Paraglacial Driftmantled Slope Landsystems

Retreat of glacier ice in mountain areas often exposes valley-side slopes mantled by thick glacigenic deposits, usually composed of stacked lateral moraines (Mattson and Gardner, 1991; Ballantyne and Benn, 1994, 1996). Such deposits are susceptible to erosion by translational slope failure (slumping), debris flow, snow avalanches, streamflow and surface wash. These processes may completely rework drift-mantled slopes within a few decades or centuries, forming a paraglacial landsystem of intersecting gullies, coalescing debris cones and valley-floor deposits of reworked sediment (Fig. 17.7).

Paraglacial Fan

Figure 17.7 Gullying of drift-mantled slopes near the snout of Fabergst0lsbreen, Jostedalen, Norway. Photographic evidence shows that as late as AD 1943 this slope was ungullied and supported a prominent Little Ice Age lateral moraine. By 1988 the slope was extensively gullied, the lateral moraine had been completely removed, and extensive areas of fresh bedrock were exposed as a result of annually recurrent debris-flow activity. Most reworked sediment has accumulated in coalescing debris cones at the slope foot.

Figure 17.7 Gullying of drift-mantled slopes near the snout of Fabergst0lsbreen, Jostedalen, Norway. Photographic evidence shows that as late as AD 1943 this slope was ungullied and supported a prominent Little Ice Age lateral moraine. By 1988 the slope was extensively gullied, the lateral moraine had been completely removed, and extensive areas of fresh bedrock were exposed as a result of annually recurrent debris-flow activity. Most reworked sediment has accumulated in coalescing debris cones at the slope foot.


17.4.1 Drift-Mantled Slopes: Processes

The dominant agent of sediment reworking on recently deglaciated drift-mantled slopes is debris flow, the rapid downslope movement of a poorly sorted mixture of boulders, fine sediment and water (Zimmermann and Haeberli, 1992; Evans and Clague, 1994; Owen, 1994; Solomina et al., 1994). Snow avalanches and streams generally play a secondary role in redistributing glacigenic sediment downslope. On the foreland of Fabergstelsbreen in Norway, for example, Ballantyne and Benn (1994) recorded an average of five debris flows per year per kilometre of slope, many of which had been triggered by rapid snowmelt at gully heads. Numerous flow tracks marked by parallel levées of debris descend from gullies, often cross-cutting earlier flows to produce a complex hummocky microtopography of dissected levées. By no means all recently deglaciated drift-mantled slopes experience extensive paraglacial modification, however. Research by Curry (2000a) suggests that initial gradients over 30° are essential for extensive slope erosion by debris flows, and that on such slopes a high density of gullies (>20 km-1) is associated with thick drift cover and sediments with high void ratios. Local hydrological controls (particularly focusing of runoff by rock gullies upslope) may be critical in initiating widespread reworking of drift-mantled slopes by debris flow activity.

In valleys where recent ice downwastage has exposed steep-sided lateral moraines, proximal moraine slopes may be extensively modified by slumping, debris falls and debris flows. Widespread failure of moraine walls has occurred along the flanks of Tasman Glacier in New Zealand at sites where moraine relief exceeds 120 m (Blair, 1994). Ice-cored lateral moraines are particularly susceptible to failure as the underlying ice melts, reducing the strength of the overlying sediment so that debris is released through a combination of slumping and flow (Fitzsimons, 1996b; Bennett et al., 2000a; Etzelmuller, 2000). At Boundary Glacier in Alaska, Mattson and Gardner (1991) recorded 25 slope failures incorporating ~35,000 m3 of debris from ice-cored moraines over two summers. Most involved failure at the ice-sediment boundary, and the majority occurred near the glacier snout, indicating rapid modification of moraine slopes following deglaciation. The distal slopes of steep terminal moraines may also be affected by slumping and debris flow (Palacios et al, 1999).

17.4.2 Drift-Mantled Slopes: Landforms

The morphological consequences of recent drift-slope modification have been intensively studied on the foreland of Fabergstelsbreen (Norway), where steep drift-mantled slopes have been so extensively modified by debris flows that little of the original slope remains. The modified slope comprises two zones. The upper comprises broad gullies up to 25 m deep and 80 m wide that are incised into valley-side drift, locally exposing areas of underlying bedrock, and separated by 'arêtes' of drift (Fig. 17.7). The lower consists mainly of reworked sediment, mainly in the form of coalescing debris cones that overlie bedrock or till. Ballantyne and Benn (1994) found that within 50 years, gullying of upper drift slopes resulted in a reduction in slope gradient from ~35° to ~30°, the latter probably representing the minimum gradient for debris-flow initiation. Curry (1999) has shown that gully incision occurs rapidly after deglaciation, and that gullies thereafter undergo progressive widening until sidewall slopes have declined to a gradient of ~25°, after which parallel retreat of gully sides predominates until inter-gully arêtes are consumed or gully-side slopes attain stability. The final form of the driftslope landsystem comprises an upper, bedrock-floored source area, a midslope zone of broad gullies with sidewalls resting at stable, moderate gradients, and a lower zone of coalescing debris cones and fans, a landform assemblage common in many upland valleys that were deglaciated in Late Pleistocene times (Miller et al., 1993; Ballantyne and Benn, 1996). This assemblage achieves spectacular dimensions in the Karakoram Mountains and Lahul Himalaya, where paraglacial debris-flow deposits form sediment sequences up to 90 m thick (Owen and Derbyshire, 1989; Owen, 1991; Owen et al., 1995).

