The Deforming Bed Characteristics of a Stratified Till Assemblage in North

ARTICLE IN PRESS

Quaternary Science Reviews 24 (2005) 123–140

The deforming bed characteristics of a stratiﬁed till assemblage in north East Anglia, UK: investigating controls on sediment rheology and strain signatures David H. Robertsa,*, Jane K. Hartb a Department of Geography, Environmental Research Centre, University of Durham, Science Site, South Road, Durham DH1 3LE, United Kingdom b Department of Geography, University of Southampton, Highﬁeld, Southampton SO17 1BJ, United Kingdom Received 24 June 2003; accepted 18 March 2004

Abstract

The glacial coastal exposures of north Norfolk are a type site for subglacial glaciotectonic deforming bed sediments. This investigation of the lower stratified diamict within the North Sea Drift at West Runton reveals two distinct lamina types. Type 1 laminae are the product of primary extensional glaciotectonism, with ductile, intergranular pervasive shear predominating over brittle shear. Type 2 laminae also exhibit structures that can be attributed to ductile, intergranular pervasive shear and brittle shear, but the lateral continuity of Type 2 laminae and the presence of dropstone—like structures supports a primary subaqueous origin with secondary subglacial deformation. When coupled with micromorphological analysis, these findings show that ductile, viscous creep mechanisms control sedimentary architecture, and that ‘shear stratification’ in particular, has the potential to affect the rheological properties of the sediment pile and the hydraulic routing of basal water, ultimately influencing critical effective pressure fluctuations and the thresholds controlling the subglacial drainage system. r 2004 Elsevier Ltd. All rights reserved.

1. Introduction most field (Boulton and Jones, 1979; Boulton and Hindmarsh, 1987; Fischer and Clarke, 1994; Iverson 1.1. Properties of the deforming bed et al., 1994) and laboratory (Iverson et al., 1997, 1998) based investigations of till deformation subscribe to a The identification and understanding of glacial plastic rheological model. Other field and laboratory deforming bed processes has revolutionised glacial experiments have suggested a viscous and viscoplastic geological theory in the past 20 years, yet the exact rheological model for till behaviour however, and recent nature of deforming bed mechanisms remain only micromorphological studies point to the presence of a partially understood (Boulton and Jones, 1979; Alley range of plastic to viscous rheological states within the et al., 1987; Blakenship et al., 1987; Boulton and deforming bed at smaller, subcentimetre, scales (van der Hindmarsh, 1987; Hart et al., 1990; Clark, 1995; Meer, 1993, 1997; Iverson et al., 1995; Carr, 1999, 2001). Hindmarsh, 1997; Murray, 1997; Iverson et al., 1999). The scale at which different subglacial deformation Of particular interest is the ‘rheological state’ of the regimes operate is a critical consideration in under- deforming bed material. Many researchers have sug- standing the rheological state of deforming bed material. gested that subglacial sediment deformation occurs in a van der Meer (1993, 1997) has described both brittle plastic manner through discrete failure and does not (plastic) and ductile (viscous), microscale rotational deform in a viscous manner at smaller scales. Indeed, deformation features within subglacial tills and has related specific features to depth within the deforming *Corresponding author. Tel.: +44-191-334-1935; fax: +44-191-334- bed. This approach suggests that the lower parts of the 1801. deforming layer are characterised by drier, more brittle E-mail address: [email protected] (D.H. Roberts). conditions, while the upper part of the bed deforms via

0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.03.004 ARTICLE IN PRESS 124 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 intergranular, rotational, ductile shear induced by more It is the latter that forms the basis of the research saturated conditions. Others have also described micro- presented here. scale structures relating to brittle and ductile, inter- From a sedimentological standpoint, the North Sea granular shear conditions (Menzies and Maltman, 1992; Drift (NSD) has been interpreted as a glacioterrestrial Menzies and Woodward, 1994; Menzies and van der facies, with glaciotectonic (Banham, 1977, 1988; Hart, Meer, 1998), with Menzies et al. (1997), for example, 1990; Hart et al., 1990; Hart and Boulton, 1991a ,b; describing folds, boudins, shadows, rotations, and Kwatwa and Tulaczyk, 2001), glaciolacustrine and squeeze and diffusion structures as ductile in nature, glaciofluvial (Lunnka, 1988; Hart, 1992; Lunkka, 1994; while brittle deformation is characterised by faults and Lee, 2001) activity producing a structurally complex shear planes/zones. Clearly, there is widespread evidence sequence of glaciogenic deposits. Although Eyles et al. for both viscous and plastic deformation regimes (1989) re-interpreted the sediments as glaciomarine in occurring in close proximity over short distances within origin, Hart and Roberts (1994) clearly demonstrated the deforming bed. that the macroscale, sedimentary, architectural signa- Boulton et al.’s (2001) recent review of the controls on ture of the NSD, and in particular the Laminated till rheology highlights the importance of subglacial Diamict facies typical of West Runton, was that of a water pressure and till granulometry in till deformation. deforming bed environment. The effects of large clasts within the deforming bed and their ability to transmit strain through interlocking, 1.3. Research aims ploughing, and rotational movement deep in to the subglacial substrate (even at low effective pressures) This paper develops the subglacial model proposed by point to and support ductile/viscous sediment behaviour Hart and Roberts (1994) with specific reference to the (Tulaczyk, 1999). In contrast, the deforming layer is microscale signature found within the Laminated likely to be thinner within fine grained sediments, Diamict facies at West Runton and the mechanisms by whether it behaves plastically or viscously (Boulton which deformation and rheological contrasts have et al., 2001), as smaller grains cannot transmit strain as controlled sediment architecture. In particular, it far through the sediment medium. Thus, although there attempts to: (i) determine the mechanisms responsible is widespread evidence that till has plastic rheological for stratification within the Laminated Diamict facies, properties, it can also behave in a quasi-viscous fashion (ii) interpret the strain signature within the Laminated due to local lateral and vertical variations in its hydaulic Diamict facies, (iii) understand the influence of the regime (drainage, porewater pressure, effective pressure) subglacial substrate on the effective pressure system, (iv) and granulometry through time (Hindmarsh, 1997). investigate the nature of sediment entrainment along the lower deforming bed decollement! surface, and (v) understand the influence of shear stratification on the 1.2. Deforming bed environments during the Anglian rheological state of the till and the effective pressure Glaciation system.

