Glacial Geology and Geomorphology

3.3.1 Glacitectonic Landforms

Some of the most impressive landforms produced at the margins of sub-polar glaciers of the Canadian and Greenland high arctic are thrust-block moraines or composite ridges (Fig. 3.5) (Kalin, 1971; Evans, 1989b; Evans and England, 1991; Lemmen, et al. 1991; Lehmann, 1992). These landforms are constructed by proglacial glacitectonic disturbance of glacilacustrine, raised glacimarine or glacifluvial sediments on valley floors where the compressive stresses in the glacier snout are transmitted to unconsolidated sediments. Most thrust-block moraines of the region occur below local marine limit and in many cases record late Holocene readvances into recently emerged marine silts and/or recently deposited glacifluvial sediments (Blake, 1981; Evans and England, 1992). Because the moraines occur well below marine limit, the emergence and re-aggradation of permafrost likely date from the mid- to late Holocene. Occasionally, thrust-block moraines are constructed in glacilacustrine sediments that record former ice-marginal lakes.

Thrust-block moraines of the region are typically of the composite ridge form (Aber et al., 1989), comprising relatively intact blocks of sediment displaced in en-echelon arcs by proglacial

SO GLACIAL LANDSYSTEMS

Ice Glacial Marine Sediment
Figure 3.5 Thrust-block moraine incised by proglacial meltwater channels, Axel Heiberg Island. The prominent arcuate moraine marks the terminus position contemporaneous with thrusting. Small lakes have become ponded on the glacier-proximal side of the thrust-block ridges.

thrusting. Thrust blocks are commonly tens of metres high (sometimes as great as 70 m), hundreds of metres wide, and en-echelon they may be hundreds of metres long. Bedding in the displaced blocks generally dips back towards the glacier snout, suggesting that they are imbricately stacked scales or deep-seated blocks partially rotated during thrusting. However, some moraines comprise blocks with bedding dipping away from the snout, indicating that the glacier was responsible for deep-seated wedging of the proglacial materials (Fig. 3.5) (Evans and England, 1991).

The role of permafrost in the thrusting process has long been debated (cf. Mathews and Mackay, 1960; Mackay and Mathews, 1964; Klassen, 1982). Furthermore, the coincidence of thrust-block moraines and former deep-water sediments or low-altitude outwash suggests that the sea level history of the region may also be significant in moraine construction. It has been shown that during the last glaciation of the Canadian high arctic, permafrost was removed by geothermal heat beneath warm-based glaciers in most valleys where the glaciers were undergoing extending flow (Dyke, 1993). During subsequent deglaciation, the sea transgressed these valleys to marine limit. Due to the high heat capacity of water bodies, permafrost development was retarded. Thus, permafrost re-aggradation was prevented until glacioisostatic emergence occurred and re-aggradation is progressively less from marine limit to modern sea level. Permafrost thickness ranges from hundreds of metres near marine limit to tens of metres at lower elevations. Therefore, decollement surfaces suitable for the mass displacement of large sediment bodies occur at the base of the aggrading permafrost. These surfaces would provide natural planes of weakness. It has also been suggested that thrust-block moraines are a major landform of the surging glacier landsystem (Evans and Rea 1999, Chapter 11).

Recent glacier advances in some valleys have resulted in the thrusting of former ice-contact deltas deposited at marine limit during Late Wisconsinan deglaciation. An excellent example of this process is the margin of Hook Glacier, Makinson Inlet, Ellesmere Island, which has proglacially thrust an existing delta and is presently incorporating the topset gravels by apron overriding (Fig. 3.6). The role of thrust blocks in providing sediments for later entrainment by the overriding glacier has also been stressed by Evans (1989a, b) and by Evans and England (1991), although most examples in the Canadian high arctic have been only partially overridden. This usually gives rise to a zone of controlled moraine ridges (see below) being superimposed on the inner blocks of thrust-block moraines during downwasting of the glacier snout (Fig. 3.7). Thrust-block moraines may also act to dam small lakes into which glacilacustrine sedimentation can take place (Fig. 3.5).

