Downstream changes of large-scale bedforms in turbidites around the Valencia channel mouth, north-west Mediterranean: implications for palaeoflow reconstruction

Downstream changes of large-scale bedforms in turbiditesaround the Valencia channel mouth, north-west Mediterranean:implications for palaeo¯ow reconstruction

S. A. MORRIS*1, N. H. KENYON , A. F. LIMONOVà and J. ALEXANDER§*Department of Earth Sciences, University of Wales, PO Box 914, Cardiff, CF1 3YE, UK1Present address: Badley Ashton and Associates Ltd, Winceby House, Winceby, Horncastle,Lincolnshire LN9 6PB, UK (E-mail: [email protected]) Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, UK(E-mail: [email protected])àFaculty of Geology, Moscow State University, Lenin Hills, 119899 Moscow, Russia(E-mail: main@¯uns.geol.msu.su)§School of Environmental Sciences, The University of East Anglia, Norwich, NR4 7TJ, UK(E-mail: J. [email protected])

ABSTRACT

Side-scan sonar, seismic and core data are used to identify mega-¯utes, transverse and

`V' shaped bedforms in turbidites around the Valencia channel mouth, north-west

Mediterranean. Long-range side-scan sonar data reveal a broad, curved, asymmetric,

channel, that widens and terminates downfan. The western channel bank near the

channel mouth has been partly eroded by turbidity currents that spilled out of the

channel. Transverse bedforms on the east of the channel ¯oor are interpreted as

antidunes and, if this interpretation is correct, they indicate that the ¯ow was

probably supercritical at least locally within the channel. Trains of mega-¯utes, are

incised into coarse-grained sediments of the channel ¯oor near the channel mouth.

The association of mega-¯utes and antidunes is thought to be diagnostic of channel±

lobe transitions on deep-sea fans. The mega-¯utes pass downfan into an area of streaks

that diverge at up to 45° and indicates ¯ow expansion from the channel mouth. About

75 km downfan from the channel mouth, deep-towed side-scan data record transverse

bedforms (interpreted as antidunes) passing downfan into an area covered by `V'

shaped bedforms with up¯ow pointing apices (named chevrons here). The chevrons

are commonly c. 200 m from limb to limb and c. 2 m in amplitude with ¯ow-parallel

wavelengths of c. 400 m. We propose that chevrons were formed by a strong, probably

supercritical (or near critical) turbidity current spreading from the channel mouth and

¯owing towards the Balearic Abyssal Plain. Thinning of the turbidity current,

resulting from ¯ow spreading would allow the Froude number to remain high up to

100 km from the channel mouth and could explain the observed reduction in

antidune wavelength.

INTRODUCTION

The Gulf of Valencia extends from the EbroMargin of eastern Spain to the Balearic AbyssalPlain. The Valencia sediment transport system

includes the Ebro Delta and the Valencia Canyon,channel and fan (Alonso et al., 1991; Fig. 1). TheValencia Canyon passes down-fan into the Va-lencia channel which widens onto the relatively¯at, lower parts of the fan that extend south-east

Sedimentology (1998), 45, 365±377

Ó 1998 International Association of Sedimentologists 365

towards the Balearic Abyssal Plain (Palanques &Maldonado, 1985; Palanques et al., 1994; Fig. 1).The fan is primarily supplied with sediment fromthe Ebro Margin of eastern Spain (Alonso et al.,1985; Fig. 1). Sediment may also have reached theValencia channel mouth area from the northernslopes of the Balearic Islands via the deeplyincised North Menorca Channel (Fig. 1). Currentsfrom the RhoÃne Fan may have entered the lowerpart of the Valencia Channel (Maldonado et al.,1985; Fig. 1).

Palanques et al. (1994) used seismic data toidentify phases of fan progradation and retreatresulting from changes in sediment supply, whichthey relate to sea-level changes: periods of sealevel rise caused sites of sediment accumulationto migrate up-fan, and sea level fall resulted inprogradation towards the Balearic Abyssal Plain.Palanques et al. (1996) used side-scan sonar andseismic data to interpret zones of erosional anddepositional bedforms in the channel-lobe transi-tion area. In this paper we present new data andpropose a different interpretation of bed featuresand behaviour of turbidity currents ¯owingthrough the Valencia channel mouth towards theBalearic Abyssal Plain. In addition, we commentupon how these kinds of data can be used toenhance models of turbidity current behaviour.

