Evolution of tunnel valleys – contrasting examples from western Poland (Scandinavian Ice Sheet)

Tunnel valleys are commonly found in beds of palaeo-ice sheets. They indicate subglacial meltwater pathways in near marginal environment. Their formation mechanisms are still debated, with hypotheses ranging from gradual, time-transgressive origin to catastrophic. The aim of the study is to contribute to the discussion by comparative analysis of tunnel valleys footprint that was formed during the deglaciation of Scandinavian Ice Sheet from its southernmost sector.

The context of the study area comprises quasi-regular set of tunnel valleys located in close proximity to anastomosing network of tunnel valleys. From the former pattern, two neighbouring tunnel valleys (eastern and western) located ca 7 km away were selected for detailed landform analysis, performed using a Digital Elevation Model (DEM) based on high-resolution LiDAR data.

Both tunnel valleys are ca 12-14 km long. The proximal parts of both valleys have similar width as well – ca 1 km, though the western tunnel valley gets much wider in the distal part, compared to the eastern one. The depth of incision of the western tunnel valley is smaller ( >20 m) compared to the eastern one ( >40 m). The eastern one ends with an extensive outwash fan, the other, western one, not – its southern (distal) part gets wider and shallower down-ice, with an array of landforms related to glacial meltwater flow. The western tunnel valley seems only half-developed, with its southern part much wider, shallower and less pronounced: the valley gets less sharply defined down-ice.  The distal part of the western valley contains an array of landforms formed under high energy turbulent flow, possibly evidence of subglacial flood: mega-scale current ripples (giant current ripples - several ridges with arcuate crests arranged more-less perpendicular to the tunnel valley axis), circular incision, scours/furrows, and potholes.

The composite sequence of landforms comprising the tunnel valleys suggest they were forming in highly dynamic environment and switching between steady-state conditions to catastrophic basal flooding events. Both tunnel valleys analysed here reveal similar evolution history to an extent - with a different ending.

This contribution presents the findings of an initial study, which will be continued and complemented with sediment lithofacies analysis.