Sedimentology and depositional history of Holocene sandy contourites on the lower slope of the Faroe-Shetland Channel, northwest of the UK

Masson, D.G., Plets, R.M.K., Huvenne, V.A.I., Wynn, R.B. and Bett, B.J. (2010) Sedimentology and depositional history of Holocene sandy contourites on the lower slope of the Faroe-Shetland Channel, northwest of the UK. Marine Geology, 268 (1-4), 85-96. (doi:10.1016/j.margeo.2009.10.014).

Abstract

Thermohaline ocean currents with near-bed velocities of a few tens of cm s-1 have the ability to transport and sort sand, which is often deposited in thin sheets referred to as ‘contourite’ sand sheets. We examine a small sandy contourite sheet located in 800-1000 m water depth on the west Shetland slope in the Faroe-Shetland Channel. We describe the sedimentology of the contourite, and the relationship between sediment transport processes and contourite distribution through the last deglaciation and Holocene, based on a set of 17 short cores. Four sedimentological units ranging from very poorly sorted mud at the base (unit 1) to moderately sorted, slightly muddy, fine sand at the top (unit 4) are recognised. Ice-rafted detritus (IRD) occurs in the lower three units, but is absent in unit 4. The upper two units, 3 and 4, make up the contourite sand sheet. Analysis of the sand fraction of units 3 and 4 shows that the mean sand grain size is remarkably constant at around 3 throughout, except in some unit 3 samples with unusually high IRD content, where the mean can be coarser. IRD content correlates strongly with mean grain size, sorting and skewness throughout unit 3 even when the IRD content is small, showing that IRD input is one of the key processes controlling the sedimentology of unit 3. We conclude that the muddy sand contourites form by incomplete winnowing of the underlying glaciomarine sediments. The mud component is transported with the sand from the same source, either as small particles attached to sand grains or as sand-sized mud aggregates. Thickness maps of units 3 and 4 show that they have different depocentres, with unit 4 displaced northwest (downslope) relative to unit 3. Sedimentological trends in unit 3 are inconsistent and show no consistent transport direction. In contrast, clear across slope trends are seen in unit 4, with weak alongslope trends from the centre to both ends of the elongate contourite body. This suggests that the upper part of the contourite, unit 4, is derived from the downslope reworking of unit 3 sediment. There is no evidence for significant alongslope transport in either unit 3 or unit 4. The patchy occurrence of sandy contourites along the west Shetland slope is interpreted to result from localised export of sand from further upslope. The usefulness of the contourite sand in palaeoceanographic terms is limited. No meaningful dates can be derived from the sediments because of reworking and bioturbation. However, the presence of IRD within unit 3 shows that the Holocene circulation pattern, with strong bottom water flow through the Faroe-Shetland Channel, was established while there was still significant floating ice in the area.

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Contributors

Author: V.A.I. Huvenne

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