ALGERIAN MARGIN SEDIMENTATION PATTERNS (ALGIERS AREA ...

69ALGERIAN MARGIN SEDIMENTATION PATTERNS

ALGERIAN MARGIN SEDIMENTATION PATTERNS (ALGIERS AREA,SOUTHWESTERN MEDITERRANEAN)

GABRIELA DAN-UNTERSEHIFREMER, Géosciences Marines, Laboratoire Environnements Sédimentaires, Plouzané, France

AND

Université de Bretagne Occidentale, IUEM-CNRS UMR6538, 29280 Plouzané, [email protected]

BRUNO SAVOYE (DECEASED)VIRGINIE GAULLIER

LEGEM, Université de Perpignan, 66860 Perpignan, FranceANTONIO CATTANEO

IFREMER, Géosciences Marines, Laboratoire Environnements Sédimentaires, Plouzané, FranceJACQUES DEVERCHERE

Université de Bretagne Occidentale, IUEM-CNRS UMR6538, 29280 Plouzané, FranceKARIM YELLES

CRAAG, Centre de Recherche en Astronomie, Astrophysique et Géophysique, Bouzaréah, Alger, AlgérieAND

MARADJA 2003 TEAM

Abstract: The present study provides an overview of recent sedimentation patterns on the central Algerian continental margin. Recentsedimentation patterns were assessed from morphological analysis, which is based on swath bathymetry and echo-facies mapping. Itappears that sedimentation along the Algerian margin is controlled by two processes: (1) gravity-induced processes, including both mass-transport deposits and turbidity currents, and (2) hemipelagic sedimentation. Mass-transport deposits occur on the Algerian margin at thecanyon heads and flanks, in the interfluve areas between canyons, along the seafloor escarpments, and on the flanks of salt diapirs. Mass-transport deposits (MTDs) sampled by coring consist of a variety of soft and hard mud-clast conglomerate and turbidite deposits. MTDsare mostly localized at the toes of steep slopes, where thrust faults were previously identified and mapped. Analysis of the spatialdistribution of MTDs and their recurrence in time help reconstruct the main predisposing factors and triggering mechanisms, and evaluatetheir impact on evolution of the Algerian margin.

KEY WORDS: Algerian margin, backscatter, Boumerdès earthquake, diapirs, echo facies, mass-transport deposits, submarine canyons,sediment waves

Mass-Transport Deposits in Deepwater SettingsSEPM Special Publication No. 96, Copyright © 2011SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-287-9, p. 69–84.

INTRODUCTION

Occurrences of MTDs involving large volumes of sedimentare known across continental slopes worldwide, especially alongpassive margins and volcanic islands (e.g., Masson et al., 1998;Bryn et al., 2003; Haflidason et al., 2004; Canals et al., 2004).However, MTDs were also documented along active margins,where tectonic activity may be one of the most relevant factors ingenerating sediment instabilities (e.g., von Huene et al., 1989; vonHuene et al., 2000; Collot et al., 2001). A well-documented earth-quake-induced MTD is the 1929 Grand Banks event, whichoccurred between Newfoundland and Nova Scotia off AtlanticCanada (Rupke, 1978; Piper and Asku, 1987; Piper et al., 1999).Earthquakes are known to trigger MTDs and generate tsunamis,which can seriously damage coastal and offshore infrastructures.This is the case of the Algerian margin, where several devastatingearthquakes occurred during the last century (Heezen and Ewing,1955; El-Robrini et al., 1985). The most violent instrumentallyrecorded earthquake (7.1 Mw) occurred on 10 October 1980 in ElAsnam. More recently on 21 May 2003 an earthquake with amagnitude of 6.8 (Mw) struck the city of Boumerdès, on the coastnear Algiers, and generated significant turbidity currents, con-firmed by numerous submarine-cable breaks. Following the 2003

seismic event, swath bathymetry, chirp subbottom profiles, andsediment cores were acquired in the area affected by theBoumerdès earthquake.

The present study describes geomorphological features andcharacterizes sedimentary processes on the central Algerianmargin (Fig. 1), located offshore the cities of Tipasa, Algiers, andDellys. The main objectives of this study are to:

• Highlight the main geomorphological features existing alongthe central Algiers margin, and provide a detailed descriptionof the seafloor characterized by many MTDs.

• Describe the main subsurface features with high-resolutionseismic data and document the most significant echo facies.

• Integrate different data types to better understand regionalsedimentary dynamics along the central Algerian margin.

TECTONIC AND GEOLOGIC CONTEXT

Since the early Cenozoic, the Algerian margin has been undera compressional regime with a northwest–southeast conver-gence (Stich et al., 2003). This active zone absorbs approximately

G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM70

5 mm/year of crustal shortening (Calais et al., 2003; Nocquet andCalais, 2004), with the onshore part accommodating only 50% ofthe long-term convergence between the European and Africanplates (Meghraoui and Doumaz, 1996). It appears that activedeformation offshore northern Algeria is expressed by 2 to 3mm/year of shortening, and is likely related to occurrence ofearthquakes. Numerous studies focused on tectonic activity orsedimentary processes in the marine domain were conductedafter the Boumerdès earthquake in 2003. As a consequence, thefault believed responsible for the Boumerdès earthquake wasidentified between 6 and 16 km below the seafloor (Yelles et al.,2004; Meghraoui et al., 2004; Semmane et al., 2005). The faultimprint on the seafloor was mapped on the lower part of thecontinental slope offshore the city of Dellys (Déverchère et al.,2005; Domzig et al., 2006) (see fault trace in Figure 2).

