Department of Geography, Queen's University, Kingston, Ontario K7L 3N6, Canada ... turbulent subglacial meltwater (Shaw, 1983; Shaw and Kvill, 1984; Shaw.
Drumlins, subglacial meltwater floods, and ocean responses John Shaw Department of Geography, Queen's University, Kingston, Ontario K7L 3N6, Canada
ABSTRACT Drumlins and erosional marks in bedrock give evidence for broad, subglacial meltwater floods that have discharge-rate estimates of about !C 6 m 3 /§. Similar discharge rates are obtained for other late glacial catastrophic floods. The total volume of meltwater that is thought to ¡have formed the Livingstone Lake, Saskatchewan, drumlin field is estimated at 8.4 x 10 4 km 3 . This volume is equivalent to a eustatic rise of U.23 m in global sea level. Meltwater release and roughly contemporaneous formation of drumlin fields in North America and Europe could have involved several metres of sea-level rise in a few years. Ttoe implications of such floods for the generation of myths and tine ¡interpretation of the oxygen isotopic record of the oceans are discussed. High meltwater discharges are of potential importance to the generation of a lid of cold, fresh water over the North Atlantic and ¿is effects on late glacial climate. INTRODUCTION Recent work on drumlins of the Livingstone Lake drumlin field, northern Saskatchewan, and bedrock drumlins at Beverley Lake, Northwest Territories, Canada, leads to the conclusion that vast fields of depositional and erosional drumlins resulted from broad sheetfloods of turbulent subglacial meltwater (Shaw, 1983; Shaw and Kvill, 1984; Shaw
Figure 1. Drumlins of Livingstone Lake field, northern Saskatchewan. Flow irom northeast Spindle (Sp), parabolic (P), parabolic with extended arms (Pa), and shield complexes (S) have counterparts in erosional marks produced by turbulent (lows. These landforms are interpreted as fills of glaciofluvially eroded cavities in underside of glacier. Courtesy of Government of Canada, Department of Energy, Mines and Resources. Airphoto A14509-3. GEOLOGV, v. 17, p. 853-856, September 1989
and Sharpe, 1987; Shaw et al., 1989). In addition, erosional marks in bedrock in the Kingston area, Ontario, are also interpreted to have resulted from broad subglacial meltwater flows (Shaw, 1988). The extraglacial implications of such floods will be considered here. In particular, the effects on sea level and isotopic composition of ocean water are discussed. EVIDENCE FOR SUBGLACIAL MELTWATER FLOODS Drumlins in northern Saskatchewan (Fig. 1) appear like flute casts on the bases of turbidite beds and contain sediment deposited primarily by meltwater (Shaw, 1983; Shaw and Kvill, 1984). They also occur in close association with tunnel valleys and eskers, landforms confidently ascribed to the action of subglacial meltwater (Wright, 1973; Grube, 1983). Other drumlins, such as those in the Beverley Lake area, Northwest Territories, and some in the Peterborough area, Ontario, appear to be erosional forms left as residual ridges after meltwater erosion of surrounding areas (Fig. 2; Shaw and Sharpe, 1987). Other authors have ascribed drumlins to erosional processes, but have favored direct glacial rather than glaciofluvial erosion (Gravenor, 1953; Hillefors, 1969; De Jong et al., 1982). However, the form similarity to erosional forms produced by turbulent flows and the landform associations of the drumlins favor a meltwater origin (Shaw et al, 1989). The drumlins in the fields illustrated here have axes indicating a coherent flow pattern over the whole field (Shaw and Kvill, 1984). There is no sign of crosscutting or discordant flows, which would be expected
Figure 2. Drumlins of Beverley Lake field, Keewatin, Northwest Territories. Flow from southeast. These erosional drumlins are cut from bedrock and are defined by crescentic scours (A) that wrap around proximal ends of drumlins. Courtesy of Government of Canada, Department of Energy, Mines and Resources. Airphoto T300C-39. 853
had the drumlins been formed by a fluid flow that shifted laterally with time. Furthermore, abrupt changes do not occur in drumlin scale and form. Rather, there are gradual changes between areas with distinct forms. These observations support the conclusion, to be used later, that the formative flows were at least as wide as the drumlin fields themselves. Concerning the depth of flow, it is clear that the drumlins of Figure 1, particularly the shield complexes with superimposed smaller-scale features, must have been submerged in the formative flow. The alternative, that they were produced by flows much shallower than drumlin height, is akin to suggesting that ripples or barchan dunes could be produced as such distinctive forms with their upper parts projecting above the surface of the formative fluid.
Erosional marks in bedrock in the Kingston area, Ontario, indicate subglacial meltwater flows that have widths of more than 60 km (Murray, 1988; Shaw, 1988). Spectacular erosional marks along the north shore of Georgian Bay, Ontario, also indicate broad subglacial meltwater flows (Fig. 3). On a helicopter traverse along the north shore of Georgian Bay, a single field of bedrock erosional marks was noted that had a width of at least 50 km. These marks closely resemble classical flutes found on the beds of turbidites and in a number of settings associated with turbulent flows (Fig. 4; Allen, 1982). The drumlins and erosional marks outlined above indicate subglacial meltwater floods that were competent to remove the largest boulders appearing on the bed. Absolute minimum velocities of 2 m/s were required (Elfstrom, 1987). Flow widths, equal to the widths of drumlin and erosional-mark fields, were in the range of 60 to 150 km. Flows were of sufficient depth to submerge drumlins and overtop escarpments (Shaw, 1988); minimum depths of about 20 m were required. Using Q = w • d • v,
Figure 3. Erosional marks in bedrock, French River, Georgian Bay, Ontario. These marks are sichelwannen or sickle troughs with crescentic rim around proximal part. They resemble classical flutes (Fig. 4). Width of foreground is approximately 50 m; background is approximately 80 m.
