SUMMARY
Site 1150All of the cores from 0 to 722.6 mbsf from Hole 1150A are dominated by diatomaceous silty clay. The upper 200 m of sediment consist of interbedded diatomaceous ooze and clay, with siliceous biogenic grains, diatoms, radiolarians, and sponge spicules occurring commonly, and foraminifers and nannofossils rarely. Volcanic glass and siliciclastic grains are also observed in smear slides, though they compose less than 10% of the total sediment. From 260 to ~620 mbsf, the sediments gradually become firmer and more hemipelagic with increasing depth. Compared with cores from the upper 200 m, volcanic ash, reworked ash layers, volcanic pebbles, thin turbiditic sand, and silt layers are rare but present in the lower section. Below ~620 mbsf, cores from both Hole 1150A (620-722 mbsf) and Hole 1150B (703-1181.6 mbsf) consist mainly of well-lithified diatomaceous silty claystone. Authigenic glauconitic sand is interbedded with the dominant clayey lithology below 430 mbsf in Hole 1150A and from the start of the cored interval (703 mbsf) at Hole 1150B. Local occurrences of detrital glauconite in minor amounts are rare within both holes. Bioturbation (Chondrites, Zoophycos, and Planolites) is abundant in Hole 1150B. A few carbonaceous layers and nodule-type accumulations are also found interbedded in the largely homogeneous diatomaceous silty claystone in Hole 1150B. Volcanic ash layers are rare in this hole.
Twelve diatom zonations were identified from core-catcher samples from Holes 1150A and 1150B. The age of the lowermost sediment is interpreted to be younger than 9.9 Ma, because the first occurrence of Denticulopsis dimorpha was not observed in the studied interval. The average sedimentation rate is 119 m/m.y., with higher sedimentation rates (>200 m/m.y.) occurring between 6.65-3.74 Ma and between 0.3 and 0.0 Ma. The lowest sedimentation rate occurs between 2.0 and ~1.24 Ma (18 m/m.y.). The Pliocene/Pleistocene boundary lies at ~110 mbsf and Pliocene/Miocene boundary at ~500 mbsf. Datums of calcareous nannofossils were difficult to determine accurately because of poor preservation and low abundance of these fossils. Of the 11 nannofossil datums identified, seven gave ages younger than those indicated by the diatom datums.
Chemical analyses of pore waters from Hole 1150A cores show that chlorinity gradually decreases with depth from ~550 mM at the top of the hole to 500 mM at ~200 mbsf. Chlorinity concentrations remain at about this value for 350 m downhole. From ~550 mbsf, values abruptly decrease with depth to reach a minimum of 350 mM at ~700 mbsf. A similar trend is observed in the magnesium, potassium, and alkalinity profiles. Values gradually decrease in the upper 200 m (from ~50 to 28 mM for magnesium; from ~11 to 8 mM for potassium; and from ~50 to 26 mM for alkalinity). Values remain fairly constant from ~200 to 500 mbsf, and drastically decrease with depth to reach minimum values of 11 mM for magnesium, 6 mM for potassium, and 11 mM for alkalinity. Calcium concentrations increase from ~2 to 8 mM in the upper 200 m, remain fairly constant to a depth of 400 mbsf, and then increase to 10 mM at 600 mbsf.
Bulk-elemental analyses for the upper 350 m of samples from Hole 1150A show relatively high concentrations of organic carbon. Values fluctuate with depth between 1.8 and 0.5 wt%. Typical values are ~1 wt%. Sulfur abundances also are relatively high in this interval. Typical values are ~0.9 wt%, fluctuating between 1.3 and 0.5 wt%. Nitrogen abundances are typically ~0.13 wt%, fluctuating between 0.6 and 0.1 wt%.
Gas analyses for Hole 1150B indicate that methane concentrations are between 2% and 8% as measured from headspace gas analysis. Ethane concentrations are typically ~6 ppm from 700 to 900 mbsf. Values then slightly increase with depth to typical values of ~14 ppm. Methane/ethane ratios tend to decrease gradually with depth from ~5000 at 700 mbsf to 2800 at the final depth of ~1200 mbsf. Other hydrocarbon gases are below the detection limit.
