Site 1176 was drilled to retrieve both slope-basin and accreted sediments. The deformational features of the slope-basin sediments are very similar to those at Site 1175, whereas deformational structures are commonly lacking in the accreted sediments, although core recovery was very poor. As shown in Table T7 and Figures F9 and F10, attitudes of bedding are quite different between lithostratigraphic Units I and II (upper and middle slope-basin facies) and III (accretionary prism).
In Units I and II (0-224 mbsf), horizontal and inclined bedding intervals alternate (Fig. F9). In the intervals of inclined bedding, the orientation of bedding exhibits a high degree of scatter (Fig. F10). Some of these inclined bedding intervals include slump folds (Fig. F2) and contorted and chaotically mixed bedding, indicating deformational features similar to the equivalent sedimentary facies at Site 1175. In addition, high-angle small faults are sporadically developed in Units I and II (Fig. F9). They are characterized by dark seams <6 mm across (Fig. F11). These small faults show both normal and reverse senses of movement and displacements up to 1 cm (Fig. F9), but displaced markers are commonly absent. Deformation bands displaying faultlike features are observed at 130, 146, and 149 mbsf (Table T7; Fig. F9). They consist of planar dark zones no more than 3 mm across, but no displaced markers are observed across them. A remarkable aspect of these features is that they are developed in quite soft, muddy sediments.
We again interpret that the inclined bedding intervals bounded by horizontal bedding intervals above and below resulted from slumping due to tectonic tilting and uplift of the slope basin associated with displacement along the seismically imaged thrust. Our interpretation of the small faults is the same as at Site 1175; they record burial compactional strains and/or extensional response related to tilting and uplifting of the slope basin.
Zones of locally intense fracturing and faulting, ~30 cm in thickness, are present across the facies boundary between Units II and III (Fig. F9). Unlike those at Site 1174, these zones are not inclined, organized fracture zones but instead consist of irregular fractures with various orientations, predominantly vertical or high angle. Below this facies boundary, core recovery was extremely poor and XCB biscuiting precluded observation of deformation structures. Where observed, bedding dips were consistently horizontal (Fig. F9). As Unit III probably includes accretionary prism sediments (see "Lithostratigraphy"), such flat-lying bedding was unexpected. One possible explanation for this horizontal orientation is that these accreted sediments at Site 1176 may correspond to the flat part of a hanging-wall anticline formed in association with the underlying thrust. The main thrust fault beneath Site 1176 appears to show flat-ramp-flat geometry in the three-dimensional (3-D) seismic reflection data (see "Seismic Stratigraphy"), and the few observed fractures with weakly developed slickensides could reflect the only modest deformation typical of a flat. Unfortunately this remains speculative, as the extremely poor recovery and high degree of XCB biscuiting prevent any unequivocal interpretation of deformation in the accreted sediments.
Results from the gas permeameter at Site 1176, again subject to the provisos outlined in "Structural Geology" in the "Explanatory Notes" chapter, are slightly different from those at other sites in that the range is even greater and there is no general decrease with depth (Fig. F12). The hemipelagic clays show greater scatter than at previous sites, between 10-15 and 10-17 m2, and the apparently high-permeability materials are more varied in nature. A black gravel at 34.13 mbsf is so permeable it was impossible to measure. Sandy layers high in the section also gave large values, but the greatest apparent gas permeabilities above 200 mbsf were yielded by bands of volcanic ash, typically between 10-11 and 10-13 m2.
In the lower half of the hole, from 200 mbsf to its base, the silty clays continue to range irregularly between 10-15 and 10-17 m2, but here the high values are given by sands and gravels. Many of the sand laminae are not represented in the data because they had been somewhat washed away during core preparation, giving a recessed surface to the split core face that was impractical to measure. However, sufficient determinations were possible to show that values in excess of 10-13 m2 were common even at these depths. In fact the deepest material recovered here, polymict gravels at 440 mbsf, gave results among the highest at the site.