Dan_Unterseh12

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 g.unterseh@fugro.com 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 masstransport 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. Masstransport 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 Occurrences of MTDs involving large volumes of sediment are known across continental slopes worldwide, especially along passive 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 in generating sediment instabilities (e.g., von Huene et al., 1989; von Huene et al., 2000; Collot et al., 2001). A well-documented earthquake-induced MTD is the 1929 Grand Banks event, which occurred between Newfoundland and Nova Scotia off Atlantic Canada (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 devastating earthquakes occurred during the last century (Heezen and Ewing, 1955; El-Robrini et al., 1985). The most violent instrumentally recorded earthquake (7.1 Mw) occurred on 10 October 1980 in El Asnam. More recently on 21 May 2003 an earthquake with a magnitude of 6.8 (Mw) struck the city of Boumerdès, on the coast near Algiers, and generated significant turbidity currents, confirmed by numerous submarine-cable breaks. Following the 2003

seismic event, swath bathymetry, chirp subbottom profiles, and sediment cores were acquired in the area affected by the Boumerdès earthquake.

The present study describes geomorphological features and characterizes sedimentary processes on the central Algerian margin (Fig. 1), located offshore the cities of Tipasa, Algiers, and Dellys. The main objectives of this study are to: • Highlight the main geomorphological features existing along the central Algiers margin, and provide a detailed description of the seafloor characterized by many MTDs. • Describe the main subsurface features with high-resolution seismic data and document the most significant echo facies.

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

TECTONIC AND GEOLOGIC CONTEXT Since the early Cenozoic, the Algerian margin has been under a compressional regime with a northwest–southeast convergence (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

Great Kabylia

Study area Tenes

3.0°E

3.2°E

3.4°E

3.6°E

3.8°E

0

Internal/external domain boundary

37.4°N

2.8°E

2.6°E

2.4°E

4.0°E

20 km 37.4°N

Lesser Kabylia Annaba

Algiers

El Marsa

KMDJ-04 MD04-2798

37.2°N

250 0

37.2°N

m

MD04-2799 2000

MD04-2800

37.0°N

37.0°N

KMDJ-02

KMDJ-03 1500

m

KMDJ-01

m Dellys

500 m 36.8°N

10

00

m

36.8°N Algiers

Sebaou River Boumerdès Isser River

Mazafran River 36.6°N Tipasa

2.2°E

2.4°E

2.6°E

2.8°E

3.0°E

3.2°E

3.4°E

Rivers

Tellian units (External zones)

Bathymetric contours

Volcanism

Tracklines

Kabylian basement + Dorsale Kabyle

Sediment core

Kabylian Oligo-Miocene

Boumerdès epicenter

Flyschs

3.6°E

3.8°E

36.6°N

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 Calais, 2004), with the onshore part accommodating only 50% of the long-term convergence between the European and African plates (Meghraoui and Doumaz, 1996). It appears that active deformation offshore northern Algeria is expressed by 2 to 3 mm/year of shortening, and is likely related to occurrence of earthquakes. Numerous studies focused on tectonic activity or sedimentary processes in the marine domain were conducted after the Boumerdès earthquake in 2003. As a consequence, the fault believed responsible for the Boumerdès earthquake was identified between 6 and 16 km below the seafloor (Yelles et al., 2004; Meghraoui et al., 2004; Semmane et al., 2005). The fault imprint on the seafloor was mapped on the lower part of the continental 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) the external domain (Tellian units), composed of sedimentary units, mainly marls and limestones; (2) the flysch nappes thrusting the external zones; and (3) the internal domain, composed of Hercynian basement sometimes associated with its sedimentary cover (e.g., Bracene et al., 2003). Thus, the study area covers the internal domain (Fig. 1). The central Algiers slope is composed of Oligo-Miocene sediments, consisting mostly of flysch and volcanic 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 is influenced by the Algerian climate, which is typically arid and hot, although northern coastal Algeria is part of the temperate

