Tsunami Deposits - Springer Link

Pure appl. geophys. 157 (2000) 875–897 0033–4553/00/080875–23 $ 1.50+0.20/0

Tsunami Deposits ALASTAIR G. DAWSON1 and SHAOZHONG SHI2

Abstract—Geological investigations of coastal sediments indicate that prehistoric tsunamis can be identified. Their characterisation has altered our knowledge of the past frequency and magnitude of tsunamis for different areas of the world. Yet there have been relatively few geological studies of modern tsunamis with virtually no direct observations of the processes associated with tsunami sediment transport and deposition. This paper discusses these issues and draws on the results of recent research to summarise our current knowledge on the nature of tsunami deposits. Key words: Tsunami deposits, boulders, sedimentation processes, microfossils, sediment supply.

Introduction Approximately twelve years ago, the study of tsunamis was mostly the province of seismologists, numerical modellers, geophysicists and historians. Historians played a particularly valuable role since they were able to provide detailed documentary information on former tsunamis for different areas of the world. In this way, tsunami scientists sought to estimate frequency-magnitude relationships for past tsunami events and hence provide information on future tsunami risk for different areas. By contrast, tsunamis received little attention from geologists. In 1987 and 1988, the publication of two papers resulted in a ‘‘sea change’’ in the way in which tsunami risk is evaluated. First, ATWATER (1987) described geological evidence for prehistoric earthquakes along the outer coast of Washington State, U.S.A. Part of the geological evidence described by Atwater included sheets of sediment visible in coastal stratigraphic sequences and which were interpreted as prehistoric (palaeo-) tsunami deposits. At about the same time, DAWSON et al. (1988) described an unusual sand deposit contained within uplifted coastal sediment sequences in Scotland. These authors argued on several grounds that this layer of sediment was a prehistoric tsunami deposit, produced as a result of one of the world’s largest submarine slides that took place approximately 7100 14C years ago on the continental shelf edge west of Norway. 1 Centre for Quaternary Science, William Morris Building, Coventry University, Priory Street, Coventry, UK, CV1 5FB. E-mail: [email protected] 2 Inverness College, Longman Road, Inverness, Scotland, IV2 3NF. E-mail: shaozhong – shi/ [email protected]

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Pure appl. geophys.,

Both ATWATER (1987) and DAWSON et al. (1988) faced particular difficulties in their interpretations since it was not possible at that time to argue that the fossil sediments resemble those deposited by a modern tsunami, since no such investigations had ever been made. Thus ATWATER (1987) used geological evidence for the occurrence of a major earthquake in support of his contention that a widespread tsunami had taken place. By contrast, the palaeotsunami interpretation of the coastal deposits in Scotland was closely linked to a correspondence between the radiocarbon ages of the flood deposits and those dating the underwater slide. Critics of the tsunami hypothesis for the deposits in Scotland, however, maintained that the sediments could be equally explained as the product of a major storm surge. Yet, the scientific literature provided rare tantalising glimpses of the possibility that tsunamis deposit sediment. For example, WRIGHT and MELLA (1963) observed that the May 1960 Chilean earthquake resulted in the deposition across certain coastal lowlands by a tsunami of a 1–2 cm veneer of marine sand. Probably one of the most graphic eye-witness accounts of a tsunami was by Lieutenant Billing on the U.S. Postal steamer Wateree, that on August 8th 1868 was transported onshore by a large tsunami generated by an offshore earthquake at Arica, Chile. He described how the tsunami engulfed the ship that was subsequently ‘‘. . . buried under a half-liquid, half-solid mass of sand and water . . .’’, that the ship was transported two miles inland and that the wave ‘‘. . . had carried us at an unbelievable speed over the sand dunes . .’’ and that ‘‘. . the town had disappeared and where it had stood there stretched an even plain of sand . .’’ (MYLES, 1985). Geological investigations of former tsunamis is thus a relatively new research area. During the last ten years a proliferation of academic papers have described a range of sediments that have been attributed to a series of former tsunamis (MINOURA and NAKAYA, 1991; PASKOFF, 1991; ATWATER, 1992; SATAKE et al., 1993; YOUNG and BRYANT, 1993; MINOURA et al., 1994; NISHIMURA, 1994; DAWSON et al., 1996a; PINEGINA et al., 1997; RANGUELOV, 1998). Many authors have maintained that tsunami deposits are distinctive (DAWSON et al., 1996a,b; BOURGEOIS and MINOURA, 1997; PINEGINA et al., 1997; YOUNG and BRYANT, 1992, 1998). Several authors have argued that tsunamis are frequently associated with the deposition of continuous and discontinuous sediment sheets across large areas of the coastal zone, provided that there is an adequate sediment supply (e.g., DAWSON et al., 1996a,b; HINDSON et al., 1996). Although tsunami deposits are mostly characterised by sheets of sediment, they are frequently represented by boulder accumulations. In addition, microfossil assemblages of diatoms and foraminifera contained within tsunami-deposited sand sheets may provide additional information on the nature of onshore transport of sediment from deeper water (HEMPHILL-HALEY, 1995a,b, 1996; DOMINEY-HOWES, 1996).

