Under pressure: clastic dykes in glacial settings

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

“…At the first site, site A (4m, Figure 4), the stratified silts and sands of Lithofacies B are cut by a 1.8m long steeply-inclined (35-52° south), upwards tapering fracture (AF1) filled by weakly laminated to massive reddish brown (2.5Y 4/2) sand and dark greyish brown (2.5Y 4/2) sandy silt (Figure 7a). The stratification within this fill occurs parallel to sub-parallel to the margins of the fracture, consistent with either the pulsed injection of sediment-laden fluid upwards through the fracture, or repeated reactivation of this hydrofracture system (van der Meer et al, 1999Meer et al, , 2009Phillips & Merritt, 2008;Phillips et al, 2012). The margins of the hydrofracture vary from sharp to diffuse; the latter potentially resulting from the infiltration of water and sediment into the adjacent walls of the fracture.…”

“…[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.…”

“…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). The behaviour of glacial hydrogeological systems is known to vary 17 Lee, J.R., Wakefield, O.J.W., .…”

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…Hydrofracturing and injection of water or of fluidised sediments occur when the fluid pressure within the sediment, or at the glacier/sediment interface, exceeds the sediment shear strength. Clastic dykes, but also pipes or sills, are common attributes of the Quaternary glacial record and various types have been recognised and interpreted (Dreimanis and Rappol, 1997;Le Heron and Etienne, 2005;Phillips et al, 2008;Denis et al, 2009;Van der Meer et al, 2009;Clerc et al, 2012;Phillips and Lee, 2013;Knight, 2015): down-tapering dykes vs. upward propagating dykes, sand dykes vs. conglomeratic wedges, either connected or not to the glacier/sediment interface. Similar examples are known from the prePleistocene glacial record (Jones and Fielding, 2008), especially those related to the end-Ordovician ice sheets (Ghienne, 2003;Deynoux and Ghienne, 2004;Denis et al, 2010;Douillet et al, 2012).…”

A new ‘microstructural mapping’ methodology for the identification, analysis and interpretation of polyphase deformation within subglacial sediments

“…Micromorphology is increasingly being used by glaciologists and Quaternary geologists as a primary tool for the analysis of diamictons deposited in different sedimentary environments (van der Meer 1987;Harris 1998;Lachniet et al, 1999;Carr et al, 2000;Carr 2001;Lachniet et al, 2001;Menzies and Zaniewski 2003;Carr et al, 2006;Phillips 2006;Menzies et al, 2006;Reinardy and Lukas 2009;Kilfeather et al, 2010), as an aid to understanding the processes occurring beneath glaciers (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;Baroni and Fasano 2006;Larsen et al, 2006;Hart 2007;Larsen et al, 2007), to *Manuscript Click here to view linked References help unravel the often complex deformation histories recorded by glacigenic sequences (van der Meer 1993;Phillips and Auton 2000;van der Wateren et al, 2000;Phillips et al, 2007;Lee and Phillips 2008;Denis et al, 2010), and investigate the role played by pressurised pore-water/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). In the majority of these published studies the terminology used to describe the various microtextures in thin section follows that proposed by van der Meer (1987van der Meer ( ,...…”