Northern Algeria is part of the Maghrebian mountain chain,which can be divided from south to north into three units: (1) theexternal domain (Tellian units), composed of sedimentary units,mainly marls and limestones; (2) the flysch nappes thrusting theexternal zones; and (3) the internal domain, composed ofHercynian basement sometimes associated with its sedimentarycover (e.g., Bracene et al., 2003). Thus, the study area covers theinternal domain (Fig. 1). The central Algiers slope is composed ofOligo-Miocene sediments, consisting mostly of flysch and volca-nic deposits (Domzig et al., 2006).

There are three main rivers in the study area: from east to west,the Sebaou River, the Isser River, and the Mazafran River (Fig. 1),which supply sediment to the study area. The river regime isinfluenced by the Algerian climate, which is typically arid andhot, although northern coastal Algeria is part of the temperate

zone and enjoys a mild Mediterranean climate. Rainfall is fairlyabundant along the coastal area, ranging from 0.3 cm/monthduring summertime to 10 to 12 cm/month during wintertime inthe Algiers area. As a consequence, rivers have a seasonal regimewith significant flood periods.

DATA SET AND METHODS

The MARADJA survey took place on the R/V Le Suroît(IFREMER) in August through September 2003. In order toinvestigate the imprint of recent and past earthquakes, this sur-vey focused on the part of the Algerian margin affected by theBoumerdès earthquake (Fig. 1). Data gathered during the surveyconsist of swath bathymetry and backscatter data (KongsbergSimrad EM-300 and EM 1000 echosounder), subbottom profiler(chirp), and Kullenberg cores (Fig. 1). A 50-m-resolution digitalelevation model was created using the IFREMER CARAÏBESsoftware, and the backscatter data provided a mosaic with 12.5 mresolution. The chirp subbottom profiler of the R/V Le Suroît usesfrequencies between 1.8 and 5.3 kHz, reaching a maximumvertical penetration of 80–100 m in muddy sediment. More than2800 km of chirp profiles were acquired for the study area (Fig. 1).Four gravity cores (designated with letters KMD) were collectedin the study area during the MARADJA cruise in 2003, and havea maximum length of about 7 m (Table 1). In addition, three giantCalypso cores (designated with letters MD) were collected dur-ing the PRISMA survey (May 2005) from the R/V Marion Dufresne(Table 1). The sediment cores were analyzed in the SedimentaryEnvironments Laboratory at IFREMER. Unopened core sectionswere analyzed using the GEOTEK core logger (MSCL, http://

Dellys

Mazafran River

Isser River

Sebaou River

0 20 km

Tipasa

Algiers

Boumerdès

36.8°N

36.6°N

37.2°N

37.4°N

2.8°E 2.6°E 3.0°E 3.2°E 3.4°E 3.6°E 3.8°E 4.0°E

500 m

1000 m

1500 m

2500 m

2000 m

KMDJ-04

MD04-2798

MD04-2799

KMDJ-01KMDJ-02

KMDJ-03MD04-2800

Sediment core

Rivers

Kabylian basement + Dorsale Kabyle

Kabylian Oligo-Miocene

Flyschs

Tellian units (External zones)

Volcanism

Tracklines

Boumerdès epicenter

2.4°E 2.2°E

Annaba

37.0°N

Bathymetric contours

Algiers

Internal/external domain boundary

Tenes

GreatKabylia

Lesser Kabylia

El Marsa

Study area

36.8°N

36.6°N

37.2°N

37.4°N

37.0°N

2.8°E 2.6°E 3.0°E 3.2°E 3.4°E 3.6°E 3.8°E 4.0°E 2.4°E 2.2°E

FIG. 1.—Location of the study area on the Algerian margin and the 2003 Boumerdès earthquake epicenter (red star). Gray track lineare the seismic chirp profile; black diamonds are either sediment cores (KMDJ-01, -02, -03, and- 04) collected during theMARADJA cruise (2003), or sediment cores (MD04-2798, -2799, and -2800) collected during PRISMA cruise (2004). Bathymetriccontour interval is 500 m. Onshore geology illustrates the main units of the Maghrebian chain (from Domzig et al., 2006).


www.geotek.co.uk). Sections of split cores were photographedand then X-rayed using a SCOPIX X-ray device (University ofBordeaux I). Grain-size analysis was performed by the lasertechnique (Coulter LS130).

Chirp profiles were used to define echo facies in the studyarea. The echo-facies methodology, first described by Damuth(1975, 1980), and more recently refined by other workers (Gaullierand Bellaiche, 1998; Loncke et al., 2002), was adapted for thepresent study. First, we listed and classified different echo facies,followed by the mapping of each echo facies along ship tracks andinterpolation between the lines. This last step was facilitated byalso considering bathymetric and backscatter maps.

RESULTS

Physiography of the Algiers Area

The study area is along the central Algerian margin (an areameasuring 200 km x 50 km), located between 2.2° E (west ofTipasa) and 4.1° E (east of Dellys). The following major physi-ographic domains were defined in the study area: (1) continentalshelf; (2) continental slope delimited by different slope breaks (B1to B5); (3) major canyons; (4) Khayr al Din Bank; (5) abyssal plain(basins D1 to D4); and (6) deep curvilinear escarpments (S1 andS2). The following discussion examines each one of these do-mains separately.

Continental Shelf.—

The continental shelf has a variable width. The shelf widthranges from 11 to 30 km west of Algiers, and becomes narrow (1

to 8 km) in the eastern part (Fig. 2). Bathymetry on the continentalshelf is not available for this study; only the shelf break, rangingbetween 100 and 150 m water depth, is locally imaged within theMARADJA data.

