Shetlands Islands field trip May 2014 : summary of results

This report provides a record of a field excursion to the Shetland Islands in May 2014 to investigate sediments deposited from tsunamis generated from submarine landslides mainly located off the coast of Norway. The research was funded under a NERC Consortium Grant for a project entitled ‘Will climate change in the Arctic increase the landslide-tsunami risk to the UK?’ It was 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’? The best known and most studied tsunami from the Norwegian submarine landslides is the Storegga event dated at 8,200BP. Sediments deposited from this tsunami are commonly found along the west coast of Norway, east coast of mainland Scotland, and also on the Shetland and Faeroe islands. However, there are other landslides off of Norway for which no associated tsunami has been identified, which poses the question as to whether these events did not generate a tsunami or whether the evidence for a tsunami has not yet been found. Although evidence for seabed slumping off Norway was first discovered in the 1950’s (Holtedahl, 1955, 1971) and the scale and morphology of a massive submarine landslide, subsequently termed Storegga, mapped in the 1970’s (Bugge, 1983), it was not until 1985 that an associated tsunami was first proposed (Svendsen, 1985). The first supporting sedimentary evidence of the tsunami was first identified on mainland Scotland in 1988 (Dawson et al., 1988) then, subsequently, similar sediments were identified on the Shetland Islands (Smith, 1993a). The Storegga Slide has been dated to 8,150BP (Haflidason et al., 2005), however more recent research on the deposits on the Shetlands suggests that some may not all be from Storegga, because 14C age dating gives younger ages of ~5,000 and 1,500 cal yr BP (Bondevik et al., 2005). A major challenge posed by the ages of these younger dates is that they are confined to the Shetlands; there is no indication of these younger tsunamis on mainland Scotland. If the dating is correct and the sediments are indeed from tsunamis, then the submarine landslides off Norway would be an unlikely source, so a local source seems most likely, but none has yet been identified. Alternatively a non-tsunami source for the sediments may explain their presence. The objectives of WB2 therefore are to investigate the tsunami deposits on Shetland that post-date the Storegga Slide, to validate their ages and, if possible, identify possible source locations of the submarine landslides that generated the tsunamis. On Shetland research on tsunami sediments was mainly based on evidence from coastal exposures around Sullom Voe where tsunami sands are dated as coeval with Storegga. The younger sands are mainly preserved in lake cores at locations on Shetland Mainland (Bondevik et al., 2005) where those of 5,000 BP overlie sands of Storegga age at 8,200 years BP age. At coastal sites along Basta Voe on Yell and at a mainland site at Dury Voe very young age dates of ~1,500 BP suggest an additional and very recent, late Holocene event (Bondevik et al., 2005; Dawson et al, 2006). A preliminary field excursion to the Shetlands carried out in 2013 discovered possible new tsunami deposits preserved in peat on central Yell at Whale Firth, Mid Yell Voe and Kirkabister. Subsequent 14C age dating of these deposits resulted in a variety of ages, many much younger than that of Storegga. The 14C method is known to be subject to major uncertainties because of contamination, for example initial age dating in the 1990’s at sites around Sullom Voe returned ages of around 5,000 years BP, although these were subsequently rejected in favour of the earlier, 8,200 BP Storegga event. Thus, validating the ages of the deposits on Yell, prospectively from a number of deposits laid down successively at one site (thereby reducing the sole reliance on 14C dating) was critical in validating the presence of more than one tsunami event on Shetland. The objective of the 2014 field visit to the Shetlands, therefore, was to return to Yell and validate the preliminary results from 2013; revisiting the sites at Whale Firth, Mid Yell and Kirkabister and searching the coastlines of Unst, Fetlar, Yell and north Mainland for additional sites where tsunami sediments might be preserved. Just before the visit new 14C dates from Mid Yell from samples collected in 2013 confirmed the previous results from other locations that had given a wide range of ages; at Whale Firth a single date gave a ‘young’ age of ~5,000 years BP, a range of ages with the oldest at 8,200 years BP were returned from Mid Yell Voe. We first visited sites on north Mainland around Sullom Voe, as it was here that the first indications of the Storegga tsunami were identified on Shetland in 1992. The deposits are classic as they contain rip-up clasts characteristic of tsunami deposits elsewhere. We then visited the sites at Basta Voe, Whale Firth, Mid Yell and Kirkabister. We carried out reconnaissance surveys on Unst, Fetlar, Yell and north Mainland. Preliminary results: 1. The new evidence supports the presence of tsunami sediments on Yell at Mid Yell Voe and Whale Firth, but the age of these sediments requires further research to confirm previous dating and their possible sources, 2. The youngest dated sediments (~1,500 BP) at Vasta Voe are most likely from a tsunami, but their limited areal extent suggests a local source, as yet undetermined, 3. The presence of three events at Mid Yell Voe based on surveys in 2013 was not confirmed, 4. The similarity of the deposits on Mid Yell with those around Sullom Voe on Mainland are suggestive of a similar source, 5. The wide range of the preliminary age dating at the Mid Yell sites (Whale Firth and Mid Yell Voe) is analogous to the early age dating of coastal deposits around Sullom Voe, suggesting the possibility of contamination of the peat material dated, 6. Whereas the 5,500BP event is identified in lake cores, no strongly supportive evidence for sands of this age were identified in the coastal sections, 7. Of the proposed three tsunami events proposed for Shetland only one, Storegga, has a confirmed source, 8. Further analysis of the peat stratigraphy at the coastal sites, reflects vegetation changes over the past ~8,000 years related to climate change, and these could be used to provide a broader context for the 14C age dating that may resolve the present dating issues, 9. Newly discovered sediments at Kirkabister require further research to determine their origin, 10. The origin(s) of the laminated deposits at Whale Firth, Mid Yell and Vatsetter is/are uncertain, but they are probably not from a tsunami, 11. No additional coastal exposures of peat with tsunami sands were located during the reconnaissance surveys on Mainland, Yell, Unst and Fetlar. Postscript; Immediately after this report was finalised, age dating of peat sections at Whale Firth and Mid Yell Voe confirmed that the sands preserved in the woody peat here are of Storegga age, ~8,200 cal yr BP

