Consolidation Properties of Equatorial Pacific Ocean Sediments and Their Relationship to Stress History and Offsets in the Leg 138 Composite Depth Sections

Abstract

One-dimensional consolidation tests were run on 20 samples recovered from Sites 844 through 847 during Ocean Drilling Program (ODP) Leg 138. The samples consist of siliceous and calcareous oozes and siliceous clays. The consolidation behavior of these samples is strongly related to their composition. The elastic rebound and compressibility are lowest for calcareous (>70% CaCO3) oozes and increase with increasing silica and clay content, with the larger values attributed primarily to high clay content. The consolidation behavior of the sediments covers the full range of stress history from overconsolidated to underconsolidated. Shipboard laboratory porosity and bulk-density measurements have been corrected to in-situ values using elastic rebound values determined from the consolidation test results. Corrected laboratory density and porosity values correlate well with downhole logging data. The elastic rebound value, the change between laboratory and corrected void ratio, and the vertical effective overburden stress were used to estimate core length expansion caused by stress relief during sampling. This expansion correlates well with recovered core lengths after rebound. The elastic response of the sediment contributes to offsets between a composite depth scale, produced by splicing together core data from holes at the same site, and the standard ODP depth scale of meters below seafloor (mbsf). A good correlation was found between the total recovered core in one hole at a site summed with the total rebounded sediment from all cores at one site and the composite section. This correlation and the good match between downhole logs and core data suggest that offsets between the composite and the ODP mbsf depths result primarily from sediment expansion, not coring gaps. However, only up to one-third of the modified core depth (mcd) offset can be accounted for by elastic sediment rebound. The remaining offset is likely associated with errors in the construction of the composite stratigraphic section.