ANISOTROPY OF MAGNETIC SUSCEPTIBILITY TILL FABRICS AND THE ACCRETIONARY ORIGIN OF DRUMLINS IN NEW YORK STATE

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 well-tested tool for investigating kinematics of the subglacial system. Recent experimental and field research has further expanded till fabric analysis to include measurements of anisotropy of magnetic susceptibility (AMS). AMS analysis measures the magnetic fabric (‘easy’ axis orientation) of elongate, microscopic ferrimagnetic grains, such as magnetite. We applied this technique to the problem of drumlin genesis in the Weedsport Drumlin Field (WDF) of New York State. The genesis of these glacial bedforms is controversial, owing, in part, to a wide variability in the composition and internal structures observed in a suite of morphologically-similar landforms all referred to as drumlins. Drumlins of the WDF are composed of a relatively uniform, overconsolidated brown to pink subglacial till. Twenty AMS fabrics were systematically measured at approximately two meters depth within one drumlin and the adjacent interdrumlin low near Cato, NY. Fabrics are typically strong (S₁ eigenvalues 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 paramagnetic clays. 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 (S₁ eigenvalues 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 depths exceeding estimates of deforming bed thickness. Additionally, characteristically weak interdrumlin fabrics are inconsistent with streaming ice 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.