Glacio-isostatic age modelling and Late Weichselian deglaciation of the L€ ogurinn basin, East Iceland HREGGVIÐUR NORÐDAHL ,  OLAFUR ING  OLFSSON, ERLA D. VOGLER, BENEDIKT  O. STEINGR  IMSSON AND  ARNI HJARTARSON Norðdahl, H., Ing olfsson,  O., Vogler, E. D., Steingr  ımsson, B.  O. & Hjartarson,  A. 2019 (July) : Glacio-isostatic age modelling and Late Weichselian deglaciation of the L€ ogurinn basin, East Iceland. Boreas, Vol. 48, pp. 563–580. https://doi.org/10.1111/bor.12366. ISSN 0300-9483. Knowledge of the glaciation of central East Iceland between 15 and 9 cal. ka BP is important for the understanding of the extent, retreat and dynamics of the Icelandic Ice Sheet. Crucially, it is not known if the key area of Flj otsdalur-  Uth erað carried a fast-flowing ice stream during the Last Glacial Maximum; the timing and mode of deglaciation is unclear; and the history and ages of successive lake-phases in the L€ ogurinn basin are uncertain. We use the distribution of glacial and fluvioglacial deposits and gradients of former lake shorelines to reconstruct the glaciation and deglaciation history, and to constrain glacio-isostatic age modelling. We conclude that during the Last Glacial Maximum, Flj otsdalur-  Uth erað was covered by a fast-flowing ice stream, and that the L€ ogurinn basin was deglaciated between 14.7 and 13.2 cal. ka BP at the earliest. The Flj otsdalur outlet glacier re-advanced and reached a temporary maximum extent ontwo separate occasions, duringthe Younger Dryas andthe Preboreal.In the Younger Dryas, about 12.1 cal. ka BP, the outlet glacier reached the Tjarnarland terminal zone, and filled the L€ ogurinn basin. During deglaciation, a proglacial lake formed in the L€ ogurinn basin. Through time, gradients of ice-lake shorelines increased as a result of continuous but non-uniform glacio-isostatic uplift as the Flj otsdalur outlet glacier retreated across the Valþj ofsstaður terminal zone. Changes in shoreline gradients are defined as a function of time, expressed with an exponential equation that is used to model ages of individual shorelines. A glaciolacustrine phase of Lake L€ ogurinn existed between 12.1 and 9.1 cal. ka BP; as the ice retreated from the basin catchment, awholly lacustrinephase of Lake L€ ogurinn commenced and lasted until about 4.2 cal. ka BP when neoglacial ice expansion started the current glaciolacustrine phase of the lake. Hreggviður Norðdahl (hreggi@hi.is) and  Olafur Ing olfsson, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjav  ık IS-101, Iceland; Erla D. Vogler, The Icelandic Road and CoastalAdministration, Miðh usavegur 1, Akureyri IS-600, Iceland; Benedikt  O. Steingr  ımsson, Mannvit hf, Urðarhvarf 6, K opavogur IS-203, Iceland;  Arni Hjartarson, Iceland GeoSurvey (ISOR), Grens asvegur 9, Reykjav  ık IS-108, Iceland; received 17th November 2017, accepted 9th November 2018. At the Last Glacial Maximum (LGM) the Icelandic Ice Sheet (IIS) reached the edge of the Iceland shelf, more than 100 km off North and East Iceland, and the deglaciation, at least partly controlled by rising global sea level, was extremely rapid and marked by a collapse of the marine- based sections of the IIS between 15.0 and 14.7 cal. ka BP (e.g. Norðdahl et al. 2008; Norðdahl & Ing olfsson 2015; P etursson et al. 2015; Patton et al. 2017). Subsequently, there were two re-advances that terminated in the Younger Dryas and Preboreal (c. 12.1 and c. 11.2 cal. ka BP, re- spectively), separated by a regression and subsequent transgression of relative sea level (RSL) (Norðdahl et al. 2008; P etursson et al. 2015). Reviews of the Late Weich- selian glacial history of Iceland (e.g. Norðdahl et al. 2008; Geirsd ottir et al. 2009; P etursson et al. 2015) show that there is a scarcity of well-constrained data highlighting the glacial history of East Iceland. During the LGM, Vopnafj€ orður and Flj otsdalur-  Uth erað in northeast Ice- land (insert map in Fig. 1) were possibly occupied by fast- flowing ice streams and outlet glaciers (Norðdahl 1983; Sigbjarnarson 1983; Sæmundsson 1995; Bourgeois et al. 1998; Hubbard et al. 2006; Patton et al. 2017). The deglaciation of East Iceland is not well known, as the few studies that have been carried out mainly focused on post-Younger Dryas deglaciation and asso- ciated shoreline displacements (Norðdahl & Hjort 1995; Sæmundsson 1995; Richardson 1997; Norðdahl & Einarsson 2001). Patton et al. (2017) modelled a rapid marine deglaciation from 21.8 to 15 cal. ka BP with a sudden Bølling warming that forced an onshore ice-sheet collapse. The coastal areas of  Uth erað were ice-free after c. 17 cal. ka BP, and by 13.2 cal. ka BP Flj otsdalur was ice-free (Patton et al. 2017). Their model recognizes a Younger Dryas glacier re-advance, with increased ice-flow velocities towards the coast in Vopnafj€ orður and  Uth erað (insert map in Fig. 1) prior to 11.7 cal. ka BP. The chronology of glacial oscillations during the deglaciation of East Iceland in general and Flj otsdalur-  Uth erað in particular is not well constrained, and the pattern of ice retreat, glacier configuration and dynam- ics of outlet glaciers/ice streams draining the IIS is poorly understood. The aim of this study is to elucidate the deglaciation historyof the Flj otsdalur-  Uth erað area of Iceland (Fig. 1) in order to better understand the dynamics of the Late Weichselian IIS and its deglaciation pattern. This is also essential for providing boundary conditions for glacio- logical modelling of the IIS, an increasingly important tool for synthesizing data and testing palaeoglaciological reconstructions. DOI 10.1111/bor.12366 © 2018 Collegium Boreas. Published by John Wiley & Sons Ltd