The Plateau Icefield Landsystem

This section integrates information from contemporary environments in the Canadian Arctic, north Norway and Iceland and provides the main criteria for identifying the former existence of plateau icefields in deglaciated landscapes. As mentioned above, glacierization may be experienced at both a smaller, local scale and at a larger, continental scale. If affected by both, the plateau icefield landsystem may comprise a composite signature. It is climate reconstructions of the smaller, local scale glaciation that are most sensitive to the non-identification of former plateau glaciers.

During successive glaciations the combination of glacial overdeepening and interglacial fluvial and slope activity results in the dissection of the landscape into isolated, remnant high altitude surfaces and incised valleys and fjords. In suitable tectonic settings, regional faulting may have been instrumental in the initial production of relief through the development of grabens (e.g. England, 1987a). In Iceland it is the juxtaposition of active volcanism and ice coverage that combined to produce plateaux. Once isolated, each plateau is an ideal collection area for permanent snowfields at the onset of glacial conditions ('instantaneous glacierization'; Ives et al., 1975). In areas where the plateau margins have precipitous drops into fjords or troughs, 'fall glaciers' (cf. 'reconstituted glaciers'; Benn and Lehmkuhl, 2000) are nourished below the regional ELA by dry calving at the plateau cliff edge (Gellatly et al., 1986). This is representative of most of the valley glaciers surrounding the Lyngen plateau icefields. During the more advanced stages of local scale glaciation, glaciers develop in the valley bottoms between plateaux and it is at this stage that the most significant landforms are likely to be produced.

Under conditions of pervasive ice sheet cover associated with full glacial conditions, the role of the topography in controlling ice discharge routeways can be reduced or overcome, and through-valleys may be excavated by transection glaciers. Conversely, valleys cut during plateau-centred glaciations will possess a radial drainage pattern (see Figs 16.17 and 16.19 later). Plateau tops may show no evidence of subglacial erosion if they hosted cold-based ice (Dyke, 1993). If there is evidence of erosion, indicating that the ice crossing the plateau was warm-based, then the erosional forms will again be aligned parallel to sub-parallel across the area or will radiate out from the plateau centre, depending upon the pattern of ice cover. Regional ice sheet imprints may be recorded as major landscape features such as through-valleys and fjords separating plateaux. Such troughs often contain adornments such as lateral meltwater channels and moraines associated with their occupation by regional ice (e.g. the fjords of southern Ellesmere Island; see above). Erratics transported onto plateaux from outside the local area often further demonstrate regional ice sheet coverage.

More localized styles of glacierization are characterized by the growth of plateau-centred icefields. As mentioned above, the climate reconstructions based upon this stage of ice coverage are most sensitive to erroneous interpretations of the valley glacier — plateau icefield configuration. The various landform units diagnostic of plateau icefield glaciation can now be isolated and reviewed

Provided some parts of the ice are warm-based, and thus erosive, moraines may be found on top of the plateau or leading on to the plateau from the surrounding valley-head outlets (Whalley et al., 1995a). A number of good examples occur around 0ksfjordj0kelen (Fig. 16.11). If the ice is cold-based throughout, lateral and frontal moraines will be found in the valleys only. Valley moraines tend to be dominated by large, cobble to boulder size, angular material even where there is evidence of basal sliding, indicating the dominant debris source is rock fall (Fig. 16.16). Many latero-frontal moraines in the valley heads that surround the plateaux are ice-cored and display within-valley asymmetry (Benn, 1989a). This reflects the distribution of bedrock free faces and concomitant variability in rockfall/avalanche and debris flow activity. Active free faces may provide sufficient debris to produce supraglacial lateral moraines. In some circumstances these may develop into rock glacierized ice-cored moraines that may persist after deglaciation of the valley heads. Where considerable thicknesses of valley floor sediments are available, plateau outlet glaciers may construct large end moraine sequences, particularly where proglacial thrusting takes place (Figs 16.5 and 16.15). This appears to be most effective where glacio-isostatic uplift has resulted in the recent aggradation of permafrost and the production of a shallow décollement surface within the valley floor sediments (e.g. northwest Ellesmere Island; see O Cofaigh et al., 2003).

Figure 16.15 A schematic diagrammatic representation of the various elements produced by the plateau icefield landsystem. It is assumed that the undulating plateau surface has a blockfield cover. Higher, smaller summits (not depicted) that show no signs of glacierization must be evaluated using other means.

Figure 16.15 A schematic diagrammatic representation of the various elements produced by the plateau icefield landsystem. It is assumed that the undulating plateau surface has a blockfield cover. Higher, smaller summits (not depicted) that show no signs of glacierization must be evaluated using other means.

16.5.2 Ice-Contact Deltas

In areas where the glacier terminated in contact with standing water, ice-contact deltas and/or grounding-line fans are used as evidence for ice-marginal locations (Fig. 16.6). In marine environments where sea level history is known, associations between the deltas and associated shorelines can be used as a proxy dating technique for reconstructing the deglaciation chronology (Evans 1990a; Evans et al., 2002).

