Marine Geology 209 (2004) 223 – 244
www.elsevier.com/locate/margeo
Reconnaissance study of the ancient Zaire (Congo)
deep-sea fan (ZaiAngo Project)
Zahie Anka, Michel Séranne *
Laboratoire Dynamique de la Lithosphère, CNRS/Université Montpellier II, UMR 5573, Case Courier 060, 34095 Montpellier, France
Received 21 November 2002; received in revised form 28 May 2004; accepted 11 June 2004
Abstract
Analysis of more than 19,000 km of multichannel seismic reflection data from the ZaiAngo Project, covering the Zaire
deep-sea fan, allowed us to identify the oceanic crust and five seismostratigraphic units on the basin floor and abyssal
plain. In the absence of a direct stratigraphic tie, the time frame is provided by long distance correlations. A stratigraphic
model for the evolution of the Zaire deep-sea fan is proposed. A widespread major regional unconformity, representing
probably the Eocene – Oligocene transition, marks a drastic change of sedimentation pattern in the basin floor: a pelagic
aggradational sequence (Albian – Eocene) overlying the oceanic crust gives way to an onlapping prograding sequence of
turbidite deposits (Oligocene – Recent). This change marks the onset of the ancient Zaire deep-sea fan, triggered by the
climatic change related to the greenhouse – icehouse shift at the Eocene – Oligocene transition, which was responsible for a
drastic increase in continental erosion and terrigenous sedimentary supply to the margin. The volume of the fan is at least
0.7 Mkm3, much broader and thicker than formerly assumed. A short-lived episode of rapid facies progradation/
retrogradation is recorded sometimes during the Miocene; triggering factors for this event are likely to have a tectonic and/
or climatic origin.
D 2004 Elsevier B.V. All rights reserved.
Keywords: West Africa Margin; Congo; Angola basin; Zaire deep-sea fan; stratigraphy and climate change
1. Introduction
The Zaire deep-sea fan, situated off the mouth of
the second largest river drainage basin in the world
(3.7 106 km2 compared to the Amazon’s 5.9 106 km2)
represents the most important depocentre in the
Angola oceanic basin (eastern South Atlantic) and is
one of the largest deep-sea fan systems in the world,
* Corresponding author. Fax: +33-467-523908.
E-mail addresses: anka@dstu.univ-montp2.fr (Z. Anka),
seranne@dstu.univ-montp2.fr (M. Séranne).
0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.margeo.2004.06.007
with a surface equivalent to the Amazon’s (approximately 300,000 km2) and a volume of at least 0.7
Mkm3. It is located offshore of the Congo– Angola
continental passive margin, between the Guinea
Ridge and the Walvis Ridge, and the system is fed
by the Zaire canyon (Fig. 1a). The canyon deep-sea
fan system is the only one currently active in the
Atlantic and thus provides a natural laboratory for the
study of turbidite systems on mature continental
margins (Droz et al., 1996) in relation to the erosion
and transport processes on the continental drainage
basin. Because erosion on the continent is under the
influence of tectonics and climate, analysis of the
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stratigraphic record of deep-sea fans should reveal
the evolution of both parameters on the adjacent
continent (e.g., Clift et al., 2001). Unfortunately, such
studies are still at an early stage due to the extensive
size of the fans and their poor accessibility for direct
study. Some studies favour high stratigraphic resolution controlled by the Ocean Drilling Program (ODP)
borehole data but restricted to latest Neogene (e.g.,
Fig. 1. (a) Extension of the present-day Zaire deep-sea fan within the Angola oceanic basin. The fan is currently being fed by the Zaire
submarine canyon, which cuts through the shelf and is connected onshore to the Zaire River. Sea-floor and land topography from GTOPO30
digital elevation model. (b) Location of the study area and 2D seismic grid acquired by the ZaiAngo project and analyzed in this work.
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
225
Fig. 1 (continued).
Amazon Fan, Lopez, 2001), while we intend to
reconstruct the long-term evolution of the Zaire
deep-sea fan based on extensive seismic reflection
data but in the absence of well data.
Although ODP-related studies on passive margins
have showed the volumetric importance of the deepbasin floor and abyssal plain stratigraphic records
(Carter et al., 2000; Exon et al., 2002), these distal
domains in the Angola basin have generally been
disregarded when dealing with the reconstruction of
the Congo– Angola continental margin. Except for the
studies carried out in light of the International Decade
of Ocean Exploration program (IDOE; Emery et al.,
1975) and some following regional ones (i.e., Emery
and Uchupi, 1984; Musgrove and Austin, 1984;
Uchupi, 1992), current studies along the Congo –
Angola margin are concentrated on the shelf and slope
(i.e., Karner and Driscoll, 1999; Marton et al., 2000;
Lavier et al., 2001; Valle et al., 2001). Although the
stratigraphy of these proximal provinces undoubtedly
records much of the margin evolution, we believe that
in the case of the Zaire, this record is not all-inclusive.
The Zaire canyon presently deeply incises the shelf
and slope so that terrigenous sediments bypass these
provinces and are delivered directly onto the basin
floor and even in the abyssal plain. The stratigraphic
record of the deep-sea fan is therefore complementary
of that of the adjacent continental margin and constitutes a crucial part of the system for geological
reconstructions.