zone and enjoys a mild Mediterranean climate. Rainfall is fairly abundant along the coastal area, ranging from 0.3 cm/month during summertime to 10 to 12 cm/month during wintertime in the Algiers area. As a consequence, rivers have a seasonal regime with 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 to investigate the imprint of recent and past earthquakes, this survey focused on the part of the Algerian margin affected by the Boumerdès earthquake (Fig. 1). Data gathered during the survey consist of swath bathymetry and backscatter data (Kongsberg Simrad EM-300 and EM 1000 echosounder), subbottom profiler (chirp), and Kullenberg cores (Fig. 1). A 50-m-resolution digital elevation model was created using the IFREMER CARAÏBES software, and the backscatter data provided a mosaic with 12.5 m resolution. The chirp subbottom profiler of the R/V Le Suroît uses frequencies between 1.8 and 5.3 kHz, reaching a maximum vertical penetration of 80–100 m in muddy sediment. More than 2800 km of chirp profiles were acquired for the study area (Fig. 1).

Four gravity cores (designated with letters KMD) were collected in the study area during the MARADJA cruise in 2003, and have a maximum length of about 7 m (Table 1). In addition, three giant Calypso cores (designated with letters MD) were collected during the PRISMA survey (May 2005) from the R/V Marion Dufresne (Table 1). The sediment cores were analyzed in the Sedimentary Environments Laboratory at IFREMER. Unopened core sections were analyzed using the GEOTEK core logger (MSCL, http://

71

ALGERIAN MARGIN SEDIMENTATION PATTERNS

2.2°E

37.4°N

2.4°E

2.6°E

2.8°E

Canyons

Sediment wave

Rivers

Diapirs

3.0°E

3.2°E

3.4°E

3.6°E

3.8°E

4.0°E

37.4°N Fig. 5a Sebaou Canyon

Mass-transport deposits

Slope break ADSF

Thrust fault

D2

Algiers Deep-Sea Fan

37.2°N

Fig. 8B

S2

37.2°N Dellys Canyon

S1

D4

D1

D3 ADSF

Inset Fig. 7

B1

Fig. 8A

B5

F

37.0°N

s

Ci

B3

B4

37.0°N

Fig. 4a

B2 Dellys

Pockmarks area

Khayr al Din Bank 36.8°N

36.8°N

Algiers Canyon

Algiers

Sebaou River

Boumerdès Isser River

36.6°N

36.6°N Tipasa Mazafran River

0 36.4°N 2.2°E

20 km 36.4°N

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 and then X-rayed using a SCOPIX X-ray device (University of Bordeaux I). Grain-size analysis was performed by the laser technique (Coulter LS130).

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

RESULTS Physiography of the Algiers Area The study area is along the central Algerian margin (an area measuring 200 km x 50 km), located between 2.2° E (west of Tipasa) and 4.1° E (east of Dellys). The following major physiographic domains were defined in the study area: (1) continental shelf; (2) continental slope delimited by different slope breaks (B1 to B5); (3) major canyons; (4) Khayr al Din Bank; (5) abyssal plain (basins D1 to D4); and (6) deep curvilinear escarpments (S1 and S2). The following discussion examines each one of these domains separately.

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

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

Continental Slope.— The continental slope is steep, with an average angle of 11° (Fig. 3). The slope is defined by northeast–southwest abrupt slope breaks (B1 to B5) and by intermediate breaks forming flat areas (F) or circular suspended basins (Ci) (Fig. 2). Well-developed canyon systems and numerous ravines incise the slope and have an enhanced expression on the slope map. MTDs occur on the slope especially at canyon heads and flanks, in the interfluve areas between canyons, and particularly on the lower part of the slope.

For instance, a large area west of the Algiers Canyon is affected by submarine slides, which occur at various water depths: on the upper slope at 500 m, on the middle slope between 1000 and 1200 m, and on the lower slope between 1600 and 1700 m. The lower part of the continental slope offshore Dellys is also affected by numerous MTDs. A particular feature showing a head scarp of more than 200 m in height and approximately 1.5 km in width is visible on the shaded relief map (Figs. 4A, B). Part of the failed sediment seems deposited on the slope (Fig. 4B), ranging between steep lateral walls of the failure (Fig. 4C). The MTD covers a significant surface of 4 km2, with an estimated volume of approximately 0.20 km3.

