OR/15/017 Introduction

Tappin, D R, Long, D, Carter, G D O. 2015. Shetland Islands Field Trip May 2014 - Summary of Results. British Geological Survey Internal Report, OR/15/017.

There is general agreement that the present day exposed Storegga Slide is the last of a series of large-scale submarine slides that have affected the mid-Norwegian margin over the past million years. It disturbed an area of approximately 95 000 km² and displaced a sediment volume of 2400–3200 km³, making it one of the largest submarine slides in the world. It has been dated to 8150 cal yr BP offshore (Haflidason et al., 2005). According to many AMS 14C dates of plant fragments the most accurate age for the tsunami onshore Norway is 7250–7350 14C yr BP (Bondevik et al., 1997), the youngest age of which correlates with the 8150 cal yr BP date from Haflidason et al. (2005).

Between the 11th and 24th May 2014 scientists from the BGS and Dundee and Aberdeen Universities visited the Shetland Islands to research tsunami deposits generated by the Storegga submarine landslide and two other recently discovered sandy ‘event’ beds dated at ~5000 BP and 1,500 cal yr BP (Bondevik et al., 2005). The research was funded under the NERC consortium project, ‘Will climate change in the Arctic increase the landslide-tsunami risk to the UK?’ and was a part of Work Block 2 (WB2): ‘What is the timing of tsunami deposits on the UK coastline, and how is it related to the age of major Arctic slides’? WB2 aims to investigate possible tsunami deposits in Mainland Scotland and Shetland that post-date the Storegga Slide (such as those dated 5500 and 1500 cal yr BP).

evidence for tsunamis in the UK

Figure 1 Google Earth Image of mainland Scotland and northern England with locations of Storegga tsunami deposits.

The UK’s north and east coasts (mainly Scotland) are vulnerable to tsunamis from the Norwegian Sea and there is good evidence for tsunamis from Arctic sources striking the UK (Long and Wilson, 2007). The bulk of the evidence in Scotland (Fig. 1) relates to the tsunami associated with the Storegga Slide of 8150 cal yr BP (Dawson et al., 1988). However, at a few sites, sedimentary deposits attributed to tsunamis younger than the Storegga event have recently been located in Shetland and dated at 5500 and 1500 cal yr BP (Bondevik et al., 2005). They include sites where both Storegga and one of the younger events have been recorded (Bondevik et al., 2005). The young age of these more recent events is of considerable importance because if validated, the mechanism of tsunami generation during post glacial climate change proposed to explain the Storegga event maybe compromised. In this context the research is of vital importance to planners and engineers concerned with estimating the magnitude and frequency of coastal flooding from tsunamis and how this should be mitigated. Proving the presence of younger tsunami event layers within the last 10 000 years, later than the proven Storegga event, that increases the frequency of tsunamis, would require a re-evaluation of the hazard from tsunamis to the northern UK and a reconsideration of the potential risk. It is thus essential to check the validity of published tsunami sediment layer interpretations and if possible to locate further examples to validate those discovered and constrain the spatial extent of known events and their possible source(s).

On Shetland, Mainland (Fig. 2), tsunami event layers are found in peat at Sullom Voe and in lake sediments in cores from Garth Loch, a marine embayment, and Loch of Benston, a freshwater body about 0.5 km to the west. Tsunami sediments from both these sites have been dated at ~8200 and ~5500 cal yr BP (Bondevik et al., 2005). A third, younger, sand layer preserved in peat outcrops in Dury Voe (Mainland) and 40 km north at Basta Voe on Yell is dated at ~1500 cal yr BP (Bondevik et al., 2003). All deposits are laterally extensive, landward thinning beds of marine origin and exhibit sedimentary features characteristic of tsunami deposition, including; rip-up clasts, sand layers, redeposited material and marine diatoms. An aeolian origin is considered unlikely because of the presence of boulders at one site and the deposits undoubted marine affinities. The absence of other non-marine event beds in an area that is subjected to frequent strong winds supports a non-aeolian origin. Within the Holocene record of sediments on Orkney and mainland Scotland there are no deposits coeval with those on Shetland. Only the ~5500 cal yr BP event has a possible equivalent in a deposit near Bergsøy in Norway (Bondevik et al., 2005). The ~1500 cal yr BP deposit at Dury and Basta votes is unique to Shetland. Neither of the younger events correlates with any other dated large-volume landslide off Norway, such as Traenadjupet (4000 cal yr BP). A few minor slide/slump events have been identified along the northern Storegga Slide escarpment and, dated at 5700 and 2200–2800 cal yr BP (Haflidason et al., 2005) might be the source of these sediments except that as the total volume of these events is less than 1 km³ they are far too small to generate a tsunami that would propagate so far . An absence of a correlative source, therefore suggests that either the young dates are artefacts and the result of contamination, or that there is a local source, such as a submarine landslide within the Shetland archipelago.

Figure 2 Google Earth Image of Shetland with locations of coastal tsunami deposits (white) and lake cores (red).

Survey objectives and plan

The objectives of the 2014 field research therefore were to:

• re-examine coastal sediment sequences in Shetland to determine the regional extent of the possible tsunami deposits dated at ~1500 and 5500 cal yr BP and possibly younger than the Storegga Slide (8200 cal yr BP), and

• collect further samples for sedimentary analysis and age dating to confirm (or not) the tsunami origin of the deposits.

The work plan was to carry out an initial field visit on Shetland to determine the extent of the younger events, followed by a coring and sampling campaign to recover good quality material for analysis. Palynological and sedimentary analysis of the recovered sediments will validate or not their tsunami origin, and 14C dating of material, either within, above or below the tsunami deposits will provide further evidence on their ages and hence the possible links with the Dury Voe (1500 cal yr BP) and or Garth (5500 cal yr BP) events on Mainland.