Continental Slope.—

The continental slope is steep, with an average angle of 11° (Fig.3). The slope is defined by northeast–southwest abrupt slopebreaks (B1 to B5) and by intermediate breaks forming flat areas (F)or circular suspended basins (Ci) (Fig. 2). Well-developed canyonsystems and numerous ravines incise the slope and have anenhanced expression on the slope map. MTDs occur on the slopeespecially at canyon heads and flanks, in the interfluve areasbetween canyons, and particularly on the lower part of the slope.For instance, a large area west of the Algiers Canyon is affected bysubmarine slides, which occur at various water depths: on theupper slope at 500 m, on the middle slope between 1000 and 1200m, and on the lower slope between 1600 and 1700 m. The lower partof the continental slope offshore Dellys is also affected by numer-ous MTDs. A particular feature showing a head scarp of more than200 m in height and approximately 1.5 km in width is visible on theshaded relief map (Figs. 4A, B). Part of the failed sediment seemsdeposited on the slope (Fig. 4B), ranging between steep lateralwalls of the failure (Fig. 4C). The MTD covers a significant surfaceof 4 km2, with an estimated volume of approximately 0.20 km3.

Major Canyons.—

Three well-developed canyon systems were identified on thecontinental slope: from east to west, Dellys Canyon, Sebaou Can-

Algiers

Mazafran River

Isser River

Sebaou River

B1

B2B3

B4

B5

S1

S2

Ci

D1

D2

D4

Mass-transport deposits

Canyons

Slope break

Diapirs

Sediment wave

Rivers

Khayr al Din Bank

s

F

Algiers Canyon

Inset Fig. 7

Tipasa

Boumerdès

Dellys

D3ADSF

Fig. 8B

2.2°E 2.4°E 2.6°E 2.8°E 3.0°E 3.2°E 3.4°E 3.6°E 3.8°E 4.0°E

2.2°E 2.4°E 2.6°E 2.8°E 3.0°E 3.2°E 3.4°E 3.6°E 3.8°E 4.0°E

37.0°N

36.8°N

36.6°N

37.2°N

37.4°N

37.0°N

36.8°N

36.6°N

37.2°N

37.4°N

36.4°N 36.4°N

Fig. 8A

Thrust fault

Pockmarks area

Sebaou Canyon

Fig. 4a

Fig. 5a

Dellys Canyon

0 20 km

ADSF Algiers Deep-Sea Fan

FIG. 2.—Shaded relief map showing the main morphological features in the study area. Slope breaks delimiting the continental slope,B1 to B5; slope breaks delimiting the deep curvilinear escarpments, S1 and S2; Deep basins, D1 to D4; Suspended basins, Ci(circular) and F (flat); Smooth area on the continental slope, s. Location of thrust fault is from Déverchère et al. (2005).


0 °2°

4°6°

8 °10

°12

°14

°16

°18

°20

°22

°24

°26

°28

°30

°

0

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Tip

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Alg

iers

Bou

mer

dès

Del

lys

2.2°

E

2.4°

E

2.6

°E

2.8

°E

3°E

3.2°

E

3

.4°E

3

.6°E

3.8

°E

4°

E

2.2°

E

2.4°

E

2.

6°E

2

. 8°E

3°E

3.2

°E

3

.4°E

3.6°

E

3.8°

E

4°E

37.2

°N

37°N

36.8

°N

36.6

°N

37.2

°N

37°N

36.8

°N

36.6

°N

Slo

pe (

degr

ees)

B1

B2

B3

B4

B5

S1

S2

D1D

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D3

AD

SF

D4

F IG

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Sedi

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Wat

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(m)

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J-01

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Fig

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6.


yon, and Algiers Canyon. The Dellys Canyon drainage area con-sists of two main branches, which collect several tributaries (Fig.5A). This canyon incises the slope at 100 m deep on the upper slopeand at 350 m deep on the middle slope. Canyon flanks are steep,with slope angles of 15 to 25° (Fig. 3). In its lower part, threeescarpments as high as 70, 120, and 200 m from the canyon floor areobserved, together with a plateau probably of tectonic origin (Fig.5B). Beyond the plateau, Dellys Canyon is no longer visible on theseafloor.

Sebaou Canyon is characterized by a rectilinear morphology,and is fed by several tributaries, probably connected with theSebaou River (Fig. 5A). Slope gradient ranges between 15 and 25°for the canyon head and flank (Fig. 3). Seaward of the B1 slopebreak, Sebaou Canyon becomes wider (approximately 3 km) withmoderately high flanks. Two asymmetric branches exist morethan 26 km from the canyon head (Fig. 5A). The primary branchfollows a northward course, while the secondary branch, consist-ing of a smaller canyon incision of approximately 30 to 50 mdepth, follows a northwestward direction. Large depressions/scours, 1 km in width and more than 40 m in depth, exist on theseafloor along the Sebaou Canyon, and are considered as strongevidence for significant erosion (Fig. 5C).

Algiers Canyon consists of two main meandering tributaries,with their heads located on the shelf break (Fig. 2). The Algierswestern tributary is sinuous, highly incised (200 to 300 m deep),and collects three other branches, each one with small tributaries.In contrast, the Algiers eastern tributary has a rectilinear mor-phology and consists of only two tributaries.

West of Algiers Canyon, the continental slope is incised bywell-developed canyons, with numerous gully-like tributaries.These tributaries connect in the middle part of the slope, creatinga large canyon with an average width between 1.5 and 3 km (Fig.2). These canyons have steep flanks, with an average slope of 18°(Fig. 3). The morphological path of these canyons is difficult tofollow beyond slope breaks B3 and B4 (Fig. 2).

