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

Abstract

The sliding of an ice mass over its bed represents one of the main mechanisms for the forward motion of glaciers and ice sheets, facilitated by the periodic introduction of meltwater along the ice-bed interface or regelation of the overriding ice. In the geological record, however, physical evidence of this process having occurred beneath former ice masses is apparently limited. This paper presents the results of a detailed micromorphological study of thinly stratified subglacial tills exposed at two sites: (i) Galmis in Switzerland and (ii) Plumpe Farm, near Gretna in SW Scotland. The stratification within these tills comprises alternating layers of massive to weakly foliated diamicton and variably deformed (folded, faulted) laminated silt and clay. Microtextural and microstructural evidence is interpreted in terms of repeated phases of basal sliding as the ice overrode a soft-sediment bed. Elevated meltwater contents/pressures encountered immediately prior to, and during basal sliding promoted localised liquefaction within the bed. Decoupling of the ice mass from its bed enabled the injection of the liquefied diamicton along the ice-bed interface and/or into the laminated sediments immediately adjacent to this boundary. The laminated silts and clays record the settling out of fines (clay, silt) from meltwater trapped along the ice-bed interface after an individual phase of basal sliding has ceased. Injection of till into the locally water saturated silts and clays resulted in partial liquefaction and incomplete mixing of these finegrained sediments with the diamicton. Density contrasts between the two liquefied sediments led to the development of a complex 'vinaigrette-like' texture comprising rounded to irregular till pebbles within a matrix of variably homogenised silty clay. Recoupling of the ice with its bed led to localised folding and thrusting within the laminated silts and clays, hydrofracturing and injection of a network of sand-filled veins, and the imposition of a variably developed clast microfabric in the diamicton layers. Analysis of the clast microfabrics indicates that the intensity of these fabrics is highly variable reflecting the variation in the intensity of deformation imposed by the overriding ice.

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

“…[ACCEPTED MANUSCRIPT] flow associated with fluctuating porewater availability (Phillips and Merritt 2008;Phillips et al, 2013b). Hydrofractures provide clear evidence for the passage of over-pressurised meltwater through glacial environments and are thought to be mainly developed in ice-marginal, sub-marginal to subglacial settings where the overburden pressure exerted by the ice leads to the required periodic over-pressurisation of the hydrogeological system (van der Meer et al, 1999Meer et al, 2009Roberts et al, 2009;Phillips et al, 2013b;Phillips & Hughes, 2014). This suggests that Lithofacies C was probably developed in an ice-marginal to proglacial setting with the periodic build-up of the hydrostatic pressures possibly reflecting a seasonal (spring-summer) increase in meltwater production.…”

“…Loading and consolidation of the sediment pile is indicated by the presence of syn-sedimentary extensional faulting (Sites A and B) that variably truncate Lithofacies B and C. This includes a kettle-hole (Site C) formed by the melting of an isolated block of buried ice. Subsequent fault reactivation as hydrofracture systems provide clear evidence for variable and fluctuating groundwater conditions and periodic over-pressurisation of the local hydrological system leading to the rapid expulsion of water from the substrate (Phillips & Merritt 2008;Phillips et al, 2013b). Hydrofracture systems have been widely recognised within subglacial to ice-marginal and proglacial settings (van der Meer et al, 1999(van der Meer et al, , 2009Roberts et al, 2009) and are important regulators of the hydrogeological system in glacial areas (Boulton et al, 1993).…”

Response of salt structures to ice-sheet loading: implications for ice-marginal and subglacial processes

“…The more rapid decay of permafrost in the vicinity of salt structures may increase the drainage capacity of the substratum and thus increase the ice-bed coupling. Increased ice-bed coupling slows down the sliding ice sheet (Narloch et al, 2013;Szuman et al, 2013) and favours glacitectonic thrusting at the ice margin (Van der Wateren, 2002;Passchier et al, 2010;Phillips et al, 2013). On a regional scale, the influence of the thermal conditions above salt structures will probably have a greater influence on the behaviour of an ice sheet than the localised obstacles formed by rising salt structures.…”

Micromorphology of modern tills in southwestern Spitsbergen – insights into depositional and post-depositional processes

“…The inclusion of micromorphology in till research has contributed to a better understanding of till deposition conditions and the identification of a number of factors that influence the deposition mode: glacier velocity, water content, clay content, etc. (Khatwa and Tulaczyk 2001;van der Meer et al 2003;Menzies et al 2010;Phillips et al 2013). In addition to qualitative analyses, quantitative micromorphological analyses have recently been developed (e.g., Phillips et al 2011;Zaniewski and van der Meer 2005;Tarplee et al 2011;Narloch et al 2015).…”