Continental Shelf Research 30 (2010) 1802–1813

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Continental Shelf Research

0278-43

doi:10.1

n Corr

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Research papers

Large-scale bedforms along a tideless outer shelf setting in thewestern Mediterranean

Claudio Lo Iacono a,n, Jorge Guillen b, Pere Puig b, Marta Ribo b, Maria Ballesteros c, Albert Palanques b,Marcel lı Farran b, Juan Acosta c

a Unidad de Tecnologia Marina, CSIC, Centro Mediterraneo de Investigaciones Marinas y Ambientales (CMIMA), Paseo Marıtimo de la Barceloneta, 37–49, 08003 Barcelona, Spainb Instituto de Ciencias del Mar, CSIC, Centro Mediterraneo de Investigaciones Marinas y Ambientales, 08003 Barcelona, Spainc Instituto Espanol de Oceanografıa, Corazon de Marıa 8, 28002 Madrid, Spain

a r t i c l e i n f o

Article history:

Received 11 March 2010

Received in revised form

9 August 2010

Accepted 10 August 2010Available online 14 August 2010

Keywords:

Continental shelf

Subaqueous dunes

Geomorphology

Swath mapping

Western Mediterranean

43/$ - see front matter & 2010 Elsevier Ltd. A

016/j.csr.2010.08.005

esponding author: Tel.: +34 93 230 95 00; fa

ail address: loiacono@utm.csic.es (C. Lo Iacon

a b s t r a c t

High-resolution multibeam swath-bathymetry and sediment samples were collected across the outer

shelf region of the Columbretes Islands (southern Ebro continental shelf, western Mediterranean Sea).

Bathymetric data from the submerged part of the Columbretes volcanic system revealed the presence of

three main relict sand bodies along the outer shelf, at 80–116 m depth range, above which asymmetric

and slightly asymmetric large and very large 2D and 3D subaqueous dunes were observed. These bed

features were recognized, mapped and quantified with the aim of evaluating their potential formation

mechanisms in relation to the local hydrodynamic and morphologic settings of the area. Dunes range

from 150 to 760 m in wavelength and from tens of centimeters to 3 m in height, and are among the

longest ever recognized in an outer shelf region. These bedforms are mostly composed of medium-sized

sandy sediments, presumably coming from the degraded relict sand bodies on top of which they have

developed, mixed with fine fractions from the recent draping holocenic sediments. The orientation of

the dunes is SSW, progressively turning W towards the southernmost sector of the area, following the

trend of the shelf-edge. Contemporary hydrodynamic measurements at the Ebro continental shelf-edge

show that recorded currents are insufficient to form the observed bedforms and that stronger currents

are required for sediment mobilization and dune formation. Based on their morphology and orientation,

it is proposed that these bedforms are produced by the action of the southward-flowing Liguro-Provenc-

al-Catalan (LPC) geostrophic current. The LPC probably reaches high near-bottom currents during

energetic hydrodynamic events through interactions with the seafloor morphology of the study area.

Subaqueous dunes are expected to be basically inactive features with respect to present-day processes,

although they can be reactivated during high-energy events. The small Dh/l ratio measured in the dune

fields of the Columbretes shelf revealed that the dune heights fall below the values predicted by the

Flemming (1988) global equation, as observed in other outer shelf settings also dominated by

unidirectional flows. This may suggest a different morphodynamic character of large dunes formed on

outer shelves in a micro-tidal regime.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The seafloor of the outer continental shelf is frequentlycovered by relict sandy sediments deposited during the lastglacial maximum and the subsequent sea-level rise (Emery,1968). These sediment sheets can potentially be reworked,allowing the formation of large subaqueous dunes on their top(Ikehara, 1993; Bassetti et al., 2006; Gardner et al., 2007). Duringthe last few years, the increasing application of submarineacoustic methodologies for studying the seafloor, such as

ll rights reserved.

x: +34 93 230 95 55.

o).

swath-bathymetry and sidescan sonar systems, has frequentlyrevealed the occurrence of extensive fields of sedimentarybedforms on the outer shelves of many continental marginsaround the world (Harris, 1988; Bassetti et al., 2006; Gardneret al., 2007; Lo Iacono et al., 2008; Barrie et al., 2009). Dunesobserved on outer shelves usually display large average dimen-sions with wavelengths ranging from 75 to 435 m and heightsranging from 1 to 30 m (Keller and Richards, 1967; Gardner et al.,2007; Van Landeghem et al., 2009). Forcing mechanisms able toinduce their formation have been associated with strong bottomcurrents related to tidal variations, especially on high-latitudemargins (Barrie et al., 2009; Van Landeghem et al., 2009), and towater masses flowing under geostrophic conditions (Ikehara,1998; Gardner et al., 2005, 2007; Bassetti et al., 2006). Bottom

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–1813 1803

currents related to wave action reach low orbital velocities atthese water depths, and are generally not strong enough togenerate large bedforms. Nevertheless, wave energy can representa secondary component in the evolution of outer shelf bedforms(Bassetti et al., 2006). In some areas, few or null variationsobserved in the bedform morphology over periods of up to tens ofyears suggest a long-term stability of these features, althoughhigh-energy events can episodically mobilize them (Ikehara andKinoshita, 1994; Kubicki, 2008; Lo Iacono and Guillen, 2008;Whitmeyer and FitzGerald, 2008).

The local configuration of the seafloor can play a crucial role insediment transport dynamics. Geomorphological restrictions,such as sills, ridges, narrowing channels and moats, are frequentlydescribed as relict features on outer shelves. These features cansteer the patterns of bottom current flows and enhance theirvelocity, favouring the formation of large sandwaves, wheresandy and gravely sediments are available (Keller and Richards,1967; Harris 1988; Ikehara, 1998; Barrie et al., 2009).

