Glacial Landsystems of the northern European Plains

The last section concluded that the palaeogeography and, more particularly, the distribution of ice-marginal drainage systems and ice-dammed lakes, is another strong controlling factor in the formation of thrust moraines. This section discusses three regions where these aspects combined in different ways with glacial dynamic influences. The first is an area in central Germany where the three Mid- and Late-Pleistocene glacial margins partly coincide in a small area on the north side of the Variscan highlands. The second is an area in northern Germany along the Baltic coast — the classical area of the Weichselian end moraines. The third is the area shaped by the Saalian glaciation in The Netherlands and adjacent western Germany.

8.10.1 Central Germany

In this central part of the marginal area of the Southern Scandinavian Ice Sheets (Fig. 8.16) the Elsterian, Saalian and Weichselian ice-sheet margins come together within less than 100 km of each other. The Elsterian was the most extensive glaciation, invading the highlands to elevations of up to 500 m above sea level (Eissmann et al., 1995). The Saalian was almost as extensive as the Elsterian, their margins coinciding in the northern foothills of the Harz, but diverging in the Thüring Basin, and almost merging again near the German-Polish border (Fig. 8.16). The area comprises the Mesozoic/Cenozoic Altmark-Brandenburg Basin (Ziegler, 1990) north of the Harz, Thüringer Wald, Erzgebirge and Sudetes — the Variscan highlands alongside the German-Polish border. The distribution of end moraines, their size, glacitectonic style, lithostratigraphy and sedimentology are largely controlled by the geological structure of the area.

The maps in Fig. 8.16 (after Eissmann, 1995; Knoth, 1995) demonstrate that thrust moraines are limited to the central parts of the basins where Cenozoic sediments are sufficiently thick and suitable décollement layers (lignite, clay) are available. The glacitectonic deformations reached depths of up to 175 m below the surface (Eissmann et al., 1995). Closer to and within the highlands, glacial margins are defined by dump end moraines consisting of bouldery and sandy till and glacifluvial outwash. In Poland there is a clear correlation of thrust moraine size and distance from the highlands, with the largest ones occurring in the central part of the basin (Brodzikowski, 1995). While the shape of the Elsterian and Saalian outer ice margins is

Figure 8.16 End moraines in central Germany. Within a relatively small area, just north of the central German Variscan highlands - Harz, Thüringer Wald, Erzgebirge - the Elsterian, Saalian and Weichselian ice sheet margins coincide and partly overlap. Thrust moraines are confined to the area underlain by unlithified Cenozoic sediments (compare with geological map, Fig. 8.2). Margins beyond this area are defined by dump end moraines consisting of bouldery and sandy till and glacifluvial outwash (sandur). Glacial limits: El, E2 = Elsterian 1, 2, SD = Saalian, Drenthe Stage, SW = Saalian, Warthe Stage, WB = Weichselian, Brandenburg end moraine, WF = Weichselian, Frankfurt end moraine. (After Eissmann, 1995; Knoth, 1995).

determined by the highland valleys and ridges, end-moraine lines in the sedimentary basins show remarkable lobate patterns. This is a positive sign of ice stream activity, which according to a theory developed by Boulton and Hindmarsh (1987) is promoted by a substratum of deformable sediments. The maps in Fig. 8.16 show a series of ice-marginal valleys, which are part of the Weichselian retreat sequence (Ehlers, 1996; Marks, 2002). The Elbe River, which also acted as a late Saalian ice-marginal system, was previously dammed by the Elsterian ice, producing a large lake south of Dresden (Fig. 8.16).

Most thrust moraines in the area to the east of Leipzig and north of Dresden originated during the Elsterian (Eissmann, 1995). They were subsequently covered and possibly modified to some extent by the Saalian Drenthe and Warthe glaciations. The Muzakow Hills/Muskauer Faltenbogen (Fig. 8.17) — along the Warthe end moraines — are the most prominent example of the inheritance of older landforms in younger landscapes.

Central Germany and bordering Poland are underlain by sedimentary basins of which the Upper Cenozoic sequence is dominated by Miocene and Pliocene lignites, and to a lesser extent clays, alternating with medium- and fine-grained shallow-marine and freshwater sands. As was explained above this kind of lithology promotes the formation of thrust moraines of the Holmströmbreen type (Fig. 8.5), characterized by concentric and inclined folds, occasional diapirs and folded nappes. Diapirs have formed as a result of two mechanisms (Eissmann et al., 1995). In some areas loading by glacial ice of a saturated substratum produced diapir-like upright and steeply inclined folds. They sometimes developed from anticlines, or, alternatively, developed into antiforms. Many lignite diapirs occur in isolated positions and are interpreted as the result of glacial loading (Eissmann et al., 1995). In areas outside of the influence of continental ice sheets diapirs have been ascribed to permafrost processes.

