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The sedimentology of Middle Holocene tsunami facies in northern Sutherland, Scotland, UK
Marine Geology 170 (2000) 69±79
www.elsevier.nl/locate/margeo
The sedimentology of Middle Holocene tsunami facies in northern Sutherland, Scotland, UK S. Dawson*, D.E. Smith Centre for Quaternary Science, Geography, School of Natural and Environmental Sciences, Coventry University, Priory Street, William Morris Building, Coventry CV1 5FB, UK Received 1 February 1999; accepted 1 September 1999
Abstract Lagoonal sediments attributed to the main Holocene marine transgression in Strath Halladale, northern Sutherland, contain a complex coarser layer believed to have been deposited during the tsunami associated with the Second Storegga Slide off South West Norway. The coarser sequence is dated at between 7590 ^ 50 and 7290 ^ 50 radiocarbon years BP (6507±6260 cal BC and 6228±6029 cal BC). Detailed stratigraphical analysis has determined a distinctive suite of sedimentary sub-units within the coarser layer in marked contrast to the sediments, which occur above, and below. A pronounced erosional unconformity with the underlying sediments is recorded with the base of the tsunami layer characterised by eroded material from the underlying peat. The presence of a mixed diatom assemblage, although fragmentary, indicates a chaotic accumulation of the deposit with all habitats represented. Variations in particle size within the sequence disclose striking similarities with those from contemporary tsunami deposits. The run-up of the tsunami sediments is calculated at a minimum of 4.6 m. This is the ®rst occasion on which a deposit of the Second Storegga Slide tsunami has been found outside the North Sea basin and indicates that the area affected by the tsunami may have been larger than has been previously described. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Tsunami; Holocene; Diatoms; Sedimentology
1. Introduction Tsunamis are seismic sea waves caused by disturbance of the sea ¯oor during earthquakes, volcanic eruptions or submarine landslides. These long waves (up to 200 km) travel over the ocean at great velocity. Within the open ocean, the wave height is low, but upon reaching shallow water, in the vicinity of the coastline, it becomes greatly ampli®ed. Therefore, the impact at the coast is often catastrophic (e.g. the
* Corresponding author. E-mail addresses: [email protected] (S. Dawson), [email protected] (D.E. Smith).
Papua New Guinea tsunami of July 1998 led to the death of at least 2000 people). Within a geological timescale, a tsunami is a lowfrequency high-magnitude event. Several studies of sedimentation associated with contemporary tsunami inundation have been undertaken, for example by Yeh et al. (1993) and Dawson (1996). These disclose complex patterns with large-scale movement of sediment both in onshore and seaward direction. Recent years have seen a proliferation of sedimentological studies of inferred tsunami sediments from the geological record (e.g. Dawson et al., 1988, 1995; Bourgeois et al., 1988; Minoura et al., 1994; Shi, 1995; Dawson et al., 1996; Dawson and Smith, 1997; Goff and Chague ±Goff, 1999b; Goff et al., 2000).
0025-3227/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(00)00066-9
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S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
Fig. 1. Geomorphological and borehole location map of lower Strath Halladale, northern Sutherland. Inset shows the location of the study area in northern Scotland.
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
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Table 1 Radiocarbon dates from Strath Halladale. Ages in conventional radiocarbon years (1s ) and calibrated ages (cal BC, 2s ) Laboratory code
14
Calibrated age (cal BC)
Altitude (m OD)
Depth (m)
Material dated
Beta-105030 (AMS)
7290 ^ 50
24.25
7.19
Peat
Beta-105031 (AMS)
7590 ^ 50
6228±6054 6043±6029 6507±6373 6364±6346 6311±6260
24.98
7.92
Peat
C age ^ 1s BP
Research by Smith et al. (1985) has established the widespread accumulation of a distinctive horizon of marine sand within Holocene coastal sediments in eastern Scotland. This horizon was subsequently interpreted as having been laid down by a tsunami triggered by the Second Storegga Slide on the continental slope off western Norway (Dawson et al., 1988) This paper describes the ®rst occurrence of the sand horizon outside the North Sea basin. The sedimentology of this deposit is the focus of the present paper. 2. Geological context The coastline of northern Sutherland is intersected by the northward draining river valleys. Strath Halladale (Fig. 1) is one of the largest of these valleys and appears to have been a major corridor for the deposition of outwash material during the deglaciation of the last Scottish ice sheet. Much of this material has been eroded and redeposited by subsequent ¯uvial action. At the mouth of the valley, in Melvich Bay, granitic rocks of the western headland and ¯agstones of Old Red Sandstone to the east form impressive cliffs. A large boulder beach occurs at the western extremity of the bay. At the eastern side a terraced intertidal rock platform occurs. The study area comprises the lower 3 km of Strath Halladale, where the river ¯ows across a wide ¯oodplain. Throughout this area, the river is tidal and the ¯oodplain and saltings are frequently inundated by spring tides. The present altitude of the High Water Mark of Ordinary Spring Tides (HWMOST) at Strath Halladale is 12.30 m OD (Admiralty Tide Tables, 1996). The surface of the ¯oodplain declines consistently down-valley from
an altitude of 3.5 m above Kirkton before levelling out beyond the roadbridge (Fig. 1), where it gradually merges with the modern saltings at ca. 2 m. Towards the mouth of the Halladale River, the vegetated saltings have an upper surface at 2.11± 2.61 m OD and a lower surface of 1.95±2.08 m OD. The surfaces are ¯ooded by spring tides. At the edge of the saltings, a small bluff overlooks sand¯ats, which are exposed during the tidal cycle at between 1.74 and 1.90 m OD. 3. Methodology Sediments have been traced across the ¯oor of Strath Halladale using an Eijkelkamp gouge of 2 cm diameter to determine the overall sedimentary sequences and inter-relationships. Samples for laboratory analyses were collected using a 50 mm diameter Stitz piston corer, which provided undisturbed cores up to 1.3 m long. In the laboratory, a photographic record of each core was taken before detailed stratigraphical descriptions were made, followed by sub-sampling for microfossil and particle size analysis. Preparation for diatom analysis followed wellestablished techniques (Barber and Haworth, 1981) and a minimum count of 300 valves was made where possible. A Malvern Mastersizer 2600 involving laser diffraction spectroscopy was used to determine particle size of the minerogenic sediments. The mean particle size and standard deviation were calculated using Malvern software. Radiocarbon dating was carried out at Beta Analytic, Miami. Table 1 lists the dates obtained, calibrations use Calib 4 according to the method of Stuiver and Reimer (1993). All dates referred to in the text are quoted in radiocarbon years BP.
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Fig. 2. Section along lower Strath Halladale showing the sedimentary sequence examined.
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
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Fig. 3. Cross-valley stratigraphic transect of lower Strath Halladale.
4. Stratigraphy The sediments have been classi®ed into two major facies groups based on their sedimentary characteristics. Group 1 facies incorporate the underlying and overlying sedimentary suite, and group 2 facies incorporate the distinctive suite of sediments within the stratigraphy of Strath Halladale. Group 1 facies. The facies in group 1 incorporate intercalated peats and clastic sediments with graded transitional facies, which represent sediments associated with long-term sea surface change (Dawson, 1999). Representative stratigraphical pro®les from north to south and from east to west within the valley are shown in Figs. 2 and 3. Organic sediments both under- and overlie the depositional unit under investigation. The overlying peats grade into a black
organic gyttja, which, from an analysis of the microfossils, indicates deposition within a shallow tidal lagoon (Dawson, 1999). This quiet water sedimentation and distinct organic facies is well suited to the identi®cation and preservation of a contrasting sediment body accumulated during a high-energy event. Group 2 facies. Detailed stratigraphical analyses were restricted to the sediments retrieved from core SH 2, the deepest borehole undertaken in the valley. Examination of Fig. 4 shows in detail the major elements of the distinctive suite of sub-units contained within the sediment. The sediment succession starts with a pronounced erosional unconformity with the underlying organic deposit. This is overlain by alternating sand layers of varying particle size, and redeposited organic material (mostly eroded peat), from the underlying organic deposits. Some of the
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S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
Fig. 4. Detailed stratigraphy of coarser unit within the sediment sequence of SH 2 and the mean particle size pro®le in microns at 1 cm contiguous intervals. Fining upwards sequences are shown to the right of the pro®le.