The ZaiAngo Project was developed by the
IFREMER and TOTAL to study the development
of the Zaire fan system. It included two multichannel
seismic surveys in 1998 (Savoye et al., 2000). Much
of the research, currently carried out in this project,
consists of analyzing the Quaternary morphology
and evolution of the Zaire Fan – Canyon system
(i.e., Babonneau et al., 2002; Droz et al., 2003).
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Fig. 2. Generalized and simplified stratigraphic column for the West Africa shelf and upper slope (compiled from Séranne et al. (1992);
Anderson et al. (2000); Mougamba, 1998).
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However, the time for the system’s initial onset and
the dimensions of the ancient deep-sea fan, as well
as its long-term evolution, are still unclear. In order
to address these questions regarding the proto-Zaire
fan system and better understand the postrift evolution of the western African continental passive margin, we have analysed more than 19,000 km of
seismic profiles from the ZaiAngo data set (Fig.
1b). Our work has allowed us to study the stratigraphic sequences present in the most distal and
deepest parts of the basin.
In this contribution, we present a seismostratigraphic framework for the Zaire deep-sea fan and
discuss the possible correlation with the continental
shelf/slope. We finally discuss the long-term evolution
of the Zaire deep-sea fan in relation with the global
climate change and the geodynamic evolution of the
margin and the adjacent continental drainage basin.
2. Geological setting
The mature continental passive margin of Western
Africa results from the early Cretaceous continental
breakup between South America and Africa, with
further drifting of the two plates from Aptian time
(Brice et al., 1982). The continental margin comprises
(i) a rifted continental crust overlaid by a wedge of
synrift to postrift sediments, and (ii) the base of the
slope, characterized by the ‘‘Angola Escarpment’’,
beyond which, the deep sea fan extends over the
oceanic crust (Fig. 1a).
2.1. Shelf and upper slope
The stratigraphic succession is constrained by
borehole data only on the shelf and upper slope. A
generalized stratigraphic column for these domains is
shown in Fig. 2. Following the Neocomian rifting of
the continental environment, restricted marine conditions set up in the basin and evaporitic sediments
accumulated during late Aptian times (Emery et al.,
1975; Teisserenc and Villemin, 1989). These thick
evaporitic packages later intruded the overlying sequence as salt diapirs, whereas the salt layer acted as a
detachment level responsible for the thin-skinned
extension and consequent down-dip compression that
affect much of the postrift sequences (Duval et al.,
227
1992; Spathopoulos, 1996; Cramez and Jackson,
2000). During the Albian, a shallow carbonate shelf
developed and, as the sea floor is spreading and
thermal subsidence went on, relative sea level rose
leading to the deposition of mudstones that indicate
the establishment of deep open marine conditions
during late Cretaceous (Brice et al., 1982; Uchupi,
1992).
The lower Tertiary succession is remarkably thinner than the Neogene’s. Well data suggest that this
small thickness represents a condensed section (Valle
et al., 2001), and therefore a period of basin starvation was established in this part of the African
continental margin during this period (Anderson et
al., 2000). However, stratigraphically equivalent
sediments could be located in the distal cone.
During the late Eocene, postrift sediments were
disrupted by a major erosional event (Séranne et
al., 1992; McGinnis et al., 1993). A submarine
erosional surface was developed on the shelf and
the upper slope in association with a hiatus of
varying duration (up to 15 My off southern Gabon;
Teisserenc and Villemin, 1989). The origin of this
unconformity is still an object of controversy.
According to Lavier et al. (2000), it results from
submarine erosion in intermediate water depths of
500– 1500 m triggered by changes in the oceanographic conditions.
In the early Oligocene, an important change in the
sedimentation pattern in the margin occurred, which
switched drastically from aggrading platform carbonates to prograding terrigenous siliciclastics (Séranne
et al., 1992). Increased terrigenous supply to the
margin is reflected by the development of a prograding deltaic system, probably related to an ancient
Zaire River. This stratigraphic switch has been previously attributed to epeirogenic motions and the
uplift of southern Africa that enhanced onshore
erosion and increased Zaire River sediment input
(Bond, 1978; Walgenwitz et al., 1990; Walgenwitz et
al., 1992). More recent works point to a global
climate-driven process, such as global cooling in
the Oligocene and Miocene (Séranne, 1999; Lavier
et al., 2001; Anka, 2004). The increase in sediment
input to the margin enhanced salt-related tectonics
including salt withdrawal, diapirism, and growth
faulting in the shelf and slope during late Oligocene – Miocene, as well as thicker and more frequent
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turbidite and debris flow deposits (Lundin, 1992;
Spathopoulos, 1996; Anderson et al., 2000; Cramez
and Jackson, 2000; Marton et al., 2000; Valle et al.,
2001).
In the early Neogene, another important erosional
phase is registered in the West African margin.
Apatite fission track chronothermometry on core
samples from wells in the West African margin
indicates that cooling to values close to present-day
temperature occurred around 22 Ma (Walgenwitz et
al., 1992). This cooling event is interpreted as a result
of denudation, associated with Miocene uplift and
seaward tilting of the margin (Brice et al., 1982;
Lunde et al., 1992; Valle et al., 2001). These events
might be related to the opening of the East African
and Suez – Red Sea rifts, with a possible southwestern
extension across southern Africa and the Walvis
Ridge (Sahagian, 1988).
