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doi:10.2204/iodp.proc.322.104.2010 LithologyAt Site C0012, seven lithologic units were identified on the basis of sediment composition, sediment texture, and sedimentary structures (Fig. F2, Table T2):
Lithologic Unit I (upper Shikoku Basin)
Lithologic Unit I is 150.86 m thick and extends from the seafloor (0.0 m CSF) to 150.86 m CSF, where the first volcaniclastic sandstone was recovered. Poor core recovery together with very intense core disturbance in this unit means that it is possible that the first sandstone occurrence is at a shallower depth. The dominant lithology is green-gray, intensely bioturbated silty clay to silty claystone with thin interbeds of volcanic ash. Most of the silty claystone is rich in nannofossils. Within the silty claystone intervals, dark green silty claystone layers are typically <0.5 cm thick. The uppermost 11 cm of Core 322-C0012A-1R, which was jetted into the formation, contains a poorly consolidated light brown–colored, oxidized clayey silt layer (interval 322-C0012A-1R-1, 0–11 cm). The uppermost fine- to coarse-grained ash layers range from light gray to olive-gray (to interval 322-C0012A-9R-1, 11–15 cm). In Core 322-C0012A-9R, the color and composition of the ash layers changes to black and mafic components, respectively (e.g., the first dark ash is at interval 322-C0012A-9R-1, 136.5–138 cm). Below interval 322-C0012A-9R-7, 48.5–57 cm, the color of the ash layers alternates between off white/olive-gray and black. Lithologic Unit II (middle Shikoku Basin)
Lithologic Unit II is 68.95 m thick and extends from 150.86 to 219.81 m CSF. Poor core recovery, together with very intense drilling disturbance, precludes any division of this unit. The dominant lithology is green-gray silty claystone, which alternates with medium- to thick-bedded tuffaceous/volcaniclastic sandstone interbedded with dark gray clayey siltstone. The silty claystone appears to be the "background sedimentation" and typically occurs as intensely bioturbated intervals from centimeters to several meters thick. Within the silty claystone intervals, dark green silty claystone layers are typically <0.5 cm thick and, where bioturbation has not obscured their presence to the naked eye, are commonly spaced 5–10 cm apart. The volcaniclastic sandstone beds range from granular to fine grained, are 5–50 cm thick, and are typically normally graded with planar-parallel lamination (Fig. F3). A minor lithology in Unit II is thin-bedded, dark gray clayey siltstone (e.g., intervals 322-C0012A-17R-1, 33–47.5 cm; 17R-1, 64–72 cm; and 18R-2, 54–57 cm). Unit II also contains chaotic deposits that are at least 0.31 and 3.1 m thick, extending from 150.86 to 151.17 m CSF (Section 322-C0012A-12R-2, 43 cm, through 12R-CC, 15 cm) and from 178 to 181.1 m CSF (Sections 322-C0012A-15R-1, 0 cm, through 15R-CC, 18.5 cm), respectively. These deposits are composed of disaggregated pieces of volcaniclastic sandstone and bioturbated silty claystone that show folding, thinning, and attenuation of original bedding (Fig. F4). Drilling-induced deformation throughout Unit II precludes an accurate assessment of the amount and thickness of other chaotic intervals that may be present. Figure F5 (see also "Site C0012 smear slides" in "Core descriptions") shows the total volcaniclastic components for Unit II, and Figure F6 shows representative photomicrographs from the smear slides. These results were obtained by point counting (200 points). The volcaniclastic sandstone lithology includes both volcaniclastic and terrigenous siliciclastic grains. Smear slides show an estimated >25% volcaniclastic grains and <25% pyroclastic grains. These sandstones have large amounts of minerals, with relative abundances of plagioclase > pyroxene > quartz > amphibole. The quartz content is estimated to be between 4% and 10%, most of which is polycrystalline. Additionally, there are large amounts of sedimentary lithic grains, mainly as reworked siltstone, sandstone, and chert grains. These sedimentary lithic grains increase in abundance toward the base of Unit II. Unit II differs from the units above and below by a higher total mineral content (on average), especially feldspar and heavy minerals, and an increase in the average ratio of volcanic lithic grains versus total lithic grains. Scatter among individual sample compositions, however, is considerable. Lithologic Unit III (lower Shikoku Basin hemipelagites)
