Chapter 25 The Significance of Contourites for Submarine Slope Stability

Slope instabilities in areas of contouritic deposits have been identified on continental slopes, on rises and in ocean gateways, including some of the largest submarine landslides known. Contouritic sediments are prone to failure because (1) their composition, geometry and location, well-sorted muddy or sandy sediments (i.e. weaker compared with poorly sorted sediments) forming sediment mounds on the continental slope and rise may experience liquefaction in response to cyclic loading (earthquake); (2) they are often characterized by high sedimentation rates implying high water content and under-consolidation and thus low shear strength; a substantial component of microfossils may also prevent normal sediment consolidation; (3) they could, due to their location on continental slopes, be subjected to rapid loading; on high-latitude margins, excess pore pressure may develop within contourites sandwiched between glacigenic sediments; (4) excess pore pressure could also develop from gas migration and gas-hydrate dissociation due to a relatively high organic-carbon content from often productive water masses along continental margins. When failing, the area affected by mass wasting can be large because the contourites may have a very large areal distribution due to the intrabasinal extent of the thermohaline and geostrophic current systems. Their weakness and resulting low shear strength may also, at least partly, explain why some of the large landslides involving contouritic sediments also show a very long run-out (up to hundreds of kilometres) on gentle slopes. © 2008 Elsevier B.V. All rights reserved.