Khayr al Din Bank.—

A major change in orientation of the Algerian margin (strikingwest-southwest to east-northeast) is observed west of the city ofAlgiers (Fig. 2). Khayr al Din Bank is an elongated area of highrelief (500 m water depth), facing towards a deep basin (2700 mdeep; Domzig et al., 2006). A first slope break occurs at 600 to 650m of water depth, followed by change of orientation towards thewest and change in slope angle from 2° to 5° (Fig. 3). SuperficialMTDs affect the western and northern part of Khayr al Din Bank,and an alignment of pockmarks occurs in its northern part (Fig.2). The pockmarks are 300 m to 450 m in diameter and up to 17 min depth. The eastern slope exhibits gullies and small MTDs. Incontrast, the western slope is much gentler, probably affected bya significant erosive process (see s on the western end of Figure 2).

Abyssal Plain.—

There are four sedimentary basins in the study area (Fig. 2).They are delimited by continental slope breaks and deep escarp-ments. In the eastern part of the study area, the D1 basin is 30 kmlong and 15 km wide in 2300 to 2400 m water depth. The D2 basinis located seawards of the curvilinear escarpment (S1). The D3basin corresponds to an elevated area downslope of the AlgiersCanyon. The D3 basin is interpreted as the Algiers deepsea fan(ADSF), which is confined on its northern part by salt diapirs anda curvilinear escarpment (S2 in Figure 2). Sediment across theADSF may be sourced by turbidity currents or bottom currents,since sediment waves occur across the ADSF. A large MTD ofapproximately 2 km width exits on the northern part of the ADSF.Salt diapirs form elongated walls or rounded ridges with variablelength (1 to 7 km), and a maximum height of 100 m above theseafloor. Small (0.2 km x 0.5 km across) subcircular diapirs alsoexist at the foot of the ADSF. Several MTDs are identified on thediapir flanks. A convex-upward area occurs on the abyssal plain

FIG. 4.—A) Shaded relief map showing mass-wasting deposits on the lower part of the continental slope. Thin black lines markprominent scarps associated with mass-wasting deposits. Location of map area is indicated in Figure 2. B) Dip bathymetric profilethrough the slide area. C) Strike bathymetric profile through the slide area. Location of both profiles is indicated in Part A.

0 1000 2000 3000 4000 5000 6000

Down dip distance (m)

2500

2300

2100

1900

1700

Slip surface

0 400 800 1200 1600 2000 2400

2180

2160

Along strike distance (m)

MTD

MTD

Dep

th (

m)

Dep

th (

m)

W E

B

Side-wall Side-wall

Max. head-wallheight

SE NW

C

Initial slope

V.E. = 2.6x

V.E. = 7.3x

i

i

A

2000

1900

2200

1700

2300

-1600

1500

1400

2400

2400

3°44'E

3°44'E

3°45'E

3°45'E

3°46'E

3°46'E

3°47'E

3°47'E

3°48'E

3°48'E

37°N 37°N

37°1'N 37°1'N

37°2'N 37°2'N

37°3'N (B)

(C)

i

37°3'N

Headscarp

Lateral scarp

MTD 0 1 km

Thrust fault

iIntersectionbetween profiles


downslope of the Khayr al Din Bank (D4 basin). This morphologi-cal feature probably corresponds to a significant MTD accumu-lated at the foot of the slope (Fig. 2).

Deep Curvilinear Escarpments.—

The abyssal plain exhibits two curvilinear escarpments thatare probably the seafloor expression of deformation repre-sented by buried thrust folds (Déverchère et al., 2005; Domzig etal., 2006) (S1 and S2 in Figure 2). The S1 escarpment is steep (10°to 15°) and is approximately 30 km in length and 350 to 450 m inheight. Numerous small MTDs, approximately 0.5 to 3 km wide,occur on the S1 escarpment (Dan et al., 2009). The majority ofthese MTDs exist on the mid slope, although two corridors,formed by several MTDs, occur on the upper part of the escarp-ment in approximately 2300 m water depth. A second deepcurvilinear escarpment, delimited by the S2 slope break, occursnorth of the ADSF between several salt diapirs (Fig. 2). Just likethe previous one, the S2 escarpment is affected by MTDs lessthan 0.5 km in width.

Echo-Facies Analysis

Echo-Facies Classification and Mapping.—

Definition of echo facies relies on acoustic properties and oncontinuity of the bottom and sub-bottom seismic reflections.Eleven distinctive echo facies exist in the study area, grouped intofour major categories: layered (L), non-penetrative, or rough (R),

chaotic (C), and transparent (T). All echo facies are illustrated inTable 2, displayed on the echo-facies distribution map (Fig. 6),and discussed in detail below. Due to the very high slope gradi-ents and considerable change in the seafloor morphology, thecontinental slope has not been well imaged on chirp profiles inthe study area (white area in Figure 6).

Layered Echo Facies (L).—Three subclasses are distinguished:(1) parallel, continuous reflectors (L1 and L2); (2) discontinuousreflectors or undulations (L3, L4); and (3) parallel reflectorsoverlying the rough acoustic basement (L5) (Table 2). At the sametime, two variants with a subclass exist in the first and secondsubclasses: high-energy reflections (L1 and L3), and low-energyreflections, corresponding to a transparent superficial layer (L2and L4) (Table 2). Based on previous work, layered echo faciesusually correspond to alternating hemipelagic intervals and tur-bidites (Damuth, 1980). However, the same echo facies could beattributed to hemipelagic intervals (Pratson and Laine, 1989).Discontinuous or undulated reflections are probably shaped bycontour currents or turbidity currents and are associated withsediment waves (Heezen et al., 1966).

The L1 echo facies occurs in the D1 and D4 basins, whereas theL2 echo facies is observed mostly in the shallow part of the studyarea and on Khayr al Din Bank (Fig. 2). The field of sedimentwaves on ADSF corresponds to the L4 echo facies. Another areacharacterized by the same echo facies (L4) exists at the foot of thecontinental slope west of Algiers deep-sea fan. The L3 echo faciesoccurs only in two narrow areas north of ADSF. The L5 echo faciesoccurs on the continental shelf.

