Megagrooves and streamlined bedrock in NW Scotland: The role of ice streams in landscape evolution

Abstract

New multibeam bathymetry data, onshore high-resolution elevation data (NEXTMap) and fieldwork in the Ullapool area of NW Scotland reveal large-scale megagrooves and streamlined bedrock forms in a well-defined ~20-km wide zone. The landsystem is typical of a coherent flow corridor within a grounded ice sheet on bedrockdominated terrain. We describe the morphology of the large-scale features, discuss their likely formation, and consider the wider implications for ice-sheet dynamics. Based on the strongly convergent bedform distribution, the presence of megagrooves and highly elongate bedrock forms, we interpret the erosional landscape to be the signature of a fast-flowing tributary that once fed the The Minch palaeo-ice stream -a major artery of the last British-Irish ice sheet. The exact genesis of bedrock megagrooves remains uncertain, although focused subglacial abrasion is likely to have carved most of the shallow, strongly parallel, features; whilst glacial meltwater may have carved or modified others. Bedform morphometry is used to discriminate zones reflecting the degree of glacial streamlining (elongation ratios <5:1 or >5:1). We interpret these zones to represent the transition from potentially cold-based slow ice-sheet flow to warm-based fast flow. Based on these results, and the presence of ribbed moraines, we suggest a bedform continuum model for onset zones of palaeoice streams on rigid beds. Rapid spatial bedform evolution is suggested to reflect an increase in subglacial erosive power that may be diagnostic of palaeo-ice-sheet thermal boundaries (i.e. from cold-to warm-based), and is also consistent with the expected downstream increase in ice velocity within an ice-stream onset zone. Finally, this study speculates on the role played by basal meltwater in ice-stream initiation and the role of ice streams and their tributaries in landscape evolution.

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

“…Importantly, these and other data have been used to develop a model of bedform variability under Antarctic palaeoice streams (Wellner et al, 2001), whereby small features eroded into bedrock progressively evolve downflow into more elongate sedimentary bedforms. In this model, and in similar studies of ice stream landsystems (Wellner et al, 2001(Wellner et al, , 2006Mosola and Anderson, 2006;Bradwell et al, 2008;Ottesen et al, 2008), changes in subglacial lithology were proposed as the main control on ice dynamics, and hence on the along-flow geomorphic progression. However despite conceptual advances associated with such models, the majority of Antarctic studies used only limited swath bathymetric coverage and often simplified observations as single 'snapshots' in time, impeding more detailed geomorphic interpretations of entire palaeo-ice stream systems.…”

“…(4) Lastly, some northern hemisphere palaeo-ice streams exhibit strong substrate controls on flow (e.g. Bradwell et al, 2008;Ottesen et al, 2008), but many do not cross a prominent bedrock-sediment boundary like that on the West Antarctic shelf. Indeed, in some cases, subglacial geology had no apparent control on northern hemisphere ice streams (Stokes and Clark, 2003b).…”

Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion

“…Bradwell et al, 2008a;Bradwell, 2013;Eyles, 2012;Roberts and Long, 2005;Stokes and Clark, 2003), which cannot be explained by the 'deforming-bed model', which requires a deforming layer of till to facilitate fast ice sheet flow (e.g. Alley, 1991;Boulton and Hindmarsh, 1987).…”

“…Eyles, 2012). It is increasingly recognised that 'hard-bed landform assemblages' are varied and influenced by bedrock type and structure, the relation between dominant bedrock structure and ice-flow direction, as well as glaciological controls such as ice thickness, ice velocity and thermal regime (Bradwell et al, 2008a;Bradwell, 2013;Eyles, 2012;Gordon, 1981;Johansson et al, 2001a, b;Krabbendam and Bradwell, 2011;Krabbendam and Glasser, 2011;Phillips et al, 2010;Zumberge, 1954).…”

Lake bed geomorphology and sedimentary processes in glacial lake Windermere, UK

“…Elsewhere, multibeam bathymetry has been used to examine the distribution of gas hydrates within lacustrine environments, such as Lake Baikal in south-central Siberia (Naudts et al, 2012). Within the UK, although a number of multibeam surveys have been completed within the marine environment (Stoker et al, 2006;Bradwell et al, 2008), there are very few examples of high-resolution bathymetry surveys within lacustrine settings. In Loch Ness, multibeam data acquired by Guillot (2004) has been analysed in combination with geophysical records to interpret the glacial history of the Ness Basin (Turner et al, 2012) and in Loch Lomond a high-resolution multibeam survey has been completed by the British Geological Survey in 2008 (BGS).…”