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Subglacial Drainage, Eskers, and Deforming Beds Beneath the Laurentide and Eurasian Ice Sheets

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Subglacial Drainage, Eskers, and Deforming Beds Beneath the Laurentide and Eurasian Ice Sheets Subglacial drainage, eskers, and deforming beds beneath the Laurentide and Eurasian ice sheets PETER U. CLARK Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506 JOSEPH S. WALDER U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661 ABSTRACT over a deforming bed and that developed over a rigid, nondeforming substrate (Rothlisberger, 1972; Shreve, 1972). In particular, Walder Glatiological theory predicts that the subglacial drainage network and Fowler showed that if subglacial water pressure is close to the at the base of gently sloping ice sheets resting on deforming sediment ice-overburden pressure and the hydraulic gradient (largely controlled should consist of many wide, shallow, probably braided "canals" dis- by ice-surface slope) is low (such as at the base of a gently sloping ice tributed along the ice-sediment interface, rather than an arborescent sheet), then the drainage network over a deforming bed should consist network of relatively few large tunnels, as would develop over a rigid of numerous wide, shallow, braided channels along the ice-sediment substrate. A corollary prediction examined here is that eskers, which interface. Meltwater would flow relatively sluggishly through such a form in large subglacial tunnels, should be rare where subglacial bed drainage network. In contrast, meltwater flowing over a rigid sub- deformation occurred, but they may be relatively common where the strate or over a deformable bed at relatively high hydraulic gradient bed was rigid. Bed deformation would be most likely where subglacial would drain through an arborescent network of relatively few large till was relatively continuous, fine-grained, and of low permeability— tunnels at substantially higher flow velocity. that is, in regions where till is derived primarily from underlying sed- The Walder-Fowler drainage theory implicitly predicts aspects imentary bedrock—but unlikely where discontinuous, coarse-grained, of the geomorphic and sedimentological record of the subglacial drain- high-permeability till was derived from underlying crystalline bedrock. age system. In this paper, we summarize salient aspects of the theory The observed distribution of eskers in areas covered by the Laurentide and show that one implication of the theory is that eskers, which form and Eurasian (British, Scandinavian, and Barents Sea) ice sheets during by sedimentation within large subglacial tunnels, should be favored the last glaciation shows that most eskers occur over crystalline bedrock where the bed is rigid and the subglacial drainage system comprises overlain by discontinuous, high-permeability till, but are rare or absent a network of arborescent tunnels, but unlikely to occur where there over sedimentary bedrock overlain by fine-grained, low-permeability is pervasive bed deformation and the drainage system consists of till, thus matching reasonably well our prediction. Glaciological theory shallow, braided channels. We then discuss characteristics of eskers and geologic evidence indicate that esker systems on a subcontinental and their mode of formation, and we examine the distribution of es- scale are time-transgressive. Sedimentological evidence for a "canal" kers from areas formerly covered by the Laurentide and Eurasian drainage system appears to be present in fine-grained tills where eskers (British, Scandinavian, and Barents Sea) ice sheets during the last are largely absent. glaciation. We find that eskers are indeed rare or absent where the ice-sheet bed would have been most susceptible to subglacial INTRODUCTION deformation. Recognition that subglacial sediment deformation by pervasive SUBGLACIAL WATER FLOW AND THE ORIGIN OF ESKERS shear plays an important role in the behavior and dynamics of some modern glaciers (Boulton, 1979; Alley and others, 1986,1987; Boulton Glaciological Considerations and Hindmarsh, 1987; Engelhardt and others, 1990) has directed much attention to the potential contribution of this process to dynam- The traditional picture of subglacial hydrology (Rothlisberger, ics of the former Northern Hemisphere ice sheets (Boulton and Jones, 1972; Shreve, 1972) was built around the assumption that the glacier 1979; Boulton and others, 1985; Fisher and others, 1985; Hughes, bed is rigid. Water pressurepw in a channel is less than the ice over- 1992; MacAyeal, 1993; Clark, 1994). Evidence of pervasive till de- burden pressure pt, and inward ice flow therefore tends to close the formation comes from direct observation beneath several glaciers channel. Counteracting this tendency is the melting caused by energy (Boulton, 1979; Boulton