5 km

Isser River

Sebaou Canyon

Dellys Canyon

Dellys Canyon

Sebaou Canyon

Scours

Escarpments

37.4°N

Dellys

3.8°E 3.9°E 4°E 4.1°E

37.2°N

37°N

37.4°N

37.2°N

3.8°E 3.9°E 4°E 4.1°E

Distance (m)

Distance (m)

Dep

th (

m)

Dep

th (

m)

Secondary branch

(C)

Thrust fault

Escarpments

36.8°N 36.8°N

0 2000 4000 6000 8000 10000 12000 14000

2600

2500

2400

2300

2200

2100

(B)

0 5000 10000 15000 20000 25000

2700

2600

2500

2400

2300

Scours

Canyon

S N

S N

V.E. = 8.5x

V.E. = 18.5x

A B

C

37°N

FIG. 5.—A) Shaded relief map in Dellys and Sebaou canyons. Location of map area is indicated in Figure 2. B) Bathymetric profilethroughout Dellys Canyon showing three escarpments on the distal part (thin blue line B in Panel A). C) Bathymetric profilethroughout Sebaou Canyon showing scours on the canyon floor (thin blue line C in Panel A).


2500

1500

Tip

asa

Inse

t F

ig. 7

Fig

. 8B

2.2

°E2

.4°E

2.6

°E2

.8°E

3°E

3.2

°E3.

4°E

3.6

°E3

.8°E

4°E

2.2

°E2

.4°E

2.6

°E2

.8°E

3°E

3.2

°E3.

4°E

3.6

°E3

.8°E

4°E

36.

7°N

36.

9°N

37.

1°N

37.

3°N

36

.7vN

36

.9°N

37

.1°N

37

.3°N

2500

2000

500

2000

D2

D1

B1

S1

B2

B3

D3

AD

SF

D4

B4

B5

S2

Dellys

Alg

iers

500

2500

0

20 k

m

L1 L2 L3 L4

RC

T1

T2 T3

no d

ata

on s

helf

or s

lope

L5

T4

Fig

. 8A

F IG

. 6.—

Seis

mic

ech

o-fa

cies

dis

trib

uti

on m

ap f

or A

lgie

rs a

rea,

bas

ed o

n th

e ch

irp

-pro

file

ana

lysi

s, c

ombi

ned

wit

h th

e ba

thym

etri

c an

d b

acks

catt

er d

ata.

Con

tou

rin

terv

al is

100

m. E

cho

faci

es c

lass

es a

re d

iscu

ssed

in te

xt a

nd d

efin

ed in

Tab

le 2

. Alp

hanu

mer

ic d

esig

nati

ons

and

sym

bols

on

the

map

are

def

ined

in th

e ca

ptio

nof

Fig

ure

2.

G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM76T

AB

LE 2

.—C

hirp

sei

smic

ech

o-fa

cies

cla

ssif

icat

ion.

Eac

h ec

ho fa

cies

is in

terp

rete

d in

term

of s

edim

enta

ry p

roce

sses

.E

cho

faci

es a

re d

escr

ibed

in th

e te

xt, a

nd th

eir

dis

trib

uti

on in

the

stu

dy

area

is s

how

n in

Fig

ure

6. C

orre

lati

onbe

twee

n ec

ho fa

cies

and

bac

ksca

tter

is d

escr

ibed

in th

e te

xt. L

ocat

ion

of b

acks

catt

er is

ind

icat

ed in

Fig

ure

9.


Ech

o-fa

cies

Cla

sses

Des

crip

tion

Exa

mpl

eC

ore

sam

ple

X-r

ayR

adio

grap

hyIm

age

Bac

ksca

tter

Inte

rpre

tati

on

Cha

otic

C

C: C

haot

icin

tern

alse

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ic f

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s-

-M

ass-

tran

spor

t dep

osits

T1:

Alte

rnat

ing

tran

spar

ent a

ndla

yere

dre

flec

tions

KM

DJ-

04

MD

04-2

798

Hem

ipel

agic

inte

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san

d tu

rbid

ite s

eque

nces

T2:

Tra

nspa

rent

,w

ithou

t int

erna

lse

ism

ic f

acie

sK

MD

J-01

Mas

s-tr

ansp

ort d

epos

its

T3:

Tra

nspa

rent

lens

. Pre

sent

on

seaf

loor

or

buri

edM

D04

-280

0M

ass-

tran

spor

t dep

osits

Tra

nspa

rent

T

T4:

Tra

nspa

rent

over

lyin

g ro

ugh

acou

stic

base

men

t

--

-

Dep

osits

for

med

by

curr

ents

and

hem

ipel

agic

sedi

men

tatio

n on

the

cont

inen

tal s

helf

TA

BL

E 2

(con

tinu

ed).—


Non-Penetrative Echo Facies (Rough, R).—Non-penetrative echofacies characterize areas where the seismic reflection signal doesnot penetrate below the seafloor. Generally, the R echo facies existsin the axes of canyons, where the eroded seafloor is mostly coveredby coarse-grained deposits (Damuth, 1975). As an example, SebaouCanyon consists of the R echo facies, covering an area of approxi-mately 22.5 km long and greater than 10 km wide (Fig. 6). The Recho facies also occurs along the curvilinear escarpments and thesmooth area described on the north slope of Khayr al Din Bank,which seem associated with MTDs along the slopes (Fig. 6).

