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doi:10.2204/iodp.pr.322.2009

Principal results

Site C0011

Core-Log-Seismic integration

Near the end of Expedition 319, measurement-while-drilling (MWD) and LWD data were collected at proposed Site NT1-07 (Hole C0011A). Although the time allocation for this operation was not long enough to reach basement, acquisition of these data proved to be extremely beneficial. MWD data include rate of penetration (ROP), rotational speed, and stick-slip. LWD data (acquired using Schlumberger geoVISION tool) include natural gamma radiation, five different resistivity readings, and resistivity images from button resistivity measurements. Generally, the data quality was good and we were able to make confident correlations between the logs and subsequent core description and multisensor core logger (MSCL) data, with a vertical offset of ~4 m between the coring hole and the logging hole. LWD data were divided into five logging units on the basis of visual inspection of the gamma ray and ring resistivity responses (Fig. F5). These divisions also correlate reasonably well with the seismic stratigraphy (Fig. F6). The log characteristics attributed to several secondary lithologies can be discriminated from a dominant background lithology interpreted to be hemipelagic mud(stone).

Logging Unit 1 (0–251.5 m LWD depth below seafloor [LSF]) exhibits minor fluctuations in gamma ray and a very gradual decrease in ring resistivity. These attributes are consistent with a lithology of hemipelagic mud (silty clay to clayey silt). Logging Unit 2 (251.5–478.5 m LSF) corresponds to a series of high-amplitude seismic reflections, which are laterally variable and discontinuous. The logging Unit 1/2 boundary is marked by steplike offsets of both gamma ray and resistivity values. The most striking features within logging Unit 2 are the high resistivity spikes, which coincide with low gamma ray peaks. The blocky log signature of this unit is indicative of local sand-filled channels (Fig. F7), and the higher resistivity values may be due to a high concentration of volcanic glass shards within the sand-size fraction (see "Lithostratigraphy"). Logging Unit 3 (478.5–736.0 m LSF) exhibits increases in both the gamma ray trendline and ring resistivity, with little variation. At ~650 m LSF, the log character changes to include low gamma ray peaks and low ring resistivity spikes (Subunit 3C), which we interpret to be intervals of terrigenous sand. Logging Unit 4 (736.0–867.0 m LSF) exhibits a series of four inferred fining- and coarsening-upward sequences that correlate with a series of high-amplitude seismic reflections with excellent lateral continuity. We interpret these intervals as packets of sand-rich turbidites. Logging Unit 5 (867.0–950.5 m LSF) is characterized by a sharp increase in the ring resistivity log, followed by minor fluctuations around a constant trendline. Gamma radiation decreases slightly with depth. Our interpretation of this unit is heavily consolidated hemipelagic mudstone.

Structural analysis of the borehole resistivity images shows that bedding dips <20° toward the north, which is consistent with the gentle dip observed in the seismic profiles down the seaward slope of the trench. Analysis of borehole breakouts indicates that the maximum horizontal stress field (S_Hmax) is orientated north-northeast–south-southwest, roughly perpendicular to the convergence direction of the Philippine Sea plate.

Lithostratigraphy

Because of the time constraints imposed by 11.5 days of operational time set aside for contingencies (e.g., typhoon evacuation), coring at Site C0011 (Hole C0011B) began at 340 m CSF rather than the mudline (Fig. F8). For the most part, core recovery was modest to poor. We identified five lithologic units in Hole C0011B (Fig. F9). Because Unit I was not cored, its inferred character is based on LWD data and analogy with the upper part of the Shikoku Basin at several other drilling sites (ODP Sites 808, 1173, 1174, and 1177). The dominant lithology of the upper Shikoku Basin facies is hemipelagic mud (silty clay to clayey silt) with thin interbeds of volcanic ash. Discrimination among all of the other lithologic units was based on a combination of visual core description (VCD), smear slide petrography, bulk powder X-ray diffraction (XRD), and X-ray fluorescence (XRF).

Lithologic Unit II is late Miocene (>7.07 to ~9.0 Ma) in age and extends from 340.0 to 479.06 m CSF. Because of its unique combination of detrital composition and age, we have designated this unit as the middle Shikoku Basin facies. This unit is divided into two subunits (Fig. F8). Subunit IIA (340.0–377.61 m CSF) consists of moderately lithified bioturbated silty claystone with interbeds of tuffaceous sandstone, whereas Subunit IIB (377.61–479.06 m CSF) contains bioturbated silty claystone, volcaniclastic sandstone, and dark gray siltstone without appreciable bioturbation. The distinction between tuffaceous and volcaniclastic sandstone is based on the abundance of volcanic glass shards (Fig. F10). Subunit IIB also contains a chaotic interval of intermixed volcaniclastic sandstone and bioturbated silty claystone (mass transport deposit). Bulk powder XRD data show scattering within Unit II as a function of grain size; sandy specimens are enriched in feldspar, whereas hemipelagic mudstones contain higher percentages of total clay minerals (Fig. F11). Bulk powder XRF data show an intriguing gradient of Al₂O₃ within Subunit IIA and an abrupt shift to higher Al₂O₃ below the subunit boundary (Fig. F12). This shift in bulk geochemistry may have been caused by a change in the clay-mineral assemblage. Judging from smear slide petrography, we suggest that the volcanic-rich sands were derived from an active volcanic arc as a mixture of primary eruptive products and reworking of pyroclastic and sedimentary deposits. The closest volcanic source was probably located along the northeast margin of the Shikoku Basin (Izu-Bonin arc), but shore-based analyses of the volcaniclastic grains will be needed to pinpoint the detrital provenance. Channellike sand-body geometry is evident in both LWD data (Fig. F7) and seismic character, and transport/deposition probably occurred in the distal part of a submarine fan.

