A 1181.6-m-thick sedimentary section of Holocene to middle Miocene age was cored from Holes 1150A and 1150B (Fig. F7). The major lithology of the recovered sediments predominantly consists of homogeneous diatomaceous silty clay and diatomaceous clay and its lithified equivalents, which are variable mixtures of biogenic siliceous microfossils, siliciclastic grains, and volcaniclastic grains (Figs. F8, F9).
Primary and reworked ash, and pumice layers are intercalated with the dominant lithology. Rounded to subangular heterogeneous pebbles and granules are locally distributed, and reworked sand and silt-sized grains and layers are disseminated and intercalated in the section (see "Minor Lithologies"). These minor components are common in the upper and lower parts of the sedimentary section. Dolomitic layers and concretions are present in a few intervals. Authigenic and reworked glauconitic sand and silt-sized grains are also found in the middle to lower parts of the section.
The sedimentary section at Site 1150 was divided into four lithologic units with subunits (Fig. F7). The unit and subunit boundaries are not sharp because major and minor components of the sediments change gradually with depth. We subdivide units and subunits mainly based on the composition of the major lithology, determined by visual observation of smear slides (see "Site 1150 Smear Slides"), bulk mineralogy from X-ray diffraction (XRD) analysis (Table T2, also available in ASCII format), amount and composition of minor lithologies (Tables T3, T4, both also available in ASCII format), variations in color reflectance spectrophotometry, and degree of lithification. In addition, magnetic susceptibility, natural gamma radiation (NGR), and gamma-ray attenuation (GRA) bulk density data from multisensor track (MST) measurements were also used to help in the division of units. Even though the color reflectance spectrophotometry data contain an error (see "Lithostratigraphy" in the "Explanatory Notes" chapter), they proved useful for identifying lithologic changes. Occurrences of minor lithologies such as ash layers, fractures, and silty layers are referred to as sparse or low (<0.4 occurrences/m), common or intermediate (<1 occurrence/m), and abundant or high (>1 occurrence/m). Lithification of the sediments is based on visual core description (VCD) and resistivity logging data from downhole measurements.
Unit I consists primarily of homogeneous diatomaceous ooze, diatomaceous clay, and silty clay with rare bioturbation. The color of the sediments in Unit I is dominantly dark olive gray and olive gray. Unit I is also characterized by volcanic grains and discrete layers such as pumice, primary and reworked ash, and bioturbated ash, typically intercalated with the dominant diatomaceous sediments (Fig. F9). Single grains or layers of rounded to subangular pebbles and granules interpreted to be reworked or, less likely, ice-rafted grains of volcanic origin are also commonly disseminated within the unit. Sandy and silty intervals and layers are abundant. These minor lithologies result in abrupt peaks and background variation of magnetic susceptibility (Fig. F9). Pyrite is often disseminated in cores, and authigenic framboidal pyrite with siliceous microfossils is always observed in smear slides.
The basal boundary of Unit I is taken at the interval where ash and sand layers decrease to the background level as constrained by VCD and by magnetic susceptibility, NGR activity, and resistivity data. Unit I is divided into two subunits. The boundary between Subunits IA and IB is determined at the interval where opal-A begins to increase and quartz and clay minerals start to decrease downhole, as documented by XRD intensities and magnetic susceptibility variations.
Subunit IA is characterized by intercalation of hemipelagic diatomaceous ooze and diatomaceous clay. Tephra and reworked tephra are commonly intercalated with sandy silt and silty clay. Pebbles and granules, and sand- and silt-rich layers are common in the cores. Biogenic siliceous components at the top and base of Subunit IA are relatively high compared with the middle part due to the increased siliciclastic components. Calcareous microfossils account for a small part of the biogenic components, but foraminifers are relatively common in the upper and lower parts of the unit and nannofossils are common in the middle part of the unit. High-amplitude variations of biogenic and detrital components correspond to intercalation of large amounts of biogenic and siliciclastic components.