Lithologic Unit III is 112.0 m thick and extends from 219.81 to 331.81 m CSF. The top of Unit III is defined at the base of the last sandstone in Unit II. The base of Unit III is just above the first appearance of dark gray siltstone. Unit III contains an unusual interval of steeply inclined bedding, typically at 30°–45° (Sections 322-C0012A-22R-1, 73 cm, through 23R-CC, 17 cm) (Fig. F7) (see "Structural geology"). The lithology is characterized by bioturbated silty claystone. Zoophycos isp., Phycosiphon incertum, and Chondrites isp. are commonly observed in the silty claystone. The intensity of bioturbation mostly corresponds to ichnofabric Index 5 (see the "Methods" chapter). Minor lithologies in this unit are dark gray silty claystone, lime mudstone, and very thin ocher-colored calcareous claystone (-filled burrows). Unit III also contains dark green layers, typically <0.5 cm thick, which recur 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, yellowish carbonate layer is present at interval 322-C0012A-20R-4, 115–122 cm. We note a similarity between this layer and a potentially correlative carbonate layer at Site C0011 (interval 322-C0011B-19R-5, 132–137 cm) (Fig. F8). Minor amounts of ocher-colored burrows occur throughout this unit. Although nannofossils are rare in parts of Unit III, some burrows tend to contain concentrations of nannofossils. Smear slide data for Unit III show relatively low proportions of quartz, accessory minerals (i.e., all other minerals except clay minerals), and volcaniclastic components. Within Unit III in Hole C0012A are peaks in the amount of feldspar and sedimentary lithic grains at ~275 m CSF (Fig. F5; see "Site C0012 smear slides" in "Core descriptions"). Lithologic Unit IV (lower Shikoku Basin turbidites)
Lithologic Unit IV is 83.77 m thick and extends from 331.81 to 418.29 m CSF. The top of Unit IV was defined by the appearance of dark gray siltstone; the base of this unit was placed above a bed of tuff. Unit IV is characterized by alternations of silty claystone, clayey siltstone, and normally graded siltstone. The clay-bearing siltstone occurs in beds ~5–50 cm thick, but typically 5–20 cm thick, characterized mainly by sharp bases, diffuse plane-parallel lamination in the lower part, and normal grading into silty claystone. Bioturbation in the clayey 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 siltstones are 10–80 cm thick and typically show plane-parallel lamination. These are also interpreted as turbidites. Unit IV contains an 82 cm thick interval of steeply inclined bedding, typically at ~40° (Sections 322-C0012A-33R-5, 35 cm, through 33R-CC, 18.5 cm). Smear slides show that the clayey siltstones have a larger terrigenous component than deposits in Unit III, with more detrital quartz (Fig. F5; "Site C0012 smear slides" in "Core descriptions"). Most grains show undulose extinction and polycrystalline quartz. Metamorphic rock fragments are also typical (i.e., sericite-quartz intergrowths, typical of low-grade metamorphic terranes). Lithologic Unit V (volcaniclastic-rich)
Lithologic Unit V is 112.93 m thick and extends from 415.58 to 528.51 m CSF. The Unit IV/V boundary is defined by a bed of tuff. The Unit V/VI boundary is defined by the first appearance of red calcareous claystone. The main lithologies in the upper part of Unit V are silty claystone alternating with tuff, and in the lower part volcaniclastic siltstone/sandstone beds alternate with siliciclastic sandstone/siltstone and rare tuff layers. This change occurs immediately beneath a tuff layer at 435.68 m CSF (Section 322-C0012A-42R-3, 102 cm). The sandstone and tuff beds typically show normal grading, plane-parallel lamination, and current-ripple lamination (Fig. F9A). The volcaniclastic deposits show load structures, flame structures, and current-ripple lamination (Fig. F9B, F9C). These laminated strata are involved in a 50.5 cm thick chaotic interval (Sections 322-C0012A-45R-1, 71 cm, through 45R-1, 121.5 cm), showing isoclinal folding, attenuation of beds, and sandstone injections (Fig. F9C). This chaotic interval contains at least three discrete zones, distinguished by sharp detachment surfaces with an abrupt change of bedding dip above them, with the uppermost zone having the most intense internal deformation. The volcaniclastic sandstones are characterized by abundant dark-colored glass shards up to several millimeters in a-axis, which are oriented parallel/subparallel to bedding (Fig. F9D, F9E). One of the very coarse grained volcaniclastic sandstones contains large (>6 cm) angular