Box 114 Moraine Banks And Cyclic Glacier Fluctuations

The behaviour of individual Alaskan tidewater or fjord glaciers has often been observed to be asynchronous with other tidewater glaciers and with those on land. It has been suggested that tidewater glaciers may behave in a cyclic fashion due to the inherent instability of a calving glacier margin. The rate of glacier calving is strongly influenced by water depth. As a tidewater glacier advances down a fjord in equilibrium with its mass balance a moraine shoal or moraine bank will form along its margin and will advance with the glacier. This bank will help stabilise the ice margin by reducing the water depth and therefore the rate of glacier calving. If this balance is disturbed and the ice margin retreats away from this bank into deep water, rapid calving and retreat may occur. This retreat may be triggered by a small rise in the equilibrium line of a glacier or by some non-climatic event such as a catastrophic glacier flood. Either way, once the glacier terminus is uncoupled from its moraine bank, calving can proceed rapidly in the deep water behind. Rapid calving may cause the glacier to retreat out of proportion with the magnitude of the event that triggered it, and may continue until the glacier reaches shallow water. At this point the position of the ice margin is likely to be out of equilibrium with its mass balance and it will start to advance again forming a new moraine bank. The glacier, therefore, behaves in a cyclic fashion and is uncoupled from climate. The margin is not advancing and retreating in response to climate but as a consequence of the inherent instability of a calving glacier margin. The presence of a moraine bank, therefore, is important to the stability of the ice margin.

B: Retreating phase

Ice - margin becomes uncoupled from marine shoal leading to rapid calving

B: Retreating phase

Ice - margin becomes uncoupled from marine shoal leading to rapid calving

Source: Mayo, L.R. (1988). Advance of Hubbard glacier and closure of Rusell Fjord, Alaska—environmental effects and hazards in the Yakutat area. United States Geological Survey Circular, 1016. [Modified from: Warren (1992) Progress in Physical Geography 16, figure 11.4, p. 265]

morphology of the debris plume (Figure 11.5). At low water discharges the plume rises rapidly to the surface. Coarse tractional deposits are dumped close to the ice margin at the meltwater portal. Most of the fan is deposited on the distal slope by settling from the debris plume and by debris slumping or sliding down the distal face. This slumping gives rise to a series of foresets. At higher water and sediment discharges the plume remains in contact with the fan for longer and flattens or planes off the apex of the fan. Coarse sediment is also deposited at this point in sheets, and as scour and fill structures. Deposition by settling and by sediment gravity flows again builds up the distal part of the fan. At very high discharges deposition from the plume while it is in contact with the fan may deposit a barchanoid bar at the point at which the plume is detached from the fan (Figure 11.5).

A: Low discharge

A: Low discharge

B: Moderate water discharge, high sediment discharge

B: Moderate water discharge, high sediment discharge

C: High discharge

Barchanoid bar

Figure 11.5 Schematic representation of the variation in the discharge plume in front of a calving glacier in relation to water discharge and the impact of this variation on the morphology of the grounding line fan produced. (A) At low water discharges the fan grows progressively with coarse traction deposits dumped at the meltwater portal and by deposition as a series of large foresets on the distal flank from settling, slumping/sliding and sediment gravity flows. (B) At moderate sediment discharges the base of the sediment plume remains in contact with the fan longer. As a consequence a series of sheet-like deposits of sand and gravel with crude scour and fill structures is deposited on the top of the fan. Settling and sediment gravity flow deposits make up more distal slopes of the fan. (C) At high water discharges the plume remains in contact with the fan much longer and may deposit a migrating barchanoid bar along line of detachment between the plume and the sea bed. [Reproduced from: Powell (1990) in Glacimarine Environments: Processes and Sediments (eds J.A. Dowdeswell and J.D. Scourse), Geological Society Special Publication No. 53, figure 11.5, p. 58. Copyright © 1990, Geological Society

Publishing House].

