Till deposition by glacier submarginal, incremental thickening

Abstract

Macro-and micro-scale sedimentological analyses of recently deposited tills and complex push/squeeze moraines on the forelands of Icelandic glaciers and in a stacked till sequence at the former Younger Dryas margin of the Loch Lomond glacier lobe in Scotland are used to assess the depositional processes involved in glacier submarginal emplacement of sediment. Where subglacial meltwater is unable to flush out subglacial sediment or construct thick debris-rich basal ice by cumulative freeze-on processes, glacier submarginal processes are dictated by seasonal cycles of refreezing and melt-out of tills advected from up-ice by a combination of lodgement, deformation and ice keel and clast ploughing. Although individual till layers may display typical A and B horizon deformation characteristics, the spatially and temporally variable mosaic of subglacial processes will overprint sedimentary and structural signatures on till sequences to the extent that they would be almost impossible to classify genetically in the ancient sediment record. At the macro-scale, Icelandic tills display moderately strong clast fabrics that conform to the ice flow directions documented by surface flutings; very strong fabrics typify unequivocally lodged clasts. Despite previous interpretations of these tills as subglacial deforming layers, micro-morphological analysis reveals that shearing played only a partial role in the emplacement of till matrixes, and water escape and sediment flowage features are widespread. A model of submarginal incremental thickening is presented as an explanation of these data, involving till slab emplacement over several seasonal cycles. Each cycle involves: (1) late summer subglacial lodgement, bedrock and sediment plucking, subglacial deformation and ice keel ploughing; (2) early winter freeze-on of subglacial sediment to the thin outer snout; (3) late winter readvance and failure along a decollement plane within the till, resulting in the carriage of till onto the proximal side of the previous year's push moraine; (4) early summer melt-out of the till slab, initiating porewater migration, water escape and sediment flow and extrusion. Repeated reworking of the thin end of submarginal till wedges produces overprinted strain signatures and clast pavements.

Multiple subglacial till deposition: A modern exemplar for Quaternary palaeoglaciology

“…Evans & Hiemstra 2005). Macrofabric measurements of the dip and azimuth 72 (orientation) of the A-axis and A/B plane of clasts were taken using a compass clinometer, aiming to use 73 predominantly clasts in the range of 30-125 mm (A-axis length) to allow comparison with other studies 74 (Benn 1994a al.…”

“…This further enabled Benn 42 (1995) to assign clast macrofabric signatures as well as textural characteristics to the different styles of 43 subglacial sediment deformation. Similarly, Evans and Hiemstra (2005) reported sedimentological and 44 clast macrofabric data collected from sub-marginal till wedges that had been stacked to produce a 45 complex push moraine during a period of glacier readvance in southern Iceland during the 1990s. Both 46 studies have proposed potentially diagnostic sedimentological criteria for the identification of subglacial 47 processes such as brittle and ductile deformation and lodgement.…”

Snæfellsjökull volcano-centred ice cap landsystem, West Iceland

“…Localized exposures through the push ridges reveal deformed stratified sediments indicative of recent glacier readvance into proglacial outwash, potentially driven by the mid to late 1990s positive mass balance trends recognized throughout Iceland (e.g. Bennett & Evans, 2012;Bradwell, Dugmore, & Sugden, 2006;Evans & Hiemstra, 2005;Evans, Shand, & Petrie, 2009, 2015b.…”

Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK

“…This is because of the critical role subglacial meltwater plays in regulating the dynamics of ice masses, ice-marginal behaviour and associated hydrogeological systems within subglacial, marginal and proglacial areas (Boulton & Hindmarsh, 1987;Piotrowski et al, 2004Piotrowski et al, , 2006Evans & Hiemstra, 2005;Evans et al, 2006;Robinson et al, 2007;Boulton et al, 2009;Bartholomew et al, 2010). Put simply, subglacial meltwater deposits are the product of a hydrological regime that is not unique to subglacial environments, with similar regimes documented from a range of other sedimentary environments including intertidal (Cheel & Middleton, 1986), fluvial (Cheel, 1990, Røe, 1987Alexander et al, 2001) and channelised turbidity currents (Ito & Saito, 2006).…”

“…Changes are driven by standard processes that control groundwater systems, plus processes that operate within glacial environments including seasonal variations in temperature and meltwater flux, glacier surges and outburst floods (Robinson et al, 2007). Several studies have demonstrated that such groundwater variability can exert a significant influence on subglacial and ice-marginal processes (Sharp, 1984;Krüger, 1994;Evans & Hiemstra, 2005) and this could have facilitated -at least locally, an ice-marginal readvance and deposition of the upper diamicton. This diamicton has been attributed elsewhere around the Irish Sea Basin with enhanced rates of basal sliding associated with the Late Weichselian Irish Sea Ice Stream (Ó Cofaigh & Evans, 2001a,b;Phillips et al, 2010).…”