Lithologic Unit III is middle–late Miocene (~9.0 to ~12.0 Ma) in age and extends from 479.06 to 673.98 m CSF. The dominant lithology is bioturbated silty claystone, typical of the hemipelagic deposits in the Shikoku Basin. Secondary lithologies include sporadic dark gray silty claystone, lime mudstone, and very thin beds of ochre-colored calcareous claystone. The unit's boundaries are defined at the top by a thin bed of cemented terrigenous sandstone and at the bottom by the appearance of dark gray clayey siltstone (mud turbidites). The most interesting aspect of this unit is a change in the rate of hemipelagic sedimentation at ~11 Ma (Fig. F13), which is also evident from both biostratigraphic and paleomagnetic data uncorrected for compaction (see "Biostratigraphy" and "Paleomagnetism"). Bulk powder XRD and XRF data show monotonous compositions within Unit III, punctuated by scattered layers of carbonate-rich mudstone (Figs. F11, F12).

Lithologic Unit IV is middle Miocene (~12.0 to ~14.0 Ma) in age and extends from 673.98 to 849.95 m CSF. Core recovery within this interval was particularly poor, and our interpretations were further hampered by poor core quality and the decision to wash down without coring from 782.6 to 844.0 m CSF. The dominant lithology of Unit IV is bioturbated silty claystone with abundant interbeds of dark gray clayey siltstone (deposited by muddy turbidity currents) and fine-grained siliciclastic sandstone (deposited by sandy turbidity currents). The sandstone beds are 10–80 cm thick and typically display plane-parallel laminae. Small wood fragments are common, as are detrital grains of polycrystalline quartz and metamorphic rock fragments. We interpret the terrigenous source of this sandy detritus to be somewhere along the outer zone of southwest Japan, where such tectonostratigraphic units as the Sanbagawa metamorphic belt and the Shimanto Belt crop out across the strike length of the Kii Peninsula, Shikoku, and Kyushu (e.g., Taira et al., 1989; Nakajima, 1997). Superficially similar sand deposits with overlapping ages have been documented on the west side of the Shikoku Basin at Site 1177 and DSDP Site 297 offshore the Ashizuri Peninsula of Shikoku (Marsaglia et al., 1992; Fergusson, 2003; Underwood and Fergusson, 2005).

The age of lithologic Unit V is poorly constrained with in the range of middle Miocene (~14.0 Ma). The unit extends from 849.95 to 876.05 m CSF, but our ability to characterize these strata was hampered by poor core recovery. The unit's upper boundary is defined by the first occurrence of tuff, and the lower boundary coincides with the destruction of the drill bit, which led to cessation of coring. The dominant lithologies are tuffaceous silty claystone and light gray tuff with minor occurrences of tuffaceous sandy siltstone (Fig. F14). XRD data show an abundance of zeolites (undifferentiated clinoptilolite/heulandite and analcime) and smectite within this unit as alteration products of volcanic glass. The tuff deposits are probably correlative with the thick rhyolitic tuffs that were recovered at Site 808 from the Muroto transect of the Nankai Trough, which yielded an age of ~13.6 Ma (Taira, Hill, Firth, et al., 1991).

Structural geology

More than 300 individual structural features were described and measured in the cores from Hole C0011B. Whenever possible, these features were reoriented to a geographic reference frame using shipboard paleomagnetic data. Most of the features are characterized by subhorizontal to gently dipping bed planes and small faults (Fig. F15). Synsedimentary creep structures and layer-parallel faults (referred to as deformation bands at Sites 1174 and 808) developed in lithologic Units II and III, whereas a high-angle normal fault/fracture system exhibiting brittle features is pervasive in lithologic Units IV and V. Deformation-fluid interactions were also deduced from mineral-filled veins precipitated along faults in the lowermost part. Although the numbers of paleomagnetic correction are limited, attitudes of these structures seem to be controlled by bathymetry of the trench-accretionary prism system; poles to these structures are distributed along a north-northwest–south-southeast trend, perpendicular to the present trench axis. The structural distributions in cores correlate nicely with the logging-based measurements of planar orientations (Fig. F5).

Biostratigraphy

Preliminary analysis of samples from Hole C0011B revealed assemblages of calcareous nannofossils and planktonic foraminifers. Biostratigraphic datums are taken mainly from coccoliths. According to these datum events, the composite sequence for Hole C0011B ranges from middle to upper Miocene, roughly equivalent to 13.65 Ma at the lowest datum up to 8.52 Ma at 425 m CSF (Fig. F9). In spite of dissolution and numerous barren cores, the floral and faunal assemblages provide some insights into paleoceanographic conditions. In the upper Miocene, for example, the sediments contained warm-water fossils such as the calcareous nannofossil genus Discoaster.

