MAGNETIC FABRIC OF STRATIFIED BASAL ICE: MATANUSKA GLACIER, ALASKA

Herein we present the results of an anisotropy of magnetic susceptibility (AMS) investigation of the fabric of the stratified facies of the debris-rich basal zone which is common to many glaciers and ice streams - most notably at the Matanuska Glacier, Alaska, where it was first described. At the Matanuska glacier, stratified basal ice occurs as a typically thin (3 to 15m) zone at the base of the glacier best exposed at or near the terminus during glacial advance in the winter. This facies is composed of distinct, semi-continuous, alternating layers of debris-poor and debris-rich ice that dip up-glacier, typically on the order of 15°. It is thought to be genetically related to debris-rich frazil ice forming subglacially as supercooled water emerges from overdeepenings at the glacier bed and underplates the overlying glacial ice; however, the mechanism by which the randomly distributed and randomly oriented debris within the frazil ice becomes separated from the ice and organized into sub-parallel up-glacier dipping strata is not well understood. It is well-known that preferred particle orientation, which can be precisely and without bias determined for microscopic magnetic grains using AMS analysis, is a useful indicator of deformation, and as such has the potential to elucidate the processes leading to the stratification present in this zone. In the winter of 2014-2015 we collected oriented samples of the stratified basal ice from one vertical column, as well as two associated diamictons, for AMS investigation to assess the presence of deformation within the stratified facies. Anisotropy ellipsoids within the basal ice are dominantly oblate (T=0.34) with mean anisotropies (P') of 1.06. Fabrics vary from weakly (S₁ = 0.6) to strongly developed (S₁ = 0.86); however, in all cases fabric maxima (V₁) parallels known ice flow direction (NW). Interestingly, fabric maxima do not consistently plunge within the plane of the stratification; instead, these fabrics tend to be horizontal. In contrast, AMS fabrics from basal diamicton sampled nearby are considerably stronger (S₁ = 0.96), oriented parallel to transport direction as indicated by slickenlines, and plunge up-glacier approximately 30°. These fabrics display patterns reflecting possible variations in the distribution of deformation within the stratified basal ice.