Buried and open tunnel valleys in Denmark—erosion beneath multiple ice sheets

Abstract

Tunnel valleys are large, elongate and irregular depressions cut beneath the margin of former ice sheets. They are generally believed to play a substantial role for the entire hydraulic system beneath ice sheets and thus also for ice sheet behaviour. Their origin, however, remains poorly understood. Examination of newly collected Danish hydrogeophysical and lithological data has revealed numerous systems of buried valleys 50–350 m deep and 0.5–4 km wide. These valleys are similar to the open tunnel valleys found in the present-day landscape. Infills comprise a variety of sediments often with subglacial clay till and meltwater deposits. The incised substratum consists of limestone, heavy Paleogene clay, sand and silt. The tunnel valleys have preferred orientations and can be divided into several generations, each ascribed individual ice advances or ice sheets occurring during multiple glaciations. The processes forming the valleys appear to prefer pre-existing (open and buried) valleys for the renewed erosion. Thus, old subglacial erosion pathways have been re-used several times, and some of the present tunnel valleys may have been established in the middle or early Quaternary or even earlier. This valley-re-use effect causes apparently anastomosed valley systems to emerge in the data images. 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 jökulhlaups 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.

Introduction

Tunnel valleys are large elongated depressions of subglacial origin cut into both unconsolidated sediments and bedrock (e.g. ÓCofaigh, 1996). They occur with undulating longitudinal profiles and contain sections that slope upwards, sometimes to be terminated at the apex of an outwash fan. They are generally believed to have served as major subglacial drainage pathways for large volumes of meltwater, and are thus supposed to play a substantial role for the entire hydraulic system beneath glaciers. Because glacier behaviour largely reflects the subglacial hydraulic regime, the understanding of how tunnel valleys form and act is of crucial importance for the reconstruction and understanding of former ice sheets.

Tunnel valleys have been discussed intensely for more than a century, and agreement as to how they were formed remains poor. However, recent years have seen new research into this area owing both to a growing interest in subglacial processes and to the growing sophistication of investigation methods (e.g. geophysical and computational). Tunnel valleys are found in the present-day landscape and in the subsurface where they were created by the European Pleistocene ice sheets (e.g. Woodland, 1970; Ehlers et al., 1984; Ehlers and Linke, 1989; Huuse and Lykke-Andersen, 2000) and by the Laurentide Ice Sheet of North America (e.g. Wright, 1973; Mullins and Hinchey, 1989; Patterson, 1994; Clayton et al., 1999). However, tunnel valleys have also been formed elsewhere in the world, for instance beneath Paleozoic ice sheets as reported by e.g. Visser (1988), Hirst et al. (2002), Eyles and de Broekert (2001) and Ghienne and Deynoux (1998). The difficulty of investigating tunnel valley formation processes lies mainly in the inaccessibility of the subglacial environment, but also in the fact that the conditions favouring these processes seem to be restricted under the present climatic conditions. The discussion of the tunnel valley formation processes mainly focuses on whether the valleys were eroded by subglacial meltwater or by a combination of meltwater and glacier ice erosion, and whether the eroding meltwater was discharged in catastrophic outbursts or by steady-state flows (cf. ÓCofaigh, 1996). Strong evidence backs these theories, and it is likely that there is no universal tunnel valley formation mode and that the valleys have a polygenetic origin (e.g. Huuse and Lykke-Andersen, 2000).

In this paper, we show the characteristics of buried valleys in Denmark (Fig. 1) by using newly collected hydrogeophysical data combined with lithological data. These are compared with the open tunnel valleys as described by other authors (e.g. Ussing, 1903, Ussing, 1907; Milthers, 1935b, Milthers, 1948; Nordmann, 1958; Smed, 1962, Smed, 1998; Nielsen, 1967; Sjørring, 1979). We find that the buried valleys and the open tunnel valleys share the same characteristics and most likely also the same origin. Tunnel valley genesis is discussed and a model for their origin is proposed.