Paleomagnetism

Because of the failure of the cryogenic magnetometer (super-conducting quantum interference device), paleomagnetic studies for Hole C0011B consisted of natural remanent magnetization (NRM) measurements and alternating-field and thermal demagnetizations on discrete samples. We noted that tuffaceous sandstone and volcanic sand layers within lithologic Unit II display NRM intensity peaks. There is a broad NRM intensity high from 530 to 570 m CSF that appears to correspond to changes in P-wave velocity and sedimentation rate at ~11 Ma. Changes in magnetic susceptibility (from both whole-core pass-through measurements by the whole-round multisensor core logger [MSCL-W] and discrete paleomagnetic samples with a Kappabridge) are largely consistent with the variations in NRM intensity. Both responses are probably caused by fluctuations in the type and/or abundance of magnetic minerals. Several relatively well-defined polarity intervals were identified in downhole magnetostratigraphic records, in spite of the presence of unstable and ambiguous magnetization, and these reversals were correlated with the geomagnetic polarity reversal timescale.

Integrated age-depth model

Using biostratigraphic data, we were able to correlate certain parts of the magnetic polarity interval with the geomagnetic polarity reversal timescale and create an integrated age-depth model (Fig. F16). Overall, the paleomagnetic data indicate ages ranging from ~7.6 to ~14.1 Ma. The composite age-depth model yields rates of sedimentation (uncorrected for either compaction or rapid event deposition by gravity flows) ranging from ~4.0 cm/k.y. in the upper part of Unit III to 9.5 cm/k.y. in the lower part of lithologic Units III and IV (Fig. F16). The average rate for lithologic Unit II is 9.4 cm/k.y. The sedimentation rate within Unit III changed at ~11 Ma. Magnetostratigraphic records also suggest the existence of a geomagnetic excursion or short reversed polarity event within lithologic Unit V. This event may have been caused by rapid deposition of tuffaceous material with a thickness of ~60 cm.

Physical properties

Physical property measurements at Site C0011 provide indications of bulk formation properties, which we were able to correlate with lithologic variation and compare with profiles from other drill sites in the Shikoku Basin. For example, sharp increases in magnetic susceptibility (from the MSCL) correlate nicely with individual sandstone beds in lithologic Unit II (Fig. F17). In addition, a shift in magnetic susceptibility in Unit III near 575 m CSF correlates with a change in sedimentation rate at ~11 Ma (Fig. F16). Calculated bulk density and porosity show a downhole increase in bulk density (1.5–2.1 g/cm³) and decrease in porosity (0.55–0.32), indicative of sediment consolidation (Fig. F18). These trends correlate with increases in compressional velocity (V_P) and electrical resistivity. The velocity-porosity relation is consistent with previous observations from Shikoku Basin sediments (Hoffman and Tobin, 2004). Although the trend for V_P increases downhole, this increase is subdued from 575 m CSF to the base of Unit III, thereby correlating with the change in magnetic susceptibility mentioned above. Velocity anisotropy changes from isotropic to anisotropic (i.e., horizontal velocity faster than vertical velocity) near 440 m CSF (Fig. F19), and we attribute this change to compaction-induced alignment of mineral grains. Thermal conductivity varies between 0.98 and 1.77 W/(m·K) in mudstone and between 1.11 and 1.75 W/(m·K) in sandstone.

Although crude first-order trends are clear in the profiles, drilling and coring disturbance perturbed the results of bulk physical property measurements in several important ways. To begin with, decreased core diameter from rotary core barrel (RCB) coring resulted in MSCL-W calculated values that are not representative of in situ conditions; these volume-related artefacts can be remedied through advanced processing involving integration of X-ray computed tomography data. Another likely source of error is inaccurate grain density measurements, as those values also show an unusually high range of scatter. A more serious problem is that many cores were damaged by microscale cracks and fractures induced by drilling, coring, and/or the recovery processes, particularly during periods of high heave. Our sampling procedure tried to avoid regions that had visible fractures; however, the data show evidence of pervasive internal disturbance. This disturbance is most obvious in the unusually high variability in bulk density and porosity and in P-wave velocity measured on cube samples (Figs. F18, F19). The observed trend of thermal conductivity, which decreases as porosity decreases, is counter-intuitive and also confirms the existence of small water-filled fractures, leading to measured values that are lower than in situ values.

Downhole measurements

Because of time constraints and the absence of hydraulic piston coring system coring at both sites, we failed to make any temperature measurements during Expedition 322. The sediment temperature-pressure (SET-P) tool was successfully tested in the drill string. Prior to deployment, the SET-P tool was connected to the colleted delivery system (CDS) and placed into the RCB bottom-hole assembly (BHA) to ensure that the landing mechanism engaged correctly. The test consisted of lowering the SET-P tool downhole with stops at 988 and 1989 m below sea level for reference measurements. Measured pressure data confirmed hydrostatic pressure within the drill string, and the temperature data correctly recorded temperature variation. Based on the observations, the test deployment was successful. The SET-P tool was not deployed in the formation, however, because of ambiguity in correlating LWD data with MSCL-W data, the sporadic presence of hard layers, and the lack of sand(stone) recovery in previous cores. This combination of factors led to large uncertainties in the location and in situ properties of our primary targets (thick sand beds), which elevated the risk of deployment to unacceptable levels.

Inorganic geochemistry

The inorganic geochemistry objectives at this site were partially achieved by collecting 46 whole-round samples for interstitial water analysis. Routine sampling density was one per core, but collection of good-quality samples was difficult throughout the hole because of extreme core disturbance and low water content of the formation. Contamination by seawater is evident in all data profiles and corrections were made on the basis of sulfide concentrations.

