Quaternary development of migrating sediment waves in the Var deep-sea fan: distribution, growth pattern, and implication for levee evolution

A field of sediment waves built by turbidity currents on the Var deep-sea fan has been studied using a large amount of seismic-reflection data and cores. To understand the spatial organisation and evolution of the sediment waves, maps of both surficial and an older subsurface wave field were prepared. Three different sediment wave geometries (symmetrical to asymmetrical cross-section) were recognised and can be linked to particular styles of evolution through time (from simple vertical aggradation to upslope and upcurrent progradation). Each geometry appears to be linked with: (i) a particular location on the Var Sedimentary Ridge; (ii) the type of turbidite deposits; (iii) the local gradient slope; and (iv) the height of the Ridge. Several factors control the nature and distribution of sediment waves and these factors do not change significantly through time, as suggested by the common geometries, evolution and distribution of the sediment waves between the present and fossil fields. Supercritical flow conditions and high sediment supply allow the development of well-developed and prograding sediment waves, while subcritical flow conditions and low sediment supply allow the formation of smaller and aggrading sediment waves. The impact of these two factors is also influenced by the morphology of the fan valley, the height of the Ridge, and the type of transported sediment. The evolution of sediment wave amplitude is also strongly influenced by the distance of sediment waves from the Ridge crest, while wavelength evolution is more influenced by slope gradient. The building of the Var Sedimentary Ridge and particularly the asymmetry and the overall morphology of the levee are directly controlled by the growth pattern of the sediment waves.