Bottom current accumulation and sediment fluxes on the Hebridean slope

Abstract

Detailed sedimentological and geophysical analyses have been undertaken on the Barra Fanregion of the Hebridean continental margin, NW of Scotland. These analyses have revealed good morphological evidence for both downslope and along slope sedimentary processes ona typical high latitude slope-front fan. The diversity of sedimentary and seismic facies on this composite slope-front fan shows the variable nature of sedimentation styles and processes across the region. The distribution of surficial and subsurface sediment facies since the last glacial period from 30 000 yrs BP show three discrete episodes of development during which different processes and process combinations have been dominant. Bottom current activity was suppressed during the last Glacial period resulting in the deposition of hemipelagites, glaciomarine deposits and minor turbidites in a completely or partially open sea environment. Deglaciation and an increase in global temperature led to the increment of bottom current activity and the deposition of muddy and silty contourites across the region. A mid-slope sandy contourite sheet illustrates the long-term time-integrated signature of processes since deglaciation and represents the dominant processes during interglacial conditions. Modern textural distributions of surficial sediments on the shelf and slope reveal a distinct three-fold oceanward fining sequence. The sandline and mudline mark the approximate boundaries between three texturally distinct groups on the margin and represent a combined physiographically/hydrographically controlled boundary on the upper slope and hydrographically controlled boundaries lower slope. High-resolution seismic facies mapping of the margin demonstrates the complexity of facies and processes that have moulded and re-moulded the Barra slope-front fan system. The outer-shelf upper slope is characterised by a smooth to deeply scoured glaciomarine surface coated by gravel-rich sediments. The upper to mid-slope in the central and southern parts of the Fan are dominated by episodic mass-wasting events. The upper and mid-slope on northern part of the Fan is incised by steep-sided erosive canyons and channels, which in some cases coalesce to form low-relief levee on the lower slope. The lower slope is dominated by hemipelagites interbedded with minor turbidites and contourites.

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