Application of Landsystem Models

Hypothesized palaeo-ice streams from the major ice sheets have been reviewed (Stokes and Clark, 2001), and elements of the criteria and landsystems models above are inherent to many of the arguments that authors have used for demonstrating the existence of ice streams. From the above consideration of marine and terrestrial ice streams, and those that leave a rubber-stamped or smudged imprint, we construct four landsystem models that we expect ice streams may leave behind as evidence (Fig. 9.12). It is hoped that these conceptual models will help in deciphering the bedform record of palaeo-ice streams. In the following, we apply our knowledge of the four main landsystems to actual examples of palaeo-ice streams.

Figure 9.10 A terrestrial ice stream may leave behind a diagnostic landsystem of bedforms and ice-marginal landforms. If the ice stream switched off and retreated back without much remoulding of the landscape we would expect to find an imprint as in (a), the 'rubber-stamped' imprint. However, if the ice stream retreated, while continuously operating, then we should expect reorganization of the bed during retreat, producing a more 'smudged' or overprinted landsystem as in (b). This is schematized into fainter systems overprinted and modified by later margin positions and flow patterns, whereby some parts of the corridor exhibit singular flow directions (i.e. down the trunk) and other parts exhibit cross-cutting bedform populations, particularly on the lobe flanks (c).

Figure 9.10 A terrestrial ice stream may leave behind a diagnostic landsystem of bedforms and ice-marginal landforms. If the ice stream switched off and retreated back without much remoulding of the landscape we would expect to find an imprint as in (a), the 'rubber-stamped' imprint. However, if the ice stream retreated, while continuously operating, then we should expect reorganization of the bed during retreat, producing a more 'smudged' or overprinted landsystem as in (b). This is schematized into fainter systems overprinted and modified by later margin positions and flow patterns, whereby some parts of the corridor exhibit singular flow directions (i.e. down the trunk) and other parts exhibit cross-cutting bedform populations, particularly on the lobe flanks (c).

222 GLACIAL LANDSYSTEMS

Parallel Lines

Figure 9.11 Distinguishing between rubber-stamped and smudged bedform imprints can be aided by careful observation of lineation morphometry, discordances in flow direction and cross-cutting relationships between bedforms. A) A rubber-stamped flow pattern should contain lineations of high parallel conformity, no cross-cuts and with gradual variations in morphometry. B) Conversely for the smudged or time-transgressive case, we expect low parallel conformity, possible occurrence of cross-cuts and abrupt discontinuities in lineation morphometry. Superimposed ice marginal landforms such as end moraines would provide further evidence.

Figure 9.11 Distinguishing between rubber-stamped and smudged bedform imprints can be aided by careful observation of lineation morphometry, discordances in flow direction and cross-cutting relationships between bedforms. A) A rubber-stamped flow pattern should contain lineations of high parallel conformity, no cross-cuts and with gradual variations in morphometry. B) Conversely for the smudged or time-transgressive case, we expect low parallel conformity, possible occurrence of cross-cuts and abrupt discontinuities in lineation morphometry. Superimposed ice marginal landforms such as end moraines would provide further evidence.

9.8.1 Marine-Terminating, Rubber-Stamped, Palaeo-Ice Stream (Fig 9.12a)

The best example is the Western Bransfield Basin Palaeo-Ice Stream, discovered by Canals et al. (2000). It was found by marine geophysical investigation of the continental shelf just off the northern tip of the Antarctic Peninsula, and is about 100 km in length and up to 21 km wide. The imprint is remarkable for its clarity and completeness, and comprises:

• converging flow pattern

• appropriate dimensions (100 by 25 km) abrupt margins, and

• a bedform signature of MSGLs.

The well-organized pattern of MSGLs, without any sign of discordance or cross-cutting, is a strong indicator that this is a snapshot record of the glacier bed, which has survived deglaciation with little modification. It represents a classic example of the rubber-stamped imprint, and almost

Marine-based ice stream (eg Ice stream B)

Terrestrial ice lobe/stream (eg Des Moines I Lake Michigan lobes ?)

CONTINENTAL SHEli

Time transgressée

IN MUM V

3 O f INFWRED MAXIMUM H) G? EXTENT OF 1 GROUNDING LINE

KEY:

Streamlined bedform

Highly-attenuated streamlined bedform (e.g. flutes, drumllns, megalineations)

Ice stagnation or déglaciation landform (e.g. hummocky moraine)

— Inferred ice-stream margin

-^J Offshore sediment accumulation (TMF)

Ice-marginal landform (e.g. lateral / terminal moraine, push moraine)

Figure 9.12 The four types of landsystem signature produced by palaeo-ice streams. Ice streams may be marine or terrestrially terminating (top diagrams), and may lay down their geomorphological imprint either isochronously (rubber-stamped) or time-trangressively (smudged). This yields four landsystem models (a, b, c and d).

looks as if the ice sheet has just been lifted off, preserving a perfect record. Indeed, on this latter point there is the possibility that it did in fact lift off as sea level rose.

