Compaction trends of clay-rich deep sea sediments

Analysis of published porosity data for clay-rich (> 50%) sediments from the Ocean Drilling Project, the Deep Sea Drilling Project and deep wells in Recent and Tertiary sediments in Japan and Italy indicate that there is no single, simple compaction (porosity-depth) law for these sediments at all depths. The deep sea, clay-rich sediments have an initial porosity of 75–90% decreasing to near 40% at about 500 m depth. Deeper clay-rich sediments show a reduction in porosity at a different rate from 40 to 10% porosity between 500 and 4000 m. These published data show significant scatter in the shallow samples (< 500 m burial depth) indicating the importance of variables such as sedimentation rate, provenance, bio-perturbation, etc. Nevertheless, the general compaction trend for these data points (median line) does not depend on sediment age and thus the trend can be considered to be for the most part due to physical compaction, i.e. rearrangement of clay particles that have organised themselves upon sedimentation in oceanic waters. The difference in compaction law for the upper sediments is considered to reflect the resistance of the clay flocculation structure formed upon or just after sedimentation.