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doi:10.2204/iodp.proc.319.103.2010 LithologyBased on a range of data available from cuttings and core and wireline logging, we defined four lithologic units at Site C0009 that were differentiated using geological, geophysical, and geochemical characteristics (Fig. F13). Unit boundaries and lithologies are constrained by
We also compared cuttings and cores in the cored interval from ~1510 to ~1594 m core depth below seafloor (CSF) to quantify possible artifacts and uncertainties related to the use of cuttings as stratigraphic indicators. At Site C0009, the cuttings show a progressive change in the degree of cohesion with depth (see LITHOLOGY in "Supplementary material"). Between 707.7 (where the first cuttings were retrieved) and 812.7 m mud depth below seafloor (MSF), the cuttings were noncohesive, and individual grains were dispersed in the drilling mud. At 812.7 m MSF, the first relatively coherent chips of sediment were identified, and we interpret this to approximately mark the change from silty mud to silty mudstone in the stratigraphic section, although the chips were still relatively soft and might normally be classified as mud rather than mudstone. At 1037.7 m MSF, the sediment chips were noticeably harder and more lithified and appeared more like silty mudstones. We therefore consider the fine-grained material in the wall rock below 812.7 m MSF to be silty mudstone, although locally partially disaggregated by drilling. Limitations using cuttingsDrilling and drilling mud circulation, including the retrieval of cuttings, significantly disturbs and mixes the formation in the vicinity of the drill bit; thus lithologic variations that are normally observed and documented in cores cannot be recognized. For example, drilling typically disaggregates partially lithified sediments, generally making it impossible to differentiate muds from mudstones or sands from sandstones. Spalling or collapse of wall rock into the drilling mud, producing "cavings," also results in vertical mixing of lithologies that makes it difficult to accurately reconstruct stratigraphic relations. Finally, infiltration of drilling fluid into high-porosity cuttings and precipitation of spurious solid phases significantly modifies the physical properties of the cutting chips. The unit boundaries described below are defined primarily from log data, which were determined to be more accurate than depths of cuttings intervals. However, consistency between the two data sets (i.e., boundaries from cuttings and log data) is good in terms of depth, with typical mismatches of less than ~10 m. Lithologic Unit I
The initial 54.5 m DSF was drilled and cased to set the conductor pipe. Below this, a 26 inch hole was drilled to 707.7 m LSF in riserless mode. No cuttings were recovered in this interval and no wireline logging data are available from this interval. Recorded parameters included weight on bit, rate of penetration (ROP), and gamma ray data, the latter from MWD. Lithologic interpretation was based on the gamma ray logs in this interval. Unit I is composed of silty mud with abundant sand-rich beds ranging up to 40 m in thickness. In the upper part of the unit (above ~360 m LSF), gamma ray values gradually decrease upsection and are capped by an abrupt increase, suggesting seven to eight sequences with increasing sand fractions uphole. High gamma ray values >150 gAPI (American Petroleum Institute units) at 233 and 244 m LSF suggest the presence of ash layers or very clay rich layers. Between 360 and 467 m LSF, some low gamma ray zones record the presence of sand, but these are either less abundant or thinner relative to the overlying section. In addition, the cycles in this interval appear to show upward increases in the proportion of mud (i.e., the gamma ray values gradually increase upward and then are capped by an abrupt decrease). Gamma ray values range between 50 and ~110 gAPI, with some cycles of decreasing sand fractions uphole and fewer cycles of increasing sand fractions uphole, though these cycles are shorter and more muted than those in the upper part of the hole. The most prominent feature is a substantial excursion to low gamma ray values from 454 to 467 m LSF; here, the lowest values in the logged interval were recorded (30 gAPI). This excursion is consistent with a sandy interval: sharp at the base and fining upward, which marks the Unit I/II boundary. Lithologic Unit II
