th

n a

University

icalsedimwave

rarely been described from the longer, more ancient,geological record. There may be two obvious reasons

1996). Secondly, it may reflect a tendency for the geo-

Quaternary record and consider the likely geologicalimprints of such palaeotsunamis.

and (3) inundation of onshore areas (Gonzalez, 1999).To this we add an important (but largely neglected)fourth tsunami-related process: traction, the tsunami-

Sedimentary Geology 200 (20for this apparent paucity of ancient tsunami deposits.Firstly, it may indicate a tendency toward erosion oftsunami traces in the rock record. Tsunamis typicallyaffect environments subject to permanent or frequentcurrent reworking, such as floodplains, coastal areas,shallow seas and submarine canyons, and theseenvironments generally have a low preservation poten-tial for event deposits (Clifton, 1988; Einsele et al.,

It is important that any study of the geological ex-pression of tsunamis consider their depositional signa-tures in terms of their physical genesis. In this regard,the physics of tsunamis generally link three overlappingbut quite distinct processes: (1) generation by any forcethat disturbs the water column, (2) propagation, eitherfrom open ocean to more restricted coastal waters, en-tirely within shallow nearshore waters or within lakes;offshore zone. 2007 Elsevier B.V. All rights reserved.

Keywords: Tsunami deposits; Geology; Offshore; Sedimentation; Run-up

1. Introduction

Tsunami deposits are well documented from modern,historical and late Quaternary times (e.g. Dawson andShi, 2000; Scheffers and Kelletat, 2003), but they have

logical characteristics of tsunami processes (tsuna-miites) to mimic those produced by other abrupt, high-energy marine and littoral processes (Shiki, 1996; Shikiet al., 2000). To examine these issues, we review thereported incidence and effects of tsunamis in the pre-sedimentary processes associated with modern tsunamis haveprocesses associated with tsunami-triggered gravity backwash flows and comparisons are made with turbidity current action. Weobserve that despite sedimentary evidence for recent tsunamiites, geological research on ancient tsunamis has not identifiedstratigraphic units associated with onshore tsunami sedimentation. Equally, it is noted that nearly all published studies of

not considered patterns of sediment transport and deposition in theTsunami deposits in

Alastair G. Dawsoa Aberdeen Institute for Coastal Science and Management (AICSM),

b School of Earth, Ocean and Environmental Science, Unive

Abstract

A review is presented here of tsunami deposits in the geologthe processes of tsunami generation and propagation and thesedimentary processes associated with the passage of tsunami Corresponding author.E-mail address: eng731@abdn.ac.uk (A.G. Dawson).

0037-0738/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2007.01.002e geological record

,, Iain Stewart b

rsity of Aberdeen, Aberdeen, AB24 3UE, Scotland, United Kingdomof Plymouth, Plymouth PL4 8AA, England, United Kingdom

record. It begins with a discussion of the relationships betweenentary responses. This is followed by a consideration of thes across coastlines. Attention is also given to the sedimentary

07) 166183www.elsevier.com/locate/sedgeogenerated backwash current from shoreline into deeper-waters (Einsele et al., 1996) (Fig. 1).

This paper considers the nature of each of these fourtsunamiphases in turn.The generationphase concerns thenature of the tsunamigenic source mechanism (earthquakefaulting, impact cratering, landslide etc.) and although this

placements of the seabed (earthquakes, volcanic erup-tions, and submarine landslides), or by top-downstrikes onto the sea-surface (asteroid and comet impacts,coastal landslides) (Fig. 2). In simple terms, the larger

Fig. 1. Schematic illustration of principal pathways of tsunami sediment transport and deposition (after Einsele et al., 1996).

167A.G. Dawson, I. Stewart / Sedimentary Geology 200 (2007) 166183has implications for the magnitude and extent of thetsunami generated, it is not expected that it will have anyrecognisable sedimentary signature (beyond any physicaltrace of the causativemechanism, e.g. sea floor fault scarp,crater, landslide scars and debris fields). By contrast, theother three phases are thought likely to leave a potentialimprint in sedimentary depositional environments. Duringthe propagation of a tsunami from the open ocean acrossareas of continental shelf and shelf edge towards the coast,sea-bottom sediment can be disturbed and mobilised. Inthe inundation phase, tsunamis erode, transport and de-posit sediment onshore during run-up. During the tractionphase, pulses of tsunami backwash may generate turbiditycurrents that move seaward towards the abyssal zone viasubmarine gullies and canyons.

2. Generation

Tsunamis arise from any significant disturbance ofthe marine water column, either by bottom-up dis-Fig. 2. Cartoon showing three main mechanisms for generating tsunamiDisplacement of the sea floor by large earthquakes produce an initial wave pseabed displacement. Sliding masses from steep continental shelves of coasleading edges. Bolide impacts larger than 1 km in size can penetrate to the deein the first moments of the impact tsunami amplitudes equivalent to the ocethe disturbance the larger the consequent wave, but thewave height in the open ocean (tsunami height)increases as it enters more restricted coastal waters,and again as it arrives at the shore (inundation height)and surges on land (run-up height). For instance,satellite altimetry measured the wave height of the 26December 2004 tsunami midway across the IndianOcean at about 1 m (Gower, 2005), while post-eventfield surveys in Sumatra, Thailand and Sri Lankarecorded inundation heights of up to 13 m (Lay et al.,2005). The degree of amplification (shoaling) of tsu-namis in the nearshore zone (typically threefold tosixfold over a wide range of conditions Ward, 2001)is a crucial element of geological studies of past tsu-namis. A general rule-of-thumb noted by Lowe and deLange (2000) is that a minimum inundation height of5 m is needed to leave a recognisable deposit in theonshore sedimentary and geomorphological record. Alltsunami wave heights, however, are a function of theirinitial amplitude of the wave at source, and this variess: great earthquakes, giant slope failures and large bolide impacts.ulses several metres high, roughly equivalent to the amount of verticaltal and island volcanoes build waves tens of metres high above theirp-ocean floor and instantly displace the entire water column, generatingan depth.

dramatically with the causal mechanisms for tsunamiexcitation.

2.1. Earthquake tsunamis

Most transoceanic tsunamis are generated by massivesubduction-zone earthquakes whose rupture zonesextend for several hundreds of kilometres along oceantrenches (Ward, 2002; Fryer et al., 2004). All else beingequal, the larger the seismic rupture (as measured byenergy released-seismic moment), the larger the tsunami(Fig. 2). However, the amplitude of the initial sea-surface disturbance can not significantly exceed thevertical deformation of the sea floor at source, giving arule-of-thumb in which maximum tsunami amplitudein the open ocean can not be much greater than anearthquake's mean vertical slip (Ward, 2002; Okal andSynolakis, 2004). This upper limit to the size ofseismogenic tsunami means that, in the open ocean,maximum tsunami heights vary from a few centimetres

(Ward, 2002). In other words, a seismic dislocation ofthe ocean floor does not produce run-up much in excessof its own amplitude on an idealized simple shoreline(Okal and Synolakis, 2004).

