Morphology of the Petit-Rhone Canyon and Deep-Sea Turbidite System and Last Glacial and Holocene Sedimentary Facies, Gulf of Lions, Western Mediterranean

During the RHOSOS cruise aboard R/V Le Suroit (September 2008) swath bathymetry, chirp profiles and sediment coring was carried out on the Petit-Rhône canyon and deep-sea turbidite system. This dataset complements data acquired over the last ten years and allows drawing a complete view of the system with high-resolution bathymetry (50 m digital terrain model). Sediment cores retrieved on channel-levee transects together with the pre-existing core dataset give an overview of the sedimentary facies across and along the canyon and turbidite system. The canyon head shows a flat floor that is attributed to an important infill that gradually decreases downstream. The morphology of the flanks of the canyon head are dominated by numerous gullies. Further downstream, at mid-slope, flat-floored terraces dominate the morphology of the 11 km wide and 530 m deep canyon. These terraces disappear gradually to the base of slope where the canyon develops a 7-8 km wide flat-floored valley. The valley becomes gradually narrower and the canyon is prolonged by a perched turbiditic channel-levee system. Along the canyon and channel-levee system runs a narrow thalweg of rather constant depth (150 m) and width (900-600 m). A knicpoint at the bottom of the thalweg corresponds to the bifurcation and the abandonment of the perched channel-levee. This thalweg downstream the bifurcation point is the so-called Rhône “neochannel”. The canyon and the valley show high amplitude sinuosity (10 km). The thalweg shows several orders of sinuosity, 10 km, 5 km, and a remarkable well-developed sinuosity of less than 0.5 km. The thalweg becomes poorly sinuous after the bifurcation point. Terraces and the valley show multiple evidences of slope instability such as scars and displaced blocks. In the canyon, sedimentary facies show that turbidite currents are not recorded at 370 m above the bottom, where hemipelagic deposits occur, but are dominant at 150 m. Hemipelagic sedimentation replaces turbidite sedimentation at the top of cores. This change of sedimentation is related to the disconnection between the Rhone River delta and the Petit-Rhone canyon head at the beginning of the sea-level rise at the end of the last glacial maximum. Lateral changes in the turbidite facies are attributed to changes in the overflow processes related to the morphology (height and width) of the channel-levee and channel-valley. Few sites show evidence of post-glacial sandy turbidites attributed the reworking of sand bodies at the outershelf. Mass transport deposits are frequent in the thalweg.