Physical properties data show several systematic trends that correlate with downhole chemical and lithologic changes, appearing to indicate variations in hydrological and mechanical conditions. Gas expansion and drilling disturbance (formation of drilling biscuits) affects the physical properties of most cores from Hole 1150A. The interval from 80 to 200 mbsf consists of pelagic ooze and clay, and inverse trends in index properties are observed, such as the bulk density decreases from ~1.5 to 1.4 g/cm3 and porosity increases from ~65% to 75%. Constant and uniform values of index properties are observed from ~200 to 620 mbsf. The top of this interval coincides with a small change in lithologic composition, whereas the bottom corresponds to the change from firm sediments to sedimentary rocks. Bulk density and porosity range from ~1.35 to 1.55 g/cm3 and 65% to 76%, respectively.
P-wave velocity gradually increases downhole from 1.55 at 300 mbsf to 1.60 km/s at 620 mbsf. There may be a bias in index properties and P-wave velocity measurements in this interval because only coherent and undisturbed pieces were sampled. Measurements in sedimentary rocks show a wide scatter in porosity, bulk density, and P-wave velocity, but suggest generally decreasing porosity and increasing bulk density and P-wave velocity trends. Preliminary analyses of sonic anisotropy combined with paleomagnetic declination data indicate that the sedimentary rock from 730 to 1180 mbsf is anisotropic, with maximum, intermediate, and minimum principal axes along the west-northwest to east-southeast, north-northwest to south-southwest, and vertical orientations, respectively. In the interval where the borehole instruments were installed, the porosity, bulk density, and P-wave velocity are 55%, 1.65 g/cm3, and 2.0 km/s, respectively.
The magnetization of the first eight cores has an unambiguous normal polarity direction, with a steep downward inclination and a northward declination after applying the Tensor-tool orientation correction. The Brunhes/Matuyama boundary is most likely located in Core 186-1150A-10H at ~84-88 mbsf, though coring disturbance makes interpretation tenuous in this interval. Above 850 mbsf, the position of several reversals are evident following alternating-field demagnetization. These should be useful in establishing a few magnetostratigraphic tie points in the Pliocene Miocene section at this site. The magnetization of the RCB cores from Hole 1150B is fairly stable, and polarity is dominantly normal below 850 mbsf. The declinations from the Hole 1150B cores have proved useful for reconstructing structural orientations of the numerous microfaults and fractures observed in the core. For example, after reorienting fracture and fault planes into geographic coordinates, we find that most in the depth range from 703 to 940 have north-south strikes and dips of 45° to 80°, with a clear preference for eastward-dipping planes. Normal offset is observed on most of the fault planes, suggesting an east-west extensional stress field is responsible for the deformation observed in this interval. The extensional stress direction changes downhole, so that below 1080 mbsf the dominant direction is west-northwest to east-southeast.
Equilibrium temperatures obtained from the APC temperature tool (Adara) and the Davis Villinger temperature probe (DVTP) in the interval from 0 to 154.8 mbsf give a geothermal gradient of 28.9°C/km. The calculated heat flow is 20.1 mW/m2, which, though low relative to global values, is typical for the tectonic environment.
Three logging tool combinations were used in Hole 1150B: the triple combination tool string, the Formation MicroScanner (FMS)-Digital Sonic tool string, and the borehole televiewer (BHTV) tool string.
Although operational difficulties prevented logging at ~650 mbsf, three strings were deployed down to ~1170 mbsf after a wiper trip operation. Data quality is good throughout the logged intervals. The whole logged section can be divided into six units using the resistivity log in conjunction with other logs. These units are consistent with core descriptions and core measurements. The FMS data show borehole geometries to be oval below ~750 mbsf with east-west elongation (20 cm in north-south and 30-35 cm in east-west). The in situ physical properties in the lowermost 100 m (i.e., in the interval where the borehole instruments were installed) are ~1.95 km/s for P-wave velocity, ~1.7 g/cm3 for bulk density, 55%-60% for porosity, 1.2 ‡m for resistivity, and ~50 cps for spectral natural gamma ray.
