Mega deposits and erosive features related to the glacial lake Nedre Glomsjø outburst flood, southeastern Norway

Abstract

Abstract In this paper we present a suite of erosional remnants, mega deposits and subtle bar morphology that we relate to the outburst flood from the glacial lake Nedre Glomsjo at the end of the last Ice Age. By using large datasets of airborne LiDAR data implemented in a geographic information system (GIS), we have mapped flood related features along the Glomma and Vrangselva rivers in southeastern Norway. The unprecedented overview of the valley reaches obtained by the vegetation-free LiDAR-derived digital elevation models (DEM) has revealed a set of hitherto undocumented landforms. Persisting erosive lines – indicators of the uppermost flooded level – are carved into surficial deposits in the hillsides and are found as high as 80–90 m above the modern valley floor. By using the indicators as an upper flood boundary, we have computed cross-sectional profiles showing that the flood in some reaches inundated more than 120 000 m2 of the valley. Large, streamlined bed forms, which we interpret as flood bars, drape sections of the valley floor, some several kilometers long. The most morphologically striking – pendant bars – are developed behind flood flow projections, such as bedrock knolls or in lee of a valley bend. Flood bars occur in the entire study area, but are more widespread in the north and generally decrease in size moving in a downstream direction. Kettle holes and ice-block obstacle marks from icebergs arrested during the flood are common. These features support the theory of a catastrophic drainage event, but also indicate a pattern of differential erosion and deposition that allowed us to interpret palaeoflow on individual bars. Vast aeolian dune fields in the region are interpreted as a secondary product of the flood, as deposits related to the event were mobilised by northerly winds momentarily after the flood waned. The dune fields cover an excess of 50 km2 and reveal that the region was a highly active periglacial desert after the flood. Our mapping highlights the outburst flood's role as a landscape-defining event. Morphological evidence determine the southern fringe of the Scandinavian ice sheet to c. 15 km north of Elverum at the time of the outburst flood. From calibrating existing 14C dates we postulate an age of c. 10–10.4 cal ka BP for the Nedre Glomsjo drainage event and the contemporaneous margin of the ice sheet.