Sedimentologie articol

Tsunami-induced large-scale scour-and-®ll structuresin Late Albian to Cenomanian deposits of theGrajauÂ Basin, northern Brazil

DILCE DE FAÂ TIMA ROSSETTI*, ANA MARIA GOÂ ES , WERNER TRUCKENBRODT and JOSEÂ ANAISSE Jr *Museu Goeldi, Departamento de Ecologia, Av. MagalhaÄes Barata, 376, CP399, Cep 66040-170,BeleÂm, ParaÂ, Brazil (E-mail: [email protected]) Centro de GeocieÃncias, Universidade Federal do Para, Campus do GuamaÂ S/N, Cep 66075-900,BeleÂm, ParaÂ, Brazil

ABSTRACT

Late Albian to Cenomanian upper shoreface deposits from the GrajauÂ Basin,

northern Brazil, consist of well-sorted, very ®ne- to ®ne-grained sandstones

with swaley, trough, tabular and minor hummocky cross-strati®cation.

A striking feature of these deposits is the abundance of large-scale scour-and-

®ll structures, which consist of regularly spaced, repetitive, very shallow

swales with either symmetrical or asymmetrical pro®les, arranged along an

undulose surface or as a succession of superimposed troughs. The sediment

®lling these scours is characterized by very ®ne-grained sandstone with gently

undulose, near-parallel lamination to very low-angle dipping cross-

strati®cation intergraded with swaley and hummocky cross-strati®cation. The

nature of the scours and the sedimentary structures of their ®lls reveal

the action of combined ¯ows, which are hydrodynamically similar to those

developed during storms. However, it is speculated that the combined ¯ows

responsible for the genesis of these structures were formed by tsunami waves

enhanced by tsunami-induced ebb currents and/or tidal currents. This

interpretation is proposed on the basis of several lines of reasoning: (1)

palaeogeographic reconstructions of the study area during the late Cretaceous

show that it was outside the belt favourable for the development of storms; (2)

comparison of the scour-and-®ll structures with stratigraphically correlatable

deposits exposed north of the study area, where similar features occur in

association with abundant seismically induced, soft-sediment deformation

structures; and (3) the presence of several styles of soft-sediment deformation

features (i.e. convolute lamination, bed collapse, large-scale folds, massive

bedding, sand-®lled fractures and diastasis cracks) are suggestive of

synsedimentary seismic activity in Cretaceous deposits located in and near

to the study area. This study proposes that episodic, high-amplitude tsunami

waves, enhanced by tsunami-induced ebb currents, develop powerful ¯ows

capable of producing complex patterns of erosion and sedimentation, which

may be represented by scour-and-®ll structures similar to those described here.

Keywords Cretaceous, northern Brazil, scour-and-®ll structures, tsunamis.

INTRODUCTION

Flows with both a unidirectional and an oscilla-tory component (i.e. combined ¯ows) control

sedimentation patterns in many shallow marinesettings during storms (Dott & Bourgeois, 1982;McCrory & Walker, 1986; Duke et al., 1991; Cheel& Leckie, 1993). Modern continental shelf and

Sedimentology (2000) 47, 309±323

Ó 2000 International Association of Sedimentologists 309

nearshore sedimentary environments have beeninvestigated, and ¯ume experiments have beenconducted in order to understand the dynamicsof combined ¯ows (Bowen & Inman, 1969; Grant& Madsen, 1979; Brevik & Aas, 1980; Swift et al.,1983; Vincent et al., 1983; Davies et al., 1988;Arnott & Southard, 1990; Southard et al., 1990).These studies have led sedimentologists generallyto associate several styles of undulose lamina-tions documented in the sedimentary record (e.g.quasi-planar strati®cation, and hummocky andswaley cross-strati®cation) with combined ¯ows(Harms et al., 1975; Brenchley, 1985; Nùttvedt &Kreisa, 1987; Yagishita et al., 1992; Arnott, 1993;Cheel & Leckie, 1993). However, the characteris-tics and hydrodynamic interpretation of the sedi-mentary structures produced under combined¯ows are still the focus of ongoing discussion(e.g. McKie, 1994; Amos et al., 1996; Banerjee,1996; Rossetti, 1997a), with particular emphasisbeing placed on erosional structures (Myrow& Southard, 1996). The origin of combined¯ows in marine settings has commonly beenattributed to the interaction of oceanic wavesand unidirectional currents but, in areas affectedby earthquakes, high-amplitude tsunami wavesmay be associated with currents in order toproduce powerful combined ¯ows. Tsunami-induced ¯ows may reach suf®cient magnitude totransport large volumes of sediment (e.g. Shiki &Yamazaki, 1996), but the sedimentary recordresulting from this process has not yet beenwidely investigated.

This paper documents a type of scour-and-®llstructure exceptionally well preserved in Creta-ceous deposits of the GrajauÂ Basin, northernBrazil. This structure bears some resemblance toswaley cross-strati®cation (Dott & Bourgeois,1982; Leckie & Walker, 1982; Allen & Underhill,1989), because of the superimposed, shallowscours draped by low-angle dipping laminae thatthicken from the margins into the troughs. How-ever, the size and internal sedimentary features ofthese mega-swales suggest that the physicalprocesses responsible for their genesis acted ona much larger scale than those required for theorigin of ordinary swaley cross-strati®cation.Based on detailed sedimentological analysis,these large scour-and-®ll structures are proposedto be formed by strong bed erosion, followed bydeposition under the in¯uence of combined ¯owconditions probably induced by high-amplitudetsunami waves.

GEOLOGICAL SETTING

The GrajauÂ Basin (Fig. 1A and B) was formed bynorthward-dipping normal faults that developedduring the last stages of Gondwana break-up. Thebasin is bounded (Fig. 1B) by the XambioaÂ-Teresina Arch (south), the Rio ParnaõÂba Linea-ment (east) and the Tocantins-Araguaia Arch(west). From the late Albian onwards, this basinwas connected to the north with the SaÄo LuõÂs andBarreirinhas Basins. The GrajauÂ Basin was ini-tially ®lled during Aptian time by dominantlylacustrine black shales, anhydrites and lime-stones of the CodoÂ Formation, and ¯uvio-deltaicsandstones and conglomerates of the GrajauÂFormation (Mesner & Wooldridge, 1964; Aguiar,1971). These units overlay Palaeozoic sedimen-tary deposits of the ParnaõÂba Basin, as wellas metasedimentary and granitic rocks of theTocantins-Araguaia Belt and SaÄo LuõÂs Craton(Rodrigues et al., 1994). The CodoÂ and GrajauÂFormations are overlain by sandstones and minormudstones, traditionally referred to as Itapecuruand Ipixuna Formations, which were previouslyconsidered to be deposits of ¯uvial and lacustrineenvironments (Campbell et al., 1949; Franciscoet al., 1971). The age of these deposits is betweenAlbian and early Tertiary (?), based on theirstratigraphic relationship with the underlyingAptian CodoÂ Formation and the overlying Oligo-cene±Miocene Pirabas and Barreiras Formations.

