We recognized five lithostratigraphic units at Site 1177 (Fig. F1). Table T3 shows their relation to correlative units at ODP Sites 808, 1173, and 1174.
Unit I is Pliocene in age and at least 101 m thick. The unit extends from 300.20 mbsf, where coring began, to 401.76 mbsf (Section 190-1177A-11R-5, 6 cm). Unit I consists of weakly indurated hemipelagic mudstone (silty claystone to clayey siltstone) interlayered with thin beds of volcanic ash (Fig. F2). The ash contains fresh or partially altered shards of volcanic glass. The base of Unit I is located below the deepest volcanic ash with minimal alteration. Unit I at Site 1177 is equivalent to the upper Shikoku Basin facies at Sites 808, 1173, and 1174 (Table T3).
Unit II is early Pliocene(?) to late Miocene in age and 47.54 m thick (Table T3). An interval barren of microfossils makes identification of the Pliocene/Miocene boundary uncertain. We placed the lithofacies boundaries at 401.76 mbsf (Section 190-1177A-11R-5, 6 cm) and 449.30 mbsf (Section 16R-4, 90 cm). Unit II consists of hemipelagic mudstone (silty claystone to clayey siltstone) with a few scattered laminae of siliceous claystone. The transition into Unit II coincides with a sharp decrease in amount of volcanic ash, but there is also a superimposed effect of mudstone compaction and diagenesis, as shown by porosity data (see "Physical Properties"). The mudstone is more strongly indurated than equivalent strata in Unit I, and X-ray diffraction (XRD) data show higher concentrations of expandable clay minerals (Table T4). The lower unit boundary occurs at a contact between hemipelagic mudstone and sandy turbidites (Fig. F1). Unit II at Site 1177 is equivalent to the upper part of the lower Shikoku Basin facies at Sites 808, 1173, and 1174 (Table T3).
Unit III is 327.28 m thick and early to late Miocene in age (Fig. F1). Microfossils are sparse. The oldest dated horizon (691 mbsf) is 17.3-17.9 Ma (see "Biostratigraphy"). An equivalent turbidite facies does not exist at Sites 808, 1173, or 1174 (Table T3). The boundaries of Unit III are located at the top and bottom of two turbidite sand packets (Sections 190-1177A-16R-4, 90 cm, and 47R-5, 25 cm). Interbeds of sand, silty sand, and hemipelagic mudstone are the most characteristic feature of Unit III; these couplets occur together with scattered beds of gravel, mudstone-clast conglomerate, carbonate-cemented claystone, and siliceous claystone. The sandy deposits are compacted but not cemented; this weak state of induration inhibited core recovery. Four discrete packets of sand-rich strata are present at depths of 449.30-472.80, 483.90-521.50, 540.10-569.06, and 598.16-748.35 mbsf (Fig. F1).
The hemipelagic mudstone typical of Unit III (silty claystone to clayey siltstone) has pronounced color banding, with colors varying from greenish gray to brownish gray, brown, and green. The colors, in part, reflect variations in the content of expandable clay minerals (Table T4). Higher contents of smectite also resulted in considerable amounts of sediment expansion once cores were split. Strata range from structureless to plane-parallel laminated or mottled by bioturbation. Zoophycos (Fig. F3) and Chondrites trace fossils are present locally. The mudstone is composed predominantly of clay minerals, silt-sized quartz, feldspar, sedimentary or metasedimentary lithic fragments, and minor amounts of volcanic glass (see "Site 1177 Smear Slides"). Calcareous nannofossils are scarce. Disseminated wood and plant fragments are also present (Fig. F4), especially in the lower part of the unit. Laminae and thin beds of siliceous claystone vary in color from dark gray to dark green and probably formed by alteration of volcanic glass. Also scattered throughout the unit are thin beds of pale brown carbonate-cemented silty claystone and sand. Some of these zones have sharp contacts and are probably layered concretions.
Deposits of silty sand, sand, gravel, and muddy sand range from laminae to thick beds. Lower contacts are sharp, and upper contacts are diffuse. Normal size grading is typical. Plane-parallel laminae are common in the upper parts of beds, and ripple cross-laminae are present locally (Fig. F5). The sandy deposits contain subrounded to angular grains of quartz, feldspar (including microcline), fine-grained metamorphic and sedimentary rock fragments, volcanic rock fragments, rare volcanic glass, and nannofossils (see "Site 1177 Smear Slides"). Perhaps the most striking characteristic of Unit III turbidites is the unusual abundance of woody plant material. The organic matter usually is concentrated in the upper parts of sand beds together with intraformational mud clasts (Figs. F5, F6, F7). Core 190-1177A-46R contains a noteworthy example of mudstone-clast conglomerate with angular clasts, disorganized fabric, no size grading, and sandy matrix. Maximum clast size is 5-6 cm. Immediately above the conglomerate, numerous rip-up clasts of brown and green mudstone are dispersed through beds of sandy silt and sandy mud.
