An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin

Abstract

2016 (January): An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin. This paper describes the results of a spatially dense anisotropy of magnetic susceptibility (AMS) till fabric study of a single drumlin in the Weedsport Drumlin Field, New York State, USA. AMS till fabrics provide a robust, quantitative and unbiased approach to assess subglacial till kinematics and infer ice-flow dynamics. The drumlin selected for this detailed investigation was systematically sampled at 18 locations to evaluate the patterns of ice flow and associated till kinematics within a drumlin and to test erosional vs. depositional models for its formation. AMS till fabric analysis yielded strong fabrics that increase in strength towards the drumlin crest, indicating that bed deformation occurred during till deposition and that deformation within the drumlin was greater than that in the interdrumlin low. Fabric orientations reveal drumlin convergent, divergent and parallel ice-flow paths that illustrate a complex interaction between ice flow and the drumlin form; fabric strength and shape reveal systematic differences in bed deformation between the interdrumlin and drumlin regions. These observations are inconsistent with purely erosional models of drumlin genesis; instead, these observations are more consistent with syndepositional streamlining of till transported, probably locally as a deforming bed, from the interdrumlin low towards the drumlin locality.

An anisotropy of magnetic susceptibility (AMS) fabric record of till kinematics within a Late Weichselian low Baltic till, southern Sweden

“…; Hopkins et al . ); the same is observed in laboratory experiments (Hooyer et al . ; Iverson et al .…”

“…; Hopkins et al . ). At this exposure, lateral variation appears minor, at least at the scale of metres.…”

“…; Hopkins et al . ). For non‐equant grains, AMS can be controlled largely by shape, where the maximum susceptibility lies along the long axis of the grains (Tarling & Hrouda ); thus, AMS provides an effective proxy for the measurement of grain orientation.…”

“…Methods AMS provides a quantitative, robust and unbiased assessment of the orientation of non-equant magnetic mineral grains, and has been demonstrated to be a useful and reliable method for characterizing till fabric and interpreting ice-flow history and till kinematics in numerous field and laboratory settings (e.g. Hooyer et al 2008;Iverson et al 2008;Shumway & Iverson 2009;Thomason & Iverson 2009;Gentoso et al 2012;Vreeland et al 2015;Hopkins et al 2016). For nonequant grains, AMS can be controlled largely by shape, where the maximum susceptibility lies along the long axis of the grains (Tarling & Hrouda 1993); thus, AMS provides an effective proxy for the measurement of grain orientation.…”

An erosional origin for drumlins of NW Poland

“…Presence of primary intact meltwater sediments excludes significant bed deformation whereas minor (e.g. Multiple studies propose deformation and moulding of subglacial deposits (mainly tills) and debris-laden basal ice as the dominant process of drumlin formation (Boulton, 1987;Hart 1997;Fowler, 2000;Stokes et al, 2013a;Hopkins et al, 2016;Menzies et al, 2016;Spagnolo et al, 2016; whereas coexistence of drumlins with varying internal compositions within the same drumlin field (e.g. Menzies et al, 1997;Lamsters and Zelčs, 2015) deformation structures indicate different levels of sediment mobility.…”