In situ and laboratory studies of permeability, conducted by Ocean Drilling Program scientists from Leg 156, provide constraints on parameters controlling the hydrogeologic system in the Barbados accretionary prism. Results from these studies indicate that core-scale and formation-scale permeability values differ by at least several orders of magnitude and are dependent on pore-fluid pressure and effective stress conditions. Direct measurement from packer experiments and indirect evidence from consolidation tests suggest that pore-fluid pressures are commonly above hydrostatic values and approach lithostatic values within the décollement zone. Permeability and fluid pressure conditions in the Barbados accretionary prism reflect the complexity of the hydrogeologic system of such an active tectonic environment.

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% CaCO 3) 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.

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Low-gradient flow tests and one-dimensional consolidation tests were performed on four samples from Ocean Drilling Program (ODP) Leg 156, northern Barbados Ridge, Hole 949B, to provide direct as well as indirect measures of permeability and stress history. All samples tested show a significant degree of underconsolidation, with overconsolidation ratio (OCR) values decreasing with depth from 0.4 to 0.2 to 0.1. These low ratios were used to approximate the excess pore pressures within the accreted sediment and the décollement. The largest estimated excess pore-pressure values lie within the interpreted décollement zone at Site 949 and range from 1600 to 1900 kPa. Combining results from this study with previous data from Leg 110, two generalized functions of void ratio vs. hydraulic conductivity for high and low permeability sediments are developed that are characteristic for sediments of low and high smectite content. By applying these functions to the Site 671 (ODP Leg 110) porosity profile, it is possible to demonstrate the occurrence of a zone of low permeability immediately above the décollement. This is consistent with the concept that low permeability sediments will develop higher excess pore pressures, and therefore, are more susceptible to shear failure.