Subunit IB also consists of hemipelagic diatomaceous ooze and diatomaceous clay. However, detrital and volcanic minor lithologies decrease with depth and with increasing amount of biogenic opal-A. There is a downhole trend in Subunit IB toward reddish and slightly yellowish color (based on color reflectance data), though the dominant color throughout the section is olive green. There is also an increase of biogenic silica, a decrease of siliciclastic components, and decreasing ash and sand/silt accumulations.
Unit II consists of homogeneous diatomaceous clay and diatomaceous silty clay, characterized by an abundance of biogenic components and a paucity of siliciclastic components. Biogenic components increase from the top of Subunit IIA with little variation, attaining the highest value in the entire section at the lower part of Subunit IIA. Biogenic components are also high at the bottom of Subunit IIB. The division of subunits in Unit II is based on the degree of lithification of the major lithology, which increases from soft to firm. The "coring biscuits" are obviously induced by XCB coring in this unit (Fig. F10). Visual core descriptions of recovered sediments focus on the firm biscuits, not on interbiscuit soft slurry mud. Ash and pebbles are rare in this unit. The basal boundary of Unit II is drawn at the interval where enrichment of sand and silt begins and biogenic opal starts to decrease. Lithofacies are generally homogeneous in Unit II, and the intensity of bioturbation becomes rare to moderate in Subunit IIB. Based on L*, a*, and b* data, respectively, the dominantly dark olive sediments become quantitatively darker and have more light reddish and bright yellowish color than other lithologic units (Fig. F7).
Dolomitic layers are intercalated in intervals 186-1150A-47X-2, 42-59 cm (436.02-436.61 mbsf), and 63X-CC, 22-26 cm (589.85-589.89 mbsf), in Subunit IIB. Dolomitic angular or subangular fragments are recovered at the top of Core 52X (482.3 mbsf), identified by peaks in XRD measurements (Table T2) and peaks in the resistivity and bulk density data from downhole measurements (Table T3). Other carbonate fragments are recovered with pumice grains and volcanic fragments at the tops of Cores 50X (462.9 mbsf), 53X (492 mbsf), 57X (530.6-540.3 mbsf), 58X (540.3-549.9 mbsf), and 60X (559.5-569.1 mbsf), which are all low-recovery cores. The concentration of these fragments at the top of the cores is an artifact of drilling. Glauconite-rich layers are present in intervals 186-1150A-45X-4, 102 cm, to 45X-5, 100 cm (420.32-421.8 mbsf), and in Core 46X (424.4-434.1 mbsf).
Unit III is composed of hard hemipelagic diatomaceous silty claystone and clayey siltstone with moderate to common bioturbation. Ichnofossils are dominated by Planolites (Fig. F11A), Chondrites (Fig. F11B), and Zoophycos (Fig. F11C). Primary and bioturbated ash layers, pumice grains, and pumice layers are sometimes interbedded in the dominant lithology, as are silt- and sand-sized accumulations and layers and glauconite-bearing sand and siltstone. Glauconite-rich layers are especially common in interval 186-1150A-64X-2, 0-150 cm (599.5-601 mbsf). Carbonate fragments are recovered from the tops of Cores 186-1150A-64X, 65X, 75X, and 76X. Fractures, faults, and joints frequently disturb the sedimentary sequence (see "Structural Geology"). Dolomitic layers were recovered from the top part of each subunit. White sponge spicule aggregates occur very often throughout the unit.
Subunit IIIA is characterized by a gradual increase in silt- and sand-sized particles with decreasing biogenic components. Intensity of bioturbation varies over short intervals from rare to common.
Subunit IIIB is characterized by a paucity of biogenic silica and an abundance of silt and sand accumulations and layers. A few carbonate-rich layers and dolomitic nodules are intercalated with the dominant lithology, as are pumice grains and layers and primary and bioturbated ash layers and patches. Disseminated pyrite and pyrite patches are common. Bioturbation is moderate to common throughout the subunit.
