Reconnaissance study of the ancient Zaire (Congo) deep-sea fan (ZaiAngo Project)

Transcription

1 Marine Geology 209 (2004) 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, 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 Mkm 3, 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 ( km 2 compared to the Amazon s km 2 ) 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: addresses: anka@dstu.univ-montp2.fr (Z. Anka), seranne@dstu.univ-montp2.fr (M. Séranne). with a surface equivalent to the Amazon s (approximately 300,000 km 2 ) and a volume of at least 0.7 Mkm 3. 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 /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.margeo

2 224 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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.

3 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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).

4 226 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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).

5 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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) 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., 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 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

6 228 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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) 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; Uenzelmann- Neben, 1998) 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).

7 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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) 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 km 2 from the Zaire canyon to the abyssal plain (Fig. 1b). 4. Definition of seismostratigraphic units 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) Acoustic basement Source Streamer Sampling (Hz) Zai profiles 9 Air gun 2 GI 6 Channel 500 Z2 profiles 5 Air gun 6 GI 96 Channel 1000 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 produces 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 M 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 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) 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 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.

8 230 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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).

9 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). Z. Anka, M. Séranne / Marine Geology 209 (2004)

10 232 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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 Unit C3 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 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 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 Unit C4 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.

11 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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 ( 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) 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). 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 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) 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 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

12 234 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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).

13 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. Z. Anka, M. Séranne / Marine Geology 209 (2004)

14 236 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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) 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) 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) 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

15 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. Z. Anka, M. Séranne / Marine Geology 209 (2004)

16 238 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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 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

17 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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 y 18 O measured in benthic and planktonic foraminifera (Miller et al., 1987), the 87 Sr/ 86 Sr ratios (Elderfield, 1986), and the sea-level profile (Haq et al., 1987; Fig. 11). y 18 O 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) 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 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 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 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

18 240 Z. Anka, M. Séranne / Marine Geology 209 (2004) Fig. 11. Comparison between the proposed chronostratigraphic evolution of the Zaire deep-sea fan and global climate as indicated by the variation in y 18 O (bold line; Miller et al., 1987). 87 Sr/ 86 Sr (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 y 18 O 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 (y 18 O and 87 Sr/ 86 Sr 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.

19 Z. Anka, M. Séranne / Marine Geology 209 (2004) 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 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 y 18 O (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 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-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 ( 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-

20 242 Z. Anka, M. Séranne / Marine Geology 209 (2004) bilised the salt decollement. Better seismic data covering the shelf and slope should allow testing any of these hypotheses in the near future 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 (y 18 O and 87 Sr/ 86 Sr 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 Mkm 3. 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. References Anderson, J.E., Cartwright, J., Drysdall, S.J., Vivian, N., Controls on turbidite sand deposition during gravity-driven extension of a passive margin: examples from Miocene sediments in Block 4, Angola. Mar. Pet. Geol. 17, Anka, Z., Evolution de l éventail sous-marin du Zaire (Congo) depuis le Crétacé. Interactions avec la marge continentale du Golfe de Guinée. PhD Thesis, Université Montpellier II, Montpellier, France, 205 pp. Babonneau, N., Savoye, B., Cremer, M., Klein, B., Morphol-

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