Icemarginal Moraines

Ice-marginal landforms produced directly by the action of a glacier are known as ice-marginal moraines. They may form by the action of four main processes, none of which need to be mutually exclusive: (i) ice-marginal or submarginal glaciotec-tonics; (ii) ice-marginal dumping of debris via a range of processes including rockfall and debris flow; (iii) ice surface/marginal meltout; and (iv) subglacial transport and meltout of debris. As a consequence, a practical and universally accepted taxonomy for ice-marginal moraines does not exist and a plethora of terms are used in the literature ranging from non-genetic descriptors such as lateral, terminal, or recessional moraines to the genetic terms such as push moraines, thrust-block moraines, dump moraines and ablation moraines. For simplicity we recognise three broad categories of ice-marginal moraine: (i) glaciotectonic moraines; (ii) dump moraines; and (iii) ablation moraines.

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd

Table 9.1 Classification of terrestrial glacial landforms.

Ice-marginal

Subglacial

Glacial

Glaciofluvial

Glaciotectonic moraines Dump moraines Ablation moraines

Outwash fans Outwash plains Kame terraces Kames

Kame and kettle topography

Flutes

Megaflutes

Drumlins

Rogen/ribbed moraine Megascale glacial lineations Geometrical ridge networks (crevasse-squeeze ridges)

Eskers

Braided eskers

9.1.1 Glaciotectonic Moraines

Glactioteconic moraines (push moraines) have been variously classified within the literature, but here we define them as the product of constructional deformation of ice, sediment and/or rock to produce a ridge or ridges, transverse or oblique to the direction of ice flow in front of, at, or beneath an ice margin. Moraines of this sort display a wide range of different morphologies at a range of scales from landforms that are just a few metres high and wide to those that extend into the proglacial foreland for several kilometres and are composed of a diverse range or rock and sediment. In general, however, there is a morphological continuum from small, discrete ice-marginal ridges formed by seasonal readvances of an ice margin to multi-crested moraines with proximal-distal widths of several hundreds of metres formed by more sustained advances or glacier surges. It is worth exploring the end-members of this continuum by first looking at seasonal push moraines, before considering large composite push moraines formed during more sustained readvances.

Seasonal readvances occur at glaciers that are either stationary or experiencing net retreat - those with a balanced or negative mass balance (Figure 9.1A). Seasonal fluctuations of the ice margin occur where winter ice flow exceeds winter ablation and in these circumstances the ice margin advances during the winter (Figure 9.1B). In the summer months ablation exceeds glacier flow and the ice margin will retreat. The presence or absence of seasonal fluctuations therefore depends on the relative magnitude of winter ablation and ice flow. If winter ablation exceeds ice flow there will be no advance. Seasonal push moraines therefore tend to form in maritime areas where glaciers have relatively high ablation gradients and levels of glacier activity. In more continental climates with lower mass balance gradients and levels of glacier activity seasonal push moraines are usually absent.

400 600

1000 1200 Seasonal push moraine

1400

400 600

1000 1200 Seasonal push moraine

1400

Larger ice-marginal moraine

Cumulative velocity of the glacier terminus

Cumulative velocity of the glacier terminus

Displacement of the glacier terminus

Cumulative ablation at the glacier terminus

Displacement of the glacier terminus

Cumulative ablation at the glacier terminus

Distance from datum (15 December 1972) (m)

Net monthly ablation at

?lacier margin n of water equivalent)

Figure 9.1 Seasonal push moraines. (A) Seasonal push moraines in front of Slettjokull mapped from aerial photographs. The bar below indicates the presence of moraines identified in the field - long dash for prominent examples - along two transects marked by the dashed lines on the map. Note the greater number of moraines identified in the field. Also of note is the variable moraine spacing, which correlates with the rate of glacier recession and therefore the glacier's mass balance. (B) Data for BrieCamerkurjokull to illustrate the balance between ablation and ice velocity at the ice margin that determines the presence or absence of a seasonal readvance. Also note the similarity between monthly ablation and mean monthly temperature. [Modified from: (A) Kruger (1994) Folia Geographica Danica Tom, XXI, figure 37, p. 53. (B) Boulton (1986) Sedimentology, figure 3, p. 680]

At modern glacier margins seasonal push moraines are typically 1-5 m high and tend to be asymmetric in cross-section with a shallow proximal and a steep distal flank. They are frequently lobate in plan with the intervening re-entrants marked by a linear concentration of boulders, which collect in longitudinal crevasses on the ice margin (Figures 9.2 and 9.3). The continuity of the moraine and its detailed plan form is determined by the extent of

Figure 9.2 Push moraine in front of Briedamerkurjokull in Iceland. Note the complex crenulated planform caused by the crevassed and uneven ice margin. [Photograph: G. S. Boulton]