2.2. Basin floor and abyssal plain
The Angola escarpment marks the seaward limit
of the evaporite basin responsible for the intense salt
tectonics (Cramez and Jackson, 2000) and the landward boundary of the Zaire deep-sea fan. Unlike the
margin, the geological evolution of the Zaire deepsea fan is very poorly constrained. Turbidite deposition in the basin floor may be as old as the Oligocene
(Brice et al., 1982; Uchupi, 1992). Recent studies
reveal that the present-day deep-sea Zaire fan extends
basinward for more than 800 km from the base of
slope, reaching the Angola abyssal plain at depths
between 5200 and 5600 m (Droz et al., 2003). A lack
of borehole data: the limited access to industrial
commercial data, and the necessity of very distant
correlations have not allowed a comprehensive understanding of the Zaire deep-sea fan evolution
(Uenzelmann-Neben et al., 1997; UenzelmannNeben, 1998).
2.3. Model of the turbidite system
One of the main objectives of the ZaiAngo Project
was to evaluate the present (Pleistocene – Holocene)
turbidite system (Savoye et al., 2000; Babonneau et
al., 2002; Droz et al., 2003). Preliminary results of
ongoing studies based on the analyses of lateral
multibeam imaging, seismic reflection, and piston
cores indicate a particular spatial distribution of sedimentary facies within the turbidite system. Although
the model is continually being refined by studies in
progress, the basic features of the sedimentary facies
distributed across the deep-sea fan are summarized in
Fig. 3. We have used the seismic signature of this
Recent sedimentary facies association as a guide for
our interpretation of the older intervals.
Fig. 3. Block diagram showing an idealized model for the spatial distribution of facies in the Plio-Quaternary turbidite system within the Zaire
deep-sea fan inspired from Droz et al. (2003) and Turakiewicz and Lopez (2003).
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
3. Data set
The data were acquired during the first two
ZaiAngo cruises in 1998 and correspond to two
different configurations of seismic acquisition.
ZaiAngo’s
seismic
data set
Speed of
acquisition
(knots)
Source
Streamer
Sampling
(Hz)
Zai profiles
Z2 profiles
9
5
Air gun 2 GI
Air gun 6 GI
6 Channel
96 Channel
500
1000
Although intended to image near-bottom sedimentary structures in order to identify the extent of the
active turbidite system, the 6-channel seismic allowed
an initial reconnaissance of the Zaire deep-sea fan.
Penetration down to 2 s (twt) permitted the identification of deep seismic reflections down to the oceanic
basement, especially beneath the distal deep-sea fan.
The 96-channel high-resolution seismic offers an
improved image as well as a better penetration of
the thick sedimentary wedge. Due to energy dissipation within the channel/levee complexes characterizing the deep-sea fan, the seismic signal becomes
attenuated, although it remains coherent and preserves
the geometrical features. Together, the two surveys
exceed 19,000 km of profile, they are interwoven
resulting into a spacing of circa 15 km, and they cover
almost 200,000 km2 from the Zaire canyon to the
abyssal plain (Fig. 1b).
229
ces a discontinuous high-amplitude reflector termed
AB (Fig. 4). Due to the absence of magnetic anomalies in the study area, the age of the oceanic crust in
this region is not quite established; however, it is
assumed to be either Aptian (based on the proximity
to Chron M0 118.7 My; Nürnberg and Müller, 1991)
or even older: Barremian to Aptian (Marton et al.,
2000). In the westernmost parts of the seismic grid,
the oceanic crust is found at depth of approximately
8 s (twt), but it deepens eastwards due to the load of
the overlying sedimentary cover.
4.2. Unit C1
The oldest sedimentary unit immediately overlies
the oceanic crust (Fig. 4). It displays a fairly constant
thickness of 0.1 s (twt) throughout the distal part of the
deep-sea fan, draping the oceanic basement. C1 reflectors can be observed locally onlapping oceanic crust
topographic highs. Internally, this unit is nearly transparent but presents a few continuous, parallel and very
low-amplitude reflectors. The top of the unit is marked
by a continuous and moderate-amplitude reflector
(SB) that can be traced through most of the basin.
These acoustic characteristics imply that the first sediments deposited in this part of the Angola basin
resulted from normal pelagic and hemipelagic sedimentation. Therefore, we interpret this seismic facies
as representative of homogenous deep-marine pelagic/
hemipelagic deposits (clays and oozes; Sangree and
Widmier, 1979).
4. Definition of seismostratigraphic units
4.3. Unit C2
Because there was no detailed stratigraphic interpretation available for the ultradeep offshore, we have
established a seismostratigraphic chart for the deepsea fan. Analysis of the seismic data set in the distal
deep-sea fan has allowed us to identify the oceanic
basement and five overlying seismostratigraphic units
(Fig. 4).
This unit overlies Unit C1 and the basal reflectors
onlap the relatively high-amplitude reflector SB
(Fig. 4). In the distal deep-sea fan, this unit presents a
thickness of approximately 0.5 s (twt); east – west
profiles show that the thickness increases landwards.
Internally, it consists of packages of semitransparent to
low-amplitude reflectors alternating with occasional
subparallel or slightly wavy, semicontinuous, and
moderate-amplitude reflectors. This amplitude variation suggests pulses of different material. The top of the
unit is marked by an abrupt increase in reflection
amplitude (Fig. 4). However, a distinctive top reflector
could not be found due to the highly discontinuous
nature of reflections.
