We recognized three lithostratigraphic units at Site 1176 (Fig. F1). In a general sense, all three units are equivalent to what was cored at Site 1175 (Table T3).
Unit I is Quaternary in age, extends from the seafloor to a sub-bottom depth of 195.79 mbsf, and is equivalent to Unit I and part of Unit II at Site 1175 (Table T3). This unit consists predominantly of nannofossil-rich hemipelagic mud (silty clay to clayey silt) interlayered with volcanic ash, thin beds of sand, silty sand, clayey sand, silt, and rare sandy mud. In contrast to Unit I at Site 1175, chaotic bedding with recognizable fold hinges (e.g., Fig. F2) is present in only one core (interval 190-1176A-7H-2, 115 cm, to 7H-4, 120 cm). Steeply dipping strata are present in a number of other cores (190-1176A-9H, 16H, and 19H), and these deformed intervals may have been caused by slumping.
The mud in Unit I is greenish gray in color and homogeneous, faintly laminated, or mottled (see "Lithostratigraphy" in the "Site 1175" chapter for a more detailed description). Diatoms become scarce in the lower part of the unit, but nannofossils are abundant throughout (see "Site 1176 Smear Slides"). Volcanic ash beds are common (Fig. F3). The thickest ash recovered (1.67 m) is located at the base of Core 190-1176A-18H (159.4 mbsf). The ash layers are similar to those at Site 1175 (Fig. F4). Sand, clayey sand, and silt are present mainly as thin beds, laminae, and a few thick beds (Fig. F5).
Processes responsible for the sedimentation of Unit I include hemipelagic settling, occasional turbidity currents, and submarine mudflows, together with air falls of volcanic ash. Chaotic deposits are less prevalent than at Site 1175; this is probably because Site 1176 is located on a nearly flat-lying part of the slope basin with modest relief nearby.
At Site 1175, Unit II was defined primarily by the presence of sandy mudstone throughout. This unusual lithology also occurs in Unit II at Site 1176, but in only two cores (intervals 190-1176A-22X-4, 139 cm, to 22X-7, 48 cm, and 25X-3, 67 cm, to 25X-4, 34 cm). The other lithologies of Unit II are a more typical hemipelagic mudstone (silty claystone to clayey siltstone) and rare beds of volcanic ash. Unit II is also considerably thinner at Site 1176 (27.75 m) than at Site 1175 with the top and base at 195.79 and 223.54 mbsf, respectively (Table T3). Sedimentation occurred by hemipelagic settling, muddy debris flow, and volcanic ash falls.
The most characteristic feature of Unit III is the common occurrence of thin- to medium-bedded silty or sandy turbidites. The top of Unit III is located where thin sand beds become abundant at 223.54 mbsf (Section 190-1176A-25X-4, 34 cm). Other lithologies include pebbly mudstone and gravel, sandy mudstone, and silty claystone. This unit is equivalent to Unit III at Site 1175 but contains more gravel and woody material. The deepest core within this unit is from 440.36 mbsf (Section 190-1176A-48X-CC, 36 cm).
Sand to silty sand is thin to thick bedded and moderately indurated. Typical internal sedimentary structures include plane-parallel laminae and/or cross-laminae (Fig. F6). Cross-stratification is particularly common in Cores 190-1176A-37X to 44X. Most such beds also display sharp to scoured bases, normal size grading, and gradational tops. All of these characteristics are consistent with transport by turbidity currents.
The gravel to pebbly mudstone of Unit III is present in Cores 190-1176A-34X to 36X and 43X to 48X. These poorly indurated deposits are poorly sorted throughout and contain clay- to pebble-sized particles (Fig. F7). The matrix is composed of silty clay, similar in composition to the finer-grained interbeds. Larger clasts are rounded to subangular and up to 5 cm across. Clast lithologies include abundant quartz, chert, sedimentary and metasedimentary lithic fragments, feldspar, and woody material. In addition, poorly lithified fragments of green siltstone probably originated as locally derived rip-up clasts.
The sandy mudstone of Unit III is typically greenish gray, gray, or green and homogeneous to faintly laminated or mottled by bioturbation. Nannofossils are less abundant than in the silty clays of Unit I (see description in "Lithostratigraphy" in the "Site 1175" chapter for more details). Unit III also contains rare laminae and thin beds of gray to brown ash (Fig. F3). These strata are composed predominantly of fresh volcanic glass. In addition, a single bed of carbonate-cemented claystone is present in interval 190-1176A-43X-2, 55-70 cm. This layer is probably a product of diagenetic alteration of nannofossil-rich claystone.
The depositional mechanisms and depositional setting for Site 1176 are basically the same as those described for Unit III at Site 1175. Based solely on lithologic characteristics, we are not able to draw a definitive distinction between lowermost trench-slope deposits and the uppermost part of the accretionary prism. Several interpretations need to be considered. One possibility is that the thin sand beds within the upper part of Unit III settled onto a sedimentary carapace above the uplifting prism as thicker turbidity currents moved across the trench floor and lapped onto the landward wall. Interpretations of seismic reflection data, on the other hand, suggest that the unconformity between slope facies and accretionary prism occurs at a depth of ~230 mbsf at Site 1176. This depth coincides reasonably well with the upper boundary of Unit III (223.54 mbsf). The preponderance of coarse-grained gravity-flow deposits within Unit III also favors a depositional site with subdued seafloor gradients to promote rapid deceleration and trigger deposition. The flat floor of the trench satisfies this expectation better than a steeply inclined lower slope. Another way to accommodate the high influx of coarse-grained detritus would be to initiate frontal accretion close to the mouth of a submarine canyon, then to sustain the flow path of the canyon through the newly formed slope basin as uplift of the accretionary prism continued. This uplift pattern would prevent the development of a clear facies change from trench to trench-slope environments.
The results of X-ray diffraction (XRD) analysis of bulk-sediment samples from Site 1176 are shown in Figure F8 and Table T4. Within Unit I, the average relative percentages of total clay minerals, quartz, plagioclase, and calcite are 38%, 27%, 11%, and 23%, respectively (Table T5). The content of total clay minerals does not change appreciably downsection. Quartz and plagioclase abundances within Unit III increase to average values of 41% and 17%, respectively. We attribute these changes in composition to a greater proportion of siliciclastic influx and coarser grain size within Unit III. Conversely, with one exception of carbonate-cemented claystone, the calcite content drops sharply below the upper boundary of Unit III (Fig. F8). Depletion of calcite is probably a function of deposition below the calcite compensation depth and dilution of the biogenic pelagic component of suspended sediment influx by terrigeneous silt and clay. Deposition of Unit III in deeper water (relative to Units I and II) is also consistent with the interpretation of the Unit II/III boundary as an unconformity between accreted trench-wedge deposits and slope sediments.
XRD analysis was completed on four volcanic ash samples from Site 1176 (Table T6). Three of the analyses produced unusual results. One sample (190-1176A-13H-3, 12-13 cm) contains an unidentified mineral with a high-intensity peak that corresponds to a d-value of 3.648 Å. A second sample (190-1176A-17H-3, 52-53 cm) is a crystal tuff with abundant plagioclase, quartz, and pyroxene; a relatively low intensity background on the diffractogram indicates that the content of volcanic glass is minor. The third unusual sample (190-1176A-18H-2, 68-69 cm) contains a large amount of well-crystallized mica in addition to crystals of quartz and plagioclase and abundant glass.