Distribution of ribbed moraines and their connection to subglacial water - in the Oppland region of Norway

Abstract

The origin of ribbed moraines (Rogen moraine) is a long-discussed topic in the field of geomorphology. As per today, no consensus on their formation has been reached apart from a subglacial origin. This study will test the hypothesis that water presence in the form of water-saturated till or possibly a film of stagnant water under the ice sheets of the last glaciations may have caused the formation of ribbed moraines. This hypothesis, suggested in a similar form by Sollid and Sørbel (1994), has not been investigated or tested so far. The hypothesis is investigated in two steps. Firstly, ribbed moraine fields and other subglacial landform fields were mapped across the Oppland Region of Norway using new LiDAR elevation data. In a smaller area around the Langsua National Park, a detailed map and a field trip were completed to further investigate the morphology of ribbed moraines and their spatial relations to other landforms. Secondly, a model of the subglacial hydrology was constructed for the region. The model computes areas of low hydraulic gradient, possible subglacial lake positions and theoretical flow paths of subglacial streams at high resolution. High-resolution output is achieved using present-day topography data in conjunction with down-scaled ice sheet thickness data from an ice sheet reconstruction model. Mapping of subglacial landforms shows a clear continuum between ribbed moraines and hummocky bedforms across the study area, with indications for an extended continuum including glacial lineations in some locations. Glaciofluvial channels were found entering and exiting ribbed moraine fields. A widely quoted map of the ribbed moraines distribution in Southern Norway was deemed incomplete. The constructed model of subglacial hydrology was found to deliver a resolution highly suitable for interpretation with only limited and easily identifiable limitations. Modelled flow paths correspond well with flow directions interpreted from glaciofluvial traces. The modelled hydraulic gradient shows that ribbed moraine fields are consistently found in areas of low hydraulic gradient throughout the study area. This indicates that these results indicate that ribbed moraine formation is triggered by water presence at the glacial bed in the form of water-saturated till. This study suggests that water presence may facilitate instabilities at the till-ice interface, forming ribbed moraines and hummocky bedforms. Larger amounts of water are expected to have flowed at low velocity in a spread-out flow pattern between the formed ribbed moraines and hummocky bedforms during the deglaciation.