Research on landform and sediment assemblages formed by glaciers is dominated by studies of temperate glaciers in which sedimentary products reflect the influence of basal sliding, subglacial sediment deformation and subglacial hydrological systems (e.g. Boulton, 1972a and b). In contrast there have been relatively few studies of landform and sediment assemblages of polar and polythermal glaciers (e.g. Fitzsimons, 1997b; O Cofaigh et al., 1999). The objective of this chapter is to synthesize recent investigations of polar continental glacier margins and move toward a depositional model for ice-marginal environments that links our understanding of glaciology and geomorphology. This review is based on field observations in East Antarctica (Vestfold Hills, Bunger Hills, Larseman Hills and Windmill Islands) and in south Victoria Land (McMurdo dry valleys and Ross Island). This chapter begins with a definition and review of the physical conditions that control depositional processes at polar glacier margins. The review is followed by a summary of the morphology and structure of modern polar glacier margins and associated landforms and sediments in low- and high- relief environments. The chapter concludes with a synthesis of ideas that form the basis of a depositional model for polar continental glacier margins.
5.6 Towards A Depositional Model
Our knowledge of polar continental landform and sediment assemblages is incomplete. Consequently a comprehensive depositional model cannot yet be assembled. However, several elements of a model can be identified.
- Relatively low volumes of sediment are produced by polar glaciers. Consequently landforms and sediment assemblages have modest volumes and the preservation potential of ice-contact landforms is low.
- A variety of constructional moraines form at stable ice-margins. The most common constructional landforms are ice-contact fans and screes. These form adjacent to steep ice margins and inner moraines (sensu Hooke, 1973a), which form where basal debris crops out on the glacier surface.
- Thrust-block moraines and push moraines form where cold ice contacts saturated unfrozen sediment.
- In some circumstances dry-based glaciers appear to be capable of bed deformation and production of structural landforms.
- Glacifluvial landforms are generally poorly developed elements of the depositional landscape.
Three high-order controls on the nature of polar landforms and sediment assemblages are glacier thermal regime, climate of the terminus area and the topography of the landscape. Comparison of landform and sediment assemblages in polar maritime environments, such as Vestfold Hills, with polar continental environments, such as the McMurdo dry valleys, suggests that the availability of meltwater is the primary control on depositional processes in ice-marginal landscapes. If the summer is sufficiently warm and/or long enough for moderate quantities of meltwater production, glacial deposits are strongly influenced by remobilisation after release from the ice. Given the critical role of meltwater, the wide climatic variability within polar environments, and the realization that the elements of the model summarized above are not inherently different from many other glacial environments, it seems that subdivision of landform and sediment assemblages based on glacier thermal regime is unsatisfactory.
This review points to a striking gap in our knowledge of polar landform and sediment assemblages. The gap in our knowledge concerns subglacial processes and resultant landform-sediment assemblages. Very little is known about polar subglacial landform and sediment assemblages because subglacial landscape elements such as streamlined forms or eskers are not preserved in the land areas that fringe polar ice masses. The main exception is the special case of Taylor Valley where subglacial landforms and sediments associated with an expanded and grounded Ross Ice Shelf are preserved. It appears that the absence of subglacial landform and sediment assemblages could be due to two factors. First, it is likely that subglacial landscapes are eliminated or modified because of the destructive thermal transition from warm-based to cold-based marginal areas and, second, it is possible that we do not yet recognize the sedimentary imprint of subglacial processes, particularly those associated with cold-based ice, which could be quite subtle. The main prospects for improving out understanding of polar subglacial landscapes are direct observations and measurements of basal processes under thick ice where the bed is at pressure melting point (e.g. Engelhardt and Kamb, 1998) and where glaciers are thing and dry-based (e.g. Fitzsimons et al., 1999).
Boulton, G.S. 1972a. Modern arctic glaciers as depositional models for former ice sheets. Journal of Geological Society of London 128, 361-393.
Boulton, G.S. 1972b. The role of the thermal regime in glacial sedimentation. In: Price, R.J. and Sugden, D.E. (eds) Polar Geomorphology. Institute of British Geographers, Special Publication 4, 1-19.
Engelhardt, H.F. and Kamb, B. 1998. Basal sliding of ice stream B, West Antarctica. Journal of Glaciology 44, 223-230.
Fitzsimons, S.J. 1997b. Entrainment of glaciomarine sediments and formation of thrust-block moraines at the marine of Sørsdale Glacier, East Antarctica. Earth Surface Proccesses and Landforms 22 (2), 175-187. doi:10.1002/(SICI)1096-9837(199702)22:2< 175::AID-ESP694>3.0.CO;2-G
Fitzsimons, S.J., McManus, K.J. and Lorrain, R.D. 1999. Structure and strength of basal ice and substrate of a dry-based glacier: evidence for substrate deformation at sub-freezing temperatures. Annals of Glaciology 28, 236-240.
Hooke, R. LeB. 1973a. Flow near the margin of the Barnes Ice Cap, and the development of ice-cored moraines. Geological Society of America Bulletin 84, 3929-3948.
O Cofaigh, C., Lemmen, D.S., Evans, D.J.A., Bednarski, J. 1999. Glacial landform/sediment assemblages in the Canadian High Arctic and their implications for late Quaternary glaciation. Annals of Glaciology 28, 195-201.
Fitzsimons, S.J. 2003. Ice-marginal terrestrial landsystems: polar continental glacier margins. In Evans, D.J.A. (ed.) Glacial landsystems. Arnold, London, UK. 89-110.
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