The 3D microscopic ‘signature’ of strain within glacial sediments revealed using X-ray computed microtomography

Quaternary Science Reviews 30 (2011) 3501e3532

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The 3D microscopic ‘signature’ of strain within glacial sediments revealed using X-ray computed microtomography Mark F.V. Tarplee a, *, Jaap J.M. van der Meer a, Graham R. Davis b a

Centre for Micromorphology, School of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, UK Institute of Dentistry, Barts and The London School of Medicine and Dentistry, Dental Biophysics, 2nd Floor, Francis Bancroft Building, Queen Mary University of London, Mile End Road, London E1 4NS, UK b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 December 2010 Received in revised form 19 May 2011 Accepted 25 May 2011 Available online 9 September 2011

X-ray computed microtomography (mCT), a non-destructive analytical technique, was used to create volumetric three-dimensional (3D) models representing the internal composition and structure of undisturbed pro- and subglacial soft sediment sample plugs for the purposes of identifying and analysing kinematic indicators. The technique is introduced and a methodology is presented addressing specific issues relating to the investigation of unlithified, polymineralic sediments. Six samples were selected based on their proximity to ‘type’ brittle and ductile deformation structures, or because of their perceived suitability for successful application of the technique. Analysis of a proglacial ‘ideal’ specimen permitted the 3D geometry of a suite of micro-faults and folds to be investigated and the strain history of the sample reconstructed. The poor contrast achieved in scanning a diamicton of glaciomarine origin is attributable to overconsolidation under normal loading, the sediment demonstrated to have undergone subsequent subglacial deformation. Another overconsolidated diamicton contains an extensive, small scale (<20 mm) network of fractures delineating a ‘marble-bed’ structure, hitherto unknown at this scale. A volcanic lithic clast contrasts well with the surrounding matrix in a ‘lodgement’ till sample containing mCT (void) and thin-section evidence of clast ploughing. Initial ductile deformation was followed by dewatering of the matrix, which led to brittle failure and subsequent emplacement. Compelling evidence of clast rotation is located in the top of another sample, mCT analysis revealing that the grain has a proximal décollement surface orientated parallel to the plane of shear. The lenticular morphology of the rotational structure defined suggests an unequal distribution of forces along two of the principal stress axes. The excellent contrast between erratics contained within a sample and the enclosing till highlight the considerable potential of the technique in permitting the rapid (semi-)quantitative analysis of large datasets. The subglacial sample evidence indicates that complex, polyphase (brittle/ductile) deformation histories are common in such diamictic soft sediments and that the local (micro-scale) environment (composition, structure, shear forces and effective pressure) controls rheology. The sediment void ratio is a key indicator of strain. Three of the samples are tentatively placed at different points on the strain cycle for subglacially deforming soft sediment, based on their void ratio, characteristics and distribution. It has been demonstrated that mCT offers significant potential for elucidating glacial soft sediment kinematics. The ability of the technique to both test a variety of hypotheses pertaining to mechanisms operating within the subglacial environment and evaluate efforts to replicate those processes under controlled, laboratory conditions is therefore discussed along with solutions to the problems encountered within this project. mCT also permits the seamless linking of analytical techniques applied at the hand specimen (mm), micromorphological (mm-mm), scanning electron microscopy and X-ray diffraction/fluorescence (nm-mm) scales and the archiving, duplication and dissemination of sample volumetric 3D data. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: X-ray computed microtomography Glacial sediments Subglacial soft sediment deformation Sediment rheology Sediment kinematics Micromorphology

* Corresponding author. Tel.: þ44 (0) 20 7882 2777; fax: þ 44 (0) 20 8981 6276. E-mail addresses: [email protected] (M.F.V. Tarplee), [email protected] (J.J.M. van der Meer), [email protected] (G.R. Davis). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.05.016

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1. Introduction Knowledge of the three-dimensional (3D) signature of strain is a prerequisite for fully understanding the kinematics of any material that has undergone stress, though it is particularly important in relation to composite substances, such as sub- and proglacial sediments, where the rheological response is likely to be complex (van der Meer et al., 2003; Benn et al., 2004; Evans et al., 2006; Phillips et al., 2007; Twiss and Moores, 2007; Fasano et al., 2008). To date, attempts at identifying and quantifying 3D evidence of strain within sub-/proglacial sediments have concentrated on macroscopic structures such as fold and fault orientations and till fabrics (e.g. Croot, 1988 and references therein; Hart and Boulton, 1991; Hart, 1994, 2007; Benn, 1995, 2004b; Boulton et al., 1996, 1999; Harris et al., 1997; Hooyer and Iverson, 2000; Phillips and Auton, 2000; van der Wateren et al., 2000; Müller and Schlüchter, 2001; McCarroll and Rijsdijk, 2003; Hiemstra et al., 2005; Roberts and Hart, 2005; Piotrowski et al., 2006; Phillips et al., 2007; Benediktsson et al., 2008, 2010; Lee and Phillips, 2008; Rijsdijk et al., 2010) and microscopic analyses (e.g. Evenson, 1971; van der Wateren et al., 2000; Stroeven et al., 2002; Carr and Rose, 2003; Carr, 2004; Thomason and Iverson, 2006; Phillips and Auton, 2007; Phillips et al., 2007; Hooyer et al., 2008; Iverson et al., 2008; Shumway and Iverson, 2009). Fasano et al. (2008) analysed various elements within a tillite using X-ray computed microtomography (mCT), an entirely non-destructive technique that can be used to (re)construct digital volumetric 3D models (‘volume renderings’) representing the composition and structure of material samples (Ketcham and Carlson, 2001; Mees et al., 2003; Long et al., 2009). Kilfeather and van der Meer (2008) quantified till sample porosity using the same technique. The purpose of this article is to report the findings of further applications of mCT to glacial soft sediment samples containing what may be regarded as ‘type’ examples of structures considered to be indicative of soft sediment deformation. The heterogeneous lithological characteristics common to many glacial diamictons present particular challenges in the application of mCT. Thus the main thrust of the research project was on optimising the quality of the models acquired, with a consequent restriction in the number of samples scanned. Though the insights presented are based on relatively few samples, they are considered highly significant and hence the technique worthy of further application to glacial sediment samples as well as many (all?) other solid materials of interest to the Quaternary research community, e.g. snow and ice (Kerbrat et al., 2008; Heggli et al., 2009; Obbard et al., 2009; Chen and Baker, 2010; Srivastava et al., 2010), plant and animal tissue (particularly archaeological specimens) (Domínguez Alonso et al., 2004; Donoghue et al., 2006; McColl et al., 2006; Matzke-Karasz et al., 2009; Johnstone et al., 2010, 2011), (palaeo)soils, (Adderley et al., 2001; Delerue et al., 2003; Rogasik et al., 2003), sediment (Akin and Kovscek, 2003; Van Geet et al., 2003; Flisch and Becker, 2003; Erdogan et al., 2006) rock etc. (Sahagian et al., 2002; Carlson et al., 2003; Mees et al., 2003; Ketcham, 2005a,b; Jerram et al., 2009; Long et al., 2009; Renard et al., 2009). In addition mCT permits the imaging of voids, the 3D structure of which are otherwise difficult to analyse (Sellers et al., 2003; Heggli et al., 2009) and has considerable potential in geotechnical (Thomson and Wong, 2003; Oda et al., 2004; Zabler et al., 2008; Pender et al., 2009) and hydrological research (Van Geet et al., 2000; Hirono et al., 2003; Ketcham and Iturrino, 2005; Kettridge and Binley, 2008; Porter and Wildenschild, 2010; Dautriat et al., 2011). Parfitt et al. (2010) have demonstrated how mCT can be used to create a virtual archive of volumetrically accurate reconstructions of unique specimens for study by other researchers, negating the need for access to the original materials. Not only does this protect

such specimens from wear and tear but also permits additional tests, both qualitative and quantitative, to be conducted. Further examples of this approach are provided in section five of this paper. The objectives of the article are: - To provide an introduction to the principles of mCT, some of the problems inherent to the technique, with specific reference to glacial sediments, and how these issues can be either overcome or mitigated. - To outline the two-dimensional (2D) characteristics of the samples that were selected for analysis using mCT, explain why they were chosen and provide detail of the results of the investigation. - To evaluate what additional information mCT has provided and how this can be related to existing hypotheses regarding kinematics within selected glacial environments. - Propose further lines of investigation based upon the findings of this research.

2. X-ray computed microtomography (mCT)

mCT is based on the principle of attenuation of X-rays by matter (Duliu, 1999; Mees et al., 2003; Van de Casteele et al., 2004; Davis and Elliott, 2006). The level of attenuation is principally dependent on the density, but also the effective atomic number (compositional element average) of the material(s) through which the X-ray passes (Denison et al., 1997; Ketcham and Carlson, 2001; Schreurs et al., 2003; Long et al., 2009). A sample is placed between an X-ray source and a detector (commonly either a scintillator attached to a charge-coupled device (CCD) camera or a large flat panel detector array) (SkyScan, 2001; Masschaele et al., 2007; Metris, 2009). Density variations within the sample are manifest as bright areas within the resultant image where a relatively weak beam reaches the detector (reflecting dense material), dark areas representing receipt of the majority of the beam (indicating a low density pathway) and a continuum between those two end-members (Fig. 1a) (Remeysen and Swennen, 2008; Long et al., 2009). The image is a 2D ‘X-ray’ radiograph of the type familiar to many hospital patients. By rotating either the sample or source/detector combination a predetermined angular increment, capturing another image, making another relative movement and so on, a composite picture of the composition and structure of the material under investigation can be built up (Fig. 1a,b) (Van Geet et al., 2000; Remeysen and Swennen, 2008). Using perpendicular radiographs stereological techniques could be used to generate fabric data on suitable samples (Stroeven et al., 2002; Ketcham, 2005b). However, by acquiring n images a reconstruction algorithm can be used to generate the third, cross-sectional ‘image stack’. This dataset or selected subsections/individual slices are commonly quantitatively analysed and presented in the literature (Fig. 1c) (SkyScan, 2008a; Buffiere et al., 2010). This third set of images is required for volumetric 3D visualisation (Sellers et al., 2003; Ketcham, 2005b; Heggli et al., 2009). The size of sample that can be analysed is dependent on the physical size of the X-ray scanner, the density of the material and the energy output of the source. Medical science (hospital) scanners have a large capacity, but relatively low energy sources and thus have a limited range of applications in the geosciences (Ketcham and Carlson, 2001; Akin and Kovscek, 2003). Industrial tomographs generally have a smaller capacity, but much more powerful X-ray sources and are thus able to penetrate higher density samples (Ketcham and Carlson, 2001; Mees et al., 2003; Ketcham, 2005b). Spatial resolution is primarily dependent on the size of the sample, though recent technical advances are beginning to overcome this restriction (Tarplee and Corps, 2008;

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Long et al., 2009). mCT was developed specifically to achieve very high spatial resolution, e.g. circa 5 mm for a sample 5 mm in diameter (Ø), and it is such an instrument that has been used in this study (Davis, 1997; SkyScan, 2001; Masschaele et al., 2007). Unfortunately the actual spatial resolution that can be realised is less than the theoretical value (normally between 50 and 150% higher, e.g. 7.5e15 mm for a 5 mm Ø sample) due to partial volume and penumbra effects, which also create blurring at material boundaries, imprecision in sample mounting and stage rotation (Van Geet et al., 2000; Ketcham, 2005a; Sasov et al., 2008; Porter and Wildenschild, 2010; Davis et al., 2010). 3. Methodology A SkyScan 1072 desktop X-ray microtomograph, utilising a 20e100kV/0e250 mA source to produce a cone-beam geometry of X-rays detected by a Gadolinium oxysulphide phosphor scintillator connected to a one megapixel, 8 bit CCD camera, can scan samples up to 20 mm Ø (SkyScan, 2001; Van de Casteele, 2004; Masschaele et al., 2007). Such a sample size yields a pixel (spatial) resolution of 19.15 mm. There is an approximately linear relationship between sample size and spatial resolution using this instrument, e.g. a 10 mm Ø sample permits a pixel resolution of 10.46 mm to be achieved. The reconstructed dataset is comprised of interpolated cubic volume elements termed ‘voxels’ (Denison et al., 1997; Buffiere et al., 2010). An 8 bit CCD camera produces radiographs comprised of pixels assigned a grey-scale value between 0 (black) and 255 (white), dependent on the level of attenuation of the X-rays detected (Long et al., 2009). During reconstruction, relevant pixel values are used to calculate individual voxel grey-scale values. Therefore the contrast resolution of an instrument, i.e. its ability to detect ‘subtle’ variations in sample composition, is partially reliant on the detector’s number of bits per pixel. Newer instruments than the one used in the project (which is over five years old) have 12 bit (4096 greyscale), 14 bit (16384 grey-scale) or 16 bit (65,536 grey-scale) detectors, which offer greater sensitivity and hence contrast resolution. Three ways were identified in which to enhance the contrast resolution of the images stacks: a) The ‘signal to noise’ ratio was maximised by adjusting a variety of parameters accordingly. b) The densest elements in all the samples studied comprised a very low percentage of the specimen. Consequently the greyscale histograms generated were negatively skewed. Therefore, the effect of the presence of the dense particles was to lower the contrast resolution possible over the majority of the crosssectional images. By excluding the high density tail of the histogram and hence narrowing the ‘dynamic range’ to be covered by the 255 grey-scales, the overall contrast could be enhanced without adversely affecting the accuracy of the image (Davis, 1997). c) The ‘Drishti’ software package (Limaye, 2006) can be used to exaggerate the grey-scale gradient between adjacent voxels, i.e. in all three dimensions. As well as accentuating the obviously contrasting components of a volume rendering, e.g. voids within a solid, it can also be used to highlight more subtle variations in density and/or effective atomic number. Fig. 1. (a) A 2D radiograph of subglacial soft sediment sample BO/03/07 (subsection of a vertically orientated plug, 10 mm Ø, XZ plane). The white areas are very dense (metalliferous) grains, the darker areas clasts, some hosting metalliferous particles, or matrix. (b) A radiograph orthogonal to (a), i.e. YZ plane, illustrating that the large dense grain in the centre of that image is located at the edge of the sample (the dashed line shows the position of Fig. 1c). (c) A cross-sectional (reconstructed) image slice of the sample.

Commercial X-ray source tubes normally produce a polychromatic beam, i.e. a spectrum of beam energies is generated rather than the monochromatic (mono-energetic) radiation supplied by a synchrotron (particle accelerator) and radioisotope sources (Cnudde et al., 2008; Zabler et al., 2008). Polychromatic

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beams are relatively cheap to generate and the energy of the beam can be easily altered to suit the sample. However, the beam emitted from an X-ray tube will always contain a relatively ‘soft’ element, i.e. from zero kV upwards. In many circumstances this softest part of the beam creates a phenomenon referred to as ‘beam hardening’ (Ketcham and Carlson, 2001; Mees et al., 2003; Davis and Elliott, 2006; Zabler et al., 2008). When a polychromatic beam encounters an object the softer part of the energy spectrum is either blocked or significantly attenuated, producing a higher mean energy beam to continue through the remainder of the specimen. The main artefact associated with this process is a corona (in cylindrical samples) of falsely elevated density values around the edge of the object (Fig. 2a,b) (Akin and Kovscek, 2003; Davis and Elliott; 2006; Tarplee and Corps, 2008). In monomineralic samples this is the only manifestation of beam hardening (Van Geet et al., 2003; Remeysen and Swennen, 2008). However, in polymineralic specimens, such as many glacial sediment samples, beam hardening can occur wherever there is a significant density phase change, i.e. between a dense clast and the surrounding (potentially) relatively less dense matrix or at the edge of a void (Ketcham and Carlson, 2001; Van Geet et al., 2003; Van de Casteele et al., 2004). The inaccuracies that beam hardening creates within mCT scans, together with its pervasive nature, present significant obstacles to generating representative sample reconstructions (Davis, 1997; Akin and Kovscek, 2003; Van Geet et al., 2003; Long et al., 2009). Beam hardening can be mitigated in a number of ways. The two primary methods applied in this project, combined, were

Greyscale profile across Fig. 3a 0.3

d Greyscale values (x100)

Greyscale values (x100)

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considered to provide satisfactory results as indicated by a near horizontal grey-scale profile across the cross-sectional images (Fig. 2) (see Appendix A). Reconstruction algorithms are based on the linear attenuation of a monochromatic X-ray beam by a sample. The polychromatic beam produced by an X-ray tube complicates such a relationship significantly (Davis et al., 2010). However, as a general rule assuming linear attenuation leads to acceptable margins of error considering the substantial saving in both operator input and computer processing time required to resolve such issues (Brunke et al., 2008). Unfortunately wide compositional variations sometimes contained within glacial and other lithological specimens, e.g. sedimentary rocks, compromise this assumption resulting in inaccuracies within both the cross-sectional images and the volume renderings based upon them. Where relatively very dense particles align parallel to the beam during sample rotation, pronounced ‘streak’ artefacts are generated creating a false low density zone or void (Davis, 1999; Thomson and Wong, 2003; Davis and Elliott, 2006). This phenomenon can be more pervasive, forming concentric circular zones within associated portions of the sample, both in the cross-sectional slices containing the dense particles and adjacent images (Fig. 3). Such circles are very similar to ‘ring’ artefacts, a product of varying detector pixel sensitivity (Davis, 1997). However, unlike ring artefacts which can be significantly compensated for during reconstruction or removed using advanced scanning techniques (see Section 5.6), there is no straightforward solution to remove streak artefacts at present. Such

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Fig. 2. (a) A cross-sectional image slice from sample Mi.595, illustrating the effect of beam hardening on the reconstructed dataset (5 mm Ø). The white corona represents (artificially) increased material density values. The dashed line indicates the position of the profiles in Fig. 3b and d. (b) Greyscale profile across sample Mi.595 highlighting the effect of beam hardening on grey-scale values at the sample edge. (c) The cross-sectional image slice in (a) corrected (beam filtering and linearisation, see Appendix A), revealing significant compositional detail. (d) Greyscale profile across (c) demonstrating the more balanced, representative distribution of X-ray attenuation values. N.B. Linearisation reduces the statistical spread of the grey-scale values (by w66% in this case), consequently reducing the contrast resolution of the dataset (Van Geet et al., 2000).

