Richard I. Waller

Glacigenic clast fabrics: genetic fingerprint or wishful thinking?

The interpretation of glacigenic diamictons is a subjective process, for which quantitative support is frequently sought from parameters such as clast shape and fabric. It has been widely suggested that different glacigenic diamicton facies possess distinct clast-fabric signatures. This paper examines this concept using a data set of 111 clast fabrics, and a synthesis of published results. Eigenvalues are calculated and compared for a variety sedimentary facies. It is concluded that clast fabric alone is not able to discriminate between different glacigenic facies, and it is argued that clast fabric offers little quantitative support in the interpretation of glacigenic sediments. It is suggested, therefore, that although clast fabric may continue to have a role as an indicator of relative strain at specific sites, its use in the discrimination of glacigenic facies is limited. Consequently, we should be much more selective in undertaking such analyses in the future. Copyright

Stratigraphy and glaciotectonic structures of permafrost deformed beneath the northwest margin of the Laurentide ice sheet, Tuktoyaktuk Coastlands, Canada

The upper 5-20 m of ice-rich permafrost at three sites overridden by the northwest margin of the Laurentide ice sheet in the Tuktoyaktuk Coastlands, western Arctic Canada, comprise massive ice beneath ice-rich diamicton or sandy silt. The diamicton and silt contain (1) truncated ice blocks up to 15 m long, (2) sand lenses and layers, (3) ice veins dipping at 20-30°, (4) ice lenses adjacent and parallel to sedimentary contacts, and (5) ice wedges. The massive ice is interpreted as intrasedimental or buried basal glacier ice, and the diamicton and silt as glacitectonite that has never thawed. Deformation of frozen ground was mainly ductile in character. Deformation was accompanied by sub-marginal erosion of permafrost, which formed an angular unconformity along the top of the massive ice and supplied ice clasts and sand bodies to the overlying glacitectonite. After deformation and erosion ceased, postglacial segregated ice and ice-wedge ice developed within the deformed permafrost.

The influence of basal processes on the dynamic behaviour of cold-based glaciers

Abstract There exists within glaciology a widely held assumption that basal sliding and bed deformation do not operate beneath cold-based ice and that their basal velocity is therefore zero, irrespective of bed conditions. Consequently, their ability to erode, entrain and transport sediment and thereby alter the landscape is assumed to be limited. Consequently, very little research has been focused towards describing and understanding the motion of cold-based ice-masses and our knowledge of their behaviour remains poor and the assumption of zero basal velocity, unchallenged. In this review paper, it is argued that this assumption is not universally applicable and that in certain circumstances, basal processes not only remain active at sub-freezing temperatures, but can significantly influence glacier motion. This is particularly the case in glaciers where sub-freezing basal thermal conditions coincide with the presence of fine grained, ice-rich subglacial sediments. Due to the lack of work undertaken on contemporary cold-based glaciers, much of the information used to support this argument is derived from field research in permafrost areas and on Quaternary glacial sediments, and from the laboratory testing of ice/sediment mixtures. It is concluded that the assumption of zero basal velocity beneath cold-based ice is overly simplistic and that in reality, the situation is likely to be much more complex. Work is therefore urgently required: firstly, to investigate the circumstances in which basal motion remains active at sub-freezing temperatures and to determine their influence on glacier motion and; secondly, to examine the likely extent of subglacial permafrost in both the contemporary and Quaternary glacial environment.

Discharge of debris from ice at the margin of the Greenland ice sheet

Sediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12–45 m 3 m −1 a −1 . This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glaciofluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

Glaciohydraulic supercooling: the process and its significance

Glaciohydraulic supercooling is a process that allows water at the base of a glacier to remain liquid at a temperature below its freezing point in response to the geometry of water flow and subglacial pressure. Supercooling, and subsequent freezing, of subglacial water has implications for glacier dynamics, sediment transfer and landform evolution, and an understanding of the process is important both for understanding modern glacial environments and for reconstructing glacial environments of the past. However, recent research on glaciohydraulic supercooling has raised controversy both about the significance of the process and about the way in which it has been applied within the discipline. In this paper, we review recent work on supercooling in glaciers, assess its significance to glaciology, geomorphology and Quaternary science, and identify key issues requiring further research in order to resolve some of the controversy surrounding the topic. We suggest that, while glaciohydraulic supercooling is a very significant process, its adoption as an explanation of some phenomena has been premature, and that further research is required to test its true significance both in modern settings and in the glacial geologic record.