Source of the tsunami generated by the 1650 AD eruption of Kolumbo submarine volcano (Aegean Sea, Greece)

The 1650 AD explosive eruption of Kolumbo submarine volcano (Aegean Sea, Greece) generated a destructive tsunami. In this paper we propose a source mechanism of this poorly documented tsunami using both geological investigations and numerical simulations. Sedimentary evidence of the 1650 AD tsunami was found along the coast of Santorini Island at maximum altitudes ranging between 3.5 m a.s.l. (Perissa, southern coast) and 20 m a.s.l. (Monolithos, eastern coast), corresponding to a minimum inundation of 360 and 630 m respectively. Tsunami deposits consist of an irregular 5 to 30 cm thick layer of dark grey sand that overlies pumiceous deposits erupted during the Minoan eruption and are found at depths of 30–50 cm below the surface. Composition of the tsunami sand is similar to the composition of the present-day beach sand but differs from the pumiceous gravelly deposits on which it rests. The spatial distribution of the tsunami deposits was compared to available historical records and to the results of numerical simulations of tsunami inundation. Different source mechanisms were tested: earthquakes, underwater explosions, caldera collapse, and pyroclastic flows. The most probable source of the 1650 AD Kolumbo tsunami is a 250 m high water surface displacement generated by underwater explosion with an energy of ~ 2 × 1016 J at water depths between 20 and 150 m. The tsunamigenic explosion(s) occurred on September 29, 1650 during the transition between submarine and subaerial phases of the eruption. Caldera subsidence is not an efficient tsunami source mechanism as short (and probably unrealistic) collapse durations (< 5 min) are needed. Pyroclastic flows cannot be discarded, but the required flux (106 to 107 m3 · s− 1) is exceptionally high compared to the magnitude of the eruption

DTI Strategic Environmental Assessment Area 6, Irish Sea, seabed and surficial geology and processes