Chaotic Echo Facies (C).—The chaotic echo facies (C) representhighly disorganized sediments induced by gravity-driven pro-cesses (Pratson and Laine, 1989, Damuth, 1994). C echo faciesexist in various locations along the Algerian margin: at the foot ofthe circular area (Ci in Figure 2), downslope of the S1 escarpment,and in several limited areas in the D4 deep basin north of ADSF(Fig. 6). Scattered areas corresponding to the C echo facies areobserved on the Khayr al Din Bank and the western slope of thestudy area.

CHIRP profile - MDJ 04

N

MD04-2800

15 m

1 km

CHIRP profile - MDJ 03CHIRP profile - MDJ 30

10 km

MDJ 04MDJ 03

MDJ 30

V.E. = 34x

T3 T3

T3

T3

FIG. 7.—Fence diagram of the chirp seismic profiles showing the extent of T3 echo facies and the location of the sediment core MD04-2800. See Table 2 for further description of the T3 echo facies. Also see Figure 2 for location of inset map on this figure.

Transparent Echo Facies (T).—Four transparent echo facies (T)are distinguished on the chirp profiles. The first type consists of atransparent acoustic body, with an irregular base on layered echofacies (T1). The second type corresponds to alternating transparentand layered echo facies (T2). The third type consists of a transpar-ent lens, observed at the surface or buried (T3). The fourth type, theT4 echo facies, consists of transparent echo facies overlying roughecho facies. This echo facies exists only on the continental shelfwhere a rough paleotopography is covered by younger sediment.

Based on previous work, transparent echo facies are attrib-uted to MTDs (e.g., Damuth et al., 1983). The T1 echo facies isidentified at the foot of the S1 escarpment, characterizing theentire D2 basin (Fig. 6). Small areas characterized by the T2 echofacies were mapped at the edge of the eastern slope, while anextended area characterized by the T3 echo facies was observeda the foot of the Khayr al Din Bank. A fence diagram of intersect-ing seismic lines shows several MTDs. The maximum estimatedthickness of the MTD sampled in core MD04-2800 is approxi-mately 11 m (Fig. 7). Successive appearances of MTD throughoutthe subsurface interval suggest a recurrent process.


Correlation of Echo Facies with Seafloor Imagery.—

Three distinct echo facies are revealed on the chirp profile(MDJ08) acquired at the foot of the continental slope: fromsouthwest to northeast, L1, T1, and R (Fig. 8A). The T1 echo faciesis a MTD, accumulated in a local depression and showing anerosional base. The R echo facies along the ravines are most likelyindicating the presence of coarser sediments. The second chirpprofile (MDJ03) extends throughout the D2 basin and SebaouCanyon (Fig. 8B). Three echo facies occur on the seismic line, fromsouthwest to northeast, C on the flank of the salt diapir, T1 and L1in the D2 basin, and R on the floor of Sebaou Canyon. Here, thesecond branch of Sebaou Canyon, described as a small incision, isidentified on the profile (Fig. 8B).

The acoustic mosaic of the entire continental slope showsrelatively highly backscatter on the slope and Sebaou Canyon andmoderate backscatter in the deep basins and in the western part ofthe study area (Fig. 9, Table 2). The distribution of echo facies andthe backscatter imagery correlate well. In particular, all canyonsrecognized on the bathymetric map are clearly identified on thebackscatter map. Correlation between high reflectivity and R echofacies is clear in canyon axes. It is possible to infer that dark shadedareas on the map (high reflectivity) in seafloor backscatter corre-spond to areas actively swept by bottom submarine currents in thecanyon floors (Fig. 9, Table 2). Sebaou Canyon reveals the mostwidespread and darkest shades, since it seems to be the most activesediment-transport system. In the deep basin offshore the city ofAlgiers, the imagery map shows variable shades of gray (Fig. 9). Atthe foot of the escarpment delimited by the S1 slope break, a largearea characterized by low backscatter exists, and correlates withthe presence of T1 echo facies (compare Figures 4 and 9).

Analysis of Sediment Cores

A total of seven cores were used to calibrate the echo faciesand to explain the distribution of echo facies in term of sedimen-tary processes. Sedimentary facies based on geological descrip-tions and X-ray images are compared with corresponding echofacies (Table 2). L1 echo facies consist of an alternation of hemipe-lagic and turbidite sequences (core KMDJ-03). The superficialtransparent low-energy reflectors (L2) correspond to normallyconsolidated clay deposits (core MD04-2799).

Three cores substantiate the presence of MTDs (other thanturbidites) (cores KMDJ-01, KMDJ-02, and MD04-2800). CoreKMDJ-01 is located at the toe of the continental slope, anddisplays a 2-m-thick MTD. The deposit consists of hard, largegray indurated mud clasts with variable length (2 to 25 cm long)supported by a brown clay matrix. Core KMDJ-02 reveals a 1.8-m-thick MTD (T3 echo facies), characterized by soft mud clastsand deformed laminae, and core MD04-2800 reveals a MTDburied 7 to 8 m below the seafloor. This MTD is up to 8 to 9 m thickand consists of hard, consolidated gray mud clasts and highlydeformed laminae supported by a muddy matrix. Turbiditesequences are observed throughout the study area. These depos-its are characterized by high variability in terms of grain size,thickness, and structures (Table 1).

DISCUSSION

Sediment Supply

Siliciclastic sediment supply on the Algerian margin appearsto be a function of two key factors, as in the case of many other

3010

3110

3210

2 kmSW NEMDJ 08

1200 1400 1600 1800 2000 2200Trace

T1

R

L1

3420

3520

Two-

way

trav

el ti

me

(ms)

2 km NESW MDJ 03

Trace7400 7600 7800 8000 8200 8400 8600 8800

T2

R

L2C east branch

L1

B

D1 basin Sebaou Canyon

D2 basin Sebaou Canyon

Two-

way

trav

el ti

me

(ms)

A

FIG. 8.—A) Chirp seismic profile MDJ 08 showing the echo facies distribution at the foot of the continental slope. B) Chirp seismicprofile MDJ 03 showing the echo-facies distribution in the deep basin D2. Echo-facies types are discussed in text and defined inTable 2. Tracklines for both profiles are indicated in Figure 2.


