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doi:10.2204/iodp.proc.314315316.134.2009 LithologyThree lithologic units were recognized during examination of cores from Site C0006 (Fig. F3; Table T3). 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 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, and interpretations of LWD and seismic results obtained during Expedition 314 (see the “Expedition 314 Site C0006” chapter). Numerous thrust faults offset the succession at Site C0006 with duplication of stratigraphic intervals. Thus, true stratigraphic thicknesses are difficult to determine and the estimates given here are maxima. Unit I (trench to slope transition facies)
The dominant lithology of Unit I is greenish gray silty clay that contains clay, quartz, feldspar, lithic fragments, vitric fragments, and calcareous nannofossils (Fig. F3; Table T3). Calcite content is an important defining characteristic of this unit, ranging from 0% to 7.5% and averaging 2%. In LWD data from Expedition 314, the corresponding interval was not logged with the resistivity tool and was included in a logging unit that largely corresponds to lithologic Unit II. Unit I consists of a fining-upward succession of silty clay, sand, silty sand, and rare volcanic ash layers. The mud is structureless and locally has green color banding. Sand and silty sand are extensively disrupted by a coring disturbance that locally creates mixtures of sand and mud. Lower in the unit, thin to thick beds with sharp bases and graded tops fine upward into the overlying mud. Generally, sand layers are thinner in the upper part of the unit. The sand component is mainly fine to very fine grained and consists of quartz, feldspar, lithic grains, and vitric fragments. A 1.25 m thick layer of light olive-gray volcanic ash with abundant pumice fragments occurs near the base of the unit. The contact between Unit I and underlying Subunit IIA is distinguished by the first appearance of thicker beds (>50 cm; Fig. F3B) of black sand below and the increase in mud content above the unit boundary. Deposition occurred on the lowermost slope above the trench floor by hemipelagic settling, onlap of axial turbidity currents that decreased through time, and accumulation of a thick ash. Unit II (trench deposits)
Unit II is divided into four subunits based mainly on variations in silt and sand content (Fig. F3; Table T3). Lithologic variations in the cores, however, do not precisely correspond to these arbitrary or gradational divisions, and it was considered more appropriate to group them into a single unit with a general coarsening upward trend relating to a progressive increase in silt and sand content upsection. A similar coarsening upward trend is noted in the accreted trench deposits of the Nankai margin described by Moore and Karig (1976). The large variations in X-ray diffraction (XRD) data throughout this section (Tables T4, T5) can be partially attributed to the varied lithologies sampled (sand to silty clay; Figs. F3B, F4). Unit II is also structurally complex, with numerous thrust faults causing significant repetition of the sequence (see “Structural geology” and “Biostratigraphy”). Although the exact positions of these faults are uncertain, they have been tentatively placed on the summary stratigraphic section (Fig. F3). Subunit IIA (sand-dominated trench wedge)
The dominant lithology of Subunit IIA is dark gray to black fine-grained sand consisting of metamorphic and volcanic lithic fragments with secondary quartz and feldspar. Individual beds are generally 1–7 m thick and massive, although any original sedimentary structures may have been destroyed by coring. Sand typically grades upward into silt and locally silty clay with indistinct boundaries between the different lithologies. Sand beds have sharp bases (Fig. F5). Silty clay interbeds are typically <1 m thick with minor sandy silt and clayey silt intervals also present. Clay content ranges from 28.5% to 55% and averages 42%. Quartz content ranges from 20% to 29% and averages 24.1%, and plagioclase ranges from 21% to 46% and averages 33%. Calcite in this unit is below detection by XRD in most samples (only four samples contain calcite), ranging up to 3% (Fig. F4). The high sand content of this subunit is suggestive of an axial channel deposit. Subunit IIB (mixed sand-mud trench wedge)
