New Insights Reflecting the Flow and Depositional Regime of the 8200 yr BP Storegga Tsunami in Eastern Scotland (UK)

Sedimentary evidence of the 8200 yr BP Storegga submarine slide tsunami is found across the coastline of the North Atlantic, including the UK. Five sites surrounding the Montrose Basin in east Scotland contain an anomalous silty fine sand layer, thought to be evidence of inundation and return flow from this high magnitude wave event (HMWE). A range of techniques are used to examine this layer (sediment particle size, geochemical (X-Ray Fluorescence) and geophysical (Electrical Resistivity Tomography). This multi-proxy approach supports the interpretation of formation by a HMWE. Physio-chemical features of this deposit, including overall thickness, particle size grading, geochemistry and sedimentary structures, vary across the area alluding to various flow and depositional regimes. The preserved thickness of the deposit is highly variable, from 0.5 cm in Fullerton to 105 cm in Puggieston, representing one of the thickest Storegga tsunami deposits thus far found in the UK. While some sites exhibit normal grading in deposits (e.g. Maryton), inverse grading is found at most sites (e.g. Fullerton and Puggieston), reflecting sediment transport and deposition from a traction carpet. Element ratios, such as Ti/Ca and Ti/Sr, confirm heavy minerals are present within the deposit, providing further evidence of a HMWE. On the south west margin of the Basin, flame structures (FS) are found, which represent upward projections of an underlying mud within the tsunami sand deposit. FS are rare in deposits from the HMWE and signify a unidirectional flow with a high shear velocity that deformed the underlying mud layer. The high resolution study of the silty fine sand provides a new interpretation of sediment being transported and deposited by a traction carpet, evident by the grading of the deposit. New geochemical data and sedimentary structures also support formation by the HMWE. Furthermore, a potential new site identified at Torness Nuclear Power Station (south east Scotland) is thought to contain further sedimentary evidence of the HMWE. This investigation thus allows for the new characterisation of the spatial variability of the HMWE across the East Coast of Scotland.