SUBGLACIAL SEDIMENT TRANSPORT AND DRUMLIN GENESIS: INSIGHTS FROM ANISOTROPY OF MAGNETIC SUSCEPTIBILITY TILL FABRICS

Till fabric analysis - the measurement and interpretation of the orientations of pebble and sand-sized clasts embedded within the till matrix - is an old and time-tested tool for investigating kinematics of the subglacial system. Recent experimental and field research has further expanded till fabric analysis to include anisotropy of magnetic susceptibility (AMS). AMS analysis measures the fabric (long axis orientation) of elongate, microscopic ferrimagnetic grains, such as magnetite or maghemite. We applied this technique to the problems of sediment transport and drumlin genesis in the Weedsport Drumlin Field (WDF) of New York State. In 2012, twenty AMS fabrics were systematically measured at approximately two meters depth within one drumlin and the adjacent inter-drumlin low near Cato, NY. Fabrics are typically strong (alignment values greater than 0.85), with the mean fabric plunging north and parallel to the drumlin long axis. Anisotropy ellipsoids are weakly prolate, reflecting the presence of both elongate ferrimagnetics and platy, paramagnetic clays. Spatially, fabrics show divergence (SW) on the up-ice end of the drumlin and parallel to convergent (SE) fabrics along drumlin flanks. Down-ice fabrics are dominated by oblique down-slope (SSW) directions. Interdrumlin fabrics are typically weaker (alignment values less than 0.8) and roughly parallel drumlin long axis. Both fabric strength and anisotropy increase with elevation, with peak values occurring at the drumlin crest. The spatial patterns in AMS fabrics are consistent with ice flow around an obstacle, indicating the interaction of ice with the drumlin form at up to two meters beneath the modern drumlin surface. These depths far exceed most estimates of deforming bed thicknesses. Additionally, characteristically weak interdrumlin fabrics are inconsistent with fast-ice flow and enhanced erosion, thus making the erosional model of drumlin genesis unlikely. Thus, we conclude that drumlins of the WDF are formed through accretionary processes at the ice-till interface occurring simultaneously to bedform streamlining.