LINKS TO DANISH PAGES > Baggrund Kortlęgningen Resultater Kortlagte dale Download Links

English Version

 

 
  Project background
  Mapping procedures
  Mapped valleys
  Origin of the buried valleys
  Preferred orientations
  Buried valleys and groundwater
  General conclusions
 
Origin of the buried valleys

The buried valleys are predominantly formed as tunnel valleys underneath the glacier ice during the ice ages. With respect to morphology and dimensions the mapped buried valleys are comparable with open tunnel valleys found in the present-day Danish landscape (see Figure to the right).

The valleys were mainly eroded by meltwater supposed to have drained from subglacially stored reservoirs, probably behind a frozen margin. The water was most likely released in repeated jokulhlaups and flowed in relatively small channels on the floors of the tunnel valleys, which gradually became ice-filled with the lowering of the bed. Selective linear, glacial erosion is also believed to have contributed to the erosion. This, however, was most pronounced for the widest valleys and is supposed to have played a secondary role.

Tunnel valleys/buried valleys have a tendency to be re-used during the repeated cycles of glaciation producing separate valley generations and multiple cut-and-fill structures.

The mapped buried valleys vary in depth with the deepest features exceeding 350 m. The width is generally between 0.5 and 1.5 km, but widths of up to 4 km occur (see Figure, upper right). The lengths of the valleys are difficult to evaluate, because many of the surveyed areas are small. However, the length of some valleys in larger survey areas exceed 25–30 km. Striking features of the valleys are that they often terminate abruptly and that they are highly irregular with depressions and thresholds along the valley floors.

The buried valleys can be divided into different generations that were formed underneath glaciers during the Quaternary glaciations (see Figure, middle right). The valleys often cross-cut each other and sometimes have varying preferred orientations. A complicated pattern with three to five generations of valleys can often be distinguished in areas with high data resolution. If the ice flows were parallel or near-parallel to pre-existing valleys in the subsurface, these valleys would be liable to repeated erosion and re-filling producing a complicated cut-and-fill setting within the valleys. Multiple generations of valley erosion are therefore often found within the buried valleys themselves (see Figure, bottom right). The valley architecture is normally complex due to this cut–and–fill history, but also due to glaciotectonic disturbances. This complexity can be observed in both seismic data and TEM data. The complex fill is also often indicated by borehole data that show strong lithological variations over short distances.

The valley fill is deposited in sub-glacial and pro-glacial environments. Statistical analyses of borehole data in all the mapped valleys show that by far the most common infill sediment types are tills and meltwater deposits. In this group, 52% are coarse meltwater deposits (sand and gravel), 30% are clay-rich till and 18% are fine-grained meltwater deposits (silt and clay).

The buried tunnel valleys are isolated features which are not interconnected in anastomosing systems. It is unclear whether all valleys in each generation were formed simultaneously or if the individual valleys were formed separately. It is however evident, that many tunnel valleys became ice-filled by creeping ice following the erosion of the bed. Steady subglacial meltwater flow or recurring meltwater releases may continuously have eroded deeper into the substratum followed by ice creep closure of the meltwater channels during periods of decreasing meltwater pressure.

It is difficult to determine the ages of the buried valleys. The infill sediments can be dated in some cases, but this is not necessarily the time of valley erosion. However, based on the time gap between the age of the eroded sediments and the age of the infill sediments, some of the buried valleys can be shown to have formed during the Weichselian, the Saalian and the Elsterian glacial stages. Some of the valleys were probably formed during older glacial stages.
 

Width of open tunnel valleys vs. buried valleys (From Jørgensen & Sandersen 2006)

Three buried valley generations NW of the city of Aarhus (From Jørgensen & Sandersen 2006)

Selected resistivity maps and a cross section of a selected buried valley at Boulstrup-Hundslund (From Jørensen & Sandersen 2006)