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

Abstract

A 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. Waterescape structures include numerous (sub)vertical pipes. The present study shows that outburst flood-generated, large-scale dunes can develop in wellsorted, 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.

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

“…In subaerial settings upper‐flow‐regime bedforms generally record high‐discharge events and/or high discharge variations during the melt season (Kjær et al ., 2004; Pisarska‐Jamroży & Zieliński, 2014), jökulhlaups (Burke et al ., 2008, 2010a,b; Duller et al ., 2008; Girard et al ., 2015) or the catastrophic drainage of proglacial lakes (Johnsen & Brennand, 2004; Winsemann et al ., 2011, 2016; Lang & Winsemann, 2013; Hansen et al ., 2020). High rates of aggradation under supercritical flows in subaerial glacigenic deposits allow for the more complete preservation of bedforms in thicker successions than those in other types of fluvial systems (Alexander & Fielding, 1997; Fielding et al ., 2010; Froude et al ., 2017).…”

“…Flow conditions during glacial lake‐outburst floods are dominantly subcritical, due to the great flow depths. Supercritical flow conditions will commonly be attained due to local flow constrictions and shallow flows over valley margins or substratum obstacles (Carling et al ., 2009; Alho et al ., 2010; Bohorquez et al ., 2016; Winsemann et al ., 2016; Lang et al ., 2019; Hansen et al ., 2020).…”

“…Deposits of cyclic steps, chutes‐and‐pools and antidunes have been documented from successions related to glacial lake‐outburst floods and jökulhlaups (Figs 3, 5 and 6) (Duller et al ., 2008; Burke et al ., 2010a,b; Winsemann et al ., 2011; Lang & Winsemann, 2013; Le Heron et al ., 2013; Girard et al ., 2015; Winsemann et al ., 2016; Hansen et al ., 2020). Successions related to supercritical flows accumulated in the lee of glacitectonic ridges by a Pleistocene glacial lake‐outburst flood were studied by Lang & Winsemann (2013) and Winsemann et al .…”

Timing and maximum flood level of the Early Holocene glacial lake Nedre Glomsjø outburst flood, Norway

“…A suite of large bar bedforms, palaeochannels, sandy bedforms and other distinct geomorphological traces have been mapped in the flood‐inflicted area south of the ice sheet (Longva & Thoresen 1991; Longva 1994; Høgaas & Longva 2016; Hansen et al . 2020). A distinct, light‐coloured silt bed encountered in downstream trenches and basin records near Romerike was proposed to have been deposited during the outburst flood (Longva 1984; Longva & Bakkejord 1990; Longva & Thoresen 1991).…”

“…The outburst flood deposited several large bedforms, such as coarse‐grained pendant bars and sand dunes, and formed contiguous palaeochannels and erosive scarps in till on its course south (Longva 1994; Høgaas & Longva 2016; Hansen et al . 2020). The outburst flood also carried large icebergs far downstream from the collapsed ice margin, which in turn produced delicate plough marks and ice‐block obstacle marks as the icebergs eventually were stranded when the flood level decreased (Longva & Thoresen 1991; Høgaas & Longva 2016).…”