Progress in Physical Geography

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A qualitative view of sub-ice-sheet landscape evolution John Shaw Progress in Physical Geography 1994 18: 159 DOI: 10.1177/030913339401800201 The online version of this article can be found at: http://ppg.sagepub.com/content/18/2/159

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159

A qualitative view of sub-ice-sheet landscape evolution John Shaw Department of Geography, University of Alberta, Edmonton, Alberta, Canada, TG2 2 H4

Abstract: Qualitative analysis of sub-ice-sheet landscapes is based on processes inferred from landform and sediment. Striation and plucking give information on the dynamic and thermodynamic conditions at the ice bed. Lodgement and melt-out tills indicate that basal accretion and freezing conditions were followed by melting during till deposition, under sliding ice in the case of lodgement and stagnant ice in the case of melt-out. Surging is inferred from evidence of low icesheet profiles. Melt-out appears to have taken place over large areas in periods of stagnation following surges. Interbedded stratified sediment and melt-out till indicate decoupling of the ice from its bed during relatively minor subglacial drainage events. The bulk of glacial erosion, transport and deposition took place during ice-sheet advance. Over-ridden proglacial sediment, lodgement/deformation and melt-out tills, with subglacial sorted beds, were eroded by cataclysmic meltwater outburst floods that produced erosional and depositional drumlins, flutings, Rogen moraines and vast tracts of scoured bedrock. These floods marked a dramatic change in glacier regime over large areas of ice sheets. Low ice-sheet profiles caused widespread stagnation; glacial erosion and transport came to a halt. Extensive esker systems attest to a regional, integrated subglacial drainage network. Tunnel channels, glaciotectonic landforms and hummocky moraine are other elements of the subglacial landscape related directly or indirectly to meltwater storage and outburst floods. Models of landscape evolution are based on the conclusions derived from the analysis of sediment, form and process.

Keywords: Subglacial landscape, ice sheet, erosion, entrainment, transport, deposition, outburst floods, thermal regime, landform.

I

Introduction

and sedimentological processes beneath large modem ice sheets have been observed directly and are poorly understood. At the same time, the behaviour of the Greenland and Antarctic ice sheets is fairly well documented by measurement of their mass balance, flow characteristics and thermal regime. Such measurements combined with an extensive body of theory on ice physics provides the basis for the scientific explanation of these modem ice sheets (Paterson, 1981). While there is abundant evidence on

Geomorphological not


160

processes at the beds of past ice sheets, there is little but guesswork on the nature of the ice sheets at the time of landscape formation. Not surprisingly, then, intepretation of glaciated landscapes attracts controversy and does not stand on the same quantitative footing as the study of modem ice sheets. Our understanding of glacial landscape depends more on qualitative reasoning, which attempts to relate landform genesis to ice flow, deforming sediment beneath the ice sheet and meltwater processes. Thermodynamic processes controlling freezing and melting at the bed are also of significance to glacial erosion and deposition. Thus, inductive, qualitative process analysis, stemming from observations on glacial landforms and sediment, is the starting point for this article. Glacial landscapes are then considered in terms of changing ice-sheet conditions and processes inferred from landforms and sediment. This emphasis on sedimentary and geomorphological observation gives a clear indication of what it is we wish to explain, and acts as a guide to quantitative approaches. From this standpoint, the qualitative analysis presented here summarizes our understanding of subglacial landscapes and is a precursor to quantitative studies.

II

Subglacial processes

landscapes, like most landscapes, reflect both erosion and deposition. Consecareful study of glacial landforms and sediment provides a wealth of information on these processes. A brief review illustrates the use of such information in qualitative theory. Glacial

quently,

1

Glacial erosion

Striation

gives unequivocal evidence for abrasion by which scratches are produced as ice tools across subglacial rock beds. As well, the irregular, steep lee-side faces of drags roches mouton6es are confidently explained by the plucking of large blocks. Yet these apparently straightforward processes are qualitatively more complex than is suggested by their simple descriptions. Hallet ( 1981 ) noted that erosion by individual clasts, as they are drawn across a rock bed by sliding ice, depends on the basal melting rate. The physical reasoning is that a downward flow component caused by basal melting exerts drag on stones in contact with the bed, increasing the contact pressure. This qualitative insight significantly changes quantitative estimates of abrasion rates (Hallet, 1981 ) . For plucking, R6thlisberger and Iken (1981) noted the inability of simple theory to explain how ice of low tensile strength could detach much stronger rock. They introduced a transient hydraulic jacking mechanism related to rapid variation in subglacial cavity water pressure to explain the effectiveness of plucking. Thermodynamically, abrasion requires melting conditions at the glacier bed where striae are forming (R6thlisberger, 1968). Plucking requires a more complicated thermal regime with areas of frozen and areas of thawed bed (Robin, 1976; R6thslisberger and Iken, 1981). These conclusions are fundamental to the interpretation of subglacial landscapes. It is difficult, for example, to imagine the entrainment of large blocks unless the bed was frozen (Mackay and Mathews, 19 64). stone


161

2

Glacial entrainment

Our

understanding of landscape evolution is relatively poor where the combined effects of erosion, entrainment, transport and deposition were at play. We know very little about how the large continental ice sheets entrained debris, or how much was entrained in terms of absolute volume and volume concentration. Nevertheless, we do know that subglacial entrainment, whereby debris is eroded and incorporated into the body of a glacier, must involve freezing (Weertman, 1966; Drewry, 1986). Freezing involves net removal of heat and accretion requires a supply of water at the bed. Given these two qualitative requirements - net heat loss at the site of accretion and a supply of water - insight about thermodynamic and drainage conditions beneath vanished ice sheets is gained where subglacial entrainment is inferred. 3

