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doi:10.2204/iodp.proc.322.103.2010 LithologyAt Site C0011, five lithologic units were identified on the basis of sediment composition, sediment texture, and sedimentary structures (Fig. F2; Table T3):
Lithologic Unit I (upper Shikoku Basin)
Lithologic Unit I is defined from LWD and seismic data (see "Logging and core-log-seismic integration") as extending from the seafloor to 340 m CSF (Section 322-C0011B-1R-1, 0 cm). The age of the uppermost part of Unit II indicates that Unit I ranges from Holocene to late Miocene (~0–7.6 Ma). Judging from the LWD data and correlation to other Ocean Drilling Program (ODP) sites in this region, Unit I is probably composed of silty clay(stone) with rare intercalation of volcanic ash layers. Lithologic Unit II (middle Shikoku Basin)
Lithologic Unit II (middle Shikoku Basin) is 139.06 m thick and extends from 340 to 479.06 m CSF (Fig. F2; Table T3). The dominant lithology is green-gray silty claystone, which alternates with medium- to thick-bedded volcaniclastic and tuffaceous sandstone interbedded with dark gray clayey siltstone. The green-gray silty claystones appear to be the "background sedimentation" and typically occur as intensely bioturbated intervals from centimeters to several meters thick (Fig. F3A). Within the silty claystone intervals, dark green claystone layers are typically <0.5 cm thick, and, where bioturbation has not obscured their presence to the naked eye, they are commonly spaced 5–10 cm apart. Partly based on LWD data (see "Logging and core-log-seismic integration"), the tuffaceous sandstones occur in intervals 2–10 m thick (Fig. F4). The tuffaceous sandstone beds commonly range from granular to fine-grained sandstone, and they are typically normally graded with plane-parallel lamination (Fig. F3B). Smear slide estimates show that the tuffaceous sandstones are composed of >25% pyroclasts, with magmatic minerals showing relative abundances of plagioclase > pyroxene > amphibole. The volcanic glass appears very fresh, with pumiceous glass as the dominant component. The siliciclastic component of the sandstone is mainly fragments of reworked chert, altered tuff fragments, siltstone, and sandstone. Interbeds of volcaniclastic sandstone, silty claystone, and siltstone characterize the lower part of lithologic Unit II. The silty claystone typically occurs as intensely bioturbated intervals from centimeters to several meters thick. The first sandstone with estimates of <25% pyroclasts occurs at 435.99 m CSF (smear slide data from Section 322-C0011B-11R-5, 75 cm). The volcaniclastic sandstones occur in packets 2–4 m thick (based on the LWD data; see "Logging and core-log-seismic integration") and are typically normally graded and show gradational contacts into dark gray siltstone. The dark gray clayey siltstone also occurs as discrete beds that are typically 0.1–1 m thick (Fig. F5) and display minimal amounts of bioturbation. The volcaniclastic sandstones include both volcaniclastic and terrigenous siliciclastic grains. Smear slide estimates show >25% volcaniclastic grains and <25% pyroclastic grains. These sandstones have a large amount of minerals, with relative abundances of plagioclase > pyroxene > quartz > amphibole. The estimated quartz content is typically between 2% and 6%, most of which is chert. Additionally, there are large amounts of sedimentary lithic grains, mainly as reworked siltstone, sandstone, and chert grains. In the lower part of lithologic Unit II, volcaniclastic components decrease below an estimated 25% of the total grain population. The lower part of lithologic Unit II has the first occurrence of diopside in Section 322-C0011B-5R-3, 63 cm (as an accessory mineral, <1%), but from Section 322-C0011B-7R-6, 39.5 cm, diopside is relatively common (up to 3%). From Core 322-C0011B-7R, the volcanic glass appears altered, and there is a mixture of mafic and felsic glass shards. Unit II also contains a chaotic deposit that is 10.29 m thick, extending from 400.86 to 411.15 m CSF (Sections 322-C0011B-7R-CC, 16.5 cm, through 8R-7, 92 cm). This deposit is composed of