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doi:10.2204/iodp.proc.314315316.135.2009 LithologyFour lithologic units were recognized during examination of cores from Site C0007 (Fig. F3; Tables T3, T4). Four subunits are recognized in Unit II. Units and subunits are differentiated based on contrasts in grain size, mineralogy, composition, and presence (and thickness) of gravel, sand, and ash layers. In choosing unit boundaries, we also considered biostratigraphic information, paleomagnetic data, X-ray computed tomography (CT) images, observations of structural style, interpretation of seismic data, and LWD data from nearby Site C0006. Deformation within the succession makes the original stratigraphic thickness difficult to determine. Unit I
The dominant lithology of Unit I is greenish gray silty clay that contains clay minerals, quartz, feldspar, lithic fragments, vitric fragments, and calcareous nannofossils (Fig. F3A; Table T3). Unit I consists of a fining-upward succession of silty clay, sand, silty sand, and rare volcanic ash layers. Calcite content determined by X-ray diffraction (XRD) is a defining characteristic of this unit and ranges from 0% to 5.5% and averages ~1%. The mud is internally structureless and locally has green color banding. Sand and silty sand layers are extensively disrupted by probable coring disturbance that in some cases creates mixtures of sand and mud in the upper part of Unit I. Steepened bedding and faults are present throughout most of this unit (Fig. F4; see also “Structural geology”). Lower in the unit, thin to thick sand beds with sharp bases and graded tops fine upward into the overlying mud (Fig. F4). Generally, sand layers become thinner upward. The sand component is mainly fine to very fine grained and consists of quartz, feldspar, lithic grains, and vitric fragments. Several thick layers (~20 cm) of light olive-gray volcanic ash with abundant pumice fragments are present near the base of the unit. A dark brown ash is present higher in the unit (Fig. F4). The contact between Unit I and underlying Unit II is distinguished by the appearance of thick beds of dark gray sand below and the increase in mud content above the unit boundary. Deposition of this unit occurred on the lowermost slope above the trench floor by hemipelagic settling, turbidite deposition, and possibly subsequent soft sediment slumping on an oversteepened slope. Unit II
Unit II is divided into four subunits based mainly on variations in silt, sand, and gravel content (Fig. F3A; Tables T3, T4). The unit displays a general coarsening-upward trend marked by a progressive increase in silt, sand, and gravel content upsection. Subunits IIA and IIC reoccur at discrete intervals in Holes C0007C and C0007D (Subunits IIAi, IIAii, IIAiii, IICi, and IICii; Fig. F3A, F3B). Heterogeneity in XRD data (Tables T5, T6, T7, T8) throughout this section can be attributed to the varied lithologies sampled (sand to silty clay; Fig. F5). The repeated subunits within Unit II may be either structural repetitions of the same stratigraphic intervals or recurrences of equivalent facies deposited at different times. For example, spatial concentration of sand and gravel could have been affected by migration of an axial channel across the floor of the trench and/or temporal changes in sediment supply due to fluctuations in sea level. Subunit IIA
Subunit IIA is recognized in three intervals. The upper interval (Subunit IIAi) is present only in Hole C0007C and extends from 33.94 to 73.99 m CSF. The middle interval (Subunit IIAii) is present in Hole C0007C, where it extends from 109.59 to the base of core recovery at 159.13 m CSF, and in Hole C0007D from 175.00 to 190.05 m CSF. The deepest occurrence of this subunit (Subunit IIAiii) is in Hole C0007D and extends from 266.20 to 303.50 m CSF. The defining characteristic of Subunit IIA is the presence of thick sand and/or gravel. The dominant lithology of Subunit IIAi is olive-gray to dark gray fine-grained sand, consisting dominantly of metamorphic and volcanic lithic fragments with secondary quartz and feldspar. Calcareous nannofossils are almost completely absent from Subunit IIA as shown by the calcite XRD plot (Fig. F5). Only two samples contained detectable calcite (0.1% and 0.3%) by XRD. Individual beds are generally 1–6 m thick and structureless although any original sedimentary structures may have been destroyed by coring. The thickest bed is at least 6.5 m thick (34.00–40.05 m CSF) and contains abundant pumice and scoria fragments (Fig. F6). Sand beds typically have sharp bases and grade into olive-gray silt and sometimes greenish gray silty clay with indistinct boundaries between the different lithologies. Silty clay interbeds are typically <1 m thick with minor sandy silt and clayey silt intervals. The dominant lithology of Subunit IIAii is dark gray fine to very coarse grained sand, consisting dominantly of metamorphic and volcanic lithic fragments with secondary quartz and feldspar. Sand is interbedded with thin olive-gray silty clay and clayey silt. At least one thick (1.7 m) normally graded gravel sequence is present at the base of Hole C0007C with a grain size changing from medium-grade