Paraglacial Alluvial Landsystems

Paraglacial alluvial landsystems comprise three main categories of landform, namely debris cones, alluvial fans and valley fills. All three may be regarded as paraglacial sediment stores that form within a few centuries or millennia after deglaciation but which frequently experience later fluvial erosion due to decreased sediment supply and/or base-level lowering. Research on recently deglaciated terrain has shown that small and intermediate-sized debris cones and alluvial fans may form, stabilize and decay within a few decades or centuries (Broscoe and Thompson, 1969; Ballantyne, 1995; Harrison and Winchester, 1997). In the Garwhal Himalaya, for example, glacier retreat over the past 200 years was accompanied by the development of paraglacial fans composed of reworked morainic debris, but most have now ceased to accumulate and exhibit fan-head entrenchment and fluvial erosion (Owen and Sharma, 1998). Most research on paraglacial alluvial landsystems, however, has been carried out in the context of fans and valley fills that accumulated after Late Pleistocene deglaciation, particularly in British Columbia and Alberta.

17.6.1 Paraglacial Fans of Late Pleistocene and Early Holocene Age

Ryder (1971a) showed that relict, vegetated alluvial fans in British Columbia owe their origins to reworking of glacigenic sediments (till, glacifluvial and glacilacustrine deposits) by streams and debris flows in tributary valleys. Stratigraphic evidence suggests that fan accumulation commenced soon after deglaciation and continued until shortly after the deposition of a near-surface tephra layer at c. 6.6 ka BP. Many fans were subsequently dissected by fan-head trenching or incision due to lowering of local base level, although in locations where fan accumulation continued during base-level lowering, nested multi-level fans were formed. The paraglacial fans described by Ryder are composed of fluvial gravels and debris-flow diamictons with occasional intercalated lacustrine or aeolian sediments. Debris-flow-dominated fans tend to have higher gradients and to occur at the outlets of small, steep tributary catchments (Ryder, 1971a, b). The volumes of most fans imply tributary denudation rates of the order of 0.25—2.0 m ka-1 (Church and Ryder, 1972).

Subsequent work in Alberta (Roed and Waslyk, 1973; Kostaschuk et al., 1986; Beaudoin and King, 1994) has confirmed the generality of Ryder's findings, although in some instances at least, fan accumulation was episodic. Radiocarbon dating of fan deposits in the Lower Seymour Valley of British Columbia shows that paraglacial sedimentation commenced prior to c. 11.4 ka BP and was largely complete by c. 9.0 ka BP, but within this period renewed sediment accumulation occurred at c. 10 ka BP and accompanied climatic warming and a reduction in precipitation; charcoal-rich beds within fan sediments suggest that later depositional events may reflect slope instability triggered by fire in tributary catchments (Lian and Hickin, 1996). The lower Cheekye fan, which occupies an area of 8.3 km2 in southwest British Columbia, reached roughly its present dimensions before c. 6.0 ka BP, but deposition continued intermittently until c. 1.3 ka BP. Friele et al. (1999) calculated that approximately 14 X 108 m3 of sediment accumulated on this fan prior to c. 6. 0 ka BP, but an order of magnitude less (~1.4 X 108 m3) since then.

Particularly impressive paraglacial fans occur in the valleys of the Karakoram and Himalayan Mountains. Since deglaciation there has been massive reworking of glacial deposits by debris flows and rivers and consequent burial of till beneath thick fan sediments. In the Hunza Valley of the Karakoram, 44 per cent of the valley floor is covered by paraglacial fan deposits compared with only 14 per cent mantled by intact glacigenic deposits (Li Jijun et al., 1984), and the floor of the Gilgit Valley is similarly dominated by Late Pleistocene paraglacial fans (Owen, 1989; Derbyshire and Owen, 1990; Fig. 17.10). Fan deposits are commonly several tens of metres thick, implying paraglacial re-sedimentation on a truly grand scale.

The lithofacies architecture of large paraglacial fans is often complex. Eyles and Kocsis (1988) showed that the sediments in a fan that accumulated between c. 11 ka BP and c. 7 ka BP in British Columbia are dominated by diamict facies deposited by debris flows (48 per cent of fan volume) and sheetflood gravels (37 per cent), intercalated with occasional beds of aeolian silt and sheetwash deposits. Crude bedding within diamict facies represents superimposition of multiple debris-flow units 0.2-3.0 m thick, and alluvial gravel facies are characteristically massive, crudely bedded and poorly sorted, and thus similar to those in the proximal reaches of shallow braided rivers. Within the large paraglacial fans of the Karakoram, individual debris flow units are characterized by discrete shears and pronounced fabric anisotropy near their bases, and their upper surfaces are often draped in fine silt reflecting post-depositional slopewash (Derbyshire and Owen, 1990). Some fans in this area consist of a few thick debris-flow units interbedded with fluvial and glacifluvial sediments, the latter demonstrating fan accumulation very soon after glacial retreat. The main debris-flow deposits consist of diamict sheets 5-20 m thick that cover areas of up to 30 km2. The vast size of these units implies flow of exceptional volumes of fluidized debris on a catastrophic scale.

