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doi:10.2204/iodp.proc.314315316.123.2009 LithologyWe recognized two lithologic units during examination of cores from Site C0001 (Fig. F4). Unit I contains three subunits. Units and subunits are differentiated based on persistent contrasts in grain size, layer thickness, sedimentary structures, trace fossils, and mineralogy. We also considered the interpretations of logging-while-drilling (LWD) results during Expedition 314 (see the “Expedition 314 Site C0001” chapter) and seismic reflection data. Unit I (slope-apron facies)
The dominant lithology of Unit I is silty clay to clayey silt (Fig. F4; Table T6). 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 (Fig. F4). During interpretation of LWD data, the comparable boundary between logging Units I and II was placed at 198.9 m LWD depth below seafloor (LSF). Subunit IASubunit IA extends from 0.00 to 168.35 m CSF. The dominant lithology is greenish gray to grayish green mud (silty clay to clayey silt). The mud is generally structureless, but local wavy laminae are expressed by color change (dark green). Calcareous nannofossils are common to abundant in silty clay intervals; foraminifers, diatoms, and radiolarians are also evident in smear slides (Fig. F5). Bioturbation is patchy and relatively mild in severity (compared to Unit II). Irregular patches of silt and sandy silt are rare. Local zones of contorted bedding are similar to those observed at Ocean Drilling Program (ODP) Sites 1175 and 1176 within the slope-basin facies (Shipboard Scientific Party, 2001a, 2001b). The most common interbeds within Subunit IA consist of white or light gray to dark gray volcanic ash, which typically displays a sharp base and normal size grading; the upper contacts between ash and silty clay are less distinctive than the bases (Fig. F6). Ash layer thickness ranges from <1 to 25 cm (Fig. F7), and particle size ranges from coarse silt to medium sand. The highest concentrations of ash occur between ~75 and 180 m CSF (Fig. F7). Light-colored ash deposits contain fresh shards of clear volcanic glass and pumice, with minor amounts of fresh, euhedral plagioclase. In contrast, darker ash beds contain more fragments of altered volcanic rock, altered glass (replaced by clay minerals or coated by manganese oxide), crystals of plagioclase and iron-magnesium silicates (e.g., pyroxene and amphibole), and opaque grains. Subunit IBSubunit 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 (silty clay to clayey silt), superficially identical to the mud of Subunit IA. Calcareous nannofossils are common to abundant, and bioturbation is relatively mild. 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. Silt-size grains are composed mostly of quartz, feldspar, sedimentary rock fragments, and opaque minerals. Subunit ICSubunit 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 (Fig. F8). 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. Sand grains consist mostly of detrital quartz and feldspar with abundant sedimentary and low-grade metasedimentary rock fragments (shale, argillite, chert, and quartz-mica grains) (Fig. F8). LWD data indicate moderate bedding dips within the correlative logging unit. Interpretation of Unit IDeposition of Unit I probably occurred in a slope-apron to slope-basin environment. This interpretation is consistent with present-day bathymetry, which features a small depression or topographic bench that was created by uplift along the megasplay fault system. The dominant mechanism of sedimentation was hemipelagic settling of suspended silt, clay, and biogenic constituents. Most of the volcanic ash probably entered the site by vertical settling from air falls, although remobilization by subaqueous gravity flow is also feasible. Local zones of contorted bedding are probably manifestations of soft-sediment folding caused by gravitational failure. The sand beds of Subunit IC are somewhat enigmatic. Turbidity currents were the most likely agents of transport for sand and silt beds. Gravity flows during the early stages of Unit I deposition may have moved down the axis of the trench (i.e., toward the southwest) and lapped onto the landward slope. This transport direction would be consistent with paleoflow data from the Quaternary trench wedge in the Muroto transect area (Pickering et al., 1993a). Alternatively, turbidity currents during the late Pliocene may have moved across the forearc in a transverse direction, funneling sand through small submarine canyons. Detrital provenance analysis will be needed to discriminate between these options. In either case, the delivery of sand and silt gradually dissipated, either because the site rose above the influence of trench floor processes or because the transverse flow paths were blocked or diverted by accretion-related deformation. The interpretation of progressive uplift is reinforced by the distribution of calcareous nannofossils, which increase in concentration upsection. The pattern of fining and thinning upsection is similar to what was observed at ODP Sites 1175 and 1176, within slope basins of the Muroto transect of the Nankai Trough (Shipboard Scientific Party, 2001a, 2001b; Underwood et al., 2003). Unit II (upper accretionary prism)