Such empirical relations imply that to produce arecognisable tsunami deposit requires earthquakes of M8 or greater, which typically generate initial waveamplitudes of at least metre or two. On average there isabout one M8+ earthquake per year (Ward, 2002), butthe evidence from Holocene sediment sequences alongsubduction-zone coasts is that tsunamis that leave ageological trace appear to strike every few centuries tomillennia (Clague et al., 2000; Fujiwara et al., 2000;Pinegina et al., 2003). Clearly, not all large (MN8)earthquakes produce discernable tsunami traces. Thereare many reasons that determine the tsunamigenic ef-ficiency of an earthquake zone and so can account forthe mismatch (for a discussion, see Gusiakov, 2005).One is that large earthquakes involving dominantlyhorizontal displacements of the sea floor do not ap-

sourcspha

168 A.G. Dawson, I. Stewart / Sedimentary Geology 200 (2007) 166183to 1015 m as earthquake magnitude grows from 6.5 to9.5 (Fig. 2) (Ward, 2002). Fig. 3 highlights how theseopen ocean wave heights attenuate as tsunamis travelacross the ocean, however shoaling in shallow waterslargely recovers those near field wave heights. Thus, anM 8 earthquake might typically be expected to generatea 2 m high wave close to the seismic source, which aftercrossing about 1000 km of ocean reduces to a 0.3 mtsunami height, before hitting a shore and beingamplified back up to a 2 m high inundation height

Fig. 3. Computed peak open ocean tsunami height versus distance fromcurves based on asteroid radii of between 1 and 500 m (fromWard and A

to 9.5, with grey bands including an allowance for anomalous events (data fshoaling amplification factors are excluded.preciably deform the water column and consequentlytrigger little or no tsunamis. On the other hand, so-calledtsunami earthquakes are earthquakes which excite farlarger tsunamis than expected from their seismic waves,a discrepancy largely attributed to a slow and longrupture process (Kanamori, 1972); normally earth-quakes uplift the sea floor far faster than water columncan respond (tens of seconds), but where seismic ruptureis slow (minutes) the growing sea-surface disturbancestacks up on itself as the seabed displacement

e for (a) bolide impacts, and (b) earthquakes. Bolide impact attenuationug, 2002, Fig. 7). Earthquake attenuation curves are for magnitudes 8.0

rom Ward, 2002, Fig. 7). In both plots the ocean depth is 4000 m and

entarpropagates, considerably amplifying the resultant wave(Todorovska and Trifunac, 2001). Other complicationsarise if seismic dislocation lifts the offshore oceanbottom and lowers the land along the coast, a typicaltendency along many subduction-zone earthquakes.This type of displacement propagates waves seawardwith a leading crest and landward with a leading trough(the reason a receding wave sometimes precedes atsunami). Not only does the nearshore crustal subsi-dence facilitate tsunami penetration inland but coastalrun-up and inundation will be greater if the trough of theleading wave precedes the crest (Ward, 2002).

Although the amplitude of seismogenic tsunamismay be somewhat limited, their lateral dimension andreach are remarkably large. Simulations show that longearthquake ruptures preferentially emit tsunamis in atight beam perpendicular to fault strike, which ensures azone of sustained run-up which is comparable in size tothe lateral dimension of offshore rupture, i.e. hundredsof kilometres in length (Ward, 2002). Along a simpleuniform coastline, the areas of greatest inundation canbe expected adjacent to the areas of greatest strainrelease (Okal and Synolakis, 2004). The large dimen-sions of sea floor uplift produce long (low frequency)waves that travel fast and efficiently, such that theyattenuate far less than other exotic tsunamigenicmechanisms discussed below, and consequently aremore far-reaching (Ward, 2001). Longer wavelengthsmean longer wave periods, and this too can accentuateinundation effects. Trans-Pacific earthquake tsunami,for example, have predominant wave frequencies ofaround 1 h, a characteristic which in the 1960 Chileanearthquake coincided with the resonance response ofsome Japanese bays and greatly increased local tsunamirun-up.

2.2. Slide-generated tsunamis

Large earthquakes arguably generate most tsunamisbut their effects can be potentially dwarfed by the locallydevastating waves produced by landslides and volcanicactivity (Ward, 2001). In the case of volcanic eruptions,although they can parent tsunamis in a number of ways;submarine phreatomagmatic activity, Plinian-type air-blasts (Latter, 1981; Lowe and de Lange, 2000) the mostdestructive waves arise from large volumes of volcanicdebris abruptly entering the sea, either as largepyroclastic flows (Carey et al., 2001; Sacchi et al.,2005) or debris avalanches (Pelinoovsky et al., 2004), orvia the partial or even complete collapse of volcanicedifices (Tinti et al., 2000; Pararas-Carayannis, 2003;

A.G. Dawson, I. Stewart / SedimWard and Day, 2005). The mechanism by whichpyroclastic flows generate tsunamis isn't clear (thoughflows probably decouple into a dense basal current thatenters the sea and a dilute cloud that travels over thewater (Watts and Waythomas, 2003; Burgisser, 2005),but the subaerial or submarine failure of volcanic flanksis essentially a landslide process. In simple terms,therefore, volcanic mass movements and landslidesshare the same essential characteristics of a movingmass of material displacing the water column to build upwaves of potentially colossal scale.

Tsunamis generated by sliding masses are typicallydisastrous in the near field and comparatively benign inthe far field (Okal and Synolakis, 2004). This is becauseinitial waves at source form a wall of water that arenearly as tall as the slide block is thick, but since only asmall portion (from b1 to about 15%) of the grav-itational energy of the sliding mass goes into the waterwaves, they attenuate far more rapidly than earthquake-generated tsunamis (Ward, 2001; Ward and Day, 2005).Also, unlike the elongated beam of the seismogenictsunami, slide-generated tsunamis radiate from a strongdipolar pattern of sea-surface disturbance, with a neg-ative depression (trough) over the sliding mass and apositive swell (hump) seaward of its leading edge (Okaland Synolakis, 2004). This dipolar tsunami sourceforms because as the mass moves through the sea, wateris drawn up over the upper part of the slide and is pushedup in a broad rise over and ahead of the advancing noseto become the leading elevation of the seaward tsunami(McMurtry et al., 2004a,b). The speed at which the massmoves across the sea floor is critical for the wave heightsattained. Very fast slides (debris-flows) generate tsu-namis roughly as high as the slide is thick while veryslow moving slides produce little or no tsunamis.However, where slides move at velocities close or equalto that of the tsunami being produced, they develop inphase, building the waves up to exceptional size (Ward,2001).

The remarkable amplitude of tsunamis arising fromsubmarine or coastal instability is now well documen-ted, though the extent of their reach remains uncertain.Arguably the most famous historical tsunami is that ofthe 1883 paroxysmal eruption of Krakatoa volcano,where either massive flank failure or caldera collapsedelivered devastating waves up to 40 m high along theadjacent coast, but attenuated rapidly with distance,arriving as a 2.4 m high wave several hundred km awayin the capital Batavia (modern Jakarta) and a 0.2 m highwave in eastern Java (Pararas-Carayannis, 2003). How-ever, historical accounts of a tsunami-generated by the1998 eruption of Ritter Island volcano, Papua New

169y Geology 200 (2007) 166183Guinea, tell of destructive waves 1015 m or more in

entarheight over 500 km away, which Ward and Day (2003)simulate as the result of a moderately-fast (40 m/s)landslide triggered by submarine flank collapse. The1998 Papua New Guinea tsunami was sourced from a6 km3 slump that traveled downslope 1 km in waterdepths averaging 1400 m, and delivered waves to thenearest shoreline that averaged 10 m above sea level(Tappin et al., 2001). The 1929 Grand Banks landslide-generated tsunami-triggered a more extensive (200 km3)slope failure which had tsunami heights of 38 m andinundation heights of up to 13 m along the coast ofNewfoundland, as well as discernable cm-high waveson the other side of the Atlantic ocean (Ruffman, 2001;Tuttle et al., 2004; Fine et al., 2005). This was one of thevery few documented slide-generated teletsunamis,although other transoceanic sea waves have been arguedto have had a strong landslide component (e.g. Fryer,2004; Fryer et al., 2004; Okal, 2004). In general,however, it is the highly localized nature of tsunamiimpact that generally distinguishes a slide-generatedtsunami from that of a seismic dislocation of the seafloor.