Geophysical data were obtained from the sec-ond (1992) and fourth (1994) cruises of theRussian R/V Gelendzhik as part of the UNESCO/ESF/Training through Research programme. Thedata are from a long-range side-scan sonar survey(OKEAN) over the channel mouth area, whichwas recorded simultaneously with multi-channelseismic pro®les. Additionally, three deep-towedhigher resolution side-scan sonar (MAK-1) lineswere recorded: two across the channel mouth andone 75 km south-east of the channel mouth(Fig. 2). Side-scan sonar has been used extensive-ly to study deep-sea fans (cf. Alonso et al., 1985;O'Connell et al., 1985; Thornburg et al., 1990).The technique and method of interpretation aredescribed in detail by Belderson et al. (1971) andThornburg et al. (1990). In contrast to GLORIAand TOBI records, black tones on OKEAN andMAK-1 sonargraphs represent high acoustic back-scatter caused by coarser grained sediment (orrough surfaces) and white tones represent lowacoustic backscatter caused by ®ne sediments (orsmooth surfaces). The aspect of the bed featuresrelative to the ships track also strongly in¯uencesthe resulting image (slopes towards the ships,track appearing darker). In addition to geophys-ical data, gravity cores were taken along MAK-1lines for ground-truthing of acoustic facies.

Fig. 1. Location map of the Valencia Fan and the RhoÃne Fan in the north-west Mediterranean Sea. Water depths inmetres. Redrawn from Palanques et al. (1994).

366 S. A. Morris et al.

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 365±377

MORPHOLOGY AND BEDFORMDISTRIBUTION

The channel mouth

The seismic line PS-151 crosses the North Meno-rca Channel and the Valencia channel mouth(relevant part shown in Fig. 3a), and shows thatto the west of the channel mouth the sea ¯oorrises to the eastern levee of the adjacent NorthMenorca Channel. The western bank and ¯oor ofthe Valencia channel mouth appear to haveaggraded together and do not show the large-scale, multiple cut and ®ll sequences of the upperchannel that were observed by Palanques et al.(1994). Features in the uppermost sedimentscannot be resolved on the seismic pro®le as thescale of channel erosion at a given time is toosmall to resolve in the overall aggradationalcharacter of the lower fan.

The western channel bank was observed 15 kmnorth-west of seismic line PS-151 with the highresolution sub-bottom pro®le of MAK-1 line 2(Fig. 2). The channel ¯oor is asymmetric inpro®le, up to 25 m deep relative to the westchannel bank and deepest at the western side.The ¯at channel ¯oor rises steadily to the east

with a slope of 0á17°, and an eastern channel bankcan not be de®ned exactly from seismic or side-scan data. The slope of the western bank is 0á57°,and has three or more ledges with evidence oferosional truncation of ¯at, horizontal, sub-bot-tom re¯ectors (Fig. 3b). The ledges correspond tolinear ®elds of bedforms on the surface scan,similar to those in Fig. 5a. The sub-bottom pen-etration is up to 30 m beneath the slope of thewestern bank, but only as much as 4 m beneaththe channel ¯oor (Fig. 3b). In plan, the channelmouth bends to the east (Fig. 4) and appears as atrumpet shape, widening and ¯attening downfan.

Bedform ®elds

The OKEAN survey shows distinct bed featuresgrouped into ®elds of similar character, giving thegeneral impression of features radiating from thechannel mouth and extending south-east towardsthe Balearic Abyssal Plain. An interpretation ofthe OKEAN data is presented in Fig. 4a andcopies of the original OKEAN mosaic can beobtained from the authors. The sea ¯oor to thewest of the channel mouth (area A on Fig. 4a) is¯at and featureless with only minor variations inbackscatter resulting from isolated salt dome

Fig. 2. Bathymetry in metres around the lower Valencia Fan. Location of OKEAN and MAK-1 surveys and subse-quent ®gures.