Mediterranean margins: gravity-driven pro-cesses and river density flows. Both of theseprocesses lead to formation of submarine can-yons (Canals et al., 2006). As mentioned above,rivers on the central Algerian coast have a sea-sonal regime, with significant flood periods oc-curring after intense rainfalls. Even during peri-ods of no flooding, sediment transported byrivers may be trapped directly by canyon sys-tems, since the continental shelf is quite narrow.For example, the mouth of the Sebaou River islocated only 4 km from the head of the westerntributary of Sebaou Canyon, which allows directcapture of sediment by the canyon system. Incontrast, the Isser River is actually not connectedto the Algiers Canyon, since tectonic uplift dur-ing the Quaternary has probably diverted itspathway to the east (Boudiaf et al., 1998). How-ever, based on backscatter data, the occurrenceof MTDs, and the existence of the ADSF, theAlgiers canyon seems to be still very active (Fig.9).

Active Sedimentary Processes

Sedimentation along the Algerian marginseems to be controlled by two processes: (1)gravity-induced processes, including both mass-transport deposits and turbidity currents, and(2) hemipelagic sedimentation (Fig. 10). Hemi-pelagic sedimentation appears more widespreadwest of the city of Algiers. Both of the corescollected east of Algiers (cores KMDJ 02 andKMDJ 03) consist of over 80% of hemipelagicsediments.

Turbidity Currents.—

Turbidity currents seem very active on thecontinental slope of the Algerian margin (Fig.10). Three well-developed canyons and numer-ous ravines incise the continental slope. Thesesystems are complex, with large drainage basinsand multiple tributaries. Based on the backscat-ter image, coarser sediments seem to character-ize these canyon floors. Thick turbidite sequencesoccur within the basins, where turbidity currentswere confined. Based on the core descriptions,we estimated an average thickness and time ofrecurrence of turbidite sequences (Table 1). Atrend emerges that shows that coarser and thickerturbidite sequences are more common in theeastern part of the study area cores (KMDJ-04,MD04-2798, and MD04-2799). Many thin turbid-ite sequences occur at the location of core MD04-2800 beneath the MTD. It can be assumed that abig event, which triggered the MTD, has sub-stantially changed the slope morphology, sinceno turbidite sequence was deposited after thisevent.

Fairways for turbidity currents are not wellconstrained, since bathymetric data are not avail-able in the distal part of the study area. It is clearthat Sebaou Canyon continues onto the abyssalplain. The disappearance of morphologic evi-dence for Algiers Canyon and the other well-

2.2°

E

2

.4°E

2.

6°E

2

.8°E

3

.0°E

3

.2°E

3

.4°E

3.

6°E

3

.8°E

4

.0°E

37.2

°N

37.0

°N

36.8

°N

36.6

°N

2.2°

E

2

.4°E

2.

6°E

2

.8°E

3

°E

3

.2°E

3

.4°E

3.

6°E

3

.8°E

4

.0°E

37.2

°N

37.0

°N

36.8

°N

36.6

°N

B5

D4

B4

D3

AD

SF

B3

S2

S1

D2

D1 B

1

B2

Tip

asa

Alg

iers

Bou

mer

dès

Del

lys

0

2

0 km

L1

T3

L3

L4

T1

T2

R

L2

C

F IG

. 9.—

Bac

ksca

tter

map

of t

he A

lgie

rs a

rea.

Lig

ht to

nes

are

low

bac

ksca

tter

and

dar

k to

nes

are

high

bac

ksca

tter

. A

lpha

num

eric

des

igna

tion

s an

d s

ymbo

ls o

n th

e m

apar

e d

efin

ed in

the

capt

ion

of F

igur

e 2.

See

Tab

le 2

for

corr

elat

ion

betw

een

back

scat

ter

and

ech

o fa

cies

.


Delly

s

vv

v

vv

vv

vv

vv

vv

vv

v

vv

vv

vv

vv

vv

v

vv

vv

v

vv

v

v

vv

v

vvv

v

vv

v

De

llys

Ca

nyo

n

Kh

ayr

al D

in B

an

k

vv

Hem

ipela

gic

and/o

r tu

rbid

ite d

eposi

ts

Mass

-tra

nsp

ort

deposi

ts

Turb

idity

-curr

ent path

s

Dia

pirs

Se

dim

en

t w

ave

s

Riv

ers

0

2

0 k

m

Se

ba

ou

Riv

er

Isse

r R

ive

r

2.2

°E

2

.4°E

2.6

°E

2.8

°E

3

.0°E

3.2

°E

3

.4°E

3

.6°E

3.8

°E

4.0

°E

2.2

°E

2.4

°E

2.6

°E

2.8

°E

3.0

°E

3.2

°E

3

.4°E

3

.6°E

3.8

°E

4

.0°E

37.0

°N

36.8

°N

36.6

°N

37.2

°N

37.4

°N

36.4

°N

37.0

°N

36.8

°N

36.6

°N

37.2

°N

37.4

°N

36.4

°N

Alg

iers

Ca

nyo

n

Bo

um

erd

ès

Alg

iers

Ma

zafr

an

Riv

er

Tip

asa

Se

ba

ou

Ca

nyo

n

B3

B4

B5

S1

S2

D1

D2

D4

D3

AD

SF

B2

B1

vv

v

vv

vv

v

F IG

. 10.