Despite uncertainty over the effectiveness of theirreach, the truly catastrophic effects of slide-generatedwaves are confined to the near field zone. Lituya Bay inAlaska has geological evidence for five enormous rock-slide tsunamis in recent centuries, but all appeared to berestricted to this narrow glacial inlet and did not affectadjacent coasts (Pararas-Carayannis, 1999). In 1958, forexample, a rockslide generated waves that in the nearfield surged up to 520 m altitude, but which rapidlydissipated to tens of metres down the 12 km long bayand failed to reach neighbouring inlets (Pararas-Carayannis, 1999; Fritz et al., 2001). The enormousnear-field wave heights at Lituya Bay were fuelled bythe ability of subaerial slides to inject substantial aircavities into the water column, a phenomenon whichelsewhere is believed to be responsible for comparablemega-tsunamis.

The largest of the proposed mega-tsunamis arebelieved to accompany the giant slope failure of oceanicisland volcanoes (Keating and McGuire, 2000). Firstidentified off the flanks of Hawaii (Moore and Moore,1984, 1988) where vast fields of landslide blocks anddebris litter the sea floor, subsea surveys have nowrevealed equivalent submarine scars littering nearlyevery oceanic basin, with especially numerous lateral-collapse slides having been shed from volcanic islandsin the Atlantic ocean during the past few million years(Moore et al., 1994; Keating and McGuire, 2000).However, with no historical examples of volcanic flank

170 A.G. Dawson, I. Stewart / Sedimcollapses to guide us, attention has largely focused onnumerical simulations (Ward, 2001; Ward and Day,2001, 2003; McMurtry et al., 2004a,b). For instance, asimulation of the collapse of the Cumbre Vieja volcanoat La Palma in the Canary islands, assuming a landslidevelocity of 100 m/s, generates initial waves up to 900 mhigh close to the volcano, dissipating to 50100 m highwaves striking the African mainland and 1025 m highwaves affecting most of the Atlantic basin (Ward andDay, 2001). Since, to a first approximation, tsunamiamplitude is proportional to landslide volume for eventsof a similar mean depth, the Hawaiian slumps are arguedto be similarly capable of producing initial amplitudes of1 km high (McMurtry et al., 2004a,b). Confirmation ofsuch mega-tsunamis in the form of tsunami deposits isequivocal and controversial. In the Atlantic realm, onlyPrez-Torrado et al. (2006) report anomalous boulderdeposits on Gran Canaria that may be the product ofCanary Island flank collapse. In the Pacific realm,enigmatic coral and gravel deposits originally depositedseveral hundred metres above sea level on the Hawaiianisland of Lanai have long been attributed to a mega-tsunami (Moore and Moore, 1984, 1986). Somedeposits, which currently lie just 20 m above currentsea level, have been reinterpreted as uplifted high-energy shoreline deposits (Grigg and Jones, 1997;Rubin et al., 2000; Keating and Helsley, 2002; Feltonet al., 2006) but other field studies have reasserted theirinterpretation as a mega-tsunami deposit (McMurtryet al., 2004a,b). The debate over the possible sedimen-tary expression of these rare cataclysmic waves will nodoubt continue.

2.3. Bolide impact tsunamis

Of the 160 or so meteorite impact craters currentlyidentified on the Earth's surface, seven are found in themodern ocean, although a further twenty craters nowpreserved on land were originally marine impacts (EarthImpact Database, 2003). Terrestrial cratering rates,however, would indicate that over eight thousand majoroceanic craters could have formed in the last 3.5 billionyears (Glickson, 1999), making ocean impacts from bo-lides (asteroids and comets) a potentially important tsu-namigenic process. Like large-scale flank collapse, thelack of historical analogues and unequivocal impact-related tsunami deposits ensure that tsunamigenic poten-tial is estimated from numerical simulations.

These simulations show that bolides larger than 1 kmin diameter ought to penetrate to the floor of a 5 km deepocean, instantly displacing the entire water column andgenerating in the first moments of the impact tsunami

y Geology 200 (2007) 166183amplitudes equivalent to the ocean depth (Hills and

Goda, 1999; Ward and Asphaug, 2002, 2003). There-after, tsunami waves hundreds of metres high wouldfollow as the transient air cavity collapses, while ad-ditional tsunami-like waves could be triggered byslumps and slides along the crater highs and margins.Even at 1000 km from source, such strikes can producewaves 100 m or so in height (Ward and Asphaug, 2002).Thus the Eltanin asteroid impact at 2.15 Ma, modeled asa 4 km impactor reaching the ocean floor, is predicted tohave delivered a 200300 m high tsunami to the An-tarctic Peninsula and the southern tip of South America12001500 km away (Ward and Asphaug, 2002). Wardand Asphaug (2002) also note that an asteroid the size ofthe Chicxulub (10 km diameter) (Fig. 4), had it falleninto water deeper than 1000 m, would have dispatched100 m high tsunami out to 4000 km distance, even ifshoaling amplifications are neglected.

Impactors greater than 1 km in radius, however, arerelatively rare events, probably striking the Earth only

every 300,000 years or so (Shoemaker et al., 1990). Amore likely tsunami scenario is for small to moderatescale bolides 1 m to 500 m in radius. A census ofNear-Earth Objects suggests that one of these may strikethe Earth's oceans every 1000 to 100,000 years (Wardand Asphaug, 2002). Fig. 3 shows the open ocean waveheights attained by such impactors, and demonstratesthat tsunamis large enough to have a geological imprint(i.e. wave heights N5 m) can be readily generated bythese comparatively modest strikes. However, becauseocean asteroid impacts excite tsunami waves of allperiods and wavelengths, creating complex wave-trainswhich rapidly disperse with distance (Ward andAsphaug, 2002, 2003), the attenuation of bolide-gen-erated waves is dramatic. Nevertheless, their initial am-plitudes are such that impactors exceeding 100 m inradius can still deliver waves up to 10m high at distances1000 km from the impact point (Fig. 3). Such 100 mradius bolides can be expected to impact the Earth

icxulu

171A.G. Dawson, I. Stewart / Sedimentary Geology 200 (2007) 166183Fig. 4. Map showing CretaceousTertiary boundary sites around the Ch

mass-wasting deposits have been reported. Late Cretaceous palaeogeographysites from Claeys et al. (2002).b impact structure in the Gulf of Mexico where tsunami deposits and/or

map redrawn from Day and Maslin (2005) and tsunami/mass-wasting

anything between every 30005000 years (Hills andGoda, 1999) to 10,000 years (Ward andDay, 2005).Withtwo-thirds of the Earth covered with ocean, the waterlandings of medium-sized asteroids and comets wouldappear to be an important mechanism for generatingtsunamis detectable in the geological record.