17.6.2 Paraglacial Valley Fills

The term 'valley fill' describes unconsolidated deposits that overlie bedrock in valley-floor locations. Valley fills are often compositionally complex, but in glaciated areas it is often possible

Lacustrine Fan Lobes

Alluvial fan

Recent floodplain sediments- including fluvial sands and gravels, aeolian dunes, mudflows and pond sediments

Glacially eroded surface

Alluvial fan r ■ rill resedimented as debris flow

I ] Debris flow lobe ■■■:;■■■ Hummocky moraines '.'■.'■ ■ Till

Recent floodplain sediments- including fluvial sands and gravels, aeolian dunes, mudflows and pond sediments

Lacustrine sediments Glacial pond sediments Scarp of rock bottom Terrace scarp

Glacially eroded surface

Summit heights in metres

5 km

Figure 17.10 Landforms and surficial deposits of the Gilgit Valley, Karakoram Mountains. The surface of the valley fill is dominated by large paraglacial fans. The outcrop of in situ glacigenic deposits is extremely limited. (Adapted from Owen (1989).)

to identify a lower sequence of glacigenic deposits and, overlying these, a paraglacial sequence of debris flow, floodplain, alluvial fan, lacustrine and/or aeolian deposits in some combination (Owen, 1989; Figs 17.10 and 17.11).

An interesting sequence of paraglacial valley-fill aggradation and later incision occurs in the valley of the Bow River, which drains the Rocky Mountains in Alberta. In the upper valley a basal valley-fill deposit of proximal outwash sediments is overlain by multiple beds of massive diamict facies with a total maximum thickness of ~30 m. The diamicts have a sheet-like configuration with largely conformable bedding contacts, dip downvalley at 5—10° and were interpreted by Eyles et al. (1988) as the product of successive massive paraglacial debris flows that reworked glacifluvial and glacilacustrine sediments from up-valley and adjacent slopes. Farther down-valley, the Bow River is flanked by terraces cut in gravel fill, which diminishes in thickness from ~30 m near the mountain edge to ~10 m near Calgary, 100 km downstream. Radiocarbon dating of the terrace sediments implies that gravel aggradation occurred mainly within the period 11.5—10.0 ka BP, two millennia after the last glacial readvance reached the mountain edge. Jackson et al. (1982) therefore interpreted the gravel fill in the lower valley as the product of fluvial reworking of the debris-flow deposits in the upper valley, and thus as 'second-generation' paraglacial fluvial

Paraglacial alluvial fans

Paraglacial debris cones

Paraglacial alluvial fans

Paraglacial debris cones

Proglacial/paraglacial \ stratigraphie boundary

Paraglacial sediment facies

I Aeolian sediments

. | Alluvial (floodplain) deposits

Alluvial fan deposits

Debris cone deposits

Glacial/proglacial stratigraphic boundary

Glacial/proglacial sediment facies h-Glacifluvial outwash facies Glacilacustrine deposits . Till

Figure 17.11 Glacial, proglacial and paraglacial components of a valley fill (schematic). Often fewer sediment units are present. Interfingering of paraglacial units (particularly debris flow or alluvial fan deposits) and glacigenic sediments implies rapid paraglacial resedimentation during and immediately after deglaciation.

deposits. Since c. 4.6 ka BP, the Bow River has incised into the gravel fill in its lower reaches in response to diminished sediment supply from upstream, cutting the terraces that now flank its course.

The Bow River model of valley-fill aggradation (Fig. 17.12) is by no means the only possible sequence of paraglacial valley-fill accumulation (Owen, 1989). Lacustrine and distal glacilacustrine sedimentation may also constitute a major component of paraglacial valley fills (Clague, 1986). In the South Thompson Valley of British Columbia, for example, a distal glacilacustrine fill up to 150 m thick accumulated in only 100-200 years in the Late Pleistocene. Near the valley sides, paraglacial debris flow deposits interrupt lacustrine

Figure 17.12 Model of paraglacial valley-fill development based on the sequence in the Bow River valley, Alberta. A) Emplacement of thick accumulations of paraglacial debris-flow deposits in the upper part of a mountain catchment. B) Fluvial erosion of valley-head debris-flow deposits and deposition of a paraglacial alluvial fill farther downvalley. C) Fluvial incision and terracing of the alluvial fill as sediment input from upstream is reduced.

Proximal valley fill: paraglacial debris-flow deposits

Fluvial erosion of paraglacial debris-flow deposits

Lacustrine Fans

Dissected remnants of paraglacial debris cones

Model River Incision

rhythmites, and sand bodies were deposited where tributary streams entered the lake. After lake drainage, aeolian reworking of lake deposits produced a paraglacial loess deposit that caps terrace fragments, and river incision cut a floodplain through the lake sediments (Roberts and Cunningham, 1992). In the Walensee Valley in Switzerland, the valley fill comprises patchy till deposits overlain in turn by glacilacustrine sediments then postglacial lacustrine sediments, the latter being interrupted laterally by deltaic deposits overlain by alluvial gravels deposited below two major paraglacial fans. Müller (1999) calculated that the sediments underlying the two fans accumulated at an average rate of 70—100 mm year-1 prior to c. 12.3 ka BP, but thereafter at an average rate of only 3-4 mm year-1, attributing the rapid sedimentation prior to c. 12 ka BP to the reworking of glacigenic sediments in steep tributary valleys by powerful debris flows immediately after deglaciation.

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