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Palaeotsunami Deposits The study of tsunami deposits associated with former Cascadia earthquakes and of the tsunami produced by the Second Storegga slides have resulted in a series of extremely detailed studies of palaeotsunami deposits having been undertaken in the Pacific West Coast of Canada and the U.S.A. as well as along the Atlantic seaboard of NW Europe. (ATWATER, 1992; CLAGUE and BOBROWSKY, 1994; CLAGUE et al., 1999; DAWSON et al., 1988; BONDEVIK, 1996; BONDEVIK et al., 1997a,b; BRYANT and PRICE, 1997; DAWSON and SMITH, 1997; HUTCHINSON et al., 1997). The majority of published stratigraphic descriptions of palaeotsunami deposits stem from these areas and clearly illustrate the contrasting patterns of tsunami deposition between areas subject to earthquake-induced coseismic subsidence (Pacific West Coast) and aseismic coastal areas (Scotland and Norway) where tsunami deposition accompanied submarine landslide-generated waves. Accordingly, a brief summary of this research (given below) precedes a discussion of how tsunami deposits have been identified and interpreted within coastal stratigraphic sequences.

Tsunami Stratigraphy a) Pacific West Coast Detailed information from coastal Washington State indicates the former occurrence of a large tsunami that accompanied an episode of coseismic coastal submergence during a large earthquake which took place circa 300 years ago (ATWATER and YAMAGUCHI, 1991; SATAKE et al., 1996). The submergence event is indicated by growth position plant fossils which include western red cedar and spruce and stems and leaves of saltmarsh grasses buried within intertidal mudflat silt and clay sediments. Locally the macrofossil material is mantled by a deposit of sand attributed to a large tsunami inferred to have been triggered by an offshore earthquake (ATWATER and YAMAGUCHI, 1991). ATWATER and YAMAGUCHI (1991) described the inferred tsunami-deposited sand sheet as resting directly upon the buried soil and beneath intertidal muds, thus suggesting that the tsunami took place during or immediately after the episode of coseismic subsidence. Sedimentary evidence for a relatively recent tsunami is widespread throughout the Pacific West Coast of Canada (CLAGUE, 1997; CLAGUE et al., 1999). For example, CLAGUE and BOBROWSKY (1994) described tidal marshes near Tofino and Ucluelet, Vancouver Island, British Colombia, overlain by sand sheets containing marine foraminifera and vascular plant fossils, demonstrating rapid submergence prior to burial by marine sands (Fig. 1). CLAGUE and BOBROWSKY (1994) described the sand layers as having a sheet-like morphology, moderately well sorted, generally massive, sharply bounded at the top and bottom and ranging in

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Figure 1 Representative stratigraphy of coastal sediment in Tofino and Ucluelet tidal marshes, Vancouver Island, British Columbia, Canada. The peat that is traced between sites was submerged by an earthquake 100 – 400 years ago. The sand horizons are considered to be palaeotsunami deposits (based on CLAGUE and BOBROWSKY, 1994).