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using CTVol as the binary images were not readily compatible with Drishti. The resolution of these substantially smaller datasets could be retained at 10243 voxels. 3.2. Sample acquisition

Fig. 3. A cross-sectional image from sample BO/03/07 (10 mm Ø). The metalliferous grains present are of a very high density relative to the surrounding material. Where such objects align, this contrast causes proximal false voids/low density zones to be produced in the reconstructed images. The concentric rings and streak artefacts that are present are also a product of this phenomenon.

artefacts can compromise the accurate quantification of attenuation coefficients for the affected parts of such specimens making mineralogical identification very problematic (Long et al., 2009). That said, accurate analyses of unaffected elements are possible and other quantitative and qualitative analyses largely uncompromised. Through iteration each scanning parameter was set to an optimal value for the sample being analysed (see Appendix B). Settings were selected based on a compromise between scan duration (cost), spatial and contrast resolution sought and artefact reduction (Van Geet et al., 2000). Scan radiograph datasets were reconstructed using SkyScan’s licensed software ‘NRecon’ (version 1.6.2.0), except for sample BO/03/07 where an in-house program was used (Davis et al., 2008). All image stack processing and analysis was conducted using SkyScan’s licensed software ‘CTAn’ (version 1.10). 3.1. Volume rendering production Volumetric renderings were produced using either the open source program ‘Drishti’ (version 1.0.2) (Limaye, 2006) or SkyScan’s licensed software ’CTVol’ (version 2.1). Drishti was used where complete renderings were desired because of its versatility. Unfortunately, due to the limitations of the available hardware, the resolution of the volume renderings had to be reduced from 10243 voxels to 5123 voxels, often following subsampling of the original dataset to retain sections of interest only. Where a particular portion of a subsection was of interest, i.e. above, below or between minimum and maximum grey-scale values, thresholds were imposed using CTAn in order to isolate (segment) the relevant voxels. Binary image stacks were then produced from which new volume renderings could be generated. Each such dataset was enhanced either by removing all but the largest single, connected collection of voxels representing the main object of interest, or by removing voxel ‘clusters’ below a specified volume, e.g. 143 voxels. The models were visualised and, where appropriate, combined

The unlithified nature of much glacial sediment presents considerable issues with regards to acquiring samples suitable for mCT analysis. Beam hardening effects associated with dense materials preclude the use of containers robust enough to support the sample in an undisturbed state, but thin enough to be of practical field use, e.g. steel or aluminium boxes/tubes. While materials such as cardboard, polyethylene (plastic), poly(methyl methacrylate) (Perspex) and carbon fibre may prove to be suitable alternatives (Van Geet et al., 2000), an equally effective solution is to apply sample preparation techniques used in the production of micromorphological thin-sections, i.e. impregnation. Thin-sections are produced by mounting a polished face of a sample on a glass plate and removing all but a 25e30 mm ‘slice’ of the specimen (Fig. 4). Unconsolidated sediment samples can be examined once they have been stabilised. Details of the technique can be found in Carr (2004) and references therein, but essentially an undisturbed sediment sample is acquired in a metal ‘Kubiena’ (80  60  40 mm) or ‘mammoth’ (150  80  50 mm) box, orientated vertically (portrait), horizontally (landscape) or flat (recumbent) and any open voids are filled with a contrasting material, e.g. polyethylene sheet, quartz sand etc. (Fig. 4). The specimen can thus be transported to the laboratory in an undisturbed state where it is impregnated with a bonding agent in a vacuum, such that the fluid fills all pore spaces within the specimen entirely creating a coherent block of material. Complete impregnation is important, in order to prevent disturbance of the sample. The meticulous sample preparation procedure outlined above provides ideal specimens for mCT analysis, as nominally only two main density phases are present within the resultant block, i.e. the sediment and the bonding agent (which replaces air, the potential third phase). The sampling boxes used to acquire and transport unlithified sediments are sufficiently large that, once a thin-section has been produced, there is ample specimen left from which to derive one or more subsample ‘plugs’ suitable for mCT analysis using the SkyScan 1072 microtomograph (Kilfeather and van der Meer, 2008). Additionally, micromorphological analysis of the associated thin-sections can be used to identify target sites within the adjacent sediment block. To facilitate comparison between the sample contained within the thin-section and the proximal portion of the mCT specimen, plugs were orientated orthogonal to the polished face of the block (Fig. 4). As thin-section production involves loss of the sample section proximal to the slide specimen, there is always a 3e4 mm separation between what is observed under the microscope and what is contained within the adjacent face of the block. Consequently, the compositional element or structure of interest in thin-section may not extend to, or be contained within the adjacent remaining sample. Cylindrical samples help minimise the possibility of streak artefact production (Davis, 1997; Tarplee and Corps, 2008). A diamond drill corer, mounted in a precision bench top drill, was used to recover plugs that have a very smooth cylindrical surface. The samples analysed were either 20,10 or 5 mm Ø. Plug size selection was dependent on the scale, compositional and/or structural complexity of the feature of interest and therefore both the spatial and contrast resolution required to study it. Smaller diameter samples require a lower energy beam to achieve the necessary minimum transmission level, thus increasing the possibility of weakly attenuating materials being detected (Davis, 1997). The length of each plug was dictated by the thickness of the portion of the block sampled (Fig. 4).

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Sediment thin-section

Sediment sample plug

Sediment sample block

Fig. 4. A cartoon illustrating the relative positions of a thin-section and a mCT sample plug derived from the same resin impregnated unconsolidated sediment block. In this instance the mammoth sized sample box used to obtain the undisturbed sample would have been vertically (portrait) orientated.

4. Sample selection All specimens were derived from the sample collection contained within the Centre for Micromorphology, School of Geography, Queen Mary, University of London (further details of which can be found in van der Meer et al., 2003). One proglacial sediment sample was selected as it contains unambiguous evidence of both brittle and ductile deformation structures (faults and folds respectively) displacing/deforming thinly laminated sediments comprised primarily of quartz grains (SiO2) and therefore having essentially biphasic density characteristics, i.e. SiO2 and the bonding resin (Boulton and van der Meer et al., 1989). Additionally, some of the laminae are visually accentuated by the presence of iron (Fe) oxide precipitates, concentrated in the finer layers (Fig. 5aec,l) (see Section 5.1 for details). Fe has a significantly higher density (7.87 g/cm3) and atomic number (26) than Silicon (Si) (2.33 g/cm3 and 14 respectively). Therefore, despite the fact that the associated oxygen atoms affect the densities of the compounds present, e.g. SiO2 ¼ 2.7 g/cm3, pure Fe-oxy/hydroxides averaging 4.3 g/cm3 (Long et al., 2009), it was assumed that the contrasts between the two compositional elements could be readily differentiated using mCT. An initial sample scan demonstrated this assumption to be correct (Fig. 5d) and so a total of three plugs were analysed. Subglacial soft sediment samples were chosen based on the premise that, as certain thin-sections contain type rotational structures indicative of ductile deformation (van der Meer, 1993), the specimen blocks from which they were derived would contain either extensions of these structures or alternative, similar features. Rotational structures were targeted in preference to indicators of planar deformation (within the subglacial samples) (van der Meer, 1993), as the 3D morphology of the former remain poorly understood (Blake, 1992; van der Meer, 1997a; Phillips, 2006; Tarplee, 2006). The results of four sample scans are presented as they illustrate: i) the variability in contrast resolution, achievable using the SkyScan 1072, and hence the applicability of the technique to any particular sample;

ii) how mCT analyses can reveal hitherto unknown compositional and structural features because of the spatial resolution that can be achieved; iii) the very significant additional insights that can be achieved by analysing sediment samples in all three, complete dimensions. In addition the results of the mCT analysis of what is considered to be, in principle, an optimal glacial sediment sample for the following purposes are also presented: a) identifying the size, morphology, orientation and configuration of ‘turbate’ structures (van der Meer, 1993, 1997a; Phillips, 2006; Tarplee, 2006; Tarplee and van der Meer, 2010). b) a naturally deformed test specimen for evaluating the legitimacy of till fabrics as an analytical tool (Hart, 1994, 2007; Benn, 1995; Bennett et al., 1999; Hooyer and Iverson, 2000; Carr and Rose, 2003; Chandler and Hubbard, 2008; Iverson et al., 2008). Descriptions of the structures studied in thin-section and interpretations of their possible formative mechanisms appear elsewhere in the literature except, in part, for samples O.514aec, C.113 and BO/03/07 (see subsections 5.1, 5.4 and 5.6 respectively). However, in order to facilitate comparison with the results of the mCT scans selected thin-section images are reproduced here, where appropriate, together with a brief reprise of their interpretations along with the original reference(s). In some cases it was appropriate or desirable not to acquire a subsample from the block proximal to the ‘ideal’ structure(s) as presented within the literature, e.g. the ubiquity of features within the specimen permitted the acquisition of a plug towards the block edge thereby minimising the impact of subsampling. Where this was done a new thinsection image, proximal to the sampling location, is presented both to allow comparisons between the results of the analytical methods used and complement existing material. Where limited original thin-section descriptions are available within the published literature summaries are presented along with preliminary interpretations; detailed discussions of the 2D evidence more

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Fig. 5. (a) Mammoth thin-section O.514, annotated to highlight micro-faults and folds, a water escape structure (WES) and plug sites aec (the samples acquired from proximal positions in the impregnated block). Plane light, width of sample ¼ 70 mm (a non-annotated image of the thin-section can be found on p.358 of Boulton et al., 1999). (b) A close-up of the thin-section adjacent to sample site O.514a. Note that particles from the dark-brown (Fe stained) lamina have been dispersed along the reverse fault. Plane light, field of view ¼ 14.3 mm. (c) The part of the thin-section adjacent to sample site O.514b. The main structural elements are highlighted along with selected lamina segments and fragments. The subsidiary thrust fault zone appears to be more of a fold at the micro-scale. Plane light, field of view ¼ 14.3 mm (d) A cross-sectional image slice from sample O.514a (10 mm Ø). A number of laminae are observable, their contrast dependant on the concentration of Fe. (e) A top-down view, i.e. parallel to the z-axis (and hence the same orientation as the thin-section), of the volume rendering of sample O.514a. It is emphasised that colour lightness variations within the 3D model reflect X-ray attenuation only, the darker the brown the more attenuating the material. (f) The volume rendering of O.514a rotated 360 , sequentially sectioned perpendicular to the z-axis, rebuilt and sliced parallel to the z-axis and perpendicular to the orientation of the dark-brown lamina, reconstructed before removing all but the highest density material (the lamina), this segmented model rotated 360 . Finally the lamina model is stripped of all but the largest single structure (apart from a few isolated fragments missed by the automated process), the feature then rotated 360 . (g) The two sections of the main lamina in O.514a, connected by the particulate ‘bridge’ (the white line highlights the edge of the lamina). (h) A top-down view of the volume rendering of sample O.514b (20 mm Ø). (i) The volume rendering of O.514b rotated 360 , sequentially sectioned perpendicular to the z-axis, rebuilt and sliced parallel to the z-axis and perpendicular to the thrust fault zone, reconstructed and sliced parallel to the z-axis and the thrust fault zone, the model then restored before removing all but the highest density laminae and rotating the segmented model 360 . (j) The O.514b model orientated to highlight the reverse fault on the side of the plug. (k) The model sectioned to reveal the subsidiary reverse fault zone branching from the main thrust fault zone to the right. (l) A close-up of the thin-section containing the recumbent and overturned folds, adjacent to sample site O.514c. Plane light, field of view ¼ 14.3 mm (m) A cross-sectional image slice from sample O.514c (20 mm Ø). Note the pseudo ring artefact in the centre of the image, intersecting the sub-horizontal void. (n) A top-down view of the volume rendering of sample O.514c (20 mm Ø). (o) The volume rendering of O.514c rotated through 360 , sequentially sectioned perpendicular to the z-axis, rebuilt and sliced parallel to the axial plane of one of the main folds, reconstructed before removing all but the highest density laminae and rotating the segmented model 360 .

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Fig. 5. (continued)

appropriately combined with the 3D analyses and hence presented within the discussion. The orientation of each thin-section, and hence the associated orthogonal plug, is also provided. 5. Description and interpretation of selected mCT analyses (see also Appendix C) 5.1. O.514, Holmstrømbreen push moraine, Ekmanfjorden, Spitsbergen, Svalbard archipelago (Fig. 5aeo) (Boulton and van der Meer et al., 1989; Boulton et al., 1999) Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quascirev.2011.05.016. The push moraine is located proximal to what is assumed to be the Neoglacial maximum extent of the glacier, inferred to be the result of a surge and from which point it subsequently downwasted. The moraine is heavily folded and thrust faulted transverse to the glacier snout. The sample site (Be 32, 0.5m above the base of the exposure) was located on the eastern side of the ‘western gorge’, a river section through the moraine approximately perpendicular to the fold axis in that area. The sample is from Unit

d, a ‘chocolate brown’ mud comprised of deformed laminated fine sand e clay (supratidal mudflat deposits), approximately 2 m beneath the contact between units d and e (the overlying, deformed prograded outwash deposits). The sample site was located approximately 254 m from the ice-distal limit of the push moraine, within the ‘external zone’, containing open folds and thrusts mainly dipping away from the glacier front though a few backthrusts are also present. The sample site was located within the inner part of a limb of an open fold structure (cf. the ‘composite section’, p.351 of Boulton et al., 1999). The thin-section (portrait) is comprised of laminated fine sands, silts and clays containing an asymmetrical fold dissected by thrust and reverse micro-faults, additional minor folding and discrete, small scale (millimetre) water escape structures (WES) (Fig. 5aec). The sample can be subdivided into three general zones based on the level of deformation present: i) the top 27e38 mm of silt e clay laminae with minor levels of thrusting and pronounced, associated recumbent and overturned folds in the top left corner as well as one reverse fault near the right edge; ii) a heavily thrust and reverse faulted central zone of fine sand e clay approximately 60 mm in height; iii) the bottom 63e67 mm of fine sand e clay with minor

