Reconnaissance study of the ancient Zaire (Congo) deep-sea fan … · 2007-02-15 · submarine...

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Marine Geology 209 (2004) 223–244

Reconnaissance study of the ancient Zaire (Congo)

deep-sea fan (ZaiAngo Project)

Zahie Anka, Michel Seranne*

Laboratoire Dynamique de la Lithosphere, CNRS/Universite 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 with a surface equivalent to the Amazon’s (approxi-

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,

0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.margeo.2004.06.007

* Corresponding author. Fax: +33-467-523908.

E-mail addresses: [email protected] (Z. Anka),

[email protected] (M. Seranne).

mately 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

Z. Anka, M. Seranne / Marine Geology 209 (2004) 223–244224

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

Fig. 1. (a) Extension of the present-day Zaire deep-sea fan within the A

submarine canyon, which cuts through the shelf and is connected onshore

digital elevation model. (b) Location of the study area and 2D seismic gr

size of the fans and their poor accessibility for direct

study. Some studies favour high stratigraphic resolu-

tion controlled by the Ocean Drilling Program (ODP)

borehole data but restricted to latest Neogene (e.g.,

ngola oceanic basin. The fan is currently being fed by the Zaire

to the Zaire River. Sea-floor and land topography from GTOPO30

id acquired by the ZaiAngo project and analyzed in this work.

Fig. 1 (continued).

Z. Anka, M. Seranne / Marine Geology 209 (2004) 223–244 225

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 deep-

basin 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 con-

stitutes 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).

Fig. 2. Generalized and simplified stratigraphic column for the West Africa shelf and upper slope (compiled from Seranne et al. (1992);

Anderson et al. (2000); Mougamba, 1998).



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 evolu-

tion of the western African continental passive mar-

gin, 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 strati-

graphic sequences present in the most distal and

deepest parts of the basin.

In this contribution, we present a seismostrati-

graphic 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 con-

ditions 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 se-

quence 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.,

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 thin-

ner 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 starva-

tion 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 (Seranne 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 oceano-

graphic conditions.

In the early Oligocene, an important change in the

sedimentation pattern in the margin occurred, which

switched drastically from aggrading platform carbo-

nates to prograding terrigenous siliciclastics (Seranne

et al., 1992). Increased terrigenous supply to the

margin is reflected by the development of a prograd-

ing deltaic system, probably related to an ancient

Zaire River. This stratigraphic switch has been pre-

viously 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 (Seranne, 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 Oligo-

cene–Miocene, as well as thicker and more frequent


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 land-

ward boundary of the Zaire deep-sea fan. Unlike the

margin, the geological evolution of the Zaire deep-

sea fan is very poorly constrained. Turbidite deposi-

tion in the basin floor may be as old as the Oligocene

Fig. 3. Block diagram showing an idealized model for the spatial distributi

deep-sea fan inspired from Droz et al. (2003) and Turakiewicz and Lopez

(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 un-

derstanding of the Zaire deep-sea fan evolution

(Uenzelmann-Neben et al., 1997; Uenzelmann-

Neben, 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 sed-

imentary 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.

on of facies in the Plio-Quaternary turbidite system within the Zaire

(2003).


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 9 Air gun 2 GI 6 Channel 500

Z2 profiles 5 Air gun 6 GI 96 Channel 1000

Although intended to image near-bottom sedimen-

tary 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 identifi-

cation 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 dissipa-

tion within the channel/levee complexes characteriz-

ing 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).

4. Definition of seismostratigraphic units

Because there was no detailed stratigraphic inter-

pretation available for the ultradeep offshore, we have

established a seismostratigraphic chart for the deep-

sea 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).

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-

ces a discontinuous high-amplitude reflector termed

AB (Fig. 4). Due to the absence of magnetic anoma-

lies 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; Nurnberg and Muller, 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 reflec-

tors can be observed locally onlapping oceanic crust

topographic highs. Internally, this unit is nearly trans-

parent 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 sedi-

ments deposited in this part of the Angola basin

resulted from normal pelagic and hemipelagic sedi-

mentation. Therefore, we interpret this seismic facies

as representative of homogenous deep-marine pelagic/

hemipelagic deposits (clays and oozes; Sangree and

Widmier, 1979).

4.3. Unit C2

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 varia-

tion 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.

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).

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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; sedimen-

tation 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 depos-

its related to the ancient Zaire fan. The internal

geometry resembles that of the sedimentary facies

found in the distal, lobe-dominant part of the pres-

ent-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.

