Marine geology of the Medriff Corridor, Mediterranean Ridge

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Abstract
Thirty‐one piston and/or gravity cores not exceeding 10 m in length were raised in selected areas of a 300 km‐long transect (Medriff Corridor) crossing the Mediterranean Ridge, an accretionary complex subject to continent/continent collision, containing an important evaporitic body (Messinian evaporites), in order to ground‐truth the geological make‐up. Core location, very accurate with reference to a complex bottom configuration, was preceded by swathe mapping, seismic profiling and side‐scan sonar investigations. Most sediment cores have a pelagic facies, with biogenic marls as dominant lithology, and sapropels and tephras as minor, isochronous lithologies. A combination of isochronous lithologies and calcareous plankton biochronology permits high resolution stratigraphic correlations. Pelagic facies sediments are Middle Pleistocene to Holocene in age. Two cores associated with mounds located along thrusts contain a matrix‐supported mud breccia of deep provenance, Burdigalian‐Langhian in age, similar to that characteristic of the Mediterranean Ridge diapiric belt (Cita et al.1995). Three new brine‐filled anoxic basins (Urania, l'Atalante and Discovery) were discovered. The brines originated from submarine dissolution of Messinian evaporites and are different in the various basins. The sedimentary record strongly differs from basin to basin. These brine lakes are very young (35 000 years or less). A drastic change in sedimentation rate recorded in the Discovery Basin suggests that basin collapse was sudden and followed by progressive development of basin anoxia. Some cores were analyzed with a prototype multisensor for P‐wave velocity, magnetic susceptibility and density. Sapropels show up as abrupt decreases in P‐wave velocity and density, and tephra as sudden increases in magnetic susceptibility. Mud breccia displays P‐wave velocities greater than pelagic marls, with peaks related to lithic clasts. Anoxic sediments have high P‐wave velocities; peaks are associated with gypsum crystals.