Post on 16-Oct-2021
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, Franceg.unterseh@fugro.com
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).
71ALGERIAN MARGIN SEDIMENTATION PATTERNS
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).
G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM72
0 °2°
4°6°
8 °10
°12
°14
°16
°18
°20
°22
°24
°26
°28
°30
°
0
20 k
m
Tip
asa
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
2
D3
AD
SF
D4
F IG
. 3.—
Slop
e-gr
adie
nt m
ap s
how
ing
seaf
loor
slo
pe in
deg
rees
. Alp
hanu
mer
ic d
esig
nati
ons
and
sym
bols
on
the
map
are
def
ined
in th
e ca
ptio
n of
Fig
ure
2.
Sedi
men
tC
ore
Wat
erD
epth
(m)
Len
gth
(m)
Sedi
men
t C
ore
Sett
ing
Ech
oF
acie
sT
hick
ness
of
the
MT
Ds
(m)
Num
ber
ofT
urbi
dity
Sequ
ence
s
Max
imum
Thi
ckne
ss(c
m)
Ave
rage
Thi
ckne
ss(c
m)
Fre
quen
cy(#
of
sequ
ence
s/m
eter
)K
MD
J-01
2400
7.83
Foot
of
the
cont
inen
tal s
lope
T2
219
105.
23.
45K
MD
J-02
1619
6.36
Foot
of
the
slop
e, e
astw
ard
Alg
iers
can
yon
L1
1.8
62
1.5
5.71
KM
DJ-
0323
413.
73Fo
ot o
f th
e sl
ope,
wes
t of
Alg
iers
can
yon
L1
-14
102.
75.
38K
MD
J-04
2711
7.56
Aby
ssal
pla
in, d
owns
lope
S1
esca
rpm
ent
T1
-25
457.
73.
31M
D04
-279
827
0728
.68
Aby
ssal
pla
in, d
owns
lope
S1
esca
rpm
ent
T1
-13
011
07.
34.
53M
D04
-279
922
4825
.30
Upp
er p
art o
f th
e S1
esc
arpm
ent
L2
-85
51.
24.
16M
D04
-280
027
5627
.27
Aby
ssal
pla
in, d
owns
lope
Kha
yr a
l Din
Ban
kT
38-
910
75
1.5
9.86
TA
BL
E 1
.—Sy
nthe
sis
of s
edim
ent c
ores
and
info
rmat
ion
on tu
rbid
ity
sequ
ence
s. E
cho
faci
es a
re d
escr
ibed
in th
e te
xt a
nd in
Tab
le 2
, and
thei
r d
istr
ibut
ion
in th
e st
udy
area
is s
how
n in
Fig
ure
6.
73ALGERIAN MARGIN SEDIMENTATION PATTERNS
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
G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM74
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).
75ALGERIAN MARGIN SEDIMENTATION PATTERNS
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.
77ALGERIAN MARGIN SEDIMENTATION PATTERNS
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
ism
ic f
acie
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
rval
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).—
G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM78
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.
79ALGERIAN MARGIN SEDIMENTATION PATTERNS
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.
G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM80
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
.
81ALGERIAN MARGIN SEDIMENTATION PATTERNS
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.
G. DAN, B. SAVOYE, V. GAULLIER, A. CATTANEO, J. DEVERCHERE, K. YELLES, AND MARADJA 2003 TEAM82
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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83ALGERIAN MARGIN SEDIMENTATION PATTERNS
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