9.8.2 Marine-Terminating, Smudged, Palaeo-Ice Stream (Fig. 9.12b)

Again from the seafloor surrounding Antarctica, a good example of this type is that reported in Shipp et al. (1999), where MSGLs are found that have been overprinted by moraines during ice-stream retreat. The M'Clintock Channel Palaeo-Ice Stream is a further example and is illustrated in Figures 9.13 and 9.14. This ice stream, reported in Clark and Stokes (2001), has left terrestrial evidence of its existence. It drained the northwestern portion of the Laurentide Ice Sheet during deglaciation (Fig. 9.13) and likely terminated in an ice shelf. Figure 9.14 illustrates some of the complexity of flow patterns recorded in subglacial bedforms, and shows a strong ice-stream

reconstructed as 720 x 140 km in size with a cross-sectional area of 100 km2, and is thought to have discharged into an ice shelf. (From Clark and Stokes, 2001).

signature that has in part been overprinted by landforms relating to a subsequent readvance and retreat. The ice-stream record comprises:

• appropriate shape and dimensions

• converging flow pattern

• shear margin moraines, and

• drumlins and MSGLs recording fast-flow velocities.

While the geomorphology has been somewhat smudged by later activity, the ice-stream signature remains well-enough preserved to see an isochronous record of the bed at the time of ice-stream shut down. Although the M'Clintock Channel Palaeo-Ice Stream is a (slightly) smudged example, it is likely that tracks exist elsewhere where the smudging from later flows is more severe and the main trunk may appear much more subtle.

9.8.3 Terrestrially Terminating, Rubber-Stamped, Palaeo-Ice Stream (Fig. 9.12c)

The Dubawnt Lake Palaeo-Ice Stream (Stokes and Clark, 2003) existed entirely terrestrially and drained the northwestern portion of the Laurentide Ice Sheet during late glacial times. Figure 9.4 displays part of the bed which comprises highly parallel MSGLs, and Figure 9.15 illustrates the extent of the ice stream. Criteria demonstrating that this flow imprint is an ice stream include:

• appropriate dimensions

• converging — trunk — diverging flow, as we would expect of a terrestrial ice stream

• abrupt lateral margins

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440000 480000

Wheat Root System

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kilometres

Figure 9.14 Detail of some of the mapping of drift lineations (drumlins and mega-scale glacial lineations), for part of the M'Clintock Channel Ice Stream on Victoria Island, Arctic Canada. In the west, drumlins record early flow patterns that were obliterated by the ice-stream signature (flowing northwards through centre of image), which have then been slightly remoulded, or 'smudged' by a drumlinization event subsequent to cessation of ice-stream operation. (From Clark and Stokes, 2001).

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kilometres

Figure 9.14 Detail of some of the mapping of drift lineations (drumlins and mega-scale glacial lineations), for part of the M'Clintock Channel Ice Stream on Victoria Island, Arctic Canada. In the west, drumlins record early flow patterns that were obliterated by the ice-stream signature (flowing northwards through centre of image), which have then been slightly remoulded, or 'smudged' by a drumlinization event subsequent to cessation of ice-stream operation. (From Clark and Stokes, 2001).

Figure 9.15 Extent of the Dubawnt Lake Palaeo-Ice Stream (marked in bold), Nunavut, Canada, as defined by the imprint of subglacial bedforms. This rubber-stamped bedform pattern overprints the adjacent and earlier flow patterns. (From Stokes and Clark, 2003).

• importantly, an inferred velocity field (via elongation ratios) that exactly matches our expectation (i.e. Fig. 9.9, and Stokes and Clark, 2002b).

The ice-stream bedform signature is remarkably clear and consistent, with no discordances or cross-cuts, and is taken to be a rubber-stamped record of activity. The ice stream must have shut down, preserving the bed record, and from superimposed esker patterns at an oblique orientation to the ice stream axis, we infer that deglaciation 'unzipped' from a different direction. This example is the only known record of a rubber-stamped terrestrial ice stream; we presume them to be rare.

9.8.4 Terrestrially-Terminating, Smudged Palaeo-Ice Stream (Fig. 9.l2d)

There are many examples of this type. If an ice stream operates while the ice-sheet margin is gradually retreating then we expect continuous reorganization of the bed as the ice stream changes position and its margin back-steps. This may give rise to a hint, or partly obscured record of the trunk but with numerous lobe-shaped imprints smudged on top. It has been argued that many of the lobe imprints of the southern margin of the Laurentide Ice Sheet (Boyce and Eyles, 1991; Paterson, 1997a; Evans et al., 1999b) and the lobes of central Finland (Punkari, 1993, 1995, 1997; Dongelmans, 1996; Boulton et al., 2001b), are examples of this type. These may provide good examples of the record that actively retreating ice streams should produce but some caution is required. This is because it is hard to distinguish between the record that a retreating ice stream should produce, and that of simple retreat of an ice lobe. We would never expect a retreating ice margin to be straight over large distances. Therefore, simply citing a lobate pattern as evidence for an ice stream is fundamentally flawed. The key, as ever, must be in demonstrating evidence for spatially restricted fast flow extending into the ice sheet, which can be adequately demonstrated by finding the trunk zone. If only smudged imprints of the marginal lobe pattern are recorded, then there is less certainty that this was produced by an ice stream.

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