Controls on Formation and Extent of Proglacial lakes

The main factors that controlled the extent and depth of Pleistocene ice-marginal lakes and their sedimentary and geomorphic landsystems were (Teller, 1987):

1. location of the glacier margin

2. elevation and topography of the newly emergent glacial landscape

3. location and elevation of the overflow channel

4. differential isostatic rebound, and

5. volume and nature of sediment supply.

When the glacier margin lay in the drainage basin of a lake, the hydrology, sediment load, and chemistry of inflow were strongly influenced by the proximity of the glacial source. In general, when the margin was close, lake turbidity (suspended sediment in the water column) and sedimentation rates were higher, and light penetration and temperatures were lower. As a result, animal and plant populations in the lake tended to be lower, as did taxonomic diversity (e.g. Risberg et al., 1999; Bjorck, 1995; cf. Warner, 1990). When there was a glacial advance into the lake, some or all of the older lacustrine sediments and landforms were destroyed or buried. Glacier retreat in an ice-marginal lake was expedited by calving, and the typical convex outline of the ice margin on land probably became concave in deeper water, as waters promoted its retreat (see discussion in Benn and Evans, 1998). Both rapid retreat and advance (surging) of the ice margin is known to have occurred in large ice-marginal basins (e.g. Dredge and Cowan, 1989b; Clayton et al., 1985; Evans and Rea, Chapter 11).

Of course the elevation of the lake's outlet controlled the level of the lake. Outbursts of overflow occurred when new, lower outlets were opened, which led to a draw down in the level of the lake (e.g. Teller et al., 2002). The initial overflow through an outlet is the most likely time for channel deepening and further lake level lowering, mainly because discharge and flow energy were greater at this time, and because there usually was a cover of erodable glacial sediment over the bedrock.

Of particular importance to the formation of large Pleistocene proglacial lakes across North America, Asia and northeastern Europe is the fact that the continental land surface slopes northward toward the Arctic Ocean. This slope was accentuated by glacio-isostatic depression. As high latitude ice sheets expanded, drainage into the northern oceans was disrupted, resulting in large ice-marginal lakes and an extensive re-arrangement of northward drainage (Teller, 1987).

Differential isostatic loading of the northern (downslope) part of basins increased the potential storage capacity of proglacial basins. In North America and Scandinavia, the differential depression from south to north, as measured by now-deformed beaches, amounted to 200 m in some areas (e.g. Teller and Thorleifson, 1983; Andrews and Peltier, 1989; Dredge and Cowan, 1989b; Lemmen et al., 1994b), although because beaches only measure rebound since ice retreated from a region, the total difference must have been greater. Measurements of modern isostatic rebound in the northern part of these basins, such as in the former Lake Agassiz and Baltic Ice Lake basins, indicate that uplift continues at more than 0.6 m per century (Barnett, 1970; Hunter, 1970; Andrews and Peltier, 1989; Eronen, 1983; Sjorberg, 1991), shifting residual lakes on their ancient floors southward through time.

When the ice margin retreated, the extent of the lake expanded. As new lower outlets opened, lake level dropped and the beach that had formed was abandoned. Subsequent differential isostatic rebound deformed the old water planes (beaches), raising those closest to the ice load more than those beyond (Fig. 14.1). And, because the rate of differential rebound decreased through time, younger (lower) strandlines developed gentler slopes.

After a draw-down in lake level, differential isostatic rebound resulted in the lake margin transgressing in the basin, south of the isobase line (line of equal isostatic rebound) that extended through the outlet, until a new outlet was opened. North of that isobase, the lake margin regressed (Larsen, 1987; Teller, 2001). A simple bathtub model in Fig. 14.2 shows a series of snapshots of changing lake levels related to overflow through three different outlets. Overflow through the southern outlet results in a gradual regression of the shoreline throughout the basin, shown in

Figure 14.1 Ideal sequence of strandline formation resulting from ice retreat and overflow through an outlet at the southern end of the basin. Three different lake levels are shown (after Flint, 1971, fig. 13-11). Note different curvatures of the three ages of strandlines.

Fig. 14.2A by three lake levels. If overflow changes to the middle of the basin, differential isostatic rebound results in regression north of the isobase through the outlet, but transgression to the south (Fig. 14.2B). Overflow through an outlet in the northern part of the basin results in transgressing waters everywhere (Fig. 14.2C). During the life of most of North America's great proglacial lakes, two or three of these outlet scenarios occurred. Figure 14.3 shows how a change in outlets from south (S) to two different outlets in the middle of the basin (M1 and M2) could result in a complex series of beach levels. Superposed on all of this was outlet erosion (not shown in Fig. 14.3), which lowered lake level and was followed by continuing transgression and/or regression in the basin.

Thus, the elevation of the outlet controlled water level, and the geographic location of the overflow outlet controlled where transgression and regression of the lake margin occurred. As outlets changed during the life of a proglacial lake, mainly as a result of ice-marginal retreat (or advance), new strandlines formed. Because isostatic rebound varied exponentially from south to north, resultant strandlines are curved, not straight (Fig. 14.1).

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