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Marine and Petroleum Geology 26 (2009) 673–694

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Marine and Petroleum Geology

journal homepage: www.elsevier .com/locate/marpetgeo

Deposition processes from echo-character mapping along the westernAlgerian margin (Oran–Tenes), Western Mediterranean

A. Domzig a,b,*,1, V. Gaullier c, P. Giresse c, H. Pauc c, J. Deverchere a,b, K. Yelles d

a Universite Europeenne de Bretagne, Franceb Universite de Brest; CNRS, UMR 6538 Domaines Oceaniques; Institut Universitaire Europeen de la Mer, Place Copernic, 29280 Plouzane, Francec Laboratoire IMAGES – E.A. 4218, Universite de Perpignan, Via Domitia, 52 avenue Paul Alduy, 66860 Perpignan, Franced CRAAG, Route de l’Observatoire, BP 63, Bouzareah, Alger, Algeria

a r t i c l e i n f o

Article history:Received 23 October 2006Received in revised form 10 March 2007Accepted 20 May 2008Available online 23 July 2008

Keywords:Algerian marginSedimentary processesEcho characterSeismic faciesMass-transport depositsTurbidity currents

* Corresponding author. Tel.: þ33 2 98 49 87 47; faE-mail address: anne.domzig@univ-nantes.fr (A. D

1 Present address: UMR 6112, Laboratoire de PlaUniversite de Nantes, 2 rue de la Houssiniere, BP 92France.

0264-8172/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.marpetgeo.2008.05.006

a b s t r a c t

The westernmost Algerian margin (south Algero-Provençal basin) depicts a few offshore active faults,moderate to rare seismicity, and generally very steep slopes (>16�). We classified and mapped 12 echotypes according to their sub-bottom acoustic facies observed on this margin on 2–5.2 kHz Chirp echo-sounder data (MARADJA 2003 cruise). The echo-character maps are interpreted in terms of sedimentaryprocesses: the B1 echo type (parallel to subparallel high- to low-amplitude sub-bottom reflections),mainly in the deep basin, corresponds to hemipelagic sedimentation; R1 (prolonged single echo with nosub-bottoms) and R2 (small irregular overlapping hyperbolae) echo types, generally near or in canyonsystems, are associated with turbidity currents or more rarely to contour currents or mass-transportdeposits such as slumps, slides and debris flows; the transparent echo types (T1–T5) and R3 (chaotic lensof low-amplitude reflections on top of higher amplitude), often located at the foot of the slope or canyonswalls, typically indicate mass-transport deposits (like slides) or turbidites. Large zones that displaya large variety of echo types are evidenced in the study area and are generally associated with turbiditycurrents, but could also be associated with bottom currents. It appears that active tectonics playsa significant role in this part of the margin which presents a few active faults offshore but also a strongand relatively frequent seismicity onland. The general pattern of the distribution of mass-transportdeposits is particular – i.e. many but small slides all along the margin – and suggests a probabletriggering by recurrent earthquake shakings. However, active tectonics is not the only factor influencingthe deposition pattern, as some zones seem characterized by predominant strong turbidity currentstransporting sediments far away from the foot of the margin, whereas others depict retrogressive erosionfeatures on the slope, i.e. small slides scarps in gullies rather than transport by turbidity currents. Inparticular, the rivers sediment discharge fluxes and the geomorphologic characteristics of the marginseem to be very important factors too.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The western Algerian continental margin is located in theWestern Mediterranean, south of the Algero-Provençal basin, at theeastern limit of the Alboran Sea (Fig. 1). Before 2003, almost no datawere available for this offshore part of Algeria. Oceanographiccruise MARADJA in August–September 2003 identified for the firsttime the main morphological and structural characteristics of theslope and the deep basin of the Algerian margin (Domzig et al.,

x: þ33 2 98 49 87 60.omzig).netologie et Geodynamique,208, 44322 Nantes Cedex 3,

All rights reserved.

2006). By using the data from the MARADJA 2003 cruise (Fig. 1), i.e.Chirp data set, high-resolution bathymetry and acoustic back-scatter patterns plus several sediment cores, the objective of thisstudy is now to define the sediment deposition patterns and thecorresponding sedimentary processes along this margin. In thisstudy we interpret the high-resolution 2–5.2 kHz Chirp lines byclassifying and mapping the different types of echo characters.Then, we interpret the depositional processes based on thedifferent types and distribution of echo characters as well asadditional geophysical, geomorphological and core data sets fromthe Maradja 2003 cruise. Finally we try to assess the role of activetectonics on the echo types depositional pattern in this part of themargin.

This paper only focuses on the western portion of the Algerianmargin between 1�400 W and 2�150 E: the Oran and Tenes zones.The central-eastern zone (Algiers) is discussed in a separate paper

Fig. 1. Locations of the study area showing ship tracks of the Maradja 2003 cruise. The core sites (KMDJ05, KMDJ06, KMDJ07 and KMDJ08, and MD04-2801) are also shown. Boldblack boxes show locations of maps in Figs. 3–8, 11, 12, and 22. Inset shows regional location.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694674

(Dan et al., in press) because of the need to emphasize the effects ofthe recent Boumerdes earthquake (Mw: 6.9) that affected theAlgiers region in May 2003 (Ayadi et al., 2003).

Preliminary results of the Maradja 2003 cruise showed twodifferent deep-seated tectonic styles from east to west (Fig. 2): theeastern Algerian margin (mainly from Tenes to the Tunisian border,Domzig et al., 2006; Yelles and the Maradja2 team, 2006) displayspurely compressive finite strain (series of thrusts identified,Deverchere et al., 2005; Domzig et al., 2006) and seismic activity,whereas the western margin (mainly from Oran to Tenes) displaysa pattern of active or inherited strike-slip structures (en echelonfaults in the deep basin off Oran, see Domzig et al., 2006). Appar-ently, the zone between Oran and Tenes has not experienced largeearthquakes offshore, at least since recording instruments havebeen settled, and the most active regions seem to be onshore (seea review in Yelles-Chaouche et al., 2006). In this paper, we try toestimate the relative importance of tectonic activity in the depo-sitional processes along this margin transect, and to check how andto what extent the heterogeneous sediment deposition pattern isrelated to local tectonics. We also aim to take into account otherfactors which may influence the sedimentary processes, such as themorphology of the margin or hydrodynamic processes.

2. Tectonic setting

Northern Algeria is a composite margin with a complex Ceno-zoic geodynamical history. Northern Algeria is mainly an orogen,the Maghrebian belt (Fig. 2), which is characterized from south tonorth by (1) the external domain composed of sedimentary units

Ain Temouchent22/12/99 Mw: 5.7

Bo21/3

?

Kabylian basement"Dorsale Kabyle"Kabylian Oligo-Miocene

FlyschsTellian units (External zones)Volcanism

Internal zones:

Internal/external domain boundary

Atlas

Tell-Rif foredeep and Neogene-Quaternary basins

YRTell

Rif

HabibasIslands

Oran

Cape Tenes Che

N36°

N35°

N37°

O°W5°

Ceuta

JebhaBokoya

El Marsa Che

Chelif rivermouth

Dahra

Arzew

study zone

MOROCCO

Fig. 2. Synthesis of the main geological units of the Maghrebian chain, northern Algeria (mopresent plate convergence direction between Africa and Eurasia according to Nocquet and Cathe NEIC catalog, 2006).

(mainly marls and limestones), also called the Tellian units, withfolds and thrusts verging to the south; (2) the flysch nappes, whichthrust the External zones and are composed of former sediments ofthe Maghrebian Tethys ocean that were subsequently subductedlater on; and (3) the Internal domain, composed of hard Hercynianbasement sometimes associated with its sedimentary cover, the‘‘Dorsale Kabyle’’ unit, which is a relic of the AlKaPeCa domain (forAlboran, Kabylia, Peloritan, and Calabria, Bouillin et al., 1986),a fragment of European continent. The latter detached andmigrated towards the southeast during the early to middleMiocene. This drift was enabled by the subduction of the oldTethyan ocean and the opening of the back-arc Algero-Provençalbasin. In the study area, the Kabylian micro-continent movedgenerally in a north–south direction and collided with the Africanplate between 18 and 15 Ma (Frizon de Lamotte et al., 2000;Lonergan and White, 1997), and now constitutes a piece of the‘‘Internal zones’’ (Fig. 2). Whereas the collision was probably moreor less frontal in north-central Algeria, a westward shift accompa-nied by dextral strike-slip movement occurred towards Alboran,offsetting some parts of the Internal zones towards the west, e.g.the Bokoya and Jebha massifs in the Rif (e.g. Bouillin et al., 1986).

Recent GPS measurements show an Africa–Eurasia convergenceof w5.1 mm/yr in a N60� W direction at the longitude of Algiers(Nocquet and Calais, 2004). The present-day seismic activity andthe recent new offshore tectonic maps of the region (Deverchereet al., 2005; Domzig et al., 2006) show a progressive change fromthe east, which is characterized by active NW-verging thrusts onthe slope and at the foot of the margin (one of them beingresponsible for the 2003 Mw 6.8 Boumerdes earthquake (Ayadi

El Asnam10/10/80Mw: 7.3

umerdes/03 Mw: 6.9

?ALGIERS

noua

Annaba

Constantine

E5° E10°

N45° 0°

AFRICA

EUROPE

APULIA

IBERIA

N35°

E10°

Great Kabylia

Lesser Kabylia

?

rchell

100 km

~5.1 mm/yr

TUNISIA

ALGERIA

dified from Domzig et al., 2006). YR: Yusuf Ridge. The arrow with an ellipse shows thelais (2004). The light grey circles show earthquake epicenters from 1973 to 2006 (from

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 675

et al., 2003)), to the west, which is seismically less active, butdisplays east–west oriented strike-slip features offshore. However,several large earthquakes occurred onland in this western area, asfor instance the El Asnam and Orleansville earthquakes, respec-tively, 10/10/1980, Mw: 7.1 and 9/9/1954, Mw: 6.7 (Bezzeghoudet al., 1995), or the Ain Temouchent event (22/12/1999, Mw: 5.7,Yelles-Chaouche et al., 2004), for the Oran region, which provideevidence of a combination of reverse and strike-slip faulting. Manyof these earthquakes are known to have triggered tsunamis and/orlarge submarine landslides or turbiditic currents (e.g. Orleansvilleearthquake, Heezen and Ewing, 1955). We therefore suspect thatactive tectonics influences the sedimentary processes also in thewestern offshore margin, so we will study the deposition pattern ofnear-bottom sediments and their possible link with the differentprocesses shaping the margin.