Major Canyons.— Three well-developed canyon systems were identified on the continental slope: from east to west, Dellys Canyon, Sebaou Can-

2.2°E

20 km

2.4°E

T ipasa

2.4°E

7.83 6.36 3.73 7.56 28.68 25.30 27.27

Length (m)

2.6°E

B5

2.6°E

2.8°E

2.8°E

B4

D4

3°E

Algiers

3°E

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

Sediment Core Setting

3.2°E

D3 ADSF

S2

3.2°E

T2 L1 L1 T1 T1 L2 T3

Echo Facies

B3

3.4°E

0°

2°

4°

6°

3.6°E

8°

3.6°E

19 6 14 25 130 85 107

10°

S1

D1

D2

3.8°E

10 2 10 45 110 5 5

Maximum Thickness (cm)

12°

14°

3.8°E

16°

18°

Slope (degrees)

B1

Number of Turbidity Sequences

B2

Boumerdès

3.4°E

2 1.8 8-9

Thickness of the MTDs (m)

20°

Dellys

22°

4°E

24°

4°E

5.2 1.5 2.7 7.7 7.3 1.2 1.5

26°

Average Thickness (cm)

28°

30°

36.6°N

36.8°N

37°N

37.2°N

Frequency (# of sequences/ meter) 3.45 5.71 5.38 3.31 4.53 4.16 9.86

FIG. 3.—Slope-gradient map showing seafloor slope in degrees. Alphanumeric designations and symbols on the map are defined in the caption of Figure 2.

36.6°N

36.8°N

37°N

37.2°N

0

2400 1619 2341 2711 2707 2248 2756

KMDJ-01 KMDJ-02 KMDJ-03 KMDJ-04 MD04-2798 MD04-2799 MD04-2800

2.2°E

Water Depth (m)

Sediment Core

TABLE 1.—Synthesis of sediment cores and information on turbidity sequences. Echo facies are described in the text and in Table 2, and their distribution in the study area is shown in Figure 6.

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

73

ALGERIAN MARGIN SEDIMENTATION PATTERNS

A 3°44'E

3°45'E

3°46'E

3°47'E

3°48'E

B 37°3'N

SE

37°3'N

0

240

(B)

NW

Max. head-wall height

1700

2400

37°2'N

37°2'N

Depth (m)

1900

Initial slope i

2100

MTD 2300

i

23

00

Slip surface

00

20

V.E. = 2.6x

2500

22

00

0

2000

3000

4000

5000

37°1'N

C Depth (m)

W

1900 17

E

i

Side-wall

2160

Side-wall MTD

2180

V.E. = 7.3x

00

37°N

Thrust fault

Headscarp

i

Lateral scarp MTD-1600

0

6000

Down dip distance (m)

(C)

37°1'N

1000

37°N

0

400

1200

1600

2000

2400

Along strike distance (m)

0

150

Intersection between profiles

800

1 km 00

14

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 consists of two main branches, which collect several tributaries (Fig. 5A). This canyon incises the slope at 100 m deep on the upper slope and 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, three escarpments as high as 70, 120, and 200 m from the canyon floor are observed, together with a plateau probably of tectonic origin (Fig. 5B). Beyond the plateau, Dellys Canyon is no longer visible on the seafloor.

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

Algiers Canyon consists of two main meandering tributaries, with their heads located on the shelf break (Fig. 2). The Algiers western 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 morphology and consists of only two tributaries.

West of Algiers Canyon, the continental slope is incised by well-developed canyons, with numerous gully-like tributaries.

These tributaries connect in the middle part of the slope, creating a 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 to follow beyond slope breaks B3 and B4 (Fig. 2).

