Messinian Facies and Oligo-Miocene Slumping in Offshore Sirt Basin Libya and Sub-Messinian Seismic Imaging

During the Messinian (6.3-5.6 Ma) the Gibraltar Straits temporarily closed as a result of the continued collision of Africa and Eurasia. The relict Mediterranean ocean basin quickly evaporated and the sea level fell dramatically. As a result an erosional and karst surface was formed which can be identified in both outcrops and seismic data in the Sirt basin. At the base of the slope, the erosional surface becomes a bright seismic event which marks the Messinian evaporate basin. Well data indicate that anhydrite and gypsum were deposited in a sabkha environment. The central part of the Sirt Basin became isolated between the karstified slope and the outer basin high. The interval is characterized by a seismic facies of laterally continuous bright events onlapping the erosional surface. This facies is interpreted as clastic deposits within a Late Messinian enclosed lake (Lake Sirt). There is a strong correlation between the seismic facies and seismic data quality. The areas with best data quality correspond to the mapped extent of Lake Sirt characterized by clastic sedimentation. The areas with the poorest data quality correspond to the parts of the basin with sabkha depositional environments and anhydrite, largely surrounding the lacustrine environments.