Hydrocarbons prospectivity • The East Irish Sea Basin is at a mature exploration phase. • The hydrocarbons-prospective sedimentary basins are characterised by source rocks, an abundance of structured regional petroleum reservoir and seal rocks and by suitable timing of geothermal events for generation and transfer of petroleum products from the source rocks to the reservoir rocks. • SEA6 has regionally diverse seabed habitats which vary significantly from area to area in currently licensed acreages in the eastern Irish Sea. The variations in seabed habitats have been systematically related to the sedimentary processes driven by quantifiable patterns of mean seabed stress. Sedimentary Processes • Open shelf seabed sedimentary processes are driven by seabed stress originating from interaction of the seabed with strong currents generated by tidal streams and by waves. • The scale of the stress imposed on the seabed and the related seabed habitat variability varies from regional (between mainlands, varying shelter around headlands) to macroscopic (around boulders and pebbles). • During the fair to moderate weather conditions characteristic of the late spring, summer and early autumn seasons, the seabed sediment types on the open continental shelf are dominated by the stress imposed on the seabed by the strengths and flow directions of the peak tidal currents. In this setting most of the regional variations in seabed sediment types are dominated by the effects of the coastal configurations on the tidal streams. • In areas where the seabed stress from waves is dominant, the seabed sediments coarsen with exposure to the increasing seabed stress generated when the waves interact with the seabed. Seabed stress from waves is dependent on wave power that varies with weather, wave fetch, seabed slope, wave direction and water depth. • There are knowledge gaps on possible regional variations of seabed properties when the seabed is stressed during extreme weather events associated with storm surge and storm waves. • In the most highly stressed seabed environments, exposed bedrock and strongly cohesive unsorted gravelly, sandy and muddy sediments are often swept clean of unconsolidated muds, sands, granular gravel and pebbles. Parts of the seabed in these areas may consist of cobbles and boulders. Environments of least seabed stress are characterised by fine-grained muddy sediments. Mobile sandwaves are characteristic of areas where sediments are being transported along the seabed in environments that are situated between the areas of extremely high seabed stress and very low seabed stress. The sense of regional seabed sediment transfer is from and across areas of high seabed stress to areas of lower seabed stress. The observations summarised above indicate that if large-scale disruptions to the natural seabed habitat are to be avoided, new development scenarios should avoid barriers that could have a significant effect on the regional patterns of seabed stress. • As elsewhere on the UKCS, glacigenic sediments and relict static glacigenic bedforms have had significant regional and local effects on the patchiness of the distribution patterns of seabed sediments and seabed habitats. • There is a knowledge gap in the research evidence required to securely link subregional increases in the percentage of biogenic carbonate in the sand fraction of the Irish Sea Mud Belt with increased biological productivity of surface waters, with methane expulsion from shallow and seabed sediments or with processes of bedload carbonate transport. • Investigations of shipwrecks and artificial continuous barriers indicate that the amount of seabed scour is much larger than the profile of large seabed obstacles presented to near-bed current flow. The observations reveal patterns of scour asymmetry consistent with model predictions of mean peak tidal current speeds and the interpretations of the directions of regional sediment transport based on the geometries of seabed bedforms. Wreck studies could therefore be used to calibrate modelling on the likely long term effects of future seabed development scenarios. Shipwrecks have also contributed to seabed diversity. • Active pockmarks, bioherms, banks in less than 20m water depth and some shipwrecks are already regulated by conservation measures. The following geological features are also worthy of consideration for preservation because they are irreplaceable: Static bedforms — sarns, pingos, upstanding rock outcrops in mud belts Mobile bedforms — banner banks, estuary banks and spits • A gateway for sand exchange between the open shelf and the eastern Irish Sea coast off England appears to be defined by a zone situated between North Wales and the southern limit of the Eastern Irish Sea Mudbelt. Although the current prospects for large oil and gas developments in this area are very small, any developments that could the patterns of sand exchange through this environmentally sensitive area should be avoided

The Sissano, Papua New Guinea tsunami of July 1998 — offshore evidence on the source mechanism

The source of the local tsunami of 17th July 1998 that struck the north shore of Papua New Guinea remains controversial, and has been postulated as due either to seabed dislocation (fault) or sediment slump. Alternative source mechanisms of the tsunami were addressed by offshore investigation using multibeam bathymetry, sub-bottom profiling, sediment sampling and observation from the JAMSTEC Dolphin 3 K Remotely Operated Vehicle and Shinkai 2000 Manned Submersible. The area offshore of Sissano is a complex active convergent margin with subduction taking place along the New Guinea Trench. Dominant transpressional convergence results in diachronous collision of the highstanding North Bismarck Sea Plate in a westerly direction. The result is a morphological variation along the Inner Trench Slope, with the boundary between eastern and western segments located offshore Sissano in an area of on- and offshore subsidence. This subsidence, together with nearshore bathymetric focusing, is considered to increase the tsunamigenic potential of the Sissano area. The offshore data allow discrimination between tsunami generating mechanisms with the most probable source mechanism of the local tsunami as a sediment slump located offshore of Sissano Lagoon. The approximately 5–10 km3 slump is located in an arcuate, amphitheatre-shaped structure in cohesive sediments that failed through rotational faulting. In the area of the amphitheatre there is evidence of recent seabed movement in the form of fissures, brecciated angular sediment blocks, vertical slopes, talus deposits and active fluid expulsion that maintains a chemosynthetic vent fauna. Dating of the slump event may be approximated by the age of the chemosynthetic faunas as well as by a seismic signal from the failing sediment mass. Faults in the area offshore Sissano are mainly dip–slip to the north with recent movement only along planes of limited lateral extent. A possible thrust fault is of limited extent and with minimal (cm) reverse movement. Further numerical modelling of the tsunami also supports the slump as source over fault displacements

