Summary Landform Sediment associations of the Southern Scandinavian Ice Sheet

The morphology and distribution of landforms of the areas glaciated by the Scandinavian Ice Sheets are strongly controlled by the structure of the substratum. This effect is most prominent for the distribution of thrust moraines, which for their formation depend on the availability of a suitable décollement: usually Cretaceous, Tertiary or early Pleistocene clays or Tertiary lignites. These units of low shear strength, shallow near the margins of the sedimentary basins of the Northern European Plains and along salt diapirs or fault blocks. Coupling of the glacial stress field to the foreland is promoted by coarse-grained sediments in proglacial fans or in ice-marginal drainage systems, which subsequently make up the bulk of the thrust masses, sliding on a finegrained décollement layer. This explains why the largest thrust moraines form chains parallel to the boundaries of the Variscan highlands and major neotectonic structures. In view of the large influence of structural and palaeogeographical conditions on their formation, we must be careful when using thrust moraines to reconstruct stable ice sheet margins.

The majority of the tills in the area outside the Scandinavian shield consists of reworked locally derived sediments with a relatively minor component of far-travelled erratic material. Subglacial tills are usually characterized by a fabric indicating high-strain simple shear, characterized by recumbent structures, which are strongly attenuated in the direction of shearing (= ice flow direction). In an ideal section, tills show an upward increase in finite shear strain and therefore of sediment transport distance. The lower portion of a till may be almost entirely made up of local sediment, grading from an undeformed footwall to a laminated till, which represents a glacitectonic lamination (transposed foliation) rather than sedimentary layering. The next higher unit, the more massive appearing part of the till, nearest to the former glacier sole, has undergone the highest amount of shear strain and contains the most far-travelled material. The resulting petrographic layering may resemble a stratigraphic sequence of different tills. Combining a careful structural analysis with petrographic and sedimentological observations helps to avoid this pitfall.

Thick till sequences forming plateaux inward of the major end moraines indicate temporary stagnation of the ice margin during an advance or a readvance during glacial retreat. Where thrust moraines are absent (lacking the necessary preconditions for their formation), till plateaux are rimmed by stacked till sheets produced by marginal compression. These may line up with dump end moraines and proglacial alluvial fans, thus reliably indicating a stable ice-sheet margin.

Landform-sediment associations, which can be used for regional mapping and palaeo-glaciological reconstructions, show a characteristic sequence of glacitectonic styles. From the foreland to the interior of an ice sheet these are:

A. undeformed foreland, including dump end moraines and outwash fans overlying older sediments

B. high-angle structures: Jura-style concentric folds, box folds and steeply dipping reverse faults; minimal horizontal glacitectonic 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

E. (deformation) tills; boudins, boudinaged folds and folded boudins, subhorizontal shear planes, transposed foliation; extremely high finite shear strain and horizontal extension; includes 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.

Thrust masses B, C and D are unconformably overlain by glacifluvial, glacideltaic and glacilacustrine sediments that show varying degrees of deformation, depending on when, during the evolution of the thrust moraine, they had been emplaced.

A sequence of glacitectonic styles that formed during a glacial advance shows overprinting of style E (tills) over styles B, C and D. In a readvance sequence styles B, C and D overprint style E in tills from the previous advance. These overprinting relationships thus offer a reliable method to relatively date successive advances and readvances. They can be identified on all scales, from the regional to outcrop and microscopic scale. Eskers are relatively rare in the area outside the Scandinavian shield. They are confined to areas where bedrock is shallow and where the glacial bed cannot accommodate large meltwater fluxes.

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