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thickness from a few millimetres to 0.3 m. They noted that locally the sand layer extends from the seaward edge of the marsh to the forest margin and in places can be traced up to several tens of metres into the forest, where it thins and becomes finer inland. These authors also note that the sand commonly includes fragments of bark, twigs, branches, logs, cones and other plant material. CLAGUE and BOBROWSKY (1994) observed that the sediments occurring immediately below the sand layer exhibit evidence of wave erosion whilst in some places, plant leaves rooted in the underlying peat extend upward into and through the sand or on occasions are matted down at the base of the sand indicating burial by flooding (ATWATER and YAMAGUCHI, 1991). Similar sheets of sand attributed to the 1964 great Alaska earthquake and tsunami have been described by CLAGUE et al. (1994) for Port Alberni, British Columbia. Here the sand occurs as a thin sheet typically 1–2 cm in thickness but locally up to 15 cm. The sand is described as massive, moderately well-sorted and fine grained, and contains scattered fragments of branches, sticks, seeds and cones; in places these form a capping layer of woody detritus. CLAGUE et al. (1994) also observed that the sand contains tests of foraminifera species generally found only in the peaty and muddy intertidal material. ATWATER and MOORE (1992) also described stratigraphical evidence from the Puget Sound area of Washington State for a palaeotsunami that flooded coastal areas circa 1000 years ago. At Cultus Bay a sand sheet between 5–15 cm in thickness containing marine foraminifera is enclosed within peat. The deposit typically has a medium grain size of circa 0.1 mm and exhibits a progressive fining inland. At West Point tsunami deposits of a similar age are exposed along a 150 metre section. Here the tsunami deposit is represented by a tabular sheet of sediment covering a large area that becomes progressively more fine-grained as it is traced landward. Here the sand sheet is between 4–6 cm in thickness, exhibits little or no evidence of basal erosion, forms a tapering wedge that rises in altitude inland, exhibits a progressive fining upwards in mean grain size and includes transported marine bivalves and barnacles. Stems of salt marsh plants are rooted just below the sand and are buried by it. Evidence for backwash during the tsunami is indicated at West Point by the incorporation within the tsunami deposit of sediments derived from adjacent slopes. ATWATER (1987) described from Willapa Bay several marine-deposited sand sheets attributed to palaeotsunamis. One of these sand sheets in particular, typically no thicker than 7 cm, extends 3 km up valley from the seaward edge of the buried saltmarsh. Atwater noted that the sand sheet becomes increasingly difficult to trace landward and it also becomes generally thinner and finer grained in that direction, consistent with a marine source. DARIENZO and PETERSON (1990) provided evidence for palaeotsunami deposition across a series of salt marshes along the northern Oregon coastline. They described a series of sediment sheets occasionally containing clay/silt units, marine-

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brackish diatoms and generally with massive structures. These authors argued that the sands were transported and deposited out of turbulent suspension rather than by small-scale currents that produced ripples and dunes on the sea bed. However, they also noted that the extent of the sediment sheets indicated that the tsunami surges were capable of transporting fine sands over distances greater than 0.75 km despite being associated with bottom shear stresses that were insufficient to disturb/or remove the stems of plants rooted in the underlying marsh surfaces. b) Scotland and Norway Detailed sedimentological information is available for a palaeotsunami generated by the Second Storegga submarine slide that took place in the Norwegian Sea and North Atlantic at circa 7,100 14C years BP (BUGGE, 1983; BUGGE et al., 1987; HARBITZ, 1991, 1992; DAWSON et al., 1988). Most of the field evidence for the tsunami occurs along the eastern and northern coastlines of Scotland as well as from isostatically-uplifted lake basins in western Norway (BONDEVIK, 1996) (Fig. 2). Evidence for a regionally extensive layer of marine sediments within isostatically-uplifted Holocene coastal sediment sequences has been known for over 20 years and for a long time was attributed to a major storm surge (SMITH et al., 1985). The sediment layer exhibits stratigraphical complexity. In most cases, the layer consists of a grey micaceous silty fine sand that frequently occurs as a tapering wedge of sediment that rises in altitude inland. To the seaward the sediment layer occurs within estuarine sediments whereas farther inland it is enclosed within peat. The deposit generally rests upon an erosional surface and in many areas this stratigraphic unconformity is marked by rip-up structures and the incorporation of many intraclasts of eroded material within the sediment wedge. The sediment layer is also distinctive owing to its regional uniformity of grain size while boulders have never been observed within the deposit. Individual deposits are generally wellsorted and characterised by sets of fining-upwards sediment sequences that were interpreted by SHI (1995) as indicative of deposition by individual tsunami waves. Deposits attributable to the same tsunami have been identified in uplifted coastal lake basins in Norway (BONDEVIK, 1996; BONDEVIK et al., 1997a,b). Here, the deposits indicate patterns of chaotic sedimentation with marine sediments resting adjacent to layers of terrestrial turf and twigs. In several basins tsunami deposition was accompanied by erosion of underlying lake sediments and sediment redeposition within the suite of tsunami sediments. BONDEVIK et al. (1997a) observed that lake sediment cores of tsunami deposits sampled only 1 m apart are markedly different, and attributed this variability to the complicated hydrodynamic behaviour of the tsunami due to local variations in lake bathymetry. BONDEVIK et al. (1997a) proposed a model of tsunami deposition within lake basins based on the Norwegian data. They concluded that the first bed to be deposited is a graded bed upon an eroded lake floor surface. After the coarsest

