Largescale erosional marks

Large-scale erosional marks in bedrock might be loosely delimited as those with lengths >10 m. There are three main kinds: streamlined bedrock hills (Sawagaki & Hirakawa, 1997), crescentic scours and rock drumlins (Kor et al., 1991), and furrows (Kor et al., 1991). These large-scale forms all carry superimposed smaller forms.

Figure 4.5 Erosional drumlins and horseshoe vortices. Drumlins near Prince George, British Columbia (a) are clearly defined by hairpin scours related to horseshoe vortices (b). The geometry of the horseshoe vortex and the amalgamation of vortices (b) explains the absence of cross-cutting hairpin scours in (a) and (c). Erosional marks on Kellys Island, Lake Erie (c, photograph by Mandy Munro-Stasiuk). These erosional marks show the integration of crescentic scour and linear furrows and several identical elements appear in (a) and (c), particularly hairpin scours and the way in which crescentic scours define the pointed, proximal parts of rock drumlins.

Figure 4.5 Erosional drumlins and horseshoe vortices. Drumlins near Prince George, British Columbia (a) are clearly defined by hairpin scours related to horseshoe vortices (b). The geometry of the horseshoe vortex and the amalgamation of vortices (b) explains the absence of cross-cutting hairpin scours in (a) and (c). Erosional marks on Kellys Island, Lake Erie (c, photograph by Mandy Munro-Stasiuk). These erosional marks show the integration of crescentic scour and linear furrows and several identical elements appear in (a) and (c), particularly hairpin scours and the way in which crescentic scours define the pointed, proximal parts of rock drumlins.

4.4.5.1 Streamlined hills

Streamlined hills in Antarctica are up to 300 m high and they are aligned with subglacial meltwater flow rather than ice flow (Sawa-gaki & Hirakawa, 1997). Similar streamlined hills on the Antarctic continental shelf have a distinctive crescentic trough wrapped around their proximal end and extending down-flow as two parallel furrows (Shipp et al., 1999; Lowe & Anderson, 2003). A similar arrangement is observed at two classic erosional mark sites, Cantley near Ottawa (Sharpe & Shaw, 1989) and Kellys Island, Lake Erie (Fig. 4.5; Goldthwait, 1979).

4.4.5.2 Crescentic troughs

Crescentic troughs are wrapped around the upstream ends of rock bosses or rock drumlins. The knobs and drumlins usually carry stoss side furrows and the trough extends downstream as furrows before bifurcating in sichelwannen. The sichelwannen in turn are eroded into the residual rock, producing a long, tapered tail, such that the residual downstream from the crescentic scour takes on the form of a rock drumlin (Fig. 4.5). This arrangement of forms corresponds to erosion by horseshoe vortices (Fig. 4.5; Shaw, 1994) and is noted over a wide range of scales in areas of aeolian erosion, producing yardangs (Shaw, 1996). Consequently, these large-scale features are as expected for erosion of relief features submerged in a subglacial meltwater flow. As their morphology supports this explanation and there are analogies of similar scale in nature, application of the meltwater hypothesis to these large-scale crescentic scours and rock drumlins could only be denied if there was a strong argument showing that flows of the required scale could not exist.

4.4.5.3 Furrows

Furrows are large-scale subglacial features carved as troughs into bedrock. Furrow walls and floors are ornamented by all manner of erosional forms, which at a small scale replicate the furrows themselves. Probably the most spectacular example of furrow ornamentation is at Kelleys Island in Lake Erie (Fig. 4.5). This site illustrates the many nuances of meltwater erosion, particularly the importance of vortex interaction with bed features and the resultant crescentic scour/furrow sequences.

4.5 Bedform extent

The above brief commentary on bedrock features leaves unstated much of the detailed discussion of bedrock erosion by subglacial meltwater. At the same time it highlights the uncomplicated correspondence between analogous forms and, in a more sophisticated way, the correspondence between form and expected flow process in the meltwater hypothesis. In other words, the hypothesis is not simply based on form analysis. The brief comment on scale in the preceding section promises a telling test for the hypothesis. How can we determine the scale of the postulated meltwater flows?