4.1. Acoustic basement
The acoustic basement in the deep basin represents
the boundary between the oceanic crust and the
sedimentary cover. This boundary is marked by a
strong contrast in acoustic impedance, which produ-
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Fig. 4. Internal reflections and geologic interpretation of the seismostratigraphic units identified in the distal part of the deep-sea fan (see location in Fig. 1b).
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
Fig. 5. External morphology of high-amplitude reflectors from Unit C3 showing a distinctive channel – levee structure. Onlapping reflectors against the levees and onlapping infill are
common (see location in Fig. 1b).
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The distal onlapping of previous topography and
the interbedding of high-amplitude reflections indicate
a drastic change in the depositional system; sedimentation in this unit was (at least in part) probably
controlled by gravitational processes. We suggest that
turbidity currents carried terrigenous material derived
from the continent or the shelf to form interbeds
within the ambient hemipelagic sedimentation. This
unit could thus represent the onset of turbidite deposits related to the ancient Zaire fan. The internal
geometry resembles that of the sedimentary facies
found in the distal, lobe-dominant part of the present-day deep-sea fan (Fig. 3), which in the modern
Zaire system is characterized by a mixture of coarser
material from terminal lobes and pelagic/hemipelagic
sediments.
base of C4 (Fig. 4). The base of this unit is not marked
by a distinctive reflector but rather by a change in
reflection pattern and amplitude from underlying Unit
C2. This unit appears as a thin, high-amplitude
interval of 0.10– 0.15 s (twt) thick, overlain by onlapping reflectors. Internal reflections depict the typical
pattern of channel– levee structures, whose size is
similar to the channel – levee complexes found in the
present-day upper deep-sea fan (Fig. 5).
We interpret the higher amplitude C3 unit as the
result of a temporally limited interval characterised by
important lithological contrast, consistent with the
development of channel –levee complexes. C3 interval
characterised by proximal channel – levee overlies
more distal fan sediments of Unit C2, which reflects
an important basinward progradation of the fan system.
4.4. Unit C3
4.5. Unit C4
In contrast to Unit C2, reflection amplitude for
Unit C3 is predominantly high. It is embedded between two low-amplitude bands: the Unit C2 and the
This unit presents highly diversified reflection
patterns. However, it shows an overall upward increase in seismic amplitude, which is not solely the
Fig. 6. Seismic Unit C4 showing a general upward increase in amplitude/frequency of internal reflections while continuity decreases (see
location in Fig. 1b). Architectural elements of Unit C4. C4a: intercalation of high-amplitude, mounded reflectors and packages of lowamplitude, mainly parallel reflections. C4b: packages of discontinuous, chaotic, semitransparent, reflectors.
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
result of the downward dissipation of seismic energy.
It can be divided into two subunits: C4a and C4b
(Fig. 6). The lower subunit C4a has a seismic
signature similar to Unit C2. It consists of few
high-amplitude, mounded reflectors interbedded with
packages (0.125 – 0.2 s twt) of low-amplitude to
semitransparent, parallel reflections. The upper subunit C4b presents packages of highly discontinuous
and chaotic reflections delimited by, and alternated
with, continuous and high-amplitude reflectors. Both
subunits show occasional reflector packages including angular relationships characterising the channel –
levee system.
We interpret the high-amplitude, mounded reflectors of C4a as distal lobes interbedded with hemipelagic deposits. The higher amplitude of subunit C4b
suggests an increase in the ratio of turbidite/hemipelagic deposits. The alternation of chaotic and continuous facies is similar to debris flows and sand lobes
found in the coalescing terminal lobes from the
present-day deep-sea fan (Savoye et al., 2000; Droz
et al., 2003).
4.6. Unit C5
Unit C5 is the youngest interval whose top is the
ocean floor (Fig. 4). In the distal deep-sea fan, it
appears as a low amplitude interval with continuous
subparallel reflectors. It also includes channel – levee
systems from abandoned channels, as well as the
Zaire active channel. Detailed study of the presentday Zaire deep-sea fan system is beyond the scope of
this work, and it is being developed in other ongoing
projects (i.e., Turakiewicz and Lopez, 2003).
233
oceanic basement is not observed. This implies a
landward thickening of C1.
Because it overlies the oceanic crust, whose age is
assumed as Aptian in this area (Nürnberg and Müller,
1991), the base of sequence C1 can be as old as
Albian. The Angola escarpment corresponds to (i) a
deformed area where reflectors are disrupted and (ii) a
physiographic boundary, across which sedimentary
facies (and therefore seismic signature) drastically
change. Consequently, it is not possible to directly
correlate sequence C1 in the deep-sea fan with dated
formations on the margin.
Very few industrial commercial seismic sections
extending west of the Angola escarpment have been
published (i.e., Cramez and Jackson, 2000) that could
be compared with the seismic signature of Unit C1.