clasts of bioturbated, green-gray silty claystone (Fig. F9F). Disseminated and framboidal pyrite (as long as 2 cm) is common through Unit V. The larger pyrite nodules appear as the infill of burrows. Smear slide analyses show background mud sedimentation with 5%–15% glass, and the tuffs contain an estimated 40%–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. F5; see "Site C0012 smear slides" in "Core descriptions"). In the upper part of Unit V, the glass components appear to be derived from highly evolved explosive volcanism. However, the sandstones in the lower part of Unit V have high amounts of fresh, dark brown, blocky, and vesicle-free glass shards, suggesting a hyaloclastic origin. The sandstones in Unit V appear to have two separate detrital provenances: a source with fresh volcanic glass, together with relatively large amounts of feldspar, and a source enriched in sedimentary lithic grains, quartz, and heavy minerals (including pyroxene, zircon, and amphibole) (Fig. F10; "Site C0012 smear slides" in "Core descriptions"). Lithologic Unit VI (pelagic claystone)
Lithologic Unit VI is 9.3 m thick and extends from 528.51 to 537.81 m CSF. Unit VI is characterized by red calcareous claystone and is rich in nannofossils with minor amounts of radiolarian spines. Locally, particularly in the tens of centimeters overlying the basaltic basement of Unit VII, the claystones have a mottled red-green coloration, probably due to iron reduction. Immediately above the basalt, the red calcareous claystone appears much more indurated than in the overlying part of Unit VI (Fig. F11). Lithologic Unit VII (Kashinosaki Knoll basement)
Basement is composed of (1) pillow lava basalt, (2) basalt, (3) basaltic hyaloclastite breccia, and (4) mixed rubble pieces of basalt caused by drilling disturbance (Fig. F12). Successful recovery of volcanic basement in Hole C0012A defined lithologic Unit VII. On the basis of the nannofossils in the overlying red claystone, the volcanic rock is older than 18.9 Ma. The cored interval extends from 537.81 to 576 m CSF, and a sharp contact between red claystone and basalt is well preserved in Core 322-C0012A-53R (Fig. F11). The 38.2 m of basement coring resulted in 18% recovery. Pillow lavas were identified by their curved, chilled margins oriented oblique to the vertical axis of the core. Where such margins were absent, identification of pillow lavas was made by their variolitic textures, curved fractures, and microcrystalline or cryptocrystalline grain size. The name "basalt" was assigned to rocks that did not present these features, even though some pieces may be too small to be representative of the rock morphology. Basalts and pillow basalts have aphanitic to porphyritic textures (Fig. F13). Phenocryst abundance is highly variable, from slightly to highly phyric textures. This variability is not restricted to separate intervals but is also observed across diffuse boundaries between highly phyric and sparsely phyric basalts (Fig. F14). Phenocrysts are composed mostly of plagioclase, pyroxene, Fe-Ti oxides, and sparse altered olivine. Orthopyroxene is dominant compared to clinopyroxene, suggesting low CaO content in the parent magma. Pyrite is present as an accessory mineral in some basalt. Vesicularity is highly heterogeneous. In a single unit, basalts can have sparse (1%–5%) to high (>20%) vesicle content. Visual core description and thin section analyses show that two different types of basalts with different color, vesicle content, and phenocryst proportion are present. The boundaries between the two types of basalt are diffuse. This may be due to either magma mixing within the magmatic chamber or lava reworking during the emplacement of the basalts (Fig. F14). Alteration ranges from moderate to very high, with a large proportion of basalt being highly altered (Fig. F15). Alteration styles include interstitial groundmass replacement, vesicle fill, vein formation (with associated alteration halos), and complete replacement of pillow lava glass rims by alteration materials. Phenocrysts are well preserved. Phyllosilicates are among the main products of alteration and occur as saponite, celadonite, and mixtures of these two phases (Fig. F16). Other secondary mineralogical phases include zeolite (fibrous and isometric), pyrite, iddingsite, quartz, and calcite (Fig. F17). Therefore, basalts from Site C0012 exhibit the effects of several stages of alteration from relatively high temperature facies (>200°C for zeolite and saponite) to