Evelyn Murray (1988) mapped s-forms in the Kingston area and inferred a flood several tens of kilometres wide. Kor et al. (1991)

used the same approach in the French River area, Georgian Bay. Ground mapping of small-scale features and aerial photograph analysis of large-scale features illustrate a flow at least 70 km wide. Subsequent mapping west of Kilarney increased the width to about 150 km. In the absence of cross-cutting relationships of the erosional marks, what we see is a synoptic view of an enormous meltwater flow. The width scale of this flow preempts the argument that meltwater floods on the scale of drumlin, Rogen and hummock fields are impossible.

4.6 Flow magnitude

From information garnered to this point we are close to being able to estimate the instantaneous discharge of the French River event. The relief of rock drumlins in the area is in excess of 20 m and the flow must have submerged these landforms. The mapped width of s-forms gives a minimum width of the flow. We take the conservative estimate of 70 km. It then remains to estimate the velocity of the flow to obtain the instantaneous discharge. Rounded boulders rest on the erosional surface and are found as boulder deposits in sheltered locations. Many of these boulders carry percussion marks indicating violent transport. Using the range of velocities required to transport such boulders (Kor et al., 1991), we assume a conservative velocity of 10ms-1. Thus we can obtain the instantaneous discharge using the continuity equation Q = wdv, where Q is discharge, w is width, d is depth and v is velocity. The estimated discharge of 1.4 x 106m3s-1 would drain Lake Ontario (volume 1640 km3) in about 13 days.

4.7 Flow paths—a bigger picture

The existence of these enormous flow paths raises the obvious questions about their number and extent, the ways in which they affected the ice sheets, their timing and their extraglacial and climatic effects. Regarding climate, dramatic climatic change at the time of the Younger Dryas and the so-called 8.2 ka event recorded in the Greenland ice-cores are confidently attributed to meltwater outbursts from lakes (e.g. Clark et al., 2003b), yet the potential importance of outbursts from beneath the ice itself are seldom considered. Blanchon & Shaw (1995) proposed that the sea-level and climatic changes around the time of the drainage of Lake Barlow Ojibway and Heinrich events H0 and Hj (ca. 12ka and ca. 15ka) were related to outburst floods from beneath the Laurentide ice sheet destabilizing ice grounded on continental shelves.

More recent work shows there is a time correlation between Laurentide and Cordilleran events. For example, there is a coincidence of the Laurentide outburst at ca. 15 ka and flooding from the Scablands at about 15.4C14kyr BP (Normark & Reid, 2003). The date is from mud that precedes deposition of a 57-m-thick turbidite said to have originated in a Lake Missoula flood. Until recently, the Cordilleran and Laurentide ice behaving synchronously would have been related to some external forcing such as climate. However, Shaw et al. (1999) suggested that the Scabland floods were connected to subglacial drainage from the Cordilleran Ice Sheet of interior British Columbia. Recent field work shows that the Cordilleran and Laurentide ice sheets were linked hydraulically during drumlin forming events. Consequently, linked, subglacial drainages involving the Cordilleran and Laurentide ice sheets could well have been responsible for the climate change and rapid sea-level rise event at about 15 ka (Blanchon & Shaw, 1995). As well, the meltwater events that triggered later, abrupt climatic change may have included a contribution from the Laurentide Ice Sheet itself. Shaw (1996) suggested that the sudden diversions and outbursts of Lake Agassiz might well have been a cascade effect, triggered by subglacial outbursts.

The NASA Shuttle Radar Topography Mission provides the evidence on hill shade maps, based on radar interferometry with a horizontal resolution of 938 m, for continent-wide, concurrent outbursts (Fig. 4.4). These images show distinct flow paths, marked by sharply defined erosional margins and streamlined bedforms. The paths are anabranching and the absence of cross-cutting relationships amongst the streamlined forms indicates that the flow patterns they represent are synoptic, that is the flows were part of a concurrent, continent-wide drainage system beneath the Laurentide Ice Sheet (Fig. 4.4). Figure 4.4 shows the flow paths as sets of arrows. The flow has been mapped directly from directional forms visible on the image.