Coarse sand jjme 1

Sediment flowage

0 25 m Sand drape over underformed

-1 lodgement till, with clast pavement

Ice flow

0 25 m Sand drape over underformed

-1 lodgement till, with clast pavement

Ice flow

Push moraine crest \

Supraglacial crevasse-fill crevasse-fill

Push moraine

Increasing density of ice-marginal crevasses Increasing modification of push moraines

Push moraine

Time 4

Lodgement till now^^L

Lodgement till

Distance b

Distance

Figure 9.3 Seasonal push moraines. (A) Some examples of the internal structure of seasonal push moraines. (B) Internal structure of a seasonal push moraine formed from tectonic slabs of lodgement till. (C) Plan form of seasonal push moraines, showing increasing modification with the intensity of ice-marginal crevasses. (D) The formation of a moraine bifurcation, due to differential ice retreat along an ice margin. Different sections of ice front commonly retreat at different rates. [Modified from: Bennett (2001) Earth Science Reviews 53, figure 4, p. 203]

Time 2

Time 4

Time

Distance

Time b

Distance ice-marginal crevassing. The greater the crevasse density, the more fragmentary the moraine will be (Figure 9.3C). Moraines may merge or bifurcate along the ice margin, as different sections advance by different amounts each season (Figure 9.3D). The spacing between push moraines is a function of the distance of ice-marginal retreat each summer and is therefore sensitive to climate; the greater the spacing between moraines the greater the summer ablation.

Seasonal push moraines are typically composed of subglacial till, although outwash sediments and other proglacial debris may also be incorporated. Sedimentary structures, such as folds and thrusts, are not usually well preserved in these moraines due to the coarse nature of the sediment involved, although folds, thrusts and faults can form where finer material is incorporated (Figure 9.3A). In locations where meltwater flows or seeps through the moraine, fines may be washed out and deposited in a small fan in front of the moraine. The detailed formation of these ridges is both complex and highly variable. Field observations indicate that: (i) water-soaked till is extruded from beneath the glacier as it advances; (ii) simple pushing sweeps surface sediment together; and (iii) imbricate slabs of subglacial and proglacial sediment are stacked together.

Larger, more complex, glaciotectonic moraines may form where a sustained advance takes place, as a result of a positive mass balance or a glacier surge (see Section 3.5; Figure 9.4). A good example of this type of ridge is found in front of Holmstrombreen in Svalbard, where the ridge was formed during a glacier surge into an outwash fan and is composed of a stacked sequence of tectonic slices of glacier foreland, each composed of folded till and outwash sediments, rising from a basal slip or decollement plane (Figure 9.4B). Deformation appears to have involved a thin slab of permafrozen foreland moving over a decollement lubricated by high ground-water pressures trapped beneath the impermeable permafrozen layer. Deformation of a perma-frozen foreland is also evident in a glaciotectonic moraine in front of the Thompson Glacier on Axel Heiberg Island. Here blocks of frozen outwash form a stacked imbricate moraine complex in which each block is separated from the next by a thrust fault. In northern Europe there are several good examples of large push moraines involving the movement of thin slabs (nappes) of outwash and rock such as that found at Hanklit in Denmark or on the Kanin Peninsula in northwest Russia (Box 9.1). Some large composite glaciotectonic moraine systems, such as the one at Uversbreen in Svalbard, appear to incorporate compressional tectonics within both the snout and the proglacial area (Figure 9.5). Here the glacier snout is firmly coupled to a permafrozen foreland, which collectively experiences compression, due to flow deceleration across a thermal boundary - warm fast-flowing ice to cold slow-flowing ice - such that tectonic shortening occurs both in the glacier snout and proglacial area. There are also examples of glaciotectonic moraines formed by transport of sediment to the ice margin by subglacial deformation. A surge of Sefstrombreen in the 1890s in Svalbard provides a good example where marine muds within a deforming layer have been transported during the surge and deposited as a moraine system on a series of islands on which

Figure 9.4 (A) The push moraine complex at Dinas Dinlle in North Wales. The subsurface geometry of this push moraine was revealed by a seismic line. (B) Large composite push moraine in front of Holmstrombreen in Svalbard. The two insets illustrate the potential impact on the groundwater hydrology of a thin permafrozen glacier foreland and the progressive evolution of the push moraine during the surge. [Modified from: (A) Harris et al. (1997) Quarternary Science Reviews, 16, figures 7-8, pp. 116-117. (B) Boulton et al. (1999) Quaternary Science Reviews, figures 24, 27 and 29, pp. 171,174 and 177]

Figure 9.4 (A) The push moraine complex at Dinas Dinlle in North Wales. The subsurface geometry of this push moraine was revealed by a seismic line. (B) Large composite push moraine in front of Holmstrombreen in Svalbard. The two insets illustrate the potential impact on the groundwater hydrology of a thin permafrozen glacier foreland and the progressive evolution of the push moraine during the surge. [Modified from: (A) Harris et al. (1997) Quarternary Science Reviews, 16, figures 7-8, pp. 116-117. (B) Boulton et al. (1999) Quaternary Science Reviews, figures 24, 27 and 29, pp. 171,174 and 177]

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