Algerian Margin Sedimentation Patterns (Algiers Area, Southwestern Mediterranean)

ALGERIAN MARGIN SEDIMENTATION PATTERNS 69

ALGERIAN MARGIN SEDIMENTATION PATTERNS (ALGIERS AREA, SOUTHWESTERN MEDITERRANEAN)

GABRIELA DAN-UNTERSEH IFREMER, Géosciences Marines, Laboratoire Environnements Sédimentaires, Plouzané, France AND Université de Bretagne Occidentale, IUEM-CNRS UMR6538, 29280 Plouzané, France [email protected] BRUNO SAVOYE (DECEASED) VIRGINIE GAULLIER LEGEM, Université de Perpignan, 66860 Perpignan, France ANTONIO CATTANEO IFREMER, Géosciences Marines, Laboratoire Environnements Sédimentaires, Plouzané, France JACQUES DEVERCHERE Université de Bretagne Occidentale, IUEM-CNRS UMR6538, 29280 Plouzané, France KARIM YELLES CRAAG, Centre de Recherche en Astronomie, Astrophysique et Géophysique, Bouzaréah, Alger, Algérie AND MARADJA 2003 TEAM

Abstract: The present study provides an overview of recent sedimentation patterns on the central Algerian continental margin. Recent sedimentation patterns were assessed from morphological analysis, which is based on swath bathymetry and echo-facies mapping. It appears 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 the canyon 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. MTDs are mostly localized at the toes of steep slopes, where thrust faults were previously identified and mapped. Analysis of the spatial distribution of MTDs and their recurrence in time help reconstruct the main predisposing factors and triggering mechanisms, and evaluate their impact on evolution of the Algerian margin.

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

INTRODUCTION seismic event, swath bathymetry, chirp subbottom profiles, and sediment cores were acquired in the area affected by the Occurrences of MTDs involving large volumes of sediment Boumerdès earthquake. are known across continental slopes worldwide, especially along The present study describes geomorphological features and passive margins and volcanic islands (e.g., Masson et al., 1998; characterizes sedimentary processes on the central Algerian Bryn et al., 2003; Haflidason et al., 2004; Canals et al., 2004). margin (Fig. 1), located offshore the cities of Tipasa, Algiers, and However, MTDs were also documented along active margins, Dellys. The main objectives of this study are to: where tectonic activity may be one of the most relevant factors in generating sediment instabilities (e.g., von Huene et al., 1989; von • Highlight the main geomorphological features existing along Huene et al., 2000; Collot et al., 2001). A well-documented earth- the central Algiers margin, and provide a detailed description quake-induced MTD is the 1929 Grand Banks event, which of the seafloor characterized by many MTDs. occurred between Newfoundland and Nova Scotia off Atlantic Canada (Rupke, 1978; Piper and Asku, 1987; Piper et al., 1999). • Describe the main subsurface features with high-resolution Earthquakes are known to trigger MTDs and generate tsunamis, seismic data and document the most significant echo facies. which can seriously damage coastal and offshore infrastructures. This is the case of the Algerian margin, where several devastating • Integrate different data types to better understand regional earthquakes occurred during the last century (Heezen and Ewing, sedimentary dynamics along the central Algerian margin. 1955; El-Robrini et al., 1985). The most violent instrumentally recorded earthquake (7.1 Mw) occurred on 10 October 1980 in El TECTONIC AND GEOLOGIC CONTEXT Asnam. More recently on 21 May 2003 an earthquake with a magnitude of 6.8 (Mw) struck the city of Boumerdès, on the coast Since the early Cenozoic, the Algerian margin has been under near Algiers, and generated significant turbidity currents, con- a compressional regime with a northwest–southeast conver- firmed by numerous submarine-cable breaks. Following the 2003 gence (Stich et al., 2003). This active zone absorbs approximately

Mass-Transport Deposits in Deepwater Settings SEPM Special Publication No. 96, Copyright © 2011 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-287-9, p. 69–84. 70 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM

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

0 20 km Internal/external 37.4°N domain boundary Great Lesser Kabylia 37.4°N Study area Kabylia Annaba Algiers Tenes El Marsa KMDJ-04

37.2°N MD04-2798 37.2°N 2500 m

MD04-2799 m 2000 KMDJ-01 MD04-2800 37.0°N 37.0°N KMDJ-02 KMDJ-03 m 1500 Dellys

500 m 36.8°N 36.8°N 1000 m Algiers Sebaou River Boumerdès

Isser River

Mazafran River Tellian units (External 36.6°N 36.6°N Rivers zones) Tipasa Bathymetric contours Volcanism Tracklines Kabylian basement + Dorsale Kabyle Sediment core Kabylian Oligo-Miocene

