We recognized three lithostratigraphic units at Site 1175 (Fig. F1). All units represent stages of sedimentation during the creation and deformation of a small basin on the lower trench slope.
Unit I is Quaternary in age and extends from the seafloor to a depth of 224.75 mbsf (Table T3). This unit consists predominantly of nannofossil-rich hemipelagic mud (silty clay to clayey silt) interlayered with volcanic ash and a few thin beds of sand, silty sand, clayey sand, and silt. Contorted stratification is conspicuous. The base of the unit is above a 55-cm-thick clayey sand (Section 190-1175-25X-1, 15 cm).
The hemipelagic mud in Unit I is gray, greenish gray, or olive green in color and homogeneous, faintly laminated, or mottled as a result of bioturbation. The composition includes abundant clay minerals and biogenic debris, together with lesser amounts of volcanic glass and lithic fragments (see "Site 1175 Smear Slides"). Diatoms and sponge spicules are more common in the upper part of the unit, whereas nannofossils are more abundant in the lower part.
Volcanic ash beds in Unit I vary in thickness from <1 cm to >2 m (Fig. F2). The thickest ash deposit occurs near the top of Core 190-1175A-11H (92 mbsf) but was not recovered completely. Ash layers typically have sharp plane-parallel to irregular lower contacts and gradational upper contacts (Fig. F3). Color variations among and within individual ashes range from pale gray to dark gray, white with dark grains (salt and pepper), brown, pink, and greenish gray. Grain size varies from lapilli to fine ash; a grain-size range of 0.1-2.0 mm is most common. The volcaniclastic sediment is composed primarily of fresh glass shards or pumice plus variable amounts of nannofossil-rich silty clay, lithic clasts, and crystals of quartz, plagioclase, and pyroxene (see "Site 1175 Smear Slides").
Deposits of sand, clayey sand, and silt are present mainly as thin beds to laminae, but there are also a few thick-bedded units. These coarser-grained deposits have sharp bases, gradational tops, and normal grading (Fig. F4). The sand is coarse to fine grained and poorly sorted and includes foraminifers, siliceous and calcareous microfossils, quartz, feldspar, pyroxene, amphibole, volcanic glass, chert, and low-grade metasedimentary rock fragments (see "Site 1175 Smear Slides").
Contorted and disrupted stratification is the most noteworthy feature of Unit I. Common manifestations of soft-sediment deformation include steeply inclined stratification (beds and laminae) and small-scale recumbent to inclined folds with overturned beds (Fig. F5). In some cases, the hinge zones of these folds are exposed across the core face. Extreme cases of stratal dismemberment have resulted in curviplanar to irregular-shaped fragments of nannofossil-rich mud and ash engulfed in a matrix of contorted mud that is slightly different in color (Fig. F6). There are eight discrete zones of soft-sediment deformation and chaotic folding in Unit I ranging from 3 to >10 m in thickness.
The overall facies character of Unit I is consistent with conceptual models of deposition in a trench-slope basin that is isolated from the influx of coarse terrigenous sediment (e.g., Underwood and Moore, 1996). Sedimentation occurred initially through hemipelagic settling of nannofossil-rich mud, occasional turbidity currents, and air falls of volcanic ash. A higher concentration of calcareous nannofossils in the hemipelagic mud is consistent with deposition above the calcite compensation depth. The impressive zones of chaotic stratification evidently were caused by local remobilization of hemipelagic sediment. One likely triggering mechanism for such intraformational mass wasting is seismogenic loading. Another factor is the intrinsic gravitational instability of mud accumulating on steeply inclined bounding ridges of the slope basin, especially during episodes of uplift and tilting associated with offsets along out-of-sequence faults. Following the classification scheme of mass movements described by Martinsen (1994), we recognize a range of core-scale features that span a continuum from slump (coherent mass with considerable internal deformation) to debris flow (remolded mass with plastic behavior). These designations are scale dependent, however, and there is no evidence of internal deformation within the failed masses at the scale depicted by seismic reflection profiles.
