Heavy mineral assemblages of the Storegga tsunami deposit

Sedimentary Geology 334 (2016) 21–33

Contents lists available at ScienceDirect

Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo

Heavy mineral assemblages of the Storegga tsunami deposit J. Cascalho a,b, P. Costa b,⁎, S. Dawson c, F. Milne c, A. Rocha b a b c

Museu Nacional de História Natural e da Ciência (MUHNAC-UL), Rua da Escola Politécnica 56/58, 1250-102, Lisboa, Portugal Instituto Dom Luiz and Departamento de Geologia, Faculdade de Ciências da Universidade de Lisboa, Edifício C6, Campo Grande, 1749-016, Lisboa, Portugal Geography, School of Social Science, University of Dundee, Nethergate, Dundee DD1 4HN, Scotland

a r t i c l e

i n f o

Article history: Received 19 November 2015 Received in revised form 12 January 2016 Accepted 15 January 2016 Available online 21 January 2016 Editor: Dr. B. Jones Keywords: Sediment transport Mineral sorting Tsunami erosion Tsunami deposition Shetland Islands Scotland

a b s t r a c t This study applies heavy mineral analysis to the Storegga tsunami deposit across a range of locations (Whale Firth, Maggie's Kettle Loch and Scatsta Voe) in Shetland (Scotland). The usefulness of this proxy is tested in the identification and characterization of these palaeotsunami units. Furthermore, provenance relationships are established based on the mineralogical content of tsunami deposits and their potential source. Finally, the capability of identifying different phases of tsunami inundation in an 8200 years old tsunami deposit is attempted. Our results show that, overall, tsunamigenic samples presented a clear dominance of garnets and amphiboles. While Whale Firth presented a more balanced distribution between these two mineral groups, in Maggie's Kettle Loch and Scatsta Voe the tsunamigenic samples are dominated by amphiboles (N 90% of transparent heavy minerals). Focusing on the two dominant heavy minerals (garnets and amphiboles) and their vertical variation, one could observe that garnets mimic the heavy mineral concentration variability — higher values at the base and decreasing values to the top. This effect of concentration of the heaviest of the heavy minerals assemblage presents similarities with the formation of beach placer deposits. In fact, based on the heavy mineral vertical variation of the tsunami deposits in Maggie's Kettle Loch, Scatsta Voe and Whale Firth it is possible to conclude that hornblende (most likely amphibole of the assemblage) has the lowest concentration factor indicating that its transport process is more efficient and consequently most of its particles eventually may have moved offshore in the backwash phase of the tsunami. Furthermore, the more platy shape of amphiboles also favours a slower deposition. The opposite can be observed for garnets, which require more energy to be transported (i.e. they are more difficult to entrain by the tsunami waves) and tend to be more easily preserved in the formation of a tsunamigenic (placer) deposit. The work presented here is of particular relevance for future high resolution sedimentological studies aiming to distinguish different inundation phases of the Storegga tsunami, and assess the degree of preservation of these deposits, especially considering the specific geomorphological and stratigraphic depositional setting of Scotland. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The Storegga submarine landslide located on the continental slope west of Norway took place ca. 8200 years BP (Bondevik et al., 1997; Dawson et al., 1988) and was responsible for the biggest tsunami that affected the shores of Scotland. Due to the age of this event, geological evidence is the only source of relevant information to help understand its intensity and impact. Deposits from this tsunami have been detected across the North Atlantic: in the Faroe Islands (Grauert et al., 2001), along the coast of Norway (Bondevik et al., 1997) and Greenland (Wagner et al., 2007). However, most of the deposits have been studied in the United Kingdom: in Scotland mainland (Dawson et al., 1988; Dawson and Smith, 2000; Dawson et al., 1996; Tooley and Smith, ⁎ Corresponding author. E-mail address: [email protected] (P. Costa).

http://dx.doi.org/10.1016/j.sedgeo.2016.01.007 0037-0738/© 2016 Elsevier B.V. All rights reserved.

2005), in the Shetland Islands (Bondevik et al., 2005; Bondevik et al., 2003) and in NE England (Horton et al., 1999; Shennan et al., 2000; Smith et al., 2004) (Fig. 1). The tsunami sediments associated with the Storegga slide can be generally described as consisting mainly of fine or fine to medium sand, sometimes with silt and clay and occasionally containing gravel or stones in the basal layers. They often contain fragments or intraclasts of organic material and sometimes intraclasts of silt while rip-up clasts of peat have also been identified. Typically the tsunami units present at least one fining upwards sequence, commonly with bi-modal size distribution, and present a maximum thickness of 1.56 m but the deposit is normally ca. 10–30 cm thick (Smith et al., 2004). The microfossil content (diatoms and Foraminifera) of the tsunamigenic layer provides grounds to support a high-energy event of marine origin involving sediments of probably local provenance (Smith et al., 2004).

22

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

Fig. 1. Image displaying the source area of the Storegga Slide who generated the tsunami that affected the Shetland archipelago and most of the (eastern) North Atlantic approximately 8200 Cal years BP (adapted from Bondevik et al., 2005). Red dots represent studied sites by Bondevik et al. (2005). Black and white bars represent the estimated run-up (Bondevik et al., 2005).

Grauert et al. (2001) identified within a lagoonal stratigraphy (in the island of Suouroy, Faroe Islands) a major erosional and re-depositional event that was attributed to the Storegga tsunami inundation The deposit contained sand and sandy gyttja with marine shell fragments and foraminifera, gyttja with rip-up clasts, wood fragments and thin sand layers. Results from diatoms indicated that the deposit contained 5 to 8% of full marine species, decreasing to 1 to 2% in the overlying undisturbed lacustrine gyttja. The authors recognised two phases of tsunami inundation responsible for the deposition of two sand layers overlain by organic conglomerates. In Loon Lake (East Greenland), located at 18 m above msl, Wagner et al. (2007) identified a 0.72 m thick sandy horizon with erosive basis and distinct variations in the grain-size distribution. According to radiocarbon dates, this sandy horizon was deposited after 8500–8300 cal. Years BP and was interpreted as originated from the Storegga tsunami. Bondevik et al. (1997) extensively studied a group of coastal lakes in Norway and detected a marine input of water and sediment that was associated with the Storegga tsunami. A chaotic sedimentation pattern was observed in the deposits with marine sediments and layers of terrestrial peat and twigs side-by-side. In several basins tsunami deposition was accompanied by erosion of underlying lake sediments followed by its re-deposition. Near Berwick-upon-Tweed (NE England) Horton et al. (1999) and Shennan et al. (2000) described a unique sand layer in a coastal peat moss sequence. The association with the Storegga event was supported by its marine provenance, its unconformable lower contact, its rapid rise up-valley that exceeds the height of more recent extreme marine inundation deposits and its well-constrained age range. In Scotland mainland, in a site near Montrose presented the first traces of the Storegga tsunami imprints in coastal stratigraphy that