3.3.2 Glacial Debris-Release Processes and Moraine Deposition

Ice-marginal recession in the Canadian and Greenland high arctic appears to be characterized by thinning and the release of debris-rich basal ice, as suggested by Goldthwait (1960, 1961, 1971). In piedmont lobes and valley outlet glaciers this process results in the supraglacial melt-out of debris-rich folia until the snout is covered by a debris layer that exceeds the active layer thickness (<0.5 m). The release of sediment from debris-rich basal ice of a receding and thinning glacier snout often produces transverse supraglacial debris concentrations referred to as controlled moraine (Benn and Evans, 1998). Debate continues as to the origin of the debris-rich folia, centring on the transfer of the material through the ice by shearing ('shear moraine'; Goldthwait, 1951; Bishop, 1957). The shearing mechanism was rejected by Weertman (1961) and Hooke (1968) prompting the names 'Thule-Baffin moraine' and 'ice-cored moraine' (Ostrem, 1959, 1963; Hooke, 1970).

The exposure of the complex debris-rich basal ice in many sub-polar glacier snouts, often with intense folds and thrusts, during snout downwasting leads to the construction of numerous transverse septa which ultimately control the pattern of differential ablation, meltwater flow and sediment reworking. The preservation potential of these moraines is low due to sediment redistribution during melt-out. At best the moraines may be represented in the landform record by discontinuous transverse ridges with intervening rubble hummocks. Even in situations where the buried glacier ice becomes part of the permafrost (see Dyke and Evans, Chapter 7), sediment reworking in the active layer will remove much of the inherited englacial structure. Hummocky till veneers interspersed with glacifluvial outwash tracts and occasional kames occur on valley floors where piedmont glaciers have receded onto surrounding uplands, leaving buried glacier snouts at lower elevations. Such buried snouts may be completely detached or remain connected to the outlet glacier via a debris-covered ramp.

At the margins of ice fields and upland outlet glaciers, particularly those associated with plateaux (Rea et al., 1998; Rea and Evans, Chapter 16), debris turnover is low and moraines are rare. Recent glacier recession in the eastern Canadian arctic is documented by the occurrence of lines of boulders and associated rubble veneers that form conspicuous trimlines (Fig. 3.8). Such features attest to very low debris turnover in these glacial systems, as does the lack of thick unconsolidated sediments in the valleys in which the glaciers terminate (Evans 1990a). However, the major outlet glaciers that occupied the fjords and trunk valleys of the region during the last glaciation have deposited extensive lateral moraines (e.g. Lemmen et al., 1991, 1994a; England et al., 2000) and widespread till sheets (e.g. Bednarski, 1998). This is predominantly a function of the thermal l/l M

Figure 3.6 Main photograph: Holocene ice-contact delta being proglacially thrust by the advancing margin of Hook Glacier, Makinson Inlet, southwest Ellesmere Island. Inset top photograph: view across thrust blocks of delta showing contorted bedding of fine-grained bottomsets and gravelly foresets. Inset bottom photograph: detail of topset gravels in state of partial entrainment by apron overriding of the thrust-block moraine.

Figure 3.6 Main photograph: Holocene ice-contact delta being proglacially thrust by the advancing margin of Hook Glacier, Makinson Inlet, southwest Ellesmere Island. Inset top photograph: view across thrust blocks of delta showing contorted bedding of fine-grained bottomsets and gravelly foresets. Inset bottom photograph: detail of topset gravels in state of partial entrainment by apron overriding of the thrust-block moraine.

Figure 3.7 Controlled moraine ridges superimposed on the inner blocks of a thrust-block moraine at the margin of the Eugenie Glacier, Dobbin Bay, Ellesmere Island. This area was overlain by glacier ice containing discrete ice margin-parallel debris-rich folia on 1959 aerial photographs. The melt-out of the buried ice and retrogressive flow sliding at this site is gradually destroying the controlled moraine ridges.

Figure 3.7 Controlled moraine ridges superimposed on the inner blocks of a thrust-block moraine at the margin of the Eugenie Glacier, Dobbin Bay, Ellesmere Island. This area was overlain by glacier ice containing discrete ice margin-parallel debris-rich folia on 1959 aerial photographs. The melt-out of the buried ice and retrogressive flow sliding at this site is gradually destroying the controlled moraine ridges.

characteristics of larger glaciers, the soles of which reached pressure melting point in most fjord/trunk valley systems.