and Hindmarsh, 1987; Blake, 1992; Hum- dissipation within the flowing meltwater. Drainage is predicted to oc- phrey and others, 1993) and interpretation of geophysical data (Blan- cur in an arborescent network of channels (commonly called Roth- kenship and others, 1986, 1987). Criteria for identifying evidence of lisberger or R channels) cut into the basal ice. former subglacial sediment deformation remain elusive (Clayton and Walder and Fowler (in press) have shown how the classical view others, 1989; Alley, 1991) but are clearly needed in view of the po- of subglacial drainage must be modified for the case of a glacier or ice tential significance of this process to the behavior of former and sheet resting on deforming sediment. They demonstrated that the rhe- present ice sheets. ology of the sediment and the mechanics of sediment transport by Theoretical analysis by Walder and Fowler (in press) indicates a subglacial streams strongly affect the shape of subglacial drainage fundamental difference between the subglacial hydrologic system conduits, and they concluded that two distinct types of drainage con- Geological Society of America Bulletin, v. 106, p. 304-314, 8 figs., February 1994. 304 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/106/2/304/3382138/i0016-7606-106-2-304.pdf by guest on 24 September 2021 DRAINAGE, ESKERS, AND BEDS BENEATH ICE SHEETS R-channel canal general, the drainage system over deforming sediment should consist of R channels only if the ice-surface slope is large, on the order of x-section x-section about 10_1, as for a valley glacier. Ice-surface slopes associated with ice sheets are much lower, say a few times 10~3, except near the margins. ICE Pervasive till deformation and thus canal drainage will also be I favored if the sediment is "soft"; that is, if it creeps readily under BED applied stress. Creep is facilitated by the presence of clay (Mitchell, 1976), and so the drainage system over deforming till containing mod- erate amounts of clay is likely to be canal-like, whereas drainage over clay-poor, "stiff" till is likely to be in R channels. The Walder-Fowler theory thus predicts that, in general, melt- water drainage over pervasively deforming sediment beneath ice sheets should be in a non-arborescent system of canals instead of an arborescent tunnel network. Such a canal system at the base of an ice plan plan sheet should manifest an effective pressure pe typically about 1 bar, in accord with geophysical inferences (Blankenship and others, 1986, Figure 1. Schematic illustrations (cross section and plan views) of 1987) and bore-hole measurements (Engelhardt and others, 1990) at drainage types developed over rigid (R channel) versus deforming (ca- Ice Stream B, West Antarctica, which overlies weak, dilated sedi- nals) beds. Drainage through an R channel develops over a rigid bed ment thought to be deforming (Alley and others, 1986,1987). characterized by relatively high effective pressures (low water pressure). Drainage through a canal develops over a deforming bed where effective Characteristics and Formation of Eskers and Esker Systems pressures are low. Eskers are straight-to-sinuous ridges of stratified sediment, up to several hundred kilometers long, that are widespread on formerly duits may exist over a deforming substrate (Fig. 1). One type is ba- glaciated landscapes of the Northern Hemisphere (Prest and others, sically like the classical R channel, and the predicted relationship (in 1968; UNESCO, 1967-1980). They have been extensively studied the steady state) between discharge Q and effective pressure pe (ice with respect to their sedimentology, morphology, and petrography pressure minus water pressure) is of the form (compare with Lli- (Flint, 1971; Embleton and King, 1975; Shreve, 1985; Drewry, 1986). boutry, 1983): Shreve (1985) elegantly demonstrated that esker paths are explicable in terms of the physics of subglacial water flow at the base of active ,1/15 ice, where "active" means (Shreve, 1985, p. 644) that the ice is flow- Pe <* Q (1) ing forward everywhere. Eskers form by sedimentation within an Thus, as discharge increases, p(, increases; hence, water pressure pw arborescent network of drainage channels, which are incised into the decreases. Larger channels are therefore at lower pw than smaller base of active ice and follow hydraulic-potential lows on the glacier channels and tend to capture the drainage of those smaller channels, bed. Because the dominant control on the gradient of hydraulic equi- forming an arborescent network with relatively few main trunk chan- potential is the ice-surface slope (Rothlisberger, 1972; Shreve, 1972), nels, just as in subaerial stream networks. this means that channels generally trend in the same direction as the Distinct from R channels are what Walder and Fowler termed ice-surface
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