3. Propagation

In open water, tsunamis, unlike wind-generatedwaves, can feel the deep seabed (Ward, 2001). Thisis because, with wavelengths at least three times greaterthan the ocean depth, the passage of a tsunami waveinvolves the entire water column (Fig. 5). The orbitalmotion of a water particle in a passing tsunami wave hasa vertical component that is greatest at the ocean surfaceand decreases to zero at depth, but its horizontal com-ponent is constant throughout the ocean column. Aslong as the length of the wave exceeds the ocean depth,such horizontal motions will outpace vertical motions.During the passage of one tsunami wave, the horizontalcurrent velocity increases from near zero to a maximum(Umax), thereafter decreases to zero, increases back toUmax in the opposite direction and drops again to zero

172 A.G. Dawson, I. Stewart / SedimentarFig. 5. Orbital motions of water particles in tsunamis with differentperiods (modified from Ward, 2002). At long periods (e.g. 1500 s) theenormous wavelengths (hundreds of kilometres) ensure that the orbitalmotions reach to the seafloor. The vertical displacement of waterparticle peaks at the ocean surface and drops to zero at the seabed, butthe horizontal displacement is constant through the ocean column. Atwave periods of about 150 s, tsunami wavelengths are about threetimes the ocean depth, at which point horizontal and vertical motionsmore closely agree in amplitude. At 50 s period, the waves move incircles that decay exponentially from the surface and so these shortwaves do not feel the seafloor. The ability of near-bottom tsunamicurrents to erode and transport sediment are poorly known, thoughsome argue that their shear velocities are sufficient to generateerosional scour and the cyclic pressure wave can produce spontaneous

liquefaction on slopes, both of which can stir soft sediments intosuspension.(Fig. 5). The key question is whether this transitorysloshing back and forth is capable of leaving asedimentary imprint on the seabed.

Pickering et al. (1991) used shallow-wave theory anddata from the 1960 Chilean tsunami along the SE coastof Japan to calculate likely seabed velocities and relatedshear stresses in the open ocean. Recorded wave heightsof 6 m in the coastal waters were used to infer a deepwater wave height of 0.44 m, which in a water depth of500 m would give maximum horizontal orbital velo-cities on the sea floor of 0.16 m/s. Calibrations usingreasonable values of tsunami wave height in coastalwaters yielded a series of graphs for maximum hori-zontal orbital velocities against water depths. Pickeringet al. (1991) noted that the wavelength of any bedformsassociated with the passage of tsunamis over the deepocean bed would typically be of the order of 50100 m.Their principal conclusion, however, was that, assumingreasonable values of tsunami period and height, seafloor velocities at depths greater than the shelf breakwould be insufficient to entrain and transport sedimentcoarser than silt-grade; cohesive muds would besimilarly difficult to erode.

As discussed earlier, however, other mechanisms cangenerate at source tsunami heights far greater than thereasonable amplitudes derived by Pickering et al.(1991) from earthquakes. The cataclysmic late BronzeAge eruption of Thera (Santorini) in the central AegeanSea is estimated to have generated wave heights of 717 m passing over 3000 m of water in the open Med-iterranean waters, generating near-bottom current ve-locities of 0.20.5 m/s (Kastens and Cita, 1981).Kastens and Cita (1981) compare these velocities tothreshold erosion velocities for abyssal sediments estim-ated from flume experiments, which suggest that cal-careous muddy sediment is eroded under a currentexceeding 0.15 m/s. Although abyssal sediments can bepotentially stirred into suspension by tsunami passage,the shear stresses act only on the free upper surface ofthe sediment column, thereby making the processincapable of exciting significant volumes of sedimentinto suspension. An extensive, metre-thick homoge-nite that blankets much of the central Mediterranean asa result of the Thera tsunami is interpreted by Cita et al.(1996) to arise in part from erosional scour by oscil-latory near-bottom currents and in part from theinjection of sediment into the water column by thespontaneous liquefaction of seabed slopes loaded bycyclic pressure pulses of the passing tsunami.

A paucity of studies on the deep water effects oftsunami, however, make these inferences about the scale

y Geology 200 (2007) 166183and extent of sedimentary signatures of tsunami

entarpropagation largely speculative. In general it seemsunlikely that most earthquake-generated tsunamis caninduce a deep water sedimentary imprint, but thosemechanisms generating larger amplitude waves may doso close to source, a point that we shall return to in alater section on evidence from the geological record.What is clear, however, is that the main sedimentaryeffects of tsunamis come from their passage from theopen sea and into restricted coastal waters.

4. Run-up

As a tsunami travels into shallow water (acrosssubmarine ridges or the coastal zone), each individualwave must slow down and its amplitude dramaticallyincrease. Here, tsunami hydraulics depend on the natureof the nearshore marine environments. Thus wavestravelling across a wide continental shelf may becomemore attenuated and decrease in velocity earlier thanthose breaking close to the submerged flanks of theHawaiian island chain of volcanoes and seamounts oracross subduction trenches. Although it remains uncer-tain whether the passage of a large tsunami across ashallow shelf leaves a laterally extensive sedimentdeposit diagnostic of a tsunami, the nearshore zoneprovides the setting for major hydraulic action (Shuto,1993; Inamura et al., 1993). In particular, as a tsunamiwave approaches the coast, seabed sediments aresuspended and a basal erosion surface is cut (Coleman,1978). The amount of sediment available for mobiliza-tion will be an important determinant on the resultingtsunami signature, since nearshore zones barren ofsediment making it possible for a tsunami to strike acoastline but leave no trace of its passage. Alternatively,the much greater accumulations of unlithified sedimentresting on continental shelves versus volcanic flanksmay offer an environment more suitable for landwardsediment transport by tsunamis. The manner in whichthe tsunami arrives at the shore may also be important,with drawdown of the sea-surface prior to the arrival of atsunami crest exposing a source of nearshore sedimentthat can then be eroded and transported onshore duringrun-up.

Whilst in the open ocean the flow velocity is afunction of water depth and typically measures manytens to several hundred metres per second, once waterdepths decrease and the flow becomes turbulent, thelandward movement of water and sediment slows toabout 1020 m/s, the usual velocity of tsunami run-uponto coast (Nanayama and Shigeno, 2006 and refer-ences cited therein). Even at these reduced current

A.G. Dawson, I. Stewart / Sedimvelocities tsunamis can transport the full range ofsediments, from fine clays to large boulders (Yeh et al.,1993). As well as directly eroding large volumes ofsediment from the coastal plain, tsunamis can also causethe partial collapse of beach cliff and submarine canyonwalls, releasing fresh coarse rock debris and resulting ina dense flow with a far higher capacity to transportboulders than clear seawater.

As this dense turbid flow sweeps landward as abreaking wave, a wall of water or a tide-like flood, itsvelocity slows to below 5 m/s, though this is highlydependent on the coastal morphology (Nanayama andShigeno, 2006). As it wanes, its erosive capacityweakens and deposition of coarse sediment increases.Vertical run-up can reach tens of metres, and horizontalinundation, if unimpeded by coastal cliffs or other steeptopography, can penetrate several km inland (Hindsonet al., 1996). Once the maximum landward inundationhas taken place through run-up, the water bodyeverywhere changes direction and, as it does so,everywhere passes through a point of zero velocityprior to backwash flow. Dawson (1994) proposed that itis this unique characteristic of tsunami wave behaviour(in distinction to stormwaves) that produces a distinctivestyle of sedimentation across the coastal zone. Duringthis quasi-stillstand, material coarser than silt can bedeposited out of the water column while the finer-grained silts and clays remain in suspension (Shi andDawson, 1995). With the backwash from the first wave,part of this sediment remains in situwhile other areas aresubject to scour and the seaward transport of sediment.