Large-scale turbidite bedforms around the Valencia channel mouth 367


topography which is recorded also on seismicpro®les. The western channel bank is observed asa curved lineation (B on Fig. 4a) dividing therelatively featureless area from the streaks on thechannel ¯oor (area C in Fig. 4a). The lineationmarking the channel bank becomes weaker to-wards the south-east and appears to be overprint-ed by faint streaks at an angle of 30° to the bank(Fig. 4a). The eastern channel bank was notcovered by the OKEAN survey. Just beyond theeast of the channel is an area of large, irregular,angular blotches with a featureless area furthereast (D on Fig. 4a). The sea bed within thechannel mouth is covered by short streaks ofcontrasting high and low acoustic backscatter(Palanques et al., 1996; C on Fig. 4a). These passdownfan into a ®eld of long, narrow divergingstreaks of contrasting high and low backscatter(area E in Fig. 4a). The ®eld of streaks extends

75 km from the channel mouth and passes into a®eld of `V' shaped features, here termed chevrons,that are just resolved on the OKEAN sonargraphs(area F in Fig. 4). The mean bed slope varies verylittle from the channel mouth to the chevron ®eld.Downfan of the chevron ®eld, no bed features arevisible on the OKEAN data (area G on Fig. 4a).The position of the North Menorca Channel isclear from seismic line PS-151 (Fig. 3a), althoughno bedforms are observed on the OKEAN dataover the associated area.

Features on the channel ¯oor

There are a number of different types of regularbedforms on the channel ¯oor that can be dividedinto zones where one type of bedform is dominant.The western side of the channel has areas of largescours inter-spaced with smooth, unscoured

Fig. 3. Channel mouth morphol-ogy. (A) Multichannel seismicre¯ection pro®le across thechannel mouth. Palaeo¯ow to-wards observer. North MenorcaChannel, western Valenciachannel levee, multiple leveeaccumulations and westernchannel ¯oor are shown. Re¯ec-tors indicate that sediment onthe channel ¯oor and levees ag-graded simultaneously. One saltdome is truncated at the sea¯oor. Pl-Q � Plio-Quaternarysediments; ME � Messinianevaporites. (B) 5 kHz sub-bottompro®le across the lower westernchannel bank and the deepestpart of the channel ¯oor. Deepacoustic penetration (up to 30 m)beneath the lower bank reveals¯at re¯ectors indicating domi-nantly muddy sediments. Inter-nal re¯ectors are truncated at thebank by erosion. Penetration be-neath the channel ¯oor is onlyabout 4 m.



Fig. 4. (A) Interpretation of bedform ®elds from OKEAN (9á5 kHz) survey. The area is divided into seven ®elds (A±G): A. area to the west ofthe western channel bank; B. west channel bank; C. channel ¯oor; D. area to the east of the channel; E. diverging streak bedform ®eld,down¯ow of the channel mouth; F. chevron bedform ®eld; G. no bedforms. Gravity core localities are mainly concentrated along MAK-1lines for purpose of ground-truthing. Interpretations of palaeo¯ow conditions are included. (B) Part of the OKEAN survey around thechannel mouth. The western channel bank is visible as an area of high backscatter (dark tones) and streaks on the channel ¯oor diverge frominside the channel mouth.

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areas. Figure 5a shows part of a scour ®eld nearthe western bank with up to 8 m of erosion belowthe surrounding smooth areas (part of smooth areashown to SSW in Fig. 5a). The large scours andparticularly the scour edges facing towards theship's track produce higher acoustic backscatter(darker tones on side-scan sonar records). A higherresolution MAK-1 pro®le across an area c. 5 km tothe east (Fig. 5b) shows similar scours are `spoon-shaped' mega-¯utes that are up to 80 m across.The mega-¯utes are arranged in linear `trains'. Thesub-bottom pro®le reveals eroded and truncatedinternal re¯ectors in the walls of the scours.

On the eastern channel ¯oor fewer erosionalbedforms are observed (Fig. 5c). Trains of trans-verse, straight-crested bedforms (dark tones onsonargraphs) are present on a sedimentary surfacewith low acoustic backscatter (white on sonar-graphs). These bedforms commonly have crestlengths of up to 600 m and wavelengths of 80 m,though the amplitude is not resolvable on theavailable data.