3. Seismostratigraphic setting

According to Domzig et al. (2006), the acoustic basementoffshore Oran is composed of Tellian units or flysch units. Somevolcanic material is also found near the Habibas Islands andrepresents the offshore prolongation of recent volcanic activityfound on land (Louni-Hacini et al., 1995). Conversely, the offshorepart of Tenes is probably composed mostly of Kabylian basement.On top of the basement, the typical seismic stratigraphy of theAlgerian margin has been known for a long time (Auzende, 1978, forthe Algero-Provençal basin, or Rehault et al., 1984, for the stratig-raphy of the whole Western Mediterranean). Miocene pre-saltseries, older than 5.96 Ma, directly overlie the basement. We do notknow their composition because they have never been drilled. Thesalt layer, composed mainly of halite, and the Upper Evaporiteswere deposited as a result of the Messinian salinity crisis between5.96 and 5.33 Ma (Ryan et al., 1973; Rouchy, 2001). This crisisresulted from a drop of the sea level in the Mediterranean Sea. Thisevent induced huge erosion of the continental margin, and theaccompanying evaporation of the Mediterranean Sea resulted inthe deposition of large quantities of evaporites in the basin. Aftersea-level rise led to the refilling of the Mediterranean basin duringthe Pliocene, the Pliocene to Quaternary sediments have beendeposited, from 5.32 Ma ago to present. The mobile salt formed

Swamp

1° 20' W 1° 00' W 0° 40' W

35° 40' N

36° 00' N

36° 20' N

OHabibas islands

O. Atchane

Rio Salado

Oueds

2

2200

600

Canyon "des moules"

Fig. 3. Shaded bathymetry (50 m resolution DEM) with 400-m contours of the Oran area (l(KMDJ05–KMDJ08) are shown.

diapirs, which sometimes outcrop in the deep basin, and inducedthe deformation of the overlying layers in some places.

The present sea-floor sediments have different compositions:according to Leclaire (1970), sedimentary cover on the Dahracontinental slope is globally thin and composed of muds with somegravel layers, whereas the deep basin is composed of fine sedi-ments (typically hemipelagic). The Arzew Bay (Fig. 3) is composedof sediments coming from the Cheliff and Macta rivers. The OranBay is composed of grey marls with sandy layers, the same as theneighbouring coast. North of the Dahra mountains (Fig. 2), thecontinental platform is generally rocky.

The drainage system onshore is rather sparse with rare largerivers, i.e. the Cheliff River (Fig. 3); however, several small inter-mittent streams (‘‘oueds’’¼wadis, in English) commonly haveepisodic torrential flows.

4. Data and methods

The MARADJA cruise took place aboard the R/V ‘‘Suroıt’’(Ifremer) from August 21 to September 18, 2003 on the Algerianmargin, from Oran to Dellys. Continuous bathymetric and back-scattering data using a Kongsberg EM300 Simrad multibeam echo-sounder (and EM1000 for the continental shelf) were collected.Simrad EM300 is a 32-kHz multibeam system, which allows fora swath coverage of w5 times water depth, increasing with depthto a maximum width of 5000 m at 1000 m. The horizontal resolu-tion for the bathymetry is of 15� 35 m at 1000 m depth witha vertical accuracy ranging from 2 m (central beam) to 10 m (lateralbeam). Sound speed was interpolated from regularly spacedvelocity profiles for accurate depth conversions. The bathymetricand backscattering data were processed with the Caraibes� soft-ware (Ifremer). We produced a digital elevation model for the sea-floor topography, with a resolution of 50 m for the regions of Oranand Tenes. The backscattering data were processed in order toreduce the noise of the central beam. The effects of the differentsounder modes were corrected, using the Caraibes� software. Weconstructed maps with a 25 m pixel resolution for the regions ofOran and Tenes (e.g. Figs. 3 and 6). Additionally, two types of high-resolution seismic-reflection data were collected (6- and24-channel, w50–55 kHz and 50–150 kHz, respectively). These

0° 20' W 0° 00' 0° 20'E

ARZEW

RAN

Oued Magoun

Oued

el Hammam

Oued Melah

Oued Cheliff

MOSTAGANEM

O. el Abid

Oued

Macta

Cape Ivi600

600

200

10 km

Salt ridges

KMDJ05

KMDJ06

KMDJ08

KMDJ07 Kramis Deep-Sea FanKramis

canyon

Fig. 9

Fig. 10

ocation in Fig. 1). Location of Figs. 9 and 10 and positions of the four Kullenberg cores

1° 20' W 1° 00' W 0° 40' W 0° 20' W 0° 00'

35° 40' N

36° 00' N

36° 20' N

0° 20'E

ARZEW

ORAN

Oued Magoun

Habibas islands

Oued

el Hammam

Oued Melah

Oued Cheliff

MOSTAGANEM

O. el Abid

Oued

Macta

Cape Ivi

O. Atchane

Rio Salado

Swamp

Oueds

Slope gradientin degrees

10 km

40383632302826242220181614121086420

Fig. 4. Slope gradient map of the study zone off Oran. Location in Fig. 1.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694676

seismic data helped us to image deep structures and to buildtectonic maps for the region (Deverchere et al., 2005; Domzig et al.,2006). A 2–5.2 kHz Chirp echo-sounder was also used during theentire cruise: the Chirp lines were used to construct echo-characterdistribution maps to identify the near-bottom sediment patterns inthe study area and to interpret the main depositional processes.Penetration is a maximum of 150 m with the softer the sedimentthe better the penetration. Finally, five cores (KMDJ05, 06, 07, 08,w7 m of penetration by each, and MD04-2801, 25 m) weresuccessfully obtained in the study zone by a Kullenberg piston corer(for the KMDJ cores, Maradja 2003 cruise) and a Calypso piston core(MD04-2801, PRISMA cruise, 2004, R/V Marion Dufresne).

Swamp

1° 20' W 1° 00' W 0° 40' W

35° 40' N

36° 00' N

36° 20' N

OHabibas islands

O. Atchane

Rio Salado

Oueds

5

4

6

7

Fig. 5. Backscattering imagery (Ifremer’s ‘‘belle-image’’ corrections) of the study zone off Ogrey. Bold numbers correspond to positions of backscattering signals indicated in Table 1: 1:and 7: C echoes. Position of the four Kullenberg cores is also shown.

Chirp profiles were used to identify, classify and map theacoustic characteristics (i.e. echo character or seismic facies) of thenear-bottom uppermost sea-floor sediments, and to interpret thesediment types and sedimentary processes (e.g. Damuth, 1980a,1994). In order to identify the sedimentary processes responsiblefor the near-bottom sedimentation, several steps were required.First, the different echo types observed on the data set wereidentified and classified. Then, using this classification, the distri-bution of each echo type was recorded along each ship track. Then,the data were interpolated between the ship tracks, which arespaced of about w10 km (Fig. 1), and an echo-character map wasconstructed (Figs. 11 and 12). In addition, the bathymetric and the

0° 20' W 0° 00' 0° 20'E

ARZEW

RAN

Oued Magoun

Oued

el Hammam

Oued Melah

Oued Cheliff

MOSTAGANEM

O. el Abid

Oued

Macta

Cape Ivi

10 km

1

KMDJ07

KMDJ08

KMDJ06

KMDJ052

3

ran. Location in Fig. 1. High backscatter values: dark grey, low backscatter values: lightB1 echoes, 2: B2 echoes, 3: T1 and T2 echoes, 4: T3 echoes, 5: R1 echoes, 6: R2 echoes,

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 677

backscattering maps were used in echo-character map construc-tion in order to follow as accurately as possible the contours of thesedimentary deposits (e.g. debris flows), and therefore constrainour interpretations. However, many E–W linear patches of data(along ship track) are visible, because the interpolation betweenthe lines was not always possible, when no correspondence wasfound on bathymetry or backscattering maps.

The correlations with EM300 data also helped understand whatprocesses are responsible for each echo type. In order to associateaccurately each sedimentary process to each echo type, the Chirplines were also correlated with the lithologies identified in fourpiston cores.

Specific analyses on the Kullenberg piston cores included theuse of high-resolution digital photographs and X-rays’ radiographsto reveal internal structures. Based on the lithological changesobserved, samples were taken at 1–10 cm variable intervals, thenanalysed by standard methods including measurements of watercontent, grain size analysis by wet-sieving through a 315 mm and40 mm mesh, calcimetry and microscopic study of microfaunal andmineralogical sand contents. These analyses were focused on thecharacteristic components of sands in order to identify depositionalmechanisms (i.e. gravity versus hemipelagic sedimentation).Coarser sediment fractions were examined under a binocularmicroscope and the abundance of several markers was recognizedthrough various tracers of coastal low-stand deposits originatingfrom the shelf break including preserved tests of coastal bottomforaminifers (Elphidium crispum, Ammonia beccarii, Quinquelocu-lina), evolved glauconitic grains or oxidised debris. The generallithostratigraphic interpretation was supported by 10 acceleratormass spectrometry (AMS) datings of selected pelagic foraminifers.Measurements were done at Poznan-Radiocarbon Laboratory.Calibration into calendar scales was calculated using the northernhemisphere calibration curve (software Calib 5.0.2) (Stuiver andReimer, 1993). Relative sediment accumulation rates were esti-mated through the well-dated layers.

5. Physiography of the continental margin

Our study area corresponds to a zone offshore between 1�300 W(West of Oran) and 2�100 E (Cherchell). We have divided the studyarea into two zones, respectively, the Oran (from 1�300 W to 0�20 E)and Tenes (from 0�100 E to 2�100 E) zones (boxes in Fig. 1), for theconvenience of the paper and also to produce detailed maps.

5.1. Oran

In this area the mean distance between the continental shelfedge and the deep basin floor is less than 20 km (Fig. 3). Thecontinental shelf continuously deepens from Arzew (200 m deep)to the north of Mostaganem (800 m depth). The shelf width isrelatively narrow in front of the capes (less than 10 km) but widensin the bays (up to 40 km in the Arzew Bay). The continental slopebetween 1�300 W and 0�200 E is very linear and is cut by numerouscanyons (Fig. 3). This steep (w16% declivity) slope (Fig. 4) is thoughtto represent a former sheared margin (Domzig et al., 2006). Thisformer strike-slip activity is thought to be associated with theMiocene westward shift of the Alboran block. The foot of themargin is particularly linear and well marked, but the local seis-micity is very scarce and Domzig et al., 2006, have found noevidence for sub-surface breaks or fault activity at depth. Therefore,in this study, we will consider this segment of the offshore marginas tectonically inactive. Northeast of Arzew, the canyons are mostlyperpendicular to the slope, and generally display several smalltributaries. However, only one major river (the Cheliff river) on thefacing coast is feeding this part of the margin. Therefore, many ofthe canyons were probably formed during the sea-level drop

associated with the Messinian salinity crisis or during subsequentQuaternary glacio-eustatic low stands of the sea level. During thepresent high sea-level stand, only few canyons appear to be activeaccording to the reflectivity pattern observed (Fig. 5), if we considerthat only the highly reflective canyons are active, which we willdiscuss later.

