Geomorphology and Sedimentology of Receding Svalbard Glaciers

4.3.1 Moraine Complexes

Modern glaciers in Svalbard are invariably associated with large end-moraine complexes formed during the Neoglacial maximum in the late nineteenth or early twentieth century (Fig. 4.7). Some glaciers have receded 1—2 km from these complexes, leaving smaller moraine systems in the intervening zone. Some authors have referred to these moraine complexes as 'push moraines' (e.g. Croot, 1988a; Hagen, 1987, 1988; Van der Wateren, 1995; Boulton et al., 1996, 1999) although not all Svalbard moraine complexes are produced by ice-push and in many cases thrusting of sediment and ice is the primary mechanism (Bennett, 2001). The descriptive term 'moraine-mound complex' should therefore be used to describe the large moraine systems in front of receding Svalbard glaciers (Hambrey and Huddart, 1995; Huddart and Hambrey, 1996; Hambrey et al., 1997; Bennett et al., 1999). The genetic term 'thrust-moraine complex' should only be used where this mechanism has been clearly demonstrated.

4.3.2 Moraine Complexes Produced by Thrusting 4.3.2.1 Morphology

Svalbard moraine-mound complexes commonly comprise arcuate belts of aligned hummocks or mounds comprising a wide variety of morphological types. They include linear ridges some 100 m long, short-crested ridges several metres long, and near-conical mounds reaching elevations of several metres. Irrespective of size, before degradation, they show ice-proximal rectilinear or curvilinear slopes with consistent angles of around 30°, and irregular distal slopes that are commonly steeper, formed by the stacking of different sedimentary facies (Fig. 4.7a). Stacking indicates thrusting in proglacial, ice-marginal and englacial positions (Hambrey et al., 1997). Ice cores may be present, leading to the degradation of many mounds by mass-movement processes (Fig. 4.7b). Thrusting and stacking of thick sediment wedges promotes the survival of the initial moraine morphology, particularly if the sediments are not prone to subsequent reworking by glacigenic sediment flows (e.g. free draining gravels). However, large-scale degradation is likely if

Glacial Flutes

Figure 4.7 Landform-sediment assemblages at Svalbard glaciers. A) Imbricate stacking of moraine mounds in front of Kronebreen (figure for scale). Thrust planes between the individual mounds are shown up by shadows between the mounds. Former ice flow from left to right. B) Moraine-mound complex ('hummocky moraine') in front of Kronebreen. Here the moraine-mounds are more degraded. Note the reworking of moraines by glacifluvial processes in the foreground of the photograph. C) Group of small flutes at the margin of Midtre Lovenbreen. Former glacier flow is from bottom left to top right of the photograph. Note moraine-mound complex beyond the flutes.

substantial quantities of ice are buried during formation, and if the sediments are composed of clay-rich diamicton that is prone to flowage when wet.

4.3.2.2 Sedimentary Facies

The sedimentary facies in moraine-mound complexes are as varied as the material over which the glacier flows (Fig. 4.8). Most of the terrestrial glaciers flow over ground occupied by extensive sheets of glacifluvial sediment and diamicton of basal glacial origin. These facies dominate the moraine assemblages at Uversbreen (Hambrey and Huddart, 1995), Pedersenbreen (Bennett et al., 1996a) and Midtre Lovenbreen (Table 2.1; Glasser and Hambrey, 2001a). Tidewater glaciers, such as Comfortlessbreen (Huddart and Hambrey 1996) and Kongsvegen/Kronebreen (Bennett et al., 1999) tend to rework glacimarine deposits, ranging from ice-contact facies (diamictons, coarse gravels), through ice-proximal laminites (cyclopsams and cyclopels) to distal muds with dropstones up to boulder-size. Mixing with terrestrial sediments occurs where tidewater glaciers advance from the sea onto adjacent land. The wide variety of facies, however, is often organized systematically. A single ridge is commonly composed of one facies, but may be stacked as an inclined slab on another ridge of a different facies.