Straddling the Polish-German border, the Muzakow Hills (Brodzikowski, 1995), or Muskau thrust moraine (German: Muskauer Faltenbogen; Kupetz, 1997), is a representative example of the glacitectonic style in this region (Fig. 8.17). The structural style of this thrust moraine, one of the largest in Europe, is strongly controlled by the rheology of the sequence of evenly bedded Tertiary clays, sands and lignites prevailing in this region. The main detachment is approximately 150 m below the surface in Tertiary clays and lignites. Horizontal shortening is accomplished by steeply inclined concentric folds and reverse faults, which with increasing strain grade into overturned disharmonic similar style folds. Detachment surfaces in lignite beds at intermediate levels produce heterogeneous shortening, which increases from the lowest décollement to the highest level of the structure. In terms of glacitectonic style terminology (Fig. 8.7), this and similar thrust moraines from this region classify as style B and C. Horizontal shortening is considerably less than for style D thrust moraines, which are built of subhorizontal stacks of nappes.

8.10.2 Mecklenburg-Vorpommern

This type area of the classical Weichselian end-moraine landscapes (Woldstedt, 1925; Ehlers, 1996), lies along the southern Baltic coast, immediately north of the region discussed in the previous section (Fig. 8.18a, b, after Müller et al., 1995; Rühberg et al., 1995). All main late Weichselian end moraines from the European mainland are represented in this relatively small area. They mark readvances during the retreat after the last glacial maximum. Up to 100 m thick tills from the Saalian Drenthe and Warthe glaciations have been found in drillings underlying the Weichselian sediments (Müller et al., 1995).

Figure 8.17 Profile of the MuZakow Hills on the Polish-German border. (After Brodzikowski, 1995). The German part of this thrust moraine, which dated from the Saalian Warthe stage, is called the Muskau thrust moraine (Kupetz, 1997). For location, see Fig. 8.16. The profile is based on drilling and mapping around brown coal mines and has an unknown, but less than 2-times, vertical exaggeration. The structural style of this thrust moraine is strongly controlled by the rheology of the sequence of Tertiary clays, sands and lignites prevailing in this region. In terms of glacitectonic style terminology (see Fig. 8.7) this and similar thrust moraines from this region classify as style B and C.

Figure 8.17 Profile of the MuZakow Hills on the Polish-German border. (After Brodzikowski, 1995). The German part of this thrust moraine, which dated from the Saalian Warthe stage, is called the Muskau thrust moraine (Kupetz, 1997). For location, see Fig. 8.16. The profile is based on drilling and mapping around brown coal mines and has an unknown, but less than 2-times, vertical exaggeration. The structural style of this thrust moraine is strongly controlled by the rheology of the sequence of Tertiary clays, sands and lignites prevailing in this region. In terms of glacitectonic style terminology (see Fig. 8.7) this and similar thrust moraines from this region classify as style B and C.

A landscape of hummocky moraines and glacifluvial outwash fans (sandur) occurs mainly between the outer margin (W1B, Brandenburg end moraine) and the Pomeranian end moraine (W2). Depressions in this landscape are occupied by numerous lakes and peat bogs. From the highest crests — the Pomeranian end moraine — the landscape slopes gently towards the Baltic coast. These proximal slopes are scattered with eskers, which run approximately parallel to the palaeo-ice flow direction. Thrust moraines occur mainly in the Pomeranian and Mecklenburg end moraines. They originated partly from Saalian advances (Rühberg et al., 1995). The large thrust moraine on the Island of Rügen contains thrust sheets of Cretaceous chalk, similar to those on the Island of Mon, Denmark.

8.10.3 The Netherlands and West Germany

This area (Fig. 8.19a, b) has been shaped mainly by the Drenthe and Warthe stages of the Saalian Glaciation. Here, the various structural and palaeogeographic controls on the formation of Saalian glacial landsystems are well documented. The Elsterian margin runs ESE-WNW across the area, but none of its landforms survive at the surface. Drilling and mapping have

demonstrated the existence of Elsterian deep tunnel valleys in the northern Netherlands and Germany (Kuster and Meyer, 1979; Ter Wee, 1983; Huuse and Lykke-Andersen, 2000). Unlike central Germany, this area does not contain evidence of inheritance of Elsterian landforms in Saalian glacial landsystems.

