Reconstructing the Last Pleistocene (Late Devensian) Glaciation on the Continental Margin of Northwest Britain
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Date
06/2005Author
Davison, Stephen
Metadata
Abstract
The continental margin in the area west of Shetland was subjected to repeated and
extensive ice sheet advances during the Late Pleistocene. Seabed imagery, seismic
survey and borehole core data show the Late Devensian ice sheets expanded across
the continental shelf three times, two of these advances reaching the shelf edge. On
the inner shelf, where present-day water depths are generally less than 100m, only
thin sediments from the last retreat phase and exposed rock surfaces remain, all other
deposits from earlier phases having been removed by the last advance. On the mid to
outer shelf elements of all three phases are preserved, including lodgement and
deformation tills, melt-out and water-lain till sheets, in-filled hollows left by stagnant
ice decaying in situ and a series of large recessional and terminal moraines. In
addition, there is evidence of shallow troughs and overdeepend basins which indicate
preferential ice-drainage pathways across the shelf which were formerly occupied by
ice streams. At the shelf edge, a thick wedge of glacigenic sediment forms a
transition from the till sheets and moraines of the shelf to debris flows composed of
glacigenic sediments on the upper slope. Shelf-edge moraines show an architecture
indicating floating ice in modern water depths over approximately 180m, suggesting
the West Shetland ice sheet was no more than about 250m thick. The upper and
middle slope is dominated by glacigenic debris flows which are focused in the slope
areas below the proposed ice stream discharges at the shelf edge. The mid-to-lower
slope has been subjected to contour current activity which has re-worked much of the
glacigenic sediment in this position. The lower slope and floor of the Faroe-Shetland
Channel are marked by either large debris flow lobes of glacigenic sediment or thin
glacimarine muds deposited from suspension. A conceptual model of the glacigenic
development of a passive continental margin based upon the West Shetland example
shows the deposited sequence for both advance and retreat phases of a glacial cycle,
and the actual preserved sequence which might be expected in the rock record. The
model also shows that ice sheet buoyancy, thickness, and to a lesser extent, basin
subsidence, are the most important factors in the deposition and preservation of a
glacially-influenced marine sequence.