Cretaceous deposits are well exposed along theCarajas±SaÄo LuõÂs railroad, 15 km north-east ofAcËailaÃndia, MaranhaÄo State (Fig. 1C). Althoughthe exact age of these rocks is unknown becauseof the lack of biostratigraphic data, a possible lateAlbian to Cenomanian age is inferred based oncorrelation with the AlcaÃntara Formation of theItapecuru Group in the adjacent SaÄo LuõÂs Basin(Rossetti & Truckenbrodt, 1997; Rossetti, 1997a).Facies analysis suggests that the deposits exposedin the study area record environments transition-al from ¯uvial to shallow marine (Fig. 2). Themarine in¯uence increases upwards in the sec-tions, suggesting a transgressive nature. Similarfacies have been observed in the correlatableAlcaÃntara Formation, which has been attributedto a transgressive incised valley ®ll (Rossetti,1996). The sedimentary attributes of individualfacies associations will be described brie¯y toprovide the palaeoenvironmental context for thelarge-scale scour-and-®ll structures that occur inthe shoreface deposits.

310 D. de F. Rossetti et al.

Ó 2000 International Association of Sedimentologists, Sedimentology, 47, 309±323

DESCRIPTION OF FACIESASSOCIATIONS

A total of four intergrading facies associations(A±D) in the late Albian to Cenomanian sectionsof the study area have been identi®ed and

interpreted as deposits of ¯uvial to nearshoremarine settings (Fig. 2; Table 1).

Facies association A consists of poorly sorted,®ne- to pebbly grained, kaolinized, feldspathicsandstones interbedded with minor conglomer-ates. The sandstone displays small- to large-scale,

Fig. 1. (A and B) Location map of the GrajauÂ Basin showing the main structural elements de®ning its margins. (C)Study area in north-eastern AcËailaÃndia, MaranhaÄo State (I±III locate the pro®les shown in Fig. 2).

Tsunami-induced large-scale scour-and-®ll structures 311


tabular and trough cross-strati®cation withnorth-east- to south-east-oriented azimuth dipangles (Fig. 3A). Thin (i.e. few centimetres thick),parallel-laminated strata inter®nger locally withthe cross-strati®ed beds. The deposits are arranged

into mainly ®ning- and thinning-upward cyclesup to 6 m thick, which are characteristicallymarked by erosional basal surfaces mantledby quartz pebbles. Facies association A gradeslocally into facies association D through an

Fig. 2. Representative lithostratigraphic pro®les of the Cretaceous deposits in the AcËailaÃndia area, with the inter-preted depositional environments and location of the scour-and-®ll structures discussed in the text (see Fig. 1 forlocation of sections I±III). A±D, facies associations.

Table 1. Summary of facies associations and interpreted depositional environments of late Albian to Cenomaniandeposits in the study area, near AcËailaÃndia.

Faciesassociation Description environment Depositional

A Poorly sorted, ®ne-sand to pebbly sandstone with tabular and troughcross-strati®cation, basal erosive surface with lags, and ®ning/thinningupward cycles

Bedload-dominated¯uvial channel

B Sharp-based, moderately sorted, ®ne- to medium-grained sandstone withbidirectional trough and tabular cross-strati®cation characterized byreactivation surfaces and mud drapes

Tidal channel/inlet

C Horizontally laminated, heterolithic beds with rootlets and lenses of rippleand climbing ripple cross-laminated sandstone

Lagoon complex

D Well-sorted, very ®ne- to ®ne-grained sandstone with large-scale scour-and-®ll structures that internally display intergrading undulose to low-angletruncation lamination, swaley, trough, tabular and hummocky cross-strati®cation and parallel to subhorizontal lamination. The trace fossilsOphiomorpha, Skolithos, Planolites and Thalassinoides are present in thisfacies association

Nearshore



interval of low-angle, sigmoidal cross-strati®edsandstones.

Facies association B consists of moderately towell-sorted, ®ne- to medium-grained, kaolinized,feldspathic sandstone beds with sharp basalsurfaces draped by lags of quartz pebbles andmud intraclasts. The sandstone displays eithersouth-east- or north-west-oriented (Fig. 3B)trough and tabular cross-strati®cation with abun-dant reactivation surfaces and mud drapes thatde®ne 2- to 5-cm-thick foreset packages. Setthickness ranges from a few centimetres up to5 m. Large-scale sets form compound cross-strat-i®cation characterized by superimposed, small- tomedium-scale cross-beds. Grain size and setthickness commonly decrease upwards.

Facies association C can be subdivided into: (a)heterolithic beds; and (b) mudstone with lenses ofvery ®ne-grained sandstone. The heterolithic bedsare up to 2 m thick and arranged into horizontal-laminated mudstone that alternates with siltstoneand very ®ne- to ®ne-grained sandstone. Mudstonelayers contain desiccation cracks. The uppermostpart of the heterolithic beds is massive andcontains rootlets. The mudstone facies eitherdisplays horizontal lamination or is massive.Bioturbation is locally so abundant that individualtrace fossils could not be identi®ed. Soft-sedimentdeformation structures are also very commonlocally. The sandstone lenses that occur withinthe mudstone are, in general, less than 0á5 m thickand grade into either ®ning- or coarsening-upwardsuccessions showing abundant trough and climb-ing cross-lamination. These lenses also showundulose to low-angle cross-lamination and

medium-scale, swaley cross-strati®cation; sym-metrical ripple marks are present locally.

Facies association D is characterized bywell-sorted, very ®ne- to ®ne-grained sandstonewith a variety of intergrading structures, includ-ing swaley, trough, tabular and minor hummockycross-strati®cation, as well as undulose to low-angle truncating lamination. These beds gradeinto ®ne- to medium-grained, subhorizontallylaminated sandstones, forming packages up to20 m thick. Trace fossils (e.g. Ophiomorpha,Skolithos, Planolites, Thalassinoides) are rarewithin these beds, although some sandstone beds,1±2 m thick, possess pervasive burrowing so thatthe original structure has been completelydestroyed. Palaeocurrent data from the swaleycross-sets indicate north-east-oriented palaeo-¯ows (Fig. 3C).

