Ribbed moraine formed by subglacial folding, thrust stacking and lee‐side cavity infill

Abstract

Transverse‐to‐iceflow ribbed moraine occurs in abundance in the coastal zone of northern Sweden, particularly in areas below the highest shoreline (200–230 m a.s.l.), but occasionally also slightly above. Based on detailed sedimentological and structural investigations of machine‐dug sections across five ribbed moraine ridges, it is concluded that these vertically and distally prograding moraine ridges were formed as a result of subglacial folding/thrust stacking and lee‐side cavity deposition. The proximal part of the moraines (Proximal Element) was formed by subglacial folding and thrust stacking of sequences of pre‐existing sediments, whereas the distal part (Distal Element) was formed by glaciofluvial and gravity‐flow deposition in lee‐side cavities. The initial thrusting and folding is suggested to be a result of differences in bed rheology at the ice‐marginal zone during the early or late melt season, and that generated a compressive zone transverse to ice flow as a result of a more mobile bed up‐glacier compared to a less mobile bed down‐glacier. It is considered that the lee‐side cavities were formed as a result of ice‐bed separation on the distal slope of the thrust/fold‐created obstruction. The lee‐side cavities formed an integral part of a subglacial linked‐cavity drainage network regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal iceflow velocity. The proximal and distal elements of the ribbed moraine ridges are erosively cut and/or draped with a consistently more homogeneous deforming bed till (Draping Element) marking the final phase of ribbed moraine formation considered to be contemporaneous with De Geer moraine formation further down‐flow at the receding ice‐sheet margin.

Morphological characteristics, formation and glaciological significance of Rogen moraine in northern Scotland

“…Since the late 1970s, four main mechanisms for Rogen moraine formation have been proposed: (i) cavity infilling of megaripples eroded into basal ice during subglacial outburst floods (Fisher and Shaw, 1992;Shaw, 2002); (ii) remoulding of preexisting landforms such as ice marginal moraines (Möller, 2006), or drumlins and flutes following a ~ 90° change in iceflow direction under deforming bed conditions (Boulton, 1987); (iii) shearing and stacking or folding of debris-rich basal ice or preexisting sediments during compressive ice flow (Shaw, 1979;Aylsworth and Shilts, 1989;Bouchard, 1989;Lindén et al, 2008); and (iv) fracturing and extension of sediment sheets during a transition from cold-to warm-based ice conditions (Lundqvist, 1969;Hättestrand, 1997;Hättestrand and Kleman, 1999;Sarla, 2006). This range of hypotheses currently restricts confidence in palaeoglaciological reconstructions based on any one mechanism of formation (e.g., Kleman and Hättestrand, 1999).…”

Instability modelling of drumlin formation incorporating lee-side cavity growth

“…Two other forms are noteworthy. The first is ribbed moraine, which takes the form of approximate two-dimensional parallel ridges aligned transverse to the direction of ice flow (Dunlop & Clark 2006;Lindén et al 2008). The second is that of mega-scale glacial lineations (MSGLs) that are extremely long sinuous ridges aligned parallel to the ice flow, which are thought to have underlain former ice streams (Clark 1993).…”

An instability theory for the formation of ribbed moraine, drumlins and mega-scale glacial lineations

“…Indeed, apart from the HindmarshFowler instability theory, a number of alternative explanations have been put forward to explain ribbed moraine, drumlins and MSGLs (e.g. [19][20][21][22][23][24][25][26]). An early review of drumlin formation theories is that of Menzies [27], and a more recent one is that of Clark [28].…”