The boundary between Units I and II is an unconformity, with an associated hiatus, as shown by both paleomagnetic data and biostratigraphy. The occurrence of a geometric unconformity is also obvious from seismic reflection and LWD data (see the “Expedition 314 Site C0001” chapter). Within Unit II, Expedition 314 shipboard scientists recognized three logging subunits (IIA, IIB, and IIC) (Fig. F4). Lithologic variations in the cores, however, are not distinctive enough to warrant division of lithologic Unit II on the basis of texture, composition, or sedimentary structures. The dominant lithology of Unit II is greenish gray to grayish green bioturbated mud (silty clay to clayey silt). Thin layers of silt or silty sand are rare and limited to cores near the top of the unit. Volcanic ash layers are also rare, thin, and limited to cores near the top of the unit. (Fig. F7). Mud contains local wavy laminae, as defined by a darker green color and higher clay content. Concentrations of calcareous nannofossils are significantly lower here than they are within comparable mud deposits of Unit I (Fig. F5). Bioturbation, as characterized by the association of Zoophycos and Chondrites, is moderate to heavy and increases in severity with depth. Pyrite grains are moderate to abundant. The depositional environment of Unit II is difficult to interpret with confidence. Seismic reflection data indicate that the contact between Units I and II is a boundary between the slope apron and the accretionary prism, which means that the most likely depositional environment prior to accretion is trench wedge. This part of the accretionary prism, however, does not display continuous high-amplitude seismic reflectors, so lithology is evidently homogeneous throughout. Low concentrations of calcareous nannofossils point to a depositional environment below the carbonate compensation depth (CCD), probably near the base of the trench slope. Elsewhere along the Nankai margin, the preponderance of hemipelagic mud is unusual within the trench-wedge environment, although the facies becomes muddier near its seaward margin (Pickering et al., 1993b; Moore, Taira, Klaus, et al., 2001). Farther outboard, the upper Shikoku Basin facies contains abundant beds of volcanic ash, at least where it has been cored in the Muroto and Ashizuri transects (Moore, Taira, Klaus, et al., 2001). Tectonic kneading of the muddy slope apron, or a slope basin between imbricate thrusts, into the accretionary prism is also possible, although the paucity of ash beds and biogenic carbonate argues against this interpretation. Thus, although the match between lithology and environment is not definitive, our preferred interpretation for the depositional setting is outer (marginal) trench wedge during a time interval of low sand-silt influx. X-ray diffraction mineralogyBulk powder X-ray diffraction (XRD) results provide useful constraints on relative abundances of total clay minerals, quartz, plagioclase, and calcite. Data are shown in Figure F9 and tabulated in Table T7. As a measure of accuracy for these estimates, relative to absolute percentages, we completed regression analysis of percent calcite from XRD versus percent calcium carbonate from coulometric analysis (see “Organic geochemistry”). For XRD relative values above ~10%, this comparison shows a progressive overestimation of calcite relative to the coulometric data, which is to be expected if the coulometric concentration is expressed as a percentage of the total solid mass (weight percent) (Fig. F10). Conversely, the XRD method underestimates relative percent calcite for absolute values below ~5%, and the detection limit for calcite is ~3% (true weight percent). Values of quartz (XRD) at Site C0001 range from 10.2% to 28.2%. Average relative values are 20.3% and 20.6% for Units I and II, respectively. Relative percentages of plagioclase vary between 11.2% and 38.5%. Unit I contains an average of 19.6% plagioclase, whereas Unit II contains an average of 15.6%. Relative concentrations of total clay minerals are between 28.3% and 67.6%. Total clay significantly increases in Unit II (average = 62.5%) compared to Unit I (average = 42.9%). Calcite shows the largest stratigraphic variation, and its abundance is controlled mostly by the concentration of calcareous nannofossils. Unit I contains between 1.7% and 45.9% calcite (average = 17.2%), whereas relative values in Unit II drop from 9.0% to 0.0% (average = 1.3%). This shift in composition across the unit boundary is consistent with uplift of the depositional site from below the CCD (Unit II) to above the CCD (Unit I). The same compositional trend is evident at ODP Sites 1175 and 1176 across the same type of facies change (Shipboard Scientific Party, 2001a, 2001b; Underwood et al., 2003). |
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