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doi:10.2204/iodp.pr.315.2008

Principal results

Site C0001

Site C0001 (proposed Site NT2-03) is located on the hanging wall of the megasplay fault where future riser drilling is planned to 3.5 km depth. LWD data taken during Expedition 314 in Hole C0001D delineated four lithologic units (Units I–IV, from top to bottom) and northeast–southwest borehole breakouts suggesting a northwest–southeast compressional stress field (Kinoshita et al., 2008).

Coring in Holes C0001E and C0001F was conducted from the seafloor to 248.8 m CSF with the HPCS and ESCS. ESCS coring recovered only two cores because of severe drilling disturbance. RCB coring was done from 230.0 to 458.0 m CSF in Hole C0001H. Bad hole conditions did not allow continued coring to the original TD of 1000 mbsf. In addition to cores recovered during this expedition, we also described and analyzed Hole C0002B cores (~30 m long) recovered during Expedition 314. These cores were taken for geotechnical investigations for future riser well planning and remained unprocessed after nondestructive whole-round measurements and whole-round core sampling were completed.

We identified two lithologic units at Site C0001 (Fig. F5). Unit I is Quaternary to late Pliocene in age and extends from the seafloor to 207.17 m CSF. The dominant lithology of Unit I is silty clay and clayey silt. Secondary lithologies include thin interbeds and irregular patches of sand, sandy silt, silt, and volcanic ash. Overall, the siliciclastic interbeds define a trend of fining and thinning upward. Unit I can be divided into three subunits based on grain size, layer thickness, sedimentary structures, trace fossils, and mineralogy. Subunit IA extends from the seafloor to 168.35 m CSF. The dominant lithology is structureless greenish gray to grayish green mud with local wavy and dark green laminae. The most common interbeds consist of white or light gray to dark gray volcanic ash. Subunit IB extends from 168.35 to 196.76 m CSF. The subunit boundary is defined by the first occurrence of multiple closely spaced silt beds. The dominant lithology is greenish gray to grayish green mud. Dark gray silt to sandy silt is the characteristic minor lithology of this subunit. These thin beds display sharp bases, faint plane-parallel laminae, normal size grading, and diffuse tops. Such features are typical of fine-grained turbidites. Subunit IC extends from 196.76 to 207.17 m CSF. The dominant lithology is gray to dark bluish-gray sand, intercalated sporadically with greenish gray to grayish green mud. Grain size varies from silt to medium sand but is predominantly fine sand. Unconsolidated sand appears to be structureless and soupy, but this is probably an artifact caused by coring disturbance. Bed thickness is impossible to recognize because of flow within the core liner. The sand grains consist mostly of detrital quartz and feldspar with abundant sedimentary and low-grade metasedimentary rock fragments.

The boundary between Units I and II is an unconformity, with an associated hiatus, as shown by both paleomagnetic and biostratigraphic data. Strata below the unconformity are late Pliocene to late Miocene in age and extend from a depth of 207.17 m CSF to the bottom of Hole C0001H at 457.8 m CSF. Within Unit II, three subunits were recognized based on LWD data, but lithologic variations in the cores are not distinctive enough to warrant subdivision on the basis of texture, composition, or sedimentary structures. The dominant lithology of Unit II is greenish gray to grayish green bioturbated mud. The mud contains local wavy laminae, as defined by darker green color and higher clay content. Pyrite grains are moderate to abundant.

