A multiproxy study of a basal till: a time-transgressive accretion and deformation hypothesis

A two‐part basal till at Knud Strand, Denmark reveals a uniform fabric pattern and strength, petrographical composition and clay mineralogy. The nature of the contact with the underlying sediments, ductile deformation structures, partly intact soft sediment clasts, small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water. A time‐transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds. It is suggested that, although strain indicators occur throughout the entire till thickness, the deformation at any point of time encompassed the uppermost part of the till only, allowing preservation of fragile clasts below. The substantial thickness of the till (up to 6 m) coupled with a much smaller (by more than one order of magnitude) inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition. The lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path. Copyright © 2004 John Wiley & Sons, Ltd.