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79 Table 2 Diatom species and total number of valves (24.25 to 25.00 m OD) Diatom species
Total number
Paralia sulcata (broken and eroded) Grammatophora oceanica Auliscus sculptus Rhaponeis surirella Cocconeis scutellum Diploneis interrupta Diploneis ovalis Navicula forcipata Diploneis smithii Navicula abrupta Navicula pusilla Pinnularia sp. Fragilaria construens Epithemia sp.
135 40 7 12 22 17 12 4 9 5 25 34 36 6
Total no. of valves
364
sand layers are graded, whilst others are unsorted. The organic horizons contain clasts, twigs, stems and macrofossils as well as other plant detritus, which are often found in a chaotic matrix of silt, clay and sands. The base of the sedimentary succession is erosive, whereas the upper boundary with the overlying shelly organic material becomes ®ner and more gradual. The sediments can be divided into ®ve sub-facies: 1. Graded sands (sub-facies a). These are composed of very coarse sand with ®ne gravel at the base, grading upwards to medium and often ®ne grey sand. The ®ne sand often has detrital plant material present with occasional small twigs. 2. Massive sand (sub-facies b). The massive sand is generally coarse to medium with de®ned boundaries. The unit is thinner than the graded beds and has no apparent internal structure. 3. Organic inclusions (sub-facies c). This unit is composed of a variety of materials, with clasts of different origin including peat, silt, undifferentiated organic matter, sand, gravel, twigs and plant matter. Sand and silt laminae are also present. Typically, the twigs are of Betula sp. and approximately 1±2 cm in length. Where present they are orientated parallel to the silt and sand laminae. 4. Organic detritus (sub-facies d). Subfacies d resembles subunit c and the boundary between them is
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transitional. The layer is characterised by ®ner plant debris, twigs and detrital matter. The minerogenic fraction of the unit is typically ®ne sand. 5. Light grey silt (sub-facies e). Grey silt laminae are often present lying stratigraphically above the organic detritus (sub-facies d). 4.1. Microfossils The sedimentary facies was examined for diatoms and foraminifera. While no foraminifera were found (J. Wells, pers. commun), diatom species were present in small numbers within the ®ner sands and clays of the intercalating sequence. In view of the small number of individuals present, no viable counts were possible. Identi®able species include a range of marine and brackish species, including Paralia sulcata, Cocconeis scutellum, Grammatophora oceanica, Rhaphoneis surirella, Diploneis smithii, Diploneis interrupta and Auliscus sculptus. In addition, there are many freshwater species present, identi®able only by their intact central areas, such as Pinnularia sp. and Fragilaria construens species. Diatom valves were often broken and therefore, identi®cation was dif®cult. A full count was not undertaken. Table 2 details the main taxa present, together with the total number of valves identi®ed. One sample was taken from the ®ner clastic sediments within the unit, and discloses a complex suite of diatoms. This is dominated by the polyhalobous planktonic Paralia sulcata, the valves of which are of a very broken and fragmentary nature. Other fully marine indicators are present including Rhaphoneis surirella, a sand-¯at living benthon (Vos and De Wolf, 1993). The sample also contains a range of mesohalobous and oligohalobous species including Diploneis and Fragilaria species. The assemblage contains a range of species from many differing habitats within the coastal zone, from fully marine subtidal to intertidal sand¯at and supratidal dwelling species, and has a clear allochthonous component. 4.2. Particle size analysis The entire sediment sequence was also analysed for particle size and comprised 74 cm of material, which was sampled contiguously with 1 cm thick samples
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and the results are shown in Fig. 4. Mean particle size is determined in microns and discloses a multi-modal particle size distribution. From the base of the sequence upward, variation in mean particle size is complex. However, certain trends are identi®able. The sequence is characterised by several ®ningupward sequences. This can be determined both visually, from inspection of the detailed stratigraphy, and from the trend of mean particle size. Five individual coarse±®ne suites can be seen, with a range of particle sizes from ca. 500 to 40 mm. At a depth of 765±769 cm, a unit of unsorted sand occurs, and particle size through this unit exhibits no change. In addition to the individual ®ning-upwards sequences, the overall trend of mean particle size throughout the unit is from predominantly coarse at the base, at 792 cm depth to ®ne at the top, at 719 cm depth. 4.3. Chronology Radiocarbon dates on samples at the upper and lower contacts of the deposit have been determined from borehole SH 2. The dates were obtained by the Accelerator Mass Spectrometer method. The date obtained on organic deposits immediately beneath the deposit is 7590 ^ 50 14C yr BP (24.98 m OD). and the overlying organic sediment is dated at 7290 ^ 50 14C yr BP (24.25 m OD), calibrated ages 6507±6260 cal BC and 6228±6029 cal BC, respectively. The ®rst date is thought to represent a maximum age, because the underlying peat surface, from which it was taken, appears to have been eroded from the sharp contact and presence of peat intraclasts in the sand above. The upper date is thought to be a more accurate re¯ection of the age of the event in the valley, since a more gradual change occurs across the upper boundary with no apparent erosion. 5. Facies inter-relationships The presence of an erosional unconformity at the base of group 2 facies implies that the time the deposit took to accumulate was less than the span of the radiocarbon dates, and that some of the underlying deposit has been removed. The unit of graded sand, which overlies the unconformity, suggests rapid deposition from a suspended state. The presence of ripup clasts of peat and gyttja indicate high-energy erosive
processes. The greatest accumulation of organic clasts is concentrated towards the lower parts of the group 2 facies re¯ecting the initial erosion of the underlying deposits. The presence of alternating graded sand beds and organic facies throughout the sedimentary succession possibly indicates several pulses of erosion and re-deposition with the decreases in grain size indicating the decreasing velocity of each pulse of erosion and a pattern of episodic deposition. Thus, the sand beds re¯ect higher energy conditions and the organic facies and silt lamineae re¯ect intervals of deposition from suspension under quieter conditions. 6. Discussion Several processes can be considered as responsible for the deposition of the sedimentary sequence. These include: ¯ooding by the river, slumping, possible long-term sea surface change, storm surge or tsunami inundation. The overall form of the deposits precludes the effects of ¯uvial ¯ooding and possible slumping from the valley sides. The presence, however fragmentary, of marine microfossils and the geometry of the base of the deposit suggest deposition under marine conditions. The deposits are not attributed to a progressive secular marine transgression into the valley because they contrast sharply with the lagoonal sediments both under and overlying them. A storm surge hypothesis is precluded because only one extreme event has been identi®ed within the Holocene sequence and many storms have impacted the north coast of Scotland over the last century with no apparent trace within the stratigraphic record. It is believed that the deposits incorporating the alternating coarse to ®ne sedimentary suites and the intercalated organic facies represent a deposit of a high-energy event. Several factors support this view. Firstly, the diatoms, in contrast to those of other layers analysed, are generally eroded with up to 90% of the pennate species broken and many centric species damaged. Secondly, there is evidence that the basal peat was eroded as the initial coarse sand accumulated. At borehole SH 2, fragments of peat occur in the lower part of the grey silty ®ne sand and at all boreholes, the basal contact is particularly sharp in appearance. Thirdly, the sand unit rises sharply up-valley from between 24.98 (borehole SH 2) to ca. 20.50 m
