Sedimentation In Lacustrine Environments

Lacustrine ice margins develop in a variety of different situations. Glaciers may dam lakes, lakes may develop in front of a glacier due to the melting of stagnant ice beneath the proglacial surface, lakes may be dammed between moraines and an ice front, or a glacier may simply drain into a rock-confined basin. During the last glacial cycle both the Laurentide and Fennoscandinavian ice sheets terminated in large proglacial lakes formed in part by the glacio-isostatic depression of the crust in front of the ice margin. The weight of the ice sheets caused depression of the crust, which was compensated for by broad topographic bulges (forebulges) several tens of kilometres beyond the ice margin. Proglacial lakes formed between these

Glacial Geology: Ice Sheets and Landforms Second Edition Matthew R. Bennett and Neil F. Glasser © 2009 John Wiley & Sons, Ltd bulges and the ice margin. For example large lakes formed in front of the Laurentide Ice Sheet in the area of the Great Lakes today and, as a consequence, glaciolacustrine sediments occur extensively in this area.

The pattern of sedimentation within glacial lakes is controlled by the density stratification within the water body. Water density is controlled primarily by temperature and secondarily by salinity and the suspended sediment content. In the summer most lakes develop a strong water stratification in which a surface layer of warm, and therefore less dense, water (epilimnion) sits above a lower body of cold denser water (hypolimnion; Figure 10.1). In the autumn this surface layer is cooled rapidly, becomes denser and therefore sinks to the bottom of the lake causing the water body as a whole to mix. This stratification controls the resulting processes of sedimentation, which may occur through any or all of the following eight processes.

Figure 10.1 The thermal stratification of lake waters. [Modified from: Drewry (1986) Glacial Geologic Processes, Arnold, figure 11.3, p. 169]

1. Deposition from meltwater flows. The manner in which meltwater enters into a lake depends upon the density of the meltwater relative to that of the lake. If there is a significant difference in density, the sediment-laden meltwater will maintain its integrity as a plume and enter in one of three ways: (i) as an underflow, where the sediment plume is denser than the lake water and therefore sinks to the base

Water temperature

High

Water temperature

High

Figure 10.1 The thermal stratification of lake waters. [Modified from: Drewry (1986) Glacial Geologic Processes, Arnold, figure 11.3, p. 169]

of the lake; (ii) as an interflow, where the sediment plume is of similar density to the surface water (epilimnion) but less dense than the basal water (hypolimnion) and the plume enters at intermediate depth; and (iii) as an overflow, where the sediment is less dense than the surface lakewater and therefore rises to the surface. Meltwater introduced as an overflow or interflow loses immediate traction with the bed and rapid deposition of bedload will occur. These conditions are ideal for the formation of a delta - a body of sediment deposited in a water body at a river or stream mouth. As sedimentation occurs a delta will grow away (prograde) from the shore giving a distinctive internal architecture of topsets, foresets and bottomsets (Figure 10.2). The foresets are formed as sediment avalanches down the front face of the delta, while the bottom sets form as a result of fine-grained sediment flows initiated by this process of avalanching and by suspension settling from the water column.

In lakes where there is a pronounced underflow, sediment-laden meltwater descends rapidly into the lake basin as a series of sediment-rich currents. These currents may inhibit the development of large deltas close to the lake margin as they carry much of the sediment into deep water. Persistent underflow eventually develops a subaqueous or lake floor fan. These fans consist of a system of channels, often with lateral levees, which cut into the delta front and feed a series of broad lobes that reach out towards the centre of the lake basin.

2. Direct deposition from the glacier front. Material may simply be dumped into the water body from the glacier front. The degree of disaggregation that occurs will depend on the water depth and upon the current activity within the lake. This will result in irregular shaped deposits of diamict.

3. 'Rain-out' from icebergs. Calving icebergs may float debris out into the body of a lake where it may be released as the iceberg melts (Figure 10.3). The deposits produced are variable, depending upon the density of the debris concentration within the icebergs and the number of icebergs present. Low concentrations of debris and icebergs may result in the deposition of occasional dropstones, out-sized clasts set in fine-grained sediments (Figure 10.4), whereas high concentrations may result in thick laterally extensive deposits of diamict. Irregularly shaped packages of debris may also be associated with dropstones and are formed by the dumping of sediment from the icebergs as they capsize or as pockets of debris meltout.

4. Settling from suspension. Sediment within the main body of a lake will gradually settle to form thin layers of mud and clay that drape other sediments. This type of sedimentation dominates over much of the lake floor away from its margins.

5. Resedimentation by gravity flows. Sediment within a lake may become unstable on steep slopes, particularly where rapid deposition occurs. Slumping or flow of this sediment may give rise to a range of diamicts, the properties of which depend on the fluidity of the flow. The greater the flow mobility, the greater the sorting and fabric development within the diamict.

6. Current reworking. Currents within the lake may rework and sort sediment that has already been deposited

A: Components of a delta

Upper Delta

A: Components of a delta

Upper Delta

Topsets

Bottom sets

Topsets

B: Delta response to rising lake levels

B: Delta response to rising lake levels

Lake level 2 Lake level 1

C: Delta response to falling lake levels

Down cutting

Down cutting

Lake level 1 Lake level 2

Figure 10.2 Sedimentological components of deltas. (A) Delta components, topsets, foresets and bottom sets. Note the two superimposed deltas and thatthe lower one has been deformed by a glacial advance. (B) Delta response to rising lake levels. (C) Delta response to falling lake levels. (D) Dormation of a supraglacial delta. [Modified from: (A) Thomas (1984) Geological Journal,

Lake level 1 Lake level 2

Figure 10.2 Sedimentological components of deltas. (A) Delta components, topsets, foresets and bottom sets. Note the two superimposed deltas and thatthe lower one has been deformed by a glacial advance. (B) Delta response to rising lake levels. (C) Delta response to falling lake levels. (D) Dormation of a supraglacial delta. [Modified from: (A) Thomas (1984) Geological Journal,

7. Shoreline sedimentation. The action of waves on lake shorelines may modify material deposited here. The importance of wave action is limited by the size of the lake and the presence of a seasonal ice cover. The addition of hillside debris from surrounding slopes may also be important in building up shoreline deposits and non-glacial streams entering the lake may build small shoreline deltas.

