DEEP-SEA AVULSION AND MORPHOSEDIMENTARY EVOLUTION OF THE RHONE-FAN VALLEY AND NEOFAN DURING THE LATE QUATERNARY (NORTH-WESTERN MEDITERRANEAN SEA)

The Petit-Rhone Fan Valley (north-western Mediterranean) is a broad, s inuous, filled valley that is deeply incised by a narrow, sinuous thal weg. The valley fill is differentiated into three seismic subunits on high-resolution seismic-reflection profiles. The lower chaotic subunit probably consists of channel lag deposits that seem to be in lateral continuity with high-amplitude reflections representing levee facies. The intermediate transparent subunit, which has an erosional base and clearly truncates levee deposits, is interpreted to be mass-flow depos its resulting from the disintegration of the fan-valley flanks. The up per bedded subunit shows an overall lens-shaped geometry and the seism ic reflections onlap either onto the top of the underlying transparent subunit or onto the Rhone levees. Piston core data show that the uppe r few meters of this upper subunit consist of thin turbidites, probabl y deposited by overflow processes. The few available C-14 ages suggest that the upper stratified subunit filled the Petit-Rhone Fan Valley b etween 21 and 11 kyr BP. The upper bedded subunit is deposited within the Petit-Rhone Fan Valley downslope of a major decrease in slope grad ient. This upper subunit and the thalweg are genetically related and r epresent a small channel/levee system confined within the fan valley. Previous studies interpreted this thalweg to be an erosional feature r esulting from a recent avulsion of the major channel course. Our inter pretation implies that the thalweg is not a purely erosional feature b ut a depositional/erosional channel. This small channel/levee system i s superimposed an a large muddy channel/levee system after the sedimen t supply changed from thick muddy flows during the main phase of aggra dation of the Rhone Fan levees, to thin, mixed (sand and mud) flows at the end of Isotope Stage 2 (approximate to 16-18 ka BP). The pre-exis ting morphology of the Petit-Rhone Fan Valley played a determinant rol e in the sediment dispersal leading to the creation of this small and confined channel/levee system. These mixed flows have undergone flow s tripping resulting from the changes in the slope gradient along the th alweg course. The finer sediment overflowed from the thalweg and were deposited in the Petit-Rhone Fan Valley. Coarser channelled sediment r emaining in the thalweg were deposited as a 'sandy' lobe (Neofan). As indicated by C-14 dating, sedimentation on this lobe continued until v ery recently, suggesting a further evolution of the turbidity flows fr om small mixed flows to small sandy flows, The deposition of this sand y lobe and the sedimentary fill of the Petit-Rhone Fan Valley may be r elated to widespread shelf edge and canyon wall failures with a result ing downslope evolution of failed sediment into turbidity currents.