Because of the long periods of tsunami waves, anycoastal zone subject to tsunami inundation willexperience a series of tsunami waves over a period ofhours (days?). Large tsunamis are typically associatedwith wave periods between ca. 20 min and 1 h,modelling of the well-known Storegga Slide tsunamiindicates a likely wave period for this event of ca. 2 h(Harbitz, 1991). Although it is recognized that tsunamiwave period is highly variable between different events,the time interval between the major waves is significantin terms of sedimentation processes. Thus, the secondincoming wave, if it is larger than the first, may erodeand redeposit sediment produced as a result of the firstwave. Equally, the second incoming wave is capable ofdepositing new material on top of that laid down by thefirst (Dawson, 1994). Since major tsunamis can beassociated with numerous major waves, it is no surprisethat the depositional ensemble may be extremelycomplex. Although such deposits are typically massive,it may be possible through detailed grain-size analysesto attribute individual sediment to specific episodes of

173y Geology 200 (2007) 166183run-up and backwash (cf. Dawson and Shi, 2000).

Tsunami deposits produced as a result of onshorerun-up and backwash can be very distinctive (Nanayamaet al., 2000; Dawson and Shi, 2000). The change inwater flow direction between episodes of run-up andbackwash may lead to the production of massivedeposits of typically sand, silt and fine gravel thatcontain isolated boulders (the boulder floats of Youngand Bryant, 1992). Sedimentary analysis of the 1993Hokkaido-nansei-oki tsunami deposit (Nanayama et al.,2000) show that it could be divided up into four layerscorrelative with landward and seaward flows from thetwo main tsunami waves, with the run-up currentsdepositing marine sand and rounded gravel and thebackwash currents depositing a poorly sorted mixture ofsoil, non-marine sand and stream gravel with plantfragments.

Generally, in wave-dominated shallow-marine set-tings, it is difficult to distinguish tsunami deposits fromthe normal background storm deposits, firstly becausethey resemble tempestites and secondly because subse-

thickness and both thin landwards, the higher-energytsunamiite sheets often extend further inland and risehigher in altitude inland, as noted by Dawson (1994).Landward of these continuous tapering sedimentwedges there may be discontinuous lenses of tsunami-deposited sediment (Dawson, 1994).

Tsunami sediment sheets also contain distinctivemacro- and microfauna. The macro-faunal content canrange from fish remains to a wide range of shell debris.One of the major characteristics of diatoms andforaminifera found in tsunami deposits is the presenceof a majority of broken individuals due to the turbulentwater transport (Dawson and Shi, 2000). In someinstances, coral debris, up to boulder size, is present(Fig. 6), while boulders containing marine molluscaembedded in bioerosional solution hollows on the rocksurface testify to onshore tsunami transport from deepwater. Microfauna present within tsunami sedimentsheets include a wide species range of diatoms andforaminifera (e.g. Dawson et al., 1996) several of which

174 A.G. Dawson, I. Stewart / Sedimentary Geology 200 (2007) 166183quent storms can modify or remove their trace (Einseleet al., 1996). However, several recent studies have di-rectly compared the two. Nanayama et al. (2000), forexample, distinguished the 1993 tsunamiite packagefrom a typhoon-related storm deposits in that thetempestite showed foreset bedding and comprisedbetter-sorted marine sands while the tsunamiite com-prised both landward- and seaward-directed layers.Similarly Goff et al. (2004) note that whilst storm andtsunami deposits may be locally comparable inFig. 6. Large coral-reef fragments deposited on Pakarang Beach, near Khaoneeded to remove these boulders may ensure they have a higher preservatiooriginate in deep water. Nanayama and Shigeno (2006)report deep water marine benthic foraminifera within the1993 Hokkaido tsunamiite, indicating that the tsunamiwas picking up foraminifera tests on the seabed atdepths of between 45 and 90 m (and implying seabedcurrent velocities of 0.20.5 m/s). As they note, as wellas providing valuable data on the depth to which the seais agitated by tsunami waves, the presence of benthicforaminifera ought to be an important criterion that canbe used to identify past tsunami deposits.Lak, Thailand, by the 26 December 2004 tsunami. The high energiesn potential than other tsunami-related onshore sediments.

entar5. Backwash and offshore traction

Backwash flows follow the maximum landward in-undation of individual tsunami waves as water recedesseaward, but there is comparatively little information onthe nature of this process (Dawson and Stewart, inpress). Video footage of the 26 December 2004 IndianOcean tsunami, which showed great plumes of turbidwater moving offshore, suggest that tsunami backwashflow velocities can be exceptionally high. There is littleor no empirical data on backwash hydraulics, althoughNanayama and Shigeno (2006) estimate the outflowvelocity of the 1993 Hokkaido tsunami as about 2.3 m/s.The carrying capacities of these sediment-laden back-flows is unknown, but they are clearly erosive enough torework the onshore deposits deposited during run-up(Dawson, 1994; Nanayama et al., 2000) and may evenbe able to induce the corrosion and cavitation of bedrockplatforms (Aalto et al., 1999). Their erosivenessundoubtedly results from the fact that coastal topogra-phy and bathymetry serve to concentrate the backwashinto channelised flows (Einsele et al., 1996; Le Rouxand Vargas, 2005).

Because backwash flows are routed by coastal mor-phology, they are potentially more erosive and powerfulthan run-up flows (Le Roux and Vargas, 2005),particularly where flow velocities are increased byrebound from landward cliffs (Massari and D'Alessan-dro, 2000). The erosive potential of tsunami backwashhas been questioned, however, with Murty (1982,p.489) arguing that storm surges were likely to be ofgreater effect since they usually have stronger offshoremovements. Nevertheless, the expectation is that tsu-nami rip currents will have an increased erosionalcapacity from catching the large amounts of debriseroded from shoreface settings on upsurge. Thismaterial is then likely to be transported seaward assome kind of sediment-gravity flow, with hyperconcen-trated flows expected in the deepest parts of thenearshore channels (Le Roux and Vargas, 2005).

It has long been recognized that tsunami-drivensediment flows moving from the nearshore towardsdeeper-water ought to be capable of transporting a widerange of grain-sized debris. Coleman (1968), for ex-ample, noted that tsunamis have an offsurge capacityof sediment transport and envisaged that the off-surgerolls in a turbid flow along the sea floor and as it losesenergy drops its load progressively. The rapid loss ofenergy ought to result in the deposition of poorly sortedsediments that are literally slopped forth in turbidmasses relatively close to the shore (Coleman, 1968).

A.G. Dawson, I. Stewart / SedimAccording to Coleman, such deposits are characterisedby: (1) graded rudaceous beds with basal clasts ofpebble or even boulder size; (2) chaotic sediments withboulders set in a fine-grained matrix; (3) layers ofunfossiliferous, non calcareous, poorly sorted sediment,alternating with calcareous fossiliferous layers; (4)layers or mats of concentrated vegetation with inter-beds of both fine and coarse sediment; (5) featuresindicating current direction at right angles or evenopposed to the determined original seabed slope; and (6)the presence of undoubted shallow water indicators suchas mudcracks, large-scale cross bedding in coarse sands,profusion of scour channels. Furthermore, he recognizedthat the bulk of deposition may take place either inshallow water within a few km of the coast or, para-doxically, in the deeper offshore, even bathyal, waterbeyond the shelf.