disaggregated pieces of volcaniclastic sandstone and bioturbated silty claystone that show tight to isoclinal folding, thinning and attenuation of original bedding, and subhorizontal small-scale faults with a normal (extensional) sense of displacement in the uppermost part (Fig. F6). Apparently, an in situ volcanic sandstone bed, immediately underlain and overlain by chaotic deposits, is at 405.29–406.01 m CSF (interval 322-C0011B-8R-3, 70–142 cm). Given the sediment deformation in the adjacent deposits, it seems most likely that this bed is also part of the chaotic deposit. An alternative interpretation is that two thinner chaotic units are separated by a single volcaniclastic sandstone bed. Figure F7 and smear slides (see "Site C0011 smear slides" in "Core descriptions") show the total volcaniclastic components for Unit II, and Figure F8 shows representative photomicrographs from the smear slides. These results were obtained by point counting (200 points). Pyroclast abundance was used to define the first occurrence of tuffaceous sandstone. Overall, sedimentary lithic fragments are more abundant in the lower part of lithologic Unit II (Fig. F7). Lithologic Unit III (lower Shikoku basin)
Lithologic Unit III is 194.92 m thick and starts at 479.06 m CSF, immediately below an unusual tightly cemented terrigenous sandstone, and ends with the first appearance of dark gray clayey siltstone at 673.98 m CSF. Unit III is characterized by bioturbated silty claystone. Zoophycos isp., Phycosiphon incertum, and Chondrites isp. are common in the silty claystone, and the intensity of bioturbation mostly corresponds to bioturbation Index 5 (see the "Methods" chapter). Unit III contains minor amounts of dark gray silty claystone and lime mudstone (Fig. F9). Unit III also contains dark green layers, typically <0.5 cm thick with variable spacing, but as repetitive as one every 5–10 cm. These dark green layers are intensely bioturbated and contain altered mafic glass, silica, and plagioclase. The off white–colored lime mudstone lithology is rich in altered nannofossils. A cemented, laminated carbonate layer (very poor recovery) is present at interval 322-C0011B-44R-1, 0–61 cm. Minor amounts of ocher-colored burrows occur throughout Unit III. Although nannofossils are rare in parts of Unit III, some burrows contain nannofossils. Small amounts of pyrite are also present, with a few pieces up to 2 mm in diameter. A thin chaotic deposit was identified at 570.91–571.42 m CSF (Section 322-C0011B-28R-3, 92–143 cm). Possible creep structures are recognized at intervals 322-C0011B-21R-6, 50–54 cm; 26R-5, 44–46 cm; and 31R-3, 111–116 cm (see "Structural geology"). Smear slide estimates for Unit III show relatively low amounts of quartz, accessory minerals (i.e., all other minerals except clay minerals), and volcaniclastic components. Within Unit III, there is an overall uphole increase then decrease in the amount of feldspar, with a maximum estimated value near 575 m CSF (Fig. F10; "Site C0011 smear slides" in "Core descriptions"). Lithologic Unit IV (lower Shikoku Basin)
The top of lithologic Unit IV is gradational from Unit III, at 673.98 m CSF, starting with the first core recovery of clayey siltstone. The base of Unit IV is placed at 849.95 m CSF, above the first appearance of tuff. Although the overall core recovery statistic in Unit IV was ~86%, much of the section is unsampled in the 56.7 m thick washdown interval between 786.3 and 844.0 m CSF. Unit IV is characterized by alternations of silty claystone, clayey siltstone, and normally graded sandstone (Fig. F11). The clayey siltstone typically occurs in beds 5–60 cm thick (Fig. F4), mainly with sharp bases, diffuse plane-parallel lamination in the lower parts, and normal grading into silty claystone at the top. Bioturbation in the clay-bearing siltstone is slight to moderate, compared with the intense bioturbation observed in the silty claystone. We interpret these deposits as muddy turbidites. The sharp-based, normally graded sandstones are 10–80 cm thick and typically show plane-parallel lamination. The sandstones commonly contain small, disseminated