were discussed by Dawson et al. (1988) describing an unusual mainly fine or medium sand layer, occasionally coarser, but with some silt and sometimes containing intraclasts of peat within uplifted coastal sediment sequences. In NE Scotland other deposits sharing the same textural features and age have also been associated with the Storegga event (Tooley and Smith, 2005). In Inverness, excavations revealed a Mesolithic horizon covered by a layer of marine sand that was ascribed to the Storegga tsunami (Dawson et al., 1990). In northern Sutherland (North Scotland), a coarser layer in marked contrast with the under and overlying sediments has been described and associated with the Storegga tsunami (Dawson and Smith, 2000; Dawson et al., 1996). These authors identified a mixed diatom assemblage, with all coastal and some offshore habitats represented. Vertical variations in particle size shared strong similarities with tsunami deposits associated with the Storegga slide and described elsewhere in the North Sea basin. The most extensive and relevant Storegga tsunami deposits have been identified in the Shetland Islands. They have been generically characterized as a sandy layer in peat (Bondevik et al., 2003). However, recent work (Bondevik et al., 2005; Dawson et al., 2006) detected two other tsunamigenic units that were attributed to more tsunamigenic events. One of these events, detected in Basta Voe and dated to ca. 1500 years BP was characterised as a fine-medium sandy unit sandwiched in a coastal peat moss, with frequent bimodal distribution and with clear marine micropalaeontological assemblage. In Shetland Mainland, within two coastal lakes, deposits from a tsunami dated to ca. 5500 cal years BP were also detected (Bondevik et al., 2005). The sediment facies of both events were similar texturally to those of the Storegga tsunami. Palaeoevent analyses, including tsunami events, are used to predict event recurrence and to conduct vulnerability and risk assessments.

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

In that sense, the enhancement of the recognition and differentiation of tsunami sedimentological signatures through the application of diverse techniques is a pertinent approach. One of those sedimentological techniques applied to study of tsunami deposits has been the analysis of heavy minerals (minerals denser than 2.89 g/cm3). This technique has been successfully used to establish provenance of tsunamigenic sediment or to infer phases of tsunami inundation (Babu et al., 2007; Bahlburg and Weiss, 2007; Costa et al., 2015; Cuven et al., 2013; Higman et al., 2008; Jagodziński et al., 2012; Jagodzinski et al., 2009; Morton et al., 2007; Morton et al., 2008; Nakamura et al., 2012; Narayana et al., 2007; Putra et al., 2013; Switzer and Jones, 2008; Switzer et al., 2005; Szczucinski et al., 2005). However, this technique has not been applied to the Storegga tsunami deposits. In fact, most of these studies were focused on contemporaneous tsunamis and/or specific locations affected by these events. In contrast, the interpretation of heavy minerals assemblages to palaeotsunami deposits across a range of locations has been sparsely attempted and essentially in the context of broader sedimentological multidisciplinary approaches. Costa et al. (2015) studied heavy mineral assemblages of tsunami deposits from a wide range of locations (including Scotland) and ages (1500 years BP Basta Voe Tsunami, AD 1755 tsunami, 2004 Indian Ocean Tsunami). Results from the Portuguese studied sites revealed that palaeotsunami samples shared fewer similarities with nearshore materials and more resemblance with local dune and beach sediment, thus indicating these as the more likely source areas. In the Indian Ocean Tsunami case study the preservation of distinct depositional episodes, including run-in and backwash, was observed. Furthermore, in the (palaeo)tsunami samples it was possible to infer an efficient grain sorting process promoted by the backwash, through the variation in the heavy minerals assemblages (i.e. higher frequency of the denser minerals). This, demonstrated the influence of the backwash process in the composition of the (palaeo)tsunamigenic units. The authors suggested this mechanism was pivotal in the determination of the (palaeo)tsunami composition. A contemporaneous tsunami event (2004 Boxing Day Tsunami) was studied by Jagodzinski et al. (2009) in Thailand. They tried to establish provenance relationships relying on the study of the heavy mineral assemblages of tsunami deposits, beach sediments and pre-tsunami soils. The authors mainly described differences in the content of mica and tourmaline (tsunami samples exhibiting more mica – especially in the upper part of the units – and less tourmaline than beach and pre-tsunami soils). The abundance of mica in the tsunamigenic deposit was attributed to its lower density, and to its transport and deposition mode. A similar approach was followed by Jagodzinski et al. (2012) studying sediments deposited by the Tohoku-oki tsunami in the Sendai plain. The heavy mineral content of the fine sand grain size fraction (2–3 Φ) presented a percentage of heavy minerals ranging from 7.4 to 69.7%. In this work, it was concluded that tsunami waves did not transported a significant amount of offshore sediments inland. Moreover, coastal sediments and tsunamigenic sediments shared similar heavy minerals assemblages. Studying the same event on the Misawa coast (Japan), Nakamura et al. (2012) detected a high percentage of heavy minerals in the tsunami deposits, varying from 31 to 89% of total dry sediment and often recognized as thin laminae. It was also observed a lateral, alongshore and non-systematic variation of the percentage of heavy mineral content and an overall decrease inland. The aim of this work is to apply heavy mineral analysis to the Storegga tsunami deposits across a range of locations in Scotland and test if this proxy is useful in the identification and characterisation of these palaeotsunami units. Moreover, we also attempt to discuss possible sources that provided sediment to these deposits. Finally, we tested if an (8200 years old) tsunami deposit was capable of preserving the different phases of tsunami inundation and assess the degree of preservation of these deposits, especially having in consideration the specific

23

geomorphological, stratigraphic and climatological conditions of Scotland. This is of particular relevance for future high resolution sedimentological studies aiming to distinguish the different inundation phases of the event. 2. Study areas Shetland geology is complex, principally as a result of a large number of major north–south running faults (Fig. 2). The rocks outcropping in Shetland are supported by a platform of Late Archean (2500–3000 Ma) Lewisian basement gneiss which is the northward extension of the Hebridean Craton. The Shetland metamorphic rocks belong to the the East Mainland Succession with the Yell Sound Division (Moine) at its base, overlain by the Scatsta, Whiteness and Clift Hills Divisions (Dalradian) (Johnson et al., 1993). The latest ice sheet disappeared from Shetland around 14,000 years ago, although a cold phase a few thousand years later conveyed a reappearance of glaciers, finally disappearing ca. 10,000 years ago (Johnson et al., 1993). The coastal configuration of Shetland at the end of the Ice Age was very different from today, sea levels being around 100 m lower due to the amount of water locked up in the ice (Johnson et al., 1993). However, since then, melting ice has caused sea levels to rise and the Shetland landmass has also been depressed as the massive weight of the ice over the continental landmass to the east was removed (Johnson et al., 1993). Around 7000 or 8000 years ago the first Mesolithic hunter gatherers arrived and sea level was around 15 m below present levels — peat has been found at this level at this period on the Unst coast (Bondevik et al., 2005; Johnson et al., 1993). The outer coast of Shetland is essentially formed of cliffs, reflecting rapid erosion along one of the highest energy coastlines in the world. The middle coast is much more sheltered and includes the anchorages of Sullom Voe and Bressay Sound, where deep water fills drowned glacial valleys. The sands and gravels along the inner coast are highly mobile, with beaches, spits and bars adapting to sea level rise and being reworked during major storms. 2.1. Scasta Voe Scatsta Voe is a small inlet on the southern shore of Yell Sound, within Sullom Voe in Shetland Mainland and is located immediately to the northeast of Scatsta Airport (Fig. 3). The middle region of Sullom Voe (where Scatsta Voe is located) is a sheltered and varied geomorphologically, indented in places by small voes and houbs (lagoons). Intertidal sediments are confined to the houbs and the heads like for example Scatsta Voe. These sediments are variable and poorly sorted, consisting mainly of coarse sand, gravel, shell debris and peat fragments. 2.2. Maggie's kettle loch Maggie's Kettle Loch is located on the western shore of Sullom Voe (Fig. 3). A shingle barrier and lochan protect the sediments exposed along a coastal bluff around the embayment. The study site occurs along a small stream section perpendicular to the shore which has cut down through a coastal peat moss to expose an extensive outcrop of sands and gravels intercalated within peat. The outcrop at Maggie Kettle's Loch shows extensive evidence of erosion of the peat and rip-up clasts of peat are occur in the lower reaches of the section (Bondevik et al., 2003) (Fig. 4). 2.3. Whale Firth Whale Firth is a north-facing voe on the Island of Yell facing the western shore of the outer reaches of Sullom Voe (Fig. 5). At the head of the firth extensive coastal peat mosses extend to present sea level. In the exposed coastal bluffs a widespread sand layer was observed in

