49. RELATIONSHIP BETWEEN THE VARIATION OF UNDRAINED SHEAR STRENGTH, ORGANIC CARBON CONTENT, AND THE ORIGIN AND FREQUENCY OF ENIGMATIC NORMAL FAULTS IN FINE-GRAINED SEDIMENTS FROM ADVANCED PISTON CORES FROM THE EASTERN MEDITERRANEAN1Achim Kopf,2 M. Ben Clennell,3 and Rachel Flecker4 |
ABSTRACTLayered 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 (phi ´p 11 ± 1°; phi ´r 10 ± 1°) whereas friction angles for the nannofossil ooze are higher (phi ´p 28 ± 1°, phi ´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. |
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