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Mud rather than sand is the predominant sediment type in submarine trenches, and most studies attribute fine-grained layers to pelagic and hemipelagic mechanisms or to turbidity-current and slump emplacement. Analysis of 28 piston cores in the eastern Hellenic Trench (eastern Ionian basin) in the eastern Mediterranean Sea sheds light on thick muds that form a major component of late Pleistocene-Holocene trench lithofacies. Several 600-cm-long core sections consist entirely of clayey silt that in X-radiographs appears structureless or displays vague lamination or subtle graded bedding. Size analysis reveals an upward increase in the relative percentage of the clay fraction. The sand-size fraction, generally less than 2%, includes planktonic and benthonic forams, shell frag ents, and terrigenous components (light and heavy minerals, mica and plant debris). Carbonate content ranges from 20 to 40%, and that of organic matter from 3 to 5%. A 3.5-kHz subbottom survey of the region reveals discontinuous, acoustically transparent layers in trench basins and locally on slopes and depressions that bound the Hellenic Trench. A mudflow-turbidity-current origin is proposed, and is substantiated by radiocarbon dates showing rapid emplacement.
It would appear that "giant" mudflow deposits of this type are a particularly characteristic component of the typical trench lithofacies. Transport in many cases involved displacement of 1 cu km (e.g., a 10-m-thick layer covering 100 sq km). However, it should be noted that comparable or even greater volumes of mud have been displaced by single-event processes such as turbidity currents in almost all large basins of the Mediterranean such as the Algero-Balearic, Ligurian, and Tyrrhenian basins on the west and the Ionian, Sea of Crete, and Herodotus basins in the eastern Mediterranean. The marked thickness of mudflow deposits in the Hellenic Trench and other trenches primarily reflects ponding in structurally mobile and topographically restricted depressions
rather than a transport process unique to the trench environment.
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