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Fig. 5. (continued)

levels of reverse faulting (Fig. 5a). The reverse faults in all three sections are subparallel, displacement was towards the top left corner of the slide (sinistral shear) at angles between 45 and 55 , i.e. upward and towards the glacier-proximal end of the push moraine. However, the level of displacement varies between sections; top and centre ¼ <1.0 mm, bottom ¼ <1.5 mm (Fig. 5aec). The thrust faults (central section) trend upward at approximately 20 e25 towards the right (glacier-distal, dextral shear) side of the slide with displacements of up to 11 mm (Fig. 5c). Particles derived from distinctive laminae are located along some of the fault lines, contained within slivers of massive sands, silts and clays (Fig. 5b,c) (E. Phillips, pers. comm., 2011). Nowhere in the slide are faults associated with planar voids/fractures discernable at micromorphological scales, i.e. 30 mm. The apparent (2D) dextral shear evident in both the asymmetric fold and the thrust faults in the central portion of the specimen indicate production during a single, compressive deformation phase (D1) (nomenclature adopted from Phillips and Auton, 2000). Initial ductile folding was followed by brittle faulting. A number of reverse faults dissect the lower part of the syncline. Their (apparent) sinistral shear and relatively short displacements, general inclination and wider distribution compared to the thrust faults, indicate a separate (D2) compressive event. Whether or not the two phases were penecontemporaneous cannot be established from the thin-section evidence (E. Phillips, pers. comm., 2011). The isolated nature and unusual 2D morphology of the recumbent and overturned folds suggest that they are not associated with either deformation phase. Their position at the edge of the specimen limits the conclusions that can be reached regarding their genesis. As the lower limb of the overfold is dissected by a D1 thrust and the 2D orientation of the

overall feature is opposite to the direction of displacement of the D2 reverse faults, it would appear to pre-date those phases of deformation. Small mud volcanoes have been observed in the modern Holmstrømbreen proximal mudflats, an associated mudflow providing a logical alternative, non-glaciotectonic, explanation for such a feature. The isolated WES could be either syn-depositional products of dewatering due to compaction or syn-deformational, driven by porewater expulsion during; i) lateral compression of the sediment pile, ii) glaciotectonic thickening and hence an increased normal loading (E. Phillips, pers. comm., 2011). 5.1.1. O.514a A 10 mm Ø plug was acquired near the top left of the lower, reverse microfaulted (D2) zone of the sample block as it contains a discrete, thin (<400 mm) dark-brown lamina indicative of high Fe concentrations and hence would be relatively highly X-ray attenuating, confirmed in the reconstructed cross-sectional images (Fig. 5d). The plug also contained one reverse fault, the 3D geometry of which could be investigated by volume rendering the displaced sections of the lamina. The grey-scale image stack reflects the compositional and structural variation within the sample, which can be readily enhanced by employing the gradient accentuation and false colour facilities in Drishti (Fig. 5d,e and f, a movie available via the web and electronic version) (Limaye, 2006). The partial rings present within the 3D model (particularly apparent on the left side of the cross-sectional image in Fig. 5e,f) are ring artefacts, also accentuated by the gradient facility. Serial sectioning of plug O.514a was conducted to study the 3D morphology and geometry of the fault(s?) (Fig. 5f). It was established

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that the sample contained only one, discrete fault as represented by the displaced lamina sections and so the surrounding, lower density, material (voxels) was removed to enhance visualisation of that structure (Fig. 5f). As the low density material ‘stripping’ process was automated an incomplete visualisation of the lamina remains, i.e. only the densest voxels (probably representing individual grains, particle aggregates and the densest portions of both) are rendered (Fig. 5f,g). However, the planar morphology and geometry of the lamina can be unambiguously discerned (Fig. 5g). The dark-brown lamina are sub-horizontal. The strike of the microfault diverges approximately 10 from the plug z-axis, something that cannot be established from the thin-section (Fig. 5f). As the plug (z-axis) is orientated approximately perpendicular to the glaciotectonic trend of the push moraine, assuming structural isomorphism (Hiemstra and Rijsdijk, 2003) such a geometry is unsurprising (Boulton and van der Meer et al., 1989; Boulton et al., 1999). However, further microfault strike data would be required to test this hypothesis. Of note is that the lamina particles, eroded during displacement, form a thin (<100 mm), intermittent ‘bridge’ between the two segments of the layer (Fig. 5f,g). The absence of any, relatively very low density, voids within the scan dataset indicates that the intermittent nature of the bridge can be attributed to the automated thresholding process. 5.1.2. O.514b A 20 mm Ø plug was acquired at the bottom left edge of the central, heavily microfaulted, section as it contains an Fe rich lamina displaced 11 mm by a thrust fault (D1) zone as well as a subparallel, subsidiary thrust fault zone approximately 4 mm above (Fig. 5a,c). The main fault zone consists of three sediment lenses, the main lens up to 1.5 mm wide, separated from an adjacent sliver, proximal to the hanging wall, by a minor fault. The sediment within the zone has little discernible structure apart from three fragments of the lamina, one of which appears to be undeformed (subjacent to the minor fault), the other two pieces apparently moderately stretched or smeared out (Fig. 5c). The objective of the mCT analysis was to investigate the 3D morphology and geometry of the thrust fault zone and identify any evidence of reverse faulting (D2) within the sample. The volume rendering has a voxel size of 38.30 mm3, the model therefore having a rather ‘granular’ appearance in contrast to the more realistic model of O.514a which is comprised of 20.92 mm3 voxels (Fig. 5e,h). The contrast resolution of the volume rendering is very good, the laminae and associated structures apparent in thin-section clearly represented in the virtual model (Fig. 5h and i, a movie available via the web and electronic version). Binning voxels (combining eight (23) or more elements to form a single, proportionately larger voxel) has the advantage of increasing the signal to noise ratio of the scan dataset (Davis and Elliott, 2006; Porter and Wildenschild, 2010). The thrust fault zone extends to (i.e. from the location of the thin-section) and throughout the length of the plug parallel to the z-axis, showing no discernible fluctuations in orientation and minor changes in dimensions and morphology only, e.g. in the bottom approximately 3 mm the zone widens/flares proximal to the side of the plug (Fig. 5i). A continuance of the undeformed lamina fragment is evident within the plug, though its larger size/ wider distribution suggests that it may have been deformed/fragmented in that part of the fault zone (Fig. 5i). The other, smeared fragments cannot be discerned. However, by sequentially removing slices of the volume rendering parallel to the z-axis and perpendicular to the trend of the fault zone, it can be seen that fragments and smeared out elements of the laminae are sporadically distributed throughout the main thrust, in parts still clearly linked to their footwall source (Fig. 5i). It can thus be concluded that the thrust fault sediment is autochthonous.

On the side of the plug the dark-brown lamina has another obvious structural element, either an overturned, close, sharp fold or reverse fault (Fig. 5i,j) (Twiss and Moores, 2007). The simplest explanation, based on both the thin-section and volume rendering evidence, is that it is a reverse fault, the fracture containing particles derived from the lamina which appear to form the connecting limb of a pseudo overturned fold. The fault closes before it reaches the top of the plug. Alternatively it could be a fold located at the tip of a thrust fault (Brandes and LeHeron, 2010; E. Phillips, pers. comm., 2011). Deconstructing the model reveals evidence favouring the latter scenario (Fig. 5i). By projecting the fault plane through to the top of the plug, and hence onto the thin-section, it can be estimated to have a dip of approximately 25 , unusually low in comparison to the reverse faults (Fig. 5a,b,e,f,i). Such a localised offset in dip, the geometry and relationship of any additional fault swarms being unknown, may be the reason for the limited extent of the structure, i.e. the micro-glaciotectonic trend towards higher angle (reverse) faulting inhibited propagation of this relatively low angled fracture. Another fault/pseudo overturned fold can be observed dislocating a secondary dark-brown lamina which has a very minor expression in the lower left of the plug top and in the thin-section, the fault plane only becoming apparent during sequential slicing of the specimen perpendicular to the thrust fault zone (Fig. 5c,h,i,k). The strike of the fault is parallel to the thrust fault zone and by projecting its plane to the top of the plug it becomes apparent that the fracture is in fact the subsidiary thrust fault detected in thinsection, less readily discerned in the volume rendering because of the lower spatial resolution (mCT >38.30 mm compared to 25e30 mm for the thin-section). This subsidiary fault projected downwards intersects the main thrust fault zone at the base of the plug, verified by sequential slicing (Fig. 5i,k). The subsidiary fault therefore branches/splays from the main fault zone, explaining its limited expression in the thin-section. The main and subsidiary thrust faults enclose a prism of displaced sediment that appears undisturbed (see also Appendix Di). 5.1.3. O.514c A 20 mm Ø plug was acquired containing the recumbent and overturned folds dissected by a D1 thrust at the top left edge of the specimen, primarily to study the 3D morphology and orientation of the ductile deformation. Once again the scan was highly successful, the laminae and structures (including the D1 thrust) clearly discernible in the volume rendering (Fig. 5leo, a movie available via the web and electronic version). There are clear pseudo ring artefacts in the centre of the volume rendering which extend throughout the length of the plug, less obvious and narrower real ring artefacts extending to the edge of the cylinder (Fig. 5n,o). The reason for the pronounced nature of the pseudo ring artefacts in the centre of the object is unknown, but the proximal fracture appears to be the cause, its particular geometry possibly exaggerating the effect (see also Appendix Ei). By rotating the 3D model through 180 about the x-axis, i.e. turning the plug upside down, it becomes obvious that the orientation of the folds diverge significantly from the z-axis of the sample (Fig. 5o). It is also possible to sequentially section the sample at exactly the right orientations to cut through each of the folds parallel to their axial planes (see Fig. 5o for one example). Using this technique an overall fold orientation of approximately 50 from the z-axis and subparallel to the XZ plane was established. This general orientation contrasts with that of the thrust and reverse faults, subparallel to the glaciotectonic trend, thereby supporting the hypothesis that these folds pre-date the glacial surge and associated deformation phases. Furthermore the XZ plane equates to the ground surface, indicating that the folds were

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created by gravity driven stresses, e.g. slumping; their small size suggesting a mud volcano origin. However, the folds demonstrate dextral shear concurring with the D1 phase evidence, the divergence from the main structural trend possibly attributable to a highly non-cylindrical form (E. Phillips, pers. comm., 2011). Therefore, a glaciotectonic origin cannot be ruled out. 5.2. Mi.595, Irish Sea Till, Fassaroe, Enniskerry, Ireland (Fig. 6aec) (van der Meer, 1993; van der Meer et al., 1994) Two micromorphological samples were acquired of ‘Irish Sea Till’, part of the Ballycroneen Formation (Warren, 1985), as the genesis of the deposits have been the subject of much conjecture and debate regarding their glaciomarine (Eyles and McCabe, 1989; McCabe, 2008) or terrestrial basal glaciotectonic (van der Meer et al., 1994; McCarroll, 2001; McCarroll and Rijsdijk, 2003; Rijsdijk et al., 2010) origin. The sample site was 2.5 km inland and southewest of the southern end of the 5.5 km long Killiney section studied by Eyles and McCabe (1989) and Rijsdijk et al. (2010). As such it is an important additional source of information pertaining to the debate, providing 3D control on the minimum areal/volumetric extent of the sedimentological and structural evidence for either hypothesis. . The Irish Sea Till is mainly a brown clay-silt rich diamicton that contains shell fragments (Eyles and McCabe, 1989; Rijsdijk et al., 2010). At Fassaroe the till also contains ‘silty, laminated deposits with a large number of clay pebbles [.] uniform and clayey bands occur as well’ (Fig. 6a,b) (van der Meer et al., 1994). The two samples were located either side of a slightly overturned fold core, the whole unit ‘strongly deformed’ (van der Meer et al., 1994). Both thin-sections contain a significant number of folded and faulted laminations of various mean (fine) grain sizes and many intraclasts composed of clay, silt and diamicton (Fig. 6a,b). There is a common association between faulting and WES (van der Meer et al., 1994). Thin-section Mi.595 (portrait) contains a good example of a ‘Type III Pebble’ (Fig. 6a) (van der Meer, 1993). The intraclast contains a turbate structure and has a rounded morphology (2D), indicative of internal and external deformation respectively in response to the rotational forces acting on the pebble. van der Meer (1993) proposed that such a feature is created initially by ductile deformation which produced the internal turbate structure. Subsequently brecciation (brittle failure) of the body of material containing the turbate created angular fragments which were then dispersed and rounded by erosion of their corners during further, pervasive, deformation. It was postulated that these events took place in a subglacial (terrestrial basal glaciotectonic) environment, though they could also have occurred within a debris flow (Menzies and Zaniewski, 2003; Phillips, 2006) consistent with subaerial, subaquatic or sub(glacio)marine conditions. The intraclast is approximately 1.2 mm Ø and therefore, unsurprisingly, no evidence of it extending into the adjacent sediment block was found. However, other intraclasts of a similar type were identified within the block and a 5 mm Ø plug subsample acquired accordingly for mCT analysis, to try to elucidate the probable genesis of the features. By comparing the portion of the thinsection proximal to the mCT sample site with a 2D cross-sectional slice reconstructed from the scan dataset (Fig. 6b,c), it can be seen that the analysis has been partially successful. Two adjacent intraclasts, approximately 1e2 mm Ø, are present at the top of the plug, but are only vaguely discernible within the cross-sectional image (Fig. 6c). What is also noticeable is the lack of compositional variation and structure in the X-ray cross-section in comparison to the thin-section (see Appendix Eii). What can be observed in thin-section and is verified by the mCT analysis, is the paucity of voids both within the intraclasts and throughout the associated silty layer which is >15 mm across (an

Fig. 6. (a) An intraclast containing skeleton grains aligned so as to define a rotational (galaxy) structure, thin-section sample Mi.595. Plane light, field of view ¼ 3.25 mm (modified from van der Meer, 1993). (b) The thin-section proximal to the location of the mCT plug cross-sectional image slice featured in Fig. 6c. Note the intraclasts and arcuate alignment of grains. Plane light, field of view ¼ 5 mm (c) A cross-sectional slice from plug Mi.595. Two intraclasts can be discerned in the upper left (i) and lower right (ii) quadrants, delineated in part by the dashed lines. The small white spots indicate individual grains of unusually high density in comparison to the majority of the clasts and matrix. No voids can be discerned at the spatial resolution of the scan, i.e. approximately 10 mm based on a voxel size of 7.15 mm and allowing for partial volume effects.

exact measurement being impossible because the laminations are so heavily deformed) (Fig. 6aec). Indeed, the majority of voids contained within the mammoth thin-section may be attributable to poor impregnation and consequent loss of sample during

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production. The few unquestionably natural voids are isolated and appeared unconnected, based on 2D evidence. To investigate void characteristics further a ‘volume of interest’ (VOI) covering the largest possible volume, but excluding imperfections at the extremities of the plug, was created from the reconstructed dataset to undertake 3D quantitative analyses (Ketcham and Iturrino, 2005). The volume of the VOI was 142.08 mm3, the volume of the voids 0.03 mm3, or 0.02%. Though such a small sediment sample volume cannot be considered to accurately represent the percentage voids within the whole specimen block, let alone the wider sediment assemblage, it does provide a semi-objective evaluation of the consolidation of the deposit, in part corroborated by the thin-section evidence (Pender et al., 2009). Kilfeather and van der Meer (2008) conducted detailed mCT analyses of effective till porosity, concluding that values varied from <1% for ‘well-consolidated limestone till’ to >15% for a ‘poorly consolidated [.] flow till’. The analyses were conducted using the same instrument and software as in the current project and are therefore considered comparable. It can be concluded that the mCT evidence clearly indicates (over)consolidation of the Irish Sea Till sample contained within the plug obtained from specimen Mi.595. 5.3. O.604. de Woude, Holland (Fig. 7aeg) (van der Meer, 1993) Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quascirev.2011.05.016. The thin-section (portrait) was acquired from a 40 mm Ø core, the sample located at 34.50e35.60m depth beneath ground surface, within a bed of Saalian till. No surface exposures of this deposit exist proximal to the site, the core drilled for the purposes of facilitating stratigraphic correlation. The lack of macroscopic contextual evidence accentuates the value of detailed microscopic analyses. The sample is 91 mm long and comprised of a single, clast rich diamicton. Grain size varies from <30 mm to 4 mm. The majority of clasts are between 100 and 300 mm (fine sand), are subangular subrounded and evenly distributed (Fig. 7a,b). The matrix is silt rich and has a medium density (after Carr, 2004), and is also evenly distributed. The diamicton is matrix supported. There are several subvertical planar voids up to 55 mm long and 1 mm wide. They have subparallel sides with little evidence of precipitates or sedimentation, except a small amount of coarse silt at the base of the largest fracture (Fig. 7a,b). The proximity of the fractures to the sides of the core and subparallel orientation indicate an artificial origin, probably associated with the handling and splitting of the core prior to impregnation. The sedimentation at the base of the largest planar void indicates that existing pores were probably the inception points from which the fractures propagated. However, at the maximum magnification possible using a standard petrological microscope (in this case 320), a number of short subparallel and associated subperpendicular planar voids (<<100e500 mm long) can also be observed (Fig. 7b). As they are commonly <<50 mm wide, they are difficult to detect under plane polarised light and often become obscured under cross-polarised light, making differentiation from small grains problematic. Those planar voids that are large enough to be discerned conclusively generally bisect matrix rather than following grain boundaries. The specimen contains a number of turbate structures both with and without corestones, indicative of rotational deformation in response to an applied shear stress (Fig. 7a) (van der Meer, 1993, 1997a; Phillips, 2006; Kilfeather and van der Meer, 2008). This was the reason for selection and mCT analysis of a subsampled 5 mm plug, orientated orthogonal to the core. N.B. The core sample had no cardinal orientation, only ‘this way up’. The fine sand clast rich nature of the sample is not reflected in the mCT cross-section (Fig. 7c). A number of ‘exotic’ clasts (different