Glacial

deposition

Subglacial depositional processes are potentially easier to interpret than processes of erosion and transport because sedimentary evidence of them is preserved. But, although the processes of deposition are well understood from observations at the margins of modem ice sheets, the depositional processes of past ice sheets remain a matter of inference. Interpretation becomes more difficult as new processes, which compete with traditional inferences, are hypothesized. Thus, the popular concept of subglacial deformation of low strength substrates competes with and brings into question the traditional concept of lodgement (Boulton and Hindmarsh, 198?; Alley et al., 1989). The replacement of lodgement till by deformation till is much more than a simple revision of terminology; it signifies a new interpretation of the genesis of glacial sediment and landforms which holds dramatic consequences for the reconstruction of ice sheets (Boulton, 1986). Release of debris at the base of a moving glacier is caused mainly by basal melting (Muller, 1983). But beyond this assertion on the importance of melting, understanding of subglacial processes is largely by inductive inference. In reviewing these inferred processes, it is convenient to consider deposition below moving ice and stagnant ice separately.

(a) Lodgement and subglacial deformation: Sliding ice must also be melting, and a downward flow component drives clasts into a soft bed. They plough through the bed with a retarded velocity relative to the overlying ice (Boulton, 1974; Brown et al., 1987). Grooves in undeformed soft beds at the base of till attest to this process beneath ice sheets (Figure 1) (Westgate, 1968; Ehlers and Stephan, 1979; Shaw, 1982). Eventually, clasts dig deeper and the retarding forces exerted on them by the bed causes lodgement. Abrasion results in the faceting and parallel striation commonly observed on the upper and lower surfaces of lodged boulders (Dreimanis, 1989). In time, further deposition by lodgement buries these clasts. Alternatively, if the till itself is deforming clasts continue to move, even when buried. Moreover, if most of the basal movement is by sediment deformation rather than sliding, the potential for abrasion and for the formation of grooves by ploughing is greatly reduced. Consequently, the resultant deformation till is expected to differ considerably from lodgement till, not just in its properties but also in terms of its contribution to glacier flow (Alley et al., 1987; Boulton and Hindmarsh, 1987; Hicock and Dreimanis, 1992).

(b) Melt-out: Deposition by release of debris as stationary ice melts is a relatively passive


162

Underside of till sheet with lodged stones and ridges. The ridges are of grooves ploughed in the underlying sand by stones. Hugget, near Edmonton, Alberta

Figure

till

1

casts

directly at the margins of modem glaciers (Boulton, 1971; Lawson, on inferred 1979), sedimentary arguments (Goodchild, 1875; Shaw, 1982), but on theoretical questioned grounds (Paul and Eyles, 1990). The properties of melt-out till are so distinct when compared with those of lodgement and deformation till that there is no difficulty in distinguishing them providing that careful observations are made (Haldorsen and Shaw, 1982). process observed

(c) Sedimentary characteristics: The following sedimentary characteristics of glacigenic deposits are fundamental to their correct genetic interpretation: 1) Some clasts were transported enormous distances. 2) In some cases, the amount of mixing of clasts derived hundreds of kilometres upglacier with clasts derived close to the location of deposition is relatively small. Thus, in northern Ontario, tills with mainly far-travelled carbonates are distinct from tills containing local rocks in the same stratigraphic column (cf. Hicock, 1987; 1988). 3) Clast abrasion is common and multiple directions of striae indicate that in some cases abrasion accompanied clast rotation during transport. 4) Extremely large bedrock ’rafts’, with length scales on the order of kilometres and thicknesses of tens of metres, attest to wholesale detachment and transport of slabs of the glacier substrate (cf. Stalker, 1963). 5) Boulder pavements in basal diamicton (White, 1974; Eyles et al., 1982; Hicock, 1991) imply a sorting process which is not well understood. 6) Clasts in basal tills have distinctive fabrics (Boulton, 1971; Lawson, 1979; Shaw, 1982; Dowdeswell and Sharp, 1986; Dreimanis, 1989). 7) Basal tills are intimately associated with sorted and stratified water-lain deposits (Haldorsen and Shaw, 1982; Dreimanis et al., 1987; Shaw, 1982, 1987). 8) Deformation structures involving diapirism and unidirectional strain are common in

glacigenic deposits.


163

4

Glaciofluvial processes

Subglacial meltwater increases the geomorphological and sedimentary possibilities enormously. Thin meltwater films (Weertman, 1972) and slow drainage of till during deposition (Muller, 1983) may remove the finest sediment, silt and clay, at the ice bed. As well, the propensity for a thin, slowly moving film of meltwater to dissolve soluble material and to refreeze at the glacier bed may cause precipitation of carbonates (cf. Hallet, 1976). Meltwater erosion and deposition produced dramatic landforms and sediments in tunnels and cavities beneath past ice sheets. Eskers and tunnel channels are confidently attributed to subglacial fluvial action (Ringrose, 1982; Shaw, 1983a; Boyd et al., 1988; Sharpe, 1988). Nevertheless, there is vigorous debate on whether they are products of steady-state flows and incremental formation as suggested by Mooers (1989), or are products of catastrophic drainage and were formed synchronously along their full lengths (Wright, 1973; Boyd et al., 1988; Sharpe, 1988; Shaw and Gilbert, 1990). Other tunnels and cavities did not give rise to landforms, but sediment deposited within them is now part of complex stratigraphic sequences (Dreimanis et al., 1987; Shaw, 1987). Shaw (1983a) suggested that large-scale meltwater floods were responsible for some drumlins. Later, erosional drumlins, bedrock erosional marks, tunnel channels and Rogen moraine were added to the forms resulting from catastrophic floods (Shaw and Sharpe, 1987; Sharpe, 1988; Sharpe and Shaw, 1989; Shaw et al., 1989; Kor et al., 1991; Fisher and Shaw, 1992). The meltwater flood hypothesis, like the concept of subglacial deformation, has the potential to change glacial geomorphology dramatically. Both necessitate radical revision of ideas on how ice-sheet form and behaviour are reconstructed and on how subglacial erosional and sedimentary processes are modelled. 5

