Summary of Development of large proglacial lakes

The advance of continental ice sheets led to the formation of large proglacial lakes because glaciers blocked river systems. As these ice sheets expanded, the fringe of ice-marginal lakes expanded upslope, forcing them to seek new outlets; eventually some lakes were completely displaced by the ice. The morphological and sedimentary record of these lakes is incomplete.

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Miles from the southern outlet

Figure 14.19 Isostatically deformed strandlines of Lake Agassiz. Each line represents a water plane that was once horizontal. Each lower strandline is younger and relates to a new transgressive level of water reached by the lake following the opening of a lower outlet (outlet channels represented by short vertical lines). (After Teller and Thorleifson, 1983, fig. 2).

0 50 100 150 200 250 300 350 400 450 500 550 600 650

Miles from the southern outlet

Figure 14.19 Isostatically deformed strandlines of Lake Agassiz. Each line represents a water plane that was once horizontal. Each lower strandline is younger and relates to a new transgressive level of water reached by the lake following the opening of a lower outlet (outlet channels represented by short vertical lines). (After Teller and Thorleifson, 1983, fig. 2).

As ice retreated, ice-marginal lakes redeveloped within the ice-dammed lowlands. Smaller lakes (e.g. Fig. 14.20, lakes A, B, and C) generally left only a thin record of lacustrine sediment without distinct shorelines (cf. Bluemle, 1974; Fenton et al., 1983; Hobbs, 1983; Benn and Evans, 1998), and many of them soon drained and re-formed in new locations along the retreating ice margin (Kehew and Teller, 1994). Others expanded along the ice margin and, during each period of global deglaciation, a vast and interconnected system of proglacial lakes developed across Canada and the adjacent USA, and in northern Europe and Asia (e.g. Vincent and Hardy, 1979; Grosswald, 1980; Teller and Clayton, 1983; Karrow and Calkin, 1985; Dyke and Prest, 1987; Teller, 1987, 2003; Teller and Kehew, 1994; Mangerud et al., 1999, 2002; in the Baltic Sea basin, both the Scandinavian ice margin and isostasy helped create a large proglacial lake, which was eventually replaced by the ocean (e.g. Bjorck, 1995).

Glacier margins in large and deep proglacial lakes were less stable than when they were on land (cf. Benn and Evans, 1998; Eyles and McCabe, 1991), and surging and iceberg calving made it difficult for an ice sheet to establish a fixed boundary of equilibrium. This, plus the high potential for subsequent wave erosion, means that there are few end moraines in large proglacial lake basins except where the water was shallow, so the details of the history of ice retreat across these basins is not well established. However, inferences about the glacial boundary are possible using strandlines (Fig. 14.1), the history of outlet use during retreat (Fig. 14.19), and lithostratigraphy (see Teller, 2001).

In deeper parts of proglacial lake basins, fine sediments were deposited, partly from density underflows and partly from suspension. In some areas, varves composed of clays and silts

Figure 14.20 Proglacial lakes along southwestern margin of the Laurentide Ice Sheet (stippled) about 11.3 ka; dashed line is ice margin several hundred years later (after Teller, 1987, fig. 15). Overflow linked many of these lake basins, and retreat of the confining ice margin led to northward expansion and periodic catastrophic releases of waters that impacted on rivers and lakes downstream, as well as to a dramatic change in their size and extent (e.g. Kehew and Teller, 1994; Teller, 1987; Teller et al., 2002). Lake Agassiz (D) eventually expanded more than a thousand kilometres north and east but smaller lakes to the west drained and re-formed along the retreating ice front.