C: High discharge

Barchanoid bar

Figure 11.5 Schematic representation of the variation in the discharge plume in front of a calving glacier in relation to water discharge and the impact of this variation on the morphology of the grounding line fan produced. (A) At low water discharges the fan grows progressively with coarse traction deposits dumped at the meltwater portal and by deposition as a series of large foresets on the distal flank from settling, slumping/sliding and sediment gravity flows. (B) At moderate sediment discharges the base of the sediment plume remains in contact with the fan longer. As a consequence a series of sheet-like deposits of sand and gravel with crude scour and fill structures is deposited on the top of the fan. Settling and sediment gravity flow deposits make up more distal slopes of the fan. (C) At high water discharges the plume remains in contact with the fan much longer and may deposit a migrating barchanoid bar along line of detachment between the plume and the sea bed. [Reproduced from: Powell (1990) in Glacimarine Environments: Processes and Sediments (eds J.A. Dowdeswell and J.D. Scourse), Geological Society Special Publication No. 53, figure 11.5, p. 58. Copyright © 1990, Geological Society

Publishing House].

The morphology of grounding line fans is also controlled by the stability of the ice margin and the rate of discharge. At advancing ice margins grounding line fans will be small and heavily tectonised. In contrast, at retreating ice margins the fan morphology will depend upon: (i) the rate of ice-marginal retreat; (ii) the stability of the position of the meltwater portal; and (iii) the rate of the sediment discharge. Given these variables the following morphologies may develop.

1. Advancing ice margin. In this situation a small fan would develop in front of the ice margin during the summer ablation season when calving is high. These fans will be overrun and tectonised each winter when the glacier advances more rapidly due to lower calving rates.

2. Stationary ice margin. In this situation a single large fan will develop. If the rate of sedimentation is high this fan may grow to sea level and form an ice-contact delta.

3. Slowly retreating ice margins with a strong seasonal readvance. In this situation a series of fans, commonly merging in the direction of retreat, form as the ice margin withdraws. Merged fans give an esker-like ridge (paraesker) in front of the retreating meltwater portal. This ridge may be associated with closely spaced push moraines associated with seasonal readvances. If the position of the meltwater portal is not constant during retreat, a series of closely spaced fans will develop that are off set from one another in the direction of retreat to form a zigzag-type pattern.

4. Rapidly retreating ice margin with a seasonal readvance. In this situation widely spaced, in the direction of retreat, ice-marginal fans develop associated with well spaced push moraines. These widely spaced fans often look like a string of beads, each fan being an individual bead, when viewed from above and have consequently been described as beaded eskers.

5. Rapidly retreating ice margin with no seasonal readvance. In this situation, fan and push-moraine development is minimal and a broad subaqueous outwash plain or surface develops if sediment discharge is high.

In general, subaquatic outwash fans (marine or lacustrine) may be distinguished from terrestrial outwash fans on the basis of the following:

1. Dead-ice collapse features are common on the proximal flanks of terrestrial forms but are absent in subaqueous settings.

2. In marine forms, coarse-grained material is concentrated at the point of ice-contact, with rapid distal reduction in grain size. This gradient in particle size is less pronounced in terrestrial fans.

3. The thickness of marine fans is limited only by the depth of the water body. The thickness of terrestrial fans is limited by local base level and they therefore tend to be thinner.

Another type of grounding line fan known as a till delta may develop. These landforms are more common beneath ice shelves and broad glaciomarine margins than in the restricted space of a fjord. They are fans developed by the delivery of glacially transported debris to the grounding line as opposed to fluvial sediment. Transport and deposition may occur in basal or englacial ice that melts out at the grounding line, or alternatively through subglacial deformation. Either way, sediment is delivered to and deposited along the grounding line. As material is brought to the grounding line it avalanches and flows forward to form a series of foresets (Figure 11.6). If the glacier is advancing the till delta will advance and the foresets will be capped by basal till. Bottomsets will be formed by settling from suspension. In contrast to grounding line fans (which develop through meltwater and are focused by portals), till deltas form along the whole length of the grounding line.

