Landsystems as modern analogues

The landsystems approach is given more credibility and applicability wherever landscape evolution can be monitored, thereby providing modern analogues for the interpretation of ancient glaciated terrain (e.g. Price, 1969; Gustavson & Boothroyd, 1987; Kruger, 1994; Dyke & Savelle, 2000; Kjsr & Krüger, 2001; Evans & Twigg, 2002). This type of research on modern glaciers has led to the identification of landform-sediment suites indicative of specific styles of glaciation (e.g. plateau icefields (Rea et al., 1998; Rea & Evans, 2003) and arid polar glacier margins (Fitzsimons, 2003)) and ice dynamics (e.g. surging glaciers (Evans & Rea, 1999; 2003) and ice streams (Stokes & Clark, 1999, 2001; Clark et al., 2003a)). Once a landform-sediment suite pertaining to a single period of glacier occupancy or activity can be identified, it often becomes possible to differentiate overprinted signatures (e.g. Dyke & Morris, 1988; Clark, 1993, 1999; Krüger, 1994) and landscape palimpsests.

Examples of terrestrial landsystems models that have recently elaborated upon earlier landsystems classifications by incorporating modern process research include those of the glaciated valley (Spedding & Evans, 2002; Benn et al., 2003) and active temperate glacier margin (Evans & Twigg, 2002; Evans, 2003b). These landsystems represent particular styles of glaciation according to glacier morphology and environmental controls. For example, it has been recognized that the details of glaciated valley landsystems will reflect the relative relief, bedrock, climatic regime and debris supply and transfer rates of the mountainous terrain in which they are located (e.g. Boulton & Eyles, 1979; Owen &

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Derbyshire, 1989; Evans, 1990; Spedding & Evans, 2002; Benn et al., 2003). These interrelated controls are emphasized by Benn et al. (2003) in the development of a conceptual continuum of glaciated valley landsystems (Fig. 18.2). Specifically, Benn et al. (2003) identify distinct landsystem associations that are defined by the degree of coupling between glacial and proglacial environments via the glacifluvial system. Water discharges from a glacier are a function of catchment area, climatically determined catchment water storage and mean precipitation. Therefore, although there is a climatic significance attached to the glacial geomorphology, it is modulated by catchment size and topography. Coupled and decoupled glacier snouts are thereby defined respectively as those with efficient and inefficient glacifluvial transport between glacial and proglacial systems. At coupled glacier margins, due to the effective sediment transfer from the snout to the fluvial system, moraine development is limited and large amounts of sediment pass through the proglacial zone. This gives rise to the dominance of the outwash head/aggrading sandur landsystem. At decoupled glacier margins there is insufficient meltwater discharge to transfer sediment away from the snout, giving rise to the build up of debris around the glacier perimeter to form moraines. The repeated superposition of moraines around debris-covered glacier margins leads to the construction of giant bounding moraines and moraine-dammed glaciers and also rock glaciers in some settings.

The active temperate glacial landsystem (Evans & Twigg, 2002; Evans, 2003b) is a development of the most prominent terrestrial component of the subglacial landsystem of Eyles & Menzies (1983) in that a modern lowland glacier snout, Breidamerkur-jokull in Iceland, overlying a varied substratum is used as a central case study. Recent research on process-form relationships at modern glacier margins highlights three depositional domains (Fig. 18.3). First, the marginal morainic domain consists of extensive, low-amplitude marginal dump, push and squeeze moraines derived largely from material on the glacier foreland. These moraines often record annual recession of active ice but some may be superimposed during periods of glacier stability (Price, 1970; Sharp, 1984). Subglacial fluting and push moraine production is genetically linked wherein the deformation/ploughing of subglacial material into flutings results in the advection of sub-glacially deforming sediment to the ice margin where squeezing and pushing then combine to create moraines. Larger push moraines are constructed by stationary glacier margins involving either the stacking of frozen sediment slabs, the prolonged impact of dump, squeeze and push mechanisms at the same location, or the incremental thickening of an ice-marginal wedge of deformation till. Overridden push moraines are recognizable as arcuate, low-amplitude ridges draped by flutings and recessional push moraines. The paucity of supraglacial sediment in active temperate glaciers generally precludes the widespread development of chaotic hummocky moraine, although low-amplitude, bouldery hummocks are produced by the melt out of medial moraines and by the melting of debris-charged glacier snouts in settings where marginal freeze-on produces debris-rich ice facies. Second, the subglacial domain includes assemblages of flutings, drumlins and overridden push moraines on surfaces that lie between ice-marginal depocentres. Subglacial materials often display a vertical continuum comprising glacitectonized stratified sediments capped by subglacial till. The flutings are traditionally explained as the products of till squeezing into cavities on the down-glacier sides of lodged boulders. Larger drumlins are explained as the streamlined remnants of coarse-grained sandur fans (Boulton, 1987). Third, the glacifluvial and glacilacustrine domain is characterized by sandur fans (both ice-contact and spillway fed), ice-margin-parallel outwash tracts and kame terraces, topographically channelized sandar, pitted sandar (ice-marginal and jokulhlaup types), and eskers of single and more complex anabranched forms. Although hummocky terrain located at receding glacier margins is often referred to as 'kame and kettle topography' it can evolve through time due to melt out of underlying ice into complex networks of anabranched eskers. The clear landform-sediment signatures of active temperate glacier recession have been recognized in some ancient glaciated terrains (e.g. Evans et al., 1999), thereby demonstrating the potential of the landsystems approach for deciphering palaeoglacier dynamics and their linkages to climate change.

