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...

No abstract

Analysis of new high-resolution seismic-reflection profiles, chirp profiles and previously published sidescan data, together with piston cores on the Danube Fan provide new insight into the recent sedimentation processes in the deep northwestern Black Sea. The latest channel-levee system on the Danube Fan developed probably during the Neoeuxinian lowstand (oxygene isotope stage 2) in a semi-freshwater basin with a water level about 100 m lower than today. Sediment supplied by the Danube was transported to the deep basin through the Viteaz Canyon, which was directly connected to the leveed channel of this system on the middle slope. Channel avulsion was common in the middle fan, as indicated by four main phases of bifurcation. Each phase developed after the same pattern: breaching of the lower and narrower left levee by turbidity currents, building of a unit of High Amplitude Reflection Packets (HARP) by the unchannelized flow while the former channel was abandoned, followed by initiation of a new meandering leveed channel. The northward migration through successive bifurcations is influenced by the asymmetry between levees, hence by the Coriolis effect. In the lower fan where the levees became too low to maintain a stable pathway for the turbiditic flows, channel migration occurred. Locations of HARPs and channels after bifurcation are controlled by the pre-existing bathymetry. Sedimentary deposits are confined between the high levees of unit 0 (the initial phase of the youngest channel-levee system) to the south, and the steep relief of the Dniepr Fan to the north. The HARPs of the most recent phase of avulsion are the most severely constrained by local topography and form a very narrow elongate structure that is at most half as thick as the previous HARPs. Their distal part is not covered by channel-levee systems and is visible both on sidescan mosaics and on chirp profiles and was sampled in core BLKS 98-20. Sea level controlled fan activity but the evolution of the last channel-levee system with several bifurcations during a single sea level lowstand suggests that the primary control of channel avulsion and sand delivery is probably autocyclic. The presence of important HARP sand bodies in the mud-rich Danube Fan is presumed by analogy with a similar seismic facies on the Amazon Fan and indicated by the sands cored in BLKS98-20. However, only drilling of the HARP units could verify this interpretation.