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

• Published in: Boreas
• Main Authors: LINDÉN, MATTIAS, MÖLLER, PER, ADRIELSSON, LENA
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2007
• Subjects: Ice Sheet; Northern Sweden
• Online Access: http://dx.doi.org/10.1111/j.1502-3885.2007.00002.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1502-3885.2007.00002.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1502-3885.2007.00002.x

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.