Joint Oceanographic Institutions, United States

At the Leg 204 sites located on southern Hydrate Ridge, offshore the Oregon margin, detailed measurements of physical properties in the upper 10-20 m were carried out to characterize the physical nature of the near-surface sediments. Measurements included P-wave velocity, magnetic susceptibility, density, porosity, electrical resistivity, and shear strength. At Sites 1244, 1245, and 1246, P-wave velocity was also determined on the split cores using the Hamilton frame. Electrical resistivity was measured with the new noncontact resistivity tool from Geotek on whole cores as part of multisensor track core logging at Sites 1244, 1245, 1246, 1247, 1248, and 1252. These data were then used to calculate porosity using moisture and density measurements to develop appropriate transfer functions by using Archie's relation. This data set is important because only limited reliable logging data were obtained in the upper 10-20 m as a result of poor borehole conditions. Sediment porosities appear to be highest at Sites 1251 and 1252 in the slope basin, with values of approximately 75% in the upper 5 m below seafloor. Porosity is generally approximately 10% lower at all other sites. Porosity profiles were compared with P-wave velocity and sediment shear strength. Apparent overconsolidation of near-surface sediments was observed at Sites 1244, 1245, and 1246. At Site 1251, no such overconsolidation is observed. Sites 1244, 1245, and 1246 are located on the flanks of the anticline at southern Hydrate Ridge and have probably been exposed to erosion, whereas Sites 1251 and 1252, which are located in the eastward basin, have a much higher recent sedimentation rate as a result of turbidite deposition.