Sedimentary processes and origin of sediment

Marine and Petroleum Geology 26 (2009) 695–710
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Marine and Petroleum Geology
journal homepage: www.elsevier.com/locate/marpetgeo
Sedimentary processes and origin of sediment gravity-flow deposits on the
western Algerian margin during late Pleistocene and Holocene
Pierre Giresse a, *, Henri Pauc a, Jacques Déverchère b, c, the Maradja Shipboard Scientific Party
a
IMAGES EA4218, Laboratoire d’Études des Géo-Environnements Marins, Université de Perpignan, 52 Avenue Paul Alduy, 66860 Perpignan Cedex, France
Université Européenne de Bretagne, France
c
Université de Brest; CNRS, UMR 6538 Domaines Océaniques, Institut Universitaire Européen de la Mer, Place Copernic, 29280 Plouzané, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 October 2006
Received in revised form 29 February 2008
Accepted 24 March 2008
Available online 22 July 2008
Seven piston cores retrieved from the Algerian margin from Oran to 80 km east of Algiers were studied to
identify sediment gravity-flow deposits and their sources. At the foot of the slope, five sediment cores
indicate a decreasing frequency of turbidite sequences from the transgressive systems tract to the
highstand systems tract resulting in lower off-shelf sediment fluxes during the last highstand episode.
There is an approximately log-normal frequency distribution of bed thickness that increases for larger
grain-size class, but this relationship is frequently altered by truncation of the top of the turbidite
sequence. In the deep basin off Algeria, two sediment cores indicate that turbidite sequences are both
thicker and more preserved than at the foot of the slope and are observed through the entire sediment
core implying various origin of the gravity flow (eustatic change, seismicity).
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Turbidites
Debris flow
Algerian margin
Late Quaternary
Sediment cores
Benthic foraminifera
1. Introduction
In various studies of ancient turbidite systems (Johnson et al.,
2001), sandy growth packages are interpreted as mostly belonging
to lowstand system tracts with intervening fines representing
transgressive and highstand systems tracts (Van Wagoner et al.,
1988). This concept derives from the extension of classical
sequence-stratigraphic models to deep-water clastic systems
implying all important turbidite successions are fans or leveedchannel complexes deposited during or shortly after an episode of
relative low sea-level (Mitchum, 1985; Mutti, 1985; Pickering et al.,
1995; Den Hartog Jager et al., 1993). However, this concept has been
questioned because there are active deep-sea fans around the
world today coinciding with a highstand condition: the Var Fan
represents a Mediterranean example (Piper and Savoye, 1993).
Similarly, in the deep basin of the Gulf of Lions, sand beds were
deposited between 15.1 and 4.4 ka BP (radiocarbon ages) and are
believed to be related to the erosion of sand banks at the canyon
heads of the shelf breaks (Dennielou et al., 2003; Bonnel et al.,
2005; Carvajal and Steel, 2006).
Seismic activity in the western Mediterranean is concentrated in
northern Africa and especially in northern Algeria, where recent
destructive earthquakes have occurred. In a recent study (Rothwell
* Corresponding author. Tel.: þ33 4 68 66 88 48.
E-mail address: [email protected] (P. Giresse).
0264-8172/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.marpetgeo.2008.03.011
et al., 2006) on the southern Balearic Abyssal Plain, it was indicated
a little evidence for large-scale seismogenic turbidites, despite
proximity to the seismically active Algerian margin, 100 km to the
south. This suggested to the authors that seismogenic turbidites
must largely bypass this part of the plain.
The current paper reports a new high-resolution sedimentological study of seven piston cores from the Algerian margin from
Oran to 80 km east of Algiers with the main target to identify the
sedimentary record related to sediment gravity flows offshore
Algeria. The same general dynamic governs all turbidites: a waning
turbulent sediment gravity flow, but the resulting sedimentary
depositional structures do not yield direct clues about the triggering force. In spite of the seismogenic character of this margin,
the occurrence of seismo-turbidites such as those described from
the eastern margin of Japan Sea (Nakajima and Kanai, 2000) is only
a working hypothesis.
The term turbidite as used here is restricted to a deposit which
displays the vertical sequence (partial or complete) of Ta to Te
divisions (as defined by Bouma, 1962) plus at least a minimal
amount of upward-fining, graded bedding. There is usually a sharp
basal contact with the underlying muddy layer and less welldefined upper contact with the overlying muddy layer. Recurrence,
frequency distribution of bed thickness, and composition of turbidite sequences are analysed according to distance from the slope
(foot of slope and deep basin) and according to the chronostratigraphy (transgressive systems tracts or highstand systems tracts).
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Other gravity-induced processes as debris flows and density
cascading are also considered.
An improved understanding of the sources of the different
sediment fluxes should help interpreting the sedimentary record.
In this way, various indicators of the erosion of the last lowstand
deposits lying on the edge of the shelf are identified and semiquantified. From a statistical point of view, frequency distribution
of entire turbidite sequences and of sandy-bed thicknesses and the
relationship between grain-size and bed thickness according to the
work of Talling (2001) should help (1) interpreting the rheologic
changes of settling, and (2) distinguish preserved depositional
sequence from truncated sequences. This paper attempts to discuss
the eustatic or seismogenic or random causes of the initiation of
sediment gravity-flow processes on the western Algerian margin.
2. Background
2.1. Geological and tectonic setting
Northern Algeria is mainly a segment of the Alpine belt called
Maghrebides (Durand-Delga and Fonboté, 1980; Wildi, 1983). In the
south, the External zones (Tellian zones) are characterised by
Miocene folds and thrusts and overthrust the less deformed Atlasic
foreland. Further north, two other successive domains are overthrusting the External zones from South to North: (1) the flysch
units of the Maghrebian Tethys and (2) the Internal zone, made of
an old Hercynian basement.
Seismic activity of the area is related to the w5 mm yr1 oblique
convergence between the African and European plates (see Domzig
et al., 2009). This seismicity is distributed over a broad area, from
the Atlas front to the offshore margin. The active seismogenic fault
system is attested by the 1980, Ms 7.3 El Asnam and 2003, Mw 6.9
Boumerdes destructive earthquakes (CRAAG, 1994).
The shelf is generally narrow, width values ranging from 8 km
(east of Algiers) to 30 km in the western part (Fig. 1). The new
accurate bathymetric map indicates that the continental slope is
strongly irregular with an intermediate bench of varied width
(Domzig et al., 2006).
In the Algiers zone (Fig. 2A), there are numerous canyons. The
two main ones are the Algiers canyon, formerly draining the Isser
River, and the Dellys canyon, probably draining the Sebaou River.
The slope is steep, especially next to the Algiers massif, where it is
as much as 15%. East of Algiers, two major E-W slope breaks were
identified resulting mostly from the Plio-Quaternary tectonic
activity (Déverchère et al., 2005).
The Chenoua–Tenes zone (Fig. 1) is characterised by an E-W
linear continental slope which is quite steep (10%). In Cape Tenes,
the cliffs almost overhang the abyssal plain. West to the Kair Al Din
block, the numerous canyons strike generally N-S and depict linear
and narrow paths. The crests between the canyons are sharp suggesting evidence for a resistant lithology NW of El Marsa, where
a large deep-sea fan with sediment waves marks the western end of
this part of the margin.
In the Oran zone, there are two distinct geomorphic areas. From
Oran to El Marsa, there is a change of structural direction from E-W
to NE-SW. The shelf appears to deepen continuously from Arzew
(2500 m deep) to north of Mostaganem, where it reaches 800 m
depth, on its edge. Because of the smooth topography of the slope
and the mature drainage pattern, it is suggested that this part of the
margin is made of the same lithological units found nearby on land,
i.e. soft marls and flysch nappes of the Tellian units. West of Oran,
the margin trends generally E-W. Variations in the slope
morphology might indicate the presence of volcanic material, as it
was previously described onshore and recovered in some offshore
outcrops (Leclaire, 1972; Duggen et al., 2004).
2.2. Oceanography and sediment supply
The oceanographic regime in the western Mediterranean upper
water layer is dominated by the input of Modified Atlantic Water
(MAW) which extends from the surface to 100–200 m depth and
spreads over the Levantine Intermediate Water. The main MAW
flow generally leaves the Spanish coast near Almeria and forms the
Algerian Current (AC) that moves eastward along the Algerian
margin. This AC is markedly unstable and commonly gives rise to
clockwise and counter-clockwise coastal eddies, 50–100 km in
diameter and to upwelling (Millot, 1999; Millot et al., 1990). The
sea-surface swell is characterised by two main directions: WNW
and NNE. The NNW ones are the most efficient and mainly (80%) are
acting during the winter.