mineralogy) have a considerably higher density and/or atomic number than the majority of the grains, but otherwise individual particles cannot be readily differentiated from the matrix. It is reasonable to assume that a dominant proportion of the matrix particles would have been produced through erosion of the adjacent clasts, the subangular e subrounded morphology of the grains supporting this assumption (Benn, 2004a). Therefore, both the clasts and matrix will have approximately the same X-ray attenuation coefficient values, the only reason for a diversion from approximate parity being the partial volume effect caused by the presence of voids at the subvoxel scale. It can thus be concluded that the diamicton is overconsolidated, but this cannot in itself be considered indicative of a subglacial origin for the deposit. The overlying >34m of sediment will have exerted a significant normal stress upon the sample and thus could be the sole cause of consolidation. What mCT analysis reveals very clearly is the significant number of short, narrow, curvilinear fractures within the sample (Fig. 7c and d, a movie available via the web and electronic version), far more than can be detected easily in thin-section. Each crosssectional slice is one voxel thick, in this case 7.15 mm, compared to a thin-section which is approximately four times thicker. Many of the fractures are <20 mm wide and hence cannot be observed using low magnification transmitted light microscopy (Kilfeather and van der Meer, 2008). A volume rendering of the voids was constructed, both to conduct 3D quantitative analyses and qualitative evaluation of the density and distribution of the pores (Van Geet et al., 2000) (see Appendices Di and Ei). The volume of the VOI was 69.06 mm3, the volume of the voids 2.53 mm3, or 3.66%. The 3D model reveals the apparently very high density of pores within the sample (Fig. 7e). However, this is a composite image looking ‘through’ a sample that is 5 mm Ø, comprised of normally >10 individual voids along the viewer’s line of sight, the model therefore appearing ‘crowded’ (Fig. 7c,d). The removal of pores <100.1 mm3 (143 voxels) was found to clarify the model sufficiently to permit an evaluation of void distribution (Fig. 7f and g, a movie available via the web and electronic version) (see Appendix Dii). There is some clustering of voids within the sample, particularly at the top (Fig. 7f,g). The top of the specimen plug is actually the cut surface of the core sample from which the thin-section was derived. The core would have been air dried prior to impregnation and crazing (small scale fracturing) of a sample’s exposed surface is common under such conditions (E. Phillips, pers. comm., 2009). The fact that the crazing is restricted to the end of the plug provides confirmation that air drying did not lead to significant contraction of the sediment, which would have potentially created additional, artificial voids (see also Appendix Ei). In both 2D and 3D the fractures can be seen to be often connected, forming individual ‘Type I pebbles’ very similar in nature to those observed at the millimetre scale by van der Meer (1993) and Hiemstra and van der Meer (1997), which together comprise a ‘marble-bed’ structure (Fig. 7aeg). The component ‘blocks’ of the marble-bed have approximately the same dimensions along all three axes (roughly 100e200 mm) and appear to be offset from each other, reminiscent of a honeycomb structure (Fig. 7bed,f,g). A marble-bed structure is proposed to represent brecciation of the till followed by rotation of the individual elements, as evidenced by the curvilinear form of the fractures (Menzies, 1990; van der Meer, 1993; Kilfeather and van der Meer, 2008). The fractures are also evidence of sediment dilation associated with brittle deformation (Murray and Dowdeswell, 1992; Oda et al., 2004; Lee and Phillips, 2008). It has been suggested that the fracturing is the product of unloading of the sediment during deglaciation (E. Phillips, pers. comm., 2011), presumably similar in nature to the ‘elastic relaxation’ recorded by Iverson et al. (2003) and Hart et al. (2011) though over

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Fig. 7. (a) Skeleton grains defining arcuate features (Phillips, 2006), subsections of galaxy structures (one highlighted), thin-section sample O.604. Plane light, field of view ¼ 5.0 mm (b) A close-up of a portion of the thin-section where planar voids can be discerned. Plane light, field of view ¼ 2.64 mm (c) A cross-sectional image from sample O.604. The dark, short, narrow lineaments are planar voids. (d) A stack of cross-sectional images from O.604. The subparallel and associated subperpendicular planar voids define

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much greater time scales. However, such a scenario seems unlikely considering the already dilated state of sediment observed in-situ directly beneath active glaciers overlying deformed beds (e.g. Boulton et al.,1974; Engelhardt et al.,1978; Benn,1995; van der Meer, 1997b). Unsupported by elevated porewater pressures dilated sediment will collapse upon unloading, as observed by van der Meer (1997b), rather than expanding further. When taken as a whole the evidence contained within sample O.604 indicates a polyphase deformation history, i.e. brittle failure/ dilation (brecciation), followed by ductile rotation of portions of the sediment at both micron and millimetre scales producing both the marble-bed and turbate structures respectively. 5.4. C.113. Lower Fleming till, Sirius Group, Mount Fleming, Dry Valleys, South Victoria Land, Antarctica (Fig. 8aei) (Stroeven and Prentice, 1997) Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quascirev.2011.05.016. The Sirius Group is a suite of unconsolidated deposits widespread within the Transantarctic Mountains, their provenance pivotal in a debate on the extent of the East Antarctic Ice Sheet during the Pliocene. Stroeven and Prentice (1997) studied Sirius Group lodgement (their terminology) tills and concluded that the sediments were deposited by local ‘wet-based alpine ice’, based on geomorphologcal and sedimentological evidence including clast fabrics. A number of undisturbed samples were acquired from the Lower (altitudinal, not stratigraphic) Fleming till which is comprised of ‘three patches of sediment veneer’ covering approximately 0.1 km2 on a ‘small promontory’. Macroscopically the tills were subdivided into a ‘moderately consolidated light gray sediment over massive, non-stratified, consolidated, matrix supported dark gray sediments’. The deposit was classified as a ‘poorly sorted, pebbly muddy medium sand’. All the ice-flow directional evidence, including ‘striae, grooves, crescentic fractures and chattermarks’ showed a clear WSW-ENE orientation. Sample C.113 was acquired from the bottom, dark gray (unweathered) till in pit ASE 91-054, approximately 20 cm above the base of the excavation. Unfortunately the sample collapsed during transport and the orientation could not be securely established for any of the fragments. However, a relatively large fragment was selected for micromorphological analysis anyway, in order to further characterise the Sirius Till. The thin-section contains grain size ranges from <30 mm to 10 mm, the clasts angular - rounded and generally uniformly distributed throughout the sample, though some clustering does occur (Fig. 8a,b). The matrix is fine silt/clay rich, has a fine - medium density and is evenly distributed apart from a few small, brown clay units. The diamicton is matrix supported and contains few unambiguously natural voids at thin-section scale. The grains appear to have preferred orientations delineating lineations, though no one direction dominates, turbate structures also present. There is some evidence of grain crushing and both Mn and Fe nodules (precipitates). The specimen contains an elongate volcanic lithic clast, a large proportion of which is enclosed within a very fine-grained dark matrix (Fig. 8a). Originally the clast was considered to have rotated, dragging/pushing a portion of the black matrix material with/in front of it. However, the fine-grained black material has an extensive, reasonably well developed masepic plasmic fabric indicative of pervasive planar shear (Fig. 8b) (Carr, 2004). A section of the clast is contained within the specimen block proximal to the thin-section,

making plug (10 mm Ø) sample site selection and identification within the relevant mCT cross-sectional slices straightforward (Fig. 8c). Adjacent, relatively large clasts and a number of voids are also clearly visible. Unfortunately many of the smaller clasts have similar attenuating values to the black matrix and therefore cannot be differentiated. A number of planar voids are located either proximal to, or emanate from, the edges of the elongate clast (Fig. 8aec,d, a movie available via the web and electronic version). The planar nature of the voids indicates brittle fracture associated with movement of the clast through the matrix. In order to investigate this association further, ‘voids only’ and ‘dense objects (clasts) only’ volume renderings were constructed. All but the largest individual feature in both the solid and void volume renderings was removed using the automated sweep function in CTAn, to facilitate interpretation of the features of interest (Fig. 8eeh and i, a movie available via the web and electronic version) (see Appendix Dii). In 3D the majority of the voids present within the 2D thinsection and complete volume rendering are demonstrated to be one continuous fracture wrapped around the ‘nose’ of the elongate grain (Fig. 8aee, i). It can be observed that a number of connected subfractures also emanate at various angles from the nose of the grain, indicating a complex pattern of dilation (Fig. 8f). The main fracture closely follows the morphology of the elongate clast before closing in the approximate centre of the sample (Fig. 8g). Conversely, the subfractures are approximately perpendicular to the main fracture. By viewing the ‘underside’ of the model a close association between the elongate clast and the composite planar void structure within the sample is confirmed (Fig. 8h). The subparallel nature of the main fracture to the elongate clast indicates a failure plane at the contact between the grain and the surrounding matrix. This is unsurprising as the rheological difference between a clast and the surrounding matrix will focus an applied stress at the boundary, leading to failure plane development and propagation (Johnson et al., 2009; Renard et al., 2009). While the obvious conclusion to be drawn is that the main planar void is the product of brittle deformation of the sediment, a ductile origin could also be invoked, rotation of the sample occurring over a décollement surface (represented by the fracture). The antithetic subfractures indicate brittle failure, but such features are normally associated with rotation (ductile deformation) of blocks and could be the precursor to the formation of a marble-bed structure (van der Wateren et al., 2000; Twiss and Moores, 2007; Kilfeather and van der Meer, 2008). However, the fact that the main fracture wraps around the nose of the elongate clast, together with the orientation of the antithetic subfractures perpendicular to the long axis of the grain, indicate that the clast ploughed through the surrounding matrix. This scenario is corroborated by the distribution of, and masepic plasmic fabric within, the black matrix material around the nose of the clast (Fig. 8b). Ploughing is considered a significant mechanism for the transport and deposition of sediment within the subglacial environment (Brown et al.,1987; Clarke and Hansel,1989; Evans et al., 2006; Piotrowski et al., 2006; Rousselot and Fischer, 2007). ‘Comet structures’, observed in thin-section, may be evidence of this process (Menzies, 2000). 5.5. Mi.212. Broomfield, Chelmsford, Essex, England (Fig. 9aei) (Whiteman, 1987; van der Meer, 1993, 1997a) Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quascirev.2011.05.016.

‘Type I pebbles’, i.e. a ‘marble-bed’ structure (van der Meer, 1993). Note that some zones within the sample have fewer, more widely dispersed fractures. (e) A complete volumetric 3D model of voids contained within sample O.604, viewed along the YZ plane. (f) A simplified version of (e), with all individual features <100.1 mm3 removed. Note the relatively high density of voids at the top of the model. (g) The simplified volume rendering of the voids contained within O.604 rotated 360 .

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Broomfield quarry exposes glacial deposits proximal to the southern margin of the Anglian Ice Sheet. Three diamictic units are present, the conclusive genetic interpretation of which has not been possible based upon available evidence, though all the deposits were considered to have been emplaced within the subglacial environment (Whiteman, 1987). The thin-section sample (portrait), acquired from one of the units, is a grain-rich diamicton, comprised of angular - subrounded clasts up to 10 mm, the modal size around 150 mm. Clasts are unevenly distributed in indistinct sub-horizontal layers of <10 mm thickness producing a crude stratification. The matrix has a silt rich medium texture, distributed inversely to the clasts and displays a weak to strong skelsepic plasmic fabric, primarily associated with the larger grains. There are a large number of relatively short (<10 mm), narrow (<1 mm) planar voids distributed throughout the sample, some curvilinear in form and hence reminiscent of a well developed marble-bed structure, though at a slightly larger scale than observed in sample O.604. There are a number of ‘Type II pebbles’ (van der Meer,1993) also present within the sample, some partially delineated by the voids (Fig. 9a). A gravel sized clast (approximately 5.5 mm Ø) within the sample shows clear evidence of rotational deformation. The bottom left corner of the clast has become separated from the main body of the grain (Fig. 9a,b). The morphology, orientation and proximity of the fragment all indicate brittle failure following application of torsion forces (friction acting in opposition to the movement of the clast), probably immediately prior to cessation of ductile deformation. A number of ‘spiral arms’, together comprising a ‘galaxy structure’, are rooted at various points around the grain boundary, together with ‘arcuate clusters’ located within the diamicton surrounding the clast (Fig. 9b) (van der Meer, 1993, 1997a; Phillips, 2006). They indicate anti-clockwise rotation of the clast concurring with the relative movement of the fragment. The matrix surrounding the clast has a clear skelsepic plasmic fabric, also indicative of ductile deformation. It should be noted that this evidence is contained entirely within a slice of the sample <30 mm thick, suggesting that the thin-section is subparallel to the plane of shearing (van der Wateren et al., 2000; Iverson et al., 2008). A significant proportion of the clast is contained within the 10 mm Ø plug derived from the proximal portion of the sediment block (Fig. 9c). Unfortunately the associated fragment is not present, individual grains located within the surrounding matrix are poorly defined and there is no evidence of a discernible density variation associated with the skelsepic plasmic fabric surrounding the main clast. However, of note is the significant void proximal to the bottom right corner of the clast (Fig. 9c). A complete volume rendering of the mCT image stack revealed a network of fractures proximal to the rotated clast (Fig. 9d, a movie available via the web and electronic version). Using the same methodology as in sample C.113 a simplified model was constructed, again proving to be a highly effective way of facilitating the visual analysis of the objects and their relationship (Fig. 9eeh and i, a movie available via the web and electronic version). Many of the planar fractures extending through parts of the sample are connected forming a relatively large void, two elements of which are orientated perpendicular to the back face of the clast (the front face being exposed at the top of the plug)

(Fig. 9e). By rotating the model it becomes clear that the main clast is actually proximated by a subparallel planar void, from which emanate two antithetic planar fractures (Fig. 9f,g). The antithetic voids have a complex morphology, extending to the outer edge of the plug at two points only (Fig. 9h). By combining the thin-section and mCT evidence it becomes apparent that the clast proximal, subparallel planar void formed a décollement surface permitting the grain to rotate anti-clockwise. Consequently the clast experienced minimal friction parallel to the fracture, but exerted a ‘drag’ force upon the sediment proximal to the plane of shear. The clast proximal planar void extends over the sample diameter, well beyond the dimensions of the grain, and therefore would have focussed the drag force around the plane of shearing (Fig. 9d). The connectivity and antithetic nature of the planar voids can be partially explained by invoking ductile deformation, the second stage in marble-bed development. However, the non-linear form of some sections of the fractures and the nature of the intersections between them indicate a complex, probably polyphase, rheological response to what could have been one or more applications of shear stress. 5.6. BO/03/07, Boleynaminna Townland, nr Tynagh, County Galway Ireland (Fig. 10aef) (Tarplee, 2006; Tarplee and van der Meer, 2010) Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quascirev.2011.05.016. The Tynagh Pb, Zn, Cu, Ag hydrothermal mineral deposit and surrounding area is (in part was) overlain by several metres of subglacial till, an associated erratic plume extending 5.75 km eastwards from the source. The plume is comprised primarily of Pb and Zn oxide ‘metalliferous’ grains, both contained within limestone and sandstone host lithology clasts and as individual particles within the matrix (Donovan and James, 1967; Tarplee and van der Meer, 2010). In common with a number of other erratic assemblages, the ribbon type plume appears to rise through the till profile down-glacier of its source (Nurmi, 1976; Drake, 1983; Miller, 1984; DiLabio, 1990; Charbonneau and David, 1993, 1995; Lett, 2001; Tarplee, 2006). Detailed geomorphological, geochemical, sedimentological and micromorphological analyses revealed that the Tynagh erratic plume was emplaced within a deforming soft sediment bed (Tarplee, 2006; Tarplee and van der Meer, 2010). A plug was acquired from one of the undisturbed sediment sample blocks, in order to investigate further the dispersal mechanisms operating within this particular subglacial environment. A 2.9m deep pit was excavated approximately 400m down-glacier of the known limit of the mineral deposit’s sub-till expression. The sample was acquired from a massive, clast rich, matrix supported, normally consolidated, light grey diamicton containing  cobble sized, angular - rounded clasts. The thin-section (portrait) is orientated perpendicular to the longitudinal trend of the erratic plume and was acquired at 2.43m below the ground surface within the soil C-horizon, i.e. beneath the water table, thus precluding secondary precipitation as a source of the metal particles (Tarplee, 2006). The slide contains  pebble sized, angular - rounded clasts, distributed evenly and consisting of primarily limestone and sandstone (Fig.10a). The matrix is light e mid grey, fine silt/clay rich, has a fine - medium