Limitations of actualistic models

In neither Hallet’s (1981) explanation of abrasion nor R6thlisberger and Iken’s (1981) explanation of plucking were experiments carried out demonstrating that the suggested processes actually take place in nature. This absence of experimental field verification is common in glacial geomorphology. Moreover, it may well be that conditions beneath Pleistocene ice sheets, where much of the Earth’s glacial landscape originated, were unlike those beneath modem ice sheets. It is reasonable to think this way in view of the probable climatic contrasts between the enormous ice sheets of the past, centred over relatively low latitudes, and high latitude, modem ice sheets. This view is supported by very low estimated ice-surface slopes for the Laurentide ice sheet (Mathews, 1974; Beget, 1986;

Shoemaker, 1992). 111

1

Subglacial landscapes

.

Introduction

Following our brief review of subglacial processes, with comments on ice-sheet conditions, the challenging but difficult topic of whole landscape interpretation; challenging because it is the ultimate goal - difficult because the available information is incomplete. Our aim is to explain landscape using available theory and observations. The subglacial landscape results from a set of processes: in theory bedrock and sediment may be eroded we turn to


164

by abrasion or plucking; entrained by accretional ice at the bed; transported englacially or by deformation processes subglacially; and deposited by lodgement, or melt-out, or when a deforming bed stops moving. Meltwater also erodes ice, bedrock and drift, and transports and deposits sediment. In practice, we aim to observe landforms and sediment, infer environments of formation and draw conclusions on genesis. If there were no agreement that the subglacial landscape can be recognized, this exercise would be futile. Fortunately, most subglacial landforms are easily identified as such, although there is lively debate on their genesis. Consequently, the subglacial landscape is easily recognized to the satisfaction of most geomorphologists. But this does not mean that all glacial landforms can be easily classified. Hummocky moraine, for example, remains enigmatic. It is interpreted by some as supraglacial (cf. Gravenor and Kupsch, 1959; Boulton, 1972; Moran et al., 1980), and by others as subglacial (cf. Hoppe, 1952; Stalker, 1960; Rains et al., 1993). 2

Subglacial

landforms and their

composition

glacial heritage of Canada in photographs (Mollard, 1982; Prest, 1983; Smith, 1987). Notably, many of these photographs are of almost pristine subglacial landscapes, with little evidence of former ice margins or supraglacial sediment (Figure 2). Authors studying drumlins, flutings and Rogen moraine at a regional scale (Prest, 1968; Bouchard, 1986; Sharpe, 1988; Aylsworth and Shilts, 1989) described similarly impressive tracts of subglacial landscape. This landscape is represented on the prairies by extremely subtle flutings, detectable on satellite images with favourable sun angles and a ground cover of snow (Skoye and Eyton, 1992). The flutings are cut into bedrock, with occasional rounded erratic boulders testifying to glaciation of the area. Immense channel complexes are also eroded into the prairie bedrock. Isolated streamlined hills of glacigenic sediment in the channel complexes are all that remain of a once broad sheet of thick drift (Rains et al., 1993). Elsewhere, flutings are eroded into relatively undisturbed glacigenic sediment, yet within a few kilometres highly deformed and brecciated diapiric sediment lies beneath the same fluted erosional landscape (Figures 3 and 4). Some drumlins around Peterborough and Trenton, Ontario, are also mainly erosional (Figure 5) (Gravenor, 1957; Shaw and Sharpe, 1987; Boyce and Eyles, 1991); their internal stratigraphy is relatively undisturbed (Figure 6) (Sharpe, 1987). Armstrong and Tipper (1948) noted that drumlins, formed in till, in the interior of British Columbia are defined by linear erosional scours. They proposed two glacial advances to explain the sediment and morphology. In this interpretation, while till was deposited during the first advance, erosion during the second advance was thought to have been a consequence of a lightly loaded glacier. Alternatively, given that evidence from single glacial events elsewhere records deposition, followed by a drumlin-forming erosional event, the British Columbia drumlins and their sediment probably record changing conditions within a single event. Drumlins in Ireland contain complex glacigenic sequences, including undisturbed, finegrained laminated sediment deposited in still water and large-scale cross-bedded gravels laid down by high energy flows (McCabe and Dardis, 1989). There is no evidence suggestive of wholesale deformation of the internal stratigraphy. The form of these Irish drumlins, like the Canadian examples discussed above, is almost entirely erosional. Leeside and flanking glaciofluvial sediment resting against some drumlins add a minor A number of books illustrate the


165

2a Subglacial landscape dominated by depositional drumlins. These drumlins around Snare Lake are part of the Livingstone Lake drumlin field, northern Saskatchewan. Flow from bottom left (northeast). Airphoto A14509-2, courtesy of the National Air Photo Library, Ottawa; b Subglacial landscape with Rogen moraine and superimposed fluting (centre) and bedrock fluting (centre left). Flow from the northeast. Airphoto A12798-35, courtesy of the National Air Photo Library, Ottawa

Figure


166

component (Hanvey, 1987; McCabe and Dardis, 1989). But not all drumlins are erosional. The internal structure and forms of the Livingstone Lake drumlins, northern Saskatchewan (Figure 2a), point to a depositional origin. These drumlins are considered to be infills of subglacial cavities (Shaw and Kvill, 1984). Like their erosional counterparts, the cavity-fill drumlins represent exquisite preservation of the

depositional

subglacial landscape. Rogen moraines, some with delicate, superimposed flutings, are also extensive in the Canadian landscape (Figure 2b). They are associated with drumlins and flutings; the three are commonly complementary elements of a complex landscape (cf. Aylsworth and Shilts,

3 Fluting eroded into till and stratified Flow from northeast. Landsat TM image

Figure

sediment, Athabasca, Alberta.