low-temperature facies (<30°C for celadonite). One thin layer of basaltic hyaloclastite breccia was also recovered. Additionally, many fractures were filled with brecciated material. These breccias are composed of clasts that are similar to the surrounding basalts and are sealed with celadonite and saponite. Given the low recovery of these intervals, it is not possible to determine the relationship between the hyaloclastite portions and underlying lavas. Major elements were measured postexpedition at University Joseph Fourier on 20 bulk rock samples of basalt (Fig. F18). The variable extent of alteration is confirmed by loss on ignition values, which range from 3.1 to 18.0 wt%. SiO2 content ranges from 45.5 to 53.3 wt%, and MgO content ranges from 6.6 to 9.0 wt%. Alkaline content is high with Na2O + K2O ranging from 3.6 to 6.9 wt%. Na2O increases with depth while K2O decreases with depth. This may be related to interactions between the basalts, the overlying sediments, and any pore fluids from other sources. Further study of trace element contents is necessary to explain these characteristics. X-ray diffraction analysesFor Site C0012, the results of X-ray diffraction (XRD) analysis of bulk sediment samples are shown in Figure F19 and Table T3. In Unit I, quartz values range from 14% to 24% (average = 17%), with a slight decrease toward the bottom. Total clay mineral content increases from 51% to 72%, with an average of 65%. Feldspar content ranges from 10% to 33%, with a positive excursion at ~90 m CSF and high values (~33%) in the lower part of Unit I. The feldspar and clay mineral content are inversely correlated. Calcite content is a relatively high in the lower part of Unit I (up to 12%), with an average of 3%. In Unit II, XRD analyses of bulk samples show that calcite is low throughout (typically <0.5%, but with one value as high as ~32%). Quartz values are somewhat scattered without a significant trend and an average value of 16%. Clay mineral content ranges from 43% to 77% but with a slight increase downhole. Feldspar content, however, appears fairly uniform (~17%) throughout Unit II. Throughout Units III and IV, there are no significant mineralogical variations with depth. The average values are 71% total clay minerals, 15% quartz, 11% feldspar, and 2% calcite for Unit III and 69% total clay minerals, 18% quartz, 11% feldspar, and 2% calcite for Unit IV. Four samples contain unusually high concentrations of total clay minerals and are probably bentonites. Unit V has the most scatter in mineral values throughout Hole C0012A, possibly due to a grain-size effect. Quartz values range from 3% to 37%, with an average of 15%. Feldspar content varies between 1% and 31%, with an average of 16%. Quartz and feldspar show similar variations but show an inverse relation with total clay mineral content. Clay mineral content ranges from 20% to 91%, with an average of 64%. Calcite values are low (typically <0.5), except for several outlying samples of calcareous claystone (up to 74%). X-ray fluorescence analysesIn order to characterize compositional trends with depth and/or lithologic characteristics of the sediments in Hole C0012A, X-ray fluorescence (XRF) analysis was undertaken for 83 sediment samples from all of the cored units (Fig. F20; Table T4). The analyzed sediments span a relatively small range of major element composition, compatible with a dominant crustal composition (both high SiO2 and Al2O3, with average of 62.51 and 16.17 wt%, respectively) (Taylor and McLennan, 1985). Throughout Units I, II, III, and IV, Al2O3, SiO2, Fe2O3, MgO, CaO, and K2O content show no significant variations, with an average of 16.17 wt% Al2O3, 62.51 wt% SiO2, 6.27 wt% Fe2O3, 2.26 wt% MgO, 4.66 wt% CaO, and 2.88 wt% K2O. These variations are consistent with those uniform values for total clay minerals, quartz, feldspar, and calcite in the XRD analyses (Fig. F20). Several outlying samples of calcareous claystone have high CaO content and low Al2O3 and SiO2. At 332.12 m CSF (Section 322-C0012A-31R-4, 105 cm), the samples have high CaO values (up to 47.49 wt%) together with high P2O5 and MnO concentrations (up to 19.87 and 4.57 wt%, respectively) (Table T4). The high P2O5 content in the calcareous claystone 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). Sodium content shows a gradually decreasing trend (from 4.09 to 1.43 wt%) from Unit I to Unit IV. In Unit V, all major element concentrations show considerable scatter, and their values range from 8.25 to 19.15 wt% (Al2O3), 