Summary of the Landsystems Model for Surging Glaciers

It has been demonstrated above that no single landform can be used to identify palaeo-glacier surging in the landform record. However, landform-sediment assemblages on the forelands of contemporary surging glaciers provide powerful diagnostic criteria for the recognition of surge imprints on the landscape (Figs. 11.15 and 11.16). Based upon a combination of observations from contemporary surging glacier margins, specifically Bruarjokull and Eyjabakkajokull, Iceland (Evans et al., 1999b; Evans and Rea, 1999), and the published literature, a landsystems model is presented which includes the geomorphic and sedimentological signature of glacier surging (Fig. 11.16). The geomorphology is arranged in three overlapping zones: an outer zone (zone A) of thrust-block and push moraines, grading up-flow into patchy hummocky moraine (zone B), and then into flutings, crevasse-squeeze ridges and concertina eskers with areas of pitted, channelled and/or hummocky outwash and occasional overridden thrust and push moraines (zone C). Cyclic surging by a single glacier snout will often result in the overprinting of one surge event signature by another (Figs. 11.15 to 11.17). Further detail of the landsystems model based upon the signature of a single surge is presented below, although production of the hummocky moraine requires the pre-existence of stagnating ice from earlier surges.

The outer zone (zone A) represents the limit of the surge and is composed of weakly consolidated pre-surge sediments, proglacially thrust or pushed by rapid ice advance. Structurally and sedimentologically these thrust-block and push moraines comprise interbedded thrust slices or folded and sheared proglacial and older glacigenic deposits (glacitectonites; Benn and Evans, 1996). There may be organic layers found within these stacked sequences which represent pre-surge ground surfaces and so can be used to date individual surge events and/or sequences of multiple surge events (Clayton et al., 1985; Dredge and Cowan, 1989b). The development of major thrust-block moraines (e.g. composite ridges and hill-hole pairs) is restricted to topographic depressions that are large enough to collect sufficient sediment during the quiescent phases. Specifically, in the case of Bruarjokull and Eyjabakkajokull it appears that they are confined to braided outwash deposits. Hence, topographically constrained surging glacier snouts like those on Svalbard commonly produce thrust-block moraines that stretch across most of the snout area (Fig. 11.1). In contrast, the topographic hollows along the wider glacier snouts such as Bruarjokull dictate the location of thrust-blocks versus push moraines during surges. In the event that more than one surge terminates in the same place, the overprinted tectonic signatures will produce a complex kinetostratigraphy (cf. Berthelsen, 1978).

The intermediate zone (zone B) consists of patchy hummocky moraine located on the down-glacier sides of topographic depressions and often draped on the ice-proximal slopes of the thrust-block and push moraines. Although hummocks may be the products of supraglacial melt-out and flowage of debris derived from the incorporation of stagnant ice into the glacier and material transported from the glacier bed along shear planes and via crevasses during and immediately after the surge, the relief of such forms is likely to be subdued. High-relief

Proglacial Sediments

hummocky moraine composed of intensely glacitectonized, fine-grained stratified sediments and diamictons or poorly sorted gravels are the product of thrusting, squeezing and bulldozing of proglacial lake sediments and outwash over pre-existing stagnant ice. Evidence of overriding by the surging snout in the form of faint flutings precludes a supraglacial origin for such hummocks. Small pockets of interbedded mass flow diamictons and crudely bedded stratified sediments, disrupted by normal faulting and low-amplitude folds and occupying small depressions on the hummocky topography, have been produced by the most recent surge snout of Bruarjokull.

The inner zone (zone C) consists of subglacial deformation tills and long, low-amplitude flutings, produced by subsole deformation during the surge, and crevasse-squeeze ridges, documenting the filling of basal crevasses at surge termination. Concertina eskers can also occur in this zone where they are draped over the flutings and crevasse-squeeze ridges. The preservation potential of concertina eskers is likely to be poor although discontinuous gravel spreads and mounds normally referred to as kames or 'moulin kames' in the ancient landform record may represent concertina eskers. Evidence of the production of patchy boulder spreads and thin mass-flow diamictons from the melt-out of up-glacier dipping crevasse-squeeze ridges has been observed (Fig. 11.10). This process may help to explain some areas of low-amplitude hummocky moraine and boulder spreads preserved in the historical and ancient landform record.

Some diagnostic forms of surging are intrazonal, because either they are palimpsests of older surges (e.g. overridden moraines), or they relate to the location of proglacial outwash fans and streams (ice-cored, collapsed outwash), or they occur in ponded topographic depressions on the foreland (collapsed lake plains). Specifically, collapsed glacilacustrine sediment bodies and ice-contact fans may occur within topographic depressions where the stagnating glacier snout became buried during the quiescent phase. The locations and complexities of overridden moraines and multiple till sequences are dictated by the extents of subsequent surges in a single glacier basin.

1 1.4 APPLICATION OF THE SURGING GLACIER LANDSYSTEM

The surging glacier landsystem outlined above encompasses the landform-sediment associations typical of contemporary surging glacier margins and, because reconstructions of palaeo-ice dynamics rely on suitable analogues from glacierized landscapes where form is necessarily linked to process, it can be utilized by glacial geomorphologists/geologists when identifying possible surges in the Pleistocene glacial record. We now provide an example of the application of this model to the glacial geomorphology of part of the former southwest Laurentide Ice Sheet in western Canada.

