Influence of sea level and basin physiography on emplacement of the late Pleistocene Herodotus Basin Megaturbidite, SE Mediterranean Sea

Radiocarbon 14C dates from pelagic intervals above a megaturbidite in the Herodotus Basin give direct evidence of emplacement at the beginning of the last glacial period, approximately 27,125 calendar years before present, as sea level lowered rapidly and entered a low stand phase. The Herodotus Basin Megaturbidite is a basinwide deposit that forms a recognisable acoustically-transparent layer on 3.5 kHz high-resolution seismic profiles and covers an area of approximately 40,000 km2. It thins from about 20 m in thickness proximally to some 10 m distally over a basin length in excess of 400 km. The total volume is estimated at around 400 km3, which is significantly more than the volume of sediment that could have been displaced from its most likely source area, the funnel-shaped marginal embayment of the Gulf of Salûm to the west of the Nile Cone. The additional material may have been derived, in part, from synchronous failures on other parts of the Libyan/Egyptian shelf and slope, but most is believed to have come from large-scale erosion at the base of the very large-volume turbidity current that was generated from the original slide. Detailed sedimentary analyses of cores recovered from the megaturbidite show its distinctive characteristics: graded sand, silt, mud and bioturbated mud units; poorly developed structures proximally becoming more distinct and ordered distally; variation in grain size and structures that suggest either flow separation/reflection around topographic highs and/or an inherently unstable flow; and a mixed-source bioclastic-terrigenous composition. These features, together with its size and lateral extent, would make any similar megabed an excellent marker horizon in basin analysis. A combination of factors was responsible for triggering the initial slide-debris flow event that evolved downslope into this mega-turbidity current. These include lowered sea level that destabilised the outer shelf — upper slope sediments, tectonic oversteepening of the margin, relatively high rates of sedimentation, and seismic activity