gravel (8–16 mm) at the base to fine sand at the top (Fig. F7). The gravel is clast-supported with no obvious matrix and is moderately sorted. Clasts are subrounded to subangular. The clast assemblage is polymictic with grain types including chert, sandstone, mudstone, mylonitic quartzite, quartz-rich metamorphic, volcanic, and quartzo-feldspathic plutonic lithic fragments (Figs. F8, F9). Some clasts have a partial coating of pyrite (Fig. F7) that locally appears to have been abraded, indicating that clasts were transported and deposited after the pyrite coating had formed. Assessment of Subunit IIAii was hampered by poor core recovery, possibly indicating substantial loss of sand and gravel during coring. Subunit IIAiii has a dominant lithology of fine-grained dark gray polymictic gravel (grains 2–5 mm; Fig. F10), which fines upward into olive-gray sand and silt. Silty clay interbeds are thin and rare. The clasts in this gravel are similar to those in Subunit IIAii and may be a lateral equivalent of this facies or an older unit of similar source deposited in a similar depositional environment. Overall, poor core recovery makes it difficult to determine if the repetition of Subunit IIA is related to temporal flucuations in channel position or sediment supply or is, in part, structural in nature. Subunit IIB (outer trench wedge)
Subunit IIB consists of interbedded fine-grained sand, silty sand, and silty clay in approximately equal abundance. Dark gray fine-grained sand and dark olive-gray silt are dominated by metamorphic and volcanic lithic grains with secondary quartz and feldspar, although the proportion of lithic fragments decreases in very fine sand and silt. Calcite content by XRD is minimal, ranging from 0% to 0.1%. Sand is typically <1 m thick and normally graded with indistinct upper boundaries that grade into silty clay. Silty clay is greenish gray and is only slightly bioturbated or mottled in places. Ash horizons are either absent or not recovered in this subunit. Subunit IIC (marginal trench wedge)
Subunit IIC is recognized in two intervals in Hole C0007D. The upper interval (Subunit IICi) extends from 190.05 to 266.20 m CSF, and the lower interval (Subunit IICii) extends from 303.50 to 326.02 m CSF (Fig. F3B). The dominant lithology is greenish gray silty clay, which is interbedded with olive-gray clayey silt and volcanic ash. Many of the silt beds have parallel lamination and diffuse upper boundaries. Ash layers are relatively abundant in comparison with overlying units (Fig. F11), particularly near the base of the Subunit IICi, although there may be some repetition of these (see “Structural geology”). Mottling and bioturbation are common in both intervals, and Chondrites burrows are present in places (Fig. F12). A downhole increase in total clay mineral content is observed in Subunit IICi with a corresponding decrease in the relative proportions of quartz and feldspar (Fig. F5). Calcite content is also slightly higher (locally up to 4%, averaging ~1%) in Subunit IICi because of the presence of calcareous nannofossils. In comparison to Subunit IICi, Subunit IICii has a slightly higher clay mineral content (average = 52% versus 48% in Subunit IICi), less quartz and plagioclase (22% and 25%, respectively, versus 24% and 27% in Subunit IICi), and calcite is rarely present except at the base of the interval. Incomplete recovery in the upper part of Subunit IICii has hampered this assessment. Subunit IID
The dominant lithology of Subunit IID is greenish gray silty clay, and minor lithologies include silt and ash. It is discriminated from the overlying subunits of Unit II in part by its higher clay content, averaging 54%. In addition, silt layers are thin-bedded and relatively rare and sand is completely absent. Ash layers are relatively abundant in comparison with overlying units (Fig. F11). Silty clay exhibits green (glauconitic?) color bands which increase in frequency toward the base of the subunit. Mottling and slight to moderate bioturbation are common, but positive identification of Chondrites burrows is limited to the upper part of the subunit. Unit II is interpreted as having been deposited in a trench setting with increasing proximity to the axial portion of the trench upsection. The repetition of the axial-channel fill facies (Subunit IIA) indicates either a repetition of the sequence because of thrust faulting or shifting of the position of the trench channel. Unit III
