Box 112 De Geer Moraines

The increasing availability of detailed three-dimensional bathymetry from continental shelves provides an opportunity to study the glacial landsystems formed by retreating tidewater glaciers. A particularly fine example is provided by Todd et al. (2007), who mapped an area of the Scotian Shelf revealing a bathymetric assemblage consisting of a series of superimposed landform suites. At the base is a series of subglacial flutes and drumlins, above which there are crevasse-squeeze ridges, De Geer moraines and larger moraine banks charting the pattern of ice-marginal retreat across the shelf. These landforms are in turn cross-cut by iceberg scour marks formed by the grounding of iceberg keels. Linden and Moller (2005) investigated the internal sedimentary architecture of De Geer moraines in the coastal zone of northern Sweden, submerged during deglaciation by a freshwater lake occupying much of the Baltic Sea. They suggest that the proximal part of the moraines were built up in a submarginal position as stacked sequences of deforming bed diamicts, intercalated with glaciofluvial sediments formed in subglacial channels/canals, whereas the distal part of the ridges are formed from prograding layers of sediment deposited by sediment gravity flows moving forward of the grounding line. These sediment flows interfinger in a distal direction with glaciolacus-trine sediments. In contrast, Blake (2000), working in northern Norway, emphasised the importance of subglacial and ice-marginal deformation of sediment deposited at the grounding line. No single model is likely to be appropriate at all localities due to the dynamic nature of a grounding line and its seasonal stability, which will be a function of ice velocity and the calving rate.

Sources: Blake, K.P. (2000) Common origin for De Geer moraines of variable composition in Raudvassdalen, northern Norway. Journal of Quaternary Science, 15, 633-44. Linden, M. and Moller, P. (2005) Marginal formation of De Geer moraines and their implications to the dynamics of grounding-line recession. Journal of Quaternary Science, 20, 113-33. Todd, B.J., Valentine, P.C., Longva, O. and Shaw, J. (2007) Glacial landforms on German Bank, Scotian Shelf: Evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines. Boreas, 36,148-69.

formed by the forward flow of the debris as it is released at the grounding line. Alternative explanations for De Geer moraines include the formation of ridges by the squeezing of saturated till into basal crevasses behind a calving ice margin. Similar ribbed moraines have also been described as forming well behind the ice margin in a subglacial position as a result of changes in the reheology of a deforming bed (see Section 3.33). Small sublacustrine fans can form at a steep calving ice margins wherever a meltwater portal exists. These small fans typically have steep gradients because deposition of the coarse fraction

A: Formation during calving

C: Formation by basal melting and debris accumulation

A: Formation during calving

C: Formation by basal melting and debris accumulation

Debris accumulation due to basal melting

Debris accumulation due to basal melting

Rofion of iceberg D: Formation by glacial thrusting during calving

Rofion of iceberg D: Formation by glacial thrusting during calving

B: Formation by the release of debris from a deforming layer

B: Formation by the release of debris from a deforming layer

Deforming layer

Transport within deforming layer

Foresets formed by debris avalanching into the lake as it is released from the deforming layer

Figure 11.4 Possible modes of formation for De Geer moraines and other similar transverse sublacustrine ridges. (A) Formation during iceberg calving. (B) Formation by the release of debris from a deforming layer. (C) Formation by basal melting and debris accumulation. (D) Formation at the base of englacial thrusts.

Ice occurs rapidly as the meltwater enters the standing water body. The position of meltwater portals is relatively unstable due to calving and the point of fan formation may move laterally along an ice margin. In this way a series of small coalescing fans may form along the margin. Their steep ice-contact faces may merge to produce a ridge-like form. In some cases these coalescent fans may be mistaken for cross-valley or De Geer moraines. Small fans may also develop at the sides of a lake, where water is channelled along the ice margin. The sediment in these fans is typically much coarser, being derived from the valley sides and lateral moraines of the ice margin, than sediments in the centre of the lake dominated by subglacial meltwater debris.

3. Glaciolacustrine landforms not associated with an ice margin. Where lake levels are stationary for some time shorelines or strandlines may develop. Fossil shorelines can be identified on hillsides as horizontal lines with a faint terrace-like form. The shoreline terrace may be purely depositional in character or may also be eroded into the hill side. Depositional elements involve the reworking of slope debris by wave action moving down the hillside toward the lake. The amount of sediment modification will largely depend on the size of the lake because this controls the available fetch and therefore the size of the lake because this controls the available fetch and therefore the size of waves acting upon it. Waves generated by the calving of icebergs are often of greater magnitude and therefore more significant than wind-generated waves. Small berms produced by wave action are commonly superimposed upon the beach terrace. These features are transitory and unlikely to be preserved. Sediment on the shoreline is often deformed or pushed up by lake ice and a variety of pushed ridges may form. Some shorelines are formed not only of depositional terraces but have also been eroded into bedrock or slope debris. Given that most ice-dammed lakes are transitory features and the wave action upon them is likely to be limited, it seems probable that erosion is the result of the action of freeze-thaw weathering and of icebergs rather than associated with wave action. It has been suggested that lake shorelines may be an ideal location for intense freeze-thaw activity (Box 11.3). The size of a shoreline terrace may be increased where small streams enter the lake and deposit sediment in small fans or deltas. The lake floor may also be subjected to scour by grounded icebergs, especially towards the lake margins. The keel or bottom of an iceberg may drag along the lake floor to form linear plough marks.

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