24

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

Fig. 2. Geological map of Shetland (obtained on the British Geological Survey website — http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1003864 viewed on the 9th November 2015).

peat cuts, coastal sections, trenches and cores (Fig. 4). This was particularly visible in an actively eroding coastal peat bluff above the modern day coarse clastic beach at the head of the fjord. 3. Methods Departing from a larger sample database retrieved from several locations in the Shetland archipelago, an attempt was done to study heavy mineral assemblages focusing on sites where the Storegga tsunami deposit was more preserved and had a stronger presence in the coastal stratigraphy (both vertically and horizontally). A total of thirty three Storegga tsunami samples were collected from trenches excavated in coastal outcrops in the locations presented in Figs. 3, 5: twelve samples were obtained in Maggie's Kettle Loch; seventeen samples were obtained from four different trenches in Scatsta Voe (SHT1, SHT3, SHT8 and SHT9); and finally, four samples were collected from one trench located in Whale Firth. Additionally, to discuss provenance relationships six samples were collected from Mid-Yell Voe and the adjacent voe of Whale Firth, corresponding to three glacial samples and two beach samples. Additionally, another beach sample was collected from Kirkabister (at the outer edge of Mid Yell Voe). Grain size analysis was conducted using a set of sieves at 0.5 Φ interval. Initially, the bulk samples were washed with tap water and two fractions were separated (N63 μm and b63 μm) using a 4 Φ mesh.

The finer fraction (b63 μm) was analysed using the laser granulometer Malvern Mastersizer Hydro 2000 MU after deflocculating with 30% Sodium hexametaphosphate. The coarser fraction (N63 μm) was analysed exclusively through sieving, with exception of samples that had a coarser fraction weighting less than 10 g. Results derived from both methods were assumed to be comparable. Statistical parameters of the grain size distribution (e.g. mean grain size, Φ10, Φ90, median, mode, standard deviation, kurtosis and skewness) of the samples were calculated using the graphic method (Folk and Ward, 1957). For heavy mineral analysis samples were grouped in the1 to 3 Φ fraction and prepared in order to observe them in slides under the petrographic microscope. Heavy minerals were separated using bromoform and then mounted using Entelan resin on glass slides (Costa et al., 2015). The required amount of grains for each slide was obtained using a micro-splitter. About 300 heavy minerals per slide were identified and counted under an Olympus BX40 petrographic microscope in each sample. The percentage of heavy minerals was recalculated to solely consider the transparent grains (examples in Fig. 6). Statistical analysis (Principal Component Analysis –PCA-) were conducted to facilitate interpretation of the results. This analysis consists in the reduction of the original variables (expressed by the relative frequency of the minerals) into fewer unobservable variables (principal components) that are linear combinations of the original ones. The main

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

25

Fig. 3. Location map of Maggie's Kettle Loch and Scatsta Voe with the Sullom Voe area. Samples were retrieved from areas adjacent (b200 m) to the present-day coast line.

objective of PCA is the explanation of as much of the variability of the original data with as few of the extracted components as possible. In the interpretation of the results we used mean values for the density for each mineral species (the range of densities is presented in Table 1 according to Mange and Maurer (1992). This criteria was selected based in the knowledge of the more common types of minerals for each heavy mineral species/group and in the common practice used in heavy mineral studies (Mange and Wright, 2007).

4. Results 4.1. Scatsta Voe At Scatsta Voe seventeen samples from the tsunami deposit were analysed (SHT1, SHT3, SHT8 and SHT9) (Table 2). Vertical analysis

focused on samples from trench SHT9 where the sampling resolution was higher. In this location, the erosion of peat by the sea exposed along the coast a cliffed outcrop of ca. 2 m high (Figs. 3, 4). Within the lower part of that peat, there is an approximately 0.10 m thick, wide-spread massive sand layer, which overlies 0.30 m of peat. The sand layer is more or less continuous for more than 150 m both alongshore and inland. The exposed trench walls revealed a basal peat unit of ca. 0.30 m overlying a heterometric glacial deposit. This unit is overlaid by a medium to coarse sand layer with an erosional contact at the base and an unconformity at the top of the unit. Moreover, this unit also exhibited some centimetric pebbles and rare cobbles although it did not present a very rich shell content. On top of this sandy unit, a peat layer is present. The top 0.10 m are composed of dark brown soil with many roots and plant fragments. The association of this deposit with the Storegga tsunami has been discussed previously by Costa (2012).

26

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

Fig. 4. Field and laboratory photographs of the Storegga tsunami deposit. Images from the tsunami deposit retrived from Maggie's Kettle and stored in a box-core (A, C). Images obtained in the coastal area in Scasta Voe (B) and Whale Firth (D) displaying the coastal trenches stratigraphy (approximately 1 m of exposed sediment) and the uniqueness of the tsunamigenic sandy deposit over and underlay by the dark brown/black peat.

The tsunami layer presents a median (D50) average of 1.59 Φ (medium sand), with a maximum of 0.57 Φ (coarse sand) and a minimum of 2.33 Φ (fine sand). The percentage of heavy mineral in total sediment corresponds to an average of 0.11% with a maximum of 0.20% and a minimum of 0.02%. The analysis of the heavy mineral assemblage revealed a high dominance of amphiboles with an average of 93.83%, a maximum of 97.10% and a minimum of 89.55%. This mineral group was followed, at a great distance, by garnets that exhibit an average value of 2.27% with a maximum of 5.63% and a minimum of 0.32%. Other minerals were also identified but its presence is residual: mica, apatite, clinopyroxene, andalusite, epidote, tourmaline and zircon. Of these, only mica and apatite revealed a frequency above 1% (1.08 and 1.06%, respectively) (Table 2).