Boumerdès epicenter Flyschs

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

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

5 mm/year of crustal shortening (Calais et al., 2003; Nocquet and zone and enjoys a mild Mediterranean climate. Rainfall is fairly Calais, 2004), with the onshore part accommodating only 50% of abundant along the coastal area, ranging from 0.3 cm/month the long-term convergence between the European and African during summertime to 10 to 12 cm/month during wintertime in plates (Meghraoui and Doumaz, 1996). It appears that active the Algiers area. As a consequence, rivers have a seasonal regime deformation offshore northern Algeria is expressed by 2 to 3 with significant flood periods. mm/year of shortening, and is likely related to occurrence of earthquakes. Numerous studies focused on tectonic activity or DATA SET AND METHODS sedimentary processes in the marine domain were conducted after the Boumerdès earthquake in 2003. As a consequence, the The MARADJA survey took place on the R/V Le Suroît fault believed responsible for the Boumerdès earthquake was (IFREMER) in August through September 2003. In order to identified between 6 and 16 km below the seafloor (Yelles et al., investigate the imprint of recent and past earthquakes, this sur- 2004; Meghraoui et al., 2004; Semmane et al., 2005). The fault vey focused on the part of the Algerian margin affected by the imprint on the seafloor was mapped on the lower part of the Boumerdès earthquake (Fig. 1). Data gathered during the survey continental slope offshore the city of Dellys (Déverchère et al., consist of swath bathymetry and backscatter data (Kongsberg 2005; Domzig et al., 2006) (see fault trace in Figure 2). Simrad EM-300 and EM 1000 echosounder), subbottom profiler Northern Algeria is part of the Maghrebian mountain chain, (chirp), and Kullenberg cores (Fig. 1). A 50-m-resolution digital which can be divided from south to north into three units: (1) the elevation model was created using the IFREMER CARAÏBES external domain (Tellian units), composed of sedimentary units, software, and the backscatter data provided a mosaic with 12.5 m mainly marls and limestones; (2) the flysch nappes thrusting the resolution. The chirp subbottom profiler of the R/V Le Suroît uses external zones; and (3) the internal domain, composed of frequencies between 1.8 and 5.3 kHz, reaching a maximum Hercynian basement sometimes associated with its sedimentary vertical penetration of 80–100 m in muddy sediment. More than cover (e.g., Bracene et al., 2003). Thus, the study area covers the 2800 km of chirp profiles were acquired for the study area (Fig. 1). internal domain (Fig. 1). The central Algiers slope is composed of Four gravity cores (designated with letters KMD) were collected Oligo-Miocene sediments, consisting mostly of flysch and volca- in the study area during the MARADJA cruise in 2003, and have nic deposits (Domzig et al., 2006). a maximum length of about 7 m (Table 1). In addition, three giant There are three main rivers in the study area: from east to west, Calypso cores (designated with letters MD) were collected dur- the Sebaou River, the Isser River, and the Mazafran River (Fig. 1), ing the PRISMA survey (May 2005) from the R/V Marion Dufresne which supply sediment to the study area. The river regime is (Table 1). The sediment cores were analyzed in the Sedimentary influenced by the Algerian climate, which is typically arid and Environments Laboratory at IFREMER. Unopened core sections hot, although northern coastal Algeria is part of the temperate were analyzed using the GEOTEK core logger (MSCL, http:// ALGERIAN MARGIN SEDIMENTATION PATTERNS 71

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

37.4°N Canyons Sediment wave 37.4°N Fig. 5a Rivers Diapirs Sebaou Canyon Slope break Mass-transport deposits

Thrust fault ADSF Algiers Deep-Sea Fan D2

B Fig. 8 37.2°N 37.2°N S2 Dellys Canyon S1 D4 D1 D3 B1 ADSF Inset Fig. 8A Fig. 7 B5 37.0°N F 37.0°N Ci Fig. 4a B3 s B4 B2 Dellys Pockmarks area

Khayr al Din Bank 36.8°N Algiers Canyon 36.8°N Algiers Sebaou River Boumerdès

Isser River

36.6°N 36.6°N Tipasa Mazafran River

0 20 km

36.4°N 36.4°N 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

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). www.geotek.co.uk). Sections of split cores were photographed to 8 km) in the eastern part (Fig. 2). Bathymetry on the continental and then X-rayed using a SCOPIX X-ray device (University of shelf is not available for this study; only the shelf break, ranging Bordeaux I). Grain-size analysis was performed by the laser between 100 and 150 m water depth, is locally imaged within the technique (Coulter LS130). MARADJA data. Chirp profiles were used to define echo facies in the study area. The echo-facies methodology, first described by Damuth Continental Slope.— (1975, 1980), and more recently refined by other workers (Gaullier and Bellaiche, 1998; Loncke et al., 2002), was adapted for the The continental slope is steep, with an average angle of 11° (Fig. present study. First, we listed and classified different echo facies, 3). The slope is defined by northeast–southwest abrupt slope followed by the mapping of each echo facies along ship tracks and breaks (B1 to B5) and by intermediate breaks forming flat areas (F) interpolation between the lines. This last step was facilitated by or circular suspended basins (Ci) (Fig. 2). Well-developed canyon also considering bathymetric and backscatter maps. systems and numerous ravines incise the slope and have an enhanced expression on the slope map. MTDs occur on the slope RESULTS especially at canyon heads and flanks, in the interfluve areas between canyons, and particularly on the lower part of the slope. Physiography of the Algiers Area For instance, a large area west of the Algiers Canyon is affected by submarine slides, which occur at various water depths: on the The study area is along the central Algerian margin (an area upper slope at 500 m, on the middle slope between 1000 and 1200 measuring 200 km x 50 km), located between 2.2° E (west of m, and on the lower slope between 1600 and 1700 m. The lower part Tipasa) and 4.1° E (east of Dellys). The following major physi- of the continental slope offshore Dellys is also affected by numer- ographic domains were defined in the study area: (1) continental ous MTDs. A particular feature showing a head scarp of more than shelf; (2) continental slope delimited by different slope breaks (B1 200 m in height and approximately 1.5 km in width is visible on the to B5); (3) major canyons; (4) Khayr al Din Bank; (5) abyssal plain shaded relief map (Figs. 4A, B). Part of the failed sediment seems (basins D1 to D4); and (6) deep curvilinear escarpments (S1 and deposited on the slope (Fig. 4B), ranging between steep lateral S2). The following discussion examines each one of these do- walls of the failure (Fig. 4C). The MTD covers a significant surface mains separately. of 4 km2, with an estimated volume of approximately 0.20 km3.