Cores taken from Holes C0001B, C0001E, and C0001F were cross-correlated based on whole-round multisensor core logger (MSCL-W) magnetic susceptibility data. The interval below 6 m CSF in Hole C0001B correlates with the 1.5 m deeper interval of Hole C0001E, although there seems to be no shift at the core top. The lowermost core in Hole C0001E and the uppermost core in Hole C0001F overlap. A distinctive pair of magnetic susceptibility peaks, corresponding to ash layers, indicates a 2.7 m deeper offset of Hole C0001F to Hole C0001E. The intercalated mud intervals between 196.76 and 206.93 m CSF in cores from Hole C0001F are well imaged as conductive layers in the resistivity-at-the-bit image between 192 and 199 m logging-while-drilling (LWD) depth below seafloor (LSF) in Hole C0001D. This imaging constrains the stratigraphic offset between Holes C0001E and C0001D to 4.75 m at the top of the sand and 8 m at the base. A good fit between the MSCL and LWD gamma ray data is obtained in the lower part of Hole C0001D with offsets of 7–9 m. This suggests a lateral variation of sand thickness from 7 m in Hole C0001D to ~10 m in Hole C0001F. Taking into account this 7 m shift of the base of Hole C0001F, it is logical to fit the magnetic susceptibility peaks at 231.08 and 233.04 m CSF in Hole C0001H with those located ~8 m deeper at 239.49 and 240.37 m CSF in Hole C0001F and to infer an upward shift of ~1 m of the uppermost part of Hole C0001H with respect to the LWD hole (C0001D).

Nannofossils and planktonic foraminifers were recovered from the sedimentary succession at Site C0001. The combined results suggest that there is at least one time break in the sequence in the interval from the late Quaternary to the Miocene/Pliocene boundary. Foraminifers and nannofossils are generally abundant within Subunit IA, with preservation ranging from moderate to good; best preserved assemblages are found in the Pleistocene section. The Pliocene/Pleistocene boundary is placed at ~190 m CSF based on planktonic foraminiferal evidence. The existence of Pliocene nannofossil assemblages just below this depth is consistent with the foraminifers defined. A marked change in both nannofossil and foraminifer assemblages occurs in the interval between 207.43 and 213.39 m CSF; mixed assemblages of late Pliocene to late Miocene age were encountered. The Miocene/Pliocene boundary is placed below 419.5 m CSF based on foraminiferal evidence. Nannofossil results, however, suggest that the boundary is located between 448 and 458 m CSF. The exact position of the Miocene/Pliocene boundary at Site C0001 remains equivocal and warrants more detailed investigation.

Natural remnant magnetization was measured at 5 cm intervals in each core section, followed by alternating-field (AF) demagnetization at 5, 10, 15, and 20 mT peak fields. Cores from Hole C0001H (240–458 m CSF) were excluded because of the lack of intact sections long enough for pass-through magnetometer analysis. The polarity pattern was determined by the magnetic inclination after AF demagnetization at 20 mT (Fig. F6). The magnetic polarity record was then identified using biostratigraphic datums and correlated with the Geomagnetic Polarity Time Scale of Gradstein et al. (2004). In Hole C0001E, the magnetic inclination is dominantly positive above 86 m CSF. A polarity change from normal to reversed at this depth is interpreted as the Brunhes/Matuyama Chron boundary, in agreement with the identified nannoplankton zone. The Matuyama Chron (C1r) is characterized by the predominance of reversed polarity. A short normal polarity interval observed between ~127 and ~131 m CSF is interpreted as a part of the Jaramillo Subchron (C1r.1n), although the upper and lower limits are missing. In Hole C00001F, we could reliably determine the top of the Olduvai subchron (C2n) at 174.7 m CSF. The bottom of the Olduvai occurs at 193.5 m CSF, although the subsequent older Matuyama Chron is only represented by 3.3 m of sediment. Below, we encountered thick sandy Subunit IIB, which is completely disturbed, preventing any continuous reliable paleomagnetic analysis.

