The compressional-wave velocity of Amazon Fan sediments: Calculation from index properties and variation with clay content

Abstract

Acoustic properties of unconsolidated marine sediments contain important information about sedimentary materials, and velocity structure is important for relating time on seismic profiles to depth in the sediments. We have analyzed the velocity structure of the Leg 155 Amazon Fan sediments from two independent perspectives. In situ compressional-wave velocities were calculated following the Biot model for sediment acoustic properties from shipboard measurements of porosity and grain density. Some of the parameters for this calculation were derived from the comparison of calculated velocities with those deter- mined from wireline logs and applied to other sites. This approach suggests that surficial sediment deposits are normally con- solidated, but that debris-flow deposits and some buried levees are overconsolidated. We also identified some buried levees where underconsolidated sediments directly underlie the debris-flow deposits. The velocity-depth information was used to determine a time-depth curve at each site. The curves are similar above 200 meters below seafloor, but diverge at greater depths. Sediment depths for deeper layers determined aboard Leg 155 are 10–20 m less than initially determined. An analysis of the relationships between compressional-wave velocity and clay volume and between porosity and clay volume suggests that clay minerals and framework grains bear the overburden stress in surficial levees and sand units (a matrix-supported structure or isostress condition). Framework grains bear the overburden stress in deeper levees and debris-flow (a grain-supported struc- ture or isostrain condition). These observations are all consistent with the upper ~400 m of the Amazon Fan being uncemented and unlithified, except for some restricted zones where iron sulfides (e.g., hydrotroilite) or gas hydrates may affect the sediment acoustic properties.