The glacial deposits of northeast Norfolk have traditionally been interpreted as Anglian in age (MIS 2. Field description 12) and were deposited by a number of ice sheet advances from both the North Sea basin and the British 2.1. Macroscale sedimentary characteristics mainland (West, 1980; Bowen et al., 1986; Hart and Peglar, 1990)(Fig. 1). Workers such as Hart and The NSD at West Runton is exposed along a three Boulton (1991a), Lunkka (1994), Fish et al. (2000) and kilometre coastal cliff section running approximately Fish and Whiteman (2001) support a two phase Anglian northwest to southeast (Fig. 2). Geologically, the age glaciation. More recently, Rose et al. (2000) and Lee sequence overlies Cretaceous chalk bedrock and a pre- et al. (2002) have proposed a much longer time scale, glacial sequence comprising Wroxham Crag, the Cro- four stage glacial history for north Norfolk based on mer Forest Bed and estuarine and glacioﬂuvial sands. lithostratigraphic analysis. This shows the area to be Overlying the pre-glacial sequence is the Laminated glaciated during MIS 16 (Happisburgh glaciation), MIS Diamicton facies of the NSD (Hart and Boulton, 12 (Anglian glaciation), MIS 10 (Oadby glaciation) and 1991a). It has a laterally and vertically deformed aspect, MIS 6 (Britons Lane glaciation). Hence, there is much and consists of a melange! of stratiﬁed diamict which debate as to the chronological context of the glacial envelopes large (ca 100 Â 30 m2) chalk bedrock and sand deposits in the area. In broad lithostratigraphic terms, rafts and which in turn are overlain by deformed and three distinct glacial lithofacies have been recognised; undeformed sand basins (Fig. 2). The lower junction Lowestoft Till, the Marly Drift and the North Sea Drift between the Laminated Diamicton facies and the pre- (Reid, 1882; Banham, 1968; Ehlers and Gibbard, 1991). glacial sequence is abrupt. ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 125

WWestest RRuntonunton

CCromerromer TTrr iminghamimingham ˆˆ ˆ ˆˆˆˆˆ ˆˆ ˆˆˆˆˆ ˆˆ HHappisburghappisburgh

Wens um NNorwichorwich

Ya re

Ice contact face

Esker

Conical mounds

Hummocky topography ˆˆˆˆˆˆˆ

Land over 50m

0 kilometres 200 Southern limit of Anglian glaciation

Fig. 1. Location of the North Sea Drifts, West Runton, north Norfolk, UK (Modiﬁed from Hart, 1990).

2.1.1. Sedimentary architecture instances strata and laminae originate as stringers and The section at 820 m (Fig. 2) typifies the West Runton folds from streamlined pods. Internal pod structure (e.g. sequence. At the base of the section a matrix supported, stratified sands) shows no relationship to pod shape. silt/sand, grey/brown, chalky stratified diamict truncates Attenuation (boudins) and rotational (augens) struc- the underlying sand along a sharp decollement! surface. tures are also prevalent throughout (Figs. 6 and 7). The lower part of the diamict is massive, exhibiting a In other parts of the section, laminae within the lower few discontinuous chalky laminae and occasional hard stratified diamict facies have a laterally continuous rock and chalk clasts. This diamict grades up into a aspect. At 340 m (Figs. 2 and 8) a matrix supported, silt/ variably stratified chalky diamict. In areas adjacent to sand, stratified, grey/brown diamict underlies a chalk the overlying chalk raft the diamict is intensely raft. Diamict stratification is produced by a series of laminated, with chalk laminae originating from the subhorizontal, laterally continuous, poorly sorted lami- lower surface of the raft. The chalky laminae have a nae, with dropstone-like structures, where lower laminae subhorizontal, discontinuous stringer-like aspect and are depressed beneath the overlying stone and upper are ungraded. These types of laminae/stringers are here laminae lie concentrically over them. Laminae are termed ‘‘Type 1’’ laminae and are the commonest form generally composed of variably sorted diamict, and are of laminae within diamict facies throughout the whole laterally continuous over 5–10 m before the section is section (Figs. 3 and 4). lost due to slumping. Such laminae are termed ‘‘Type There are also pods of chalk and sand in the section. 2’’. The laminae can be both continuous and discontin- Pod shape varies from tabular rafts of chalk and sand uous and exhibit brecciated, reworked, soft sediment (10s m) to small spherical and streamlined forms clasts. (o1 m). Where the surrounding diamict is stratified, Sand basins are prevalent at a number of locations laminae and strata often flow around pods sometimes along the section (Fig. 2). Typically the basins exhibit forming folds in pressure shadows (Fig. 5). In other laterally continuous, planar bedded and planar cross- ARTICLE IN PRESS 126 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140

