Formation

Kj^r et al. (2006) describe a series of observations from Bruarjokull in Iceland, a northern outlet glacier of the Vatnajokull Ice Cap in Iceland. The glacier has a flow regime which oscillates between major winter flow events that last up to 3 months, punctuated by quiescent periods that last between 70 and 90 years; surges occurred in 1890 and 1963 and involved ice-marginal advances of between 10 and 8 km in each case. Since 1964 ice flow has been negligible and the ice margin retreated at a rate of up to 250 m per year between 2003 and 2005, with a surface lowering of 6-7 m per year. Kj^r et al. (2006) argue that fast ice flow during the 1890 surge occurred because of decoupling between the deforming bed and the bedrock as evidenced by thin layers of waterlain sediments along this interface. Subglacial drainage was unable to escape through the impermeable basalt bedrock and consequently pore-water pressures built up at the sediment-rock interface, leading to rapid deformation and or sliding. The limit of the glacier advance is determined by locations at which this pressurised groundwater could escape.

Flow compression

Glaciotectonic moraine built by subglacial deformation

| Rapidly deforming uppper layer

Slowly deforming lower layer J Impermeable bedrock

Water escape

Flow compression

Glaciotectonic moraine built by subglacial deformation

Water escape

| Rapidly deforming uppper layer

Slowly deforming lower layer J Impermeable bedrock

Major décollement along sediment rock interface

Major décollement along sediment rock interface

This limit is associated with a large glaciotectonic moraine, which is described by Benediktsson et al. (2008). As a result of substrate-bedrock decoupling during the surge, subglacial sediment was advected over bedrock and deformed compressively, leading to a gradual thickening toward the ice margin and the formation of a sediment wedge in the marginal zone. Water escape in these areas at the end of the surge led to a substrate-bedrock coupling and stress transfer into the sediment sequence below the ice margin, causing brittle deformation in areas of sand and gravel and more ductile deformation in areas of finer-grained sediments. The glacier increasingly ploughed into these sediments, leading to marginal moraine formation; narrow ridges occur where fine-grained incompetent sediments crop out and were deformed in a ductile fashion, whereas in areas of coarse-grained sediment deformation occurred via brittle failure along thrusts to give a wider and topographically more complex moraine. Rapid emplacement of the moraine is envisaged, taking just a few days towards the end of the surge. Moraine formation occurred therefore as a two-stage process, first the sediment wedge was built up by sediment transport within the subglacial deformating layer, before the ice-marginal sediment pile was tectonised further as the ice bed coupled with the deformating layer more effectively once it ceased to move freely over the underlying bedrock upon dewatering.

Sources: Kj^r, K.H., Larsen, E., van der Meer, J., et al. (2006) Subglacial decoupling at the sediment/bedrock interface: A new mechanism for rapid flowing ice. Quaternary Science Reviews, 25, 2704-12. Benediktsson, I.O., Moller, P., Ingolfsson, O. et al. (2008) Instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of Bruarjokull, Iceland. Quaternary Science Reviews, 27, 209-34. [Reproduced from: Benediktsson et al. (2008) Quaternary Science Reviews, 27, 3figure 14, p. 232. Copyright © 2008, Elsevier Ltd].

loading of the competent sandstone bedrock and the saturated incompetent mud-stone and clay-rich strata beneath caused the sandstone to fracture and be thrust-up in a complex series of blocks along the margins of the advancing ice sheet.

The above review illustrates the range of different morphologies and scales at which glaciotectonic moraines are encountered at past and present ice margins. It is worth exploring further some of the variables that control the development of these moraines. Figure 9.7 defines some of the key variables, which include the following.

1. Application of stress. A glacier can transfer stress to the foreland in several different ways, each having a different impact. At its simplest a glacier can be viewed as a bulldozer pushing from the rear. However, in practice it may also apply stress via gravity-spreading. A glacier increases in thickness away from the ice margin and consequently the load applied to subglacial sediments also increases away from the margin. This effectively sets up a lateral stress gradient from areas of thick ice and high load to areas at the margin of thin ice and little load. This may be as important in driving proglacial deformation as the force applied directly by the glacier.

2. The geometry of the foreland wedge. This can be defined in terms of the size, both the proximal to distal width and depth of the proglacial area (foreland) that was, or is, being tectonised to produce the moraine. In the case of a small seasonal push moraine it is likely to consist simply of a thin veneer of surface debris, alternatively it might involve a significant part of a glacier proglacial area or perhaps part of the ice margin itself. Prior to deformation the foreland wedge can be defined as a slab of known width (measured

Glacial foreland

Uice

Umoraine

Glacial foreland

Uice

Umoraine

Push moraine: sub-horizontal nappes

Figure 9.7 (A) The anatomy of a glaciotectonic moraine showing some of the key dimensions and properties involved in their formation. (B) Alternative tectonic models for the formation of glaciotectonic moraines.

