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ABSTRACT

The early Holocene S-1 sapropelic sequence in the northwest Hellenic Trench has been studied in six piston cores from the Zakinthos and Strofadhes basins. The S-1 sequence, 0.7-3.5 m thick, consists principally of silt to mud turbidites, with rare, thick, disorganized, sandy turbidites. These lithofacies are described and compared with fine-grained turbidites from the literature. Petrographical data, including the abundance of organic carbon and planktonic microfossils, indicate that the principal source of sediment to the turbidites was from the continental slope. On the basis of composition and texture, five turbidite units can be correlated between the two basins. These basins are fed by separate but adjacent drainage systems. The apparently synchronous occurrence of turbidites in the two drainage systems suggests that the turbidity currents were seismically triggered. Some of the turbidites show poorly organized beds which may reflect the slump origin and the short (30 km) distances of travel. Turbidites were deposited more frequently in the S-1 sapropelic interval than in the over- and underlying sediments. Application of slope stability analysis shows that on the 8° slopes above the basins, a 10-cm-thick sapropel would have a factor of safety of about 2, and would fail with earthquake accelerations in excess of 0.08 g. The frequency of earthquakes likely to produce such accelerations is similar to the observed frequency of turbidites. The low strength of the sapropelic sediment makes it particularly susceptible to such failure. Similar thin-skinned slumping may be an important process for the initiation of turbidity currents in other environments where there are steep slopes or high sedimentation rates.