We recognized five lithostratigraphic units at Site 1174 (Fig. F1) and were able to correlate these with units previously described at Sites 808 and 1173 (Table T3). Unit boundaries are diachronous between the sites.
Unit I is Quaternary in age and extends from the seafloor to a sub-bottom depth of 4.00 mbsf (Fig. F1; Table T3). The dominant lithology is brown to gray mud (silty clay to clayey silt). A pale brown glass-rich ash layer at 3.38 mbsf is normally graded and 22 cm in thickness. The mud is generally structureless and contains clay minerals with less abundant glass, lithic fragments, quartz, feldspar, siliceous microfossils (mostly diatoms), and calcareous nannofossils (mostly coccoliths) (see "Site 1174 Smear Slides"). Deposition occurred ~50 m above the trench floor by way of hemipelagic settling and, perhaps, muddy turbidity currents.
Unit II is Quaternary in age and 479.23 m in thickness (Fig. F1; Table T3). Subunit IIA (axial trench-wedge facies) consists of 310.55 m of sand, silty to muddy sand, silt to sandy silt, silty clay to clayey silt; one very thin bed of pale gray volcanic ash is at 257 mbsf. We drilled through an inferred sand-rich interval without coring from 67.43 to 143.70 mbsf. Recovered beds of black, dark gray, and greenish gray sand and silty sand range from thick (30-100 cm) to very thick (>100 cm), typically with normal grading (Fig. F2), sharp bases, and gradational tops. Grain size varies from silt to granule (Fig. F3) but is predominantly medium to coarse sand. Thicker intervals of sand are typically soupy. Layers of sandy silt and silt range from medium bedded (10-30 cm) to very thin bedded (1-3 cm) or laminated (<1 cm). Lower contacts are sharp, scoured, or loaded, whereas tops are gradational. Grain size typically fines upward, and plane-parallel laminae are common in the upper parts of beds. 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 1174 Smear Slides"). Fine-grained sulfide and framboidal pyrite are present throughout Subunit IIA as a black mottling.
Hemipelagic settling and fine-grained turbidity currents probably caused sedimentation of muddy interbeds in Subunit IIA. The sandy beds are turbidites. Their overall seismic-stratigraphic position and textures and bed thicknesses are all consistent with deposition in the axial portion of the trench wedge. The upper 18 m of the trench wedge is lithologically similar to Unit I at Site 808 (Shipboard Scientific Party, 1991) but does not contain contorted or overturned beds indicative of slump folding. Incipient deformation in the protothrust zone has lifted strata ~50 m above the trench floor. Thus, we cannot rule out the possibility that some of the silty sand beds assigned to Subunit IIA were deposited from axial flows that lapped onto the lowermost slope, and the change to Unit I may be transitional.
The top of Subunit IIB is at the base of a thick silty sand bed at 314.55 mbsf (Section 190-1174B-19R-1, 55 cm). This subunit consists predominantly of silty clay to clayey silt (Fig. F4) with laminae to medium beds of silt and sandy silt. Laminae of volcanic ash are also present. Subunit IIB is very similar to Subunit IA at Site 1173; both probably were deposited in the outer trench-wedge environment.
The top of Subunit IIC (trench to basin transition) is located at the top of a prominent ash bed at 431.55 mbsf (Section 190-1174B-31R-2, 105 cm). This subunit consists of silty claystone to clayey siltstone, volcanic ash, and thin beds of laminated silt to sandy silt. This subunit is equivalent to Subunit IB at Site 1173 and Unit III at Site 808. The base of Subunit IIC is defined by the deepest occurrence of a discrete silt bed (>1 cm) at 483.23 mbsf (Section 190-1174B-36R-5, 73 cm). Thinner silt laminae are sporadically present deeper in the hole.
Unit III is Pliocene to Quaternary in age and 177.76 m in thickness (Fig. F1; Table T3). This unit consists predominantly of silty claystone to clayey siltstone with interbeds of volcanic ash (Fig. F5) and is equivalent to Unit II at Site 1173 and Subunit IVA at Site 808. The deepest unequivocal ash bed at 660.99 mbsf (Section 190-1174B-55R-2, 49 cm) defines the base of Unit III. This particular ash bed is somewhat unusual in that calcareous nannofossils are mixed with unaltered and partially altered volcanic glass. One other deposit of mixed nannofossil-rich mud and volcanic glass is present in Unit III. Many of the ash beds in Unit III contain partially altered volcanic glass, and the finer-grained examples display more alteration than coarser-grained beds. This alteration pattern differs from that at Site 1173, where only the lower portion of the upper Shikoku Basin facies showed signs of ash alteration.