Push moraine: sub-horizontal nappes

Figure 9.7 (A) The anatomy of a glaciotectonic moraine showing some of the key dimensions and properties involved in their formation. (B) Alternative tectonic models for the formation of glaciotectonic moraines.

perpendicular to the ice margin) and depth. Many large push moraines appear to involve slabs with a high aspect ratio (i.e. slabs that are thin, but laterally extensive). The topographic geometry of the foreland may also be important. For example, if a glacier is advancing up a reverse slope it will transfer the forward stress effectively into the slope, pushing up a larger moraine; a glacier on a horizontal surface will not have the same impact because much of the forward stress is not transferred into the proglacial sediment (Figure 9.8). An obvious nucleus against which an ice

Figure 9.8 Push-moraine-forming episodes within a glacial cycle. (1) Glacier decay is halted by a period of positive mass balance and an outwash fan forms, which is subsequently pushed up by a glacier advance due to the continued positive mass balance. As a negative mass balance re-establishes itself and ice retreat continues, the glacier decays away from the moraine and meltwater is trapped between it and the moraine to form kame terraces. (2) A period of glacier advance is halted by a short spell of negative mass balance. An outwash fan first forms at the stationary ice margin before being pushed up into a push moraine as a positive mass balance regime re-establishes itself and the glacier advances. The implication is that during a glacier advance push moraine formation is initiated by a climatic amelioration, whereas during decay it is stimulated by a deterioration in climate. [Modified from: Boulton (1986) Sedimentology, 33, figure 17, p. 695]

Figure 9.8 Push-moraine-forming episodes within a glacial cycle. (1) Glacier decay is halted by a period of positive mass balance and an outwash fan forms, which is subsequently pushed up by a glacier advance due to the continued positive mass balance. As a negative mass balance re-establishes itself and ice retreat continues, the glacier decays away from the moraine and meltwater is trapped between it and the moraine to form kame terraces. (2) A period of glacier advance is halted by a short spell of negative mass balance. An outwash fan first forms at the stationary ice margin before being pushed up into a push moraine as a positive mass balance regime re-establishes itself and the glacier advances. The implication is that during a glacier advance push moraine formation is initiated by a climatic amelioration, whereas during decay it is stimulated by a deterioration in climate. [Modified from: Boulton (1986) Sedimentology, 33, figure 17, p. 695]

margin can push is an outwash fan and the association of glaciotectonic moraines with the former location of outwash fans has been widely noted. This observation has been used to build a general model of where push moraines may occur on a glacial advance-retreat cycle (Figure 9.8). Consider a glacier experiencing a period of long-term advance, due to a positive mass balance, where continuous frontal advance would plough up only a small push moraine given a horizontal glacier forefield. Its size will remain limited by the loss of material beneath the advancing glacier and it will grow only where there is an upstanding or resistant sediment mass against which the glacier may push effectively. If, however, the ice-marginal position stabilises due to a short amelioration in climate, ice-contact out-wash fans may develop from meltwater activity. A renewed glacier advance would deform these fans into large push moraines, because the glacier now has material against which to push. The push moraines would ultimately be overridden given continued glacier advance (Figure 9.8). In contrast, at a glacier receding due to a negative mass balance, temporary stabilisation of the ice margin would be required for ice contact fans to develop. A read-vance, due to a short deterioration in climate, would cause these fans, or at least their proximal faces, to be deformed into push moraines (Figure 9.8). It follows, therefore, that widespread push moraine formation during a phase of glacier advance would require a temporary amelioration of climate, whereas deterioration in climate is required to produce push moraines during a period of prolonged glacier retreat.

3. Foreland rheology. Some materials behave in a ductile fashion (i.e. they fold) when they are subject to an applied stress such as an advancing ice margin, whereas other materials behave in a brittle fashion (i.e. they break up along thrust faults into distinct slabs). The presence or absence of proglacial permafrost may play a role in determining foreland rheology. The strain response of a material is also determined by the rate at which stress is applied; for example materials tend to behave in a brittle fashion when the stress is applied rapidly.

4. Decollement friction. The presence of a weak basal horizon along which deecol-lement can occur is important. In parts of northern Europe this may control the distribution of large complex push moraines, which tend to occur where soft Tertiary and Cretaceous clays occur at depth below more competent rocks. The Dirt Hills in Saskatchewan also provide an illustration of this point. The hydro-geological setting is also important and elevated groundwater pressure below thin surface permafrozen sediment is important in the formation of some Svalbard moraine systems, such as Holmstrambreen (Figure 9.4B). The degree of friction along the basal decollement plane may control the style of deformation and the degree to which stress is propagated into a glacier foreland. At one end of the spectrum are thrust-dominated moraines, formed of stacked nappes or imbricate thrust blocks, where the friction along the decollement is high. As the friction on the decollement increases one passes to fold-thrust dominated systems, and in zones of high basal friction fold-dominated systems occur, with compression concentrated close to the ice margin.

There is increasing evidence from both Iceland and Norway of a mechanism of moraine formation which involves the basal freeze-on of debris. Although strictly speaking not a tectonic process it gives an internal moraine architecture that is very similar to an imbricate stack of thrust foreland slabs (Box 9.3). A good example has been described from Myrdalsjokull in south Iceland; here the ice margin is stationary, although subject to seasonal readvances. Each winter the thin ice at the glacier margin becomes cold-based and freezes to the underlying layer of till. As the glacier advances at the end of each winter it rips up this slab of till and moves it forward

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