Bedforms down¯ow of the channel mouth

The OKEAN data show streaks of varying acousticbackscatter intensity, with a regular spacing,

diverging downfan from the channel mouth (areaE in Fig. 4a). The streaks are up to 15 km inlength with spacing of 2 km. They are interpretedas bedforms and apparently record palaeo¯owdirections of currents spreading from the channelmouth.

Downfan from of the streak bedforms, the areaof chevron bedforms (resolved on the OKEANdata) were also imaged using deep-towed side-scan sonar. At the upfan end of the chevronbedform area, the sea ¯oor is smooth and coveredby a c. 1 km2 ®eld of transverse bedforms, withindividual crest-lengths as much as 100 m andwavelengths of 70 m (Fig. 6a). Towards the south-east, these bedforms develop into conjugate, en-echelon trains forming `V' shaped patterns. Thetransverse bedforms become less distinct down-fan and the V shaped patterns more pronounced,passing into fully developed chevrons. These are`V' shaped bedforms with apices facing upfan andare up to 200 m across from limb ends, with limbseparation angles that are close to 45° (Fig. 6b).These chevrons are up to 2 m in amplitude with awavelength of around 400 m. The chevrons con-sistently display sharp upfan facing edges and`blurred' downfan edges; their high backscatter-ing pattern suggests that they are individual,

Fig. 5A. Bedforms on the channel ¯oor. (A) A deep scoured area on western side of channel ¯oor revealed by 30 kHzMAK-1 side-scan sonar. High acoustic backscatter is shown by darker tones. The corresponding 5 kHz sub-bottompro®le shows scours of vertical magnitude up to 8 m. The scour area is bordered by ¯atter sea ¯oor with low acousticbackscatter.



relatively coarse-grade sediment bodies encasedby mud and the sub-bottom pro®le shows thatthey may be buried to a depth of 1 m (Fig. 6b). Agravity core through the limb of a chevron (core152G; Fig. 4) suggests that these bedforms arecomposed of pteropod fragments and sand (A.Akhmetjanov, personal communication).

Within the chevron ®eld, a linear feature wasresolved by deep-towed side-scan sonar (Fig. 6b).The line is a small fault escarpment and is shownin the corresponding sub-bottom pro®le (lowerpart of Fig. 6b). The fault throws c.1á1 m down tothe south-west and creates an escarpment facing

upfan. The chevrons upfan of the fault terminateabruptly against it, and re-establish downfan,beyond a smooth area with no bedforms. Thefacing direction of the chevrons and chevronspacing and size remains constant across the faultscarp. Morris (1998) describe the sea ¯oor featuresin the area around the fault and discuss thecauses of variations.

Sediment cores

There was poor recovery of gravity cores from thechannel ¯oor and downfan of the channel mouth

Fig. 5B,C. Bedforms on the channel¯oor. (B) Higher resolution image ofpart of the scour area on westernchannel ¯oor from 100 kHz MAK-1side-scan sonar. A linear ®eld ofmega-¯utes is shown by the surfacescan bordered by ¯at, featurelessareas. The corresponding 5 kHz sub-bottom pro®le shows deeper acous-tic penetration than that in Fig. 3B,as a result of the side-scan ®sh beingtowed closer to the sea ¯oor. Part ofthe mega-¯utes correspond to ero-sion which truncates ¯at, internal,sub-bottom re¯ectors. (C) Straight-crested sediment waves, orientatedtransverse to ¯ow, with a wave-length of 80 m on the channel ¯oorto the east of the scoured zone.



which may have been caused by the presence ofimpenetrable, near surface, sand or gravel layers(Fig. 4). Core 157G, from the middle of thechannel mouth, recovered a 70 cm thick sandyturbidite covered by 20 cm of pteropod rich mudand other, thinner, sandy turbidites. The ptero-pod shells have been dated as Holocene (Bella-iche & Thiriot-Quieveux, 1982). Cores from areasF and G recovered small amounts of mud andonly Core 152G contains a sandy turbidite. Core155G, from the east side of the channel ¯oor,consists of 5 m of mud with two, thin silty layers.Core 158G on the channel ¯oor, near to the base of

the west bank, recovered 1 m of mud with only athin silt layer.