Site 1151
Site 1151 is located 48 km south of Site 1150, in the deep-sea terrace
of the Japan Trench, a similar geological setting as at Site 1150. The main
objective at this site was to establish another borehole geophysical
observatory to monitor active processes in a plate subduction zone with
strain, tilt, and seismic sensors. A key difference is that this area is
above an aseismic portion of the seismogenic zone. The strainmeter at
this site measures volumetric strain changes. This second seafloor
borehole geophysical observatory (NEREID-2) was successfully installed in
Hole 1151B with much less downtime than for the first one. The sensor
string was set in a section with a density of ~1.9 g/cm3 and
P-wave velocity of ~2000 m/s. The target depth is slightly shallower at
1095 mbsf for the sensor string bottom, but the physical properties
suggest a more competent rock environment than at Site 1151. The bottom
of the open hole was filled with cement up to ~50 m into the cased hole
section. The observatory sites will be visited by the ROV Dolphin 3K of
JAMSTEC between 2 and 10 September with a mission to start and check
the system.
At Site 1151, the sedimentary section from 0 to 1113 mbsf was cored with APC (Holes 1151C and D) and RCB (Hole 1151A) coring systems. The recovered sequence ranges from Holocene to middle Miocene age. The average recovery was 68% for Hole 1151A and ~100% for Holes 1151C and 1151D. The common major lithology at this site is diatomaceous silty clay with intercalations of minor lithologies such as volcaniclastic ash, pumice, silt, and sand. Brittle deformational structures dominate below 400 mbsf. Bioturbation can be seen in most cores below 300 mbsf. Detrital glauconite occurs as sand-sized grains distributed throughout the section. Authigenic glauconite, found in both the major and minor lithologies, is of fine silt size. The detrital glauconite is always found in association with the finer-grained authigenic glauconite, but the latter may occur in the absence of the former. Alteration to limonite locally occurs. The following lithostratigraphic units were identified:
At Site 1151, 24 diatom datum levels were identified from core catcher samples, the lowest being in middle Miocene (< 16.3 Ma). Calcareous nannofossils are generally barren to abundant throughout, with variable preservation. As at Site 1150, of the 11 nannofossil datums identified, seven gave ages younger than those indicated by the diatom datums. Except for some ash layers and dolomite layers, the sequence contains few to abundant diatoms throughout. Diatom assemblages from all samples consist almost entirely of oceanic species, mainly from the subarctic North Pacific Ocean.
Sedimentation rates at Site 1151 were estimated using a combination of biostratigraphy and magnetostratigraphy. The upper 200 m has a relatively low rate (20 to 152 m/m.y.). It increases between 200 and 450 mbsf, reaching ~240 m/m.y., and remains at that level down to 800 mbsf, below which the rate gradually decreases. At 1027 mbsf, there is a hiatus of more than 0.2 m.y., and the rate then gradually increases downhole to 43 m/m.y. Site 1151 had high rates in the latest Miocene and low rates before and after this, a pattern similar to that at Site 1150. The intervals of low rate correspond to the early late Miocene (before 8.5 Ma) and the early to middle Pleistocene (2.0-0.78 Ma).
Gas analyses at Site 1151 indicate that methane concentrations are between 0.4% to 5% with an average concentration of ~2%. Ethane concentrations fluctuate between 1 and 12 ppmv and typically are of ~4 ppmv. Methane/ethane ratios are ~4400 throughout. Other hydrocarbon gases are below the detection limit.
No trend is present in the distribution of carbonate abundances with depth. Abundances range from 0.08 to 79 wt%, with an average value of 3.3 wt%. Most values, however fall between 2 and 4 wt% with excursions having peak values up to 15 wt%. Low carbonate abundances are in agreement with low occurrences of calcareous fossils in the sediments.
Sediments exhibit relatively high abundances of organic matter (OM) with characteristic low (<10) C/N ratios. Abundance of organic carbon (Corg) fluctuates between 0.2 and 1.4 wt%, with an average value of ~0.9 wt%. Total sulfur abundances irregularly fluctuate between 0.35 to 1.5 wt% with an average value of 0.85 wt%.
