Regional Geology and Glacial Landsystems

The Scandinavian shield (Fig. 8.2), the core region of the Fennoscandian Ice Sheets, consists of Precambrian and Early Palaeozoic metamorphic and intrusive rocks (the same rocks that are found as erratics in all Pleistocene tills). These are covered at most with a sparse cover of Quaternary sediments. The Southern Scandinavian Ice Sheets covered areas almost completely underlain by large sedimentary basins. These basins separate the Scandinavian craton from the Variscan and Alpine orogens in the south. They are filled with up to 14 km of Palaeozoic to Cenozoic sediments (Ziegler, 1990). The upper few hundred metres consist of unlithified and weakly lithified sediments, which may deform when subjected to glacial stresses. Under the right conditions deformation by ice-marginal stress fields may produce thrust moraines. Subglacial shearing may produce tills, which are a mixture of local sediments and far-travelled Scandinavian material.

The margins of the Elsterian and Saalian ice sheets were located in the northern foothills of the Variscan uplands. The shaded topography map (Fig. 8.1a) clearly shows that major end moraines occur in narrow belts parallel to the uplands, for example the moraines within the Drenthe

Figure 8.2 Geological sketch map of northwest Europe (data courtesy of Cornell University Interactive Mapping Tool; Kirkham, 1995) including Elsterian, Saalian and Weichselian ice sheet margins. Cenozoic sedimentary basins separate the Scandinavian Precambrian craton in the north from the Variscan and Alpine orogens in the south. They form an area of extended and thinned lithosphere and are filled with up to 14 km of Palaeozoic to Cenozoic sediments. Major neotectonically reactivated basement faults (sketched) and neotectonic structural subdivision are based upon Garetsky et al. (2001).

Figure 8.2 Geological sketch map of northwest Europe (data courtesy of Cornell University Interactive Mapping Tool; Kirkham, 1995) including Elsterian, Saalian and Weichselian ice sheet margins. Cenozoic sedimentary basins separate the Scandinavian Precambrian craton in the north from the Variscan and Alpine orogens in the south. They form an area of extended and thinned lithosphere and are filled with up to 14 km of Palaeozoic to Cenozoic sediments. Major neotectonically reactivated basement faults (sketched) and neotectonic structural subdivision are based upon Garetsky et al. (2001).

glaciated area (SD in Fig. 8.1b), or the prominent belt of Warthe (SW) end moraines, which runs from northern Germany across Poland into Belarus. These end moraines are mainly thrust moraines built of unlithified and weakly lithified preglacial sediments, deposited in Mesozoic/Cenozoic sedimentary basins. The coincidence of moraine lines with the shallow margins of these basins draws attention to a possible relationship between the formation and distribution of glacial landforms and geological structure. This correlation of substratum geology and glacial landforms has been noted by many workers in Europe as well as North America (see the Introduction in Van der Wateren, 1995).

In The Netherlands the distribution of Saalian thrust moraines and till sheets is evidently controlled by Cenozoic crustal scale structures in the substratum (De Gans et al., 1987). The Saalian limit, which is marked by large thrust moraines, exactly follows the northern margin of major structural highs. Other thrust moraine belts north of the Saalian limit in The Netherlands can likewise be shown to correlate with subsurface structures. Most of the larger faults have been active throughout the Neogene up to the present. The large Saalian thrust moraines of the Rehburg line in Lower Saxony, west Germany, and those in central Germany occur in areas near the margin of sedimentary basins where Mesozoic and Tertiary clay and silt beds shallow and crop out. Within the Weichselian glaciated area, substratum control is less obvious, except for the Pomeranian end moraines, which tend to follow the —50 and —100m contours of the Quaternary base in Germany as well as in Poland (Stackebrandt et al., 2001).

