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

Abstract

X-ray computed microtomography (μCT), 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 μm) 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 μCT (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, μCT 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 μCT 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. μCT also permits the seamless linking of analytical techniques applied at the hand specimen (mm), micromorphological (μm-mm), scanning electron microscopy and X-ray diffraction/fluorescence (nm-μm) scales and the archiving, duplication and dissemination of sample volumetric 3D data.