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Figure 2 Stratigraphy of lake sediments in western Norway showing palaeotsunami deposits associated with Second Storegga Submarine Slide of circa 7100 14C years BP (based on BONDEVIK, 1996; BONDEVIK et al., 1997a,b).

grains were deposited out of suspension, organic detritus was deposited. In the lower part of the organic detritus facies, large and heavy fragments including rip-up clasts, water-logged wood fragments and sand grains were concentrated. The authors noted that fine organic matter, silt and fine sand appear to have been

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deposited after the withdrawal of each major wave. They noted also that the tsunami sedimentary unit as a whole generally fines and thins upwards and is possibly indicative of decreasing shear velocities associated with successive waves and a decrease over time in the ability of individual waves to erode sediments. After the final withdrawal of the tsunami, sedimentation of fine silt laminae appears to have taken place, the amount of sedimentation being proportional to the depth of the lake and the amount of suspended material contained within the water column.

Estimating Runup Height from Tsunami Deposits It has been shown in several studies that the highest altitudes reached by individual tsunami deposits cannot be used as indicative of the upper limit of runup reached by that tsunami (cf., SHI, 1995). For example, in the Algarve, Portugal, tsunami deposits produced during the great Lisbon earthquake of November 1st 1755 AD occur as a continuous sheet of sediment up to 600 m inland from the coast but farther inland are replaced by discontinuous and eventually sporadic sediment sheets until a point is reached when there is no sedimentary trace of the tsunami, despite historical observations that tsunami flooding took place at considerably higher altitudes and farther inland (ANDRADE, 1992; DAWSON et al., 1995; HINDSON et al., 1996). A recent study of coastal sediments deposited by a tsunami in Java, 1994 showed that tsunami sediment deposition is frequently associated with the deposition of sediment sheets that rise in altitude inland as tapering sediment wedges although to considerably lower altitudes than the observed upper limit of runup (DAWSON et al., 1996a,b). Similarly, SHI et al. (1993, 1995) demonstrated that the Flores tsunami of 12th December 1992 was associated with the deposition of extensive sheets of sediments up to 1 m thick, and these are continued landward by discontinued sediment accumulations that always occur at least 10 m below the upper limit of tsunami runup. Therefore palaeotsunami deposits cannot provide useful information on former runup limits. However, study of the sedimentology of the palaeotsunami deposits may provide information on the characteristics of the various waves within tsunami wave trains that affected specific coastal zones.

The Role of Sediment Reworking and Backwash Flow The deposition and the preservation of any tsunami deposit is dependent principally upon an adequate supply of sediment from the nearshore and offshore zones. Frequently the overtopping of coastal dunes results in the formation of relatively extensive tsunami-deposited sand sheets (e.g., HINDSON et al., 1996). The interpretation of any tsunami deposit, is, however, complicated not only by the

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occurrence of a series of long-period waves each of which may be responsible for localised littoral deposition, but also by episodes of sediment erosion. Successive tsunami waves may result in the erosion of pre-existing tsunami deposits and, on occasion, their complete removal. The pattern and processes of tsunami sedimentation are additionally complicated by episodes of backwash flow that follow the maximum landward inundation of individual tsunami waves. Under such circumstances strong currents may flow from landward to seaward and may cause additional fluvial erosion and sediment re-deposition. In some cases water flow may even take place sub-parallel to the coastline rather than perpendicular to it. Velocities of backwash flow are greatly influenced by the nature of the local coastal topography and this factor may often play an important part in determining the nature and scale of backwash sediment transport and reworking. In addition, this process may introduce terrestrial sediments including plant macrofossils into the sediment assemblage, thus complicating palaeoenvironmental interpretation. For example, within lake sediments in uplifted isolation basins in western Norway inundated by the tsunami associated with the Second Storegga Slide. BONDEVIK et al. (1997a,b) noted large quantities of roots and pieces of wood that were interpreted to have been transported into individual lake basins (together with sand entrained at the shore) as a result of local backwash flow (Fig. 2).