In the area occupied by the Southern Scandinavian Ice Sheets, including the sedimentary basins of the North German Lowlands and adjacent Polish lowland, the Quaternary base has a relief of more than 1100 m, ranging from more than 600 m below sea level in the central parts of the basins to more than 500 m in the highlands south of them (Stackebrandt et al., 2001). The deepest depressions are produced by Elsterian subglacial erosion and constitute tunnel valleys reaching up to 500 m in depth. Many of these, particularly those in the Central European Subsidence Zone (Fig. 8.2), follow NNE-SSW trending basement faults, which have been neotectonically active. Neotectonics in this area appears to have played a major role in channeling Elsterian ice stream flow (Stackebrandt et al., 2001). In the transition zone between the Central European Subsidence Zone and the Central European Uplift (including the Variscan highlands) the Quaternary base contours are affected by another set of NW—SE trending neotectonically reactivated faults (Garetsky et al., 2001; Stackebrandt et al., 2001).

In Poland, Brodzikowski (1995) demonstrated that glacitectonic styles, intensity and dimensions of the glacitectonic deformation of the pre-Vistulian (pre-Weichselian) glaciated area in Poland is strongly related to the geological structure of the substratum. In Belarus, Matveyev and Nechiporenko (1995) showed that Pleistocene sediment thickness and the distribution of glacigenic landforms are strongly correlated with the structure of the pre-Quaternary basement as well as with neotectonic structures.

In all these cases the depth of fine-grained sediments (e.g. Cretaceous and Tertiary marine clays and silts, Tertiary lignites, glacial and interglacial lacustrine clays and silts), appears to be the main controlling factor for the formation of thrust moraines. These layers formed a low-friction décollement on which the pushed sediment sheets moved. In areas where they were lacking, or out of reach of the ice-marginal stress field, thrust moraines are absent or very small. A discussion of the conditions and mechanics of thrust moraine formation can be found by Van der Wateren (1994a, 1995, 2002a). Deformation of sediments in the glacier bed rather than formation of thrust moraines can be expected where the surface before the arrival of the Pleistocene ice sheets consisted of clay and silt. Here the décollement was very shallow, within the top layer of the bed immediately beneath the glacier sole. The tectonic style is dominated by a shear zone fabric of extremely high shear strain, as described below. The properties of glacial sediments and landforms are to a large degree controlled by the structure of the substratum. In the plains underlain by Mesozoic/Cenozoic sedimentary basins, thrust moraines form predominantly where a suitable décollement — in fine-grained clastic and/or lignite layers — is sufficiently shallow. The schematic profile across the Northern European Plains (Fig. 8.3) illustrates the association of subsurface geology and thrust moraine structure/lithostratigraphy.

Local geological conditions determine whether a décollement is provided by either relatively young, or old lithostratigraphic units. In the centre of a sedimentary basin, salt diapirs may bring up layers, which in other parts of the basin are out of reach of the glacial stress field. If these contain fine-grained sediments these may act as a décollement for a thrust moraine ('a' in Fig. 8.3). Shallowing of the basin towards its margin brings up a potential décollement in stratigraphically higher units (e.g. upper Neogene marine clay and silt and lower Pleistocene fluvial and lacustrine clays; 'b' in Fig. 8.3). Near the margin of the basin again stratigraphically deeper units may be incorporated in thrust moraines (e.g. Mesozoic clays and Tertiary lignites; 'c' in Fig. 8.3). Thrust moraines formed close to the highlands tend to be relatively small, because sedimentary units thin out towards the basin margin. The highlands are fringed with zones which are generally free of

Lower Saxony Variscan

Lower Saxony Variscan

Figure 8.3 Schematic profile of the northern European plains north of the Variscan highlands showing the association of substrate geological structure and thrust-moraine structure/lithostratigraphy. In the plains underlain by Mesozoic/Cenozoic sedimentary basins, thrust moraines form predominantly where a suitable décollement in fine-grained clastic and/or lignite layers is sufficiently shallow. Depending on local geological conditions a décollement is provided by stratigraphic units of varying age. a = salt diapir brings up layers, which are too deep elsewhere, to act as a décollement for the thrust moraine, b = shallowing of the basin towards its margin provides a décollement in stratigraphically higher units (e.g. upper Neogene or lower Pleistocene clays), c = near the margin of the basin stratigraphically deeper units are incorporated in the thrust moraine (e.g. Mesozoic clays and Tertiary lignites). Thrust moraines formed close to the highlands (shallowest part of the basin) tend to be smaller than those further away from it. Dump end moraines (till and outwash) form at the ice margin where a suitable décollement is lacking ((d) and (e)). Dashed lines indicate successive ice sheet profiles. Vertical scale exaggerated.

