ALBERT

Description: 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.

Description: Multichannel seismic data newly acquired during two ZaïAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood. Laurence Droz is a researcher at the Centre National de la Recherche Scientifique, currently based at the University of Brest. She received her Ph.D. in 1983 and the Habilitation à Diriger les Recherches in 1991, both from the University of Paris 6. She is a specialist in turbidite systems on passive margins and in seismic stratigraphy.Tania Marsset is a researcher at Ifremer (Institut Français de Recherche et d'Exploitation de la Mer, Brest, France). She received her Ph.D. in geology in 1988 (University of Paris 6) and a Master's degree in computer sciences in 1990 (Institut d'Informatique Industrielle of Brest). She is a specialist in seismic stratigraphy of sedimentary bodies (shelf, slope, and deep-sea environments). Hélène Ondréas is a geophysicist at Ifremer, currently working on oceanic spreading processes and margin cold seeps. She received her DEA from the University of Brest, France in 1992. She has in-depth experience of in-situ observation of midoceanic ridges via submersibles and Remotely Operated Vehicle “ Victor 6000”. Michel Lopez is an associate professor at the University of Montpellier 2. He received his Ph.D. in mixed siliciclastic-carbonate platforms in 1992. Since 1994, he has focused on passive margin deep-sea fans and outcrop analogs. He spent four years (1995–1999) with Total-Fina-Elf developing turbidite understanding and reservoir prediction in deep-sea fans. Bruno Savoye is a researcher in marine sedimentology at Ifremer in Brest. He graduated as a geology engineer in 1982 (ENSG, Nancy) and obtained a geophysics engineering diploma in 1983 (IFP). He specializes in submarine avalanches and deep turbidite systems. He is currently the head of the ZaïAngo project and head of the sedimentary environments laboratory at Ifremer. Frances-Lucie Spy-Anderson received a PhD in geology from the University of Lyon, France. She joined Elf Aquitaine Exploration Production in 1982 and is currently a senior exploration geologist in Total-Fina-Elf New Business Department in Paris.