Grain size data (D50) of the tsunami deposit (twelve samples) indicates that this unit consists of coarse sand (median value of 0.68 Φ), with a maximum value of — 0.05 Φ (very coarse sand) and a minimum of 0.88 Φ (coarse sand). The heavy mineral weight in total sediment reveals an average of 0.13% with a maximum of 0.29% and a minimum of 0.08%. Considering the identified heavy minerals the samples reveal high dominance of amphiboles with an average of 93.31% correspondent to a maximum of 97.28% and to a minimum of 89.13%, followed, by far, by the garnets group that exhibit an average value of 2.23% with a maximum of 4.78% and a minimum of 1.15%. Other minerals were also identified but only in very small numbers: epidote, apatite, rutile, mica, clinopyroxene, tourmaline, zircon and andalusite. From these the epidote is the unique mineral with frequency above 1% (1.56%) (Table 2).

4.2. Maggie's kettle loch

4.3. Whale Firth

The tsunami deposits along the stream section at Maggie Kettles Loch was traced across the present shore and up to 9.2 m above high tide level over a total distance of 150 m. Close to the present shore, the tsunami deposit is 30–40 cm thick and shows large rip-up clasts of peat embedded in the sand. Many of the clasts are 10–30 cm in diameter with sharp edges. The sand, which is medium to very coarse, contains pebbles and cobbles. The sand thins and fines inland. Close to the sea, the sand is 30–40 cm thick. From about 18 m from the shore and inland, the sand thins from 10 cm to less than 1 cm at the maximum elevation. Between 0.8 and 4 m above high tide, the sand is normal graded, from very coarse sand with fine gravel particles at the bottom, to medium sand at the top. From 6 m above high tide and inland, the sand is massive—between 4 and 1 cm thick—and discontinuous, and it ends 9.2 m above high tide (Bondevik et al., 2005). In Maggie's Kettle Loch (Figs. 3, 4), twelve tsunami samples were analysed after being collected (at different depths) in a single beach outcrop. –vertical variation results are discussed below. The tsunami deposit here is texturally heterometric. It is a massive sandy unit embedded in a peat sequence (Fig. 4). The tsunami unit presents several impressive and unique rip-up clasts and is in sharp contrast with the lower energetic sedimentation pattern responsible for the deposition of peat. At the base of the sequence the heterometric sand-supported glacial deposit (Younger Dryas) is visible.

At Whale Firth a widespread sand layer was observed in peat cuts, coastal sections, trenches and cores. This was particularly visible in an actively eroding coastal peat bluff above the modern day coarse clastic beach at the head of the fjord. A length of this coastal section was cleaned up for a distance of 65 m to enable the stratigraphy to be recorded and samples taken. Along this section the deposit exhibited as an organic, micaceous sand layer up to 10 cm thick within woody peat (Figs. 4, 5) The base of the deposit was visibly coarser than the top, containing very coarse sand and a large number of granules and small pebbles. The deposit also was characterised by a sharp undulating and unconformable contact with the underlying peat. In Whale Firth the grain size of the 7 samples presents a D50 average of 1.67 Φ (medium sand), with a maximum of 1.13 Φ (medium sand) and a minimum of 2.48 Φ (fine sand). The heavy mineral percentage total sediment displays an average of 0.05% with a maximum of 0.06% and a minimum of 0.03%. The transparent heavy minerals identified reveal the preponderance of amphiboles with an average of 46.36%, with a maximum of 64.19% and a minimum of 31.78%, followed, with very similar values, by garnets that exhibit an average value of 41.17% with a maximum of 62.02% and a minimum of 25.38%. Other minerals were also identified: andalusite, clinopyroxene, tourmaline, rutile and zircon. From these only andalusite and clinopyroxene have frequencies above 1% (9.46% and 1.56%, respectively) (Table 2).

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

27

Fig. 5. Location map of Whale Firth. Vertical variation analysis was studied on samples retrieved from trench Whale 4.Samples G1 and G2 correspond to the analysed glacial sediments while B1 and B2 correspond to beach (sand flat) sediments.

Beach and glacial samples were retrieved to assess possible provenance relationships (Fig. 3). In terms of the percentage of heavy minerals in total sediment fraction, the glacial samples presented a

mean value of 0.14% whereas the beach samples presented a lower value of 0.03%. The heavy mineral assemblage of both sample sets is essentially dominated by garnets and amphiboles however, while in

Fig. 6. Image of main heavy mineral species observed in the petrographic microscope under plane-polarized light: A — Amphibole, B — Andalusite, C — Garnet and D — Mica. Scale in the image can be applied to each mineral grain.

28

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33 Table 1 Range of densities of the main heavy mineral species identified (according to Mange and Maurer, 1992). Heavy mineral

Range of densities (g/cm3)

Amphiboles Garnets Apatite Andalusite Epidote Mica Pyroxene Tourmaline Zircon Rutile

2.85–3.57 3.40–4.30 3.10–3.35 3.13–3.16 3.38–3.49 2.40–3.30 2.96–3.96 3.03–3.25 4.60–4.70 4.23–5.50

all palaeotsunami samples this association (garnets + amphiboles) presented values between 83 and 98%, the glacial sediment exhibited values between 57 and 94% while beach samples showed results between 67 and 87%. These results suggest a richer and more diverse heavy mineral assemblage for the potential source sediment (as can be seen in the percentage of the other minerals — Table 2). 5. Discussion In this work we apply, for the first time, the analysis of heavy mineral assemblages to the study of the Storegga tsunami depositional

signature. Typically heavy minerals are used to define sediment sources and transport pathways. As summarized by Garzanti and Andò (2007), the concentration of heavy mineral grains in sand-sized terrigenous sediments may fluctuate considerably because of several factors including provenance, sedimentary processes, source effect, syn and post-depositional dissolution (Mange and Maurer, 1992; Morton and Hallsworth, 1999). The anticipated effect of this process in tsunami deposits is likely responsible for the increase in weathered minerals (i.e. alterites). In tsunami deposits another relevant factor to be considered is the hydraulic sorting (Costa et al., 2015) that can disturb the heavy mineral assemblage relationship between source and deposit. Although the total number of samples is limited (N = 40) and not well balanced (e.g. Whale Firth has eleven samples – but just 5 tsunamigenic – whereas Scatsta Voe has seventeen), from Table 2 it is possible to observe that the tsunamigenic samples present an equivalent median value for the percentage of heavy minerals in total sediment fraction when compared with its potential source material with the exception of two glacial sediment samples collected in Whale Firth. These exceptions are justified essentially by the larger percentage of mica which is influenced by the primary source — mica-plagioclase gneiss of Yell (i.e. Whale Firth) (Fig. 2). In Scatsta Voe and Maggie's Kettle Loch the percentage of heavy minerals in total sediment fraction is very high (about 2 times higher than other studied samples) eventually because of the coastal exposure to the Storegga tsunami waves (originated NE of Shetland) and the local geomorphological features

Table 2 Detailed mineralogical data from the samples. Location and comment on the origin of the sample. Percentages of heavy mineral in total sediment fraction, 1 to 3 phi fraction, transparent heavy minerals in the 1 to 3 phi fraction and total sediment fraction. Individual heavy mineral composition from all studied samples. HM — heavy minerals; D50 — median grain size; AM — amphiboles; GA — garnets; APA — apatite; AND — andalusite; EP — epidote; MI — micas; CPX — clinopyroxenes; TOU —tourmaline; ZI — zircon; RU — rutile. Samples