The relief of the canyons on the slope between Arzew and CapeIvi (eastern part of the Fig. 3) is rather smooth and canyons do notdeeply incise the slope. The canyons abruptly stop at the foot of thecontinental slope. West of Arzew, the margin trends in an E–Wdirection, and the canyon interbeds, which show a sharper relief insome places, continue further across the deep basin floor andappear larger than the canyons on the slope between Arzew andMostaganem (Fig. 3). Some volcanic outcrops exist in this area(Leclaire, 1970), which would explain heterogeneous erosionpatterns within the slope. The basin west of Oran is shallower(w2400 m) and is mainly filled by the material drained by the largecanyons there. These canyons are cut by NW–SE structures, whichwere interpreted as branches of a flower structure associated withthe eastern prolongation of the strike-slip Yusuf fault (Domzig et al.,2006). In one of these branches, the southern and most importantone, the ‘‘canyon des moules’’ (El Robrini, 1986; Fig. 3) trends in anESE–WNW direction.

The edge of the deep basin lies at w2700 m offshore of Oran toEl Marsa, whereas west of Oran, the continental rise, constructed bydeposition from the large canyons there, continues farther in thebasin (Fig. 3). A few salt ridges outcrop at 65 km from the coast,offshore Cape Ivi along a SW–NE trend parallel to the slope.

5.2. Tenes

This part of the margin trends east–west and the continentalshelf is absent or very narrow (<10 km) (Fig. 6). The continentalslope is particularly linear and rather steep (10% declivity, Fig. 7), itis narrow on the east (20 km) but widens westward to 40 km. Thedense network of linear canyons is perpendicular to the coast andthe canyons generally display numerous tributaries. The inter-canyon areas have a sharp morphology. The canyon walls are verysteep with slope gradients up to more than 40% (Fig. 7). Thecanyons seem still active, as suggested by the high reflectivity intheir axes (Fig. 8).

East of 1�500 E, the shelf break is 20 km from the coast. Justseaward is the western end of the Khayr al Din Bank (KADB onFig. 6). The bank is in fact a perched basin limited to the north bya south-dipping thrust, and to the south by an accommodatingnormal fault (Domzig et al., 2006). The bank deepens westward(from 2000 to 2500 m deep), and gently sinks towards the deepbasin around 1�500. Its western side seems to be strongly destabi-lized. West of the bank, in the deep basin, we observe roughly E–Woriented sediment waves probably formed by the action of deepbottom currents or turbidites (compare with Migeon et al., 2001and Savoye et al., 1993 for the Var ridge). Further west, our data setterminates at the foot of the slope, so we have no information onthe deep basin sediments.

Northeast of El Marsa, we observe some sinuous canyons. Thechanges in canyon sinuosity are located along straight E–W linea-ments, which suggests that they are caused by faults (Domzig et al.,2006). However, we do not see discontinuities in the basement, sowe suggest that these lineaments could also be the result ofdetachment and sliding towards the basin of the sedimentarycover.

At the western end of the Tenes map near 0�200 E, a roughlyE–W oriented field of sedimentary waves forms the highly devel-oped northern levee of the Kramis Deep-Sea Fan (also called ElMarsa Deep-Sea Fan, see El Robrini, 1986; Domzig et al., 2006;Mauffret, 2007), which is 40 km long and 20 km wide. This fan is

-2400

0° 20' E 0° 40' E 1° 00' E 1° 20' E 1° 40' E 2° 00' E

36° 20' N

36° 40' N

37° 00' N

O. Kramis

EL MARSA

TENESGOURAYA

CHERCHELL

Mount Chenoua

Oued D

amou

s

Oued Cheliff

Cape TenesOued

Allalah

Oued Tarzout

O. Aradj

O. el Abid

Oued Malah

O. MesselmounO. Es Sebt

Canyon Guelta

CanyonKhadra

Canyon Kramis

Kramisdeep-sea fan -2400

-2000

-2000

-2000

-1600

-1600

KADB

10 km

Fig. 15

Fig. 14

Fig. 13

Salt ridges

MD04-2801 core

Fig. 16

Fig. 21

Fig. 6. Shaded bathymetry (50 m resolution DEM) with 400-m contours for the Tenes area. KADB: Khayr al Din bank. The names of the canyons are from El Robrini (1986). Locationin Fig. 1. Positions of Chirp lines in Figs. 13–16 and 21, and position of Calypso piston core MD04-2801 are shown.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694678

fed by the E–W Kramis Canyon, and bounded to the east by theKhadra Canyon (Fig. 6). As shown by Domzig et al. (2006), theKramis canyon is currently developed on top of an E–W strike-slipfault. However, this fault does not seem to have been active in theQuaternary. Upslope, two tributaries feed the Kramis Canyon: oneoriented E–W, north of Oued Kramis, and the other N–S, north ofOued Malah (Fig. 8). In front of the N–S branch of the canyon, thesediment waves of the fan are higher (more than 100 m high) thanthe surrounding sediment waves (less than 100 m high). Some E–Woutcropping salt ridges with crestal grabens occur north of thedeep-sea fan in the deep basin (Fig. 6).

6. Echo character

6.1. Classification of Echo Types

The 2–5.2 kHz echoes observed in the study area are classifiedinto 12 different echo types according to their reflection character(our classification is inspired from Damuth, 1975, 1980a; Gaullierand Bellaiche, 1998; Mitchum, 1977; Vail et al., 1977) (Table 1).

0° 20' E 0° 40' E 1° 00' E

36° 20' N

36° 40' N

37° 00' N

O. Kramis

EL MARSA

TENE

Oued AllaO. Tarzout

O. Aradj

O. el Abid

Oued Malah

Slopegradient

in degrees

10 km

Oueds

4038363432302826242220181614121086420

Fig. 7. Slope gradient map of the study

6.1.1. Echoes with parallel to subparallel internal reflectionsThe typical parallel or subparallel bedded echo type (B1) (Table

1) is an alternation of parallel continuous thin high- and low-amplitude reflections. For this echo type, sea-floor penetration ofthe Chirp system is generally highest (up to 150 m). NB: when B1echo type occurs on the continental slope, the reflections havea slightly higher amplitude than in the deep basin. This could bedue to the difference of incidence angle of the sounder signal, or thedifference of water depth, that produces higher amplitude echoeson the slope than in the deep basin. However, this pattern couldalso correspond to different types of sediments on the slope versusthe deep basin. This phenomenon is also observed in other zoneswhere the same echo-sounder has been used (i.e. Tahchi et al.,submitted for publication; Dan et al., in press).

According to the literature, B1 echo type generally correspondsto alternations of muddy/silty (hemipelagic) and coarser (turbi-dites) deposits (e.g. Damuth, 1980a). Sometimes this echo isdirectly correlated with sedimentary channel levees, built byoverbank deposits of turbidity currents (Damuth, 1980a). Similarlayers of muds interbedded with silts, have been identified in theGulf of Lions (Gaullier and Bellaiche, 1998). The B1 echo has also

1° 20' E 1° 40' E 2° 00' E

SGOURAYA

CHERCHELL

Mount Chenoua

Oued

Dam

ous

Oued Cheliff

Cape Teneslah

O. MesselmounO. Es Sebt

zone off Tenes. Location in Fig. 1.

0° 20' E 0° 40' E 1° 00' E 1° 20' E 1° 40' E 2° 00' E

36° 20' N

36° 40' N

37° 00' N

O. Kramis

EL MARSA

TENES

GOURAYA

CHERCHELL

Mount Chenoua

Oued

Dam

ous

Oued Cheliff

Cape TenesO. Allalah

Oued Tarzout

Oued Aradj

O. el Abid

Oued Malah

O. MesselmounO. Es Sebt

10 km

MD04-2801core

Fig. 8. Backscattering imagery (Ifremer’s ‘‘belle image’’ corrections) of the study zone off Tenes. Location in Fig. 1. High backscatter values: dark grey, low backscatter values: lightgrey. Position of Calypso piston core MD04-2801 is shown.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 679

been attributed to pelagic or hemipelagic sediments (e.g. Damuth,1980a, Le Cann, 1987; Pratson and Laine, 1989; Yoon et al., 1996).

When this echo type displays more discontinuous to hummockyreflections in the layering (but layering is still visible), it is classifiedin the B2 category (Table 1). When the layers undulate and occurnear a channel, they likely represent migrating sediment waves onsedimentary channel overbank deposits. Alternatively, theseundulating bedforms can be small-scale sediment waves created bycontour currents (e.g. Heezen et al., 1966; Hollister et al., 1974;Damuth, 1980a) or creep/slump deposits (e.g. Syvitski et al., 1987;Lee and Chough, 2001). We did not identify regular undulations inour chaotic bedded echo type (B2), thus there appears to be anothercause for the formation of B2 echoes. The degree of disorganisationof the bedding suggests deformed turbidites (e.g. Damuth, 1980b)and/or slumping (Embley and Jacobi, 1977; Chough et al., 1985;Pratson and Laine, 1989; Damuth, 1994). The discontinuous char-acter on the bedding could also correspond to coarser turbidityflows (Mear, 1984) or debris flows.

6.1.2. Transparent echo typesWe distinguished four different echo types containing one or

several transparent lenses: transparent lens on top of parallel tosubparallel reflections (T1), transparent lens with no reflectionsbelow (T2), alternation of transparent lenses and parallel tosubparallel reflections (T3), and transparent lens buried in B1 echotype (T4), transparent lens with rough sea-floor and irregular high-amplitude reflections at the surface (T5) (Table 1).

T1 echo type shows a homogenous transparent lens overlying(and commonly truncating) parallel to subparallel continuouslayers. The base of the lens is generally irregular and suggests anerosion surface. T2 echo type also shows a transparent lens, but itoverlies an irregular and highly reflective sediment with no internalreflections. The T3 echo type corresponds to transparent lenses ontop of each other, separated by parallel to subparallel reflections(B1). T4 type is like T1, except that the transparent lens(es) arecovered by B1 facies at the surface. We will see later that this willhave some implications in the backscatter signal. Finally, T5corresponds to transparent lenses at depth, with rough sea-floorand chaotic high-amplitude reflections on top of the transparentlens.