SEDIMENT DISTRIBUTION

The seismic pro®le PS-151 shows that the chan-nel mouth and lower fan have aggraded verticallythrough time and have not stacked laterally withcut-and-®ll events, as in the upper parts of thechannel. The 5 kHz sub-bottom pro®le across thechannel bank (Fig. 3b) shows this earlier periodof aggradation with well-bedded, possibly muddy

Fig. 6. Bedforms down¯ow of thechannel mouth, recorded by a30 kHz MAK-1 side-scan sonar sur-vey. (A) 1 km square ®eld of large,transverse bedforms which passdown¯ow into V-shaped trains ofen-echelon transverse bedforms andthen into chevron bedforms. (B)Fully developed chevron bedforms.The chevrons terminate abruptlyagainst a line across the surfacescan. The 5 kHz sub-bottom pro®leshows the line as a scarp producedby faulting, related to sub-surfacesalt movements (Morris, 1998). Thechevrons re-establish down¯ow ofthe fault, beyond an area with nobedforms. The corresponding sub-bottom pro®le shows deep penetra-tion with ¯at re¯ectors, indicating amud dominant area. The acousti-cally transparent horizon above 8 msub-surface is believed to be a sev-eral metre-thick `homogenite' at agreater depth than the chevronbedforms.



overbank sediments, allowing deep acoustic pen-etration. In contrast, the channel ¯oor is com-posed of coarser sediment with very poor acousticpenetration and weak sub-bottom re¯ectors. Sub-bottom penetration increases from west to eastacross the channel ¯oor (along with a change tomainly non-erosive bedforms) and may representa bulk change to ®ner sediment. The few sedi-ment cores recorded mud above any sand layerson the channel ¯oor and in the chevron bedformarea that accumulated after the formation of thebedforms. The mud is thicker in the lower fanarea (overlying the chevrons).

The transverse bedforms (Fig. 5c) are isolatedsediment waves, and the sidescan image can beexplained by coarse grade sediment ridges sepa-rated by areas of ®ner grained sediment. Thebedforms are non-erosive as there are no scours.The 5 kHz sub-bottom pro®le in the area ofchevrons (Fig. 6b) shows deep acoustic penetra-tion and ¯at re¯ectors indicating dominantlymuddy sediments. The chevrons are buried upto 1 m and are isolated coarse sediment bodies,encased in mud.

DISCUSSION

The data presented here record the passage of oneor more powerful turbidity currents. The bed-forms of the most recent powerful turbiditycurrent have been buried by ®ne grained sedi-ment that thickens away from the channel mouth.Millot and Monaco (1984) described strong, deepcurrents in this area of the Mediterranean thatmay explain in part the mud distribution but cannot explain the scale or orientation of the largebedforms. The distribution of the mud may alsobe explained by low powered turbidity currents¯owing down the channel and remobilizing ®nesediment. The biostratigraphic interpretations ofthe sediment taken in gravity cores implies thatthe youngest sandy turbidites are of late Pleisto-cene±Holocene age (Limonov et al., 1995). It ispossible that the thin silty layers in cores 155Gand 158G were deposited from the dilute mixingclouds associated with the more powerful, basalcomponents of currents in the deeper part of thechannel mouth. However, biostratigraphic reso-lution is not high enough to allow direct correla-tion of turbidites from the channel ¯oor with thethinner, ®ner-grained layers on the channelbanks. The change from sand-transporting turbid-ity currents to mud-dominated sedimentationmay represent the sedimentary response of the

Valencia system to the last sea-level rise (cf. Kolla& Macurda, 1988; Palanques et al., 1994). Thewell-bedded sediment beneath the channel bankssuggests an earlier stage of sediment accumula-tion preceded the turbidity currents which erod-ed the western channel mouth. The exact age ofthe channel bank sediments and the turbiditebedforms are as yet unknown.

