Geomorphology and sedimentology of features formed at the outlet during the final drainage of the Baltic Ice Lake

• Published in: Boreas 2022-01, Vol.51 (1), p.20-40
• Main Authors: Johnson, Mark D., Öhrling, Christian, Bergström, Adrian, Dreyer Isaksson, Oskar, Pizarro Rajala, Emma
• Format: Article
• Language: English
• Subjects: agassiz; Bars; basin; Bed load; Cambrian; climate-change; debris flows; deglaciation; deposits; Drainage; Floods; Floodwater; Floodways; Fysisk geografi; Geologi; Geology; Geomorphological mapping; Geomorphology; Grain size; hyperconcentrated flows; Ice; Icebergs; Lakes; middle; mt-billingen; outburst floods; Physical Geography; Quaternary; Sediment; Sediment transport; Sedimentology; Sediments; Tunnels
• ISSN: 0300-9483; 1502-3885
• DOI: 10.1111/bor.12547

We present a conceptual model for the character of the 11.6 cal. ka BP drainage of the Baltic Ice Lake in the outlet area based on a new geomorphological map as well as new sedimentological information. We combine this new information with a review of the previous work on this well‐studied area. The model includes the premise of remnant ice in the Lången valley as suggested by previous workers and indicates the drainage began subglacially. Cobbly sediment on northeast Billingen formerly interpreted as beach sediment is interpreted to be a subglacial lag deposit. The ridge at Timmersdala is composed of drainage sediment from the initial phases of the drainage, deposited in a subglacial tunnel that emptied out on to central Klyftamon. Continued drainage widened the tunnel by frictional melting, eventually lifting and fracturing the ice. Pendant bars, expansion bars and boulder sheets on central and southern Klyftamon formed during the drainage event. Cobbly and bouldery drainage sediment in the bars, which is massive and poorly sorted, was deposited as bedload from sediment‐charged water. Imbrication, bar orientation, and changes in grain size and Cambrian‐clast content indicate flow to the west–northwest. Most sediment in the bars was derived from erosion on Klyftamon, but Cambrian rock from Billingen is present, delivered directly by the flood waters and/or from debris‐rich icebergs from the broken‐up remnant ice. This flood event is distinguished from other Quaternary megafloods by being sediment‐supply limited and having a short, subaerial floodway (20 km).