Reconstructing Indi6idual Tsunami Wa6es Any tsunami deposit, although providing in most cases conclusive evidence that a palaeotsunami has taken place, presents stratigraphic information for a series of alternating episodes of sediment erosion, transport and deposition associated with individual waves. For example, as a result of the effects of wave resonance it is theoretically possible that one of the last waves within a tsunami wave train may be capable of eroding most of the sediments deposited by the preceding waves. Detailed studies of vertical variations in grain size distribution together with studies of related sedimentary structures can provide information on the approximate number of tsunami waves associated with sediment deposition. For example, SHI (1995), in an investigation of palaeotsunami deposits associated with the Second Storegga Slide in NE Scotland, noted the presence at several locations of a series (up to 5) of fining-upwards sequences and he proposed that these may represent separate tsunami waves. By contrast ANDERSON et al. (1995) and DAWSON et al. (1996b) in an investigation of vertical variations in grain size of a tsunami deposit produced by the Grand Banks earthquake of 1929, noted the presence of a structureless sand deposit that exhibits evidence of a single fining-upwards sedimentary sequence (Plate 1). Similarly, grain size investigations of the palaeotsunami deposit at Seattle, associated with the circa 300 years BP Cascadia earthquake, exhibits evidence of only a single fining-upwards sequence (DAWSON, unpublished).

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Plate 1 Sand horizon (within peat) deposited at Taylors Bay, Newfoundland, Canada, as a result of tsunami associated with Grand Banks offshore earthquake of 1929 (photo courtesy of Alan Ruffman, also see ANDERSON et al., 1995).

Boulder Accumulations A characteristic of many tsunamis is their ability to deposit boulders across the coastal zone (Plate 2). Several accounts of former tsunamis describe boulders the size of houses (e.g., PASKOFF, 1991) while other descriptions provide reports of boulders embedded within sheets of tsunami-deposited sand (e.g., YOUNG and BRYANT, 1992; BRYANT et al., 1997a). Clearly, therefore, the processes of tsunami

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boulder transport and deposition are complex and poorly understood. Empirical evidence of coastal boulder deposition during a tsunami was provided by YEH et al. (1993) in their study of the Flores tsunami of December 1992. In this area sections of coral boulders derived from adjacent areas of reef were observed to have been transported by the tsunami and deposited farther inland. Similarly YOUNG and BRYANT (1998:105) describe an observer in NW Australia who witnessed the arrival at the coast of a tsunami generated by the June 1994 Java earthquake, ‘‘. . . a surge estimated at 3 to 4 m carried . . . rocks and coral inland for a distance of two to three hundred metres . .’’ BOURROUILH-LE JAN and TALANDIER (1985) described areas of coral reef in the Tuamoto archipelago, SE Pacific, where large numbers of giant boulders (up to 750 m3) appear to have been transported across the atoll rim and into the lagoon. However, although palaeotsunamis have been considered in this case as a possible depositional agent, similar boulders were also considered to have been deposited as a result of storm surges (TEISSIER, 1969; PIRAZZOLI et al., 1988). Boulder fields attributable to tsunamis have been observed elsewhere in the world (e.g., Japan) (OTA et al., 1985; KAWANA and PIRAZZOLI, 1990; NAKATA and KAWANA, 1993). In southern Portugal, Hindson and Andrade (unpublished) observed boulders deposited by the tsunami caused by the Great Lisbon earthquake of 1755 AD (Plate

Plate 2 Isolated boulder contained with tsunami deposit of circa 2300 – 2500 14C years BP at Basta Voe, Yell, Shetland Isles (photo courtesy of Stein Bondevik).

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4). Hindson noted that at several locations on the Algarve coastline the palaeotsunami was associated with the deposition of both continuous and discontinuous sediment sheets, some of which contained boulders. The individual boulders were frequently pitted with bioerosional hollows in which were found marine mollusca. Since both the boulders and the sediment sheets were attributed to the former tsunami, it was concluded the individual boulders were transported during the tsunami from the seabed offshore. Similarly YOUNG and BRYANT (1998) attributed the deposition of sheets of lateritic cobbles near Darwin, Australia, to a series of tsunamis that took place during the Holocene. On the island of Molokai, Hawaii, boulder accumulations considered to have been deposited by a palaeotsunami during the last interglacial, have been described by several authors (MOORE and MOORE, 1984, 1988; GRIGG and JONES, 1997) (Plate 3). In this area, accumulations of limestone and basalt boulders together with fragments of coral and other reef fragments known as the Hulopoe Gravel and which occurred up to 375 m above sea level have been interpreted as tsunami deposits. The Hulopoe Gravel is unique among deposits attributed to tsunamis, owing to its great thickness (cf., STEARNS, 1940). For example, MOORE and

Plate 4 Boulder deposited at Bo do Rio, Algarve, Portugal by tsunami associated with Lisbon earthquake of 1755 AD. The boulder surface is pitted as a result of the effects of bioerosion while several of the hollows contain marine mollusca (photo courtesy of Cesar Andrade and Robert Hindson).