Turbidity currents passing through the channelmouth eroded and overtopped the western chan-nel bank, and overprinted streaks on the channelmargin to the west of the channel lineation(OKEAN survey, Fig. 4a). Bedforms indicate lat-eral variability in ¯ow conditions across thechannel mouth. The asymmetric shape of thechannel, and the non-uniform distribution ofbedforms and sediment on the channel ¯oorsuggest that the current was strongest towardsthe western bank on the concave side of thechannel bend. Erosion is more pronouced on thewestern side of the channel ¯oor (and the westernchannel bank) and arranged in large, elongatedareas, that may correspond to streaks of erosiveturbulence within the ¯ow(s) (cf. Karcz, 1970;McLean, 1981). Strong divergence of the streakson the OKEAN survey suggest that the currentsstarted to expand up¯ow of the channel mouthand continued to expand towards the BalearicAbyssal Plain.

The mega-¯utes on the ¯oor of the channelmouth were formed by powerful turbidity cur-rents. They are similar to the ®eld of large scoursmapped in the Bering Sea (Kenyon & Millington,1995) that are concentrated on the right-handside of a channel, where it starts to expand intoa trumpet-shaped mouth. They are also similarto features observed on the Navy Fan (Normarket al., 1979), to giant ¯utes formed by the GrandBanks turbidity current (Shor et al., 1990) and togiant slates on the Rhone Fan (Kenyon et al.,1995). Mega-¯utes are believed to be caused byprocesses controlled by ¯ow expansion or sec-ondary ¯ow (Normark & Piper, 1991), althoughthey may not be restricted to channel-lobetransition areas of deep-sea fans. The generationof erosive turbulence in ¯ows expanding throughthe Valencia channel mouth may have beenaccentuated by intensi®ed mixing at the back ofthe head of the expanding current (cf. Alexander& Morris, 1994).

Deep-sea channel mouths are important areasfor coarse sediment deposition, but they will bedif®cult to recognize in outcrop due to their scale(Valencia channel mouth is c. 20 km wide). Therecognition of channel-lobe transition areas in



ancient, high ef®ciency deep-sea fans may onlybe possible from diagnostic patterns of bedforms.This study con®rms the ®ndings of Vicente Bravoand Robles (1995), that mega-¯utes, if they can berecognized, may be used to help identi®cation ofchannel-lobe transition areas in the rock record.Vicente Bravo and Robles (1995) described mega-¯utes in the Black Flysch, Spain which theyattributed to a channel-lobe transition. If thetransverse and chevron bedforms could be recog-nized in ancient turbidites they may also be of usein identifying channel-lobe transition deposits.Little is known of the internal character of eitherthe transverse or chevron bedforms so at presentthey cannot be reliably identi®ed in the rockrecord and consequently cannot be used asdiagnostic features. It is also possible that theyare transient features that may have poor preser-vation potential.

Transverse, long wavelength, straight-crestedbedforms are preserved on the eastern side of thechannel ¯oor and in the area between the streakand chevron ®elds. The low amplitude makestheir interpretation dif®cult, but one possibleexplanation of the MAK-1 image is that thesebedforms are coarse sediment ridges separated by®ne sediment. If this is the case then they arepossibly in an area that was relatively starved ofcoarse sediment. Their location, postulatedcoarse sediment composition and associationwith mega-¯utes (if the same age) suggest thatthey formed in a strong current. Similar scalebedforms have been identi®ed on the lower slopearea of the modern Noeick fan delta, BritishColumbia, where they are also seen in associa-tion with mega-¯utes (Bornhold & Prior, 1990)and are interpreted there as antidunes. Normarket al. (1980) attempted to calculate turbiditycurrent ¯ow characteristics from similar bed-forms interpreted as antidunes on the backside oflevees on the Monterey deep sea fan using therelationship

V � gkDq2pq

where V is the mean ¯ow velocity, k is thebedform wavelength, q is the water density, Dq isthe excess density of the turbidity current, and gis the acceleration due to gravity, assuming thatthe turbidity current was ¯owing under a station-ary upper water layer. Assuming the water phasein the turbidity current has the same density asthe ambient water (which may not be a realisticassumption in many cases according to Rimoldi

et al., 1996) and that the speci®c gravity ofsediment particles approximates to 2á6, then theexcess density (Dq) is c.1á6C where C is the non-dimensional sediment concentration (Normarket al., 1980) and thus

V � 1�6gkC1=2

2p

Therefore the wave length of antidunes is depen-dent on sediment concentration and currentvelocity, neither of which is known for theValencia turbidity currents.