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
between boreholes SH 13 and SH 12, a rise of ca. 4.5 m for the base of the deposit over a distance of 700 m. Indeed, the rise of 4.5 m is actually a minimum, since the layer is apparently still descending seaward at borehole SH 2. Fourthly, the radiocarbon dates imply a relatively short period of deposition. The sequence probably accumulated closer to the upper date of 7290 ^ 50 BP (6228±6029 cal BC) and the calibrated dates at 2s almost overlap. Taken together the microfossil, stratigraphical and radiocarbon evidence supports relatively rapid accumulation under high-energy conditions. The sedimentary signature within the stratigraphy of the Strath Halladale valley is thought to be representative of the passage of a tsunami generated by the Second Storegga Slide, dated elsewhere at between ca. 7400 and ca. 6900 radiocarbon years BP. (Dawson et al., 1988). Where the deposit is found, the base of the sequence is normally marked by an erosional unconformity. The sand bed, which overlies the unconformity, is graded, indicative of rapid deposition of sand settling out from the tsunami waves. The organic clasts in subunits c and d are probably indicative of erosion of both the underlying organic material as well as the presence of organic detritus in the vicinity. The presence of sub-unit d (overlying b and c) re¯ects the settling out of the ®ner sands and detritus as the wave energy dissipated and the presence of silt laminae above the organic horizons re¯ects the ®nest material settling out of suspension. Within the sedimentary sequence at site SH 2 the presence of ®ve units of coarse to ®ne sediments and associated organic and silt sub-units implies several pulses of erosion and deposition, with the alternation between sand and organic fractions suggesting an episodic pattern of deposition. In general, the sand units record higher energy events and the ®ner laminated units (incorporating the organic fractions) record intervals of deposition from suspension under possibly quieter conditions. Diatom evidence, although fragmentary, indicates a fairly disturbed accumulation of the deposit with all habitats represented from open marine, subtidal mud¯ats, intertidal mud¯ats, saltings and freshwater environments. This is also characteristic of tsunami sediments in that the waves will inundate a whole range of habitats and thus produce a fairly chaotic assemblage (Dawson et al., 1996; Hemphill-Haley,
77
1996; Goff et al., 2000). This is in marked contrast with marine sediments accumulating under conditions of secular, relative sea level rise, in which gradual transitions are often noted from freshwater, through brackish to fully marine assemblages (e.g. Smith et al., 1992; Dawson and Smith, 1997). The particle size analysis results (Fig. 4) disclose striking similarities with those from study of the sedimentology of contemporary tsunami deposits, for example those from Indonesia (Shi, 1995), in showing an upward progression of variations in multi-modal distribution as well as an upwards ®ning of all modal sub-populations within the sediment. This lends further support for a tsunami hypothesis for the Strath Halladale sedimentary sequence. Sedimentological studies of recent tsunamis show a very complex pattern of erosion and deposition (Yeh et al., 1993; Shi, 1995). Incoming tsunami waves may erode and transport material landwards, and the backwash carries material seawards (Minoura and Nakaya, 1991). Therefore, the sedimentary sequences are often highly localised due to the local bathymetry inducing complex hydrodynamic conditions. Nevertheless, the sedimentary units are similar in Scotland and Norway (Dawson et al.,1993; Bondevik, 1996) and in Indonesia (Yeh et al., 1993). The rise in the height of the deposit up-valley of between ca. 25 and 20.5 m over a distance of 800 m is likely to have been accomplished by a ¯ow of water inland, which reached greater altitudes. Circumstantially, the age of the Strath Halladale valley deposit is similar in age, microfossil content, preservation and stratigraphical position to similar sand layers described from a number of coastal sites in eastern Scotland (e.g. Smith et al., 1980, 1983, 1985, 1999; Smith and Cullingford, 1985; Dawson and Smith, 1997) interpreted as having been laid down following the Second Storegga Slide tsunami (Dawson et al., 1988). In accord with recent evidence from Norway, in which the time of year the tsunami is thought to have struck the coastline has been alluded to from macrofossil and faunal evidence, further re®ning of the timing of the event has been possible from Strath Halladale. Within the subunit c (organic clasts) a macrofossil of Prunus avium L. (wild cherry) has been identi®ed (M Field, pers. commun.). The fruit occurs commonly on the trees in the Autumn time (late September 2 October) and although its provenance in
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S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
uncertain due to a lack of additional macrofossils, it gives added support to the autumn age for the event determined by Bondevik et al. (1997a,b). Within a Norwegian Lake basin, ®sh bones deposited with the tsunami sediments were attributed to the Autumn based on their size. Only a minimum value can be given for the run-up of the tsunami at Strath Halladale. This is due to the selective preservation of the tsunami deposits within the valley; the fact that lower seaward deposits may occur and because the sedimentary signature may not represent the ®nal elevation attained by the tsunami waves. Nevertheless, the evidence presented suggests at least a 4.5 m run-up (even accounting for compaction of the underlying peat, which is calculated to 22 cm the run-up is in the vicinity of 4 m) for the event in Strath Halladale, as determined from the stratigraphic extent of the deposit within the Strath Halladale valley. This is currently greater than the value for run-up from available numerical models (Harbitz, 1991), although these use relatively simple bathymetric information. The presence of tsunami deposits attributed to the Second Storegga Slide in Strath Halladale provides the ®rst evidence for the event outside the North Sea basin, and the highest minimum run-up of this tsunami recorded to date, despite the possible sheltering effect of the Orkney and Shetland Islands located immediately in the path of an incoming tsunami from the Norwegian continental slope. It is possible that the magnitude of the run-up may re¯ect particular local conditions, including the point on the tidal cycle when the tsunami struck. 6.1. Tsunami inundation into a shallow lagoonal and estuarine environment Studies of recent tsunamis exhibit a highly complex pattern of erosion and deposition (Dawson, 1994; Shi, 1995; Sato et al., 1995, 1996). Processes of tsunami backwash are poorly understood (Dawson, 1994), but are thought to re¯ect the redeposition of much of the eroded clasts and sand sheets. The deposits attributable to tsunami inundation range from single sand sheets of a few centimetres in thickness (e.g. Reinhardt and Bourgeois, 1989; Minoura and Nakaya, 1991) to multiple graded sand units and intercalated organic facies (e.g. Shi, 1995; Bondevik et al., 1997a,b)