In general, the transparent facies is due either to the dis-organisation of the bedding with incorporation of water during thesediment transport, or simply to the original absence of

organisation in the sediment. These echo types generally representthe acoustic expression of mass-transport deposits (debris flows,mud flows) (Embley, 1976, 1980; Jacobi, 1976; Damuth, 1980a,b;Damuth et al., 1983). These mass-transport processes (slump, slide,and debris flow) can occur on slopes, canyon flanks, levees, or saltdiapirs (see examples in Gaullier and Bellaiche, 1998) when thesediment becomes destabilized. The hypothesis that these echotypes can also correspond to muds or hemipelagic deposits hasbeen mentioned in Loncke et al. (2002) and Tripsanas et al. (2004b),but generally, the physiographic position, the erosive base and thefact that the transparent lenses are of variable thickness identifythe sedimentary process as a mass-transport. Hemipelagic sedi-ments are generally conformable over the underlying surface and ofconstant thickness.

Transparent lenses separated by sequences with regularbedding (T3 or T4) correspond generally to interlayered debrites(transparent mass-transport deposits) and turbidites or hemi-pelagites (bedded layers) (Embley, 1976; Chough et al., 1997).

6.1.3. Single to overlapping hyperbolic echoes with poor orno sea-floor penetration

Four different types of echoes are recorded from areas that showno, or very poor, sea-floor penetration (Table 1). R1 echo typeshows a smooth, prolonged and very reflective sea-floor with nosub-bottom penetration. We cannot distinguish any structurebelow the surface. This echo type is generally located in the axis ofthe submarine canyons, which corresponds to particularly hardsea-floor (highly eroded) covered with heterogeneous and coarse-grained turbidite deposits (Damuth, 1975).

R2 echo type also shows little to no sea-floor penetration and ischaracterized by small irregular overlapping hyperbolae withchaotic sub-bottom reflections of relatively high amplitude. Inother studies, this type corresponds to rough sea-floor coveredwith coarse sediments like large rafted blocks or mass-transportdeposits (Damuth, 1975; Jacobi, 1976; Le Cann, 1987), also calleddebrites (Nardin et al., 1979; Damuth and Embley, 1981; Lee et al.,1999).

R3 echo type shows lens(es) of low-amplitude chaotic reflec-tions, which overlie reflections (often chaotic) of higher amplitude.Small hyperbolae are sometimes found at the surface, like on theexample shown in Table 1.

The large irregular hyperbolae of echo type R4 are recordedfrom rugged, high-relief slope areas where the echo-sounder is

Table 1Classification of echo types observed and mapped in the study area

Echo type Typical 2–5 kHz echo Examples ofassociatedbackscattering

Occurrence Sedimentary processes

B1: parallel to subparallel high- to low-amplitude sub-bottom reflections

1Deep basin or upslope, away fromcanyon axis

- Hemipelagic sedimentation(pelagic sediments withinterbedded thin turbidites)

- Turbiditic currents deposits

B2: discontinuous parallel to hummocky high-amplitudesub-bottom reflections

2

Mid-slope, canyon walls Slides, slumps

Foot of slopes- Mass-transport deposits (like

slumps)- Turbidity currents

Deep-sea fan Turbidity currentsDeep basin, areas not connectedwith canyon systems

- Contour currents- Non-sedimentary process:

active faulting

T1: transparent lens over continuous parallel or subparallelbedding

3

Distal part of canyons, foot of slope- Mass-transport deposits- Turbidity currents deposits

Continental shelf, next to a rivermouth

River discharge

T2: transparent lens over irregular sub-bottom with noreflections below

3

Canyon flanks or axis, salt diapersMass-transport deposits (slides,debris flows)

Distal part of canyons Turbidity currents depositsSediment waves in the deep basin Turbidity currents, mass-transport

deposits, or contour currents

T3: alternations of transparent and bedded 4 Foot of slope, downslope canyons- Mass-transport deposits- Turbidity currents deposits

T4: transparent lens(es) buried in parallel to subparallelcontinuous reflections

See B1 echotypesbackscattering

Foot of slopes

- Mass-transport deposits- Turbidity currents deposits

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694680

Table 1 (continued )

Echo type Typical 2–5 kHz echo Examples ofassociatedbackscattering

Occurrence Sedimentary processes

T5: buried transparent lens(es) with rough sea-floor andchaotic reflections at surface

No typicalbackscattering

Foot of slope, downslope canyons,or canyon flanks, sediment wavesin the deep basin

Mass-transport or turbidites, withor without active faulting orcontour currents

R1: prolonged single echo with no sub-bottoms 5

Canyon axis Turbidity current erosionSediment waves in the deep basin,far from canyons systems

Mass-transport turbidity orcontour currents (rough sedimentdeposits)

Continental shelf Outcropping bedrock (no sedimentdeposition)

R2: small irregular overlapping hyperbolae 6

Canyons, sediment waves in thedeep basin, slope

- Turbidity currents- Mass-transport

Salt ridges Mass-transport deposits

R3: chaotic lens of low-amplitude reflections on top ofhigher amplitude chaotic reflections

No typicalbackscattering

Foot of slopes, downslope ofcanyons or canyon flanks, sedimentwaves in the deep basin

Mass-transport or turbiditycurrents, with or without activefaulting or contour currents

Continental shelf next to a rivermouth

River discharge

R4: large overlapping hyperbolae or no dataNo typicalbackscattering

Slope /

C: semi-transparent lens with discontinuous low-amplitude sub-bottom reflections over irregular sub-bottom with no penetration below

7

Continental shelf, next to a riverdelta

River discharge

Continental shelf, far from a riverdelta

Coastal currents deposits?

Pictures show examples of each echo type. Typical corresponding backscattering signal is shown in column 3: the numbers refer to positions in Fig. 5. The last column showsthe possible sedimentary processes associated with the echo types according to their location.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 681

3450

3500

tw

tt (m

s)

7.5 m

1 km

NW SE

B1

B2

B1

Fig. 10. Chirp line illustrating continuous parallel bedding (B1 echoes) with, locally,B2 echoes (see explanation in the text). Position of the Chirp line is shown in Figs. 3and 11.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694682

unable to penetrate. Therefore we consider this echo typerepresents areas with no data.

6.1.4. Continental shelf echo characterOn the continental shelf, where the water depth is less than

200 m, we found a particular echo type (C in Table 1) that differs inmany ways from the previous categories. First, it displays a highlyreflective sea-floor bottom reflector, which is due to the smalldistance between the sounder and the bottom, and the frequency ofthe reflections is also higher than in the deep basin. Secondly, itshows a layer of high-frequency and low-amplitude (almosttransparent, in some cases) continuous to discontinuous reflectionscovering a rugged and highly reflective sediment with no pene-tration below.

6.1.5. Ambiguities between different echo typesAssigning specific echo character to a specific type in the clas-

sification is not always straightforward. The classification of echotypes represents end members and it is sometimes difficult todecide which type a particular echo falls into. In particular, thedifference between R2 and R3 echoes is often small, especiallywhen the lower amplitude lenses in R3 echoes are not large(sometimes the size of one or two hyperbolae). In the same way,when the reflections of B2 echoes are very discontinuous and tendto be chaotic, the boundary between B2 and R2 echoes is hard todistinguish. Sometimes we also observe a progressive transitionfrom one echo type to another. In Fig. 9, we show the example ofa transparent lens that thins to the north–west. In this case, thegeographical limit between the two echo types (¼the limit of thetransparent lens) is not easy to set.

Another ambiguity commonly encountered is based on the factthat B1 echo type often shows low-amplitude reflections at thesurface. In some places, when the parallel sub-bottoms are slightlychaotic, the low-amplitude reflections at the surface look more likea transparent layer. However, we should not classify it in T1 type,because we still distinguish the layers of B1 echoes, and we do notidentify an unconformity, so we see that these sediments areautochtonous and do not represent any mass-transport deposit(such as typical transparent layers). These echoes must then beclassified in the B2 type (Fig. 10).

6.1.6. Correlation between backscattering imagery and echo typesAs the reflectivity of a sediment depends on the nature of its

uppermost layers and the sea-floor roughness, we are able tocorrelate many of the echo types with specific backscatteringsignature recorded with the EM300 sounder (Table 1, Figs. 5 and 8).

The B1 echo type shows relatively homogeneous intermediatereflectivity values (grey on the backscatter map, Fig. 5). B2 echoeshardly show a typical backscattering signature, because they aresimilar to B1, except that their reflection pattern shows some

3450

3500

7.5 m1 km

tw

tt (m

s)

T4 B1

NWSE

Fig. 9. Chirp line illustrating a progressive lateral transition of echo types from T4 (left)to B1 (right). Position of Chirp line is shown in Figs. 3 and 11.

variations of intensity. However, we chose to show an example ofbackscattering signal associated with this echo type (see Table 1and Fig. 5).

The continental shelf echo type (C) correlates with a veryhomogeneous backscattering pattern, which is relatively similar tothe pattern associated with the B1 echo type, but with higherreflectivity than the B1 echo type, and the central part of each trackis highly reflective.

The transparent lenses at the surface (T1–2 echo types) gener-ally display low reflectivity. They are not easily recognized on thebackscattering maps, because if the transparent lenses are coveredby a thin layer of hemipelagic material or coarse sediments (thinenough to be not detected on the Chirp lines), the reflection patternwill change (more reflective). T3 echo types display higher reflec-tivity than T1–2 echoes. This may be due to the fact that the onlyexamples of T3 echoes in our study zone are perhaps covered bythin layers of coarser sediments that make this echo type morereflective.

R1 echo type, generally in the canyon floors, is typically char-acterized by high to very high reflectivity, which permits an easyidentification of the canyon paths (Figs. 5 and 8). R2 echo typedisplays various backscattering values, which produce light anddark patches on the maps (Fig. 5). T4 echoes show the samereflective pattern as B1 echoes. T5 and R3 echoes backscatter signalis similar to the one of B2 or R2, and is very variable, so we did notassign a typical backscattering pattern. R4 echo types have notypical backscattering signal.

6.2. Echo-character maps, and possible associated sedimentaryprocesses according to the location of echoes

Echo-character maps showing the distribution of echo typesthroughout the study area are shown in Figs. 11 and 12, for Oranand Tenes, respectively. The general geographic occurrences of eachecho type as well as their possible sedimentary processes accordingto their location are listed in Table 1.

