Turbidites and megaturbidites from the Herodotus abyssal plain (eastern Mediterranean) unrelated to seismic events

Abstract

A transect of 9 piston and gravity cores was taken across the southeast flank of the Mediterranean Ridge. The transect extends from the abyssal plain to a location 290 m shallower than the abyssal plain and 50 km north of the edge of the ridge. Each core contains two or more turbiditic layers, which belong to two discrete types, here called A and B. Type A essentially consists of black plastic clays rich in smectite, with silt fraction of terrigenous nature; source area is the Nile cone, and age is middle or late Pleistocene. Type B turbidites are pale brown to whitish in color, coarse-grained, carbonate-rich and bioclastic in composition with shallow-water fossils dominant. They are poor in smectite, and the silt fraction is very rich in carbonates (calcite and aragonite). The source area is the North African shelf, south of the abyssal plain. A megabed belonging to Type B turbidites is correlatable from core to core, with a thickness in excess of 10 m in the abyssal plain, and an estimated volume of 10 km 3. Its stratigraphic position in the latest Pleistocene suggests that its origin is related to a sudden collapse of bioclastic sediments accumulated in a canyon head, during a low sea-level stand. The source areas of the two discrete turbidite types belong to the essentially aseismic North African continental margin. Although they are presently recorded on the tectonically active Mediterranean Ridge, no seismic events are required to account for the presence of either type of turbidite.