Fig. 2. Lithofacies overview of the West Runton section. ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 127

2.1.3. Macrofabrics The macrofabric measurements at West Runton are derived from the lower parts of the Laminated Diamict Dark brown grey diamict facies. The fabrics generally show a northeast to with streamlined chalk pod + discontinuous southwest girdle distribution. Exceptions to this general sub-horizontal chalk pattern occur at 340 m, where two fabrics in Type 2 stringers diamict display a distinct northwest to southeast a-axes orientation with consistent southerly dips (Fig. 10). Isotropy values are generally very low, while elongation values range between 0.17 and 0.77. Sharp decollement

2.2. Micromorphology

Preglacial sands The samples represent the range of sedimentary architectural styles found within the Laminated Dia- micton facies. These include stratified and unstratified diamicts and a variety of deformation structures. Sample positions along the section are shown in Fig. Fig. 3. The base of the Laminated Diamict Facies, North Sea Drifts. 2. Grain size and shape is described and microfabric Note the variably stratified chalky diamict and the sharp lower contact described in terms of anisotropy; high anisotropy being to the underlying pre-glacial sands. a high degree of alignment between individual grains, low anisotropy being completely random grain orientations. The microfabric data is predominantly derived from the sand size fraction of the sediment matrix and a minimum of 30 grain orientations were counted. Porosity is expressed as low, medium or high and determined as a percentage of a field of view during optical thin section and SEM analysis. Plasmic fabrics (van der Meer, 1993) are described where visible, but are generally poorly represented due to the high calcium carbonate content of the samples.

2.2.1. Massive, unstratified diamicts Fig. 4. Type 1 chalky stratified diamict. Note the stringer-like, Sample WRX7 is a massive, silty, sandy, diamict (2– discontinuous morphology of the laminae. 500 mm; SA-SR). It is generally structureless, but does have occasional rotational structures. Fig. 11 clearly stratified beds, that are conformably up-warped with the exhibits a soft sediment clast undergoing rotational edges of the basin. The underlying diamict facies are disintegration to form a clast and tail structure. There is also conformably up-warped forming diapiric struc- also a suggestion of partial alignment of skeletal clasts tures. (Fig. 11). Microfabric anisotropy throughout the sample is generally low, as is porosity.

2.1.2. Folds 2.2.2. Stratified ‘type 1’ diamicts Folds within the lower parts of the stratified diamict Sample WRX6 is a chalky stratified diamict (Type 1). are generally isoclinal and recumbent in nature. Fig. 9 The sample is composed of a mixture of subhorizontal, illustrates a fold at 2375 m along the section, with the discontinuous, chalky (o5 mm; SR) and silty, sandy (10– main fold structure being 2–3 m in length and formed in 500 mm; SA-SR) stringers (Fig. 12). Contact boundaries chalky, stratified diamict. The core of the fold displays sharp and undulatory (Fig. 12i). The chalk stringers are ‘‘S’’-type structures on the upper fold limbs indicative of comprised of reworked chalk with secondary inclusions extension, but corresponding ‘‘Z’’ folds indicative of such as quartz and feldspar and attenuation of stringers compression are absent on the lower limbs. The main is apparent (Fig. 12ii). Isoclinal, recumbent microfolds fold has a slight upwards trend with the fold axis are also evident in pressure shadows (Fig. 12iii). Chalk striking approximately northeast to southwest. Hart clasts are both deformed and undeformed and drop- (1987) showed the general fold strike of these features to stone-like structures with draped and downwarped be approximately north-northeast to south-southwest. contact boundaries occur (Fig. 12i). Some clay/silt ARTICLE IN PRESS 128 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140

Fig. 5. Overview of the chalky stratiﬁed Laminated Diamict facies at Fig. 6. Boudin structures in Laminated Diamict facies at 320 m in the West Runton. Note the laterally discontinuous laminae, which West Runton section. originate from, and ﬂow around, streamlined chalkpods within the diamict.