Unit IV is Miocene to Pliocene in age and consists of 441.46 m of mostly bioturbated silty claystone to clayey siltstone (Figs. F6, F7), with minor calcareous and siliceous claystone (Fig. F1; Table T3). Most of the siliceous claystones are probably altered ash beds because they contain particles of cryptocrystalline silica, smectite, zeolite, and opaque minerals (Table T4). The boundary between Unit III and Unit IV, therefore, is controlled partially by diagenetic alteration, as also documented at Sites 1173 and 808. Attempts to correlate ash stratigraphy at Sites 1173 and 1174 met with limited success (Fig. F8). The total thickness of ash-rich Subunit IB and Unit II at Site 1173 (261 m) is greater than the equivalent thickness of Subunit IIC and Unit III at Site 1174 (229.44 m). This discrepancy, together with the smaller number of identifiable ashes at Site 1174, indicates that the diagenetic front of ash alteration migrates upsection as the Shikoku Basin strata are buried progressively beneath the axial trench wedge. Thickness variations also may have been affected by variable sedimentation rates within Shikoku Basin.
Unit V begins at a depth of 1102.45 mbsf (Section 190-1174B-102R-CC, 26 cm) and consists of 8.86 m of variegated silty claystone (Fig. F1; Table T3). The probable age is middle Miocene, but recovery from this interval was very poor. The claystone ranges in color from greenish gray to mottled green and red. Even though silicic tuff beds were not recovered, we believe this variegated hemipelagic claystone represents the top of the same volcaniclastic facies as encountered at Site 808 (Shipboard Scientific Party, 1991).
The results of X-ray diffraction (XRD) analyses of randomly oriented bulk-sediment powders are shown in Figure F9, and all data are listed in Tables T5 and T6. The average values of normalized relative mineral abundance in Subunit IIA are quartz = 35%, plagioclase = 19%, calcite = 1%, and total clay minerals = 45%. These values are typical of the entire trench-wedge facies. Quartz content decreases slightly within Unit III (mean = 37%), whereas calcite content increases (mean = 4%). A subtle increase in quartz content occurs below the Unit III/IV boundary; this compositional gradient is followed by a reduction in quartz below ~700 mbsf. Total clay mineral content, conversely, increases gradually below the trench-wedge deposits. Average contents of total clay minerals are 46% in Unit III and 49% in Unit IV. This modest increase in clay mineral abundance is probably a consequence of both diagenetic alteration of disseminated volcanic glass and a decrease in particle size within the hemipelagic mudstones of the Shikoku Basin. Plagioclase content decreases steadily beneath the base of Unit II, probably in response to an overall decrease in grain size. Calcite content is erratic in Unit IV as a result 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. F9). The pattern of silica alteration is more complicated at Site 1174 than at Site 1173 because the effects of burial beneath the Nankai trench wedge are superimposed on trends inherited from burial and heating in the Shikoku Basin. Smear-slide observations show that diatoms become increasingly scarce in Cores 190-1174B-18R and 19R (305-315 mbsf) and disappear in Core 20R (~325 mbsf) and below. The inferred temperature range for that depth interval is 50°-60°C. The cristobalite to quartz ratio decreases toward and beneath the base of Unit II then increases at ~605 mbsf. The peak-area ratio decreases again at ~660 mbsf just below the boundary between Units III and IV.
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 or bentonite (Table T4). Some of the samples analyzed are nearly pure smectite (Fig. F10). A second common alteration product within the lower portion of the upper Shikoku Basin facies is undifferentiated clinoptilolite-heulandite. Similar alteration products were documented at Site 808 (Shipboard Scientific Party, 1991). It is interesting to note, however, that zeolites are not present in the altered ash layers from Hole 1173A (see "Site 1173 Smear Slides"). This difference may be a function of either reaction temperature or reaction time.