Subunit IIB consists of interbedded fine-grained sand, silty sand, and silty clay in approximately equal abundances. The sand content of this subunit is markedly diminished compared to overlying Subunit IIA (Fig. F3B). Dark gray fine-grained sands and dark olive-gray silts are dominated by metamorphic and volcanic lithic grains with secondary quartz (averaging 25% by XRD) and plagioclase (averaging 31% by XRD), although the proportion of lithic fragments decreases in very fine sand and silt. Sand beds are typically normally graded with indistinct upper boundaries that grade into silty clay. The silty clay is greenish gray and is only slightly bioturbated or mottled in places. Carbonate content is below detection by XRD. Minor ash and carbonate-cemented tuff exist in the upper portions of this subunit (see description of carbonate-cemented ash below). Interpretation of this subunit was hampered by poor core recovery, possibly indicating substantial loss of the sand units shown by LWD logs (see the “Expedition 314 Site C0006” chapter). Subunit IIC (mud-dominated trench wedge)
The dominant lithology of Subunit IIC is greenish gray silty clay and minor lithologies include normally graded silt, sand, and rare ash. Within the coarser siliciclastic beds, silt is generally more abundant than sand, particularly when compared to Subunit IIA (Fig. F3B). The silty clay is slightly bioturbated and mottled in places, with local Chondrites burrows. Sand and silt beds are typically graded with indistinct upper boundaries and commonly exhibit parallel laminae and locally well-formed cross-laminae. They are typically 5–25 cm thick, although sand beds up to 5 m thick have also been identified (with their tops at 230, 247, 326, and 369 m CSF). The lowermost of these contains abundant organic matter and wood fragments (up to 50%) with individual fragments up to 3 cm across (Fig. F6). This bed coincides with the base of a significant fault and shows various structures indicating sand remobilization and injection. Compositionally, this subunit is similar to Subunit IIB with the exception of a slightly greater calcite content (ranging from 0% to 12% and averaging ~1%) as determined by XRD. Subunit IID (deep marine basin to mud-dominated trench transition)
The dominant lithology of Subunit IID is greenish gray silty clay, and minor lithologies include silt and ash. Silt layers are thin-bedded; they are relatively rare in this section and absent below 405 m CSF. Sand beds are completely absent from this subunit. Ash layers are relatively abundant in comparison with overlying units (Fig. F7). The silty clay typically 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. A pronounced increase in the relative abundance of calcite in the XRD data (up to 13%; Fig. F4) appears to correspond to a zone of abundant foraminifer tests in the core (up to 8% in smear slides) in the lowermost part of the subunit. Except for minor variations in calcite content, Subunits IIB, IIC, and IID manifest no prominent depth-dependent or age-dependent trends in composition as determined from XRD (Fig. F4). X-ray fluorescence (XRF) analysis was applied to a subset of samples spanning these three subunits in an effort to find compositional trends related to the clay fraction (see “X-ray fluorescence” in the “Expedition 316 methods” chapter). Variations in the silt/clay ratio potentially provide a source of elemental variation that is large enough to mask trends related specifically to clay minerals, so the sample set for XRF (33 samples) was selected to include only a narrow range in the total clay content (48.0%–54.0% clay minerals). Despite this effort to constrain sources of elemental variation, no depth trends were detected in 10 of the 11 elements analyzed. Only P (Fig. F8) shows a subtle decreasing trend with depth that may reflect decreasing environmental oxygenation in areas more remote from persistent bottom currents, an interpretation that is consistent with the abundant pyrite observed in deeper mudstones. Unit II is interpreted as having been deposited in a trench setting, with increasing proximity to the axial portion of the trench upsection. Unit III (deep-marine basin)