Fig. 8. Close up of Type 2 laminae at 340 m. Note the lateral Fig. 7. ‘Clast and tail’ development in a chalky pod resulting from continuity and uniform aspect of the strata. rotational, extensional shear. coatings can also be seen on skeletal grains. Microfabric lenses of finer material within the sandy matrix, and anisotropy is generally low, as is porosity. subhorizontal partings similar to those seen in Zone 2. Sample ER1 is a sandy stratified diamict (Type 2) (2– 2.2.3. Stratified ‘type 2’ diamicts 600 mm; SA-SR). Individual lamina are characterised by ER3 is a crudely stratified diamict from 85 m along distinct, diamictic compositions and sharp contact the section. It has three distinct subhorizontal zones. boundaries. Lamina exhibit reworked and brecciated Zone 1 is characterised by a silty, sandy diamict (5– soft and metamorphic skeletal clasts. In some cases 500 mm; platy to SA-SR), with a low porosity and low fragments of brecciated metamorphic clasts show some microfabric anisotropy in the sand size fraction of the preferrential orientation subparallel to clast edges (i.e. matrix. There is a well-developed lattesepic plasmic skelsepic plasmic fabric; van der Meer, 1993)(Fig. 13). fabric. Zone 1 is underlain by a distinct iron stained Chalk clasts within the matrix are both deformed and horizon, which separates it from Zone 2, which has undeformed, and in some cases skeletal clasts have similar characteristics to Zone 1. There are also a asymmetric lamina drapes over their top surfaces (Fig. number of faint subhorizontal lineations within Zone 2, 13). Microfabric anisotropy throughout the sample is characterised by a lack of fine matrix. Zone 3 is generally low. Porosity is low. demarcated by a sharp, dipping boundary and char- Sample WRX2 is a silty, sandy, stratified diamict acterised by a loss of fine matrix, with the diamict (Type 2) (Fig. 14). Diamictic laminae (2–600 mm.; SA- becoming sandy (50–150 mm; SA-SR) and moderately SR) tend to be continuous in nature and exhibit sorted. As a result porosity is high, although microfabric reworked soft sediment clasts. Silty laminae (5–20 mm; anisotropy remains low. There are occasional pods and platy) have a more discontinuous, attenuated nature and ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 129

Fig. 9. Isoclinal recumbent fold structure in the Laminated Diamict facies at 2375 m. Close up of the fold at 2375 m exhibiting well developed ‘S’-type structures on upper fold limbs.

Fab 340m Fab 340m Fab 820m Fab 2735m (Upper Dms (Lower Dmu (WRX6) (WRW2) WRX2) WRX5)

Fab 2375m Fab ca. 2100m Fab 2735m Fig. 11. Sample WRX7 (Plane polarised light)(PPL)—massive silty, (WRW4) (WRW1) (WRW3) sandy diamict. Note central rotated clast and tail structure and grain alignments to the right of the micrograph. Fig. 10. Macrofabric data from West Runton. Fabric locations are shown shown in Fig. 2.

3. Discussion also exhibit reworked soft sediment textures. Contact boundaries are sharp and conformable, and dropstone 3.1. Stratification mechanisms within the laminated structures are evident (Fig. 14). Microfabric anisotropy diamict facies is low, but under cross polarised light there are some areas of high birefringence subparallel to silty lamina A subglacial deformation model has been proposed contact boundaries. Porosity is low. by a number of researchers to describe the complex Sample WRW1 is a chalky, stratified diamict (Type 2) sedimentology of the NSD (Banham, 1988; Hart et al., with sharp to indistinct contact boundaries. The laminae 1990; Hart and Boulton, 1991b). The macroscale field are discontinuous and indistinct. In the centre of the evidence described herein reinforces the most recent sample there is a sock-fold formed in a southeasterly model put forward by Hart and Roberts (1994) direction over an undeformed chalk clast (Fig. 15). The (Fig. 17). Perturbation disturbance and stringer initia- chalk clasts within the matrix are undeformed. Micro- tion occur throughout the Laminated Diamict facies. fabric anisotropy throughout the sample is generally Stringers clearly emanate from large chalk and other low, as is porosity. soft sediment point sources and often develop into Sample WRW4 is a chalky, sandy, stratified, (Type 2) isoclinal recumbent folds. Pods and rafts sediment diamict (2–500 mm; SA-SR), with interlaminated chalky within the diamict matrix are clearly deformed, attenu- and diamictic laminae forming a laterally attenuated ated and streamlined. In many cases pods and rafts form and disintegrated fold nose (Fig. 16). Microfabric boudins or syntectonically rotated augens with asso- anisotropy is medium to low, with some particle long ciated pressure shadows. Folds formed in pressure axes orientated subparallel to the fold axial plane. shadows demonstrate flow deceleration and rheological Porosity is low. heterogeneity in the lee of deforming bed perturbations. ARTICLE IN PRESS 130 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140

3.1.1. Microscale evidence of brittle and ductile shear mechanisms On the microscale, structures indicative of pure brittle shear are poorly represented, with ductile, deforming mechanisms predominating in both Types 1 and 2 laminae. Evidence for some brittle shear can be seen in sample ER1. The brecciated laminae in ER1 (Fig. 13: Zones 2a and 2b) support a brittle deformation mechanism, with metamorphic and soft sediment skeletal grains having suffered lateral, discrete shear. Evidence for the mechanical breakdown of hard rock clasts is rare at West Runton, although Hiemstra and van der Meer (1997) and Carr (1999) have described crushed grains resulting from the build up shear stress in many deforming bed examples. Ductile deformation signatures dominate the West Runton samples. The microclast and tail structure in sample WRX7 is evidence of syntectonic rotation of the clast in a ductile, shearing medium, causing attenuation of the clast and stringer initiation (Fig. 11). The skeletal grain alignments relate to the turbation of the diamict matrix during grain rotation (Fig. 18). This is analogous to the rotational/galaxy structures reported by van der Meer (1993, 1997) and Carr (1999). Further support for this is provided by the skelsepic and lattesepic plasmic fabrics in samples ER1 and ER3, where rotation of the skeletal components of the matrix has caused the preferential alignment of fine particles through the transmission of stress perpendicular to grain edges. Hence, the combination of lateral shear and rotational movement causes different sized components of the sediment matrix to respond differently to the applied stress field (cf. Roberts, 1995; Carr and Rose, 2003). The integranular, rotation of particles plays a part in the ductile ‘‘smearing’’ or ‘‘drawup’’ mechanism that can be inferred from the microstringer development in WRX6. Attenuation of the chalk matrix was caused by ductile smearing as one sediment matrix has sheared past the other (Fig. 19). For this to occur, the yield strength of the chalk matrix in WRX6 will have been significantly reduced, as chalk is highly coherent in its natural state. This is not unlikely given the saturated state of the deforming bed, and the possible freezing and thawing of the bedrock prior to glacial over-ride, as well as at the ice/bed interface. The discontinuous stringers of chalk and silt in many samples are a continuation of the deformation continuum shown in Fig. 17, whereby stringers have become detached from their sources to form isolated drawn out stringers. Microfold structures also support the existence of a lateral deformation continuum. Sample WRW1 exhibits a sock fold in its early stages of development (Fig. 15), Fig. 12. Micromorphological structures in sample WRX6 (PPL): (i) as lateral shear has caused upward attenuation in to the dropstone-like structure with a down-warped lower contact boundary, overlying sediment matrix, in a manner similar to that (ii) chalky stringers emanating from the upper contact boundary of a envisaged for stringer initiation in WRX6 (Fig. 19). The chalk lamina and (iii) microfold structure in the lee of a chalk clast. laterally attenuated fold nose in sample WRW4 is a ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 131