167

Figure 4a Undeformed melt-out till and intratill glaciofluvial sediment (cavity or tunnel fills). These sediments were eroded in the formation of the fluting shown in Figure 3; b Diapiric breccia with rafts (selected ones arrowed) of stratified fluvial sediment ’floating’ in injected diamicton. The undeformed, original sequence of diamicton and fluvial sediment was probably similar to that shown in Figure 4a. Diapirism marks vertical loading and not unidirectional strain. These sediments also underlie the fluting of Figure 3


168

1989). As with drumlin fields, Rogen moraine fields may be crossed by subglacial eskers, but recessional or supraglacial features are rarely seen within them (Lundqvist, 1969; Aario, 1977; Shaw, 1979; Bouchard, 1986; Aylsworth and Shilts, 1989; Fisher and Shaw, 1992). Broad areas of scoured bedrock cover much of Canada, especially on the Canadian Shield. They are traditionally considered to be zones of pronounced glacial erosion (Sugden, 1978; Aylsworth and Shilts, 1989; Bouchard, 1989). Despite this view, meltwater erosion best explains the sculpted forms and absence of drift around the north shore of Georgian Bay (Figure 7) (Kor et al., 1991), around Kingston (Murray, 1988; Shaw and Gilbert, 1990), and in broad swaths on the Prairies (Rains et al., 1993). As with drumlin and Rogen moraine landscapes, these bedrock areas are parts of delicately

preserved primary subglacial landscapes. The genesis of these subglacial landforms

remains controversial, despite hundreds of scientific studies. Still, one fundamenntal conclusion emerges from the fine preservation of subglacial landforms. It tells us that over vast regions, drumlins, Rogen moraine and eroded bedrock represent the last major geomorphic activity of the continental ice sheet. The moraines, eskers and tunnel channels which formed later are on a smaller spatial and

5 Erosional drumlins and fluting, Peterborough drumlin field, flow from the northeast. Airphoto A24314-30, courtesy of the National Air Photo

Figure

Library


169


170

Figure Kor

et

7 al.

Scoured bedrock with s-forms (erosional marks) in the study area of ( 1991 ), French River, Georgian Bay, Ontario. Flow from bottom left

volumetric scale than these subglacial bedforms and the glacial sedimentation that preceded them. Evidently, the bulk of erosion and transport of debris and deposition of till by continental ice sheets preceded or accompanied formation of these landforms. The formation of drumlins marked a drastic change in ice-sheet behaviour. Understanding why this was so is of fundamental importance to understanding subglacial processes and their cumulative effects on landscape. 3

Glaciotectonic landforms

Drumlins and associated bedforms are not the only landforms offering clues on subglacial conditions. Glaciotectonic depressions and ridges, which are widespread on the prairies, involve deformed bedrock and drift (Kupsch, 1962; Clayton and Moran, 1974; Moran et al., 1980). The evidence for glaciotectonics is unequivocal and their mechanics, founded in structural geology and soil mechanics, are well understood. However, there is no clear explanation why glaciotectonic landforms are local rather than ubiquitous (Moran et al., 1980; Bluemle and Clayton, 1984). 4

The

sedimentary evidence

.

The genesis of sediment in erosional drumlins is of interest for the information it gives on the predrumlinization phase. Bouchard (1989) gave the most comprehensive analysis of subglacial sedimentary facies related to glacial bedforms. He observed a relatively finegrained diamicton, with prominent joints and resting on a striated pavement, within elongated drumlins in northern Quebec. Clasts are striated and show both parallel and transverse preferred orientations. The proportion of far-travelled clasts increases upwards