Regional seismic reflection data in the southern
Angola Basin (Musgrove and Austin, 1984; Uchupi,
1992) point to the existence of high-amplitude, continuous, parallel reflectors that are in the same stratigraphic position and have a similar seismic signature
as the top of our Unit C1. Based exclusively on these
similarities, we could preliminarily correlate C1 with
Unit 2 and Unit 1 identified by Musgrove and Austin
(1984) in seismic profiles about 250 km to the
southeast of our area. On the other hand, the Deep
Sea Drilling Project (DSDP) Leg 75 Site 530 near the
Walvis Ridge, at 50 km to the northwest of Musgrove
and Austin’s seismic profile, revealed that (a) the lowamplitude discontinuous reflectors of Unit 1 correspond to Albian to Campanian pelagic clay and
nannofossils ooze; (b) Unit 2 corresponds to Late
Campanian to Eocene marlstone and mudstone (Hay
et al., 1982).
5.2. Reflector SB
5. Temporal and spatial correlation of
seismostratigraphic units
5.1. Unit C1
From the western boundary of the data set, Unit C1
and the top of the oceanic crust deepen landwards. C1
can be identified in the middle deep-sea fan, between
7.8 and 8 s (twt) by the presence of continuous,
parallel reflectors beneath the onlapping basal reflectors of Unit C2 (Fig. 7). Close to the Angola escarpment, C1 unit is thicker than 0.5 s (twt), because the
Reflector SB is the major unconformity in the
area. It can be traced eastwards where the onlap
surface progressively gives way to a conformable
surface. This suggests that in the outer part of the
deep-sea fan, in the abyssal plain, SB corresponds to
a condensed surface onto which progrades a sedimentary wedge. SB is therefore a diachronic marker.
In the central part of the deep-sea fan, reduce seismic
penetration disallows imaging SB, but it can be
identified to the north and south of the deep-sea
fan (Fig. 8). Reflector SB deepens towards the
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Fig. 7. Alternating low- and high-amplitude internal reflections in seismic Unit C2. Note how the unit thickness increases landwards as the oceanic crust and basal reflector SB
deepens in this direction as well (see location in Fig. 1b).
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Fig. 8. Isochron map in seconds (twt) displaying the bathymetry of basal reflector SB. Depth increases towards the central area of the present-day deep-sea fan.
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central area of the fan, where it reaches a depth of at
least 8.4 s (twt).
Following the correlation with DSDP Leg 75 Site
530, SB should correspond to the Eocene –Oligocene
transition. However, in the area of the deep-sea fan,
SB’s amplitude, continuity, and depth correlate with
the regional ‘‘Horizon A’’ of Uchupi, 1992, which is
interpreted by this author as a condensed, onlapping
surface of middle Eocene – middle Oligocene age.
Moreover, industrial seismic data from the southeast
of the fan on the salt basin (courtesy of Western
Geophysical) allows correlation with DSDP Leg 40
Site 364, and, in spite of the uncertainties due to the
long-distance correlation, it seems that SB may correlate with a major erosion surface of middle – late
Eocene to middle Oligocene age (Bolli et al., 1978). It
thus appears that the condensed surface identified in
the deep-sea fan (SB in the abyssal plain) is the
correlative equivalent to a major regional unconformity in the slope and shelf, where it is expressed by a
large-amplitude submarine erosion (Séranne et al.,
1992; McGinnis et al., 1993).
The isopach map (in twt) for the interval SB-sea
floor (Fig. 9) depicts a clear fan-shaped morphology,
and the depocentre is located around the cone apex.
Because thickness decreases almost evenly to the
west, north, and south of the Zaire canyon, it is
evident that the main sediment conduit for this interval is the Zaire canyon, and the sediment source is
located in the continent. This also proves that the
Zaire deep-sea fan started to deposit right after SB,
that is, in Oligocene times. Fig. 9 further indicates that
the smaller coastal river (Kouilou) located north of the
Zaire has not significantly contributed to the depocentre, in contrast to the suggestion previously pointed out by Droz et al. (1996).
5.3. Unit C2
Given that the base of Unit C2 is the marker SB,
the former has a diachronic base possibly as old as
Oligocene in the east and younger westward. The
available data do not permit a better chronological
resolution. At the western end of the ZaiAngo data,
Unit C2 onlaps SB over more than 50 km, and it
thickens rapidly eastwards, suggesting that after deposition of C1, a drastic increase in flexural subsidence occurred centred on the apex of the deep-sea
fan. A simple extrapolation of C2 boundaries indicates that Unit C2 extends some 100 km west of the
surveyed zone (about 900 km off the present coastline).
5.4. Unit C3
Unit C3 corresponds to a short interval of channel/
moundlike levees (Fig. 5). It is well exposed in the
distal and middle deep-sea fan but cannot be distinguished further east due to the resolution of the
seismic records, and to the fact that landwards, the
seismic character of C2 and C4 becomes similar to C3
because the more proximal channel –levees complex
are dominants in this direction (Fig. 10a). Therefore,
C3 represents the arrival of channelized sediment
delivery system onto the abyssal plain, and thus, it
marks a sudden basinward shift of more proximal
facies.
Unit C3 may be correlated via its depth and seismic
amplitude with ‘‘Horizon A’’ of the original work of
Emery et al. (1975). It must be pointed out that in later
works, ‘‘Horizon A’’ in the deep-sea fan area is placed
much deeper than its original interpretation (Uchupi,
1992), where it correlates with our SB marker and the
Eocene– Oligocene transition. It results that Unit C3 is
younger than Oligocene and may represent an event
comprised in the Miocene. Objective data are missing
to improve this chronology. On the basis of the
relative thicknesses of sequences below and above
C3, it could be speculated that the later belongs to the
early-middle Miocene (Fig. 10b).