Fig. 13. Sample ER1 (PPL)—note the central lamina composed of a crystalline groundmass exhibiting a skelsepic plasmic fabric, the brecciated soft sediments clasts and asymmetric lamina drape.

Fig. 14. Sample WRX2 (PPL)—stratiﬁed diamict (Type 2). Note laterally continuous laminae, as well as discontinuous, attenuated stringers and dropstone-like structures denoted by laminae drapes. further development of this, with fold disintegration The spatial variability in laminae type is not resulting from further extensional strain (Fig. 16). The unexpected. High porewater pressures, combined with small fold in the lee of a clast in sample WRX6 varying lithologies, would have generated highly vari- (Fig. 12iii), points to the preservation of folds in leeside able rheological conditions. Menzies and Maltman pressure shadows indicating ﬂow deceleration and (1992) discuss the critical role of sediment saturation, rheological heterogeneity over very small distances. inferring brittle deformation to be characteristic of drier Evidence from Types 1 and 2 laminae indicate that areas within the deforming layer, while saturated areas simple shear occurred predominantly as ductile, inter- are prone to more ductile, intergranular pervasive shear. granular pervasive shear, and hence, the deformation They associate ductile, intergranular pervasive shear occurred mainly through a viscous mechanism, with zones with dilation zones, which often exhibit reworked, only limited evidence for brittle, plastic shear. rip-up soft sediment clasts. The faint subhorizontal ARTICLE IN PRESS 132 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140

Fig. 15. Sample WRW1 (PPL)—stratiﬁed diamict (Type 2). Note the sock fold deforming around a chalk clast and the undeformed nature of other chalk clasts.

Fig. 16. Sample WRW4 (PPL)—chalky, sandy stratified diamict (Type 2) exhibiting attenuated fold nose. partings associated with the loss of fine matrix and sorted, diamictic nature of each lamina also requires the improved sorting in sample ER3 provides some evidence cyclical generation of discrete diamictic sediment that dilation was occurring in conjunction within packages within the deforming layer. Such strata must intergranular shear. then undergo extensional, lateral deformation over tens of metres in order to produce intact subhorizontal 3.1.2. A subaqueous signal? laminae. The pseudo-planar, bedded nature of the Despite exhibiting a number of ductile deformation strata, with their sharp contact boundaries are however, structures, the subhorizontal, lateral continuity of Type characteristic of underflows and subaqueous debris 2 laminae and dropstone-like structures that exhibit flows, with each lamina representing a discrete deposi- both down-warped lower contacts and draped upper tional event (Visser, 1983a ,b; Visser et al., 1984; contacts (Figs. 14 and 20), support a primary subaqu- Gravenor, 1985). eous origin with secondary subglacial deformation for Type 2 laminae also exhibit subrounded, reworked these sediments. Moreover, the lateral continuity of soft sediment clasts and poorly developed microfabric Type 2 lamina over tens of metres is difficult to visualise orientations that are often reported from underflow in a subglacial glaciotectonic model, as other evidence sediments, which are derived from mixtures of sus- from West Runton and other glaciotectonic sites (e.g. pended sediment load and eroded secondary bed load Menzies and Maltman, 1992), points to lateral breccia- (Visser et al., 1984; Gravenor, 1985). Visser et al. (1984) tion of tectonically formed strata. The distinct, variably describe well defined boundaries, rip-up structures, ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 133

Fig. 19. Stringer development resulting from the ductile, intergranular ‘drawdown’ of a chalk stringer in sample WRX6.

Fig. 17. The subglacial deformational continuum as observed at West Runton (from Hart and Roberts, 1994).