Preservation of sedimentary structures, for example cross-bedding and grading in sands and gravels or laminations with dropstones in fine-grained distal sediments, allows determination of the

Glacial Lithofacies PicturesGlacial Lithofacies Pictures

Figure 4.8 Sedimentary facies produced by Svalbard valley glaciers. A) Clast-rich diamicton on the forefield of Kronebreen, interpreted as a basal till. B) Diamicton with anastomosing planar fabric interpreted as a shear fabric in front of Austre Lovénbreen. C) Muddy cobble/boulder gravel in front of Midtre Lovénbreen. This lithofacies is interpreted as an ice-marginal facies, created by the reworking of glacifluvial deposits and subsequent mixing with basal glacial material.

D) Reworking of sediments on the forefield of Austre Lovénbreen by proglacial streams.

E) Glacifluvial facies in moraine mounds at Finsterwalderbreen. F) Glacimarine sand-mud laminites in moraine mound, Comfortlessbreen. G) Glacimarine shelly mud in moraine mound, Comfortlessbreen.

Figure 4.8 Sedimentary facies produced by Svalbard valley glaciers. A) Clast-rich diamicton on the forefield of Kronebreen, interpreted as a basal till. B) Diamicton with anastomosing planar fabric interpreted as a shear fabric in front of Austre Lovénbreen. C) Muddy cobble/boulder gravel in front of Midtre Lovénbreen. This lithofacies is interpreted as an ice-marginal facies, created by the reworking of glacifluvial deposits and subsequent mixing with basal glacial material.

D) Reworking of sediments on the forefield of Austre Lovénbreen by proglacial streams.

E) Glacifluvial facies in moraine mounds at Finsterwalderbreen. F) Glacimarine sand-mud laminites in moraine mound, Comfortlessbreen. G) Glacimarine shelly mud in moraine mound, Comfortlessbreen.

original sedimentary facies. Intact and broken shells are a feature of reworked glacimarine sediment. Diamicton, on the other hand, being more susceptible to ductile deformation rarely preserves its original fabric. Typical basal till fabrics, with strong preferred alignment of clasts, are often (though not always) destroyed during thrusting.

4.3.2.3 Internal Structure

The internal structure of individual mounds has been documented in a number of cases, indicating evidence of deformation (Hambrey and Huddart, 1995; Bennett et al., 1999). In mounds containing mud and sand, typically structures are low-angle thrust-faults, various normal faults and recumbent folds with sheared-off lower limbs. Such a combination of structures develops under strong longitudinal compression. Sometimes, however, the moraines show an earlier phase of longitudinal extension, such as boudinage. In mounds dominated by reworked sand and gravel, the degree of internal deformation is limited. Sedimentary bedding may be rotated, but otherwise survives intact within thrust slices, as in the gravels of Uversbreen. Alternatively, bedding may be strongly modified, as in the fine-grained glacimarine sediments of Kronebreen and Comfortlessbreen.

Lithofacies (and interpretation)

Abundance

% clasts

Matrix (%)

Sorting coefficient

Sorting category

Sand

Silt

Clay

Clast-poor intermediate diamicton (basal glacial)

*

5

48

33

19

4.05

Extremely poorly sorted

Clast-rich sandy diamicton (basal glacial)

**

30

75-90

8-20

2-8

1.5-2.87

Very poorly sorted

Clast-rich intermediate diamicton (basal glacial)

***

25-35

41-83

5-43

11-20

2.91-4.27

Very poorly sorted

Clast-rich muddy diamicton (basal glacial)

*

30

15

78

7

1.22

Poorly sorted

Sandy gravel (glacifluvial)

**

10-40

92-98

1-6

1-2

0.87-1.61

Poorly sorted

Gravel (type 1) (fluvial)

**

80-95

75-90

6-19

2-8

1.78-2.96

Very poorly sorted

Gravel (type 2) (subglacial fluvial)

*

100

-

-

-

-

Well sorted

Gravel with sand (glacifluvial)

*

70-90

90-98

1-7

1-3

0.73-2.82

Moderately sorted to poorly sorted

Sand and mud (lacustrine)

**

0-5

76-99

1-19

1-5

0.88-1.12

Moderately sorted to poorly sorted

Table 4.1 Summary of lithofacies identified on the forefield of Midtre Lovenbreen. Key to abundance: *** = dominant, ** = prevalent, * = rare.