5.5. Unit C4
In the distal deep-sea fan, Unit C4 is rather uniform
as far as thickness (about 0.7 s twt) and seismic facies
are concerned. Although individual reflections are
discontinuous, their envelopes are parallel. Basinwards, the boundaries of C4 clearly show that the
unit extends well beyond the ZaiAngo zone (about
200 to 300 km to the west), indicating a much broader
ancient deep-sea fan than initially assumed (Fig. 10a).
This result agrees with the regional seismic data of
Emery et al. (1975), which suggested the presence of
a deep-sea fan some 700 to 800 km seaward of the
Angola Escarpment. Towards the middle deep-sea
fan, the lower boundary of C4 deepens beneath the
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
Fig. 9. Isopach map (in twt) for the interval reflector SB-present-day sea floor (bold lines are data-controlled). Data interpolation shows that the depocentre is located around the cone
apex and suggests a fan-shaped morphology.
237
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Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
Fig. 10. (a) Line drawing from a regional east – west seismic correlation showing spatial distribution of distal seismic facies and their temporal
relation with the base of the Pliocene reflector identified at the base of the Angola Escarpment. Note that reflector continuity gradually decreases
eastwards as seismic character of Units C2 and C4 becomes similar to channel – levees of Unit C3 (see location in Fig. 1b). (b) Proposed model
for the chronostratigraphic evolution of facies within the Zaire deep-sea fan. The onset of the turbidite system would have taken place in early
Oligocene, and it is marked by a drastic switch in sedimentation pattern. The Oligocene – Present period is characterized by a basinward
continuous facies progradation. A sudden short-time progradation/retrogradation event is represented by Unit C3. Although this event is poorly
age-constrained, it may be placed in the early – middle Miocene.
sea floor, and the distinction between C4a and C4b
becomes less obvious (Fig. 10a).
The age of unit C4 is very poorly constrained in the
abyssal plain. In the slope, there exists a remarkable
reflector which has tentatively dated as the base of the
Oligocene (Uenzelmann-Neben et al., 1997; Uenzelmann-Neben, 1998). However, correlation with borehole data in the shelf of southern Gabon indicates an
age earliest Pliocene or latest Miocene (Nzé Abeigne,
1997). This reflector, whose seismic expression
changes across the Angola Escarpment, can be extended basinward, where it corresponds to the upper
part of C4 (Fig. 10b). As data from boreholes in the
slope will be eventually available, the age of C4 will
be estimated with more precision.
5.6. Unit C5
This unit displays important seismic facies changes
across the deep-sea fan. In the western end of the
survey, it consists of high-amplitude, continuous
reflectors interpreted as distal lobes (Savoye et al.,
2000). There is a N –S partition of Unit C5 across the
fan, with thick blanket of hemipelagic drape in the
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
north, a thinner hemipelagic drape in the south, and a
central part displaying a number of paleochannels as
well as the active Zaire channel. This is interpreted as
resulting from a three-stage migration of the recent
Zaire system (Droz et al., 2003). Unit C5 is Pleistocene
in age through correlation with the ODP site 1075,
located on the slope, northeast of the fan (Uenzelmann-Neben et al., 1997; Shipboard-Scientific-Party,
1998).
6. Evolution of the paleo-Zaire deep-sea fan
We have compared our proposed stratigraphic
model for the Zaire deep-sea fan with the evolution
of variables, such as the y18O measured in benthic and
planktonic foraminifera (Miller et al., 1987), the
87
Sr/86Sr ratios (Elderfield, 1986), and the sea-level
profile (Haq et al., 1987; Fig. 11). y18O and Sr isotope
ratios can be used as proxies for ocean temperature/ice
volume and continental erosion, respectively. Because
oxygen isotopes are proxy of ice volume, and by
association of sea level, they provide more detail than
the sea-level curve of Haq et al. (1987).
6.1. C1: Cretaceous – Eocene basin floor/abyssal
plain
The first postrift sedimentary record in the distal
basin consists of a pelagic drape overlying the
oceanic crust. However, the landward thickening of
Unit C1 suggests that part of the sedimentation was
originated on the continental margin. Assuming a
velocity of 2500 to 3000 m/s for the interval
(velocity values courtesy of TOTAL), we find about
250 –300 m of sediment in the distal part of the fan,
equivalent to about 3.5 m/My. While in the upper
deep-sea fan, C1 minimum thickness is in the range
of 1500 m corresponding to an average accumulation
rate of some 20 m/My. This shows an important
increase in sedimentation rates landwards and hence
an increased terrigenous flux in that direction. Therefore, to the east, Unit C1 may represent turbidite
deposits similarly to the contemporaneous record of
occasional turbidites off the Walvis Ridge (Musgrove and Austin, 1984), while its chronological
distal equivalents are mainly pelagic/hemipelagic
deposits.