Fig. 20. Dropstone structure in Type 2 laminae exhibiting both downwarped contact and draped upper contact.

could also be derived from suspension rainout. The lack of other definitive in situ debris flow criteria such as interbedded, graded strata and localised, compressional Syntectonic rotational shear of soft sedimental clast folds restricted to distinct horizons (Visser et al., 1984), suggests that the conditions associated with Type 2 Skeletal grain alignment caused by rotational Ductile intergranular shear lamina deposition remained fairly constant, although turbation reworked soft sediment clasts in other samples do Fig. 18. Syntectonic rotational deformation of a soft sediment clast exhibit graded, sorted, rhythmic laminae suggesting and preferential alignment of skeletal grains in its ‘wake’ caused some localised variablility in subaqueous conditions through rotational turbation. Sample WRX7. prior to secondary deformation. The dropstone-like structures exhibit morphology microboudins and thin shear laminae within the basal typical of ice rafted dropstones, with downwarped lower shear zones of debris flows and these are certainly laminae and draped upper contacts (Figs. 14 and 20). evident in some Type 2 samples such as WRX2 (Fig. 14), Possible tectonic relationships, such as pierced or although secondary subglacial deformation has also slightly down-warped overlying laminae (Roberts, played a role. Fine laminae such as those in WRX2 1995), are not observed, although assymetic drapes ARTICLE IN PRESS 134 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 may well be a function of secondary deformation Alternatively, if the clast fabric is relict, ice could have (Fig. 13). The characteristics of these sediments and arrived from the northeast first and the northwest later. their lack of glaciomarine microfossils suggest they are The strain signal within the sediment is also related to glaciolacustrine in origin, as has been previously the marginal position of the ice sheet. During its suggested (Hart, 1990; Lunkka, 1994; Roberts, 1995). maximum southerly extent subglacial environments beneath the ice sheet at West Runton would have been 3.2. The strain signature with the Laminated Diamict experiencing highly extensional lateral shear imparting a facies high strain signal to the sediments. In contrast, during a later period associated with the formation of the Cromer In the light of a predominant northeast to southwest Ridge push moraine complex (1–2 km down ice of West fold axes strike direction within the lower Laminated Runton, Fig. 1), marginal subglacial ice sheet locations Diamict facies, it can be assumed that the final would have been experiencing a lower strain, compres- depositional/deformational mechanism occurred as a sional regime (Hart, 1990). This latter scenario would result of ice moving from the northwest. Thus, most of correlate well with the low-medium strain fold signal the clast fabrics have a girdle distribution transverse to within the Laminated Diamict facies. final ice flow (Fig. 10). Following the rationale of Hart The variable nature of the subglacial substratum et al. (1990) this sedimentary strain signal suggests the between advances is also critical. The first ice advance Laminated Diamict facies suffered only a low to into the area deposited a deformation till over the pre- medium deformational strain. If strain had been higher glacial sequence of estuarine and glaciofluvial sands, the stratified and folded form of the diamict would have although there may have been areas of glaciolacustrine been partially obliterated resulting in a more homo- sediments. Conversely, following initial retreat, subse- geneous, massive diamict. In contrast, the clast fabric quent ice advances moved over a very different substrate eigenvalues could be used to suggest a medium to high with a patchwork of outwash sands, glaciolacustrine cumulative shear strain history (S1=0.6–0.65; and deformation diamicts. As a result, the subglacial S3=0.08–0.15) (Benn, 1994; Hart, 1994). effective pressure system will have been significantly Such a discrepancy in the strain signals could be due to different between advances. a number of factors. Firstly, and most simply, the transverse fabric could be the result of low to medium strain 3.3. The influence of the subglacial substrate on the sediment attenuation and folding as ice moved in from the effective pressure system northwest. Alternatively, if the fabric records a medium to high shear strain it may be a relict feature within the Given that the pre-glacial land surface of West sediment, related to an earlier ice advance or subaqueous Runton was composed of contrasting areas of perme- gravity flow mechanism (subparallel to flow). Its preserva- able sands and other areas of impermeable glaciolacus- tion as a transverse fabric is due to low strain conditions trine silts and clays and diamicts, the hydraulic during a final ice advance from the northwest. However, it conductivity of a deforming bed would have been highly should be stressed that determination of cumulative shear variable (cf. Hart et al., 1990). During initial ice sheet strain using eigenvalues must be viewed with caution, as advance, basal groundwater pressures will have been there can be large quantitative variations within and elevated due to a high regional hydraulic gradient. In between clast fabric data sets (Bennettetal.,1999). areas of sandy substrate basal water pressures would On a large scale, the highly complicated palaeo- have been directed laterally and upwardly (Boulton and glaciodynamics of the area would have affected the Caban, 1996; Boulton et al., 2001). In localised areas of subglacial sediment strain signal in two main ways; (i) less permeable glaciolacustrine sediments, the substrate the number of glacial cycles and (ii), the submarginal will have acted as an acquiclude, limiting the vertical context of the site. Irrespective of the chronological routing of water, but focussing lateral routing of water context of ice advance in the area, it is clear from the towards the ice/bed interface. This will have enhanced structural evidence that the site has been overrun twice. local hydraulic gradients above, below and within the Hart and Boulton (1991a) and Roberts (1995) suggest deforming bed. In high pressure areas of lateral and ice arrived from the northeast initially, but was followed upward groundwater migration, sediment failure will by ice from the west/northwest. Following the alter- have occurred at depth, promoting a thick, deforming native model of Rose et al. (2000) and Lee et al. (2002), layer characterised by a lower decollement! surface West Runton records the arrival British ice during MIS (Fig. 21i). 12 and MIS 10 with ice flowing from the northwest. If the porewater pressures (Pwp) were high and Importantly, the superimposition of one strain signal effective pressures low (PooPc1), ductile intergranular over another is possible under both models. Under an in shear would have operated causing sediment attenua- situ tranverse clast fabric scenario, ice could have tion, stringer initiation and folding. This is especially arrived from the northwest during both advances. true in tills with higher grain sizes (cf. Murray, 1997; ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 135