Unit II is composed dominantly of silty clay, with silt and sand interbeds and minor interbeds of volcanic ash. It is distinguished from Unit I by a distinct drop in gamma ray values. Below 467 m LSF, there is a variable but generally low gamma ray interval, suggesting continuing occurrence of sand layers. The log has little character and is interpreted to be similar to the upper unit. Cuttings, which start at 707.7 m MSF, comprise coarse to fine sand grains dispersed in the drilling mud, with a few weakly consolidated clayey agglomerates (Figs. F14, F15, F16). Drilling contamination is high in this area. The typical grain-size distribution in the cuttings based on the Atterberg method is given by representative Sample 319-C0009A-23-SMW with ~81% sand, 9% silt, and 10% clay fractions. However, the sand fraction may be overestimated by this method and/or by effects from the riser drilling process. Sand- and silt-sized grains (45 µm–1 mm) are angular to rounded and moderately well sorted. Quartz is the dominant mineral, followed by lithic fragments (mostly metamorphic, some sedimentary), partly reworked volcanic glass and pumice, feldspar, partly broken shallow-water and planktonic fossils, rare wood/lignite fragments, mica, and heavy minerals (e.g., pyrite, amphibole, apatite, and zircon). In the interval from 712.7 to 715.7 m MSF (Sample 319-C0009A-3-SMW), the amount of volcanic glass markedly increases and likely corresponds to local ash-rich beds. Detailed observations on cuttings from all units are summarized in C0009_T1.XLS and C0009_T2.XLS in LITHOLOGY in "Supplementary material" (see also Figs. F17, F18). Unit II is characterized by high density (up to 2.2 kg/cm3) and P-wave velocity (up to 2300 m/s) compared to Unit III below (Fig. F19). SP increases slightly through the unit up to –110 mV, and resistivity values range from 1.2 to slightly over 1.4 Ωm. Gamma ray values increase slightly with depth from 75 to >80 gAPI, whereas PEF decreases steadily from 3.2 to <2.6 b/e–. The change in character of the gamma ray log at >100 m LSF coincides with the change from riserless drilling above to riser drilling with mud below. We do not know whether the change in log character reflects a change in lithology or drilling parameters and/or log processing. Lithologic Unit III
Unit III is composed of abundant silty clay and poorly lithified silty claystone, with interbeds of silt and fine sand layers. It is distinguished from Unit II by generally having (Figs. F17, F19)
The Unit II/III boundary is defined by localized occurrence of silt-rich samples at ~802.7 to ~812.7 m MSF (Samples 319-C0009A-25-SMW and 26-SMW), change in cohesion of the cuttings, reduced sorting of the sand fraction (poorly sorted) in the upper part of Unit III, which then progressively increases again downhole, and sharp peaks and shifts in logging data values (see below). Cuttings recovered from Unit III consist mainly of fine sand, silt, clay, and soft chips of silty clay. These chips are progressively more lithified downhole (Fig. F17). Differences in mineral and organic composition (see below) allow further subdivision into two lithologic subunits. Log data in Unit III also display differences from Unit II (Fig. F19). The Unit II/III boundary is sharp and marked by distinct decreases in P-wave velocity, density, and PEF and an increase in SP. PEF values are particularly high (>3 b/e–) in contrast to 2.5 b/e– in Unit II. This difference suggests a lithologic change, such as increasing silt content, and is consistent with cuttings characteristics (see below). Additionally, an increasing proportion of mud is suggested by a gradual increase of gamma ray values through most of Unit III. The largest variation in P-wave velocity is observed between 1010 and 1285 m WMSF and corresponds approximately to Subunit IIIB (see below). This region, which also correlates with slightly higher density and resistivity, is coincident with the highest mud gas concentrations and the largest and most abundant wood/lignite fragments in cuttings samples (see below and "Geochemistry"). Subunit IIIA
Subunit IIIA extends from ~802.7 to ~1037.7 m MSF, with a typical grain size distribution given by representative Sample 319-C0009A-46-SMW with ~17% sand, 76% silt, and 7% clay fractions. It is composed dominantly of silty clay, with rare interbeds of silt and fine sand and minor interbeds of fine ash. The silt/sand fraction contains subangular-to-rounded grains that are moderately well sorted (Figs. F14, F17). The composition of these grains is broadly similar to Unit II (i.e., quartz is the dominant component, followed by lithic fragments, partly reworked volcanic glass and pumice, feldspar, partly broken fossils, wood/lignite fragments, and heavy minerals; Fig. F15). Rounded glauconite grains complement the grain assemblage from ~887.7 to 892.7 m MSF (Sample 319-C0009A-43-SMW) and are an additional distinctive feature of Subunit IIIA. A similar increase in glauconite was detected at Site C0002 in Unit III. Partly reworked fossils are common and most are replaced by pyrite (Figs. F14, F15, F16). Subunit IIIB