Bottom sets

Figure 10.3 Icebergs on a proglacial lake in Alaska. [Photograph: M.R. Bennett]

8. Biological sedimentation. This is relatively unimportant as the biological component of glacial lakes is small due to the long freezing season, the ephemeral and unstable nature of many glacial lakes and the high sediment load. However, the importance of biological sedimentation may increase with the size of the lake.

The distribution of these eight processes within a lacustrine environment will determine the facies pattern that develops. In small lakes this will reflect the dominance of, and fluctuation between, underflow, interflow and overflow conditions. Despite this variation there is a general drop in flow competence away from the point of water inflow into the lake. This produces a broad size grading within glaciolacustrine environments: coarse deposits occur near the point of water inflow or glacier,

Figure 10.4 Dropstones in glaciolacustrine deposits in Patagonia. [Photograph: P. Doyle]

whereas fine-grained deposits dominate in the lake centre. In fact we can identify two broad facies within a lake: (i) the basin-margin facies; and (ii) the lake-floor facies.

The basin-margin environment and the associated sedimentary facies are dominated by the inflow of water into the lake. Where water enters as an underflow a higher proportion of coarse sediment is carried further into the lake basin, inhibiting the size of the delta produced and increasing the basinward transport of sediment. This prodelta area may be associated with lobes and fans formed by deposition from subaqueous sediment gravity currents such as turbidity currents. In contrast, where meltwater enters as an overflow or inflow, much of the sediment load is deposited quickly on the delta and the degree of basinward transport is more limited. In practice this may vary seasonally as the temperature and density of the inflow varies along with the degree of water stratification in the lake.

The nature of the processes of sedimentation away from the lake margin depends upon the amount of ice-rafted debris being introduced into the system -in effect the degree to which the lake is connected to the ice margin. The greater the length of the ice margin that is in contact with the lake, the greater the potential for iceberg calving and basinward transfer of ice-rafted debris. We can identify a broad continuum from those lakes in which the input of ice-rafted debris is small, and therefore sedimentation is dominated by settling from suspension and the inflow of meltwater, to those lakes that are dominated by ice-rafted sedimentation. If the ice-rafted component is small then rhythmic fine-grained sediments produced by settling of material out of suspension tend dominate, giving rise to fine laminations of silt and clay (Figure 10.5). This process is interrupted by the deposition of coarser laminations from material introduced by sediment gravity flows (e.g., turbidity

Figure 10.5 A range of glaciolacustrine sediment types. (A) Finely laminated silt and sand units forming rhythmic beds. (B) Dropstone within laminated silts and sands. (C) Waterlain diamict, showing different degrees of current winnowing. (D) Waterlain diamict. [Photographs: M.R.

Bennett]

Figure 10.5 A range of glaciolacustrine sediment types. (A) Finely laminated silt and sand units forming rhythmic beds. (B) Dropstone within laminated silts and sands. (C) Waterlain diamict, showing different degrees of current winnowing. (D) Waterlain diamict. [Photographs: M.R.

Bennett]

currents), generated either by underflow or by slumping on the delta front or lake margin. This forms rhythmic sediment in which there are coarse- and fine-grained laminations giving a distinctive couplet (Figure 10.5B). When or where underflow is not active, for example during winter months when the inflow or discharge of water into the lake is low, settling from suspension will dominate. Laminations produced in this way typically grade in size from silt and clay particles at the base to fine clay at the top. They are usually terminated by a sharp contact formed by the influx of a new underflow of coarse material. The relative thickness and importance of these two components changes with distance away from the sediment source or point of water inflow. Close to the delta the coarse sand/silt layer, formed by underflow, will be thick. In the basin centre the coarse layers will be much thinner and may be absent. In such cases a continuous deposit of homogeneous fine clay may result. In specific circumstances laminated or rhythmic sediment may have an annual pattern. Underflow and therefore coarse lamination dominate in the summer when the influx of meltwater to a lake is high, whereas suspension settling and fine lamination tend to dominate in the winter when the influx of turbid flows is small or absent. Where an annual cycle can be identified within the rhythmic sediments they are known as varves. It is important to emphasise, however, that not all rhythmic lake sediments necessarily contain an annual signature. These rhythmic deposits are easily disturbed or deformed during or after deposition and may show a variety of deformation structures.

In contrast, if the ice-rafted debris component is large, sedimentation is dominated by coarse debris melting from icebergs (Figure 10.5C,D). In this case the deposition of diamicts and resedimentation by gravity flows will dominate. The type of diamict produced in these environments will depend on the interplay between three processes: (i) the rate of debris rainout; (ii) the degree of downslope, basinward remobilisation via sediment gravity flows; and (iii) the degree of current winnowing and reworking on the lake floor. In some of the large glaciolacustrine lakes along the margins of the former mid-latitude ice sheets extensive deposits of diamicts have been deposited in this way (Box 10.1).

0 0

Post a comment