wood fragments <5 mm in diameter. Dark-colored anastomosing, millimeter-scale deformation bands occur at the base of some clayey siltstone beds (see "Structural geology"). Smear slide estimates show that the clay-bearing siltstones in Unit IV have a larger terrigenous component than hemipelagic deposits in Unit III, with more abundant detrital quartz (Fig. F10; see "Site C0011 smear slides" in "Core descriptions"). Most quartz grains show undulose extinction. Also more abundant are grains of polycrystalline quartz and vein quartz (with chlorite inclusions as rhipidolith chlorite). Metamorphic rock fragments are also typical (i.e., sericite-quartz intergrowths) and characteristic of low-grade metamorphic terranes. Lithologic Unit V (lower Shikoku Basin)
Lithologic Unit V starts at 849.95 m CSF and extends to 876.05 m CSF at the premature termination depth of Hole C0011B. The Unit IV/V boundary is at Section 322-C0011B-57R-3, 73 cm, with the first appearance of tuff. The integrated age-depth model gives an age of 14.0 Ma at the lithologic unit boundary (see "Paleomagnetism"). The main lithology in Unit V is tuffaceous sandy siltstone, with minor amounts of silty claystone and tuff (Fig. F12). The beds of tuffaceous sandy siltstone show normal grading, load structures, flame structures, and current-ripple lamination in their uppermost parts (Fig. F12). Smear slide estimates show background mud sedimentation with 10%–40% glass and the tuffs containing 60%–80% glass, juvenile minerals, and minor amounts of pelagic sediments (claystone, chert, and siltstone grains). Within Unit V, there is a marked increase in the feldspar content (Fig. F10; "Site C0011 smear slides" in "Core descriptions"). Most of the tuffs show considerable alteration of the primary mineral components, also expressed in the occurrence of zeolites (e.g., analcime and clinoptilolite). The glass components appear to be derived from highly evolved, explosive volcanism. Mafic volcanic components are minor. Lithification of the tuffs has resulted from an increase in diagenetic carbonate cement and zeolites. X-ray diffraction analysesFor Hole C0011B, the results of X-ray diffraction (XRD) analyses of bulk sediment samples are shown in Figure F13 and Table T4. In Unit II, XRD analyses of the bulk samples show that calcite is low throughout (up to ~6%, but typically <0.5%). Quartz appears fairly uniform (~15%) throughout the entire Unit II but with a slight increase toward the bottom (from ~13% to ~19%). Clay mineral content scatters in the upper part of Unit II (from ~90% to 40%) and is approximately constant in the lower part of Unit II (~64%). Feldspar shows a minimal increase downhole in Unit II and is typically 13%–14%; outlying high values in the upper and lower part of Unit II exceed 30%. The feldspar and clay mineral content are inversely correlated. This is probably a grain-size effect with more feldspar in the sand-size fraction. Throughout Unit III, there are no significant variations in bulk mineralogy, with an average of 68% total clay minerals, 17% quartz, 12% feldspar, and 2% calcite. Departures from this typical composition are associated with thin beds of lime mudstone and calcareous claystone, which naturally show elevated relative percentages of calcite. In Unit IV, quartz values range from 0.1% to 39%, with an average of 18%. Feldspar content varies between 1% and 40%, with an average of 12%. Quartz and feldspar show similar variations, with a negative shift between 758.40 and 762.55 m CSF (Sections 322-C0011B-52R-2, 11 cm, through 52R-5, 34 cm) but increasing below 774.68 m CSF (Section 322-C0011B-54R-5, 44 cm) because of grain-size effects. Clay mineral content averages 64%. Quartz and clay mineral content are inversely correlated. Calcite values are low (<0.5%), except for several scattered samples of calcareous claystone. In Unit V, clay mineral content decreases from 73% to 47%, with an average of 61%. However, quartz and feldspar content increase from 8% to 40% and 11% to 23%, respectively, with average values of 23% and 15%, respectively. In Unit V, calcite values were below detection