Continental Shelf.— Major Canyons.—

The continental shelf has a variable width. The shelf width Three well-developed canyon systems were identified on the ranges from 11 to 30 km west of Algiers, and becomes narrow (1 continental slope: from east to west, Dellys Canyon, Sebaou Can- 72 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM

36.8°N

36.6°N

37°N

37.2°N

3.45

5.71 5.38

3.31 4.53 4.16

9.86

(# of

meter)

sequences/

Frequency

4°E

5.2 1.5

2.7 7.7 7.3 1.2 1.5

(cm)

Average

Thickness

Dellys

caption of Figure 2.

8°E 4°E

10 45

110

(cm)

Thickness

Maximum

Slope (degrees)

14 25

130

107

Turbidity

Sequences

Number of

Boumerdès

- -

1.8

8-9

Thickness of

the MTDs (m)

L1 L1

T1 L2

ADSF

Echo

Facies

Algiers

8°E 3°E 3.2°E 3.4°E 3.6°E 3.

8°E 3°E 3.2°E 3.4°E 3.6°E 3.8°E

Sediment Core Setting

in the text and in Table 2, and their distribution in the study area is shown in Figure 6.

1.—Synthesis of sediment cores and information on turbidity sequences. Echo facies are described

Foot of the continental slope Foot of the slope, eastward Algiers canyon Foot of the slope, west of Algiers canyon Abyssal plain, downslope S1 escarpment Abyssal plain, downslope S1 escarpment Upper part of the S1 escarpment Abyssal plain, downslope Khayr al Din Bank

ABLE

ipasa

(m)

7.83 6.36 3.73 7.56

28.68 25.30 27.27

Length

(m)

0 20 km

2400 1619 2341 2711 2707 2248 2756

Depth

Water

2.2°E 2.4°E 2.6°E 2.

Core

37°N

Sediment

KMDJ-01 KMDJ-02 KMDJ-03 KMDJ-04

. 3.—Slope-gradient map showing seafloor slope in degrees. Alphanumeric designations and symbols on the map are defined in the

MD04-2798 MD04-2799 MD04-2800

37.2°N

36.8°N

36.6°N

F ALGERIAN MARGIN SEDIMENTATION PATTERNS 73

A 3°44'E 3°45'E 3°46'E 3°47'E 3°48'E B

37°3'N SE NW (B) 2400 37°3'N 1700 Max. head-wall height

1900 Initial slope

i 2100 2400

37°2'N 37°2'N Depth (m) MTD 2300 Slip surface 2300 i 2000 2500 V.E. = 2.6x 2200 0 1000 2000 3000 4000 5000 6000 Down dip distance (m) (C) 37°1'N 37°1'N C W i E

2160 Side-wall Side-wall 1900 2180 MTD 1 700 Depth (m) V.E. = 7.3x

37°N Headscarp Thrust fault 37°N 04008001200 1600 2000 2400 Along strike distance (m) Lateral scarp Intersection 1500 i between profiles -1600 MTD 0 1 km

