The role of deformation processes on the geometry of mud-dominated turbiditic systems, Oligocene and Lower?Middle Miocene of the Lower Congo basin (West African Margin)

Abstract

The variation of sediment supply through time is assumed to be the most critical parameter for the sequential evolution of deep-sea fans. It includes sea-level fluctuations (eustatic or tectonically induced), and the rate and source of sediment supply. The deformation is most of the time only described as critical for gravity flow sediment confinement. Although the effect of the deformation on a single flow is well established, only few studies investigate the influence of a deforming seafloor on turbiditic sedimentation at larger scale of space (10–100 km in length) and time (0.5–10 My). The main objective of this study is to document the influence of seafloor deformation on the morphology of gravity flow deposits, both at regional (×100 km) and local (1–10 km) scales and for 0.5–10 My periods, taking into account the tectono-stratigraphic evolution of the basin. Using seismic (2D and 3D) data calibrated on well data, the parameters controlling the morphology of two major turbiditic systems of the Lower Congo basin are investigated over a large area (5000 km2) and for a large period of time (Oligocene to Middle Miocene, about 10 My). Studied turbiditic systems can be described by three main depositional architectures: (1) an erosive morphology characterized by a well-developed basal erosional surface, (2) a constructive morphology characterized by well-developed external levees, and (3) a depositional morphology characterized by lobes. The first turbiditic system (Chattian) is almost exclusively composed of turbiditic deposits (stacked channel-levee complexes). The geometry of the channel-levee complexes is mainly constructive and their spatial distribution is controlled, at the scale of the study area, by autocyclic sedimentary processes (mostly compensation phenomenon). In contrast, the second turbiditic system (Burdigalian-Langhian) is made up of erosive channel complexes and lobes isolated within a hemipelagic background. The geometry of turbiditic deposits is partly controlled, at the scale of the study area, by seafloor deformation related to growth structures (major and secondary faults, turtle-back anticline) rather than sedimentary topography. The two turbiditic systems are separated by hemipelagic deposits of Aquitanian age (Lower Miocene), related to a high eustatic sea-level, whereas turbiditic systems are related to eustatic sea-level lows. However, even though both are associated with eustatic sea-level lows, similar climatic contexts, and similar distances from source, the Chattian and Burdigalian–Langhian systems exhibit different morphology. We relate this difference to both regional (×100 km) and local deformation (×10 km). The regional deformation, related to the Miocene West African margin uplift, explains the more erosive character of the Burdigalian–Langhian system. We propose a model based on the ratio of sedimentation rate versus local deformation (S/D ratio) to explain the origin of the topography that controls the morphology of the systems. During periods of high S/D ratio (equal or larger than 1), sedimentation may smooth seafloor topography and, consequently, turbiditic complexes are controlled by the topography of previous complexes (compensation phenomenon). During periods of low S/D ratio (lower than 1), deformation related to growth structures is not obliterated by sediment deposition (Aquitanian) and may control turbiditic complex geometry (Burdigalian–Langhian).