Deformation structures at Site 1177 are very sparse oceanward of the prism, as expected for a site designed to be a reference site. Bedding dips are <10° in the entire hole (Table T7). Intralayer folds and sedimentary breccias, seen mostly between 748 and 831 mbsf (lithostratigraphic Unit IV) (see "Lithostratigraphy"), are interpreted as early soft-sediment structures probably related to slope instabilities. The main later structure observed is a faulted interval from 579.45 to 581.10 mbsf that includes a healed normal fault and a few slickenlined fractures with greenish alteration walls (Fig. F12). Minor structures include weblike features with normal displacements at 502.45 mbsf in silty sand and stylolites at 537.00 mbsf in carbonate-cemented silty claystone.
The most surprising fact at Site 1177, compared with the other reference sites at the Nankai Trough (DSDP Site 582, Shipboard Scientific Party, 1986; also see "Structural Geology" in the "Site 1173" chapter), is the near absence of structures and dipping beds. The extreme scarcity of faults may be due to a differing response to compaction, either because of a slower rate of sedimentation above Unit III, slight differences in lithology, or differences in topography of the substrate. The virtually flat-lying beds throughout the hole and the lack of strongly disrupted intervals could be due to Site 1177 being farther oceanward with respect to the Nankai Trough compared to previous sites.
Approximately 1 m of basalt was recovered at the bottom of Hole 1177A at 831 mbsf. From 831.08 to 831.38 mbsf, the basalt pieces exhibit up to 1-cm-thick glassy rinds at the contact with overlying light green claystones (Fig. F10) and underlying coarser grained basalt. These uppermost basalt pieces show a set of fractures perpendicular to the glassy boundaries. In addition, a network of veins filled by varying proportions of calcite and chlorite is present in all the recovered basalt. Calcite veins tend to be subhorizontal or shallowly dipping but in places occupy higher angle fractures (Fig. F13). One of the veins reaches and crosscuts the contact with the sediments (Fig. F10). Calcite and chlorite veins and seams within the veins have crosscutting relationships, suggesting successive episodes of mineralization, but we were unable to determine a clear relative timing.
Data collected from the gas permeameter at Site 1177 are shown in Figure F14. Plotting the data on a logarithmic scale (Fig. F14A), as was done for the other sites, reveals the relatively fine-scale variations among the poorly gas-permeable materials. However, a linear plot shows that most measurements differ little from 10-16 m2 (Fig. F14B). The change in other physical properties reported at 410 mbsf is not reflected in the gas-permeability values. The appearance of sand with the facies change from Unit II to Unit III accounts for all the high measurements at the site. The carbonate-cemented claystones at 540 and 591 mbsf give slightly higher values, as do the altered ashes of Unit IV, but the increase is small. Numerous wood-bearing silty sands were measured in the lower part of Unit III but give identical results to the background hemipelagites. The one exception, at 696 mbsf, is unusually coarse, with wood fragments more than a centimeter long.