The highest particulate supply is from the Cheliff River
(0.6 106 t yr1), because the highest suspended matter concentration, followed by the others, Sebaou (0.27 106 t yr1), Mazafran
(0.13 106 t yr1) and Isser (0.1 106 t yr1) rivers (Leclaire, 1972;
Pauc et al., 1997).
3. Sampling and methods
The Maradja (‘‘MARge Active DJAzaı̂r’’) oceanographic campaign
was aimed at identifying active faults and sedimentary instabilities
offshore Algeria, from Oran (0 400 W) to Dellys, w80 km east of
Algiers (Fig. 1). During the cruise (21 August–18 September 2003),
Ifremer R/V Le Suroit obtained continuous seafloor multibeam
imagery, very-high to high-resolution profiling, and sediment
Fig. 1. Study area, bathymetric data were obtained using a Konsgberg EM300 Simrad multibeam echosounder. Boxes show locations of Fig. 2A and 2B.
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
697
Fig. 2. (A). Detailed maps of the Algiers area locating position of analysed sediment cores. (B). Detailed maps of the Chenoua–Tenes area locating position of analysed sediment
cores.
samples using a Kullenberg corer (Domzig et al., 2006; Dan et al.,
2008). In this paper, we provide the first detailed sedimentological
overview of the offshore area.
Seven cores (Table 1) were taken on the deep basin off Algiers
(Fig. 2A) and off the Cheliff River mouth (Fig. 2B). Before opening,
the cores were logged for magnetic susceptibility using an MSCL
(Multi-Sensor Core Logger) based in IFREMER Brest Laboratories.
Samples were collected with an average sampling interval of
1 sample/5 cm, but this interval was reduced to 0.5 or 1 cm
according to colour and texture changes of the units of the turbidite
sequences. Samples were dried in the laboratory at 50 C until
constant weight and water content were achieved. After wetand
sieving,
the
two
coarser
fractions
(d > 315 mm
40 mm < d < 315 mm) were dried at 40 C, then examined with
a binocular microscope.
Sediment gravity-driven inputs were recognized through
concentration of various tracers of relict lowstand deposits originating from the shelf edge: altered or preserved tests of coastal
benthic foraminifers (Elphidium crispum, Ammonia beccarii, Quinqueloculina seminulum), iron-stained Mollusc debris, black debris
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Table 1
Overview over the cores used in this study
Core number
KMDJ01
KMDJ02
KMDJ03
KMDJ04
KMDJ06
KMDJ07
KMDJ08
Latitude N
0
00
37 02 76
36 570 8300
36 560 8800
37 150 6900
36 170 6400
36 320 4200
36 2109700
Longitude E
0
00
03 43 01
03 3104900
03 170 1900
03 420 5300
00 080 1200
00 070 6000
00 020 7700
Water Depth (m)
Core length (cm)
Geologic setting
Mean sed. acc. rate (mm yr1)
2400
1619
2341
2711
2651
2631
2631
785
637
337
729
818
636
764
Base of slope
Bench on the slope
Base of slope
Deep basin
Base of slope
Distal deep-sea fan
Base of slope
0.51
0.49
0.35
0.50
0.64
0.51
0.56
In the particular case of KMDJ01, the 230-cm thick debris flow deposit was not included in the calculation of the mean sediment accumulation rate.
from lagoonal deposit, pieces of calcareous sandstone or slaty
flysch debris. Glauconitic grains, especially cracked and dark-green
in colour, originate from the outer shelf or the upper slope where
the relict sediment remains unburied, implying cationic exchanges
with the sea-water reservoir (Giresse and Odin, 1973; Odin, 1988).
Such glauconitization processes were recently described in the
outer shelf of Gulf of Lions (Giresse et al., 2004). These markers
were previously used in various Mediterranean slope or basin
deposits (Giresse et al., 2001, 2003). It was proceeded to the
counting of these particles, the amount is plotted on a 10-g sand
fraction reference.
The time control is based on AMS 14C analyses (Radiocarbon
Laboratory of Posnan) of selected planktonic foraminifers collected
in the hemipelagic intervals. However, when it was impossible to
collect sufficient sample weight of mono-specific test samples,
depending on the amount of material available, datings were made
on bi-specific or on mixed planctonic foraminifers for some
samples. Calibration into calendar scales was calculated using the
northern hemisphere calibration curve (software Calib 5.0.2)
(Stuiver and Reimer, 1993). Calendar ages are given with 1 standard
deviation (Table 2).
Calcium carbonate content was measured with a Bernard calcimetre. Lastly, in some intervals, the mineral composition was
studied by means of X-ray diffraction using Co Ka1 radiation and
heavy minerals optic determination following heavy liquid separation, but for reasons of volume these analyses are generally not
reported in this study.
4. Lithostratigraphic analysis of the sediment cores
4.1. Eastern area (Algiers)
4.1.1. Core KMDJ01
This 7.85-m long KMDJ01 core was collected at 2400 m water
depth, on the lower continental slope. This site is located seaward
of two submarine valleys in apparent prolongation of Sebaou and
Isser mouths (Fig. 2A). Chirp profile shows a transparent to chaotic
sedimentary body overlying a typical bedded echo-type (Fig. 3).
The transparent unit at the surface may correspond to mass
deposits such as a debris flow deposit, when they are located at the
foot of a slope, as evidenced in the Oran–Tenes area (Domzig et al.,
2009).
A Holocene succession of hemipelagite and turbidite deposits is
overlain by a recent debris flow at 2.30–0.06 m below seafloor
(mbsf). Four radiocarbon ages were obtained: 12,708 cal yr BP at
7.85 mbsf, 5498 cal yr BP at 4.98–4.89 mbsf, 1359 cal yr BP at
3.37–3.77 mbsf (Table 2). The 11,836 cal yr BP age at 2.79–2.69 mbsf
is related to the deposit immediately underlayering the debris flow,
indicating that a part of the hand-picked planktonic foraminifers
are probably old and reworked during the movement of the debris
flow.
Between 7.85 m (base of the sediment core) and 2.30 mbsf, 17
thin-bedded turbidites of mm-scale were observed (Fig. 4a). These
turbidites correspond to peaks of 20–40 wt% fine sand, and at times
of 5 wt% coarse sands. Where the thickness is rather important, one
Table 2
Chronology of the seven cores
14
Core number
Depth core (cm)
Lab. no.
Conventional
Mean
Standard deviation
KMDJ01
269–279
337–347
489–498
785
Poz-9092
Poz-11977
Poz-9094
Poz-9093
10,160 50
1480 30
4750 40
10,790 60
11,836
1359
5498
12,708
111
27
70
58
KMDJ02
180–190
390–400
817–827
Poz-9095
Poz-10991
Poz-9096
2280 35
7330 40
12,860 60
2301
8149
14,983
66
47
111
KMDJ03
333–337
Poz-10992
9690 50
11,134
82
KMDJ04
255–265
715–724
Poz-9101
Poz-9104
9860 50
14,340 70
11,249
16,864
37
143
KMDJ06
201–212
551–559
791–800
Poz-10997
Poz-10998
Poz-10999
10,120 50
12,380 60
12,780 80
11,748
14,232
14,891
124
129
107
KMDJ07
102–103
391–397
586–589
Poz-9097
Poz-11000
Poz-9099
3950 40
12,960 50
12,290 70
4410
15,098
14,094
48
110
110
KMDJ08
141–151
735–740
Poz-9100
Poz-9103
4480 40
13,470 70
5094
15,681
108
134
C age (yr BP)
Calendar age (cal yr BP)
In the first row, italics characters indicate the apparent age of the last bed below the debris flow accumulation. Calibration into calendar scales was calculated using the
northern hemisphere calibration curve (software Calib 5.0.2) (Stuiver and Reimer, 1993). Calendar ages are given with 1 standard deviation and include a 400 years Reservoir
age correction. On the Algerian Margin, there are only four measures radiocarbon reservoir age data. But they are all located in the same zone close to the Bay of Algiers and,
nevertheless, present values scattered enough going from 376 to 516 (Butzin et al., 2005).