The top of the sampled sediment section (340 m CSF) lies beneath the sulfate/methane interface; thus, the shallow processes associated with organic carbon diagenesis were not sampled here. Cl concentration in the sampled fluids decreases from ~550 mM to ~7% less than seawater (Fig. F20). This freshening trend is superficially consistent with the trend observed at Site 1177, which was drilled seaward of the deformation front in the Ashizuri transect during Leg 190 (Moore, Taira, Klaus, et al., 2001). Judging from the similarity of the two profiles, we suggest that the sampled fluids may have originated from greater depth by clay dehydration reactions. If this is true, then fluid migration toward Site C0011 must have occurred updip along permeable conduits of turbidite sandstone. Unfortunately, recovery of the sandstone was insufficient to characterize their hydrogeologic properties and vertical extent. The lack of reliable temperature constraints also hinders a more definitive interpretation of the chloride data (i.e., in situ versus deeper seated dehydration). The distributions of major and minor cations document extensive alteration of volcanogenic sediments and oceanic basement, including the formation of zeolites and smectite-group clay minerals. These reactions lead to consumption of silica, potassium, and magnesium and production of calcium (Fig. F20). The very high calcium concentrations (>50 mM) favor authigenic carbonate formation, even at alkalinity of <2 mM. Those results are consistent with the recovery of scattered lenses/beds of lime-rich mudstone (Fig. F11).

Organic geochemistry

Concentrations of nitrogen and sulfur are very low in all lithologic units. Organic carbon, total sulfur, and total nitrogen contents are, on average, equal to 0.31 ± 0.17 wt%, 0.37 ± 1.47 wt% and 0.06 ± 0.02 wt%, respectively. Organic carbon contents show more scatter in lithologic Unit II than in Unit III (Fig. F21). C/N ratios ~6.0 ± 3.1 indicate a marine origin of the sedimentary organic matter, but two elevated values in Units II and IV suggest increased input from terrigenous sources. In general, inorganic carbon contents (0.35 ± 0.93 wt%) are only slightly larger than organic carbon contents and correspond to a mean calcium carbonate content of 2.95 wt%. Elevated carbonate contents in Unit III reach up to 61.7 wt% and coincide with thin beds of lime-rich mudstone.

In spite of the sediment's very low organic carbon content, dissolved hydrocarbon gas concentrations in the interstitial water increase with depth. Methane is present as a dissolved phase in all samples (Fig. F22). Ethane was detected in all but one core taken from depths >422 m CSF. Dissolved propane was first observed at 568 m CSF and is present in almost all deeper cores. Butane occurs sporadically deeper than 678.5 m CSF. The occurrence of ethane below 422 m CSF results in low C₁/C₂ ratios ~277 ± 75 (Fig. F22). The very low C₁/C₂ ratios are unusual for sediments with organic carbon contents of <0.5 wt%. Without better constraints on temperature at depth, these results are difficult to interpret with confidence. In situ production of heavier hydrocarbons is possible at Site C0011, but that would require burial temperatures warmer than predicted by the erratic values of near-surface heat flow near the Kashinosaki Knoll (Yamano et al., 2003). In addition, unlike the monotonic increases in headspace gas concentrations at Site 1173 (Moore, Taira, Klaus, et al., 2001), the dissolved hydrocarbon concentrations increase within lithologic Units III and IV but then decrease within lithologic Unit V (Fig. F22). We attribute the pattern to fluids originating from deeper/hotter sources and migrating along intervals of higher permeability (see "Inorganic geochemistry").

Microbiology

The microbiology component of Site C0011 was limited to whole-round sampling for molecular (phylogenetic) studies, fluorescence in situ hybridization (FISH), and cell counting studies. Interstitial water samples were also obtained for shipboard spectrophotometric analyses of ferrous iron and acid volatile sulfide and measured using a third-party tool. The results of these measurements are tabulated along with inorganic geochemistry data.

Site C0012

Lithostratigraphy

At Site C0012, we identified six sedimentary lithologic units on the basis of sediment composition, sediment texture, and sedimentary structures (Fig. F23). Unit VII is composed of basalt (igneous basement). For the most part, core recovery was modest to poor (Fig. F24) and damage to the core was typically severe. Nevertheless, several correlations of distinctive marker beds were made between this site and Site C0011. For example, we identified a distinctive cemented yellow carbonate at ~229 m CSF in Hole C0012A (7 cm thick), with a similar 5 cm thick carbonate layer at ~229 m CSF in Hole C0011B. We also recognized several lithologic units common to both sites.

Lithologic Unit I was not cored between 0.81 and 60 m CSF. It is 150.86 m thick and extends from the seafloor to 150.86 m CSF, below which we observed the first occurrence of volcaniclastic sandstone. The age of this interval ranges from Quarternary to late Miocene (0 to ~7.1–8.5 Ma). The dominant lithology is green-gray intensely bioturbated silty clay, typical of hemipelagic mud deposits. Thin layers of volcanic ash are scattered throughout. The results of bulk powder XRD show modest amounts of calcite within this unit, consistently above trace quantities (Fig. F25). This retention of biogenic carbonate is compatible with deposition on top of Kashinosaki Knoll at a water depth close to (but above) the calcite compensation depth. This lithologic unit is correlative with logging Unit 1 at Site C0011.