4.4. Unit C3

In contrast to Unit C2, reflection amplitude for

Unit C3 is predominantly high. It is embedded be-

tween two low-amplitude bands: the Unit C2 and the

Fig. 6. Seismic Unit C4 showing a general upward increase in amplitud

location in Fig. 1b). Architectural elements of Unit C4. C4a: intercala

amplitude, mainly parallel reflections. C4b: packages of discontinuous, ch

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 onlap-

ping 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.

4.5. Unit C4

This unit presents highly diversified reflection

patterns. However, it shows an overall upward in-

crease in seismic amplitude, which is not solely the

e/frequency of internal reflections while continuity decreases (see

tion of high-amplitude, mounded reflectors and packages of low-

aotic, semitransparent, reflectors.


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 sub-

unit 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 includ-

ing angular relationships characterising the channel–

levee system.

We interpret the high-amplitude, mounded reflec-

tors of C4a as distal lobes interbedded with hemi-

pelagic deposits. The higher amplitude of subunit C4b

suggests an increase in the ratio of turbidite/hemi-

pelagic deposits. The alternation of chaotic and con-

tinuous 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 present-

day 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).

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 reflec-

tors of Unit C2 (Fig. 7). Close to the Angola escarp-

ment, C1 unit is thicker than 0.5 s (twt), because the

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 (Nurnberg and Muller,

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, con-

tinuous, parallel reflectors that are in the same strati-

graphic 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 low-

amplitude discontinuous reflectors of Unit 1 corre-

spond 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

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 sedi-

mentary 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

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 cor-

relate 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 unconfor-

mity in the slope and shelf, where it is expressed by a

large-amplitude submarine erosion (Seranne 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 inter-

val 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 depo-

centre, in contrast to the suggestion previously point-

ed 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 de-

position of C1, a drastic increase in flexural subsi-

dence occurred centred on the apex of the deep-sea

fan. A simple extrapolation of C2 boundaries indi-

cates that Unit C2 extends some 100 km west of the

surveyed zone (about 900 km off the present coast-

line).

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 distin-

guished 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. Basin-

wards, 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

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.

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eGeology209(2004)223–244

237

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; Uenzel-

mann-Neben, 1998). However, correlation with bore-

hole data in the shelf of southern Gabon indicates an

age earliest Pliocene or latest Miocene (Nze Abeigne,

1997). This reflector, whose seismic expression

changes across the Angola Escarpment, can be ex-

tended 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. Seranne / Marine Ge

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 (Uenzel-

mann-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), the87Sr/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. There-

fore, to the east, Unit C1 may represent turbidite

deposits similarly to the contemporaneous record of

occasional turbidites off the Walvis Ridge (Mus-

grove and Austin, 1984), while its chronological

distal equivalents are mainly pelagic/hemipelagic

deposits.

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 (Seranne and Nze Abeigne, 1999). Never-

theless, 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 grav-

ity sediments would be located to the east and corre-

sponds 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 sea-

level 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 (Seranne 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 en-

hanced seasonality and thus continental erosion and

weathering (Seranne, 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

ology 209 (2004) 223–244 239

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 y18Ovalues 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.

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eGeology209(2004)223–244

240


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 sur-

veyed 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 avul-

sions. Given the quality of the available data set, we

can only speculate. Nevertheless, this event character-

ises 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 pelag-

ic-draped facies of overlaying subunit C4a suggests

that the C3’s driving mechanism was transient. In this

respect, it strongly differs from the Oligocene long-

term 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 strati-

graphic 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 gla-

cioeustatic lowering of about 45–70 m (Miller et al.,

1991; Zachos et al., 2001). On the other hand, Haq et

al.’s sea-level profile display at least four large-ampli-

tude 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 retrograda-

tion. 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 con-

strains this erosion to the Burdigalian (21.8–16.6 Ma;

Seyve, 1990). Accordingly, thermochronological stud-

ies (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 backstrip-

ping 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 sedi-

ment 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 facili-

tated 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 (Spa-

thopoulos, 1996; Marton et al., 2000). The latter is

responsible for the development of the Angola es-

carpment, which dams part of the deposits in inter-

diapir 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 (Wal-

genwitz et al., 1992; Nze Abeigne, 1997) and remo-


bilised the salt decollement. Better seismic data cov-

ering the shelf and slope should allow testing any of

these hypotheses in the near future.

6.4. C4/C5: Late Miocene–Holocene continue pro-

gradation 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 pro-

gradation 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 unrav-

elling 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 an-

other. Consequently, the widespread practice of esti-

mating 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 dia-

chronous 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 con-

ditions 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 pres-

ent-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 represen-

tative 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 deep-

sea 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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Reconnaissance study of the ancient Zaire (Congo) deep-sea fan … · 2007-02-15 · submarine...

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