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Fig. 3. Examples of Chirp sections at the location of cores KMDJ01, KMDJ02, KMDJ03, KMDJ04, KMDJ06, KMDJ07 and KMDJ08.
699
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Fig. 4. Vertical profiles of sediment cores from the Algiers area: KMDJ01 (a), KMDJ02 (b), KMDJ03 (c), KMDJ04 (d). Basic sedimentological parameters, magnetic susceptibility, are
shown with relative abundance of bio- and geo-indicators of sediment’s provenance. Ages are calendar 14C ages.
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
can note a normal fining-upward, especially at 7.25, 6.50 and
5.25 mbsf. In such occurrences, the basal sand layer (Ta and/or Tb
divisions of Bouma, 1962) is both quartzous and shelly. Where the
carbonate content of these layers reaches or exceeds 30 wt%,
the magnetic susceptibility becomes slightly discriminated from
the background level. The layers are rich enough in coastal benthic
foraminifers (E. crispum, A. beccarii) and in glauconitic grains
reworked from the coastal lowstand systems tract lying on the
outer shelf edge.
In a regular sequence succession, the lowest deposits show
a more or less dark pigmentation associated generally with highly
altered vegetal black fibres or debris probably reworked from the
old coastal deposits lying on the outer shelf (Leclaire, 1972) (6.50,
5.93, 4.70, 4.12, 3.15, and 0.08 mbsf). Then, some dark laminae
(equivalent of Tc division of Bouma) are observed and the
pigmentation becomes clearer and trends to a greyish beige,
characteristic of hemipelagic muds (Te division of Bouma).
Commonly, small and oxidized debris (especially very small worm
burrow hole fillings) were noted toward the top of the sequence
where the sedimentation rate became probably lower. If the
deposition of the next sequence erodes the top of the previous one,
winnowing induces a secondary concentration of this oxidized
debris. As a consequence, such debris can be observed both at the
top (3.40 mbsf) or at the base (6.40 and 3.35 mbsf) of the sequence.
In some cases, planktonic foraminifers of the underlying sequence
are also affected by the reworking process. The frequency and the
thickness of the 17 thin-bedded turbidites are slightly variable;
some of them recur on short intervals. Various incomplete
sequences (TaTb or Ta) suggest the occurrence of common episodes
of erosion. However, the thickest and coarsest sequences are
deposited during transgression (Fairbanks, 1989), i.e. between
w13,000 and w6000 cal yr BP (especially at 7.20, 6.50 and
5.24 mbsf). The 2.30 m of debris flow deposit are well identified by
1–20 wt% of coarse sand and 5–30 wt% of fine sand. This debris flow
contains chaotically arranged, stiff (d w 2.2), blue mud clasts of
various size. They are included in a beige mud matrix that is poorly
compacted (d w 1.80). The mud clasts include shell debris of coastal
molluscs (Venus, Glycimeris, Turritella), 50% of which are stained by
bluish iron sulfides. Some clasts are armoured with mm-scale
subrounded quartz grains and white or ochre shell debris. All these
very rounded gravels are also present in the mud matrix associated
with mud chips, gritty small pebbles, coastal benthic foraminifers
(E. crispum, A. beccarii, Quinqueloculina) and cracked glauconitic
grains. This deposit suggests a proximal high-concentration debris
flow. It includes at once a slightly old blue mud (probably isotopic
stage 3) with various coastal deposits of the last lowstand (stage 2)
as commonly described on the outer edge of the Algerian shelf
(Leclaire, 1972). The down-slope movement was probably initiated
at the contact between fluid deposits and overconsolidated
deposits as it is frequently observed (Shanmugan, 2002).
At the core top, 6-cm thick hemipelagic beige-ochre and fluid
mud that includes trace amounts of sand and abundant vegetal
fibres overlies the debris flow, but did not indicate traces of 210Pb
(Jouanneau, personal communication). Taking into account the
mean Holocene accumulation rate, these 6 cm would be equivalent
to a little more than one century deposition (w128 years). So, it is
suggested that the underlying debris flow would be contemporaneous with the major 1891 earthquake (Ms 7.5, Io X and macroseismic radius of 200 km) felt in the towns of Gouraya, Cherchell,
and Blida. During a period of 25 years before and after the 1891
earthquake, the other earthquakes were weaker and more distant,
especially in the Oran area (CRAAG, 1994).
4.1.2. Core KMDJ02
Sediment core KMDJ02 is 6.37-m long and was sampled at
1619 m water depth on an intermediate bench of the slope where
701
a transparent echo-type overlying a bedded echo-type (Fig. 3).
Thus, this site appears isolated from the main paths of turbidity
currents. Turbidites are effectively scarce and only occur as discrete
fine-grained beds over most of the sedimentary column, decreasing
above 4 mbsf (Fig. 4b). Three radiocarbon ages were obtained and
indicate successively 14,983 cal yr BP at the core base,
8149 cal yr BP at 4–3.90 mbsf, and 2301 cal yr BP at 1.90–1.80 mbsf.
From the base to 4 mbsf (about 8000 cal yr BP), the deposits
include mm-scale laminae rich in organic matter (at 6.10, 5.75 and
4.10 mbsf) that are characterised by fine sand concentrations rising
up to 15–20 wt%. From 4 mbsf to the top, very rare discontinuities
are restricted to a few organic and sandy laminae (7.50, 2.70 and
1.60 mbsf) including trace amounts of fine sand (5–8 wt%) (Fig. 4b).
Carbonate abundance decreases upward more or less linearly
from 20 to 15 wt%. The layers with fine sand are slightly richer in
calcareous bioclasts derived from the shelf edge. The general trend
of the carbonate curve seems controlled by the increasing scarcity
of these bioclasts through the upper 4 m.
There is little evidence for tracers of the erosion of the shelf
edge: E. crispum and A. beccarii are nearly absent despite proximity
of the source. Only, Quinqueloculina is commonly recorded from the
base up to 5.65 mbsf and disappears upward. Glauconitic grains,
especially dark and cracked grains, are the most frequent tracers of
gravity-induced processes, especially below 4 mbsf; the higher
contents are linked to the main peaks of sand attesting reworking
processes; however, these grains are locally found in muddy
intervals.
4.1.3. Core KMDJ03
This short KMDJ03 sediment core was collected at 2341 m water
depth at the foot of the slope, seaward of several submarine valleys
linked to Algiers canyon. The core is only 3.37 m in length, but it
nearly spans the entire Holocene as the basal layer was dated to
11,134 cal yr BP. It roughly corresponds to the 3–4 m transparent
echo-type of the upper part of the Chirp profile (Fig. 3).
The lithologic column consists entirely of grey-beige mud.
However, some very thin turbidites, including 20–40 wt% of fine
sands, are interbedded (Fig. 4c). Twelve thin turbidites are regularly
represented from bottom to top of the sedimentary column. Some
of them are isolated; others form a doublet or a triplet. The only
relatively thick turbiditic sequence is located between 1.70 and
1.60 mbsf consisting of a 1 cm sandy layer overlain by several
laminites. Through this 10-cm thick interval, the sand content
reaches 50 wt% whereas it is less than 20 wt% in the other intervals.
Some dark laminae are organic matter-rich and do not include
significant sand concentrations.
CaCO3 contents, varying between 20 and 25 wt%, are fairly
constant. The highest values are generally associated with sandy
layers, particularly the coarser one at 1.7–1.6 mbsf. However, this
relationship is not confirmed, especially through the upper 160 cm
where several dark laminae show very few calcareous bioclasts.
4.1.4. Core KMDJ04
The 7.29-m long KMDJ04 sediment core was collected in the
deep basin away from the outer break of the slope. With a water
depth of 2711 m, this site is the deepest along this eastern part of
Algerian coast. The Chirp profile indicates a transparent echo-type
scattered in few places, lying on an important bedded sedimentary
body (Fig. 3).
Radiocarbon measurements indicate 16,864 cal yr BP at
7.24–7.15 mbsf and 11,249 cal yr BP at 2.65–2.55 mbsf suggesting, as
at foot of the slope, an accumulation rate higher during the transgressive episode than during the highstand episode.
Turbidites are numerous (35 sequences within the 7.29 m) and
are equally spread through the sedimentary column (Fig. 4d). A
similar distribution was observed in the nearby core MD04-2798
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
that was recently collected during 2004 PRISMA cruise (Dan et al.,
2008).