Khayr al Din Bank.— A major change in orientation of the Algerian margin (striking west-southwest to east-northeast) is observed west of the city of Algiers (Fig. 2). Khayr al Din Bank is an elongated area of high relief (500 m water depth), facing towards a deep basin (2700 m deep; Domzig et al., 2006). A first slope break occurs at 600 to 650 m of water depth, followed by change of orientation towards the west and change in slope angle from 2° to 5° (Fig. 3). Superficial MTDs 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 m in depth. The eastern slope exhibits gullies and small MTDs. In contrast, the western slope is much gentler, probably affected by a 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 escarpments. In the eastern part of the study area, the D1 basin is 30 km long and 15 km wide in 2300 to 2400 m water depth. The D2 basin is located seawards of the curvilinear escarpment (S1). The D3 basin corresponds to an elevated area downslope of the Algiers Canyon. The D3 basin is interpreted as the Algiers deepsea fan (ADSF), which is confined on its northern part by salt diapirs and a curvilinear escarpment (S2 in Figure 2). Sediment across the ADSF may be sourced by turbidity currents or bottom currents, since sediment waves occur across the ADSF. A large MTD of approximately 2 km width exits on the northern part of the ADSF. Salt diapirs form elongated walls or rounded ridges with variable length (1 to 7 km), and a maximum height of 100 m above the seafloor. Small (0.2 km x 0.5 km across) subcircular diapirs also exist at the foot of the ADSF. Several MTDs are identified on the 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

3.8°E 37.4°N

3.9°E

4°E

B

4.1°E 37.4°N

5 km

Dellys Canyon S

2100 2200

Depth (m)

Sebaou Canyon (C) Secondary branch

2300

Escarpments

2400 2500

Scours

2600

37.2°N

37.2°N

N

V.E. = 8.5x

Dellys Canyon 0

(B)

2000

4000

6000

8000

10000

12000

14000

Distance (m) Escarpments

C

Sebaou Canyon S

37°N

Dellys

N

2300

Depth (m)

37°N

2400

Scours 2500 2600

Canyon

Isser River Thrust fault

V.E. = 18.5x

2700 0

36.8°N 3.8°E

3.9°E

4°E

36.8°N 4.1°E

5000

10000

15000

20000

25000

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 morphological feature probably corresponds to a significant MTD accumulated at the foot of the slope (Fig. 2).

Deep Curvilinear Escarpments.— The abyssal plain exhibits two curvilinear escarpments that are probably the seafloor expression of deformation represented by buried thrust folds (Déverchère et al., 2005; Domzig et al., 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 in height. Numerous small MTDs, approximately 0.5 to 3 km wide, occur on the S1 escarpment (Dan et al., 2009). The majority of these MTDs exist on the mid slope, although two corridors, formed by several MTDs, occur on the upper part of the escarpment in approximately 2300 m water depth. A second deep curvilinear escarpment, delimited by the S2 slope break, occurs north of the ADSF between several salt diapirs (Fig. 2). Just like the previous one, the S2 escarpment is affected by MTDs less than 0.5 km in width.

Echo-Facies Analysis Echo-Facies Classification and Mapping.— Definition of echo facies relies on acoustic properties and on continuity of the bottom and sub-bottom seismic reflections.

Eleven distinctive echo facies exist in the study area, grouped into four major categories: layered (L), non-penetrative, or rough (R),

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

Layered Echo Facies (L).—Three subclasses are distinguished: (1) parallel, continuous reflectors (L1 and L2); (2) discontinuous reflectors or undulations (L3, L4); and (3) parallel reflectors overlying the rough acoustic basement (L5) (Table 2). At the same time, two variants with a subclass exist in the first and second subclasses: high-energy reflections (L1 and L3), and low-energy reflections, corresponding to a transparent superficial layer (L2 and L4) (Table 2). Based on previous work, layered echo facies usually correspond to alternating hemipelagic intervals and turbidites (Damuth, 1980). However, the same echo facies could be attributed to hemipelagic intervals (Pratson and Laine, 1989).

