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doi:10.2204/iodp.proc.314315316.124.2009 LithologyWe recognized four lithologic units during examination of cores from Site C0002 (Fig. F4; Table T4). Units are differentiated based on persistent contrasts in grain size, layer thickness, sedimentary structures, trace fossils, and mineralogy. Because shipboard examination of split cores began at a coring depth of 479.40 m CSF, we also considered seismic reflection data and facies interpretations of logging-while-drilling (LWD) data to provide a broader stratigraphic context (see the “Expedition 314 Site C0002” chapter). Core recovery was generally poor to moderate and was particularly bad within intervals in which the log character is indicative of high sand content. Thus, we believe that LWD data provide a more accurate picture of facies character. Unit I
Beginning at the seafloor, lithologic Unit I was sampled only for interstitial water and microbiology and was imaged using the multisensor core logger (MSCL) and X-ray computerized tomography. The cores were split and described at the Kochi Core Center after the expedition had concluded. The lithologic boundary is located at the bottom of Section 315-C0002D-15X-CC. The correlative boundary between logging Units I and II was placed at 135.5 m LWD depth below seafloor (LSF). The dominant lithology of Unit I is dark olive-gray and greenish gray to grayish green mud (silty clay to clayey silt) (Fig. F4; Table T4). Locally, mud is enriched in foraminifers and there are local dark color bands. Secondary lithologies include thin interbeds and irregular patches of dark greenish gray medium to fine sand, silty sand, sandy silt, and silt. Thin layers of light gray volcanic ash are also common. The coarser interbeds are typically 1 to 15 cm thick, with sharp bases, and many such beds display normal size grading and diffuse tops. Cross-laminae occur in some silt beds. These sedimentary structures are characteristic of turbidites. The thickest sand bed is ~1.86 m. Overall, the vertical trend for the unit thins and fines upward. Interpretation of Unit IDeposition of Unit I occurred in the distal reaches of the Kumano Basin, as did the deposition of Unit II below. The facies character is typical of a basin-plain type environment, with rhythmic interlaying of fine-grained turbidites and hemipelagic mud. Turbidity currents entered the basin frequently via submarine canyons and gullies along the upper slope and shelf. After examining the cores, we conclude that the differences in log character between logging Units I and II are related more to consolidation state than changes in lithology. Unit II (forearc basin facies)
After a coring gap of ~15 m, Unit II begins at a depth of 150.00 m CSF at the top of Section 315-C0002D-16H-1. There is a substantial gap in coring between 203.52 m CSF and the first core from Hole C0002B at 479.40 m CSF. The lower unit boundary based on coring data is located between the bottom of Section 315-C0002B-39R-CC (826.34 m CSF) and the top of Section 315-C0002B-40R-1 (834.00 m CSF). This pick is based on the occurrence of silty beds with plane-parallel laminae above the boundary. During LWD data interpretation, the equivalent contact between logging Units II and III was placed at 830.4 m LSF. The dominant lithology recovered from Unit II is greenish gray to grayish green mud (silty clay to clayey silt) (Fig. F4; Table T4). Secondary lithologies include thin interbeds and irregular patches of sand, sandy silt, and silt. Volcanic ash is rare. Zones of contorted bedding and soft-sediment folding occur locally. Logs show a much higher proportion of sandy layers than what was recovered by coring. Direct comparisons between core and log intervals indicate that recovery of sand and silt beds was <10%. Unfortunately, we were unable to recognize depositional cycles in the cores or systematic changes in the frequency of occurrence of turbidites. In general, the mud in this unit is noticeably coarser than comparable deposits at greater depth. Mud is locally structureless but more commonly shows plane-parallel laminae and incipient fissility. Orientation of this fabric is horizontal to gently inclined. Calcareous nannofossils are sparse in the clayey silt intervals. Bioturbation is patchy and relatively mild in severity (compared to Unit III). The ichnofauna is dominated by Chrondrites. Dark gray silt, sandy silt, and silty sand are the characteristic interbeds of Unit II (Table T5). These beds are typically <5 cm thick and display sharp bases, faint plane-parallel laminae, normal size grading, and diffuse tops. Such features are typical of fine-grained turbidites. Silt and sand grains are composed mostly of quartz, feldspar, sedimentary and metasedimentary rock fragments (shale, argillite, chert, and quartz-mica grains), and a rich diversity of heavy minerals. Interpretation of Unit IIDeposition of Unit II occurred in the distal reaches of Kumano Basin. The facies character is typical of a basin-plain type environment and does not significantly differ from that of Unit I. This interpretation is consistent with present-day bathymetry and the proxies for abundant silt and sand turbidites in the logs. Nannofossil datums show that sedimentation rates were high (400–800 m/m.y.) throughout the history of Unit II (see “Biostratigraphy”). Turbidity currents entered the basin frequently via submarine canyons and gullies along the upper slope and shelf. Arrival of the first turbidites, which began at ~1.6 Ma, depended on incision of through-going erosional conduits and sufficient uplift along the basin’s seaward edge to form a deep depocenter. Unit III (basal forearc basin)