Antidunes generally form in ¯ows with Froudenumbers (Fr) around 1 (greater than 0á84; seediscussion in Alexander & Fielding, 1997). Thebulk Froude number for a turbidity current isgiven by the equation

Fr � V=�ghDq=q�1=2

where h is the ¯ow thickness. Sediment concen-tration can be substituted for density difference inthe same way as above, assuming (probablyunrealistically) thorough mixing in the current.The wavelength of antidunes therefore is con-trolled by the ¯ow velocity, sediment concentra-tion and ¯ow thickness.

Transverse bedforms in the Valencia channelmouth (Fig. 5c) have longer wavelengths com-pared with those near the chevrons (Fig. 6a). Ifthese transverse bedforms are antidunes, then thedownfan reduction in wavelength may be causedby decreased velocity, increased sediment con-centration or decreased ¯ow thickness. There isevidence of sediment entrainment (mega-¯utes)but it is considered that this is unlikely to besuf®cient on its own to account for the decreasedwavelength near the chevrons (the concentrationwould have to be 1á14 times greater, assumingeverything else stays constant). The currentwould only need to be 1á07 times faster in thechannel than over the streak-to-chevron transi-tion to explain the change in antidune wave-lengths (with constant concentration). The ab-sence of antidunes on the western side of thechannel may be explained by the greater depth ofthe channel (and therefore ¯ow thickness) andconsequently lower Froude number.

If the antidunes formed in the channel whenthe streaks were formed crossing the lower mostwestern bank (Fig. 4), then the ¯ow may havebeen more than channel full just upstream of thepoint where the streaks overprint the channeledge. The depth of the channel at this point is



c. 25 m below the area to the west of the channel,and the current depth over the antidune ®eld wasc. 12á7 m (Normark et al., 1980; Eq. 6). Assuminga horizontal upper surface of the ¯ow, the currentdepth in the deepest part of the channel wouldhave been 37 m. Making these bold assumptions,the depth of the ¯ow over the streak-to-chevrontransition zone would have been 11 m, if therewas no change in velocity or sediment concen-tration. It is unrealistic to suggest that a change inonly sediment concentration or velocity or depthwould have been responsible for the change inwavelength as these factors in a turbidity currentare dependent on each other and the valuescalculated are therefore maximum values.

Chevrons have not been described from deepsea fans before. We suggest that the chevronswere deposited from bedload traction in regionsof reduced bed shear stress beneath stationarywaves in a turbidity current. The chevrons aresimilar in morphology to very much smaller-scale`rhomboid ripples' seen on beaches (Hoyt &Henry, 1963; Otvos, 1965; Strauffer et al., 1976)and produced experimentally by supercritical¯ows (Karcz & Kersey, 1980). By analogy weconsider that the chevrons may be the result ofdeposition under supercritical (or near critical)¯ow conditions, maintained by thinning of thespreading current.

Komar (1971) advocated an hydraulic jumpoccurring at channel-fan transitions, which con-verts the ¯ow to a depositing, subcritical state.This is also implied by Palanques et al. (1996) andothers for channel-lobe areas of deep-sea fans. Atthe Valencia channel mouth, however, as there isno signi®cant decrease in slope and spreadingfavours ¯ow thinning (supported by the reductionin antidune wavelength), it is less likely that anhydraulic jump formed at the channel mouth. Inaddition, the interpretation of bedforms on theValencia fan suggests that the turbidity currentremained supercritical (or at least Fr > 0á84) up to100 km down¯ow of the channel mouth. Anhydraulic jump could have occurred downfan ofthe chevrons and resulted in an area of reducedbed shear and deposition. The ®eld of chevronbedforms passes down¯ow into ¯at sea ¯oor withno bedforms or bedforms too small to resolve withthe techniques used here. The chevrons at thisboundary become sparse but do not change insize or backscatter intensity, although the turbid-ity currents must have ¯owed farther towards theBalearic Abyssal Plain. This may imply that theturbidity current was transporting coarse-grainedsediment as a bedload and that as this sediment