Table 4.1 Summary of lithofacies identified on the forefield of Midtre Lovenbreen. Key to abundance: *** = dominant, ** = prevalent, * = rare.

4.3.3 Moraine Complexes Resulting from Deformation of Permafrost

Drawing on data from two Svalbard valley glaciers, Usherbreen and Erikbreen, Etzelmuller et al. (1996) have suggested that the deformation of permafrost is important in the formation of ice-cored moraines and push moraines. Usherbreen is a surge-type glacier (Hagen, 1987), while Erikbreen is not. Stresses beneath the advancing glaciers are transmitted to the proglacial sediments and can be sufficient to cause proglacial deformation of the permafrost layer. Folding, thrust-faulting and overriding of proglacial sediments are possible under these conditions (Fig. 4.9). The nature of the deformation is controlled by the mechanical properties of the sediment, which is influenced by the water content and thermal condition (frozen/unfrozen). Typical landform/sediment associations are:

• an outermost push-moraine system

• an arc of ice-cored moraines, and

• an innermost area dominated by glacigenic sediment flows.

The deformed material can consist of a wide variety of sedimentary facies, including glacifluvial, glacilacustrine, glacimarine deposits and subglacial deposits.

Elsewhere in Svalbard, notably at Uvêrsbreen (Hambrey and Huddart, 1995) and Comfortlessbreen (Huddart and Hambrey, 1996) the outer parts of the moraine complexes represent deformation beyond the ice margin as stress was transmitted into frozen glacifluvial sediments in front of the glacier during the Neoglacial maximum. In such circumstances a basal décollement surface must have propagated at depth to form the outermost thrust.

4.3.4 Other Constructional Landforms

At least four other types of constructional landforms exist in the proglacial areas of Svalbard glaciers: linear debris stripes, foliation-parallel ridges, geometrical ridge networks and streamlined ridges/flutes.

Linear debris stripes are produced by the folding of supraglacially-derived debris layers in the ice, as described above. These are derived from folded stratified layers and emerge at the glacier surface as medial moraines as a result of ablation near the snout (Hambrey et al., 1999). Debris is released from the ice as regular stripes of angular debris extending for considerable distances across the proglacial area (see Figs. 4.3 and 4.4). Commonly, linear debris stripes drape moraine-mound complexes. Individual debris stripes can often be traced to their source areas in the headwall of a glacier, where they are fed by rockfall material. These debris stripes are recognizable by their angular, unilithological nature and lack of fine matrix. Debris stripes survive as prominent features on the forefield following deposition because the large blocks and lack of associated fine sediment fails to support extensive vegetation.

Foliation-parallel ridges are ridges of basally-derived debris (Bennett et al., 1996b; Glasser et al., 1998a). They are particularly well-developed at surge-type Kongsvegen, where significant quantities of basal material are observed on the glacier surface parallel to longitudinal foliation. Other degraded examples occur at Vestre Lovénbreen, a non-surge-type glacier. Although the ridges are typically 1—2 m wide and up to 1.5 m high, the source debris layers in the ice below are rarely greater than 0.1 m wide. The dispersion of material associated with these features is a result of the melting of their ice core. Some ridges are composed of a clast-rich sandy diamicton,

Clastic Dispersion

Figure 4.9 Structures and sedimentary facies associated with the deformation of permafrost at Erikbreen: A) The snout and proglacial area of the glacier. Solid rectangles indicate the locations of Sections 1 and 2. B) Section 1, showing an example of an ice-cored moraine. Unit I = foliated stratified sediments of glacifluvial origin, Unit II = stratified sands of glacifluvial origin, Unit III = horizontally stratified sandy silts, Unit IV = glacigenic sediment flow. C) Section 2, showing folded and thrust glacifluvial and glacimarine sediments in a push moraine. (Modified from Etzelmuller et al. (1996).)