The boundary between lithologic Units II and III is defined by a shift in lithofacies from turbidites above to condensed mudstone below. As defined by log character, the unit boundary is positioned at 830.4 m LSF, which coincides with a coring interval of no recovery between Section 315-C0002B-39R-CC and the top of Section 315-C0002B-40R-1 (834.0 m CSF). Strata below the boundary are Pleistocene to late Miocene in age, and rates of sedimentation within that interval slowed to 18–30 m/m.y. (see “Biostratigraphy”). Lithologic variations in the cores are not distinctive enough to warrant further subdivision on the basis of texture, composition, or sedimentary structures. However, on the basis of lithology and nannofossil events, we recognize a pronounced unconformity at the base of Unit III at a depth of ~923 m CSF; the hiatus spans from ~3.8 to 5.0 Ma (see “Biostratigraphy”). A second possible hiatus occurs within lithologic Unit III at ~865 m CSF. The base of logging Unit III occurs at 935.6 m LSF. Fine-tuning of the biostratigraphy will be needed to pinpoint the position and duration of the hiatus. Our provisional pick for the unconformity is 921.73 m CSF, where a sharp color change signals a shift in carbonate and nannofossil abundance. The dominant lithology of Unit III is greenish gray, gray, and gray-brown mudstone (silty claystone). Calcareous nannofossils are abundant in the mudstone. The mud texture is finer grained than equivalent deposits in Unit II, and bioturbation is more widespread and diverse. The ichnofauna is dominated by Chrondrites, with lesser Zoophycos. Secondary lithologies are limited to sparse beds and irregular lenses of volcanic ash. Locally, zones of intense green mineralization display gradational bases and sharp tops. Green particles, which range in size from sand to gravel, are widely dispersed through these intervals. Smear slide analysis indicates that the green particles are composed of glauconite. Sharp-topped zones may be firmgrounds, with evidence of scouring and reworking of compacted clay-rich sediment. Another striking feature within Unit III is subvertical sigmoidal clay-filled "vein structure" (see “Structural geology”). Development of these dewatering structures is clearly affected by lithology and firmness; veins are spaced more widely in the softer gray and gray-brown mudstone and are more closely spaced within the harder mineralized green intervals. Interpretation of Unit IIIWe regard Unit III as a product of forearc or trench-slope deposition above the carbonate compensation depth (CCD), both prior to and during the early stages of formation of the Kumano Basin. Sediment-starved conditions were accompanied by a diverse assemblage of infauna. Localized cementation of the sediment surface (by glauconite and possibly phosphates and carbonates) was fostered by slow sedimentation and exposure to oxygenated seawater circulation. The stratigraphic occurrence of clay-filled vein structures seems to be uniquely tied to this facies. Rapid dewatering of firm mud may have been induced by shaking during earthquakes within an environment that was modulated by unusually slow rates of sedimentation. Similar structures have been documented on land and at several Ocean Drilling Program (ODP) sites (e.g., Ogawa and Miyata, 1985; Pickering et al., 1990; Lindsley-Griffin et al., 1990; Ogawa et al., 1992; Brothers et al., 1996). As discussed below, we suggest that the base of Unit III (921.73 m CSF; 935.6 m LSF) is a depositional contact between accreted trench-wedge sediment and the initial deposits of hemipelagic mud on the lowermost trench slope. Seismic reflection profiles show complicated geometries with angular discordance and contrasts in structural style across the boundary. We regard the dramatic unconformity at ~922 m CSF as a manifestation of uplift along a system of out-of-sequence (splay) faults at ~5 Ma. Whether the uplift triggered erosion of accreted strata or just very slow sedimentation above the prism cannot be resolved without higher resolution biostratigraphy. This phase of tectonic activity led to bathymetric blockage along the seaward edge of an incipient Kumano Basin, creating a large depocenter. It is noteworthy that the depositional environment remained starved of significant terrigenous influx for >3 m.y. As discussed above, delivery of silt and sand turbidites into the basin began at ~1.6 Ma, signaling the arrival of Unit II deposition. Unit IV (upper accretionary prism)
