Sand dunes and valley fills from Preboreal glacial‐lake outburst floods in south‐eastern Norway – beyond the aeolian paradigm

Abstract

AbstractA major glacial‐lake outburst flood in the Glomma valley, south‐eastern Norway, took place during the final decay of the Scandinavian Ice Sheet. A combined morphological, geophysical and sedimentological study provides new insight into the variety of processes and deposits of the flood. The studied succession, some tens of metres in thickness, comprises the fill of a major flood basin that developed during hydraulic ponding. Large‐scale sand dunes and bars accumulated downstream of locations with expanding flow. Most notable are 10 m high, concentric dune ridges that accumulated downstream of a topographical constriction hosting a high‐velocity flow. Flow expansion at the outlet generated intense turbulence and scouring. The sand‐loaded eddies helped feeding the semi‐stationary dune ridges that grew vertically and downflow under high aggradation rates. Internal structures vary but reflect an overall shift in sedimentation from prevailing supercritical flow to overall subcritical flow conditions at high flood levels. Loading by water and sediment caused large‐scale, synsedimentary deformation, increasing local accommodation space. Fast‐falling flood levels caused stronger flow locally, while mud and fine sand settled in stagnant pools. The fall caused a significant drop in hydrostatic pressure. This triggered a release of excess pore pressure causing massive dewatering and fluidisation. Water‐escape structures include numerous (sub)vertical pipes. The present study shows that outburst flood‐generated, large‐scale dunes can develop in well‐sorted, fine sand and are thereby easily confused with aeolian deposits. Several dune fields in south‐eastern Norway are here reinterpreted as the product of at least two major flood events. Sandy dune fields with similar characteristics elsewhere in Scandinavia should likely also be reinterpreted, and the role of outburst floods during the final deglaciation of Scandinavia has seemingly been underestimated. The study emphasises the importance of ponding, flow expansion, sorting, rapidly changing pressure conditions and deformation for outburst flood‐related sedimentation.