Lithologic Unit II is late Miocene (~7.1–8.5 to ~8.8–9.6 Ma) in age. We applied a new designation of middle Shikoku Basin facies because of this unit's unique composition (i.e., volcanic rather than siliciclastic sand) and age relative to broadly equivalent Miocene turbidites cored along the Ashizuri transect. It is 68.95 m thick and extends from 150.86 to 219.81 m CSF. Stratigraphic equivalents to the basal part of Subunit IIA (as defined in Hole C0011B) appear to be present here as very coarse- to fine-grained tuffaceous to volcaniclastic sandstone. The dominant lithology is green-gray silty claystone, alternating with medium- to thick-bedded tuffaceous/volcaniclastic sandstone and dark gray clayey siltstone. Lithologic Unit II contains two chaotic deposits that are 0.3 and 3.1 m thick (150.86–151.17 and 178.00–181.10 m CSF, respectively). As observed at Site C0011, these deposits consist of disaggregated pieces of volcanic-rich sandstone and bioturbated silty claystone that show folding, thinning, and attenuation of primary bedding. They are interpreted to have formed by gravitational sliding on the relatively steep north-facing slopes of Kashinosaki Knoll. XRD data show scatter in concentrations of total clay minerals and feldspar because of lithologic heterogeneity (Fig. F25). The closest source of volcanic sand was probably the Izu-Bonin volcanic arc (Taylor, 1992). Unlike Site C0011, however, deposition of sand on top of the knoll must have required upslope transfer by turbidity currents (e.g., Muck and Underwood, 1990) and/or postdepositional uplift of the basement high.

Lithologic Unit III is middle Miocene (~8.5 to ~12.5 Ma) in age. It is 112.0 m thick and extends from 219.81 to 331.81 m CSF. The interval is characterized by bioturbated silty claystone, typical of hemipelagic deposition. With the exception of scattered carbonate beds, the homogeneity of this unit results in very consistent XRD-deduced mineral abundances and major oxides (Figs. F25, F26). Other exceptions are related to beds with unusually high concentrations of clay minerals (possible bentonites). Unit III also contains an interval at least 15.2 m thick with steeply inclined bedding, typically at an angle of 40°–45° (see "Structural geology"). From seismic evidence, this disruption appears to be associated with rotational normal faulting. Biostratigraphic data also yield evidence of a possible hiatus near the top of this interval.

Lithologic Unit IV is middle Miocene (>12.0 to <13.6 Ma) in age. It is 86.48 m thick and extends from 331.81 to 415.58 m CSF. This interval is characterized by alternations of silty claystone, thin clayey siltstone, and fine-grained, normally graded volcaniclastic sandstone. We interpret the siltstone and fine sandstone beds to have resulted from muddy and sandy turbidity currents, respectively. This unit shows a trend of increasing Fe₂O₃ and decreasing Na₂O with depth (Fig. F26), and XRD data show some shifts in mineralogy because of grain size effects (Fig. F25).

The age of lithologic Unit V ranges from early to middle Miocene (>13.6 to 17.7–18.9 Ma). It is 112.93 m thick and extends from 415.58 to 528.51 m CSF. The Unit IV/V boundary is subtle and defined by an appearance of volcanic tuff. The main lithology in Unit V is silty claystone; coarser interbeds in the lower part of the unit consist of siliciclastic sandstone, volcaniclastic sandstone, and rare tuff. The sandstones, some of which display spectacular cross-laminae, plane-parallel laminae, convolute laminae, and soft-sediment sheath folds (Fig. F27), appear to have two separate detrital provenances judging from point counts of smear slides: (1) a volcanic source with fresh volcanic glass, together with relatively large amounts of feldspar, and (2) a siliciclastic source enriched in sedimentary lithic grains, quartz, and heavy minerals (including pyroxene zircon and amphibole). The outer zone of southwest Japan is the likely siliciclastic source. Bulk XRD confirms that relative percentages of quartz and feldspar increase significantly in the coarse-grained strata (Fig. F25), and XRF data show appreciable scatter in most of the major oxides because of lithologic/textural heterogeneity (Fig. F26). This range of compositions, textures, and ages is also reminiscent of the older turbidite intervals and volcaniclastic-rich facies at Site 1177 (Moore, Taira, Klaus, et al., 2001).

Lithologic Unit VI is early Miocene in age (>18.921 Ma). It is only 9.3 m thick and extends from 528.51 to 537.81 m CSF. This unit is characterized by variegated red, reddish brown, and green calcareous claystone, rich in nannofossils, with minor amounts of radiolarian spines. Carbonate content is ~20 wt% based on bulk powder XRD analysis (Fig. F25). We interpret this unit as a pelagic clay deposit in direct contact with igneous basement.

Igneous petrology

Successful recovery of volcanic basement in Hole C0012A defined lithologic Unit VII. On the basis of the nannofossils in overlying red claystone, the volcanic rock is older than 18.921 Ma. The cored interval extends from 537.81 to 576 m CSF, and a sharp contact between red claystone and basalt is beautifully preserved in Core 322-C0012A-53R (Fig. F27). The 38.2 m of basement coring resulted in 18% recovery, consisting of (1) pillow lava basalts, (2) basalts, (3) basaltic-hyaloclastite breccia, and (4) mixed rubble pieces of basalt caused by drilling disturbance. Basalts and pillow basalts have aphanitic to porphyritic textures. 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 limits between highly phyric and sparsely phyric basalts. Phenocrysts are composed mostly of plagioclase, pyroxene, and sparse altered olivine. Othopyroxene is dominant compared to clinopyroxene. Pyrite is present as an accessory mineral in some basalts. Vesicularity is highly heterogeneous. In a single sequence, basalts can have sparse (1%–5%) to high (>20%) vesicle content. VCD and thin section analyses show evidence of magma mixing.

