Deep-reaching liquefaction potential of marine slope sediments as a prerequisite for gravity mass flows? (Results from the DSDP)

Abstract

Plots of water content ( w) and Atterberg liquid limits ( w L) versus depth, coupled with measurements from young submarine mass flows, support the concept that many fine-grained, but low-cohesive slope sediments are susceptible to liquefaction and can, as a result of dynamic loading (earthquakes and storm waves), be transformed into mud flows without the uptake of additional water. The in-situ water content w can be higher or approximately equal to w L (boundary between the plastic and liquid state of a soil) usually in the uppermost few meters, but frequently also tens or even some hundreds of meters below the sea floor. Such a substantial liquefaction potential is particularly common in sediments rich in silt-size biogenic particles and in regions of high sedimentation rates, causing underconsolidation. It is not clear whether changes in the salt concentration and chemistry of interstitial water versus depth in core exert some influence on w L and the liquefaction potential. The average “field liquid limit” of the total mass flow can be even lower than w L of small, only fine-grained homogenized samples, if part of the dislocated material is carried unchanged as lumps of mud by a liquefied matrix. w and w L often decrease in a similar way versus depth in sediments rich in microfossils due to the onset of slight chemical diagenesis with increasing burial depth. Thus, in spite of increasing compaction, a high liquefaction potential may be maintained, until finally substantial lithification reduces w to values lower than the plastic limit, w p, and completely alters all the mass physical properties.