and even boulder deposition (Dawson, 1996). This re¯ects, in the main, the site speci®c antecedent conditions and the hydrodynamic nature of the tsunami to variable offshore bathymetry. Nevertheless, tsunami inundation into quiet-water lagoonal sedimentation holds great potential for preservation of the tsunami deposits and may lead to the deposition of a sedimentary succession, which can be observed in many other localities, both in other sites around Scotland and further a®eld, for example in coastal lagoonal deposits and isolation basins at sites in western Norway (Bondevik et al., 1997a,b). 7. Conclusions Detailed analysis of a sedimentary sequence within deposits attributed to middle Holocene lagoonal conditions in Strath Halladale, northern Sutherland, reveals the presence of a distinctive sedimentary suite, which contrasts strongly with the enclosing sediments. The sediments have been classi®ed into ®ve predominant facies, and have been attributed to a tsunami, probably triggered by the Second Storegga Slide dated to ca. 7100 radiocarbon years BP. The main characteristics of the deposit are that it is underlain by an erosional unconformity with an overlying distinctive graded or poorly sorted medium to coarse sand, which contain eroded organic sediments, redeposited gyttja, plant and twig fragments eroded from the underlying sediments. A sequence of graded sand from coarse to ®ne lies stratigraphically above the redeposited organic fragments. Within the Halladale valley, the coarse to ®ne sequences are repeated and up to four ®ning upwards sequences are present, and ®ne silts are characteristic immediately overlying the ®ner sand sediments. The presence of a deposit of the Second Storegga Slide tsunami in the Strath Halladale valley taken with the evidence of the event elsewhere demonstrates that the tsunami not only affected the low-lying eastern coast of mainland Scotland but also the coastline of the Pentland Firth. Acknowledgements The authors are grateful to Lucy Holloway, James Wells and Anne de la Vega for assistance with the
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®eldwork and to Mike Field for identifying the macrofossils. Thanks are extended to Kirsty Handley and Erica Milwain for cartographic assistance. Roland Gehrels and James Goff are thanked for their constructive advice. The research was partially funded by the European Union under Contract EV5C-CT93-0266 and radiocarbon dates were funded by the Centre for Quaternary Science, Coventry University. This work is a contribution to IGCP 367 Rapid coastal changes during the Late Quaternary. References Admiralty Tide Tables, 1996. European waters including the Mediterranean Sea, vol. 1. Hydrographer of the Navy, Admiralty Hydrographic Department. Barber, H.G., Haworth, E.Y., 1981. A guide to the morphology of the diatom Frustule, with a key to the British freshwater genera, Freshwater Biological Association, Ambleside (109pp.). Bondevik, S., 1996. The Storegga tsunami deposits in western Norway and Postglacial sea level margin on Svalbard. DSc thesis, Department of Geology, University of Bergen. Bondevik, S., Svendsen, J.I., Mangerud, J., 1997a. Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway. Sedimentology 44, 1115±1131. Bondevik, S., Svendsen, J.I., Johnsen, G., Mangerud, J., Kaland, P.E., 1997b. The Storegga tsunami along the Norwegian coast, its age and runup. Boreas 26, 29±53. Bourgeois, J., Hansen, T.A., Wiberg, P.L., Kauffman, E.G., 1988. A Tsunami deposit at the Cretaceous-Tertiary Boundary in Texas. Science 241, 567±570. Dawson, A.G., 1994. Geomorphological effects of tsunami run-up and backwash. Geomorphology 10, 83±94. Dawson, A.G., 1996. The geological signi®cance of tsunamis. Z. Geomorphol. N.F. 102, 199±210. Dawson, S., 1999. Flandrian relative sea level changes in northern Scotland. Unpublished PhD dissertation, Coventry University. Dawson, S., Smith, D.E., 1997. Holocene relative Seal-level changes on the margin of a glacio-isostatically uplifted area: an example from Caithness, Scotland. The Holocene 7 (1), 51±77. Dawson, A.G., Long, D., Smith, D.E., 1988. the Storegga Slides: evidence from eastern Scotland for a possible tsunami. Mar. Geol. 82, 271±276. Dawson, A.G., Long, D., Smith, D.E., Shi, S., Foster, I.D.L., 1993. Tsunamis in the Norwegian Sea and North Sea caused by the Storegga submarine landslides. In: Tinti, S. (Ed.). Tsunamis in the World, Kluwer, Dordrecht, pp. 31±42. Dawson, A.G., Hindson, R., Andrade, C., Freitas, C., Parish, R., Bateman, M., 1995. Tsunami sedimentation associated with the Lisbon earthquake of 1 November AD 1755: Boco do Rio, Algarve, Portugal. The Holocene 5 (2), 209±215.
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Dawson, S., Smith, D.E., Ruffman, A., Shi, S., 1996. The diatom biostratigraphy of a tsunami sediments: examples from recent and middle Holocene events. Phys. Chem. Earth 21 (12), 87±92. Goff, J.R., ChagueÂ-Goff, C., 1999b. A Late Holocene record of environmental changes from coastal wetlands: Abel Tasman National Park, New Zealand. Quatern. Int. 56, 39±51. Goff, J.R., Rouse, H.L., Jones, S., Hayward, B., Cochran, U., McLea, W., Dickinson, W.W., Morley, M.S., 2000. Reconnaissance of the Late Holocene Evolution of Okupe Lagoon, Kapiti Island, New Zealand. Mar. Geol. (in review). Harbitz, C.B., 1991. Model simulations of tsunamis generated by the Storegga slide. Institute of Mathematics, University of Oslo, series 5, 30pp. Hemphill-Haley, E., 1996. Diatoms as an aid in identifying late Holocene tsunami deposits. The Holocene 6 (4), 439±448. Minoura, K., Nakaya, S., 1991. Traces of tsunami preserved in inter-tidal lacustrine and marsh deposits:some examples from Northeast Japan. J. Geol. 99, 265±287. Minoura, K., Nakaya, S., Uchida, M., 1994. Tsunami deposits in a lacustrine sequence of the Sanriku coast, northern Japan. Sediment. Geol. 89, 25±31. Reinhardt, M.A., Bourgeois, J., 1989. Tsunami favoured over storm or seiche for sand deposits overlying buried Holocene peat, Willapa Bay, Washington. EOS Trans., 1331. Sato, H., Shimamoto, T., Tsutsumi, A., Kawamoto, K., 1995. Onshore tsunami deposits caused by the SW Hokkaido and 1993 Japan Sea earthquakes. Pageoph 144 (3/4), 693±717. Shi, S., 1995. Observational and theoretical aspects of tsunami sedimentation. Unpublished PhD dissertation, Coventry University. Smith, D.E., Cullingford, R.A., 1985. Flandrian relative sea level changes in the Montrose Basin area. Scott. Geogr. Mag., 91±105. Smith, D.E., Morrison, J., Jones, R.L., Cullingford, R.A., 1980. Dating the Main Postglacial Shoreline in the Montrose area. In: Cullingford, R.A., Davidson, D.A., Lewin, J. (Eds.), Timescale in Geomorphology. Riley, pp. 225±245. Smith, D.E., Cullingford, R.A., Brooks, C.L., 1983. Flandrian relative sea level changes in the Ythan Valley, North-East Scotland. Earth Surface Processes Landforms 8, 423±438. Smith, D.E., Cullingford, R.A., Haggart, B.A., 1985. A major coastal ¯ood during the Holocene in eastern Scotland. Eiszeitalte Gegenwart 35, 104±118. Smith, D.E., Firth, C.R., Turbayne, S.C., Brooks, C.L., 1992. Holocene relative sea-level changes and shoreline displacement in the Dornoch Firth area, Scotland. Proceedings of the Geologists' Association, vol. 103, pp. 237±257. Smith, D.E., Firth, C.R., Brooks, C.L., Robinson, M., Collins, P.E.F., 1999. Relative sea level rise during the Main Postglacial Transgression in North East Scotland, UK. Trans. R. Soc. Edinb. Earth Sci. 90, 1±27. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215± 230. Vos, P.C., De Wolf, H., 1993. Diatoms as a tool for reconstructing sedimentary environments in coastal wetlands; methodological aspects. Hydrobiologia 269/270, 285±296. Yeh, H., Imamura, F., Synolakis, C., Tsuji, Y., Liu, P., Shi, S., 1993. The Flores Island tsunamis. EOS Trans. AGU 74, 369±373.