INTERPRETATION OF THEDEPOSITIONAL SETTING

A depositional setting ranging from ¯uvial to tideand shallow marine is proposed for the depositsof the study area. Within this depositional setting,four facies associations have been recognized.Facies association A is attributed to bedload-dominated channels based on the poorly sortedand coarse-grained deposits, their original felds-pathic (now kaolinitic) composition, internalarrangement in ®ning-upward cycles and basalerosional surfaces mantled by lags. The sigmoidalbeds that occur laterally between ¯uvial andmarine deposits are interpreted as deltaic lobes

Fig. 3. Palaeocurrent distribution for the Cretaceous deposits near AcËailaÃndia measured from (A) ¯uvial cross-strati®cation; (B) tidal cross-strati®cation; and (C) swaley cross-strati®cation.



that entered into the nearshore setting. The mainnorth-east/south-east orientation of ¯uvial cross-strata suggests a local, roughly north- to south-oriented palaeoshoreline.

Facies association B is attributed to a tidalsetting, as suggested by the trough and tabularcross-beds with abundant reactivation surfacesand mud drapes (e.g. Kreisa & Moiola, 1986;Leckie & Singh, 1991; Nio & Yang, 1991). Thesefeatures, together with the sharp base with lagsand the internal sedimentary architecture, suggesta tidal channel or inlet depositional setting. Thecompound cross-strati®cation with opposingcross-sets is attributed to the migration of large-scale bedforms within the channels during alter-nating dominant and subordinate tidal currents(e.g. Allen, 1980). The south-east and north-westorientation of these cross-sets reinforces theproposed N±S palaeoshoreline orientation, givena simpli®ed model in which the tidal currents arenormal to the coastline.

The genetic relationship of facies association Bwith facies association C suggests that the tidalchannels/inlets were probably surrounded bylagoonal deposits. The mudstone in facies asso-ciation C is interpreted as representing low-energy, lagoonal conditions, while the internalsandstone lenses probably formed as a result oftidal deltas and wash-over fans during stormevents (e.g. Carter et al., 1989; Murakoshi &Masuda, 1991). The associated laminated hetero-lithic deposits with desiccation cracks and rootmarks are interpreted as recording sedimentationon tidal ¯ats that surrounded the lagoon.

Facies association D corresponds to more openmarine conditions. The abundance of swaley andminor hummocky cross-strati®cation point to ahigh-energy, upper shoreface environment (e.g.Dott & Bourgeois, 1982; Walker, 1984; McCrory &Walker, 1986; Hadley & Elliot, 1993). Minordeposition under foreshore conditions is also

suggested by the occurrence of parallel to sub-horizontal laminated sandstones. The large-scalescour-and-®ll structure, which is typical ofthis facies association, is attributed to sea bederosion and subsequent deposition under severecombined ¯ow conditions, as shown in thefollowing discussion.

DESCRIPTION OF THE SCOUR-AND-FILL-STRUCTURE

A distinctive sedimentary feature of facies asso-ciation D is the large-scale scour-and-®ll struc-tures (Fig. 4), which form the main focus of thispaper. The scours occur either distributed alongan undulose surface or, more commonly, as asuccession of superimposed, intercutting troughs(Fig. 5A and B). The scoured surface is charac-terized by a series of regularly spaced, large-scaletroughs separated by intervening hummocks withsizes similar to the adjacent scours. Trough-to-trough or hummock-to-hummock distancesaverage 15±20 m, although scours up to 40 mwide were observed in several localities (Fig. 5A).The relief produced by the scours and hummocksalong the surface ranges from approximately 0á3to 1á2 m; thus, wavelength to height ratios are upto 50. The complete geometry of the scours couldnot be de®ned, as three-dimensional exposureswere not available. In the sections studied, thescours show either symmetrical or asymmetricalshapes. The latter is far more common, beingcharacterized by an abrupt side, dipping at anglesthat range from 20° up to 30°, and an opposite,more gently dipping (10±15°) side. Both the ¯oorof the scours and the top of the hummocks showsuperimposed smaller scale scours (Fig. 6), whichare either symmetrical or asymmetrical in shapeand have a wavelength and height averaging0á7 m and <0á1 m respectively. A striking feature

Fig. 4. Large-scale scour-and-®ll structures that occur near AcËailaÃndia, northern Brazil (dashed lines indicate theundulose surface at the base of these structures).



of the large-scale and superimposed, smallerasymmetrical scours is that, in both cases, thegently dipping sides dip in the same, north-eastward direction.

The sediment that ®lls the scours is well sortedand characterized by subrounded to well-roun-ded, very ®ne sand. The main sedimentarystructure consists of gently undulose, horizontalto very low-angle dipping laminae that typicallythicken into troughs and thin towards the hum-mocks (Fig. 5A and B). The undulose laminationclosely follows the morphology of the scours,giving the impression that internal laminae arevirtually continuous and parallel to the basalsurface. However, close observation reveals that,although individual laminae drape the hum-mocky topography conformably, they gentlyonlap against the basal troughs. The unduloselaminae are marked by a large number of verygently dipping erosional surfaces that truncatethe underlying lamination and are mantled bylaminae that are conformable to, or end against,such surfaces (see u in Fig. 7; Fig. 8). Thesetruncation surfaces eventually become more

continuous and undulose until they becomeparallel to bedding.

A striking feature of the sediment in®ll is itsinternal arrangement into cycles, each up to 1á5 mthick and consisting of undulose lamination thatgrades upwards into small- to medium-scale,nearly north-eastward dipping, low-angle cross-strati®cation. Some of these cross-sets have topsmarked by either symmetrical or asymmetricalripples (Fig. 9). Where cycles are absent, theundulating lamination forms amalgamated setsbounded by sharp surfaces.

The undulose laminae merge laterally intonested, small and medium-scale swaley, troughand minor, hummocky cross-strati®cation (Fig. 7).Palaeocurrent measurements from the swaley andtrough cross-strati®cation indicate a dominantnorth-eastward (i.e. offshore) ¯ow orientation(Fig. 3C). Occasionally, cosets of small- to me-dium-scale cross-strati®cation, with a number ofreactivation surfaces, mud drapes and/or mudchips, also inter®nger with the undulating lami-nae. These cross-sets indicate palaeocurrent ori-entation similar to that of the structures with

Fig. 5. Lateral sections with large-scale, scour-and-®ll structures showing (A) symmetrical to slightly asymmetricalscours distributed along an undulose surface (arrows); and (B) a superimposed intercutting relationship betweenscours and ®lls. Note the undulose lamination (u) of the sediment ®ll.