Subunit IIIC is rich in opal-A and poor in silt and sand accumulations and layers. Bioturbation is moderate to common throughout the subunit. A few carbonate-rich layers and dolomitic nodules are intercalated with the dominant lithology, as are pumice grains and layers. Primary and bioturbated ash layers and patches are also common.
Unit IV is characterized by hard diatomaceous silty claystone and clayey siltstone. The boundary between Units III and IV is marked by a distinct downhole increase in resistivity. Within Unit IV, biogenic tests of diatoms are rare, but the ratio of opal-CT to opal-A is high. All of the detrital minerals are common, and sand and silt accumulations and layers are rich in glauconite. Thick, coarse sand turbidites occur in interval 186-1150B-38R-1, 93-120 cm (1057.23 -1057.50 mbsf). Pumice grains, ash layers, and bioturbated patches are commonly intercalated with the dominant lithology. Bioturbation is moderate to common and is dominated by Planolites, Chondrites, and Zoophycos. Fractures, faults, and joints are common throughout the unit.
Key factors used to subdivide the recovered sedimentary section are compositional variations of major components and the degree of diagenetic lithification.
The recovered sediments at Site 1150 consist of biogenic, siliciclastic, and volcaniclastic components determined from smear slides (Figs. F8, F9; see "Site 1150 Smear Slides"). Siliciclastic material typically is the dominant component, but with variable composition with depth (Fig. F9).
The biogenic component varies from 8% to 75% with an average of 32%. The biogenic component is dominated by siliceous microfossils (mainly diatoms), siliceous sponge spicules, and a small number of radiolarians and silicoflagellates (Fig. F12). The biogenic component is high in the upper part of Units I and II and lower in the middle part of Units I, III, and IV. Calcareous biogenic components such as foraminifers and nannofossils are generally few to rare, though they are abundant in a few short intervals in Units I, III, and IV (Fig. F12).
Siliciclastic components mainly consist of clay minerals and sand-, silt-, and clay-sized quartz and feldspar grains with small amounts of mica, glauconite, hornblende, and clinopyroxene grains (Fig. F12). The siliciclastic components range from 15% to 71%, with an average of 52%. Grains of feldspar and hornblende are often extremely common in thin silty or sandy layers. The siliciclastic components are high in Unit I, Subunit IIIC, and Unit IV and poor in Unit II.
Volcaniclastic components in the major lithology are almost constant, with an average of 11%, but increase slightly below 650 mbsf (Fig. F12). The components are predominantly volcanic glass. Other minor components such as authigenic pyrite, hematite, volcanic and rock fragments, and carbonate grains are observed in the sediments.
Minerals in the dominant lithology, determined by XRD analysis, consist of opal-A, quartz, feldspar, hornblende, clay minerals, and calcite; this analysis is consistent with smear-slide observations (Fig. F13A). Opal-A input is predominantly from diatoms. Minor silt- and sand-rich layers have a tendency to be rich in feldspar and hornblende (Fig. F13B). Carbonate-rich layers and concretions consist mainly of dolomite (Fig. F13C). Halite, which is always observed, is precipitated from interstitial water during the drying procedure. Variations of these major minerals with depth are compiled as a series of XRD intensities of each mineral as counts per second (cps) (Fig. F14). These results enable us to determine the semiquantitative variation of each mineral with depth.
The coupled variation of biogenic opal-A and detrital minerals is observed over the entire sedimentary section. The opal-A hump is high in the top 50 mbsf, decreasing down to 70 mbsf in Unit I and then increasing with broad peaks of variation at ~200, 400, and 550 mbsf in Unit II, with maximum values between 400 and 600 mbsf in Unit II. Below these intervals, opal-A generally decreases with depth, although it locally increases in Subunit IIIC and in the bottom of Unit IV.