Three main turbidite sequences were recovered:
From 6.45 to 6.20 mbsf, a 10-cm thick bed of coarse sand with
calcareous bioclasts is buried by a 1-cm thick layer slightly more
silty and then by fining-upward deposits. The basal 10 cm
suggest an ‘‘en masse’’ deposition without detectable graded
bedding (Fig. 6). Because of its abundance of calcareous clasts,
this sequence has low values on the magnetic susceptibility
profile.
From 2.00 to1.60 mbsf, a 20-cm thick coarse deposit is overlain
by a fining-up sequence constituted by a succession of dark and
silty laminites. This slightly siliceous deposit is well marked on
the magnetic susceptibility profile.
From 0.4 to 0.14 mbsf, a relatively thin sequence shows
a marked fining-upward deposition. From the base, the
sequence shows alternating dark and sandy laminae and light
and clayey laminae. The coarse sand intervals are restricted to
the base of the sequence. The abundance of calcareous clasts
results in a moderate negative rise of the magnetic susceptibility
value.
The other numerous sequences within the sediment core
correspond on average to mm-scale layers composed with fine
sand and organic matter, or simply with organic matter as at
0.8 mbsf where very dark laminae are observed. When the interval
between two sandy events is thicker, one can observe, through
40–50 cm, the gradual transition from a dark grey-beige mud to
beige mud and then an increasing lightening with ochre shade at
the top. This complete sequence, similar to those at the foot of the
slope, indicates the decreasing trend of the accumulation rate going
to a marked oxidation of the top. In other cases, the continuity is
broken by the new overlying sequence.
Microscopic observation indicates the abundance of very altered
vegetal fibres and charcoal grains at the base of each coarser
sequence and in the overlying dark layers. Such concentrations are
particularly high in several layers (6.14, 6.12, 5.05, 4.70–4.73, 3.67,
3.63, 1.82, 1.66, 1.60, 1.50, 0.41 and 0.22 mbsf). The pronounced
weathering of this debris suggests reworking of shallow deposits of
the last lowstand lying on the shelf edge.
Coastal benthic foraminifers, other extraneous material from the
shelf edge, are frequently observed. They exhibit higher concentrations in the sandy beds of the two main turbidites (between 6.36
and 6.26 mbsf and between 1.97 and 1.82 mbsf). They appear less
abundant in the last main sequence suggesting another provenance, possibly from the upper slope.
Schistic rock debris appear particularly abundant in the lower
part of the column resulting from a more active wasting process of
the shelf edge outcrops (Leclaire, 1972).
4.2. Western area (Tenes–Chenoua)
4.2.1. Core KMDJ06
This KMDJ06 sediment core is 8.18 m in length and was taken in
2651 m water depth on the floor of the foot slope off Cape Ivi, i.e.,
a little east off the Cheliff Oued mouth. This site lies down slope
from a well incised submarine valley (Fig. 2B). Chirp profile indicates a transparent sedimentary body overlying a very well
expressed bedding (Fig. 3). The question is to know if the core
reached the top of the bedded body.
The sedimentary column appears as uniform homogeneous
grey-beige mud except in the lowermost 2 m. Grey colour
throughout the entire sediment core indicates the abundant iron
sulphides derived from Neogene flysch units outcropping in the
Cheliff Oued catchment basin. This particular pigmentation is
characteristic of the submarine deposits of this area.
Three 14C ages were obtained; 14,891 cal yr BP at 8.00–
7.91 mbsf, 14,232 cal yr BP at 5.59–5.51 mbsf, and 11,748 cal yr BP at
2.12–2.01 mbsf, indicating a marked decrease of the sediment
accumulation rate during the Holocene (Fig. 5a).
A total of 39 individual turbidites were identified within basal
sands commonly making up 10–20 wt%. The highest thicknesses
and emplacement frequencies are located over the lower 2 m of the
sedimentary column. Each individual turbidite sequence is identified by the presence of organic matter and iron sulphide pigments.
Generally, silty and/or sandy bases are dark grey or medium grey.
They are underlain by paler grey muds and then by beige grey muds
as a result of oxidation by bottom water. These chromatic gradients
occur throughout the column studied, although they are evident
especially between 8.17 and 7 mbsf and through the upper 3 m
(Fig. 6). The thickest turbidite is recorded between 6.70 and
6.30 mbsf with a sand content reaching 90 wt%. The lower 27 cm of
the sequence is graded and several dark laminations occur through
the upper few decimetres of the sequence (Td division of Bouma).
Two intervals of low accumulation rates are probably identified
between 5.20 and 3.20 mbsf and in the upper metre that show
moderate or intense bioturbation without evidence of new nutriment input.
Numerous cm-scale pebbles of grey shale are found included in
the sandy units of turbidites. However, some rare cm-scale schistous debris are noted even in the interbedded hemipelagites.
Average carbonate contents are 30 wt% with some peaks up to
40 wt%, which is higher than in the eastern study area. Generally, there
is a positive correlation between sand content of coarse bases of
a turbidite and the CaCO3 wt%. As noted for KMDJ02 and 03, one
observes a slight decrease of CaCO3 in the upper half of the sediment
core. The magnetic susceptibility is approximately in inverse proportion to CaCO3 content of the sandy intervals. But in the upper 3 m, this
susceptibility increases markedly: the peaks underline fairly well the
sandy bases suggesting a more abundant siliceous component.
Most of the sand-rich turbidites are characterised by the presence of dark-green glauconitic grains as the main turbidites identified near 6.50 mbsf. However, some other glauconitic grains are
also observed throughout hemipelagic deposits attesting the
recurrence of gravity-driven supply. The best markers of Ta or Tb of
turbidite are subrounded shell debris stained with sulphides, and
associated to slate debris. This relationship is especially shown in
the 6.50 mbsf turbidite. All these markers decrease markedly above
3.00 mbsf. Generally, E. crispum, A. beccarii as Quinqueloculina are
scattered within the sedimentary column and appear irregularly in
the turbidite sequences, but there is an upward decreasing trend
similar to turbidite distribution.
Degraded vegetal debris show generally higher concentrations
in the base of each turbiditic sequence, but appear to generally
increase in the upper 3 m. In this way, some concentrations of
quartz grains in the same interval would indicate Saharan aeolian
supply as a potential source during the second half of the Holocene,
this period was the scene of the increasing aridity of the Saharan
Desert (Gasse, 2002).
4.2.2. Core KMDJ07
This KMDJ07 sediment core was taken down to the deep-sea fan
of El Marsa in 2631 m water depth and is 6.36 m long. This is the
most distal site within the western study area. Chirp section at the
location of KMDJ07 shows bedded echo-type, however, the echocharacter belongs to the darker echo-type at the deeper bedded
part (Fig. 3). Turbidites were identified all along the sediment core
(38 sequences for 6.36 m), but their distribution frequency as well
as their thicknesses appear to diminish through the upper 2/3 of
the sediment core (Fig. 5b).
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
703
Fig. 5. Vertical profiles of sediment cores from the Chenoua–Tenes area: KMDJ06 (a), KMDJ07 (b), KMDJ08 (c). Basic sedimentological parameters, magnetic susceptibility, are
shown with relative abundance of bio- and geo-indicators of sediment’s provenance. Ages are calendar 14C ages.
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Fig. 6. Examples of truncation of the upper divisions of the turbidite sequences. KMDJ04: sharp contact of the basal sandy division (Ta) above the upper division (Te) of the previous
sequence; KMDJ06: a nearly complete sequence (TaTbTcTe), 1-cm thick coarse basal sand (Ta) prints its mark on the strongly eroded underlying sequence (thin-bedded turbidite);
KMDJ07: a fine-grain sequence (thin-bedded turbidite) is almost completely eroded by the coarse 15-cm bed of the following sequence (TaTb).
Three radiocarbon ages are available: 14,094 cal yr BP at
5.89–5.86 mbsf,15,098 cal yr BP at 3.97–3.91 mbsf, and 4410 cal yr BP
at 1.03–1.02 mbsf. The two nearly similar ages measured between
15,000 and 14,000 cal yr BP would indicate, a high accumulation rate
during the late Pleistocene and a slower accumulation during the
Holocene.