Discontinuous or undulated reflections are probably shaped by contour currents or turbidity currents and are associated with sediment waves (Heezen et al., 1966).

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 area and on Khayr al Din Bank (Fig. 2). The field of sediment waves on ADSF corresponds to the L4 echo facies. Another area characterized by the same echo facies (L4) exists at the foot of the continental slope west of Algiers deep-sea fan. The L3 echo facies occurs only in two narrow areas north of ADSF. The L5 echo facies occurs on the continental shelf.

2. 2°E

0

2500

2.4 °E

20 km

2.4 °E

Tipasa

Inset Fig. 7

50

2. 6°E

B5

0

2. 6°E

2.8 °E

B4

D4

2.8 °E

2500

3°E

Algiers

3°E

3.2 °E

200 0

D3 ADSF

S2

3.2 °E

B3

3. 4°E

3. 4°E

3.6 °E

0 50

B2

200 0

Fig. 8A

S1

Fig. 8B

3.6 °E

1500

T4

no data on shelf or slope L4

4°E

T3

T2

T1

2500

R

C

L2

4°E

L3

L5

Dellys

B1

L1

3.8°E

D1

D2

3.8°E

36.7 vN

36.9 °N

37.1 °N

37.3 °N

FIG. 6.—Seismic echo-facies distribution map for Algiers area, based on the chirp-profile analysis, combined with the bathymetric and backscatter data. Contour interval is 100 m. Echo facies classes are discussed in text and defined in Table 2. Alphanumeric designations and symbols on the map are defined in the caption of Figure 2.

36. 7°N

36. 9°N

37. 1°N

37. 3°N

2. 2°E

ALGERIAN MARGIN SEDIMENTATION PATTERNS

75

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

Echo facies are described in the text, and their distribution in the study area is shown in Figure 6. Correlation between echo facies and backscatter is described in the text. Location of backscatter is indicated in Figure 9.

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

Transparent T

Chaotic C

Echo-facies Classes

T4: Transparent overlying rough acoustic basement

T3: Transparent lens. Present on seafloor or buried

T2: Transparent, without internal seismic facies

T1: Alternating transparent and layered reflections

C: Chaotic internal seismic facies

Description Example

-

MD04-2800

KMDJ-01

MD04-2798

KMDJ-04

-

Core sample

-

-

X-ray Radiography Image

TABLE 2 (continued).—

-

Backscatter

Deposits formed by currents and hemipelagic sedimentation on the continental shelf

Mass-transport deposits

Mass-transport deposits

Hemipelagic intervals and turbidite sequences

Mass-transport deposits

Interpretation

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Non-Penetrative Echo Facies (Rough, R).窶年on-penetrative echo facies characterize areas where the seismic reflection signal does not penetrate below the seafloor. Generally, the R echo facies exists in the axes of canyons, where the eroded seafloor is mostly covered by coarse-grained deposits (Damuth, 1975). As an example, Sebaou Canyon consists of the R echo facies, covering an area of approximately 22.5 km long and greater than 10 km wide (Fig. 6). The R echo facies also occurs along the curvilinear escarpments and the smooth 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).窶乃he chaotic echo facies (C) represent highly disorganized sediments induced by gravity-driven processes (Pratson and Laine, 1989, Damuth, 1994). C echo facies exist in various locations along the Algerian margin: at the foot of the 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 are observed on the Khayr al Din Bank and the western slope of the study area.

Transparent Echo Facies (T).窶認our transparent echo facies (T) are distinguished on the chirp profiles. The first type consists of a transparent acoustic body, with an irregular base on layered echo facies (T1). The second type corresponds to alternating transparent and layered echo facies (T2). The third type consists of a transparent lens, observed at the surface or buried (T3). The fourth type, the T4 echo facies, consists of transparent echo facies overlying rough echo facies. This echo facies exists only on the continental shelf where a rough paleotopography is covered by younger sediment.

