Sandy submarine canyon-mouth lobes on the western margin of Corsica and Sardinia, Mediterranean Sea

Long-range, low-resolution and deep-towed, high-resolution side-scan sonar records, high-resolution seismic profiles and core samples were used to study the relatively small canyon fed turbidite systems west of Corsica and Sardinia. The margin west of Corsica is dissected by deep (up to 1500 m), straight canyons that have steep axial gradients (10° slopes are common) and that extend from land to sea without a break in gradient. The submarine canyon axes are readily mapped by their stronger acoustic backscatter. The axes have scour holes and trains of gravel or pebble waves. Canyoned slopes have widespread, shallow sediment failures. Five separate depositional lobes are recognised, extending beyond the canyon mouths. Deep-towed, high-resolution seismic profiles across part of one lobe show stacked sedimentary sheets, a few tens of kilometres wide. Cores from these sheets contain coarse to medium sand beds that are up to 3 m thick, with some mud clasts in the middle of the beds and up to 3% clay in the sand matrix. A drape of nannofossil ooze on top of cores indicates that the main activity through the canyons is at times of low sea level. The lobes tend to appear as weak backscatter, with fringes of a braid-like pattern of stronger backscatter. The reasons for this acoustic pattern are not fully understood, though in general the sand bodies are found where backscatter is relatively weak. The size of the canyon-mouth lobes is proportional to the size of the subaerial drainage basins. The limited sediment supply accounts for the absence of a well-developed submarine ramp despite the sand-dominated input from multiple sources.