Nathan R Hopkins*, Edward B Evenson*, Kenneth ...

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Tom Pasquini, ExxonMobil, and the Lehigh EES Dept. for funding;. Brian Byrd and Michael Clifford for field assistance; Anheuser-. Busch and Molly's Irish Grill ...
ANISOTROPY OF MAGNETIC SUSCEPTIBILITY TILL FABRICS: AN ACCRETIONARY ORIGIN OF DRUMLINS IN NEW YORK STATE Nathan R Hopkins*, Edward B Evenson*, Kenneth Kodama*, and Andrew Kozlowski** *Dept of Earth and Environmental Science, Lehigh University, Bethlehem, PA;**Geological Survey, New York State Museum, Albany, NY

Till fabrics - 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 non-equant, microscopic magnetic grains. The maximum ‘easy axis’ (k1), parallels the long axis of the grains, which become oriented in response to shear such that the k 1 orientation parallels shear (Shumway & Iverson, 2009) direction and plunges up-shear (RIGHT). We applied this technique to the problem of drumlin genesis in the Weedsport Drumlin Field (WDF) of New York State (ABOVE). 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 collectively referred to as drumlins. Drumlins of the WDF are composed of a relatively uniform, over-consolidated brown to pink subglacial till. AMS fabrics were systematically measured at two meters depth from 18 pits within one drumlin and the adjacent interdrumlin low near Cato, NY (RIGHT). Pits were excavated via back-hoe, and 20-25 oriented 8cm3 samples were hand-carved from the exposure. AMS was measured using a KLY-3S Kappa-bridge and plotted on equal-area stereonets. Ice flow direction (red arrow) is inferred from the principle eigenvector orientation (red square) calculated from the AMS k1 orientations (black dots). Oblate

Prolate

Anisotropy ellipsoids vary in the degree of anisotropy (P’) and shape (T). Generally, site-mean shape parameters indicate triaxial to oblate fabrics, reflecting the presence of both elongate ferrimagnetics and paramagnetic clays.

AMS k1 orientation (black dots) are asymmetrically distributed about the drumlin long axis. The primary orientation (000°, 20°) is deflected east of the drumlin orientation; a secondary mode (295°, 20°) is deflected west. The total mean fabric (red square) is oriented parallel to the drumlin and plunges 23° towards the north.

Conclusions The spatial patterns in AMS fabrics are consistent with ice flow (solid black lines) around an obstacle (LEFT). The observed relationships between the drumlin form and the shape and orientation of fabrics indicate interaction of ice flow with the drumlin form at depths exceeding most estimates of deforming bed thickness (typically less than 1 meter). Additionally, weak inter-drumlin fabrics are inconsistent with streaming ice and enhanced bed deformation and erosion. Based upon these two observations, the purely erosional model of drumlin genesis proposed for the WDF is unlikely. Thus, we conclude that drumlins of the WDF are formed through accretionary processes at the ice-till interface occurring simultaneously to bedform streamlining. Additionally, our data supports hypothesis of bed deformation as the primary sediment transport agent occurring during drumlin formation.

Acknowledgements Mean degree of anisotropy (P’) and fabric strength (S1) display increasing, nonlinear trends between the interdrumlin low (440 ft) and the drumlin crest (550 ft). These trends illustrate a relationship between the drumlin form and ice flow, and imply a change in the style or magnitude of bed deformation in these two regions.

Larry Waterman for field access and assistance with excavation; Tom Pasquini, ExxonMobil, and the Lehigh EES Dept. for funding; Brian Byrd and Michael Clifford for field assistance; AnheuserBusch and Molly’s Irish Grill & Pub for moral support.