The landscape is dominated by vast till plateaux ('Geeste' in German) and thrust moraines. Few eskers are found in areas of shallow bedrock and some of the till plateaux are drumlinized (Fig. 8.19b). The largest thrust moraines occur in the E-W trending Rehburg line, Drenthe stage. The German Rehburg line probably correlates with a line of thrust moraines and stacked till sheets (style Ec) in The Netherlands at 52.5°N to 53°N (Van den Berg and Beets, 1987; Van der Wateren, 1995). The chain of large thrust moraines in the central Netherlands at the Saalian outer margin formed after overriding of the Rehburg line.

In Germany thrust moraines frame the Variscan highlands of the Teutoburgerwald and Wiehengebirge at the southern margin of the Lower Saxony Basin. In The Netherlands those at the Saalian margin follow a major NW-SE trending fault system delineating the uplifted flank of the Roer Vally Graben, the northwest branch of the Rhine Valley rift system (Fig. 8.19a). In The Netherlands the distribution of thrust moraines and till sheets is quite clearly controlled by Cenozoic structures in the substratum (De Gans et al., 1987). To clarify this relationship Fig. 8.19a includes contours of the Quaternary base (after Zagwijn and Doppert, 1978) that are strongly influenced by Cenozoic faulting. Quite remarkably, the Cenozoic Zuiderzee Low is rimmed on three sides by thrust moraines, which is a clear indication of the influence of subsurface structural and hydrological conditions on glacitectonic processes.

Van den Berg and Beets (1987) demonstrated the close link of the various tectonic styles with zones in which strata of contrasting hydraulic conductivities are close to the surface. The Rehburg line in The Netherlands follows a zone where fine-grained Elsterian glacilacustrine deposits reach the surface. The detachment is in Holsteinian fluvial clays, while thrust moraines near the German border and further east regularly have their décollement in Tertiary and Cretaceous formations.

A reconstruction of the glacier bed just before the Drenthe ice sheet reached The Netherlands indicates it is likely that the Saalian ice sheet expanded in the northern and eastern parts of the country over a surface of fine-grained sediments. South of the line (53.0°N, 4.0°E - 52.7°N, 6.7°E), running eastward from the island of Texel, the surface consisted of coarse-grained

Figure 8.18 The Weichselian glaciated area of Mecklenburg. All main late Weichselian end moraines from the European mainland are represented in this relatively small area. A) Shaded relief map (GTOPO30). Numerous lakes occur on hummocky moraine and glacifluvial outwash landscape between successive Weichselian end moraines. From the crests of the end moraines the landscape slopes gently towards the Baltic coast. B) The various margins mark readvances during the retreat after the Last Glacial Maximum. They are associations of dump end moraines, thrust moraines, eskers and glacifluvial outwash fans. WIB = Brandenburg advance, WIF = Frankfurt readvance, W2max = limit of Pomeranian till, W2 = Pomeranian readvance, W3 = Mecklenburg readvance, W3V = Velgast readvance. (After Müller et al., 1995; Rühberg et al., 1995).

A; dump endmoraine

■i?

E; till plateaus

outwash fan

9

Ee, glacial basin

B, C

Ec

E/B,C

\

drumlin

D

esker

«u»

E/D

ice-marginal valley

sediments. A deforming bed of fine-grained deposits north of this line was reworked into thick accumulations of till (Rappol, 1984; Rappol and Stoltenberg, 1985). Thrust moraines formed once the ice reached areas covered with coarse-grained fluvial material and where a fine-grained layer in the lower substratum could act as décollement. These coarse-grained sediments filled ice-marginal valleys, once more stressing the impact of palaeogeographical conditions on glacial landsystem formation. Where a suitable décollement was lacking, stacked till sheets rather than thrust moraines formed along the stagnating ice margin (Fig. 8.19b).

A similar relation seems to exist between salt structures and pushed zones. Most of the domes in Zechstein salt originated as Hercynian anticlinal structures that intruded into the overlying strata (Jaritz, 1973). Their trends closely follow those of the Variscan and Alpine orogenies in the area. They remain usually well below the deepest level of glacial deformation and are only cut by the deepest Elsterian tunnel valleys (Kuster and Meyer, 1979). However, some have been mobile in the Tertiary and even up to the present time (Gripp, 1952; Jaritz, 1973; Picard, 1964), which is why in those areas fine-grained Mesozoic and Tertiary strata may be found on higher levels than elsewhere. This relationship is demonstrated by the Lamstedt thrust moraine (L in Fig. 8.19b) and the Altenwald thrust moraine to the northwest. These ridges run more or less parallel to a series of long N-S trending salt domes that crop out locally (Kuster and Meyer, 1979).