The magmatic and eruptive evolution of the 1883 caldera-forming eruption of Krakatau: Integrating field- to crystal-scale observations

Explosive, caldera-forming eruptions are exceptional and hazardous volcanic phenomena. The 1883 eruption of Krakatau is the largest such event for which there are detailed contemporary written accounts, allowing information on the eruptive progression to be integrated with the stratigraphy and geochemistry of its products. Freshly exposed sequences of the 1883 eruptive deposits of Krakatau, stripped of vegetation by a tsunami generated by the flank collapse of Anak Krakatau in 2018, shed new light on the eruptive sequence. Matrix glass from the base of the stratigraphy is chemically distinct and more evolved than the overlying sequence indicating the presence of a shallow, silicic, melt-rich region that was evacuated during the early eruptive activity from May 1883 onwards. Disruption of the shallow, silicic magma may have led to the coalescence and mixing of chemically similar melts representative of a range of magmatic conditions, as evidenced by complex and varied plagioclase phenocryst zoning profiles. This mixing, over a period of two to three months, culminated in the onset of the climactic phase of the eruption on 26th August 1883. Pyroclastic density currents (PDCs) emplaced during this phase of the eruption show a change in transport direction from north east to south west, coinciding with the deposition of a lithic lag breccia unit. This may be attributed to partial collapse of an elevated portion of the island, resulting in the removal of a topographic barrier. Edifice destruction potentially further reduced the overburden on the underlying magmatic system, leading to the most explosive and energetic phase of the eruption in the morning of 27th August 1883. This phase of the eruption culminated in a final period of caldera collapse, which is recorded in the stratigraphy as a second lithic lag breccia. The massive PDC deposits emplaced during this final phase contain glassy blocks up to 8 m in size, observed for the first time in 2019, which are chemically similar to the pyroclastic sequence. These blocks are interpreted as representing stagnant, shallow portions of the magma reservoir disrupted during the final stages of caldera formation. This study provides new evidence for the role that precursory eruptions and amalgamation of shallow crustal magma bodies potentially play in the months leading up to caldera-forming eruptions

Insights on the source of the 28 September 2018 Sulawesi tsunami, Indonesia based on spectral analyses and numerical simulations

The 28 September 2018 Sulawesi tsunami has been a puzzle because extreme deadly tsunami waves were generated following an Mw 7.5 strike-slip earthquake, while such earthquakes are not usually considered to produce large tsunamis. Here, we obtained, processed and analyzed two sea level records of the tsunami in the near-field (Pantoloan located inside the Palu Bay) and far-field (Mamuju located outside the Palu Bay) and conducted numerical simulations to shed light on the tsunami source. The two tide gauges recorded maximum tsunami trough-to-crest heights of 380 and 24 cm, respectively, with respective dominating wave periods of 3.6-4.4 and 10 min, and respective high-energy wave duration of 5.5 and [14 h. The two observed waveforms were significantly different with wave amplitude and period ratios of *16 and *3, respectively. We infer tsunamigenic source dimen19 sions of 3.4–4.1 km and 32.5 km, for inside and outside of the Palu Bay, respectively. Our numerical simulations fairly well repro21 duced both tsunami observations in Pantoloan and Mamuju; except for the arrival time in Mamuju. However, it was incapable of reproducing the maximum reported coastal amplitudes of 6–11 m. It is possible that these two sources are different parts of the same tectonic source. A bay oscillation mode of *85 min was revealed for the Palu Bay through numerical modeling. Actual sea surface disturbances and landslide-generated waves were captured by two video recordings from inside the Palu Bay shortly after the earthquake. It is possible that a large submarine landslide contributed to and intensified the Sulawesi tsunami. We identify the southern part of the Palu Bay, around the latitude of -0.82o S, as the most likely location of a potential landslide based on our backward tsunami ray tracing analysis. However, marine geological data from the Palu Bay are required to confirm such hypothesis