In northern Alberta, the Livingstone Lake Event(s) scoured huge channels and left behind residual hills, several hundred metres above the channel level. These highlands stand out clearly on the hill shade, particularly the Caribou Hills and Birch Mountains (Fig. 4.4). Nevertheless, these hills were also overtopped by the enormous flows that sculpted their streamlined form. Trying to picture this is next to impossible; the scale is unimaginable! From the image we see that the flow to the east of the Caribou Mountains continued southwards and exited Alberta to the south of Calgary, east of the Cypress Hills. This flow is just part of the so-called Livingstone Lake Event (Rains et al., 1993) with its source in the Keewatin Ice Divide zone (Shaw, 1996). It is a mere filament in a much wider flow.

The Livingstone event path is clearly identified and there is a wealth of detailed study supporting meltwater formation of features along this flood path [e.g., fluting (Shaw et al., 2000); bedrock s-forms, cavity fill drumlins (Shaw & Kvill, 1984; Shaw et al., 1989); sedimentary architecture and lithological composition (Shaw et al., 1989), tunnel channels (Beaney, 2002), scablands and broad-scale erosion (Sjogren & Rains, 1995); hummocky terrain (Sjogren et al., 1990; Munro & Shaw, 1997); lake systems (Shoemaker, 1991; Munro-Stasiuk, 2000)]. Shaw (1996) and Rains et al. (1993) present more general overviews which were designed to paint the bigger picture.

4.8 Earth system effects

The scale and coherence of the Livingstone Lake flow path are stunning, yet they pale in the larger scale image. The Shuttle Radar Topography Mission (SRTM) (NASA/JPL PIA03377) shows the true magnitude of these flows (Fig. 4.4). Probably the most exciting aspect of the flows is that, with few minor exceptions, where local relief dictates flow changes, the flows were simultaneous. The evidence for this is both simple and compelling. Rather than one set of forms cross-cutting another, the flow tracts merge and even carry interference patterns.

The Livingstone Lake event dominated the early thinking on flood tracts or paths, although it was clearly smaller than the drainage south of Winnipeg to the Mississippi. Leventer et al. (1982) provided independent evidence for just this kind of flood. Only the limitations of dating resolution prevented them from proposing extremely short-lived, high-magnitude drainage outbursts. With the continent-wide synoptic flow (Fig. 4.4a), the Livingstone Lake event pales to insignificance (Fig. 4.4a). We can make a rough estimate of the total drainage by extrapolating the flow estimates from French River (Kor et al., 1991); the total discharge to the Gulf is about 2.7 x 107m3s-1. The total volume of meltwater added to the Gulf of Mexico is more difficult to estimate. Taking the calculations for the Livingstone Lake event, V = 84,000km3, where V is volume of flow (Shaw et al., 1989), then extrapolating that figure to include the full width of flow, gives total rise in sea level of about 3.7m attributed to water flowing to the Gulf of Mexico. This does not include contributing outlets via Hudson Strait, flows to the Arctic Ocean, or flow through the St Lawrence. The Arctic outlets must have been at least equivalent to those to the Gulf. As well, other continental ice sheets may have contributed meltwater directly to the Catastrophic Rise Event (CRE) at about 15ka (Blanchon & Shaw, 1995). Consequently, the floods were capable of producing the rates and amounts of sea-level rise discussed by Blanchon & Shaw (1995). The predominant rise is at about 15ka. The timing of this rise is close to that for the double peak of meltwater input to the Gulf of Mexico (Leventer et al., 1982). In addition, sea-level rise might have destabilized ice resting on continental shelfs, causing the iceberg armadas of Heinrich events.

4.9 Conclusions

There is much more to write about the meltwater effects discussed here, both from the point of view of landscape and also from the large-scale, global effects of such high volumes of cold, sediment charged, freshwater. These global effects are expected to be extreme and the floods are expected to play an important part in explaining the various scales of abrupt climate change associated with Quaternary glaciations.

As many earth scientists do not consider the meltwater hypothesis credible, work must continue at the scale of landforms and landform associations. Although many are unlikely to be persuaded by such work, it must be done if the hypothesis is to be supported. The recent flurry of papers on Antarctic subglacial outbursts supports the concept of megafloods in the warmer ice sheets of the mid-latitude ice glaciers.

Acknowledgements

I am grateful to NSERC Canada for supporting this work from the beginning. Graduate students and colleagues have done much of the research cited here. I owe them an enormous debt. Above all, I could not have persevered in the face of often bitter criticism without the contribution and friendship of Bruce Rains. I am thankful to Peter Knight for his encouragement and generosity as this paper evolved.

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