Top

Base

MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S MAGGIE'S SHT1 SHT1 SHT3 SHT3 SHT 8 SHT 8 SHT 8 SHT 9 SHT 9 SHT 9 SHT 9 SHT 9 SHT 9 SHT 9 SHT 9 SHT 9 SHT 9 Whale4 Whale4 Whale4 Whale4 Mid Yell Tsu Mid Yell 2 Glacial WF Glacial WF Glacial 2 Kirkabister beach WFT Flat 1 WFT Flat 2

4.5 5.5 5.5 6.5 6.5 7.5 7.5 8.5 8.5 9.5 9.5 10.5 10.5 11.5 11.5 12.5 12.5 13.5 13.5 14.5 14.5 15.5 15.5 16.5 22 24.0 26 28.0 22 25.0 25 28.0 7 9.0 13 15.0 19 21.0 0 1.0 1 2.0 2 3.0 3 4.0 4 5.0 5 6.0 6 7.0 7 8.0 8 9.0 9 10.0 71 73.0 73 75.0 75 77.0 77 79.0 Tsunami Glacial Glacial Glacial Beach Beach Beach

% Transparent HM

% 1-3phi in total sed

% HM in 1-3phi

% HM in total sed fraction

D50

AM

GA

APA

AND

EP

MI

CPX

TOU

ZI

RU

75.36 82.57 74.49 81.19 77.46 75.66 75.82 78.74 80.47 74.48 86.59 79.47 90.82 93.09 90.55 90.00 88.37 87.50 86.82 88.83 89.53 95.50 92.20 88.83 91.09 90.30 90.87 92.78 88.26 90.38 86.08 90.56 84.29 84.47 79.38 91.67 92.81 85.53 94.33 96.73

34.38 27.37 29.22 28.81 25.58 36.65 39.03 31.57 30.27 17.94 12.46 15.77 52.15 39.64 45.84 57.71 57.68 31.12 27.78 42.85 42.88 33.95 30.55 27.97 28.48 26.80 24.43 27.28 34.29 74.12 69.85 68.99 66.88 59.20 56.22 41.94 36.59 89.81 92.07 79.06

3.42 3.14 2.94 2.57 2.46 3.06 3.39 3.47 4.09 3.46 3.57 3.35 3.45 2.65 2.18 1.43 1.89 2.10 4.19 3.66 4.05 4.44 3.98 4.30 3.39 4.47 4.36 5.52 6.07 3.19 2.71 3.64 4.21 1.58 2.40 7.08 7.24 4.61 1.64 2.68

0.10 0.11 0.10 0.09 0.10 0.08 0.09 0.11 0.13 0.19 0.29 0.21 0.07 0.07 0.05 0.02 0.03 0.07 0.15 0.09 0.09 0.13 0.13 0.15 0.12 0.17 0.18 0.20 0.18 0.04 0.04 0.05 0.06 0.03 0.04 0.17 0.20 0.05 0.02 0.03

0.827 0.761 0.759 0.769 0.706 0.848 0.875 0.723 0.804 0.576 −0.055 0.532 1.166 0.572 0.885 1.283 1.385 0.886 0.864 2.323 2.331 1.717 2.070 1.994 1.884 1.916 1.860 1.895 2.048 1.148 1.129 1.357 2.179 1.252 2.480 1.367 1.083 2.179 2.172 1.738

93.85 93.43 97.28 93.38 92.36 92.64 93.91 93.92 89.13 90.84 94.28 94.70 96.86 96.13 94.61 95.96 94.98 94.72 91.75 97.10 95.38 92.80 94.04 93.30 93.88 89.55 94.18 89.63 90.28 50.00 54.04 37.79 44.80 64.19 31.78 39.94 38.71 41.92 54.75 38.46

1.15 1.38 1.17 1.47 1.45 2.71 2.15 1.90 2.90 4.78 4.04 1.66 0.70 0.32 1.01 0.34 1.88 3.42 4.62 1.29 1.54 2.77 3.76 2.23 3.67 1.79 1.06 2.59 5.63 42.91 29.41 51.14 47.67 29.68 62.02 36.36 18.06 25.38 30.06 48.62

0.38 2.08 0.00 1.47 1.09 0.78 0.72 1.14 0.72 0.00 0.34 0.66 1.74 2.26 3.03 2.69 0.94 0.00 0.33 0.32 0.62 1.11 0.31 0.00 0.61 0.60 0.79 1.44 1.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.95 0.31

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.33 0.00 0.00 0.00 0.00 0.31 0.31 0.99 0.00 0.31 0.28 0.00 0.28 0.61 0.30 0.26 0.86 1.53 6.38 11.40 9.45 6.45 5.16 5.43 1.62 2.58 21.92 1.58 1.54

1.15 2.42 0.39 2.57 1.82 1.94 1.43 1.52 3.62 0.80 0.00 0.99 0.00 0.00 0.00 0.00 0.31 0.00 0.99 0.00 0.62 0.00 0.31 0.28 0.61 0.30 0.00 1.15 1.02 0.00 0.00 0.00 0.00 0.00 0.00 0.32 0.00 0.00 0.00 0.00

0.00 0.00 0.39 0.00 0.36 0.39 0.00 0.76 0.72 2.79 0.34 0.66 0.00 0.00 0.00 0.00 1.57 0.31 0.33 1.29 0.00 2.49 0.63 3.63 0.00 3.58 1.32 3.17 0.00 0.00 0.00 0.00 0.00 0.00 0.00 21.75 40.65 0.00 12.66 11.08

2.31 0.35 0.39 0.00 0.36 0.39 0.36 0.00 0.00 0.00 0.34 0.66 0.35 0.97 1.01 0.67 0.00 1.24 0.66 0.00 1.54 0.28 0.63 0.00 0.31 2.69 2.38 0.86 0.00 0.35 2.94 0.00 0.72 0.00 0.00 0.00 0.00 6.92 0.00 0.00

0.77 0.00 0.39 0.00 0.36 0.39 0.36 0.00 0.72 0.00 0.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.33 0.00 0.00 0.28 0.31 0.28 0.31 0.60 0.00 0.29 0.26 0.00 2.21 0.33 0.00 0.00 0.00 0.00 0.00 3.85 0.00 0.00

0.38 0.35 0.00 0.74 0.00 0.00 0.00 0.00 0.36 0.40 0.00 0.00 0.35 0.32 0.34 0.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.60 0.00 0.00 0.00 0.00 0.00 0.65 0.36 0.00 0.39 0.00 0.00 0.00 0.00 0.00

0.00 0.00 0.00 0.37 2.18 0.78 1.08 0.76 1.81 0.40 0.34 0.33 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.35 0.00 0.65 0.00 0.97 0.39 0.00 0.00 0.00 0.00 0.00

