Late Glacial to Preboreal sea-level rise recorded by the Rhône deltaic system (NW Mediterranean)

International audienceA unique late Glacial–Preboreal record of changes in sea-level and sediment fluxes originating from the Alps is recorded in the Rhône subaqueous delta in the Western Mediterranean Sea. The compilation of detailed bathymetric charts, together with high-resolution seismic profiles and long cores, reveals the detailed architecture of several sediment lobes, related to periods of decreased sea-level rise and/or increased sediment flux. They are situated along the retreat path of the Rhône distributaries, from the shelf edge and canyon heads up to the modern coastline. They form transgressive backstepping parasequences across the shelf, the late Holocene (highstand) deltas being confined to the inner shelf. The most prominent feature is an elongated paleo-shoreface/deltaic system, with an uppermost sandy fraction remolded into subaqueous dunes. A long piston core into the bottomsets of this prograding unit allows precise dating of this ancient deltaic system. In seismic data, it displays aggradation, starting at not, vert, similar 15 cal kyr BP, followed by progradation initiated during the first phase of the Younger Dryas, a period of reduced sea-level rise or stillstand. The delta kept pace with resumed sea-level rise during the Preboreal (which is estimated at about 1 cm/yr), as a result of increased sediment supply from the Alps (melting of glaciers and more humid climate “flushing” the sediment down to the sea). Abandonment of the delta occurred around 10,500 cal yr BP, that is to say about 1000 yr after the end of the Younger Dryas, probably because of decreased sediment flux

Evidence for pre-Messinian submarine canyons on the Gulf of Lions slope (Western Mediterranean)

International audienceOn the Gulf of Lions margin, the Messinian Salinity Crisis (MSC) event led to a significant and widespread erosion of the Miocene shelf and slope. It is thus difficult nowadays to restore the morphology of the margin at the beginning of the crisis, and to study the Middle-Upper Miocene series rarely preserved from erosion. In this study, thanks to the use of multichannel seismic reflection data from the Gulf of Lions western slope, it has been possible to image the Miocene slope deposits. Interpretation of the data reveals several diachronic sediment bodies just beneath the Miocene/Pliocene boundary. These bodies are related to slope erosional-depositional complexes, underlain by erosional unconformities outlined by seismic reflector truncations. They are V- or U-shaped in cross section. Over their mapped extent, these bodies are typically 8-12 km wide and incise up to 600 ms TWTT into the Miocene slope deposits. in plan view, they are roughly elongated NW-SE, perpendicularly to the present day shelf break, and they progressively migrate towards the South-East. We interpret these seismic bodies as buried Miocene submarine canyons pre-dating the dramatic drawdown related to the MSC. In this study we try to constrain their age, and we discuss the events that may be at their origin. We also discuss the long-term impact of these slope canyons on the evolution of the margin