The relationship of undrained shear strength and porosity with depth in shallow (< 50 m) marine sediments

In this paper we review the relationship between undrained shear strength and porosity with depth of marine sediments in the upper 50 m below the seafloor. Based on a compilation of data from 46 Ocean Drilling Program (ODP) legs, we find that only about half of the drill sites show the anticipated increase in strength (59%) and associated decrease in porosity (49%) down hole. Moreover, a significant correlation coefficient for both strength and porosity with depth is found at only 31% of the sites. Four categories of sediments are distinguished based on the significance of correlation coefficients. We observe an influence of sediment composition and texture on the relationship between strength and porosity with depth. Fine-grained siliciclastic sediments (clayey and silty sediments and nannofossil oozes with less than 50% carbonate) frequently show significant correlation coefficients for strength and porosity with depth while fine-grained carbonates with high carbonate concentrations (calcareous muds and nannofossil oozes with an average carbonate content of 87%) as well as reef-facies carbonates lack significant trends down hole. Moreover, correlation patterns vary considerably with geological or tectonic setting. For instance, delta deposits of the Amazon Fan and sediments from the Atlantic passive continental margins frequently show significant correlation coefficients for both properties, extensional arcs often show significant correlation only for one property, shear strength or porosity. Correlation patterns are discussed in the light of sediment composition, geological and tectonic setting, the methods used for shear strength measurement, and previous work published on compaction behaviour.