Several geochemical parameters exhibit similar distributions with depth. K+, Na+, Mg2+, chlorinity, and salinity show a characteristic decreasing trend with depth. Salinity gradually decreases with depth from a value of ~32 at the top of the borehole to a value of 18 at ~900 mbsf. Below this depth, salinity remains constant at 18 to the final depth. Chlorinity concentrations remain constant at ~500 mM in the upper 200 m of the borehole and then steadily decrease to 320 mM at the final depth.
Alkalinity values gradually decrease downhole in the upper 200 m from 31 to 17 mM and then increase to 25 mM at ~450 mbsf. Below this depth, values decrease steadily downhole to 2 mM at the bottom. Dissolved sulfate concentrations exhibit values lower than 2 mM throughout.
Concentrations of dissolved calcium (Ca2+) in pore waters slightly increase downhole from ~3 mM at the top to 18 mM at the bottom. Concentrations of dissolved strontium (Sr2+) fluctuate between 100 and 130 µM down to 550 mbsf and then increase to reach a maximum value of 232 µM at the bottom. Concentrations of dissolved lithium (Li+) gradually increase from 20 to 480 µM in the upper 850 m and then decrease to ~280 µM at the bottom.
The gamma-ray attenuation porosity evaluator density in Hole 1151A ranges from ~1.3 to 2.0 g/cm3. Lithostratigraphic Unit II is characterized by rather constant values, averaging ~1.4 g/cm3. Density varies from 1.3 to 1.7 g/cm3 along an oscillating trend in Unit III. Similar oscillating trends increase in magnitude in Units IV (1.3-1.8 g/cm3) and V (1.4-2.0 g/cm3). Such an oscillating trend is also observed in natural gamma-ray activity. The average thermal gradient, which was obtained from three measurements, is 35.9°C/km.
In Hole 1151A, P-wave velocity (horizontal) ranges from 1540 to 5290 m/s, with most values being less than 2150 m/s. The highest velocities were measured in thin beds of carbonate-rich sediments (i.e., dolomite layers or dolomite concretions). The maximum velocity in Hole 1151A of 5290 m/s was measured on a dolomite concretion at 1108 mbsf.
The ranges of porosity, bulk density, and grain density in Hole 1151A are 10%-77%, 1.32 to 2.42 g/cm3, and 2.09 to 3.91 g/cm3, respectively. The section from 78 to 295 mbsf (Unit I) has scattered, but slightly inverse trends of porosity and bulk density: porosity and bulk density generally range from 57% to 77%, and 1.32 to 1.59 g/cm3, respectively. The top of Unit IV (825 mbsf) coincides with a shift to higher porosity, and lower bulk density values. Yet another significant shift to higher porosity, by ~13%, occurs from 963 to 977 mbsf. The corresponding decrease in bulk density is ~0.22 g/cm3. Apart from these two shifts, index properties show normal trends across Units IV and V. At the base of Hole 1151A, porosity, bulk density and grain density are 49%, 1.75 g/cm3, and 2.46 g/cm3, respectively.
The remanent magnetization at Site 1151 is very similar in behavior to that at Site 1150. The upper 78 m has an unambiguous normal polarity direction, with a steep downward inclination. The location of the Brunhes/Matuyama reversal appears to be well resolved by the abrupt downhole change to negative inclinations at ~78.2 mbsf in Hole 1151C, 80.1 mbsf in Hole 1151D, and at 82-84 mbsf (between Cores 186-1151A 2R and 3R) in Hole 1151A. However, the reversals lower in the section are not easily correlated with the geomagnetic polarity time scale. Below 700 m, virtually the entire section has very stable positive inclinations that average ~60°. We interpret this interval to be partially or totally overprinted by a recent normal polarity field direction. The stable declinations from these cores have proved useful for reconstructing structural orientations of the microfractures and bedding planes. We have found that the orientation of fracture planes changes downhole with dip azimuths dominantly to the west-northeast and east-southeast in the upper domain, but dominantly east and west in the middle and lower domains. Below 900 mbsf, the dip angles of bedding planes are more than 10° and preferentially dip toward the east.
Three logging runs (one triple combo and two FMS/Sonic runs down to 850 mbsf) were achieved in Hole 1151D by extending the second APC/XCB hole (1151D). The hole condition (caliper log) was much more stable at Site 1151, and logging was accomplished without difficulty.