The conspicuous lobate shape of many of the Saalian thrust moraines is a clear indication of the activity of ice streams. These were particularly active following the advance beyond the Rehburg line to the Drenthe margin. An ice stream, which occupied the Nordhorn basin (N, Fig. 8.19b), advanced into the embayment south of the Teutoburgerwald (Skupin et al., 1993). The orientation of drumlins and eskers in this area indicate a strongly diverging ice flow with even northeastward flow in the eastern part of the embayment (Fig. 8.19b).

Although thrust moraines are commonly considered to consist of steeply imbricated thrust sheets ('Schuppen'), in reality they are built of thin subhorizontal nappes (Van der Wateren, 1995). Figures 8.20 and 8.21 show two examples, the Blankenese thrust moraine west of Hamburg (after

Figure 8.19 Glacial landsystems in The Netherlands and western Germany - thrust moraines and till plateaux from the Saalian glaciation. The prominent moraines trending east-west across the middle of the map belong to the Rehburg line, Drenthe stage (SD). End moraines and till plateaux from the Saalian Warthe stage (SW) are in the northeastern part of this map. In Germany (eastern half of the map) thrust moraines rim the Variscan highlands, while in The Netherlands, these formed along a major NW-SE trending fault system. A) Shaded topography. Morphological features are strongly related to the structure of the substratum as shown by depth contours of the base of the Tertiary (stippled lines, from Zagwijn and Doppert, 1978) and major faults. Box outlines the area of the Dammer Berge (Fig. 8.4). B) Distribution of glacitectonic styles (see also Fig. 8.7). M = Münsterland esker ridge, N = Nordhorn glacial basin, D = Dammer Berge, H = Hamburg-Blankenese thrust moraine (Fig. 8.20), L = Lamstedter Berge thrust moraine (Fig. 8.21). The Lamstedter Berge (L) show a change in style from north to south from E/D to E/C to A/dump end moraine, implying that the strongest shortening occurred in the northern part of the thrust moraine. The Warthe advance overrode the thrust moraine, reaching its limit at approximately the same latitude as the southern tip of the thrust moraine.

Figure 8.20 Profile across the Blankenese thrust moraine west of Hamburg. The thrust moraine formed during the Warthe stage, Saalian glaciation. Décollement is in Upper Miocene micaceous clay (black) which is overlain by Pliocene, Elsterian and Holsteinian silts and sands (white, stippled lines indicate bedding). Till of the Drenthe stage (white triangles, grey shading) is incorporated in the nappe structures. The thrust moraine is covered on its proximal side by Warthe stage till (black triangles, stipple shading). Although this and other Saalian thrust moraines have been commonly cited to consist of steeply imbricated thrust sheets ('Schuppen'), the profile without vertical exaggeration shows they are really built of thin nappes extending more than 1 km (Van der Wateren, 1995). In terms of glacitectonic style terminology (Fig. 8.7) this and similar thrust moraines from this region classify as style D. Horizontal shortening is much greater than for style B and C thrust moraines. (After Wilke and Ehlers, 1983).

Figure 8.20 Profile across the Blankenese thrust moraine west of Hamburg. The thrust moraine formed during the Warthe stage, Saalian glaciation. Décollement is in Upper Miocene micaceous clay (black) which is overlain by Pliocene, Elsterian and Holsteinian silts and sands (white, stippled lines indicate bedding). Till of the Drenthe stage (white triangles, grey shading) is incorporated in the nappe structures. The thrust moraine is covered on its proximal side by Warthe stage till (black triangles, stipple shading). Although this and other Saalian thrust moraines have been commonly cited to consist of steeply imbricated thrust sheets ('Schuppen'), the profile without vertical exaggeration shows they are really built of thin nappes extending more than 1 km (Van der Wateren, 1995). In terms of glacitectonic style terminology (Fig. 8.7) this and similar thrust moraines from this region classify as style D. Horizontal shortening is much greater than for style B and C thrust moraines. (After Wilke and Ehlers, 1983).

Wilke and Ehlers, 1983) and the Lamstedter Berge (after van Gijssel, 1987), respectively. Thrust moraines in this region are predominantly built of large nappe structures, glacitectonic style D (Fig. 8.19b). Investigations in the Dammer Berge (Van der Wateren, 1987, 1995) and the Itterbeck-Uelsen thrust moraine (Dutch-German border, (Kluiving, 1994) showed that these nappes may measure several km2 up to 12 km2. This style, contrasting with that of central European thrust moraines, is controlled by thick sequences of coarse-grained fluvial sediments derived from the ice-marginal valley systems across which the ice sheets advanced.

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