More than 550 individual structural features were documented and described. Most of these features were reoriented to a geographic reference frame using shipboard paleomagnetic data. The main features were faults, shear zones, vein structures, breccia, and steepened bedding. Faults are relatively planar narrow zones of deformation characterized by a single band of concentrated deformation that is slightly bright in X-ray CT images, suggesting a higher density than the adjacent wall rock. Shear zones (or deformation bands) are generally wider than faults and are often composed of multiple sets or bands of concentrated deformation. Vein structures include a variety of structures, many of which have been recognized during previous ocean drilling and have been interpreted as dewatering structures. This category of structures includes the classic sigmoidal-shaped suite of thin mud-filled veins originally described by Ogawa (1980) and Cowan (1982). Breccia is marked by a relatively high concentration of small polished and slickenside lens-shaped fragments of clayey silt or silty clay a few centimeters to 2 dm in thickness. Figure F7 shows the vertical distribution and dip angle of the dominant deformation structures at Site C0001. Bedding generally dips gently with only three anomalous zones: (1) ~80–100 m CSF, (2) 140 m CSF, and (3) ~210 m CSF. Faults generally occur at all depths at Site C0001, but only normal faults and a few relatively minor thrust faults occur above the zone of deformation at 220 m CSF. A cluster of normal faults is present in the interval 150–160 m CSF (Fig. F7), just above the breccia zone. The orientations of faults strongly contrast above and below the highly deformed zone at 220 m CSF. In the slope apron and above the deformed zone, 90% of the faults described are conjugate sets of normal faults dipping at 60° and striking southeast, indicating northeast–southwest extension. Exceptionally, a few thrust faults dipping at 50° encountered just above the deformed zone suggest northwest–southeast shortening. The geometry and kinematics of planar structures display greater variation below the deformed zone. The fault plane solutions computed from normal and thrust faults are respectively consistent with the northeast–southwest extension and northwest–southeast shortening.

A total of 48 whole-round sections were collected for interstitial water analyses from Holes C0001E, C0001F, and C0001H. Routine sampling density was approximately one per core; denser sampling was done from the first core just below the seafloor (three samples), where the considerable mixing of interstitial water with seawater and anaerobic methane oxidation occur. SO₄ concentration decreases linearly from the seawater value at a rate of ~2.5 mM/m to nearly 0 mM at 14 m CSF at Site C0001. Alkalinity and dissolved PO₄ concentration consistently increase to maximum values at ~50 m CSF, reflecting the activity of interstitial microbes that metabolize sedimentary organic matter through a reaction that ultimately produces inorganic metabolic byproducts in the shallow interval (e.g., Berner, 1980). Dissolved NH₄ concentration also reaches a maximum constant value between 105 and 175 m CSF, resulting presumably from anaerobic deammonification of organic matter during early diagenesis in this interval. The concentration of Cl rapidly decreases from the seafloor to 100 m CSF to a local minimum and then increases continuously below this depth (Fig. F8). Chlorine is one of the most conservative elements in marginal settings; therefore, the 3.6% dilution at 100 m CSF reflects input of freshwater. The coincidence of the Cl minimum with the NH₄ maximum at 100 m CSF suggests that water input from a deamonification reaction could be responsible for this freshening. This question will be resolved by future oxygen and hydrogen isotopic analysis, which may record the presence of hydrate-sourced fluids as another candidate. Sodium concentration is scattered because of analytical error, but there is a slight increase in concentration with depth and a clear decrease at the seafloor (Fig. F8). The monotonous decrease of K with depth is the result of continuous reaction with clay mineral phases in thermal equilibrium. There is a pronounced breakout of the trend at 112 m CSF, resulting from interaction with a volcanic ash layer. There is also a localized minimum in Mg and maximum in Ca concentration at this depth. The increase in Ca at greater depths suggests that Ca is being released from clay minerals at all depths, but the amount of removal exceeds the amount of production in the upper zone.

Concentrations of methane, ethane, and their ratio (C₁/C₂) are shown in Figure F9. Methane concentration increases rapidly from 3.7 ppmv in the near surface to 37,204 ppmv at 34 m CSF. It decreases downhole to 100 m CSF and remains constant until 231 m CSF. Below this depth, methane concentration shows wider variation but is relatively high on average; the highest value occurs at 278 m CSF. Vertical distribution of ethane is similar to that of methane. C₁/C₂ ratios gradually decrease with depth in Unit I and slightly increase with depth in Unit II. The increase of methane concentration and C₁/C₂ ratio in Unit II indicate contribution of biogenic methane. The majority of C₁/C₂ ratios >1000 suggests that the methane at Site C0001 is of biogenic origin. Initial results for calcium carbonate (CaCO₃), total organic carbon (TOC), organic carbon to total nitrogen (C/N) ratio, and total sulfur in the sediments are presented in Figure F9. Calcium carbonate, TOC, and C/N ratios generally decrease with depth in Unit I and remain constant in Unit II except for some high values of TOC and C/N ratio below 375 m CSF. Total sulfur content is fairly uniform with two spikes at 103 and 181 m CSF and a cluster showing high values between 200 and 260 m CSF.