Figure 11.15 Aerial photograph (upper: Landmaelingar Islands, 1993) and general geomorphology zonation map (lower) associated with the recent surging of Eyjabakkajokull, Iceland, displaying the juxtaposition/overlapping of thrust moraines, flutings, hummocky moraine, crevasse-squeeze ridges and concertina eskers (after Evans et al., 1999b). Note that the concertina esker and associated crevasse-squeeze ridges on the west side of the snout were produced during the 1972 surge, whereas the outer zone of landforms were produced during the 1890 surge.

Some lobate margins of the Laurentide Ice Sheet have been explained in palaeo-glaciological models as the products of ice streams, possibly subject to surging over deformable substrates, based upon theoretical reasoning or selected geomorphological and sedimentological criteria (e.g. Boulton etal, 1985; Clayton etal, 1985; Fisher etal., 1985; Clark, 1994b; Marshall etal., 1996). However, the palaeo-geographical settings are also favourable for deforming bed development and the accretion of glacitectonites and deformation tills under non-surging conditions (e.g. Hicock, 1992; Boulton, 1996a, b; Evans, 2000a). Furthermore, a variety of genetic explanations have been provided for individual landform assemblages in this region that are not surge-related (e.g. hummocky terrain, thrust moraines, fluting and drumlin swarms). The surging glacier landsystem model presented above, is now applied to an area of east-central Alberta, Canada, that has previously been reported as having a strong palaeo-surge signature (Evans et al., 1999b; Evans and Rea, 1999).

The glacial geomorphology of part of east-central Alberta, specifically part of National Topographic Survey (NTS) map area 73E, is summarized in Fig. 11.18. In the northern half of the map, mega-flutings document the passage of the Lac La Biche ice stream during recession of the Laurentide Ice Sheet. These have traditionally been related to ice streaming (Jones, 1982),

Icelandic Glacial Advert Model

Figure 11.16 A) Aerial photograph (Landmaelingar Islands, 1993) and B) geomorphology map of part of the margin of Bruarjokull, Iceland (after Evans et al., 1999b). The prominent features include flutings, crevasse-squeeze ridges, concertina eskers, small patches of hummocky moraine and extensive thrust-block and push moraines. The outermost moraine dates to the AD 1890 surge and the innermost moraine dates to the 1964 surge. An overridden moraine dating to a pre-1890 glacier margin lies between the two prominent moraines. Buried glacier ice from this pre-1890 surge is most likely responsible for the melt-out features in the hummocky moraine developed after the 1890 surge.

Figure 11.16 A) Aerial photograph (Landmaelingar Islands, 1993) and B) geomorphology map of part of the margin of Bruarjokull, Iceland (after Evans et al., 1999b). The prominent features include flutings, crevasse-squeeze ridges, concertina eskers, small patches of hummocky moraine and extensive thrust-block and push moraines. The outermost moraine dates to the AD 1890 surge and the innermost moraine dates to the 1964 surge. An overridden moraine dating to a pre-1890 glacier margin lies between the two prominent moraines. Buried glacier ice from this pre-1890 surge is most likely responsible for the melt-out features in the hummocky moraine developed after the 1890 surge.

however, because of their association with other elements of the surging glacier landsystem, are linked here with former surge activity. The higher topography in the western half of the map is composed of large thrust-block moraine ridges (TM). Most of these ridges were formed at the margin of ice moving from the north-northwest, but the most recent features (marked by the southernmost TM on Fig. 11.18 and aligned NW-SE) were constructed during the surge of the Lac La Biche ice stream as it flowed from the northwest towards Lloydminster (L) and then from the northeast towards the south of the map area. All of the thrust-block moraines are

Figure 11.17 A landsystems model for surging glacier margins (after Evans et al., 1999b; Evans and Rea, 1999): 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 11.17 A landsystems model for surging glacier margins (after Evans et al., 1999b; Evans and Rea, 1999): 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.

truncated or overprinted by the surge geomorphology to the east. The eastern half of the map is dominated by a dense network of crevasse-squeeze ridges, which overwhelm the mega-flutings in a down-ice direction towards the centre of Fig. 11.18, and document a surge of the Lac La Biche lobe as it breached the large thrust-block moraines in the south. A small area of hummocky moraine and kame and kettle topography occurs amongst the crevasse-squeeze ridges in the southernmost part of the map. Stratigraphic sequences in the map area include severely glacitectonized deposits in the thrust-block moraines and multiple tills with interbedded stratified sediments at the centre of the former Lac La Biche lobe (Andriashek and Fenton, 1989; Mougeot, 1995).

It is evident that the landform-sediment assemblages shown in Fig. 11.18 exhibit all the characteristics (with the exception of concertina eskers) of the surging glacier landsystem model (Fig. 11.17). The juxtaposition of these landforms and sediments at the margin of the surging palaeo-ice stream demarcated by the Lac La Biche fluting field strongly suggests that surging affected this margin of the Laurentide Ice Sheet during its recession from western Canada.

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