Fig. 6. Detail of the large-scalescours with superimposed, sym-metrical and asymmetrical smallerscale scours (arrows). See Fig. 5Afor location.



reactivation surfaces and mud drapes observed infacies association B.

COMBINED FLOW GENESIS OF THESCOUR-AND-FILL STRUCTURE

Disregarding the scale and internal ®ll, thesescour-and-®ll structures resemble swaley cross-strati®cation, which was formally de®ned by aseries of superimposed, shallow scours about 0á5±2á0 m wide and a few tens of centimetres deep(Leckie & Walker, 1982). Although the physicalprocesses responsible for generating swaley cross-

strati®cation are still debated (e.g. Nùttvedt &Kreisa, 1987; Allen & Underhill, 1989), thisstructure has generally been interpreted as avariety of hummocky cross-strati®cation (e.g.Leckie & Walker, 1982; Duke, 1985; McCrory &Walker, 1986) formed by migration of low-reliefbedforms under combined ¯ows in which theunidirectional component was more effectivethan the oscillatory component (e.g. Arnott,1992).

Several features, including the depositionalcontext, the nature of the scours and the variedfeatures of their sediment in®ll, strongly suggestthat the scour-and-®ll structures described here

Fig. 7. Superimposed large-scale, scour-and-®ll structures illustrating their internal features represented by undu-lose lamination (u) with several internal low-angle truncation surfaces (arrows), swaley (s) and hummocky cross-strati®cation. The photograph and its line drawing show details of hummocky cross-strati®cation (arrows above thehammer).

Fig. 8. Detail of a deposit ®lling alarge-scale scour, illustrating theundulose lamination with gentlydipping truncation surfaces(arrows). The dashed line at thelower portion of the photographmarks the base of the scour (hammeras scale on the left of thephotograph).



also had an origin related to combined ¯ows.Hence, the regular spacing, repetitive nature, co-existence of asymmetrical and symmetrical ge-ometries and intercutting relationships of thescours are attributed to the erosive effect of wavesenhanced by current action. However, despite theresemblance to swaley cross-strati®cation (Dott &Bourgeois, 1982; Leckie & Walker, 1982; Allen &Underhill, 1989), the scale of these scour-and-®llstructures up to 40 m wide suggests waves withmagnitudes several times greater than thosenecessary to form ordinary swaley cross-strati®-cation.

Many modern and ancient shallow marinedeposits record the enhancement of storm wavesby either local or storm-generated currents (e.g.Dott & Bourgeois, 1982; Nùttvedt & Kreisa, 1987;Snedden et al., 1988; Duke et al., 1991; Cheel &Leckie, 1993). However, as discussed in detail inthe next section, the combined ¯ows responsible

for the scour-and ®ll-structures described in thispaper are attributed to the interaction of currentsand high-amplitude tsunami waves. Despite thisproposed alternative mechanism, the scour to-pography is similar to that produced by storm (i.e.oceanic pressure gradient) ¯ows (Swift et al.,1983). Thus, the ¯ow hydrodynamics of tsuna-mi-induced scours may be understood by com-parison with storm-generated combined ¯ows. Atstorm peak, when strong erosion is promoted bythe effect of storm waves touching the bottom,their impact on the shoreface causes maximumbed shear stresses, resulting in frequent and deepbed erosion (Amos et al., 1996). For instance,storm-related erosion in the modern shoreface offFire Island (Long Island, New York, USA) hasresulted in sea-¯oor scouring in the range of 1±2 m (Kumar & Sanders, 1976). This is nearly thedepth of erosion recorded by the scours in thepresent study area. Hence, independent of

Fig. 9. Large-scale, nearly symmetrical scour-and-®ll structure with internal cycles of undulose lamination and low-angle, ripple cross-strati®cation. Note, in the uppermost line drawing, the undulose lamination displays a largenumber of very gently dipping truncation surfaces (t), which grade laterally into swaley cross-strati®cation (s). Thephotograph in the centre and the lowermost line drawing show details of the cycles with undulose lamination (u)and low-angle cross-strati®cation (rs). Note that individual cycles are marked by long, vertical arrows on the linedrawing. Note also that some of the cross-sets have tops marked by either symmetrical or slightly asymmetricalrippled morphology (short arrows in the line drawing).



the mechanism inducing the combined ¯ows,these large-scale scours are most probably attrib-utable to maximum erosion during periods ofstrong combined ¯ow conditions that are similarto the ¯ow patterns observed during storms. Thesuperimposed smaller scale scours developed onthe surfaces of the large scours record less intenseerosion, probably during waning ¯ows.

Combined ¯ows are further inferred fromseveral sedimentary features associated with thesediment ®ll of these scours in the GrajauÂ Basin.These structures may also be interpreted incomparison with combined ¯ows developed onmodern storm-dominated settings (e.g. Kumar &Sanders, 1976). Large volumes of sand erodedfrom the shoreface at the storm peak either go intosuspension or are carried as a dispersed massnear the bottom. After this period of maximum¯ow strength, sediment settles out from suspen-sion, resulting in aggradation and the formation ofgently undulose parallel lamination under upper-regime ¯ow conditions (e.g. Arnott & Southard,1990). Experimental work developed by theseauthors has shown that gently undulating parallellamination, which is a structure observed perva-sively in the study area, records the equilibriumcombined ¯ow bed con®guration developedunder sheet ¯ow when the oscillatory velocity islarger than the unidirectional velocity compo-nent. During waning ¯ow stages, various types oflow-angle cross-strati®cation form by migration oflow-relief bedforms (e.g. Nùttvedt & Kreisa, 1987;Arnott, 1993; Amos et al. 1996). In the study area,these structures are represented by laterallyintergrading swaley and minor hummockycross-beds, which, according to Nùttvedt & Kreisa(1987), re¯ect the complex interaction of oscilla-tory and unidirectional ¯ow components. Similarfeatures from sandstone beds of the Lower Creta-ceous Bootlegger Member in north-centralMontana, USA, have also been attributed tocombined ¯ows by Arnott (1993). The lateralpassage from undulose lamination to varioustypes of low-angle cross-strati®cation suggestsgradually reducing ¯ow velocities and aggrada-tion simultaneous with migration of low-reliefbedforms (Dott & Bourgeois, 1982; Nùttvedt &Kreisa, 1987). Low-angle, swaley and hummockycross-strati®cation are generated, in this order, byincreasing oscillatory velocity relative to theunidirectional velocity component (Arnott &Southard, 1990). If this scheme of bedformevolution is accepted, the abundance of swaleyand low-angle cross- strati®cation, added to thepaucity of hummocky cross-strati®cation, might

be attributed to the dominance of unidirectionalcomponent over oscillatory motion. The amal-gamated beds with undulose lamination, whicheventually lose the cross-bedded interval at thetop of their sets, formed as a result of eitherpronounced erosion that was suf®cient to destroythe underlying waning ¯ow cross-sets or main-tenance of high-energy ¯ow conditions betweenperiods of erosion.

