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

Radiocarbon C-14 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, a s sea level lowered rapidly and entered a low stand phase. The Herodotus Ba sin Megaturbidite is a basinwide deposit that forms a recognisable acoustic ally-transparent layer on 3.5 kHz high-resolution seismic profiles and cove rs an area of approximately 40,000 km(2). It thins from about 20 m in thick ness proximally to some 10 m distally over a basin length in excess of 400 km. The total volume is estimated at around 400 km(3), which is significant ly 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 Salum to the west of the Nile Cone. The additional material may have bee n 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-scal e erosion at the base of the very large-volume turbidity current that was g enerated from the original slide. Detailed sedimentary analyses of cores re covered from the megaturbidite show its distinctive characteristics: graded sand, silt, mud and bioturbated mud units; poorly developed structures pro ximally becoming more distinct and ordered distally; variation in grain siz e and structures that suggest either flow separation/reflection around topo graphic highs and/or an inherently unstable flow; and a mixed-source biocla stic-terrigenous composition. These features, together with its size and la teral extent, would make any similar megabed an excellent marker horizon in basin analysis. A combination of factors was responsible for triggering th e initial slide-debris flow event that evolved downslope into this mega-tur bidity current. These include lowered sea level that destabilised the outer shelf - upper slope sediments, tectonic oversteepening of the margin, rela tively high rates of sedimentation, and seismic activity. (C) 2000 Elsevier Science Ltd. All rights reserved.