Fluvial Sedimentation

Glacial meltwater entrains and transports sediment, which is subsequently deposited on, within, beneath or beyond the glacier. Sedimentation on or within the glacier may occur in surface channels and in either englacial or subglacial tunnels. Sedimentation in surface (supraglacial) channels is inhibited by the steep channel gradients and the smooth ice walls, which provide little frictional drag on the flow and its sediment load (Figure 4.1A). Deposition does, however, occur due to changes in the discharge of such channels, because sediment may lodge temporally in active channels as discharges fall. Sediment may also be deposited in channels where the gradient falls and the sediment load is high. Supraglacial channels are often ephemeral and when they are abandoned, sediment can be stranded within them.

Frictional drag is greater in subglacial tunnels that are in contact with the glacier bed. Sediments deposited within tunnels consist of sheet-like units of stratified sand and gravel in which secondary bedforms such as ripples, dunes and graded beds may occur. Deposition within such tunnels can be understood to some extent by the application of theory developed for the deposition of solids within pipes. Four flow regimes have been identified within pipes: (i) at low flow velocities a stationary bed with little or no transport occurs; (ii) as flow velocities increase material begins to slide over the bed as a single unit; (iii) at higher velocities suspension of all particles occurs, although the coarser fraction is still transported close to the tunnel floor; and (iv) at very high velocities all particles move in suspension and no size sorting is present. If the flow velocity was to fall rapidly during this final flow regime a massive, heterogeneous non-sorted sediment would result. Despite this work, very little is known about the processes of sedimentation within subglacial tunnels.

Subglacial meltwater may also deposit thin coatings of precipitated solutes in subglacial rock cavities (Figure 8.17). As we saw in Section 4.8.2 subglacial meltwater may dissolve soluble components and transport them. These may in turn be precipitated to form thin coatings. These coatings may either infill shallow depressions or be concentrated into small linear ridges. These precipitates are best developed on carbonate rocks such as limestone or chalk. In Norway, areas of bedrock that were once covered by basal cavities frequently contain a brown staining, which results from the precipitation of iron oxides from subglacial meltwater in the cavities.

Figure 8.17 Photograph of a subglacial calcium carbonate precipitate in front of the Glacier de Tsanfleuron, Switzerland. These precipitates are strung out parallel to the former ice flow, which was from right to left. [Photograph: M. Sharp]

The precipitation of silica has also been noted. In general these precipitates are confined to former subglacial cavities and are believed to be the product of regelation. As ice flows against a bedrock obstacle, pressure melting occurs on the upstream side and refreezing or regelation occurs on the downstream side (see Section 3.3.2). Refreezing concentrates the solutes within the meltwater, leading to their eventual precipitation in the lee of the bedrock obstacles. The linear form of many precipitates is due to smearing out of solute-rich meltwater by the flowing ice. These coatings are quickly dissolved and removed by weathering when the rock surfaces are exposed on deglaciation. Their long-term preservation potential is therefore small.

The processes of sedimentation beyond the glacier margin are much better understood. Sedimentation beyond the glacier occurs in the same way as conventional fluvial deposition, with the following exceptions.

1. The water is generally colder, denser and therefore more viscous. The viscosity of water increases with a fall in temperature. Increased viscosity reduces the settling rate for particles in suspension and allows a greater volume of suspended sediment to be transported.

2. The water and sediment discharge is highly seasonal. Water discharge beneath the Glacier d' Argentiere varies, for example, between 0.1 m3 s-1 in the winter and 11 m3 s-1 in the summer. On Nisqually Glacier in North America the sediment transported during just 5 minutes in the month of June is equal to the whole sediment yield for the month of January. Similarly 60% of the annual sediment load from the Decade Glacier on Baffin Island was discharged during just 24 hours in 1965. Sediment discharge is therefore highly seasonal. It also varies diurnally. Most discharge occurs during the period of nival floods early in the melt season (see Section 4.7).

Sediment is transported both in suspension and as bedload (traction and saltation). A number of studies have attempted to record the sediment load of meltwater streams and its variation through time. Suspended sediment can be measured relatively easily by water sampling; the water sample is then filtered or evaporated to determine the sediment content. Results show that during the winter, when discharge is negligible, meltwater contains only a few milligrams of sediment per litre. During summer, this rises to several grams per litre. Suspended sediment content reaches a peak early in the summer as the fluvial system within the glacier is flushed clean (see Section 4.8.2). Suspended sediment content also varies diurnally and peaks prior to the maximum daily discharge on many glaciers (Figure 4.10). In contrast accurate estimates of the sediment moving as bedload are much more difficult to obtain and seasonal variations are less well understood at present (Box 8.5). Estimates of the relative importance of the two components (suspended versus bedload) vary from as little as 40% suspended load to over 90% of the total sediment discharge, depending on the particular characteristics of the glacier. Sedimentation in front of a glacier can be divided into three zones, although the boundaries between each are somewhat unclear. The three zones are: (i) the proximal zone; (ii) the medial zone; and (iii) the distal zone.

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