This is the case of the southern sector of the Ebro continentalshelf, NW Mediterranean Sea, where the outer shelf is constrainedby the presence of the Columbretes Islands, a volcanic archipelago(Aparicio et al., 1991; Munoz et al., 2005). The outer shelfphysiography of this area has already been described by Dıaz et al.(1990) and Munoz et al. (2005). These authors conducted ageomorphological study of the area and described large sedimen-tary features, such as ridges, furrows and sandwaves, pointing outthat the geostrophic current affecting this region could play a rolein controlling their evolution.

In this paper, recently acquired data have been integrated inthe dataset compiled by Munoz et al. (2005). The results cover awider area of the Columbretes Islands continental shelf andcharacterize several large subaqueous dune fields that were onlypartially observed in the previous study. Here, we present a verydetailed morphologic analysis of these subaqueous dunes, usingmultibeam bathymetry imagery supported by surface grain sizedata. Our aims are: (1) to present a morphodynamic andsedimentological characterization of the subaqueous dune fields

Fig. 1. Bathymetric map of the Ebro Continental Margin (www.icm.csic.es/geo/gma/M

bathymetric map of the southwestern Mediterranean and of the south Iberian margin. T

references to colour in this figure legend, the reader is referred to the web version of t

in the area; (2) to discuss the potential mechanism of formation ofthe observed dunes, in relation to the local geomorphology andthe hydrodynamic setting of the area; and (3) to interpret thedynamics of these dunes by comparing their morphology withthat of similar bedforms formed in similar outer shelf settingsworldwide.

2. The study area

2.1. Geological setting

The Ebro continental margin, located in the NW Mediterra-nean, was formed during the Meso-Cenozoic southern Alpinemega-suture in relation to the rifting of the Valencia Trough, adeep depression that separates the Iberian margin from theBalearic Islands (Dur�an-Delga and Fontbote, 1980; Danobeitiaet al., 1990).

The Ebro continental shelf covers an area of almost 10,000 km2

and extends from Cape Salou in the north to Cape San Antonio inthe south (Fig. 1). Its width ranges from 7 km at Cape Salou to amaximum of 60 km south of the Ebro delta, and the averagegradient ranges from a maximum of 0.51 recorded along theprodelta to less than 0.11 along the outer shelf. The shelf-edge,ranging in depth from 100 to 140 m, is oriented almost parallel tothe coast, and turns landwards south of the Columbretes Islands,reducing the extension of the shelf to almost 20 km off Valencia(Fig. 1). The seafloor configuration of the Ebro continental shelfis generally smooth, low-gradient and sub-horizontal, and issporadically interrupted by isolated features, such as biohermsand sand shoals (Dıaz et al., 1990; Farran and Maldonado, 1990).

Based on seismic data, the Plio-Quaternary succession of theEbro continental shelf is composed of stacked units separated bydisconformities, and lies on a wide erosional surface formedduring the Messinian Mediterranean desiccation (Alonso et al.,1990). The construction of the Plio-Quaternary units has beenrelated to the interplay of glacioeustatic sea-level oscillations,

CB). The red rectangle corresponds to the study area, depicted in Fig. 2. Inset:

he red rectangle outlines the area depicted in this figure. (For interpretation of the

his article.)

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–18131804

subsidence, changes in sediment supply and local currentpatterns, favouring the formation of shelf deltas and shelf-margindeltas along the middle and outer shelf during the sea-levellowstands (Farran and Maldonado, 1990). The uppermost part ofthe Plio-Quaternary succession is composed of a prodeltaic unit,formed during the last 10 kyrs, which borders the actual Ebrodelta along the inner shelf and progrades southward over a lowertransgressive unit (Verdaguer, 1983; Dıaz et al., 1990). Thistransgressive unit (the Coves Unit according to Farran andMaldonado (1990)) was formed during the Late Pleistocenetransgression (14–10 kyrs BP) in estuarine, littoral and delta plainenvironments (Farran and Maldonado, 1990) and at presentcovers most of the outer shelf. It is mainly composed ofterrigenous and bioclastic relict sands, although abundantvolcanic debris are observed near the Columbretes Islands (Dıazet al., 1990). Positive reliefs observed along the outer shelf of thearea, such as shoals, hardgrounds and sand ridges have beeninterpreted as shoreline-related relict features formed in shallowenvironments during the Late Pleistocene transgression (Dıazet al., 1990).

The Ebro continental shelf is characterized by active fluvialsediment inputs mainly originating from the Ebro River. Thespatial distribution of bed surface sediment is characterized bythe transition from the nearshore sand to the prodeltaic mud thattakes place across a narrow zone of sandy mud, which can belocated at a water depth ranging from 6 to 20 m. Offshore, themuddy prodeltaic deposit surrounds the deltaic front and extendsalong the shelf between 20 and 80 m water depth, up to 110 kmsouthward from the present river mouth (Dıaz et al., 1996). Themean grain size of the prodeltaic mud ranges from 1.0 to 4.0 mm,with the finest sediments accumulated in a muddy belt betweenabout 60 and 80 m water depth. The outer edge of the prodeltaicmud belt is defined by a narrow transitional boundary betweenthe mud deposit and the outer shelf transgressive sand, which is arelict deposit with a mean grain size ranging from 100 to 150 mm(Dıaz et al., 1990).

