Effects of Messinian Salinity Crisis In The Western Sardinian Margin Sedimentation.

Between 5.95 and 5.32 My ago, the sea level of the Mediterranean Sea dropped some hundreds of meters below the present one. This period, Messinian Salinity Crisis, was accompanied by deposition of a vast salt layer in the basin and a huge erosion on the slope. This period of drying was followed by a fast rising of the sea level. SESAME Cruise (N/O TETHYS II) was conducted on the Western Sardinian Margin in September 2001, in order to characterize the sedimentation during the salinity crisis. During the cruise 512 km of 8-channel seismic lines were collected. These seismic profiles complete seismic profiles collected during the SARD'85 cruise. Over the acoustic basement, the acoustic Unit C is characterized by strong low- frequency reflectors, inclined towards the basin. The thickness of Unit C has its maximum in the messinian canyons, where it reaches 500 mstwt (~ 800m). In its upper part, this unit lies over the messinian erosional surface, which is observed as deep as 2.3 stwt (~ 2500m). Unit C is overlaid in discordance by unit T showing strong parallel reflectors getting more chaotic in the deep parts of canyons. The thickness of unit T is around 100 mstwt between the messsinian canyons, but reaches 250 mstwt (~ 400m) in the canyons. In the basin, the same facies are observed, but here in concordance. The connection between the basin and the margin is marked by a normal fault, which has a fault slip of 500­700 ms. This fault mainly results of the migration of the salt. The acoustic facies, as well as the lateral extend and thickness of both acoustic facies C and T are strongly related to the canyon positions. We thus interpret such acoustic facies as results of the messinien Salinity Crisis : unit C is interpreted as a prograding detritic delta fed by the margins erosion; it is probably of same age as the salt and Upper Evaporites deposition within the deep basin. Unit T is probably deposed during the beginning of the sea level rising, and appears to be also of detritical nature.