Examples of landsystems that incorporate the landform and sediment suites indicative of particular glacier dynamics are the palaeo-ice stream (see Stokes & Clark, this volume, Chapter 26) and surging glacier landsystems (Evans & Rea, 1999,2003). These landsystem signatures are contained within the larger imprints of ice sheets or icefields. For example, Evans & Rea (1999,2003) recognize a suite of landforms and sediments produced by modern glacier surging at the margins of upland icefields in Iceland and

Figure 18.1 Examples of glaciated valley landsystems. (a) High-relief terrain, from Owen & Derbyshire (1989) based upon glaciers located in the Karakoram Mountains where rates of debris supply are extremely high. Number codes are: (1) truncated scree; (2) termino-lateral dump moraine; (3) lateral outwash channel; (4) glacifluvial fan; (5) slide moraine; (6) slide-debris flow cone; (7) slide-modified lateral moraine; (8) lateral outwash fan; (9) meltwater channel; (10) meltwater fan; (11) abandoned meltwater fan; (12) bare ice; (13) trunk valley river; (14) debris flow; (15) flow slide; (16) gullied lateral moraine; (17) lateral moraine; (18) ablation valley lake; (19) ablation valley; (20) supraglacial lake; (21) supraglacial stream; (22) ice-contact terrace; (23) lodgement till; (24) roche moutonnée; (25) fluted moraine; (26) diffluence col; (27) high-level till remnant; (28) diffluence col lake; (29) fines from supraglacial debris; (30) ice-cored moraines; (31) river alluvium; (32) supraglacial debris; (33) dead ice. (b) Low-relief mountain terrain, from Benn & Evans (1998) based upon northwest Europe where supraglacial inputs of debris are relatively low. Number codes are: (1) supraglacially entrained debris; (2) periglacial trimline above ice-scoured bedrock; (3) medial moraine; (4) fluted till surface; (5) paraglacial reworking of glacigenic deposits; (6) and (7) lateral moraines, showing within-valley asymmetry.

Figure 18.2 Constraints on landsystems development around valley glaciers, showing development pathways to four landsystem associations (from Benn et al., 2003).

Medial moraine

Medial moraine

Figure 18.3 The active temperate glacial landsystem (after Krüger, 1994; Evans & Twigg, 2002; Evans, 2003b). Landforms are numbered according to their domain (1, morainic domain; 2, glacifluvial domain; 3, subglacial domain): (1a) small, often annual, push moraines; (1b) superimposed push moraines; (1c) hummocky moraine; (2a) ice-contact sandur fans; (2b) spillway-fed sandur fan; (2c) ice-margin-parallel outwash tract/kame terrace; (2d) pitted sandur; (2e) eskers; (2f) entrenched ice-contact outwash fans; (3a) overridden (fluted) push moraines; (3b) overridden, pre-advance ice-contact outwash fan; (3c) flutes; (3d) drumlins. The idealized stratigraphical section-log shows a typical depositional sequence recording glacier advance over glacifluvial sediments, comprising: (i) undeformed outwash; (ii) glacitectonized outwash/glacitectonite; (iii) massive, sheared till with basal inclusions of pre-advance peat and glacifluvial sediment; (iv) massive sheared till with basal erosional contact.

