The Messinian event on the margin of the Mediterranean Sea in the Nice area, southern France

Abstract

Seismic reflection profiles have been used to map the Messinian erosion surface and associated basin-margin deposits on the continental margin off Nice. A palaeovalley extends seaward from the incised Var palaeovalley mapped on land. The lower part of this valley is filled by an acoustically incoherent unit with a flat top and steep slope into the basin: this unit is interpreted as a coarse-grained deltaic deposit that accumulated synchronously with the upper Messinian evaporites in a ponded lake basin. Two submersible dives on strategic outcrops supported the interpretation of the seismic profiles. We examined and sampled outcrops of probably early Tertiary sandstones underlying the Messinian erosion surface and of a fining-up, predominantly conglomeratic sequence that fills Messinian channels on the continental slope and probably overlies the deltaic deposits. These conglomerates pass up into Early Pliocene marls and have turbidite sedimentary structures. These are prodeltaic deposits formed during the transgression at the end of the Messinian. Based on seismic facies mapping, sampling and submersible observations, a palaeogeographic reconstruction of the area is proposed. A basal erosion surface, including the Var palaeovalley, represents extreme dessication of the western Mediterranean and was followed by accumulation of the main Messinian salt. Following this, a lake was ponded in the Ligurian Basin, with accumulation of evaporites and shales. A Gilbert-type delta prograded into the basin from the Var palaeovalley. Relative sea (lake)-level lowering is estimated to be about 1200 m at this time. There has been little faulting on the margin since the Early Pliocene transgression, but there is estimated to have been about 800 m of flexural subsidence due to both thermal subsidence and water loading.