171

in the diamicton

(DiLabio, 1981; Bouchard, 1989). Sorted sand lenses within the diamicton are only slightly disturbed and show preservation of delicate crosslamination in fine sand. At one site, a vertical clastic dike, which penetrates the diamicton, shows no sign of having been overturned in the direction of flow indicated by the drumlins. DiLabio (1981) and Bouchard (1989) interpreted the diamicton as lodgement till. Alternatively, it may be deformation till, in which case - since it is not deformed - the clastic dike must have been emplaced after deformation had ceased. Hicock (1987; 1988) described a compact diamicton in the Hemlo district of Ontario in which stones are subangular to rounded and dominantly of far-travelled lithologies. Parallel and transverse alignments of stones, extensional fractures and diamicton wedges are consistently oriented with respect to striations on stones in the diamicton. The fractures and wedges are transverse to the striations. Again, genesis by lodgement is preferred, because deposition was clearly subglacial and it is unlikely that slurry-like subglacially deforming sediment could fracture in a brittle fashion to produce the extensional fractures and till wedges. Alternatively, deformation may have transported and deposited till prior to fracturing. Both Bouchard (1986) and Hicock (1987; 1988) reported a second till facies overlying the first. This second facies shows classical characteristics of melt-out till: strong preferred orientation parallel to the former ice-flow direction, interbedded sorted sediment in scours below boulders and in sorted beds draped over clasts, far-travelled clasts many of which are striated and faceted, and a coarser grain size than the associated lodgement or deformation till. This facies has the properties of Sveg till of central Sweden, which is interpreted as a melt-out till (Shaw, 1979). Sveg till is also associated with a fine-grained, compact till resembling Bouchard’s (1986) lodgement facies. The fine-grained till is regionally extensive in central and northern Sweden, shows preferred orientation of clasts aligned with striations on underlying rock and is overlain by sandier tills (cf. Bjornbom, 1979). Fluting around Athabasca, Alberta (Figure 3), is also underlain by diamicton, which includes more sand lenses and partings towards the top while, near the base, it is massive and compact, with a preferred clast orientation at variance with the flutings (Shaw and Freschauf, 1973), The stratigraphy is truncated and there are in situ undeformed primary current bedded deposits near the top of the sequence. To the south, around Edmonton, diamicton is complexly interbedded with sorted sediment, yet it shows a regionally consistent preferred orientation of clast long axes aligned with grooves cut into the underlying sand (Figure 1 ) (Westgate, 1968; Rains, 1969; Ramsden and Westgate, 1971; Shaw, 1982). Shaw (1987) related the stratigraphy to changing subglacial conditions. The grooves represent a short period of ploughing (Brown et al., 1987) upon reattachment of glacier ice and frozen sediment, which had been separated from the unfrozen bed by meltwater under high pressure. Basal ice would have been in thermal equilibrium with meltwater at the time of reattachment and, therefore, would have been at pressure melting point. During decoupling, the glacier - including the frozen part of the sand substrate - was lifted from its bed, tearing away slabs of frozen sand. Preservation of these slabs in the till (Figure 8), in situ intratill and subtill sand bodies, preferred orientation of clasts and diapiric loading of the diamicton without unidirectional deformation of injected dikes and domes indicate deposition by passive melt-out (Shaw, 1987). A similar interpretation applies to the sequences below the Athabasca flutings (Figures 4a and b). While Dardis (1985) also found melt-out till sequences within drumlins in the interior of


172

Ireland, around the coast, Hanvey (1987) and McCabe and Dardis (1989) found drumlins containing fine-grained laminated sediment interbedded with gravel and muddy diamicton. The diamicton units have poor preferred clast orientation. They interpreted these complex sequences as primary proglacial lacustrine or marine deposits and stressed the absence of systematic deformation of the drumlin sediment. Very similar sequences in some of the drumlins around Peterborough, Ontario, are probably ice-marginal lacustrine deposits (Figure 6). Sharpe (1988) also describes undeformed sorted sediment in drumlins on Victoria Island, Northwest Territories. For each drumlin or fluting field from which these sediments are described, there are adjacent areas in which scoured bedrock is exposed. It is most unlikely that the melt-out process which deposited tills around Edmonton and the lodgement/deformation and meltout processes which deposited tills in Ontario and Quebec (Hicock, 1987; 1988; Bouchard, 1989) selectively deposited thick sediment in one area and none in adjacent bedrock areas. Similarly, subaqueous deposition in Ireland, Ontario and the Northwest Territories is unlikely to have been spatially selective. Meltwater erosion is inferred from detailed morphological studies in areas of exposed bedrock (Figure 7) (Murray, 1988;

Shaw, 1988; Kor et al., 1991; Rains et al., 1993). IV

Subglacial geomorphic and sedimentary phases

We infer that glacial sedimentary sequences in erosional drumlins commonly contain either proglacial lacustrine or marine sediments, or lodgement/deformation till below melt-.-

.--

--..-

-

.-....

Figure 8 Sand slabs (below lens cap) eroded from subtill deposits in melt-out till. Note the necking of the slabs caused by tensile strain. Villeneuve, near Edmonton, Alberta


173

out till, with associated glaciofluvial deposits. These sediments were obviously deposited prior to drumlin erosion, though probably during the same glaciation. Where the lodgement/deformation till rests on hard bedrock, the bedrock is invariably striated. Depositional drumlins and Rogen moraine ridges contain glaciofluvial and massmovement sediment deposited in cavities. A broad synthesis emerges from these interpretations (Table 1).

1

Ice-sheet advance

phase: thermal regime and process

Where hard bedrock underlies till of the Laurentide ice sheet it is plucked and striated. Thus, since these processes require a thermally complex bed with freezing and melting, this ice sheet was either once warm based everywhere, or spatially restricted warm-based conditions migrated across the bed with ice advance. Very different reconstructions emerge from theoretical modelling of ice-sheet thermal regime. Arnold and Sharp (1992) presented a model based on glaciology, climate and topography, in which a marginal melting zone sweeps slowly across the landscape with glacial advance and retreat. The melting zone occupies the area close to the ice maximum once, and other areas twice - once during advance and then again during retreat. Moran et al. (1980) argued on geological grounds for a narrow marginal, frozen-bed zone and a broad warm-bed zone. They used flow parameters to calculate ice-sheet profiles and flowlines and, by adjusting accumulation rates and mean annual marginal temperatures, showed that a 2 km wide marginal zone of subfreezing basal temperatures was plausible. In this case, beneath most of the ice sheet and for most of the time the bed would have been warm based, with cold conditions only where an area was in the marginal zone during advance and retreat. What can geomorphology and sedimentology contribute to these reconstructions? First, the great contrast in geomorphic activity before and after drumlin formation speaks against a symmetrical, cyclical set of processes related simply to ice-sheet advance and retreat. Special conditions that formed drumlins appear also to have caused a dramatic change in glacier regime. Secondly, the melt-out tills, which succeed lodgement or deformation tills (Shaw, 1979; Hicock, 1987; Bouchard, 1989), indicate prior entrainment in a freezing zone effected by meltwater from an upglacier melting zone (Table 1) (VTleertman, 1961 ) . Striations and lodgement/deformation tills below melt-out facies imply that the accreted, debris-rich ice subsequently began to melt. The simplest way to explain this sequence is to assume that, with ice advance, the upglacier melting zone expanded downglacier into areas where previously freezing and accretion took place (Table 1). Thus, in areas where there is an upward succession, striated bedrock, then lodgement/deformation till, then melt-out till, the geological evidence favours a thermal model of advancing ice with an inner thawed bed and an outer frozen zone (Moran et al., 1980; Mooers, 1990). This conclusion relies heavily on the correct recognition of regionally extensive melt-out till. This thermal configuration and other process interpretations of landforms and sediment leave few options for general interpretation of the subglacial environment. Sediment entrained by ice accretion would have been transported towards the ice margin. As the melting zone migrated with ice advance, it would have extended beneath previously accreted ice and debris, causing sliding of debris-rich basal ice. First, abrasion would have produced striation (Hallet, 1981), then with further melting, till would have been deposited by lodgement and, possibly, transported by subglacial deformation (Table 1) (Boulton and Hindmarsh, 1987; Alley et al., 1989).


