We recognized five lithostratigraphic units at Site 1173 (Fig. F3) and correlated them with the lower five of six units at Site 808 (ODP Leg 131), located 13.5 km to the northwest (Table T3).
Unit I is Quaternary in age and extends from the seafloor to Section 190-1173A-12H-1, 0 cm, at a sub-bottom depth of 102.14 mbsf (Fig. F3). We recognized two subunits within this part of the stratigraphic succession. Subunit IA (outer trench-wedge facies) consists of 83.37 m of silty clay interbedded with silt, sandy silt, silty sand, and rare beds of volcanic ash (Fig. F4). The dominant lithology is greenish gray silty clay to clayey silt. This muddy sediment is typically homogeneous but may be faintly laminated or mottled as a result of bioturbation. The mud includes abundant clay minerals, quartz, and feldspar, with lesser amounts of volcanic glass, lithic fragments, and a variety of diatoms, sponge spicules, and radiolarians (see "Site 1173 Smear Slides"). Layers of silty sand and silt range from medium bedded (10-30 cm) to very thin bedded (1-3 cm) or laminated (<1 cm); thick beds (>30 cm) are unusual. Lower contacts are sharp, plane parallel, scoured, or loaded. Grain size typically fines upward from medium sand to silt, and plane-parallel laminae are common in the upper parts of beds. Tops are gradational into overlying silty clay. The sand and silt deposits contain subrounded to subangular grains of quartz, feldspar, and lithic fragments together with lesser amounts of ferromagnesium minerals, volcanic glass, microfossils, and mica (see "Site 1173 Smear Slides"). Fine-grained sulfide and framboidal pyrite are present throughout Subunit IA as a black mottling. Laminae to thin beds of volcanic ash are rare; one notable example of crystal-rich ash (at 36 mbsf) is a 31-cm-thick bed with normal grading.
Hemipelagic settling and fine-grained (muddy) turbidity currents probably deposited the silty clay to clayey silt of Subunit IA. Calcareous nannofossils are common in the muddy turbidites because of their rapid deposition and burial near the calcite compensation depth; hemipelagic mud, in contrast, contains few preserved nannofossils. We also interpret the silt and sand beds as thin tubidites. The overall character of Subunit IA is consistent with its depositional position on the outer trench floor, marginal to the trench axis. The unit is equivalent to Subunits IIB and IIC documented at Site 808 (Shipboard Scientific Party, 1991).
The base of Subunit IB (trench to basin transition facies) is defined by the deepest occurrence of medium-bedded silty sand (Section 190-1173A-12H-1, 0 cm). The top of the subunit is defined by the uppermost interval containing multiple ash layers (Section 190-1173A-10H-1, 23 cm). Subunit IB is 18.77 m thick and contains silty clay with scattered interbeds of silt- to clay-sized volcanic ash and rare volcanic lapilli (Fig. F3). The greenish gray silty clay to clayey silt is typically structureless with faint laminae of darker green color and rare, normally graded laminae. Ash layers range from laminae to very thin beds, with the latter typically showing normal grading (Figs. F4, F5). Contacts with overlying clayey silt are gradational.
Subunit IB is stratigraphically transitional between the upper Shikoku Basin facies and the outer trench-wedge facies. Deposition occurred predominantly by hemipelagic settling interrupted by sporadic influx of pyroclastic particles and siliciclastic turbidites. Subunit IB is equivalent to Unit III at Site 808 (Shipboard Scientific Party, 1991).
Unit II is Pliocene to Quaternary in age and 343.77 m thick (Fig. F3). The most common lithology ranges in texture from silty clay to clayey silt and changes with increasing compaction to silty claystone and clayey siltstone. Interbeds of volcanic ash and tuff are common (Fig. F4). The deepest unequivocal ash bed (Section 190-1173A-37X-3, 3 cm) defines the base of Unit II (343.77 mbsf). Silty claystone to clayey siltstone in Unit II is typically massive or faintly laminated. Mottling caused by bioturbation is present throughout; Zoophycos trace fossils are common, and Chondrites trace fossils are rare. There are also scattered green laminae, pyrite nodules, clasts of pumice and scoria, and bundles of sponge spicules. The silty claystone of Unit II appears to be finer in grain size than equivalent lithologies of Unit I, and it contains fewer siliceous microfossils.
Volcanic ash and tuff layers are typically very thin bedded (1-2 cm) but are present in beds up to 25 cm thick with many beds 5 to 15 cm thick (Fig. F4). The ash layers have sharp, plane-parallel to irregular lower contacts and gradational upper contacts. Color varies considerably among and within volcanic ashes, ranging from pink, gray, and brown to green (Figs. F6, F7). Particle size varies from silt to coarse sand and gravel-sized lapilli. Many of the ash beds are normally graded although some coarsen upward and then fine upward (Fig. F6). Diffuse plane-parallel laminae are present in some ash layers. The volcanic debris is composed predominantly of clear, unaltered glass shards together with variable amounts of quartz, plagioclase, amphibole, opaque minerals, and pumice fragments (see "Site 1173 Smear Slides").
Unit II represents the upper part of the Shikoku Basin succession. Deposition probably occurred by hemipelagic settling interrupted by pyroclastic particle settling from air falls. The most likely sources of ash are the volcanic centers of Kyushu and/or Honshu. This unit is equivalent to Subunit IVA at Site 808 (Shipboard Scientific Party, 1991).
Unit III is Pliocene to middle Miocene in age and consists of 344.22 m of bioturbated silty claystone and minor calcareous and siliceous claystone (Fig. F3). The top of the unit is located immediately beneath the deepest unequivocal ash bed that contains fresh glass shards (343.77 mbsf). As at Site 808 (Shipboard Scientific Party, 1991), the upper tens of meters of the lower Shikoku Basin facies displays a gradational change from ash to siliceous claystone. Thus, the facies boundary is partially controlled by diagenesis.
