The Congo deep-sea fan is one of the largest fans in the world still affected by presently active turbidity currents. The present activity of deep-sea sedimentary processes is linked to the existence of a direct connection between the Congo River estuary and the Congo canyon head that allows relatively continuous sediment feeding of the deep-sea environment, in spite of a wide continental shelf (150 km). Because of this important activity in terms of sedimentary processes, the deep-sea environment of the Congo-Angola margin presents major interests concerning physical, chemical and biological studies near the sea floor. The main aim of this paper is to present the initial geological context of the BioZaire Program, showing a synthesis of the major results of the ZaïAngo Project including (1) the brief geological setting of the Congo-Angola margin, (2) the structure of the modern Congo deep-sea fan, (3) the sedimentary architecture of the recent Congo turbidite system (from the canyon to the distal lobes), and (4) the recent and present turbidite sedimentation. In order to provide useful information and advice relevant to biological and geochemical studies across the Congo sedimentary system, this article focuses on the present sedimentary processes and the present activity of turbidity current along the Congo canyon and channel.Keywords : Congo-Angola margin, Congo channel, sedimentary deposits, turbidite .1 IntroductionThe accumulation of marine sediment seaward of the Congo River estuary is probably one of the greatest in the world for a submarine system that is still active today. In the continental domain, the Congo sedimentary system is composed by a wide drainage basin, the river and its tributaries. In the nearshore and marine domain: it consists of the estuary, the canyon, which acts mainly as a by-pass zone, and finally an area of preferential sedimentation at the foot of the continental slope and in the deep basin. The Congo submarine system has been geologically studied since Buchanan in 1887 (submarine cable survey) and was the focus of the ZaïAngo project team from 1998 to 2005. Several types of evidence of Holocene and very recent activity in the canyon have been described and linked to sedimentary processes such as turbidity currents, river floods, slides, gas hydrate formation and fluid escape. The aim of this paper is to synthesize the geological knowledge of the area and in particular the results of the ZaïAngo project in order to provide useful information and advice relevant to biological and geochemical studies across the Congo sedimentary system. Marine biologists and oceanographers are usually not familiar with turbidite deposits. These sedimentary deposits were first recognised and described in outcrops of ancient sedimentary series (Bouma, 1962;Walker, 1978;Mutti, 1992). They are generated by submarine gravity processes and they build large sedimentary systems in deep-water environments, termed deep-sea fans. Deep-sea fans are well developed from the foot of the continental slope...

International audienceSinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a "reservoir" scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel mig...

The role of internal and external forcing of sedimentation in turbidite systems remains a subject of debate. Here we propose new insights from the quantitative analysis of architectural parameters of the Congo Axial Fan.Fifty-two channel-levee-lobe systems, spanning the last ca. 200 ka, are visible on the seafloor, most of them having slightly elongated lobe complexes at their termination. Volumes of lobe complexes (usually 3 to 196 km3) are highly variable in time and space. The cumulative volume of the lobe complexes represents approximately 30% of the volume of the Axial Fan.The Axial Fan is sequentially divided into periods of increasing/decreasing channel lengths and basinward/landward migrations of avulsion points, representing successive prograding/retrograding architectural patterns called architectural cycles. These cycles are either symmetrical saw toothed and bell-shaped with progressive progradation and retrogradation phases, or asymmetrical, with long-lasting progradation phases and abrupt retrogradation phases that correspond to channel avulsions occurring high up on the fan.Our study points to the interplay between internal and external factors controlling the architecture of the Congo Axial Fan. The local topographic constraint is a major factor in the fan's stacking pattern. However, cyclic evolution of the architecture reveals major shifts in the deposition site that are linked to very upfan avulsion events. These events are interpreted to be driven by external factors (e.g. climate and/or eustatic sea-level change) that were able to drastically increase and/or coarsen the sediment supply to the fan. Highlights ► Exhaustive architectural evolution of the Congo turbidite system since 200 ka. ► Shifts of channellevee-lobe systems reveal cyclic patterns at different time scales. ► Dimensions of terminal lobe complexes do not exhibit cyclic evolution. ► Pointing to permanent internal control by inherited topography. ► Revealing possible exceptional periods of increased or coarsening Congo River inputs. At a system scale, for instance on the Amazon Fan, the chrono-stratigraphic framework provided by Leg 155 ODP drillings (Flood and Piper, 1997) delivered clues that uncovered a link between sedimentation and architectural evolution to climate and eustasy. Maslin et al. (2006) assumed that channel avulsions in the Amazon Fan could be triggered by pulses of sediment flux, and therefore be externally forced by factors such as sea level and/or climate fluctuations. Lopez (2001) also suggests that sea level variations influence the occurrence of the avulsion process, which is more frequent during periods of sea level lowering. Additionally, at the levee scale it was demonstrated that external forcing mechanisms such as sediment flux pulses or sea level fluctuations control the growth of the levees (Bonneau et al., 2014; Jorry et al., 2011; Toucanne et al., 2012) therefore potentially playing a role in triggering channel avulsions and thus influencing the distribution pattern of the channel-levee-lobe syste...

Terraces have been frequently observed and described along turbidite valleys.Many interpretations have been aimed at determining the origin of these structures, including a tectonic origin, succession of infilling and incision processes, channel-wall slumps, or inner levee aggradation. The Zaire submarine valley presents a complex structure with multiple terraces bordering a deep incised meandering thalweg. The detailed analysis of the morphology, the seismic structure and the recent sedimentation (in cores) along the Zaire upper-fan valley show that terraces are inner levees confined within the incised valley. Many terraces correspond to the infilling of abandoned meanders, and aggrade by deposition of turbidite sequences due to current overflows. The major process affecting the initiation and the development of terraces inside the valley is the vertical incision of the thalweg, simultaneously with meander migration. 1997), or in terms of slump or slide as for the Kaoping canyon (Liu et al. 1993).

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