3.3.4 Glacifluvial Processes and Forms

The most extensive evidence of glacier recession in the Canadian and Greenland high arctic are the numerous inset lateral meltwater channels cut along glacier margins. The nested patterns of such channels excavated in bedrock document the successive recessional positions of glacier snouts confined by topography (e.g. Hodgson, 1985; England, 1986, 1990; Lemmen, 1989; Evans, 1990a, b; Dyke, 1993; Bednarski, 1998; Smith, 1999; O Cofaigh et al., 1999, 2000; England et al., 2000) (Fig. 3.9a). Channel gradients are related to the gradient of the former ice margin. For example, low-gradient channels indicate similar low-gradient ice-surface profiles, probably related to rapid ice retreat and extensional flow, common in fjords, which would act to flatten the glacier profile (Fig. 3.9b). Steeper channels record a steeper glacier snout related to slower retreat, and are common inland of fjord heads where glacier margins become terrestrially based (Lemmen et al., 1994a; O Cofaigh, 1998).

Although subglacial meltwater has been reported in the sub-polar glaciers of the region, eskers are rare. This probably reflects both the restricted nature of subglacial drainage and the sparsity of debris available for meltwater transport. Conical mounds of gravel in some valley bottoms are interpreted as kames. Because large volumes of meltwater are directed along the frozen margins of sub-polar glaciers, any sediment carried by such meltwater can be deposited as kame terraces allowing the reconstruction of former ice margins (e.g. Lemmen, 1989; Evans, 1990b; Smith, 1999). Where the retreating glaciers are in contact with the sea or lakes, meltwater draining through these channels routinely forms deltas (O Cofaigh, 1998; England et al., 2000).

S4 GLACIAL LANDSYSTEMS

Figure 3.8 A trimline moraine comprising a line of boulders and a weakly developed moraine ridge, near Dobbin Bay, eastern Ellesmere Island. Inset photograph shows detail of the bouldery veneer that comprises the trimline moraine.

3.3.5 Rock Glacierization

Piedmont or tongue-shaped rock glaciers occurring at the base of cirque and valley glaciers, have not been reported from the Canadian and Greenland high arctic probably because of a lack of sufficient debris from surrounding slopes. Talus-foot or valley side rock glaciers are common, however, and have been subdivided by Evans (1993) into glacier ice-cored (glacial) and permafrost-related (periglacial) categories. The glacial ice-cored rock glaciers represent the former margins of outlet glaciers. Specifically, the lateral margins of outlet glaciers occupying major valleys are often characterized by supraglacial lateral moraines. These moraines later form discontinuous rock glaciers when the valley becomes deglaciated (England 1978). Paraglacial activity can also contribute to the production of rock glaciers in such settings where talus buries parts of ice margins during glacier downwasting (Fig. 3.10). Indeed, this relationship is so clear that extensive talus foot rock glaciers are often employed in the reconstruction of former glacier margins where they are thought to represent rock glacierized lateral moraines (England, 1978; Evans, 1990a, b, 1993).

3.3.6 Ice-Contact Glacimarine and Glacilacustrine Landforms

In the Canadian high arctic, research on modern marine-terminating glaciers and their sedimentary processes has been largely overlooked (Lemmen, 1990). Consequently, our understanding of glacimarine and glacilacustrine sedimentation from sub-polar glaciers in this region is largely based on investigations of emergent Holocene glacimarine, and to a lesser extent glacilacustrine, sediments (e.g. Bednarski, 1988; Evans, 1990a; Stewart, 1991; O Cofaigh, 1998; O Cofaigh et al., 1999; Smith, 2000). It is likely that many modern outlet

Figure 3.9 Examples of lateral meltwater channels formed by meltwater erosion along the frozen lateral margins of valley/fjord glaciers. A) Nested lateral meltwater channels, Phillips Inlet, Ellesmere Island. B) Low-gradient lateral meltwater channels recording rapid Early Holocene retreat of fjord glacier with cold-based margins, Blind Fiord, Ellesmere Island. Note that postglacial streams have cut gorges oblique to the meltwater channels.

Figure 3.10 Glacier ice buried by talus in inner Dobbin Bay, eastern Ellesmere Island.

glaciers produce subglacial meltwater where they enter the sea given the evidence for subglacial meltwater in their terrestrial counterparts (Iken, 1972; Skidmore and Sharp, 1999). By contrast, much more is known about contemporary glacimarine sedimentation in the fjords of East and West Greenland (e.g. Dowdeswell et al., 1994; Gilbert et al., 1998; O Cofaigh et al., 2001; Syvitski et al., 2001). Studies of glacimarine sedimentation associated with fast-flowing outlet glaciers in East Greenland demonstrate that deposition by subglacial meltwater is significant in this environment (O Cofaigh et al., 2001).