A genetically similar landform, a trough-mouth fan, is found at the edge of glaciated continental shelves. Most northern hemisphere continental shelves have a simple geometry that consists of shallow banks, of 100-200 m depth, crossed by

Figure 11.6 The formation of diamict deltas and trough mouth fans. (A) The formation of a diamict delta. Steps 1-3 show the response of the diamict delta to an increase in sea level. (B) Formation of a trough mouth fan. [Reproduced from: Boulton (1990) in Glacimarine Environments: Processes and Sediments (eds J.A. Dowdeswell and J.D. Scourse), Geological Society Special Publication No. 53, figure 6, p. 21. Copyright © 1990, Geological Society

Publishing House].

L-

"I

-prr

Sea-level

rise

Figure 11.6 The formation of diamict deltas and trough mouth fans. (A) The formation of a diamict delta. Steps 1-3 show the response of the diamict delta to an increase in sea level. (B) Formation of a trough mouth fan. [Reproduced from: Boulton (1990) in Glacimarine Environments: Processes and Sediments (eds J.A. Dowdeswell and J.D. Scourse), Geological Society Special Publication No. 53, figure 6, p. 21. Copyright © 1990, Geological Society

Publishing House].

glacially eroded troughs that descend 300-400 m below sea level. These troughs probably reflect the location of lines of fast-flowing ice, or ice streams, within ice sheets. At the glacial maximum, ice was located at the mouths of these troughs on the edge of the continental shelf. The glacier would not be able to advance into the deep water beyond the edge of the shelf. As a result, large fans (trough-mouth fans) build out over the continental slope. Sediment is transported to the grounding line: (i) by subglacial deformation; (ii) by meltwater; and (iii) in the basal ice. This sediment is then deposited as giant foresets down the continental slope by sediment gravity flows (Figure 11.6B). These foresets are prone to slumping and further downslope movement. While the ice margin is located close to the edge of the continental shelf progradation continues, producing steep foresets. However, as the glacier retreats away from the edge of the continental shelf, slumping and down-slope movement of material may reduce the gradient of these foresets.

Other glaciomarine landforms include iceberg-grounding structures, which form wherever the keel of iceberg makes contact with the sea floor. Plough marks produced by icebergs are common wherever an iceberg runs aground and can be observed as both relict and contemporary features on many continental shelves today. The morphology of an individual plough mark depends upon: (i) the sedimentological characteristics of the sea-floor sediment; (ii) the geometry of the iceberg keel; and (iii) the motion of the iceberg. In stiff, cohesive sediments ice keels may create irregular grooves flanked by low ridges. Blocks of sediment may be dislodged and pushed over the sea bed in front of the keel. In less cohesive soft sediments plough marks consist of more regular and continuous grooves, which are rapidly modified by currents and wave action. The keels of icebergs may either be single- or multi-pronged. Multi-pronged icebergs give rise to complex marks that consist of semi-parallel troughs. The motion of the iceberg is also important and is controlled by the surface wind, currents and the interaction of one iceberg with another. For example, the rotation of an iceberg keel during ploughing may change the shape of a plough mark. Alternatively, unstable or 'wobbly' icebergs may form grooves with tread - ridges perpendicular to the groove orientation, known as 'sprag' or 'jigger' marks. Small icebergs may also produce grooves with tread if they are lifted by waves in shallow water. Ploughing of icebergs through glaciomarine sediment may also cause tectonic disturbance, leading to faults and thrusts, in the underlying glaciomarine sediments. A grounded or stationary iceberg that settles on the sea bed but does not plough forward may produce a gravity crater. This may form partly under the weight of the iceberg and partly by current scour around the keel.

Other glaciomarine landforms include striated boulder pavements, which develop as ice advances over marine sediment. Boulder lags from on shallow continental shelf areas due to winnowing of diamicts, produced by the 'rain-out' of debris from icebergs, by waves and tidal currents. The top surfaces of these boulder lags are then striated and planed as the glacier advances over them (Figure 11.7).

Figure 11.7 Boulder pavements, South Shetland Islands. [Photograph: J. Hansom].

Finally it should be noted that normal subglacial processes continue beneath grounded ice, and subglacial landforms may develop. Flutes, drumlins and mega-scale glacial lineations may therefore be identified beneath marine margins provided the glacier was grounded. The morphology of these landforms may of course have been modified by current activity and by the deposition of grounding-line fans or the gentle 'rain-out' of glaciomarine sediment.

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