Progressive soft sediment deformation within a subglacial shear zone—a hybrid mosaic–pervasive deformation model for Middle Pleistocene glaciotectonised sediments from eastern England

Abstract

This paper presents a hybrid 'pervasive' and 'mosaic' model for the progressive deformation of a stratified diamicton complex in eastern England within an active subglacial shear zone. Sedimentary evidence from undeformed low strain zones towards the base of the sediment pile indicate that the tectonised sediments were originally deposited as a series of subaqueous flows in a glaciolacustrine basin. These sediments have subsequently been deformed during a progressive subglacial shearing event. This event is divided into three stages: ductile folding and sediment remobilisation (D1), brittle faulting in the form of Reidel shears (D2); and hydrofracturing and sediment remobilisation (D3). The dominant control on the pattern and style of deformation appears to relate to the rate of thrust-induced till accretion, and the aquifer properties and pore water content and / or pressure of the deforming sediments.

A polyphase glacitectonic model for ice-marginal retreat and terminal moraine development: the Middle Pleistocene British Ice Sheet, northern Norfolk, UK

“…The stratigraphy of this sequence is now well-established with deposits linked into a succession of advances of the British Ice Sheet across north Norfolk (Reid, 1882;Banham, 1968;Hart and Boulton, 1991a;Lunkka, 1994;Lee et al, 2004;Pawley, 2006). The repeated advances of ice across the area has left a complex glacitectonic overprint on the preexisting sediment pile, informally referred to as the 'Contorted Drift' (Reid, 1882;Slater, 1926; Dhonau and Dhonau, 1963;Banham, 1975), with deformation having occurred in proglacial, submarginal and subglacial settings (Banham, 1966(Banham, , 1975(Banham, , 1977aHart, 1990;Hart and Boulton, 1991b;Fish et al, 2000;Roberts and Hart, 2005;Hart, 2007;Lee and Phillips, 2008;Phillips et al, 2008;Burke et al, 2009;Vaughan-Hirsch et al, 2013; Fleming et al, this volume; Phillips and Lee, this volume). A detailed tectonostratigraphy erected for the north Norfolk area has revealed a succession of structural events which have been correlated with repeated ice advances across the region (Banham, 1988a;Phillips et al, 2008;Lee et al, 2011a).…”

“…The Bacton Green Till was deposited by an earlier oscillation (A4) of the North Sea lobe of the British Ice Sheet into north Norfolk and consists of a complex sequence of subglacial and subaqueous diamictons (Lunkka, 1994;Lee and Phillips, 2008). As the margins of the North Sea lobe of ice retreated northwards, north Norfolk was crossed by a lobe of British ice that crossed the region broadly from west to east (Perrin et al, 1979;Ehlers et al, 1987;Scheib et al, 2011).…”

“…As the margins of the North Sea lobe of ice retreated northwards, north Norfolk was crossed by a lobe of British ice that crossed the region broadly from west to east (Perrin et al, 1979;Ehlers et al, 1987;Scheib et al, 2011). This A5 advance led to the development of a variably pervasive glacitectonite which partly remobilised the underlying Bacton Green Till, and in places deposited a highly chalky till of western provenance, called the Weybourne Town Till (Dhonau and Dhonau, 1963;Banham and Ranson, 1965;Banham, 1966;Ehlers et al, 1987;Hart, 2007;Lee and Phillips, 2008;Phillips et al, 2008). The highly ductile soft-sediment style of deformation indicates that pore-water content between A4 and A5 was largely maintained, suggesting that the two events followed in quick succession .…”

“…Each till is separated by variable thicknesses of glacilacustrine silts, clays and sands including the Mundesley Sand Member which crop-out between the Walcott and Bacton Green Till members. The sediment pile was then overridden and disrupted by ice advancing from the west/southwest (A5) leading to the formation of a glacitectonic mélange (Banham, 1965;Banham and Ranson, 1965;Lee and Phillips, 2008;Phillips et al, 2008; Fleming et al, this volume; Phillips and Lee, this volume), and the local deposition of a chalk-rich diamicton (Perrin et al, 1979; Ehlers et al, 1987; Ehlers et al, 1991;Pawley, 2006;Scheib et al, 2011). Recent work by Waller et al (2009) andWaller et al (2011) has suggested that an extensive permafrost developed prior to/during A5, with overriding of this 'warm permafrost' having influenced conditions within the bed of the ice sheet and the style of subglacial deformation.…”

Subglacial till behaviour derived from in situ wireless multi-sensor subglacial probes: Rheology, hydro-mechanical interactions and till formation

“…However, once the importance of pore-water pressure on subglacial processes was demonstrated (Boulton and Jones, 1979;Alley et al, 1986;Clarke, 1987), it was suggested that lodgement and deformation till were actually part of a pore-water pressure continuum (Brown et al, 1987;Hart and Boulton, 1991;Hicock, 1992;Hicock and Dreimanis, 1992;Iverson et al, 1995), which varies temporally and spatially in the form of mobile beds and sticky spots (Alley, 1993;Piotrowski et al, 2004;Stokes et al, 2007Stokes et al, , 2009Smith and Murray, 2009). Deformation within this layer has been modelled as a shear zone (Hart and Boulton, 1991;Scherer et al, 2004;van der Meer et al, 2003;Lee and Phillips, 2008).…”