www.elsevier.nl/locate/margeo
The sedimentology of Middle Holocene tsunami facies in northern Sutherland, Scotland, UK S. Dawson*, D.E. Smith Centre for Quaternary Science, Geography, School of Natural and Environmental Sciences, Coventry University, Priory Street, William Morris Building, Coventry CV1 5FB, UK Received 1 February 1999; accepted 1 September 1999
Abstract Lagoonal sediments attributed to the main Holocene marine transgression in Strath Halladale, northern Sutherland, contain a complex coarser layer believed to have been deposited during the tsunami associated with the Second Storegga Slide off South West Norway. The coarser sequence is dated at between 7590 ^ 50 and 7290 ^ 50 radiocarbon years BP (6507±6260 cal BC and 6228±6029 cal BC). Detailed stratigraphical analysis has determined a distinctive suite of sedimentary sub-units within the coarser layer in marked contrast to the sediments, which occur above, and below. A pronounced erosional unconformity with the underlying sediments is recorded with the base of the tsunami layer characterised by eroded material from the underlying peat. The presence of a mixed diatom assemblage, although fragmentary, indicates a chaotic accumulation of the deposit with all habitats represented. Variations in particle size within the sequence disclose striking similarities with those from contemporary tsunami deposits. The run-up of the tsunami sediments is calculated at a minimum of 4.6 m. This is the ®rst occasion on which a deposit of the Second Storegga Slide tsunami has been found outside the North Sea basin and indicates that the area affected by the tsunami may have been larger than has been previously described. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Tsunami; Holocene; Diatoms; Sedimentology
1. Introduction Tsunamis are seismic sea waves caused by disturbance of the sea ¯oor during earthquakes, volcanic eruptions or submarine landslides. These long waves (up to 200 km) travel over the ocean at great velocity. Within the open ocean, the wave height is low, but upon reaching shallow water, in the vicinity of the coastline, it becomes greatly ampli®ed. Therefore, the impact at the coast is often catastrophic (e.g. the
* Corresponding author. E-mail addresses: [email protected] (S. Dawson), [email protected] (D.E. Smith).
Papua New Guinea tsunami of July 1998 led to the death of at least 2000 people). Within a geological timescale, a tsunami is a lowfrequency high-magnitude event. Several studies of sedimentation associated with contemporary tsunami inundation have been undertaken, for example by Yeh et al. (1993) and Dawson (1996). These disclose complex patterns with large-scale movement of sediment both in onshore and seaward direction. Recent years have seen a proliferation of sedimentological studies of inferred tsunami sediments from the geological record (e.g. Dawson et al., 1988, 1995; Bourgeois et al., 1988; Minoura et al., 1994; Shi, 1995; Dawson et al., 1996; Dawson and Smith, 1997; Goff and Chague ±Goff, 1999b; Goff et al., 2000).
0025-3227/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0025-322 7(00)00066-9
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S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
Fig. 1. Geomorphological and borehole location map of lower Strath Halladale, northern Sutherland. Inset shows the location of the study area in northern Scotland.
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
71
Table 1 Radiocarbon dates from Strath Halladale. Ages in conventional radiocarbon years (1s ) and calibrated ages (cal BC, 2s ) Laboratory code
14
Calibrated age (cal BC)
Altitude (m OD)
Depth (m)
Material dated
Beta-105030 (AMS)
7290 ^ 50
24.25
7.19
Peat
Beta-105031 (AMS)
7590 ^ 50
6228±6054 6043±6029 6507±6373 6364±6346 6311±6260
24.98
7.92
Peat
C age ^ 1s BP
Research by Smith et al. (1985) has established the widespread accumulation of a distinctive horizon of marine sand within Holocene coastal sediments in eastern Scotland. This horizon was subsequently interpreted as having been laid down by a tsunami triggered by the Second Storegga Slide on the continental slope off western Norway (Dawson et al., 1988) This paper describes the ®rst occurrence of the sand horizon outside the North Sea basin. The sedimentology of this deposit is the focus of the present paper. 2. Geological context The coastline of northern Sutherland is intersected by the northward draining river valleys. Strath Halladale (Fig. 1) is one of the largest of these valleys and appears to have been a major corridor for the deposition of outwash material during the deglaciation of the last Scottish ice sheet. Much of this material has been eroded and redeposited by subsequent ¯uvial action. At the mouth of the valley, in Melvich Bay, granitic rocks of the western headland and ¯agstones of Old Red Sandstone to the east form impressive cliffs. A large boulder beach occurs at the western extremity of the bay. At the eastern side a terraced intertidal rock platform occurs. The study area comprises the lower 3 km of Strath Halladale, where the river ¯ows across a wide ¯oodplain. Throughout this area, the river is tidal and the ¯oodplain and saltings are frequently inundated by spring tides. The present altitude of the High Water Mark of Ordinary Spring Tides (HWMOST) at Strath Halladale is 12.30 m OD (Admiralty Tide Tables, 1996). The surface of the ¯oodplain declines consistently down-valley from
an altitude of 3.5 m above Kirkton before levelling out beyond the roadbridge (Fig. 1), where it gradually merges with the modern saltings at ca. 2 m. Towards the mouth of the Halladale River, the vegetated saltings have an upper surface at 2.11± 2.61 m OD and a lower surface of 1.95±2.08 m OD. The surfaces are ¯ooded by spring tides. At the edge of the saltings, a small bluff overlooks sand¯ats, which are exposed during the tidal cycle at between 1.74 and 1.90 m OD. 3. Methodology Sediments have been traced across the ¯oor of Strath Halladale using an Eijkelkamp gouge of 2 cm diameter to determine the overall sedimentary sequences and inter-relationships. Samples for laboratory analyses were collected using a 50 mm diameter Stitz piston corer, which provided undisturbed cores up to 1.3 m long. In the laboratory, a photographic record of each core was taken before detailed stratigraphical descriptions were made, followed by sub-sampling for microfossil and particle size analysis. Preparation for diatom analysis followed wellestablished techniques (Barber and Haworth, 1981) and a minimum count of 300 valves was made where possible. A Malvern Mastersizer 2600 involving laser diffraction spectroscopy was used to determine particle size of the minerogenic sediments. The mean particle size and standard deviation were calculated using Malvern software. Radiocarbon dating was carried out at Beta Analytic, Miami. Table 1 lists the dates obtained, calibrations use Calib 4 according to the method of Stuiver and Reimer (1993). All dates referred to in the text are quoted in radiocarbon years BP.
72 S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
Fig. 2. Section along lower Strath Halladale showing the sedimentary sequence examined.
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
73
Fig. 3. Cross-valley stratigraphic transect of lower Strath Halladale.
4. Stratigraphy The sediments have been classi®ed into two major facies groups based on their sedimentary characteristics. Group 1 facies incorporate the underlying and overlying sedimentary suite, and group 2 facies incorporate the distinctive suite of sediments within the stratigraphy of Strath Halladale. Group 1 facies. The facies in group 1 incorporate intercalated peats and clastic sediments with graded transitional facies, which represent sediments associated with long-term sea surface change (Dawson, 1999). Representative stratigraphical pro®les from north to south and from east to west within the valley are shown in Figs. 2 and 3. Organic sediments both under- and overlie the depositional unit under investigation. The overlying peats grade into a black
organic gyttja, which, from an analysis of the microfossils, indicates deposition within a shallow tidal lagoon (Dawson, 1999). This quiet water sedimentation and distinct organic facies is well suited to the identi®cation and preservation of a contrasting sediment body accumulated during a high-energy event. Group 2 facies. Detailed stratigraphical analyses were restricted to the sediments retrieved from core SH 2, the deepest borehole undertaken in the valley. Examination of Fig. 4 shows in detail the major elements of the distinctive suite of sub-units contained within the sediment. The sediment succession starts with a pronounced erosional unconformity with the underlying organic deposit. This is overlain by alternating sand layers of varying particle size, and redeposited organic material (mostly eroded peat), from the underlying organic deposits. Some of the
74
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
Fig. 4. Detailed stratigraphy of coarser unit within the sediment sequence of SH 2 and the mean particle size pro®le in microns at 1 cm contiguous intervals. Fining upwards sequences are shown to the right of the pro®le.
