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A TIME-TRANSGRESSIVE TILL ACCRETION AND BED DEFORMATION HYPOTHESIS

LARSEN, NICOLAJ K  University of Aarhus, Denmark .
Piotrowski, Jan A  University of Aarhus, Denmark .
Kronborg, Christian  University of Aarhus, Denmark .
Wysota, Wojcieck  Nicholas Copernicus University, Poland.

Despite a concentrated effort over the last decades the formation of basal tills and the glacier dynamics involved are still controversial. This paper describes two basal tills of the last glaciation in Denmark and Poland (Larsen and Piotrowski, 2003, in prep; Larsen et al., 2004). It is shown that fabric orientation and -strength, grain-size distribution, clay mineralogy, clast roundness, crushing index and petrographic composition are highly uniform through the entire till thicknesses (Fig. 1 and 2). The boundary between the till and the underlying sediments is a mosaic of undeformed areas interspersed with areas exhibiting bed deformation (Fig. 3). Both tills are characterised by numerous inclusions of soft sediments and mm-to-cm thick sand lenses with microscale rip-up clasts. Small meltwater channels were observed at the base of the till in DK. Preliminary micromorphological studies reveal both depositional and deformational structures dispersed throughout the tills.

A time-transgressive model is suggested to explain the lack of vertical change in till properties and the preservation of soft sediment clasts, in which the debris released from the sole of an active glacier is sheared in a thin (<40 cm) zone moving upward as till accretion proceeds (Fig. 4). Small cumulative strain of the till allowed preservation of fragile clasts. The surprising lack of petrographical gradation in the tills is probably due to multiple reworking and homogenisation of different sediments over several glacial cycles. During deposition and deformation, pore-water pressures were high as indicated by the presence of small meltwater channels, the mosaic of deforming non deforming areas and the sand lenses formed during multiple re-coupling events.

REFERENCES
Larsen, N.K. and Piotrowski, J.A., 2003, Fabric pattern in a basal till succession and its significance for reconstructing subglacial processes: Journal of Sedimentary Research, v. 73, p. 725-734.



Larsen, N.K. and Piotrowski, J.A., in prep, Estimating intensity of subglacial bed deformation using crushed grains and fabric strength as proxies.



Larsen, N.K. Piotrowski, J.A., and Kronborg, C., 2004, A multiproxy study of a basal till: a time-transgressive accretion and deformation hypothesis: Journal of Quaternary Science, v. 19, p. 9-21.



Piotrowski, J.A., Larsen, N.K. and Junge, F.W., in press, Reflections on soft subglacial beds as a mosaic of deforming and stable spots: Quaternary Science Reviews.



Figure 1. Lumped sedimentological data from Knud Strand, Denmark including mean till fabric eigenvectors (V1) from the same levels in three profiles. Note the uniform distribution of lithological components (modified from Larsen et al., 2004).


Figure 2. Sedimentary log and lumped sedimentological data from Kurzetnik, Poland including mean till fabric eigenvectors (V1) from the same levels in three profiles. Note that, except for the transition from sand to till on the grain-size scale, the distribution of lithological components in both sand and till units is consistent (from Larsen and Piotrowski, in prep.).


Figure 3. Glaciolacustrine sediment covered by Weichselian till at Knud Strand, Denmark. (A) Largely intact, layered fine-grained sand, silt and clay overlaid by till along a sharp contact with no apparent diffusive mixing of the two units. The till has very few local components indicating low degree of ice-bed interaction (Larsen et al., 2004). (B) Same outcrop (ca. 150 m from (A)) showing a deforming spot with mobilised and heavily disturbed top part of the glaciolacustrine sediment. Multiple transitions between (A) and (B) occur along the section. Ice movement to the right; coin for scale (from Piotrowski et al., in press).


Figure 4. Time-transgressive model of till deposition and deformation. Note that the depth of deformation at any time is relatively small but its time-transgressive nature results in deformation structures spread throughout the entire till thickness. Till accretion is mainly by lodgement (from Larsen et al., 2004).


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