Most of the turbidites cored in the column are very sandy over
a thickness of several centimetres: 28 cm between 6.32 and
6.05 mbsf, 2–3 cm between 4.99 and 4.97 mbsf, 30 cm between
4.32 and 4.14 mbsf, 4 cm between 3.98 and 3.94 mbsf, 4 cm
between 1.88 and 1.84 mbsf (Fig. 5b). The grey-beige muds with
a metallic shade probably induced by iron sulphides show an
ordered chromatic sequence as in KMDJ06: dark grey, medium
grey, pale grey, and beige to ochre. The top of the sequence was
commonly erosionally truncated during the deposition of the
successive sandy base. Normal grading within turbidites is generally difficult to distinguish, except between 4.32 and 4.14 mbsf.
Between the turbidites, the intervals are still sand/silt rich.
There is no linear relationship between sand and CaCO3
contents. The correlation tends toward negative values for the
sand-rich beds including more siliceous or silico-aluminous debris
than shell clasts. Magnetic susceptibility values are somewhat
variable considering irregular CaCO3 contents of the sand-enriched
beds.
Considering the high distribution frequency of this core sediment and the presumed high erosion frequency of the top of the
sequence, the markers of the provenance from relict deposits of the
shelf edge are observed only irregularly. Coastal benthic foraminifers (E. crispum, A. beccarii, Quinqueloculina) are generally associated and characterise several sandy turbidites, but not all of them.
Their abundance decreases markedly in the Holocene deposits
above 2.30 mbsf leaving one to presume another provenance.
Glauconitic grain distribution is slightly erratic, but tends to
decrease in the upper 2 m of the cores. Oxidized debris (pieces of
small burrow fillings) are not systematically included in the upper
boundary of the turbidite bed, because they are commonly
reworked within the overlying unit. So, they can help to the
recognition of the transition between sequences.
4.2.3. Core KMDJ08
The sampling site for KMDJ08 sediment core is located at foot
of the slope, in the easternmost area below several submarine
valleys and a very narrow shelf (<20 km wide). The 7.64-m long
core was collected at 2631 m water depth. A darker bedded echotype was recorded on the Chirp line, with at places, a more
transparent superficial layer (Fig. 3). The sediment core contains
a series of grey-beige muds with irregular sand percentages and
with the characteristic grey of this western area. This sedimentary
column displays 20 turbiditic beds. Sand-based turbidites (Ta-Tb
divisions) are generally thinner than 1–2 cm. Consequently, it can
be difficult to ascertain if there is grading. Coarse-grained-based
turbidites are apparent only in some beds: near 2.10, 3.20, 5.20,
and 7.60 mbsf. Fine-grained turbidites are characterised, once
again, by upward colour grading: dark grey, pale grey with black
organic spots, and oxidized sediments (greyish beige). As in
KMDJ06, the occurrence of sand-based turbidite is significantly
higher through the lowermost 2 m of the sediment core.
Two radiocarbon ages were measured: 15,681 cal yr BP at 7.35–
7.40 mbsf, near the base of the core, and 5094 cal yr BP at 1.41–
1.51 mbsf. As noted earlier, these two ages point a slightly more
active accumulation during the late Pleistocene and decreasing
sedimentation rate during the Holocene. This contrasting frequency
distribution seems dependent on the upward decreasing occurrence of turbidites.
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
There is not a consistent relationship between contents of
carbonate and sand (Fig. 5c), because the coarser beds include
calcareous bioclasts, but also weathered debris of schist and
sandstone. However, several turbidite bases are relatively shell-rich
and are identified as having a proximal source from relict deposits
of the shelf edge (w7.80, 7.71, 6.72, 6.60, 6.46, 5.75 and 4.40 mbsf).
Coastal benthic foraminifers (E. crispum, A. beccarii, Quinqueloculina) follow a consistent relationship and point the sand-rich layer
of each turbidite. Their general distribution through the sediment
core emphasizes the upward decrease of turbidites. The presence of
glauconitic grains is correlated with shallow-water benthic foraminifers only in some beds of the lower part of the sediment core.
Oxidized debris occur in the upper part of the turbidite sequence
and underline the slowing down of the deposition rate; these
debris are sometimes reworked and amalgamated in the first
deposits of the next sequence.
5. Statistic analyses and sedimentological implications
5.1. Sediment accumulation rates, fluxes, and emplacement
frequency of turbidites
To the scale of each sediment core, mean sediment accumulation rates are found relatively homogeneous ranging between 34.6
and 62.6 cm/103 yr. They are slightly higher in the western part of
the studied margin (55.1 cm/103 yr) than in the eastern area near
Algiers (45 cm/103 yr). The rates in the deep basin (KMDJ04 and 07)
are slightly lower (respectively, 45 and 47.9 cm/103 yr) than the
rates recorded nearby the foot of the slope (on average 50.4 cm/
103 yr).
On the graph reported on Fig. 7, the slope of the sediment
accumulation rate specific to each sediment core tends to diminish
during the Holocene, in particular since 6000 cal yr BP, when the
shoreline reached the present position. This slope change is verified
on KMDJ04, 06, 07, and 08 curves, but less distinctly on 01. The only
exception is KMDJ02, located on a mid slope bench out of the main
paths of the turbidity currents.
Taking into account the higher compaction of the deeper buried
deposits of the sediment cores, calculation of sediment fluxes
allows to depict the following chronological trend: fluxes were
particularly accelerated when the shoreline and its zone of surf
Fig. 7. Comparison of accumulation sediment rates of seven studied sediment cores.
Thickness of debris flow interval of KMDJ01 was not taken into account.
705
were still at a short distance from the shelf break, but were roughly
reduced by half of the transgressive value during highstand stage
(Fig. 8). KMDJ02, here too, is an exception. The nearby KMDJ01
sediment core shows only a slightly moderate slowing down during
the highstand. For this core, the paroxysmal acceleration
(w25 mg cm2 yr1) recorded near 12,000 cal yr BP, in the lower
part of the section, coincides with a high recurrence of the turbidite
sequences (1 sequence/108 years).
Table 3 gives the frequency distribution of turbidite beds for
each layer between radiocarbon dates. The mean intervals between
each turbidite sequences are expressed both in centimetre and
extrapolated years BP. It is apparent that in most cases (cores
KMDJ01, 04, 06, 07, 08), hemipelagic intervals are thinner and
shorter during the lowstand stage and the transgression, but tend
to thicken during highstand stage. KMDJ02 shows a growing scarcity of thin-bedded turbidite laminations after w7000 cal yr BP.
Regardless to this distribution of turbidites, which is probably
controlled here by the increasing distance between the shelf edge
and the transgressive shoreline, the emplacement frequency of the
turbidite laminations in both areas is relatively greater in the core
section from the deep basin. This is verified for KMDJ04 in the
eastern area and for KMDJ07 in the western area. This last site
shows the highest emplacement frequency of this margin
(1 turbidite/11 cm between w12,000 and w4000 cal yr BP).
5.2. Frequency distribution of turbidite thickness
In Cenozoic deposits, thick-bedded (>20 cm) turbidites have
been shown to cover much of their basin plain, whereas thinbedded turbidite were significantly less extensive (Ricci Lucchi and
Valmori, 1980; Talling, 2001). However, this model result may be
especially applicable to the history of old basins where turbidite
deposition was mainly controlled tectonically. On the Algerian
margin, it is suggested that thick-bedded turbidites are restricted
along the channels of the canyons, but we have insufficient information. Then the turbidity currents continue widely their course in
the abyssal plain where the sedimentation ends.
Many studies have shown an approximately log-normal
frequency distribution of the sandy component of turbidite thickness (Hiscott and Middleton, 1979; Beeden, 1983; Drummond and
Wilkinson, 1996; Murray et al., 1996; Ishihara et al., 1997). In
addition, a power-law trend has been suggested for the cumulative
frequency curve of sand layer thickness (Rothman et al., 1994.
Beattie and Dade, 1996; Rothman and Grotzinger, 1996;
Malinverno, 1997; Pirmez et al., 1997; Chen and Hiscott, 1999;
Winkler and Gawenda, 1999; Carlson and Grotzinger, 2001). This
power-law distribution suggests that flow initiation is controlled by
continental-slope failure induced by earthquake shaking, because
earthquake magnitude also has a power-law frequency distribution. Commonly, the cumulative frequency distribution of turbidite
thickness follows a segmented power-law relationship that is
equivalent to the summation of log-normal distributions. Each lognormal distribution is associated with a characteristic basal Bouma
Ta, and Tb divisions that are deposited by flows with high sediment
concentrations, whereas Tc, Td, and Te originate from dilute flow
components. Such summation produces a step in the trend of the
probability plot for the entire bed population.
