Structures Developed in Response to

SUBGLACIAL DEFORMATION

The developing discussion within the field of glacial research regarding deforming beds and their possible widespread occurrence (e.g. Boulton, 1987, 1996a; Boulton and Hindmarsh, 1987; Hart and Boulton, 1991; Hart and Roberts, 1994; Benn and Evans, 1996; Boulton et al., 1996; Hart et al., 1996, 1997; Piotrowski et al., 1997, 2001; Piotrowski and Tulaczyk, 1999; Van der Wateren, 1999, 2002b; Van der Wateren et al., 2000; O Cofaigh and Evans, 2001a, b; Stephan, 2002;) calls for reliable criteria to distinguish subglacially deformed from undeformed sediments.

Where meltwater drains subglacially, deformation of the bed is the most likely mechanism to produce tills in areas such as the Northern European Plains, which are underlain by unlithified and weakly lithified sediments (Boulton, 1996a). Identifying the products of subglacial deformation thus remains vitally important in the reconstruction of glacial landsystems. The simplest model to describe subglacial deformation is progressive simple shear (Van der Wateren et al., 2000). This describes the deformation history of most subglacial tills with sufficient accuracy, while the geometry of its fabric is relatively easy to understand in terms of the model. In reality, both simple and pure shear (flattening and horizontal compression) usually act together. Compaction by dewatering and ice loading are examples of shear plane normal compression that result in volume loss. Flow within the deforming bed is generally not uniform. Large boulders or lenses of stratified sediment that move relatively slowly tend to have sectors of compressed sediment on their upstream side and extended sectors on their downstream side. Shear plane parallel/subhorizontal compression occurs at the ice-sheet margin, leading to stacking of till sheets. Although far from uncommon, these departures from the ideal simple shear model are minor compared with the vast amounts of shear strain in subglacial shear zones. Usually they do not present problems for the interpretation of shear zone structures. For a comprehensive treatment of the kinematics of subglacial shear zones and their structural characteristics, see Van der Wateren et al. (2000).

The genesis of laminated diamicts is the most controversial issue in the deforming bed debate. Layers, lenses and boudins of sorted sediments interbedded with diamict are common features of

Figure 8.9 Examples of map patterns of glacitectonic styles indicating stable ice sheet margins, advance and retreat sequences. A) Stable ice sheet margin (arrows) located along the dump end moraines (A) downstream of the overridden thrust moraine (E/D, E/C). B) Stable ice sheet margin was located along the (B)-, (C)- and (D)-style thrust moraines which had not been overridden. C) Advance sequence. Overridden thrust moraines ((E/D), (E/C)) upstream from the advance limit (arrows). D) Readvance sequence (arrows: limit of readvance). Stacked sheets of till (Ec) deposited during the previous advance, which has its limit further downstream. See Fig. 8.19 for an application of this method of mapping and identifying different glacial margins.

Figure 8.9 Examples of map patterns of glacitectonic styles indicating stable ice sheet margins, advance and retreat sequences. A) Stable ice sheet margin (arrows) located along the dump end moraines (A) downstream of the overridden thrust moraine (E/D, E/C). B) Stable ice sheet margin was located along the (B)-, (C)- and (D)-style thrust moraines which had not been overridden. C) Advance sequence. Overridden thrust moraines ((E/D), (E/C)) upstream from the advance limit (arrows). D) Readvance sequence (arrows: limit of readvance). Stacked sheets of till (Ec) deposited during the previous advance, which has its limit further downstream. See Fig. 8.19 for an application of this method of mapping and identifying different glacial margins.

subglacial tills. They produce a lamination, which may sometimes be confused with sedimentary layering. In this context, intraclasts of stratified and sorted sediment are commonly seen as evidence of subglacial meltwater discharge (Menzies and Shilts, 2002). Where these can be correlated with footwall sediments that are older than the diamict, subglacial melt-out and/or water flow cannot account for the laminations. In these, indeed very common, cases subglacial deformation of the sediments together with the diamict is the most obvious explanation (Van der Wateren, 1995, 1999, 2002a; Van der Wateren et al., 2000).

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