Coleman (1968) stressed the important process linksbetween tsunami sediment transport, turbidity currentflow and the long-term evolution of submarine canyons,arguing that canyon development may be closely linkedto tsunami-triggered turbidity current flow. Even earlier,Bailey (1940) had highlighted possible links betweenturbidity current activity and the evolution of submarinecanyons. However, this association between tsunamibackwash and offshore turbidity currents has receivedrelatively little modern attention, though an interestingexception is the report by Nanayama and Shigeno(2006) that as the outflow from the 1993 Hokkaidotsunami moved down the submarine shelf slope, it mayhave generated a turbidity current that deposited aturbidite. The possible relationships between submarinecanyon development and tsunami sediment transport arebeyond the scope of this paper, though we recognize thatonce formed such canyons may frequently act as con-duits through which tsunami-triggered turbidity currentstransport and deposit sediments. In so doing, such cur-rents are likely to bring with them indicators of sedimenttransport from shallow to deep water (cf. Shiki andYamazaki, 1996).

It is important to stress that much of the abovediscussion of tsunami backwash effects remainsspeculative because there are no reliable measurementsof this process in action and, more surprisingly, fewrecent sedimentary deposits that are attributed to it.Indeed, sedimentary indicators of modern backwashactivity are confined to apparent seaward-directedsediment layers or reworked debris within supralittoraldeposits (Dawson, 1994; Nanayama et al., 2000; LeRoux and Vargas, 2005; Nanayama and Shigeno,2006). By comparison, we have been unable to findany reports of the effects of tsunami backwash in the

175y Geology 200 (2007) 166183nearshore zone.

Bailey and Weir (1932) described an Upper Jurassic(Kimmeridgian) marine sediment sequence in easternSutherland, Scotland characterised by extensive beds ofboulder deposits, often less than 5 ft in individualthickness. The boulder beds alternate with beds of shalewhile the top of each boulder bed is levelled downwith rubble (an abundance of shell fragments) andshelly sand and present a flat surface to the succeedingshale. By contrast the basal part tends to contain largerboulders and to be irregular and transgressive (Baileyand Weir, 1932, p.450) (Fig. 7). They attributed thesecharacteristics to tsunami sediment transport, triggeredby seismic dislocation on a large and active submarinefault scarp. Later, Pickering (1984) would describe thesame boulder beds in terms of debris-flow processes,though with no reference to Bailey and Weir's (1932)palaeotsunami hypothesis. This raises the key questionas to precisely what do offshore tsunami deposits looklike? It is a question that has been resurrected in recentyears with the ongoing debate over the geologicalsignatures of impact-derived tsunamis.

entary Geology 200 (2007) 166183One possible sedimentary distinction of tsunamibackwash in the nearshore realm could lie in the long-duration wave periods often associated with tsunamis(between 20 min (e.g. the Great Lisbon earthquake andtsunami of 1755 AD) and 2 h (modelling of the StoreggaSlide and tsunami of ca. 8000 yr BP). During thepassage of such wave-trains, episodes of high-energysediment transport alternate with intervening quiescentperiods during which finer-grained sediments can bedeposited out of suspension. Estimates of sedimentsettling rates are rarely considered in palaeotsunamistudies, though Bourgeois et al. (1988) argued that afterinitial turbulence, sand (settling velocity 0.25 cm/s)would settle out of 100 m of water in about 12 h whilethe silt and clay would take a few days to settle out,depending on the residual turbulence. At these rates itseems likely that the bulk of agitated sediment wouldremain suspended between tsunami waves and only afraction of the stirred sediment would settle during thequiet interlude to form a thin low-energy layer. Anadditional effect may be that the density gradient of avery turbid water column stirred by very large tsunamiscould conceivably cause sediment advection, drawingsandy material from close to shore toward the site ofdeposition (Kastens and Cita, 1981). As we discusslater, geological analogues may provide some indica-tions of the sedimentary structures that we ought to findin modern seabeds agitated by tsunamis.

6. Tsunamiites in the geological record

Geological studies of tsunamis emerged with thedescription by Atwater (1987) of widespread sand sheetsfrom the marshes of Oregon that he attributed to onshoretsunami deposition linked to a large Cascadia offshoreearthquake at ca. AD 1700, and an account by Dawsonet al. (1988) of similar sediment sheets from easternScotland that they attributed to a large slide-generatedtsunami-triggered west of Norway ca. 8000 yr BP. Theseaccounts have subsequently been followed by a prolifer-ation of papers that describe onshore tsunami depositsfrom different areas of the world. This wealth of modernpalaeotsunami investigations appear to have neglected amuch earlier debate concerning the offshore expression oftsunamis and their possible association with turbiditycurrents (Bailey, 1940; Coleman, 1968). Ironically, as wediscuss in the following section, reports of tsunamispreserved in the more ancient (pre-Quaternary) strati-graphic record largely relate to deposits in amarine setting,with few descriptions of onshore tsunamiite deposits.Indeed, one of the earliest accounts of a geological

176 A.G. Dawson, I. Stewart / Sedimtsunamiite relates to the role of tsunami backwash.Fig. 7. Upper Jurassic sediments of Helmsdale, northern Scotlandproposed by Bailey and Weir (1932) as of tsunamigenic origin a)cross-section of boulders beds resting with unconformity with adjacent

shale unit, b) stratigraphic relationship between shale laminae and asingle boulder bed.

entar6.1. Impact-related tsunamiites

Following Alvarez et al. (1980) a considerable lit-erature has emerged on aspects of tsunami sea floorsedimentation associated with the K/T impact. Arguablythe earliest account is that of Bourgeois et al. (1988)who at the Brazos River section in Texas described acoarse-grained sandstone with large clasts of mudstone,reworked carbonate nodules and wood fragments thatgrade upwards to wave ripple-laminated, very finegrained sandstone that they consider were deposited atthe shelf break and on the shelf in ca. 50100 m waterdepths. Their analysis concludes that these character-istics were consistent with the passage of a tsunami witha wave height between ca. 50100 m. In particular, sucha scenario could explain:

The transport and redeposition of shelf sediments andthe concentration of older lag material includingbored limestone clasts and glauconite.

The absence of shallow water microfauna in the sandunits.

The inclusion of mud clasts inferred to have beentransported at water depths of between 50100 m.

The presence within the sediment of tsunami-inducedwave ripples rather than the asymmetric ripplescharacteristic of a turbidity current.

Patterns of sandstone deposition that require initialshear velocities of ca. 15100 cm/s.

The presence of wave ripples within fine sand thatrequire a rapid drop in shear velocities to less than1 cm/s under conditions of oscillatory flow.

Bourgeois et al. (1988) pay considerable attention tothe sedimentary detail, but there is no explicit discussionof the direction of former water movement (i.e. landwardor basinward). The difficulties in distinguishing the twodifferent phases have been taken up by more recentstudies of the so-called K/T tsunamiites. Smit et al.(1992), in a discussion of K/T tsunami deposits inMexico, draw attention to the presence within the depositof wood debris and hence argue that the sediments inquestion were deposited as a result of tsunami-inducedgravity flow. Olsson et al. (2002) interpreted a layer ofsmall (mmcm) clay clasts overlying a spherule bed(fallout from the ejecta vapour cloud) as rip-up featuresfrom the erosional scour of a tsunami moving shorewardfrom the deeper parts of the Late Cretaceous sea floor.Takayama et al. (2000) describe a 180 m thicksedimentary sequence formed in association with theK/T boundary in which a massive, poorly sorted, grain-

A.G. Dawson, I. Stewart / Sedimsupported unit contains large shallow-marine fossils andoccasional large intraclasts. The unit was interpreted as agrain flow, whilst an overlying upwards-fining homo-geneous unit (akin to the homogenite of Cita et al.,1996; Cita and Aliosi, 2000) with abundant water-escapestructures and reworked fossils, was regarded as theresult of a high-density suspension caused by tsunamis.Lawton et al. (2005) has highlighted the possiblecontribution of high-discharge, supercritical offshore-directed flows associated with the KT impact atChicxulub in delivering voluminous amounts of ejecta-bearing strata into deep water Gulf of Mexico. Theirstudy interpreted the KT impact-related conglomeraticstrata occupying valley-like features at the shelf edge asthe result of the turbulent backflow of one or severaltsunami. Key characteristics of the valley fill deposit thatargue against a debris-flow origin are suggested to begrain support, normally-graded beds, tightly packedintraclast clusters, scours and crude imbrication. Thisinterpretation of tsunami-induced backflow is stronglycontested, however, with other high-energy processesbeing invoked to explain the same sedimentary sig-natures (Keller et al., 2003).

