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

Abstract

Although integrated clast macro-and microfabric studies have been used to provide evidence of palaeo-ice flow directions, to discriminate between diamictons deposited in differing sedimentary environments, and to model the response of tills to subglacial deformation, such studies have typically only examined the "bulk" fabric developed within the diamicton, largely ignoring the potential for several phases of fabric development within these typically polydeformed deposits. We present a new graphical method for the identification, description and analysis of clast microfabrics in polydeformed tills. This method utilises high resolution digital scans of orientated thin sections imported into a standard computer graphics package to measure the orientation of long axes of detrital (skeleton) grains included within the till. These data are then used to identify the main clast microfabrics present within the till, allowing the delineation of clast microfabric domains in which elongate grains share a similar preferred orientation. This process leads to the identification, characterisation and, most importantly, the interrelationships between the various generations of microfabrics. The relationships between these clast microfabrics and other microstructures (e.g. plasmic fabrics, turbate structures, shears, folds, faults…etc) can also be examined, providing a detailed microfabric-microstructural map of the thin section. Clast long axis orientation data collected for different parts of a thin section are plotted on a series of rose diagrams allowing the variation in clast microfabric orientation to be displayed graphically. A new set of terminology for the description of clast microfabrics in tills, which is based upon the system used by metamorphic petrologists, is also presented. Three case studies of the fabrics present within subglacial sediments of Pleistocene age as well as the present are used as examples of how to apply this methodology. IntroductionMicromorphology is increasingly being used by glaciologists and Quaternary geologists as a primary tool for the analysis of diamictons deposited in different sedimentary environments (van der

Hydrofracturing in response to the development of an overpressurised subglacial meltwater system during drumlin formation: an example from Anglesey, NW Wales

“…The thin sections were examined using a standard Zeiss petrological microscope and Zeiss projector, the latter allowing detailed study of the range of microstructures at very low magnification. The microstructural mapping technique used to analyses the thin sections is described by Phillips et al, (2010a). The terminology used to describe the various microtextures developed within these sediments in general follows that proposed by van der Meer (1987van der Meer ( , 1993van der Meer ( , 1996 and Menzies (2000).…”

Repeated advance and retreat of the East Antarctic Ice Sheet on the continental shelf during the early Pliocene warm period

“…The relationships between the clast microfabrics and other microstructures (e.g. plasmic fabrics, rotation structures, shears, lineations, and water escape structures) can also be examined, providing a detailed microfabric-microstructural map of the thin section (Watling, 1988;Phillips et al, 2011;Vaughan-Hirsch et al, 2012). Individual grains that can be observed in a thin section are known as skeleton grains.…”

Micromorphology of diamicton affected by iceberg-keel scouring, Scoresby Sund, East Greenland

“…7-9), including human-made materials such as concrete and mortar (Menzies et al, 2010;van der Meer and Menzies, 2011 -and references within), is undeniable. In addition, quantification of microstructures (Carr and Goddard, 2008;Menzies and Whiteman, 2009;Patel, 2011;van der Meer and Menzies, 2011;Linch and van der Meer, 2013;Neudorf et al, 2013;Lea and Palmer, 2014;Narloch et al, 2015) and new methods in 'microstructural mapping' (Phillips et al, 2011;Gehrmann et al, 2016), are developing with promising results. Finally, in combination with thin sections, 3D microtomography studies of microstructures (Kilfeather and van der Meer, 2008;Tarplee et al, 2011;Bendle et al, 2015;Groves, in prep) and development of the Metripol microscope system to automatically measure and record birefringence (Linch and van der Meer, 2013;Hiemstra and Carr, in prep) should be even more rewarding.…”