Evolution of the Rhône delta plain in the Holocene

Abstract

The delta plain of the Rhone shows many sandy beach ridges cropping out at the surface. We propose here a radiocarbon chronology for the accumulation of the sedimentary bodies and an interpretation of the morphology of the mouth lobes that they form. Morphologies of the lobes depend on four principal factors: (1) the variation of the relative sea level, which constitutes the morphogenic base level, (2) the fluvial input (volume of water discharge and sedimentary input, number and position of the mouths), (3) marine dynamics (volume and direction of the longshore drift, dominant wave direction) (4) the accommodation space created by the rise of relative sea level, along with the paleobathymetry onto which the lobe will prograde immediately in front of the delta and at sea. Our conclusions are: the main Holocene paleo-delta lobes of Ulmet and Saint Ferréol were contemporaneous, being deposited from 4000 to 2000 yr BP. However, the fluvial style of their downstream channel (meandering for Ulmet channel and linear for Saint-Ferréol) and the lobe morphologies of both channels are contrasted: rounded in the case of Ulmet and elongated for Saint-Ferréol. The role of the accommodation space is confirmed by two recent drillholes traversing the entire Holocene succession, which reveal a prior stage of sedimentary filling in the case of the Saint-Ferréol lobe, thus limiting the accommodation space and favouring a rapid and pointed progradation of this lobe. The decoupling between hydrological activity and progradation of lobe indicates the complexity of mechanisms in the large catchment basin of Rhône. However, the high rate of progradation of Saint-Ferréol lobe after 2900 BP is probably a consequence of the increase of human activity after this date. The variation of relative sea level remains an important factor controlling the evolution of the Rhone delta after 6000 BP. The local (?) stabilization of the relative sea level between 4585 BP and 3520 BP played an important role in the triggering of coastal progradation (Saint-Ferréol lobe, eastern margin Rhône delta).