Seabed Sediments

A revised map of the seabed sediments within the area around St Kilda has been prepared especially for this report by the British Geological Survey (BGS). This new interpretation is based on the BGS 1:250 000 scale seabed sediment map and new marine survey data provided by Scottish Natural Heritage, including swathe bathymetry, over 100 seabed sample descriptions, Acoustic Ground Discrimination System data and interpreted marine habitats. In common with the published seabed sediments map series, this new interpretation uses a sediment size classification based on the system proposed by Folk. However, it should be noted that in this instance the sediment divisions illustrated on the revised map are not based on a rigorous grain size analysis of each sample, but on the general descriptions provided by Scottish Natural Heritage. Consequently, the revised map should be viewed as a preliminary reinterpretation. It is also notable that in areas very close to the coast of St Kilda, seabed sediments commonly are not defined because of a lack of data.

Figure 3.9: Map of the seabed sediments.

Within the area of interest encompassing the St Kilda archipelago, large areas of bedrock crop out at the sea bed and comprise the St Kilda Central Complex. This complex makes up the bathymetric platform, known as the St Kilda Platform, from which the islands arise. Water depths range up to about 80 m on the St Kilda Platform to over 140m beyond the platform’s cliff-like edge. The relatively few BGS sample sites suggest that only a thin veneer of sediments (commonly 0.1m in thickness) is developed on the submarine platform around St Kilda. Within these deposits, gravel is the most widespread grade of sediment.
Numerous gravel patches exist on the bedrock ‘ring’ encompassing the archipelago, with an extensive gravel deposit located in the centre of the platform between the islands of Hirta and Boreray. Samples located within this deposit indicate the presence of boulders and cobbles with small amounts of mobile sand. Within this gravel accumulation areas of exposed bedrock are present and are associated with bathymetric highs located near the centre of the platform. Gravel deposits associated with exposed bedrock occur off the coasts of Soay and Hirta and to the east of Dun and extensive deposits of sandy gravel and gravelly sand transect the St Kilda Platform.

The finest grade of sediment encountered in the area of interest is sand, which is found in Village Bay, on the eastern flank of the platform and to the south-west of the platform. These deposits commonly display ripple bedforms indicating active sediment transport. The extensive sand body located on the eastern flank of the platform is thought to have formed due to the deflection of a northward flowing current around the topographic high of the St Kilda archipelago. The sand body in Village Bay, Hirta may have accumulated under the relative shelter provided by Dun and the Oiseval promontory. Glen Bay also hosts sandy sediment (gravelly sand and sandy gravel), but this area is more exposed than Village Bay and a larger proportion of the finer sediments have been transported offshore.

The sea floor surrounding the St Kilda Platform is relatively flat and the seabed sediments in this area have been sparsely sampled and are relatively poorly known.