30.87 to 76.25 wt% (SiO2), 2.19 to 10.55 wt% (Fe2O3), 0.86 to 4.2 wt% (MgO), 1.24 to 50.32 wt% (CaO), 0.9 to 2.64 wt% (Na2O), and 0.5 to 3.31 wt% (K2O). The average values are 14.72 wt% for Al2O3, 63.24 wt% for SiO2, 6.03 wt% for Fe2O3, 2.01 wt% for MgO, 7.09 wt% for CaO, 1.94 wt% for Na2O, and 2.55 wt% for K2O (Table T4). Lithofacies at Site C0012At Site C0012, eight major lithologies are recognized (silty claystone, clayey siltstone, siltstone, tuffaceous sandstone, volcaniclastic sandstone, sandstone, red calcareous claystone, and basalt). Several minor lithologies such as volcanic ash/tuff, tuffaceous siltstone, calcareous claystone, lime mudstone, and chaotic deposits also occur (Table T2). These lithologies are grouped into six broad "lithofacies:" pelagic, hemipelagic, turbidite, chaotic deposits, tuff, and igneous rocks. The hemipelagic lithofacies is mostly composed of green-gray silty claystone, the dominant lithology throughout deposition at Site C0012. This lithology is intensely bioturbated and commonly intercalated with green silty claystone layers (<1 cm thick). It is interpreted to represent the "background" fallout from suspended sediment. The approximately constant sediment accumulation rate supports this view (see "Sediment accumulation rates at Site C0012"). Some minor lithologies, such as calcareous claystone and lime mudstone, contributed as nannofossils are mixed with hemipelagic sediment influx, although some carbonates may also be produced by diagenesis. Coarser grained lithologies (clayey siltstone, 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, mostly from turbidity currents. Together, we group these lithologies into the turbidite lithofacies. The volcaniclastic and siliciclastic sandstones in the lower part of Unit V, together with ash in Unit I, are interpreted as the deposits from various sediment gravity flows and belong to the turbidite lithofacies. Chaotic deposits occur in Units II, III, IV, and V. Throughout Cores 322-C0012A-1R through 22R there is severe drilling-induced deformation, which precludes any accurate assessment of the actual amount and thickness of the chaotic deposits. These chaotic deposits 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) or in response to essentially in situ deformation liked to seismicity on the normal fault flanking Kashinosaki Knoll (Fig. F21; see "Logging and core-log-seismic integration"). The steeply dipping beds in Unit III were probably formed by rotational normal faulting (see "Structural geology"). Together, these deposits constitute the chaotic lithofacies. The tuffs in the upper part of Unit V are interpreted as fallout pyroclastic deposits because they are estimated to contain >75% glass shards, show normal grading, and are texturally well sorted. These deposits form the tuff lithofacies. The red calcareous claystone in Unit VI is interpreted as pelagic deposition and is, therefore, the sole constituent of the pelagic lithofacies. The basalt in Unit VII belongs to the igneous oceanic crust, including pillow lavas from effusive eruptions and hyaloclastic breccias. Sediment accumulation rates at Site C0012An analysis of the sediment accumulation rates for the hemipelagites and turbidites show differences among lithologic units. For sedimentation accumulation analysis, 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. F22A). 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 turbidites was plotted against that of the hemipelagites (Fig. F22B). Although the recovery rate of most cores was considerably <100%, 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. There are two likely hiatuses at this site (see "Biostratigraphy" and "Paleomagnetism"). On the basis of the integrated age model, the youngest of the two hiatuses occurs (1.35 m.y.) within the uppermost interval of Unit III. In our analysis of the sediment accumulation rate, we followed the age model of the biostratigraphy because it is more consistent with the lithologic and structural observations of the inclined bedding (as described for Unit III). A large hiatus (up to ~4 m.y.) probably exists within the basal part of Unit V, and, therefore, we excluded the data obtained from the lower part of Unit V from our analysis. In addition, core recovery at this site was low (average = 43%) as compared with Site C0011 (average = 71%). Therefore, this analysis is less reliable than that for Site C0011, although both sets of results for the sediment accumulation rates show concordant trends. The