Figure 14.20 Proglacial lakes along southwestern margin of the Laurentide Ice Sheet (stippled) about 11.3 ka; dashed line is ice margin several hundred years later (after Teller, 1987, fig. 15). Overflow linked many of these lake basins, and retreat of the confining ice margin led to northward expansion and periodic catastrophic releases of waters that impacted on rivers and lakes downstream, as well as to a dramatic change in their size and extent (e.g. Kehew and Teller, 1994; Teller, 1987; Teller et al., 2002). Lake Agassiz (D) eventually expanded more than a thousand kilometres north and east but smaller lakes to the west drained and re-formed along the retreating ice front.

accumulated, although massive to poorly laminated sediment is common in some basins. Closer to the margin of the ice sheet and the mouths of rivers, the clay-rich sequence may be interrupted by sand and coarse silt units deposited from sediment-rich subaqueous density underflows; these may display climbing ripples as well as graded bedding. Close to the retreating ice margin, iceberg-rafted detritus is abundant, but the concentration varies temporally at any one site, reflecting the proximity of the ice margin. Along the ice margin, fans of gravel and sand were deposited in places where subglacial conduits delivered sediment to the lake. Beds in these fans dip lakeward and coalesced into asymmetric ridges in some areas that are commonly called grounding-line fans (Figs. 14.6, 14.10B and C). Diamicton was commonly deposited on the up-ice side of this ridge and may also be interbedded with the fan complex. Traditional Gilbert-type deltas with topset, foreset and bottomset beds may also form at the mouth of a subglacial tunnel (Fig. 14.10A).

The floors of glacilacustrine basins are generally very flat, as original relief was masked by sediment accumulation and as elevated areas on the floor were eroded by waves. Distinct large, but very low relief, linear and curvilinear scour marks and plough ridges criss-cross the surface of many proglacial lake floors, reflecting the influence of icebergs as they dragged across the lake floor (Fig. 14.4); a variety of other low-relief patterns formed in some areas (Fig. 14.5).

Toward the shallower parts of the basin, glacilacustrine sediments are more silty and sandy. The absence of lacustrine sediment over large regions of some basins, such as in the Lake Agassiz basin (see Dredge and Cowan, 1989, Fig. 3.18), suggests that, in some cases, current strength on the lake floor prevented sediment from being deposited or that previously deposited sediments were eroded by waves or perhaps by subaqueous outbursts of water.

Around most proglacial lakes, sand and gravel beaches and wave-cut cliffs outline old lake levels, with younger beaches assuming progressively lower positions on the landscape (Figs. 14.17 and 14.19). Although small beaches may reflect storms or short episodes in the life of the lake, large beaches were formed by long-term transgressive events that allowed wave action to accumulate sediment over time and move it upslope. Because of differential isostatic rebound, large beaches only formed south of the isobase through the outlet carrying overflow during a particular phase in the lake's history, and reflect the maximum extent to which waters rose before the beach was stranded (fixed in place) when a new outlet began to carry the overflow (Fig. 14.3D); the new outlet may have been one opened by ice retreat or simply have been the new low point on the divide reached by transgressing waters. As shown in Fig. 14.2, north of the overflow outlet, the shoreline regressed, and a relatively thin and smooth blanket of shoreline sediment was deposited, interrupted in places by low beach ridges related to storms. Lake levels in most proglacial lakes rose and fell repeatedly because outlet elevation changed through time as a result of glacial advance and retreat, differential isostatic rebound, and outlet erosion. Each time the level of a lake declined, waters immediately began to rise throughout the region south of the overflow channel (Figs. 14.3 and 14.17). Today, because of isostatic rebound, all of these beaches rise in elevation toward the former centres of maximum ice thickness (Fig. 14.19).

Glacilacustrine sediments extend across a large part of continents in high northern latitudes, and their stratigraphic and morphological records are integral in our interpretation of continental glacial history. Additionally, these lakes played a major role in landsystem development far beyond their own boundaries, with their overflow influencing rivers, lakes, and oceans downstream. The potential impact of overflow from these lakes on oceans and climate has been discussed by Bjorck et al. (1996), Clark et al. (2001), Teller et al. (2002), and others.

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