Fig. 8. (a,b) A relatively large, elongate clast (centre of the image) almost completely surrounded by a dark, fine-grained matrix material. Sample C.113, (a) plane light, (b) crosspolarised light; field of view ¼ 14.3 mm. Note the planar voids in (a) and the masepic plasmic fabric (medium birefringence) surrounding the clast, observable in (b). (c) A crosssectional image from sample C.113. A section of the elongate clast and the surrounding fine-grained matrix are discernible easily, along with a number of planar voids. (d) A volume rendering of C.113 rotated 360 , sequentially sectioned perpendicular to the z-axis, rebuilt and sliced parallel to both the z-axis and the elongate clast, reconstructed and sliced parallel to the z-axis and perpendicular to the elongate clast. Note the proximity of the planar voids to the ‘nose’ of the clast. (eeh) Stills of the volumetric 3D models of the elongate clast (yellow) and the largest single void structure (blue), further illustrating their proximity. N.B. The combined model has been tilted by 45 towards the viewer: (e) ¼ 0 rotation, i.e. aligned with images 8c,d; (f) ¼ 60 rotation; (g) ¼ 90 rotation; (h) ¼ 180 rotation. (i) The combined volume renderings of the clast and void structure in C.113 rotated 360 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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density and is distributed uniformly apart from some small pockets of clay. There is some evidence of expansion of natural voids associated with desiccation and subsequent shrinkage of the sample, as the saturated specimen was allowed to drain for a number of days before sealing for transport. However, undisturbed planar voids (20 mm long and 2 mm wide), channels and chambers, vughs and vesicles are also present throughout the thin-section as indicated by finegrained sedimentation located within them. The sample contains a significant number of turbate structures, both with and without corestones, and a small number of discrete shears (Fig. 10a). The metalliferous grains within the thin-section are opaque under transmitted light, but have a metallic, highly reflective appearance under incident light making them readily identifiable (Fig. 10a,b). Of the 244 metalliferous grains identified within the thin-section, only six are larger than 500 mm and nine are located within clasts. As the slide contains the highest concentration of Tynagh mineral deposit erratics within the whole sample suite, the associated block was selected for mCT analysis. The distribution of metalliferous grains should provide insights regarding the size, morphology, orientation, distribution and connectivity (if any) of a variety of microstructures, particularly turbates. Plug (10 mm Ø) sample site selection was based entirely on examination of the specimen block rather than via thin-section. Only one cluster of rotational structures defined by metalliferous grains has been identified in thin-section which, as each turbate is <1 mm Ø, is highly unlikely to have an expression within the adjacent sample block (Tarplee, 2006). The chosen sample site contains both relatively large clasts and a significant proportion of fine-grained/ matrix material, in anticipation that metalliferous grains could define alignments around clasts, turbates and galaxy structures. The significant X-ray attenuating properties of Pb and Zn oxides means that metalliferous grains are easily differentiated from the limestone, sandstone, shale and conglomerate particles (Fig. 1aec). However, this contrast can also compromise the quality of the resultant dataset and images as previously described. To evaluate whether such issues could be overcome the plug was scanned using the leading edge technology contained within the ‘MuCat 2 system’ (Fig. 10c and d, a movie available via the web and electronic version) (Davis et al., 2010) (see Appendix Eiv). The limestone and sandstone clasts, which have similar X-ray attenuation properties and are therefore not easily differentiated within the SkyScan 1072 cross-sectional images (cf. Remeysen and Swennen, 2008), can be separated based on both their compositional differences and internal features, e.g. shell fragments, in the MuCat 2 scan image stack. The clast and void boundaries are also much more sharply defined and the noise minimal. Unfortunately no obvious microstructures, associations between large clasts and metalliferous grains or other patterns within the distribution of Pb and Zn particles could be discerned within the volumetric 3D model (Fig.10e and f, a movie available via the web and electronic version). There are almost 0.5 million host lithology clasts and >29,500 metalliferous grains within the respective volume renderings (Table 1). As a consequence the combined model is extremely crowded, much more so than for sample O.604. The model could be simplified to facilitate visual analysis by removing clasts below, between or above predetermined volumes. However, doing so would mean the partial deconstruction of the features of interest, any subsequent discernible structures being incomplete or potentially false. It should be noted that, in addition to the thin-section, the image stack contains 2D evidence of microstructures, though they are not apparently defined by metalliferous grains (Fig.10d). The inability to identify such structures is attributable to both the limits of human visual perception and the visualisation techniques used, i.e. volumetric 3D models presented via a two-dimensional medium. It is possible to produce anaglyphs (stereoscopic 3D animations) (Parfitt

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et al., 2010), but such an approach was deemed unnecessary because the complexity of this particular model would most likely preclude any useful insights from being gained. The ultimate solution to this problem would be to place analysts within the model, i.e. using virtual (sensory) immersion techniques, which would allow the person(s) to navigate their way around the volume rendering facilitating structure identification (Jones et al., 2009). Quantitative analyses negate the limits of human perception. A series of subjectively selected threshold values were applied to segment the voids and matrix, in addition to the host lithologies and metalliferous grains, and (semi-)quantitative data derived (Table 1). The seemingly very high numbers of individual matrix elements and voids detected suggest that partial volume effects, in combination with streak artefacts and subtle particle composition variations, have served to create artificially fragmented binary volume renderings (see Appendix Eiii). The alternative, that there are a large number of relatively small voids distributed throughout the matrix, is not borne out by visual inspection of the image stack (Fig. 10c,d). The voids comprise just over 3% of the specimen volume, very similar to sample O.604. However, in contrast to the de Woude specimen, the unambiguously natural voids in sample BO/03/07 are predominantly vughs and vesicles, channels and chambers, a small number of planar voids also present (Fig. 10c,d). Unfortunately a simplified volume rendering of the voids could not be created in this case. Void volumes have a wide distribution and hence no minimum value could be used to reduce the number of pores represented to a satisfactory level without oversimplifying the resultant model. The matrix comprises over twice as much of the sample volume as the clasts, complementing the field and thinsection evidence. In contrast the 1.45% sample volume comprised of metalliferous grains conflicts with the thin-section evidence of <<1% Pb and Zn oxide particle content. This reflects the basic method of grain identification applied in the thin-section analysis and both the very sensitive nature and high spatial resolution of dense particle detection achievable using mCT. The CTAn software can be used to quantify a number of additional morphometric parameters both for each individually defined object and groupings thereof, two examples provided in Table 1 (SkyScan, 2008b). The morphology index provides an indication of particle shape (Benn, 2004a). The ‘Degree of Anisotropy’ value is based on mean intercept length and eigenvector/value calculations and is therefore applicable directly in this research (SkyScan, 2008b). As all defined objects within each segmented model were analysed, regardless of morphology, the relatively weak resultant values are unsurprising (see also Appendix Diii). 6. Discussion The additional evidence provided by volumetric 3D modelling of structures associated with 2D type examples of microscopic glacial soft sediment deformation permit an evaluation of our understanding of the nature, dimensions and extent of the mechanisms invoked to explain these features. 6.1. O.514, Holmstrømbreen push moraine, Ekmanfjorden, Spitsbergen, Svalbard archipelago (Fig. 5aeo) The evidence for the sequential (D1 and D2) phases of compressive deformation contained within samples O.514a and b can be interpreted in three ways: (1) The sediment was unfrozen, containing little or no interstitial ice during all phases of deformation. Sediment rheology and hence failure mechanism was controlled by porewater content

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Fig. 9. (a) A relatively large clast (centre of the image) displaying three attributes indicative of rotation in thin-section sample Mi.212. Plane light, field of view ¼ 13.8 mm (from van der Meer, 1993, 1997a). (b) A manipulated image of (a), containing only the main clast and the larger quartz grains contained in the surrounding sample. The main structural features are annotated (from van der Meer, 1997a). (c) A cross-sectional image from sample Mi.212. A section of the rotated clast (centre) and associated planar voids (dark areas) can be

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and pressure (Boulton et al., 1974; Alley, 1989; Menzies, 1989). Initially the porewater content and pressure was relatively high, lowering effective pressure such that the sediment failed through pervasive deformation i.e. the ductile part of the D1 phase that produced the asymmetric fold. A decrease in the porewater pressure led to an increase in sediment shear strength, the localising of shear stress at points/zones of weakness and hence discrete, brittle deformation, i.e. the thrust faulting portion of the D1 event. Both elements of D1 were orientated ice-distal. Fracture development enhanced the permeability of the sediment, leading to dewatering and a further increase in effective pressure/sediment shear strength (Benediktsson et al., 2010). This led to the redistribution of strain over a larger volume of sediment producing the discrete, but dispersed, ice-proximal (D2) reverse faults. The presence of WES can be readily explained within the context of such a scenario (van der Meer et al., 2003; Menzies et al., 2006, 2010; Menzies and Whiteman, 2009). (2) The sediment was frozen (permafrost) during all phases of deformation. Interstitial ice would have bonded the grains together creating a cohesive block of sediment (Williams and Smith, 1989; Menzies and Whiteman, 2009) at least at the metre scale, i.e. unit thickness, though the continuity of structural deformation evidence presented in Boulton and van der Meer et al. (1989) and Boulton et al. (1999) indicates that such conditions would have extended throughout the area of the push moraine. During the glacier surge the sediment buckled initially, producing the folding observed in the western gorge field section. The sample site was located within the contracting inner zone of a limb of one of the developing open folds. Therefore reorientation and micro-scale compression of the sediment would have occurred. The compressive force was dissipated through ductile and, subsequently, brittle failure, i.e. the D1 phase. The strengthening/stiffening effect of the permafrost limited ductile deformation, e.g. the asymmetric fold and hence the continuing surge led to an increase in undissipated driving stresses (Brandes and Le Heron, 2010). Eventually the sediment failed through discrete shearing at a relatively low angle and in both directions parallel to the axis of principle force, i.e. the thrusts and backthrusts observed in field section, the former also manifest in the thin-section and O.514b. Continued reorientation of the volume of sediment comprising sample O.514, during larger scale deformation, would have changed the orientation of the principle stresses acting upon it. This led to further dissipation of stress occurring through failure of the sediment along short planar shears distributed throughout the sample, i.e. the reverse faulting (D2) phase. Thus there would be no requirement for a pervasive reduction in volume of the sediment pile, manifest as a contraction of pore space, pressure-melting and subsequent expulsion of water. (3) The presence or absence of permafrost had little bearing upon the D1 phase of sediment deformation. The folding and thrust faulting may have been syn-depositional; compaction driven collapse of a nearby zone of sediment (outside and iceproximal to the sample block) applying a compressive lateral force, coincidentally subparallel to the orientation of the subsequent glaciotectonic trend. Such compression could have created a minor porewater pressure wave that led to the

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production of the WES. The subsequent application of a driving stress by the glacier surge produced the more dispersed D2 reverse faults under either frozen or unfrozen conditions. Scenarios one and three have both merits and shortcomings. The unfrozen (‘wet’) scenario (1) permits dynamic rheological responses due to fluctuations in porewater content and pressure (Menzies, 1989; Boulton and Dobbie, 1998), affecting both the extent (discrete versus distributed) and orientation (towards or away from the glacier) of the strain signature. The width of the main thrust fault zone in O.514b also supports a wet scenario. If ice were present to bond grains together and create an impermeable barrier to pressurised porewater ingress (Freeze and Cherry, 1979), it is difficult to envisage how a volume of sediment many times thicker than the average grain size (>50 mm) could be eroded from the fault walls, thus creating the sediment lenses. Instead an initial thin layer of sediment (one or two grain diameters wide) would likely act as a suite of ‘bearings’, facilitating movement and thus preservation of the fracture surfaces. However, the absence of permafrost approximately 254m inside the ice-distal limit of the push moraine does not fit with the field and theoretical evidence presented in Boulton and van der Meer et al. (1989) and Boulton et al. (1999). Permafrost extending throughout the area of the push moraine was a prerequisite for the hydrological conditions that led to the development of the underlying décollement surface, necessary for the extensive propagation of the glaciotectonic feature and its structural components. The syn-depositional compaction driven D1 phase hypothesis (3) requires an element of coincidence, though that caveat does not automatically invalidate it. Conversely the permafrost hypothesis (2) explains the very good preservation of the laminae and localised discrete, brittle nature of much of the deformation; the former difficult to reconcile readily with the other two scenarios. It also conforms with the wider field evidence and is therefore the preferred scenario. The width of the D1 thrust fault zone and WES can be attributed to the presence of meltwater, produced through frictional heating associated with fault movement. This water would have been contained within the fractures, but have penetrated into the fault walls thus weakening them, thereby facilitating lateral expansion of the fault zone. The individual sediment lenses contained within O.514b indicate reactivation of the thrust fault at least once, the second event mobilising a narrower volume of sediment than the first. Alternatively the lenses are the product of the penultimate and final fault movements. All three scenarios could be tested in the laboratory using an artificial sediment deformed in a suitable apparatus (triaxial rig) under both frozen and unfrozen conditions; specialist apparatus allowing control of water pressure (Altuhafi et al., 2009) and realtime scanning of deformation using mCT (Razavi, 2006). However, even without such tests it has been unequivocally demonstrated that mCT can both enhance 2D thin-section evidence and provide invaluable additional information, in this case regarding the 3D orientation of faults. 6.2. Mi.595, Irish Sea Till, Fassaroe, Enniskerry, Ireland (Fig. 6aec) The validity of the glaciomarine and terrestrial basal glaciotectonic hypotheses can be in part evaluated by considering the porosity of sample Mi.595. As Menzies and Zaniewski (2003),

differentiated. (d) A volume rendering of Mi.212 rotated 360 , sequentially sectioned perpendicular to the z-axis, rebuilt and sliced parallel to both the z-axis and the clast apparent a-axis, reconstructed and sliced parallel to the z-axis and perpendicular to the clast a-axis. Note the proximity of the planar voids to the clast. (e-h) Stills of the volumetric 3D models of the clast (grey) and the largest single void structure (blue), further illustrating their proximity. N.B. The combined model has been tilted by 60 towards the viewer: (e) ¼ 0 rotation, i.e. aligned with images 9c,d; (f) ¼ 67 rotation; (g) ¼ 90 rotation; (h) ¼ 180 rotation. (i) The combined volume renderings of the clast and void structure in Mi.212 rotated 360 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 10. (a), (b) The portion of thin-section BO/03/07 adjacent to the plug sample site. Note the channel at the bottom centre of the image and the deformation structures. The position of (b) is indicated by the dashed rectangle. Plane light, field of view ¼ 14.3 mm (b) A close-up of the thin-section, highlighting the presence of metalliferous grains. Incident light, field of view 2.64 mm (c) A cross-sectional image from sample BO/03/07. The white spots indicate individual metalliferous grains, light grey forms are clasts, black areas are voids. Note the difference in contrast resolution compared to 1c. (d) A stack of cross-sectional images from BO/03/07. (e) A volumetric 3D model of BO/03/07 sectioned to illustrate the random distribution of metalliferous grains (red). (f) The volume rendering of BO/03/07 rotated 360 , sequentially sectioned perpendicular to the zaxis, rebuilt and sliced parallel to the z-axis, reconstructed before removing all but the highest density material (the metalliferous grains), this segmented model rotated 360 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Phillips (2006) and Menzies and Whiteman (2009) have established, turbate structures and rounded intraclasts can develop in both subglacial soft sediment deforming beds and debris flows. However, the confining (normal) pressure will differ very significantly between these two environments, especially if the debris flow were to occur within a glaciomarine environment where buoyancy effects will counter gravitational forces (Maltman and Bolton, 2003). Porewater, common to both environments, will be expelled as soon as the overburden pressure, either the weight of the overlying sediment for a submarine debris flow or sediment plus

ice-mass for a subglacial environment, exceeds the porewater pressure within the deposit (Boulton and Dobbie, 1993, 1998; DeBlasio et al., 2004). For a submarine debris flow the overlying water column will help maintain porewater pressure, conversely an ice-mass will accelerate water expulsion from the sediment (van der Meer et al., 1999, 2009; DeBlasio et al., 2004; Kjær et al., 2006; Menzies and Whiteman, 2009; O’Regan et al., 2010). A reduction in pressurised water content will lead to increased grain to grain contacts, the failure of the weakest connections, reconfiguration of grains and hence normal consolidation through a reduction in the

M.F.V. Tarplee et al. / Quaternary Science Reviews 30 (2011) 3501e3532 Table 1 Compositional data and selected morphometric parameters derived from the volumetric 3D model of sample BO/03/07. Sample component

Number of Percentage Morphology index individual of volume (0 ¼ plate, elements 3 ¼ cylinder, 4 ¼ sphere

Voids 328893 Matrix 257494 Host lithologies 469912 Pb and Zn oxides 29531

3.09 66.26 29.20 1.45

2.33 e 2.36 6.53

Degree of anisotropy (0 ¼ isotropic, 1 completely anisotropic) 0.21 e 0.20 0.20

percentage of voids, i.e. porosity (Clarke, 1987; Boulton and Dobbie, 1993, 1998; Sane et al., 2008; Menzies and Whiteman, 2009). Consequently, a diamicton deposited and/or deformed within a subglacial setting is far more likely to be well-consolidated than one emplaced during a submarine debris flow. By reference to the observations of Kilfeather and van der Meer (2008) it can be seen that the very low porosity (0.02%) of sample Mi.595 is below that of a well-consolidated till, i.e. it could be classified as a very well-consolidated diamicton, and around 2.75 orders of magnitude less than that of a flow till. This finding is corroborated by the apparently very low void content contained within the thin-section. Both lines of evidence support a terrestrial basal glaciotectonic origin for the Irish Sea Till as compared to a glaciomarine (debris flow) scenario (McCarroll and Rijsdijk, 2003). Rijsdijk et al. (2010) reached the same conclusion based upon an entirely macroscopic, predominantly glaciotectonic structural approach. Though the mCT porosity/consolidation data supplements the thin-section evidence and corroborates both the findings of Kilfeather and van der Meer (2008) and the m-km scale studies by Rijsdijk et al. (2010), it cannot be considered in isolation diagnostic of a particular depositional environment. However, where information is limited, such as in core samples, it can provide significant additional insights and can be conducted prior to the application of destructive investigative methods.