The study area is located in an outer shelf sector southward ofthe Columbretes Islands, which are part of a volcanic complexformed during the Quaternary and associated with the Trans-Moroccan–western Mediterranean–European mega-shear (Biju-Duval et al., 1978). This volcanic field displays a rough morphologyof the seafloor at water depths of about 80–100 m, and is partiallycovered by sediment sheets, active sediment ridges, swells,sandwaves and erosional furrows (Munoz et al., 2005).

2.2. Oceanographic setting

The Ebro continental margin is characterized by a microtidal(o0.3 m) regime and a fetch-limited wave climate. The direc-tional distribution of waves shows a predominance of NW and Ewave conditions, although the largest waves come from the E orE-NE (Sanchez-Arcilla et al., 2008). Storms with a significant waveheight of 6.6 m are expected to have a return period of 100 yearsin this area (7.6 m for a 500-year return period) and peak waveperiods can reach 12 s during extreme storms (Gomez et al.,2003).

Over the mid-continental shelf, the near-bottom mean currentspeed is �8 cm/s, while over the outer shelf it is �15 cm/s (Fontet al., 1990; Palanques et al., 2002). According to the authors ofthese findings, this increase in the mean current over the outershelf has been associated with the steady unidirectional flowsinduced by the jet of the along-slope geostrophic current, alsoknown as the Liguro-Provenc-al-Catalan (LPC) current, when itapproaches the shelf-break. This geostrophic southward flow islittle affected by local wind variations and can also extend over

the mid-shelf, but with decreased velocities (Font et al., 1990).The highest near-bottom current speeds over the mid-shelf aretypically associated with storm events, reaching 50 cm/s (Puiget al., 2001; Palanques et al., 2002), whereas over the outer shelfmaximum currents can reach 35 cm/s during periods of steadygeostrophic southward flow (Palanques et al., 2002).

As stated above, the formation of sandwaves, ridges andfurrows on the outer shelf around the Columbretes Islands hasbeen previously related to the interaction between the seafloorand the LPC geostrophic southward flow, with the peculiarseafloor configuration of the area, probably enhancing the effectof bottom current action (Munoz et al., 2005). However, nospecific analysis has yet been conducted to validate such acausative relationship.

3. Material and methods

The present study is based on an integration of multibeam andcoring data acquired during the BALCOM cruise carried out onboard the R/V Vizconde de Eza in June 2002, and during twosurveys carried out on board the R/V Garcia del Cid within theframework of the CASCADES Project in October 2008 and March2009.

Swath-bathymetry was collected using the full water depthSimrad EM-300 30 kHz (R/V Vizconde de Eza) and the shallowdepth Elac Seabeam 1050D 180 kHz (R/V Garcia del Cid) multi-beam echo-sounders. Sound velocity profiles were calculatedfrom multiple CTD data collected during cruises. Post-processingof multibeam data was performed with the HIPS system, asubmarine mapping software developed by CARIS. Once correctedfor water column sound velocity variations and cleaned with aping graphical editor, gridding of the filtered soundings wascarried out to obtain the final digital terrain model. Footprintresolution was 10 m for the whole insonified area. Visualization ofbathymetric data was obtained, using the IVS Fledermaus systemand the Golden Software Surfer 9, including shaded relief maps,3D images and bathymetric cross sections. The UTM coordinatesystem (units in metres) was chosen to represent the plan viewsof the bathymetric images, except for Fig. 1, for which thegeographic coordinate system was adopted.

Surface sediment samples were collected using a van Veen graband a HAPS corer and grain size of sediment samples was analyzedby laser diffraction using the Horiba Partica LA-950 analyzer. Onlysurface deposits of the collected cores were analyzed.

Seismic profiles, used as supplementary data for this work,were acquired in 1985 during the GC-85-4 survey, using anOretech-Geopulse Boomer with a source of 450 J.

The dunes were classified according to Ashley (1990), thewavelength being considered as the main classification parameterand very large dunes being considered those longer than 100 m.Asymmetry of the dunes was calculated using the symmetryindex (SI) of Allen (1980), taking the value 0.6 as the minimum SIvalue for classifying the dunes as asymmetric.

4. Results

4.1. Geomorphology of the area

The studied portion of the Columbretes Islands continentalshelf covers an area of 855 km2, with water depths ranging from20 m, observed in the areas close to the subaerial volcanicedifices, to 550 m, within submarine canyons in the upper sloperegion (Fig. 2a). Average gradient along the continental shelf is0.11, with maximum slope values of 331 observed along the walls

Fig. 2. (a) 3D shaded relief bathymetric map of the Columbretes shelf based on data acquired during the BALCOM 2002 and the CASCADES 2008 and 2009 surveys.

(b) Geomorphological map showing the main morphological and sedimentary features. Rose diagrams (radius is 800 m) display the flow direction and the length of the

very large dunes. (Keys: NSB: northern sand body; CSB: central sand body; SSB: southern sand body; PBA: Placer de la Barra Alta; BC: Barreta dels Colls; NCC: Northern

Columbretes Canyons; SCC: Southern Columbretes Canyon. Columbretes Islands: CG: Columbrete Grande; F: La Ferrera; H: Horadada and B: El Bergantın.)

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–1813 1805

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of the volcanic cones. The upper slope shows average gradients of0.61, with maximum values of 431 along the walls of the Northernand Southern Columbretes Canyons.

