From glaciations to tsunami – a complex history of Shetlands recorded on the surface of quartz grains

Understanding the North Sea Pleistocene succession requires identifying the sequence of glacial events and sea-level fluctuations. The presence of the Last Glacial Maximum is supported by numerous geomorphic evidence in the Shetland archipelago. However, the presence of older ice sheets and the dominance of a locally sourced ‘Shetland ice cap’ or an invasive Fennoscandian Ice Sheet over the islands is still debated. Microtextures of quartz grains originating from the North Sea tsunami deposits retrieved at four offshore sites in Dury Voe (E Shetland) were analysed. The results reveal a complex history of the North Sea Pleistocene succession evidenced in overlapping micro-scale features encountered on the surface of quartz grains – from the primary features indicative of the crystallisation of quartz grains to the last processes affecting the grains. The majority of grains represent well-crystallised euhedral silica crystals. This indicates that the studied quartz grains originated and were delivered by glaciers from a single source area rich in quartzite or quartzite sandstone. The surficial characteristics of the studied grains are dominated by the microtextures formed due to glacial and subaqueous processes, including palaeo-tsunami events. Glacially-originated microtextures include sharp angular features, such as large-sized (> 10 µm) conchoidal fractures with minor microtextures imprinted on their surface, parallel ridges, arc-shaped steps, linear steps and subparallel linear fractures. Mechanically-induced chattermarks were also observed. Subaqueous processes induce the smoothing of the grain surface and rounding of the grain edges and protrusions. Moreover, a microtexture induced by the oscillation movement of water and thus peculiar for coastal processes, i.e. V-shaped percussion cracks, was commonly observed on the grain surface. Tsunami events are evidenced by the presence of single, small-sized (< 5 µm) conchoidal fractures encountered on the most convex parts of the grains.

Based on the freshness and overlapping of glacial and subaqueous microtextures observed on the surface of the studied quartz grains, three glacial events followed by sea-level changes, including at least two tsunami events, were inferred in Dury Voe (E Shetland). A mineralogical homogeneity of the studied quartz grains denies the hypothesis suggesting that the Shetland archipelago was covered by the Fennoscandian ice sheet during the last glacial cycle. The studied quartz grains reveal a multi-cycle history of sediment redeposition with no sediment supply from outside the Shetland area (e.g. quartz grains from the Fennoscandian magmatic rocks) to the system. The prevailing nature of the Shetland Pleistocene glaciation was therefore dominated by the presence of a locally sourced ‘Shetland ice cap’.

The research was supported by The Belgian Science Policy Office (BELSPO); NORSEAT project - Storegga and beyond – North Sea tsunami deposits offshore Shetland Islands and The Polish National Agency for Academic Exchange (NAWA); BPN/BEK/2023/1/00319 project - Micro-scale perspective of tsunami events – traces recorded on quartz grains and coastal risk prediction.