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

(Whale Firth is a narrower and longer voe while Scatsta and Maggie's Kettle Loch are wider thus dissipating less wave energy). The importance of mineral composition of the source region is also evident in the tsunamigenic samples analysed. Overall, tsunamigenic samples presented a clear dominance of garnets and amphiboles (Table 2). However, Whale Firth presented a more balanced equilibrium between these two mineral groups, in contrast with Maggie's Kettle Loch and Scatsta Voe tsunamigenic samples where amphiboles are dominant (N 90% of transparent heavy minerals), this reflects local source material (namely hornblende gneiss or schist from the Moine Formation in Shetland Mainland that are richer in amphiboles). By the application of the principal component analysis we observe that the sum of the first two components variance correspond to approximately 80% of total variance (Fig. 7). Applying the PCA to the data matrix we obtain a set of four components where the first two explain about 80% of the variance. Of these, the first one (explaining about 60% of the variance) mostly exhibiting the opposition between garnets and amphiboles while the second one (explaining about 20% of the variance) essentially shows the opposition between the median grain size and the heavy mineral percentage in total sediment fraction. The regional source preponderance is clear in Fig. 7 where it is possible to observe that each quadrant presents specific grouping of samples. The left hemisphere of Fig. 7 is dominated by Maggie's Kettle Loch and by geographically near-by Scatsta Voe samples while the lower right hemisphere presents samples from Whale Firth. Component 1 along the x-axis (Fig. 7) demonstrate the opposite loading between garnets and amphiboles, which reflects the source effect (Whale Firth sources are richer in garnets). Component 2 along the y-axis (Fig. 7) is controlled by the influence of median grain size and, to a lesser extent by the percentage of heavy minerals in total sediment fraction. In fact, the coarser sand was detected in Maggie's Kettle Loch (D50 = 0.68 Φ —coarse sand). The analysis of this component per se could allow careless speculation regarding the decrease of energy involved in the depositional process and its reflection on the grainsize. However, local geomorphological and sedimentary settings and characteristics need to be taken into account which limit discussion solely based on the data presented here.

29

The vertical variation of the mean grain-size (in samples from all studied locations) does not show a clear positive or negative grading (Figs. 8B, 9B). In fact, the oscillating pattern is reflected in its medium sand composition, with a few exceptions (e.g. coarser at the base in Maggie's Kettle Loch or finer at the base in Scatsta Voe). On the other hand, the percentage of heavy mineral in total content presents a different vertical variation. Overall, the higher concentrations are detected at the base of the deposit which could indicate higher transport energy involved. In Maggie's Kettle Loch the basal section of the deposit (3 basal samples) presents about 3 times higher concentration in the heavy mineral content when compared with the upper section of the deposit (Fig. 8A). In Scatsta Voe (trench SHT9) this ratio is about 2 times while in Whale Firth the vertical variation is less noticeable but still present (Fig. 9A and Table 2). One can conclude that all studied sites presented a largely similar tendency in the vertical variation of heavy mineral content which implies that the initial phase of the tsunami event left a clear signature in the deposits which reflected its higher energetic capability contrasting with the subsequent phase of the tsunamigenic inundation (and its depositional signature — with lower heavy mineral content). Focusing in the two dominant heavy minerals (garnets and amphiboles) and their vertical variation, one can observe that garnets mimics the heavy mineral concentration variation — higher values at the base and decreasing values to the top (Figs. 8C, D, 9C, D) this might reflect a dominance of bed load sorting process. In Maggie's Kettle Loch and Scatsta Voe (trench SHT9) the garnets are about 4 times higher at the basal section while in Whale Firth the discrepancy in the values are not so marked (but the same pattern is still visible). By contrast, amphiboles slightly increase towards the top of the deposit with the exception of the basal section in Maggie's Kettle Loch. In this specific basal sector, it is also detected the coarsest sample (14.5 cm) and in this sector the concentration of heavy minerals is the highest. This reasoning might suggest a dominance of bedload transport in the basal sector followed by the subsequent sediment settling by suspension. The first mode of transport is commonly responsible for massive or inverse grading (Jaffe et al., 2012) while the second typically produces suspension or positive

Fig. 7. Principal components analysis conducted for all samples studied. Exhibiting the relationship between the different variables and also displays the sample distribution based in the two main components. The first one essentially shows the opposite loading between garnets (GA) and amphiboles (AM) while the second one highlights the opposition between the median grain size (D50) and the percentage of heavy minerals in total sediment fraction (%HM tsf). Red circles — Scatsta Voe (S), blue circles — Maggie's Kettle Loch (M) and green circles — Whale Firth (W) and MYt. Potential source sediments are represented by Kb, B1 and B2 (beach samples) and MYg, G1 and G2 (glacial samples).

30

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

Fig. 8. Vertical variation of heavy mineral percentage in total sediment fraction (A), median grain size (B), percentage of amphiboles (C) and percentage of garnets (D) in samples collected in Maggie's Kettle Loch. The two lower images (E and F) display the principal components analysis conducted in these samples.

grading in the tsunami deposits. This allows an analogy with Maggie's Kettle Loch tsunami deposit can be established therefore indicating a dominance of bedload transport in the basal sector of the deposit, this is further supported by grain-size data. Further support is provided by the seminal work of Komar (2007) where the settling velocities and the hydrodynamic and sorting properties

of heavy minerals were studied. Komar (2007) analysed the mineral sorting by waves and the formation of heavy mineral concentration (placers) and looked at the variation of heavy mineral assemblages and percentages focusing in the development of a concentration factor that allows comparison between different heavy mineral species. The concentration factor of heavy minerals increases with

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

31

Fig. 9. Vertical variation of heavy mineral percentage in top sediment fraction (A), median grain size (B), percentage of amphiboles (C) and percentage of garnets (D) in samples collected in Scatsta Voe (trench SHT9).

Fig. 10. Concentration factors of minerals in an Oregon-beach black sand (with high percentage of heavy minerals) versus the flow stresses required for their entrainment — (Komar and Wang, 1984) — A. Along-profile variations in heavy-mineral percentages in the swash zone of an Oregon beach - (Komar and Wang, 1984) — B.

