Morphological properties of tunnel valleys of the southern sector of the Laurentide Ice Sheet and implications for their formation

Abstract

Abstract. Tunnel valleys have been widely reported on the bed of former ice sheets and are considered an important expression of subglacial meltwater drainage. Although known to have been cut by erosive meltwater flow, the water source and development of channels has been widely debated; ranging between outburst flood events through to gradually occurring channel propagation. We have mapped and analysed the spatial pattern and morphometry of tunnel valleys and associated glacial landforms along the southern sector of the former Laurentide Ice Sheet from high-resolution digital elevation models. Around 2000 tunnel valleys have been mapped, revealing an organised pattern of sub-parallel, semi-regularly spaced valleys that form in distinctive clusters. The tunnel valleys are typically < 20 km long, and 0.5-3 km wide, although their width varies considerably down-valley. They preferentially terminate at moraines, which suggests that formation is time dependent; while we also observe some tunnel valleys that have grown headwards out of hill-hole pairs. Analysis of cross-cutting relationships between tunnel valleys, moraines and outwash fans permits reconstruction of channel development in relation to the retreating ice margin. This palaeo-drainage reconstruction demonstrates incremental growth of most valleys, with some used repeatedly or for long periods, during deglaciation, while others were abandoned shortly after their formation. Our data and interpretation support gradual (rather than a single-event) formation of most tunnel valleys with secondary contributions from flood drainage of subglacial and or supraglacially stored water down individual tunnel valleys. The distribution and morphology of tunnel valleys is shown to be sensitive to regional factors such as basal thermal regime, ice and bed topography, timing and climate.

Subglacial drainage patterns of Devon Island, Canada: detailed comparison of rivers and subglacial meltwater channels

“…The systems identified by Dyke (1999) as subglacial meltwater channels on Devon Island are approx. 10-20 m wide and 3-6 m deep, which is consistent with subglacial channels observed elsewhere, and 1 to 2 orders of magnitude smaller than tunnel valleys (e.g., Cofaigh, 1996;Kehew et al, 2012;Livingstone and Clark, 2016). Although subglacial channels have been described in detail in the field in northern Europe (e.g., Kleman, 1992;Clark et al, 2004;Piotrowski et al, 2006), the Antarctica Dry Valleys (e.g., Denton et al, 1984;Sugden et al, 1991), Canada (e.g., Kor et al, 1991;Beaney and Shaw, 2000;Shaw, 2002), and the United States (e.g., Walder and Hallet, 1979;Booth and Hallet, 1993), the number of studies addressing their identification from other systems from remote sensing data is limited (Greenwood et al, 2007).…”

“…At lower resolution, in addition, differences among channel direction and local topographic gradients can also be indicative of subglacial erosion, as long as ice erosion rate by sliding is lower than meltwater erosion rate (e.g., Weertman, 1972;Shreve, 1972;Paterson, 1994). Observations of channels incised oblique to topographic gradients are common in the literature (e.g., Sissons, 1961;Walder and Hallet, 1979;Sugden et al, 1991;Livingstone et al, 2017). Quantifying and measuring these deviations in a set of subglacial channels to establish a quantitative base for channel categorization has, however, not been done.…”

Examining the geometry, age and genesis of buried Quaternary valley systems in the Midland Valley of Scotland, UK

“…It has been suggested that buried valleys are rare outside areas underlain by poorly consolidated Mesozoic and Cenozoic sediments (Huuse & Lykke-Andersen 2000). However, detailed analyses undertaken as part of this study and others (Lee et al 2015;Livingstone & Clark 2016) have shown this not to be the case. Phillips et al (2010) identify the link between bedrock lithology and the velocity and/or location of faster flowing zones in the overriding ice streams.…”

“…Kehew et al (2012) recognize four criteria for identifying tunnel valleys: (i) that they are parallel to ice flow; (ii) they have an undulating convex upward long profile; (iii) they terminate close to or near former ice margins and they are associated with eskers and/or other types of subglacial landforms. This is an idealized list and studies such as those of van der Vegt et al (2012) and Livingstone & Clark (2016) rely more on the morphology of the valleys themselves. Livingstone & Clark (2016) suggest that tunnel valleys often startandendabruptlyandtheup-glacierendoftunnelvalley tends to be rounded.…”

“…This is an idealized list and studies such as those of van der Vegt et al (2012) and Livingstone & Clark (2016) rely more on the morphology of the valleys themselves. Livingstone & Clark (2016) suggest that tunnel valleys often startandendabruptlyandtheup-glacierendoftunnelvalley tends to be rounded. In the Midland Valley of Scotland, the Kelvin and Ochils palaeo-valleys ( Fig.…”