limits. XRD analyses of tuff samples from Unit V show the presence of zeolites, which corroborates the presence of analcime in smear slides. Zeolites include undifferentiated clinoptilolite/heulandite and analcime (Fig. F14). Also, there are considerable amounts of authigenic smectite in the altered tuffs. X-ray fluorescence analysesIn order to characterize compositional trends with depth and/or lithologic characteristics of the sediments from Hole C0011B, X-ray fluorescence (XRF) analysis was undertaken for 96 samples, which sampled all of the cored units (Fig. F15; Table T5). The analyzed sediments span a relatively small range of major element composition, compatible with a dominant crustal composition (both high SiO2 and Al2O3, Fig. F15A). With depth, Al2O3 content (Fig. F15B) appears to decrease throughout the upper part of Unit II and appears constant in the lower part of Unit II and in Unit III. These variations reflect variations of clay mineral proportions as seen in XRD analyses (see "X-ray diffraction analyses"). Unit IV and V sediments have variable Al2O3 composition. Sodium content has local maxima in Unit II and V sediments (up to 4.43 wt%, Fig. F15C) but shows an overall uphole increasing trend. Differentiating between lithologic units (Fig. F15D) shows that volcanic components result in higher Na2O content, compatible with the higher amount of feldspar observed in these sediments (see "X-ray diffraction analyses"). Na2O content (Fig. F15D), together with MgO and K2O (not shown) in the silty claystones, shows an uphole increasing trend. This is probably related to dilution by silica from quartz minerals as seen in the slightly increasing trend with depth in XRD analyses (see "X-ray diffraction analyses"). K2O content (Fig. F15E) is variable throughout Hole C0011B. Most of this variability can be explained by carbonate- and volcaniclastic-rich sediments, but the silty claystones also have a large scatter in values, which may be related to the presence of different types of clay minerals (e.g., illite). In Unit II, the sediments that include a volcanic component appear to have a smaller K2O content (K2O from 1.07 to 2.16 wt%) compared with the sediments that include a volcanic component in Unit V (K2O from 1.23 to 3.68 wt%). This difference could be related to different volcanic sources and/or the chemical evolution of a single source. The few lime mudstones that were analyzed have very high concentrations of both P2O5 and MnO (up to 8.4 and 11.2 wt%, respectively, Fig. F15F). The high P2O5 content in the carbonate samples is probably related to the presence of authigenic apatite (Ca5[PO4]3[F,Cl,OH]). The MnO content can be explained by the presence of rhodochrosite (MnCO3). Rhodochrosite is typical of anoxic environments and is indicative of geochemical conditions where ferric iron oxides usually dissolve (Canfield et al., 1992). Interpretation of lithologies and lithofacies at Site C0011At Site C0011, six major lithologies are recognized (silty claystone, clayey siltstone, tuffaceous sandstone, volcaniclastic sandstone, sandstone, and volcanic ash/tuff) together with several minor lithologies such as tuffaceous siltstone, calcareous claystone, lime mudstones, and chaotic deposits. These lithologies compose four broad "lithofacies:" hemipelagic, turbidite, chaotic deposits, and tuff. The hemipelagic lithofacies is composed mostly of silty claystone. Green-gray silty claystone is the dominant lithology throughout Site C0011. This lithology is intensely bioturbated and commonly intercalated with green silty claystone layers (<1 cm thick). We interpret the lithofacies to represent the background sedimentation from suspended sediment. The approximately constant rate of sediment accumulation supports this interpretation (see "Sediment accumulation rates at Site C0011"). Minor lithologies, such as calcareous claystone and lime mudstone, contributed through high concentrations of nannofossils, although some such carbonates were probably produced by diagenesis. Coarser grained lithologies (clayey siltstone, tuffaceous sandstone, volcaniclastic sandstone, and sandstone) all