Unit II is Quaternary in age and extends from 224.75 mbsf (Section 190-1175A-25X-1, 15 cm) to 301.64 mbsf (Section 33X-1, 4 cm). Lithification of the fine-grained strata is more advanced than in Unit I, and there is an absence of features ascribed to soft-sediment deformation. The most distinctive lithology within Unit II is greenish gray sandy mudstone (Table T3). This relatively unusual type of sediment is poorly sorted and generally structureless. Faint dark green bands are present locally. Grain constituents include abundant nannofossils, clay minerals, monocrystalline quartz, volcanic glass, feldspar, metasedimentary lithic fragments, and polycrystalline quartz (see "Site 1175 Smear Slides"). Other lithologies within Unit II include a more typical hemipelagic mudstone (silty claystone to clayey siltstone), volcanic ash, and rare beds of sand. The hemipelagic mudstone is greenish gray to gray and massive, faintly laminated, or mottled as a result of bioturbation, with scattered Zoophycos and Chondrites trace fossils. Pyrite nodules, rounded pumice fragments, and sand patches are present locally. Diatoms, radiolarians, silicoflagellates, and sponge spicules are also present, but the percent of carbonate is generally lower than in Unit I. Thin beds and laminae of volcanic ash are scattered throughout Unit II (Fig. F2) and vary in color from pale brown and pale gray to dark gray. Most of the pyroclastic particles are composed of fresh volcanic glass with some crystals of plagioclase, quartz, amphibole, and pyroxene (see "Site 1175 Smear Slides"). In some cases the ash is intermixed with terrigenous silt, clay, and calcareous nannofossils; we view this mixing as indicative of remobilization during and/or after ash-fall events.
The most obvious distinction between Units I and II is the absence of chaotic stratal disruption within the middle slope-basin facies. Based on this contrast, we infer that the relief and seafloor gradients of nearby slopes were more subdued during the basin's intermediate phase of sedimentation. The distinctive sandy mudstone deposits were probably transported as relatively fine-grained debris flows or mudflows, with the sand-sized clasts fully supported by the muddy matrix. Delivery of the siliciclastic sand from detrital sources on Shikoku appears likely, based on compositional similarities with rock types currently exposed in the Outer Zone of Japan. Transport of sandy mud beyond the shelf edge could have occurred within transverse channels or as unconfined mudflows.
The most distinctive feature of Unit III is the common occurrence of thin- to medium-bedded silt and sand turbidites (Table T3; Fig. F1). The top of Unit III is located just above a package of seven such sand beds (Section 190-1175A-33X-1, 4 cm). The deepest core within this unit is from 435.4 mbsf (Section 190-1175A-47X-CC, 30 cm). The most noteworthy lithology, although less common, is muddy gravel to pebbly mud. Hemipelagic mudstone and/or sandy mudstone is typically present between the coarse-grained beds.
The sandy mudstone and hemipelagic mudstone lithologies in Unit III are typically greenish gray, gray, or green in color, and they may be homogeneous, faintly laminated, or mottled as a result of bioturbation. Zoophycos and Chondrites are present. Microfossils are generally less common than in comparable deposits of Unit II, although nannofossils are abundant locally. Rare laminae and thin beds of light gray vitric ash are scattered through the unit.
The sand and silty sand beds of Unit III are poorly indurated and thin to medium bedded. Plane-parallel laminae are the most common internal sedimentary structure. Cross-laminae and normal grading are less common. Color ranges from medium gray to greenish gray. These deposits are poorly sorted; grain size varies from very coarse sand to silt. Grains are subrounded to angular. Lower bed surfaces are sharp, and most are plane parallel; upper bed contacts are diffuse and gradational. Compositionally the sand is quartz rich, with subordinate amounts of sedimentary or metasedimentary lithic clasts, chert, feldspars, and minor to rare volcanic glass (see "Site 1175 Smear Slides"). Woody plant fragments are common in some sands.
Five distinctive beds of poorly indurated muddy gravel to pebbly mud are present between 340 mbsf (Section 190-1175A-37X-1, 0 cm) and 347 mbsf (Section 37X-5, 118 cm). The pebbly mud is poorly sorted throughout, with a disorganized clast fabric, lack of internal stratification, and partial to complete support of clasts by a muddy matrix (Fig. F7). The matrix is composed of nannofossil-bearing silty clay, similar in composition to the typical hemipelagic deposits of Unit III. The clasts are rounded to subrounded and as large as 5.5 cm across. The polymictic population of clast lithologies includes abundant quartz, chert, and sedimentary to metasedimentary lithic fragments together with feldspar and minor volcanic clasts.
