TIME-TRANSGRESSIVE THIN-SKINNED DEFORMATION IN A HOMOGENEOUS BASAL TILL AT KNUD STRAND, DENMARK

Basal tills yield information about ice sheets’ dynamics including their ability to transport and deposit soft sediments. However, the processes generating basal tills remain controversial despite intensive studies over the last decades. Here, we describe a basal till from the last glaciation at Knud Strand, western Denmark, containing soft intra-clasts and seek a tentative explanation for its texture and structure.

Fabric orientation and -strength, grain-size distribution, clay mineralogy and petrographic composition are highly uniform throughout the entire till thickness studied in detail in three vertical profiles. The boundary between the till and the underlying sediments varies between planar/sharp and deformed/diffuse. Numerous inclusions of peat and other soft sediments are present in the till.

A time-transgressive model is used to explain the lack of vertical change in till components and the preservation of soft sediment clasts randomly dispersed throughout its thickness. After the debris was lodged from the sole of an active glacier, it was sheared in a thin (<40 cm) zone moving upward as till accretion proceeded. The depth of deformation at any time was relatively small but its cumulative result from the ice advance is deformation structures spread throughout the entire till thickness. The small strain of the till allowed preservation of fragile clasts. The Knud Strand till probably owes its surprising lack of petrographical gradation to multiple reworking and homogenisation of different sediments over several glacial cycles.