239
6.2. C2: Oligocene onset of the Zaire deep-sea fan
Reflection patterns in the abyssal plain changed
significantly across the SB marker, from primarily
parallel drape to onlapping reflectors which point to
a dramatic change in sedimentation style over the
basin. It could be argued that the onlapping results
from sediment remobilisation by bottom flow currents,
because the latter may have been initiated in the
Oligocene (Séranne and Nzé Abeigne, 1999). Nevertheless, the westward direction of the onlapping basal
reflectors and the eastward thickening of C2 suggest
that in the distal basin, this unit rather represents
gravity deposits in association with the background
hemipelagic sedimentation. The source of these gravity sediments would be located to the east and corresponds to a paleo-Zaire River. Timing of this event
coincides with the global climate change observed at
the Eocene – Oligocene transition. This climate change
represents the first occurrence of a major ice-cap in
Antarctica, which led to an estimated worldwide sealevel drop of 30 –90 m and marks the transition from
greenhouse to icehouse conditions (Fig. 11; Miller et
al., 1987, 1991; Zachos et al., 1992).
The turbidite material could be derived from the
continental margin through shelf exposure and erosion
of incised valleys due to successive sea-level drops.
However, (a) the depocentre is clearly located off and
centred on the Zaire River system, (b) the ramp profile
of the margin until Eocene (Séranne et al., 1992;
Lavier et al., 2000, 2001) implied spatially restricted
shelf exposure and erosion and thus limited volumes
of reworked sediments. By contrast, we propose that
the onset of turbidite accumulation in the distal part of
the African margin, on the oceanic basement, requires
the establishment of a large-scale river system with a
localised outlet, i.e., a paleo-Zaire.
The shift to icehouse conditions considerably enhanced seasonality and thus continental erosion and
weathering (Séranne, 1999) that provided terrigenous
material to the margin. The paleo-Zaire collected the
material in the already probably wide drainage basin,
transported it, and delivered it to the continental
margin. The process would also have been facilitated
by further sea-level fluctuations, as indicated by
oxygen isotope curves and by uplift on the continent.
However, timing of Tertiary uplifts associated to (a)
the development of the west flank of the East African
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Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
Fig. 11. Comparison between the proposed chronostratigraphic evolution of the Zaire deep-sea fan and global climate as indicated by the variation in y18O (bold line; Miller et al.,
1987). 87 Sr/86Sr (dotted line; Elderfield, 1986) and sea level (dashed line; Haq et al., 1987). Note that the onset of the turbidite system correlates with the major increase in y18O
values during the transition from greenhouse to icehouse conditions at the Eocene/Oligocene transition. Oligocene to Present basinward progradation of the fan is consistent with the
long-term sea-level fall during the Neogene and the continuous cooling of the global climate and the increasing efficiency of continental erosion (y18O and 87Sr/86Sr increase,
respectively). The episode of sudden progradation represented by Unit C3 does not correlate with any special changes in the global proxies, which suggests a possible local tectonic
origin.
Z. Anka, M. Séranne / Marine Geology 209 (2004) 223–244
Rift and (b) the general uplift of the African continent
are both recorded during the Miocene (Frostick and
Reid, 1989; Lavier et al., 2001; Leturmy et al., 2003).
That is, they took place after the onset of the deep-sea
fan at the Oligocene. Hence, the early Oligocene
global climatic change could be the primary triggering
factor for the onset of the Zaire ancient deep-sea fan.
6.3. C3: intra-Miocene progradation/retrogradation
event
Following the proposed Oligocene establishment
of the Zaire deep-sea fan, a major break occurred
during the Neogene. Unit C3 reflects a major sudden
basinward progradation of more proximal facies
(channel/levees dominant) over the distal lobes and
hemipelagic sediments (Fig. 11). The channel – levee
facies progradation occurred across the whole surveyed area, suggesting (a) basinward shift of the
whole depositional system, including presence of
distal lobes several hundred of kilometres to the west,
and/or (b) a drastic change in the depositional system,
characterised by prevalence of channels and avulsions. Given the quality of the available data set, we
can only speculate. Nevertheless, this event characterises the onset of a channelized sediment delivery to
the most distal part of the abyssal plain. Moreover, the
abrupt switch back to distal lobe-dominant and pelagic-draped facies of overlaying subunit C4a suggests
that the C3’s driving mechanism was transient. In this
respect, it strongly differs from the Oligocene longterm progradation of the deep-sea fan.
The driving mechanism for this event must account
for the sudden and reversible character of the record;
it must also induce a short-lived facies progradation in
the deep sea fan. The missing key parameter is the
precise age of this intra-Miocene event.
bThe deposition of Unit C3 differs from the stratigraphic switch corresponding to the long-term climatic
change of the Eocene –Oligocene transition. Neither
y18O (proxy of ocean temperature) nor Sr isotopic ratio
(proxy of continental crust-derived material) shows a
distinct excursion in the early Miocene, which could
account for the C3 event (Fig. 11). Conversely, if C3 is
middle Miocene, it could be related to Miller’s events
Mi3 (circa 13.6 Ma) or Mi4 (circa 12.6 Ma), which
represent substantial ice volume increases and glacioeustatic lowering of about 45 –70 m (Miller et al.,
241
1991; Zachos et al., 2001). On the other hand, Haq et
al.’s sea-level profile display at least four large-amplitude sea-level drops during the Miocene, making it
difficult to relate them to the unique C3 event.