32

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

grain density and grain diameter. Here we analysed the 1–3 Φ fraction, so decreasing the relevance of the diameter and focusing on the influence of density in the deposition of sediments. In Fig. 10A one can observe the relationship between the concentration factor and threshold stress and the percentage of heavy minerals and the distance travelled offshore (Fig. 10B). Based in this data and the heavy mineral vertical variation of the tsunami deposits in Maggie's Kettle Loch, Scatsta Voe and Whale Firth it is possible to conclude that hornblende (most likely amphibole of the assemblage — see Section 2 — Study areas) has the lowest concentration factor indicating that sediment transport is more efficient and the vast majority of amphiboles move offshore. Furthermore, the more platy shape of amphiboles also favours its slower deposition and it is likely that higher concentrations of these amphiboles are found at the deposit inlandmost limit especially in cases where very limited backwash occur. The opposite can be observed for garnets, which are more difficult to entrain by the tsunami waves and tend to be more easily preserved in the formation of a beach placer deposit (Komar, 2007) thus likely deposited in higher concentrations closer to the original sediment source and away the inlandmost deposit limit. The resemblance with tsunami deposits is obvious and stresses the relevance of the study of heavy minerals assemblages in the identification of tsunami deposits and in the understanding of the sediment transport modes associated with tsunami deposition. In the studied cases, garnets is the (main) heavy mineral providing insights regarding sediment transport modes although amphiboles are dominant, their constant presence, smaller vertical variations and lower concentration factors. When samples from the tsunami deposit in Whale Firth and their likely source material (beach and glacial sediment) are compared, it can be hypothesize that both, the beach and the glacial sediments, could be the likely sediment source. The beach sediment (if presentday beach is assumed as an analogue) could be the sand-source of the deposit due to a compatible mineral assemblage (although more diverse) and similar heavy mineral content in total sediment fraction. However, the more likely source of sand is the heterometric and chaotic glacial sediment underlying peat and the tsunamigenic sandy unit. This is supported by the content of both garnets and amphiboles and was previously addressed by Costa et al. (2015) for the more recent Basta Voe tsunami in the Shetland Islands. The reasoning applied for that event could and should be extrapolated to the Storegga deposits studied here. This also implies that during the tsunami inundation large sections of the “coastal” glacial sediment were eroded and transported and deposited farther inland by the incoming waves. This can easily be verified in the field where the outer sectors of the voes present a direct stratigraphic contact between tsunami sand and glacial drift. Once more it is evident that tsunami deposits are essentially composed of local material that is available at the time of the inundation. In fact, this interpretation is fundamental to validate future sediment transport modelling exercises because tsunami waves transport capability might be more constrained geographically than initially thought. The Storegga tsunami deposits across Shetland are essentially massive which does not allow inferences in terms of the number of waves during the event. In more recent events (Costa et al., 2015; Jagodziński et al., 2012; Nakamura et al., 2012) it was possible to differentiate different inundation phases/pulses based on analysis of the mineralogical data. For example, Costa et al. (2015) associated an increase in (euhedral) zircon with the backwash phase of the Indian Ocean tsunami. In the case of the Storegga deposit to discriminate these phases of inundation would allow great progress to be made in the understanding of this event. However, the relevance of post-depositional dissolution must also be considered in future palaeotsunamigenic studies. In this work, both garnet and amphiboles present similar relative stability to chemical dissolution as demonstrated in the experimental work of Nickel (1973) in a wide range of pH conditions (3.6 to 10.6) thus not constraining sediment transport interpretations. The geological

record is, even more for this tsunami, the only source of reliable information. However, this challenging task faces difficulties as well as the issues regarding preservation. For example, the monomineralic character of Maggie's Kettle Voe and Scatsta Voe limit interpretation in terms of variability within the deposit and creates major difficulties with reference to the recognition of different inundation pulses (inundation and backwash). A possible way to overcome this would be to intensively use microprobe analyses that would allow a clearer differentiation within monomineralic groups. Another possible explanation for the massive character of the deposit might rely on its source mechanism. Typically, landslide-tsunamis present an initial (uni)directional wave alignment when compared with more radial wave dispersion in earthquake-generated tsunamis. Furthermore, landslide-tsunamis also tend to present a higher initial wave energy followed by much lower waves, more likely incapable of imprinting coastal stratigraphy and this could be translated in a single sedimentological signature (massive unit) in contrast with other observations in earthquake-generated tsunami (e.g. Lhok Nga, Indian Ocean Tsunami — Costa et al., 2015). This unidirectional character could provide an alternative explanation to the massive character of the deposit, something that can only be proved by high-resolution wave and sediment transport modelling — beyond the scope of this work but an interesting challenge for future research. 6. Conclusions Heavy mineral analysis have been scarcely used in the study of tsunami deposits although recent works have indicated that this sedimentological technique can provide useful insights in the interpretation of different phases of tsunami inundation and in the establishment of provenance relationships. In this work, we analyse the heavy mineral assemblages of the Storegga tsunami deposit (app. 8200 years old) across a range of locations in the Shetland archipelago (Scotland). Thirty four palaeotsunami and six likely sediment source samples were prepared in order to observe heavy minerals (1 to 3 Φ sediment size-fraction) under the petrographic microscope. Results show that these samples have a mineral assemblage dominated by garnets and amphiboles. The later are dominant (N 90% of transparent heavy minerals) in Maggie's Kettle Loch and Scatsta Voe while in Whale Firth these minerals appear in similar proportions. The observation of the vertical variation of the main heavy minerals (garnets and amphiboles) displays a similarity between garnets and total heavy mineral concentration – higher values at the base and decreasing values to the top – mimicking a process common in the formation of beach placer. The detailed analysis of the heavy mineral vertical variation of the tsunami deposits in Maggie's Kettle Loch, Scatsta Voe and Whale Firth demonstrates that hornblende (most likely amphibole of the assemblage) has the lowest concentration factor suggesting that sediment transport is more competent and many amphiboles particles are transferred to the offshore in the backwash phase of the tsunami. Moreover, shape characteristics also allowed sediment transport inferences. The more platy shaped heavy mineral grains (amphiboles) require less energy to be transported and are more likely to be transported as suspended load. By contrast, garnets are more difficult to be entrained and transported by tsunami grains and are more easily preserved in the formation of tsunamigenic (placer) deposit. Future palaeotsunamigenic research can benefit from the work presented here especially differentiating inundation phases and in the assessment of post-depositional changes in these deposits (namely chemical dissolution). However, the full understanding of the sediment processes involved in tsunami inundation and backwash, responsible for the deposition of these peculiar sedimentary units and its heavy mineral assemblages, will benefit from specifically designed physical experiments (involving wave flumes and tanks). Coupling results from

J. Cascalho et al. / Sedimentary Geology 334 (2016) 21–33

field and laboratory data will provide the decisive contribution to fully perceive palaeotsunami sedimentological signatures. Acknowledgements The work presented here was supported by NERC project “Will climate change in the Arctic increase the landslide-tsunami risk to the UK?”. P.Costa benefited from an FCT Post-Doctoral Fellowship SFRH/BPD/84165/2012. The authors would like to express their gratitude to Dr. Brian Jones (Editor of Sedimentary Geology) and two anonymous reviewers for their very constructive comments. References Babu, N., Suresh Babu, D.S., Mohan Das, P.N., 2007. Impact of tsunami on texture and mineralogy of a major placer deposit in southwest coast of India. Environmental Geology 52, 71–80. Bahlburg, H., Weiss, R., 2007. Sedimentology of the December 26, 2004, Sumatra tsunami deposits in Eastern India (Tamil Nadu) and Kenya. International Journal of Earth Sciences 96 (6), 1195–1209. Bondevik, S., Svendsen, J.I., Mangerud, J., 1997. Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, Western Norway. Sedimentology 44, 1115–1131. Bondevik, S., Mangerud, J., Dawson, S., Dawson, A., Lohne, Ø., 2003. Record-breaking height for 8000-year-old tsunami in the North Atlantic. EOS. Transactions of the American Geophysical Union 84, 289–293. Bondevik, S., Mangerud, J., Dawson, S., Dawson, A., Lohn, Ø., 2005. Evidence for three North Sea tsunamis at the Shetland Islands between 8000 and 1500 years ago. Quaternary Science Reviews 24, 1757–1775. Costa, P.J.M., 2012. Sedimentological Signatures of Extreme Marine Inundations. Lisbon, Lisbon 245 pp. Costa, P.J., Andrade, C., Cascalho, J., Dawson, A.G., Freitas, M.C., Paris, R., Dawson, S., 2015. Onshore tsunami sediment transport mechanisms inferred from heavy mineral assemblages. The Holocene. Cuven, S., Paris, R., Falvard, S., Miot-Noirault, E., Benbakkar, M., Schneider, J.-L., Billy, I., 2013. High-resolution analysis of a tsunami deposit: case-study from the 1755 Lisbon tsunami in southwestern Spain. Marine Geology 337, 98–111. Dawson, S., Smith, D.E., 2000. The sedimentology of Middle Holocene tsunami facies in northern Sutherland, Scotland, UK. Marine Geology 170 (1–2), 69–79. Dawson, A.G., Long, D., Smith, D.E., 1988. The Storegga slides: evidence from eastern Scotland for a possible tsunami. Marine Geology 82 (3–4), 271–276. Dawson, A.G., Smith, D.E., Long, D., 1990. Evidence for a tsunami from a mesolithic site in Inverness, Scotland. Journal of Archaeological Science 17 (5), 509–512. Dawson, S., Smith, D.E., Ruffman, A., Shi, S., 1996. The diatom biostratigraphy of tsunami sediments: examples from recent and middle holocene events. Physics and Chemistry of the Earth 21 (1–2), 87–92. Dawson, A., Dawson, S., Bondevik, S., 2006. A Late Holocene tsunami at Basta Voe, Yell, Shetland Isles. Scottish Geographical Journal 122 (2), 100–108. Folk, R.L., Ward, W.C., 1957. Brazos river bar: a study of significant of grain size parameters. Journal of Sedimentary Petrology 27, 3–26. Garzanti, E., Andò, S., 2007. Heavy-mineral concentration in modern sands: implications for provenance interpretation. In: Mange, M.A., Wright, D.T. (Eds.), Heavy Minerals in Use. Elsevier, Amsterdam, pp. 517–545. Grauert, M., Bjork, S., Bondevik, S., 2001. Storegga tsunami deposits in a coastal lake on Suduroy, the Faroe Islands. Boreas 30, 263–271. Higman, B., Bourgeois, J., Shiki, T., Tsuji, Y., Yamazaki, T., Minoura, K., 2008. Deposits of the 1992 Nicaragua Tsunami, Tsunamiites. Elsevier, Amsterdam, pp. 81–103. Horton, B.P., Innes, J.B., Shennan, I., Gehrels, W.R., Lloyd, J.M., McArthur, J.J., Rutherford, M.M., 1999. The Quaternary of North-East England. Field Guide. Quaternary Research Association 146–165 pp.

