Box 93 Formation Of Stacked Till Sheets And Icemarginal Moraines

Evans and Hiemstra (2005) reported observations made at a number of Icelandic glaciers. They identify the presence of stacked sequences of till layers within ice-marginal settings forming moraine systems. Study of both microscopic and macroscopic processes indicate that the tills were deposited by a range of processes and not just subglacial deformation. They suggest that each layer of sediment was deposited by a distinctive cycle of events:

1. During the late summer subglacial lodgement, and bedrock and sediment plucking occur, along with subglacial deformation and ice-keel ploughing, to create a layer of daimict showing a range of depositional and tectonic signatures.

2. Early in the winter this layer freezes onto the thin outer snout of the glacier, a process that has been widely documented from both Icelandic and Norwegian glaciers.

3. During the late winter, a readvance moves this frozen layer of sediment forward; failure occurs along a decollement plane within the till; movement occurs onto the proximal side of the previous year's push moraine.

4. Early summer meltout of the till slab initiates pore-water migration with water escape and sediment flowage and extrusion under the weight of the ice. This sediment may also be subject to shear during summer ice flow and associated sediment deformation.

Over time an ice marginal till layer is thickened incrementally and may also be linked to the formation of an ice-marginal moraine at a largely stationary ice margin. This type of process - the freezing on of basal slabs of diamict during winter and their transport during winter advances - has been described from a number of locations in Iceland and Norway (Kriiger, 1993; Matthews et al. 1995). The work by Evans and Hiemstra (2005) also demonstrates how a range of different depositional and tectonic processes are involved in the formation of a subglacial till layer and that it is difficult to ascribe the formation of subglacial tills to any single process, such as lodgement or deformation.

Sources: Evans, D.J.A. and Hiemstra, J.F. (2005) Till deposition by glacier submarginal, incremental thickening. Earth Surface Processes and Landforms, 30,1633-62. Kriiger, J. (1993) Moraine-ridge formation along a stationary ice front in Iceland. Boreas 22, 101-9. Matthews, J.A., McCarroll, D. and Shakesby, R.A. (1995) Contemporary terminal moraine ridge formation at a temperate glacier: Styffedalsbreen, Jotunheimen, southern Norwary. Boreas, 24,129-39.

across a decollement surface. During the summer the slab detaches from the ice margin as the snout and subsurface warms. The following year another slab is deposited on top of the first, such that a moraine is built up sequentially from a stack of sediment slabs.

9.1.2 Dump Moraines

Dump moraines form where debris is delivered to an ice margin and accumulates along the side or in front of the glacier to form a ridge (Figure 9.9). Their formation requires a stationary ice front. The size of moraine produced is a function of: (i) ice velocity, because the faster the ice flow the higher the rate of debris delivery to the ice margin; (ii) debris content within the ice, because the greater the debris content the larger the moraine formed; and (iii) the rate of ice-marginal retreat, because if the ice margin recedes quickly then any debris will be distributed widely, but if recession is slow or punctuated by long stillstands then ice-marginal debris becomes concentrated into moraines. Large dump moraines will therefore form when: (i) ice velocity is high; (ii) the rate of retreat is slow; and (iii) the debris content in the glacier is high. A steep ice margin is also important to ensure the effective transfer of the debris away from the glacier so that it does not simply accumulate on the ice surface to form an ablation-type moraine (see Section 9.1.3). At a glacier terminus these

Figure 9.9 Dump moraine forming at the margin of Skeidararjokull, Iceland. [Photograph: M. R. Bennett]
Figure 9.10 Small valley glacier in Arctic Sweden, showing lateral and frontal moraines. Note how paraglacial process have modified the lateral moraine [Photograph: N.F. Glasser]

circumstance may not always be met, particularly as stationary ice margins often have low gradients and supraglacial debris consequently tends to accumulate on the glacier surface. However, the lateral margins of a glacier are usually steep and relatively stationary, because ice flow is parallel to the margin not perpendicular to it. Here lateral moraines (Figure 9.10) develop primarily by the dumping of glacial debris, although some debris may also be derived from the valley sides. The formation of these moraines is illustrated in Figure 9.11. The dumping process tends to develop a strong fabric and crude bedding of platy boulders within the moraine, similar to that found in talus slopes (Figure 9.11). Typically this fabric and bedding dips towards the valley wall at between 10° and 40°. The dumping process may be seasonal, material being stored on the glacier during summer and deposited in a single pulse during a winter readvance. Debris in lateral moraines is derived from both supraglacial and subglacial transport pathways and is consequently highly variable. The detailed morphology of the moraine and the sedimentary facies present within it depend upon the relative quantities of debris derived from the glacier and the valley side (Figure 9.11). If the valley side component is high then the moraine will tend to have a more bench-like form. This is particularly true where a debris or talus cone delivers material directly to the lateral moraine. In these situations the debris cone is often drawn out or extended in the direction of glacier flow. In contrast, if the direct supply of valley-side debris is less important then a distinct ridge bordering the glacier may develop (Figure 9.11). The morphology of the moraine may also be

Figure 9.11 Morphology and sedimentary facies of lateral moraines. (A) Formation, morphology and facies of a lateral moraine where the supply of supraglacial debris is greater than the supply of valley-side debris. (B) Formation, morphology and facies of a lateral moraine where the supply of supraglacial debris is less than the supply of valleyside debris. (C) Typical sedimentary facies of a lateral moraine. (D) Possible modes of growth for a lateral moraine formed by successive ice advances.

Figure 9.11 Morphology and sedimentary facies of lateral moraines. (A) Formation, morphology and facies of a lateral moraine where the supply of supraglacial debris is greater than the supply of valley-side debris. (B) Formation, morphology and facies of a lateral moraine where the supply of supraglacial debris is less than the supply of valleyside debris. (C) Typical sedimentary facies of a lateral moraine. (D) Possible modes of growth for a lateral moraine formed by successive ice advances.

modified by ice-marginal meltwater, which can deposit material between the lateral moraine and the valley wall. Small kame terraces (see Section 9.3) may be superimposed upon the proximal slopes of lateral moraines as the ice retreats (Figure 9.11). Lateral moraines often display cross-glacier asymmetry; the lateral moraine on one side of a valley being larger than that on the other. This asymmetry reflects the distribution of debris within the glacier, which is normally a function of the distribution of bare rock faces in the upper reaches of the glacier.

Ice-marginal fans composed predominately of diamicts represent one type of dump moraine. Here a stationary ice margin subject to seasonal or more prolonged stillstands becomes the focus for the build up of an ice-marginal fan consisting of a diverse range of materials delivered by a range of different processes. Sediment may be delivered by debris flows originating from supra-glacial sediment, by direct avalanching of debris, or by small seasonal melt-streams, building up a layered internal stratigraphy. The pile of debris may become a focus for deformation and ice pushing, but it essentially accumulates through a diverse range of processes transferring sediment in an off-ice direction. These grade with increasing glaciofluvial input into outwash fans (see Section 9.3). These fans have been used to explain one component of the landform assemblage commonly found in the hills of upland Britain and associated with the Younger Dryas (Box 9.4).

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