Ribbed moraines formed during deglaciation of the Icelandic Ice Sheet : implications for ice-stream dynamics

Abstract

Investigations of the geomorphological fingerprints of palaeo-ice streams are essential for enhancing our understanding of ice-stream behaviour. Cross-cutting flowsets of palaeo-ice streams, during and following the Last Glacial Maximum, have been suggested in northeast Iceland based on the mapping of streamlined subglacial bedforms (SSBs). To increase our understanding of the dynamics of the Icelandic Ice Sheet (IIS) during deglaciation, we investigate transverse ridges, together with glacifluvial and ice-marginal landforms, within the largest flowset. The main emphasis is on the transverse ridges, which are primarily interpreted as ribbed moraines – the first to be described in Iceland. Morphological data are combined with sedimentological analyses of the ribbed moraines. Our results show that the ribbed moraines are composed of pre-existing material, often exhibiting a base of stratified glacifluvial sediments draped with subglacial till. Deformation and thrust structures within both units suggest compressional flow during the formation of the ridges. Our results suggest the ribbed moraines develop due to velocity gradients under the lateral shear margin of an ice stream based on their distribution and often oblique orientation in relation to SSBs. The ribbed moraines superimpose and thus post-date the SSBs, signifying the waning stage of ice streaming. We suggest that the ridges formed during ice-stream shutdown following the Younger Dryas and/or Preboreal re-advances. Eskers and ice-marginal landforms imply channelized water drainage during the final deglaciation interrupted by short-lived re-advances. This study has implications for the style of deglaciation associated with ice streaming in the northeastern part of the IIS. We characterize the role of ribbed moraines within the ice sheet and identify directions for future work.