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6.4. C.113. Lower Fleming till, Sirius Group, Mount Fleming, Dry Valleys, South Victoria Land, Antarctica (Fig. 8aei) The style of brittle failure associated with the ploughing clast in sample C.113 demonstrates the complex rheological response of a diamicton to a locally applied shear stress (Rousselot et al., 2008; Thomason and Iverson, 2008). The fact that the elongate volcanic fragment was able to plough through the till means that the sediment’s shear strength would have been lower than that of the shear stress. The wet-based subglacial conditions inferred by Stroeven and Prentice (1997) would have provided the elevated porewater pressure necessary to weaken the sediments to that extent, in particular softening the proximal fine-grained matrix which in turn would have permitted the development of the masepic plasmic fabric it displays (Alley, 1989; van der Meer, 1993; van der Meer et al., 2003). In order for the sediment in front of the clast to fracture it must have dewatered, thereby increasing the shear strength of the material (Menzies and Zaniewski, 2003; Rijsdijk et al., 2010). Rousselot et al. (2008) and Thomason and Iverson (2008) observed that porewater pressure increased immediately in front of two different types of instrumented apparatus ploughing through till under laboratory controlled conditions. It is therefore assumed that the water escaped via a pre-existing conduit (a void or an interlinked number thereof) within the sediment, which the volcanic lithic fragment encountered as it ploughed. A number of vughs are present within the sample, one connected to the fracture network, supporting this theory (Fig. 8c,dei) As the shear strength of the till increased, the style of deformation changed from pervasive ductile to discrete brittle, i.e. the fracturing phase. Further dewatering and hence strengthening of the matrix led to lodgement of the clast. In this respect Stroeven and Prentice (1997) are correct in using the term ‘lodgement till’ sensu stricto. However, as there is considerable additional evidence of deformation, both brittle and ductile, within the thin-section it would be more appropriate to refer to the deposit as a deformation till (Dreimanis, 1988), a tectomict (van der Meer et al., 2003; Menzies et al., 2006) or a traction till (Evans et al., 2006). 6.5. Mi.212. Broomfield, Chelmsford, Essex, England (Fig. 9ei)

6.3. O.604. de Woude, Holland (Fig. 7aeg) Using the findings of Kilfeather and van der Meer (2008) as a guideline of likely percentage porosity for different types of glacial sediment, it can be seen that sample O.604 has a similar value (3.66%) to that of ‘well-consolidated fissile till’, i.e. 1.70e4.25%. This comparison is based on four samples, none of which is greater than 20 mm Ø, and so it is inappropriate to draw substantive conclusions from the data. However, it is clear that mCT creates the opportunity to further enhance our understanding of void structure, its development and influence on porosity, permeability and hence (sub)glacial sediment hydrology (Oda et al., 2004; Pender et al., 2009). The evidence for a marble-bed structure, at a scale hitherto unrecognised because of the limitations associated with the thickness of thin-sections, highlights the possibility that this particular microstructure is far more prevalent in subglacial soft sediments than previously considered. The structure indicates initial brittle failure followed by ductile deformation, evidence for a polyphase response to an applied shear stress (Hiemstra and van der Meer, 1997; Kilfeather and van der Meer, 2008). The latter deformation phase concurs with the thin-section evidence of larger scale turbates without corestones (van der Meer, 1993; Phillips, 2006), the individual marbles/blocks theoretically also deforming internally leading to individual grain alignments forming the spiral arms of galaxy structures.

The plug derived from Mi.212, proximal to the rotated grain, contains an arguably even more complex strain signature than that of C.113, particularly relating to brittle deformation as the evidence for ductile deformation is unambiguous (Fig. 9a,b). However, though a possibly unique scenario, the evidence does provide a first insight into the 3D morphology of rotational structures. The position of the décollement surface proximal to the clast and subparallel to the inferred plane of shearing suggests that rotational structures are areally restricted along the axis of rotation, i.e. the axis of smallest principal effective stress (Hiemstra and van der Meer, 1997; van der Wateren et al., 2000; Twiss and Moores, 2007). The extension of ductile deformation into the diamicton surrounding the clast, as evidenced by the spiral arms present in the thin-section, indicates that rotational structures are more areally extensive perpendicular to the axis of rotation, i.e. along the axis of largest principal effective stress (Hiemstra and van der Meer, 1997; Twiss and Moores, 2007). This would imply that the rotational structure analysed has a lenticular disc (discus) shape, intuitively a probable morphology considering the direction of the applied shear stress and confining nature of the surrounding diamicton (Fig. 11). It has been suggested that a lenticular disc does not reflect the fact that any such structure will be deformed by the simple shear stress that creates it (McCarroll and Rijsdijk, 2003; K. Rijsdijk, pers. comm., 2009). This is a valid point and it could therefore be argued

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would be aligned along or proximal to fracture planes (Miller, 1984; van der Meer, 1997a; Tarplee, 2006). Complex microfracturing of the types outlined previously and elsewhere, e.g. Phillips et al. (2007) would undoubtedly disrupt such a pattern. However, under such conditions it seems reasonable to expect the presence of some discernible evidence of Pb and Zn grain alignment, associated with the final phase of deformation within the sample. An alternative scenario is that discrete and pervasive shearing, both brittle and ductile, occurred throughout the sample intermixing thoroughly the metalliferous erratic material with autochthonous as well as less obviously allochthonous clasts (van der Meer, 1997a; Tarplee, 2006). The paucity of discernible deformation structures defined by metalliferous grains, both in 2D and 3D, can be in part attributed to the ephemeral nature of these features; disrupted/destroyed by subsequent different modes of deformation as proposed for such forms within debris flows (Menzies and Zaniewski, 2003; Phillips, 2006; Tarplee, 2006). 6.7. A comparison of void ratios between selected samples; evidence for pore evolution during soft sediment deformation?

Fig. 11. A schematic representation of the lenticular disc morphology rotational structure contained within Mi.212 (adapted from van der Meer, 1993, 1997a).

that the shape should be adjusted to a lenticular ellipsoid. However, the available evidence, both within the sample Mi.212 and other specimens, does not support such a modification, rotational structures being predominantly (near) circular in form. By way of explanation it has been proposed that the majority of turbate structures, particularly those without corestones, are highly ephemeral features that are rapidly destroyed by the torsion forces that produce them (Tarplee, 2006). Phillips (2006) proposed that a rotating clast, such as the one contained within sample Mi.212, entrains the surrounding material (smaller clasts and matrix) forming a ‘spherical to ellipsoidal envelope (cell)’, indicating that either effective stress is equal in all directions (spherical) or that such stress is concentrated/least well constrained along the axis of rotation (ellipsoid), i.e. counter to the scenario outlined above. It is probable that these two scenarios are end-members, the majority of rotational structure forms in-between them. It is impossible to reconstruct the strain history of the portion of sample Mi.212 containing the rotational structure from the available evidence. However, the arguably intrinsic association of the rotational structure (ductile shear) and décollement surface (brittle shear) indicate a proximal, contemporaneous polyphase style of deformation, counter to previous concepts of a strain response where discrete and pervasive forms of shearing occur sequentially dependent on fluctuations in water content/pressure (van der Meer et al., 2003; Menzies et al., 2006; Phillips et al., 2007; Lee and Phillips, 2008). 6.6. BO/03/07, Boleynaminna Townland, nr Tynagh, County Galway Ireland (Fig. 10aef) The inability to discern patterns within the metalliferous grain fraction of sample BO/03/07 may well be significant. If transport/ dispersal of such material (and, by association, the enclosing deforming soft sediments) was the product of discrete brittle shearing alone, in the simplest scenario the metalliferous grains

The percentage volume of voids within plug O.604 (3.66%) is similar to that contained within the specimen BO/03/07 (3.09%); unsurprising as both samples are derived from subglacial sediments that have undergone polyphase deformation. However, the characteristics of the voids in each sample are very different; the de Woude specimen containing connected planar voids indicative of discrete, brittle shear while the Tynagh sample has more amorphous, poorly connected pores such as vughs and channels/chambers. BO/03/07 was acquired from a normally consolidated till, the consolidation unrecorded of the core from which sample O.604 was taken. However, if the reasonable assumption is made that the matrix has been derived primarily from the proximal clasts and therefore has a similar effective atomic number, the mCT data indicates that the de Woude till is overconsolidated because the fine-grained material has a similar density to the enclosed grains, i.e. inter-particle porosity is minimal (Fig. 7c). This finding is supported by the mCT data derived from scanning the Tynagh sample using the same instrument, i.e. the SkyScan 1072 (Fig. 1c). Within the cross-section through plug BO/03/ 07 the grains can be readily differentiated from the matrix, complementing the field evidence of normal consolidation. Upon application of a shear stress a sediment package will initially undergo compaction/strain hardening, i.e. a reduction in the void ratio, often leading to overconsolidation (Boulton and Dobbie, 1993, 1998; Altuhafi et al., 2009). When the volume of the deposit cannot be reduced further, continued application of the shear stress can cause the shear strength of the sediment to be exceeded and consequent failure, leading to dilation/strain softening often manifest as fractures, e.g. fissility (Boulton et al., 1974; Murray and Dowdeswell, 1992). However, in the subglacial environment pressurised porewater also controls sediment shear strength and rheology, an increasing percentage changing the nature of shear strain from brittle through ductile to liquefaction potentially (Boulton et al., 1974; Alley, 1989; Menzies, 1989; van der Meer et al., 2003; Evans et al., 2006; Menzies et al., 2006). Therefore a sediment package that has failed and fractured may subsequently experience ingress of pressurised porewater along those shear planes, which can initiate either ductile deformation or liquefaction consequently disrupting or destroying the fissility. The cyclical occurrence of this process is evidenced by the often fragmented nature of deformation structures (Menzies and Zaniewski, 2003; Phillips, 2006; Tarplee, 2006; Menzies et al., 2010) It is proposed that the void characteristics of three samples represent stages within the cycle of void development, reduction and near eradication within the subglacial soft sediment substrate, i.e.

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O.604, BO/03/07 and Mi.595 respectively. Void development is addressed first because the nature of this cyclical process means that any evidence of a pre-existing void structure will have been overprinted. The marble-bed structure contained within sample O.604 represents a well developed fissility that would act as a highly effective conduit for pressurised water ingress, the vertical fracturing component facilitating the movement of water between the subhorizontal shear planes (Menzies, 1989; Murray and Dowdeswell, 1992). The presence of pressurised porewater would promote ductile deformation which would in turn disrupt or destroy the marble-bed structure, impeding further water throughflow and potentially leading to sediment liquefaction (Menzies, 1989; van der Meer et al., 2003). A more distributed, less well connected void structure would result, i.e. that observed within sample BO/03/07. During times of relatively high shear stress and low porewater pressures strain hardening would occur, reducing the void ratio to near zero depending on the nature of the sediment (e.g. modal clast and matrix sizes etc.), i.e. the very low void ratio detected in sample Mi.595 (Boulton and Dobbie, 1993, 1998; Altuhafi et al., 2009). 7. Summary

mCT has permitted insights into the characteristics of glacial sediments otherwise impossible using existing, destructive analytical techniques. The type of information that can be acquired varies between sediment samples, being heavily dependent on both the density/effective atomic number of the constituent lithologies and the compaction of the specimen components. However, quantitative analyses are always possible and, where detectable, a variety of void/pore characteristics can be objectively evaluated. Most importantly, all such analyses are conducted on volumetric 3D datasets. Scanning of a suite of samples acquired from a deformed proglacial sediment deposit (O.514), the constituents of which comprise an ‘ideal’ specimen for analysis using mCT, has confirmed the potential of the technique. The composition and structure of the sample observable in hand specimen (a resin impregnated block) and thin-section are readily discerned in the digital models created from the mCT scan datasets. Indeed, in many ways images of the virtual volume renderings appear near identical to the proximal portions of the thin-section, but may have better contrast resolution. The morphology, orientation and relationships of all the structural elements contained within the three specimen plugs derived from sample O.514 were analysed. The 2D and 3D evidence combined indicate that deformation phases D1 and D2 are directly associated with formation of the Holmstrømbreen push moraine, the recumbent and overturned folds preceding that event potentially. By analysing kinematic indicators in volumetric 3D it has been demonstrated that the strain response of subglacial sediment to an applied shear stress tends towards polyphase (brittle/ductile) and can be structurally complex (van der Meer, 1993, 1997a; Phillips and Auton, 2000; van der Wateren et al., 2000; Menzies and Zaniewski, 2003; van der Meer et al., 2003; Menzies et al., 2006; Phillips et al., 2007; Menzies and Whiteman, 2009). The polyphase deformation history of sample Mi.595 has been established unequivocally based on 2D evidence alone (van der Meer, 1993; van der Meer et al., 1994). However, mCT analysis has provided compelling evidence regarding within which of two possible environments the sequence of deformation events took place, i.e. glaciomarine (Eyles and McCabe, 1989) or terrestrial basal tectonic (Rijsdijk et al., 2010). The very low porosity of the sample indicates significant overconsolidation, the product of compaction associated with the presence of an overlying ice-mass and low porewater pressure conditions, corroborating the conclusions reached by van der Meer et al. (1994) and Rijsdijk et al. (2010).

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The subglacial till from which sample O.604 was acquired is also known to have undergone polyphase deformation, based on thinsection evidence only (van der Meer, 1993). However, considerable additional evidence of sediment kinematics contained within the sample laid beyond the spatial resolution possible using micromorphological techniques. mCT analyses have revealed an extensive network of connected planar voids, many <20 mm wide. The configuration of the fractures delineates a marble-bed structure, many of the individual elements near equidimensional. This information both complements and adds to the 2D evidence for a polyphase deformation history. It also indicates that the marblebed may be far more prevalent than previously considered, the formative processes primarily controlled and/or constrained by forces acting equally along all three principal effective stress axes. The micromorphological kinematic evidence regarding the emplacement of the volcanic lithic clast contained within sample C.113 is ambiguous, indicating either rotation or ploughing of the grain through the surrounding fine-grained matrix. By imaging the void structure proximal to the clast using mCT, it has become apparent that the grain ploughed into position following a linear path immediately prior to cessation of movement. Dewatering of the surrounding material led to compaction/strain hardening and hence a change from pervasive planar shear to brittle failure, followed by emplacement of the clast. The presence of water, essential to this interpretation of the evidence, corroborates Stroeven and Prentice’s (1997) conclusion that the Sirius Group tills were emplaced by a local subpolar (wet-based) glacier. However, it is appropriate to consider the deposits a product of subglacial soft sediment deformation rather than lodgement. Thin-section sample Mi.212 contains unequivocal evidence of ductile, rotational deformation. mCT has revealed that brittle, planar fracturing was also intrinsically associated with rotation of the clast immediately prior to emplacement. Though the sequence of events, ductile to brittle deformation or visa versa, cannot be determined based on the available evidence, once again a polyphase response to an applied shear stress is unequivocally demonstrated. Furthermore a first insight into the 3D morphology of rotational structures has been gained suggesting that such features approximate a lenticular disc in form, thereby indicating unequal force magnitudes along two of the principal effective stress axes. This contrasts with the 3D evidence contained within sample O.604 and points towards a continuum of force distribution parallel and perpendicular to the plane of shear, from that contained within sample Mi.212 to the converse elliptical state as proposed by Phillips (2006). The mCT analysis of sample plug BO/03/07 highlights the current, though largely surmountable, limitations of the technique and its considerable potential. Ostensibly well over one million individual objects are contained within the volume rendering of the scan dataset, making subjective analyses impossible. However, virtual serial thin-sectioning of the sample can be performed and each image analysed using micromorphological techniques; suites of objects can be differentiated semi-objectively in both 2D and 3D and a variety of quantitative morphometric and statistical analyses conducted. The scan also demonstrates the (seemingly) complex nature of subglacial sediment dispersal processes at the microscale. However, the other case studies have demonstrated that the mechanisms involved in soft sediment deformation within this environment are relatively simple and reasonably well understood in many respects. It is most likely that multiple reactivations of the same mechanism(s), the signatures of which partially overprint each other, generate the complex particle distributions observable in samples such as BO/03/07. By combining mCT evidence of consolidation with pore morphology and connectivity, it has been possible to provisionally place a number of the studied samples at different points within

