Controls on canyon formation along mud-rich continental margins

Submarine canyons are conduits for transferring land-derived materials from continents to the deep ocean. Regional and global studies show that canyon occurrence correlates positively with seafloor gradient and supply of coarse sediment, yet the processes governing their initiation remain elusive. Here, we investigated the northern South China Sea (SCS) where the spatial variations in fine-grained input from the Red River, delivered to the shelf and slope by buoyant sediment plumes, exert a critical control on canyon development. From SW to NE, the northern SCS transitions from an aggradational-progradational margin with a smooth seafloor devoid of canyons to a purely aggradational margin deeply incised by canyons, gullies, and landslide scars. Seismic and well data show mass-transport deposits are pervasive across the margin, even in the western sector where canyon headscarps are absent at the seafloor. We posit that where the accumulation rate of fine-grained sediment at the shelf edge exceeds tectonic subsidence, rapid mud deposition heals landslide scars and mantles the seafloor, thereby inhibiting canyon formation. By contrast, where sediment supply is lower and balances tectonic subsidence, steeper gradients favor erosion by gravity flows and canyon incision. These results highlight the need to consider sediment supply, caliber, and lateral advection by coastal currents together with slope gradient when predicting canyon distribution.