Figure 18.3 The active temperate glacial landsystem (after Krüger, 1994; Evans & Twigg, 2002; Evans, 2003b). Landforms are numbered according to their domain (1, morainic domain; 2, glacifluvial domain; 3, subglacial domain): (1a) small, often annual, push moraines; (1b) superimposed push moraines; (1c) hummocky moraine; (2a) ice-contact sandur fans; (2b) spillway-fed sandur fan; (2c) ice-margin-parallel outwash tract/kame terrace; (2d) pitted sandur; (2e) eskers; (2f) entrenched ice-contact outwash fans; (3a) overridden (fluted) push moraines; (3b) overridden, pre-advance ice-contact outwash fan; (3c) flutes; (3d) drumlins. The idealized stratigraphical section-log shows a typical depositional sequence recording glacier advance over glacifluvial sediments, comprising: (i) undeformed outwash; (ii) glacitectonized outwash/glacitectonite; (iii) massive, sheared till with basal inclusions of pre-advance peat and glacifluvial sediment; (iv) massive sheared till with basal erosional contact.

Figure 18.4 Landsystems model for surging glacier margins (after Evans et al., 1999; Evans & Rea, 1999, 2003): (a) outer zone of proglacially thrust pre-surge sediment which may grade into small push moraines in areas of thin sediment cover; (b) zone of weakly developed chaotic hummocky moraine located on the down-ice sides of topographic depressions; (c) zone of flutings, crevasse-squeeze ridges and concertina eskers; 1, proglacial outwash fan; 2, thrust-block moraine; 3, hummocky moraine; 4, stagnating surge snout covered by pitted and channelled outwash; 5, flutings; 6, crevasse-squeeze ridge; 7, overridden and fluted thrust-block moraine; 8, concertina esker; 9, glacier with crevasse-squeeze ridges emerging at surface.

Figure 18.4 Landsystems model for surging glacier margins (after Evans et al., 1999; Evans & Rea, 1999, 2003): (a) outer zone of proglacially thrust pre-surge sediment which may grade into small push moraines in areas of thin sediment cover; (b) zone of weakly developed chaotic hummocky moraine located on the down-ice sides of topographic depressions; (c) zone of flutings, crevasse-squeeze ridges and concertina eskers; 1, proglacial outwash fan; 2, thrust-block moraine; 3, hummocky moraine; 4, stagnating surge snout covered by pitted and channelled outwash; 5, flutings; 6, crevasse-squeeze ridge; 7, overridden and fluted thrust-block moraine; 8, concertina esker; 9, glacier with crevasse-squeeze ridges emerging at surface.

Svalbard (Fig. 18.4). This includes an outer zone of thrust-block moraines and push moraines produced by rapid ice advance into proglacial sediments and the consequent failure and stacking of large contorted and faulted blocks due to high proglacial and submarginal compressive stresses. Evidence of earlier surges is often manifest in the form of overridden thrust-block moraines. These are recognizable as ice-moulded (fluted) hills on the down-ice side of topographic depressions from which they were displaced by thrusting. Inside the thrust-block moraines of an individual surge lies a zone characterized by 'concertina' or 'zig-zag' eskers, crevasse-squeeze ridges, flutings and pockets of hummocky moraine and ice-cored outwash. The zig-zag eskers were produced englacially or supraglacially by meltwater that exploited the extensive network of crevasses created during the surge. The crevasse-squeeze ridges are the product of subglacial sediment squeezing into the widespread cross-cutting basal crevasses produced during the surge. The intersection points of flutings and crevasse-squeeze ridges show that the sediments of both land-forms were displaced upwards into crevasses, demonstrating that the flutings are also surge features. Moreover, the great length of the flutings verifies that they were produced by a fast-flow event. The hummocky moraine is produced by the overriding, over-thrusting and incorporation of debris-rich stagnant ice dating to a previous surge. Additionally, thrusting, squeezing and bulldozing of proglacial lake sediments and outwash lying over pre-existing stagnant ice also occur. The post-surge evolution of such supraglacial lake and outwash sediments is manifest in tracts of ice-cored outwash fans and glacilacustrine sediment bodies with surfaces that become increasingly kettled and incised through time, exposing stagnant ice in the sides of ice-walled channels. The surging glacier landsystem has been applied to the regional geomorphology of western Canada (Evans et al., 1999; Evans & Rea, 2003) by highlighting the spatial association of thrust-block moraines, crevasse-squeeze ridges and long flutings produced by an ice stream within the receding Laurentide Ice Sheet.

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