1400 3°44'E 3°45'E 3°46'E 3°47'E 3°48'E

FIG. 4.—A) Shaded relief map showing mass-wasting deposits on the lower part of the continental slope. Thin black lines mark prominent scarps associated with mass-wasting deposits. Location of map area is indicated in Figure 2. B) Dip bathymetric profile through the slide area. C) Strike bathymetric profile through the slide area. Location of both profiles is indicated in Part A. yon, and Algiers Canyon. The Dellys Canyon drainage area con- Khayr al Din Bank.— sists of two main branches, which collect several tributaries (Fig. 5A). This canyon incises the slope at 100 m deep on the upper slope A major change in orientation of the Algerian margin (striking and at 350 m deep on the middle slope. Canyon flanks are steep, west-southwest to east-northeast) is observed west of the city of with slope angles of 15 to 25° (Fig. 3). In its lower part, three Algiers (Fig. 2). Khayr al Din Bank is an elongated area of high escarpments as high as 70, 120, and 200 m from the canyon floor are relief (500 m water depth), facing towards a deep basin (2700 m observed, together with a plateau probably of tectonic origin (Fig. deep; Domzig et al., 2006). A first slope break occurs at 600 to 650 5B). Beyond the plateau, Dellys Canyon is no longer visible on the m of water depth, followed by change of orientation towards the seafloor. west and change in slope angle from 2° to 5° (Fig. 3). Superficial Sebaou Canyon is characterized by a rectilinear morphology, MTDs affect the western and northern part of Khayr al Din Bank, and is fed by several tributaries, probably connected with the and an alignment of pockmarks occurs in its northern part (Fig. Sebaou River (Fig. 5A). Slope gradient ranges between 15 and 25° 2). The pockmarks are 300 m to 450 m in diameter and up to 17 m for the canyon head and flank (Fig. 3). Seaward of the B1 slope in depth. The eastern slope exhibits gullies and small MTDs. In break, Sebaou Canyon becomes wider (approximately 3 km) with contrast, the western slope is much gentler, probably affected by moderately high flanks. Two asymmetric branches exist more a significant erosive process (see s on the western end of Figure 2). than 26 km from the canyon head (Fig. 5A). The primary branch follows a northward course, while the secondary branch, consist- Abyssal Plain.— ing of a smaller canyon incision of approximately 30 to 50 m depth, follows a northwestward direction. Large depressions/ There are four sedimentary basins in the study area (Fig. 2). scours, 1 km in width and more than 40 m in depth, exist on the They are delimited by continental slope breaks and deep escarp- seafloor along the Sebaou Canyon, and are considered as strong ments. In the eastern part of the study area, the D1 basin is 30 km evidence for significant erosion (Fig. 5C). long and 15 km wide in 2300 to 2400 m water depth. The D2 basin Algiers Canyon consists of two main meandering tributaries, is located seawards of the curvilinear escarpment (S1). The D3 with their heads located on the shelf break (Fig. 2). The Algiers basin corresponds to an elevated area downslope of the Algiers western tributary is sinuous, highly incised (200 to 300 m deep), Canyon. The D3 basin is interpreted as the Algiers deepsea fan and collects three other branches, each one with small tributaries. (ADSF), which is confined on its northern part by salt diapirs and In contrast, the Algiers eastern tributary has a rectilinear mor- a curvilinear escarpment (S2 in Figure 2). Sediment across the phology and consists of only two tributaries. ADSF may be sourced by turbidity currents or bottom currents, West of Algiers Canyon, the continental slope is incised by since sediment waves occur across the ADSF. A large MTD of well-developed canyons, with numerous gully-like tributaries. approximately 2 km width exits on the northern part of the ADSF. These tributaries connect in the middle part of the slope, creating Salt diapirs form elongated walls or rounded ridges with variable a large canyon with an average width between 1.5 and 3 km (Fig. length (1 to 7 km), and a maximum height of 100 m above the 2). These canyons have steep flanks, with an average slope of 18° seafloor. Small (0.2 km x 0.5 km across) subcircular diapirs also (Fig. 3). The morphological path of these canyons is difficult to exist at the foot of the ADSF. Several MTDs are identified on the follow beyond slope breaks B3 and B4 (Fig. 2). diapir flanks. A convex-upward area occurs on the abyssal plain 74 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM

A B Dellys Canyon 3.8°E 3.9°E 4°E 4.1°E 37.4°N 37.4°N S N 5 km 2100

2200 Sebaou Canyon (C) 2300 Escarpments

Secondary 2400 branch Depth (m) 2500 Scours

37.2°N 37.2°N 2600 V.E. = 8.5x Dellys Canyon (B) 0200040006000800010000 12000 14000 Distance (m)

Escarpments

C Sebaou Canyon

S N

37°N 37°N 2300

2400 Scours 2500 Dellys Depth (m) 2600 Canyon Isser River 2700 V.E. = 18.5x Thrust fault 0500010000 15000 20000 25000 36.8°N 36.8°N 3.8°E 3.9°E 4°E 4.1°E Distance (m)

