Holocene Shelf-Margin Submarine Landslides, Donegal Fan, Eastern Rockall Trough

Abstract

The Donegal Fan is a large Neogene to Pleistocene composite fan which, together with the Barra Fan, has built out into the deep water basin of the eastern Rockall Trough (Fig. 1). Modern fan morphology has been influenced by alongslope and downslope translation processes. The most catastrophic of these originated as submarine landslides, the largest of which have translated slopefront glacigenic, hemipelagic and contouritic sediments for more than 100 km towards the axis of the Rockall Trough (Holmes et al. 1998). Fan morphology has also been remoulded by spatial and secular changes to Holocene near-bottom slope currents. Their effects on the Barra Fan and Donegal Fan have been overall seabed erosion from the shelf edge to the upper slope, and the development of a sandy and gravelly sheeted contourite drift across the middle slope (Armishaw et al. 2000). In both of these environments, hemipelagic and drifted sediments are preferentially preserved within the excavation zones of former submarine landslides (Fig. 2) (Holmes et al., in press) which, in effect, form modern intra-slope basins.Fig. 1.Location and regional Pliocene-Pleistocene setting of the Donegal Fan shelf-margin submarine landslides. Thickness Pliocene-Pleistocene modified after Evans (1997)Fig. 2.Shaded relief from EM12 swath bathymetry. Headwalls and side-walls define the excavation zones of the Pleistocene shelf-margin submarine landslides which extend east towards iceberg scour on the upper slope and the continental shelf. The excavation zones are now intra-slope basins in which post-slide hemipelagic and contouritic sediments have preferentially accumulated. When with a low gravel content, or gravel free, these sediments return very low acoustic backscatter on 32 kHz side-scan sonar images