We infer that the sediments within Unit III were deposited by hemipelagic settling, mudflows, muddy turbidity currents, sandy turbidity currents, and pebbly debris flows. The clast population of the debris-flow deposits is consistent with a source that includes rock units currently exposed in the Outer Zone of southwest Japan (e.g., the Shimanto Belt). Deposition occurred in the lower part of a trench-slope environment, according to interpretations of seismic-reflection data. The original basin geometry and contact relations, however, have been obscured by subsequent tectonic disruption and intermixing across the basal unconformity of the slope basin. This process has been referred to as tectonic kneading (Scholl et al., 1980). Interpretation of the depositional environment for the lower part of the unit, therefore, remains ambiguous. Careful petrographic examination may reveal compositional differences between transverse and axial turbidite systems in the trench-wedge facies, although probably not between transverse-trench and transverse-slope turbidites.
Proportions of sandy turbidites and pebbly mud were high relative to hemipelagic setting during deposition of Unit III. Evidently, Site 1175 was adjacent to a through-going shelf to trench sediment conduit, perhaps a submarine canyon and trench fan system, early in its history. In the present day physiography of Nankai Trough, one nearby canyon and fan system of this type is Murotomisaki Canyon, ~10 km to the west of the Leg 190 drilling transect. A second example is Shionomisaki Canyon, which begins to the northeast off shore from the Kii Peninsula (Taira and Ashi, 1993). Submarine canyons are important for controlling sediment transport into both slope basins and trench-wedge environments (Underwood and Karig, 1980; Taira and Ashi, 1993). In general, however, newly formed basins lower on the trench slope are less likely to connect directly to the shoreline via submarine canyons; their facies associations have been described as immature and consist mostly of hemipelagic mud and locally derived slumps and mudflows (Underwood and Bachman, 1982). Conceptual models of slope-basin evolution predict that uplift and downslope canyon-channel erosion will combine to increase coarse-grained siliciclastic influx to basins as they mature (Underwood and Bachman, 1982). As a consequence, stratigraphic successions should thicken and coarsen upward (Moore and Karig, 1976; Underwood and Moore, 1996). Rerouting of the transverse sediment delivery system must have occurred upslope of Site 1175 during uplift and rotation of the slope basin. Thus, rapid growth of the accretionary prism resulted in termination of sandy siliciclastic input to the slope basin and an overall upward-thinning and upward-fining megasequence.
Based solely on the lithologic characteristics of Unit III, we are not able to locate a definitive boundary between lowermost trench-slope deposits and the uppermost accretionary prism. The preponderance of coarse-grained gravity-flow deposits, especially the gravel and pebbly mudstone from 340 to 347 mbsf, supports the notion of a relatively gentle seafloor gradient to promote deceleration and trigger deposition. The flat floor of the trench satisfies this notion better than a steeply inclined lower slope. On the other hand, some of the thin sand beds within the upper part of Unit III could have settled onto a sedimentary carapace several tens of meters above the uplifting prism as thicker turbidity currents moved across the trench floor and lapped onto the landward wall. To complicate the interpretation further, the slope to prism transition is not defined clearly by seismic reflection data. Thus, we suggest that the base of the trench-slope deposits is probably within the upper 20-40 m of Unit III.
The results of X-ray diffraction (XRD) analysis of bulk-sediment samples from Site 1175 are shown in Figure F8 (see Table T4 for peak-intensity and peak-area data). Calcite is the only mineral to show significant changes in relative abundance with depth. Carbonate content increases downsection within the nannofossil-rich hemipelagic deposits of Unit I (average = 25%). The average contents of total clay minerals, quartz, and plagioclase within Unit I are 36%, 28%, and 11%, respectively. Similar averages occur within Unit II (Table T5). Below the uppermost part of Unit III, calcite content decreases sharply to relative percentages of <12% (average = 10%). This reduction of carbonate within the turbidite-rich facies is probably due to dilution of biogenic-pelagic input by terrigenous silt and clay, as well as to deposition closer to the calcite compensation depth. Average contents of quartz and plagioclase increase within Unit III to 35% and 16%, respectively.
XRD analysis of selected ash layers shows that the degree of alteration of volcanic glass is minimal. Common crystalline minerals within the ash beds include quartz, plagioclase, amphibole, and pyroxene (Table T6). There are also small amounts of pyrite, calcite (from nannofossils), and halite (an artifact from evaporation of pore water). In general, unambiguous recognition of cristobalite is precluded by interference from a relatively strong plagioclase peak.