Unit III is 140.06 m thick (Fig. F3; Table T3), although this thickness is exaggerated by repetition of parts of the unit along thrust faults (see “Structural geology”). The unit (Fig. F9) consists of greenish gray to grayish silty clay with some interbedded volcanic tuff layers, including dolomite-cemented and calcite-cemented ash (see below). Bioturbation is widespread, particularly in the upper part of the unit with Zoophycos, Chondrites, other types of burrows, and color mottling. Green color bands are also abundant in the upper part of the unit. The silty clay consists mainly of clay with quartz, feldspar, and minor to rare calcareous nannofossils. Clay content of the silty clay (averaging 51% for the unit) increases downhole, and its color changes from greenish gray (Fig. F9A) to gray at 561.5 m CSF (Fig. F9B). Unit III has an overall increased clay mineral content and decreased quartz and feldspar content (averaging 22% and 25%, respectively) compared to overlying Subunit IID (Fig. F4). Volcanic ash layers are white to light gray, at least partly laminated, with sharp lower and diffuse upper contacts, and are locally burrowed. They consist mainly of glass with minor pumice, quartz, feldspar, and calcareous nannofossils. Unit III was deposited by hemipelagic settling along with accumulation of volcanic ash during major volcanic eruptions. The stratigraphic position of Unit III is below the trench-wedge facies of Unit II. Its early Pleistocene–Miocene age and overall lithologic content are consistent with deposition in the Shikoku Basin. Similar deposits have been documented at Ocean Drilling Program Sites 808, 1173, and 1174 in the Muroto transect over 100 km to the west-southwest along the Nankai Trough (Shipboard Scientific Party, 2001a, 2001b; Moore et al., 2001). DiagenesisOverall, diagenetic effects observed at this site correspond almost entirely to mechanical processes, an unsurprising result given that the maximum temperature in the deeper portions of Site C0006 is not expected to exceed 40°C (see “Physical properties”). Even allowing for uplift, the detrital feldspar and clay mineral assemblages at these temperatures 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 (<1 cm) nodules is distributed widely through the mudrocks in all of the units and is appreciated most fully in X-ray CT images. Occurrences of ash cemented by calcite or dolomite were also encountered. Most samples of this material were discovered as pieces of relatively lithified rubble in the tops of cores (Table T6). Two possible in-place occurrences of this lithology are in Sections 316-C0006F-12R-1, 7–32 cm, and 13R-2, ~45–52 cm. A second less certain in-place occurrence is in Section 316-C0006F-20R-1, 10–12 cm. XRD analysis indicates that the nonash component of these rocks consists of either dolomite or calcite with minor amounts of clay, quartz, and feldspar. Measurements of resistivity and velocity in one sample (rubble collected from Section 316-C0006E-46X-1) indicate values far in excess of that observed for normal silty clay (8 cm3 cube; 3.0 × 1.9 × 3.3 kΩ; ~5.1 km/s P-wave velocity; see “Physical properties”). Resisitivity logs (see the “Expedition 314 Site C0006” chapter) show thin intervals of extremely high values and associated low gamma ray intensity that may correspond to this lithology. Further clues about the distribution of authigenic carbonate come from the drillers who reported informally having encountered extremely hard but thin layers. In thin section the dolomite appears as microcrystalline cement in well-sorted, little-compacted, and very pure volcanic ash (Fig. F7C). Millimeter-scale laminations correspond to different compactional states of the ash particles. A third minor instance of chemical diagenesis was observed in Section 316-C0006F-19R-1, 91 cm (dark gray mud of Unit III). A minute sand concentration (most likely an agglutinate test) was examined in smear slide and discovered to contain an abundance of 2–10 µm bladed crystals having low relief and extremely low birefringence (Fig. F10). These small crystals are very tentatively identified as the zeolite phillipsite. X-ray CT numberX-ray CT images were used extensively by the science party for evaluation of sedimentological and structural features. Figure F11 shows the depth trends for CT number (averaged pixel intensity for a 1 mm2 area) determined for coherent pieces and tectonic breccia clasts of silty 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 porosity. Across Unit I through Subunit IIC, CT number increases with depth. CT number decreases gradually from 1450 to 1300 between lower Subunit IID and upper Unit III. The lowest value of 1300 in this section is shown at 480 m CSF, and CT number recovers to 1450 at 530 m CSF. This relatively low CT number zone is consistent with the lower mud-dominant part of the site. |
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