174

Where ice sheets terminated in the sea or large lakes and where there was no frozen toe, example in western Ireland or the southeastern parts of the Laurentide ice sheet, subglacially deforming sediment may have extended to the ice margins, producing the observed large thicknesses of diamicton interbedded with sorted sediment (Figure 6) (Sharpe, 1987; McCabe and Dardis, 1989; Boyce and Eyles, 1991; Hicock and Dreimanis, 1992). Alley et al. (1989) noted that ice flow in these circumstances would have been extending, with appreciable rates of basal melting. Thus, englacial sediment would probably have been deposited after transport of only a few tens of kilometres: extending flow would have attenuated the basal zone of debris-rich ice and frictional heat would have caused high rates of basal melting. A minimum transport rate of englacial debris implies that deposition of diamicton would have been by mass flow rather than from floating ice. Dowdeswell and Dowdeswell (1989) drew a similar conclusion from records of low icerafted sediment fluxes relative to other sediment inputs to fiords in Spitsbergen. They argued that ice rafting is likely to be relatively unimportant in areas with high basal meltwater fluxes. for

2

Surging, stagnation

and melt-out

phase

The low profile of the Laurentide ice sheet indicates glacier flow with extremely low basal shear stresses (Mathews, 1974). It is argued that such flow is best explained by deformation of a low strength substrate and that it may have been associated with surging or fast glacier flow (Boulton and Jones, 1979; Beget, 1986; Boulton and Hindmarsh, 1987; Alley et al., 1989). But, although rapid advance of the Laurentide ice margin is supported by the radiocarbon chronology, pervasive deformation is contradicted by the geological evidence (Clayton et al., 1989). Thus, the mechanism for fast ice flow remains uncertain. Following surging, large areas of the glacier would have become quiescent (Raymond, 1987). Plausible conclusions about this quiescent phase can be inferred from detailed observations on subglacial sediment. The entrainment of sand slabs from the glacier substrate (Figure 8) implies that the freezing plane migrated downward into the bed. This is to be expected with increased heat loss by conduction through relatively thin, quiescent ice (Table 1). Boulder scours and subglacial fluvial deposits (Shaw, 1983b) indicate subsequent separation of ice and frozen bed from the unfrozen bed. The source of the meltwater causing this separation is unknown, although it is equally plausible that it was subglacial or supraglacial or even a combination of the two. Grooves beneath till (Figure 1) indicate sliding and basal melting (Hallet, 1981) - a temporary departure from general quiescence. Sliding of previously stagnant ice may have been caused by locally increased shear stresses over grounded areas and basal melting would have been facilitated by frictional heat and ice at pressure melting point. Slow melt-out of basal debris with included sand slabs (Figure 8) implies a net heat gain at the stagnant ice bed. Assuming constant geothermal heat flux, this melting must have been a consequence of reduced conduction of heat from the bed resulting either from snow accumulation at the surface or climatic warming or both. Successive drainage events may then account for complex stratigraphies involving till and sorted sediment (Dreimanis et al., 1987; Shaw, 1987). The objection of Paul and Eyles (1990) that such melt-out sequences could not survive because of low strengths associated with thaw consolidation is negated because shear stress beneath a quiescent stagnant zone, part of which had been completely supported by a subglacial meltwater sheet, would have been negligible. Loading and diapirism, without unidirectional strain, support this argument (Shaw, 1982).


175


176

Alternation of surges and quiescent periods may have been conducive to the formation of dispersal trains and associated sedimentary sequences (Dyke and Morris, 1988; Hicock, 1987; 1988). Meltwater migration below a freezing bed during quiescence provides the necessary conditions for upstream plucking and accretion. Accreted debris could then have been transported large distances englacially above a sliding bed during surges. The common succession - melt-out till overlying lodgement till - would mark deposition by lodgement in the active phase followed by melt-out during the quiescent stage. Such englacial transport explains the dominance of far-travelled erratics - a consequence of restricted mixing with local material - better than does transport in a subglacial deforming bed. A deforming bed, transporting sediment hundreds of kilometres over surface roughnesses in the form of valleys, rock knobs and small hills on the same scale as or larger than the thickness of the deforming layer, would be thoroughly mixed. Thus, it is unlikely that tills transported this way could retain a clast composition of mainly far-travelled lithologies.

