v' curves. Conversely, careful, extended experimental consolidation tests can provide valuable information not only concerning the in situ stress but also for other mechanical characteristics of marine sediments.
The consolidation tests on the single core sample available raised more questions about the stress history at Site 897 than they answered. Unfortunately, most of these questions could only have been answered with additional drilling and sampling, but the tests did strongly imply very high pore pressures at the base of the sediment section. As importantly, these tests revealed a number of differences between consolidation in the geological environment and under laboratory conditions. An understanding of these differences permits more realistic analyses of natural consolidation through laboratory testing.
The complications resulting from creep and cementation can account for many of the common deviations of observed porosity-depth curves from laboratory (experimental) consolidation curves or even from any exponential porosity-depth relationship. If uncemented or nearly so, deep marine sediments should tend to have in situ porosities less than their experimentally consolidated equivalents (Bryant et al., 1981), an effect that can be attributed to creep. However, with higher degrees of cementation, which is also a common condition, the in situ porosity of a sediment can be greater than that of its disaggregated equivalent (e.g., Hamilton, 1976). Small variations in cementation with depth are most probably the cause of large excursions from an exponential porosity-depth relationship that cannot be attributed to excess P.
This creep-cementation interaction might explain some of the porosity excursions at Site 897 but not all, and certainly not the radical porosity variations at the depths from which the test sample was collected. The sum of evidence points toward very high pore-fluid pressures over most of the stress history of the basal sediments. The hydrodynamic picture is incomplete, but fluid egress from the basement seems to be an important process during rifting.
If there is a moral to be drawn from this study, it is that there is more to the understanding of consolidation than can be derived from simple porosity or void ratio vs. log
v' curves. Conversely, careful, extended experimental consolidation tests can provide valuable information not only concerning the in situ stress but also for other mechanical characteristics of marine sediments.