The boundary between Units III and IV is defined by a sharp change in structural style and a shift in lithofacies from condensed mudstone above to interbeds of mudstone, siltstone, and sandstone below. As defined by log character, the unit boundary is positioned at a depth of 935.6 m LSF. Beginning with Section 315-C0002B-50R-1 (929.0 m CSF), the mudstone is highly fractured. Strata below the boundary are Miocene in age and extend to the bottom of Hole C0002B at 1052.50 m CSF (Fig. F4; Table T4). Lithologic variations in the cores are not distinctive enough to warrant subdivision on the basis of texture, composition, or sedimentary structures. The dominant lithology of Unit IV is gray to greenish gray and dark gray mudstone (silty claystone to clayey siltstone). Mudstone contains local wavy laminae or bands, as defined by darker green color and higher clay content. Concentrations of calcareous nannofossils are significantly lower here than within comparable mud deposits in Unit III. Bioturbation, as characterized by Chondrites, is moderate. More typically, mudstone takes on a mottled appearance. Thin layers of siltstone are rare. Sandstone also occurs locally, and in some cases sand is cemented by calcium carbonate (Table T5). Silt and sand grains are composed mostly of quartz, feldspar, sedimentary and metasedimentary rock fragments (shale, argillite, chert, and quartz-mica grains), and a rich diversity of heavy minerals (zircon, green and yellow tourmaline, epidote, green amphiole, and blue amphibole). Interpretation of Unit IVThe depositional environment of Unit IV is difficult to interpret because of poor core recovery and a strong tectonic overprint characterized by intense fracturing, scaly fabric in mudstone, and fragmentation of sandstone beds (see “Structural geology”). Seismic reflection data indicate that the contact between Units III and IV is a boundary between the forearc basin and the accretionary prism, which means that the most likely depositional environment for Unit IV is trench wedge. Low concentrations of calcareous nannofossils point to a depositional environment below the CCD, probably near the base of the trench slope. The Quaternary trench-wedge environment of the Nankai Trough is sandy (Pickering et al., 1993; Moore, Taira, Klaus, et al., 2001). As another possibility farther outboard, the upper Shikoku Basin facies contains numerous beds of volcanic ash and virtually no sand or silt beds—at least where it has been cored in the Muroto and Ashizuri transects (Moore, Taira, Klaus, et al., 2001). We regard the top of Unit IV as a boundary between accreted trench turbidites and muddy slope-apron deposits. Resolution of biostratigraphy is not precise enough to tell whether or not the lower slope sedimentation began immediately after frontal accretion. Unlike the kindred boundary at Site C0001, the facies change from accretionary prism to muddy slope apron is sharp, but this does not necessarily require erosion along the contact. X-ray diffraction mineralogyBulk powder X-ray diffraction (XRD) results provide useful constraints on the relative abundances of total clay minerals, quartz, plagioclase, and calcite. Hole C0002B data are shown in Figure F5 and tabulated in Table T6. As a measure of how accurate these estimates are relative to absolute percentages, we completed regression analysis of percent calcite from XRD versus percent calcium carbonate from coulometric analysis (see “Organic geochemistry”). This comparison shows a shift in the coulometric data above values of ~10%, which is to be expected if the concentration of CaCO3 is expressed as a percentage of the total solid mass (weight percent) (Fig. F6). The correlation coefficient is lower than that for a comparable regression for Site C0001. Two data points on the regression plot show unusually large discrepancies, which we suspect are due to operator errors in recording sample codes for XRD. Among the major minerals, values of calcite (XRD) show the greatest amount of variation at Site C0002, ranging from 0.1% (trace) to 28.2%. Average values are 2.1% for Unit II, 16.5% for Unit III, and 1.9% for Unit IV. The abrupt compositional shift across the Unit III/IV boundary is noteworthy (Fig. F5) and consistent with rapid uplift of the depositional site from below the CCD (Unit IV) to above the CCD (Unit III). The same compositional trend is evident, but more gradual, at ODP Sites 1175 and 1176 (Shipboard Scientific Party, 2001a, 2001b; Underwood et al., 2003). The relatively low concentration of calcite within Unit II is probably a consequence of dilution of biogenic carbonate by high influxes of terrigenous silt and clay into the Kumano Basin. Percentages of plagioclase at Site C0002 vary between 13.4% and 34.9%. Unit II contains an average of 23.2% plagioclase, whereas Unit III contains an average of 18.1% and Unit IV contains 17.3%. Relative concentrations of total clay minerals are between 30.9% and 65.3%, with a significant decrease in Unit III. Quartz content varies between 13.2% and 32.7%. Unit II contains an average of 27.8% quartz. Units III and IV average 20.6% and 21.5% quartz, respectively. |
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