3

Cataclysmic events:

drumlin and associated bedform creation

To this

point we have accounted for erosion of the bedrock, entrainment of debris and deposition by lodgement, from a subglacially deforming bed, and by melt-out. Details of sedimentary sequences are related to thermal and hydraulic conditions at the bed. Deformation of basal sediment may explain the ice sheet flattening. Surging and ice streams may also have resulted from this deformation. Quiescence, with snow accumulation or climatic warming, may explain the rise in basal temperatures causing melt-out beneath regions of stagnant ice. Astonishingly, all of this happened before drumlins, flutings and Rogen moraine were formed, and there was no appreciable deposition after their formation. Furthermore, erosion to bedrock with the formation of meltwater erosional marks over vast regions (Figure 7) (cf. Murray, 1988; Shaw and Gilbert, 1990; Kor et al., 1991; Rains et al., 1993) also followed the predrumlin phase of deposition. Since these scoured areas remain essentially free of sediment, there was virtually no posterosional event deposition. Depositional landforms, Rogen moraines and cavity-fill drumlins, which were formed at the same time as the erosional events, also mark the end of areally significant subglacial sedimentation. Two main concepts compete to explain the formation of drumlins, flutings and Rogen moraine: (1) subglacial deformation of the glacier bed; and (2) erosion and deposition by catastrophic outburst floods. Observations on drumlins, Rogen moraine and their sediment, and tracts of eroded bedrock contradict the subglacial deformation hypothesis and support the meltwater hypothesis. (a) Subglacial deformation: Considering subglacial deformation first, a deformed surface layer up to 10 m thick should be visible if drumlins were produced by an erosional process which pared down the landscape (Hart et al., 1990; Boyce and Eyles, 1991). Alternatively, if drumlins accreted from a deforming bed (Menzies, 1987) or resulted from wholesale deformation (Boulton, 1987), they should contain only translocated deformed sediment. Yet detailed observations on Irish drumlins (Hanvey, 1987; McCabe and Dardis, 1989), on drumlins in Ontario (Shaw, 1983a; Sharpe, 1987) and on flutings in Alberta reveal no pervasive surface or subsurface deformation. On a recent field excursion in Ireland, a group of drumlin experts failed to find evidence for drumlin formation by deformation.


177

Rather, undisturbed fine-grained sediment near the surface of many drumlins speaks against such deformation (Figure 6) (Sharpe, 1987; McCabe and Dardis, 1989). In addition, the subglacial deformation hypothesis does not adequately explain scoured bedrock areas which are complementary to drumlin tracts and are themselves sculpted in a variety of bedforms (Figure 7) (Kor et al., 1991; Rains et al., 1993). How, for instance, could subglacial deformation remove all sediment overlying crystalline bedrock, or how could it cut into soft, cretaceous shales without leaving a surficial deformed layer? Hart et al. (1990) used subglacial deformation theory to retrodict till thickness. The absence of till, in their theory, implies either no deformation or subsequent erosion of the deformed bed by other processes. Thus, scoured bedrock with adjacent drumlins containing undeformed laminated sediment or with adjacent flutings containing undeformed melt-out till cannot be attributed to subglacial deformation. Finally, an important characteristic of many subglacial landscapes is the virtual absence of ice-marginal forms and associated sediment. Deformation is hypothesized as a ubiquitous process within a broad marginal zone, both during glacier advance and retreat (Boulton and Hindmarsh, 1987; Hart et al., 1990). Once deforming sediment reached an ice margin, the driving force for its transport, glacially induced shear, would have been removed and the sediment would have been deposited in marginal moraines. The absence of such moraines in fields of drumlins, flutings and Rogen moraine ridges contradicts the hypothesis that subglacial deformation created these landforms. (b) Meltwater outburst floods: The meltwater outburst explanation for subglacial bedforms is based less on theory and more on empirical observation. Consequently, it is not surprising that observation and hypothesis are in agreement. The essence of this concept is that fast-flowing meltwater sheets eroded upwards into the ice bed and downwards into the glacier substrate (Figure 9). Direct erosion of the substrate produced a variety of erosional marks in bedrock (Sharpe and Shaw, 1989; Kor et al., 1991) that can be replicated experimentally (Shaw et al., 1989) and are indistinguishable from the set of erosional marks produced by turbulent flows in air or water (cf. Allen, 1982). Positive forms, drumlins and Rogen moraine ridges are interpreted as either residual ridges if they are erosional, or as infills of cavities eroded upwards into glacier beds by outburst floods if they are depositional. Residual ridges are explained in terms of erosion by horseshoe vortices. The proposed cavities of cavity-fill drumlins and Rogen moraine ridges are identical in form to a variety of erosional marks produced by turbulent flows (Shaw, et al., 1980; Fisher and Shaw, 1992). Sediment within these landforms was deposited by glaciofluvial and mass-movement processes, which is as expected for subglacial cavity fills (Shaw and Kvill, 1984; Fisher and Shaw, 1992). The sedimentary architecture within ridges shows that they are accretional rather than erosional. Given the huge quantities of sediment eroded and transported to form drumlin fields, the meltwater hypothesis involves enormous amounts of floodwater (Shaw et al., 1989). Since discharge rates of about 106 m3s were more than could be supplied directly by steady-state melting of the ice sheet, reservoir storage must have occurred. The geomorphological and sedimentological evidence for floods is so strong, the former existence of such reservoirs is simply assumed in much the same manner as Bretz (1923) assumed a reservoir for the Missoula floods. Yet there is no body of observation, similar to that for the outburst floods themselves, giving information on the extent and duration of the reservoirs, on the source of water, or on the mechanisms of containment and drainage.


178

In the absence of observation, Shoemaker (1991; 1992) has treated the question of large subglacial reservoirs theoretically. He presents a plausible picture of an inefficient drainage system and a large, steadily growing subglacial ’lake’ beneath the central part of the Laurentide ice sheet. The postulated effects of this lake on the ice-sheet profile, the proposed catastrophic drainage, and the link between drainage events and surging accord well with conclusions drawn from landforms and sediments. Of course, there may be other plausible explanations for these effects and direct evidence on the reservoir characteristics is needed to answer questions on the meltwater source for outburst floods.

