Layered successions of hemipelagic sediments from different sites drilled during Ocean Drilling Program Leg 160 investigating the Eastern Mediterranean, are intersected by numerous high-angle normal faults. The formation and frequency distribution of these faults is a function of lithologic variation and thus different sediment physical properties, such as their strength and brittleness. Sediment failure is generally observed where organic-rich layers (sapropels or sapropelic beds of >2% organic carbon) are interbedded with the predominant nannofossil ooze. Undrained shear strength measurements (vane, pocket penetrometer) on split deep sea cores show that sapropels are significantly firmer than the surrounding oozes and clays (up to 40 kPa stronger within the same depth interval). Values >220 kPa were obtained for the most resistant sapropel beds. Drained shear box tests conducted at 1 MPa normal stress on remolded sediment, however, revealed lower peak and residual shear strength for the sapropel (253 and <226 kPa, respectively) compared with nannofossil ooze (621 and >580 kPa, respectively). The internal angle of friction of the sapropel is comparable to that of the weakest clay minerals (φ ′ p 11 ± 1°; φ ′ r 10 ± 1°) whereas friction angles for the nannofossil ooze are higher (φ ′ p 28 ± 1°, φ ′ r 30 ± 1°) and controlled by its high carbonate content.The influence of organic matter on sediment shear strength is the subject of an ongoing debate. The structural observations and shear strength determinations on sediments recovered from the Eastern Mediterranean favor a positive correlation between the abundance of organic carbon and sediment strength. The effect of particle bonding is lessened when the original sedimentary fabric is destroyed (e.g., remolding before shear box test). Faulting takes place adjacent to sapropels and fault occurrence in the stratigraphic succession can be positively correlated with abundance of sapropels.
Three sites (Sites 965, 966, 967) drilled into the Eratosthenes Seamount during Ocean Drilling Program Leg 160 recovered a Cretaceous to Pleistocene succession in which several stratigraphic gaps were identified. Some of these are associated with changes in depositional environment, whereas others have apparently identical lithofacies on either side of the hiatus. Four independent structural data sets were generated to ascertain the tectonic characteristics of each hiatal gap. These are bedding and fracture measurements on downhole formation microscanner data; bedding and fracture measurements on recovered core; strain data collected from individual marker particles in thin section; and axial ratio measurements of cross sections through burrows exposed on the split-core face. These data, in combination with lithostratigraphic and biostratigraphic information, indicate that, whereas most of the hiatuses were generated by tectonic events, some were generated by a combination of slow sedimentation, reworking, and possibly sediment bypassing. Furthermore, each drill site appears to be located on a different structural block that was affected by differential movement at least as far back as middle Eocene time.
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