Figure 4.9 Structures and sedimentary facies associated with the deformation of permafrost at Erikbreen: A) The snout and proglacial area of the glacier. Solid rectangles indicate the locations of Sections 1 and 2. B) Section 1, showing an example of an ice-cored moraine. Unit I = foliated stratified sediments of glacifluvial origin, Unit II = stratified sands of glacifluvial origin, Unit III = horizontally stratified sandy silts, Unit IV = glacigenic sediment flow. C) Section 2, showing folded and thrust glacifluvial and glacimarine sediments in a push moraine. (Modified from Etzelmuller et al. (1996).)

characterized by subangular and subrounded clasts, which are occasionally striated. Lithologically, these foliation-parallel ridges are highly variable. The ridges can often be traced onto the glacier forefield as low (<1 m high) ridges. The foliation-parallel ridges are important as the incorporation of basal debris along longitudinal foliation is not a universally acknowledged process, and similar ridges elsewhere may have been mistaken for flutes. The mechanism invoked to explain this process is one where lateral compression of ice leads to the development of a transposition foliation parallel to flow, combined with the incorporation of basal debris-rich ice or soft basal sediment in the fold complex (Glasser et al., 1998a; Hambrey et al., 1999; Fig. 4.6). The base of the deforming layer represents a décollement surface that may represent the contact with bedrock. Incorporation of debris must take place where the ice is wet-based. The incorporation of debris by this process clearly precedes most thrusting, as foliation-parallel ridges are truncated by thrust moraines in the proglacial area. Their preservation potential appears to be low, because of destruction or burial by mass-movement and fluvial processes.

Geometrical ridge networks are created when both longitudinal and transverse debris accumulations melt out of the glacier and become superimposed. This landform-sediment assemblage has been described in the proglacial area of Kongsvegen as a result of the 1948 surge (Bennett et al., 1996b). Here, small (4—8 m high) thrust ridges intersect low (<1 m) debris ridges or foliation-parallel ridges to form a complex of cross-cutting ridges on the glacier forefield. Although so far only observed at this glacier, there is no reason to suppose that they should not also occur at non-surge-type glacier margins. However, the preservation potential of these networks is probably low, as they are continually modified by slope processes and glacigenic sediment flows.

Flow-parallel ice structures can also incorporate large quantities of glacifluvial sediment, helping to redistribute sediment within the glacier. On Marthabreen, for example, two types of longitudinal debris-rich structures have been described on the glacier surface: 'longitudinal sediment structures' and 'longitudinal ridge accumulations' (Glasser et al., 1999). Longitudinal sediment structures are ridges of sand and gravel, commonly 1—6 m long and 0.5 to 1.0 m high. They vary in width from 0.05 to 0.15 m and are always sub-parallel to the foliation. Their sediment fill consists of fine sand and granule gravel, and they are interpreted as former englacial channels formed parallel to longitudinal foliation (Glasser et al., 1999). Longitudinal ridge accumulations are larger ridges that attain dimensions of between 20 and 30 m in length and 1—3 m in height. They have crests of in situ sand and gravel, while the ridge flanks contain slumped debris. These ice-cored sediment ridges frequently occur downstream of sediment structures or other debris pinnacles and have been interpreted as the product of sediment reworking by englacial or supraglacial streams flowing sub-parallel to the longitudinal foliation (Glasser et al., 1999). The preservation potential of both types of structurally-controlled supraglacial and englacial fluvial deposits is probably low and their landform manifestation has yet to be observed in the proglacial areas of Svalbard glaciers.

Streamlined ridges have been described from the forefield of both Austre and Midtre Lovénbreen (Glasser and Hambrey, 2001a). They are between 25 and 50 m wide, up to 200 m in length, and reach 7 m in height. The ridges are elongated in the direction of glacier flow and, at the glacier margin, the ridges emerge from beneath the receding glacier. They generally comprise, from bottom to top: muddy-sandy gravel, diamicton, variable gravel and angular gravel. Flutes and fluted surfaces are also developed on the flat areas of the forefields at Midtre and Austre Lovénbreen (see Fig. 4.7c). Close to the ice margins, the ridges form low (<0.5 m high) and elongated (>10 m) fluted ridges composed of diamicton. The majority of these flutes commence in the lee of boulders, indicative of a subglacial origin. Further from the glacier margins, the flutes degrade rapidly and lose much of their surface relief. These small and relatively fragile landforms probably have a low preservation potential in the landform record.

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