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79 Table 2 Diatom species and total number of valves (24.25 to 25.00 m OD) Diatom species
Total number
Paralia sulcata (broken and eroded) Grammatophora oceanica Auliscus sculptus Rhaponeis surirella Cocconeis scutellum Diploneis interrupta Diploneis ovalis Navicula forcipata Diploneis smithii Navicula abrupta Navicula pusilla Pinnularia sp. Fragilaria construens Epithemia sp.
135 40 7 12 22 17 12 4 9 5 25 34 36 6
Total no. of valves
364
sand layers are graded, whilst others are unsorted. The organic horizons contain clasts, twigs, stems and macrofossils as well as other plant detritus, which are often found in a chaotic matrix of silt, clay and sands. The base of the sedimentary succession is erosive, whereas the upper boundary with the overlying shelly organic material becomes ®ner and more gradual. The sediments can be divided into ®ve sub-facies: 1. Graded sands (sub-facies a). These are composed of very coarse sand with ®ne gravel at the base, grading upwards to medium and often ®ne grey sand. The ®ne sand often has detrital plant material present with occasional small twigs. 2. Massive sand (sub-facies b). The massive sand is generally coarse to medium with de®ned boundaries. The unit is thinner than the graded beds and has no apparent internal structure. 3. Organic inclusions (sub-facies c). This unit is composed of a variety of materials, with clasts of different origin including peat, silt, undifferentiated organic matter, sand, gravel, twigs and plant matter. Sand and silt laminae are also present. Typically, the twigs are of Betula sp. and approximately 1±2 cm in length. Where present they are orientated parallel to the silt and sand laminae. 4. Organic detritus (sub-facies d). Subfacies d resembles subunit c and the boundary between them is
75
transitional. The layer is characterised by ®ner plant debris, twigs and detrital matter. The minerogenic fraction of the unit is typically ®ne sand. 5. Light grey silt (sub-facies e). Grey silt laminae are often present lying stratigraphically above the organic detritus (sub-facies d). 4.1. Microfossils The sedimentary facies was examined for diatoms and foraminifera. While no foraminifera were found (J. Wells, pers. commun), diatom species were present in small numbers within the ®ner sands and clays of the intercalating sequence. In view of the small number of individuals present, no viable counts were possible. Identi®able species include a range of marine and brackish species, including Paralia sulcata, Cocconeis scutellum, Grammatophora oceanica, Rhaphoneis surirella, Diploneis smithii, Diploneis interrupta and Auliscus sculptus. In addition, there are many freshwater species present, identi®able only by their intact central areas, such as Pinnularia sp. and Fragilaria construens species. Diatom valves were often broken and therefore, identi®cation was dif®cult. A full count was not undertaken. Table 2 details the main taxa present, together with the total number of valves identi®ed. One sample was taken from the ®ner clastic sediments within the unit, and discloses a complex suite of diatoms. This is dominated by the polyhalobous planktonic Paralia sulcata, the valves of which are of a very broken and fragmentary nature. Other fully marine indicators are present including Rhaphoneis surirella, a sand-¯at living benthon (Vos and De Wolf, 1993). The sample also contains a range of mesohalobous and oligohalobous species including Diploneis and Fragilaria species. The assemblage contains a range of species from many differing habitats within the coastal zone, from fully marine subtidal to intertidal sand¯at and supratidal dwelling species, and has a clear allochthonous component. 4.2. Particle size analysis The entire sediment sequence was also analysed for particle size and comprised 74 cm of material, which was sampled contiguously with 1 cm thick samples
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S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
and the results are shown in Fig. 4. Mean particle size is determined in microns and discloses a multi-modal particle size distribution. From the base of the sequence upward, variation in mean particle size is complex. However, certain trends are identi®able. The sequence is characterised by several ®ningupward sequences. This can be determined both visually, from inspection of the detailed stratigraphy, and from the trend of mean particle size. Five individual coarse±®ne suites can be seen, with a range of particle sizes from ca. 500 to 40 mm. At a depth of 765±769 cm, a unit of unsorted sand occurs, and particle size through this unit exhibits no change. In addition to the individual ®ning-upwards sequences, the overall trend of mean particle size throughout the unit is from predominantly coarse at the base, at 792 cm depth to ®ne at the top, at 719 cm depth. 4.3. Chronology Radiocarbon dates on samples at the upper and lower contacts of the deposit have been determined from borehole SH 2. The dates were obtained by the Accelerator Mass Spectrometer method. The date obtained on organic deposits immediately beneath the deposit is 7590 ^ 50 14C yr BP (24.98 m OD). and the overlying organic sediment is dated at 7290 ^ 50 14C yr BP (24.25 m OD), calibrated ages 6507±6260 cal BC and 6228±6029 cal BC, respectively. The ®rst date is thought to represent a maximum age, because the underlying peat surface, from which it was taken, appears to have been eroded from the sharp contact and presence of peat intraclasts in the sand above. The upper date is thought to be a more accurate re¯ection of the age of the event in the valley, since a more gradual change occurs across the upper boundary with no apparent erosion. 5. Facies inter-relationships The presence of an erosional unconformity at the base of group 2 facies implies that the time the deposit took to accumulate was less than the span of the radiocarbon dates, and that some of the underlying deposit has been removed. The unit of graded sand, which overlies the unconformity, suggests rapid deposition from a suspended state. The presence of ripup clasts of peat and gyttja indicate high-energy erosive
processes. The greatest accumulation of organic clasts is concentrated towards the lower parts of the group 2 facies re¯ecting the initial erosion of the underlying deposits. The presence of alternating graded sand beds and organic facies throughout the sedimentary succession possibly indicates several pulses of erosion and re-deposition with the decreases in grain size indicating the decreasing velocity of each pulse of erosion and a pattern of episodic deposition. Thus, the sand beds re¯ect higher energy conditions and the organic facies and silt lamineae re¯ect intervals of deposition from suspension under quieter conditions. 6. Discussion Several processes can be considered as responsible for the deposition of the sedimentary sequence. These include: ¯ooding by the river, slumping, possible long-term sea surface change, storm surge or tsunami inundation. The overall form of the deposits precludes the effects of ¯uvial ¯ooding and possible slumping from the valley sides. The presence, however fragmentary, of marine microfossils and the geometry of the base of the deposit suggest deposition under marine conditions. The deposits are not attributed to a progressive secular marine transgression into the valley because they contrast sharply with the lagoonal sediments both under and overlying them. A storm surge hypothesis is precluded because only one extreme event has been identi®ed within the Holocene sequence and many storms have impacted the north coast of Scotland over the last century with no apparent trace within the stratigraphic record. It is believed that the deposits incorporating the alternating coarse to ®ne sedimentary suites and the intercalated organic facies represent a deposit of a high-energy event. Several factors support this view. Firstly, the diatoms, in contrast to those of other layers analysed, are generally eroded with up to 90% of the pennate species broken and many centric species damaged. Secondly, there is evidence that the basal peat was eroded as the initial coarse sand accumulated. At borehole SH 2, fragments of peat occur in the lower part of the grey silty ®ne sand and at all boreholes, the basal contact is particularly sharp in appearance. Thirdly, the sand unit rises sharply up-valley from between 24.98 (borehole SH 2) to ca. 20.50 m