(c) Glaciotectonics and hummocky moraine: Since both glaciotectonics and hummocky moraine are widespread in western Canada (Shetsen, 1987; 1990), a landscape model for the prairies would be incomplete without them. Glaciotectonics around the ice margin (Moran et al., 1980) probably required a frozen bed and high pore-water pressures in the unfrozen substrate. The frozen bed would have allowed transmission of shear stress from the glacier to the bed, and the high pore-water pressures would have reduced the strength of the substrate by reducing effective pressure (Moran et al., 1980; Bluemle and Clayton, 1984). As an alternative, high but short-lived shear stresses in areas of grounded ice adjacent to ice supported by meltwater may have played a part in glaciotectonic processes. For example, some streamlined or fluted glaciotectonic hills show that glaciotectonics preceded streamlining (Bluemle and Clayton, 1984). This is as expected if the tectonics were a result of high shear stresses generated by bed decoupling and the streamlined hills

Figure 9 Subglacial landforms related to outburst floods


179

are residual ridges preserved behind obstacles - the glaciotectonic hills - submerged in a subsequent outburst flood. Hummocky moraine remains enigmatic; both subglacial and supraglacial origins have strong support. Yet, vast extents of hummocky moraine in western Canada, with thick glacigenic deposits, stand as islands within scoured zones (Rains et al., 1993). Deformed and in situ bedrock in hummocks around Stettler and Hanna, Alberta, and subglacial eskers without superimposed hummocks in the same area (Shetsen, 1987) indicate a subglacial origin for this hummocky moraine. Rains et al. (1993) argue that the hummocks were formed subglacially by meltwater floods in areas where the ice sheet reattached to the bed relatively early in a flood event. Streaming of meltwater around these areas of grounded ice and hummocky moraine produced scour zones between which the hummocky moraine stands as residual islands. These preliminary interpretations integrate hummocky moraine into a unified outburst flood landscape model which incorporates drumlins, Rogen moraine and bedrock erosional marks (Figure 1 Oa) .

4

Regional stagnation and esker phase

On the basis of careful study of the landforms themselves, the subglacial landscape with drumlins, fluting, Rogen moraine, tracts of water-scoured bedrock and anastomosing tunnel channels (Figures 9 and 10) is best explained by cataclysmic floods which, by causing surging and by further reducing already low ice-surface gradients, brought about over broad regions. This effectively terminated glacial geomorphic activity in these areas. Glaciofluvial processes then dominated, depositing eskers in subglacial tunnels, commonly located in tunnel valleys (Table 1, Figure lOb). These eskers, many of them hundreds of kilometres long (Prest, 1968), mark the melting away of vast regions of clean ice with low gradients. Varves deposited in ice-marginal lakes indicate seasonal flow in the esker tunnels (Ringberg, 1979). Seasonal discharge implies that meltwater was largely from supraglacial sources. We may speculate that some catastrophic sedimentation events recorded in icemarginal fans (Gorrell and Shaw, 1991), fed by subglacial tunnels, resulted from sudden drainage of supraglacial lakes. Such lakes would have been probable on stagnant ice

stagnation

covering vast regions. V

Discussion

This article presents a brief review of subglacial processes from the viewpoint of how they might be inferred from sediment and landform and used in a qualitative analysis of subglacial landscape. The approach is necessarily inductive; the characteristic properties of landforms and their associated sediment are observed and possible explanatory processes are considered as working hypotheses. At the first level, we consider how well given processes explain individual forms or sediment facies. In some cases it is easy to draw conclusions. Thus, for the case of a simple form and a relatively simple process, there is strong agreement that striation results from abrasion. By contrast, for genetically more complex forms, such as drumlins, which may be mainly erosional or depositional, there is little agreement on the formative agent, let alone the actual processes. Qualitative analysis is particularly useful here and the properties of drumlin forms and associated sediment


180

a meltwater origin for drumlins and contradict formation by subglacial deformation. At the second level, we consider how processes explain an assemblage of forms. Thus the association of drumlins, flutings, Rogen moraine and areas of scoured bedrock can be best related to meltwater outburst floods in an integrated landscape model (Figure 9). But glaciotectonic landforms, which also appear in this assemblage, require bed failure under high shear stresses. Significantly, high local stresses are to be expected in grounded areas of a partially decoupled ice sheet. In this way, subglacial processes and the properties of

strongly support

Preglacial and glacier advance sedimentary sequences and outsubglacial landforms; b Superimposition of eskers from the late icestagnation phase on outburst flood landforms

Figure

l0a

burst flood


181

subglacial landforms come together in a unified explanation of subglacial landscape. An inferred pattern of subglacial landscape evolution derives from this approach (Table 1). In an early, highly active geomorphic phase, thermal and dynamic conditions were conducive to high rates of glacial erosion and deposition. Most subglacial and ice-marginal deposition of continental ice sheets took place in this phase. Surging at this time, possibly caused by subglacial deformation, extended and flattened ice sheets. Stagnation and surge/ quiescent episodes then caused meltwater storage, accretion and englacial transport. Stagnation and basal warming produced extensive melt-out tills. Meltwater accumulated in enormous reservoirs. Catastrophic release of the stored water eroded drumlins, flutings and other erosional marks, deposited drumlins and Rogen moraines, and transported vast quantities of sediment to proglacial river valleys and, eventually to the sea. There were many flood events before the last phase in which a well integrated ice-sheet drainage system, in virtually stagnant ice, produced eskers and ice-marginal fans and delivered meltwater and sediment to ice-marginal lakes.

Acknowledgements I appreciate the advice, encouragement and constructive criticism Mike Church and Garry Clarke.

given generously by

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