was deposited the current continued to ¯ow butthe lack of coarse sediment prevented develop-ment of recognizable bedforms. As the ¯owwaned the hydraulic jump would have movedupstream across the chevrons, but during thiswaning period relatively little deposition ofcoarse-grained sediment appears to have oc-curred. Continued ¯ow expansion suggests thatcurrents were in a depletive state at this stage (cf.Kneller, 1995). The ¯ows were capable of trans-porting coarse sediment, but may have beendominantly muddy currents that had long run-out distances (high ef®ciency). Sheet sands areunlikely to have been deposited in the channel-lobe transition area of the Valencia fan during thisstage of fan sedimentation. However, they may bepresent further downfan, but have so far not beenmapped on this turbidity current pathway.

There are not enough data to interpret eitherthe exact age of the large turbidity currentresponsible for the features we report here, or toreliably distinguish its origin. Given the scale ofthe features observed we consider that the ¯owmay have been triggered either by a major ¯ood,landslide or be an earthquake/tsunami induced¯ow (as discussed by Mulder et al., 1997).

CONCLUSIONS

The new data from the Gulf of Valencia presentedhere have allowed examination of previouslyundocumented bedforms and consideration ofpalaeo¯ow conditions in turbidity currents. Thisstudy leads to the following major conclusions.

1. Bed features around the Valencia channelmouth prove that one or more powerful turbiditycurrents travelled down the channel in the LatePleistocene or Holocene.

2. The mega-¯utes (up 200 m across) occur inthe channel mouth and in association with otherfeatures may be diagnostic of channel-lobe tran-sition areas of deep sea-fans. They may be relatedto onset of rapid ¯ow expansion.

3. Transverse bedforms have been identi®ed inshallower areas of the channel and downfan ofthe channel mouth and these are interpreted asantidunes. They suggest that the ¯ow was super-critical or near critical (certainly Fr > 0á84) in thechannel and to a distance of at least 75 km fromthe channel mouth. The wavelength of antidunesdecreases down fan, implying an increase insediment concentration, decrease in velocity,decrease in ¯ow thickness, or combination ofthese three mutually dependent factors. Flow



thinning can be explained by ¯ow spreading fromthe channel mouth and so may be the primarycontrol on the change in wavelength as there isinsigni®cant change in slope.

4. A newly documented type of bedform,named a chevron, occurs in a distinct ®elddownfan from the channel mouth. There is anapparent transition from transverse bedformtrains (interpreted as antidunes) to chevrons.The chevrons are thought to form under super-critical (or near critical) ¯ow conditions and maybe associated with the continuing ¯ow thinning.

5. Variations in bedforms (including mega-¯utes, antidunes and chevrons) in the channelmouth and lobe, indicate varying ¯ow power anderosion across- and down-¯ow during the passageof a powerful turbidity current. The morphologyand distribution of bedforms suggest that anhydraulic jump did not form at the channelmouth but instead, ¯ow thinning allowed thecurrents to remain supercritical some 100 kmfrom the channel mouth.

6. The Valencia fan, during the period of eventsrecorded with these data, may have been a highef®ciency fan with transportation of coarse sed-iment a long distance from the channel mouthand development of discontinuous sand sheets inthe channel-lobe transition area.

ACKNOWLEDGMENTS

We would like to thank the UNESCO/ESF/Training through Research programme for theuse of these data for publication. The Trainingthrough Research cruises are funded by UNESCO,ESF and a consortium of universities (includingthe `Floating University' of Moscow State Univer-sity, Free University, Amsterdam and Universityof Wales, Cardiff). Thanks to Thierry Mulder,Bryan Cronin and Tim Davies for comments on anearlier version of this paper. The ®nal version ofthe paper was greatly improved following reviewsby Brian Jones and Ben Kneller.

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Manuscript received 10 July 1996; revision accepted 14July 1997.



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Downstream changes of large-scale bedforms in turbidites around the Valencia channel mouth, north-west Mediterranean: implications for palaeoflow reconstruction