Alteration ranges from moderate to very high, with a large proportion of basalts being highly altered. Alteration styles include interstitial groundmass replacement, vesicle fill, vein formation (with associated alteration halos), and the complete replacement of pillow lavas glass rims by alteration materials. Secondary mineralogical phases are comprised of saponite, celadonite, zeolite, pyrite, iddingsite, quartz, and calcite. 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 recovered, but many fractures are 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 potions and underlying lavas.

Structural geology

Structures measured in cores from Hole C0012A consist of bedding planes, minor faults, and fractures. The beds usually dip gently to the north; steep bedding dips occur only in lithologic Unit III (Fig. F28). Rotation of bedding to higher angles probably occurred by large block sliding on the north-inclined seafloor of Kashinosaki Knoll. The sliding apparently caused a time gap (angular unconformity) as deduced by nannofossil datums (see "Biostratigraphy"). Small high-angle faults and fractures strike north-northwest–south-southeast, and the poles show girdle distribution trending east-northeast–west-southwest. Because most slickenlines on the fault surfaces exhibit dip-slip movements, the direction of intermediate principal stress apparently trends north-northwest–south-southeast, perpendicular to the trench. Deformation-fluid interactions were also deduced from calcite-filled veins precipitated along faults in the lower part of the sedimentary section.

Biostratigraphy

Preliminary analysis of the core catcher and additional samples from Cores 322-C0012A-1R through 52R reveal assemblages of calcareous nannofossils and planktonic foraminifers. Biostratigraphic datums are recognized mainly from calcareous nannofossils. The oldest depositional age at Site C0012 is early Miocene (18.921–20.393 Ma; that event has a depth range of 528.67–530.32 m CSF). The average calculated sedimentation rate (uncorrected for compaction) changes from 3.07 cm/k.y. above 216 m CSF to 6.42 cm/k.y. below 235 m CSF. That change occurred within lithologic Unit III at ~9.56–10.88 Ma, and it may be associated with a brief hiatus in sedimentation (i.e., the angular unconformity described in "Structural geology"). Another possible hiatus exists above the Unit V/VI boundary (14.914–18.921 Ma).

Paleomagnetism

Magnetic property variations among various lithologies are similar to those observed at Site C0011. For example, the typical silty claystones yield relatively low NRM intensity compared to sandstone and volcaniclastic sandstone. In lithologic Unit III, both NRM intensity and magnetic susceptibility show a steady downhole decrease. We collected four basalt samples from the basement unit and subjected them to detailed alternating-field (AF) demagnetization experiment. Apparent self-reversal of remanence is observed during the AF demagnetization on two of the basaltic samples. This behavior could be due to a self-reversal of pyrrhotite and needs to be further examined during shore-based studies. Several relatively well-defined polarity intervals were identified in downhole magnetostratigraphic records. Using biostratigraphic data, we were able to correlate patterns of the magnetic polarity interval recorded in the sediments against the standard geomagnetic reversal timescale. In particular, the polarity interval between 5 and 8 Ma can be correlated with confidence and with somewhat less confidence back to 11 Ma.

Integrated age-depth model

The integrated age-depth model from biostratigraphy and magnetostratigraphy gives an age of ~7.8 Ma at the Unit I/II boundary (Fig. F29) and indicates a significant increase in sediment accumulation rate across the boundary (from ~2 cm/k.y. above to ~6 cm/k.y. below). The composite model yields an age of ~10.2 Ma at the Unit II/III boundary. The age model for older parts of the sedimentary section is less certain, especially where core recovery was poor. The calculated sedimentation rate is fairly uniform to 430 m CSF, below which the rate increases significantly to ~46 cm/k.y. This acceleration is a response to rapid accumulation of thick turbidite sand and silt beds (see "Lithostratigraphy"). The integrated model indicates an age of ~14.4 Ma for the Unit IV/V boundary (Fig. F29). The calculated sedimentation rate below 470 m CSF decreases to ~8 cm/k.y. down to 500 m CSF.

Physical properties

The physical properties of sedimentary strata at Site C0012 are similar to those documented at Sites C0011 and 1177. Profiles show downhole increases in bulk density, electrical resistivity, thermal conductivity, and P-wave velocity, together with a downhole decrease in porosity (Fig. F30). These trends are consistent with conditions of normal consolidation. It is important to note, however, that multiple forms of disturbance from drilling and coring processes affected core quality in a negative and widespread way as did the scatter in grain density values. These artifacts reduced the quality of data and should be considered as additional research progresses, including shore-based geotechnical experiments. Typical symptoms of coring damage include decreased core diameter, microscale water-filled cracks and fractures, and abnormally low thermal conductivity. On the positive side, Hole C0012A did yield a few intervals of good core recovery where data are of high quality. Furthermore, it is possible to recognize and interpret the general depth-dependent trends in physical properties through the cloud of disturbance effects.

