mass movements may play a more significant role in the paraglacial landscape adjustment than previously recognized. Similar mass-wasting processes have been documented within buried tunnel valleys of the central North Sea (Kirkham et al., 2024) and many other potential paleo-landslides are present in the Danish landscape.
References:
Adamczyk, A., Wysota, W., & Piotrowski, J. A. (2022). Inventory of glacial curvilineations (GCLs) at the southern periphery of the last Scandinavian Ice Sheet. Geomorphology, 400, 108094. https://doi.org/10.1016/j.geomorph.2021.108094
Ballantyne, C. K. (2002). Paraglacial geomorphology: Quaternary Science Reviews, v. 21. Doi, 10, S02773791.
Clark, C. D., & Livingstone, S. J. (2018). Glacial curvilineations found along the southern sector of the Laurentide Ice sheet and a hypothesis of formation involving subglacial slope failure in tunnel valleys and subglacial lakes. Earth Surface Processes and Landforms, 43(7), 1518–1528. https://doi.org/10.1002/esp.4324
Hooke, R. L., & Jennings, C. E. (2006). On the formation of the tunnel valleys of the southern Laurentide ice sheet. Quaternary Science Reviews, 25(11–12), 1364–1372.
Kehew, A. E., Piotrowski, J. A., & Jørgensen, F. (2012). Tunnel valleys: Concepts and controversies—A review. Earth-Science Reviews, 113(1–2), 33–58.
Kirkham, J. D., Hogan, K. A., Larter, R. D., Arnold, N. S., Ely, J. C., Clark, C. D., Self, E., Games, K., Huuse, M., & Stewart, M. A. (2024). Tunnel valley formation beneath deglaciating mid-latitude ice sheets: Observations and modelling. Quaternary Science Reviews, 323, 107680.
Svennevig, K., Luetzenburg, G., Keiding, M. K., & Pedersen, S. A. S. (2020). Preliminary landslide mapping in Denmark indicates an underestimated geohazard. GEUS Bulletin. https://geusbulletin.org/index.php/geusb/article/view/5302