The sedimentology of Middle Holocene tsunami facies in northern Sutherland, Scotland, UK

Lagoonal sediments attributed to the main Holocene marine transgression in Strath Halladale, northern Sutherland, contain a complex coarser layer believed to have been deposited during the tsunami associated with the Second Storegga Slide off South West Norway. The coarser sequence is dated at between 7590 plus or minus 50 and 7290 plus or minus 50 radiocarbon years BP (6507-6260 cal BC and 6228-6029 cal BC). Detailed stratigraphical analysis has determined a distinctive suite of sedimentary sub-units within the coarser layer in marked contrast to the sediments, which occur above, and below. A pronounced erosional unconformity with the underlying sediments is recorded with the base of the tsunami layer characterised by eroded material from the underlying peat. The presence of a mixed diatom assemblage, although fragmentary, indicates a chaotic accumulation of the deposit with all habitats represented. Variations in particle size within the sequence disclose striking similarities with those from contemporary tsunami deposits. The run-up of the tsunami sediments is calculated at a minimum of 4.6 m. This is the first occasion on which a deposit of the Second Storegga Slide tsunami has been found outside the North Sea basin and indicates that the area affected by the tsunami may have been larger than has been previously described.