Whole-round cores were taken from Holes C0001E, C0001F, and C0001H for microbiological analysis. Some of the whole-rounds were subsampled on board for cell fixing, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) extraction, and culturing studies. A large part of the microbiological work will be done on shore. Selected samples of fixed cells were stained on board with double-stranded DNA-binding SYBR green-I-stain. Cells were detected in core samples from 0.5 to 448 m CSF. More than 200 enrichment cultures were established with the aim to culture sulfate-reducing microorganisms. The cultures were monitored for sulfate reduction based on the formation of black iron sulfide precipitate. During the expedition, blackening of the growth medium was observed in only a few sediment cultures incubated at 9° or 37°C and in drilling fluid (seawater gel) cultures incubated at 37°C.

Moisture and density (MAD) was measured on discrete subsamples collected from working-half cores as well as from "clusters" adjacent to whole-round samples (Fig. F10). MAD porosity gradually decreases with depth throughout Subunits IA and IB. Subunit IC shows extremely low porosity, probably due to a highly disturbed sandy sequence. There is a large gap of porosity between Subunit IC and Unit II. The entire porosity profile in Unit II is shifted higher than those of Subunits IA and IB. The cause of this shift is not evident and should be resolved in terms of different backgrounds of compaction and digenesis of both units. Wet bulk density data show a reverse correlation with porosity data. Grain density values show no specific trend with depth, although there is subtle lack of low values at Subunit IB. Undrained shear strength measurements were determined using a miniature vane shear device and a pocket penetrometer. Shear strength increases with depth except for some low values below 228 m CSF. These data are derived from ESCS core samples, suggesting heavy drilling-induced disturbance. Thermal conductivity measured on whole-round samples from HPCS cores (<230 m CSF) and on split samples from RCB cores (>230 m CSF) are widely scattered but generally increase with depth.

Downhole temperature measurement was conducted at seven depths with the APCT3 in Holes C0001E and C0001F. Measurement was done on every third core from 13.6 to 170.98 m CSF. Temperatures increase almost linearly with depth with a gradient of ~0.042°C/m.

Site C0002

Site C0002 is located at the southern margin of the Kumano forearc basin where future riser drilling to 6 km depth is planned. LWD data of the thick forearc basin sequence and the top of the underlying accretionary prism were obtained in Hole C0001A during Expedition 314 and identified four lithologic units: Units I–IV (Kinoshita et al., 2008).

Coring in Hole C0002B was conducted from the middle of the forearc basin (475 m CSF) to the top of the accretionary prism (1057 m CSF). Hole C0002D was mainly dedicated to geotechnical assessments to 70 m CSF for future riser drilling. After recovering these cores, we continuously cored to 129.15 m CSF and conducted spot coring at 150 and 200 m CSF in the same hole. Only time-critical whole-round sampling and minimum measurements were conducted, such as pore fluid geochemistry, X-ray CT scan, MSCL-W, MAD, and thermal conductivity. In order to complement missing intervals by dense whole-round sampling in the upper 10 m, we took two cores in Hole C0002C to 13.77 m CSF. Therefore, data presented here are mainly from Hole C0002B cores.

Three lithostratigraphic units were identified in cores taken from below 479.40 m CSF in Hole C0002B on the basis of grain size, layer thickness, sedimentary structures, trace fossils, and mineralogy (Fig. F11). Core recovery was generally poor to moderate and was particularly bad within intervals in which the log character was indicative of high sand content. We therefore adopted positions of unit boundaries determined by LWD during Expedition 314 (Kinoshita et al., 2008). Hole C0002B is located 60 m south–southwest of Hole C0002A for LWD. The forearc basin sequence has a dip of 6.5° to 7.0°; hence, a stratigraphic offset of 6.0 to 7.5 m is expected between the boreholes in Lithologic Unit II. Core-log integration between LWD bit resistivity and core resistivity of discrete samples shows a 11–12 m deepening of forearc basin strata from Hole C0002B to Hole C0002A.

