At Site 808, combining data from logs (intermediate-resolution measurements at in situ conditions,) and cores (high-resolution measurements at surface conditions) provides unique views into the processes operating in this active deformation zone (Fig. F1). Here we discuss preliminary interpretations from the logging results. These insights will undoubtedly be more focused once the data have been thoroughly processed, especially the density (and derived porosity) and sonic velocity measurements.
The borehole breakouts apparent in the RAB images suggest a maximum principal horizontal stress oriented at ~315°. This correlates well with both the shortening direction determined from the inversion of core-scale faults oriented at 305°-315° (Lallemant et al., 1993) and the convergence direction of ~310°-315° (Seno et al., 1993). The borehole breakouts are best developed bracketing the frontal thrust zone from 270 to 530 mbsf in log Unit 2. The breakouts are thus concentrated in a silty turbidite and hemipelagic mud unit, suggesting that their occurrence is in part lithologically controlled.
The gamma ray log shows two prominent lows at the base of log Unit 1 and near the base of the frontal thrust zone at ~414 mbsf. These log signatures correlate with a conglomerate bed that defined a frontal thrust zone offset of 145 m in the cores (Shipboard Scientific Party, 1991). The offset defined by the gamma ray log lows is ~150 m. The gamma ray low at ~410 mbsf is thinner than the low at 260 mbsf, probably because the former is partially cutoff by the frontal thrust zone that lies in a zone extending from 389 to 414 mbsf.
The frontal thrust zone shows a sharp and sustained increase in resistivity. Because the pore water shows no significant variations in composition through this zone (Shipboard Scientific Party, 1991) it is unlikely that the resistivity increase is caused by differing fluid composition. Rather, the increase in resistivity may indicate a densification of the rock unit caused by compactive deformation. The density curve in this interval shows large variations, with the lower values attributable to anomalously large hole size. However, the higher density values are not suspect and are consistent with densification occurring in the fault zone due to compactive deformation associated with thrusting.
The RAB images show concentrations of high- and low-angle apparent fractures in the frontal thrust zone. The apparent fractures show a mean strike perpendicular to the principal stress orientation estimated from the breakouts.
The top of the décollement zone, as determined from the RAB images, occurs approximately where the resistivity trend changes from gradually increasing to gradually decreasing. The pore water chemistry around the décollement zone shows no anomalies that would explain the decreasing resistivity trend. The decreasing resistivity trend is presumably a response to the increased amount of fluid-filled, unhealed fractures. Density also decreases over this interval, but part of this change is due to a slight enlargement of the hole and must be interpreted cautiously. Nevertheless, density measurements are consistent with the resistivity data, indicating an increase in bulk porosity over this interval. In contrast, core measurements of density increase in the décollement zone. Therefore, a combination of log and core measurements suggests that the décollement zone is an interval of enhanced porosity, probably fracture porosity, that encompasses blocks of sediment of relatively low porosity and high density. These lower porosity, higher density sediment blocks could have developed during episodic periods of deformation and drainage. Currently the fracture porosity of the décollement zone is probably held open by high fluid pressure, in contrast to the frontal thrust zone, which shows an overall densification and may not be as highly overpressured at present.
The base of the décollement zone and the underthrust sediment contact is marked by a sharp porosity increase that is also observed in cores. This porosity increase is apparent in both density (and derived porosity) and neutron porosity logs, although it is quantitatively suspect because of poor hole conditions.