The dominant lithology of silty claystone in Unit III is gray to greenish gray with local faint laminae. A characteristic mottled appearance was imparted by bioturbation. There are common Zoophycos and rare Chondrites trace fossils. Silty claystone of Unit III is devoid of siliceous microfossils, but calcareous nannofossils are locally abundant. Laminae to thin beds of siliceous claystone vary in color from pale gray to green and dark gray, and beds have sharp to diffuse or irregular contacts (Fig. F8). The siliceous claystone contains variable amounts of opaque grains, amphiboles, and zeolites in a groundmass of cryptocrystalline silica (see "Site 1173 Smear Slides"). In samples from the upper few tens of meters of Unit III, smear-slide observations show that some fragments of cryptocrystalline silica retain relict shapes indicative of altered volcanic glass shards. Also scattered throughout the middle and lower part of Unit III are pale brown carbonate-cemented intervals up to a few centimeters thick, as well as smaller nodules of calcite, dolomite, and siderite. We classified the layered intervals as carbonate-cemented claystone where they are continuous across the core width. The carbonate-rich beds have diffuse to sharp margins.
We regard the silty claystone of Unit III as a typical hemipelagic deposit that accumulated in the lower part of the Shikoku Basin. Two origins are likely for the carbonate-rich claystones: (1) diagenetic precipitation of carbonate cement to form nodules and (2) partial recrystallization of primary beds of nannofossil-rich pelagic ooze. Such primary beds of nannofossil chalk and calcareous claystone are relatively common in more distal portions of the Shikoku Basin (Shipboard Scientific Party, 1980), as well as on the nearby Kyushu-Palau Ridge to the west (Shipboard Scientific Party, 1975). The siliceous claystone also could have formed in more than one way. One possibility is diagenetic alteration of volcanic ash. We favor this interpretation where such components as opaque grains are common. Some of the more heavily altered examples, however, could be recrystallized beds of biogenic silica, as exemplified by coeval deposits in the Sea of Japan (Tamaki et al., 1990) and the Japan Trench (Shipboard Scientific Party, 1986b).
Unit IV begins at a depth of 687.99 mbsf (Section 190-1173A-73X-2, 15 cm) and consists of 36.90 m of variegated siliceous claystone and silty claystone (Fig. F3). The probable age is middle Miocene, but recovery from this interval was very poor and heavily disrupted by drilling disturbance. Greenish gray to mottled green, maroon, and red silty claystone is interlayered with slurried intervals of siliceous claystone that range from very thin to medium bedded, green to pale gray, and clay sized to medium sand sized. The silty claystone contains calcareous nannofossils, clay minerals, and cryptocrystalline silica. In contrast, siliceous intervals contain more cryptocrystalline silica and opaque minerals. Thin bands of claystone at the base of Unit IV (interval 190-1173A-77X-CC, 20-25 cm) are variably and irregularly colored from pale gray to yellow to reddish brown (Fig. F9), partly a result of the formation of iron oxide cements.
Silty claystone of Unit IV is probably a hemipelagic deposit, whereas the light gray siliceous intervals appear to be altered volcaniclastic deposits. This interpretation is consistent with the recovery of 47 m of thick-bedded silicic volcaniclastic deposits at Site 808. This volcanism was linked to a middle Miocene episode of anomalous near-trench magmatism, as currently exposed in the Outer Zone of southwest Japan (Shipboard Scientific Party, 1991).
The core catcher of Core 190-1173A-77X (724.89 mbsf) contains a 2-cm fragment of basalt (Fig. F9). By analogy with basalt from Site 808 (Shipboard Scientific Party, 1991), the probable age is middle Miocene (13-15 Ma). It is unclear whether or not this fragment was part of a basaltic breccia or lava flow. The basalt is porphyritic with euhedral plagioclase crystals up to 3 mm long in a partially altered glassy groundmass with abundant fine-grained radiating plagioclase crystals and clusters of oxides. Silty claystone of Unit IV, where adjacent to the basalt clast, displays a considerable amount of alteration, similar to sediment-basalt contacts documented in ridge-flank environments elsewhere (e.g., Davis et al., 1997).
The results of X-ray diffraction (XRD) analyses of randomly oriented bulk-sediment powders are shown in Figure F10 (and listed in Tables T4 and T5). The mean values of relative mineral abundance in Unit I are quartz = 35%, plagioclase = 17%, calcite = 2%, and total clay minerals = 46%. Quartz content decreases slightly within Unit II (mean = 33%), whereas calcite content increases (mean = 9%). A gradual increase in quartz content begins near the boundary between Units II and III; this compositional gradient is followed by a reduction in quartz below 515 mbsf. Total clay-mineral content increases abruptly below the Unit II/Unit III boundary (mean = 54%). Calcite content is erratic in Unit III because of scattered nannofossil-rich beds and nodules of carbonate.
The peak-area ratio of (101) cristobalite to (100) quartz also changes with stratigraphic position (Fig. F10). The cristobalite to quartz ratio increases at ~200-220 mbsf. This depth coincides with a temperature of ~40°C, and the change in mineralogy is probably due to the transition from opal-A (amorphous silica) to opal-CT (cristobalite). The cristobalite to quartz ratio decreases at ~320-340 mbsf near the boundary between Units II and III. The depth of this shift occurs at a temperature of ~60°C.
XRD analysis of representative volcanic ash beds shows a clear transformation downsection from glass-rich deposits with crystals of plagioclase and quartz to smectite-rich claystone (Table T6). Other common minerals include cristobalite, calcite (from nannofossils), pyroxene, halite (from pore water), and pyrite.