show sharp-based beds, normal grading, and plane-parallel lamination. Such features are typical in turbidites, and, therefore, we interpret these deposits as the products of sediment gravity flows, most likely from turbidity currents. Together, we group these lithologies into the turbidite lithofacies. The tuffaceous turbidity currents may have been triggered either by submarine slides or flow transformations from subaerial pyroclastic flows entering the ocean (e.g., Stow et al., 1998; Freundt, 2003; Freundt and Schmincke, 1998; Trofimovs et al., 2006) (Fig. F16). Terrigenous sand may have been deposited from turbidity currents that originated by flow transformations from debris flows or sediment slides (e.g., Felix and Peakall, 2006), hyperpycnal flow (Mulder and Syvitski, 1995), continuous breaching of an unconsolidated sand cliff (Mastbergen and Van Den Berg, 2003), mobilization of sand by storm waves, storm-surge ebb currents, or a seismic trigger. The clay-bearing siltstone also shows characteristic features of turbidites, such as normal grading and lamination, and they are less bioturbated than the hemipelagic lithofacies. The siltstone commonly overlies turbiditic sandstone with a gradational contact, and, therefore, it was probably deposited as muddy turbidites from the finest grain-size fraction. The tuffaceous and volcaniclastic turbidites contain small amounts of terrigenous clasts. This suggests reworking of older sedimentary strata, the source of which remains unclear. The longevity and distance traveled by turbidity currents is derived from self-sustaining flow (i.e., the process by which the erosion of sediment from the bed increases current density) and, therefore, the downstream pull of gravity (Pantin, 1979; Parker, 1982). It is, therefore, reasonable to expect natural turbidity currents to develop from relatively weak initial upstream conditions (Naruse et al., 2008). Many of the sandstones at Site C0011 are mixtures of pyroclastic and terrigenous siliciclastic material, so this self-sustaining process can help explain the composition of turbiditic deposits. Chaotic deposits occur in both Units II and III. The chaotic deposits of Unit II formed either during long-distance transport in cohesive debris flows or more locally as sediment slides (e.g., along the inner margins of submarine channels). The presence of coarse-grained sandstone injection features within the chaotic deposits, together with the presence of small-scale, near-horizontal normal faults at the top, favors an origin that was relatively close to Site C0011. The rotation of more steeply dipping faults during the final stages of sediment slumping and sliding, for example, could have led to liquefaction and fluidization. Together, these deposits constitute the chaotic lithofacies. The tuffs in Unit V are interpreted as air fall pyroclastic deposits because they contain >75% glass shards, show normal grading, and are texturally well sorted. These deposits, with hemipelagic interbeds, form the tuff lithofacies. Sediment accumulation rates at Site C0011Sediment accumulation rates for the hemipelagic and the turbiditic deposits at Site C0011 were compared. The hemipelagic accumulation rate appears to have changed abruptly in the middle of Unit III (Fig. F17C). Units II and IV are characterized by high sediment accumulation rates for the turbiditic deposits (Fig. F17B). To define turbidite accumulation rates, the thickness ratio between the hemipelagites (silty claystone, calcareous mudstone, and lime mudstone) and turbidites (sandstone, tuffaceous sandstone, volcaniclastic sandstone, and clayey siltstone) was measured in each core (Fig. F17A). Then, the cumulative thickness of the hemipelagic and turbiditic deposits was calculated on the basis of the thickness ratio and core length. Next, the cumulative thickness of the turbiditic deposit was plotted against that of the hemipelagic deposit (Fig. F17B). Although most cores were not 100% recovery, as an approximation, the thickness ratio between two lithofacies was assumed to be constant within each core. Cores showing low recovery rates (<15%) were