Glacitectonic Styles

The discussion by Hart et al. (1996, 1997) and Piotrowski et al. (1997) is a clear illustration of how different interpretations of sedimentary structures, and whether or not they have been significantly deformed by subglacial shearing, may lead to widely disparate interpretations of glacial landsystems. The reconstruction of the dynamics of former ice sheets requires that it can be determined whether sediments have either been sheared by overriding ice, or deposited by lodgement or subglacial melting. Consequently, it is of critical importance to establish an unambiguous set of criteria that can be used to identify subglacial deformation.

Slater (1926), Gry (1942) and Banham (1975, 1977) were among the first to recognize the similarities between the deformation structures present in tills and those in regionally deformed metamorphic rocks (e.g. slates, schists) and shear zones (e.g. mylonites). Shear tests with clay by Maltman (1987) produced structures very similar to cleavages in low-grade metamorphic rocks (e.g. slates), while later studies of actively deforming accretionary wedges (Byrne, 1994; Maltman, 1994) demonstrated that these structures indeed form in unlithified sediments under natural conditions. Alley (1991a), Boulton (1996a, b), Boulton and Hindmarsh (1987) and Boulton and Jones (1979) showed that the dynamics of mid-latitude ice sheets are, to a high degree, controlled by deformation of the bed. As we have seen, outside their core regions the Southern Scandinavian Ice Sheets advanced over a bed of unlithified and weakly lithified sediments.

As a consequence of ice flow, sediment is continuously being transported from the internal parts of an ice sheet towards its margin. Under the right conditions basal shear stresses may cause the sediments in the bed to deform by simple shearing, which moves them towards the margin. Slowing down of the sediment flow in the marginal zone leads to horizontal compression and deposition. This pattern of deformation of the bed is similar to that in the overlying ice, where extending flow gives way to compressive flow near the margin. In sediments within a glaciated region we can therefore distinguish two glacitectonic regimes, the subglacial shear zone and the marginal compressive belt (Van der Wateren, 1995).

Two types of thrust moraine serve as models for the interpretation of structures forming at the ice margin. The first (Fig. 8.5) is based on the Holmstrombreen thrust moraine in Spitsbergen (Van der Wateren, 1995; Boulton et al., 1999). The structure of this thrust moraine is dominated by folds and relatively thick fold nappes (aspect ratios of 5:1 to 8:1). This style is typical of thrust moraines where the bulk rheology is controlled by relatively ductile fine-grained sediments. A unit of coarse-grained sediments overlies a unit of roughly equal thickness of fine

Figure 8.5 Model of a thrust moraine in which the glacitectonic style is controlled by relatively ductile fine-grained sediments. Nappes have low aspect ratios of 5:1 to 8:1 (up to 20 m in thickness and 100 m long in cross section). The model is based upon the thrust moraine which formed during a late 19th century surge of the Holmstrombreen glacier in Spitsbergen (Van der Wateren, 1995; Boulton et al., 1999). Vertical scale is exaggerated. Volume of the glacial basin, from which the sediments have been removed, does not match the volume of the thrust

Figure 8.5 Model of a thrust moraine in which the glacitectonic style is controlled by relatively ductile fine-grained sediments. Nappes have low aspect ratios of 5:1 to 8:1 (up to 20 m in thickness and 100 m long in cross section). The model is based upon the thrust moraine which formed during a late 19th century surge of the Holmstrombreen glacier in Spitsbergen (Van der Wateren, 1995; Boulton et al., 1999). Vertical scale is exaggerated. Volume of the glacial basin, from which the sediments have been removed, does not match the volume of the thrust moraine.

grained sediments — in this case, sand and gravel of several glacifluvial outwash fans, and silt and clay of a proglacial delta, respectively. This type of thrust moraine is characteristic of areas in central Europe where Tertiary lignites crop out. In the Holmstrombreen thrust moraine the entire structural succession is represented in a section from the undeformed foreland to the glacier margin (Van der Wateren, 1995). The central European thrust moraines are mainly built of one unit of folds and thrust sheets above a detachment surface, while piles of nappes are rare.

The second type (Fig. 8.6) is based on the Dammer Berge thrust moraine, Germany, Drenthe Stage, Saalian Glaciation (Van der Wateren, 1995). It comprises a stack of up to six subhorizontal nappes measuring up to 12 km2. Their aspect ratios range from 20:1 to 50:1. They are composed of relatively thick coarse-grained sediments (typically 20—50 m, 150 m maximum) overlying a thin layer of silt and clay (0.5—3.0 m). This is characteristic of areas where glacifluvial outwash fans and fluvial sands and gravels overlie Pleistocene fluvial clays or Tertiary marine clays. Such sedimentary sequences are relatively stiff and favour transmission of stress over great distances. This leads to a wide spacing of folds and thrusts and the formation of relatively thin nappes, which each may contain several high-angle thrusts and folds.

Terrains within the marginal zone experience a different style of deformation resulting from horizontal shear stresses applied by the overlying ice (Fig. 8.7):

A. Undeformed foreland. Dump end moraines and outwash fans, consisting mainly of proglacial outwash and supraglacial flow tills, directly related to the latest pushing event, overlying older sediments.

Figure: 8.6 Model of a thrust moraine in which the glacitectonic style is controlled by relatively stiff and brittle coarse-grained sediments. Based upon the Dammer Berge thrust moraine, Germany, Drenthe Stage, Saalian Glaciation (Van der Wateren, 1995). This type of thrust moraine is built of nappes and thrust sheets of high aspect ratios - between 20:1 and 50:1. Nappes are typically between 20 and 50 m in thickness and may measure up to 12 km2. Vertical scale exaggerated and glacial basin not to scale.