16.5.3 Sediments

Sediments found on the plateau are most likely to be allochthonous or autochthonous weathering products described variously as blockfields, felsenmeer, weathering residuum and/or highly weathered bedrock exposures (Fig. 16.12). The depths, features and mineralogy of the weathering have been used to infer significant ages for these sediments and associated landforms (e.g. Sugden and Watts, 1977; Rea et al., 1996a and b). As mentioned above, moraines may be found on plateaux that contain material indicative of active subglacial transport. Significant till cover does not form, due to the commonly short transport distances to plateau outlets and the dynamics of the plateau ice cover.

The valley floors are characterized by thin, generally patchy tills, which locally thicken in depressions. Proglacial reworking of sediments occurs to some degree, and in places large braided river networks produce extensive spreads of reworked material. Lake sediments may be deposited in ice or moraine dammed lakes and in overdeepened basins.

Figure 16.16 The bouldery latero-frontal moraine found at the mouth of Storelvdalen (see Figure 16.19) produced during the Younger Dryas. Note the steep valley head outlets exiting the icefield in the background.

16.5.4 Meltwater Channels

Lateral meltwater channels produced by the retreat of outlet glaciers on to plateaux (Figs 16.3 and 16.4) are typical of large areas of deglaciated upland terrain throughout the Canadian and Greenland high arctic, where they are interpreted as the products of recession by cold-based ice (e.g. Maag, 1969, 1972; Dyke, 1978, 1993; Edlund, 1985; O Cofaigh et al., 2003). Meltwater channels have not been found around Lyngsdalen where plateaux are small and high and ice is cold-based. Melting and downwasting has been observed to produce meltwater that ponds around the ice margins (Gellatly et al., 1988) but the quantities appear to be insufficient to produce channelized drainage. The larger, lower icefields in north Norway (e.g. Troms-Finnmark) tend to have warm-based outlets with basal ice at the PMP, producing subglacial meltwater and allowing surface meltwater to percolate to the bed. The subglacial bedrock topography and ice surface slope act to drain the meltwater towards the outlets, invariably in channelized subglacial meltwater streams (Fig. 16.11). Meltwater at the margins of the Icelandic plateau icefields is directed by both subglacial and ice marginal drainage pathways and ultimately feeds into proglacial channels that connect to steep alluvial fans at the base of plateau edge gullies.

16.5.5 Glacial Erosion

On plateaux the extent and duration of the coverage by warm-based ice will influence the quantity of erosion and thus the landforms produced. If only limited erosion occurs, there may only be a clearing out of pre-existing regolith or blockfield, forming a moraine if the ice terminates on top of the plateau. However, if the ice margin avalanches off the plateau edge then moraines may not form (Fig. 16.11), but the presence of bedrock surfaces cleared of regolith or blockfield with striae and roches moutonnées formed when the erosion is more extensive, indicates the former presence of warm-based ice. If extensive evidence of erosion is available then ice flow directional indicators can be investigated. If the erosion is the product of a local ice cover, erosional forms should indicate that ice flowed out radially from a central ice accumulation centre. Alternatively, they may occur as parallel suites of forms cutting obliquely across the plateau if produced under a regional glacier cover. Striated bedrock and roches moutonnées may be exposed in the surrounding valleys where the sediment cover was thin (Fig. 16.15).

16.5.6 Bedrock Weathering Zones

During periods when the ice does not cover the whole of the plateau, exposed bedrock is subjected to subaerial weathering. Providing sufficient time elapses between successive exposure periods (the time required being site-specific and dependent upon the characteristics of the rock exposed), then it may be possible to identify a progressively 'younging' zonation towards the centre of the plateau. Gellatly et al. (1988) reported finding two distinct bedrock weathering zones around parts of the margins of Oksfjordjekelen. Cosmogenic exposure history studies could potentially identify younging zonation, provided the difference between zones is greater than the resolution of the dating.

16.5.7 Erratics

The presence of erratics on plateaux (Fig. 16.15) provides evidence of former glacier coverage and may be used to identify palaeo-iceflow directions. They are likely to represent large-scale ice cover when the topography is submerged and ice surface slopes can force ice to flow over the summits.

It must be remembered that as full glacial conditions give way to regional ice cover, plateau glaciers may become erosive and so local erratic transport may occur. Erratics, which are sourced from beyond the plateau, can be used to infer ice flow directions during large-scale ice inundation, and local erratic transport may be used to reconstruct former ice flow patterns of the plateau icefield.

16.5.8 Plateaux With No Evidence of Former Glacier Cover

Such plateaux provide the most problems in reconstructing regional glaciation style but are obviously very important in climate reconstructions if they were supplying ice to valleys where evidence of glaciation can be found. Providing that evidence of glaciation can be found on surrounding summits, a relationship of the form shown in Fig. 16.1 can be used to assess whether or not the plateau is broad enough to have sustained ice. If there is no indication of glaciation on any plateaux, evidence from the valleys must be employed to constrain glaciation style. The regional palaeo-ELA/firn line can be reconstructed from alpine style valley glaciers and cirques that would have had no plateau ice contributions, and the equation above (see section 16.2) can again be employed for icefield detection. Also, if valley glaciers below plateaux, when reconstructed without accounting for any accumulation input from the plateau above, show lower ELAs than for the alpine and cirque glaciers, then additional accumulation from surrounding plateaux is most likely. Conversely, when reconstructed without accounting for any accumulation input from the plateau above they show similar ELAs to those calculated for the alpine-type valley glaciers; then additional accumulation from above was insignificant. However, a plateau ice cover with no connections to valley glaciers at lower altitudes is still possible.

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