Thickness data of the sandy basal interval and of the entire
turbidite are shown on probability plots with logarithmic axis
(Fig. 9).
KMDJ01. The frequency distribution of the entire turbidite
thickness is typically segmented. It is suggested that the
sharp crossover in the scaling exponent is related to
flow rheology (Talling, 2001). In this case, the three
turbidites with a thick sandy layer are shown. However,
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Fig. 8. Successive fluxes of KMDJ01, 02, 04, 06, 07 and 08 calculated on the basis of
a log-normal frequency is observed for each sandy
interval thickness. Debris flow deposit is not represented because its thickness distribution deviates
systematically from a log-normal distribution.
14
C ages. In KMDJ01, debris flow thickness was not taken into account.
KMDJ03. In this short section (350 cm), a single sandy layer was
observed. The other events are thin-bedded turbidites.
The same log-normal frequency distribution is recorded
for the entire turbidite thickness and for the sandy layers.
Table 3
Mean sediment thickness and extrapolated time between two successive turbidites: location of radiocarbon ages allowing the calculation
The bottom row indicates the average sediment thickness and the average time between turbidity currents for entire core.
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
707
Fig. 9. Probability plots of entire turbidite thickness and sandy basal interval. Segmented lines are related to y ¼ ax þ b calculation; continuous lines show segmented frequency
distributions.
KMDJ04.
KMDJ06.
KMDJ07.
KMDJ08.
This means a rather homogeneous flow rheology and
a good preservation of the deposits (without toptruncation) (Talling, 2001).
The distribution of the entire turbidite thickness is
a typically segmented distribution. The >10 cm and
<10 cm thicknesses are distinguished. The <10 cm can be
produced by the occurrence of thin-bedded rheology or
because the pristine upper part was eroded.
This sediment core shows the contrast between the
dominant thin-bedded turbidite from the upper 6 m and
the significant thick turbidite from 6.50 mbsf core. The
two thickness records show a segmented distribution
caused by a single point, whereas the other turbidites
shown are log-normal frequency distribution.
Turbiditic layers are widespread through the entire
sediment core. The entire turbidite sequence has an
approximately log-normal frequency distribution. A
sharp crossover in the scaling component is related to the
four sequences thicker than 20 cm. The sandy layer
thickness distribution shows a series of distinct lognormal distributions. The three log-normal distributions,
respectively, corresponds to very thin-bedded (0.5–1 cm),
to thin-bedded (2–4 cm), and to thick-bedded (15–30 cm)
units. The two latter divisions are characterised by normal
grading. Thus, the occurrences of three flows with distinct
rheology are recorded, suggesting a good preservation of
each sequence.
Most of the turbidites are located within the deepest
metre of the core in which sand-rich turbidites are
frequent. The entire turbidite thickness distribution is
segmented. Most of the thin-bedded (<25 cm) units are
along the first slope, the second slope corresponds the
three thicker layers (>35 cm) the top of which were
probably truncated. Sandy interval thickness has a log-
normal frequency distribution (except a 0.3-cm thick
layer).
Most of the turbidite-bed thicknesses along the western Algeria
margin are bimodal with mode representing, respectively, thickbedded (>20 cm) and thin-bedded units. However, small-volume
beds are probably underrepresented in the deepest sites of the
basin as probably they do not extend across the entire abyssal plain.
The small-volume beds indicate distal deposit from the channel
and levees sedimentation.
5.3. Relationship between grain-size and bed thickness
The logarithmic plots of bed thickness and basal grain-size show
a strong positive correlation between the two variables (Sadler,
1982). Talling (2001) later showed that basal grain-size influences
the frequency distribution of bed thickness. Each grain-size class
has an approximately log-normal frequency distribution of bed
thickness with a median thickness that increases for larger grainsize classes. However, distinctly different gradients are observed for
TaTb to Te, TcTd to Te and just Te Bouma divisions. These differences
are inferred as indicating a change from viscous settling to inertial
settling as grains became larger. To assess this relationship, the
logarithmic plot of entire turbidite thickness and of sandy-bed
thickness are presented against total sand content (>40 mm)
(Fig. 10).
KMDJ01. There is positive correlation of sand percentage with both
entire turbidite and sandy-bed thickness. However, this
relationship is weak. Such results suggest rather marked
erosion of both the tops of the entire sequence
(TaTbTcTdTe) and of the top of basal interval (Ta).
KMDJ03. Basal division turbidites indicate a good positive correlation that is statistically driven by the thicker turbidite in
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P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Fig. 10. Logarithmic plots of entire turbidite thickness and of sandy-bed thickness presented against total sand content (>40 mm).
KMDJ04.
KMDJ06.
KMDJ07.
KMDJ08.
this sediment core. However, the thickness of the entire
sequence does not exhibit any correlation, indicating
irregular erosion of the top of the sequences through
recurrent turbiditic events.
With a deeper water depth and a remote location from
the slope, turbidite beds occur throughout the entire
section with three main thick sequences that drive the
correlation. Both regression coefficients are positive. The
sandy basal unit regression coefficient (0.7) is higher than
the entire sequence correlation (0.53). We deduce that
the upper beds have undergone more or less severe
truncation (one example is provided in Fig. 6).
Despite the core location at foot of the slope, these units
seem to be well preserved. A mean regression coefficient
is calculated for the entire sequences as for the sandy
bases; the entire sequences (0.53) being a little lower
than the sandy base (0.63).
A large number of turbidites implies a frequent erosion of
the upper division of the sequences and accounts for two
poor positive correlations. In some places, sharp contacts
would indicate these erosions (Fig. 6).
This record does not indicate any significant correlation
between thickness and sand content. Most of the turbidite beds are concentrated within the 7.50–5.75 mbsf
interval. The high number of basal sandy divisions could
be interpreted as a frequent truncation of the top of the
sequences. TcTdTe divisions, and even Tb divisions were
commonly eroded. The result of this is that the two
variables are not related.
This relationship analysis supports the view that thick summit
sequences were commonly truncated at the foot of the slope. These
erosive processes were particularly significant during the shoreline
change of the last transgressive characterised by a high
emplacement frequency of the turbidites. Basal sequence boundaries are commonly sharp and probably erosive (Fig. 6). However,
KMDJ06 site suggests a relatively confined depositional environment preserved from sediment gravity-flow processes.
In contrast, the two deepest sites generally indicate that accumulation processes appear to take precedence over erosion
processes. Consequently, sequences were regularly accumulated
and preserved. In the two areas, this depositional results in a positive regression coefficient between sand content and the two
measured bed thickness.
6. Discussion and conclusion
(1) On the western Algerian margin, sediment accumulation rates
during the last ca. 15,000 years are generally rather high
(35–60 cm/103 yr), even if off-shelf sediment fluxes were lower
during the current highstand (last ca. 6000 years). This lower
flux is common in deep-water basins as, for example, off the
Gulf of Lions, where very low sediment accumulation rates
(5–10 cm/103 yr) were recorded (Dennielou et al., 2003). On
the Algerian margin, the flux calculations indicate that this
decrease is strong, even reduced by half, during the transgressive interval. At the foot of the slope, but also in the deep
basin, this decrease is commonly associated with a lower
emplacement frequency of turbidites. Off-shelf sediment
transport was generally enhanced by the narrowness of the
continental shelf and by the steepness of the slope.
(2) At the foot of the western Algerian slope, most of the turbidites
are fine-grained thin beds (TcTdTe divisions). In each sequence,
on the basis of an upward decrease of organic carbon content
and increase in oxidation, the divisions are approximately
recognized by a colour trend from dark grey to beige grey.
These thin-bedded turbidites illustrate many events with
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
a probable small initial volume and a distal depositional environment (dilute or low-density turbidity currents probably
resulting from the nearby channels). Because of its topographic
location, the KMDJ02 sampling site was largely sheltered from
the gravity-induced flows.
Some thick-bedded turbidites were observed within the
transgressive systems track of the lower part of the sections.