K/T tsunamiites have now been described from as farafield as Mexico (Smit et al., 1992; Lawton et al., 2005),northeastern Brazil (Alberto and Martins, 1996), theCaribbean (Takayama et al., 2000; Tada et al., 2002,2004), but the nature of the CretaceousTertiary impactand of any associated tsunami deposits remains highlycontroversial. One source of controversy relates whetherthe tsunamis were triggered directly by the bolidelanding (e.g. Bourgeois et al. (1988) or by associatedimpact-triggered slope failures (e.g. Olsson et al., 2002).Day and Maslin (2005) have argued that the shallownessof the Chicxulub impact site made it an ineffectivetsunamigenic source, and instead the impact may haveproduced a regional seismic shock which triggeredwidespread submarine slide activity and, in turn, gashydrate release, a mechanism also suggested by Max etal. (1999). For those who favour an impact origin, itseems clear that any tsunamiite associated with theChicxulub event horizon could have had multiplegeneration mechanisms, and may therefore be moreproperly considered as a series of near-simultaneoustsunamis that combined to produce highly complexpatterns of water displacement (e.g. Sculte et al., 2006).However, a more serious source of controversy relates towhether the deposits themselves are generated bytsunamis in the first place (e.g. Keller et al., 1993;Fourcade et al., 1997; Keller et al., 2003).

While the debate over K/T tsunamiites has raged,other impact-related tsunami deposits have been pro-

177y Geology 200 (2007) 166183posed. Poag et al. (1992) and Poag (1997) attributed part

of a thick breccia unit (Exmore Breccia) to a super-tsunami-generated by the late Eocene (35.5 Ma) Che-sapeake Bay bolide impact, and contemporaneousreworked ejecta layers on the New Jersey continentalmargin are similarly attributed to those tsunamis trig-gered directly by that impact and its associated slopefailures (McHugh et al., 1998). Masaitis (2002) sug-gested that a middle Devonian angular breccia deposit(the Narva Breccia) found in the Baltic countries andadjacent areas of northwestern Russia and Belarus maybe a possible tsunamiite from the Kaluga impact, whichstruck shallow (300500 m deep) seas of the EastEuropean platform 380 million years ago. Deconnicket al. (2000) tentatively link an erosional conglomerateunit in northern France to the Late Jurassic bolideimpact that created the Mjolnir impact crater (142 Ma)in the Barents Sea. The most ancient evidence fortsunami impact would appear to be the anorbital waveripples within a spherule bed in Late Archean strata inthe Hammersley basin of Western Australia, whichHassler et al. (2000) interpret as the product of abyssal

scour by giant tsunami waves rippling out from anoceanic bolide impact several thousand kilometresaway. A possible other more ancient example is reportedfrom a Precambrian carbonate platform by Pratt (2001),who attributed hummocky cross-lamination in ooidalgrainstones to oscillatory bottom currents induced bytsunami advance and offsurge, and basin-directed toolmarks to tsunami backflow, though no explicit tsunamimechanism was offered.

Few of these geological reports above are explicitabout the precise mechanisms envisaged for tsunamideposition, though there are exceptions (e.g., Dypvikand Jansa, 2003). In the case of the K/T examples, forexample, some deposits are envisaged to be associatedwith a set of tsunami waves approaching the continentalshelf from deep water (e.g. Bourgeois et al., 1988;Alberto and Martins, 1996), while others relate tobasinward-directed backflows of tsunamigenic turbidsediment across the shelf (e.g. Bralower et al., 1998;Lawton et al., 2005). Clearly, a key question to beresolved by tsunami studies is how the marine

Table 1Generic diagnostic criteria of offshore palaeotsunami deposits, summarized from the following sources: (1) Ballance et al. (1981), (2) Takashimizuand Masuda (2000), (3) Massari and D'Alessandro (2000); (4) Cantalamessa and Di Celma (2005); (5) Le Roux and Vargas (2005); (6) Fujino et al.(2006); (7) D. R. Tappin (pers. comm.)

Characteristic Interpretation Source

Unusually coarse sediment compared with theoverlying and underlying deposit

i.e. The bed is an event horizon 1, 4

nusu

atesch, taates vrapidion bular iadationg pe

ositionespreahort-pal deparserositionamonant aositedsporte tsunid dep

ates r

178 A.G. Dawson, I. Stewart / Sedimentary Geology 200 (2007) 166183The bed includes many exotic fragments (e.g. plants, coconuts,beachrock, corals) from beach environment, which areabsent from the overlying and underlying deposits

An u

Admixture of clasts poorly sorted angular clasts mixedwith well-rounded beach pebbles and beach sands

Indic(bea

A liquefied zone below or in the lower part includesrip-up clasts, injection and deformation structures

Indicand

Irregular undulating erosional base, and flat-ish top(tempestites mainly have mostly sharp, flat-ish basesand irregular upper surfaces)

Erosirregaggr

Inversely-directed imbrications: palaeocurrents alternatebetween landward and seaward directions

A lo

Cross-stratification includes mud drapes DepBed geometry should be more sheet-like, (rather than thetypical pinch-and-swell (hummock-and-swale) of tempestites)

Widby s

Inverse- to normally-graded coarse-grained,clast-supported basal carpet

Initiof co

Condensed mud or organic bed in the upper part Deplarge

Scour-and-grading structure StagAntidune-like deposits (?) DepMultiple upward-fining units Tran

of thBioturbation is absent in the bed, although this iscommon in the overlying and underlying deposits

Rap

Goodexcellent preservation of fossils Indic

by later stal influx from a subaerial source 2, 6

erosion from shoreface and emergent coastal environmentslus slopes at cliff bases, nearby alluvial fans etc.)

4

ery high dynamic pressures from vibrationdeposition

2, 5

y strong currents in the early upflow stage results innfilling of a scoured substrate (rather than irregularn under combined-flow conditions typical of storms)

6

riod of oscillatory current reversals 2, 3, 6

from calm conditions during long-period oscillatory flows 2d runout (rather than substrate mobilisationeriod storm waves)

7

osition via basal traction flow followed by late-stage settlingmaterial from a laminar, inversely-graded debris-flow

4

from water with a high mud content and withunt of organic debris washed from the land

6

nd brisk flow velocities alternated repeatedly 6from relatively thin but high velocity backwash flows 7energy decreases with time during depositionamiite multiple units reflect successive waves

4

osition from strong currents 2

apid deposition and minimal reworking 6

orms, longshore currents and other processes

seawards. Rossetti et al. (2000) propose that high-am-plitude waves, enhanced by tsunami-induced ebb cur-rents, created repeated regularly spaced, very shallowswales (both symmetrical and assymetrical in profile)within Late Albian to Cenomanian upper shorefacedeposits in northern Brazil; soft-sediment deformationstructures in association with the scours are suggestiveof seismic shaking at the time of the erosion.