Preservation of basal-ice sediment texture in ice-sheet moraines

Ice-sheet moraines near Kangerlussuaq in west Greenland inherit distinctive particle-size distributions from basal ice, although debris structures from the basal ice are commonly destroyed by deposition and resedimentation processes. The abundance of clay and silt in the ‘dispersed faciesa basal ice at the ice-sheet margin is clearly re#ected in the sedimentology of the ice-sheet moraine. Geographical variations in the texture or grain size of moraine sediments may thus re#ect geographical variations in basal ice. This o!ers a new approach to reconstructing the basal-ice characteristics, and hence the thermal and dynamic properties, of former ice sheets. ( 2000 Elsevier Science Ltd. All rights reserved.

Subglacial deformation at sub-freezing temperatures? Evidence from Hagafellsjökull-Eystri, Iceland

Abstract We report evidence of deformation at sub-freezing temperatures beneath Hagafellsjokull-Eystri, an Icelandic surge-type glacier. The bed of a piedmont lobe that advanced during the 1999 surge comprises deformed blocks of glacier ice set within frozen sediment. This material has also been injected through overlying ice to form a network of crevasse-squeeze ridges. This layer contains evidence for two phases of deformation under contrasting rheological conditions: (1) deformation under relatively warm conditions, when the blocks of glacier ice acted as competent clasts within an unfrozen deforming matrix and (2) subsequent deformation at sub-freezing temperatures when the ice blocks were attenuated into the surrounding frozen matrix along fractures and planar shears enriched with excess ice. This suggests that the basal thermal regime of the advancing ice margin changed from warm-based to cold-based during the surge event. The persistence and potential prevalence of subglacial sediment deformation at sub-freezing temperatures has fundamental implications for our understanding of the dynamic behaviour, sediment flux and geomorphic ability of cold-based glaciers.

Glacier advance, ice-marginal lakes and routing of meltwater and sediment: Russell Glacier, Greenland

The ice-sheet margin at Russell Glacier, West Greenland, advanced ∼7 m a −1 between 1968 and 1999. As the ice advanced over moraine ridges, small changes in position caused major changes in the routing of proglacial water and sediment. These included changes in the distribution of ice-marginal lakes, in the periodic drainage of ice-dammed lakes, in the routing and sediment content of meltwater draining into the proglacial zone, and in the release of sediment from the moraines by erosion and mass movements. Proglacial hydrology and sediment flux appear to be controlled not simply by glacier mass balance, but by evolving ice-marginal geomorphology, which must be accounted for in palaeoenvironmental interpretation of proglacial sediments.

Basal glacier ice and massive ground ice: Different scientists, same science?

Abstract Whilst glaciologists and permafrost researchers investigate ice bodies using similar techniques, there has been surprisingly little collaboration between the two communities. This paper examines the potential benefits of interdisciplinary research into the formation of basal ice beneath glaciers and the origin of massive ice in glaciated permafrost regions. Active collaboration in these areas has already improved our understanding of the formation of basal ice beneath cold-based glaciers, the critical role played by basal freezing in controlling the dynamic behaviour of stagnating ice streams and the significance of glacier–permafrost interactions at the margins of Pleistocene ice sheets. However, in order to promote future collaboration certain obstacles need to be overcome. The contrasting ice-classification schemes employed by glaciologists and permafrost scientists, for example, need to be unified in order to allow detailed comparisons of ice-rich sequences in both environments. This could, in turn, enable exciting research advances, most notably by facilitating the identification of preserved remnants of Pleistocene ice sheets within permafrost regions that provide a potentially invaluable and currently largely untapped source of palaeoglaciological information.

Glacier-permafrost interactions and glaciotectonic landform generation at the margin of the Leverett Glacier, West Greenland

Abstract This paper describes the key characteristics of a proglacial moraine complex at the Leverett Glacier, western Greenland. The presence of a large stream-cut exposure allowed the examination of its internal structure, as well as its surface geomorphology. It is composed of a variety of ice and sediment facies, including debris-poor ice, ice-rich diamicton and ice-rich gravel. These units are glaciotectonized, with the exposure featuring a major fault and associated drag fold, a planar, erosional unconformity, and a variety of small-scale folds. Various interpretations are considered, including the possibility that the sequence represents a buried basal ice layer. However, it is argued that the structural characteristics are best explained by a two-phase model involving ice advance and proglacial or ice-marginal compression, followed by overriding and subglacial deformation and erosion, tentatively related to ice advance after the Holocene Hypsithermal (c. 4900–3000 calendar years bp). The polygenetic origin of this ice-marginal, glaciotectonic landform contrasts with the majority of Arctic push-moraines, which are largely considered the result of proglacial deformation and the stacking of imbricate thrust sheets of frozen sediment. This contrast may reflect differences in the thickness and spatial continuity of permafrost within the glacier foreland, and adds to the range of ice-marginal landforms associated with glacier-permafrost interactions.