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.

1. Relatively low volumes of sediment are produced by polar glaciers. Consequently landform and sediment assemblages have modest volumes and the preservation potential of ice-contact landforms is low.

2. 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.

Figure 5.14 Aerial photograph of Canada Glacier showing a prominent end moraine loop (CM) and small moraines (RM) and eskers (RE) of the Ross Sea Drift deposited as the Ross Ice Shelf grounded and advanced into the Taylor Valley.

3. Thrust-block moraines and push moraines form where cold ice contacts saturated unfrozen sediment.

4. In some circumstances dry-based glaciers appear to be capable of bed deformation and production of structural landforms

5. Glacifluvial landforms are generally poorly developed elements of the depositional landscape.

Three high-order controls on the nature of polar landform 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 remobilization 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 landformsediment 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 our 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 thin and dry-based (e.g. Fitzsimons et al., 1999).

Acknowledgements

This work was supported by the Australian Antarctic Science Advisory Committee and the Marsden Fund (New Zealand). Logistical support was provided by the Australian Antarctic Division and Antarctica New Zealand. I thank Damian Gore, Massimo Gasparon, Roland Payne, Marcus Vandergoes, Regi Lorrain, Sarah Mager and Paul Sirota for assistance in the field, Sarah Mager and Dr C. O Cofaigh for critical comments on the text, and Bill Mooney for drawing the diagrams.

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