The continental shelf alternates rough morphologic featureswith smooth and sub-horizontal regions. The Columbretesvolcanic system covers an area of 35 km2 along a main N–Sdirection. This system is composed of the Columbretes Islands anda wide submerged zone, whose main feature is a sub-circular conewith a diameter of up to 4 km, composed of three main craters(Fig. 2b) and described as DFP by Munoz et al. (2005). In the area,small ridges and cones rising from the seafloor up to 40 malternate with volcanic sub-horizontal terraces and thin sandysheets for a depth range of 60–80 m (Fig. 2b). Other volcaniccones, isolated from the main system, occur at a depth interval of80–90 m and rise up to 60 m from the seafloor (Fig. 2b). At a depthof 90 m, along the seaward-facing portion of the Columbretessystem, volcanic cones up to 45 m high act as shelter for long sandtails, which accumulate in shadow areas toward the SW and WSWdirections for a length of up to 4 km (Fig. 2b). These sand tails are10 m high on an average and display an erosive moat all alongtheir base. In the seaward region of the study area, an isolatedvolcano, named Barreta dels Colls by the local fishermen, is foundat a depth of 100 m, displaying a typical rounded cratermorphology with an average height of 30 m and covering a totalarea of 2 km2 (Fig. 2b). A 2.5-km-wide sediment tail covers theentire southern side of the Barreta dels Colls, following a NE-SWdirection for a total length of 6 km and an average height of 8 m(Fig. 2b). The shelf-edge in this area is found at a depth of 115 mand follows a NNE-SSW direction, turning progressively towardsthe SW and W (Fig. 2b). The eastern and southeastern portions ofthe outer shelf are incised by the Northern Columbretes Canyonand the Southern Columbretes Canyon, entering the shelf-edge toa depth of 110 m (Fig. 2b). The southernmost portion of the outershelf is entirely carved by gullies and rills. This area is alsocharacterized by the occurrence of small, rounded rocky outcrops,probably of carbonatic origin, aligned in a mainly NE-SW directionat a depth range of 105–120 m (Fig. 2b). Some of these rockyoutcrops shelter areas from bottom currents, resulting in smallsediment tails that develop parallel to the shelf-edge along aWSE-ENE direction (Fig. 2b). Small seafloor longitudinal lineations

Fig. 3. Sediment distribution on the Columbretes outer shelf. The isolines represent th

Coarser fractions coincide with the areas surrounding the volcanic outcrops (in blue) and

references to colour in this figure legend, the reader is referred to the web version of

found south from the Southern Columbretes Canyon give furtherevidence of the presence of strong bottom currents occurring inthe outer-shelf and upper-slope regions (Fig. 2b).

Three large sand bodies occur in the deepest sector of the shelfat water depths ranging from 85 to 115 m. The three sedimentarybodies (the northern, central and southern bodies) rise 2–9 mfrom the surrounding smooth seafloor, and are oriented along aNNE-SSW direction (Fig. 2b). Asymmetric, slightly asymmetricand symmetric very large subaqueous dunes were observed abovethese sandy bodies, with a downstream direction running almostparallel to the shelf-edge and oriented towards the SW,progressively turning to the W in the southern portion of thestudy area (Fig. 2b).

4.2. Sediment distribution

The grain size of the sediments sampled on the Columbretesshelf ranges from coarse silt to medium sand, with an exception ofcoarse or very coarse sands near the volcanic highs (Fig. 3).Sediments are poorly or very poorly sorted and fine or very fineskewed. The median grain size ranges from 7 to 1566 mm, and theskewness ranges between �0.7 and 0.4. The most abundantgranulometric class corresponds to medium and fine sands,covering 39.4% of the study area. Low percentages of gravel(1.53%) and mud (silt: 7.21% and clay: 1.85%) are also present.Sediment distribution on the Columbretes outer shelf is mainlycontrolled by the presence of the large sand bodies and volcanicedifices (Fig. 3). There is a grain-size fining trend in an offshoredirection and sediments composing the large sand bodies arecoarser than those sampled in the surrounding areas (Fig. 3).Coarser sediments mixed with the fine fractions were also presentalong the topographic highs. A high percentage of bioclasts(mainly shells) was found in the area of the rocky outcropsmapped near the southern sand body (Fig. 2b).

4.3. Sand bodies and subaqueous dunes along the outer shelf

4.3.1. The northern sand body

The northern sand body borders the outer shelf in thenorthernmost sector of the study area, with the seaward flank

e medium grain size in microns. White dots correspond to the sample locations.

above the sand bodies, where the very large dunes occur. (For interpretation of the

this article.)

Fig. 4. 3D shaded relief model and bathymetric profiles (in meters) of the very large dunes of the northern sand body.

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–1813 1807

at a depth interval of 97–120 m (Fig. 4). This sand body was onlypartially covered by multibeam acoustic imagery, so it was notpossible to follow its landward flank to estimate its total area.However, based on available data, it displays a teardrop shapeending with a narrow and sharp tip oriented towards the SW(Fig. 2b). The visible portion of the inner flank displays a NE-SWlinear trend and a straight morphology, rising from the seafloor upto 10 m and displaying a 0.5-m-deep erosive moat at its base(Fig. 2b). The inner flank borders 1-km-wide erosive trough,which is delimited by the northern sand body in the east and by asmaller sediment body in the west. The erosive trough extendsand progressively widens to the ramp of the central sand body,following a SW-NE direction, and then turning towards the west(Fig. 2b).

