Genesis of hummocks found in tunnel valleys: an example from Hörda, southern Sweden

Abstract

In the south-central sector of the former Fennoscandian Ice Sheet, imprints of the sub-glacial hydrological system are present as “glaciofluvial corridors,” formed by glacial meltwater at the ice–bed interface during the Bølling–Allerød warm period. Many of these are interpreted as tunnel valleys and are commonly characterized by hummocks on their valley floors. Contemporary ice sheets produce increased amounts of meltwater as a consequence of global warming, and occasionally it is observed that meltwater is suddenly released from supra- and subglacial lakes, suggesting a highly dynamic subglacial hydraulic system. Studies of the imprints and deposits from such systems on formerly glaciated terrain can expand our knowledge of ice-sheet response to increased meltwater production. Here, we study sediments exposed in two hummocks within the tunnel valley at Hörda, south Sweden. One of the investigated hummocks is superposed by a small esker. This hummock consists of a diamict interpreted as a subglacial traction till, observed to be overlain by esker sediment. A second hummock displays deformed sediment at its base, which is glaciotectonically intercalated with above-lying diamict, a sub-glacial traction till. The sub-till sediments, interpreted as proglacial outwash, were deformed by overriding ice. The sediment was dated using optically stimulated luminescence (OSL), inferring a late MIS 4 or early MIS 3 age, congruent with other observations of sub-till sediments in south Sweden. The investigated hummocks on the floor of the Hörda tunnel valley are interpreted to have been formed by sub-glacial fluvial erosion simultaneous with tunnel valley formation, most probably during the latest deglaciation of the area.

Keywords:

• Tunnel valley
• glaciofluvial corridor
• Subglacial meltwater corridor
• hummock
• glaciofluvial
• meltwater
• Subglacial
• esker

Acknowledgments

We acknowledge the Geological Survey of Sweden (swe. Sveriges geologiska undersökning) and the Swedish Society for Anthropology and Geography (SSAG) for funding this research. Moreover, we are grateful to the reviewers, Per Möller and Kärstin Malmberg-Persson, for valuable comments that indeed enhanced this work. Finally, thanks go to land owner Birger Karlsson and excavator driver Oskar Johansson.