The dominant lithology of Unit II is greenish gray to grayish green mud (silty clay to clayey silt). Secondary lithologies include thin interbeds and irregular patches of sand, sandy silt, silt, and rare volcanic ash. The mud is locally structureless but more commonly shows plane-parallel laminae and incipient fissility. Orientation of this fabric is horizontal to gently inclined. Dark gray silt, sandy silt, and silty sand are the characteristic interbeds of Unit II. These beds are typically <5 cm thick and display sharp bases, faint plane-parallel laminae, normal size grading, and diffuse tops. Such features are typical of fine-grained turbidites.

The boundary between Units II and III is defined by a shift in lithofacies from turbidites above to condensed mudstone below at 830.4 m LSF (Kinoshita et al., 2008). The dominant lithology of Unit III is greenish gray, gray, and gray-brown mudstone. Mud texture is finer grained than equivalent deposits in Unit II, and bioturbation is more widespread and diverse. Secondary lithologies are limited to sparse beds and irregular lenses of volcanic ash. Locally, sharp-topped zones with glauconite grains suggesting erosion and reworking of compacted and/or cemented clay-rich sediment were observed.

The boundary between Units III and IV, an unconformity between the forearc basin and the accretionary prism, is defined by a change in structural style and a shift in lithofacies from a condensed mudstone section above to interbeds of mudstone, siltstone, and sandstone below. As defined by log character, the unit boundary is positioned at a depth of 935.6 m LSF. The dominant lithology of Unit IV is gray to greenish gray and dark gray mudstone. The mudstone contains local wavy laminae or bands, as defined by darker green color and higher clay content. Thin layers of siltstone are rare. Sandstone also occurs locally, and in some cases the sand is cemented by calcium carbonate.

The preliminary biostratigraphy for Site C0002 is based on calcareous nannofossils. Nannofossils are continuously present throughout the sequence. Although the nannofossils significantly decrease in abundance downhole, a good biostratigraphic framework could be established for the entire succession. Nannofossil assemblages recovered from Unit II are Pleistocene in age. Unit III comprises a suite of Pliocene biostratigraphic events. In Unit IV sediments, only one nannofossil event was identified, assigning this unit a late Miocene age.

Remnant magnetization measurements on archive-half cores from Hole C0002B above 620 m CSF suggest artificial overprints. Excluding these intervals, the magnetic polarity record after AF demagnetization at 30 mT was correlated to the Geomagnetic Polarity Time Scale of Gradstein et al. (2004). A clear polarity change from negative to positive inclination was observed at ~850 m CSF. Referring to the biostratigraphic datum, this reversal can be correlated to the top of Olduvai and the reversed polarity above this horizon can be correlated to Matuyama Chron. Magnetic inclination values also infer tectonic tilting of bedding planes after acquisition of remnant magnetizations. Measurements of bedding dips on split cores show a gradual increase from Unit II to Unit III and steeper dips in Unit IV with high scattering distribution (Fig. F12). The inclination value in the Unit II interval is relatively close to the inclination estimated from the site latitude. In contrast, the Unit III interval reveals a significantly steeper inclination, suggesting northward (northwest–northeast) tilting of strata. Magnetic inclination in Unit IV ranges more widely toward both lower and higher angles. The wide variety of dip and azimuth in Unit IV is consistent with highly deformed strata of the accretionary prism expected from seismic reflection profiles.

More than 400 structural features were observed in cored sediments. Five main structural features and one subordinate feature were identified. The main features observed were steepened bedding, faults, breccias, shear zones, and vein structures. Fissility occurred only in the upper sections of Hole C0002B and was consistently oriented parallel to bedding. Vein structures are particularly well developed in Unit III. A wide variety of forms and shapes were observed for vein structure; however, most of them are perpendicular to the bedding. Faults are relatively rare in the upper 500 m of Hole C0002B. Although a small cluster of moderately dipping faults occurs at ~700 m CSF, most of the observed faults occur in two clusters below 900 m CSF (Fig. F12). One cluster ranges from 920 to 950 m CSF, whereas the other cluster more widely ranges from 1000 to 1050 m CSF within Unit IV. About 20% of the planar and linear features observed in the cores were reoriented to true north using nearly 50 individual paleomagnetic poles. Three deformation phases were suggested by textures and crosscutting relations. An early phase of thrust faulting (and possibly strike-slip faulting) exhibits northwest–southeast shortening. Two phases of normal faulting occurred subsequent to thrusting. The first is recorded in shear zones and indicates northeast–southwest extension. The second is recorded in normal faults and indicates north–south extension.