The tectonically mobile Chilean margin too appearsto preserve evidence for uplifted shallow-marine tsu-nami deposits. Hartley et al. (2001) (cf. Fig. 8) describeduplifted Pliocene submarine slide sediments at Hornitos(northern Chile) that includes a stratigraphic unit inter-preted as the product of deep water tsunami sedimenttransport (possibly by offshore tractive currents). A 710 m thick deposit nearby was considered in more detailby Cantalamessa and Di Celma (2005), who describe anarray of complex sedimentary signatures from shallow-

179entary Geology 200 (2007) 166183sedimentary expression of tsunamis travelling from thedeep ocean towards the coast differs from that producedas result of gravity flows triggered by tsunami backwashfrom the nearshore zone towards the deep ocean (Shikiand Yamazaki, 1996; Kiyokawa et al., 2002). As wediscuss in the following section, ancient nearshoredeposits attributed largely to earthquake tsunamis mayprovide some clues.

6.2. Tsunamites along tectonically active margins

In areas of active plate collision, long-term tectonicuplift may be of a scale sufficient to result in thepreservation of (uplifted) submarine slide and palaeotsu-nami deposit complexes. The seismically active coast ofJapan provides one such environment. Takashimizu andMasuda (2000), for example, describe a possibletsunamiite in a complex coastal sediment package in theUpper Pleistocene incised valley fill in central Japan. Thepackage comprises a lower part of seismically-shakenconvoluted and structureless sand, a middle part char-acterised by cross-laminations that indicate seaward andlandward palaeocurrent directions and so imply rapid ebband flow currents, and an upper part of suspendedsediment. Takashimizu and Masuda (2000) noted thateach of the sedimentary features of this package (Table 1)could on their own be variously interpreted, but whenfound in one set the most likely sedimentary mechanismwould be an earthquake-induced tsunami.

From a similar setting in northeastern Japan, butassociated with Lower Cretaceous gravels, Fujino et al.(2006) document a suite of sedimentary structureswhich, collectively, they argue provide sound evidencefor a tsunami origin. This array includes (1) inversely-directed imbrications and palaeocurrents consistent withseaward to landward current reversals, (2) abundantdeposition of beachrock and coral clasts that indicatehigh-energy erosion of the shoreface environment, (3)abundant organic debris in the form of a condensed bedwhich implies that the event washed onto land andsubsequently settled out; (4) exceptional preservation offossils due to rapid deposition with minimal reworking;and (5) scour and grading structures that impliesalternation between stagnant and abrupt flow velocities.

Comparable structures are described from elsewhere.Giant scour features are reported from Middle Pliocenerestricted-bay carbonate deposits in Salento, in thetectonically active heel of southern Italy (Massari andD'Alessandro, 2000). Here the wavy scours are up tomore than 50 m in wavelength, draped by graded andlaminated and locally trough cross-bedded calcarentites

A.G. Dawson, I. Stewart / Sedimwith evidence of flows directed both landwards andmarine Miocene conglomeratic beds in northern Chilewhich they argue can be explained by tsunami back-wash. This deposit appears to show many of the char-acteristics of tsunami beds, including extremely poorsorting, angular basement boulders, large (up to 10 m inlength) rip-up clasts and an erosional base. Here thetsunamigenic mechanism is unclear, though Cantala-messa and Di Celma (2005) argue for seismic dislo-cation offshore accompanied by an episode of suddencoastal uplift. Certainly other enigmatic MiocenePliocene tsunami beds described by Le Roux andVargas (2005) along this coast appear to have beenassociated with especially extreme seismic shaking,exhibiting sedimentary dykes and related structures

Fig. 8. Stratigraphic section showing inferred palaeotsunami depositsequivalent to La Portada Formation of western Chile within a

tectonically uplifted Pliocene submarine slide complex (after Hartleyet al., 2001).

entarindicative of violent fluid escape. However, Felton andCrook (2003) speculate that the Hornitos deposit ofHartley et al. (2001) may be the product of the Eltaninimpact (2.51 Ma) in the south Pacific (Kyte et al., 1988;Gersonde et al., 1997; Frederichs et al., 2002).

Cantalamessa and Di Celma (2005) argue that theexceptional preservation of these backflow tsunamideposits within a shallow-marine environment is likelyto be related to two concomitant factors: firstly, theunusual thickness of the material and the coarse size ofthe clasts foster their preservation, and secondly, theemplacement of these sediments within a restrictedbasin. Massari and D'Alessandro (2000) too contendsthat nearshore low-energy areas are sites where therecord of tsunamis may be preserved without loss,because they are efficient sediment traps where low-energy conditions inhibit subaqueous reworking.

Identifying a tsunami origin for deeper-water sedi-ments is clearly more problematic. As discussed earlier, ithas long been suspected that tsunamis can sweep debrisoff the land and onto submarine canyons, fromwhere theywould continue as turbidity currents and be discharged tooffshore basins (Coleman, 1968, 1978). Active collisionalmargins like those of Japan and Chile offer those idealconditions, but tsunami-related turbidites are not knownfrom these regions. An intriguing geological example,however, has been reported from the seismically activeaccretionary margin of New Zealand, where coconutshells in Miocene turbidites have been postulated to be alikely tsunami effect (Ballance et al., 1981). Althoughother high-energy processes can conceivably transportnormally buoyant littoral debris to such depths (Murty,1982), it seems likely that the presence of terrestrial debriswithin turbidity current sediments ought to represent a keycriterion by which one may recognize tsunami-inducedoffshore sediments produced as a result of offsurgetractive flow.

7. Conclusions

Despite the growing body of sedimentary evidencefor recent tsunamiites, geological research on tsunamideposits appears to have been largely characterised by alack of knowledge on the key physical processes ofsediment erosion, transport and deposition that takeplace when a tsunami strikes a coastline. To some extentthis no doubt reflects the paucity of studies of hydraulicsand sediment dynamics during modern tsunami events.Equally, in the small number of studies undertaken onsedimentary processes that have accompanied recenttsunami there have been no equivalent studies of con-

180 A.G. Dawson, I. Stewart / Sedimtemporaneous patterns of offshore sediment transportand deposition. Given the current attention being paid totsunami science, this will undoubtably change. Increas-ingly sophisticated tsunami models now offer a way tolink sediment deposition models to tsunami dynamicsand it is anticipated that the next few years will see thesenumerical studies shed new light on the likely scale andform of tsunamiites.

Overall, it remains unclear if the apparent absence ofonshore tsunami deposits in the rock record reflects poorpreservation or a lack of awareness that such features mayexist. In terms of onshore preservation potential, wesuggest here that boulder complexes deposited by tsunamimay represent the most promising area of scientificenquiry, with the expectation that such high-energydeposits ought to best retained in the rock record.However, it seems likely that the deposits with thegreatest preservation potential are those produced off-shore, principally as a result of tractive currents movinginto deepwater. The key research question here is whetherthe sedimentary signatures of these tsunami-relatedbottom currents can be distinguished from the turbidityand debris-flow sediment sequences that dominate thegeological literature. Tsunami science, it seems, needs torediscover the offshore realm.

Acknowledgements

We thank Peter Ballance and Dave Tappin for theirconstructive critical reviews of the manuscript.

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Tsunami deposits in the geological recordIntroductionGenerationEarthquake tsunamisSlide-generated tsunamisBolide impact tsunamis

PropagationRun-upBackwash and offshore tractionTsunamiites in the geological recordImpact-related tsunamiitesTsunamites along tectonically active margins

ConclusionsAcknowledgementsReferences