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the subglacial deforming sediment strain cycle, similar to the ‘Life cycle of granular materials’ proposed by Muir Wood (1998). The disparate nature of the three samples with respect to their composition, depth beneath ground surface (and hence consolidation due to normal loading), chronostratigraphic position, geographic location and probable strain history compromise such comparisons significantly. However, together they present a case for a more detailed investigation of the evolution of voids in response to applied pure and simple shear stresses, in turn influenced by porewater pressure fluctuations. 8. Future work 8.1. Exploiting advances in mCT and associated technologies During the period of this research project (2007-2010) continued advances in X-ray computed tomography hardware and software have facilitated the acquisition of radiographs with significantly improved contrast resolution. The ‘dual-energy’ approach has been proven to provide additional improvements in the imaging and analysis of geological samples and should be equally applicable to diamictons (see Appendix Eii) (Van Geet et al., 2000, 2003; Karacan, 2007; Remeysen and Swennen, 2008). The lithologically heterogeneous nature of much glacial sediment can compromise the assumption of linear attenuation of X-rays passing through a sample, sometimes significantly, e.g. the metalliferous grain content of specimen BO/03/07. However, micromorphological thin-sections offer a possible solution to this problem. A mCT scan of a sample could be conducted and a thin-section subsequently acquired. The slide could then be scanned, though probably only as a single (2D) radiograph as its very high aspect ratio would create significant streak artefacts in a reconstructed (3D) cross-sectional image stack (Davis, 1997). This single radiograph could be used to automatically correct its equivalent within the volume rendering, those adjustments then applied via interpolation to the remainder of the model (Foubert et al., 2009). In addition, the thin-section clast evidence could be used to identify the grey-scale range pertaining to each lithology, though overlapping distributions could well complicate/compromise such differentiation (Remeysen and Swennen, 2008; Foubert et al., 2009; Long et al., 2009; Elangovan et al., 2010). There are now a number of mCT instruments ideal for scanning sediment samples and other solid materials e.g. the Nikon Metrology HMX 225 CT recently acquired by the School of Geography, Queen Mary University of London. The large sample capacity (150 mm Ø) and greater X-ray source output of this instrument will negate the requirement for scanning only portions of either Kubiena or mammoth box samples. Dual-energy sample scanning will be possible, as will ‘tuning’ of the source output to the characteristics of either the detector or the element of interest within the specimen. The sample cabinet within such instruments is relatively large, permitting the introduction of additional equipment such as geotechnical apparatus (uni- or triaxial compression stages), a cooling stage or environment control equipment. The feasibility and significance of the types of research such adaptations permit has already been demonstrated (Thomson and Wong, 2003; Razavi, 2006; Buffiere et al., 2010). Such equipment is increasingly commonplace in universities, particularly within life and material science faculties or as dedicated facilities. 8.2. Proposed directions in glacial sediment mCT research Geotechnical sediment characterisation studies traditionally quantify consolidation by measuring the reduction in volume of the

sample, i.e. the void ratio (Boulton and Dobbie, 1993, 1998; Muir Wood, 1998; Rea, 2004; Altuhafi et al., 2009; O’Regan et al., 2010). As it is the structure of the sediment that changes, the composition remaining nominally constant, mCT offers an additional, potentially more accurate method for quantifying compaction. A consolidation index could be developed by combining geotechnical and mCT analyses, based on the principle that replacing a void with a grain, or part thereof, increases the relative density of the overall sample with respect to X-ray transmission. It should be therefore possible to use clast - matrix density ratio values as an indicator of likely genetic conditions and/or environments, as was done with the Mi.595 porosity data with reference to the work of Kilfeather and van der Meer (2008). The problem of contrast resolution in mCT images does not apply to voids within diamictons as has already been demonstrated by Kilfeather and van der Meer (2008). The distribution, size and connectivity of voids within subglacial till, i.e. its drainage network, is a fundamental control not only on the nature and extent of sediment deformation but also the dynamics of the whole glacier system (Boulton et al., 1974; Alley, 1989; Menzies, 1989; van der Meer et al., 2003; Piotrowski et al., 2004; Evans et al., 2006; Menzies et al., 2006). The morphology and orientation of voids can also reveal much about their likely genesis and history, e.g. whether a fluid has passed through and modified them (Menzies, 2000; van der Meer et al., 2003; Kilfeather and van der Meer, 2008). Most methods of void/pore analysis are either indirectly quantitative, e.g. mercury intrusion porosimetry (MIP) (Cnudde et al., 2009) or limited to 2D insights, e.g. micromorphology (Kilfeather, 2004; Kilfeather and van der Meer, 2008). mCT presents the opportunity to characterise voids in unprecedented detail, and hence the possibility of understanding how the subglacial drainage system evolves and responds to cyclical applications of varying levels of shear stress under fluctuating effective pressure regimes. Combining quantitative consolidation and void characteristic evidence acquired using mCT allows inferences to be made as to where within the ‘strain cycle’ a subglacial deformable soft sediment sample was located at the moment of emplacement (Fig. 12). Such an approach would permit ‘strain mapping’ of both active subglacial environments, via borehole drilling and sample recovery, and palaeoglaciated landscapes/landforms. In principle this would allow testing of the various postulated ‘bed mosaic’ or ‘anastomosing’ models of distributed soft sediment deformation (Menzies, 1989; Piotrowski and Kraus, 1997; Boulton and Dobbie, 1998; van der Meer et al., 2003; Piotrowski et al., 2004; Lee and Phillips, 2008; Shumway and Iverson, 2009). A small scale project applying this approach is currently being undertaken on samples acquired from a drumlin that has recently been exposed during retreat of Múlajökull, Iceland (Johnson et al., 2010). Replication of some of the boundary conditions operating within the subglacial zone has been possible under laboratory controlled conditions at relatively small scales (e.g. Iverson et al., 1997, 1998, 2007, 2008; Boulton and Dobbie, 1998; Tulaczyk et al.,

undeformed / structureless liquefaction

ductile deformation

structural overprinting

consolidation

failure / brittle deformation

Fig. 12. The ‘strain cycle’ of subglacial sediments, illustrating the ephemeral nature of micro-scale deformation structures (adapted from Muir Wood, 1998).

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2000; Müller and Schlüchter, 2001; Larsen et al., 2006; Altuhafi et al., 2009). Research on two sections of the Whillans Ice Stream, West Antarctica by Tulaczyk (2006) established that the rheology of the soft sediment substrate is scale independent, i.e. laboratory experiments can successfully replicate subglacial conditions (cf. Hooke et al., 1997). Similarly, studies by Müller and Schlüchter (2001), Larsen et al. (2006), Thomason and Iverson (2006) and Iverson et al. (2008) have demonstrated that remoulded and subsequently laboratory (ring-shear) deformed subglacial till samples contain a number of the same microstructures observable in 2D (thin-section) as do undisturbed specimens. mCT presents the opportunity to further test the validity of laboratory techniques in replicating subglacial kinematics and hence likely deformable soft sediment rheology within such an environment. Void reduction under a significant normal load and/or evolution during simple shear, i.e. subglacial soft sediment consolidation and deformation, is the obvious process to study first using mCT. Relatively low strain, undrained triaxial experiments, similar to those conducted by Thomson and Wong (2003) and Pender et al. (2009), could be undertaken to assess the role of effective pressure in void development. Drained ring-shear experiments, such as those carried out by Boulton and Dobbie (1998), Larsen et al. (2006) and Thomason and Iverson (2006), could be used to observe the evolution of void structure with increasing strain. Finally, pore evolution under undrained and low effective pressure conditions, probably the most common state leading to subglacial soft sediment deformation, could be analysed from low to high strains, again in a ring-shear apparatus (J. Piotrowski, pers comm., 2010). One of the key indicators of subglacial sediment kinematics is clast fabric, though both individual particle dynamics and analyst subjectivity remain subjects of debate (Hart, 1994; Benn, 1995; Bennett et al., 1999; Carr and Rose, 2003; Carr and Goddard, 2007; Chandler and Hubbard, 2008; Iverson et al., 2008). mCT has the potential to both remove the subjective element of the technique and create statistically far more robust datasets, based on >100000 individual clasts rather than 50. Thus an entirely automated approach can be applied to clast fabric anisotropy evaluation, i.e. eigenvector and eigenvalue calculations. Individual particle dynamics during simple shear deformation of subglacial sediment under laboratory controlled conditions have already been studied in some detail (Boulton and Dobbie, 1998; Hooyer and Iverson, 2000; Thomason and Iverson, 2006; Iverson et al., 2008; Shumway and Iverson, 2009). mCT presents the opportunity to study such processes in volumetric 3D, entirely objectively. The sample scans of BO/03/07 demonstrate how easily distinctive clasts can be isolated from the enclosing sediment. Remoulding of glacial diamicton used in such laboratory experiments would permit the ‘seeding’ of the sediment with artificial clasts of an appropriate material, i.e. to act as fiducial markers (see Appendix Ev) (Boulton and Dobbie, 1998). As scan datasets are ostensibly accurate volume renderings of both the external morphology of an object and its internal composition and structure, they can be easily converted into ‘meshes’ to which finite- and discrete-element modelling techniques can be applied (Boulton and Dobbie, 1998; Young et al., 2008, C. O’Sullivan, pers. comm., 2009). mCT scanning appropriately seeded samples of both remoulded and natural samples of subglacial diamicton, converting the datasets into meshes and hence finite-/discrete-element modelling real data presents the prospects of; a) finally being able to robustly compare data from laboratory and field investigations, b) establishing the rheological behaviour of subglacial sediment, c) reconstructing the rheology of palaeoglaciated till deposits (Muir Wood, 1998; S. Carr, pers. comm., 2010; J. Piotrowski, pers. comm., 2010).

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9. Conclusions This research has demonstrated that mCT is a highly effective technique, both for acquiring qualitative and quantitative data relating to kinematic indicators and for the evaluation of other glacially deformed soft sediment characteristics. 9.1. The technique  Is entirely non-destructive. Therefore a sample can be analysed using mCT and the volumetric 3D data archived before the specimen is further investigated using destructive approaches, where required. Thus evidence derived from micromorphological thin-sections can be placed within the context of whole samples (and used to extract further information from the scan data); clast lithological, size and morphological data can be related to spatial distribution; matrix density (void ratio) can be quantitatively evaluated, both as an average value and at the spatial and contrast resolution of individual voxels etc.  Permits virtual thin-sectioning of a sample, creating the potential for a seamless link between a hand specimen, its micromorphological derivative(s) and SEM/XRD/XRF data (cf. Remeysen and Swennen, 2008).  Allows the detailed analysis of structures otherwise impossible to image in volumetric 3D, e.g. voids. Therefore the volume, morphology and orientation of each pore within a sample can be analysed quantitatively, as can their collective configuration(s) and connectivity.  Permits virtual deconstruction of a specimen, e.g. isolation of a volume or individual object of interest, removal (segmentation) of a particular suite of objects etc.  Is still developing rapidly and has significant potential for further improvement, adaptation and innovation. Combined with the continued advancements in computer and data handling/processing technologies, the possibilities for both more extensive and involved applications of mCT, specifically within the geological sciences, appear considerable. Some of the most recent advances are to be exploited fully through acquisition of a new instrument to replace the unit used in this project.

9.2. This project has shown that  Certain sediment types, e.g. those that contain predominantly one mineralogy, parts of which are stained with precipitates that have significantly higher X-ray attenuating properties, are ideal materials to analyse using mCT. Unfortunately, both heterogeneous and homogeneous types of glacial sediment composition can also present considerable challenges to the successful application of mCT; wide lithological variations compromising the assumption of linear X-ray attenuation and a predominantly autochthonous matrix making the differentiation of enclosed clasts difficult potentially. However, optimal quality scans will almost invariably reveal some otherwise unknown characteristic(s) of the sample under investigation. Continued advances in X-ray production, detection, data analysis/reconstruction and image visualisation methods will significantly improve the effectiveness and applicability of the technique.  The kinematics of any particular subglacial sediment sample are likely to be more complex than can be evaluated by 2D analysis alone.  The samples studied contain evidence for both brittle and ductile deformation, indicative of a polyphase strain response to an applied shear stress.

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 Voids are a key indicator of both sediment compression and shear strain, the 3D characteristics of which hitherto could not be easily examined at the micro-scale. Pore characteristics can be quantitatively and qualitatively evaluated using mCT, permitting significant new insights into void development and hence the micro-scale subglacial drainage network.  The marble-bed structure may be far more prevalent than previously considered and may present a simple explanation for the relatively independent movement of individual grains within a diamicton, leading to the development of galaxy structures (van der Meer, 1993; Iverson et al., 2008; Kilfeather and van der Meer, 2008).  Rotational structures may have a vertically orientated lenticular disc morphology, intuitively conforming to the physical rules dictated by ductile deformation within the confined environment of the subglacial zone. However, because of the diamictic nature of much subglacial sediment and hence complex local influences on the distribution of force magnitudes along the principal effective stress axes, such an ideal morphology is probably quite rare.  At the small (millimetre) scale investigated local conditions, e.g. the presence/absence of relatively large clasts, composition, porewater content etc., are likely to be the key factors controlling the rheological response of the sediment to an applied shear stress (van der Meer, 1993, 1997a; Phillips and Auton, 2000; van der Meer et al., 2003; Menzies et al., 2006; Phillips et al., 2007; Iverson et al., 2008).  The polyphase nature of the strain signature contained within the samples studied can be readily accounted for by the deforming bed model of subglacial soft sediment deformation (Boulton and Hindmarsh, 1987; Boulton and Dobbie, 1998; Boulton et al., 2001; van der Meer et al., 2003; Evans et al., 2006; Menzies et al., 2006), though it could also be accounted for by lodgement or even deposition within a debris flow (Dreimanis, 1988; Menzies and Zaniewski, 2003; Phillips, 2006; Piotrowski et al., 2006; Iverson et al., 2008). It is therefore essential that mCT analyses be applied within the context of a much wider suite of analytical methods, ideally including both macro- and micro- sedimentological techniques as well as geomorphological investigations where possible.  Subglacial soft sediment deformation is almost certainly cyclical, the resultant strain signature mostly likely composite in nature (Boulton and Dobbie, 1998). Strain mapping using mCT would provide a ‘snap-shot’ of stress/strain conditions at the moment when deformation ceased/emplacement occurred. In principle such mapping would allow the testing of various distributed soft sediment deformation models, the constraining of subglacial boundary conditions in palaeoglaciated environments and hence the refining of ice sheet numerical models.  It is possible to quantitatively evaluate grain size, morphology and orientation either entirely objectively, where the clasts can be automatically isolated from the enclosing matrix, or semiobjectively through operator aided segmentation. The theory of clast fabric reorientation as a strain response mechanism, and hence method for facies differentiation, can therefore be rigorously tested both within artificial and field laboratory contexts.  The ability of laboratory geotechnical apparatus to replicate the stress and effective pressure conditions that lead to subglacial soft sediment deformation can be assessed in unprecedented detail using mCT. Furthermore, the acquisition of volumetric 3D datasets recording the real-time deformation of sediments under controlled conditions, already demonstrated as feasible, should become relatively straightforward and repeatable in the near future.

Separately, micromorphology and mCT provide powerful analytical tools with which to investigate the characteristics, history and likely genesis of both glacial sediments and a variety of other solid materials (Menzies et al., 2010). Combined, the two techniques provide invaluable insights, which are otherwise impossible or extremely challenging to acquire in-situ, and allow a permanent archive to be created. While both elements of that archive are portable, the mCT element is infinitely replicable digitally and hence can be distributed widely, as this paper and Parfitt et al. (2010) have demonstrated. There is a clear requirement for further applications of mCT to subglacial soft sediment samples, both to evaluate the validity/ accuracy of the observations recorded here and to develop a ‘taxonomy of microfabrics and microstructures within (sub) glacial sediments’ for features in 3D, as has already been produced for 2D investigations (van der Meer, 1993; Menzies, 2000; Menzies et al., 2006). The types of analyses that can be conducted, both using the technique and upon the datasets generated, also present the tantalizing prospect of gaining significant additional insights into the nature of subglacial shear strain based on controlled laboratory investigations. Furthermore, the digital data archive could be revisited, refined and tested with each software development or advancement in the understanding of subglacial soft sediment deformation processes and rheology ad infinitum. The applicability of the technique to samples acquired from other glacial sedimentary environments, and many areas of Quaternary research, has been compellingly demonstrated both here and elsewhere in the literature. Acknowledgements This research was part-funded by the School of Geography, Queen Mary University of London via a Postdoctoral Research Assistant position to which the corresponding author was appointed, continued support provided through the subsequent award of an Honorary Research Associate affiliation. Aoibheann Kilfeather’s pioneering work on the application of mCT in glacial sediment analysis was the inspiration for this project. Nick Corps (e2v Scientific Instruments) provided invaluable technical assistance, introductions and so much more throughout the research. Richie Abel and Premkumar Elangovan (Natural History Museum), Phil Salmon (SkyScan),Manuel Dierick and Bert Masschaele (Universiteit Gent), Simon Carr and Simon Dobinson (Geography, Queen Mary), Jan Piotrowski (Aarhus/Sheffield), Catherine O’Sullivan (Imperial College), together with numerous other researchers, are thanked for helpful and insightful discussions both during the research project and the preparation of this paper. Kamil Zaniewski provided a detailed description of thin-section C.113, Richard Hubers a similar record for slide Mi.212. Ed Oliver helped produce and significantly enhance the figures. Emrys Phillips, Kenneth Rijsdijk and an anonymous reviewer are thanked for their thorough and constructive reviews of both this and a previous version of the manuscript, which have led to substantial improvements. Mark Tarplee would like to thank Sue Tombs for her considerable support during the empirical phase of the project. Appendix A. Methods for mitigating beam hardening: i) Hardware filters (metal plates or foils) are placed within the line of the X-ray cone, absorbing the lower end of the X-ray energy spectrum and hence ‘pre-hardening’ the beam before it encounters the sample (Ketcham and Carlson, 2001; SkyScan, 2001; Van Geet et al., 2003; Davis and Elliott, 2006). The

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spectrum of X-ray energies absorbed depends on the composition and thickness of the filter(s), which are all >99% purity so as to minimise the introduction of any further error. Though this method can significantly reduce beam hardening artefacts, it does not wholly remove them. It also produces a more energetic beam, so it is important to select the correct combination of X-ray accelerating voltage as well as the filter material/thickness for a given specimen. Increasing the amount of filtering narrows and shifts the X-ray spectrum towards the maximum set by the accelerating potential, but also reduces the intensity (leading to increased image noise). Thus filter thickness is selected as a compromise that will give sufficient narrowing of the spectrum without excessive attenuation of the beam. It should be noted that the attenuating effect of the filter will be less through the specimen compared to the background intensity and the former should be used as an indicator of the effect on image noise. ii) Prior to reconstruction, the projection data can be linearized using a polynomial representing the relationship between the measured attenuation with polychromatic radiation and the theoretical attenuation that would be attained with monochromatic radiation (Davis and Elliott, 2006).