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
between boreholes SH 13 and SH 12, a rise of ca. 4.5 m for the base of the deposit over a distance of 700 m. Indeed, the rise of 4.5 m is actually a minimum, since the layer is apparently still descending seaward at borehole SH 2. Fourthly, the radiocarbon dates imply a relatively short period of deposition. The sequence probably accumulated closer to the upper date of 7290 ^ 50 BP (6228±6029 cal BC) and the calibrated dates at 2s almost overlap. Taken together the microfossil, stratigraphical and radiocarbon evidence supports relatively rapid accumulation under high-energy conditions. The sedimentary signature within the stratigraphy of the Strath Halladale valley is thought to be representative of the passage of a tsunami generated by the Second Storegga Slide, dated elsewhere at between ca. 7400 and ca. 6900 radiocarbon years BP. (Dawson et al., 1988). Where the deposit is found, the base of the sequence is normally marked by an erosional unconformity. The sand bed, which overlies the unconformity, is graded, indicative of rapid deposition of sand settling out from the tsunami waves. The organic clasts in subunits c and d are probably indicative of erosion of both the underlying organic material as well as the presence of organic detritus in the vicinity. The presence of sub-unit d (overlying b and c) re¯ects the settling out of the ®ner sands and detritus as the wave energy dissipated and the presence of silt laminae above the organic horizons re¯ects the ®nest material settling out of suspension. Within the sedimentary sequence at site SH 2 the presence of ®ve units of coarse to ®ne sediments and associated organic and silt sub-units implies several pulses of erosion and deposition, with the alternation between sand and organic fractions suggesting an episodic pattern of deposition. In general, the sand units record higher energy events and the ®ner laminated units (incorporating the organic fractions) record intervals of deposition from suspension under possibly quieter conditions. Diatom evidence, although fragmentary, indicates a fairly disturbed accumulation of the deposit with all habitats represented from open marine, subtidal mud¯ats, intertidal mud¯ats, saltings and freshwater environments. This is also characteristic of tsunami sediments in that the waves will inundate a whole range of habitats and thus produce a fairly chaotic assemblage (Dawson et al., 1996; Hemphill-Haley,
77
1996; Goff et al., 2000). This is in marked contrast with marine sediments accumulating under conditions of secular, relative sea level rise, in which gradual transitions are often noted from freshwater, through brackish to fully marine assemblages (e.g. Smith et al., 1992; Dawson and Smith, 1997). The particle size analysis results (Fig. 4) disclose striking similarities with those from study of the sedimentology of contemporary tsunami deposits, for example those from Indonesia (Shi, 1995), in showing an upward progression of variations in multi-modal distribution as well as an upwards ®ning of all modal sub-populations within the sediment. This lends further support for a tsunami hypothesis for the Strath Halladale sedimentary sequence. Sedimentological studies of recent tsunamis show a very complex pattern of erosion and deposition (Yeh et al., 1993; Shi, 1995). Incoming tsunami waves may erode and transport material landwards, and the backwash carries material seawards (Minoura and Nakaya, 1991). Therefore, the sedimentary sequences are often highly localised due to the local bathymetry inducing complex hydrodynamic conditions. Nevertheless, the sedimentary units are similar in Scotland and Norway (Dawson et al.,1993; Bondevik, 1996) and in Indonesia (Yeh et al., 1993). The rise in the height of the deposit up-valley of between ca. 25 and 20.5 m over a distance of 800 m is likely to have been accomplished by a ¯ow of water inland, which reached greater altitudes. Circumstantially, the age of the Strath Halladale valley deposit is similar in age, microfossil content, preservation and stratigraphical position to similar sand layers described from a number of coastal sites in eastern Scotland (e.g. Smith et al., 1980, 1983, 1985, 1999; Smith and Cullingford, 1985; Dawson and Smith, 1997) interpreted as having been laid down following the Second Storegga Slide tsunami (Dawson et al., 1988). In accord with recent evidence from Norway, in which the time of year the tsunami is thought to have struck the coastline has been alluded to from macrofossil and faunal evidence, further re®ning of the timing of the event has been possible from Strath Halladale. Within the subunit c (organic clasts) a macrofossil of Prunus avium L. (wild cherry) has been identi®ed (M Field, pers. commun.). The fruit occurs commonly on the trees in the Autumn time (late September 2 October) and although its provenance in
78
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
uncertain due to a lack of additional macrofossils, it gives added support to the autumn age for the event determined by Bondevik et al. (1997a,b). Within a Norwegian Lake basin, ®sh bones deposited with the tsunami sediments were attributed to the Autumn based on their size. Only a minimum value can be given for the run-up of the tsunami at Strath Halladale. This is due to the selective preservation of the tsunami deposits within the valley; the fact that lower seaward deposits may occur and because the sedimentary signature may not represent the ®nal elevation attained by the tsunami waves. Nevertheless, the evidence presented suggests at least a 4.5 m run-up (even accounting for compaction of the underlying peat, which is calculated to 22 cm the run-up is in the vicinity of 4 m) for the event in Strath Halladale, as determined from the stratigraphic extent of the deposit within the Strath Halladale valley. This is currently greater than the value for run-up from available numerical models (Harbitz, 1991), although these use relatively simple bathymetric information. The presence of tsunami deposits attributed to the Second Storegga Slide in Strath Halladale provides the ®rst evidence for the event outside the North Sea basin, and the highest minimum run-up of this tsunami recorded to date, despite the possible sheltering effect of the Orkney and Shetland Islands located immediately in the path of an incoming tsunami from the Norwegian continental slope. It is possible that the magnitude of the run-up may re¯ect particular local conditions, including the point on the tidal cycle when the tsunami struck. 6.1. Tsunami inundation into a shallow lagoonal and estuarine environment Studies of recent tsunamis exhibit a highly complex pattern of erosion and deposition (Dawson, 1994; Shi, 1995; Sato et al., 1995, 1996). Processes of tsunami backwash are poorly understood (Dawson, 1994), but are thought to re¯ect the redeposition of much of the eroded clasts and sand sheets. The deposits attributable to tsunami inundation range from single sand sheets of a few centimetres in thickness (e.g. Reinhardt and Bourgeois, 1989; Minoura and Nakaya, 1991) to multiple graded sand units and intercalated organic facies (e.g. Shi, 1995; Bondevik et al., 1997a,b)