17.4.3 Drift-Mantled Slopes: Sediments

Differentiation of in situ glacigenic deposits from those reworked by debris flow is often problematic, particularly in the case of glacigenic deposits that have experienced flow during deposition (Lawson, 1988; Owen and Derbyshire, 1989; Zielinski and van Loon, 1996). Comparative studies of recent paraglacial debris-flow deposits and their parent tills have demonstrated that the former retain most of the characteristics of the latter, being indistinguishable in terms of macrofabric strength or type, clast imbrication, angularity, shape and texture, matrix granulometry or void ratio (Owen, 1991, 1994; Ballantyne and Benn, 1994; Curry and Ballantyne, 1999). Significant differences occur, however, in terms of the alignment of discontinuities, stratification, shear structures and bedding, which in paraglacial debris-flow deposits tends to be parallel or sub-parallel to valley-side slope, and in terms of the aggregate preferred orientation of elongate clasts, which tends to be aligned downslope in reworked deposits but down-valley in in situ basal tills. Micromorphological analyses also show promise for distinguishing reworked from in situ glacigenic deposits. Owen (1991, 1994) detected differences in the characteristics of microshears, and Harris (1998) found that till deposits reworked by debris flows exhibited a range of diagnostic characteristics, including preferred downslope grain orientations, shear-induced birefringence, evidence for clast rotation and sheared wavy textural domains, together with the presence of wash layers and well-sorted sand and gravel lenses.

17.4.4 Drift-Mantled Slopes: Rates of Paraglacial Modification

Extensive paraglacial reworking of drift-mantled slopes may occur within a few decades or centuries of deglaciation. The drift-mantled slope exposed by retreat of Fabergstolsbreen was transformed into a badland of deep gullies within 50 years (Ballantyne and Benn, 1994; Curry, 1999), and in neighbouring Bergsetdalen paraglacial debris cones that began to accumulate between AD 1750 and AD 1908 had completely stabilized by 1965 as a result of sediment exhaustion (Ballantyne, 1995). Such rapid changes imply minimum gully erosion rates of 19—169 mm year-1. From the volume of paraglacial debris cones in the Nepal Himalaya, Watanabe et al. (1998) inferred mean catchment denudation rates of 0.4-8.0 mm year-1 over the past 550 years, but acknowledged that denudation rates were probably much higher immediately after deglaciation. Such rates imply that paraglacial drift-slope reworking is likely to be completed within a few centuries of deglaciation, a conclusion that is supported by evidence of rapid attainment of drift-slope stability following Late Pleistocene deglaciation. Miller et al. (1993) have shown that paraglacial gully erosion and concomitant debris cone formation in the Andes of northern Peru commenced during deglaciation at c. 12-10 ka BP but was complete before c. 8 ka BP. Similarly, Jackson et al. (1982) estimated that 80 per cent of postglacial debris flow activity in the Bow River valley of the Canadian Rockies took place between deglaciation at c. 13-12 ka BP and establishment of spruce forest at c. 10.4-10.0 ka BP. Drift-mantled slopes, however, may experience renewed or delayed paraglacial reworking long after the end of the initial period of paraglacial activity has ended, particularly in response to extreme storm events (Ballantyne and Benn, 1996; Curry, 2000b).


17.4.5 Glacial-Paraglacial Sediment Recycling

It was noted above that paraglacial rock-slope debris and rock weakening may provide important sources of readily entrainable sediment. This principle also applies to paraglacial sediment accumulations reworked from valley-side drift mantles. Sections exposed in the sidewalls of gullies incised in valley-side drifts exposed by retreating outlet glaciers draining Jostedalsbreen in southern Norway exhibit two distinct sediment associations (Ballantyne and Benn, 1994; Curry and Ballantyne, 1999). The upper consists of a massive diamicton that represents glacigenic deposits emplaced during recent (Little Ice Age) glacier advance. The lower exhibits crude slope-parallel stratification and preferred downslope clast orientation, and represents paraglacial reworking of much earlier (Preboreal) glacigenic deposits by debris flows. As the contact between the two is usually erosional (Fig. 17.8), it implies that Preboreal paraglacial sediments were re-entrained by the outlet glaciers during the Little Ice Age advance. This sequence therefore implies a cycle of alternating glacial and paraglacial sediment transfer, the former being dominant during glacier advance and the latter during glacier retreat. It suggests that many glacigenic deposits in mountain areas contain sediments that have undergone at least one previous cycle of glacial/paraglacial reworking.

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