sediment accumulation rates for the hemipelagites and turbidites 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. F22B). The results of this analysis suggest that the sediment accumulation rate for the hemipelagic deposits was constant (4.5 cm/k.y.) from Unit V to the lower part of Unit III, but in the middle part of Unit III, there appears to have been a significant decrease to 2.6 cm/k.y. (Fig. F22B). This change coincides with a shift in magnetic susceptibility (see "Physical properties"). The sediment accumulation rate was also constant throughout Unit II and into Unit I. The significant decrease in the sediment accumulation rate for the hemipelagites in Unit III at ~11–12 Ma suggests the possibility of some fundamental paleoenvironmental change at Sites C0011 and C0012. Our analysis shows that the sediment accumulation rate for the turbidites in Units II and IV averaged ~0.5–1.0 m per 1 m of deposition for the hemipelagites. The sediment accumulation rate of the turbidites, therefore, was ~1.3–2.6 cm/k.y., for a hemipelagic sediment accumulation rate of ~2.6 cm/k.y. At the Unit IV/III boundary, the sediment accumulation rate for the turbidites shows a substantial decrease (Fig. F22C), followed by a marked increase at the Unit III/II boundary. These trends are all similar to those observed at Site C0011. Paleogeography and sediment provenance at Site C0012Site C0012 is located 10 km south of Site C0011, and, therefore, the paleogeography and many of the related discussions are similar to those described for that site (see the "Site C0011" chapter). However, Hole C0012A reached basalt basement and allows us to interpret two lithologic units not previously discussed. The basement rock beneath the Kashinosaki Knoll is basalt (Unit VII) covered by a thin veneer of red calcareous claystone (Unit VI) that represents both pelagic and hemipelagic deposits. The igneous age of the basement is unknown, but calcareous nannofossil assemblages obtained from red calcareous claystone (Unit VI) require an age >18.9 Ma (see "Biostratigraphy"). Following cessation of pelagic sedimentation, the overlying submarine fan system(s) includes coarse-grained sandy turbidites with air fall tuff (Units V and IV) at this site. The lower part of Unit V is characterized by sandstone turbidites that show alternating compositional changes. Because these sandstones are either volcanic or terrigenous in composition, two discrete sediment sources can be assumed. One possibility is that such compositional differences may be related to separate feeder canyon and channel systems, originating from different parts of the Izu-Bonin arc (e.g., separated by a cross-chain in the Shikoku Basin). An alternative explanation is that the different sediment compositions reflect discrete contributions from both the Outer Zone of southwest Japan and the Izu-Bonin arc sources. Total organic carbon (TOC)/total nitrogen (TN) ratios in the fluid phase of the sediments in Unit V are consistent with a terrigenous component (Fig. F23; see "Organic geochemistry"). In contrast, the submarine fan system of Unit IV is composed mostly of terrigenous material, suggesting the possibility of a later change in the source area. The turbidity current influx ceased in the middle Miocene (~12.5 Ma) and was replaced by hemipelagic deposition at Site C0012 (Unit III). During that hemipelagic phase of sedimentation, the accumulation rate slowed at ~11–12 Ma, which is coincident with the cessation of voluminous forearc volcanism of southwest Japan at ~ 11 Ma (the transition from Stage II to Stage III of Kimura et al., 2005). After the deposition of hemipelagic Unit III, another turbidite system that contains abundant tuffaceous materials (Unit II) was activated during the late Miocene (~9 Ma). As discussed for Site C0011, we tentatively interpret this middle Shikoku Basin facies as the distal parts of a submarine fan that was sourced from the Izu-Bonin arc. This submarine fan system ceased during the late Miocene (~7.8 Ma), and hemipelagic sediment with air fall ash layers finally covered Site C0012 (Unit I). Seismic depth Section ODKM-22 crosses the summit of the Kashinosaki Knoll in the vicinity of Sites C0011 and C0012 and shows thinning of the Shikoku Basin lithologic units onto the basement high (Ike et al., 2008). The upper part of Unit III contains a minimum ~15 m thick interval with steeply inclined strata, typically at 40°–45° (Sections 322-C0012A-22R-1, 73 cm, through 23R-CC, 26 cm). This is interpreted as a