Where sub-polar glaciers terminate in marine or lacustrine environments as grounded or floating margins, englacial and subglacial meltwater emanating from these ice masses often constructs subaqueous depo-centres in the form of grounding line fans, ice-contact deltas and morainal banks. Spatially, the location of such ice-proximal glacimarine depo-centres exhibits a strong relationship to fjord bathymetry in that the most abundant sediment accumulations occur at topographic constrictions or areas of shallower water. Such areas acted as pinning points allowing retreating fjord glaciers to temporarily stabilize and deposit sediment. By contrast, between pinning points, glacimarine sediments are often sparse or absent, reflecting more rapid glacier retreat (e.g. England, 1987a; Lemmen et al., 1994a; O Cofaigh, 1998).

Grounding-line fans form where sediment-laden meltwater enters deep water from englacial, or more typically, subglacial conduits. The ice-proximal location of these fans dictates that their lithofacies are texturally and sedimentologically heterogeneous. Meltwater deposits formed by the settling of suspended sediment from turbid overflow plumes are characteristic of grounding-line fans in the Canadian high arctic (Evans, 1990a; Stewart, 1991; O Cofaigh et al., 1999). These sediments comprise various rhythmically interlaminated sand-mud (clay and silt) facies that have a drape-like geometry in section and contain variable amounts of ice-rafted debris (Fig. 3.11a). Mass-flow deposits are also a characteristic sedimentary component of grounding-line

Figure 3.11 Glacimarine sediments in Early Holocene grounding-line fans, Ellesmere Island.

A) Horizontally laminated sand-silt couplets with occasional small dropstones, interpreted as suspension deposits from turbid overflow plumes with background iceberg-rafted debris.

B) Channelized normally graded gravel with sharp erosional contacts, interpreted as the product of deposition from a high-concentration turbidity current, Ellesmere Island.

Figure 3.11 Glacimarine sediments in Early Holocene grounding-line fans, Ellesmere Island.

A) Horizontally laminated sand-silt couplets with occasional small dropstones, interpreted as suspension deposits from turbid overflow plumes with background iceberg-rafted debris.

B) Channelized normally graded gravel with sharp erosional contacts, interpreted as the product of deposition from a high-concentration turbidity current, Ellesmere Island.

fan sequences in high-arctic fjords (Fig. 3.11b). Resedimentation is common due to high sedimentation rates, which result in oversteepening, failure and downslope transport. Mass-flow deposits range from channelized units of massive or variably graded, gravel and sand recording deposition from high-density turbidity currents, to finer-grained (mud and sand) laminated and stratified turbidites (Stewart, 1991; Gilbert et al., 1998). Subglacial tills extruded at the ice margin or ice-rafted diamicts may also undergo resedimentation by cohesive debris flow to produce crudely stratified and massive matrix-supported diamict facies.

Subaqueous morainal banks are transverse, elongate landforms deposited along grounding lines during intervals of glacier terminus stability (Fig. 3.12a). They are commonly composed of coalescent grounding-line fans and generally range in height from about 5—30 m. Morainal bank size is controlled by the duration of grounding-line stability, sedimentation rate and availability of debris for entrainment. The elongate morphology of morainal banks reflects their origin by deposition from a series of point sources along the ice front, as well as by ice-marginal fluctuations that act to bulldoze and squeeze sediment along the grounding line. Depending on the availability of debris for entrainment and sedimentation rates, emergent morainal banks in the Canadian high arctic range from massive diamict veneers (<5 m thick) over striated bedrock to thicker accumulations of mud, sand and diamict. These include rhythmically laminated muds deposited by suspension sedimentation, and diamict facies ranging from massive tills and weakly-graded subaqueous debris flow deposits (Fig. 3.12b), to massive iceberg-rafted diamicts with in situ marine macrofauna (England, 1987b; Evans, 1990a; Stewart, 1991; O Cofaigh, 1998).