Sedimentary bedforms cover the entire surface of the northernsand body (Fig. 4). Very large dunes mainly occur along theseaward flank, at depths ranging from 88 to 116 m. The dunesoccurring along the northernmost portion of the sand body areclassified as very large 2D dunes. They are from 2 to 4 m high,have a wavelength of 400–550 m and a lateral extension of500 m–1 km, and the largest ones are found near the shelf-edge.These dunes are asymmetric to slightly asymmetric, with the flowdirection towards the SW and an average symmetry index (SI)ranging from 0.5 to 1.2. The 2D dunes progressively reduce theirlateral extension and increase their wavelength towards thesouthern portion of the sand body. The southernmost dunes canbe defined as very large 3D asymmetric dunes, with a crest lengthof up to 800 m, and the lee side facing towards the SW. Theiraverage wavelength is 600 m and the dune height is 3 m. Dunes ofthis sector are the longest observed in our study area and rankamong the longest ever observed in an outer continental shelfsetting, with a maximum wavelength of up to 780 m (Fig. 4).Moving towards the innermost sector of the sand body, the 3Ddunes evolve to small NE-SW-oriented longitudinal sand ridgesthat are 4.5 m high and up to 1 km long (Fig. 4). Smaller bedforms,120 m long and up to 20 cm high on an average, are superimposedon the larger dunes of the northern sand body.

4.3.2. The central sand body

The central sand body occurs over a depth range of 85–97 m,covering an area of up to 74 km2 (Fig. 5), and develops in aneast–west direction (Fig. 2a and b, 5). Large sediment tails alongthe central sand body have been generated by the shelteringeffect of the Columbretes volcanic edifice and the Barreta delsColls volcano (Fig. 5). Sediment tails rise up to 10 m on an average

and extend for lengths of up to 6 km in a NNE-SSW direction,following the trend of the shelf-edge. The sediment tail behind theColumbretes volcanic edifice is composed of two main portions,forming a curved body. The first portion, in contact with theColumbretes volcanic structure, rises from the seafloor up to12 m, covering a surface area of 4 km2. It displays a lenticular‘‘angel-wing’’ shape and an NE-SW orientation (Fig. 5). The secondportion forms the distal part of the sediment tail, forming amorphological step along the contact with the first one (Fig. 5). Itis 6 m high and follows an E-W direction. A 1-km-wide and 8-km-long sediment tail has been developed along the central portion ofthe central sand body in an NE-SW direction, aligned with theposition of the Northern Columbretes Canyon head located 6 kmto the north (Fig. 5).

Very large 2D dunes on the central sand body occur in clustersin the head region and on the largest sediment tails, over a depthrange of 79-92 m (Figs. 5 and 6). Their wavelength ranges from150 to 580 m and the height between tens of centimeters and2 m. Maximum dimensions were observed along the head region,where an outlier dune, with less lateral extension, reaches alength of 680 and a height of 1.8 m (Fig. 5). Crests are generallyrounded and degraded, with linear to sinuous trends and amaximum lateral extension of 1 km. The asymmetry of thesedunes clearly decreases from those occurring near the shelf-edge,with an SI of between 0.5 and 0.9, to the innermost ones occurringalong the ‘‘angel-wing’’ sediment tail behind the Columbretesvolcanic structure. These latter dunes, 200 long and 1 m high onaverage, are symmetric, with a mean SI of 0.1 (Fig. 5). Stronglydegraded dunes occur along the second portion of the sedimenttail behind the Columbretes volcanic structure. These dunesdisplay an average length of 350 m and an average height of0.4 m. Most of them are barely recognizable on the 3Dbathymetric imagery. Two main sediment tails are present behindthe Barreta dels Colls. The first one occurs at almost 90 m waterdepth along the southern edge of the sand body and is up to 2 kmlong and 4 m high (Fig. 5). The second one, with a total length ofup to 5 km, occurs behind the external and deeper sector of theBarreta dels Colls, at a depth of 110 m, and seems to be composedof different small sedimentary bodies (Fig. 5).

4.3.3. The southern sand body

The southernmost sand body covers a total surface of almost50 km2, displays a rounded deltoid shape, and is NE-SW oriented(Fig. 7). It occurs at depths ranging from 95 m, along the westernlandward side, to 105 m along the eastern seaward side, and

Fig. 6. High-resolution Uniboom profile displaying the very large dunes formed on the central sand body. (Key: yellow line displays the base of the upper Quaternary

succession; RD¼regressive deposits; VB¼volcanic basement.)

Fig. 5. Perspective view of the 3D shaded relief model and bathymetric profiles (in meters) of the very large dunes of the central sand body. NCC: Northern Columbretes

Canyon.

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–18131808

Fig. 7. 3D shaded relief model and bathymetric profiles (in meters) of the very large dunes of the southern sand body.

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–1813 1809

borders the shelf-edge along its head, at 115 m depth. The flanksof this sand body differ from each other. The innermost flank runslinearly in a N–S direction and displays an abrupt morphologyuntil it reaches the southern end, rising up to 9 m from theseafloor (Figs. 2b and 7). The deeper seaward flank, rising up toalmost 6 m, shows rounded shapes, smooth gradients and asinuous trend parallel to the shelf-edge (Figs. 2b and 7). The top ofthe sand body is characterized by the occurrence of very large 2Dasymmetric subaqueous dunes organized in a regularly spacedassemblage and with the flow direction oriented towards the west(Fig. 7). The dunes cover almost 14 km2 of the western flank of thesand body (28% of the sand body surface) with a 93–106 m depthrange. Dune wavelength ranges from 300 to 650 m, with anaverage of 440 m, and height ranges from 1.5 to 6 m, with anaverage of 3 m (Fig. 7). Observed dunes are asymmetrical, withthe highest SI observed in the study area, 2.01 on average, andasymmetry progressively increasing towards the shelf-edge.