Echo type B1 occurs mainly in the deep basin, away from canyonsystems, and more rarely at the foot of the slope. We can also find itupslope, on flat areas between canyons (Figs. 11 and 12). It alsoappears in some places on the sediment waves of the Kramis

1° 20' W 1° 00' W 0° 40' W 0° 20' W 0° 00'

35° 40' N

36° 00' N

36° 20' N

0° 20'E

ARZEW

ORAN

Oued Magoun

Habibas islands

O. el Hamm

am

Oued Melah

Oued Cheliff

MOSTAGANEM

O.el Abid

Oued

Macta

Cape Ivi

O. Atchane

Rio Salado

10 km

drainage system

SwampEcho types

R4, or no data

R2

B2

B1

T2 R3T1

R1

CT4

T5

T3 KMDJ06

KMDJ05

KMDJ08

KMDJ07

Fig. 9

Fig. 10

Fig. 11. Echo-character map of the Oran area. Location in Fig. 1. Position of the Kullenberg cores is indicated with their corresponding Chirp lines (black bold lines) (Figs. 17–20).Position of Chirp lines in Figs. 9 and 10 are shown.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 683

Deep-Sea Fan (Figs. 6 and 12). It is the most widespread echo typethroughout our study zones, we can see it all along the margin(Figs. 11 and 12).

In the deep basin, this echo type could correspond to inter-bedded pelagic sediments and thin turbidity-current deposits. Twoof our cores (KMDJ07 and 08) are located on this echo type and willhelp confirm this (see the core description below). However, whenthis echo type is recorded from a topographic high, or away fromthe canyons’ influence, it probably represents old consolidatedbedded sediments.

Echo type B2 is only found in the deep basin or rise, generally inthe distal part of canyons, or on slopes, but also in sediment waves,in particular north of Gouraya and in the Kramis Deep-Sea Fan(Fig. 12).

This echo type is likely to represent hemipelagic sediments thathave been remobilised by different processes. First, the turbiditycurrents can create irregular little waves in the sediments, giving

0° 20' E 0° 40' E 1° 00' E

36° 20' N

36° 40' N

37° 00' N

O. Kramis

EL MARSA

TENEO. Allala

O. Tarzout

O. Aradj

O. el Abid

Oued Malah

R4, or no data

R2B2B1

T2 R3T1

R1

C

drainage system

T4

T5

T3

10 km

Echo types

Kramisdeep-sea fan

MD04-2801

Fig. 15

Fig. 13

Fig. 16

Fig. 21

Fig. 12. Echo-character map of the Tenes area. Location in Fig. 1. Position of MD04-2801

the discontinuous and sometimes hummocky character to the sub-bottom reflections. However, some areas, like the eastern slope ofKramis Deep-Sea Fan, or north–west of Gouraya (Fig. 12), are notlinked with canyons. In this case, two mechanisms may be implied:mass-transport deposition (especially when we clearly identify anunconformity in the reflections, as for the slump identified on Chirpline in Fig. 13, north–west of El Marsa (Fig. 12)), or bottom currents(see Section 8.2). When this echo type is located at the foot of theslope or in canyons, mass-transport (like slumping) is the mostprobable process at the origin of the echo type.

The transparent echo types (T1–T5) are generally recorded atthe foot of slopes, but they are also observed at mid-slope oncanyon flanks, and in very few places on the continental shelf(Figs. 11 and 12).

T1 echoes are found at the foot of the slope, not far froma canyon, north of Tenes and north–west of El Marsa (Fig. 12), and inone place upslope on the continental shelf, next to a river mouth

1° 20' E 1° 40' E 2° 00' E

S

GOURAYA

CHERCHELL

Mount Chenoua

Oued

Dam

ous

Oued Cheliff

Cape Tenes

h

O. MesselmounO. Es Sebt

Khayr al Din

bank

Fig. 14

Calypso piston core and positions of Chirp lines in Figs. 13–16 and 21 are indicated.

7.5m

1 km

Slump3100

3150

Dep

th

in

m

s tw

tt

7.5m

1 km

3100

3150

Dep

th

in

m

s tw

tt

B2

Vertical exaggeration:~130

WE WE

B1

Fig. 13. Chirp line in B2 echo type illustrating a slump (discontinuous to chaotic reflections on top of continuous parallel reflections), north–east of the Kramis Deep-Sea Fan. To theleft: non-interpreted line. Location in Figs. 6 and 12. Vertical Exaggeration: w130.

Canyon axisWE

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694684

(Oued Cheliff, Fig. 11). T2 is typically located downslope of canyonsor on canyons flanks, in the two study zones. Occasionally, we findit also in the deep basin in sediment waves or next to salt domes. T3is exclusively recorded downslope of canyons in the deep basin, intwo places in our study zone: north of Oran, and NW of HabibasIslands (Fig. 11). T4 is only identified at the foot of the continentalslope, along the Arzew–Mostaganem slope (Fig. 11), north of Tenes,north of Cherchell (Fig. 14) and NW of El Marsa (Fig. 12). We foundonly one example of T5 in Oran study zone (Fig. 11): north of Arzew,at the foot of the slope, but in the Tenes zone (Fig. 12), we also find itin canyons (Fig. 15) or in sediment waves (Kramis Deep-Sea Fan, orwest of Khayr al Din bank, Fig. 12).

The transparent lenses are likely to correspond to mass-transport deposits such as debris flows or slides when they arelocated at the foot of a slope (continental slope, canyons flanks, orsalt domes), or turbidity current deposits when located downslopeof canyons (like the large transparent deposits north of Oran, or NW

7.5 m

1 km

2720

2730

2740

2750

2760

Dep

th

in

m

s tw

tt

T4

WE

Fig. 14. Chirp line illustrating a transparent layer buried between undisturbed bedding(echo type T4). Location in Figs. 6 and 12. Vertical Exaggeration: w130.

of the Habibas Islands). When T1–T5 echoes are found in sedimentwaves, they could be local small slides. The example of T1 echo typelocated on the continental shelf is likely to be correlated with fluvialsediment discharge, as it is located near the Cheliff river mouth.

The limits of an echo type are not necessarily correlated with thelimits of a deposit, as it is shown in Fig. 16: in this case, T4 and T5echoes are next to each other but represent parts of the samedeposit. This case also shows that several successive processes maybe at the origin of the present pattern: we can hypothesize a firstmass-transport deposit on top of parallel continuous reflection,which is followed by hemipelagic sedimentation (parallel contin-uous reflections) or more mass-transport deposits (e.g. to the left of

7.5 m

1 km

Mass-transportdeposit

tw

tt (m

s)

3400

3450

T5T5 R1

Fig. 15. Chirp line illustrating a T5 echo type around R1 echo type characterizing thecanyon axis. Below the canyon axis, the sub-surface is totally masked, because thecanyon floor is so reflective that the echo-sounder is unable to penetrate. As the twotransparent lenses are at the same level, it is likely to be one unique lens, as shown bythe dotted lines, its center being masked by the highly reflective canyon. Location inFigs. 6 and 12. Vertical Exaggeration: w130.

7.5 m

1 km

tw

tt (m

s)

3400

3450

WE

R1 T4 T5 B1T4

Fig. 16. Chirp line illustrating T4 and T5 echoes on a same transparent buried lens. Tothe left of the profile, we also see the superimposition of two mass-transport deposits.Location in Figs. 6 and 12. Vertical exaggeration¼w130.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 685

the profile), and finally turbidity currents creating the chaoticreflections at the surface in some places (T5 echo type).

R1 echo type is characteristic of active canyon floors and isapparently related to the high reflective properties of eroded rocks,gravel, and sand in the canyon floors. This echo type is found incanyon axes all along the Tenes margin (Fig. 12). In contrast, in theOran zone, this echo type is only found on the continental shelf, orin the canyons between the Habibas Islands and Arzew and at thenorth-eastern end of the map (Fig. 11), where the canyons are widerthan in the Arzew–Mostaganem slope (Fig. 3). The restriction of therugged (R1) echo type almost exclusively to canyon floors confirmsthe erosional action of these drainage paths and the presence ofgravels and sand (i.e. coarse sediments) there. When located on thecontinental shelf, this echo type could correspond to hard bedrockoutcrops or very reflective coarse sediments coming from coastalcurrents or river discharge. However, this echo type is also found inthe Kramis Deep-Sea Fan and in several places next to the slide areawest of Khayr al Din bank (Figs. 6 and 12), corresponding here torough areas representing eroded slide scarps, or simply deposits ofvery coarse sediments, associated with mass-transport, turbidity orbottom currents (example: NW of Gouraya).

R2 echo type is often located in canyons, slopes, and occasion-ally on sediment waves and salt domes. The chaotic character of thereflections suggests a high degree of remobilisation of the sedimentand thus deposits from turbidity currents or debris flows. The high-amplitude reflections probably testify to the presence of coarsesediments.

R3 echo type is found downslope of canyons (e.g. North ofArzew) or on canyon flanks (e.g. North of El Marsa), but also in thesediment waves NW of Gouraya (Fig. 12), and on the continentalshelf, next to the Cheliff river mouth (Fig. 11). When this echo typeoccurs downslope of canyons, it is likely to be the result of turbiditycurrents deposits or mass-transport deposit, whereas when itoccurs on canyon flanks, it would correspond to local slides incanyon flanks. On the continental shelf, this echo could be linked toriver discharge process, and on sediment waves, either turbidity orcontour currents. However, as explained in Section 6.1.5, R3 echotype is possibly a B1 echo that has been ‘‘shaken’’ by active tectonics(see Section 8.2).

Finally, C echo type is located on the continental shelf, mainly inareas near river mouths (Bay of Arzew, Fig. 11), but also sometimesin bays not fed by any river (Bay of Oran, Fig. 11). This echo is absentin the Tenes area (Fig. 12), probably mainly because we have no datafrom the continental shelf, which is very narrow. C echo type mayreflect the accumulation of the river sediments on the bedrock ordeposits brought by coastal currents. The KMDJ05 core has beensampled in this echo type and will help us to solve this.

7. Sediment cores

We studied the four Kullenberg piston cores from the Oran areato help ground truth the depositional processes represented bysome of the echo types (Table 2). We also use a long Calypso corefrom the PRISMA cruise in 2005 (Sultan et al., 2004).