excluded from this analysis, and their data were linearly interpolated with respect to data in adjacent cores. Cores of sand-prone intervals (Units II and IV) mostly showed >70% recovery rate, so this approximation has little effect on our analysis. This approximation also appears to have a minimal effect on the analysis of low-recovery rate cores because the LWD data suggest that these cores were mostly obtained from intervals of monotonous hemipelagic deposits. The only exception that could not be estimated well by this approximation is the wash down interval of Unit IV (786.3–844.0 m CSF). As there are only two age-control points, both within and below the wash down interval, the possible error introduced in our analysis in the wash down interval could be large. Thus, if we exclude all data obtained below the wash down interval, the conclusions about the sediment accumulation rate are robust. Sediment accumulation rates for the hemipelagic and turbiditic deposits were calculated on the basis of paleontologic and magnetostratigraphic data (see "Paleomagnetism"), using a least-squares fit and a central difference method with a length of two cores (Fig. F17B). The results of this analysis suggest that the sediment accumulation rate for the hemipelagic deposits was constant (7.7 cm/k.y.) from Unit V to the lower part of Unit III. Near the middle part of Unit III, there appears to have been a significant decrease to 3.9 cm/k.y. (Fig. F17C), which is maintained through Unit II. This significant decrease in the sediment accumulation rate for the hemipelagites, along with the observed changes in magnetic properties recorded for the same depth interval at Site C0011 (see "Physical properties"), suggests the possibility of some fundamental paleoenvironmental change at Site C0011. Also, our analysis shows that the sediment accumulation rate for the turbidites in Units II and IV averaged ~1–1.2 m of turbidite per 1 m of hemipelagic deposition. The sediment accumulation rate of the turbidites, therefore, was ~3.9–4.7 cm/k.y., for a hemipelagic sediment accumulation rate of ~3.9 cm/k.y. Unit III is characterized by a near-zero sediment accumulation rate of turbidites (Fig. F17B). Paleogeography and sediment provenance at Site C0011Kashinosaki Knoll is located ~100 km southeast of the Kii Peninsula and is ~150–200 km west of the Izu-Bonin arc. Figure F18 shows the calculated locations of Hole C0011B from its present position backtracked to its position at ~12 Ma (Unit V), based on estimated plate convergence rates (0–4 Ma: 4 cm/y to the northwest; 4–12 Ma: 0.9 cm/y to the north-northwest) (Kimura et al., 2005). From these rates, we interpret the initial location of Site C0011, for deposition of Unit V, as ~350 km south of its present position. During deposition of the entire section cored at Site C0011, the site's distance from the Izu-Bonin arc would have been similar with respect to the current geographic location (~150–200 km), and, therefore, sediment gravity flows could have transported volcanic/pyroclastic material from the Izu-Bonin arc over such distances. Also, at ~7–12 Ma, Site C0011 was located ~200 km south of southwest Japan, making this a possible additional sediment source. In addition, there were probably locally active volcanic seamounts within the Shikoku Basin at that time. The terrigenous sediments of Unit IV represent the distal parts of a submarine fan, possibly sourced from sedimentary and metasedimentary terranes in the Outer Zone of southwest Japan. Age-dating and geochemical analysis of the igneous rocks from southwest Japan, integrated with the tectonic development of Japan and the Philippine Sea plate, suggests the following history of volcanic activity (Fig. F19) (Kimura et al., 2005):