33

Jaffe, B.E., Goto, K., Sugawara, D., Richmond, B.M., Fujino, S., Nishimura, Y., 2012. Flow speed estimated by inverse modeling of sandy tsunami deposits: results from the 11 March 2011 tsunami on the coastal plain near the Sendai Airport, Honshu, Japan. Sedimentary Geology 282, 90–109. Jagodzinski, R., Sternal, B., Szczucinski, W., Lorenc, S., 2009. Heavy minerals in 2004 tsunami deposits on Kho Khao Island, Thailand. Polish Journal of Environmental Studies 18, 103–110. Jagodziński, R., Sternal, B., Szczuciński, W., Chagué-Goff, C., Sugawara, D., 2012. Heavy minerals in the 2011 Tohoku-oki tsunami deposits—insights into sediment sources and hydrodynamics. Sedimentary Geology 282, 57–64. Johnson, H., Richards, P.C., Long, D., Graham, C.C., 1993. United Kingdom offshore Regional Report: the Geology of the North Sea. NERC, London. Komar, P.D., 2007. The entrainment, transport and sorting of heavy minerals by waves and currents. In: Mange, M.A., Wright, D.T. (Eds.), Heavy minerals in use, pp. 3–48. Komar, P.D., Wang, C., 1984. Processes of selective grain transport and the formation of placers on beaches. Journal of Geology 92, 637–655. Mange, M.A., Maurer, H.F.W., 1992. Heavy Minerals in Colour. Chapman and Hall, London 147 pp. Mange, M.A., Wright, D.T., 2007. Heavy Minerals in Use. Developments in Sedimentology. Elsevier, Amsterdam, The Netherlands 1283 pp. Morton, A.C., Hallsworth, C.R., 1999. Processes controlling the composition of heavy mineral assemblages in sandstones. Sedimentary Geology 124, 3–29. Morton, R.A., Gelfenbaum, G., Jaffe, B.E., 2007. Physical criteria for distinguishing sandy tsunami and storm deposits using modern examples. Sedimentary Geology 200, 184–207. Morton, R.A., Richmond, B.M., Jaffe, B.E., Gelfenbaum, G., 2008. Coarse-clast ridge complexes of the Caribbean: a preliminary basis for distinguishing tsunami and storm-wave origins. Journal of Sedimentary Research 78, 624–637. Nakamura, Y., Nishimura, Y., Putra, P.S., 2012. Local variation of inundation, sedimentary characteristics, and mineral assemblages of the 2011 Tohoku-oki tsunami on the Misawa coast, Aomori, Japan. Sedimentary Geology 282, 216–227. Narayana, A.C., Tatavarti, R., Shinu, N., Subeer, A., 2007. Tsunami of December 26, 2004 on the southwest coast of India: post-tsunami geomorphic and sediment characteristics. Marine Geology 242, 155–168. Nickel, E., 1973. Experimental dissolution of light and heavy minerals in comparison with weathering and intrastratal solution. Contributions to Sedimentology (Series). Verlagsbuchhandlung, E. Schweizerbart'sche 125 pp. Putra, P.S., Nishimura, Y., Nakamura, Y., Yulianto, E., 2013. Sources and transportation modes of the 2011 Tohoku-Oki tsunami deposits on the central east Japan coast. Sedimentary Geology 294, 282–293. Shennan, I., Horton, B., Innes, J., Gehrels, R., Lloyd, J., McArthur, J.J., Rutherford, M.M., 2000. Late Quaternary sea-level changes, crustal movements and coastal evolution in Northumberland, UK. Journal of Quaternary Science 15 (3), 215–237. Smith, D.E., Shi, S., Cullingford, R.A., Dawson, A.G., Dawson, S., Firth, C.R., Foster, I.D.L., Fretwell, P.T., Haggart, B.A., Holloway, L.K., Long, D., 2004. The Holocene Storegga Slide tsunami in the United Kingdom. Quaternary Science Reviews 23, 2291–2321. Switzer, A.D., Jones, B.G., 2008. Large-scale washover sedimentation in a freshwater lagoon from the southeast Australian coast: tsunami or exceptionally large storm? The Holocene 18, 787–803. Switzer, A.D., Pucillo, K., Haredy, R.A., Jones, B.G., Bryant, E.A., 2005. Sea-level, storms or tsunami; enigmatic sand sheet deposits in sheltered coastal embayment from southeastern New South Wales Australia. Journal of Coastal Research 21, 655–663. Szczucinski, W., Niedzielski, P., Rachlewicz, G., Sobczynski, T., Zioła, A., Kowalski, A., Lorenc, S., Siepak, J., 2005. Contamination of tsunami sediments in a coastal zone inundated by the 26 December 2004 tsunami in Thailand. Environmental Geology 49, 321–331. Tooley, M.J., Smith, D.E., 2005. Relative sea-level change and evidence for the Holocene Storegga Slide tsunami at a site in Fife, Scotland, United Kingdom. Quaternary International 113-114, 107–119. Wagner, B., Bennike, O., Klug, M., Cremer, H., 2007. First indication of Storegga tsunami deposits from East Greenland. Journal of Quaternary Science 22, 321–325.