Beachrocks and lithified barriers in the Gulf of Lions (western Mediterranean Sea) as new markers of the last sea‐level rise

Abstract

AbstractThe Gulf of Lions shelf (southern part of the French Mediterranean coast) displays several occurrences of beachrocks, thus offering an exceptional opportunity to determine the stages of the last marine transgression. These beachrocks crop out especially on the outer shelf and near the modern shoreline. In addition, several other exposures are located on the inner shelf between 10 m and 25 m. The absence of exposure on the middle shelf (water depths of 25 to 90 m) is probably related to a higher rate of sea‐level rise. Most of the beachrocks occur at the top of two or three parallel and contiguous barriers. The seaward barriers are generally less lithified than the inner barriers. Each barrier displays an asymmetrical profile, the seaward slope being more accentuated. This gently‐dipping landward slope forms part of the outer trough resulting from resuspension due to enhanced turbulence at the barrier toe. Almost all of these beachrocks are coarse quartz‐rich sandstones and conglomerates, whose large pore space has been first filled by magnesian calcite (ca 11 mol.% MgCO3), which predominantly fills the voids. Secondary sparites or microsparites, also composed of high‐magnesian calcite, may develop locally in the last remaining voids. Finally, more rarely and very locally (for example, Pierres de Sète), a late‐stage cementation of low‐magnesian calcite is observed which expresses a brief episode of influence from a nearby freshwater groundwater table. The δ18O and δ13C values reflect the relative geochemical homogeneity of these Gulf of Lions beachrocks. However, they are quite distinct from other marine cements and, in particular, those of the eastern Mediterranean, expressing lower temperatures and a lack of influence of dissolved carbon linked to terrigenous fluxes. Based on the radiocarbon ages of these calcitic cements, this study proposes new index points on sea‐level positions in the Gulf of Lions from −95 to 108 m to −0.3 m water depths, between ca 18 583 a cal bp and 633 a cal bp.