7.1. Core KMDJ05

It is 768-cm long and was raised from a water depth of 75 m onthe middle part of the narrow shelf off the Cheliff River mouthwhere type C echoes are recorded (Fig. 11). The core was taken toprovide a record of the direct river input of the Cheliff River over-flows. The deposits consist mostly of dark grey and slightly organicmud with common black thin silt/sand laminae with higher organicmatter contents than the mud (Fig. 17). Sand content in the laminaeis generally <10%. These laminae are attributed to recurrent CheliffRiver overflows. However, taking into account the general beddedecho types of this upper mud unit, more sandy laminae arepresumed to occur in the shoreward lateral facies. The core pene-trated through 10 cm of the underlying high-amplitude reflections,which are a coarse, shelly deposit with >70% sand. This sandcontains reworked shallow-marine faunas. The sand is relativelywell-sorted with abundant mollusc borings in many of the verywell rounded shell fragments, which shows that they resided for atleast some time in the littoral zone. This deposit provides evidencefor a step of the last transgression dated in this study at11,210 cal. yrs BP (Fig. 17). Another high-amplitude bedded echotype was recorded on the Chirp line directly overlying the ruggedhighly reflective metamorphic bedrock (see arrow on Fig. 17). Thisolder sediment accumulation is presumed to correspond to theindurate grey–blue muds related to the transgressive tract ofoxygen isotopic Stage 3. This same facies was found included in thedebris flow of the lower slope core (KMDJ01) of the Algiers margin(Giresse et al., 2006, 2009).

7.2. Core KMDJ06

It is 818 cm in length and was raised from 2651 m water depth atthe base of the slope off Cape Ivi, northwest of the Cheliff Rivermouth (Fig. 3). The core site is at the foot of a well-incisedsubmarine canyon and displays a T4 echo (Fig. 11). Metallic greycolour throughout the entire sedimentary column indicates theabundant iron sulphides derived from Neogene flysch unitsoutcropping in the Cheliff River catchment. The upper 6 m appearas fairly homogeneous grey beige muds with thin turbiditeswhereas the lowermost 2 m include thicker and coarser turbiditeintervals (Fig. 18). The lower (coarser) part and the normal gradedpart of turbidites are easy to identify, but the limit in the gran-ulometry between the upper (finer) part of the turbidite andhemipelagic sediments (silts and muds) is not always straightfor-ward. So we will mainly talk about the base of the turbidites, whileidentifying them. The base of the thickest turbidite recorded is at670 cm depth and has a sand content up to 90%wt between 670 and630 cm. The thickest turbidites (e.g. 650, 375, and 300 cm) exhibita normal grading. Some small cm-scale schistous debris areincluded in the sandy bases of the turbidites, but their occurrence,even in the interbedded hemipelagites, indicates recurrent trans-port by turbidity currents. Average carbonate contents rise near30%wt with some peaks up to 40%wt and indicate a positivecorrelation with sand content in the coarse bases of the turbidites(Giresse et al., 2009). Three 14C ages were obtained:14,891 cal. yrs BP at 800–791 cm, 14,232 cal. yrs BP at 559–551 cm,and 11,748 cal. yrs BP at 212–201 cm, and suggest a markeddecrease of the sediment accumulation rate during the Holocene.Two intervals of relatively slower accumulation rates are identified

Table 2Sediments of the cores, corresponding echo types, and sedimentary processes interpreted from the core sediments

Core Core content Corresponding echo type Corresponding sedimentary processes

KMDJ05 Dark grey and slightly organic mud frequently interrupted by blackthin laminate of sand and organic matter

C Fluvial deposits and/or littoral currentsdepositsþ pelagic sedimentation

KMDJ06 Six first meters: homogeneous muds with few thin turbidites 2 lowermeters: muds with more frequent and coarser turbidites

T4 6 first meters: semi-transparent 2 lower meters:alternation of high- and low-amplitude parallelreflections

Alternation of pelagic sedimentationand turbidity currents deposits

KMDJ07 Thick and coarse turbiditic sequences, more frequent at depth, eachwith an erosive base

B1 higher amplitude reflections at depth Turbidity currents deposits

KMDJ08 Muds containing an upward decreasing frequency of turbidites B1 higher amplitude reflections at depth Pelagic sedimentationand turbidity currents deposits

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694686

between 520 and 320 cm and in the uppermost 100 cm based onmoderate to intense bioturbation.

The low-amplitude parallel reflections and (semi-)transparentechoes in the upper part of the Chirp line correspond to the fine-grained turbidites and hemipelagic sediments (like in Tripsanaset al., 2004b) from 0 to 630 cm. At 630 cm depth, the thick sandylayer corresponds to a high-amplitude reflection (Fig. 18). Below,the high-amplitude parallel reflections correspond to the higherfrequency of thin turbidites.

According to the location of this core (i.e. foot of slope), thetransparent lens (T4 echo, Fig. 11) is likely to be a mass-transportdeposit, however, it surprisingly corresponds to normal gradedturbidites in the core. On the Chirp line we can see that this deposithas a thickness that varies laterally. So, maybe this deposit is notclearly identified in the core because of a local thinning of thismass-transport deposit at the core site location. To identify thenature of the transparent deposit, we would need more cores in thisarea.

7.5 m

2 km

Bedrock

coarse sand percent (>315 µm)

fine sand percent (40-315 µm)

silt-clay sediment percent (<40 µm)

NW SE

Diameter of particles:

Fig. 17. Chirp line at the location of core KMDJ05, sampled in C echo type. Graph to the rightVertical exaggeration¼w260.

7.3. Core KMDJ07

It is 6.36-m long and was raised from 2631 m of water depth onthe westernmost end of the Kramis Deep-Sea Fan (Fig. 11), in the B1echo type. Thick turbidites (38 in total, identified by their coarsebase) with coarse granulometry occur throughout the core;however, the abundance, thickness and sediment coarsenessappear to diminish in the upper 2/3 of the core section (Fig. 19).Most of these turbidites are predominantly sand up to 30 cm thick(e.g. 28 cm of sand above 632 cm, 2–3 cm above 499 cm, 30 cmabove 432 cm, 4 cm above 398 cm, and 4 cm above 188 cm). Thegrey beige muds, partly hemipelagic, show a metallic shadeinduced by iron sulphides. Each turbiditic sequence is underlinedby an upward colour grading: dark grey, medium grey, pale grey,and beige to ochre. The top of each of these sequences wasgenerally truncated during the deposition of the next sandyturbidite layer, which is marked by a sharp irregular base. Threeradiocarbon ages are available: 14,094 cal. yrs BP at 589–586 cm,

00 10 20 30 40

11,210 cal yr. BP

-100

-200

-300

-400

-500

-600

-700

-800

-900

Dep

th

(cm

)

Grain content (%)

KMDJ 05

shows granulometry of the core (in blue, the coarser fraction, in red the finer fraction).

-850

-750

-650

-550

-450

-350

-250

-150

-50

Dep

th

(cm

)

14,891 cal yr BP

7.5 m

1 km

SWNE

coarse sand percent (>315 µm)fine sand percent (40-315 µm)silt-clay sediment percent (<40 µm)

Diameter of particles:

Grain content (%)

core

14,232 cal yr BP

11,748 cal yr BP

Chirp0 20 40 60 80 100

KMDJ 06

Fig. 18. Chirp line at the location of core KMDJ06 illustrating the T4 echo type. Graph to the right shows granulometry of the core (in red, the finer fraction, in blue the coarserfraction). A zoom of the Chirp line (at the same scale than the core) at the core location is also shown. Vertical exaggeration¼w130.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 687

15,098 cal. yrs BP at 397–391 cm, and 4410 cal. yrs BP at 103–102 cm. The two nearly similar ages (the apparent age inversion isonly due to the uncertainty related to hand-picked reworkedforaminifers) between 15,000 and 14,000 cal. yrs BP for the intervalfrom 589 to 391 cm indicate relatively rapid sediment accumula-tion during the late Pleistocene. The youngest date around4000 yrs BP suggests slower accumulation during the Holocene.This core site is remote from the base of the slope, but next to theKramis canyon, on its highly developed northern levee, and showsa frequency of gravity-induced events higher than near the foot ofthe slope.

The B1 echo type at the core location (Fig. 19) clearly corre-sponds here to the accumulation of turbiditic sequences, the high-amplitude reflections corresponding to the coarser part of the

7.5 m

1 km

core

coarse sand percent (>315 µm)

fine sand percent (40-315 µm)

silt-clay sediment percent (<40 µ

Diameter of particles:

NSW

Fig. 19. Chirp line at the location of core KMDJ07 illustrating B1 echo type. Graph to the rightVertical exaggeration¼w130.

turbidites, and the low-amplitude reflections to finer sediments(muds and silts).

7.4. Core KMDJ08

It was raised from the foot of the slope in 2631 m of water in anarea with several submarine valleys (Fig. 11). The correspondingecho type is here the B1 type. The 764 cm core shows a series ofgrey beige muds (mostly hemipelagic) with interbedded sands andthe characteristic metallic grey colour observed in other cores fromthis area. The sand content is variable and some 20 turbiditic beds(identified by the coarse levels) were observed (Fig. 20). As inKMDJ06, the abundance of sandy turbidites is significantly higherthroughout the lowermost 200 cm of the section. The thickest

-650

-600

-550

-500

-450

-400

-350

-300

-250

-200

-150

14,094 cal yr BP

-50

-100

Dep

th

(cm

)

m)

Grain content (%)

E

15,098 cal yr BP

4,410 cal yr BP

0 20 40 60 80 100

KMDJ 07

shows granulometry of the core (in blue, the coarser fraction, in red the finer fraction).

-800

-700

-600

-500

-400

-300

-200

-100

0

15681 cal yr. BP

Dep

th

(cm

)

7.5 m

1 km

NW SE

coarse sand percent (>315 µm)fine sand percent (40-315 µm)silt-clay sediment percent (<40 µm)

Diameter of particles:

Grain content (%)

5094 cal yr. BP

0 20 40 60 80

KMDJ 08

Fig. 20. Chirp line at the location of core KMDJ08 illustrating B1 echo type. Graph to the right shows granulometry of the core and its corresponding granulometry (in blue, thecoarser fraction, in red the finer fraction). Vertical exaggeration¼w130.

3220

3210 SN

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694688

turbidites exhibit normal grading at 700, 580, 310, and 210 cm. Thecontinuum of the fine-grained turbidite is expressed by colourgrading through C–D Bouma divisions. Two radiocarbon ages weremeasured 15,681 cal. yrs BP at 735–740 cm (near the base of thesection) and 5094 cal. yrs BP at 141–151 cm. These two ages provideevidence of relatively high sediment accumulation rates during thelate Pleistocene and followed by decreasing sedimentation duringthe Holocene. This trend results from the upward decreasingfrequency of turbidite accumulation.

The coarser beds of the turbidites include calcareous bioclasts(among them, foraminifers from shallow environments) andweathered debris of schist and sandstone. The higher abundance ofturbidites during the Last Glacial sea-level change compared to theHolocene high-stand is corroborated by the Chirp line over the coresite (type B1, Fig. 20). The low-amplitude parallel reflections in theupper part of the core appear to be the Holocene hemipelagicsediments, whereas the amplitude of the reflections increases withdepth, which emphasises the upward decrease of the gravity-driven processes.