Figure 8.3 Schematic profile of the northern European plains north of the Variscan highlands showing the association of substrate geological structure and thrust-moraine structure/lithostratigraphy. In the plains underlain by Mesozoic/Cenozoic sedimentary basins, thrust moraines form predominantly where a suitable décollement in fine-grained clastic and/or lignite layers is sufficiently shallow. Depending on local geological conditions a décollement is provided by stratigraphic units of varying age. a = salt diapir brings up layers, which are too deep elsewhere, to act as a décollement for the thrust moraine, b = shallowing of the basin towards its margin provides a décollement in stratigraphically higher units (e.g. upper Neogene or lower Pleistocene clays), c = near the margin of the basin stratigraphically deeper units are incorporated in the thrust moraine (e.g. Mesozoic clays and Tertiary lignites). Thrust moraines formed close to the highlands (shallowest part of the basin) tend to be smaller than those further away from it. Dump end moraines (till and outwash) form at the ice margin where a suitable décollement is lacking ((d) and (e)). Dashed lines indicate successive ice sheet profiles. Vertical scale exaggerated.

thrust moraines. Here, basin sediments consist mainly of coarse-grained erosion products from the Variscan and Alpine ranges (e.g. Neogene and Early Pleistocene river and alluvial fan sediments), while fine-grained units that could produce a décollement are lacking. No thrust moraines are found in the highlands. In both areas, former ice margins are marked by accumulations of till and glacifluvial outwash (dump end moraines; 'd' and 'e' in Fig. 8.3).

The Dammer Berge, one of the largest Saalian thrust moraines (Fig. 8.4), illustrates how basin structure and (neo)tectonic activity combined to create the conditions necessary for its formation (Van der Wateren, 1995). Up to 50 m thick nappes and thrust sheets in the western half of this horseshoe-shaped thrust moraine are built largely of Miocene and Pliocene units, while Pleistocene units are relatively thin or even lacking. Tertiary clays invariably form the intensely deformed basal units on which the nappes moved. These sediments originated from the glacial basin where an originally Variscan anticline just north of this part of the thrust moraine (Fig. 8.4a) has been reactivated during the Alpine orogeny (Ziegler, 1990). This has lifted the Tertiary units to shallower levels (Fig. 8.4b). The lithostratigraphy in the eastern half of the Dammer Berge is dominated by Early and Middle Pleistocene fluvial deposits with a basal shear zone usually consisting of a Pleistocene, sometimes Tertiary, clay layer. Glacial overdeepening of the glacial basin from which the nappe units had been removed enabled the ice to cut into a Campanian-

I76 GLACIAL LANDSYSTEMS

I76 GLACIAL LANDSYSTEMS

Thrust MoraineThrust Moraine

Figure 8.4 The Dammer Berge thrust moraine (Van der Wateren, 1995; grey shading, see Fig. 8.19a for location) north of the Variscan highlands in Germany. Neotectonic movements strongly affected the underlying Variscan folds and high-angle faults (A), which is reflected by the contours of the base of the Tertiary (B). The combination of shallowing of the Lower Saxony Basin towards the margin with the effect of (neo)tectonic movements carried upper Cretaceous and middle Tertiary clays close enough to the surface for them to be incorporated in the thrust moraine. The Campanian synclinal basin underlying the eastern part of the Dammer Berge is the source of the clay which has been reworked in some of the nappe shear zones and a clay-rich till (Van der Wateren, 1995). (Data courtesy of the Archive, Niedersächsisches Landesamt für Bodenforschung, Hannover.)

Maastrichtian clay. These sediments were moved to the highest flanks of the thrust moraine to produce a till, which is made up entirely of this clay with a few scattered Scandinavian erratic pebbles (Van der Wateren, 1987).

8.4 DISTRIBUTION OF GLACIAL LANDFORMS AND

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