Erosional sculpting of the Caprera confined deep-sea fan as a result of distal basin-spilling processes (eastern Sardinian margin, Tyrrhenian Sea)

The aim of this work is to study the effect of basin confinement on the Caprera deep-sea fan (CDF), the main turbidite system of the Olbia intraslope basin (OIB) located in the eastern Sardinian margin (Central Tyrrhenian Sea). The OIB is the northernmost intraslope basin of the margin, bound seaward by the Etruschi and Baronie seamounts, and landward by the Sardinian continental slope. The OIB is completely filled and sediments can spill toward the lower Tavolara basin (TB) through a system of bypass canyons located in a saddle between the two seamounts. The CDF has been imaged by high-resolution swath-bathymetric dataset acquired by the ISMAR of Bologna during the cruises TIR96 and TIR99. The multibeam data have been integrated with seismic lines in order to understand the shape, the morphology and internal organization of the CDF. The CDF faces a wide shelf sector and as a consequence canyons are presumably stranded from river supply during high stands of sea level. At the base of slope the CDF consists of a wide leveed channel. The increasing channel flank relief moving downfan, the presence of knickpoints, erosional remnants, chutes scouring the levees and erosional terraces show that the Caprera channel, at the present day, is out of grade. In addition, erosional processes also act beyond the channel mouth, where distal erosional channels are present, and in the basin plain, that is scoured by a broad shallow valley. The bypass of sediments, from the completely filled OIB to the lower TB, has promoted the lowering of the ultimate base level for sediment gravity flows which is considered the cause of the disruption of the equilibrium profile of the CDF. However, the seismic data show that, in the CDF phases of deposition also alternate with the mainly erosional ones. These are interpreted to reflect the variability in flow volume during different sea level stands. The depositional style of the CDF does not conform to simple deep-sea fan models, and emphasizes that seafloor topography is a prime control on turbidite system development also in tectonically inactive margins.