And Lakes

Meltwater plays an important role in ice-marginal landsystems. Where a glacial margin stagnates, sediment accumulates in the form of outwash fans (sandur) and glacifluvial deltas. A new advance may incorporate these sediment bodies into thrust moraines. Lakes form where glacial advances block ice-marginal valleys. These lakes may cause permafrost to decay over large areas, thus generating an important morphodynamic control on glacial advances. The area covered by the Rehburg advance during the Saalian may serve as a model to illustrate the morphodynamic relationships at an ice-sheet margin.

Rivers like the Rhine and Meuse in The Netherlands, Weser and Ems in western Germany, the Elbe in central Germany, and the Odra and Vistula in Poland, used to drain into the North Sea and Baltic Sea basins. Successive Pleistocene glaciations redirected these rivers to a westward course parallel to the ice-sheet margins. True ice-marginal valleys (in German Urstromtal, plural Urstromtäler) formed only during the Weichselian glaciation. From south to north (old to young) the major Weichselian ice-marginal valley systems are the Baruth-Glogow, Warsaw-Berlin, Warsaw-Toruii-Eberswalde and Kashubian-Pomeranian Urstromtäler (Ehlers, 1996; Marks, 2002). For the Elsterian and Saalian glaciations ice-marginal drainage can only be reconstructed by sediment-petrographical and provenance studies (Meyer, 1983; Zandstra, 1983). Valley incision into bedrock was largely inhibited because the ice margins spent relatively short intervals of time next to and in the highlands (Liedtke, 1981).

When an ice sheet advances across an Urstromtal, ice-marginal river and lake sediments may be incorporated into thrust moraines (Van der Wateren, 1994b, 1995; Fig. 8.14). The advancing ice lobe and emerging thrust moraines block river channels, producing ice-marginal lakes. Subaquatic fans build at the mouths of subglacial meltwater tunnels (Fig. 8.14b). Ice-marginal deltas and subaerial alluvial fans form on top of and adjacent to subaquatic fans, silting up the lake (Fig. 8.14c). The advancing ice lobe compresses these sediments into thrust moraines and the subaerial drainage forms a rectangular pattern between the outcropping nappe and fold structures. Subaquatic mass flows move down the partly submerged thrust moraine slopes into the lake.

Meltwater streams interact with the emergent thrust moraine forming syn- and post-tectonic terraces and channel fills (Boulton, 1986; Van der Wateren, 1987). In cross section these show as stacked sediment bodies of contrasting finite strain (Fig. 8.15). The terms pre-, syn- and post-tectonic refer to stages in the formation of a thrust moraine. The lowest of the stack, pre-tectonic folded/thrusted sediments — including glacifluvial outwash overlying preglacial sediments — have undergone the highest degree of deformation (finite strain). Syn-tectonic sediments fill the synclinal valleys on top of the thrust moraine while it was forming and therefore show varying amounts of finite strain: folding of the basal unconformities becomes progressively less tight in Fig. 8.15. They form terraces, glacifluvial deltas and channel deposits unconformably overlain by undeformed post-tectonic sediments (glacifluvial channel and alluvial fan deposits).

One of the most outstanding features of glacifluvial sediments in many thrust moraines in northwestern Europe is the large proportion of subaquatic sediments. Sedimentary sections

Features Strain Sedimentary

Figure 8.14 Model of pre-tectonic and syn-tectonic drainage of the area of the Rehburg thrust moraines based on sedimentological analyses of glacifluvial sediments in thrust moraines (Van der Wateren, 1994b, 1995). A) Pre-tectonic westward drainage of the ice-marginal braided system (Urstromtal). B) Formation of an ice-marginal lake by blocking of river channels. Subaquatic fans build at the mouths of meltwater tunnels. C) The lake fills up with subaerial alluvial fans, ice-marginal deltas, subaquatic fans and subaquatic mass flows moving down the thrust moraine slopes. These sediments make up a large proportion of thrust moraines. Note rectangular drainage pattern on the thrust moraine.