40Ar/39Ar dates of volcanic rocks from the seamount chains in the northern part of the Shikoku Basin show that volcanism began at ~17 Ma (Ishizuka et al., 2003), slightly predating the cessation of Shikoku Basin (backarc basin) spreading at 15 Ma (Okino et al., 1994, 1999) and the opening of the Sea of Japan, waxing in intensity at 12 Ma, and continuing until 3 Ma. Magmatism migrated eastward through backarc knolls to the intra-arc rift zone (Ishizuka et al., 2003). After 7 Ma, volcanism became active along the entire Izu-Bonin seamount chains, both in the western seamounts and backarc knoll zone (Ishizuka et al., 1998). The crest of Kashinosaki Knoll is located 10 km south of Site C0011. Differences in the magnetic and gravity anomalies over the Kashinosaki Knoll and the Zenisu Ridge, together with seismic reflection data, suggest that the knoll is unlikely to be the westward extension of the Zenisu Ridge and that the north-dipping thrust fault underlying the Zenisu Ridge does not continue westward under the knoll as previously proposed (Le Pichon et al., 1987; Lallemant et al., 1989). Seismic depth Section ODKM-22 crosses the summit of the Kashinosaki Knoll in the vicinity of IODP Sites C0011 and C0012 and shows thinning of the lower and middle Shikoku Basin sediments onto this basement high (Ike et al., 2008a); this seismic interval corresponds to Units III, IV, and V at Site C0011. Units IV and V thin considerably toward the knoll. It appears that much of the hemipelagic Unit III passively draped the knoll; however, the Kashinosaki Knoll retained some bathymetric relief during that phase of hemipelagic deposition. Petrographic trends interpreted from the smear slide data (Fig. F10) are consistent with a change from an essentially inactive and dissected volcanic island arc that was being reworked into deep water (Unit IV and the lower part of Unit II) to an active volcanic arc (upper part of Unit II). Unit V (~14 Ma) accumulated on the rough seafloor topography created earlier by seamount volcanism. Those strata are dominated by felsic volcanic grains, derived from either the Izu-Bonin or southwest Japan arc. At that time (~14 Ma), there was considerable magmatic activity in the southwest Japan arc (e.g., the anomalous near-trench igneous activity as seen in granite emplacement during the middle Miocene in Kii Peninsula) (Hoshi et al., 2007, and references therein). Unit IV (lower Shikoku Basin; ~14–12 Ma) comprises fine-grained, terrigenous turbidites, containing locally abundant woody material together with low-grade metamorphic grains, all suggesting a provenance from the Outer Zone of southwest Japan. Unit III is almost exclusively hemipelagic deposits, probably from several sources, including the Izu-Bonin arc, southwest Japan arc, and seamounts within the Shikoku Basin. The seismic character of Unit III is essentially a sheetlike "drape" that thins over the Kashinosaki Knoll. The abrupt change in the sediment accumulation rate in Unit III at ~11 Ma coincides with the cessation of voluminous forearc volcanism at ~11 Ma. Although there are various ways to explain this change in the sediment accumulation rate, one possibility is that sediment influx slowed down as anomalous near-trench volcanism in the main source area for sediment was supplanted by forearc cooling (the transition from Stage II to Stage III of Kimura et al., 2005) (Fig. F19). On the basis of the stratigraphy at Site C0011 and the seismic and LWD character, we interpret Unit II (middle Shikoku Basin) as the distal facies of a submarine fan (with both shallow channel and more sheetlike subenvironments) developed on the Shikoku Basin plain. The present-day topography of the Shikoku Basin shows the presence of backarc en echelon seamounts, consisting of four cross-chains in the northern part of the Izu-Bonin backarc (Machida et al., 2008). Such cross-chains would act to laterally confine sediment gravity flow deposits from relatively restricted geographic segments of the Izu-Bonin arc. Development of elongated feeder systems (canyons and channels) to very irregularly shaped submarine fans on the Shikoku Basin plain would enhance prospects for greater sediment accumulations further west in the Shikoku Basin. Additionally, submarine canyons along the Izu-Bonin arc (as is the present case) would tend to act as the principal sediment transport paths for redepositing sediment into deep water between these cross-chains. We envision a similar topography for the Shikoku Basin during