Analysis of physical properties measured on cores and on discrete samples collected by the Ocean Drilling Programme (ODP) Leg 178 on the Pacific margin of the Antarctic Peninsula reveals anomalous down-hole curves of porosity, density, water content, and P-wave velocity. These indicate an overall trend of increasing porosity with depth and suggest that the drifts are mostly undercompacted. In one of the two boreholes analysed, a sharp decrease in porosity, matching increasing bulk sediment density and increasing compressional velocity occurs towards the base of the hole, which corresponds to a bottom-simulating reflector in the seismic section. Analysis of seismic reflection, down-hole logging, geotechnical and mineralogical data from two drilling sites indicates that the observed anomalous consolidation trends are a consequence of the presence of biogenic silica (diatom and radiolarian skeletons) even with a small to moderate amount. Above the bottom-simulating reflector, intergranular contacts among whole or broken siliceous microfossils prevent normal sediment consolidation. Diagenetic alteration of biogenic opal-A to opal-CT causes a dramatic reduction of intra- and interskeletal porosity allowing sediments to consolidate at depth. This results in overpressuring and a decrease in the effective stress. Excess fluids are expelled towards the sediment surface through near vertical, small throw normal faults extending from the diagenetic front to the seafloor and affecting the stability of the submarine slope in the form of gravitational creep along a weakened surface. This work shows how physical properties of shallow fine-grained marine sediments can be analysed as basin-wide indicators of biogenic silica abundance. The diagenetic alteration of siliceous microfossils is a possible cause of slope instability along world continental margins where bottom-simulating reflectors related to silica diagenesis are present at a regional scale.
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