Crests are almost rounded and display a lateral extensionfrom 0.5 to 2 km, with a low to moderate sinuosity. Secondarydunes of smaller dimensions were observed superimposed on thelarger dunes, with wavelengths ranging from 80 to 120 m andheights from 0.50 to 0.80 m.

5. Discussion

5.1. Origin of the sand bodies

The sand bodies of the Columbretes outer shelf werepreviously documented by Dıaz et al. (1990), who referred tothem as ‘‘sand shoals’’. They were interpreted as a shoreline-related feature that developed in periods of deceleration duringthe last sea-level rise, and their formation was associated withreworking and downdrift transport of late Pleistocene shelf-margin delta sandy deposits during the last transgressive phase(Dıaz et al., 1990). It is documented that the LPC current stronglyinfluenced the growth pattern of the Plio-Quaternary successionof the Ebro continental shelf, affecting the geometry of manysedimentary bodies, which display southward oriented seismicreflectors, especially for those which compose the distal prodeltabelt (Dıaz et al., 1990; Farran and Maldonado, 1990). Based ongeomorphologic and seismostratigraphic studies from the Ebrocontinental shelf, the sand bodies could also be defined as shelf-edge deltas, because of their shape, depth and sedimentarycharacteristics, and the juxtaposition between them (Farran andMaldonado, 1990).

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Similar sedimentary features have also been observed in otherequivalent settings such as the Gulf of Mexico and NW Floridaouter shelves (Gardner et al., 2005, 2007). In the westernMediterranean, Bassetti et al. (2006) described the sand bodiesobserved along the shelf-edge of the Gulf of Lions as transgressivefossil ridges lying over a major erosive surface. According toradiocarbon dating, core lithology and inferences from glacio-eustatic sea-level curves, the relict features of the Gulf of Lionsouter shelf formed during the last post-glacial transgression,probably around the Younger Dryas (15–12 ka cal BP), duringwhich a sea-level rise deceleration was long enough to favor theformation of sedimentary bodies (Bassetti et al., 2006). Radio-carbon dating of shell fragments collected at the top of the sandydeposits of the Columbretes outer shelf give an age of about11,100 yrs BP (Dıaz et al., 1990). According to the reconstructionsof the last sea-level rise near the study area, the sea-levelunderwent a deceleration about 11,100 yrs BP, standing at apresent depth of 60 m (Somoza et al., 1998; Lambeck and Bard,2000; Lambeck et al., 2004). Hence, the observed sand deposits,from 75 to 100 m deep, must have formed in a coastal environ-ment. Their preservation on the present outer shelf is probably dueto a rapid sea-level rise subsequent to their formation, preventingthem from strong dismantling coastal hydrodynamic events.

5.2. Formation and potential dynamics of subaqueous dunes

Large subaqueous dunes were formed during a periodsubsequent to the age of the sand bodies above which theydeveloped. Hence, the dunes are presumably younger than11,000 yr BP. However, it is not clear whether they formed in ashallower environment during the last sea-level transgression, orafter the beginning of the present highstand phase, once the sea-level concluded its rapid rise 65007500 yr BP.

Following the first hypothesis, the very large dunes would havebeen generated under the influence of strong unidirectional coastalcurrents flowing parallel to the coast and acting in the samedirection for a period long enough to allow their formation.Furthermore, large dunes should withstand for more than 6 kyrsdegrading factors such as bioturbation, coastal erosion and the

Fig. 8. Pathways of the LPC current interpreted to be the main forcing mechanism in

direction (white arrows) is inferred from the orientation of the observed dunes, from th

the seafloor.

dismantling action of wind- and wave-induced currents, occurringduring the transgression. The Columbretes large dunes could hardlyhave been preserved in the above sedimentary and hydrodynamicscenario. For these reasons, we infer that the observed bedformmorphology do not correspond to that generated in a shallowerenvironment during the last sea-level transgression. Bedformsprobably formed or were reactivated several times during theactual highstand phase (since �6 kyrs BP). However, this wouldrequire the occurrence of an energetic hydrodynamic process, suchas the LPC current, that was able to generate these large dune fieldsin a tideless environment, such as the Columbretes shelf.

Large and very large dune fields have been observed world-wide in tide-dominated seas, even at significant depths along theshelf (McCave, 1971; Terwindt, 1971; Field et al., 1981; Ikeharaand Kinoshita, 1994; Valentine et al., 2002; Li and King, 2007;Nemeth et al., 2007; Van Landeghem et al., 2009). On the otherhand, the occurrence of very large subaqueous dunes in tidelessseas is less frequently observed. In an absence of strong tidalcurrents, the formation (or reactivation) of large subaqueousdunes on the outer shelf has been related to unidirectional flowsproduced by ocean currents accelerated by the presence ofmorphological constraints (Ikehara and Kinoshita, 1994), byseasonal monsoon winds (Kubicki, 2008), by strong geostrophiccurrents along the shelf-break (Gardner et al., 2007), or by sinkingcold and dense water masses (Lykousis, 2001). Described bottomunidirectional flows able to form large dunes, such as thoseobserved in the SW Japan Sea and the Vietnam outer shelf, rangefrom 0.8 to 1 m/s (Ikehara, 1998; Kubicki, 2008), whereas themaximum near-bottom currents observed on the Ebro outer shelfassociated with the LPC have been 0.35 m/s, over a period of1 year (Palanques et al., 2002). However, these measurementswere conducted 70 km upstream of the study area in an outershelf sector not affected by a rough topography.

