Metripol birefringence imaging of unconsolidated glaciotectonized and ice keel scoured sediments: identification of unistrial plasmic fabric

Abstract

In unconsolidated sediments subject to strain, clays and silts are realigned into particular optical birefringent arrangements (plasmic fabrics), which provide information on the style and intensity of sediment deformation. A relatively new, non‐destructive, optical microscopy technique for automatically recording and quantifying birefringence (previously commercialized under the name ‘Metripol’) is pioneered in this study as a valuable and innovative micromorphological tool with which to examine deformation in unconsolidated sediments. Metripol is applied to unistrial plasmic fabric in glaciotectonized and ice keel scoured sediment from the Netherlands and former Glacial Lake Agassiz (Manitoba, Canada) respectively. Colour‐coded images are produced in which colour represents relative optical retardation and thus optical anisotropy through the quantity |sinδ| and optical orientation of anisotropy through the angle Ø (also indicated by linear azimuths). In this study Metripol typically demonstrates that the better developed the unistrial plasmic fabric is, the higher the |sinδ| values, the larger the areas of high |sinδ| values, and the longer and more densely populated the azimuths. In addition, some unistrial plasmic fabrics under Metripol demonstrate lower |sinδ| than previous examples and the surrounding sediment, despite being ‘perceived’ as demonstrating higher birefringence under a standard petrographic microscope. This is particularly true in clay‐rich sediments and has implications for the way we currently describe and interpret unistrial plasmic fabrics in unconsolidated sediment. Finally, the identification and quantification of additional structures that would otherwise have gone undetected using a standard petrographic microscope (e.g. linear and circular structures that are likely to represent discrete shears and skelsepic plasmic fabric, respectively) highlight the potential for Metripol to gather information on the deformation history of unconsolidated sediments that is unavailable to standard techniques.