Neotectonic deformation in a Scottish fjord, Loch Broom, NW Scotland

Multibeam bathymetry, boomer seismic profiles and sediment core data from outer Loch Broom reveal slumping of the basin-floor fjord deposits of the Assynt Glacigenic Formation. On the swath image, the expression of slumping is manifest as two distinct sea bed depressions, at least 10 m deep and several hundred metres wide. Although the extent of displacement is constrained within the fjord, the seismic profiles reveal extensional and compressional faulting, and associated folding, within the fjord infill. The possibility that collapse of the sea bed has been partly facilitated by some kind of associated fluid release along the fault planes cannot be discounted. Local (core data) and regional stratigraphical information indicate that slumping occurred shortly after deposition of the Assynt Glacigenic Formation, between about 14 and 13 ka BP, during the deglaciation of the fjord region. It is inferred that these slumps broadly correlate with two areas of major sliding in adjacent fjord basins, and are linked to a regional phase of Lateglacial instability throughout the Summer Isles region. It is suggested that earthquake activity related to ice unloading is the most probable cause of this deformation. Holocene bottom-current activity has partially modified the shape of the depressions, and influenced the nature of the sediment infill.