The asymmetric character of the Columbretes outer shelfdunes, with the lee slope facing towards the south, suggests theLPC current as the primary forcing factor in their formation. This isalso reflected by the change of dune orientation along the centraland southern sand body, following the direction of the shelf-edgewhen it turns to the west (Figs. 2b and 8). The decrease of the SI

the generation and maintenance of the Columbretes very large dunes. Current

e seafloor lineations of the outer shelf and from the morphologic configuration of

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from the shelf-edge to the innermost portion of the study areagives further support to the hypothesis that the LPC is the primaryforcing factor in dune formation. The current is more active alongthe margin of the shelf, where bedforms maintain their asym-metry, and loses its energy along the inner and shallower sector,where dunes remain inactive features for longer periods and aremore exposed to potential reworking mechanisms (e.g. duringstrong storms).

It can be roughly estimated that the observed current (0.35 m/s)can generate a bottom shear stress of about 0.15 N/m2, whereasthe critical shear stress able to mobilize the surficial sedimentssampled on the dune fields (D50¼230 mm) is about 0.17 N/m2.Since these calculations were based on Madsen (1994) for a flatbottom, the current-induced shear stress would increase becauseof the increased turbulence due to the high roughness associatedwith the seabed micro-topography, as hypothesized by Cacchioneet al. (1990) on the Ebro Continental shelf. Despite theseuncertainties, bottom shear stress estimations suggest thatobserved bottom currents can remove part of the fine sands andthe non-cohesive silt fraction of the bottom sediments on theouter shelf, bypassing them to the slope region and preventingmud deposition on the outer shelf. However, it cannot explain theformation of the very large dunes that are observed. Furthermore,the required bottom shear stress for the formation of the observeddune fields inferred from the empirical formulation of Van Rijn(1984) should be higher than 0.3 N/m2. Both indirect approachessuggest that stronger currents are required for sediment reactiva-tion and dune formation on the Ebro outer continental shelf. Thesingular morphological configuration of the area and the action ofhigh-energy events can provide an explanation for dune genera-tion.

The local morphological configuration of the Columbretesshelf, made up of rising volcanic ridges, sand bodies andbathymetric restrictions, probably induces an increase in thesouthward-flowing bottom current intensity, when the LPCapproaches the shelf-edge and enhances the shear stress inter-acting with the rough seafloor. This could explain the formation ofthe very large dunes (Fig. 8). Flows canalized along the troughsand restrictions of the area probably help to shape the seaflooritself, interacting in a sort of positive feedback mechanism (Fig. 8).Where a fine-sediment, sub-horizontal smooth seafloor is present,currents produce long lineations that reflect their paths andsediment tails when they interact with morphologic featuresrising from seafloor (Fig. 2b). Some of the evidence for thismechanism is found along the southern sand body, where the‘‘angel-wing’’ sediment tail controls the shape and orientation ofthe contiguous sediment tail, with bottom currents turning awayto the west (Fig. 5). In the same area, the formation of the centralsediment tail is probably related to the Northern ColumbretesCanyon, which acts as a shelter disrupting the flow of the bottomcurrents coming from the north along the shelf-edge (Fig. 5).

The occurrence of lineations and sediment tails associatedwith strong bottom currents flowing parallel to the shelf-edge isalso as evident in the upper slope region. However, the absence ofspecific near-bottom hydrodynamic records in the Columbretesarea prevents any precise assessment of the increase in currentintensity.

Although the main forcing mechanism in dune formationseems to be the LPC enhanced by morphological constraints,additional processes such as extreme wave events or strong windinduced currents also deserve attention. For instance, waves withreturn periods of 100 and 500 years could generate bottom shearstresses on the dune fields of about 0.17 and 0.20 N/m2,respectively, causing sediment remobilization and enhancingthe action of currents (Gomez et al., 2003). Furthermore, basedon model estimations in the Gulf of Lion, 20 m/s easterly winds

can generate bottom shear stresses of about 0.24 N/m2 at 100 mwater depth (Bassetti et al., 2006). If equivalent events occur inthe Columbretes area, which is reasonable to expect because ofsimilar meteorological and morphological conditions, sandysediment transport and rejuvenation of the subaqueous dunesshould be frequent. Based on meteorological records acquired onthe Ebro continental shelf over a period of 43 years (1958–2001)(SIMAR-44, 2008), 20 m/s wind velocities can occur in the areawith a recurrence period of 4 years.

In spite of uncertainties, the subaqueous dunes are expected tobe inactive features with respect to present-day processes,although they can be reactivated during high-energy events. Infact, the comparison between the bathymetric images of a dunefield coming from two surveys carried out over a period of 7 yearsshow no change in dune morphologies and positions, at least at aspatial scale of 10 m, supporting the hypothesis that they areinactive morphological features at short time-scales (Fig. 9).

Nonetheless, the results suggest a polycyclic evolution of dunefields in the study area. The vertical superposition between olddunes, sediment tails and more recent dunes observed in thecentral sand body (Figs. 5 and 6) demonstrates that hydrodynamicconditions suitable for their generation occurred during morethan one discrete high-energy event over time.

5.3. Dune morphology

The good wavelength/height correlation of the Columbretesdunes (Dh¼0.934l0.0063; R¼0.75) resembles the trend of theglobal mean equation by Flemming (1988) (Fig. 10) and othertrends based on many observations in different oceanic environ-ments (Allen, 1969; Dalrymple et al., 1978; Francken et al., 2004).Subaqueous dunes seem to be similar features responding to acommon hydrodynamic control, but formed in different settings.However, our field observations show average heights falling1–6 m below the values predicted by the global equationsfor bedforms in equilibrium conditions (Fig. 10). It is unlikelythat degrading factors were the most important cause of thereductions in height. Improbable constant and widespread events(high-energy hydrodynamics, bioturbation) would have had tohave been acting for a period long enough to erode a significantamount of sediments over the entire area, at the same timepreserving the large-scale morphology of the dunes.