7.5

m

1 km

3230

3240

3250

tw

tt (m

s)

7.5. MD04-2801 Calypso core

It is 25 m long and was sampling a channel levee of the KramisDeep-Sea Fan in a water depth of 2067 m (Figs. 6 and 12). Thelithological descriptions show that the first 10 m are onlycomposed of homogeneous silty mud (Sultan et al., 2004). Thissuggests that this part of the fan has not been fed by turbidites andthus was relatively inactive for a long time, and that this mudcorresponds to hemipelagic sedimentation, or that this part of thefan only gets the finer fraction of the turbidites. The lower part ofthe core (1000–2500 cm) shows regular intercalations of thinsandy layers between the homogeneous silty mud, which indicatesan older turbiditic activity. The corresponding echo type is B1(Fig. 21).

3260

Fig. 21. Chirp line near the location of core MD04-2801, on the levee of the KramisDeep-Sea Fan, showing continuous and parallel reflections typical of B1 echo type.Location shown on Figs. 6 and 12. Vertical exaggeration¼w200.

8. Sedimentary processes and their controls

Correlation between the bathymetry, backscattering, Chirp linesand the sediment cores permitted the construction of a map

showing the main sedimentary processes throughout the studyarea (Fig. 22).

8.1. Sedimentary processes

In our study area, the deep basin, even if not well represented inthe Tenes zone data set, is characterized by undisturbed alterna-tions of hemipelagic sediments and thin turbiditic sequences (cores06 and 08), except on salt-diapir outcrops, where slope failuresoccur. Conversely, near the foot of the continental slope, severalsedimentary processes occur and they appear to be different along

Fig. 22. Map of the main sedimentary processes and structural features identified in the study zone off Oran and Tenes. See text for explanations. Location in Fig. 1. Position of thefour Kullenberg cores and the Calypso piston core MD04-2801 are shown.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 689

the margin. Many echoes at the foot of the slope display trans-parent lenses. In most cases, we have no mean to distinguishwhether they are turbidites or mass-transport deposits, because welack cores in these deposits. However, we can guess that theycorrespond to mass-transport deposits or not according to theproximity of canyons and to the shape of the transparent deposits:large areas of deposition together with regular and small thicknessare assumed to be turbidites, whereas thick and irregular depositstogether with limited geographical extent are assumed to corre-spond to mass-transport deposits (slides and debris flows). As thisinterpretation is rarely straightforward, we have sometimes chosento attribute these transparent levels to ‘‘turbidity currents and/ormass-transport deposits’’ in the sedimentary processes map(Fig. 22), as both could occur in these places.

Whatever the remaining uncertainties, we observe a strikingdifference between the Arzew–Mostaganem margin and the Oranmargin or the Tenes margin (Fig. 22). Off Arzew–Mostaganem,canyons paths display a low reflectivity, and depict only mass-transport deposits close to the foot of the slope. Conversely, offOran or Tenes, large systems of canyons, which are highlyreflective and deeply incised, transport sediments far away in thebasin, as shown by transparent layers and chaotic echo typesdownslope of canyons, up to 20 km away from the continentalslope. This provides strong evidence for turbidity currentserosion, and transport of sediments through the canyon far in thedeep basin. On the contrary, the Arzew–Mostaganem margin,with its shorter canyons, would show an example of canyongrowth by retrograding erosion, inducing slides falling at the footof the slope. This would explain the different patterns at the footof the slope in the echo-character maps, especially the map ofOran (Fig. 11).

We identified large zones of sediment waves (Kramis Deep-SeaFan and west of Khayr al Din bank) displaying a variety of echotypes, where various sedimentary processes may interplay. Thehighly developed northern levee of the Kramis Deep-Sea Fan (Figs.6 and 12, and north–east of Figs. 3 and 11) shows mass-transportdeposits (T2 echo type) but also B2, R1 and R2 echoes. Most of them

occur in a NNW–SSE corridor (just west of the MD04-2801 core)facing the N–S tributary of the Kramis canyon. Therefore, webelieve that these echoes are the result of strong eroding turbiditycurrents, like the one that occurred in 1980, after the El Asnamearthquake, which destroyed submarine cables and is supposed tohave passed there, according to El Robrini et al. (1985). Apart fromthis corridor, the fan is composed of sediment waves showing a B1echo type, which has been interpreted as the deposition of the finerpart of overflowing turbidity currents and interbedded hemipelagicsediments (see core KMDJ07 and MD04-2801). However, we cannotrule out the possible contribution of contour currents to theformation of the sedimentary waves, even if we do not believe thatthis is the main process.

The other sediment wave field is located west of the Khayr al Dinbank (Fig. 6) and displays several echo types: B1, B2, R1, and R2. Theproximity of wide canyons suggests that turbidity currents are atthe origin of this pattern, but we also cannot preclude the action ofcontour currents along the base of the slope. The western slope ofthe Khayr al Din bank seems to be completely slumped. Local smallslides (T2, T5, and R3 echoes) are superimposed to the large slump,and the R1 echoes could be indication for erosion (slide scars orturbidity currents erosion).

Concerning the mass-transport deposits alone, we observe thatsome parts of the slope are subject to recurrent slope failures, asindicated by superimposition of transparent lenses (T3 echoes, andsometimes T4 and T5) at the foot of these slopes. This may be anindication for a weak lithology of the slope, or merely that thesediment input is higher in these zones, and therefore that theslope equilibrium is broken more rapidly. The other explanation forrecurrent slope failures, which will be discussed in the next section,is recurrent seismic events.

8.2. Control parameters of sedimentary processes

In this section, we examine the different parameters that mayinfluence the distribution of the sedimentary processes revealed byecho-character studies and described above.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694690

8.2.1. Slope gradientsFigs. 4 and 7 show the slope gradients for the Oran and Tenes

areas, respectively. The regional declivity in the Tenes study area isabout 16–19% for the slope and higher in the canyons (Fig. 7). Mass-transport deposits are found all along the continental slope(Fig. 22). The largest mass-transport deposits are located at the footof relatively less steep slopes (northwest of Kramis Deep-Sea Fan(slope gradient <10�), and north of Cherchell (slope gradient<20�)). However, there is no clear relationship between the slopegradient (regional or local) and the occurrence of mass-transportdeposits. In this case, the slope failure occurs more likely because ofthe high sediment accumulation rate on the deep-sea fan levee.

In the area of Oran, the highest gradients (up to 40�) arebetween the Habibas Islands and Arzew, and there are large mass-transport deposits (or thick turbidites) at the foot of the slope(Figs. 4 and 22). The most active canyons are located in this area,according to the high backscatter values of the canyons floors, andare not linked with rivers onshore. Therefore, the role of the slopegradient is probably important for the generation of mass-transportprocesses and triggering of turbidity currents in this sector. West ofthe Habibas Islands and north of Mostaganem, where the slopegradients are lower (less than 20�, except for canyons flanks, Fig. 4),we also identify large mass-transport deposits, but the occurrenceof strong turbidity currents seems limited. This difference oferosion style along this part of the margin could be due to thedifferent slope gradients, but we cannot rule out differences inlithology of the continental slope (as it is suggested in Domzig et al.,2006).

8.2.2. Salt tectonicsAnother type of triggering factor for slope failure is salt

tectonics. Salt movements are known for long to trigger mass-transport deposits (Gaullier and Bellaiche, 1998; Tripsanas et al.,2004a; Loncke et al., 2006). In our study area, we identified saltdiapirs in several locations (Fig. 22), which have small mass-transport deposits on their sides. This shows that the slope stabilityof the flanks of the diapirs is broken, and small slides occur.

8.2.3. CurrentsThe Algerian current is the main oceanographic current flowing

eastward from Gibraltar along the continental margin (Millot, 1985,1987). This superficial current is 200–400 m thick. In addition,some mesoscale cyclonic and/or anticyclonic eddies (100–200 kmdiameter) sometimes form along the coast and cause westwardflows that create upwelling or downwelling, which can transportsediment particles down to the bottom with a velocity of 0.05 m/s(Van Haren et al., 2006). Because we observe migrating sedimentwaves on the Chirp lines, we need to understand the role of thesecurrents, if any, in the deep-basin sedimentation at depths of2000–3000 m. According to Millot et al. (1997), at 2000 m depth,the main currents are eastward and up to 4 cm/s. But according toObaton et al. (2000), it is not clear whether or not the Eddie eventsaffect layers deeper than 1000 m. However, other studies (i.e. Howeet al., 2006) have shown sediment waves similar to Algerian ones(crests parallel to the slope, as in the area west of Khayr al Din bank;Fig. 6) that were associated with bottom currents. In these studies,these features display parallel continuous reflectors, an observationwhich is consistent with our observations in several places. So it ispossible that contour currents occur along the Algerian margin. Inthe Tenes area, most sediment waves observed can also be relatedto turbidity currents because of the proximity of canyon systems.So, on our map (Fig. 22), we did not distinguish the two processes,but in absence of precise current measurements in our study zone,we cannot preclude a potential action of contour current for theformation of the sedimentary waves and the associated echoes (B1,B2, T2, T5, R1, R2, and R3).

8.2.4. Active tectonics and seismicityIf we compare the echo-character maps (Figs. 11 and 12) with

the structural maps published in Domzig et al. (2006), we note thatsome areas displaying echo types with discontinuous to chaoticreflections are located along fault zones. In particular, the zone ofthe canyon ‘‘des moules’’, NW of the Habibas Islands is located onseries of WNW–ESE strike-slip faults (Fig. 22), the eastern prolon-gation of the strike-slip Yusuf-fault system (Domzig et al., 2006). Inaddition to sedimentary processes such as turbidity currents, theactivity of the fault may be responsible for the R2, B2 echoes as wellas the T2 and T3 small spots.

In the Tenes area, the tectonic influence is not so clear becausethere is no outcropping active fault along the margin, except nearthe Khayr al Din bank. The B2 and T2 echoes north of the Khayr alDin bank could be directly linked to the activity of the faultdelimitating the bank to the north. We do not know the exactextent of the fault to the west, so it is possible that tectonics playsa role in the formation of the chaotic echo types (R2 and B2 echoes)west of the Khayr al Din bank. Elsewhere, the turbidity currents andmass-transport could be triggered by recurrent earthquakes. Futurepaleoseismological studies will help to better constrain the role ofearthquakes in triggering slope failures.