Appendix B. Scan parameters

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the SkyScan 1072 and so exposure time is no longer such an issue. - Frame averaging (the number of iterations of image capture while the sample is held in the same position, from which average pixel values are calculated (integrated) in order to reduce noise) was set at four. This parameter in part acts in the same way as increasing the exposure time, but without causing camera saturation (Van Geet et al., 2000; Tarplee and Corps, 2008). Therefore, it was prioritised over adjustments to the exposure time. - The incremental angle of rotation was set at the minimum level possible e 0.23 .  360 scans were conducted as is appropriate for laboratory instruments (Buffiere et al., 2010). Appendix C. Alternative material archive and Supplementary data Full resolution images and movies associated with this article can be found in the on-line version at doi:10.1016/j.quascirev. 2011.05.016 and are also available via a dedicated file folder which can be found at the following address: www.geog.qmul.ac. uk/microCT Username: microct Password: Tarpleeetal

Through iteration optimal parameter settings were established as follows: Appendix D. Suggestions for further exploiting datasets - Either a 0.5 mm thick aluminium (Al) or 0.038 mm thick copper (Cu) filter was used, depending on the density of and variation within the sample. The Al filter is of the minimum thickness required to remove the <10 kV section of the energy spectrum, which significantly contributes to the corona effect (see Section 3) (Tarplee and Corps, 2008). As such it was used as the default filter. The Cu filter is far more efficient at absorbing the softest X-rays and so could be used to reduce the X-ray spectral width when using the highest beam energy setting, required for the densest samples (Davis et al., 2010). - The (maximum) kV was set at the lowest level possible that achieved a minimum 5% X-ray beam transmission through the densest point within the sample. The lower the average beam energy, the better the signal to noise ratio allowing the best contrast resolution to be achieved (Davis, 1997). However, below 16% the increased contrast is outweighed by an even larger increase in the noise, so a compromise must be established based on the objectives of the scan. - The beam current was set to the maximum available level by default, to maximise the photon flux received by the detector and hence optimise the signal to noise ratio in the resultant radiographs (Davis, 1997; Davis and Elliott, 2006; Tarplee and Corps, 2008). - The exposure time was set at a minimum of 5936 ms, the default setting when using hardware filters (SkyScan, 2001; Tarplee and Corps, 2008). Increasing the exposure time increases both the overall photon flux and minimum transmission level, the longest exposure time (9968 ms) leading to an approximately 2.4% difference to the latter parameter (Tarplee and Corps, 2008). However, as such an adjustment also increases scan times by circa 40% (from potentially 36 h to 60 h) and the X-ray source on the SkyScan 1072 has a finite operational life of 5000e7000 h, such an adjustment was considered economically untenable for the marginal improvement in overall scan quality. Modern instrument scan times are approximately an order of magnitude shorter than

i) Model ‘inversion’ Meticulous identification and plotting of fault planes and related displacements, as was undertaken on thin-section slide O.514 (2D), could be conducted on the relevant volume renderings in order to reconstruct the brittle deformation history of the sample in 3D. Such detailed analyses could prove time consuming, depending on the complexity of the structure(s). However, where fault planes are delineated by planar voids the threshold function can be inverted so that fractures only are visualised in the volume rendering, the voids being used as a fault plane proxy effectively. Relative displacements calculated from the solid volume rendering could then be projected onto the objectively created void/fault proxy model, thus permitting a strain history to be deduced. Unfortunately, as neither the thin-section or any of the plug samples derived from specimen O.514 contain significant voids of this nature, such an approach was not possible (though see sections 5.3e5.5 for related applications of the void/fault proxy technique). ii) Model manipulation Volume renderings can be dissected in an almost infinite number of ways and subsequently reconstructed. Consequently virtual serial sectioning can be conducted, orientated at any angle, and related to actual micromorphological slide, other microscopic (e.g. scanning electron microscopy - SEM) and macroscopic (e.g. hand specimen, field observation) evidence. They can also be made simpler to interpret by removing objects below or above a certain volume, which through iteration can be used to achieve a compromise between detail and visual perceptibility. Being an entirely objective technique the process is unable to discriminate between objects that are connected at the spatial resolution of the mCT scan, but are separate in reality. The CTAn software has a ‘Morphological operations e erosion (object separation), dilation

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(object expansion)’ option that can be used to correct such errors, as do other such programs (Ketcham, 2005a,b; Porter and Wildenschild, 2010). iii) Quantitative analyses Morphology Index - The routine was designed for application in the medical sciences and thus is not directly applicable to the dataset in question, hence the curious value calculated for the Pb and Zn oxides. However, it does illustrate one of the possibilities for adaptation of existing software. Degree of Anisotropy - It is not currently possible to automatically isolate and analyse objects with an appropriate shape, e.g. a- and b-axis ratios of >1.5:1 (Benn, 1995, 2004b) within CTAn, but the program does permit the introduction of such ‘plug-ins’. There are a number of programs, particularly ‘BLOB3D’ (Ketcham, 2005a,b), that have been developed for, and used successfully in, geological research (e.g. Jerram et al., 2009; Porter and Wildenschild, 2010). Appendix E. Technical problems and (possible) solutions The principles underpinning mCT are now well established. However, advances in both hardware and software have been dramatic over the last 15 years and are likely to continue (Cundall, 2001; Ketcham, 2005a,b; Porter and Wildenschild, 2010). The substantial capital cost of the equipment makes keeping pace with such advancements difficult, a fact reflected in the results presented in this article. The following are a list of the problems encountered, together with potential solutions: i) Sample acquisition and preparation During sample acquisition and transport disturbance may occur, e.g. the fractures contained within plug O.514c (Fig. 5leo). Almost all of the observable voids are located near, or extend from, the edges of the sample (Fig. 5a). Their position and orientation (either within, between or cross-cutting laminae at high angles), morphology (mainly parallel sides) and clean walls (i.e. no evidence of sedimentation or precipitation of solutes associated with fluid flow) indicate that they are unlikely to be natural in origin. Other possible production mechanisms are desiccation or plucking of sample grains during thin-section cutting and polishing (Carr, 2004). Desiccation would likely have been more extensive at the edges of the sample. Plucking created the small, irregular shaped voids in the thin-section and the pitting visible in the top surface of the plug (Fig. 5len and 5o). During sample drying a small amount of desiccation/sample contraction is inevitable and can be beneficial, accentuating voids that might otherwise not be detectable at the highest spatial resolutions of certain techniques, e.g. micromorphology. It is possible to minimise such changes using acetone drying, where applicable (Carr, 2004). Any expansion of pores will lead to an overestimation of percentage porosity, but such changes can be quantified using alternative techniques, e.g. MIP analysis, of non-impregnated samples (Cnudde et al., 2009; Dautriat et al., 2011). ii) Enhancing contrast resolution Where there is either no significant difference in density and effective atomic number between the materials that comprise a sample, or the variation in density is countered by an inverse

asymmetry in effective atomic number, the contrast resolution may be poor, e.g. sample Mi. 595. The problem can be evaluated by either: a) scanning the sample using two different mean X-ray beam energies, i.e. a dual-energy approach, to exploit the differing response of each of the compositional element’s electrons to such excitation (Van Geet et al., 2000, 2003; Karacan, 2007; Remeysen and Swennen, 2008). The technique could not be applied in this project as the X-ray source output of the SkyScan 1072 was insufficiently powerful (Van Geet et al., 2003; Van de Casteele, 2004; Bugani, 2007); b) establishing the exact compositional elements of the sample, e.g. by applying X-ray diffraction (XRD) or fluorescence (XRF) techniques to the top of the specimen, and then tuning the X-ray beam to exploit the absorption edge of particular electron shells (Walden, 2004; Davis and Elliott, 2006; Porter and Wildenschild, 2010). Such tuning was prohibited by the requirement for a minimum beam transmission level to be maintained (see appendix B). The value of the latter approach is demonstrated by the very high quality of the MuCat 2 scan dataset (BO/03/07). iii) Partial volume and penumbra effects and other potential sources of inaccuracy To counter partial volume and penumbra effects the threshold grey-scale value used to segment the material of interest (including voids) was subjectively chosen to favour that component, based on how completely the largest single body composed of that substance was represented balanced against the inclusion of other sample constituents. There are sophisticated software routines that can be used to further objectify such analyses, but the scan data was considered of too poor a contrast resolution to be able to robustly apply such techniques (Ketcham, 2005a,b; Porter and Wildenschild, 2010; Roche et al., 2010). The artefacts and small inaccuracies in the X-ray attenuation values acquired for sample BO/03/07 rendered impossible the selection of threshold values that satisfactorily segmented any particular component of interest, particularly voids and matrix (Table 1). The compromise threshold value selected to isolate voids also undoubtedly permitted the inclusion of fragments of low density matrix in the subsequent binary model. Similarly the presence of voids <12.50 mm (the spatial resolution of the scan) would have reduced the averaged grey-scale value for the affected voxels, potentially causing their exclusion from the matrix volume rendering. For semi-quantitative analyses, e.g. calculations of the percentage volume for each component of a sample, such compromises are of little consequence as the effects can be generally assumed to offset each other. Kilfeather and van der Meer (2008) present a method for evaluating the accuracy and precision of such mCT data, by combining it with quantitative image analysis of thin-sections and SEM techniques. iv) The MuCat 2 system Very high contrast resolution and ring artefact eradication is achieved by tuning the X-ray beam to just above the absorption edge of the active elements in the scintillator, translating the (12bit) detector during acquisition of each radiograph and utilising a time delay integration (TDI) method of image capture (Davis and Elliott, 1997; Davis et al., 2010). In addition an advanced beam hardening correction methodology is employed to further enhance image quality (Davis et al., 2008). The MuCat 2 currently represents the gold standard in polychromatic mCT. The technology is also more readily available than that provided within a synchrotron as the TDI package is a commercial product. However, the substantially increased

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duration of each scan and the requirement for additional expert operator input, with a concomitant increase in overall cost, limited its application in this research. v) Fiducial markers Perspex or (toughened) glass have relatively low X-ray attenuating properties and hence would minimise the possibility of artefacts being created in the subsequent reconstructed image stacks, while allowing straightforward segmentation. Should these materials not prove robust enough to withstand the applied shear stress, hollow or foam filled clasts could be constructed from ‘light’ metals, such as aluminium, to achieve the same result (Buffiere et al., 2010). References Adderley, W.P., Simpson, I.A., Macleod, G.W., 2001. Testing high-resolution X-ray computed tomography for the micromorphological analyses of archaeological soils and sediments. Archaeological Prospection 8, 107e112. Akin, S., Kovscek, A.R., 2003. Computed tomography in petroleum engineering research. London. In: Mees, F., Swennen, R., Van Geet, M., Jacobs, P. (Eds.), Applications of X-ray Computed Tomography in the Geosciences. Geological Society Special Publication, vol. 215, pp. 23e38. Alley, R.B., 1989. Water-pressure coupling of sliding and bed deformation, II. Velocity-depth profiles. Journal of Glaciology 35, 119e129. Altuhafi, F.N., Baudet, B.A., Sammonds, P., 2009. On the time-dependent behaviour of glacial sediments: a geotechnical approach. Quaternary Science Reviews 28, 693e707. Benediktsson, ÍÖ, Möller, P., Ingólfsson, Ó, van der Meer, J.J.M., Kjær, K.H., Krüger, J., 2008. Instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of Brúarjökull, Iceland. Quaternary Science Reviews 27, 209e234. Benediktsson, ÍÖ, Schomacker, A., Lokrantz, H., Ingólfsson, Ó, 2010. The 1890 surge end moraine at Eyjabakkajökull, Iceland: a re-assessment of a classic glaciotectonic locality. Quaternary Science Reviews 29, 484e506. Benn, D.I., 1995. Fabric signature of subglacial till deformation, Breidamerkurjökull. Iceland. Sedimentology 42, 735e747. Benn, D.I., 2004a. Clast morphology. In: Evans, D.J.A., Benn, D.I. (Eds.), A Practical Guide to the Study of Glacial Sediments. Arnold, London, pp. 78e92. Benn, D.I., 2004b. Macrofabric. In: Evans, D.J.A., Benn, D.I. (Eds.), A Practical Guide to the Study of Glacial Sediments. Arnold, London, pp. 93e114. Benn, D.I., Evans, D.J.A., Phillips, E.R., Hiemstra, J.F., Walden, J., Hoey, T.B., 2004. The research project e a case study of Quaternary glacial sediments. In: Evans, D.J.A., Benn, D.I. (Eds.), A Practical Guide to the Study of Glacial Sediments. Arnold, London, pp. 209e234. Bennett, M.R., Waller, R.I., Glasser, N.F., Hambrey, M.J., Huddart, D., 1999. Glacigenic clast fabric: genetic fingerprint or wishful thinking. Journal of Quaternary Science 14, 125e135. Blake, E.W., 1992. The deforming bed beneath a surge-type glacier: measurement of mechanical and electrical properties. Ph.D. Thesis, Department of Geophysics and Astronomy, The University of British Columbia, Canada. Boulton, G.S., Dobbie, K.E., 1993. Consolidation of sediments by glaciers: relations between sediment geotechnics, soft-bed glacier dynamics and subglacial ground-water flow. Journal of Glaciology 39 (131), 26e44. Boulton, G.S., Dobbie, K.E., 1998. Slow flow of granular aggregates: the deformation of sediments beneath glaciers. Philosophical Transactions of the Royal Society of London A 356, 2713e2745. Boulton, G.S., Hindmarsh, R.C.A., 1987. Sediment deformation beneath glaciers: rheology and geological consequences. Journal of Geophysical Research 92, 9059e9082. Boulton, G.S., Dent, D.L., Morris, E.M., 1974. Subglacial shearing and crushing, and the role of water pressures in tills from South-east Iceland. Geografiska Annaler 56A (3e4), 135e145. Boulton, G.S., van der Meer, J.J.M., (Eds.), Beets, D.J., Castel, I.I.Y., Hart, J.K, Quinn, I.M., Riezebos, P.A., Ruegg, G.H.J., Thornton, M., van der Wateren, F.M., 1989. Preliminary report on an expedition to Spitsbergen in 1984 to study glaciotectonic phenomena. Report 37, Fysisch Geografisch en Bodemkundig Laboratorium, University of Amsterdam, pp. 185. Boulton, G.S., van der Meer, J.J.M., Hart, J., Beets, D., Ruegg, G.H.J., van der Wateren, F.M., Jarvis, J., 1996. Till and moraine emplacement in a deforming bed surge e an example from a marine environment. Quaternary Science Reviews 15, 961e987. Boulton, G.S., van der Meer, J.J.M., Beets, D.J., Hart, J.K., Ruegg, G.H.J., 1999. The sedimentary and structural evolution of a recent push moraine complex: Holmstrømbreen, Spitsbergen. Quaternary Science Reviews 18, 339e371. Boulton, G.S., Dobbie, K.E., Zatsepin, S., 2001. Sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system. Quaternary International 86, 3e28. Brandes, C., Le Heron, D.P., 2010. The glaciotectonic deformation of Quaternary sediments by fault-propagation folding. Proceedings of the Geologists’ Association 121, 270e280.

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