Formation of subglacial till under transient bed conditions: deposition, deformation, and basal decoupling under a Weichselian ice sheet lobe, central Poland

Abstract

A multi-proxy approach involving a study of sediment architecture, grain size, grain roundness and crushing index, petrographic and clay mineral composition, till fabric and till micromorphology was applied to infer processes of till formation and deformation under a Weichselian ice sheet at Kurzetnik, Poland. The succession consists of three superposed till units overlying outwash sediments deformed at the top. The textural characteristics of tills vary little throughout the till thickness, whereas structural appearance is diversified including massive and bedded regions. Indicators of intergranular bed deformation include overturned, attenuated folds, boudinage structures, a sediment-mixing zone, grain crushing, microstructural lineations, grain stacking and high fabric strength. Lodgement proxies are grooved intra-till surfaces, ploughing marks and consistently striated clast surfaces. Basal decoupling by pressurized meltwater is indicated by undisturbed sand stringers, sand-filled meltwater scours under pebbles and partly armoured till pellets. It is suggested that the till experienced multiple transitions between lodgement, deformation and basal decoupling. Cumulative strain was high, but the depth of (time-transgressive) deformation much lower (centimetre range) than the entire till thickness (ca 2 m) at any point in time, consistent with the deforming bed mosaic model. Throughout most of ice overriding, porewater pressure was high, in the vicinity of glacier floatation pressure indicating that the substratum, consisting of 11 m thick sand, was unable to drain subglacial meltwater sufficiently.

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

“…Understanding the behaviour of subglacial meltwater systems is crucial because of their influence upon substrate rheology and ice-bed coupling (Boulton & Hindmarsh, 1987;Iverson et al, 1995;Boulton, 1996;Piotrowski et al, 2004Piotrowski et al, , 2006Evans et al, 2006;Kjaer et al, 2006;Lee & Phillips, 2008;Boulton et al, 2009), and in-turn, glacier dynamics that operate over a range of temporal and spatial scales (Kamb, 1987;Bartholomew et al, 2010;Sundal et al, 2011;Robel et al, 2013). Research now recognises that these processes act to drive the expansion, break-up and collapse of major ice streams and ice masses (MacAyeal, 1993;Clark, 1994;Tulaczyk et al, 2000;Bell et al, 2007;Stokes et al, 2007;Burke et al, 2012) thus linking subglacial drainage to collapsing ice masses, sea-level change and abrupt climate change (Goezler et al, 2011;King et al, 2012;Hanna et al, 2013;Fürst et al, 2014 and references therein).…”

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

“…The shape of the sand struc tures in the con tact zone in di cates that the wa ter flow took place through a sys tem of in ter con nected, very slight cav i ties or in slightly larger dif fuse ca nals. The ge netic con nec tion of lam i nar struc tures of sorted de pos its in basal till with subglacial wa ter flows has been proved by many au thors (Brown et al, 1987;Piotrowski and Kraus, 1997;Tulaczyk, 1999, Munro-Stasiuk, 2000;Fuller and Murray, 2000;Wysota, 2002Wysota, , 2007Piotrowski et al, 2006;Lesemann et al, 2010). Af ter the flow had stopped, the rate of the ef fec tive basal pres sure increased so much that the strength of the ice-bed cou pling for a short time again be came higher than the strength of the subglacial de pos its.…”

“…This type of sorted sed i ments within the till has of ten been in ter preted in dif fer ent ways (e.g., Ruszczyñska-Szenajch, 1987;Benn and Ev ans, 1996;Piotrowski et al, 2006). In this case it seems most prob a ble that this de posit was con nected with subglacial drain age.…”

“…The li thol ogy of these sedi ments and the basal pore-wa ter pres sure, which af fected the shear strength of the de pos its of the bed and the strength of the ice-bed cou pling, were di rectly re lated (see Kamb et al, 1985;Iverson et al, 1994Iverson et al, , 1995Piotrowski et al, 2004;Ev ans et al, 2006). The subglacial wa ter pres sure de pended on the rate of melt wa ter sup ply, but also on the granulometry of the sed i ments, which in flu enced the per me abil ity of the ice sheet sub stra tum (Boulton and Jones, 1979;Boulton et al, 2001;Piotrowski et al, 2004;Rob erts and Hart, 2005;Piotrowski et al, 2006). In the study area, the im pact of the sub stra tum li thol ogy on the subglacial hy drau lic con di tions was, how ever, not al ways straight for ward.…”

“…It has become clear from nu mer ous stud ies that the sub soil of the Scandi na vian Ice Sheet was not able to drain the to tal mass of wa ter through the sed i ments at the base in the form of ground wa ter flows (Piotrowski, 1997;Piotrowski et al, 2001Piotrowski et al, , 2006. The most ef fec tive drain age took place through spe cific drain age sys tems, mainly large tun nel val leys (cf.…”