Controlling factors on the stratigraphic architecture of the Rhône delta during Holocene

Delta systems evolve as a function of interactions between hydroclimatic processes that occur at the watershed level, and coastal marine processes that reshape the coastline. The evolution of these environments is controlled by several factors, like climate, tectonics, and anthropization (by feedback on soils, vegetation, hydrosystems) yet rarely taken into account in models of evolution of sedimentary systems.

The Rhone delta has recorded the impact of climatic variations as well as the development and evolution of human societies during the Holocene period. This system has undergone a post-glacial evolution controlled by a global climatic warming punctuated by short periods of cooling, fluvial metamorphoses, and by a rapid marine transgression generated by the melting of the ice caps, which was followed by the initiation of the progradation of the delta from about 7000 BP, and which seems to accelerate during the Roman antiquity (Arnaud-Fassetta, 2002). The dynamics of the system was modified during the Little Ice Age (LIA, 1350-1850), under the combined effects of a climatic cooling that modified the dynamics of the system, and the increasing effect of anthropization forcing (land use, deforestation, damming, …). The evolution of hydroclimatic variations since the end of the LIA corresponds to a decrease in the frequency of floods and the overall decrease in rainfall, favoring the impact of human activities which became increasingly important until the 20^th century (canalization of the river, construction of dams...). The industrial revolution and mechanization led to a drastic increase in sediment flows in most major rivers, due to deforestation, deep destructuring of cultivated soils and the generalization of intensive farming (Fanget et al., 2013). In the 1950s, this signal is reversed with the proliferation of dams in many rivers, which generates a reduction in sediment load from upstream to downstream.

Sedimentological and chronostratigraphic studies of 17 coredrills complementing an existing dataset on the deltaic plain (Arnaud-Fassetta & Suc, 2015; Amorosi et al., 2013, Vella et al., 2005, 2008), as well as on the prodelta (Jouet, 2007; Fanget et al, 2012, 2014) enabled to the construction of well-constrained stratigraphic correlations, allowing to specify the stratigraphic architecture of the delta and the spatio-temporal evolution of the different lobes composing the Rhone deltaic edifice (lobes of St Ferréol, Ulmet, Peccaïs, Bras de Fer and then Roustan chronologically). The variation of sediment fluxes was evaluated for the different sequences of deltaic progradation phases. This calculation was made possible by the contribution of new dating of cores that helped at constraining the sequential evolution of the lobes. In addition, numerous geochemical data obtained by XRF are carried out on samples of these cores and make it possible to link the deposition of the various sedimentary lobes with the potential contribution of the various sub-watersheds of the Rhone. All these data highlight contrasting periods corresponding to the pre-LIA period (with an increasing impact of human activities on the landscape since the late Neolithic period), the LIA period, the post-LIA period, the industrial revolution and finally the “anthropocene".)