Microstructural evidence of low‐strain, time‐transgressive subglacial deformation

Abstract

Microstructural analyses were used to investigate the formation of a macroscalemassive till at Knud Strand in Denmark. More than 100 thin sections were examined and microstructures mapped and counted for quantitative comparison and interpretation. Microstructures indicative of both brittle (grain lineations, edge-to-edge crushed grains) and ductile (turbate structures) deformation are evenly distributed in vertical profiles through the till, suggesting that strain contributed to its formation. Discrete shears (grain lineations and plasmic fabric) probably accommodated most deformation, whereas rotational deformation was less prominent. The microshear geometry fits the predicted Coulomb-Mohr failure criterion, indicating that till behaves as a plastic material. Strain estimate of ca. 10 1 from micromorphological proxies is two-three orders of magnitude lower than expected if the till was subjected to pervasive deformation. A hybrid of lodgement and time-transgressive deformation is envisaged as the till-forming processes. Our data suggest that even abundant evidence of microscale deformation at continuing high levels of strain may only record the latest process of deposition and deformation and therefore not fully reflect the complexity of till genesis.

Reinterpreting Rotherslade, Gower Peninsula: implications for Last Glacial ice limits and Quaternary stratigraphy of the British Isles

“…We employ here clast fabric analysis (see, for example, Benn, 1994Benn, , 2004Evans et al, 2007) and micromorphology (see, for example, van der Meer, 1993;Hiemstra, 2006), a combination with proven success in differentiating between glacigenic sediments representing different depositional environments (see, for example, Hiemstra et al, 2005;Evans et al, 2006a;Larsen et al, 2004Larsen et al, , 2007.…”

Basal till and subglacial conditions at the base of the Upper Odra ice lobe (southern Poland) during the Odranian (Saalian) Glaciation

“…In a later model, soft subglacial beds were pre sented as a mo saic of de form ing and sta ble spots; the in ten sity of de for ma tion var ied in both time and space . Con cepts of spa tially var ied glacier bed char ac ter is tics have been de scribed also by other authors (see Knight, 2002;Van der Meer et al, 2003;Larsen et al, , 2007Stokes et al, 2007;Narloch et al, 2012Narloch et al, , 2013Tylmann et al, 2013). The dif fer ences in as sess ment of the role and scale of subglacial de for ma tion are largely due to the dif ferent rhe o log i cal mod els for the be hav ior of basal till, i.e.…”

Micromorphological evidence of liquefaction, injection and sediment deposition during basal sliding of glaciers

“…No obvious strain markers are present in the diamicton. Several workers have attempted to use the relative abundance of selected microstructures, such as microshears and grain stacks, as a proxy for strain in subglacial diamictons (Larsen et al, , 2007Narloch et al, 2012). However, the development of such structures can be strongly lithologically controlled (e.g.…”

“…Micromorphology is increasingly being used by glaciologists and Quaternary geologists as a primary tool for the analysis of subglacial sediments as the technique can provide far greater detail of the depositional and deformation histories recorded by these sediments than can be obtained from macro-scale studies alone (see Menzies and Maltman, 1992;van der Meer, 1997;Menzies et al, 1997;Khatwa and Tulaczyk, 2001;van der Meer et al, 2003;Roberts and Hart, 2005;Hiemstra et al, 2005;Evans et al, 2006;Baroni and Fasano, 2006;Larsen et al, 2006Larsen et al, , 2007Neudorf et al, 2013). In particular, this approach has been used to unravel the often complex deformation histories recorded by glacigenic sequences (van der Meer, 1993;Phillips and Auton, 2000;van der Wateren et al, 2000;Menzies, 2000;Phillips et al, 2007;Lee and Phillips, 2008;Denis et al, 2010;Vaughan-Hirsch et al, 2012;Narloch et al, 2012Narloch et al, , 2013 as well as investigate the role played by pressurised melt water during these deformation events (Hiemstra and van der Meer, 1997;Phillips and Merritt, 2008;van der Meer et al, 2009;Denis et al, 2010;Phillips et al, 2012;Narloch et al, 2012Narloch et al, , 2013.…”