Based on previous work, transparent echo facies are attributed to MTDs (e.g., Damuth et al., 1983). The T1 echo facies is identified at the foot of the S1 escarpment, characterizing the entire D2 basin (Fig. 6). Small areas characterized by the T2 echo facies were mapped at the edge of the eastern slope, while an extended area characterized by the T3 echo facies was observed a the foot of the Khayr al Din Bank. A fence diagram of intersecting seismic lines shows several MTDs. The maximum estimated thickness of the MTD sampled in core MD04-2800 is approximately 11 m (Fig. 7). Successive appearances of MTD throughout the subsurface interval suggest a recurrent process.

MDJ 03 MDJ 04 MDJ 30

15 m 1 km V.E. = 34x T3 10 km

ofile P pr

J 04

- MD

CHIR MD04-2800 T3

T3

T3

N

CHI

e rofil RP p

J 03 - MD

CH

IRP

pro

file

-M

DJ

30

FIG. 7.窶認ence diagram of the chirp seismic profiles showing the extent of T3 echo facies and the location of the sediment core MD042800. See Table 2 for further description of the T3 echo facies. Also see Figure 2 for location of inset map on this figure.

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ALGERIAN MARGIN SEDIMENTATION PATTERNS

Correlation of Echo Facies with Seafloor Imagery.—

Analysis of Sediment Cores

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

The acoustic mosaic of the entire continental slope shows relatively highly backscatter on the slope and Sebaou Canyon and moderate backscatter in the deep basins and in the western part of the study area (Fig. 9, Table 2). The distribution of echo facies and the backscatter imagery correlate well. In particular, all canyons recognized on the bathymetric map are clearly identified on the backscatter map. Correlation between high reflectivity and R echo facies is clear in canyon axes. It is possible to infer that dark shaded areas on the map (high reflectivity) in seafloor backscatter correspond to areas actively swept by bottom submarine currents in the canyon floors (Fig. 9, Table 2). Sebaou Canyon reveals the most widespread and darkest shades, since it seems to be the most active sediment-transport system. In the deep basin offshore the city of Algiers, the imagery map shows variable shades of gray (Fig. 9). At the foot of the escarpment delimited by the S1 slope break, a large area characterized by low backscatter exists, and correlates with the presence of T1 echo facies (compare Figures 4 and 9).

A total of seven cores were used to calibrate the echo facies and to explain the distribution of echo facies in term of sedimentary processes. Sedimentary facies based on geological descriptions and X-ray images are compared with corresponding echo facies (Table 2). L1 echo facies consist of an alternation of hemipelagic and turbidite sequences (core KMDJ-03). The superficial transparent low-energy reflectors (L2) correspond to normally consolidated clay deposits (core MD04-2799).

Three cores substantiate the presence of MTDs (other than turbidites) (cores KMDJ-01, KMDJ-02, and MD04-2800). Core KMDJ-01 is located at the toe of the continental slope, and displays a 2-m-thick MTD. The deposit consists of hard, large gray indurated mud clasts with variable length (2 to 25 cm long) supported by a brown clay matrix. Core KMDJ-02 reveals a 1.8m-thick MTD (T3 echo facies), characterized by soft mud clasts and deformed laminae, and core MD04-2800 reveals a MTD buried 7 to 8 m below the seafloor. This MTD is up to 8 to 9 m thick and consists of hard, consolidated gray mud clasts and highly deformed laminae supported by a muddy matrix. Turbidite sequences are observed throughout the study area. These deposits 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 appears to be a function of two key factors, as in the case of many other

A Two-way travel time (ms)

3010

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NE 2 km

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NE

Sebaou Canyon R

C

east branch

L2 L1

T2

3520

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8800

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

3.2°E 3°E 2.8°E 2.6°E

D4

2.8°E

2.2°E

36.6°N

36.8°N

37.0°N

37.2°N

0

L1

T3

2.4°E

20 km

Tipasa

B5

2.6°E 2.4°E 2.2°E

FIG. 9.—Backscatter map of the Algiers area. Light tones are low backscatter and dark tones are high backscatter. Alphanumeric designations and symbols on the map are defined in the caption of Figure 2. See Table 2 for correlation between backscatter and echo facies.