Spikes in magnetic susceptibility in lithologic Unit II correlate with sandstone beds. In Unit III, we observe a step decrease in magnetic susceptibility similar to the one at Site C0011, which correlates to a change in sediment accumulation rate. An interval with increasing magnetic susceptibility and decreasing natural gamma radiation from 480 m CSF to the base of Unit VI correlates with the appearance of sandstone layers containing iron-rich minerals.

The consolidation trend for Site C0012 hinges on a single near-seafloor measurement of 0.70 in Core 322-C0012-1R; values decrease to a porosity of 0.35 by 530 m CSF (Fig. F30). Compressibility is twice that interpreted for Site C0011. In a manner somewhat similar to what was documented at Sites 1173, 1177, and C0011, Site C0012 displays an anomalous interval in the shallow subsurface, although its existence is far more subtle (Fig. F30). This zone of near-constant porosity is also shallower (100–136 m CSF) than at the other sites. We identify two additional zones of anomalous porosity: scattered values throughout lithologic Unit III, and the top of lithologic Unit V. Intervals and individual specimens with abnormally high porosity indicate lower apparent compressibility, and the changes could be controlled by grain shape, sediment composition, or cement. Higher porosity near the Unit IV/V boundary may facilitate fluid migration at depth.

Increases in P-wave velocity downhole are consistent with increases in bulk density and decreases in porosity (Fig. F30). Velocity in the sedimentary section ranges from 1600 to 2100 m/s. Samples from Sites C0011 and C0012 have higher velocity for a given porosity than at Site 1173. This suggests higher bulk and/or shear moduli of sediments at Sites C0011 and C0012. P-wave velocity of basalt ranges from 3000 to 4750 m/s. Vertical-plane velocity anisotropy is nearly zero in lithologic Unit I but then increases with depth through Unit II into Unit VI. This likely reflects preferential grain orientation and enhanced grain contacts from consolidation. Around 450 m CSF, a cluster of samples near sand-rich layers has negative V_P anisotropy.

Thermal conductivity varies between 0.98 and 1.47 W/(m·K) in mud(stone) and between 1.19 and 2.10 W/(m·K) in sand(stone). Thermal conductivity increases with depth in lithologic Units I and II; this change is controlled by porosity loss. A step decrease in thermal conductivity in Unit III coincides with a marked decrease in core quality.

Inorganic geochemistry

A total of 42 pore fluid samples were collected from whole-round sections between 89.4 and 529.5 m CSF for chemical and isotopic analyses. Pore water volume decreases monotonically with depth, but in contrast to the distribution in Site C0011, there is no correlation between the volume of water and lithologic unit or sand content. Even though the strata here are moderately lithified, the core quality is significantly better than at Site C0011; thus, contamination by drilling fluid was much less severe. The results from Site C0012 come as close as we can get to a true geochemical reference site for the Nankai Trough. There are no obvious effects of abnormally high heat flow (as we see along the Muroto transect) and/or deep burial beneath the Quaternary trench wedge and accretionary prism toe.

The dissolved sulfate profile for Hole C0012A shows quite a bit of structure (Fig. F31), which is consistent with biogeochemical processes. The observed distribution precludes any correction for potential contamination because a total depletion of sulfate in the formation fluids could not be assumed. The sulfate profile clearly documents an abnormal sulfate reduction zone (Fig. F31); its depth is significantly deeper than those detected at other sites along the Nankai margin. At Site 1173, for example, sulfate is undetectable below 6 meters below seafloor (mbsf) (Moore, Taira, Klaus, et al., 2001). The subdued microbial activity in the upper sections of Hole C0012A may be due to lower sedimentation rates above the basement high, as compared to the flanks of the Kashinosaki Knoll, which receive more voluminous supplies of sediment by turbidity currents. Sulfate depletion at ~300 m CSF also coincides with an increase in dissolved methane and ethane concentrations in the interstitial water (Fig. F32). One viable explanation for the presence of these hydrocarbons, which occur in significantly lower concentrations here than at Site C0011, is updip migration of gas in solution from deeper thermogenic sources. Another possibility is in situ biogenic formation from terrigenous organic matter. Regardless, it is likely that the sulfate concentrations at Site C0012 are modulated by anaerobic methane oxidation (AMO), thereby leading to the production of hydrogen sulfide. In support of this idea, we observed a marked increase in hydrogen sulfide concentration concomitant with the peak in methane concentration. Among the iron sulfide minerals, pyrite is the most stable phase and is common in the sediments over a comparable depth range.

Unlike Site C0011, chlorinity values at Site C0012 increase by 12% relative to seawater, with uniform values of ~560 mM within lithologic Units I and II to a maximum of 627 mM at 509 m CSF (Fig. F31). This steady increase in chlorinity is probably a response to hydration reactions during alteration of both basaltic basement and dispersed volcanic ash (i.e., volcanic glass reacting to smectite and zeolites within Units IV and V). The profile for Site 0012 is unique with respect to all of the other Nankai sites; it shows no indication of the freshening patterns observed elsewhere (Taira et al., 1992; Moore et al., 2001). If the freshened pore fluids at Site C0011 did originate from deeper sources landward of the prism toe and if updip migration was focused through highly permeable horizons, then the absence of freshening at Site C0012 indicates a lack of hydrologic connectivity between the flanks and the crest of the basement high.