A total of 31 whole-round samples were collected for interstitial water analyses from Hole C0002B on a routine bases with a frequency of approximately one per every core. Sulfate concentration generally ranges between 1.0 and 4.0 mM, well above the detection limit throughout the entire interval (Fig. F13); even potential contamination of drilling fluid is taken into consideration. These concentrations suggest existence of high sulfate fluids at greater depths, inferred from previous studies (e.g., ODP Sites 1173, 1174, and 1176A; Moore, Taira, Klaus et al., 2001). Alkalinity and ammonium concentrations decrease steeply downhole, showing inflections at unit boundaries. We infer that the lithologic boundaries affect fluid migration and/or reaction. Salinity and concentrations of chlorine consistently increase downhole from levels below seawater value at ~800 m CSF and then slightly decrease toward the bottom of the hole (Fig. F13). Dilution at the top of the cored interval (475 m CSF) is also observed in all other cation profiles, indicating the presence of a freshwater source or path above this interval. A coupled change in concentrations of sodium, potassium, and calcium across 800 m CSF may suggest a progressive reaction of fluid with clay minerals through Units III and IV or deeper, probably a result of smectite illitization (Fig. F13).

Concentrations of methane, ethane, and propane are shown in Figure F14. Methane concentration is constant to 610 m CSF and increases throughout the rest of Unit II. It decreases with depth in Unit III and remains low in Unit IV except for high values at the bottom of Hole C0002B. Propane was also detected in the lower part of the hole. C₁/C₂ ratios below 900 m CSF suggest some contribution of thermogenic hydrocarbons. Initial results for inorganic carbon, CaCO₃, TOC, total nitrogen, C/N, and total sulfur in the sediments are presented in Figure F14. CaCO₃ concentration is significantly high in Unit III. C/N ratios suggest that organic matter in sediments is mainly of marine origin. Total sulfur content is constant in Unit II, increasing with depth in Unit III and showing high values with scattering in Unit IV.

Whole-round cores were taken from Holes C0002B and C0002D for microbiological analysis. Some of the whole-round cores were subsampled on board for cell fixing, DNA and RNA extraction, and culturing studies. A large part of the microbiological work will be done on shore because of time constraints and the need for special laboratory facilities. Onboard work mainly included preserving the whole-round cores and subsamples for shore-based research, fixing cells for cell detection and counting, and setting up enrichment cultures of sulfate-reducing bacteria. Selected samples of the fixed cells were stained on board with double-stranded DNA-binding SYBR green-I-stain. Cells were detected in core samples to 1020 m CSF.

MAD was measured on discrete subsamples collected from the working-half cores as well as from "clusters" adjacent to whole-round samples (Fig. F15). Porosity gradually decreases with depth within Unit II, remains constant in Unit III except for a shift to higher values in the lower part of the unit, and increases between 1005 and 1050 m CSF in Unit IV. Thermal conductivity was measured on whole-round core sections above 555 m CSF (needle probe method) and on split working-half cores (half-space method) for stiffer sediments below. Thermal conductivity values increase with depth in Unit II and show no specific trend in Unit III. The decrease of thermal conductivity below 1000 m CSF is consistent with the increase in porosity. Shear strength experiments using a penetrometer above 641 m CSF show highly scattered values, suggesting strong core disturbances caused by RCB coring.

Six measurements of downhole temperature were conducted with the APCT3 in Hole C0002D. The measurement interval is every second core above 72.38 m CSF. Measurements were also conducted at 110.38 and 158.97 m CSF. Temperature increases almost linearly with depth with a gradient of ~0.043°C/m.

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