rotational normal fault block on the flank of the Kashinosaki Knoll (see "Structural geology"). In the ~20 m above this interval of rotated strata, there appears to be a hiatus of ~1.35 m.y. (Sections 322-C0012A-21R-3, 7 cm, through 19R-1, 32–34 cm) (see "Biostratigraphy"). The occurrence of gravitational failure suggests that significant bathymetric relief existed on the Kashinosaki Knoll at the time. The older Units IV and V thin considerably toward the Kashinosaki Knoll (see "Logging and core-log-seismic integration"), which demonstrates considerable effects on seafloor bathymetry on the transport and deposition of the turbidites. Unit VI represents the thin veneer of pelagic sediments (~9.3 m thick) immediately overlying the basalt basement. Comparison between Site C0012 and ODP/IODP sitesThe seven lithologic units at Site C0012 tentatively correlate with lithostratigraphic units at Ocean Drilling Program (ODP) Sites 808, 1173, 1174, and 1177 (Table T5) (Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001; Underwood and Steurer, 2003). On the basis of lithology and age, Unit I at Site C0012 correlates with the upper Shikoku Basin facies at Sites 808 (Subunit IVa), 1173 (Unit II), 1174 (Unit III), and 1177 (Unit I) (Table T5). These deposits are all composed of hemipelagic mudstone with intercalations of thin ash. Unit I is Pleistocene–late Miocene in age at Site C0012, which also overlaps with the age of the upper Shikoku Basin facies at the other sites (above). At Sites C0011 and C0012, Unit II contains abundant tuffaceous and volcaniclastic turbidites and has no equivalent at any other ODP or IODP site (Table T5). No unequivocal ash fall deposit occurs in this unit. Unit II is assigned to the middle Shikoku Basin (see the "Site C0011" chapter for explanation) on the basis of age and lithology. Unit III at Site C0012 is older than, but correlates lithologically with, the lower Shikoku Basin hemipelagic facies at Sites 808 (Subunit IVb), 1173 (Unit III), 1174 (Unit IV), and 1177 (Unit II) (Table T5). Unit III is characterized by hemipelagic deposits with thin dark green layers (color banding), which is a common lithology in the lower Shikoku Basin facies of all correlative ODP sites in the Nankai Trough and surrounding deep-marine basins. Although ODP Leg 190 scientists did not interpret the origin of color banding, the dark gray to dark green claystone layers may be the alteration products of volcanic glass. 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 transition is also affected by a diagenetic front for ash layers (Taira, Hill, Firth, et al., 1991; Moore, Taira, Klaus, et al., 2001). Unit IV at Site C0012 roughly corresponds to the lower Shikoku Basin turbidite facies of Site 1177 (Unit III) (Table T5). Abundant muddy to sandy turbidites suggest that this unit was deposited in a submarine fan system. The lower Shikoku Basin turbidite facies of Site 1177 contains four packets of sandy turbidites, which contain abundant quartz, sedimentary and metasedimentary lithic fragments, and woody organic matter, all indicating a terrigenous origin. These features are similar to the composition of Unit IV at Site C0012, although the siltstone turbidites in Unit IV are finer grained. Unit V at Site C0012 is composed of both volcaniclastic and siliciclastic deposits, including tuff and volcaniclastic sandstone, and it is tentatively correlated with the volcaniclastic facies observed at other ODP and IODP sites (i.e., Sites 808, 1174, 1173, 1177, C0011) (Table T5). However, at Site C0012 there is only a thin interval (~25 cm) of felsic tuffs (and a slightly thicker interval of >15 m at Site C0011), which contrasts with Site 808, where ~50 m of acidic volcaniclastic deposits occur (Taira, Hill, Firth, et al., 1991). Volcaniclastic facies are widely distributed as deposits covering the basement rocks in the Nankai Trough, with ages increasing away from the axis of the Kinan Seamounts (Unit V at Sites 808, 1174, and C0011; Unit IV of Sites 1173 and 1177). Unit V at Site C0012 is ~14–19 Ma, which also overlaps the age of volcaniclastic deposits at previous ODP sites. Sites 808, 1173, and 1177 penetrated the basalt basement. Thus, the basalt basement rock (Unit VII), together with its thin veneer of red calcareous claystone (Unit VI) recovered at Site C0012, correlates with Unit VI of Site 808 and Unit V of Sites 1173 and 1177; basement ages increase with distance from the axis of the Kinan Seamounts. |
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