They include TaTcTc divisions, but rarely TaTbTcTdTe divisions,
suggesting repeated truncation processes. In various cases,
TaTb divisions show a fining-upward deposit. All the records at
foot of the slope indicate a decreasing frequency of turbidite
sequence from the transgressive to highstand systems tract. In
most sequences, the marked concentration of shallow-water
foraminifers reworked from the relict lowstand deposit of the
shelf provided information on the origin of the inputs.
(3) In the deep basin off Algeria (KMDJ04, 07), turbidite sequences
are both thicker and more complete than at the foot of the
slope. Ta and Tb divisions are generally well identified by the
markers of the shelf edge (coastal benthic foraminifers, glauconitic grains, and vegetal black debris). The slow accumulation
rate of division Te allows the development of small iron
concretions that were sometimes reworked during the deposition of the next bed. The turbidite beds are continuously
observed through the entire sediment core but this emplacement frequency decreases with rising sea-level. These two
deep-water sites (especially KMDJ04) are assumed to display
a steady accumulation from multiple submarine valleys.
KMDJ07 study suggests that some Holocene turbidite provenance would not be outer shelf but upper slope. Here, it is
suggested that the frequency of the sediment gravity flow is
not controlled entirely by the eustatic changes. These signals
might contain components of a seismically induced sedimentation, because they appear independent from sea-level
position.
(4) The turbidite-bed thickness variation is bimodal with modes
representing thick-bedded and thin-bedded turbidites. The
under-representation of small-volume beds in the two deepest sites of the basin indicates that the low-density currents
do not extend across the entire basin. Consequently, at most
sites, there is a summation of log-normal distribution that
shows a step in the trend of the probability plot. In the deep
basin, the normal log-frequency distribution of the sandy
component is the same power-law distribution as magnitude
of earthquake shaking (Talling, 2001). Consequently, these
deeper sites would reflect the seismicity of the margin
whereas the foot of the slope is more strongly controlled by
the eustatic trend.
There is an approximately log-normal frequency distribution of bed thickness with a median thickness that increase for
larger grain-size classes. Different gradients in the curves are
inferred to represent a change from viscous settling to initial
settling as grain became larger. But the relationship is
commonly altered by the ablation of the top of the turbidite
sequence or even of the basal sandy bed. The higher the
frequency of turbidites, the more the truncations are developed, and the more their relationship would expected to be
poor. At the foot of the slope, the erosions were preferentially
developed during lowstand period or during initial stages of
sea-level rise, probably because of the weak distance separating the high-energy shoreline from the shelf break. In the
two deepest sites, the greater thickness of each sequence is
probably tied to a better preservation.
(5) Only one evident debris flow deposit was observed in this
study, a 230-cm thick accumulation forming the upper part of
the section of KMDJ01. This slurried interval contains chaotically
arranged blue mud clasts and coastal sand and gravel probably
709
eroded from shelf edge. On the basis of a seismic-reflection
survey (Domzig et al., 2009), the lateral extent of this deposit
seems rather restricted. A 6-cm thick veneer overlays this debris
flow implying a deposition interval of w100–150 years.
(6) The occurrence of siliciclastic as well as calcareous grains
though to come from the relict deposits of the shelf break
within hemipelagic deposition intervals would indicate an offshelf sediment transport by density cascading. This process
rather frequent, if not permanent, may develop particularly on
the Algeria margin, especially if the density of water mass
above the shelf is subjected to strong season period imbalance
(Millot et al., 1990). Flushing by storm surge would be another
explanation for such deposition, at most during low sea-level
period.
Acknowledgments
We thank the crew and scientists aboard the R.V. Suroit for their
coring and sampling operation during Maradja 1 cruise to the
Algerian margin and IFREMER Brest Centre laboratory (GM-LES)
technicians for assistance. We wish to thank the two reviewers
Antonio Cattaneo and Bill Normark for linguistic improvements and
very helpful comments. We shall have a very strong thought for this
last one who left us after this work of revision and who looks at us
now of in the top of the sky. Contribution N 1090 of the IUEM,
European Institute for Marine Studies. This project has received
support from the following programmes: GDR Marges ‘‘Instabilités’’, ACI (Action Concertée Incitative) ‘‘Algérie’’ 2003–2006, the
French-Algerian Cooperation Project CMEP-Tassili 2004–2007 No.
041MDU619, and ANR (Agence Nationale de la Recherche) projects
ISIS and DANACOR.
References
Beattie, P.D., Dade, W.B., 1996. Is scaling in turbidity deposition consistent with
forcing by earthquake? J. Sediment. Res. 66, 909–915.
Beeden, R.D., 1983. Sedimentology of Some Turbidites and Related Rocks from the
Cloridorme Group, Ordovician, Quebec. MSc thesis, McMaster University,
Hamilton, Ontario, 254 pp.
Bonnel, C., Dennielou, B., Berné, S., Mulder, T., Droz, L., 2005. Architecture and
depositional pattern of the Rhône Neofan and recent gravity activity in the Gulf
of Lions (Western Mediterranean). Mar. Petrol. Geol. 22, 827–843.
Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits. Elsevier, New York, 168
pp.
Butzin, M., Prange, M., Lohmann, G., 2005. Radiocarbon simulations for the glacial
ocean: the effects of wind stress, Southern Ocean sea ice and Heinrich events.
Earth Planet. Sci. Lett. 235, 45–61.
Carvajal, C.R., Steel, R.J., 2006. Thick turbidite successions from supply-dominated
shelves during sea-level highstand. Geology 34 (8), 665–668.
Carlson, J., Grotzinger, J.P., 2001. Submarine fan environment inferred from turbidite
thickness distributions. Sedimentology 48, 1331–1351.
Chen, C., Hiscott, R.N., 1999. Statistical analysis of facies clustering in submarine-fan
turbidite successions. J. Sediment. Res. 69, 505–517.
CRAAG (Centre de Recherche et Astronomie, Astrophysique et Géophysique), 1994.
Les séismes en Algérie de 1365 à 1992. Département d’Études et Surveillance
sismique, Alger-Bouzareah, Algérie, 227 pp.
Dan, G., Savoye, B., Cattaneo, A., Gaullier, V., Déverchères, J., Yelles, K., MARADJA
2003 team, 2008. Recent Sedimentation Patterns on the Algerian Margin
(Algiers Area, Southwestern Mediterranean). AAPG-SEPM Special, vol. 9.
Den Hartog Jager, D.R., Giles, M.R., Griffiths, G., 1993. Evolution of Paleogene
submarine fans of the North Sea in space and time. In: Parker, J.R. (Ed.),
Petroleum Geology of Northwest Europe: Proceedings of the Fourth Conference.
Geological Society, London, vol. 1, pp. 59–71.
Dennielou, B., Bonnel, C., Sultan, N., Voisset, M., Berné, S., Droz, L., 2003. Sand
transfer into the deep basin of the Gulf of Lions: even during the sea level high
stand.! In: Ocean Margin Research Conference, Paris, p. 129.
Déverchère, J., Yelles, K., Domzig, A., Mercier de Lépinay, B., Bouillin, J.P., Gaullier, V.,
Bracène, R., Calais, E., Savoye, B., Kherroubi, A., Le Roy, P., Pauc, H., Dan, G., 2005.
Active thrust faulting offshore Boumerdes, Algeria, and its relations to the 2003
Mw 6.9 earthquake. Geophys. Res. Lett. 32, L04311.1–L04311.5.
Domzig, A., Yelles, K., Le Roy, C., Déverchère, J., Bouillin, J.P., Bracène, R., Mercier de
Lépinay, B., Le Roy, P., Calais, E., Kherroubi, A., Gaullier, V., Savoye, B., Pauc, H.,
2006. Searching for the Africa-Eurasia Miocene boundary offshore western
Algeria (MARADJA’03 cruise). C. R. Geosci. 338, 80–91.
710
P. Giresse et al. / Marine and Petroleum Geology 26 (2009) 695–710
Domzig, A., Gaullier, V., Giresse, P., Pauc, H., Déverchère, J., 2009. Deposition processes
from echo-character mapping along the western Algerian margin (Oran-Tenes),
Western Mediterranean. Mar. Petrol. Geol. 26 (5), 673–694.
Drummond, C.N., Wilkinson, B.H., 1996. Stratal thickness frequencies and the
prevalence of orderdness in stratigraphic sections. J. Geol. 104, 1–18.
Duggen, K., Hoernle, P., van den Bogaard, C., Harris, C., 2004. Magmatic evolution of
the Alboran region; the role of subduction in forming the western
Mediterranean and causing the Messinian Salinity Crisis. Earth Planet. Sci. Lett.