—Su

mm

ary

map

sho

win

g th

e re

sult

ing

dep

osit

s fr

om th

e m

ain

sed

imen

tary

pro

cess

es in

the

Alg

iers

are

a. A

lpha

num

eric

des

igna

tion

s an

d s

ymbo

ls o

n th

e m

apar

e d

efin

ed in

the

capt

ion

of F

igur

e 2.


developed canyons on the abyssal plain might be caused bychange of slope gradient. The average dip on the continentalslope is approximately 18 to 21°, whereas seaward of the continen-tal slope the dip decreases to 1 to 6° on the abyssal plain (Fig. 3).

Mass-Transport Deposits.—

MTDs occur across the Algerian margin and are preferen-tially located (1) on the steep slopes and (2) within the canyonssystem. MTDs have small size, with an average areal extent ofapproximately 0.2 km2 and a sediment volume of approxi-mately 0.01 km3. A single feature, located in the lower part of thecontinental slope offshore Dellys, is much larger than otherMTDs. In comparison with other studies dealing with morpho-logic analysis and statistics about parameters of MTDs (Booth etal., 1993; Hampton et al., 1996; McAdoo et al., 2000; Hühnerbachand Masson, 2004; Sultan et al., 2004), the size of the Algierianfeatures is considered quite small. Recent work (Domzig et al.,2009) on the western part of the Algerian margin documentedsmall-size MTDs, similar to those found in the Algiers area.MTDs were recognized on the seismic lines as three echo facies:C, T2, and T3. It seems that MTDs across the entire Algerianmargin are located at the base of the slopes, on the flanks ofdiapirs, and in association with the canyon system (head, flanks,and interfluves).

Sediment Waves.—

Sediment waves observed on the ADSF appear related to theactivity of turbidity currents, spilling over the right (eastern)levee of the Algiers turbidity system. However, the effect ofbottom currents offshore Algiers, although not well documentedin this specific case, cannot be ruled out (Millot and Taupier-Letage, 2005).

Trigger Mechanisms

As previously documented, abundant small-size MTDs occurnot only on the continental slope and the deep escarpments in theAlgiers area but also across the rest of the Algerian margin(Domzig et al., 2009). Any attempt to explain the small size ofthese features across the Algerian Margin must consider severalfactors, such as steep slopes, weak layers, salt tectonics, andearthquakes.

Head scarps of the MTDs do not coincide with the maximumslope gradient; thus slope gradient is not a significant controllingparameter for failure initiation. The minimal effect of slopegradient on initiation of MTDs is well documented in the litera-ture (Hampton et al., 1996; Booth et al., 1993; McAdoo et al., 2000;Sultan et al., 2004; Hühnerbach and Masson, 2004; Lastras et al.,2006).

Several MTDs occur on the flanks of salt diapirs, suggestingthat salt diapirism may also act as a trigger mechanism. However,this implies only local destabilization, not large-scale failureprocesses.

Numerous silt and sand layers were observed in the availablecores. These layers may act as weak layers and could be the maincause of sediment disturbance and liquefaction during earth-quakes. Recent studies highlighted the presence of reverse faultsalong the Algerian margin. The expression of an active fault onthe seafloor was mapped on the lower part of the continentalslope offshore the city of Dellys (Déverchère et al., 2005; Domziget al., 2006) (Fig. 2). The 2003 Boumerdès earthquake occurred at6 to 16 km depth halfway between the cities of Bourmedes andDellys (Fig. 2). The occurrence of various MTDs documented in

this study may suggest ongoing active deformation. However, adirect connection between the 2003 Boumerdès earthquake andthe initiation of MTDs on the lower continental slope cannot bedefinitively established at this time.

CONCLUSIONS

Bathymetry data supported by chirp seismic profiles allowsdefining the main morphometric features on the central Algerianmargin area. Echo-facies mapping, calibrated by cores, also al-lows description of the main pattern of sediment accumulationand permits reconstruction of the main sedimentary processes.Conclusions from this study are:

(1) The study area, representing approximately 200 km of thecentral Algerian continental margin, reveals a morphology-controlled tectonics, with the presence of seafloor escarp-ments, small basins, and diapirs. All of these features mayhave an important role in the transport, accumulation, anddisturbance of sediment.

(2) The continental slope is deeply incised by well-developedcanyon systems. Sediment is transported from the continentthroughout the canyon system to the deep basins, where thickturbidite sequences are confirmed by analysis of cores.

(3) The MTDs observed on the central Algerian margin are rela-tively small in size. The lower part of the continental slopeexhibits one significant-size feature that is located near thefault allegedly responsible for the 2003 Boumerdès earth-quake. Large buried MTDs, consisting of transparent lenses,occur in the western part of the study area and imply arecurrent event in the past.

(5) Earthquakes could be considered as the main triggering mecha-nism for MTDs within the study area. In particular, a possiblesignature of the 2003 Boumerdès earthquake was evaluated,even if a direct impact on the study area was not obvious.Moderate to high seismicity and high frequency of earth-quakes in the study area may explain the small size of MTDsand the lack of recent large events. Rigorous dating of recentand past MTDs is needed in order to achieve new insightsabout frequency of events and their connection with theAlgerian seismicity.

ACKNOWLEDGMENTS

This study is part of the EURODOM European Project (con-tract RTN2-2001-00281). Financial support was provided byIFREMER and the “Agence Nationale de Recherche” (ISIS-ANR-05-Catt-005-01). We thank officers and crew members fromMARADJA (2003) and PRISMA (2004) surveys. The authorsacknowledge Homa Lee and David Twichell for their criticalreviews and suggestions. We warmly thank R. Craig Shipp for hissubstantial input, which significantly improved the manuscript.The paper is dedicated to Bruno, who passed away in August,2008 at only 48 years old.

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