3.8°E 3.6°E 3.4°E

Boumerdès

L3 L4

B4

3.0°E

Algiers

D3 ADSF

3.2°E

B3

S2

3.4°E

C

B2

3.6°E

L2

T1

T2

S1

D1

B1

D2

3.8°E

R

Dellys

4.0°E

4.0°E

36.6°N

37.2°N

37.0°N

36.8°N

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 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 occurring after intense rainfalls. Even during periods of no flooding, sediment transported by rivers may be trapped directly by canyon systems, since the continental shelf is quite narrow.

For example, the mouth of the Sebaou River is located only 4 km from the head of the western tributary of Sebaou Canyon, which allows direct capture of sediment by the canyon system. In contrast, the Isser River is actually not connected to the Algiers Canyon, since tectonic uplift during the Quaternary has probably diverted its pathway to the east (Boudiaf et al., 1998). However, based on backscatter data, the occurrence of MTDs, and the existence of the ADSF, the Algiers canyon seems to be still very active (Fig. 9).

Active Sedimentary Processes Sedimentation along the Algerian margin seems to be controlled by two processes: (1) gravity-induced processes, including both masstransport deposits and turbidity currents, and (2) hemipelagic sedimentation (Fig. 10). Hemipelagic sedimentation appears more widespread west of the city of Algiers. Both of the cores collected east of Algiers (cores KMDJ 02 and KMDJ 03) consist of over 80% of hemipelagic sediments.

Turbidity Currents.— Turbidity currents seem very active on the continental slope of the Algerian margin (Fig. 10). Three well-developed canyons and numerous ravines incise the continental slope. These 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 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 eastern part of the study area cores (KMDJ-04, MD04-2798, and MD04-2799). Many thin turbidite sequences occur at the location of core MD042800 beneath the MTD. It can be assumed that a big event, which triggered the MTD, has substantially changed the slope morphology, since no turbidite sequence was deposited after this event.

Fairways for turbidity currents are not well constrained, since bathymetric data are not available in the distal part of the study area. It is clear that Sebaou Canyon continues onto the abyssal plain. The disappearance of morphologic evidence for Algiers Canyon and the other well-

v

B5

2.4°E

Tipasa

2.6°E 2.8°E

Mazafran River

Khayr al Din Bank

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3.0°E

Algiers

D3 ADSF

3.2°E

S2

B3

3.4°E

Boumerdès

Algiers Canyon

B2

S1

3.6°E

3.6°E

Isser River

B1

D1

D2

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3.8°E

0

Dellys Canyon

4.0°E

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Sebaou River

Dellys

Sebaou Canyon

4.0°E

36.4°N

36.6°N

36.8°N

37.0°N

37.2°N

37.4°N

FIG. 10.—Summary map showing the resulting deposits from the main sedimentary processes in the Algiers area. Alphanumeric designations and symbols on the map are defined in the caption of Figure 2.

2.2°E

v v v

36.4°N

v

36.6°N

v

36.8°N

v

v

37.0°N

v

v vv

v

37.2°N

v

v

v

Rivers

v

v

Turbidity-current paths

v v

v

v

Sediment waves

v

v

Mass-transport deposits

Diapirs

v

v

Hemipelagic and/or turbidite deposits

v

v

37.4°N

v

3.4°E

v

3.2°E

v

v

v v

3.0°E

v

v v v v

v

2.8°E

v v

vv v

2.6°E

v

v v v v

v

2.4°E

v

v

v

v v

2.2°E

ALGERIAN MARGIN SEDIMENTATION PATTERNS

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developed canyons on the abyssal plain might be caused by change of slope gradient. The average dip on the continental slope is approximately 18 to 21°, whereas seaward of the continental 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 preferentially located (1) on the steep slopes and (2) within the canyons system. MTDs have small size, with an average areal extent of approximately 0.2 km2 and a sediment volume of approximately 0.01 km3. A single feature, located in the lower part of the continental slope offshore Dellys, is much larger than other MTDs. In comparison with other studies dealing with morphologic analysis and statistics about parameters of MTDs (Booth et al., 1993; Hampton et al., 1996; McAdoo et al., 2000; Hühnerbach and Masson, 2004; Sultan et al., 2004), the size of the Algierian features is considered quite small. Recent work (Domzig et al., 2009) on the western part of the Algerian margin documented small-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 Algerian margin are located at the base of the slopes, on the flanks of diapirs, and in association with the canyon system (head, flanks, and interfluves).

