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 Mediterranean

Abstract

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.

Molar tooth structures of the Neoarchean Monteville Formation, Transvaal Supergroup, South Africa. I: Constraints on microcrystalline CaCO3 precipitation

“…An injection model has been proposed for both MT veins of microcrystalline calcite and ‘synaeresis‐like’ veins of mixed mineralogy, with differences in fill because of the material being injected (Pratt, 1998a,b, 1999, 2001). Seismic injection may be reasonable for some mixed‐mineralogy veins; it is consistent with results from sand box experiments on homogenized mixtures of clay, nanofossil carbonate and organic matter (Kopf et al. , 1998).…”

“…, 1998). In these experiments, shear and loading enriched granular material – carbonate with clay – along shear planes (Kopf et al. , 1998).…”

Lost tsunami

“…If we consider the upper 10 m of sediments in cores taken from sites across eastern Mediterranean (Ocean Drilling Project (ODP) leg 160), which mainly consist of nannofossil and silt ooze plus Sapropels [ Kopf et al , 1998], we obtain yield strengths versus depth relationships which give typical values for cohesion (= yield strength at the sea floor) of the order of kPa. For example, C of around 2.5 kPa is obtained at site ODP 160–964‐A, at a depth of 3650 m. Similar values are also found by Sultan et al [2004] in undrained conditions, at continental margins where a 10 – 20% sand fraction is present.…”

“…It originated from the steep slopes of the basins themselves. At site CR (Figures 1 and 2), by interpolating Kastens and Cita 's [1981] morphological data, we find that 1–7% of the basin's slopes exceed 27° (this being the internal friction angle for nannofossil/silt ooze [ Kopf et al , 1998]). Forty‐to‐fifty percent of the slopes also exceed 11°, which is the internal friction angle of the Sapropel S1 layer [ Kopf et al , 1998].…”

“…At site CR (Figures 1 and 2), by interpolating Kastens and Cita 's [1981] morphological data, we find that 1–7% of the basin's slopes exceed 27° (this being the internal friction angle for nannofossil/silt ooze [ Kopf et al , 1998]). Forty‐to‐fifty percent of the slopes also exceed 11°, which is the internal friction angle of the Sapropel S1 layer [ Kopf et al , 1998]. The name “homogenite” was assigned by Kastens and Cita [1981], because, except for their basal part, such marine deposits are quite homogeneous, with fine silt to clay grain‐sizes and no grading.…”