FIG. 5.—A) Shaded relief map in Dellys and Sebaou canyons. Location of map area is indicated in Figure 2. B) Bathymetric profile throughout Dellys Canyon showing three escarpments on the distal part (thin blue line B in Panel A). C) Bathymetric profile throughout Sebaou Canyon showing scours on the canyon floor (thin blue line C in Panel A). downslope of the Khayr al Din Bank (D4 basin). This morphologi- chaotic (C), and transparent (T). All echo facies are illustrated in cal feature probably corresponds to a significant MTD accumu- Table 2, displayed on the echo-facies distribution map (Fig. 6), lated at the foot of the slope (Fig. 2). and discussed in detail below. Due to the very high slope gradi- ents and considerable change in the seafloor morphology, the Deep Curvilinear Escarpments.— continental slope has not been well imaged on chirp profiles in the study area (white area in Figure 6). The abyssal plain exhibits two curvilinear escarpments that are probably the seafloor expression of deformation repre- Layered Echo Facies (L).—Three subclasses are distinguished: sented by buried thrust folds (Déverchère et al., 2005; Domzig et (1) parallel, continuous reflectors (L1 and L2); (2) discontinuous al., 2006) (S1 and S2 in Figure 2). The S1 escarpment is steep (10° reflectors or undulations (L3, L4); and (3) parallel reflectors to 15°) and is approximately 30 km in length and 350 to 450 m in overlying the rough acoustic basement (L5) (Table 2). At the same height. Numerous small MTDs, approximately 0.5 to 3 km wide, time, two variants with a subclass exist in the first and second occur on the S1 escarpment (Dan et al., 2009). The majority of subclasses: high-energy reflections (L1 and L3), and low-energy these MTDs exist on the mid slope, although two corridors, reflections, corresponding to a transparent superficial layer (L2 formed by several MTDs, occur on the upper part of the escarp- and L4) (Table 2). Based on previous work, layered echo facies ment in approximately 2300 m water depth. A second deep usually correspond to alternating hemipelagic intervals and tur- curvilinear escarpment, delimited by the S2 slope break, occurs bidites (Damuth, 1980). However, the same echo facies could be north of the ADSF between several salt diapirs (Fig. 2). Just like attributed to hemipelagic intervals (Pratson and Laine, 1989). the previous one, the S2 escarpment is affected by MTDs less Discontinuous or undulated reflections are probably shaped by than 0.5 km in width. contour currents or turbidity currents and are associated with sediment waves (Heezen et al., 1966). Echo-Facies Analysis The L1 echo facies occurs in the D1 and D4 basins, whereas the L2 echo facies is observed mostly in the shallow part of the study Echo-Facies Classification and Mapping.— area and on Khayr al Din Bank (Fig. 2). The field of sediment waves on ADSF corresponds to the L4 echo facies. Another area Definition of echo facies relies on acoustic properties and on characterized by the same echo facies (L4) exists at the foot of the continuity of the bottom and sub-bottom seismic reflections. continental slope west of Algiers deep-sea fan. The L3 echo facies Eleven distinctive echo facies exist in the study area, grouped into occurs only in two narrow areas north of ADSF. The L5 echo facies four major categories: layered (L), non-penetrative, or rough (R), occurs on the continental shelf. ALGERIAN MARGIN SEDIMENTATION PATTERNS 75

37.3°N

37.1°N

36.7vN

36.9°N

2500

4°E

no data on shelf or slope

Dellys

and backscatter data. Contour

1500

3.8°E

. 8B .

Fi Fig

°E 3.8°E 4°E

3.6°E 500

200

3.4°E

Alphanumeric designations and symbols on the map are defined in the caption

.2°E 3.4°E 3.6 0

ADSF

200

3°E 3.2°E

Algiers

250

2.8°E 500

pasa

Inset

Fig. 7

2500

0 20 km

2.2°E 2.4°E 2.6°E 2.8°E 3°E 3

2.2°E 2.4°E 2.6°E

interval is 100 m. Echo facies classes are discussed in text and defined in Table 2. of Figure 2.

. 6.—Seismic echo-facies distribution map for Algiers area, based on the chirp-profile analysis, combined with the bathymetric

37.3°N

37.1°N

36.9°N

36.7°N 76 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM

2.—Chirp seismic echo-facies classification. Each echo facies is interpreted in term of sedimentary processes.

between echo facies and backscatter is described in the text. Location of backscatter is indicated in Figure 9.

Echo facies are described in the text, and their distribution in the study area is shown in Figure 6. Correlation

ABLE

T ALGERIAN MARGIN SEDIMENTATION PATTERNS 77

hemipelagic

currents and

Interpretation

continental shelf

Deposits formed by

sedimentation on the

Hemipelagic intervals

and turbidite sequences

Mass-transport deposits

Backscatter

X-ray

Image

Radiography

(continued).—

KMDJ-04

KMDJ-01

MD04-2798

MD04-2800

Core sample

ABLE

Example

buried

layered

internal

acoustic

basement

reflections

C: Chaotic

seafloor or

Description

seismic facies

transparent and

T1: Alternating

lens. Present on

without internal

T3: Transparent

T4: Transparent overlying rough

T2: Transparent,

Classes

Chaotic

Transparent

Echo-facies 78 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM

Non-Penetrative Echo Facies (Rough, R).—Non-penetrative echo Transparent Echo Facies (T).—Four transparent echo facies (T) facies characterize areas where the seismic reflection signal does are distinguished on the chirp profiles. The first type consists of a not penetrate below the seafloor. Generally, the R echo facies exists transparent acoustic body, with an irregular base on layered echo in the axes of canyons, where the eroded seafloor is mostly covered facies (T1). The second type corresponds to alternating transparent by coarse-grained deposits (Damuth, 1975). As an example, Sebaou and layered echo facies (T2). The third type consists of a transpar- Canyon consists of the R echo facies, covering an area of approxi- ent lens, observed at the surface or buried (T3). The fourth type, the mately 22.5 km long and greater than 10 km wide (Fig. 6). The R T4 echo facies, consists of transparent echo facies overlying rough echo facies also occurs along the curvilinear escarpments and the echo facies. This echo facies exists only on the continental shelf smooth area described on the north slope of Khayr al Din Bank, where a rough paleotopography is covered by younger sediment. which seem associated with MTDs along the slopes (Fig. 6). Based on previous work, transparent echo facies are attributed to MTDs (e.g., Damuth et al., 1983). The T1 echo facies is Chaotic Echo Facies (C).—The chaotic echo facies (C) represent identified at the foot of the S1 escarpment, characterizing the highly disorganized sediments induced by gravity-driven pro- entire D2 basin (Fig. 6). Small areas characterized by the T2 echo cesses (Pratson and Laine, 1989, Damuth, 1994). C echo facies facies were mapped at the edge of the eastern slope, while an exist in various locations along the Algerian margin: at the foot of extended area characterized by the T3 echo facies was observed the circular area (Ci in Figure 2), downslope of the S1 escarpment, a the foot of the Khayr al Din Bank. A fence diagram of intersect- and in several limited areas in the D4 deep basin north of ADSF ing seismic lines shows several MTDs. The maximum estimated (Fig. 6). Scattered areas corresponding to the C echo facies are thickness of the MTD sampled in core MD04-2800 is approxi- observed on the Khayr al Din Bank and the western slope of the mately 11 m (Fig. 7). Successive appearances of MTD throughout study area. the subsurface interval suggest a recurrent process.