Grounding-line fans and morainal banks can aggrade to sea level and form marine limit deltas. These deltas commonly form isolated flat-topped hills in the landscape, often located on topographic highs, which acted as pinning points during glacier retreat. Distinguishing characteristics are steep ice-proximal slopes and pitted surfaces (kettle holes) due to the melt-out of buried ice (e.g. Evans, 1990b; O Cofaigh, 1998; England et al., 2000; Fig. 3.13a). Where the ice margin has retreated above sea or lake level, it is commonly separated from the sea or lake by a braided outwash plain, which forms a delta where it enters standing water (Fig. 3.13b). Such glacier-fed deltas are common at many fjord heads throughout the Canadian and Greenland high arctic (e.g. Gilbert, 1990b; Gilbert et al., 1993, 1998; England et al., 2000).

In general, ice-contact and glacier-fed high-arctic deltas have a tripartite internal structure consisting of topsets, foresets and bottomsets (Fig. 3.13c). Delta topsets are essentially braided river deposits, and consist of sub-horizontally-bedded, massive gravel, with an a-axis transverse, b-axis imbricate, clast fabric. Foreset beds range from gravel to sand facies, depending on the depositional mechanism, sedimentation rate and proximity to the glacier. Commonly, however, mass-flow sediments dominate foreset beds (Fig. 3.13d). High sedimentation rates and mixing during downslope remobilization means that some of these units may be very poorly sorted. With increasing distance downslope, foresets become finer-grained and eventually grade into proximal bottomset beds of massive and graded sands and silts (Fig. 3.13e). Distal bottomset beds are composed of finer-grained silts and clays that in the marine environment often contain well-developed macrofaunal assemblages. Ice-proximal slopes are often characterized by a range of gravitational and soft-sediment deformation structures, slumps and normal faults, which are related to melt-out of buried ice and/or collapse associated with glacier retreat. In addition, the ice-proximal sides may be glacitectonized and contain subglacially-remoulded sediments in the form of glacitectonite and deformation till.

Figure 3.12 A) Arcuate morainal bank inset between fjord side and ice-moulded bedrock highs, Ellesmere Island. B) Stacked beds of massive, matrix-supported diamict with outsized clasts at bed tops, deposited by subaqueous cohesive debris flows.

Ice-dammed lakes are common in the region due to cold-based glacier margins (Fig. 3.14) (Maag, 1969, 1972), and raised shorelines often record their drainage (e.g. Hattersley-Smith, 1969b; Maag, 1969). Glacilacustrine sediments are also routinely documented from recently deglaciated terrain in the high arctic. However, studies on glacilacustrine landform development and sedimentation are rare (e.g. Smith, 2000). Sediment input to ice-dammed lakes is dictated by the debris-content of the glaciers in the catchment, and typical landforms produced in lakes with sufficient debris supply are ice-contact deltas, beaches and incised terraces, the latter recording the downcutting by meltwater streams through lake sediments during and after lake drainage. Because lake water tends to drain over cold-based glacier barriers, rather than through or under them, the most prominent landforms produced in association with ice-dammed lakes are erosional spillway channels (Maag, 1969, 1972; Smith, 1999).

Raised Sea Bed Geology

Figure 3.13 A) Raised marine ice-contact delta, western Ellesmere Island. B) Braided outwash plain entering the sea, western Ellesmere Island. C) Detail of internal stratigraphy of ice-contact Gilbert-type raised marine gravel delta, western Ellesmere island. Note the horizontally bedded topset gravels which unconformably overlie planar crossbedded foresets. D) Delta foreset bed composed of normally-graded open-work gravels deposited by high-concentration turbidity current, Ellesmere Island. E) Delta bottomsets composed of normally-graded sands with silty mud cap, western Ellesmere Island.

Figure 3.13 A) Raised marine ice-contact delta, western Ellesmere Island. B) Braided outwash plain entering the sea, western Ellesmere Island. C) Detail of internal stratigraphy of ice-contact Gilbert-type raised marine gravel delta, western Ellesmere island. Note the horizontally bedded topset gravels which unconformably overlie planar crossbedded foresets. D) Delta foreset bed composed of normally-graded open-work gravels deposited by high-concentration turbidity current, Ellesmere Island. E) Delta bottomsets composed of normally-graded sands with silty mud cap, western Ellesmere Island.

As sub-polar glacier margins are prone to float upon contacting deep water they have the potential to deposit horizontal moraines or ice-shelf moraines at their margins (England et al., 1978). However, ice-shelf moraines tend to be rare in the Canadian and Greenland high arctic and have (to date) only been documented from northeast and east Ellesmere Island (England, 1978, 1999; England et al., 2000) and northwest Greenland (England, 1985).

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