and even boulder deposition (Dawson, 1996). This re¯ects, in the main, the site speci®c antecedent conditions and the hydrodynamic nature of the tsunami to variable offshore bathymetry. Nevertheless, tsunami inundation into quiet-water lagoonal sedimentation holds great potential for preservation of the tsunami deposits and may lead to the deposition of a sedimentary succession, which can be observed in many other localities, both in other sites around Scotland and further a®eld, for example in coastal lagoonal deposits and isolation basins at sites in western Norway (Bondevik et al., 1997a,b). 7. Conclusions Detailed analysis of a sedimentary sequence within deposits attributed to middle Holocene lagoonal conditions in Strath Halladale, northern Sutherland, reveals the presence of a distinctive sedimentary suite, which contrasts strongly with the enclosing sediments. The sediments have been classi®ed into ®ve predominant facies, and have been attributed to a tsunami, probably triggered by the Second Storegga Slide dated to ca. 7100 radiocarbon years BP. The main characteristics of the deposit are that it is underlain by an erosional unconformity with an overlying distinctive graded or poorly sorted medium to coarse sand, which contain eroded organic sediments, redeposited gyttja, plant and twig fragments eroded from the underlying sediments. A sequence of graded sand from coarse to ®ne lies stratigraphically above the redeposited organic fragments. Within the Halladale valley, the coarse to ®ne sequences are repeated and up to four ®ning upwards sequences are present, and ®ne silts are characteristic immediately overlying the ®ner sand sediments. The presence of a deposit of the Second Storegga Slide tsunami in the Strath Halladale valley taken with the evidence of the event elsewhere demonstrates that the tsunami not only affected the low-lying eastern coast of mainland Scotland but also the coastline of the Pentland Firth. Acknowledgements The authors are grateful to Lucy Holloway, James Wells and Anne de la Vega for assistance with the
S. Dawson, D.E. Smith / Marine Geology 170 (2000) 69±79
®eldwork and to Mike Field for identifying the macrofossils. Thanks are extended to Kirsty Handley and Erica Milwain for cartographic assistance. Roland Gehrels and James Goff are thanked for their constructive advice. The research was partially funded by the European Union under Contract EV5C-CT93-0266 and radiocarbon dates were funded by the Centre for Quaternary Science, Coventry University. This work is a contribution to IGCP 367 Rapid coastal changes during the Late Quaternary. References Admiralty Tide Tables, 1996. European waters including the Mediterranean Sea, vol. 1. Hydrographer of the Navy, Admiralty Hydrographic Department. Barber, H.G., Haworth, E.Y., 1981. A guide to the morphology of the diatom Frustule, with a key to the British freshwater genera, Freshwater Biological Association, Ambleside (109pp.). Bondevik, S., 1996. The Storegga tsunami deposits in western Norway and Postglacial sea level margin on Svalbard. DSc thesis, Department of Geology, University of Bergen. Bondevik, S., Svendsen, J.I., Mangerud, J., 1997a. Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway. Sedimentology 44, 1115±1131. Bondevik, S., Svendsen, J.I., Johnsen, G., Mangerud, J., Kaland, P.E., 1997b. The Storegga tsunami along the Norwegian coast, its age and runup. Boreas 26, 29±53. Bourgeois, J., Hansen, T.A., Wiberg, P.L., Kauffman, E.G., 1988. A Tsunami deposit at the Cretaceous-Tertiary Boundary in Texas. Science 241, 567±570. Dawson, A.G., 1994. Geomorphological effects of tsunami run-up and backwash. Geomorphology 10, 83±94. Dawson, A.G., 1996. The geological signi®cance of tsunamis. Z. Geomorphol. N.F. 102, 199±210. Dawson, S., 1999. Flandrian relative sea level changes in northern Scotland. Unpublished PhD dissertation, Coventry University. Dawson, S., Smith, D.E., 1997. Holocene relative Seal-level changes on the margin of a glacio-isostatically uplifted area: an example from Caithness, Scotland. The Holocene 7 (1), 51±77. Dawson, A.G., Long, D., Smith, D.E., 1988. the Storegga Slides: evidence from eastern Scotland for a possible tsunami. Mar. Geol. 82, 271±276. Dawson, A.G., Long, D., Smith, D.E., Shi, S., Foster, I.D.L., 1993. Tsunamis in the Norwegian Sea and North Sea caused by the Storegga submarine landslides. In: Tinti, S. (Ed.). Tsunamis in the World, Kluwer, Dordrecht, pp. 31±42. Dawson, A.G., Hindson, R., Andrade, C., Freitas, C., Parish, R., Bateman, M., 1995. Tsunami sedimentation associated with the Lisbon earthquake of 1 November AD 1755: Boco do Rio, Algarve, Portugal. The Holocene 5 (2), 209±215.
79
Dawson, S., Smith, D.E., Ruffman, A., Shi, S., 1996. The diatom biostratigraphy of a tsunami sediments: examples from recent and middle Holocene events. Phys. Chem. Earth 21 (12), 87±92. Goff, J.R., ChagueÂ-Goff, C., 1999b. A Late Holocene record of environmental changes from coastal wetlands: Abel Tasman National Park, New Zealand. Quatern. Int. 56, 39±51. Goff, J.R., Rouse, H.L., Jones, S., Hayward, B., Cochran, U., McLea, W., Dickinson, W.W., Morley, M.S., 2000. Reconnaissance of the Late Holocene Evolution of Okupe Lagoon, Kapiti Island, New Zealand. Mar. Geol. (in review). Harbitz, C.B., 1991. Model simulations of tsunamis generated by the Storegga slide. Institute of Mathematics, University of Oslo, series 5, 30pp. Hemphill-Haley, E., 1996. Diatoms as an aid in identifying late Holocene tsunami deposits. The Holocene 6 (4), 439±448. Minoura, K., Nakaya, S., 1991. Traces of tsunami preserved in inter-tidal lacustrine and marsh deposits:some examples from Northeast Japan. J. Geol. 99, 265±287. Minoura, K., Nakaya, S., Uchida, M., 1994. Tsunami deposits in a lacustrine sequence of the Sanriku coast, northern Japan. Sediment. Geol. 89, 25±31. Reinhardt, M.A., Bourgeois, J., 1989. Tsunami favoured over storm or seiche for sand deposits overlying buried Holocene peat, Willapa Bay, Washington. EOS Trans., 1331. Sato, H., Shimamoto, T., Tsutsumi, A., Kawamoto, K., 1995. Onshore tsunami deposits caused by the SW Hokkaido and 1993 Japan Sea earthquakes. Pageoph 144 (3/4), 693±717. Shi, S., 1995. Observational and theoretical aspects of tsunami sedimentation. Unpublished PhD dissertation, Coventry University. Smith, D.E., Cullingford, R.A., 1985. Flandrian relative sea level changes in the Montrose Basin area. Scott. Geogr. Mag., 91±105. Smith, D.E., Morrison, J., Jones, R.L., Cullingford, R.A., 1980. Dating the Main Postglacial Shoreline in the Montrose area. In: Cullingford, R.A., Davidson, D.A., Lewin, J. (Eds.), Timescale in Geomorphology. Riley, pp. 225±245. Smith, D.E., Cullingford, R.A., Brooks, C.L., 1983. Flandrian relative sea level changes in the Ythan Valley, North-East Scotland. Earth Surface Processes Landforms 8, 423±438. Smith, D.E., Cullingford, R.A., Haggart, B.A., 1985. A major coastal ¯ood during the Holocene in eastern Scotland. Eiszeitalte Gegenwart 35, 104±118. Smith, D.E., Firth, C.R., Turbayne, S.C., Brooks, C.L., 1992. Holocene relative sea-level changes and shoreline displacement in the Dornoch Firth area, Scotland. Proceedings of the Geologists' Association, vol. 103, pp. 237±257. Smith, D.E., Firth, C.R., Brooks, C.L., Robinson, M., Collins, P.E.F., 1999. Relative sea level rise during the Main Postglacial Transgression in North East Scotland, UK. Trans. R. Soc. Edinb. Earth Sci. 90, 1±27. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215± 230. Vos, P.C., De Wolf, H., 1993. Diatoms as a tool for reconstructing sedimentary environments in coastal wetlands; methodological aspects. Hydrobiologia 269/270, 285±296. Yeh, H., Imamura, F., Synolakis, C., Tsuji, Y., Liu, P., Shi, S., 1993. The Flores Island tsunamis. EOS Trans. AGU 74, 369±373.