Florida State University, United States

Sediment density and velocity at the eight sites that comprise the Ocean Drilling Program Leg 199 Pacific Paleogene transect were examined to determine the consistency of the various laboratory and logging measures of density and velocity and the extent to which these parameters vary as a function of sediment composition. Densities were provided in the laboratory by gamma ray attenuation (GRA) in whole-core sections using the GRA sensor and gravimetric and volumetric measurements of discrete samples and from the borehole by GRA using the litho-density tool. Laboratory velocities were obtained for whole-core sections using the P-wave logger and for discrete samples using insertion and contact probes. The consistency of bulk densities among the different techniques is very good; however, there is less agreement between the velocities determined by the P-wave logger and insertion and contact probes. Variation of density and velocity was compared with the results of the determination of the calcium carbonate content, mineralogy as estimated by light absorption spectroscopy, and bulk sediment geochemistry. Wet bulk density displays a significant dependence on sediment composition, with calcite content being the most important factor influencing density. The amount of opal in the sediment and depth are lesser influences. The sediment velocity does not display a clear relationship with the composition of the sediments. In situ estimates of density and velocity were determined by comparison of the laboratory and logging values of density and by using published relationships for the derivation of in situ density and velocity from laboratory measurements. Comparison of laboratory and logging measurements produces equivocal results. A clear trend of increasing elastic rebound in the nannofossil ooze with increasing burial depth is lacking. A larger sample population and greater range of densities is needed to establish a trend in elastic rebound for pelagic clay and radiolarian ooze. The in situ estimates of density and velocity display a distinct character for the different lithologies recovered during Leg 199. Pelagic clay has low bulk density and low velocity. Radiolarian ooze is characterized by uniformly low wet bulk density and unusually high velocity, and nannofossil ooze displays a larger range in bulk density and a more consistent relationship between density and velocity. Because of the variability in velocity, acoustic impedance, calculated from the in situ estimates of the sediment properties, varies primarily as a function of wet bulk density.