Results from one-dimensional modeling of Site 1108 suggest that overpressures due to sedimentation require permeabilities less than 3.4 × 10-19 m2. Although permeability values in that range would be reasonable for clay-rich sediments (Neuzil, 1994), sediments in Woodlark Basin have a greater component of medium- to coarse-grained material. The lowest permeability measured at Site 1108 was 5.0 × 10-18 m2, whereas the lowest overall permeability measured during this investigation was 1.5 × 10-18 m2. The low value was measured for Site 1115 sediments at 678 mbsf. Therefore, it does not seem likely that permeabilities below Site 1108 are lower than 3.4 × 10-19 m2, although it is possible that extensive cementation could cause low permeability in the base of the rift basin.
The modeling assessed only overpressures due to one-dimensional loading. Additional pressure-generating mechanisms, such as hydrocarbon generation, clay dehydration, or additional input of fluids from greater depths of the fault zone are likely to be active in Woodlark Basin. The high thermal gradient in this area (~94-100°C/km) (Shipboard Scientific Party, 1999) would put the base of the section in the range for smectite dehydration.
On the other hand, overpressures could be reduced by escape of fluids along lateral conduits or fault zones. The occurrence of sand layers may provide lateral conduits for fluid flow within the basin. For example, Sample 180-1108B-30R-6, 32-52 cm, at 280 mbsf, yielded permeability values of 1.0 × 10-16 m2, 20 times greater than those reported for Site 1108 samples from 121 and 218 mbsf (Bolton et al., 2000). Temperatures observed during logging of Site 1118, at the margin of the rift basin, suggested the occurrence of a thermal anomaly at depth, which may be an indication of lateral and upward fluid migration from the basin (Shipboard Scientific Party, 1999). Lateral fluid migration could lower pressures within the basin and transfer overpressures to the basin margins.