The historical and instrumental seismicity indicate that themargin has already experienced many large earthquakes that areknown to have triggered landslides onland and probably alsoinitiated mass-transport deposits and turbidity flows on thecontinental slope (El Robrini, 1986). For instance, the Tenes regionhas experienced several large events (Intensity VI–X on the Mercalliscale) during the second half of the 19th century. Although notdirectly apparent at the sea bottom, active faulting and/or folding atdepth is suspected (Domzig et al., 2006) and may thereforecontribute to trigger sedimentary instabilities. When we superim-pose earthquake epicenters from 1973 to present (NEIC catalog,2006) on top of the mass-transport deposits (T echo types) alongthe whole Algerian margin (Fig. 23), it appears that the highlyseismic zones do not correlate well with zones of mass-transportdeposition. The principal limitations of this data set are that (1) itdisplays only the brittle part of the strain field, and (2) since onlythe very recent seismicity catalog is available, the seismicity shownhere does not represent the period over which the identified mass-transport deposits have been deposited.

8.2.4.1. Quantifying mass-transport deposits. We have identifiedmore than 860 km2 of transparent echo types. The majority ofslides (corresponding to T echoes) cover areas of 10 km2 or less,whereas the largest one covers an area of 85 km2. It is difficult toestimate the volume of a mass-transport deposit because we arerarely able to identify its corresponding slide scar on the slope, andbecause the relatively low density of our Chirp lines does notprovide an accurate 3D view of the shape of the transparent bodies.Therefore we must make some assumptions to calculate mass-transport deposits volumes. For example, we assume that most ofthe slides have the shape of an ellipsoid, therefore their volumewould be 4/3pabc (with ‘‘a’’, ‘‘b’’, and ‘‘c’’ being the three radiuses ofthe ellipsoid). If we take the example of the slide in Fig. 16 (the oldertransparent deposit), assuming a¼ 5 km, b¼ 4 km, and c¼ 10 m,we obtain a volume of w0.8 km3.

We observe that in the Oran–Tenes zone the size of mass-transport deposits seem larger than in the central Algerian margin(the highly faulted Boumerdes margin, see e.g. Dan, 2007; Danet al., in press; Fig. 23), where gravity-induced processes includingboth submarine slides and turbidity currents are responsible fornumerous mass-transport deposits (including debris flows andslumps) of limited size mostly located at the foot of slopes, whichare often tectonically controlled. The same way, large mass-transport deposits appear to be relatively less common in our study

Fig. 23. Top: map of the recent mass-transport deposits in the Gulf of Lions (1: partially transparent echo character, 2: wholly transparent echo character, 3: buried transparent echocharacter (Vendeville and Gaullier, 2003, modified from Gaullier and Bellaiche, 1998)). Bottom: map of the recent mass-transport deposits on the Algerian margin, at the same scaleas map at top. Data for the Algiers region from Gaullier et al. (2004). Data for the Oran and Tenes zones from the present study. Epicenters from 1973 to 2006 (NEIC catalog, 2006) inblue. Dotted lines: limits of the continental slope.

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694 691

area compared to other regions of the Mediterranean sea, especiallynon-seismic margins (Gulf of Lions, see e.g. Gaullier and Bellaiche,1998, Fig. 23), or margins with less important slopes (like the Niledelta, see e.g. Ducassou, 2006; Loncke et al., 2009; Mascle et al.,2006). However, a major limitation for a precise comparison ofvolumes of mass-transport deposits in our study area is the lack ofdata in the deeper part of the basin. In all cases, this map (Fig. 23)shows that the Algerian margin is very often subjected toearthquakes, and as earthquakes with a magnitude of 6 or greatercan destabilize sediments 200 km away from their epicenters(El Robrini, 1986), these must be considered as an importantmechanism for triggering mass-transport deposits, even very faraway from the epicenters.

One hypothesis to explain the relatively low abundance of mass-transport deposits compared to other margins (Fig. 23) is that thesediment load is not sufficient to allow the remobilisation of largeamounts of sediments. This has been shown by Leclaire (1970) forthe region of Tenes, where many areas of the continental slope aredevoid of sediment and commonly show outcropping basement.This relative scarcity of sediment supply might also be explained bythe small number of rivers transporting sediments into the sea, andby the steep slopes (>15%) and the very narrow continental shelf

(especially in the Tenes area), which do not facilitate the sedimentaccumulation. These conditions would lead to only small amountsof sediment sliding into the deep basin, thus explaining the pres-ence of only small sized mass-transport deposits. Another addi-tional factor could be that this part of the margin is often shaken byearthquakes: consequently, the time frequency of earthquakes doesnot leave enough time to allow large amounts of sediments toaccumulate, so when the margin is shaken, only small volumes ofsediment flow into the basin (turbidites or mass-transportdeposits).

Previous studies have shown that earthquakes can triggerturbidity currents strong enough to break submarine cables (e.g.the Orleansville earthquake, Heezen and Ewing, 1955 and theBoumerdes earthquake, Ayadi et al., 2003). This suggests thatthe sediments remobilised during a large earthquake may bypassthe slope and flow very far in the deep basin beyond our study area,which would also explain why we do not find large debris flows atthe foot of the slope. Possibly, the steepness of the slopes favoursgeneration of high-energy turbidity currents.

8.2.4.2. Possible implications in terms of geological hazard. We maywonder whether tsunamis may be triggered by mass-transport

A. Domzig et al. / Marine and Petroleum Geology 26 (2009) 673–694692

processes in the study area. From the modelling of the moderatetsunami which has followed the 2003 Boumerdes earthquake eastof Algiers and has affected the Balearic margin, Alasset et al. (2006)concluded that the tsunami resulted mostly from the coseismic sea-floor deformation. Although turbidites were widespread followingthe main shock according to the numerous telecommunicationcable breaks reported, they appear to be unable to trigger thetsunami waves observed on the tide gauge records after thisearthquake. However, from the study by Dan et al. (in press) in thesame area, the maximum size of the slide reported is 0.18 km3,a volume comparable to the one estimated off the Californianmargin (Fisher et al., 2005) that appeared to be large enough totrigger moderate tsunamis (w2 m run up, Borrero et al., 2001).Therefore, although the volumes of destabilized sediments in thestudy area (maximum of w0.8 km3, see above) seems much smallerthan slides usually known to trigger large tsunamis (e.g. ten Brinket al., 2006; Hornbach et al., 2007), a tsunamogenic potential ofpast or possible future slide events in the area cannot be ruled out.Tsunami modelling taking into account the coastal bathymetry andcalculating precisely the volumes of the slides is needed if we wantto better assess the tsunami hazard potentially associated withsubmarine slides on this particular Algerian slope in the future.

9. Conclusions and perspectives

The 12 echo types identified in our study zone have beeninterpreted in terms of processes according to their physiographiccontext, e.g. localization relative to canyons or slopes. B2 echoes –discontinuous parallel to hummocky high- to low-amplitude sub-bottom reflections – the sedimentary process characteristic of thedeep basin is the hemipelagic sedimentation, i.e. alternation of thinturbidites and pelagic sediments, typically represented by B1echoes (parallel to subparallel high- to low-amplitude sub-bottomreflections). In some places some salt-diapirs outcrop, and on theirsides small mass-transport deposits are identified, typically T2echoes (transparent lens over irregular sub-bottom with no pene-tration below). Mass-transport deposits such as slumps, slides anddebris flows are located on canyons flanks or on the foot of theslope. They generally correspond to the transparent echoes (T1–T5)or R3 echoes (chaotic lens of low-amplitude reflections on top ofhigher amplitude), and more rarely to B2 (discontinuous parallel tohummocky high- to low-amplitude sub-bottom reflections) and R2(small irregular overlapping hyperbolae). In some places we seesuperimposed transparent lenses (T3, T4 or T5 echoes) that indicatemore frequent slope failures. Because of the insufficient number ofcores in our study area, an ambiguity remains about the nature ofthe transparent lenses, i.e. thick turbidites or mass-transportdeposits. However, according to the shape and location of thedeposits, we propose to attribute to turbidity currents deposits therather laminar transparent lenses at the distal part of canyons, andto mass-transport deposits, the lenses at the foot of slopes, far fromcanyon systems. The high-energy turbidity currents can also formwaves that correspond to B2 echo types downslope canyons, andthe erosion of the canyon axis by turbidity currents is characterizedby R1 (prolonged single echo with no sub-bottoms) and R2 echoes.Large sediment waves zones that display various echo types areevidenced in the study area and are generally associated withturbidity currents, but could also be associated with bottomcurrents or tectonics. We have seen that the relation betweenfaulted zones and mass-transport deposits is not always straight-forward. The active faults zones are generally surrounded by mass-transport deposits and discontinuous to chaotic reflections (B2 andR2 echoes). Elsewhere, mass-transport deposits can be also due tothe frequent (and sometimes strong) earthquakes that shake thewhole margin. For example, the M 7.3 El Asnam earthquake isknown to have triggered mass-transport deposits and turbidity

currents far away offshore. In addition, the relatively small size ofthese deposits (compared to non-seismic regions) can be partlyexplained by the frequent seismicity in the region, which does notenable the upslope margin to accumulate great amounts of sedi-ments. These arguments favour a significant role of active tectonicsin slope failures in this zone.

A major factor influencing the distribution of the different echotypes is the morphology of the margin, in particular the location ofslopes and the presence or absence of canyons.

We have shown that the slope gradients may play a role in theinitiation of high-energy turbidity currents, as in the western Oranzone. We have also seen a difference of style in the erosion of themargin, especially in the Oran area: to the west, high-energyturbidity currents deeply incise the canyons, whereas the Arzew–Mostaganem margin displays mass-transport deposits at the foot ofthe slope, and retrograding erosion in the canyons. This differencebetween these portions of the margin could be linked to slopegradients, but also to lithological variations.

As a perspective for future work, we have shown that some ofthe identified slides, by their volume, could potentially be able totrigger a tsunami. A more detailed study, including tsunamimodelling and the integration of new data to the East is nowrequired to assess the tsunami hazard for the western Mediterra-nean coasts.

Acknowledgements

We thank Lies Loncke and John E. Damuth for their helpful andvery constructive reviews. We also wish to thank Eulalia Gracia forconstructive comments. We thank Eliane Le Drezen and Jean-MarieAugustin (Ifremer) for their help in the processing of the ‘‘belle-image’’ backscattering data, Renaud Cagna for the onboard pro-cessing of the bathymetry DEM, Ifremer Brest Center techniciansfor assistance in the core-opening operations, and the crew of theR.V. Suroit during the Maradja 2003 cruise. This research is fundedby the GDR Marges (‘Instabilites gravitaires’), ESF EUROMARGINS(Westmed project), and the French ACI ‘Risques naturels’. Contri-bution no. 1085 of the IUEM. This work was led in the frame of theFrench–Algerian program TASSILI-CMEP 041-MDU619.

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