Detrital minerals are mainly represented by quartz and by 14- and 7-Å minerals, where 14-Å minerals are smectite and mixed-layer clay minerals and 7-Å minerals are kaolinite and/or chlorite. Their variation downhole is roughly inverse to that of opal-A. Variation of 10-Å minerals, which consist of illite and/or glauconite, is similar to 14- and 7-Å minerals in Units I and IV and Subunits IIIA and IIIB but differs in Unit II and Subunit IIIC. Feldspar and hornblende are locally concentrated in the sand- and silt-rich intervals. Their variation with depth appears to be similar to that of the major detrital mineral.
Calcite variation seems to be independent of other components. Peaks in calcite occur at around 70, 220, 320, 400, 500, 750, 850, 950, and 1150 mbsf, where calcareous microfossils are rich or burrows are filled by light-colored calcareous material.
Compositional variations are consistent with variations of color reflectance, NGR, GRA bulk density, and magnetic susceptibility and with the downhole measurements, such as total gamma ray, potassium, and thorium, as well as the resistivity logs. The variation of opal-A at Site 1150 appears to be similar to that of the Leg 127 sites in Japan Sea, although postcruise studies are needed to confirm the correlation and investigate possible regional influences.
A weak 100-m cyclicity in the sediments may exist in the MST and downhole measurements data sets (Fig. F15). In addition, there are several meter- and dekameter-scale variations of biogenic opal-A and detrital minerals. The diatom-rich intervals correspond to slightly high a* values (reddish), low resistivity, low NGR counts, and low density. In contrast, clay-rich intervals show slightly low a* values (light reddish or light greenish), high resistivity, high NGR counts, and high density (Fig. F16).
Induration of the sediments generally increases with depth (Fig. F17) and changes downhole from soft to firm at 424 mbsf. Below 598 mbsf, we described the lithology with the suffix "-stone" (see "Descriptive Terminology" in "Lithostratigraphy" the "Explanatory Notes" chapter). Resistivity data from the downhole measurements show shifts at 424.4, 786.5, and 1046 mbsf with smaller shifts at 598 and 918 mbsf. The calculated ratio of opal-CT to opal-A shows increasing shifts at 786.5 and 1046 mbsf. A possible interpretation of these shifts is that opal-phase transformation from opal-A to opal-CT may start in Subunit IIIC and Unit IV; however, more detailed study is required to confirm this.
Sediment induration decreases downhole in some intervals. In general, the more indurated intervals are dolomite layers, which overlie less-indurated intervals with biogenic opal-A. Several of these intervals can be observed in the resistivity data because the dolomite layers have high resistivity relative to the biogenic-rich intervals. For example, the decreasing resistivity in the lower part of Unit I and upper part of Unit II (~100-220 mbsf) is very obvious (Figs. F15, F18; see "Downhole Measurements"). The intervals in the uppermost part are resistive dolomite layers. The downhole decrease in resistivity below this can be explained by an increase of biogenic opal-A, which has high porosity, and by a decrease of detrital minerals.
Volcaniclastic minor grains and layers are intercalated with the dominant lithology. At Site 1150, we classified four kinds of volcaniclastic layers: (1) pumice grains and layers, (2) primary ash layers, (3) reworked ash layers, and (4) bioturbated ash layers, which are represented by ash patches.
The pumice is intercalated as single grains or accumulations of grains with the dominant lithology. The grains have round, subangular, or angular shape and vesicular texture (Fig. F19).
The primary ash layers typically consist of light gray to white fine-grained vitric ash (Fig. F20). The basal boundary of the ash layers is generally sharp with some scattered or mottled grains and patches in the dominant diatomaceous sediments below the basal boundary. The upper boundary, which is mostly diffuse or grades upward into diatomaceous sediments, is sometimes bioturbated.
The reworked ash layers consist of dark gray fine- to coarse-grained ash with sand-sized quartz, feldspar, hornblende, and other minerals (Fig. F21). The basal boundary of reworked layers is erosional. The upper boundary is typically irregular and grades upward to diatomaceous sediments that are sometimes bioturbated.