The Subunit IIIA/IIIB boundary at ~1037.7–1042.7 m MSF (Sample 319-C0009A-75-SMW is marked by an abrupt increase in wood/lignite fragments and rounded glauconite (Figs. F14, F15, F16, F17, F18). The wood/lignite fragment abundance remains relatively high throughout Subunit IIIB, with typical sizes of ~2 mm and some fragments as large as ~15 mm. This component defines Subunit IIIB relative to Subunit IIIA. The increased wood/lignite abundance correlates with high TOC and CH4 observed in cuttings and mud gas, respectively (see "Geochemistry"). Other detrital grains do not differ significantly in geometry, size, or composition from those of Subunit IIIA, pointing toward broadly similar lithologic assemblages in these two units (Figs. F14, F15, F16, F17, F18). The typical grain-size composition of the bulk cuttings from Subunit IIIB is given by the representative Sample 319-C0009A-85-SMW with ~35% sand, 41% silt, and 24% clay fractions. At the Subunit IIIA/IIIB boundary, the silty clay chips (cuttings) are sufficiently indurated to allow separation from drilling mud by washing, and the chips can be prepared for XRD, XRF, and other analyses (see the "Methods" chapter). Relative abundance of total clay minerals, quartz, feldspars, and calcite were obtained by XRD measurements (Fig. F20). Pyrite, glauconite, halite, and organic matter were also detected in XRD patterns, but their abundances were not determined. Quartz and feldspar content remains fairly constant throughout Subunit IIIB, with values of 22–28 wt% and 11–18 wt%, respectively (average values = 25 wt% and 14 wt%, respectively). Calcite shows modest variations (11–20 wt%) that may be related to variable calcite contamination by drilling mud. However, similar values have been detected at Site C0002 in Unit III. Total clay minerals are 41–50 wt% and show an inverse correlation to calcite abundance. Major and minor element contents (SiO2, Al2O3, TiO2, Na2O, MgO, CaO, K2O, Fe2O3, MnO, and P2O5) were analyzed by XRF and complemented by loss on ignition (LOI) measurements. In many samples, chemical composition was relatively low, and LOI was particularly high; thus, XRF values were recalculated without the LOI component (LOI-free) to remove any bias linked to high abundance of organic matter and water content (Fig. F21). The cuttings may have been contaminated by calcite precipitation because of a high pH (pH = ~10) in the drilling mud, as suggested by higher calcite values. However, as pH stayed mostly constantly high throughout drilling, pH does not appear to explain the fluctuations in calcite content (see "Geochemistry"). LOI-free CaO content downsection is poorly correlated to log data (Figs. F19, F20, F21), divergence of XRD and XRF values between core and cuttings samples in the cored interval (see discussion in "Lithologic Unit IV"), and comparison with potentially equivalent units at Site C0002 (Expedition 315 Scientists, 2009). The LOI content in Subunit IIIB is high (up to 24 wt%) and correlates with increased wood/lignite abundance and gas content in the drilling mud (see "Geochemistry"). Fe2O3 and MnO contents display a trend similar to LOI content, which may be related to increased abundance of glauconite and some heavy mineral grains. Both XRD and XRF (LOI-free) analyses indicate relatively constant quartz/SiO2 content in Subunit IIIB. Compositional variation is also observed in log data between ~1168 and ~1297.7 m MSF (e.g., gamma ray, SP, and PEF) (Figs. F19, F20). Lithologic Unit IV
Unit IV is composed dominantly of silty claystone, with common silt interbeds, few poorly consolidated sand layers, and rare interbeds of fine vitric tuff. It is distinguished from Unit III by
In addition, there are structural features preserved in the cuttings of Unit IV that are not observed in Unit III (see "Structural geology"). At the Unit III/IV boundary, we also observed changes in the composition of cuttings measured by XRD and XRF; an increase in P-wave velocity; and a decrease in bulk density, resistivity, and SP values from wireline logs (Figs. F19, F20, F21). The Unit III/IV boundary is marked by an ~1.8 Ma hiatus (3.8–5.6 Ma). Observations from coresIn Unit IV, nine cores were recovered from 1509.7 to 1593.9 m CSF. Lithologies include brown-gray silty claystone, minor interbeds of brown-gray siltstone-sandstone, and minor interbeds of light gray fine vitric tuff. Different combinations of these lithologies in the cores define four lithofacies (Fig. F22) that are representative of Unit IV based on comparison with cuttings and log data. Lithification of the sediments decreases with increasing grain size (i.e., sandstones are less consolidated than silty claystones). The clayey and silty sediments of Unit IV are generally moderately to highly bioturbated (Fig. F23). Ichnospecies include widespread Chondrites and rare Zoophycos, indicating deepwater environments. Siltstone-sandstone beds are grain supported and show common normal grading and rare inverse grading. Parallel laminae and basal erosional contacts are common in sandy siltstone beds (Figs. F16, F23). Average thicknesses of