The roughly north-eastward orientation of thelow-angle dipping and swaley cross-strati®cationassociated with the undulose lamination suggeststhat the unidirectional component of the com-bined ¯ow was directed offshore slightly obliqueto the interpreted north- to south-oriented palae-oshoreline. The second trend of south-east- tosouth-west-oriented cross-strati®cation associ-ated with the undulose lamination is attributedto reworking tidal currents. The tidal signature issuggested by the presence of abundant reactiva-tion surfaces and/or mud drapes.

Thesuccessionof superimposedscours, togetherwith internal cycles of undulose laminationand low-angle cross-strati®cation, implies thatthe scour-and-®ll structure was a product ofsuccessive pulses of combined ¯ow rather thanone speci®c event. The symmetrical ripple marksat the topmost cross-sets of some beds re¯ectperiods of prevailing orbital motion betweenindividual pulses.

DISCUSSION

Documentation of large-scale scours with ®llssimilar to those described here is rare in thesedimentological literature. Vigorous storm ¯owsare claimed to have produced large-scale com-pound swaley cross-strati®cation with superim-posed smaller scale cross-strati®cation in theChungo Member, Wapiabi Formation, Alberta,Canada (Arnott, 1992). These features, whichresemble the scour-and-®ll structures describedhere, were attributed to the migration of large-scale, symmetrical to slightly asymmetrical bed-forms with superimposed small-scale ripplesunder combined ¯ow conditions (Arnott, 1992).However, the exceptional preservation of thestructures in the AcËailaÃndia area has permittedmore complete analysis, which reveals theircombined ¯ow origin.

Similar scours attributed to storm processeshave been described by Cacchione et al. (1984),Cheel & Middleton (1993) and McKie (1994).The Silurian Whirlpool Sandstone of southern



Ontario, Canada (Cheel & Middleton, 1993) con-tains parabolic scours that share the followingcharacteristics with the scour-and-®ll structuresdetailed in the AcËailaÃndia area: (1) associationwith upper shoreface deposits; (2) sharp, wavysurfaces de®ned by repetitive, large-scale scourswith a relief of up to several decimetres; (3)occurrence of both symmetrical and asymmetricalshapes; and (4) symmetrical and asymmetricalsmaller scale scours superimposed on their sur-faces. Although the geometry of the scours in theWhirlpool Sandstone could easily be determinedin plan view, detailed analysis of the sedimentin®ll was not possible, and only one scourshowed a ®ll consisting of a basal lag of shaleoverlain by faintly horizontal lamination andunidirectional rippled cross-laminated sand-stone.

Features similar to the scour-and-®ll structuresof the study area are also known from both theLate Permian Brotherton Formation, UK (McKie,1994), and the Cretaceous deposits in the SaÄoLuõÂs Basin, northern Brazil (Rossetti, 1997a). Inthe ®rst case, the repetitive nature of the scourswas not reported. The sediment ®ll of the scoursin the Cretaceous deposits from the SaÄo LuõÂsBasin is structurally different, as it consists of avariety of large-scale, very low-angle dippingcross-sets formed by the interaction of storm¯ows with tidal currents. The present examplefrom the GrajauÂ Basin is distinctive comparedwith the others because of the arrangement of thescours as a succession of intercutting large-scaleswales and the occurrence of internal sedimen-tary structures forming cycles that are indicativeof successive combined ¯ow events.

Therefore, the sedimentological data presentedin this paper and those by Arnott (1992), Cheel &Middleton (1993), McKie (1994), and Rossetti(1997a) are mutually complementary. Together,they help to characterize and understand betterthe variable record of combined ¯ow structures.However, determining the mechanism thatinduced the development of the combined ¯owsresponsible for the large-scale scour-and-®llstructures of the AcËailaÃndia area is problematic.It is well known from observations in modernseas that combined ¯ows are commonly formedduring storms. In the study area, the palaeocur-rent patterns obtained from the combined ¯owstructures, oblique to the proposed palaeoshore-line, are consistent with geostrophic, wind-drivenstorm ¯ows in many modern and ancient settings(e.g. Nùttvedt & Kreisa, 1987; Snedden & Swift,1991; Winn, 1991; McKie, 1994). In addition,

depositional settings dominated by storms arenaturally characterized by successive pulses in-volving erosion and deposition, as recorded bythe scour-and-®ll structures of the AcËailaÃndiaarea. This information could suggest storms as theprimary mechanism inducing the development ofcombined ¯ows. However, several factors can beused to propose an alternative genesis for thesescour-and-®ll structures by tsunami-inducedcombined ¯ows, as discussed below.

The low palaeolatitudes (less than 10°) of thestudy area during the late Cretaceous (e.g. Scoteseet al., 1989) were not favourable to the develop-ment of vigorous oceanic storms, because hurri-canes and tropical summer storms are reported tooccur between 10° and 45°, and winter stormsoccur above 25° (Marsaglia & Klein, 1983; Duke,1985; Barron, 1989). Thus, an alternative mecha-nism probably induced the development of pow-erful combined ¯ows in this palaeoenvironment.Tsunami waves caused by earthquakes are wellknown as much more powerful events than stormwaves, and can be expected to cause an impact inthe sedimentation pattern even greater than thatresulting from storm processes. In addition, theapproach of tsunami waves towards the coast maygenerate return (ebb) currents, which may reachvelocities of several metres per second (Shiki &Yamazaki, 1996). These back-¯ow surges of tsu-namis might cause ¯ows characterized by com-bined motions, with an oscillatory componentproduced by the tsunami waves and a unidirec-tional component produced by the tsunami-in-duced ebb currents. Such ¯ows would be severalorders of magnitude more powerful than storm-induced ¯ows and can move all available grainsizes (Pickering et al., 1991).