Hence, other potential factors should be investigated. Thesediment grain size, water depth and bottom flow velocity arethe main environmental factors controlling the dimensions, shapesand migration rates of dunes (Mazumder, 2003). The grain size anddepth of the Columbretes dunes are environmental characteristicscommon to many other dune fields observed on several continentalshelves around the world (Kuijpers et al., 2002; Gardner et al.,2005, 2007; Barrie et al., 2009). Hence, the typology of the forcingbottom current flow may be the main discriminating factorresponsible for the reduced height of the observed dunes.

The small Dh/l ratio has also been reported in outer shelfregions of New Mexico, California, Vietnam, Japan and Scotland,which display subaqueous dunes with smaller heights than thosepredicted by the Flemming (1988) equation (Ikehara, 1998;Gardner et al., 2005, 2007; Li and King, 2007; Kubicki, 2008)(Fig. 10). On the Vietnam outer shelf, the most plausibleexplanation for the smaller dune height observed may besediment starvation and/or insufficient time for the larger dunesto fully adjust to changing flow conditions (Kubicki, 2008).However, it is difficult to extend this mechanism of formationto other regions, where different sedimentary and hydrodynamicconditions occur.

Fig. 10. Plot of dune heights (h) versus dune lengths (l) of the very large dunes of

the study area (red dots) and of similar bedforms formed in microtidal outer

shelves of many continental margins around the world (blue dots) (Ikehara, 1998;

Gardner et al., 2005, 2007; Li and King 2007; Kubicki, 2008). Regression curves are

calculated for the Columbretes dunes (red line) and for the whole data set of

bedforms observed on microtidal outer shelves, including our study area (green

line). Note the smaller heights of plotted dunes compared to the mean global trend

extracted from Flemming, 1988 (grey line).(For interpretation of the references to

colour in this figure legend, the reader is referred to the web version of this

article.)

Fig. 9. Comparative 3D shaded relief models of the ‘‘angel-wing,’’ very large dune field observed on the central sand bodies. Bathymetric data were acquired during the

BALCOM 2002 survey (a) and during the CASCADES 2008 survey (b). The comparison between the two images suggests a long-term morphological stability of the observed

bedforms for a period of at least 7 years, at least at a spatial scale of 10 m.

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–18131812

The environmental character common to all the outer shelfregions, where the smaller dune heights occur is a dominatingunidirectional bottom flow with negligible or subordinate tidalcurrents. This observation suggests that the general equation forbedform shape proposed by Fleming (1988) fits well for tide-dominated dunes, but could underestimate the bedform height inouter shelf environments dominated by unidirectional flows(Fig. 10). The wavelength/height correlation of Columbretes dunesand of dunes observed in the aforementioned outer shelvesfollows the equation Dh¼0.0077l0.528 and shows a statistically

significant correlation with R¼0.64 (Fig. 10). However, the mainreason for the occurrence of smaller dune height (or longerwavelength) in dune fields located on outer shelves dominated byunidirectional currents remains unclear.

6. Conclusions

Acoustic data acquired during different surveys have providedextensive imagery of the morphology of the Columbretes outershelf. Three relict sand bodies, interpreted as relict shoreline-related features, occur along the shelf-edge over a depth range of80–116 m. Asymmetric and slightly asymmetric very large duneshave been mapped above these relict sand bodies. Dunewavelength ranges from 150 to 760 m, whereas dune heightranges from tens of cm to 3 m. Maximum wavelength values placethe Columbretes very large dunes among the longest everrecognized in an outer shelf region.

Hydrodynamic measurements obtained on the Ebro outercontinental shelf show that the shear stresses induced by currentsalong the shelf-edge (0.15 N/m2 for a flat bed) are insufficient toform the observed bedforms. Stronger currents than thoseobserved in the study area, are required for the formation of theColumbretes very large dunes. The morphology of the dunessuggests that they could be produced by very intense near-bottom currents during sporadic energetic hydrodynamic eventsof the southward-flowing LPC geostrophic current, as it interactswith the rough seafloor of the study area. The subaqueous dunescan be expected to be inactive features with respect to present-day processes.

The wavelength/height ratio of the Columbretes dunes dis-plays a statistically significant correlation with R¼0.75, resem-bling the trend of the global mean equation by Flemming (1988).Nevertheless, the observed dunes show average heights fallingbelow the values predicted by the global equations for bedformsin equilibrium conditions. This morphologic character has beenreported in other outer shelf regions of several continentalmargins with negligible or subordinate tidal currents. This result

C. Lo Iacono et al. / Continental Shelf Research 30 (2010) 1802–1813 1813

could suggest that the height of subaqueous dunes formed intideless outer shelf environments may be underestimated by thegeneral equation for bedform shape proposed by Fleming (1988).

Acknowledgements

This research was supported by the Spanish national ProjectsCASCADES (Ref. CTM2008-01334-E/MAR) and COSTEM (Ref.CTM2009-07806). Additional support was provided by theSecretarıa General de Pesca Marıtima (MAPA), which providedship-time to carry out the BALCOM cruise. We thank the captainand the crew of the R/V Garcia del Cid for their kind assistanceduring the surveys and the UTM technicians Pablo Rodriguez,Hector Sanchez and Jose ‘‘Pou’’ Pozo for providing us withassistance during the data acquisition. We are grateful to SusanaDiez (UTM-CSIC, Barcelona) for her valuable advices during theprocessing of bathymetric data.

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