Fig. 21. (i) Reconstructed deforming bed processes under high basal porewater pressures acting laterally and upwardly. Low effective pressures pervade the entire depth of the deforming layer causing ductile, intergranular throughout (adapted from van der Meer, 1993 and Boulton and Dobbie, 1993). (ii) Reconstructed deforming bed processes under restricted basal water routing conditions. Lateral routing of high basal water pressures is focussed towards the upper deforming bed and the ice/bed interface. Low effective pressures operate here promoting sliding and pervasive intergranular shear, but lower parts of the deforming become in active (adapted from van der Meer, 1993 and Boulton and Dobbie, 1993). (iii) Variations in substrate and the hydraulic routing of basal water lead to ﬂuctuations in the effective pressure system, sediment rheology and the stress/strain regime in time and space.

Tulaczyk et al., 2000). Direct evidence of ductile, viscous support for very high basal water pressures is provided entrainment of material across the decollement! surface by the existence of massive chalk bed rock rafts within is not widespread at West Runton, but the incorpora- the deforming bed at West Runton, the detachment of tion of sand and chalk material into the Laminated which, could clearly be related to hydraulic action. Diamict facies is a result of this entrainment process The emplacement of these rafts has been partially which is driven by the upwardly directed hydraulic responsible for the establishment of discrete, stacked gradient (Figs. 21i and iii). Where entrainment is shear zones with the deforming bed at West Runton. conspicuous in the Laminated Diamict facies it is the It is worth noting that if the hydraulic gradient in the result of isoclinal, recumbent folds enveloping material areas of sandy substrate had been downward at West from below the decollement! surface (Fig. 17). This type Runton, the focus of deformation would have been of hydraulic regime is reported by Truffer et al. (2000) higher within the deforming bed towards the ice/bed from the Black Rapids glacier in Alaska, where the interface. This is because net strain would increase deforming bed is thought to be 2 m thick. Further upward rather than downward. In this case, a gradient ARTICLE IN PRESS 136 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 from brittle deformation structures in the lower parts of the deforming bed and fluctuations in the effective the deforming bed to ductile structures towards the ice pressure system through time (Boulton et al., 2001; bed interface should be discernible, as envisaged by van Fischer and Clarke, 2001). Evidence for elevated water der Meer (1993). This does not happen at West Runton pressures at the ice/bed interface causing bed separation, where a ductile/viscous deformation signal occurs and hence, enhanced basal sliding, is not common throughout the depth of the deforming layer (Fig. 21i), (Piotrowski and Tulaczyk, 1999; Piotrowski et al., 2001) which is 1–5 m thick in places, depending on the and subglacial sediment deformation has been a major distribution of chalk rafts which can create thinner, component of ice sheet motion. localised shear zones. The incorporation of sand into the deforming bed from the pre-glacial sands below 3.4. Sediment entrainment along the lower decollement! increases the grain size distribution of the diamict, and surface at west runton in turn, increases the transference of intergranular strain which may, in part, be responsible for the medium to There has been much recent debate regarding the high clast fabric strain signal and the pervasiveness of nature of the entrainment mechanism across lower shear throughout the depth the deforming bed at West decollement! surfaces. Boulton et al. (2001) suggest that Runton. entrainment without sediment mixing is possible where In areas where the subglacial substratum is composed the rheological contrasts between the two sediments are of glaciolacustrine sediments (Type 2 laminae), the less high and where the strain is concentrated in the upper permeable nature of the finer grained sediment pile medium. In contrast, Hooyer and Iverson (2000) and concentrated shear stresses towards the ice/bed Piotrowski and Hoffman (1999) assert that shear interface. Here, the depth of penetration of the entrainment must involve sediment mixing. However, deforming bed would have been reduced, with pore- the scale of observation is important to consider here. water pressure differentials between the top and bottom On the macroscale, evidence from West Runton shows of the deforming layer more exaggerated, but critical that entrainment is not ubiquitous across the lower effective pressure for sediment failure (Pc2), higher than decollement! interface, but above the decollement! in areas of sandy substrate (Pc1) as pervasive, inter- boundary discrete pods of sand and chalk clearly granular deformation would have been reduced due to demonstrate up-ice sediment excavation, extensional decreased hydraulic transmissivity (Fig. 21ii). attenuation and pod disintegration through lateral The focussing of high porewater pressure toward the shear. Hence, excavation and entrainment is very ice/bed interface could theoretically facilitate basal variable or ‘patchy’ and sediment mixing is often sliding, allowing the partial preservation of the original minimal, especially where there are strong rheological glaciolacustrine sedimentary signature below, although constrasts between materials (sand/chalk/diamict). On there is no distinct sedimentary evidence to suggest bed the microscale though, entrainment and mixing of one separation. The general lack of a brittle deformation matrix with another occurs commonly as sediment is signature and the occurrence of well preserved isoclinal, sheared and stratified (Fig. 12ii). recumbent folds in Type 2 areas, suggest that a largely The sharp nature of the decollement! interface at West ductile/viscous regime predominated and left a low Runton is therefore the product of two principle strain imprint on the sediment during both ice advances. mechanisms. The first is subglacial advective excavation The depth of penetration of the strain signal in these of chalk bedrock and pre-glacial sediments and the areas is difficult to reconstruct. There is is no distinct second is till advection without excavation. However, decollement! boundary within Type 2 sediments. Folding some parts of the decollement! interface represent a and minor lateral brecciation can occur towards the sedimentary boundary, as in situ glaciolacustrine sedi- base of Type 2 sediments, but is more prevalent higher ments are preserved immediately above the pre-glacial up the sections suggesting deformation has not been sequence. pervasive within in the lower parts of the deforming bed. These contrasting areas of substrate lead to a highly 3.5. The influence of shear stratification on the effective dynamic deforming bed environment governed by the pressure system geometry of, and fluctuations within, the subglacial effective pressure system (Fig. 21iii). Areas underlain by Shear stratification is also an important influence on sand will exhibit increased ice-bed coupling, a thicker sediment rheology. Sediment porosity, permeability and deforming layer and medium to high strain signal. Areas granulometry are altered by lateral shear through the underlain by glaciolacustrine fines may exhibit increased incorporation of new material and the breakdown of basal sliding and reduced ice-bed coupling, a thinner pre-existing material (Boulton and Dobbie, 1993; deforming layer and a low strain signal. The develop- Boulton et al., 2001). The hydraulic routing of pore- ment of stick-slip mechanisms beneath the ice is possible water is thus affected vertically as well as laterally. In under this model, due to the rheological heterogeneity of turn, this leads to the development of banded zones of ARTICLE IN PRESS D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140 137