218, 91–108.
Durand-Delga, M., Fonboté, J.M., 1980. Le cadre structural de la Méditerranée
occidentale. In: Aubouin, J., Debelmas, J., Latreille, M. (Eds.), Géologie des
chaı̂nes alpines issues de la Téthys. Coll. No. 5, 26 Congrès Géologique International, Paris, Mém. B.R.G.M., pp. 67–85.
Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea-level record: influence of
glacial melting rates on the Younger Dryas event and deep-ocean circulation.
Nature 342, 637–642.
Gasse, F., 2002. Diatom-inferred salinity and carbonate oxygen isotopes in Holocene
waterbodies of the western Sahara and Sahel (Africa). Quaternary Sci. Rev.,
737–767.
Giresse, P., Buscail, R., Charrière, B., Abassi, A., Masqué, P., Sanchez-Cabeza, J.-A.,
2001. Biotracers and geotracers of depositional events in NW Mediterranean
margin over the past two centuries. Oceanol. Acta 24 (6), 581–597.
Giresse, P., Buscail, R., Charrière, B., 2003. Late Holocene multi-source material input
into the Aegean Sea: depositional and post-depositional processes. Oceanol.
Acta 26, 657–672.
Giresse, P., Odin, G.S., 1973. Nature minéralogique et origine des glauconies du
plateau continental du Gabon et du Congo. Sedimentology 20, 457–488.
Giresse, P., Wiewióra, A., Grabska, D., 2004. Glauconitization processes in the
northwestern Mediterranean (Gulf of Lions). Clay Miner. 39, 57–73.
Hiscott, R.N., Middleton, G.V., 1979. Depositional mechanics of thick-bedded
sandstones at the base of a submarine slope. Tourelle Formation (Lower
Ordovician); Quebec, Canada. SEPM Spec. Publ. 27, 307–326.
Ishihara, Y., Miiyata, Y., Tokuhashi, S., 1997. Time-series analysis of turbidite
sequence in the upper part of the Awa Group in the Boso Peninsula, central
Japan. J. Geol. Soc. Jpn. 103, 579–589.
Johnson, S.D., Flint, S., Hinds, D., De Ville Wickens, H., 2001. Anatomy, geometry and
sequence stratigraphy of basin floor to slope turbidite systems, Tanqua Karoo,
South Africa. Sedimentology 48, 987–1023.
Leclaire, L., 1972. La sédimentation holocène sur le versant méridional AlgéroBaléares (Précontinent Algérien). Mém. Mus. Natl. Hist. Nat. Paris, C XXIV, 1–392.
Malinverno, A., 1997. On the power law distribution of turbidite bed. Basin Res. 9,
263–274.
Millot, C., 1999. Circulation in the western Mediterranean Sea. J. Mar. Syst. 20, 423–
442.
Millot, C., Taupier-Letage, I., Benzohra, M., 1990. The Algerian eddies. Earth Sci. Rev.
27, 203–219.
Mitchum, R.M., 1985. Seismic stratigraphic expression of submarine fans. In:
Berg, O.R., Woolverton, D.G. (Eds.), Seismic Stratigraphy II: an Integrated
Approach to Hydrocarbon Exploration. AAPG Memoir, vol. 39, pp. 117–138.
Murray, C.J., Lowe, D.R., Graham, S.A., Martinez, P.A., Zeng, J., Caroll, A., Cox, R.,
Hendrix, M., Heubeck, C., Miller, D., Moxon, I.W., Sobel, E., Wendebourg, E.,
Williams, T., 1996. Statistical analysis of bed-thickness patterns in a turbidite
section from the Great Valley Sequence, Cache Creek California. J. Sediment.
Petrol. 66, 900–908.
Mutti, E., 1985. Turbidite systems and their relations to depositional sequences. In:
Zuffa, G.G. (Ed.), Provenance of Arenites. D. Reidel Publishing, Dordrecht, pp.
65–93.
Nakajima, T., Kanai, Y., 2000. Sedimentary features of seismoturbidites triggered by
the 1983 and older historical earthquakes in the eastern margin of the Japan
Sea. Sediment. Geol. 135, 1–19.
Odin, G.S., 1988. Green Marine Clays, Developments in Sedimentology 45. Elsevier,
Amsterdam, 445 pp.
Pauc, H., Benslama, L., Maouche-Berkane, S., 1997. Les apports fluviatiles sur la
marge algérienne, aspects qualitatifs et quantitatifs. Leur rôle dans la pollution
du bassin méditerranéen méridional. In: 8 Rencontres de l’Agence Régionale
pour l’Environnement P.A.C.A., 10–11 Oct. 1996, Coll. ‘‘Les apports fluviatiles en
Méditerranée, leurs charges polluantes, leurs effets sur le milieu.’’, Nice.
Pickering, K.T., Clark, J.D., Smith, R.D.A., Hiscott, R.N., Ricci Lucci, F., Kenyon, N.H.,
1995. Architectural element analysis of turbidite systems, and selected topical
problems for sand-prone deep-water systems. In: Pickering, K.T., Hiscott, R.N.,
Kenyon, N.H., Ricci Lucci, F. (Eds.), Atlas of Deep Water Environments. Chapman
& Hall, London, pp. 1–11.
Piper, D.J.W., Savoye, B., 1993. Process of late quaternary turbidite current flow and
deposition on the Var deep-sea fan, north-western Mediterranean Sea. Sedimentology 40, 557–582.
Pirmez, C., Hiscott, R.N., Kronen Jr., J.D., 1997. Sandy turbidite successions at the base
of channel-levee systems of the Amazon fan revealed by FMS logs and cores
unravelling the facies architecture of large submarine fans. In: Flood, R.D.,
Piper, D.J.W., Klaus, A., Peterson, L.C. (Eds.), Proceedings of the Ocean Drilling
Program. ), vol. 155, pp. 7–33.
Ricci Lucchi, F., Valmori, E., 1980. Basin-wide turbidites in a Miocene, over-supplied
deep-sea plain: a geometric analysis. Sedimentology 27, 241–270.
Rothman, D.H., Grotzinger, J.P., 1996. Scaling properties of gravity-driven sediments.
Nonlinear Process. Geophys. 2, 178–185.
Rothman, D.H., Grotzinger, J.P., Fleming, P., 1994. Scaling in turbidite deposition. J.
Sediment. Petrol. 164, 59–67.
Rothwell, R.G., Hoogakker, B., Thomson, J., Croudace, I.W., Frenz, M., 2006. Turbidite
emplacement on the southern Balearic Abyssal Plain (western Mediterranean
Sea) during Marine Isotope Stages 1–3: an application of ITRAX XRF scanning of
sediment cores to lithostratigraphic analysis. In: Rothwell, R.G. (Ed.), New
Techniques in Sediment Core Analysis. Geological Society, London, Special
Publication, 267, pp. 79–98.
Sadler, P.M.,1982. Bed-thickness and grain-size of turbidites. Sedimentology 5, 37–51.
Shanmugan, G., 2002. Ten turbidite myths. Earth Sci. Rev. 58, 311–341.
Stuiver, M., Reimer, P.J., 1993. Extended 14C data base and CALIB 3.0 14C Age calibration program. Radiocarbon 35 (1), 215–230.
Talling, P.J., 2001. On the frequency distribution of turbidite thickness. Sedimentology 48, 1297–1329.
Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, P.R., Sarg, J.F., Loutit, T.S.,
Hardenbol, J., 1988. An overview of the fundamentals of sequence stratigraphy
and key definitions. In: Wilgue, C.K., Hastings, B.S., Kendall, C.G.St.C.,
Posamentier, H.W., Ross, C.A., Van Wagoner, J.C. (Eds.), Sea-Level Changes – an
Integrated Approach. SEPM Special Publication, vol. 42, pp. 39–45.
Wildi, W.,1983. La chaı̂ne tello-rifaine (Algérie, Maroc, Tunisie); Structure, stratigraphie
et évolution du Trias au Miocène. Rev. Géol. Dyn. Géogr. Phys. 24, 201–297.
Winkler, W., Gawenda, P., 1999. Distinguishing climatic and tectonic bearing on
turbidite bed scaling: Paleocene-Eocene of northern Spain. J. Geol. Soc. Lond.
156, 791–800.