siltstone-sandstone beds are ~5 cm in Cores 319-C0009A-3R through 8R and ~20 cm in Core 319-C0009A-9R, correlating with a slight grain size change downhole (i.e., finer grained sediments in Cores 319-C0009A-3R through 8R relative to Core 319-C0009A-9R). Cross-laminae are common in the coarsest sediments (i.e., silty sandstone-sandstone) and are mostly encountered in Core 319-C0009A-9R (Fig. F23). Interbeds of fine vitric ash are typically <2 cm thick and contain ~100% subangular-to-angular volcanic glass fragments with minor amounts of planktonic fauna (e.g., radiolarians). Increases in tuff content occur between ~1579.5 and ~1582.0 m CSF (Core 319-C0009A-8R) and possibly between ~1509.7 and ~1520.0 m CSF (Core 319-C0009A-1R). Some of the tuff beds display spectacular convolute bedding (Fig. F23). In comparison to Unit IV of Site C0002, which shows highly fractured mudstone, the rocks from Site C0009 are only moderately fractured. Bioturbation and mineralogy on the other hand, are similar between the two sites. Microscopic observations and XRD and XRF analyses provide constraints on the composition of cored sediments (Figs. F20, F21) (see below). Silty claystones are predominantly composed of a clay-sized, mostly carbonate-bearing argillaceous matrix incorporating minor amounts of quartz, feldspar, volcanic glass, planktonic fossils, coal, and accessory minerals (e.g., iron hydroxides, iron sulfides, and zircon). Some silty claystones also include a significant component of volcanic ash and can be described as tuffite. Siltstones-sandstones have a matrix composition similar to the silty claystones, with higher contents of quartz and feldspar. XRD values show in general typical differences in sand, silt, and clay-rich sections; however, the values are in general lower than in cuttings. XRD values from Site C0002 show an abrupt compositional shift in calcium between Unit III and Unit IV toward very low values. These low values in calcite content could not be observed in the core samples at Site C0009. Observations from cuttingsCuttings from Unit IV comprise fine sandy-to-silty grains and abundant lithified silty claystone fragments (up to ~2 cm in size) (Figs. F14, F15, F16). Increased lithification occurs between 1322.7 and 1372.7 m MSF (Samples 319-C0009A-135-SMW through 144-SMW). The typical distribution in terms of grain size from representative Samples 319-C0009A-123-SMW and 129-SMW is ~29%–38% sand, 38%–52% silt, and 20% clay fraction. Grains of the silty/sandy fraction are angular to subrounded and moderately well sorted (Figs. F14, F15, F16). Cuttings indicate no major lithologic variation within Unit IV. Quartz is dominant, followed by feldspar, partly reworked volcanic glass fragments, heavy minerals (similar in composition to Units II and III), glauconite, and coal. Wood/lignite fragments are rare, whereas partly reworked planktonic fossils and shell fragments are common. The occurrence of wood/lignite fragments in samples from the upper part of Unit IV points toward mixing with cavings from units above or material plucked from the formation by drilling mud as it traveled up the annulus above the bit (Figs. F14, F15, F16, F17). Hence, other grains such as lithoclasts observed in Unit IV samples may also be the result of contamination from units above. We note that comparisons of cuttings and core samples in this unit is difficult, as washing cuttings with fresh deionized water (see "Introduction" and "Lithology" in the "Methods" chapter) may have affected the chemical composition of the cuttings. Mineralogical and geochemical analyses (XRD and XRF)Mineral abundances and bulk compositions determined by XRD and XRF in core samples show distinct variations between different lithologies (Figs. F20, F21). For example, quartz content is low in tuff layers (14 wt%), higher in silty claystones (18–36 wt%), and highest in sandstones and siltstones (23–54 wt%). XRD and XRF analyses from cuttings are more homogeneous than from core samples probably because of preferential preservation of fine-grained (better-consolidated) sediments in the drilling mud with respect to coarse-grained (less-consolidated) sediments. This interpretation is further supported by similarities between samples of silty sandstone in cores and cuttings samples in terms of quartz, feldspar, and clay minerals abundances, as well as SiO2, Al2O3, TiO2, MgO, K2O, Fe2O3, MnO, and P2O5 (LOI-free) contents (Figs. F20, F21). Calcite contamination of cuttings samples by drilling mud is supported by