Major isotope shift and fall in sea level associated
with the Mi events could have triggered the sudden
facies progradation represented by Unit C3, they
cannot explain the following abrupt facies retrogradation. It seems therefore unlikely that the progradation –
retrogradation of the C3 unit was driven by global
process.
bComparison with the Angola margin record might
improve the understanding of deposition of C3. The
margin registered one large-amplitude uplift – erosion
event during the early Miocene, distinct from the
major Oligocene unconformity. Biostratigraphy constrains this erosion to the Burdigalian (21.8 – 16.6 Ma;
Seyve, 1990). Accordingly, thermochronological studies (Walgenwitz et al., 1990, 1992) concluded that the
coastal margin underwent uplift and erosion during the
early Miocene, and up to 1500 m of the onshore Congo
and Gabon margin has been eroded during this period.
Lavier et al. (2001) computed 2D flexural backstripping of profiles across the Congo and Angola shelf/
upper slope and showed an increase of uplift rate in the
interval 15 to 20 Ma. It can be argued that uplift of the
coastal margin produced erosion and increased sediment flux to the deep basin and deep-sea fan. Such an
uplift – erosion event is associated with canyon cutting
in the shelf area allowing the transit of terrigenous and
reworked sediments, which eventually may be facilitated by earlier Mi isotopic events.
bSudden progradation events in the deep-sea fan
could also be controlled by the tectonic evolution of
the upslope margin. The extensional salt tectonics in
the shelf/upper slope is balanced by contractional
tectonics around the ocean– continent transition (Spathopoulos, 1996; Marton et al., 2000). The latter is
responsible for the development of the Angola escarpment, which dams part of the deposits in interdiapir slope basins. Reactivation of the salt
decollement may have induced opening or canyon
incision of such structural barrier and allowed the
reworking of the sediments in the deep-sea fan.
The last two hypotheses may have interacted; the
early Miocene uplift of the coastal margin must have
increased the basinward tilting of the margin (Walgenwitz et al., 1992; Nzé Abeigne, 1997) and remo-
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bilised the salt decollement. Better seismic data covering the shelf and slope should allow testing any of
these hypotheses in the near future.
6.4. C4/C5: Late Miocene –Holocene continue progradation of the fan
Deposition of the Unit C4 corresponds to the
continued, long-term progradation of the deep-sea
fan, punctuated by short-term events. Basinward progradation of the fan is consistent with the long-term
sea-level fall during the Neogene, which results in
decreased accommodation on the shelf (Fig. 11). It is
also in agreement with continuous cooling of the
global climate, increasing efficiency of continental
erosion (y18O and 87Sr/86Sr increase, respectively),
and with the onset of the Zaire canyon (Babonneau et
al., 2002; Anka, 2004).
7. Conclusions
Analysis of the ZaiAngo seismic data set has
allowed us to propose a stratigraphic model for the
basin floor and abyssal plain, which accounts for the
long-term evolution of the Zaire deep-sea fan. The
main episodes in the history of the deep-sea fan are
well recorded further off the slope, therefore unravelling the stratigraphy of distal provinces is crucial
for understanding the margin evolution. In the case of
the Congo – Angola margin, due to the presence of
the Zaire canyon, the basin floor/abyssal plain and
shelf/slope stratigraphic records complement one another. Consequently, the widespread practice of estimating rates of past erosion based solely upon the
rates of sedimentation on the shelf may introduce
miscalculations.
We have identified five seismostratigraphic units,
which document the depositional history of the basin
from Cretaceous to Recent. Oldest Unit C1 (Albian –
Eocene) was deposited prior to the establishment of
the Zaire deep-sea fan, whereas overlying Unit C2
reflects the onset of the ancient Zaire deep-sea fan
probably during the Oligocene. The widespread diachronous reflector SB represents a condensed surface
that correlates with a major regional unconformity in
the slope and shelf sequences. We propose that this
event resulted from the global climatic change related
to the transition from greenhouse to icehouse conditions at the Eocene –Oligocene transition, which in
turn produced the drastic change in the sedimentation
pattern between Units C1 (aggrading hemipelagic
drape) and C2 (prograding clastic wedge). A similar
stratigraphic switch has been reported in the New
Jersey continental margin (Steckler et al., 1993).
A drastic facies progradation episode is found to
occur in the early to mid-Miocene. The cause of this
event is likely to be tectonic. Further studies should be
carried out in order to determine more precisely the
age of this event.
Our work also demonstrates that the ancient Zaire
deep-sea fan was much broader and thicker than
formerly estimated. The volume of the deep-sea fan
(Oligocene –Recent) is at least 0.7 Mkm3. The present-day Zaire River bed load is unable to account for
the volume deposited in the past, which suggests that
the present-day turbidite system is not fully representative of the fossil system. This raises the question
over the origin of the terrigenous sediments in the
past. Further studies should address this problem as
well as the effects of the sedimentary load of the deepsea fan on the flexural subsidence of the margin.
Acknowledgements
We thank the crew of the N/O Atalante. Seismic
data were acquired and processed thanks to the skills
of the GENAVIR and IFREMER staff. We are
indebted to TOTAL and IFREMER for providing
the ZaiAngo Project’s seismic data, support, and
allowing publication of this work. The manuscript
benefited from discussions with Dr. M. Lopez and
reviews by Drs. L. Carter and G. Uenzelmann-Neben.
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