Sediment Waves.— Sediment waves observed on the ADSF appear related to the activity of turbidity currents, spilling over the right (eastern) levee of the Algiers turbidity system. However, the effect of bottom currents offshore Algiers, although not well documented in this specific case, cannot be ruled out (Millot and TaupierLetage, 2005).

Trigger Mechanisms As previously documented, abundant small-size MTDs occur not only on the continental slope and the deep escarpments in the Algiers area but also across the rest of the Algerian margin (Domzig et al., 2009). Any attempt to explain the small size of these features across the Algerian Margin must consider several factors, such as steep slopes, weak layers, salt tectonics, and earthquakes.

Head scarps of the MTDs do not coincide with the maximum slope gradient; thus slope gradient is not a significant controlling parameter for failure initiation. The minimal effect of slope gradient on initiation of MTDs is well documented in the literature (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, suggesting that salt diapirism may also act as a trigger mechanism. However, this implies only local destabilization, not large-scale failure processes.

Numerous silt and sand layers were observed in the available cores. These layers may act as weak layers and could be the main cause of sediment disturbance and liquefaction during earthquakes. Recent studies highlighted the presence of reverse faults along the Algerian margin. The expression of an active fault on the seafloor was mapped on the lower part of the continental slope offshore the city of Dellys (Déverchère et al., 2005; Domzig et al., 2006) (Fig. 2). The 2003 Boumerdès earthquake occurred at 6 to 16 km depth halfway between the cities of Bourmedes and Dellys (Fig. 2). The occurrence of various MTDs documented in

this study may suggest ongoing active deformation. However, a direct connection between the 2003 Boumerdès earthquake and the initiation of MTDs on the lower continental slope cannot be definitively established at this time.

CONCLUSIONS Bathymetry data supported by chirp seismic profiles allows defining the main morphometric features on the central Algerian margin area. Echo-facies mapping, calibrated by cores, also allows description of the main pattern of sediment accumulation and permits reconstruction of the main sedimentary processes.

Conclusions from this study are: (1) The study area, representing approximately 200 km of the central Algerian continental margin, reveals a morphologycontrolled tectonics, with the presence of seafloor escarpments, small basins, and diapirs. All of these features may have an important role in the transport, accumulation, and disturbance of sediment.

(2) The continental slope is deeply incised by well-developed canyon systems. Sediment is transported from the continent throughout the canyon system to the deep basins, where thick turbidite sequences are confirmed by analysis of cores.

(3) The MTDs observed on the central Algerian margin are relatively small in size. The lower part of the continental slope exhibits one significant-size feature that is located near the fault allegedly responsible for the 2003 Boumerdès earthquake. Large buried MTDs, consisting of transparent lenses, occur in the western part of the study area and imply a recurrent event in the past.

(5) Earthquakes could be considered as the main triggering mechanism for MTDs within the study area. In particular, a possible signature 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 earthquakes in the study area may explain the small size of MTDs and the lack of recent large events. Rigorous dating of recent and past MTDs is needed in order to achieve new insights about frequency of events and their connection with the Algerian seismicity.

ACKNOWLEDGMENTS This study is part of the EURODOM European Project (contract RTN2-2001-00281). Financial support was provided by IFREMER and the “Agence Nationale de Recherche” (ISIS-ANR05-Catt-005-01). We thank officers and crew members from MARADJA (2003) and PRISMA (2004) surveys. The authors acknowledge Homa Lee and David Twichell for their critical reviews and suggestions. We warmly thank R. Craig Shipp for his substantial 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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