MDJ 03 MDJ 04 MDJ 30 15 m

1 km

V.E. = 34x T3 10 km 04 profile - MDJ CHIRP MD04-2800

T3 T3

CHIRP profile - MDJ 30 J 03 - MD ofile RP pr N CHI

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. ALGERIAN MARGIN SEDIMENTATION PATTERNS 79

Correlation of Echo Facies with Seafloor Imagery.— Analysis of Sediment Cores

Three distinct echo facies are revealed on the chirp profile A total of seven cores were used to calibrate the echo facies (MDJ08) acquired at the foot of the continental slope: from and to explain the distribution of echo facies in term of sedimen- southwest to northeast, L1, T1, and R (Fig. 8A). The T1 echo facies tary processes. Sedimentary facies based on geological descrip- is a MTD, accumulated in a local depression and showing an tions and X-ray images are compared with corresponding echo erosional base. The R echo facies along the ravines are most likely facies (Table 2). L1 echo facies consist of an alternation of hemipe- indicating the presence of coarser sediments. The second chirp lagic and turbidite sequences (core KMDJ-03). The superficial profile (MDJ03) extends throughout the D2 basin and Sebaou transparent low-energy reflectors (L2) correspond to normally Canyon (Fig. 8B). Three echo facies occur on the seismic line, from consolidated clay deposits (core MD04-2799). southwest to northeast, C on the flank of the salt diapir, T1 and L1 Three cores substantiate the presence of MTDs (other than in the D2 basin, and R on the floor of Sebaou Canyon. Here, the turbidites) (cores KMDJ-01, KMDJ-02, and MD04-2800). Core second branch of Sebaou Canyon, described as a small incision, is KMDJ-01 is located at the toe of the continental slope, and identified on the profile (Fig. 8B). displays a 2-m-thick MTD. The deposit consists of hard, large The acoustic mosaic of the entire continental slope shows gray indurated mud clasts with variable length (2 to 25 cm long) relatively highly backscatter on the slope and Sebaou Canyon and supported by a brown clay matrix. Core KMDJ-02 reveals a 1.8- moderate backscatter in the deep basins and in the western part of m-thick MTD (T3 echo facies), characterized by soft mud clasts the study area (Fig. 9, Table 2). The distribution of echo facies and and deformed laminae, and core MD04-2800 reveals a MTD the backscatter imagery correlate well. In particular, all canyons buried 7 to 8 m below the seafloor. This MTD is up to 8 to 9 m thick recognized on the bathymetric map are clearly identified on the and consists of hard, consolidated gray mud clasts and highly backscatter map. Correlation between high reflectivity and R echo deformed laminae supported by a muddy matrix. Turbidite facies is clear in canyon axes. It is possible to infer that dark shaded sequences are observed throughout the study area. These depos- areas on the map (high reflectivity) in seafloor backscatter corre- its are characterized by high variability in terms of grain size, spond to areas actively swept by bottom submarine currents in the thickness, and structures (Table 1). canyon floors (Fig. 9, Table 2). Sebaou Canyon reveals the most widespread and darkest shades, since it seems to be the most active DISCUSSION sediment-transport system. In the deep basin offshore the city of Algiers, the imagery map shows variable shades of gray (Fig. 9). At Sediment Supply the foot of the escarpment delimited by the S1 slope break, a large area characterized by low backscatter exists, and correlates with Siliciclastic sediment supply on the Algerian margin appears the presence of T1 echo facies (compare Figures 4 and 9). to be a function of two key factors, as in the case of many other

3010 SW MDJ 08 NE 2 km

D1 basin Sebaou Canyon R

3110 L1 T1 wo-way travel time (ms) T

3210 1200 1400 1600 1800 2000 2200 Trace B

3420 NE SW MDJ 03 2 km

D2 basin Sebaou Canyon R

C L2 east branch L1 T2 3520 o-way travel time (ms) Tw

7400 7600 7800 8000 8200 8400 8600 8800 Trace

FIG. 8.—A) Chirp seismic profile MDJ 08 showing the echo facies distribution at the foot of the continental slope. B) Chirp seismic profile MDJ 03 showing the echo-facies distribution in the deep basin D2. Echo-facies types are discussed in text and defined in Table 2. Tracklines for both profiles are indicated in Figure 2. 80 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM

