Bottom current accumulation and sediment fluxes on the Hebridean slope

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Description/Abstract

Detailed sedimentological and geophysical analyses have been undertaken on the Barra Fan
region of the Hebridean continental margin, NW of Scotland. These analyses have revealed
good morphological evidence for both downslope and alongslope sedimentary processes on
a 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 timeintegrated
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 gravelrich
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.