Unit III is 77.18 m thick (Fig. F3B; Table T4), although this thickness is exaggerated by repetition of the unit along thrust faults (see “Structural geology”). The unit consists of greenish gray clay and silty clay with interbedded volcanic ash layers, including one occurrence of calcite-cemented tuff. The mud has a clay content substantially higher than overlying Unit II, averaging 65% (Fig. F5). In concert with the increased clay mineral content, quartz and plagioclase are diminished, making up only 19% and 15% of the mud, respectively. In contrast to Subunit IID, Unit III has a greater proportion of volcanic glass reflecting greater ash input, perhaps compounded by lower sedimentation rates. Volcanic ashes range from white, light gray, light greenish gray, to black. They have sharp lower and diffuse upper contacts and are locally burrowed. They consist mainly of glass with minor pumice, quartz, feldspar, and calcareous nannofossils. Black ash includes abundant scoria. Widespread and diverse bioturbation includes Zoophycos, Chondrites, large (>1 cm) subvertical burrows, and other types (Fig. F12). Some bioturbation is made visible by color mottling. Green color bands (glauconization?) are also abundant throughout the unit and some display sharp tops. Sand-size glauconite particles are common and dispersed through the mudstones. Small (1 mm) tubular agglutinated tests and possible burrow linings made mostly of volcanic ash particles and concentration of heavy minerals are widely distributed through the unit (Fig. F12). Pyrite nodules and pyrite-filled burrows occur throughout the unit (Fig. F12). In contrast to Site C0006, no change in color within this unit has been noted downhole. Smear slide observations indicate that grain size decreases downhole. Unit III contains lithologies similar to those described in the deep marine Shikoku Basin succession of the western Nankai Trough (Shipboard Scientific Party, 2001a, 2001b; Moore et al., 2001) and was deposited by hemipelagic settling with accumulation of volcanic ash. Unit IV
Unit IV was encountered below a low-angle thrust fault observed on seismic sections at the site, but only limited recovery was achieved. The presence of thick unconsolidated sand is suggested by the significant increase of drilling penetration rate for the lowermost 3 m of Core 316-C0007D-29R and subsequent cores. Only 25 cm of dark gray sand (Fig. F13) and a few drilling-affected mudstone fragments were recovered from Core 316-C0007D-35R. The sand is fine- to medium-grained and consists of abundant black lithic fragments, metamorphic rock fragments, ferromagnesian minerals, quartz, feldspar, and opaque grains. From seismic data and LWD interpretation of equivalent intervals at Site C0006 (see the “Expedition 314 Site C0006” chapter), Unit IV is interpreted as the underthrust part of the Pleistocene trench wedge. DiagenesisOverall, diagenetic effects observed at this site are minimal. Given the overall low temperature (see “Physical properties”), even allowing for uplift, the detrital feldspar assemblage, heavy minerals, and clay mineral assemblage are expected to provide reasonably faithful records of provenance information. Volcanic glass observed at this site is water-clear in transmitted light and shows no discernible evidence of alteration at the scale of smear slide observation. Only minor evidence of chemical diagenesis could be detected at this site using macroscopic core observation and smear slides. Pyrite in the form of framboids, pyritized burrows, and small (up to 1 cm) nodules is distributed widely through the mud in all of the units and is appreciated most fully in X-ray CT images. One occurrence of ash cemented by calcite (see “Lithology” in the “Expedition 316 Site C0006” chapter for a description of other similar examples) was also encountered in interval 316-C0007D-31R-1, 16–24 cm. Measurements of resistivity and velocity in this sample indicate values far in excess of that observed for silty clays (see “Physical properties”). An interesting petrographic feature of uncertain significance was observed in Unit III. Locally within deformed zones (e.g., Section 316-C0007D-27R-CC; see “Structural geology” for further description of this interval), the silt particles (primarily quartz and feldspar) are coated with thin rims composed of highly oriented clay particles (Fig. F14). The isopachous rims are ~5–10 µm thick and have a high degree of orientation that is readily observed in crosspolar observation and made starkly visible by use of the gypsum plate. Clays within the silt coatings are aligned with their 001 surfaces approximately parallel to the grain surfaces. In samples that display these clay rims, almost every grain has a rim developed to some degree, though the rims are more readily observed on the larger particles. The rims do not appear to be selective to grain type and are observed on quartz, feldspar, lithic fragments, sponge spicules, heavy minerals, and opaque grains including pyrite framboids. X-ray CT numberX-ray CT images were used extensively for evaluation of sedimentological and structural features. Figure F15 shows the depth trends for CT numbers (averaged pixel intensity for a 1 mm2 area) determined for coherent rock pieces and tectonic breccia clasts of silt clay. CT number reflects the average sample density at the scale of the CT observation, a value that is affected by sample composition (mineral composition and density) and porosity (bulk density), although in these porous samples, it is largely dominated by the porosity. Throughout Units I and II CT number increases with depth. CT number decreases from 1410 to 1270 in the upper part of Unit III. The lowest value of 1240 in this section is shown at 380 m CSF, and CT number recovers to 1360 at 418 m CSF. This relatively low CT number zone is consistent with the lower clay-dominated part of the site. |
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