Clay mineral abundance determined by XRD of cuttings is higher in Unit IV than in Unit III, and quartz and feldspar abundances are reduced relative to Unit III. These values remain relatively constant throughout Unit IV. Other measures of bulk composition (XRD and XRF) of Unit IV and Subunit IIIB are also distinct (Figs. F20, F21). Observations from log dataUnit IV is characterized by a sharp increase in P-wave velocity; increased variability in caliper; and sharp decreases in SP, resistivity, and bulk density relative to Unit III (Fig. F19). Other logging data exhibit more gradual changes across the Unit III/IV boundary. Density decreases from 2.1 to 1.9 kg/cm3 and PEF increases from 2.8 to >3.2 b/e– at the boundary. Gamma ray values also increase slightly from 75 to >80 gAPI. Increased gamma ray values through this zone are consistent with the higher clay content. The increased P-wave velocity may also be related to decreased gas content below the Unit III/IV boundary. Preliminary analyses of clay mineralogyFive samples were selected from washed rock cuttings for more detailed preliminary investigation of the clay minerals illite, illite-smectite, and smectite (792.7, 902.7, 1087.7, 1267.7, and 1297.7 m MSF; Fig. F24). We separated two grain-size fractions: <2 µm and 2–64 µm. The samples contain abundant smectite, illite, and chlorite ± kaolinite. High d-spacings (~16–18 Å) were detected in all (air-dried) samples, which may be due to rehydration of smectite in drilling mud and surface exposure effects. Higher d-spacing (~17.6–18 Å) in the clay-size fraction than those in the coarser fraction (~16.7 Å) may reflect different interlayer cations in expandable clay minerals and/or different abundances of detrital and authigenic minerals. The smectite (001) peak area and peak intensity increase with depth, reflecting an increase in smectite abundance relative to other clay minerals. Illite (001) peaks slightly broaden at ~10 Å (full widths half maximum 0.2–0.3), possibly indicating reduced crystallinity with increasing depth. The different intensity of the different grain sizes is probably a crystal size effect. These changes indicate that detrital clay minerals have not yet undergone diagenetic transformation, although authigenic reactions are possible (e.g., smectite replacing dispersed volcanic glass). A more detailed investigation of the relative amount and types of illite and smectite minerals is necessary to determine the actual changes with depth. Interpretation of drilled sequencesOverall, the stratigraphic succession is interpreted as a series of mudstones, with varying sand and silt turbidite abundance filling the forearc basin and potentially prism slope basins and/or the trench. We interpret Unit I as a turbidite-rich unit deposited in the Kumano Basin that is significantly sandier than units drilled at Site C0002 (Expedition 315 Scientists, 2009) (Fig. F13). Unit II is also interpreted as a basin-fill unit deposited in the Kumano forearc basin. It is characterized by turbidites that are coarser than those in underlying Units III and IV and the units drilled at Site C0002 but markedly thinner and finer grained than those in Unit I above. Composition of detrital grains points toward sediment supply from erosion of exposed sedimentary and metasedimentary rock units within the Outer Zone of Japan, including the Shimanto Belt (e.g., Taira et al., 1988; Isozaki and Itaya, 1990). Unit III includes two subunits containing thinly bedded fine-grained turbidites (Subunits IIIA and IIIB) that were deposited above the carbonate compensation depth (CCD) and interpreted as basal Kumano forearc basin deposits. The lower subunit (IIIB) apparently had an increased supply of terrigenous organic matter (i.e., wood fragments), possibly because of a change in climate and/or topographic relief. As for Units I and II above, the detrital sediment composition of Unit III at Site C0009 indicates that the sediments were partly sourced from exposed sediments and metasediments of the Outer Zone of Japan. We interpret the fine-grained (~3.8–3.3 Ma) sediments at Subunit IIIB at Site C0009 (1168–1285 m MSF interval) as a lateral equivalent of (~3.8–3.3 Ma) Unit III at Site C0002 with a similar increase of glauconite and the occurrence of pyritized fossils. However, occurrence of wood/lignite fragments and the increased gas content (see "Geochemistry") is very different. Unit IV is a mudstone also containing thin-bedded fine-grained turbidites and was deposited above the CCD. It resembles Unit IV at Site C0002 (Expedition 315 Scientists, 2009) in terms of sedimentary facies. However, sedimentary, geochemical, or structural evidence is lacking to clearly suggest that this unit was accreted in a trench setting. Thus, Unit IV could be interpreted as a weakly deformed package of accreted trench sediments, trench-slope deposits, or sediments deposited in the distal reaches of the early Kumano Basin. The long hiatus between deposition of Units IV and III at Site C0009 (from ~5.6 to 3.8 Ma) is similar to the hiatus between Units III and IV at Site C0002 (from ~5.1 to 3.7 Ma) (Expedition 315 Scientists, 2009), suggesting a tectonic event of regional significance in this interval. |
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