Multiple subglacial till deposition: A modern exemplar for Quaternary palaeoglaciology

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The sedimentology of a vertical succession of alternating beds of massive and fissile diamictons on a 6Þorisjökull plateau icefield outlet foreland is employed to assess the evolution of subglacial traction tills 7 at the margins of active temperate glaciers with deformable substrates. Lodged boulders display strong 8A-axes and surface striae alignments which parallel surface flutings, indicating that fluting construction 9 and till emplacement was related to moulding by consistent glacier flow from the SSW during the Little 10 Ice Age. In contrast, clast macrofabrics at the sub-boulder size, not unlike those reported from other 11Icelandic tills, are not as strong as would be expected in a subglacially sheared medium, indicating shear 12 strains too low for a steady state strain signature. This separation of fabric data has isolated the strain 13 signatures of the lodgement and deformation components of subglacial traction till, whereby the 14 orientations of the largest, lodged clasts record high cumulative shear strains and those of the sub-15 boulder sized clasts record greater susceptibility to deformation of their enclosing matrix. This is likely 16 due to the effect of clast collisions in clast rich till and the perturbations set up by the numerous large 17 boulders, consistent with observations on till fabrics in flutings and around lodged clasts.

Sub-annual moraine formation at an active temperate Icelandic glacier

“…As argued in preceding sections, these shape characteristics are consistent with active transport of the clasts in the subglacial traction zone (e.g. Boulton, 1978;Benn and Ballantyne, 1994;Lukas et al, 2013;Evans et al, 2016bEvans et al, , 2018a. This initial interpretation of clast shape data is substantiated by comparison of the moraine samples with the control envelopes on covariance plots of both RA-C 40 and RWR-C 40 : the moraine samples plot closest to the subglacial/till control envelopes, and they are clearly distinguishable from the fluvial and supra/extraglacial control samples ( Figure 10).…”

Skaftafellsjökull, Iceland: glacial geomorphology recording glacier recession since the Little Ice Age

“…Evans et al 2016;Evans & Hiemstra, 2005). These features were overridden and fluted by the glacier during its advance to the Little Ice Age maximum but were not significantly modified and hence, during later ice recession, they exerted a topographic control on the routing of glacial meltwater and the deposition of glacifluvial outwash in snout-parallel linear sandar tracts.…”

Submarginal debris transport and till formation in active temperate glacier systems: The southeast Iceland type locality

“…Evans 2000;Evans & Twigg 2002;Lukas et al 2013). In contrast, the quarrying of fresh blocks from bedrock protuberances that bridge the subglacial deforming layer can introduce anomalously angular material to downglacier till deposits (Evans et al 2016b).…”

“…In the Icelandic setting, till sedimentology associated with active temperate glaciers has been reconciled with subglacial observations on deforming substrates (cf. Boulton & Hindmarsh 1987;Benn 1995) but has been, and is increasingly being related to more localized conditions associated with substrate inheritance/till overprinting (Evans 2000;Evans & Twigg 2002;Evans et al 2016b), glacitectonic disturbance and clastic dyke intrusion Evans & Twigg 2002;Le Heron & Etienne 2005), push moraine formation (Sharp 1984;Boulton 1986;Chandler et al 2016a, b) and seasonal changes to sub-marginal thermal regimes (Krüger 1993(Krüger , 1994(Krüger , 1996Evans & Hiemstra 2005). From this research we now appreciate that the subglacial to sub-marginal footprints of former glacier margins, in the Icelandic setting represented by the sediment-landform imprints of the recently deglaciated Little Ice Age forelands, record an integrated signature of till production by temperate glacier processes.…”

“…Specifically, despite the development of a "steady state fabric" (S 1 eigenvalues > 0.78) at shear strains of 7-30 in laboratory experiments (Iverson et al 2008), field sampling of Icelandic till fabrics yields relatively weak S1 eigenvalues ranging from 0.44-0.74 even though a bed deformation origin implies that shear strains should be in excess of 100. By separating out the macrofabrics of unequivocally lodged boulders, which are predictably strong (0.77-0.81), from the more weakly aligned sub-boulder sized clasts in subglacial traction tills, Evans and Hiemstra (2005) and Evans et al (2016b) have demonstrated that the weaker S1 eigenvalues likely reflect perturbation of the deforming matrix and smaller clasts in the leeside pressure shadows of the boulders (cf. Kjaer & Krüger 1998;Carr & Rose 2003).…”