where Q is discharge (m 3 /s), w is flow width (m), d is flow depth (m), and v is flow velocity (m/s), a range of minimum discharge, 2.4 to 6.0 x 106 m 3 /s, is obtained. This estimate is similar to that obtained by Boyd et al. (1988) for subglacial discharges that form the tunnel valleys of the Scotia Banks, Nova Scotia, and by Baker (1978) for the floods related to the drainage of glacial Lake Missoula. In the absence of data on reservoir size and an estimate for the duration of flow, it is not possible to determine directly the total volume of water involved in these subglacial, drumlin-forming floods. An alternative approach is to estimate the amount of ice melted in the production of the Livingstone Lake drumlins, which are related to cavities ablated into the ice bed by meltwater (Shaw, 1983; Shaw and Kvill, 1984; Shaw et al., 1989). The volume of ice melted is approximated as three times the volume of drumlins in the Livingstone Lake field. The volume of drumlins is obtained by determining the average volume of individual drumlins, = 1.57 x 107 m 3 , and the drumlin density, D = 0.6 km -2 , from topographic
Lateral
Median ridge Principal
furrow
teral Lateral
ridge
Lateral
furrow ridge Median
ridge
furrow
Figure 4. Classical flute produced by turbulent, separated flow (from Allen, 1982).
Lee side
854
Stoss side
GEOLOGY, September 1989
maps. Nye (1976) presented a relation between discharge per unit width q, pressure gradient dp/dx, and mass of ice melted by meltwater (m'): ? [ - ( ! - 7)
dx
3.0
j
= Lm',
where 7 is a dimensionless constant = 0.313 and L is the latent heat of fusion of ice. Using the low surface slopes of the southwestern part of the Laurentide ice sheet to obtain dp/dx (Mathews, 1974), a total volume of flow of 8.4 x 104 km3 is obtained for the formation of the Livingstone Lake drumlin field. This may be compared with the 2.0 x 103 km3 estimate for the Missoula floods based on the volume of glacial Lake Missoula (Baker, 1978). Clearly, such enormous discharges cannot have been produced by steady-state subglacial melting by geothermal and frictional heat sources (Wright, 1973; Shaw, 1983; Boyd et al., 1988). Catastrophic release of subglacially stored meltwater must be invoked to account for the discharge rates and total volume of meltwater (Shaw, et al., 1989).
CO
o o x m LL
O LL
_l O O
Second Major Discharge
EXTRAGLACIAL IMPLICATIONS If the meltwater hypothesis on drumlin and erosional mark formation is correct, the enormous outpouring of fresh water associated with the Livingstone Lake drumlin field represents only a small proportion of catastrophic flooding associated with drumlin fields of the late Wisconsinan Laurentide and Fennoscandian ice sheets, yet the effects of this single flood on the oceans of the world would have been significant. Taking the area of the oceans at 3.61 x 10 8 km 2 (Andrews, 1976), the volume of the northern Saskatchewan outburst would have produced a global rise in sea level of 0.23 m in a probable time period of a few weeks, and certainly less than one year. This rise in sea level would have been noted by coastal dwellers, and myths of drowned continents and catastrophic floods, recurrent in several cultures, may have had their origins in such observations (Emiliani et al., 1975). The objections on climatological grounds (Wright and Stein, 1976) to the conjecture of Emiliani et al. (1975) of massive meltwater floods are negated if, as is proposed here, the floods resulted from catastrophic release of stored meltwater. In this explanation, the trigger or forcing agent for sea-level rise becomes glaciological or glaciohydraulic rather than climatological. The suggestion by Emiliani et al. (1978) that we look again at the geological evidence from beneath the ice sheets appears prescient in the light of the meltwater interpretation for drumlin formation. Emiliani et al. (1975) provided evidence that major floods from the late Wisconsinan Laurentide ice sheet caused significant freshening of the surface waters of the Gulf of Mexico. The rapid change that they inferred for the isotopic composition of the gulf waters toward increasingly negative values of 5 l s O has also been recorded by others (Fig. 5; Kennett and Shackleton, 1975; Leventer et al., 1982). However, there is difficulty in pinpointing the duration of such inflows because of relatively poor age control on the cores from which the isotopic data were obtained. Consequently, sampling over intervals of several centimetres or several tens of centimetres has the effect of attenuating any peaks in isotopic composition occurring within sedimentary units of thickness less than the sampling interval. Thus, the peaks of Figure 5, although pronounced, are probably less so than they should be. This means that the magnitude of the floods has probably been underestimated in that their full effect is not recorded in the isotopic data. In addition to the isotopic evidence for floods. Leventer et al. (1982) described a sediment unit from 509 to 537 cm in core EN32PC6, which is massive and contains few microfossils. They attributed this bed, which corresponds to the period of low S l 8 0 content, to rapid deposition in a period of extreme meltwater discharge. Thus, there is a convergence of evidence on catastrophic meltwater GEOLOGY, September 1989
First Major Discharge Glacial Maximum
25 -
30 - 1
Figure 5. Plot of percent