The bioturbated ash layers are laterally discontinuous and occur as discrete patches of primary and reworked ash layers as mentioned above. They show light- (Fig. F22) and dark-colored (Fig. F23) discrete and diffuse shapes. Some dark greenish patches are interpreted as altered ash.
The number of pumice and ash layers were counted in each core, and then the total was divided by recovered length in meters (Table T5; Fig. F9). We recorded two maxima in volcanism at Site 1150, one broad maximum during the late Pliocene to Pleistocene spanning Unit I, and another small maximum during the late Miocene at ~700 mbsf. The timing of these two maxima is similar to the volcanism of the ODP Leg 127 sites in the Japan Sea and the DSDP sites east of Japan (Tamaki, Pisciotto, Allan, et al., 1990; Scientific Party, 1980; Kagami, Karig, Coulbourn, et al., 1986), possibly representing volcanic activity that affected the whole area of the Japan Arc System. The timing of volcanism at Site 1150 also seems to be similar to that at DSDP Leg 86 Sites 578, 579, and 580 in the Western Pacific (Heath, Burckle, et al., 1985) and ODP Leg 132 Site 810 on the Shatsky Rise (Storms, Natland, et al., 1991), although the frequency of ash layers is lower at Site 810 than at Site 1150. However, more detailed age and compositional studies on individual ash layers are needed before an accurate tephrachronology can be determined.
The intervals of thin dolomitic layers (Fig. F24) in the recovered sediments and those interpreted as dolomitic layers from the downhole measurements (see "Downhole Measurements") are listed in Table T3 and plotted in Figures F14 and F15. Turbiditic dolomitic layers have low XRD peak intensities, and authigenic dolomitic layers or concretions are completely composed of dolomite. Both types mainly occur in intervals with high amounts of detrital minerals, low biogenic opal-A, and high NGR activity and resistivity.
The major and mostly homogeneous lithology of diatomaceous silty clay is locally interbedded by rare coarser grained accumulations of sand/silt and pebbles/granules. These accumulations of variable thickness and diameter occur as layers with sharp, uncertain, or erosional lower boundaries and with gradational or diffuse upper boundaries; distributed grains over distances of varying scale; patches with sharp boundaries; and accumulations of different shapes and densities with diffuse boundaries. The sand/silt occurrences are mostly a mixture of dark and white grains in strongly varying proportions, which sometimes include considerable amounts of olive-green material.
The pebbles/granules occur as single and/or several grains, accumulations, and layers and with rounded or subrounded to angular shapes. We recognize several kinds of pebbles and granules. The white pebbles are presumably pumice or carbonate. The symbol for pumice was added to the core descriptions only if a clear visible vesicular structure was present. The dark gray and black granules and pebbles are mostly lithoclasts of volcanic rocks. The dark gray and black sand and silt grains are interpreted to be dark volcanic glass, altered grains of volcanic glass, lithoclasts of volcanic rocks, or glauconite grains. Detrital grains of feldspar and hornblende are rich in sand and silt layers. The light-colored sand/silt grains were determined under the microscope to be quartz and volcanic glass. A close relationship exists between the presence of detrital glauconite grains and sand accumulations. Detrital glauconite usually occurs in aggregates of green to greenish black sand-sized grains. In most cases, the accumulations are small in vertical extent, but locally, the thickness increases to several tens of centimeters.
The occurrences of sand/silt and pebbles/granules in each core were counted, and then the total number was divided by recovered length in meters (Table T4, Fig. F9). The upper part of Hole 1150A (Unit I) has higher values in sand/silt than the lower part below 250 mbsf, and in pebbles/granules as well. Below 700 mbsf, it shows a pattern of pebbles/granules with higher numbers in the upper part, and lower numbers in the lower part, where the sand/silt number is highly variable. Increases in sand/silt layers correspond to increases in pebbles/granules between 700 and 950 mbsf and at ~430 mbsf. Between the seafloor and 320 mbsf, the peaks of pebbles/granules are offset downhole relative to the sand/silt peaks.