Consolidation and strength of Pliocene-Pleistocene sediments of ODP Leg 105 holes (Table 1)

Based on laboratory geotechnical tests, the stress history of sediment drift deposits at two Ocean Drilling Program (ODP) sites of Leg 105 is analyzed. Geological analyses of Sites 646 and 647 indicate that sedimentation is primarily controlled by bottom currents having periodic turbidite sequences and no significant hiatuses. Consolidation tests on a limited number of good quality subsamples and other supporting data show that sediments deeper than approximately 5 to at least 150 meters below seafloor (mbsf) are significantly underconsolidated (i.e., these sediments are still consolidating under the existing overburden stresses) and have overconsolidation ratios of less than 0.4 below 50 mbsf. Possible explanations for this underconsolidation include relatively high rates of sedimentation (up to 80 m/m.y.), low permeability layers, an upper zone (~5 m) of apparently overconsolidated sediment, high concentrations of siliceous microfossils, and the existence of nonlinear flow behavior at low hydraulic gradients.