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MOORE (1984, 1988) have described three sedimentary units circa 2, 4 and 2 m in thickness attributed to successive waves within a tsunami wave train. MOORE and MOORE (1984) considered that the lower sediments were produced as a result of sedimentation during tsunami runup while the upper sub-units were deposited during episodes of backwash. The remarkably high runup altitude was attributed by MOORE et al. (1989, 1992) to a large submarine slide/debris avalanche (the Alika phase 2 event) that was considered to have taken place circa 105,000 years ago on the western flank of the Manua Loa volcano. This tsunami is also considered by YOUNG and BRYANT (1992, 1993) to have resulted in 20-m high tsunami waves that struck the coastline of New South Wales, Australia between 100–110 ka and led to the removal of large blocks of bedrock from pre-existing uplifted coastal rock platforms together with the widespread destruction of the last interglacial highstand shoreline (oxygen isotope substage 5e) along the coastline of New South Wales. This interpretation, however, has been questioned by JONES and MADER (1997). The giant wave origin for the Hulopoe Gravels has subsequently been questioned by GRIGG and JONES (1997) and FELTON et al. (this issue). YOUNG and BRYANT (1992) and BRYANT et al. (1997b) also observed in SE Australia, thicknesses of massive sands and silts attributed to tsunamis containing

Plate 3 Gulley-filling Hulopoe Gravels in Archaeology Gulley (0.15 km west of type section) on the south coast of Lanai Island, Hawaii. The outcrop (11.4 m in height) contains numerous layers of gravel. The percentage of basalt versus coral clasts varies significantly between beds as does the post-depositional cementation (photo courtesy of Barbara Keating).

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occasional isolated boulders. Some of the boulders reach 6 m in diameter and are interpreted as having been transported up to circa 30 m above sea level. Isolated boulders contained within massive sandy deposits have also been observed in other palaeotsunami deposits (e.g., Shetland Islands) (Plate 2). Their occurrence presents considerable difficulty in understanding how tsunamis deposit sediment since they require tsunamis simultaneously to be able to deposit fine-grained sediment and boulders (see below). BRYANT et al. (1997b) maintain that this is possible and cite published information regarding the hydraulics of sand waves by ALLEN (1984) and of boulder transport by COSTA (1983) in support of their arguments. However, the mathematics of such sediment and boulder transport is very much in its infancy and requires considerably more detailed analysis.

Mechanisms of Deposition Field observations of tsunami flooding usually document rapid lateral translation of water across the coastal zone. Frequently the lateral water motion associated with runup is influenced by local wave resonance. Thus the tsunami waves as they strike the coast are unlike waves associated with storms since not only are they associated with considerably greater wavelengths and wave periods, but they are essentially constructive as they move inland across the coastal zone (REINHARDT and BOURGEOIS, 1989). The rapid water velocities (provided that there is an adequate supply of sediment in nearshore zone) in most cases result in the transport of a variety of grain sizes ranging from silt to boulders. Unlike storm waves, individual tsunami waves reach a point of zero water velocity prior to backwash flow. At this point large volumes of sediment may be deposited out of the water column onto the ground surface. The processes of tsunami flooding and sediment transport associated with the passage of individual waves across the coastal zone have never been adequately modelled. Preliminary attempts have been made to simulate the movement of large boulders by tsunamis (e.g., NOJI et al., 1993) however, such studies have encountered tremendous difficulties in simulating the effects of bottom friction on boulders of different shapes and densities. Moreover, no study has attempted to simulate the processes of tsunami sediment transport and deposition. Any such study in the future would not only need to be able to explain the emplacement of boulders within sand sheets but would also need to explain why some sediment sheets are generally structureless and massive accumulations of sand while others exhibit evidence of grading. An important clue in the development of a qualitative explanation of tsunami sediment transport and deposition lies in the unique descriptions of Lt Billings during the Arica tsunami in Chile during 1868. His eye-witness description of the tsunami as it approached the shore as a, ‘‘. . . half-liquid, half-solid mass of sand

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and water . . .’’ is in many ways similar to descriptions of turbidity currents (DUFF, 1993). Whereas turbidity currents are gravity-driven currents consisting of dilute mixtures of sediment and water with densities generally between 1 and 1.4 m/cm3 that maintain their motion through internal turbulence, sediment-rich tsunami waves may be considered as analogous phenomena that are not gravity driven but which derive their energy from the tsunami-generating mechanism. In the shallow offshore zone, sediment is entrained into suspension and is subsequently followed by deposition across low-lying coastal areas. It may also be the case that the unique sequences of current reversals during episodes of tsunami runup and backwash may be of considerable importance in understanding how tsunamis deposit sediment. However, the plausibility of such a hypothesis must await the results of future research.