Fig. 22. The effect of shear stratiﬁcation on sediment structure and porewater routing.

Fig. 23. The controlling influence of shear stratification and porewater routing on local fluctuations in effective pressure within the deforming bed. differential shear, some characterised by ductile shear of the sediment pile influences the depth of the and dilation, others perhaps by brittle shear (Fig. 22). deforming layer and the role of ‘excavational’ and Such vertical rheological and hydraulic differentiation ‘constructional’ deformational mechanisms (Hart et al., ARTICLE IN PRESS 138 D.H. Roberts, J.K. Hart / Quaternary Science Reviews 24 (2005) 123–140

1990), as well as the effective pressure system and with some ice rafted input, followed by secondary subglacial drainage regimes. subglacial deformation. Their preservation is in part a Once shearing is initiated, vertical zones within the function of low strain at the lower interface of the sediment pile take on contrasting permeability, porosity deforming bed. and granulometric characteristics, and hence, effective The microscale deformation signature within the pressures will vary locally within a short vertical sediments points to both plastic and viscous behaviour, distance (Fig. 23). This is important as the critical however it is viscous, intergranular localised, pervasive effective pressures required to deform sediment in a shear that characterises lateral sediment attenuation, till brittle sense differ from the critical effective pressures advection and sediment entrainment from the lower for ductile, intergranular failure. The strong rheological decollement! surface. The strain signal at West Runton is contrasts (diamict/chalk/sand) within the deforming bed mainly related to changes in subglacial substrate and at West Runton have helped to propagate distinct shear shear stratification which directly influences local sedi- zones on a scale of microns to metres. ment rheology. Sediment porosity, permeability and On the macroscale, large chalks rafts sometimes grain size distribution are all altered by lateral shear, separate 1–2 m thick deforming bed zones, whilst on which in turn controls the structural geometry of the the microscale both intergranular pervasive shear and deforming bed. It is these rheological and structural discrete shear mechanisms operate where combinations controls that control the hydraulic routing of porewater, of low sediment permeability/porosity and poor drai- the critical effective pressures for sediment failure and nage lead to increased porewater pressures (Pwp) and the critical thresholds for switches in the subglacial thus, decreased effective pressures (P1) which approach drainage. As ice sheets are sensitive to rapid changes in zero (Boulton and Dobbie, 1993). The critical control on subglacial substrate and drainage regime, the interplay the mechanism of deformation is the critical effective between these regulatory mechanisms has a direct pressure at failure (Pc). When Pwp is very high, the influence on ice sheet stability. critical effective pressure for ductile intergranular shear (Pc1 or Pc2) will be lower than that required for brittle discrete shear, which will occur at higher effective pressure (Pc3) due to reduced Pwp. At West Runton, Acknowledgements shear stratification will clearly cause differential switches in critical effective pressure over short vertical and This work was undertaken as part of an NERC lateral distances as individual failure events occur studentship at the University of Southampton. DHR (Fig. 23). would like to thank Mr. A. Jones, Dr. J.K. Hart, Dr. D. Shear stratification has the potential to re-focus the Evans, Dr. D. McCarroll, and Dr. A. Long for support hydraulic routing of porewater through the deforming in many guises. Thanks also to Dr. L. Owen, Prof. J. bed, even promoting canalisation (cf. Evans et al., 1995), Rose and the staff of the Electron Microscopy Unit at and thus can influence critical thresholds in the basal Royal Holloway, University of London, as well as to hydraulic system (Walder and Fowler, 1994). This will BobJones and John Ford for thin section preparation at influence the stability of an ice sheet, as the structural Southampton, and the staff of the Design and Imaging geometry and aquicludal nature of an advected and Unit, University of Durham. Finally, thank you to John sheared deforming bed will affect subglacial hydraulics Menzies and Jonathan Lee for critical and constructive and drainage regime (MacAyeal, 1993; Clark, 1994; comments on the manuscript. Clark and Walder, 1994).

4. Conclusions References

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