deposition of Unit II. Comparison between Site C0011 and nearby ODP/IODP sitesWe recognize five lithologic units at Site C0011 and tentatively correlate these with lithostratigraphic units at ODP Sites 808, 1173, 1174, and 1177 (Table T6). Unit I of Site C0011 is probably equivalent to the upper Shikoku Basin facies at Sites 808 (Subunit IVA), 1173 (Unit II), 1174 (Unit III), and 1177 (Unit I) (Table T6). Seismic profiling suggests no trench-wedge facies at Site C0011. Although Unit I at Site C0011 was drilled without coring, the LWD data of Hole C0011A imply that this unit is composed mainly of hemipelagic deposits with possible intercalation of thin ash layers, which are similar to the lithologies in the upper Shikoku Basin facies at Sites 808, 1173, 1174, and 1177. Judging from the age at the uppermost interval of Unit II, the bottom of Unit I is ~7.6 Ma, which is older than the maximum age of the upper Shikoku Basin facies at previous sites. Unit II of Site C0011, with abundant sandstone turbidites, has no equivalent of comparable age at previous ODP sites (Table T6). In spite of the common occurrence of volcanic sandstone turbidites, we did not find unequivocal tuff beds formed by ash fall in this unit. Together with its Miocene age (7.6–9.1 Ma), it is reasonable to define a new stratigraphic interval for the Shikoku Basin. We therefore designate Unit II as the "middle Shikoku Basin" facies. Age-equivalent strata of the lower Shikoku Basin facies at nearby ODP sites do not contain volcanic-rich sandstone turbidites. Unit III of Site C0011 is equivalent in lithology to the lower Shikoku Basin hemipelagic facies at Sites 808 (Unit IVB), 1173 (Unit III), 1174 (Unit IV), and 1177 (Unit II) (Table T6), but its age is older (9.1–12.2 Ma). Unit III is characterized by monotonous hemipelagic deposits with thin green layers (color banding), which is common lithology in the lower Shikoku Basin facies of all correlative ODP sites in the Nankai Trough. At Site 1177, similar thin, dark green layers in mudstone were described in the hemipelagic silty claystone. The colors, in part, reflect variations in the content of clay minerals (Shipboard Scientific Party, 2001b). Generally, the upper boundary of the lower Shikoku Basin facies is assigned to the bottom of the last distinct ash layer in the upper Shikoku Basin facies, although this is also influenced by a diagenetic transition from unaltered to heavily altered ash layers (Moore, Taira, Klaus, et al., 2001). Unit III of Site C0011 contains no distinct tuff layer, so it seems reasonable using facies criteria to consider this unit as the youngest part of the lower Shikoku Basin facies. Unit IV of Site C0011 probably correlates in both facies and age with the lower Shikoku Basin turbidite facies of Site 1177 (Unit III) (Table T6). Abundant muddy and sandy turbidites suggest that this unit was deposited in the submarine fan system. The lower Shikoku turbidite facies of Site 1177 contains four packets of sandy turbidites. The gravity flow deposits contain abundant quartz, sedimentary and metasedimentary lithic fragments, and woody organic matter, indicating their terrigenous origin. These features are similar to the finer grained turbidites in Unit IV of Site C0011. Site 1173, which is located along the Kinan Seamounts between Sites 1177 and C0011, does not contain a Miocene turbidite facies. Thus, Unit III of Site 1177 and Unit IV of Site C0011 may represent independent submarine fan systems separated by a basement high. Unit V of Site C0011 is composed of volcaniclastic deposits, such as tuff and tuffaceous silty claystone, and is equivalent to the volcaniclastic facies observed in Sites 808, 1174, 1173, and 1177 (Table T6). The volcaniclastic facies are widely distributed as deposits covering the basement rocks in the Nankai Trough (Unit V of Sites 808 and 1174; Unit IV of Sites 1173 and 1177), and their maximum age increases with distance from the Kinan Seamounts. Unit V of Site C0011 is ~14 Ma in age, which also matches the age of volcaniclastic deposits in the Muroto transect area. |
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