Distinguishing Sediments Deposited by Storms and Tsunamis One of the most awkward problems in reconstructing chronologies of palaeotsunamis for different areas of the world is how to distinguish tsunami deposits from sediments deposited as a result of storm waves. Thus ATWATER and MOORE (1992) described stratigraphical evidence from the Puget Sound, Washington, for an extreme coastal flood that took place circa 1000 years ago. ATWATER and MOORE (1992) argued that the lack of other sand beds distinguishes this surge for most or all storms in the region during the past 2000 years. LIU and FEARN (1993) have shown from coastal Alabama, U.S.A., that a series of hurricanes during historical time deposited multiple sand layers across low-lying coastal wetlands. Similarly, DAVIS et al. (1989) argued that hurricanes produced graded or homogeneous deposits of sand, shell, gravel and mud within the prominently clastic sediments in the coastal lagoons of Florida. While it is accepted that storm waves result in the deposition of discrete sedimentary units, it is argued here that tsunamis, in contrast to storms, generally deposit continuous and discontinuous sediment sheets over relatively wide areas and considerable distances inland. For example, sediment sheets in the Algarve, Portugal associated with the Lisbon earthquake tsunami of 1755 AD occur in excess of 1 km inland (HINDSON et al., 1996). Similarly, palaeotsunami deposits in Scotland associated with the Second Storegga Slide of circa 7100 14C BP consist of extensive sediment sheets that locally extend several km inland (SMITH et al., 1985; DAWSON et al., 1988; LONG et al., 1989) (Fig. 3). In addition, it may be the case that tsunami deposits contain distinctive microfossil assemblages that can be differentiated from those produced by storm surges (DAWSON, 1994, 1996b). A detailed study of modern tsunami deposits and storm deposits was undertaken by NANAYAMA et al. (1998) in respect of the 1993 Hokkaido earthquake tsunami and the 1959 Miyakojima typhoon. These authors noted that the storm

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Figure 3 Stratigraphy of buried estuarine sediments in Wick River valley, Caithness, northern Scotland showing a prominent tsunami deposit layer (unit 4) enclosed within peat and generated by Second Storegga Submarine Slide (based on DAWSON and SMITH, 1997).

deposits exhibited better sorting than the tsunami sediments and that whereas storm deposits only show evidence of one direction of palaeocurrent (from offshore to onshore), tsunami deposits display evidence of sediment transport by both runup and backwash flow. Storm surges in certain areas of the world (e.g., Bangladesh) reach many kilometres inland but are rarely associated with the widespread deposition of sheets of sediment. Moreover, storm waves are quite different from tsunami waves. For example, storm waves are typically characterised by high wave height (H) to length (L) ratios resulting in the production of relatively steep waves. By contrast, tsunamis are characterised by waves of exceptionally low steepness values that reflect their relatively long wavelengths (H/L= B 0.04). It remains to be discovered if these differences in wave steepness result in distinctive styles of coastal sedimentation. The difficulties encountered when attempting to distinguish storm and tsunami sediments are well illustrated by the different interpretations attributed to the transport and deposition of boulders. Thus both storms and tsunamis have been attributed as likely boulder transport and depositional agents in the atolls of French Polynesia (cf., TEISSIER, 1969 and PIRAZZOLI et al., 1988). Similarly, YOUNG and BRYANT (1998) have highlighted the difficulties faced in attributing a tsunami or a storm origin to the relict lateritic cobble sheets that occur near Darwin, Australia.

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NELSON et al. (1996) summarised the principal stratigraphic, sedimentological and palaeontological evidence for the occurrence of tsunami deposits synchronous with episodes of coseismic submergence. They drew attention to the occurrence of thin (marine) sand layers overlain by intertidal muds and underlain by buried salt marsh surfaces. NELSON et al. (1996) argued that the lack of similar sand beds within beds of peat or mud at these protected sites indicated that sand deposition was restricted to times of rapid submergence and hence unrelated to storm waves. They noted that the presence of such sand sheets is only likely where there is a suitable sediment supply and where wave characteristics are favourable for the deposition of sediment across the coastal zone. Since it is well known that sand sheets are also deposited within the intertidal zone during episodes of channel migration, river floods and storms, criteria are needed to distinguish tsunami deposits from similar types of sediment within tidal wetland sediment sequences. They also drew attention to the fact that in open coastal settings, deposits produced as a result of infrequent storms are often difficult to distinguish from those due to tsunamis.