Earthquakes are speculated as the possibletrigger mechanism for these high-energy com-bined ¯ow scour-and ®ll-structures because oftheir similarities to sedimentary features from thestratigraphically correlatable AlcaÃntara Forma-tion exposed in the eastern SaÄo LuõÂs Basin, wherecombined ¯ow-generated deposits occur inassociation with abundant seismic-induced struc-tures. A seismically induced mechanism generat-ing these features is supported by both theiroccurrence nearby a major strike-slip fault zonethat was active during the late Cretaceous, andtheir association with laterally continuous, highlydeformed intervals displaying faulted blocks,fractures and a large variety of soft-sedimentdeformed structures that occur between undis-turbed deposits (Rossetti, 1997b). In the studyarea, soft-sediment deformation features, including



convolute lamination, bed collapse, large-scalefolds, massive bedding, sand-®lled fractures,intraformational breccia and diastasis cracks sim-ilar to those described by Cowan & James (1992),were observed locally within the deposits withscour-and-®ll structures (Fig. 10). Similar fea-tures were also observed within stratigraphicallycorrelatable strata located only a few kilometresnorth-east of the study area. Thus, it is intriguingto invoke the possibility that tsunamis might havein¯uenced the sedimentation patterns aroundAcËailaÃndia as well, causing both soft-sedimentdeformation and large-scale scour-and ®ll-struc-tures.

It is proposed that dislocation of the sea ¯oorduring fault reactivation in the eastern SaÄo LuõÂsBasin would have produced powerful tsunamiwaves, which affected sedimentation patternsover a large area (i.e. hundreds of kilometres)from the earthquake epicentre. Studies in modernsettings suggest that activity on single strike-slipfaults may produce major earthquakes withinshort time periods that range from hours to weeksor decades to millenia (e.g. Crone et al., 1997).Thus, rather than an isolated wave, tsunamis

produced by earthquakes have several pulses,which result in episodic tsunami-induced ebbcurrents (Shiki & Yamazaki, 1996). Although notstudied in detail yet, such complex ¯ow patternsmay have tremendous impacts on sedimentation.Therefore, the sedimentary record of tsunami-induced ¯ows is expected to be characterized bysuccessive pulses of erosion and depositionwithin several time ranges, which may eventuallyresult in features similar to those described here.

Although not well documented yet, the cata-strophic impact of successive tsunami waves hasbeen recorded increasingly in several modern andancient shallow marine and coastal settings (e.g.Yeh et al., 1993; Bondevik et al., 1997). Conglom-erates and high-energy laminated sandstonescon®ned within erosional troughs have beenreported in tsunami-induced deposits of the ChitaPeninsula, central Japan (Shiki & Yamazaki,1996). The complex pattern of erosion and sedi-mentation recorded by the superimposed scour-and-®ll structures of the study area might havebeen formed under the in¯uence of tsunamiwaves travelling on shallow shoreface waters.The combined nature of the ¯ow is thought to

Fig. 10. Deformational structures observed in association with the scour-and-®ll structures of the study area, illus-trating (A) vertical to subvertical, sand-®lled diastasis cracks; (B) intraformational breccia; (C) convolute lamination;and (D) locally massive (¯uidized) bed (labelled m).



have been induced by the interaction of thesewaves with either tsunami-induced ebb currentsor local (e.g. tidal) currents. The tsunami hypo-thesis proposed here is consistent with the preser-vation of thick (up to 20 m) deposits dominatedby combined ¯ow-generated features, with littlereworking by tidal currents. The preferentialpreservation of the deposits with scour-and-®llstructures in this shallow water setting mighttherefore have resulted from the fact that sedi-mentation rates were too high to keep pace withtidal reworking. Violent ¯ows promoted by thepassage of tsunami waves along the upper shore-face would have generated very large quantities ofsuspended sediment, which were then accumu-lated during waning ¯ow stages. Although thetsunami hypothesis is appealing in explaining thepreservation of these large-scale structures, fur-ther studies are still required to demonstrate itsin¯uence in the study area fully.

CONCLUSION

Late Albian to Cenomanian upper shoreface stratafrom the GrajauÂ Basin, northern Brazil, displaylarge-scale, combined ¯ow-generated, scour-and-®ll structures represented by a series of regularlyspaced, shallow troughs distributed either alongan undulose surface or as a succession of super-imposed swales. Although these scour-and-®llstructures resemble swaley cross-strati®cation,the large scale of the scours, up to 40 m wide,and the complex sedimentary in®ll, consisting ofundulose horizontal to low-angle truncating lam-ination that intergrades with swaley, trough,tabular and hummocky cross-strati®cation, sug-gest much more vigorous and variable ¯owpatterns than those required to produce ordinaryswaley cross-strati®cation. The association ofsedimentary features points to the action ofcombined ¯ows. Strong sea bed erosion tookplace at maximum ¯ow strength and formed thelarge-scale scours, while sedimentation thatoccurred during the following waning ¯owsresulted in the ®lls with various types of high-energy undulose laminations (i.e. gently undu-lose horizontal to very low-angle lamination,swaley and hummocky cross-strati®cation). Thepalaeogeographic reconstruction in low latitudeareas, the association with different styles of soft-sediment deformation and the similarities ofthese scour-and-®ll structures to other sedimen-tary features formed in association with seismi-cally in¯uenced deposits from the SaÄo LuõÂs Basin

all point to an origin related to the interaction ofhigh-amplitude tsunami waves and tsunami-induced ebb currents, rather than to pressuregradient storm ¯ows.

ACKNOWLEDGEMENTS

The Federal University of Para (UFPA), theGoeldi Museum (MPEG) and the National Coun-cil of Scienti®c and Technological Development(CNPq) are acknowledged for providing ®nancialsupport for the ®eld work. We are also grateful forthe constructive thoughtful reviews by Dr RickCheel, Dr Prave and Dr Jim Best, who madesubstantial contributions to improving the manu-script.

REFERENCES

Aguiar, G.A. (1971) RevisaÄo geoloÂgica da Bacia paleozoÂica do

MaranhaÄo. Congresso Brasileiro de Geologia, 25 Anais, SaÄoPaulo, SBG, 3, 113±122.

Allen, J.R.L. (1980) Sand waves: a model of origin and internalstructure. Sedim. Geol., 26, 281±328.