Unit III was deposited through a combination of hemipelagic settling, siliciclastic turbidity currents, and debris flows. The paucity of calcareous nannofossils in hemipelagic interbeds implies that the sediment was deposited close to or below the calcite compensation depth. Nannofossils are more common in the turbidites because of their rapid deposition. The abundance of quartz, sedimentary and metasedimentary lithic fragments, and woody organic matter indicates that the turbidites were derived from a landmass of significant size. Higher contents of disseminated smectite in the hemipelagic mudstones also indicate frequent influx of pyroclastic debris. The most likely detrital provenance is the central to southwest portion of Japan. That geologic domain was probably emergent during the middle to late Miocene as indicated by the presence of a major unconformity surface (Kano et al., 1991). Unlike the modern Nankai trench-wedge facies, however, the Miocene turbidites of the Shikoku Basin must have spread out over an impressive surface area of the abyssal floor. Transverse sediment dispersal probably occurred through a broad system of coalescing submarine fans instead of a narrow axial-channel system typical of the modern trench.
Unit III at Site 1177 is lithologically similar to Unit 4 at Deep Sea Drilling Project (DSDP) Site 297 (Shipboard Scientific Party, 1975b), but there are also some important differences. Site 297 is located ~85 km to the south, and the turbidite section there is ~240 m thick (as compared to 299 m at Site 1177). Barren specimens precluded identification of the Pliocene/Miocene boundary at Site 297, but the upper part of Unit 4 does contain early Pliocene microfossils. In comparison, the age of the turbidite succession at Site 1177 falls entirely within the Miocene. Thus, the available biostratigraphic control shows that the two turbidite successions are not the same age. Considerable overlap of correlative Miocene strata seems likely between the respective units, but this inference cannot be proven using existing biostratigraphic data.
Farther to the northeast, Sites 1173 and 1174 are located above a structural high in the igneous basement that is associated with the Kinan Seamount chain. Temporal equivalents to the Miocene turbidite facies within that transect area are composed exclusively of hemipelagic mudstone. Evidently, seafloor relief along higher segments of the seamount chain was enough to prevent upslope deposition by sandy turbidity currents. We also note, however, that seismic reflection data from southeast of Site 1173 show an expanded Miocene section above a basement low along the axis of the seamount chain (Fig. F6 in the "Data Report: Structural Setting of the Leg 190 Muroto Transect" chapter). Continuous high-amplitude reflectors within this interval of the reflection profile could represent lateral equivalents of the lower Shikoku turbidite facies, as recovered at Site 1177. If this interpretation is correct, the Kinan Seamount chain did not form a continuous barrier to sediment gravity flows moving from the paleomargin of central Japan toward the southeast.
Unit IV is early Miocene in age and 82.73 m thick. The unit extends from 748.35 mbsf (Section 190-1177A-47R-5, 25 cm) to 831.08 mbsf (Section 56R-3, 0 cm). Its upper part (748.35-766.58 mbsf) consists predominantly of silty claystone with local silt laminae. The most distinguishing characteristics of Unit IV include the common occurrence of relatively fresh volcanic ash (Fig. F8), variegated mudstone to claystone with disseminated volcanic glass, and sandy to silty siliciclastic and volcaniclastic beds without terrigenous organic matter. Small-scale recumbent folds and chaotic bedding also indicate that slumping has affected some of the finer-grained sediments (Fig. F9). Examples of soft-sediment deformation are present in the following intervals: 190-1177A-50R-1, 67-74 cm, and 110-116 cm; 50R-2, 18-31 cm; 55R-2, 112-115 cm; and 55R-3, 16-20 cm.
The hemipelagic mudstone to claystone of Unit IV varies in color from brownish gray to greenish gray and green. Variations among tones of green are vivid, and many of the boundaries among thicker color zones are sharp. As in Unit III, these color variations are caused by changes in clay mineralogy (Table T4), and high concentrations of smectite caused significant amounts of core expansion upon splitting. The fine-grained deposits range from massive to laminated or bioturbated, and Zoophycos trace fossils are present locally. The mineral constituents are predominantly clay minerals. Disseminated volcanic glass is also common, as are quartz, feldspar, and lithic fragments (see "Site 1177 Smear Slides"). Microfossils are less common, but nannofossils are present in the lower part of the unit.
Most of the volcanic ash deposits are in the middle of Unit IV between 766.58 mbsf (Section 190-1177A-49R-4, 78 cm) and 793.33 mbsf (52R-3, 3 cm). These ash layers are interbedded with massive mudstone-claystone and laminated glass-rich silt. The volcanic ash is present in beds as thick as 18 cm (Fig. F2). Color varies from white to green, grayish green, and brown. The ash beds have sharp bases, diffuse to bioturbated tops, and plane-parallel laminae (Fig. F8). The ash contains clear, fresh volcanic glass, as well as some shards that are partially altered to clay. Most of the crystal constituents are composed of quartz and feldspar (Table T5). The lower part of Unit IV, between 793.33 and 831.08 mbsf, consists of massive or laminated silty claystone interbedded with volcanic ash and both siliciclastic and volcaniclastic sand to silt (interval 190-1177A-52R-3, 3 cm, to 56R-3, 0 cm). These weakly lithified, relatively coarse-grained lithologies vary in thickness from a few millimeters to 70 cm. The thicker silt and sandy mudstone beds are typically laminated, with sharp bases and normal grading. Composition of the silt- and sand-sized fragments varies considerably. Most of the sands contain abundant volcanic glass mixed with quartz, clay minerals, and altered volcanic lithic clasts. Some of the silts also contain abundant fragmented glass shards, whereas others are rich in quartz, altered lithic clasts, and clay minerals, with only traces of volcanic glass (see "Site 1177 Smear Slides").