Tsunami Deposits and Microfossil Assemblages Relatively few investigations have been undertaken on the microfossils associated with tsunami deposits (Plate 5). HEMPHILL-HALEY (1995a,b, 1996) has described various diatom assemblages contained within tsunami deposits along the Pacific coastline of Oregon, Washington and British Columbia. HEMPHILL-HALEY described a variety of brackish-marine diatoms within tsunami deposits that could be used not only to identify tsunami deposits but also to estimate the greatest inland extent of tsunami inundation owing to the fact that certain assemblages occur beyond the landward limit of tsunami-deposited sands and silts. Furthermore she noted that in certain coastal freshwater lakes located beyond the reach of storms, the presence of marine planktonic and neritic plankton can be used to identify former episodes of tsunami inundation. Studies of diatom assemblages contained within tsunami deposits in Scotland, related to the Second Storegga Slide, have shown the presence of exceptionally large numbers of the species Paralia sulcata with many individuals exhibiting evidence of breakage (DAWSON et al., 1996b). Similarly, DAWSON et al. (1996b), in an investigation of diatom assemblages contained within the Grand Banks tsunami deposit, showed that the major diatom species present was Paralia sulcata and that the majority of individuals were broken. At first sight it may appear that large percentages of broken diatoms may be indicative of former tsunamis. However, this issue is made problematic since owing to the varying robustness of pennate (lenticular) and centric (circular) diatoms, tests of some species are more able than others to withstand fracturing.

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Plate 5 Thin section of tsunami deposit of 1755 AD at Martinhal, Cape St. Vincent, SW Portugal showing echinoid stem (sample courtesy of Stella Kortekaas).

Foraminifera also can be used to identify former tsunami deposits. The most detailed investigation of foraminifera contained within tsunami deposits has been undertaken by DOMINEY-HOWES (1996) and DOMINEY-HOWES et al. (1999) in western Crete. DOMINEY-HOWES (1996) demonstrated that tsunami deposits dating to the first century AD contain a wide variety of species. In particular the presence of certain species indicated transport from deep water offshore, and this observation was used to support the concept of a former tsunami having taken place. Studies of foraminifera contained within tsunami deposits are very much in their infancy and the value of this particular technique remains to be discovered.

Summary In recent years numerous descriptions have been provided of sediments interpreted to have been produced by former tsunamis. Although there is now a burgeoning literature on palaeotsunami deposits, relatively few studies have been undertaken of geomorphological and sedimentological changes associated with modern tsunamis. Tsunami deposits exhibit enormous variability. In some cases a former tsunami can be represented by a single layer of sand. Elsewhere, tsunami

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deposits can be represented by chaotic sediment layers containing abundant stratigraphic evidence for sediment reworking and redeposition. In rocky coastal areas where sediment supply is limited, former tsunamis can leave no trace. Given such variability in the nature of tsunami sediments, it is not surprising that it is often difficult to distinguish sediments produced by tsunamis and storms and even in some cases by changes in sea level. Solving this particular problem is a priority for future research. Reconstruction of prehistoric tsunami frequency and magnitude also needs to be understood within this context. Considered from a time scale of thousands of years, it may the case that only the stratigraphic records of a small number of former tsunamis that have taken place are preserved in coastal sediment sequences. A promising research area consists of microfossil investigations of tsunami deposits. These studies have provided relatively unambiguous information relating to the nature of former tsunamis and have also pointed to ways in which storm and tsunami deposits might be distinguished. Since the first detailed descriptions of sediments attributed to former tsunamis were published circa twelve years ago, the publication of a series of recent papers on this topic have produced a new and exciting research area. The study of tsunami deposits is very much in its infancy, with many new ideas awaiting discovery.

Acknowledgements The authors are grateful to David Smith, Sue Dawson, Dale Dominey-Howes, Robert Hindson, Ian Foster, Jim Rose, Phil Wood, Stein Bondevik, Jon-Inge Svendsen, Brian Ruffman, John Clague, Cesar Andrade, David Long and Stella Kortekaas for valuable discussions. Cartographic assistance was kindly provided by Erica Millwain, and Gillian West kindly typed the manuscript. This paper is a contribution to Project GITEC-TWO ‘‘Genesis and Impact of Tsunamis on European Coasts,’’ European Union Project ENV4-CT96-0297-GITEC-TWO.

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