Allen, P.A. and Underhill, J.R. (1989) Swaley cross-strati®ca-

tion produced by unidirectional ¯ows, Bencliff Grit (UpperJurassic), Dorset, UK. J. Geol. Soc. London, 146, 241±252.

Amos, C.L., Li, M.Z. and Choung, K.S. (1996) Storm-generated,

hummocky strati®cation on the outer-Scotian Shelf. Geo-Marine Lett., 16, 85±94.

Arnott, R.W.C. (1992) Ripple cross-strati®cation in swaley

cross-strati®ed sandstones of the Chungo Member, Mount

Yamnuska, Alberta. Can. J. Earth Sci., 29, 1802±1805.

Arnott, R.W.C. (1993) Quasi-planar-laminated sandstone bedsof the lower Cretaceous Bootlegger Member, north-central

Montana: evidence of combined-¯ow sedimentation.

J. Sedim. Res., 63, 488±494.Arnott, R.W.C. and Southard, F.J.B. (1990) Exploratory ¯ow-

duct experiments on combined ¯ow bed con®gurations, and

some implications for interpreting storm-event strati®ca-

tion. J. Sedim. Petrol., 60, 211±219.Banerjee, I. (1996) Populations, trends, and cycles in com-

bined-¯ow bedforms. J. Sedim. Res., 66, 868±874.

Barron, E.J. (1989) Severe storms during earth history. Geol.Soc. Am. Bull., 101, 601±612.

Bondevik, S., Svendsen, J.I. and Mangerud, J. (1997) Tsunami

sedimentary facies deposited by the Storegga tsunami in

shallow marine basins and coastal lakes, western Norway.Sedimentology, 44, 1115±1131.

Bowen, A.J. and Inman, D.L. (1969) Rip currents: laboratory

and ®eld observations. J. Geophys. Res., 74, 5479±5490.

Brenchley, P.J. (1985) Storm-in¯uenced sandstone beds. Mod.Geol., 9, 369±396.

Brevik, I. and Aas, B. (1980) Flume experiments on waves and

currents. I. rippled bed. Coast. Eng., 3, 149±177.

Cacchione, D.A., Drake, D.E., Grant, W.D. and Tate, G.B.(1984) Ripple scour depressions on the inner continental

shelf off central California. J. Sedim. Petrol. 54, 1280±

1291.



Campbell, D.F., Almeida, L.A. and Sivla, S.O. (1949) RelatoÂrioPreliminar sobre a Geologia da Bacia do MaranhaÄo. Boletimdo Conselho Nacional do PetroÂleo, Rio de Janeiro, 1, 159 pp.

Carter, R.W.G., Forbes, D.L., Jennings, S.C., Onford, J.D.,Shaw, J. and Taylor, R.B. (1989) Barrier and lagoon coast

evolution under differing relative sea-level regimes: exam-

ples from Ireland and Nova Scotia. Mar. Geol., 88, 221±242.

Cheel, R.J. and Leckie, D.A. (1993) Hummocky cross-strati®-cation. In: Sedimentological Review, 1 (Ed. V.P. Wright), pp.

103±122. Blackwell Scienti®c Publication, Cambridge.

Cheel, R.J. and Middleton, G.V. (1993) Directional scours on atransgressive surface: examples from the Silurian Whirlpool

Sandstone of southern Ontario, Canada. J. Sedim. Petrol.,63, 392±397.

Cowan, C.A. and James, N.P. (1992) Diastasis cracks:mechanically generated synaeresis-like cracks in Upper

Cambrian shallow water oolite and ribbon carbonates.

Sedimentology, 39, 1101±1118.

Crone, A.J., Machette, M.N. and Browman, J.R. (1997) Epi-sodic nature of earthquake activity in stable continental

regions revealed by palaeoseismicity studies of Australian

and North American Quaternary faults. Aust. J. Earth Sci.,44, 203±214.

Davies, A.G., Soulsby, R.L. and King, H.L. (1988) A numerical

model of the combined wave and current bottom boundary

layer. J. Geophys. Res., 93, 491±508.Dott, R.H. and Bourgeois, J. (1982) Hummocky strati®cation:

signi®cance of its variable bedding sequences. Bull. Geol.Soc. Am., 93, 663±680.

Duke, W.L. (1985) Hummocky cross-strati®cation, tropicalhurricanes, and intense winter storms. Sedimentology, 32,167±194.

Duke, W.L., Arnott, R.W.C. and Cheel, R.J. (1991) Shelfsandstones and hummocky cross strati®cation: new insights

on a stormy debate. Geology, 19, 625±628.

Francisco, B.H.R., Lowenstein, P.O., Silva, O.F. and Silva,G.G. (1971) ContribuicËaÄo aÁ geologia da folha de SaÄo LuõÂs(SA-23), no estado do ParaÂ. III Estratigra®a IV Recursos

Minerais. Boletim Do Museu Paraense EmõÂlio Goeldi, NovaSeÂrie, Geologia, 17, 1±12.

Grant, W.D. and Madsen, O.S. (1979) Combined wave andcurrent interaction with a rough bottom. J. Geophys. Res.,84, 1797±1808.

Hadley, D.F. and Elliot, T. (1993) The sequence-stratigraphic

signi®cance of erosive-based shoreface sequences in theCretaceous Mesaverde Group of northwestern Colorado. In:

Sequence Stratigraphy and Facies Associations (Eds H.W.

Posamentier, C.P. Summerhayes, B.W. Haq and G.P. Allen),Spec. Publ. Int. Assoc. Sediment., 18, 521±535.

Harms, J.C., Southard, J.B. and Walker, R.G. (1975) Deposi-tional Environments as Interpreted from Primary Sedimen-tary Structures and Strati®cation Sequences. SEPM ShortCourse 2, 161 pp.

Kreisa, R.D. and Moiola, R.J. (1986) Sigmoidal tidal bundles

and other tide-generated sedimentary structures of the

Curtis Formation, Utah. Geol. Soc. Am. Bull., 97, 381±387.Kumar, N. and Sanders, J.E. (1976) Characteristics of shore-

face storm deposits: modern and ancient examples. J. Sedim.Petrol., 46, 145±162.

Leckie, D.A. and Singh, C. (1991) Estuarine deposits of the

Albian Paddy Member (Peace River Formation), and low-

ermost Shaftesbury Formation, Alberta, Canada. J. Sedim.Petrol., 61, 825±849.