Figure 8.14 Model of pre-tectonic and syn-tectonic drainage of the area of the Rehburg thrust moraines based on sedimentological analyses of glacifluvial sediments in thrust moraines (Van der Wateren, 1994b, 1995). A) Pre-tectonic westward drainage of the ice-marginal braided system (Urstromtal). B) Formation of an ice-marginal lake by blocking of river channels. Subaquatic fans build at the mouths of meltwater tunnels. C) The lake fills up with subaerial alluvial fans, ice-marginal deltas, subaquatic fans and subaquatic mass flows moving down the thrust moraine slopes. These sediments make up a large proportion of thrust moraines. Note rectangular drainage pattern on the thrust moraine.

often contain structures that are typical of deltaic and lacustrine environments. Apparently, a large proportion of the glacifluvial sediments in thrust moraines has been deposited in lakes dammed off by ice or emerging thrust moraines. Glaciated lowlands form a suitable environment to produce lakes of a wide size range, silting, draining and falling dry in response to the continuously changing topography. As many of these lakes formed as a result of damming by advancing ice masses, it may be tentatively concluded that thrust moraines form preferentially along ice-marginal river systems.

Syn Tectonic

Figure 8.15 Pre-, syn- and post-tectonic sediments in a thrust moraine. (After Van der Wateren, 1987). 1 = pre-tectonic folded/thrusted sediments, 2 = syn-tectonic fill of synclinal valleys on top of the thrust moraine (glacifluvial delta and channel deposits, terraces), 3 = undeformed post-tectonic sediments (glacifluvial channel and alluvial fan deposits). Structures in unit 2 show decreasing finite strain in successively younger deposits.

Figure 8.15 Pre-, syn- and post-tectonic sediments in a thrust moraine. (After Van der Wateren, 1987). 1 = pre-tectonic folded/thrusted sediments, 2 = syn-tectonic fill of synclinal valleys on top of the thrust moraine (glacifluvial delta and channel deposits, terraces), 3 = undeformed post-tectonic sediments (glacifluvial channel and alluvial fan deposits). Structures in unit 2 show decreasing finite strain in successively younger deposits.

The deltas usually contain onlapping lobes and bodies of unstructured fine loamy sands and sandy loams alternating with sets of climbing ripple and parallel-laminated fine-grained sands (Van der Wateren, 1994b). These fine-grained units alternate with trough-shaped sets of coarse-grained gravelly sands. A likely interpretation of these sequences is that they belong to fans and fan deltas at the mouths of subglacial and englacial tunnels draining into ice-marginal lakes. The alternation of coarse-grained trough-shaped units and fine-grained ripple laminated units reflects the strong variation in competence, which is typical of meltwater streams. In this way they are quite similar to eskers described by Banerjee and McDonald (1975), Rust and Romanelli (1975) and Saunderson (1975).

While large eskers are abundant in Scandinavia, only a few and usually small eskers have been described in the areas glaciated by the Southern Scandinavian Ice Sheets. The reason is, as explained by Clark and Walder (1994), that eskers, the sedimentary fill of englacial and subglacial meltwater tunnels (R-channels, Rothlisberger 1972), have a preference for a substratum of low permeability, such as bedrock. Where most of the area under consideration is underlain by sedimentary basins filled with permeable fluvial and glacifluvial sediments, esker formation is impeded. The few examples given on the regional maps in the next section are from areas where either bedrock or impermeable clay beds are shallow, thereby confirming Clark and Walder's (1994) ideas. In the area occupied by the Southern Scandinavian Ice Sheets, eskers have been described from the Hamburg area (Homci, 1974), the Munsterland area near the Dutch-German

I92 GLACIAL LANDSYSTEMS

border (Van den Berg and Beets, 1987; Klostermann, 1995), the Barnim till plain northeast of Berlin (Chrobok and Nitz, 1995) and Mecklenburg (Schulz, 1963, 1970).

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