Mediterranean margins: gravity-driven processes and river density flows. Both of these

36.8°N

36.6°N

37.0°N 37.2°N processes lead to formation of submarine canyons (Canals et al., 2006). As mentioned above, rivers on the central Algerian coast have a seasonal regime, with significant flood periods oc- curring after intense rainfalls. Even during periods of no flooding, sediment transported by rivers may be trapped directly by canyon sys-

Dellys

R tems, since the continental shelf is quite narrow. For example, the mouth of the Sebaou River is

D2 located only 4 km from the head of the western

D1 tributary of Sebaou Canyon, which allows direct

S1 capture of sediment by the canyon system. In

signations and symbols on the map

3.8°E 4.0°E 3.8°E 4.0°E contrast, the Isser River is actually not connected T1 to the Algiers Canyon, since tectonic uplift dur-

L2 ing the Quaternary has probably diverted its pathway to the east (Boudiaf et al., 1998). How- ever, based on backscatter data, the occurrence of MTDs, and the existence of the ADSF, the Algiers canyon seems to be still very active (Fig.

B2 9).

C Active Sedimentary Processes

Boumerdès Sedimentation along the Algerian margin S2 seems to be controlled by two processes: (1) gravity-induced processes, including both mass- transport deposits and turbidity currents, and

B3 (2) hemipelagic sedimentation (Fig. 10). Hemi- pelagic sedimentation appears more widespread west of the city of Algiers. Both of the cores

ADSF collected east of Algiers (cores KMDJ 02 and KMDJ 03) consist of over 80% of hemipelagic sediments.

3.2°E 3.4°E 3.6°E Turbidity Currents.—

Algiers Turbidity currents seem very active on the continental slope of the Algerian margin (Fig.

B4 10). Three well-developed canyons and numer-

3°E 3°E 3.2°E 3.4°E 3.6°E ous ravines incise the continental slope. These

L3 L4

3.0°E

systems are complex, with large drainage basins and multiple tributaries. Based on the backscatter image, coarser sediments seem to characterize these canyon floors. Thick turbidite sequences occur within the basins, where turbidity currents

D4 were confined. Based on the core descriptions, we estimated an average thickness and time of recurrence of turbidite sequences (Table 1). A trend emerges that shows that coarser and thicker turbidite sequences are more common in the B5 eastern part of the study area cores (KMDJ-04,

pasa MD04-2798, and MD04-2799). Many thin turbid- Ti ite sequences occur at the location of core MD04- 2800 beneath the MTD. It can be assumed that a big event, which triggered the MTD, has sub- stantially changed the slope morphology, since T3 no turbidite sequence was deposited after this event. Fairways for turbidity currents are not well

0 20 km constrained, since bathymetric data are not avail- L1 able in the distal part of the study area. It is clear

2.2°E 2.4°E 2.6°E 2.8°E 2.2°E 2.4°E 2.6°E 2.8°E that Sebaou Canyon continues onto the abyssal

are defined in the caption of Figure 2. See Table 2 for correlation between backscatter and echo facies. . 9.—Backscatter map of the Algiers area. Light tones are low backscatter and dark tones are high backscatter. Alphanumeric de plain. The disappearance of morphologic evi-

37.0°N

37.2°N

36.8°N 36.6°N dence for Algiers Canyon and the other well- ALGERIAN MARGIN SEDIMENTATION PATTERNS 81

37.4°N

37.2°N

37.0°N

36.6°N

36.8°N

36.4°N

4.0°E v

4.0°E

Dellys Canyon v

Sebaou River

Dellys

0 20 k

v v

tions and symbols on the map

Sebaou Canyon

v v

Isser River v

v v

Boumerdès

Algiers Canyon

v v

D3 v

ADSF

3.0°E 3.0°E 3.2°E 3.4°E 3.6°E 3.8°E

Algiers

Diapirs

Rivers

Sediment waves

Mazafran River

Khayr al Din Bank

v v

pasa v

v v

urbidity-current paths

Mass-transport deposits

Hemipelagic and/or turbidite deposits

v v

2.2°E 2.4°E 2.6°E 2.8°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

are defined in the caption of Figure 2.

. 10.—Summary map showing the resulting deposits from the main sedimentary processes in the Algiers area. Alphanumeric designa

37.0°N

37.4°N

37.2°N

36.6°N

36.8°N

36.4°N

F 82 G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM developed canyons on the abyssal plain might be caused by this study may suggest ongoing active deformation. However, a change of slope gradient. The average dip on the continental direct connection between the 2003 Boumerdès earthquake and slope is approximately 18 to 21°, whereas seaward of the continen- the initiation of MTDs on the lower continental slope cannot be tal slope the dip decreases to 1 to 6° on the abyssal plain (Fig. 3). definitively established at this time.