Leckie, D.A. and Walker, R.G. (1982) Storm- and tide-domi-

nated shorelines in Cretaceous Moosebar±Lower Gates

interval outcrop equivalents of Deep Basin gas top in

western Canada. Bull. Am. Assoc. Petrol. Geol., 66, 138±157.

McCrory, U.C.L. and Walker, R.G. (1986) A storm and tidally-

in¯uenced prograding shoreline-Upper Cretaceous MilkRiver Formation of southern Alberta, Canada. Sedimentol-ogy, 33, 47±60.

McKie, T. (1994) Geostrophic versus friction-dominated storm

¯ow: paleocurrent evidence from the Late Permian Broth-erton Formation, England. Sedim. Geol., 93, 73±84.

Marsaglia, K.M. and Klein, G. de V. (1983) The paleogeogra-

phy of Paleozoic and Mesozoic storm depositional systems.J. Geol., 91, 117±142.

Mesner, J.C. and Wooldridge, L.C.P. (1964) MaranhaÄo Paleo-

zoic basin and Cretaceous coastal basins, north Brazil. Bull.Am. Assoc. Petrol. Geol., 48, 1475±1512.

Murakoshi, N. and Masuda, F. (1991) A depositional model

for a ¯ood tidal delta and washover sands in the late

Pleistocene Paleo-Tokio Bay, Japan. In: Clastic Tidal Sedi-mentology (Eds G.E. Reinson, B.A. Zaitlin and R.A. Rah-mani). Mem. Can. Soc. Petrol. Geol., 16, 219±226.

Myrow, P.M. and Southard, J.B. (1996) Tempestite deposition.

J. Sedim. Res., 66, 875±887.Nio, S.D. and Yang, C. (1991) Diagnostic attributes of clastic

tidal deposits: a review. In: Clastic Tidal Sedimentology(Eds D.G. Smith, G.E. Reinson, B.A. Zaitlin and R.A. Rah-

mani). Mem. Can. Soc. Petrol. Geol., 16, 3±28.Nùttvedt, A. and Kreisa, R.D. (1987) Model for the combined-

¯ow origin of hummocky cross strati®cation. Geology, 15,357±361.

Pickering, K.T., Soh, W. and Taira, A. (1991) Scale of tsunami-generated sedimentary structures in deep water. J. Geol.Soc. London, 148, 211±214.

Rodrigues, T.L.M., Facilla, C.A.A., Camozatto, E. and VerõÂs-simo, L.S. (1994) Folha SB-23-X-A (Bacabal). ProgramaLevantamentos GeoloÂgicos do Brasil, CPRM, 124 pp.

Rossetti, D.F. (1996) Sequence stratigraphic signi®cance of

two estuarine valley ®lls: the upper ItapecuruÂ Formation inthe SaÄo LuõÂs Basin, northern Brazil. Acta GeoloÂgica Leop-oldensia, 19, 111±125.

Rossetti, D.F. (1997a) Internal architecture of mixed tide- and

storm-dominated deposits: an example from the AlcaÃntaraFormation, northeastern Brazil. Sedim. Geol., 114, 163±188.

Rossetti, D.F. (1997b) Seismic-induced disturbances in late

Cretaceous rocks of the SaÄo LuõÂs Basin, MarnhaÄo State,

Brazil. Boletim do Museu Paraense EmõÂlio Goeldi, SeÂrieCieÃncias da Terra, 9, 3±20.

Rossetti, D.F. and Truckenbrodt, W. (1997) RevisaÄo estra-

tigraÂ®ca para os depoÂsitos do Albiano-TerciaÂrio Inferior (?)na Bacia de SaÄo LuõÂs, MaranhaÄo. Boletim Do Museu Para-ense EmõÂlio Goeldi, SeÂrie CieÃncias Da Terra, 9, 31±43.

Scotese, C.R., Gahagan, L.M. and Larson, R.L. (1989) Plate

tectonic reconstructions of the Cretaceous and Cenozoicocean basins. In: Mesozoic and Cenozoic Plate Reconstruc-tions (Eds C.R. Scotese and W.W. Sager), pp. 22±48. Else-

vier, The Netherlands.

Shiki, T. and Yamazaki, T. (1996) Tsunami-induced con-glomerates in Miocene upper bathyal deposits, Chita Pen-

insula, central Japan. Sedim. Geol., 104, 175±188.

Snedden, J.W. and Swift, D.J.P. (1991) Is there evidence forgeostrophic currents preserved in the sedimentary record of

inner to middle-shelf deposits? Discussion. J. Sedim. Pet-rol., 61, 148±151.

Snedden, J.W., Nummedal, D. and Amos, A.F. (1988) Storm-and fair-weather-combined ¯ow on the central Texas con-

tinental shelf. J. Sedim. Petrol., 58, 580±595.



Southard, J.B., Lambie, J.M., Federico, D.C., Pile, H.T. and

Weidman, C.R. (1990) Experiments on bed con®gurations in

®ne sands under bidirectional purely oscillatory ¯ow and

the origin of hummocky cross-strati®cation. J. Sedim.Petrol., 60, 1±17.

Swift, D.J.P., Figueiredo, A.G., Freeland, G.L. and Ortel, G.F.(1983) Hummocky cross strati®cation and megaripples: a

geological double standard? J. Sedim. Petrol., 53, 1295±1317.Vincent, C.E., Young, R.A. and Swift, D.J.P. (1983) Sediment

transport on the Long Island shoreface, North American

Atlantic Shelf: role of waves and currents in shorefacemaintenance. Cont. Shelf Res., 2, 163±181.

Walker, R.G. (1984) Shelf and shallow marine sands. In: FaciesModels (Ed. R.G. Walker), Geosci. Can. Repr. Ser., 1, 141±170.

Winn, R.D. Jr (1991) Storm deposition in shallow marine

sand sheets: Wall Creek Member, Frontier Formation,

Powder River Basin, Wyoming. J. Sedim. Petrol., 61,86±101.

Yagishita, K., Arakawa, S. and Taira, A. (1992) Grain fabric of

hummocky and swaley cross-strati®cation. Sedim. Geol., 78,181±189.

Yeh, H., Imamura, F., Synolakis, C., Tsuji, Y., Liu, P. and Shi,S. (1993) The Flores Island tsunamis. Eos Trans. AGU, 74,369±373.

Manuscript received 15 April 1998;revision accepted 21 July 1999.



Sedimentologie articol

Documents

Transcript of Sedimentologie articol