Unit IV formed through a combination of hemipelagic settling, fallout of volcaniclastic material, turbidity currents, and local remobilization through slumps. Nannofossils in the lower part of the unit also indicate that deposition occurred above the calcite compensation depth. This relatively shallow depth is consistent with deposition above juvenile igneous basement close to the Shikoku Basin spreading ridge.
The volcaniclastic-rich facies at Site 1177 appears to be correlative with Unit 5 at DSDP Site 297 (Shipboard Scientific Party, 1975b). Fine-grained vitric ash and ash-rich claystone at Site 297 is also late early Miocene to middle Miocene in age; basal sediments at Site 1177 are 18.6-23.2 Ma. On the other hand, coring at Site 297 was terminated ~80 m above igneous basement. This means that the probable thickness of the ash-rich claystone unit is nearly 210 m at Site 297, whereas the cored thickness at Site 1177 is only 82.73 m.
A true stratigraphic equivalent to Unit IV does not occur within the eastern Nankai Trough transect area that includes Sites 1173 and 1174. The rhyolitic tuff deposits at Site 808, although similar in some lithologic respects, are no older than 13.6 Ma (Shipboard Scientific Party, 1991). The volcaniclastic facies at Site 808, moreover, contains impressively thick beds of coarse-grained white to gray tuff. The early Miocene ash beds at Site 1177, in contrast, are older (by 5-9 m.y.), thinner, finer grained, and interbedded with volcanic and siliciclastic silt and sand. These differences make a rigorous stratigraphic correlation between the two units dubious.
Periodic influx of volcaniclastic material certainly affected the early stages of sedimentation within the western Shikoku Basin, but the source area of this volcanic and siliciclastic material remains uncertain. One possibility for the detrital source is the geologic domain of central to southwest Japan. Provenance of the sandy deposits of Unit IV, however, differs from the provenance of Unit III in that volcanic glass is common to abundant in many such beds and terrigenous plant debris is scarce. Another possible detrital source is the Kyushu-Palau Ridge; this remnant arc is Paleogene in age and forms the western edge of Shikoku Basin (Karig, 1971). The oldest sedimentary strata recovered from DSDP Site 296 on Kyushu-Palau Ridge consist of lower(?) to upper Oligocene volcanic tuff, lapilli tuff, volcanic sandstone, and siltstone (Shipboard Scientific Party, 1975a). Based on this similarity in overall lithology, we suggest that the quartz and lithic-rich sand-silt beds of Unit IV originated from subaerial weathering of volcanic and volcaniclastic rocks that were exposed on the same subsiding remnant arc during early Miocene time.
Basalt of Unit V is probably early Miocene in age. We cored only 1.05 m into igneous basement, from 831.08 mbsf (Section 190-1177A-56R-3, 0 cm) to 832.13 mbsf (56R-3, 105 cm). The first occurrence of basalt displays a sharp contact with altered silty claystone (Fig. F10). The basalt is dark gray to black in color and aphyric to sparsely plagioclase phyric. The upper 30 cm of basalt has upper and lower chilled margins that are ~1 cm thick and concave. We interpret this geometry to be part of a pillow structure. A third chilled margin is straight. Alteration imparted a yellowish brown and grayish purple color to the interior and rim of the uppermost basalt. Fibrous veins of calcite and chlorite are common. We believe that the basalt formed through submarine lava flows and/or injection of sills near the spreading ridge that created the oceanic lithosphere of Shikoku Basin.
The results of XRD analyses of bulk-sediment powders are shown in Figure F11. Data for two ash layers are in Table T5. Normalized relative mineral abundances within Unit I are similar to what we documented in equivalent deposits at Sites 1173 and 1174 (i.e., upper Shikoku Basin facies). Average values of total clay minerals, quartz, plagioclase, and calcite are 48%, 34%, 14%, and 3%, respectively (Table T6). The content of total clay minerals increases in Unit II to an average of 55%, and there is also an apparent decrease in the amount of cristobalite relative to quartz. This compositional boundary between Units I and II is fundamental because it also shows up as a pronounced shift in porosity values (see "Physical Properties"). The scattered values of all constituents within Unit III are a consequence of interstratification among hemipelagic mud, turbidite mud and silt, and rare carbonate beds. The average content of total clay minerals within Unit III is 53%, but smectite content varies erratically (Table T4). The total clay value increases to 59% within Unit IV. In addition, Unit IV displays a significant increase in the ratio of cristobalite to quartz. Alteration of disseminated volcanic glass within the mudstone and claystone of Unit IV probably caused the apparent increase in cristobalite and total clay, particularly smectite-group minerals.