Mass-Transport Deposits.— CONCLUSIONS

MTDs occur across the Algerian margin and are preferen- Bathymetry data supported by chirp seismic profiles allows tially located (1) on the steep slopes and (2) within the canyons defining the main morphometric features on the central Algerian system. MTDs have small size, with an average areal extent of margin area. Echo-facies mapping, calibrated by cores, also al- approximately 0.2 km2 and a sediment volume of approxi- lows description of the main pattern of sediment accumulation mately 0.01 km3. A single feature, located in the lower part of the and permits reconstruction of the main sedimentary processes. continental slope offshore Dellys, is much larger than other Conclusions from this study are: MTDs. In comparison with other studies dealing with morphologic analysis and statistics about parameters of MTDs (Booth et (1) The study area, representing approximately 200 km of the al., 1993; Hampton et al., 1996; McAdoo et al., 2000; Hühnerbach central Algerian continental margin, reveals a morphology- and Masson, 2004; Sultan et al., 2004), the size of the Algierian controlled tectonics, with the presence of seafloor escarp- features is considered quite small. Recent work (Domzig et al., ments, small basins, and diapirs. All of these features may 2009) on the western part of the Algerian margin documented have an important role in the transport, accumulation, and small-size MTDs, similar to those found in the Algiers area. disturbance of sediment. MTDs were recognized on the seismic lines as three echo facies: C, T2, and T3. It seems that MTDs across the entire Algerian (2) The continental slope is deeply incised by well-developed margin are located at the base of the slopes, on the flanks of canyon systems. Sediment is transported from the continent diapirs, and in association with the canyon system (head, flanks, throughout the canyon system to the deep basins, where thick and interfluves). turbidite sequences are confirmed by analysis of cores.

Sediment Waves.— (3) The MTDs observed on the central Algerian margin are relatively small in size. The lower part of the continental slope Sediment waves observed on the ADSF appear related to the exhibits one significant-size feature that is located near the activity of turbidity currents, spilling over the right (eastern) fault allegedly responsible for the 2003 Boumerdès earth- levee of the Algiers turbidity system. However, the effect of quake. Large buried MTDs, consisting of transparent lenses, bottom currents offshore Algiers, although not well documented occur in the western part of the study area and imply a in this specific case, cannot be ruled out (Millot and Taupier- recurrent event in the past. Letage, 2005). (5) Earthquakes could be considered as the main triggering mecha- Trigger Mechanisms nism for MTDs within the study area. In particular, a possible signature of the 2003 Boumerdès earthquake was evaluated, As previously documented, abundant small-size MTDs occur even if a direct impact on the study area was not obvious. not only on the continental slope and the deep escarpments in the Moderate to high seismicity and high frequency of earth- Algiers area but also across the rest of the Algerian margin quakes in the study area may explain the small size of MTDs (Domzig et al., 2009). Any attempt to explain the small size of and the lack of recent large events. Rigorous dating of recent these features across the Algerian Margin must consider several and past MTDs is needed in order to achieve new insights factors, such as steep slopes, weak layers, salt tectonics, and about frequency of events and their connection with the earthquakes. Algerian seismicity. Head scarps of the MTDs do not coincide with the maximum slope gradient; thus slope gradient is not a significant controlling ACKNOWLEDGMENTS parameter for failure initiation. The minimal effect of slope gradient on initiation of MTDs is well documented in the litera- This study is part of the EURODOM European Project (con- ture (Hampton et al., 1996; Booth et al., 1993; McAdoo et al., 2000; tract RTN2-2001-00281). Financial support was provided by Sultan et al., 2004; Hühnerbach and Masson, 2004; Lastras et al., IFREMER and the “Agence Nationale de Recherche” (ISIS-ANR- 2006). 05-Catt-005-01). We thank officers and crew members from Several MTDs occur on the flanks of salt diapirs, suggesting MARADJA (2003) and PRISMA (2004) surveys. The authors that salt diapirism may also act as a trigger mechanism. However, acknowledge Homa Lee and David Twichell for their critical this implies only local destabilization, not large-scale failure reviews and suggestions. We warmly thank R. Craig Shipp for his processes. substantial input, which significantly improved the manuscript. Numerous silt and sand layers were observed in the available The paper is dedicated to Bruno, who passed away in August, cores. These layers may act as weak layers and could be the main 2008 at only 48 years old. cause of sediment disturbance and liquefaction during earthquakes. Recent studies highlighted the presence of reverse faults REFERENCES along the Algerian margin. The expression of an active fault on the seafloor was mapped on the lower part of the continental BOOTH, J.S., O’LEARY, D.W., POPENOE, P., AND DANFORTH, W.W., 1993, U.S. slope offshore the city of Dellys (Déverchère et al., 2005; Domzig Atlantic continental slope landslides: their distribution, general at- et al., 2006) (Fig. 2). The 2003 Boumerdès earthquake occurred at tributes, and implication, in Schwab, W.C., Lee, H.J., and Twichell, 6 to 16 km depth halfway between the cities of Bourmedes and D.C., eds., Submarine Landslides: Selected Studies in the U.S. Exclu- Dellys (Fig. 2). The occurrence of various MTDs documented in sive Economic Zone: U.S. Geological Survey, Bulletin 2002, p. 14–22. ALGERIAN MARGIN SEDIMENTATION PATTERNS 83

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