Figure 8.7 Synthesis of glacitectonic styles in relation with the ice margin (dashed line). A = undeformed foreland: dump end moraine, outwash fan, B = steeply inclined structures: concentric style folds, reverse faults, C = overturned and recumbent structures: thrusts, folds, D = nappes, E = extensional structures: deformation tills and other subglacial tills, Ec = compressive structures in E style terrain: stacked till sheets, Ee = strong extension in E style terrain: erosional features, tunnel valleys, overdeepened glacial basins. E-type structures are particularly widespread in the extensive till plateaux well inside the glacial margins. Till on top of the thrust moraine is not shown. It may consist of strongly deformed sediments from the thrust moraine or deformed sediments from stratigraphically lower levels subcropping in the overdeepened glacial basin (e.g. Dammer Berge, Van der Wateren, 1995).

Figure 8.7 Synthesis of glacitectonic styles in relation with the ice margin (dashed line). A = undeformed foreland: dump end moraine, outwash fan, B = steeply inclined structures: concentric style folds, reverse faults, C = overturned and recumbent structures: thrusts, folds, D = nappes, E = extensional structures: deformation tills and other subglacial tills, Ec = compressive structures in E style terrain: stacked till sheets, Ee = strong extension in E style terrain: erosional features, tunnel valleys, overdeepened glacial basins. E-type structures are particularly widespread in the extensive till plateaux well inside the glacial margins. Till on top of the thrust moraine is not shown. It may consist of strongly deformed sediments from the thrust moraine or deformed sediments from stratigraphically lower levels subcropping in the overdeepened glacial basin (e.g. Dammer Berge, Van der Wateren, 1995).

B. High-angle structures. Jura-style concentric and box folds with vertical and steeply dipping axial surfaces. Thrusts are rare and steeply dipping, with small offsets. Minimal horizontal tectonic shortening.

C. Low-angle structures. Strongly asymmetric, overturned and recumbent folds. Low-angle thrusts. Medium shortening.

D. Nappes. Extensive horizontal and relatively thin thrust sheets. Maximum shortening. Nappes may contain a number of high-angle folds and reverse faults.

E. (Deformation) tills. Boudinage, boudinaged folds and folded boudins, subhorizontal shear planes, transposed foliation. Extremely high shear strain and horizontal extension. Subdivided into:

Ec (compression), comprising stacked till sheets and other compressive structures and Ee (extension), with strong erosion of the substratum, comprising overdeepened (tunnel) valleys, drumlin fields and megaflutes.

Styles B, C and D represent terrains of horizontal compression, with increasing longitudinal strain from B to D, whereas E is characterized by strong horizontal extension as a result of progressive simple shear. As the flow rate in a subglacial deforming bed decreases towards the margin, compressive structures may be expected to occur in tills, which are deposited close to the margin. Hence the division of E-style terrains in a purely extensional zone and a compressive zone, Ee and Ec, respectively.

Terrains may be expected to show different overprinting relationships if they are produced by either an advance, or a readvance during a general glacial retreat. Two sequences may be distinguished (Fig. 8.8):

Figure 8.8 Overprinting of glacitectonic styles in A) advance sequence, and B) readvance sequence during retreat. Open triangles are older till, black triangles are younger till associated with the formation of the thrust moraine. Dashed lines indicate ice sheet limits. Using simple overprinting relationships makes it possible to distinguish moraine lines produced during a glacial advance from those produced during retreat.

1. Advance sequence (Fig. 8.8a), in which style E overprints compressive structures B, C, D and undeformed foreland, A. Boulton (1987) argued that drumlins cored by coarse-grained sediments may be the result of overriding and streamlining of glacial outwash (i.e. tectonic style E overprinting A (E/A)). Many drumlins in northwestern Germany originated as thrust moraines, which have subsequently been overridden (Stephan, 1987). The Rehburg thrust moraines in western Germany are overlain by deformation till and have been streamlined by the overriding ice sheet. Those in the northern part of The Netherlands have been pushed and subsequently overridden after a temporary stagnation of the glacial advance (Rappol et al., 1989; Van den Berg and Beets, 1987). These are examples of style E/(B,C,D).

2. Readvance sequence (Fig. 8.8b). When a readvance occurs during a general retreat, tills and outwash dating from the last glacial advance may be incorporated in thrust moraines. They may be quite rare, as they are susceptible to erosion by meltwater and overriding ice. Thus B, C and D overprint E style structures. The Lamstedt thrust moraine in western Germany is an example of (C,D)/E.

Overprinting of glacitectonic styles can be observed on all scales, from the map scale to outcrop and microscopic scale, and may be a useful tool to unravel advance and retreat sequences. Overprinting relations provide a means to relatively date successive deformation episodes. This has proved to be successful in the Lamstedt and Wilsum thrust moraines, west Germany, and the adjoining area in The Netherlands (van Gijssel, 1987; Kluiving, 1994; Kluiving et al., 1991).

Figure 8.9 shows the different map patterns ensuing from overprinting of different glacitectonic styles. These allow the identification of stable ice margins, advance and retreat sequences. A stable ice-sheet margin, or an episode of long-term stagnation during either an advance or retreat (Fig. 8.9a), is associated with dump end moraines (style A, usually large outwash fans) downstream of the overridden thrust moraine (E/D, E/C). A stable ice-sheet margin is also indicated by a line of B-, C- and D-style thrust moraines, which have not been overridden (Fig. 8.9b). An advance sequence (Fig. 8.9c) shows overridden thrust moraines (E/D, E/C) upstream from the advance limit, which is indicated by patterns such as in Fig. 8.9a and 8.9b. A readvance sequence (Fig. 8.9d) is indicated by stacked sheets of till (Ec) deposited during the previous advance or, alternatively, by a thrust moraine containing till from the previous advance.

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