Mass transport events and their tsunami hazard

Mass transport events, such as those from submarine landslides, volcanic flank collapse at convergent margins and on oceanic islands, and subaerial failure are reviewed and found to be all potential tsunami sources. The intensity and frequency of the tsunamis resulting is dependent upon the source. Most historical records are of devastating tsunamis from volcanic collapse at convergent margins. Although the database is limited, tsunamis sourced from submarine landslides and collapse on oceanic volcanoes have a climate influence and may not be as hazardous as their frequency suggests. Conversely, tsunamis sourced from submarine landslides at convergent margins may be more frequent historically than previously recognized and, therefore, more hazardous

An overview of the lithostratigraphical framework for the Quaternary deposits on the United Kingdom continental shelf

This stratigraphical framework report presents a lithostratigraphical scheme for the Quaternary succession on the United Kingdom continental shelf (UKCS). The emphasis has been placed on the delineation and definition of a series of lithostratigraphical groups that provide the basis for first-order correlation between Quaternary deposits, both offshore and onshore. The proposed scheme is based on information derived from the extensive marine dataset acquired by the British Geological Survey (BGS) since the late 1960s, and published as a series of offshore maps and regional reports. The first part of the report (Chapter 1) introduces the project and in particular focuses upon the fundamental differences between onshore and offshore stratigraphical approaches. Resolving this problem is fundamental to creating a unified stratigraphical scheme that is applicable to both domains. The timescale that we use defines the base of the Quaternary System/Period and the Pleistocene Series/ Epoch at 2.58 Ma, as formally ratified by the International Union of Geological Sciences (IUGS) (Gibbard et al., 2010). This is followed in Chapter 2 by a brief description of the methodology that underpins the existing offshore stratigraphy. Although this scheme has been constructed largely on the basis of seismic stratigraphy, information on the nature and age of the stratigraphical units is provided by a wealth of borehole and short core data. Consequently, the offshore scheme is best described as a hybrid of seismic, litho- and biostratigraphy. Chapter 3 outlines the principles behind the new proposed lithostratigraphical scheme. Although the scheme is not wholly lithostratigraphical in nature, the hierarchy of lithostratigraphical nomenclature is adopted as the most practical terminology for describing a succession that is mappable at several levels, is divided by distinctive regional bounding surfaces, and displays significant lithological variation. By adopting a lithostratigraphical nomenclature we retain consistency with a recently published BGS onshore lithostratigraphical framework, thereby promoting an integrated land–sea approach to Quaternary correlation. A brief description of the new lithostratigraphical scheme is presented in Chapter 4, with emphasis at the group level. We define twelve groups from the Atlantic margin, North Sea and Celtic Sea–Irish Sea region that represent regional subdivision into predominantly non-glacial Lower–Middle Pleistocene, and glacially-dominated Middle Pleistocene– Holocene units. The proposed defining formations from each group are presented in a series of accompanying tables. Some of the larger estuaries (e.g. Moray Firth) and the English Channel–South-west Approaches region remain undivided at the present time. A comparison of the UKCS lithostratigraphical scheme with those in adjacent international sectors is presented in Chapter 5, with specific focus on the Dutch and Norwegian sectors. A major concern across the international boundaries is that the lithostratigraphical hierarchy of equivalent units varies between countries. Chapter 6 presents some recommendations for further work in order that the stratigraphical scheme be fully utilised by the scientific community and industry. This includes: 1) complete revision, update and population of the offshore entries in the BGS Stratigraphical Lexicon of Named Rock Units; 2) the production of a full framework report that details all aspects of the offshore Quaternary succession (groups, formations, members, etc); 3) a review of areas where the Quaternary stratigraphy is ambiguous or poorly defined; 4) the development of a single onshore–offshore classification scheme that can be captured seamlessly within the BGS Geological Spatial Database (GSD); and 5) the development of a unified North-west European Quaternary stratigraphical scheme. It is concluded that tasks 1 and 4 are essential corporate issues that underpin the entire BGS superficial deposits framework