Alteration of volcanic ash and volcanic sand within the middle range of the sedimentary section (lithologic Units III, IV, and V) probably controls the changes in silica, potassium, and magnesium concentrations (Fig. F31), which almost certainly are in equilibrium with montmorillonite (a common smectite group clay mineral) below 250 m CSF. Increases in dissolved calcium because of the onset of ash alteration in Unit II are probably overprinted by deep-seated reactions in oceanic basalt, including the alteration of plagioclase. The calcium profile is almost linear (Fig. F31), which indicates that the distribution of this major element is mainly controlled by a diffusion gradient between seawater values at the top and basement fluid at the bottom. Dissolved calcium concentrations are even higher than observed at Site C0011 and help explain the precipitation of CaCO₃ (observed as thin layers and nodules), even at very low alkalinity (<2 mM). Depletion of dissolved sodium is probably also due to formation of zeolites (clinoptilolite, heulandite, and analcime) from alteration of dispersed volcanic glass and plagioclase in the basalt.

In addition, however, the profiles of all major cations and sulfate show an intriguing reversal toward more seawater-like values within the lower half of lithologic Unit V (Fig. F31). We tentatively attribute this shift to fluids moving into and through the turbidites of lithologic Unit V. The hydrology responsible for this flow, including potential recharge and discharge zones within upper igneous basement, remains unidentified.

The shipboard results require refined shore-based analyses to interpret the geochemistry with greater confidence, but they point to the intriguing possibility of two distinct fluid regimes within the sedimentary strata seaward of the trench. One regime is characterized by flow toward the Shikoku Basin from subducting sediments (i.e., due to a combination of compaction and mineral dehydration reactions) together with migration through high-permeability horizons of middle and lower Shikoku Basin facies. The geochemical fingerprints for this regime are a combination of fluid freshening and the presence of heavier hydrocarbons at all sites drilled on the basement plain, including Sites C0011, 1173, and 1177. No such freshening is observed at Site C0012. Rather, the only hints of a lateral flow are small concentrations of dissolved methane and ethane, which indicate that a modest contribution of hydrocarbons did reach the top of the basement-high site. Unlike its flanks, the crest of Kashinosaki Knoll reveals a separate pore water regime driven by in situ hydration reactions, diffusion, and migration of a higher chlorinity more seawater-like fluid into the sediment pile from the underlying igneous basement. Especially noteworthy is the observed increase in sulfate below 490 m CSF, which cannot be supplied by the methane bearing fluids that moved toward the knoll from the deeper landward side. Furthermore, the fact that we see an increase in hydrogen sulfide produced by AMO in the overlying sediments argues for sustained presence of sulfate below 490 m CSF, which must be supplied by active flow in lithologic Unit V.

Organic geochemistry

At Site C0012, dissolved hydrocarbon gases were not detected in the upper 189 m of sediment. Low levels of dissolved methane were observed in all cores below this depth (Fig. F32). Methane concentrations increase to a maximum of 244 µM at 417 m CSF. In the same vicinity, dissolved ethane (C₂) was detected and reaches a concentration maximum of 3.9 µM at 417 m CSF before dropping off toward basement. Propane (C₃) and butane (C₄) are absent in all cores. The methane and ethane concentration maxima fall within a zone of sporadic increases in total organic carbon (TOC) content (Fig. F33), but the very low C₁/C₂ ratios are unusual for sediments with such generally low contents of organic carbon (<0.5 wt%). Potential sources of the higher hydrocarbon gases (i.e., in situ biogenic production versus migration from deeper hotter sources) remain to be explored by additional shore-based investigations. We regard the pattern of steady reduction of hydrocarbon concentrations toward the base of lithologic Unit V (Fig. F32) as evidence against in situ thermogenic production.

In general, nitrogen and sulfur contents are low (Fig. F33). Inorganic carbon contents (0.39 ± 0.90 wt%) are only slightly higher than organic carbon contents (0.26 ± 0.14 wt%) but show sporadic excursions toward higher values in all lithologic units. The corresponding mean calcium carbonate value is 3.26 wt%, but local occurrences of lime mudstone reach 63.6 wt% (consistent with XRD data). Total sulfur contents are on average 0.24 wt% but show distinct excursions toward higher values at the Unit IV/V boundary and reach a maximum of 4.3 wt% in Unit V. C/N ratios ~6.7 indicate a predominantly marine origin of the organic matter, but ratios >25 indicate the presence of terrigenous organic matter in Unit V.

The occurrences of higher TOC and sulfur contents in the upper part of Unit V are striking and fall within the depth range of higher dissolved methane, ethane, and sulfide. This observation gives rise to several questions. Does the presence of terrigenous organic matter in Unit V support the metabolic activity of deeply buried microorganisms, which in turn results in the in situ formation of biogenic methane and ethane, or have thermogenic hydrocarbons migrated from a deeper source? In either case, methane consumption and sulfide formation by AMO are possible wherever sulfate is available within the methanogenic zone.

Microbiology

The microbiology component of Site C0012 was limited to whole-round sampling for molecular (phylogenetic) studies, FISH, and cell counting studies. Interstitial water samples were also obtained for shipboard spectrophotometric analyses of ferrous iron and acid volatile sulfide and measured using a third-party tool. The results of these measurements are tabulated along with inorganic geochemistry data in the J-CORES database.

Logging and core-log-seismic integration

Wireline logs were not obtained from Hole C0012B because of difficult conditions in the borehole and expiration of time to prepare for a typhoon evacuation. Coring results, however, allowed some refinements in the interpretation of seismic reflection data. Six seismic stratigraphic units were recognized (Fig. F34).

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